news from glass crystallization rüssel_357 br_4... · the state-of-the-art solution for high power...

Christian Ruumlssel Christian Bocker Martina StoicaKatrin Thieme Askan Keshavarzi

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Journal of Chemical Technology and Metallurgy 50 4 2015 357-366

NEWS FROM GLASS CRYSTALLIZATION


Otto-Schott-Institut Jena University Fraunhofer str 6 07743 Jena GermanyE-mail ccruni-jenade

ABSTRACT

Procedures for preparation of nano glass-ceramics are described In principle nano-crystals can be precipitated from a homogeneous glass If the crystals contain components which act as network modifiers the viscosity increases in a shell around the crystals and hence the diffusion coefficient increases This leads to well expressed decrease of the crystal growth velocity and hence to a fairly narrow crystal size distribution Another procedure refers to a preceding phase separation This can happen following a similar scheme ie the droplet phase is enriched in network modifiers which results in small droplets formation within a narrow size distribution range In the subsequent step crystallization occurs inside the droplets

The paper describes further the application of this principle to the preparation of new photo thermal refractive glass Here UV-light irradiation and a subsequent two step tempering leads to crystallization of CaF2 in the irradiated regions No crystallization occurs in the non-irradiated one

An effect denoted as ldquonucleation inhibitionrdquo is described as well Small quantities of La2O3 Al2O3 ZrO2 or TiO2 lead to nucleation rate decrease as well as induction time increase The addition of these components results also in an increase of the glass viscosity The effect on the nucleation rate and induction time is however much too large to be explained by viscosity increase

Glass ceramics for lighting is another interesting topic of large industrial relevance The light of blue LEDs passes a conversion material which transfers it to light of other wavelengths so that the overall impression for the human eye is that of white light The state-of-the-art solution for high power applications refers to inorganic phosphor embedding in a polymer However this does not sufficiently remove the heat generated There are glass compositions which allow the crystallization of Ce3+ doped yttrium aluminium garnet thus providing a broad fluorescence in the visible range in case of blue light irradiation Different crystal morphologies can be observed depending on the chemical composition provided

Keywords crystallization nucleation nano crystals

Received 30 January 2015Accepted 20 May 2015

INTRODUCTION

Among the nano materials glass-ceramics are of high potential especially with respect to applications in optics and photonics [1 2] Some challenging properties such as up-conversion can hardly be achieved in glassy as well as in polycrystalline materials Up-conversion enables the generation of visible light from infrared one ie light of large wavelengths is transferred to light of smaller wavelengths [3 4] It is quite essential to avoid light scattering when glass-ceramics are applied in photonic technologies This can be scarcely achieved

by adapting the refractive index of the residual glassy matrix to that of the crystal because that prerequisite must be fulfilled at all wavelengths of interest Hence it is necessary to tailor the chemical composition and the preparation technology of the parent glass aiming to obtain precipitated crystals of a size much smaller than half of the wavelength of the light in a narrow size distribution range [5]

In the past decade glass-ceramic materials were of increasing interest with respect to any type of light con-version Fluorescence besides the already mentioned up-conversion offers also challenging applications [6 - 11]

Journal of Chemical Technology and Metallurgy 50 4 2015

358

These materials can be utilized in light converters [6 - 8] which are especially important for the conversion of LEDs blue light into white one At present inorganic phosphors such as Ce3+ doped yttrium aluminium garnet embedded in organic polymers like polysiloxanes are used for this purpose If higher light intensities are to be achieved the thermal conductivities of these composites are not sufficient to remove the generated heat Hence it is advantageous to crystallize appropriate phosphors directly in the glass [7 8 12 - 14] when high energy densities are required

The preparation of magnetic [15 16] or semicon-ducting phases [17 - 19] from glasses is another im-portant field especially if nano-crystals of respective compositions are crystallized from borate glasses In the latter case a subsequent dissolution of the borate glass matrix is carried out in water or diluted acid resulting in nanocrystalline powders preparation These powders usually exhibit narrower particle size distributions than those obtained by other methods [20 21] and may also possess superparamagnetic or other interesting magnetic electric and optical properties

