study of the formation of the apatite-type phases synthesized from a lanthanum oxycarbonate
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Study of the formation of the apatite-type phases synthesized from a lanthanum oxycarbonateTRANSCRIPT
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Solid State Sciences 38 (2014) 150e155
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Solid State Sciences
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Review
Study of the formation of the apatite-type phasesLa9.33þx(SiO4)6O2þ3x/2 synthesized from a lanthanum oxycarbonateLa2O2CO3
A. Pons a, J. Jouin a, E. B�echade a, *, I. Julien a, O. Masson a, P.M. Geffroy a, R. Mayet a,P. Thomas a, K. Fukuda b, I. Kagomiya b
a Science des Proc�ed�es C�eramiques et de Traitements de Surface, UMR CNRS 7315, Centre Europ�een de la C�eramique e Facult�e des Sciences et Techniques eENSCI, 12 Rue Atlantis, 87068 Limoges Cedex, Franceb Department of Environmental and Materials Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan
a r t i c l e i n f o
Article history:Received 27 May 2014Received in revised form13 October 2014Accepted 23 October 2014Available online 24 October 2014
Keywords:OxyapatitePowders-solid state reactionLanthanum carbonateIonic conductivityFuel cells
* Corresponding author.E-mail addresses: [email protected] (A.
(J. Jouin), [email protected] (E. B�echad(I. Julien), [email protected] (O. Masson), p(P.M. Geffroy), [email protected] (R. Mayet(P. Thomas), [email protected] (K. Fuk(I. Kagomiya).
http://dx.doi.org/10.1016/j.solidstatesciences.2014.10.01293-2558/© 2014 Elsevier Masson SAS. All rights re
a b s t r a c t
Lanthanum silicated apatites with nominal composition La9.33þx(SiO4)6O2þ3x/2 (�0.2 < x < 0.27) havebeen successfully synthesized by solid state reaction using a new reagent La2O2CO3 and amorphous SiO2
precursors. The formation mechanism of La2O2CO3 reagent, which cannot be purchased, has been fol-lowed by in-situ temperature depend XRD of La2O3 under CO2 atmosphere. The stability of this reagentduring the synthesis step allowed to limit the formation of secondary phase La2Si2O7 and made theweighting of the reagent easier. High purity powders could be synthesized at the temperature of 1400 �C.Dense pellets (more than 98.5%) were obtained by isostatic pressing of powders calcined at 1200 �C andthen sintered at 1550 �C. Traces of La2SiO5 secondary phase present in synthesized powder disappearedafter densification and pure oxyapatite materials were obtained for all the compositions. Electricalmeasurements confirmed that conductivity behaviors of the sintered pellets were dependent to theoxygen over-stoichiometry. Indeed, a relatively high conductivity of 1 � 10�2 S cm�1 was exhibited at800 �C for the nominal composition La9.60(SiO4)6O2.405 with low activation energy around 0.79 eV. Theionic conductivity properties were comparable with that of the earlier obtained materials.
© 2014 Elsevier Masson SAS. All rights reserved.
1. Introduction
Over the last three decades, solid oxide fuel cell (SOFC) has beenattracting considerable interest because of their low emission ofpollutants and high energy conversion efficiency [1e3]. Animportant number of oxide ion conductor materials have beendeveloped for an application as SOFC's solid electrolytes. At present,the most common materials for this kind of fuel cells are oxide ionconducting yttria-stabilized zirconia (YSZ) [2e4]. However, theneed of using SOFC at lower temperatures, from 600 to 800 �C,requires new oxide ions conductors with equivalent conductivityvalues than YSZ at 1000 �C (above 0.1 S cm�1).
Pons), [email protected]), [email protected]@unilim.fr), [email protected]), [email protected]
13served.
