Année académique 2014-2015 Service 4MAT Bourse FRIA
Amorphous phase separation and crystallization in the BaO-TiO 2 -SiO 2 system: experimental
approach and thermodynamic study
Promoteur : Prof. Stéphane Godet (ULB)
Membres du jury: Prof. Marie-Paule Delplancke (ULB) Prof. Luc Segers (ULB)
Prof. Bart Blanpain (KUL) Prof. Maurice Gonon (UMONS) Prof. Véronique Vitry (UMONS)
Dr. Jinichiro Nakano (US Department of Energy) Dr. Michel Bogaerts (AGC)
Thèse présentée parEmilie BOULAY en vue de l’obtention du diplôme
Résumé
Les vitrocéramiques sont d’un interêt grandissant grâce aux nombreuses propriétés qu’elles offrent par rapport aux verres de mêmes compositions. Le contrôle microstructural est un challenge majeur car les propriétés sont des conséquences directes des microstructures. Ces microstructures peuvent être modifiées en formant des phases cristallines spécifiques avec des morphologies spécifiques, ou en utilisant une séparation de phase amorphe. Les objectifs de cette thèse sont de montrer que les propriétés des verres silicatés peuvent être améliorées en contrôlant la génèse microstructurale par la composition et les paramètres de traitement thermique. Plus spécifiquement, deux systèmes seront étudiés et comparés : le système BaO-TiO2-SiO2et le système sodo-calcique Na2O-CaO-SiO2 utilisé dans l’industrie. Ces deux systèmes présentent une large zone d’immiscibilité, permettant l’étude de l’influence de la séparation de phase sur la cristallisation.
Le premier système, BaO-TiO2-SiO2, a sucité beaucoup d’intérêt grâce aux propriétés intéressantes qu’offre la fresnoite (Ba2TiSi2O8) : piézo et pyroélectricité, génération de seconde harmonique et photo- luminescence bleue/blanche. De nombreuses études ont été menées sur la composition stoéchiometrique et sur des compositions contenant un léger excès de SiO2, pour tenter de comprendre et d’optimiser ces propriétés prometteuses. Il a cependant été suggéré récemment que l’intensité de photoluminescence peut- être améliorée en utilisant des compositions formant de la séparation de phase. Ceci semble indiquer que l’intensité de photoluminescence peut être encore optimisée à travers un contrôle microstructural. Le rôle de la séparation de phase sur la cristallisation a été le sujet de nombreux débats durant ces dernières années, et n’a toujours pas été clarifié, particulièrement en ce qui concerne les interfaces générées par la séparation de phase. Dans cette thèse, le rôle de la séparation de phase sur la cristallisation de la fresnoite a été étu- dié. Ceci est nécessairement passé par le calcul de la zone d’immiscibilité liquide-liquide dans le diagramme de phase, dans le but de choisir des compositions propices à comparer dans une étude systématique de la cristallisation. L’étude systématique de la cristallisation de la fresnoite a permis de conclure à un mécanisme de cristallisation en surface pour toutes les compositions non-stoéchiométriques et a permis de montrer qu’il n’y a pas d’influence des interfaces. Cette étude a été complétée par une investigation des effets de la composition (i.e. un excès en SiO2), de la température de recuit ainsi que des paramètres du traitement thermique, c’est à dire la vitesse de chauffe, la vitesse de refroidissement ou un palier isotherme avant le recuit. Il est montré que des microstructures bien spécifiques peuvent être obtenues, dépendant du choix de ces paramètres. Pour terminer, des mesures d’intensité de photoluminescence ont été conduites sur un choix de compositions et paramètres thermiques guidé par les résultats précédants. Les meilleurs résultats sont obtenus pour une fraction cristallisée élevée ainsi qu’avec la présence d’interfaces dues à une microstructure fine.
Les résultats obtenus dans l’étude du système BaO-TiO2-SiO2 ont ensuite été étendus au système sodo-calcique dans le but d’étudier l’effet de la séparation de phase sur la cristallisation. Il est montré que la formation de la cristobalite en surface est inévitable et que le changement de composition qu’elle induit inhibe la séparation de phase. Il est par conséquent difficile d’observer une intéraction entre la séparation de phase et la cristallisation dans ce système. Ces études aboutissent à une discussion quant aux caractéristiques permettant d’observer des intéractions entre la séparation de phase et la cristallisation, pour les verres
Abstract
Glass-ceramics are of growing interest due to their enhanced properties compared to the base glasses.