Although nano-crystalline glass-ceramics are of great technological potential for numerous new applica-tions the studies focused on the fundamentals of their preparation were only recently reported [15 - 22] Clas-sical theories of nucleation and crystal growth mainly concern isochemical systems ie systems in which the precipitated crystals have the same chemical composition as that of the parent glass [22 23] They usually describe the nucleation and the crystal growth as two separate processes Furthermore the assumption that a nucleus formed starts immediately to grow is not justified in any case Glass-ceramics containing a large quantity of nano-sized crystals of diameters ranging from 5 nm to 50 nm within a narrow size distribution interval have been so far reported only in non-isochemical systems [25] In these multicomponent systems the chemical composi-tion of the residual glass changes during the course of crystallization Diffusion layers are formed around the growing nuclei affecting the crystal growth velocity due to the depletion of the components attached to the crystal phase Thus the concentration gradient varies with the change of the diffusion coefficient Hence the latter has to decrease continuously in the course of crystallization if nano-crystals formation is aimed at This results in a layer of increasing viscosity which acts as a barrier strongly decelerating further crystal growth [24 26]

In the past few years numerous fundamental investiga-tions on the preparation of nano-crystalline glass-ceramics from metal fluoride containing glasses eg from glasses containing alkali fluorides alkaline earth fluorides [22 - 32] rare earth fluorides [33-36] or alkali rare earth fluorides [37 - 41] as well as PbF2YbF3 solid solutions [9 42 43] have been reported All these crystals may also be doped with other rare earth elements The crystallisation of ferroelectric phases such as lithium niobate [44 45] or BaTiO3 [46 47] of oxidic semiconducting phases such as indium oxide [48 49] or tin oxide [50] as well as that of ferromagnetic phases such as magnetite [15 16] has been also described

The precipitation of α- or β-quartz or related struc-tures such as szlig-eucryptite spodumen or keatite from glasses in the systems MgOAl2O3SiO2 [51 - 57] and Li2OAl2O3SiO2 [58 - 61] is a special topic These glass-ceramics exhibit often a coefficient of thermal expansion close to zero (eg Ceranreg or Zerodurreg)

Another new and challenging topic in glass ce-ramic research refers to the improvement of nucleation processes control [62 63] Especially in case where the glass powders are to be sintered and subsequently crystallized the two production steps have to be well separated Otherwise the sintering process might not be completed prior to the formation of a notable quantity of crystals which affects the continuation of the sintering process The addition of small concentrations of the so called nucleation inhibitors is required in this case It is also of special importance in crystallizing glass seals

This paper gives a short overview on nano-crys-tallization of glass and some special applications of nano-crystalline glass-ceramics of glass ceramics for lighting technology and on the inhibition of nucleation by small concentrations of certain additives

GLASS-CERAMICS WITH NANO SIZED CRYSTALS The glass-ceramics nano-crystallinity can be achieved

by the formation of a highly viscous layer around the grow-ing crystals This layer is initially formed by diffusion ie by depletion of the crystal components near the crystal itself [22 - 26 32 33 35] This can be easily achieved if the crystals consist of components which act as network modifiers Then the glassy phase near the crystal is enriched in network formers which leads to an increase in viscosity and to a decrease in the diffusion coefficients If during the course of the crystallization process the viscosity around the crystal exceeds 1013 dPamiddots and hence the glassy layer is


359

at a temperature below its glass transition temperature then the crystallization process is totally frozen In other sections of the bulk far away from the crystal the chemical composition is that of the parent glass and the nucleation is still possible

Fig 1 shows the glass transition temperatures of sam-ples crystallized at 550degC for different periods of time The glass is of the system Na2OK2OAl2O3CaOCaF2SiO2 while the crystals formed are CaF2 [22 32] It is seen that the glass transition temperature increases with the crystal-lization time increase and in case of long crystallization times it approaches a value approximately equal to that of the sample crystallization ie 550degC The mean size of the crystals formed in the nano-meter range can easily be calculated from the X-ray diffraction line broadening Fig 2 shows the mean sizes of CaF2 crystals obtained upon crystallization for 20 h at different temperatures They are ca 95 nm at all temperatures studied (in the range from 520degC to 560degC) and are the same within the error limits