In this context, lanthanum silicates of general formulaLa9.33þx(SiO4)6O2þ3x/2 and with apatite-type structure appeared asattractive ionic conductors and are considered as potential candi-dates to operate as electrolyte for SOFC in intermediate tempera-tures (700e900 �C) [5e11]. Their structure is based on a 3-dimensional organization of isolated silica tetrahedra, forminglarge lanthanum oxide channels oriented along c-axis [12e14]. Thisexplains that they can exhibit high anisotropic conductivity in the cdirection increasing with the incorporation of interstitial oxygeninside large channel (formula with x > 0) [6e8,10,13e16]. Forexample, highly oriented La9.5(SiO4)6O2.5 apatite crystals of a fewmicrometers have been prepared from La2Si2O7 and La2SiO5 with along time heating at 1650 �C and exhibit a high conductivity valuealong c-axis of 7.9 � 10�2 S cm�1 at 800 �C with a low activationenergy value of 0.35 eV [13]. This conductivity is higher than the2.39 � 10�2 S cm�1 measured at 800 �C on a c-oriented apatitepolycrystal of La9.33(SiO4)6O2 [6]. No difference in activation energyvalues between these compositions is observed. It suggests thatboth materials present the same oxygen interstitial mechanismalong the channel [6,13,16].
Fig. 1. Thermal reaction of the La2O3 secondary phases in air according to Bernal et al.[28].
A. Pons et al. / Solid State Sciences 38 (2014) 150e155 151
Studies performed on the synthesis of apatite-type lanthanumsilicates by solid state reactions have indicated that thermal treat-ments at high temperatures and long time are required to preparepure apatite-type lanthanum silicate when starting from La2O3 andSiO2 precursors [17e20]. These high temperatures present somedisadvantages such as a low control of the particle morphology andmaterial density. Recent works [21e25] have shown that bulk andgrain boundary conductivities may highly depend on the micro-structure of the apatite sample, especially porosity and grain size. Itwas previously demonstrated that pure apatite-type structurecould be synthesized at a lower temperature of 1200 �C using anappropriate thermal treatment and synthesis process [21].
This paper deals with the synthesis of pure oxyapatites usinglanthanum oxycarbonate La2O2CO3 as a new reagent. CompositionsLa9.33þx(SiO4)6O2þ3x/2 (�0.2 < x < 0.27) were synthesized for thepresent study. La9.33(SiO4)6O2 composition was used as a “refer-ence” material because of its oxygen stoichiometry. The otherswere oxygen under- and over-stoichiometric (x < 0 and x > 0respectively) in order to study phase diagram compositions and tocharacterize evolution of electrical properties in relation to theoxygen stoichiometry (or x) in apatite phase.
2. Materials and methods
2.1. Reagents
All the samples were elaborated by solid state reaction usinghigh purity powders of La2O3 (Aldrich, 99.9%) and SiO2 (Prolabo,>99.5%) as starting reagents. The synthesis procedure is discussedin the results section.
2.2. Characterization
X-ray powder diffraction (XRPD) patterns of the calcined pow-ders were recorded with CuKa radiation in the 2q range 20e50� ona q/2q diffractometer (Siemens D5000, Germany) with a step of0.04� and exposure time of 2.8 s per step. The crystalline phaseswere identified from a comparison of the registered patterns withthe International Center for Diffraction Data (ICDD) powderdiffraction files (PDF).
A D8 advance diffractometer (Bruker, Germany) with CuKa1 ra-diation was used to characterize sintered samples at room tem-perature and to study the in-situ evolution of La2O3 reagent underCO2 flux as a function of temperature. For the former experiment,patterns were recorded in the 2q range 10e60� with a step of 0.014�
and exposure time of 87 s per step. For the latter experiment, anexposure time of 350 s per step and Anton Paar HTK1200N Furnacewith a controlled CO2 flow were used.
The carbonatation of lanthanum oxide was also investigated bythermogravimetric and differential thermal analysis (TGeDTA, TAinstruments STD2960) from room temperature to 1200 �C in plat-inum crucible with a heating/cooling rate of 10 �C min�1 and a CO2flow rate of 20 mL min�1. The sample mass was about 40 mg.
Microstructural aspects of sintered pellets were revealed byscanning electron microscopy (Philips XL30 SEM microscope).Surfaces of the pellets were treated before observation by polishingwith SiC paper and thermal etching at 1500 �C for 15 min. Therelative density of the sintered samples was evaluated by imageanalysis using ImageJ software [26].
