Ceramic Materials edited by Wilfried WunderlichSCIYOCeramic Materials Edited by Wilfried WunderlichPublished by SciyoJaneza Trdine 9, 51000 Rijeka, CroatiaCopyright 2010 SciyoAll chapters are Open Access articles distributed under the Creative Commons Non Commercial Share Alike Attribution 3.0 license, which permits to copy, distribute, transmit, and adapt the work in any medium, so long as the original work is properly cited. After this work has been published by Sciyo, authors have the right to republish it, in whole or part, in any publication of which they are the author, and to make other personal use of the work. Any republication, referencing or personal use of the work must explicitly identify the original source.Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those ofthe editors or publisher. No responsibility is accepted for the accuracy of information contained in thepublished articles. The publisher assumes no responsibility for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained in the book. Publishing Process Manager Ana NikolicTechnical Editor Sonja MujacicCover Designer Martina SiroticImage Copyright Noam Armonn, 2010. Used under license from Shutterstock.comFirst published September 2010Printed in IndiaA free online edition of this book is available at www.sciyo.comAdditional hard copies can be obtained from publication@sciyo.comCeramic Materials, Edited by Wilfried Wunderlichp.cm. ISBN 978-953-307-145-9 SCIYO.COMWHERE KNOWLEDGE IS FREEfree online editions of Sciyo Books, Journals and Videos can be found at www.sciyo.comChapter 1Chapter 2Chapter 3Chapter 4Chapter 5Chapter 6Chapter 7Chapter 8Chapter 9PrefaceVIIDevelopment of Thermoelectric materials based onNaTaO3 - composite ceramics1Wilfried Wunderlich and Bernd BaufeldGlass-Ceramics Containing Nano-Crystallites of Oxide Semiconductor29Hirokazu Masai, Yoshihiro Takahashi and Takumi FujiwaraTape Casting Ceramics for high temperatureFuel Cell applications49Alain S.ThorelAlkoxide Molecular Precursors for Nanomaterials:A One Step Strategy for Oxide Ceramics69ukasz John and Piotr SobotaNew ceramic microfltration membranes from Tunisian naturalmaterials: Application for the cuttlefsh effuents treatment87Sabeur Khemakhem, Andr Larbot, Raja Ben AmarElectron microscopy and microanalysis of the fber, matrix and fber/matrix interface in sic based ceramic composite material for use in a fusion reactor application99Tea Toplisek, Goran Drazic, Vilibald Bukosek, Sasa Novak and Spomenka KobeMechanical Properties of Ceramics by Indentation:Principle and Applications115Didier Chicot and Arnaud TricoteauxCeramic Materials and Color in Dentistry155Cludia ngela Maziero Volpato, Mrcio Celso Fredel Analcia Gebler Philippi and Carlos Otvio Petter Surface quality controls mechanical strength and fatiguelifetime of dental ceramics and resin composites175Ulrich Lohbauer, Roland Frankenberger and Norbert KrmerContentsVIChapter 10Chapter 11Re-use of ceramic wastes in construction197Andrs Juan, Csar Medina, M. Ignacio Guerra, Julia M. Morn,Pedro J. Aguado, M. Isabel Snchez de Rojas, Moiss Fras and Olga RodrguezCeramic Products from Waste215Andr ZimmerCeramicmaterialsisthetitleofthisbook,whichdescribesthestate-of-the-artofsome aspectsinthislargefeldinengineeringmaterials.Byinvitationofthepublisher,several authorsfromtencountries,mostofthemdonotknoweachother,havecollectedabunch of chapters which cover a wide area of engineering science. The frst three chapters describe thefundamentalaspectsoffunctionalceramicsforthermoelectric,semiconductorandfuel cellapplications.Chapters4,5and6describetheprocessingofnano-ceramicsandtheir characterisation.Thefollowingchaptersdescribestructuralceramics;chapter7describesa new hardness characterisation method for thin flms, and chapters 8 and 9 describe ceramic materials for dental applications. Finally, chapters 10 and 11 describe the re-use of ceramics for new structural applications. This is the frst book of a series of forthcoming publications on this feld by Sciyo publisher. The reader can enjoy both a classical printed version on demand for a small charge, as well as the online version free for download. Your citation decides about the acceptance, distribution, and impact of this piece of knowledge. Please enjoy reading and may this book help promote the progress in ceramic development for better life on earth.EditorProf.Dr. Wilfried Wunderlich Tokai University, Dept. Mat.Sci., JapanPrefaceDevelopment of Thermoelectric materials based on NaTaO3 - composite ceramics 1Development of Thermoelectric materials based on NaTaO3 - composite ceramicsWilfried Wunderlich and Bernd Baufeldx Development of Thermoelectric materialsbased on NaTaO3 - composite ceramics Wilfried Wunderlich1 and Bernd Baufeld2 1 Tokai University, Dept. Material Science., Kitakaname 1117, Hiratsuka-shi, Japan 2 Kath. Universiteit Leuven, Dpt MTM Metallurgy and Ma. Eng., Leuven, Belgium 1. IntroductionThischapterdescribesthedevelopmentofnovelthermoelectricmaterialsforhigh-temperatureapplicationslikegasburners,combustionengines,nuclearfuel,orfurnaces. Thegoalofthisdevelopmentistorecyclewasteheatforenergyharvestinginorderto contributeinsavingtheenvironment.Theresearchresultsaredescribedinthefollowing sub-chapters in four different sections.AfterageneralreviewaboutperovskitesandNaTaO3insection2,ab-initio-simulationsof theSeebeckcoefficientaredescribedinsection3.TheSeebeckcoefficientstronglydepends ontheeffectivemassandcarrierconcentration.Theelectronicband-structurecalculations showed a large electron effective mass for NaTaO3. Heavily doping changes NaTaO3s band-structureinasimilarwayasthewell-knownthermoelectricmaterialNb-dopedSrTiO3. Hence, NaTaO3, which is stable up 2083 K and which is known as a material with excellent photo-catalytic properties, was chosen as a candidate for thermoelectric materials.Section4describesthefindingofsuitabledopingelementsbysinteringNaTaO3with differentrawmaterials.Whileboth,pureNaTaO3andNaTaO3sinteredwithFe2O3,are almost insulators, it was discovered that sintering with metallic iron increases both, electric conductivityandSeebeckcoefficient.MicrostructuralcharacterizationbySEMandXRD measurementsshowedthataNaTaO3-Fe2O3compositematerialisformed.Theamountof FesolvedintheNaTaO3latticeismuchhigherwhenthestartingmaterialsconsistofFe instead of Fe2O3. Addition of several metals like Mn, Cr, Ti, Ni, Cu, Mo, W, Fe, and Ag were tested,butonlythelatertwoelementsleadtoremarkableelectricconductivityobserved above 773 K. Section 5 describes the measurement of thermoelectric properties such as Seebeck-voltage at a large temperature gradient, a method which is close to applications, but not yet commonly used,becausethethermoelectrictheoryisbasedonsmalltemperaturegradients.Thermal conductivityisnotmeasured,butonlyestimated.Thedopingisachievedbysintering metallic iron or silver together with NaTaO3. The results are high negative Seebeck voltages up to -320 mV at a temperature difference of 700 K, as well as high closed-circuit currents up to-250AforFe-dopingandpositivevaluesforAg-doping.Besidesreportingprevious results, several new findings are described here for the first time. Composites with Cu yield 1Ceramic Materials2 to a small Seebeck voltage of about -10 mV with a strong response, when heat flow direction is reversed.In section 6 the thermokinetic measurement by differential scanning calorimetry (DSC) and thermoanalysis(TA)clarifiesthereactionsinteringbetweenFeandNaTaO3.The experimentaldataobtainedatdifferentheatingrateswereanalyzedbyFriedmananalysis andshowedacharacteristicshapeintheplotofenergyversuspartialarea.Further directionsofimprovement,likeimprovingthedensificationbysintering,arementionedin the last section under discussions. 2. Perovskite structure 2.1 Functional Engineering Materials based on Perovskite Crystal structure The goal of this book chapter is to describe the development of new thermoelectric materials (TE),whosemostimportantfeaturesaredescribedfirst.Thentheperovskitestructureis reviewed, before focusing on the main topic, NaTaO3.Successfulthermoelectricshavetobesemiconductors[Sommerlateetal.2007,Nolasetal. 2001,Ryan&Fleur2002,Bulusuetal.2008],sotherearetwopossibleapproachesinTE development,onefromtheceramicside,whichhavelargeSeebeckcoefficients,andone fromthemetalside,whichhavelargeelectricconductivity,butaratherpoorSeebeck coefficient. The main goal of development for ceramics, which are the focus in this book, is theimprovementoftheelectricconductivity.TheengineeringtargetsofsuchTE-ceramics areapplicationsinanycombustionengines,gasturbines,powerplantsincludingnuclear power plants, furnaces, heaters, burners or in combination with solar cells or solar heaters as illustrated in fig. 1. Fig.1.Possibleapplicationsforhigh-temperaturethermoelectricceramics(inbluecolor)in solar cells, solar heaters, combustion engines or gas turbines. Theservicetemperaturesofsuchdevicesareusuallytoohighastobeapplicableforother TEmaterials.Thetemperaturedifference[Ryan&Fleur2002]betweenthehotchamber insideandthe(cold)ambientenvironmentisconsideredastheenergysourceforthese energy conversion devices, which have a long life time and low maintenance costs, because there are no rotating parts. The main advantage is that any waste heat can be converted into electricity.Hence,advancedthermoelectricsareboth,environment-friendlyeco-materials andenergymaterials,whichmainpurposeisproducingenergy.Forawiderangeof applications, materials with higher energy conversion efficiency than present TEs need to be found,inordertobeconsideredascleanenergysourceshelpingtosolvethesevereCO2- problem. One important indicator for efficient thermoelectric material is the figure-of-merit ZT ZT=S2T/(1) whichshouldhaveavaluesignificantlylargerthan1tobeeconomicallyreasonable. Improvement ofZTrequires ahighSeebeckcoefficientSandelectricconductivity anda lowthermalconductivity.ForincreasingZTseveralconceptsformaterialsdesignof thermoelectrics have been introduced [Nolas et al. 2001, Ryan&Fleur 2002, Bulusu et al.2008, Wunderlichetal.2009-c].Thesearephonon-glasselectron-crystal(PGEC)[Terasakiet al.1997],heavyrattlingatomsasphononabsorbers,propercarrierconcentration[Vining 1991, Wunderlich et al.2006], differential temperature dependence of density of states, high density of states at the Fermi energy, high effective electron mass [Wunderlich et al. 2009-a], superlattice structures with their confined two-dimensional electron gas [Bulusu et al. 2008, Ohtaetal.2007,Vashaee&Shakouri2004],andelectron-phononcoupling[Sjaksteetal. 