More specifically, the control of microstructures is a major challenge as the properties of glass-ceramics are the direct consequences of microstructures. Microstructures can be modified by forming specific crystal phases or by using a prior amorphous phase separation before crystallization. The PhD thesis objectives are to demonstrate that the properties of silicate glasses can be enhanced by controlling their microstructure genesis with composition and thermal process parameters. More specifically, two systems were studied and compared: the BaO-TiO2-SiO2 system and the soda-lime silica Na2O-CaO-SiO2 used industrially. Both systems exhibit a large zone of immiscibility allowing the study of the influence of phase separation on crystallization.
The first system BaO-TiO2-SiO2 has gathered interest from the interesting properties of fresnoite (Ba2TiSi2O8): piezo and pyroelectricity, second harmonic generation and blue/white photoluminescence.
Many studies on the stoichiometric composition were conducted to understand and improve those promising properties. However, it was recently suggested that the photoluminescence can be improved with composition exhibiting phase separation. This indicates that the photoluminescence intensity can be improved through a microstructural control. The possible role of a prior amorphous phase separation on the subsequent crystallization has been however the topic of vigorous debates over the last decades and has not yet been clarified, especially regarding the role of the interfaces created by the phase separation. In this PhD, the effect of phase separation on fresnoite crystallization was studied. This had to pass through the calculation of the liquid-liquid immiscibility in the phase diagram in order to select suitable compositions to compare in a systematic study. The systematic study concludes to a surface crystallization mechanism for all non- stoichiometric compositions and shows no influence between amorphous droplets and matrix crystallization.
This study was also completed with the investigation of the effect of composition (i.e. SiO2-excess), annealing temperature and prior heat treatment, i.e. heating rate, cooling rate or a prior isothermal step before annealing. It is shown that specific microstructures are obtained depending on the process parameters.
Finally, selected compositions and heat treatment show how photoluminescence intensity can be improved by a microstructural control. The highest intensity is obtained with a high crystallization fraction and a maximization of the number of interfaces.
The results obtained in the study of the BaO-TiO2-SiO2 system are extended to the soda-lime-silica system in order to study the effect of phase separation on crystallization. It is shown that cristobalite forma- tion from the surface cannot be avoided and that the involved composition shift inhibits phase separation. It is consequently difficult to observe an interplay. Those studies lead to a general discussion about the criteria allowing to observe an interplay between phase separation and crystallization in oxide glasses.
Remerciements
Une thèse est un travail personnel, une expérience en soi . . . et une expérience pour soi, faite de moments difficiles, de doutes et d’interrogations mais également de moments d’épanouissement, de fierté et de satis- faction quand, enfin, l’effort paie et que les résultats tant attendus arrivent ! !
Mais au-delà des défis personnels et scientifiques, l’environnement dans lequel se déroule la thèse ainsi que les intéractions ont également leur grande part de responsabilité dans son succès. L’ambiance, la convivalité et l’entente au sein d’un laboratoire, les collaborations et intéractions qui se créent ainsi que le soutien des proches constituent les fondations de ce travail de longue haleine. Ce présent manuscript est le résultat d’un peu plus de 4 années de travail passées dans le laboratoire 4 MAT de l’ULB et c’est un travail que je n’aurais jamais pu accomplir sans l’aide, les conseils, la confiance et le soutien d’un grand nombre de personnes. Je voudrais donc avant tout dédier ces paragraphes à toutes les personnes ayant contribué, de près ou de loin à l’aboutissement de ce travail.
Je commence bien évidemment par mon promoteur, Stéphane Godet, qui m’a efficacement encadrée et m’a permis de réaliser cette expérience de laquelle je resors grandie. Je le remercie pour sa disponibilité, les intéractions qu’on a pu avoir, son optimisme, son écoute, sa patience et son enthousiasme mais aussi et surtout pour son humanité et la confiance qu’il a pu m’accorder. En effet, tout au long de ma thèse, Stéphane m’a laissé toute l’indépendance et la liberté nécessaire à un chercheur pour se développer, et j’ai toujours pu compter sur sa présence et son soutien dans les bons comme dans les mauvais moments.