This means that the crystals do not grow in the temperature range pointed above which in turn indicates that the diffu-sion is not the only factor determining the growth process Otherwise Ostwald ripening would have occurred leading to a fairly broad crystallite size distribution By contrast the enrichment of a layer around the growing crystals which is a type of a self organisation process leads to an increased viscosity and decreased diffusion coefficients in the area Fig 3 shows the microstructure of a glass of the system Na2OK2OBaF2Al2O3SiO2 [23] The crystals have a size of ca 20 nm which is in an approximate agreement with the XRD-line broadening The crystal size distribu-tion is fairly small Fig 4 shows the size distribution of a sample annealed for 2 h at 700degC The distribution is compared to the Lifshitz-Slyozov-Wagner (LSW) distribution resulting from the Ostwald ripening [25] The distribution estimated on the ground of the Brails-ford and Wynblatt (BampW) theory is also shown Here in contrast to the Ostwald ripening an infinitely small

Fig 1 Glass transition temperatures of samples in the system Na2OK2OAl2O3CaOCaF2SiO2 crystallized at 550degC for different periods of time

Fig 2 Mean sizes of the CaF2 crystals after crystallization for 20 h at different temperatures

Fig 3 TEM micrograph of a glass in the system Na2OK2OBaF2Al2O3SiO2 crystallized for 20 h at 540degC

50 nm

Fig 4 Size distribution of a sample in the system Na2OK2OBaF2Al2O3SiO2 crystallized at 700degC for 2 h


360

concentration of the crystal phase is excluded which results in broadening of the function with concentra-tion increase In the chosen example a crystal volume fraction of 6 is assumed for the BampW-function It is seen that both distributions are significantly broader than the experimentally determined one The latter can most properly be fitted by a Gaussian curve

The formation of diffusion layers enriched in com-ponents increasing the viscosity is proved by high resolu-tion TEM applied to the crystallization of BaF2 from the glasses described above [24] In this case the layer is predominantly enriched in SiO2 In other cases such as the crystallization of rare earth fluorides similar concentration profiles are obtained The Anomalous Small Angle X-Ray Scattering (ASAXS) [26] is another experimental method which enables the diffusion layer detection Here coreshell structured glasses are crystallized in Na2OK2OBaF2Al2O3SiO2 [26] and Na2OK2OAl2O3CaOCaF2SiO2 [64] As shown in Table 1 the shell has a lower density than that of the core which can easily be explained by the depletion of the shell in barium which in this system is the rate determining step of the crystallization process The shell thickness decreases with crystallization temperature increase due to enhanced diffusion

Structures formed only from homogeneous glasses have been considered so far Another possibility however is to carry out the crystallization in a phase separated structure This is well known procedure and is reported for instance for glasses of the system SiO2middotAl2O3middotCaOmiddotP2O5middotK2OmiddotF- where calcium apatite crystallizes [65] However the crystal sizes reported in studies published more than 10 years ago are mostly in the microm range In the past few years structures as small as some nm are also obtained If a droplet phase separation occurs in a system the droplet size depends strongly on the chemical composition If the components enriched in the droplets act as network modifiers during the course of the phase separation pro-

cess the concentration of the network formers increases around the crystals which results in viscosity increase and hence diffusion coefficients decrease In principle the mechanism is fairly similar to the nano-crystallization described above with the only exception that the chemi-cal composition of the nano-sized phase depends on the temperature and is hence a function of time [66] The crys-tallization inside the droplets proceeds as a second step The two described mechanisms of nano-crystallization are visualized in Fig 5

Nano-crystallization in a phase separated glass is described for example in case of lithium alumosilicate glasses doped with ZrO2 and TiO2 as nucleating agents A droplet phase enriched in Al2O3 TiO2 and ZrO2 [59 - 61] is formed in the course of the first step It is then followed by ZrTiO4 precipitation First phase separation and subse-quent step crystallization inside the droplet occur also in the Na2OAl2O3SiO2B2O3FeOx-system [67 68] In this case the droplet phase is enriched in B2O3 and FeOx By contrast to systems where nano-crystallization starts in a homogeneous glass numerous crystals might be formed inside a single droplet leading thus to agglomerated nano-crystals In case of the Na2OAl2O3SiO2B2O3FeOx-system this gives rise to the precipitation of numerous multicore magnetite particles The residual glassy matrix is enriched in silica and hence exhibits high viscosity

Fig 5 Schematic of the different nano crystallization mechanisms The crystallization of a homogeneous glass and the crystallization of a phase separated glass