2007]. As all these factors can influence also the material focused in this chapter NaTaO3, at firstbasicprinciplesofthePervoskitecrystalstructurearebrieflyreviewed,asthis interdisciplinaryapproachissupposedtogainimportantunderstandingforfuture improvement.The interest on Perovskite structure related materials has dramatically increased in the past threedecadesafterthediscoveryofmanysuperiorsolid-stateproperties,whichmakes Perovskite materials or their layered derivatives record holders in many fields of solid state physics as shown in fig. 2. The most popular finding was the discovery of superconductivity inY1Ba2C3O7-x(YBCO)forwhichtheNobelPrize1987wasprovided.Thepresentrecord holderisBi2212withacriticaltemperatureofTC=120K.AlargescaleapplicationofYBCO since 1998 is the linear motor train using the magnetic levitation (Maglev) in Yamanashi-ken Japan, whose entire rail consists of Helium-cooled superconductors. Present portable phone technologyisallbased on layered(Ba,Sr)TiO3dielectricmaterial[Ohsato2001,Wunderlich etal.2000]duetotheirhighdielectricconstant(e>10000)andqualityfactor.Duringthe materials development detailed spectroscopic data of the electromagnetic resonance [Bobnar et al. 2002, Lichtenberg et al. 2001] have been measured, which further analysis can provide moreunderstandingofelectron-phononinteractionsasoneofthekeyissuefor thermoelectricsbasedonperovskites.PiezoelectricmaterialsonPb(Ti1-xZrx)O3(PZT)orthe environmentalbenignleadfreeK0.5Na0.5NbO3 (KNN)materials[Stegketal.2009]havean increasing application demand in actuators and sensors. Fig.2.AsPerovskite-structurebasedmate-rialsarerecordholdersinmanysolid-state properties, they might become so in thermoelectrics too. Development of Thermoelectric materials based on NaTaO3 - composite ceramics 3 to a small Seebeck voltage of about -10 mV with a strong response, when heat flow direction is reversed.In section 6 the thermokinetic measurement by differential scanning calorimetry (DSC) and thermoanalysis(TA)clarifiesthereactionsinteringbetweenFeandNaTaO3.The experimentaldataobtainedatdifferentheatingrateswereanalyzedbyFriedmananalysis andshowedacharacteristicshapeintheplotofenergyversuspartialarea.Further directionsofimprovement,likeimprovingthedensificationbysintering,arementionedin the last section under discussions. 2. Perovskite structure 2.1 Functional Engineering Materials based on Perovskite Crystal structure The goal of this book chapter is to describe the development of new thermoelectric materials (TE),whosemostimportantfeaturesaredescribedfirst.Thentheperovskitestructureis reviewed, before focusing on the main topic, NaTaO3.Successfulthermoelectricshavetobesemiconductors[Sommerlateetal.2007,Nolasetal. 2001,Ryan&Fleur2002,Bulusuetal.2008],sotherearetwopossibleapproachesinTE development,onefromtheceramicside,whichhavelargeSeebeckcoefficients,andone fromthemetalside,whichhavelargeelectricconductivity,butaratherpoorSeebeck coefficient. The main goal of development for ceramics, which are the focus in this book, is theimprovementoftheelectricconductivity.TheengineeringtargetsofsuchTE-ceramics areapplicationsinanycombustionengines,gasturbines,powerplantsincludingnuclear power plants, furnaces, heaters, burners or in combination with solar cells or solar heaters as illustrated in fig. 1. Fig.1.Possibleapplicationsforhigh-temperaturethermoelectricceramics(inbluecolor)in solar cells, solar heaters, combustion engines or gas turbines. Theservicetemperaturesofsuchdevicesareusuallytoohighastobeapplicableforother TEmaterials.Thetemperaturedifference[Ryan&Fleur2002]betweenthehotchamber insideandthe(cold)ambientenvironmentisconsideredastheenergysourceforthese energy conversion devices, which have a long life time and low maintenance costs, because there are no rotating parts. The main advantage is that any waste heat can be converted into electricity.Hence,advancedthermoelectricsareboth,environment-friendlyeco-materials andenergymaterials,whichmainpurposeisproducingenergy.Forawiderangeof applications, materials with higher energy conversion efficiency than present TEs need to be found,inordertobeconsideredascleanenergysourceshelpingtosolvethesevereCO2- problem. One important indicator for efficient thermoelectric material is the figure-of-merit ZT ZT=S2T/(1) whichshouldhaveavaluesignificantlylargerthan1tobeeconomicallyreasonable. Improvement of ZTrequires ahighSeebeckcoefficientSandelectricconductivity anda lowthermalconductivity.ForincreasingZTseveralconceptsformaterialsdesignof thermoelectrics have been introduced [Nolas et al. 2001, Ryan&Fleur 2002, Bulusu et al.2008, Wunderlichetal.2009-c].Thesearephonon-glasselectron-crystal(PGEC)[Terasakiet al.1997],heavyrattlingatomsasphononabsorbers,propercarrierconcentration[Vining 1991, Wunderlich et al.2006], differential temperature dependence of density of states, high density of states at the Fermi energy, high effective electron mass [Wunderlich et al. 2009-a], superlattice structures with their confined two-dimensional electron gas [Bulusu et al. 2008, Ohtaetal.2007,Vashaee&Shakouri2004],andelectron-phononcoupling[Sjaksteetal. 2007]. As all these factors can influence also the material focused in this chapter NaTaO3, at firstbasicprinciplesofthePervoskitecrystalstructurearebrieflyreviewed,asthis interdisciplinaryapproachissupposedtogainimportantunderstandingforfuture improvement.The interest on Perovskite structure related materials has dramatically increased in the past threedecadesafterthediscoveryofmanysuperiorsolid-stateproperties,whichmakes Perovskite materials or their layered derivatives record holders in many fields of solid state physics as shown in fig. 2. The most popular finding was the discovery of superconductivity inY1Ba2C3O7-x(YBCO)forwhichtheNobelPrize1987wasprovided.Thepresentrecord holderisBi2212withacriticaltemperatureofTC=120K.AlargescaleapplicationofYBCO since 1998 is the linear motor train using the magnetic levitation (Maglev) in Yamanashi-ken Japan, whose entire rail consists of Helium-cooled superconductors. Present portable phone technologyisallbased on layered(Ba,Sr)TiO3dielectricmaterial[Ohsato2001,Wunderlich etal.2000]duetotheirhighdielectricconstant(e>10000)andqualityfactor.Duringthe materials development detailed spectroscopic data of the electromagnetic resonance [Bobnar et al. 2002, Lichtenberg et al. 2001] have been measured, which further analysis can provide moreunderstandingofelectron-phononinteractionsasoneofthekeyissuefor thermoelectricsbasedonperovskites.PiezoelectricmaterialsonPb(Ti1-xZrx)O3(PZT)orthe environmentalbenignleadfreeK0.5Na0.5NbO3 (KNN)materials[Stegketal.2009]havean increasing application demand in actuators and sensors. Fig.2.AsPerovskite-structurebasedmate-rialsarerecordholdersinmanysolid-state properties, they might become so in thermoelectrics too. Ceramic Materials4 Themainreasonforthegoodpiezoelectric propertieswithitslarge d33 shearcomponentis thatsoftmodesinthephononspectrumappearnearthemorphotrophicphaseboundary [Stegketal.2009].Thisderivesfromthesofteningoftheatomicbondsbyaddingother elements, or from increasing of the lattice constants as described in the next sub-section. The NobelPrize2007hasbeenprovidedforthediscoveryofthegiantmagneticresonance (GMR)observedonHeusler-phases,butitalsooccursonPerovskiteinterfacesasin (La,Sr)MnO3[Coeyetal.1999].Similarly,forthermoelectricmaterials,likethelayered Perovskite-relativescalledRuddlesden-Popperphases(SrTiO3)n(SrO)m,largeZTvalues have been reported. Fig. 3. Schematic drawing of the crystal structure of the perovskite structure and of relatives, (a)erovskitestructurewithsmalllatticeconstantcomparedtoatomicradius,(b)same with large lattice constants, (c) tilted octahedron in LaTiO3, (d) layered Ruddlesden-Popper phase with uniaxial distorted TiO6-octahedron, (e) Aurivilius phase. The Perovskite structure is schematically summarized in fig 3. In pure perovskites there are two extreme structural variants, expressed by the tolerance factor f [Imada et al. 1998] O BO Ar rr rf (2) where rA, rB, rO are the atom radii or the A-(alkali or rare earth-), B-(transition metal group-elements), and O-atom in ABO3-perovskites. The first extreme with small f (fig. 3a) has small latticeconstantscomparedtotheatomicradii.Thus,theatomsfitalmostwithoutfree volumeintothecubicunitcell.Thesecondvariantwithlargef(fig.3b)haslargelattice constantscomparedtotheatomicradii.Hence,phononmodesespeciallysoftmodescan easily be excited and this is considered as a beneficial factor for many of the superior solid-statepropertiesmentionedabove[Imadaetal.1998,Stegketal.2009].Ifthespaceforthe octahedron is too large, they start too tilt as shown in fig. 3 c for LaTiO3. This is considered asbadforthethermoelectricproperties.Thisholdsalsotrueforthecaseoftheuniaxial octahedronextensionasshowninfig.3dforthelayeredRuddlesden-Popperphase [Ruddlesden&Popper1958],whichisanaturalgrownnano-compositeconsistingofSrO and SrTiO3. They are explained in the section 2.3, as well as the Auirvillius phases (fig, 3 e), but before that the findings on perovskite-based thermo-electrics are briefly summarized. 2.2 Perovskite based thermoelectrics Focusingfromnowonthermoelectricmaterials,ithasbeenshown[Yamamotoetal.2007, Sterzel & Kuehling 2002] that in the (Sr,Ba,Ca)TiO3 ternary system only specimens at the Sr-rich corner show a large Seebeck-coefficient. Because pure SrTiO3 is an insulator with a band gapof3.2eV,itneedstobedopedinordertobecomeasemiconductor.N-dopinghas successfully been demonstrated by partially substitution of Sr with La, or Ti with Nb, and a rather large thermoelectric figure of merit of 0.34 at 1000K isachieved [Ohta et al. 2005-a,b, Wunderlich et al. 2006] As shown in fig. 4, the principle is the same as doping in Si, electron donator elements from the right side of the host atoms in the period system are substituted. However,intheseoxideceramics,notonlyanelectronisreleased,butalsoduetothe valencechangeofTi-atom,oxygenatomsarereleased(fig,4b).Hence,firinginreduced atmosphereimprovesthepropertiesofNb-dopedSrTiO3,aswellasNaTaO3asexplained later.The oxygen deficit introduces an additional electronic state 300 mV below the valence band edge, as discussed elsewhere [Wunderlich et al. 2009-a]. In this paper also one of the reasons for the good thermoelectric performance of SrTi1-xNbxO3-v, has been discovered. Fig.4.N-typedopingofSrTiO3forA-andB-sideinshown(a)intheperiodtable,(b)as reactionequationwitheithercreationoxygenvacanciesorchangingtheoxidationstateof the Ti-atom. Fig.5EffectivebandmassinNb-dopedSrTiO3asafunctionoftheNb-content.Theinset showstheconductionbandfeaturesnearthebandgapfordifferentconcentrationsin-Z direction, from which the effective mass was estimated [Wunderlich et al. 2009-a]. Development of Thermoelectric materials based on NaTaO3 - composite ceramics 5 Themainreasonforthegoodpiezoelectric propertieswithitslarge d33 shearcomponentis thatsoftmodesinthephononspectrumappearnearthemorphotrophicphaseboundary [Stegketal.2009].Thisderivesfromthesofteningoftheatomicbondsbyaddingother elements, or from increasing of the lattice constants as described in the next sub-section. The NobelPrize2007hasbeenprovidedforthediscoveryofthegiantmagneticresonance (GMR)observedonHeusler-phases,butitalsooccursonPerovskiteinterfacesasin (La,Sr)MnO3[Coeyetal.1999].Similarly,forthermoelectricmaterials,likethelayered Perovskite-relativescalledRuddlesden-Popperphases(SrTiO3)n(SrO)m,largeZTvalues have been reported. Fig. 3. Schematic drawing of the crystal structure of the perovskite structure and of relatives, (a)erovskitestructurewithsmalllatticeconstantcomparedtoatomicradius,(b)same with large lattice constants, (c) tilted octahedron in LaTiO3, (d) layered Ruddlesden-Popper phase with uniaxial distorted TiO6-octahedron, (e) Aurivilius phase. The Perovskite structure is schematically summarized in fig 3. In pure perovskites there are two extreme structural variants, expressed by the tolerance factor f [Imada et al. 1998] O BO Ar rr rf (2) where rA, rB, rO are the atom radii or the A-(alkali or rare earth-), B-(transition metal group-elements), and O-atom in ABO3-perovskites. The first extreme with small f (fig. 3a) has small latticeconstantscomparedtotheatomicradii.Thus,theatomsfitalmostwithoutfree volumeintothecubicunitcell.Thesecondvariantwithlargef(fig.3b)haslargelattice constantscomparedtotheatomicradii.Hence,phononmodesespeciallysoftmodescan easily be excited and this is considered as a beneficial factor for many of the superior solid-statepropertiesmentionedabove[Imadaetal.1998,Stegketal.2009].Ifthespaceforthe octahedron is too large, they start too tilt as shown in fig. 3 c for LaTiO3. This is considered asbadforthethermoelectricproperties.Thisholdsalsotrueforthecaseoftheuniaxial octahedronextensionasshowninfig.3dforthelayeredRuddlesden-Popperphase [Ruddlesden&Popper1958],whichisanaturalgrownnano-compositeconsistingofSrO and SrTiO3. They are explained in the section 2.3, as well as the Auirvillius phases (fig, 3 e), but before that the findings on perovskite-based thermo-electrics are briefly summarized. 