Je voudrais remercier chacun des membres du jury d’avoir accepté de lire et discuter ce travail : Marie- Paule Delplancke, Luc Segers, Bart Blanpain, Maurice Gonon, Véronique Vitry, Jinichiro Nakano, Michel Bogaerts et Stéphane Godet.
Je remercie également la Région Wallonne et le FRIA pour avoir financé ce travail.
Mes remerciements vont aussi à toute l’équipe 4MAT avec qui j’ai pu passer de merveilleuses années ! ! Je remercie les professeurs et co-chefs de service Marie-Paule Delplancke, Stéphane Godet et Marc Degrez, mais aussi le professeur Luc Segers pour les nombreux conseils qu’il a pu me donner en thermodynamique et traitements hautes températures.
Tiriana, ma collègue et complice. Merci pour ta disponibilité, ton temps et ta gentillesse. Tu es une collègue en or !
Mes collègues de bureau Loïc Mallet et Matteo Caruso avec qui j’ai pu passer 4 ans . . . quoique Loïc, à la fin, tu m’as lâchée pour voir plus grand ! Ce fut un plaisir d’avoir toutes ces discussions aussi bien scientifiques que sur tout et rien. Loïc merci pour tout ce temps passer à l’EBSD et au TEM et sur les débats concernant la cristobalite. Mais non, les gouttes ne cristallisent pas ! Même si ...
Mes collègues sur qui j’ai toujours pu compter pour avoir un travail impeccable : Gilles pour les superbes microscopies au SEM (je suis hyper frustée de n’avoir jamais fait mieux !), Didier pour les super lames (ultra-)minces, Jean, pour son puit de savoir et sa disponibilité, Pierre pour ton dynamisme et pour tous les bons conseils que tu as pu me donner. Je remercie mes deux TFistes et finalement actuelles collègues également, Céline Ragoen et Gaëlle Couturiaux : vous avez fait un super boulot ! Je remercie également les autres membres du personnel de 4MAT, Shain, Colin, Suzanne, Patrizio et René mais également tous les chercheurs, anciens ou nouveaux. Merci à tous de contribuer à une ambiance aussi agréable au sein de 4MAT, grâce à votre présence, votre enthousiasme, bonne humeur et à tous les évènements organisés ensemble comme les barbecues, Sainte-Barbe, Repas de Noël ou autres 10h spéciaux pour les anniversaires.
Je remercie également l’équipe d’AGC avec qui j’ai eu le plaisir de collaborer pendant presque 2 ans : Michel Bogaerts, Pierre Carleer, Samuel Martinquet, Dominique Deleuze, . . . Michel, ne déserpérons pas, un jour on aura de la nano-crystallisation. Merci également à Raffaele Sinatra pour m’avoir appris à couler des verres.
Je remercie Jinichiro Nakano pour tout son soutien, son temps et sa disponibilité pour l’aboutissement du calcul du diagramme de phase. Sans ces cessions skypes et ces nombreux mails échangés, je n’aurais pu arriver aussi loin ! Merci également à Yves Muggianu et Jean-Claude Tedenac que j’ai eu l’occasion de rencontrer à la conférence Calphad, qui ont soutenu mon travail et ont permis d’en améliorer notablement les résultats.
Je voudrais terminer par remercier mes proches. Mon groupe d’amis dont je connais certains depuis 10 ans... 20 ans même ! Laurent Vandievoet, Coralie Jacques, Anne Rondeaux, Nicolas Sand, Julie Vander- schuren, Gilles Remy, Mélissa Hannegreefs, Jimmy Mouffok, Kevin Leempoel, Sarah Bunton, Jonathan et Linda Bion. Je remercie ma maman, mon papa, ma soeur et ma grand-mère, qui ont toujours cru en moi et évidemment Xavier, qui, disons-le a du me supporter dans les moments difficiles (et lui seul sait comme je peux être pénible !). Je lui dois beaucoup pour l’aboutissement de ce travail, grâce à sa présence et au soutien qu’il m’apporte au quotidien voici maintenant plus de 8 ans.