In order to perform electrical characterization, metallic elec-trodes were deposited on the pellets using platinum paint (Ferro,6402-1001) calcined at 1000 �C for 1 h with 5 �C/min heating rateand 2 �C min�1 cooling rate. Electrical properties were measuredusing a Solartron 1260 Impedance/Gain Phase Analyzer. The sam-ples have been characterized in static air by the complex
impedancemethod in the frequency range from 1 Hz to 5MHzwithan ac signal of 300 mV. Curve fitting and resistance calculationswere done using Z-live software [27]. The total conductivities werecalculated using the Equation (1):
s ¼ lS:R
(1)
where l is the sample thickness, S is the electrode area of the samplesurface and R is the total bulk resistance. Activation energies (Ea)were calculated by fitting the conductivity data using the Arrheniusrelation for thermally activated conduction, which is given byequation (2).
s,T ¼ s0,exp��Eak,T
�(2)
where s, s0, Ea, k, T are the conductivity, pre-exponential factor,activation energy, Boltzmann constant and absolute temperature,respectively.
3. Results and discussion
3.1. Preparation and characterization of La2O2CO3 precursor
Previous work revealed that La2O3 reagent was unstable and ledto the formation of La(OH)3 and La2(OH)4CO3 phases at ambientatmosphere. These two phases decompose when temperature rai-ses above 300 �C leading to the formation of LaOOH and hexagonalLa2O2CO3 respectively, as shown in Fig. 1.
When the La2O3/SiO2 mixture was calcined to obtain apatite,La2Si2O7 impurity was observed at 450 �C; the formation of thissecondary phase during the thermal treatment is likely due to theexistence of LaOOH phase according Equation (3).
2LaOOHþ 2SiO2/La2Si2O7 þH2O at 450 �C (3)
A preliminary La2O3 treatment (800 �C) and furnace pre-heatingat 450 �C was necessary in order to use lanthanum oxide as a re-agent and to avoid LaOOH phase formation above 300 �C.
As La2O2CO3 reagent is more stable in this range of temperature,the direct use of this reagent is a good solution to simplify thesynthesis procedure. Moreover, Kobayashi et al. [29] reported thatthe formation of lanthanum silicate apatite was in good agreementwith the decomposition of hexagonal lanthanum oxycarbonateduring solegel synthesis.
Fig. 3. In-situ temperature dependent XRD of La2O3 under CO2 atmosphere at themaximum temperature of 900 �C.
A. Pons et al. / Solid State Sciences 38 (2014) 150e155152
La2O2CO3 reagent cannot be purchased and has to be synthe-sized at the laboratory. The transformation from La2O3 to La2O2CO3was followed by TGeDTA analysis under CO2 atmosphere as shownin Fig. 2.
An endothermic feature is observed at 370 �C. It is associatedwith a small weight loss and corresponds to a partial dehydratationof the sample due to its high sensitivity towards water. Then, aprogressive carbonatation of the sample occurs from 470 �C to700 �C and results in the formation of La2O2CO3 phase [30].
This is related to a broad exothermic peak and a weight gain of12%. This carbonate phase is stable until 950 �C, and the decar-bonatation leads to La2O3 phase at 1025 �C. This decarbonatation isassociated with a sharp endothermic peak. Upon decreasing of thetemperature, the lanthanum oxide is carbonated again from 815 �Cto 660 �C with a strong exothermic effect. Finally, a last exothermicfeature is observed at 485 �C without any mass change. This is dueto the allotropic transformation of monoclinic to hexagonallanthanum carbonate as shown below.
According to Belous et al. [31], the carbonatation and decar-bonatation processes occur into two steps as proposed in thefollowing equations. This explains the slight deformation/asym-metry of corresponding DTA peaks.
2La2O3 þ CO2���! ���550 �CLa2O2CO*3La2O3 (4)
La2O2CO*3La2O3 þ CO2���! ���650 �C2La2O2CO3 (5)
In-situ X-ray diffraction as a function of the temperature wasperformed under a flow of CO2 using La2O3 as starting reagent, inorder to understand the main reactions occurring during thermaltreatments. The maximum temperature of the experiments waseither 900 �C (Fig. 3) or 1100 �C (Fig. 4).