2.2 Perovskite based thermoelectrics Focusingfromnowonthermoelectricmaterials,ithasbeenshown[Yamamotoetal.2007, Sterzel & Kuehling 2002] that in the (Sr,Ba,Ca)TiO3 ternary system only specimens at the Sr-rich corner show a large Seebeck-coefficient. Because pure SrTiO3 is an insulator with a band gapof3.2eV,itneedstobedopedinordertobecomeasemiconductor.N-dopinghas successfully been demonstrated by partially substitution of Sr with La, or Ti with Nb, and a rather large thermoelectric figure of merit of 0.34 at 1000K isachieved [Ohta et al. 2005-a,b, Wunderlich et al. 2006] As shown in fig. 4, the principle is the same as doping in Si, electron donator elements from the right side of the host atoms in the period system are substituted. However,intheseoxideceramics,notonlyanelectronisreleased,butalsoduetothe valencechangeofTi-atom,oxygenatomsarereleased(fig,4b).Hence,firinginreduced atmosphereimprovesthepropertiesofNb-dopedSrTiO3,aswellasNaTaO3asexplained later.The oxygen deficit introduces an additional electronic state 300 mV below the valence band edge, as discussed elsewhere [Wunderlich et al. 2009-a]. In this paper also one of the reasons for the good thermoelectric performance of SrTi1-xNbxO3-v, has been discovered. Fig.4.N-typedopingofSrTiO3forA-andB-sideinshown(a)intheperiodtable,(b)as reactionequationwitheithercreationoxygenvacanciesorchangingtheoxidationstateof the Ti-atom. Fig.5EffectivebandmassinNb-dopedSrTiO3asafunctionoftheNb-content.Theinset showstheconductionbandfeaturesnearthebandgapfordifferentconcentrationsin-Z direction, from which the effective mass was estimated [Wunderlich et al. 2009-a]. Ceramic Materials6 When x, the doping concentration ob Nb increases, the effective electronic mass increases as showninfig.5.Whenanalyzingthebandstructure,thisfactcanbeexplainedbythe decreaseinenergyofaflatbandasseenintheinsetoffig.5.AttheconcentrationofxNb =0.24thelow-massbandstretchingbecomestoolargeanditformsanindependentband section at the -point (inset of fig. 5, case (C)). As a result the band mass suddenly becomes small, and in the experiments the bad TE-properties have been confirmed.Thefindingexpressedinfig.5[Wunderlichetal.2009-a]canbeconsideredasakindof guidelineforanyfunctionalmaterialdevelopment.Incontrarytostructuralmaterials, whereawideconcentrationrangegivesusualgoodperformance,infunctionalmaterials onlyanarrowconcentrationrangegivesgoodproperties.Alittlebitincreasesthe performanceremarkable,butalittlebittoomuch,deterioratesthem,isaprinciple occurring often in nature, especially in organic or bio-chemistry.AnotherreasonforthesuccessofNb-dopedSrTiO3-Perovskitehasbeensuggestedbythe decrease of the bandgap due to phonons [Wunderlich W., 2008-a]. This mechanism explains theimportanceofphononsforelectronexcitationastheoriginoftheheatconversion,and ontheotherhanditexplainsthelargeSeebeckcoefficientduetoreductionof recombination.Namely,whenthe excited electronwants tojumpbacktogroundstate, the phononhastraveledawayandthebandgapislargeasitiswithoutphononmakingade-excitation unlikely.Thefollowingformula[Wunderlichetal.2009-a]relatesthecalculatedbandmassestothe effective band mass m* as determined in experiments i B em m m,** (3) bytakingmB,iwithi=1,thenextbandtothebandgapfrombandstructurecalculations,as an average of high and low band masses mB,i,hmB,i,lat two different reciprocal lattice points by

3 / 22 / 3, ,2 / 3, , , l i B h i B i Bm m m (4). Throughthesebandmasscalculationsitwasdescribedforthefirsttime[Wunderlich& Koumoto2006],thatNaTO3,KTaO3andothersarepossibleTE-candidates,becausethey possessalargeeffectivemassofm*/me=8,abouttwotimeslargerthanNb-SrTiO3.Before describing NaTO3 in section 2.4., we briefly summarize findings on layered Perovskites. 2.3 Layered Perovskites as thermoelectricsThe electron confinement at Perovskite interfaces has been demonstrated first in [Ohmoto & Hwang2004].Duetosuch2-dimensionalelectrongas(2DEG)atinterfaces,also thermoelectric propertiesare enhancedaspredictedtheoretically(seereferencesin[Bulusu &Walker2008]).TheconfinedelectrongashasbeensuccessfullydemonstratedforNb-dopedSrTiO3,andthisdiscoveryleadstoSeebeckcoefficientstentimeshigherthanbulk materials [Mune et al. 2007, Ohta et al. 2007, Hosono et al. 2006, Lee et al. 2008]. Theoretical calculations[Wunderlichetal.2008]showedthatthecontroloftheconcentrationon atomisticlevel,diffusionandstructuralstabilityisessential,asaSrTiO3-SrNbO3-SrTiO3 composite is much more effective that an embedded Nb-doped SrTiO3.Theideathataninsulatingnano-layerofSrOinsideNb-dopedSrTiO3reducesthethermal expansionofthecomposite, hasbeen demonstratedfortheRuddlesden-Popperphase[Lee etal.2007-a,Leeetal.2007-b,Wunderlichetal.2005],whicharenaturallygrown superlattices [Haeni et al.2001]. As mentioned in section 2.2, in such case structural uniaxial distortionsoftheTi-octahedroncanoccur,whichdeterioratethethermoelectricproperties duetotheirlargerTi-O-distance.Byadditionaldopingelementstheextensioncanbe restored and thermoelectric properties are improved [Wang et al. 2007].OtherPerovskiterelativesarethevariousAuriviliusphases,whichconsistsofBi2O2layers betweenPerovskite[Lichtenbergetal.2001,Perez-Matoetal.2004].Theirthermoelectric conversion power has yet been tested to a certain degree. Other Perovskite relatives are the Tungsten-bronze crystals [Ohsato 2001], which have not yet been tested. 2.4 Pure NaTaO3 is a distorted PerovskiteThe interest in NaTaO3 recently increased after the discovery of its photo catalytic properties aswatersplitting[Katoetal.1998],ordegradationoforganicmolecules,especiallywhen doped with rare earth elements like La [Yan et al., 2009]. In spite of its high melting point of 1810oC[Leeetal.1995,Suzukietal.2004]ithasalatticeenergyof-940kJ/mol,butnotas lowasTa2O5(-1493kJ/mol).Itcanbeproducedatrelativelylowtemperaturesfrom Na2C2O4andTa2O5[Xuetal.2005]anditreactiveswithSi3N4[Leeetal.1995].NaTaO3 formsaneutecticceramicalloywithCaCO3,whichlowersthemeltingpointto813K [Kjarsgaard & Mtchell 2008]. Ta in NaTaO3 can be exchanged isomorphly by Nb, relating in similar properties as NaNbO3 [Shirane et al. 1954, Shanker et al., 2009]. DetailedinvestigationsshowedthatNaTaO3possessesthePervoskitestructure(Pm-3m) onlyabove(893K)beforeitlowersitssymmetrybecomingtetragonal(P4/mbm),and orthorhombic(Cmcm,Pbnm)below843Kand773K,respectively[Kennedyetal.1999]. NaTaO3ismorestablecomparedtoNaNbO3,whichbecomestetragonalat653Kand orthorhombicat543K,orKNbO3,wherethesetransformationsoccurat608Kand498K, respectively[Shiraneetal.1954]. NaTaO3hasabandgap of4eV[Xu etal.2005].Thephase transition is caused by the octahedron tilting (fig. 2 c), which can reach up to 8o in the case of NaTaO3 [Kennedy. et al. 1999].NaTaO3hasbeensuggestedasthermoelectricmaterial[Wunderlich&Koumoto2006, Wunderlichetal.2009-a,Wunderlich&Soga2010],asitshowsalargeSeebeckcoefficient. Thefindingsarebrieflysummarized,togetherwithexplanationofnewresearchresultsin the following sections. 3. Ab-initio calculations of doped NaTaO3 First-principlecalculationsbasedonthedensity-functionaltheory(DFT)arepresentedin this chapter. They should clarify the following topics, namely which doping element sits on A-orB-siteoftheperovskitelatticeABO3,howthelatticeconstantschange,howFermi energy and bandgap change, and finally how the bandstructure and density-of-states (DOS) looks like.ThefirstprinciplescalculationswereperformedusingVASPsoftware[Kresse&Hafner 1994] in the LDA-GGA approximation with a cut-off energy E=-280eV, U=0V and sufficient Development of Thermoelectric materials based on NaTaO3 - composite ceramics 7 When x, the doping concentration ob Nb increases, the effective electronic mass increases as showninfig.5.Whenanalyzingthebandstructure,thisfactcanbeexplainedbythe decreaseinenergyofaflatbandasseenintheinsetoffig.5.AttheconcentrationofxNb =0.24thelow-massbandstretchingbecomestoolargeanditformsanindependentband section at the -point (inset of fig. 5, case (C)). As a result the band mass suddenly becomes small, and in the experiments the bad TE-properties have been confirmed.Thefindingexpressedinfig.5[Wunderlichetal.2009-a]canbeconsideredasakindof guidelineforanyfunctionalmaterialdevelopment.Incontrarytostructuralmaterials, whereawideconcentrationrangegivesusualgoodperformance,infunctionalmaterials onlyanarrowconcentrationrangegivesgoodproperties.Alittlebitincreasesthe performanceremarkable,butalittlebittoomuch,deterioratesthem,isaprinciple occurring often in nature, especially in organic or bio-chemistry.AnotherreasonforthesuccessofNb-dopedSrTiO3-Perovskitehasbeensuggestedbythe decrease of the bandgap due to phonons [Wunderlich W., 2008-a]. This mechanism explains theimportanceofphononsforelectronexcitationastheoriginoftheheatconversion,and ontheotherhanditexplainsthelargeSeebeckcoefficientduetoreductionof recombination.Namely,whenthe excited electronwants tojumpbacktogroundstate, the phononhastraveledawayandthebandgapislargeasitiswithoutphononmakingade-excitation unlikely.Thefollowingformula[Wunderlichetal.2009-a]relatesthecalculatedbandmassestothe effective band mass m* as determined in experiments i B em m m,** (3) bytakingmB,iwithi=1,thenextbandtothebandgapfrombandstructurecalculations,as an average of high and low band masses mB,i,hmB,i,lat two different reciprocal lattice points by