Crystallization Parameters 540degC 20h 600degC 20h 700degC 2h

Radius Particle (nm) 475 plusmn 05 520 plusmn 05 2418 plusmn 10

Shell Thickness (nm) 235 plusmn 01 213 plusmn 01 182 plusmn 01

Density Shell (gcm3) 220 plusmn 01 235 plusmn 01 22 plusmn 01

Density Matrix (gcm3) 255 plusmn 003 258 plusmn 003 250plusmn 003

Table 1 Results from ASAXS of samples in the system Na2OK2OBaF2Al2O3SiO2 The mean radius of the BaF2 nanoparticles the thickness and the density of the shell as well as the density of the residual glassy matrix are given


361

This is the reason preventing the droplet phase growth Multi core particles are also formed in an oxyfluoride glass [31 69 70] if the first phase separation is fol-lowed by crystallization inside the droplets This has been observed in some rare earth or lanthanum doped oxyfluoride glass compositions in NaGdF4 crystalliza-tion [1] as well as in non-phase separated oxyfluoride glasses It is reported that BaF2 and LaF3 as well as La-F structures are formed in the Ba-F amorphous matrix [27 71] Fig 5 illustrates schematically the juxtaposition of the two mechanisms for nano-crystallization

A NEW PHOTO THERMAL REFRACTIVE GLASSPhoto thermo refractive glasses (PTR) are glasses

which can be structured by light and subsequent thermal treatment [72 - 74] The first step is irradiation by UV light or better by a HeCd laser with a wavelength of ca 325 nm Then the irradiated glass is crystallized at one or two temperature values both above the glass transition temperature Tg During this treatment tiny crystals are formed but only in those parts of the glass which have previously been irradiated This leads to a change in the refractive index usually the irradiated regions exhibit a lower refractive index [72] Hence the refractive index of these glasses can be locally changed which enables the structurization of the glass by using light The pre-requisite for photonic devices utilisation refers to the absence of any or at least the presence of slight light scattering Conventional PTR glasses are based on the crystallization of NaF which provides the generation of refractive index changes of the order of 10-4

A new PTR glass formation is described below The principle of the interface controlled crystallization is applied ie the growth of the crystals is hindered by the formation of a diffusion barrier This enables the formation of small crystals with a narrow size distribu-tion For that purpose a glass of the system Na2OK2OCaOCaF2Al2O3SiO2 doped additionally with Ag2O CeO2 KBr SnO2 and Sb2O3 is studied [72]

According to the reference pointed above Ce3+ is oxidized to Ce4+ during the PTR glass irradiation and the generated electron is trapped by a silver ion which is reduced to a silver atom ieCe3+ + hυ rarr Ce4+ + e- (1)

Ag+ + e- rarr Ag0 (2)The subsequent annealing step is carried out at a

temperature few tens of Kelvin above Tg This leads to the formation of silver clusters which act as seeds during the second annealing step and trigger the crystallization of further components The absorption spectrum of the new PTR glass is shown in Fig 6 The non-irradiated base glass shows an absorption peak at 316 nm which is due to Ce3+ After irradiation with light of this wave-length (or with polychromatic light enriched in UV) the formation of Ce4+ gives rise to strongly increased absorption at wavelengths below 350 nm Fig 7 shows the X-ray diffraction patterns of glasses which are irra-diated with a xenon lamp for different periods of time and subsequently thermally treated for 1 h at 530degC and additionally for 20 h at 560degC During the first thermal step at 530degC the silver atoms form clusters which can be noticed due to the yellow coloration of the samples Besides a peak of a notable intensity is observed in the transmission spectrum at a wavelength of 440 nm During the second thermal treatment for 20 h at 560degC CaF2 crystals are formed but only in the irradiated areas of the glass Fig 7 shows that the irradiation time plays an important part Irradiation with a xenon lamp for 10 min leads to a comparatively small XRD-peak at a 2 θ value of approximately 28deg while samples irradiated for 30 min show a much stronger peak The irradiation for 60 min results in ca six fold increase of the value obtained in case of 30 min irradiation The refractive index n0 of the non irradiated glass is equal to 15484 while that of the irradiated glass subjected to two step thermal treatment is equal to 15472 The difference of 12middot10-3 is 10 times greater than that obtained in case of conventional PTR glasses (crystallization of NaF)

Fig 6 Absorption spectra of the non-irradiated base glass and the glass after irradiation


