Nanostructural Characterization of Silicon Oxycarbide Glasses and Glass-Ceramics

Silicon oxycarbide glasses were synthesized by the sol-gel process using precursors such as methyl-, propyl-, and phenyltrimethoxysilanes. The final products contained 14–38 wt% carbon. A TEM study on the nanometer scale revealed that all of the materials were amorphous and monophasic, and that it was not possible to detect any crystalline or otherwise distinct carbon phases. Hot-pressing the glasses led to the crystallization of graphite and silicon carbide within the amorphous matrix. X-ray and electron diffraction showed increasing crystallinity at the higher hot-pressing temperatures. Hot pressing at 1400°C resulted in the appearance of fine-grained silicon carbide, whereas at the highest temperature (1650°C), graphite and both hexagonal and cubic silicon carbide were produced. Subsequent heat treatment of the hot-pressed glasses under an argon atmosphere at 1400°C resulted in the formation of cristobalite. The glass-ceramics produced at the highest hot-pressing temperatures were more resistant to the crystallization of cristobalite during subsequent heat treatments.     

Transitions in Vapor-Deposited Alumina from 300° to 1200°C

The transition of amorphous alumina to α-alumina was studied by X-ray diffraction, electron diffraction, DTA, TGA, and microscopic observation. The amorphous alumina was prepared by condensing vapor from evaporating molten alumina in vacuo onto the glass envelope of the vacuum chamber. The amorphous alumina was transformed to a poorly crystalline material by heating for 16 hr between 570° and 670°C. Between 670° and 1200°C, the poorly crystalline alumina was converted to α-alumina via two parallel series of transition aluminas. The principal series was γ-alumina to δ-alumina to α-alumina. A minor amount of θ-alumina developed from the initial crystallization and persisted throughout the duration of the principal series as a parallel path. Some conversion of δ- to θ-alumina was detected above 900°C. DTA produced an unexplained exothermic peak at 320°C and a second exothermic peak at 860°C which corresponded to formation of metastable aluminas.     

Synthesis of Yttrium–Aluminum–Garnet Hollow Microspheres by Reverse-Emulsion Technique

Yttrium–aluminum–garnet (YAG, Y₃Al₅O₁₂) hollow microspheres were synthesized by reverse-emulsion (w/o) technique starting with aqua-based precursors of oxides. The non-ionic surfactant was used as the emulsifying agent. The gel powders were calcined at 700°–1200°C. The synthesized powders were characterized by differential thermal analysis (DTA), thermogravimetry, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The appearance of an exothermic peak at 932°C in the DTA curve revealed the crystallization of YAG, which was further confirmed by XRD and FTIR studies. SEM confirmed the formation of hollow microspheres.     

Nucleation and Crystallization of a Lead Halide Phosphate Glass by Differential Thermal Analysis

The nucleation and crystallization mechanisms of a lead halide phosphate glass [40P₂O₅·30PbBr₂·30PbF₂ (mol%)] were investigated by differential thermal analysis (DTA) and X-ray diffraction analysis. There were two crystalline phases in the crystallized samples: the major phase was PbP₂O₄, and the minor phase was PbP₂O₆. The average activation energy for crystallization, E , for two different particle sizes of this glass was determined to be 119 ± 4 kJ/mol by the Kissinger method and 124 ± 4 kJ/mol by the Augis–Bennett method. The Avrami constants were determined to be 1.6 and 2.5 for particle sizes of 203 and 1040 μm, respectively, by the Ozawa equation, and 1.7 and 2.4 for particle sizes of 203 and 1040 μm, respectively, by the Augis–Bennett equation. The decrease in the crystallization peak height in the DTA curve with increasing particle size suggested that the particles crystallize primarily by surface crystallization. A nucleation-rate type curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T _p, or the height of the crystallization peak, (δ T )_p, as a function of nucleation temperature, T _n. The temperature where nucleation can occur for this glass ranges from 360°–450°C and the maximum nucleation rate is at 420°± 10°C.     