3 / 22 / 3, ,2 / 3, , , l i B h i B i Bm m m (4). Throughthesebandmasscalculationsitwasdescribedforthefirsttime[Wunderlich& Koumoto2006],thatNaTO3,KTaO3andothersarepossibleTE-candidates,becausethey possessalargeeffectivemassofm*/me=8,abouttwotimeslargerthanNb-SrTiO3.Before describing NaTO3 in section 2.4., we briefly summarize findings on layered Perovskites. 2.3 Layered Perovskites as thermoelectricsThe electron confinement at Perovskite interfaces has been demonstrated first in [Ohmoto & Hwang2004].Duetosuch2-dimensionalelectrongas(2DEG)atinterfaces,also thermoelectric propertiesare enhancedaspredictedtheoretically(seereferencesin[Bulusu &Walker2008]).TheconfinedelectrongashasbeensuccessfullydemonstratedforNb-dopedSrTiO3,andthisdiscoveryleadstoSeebeckcoefficientstentimeshigherthanbulk materials [Mune et al. 2007, Ohta et al. 2007, Hosono et al. 2006, Lee et al. 2008]. Theoretical calculations[Wunderlichetal.2008]showedthatthecontroloftheconcentrationon atomisticlevel,diffusionandstructuralstabilityisessential,asaSrTiO3-SrNbO3-SrTiO3 composite is much more effective that an embedded Nb-doped SrTiO3.Theideathataninsulatingnano-layerofSrOinsideNb-dopedSrTiO3reducesthethermal expansionofthecomposite, hasbeen demonstratedfortheRuddlesden-Popperphase[Lee etal.2007-a,Leeetal.2007-b,Wunderlichetal.2005],whicharenaturallygrown superlattices [Haeni et al.2001]. As mentioned in section 2.2, in such case structural uniaxial distortionsoftheTi-octahedroncanoccur,whichdeterioratethethermoelectricproperties duetotheirlargerTi-O-distance.Byadditionaldopingelementstheextensioncanbe restored and thermoelectric properties are improved [Wang et al. 2007].OtherPerovskiterelativesarethevariousAuriviliusphases,whichconsistsofBi2O2layers betweenPerovskite[Lichtenbergetal.2001,Perez-Matoetal.2004].Theirthermoelectric conversion power has yet been tested to a certain degree. Other Perovskite relatives are the Tungsten-bronze crystals [Ohsato 2001], which have not yet been tested. 2.4 Pure NaTaO3 is a distorted PerovskiteThe interest in NaTaO3 recently increased after the discovery of its photo catalytic properties aswatersplitting[Katoetal.1998],ordegradationoforganicmolecules,especiallywhen doped with rare earth elements like La [Yan et al., 2009]. In spite of its high melting point of 1810oC[Leeetal.1995,Suzukietal.2004]ithasalatticeenergyof-940kJ/mol,butnotas lowasTa2O5(-1493kJ/mol).Itcanbeproducedatrelativelylowtemperaturesfrom Na2C2O4andTa2O5[Xuetal.2005]anditreactiveswithSi3N4[Leeetal.1995].NaTaO3 formsaneutecticceramicalloywithCaCO3,whichlowersthemeltingpointto813K [Kjarsgaard & Mtchell 2008]. Ta in NaTaO3 can be exchanged isomorphly by Nb, relating in similar properties as NaNbO3 [Shirane et al. 1954, Shanker et al., 2009]. DetailedinvestigationsshowedthatNaTaO3possessesthePervoskitestructure(Pm-3m) onlyabove(893K)beforeitlowersitssymmetrybecomingtetragonal(P4/mbm),and orthorhombic(Cmcm,Pbnm)below843Kand773K,respectively[Kennedyetal.1999]. NaTaO3ismorestablecomparedtoNaNbO3,whichbecomestetragonalat653Kand orthorhombicat543K,orKNbO3,wherethesetransformationsoccurat608Kand498K, respectively[Shiraneetal.1954]. NaTaO3hasabandgap of4eV[Xu etal.2005].Thephase transition is caused by the octahedron tilting (fig. 2 c), which can reach up to 8o in the case of NaTaO3 [Kennedy. et al. 1999].NaTaO3hasbeensuggestedasthermoelectricmaterial[Wunderlich&Koumoto2006, Wunderlichetal.2009-a,Wunderlich&Soga2010],asitshowsalargeSeebeckcoefficient. Thefindingsarebrieflysummarized,togetherwithexplanationofnewresearchresultsin the following sections. 3. Ab-initio calculations of doped NaTaO3 First-principlecalculationsbasedonthedensity-functionaltheory(DFT)arepresentedin this chapter. They should clarify the following topics, namely which doping element sits on A-orB-siteoftheperovskitelatticeABO3,howthelatticeconstantschange,howFermi energy and bandgap change, and finally how the bandstructure and density-of-states (DOS) looks like.ThefirstprinciplescalculationswereperformedusingVASPsoftware[Kresse&Hafner 1994] in the LDA-GGA approximation with a cut-off energy E=-280eV, U=0V and sufficient Ceramic Materials8 numberofk-points.TheDOSisconvolutedwithaGaussiandistributionwithaFWHMof 0.2eV,toapproximatethebroadeningatroomtemperature.Therelevantsymmetrypoints inreciprocalspacewerechosenaccordingtothestandardnotificationsofthePerovskite spacegroupPm-3m,whichwasassumedasafirstapproximationtohaveuntitled octahedra. The path in reciprocal space was focused on the three directions near the -point, seediscussionin[Wunderlichetal.2009-a].Thesupercellusedinthesecalculationsisa 2x2x2 extension of the unit cell, allowing calculations of minimal doping concentration steps of 0.125 = 1/8 for A- or B-side or 1/24 for O. Fig. 6. The energy-volume dependence for pure NaTaO3 (pink line) is shown and compared with different doping elements dissolved in NaTaO3, either on Na- or Ta-side, each for two concentration. The variants with lowest energy are (a) Fe on Ta-side, (b) Ag on Na-side, and (c) Ti, (d) Mn, (e) Cr all three on Ta-side. Dopingelement NaTa0.9Me0.1O3 un-dopedFeAgTiMnCr Lattice constant [nm]0.39090.39480.39680.39090.39520.3929 Band-gap [eV] 2.200.740.602.100.951.30 Fermi energy [eV]2.371.911.802.171.952.54 Table1.Latticeconstants,band-gapandFermienergyforTa-sitedopedNaTaO3as estimated from ab-initio calculations Theresultsoftheenergy-versus-volume(E(V))calculationsareshowninfig.6fordoping elementsFe,Ag,Ti,Mn,andCrforeitherdopingonA-orB-side.Theobtainedlattice constantsareshownintable1andexhibitonlyasmallchangecomparedtopureNaTaO3. As explained in the following section and in a previous paper [Wunderlich 2009-b], Ag and Fearethetwo dopingelements,which causethehighestSeebeckvoltagedue totheirhigh solubilityintheNaTaO3 lattice.Thedatainfig.6 showthatboth,Fe,and Ag,dopedon B-sitehaveaslightlyhigherenergy,whileaccordingtotheexperimentaldataintuitivelyone wouldexpectalowerenergythanpureNaTaO3,asitisinthecaseforallotherdoping elements.ThediscrepancycanbeexplainedbythefactthatpureNaTaO3hastilted octahedron.Furthermore,AgshowsaslightlylowerenergyfordopingonA-side,butthis makesnosense,becausevalenceandhencebandstructureremainsunchanged.Asinthe caseofNb-dopedSrTiO3[Wunderlichetal.2009-a]DFT-calculationsofthecombined defectsNaTa0.88Me0.12O3-xmightclarifythisissue.Asexplainedinfig.4bintheprevious section,anincreaseintheelectronconcentrationonB-sideisalwaysrelatedtoadeficitin oxygen. Fig.7.Bandstructureof(a)NaTaO2.8.(b)NaTa0.88Fe0.12O3(n-type)(c)NaTa0.88Ag0.12O3(p-type).Thearrowsshowthechangecomparedtoun-dopedNaTaO3.(Thebandcolorsare just for distinguishing and have no other meaning). ThecalculatedbandstructureofFe-dopedNaTaO3isshowninfig.7b,thatofAg-doped NaTaO3infig7candtheoxygen-deficitNaTaO2.8latticeinfig.7a.InallplotstheFermi energylevel,whichisshownintable1,hasbeenadjustedto0eV.Inthecaseofn-doping theTa-2egbandshaveloweredtheirenergyandthebandgapisreducedremarkablyfrom 2.2eVinpure NaTaO3to0.74eV,sothatexcitationsdue tophononsbecomepossible.The p-dopingbyAgshiftstheTa-2eg bandstowardsthevalenceband, sothatanindirectband gapwith0.6eVoccurs.Asshownintable1,thebandstructuresofotherdopingelements showlargerbandgaps.Thebandgapwidthscorrespondwelltotheelectricresistivityof Development of Thermoelectric materials based on NaTaO3 - composite ceramics 9 numberofk-points.TheDOSisconvolutedwithaGaussiandistributionwithaFWHMof 0.2eV,toapproximatethebroadeningatroomtemperature.Therelevantsymmetrypoints inreciprocalspacewerechosenaccordingtothestandardnotificationsofthePerovskite spacegroupPm-3m,whichwasassumedasafirstapproximationtohaveuntitled octahedra. The path in reciprocal space was focused on the three directions near the -point, seediscussionin[Wunderlichetal.2009-a].Thesupercellusedinthesecalculationsisa 2x2x2 extension of the unit cell, allowing calculations of minimal doping concentration steps of 0.125 = 1/8 for A- or B-side or 1/24 for O. Fig. 6. The energy-volume dependence for pure NaTaO3 (pink line) is shown and compared with different doping elements dissolved in NaTaO3, either on Na- or Ta-side, each for two concentration. The variants with lowest energy are (a) Fe on Ta-side, (b) Ag on Na-side, and (c) Ti, (d) Mn, (e) Cr all three on Ta-side. Dopingelement NaTa0.9Me0.1O3 un-dopedFeAgTiMnCr Lattice constant [nm]0.39090.39480.39680.39090.39520.3929 Band-gap [eV] 2.200.740.602.100.951.30 Fermi energy [eV]2.371.911.802.171.952.54 Table1.Latticeconstants,band-gapandFermienergyforTa-sitedopedNaTaO3as estimated from ab-initio calculations Theresultsoftheenergy-versus-volume(E(V))calculationsareshowninfig.6fordoping elementsFe,Ag,Ti,Mn,andCrforeitherdopingonA-orB-side.Theobtainedlattice constantsareshownintable1andexhibitonlyasmallchangecomparedtopureNaTaO3. As explained in the following section and in a previous paper [Wunderlich 2009-b], Ag and Fearethetwo dopingelements,which causethehighestSeebeckvoltagedue totheirhigh solubilityintheNaTaO3 lattice.Thedatainfig.6 showthatboth,Fe,and Ag,dopedon B-sitehaveaslightlyhigherenergy,whileaccordingtotheexperimentaldataintuitivelyone wouldexpectalowerenergythanpureNaTaO3,asitisinthecaseforallotherdoping elements.ThediscrepancycanbeexplainedbythefactthatpureNaTaO3hastilted octahedron.Furthermore,AgshowsaslightlylowerenergyfordopingonA-side,butthis makesnosense,becausevalenceandhencebandstructureremainsunchanged.Asinthe caseofNb-dopedSrTiO3[Wunderlichetal.2009-a]DFT-calculationsofthecombined defectsNaTa0.88Me0.12O3-xmightclarifythisissue.Asexplainedinfig.4bintheprevious section,anincreaseintheelectronconcentrationonB-sideisalwaysrelatedtoadeficitin oxygen. Fig.7.Bandstructureof(a)NaTaO2.8.(b)NaTa0.88Fe0.12O3(n-type)(c)NaTa0.88Ag0.12O3(p-type).Thearrowsshowthechangecomparedtoun-dopedNaTaO3.(Thebandcolorsare just for distinguishing and have no other meaning). ThecalculatedbandstructureofFe-dopedNaTaO3isshowninfig.7b,thatofAg-doped NaTaO3infig7candtheoxygen-deficitNaTaO2.8latticeinfig.7a.InallplotstheFermi energylevel,whichisshownintable1,hasbeenadjustedto0eV.Inthecaseofn-doping theTa-2egbandshaveloweredtheirenergyandthebandgapisreducedremarkablyfrom 2.2eVinpure NaTaO3to0.74eV,sothatexcitationsdue tophononsbecomepossible.The p-dopingbyAgshiftstheTa-2eg bandstowardsthevalenceband, sothatanindirectband gapwith0.6eVoccurs.Asshownintable1,thebandstructuresofotherdopingelements showlargerbandgaps.Thebandgapwidthscorrespondwelltotheelectricresistivityof Ceramic Materials10 suchspecimensasexplainedinthenextsection.Hence,theband-gap-reductionwillbea future engineering challenge for obtaining a large electric conductivity. Fig. 8. Band structure near the conduction band edge at the -point for Na-site doping,(a) Na0.88Ca0.12TaO2.8, , (b) Na0.88Sr0.12TaO2.8, (c) Na0.88Ba0.12TaO2.8, , (d) Na0.88Ce0.12TaO2.8. ThemechanismforelectronconductivityissimilartothatinNb-dopedSrTiO3;fordetails see the discussions in [Wunderlich et al. 2009-a]. The oxygen vacancies introduce electronic statesabout200~300meVbelowthevalencebandedge,formwhichelectronsfromthe conductionbandcanbeexcitedintothevalenceband.ComparedtopureandNb-doped SrTiO3(m*/m0=4.8and8),inpureNaTaO3(m*/m0=8)theeffectiveelectronmassincreases further(m*/m0=12),ascanbeseenfromtheflatbandsovertheentireregion-Xinall three cases of fig. 7. In un-doped NaTaO3 the hole mass is also large (m*/m0= 8). The mass of Ag-doped NaTaO3(Fig.7c)issmaller duetothe indirect bandsatZand -points,butthe large effective mass of the valence band minimum in un-doped regions (m*/m0> 20) seems to have also an large influence on the effective mass measured in experiments. Calculations for A-site doping analog to La-doped SrTiO3 [Wunderlich et al. 2009-a] are shown for NaTaO2.8 infig.8.Inall casestheDOS nearthebandedgeisincreased,butforCe-dopingitbecame especiallylargeascanbealsoseenontheincreasednumberofbands(fig.8d).Inspiteof experimental difficulties with sintering of Ce2O3 containing samples [Wunderlich et al. 2009-d],alargeTE-performancebyco-dopingmightbeexpected.Infollowingexperimental results about Ta-site doping are reported.