362

Dopand T (degC) no 1 Al2O3 2 Al2O3 1 La2O3 2 La2O3 1 TiO2 2 TiO2

449 122 14 006 459 10 367 023 017 002 151 0058 469 1633 326 089 097 0031 394 079 474 107 751 110 479 925 206 030 062 0011 661 098 489 040 0003 321 014

Table 2 Steady-state nucleation rates (in 1(mm3middots))

NUCLEATION INHIBITION IN LITHIUM DISILICATE GLASS-CERAMICS

Additives of 1 mol and 2 mol of Al2O3 La2O3 TiO2 and ZrO2 are applied to stoichiometric lithium dis-ilicate glasses [62 63] The glasses are thermally treated which results in lithium disilicate crystallization This is only phase detected in all samples studied with the exception of the sample containing 2 mol ZrO2 where lithium disilicate and lithium metasilicate are formed Optical micrographs using a Laser Scanning Microscope (LSM) show crystals of elliptic morphology (see Fig 8 left) They can be easily counted and the dimensions of each particular crystal can be also determined

Optical hot stage microscopy is used to study the kinetics of the nucleation and crystallization process The nucleation rate the induction period and the crystal growth velocities are determined as a function of tempera-ture using the micrographs obtained The crystal growth velocities of all samples studied increase steadily within the investigated temperature range from 580degC to 660degC

Additions of Al2O3 or La2O3 lead to a decrease in the crystal growth velocities while the samples doped with TiO2 show nearly the same crystal growth veloci-ties when compared to those of the undoped one The nucleation rates of all samples exhibit a maximum at ca 10 K above Tg All studied additives lead to decrease of the nucleation rate (see Table 2) and the induction period (see Table 3) In all cases the induction time decreases steadily with increasing temperature The addition of lanthanum results in the most pronounced decrease in the nucleation rate and the highest elongation of the induction time The studied additives increase also the viscosity of the glass samples Related to the same vis-cosity (ie not the temperature) the nucleation rates are still much smaller and the induction times are still much larger when compared to those of the undoped sample

Fig 7 XRD-patterns of samples first irradiated for different periods of time then annealed at 530oC for 1 h and finally annealed at 560oC for another 20 h (10AB) 10 min irradia-tion (30AB) 30 min irradiation (60AB) 60 min irradiation

Fig 8 Optical micrograph (left) and a SEM-micrograph (right) recorded from a lithium disilicate glass doped with 2 mol La2O3 crystallized at 459degC for 70 h


363

The deceleration of the crystallization process by adding Al2O3 La2O3 or TiO2 is predominantly due to the much slower nucleation process but not to a decrease of the crystal growth velocity It should be noted that the same additives act as nucleating agents in other glass systems and especially at higher concentrations The additives described above may be utilized to control the entire crystallization process (to either enhance or prevent crystallization) This is also very helpful in case of using crystallizing glass seals Nucleation and subsequent crystal growth is not desired in the temperature range suitable for sintering

The application of the nucleation inhibitors de-scribed above may shift to higher temperatures the range in which nucleation occurs

LUMINESCENT GLASS CERAMICS FOR LIGHT CONVERSION

Light technologies are rapidly developing in the past few years The most efficient light source light emitting diodes (LEDs) deliver only monochromatic light For the generation of white light blue LEDs in combination with light converters are used The most commonly used material is yttrium aluminum garnet (YAG Y3Al5O12) doped with Ce3+

The heat in glass-ceramic materials can be more effectively removed than in the conventional polymer

matrix composites in case of high power applications Hence it would be highly advantageous if doped YAG crystals can be directly crystallized in glasses Although many studies have been performed on YAG relatively few reports on the crystallization of doped YAG in glasses have been published [7 8 12 - 14]

The general problem here refers to the high tem-peratures in some cases as high as 1400degC required for YAG crystallization

The crystallization of glasses of compositions x CaO 1CeF3 (11 - 02x) Y2O3 (492 - 08x) Al2O3 288 SiO2 (with x = 10 20 30 and 40) is also described These glasses are annealed at 1200 degC which results in the crystallization of yttrium aluminum garnet (YAG) as the only crystalline phase The crystals show the morphol-ogy of dendrites with large quantities of glassy phase in between them (see Fig 9) Large cubic crystals are found in samples containing 10 mol and 40 mol of CaO

Interpenetrating dendrites are detected in samples of calcium concentrations of 20 mol and 30 mol as proved by electron backscatter diffraction (EBSD)