Je voudrais vraiment vous dire à tous : MERCI !
Emilie Boulay
List of symbols and acronyms
List of symbols
ai Activity of species i
a0 Diffusion distance m
A Surface of liquid/solid interface m2
↵ Heating rate J
↵ Convergence semi-angle rad
↵ Dilatation coefficient K 1
B,T0 Coefficients of Vogel-Flücher equation
Acceptance semi-angle rad
0 Magnetic moment J.T 1
c Composition %mol, %wt
cP Heat capacity at constant pressure J.K 1
c0 Initial composition %mol, %wt
dac Distance anion-cation m
d33 Piezo-electric coefficient/SHG coefficient pC.N 1/pm.V 1
Dn Self-diffusion coefficient m2.s 1
¢G Change in (molar) Gibbs free energy J(mol 1)
¢G00 Thermodynamic barrier for growth J
¢GD Activation energy for diffusion / kinetic barrier for nucleation J
¢Ghet Heterogeneous thermodynamic driving force J
¢Ghom Homogeneous thermodynamic driving force J
¢Gm Change in (molar)free energy of mixing J(mol 1)
¢gm Change in free energy (at Tm), per unit of volume J.m 3
¢Gs Surface contribution to the change in Gibbs free energy J
¢Gv Volume contribution to the change in Gibbs free energy J
¢gv Change of volume contribution to the Gibbs free energy , per unit of volume
J.m 3
¢Hm Change in (molar)enthapy of mixing J(mol 1)
¢H0f(298K) Standard enthalpy of formation at 298 K J
¢hm Change in enthalpy (at Tm), per unit of volume J.m 3
¢sm Change in entropy (at Tm), per unit of volume J.m 3K 1
¢Sm Change in (molar)entropy of mixing JK 1(mol 1)
¢S0f(298K) Standard entropy of formation at 298 K J.K 1
¢Scombm Change in combinatorial entropy of mixing J.K 1
¢T Undercooling °C,K
EAA Energy associated to all AA bonds J
Eact Activation energy J
EAB Energy associated to all AB bonds J
EBB Energy associated to all BB bonds J
Ed Energy required to dissociate an oxide into its composition atoms in the gaseous state
kcal/mol
e0 Elementary change C
✏r Dielectic constant of the solvent
✏0 Dielectic constant of vaccum (8.85 x 10 12 F.m 1
⌘ Viscosity Pa.s or poise
F Field strength N/C
f Jump frequency s 1
G Gibbs free energy of a system J
GHSER (molar) Gibbs energy at the standard reference state (1atm,298.15K) or lattice stability
J(mol 1)
GSERi (molar) Gibbs energy of element in its reference state (1atm,298.15K)
J(mol 1)
G© (molar) Gibbs energy of a phase© J(mol 1)
G©i (molar) Gibbs energy of element i in phase© J(mol 1)
0G (molar) Gibbs energy due to the mechanical mixing of the constituents of the phase
J(mol 1)
0GA,B (molar) Gibbs energy of pure A,B J(mol 1)
exGABC Excess Gibbs energy of mixing in the ternary ABC J
idGmix Ideal (molar) Gibbs energy of mixing J(mol 1)
exGmix Excess (molar) Gibbs energy of mixing J(mol 1)
0Gi© Gibbs energy of component i in phase©due to the mechanical mixing
J
i Activity coefficient
i Charge of ion i
H Enthalpy J
HSER (molar) Enthalpy at the standard reference state (1atm,298.15K)
J
HSERA (molar) Enthalpy of the element A in its stable state at 298.15 K and 1 atm
J(mol 1)
I Nucleation rate s 1
IMAX Maximum nucleation rate s 1
kB Boltzmann constant J.K 1
LA,B:⇤ Regular solution interact parameter for a mixing between A and B on the sublattice irrespective to the site occupation of the other sublattice
J
i Coefficients of the Margule polynomial
LABC A-B-C ternary interaction parameter J
LA,B:C Regular solution interact parameter between A and B while C is on the other sublattices
J
0LAB A-B binary interaction parameter in a regular solution J
1LAB A-B binary interaction parameter in a sub-regular solution J
2LAB A-B binary interaction parameter in a sub-sub-regular solution J m Avrami parameter (dimension)
µ Chemical potential J
µ↵A Chemical potential of A in phase↵ J
N Number of particles (atom, molecule) n Avrami parameter (mechanism)
n Number of particles (atom, molecule)/mole per unit of volume or surface
m 3 or m 2
n Number of particles crossing the interface
nl Number of mole per unit of surface in the liquid phase mol.m 2 ns Number of mole per unit of surface in the solid phase mol.m 2
⌫ Number of vibrations
⌫L Coefficient of the Redlich-Kister polynomial Nsi Number of atoms of component i on sublattice s