Classical and Differential Thermal Analysis Studies of the Glass-Ceramic Transformation in a YSiAlON Glass

Nucleation and crystallization studies were conducted on a YSiAlON glass that contained 17 equiv.% nitrogen (7.5 at.%) by using a two-stage nucleation-and-growth treatment. Classical and differential thermal analysis (DTA) techniques were both used to study the crystallization process, to ensure that the optimum heat-treatment schedule that yielded a fine microstructure and minimum residual glass was applied. The optimum nucleation and crystallization temperatures were determined from DTA traces that were recorded from isothermal heat treatments at different nucleating temperatures, ranging between T _g - 40°C and T _g+ 100°C for 1 h and crystal-growth temperatures in the range of 1170°-1310°C for 0.5 h, respectively. The activation energy for the crystallization process was determined, based on the analysis of the variation of peak temperature at five different heating rates. Specimens heat treated in a tube furnace under nitrogen gas were subjected to microscopical investigation, and results showed variations in the volume fraction of crystalline phases and crystal size with nucleation temperature. The nucleation temperature, T _g+ 40°C (1025°C), which corresponded to the maximum volume fraction of crystalline phases and minimum crystal size, was consistent with the optimum nucleation temperature, T _g+ 35°C (1020°C), as determined from DTA. The time and temperature of nucleation and crystal growth dictated the nature and size of the crystalline phases. Properties such as hardness and density were assessed and correlated to the nucleation temperature. The influence of sample specific surface on the devitrification mechanisms was estimated, and bulk nucleation was observed to be the dominant nucleation mechanism.     

Resolution of Thermal Peaks of Kaolinite in Thermomechanical Analysis and Differential Thermal Analysis Studies

The recent findings for the kaolinite metakaolinite, cubic-mullite, and orthorhombic-mullite reaction series have been thoroughly examined by differential thermomechanical analysis (DTMA) and differential thermal analysis (DTA). Metakaolinite shows two differential contraction peaks in the vicinity of 980°C caused by final dehydroxylation at the endothermic dip just before 980°C in DTA with expulsion of 35–37 wt% SiO₂, formation of a defect aluminosilicate phase and simultaneous contraction of the latter phase, and crystallization of cubic mullite at the 980°C exotherm in DTA. Mullitization takes place in two simultaneous reaction steps: (i) polymorphic transformation of cubic mullite to orthorhombic mullite during the ∼1250°C exotherm shown by DTA which coincides with the differential expansion peak in DTMA and (ii) nucleation followed by crystallization of orthorhombic mullite from the residual aluminosilicate compact phase during the ∼1330°C exotherm shown by DTA. The aluminosilicate formed during the large differential contraction at 1100°–1400°C as shown by DTMA. These results, obtained by the two physical techniques, corroborate earlier findings of the kaolinite transformation series.     

Melt Processing and Properties of Barium-Sialon Glasses

Bulk oxynitride glasses in the system Ba-Sialon were prepared by cooling homogenized melts from ∼1740°C at ∼50°C/min. The microchemistry, microstructure, and properties of these materials were studied by DTA, dilatometry, electron microscopy, and energy dispersive X-ray analysis. Up to ∼ 7 at.% nitrogen was retained in the glasses by the conventional glass-processing techniques used. Microscopically homogeneous glasses were obtained despite macroscopic segregation of millimeter-sized metallic spheres consisting of Si and Fe. Incorporation of nitrogen in the glass network led to decreasing thermal expansion coefficients, higher glass-transition temperatures, and greater values of indentation hardness with increasing nitrogen content.     

Sol—Gel Synthesis of Strontium-Doped Lanthanum Manganite

Strontium-doped lanthanum manganite powders were prepared using a peroxide acetate salt based solution. The stable sol was peptized by reacting ammonium hydroxide with the precursor solution. The amorphous dried gel powders exhibit a high energy level, due to their high cations coordination and small particles, to develop the perovskite phase. This crystalline phase development from powders containing monocarboxylate ligands was characterized by thermal analysis (TG, DTG, DTA), X-ray diffraction, and IR spectroscopy. The transformation from amorphous powders into a crystallized homogeneous oxycarbonate phase in a first stage corresponds to an exothermal DTA peak at 270°C. X-ray diffraction patterns and IR spectra showed similar behavior of the powders after complete organic removal, during the conversion into perovskite phase starting at approximately 630°C and achieved about 700°C and achieved about 700°C, as well as during the sintering process.     

Nucleation and Crystallization of New Glasses from Fly Ash Originating from Thermal Power Plants

The nucleation and crystallization kinetics of new glasses obtained by melting mixtures of a Spanish carbon fly ash with glass cullet and dolomite slag at 1500°C has been evaluated by a calculation method. These glasses, whose microstructure was examined by TEM carbon replica, were susceptible to controlled crystallization in the 800°–1100°C range. The resulting glass-ceramics developed acicular and branched wollastonite crystals or a network of dendritic pyroxene mixed with anorthite feldspar (SEM and EDX analysis). The time–temperature–transformation curves (processing of the XRD data) showed the crystallization kinetics and the critical cooling rate to be in the 12°–42°C/min range.     