The glass-ceramics with YAG crystals show intense fluorescence caused by Ce3+ They show quantum ef-ficiency comparable to that of the commercial light converters composed of YAG embedded in a polymer matrix Other glass compositions enable the crystal-lization of large blocky YAG crystals or surface layers mainly composed of YAG

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449 167 851 926 459 104 404 532 453 2824 396 557 469 25 107 336 335 1197 216 502 474 82 145 396 479 - 49 286 121 150 136 226 489 132 - 91 -

Table 3 Induction times tind (min) for steady-state nucleation in min

Fig 9 Optical micrographs recorded from a sample with the composition 288 SiO2middot412 Al2O3middot9 Y2O3middot1 CeF3middot20 CaO (right) and 288 SiO2middot332 Al2O3middot7 Y2O3middot1 CeF3middot30 CaO both crystallized at 1200degC for 6 h


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44 P Prapitpongwanich R Harizanova K Pengat C Ruumlssel Nanocrystallization of Ferroelectric Lithium Niobate from LiNbO3-SiO2 Glasses Mater Lett 63 2009 1027ndash1029

45 P Prapitpongwanich K Pengpat C Ruumlssel Phase Separation and Crystallization in LiNbO3SiO2 Glasses Mater Chem Phys 113 2009 913ndash918

46 R Harizanova C Bocker G Avdeev C Ruumlssel I Gugov Crystal Growth and Dielectric Properties of BaTiO3 Obtained in Aluminoborosilicate Glasses J Non-Cryst Solids 401 2014 191-196

47 R Harizanova C Bocker G Avdeev C Ruumlssel I Gugov Crystallization and Dielectric Properties of BaTiO3 Containing Invert Aluminoborosilicate Glass-Ceramics Bulg Chem Comm 45 A 2013 69-73

48 R Garkova I Gugov C Ruumlssel Precipitation of In2O3 Nano Crystallites from Glasses in the System Na2OB2O3Al2O3In2O3 J Non-Cryst Solids 320 2003 291-298

49 R Garkova G Voumllksch C Ruumlssel In2O3 - and Tin Doped In2O3- Nano Crystals Prepared by Glass Crystallization J Non-Cryst Solids 352 2006 5265-5270

50 R Garkova G Voumllksch C Ruumlssel Precipitation of SnO2 Nano-Crystallites from Na2OB2O3SnO2(Al2O3) Glasses J Non-Cryst Solids 351 2005 2287-2293

51 A Hunger G Carl A Gebhardt C Ruumlssel Youngrsquos moduli and microhardness of glassndashceramics in the system MgOAl2O3TiO2ZrO2SiO2 containing quartz nanocrystals Mater Chem Phys 122 2010 502-506

52 A Hunger G Carl A Gebhardt C Ruumlssel Ultra-high


366

thermal expansion glassndashceramics in the system MgOAl2O3TiO2ZrO2SiO2 by volume crystallization of cris-tobalite J Non-Cryst Solids 354 2008 5402-5407

53 A Hunger G Carl C Ruumlssel Formation of nano-crystalline quartz crystals from ZnOMgOAl2O3TiO2ZrO2SiO2 glasses Solid State Sci 12 2010 1570-1574

54 M Dittmer C Ruumlssel Colorless and high strength MgOAl2O3SiO2 glass-ceramic dental material us-ing zirconia as nucleating agent J Biomed Mater Res B 100B 2012 463-470

55 M Dittmer M Muumlller C Ruumlssel Self-organized nanocrystallinity in MgOndashAl2O3ndashSiO2 glasses with ZrO2 as nucleating agent Mater Chem Phys 124 2010 1083-1088

56 P Wange T Houmlche C Ruumlssel JD Schnapp Mi-crostructure-property relationship in high-strength MgO-Al2O3-SiO2-TiO2 glass-ceramics J Non-Cryst Solids 298 2002 137-145

57 C Patzig T Houmlche M Dittmer C Ruumlssel Temporal Evolution of Crystallization in MgOndashAl2O3ndashSiO2ndashZrO2 Glass Ceramics Cryst Growth Des 12 2012 2059-2067

58 R Wurth F Muntildeoz M Muumlller C Ruumlssel Crystal growth in a multicomponent lithia aluminosilicate glass Mater Chem Phys 116 2009 433-437