The first cycle, represented in Fig. 3, was used to understand themechanism of the carbonatation occurring with increasing tem-perature and the stability of the obtained lanthanum carbonate.The second one (Fig. 4) follows the second carbonatation observedfrom La2O3 during the temperature decrease, especially the featureobserved at 485 �C on the DTA curve.
La2O3 is highly sensitive to atmospheric water and leads to theformation of La(OH)3 which weak peaks are visible on startingroom temperature diffractograms.
Fig. 2. TGeDTA analyses of La2O3 precursor calcined under CO2.
The study of the first in-situ XRD, Fig. 3, shows that La(OH)3dehydrated at 370 �C as seen in DTA (Fig. 2). Indeed, a mixture ofLa2O3 and La(OH)3 is observed on XRD diagram measured at roomtemperature and only La2O3 can be seen at 400 �C. Single phasehexagonal La2O2CO3 (PDF No 037-0804) detected at 900 �C con-firms the total carbonatation of our compound. This phase is wellcrystallized and remains stable during cooling.
The second in-situ XRD reveals that after heating at highertemperature than 1000 �C, La2O2CO3 decarbonate to form La2O3 asobserved at 1100 �C (Fig. 4). During the cooling step toward 600 �C,recarbonatation of La2O3 isn't completed due to low kinetic of thecarbonation reaction. At 600 �C, a mixture of three phases La2O3,monoclinic La2O2CO3 and hexagonal La2O2CO3 is detected. Peaksare much broader than in the first cycle, which suggests a muchsmaller crystallite size. At 400 �C, La2O3 phase is totally transformedinto monoclinic and hexagonal oxycarbonate phases. The intensityof the hexagonal phase is higher at 400 �C than 600 �C in agreementwith the DTA peak observed at 485 �C (without mass change,Fig. 2).
Carbonatation from temperature higher than 1000 �C leads to amixture of hexagonal andmonoclinic La2O2CO3. However, pure and
Fig. 4. In-situ temperature dependent XRD of La2O3 under CO2 atmosphere at themaximum temperature of 1100 �C.
A. Pons et al. / Solid State Sciences 38 (2014) 150e155 153
stable hexagonal phase can be synthesized at lower temperature asshow in Fig. 3. The thermal stability of this phase is crucial to avoidformation of La2Si2O7 secondary phase [17] as reported in equation(3).
3.2. Synthesis of oxyapatite powders
Powders with the nominal composition La9.33þx(SiO4)6O2þ3x/2(x ¼ �0.2, 0, 0.2 and 0.27) were synthesized using stoichiometricamounts of La2O2CO3 and SiO2 in accordance with Equation (6).
ð9:33þ xÞ=2 La2O2CO3 þ 6 SiO2/La9:33þxðSiO4Þ6O2þ3x=2þ ð9:33þ xÞ=2 CO2 (6)
In order to obtain our reagent, La2O3 was calcined at 900 �Cunder CO2 for 2 h with a heating and cooling rate of 10 �C/min, onplatinum sheets under a continuous flux of 0.5 L h�1 of CO2 in orderto be carbonated into La2O2CO3 phase according to the reactionfollowing.
La2O3 þ CO2/La2O2CO3 (7)
In opposite to La2O3, the La2O2CO3 precursor is not hygroscopicand the lanthanum molar ratio could be precisely determined.Stoichiometric amounts of SiO2 and La2O2CO3 (see Equation (6)),were attritionmilled in ethanol (2 h 30min,180 rpm) using a UnionProcess 01-Lab Attritor. Then, the ethanol was evaporated undervacuum at 45 �C (P ¼ 100 mbar). During this step, hexagonalLa2O2CO3 was stable and was not transformed into La(OH)3 phaseas observed with La2O3 reagent.
Then these mixtures were calcined at different temperaturesfrom 1000 to 1500 �C during 4 h with a heating and cooling rate of10 �C/min and phases were identified by XRD (example ofLa9.33(SiO4)6O2 in Fig. 5).