Thermal Decomposition of Alkoxides in an Inert Organic Solvent: Novel Method for the Synthesis of Homogeneous Mullite Precursor

Thermal decomposition of a mixture of aluminum iso-propoxide and tetraethoxysilane (Al/Si = 3) in toluene at 300°C was examined. The reaction completely proceeded and yielded an amorphous product. The DTA profile of the product showed a sharp peak at ∼1000°C which was associated with crystallization of mullite and a small amount of the spinel phase. The mullite prepared by calcination of the product at 1300°C was composed of micro-crystals having an average diameter of 30 nm and had a large surface area of 41 m²g.     

Subsolidus Studies in the System PbO-SiO2

The subsolidus region of the PbO-SiO₂ system was studied by DTA and X-ray diffraction. X-ray diffraction analysis showed the presence of five compounds: 4PbO.SiO₂, 3PbO·SiO₂, 2PbO·SiO₂, 3PbO·2SiO₂, and PbO·SiO₂. The compound 4PbO·SiO₂ has previously been reported to have three polymorphic forms; there are two polymorphs of 2PbO·SiO₂ with the inversion at 460°±15°C. The compounds 3PbO·SiO₂ and 3PbO·2SiO₂ were unstable above 430°±10° and 585°±15°C, respectively; PbO·SiO₂ was unstable below 525°±15°C. DTA patterns were determined for glasses of the composition of each of these compounds.     

Strength and Related Characteristics of Li2O-SiO2 Glasses Crystallized After Ion Exchange

Lithium silicate glasses containing a small amount of Al₂O₃ for stabilization and traces of Pt for nucleation were ion-exchanged in an NaNO₃ bath at 350°C. The ion-exchanged glasses were heat-treated using a 2-stage crystallization schedule. The strength of these glasses, measured in 3-point loading in liquid nitrogen and in air at room temperature, showed a maximum as a function of ion-exchange time. The results are interpreted on the basis of X-ray and electron microprobe studies.     

Stable and Metastable Phase Relations in the System Alumina–Zirconia–Yttria

The Al₂O₃ZrO₂Y₂O₃ system was studied in the range of temperatures 1600°–2800°C by methods of X-ray analysis at 20°C, petrography, DTA in He at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X-ray analysis. The stable and metastable phase diagrams were constructed. The liquidus and solidus projections, crystallization paths for the alloys, and polythermal sections are presented in the article. The structure of the restricting systems defines the phase equilibria in the ternary system. No ternary compounds were found. The metastable phase relations were caused by the ambivalent behavior of Y₃Al₅O₁₂ during crystallization.     

The influence of TiO2 content on the properties of glass ceramics: Crystallization,microstructure and hardness

The effects of compositional variation, crystallization behavior, crystalline phases and microstructure formed in the SiO₂3Al₂O₃3CaO (SAC) glass system using various amounts of TiO₂ as nucleating agent were investigated by Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDAX) and Fourier transform infrared spectroscopy (FTIR) techniques. The crystallization kinetics and mechanical properties of SAC glass ceramics were studied using crystallization peak temperature (T_p) of three different glasses as obtained from DTA, the activation energy (E) and Avrami exponent (n) were also determined. The crystallization peak temperature (T_p) and activation energy (E) were found to increase with the increase in TiO₂ content. The major crystalline phases were anorthite and wollastonite along with gehlenite and titanite as the minor crystalline phases present in the glass ceramic system. The studies showed that the three dimensional crystalline structure and the microhardness increased with the increase of TiO₂ content in the glass ceramics system.     

Influence of Impurities on High-Temperature Reactions of Kaolinite

When kaolinite is heated for differential thermal analysis (DTA) the second exothermic peak (∼1275°C) is due to growth of mullite crystals accelerated by formation of a liquid phase. The third exothermic peak (∼1460°C) is due to crystallization of cristobalite. Addition or presence of impurity oxides causes the second peak to shift in temperature and accelerates the formation of cristobalite so that the third peak may appear as low as the second exothermic peak temperature.     