NsT OT Total number of sites on sublattice s
P Pressure atm
r Radius (nucleus,particle) m
ra Anion radius Å
rc Cation radius Å
rO Oxygen radius Å
r⇤ Critical radius (nucleus,particle) m
Rc Critical cooling rate °C/K. 1
R Gas constant J.mol 1.K 1
⇢ Density kg.m 3
Surface tension J.m 2
t Time s,min,h,days
T Temperature °C,K
Tc Consolute/critical temperature °C,K
Tc Curie temperature °C,K
Tg Glass transition temperature °C,K
Tg,r Reduced glass transition °C,K
Tm Melting temperature °C,K
TMAX Temperature of maximum nucleation rate °C,K
Tm,e Eutectic melting temperature °C,K
Tp Peak temperature °C,K
Tx Onset temperature °C,K
£ Geometrical factor
✓ Wetting angle
U Growth rate cm.s 1
UMAX Maximum growth rate cm.s 1
V Volume m3
Vm Volume of transformed (crystallized) phase m3
wAA Energy of AA bond J
wAB Energy of AB bond J
wBB Energy of BB bond J
W⇤ Thermodynamic barrier for nucleation J
WH,≠AB Regular solution parameter J
x Crystallized fraction
x Molar fraction %mol
xA Fraction of atoms A xB Fraction of atoms B yi site fraction
ysi site fraction on sublattice s z Number of nearest neighbour Zc,Za Valence of cation, valence of anion Z+,Z Charge number
List of acronyms
ACOM Automated computed orientation microscopy
APS Amorphous phase separation
BO Bridging oxygen
CALPHAD Calculation of phase diagram
CN Coordination number
CCT-curve Continous Cooling Transformation-curve CTE Coefficient of thermal expansion
CTN Classical theory for nucleation
CMV Cluster variation method
CEF Compound energy formalism
CI Confidence index
DTA Differential thermal analysis EBSD Electron backscattered diffraction EBSP Electron Backscattered pattern
EDX Energy dispersive X-ray
EELS Electron energy loss spectroscopy
EMF Electromotive force
EPM Electron probe micro-analysis
FIB Focused Ion Beam
FFT Fast Fourrier transform
G Granulometry / particle size
IPF Inverse pole figure
IQ Image quality
I2SL Ionic two sublattice
JMA Jonshon -Melh-Avrami
FEG-SEM Field emission gun - scanning electron microscopy/microscope HAADF/Z High angle annular dark field/Z contrast
HR-STEM High resolution scanning transmission electron microscopy/microscope HR-TEM High resolution transmission electron microscopy/microscope
NBO Non bridging oxygen
NMR Nuclear Magnetic resonance
OIM Orientation imaging microscopy ORM Optical reflection microscopy
RC cooling from the liquid state at 1500 - 1560 °C at rapid cooling rate (50°C/min)
RH heating from room temperature at rapid heating rate (20°C/min)
RT Room temperature
SAXS Small angle X-ray scattering
SC cooling from the liquid state at slow cooling rate (furnace inertia)
SHG Second harmonic generation
SH heating from room temperature at slow heating rate (2°C/min) TEM Transmission electron microscopy/microscope
TOM Transmission optical microscopy UST Ultrasonic surface treatment
VCFSE Variable Crystal Field stabilization energy
XRD R-ray diffraction
XRF Ray-ray fluorescence
General introduction
Over the course of time, what is called a glass has had various meanings. It can define a vitreous substance, a material as a window or an object as a wine glass. Glass material is used since Prehistory and is nowadays so common in the daily life that its presence everywhere is almost not noted. According to the most famous legend from the ancient-Roman historian Pliny, Phoenician traders made the first glass accidentally. They were cooking on sand along the banks of Belus river in pots supported by blocks of natron1and saw an unknown opaque liquid substance pouring from the blocks below the pots. From its discovery, compositions and processes were continuously improved to produce transparent materials, strongly resistant to devitrification, responding to numerous applications likes tools, jewellery, bowls and windows. In the current glass industry, few place is devoted to the influence of microstructures on final properties and the most striking is that, as illustrated in Table 1, soda-lime-silica glass composition has almost not changed since the Antiquity!