59 S Bhattacharyya T Houmlche J R Jinschek I Avra-mov R Wurth M Muumlller C Ruumlssel Direct Evi-dence of Al-Rich Layers around Nanosized ZrTiO4

in Glass Putting the Role of Nucleation Agents in Perspective Cryst Growth Des 10 2010 379-385

60 T Houmlche M Maumlder S Bhattacharyya G S Henderson T Gemming R Wurth C Ruumlssel I Avramov ZrTiO4 crystallisation in nanosized liq-uidndashliquid phase-separation droplets in glassmdasha quantitative XANES study Cryst Eng Comm 13 2011 2550-2556

61 T Houmlche C Patzig T Gemming R Wurth C Ruumlssel I Avramov Temporal Evolution of Diffu-sion Barriers Surrounding ZrTiO4 Nuclei in Lithia Aluminosilicate Glass-Ceramics Crystal Growth amp Design 12 2012 1556-1563

62 K Thieme C Ruumlssel Nucleation and Growth Ki-netics and Phase Analysis in Zirconia-Containing Lithium Disilicate Glass J Mater Sci 50 2015 1488-1494

63 K Thieme C Ruumlssel Nucleation Inhibitors- The

Effect of Small Concentrations of Al2O3 La2O3 or TiO2 on Nucleation and Crystallization of Lithium Disilicate J Eur Ceram Soc 34 2014 3969-3979

64 A Hoell Z Varga VS Raghuwanshi M Krumrey C Bocker C Ruumlssel ASAXS Study of CaF2 Na-noparticles Embedded in a Silicate Glass Matrix J Appl Cryst 47 2014 60-66

65 T Houmlche C Moisescu J Avramov C Ruumlssel WD Heerdegen Microstructure of SiO2-Al2O3-CaO-P2O5-K2O-F- Glass Ceramics 1 Needle Like Versus Isometric Morphology of Apatite Crystals Chem Mater 13 2001 1312-1319

66 J Avramov C Bocker C Ruumlssel Topology and Nu-merical Simulation of Phase Separation in Sodium Silicate Glass J Chem Phys Solids 78 2015 8-11

67 C Worsch P Schaaf R Harizanova C Ruumlssel Magnetisation Effects of Multicore Magnetic Nano-particles Crystallised from a Silicate Glass J Mater Sci 47 2012 5886-5890

68 R Harizanova G Voumllksch C Ruumlssel Microstructures Formed During Devitrification of Na2O sdot Al2O3 sdot B2O3

sdot SiO2 sdot Fe2O3 J Mater Sci 45 2010 1350ndash135369 C Bocker J Wiemert C Ruumlssel The Formation of

Strontium Fluoride Nano Crystals from a Phase Separated Silicate Glass J Eur Ceram Soc 33 2013 1737-1745

70 C Bocker A Herrmann P Loch C Ruumlssel The Nano-Crystallization and Fluorescence of Terbium Doped Na2OK2OCaOCaF2Al2O3SiO2 Glasses J Mater Chem C DOI 101039c4tc02858a

71 F Munoz A de Pablos-Martin N Hemono M J Pascual A Duran L Delevoye L Montagne NMR investigation of the crystallization mechanism of LaF4 and NaLaF4 phases in aluminosilicate glasses J Non-Cryst Solids 357 2011 1463-1468

72 M Stoica G N B M de Macedo C Ruumlssel Photo Induced Crystallization of CaF2 from a Na2OK2OCaOCaF2Al2O3SiO2 Glass Opt Mater Exp 4 2014 1574-1585

73 J Lumeau L Glebova LB Glebov Influence of UV-exposure on the crystallization and optical properties of photo-thermo-refractive glass J Non-Cryst Solids 354 2008 425-430

74 LB Glebov NV Nikonorov EI Panysheva GT Petrovsky VV Savvin IV Tunimanova VA Tsekhomsky New Potentialities of Photosensitive Glasses for Volume Phase Hologram Recording Opt Spektrosk 73 1992 404-412


358

These materials can be utilized in light converters [6 - 8] which are especially important for the conversion of LEDs blue light into white one At present inorganic phosphors such as Ce3+ doped yttrium aluminium garnet embedded in organic polymers like polysiloxanes are used for this purpose If higher light intensities are to be achieved the thermal conductivities of these composites are not sufficient to remove the generated heat Hence it is advantageous to crystallize appropriate phosphors directly in the glass [7 8 12 - 14] when high energy densities are required