It can be observed that the powders calcined at 1000e1200 �Cpresented apatite crystalline phase and traces of La2O3 and La2SiO5impurities. No trace of La2Si2O7 is observed, which confirms theinfluence of lanthanum oxycarbonate on the synthesis. Moreover,traces of secondary phase La2SiO5 are no longer detected from1300 �C for stoichiometric phase and from 1400 �C for the othersamples. La2O3 reagent is no longer observed from 1300 �C.
Fig. 5. XRD patterns of synthesized La9.33(SiO4)6O2 powders after calcination at1000 �C, 1100 �C, 1200 �C, 1300 �C, 1400 �C and 1500 �C for 4 h.
Apatite powders, without La2SiO5 or La2Si2O7 secondary phasesand with the nominal composition La9.33þx(SiO4)6O2þ3x/2 (x ¼�0.2,0, 0.2 and 0.26), are synthesized at 1400 �C for 4 h. This synthesizedtemperature is significantly lower than the ones previously re-ported in literature (1750 �C/4 h for Yoshioka [11] or 1700 �C/2 h forSansom [18] for example). Actually, according to ref [8,9], La10(SiO4)O3 were synthesized, but no data on the relevant atomic positionswere published up today. In our laboratory, the experimental at-tempts in that direction were unsuccessful.
3.3. Microstructural and electrical characterizations
The La9.13(SiO4)6O1.7, La9.33(SiO4)6O2, La9.53(SiO4)6O2.3 andLa9.60(SiO4)6O2.405 powders calcined at 1200 �C were manuallygrinded and isostatically pressed at 2000 bar for 1 min. Then thesamples were sintered on platinum sheets at 1550 �C for 8 h with aheating rate of 10 �C min�1 and natural cooling down to roomtemperature. The obtained pellets were ~6 mm in diameter and~2 mm thickness.
X-ray diffraction patterns of the crushed sintered samples arepresented in Fig. 6. Although traces of La2SiO5 phase in the powderscalcined below 1300 �C were observed, samples sintered at 1550 �Cwere single oxyapatite phase without any other impurity.
SEM micrographs obtained for the four pellets of differentcomposition are presented in Fig. 7. It can be seen that some poresexists in the sintered body; all of them are located at the grainboundaries or at the triple points. When the composition is over-stoichiometric in oxygen, the pores quantity is more importantwhich lead to a small decrease of the relative density. The relativedensity of these samples, calculated from SEM image analysis, wasover 98.5% after sintering at 1550 �C for 8 h (Fig. 7). Although nooptimization of the sintering time was performed, it seems thatoxygen vacancies lead to a significant increase of the grain size(6.2 mm average) as compared to stoichiometric or over-stoichio-metric compositions, due to higher ion mobility.
3.4. Electrical conductivity
Electrical characterizations were carried out on pellets obtainedfrom the experimental procedure presented below. Compleximpedance spectra were analyzed using the program Z-livedeveloped by S. Georges [26], in order to quantitatively estimatethe total conductivity of the samples. Fig. 8 shows an example ofcomplex impedance plan plot obtained for the compositionLa9.13(SiO4)6O1.7 at 320 �C. The contributions of grain bulk, grainboundaries and electrodes were deconvoluted by using a parallelequivalent circuit. The deconvolution of the different contributions
Fig. 6. XRD patterns of La9.13(SiO4)6O1.7, La9.33(SiO4)6O2, La9.53(SiO4)6O2.3 andLa9.60(SiO4)6O2.405 sintered at 1550 �C for 8 h from powders synthesized at 1200 �C.
Fig. 7. SEM micrographs of (a) La9.13(SiO4)6O1.7, (b) La9.33(SiO4)6O2, (c) La9.53(SiO4)6O2.3 and (d) La9.60(SiO4)6O2.405 synthesized at 1200 �C for 4 h and sintered at 1550 �C for 8 h.
A. Pons et al. / Solid State Sciences 38 (2014) 150e155154
is shown as dashed-line semicircle for the sample La9.13(SiO4)6O1.7presented in Fig. 8. The grain boundary contribution for the sampleLa9.13(SiO4)6O1.7 is higher than the grain bulk contribution, as re-ported in previous works [17]. However, it is not possible todistinguish the contribution of grain bulk and grain boundaries forother samples. Further studies are required to distinguish the effectof oxygen over or under-stoichiometry compositions on the grainbulk and grain boundary conductivities of apatite.