Glass manufacturing can clearly be qualified asstuctural engineering.
Table 1– Glass compositions comparison between Antiquity and nowadays [1]
Antiquity (wt%) Nowadays (wt%)
SiO2 70,5 72,5
Na2O 15,7 13
K2O 0,8 0,3
CaO 8,7 9,3
MgO 0,6 3
Al2O3 2,7 1,5
Fe2O3 0,4 0,1
From the early 50’s, a new category of materials, glass-ceramics, was discovered and extended largely the use of glass materials. Glass-ceramics involve a controlled crystallization, up to 90 % of the parent glass, by using specific compositions and thermal treatments. They exhibit contradicting properties as the parent glass has to be resistant to crystallization during forming while a controlled
play also a important role on the final properties of glass-ceramics. Depending on the crystalline phase obtained, thermal, optical, chemical, mechanical and/or electrical properties of the parent glass can be improved. Glass has both weak tensile strength and toughness properties. Figure 1 illustrates how the tensile strength and fracture toughness of a soda-lime-silica glass-ceramic are greatly improved compared to soda-lime-silica glass, due to a controlled crystallization. Indeed, glass is a non-crystalline material or an amorphous material and due to its amorphous covalent structure, the main weakness of glass is its brittleness. Glass-ceramics are often encountered in housewares (see Figure 22): (a) ceramic hobs with low thermal expansion, (b) zirconia knife that are strongly sharp or (c) dental prosthesis with an excellent bio-compatibility, high tensile strength and wear resistance.
Figure 1– (a) Tensile strength and fracture toughness in soda-lime glasses and (b) a glass-ceramic - CES 2010
Figure 2– (a) house cooking , (b) zirconia knifes , and (c) dental prosthesis
2(a) http://blog.estrading.com.au/?BBPage=1&Tag=cooktops, (b) http://blog.estrading.com.
The general aims of the PhD are to highlight the importance of the process parameters to obtain specific or tailored microstructures that will further enhance the properties of glass-ceramics in a given system. As it is illustrated in the example below, the choices of compositions and process parameters to control the microstructures are key factors to obtain desired properties like low thermal expansion, sharpness or biocompatibility with good mechanical properties and wear resistance.
This PhD focuses on the control of parameters to enhance the photoluminescence property of fresnoite in the BaO-TiO2-SiO2 system. More specifically, the BaO-TiO2-SiO2 is mostly known for the bulk crystallization of stoichiometric fresnoite (Ba2TiSi2O8), which was extensively studied for its blue/white photoluminescence property. Non-stoichiometric compositions with a small SiO2- excess exhibit strong oriented surface crystallization which can lead to properties like pyro or piezo- electricity. Fresnoite photoluminescence was studied in details on the stoichiometric composition.
However, it was recently reported by Hijiya et al. [2, 3] that compositions with a large SiO2-excess situated inside the miscibility gap exhibit also photoluminescence effect. In addition, the formation of a prior phase separation promotes bulk nucleation by the creation of amorphous/amorphous interfaces and significant enhancements were obtained. In this PhD, the role of a prior phase sepa- ration is first investigated. After thermodynamic modelling to localize the liquid-liquid immiscibility area, compositions are selected to compare the crystallization behavior of fresnoite in presence of phase separation or not. Secondly, the role of composition and heat treatment parameters, i.e.
the annealing temperature, the annealing time as well as the heating and cooling rates, are also investigated on a larger number of compositions. The change in viscosity as well as the interplay of fresnoite with other phases, could also have an impact on fresnoite crystallization behavior, lead- ing to various microstructures. A proper choice of process parameters should therefore enhance photoluminescence intensity.