4. Specimen preparation and microstructure characterization of NaTaO3 NaTaO3 composite ceramics were produced by conventional sintering. Well-defined weight ratios of fine powders of NaTaO3 (Fine Chemicals Ltd.) and each of the pure metals Fe, Ag and other metals, or Fe2O3, were mixed in different concentration ratios in a mortar for more than 10 min. The specimens were pressed with 100 MPa as pellets, 10 mm in diameter and 3 mm height, and sintered in a muffle furnace in air at 1000 oC for 5h with slow heating and coolingrates(50K/h)assketchedinfig.9.Theelectricpropertiesofthespecimenswere analyzedasexplainedinthefollowingsection.Thereafter,thesinteringwasrepeated severaltimesatthesametemperature.DuringsinteringthewhitecolorofNaTaO3 specimensturnintodarkcolorsindicatingthattheband-gaphasbeenreduced,whena largeamountofmetalswasdissolved.However,specimenscontainingmetalswithlow solubilitysuchasAl,Cu,Sn,Sb,Mo,Wremainedwhiteorturnedintolightorangeor reddishcolor(Ti).ThespecimenswerecharacterizedbySEM(Hitachi3200-N)at30kV equippedwithEDS(Noran),whichallowschemicalmapping.TheX-raydiffraction(XRD) analysiswasperformedusingaRigakuMiniflexdevicewithCo-sourcewith1.7889nm wavelength.SimulationoftheXRD-patternswasperformedwiththeCarineV3software (Cristmet). Fig. 9. Flowchart of the specimen preparation Fig.10XRDdiffractionpatternof NaTaO3 with50wt-% Ni.The letters NindicateNaTaO3 reflexes. TheanalysisofXRD-diffractionpatternofFe-andTi-dopedNaTaO3showed[Wunderlich, Soga,2010]thattheinitiallymixedFeorTi-metallicpowdergetsoxidizedasbesidesthe NaTaO3-XRD-peaksalsosuchofFeO3-orTi2O3areobserved.Hence,duringsinteringa FeO3- and Ti2O3NaTaO3 composite material is formed by reaction bonded sintering (RBS), amechanism,whichsupportsadditionalenergyforsinteringandhasbeensuccessfully appliedformanystructuralceramics[Claussenetal.1996].Weightmeasurementsof specimensbeforeandaftersinteringconfirmedtheoxidationbyweightgainevenin quantitative manner.InthecaseofAg,evidencesforoxidationhavenotyetbeenclearlyapproved,instead, coolingdownasinteredspecimen,metallicsilverballsseparatedonthespecimensurface areobserved.InthecaseofNi-addedNaTaO3,inspiteofthegreenishspecimensurface color due to NiO, the XRD pattern in fig. 10 shows that the interior of the specimen consists of a composite NaTaO3 with metallic Ni. In all specimens with Fe-, Ni-, Mn-, and Ag-doping theXRDpeakswereindentifiedasPerovskitewithspacegroupPm-3masmentionedin Development of Thermoelectric materials based on NaTaO3 - composite ceramics 11 suchspecimensasexplainedinthenextsection.Hence,theband-gap-reductionwillbea future engineering challenge for obtaining a large electric conductivity. Fig. 8. Band structure near the conduction band edge at the -point for Na-site doping,(a) Na0.88Ca0.12TaO2.8, , (b) Na0.88Sr0.12TaO2.8, (c) Na0.88Ba0.12TaO2.8, , (d) Na0.88Ce0.12TaO2.8. ThemechanismforelectronconductivityissimilartothatinNb-dopedSrTiO3;fordetails see the discussions in [Wunderlich et al. 2009-a]. The oxygen vacancies introduce electronic statesabout200~300meVbelowthevalencebandedge,formwhichelectronsfromthe conductionbandcanbeexcitedintothevalenceband.ComparedtopureandNb-doped SrTiO3(m*/m0=4.8and8),inpureNaTaO3(m*/m0=8)theeffectiveelectronmassincreases further(m*/m0=12),ascanbeseenfromtheflatbandsovertheentireregion-Xinall three cases of fig. 7. In un-doped NaTaO3 the hole mass is also large (m*/m0= 8). The mass of Ag-doped NaTaO3(Fig.7c)issmaller duetothe indirect bandsatZand -points,butthe large effective mass of the valence band minimum in un-doped regions (m*/m0> 20) seems to have also an large influence on the effective mass measured in experiments. Calculations for A-site doping analog to La-doped SrTiO3 [Wunderlich et al. 2009-a] are shown for NaTaO2.8 infig.8.Inall casestheDOS nearthebandedgeisincreased,butforCe-dopingitbecame especiallylargeascanbealsoseenontheincreasednumberofbands(fig.8d).Inspiteof experimental difficulties with sintering of Ce2O3 containing samples [Wunderlich et al. 2009-d],alargeTE-performancebyco-dopingmightbeexpected.Infollowingexperimental results about Ta-site doping are reported.

4. Specimen preparation and microstructure characterization of NaTaO3 NaTaO3 composite ceramics were produced by conventional sintering. Well-defined weight ratios of fine powders of NaTaO3 (Fine Chemicals Ltd.) and each of the pure metals Fe, Ag and other metals, or Fe2O3, were mixed in different concentration ratios in a mortar for more than 10 min. The specimens were pressed with 100 MPa as pellets, 10 mm in diameter and 3 mm height, and sintered in a muffle furnace in air at 1000 oC for 5h with slow heating and coolingrates(50K/h)assketchedinfig.9.Theelectricpropertiesofthespecimenswere analyzedasexplainedinthefollowingsection.Thereafter,thesinteringwasrepeated severaltimesatthesametemperature.DuringsinteringthewhitecolorofNaTaO3 specimensturnintodarkcolorsindicatingthattheband-gaphasbeenreduced,whena largeamountofmetalswasdissolved.However,specimenscontainingmetalswithlow solubilitysuchasAl,Cu,Sn,Sb,Mo,Wremainedwhiteorturnedintolightorangeor reddishcolor(Ti).ThespecimenswerecharacterizedbySEM(Hitachi3200-N)at30kV equippedwithEDS(Noran),whichallowschemicalmapping.TheX-raydiffraction(XRD) analysiswasperformedusingaRigakuMiniflexdevicewithCo-sourcewith1.7889nm wavelength.SimulationoftheXRD-patternswasperformedwiththeCarineV3software (Cristmet). Fig. 9. Flowchart of the specimen preparation Fig.10XRDdiffractionpatternof NaTaO3 with50wt-% Ni.The letters NindicateNaTaO3 reflexes. TheanalysisofXRD-diffractionpatternofFe-andTi-dopedNaTaO3showed[Wunderlich, Soga,2010]thattheinitiallymixedFeorTi-metallicpowdergetsoxidizedasbesidesthe NaTaO3-XRD-peaksalsosuchofFeO3-orTi2O3areobserved.Hence,duringsinteringa FeO3- and Ti2O3NaTaO3 composite material is formed by reaction bonded sintering (RBS), amechanism,whichsupportsadditionalenergyforsinteringandhasbeensuccessfully appliedformanystructuralceramics[Claussenetal.1996].Weightmeasurementsof specimensbeforeandaftersinteringconfirmedtheoxidationbyweightgainevenin quantitative manner.InthecaseofAg,evidencesforoxidationhavenotyetbeenclearlyapproved,instead, coolingdownasinteredspecimen,metallicsilverballsseparatedonthespecimensurface areobserved.InthecaseofNi-addedNaTaO3,inspiteofthegreenishspecimensurface color due to NiO, the XRD pattern in fig. 10 shows that the interior of the specimen consists of a composite NaTaO3 with metallic Ni. In all specimens with Fe-, Ni-, Mn-, and Ag-doping theXRDpeakswereindentifiedasPerovskitewithspacegroupPm-3masmentionedin Ceramic Materials12 section2.4.Hence,itcanbeconcluded,thattheoctahedrontiltingmentionedinsection2 was suppressed by the doping. Fig.12.SEMmicrographsoftheas-preparedsurfacesofdifferentNaTaO3-composites processed by adding 40 wt% of (a) Fe, (b) Ag, (c) Ti, (d) Mo, (e) Mn, (e) Cr, (g) Ni, (h) W. ThemicrostructureoftheNaTaO3compositeprocessedwith50wt%Feconsistsofa NaTaO3- 50 mol% Fe2O3 composite as shown in fig. 12 a. It consists of dark Fe2O3 particles, onaverage10minsize,andappearinginstreaks-likeshape,whichareembeddedina greyNaTaO3matrix.Detailedexplanationisprovidedinapreviouspaper[Wunderlich& Soga 2010]. When NaTaO3 is initially processed with Fe2O3 instead of Fe, the microstructure lookslikeasinteredceramiccompositewithwhiteFe2O3besideswhiteNaTaO3particles. The change from black to white color can be explained by oxygen saturation as explained in section 6. Such a micrograph is shown in a previous paper [Wunderlich 2009-b]. The white areas in fig. 12 a are pores remaining from insufficient compaction during sintering or from released oxygen as explained in section 5.In NaTaO3-composites containing Ag, Ti, Mn, and Ni the dark, metallic particles are slightly bigger (5~10 m). The particles have a volume fraction of about 30% which correspond well totheintensityratiosoftheXRD-pattern.In specimens,whichwereproducedfromFe2O3- insteadofFe-powder,theFe2O3-particlesformroundparticlesasshowninfig.3ain [Wunderlich 2009-b]. In the case of Cr the dark, metallic Cr-particles are significantly larger (20m),whichcanbeexplainedbytheirlowdiffusivity.Thesamewouldbeexpectedfor MoandWwiththeirhighmeltingpoints,butinsteadtheyleadtofacetedinterfaces.By chemicalmappinghomogeneousdistributionofNa,Ta,MoorWwasconfirmed.Thetwo elements, Mo, and W, having their location in the period system and their atomic radii close to Ta, and, hence, can inter-diffuse easily with Ta. They lower the surface energy of certain crystallographicplanes,whichisanimportantfacttobekeptinmindwhennano-layered composite materials based on NaTaO3 are desired.ThemaingoalofdopingistoincreasethecarrierconcentrationofNaTaO3inorderto increasetheconductivity.Inacompositethiscanonlybeachievedbyincreasingthe concentrationofthedissolvedelement.CompositionmeasurementsbyEDXinSEMwith lateral resolution of 1 m were performed on the NaTaO3-phase in the NaTaO3-composites processedwithdifferentmetals.ForCr,MoandWconcentrationsbelow2at%were detected, for Ag, Ti, Mn, and Ni, 5 ~ 10 at% were detected and for Fe 14 at%. This result can Is a necessity for a thermoelectric material and explains the success of Fe and Ag for the TE-performance as explained in the following section.