Total electrolyte resistance was calculated by addition of theresistances at high frequency (RHF) and at medium frequency (RMF),corresponding respectively to the bulk and the grain boundarycontributions. Fig. 9 shows an Arrhenius plot of total conductivities
Fig. 8. Example of deconvolution, using Z-live software, of complex impedance planplot measured under air atmosphere at 320 �C for La9.13(SiO4)6O1.7.
versus temperature. Extrapolated conductivities at 800 �C andactivation energy values are reported in Table 1.
The high temperatures conductivity values for apatite materialwere lower than that reported for YSZ [4]. This was probably due toa small grain size in our pellets. The microstructures and thusproperties of our materials were not optimized but can be modifiedwith an appropriate thermal treatment.
The conductivity is directly correlated with the amount of ox-ygen in the apatite structure. The activation energy is the highest(0.95 eV) for the composition with the lowest oxygen content(corresponding to oxygen under-stoichiometry composition). Forstoichiometric and over-stoichiometric compositions, the activa-tion energy (~0.8 eV) is similar, suggesting that the conductionmechanism is similar. This value is measured on polycrystalline
Fig. 9. Total ionic conductivity for the La9.13(SiO4)6O1.7, La9.33(SiO4)6O2, La9.53(SiO4)6O2.3
and La9.60(SiO4)6O2.405 compositions. Data on polycrystalline YSZ (8% mol Y2O3) [4] areshown for comparison.
Table 1Conductivities at 320 �C, 800 �C and activation energy values forLa9.33þx(SiO4)6O2þ3x/2 materials and YSZ.
Composition Conductivityat 320 �C(S cm�1)
Extrapolatedconductivityat 800 �C(S cm�1)
Activationenergy (eV)
La9.13(SiO4)6O1.7 1.3 � 10�6 0.3 � 10�2 0.95La9.33(SiO4)6O2 7.3 � 10�6 0.5 � 10�2 0.83La9.53(SiO4)6O2.3 1.3 � 10�5 1.1 � 10�2 0.84La9.60(SiO4)6O2.4 1.8 � 10�5 1.0 � 10�2 0.79YSZ 8.9 � 10�6 6.7 � 10�2 1.08
A. Pons et al. / Solid State Sciences 38 (2014) 150e155 155
material, and is higher than what can be obtained on oriented ormonocrystalline material. However these results are in goodagreement with what is reported in literature [11,13,17,32] and as-sume that the oxide-ionic conductivity in this apatite-type struc-ture is due to interstitial oxide ions [10,15,16,23]. This newelaboration route has no damaging effect on the ionic conductivity.
4. Conclusions
Homogeneous apatite-type La9.13(SiO4)6O1.7, La9.33(SiO4)6O2,La9.53(SiO4)6O2.3 and La9.60(SiO4)6O2.405 have been prepared suc-cessfully by a new process consisting in hexagonal La2O2CO3 andSiO2 solid state reaction. The use of lanthanum dioxycarbonate asreagent makes the weighting step easier. Moreover, the formationof La2Si2O7 secondary phase, which appears by chemical reactionbetween LaOOH and SiO2, can be completely eliminated by usingLa2O2CO3 instead of La2O3. Pure dense pellets were prepared bysintering at 1550 �C of isostatically-pressed powders calcined at1200 �C. The oxygen content dependence of electrical properties isin good agreement with conduction mechanisms previouslyobserved. The highest ionic conductivity s800�C¼ 1.10�2 S cm�1 andlowest activation energy 0.79 eVwere obtained for the compositionLa9.60(SiO4)6O2.405 with the highest oxygen content. This newexperimental procedure for elaboration of pure apatite dense pel-lets using lanthanum carbonate reagent seems to have nodamaging effect on the electrical properties of this series ofmaterials.
Investigations are in progress to improve the microstructure ofthe pellets and study the effect of the grain size and density on theconductivity.
Acknowledgments
The authors wish to thank R. Kaneko and Y. Shimono for theirhelp during the synthesis process. Limousin Region is gratefullyacknowledged for its financial support.
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