The Na2O-CaO-SiO2 is much more exploited industrially and exhibits an area of liquid-liquid immiscibility close to the industrial composition. The soda-lime-silica glass system is well-known to exhibit uncontrolled surface crystallization that affects its properties. Glass transparency is important, which means that compositions and processes were optimized to avoid devitrification or uncontrolled crystallization. In this PhD, it is proposed to adapt the results obtained with the BaO-TiO2-SiO2 system to the Na2O-SiO2 and Na2O-CaO-SiO2 systems and find compositions that allow controlled crystallization and increase mechanical properties such as tensile strength, quenchability or scratch resistance.
The work is presented as followed:
Chapter 1 is devoted to an introduction to glass and glass-ceramics materials. In Chapter 2, crystallization and amorphous phase separation theories are detailed as well as the different interplays between phase separation and crystallization that are suggested in literature.
In Chapter 3, all properties that make fresnoite a solid phase of interest are detailed and information concerning the ternary BaO-TiO2-SiO2, such as phase diagram and solid phases data is provided.
Chapter 4summarizes the CALPHAD method for the calculation of phase diagrams and details the Gibbs free energy description for solutions, terminal phases, stoichiometric and intermetallic compounds. This Chapter also details the formalism for the thermodynamic model chosen for the calculation: the Ionic Two Sublattice Model. Chapter 5 presents a literature review for all experimental data available in literature that are required for the calculation of the BaO-TiO2-SiO2
miscibility gap.
InChapter 6, all the general materials and experimental procedures are specified.
Chapter 8toChapter 10are devoted to the study of fresnoite crystallization.
Chapter 7details the thermodynamic modelling of the BaO-SiO2 and SiO2-TiO2 systems as well as their extension to the liquid phase in the BaO-TiO2-SiO2system required for the calculation of the miscibility gap.
Chapter 8 discusses the role of amorphous phase separation on the crystallization behavior of fresnoite. Because the possible role of amorphous/amorphous interfaces is still under debate and since amorphous phase separation is reported to enhance the photoluminescence properties in the BaO-TiO2-SiO2 system,Chapter 8 is devoted to a systematic study of the crystallization of frenoite. The crystallization mechanisms of stoichiometric and non-stoichiometric compositions are compared using differential thermal analysis (DTA) by calculating the Avrami parameters and the activation energies as a function of the particle size. Multi-scale microstructure characterization is conducted by FEG-SEM and TEM. The crystal phases and their orientations are studied at different scales using XRD, EBSD and ACOM-TEM.
In Chapter 9, the role of composition, specifically the SiO2-excess from the stoichiometric composition of fresnoite, coupled with the role of the annealing temperature on microstructure
InChapter 10, the role of prior thermal treatment, i.e, heating from room temperature and cooling from the liquid state are compared with direct annealing studied inChapter 9.
Chapter 11 is a summary of the phases encountered in Chapters 8, 9 and 10. Five phase diagrams reporting different cases of specific microstructures observed with a control of the process parameters are also presented.
InChapter 12, specific compositions and heat treatments are selected for photoluminescence measurements. The optimization of the optical properties is discussed and, more specifically, the role of the different microstructures is scrutinized.
Chapter 13uses the previous results obtained for the BaO-TiO2-SiO2system and proposes to extend them to the industrial soda-lime-silica system. A reference composition, inside the miscibility gap but close to the industrial composition is considered to study the interplay with crystallization.
Additional compositions aim also to investigate the importance of the binodal temperature, the proximity of crystal phases as well as the role of common added oxides and the phase separation morphology.
Chapter 14proposes a general discussion concerning the interplay between phase separation and crystallization, by comparing four systems: BaO-TiO2-SiO2, Na2O-CaO-SiO2, Na2O-SiO2, BaO-SiO2 and Li2O-SiO2.
Chapter 15provides general conclusions and proposes perspectives on the areas that can be improved in the future.