5. Thermoelectric characterization 5.1 Measuring device Thethermoelectricmeasurementswereperformedwithaself-manufactureddeviceas shown in the inset of fig. 13. The specimen was attached to the device, so that its left side lies on a copper block as a heat sink and its right side on a micro-ceramic heater (Sakaguchi Ltd. MS1000)withapowerof1kW,andwasheatedupto1000oCwithin3minutes.Hence,the bottompart ofthespecimen experiencedthe largetemperaturedifference,whiletheupper partwasheatedthroughtheheatconductivityofthespecimen.Thetemperature distribution as measured by thermocouples is shown in fig. 1c of [Wunderlich & Soga 2010]. Seebeckvoltagesweremeasuredonboth,thebottomandtoppartofthespecimenbyNi-wires, which were connected to voltmeters (Sanwa PC510), marked as V1 and V2 in the inset of fig. 13 b. The temperature was measured with thermocouples also attached to voltmeters. The data were recorded online by a personal computer.MostTE-literaturereportsTE-datameasuredundersmalltemperaturegradient[Bulusu& Walkner 2008], where the theory is valid for. Our device however, measures the data under large temperature gradient, which is close to applications. When comparing such measured data with literature data on similar specimens (CoTiSb, Fe), in general about 1.5 times larger values for the Seebeck voltages are obtained. Fig. 13. Temperature (on the left y-axis), Seebeck Voltage and short-circuit current (both on the right y-axis) as a function of time. The inset shows the scheme of the experimental setup formeasuringtheSeebeckvoltageandtheclosedcircuitcurrent.(a)Typicalmeasurement forNaTaO3 +50wt%Fe,(b)SeebeckvoltageresponseforNaTaO3 +50wt%Cu,whenthe heater is switched off or on (red line). Development of Thermoelectric materials based on NaTaO3 - composite ceramics 13 section2.4.Hence,itcanbeconcluded,thattheoctahedrontiltingmentionedinsection2 was suppressed by the doping. Fig.12.SEMmicrographsoftheas-preparedsurfacesofdifferentNaTaO3-composites processed by adding 40 wt% of (a) Fe, (b) Ag, (c) Ti, (d) Mo, (e) Mn, (e) Cr, (g) Ni, (h) W. ThemicrostructureoftheNaTaO3compositeprocessedwith50wt%Feconsistsofa NaTaO3- 50 mol% Fe2O3 composite as shown in fig. 12 a. It consists of dark Fe2O3 particles, onaverage10minsize,andappearinginstreaks-likeshape,whichareembeddedina greyNaTaO3matrix.Detailedexplanationisprovidedinapreviouspaper[Wunderlich& Soga 2010]. When NaTaO3 is initially processed with Fe2O3 instead of Fe, the microstructure lookslikeasinteredceramiccompositewithwhiteFe2O3besideswhiteNaTaO3particles. The change from black to white color can be explained by oxygen saturation as explained in section 6. Such a micrograph is shown in a previous paper [Wunderlich 2009-b]. The white areas in fig. 12 a are pores remaining from insufficient compaction during sintering or from released oxygen as explained in section 5.In NaTaO3-composites containing Ag, Ti, Mn, and Ni the dark, metallic particles are slightly bigger (5~10 m). The particles have a volume fraction of about 30% which correspond well totheintensityratiosoftheXRD-pattern.In specimens,whichwereproducedfromFe2O3- insteadofFe-powder,theFe2O3-particlesformroundparticlesasshowninfig.3ain [Wunderlich 2009-b]. In the case of Cr the dark, metallic Cr-particles are significantly larger (20m),whichcanbeexplainedbytheirlowdiffusivity.Thesamewouldbeexpectedfor MoandWwiththeirhighmeltingpoints,butinsteadtheyleadtofacetedinterfaces.By chemicalmappinghomogeneousdistributionofNa,Ta,MoorWwasconfirmed.Thetwo elements, Mo, and W, having their location in the period system and their atomic radii close to Ta, and, hence, can inter-diffuse easily with Ta. They lower the surface energy of certain crystallographicplanes,whichisanimportantfacttobekeptinmindwhennano-layered composite materials based on NaTaO3 are desired.ThemaingoalofdopingistoincreasethecarrierconcentrationofNaTaO3inorderto increasetheconductivity.Inacompositethiscanonlybeachievedbyincreasingthe concentrationofthedissolvedelement.CompositionmeasurementsbyEDXinSEMwith lateral resolution of 1 m were performed on the NaTaO3-phase in the NaTaO3-composites processedwithdifferentmetals.ForCr,MoandWconcentrationsbelow2at%were detected, for Ag, Ti, Mn, and Ni, 5 ~ 10 at% were detected and for Fe 14 at%. This result can Is a necessity for a thermoelectric material and explains the success of Fe and Ag for the TE-performance as explained in the following section.

5. Thermoelectric characterization 5.1 Measuring device Thethermoelectricmeasurementswereperformedwithaself-manufactureddeviceas shown in the inset of fig. 13. The specimen was attached to the device, so that its left side lies on a copper block as a heat sink and its right side on a micro-ceramic heater (Sakaguchi Ltd. MS1000)withapowerof1kW,andwasheatedupto1000oCwithin3minutes.Hence,the bottompart ofthespecimen experiencedthe largetemperaturedifference,whiletheupper partwasheatedthroughtheheatconductivityofthespecimen.Thetemperature distribution as measured by thermocouples is shown in fig. 1c of [Wunderlich & Soga 2010]. Seebeckvoltagesweremeasuredonboth,thebottomandtoppartofthespecimenbyNi-wires, which were connected to voltmeters (Sanwa PC510), marked as V1 and V2 in the inset of fig. 13 b. The temperature was measured with thermocouples also attached to voltmeters. The data were recorded online by a personal computer.MostTE-literaturereportsTE-datameasuredundersmalltemperaturegradient[Bulusu& Walkner 2008], where the theory is valid for. Our device however, measures the data under large temperature gradient, which is close to applications. When comparing such measured data with literature data on similar specimens (CoTiSb, Fe), in general about 1.5 times larger values for the Seebeck voltages are obtained. Fig. 13. Temperature (on the left y-axis), Seebeck Voltage and short-circuit current (both on the right y-axis) as a function of time. The inset shows the scheme of the experimental setup formeasuringtheSeebeckvoltageandtheclosedcircuitcurrent.(a)Typicalmeasurement forNaTaO3 +50wt%Fe,(b)SeebeckvoltageresponseforNaTaO3 +50wt%Cu,whenthe heater is switched off or on (red line). Ceramic Materials14 By putting the specimen completely above the ceramics heater, the temperature dependence oftheelectricresistivitywasmeasuredwiththesamedeviceasshownpreviously [Wunderlich2009-b,WunderlichSoga2010].Thereason,whytheSeebeckvoltageonly appears when heated above 500C, can be explained by the poor electric conductivity at low temperatures.Theroomtemperatureresistivityofsuchsamplesdecreasesfromabout10 M to 0.1 M when sintered in at least five sintering steps (1000oC, 5h) [Wunderlich & Soga 2010]. The temperature dependence of the resistivity was measured. The activation energy EAforthermalactivationofthechargecarriersneinthisn-dopedsemiconductorswas estimated according tone = N exp (-EA/2kT) by a suitable data fit. This analysis yield to an activation energy for charge carriers of about 1 eV during heating and 0.6 eV during cooling [Wunderlich 2009-b].Anotheroptionofthisdeviceisthemeasurementoftheclosedcircuitcurrent.Forthis option, the wires below the specimen are connected with resistances of 1, 10, 100, 1k, or 1M in a closed circuit condition as seen in the inset of fig. 13 a. As the measured electric current is a material dependent property, it is recorded too. As shown in fig. 13 a or fig. 3 in [Wunderlich2009-b],assoonasthecircuitisclosed,thevoltageoftheNaTaO3-30mol% Fe2O3 specimen drops down, and the current increases according to the amount of load with a short delay time of a few ms. The detection limits are about U=1mV and I=0.8A. 5.2 Time-dependence of Seebeck voltageForthemostspecimens,theSeebeckvoltageisnottime-dependentandonlydependsonthe temperaturegradient.Time-dependenteffectsoftheSeebeck-voltageoccurrencehavebeen reported for Co-based rare-earth Perovskite-composites (for example Gd2O3+CoOx) [Wunderlich &Fujizawa.2009-d]andwereexplainedasacombinedoccurrenceofpyro-electricityand thermoelectricity.InsomeCo-basedperovskitespecimesremarkablenon-linearitiesintheplot Seebeck voltage versus temperature difference appear, but not in NaTaO3.A time-dependent Seebeck voltage behavior appears at specimens NaTaO3 + x Cu, with x from 30to50wt%,asshowninfig.13bforx=50wt%.Onsuchspecimensingeneralonlyasmall Seebeckvoltageofonly-5mVismeasured,evenattemperaturesabove500 oC,whena sufficientlyhighchargecarrierconcentrationisreached.However,whenthentheheateris switched off suddenly, a sharp pulse, a few milliseconds in length, of the Seebeck voltage with a value of 20 mV is measured with a negative sign. When switching on the heater again, the sign reversestoapositivepulseofSeebeckvoltagewiththesameabsolutevalueof20mV.The Seebeckvoltageonthebacksideofthespecimen,whichexperiencesthetemperaturegradient only indirectly through heat conduction, is not so high in its absolute value (12 mV for a 5 mm thick specimen), but it appears with the same sign and at the same time. In fig. 13 b this is shown in dark-green, while the pulse of the specimen side with the large temperature gradient is shown inlight-green.ThevalueoftheSeebeckpulseisindependentonthetime-intervalbetweenthe heatflowreversals,justtheSeebeckvoltagebetweenthepulsesfluctuatesbetween2and10 times of its absolute value. Only when the temperature gradient decreases (right side of fig. 13b), the absolute value of the pulse becomes smaller.This heat flow dependent Seebeck pulse in time appears also in NaTaO3 + x Ag specimens, whichweresinteredonlyforashorttime(1000oC,5h).Thereasonisnotyetcompletely investigated, but the interface between NaTaO3 and metallic particles, which are not reactive with NaTaO3, is responsible for this effect. It is different from pyroelectricity, which showed a similar behavior like an electric capacitor. The heat-flow dependent Seebeck voltage pulse can be utilized for building a heat-flow meter, which would be able to detect the forward or backwarddirectionoftheheatflow,duetothesignofthevoltagepulse.Bymicro fabrication several such specimens could be arranged under different angles to heat flow, so thatthevectoroftheheatflowcanbe determined,andwhensuchdevicesarearrangedin an array, even the heat flow tensor can be measured. 