The preparation of magnetic [15 16] or semicon-ducting phases [17 - 19] from glasses is another im-portant field especially if nano-crystals of respective compositions are crystallized from borate glasses In the latter case a subsequent dissolution of the borate glass matrix is carried out in water or diluted acid resulting in nanocrystalline powders preparation These powders usually exhibit narrower particle size distributions than those obtained by other methods [20 21] and may also possess superparamagnetic or other interesting magnetic electric and optical properties

Although nano-crystalline glass-ceramics are of great technological potential for numerous new applica-tions the studies focused on the fundamentals of their preparation were only recently reported [15 - 22] Clas-sical theories of nucleation and crystal growth mainly concern isochemical systems ie systems in which the precipitated crystals have the same chemical composition as that of the parent glass [22 23] They usually describe the nucleation and the crystal growth as two separate processes Furthermore the assumption that a nucleus formed starts immediately to grow is not justified in any case Glass-ceramics containing a large quantity of nano-sized crystals of diameters ranging from 5 nm to 50 nm within a narrow size distribution interval have been so far reported only in non-isochemical systems [25] In these multicomponent systems the chemical composi-tion of the residual glass changes during the course of crystallization Diffusion layers are formed around the growing nuclei affecting the crystal growth velocity due to the depletion of the components attached to the crystal phase Thus the concentration gradient varies with the change of the diffusion coefficient Hence the latter has to decrease continuously in the course of crystallization if nano-crystals formation is aimed at This results in a layer of increasing viscosity which acts as a barrier strongly decelerating further crystal growth [24 26]

In the past few years numerous fundamental investiga-tions on the preparation of nano-crystalline glass-ceramics from metal fluoride containing glasses eg from glasses containing alkali fluorides alkaline earth fluorides [22 - 32] rare earth fluorides [33-36] or alkali rare earth fluorides [37 - 41] as well as PbF2YbF3 solid solutions [9 42 43] have been reported All these crystals may also be doped with other rare earth elements The crystallisation of ferroelectric phases such as lithium niobate [44 45] or BaTiO3 [46 47] of oxidic semiconducting phases such as indium oxide [48 49] or tin oxide [50] as well as that of ferromagnetic phases such as magnetite [15 16] has been also described

The precipitation of α- or β-quartz or related struc-tures such as szlig-eucryptite spodumen or keatite from glasses in the systems MgOAl2O3SiO2 [51 - 57] and Li2OAl2O3SiO2 [58 - 61] is a special topic These glass-ceramics exhibit often a coefficient of thermal expansion close to zero (eg Ceranreg or Zerodurreg)

Another new and challenging topic in glass ce-ramic research refers to the improvement of nucleation processes control [62 63] Especially in case where the glass powders are to be sintered and subsequently crystallized the two production steps have to be well separated Otherwise the sintering process might not be completed prior to the formation of a notable quantity of crystals which affects the continuation of the sintering process The addition of small concentrations of the so called nucleation inhibitors is required in this case It is also of special importance in crystallizing glass seals

This paper gives a short overview on nano-crys-tallization of glass and some special applications of nano-crystalline glass-ceramics of glass ceramics for lighting technology and on the inhibition of nucleation by small concentrations of certain additives

GLASS-CERAMICS WITH NANO SIZED CRYSTALS The glass-ceramics nano-crystallinity can be achieved

by the formation of a highly viscous layer around the grow-ing crystals This layer is initially formed by diffusion ie by depletion of the crystal components near the crystal itself [22 - 26 32 33 35] This can be easily achieved if the crystals consist of components which act as network modifiers Then the glassy phase near the crystal is enriched in network formers which leads to an increase in viscosity and to a decrease in the diffusion coefficients If during the course of the crystallization process the viscosity around the crystal exceeds 1013 dPamiddots and hence the glassy layer is


359







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449 122 14 006 459 10 367 023 017 002 151 0058 469 1633 326 089 097 0031 394 079 474 107 751 110 479 925 206 030 062 0011 661 098 489 040 0003 321 014









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449 122 14 006 459 10 367 023 017 002 151 0058 469 1633 326 089 097 0031 394 079 474 107 751 110 479 925 206 030 062 0011 661 098 489 040 0003 321 014









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365


























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