Dissolution of Alumina, Sintering, and Crystallization in Glass Ceramic Composites for LTCC

Sintering and microstructure evolution of alkali-free calcium–alumo–borosilicate glass/α-Al₂O₃ composites (mean particle size ca. 2 μm) for low-temperature cofired ceramics were studied during heating at 5 K/min by heating microscopy, thermal analysis (DTA), X-ray diffraction (XRD), and electron microscopy (SEM). Composites fully densify at ≈830°C, not essentially influenced by the dissolution of alumina and glass crystallization. Thus wollastonite, as first crystalline phase, was detectable at 840°C. Above 900°C, a pronounced crystallization of anorthite is evident, reaching 60 wt% at 1050°C. Rietveld analyses of XRD data revealed that anorthite precipitates at the expenses of alumina, which declines from ≈33 to <10 wt%, and wollastonite, which fully declines from its maximum of ≈19 wt%. Based on XRD, we discuss the evolution of crystal mass fractions, the residual glass composition, the glass viscosity, and the effective shear viscosity of the composites under study during heating.     

The Anorthite–Diopside System: Structural and Devitrification Study. Part II: Crystallinity Analysis by the Rietveld–RIR Method

The crystallization behavior of 10 binary glasses belonging to the CaO–MgO–Al₂O₃–SiO₂ quaternary system and two glasses corresponding to anorthite and diopside composition was investigated by X-ray diffraction, thermal, and thermomechanical analyses, and scanning electron microscopy. Particular emphasis is laid on the quantitative X-ray diffraction analysis by the Rietveld–reference intensity ratio combined procedure, which seems to be a useful tool to obtain time–temperature–transformation diagrams. Results showed that to obtain glass–ceramics with a significant crystalline phase presence, it is necessary to treat samples at 1000°C for 4 h or at 1100°C for 1 h.     

Determining the Nucleation Rate Curve for Lithium Disilicate Glass by Differential Thermal Analysis

The crystallization of lithium disilicate (Li₂O·2SiO₂) glass nucleated at various temperatures was studied by differential thermal analysis (DTA). A plot of the DTA crystallization peak height versus nucleation temperature closely resembles the classical nucleation rate curve for lithium disilicate glass whose maximum is at 453°C. The glass becomes saturated with internal nuclei when heated at 453°C for 10 h. The DTA technique is a rapid, alternative method for determining the temperature for maximum nucleation. The activation energy for crystallization, E , and the heat of crystallization, H , are independent of the concentration of nuclei and are 249±10 and 67±3 kJ/mol, respectively. The Avrami exponent, n , depends strongly on the concentration of nuclei in the glass.     

Effect of P2O5 on the Crystallization of Lead Silicate Glasses

The effect of P₂O_S on the devitrification of binary lead silicate glasses containing 64 and 59 mol% PbO was studied. Glasses underwent isothermal crystallization treatments at 400°, 450°, 500°, and 550°C. A polymorph of 3PbO₂SiO₂ was the major product of crystallization for all compositions of glasses. Secondary products of crystallization were found to be a polymorph of PbOSiO₂ in the 59 mol% and a low-temperature polymorph of 2PbOSiO₂ in the 64 mol% PbO glasses. Dominant mode of crystallization in both binary glasses was surface devitrification at all temperatures studied. Addition of P₂O₅ promoted internal crystallization in the form of spherulites. 400°C was found to be the most effective temperature for nucleating spherulitic growth. Crystallization at 400°C led to high concentrations of spherulites in all glasses containing at least 0.5 mol% P₂O₅. Concentrations of 0.5 mol% P₂O₅ were needed to produce detectable levels of spherulitic nucleation at r<400°C.     

Seeded Crystallization of Leucite

The leucite crystallization kinetics from a hydrothermally derived precursor seeded with nano-crystalline leucite was investigated by X-ray diffraction and non-isothermal differential thermal analysis. The nano-crystalline leucite was prepared by high-energy milling of high-purity leucite powder and the leucite precursor was prepared by the hydrothermal method of silica sol, aluminum nitrate, and potassium nitrate. After the seeds were introduced, the crystallization temperature of the precursor was lowered by 100°C and the transition phase kalsilite did not appear during the crystallization process. When the seeded precursor was heat treated at 700°C, a small amount of cubic leucite was stabilized to room temperature. The seeded precursor showed an exothermic peak between 800° and 920°C under different heating rates. The activation energy for the growth of leucite from the seeded precursor was 256(SD9) kJ/mol.