5.3 Seebeck voltage measured under large temperature gradientsThemeasurementsoftheSeebeckvoltageUSeeareshowninfig.14,whereatemperature gradientofuptoT=800KwasappliedtothespecimensandtheSeebeckvoltage measured as explained in section 5.1. The specimens with NaTaO3+x Fe were measured for x=30,40,50,60,70,80,90wt%.Thespecimenwithx=50,60,70wt%showedthehigh Seebeckvoltagesofabout-300mVasshowninfig.14a,detailsareexplainedinprevious publications[Wunderlich2009-b,Wunderlich&Soga2010].Fromtheplottemperature difference dT versus Seebeck voltage US a Seebeck coefficient S of 0.5 mV/K was estimated by the slope S = dUS/dT.As theXRDresultsshowedtheformationofFe2O3,also NaTaO3+rFe2O3specimens were sintered, were r was 30, 50, 70, 90 wt%. These specimens showed all a Seebeck voltage of +60 mVatT=800KwithaslightlynonlinearT-dependence.Hence,differentprocessing causesadifferentoxidationstateofthesecondcomponentinthiscomposite,andchanges then-typeNaTaO3+xFeintoap-typeNaTaO3+rFe2O3composite.Asmentionedabove, themicrostructurelooksslightlydifferentforbothcompositesandthethermo-kinetic measurements in section 6 too. When metallic Ni is added to NaTaO3, the sintered composites with x= 30 wt% Ni showed the highest value of -320 mV with a Seebeck coefficient of 0.57 mV/K, as shown in fig. 14 b. In this case non-linear behavior at T = 650 K during heating, and T = 600K during cooling appearsatallNi-specimens,butnotatotherelements,andisprobablyrelatedtosome phase transitions. In the case of W additions to NaTaO3 the specimens showed only a small Seebeckvoltageof-30mVforallconcentrationsintherange30to90wt%(fig.14c).A similar behavior is seen for Mo, where the 50wt% sample showed a Seebeck voltage of -10 mVduringheatingand+10mVduringcooling.TheplotsofSeebeckvoltageversus temperature difference are linear. Fig. 14. Seebeck Voltage as a function of the temperature difference for (a) NaTaO3+50 wt% Fe,(b)NaTaO3+30wt%Ni,(c)NaTaO3+50wt%W,(d)NaTaO3+50wt%Mo.Theslopeof the plots yield to the Seebeck-coefficients as mentioned.Development of Thermoelectric materials based on NaTaO3 - composite ceramics 15 By putting the specimen completely above the ceramics heater, the temperature dependence oftheelectricresistivitywasmeasuredwiththesamedeviceasshownpreviously [Wunderlich2009-b,WunderlichSoga2010].Thereason,whytheSeebeckvoltageonly appears when heated above 500C, can be explained by the poor electric conductivity at low temperatures.Theroomtemperatureresistivityofsuchsamplesdecreasesfromabout10 M to 0.1 M when sintered in at least five sintering steps (1000oC, 5h) [Wunderlich & Soga 2010]. The temperature dependence of the resistivity was measured. The activation energy EAforthermalactivationofthechargecarriersneinthisn-dopedsemiconductorswas estimated according tone = N exp (-EA/2kT) by a suitable data fit. This analysis yield to an activation energy for charge carriers of about 1 eV during heating and 0.6 eV during cooling [Wunderlich 2009-b].Anotheroptionofthisdeviceisthemeasurementoftheclosedcircuitcurrent.Forthis option, the wires below the specimen are connected with resistances of 1, 10, 100, 1k, or 1M in a closed circuit condition as seen in the inset of fig. 13 a. As the measured electric current is a material dependent property, it is recorded too. As shown in fig. 13 a or fig. 3 in [Wunderlich2009-b],assoonasthecircuitisclosed,thevoltageoftheNaTaO3-30mol% Fe2O3 specimen drops down, and the current increases according to the amount of load with a short delay time of a few ms. The detection limits are about U=1mV and I=0.8A. 5.2 Time-dependence of Seebeck voltageForthemostspecimens,theSeebeckvoltageisnottime-dependentandonlydependsonthe temperaturegradient.Time-dependenteffectsoftheSeebeck-voltageoccurrencehavebeen reported for Co-based rare-earth Perovskite-composites (for example Gd2O3+CoOx) [Wunderlich &Fujizawa.2009-d]andwereexplainedasacombinedoccurrenceofpyro-electricityand thermoelectricity.InsomeCo-basedperovskitespecimesremarkablenon-linearitiesintheplot Seebeck voltage versus temperature difference appear, but not in NaTaO3.A time-dependent Seebeck voltage behavior appears at specimens NaTaO3 + x Cu, with x from 30to50wt%,asshowninfig.13bforx=50wt%.Onsuchspecimensingeneralonlyasmall Seebeckvoltageofonly-5mVismeasured,evenattemperaturesabove500 oC,whena sufficientlyhighchargecarrierconcentrationisreached.However,whenthentheheateris switched off suddenly, a sharp pulse, a few milliseconds in length, of the Seebeck voltage with a value of 20 mV is measured with a negative sign. When switching on the heater again, the sign reversestoapositivepulseofSeebeckvoltagewiththesameabsolutevalueof20mV.The Seebeckvoltageonthebacksideofthespecimen,whichexperiencesthetemperaturegradient only indirectly through heat conduction, is not so high in its absolute value (12 mV for a 5 mm thick specimen), but it appears with the same sign and at the same time. In fig. 13 b this is shown in dark-green, while the pulse of the specimen side with the large temperature gradient is shown inlight-green.ThevalueoftheSeebeckpulseisindependentonthetime-intervalbetweenthe heatflowreversals,justtheSeebeckvoltagebetweenthepulsesfluctuatesbetween2and10 times of its absolute value. Only when the temperature gradient decreases (right side of fig. 13b), the absolute value of the pulse becomes smaller.This heat flow dependent Seebeck pulse in time appears also in NaTaO3 + x Ag specimens, whichweresinteredonlyforashorttime(1000oC,5h).Thereasonisnotyetcompletely investigated, but the interface between NaTaO3 and metallic particles, which are not reactive with NaTaO3, is responsible for this effect. It is different from pyroelectricity, which showed a similar behavior like an electric capacitor. The heat-flow dependent Seebeck voltage pulse can be utilized for building a heat-flow meter, which would be able to detect the forward or backwarddirectionoftheheatflow,duetothesignofthevoltagepulse.Bymicro fabrication several such specimens could be arranged under different angles to heat flow, so thatthevectoroftheheatflowcanbe determined,andwhensuchdevicesarearrangedin an array, even the heat flow tensor can be measured. 5.3 Seebeck voltage measured under large temperature gradientsThemeasurementsoftheSeebeckvoltageUSeeareshowninfig.14,whereatemperature gradientofuptoT=800KwasappliedtothespecimensandtheSeebeckvoltage measured as explained in section 5.1. The specimens with NaTaO3+x Fe were measured for x=30,40,50,60,70,80,90wt%.Thespecimenwithx=50,60,70wt%showedthehigh Seebeckvoltagesofabout-300mVasshowninfig.14a,detailsareexplainedinprevious publications[Wunderlich2009-b,Wunderlich&Soga2010].Fromtheplottemperature difference dT versus Seebeck voltage US a Seebeck coefficient S of 0.5 mV/K was estimated by the slope S = dUS/dT.As theXRDresultsshowedtheformationofFe2O3,also NaTaO3+rFe2O3specimens were sintered, were r was 30, 50, 70, 90 wt%. These specimens showed all a Seebeck voltage of +60 mVatT=800KwithaslightlynonlinearT-dependence.Hence,differentprocessing causesadifferentoxidationstateofthesecondcomponentinthiscomposite,andchanges then-typeNaTaO3+xFeintoap-typeNaTaO3+rFe2O3composite.Asmentionedabove, themicrostructurelooksslightlydifferentforbothcompositesandthethermo-kinetic measurements in section 6 too. When metallic Ni is added to NaTaO3, the sintered composites with x= 30 wt% Ni showed the highest value of -320 mV with a Seebeck coefficient of 0.57 mV/K, as shown in fig. 14 b. In this case non-linear behavior at T = 650 K during heating, and T = 600K during cooling appearsatallNi-specimens,butnotatotherelements,andisprobablyrelatedtosome phase transitions. In the case of W additions to NaTaO3 the specimens showed only a small Seebeckvoltageof-30mVforallconcentrationsintherange30to90wt%(fig.14c).A similar behavior is seen for Mo, where the 50wt% sample showed a Seebeck voltage of -10 mVduringheatingand+10mVduringcooling.TheplotsofSeebeckvoltageversus temperature difference are linear. Fig. 14. Seebeck Voltage as a function of the temperature difference for (a) NaTaO3+50 wt% Fe,(b)NaTaO3+30wt%Ni,(c)NaTaO3+50wt%W,(d)NaTaO3+50wt%Mo.Theslopeof the plots yield to the Seebeck-coefficients as mentioned.Ceramic Materials16 NaMgAlSiP --//0// K *CaScTiVCrMnFeCoNiCuZnGaGeAs -30//-20/-200-30-320-20-360-20//// RbSrYZrNbMoTcRuRhPdAgCdInSnSb /////-10/+10////+250//00 CsBaHfTaWReOsIrPtAuHgTlPbBi ///---20////////0 * KTaO3+Fe Fig.15.Partoftheperiodictableshowingtheelementswhichweretestedasdoping additives for NaTaO3. The vale refers to the Seebeck voltage in mV at T = 750K. In the case ofKitmeansKTaO3withFe-additions.Onlythetwoelementsinboldletters(Fe,Ag) showed a remarkable closed-circuit current. SuchmeasurementswereperformedbyaddingseveralmetallicelementsMefromthe periodicsystemNaTaO3+xMespecimenswithx=30,50,70,90wt%.Fig.15showsthe largest Seebeck voltage at T=800 K among these specimens, where the best results usually wereachievedforxaround50wt%.Alandthosesemiconductingelementswhichwere measured did not dissolve in NaTaO3 and such specimens remain white, a sign that they are still insulators.SpecimenssinteredfromNaTaO3-xAgpowderswithx=30,50,60wt%leadtop-type thermoelectrics.TheSeebeckcoefficientasdeducedfromfig.14a,Feasn-type,andthe correspondingplotforAgasp-type[Wunderlich2009-b]yieldforbothcompositesto almostthesamevalue,namely+/-0.5mV/K.InthecaseofNaTaO3+xFespecimens,the Seebeckvoltageincreasesduringthefirstthreesinteringcycles(1000 oC5h),andreached this saturation value,which wasconfirmedtobe stableevenaftereightsinteringcycles.In thecaseofAg-dopedNaTaO3,thevaluealsoincreases,however,afterthefourthsintering cycletheSeebeckvoltagedropstolessthan30mVandthecolorturnsintowhiteagain, indicatingastructuralinstabilityoftheNaTaO3-Agcompoundprobablyduetosilver evaporation.Thetemperaturedependenceoftheelectricresistivitywasshownpreviously [Wunderlich 2009-b, Wunderlich & Soga 2010] for both, n- and p-type specimens, with x= 50 wt%,whichwasfoundastheoptimumconcentrationforlowresistivity.Accordingtothe thermal activation of the carriers an activation energy in the order of the band gap (1 eV) can be estimated by fitting the data as shown in [Wunderlich 2009-b, Wunderlich & Soga 2010] by the formula = N0 exp(-E0/2kT) |e| e. with E0 activation energy.Therearefurtherpromisingdopingcandidates,notyetchecked,asNb,orrareearth.Asa conclusion, it canbestatedthatonlythe lighttransitionmetalslikeFe,Cr,Mn,Nishowed remarkableSeebeckvoltages.Amongthem,theclosed-circuitmeasurementsdescribedin the following section, lead to further restrictions. 5.4 Electric current under closed circuit conditionsForpowergenerationtheperformanceunderclosedcircuitconditionsis important.Fig. 16 shows the measured current when different electric resistances as load are connected. While both composites, the one processed from NaTaO3+x Fe and the NaTaO3- x Ag one, showed largeSeebeckvoltageintherangeofx=50to70wt%,theclosedcircuitcurrent measurementsshowedthehighestvalueonlyforthespecimenprocessedfromNaTaO3+x Fe with x= 50 wt%, which corresponds to NaTaO3+r Fe2O3 with r = 32 mol% after sintering.In the silver added composite, the specimen with 40 mol% Ag (about 50 wt%) yields to the optimumbetweenlargeSeebeckcoefficientandlowresistivity.FortheNaTaO3-xFe2O3-composite, the specimen with x = 32 mol % shows the highest current of 320 A, but for the Fig. 16. Seebeck Voltage and closed circuit current for n- and p-type NaTaO3 with Fe- or Ag-additionswiththeMol-%asshown.Thehorizontalandverticalarrowsindicatethetarget forcurrentandvoltageincrease,theinclinedonesindicatethetargetforpower improvement.P-andn-typematerialsshouldhavethesameSeebeckvoltageasexpressed by the target-line. NaTaO3-x Ag-composite, it is only 1.2 A. For the silver added composite, a part of Ag gets dissolved,anotherpartgetsoxidizedasNaTa1-xAgxO3-y+tAgOu,whensinteredinthree cycles (1000 oC 5h). While NaTaO3- Fe2O3 is a stable composite, the metallic Ag in the p-type compositewithitslowmeltingpointdecomposesintoaninsulatingoxideafterfour sintering cycles (1000 oC, 5h), and metallic silver balls at the surface.Themicrostructureofthep-typematerialneedstobestabilizedandoptimizedfor improving both, Seebeck voltage as well as resistivity. When this is realized, and the p-type material would have had the same short-circuit current as suggested by the target line in fig. 16, it is expected that modules with both n- and p-type materials work optimal. As p- and n-typematerialhasbeenfound,NaTaO3issuggestedasanewthermoelectricforpower generationsuitableforapplicationsinanupperrangeofapplicationtemperatures(500to 1100 oC), when a sufficient performance is achieved as described in the next section. 5.5 Estimation of the figure-of-meritTheabsolute valueofthenegativeSeebeck Voltageincreaseslinearlywiththe temperature andreaches-320mVatatemperaturedifferenceof800Kasshowninfig.14aforthe specimen NaTaO3-50mol% Fe2O3. From the slope of the Seebeck voltage versus temperature aSeebeckcoefficientof-0.5mV/Kwasestimated.Specimensintherangeof20molto70 mol%Fe2O3showedallaSeebeckcoefficientlargerthan-0.45mV/K.Fromthesedatathe figure of merit can be deduced, a little bit more promising as previously [Wunderlich 2009-b]. For the thermal conductivity in the worst case a high value of 5 W/(m K) was assumed Development of Thermoelectric materials based on NaTaO3 - composite ceramics 17 NaMgAlSiP --//0// K *CaScTiVCrMnFeCoNiCuZnGaGeAs -30//-20/-200-30-320-20-360-20//// RbSrYZrNbMoTcRuRhPdAgCdInSnSb /////-10/+10////+250//00 CsBaHfTaWReOsIrPtAuHgTlPbBi ///---20////////0 * KTaO3+Fe Fig.15.Partoftheperiodictableshowingtheelementswhichweretestedasdoping additives for NaTaO3. The vale refers to the Seebeck voltage in mV at T = 750K. In the case ofKitmeansKTaO3withFe-additions.Onlythetwoelementsinboldletters(Fe,Ag) showed a remarkable closed-circuit current. SuchmeasurementswereperformedbyaddingseveralmetallicelementsMefromthe periodicsystemNaTaO3+xMespecimenswithx=30,50,70,90wt%.Fig.15showsthe largest Seebeck voltage at T=800 K among these specimens, where the best results usually wereachievedforxaround50wt%.Alandthosesemiconductingelementswhichwere measured did not dissolve in NaTaO3 and such specimens remain white, a sign that they are still insulators.SpecimenssinteredfromNaTaO3-xAgpowderswithx=30,50,60wt%leadtop-type thermoelectrics.TheSeebeckcoefficientasdeducedfromfig.14a,Feasn-type,andthe correspondingplotforAgasp-type[Wunderlich2009-b]yieldforbothcompositesto almostthesamevalue,namely+/-0.5mV/K.InthecaseofNaTaO3+xFespecimens,the Seebeckvoltageincreasesduringthefirstthreesinteringcycles(1000 oC5h),andreached this saturation value,which wasconfirmedtobe stableevenaftereightsinteringcycles.In thecaseofAg-dopedNaTaO3,thevaluealsoincreases,however,afterthefourthsintering cycletheSeebeckvoltagedropstolessthan30mVandthecolorturnsintowhiteagain, indicatingastructuralinstabilityoftheNaTaO3-Agcompoundprobablyduetosilver evaporation.Thetemperaturedependenceoftheelectricresistivitywasshownpreviously [Wunderlich 2009-b, Wunderlich & Soga 2010] for both, n- and p-type specimens, with x= 50 wt%,whichwasfoundastheoptimumconcentrationforlowresistivity.Accordingtothe thermal activation of the carriers an activation energy in the order of the band gap (1 eV) can be estimated by fitting the data as shown in [Wunderlich 2009-b, Wunderlich & Soga 2010] by the formula = N0 exp(-E0/2kT) |e| e. with E0 activation energy.Therearefurtherpromisingdopingcandidates,notyetchecked,asNb,orrareearth.Asa conclusion, it canbestatedthatonlythe lighttransitionmetalslikeFe,Cr,Mn,Nishowed remarkableSeebeckvoltages.Amongthem,theclosed-circuitmeasurementsdescribedin the following section, lead to further restrictions. 5.4 Electric current under closed circuit conditionsForpowergenerationtheperformanceunderclosedcircuitconditionsis important.Fig. 16 shows the measured current when different electric resistances as load are connected. While both composites, the one processed from NaTaO3+x Fe and the NaTaO3- x Ag one, showed largeSeebeckvoltageintherangeofx=50to70wt%,theclosedcircuitcurrent measurementsshowedthehighestvalueonlyforthespecimenprocessedfromNaTaO3+x Fe with x= 50 wt%, which corresponds to NaTaO3+r Fe2O3 with r = 32 mol% after sintering.In the silver added composite, the specimen with 40 mol% Ag (about 50 wt%) yields to the optimumbetweenlargeSeebeckcoefficientandlowresistivity.FortheNaTaO3-xFe2O3-composite, the specimen with x = 32 mol % shows the highest current of 320 A, but for the Fig. 16. Seebeck Voltage and closed circuit current for n- and p-type NaTaO3 with Fe- or Ag-additionswiththeMol-%asshown.Thehorizontalandverticalarrowsindicatethetarget forcurrentandvoltageincrease,theinclinedonesindicatethetargetforpower improvement.P-andn-typematerialsshouldhavethesameSeebeckvoltageasexpressed by the target-line. NaTaO3-x Ag-composite, it is only 1.2 A. For the silver added composite, a part of Ag gets dissolved,anotherpartgetsoxidizedasNaTa1-xAgxO3-y+tAgOu,whensinteredinthree cycles (1000 oC 5h). While NaTaO3- Fe2O3 is a stable composite, the metallic Ag in the p-type compositewithitslowmeltingpointdecomposesintoaninsulatingoxideafterfour sintering cycles (1000 oC, 5h), and metallic silver balls at the surface.Themicrostructureofthep-typematerialneedstobestabilizedandoptimizedfor improving both, Seebeck voltage as well as resistivity. When this is realized, and the p-type material would have had the same short-circuit current as suggested by the target line in fig. 16, it is expected that modules with both n- and p-type materials work optimal. As p- and n-typematerialhasbeenfound,NaTaO3issuggestedasanewthermoelectricforpower generationsuitableforapplicationsinanupperrangeofapplicationtemperatures(500to 1100 oC), when a sufficient performance is achieved as described in the next section. 5.5 Estimation of the figure-of-meritTheabsolute valueofthenegativeSeebeck Voltageincreaseslinearlywiththe temperature andreaches-320mVatatemperaturedifferenceof800Kasshowninfig.14aforthe specimen NaTaO3-50mol% Fe2O3. From the slope of the Seebeck voltage versus temperature aSeebeckcoefficientof-0.5mV/Kwasestimated.Specimensintherangeof20molto70 mol%Fe2O3showedallaSeebeckcoefficientlargerthan-0.45mV/K.Fromthesedatathe figure of merit can be deduced, a little bit more promising as previously [Wunderlich 2009-b]. For the thermal conductivity in the worst case a high value of 5 W/(m K) was assumed Ceramic Materials18 accordingtotherangeofusualceramics.Thisleadstoanestimationofthefigure-ofmerit ZT at 1000oC as: 2SZ1 91 1210 105S/m 1 ) / 5 . 0 ( KK m WK mV, 5' 100010 27 . 1 CZT (5) ThisestimatedvalueofZTisatthemomentmuchlowerthanstate-of-the-artmaterials,for exampleSiGe,ortheabovementionedNb-dopedSrTiO3,butmaterialsdevelopment,like improved sintering, higher solubility of Fe, higher conductivity etc., will definitely increase the performance of NaTaO3, for which the following thermokinetic investigations are helpful. 6. Thermo-kinetic characterization InordertoclarifythesinteringbehavioroftheNaTaO3-Fe2O3compositedifferential scanningcalorimetry(DSC)andthermo-gravimetric(TG)measurementswereperformed. Thedevelopmentinthefieldofthermo-kineticsinthelastdecadeallowstheestimationof activation energies for chemical reactions, when DSC and TG are measured simultaneously withatleastthreedifferentheatingrates[Opfermannetal.1992,Opfermann2000].The analysisofthedifferentsinteringstepsofalumina[Bacaetal.2001],andtheoxidationof MagnetitetoFe2O3[Sanders&Gallagher2003]areexamples,wherethistechniquehas successfully been applied for ceramics. Fig. 17 Results of (a,b) DSC and (c,d) TGA measurements of (a,c) NaTaO3 + 50 wt% Fe-, and (b,d) NaTaO3 + 50 wt% Fe2O3-powder specimens. The numbers in the inset refer to heating rates in [K/min]. The red and blue arrows indicate heating and cooling, respectively. Simultaneous DSC-TG measurements were performed on a SDT Q600 (T.A.instruments) by heating two different sets of mixed powder samples (26 mg NaTaO3 + 50 wt% Fe, and 60 mg NaTaO3+50wt%Fe2O3)underairwithfivedifferentheatingratesupto1000 OC.The resultsareshowninfig.17.Thethermo-gravimetricmeasurementsshowedthatthe NaTaO3-50wt%Fepowdergains0.13mginweight(fig.17c,increaseof0.5%)andthe NaTaO3-50wt%Fe2O3 powderlooses0.05mginweight(weightreductionof0.2%).Thefact thattheweightgaininfig.17cisnotthesameforallheatingratescanbeexplainedby concentration inhomogeneities in each specimen.TheinterpretationoftheseresultsisthatFegetsoxidizedformingFe2O3whichwas observed in the XRD pattern, see section 4 and [Wunderlich & Soga 2010]. The experimental resultsshowedthatapartofFegetsdissolvedinNaTaO3,about14%.Assumingthatthe sameamountofFeisdissolvedinNaTaO3inbothcomposites(NaTaO3+50wt%Feand NaTaO3+50 wt%Fe2O3),thisfactcanexplainwhytheweightdecreasesfortheNaTaO3+ Fe2O3 mixture. Namely, the dissolved Fe needs to be reduced from the initial Fe2O3 and the excess oxygen is released. Fig.18. Analysisofthe(a,b) DSC-,(c,d)TG-datafromfig.17for(a,c)NaTaO3+50wt%Fe and(b,d)NaTaO3+50wt%Fe2O3usingASTMandFriedmanmethodyieldingtothe activationenergyfromtheslopeinthelogarithmicplotheatingrateversusinverse temperature.Theinsetshowsthedistributionoftheactivationenergyasafunctionofthe partial fraction. TheDSCmeasurementsshowedanexothermicpeakforNaTaO3+50wt%Fe(fig.17a), namelyapositiveheatflow,withabroadmaximumat500oCandsmallwigglesat690 oC Development of Thermoelectric materials based on NaTaO3 - composite ceramics 19 accordingtotherangeofusualceramics.Thisleadstoanestimationofthefigure-ofmerit ZT at 1000oC as: 2SZ1 91 1210 105S/m 1 ) / 5 . 0 ( KK m WK mV, 5' 100010 27 . 1 CZT (5) ThisestimatedvalueofZTisatthemomentmuchlowerthanstate-of-the-artmaterials,for exampleSiGe,ortheabovementionedNb-dopedSrTiO3,butmaterialsdevelopment,like improved sintering, higher solubility of Fe, higher conductivity etc., will definitely increase the performance of NaTaO3, for which the following thermokinetic investigations are helpful. 6. Thermo-kinetic characterization InordertoclarifythesinteringbehavioroftheNaTaO3-Fe2O3compositedifferential scanningcalorimetry(DSC)andthermo-gravimetric(TG)measurementswereperformed. Thedevelopmentinthefieldofthermo-kineticsinthelastdecadeallowstheestimationof activation energies for chemical reactions, when DSC and TG are measured simultaneously withatleastthreedifferentheatingrates[Opfermannetal.1992,Opfermann2000].The analysisofthedifferentsinteringstepsofalumina[Bacaetal.2001],andtheoxidationof MagnetitetoFe2O3[Sanders&Gallagher2003]areexamples,wherethistechniquehas successfully been applied for ceramics. Fig. 17 Results of (a,b) DSC and (c,d) TGA measurements of (a,c) NaTaO3 + 50 wt% Fe-, and (b,d) NaTaO3 + 50 wt% Fe2O3-powder specimens. The numbers in the inset refer to heating rates in [K/min]. The red and blue arrows indicate heating and cooling, respectively. Simultaneous DSC-TG measurements were performed on a SDT Q600 (T.A.instruments) by heating two different sets of mixed powder samples (26 mg NaTaO3 + 50 wt% Fe, and 60 mg NaTaO3+50wt%Fe2O3)underairwithfivedifferentheatingratesupto1000 OC.The resultsareshowninfig.17.Thethermo-gravimetricmeasurementsshowedthatthe NaTaO3-50wt%Fepowdergains0.13mginweight(fig.17c,increaseof0.5%)andthe NaTaO3-50wt%Fe2O3 powderlooses0.05mginweight(weightreductionof0.2%).Thefact thattheweightgaininfig.17cisnotthesameforallheatingratescanbeexplainedby concentration inhomogeneities in each specimen.TheinterpretationoftheseresultsisthatFegetsoxidizedformingFe2O3whichwas observed in the XRD pattern, see section 4 and [Wunderlich & Soga 2010]. The experimental resultsshowedthatapartofFegetsdissolvedinNaTaO3,about14%.Assumingthatthe sameamountofFeisdissolvedinNaTaO3inbothcomposites(NaTaO3+50wt%Feand NaTaO3+50 wt%Fe2O3),thisfactcanexplainwhytheweightdecreasesfortheNaTaO3+ Fe2O3 mixture. Namely, the dissolved Fe needs to be reduced from the initial Fe2O3 and the excess oxygen is released. Fig.18. Analysisofthe(a,b) DSC-,(c,d)TG-datafromfig.17for(a,c)NaTaO3+50wt%Fe and(b,d)NaTaO3+50wt%Fe2O3usingASTMandFriedmanmethodyieldingtothe activationenergyfromtheslopeinthelogarithmicplotheatingrateversusinverse temperature.Theinsetshowsthedistributionoftheactivationenergyasafunctionofthe partial fraction. TheDSCmeasurementsshowedanexothermicpeakforNaTaO3+50wt%Fe(fig.17a), namelyapositiveheatflow,withabroadmaximumat500oCandsmallwigglesat690 oC Ceramic Materials20 and700 oC,presentindataobtainedatallheatingratesatthesametemperature.Onthe otherhand,attheNaTaO3-Fe2O3mixturethemainexothermicpeakshiftsfrom410 OCat slowheatingrates(1K/min,3K/min)to510 oCatthefastestheatingrate(50K/min).At thesetemperaturesthecorrespondingTG-datashowedalargedecreaseinweight, indicating a chemical reaction.TheNetzschThermokineticssoftwarepackageversion3[Opfermann2000]wasusedfor dataanalysis.Allfoursetsofdatawereanalyzedseparatelyandonlythedataduring heatingwereused.Foreachset,theparameter-freeanalysisoftheactivationenergy accordingtoASTME698wasperformedasshowninfig.18.ThenFriedmananalysis [Opfermann et al. 1992, Opfermann 2000] of the activation ene