Sub-Sub-Tg Enthalpy Relaxation in a B2O3 Glass

Enthalpy relaxation processes in a B,O₃ glass were investigated calorimetrically in the temperature region from well below to near the glass-transition temperature T_g . The low-temperature ( T_aT_g- 100 K) or sub-sub- T_g anneals stabilize the glass structure. On heating, the annealed sample shows an excess endothermic peak above the annealing temperature T_a , and gradually recovers the original enthalpy without heating through T_g . The enthalpy relaxation evolves in a continuous manner with annealing time as seen in the recovery process. As T_a approaches T_g , a gradual transformation of the sub-sub- T_g behavior to that characteristic of the sub- T_g anneal occurs. The relaxation spectrum exhibits a broad distribution of relaxation times with activation energies of ∼ 10⁵ J/mol. The so-called β distribution commonly used in the analysis of the sub- T_g relaxation processes is seen to be too narrow to describe the sub-sub- T_g behavior. Possible mechanisms for the sub-sub- T_g relaxation phenomena are proposed.     

A New SiC-Whisker-Reinforced Lithium Aluminosilicate Composite

The glass-ceramic matrix of the well-known lithium aluminosilicate (LAS)/SiC composite is usually formulated near the spodumene composition. We report here a new composition which is rich in alumina (78 wt%) and lean in silica (21 wt%) and lithia (1 wt%). This formulation offers a new option of converting the glass-ceramic matrix to a mullite/alumina matrix upon annealing above 1400°C, and hence better creep resistance and other high-temperature mechanical properties. Using a transient-phase processing method that we developed previously for the superplastic forming of mullite, we are able to hot-press a composite containing 30 vol% SiC whiskers at ∼1350°C to achieve full density. Flexural strength measurements up to 1400°C have confirmed the improved high-temperature strength and creep resistance over conventional LAS. The fracture toughness is also higher than that of LAS. The results suggest that the new composition may be chosen as a better candidate matrix for SiC-fiber-reinforced composites.     

Crystallization of Nuclear Waste Disposal Glass

The rate of bulk crystallization and shear viscosity η of a zinc borosilicate simulated nuclear waste disposal glass (SNWDG) were measured above the glass transition temperature T_g . Using recent theories and experimental data for the temperature dependence of η at < T_g , the crystallization rate below T_g was calculated; well below T_g the SNWDG (and, more generally, any high-silica glass) will not crystallize appreciably over any important time period for radioactive decay of fission products.     

Rayleigh Scattering in Silica Glasses

Rayleigh-scattering properties in commercial silica glasses is clarified with relation to their fabrication methods. Fused silica and synthetic silica made by the flame hydrolysis of SiCl₄ (VAD) method exhibit similar magnitudes of scattering, regardless of fabrication method. Synthetic silica made by flame fusion of SiCl₄ exhibits ∼10% lower scattering than other silica glasses. The factors affecting scattering are evaluated based on refractive index dispersion and glass transition temperature (T_g). The results suggest that the difference in Rayleigh scattering is predominantly attributable to the difference in T_g , which is affected by impurities, such as OH ions.     

Glass Transition Temperatures at Rapid Heating Rates

Glass transition temperatures (T_g) determined at high rates of heating or cooling are essential for applications involving stress relief during cooling from firing of glass-to-metal seals and dental restorations. The measurement of T_g at heating rates as high as 600°C min⁻¹ is reported. These high-rate data, when combined with previously reported low-rate (≤20°C min_-1) values, indicate that the Arrhenius-type relation between T_g and heating or cooling rate is maintained into the high-rate regime.     

Complete Elastic Tensor for Mullite (∼2.5Al2O3·SiO2) to High Temperatures Measured from Textured Fibers

Directionally solidified mullite fibers have been grown by the laser-heated, float-zone method from starting materials with a nominal composition of 3Al₂O₃·2SiO₂. The fibers used in this study have large single-crystal regions with composition 2.5Al₂O₃·SiO₂ and (001) fiber axis orientation. The complete elastic tensor of these samples has been determined by Brillouin spectroscopy at room temperature and elevated temperatures up to 1200°C. Isotropic moduli (bulk, shear, and Young's) have been calculated using the Voigt–Reuss–Hill averaging scheme. The room-temperature values obtained are K _VRH= 173.5 ± 6.9 GPa, G _VRH= 88.0 ± 3.5 GPa, E _VRH= 225.9 ± 9.0 GPa. All moduli show gradual, linear decreases with temperature. The temperature derivatives obtained for the equivalent, isotropic moduli are d K _VRH/d T =−17.5 ± 2.5 MPa/°C, d G _VRH/d T =−8.8 ± 1.4 MPa/°C, d E _VRH/d T =−22.6 ± 2.8 MPa/°C. Substantial differences between bulk properties calculated from the single–crystal measurements in this study and the properties reported in the literature for polycrystalline sintered mullite are identified, indicating the importance of factors such as microstructure, intergranular phases, and composition to the elasticity of mullite ceramics.     

Effect of Oxygen Vacancies on the Hot Hardness of Mullite

The structure of mullite, which has a composition ranging from 3Al₂O₃·2SiO₂ to Al₂O₃·2SiO₂, contains ordered oxygen vacancies. Sillimanite, Al₂O₃·SiO₂, has a similar structure but with no vacancies. The indentation hardness of polycrystalline mullite (3Al₂O₃·2SiO₂) was measured from room temperature up to 1400°C and compared with that of single-crystal sillimanite (Al₂O₃·SiO₂) up to 1300°C. It was found that both materials show the same variation in hardness with temperature, suggesting that the structures have a similar resistance to plastic deformation, and therefore that the oxygen vacancies in the mullite structure are not the primary cause of mullite's resistance to high-temperature deformation.     

Machinable Ceramics Containing Rare-Earth Phosphates

Two-phase composites consisting of LaPO₄ or CePO₄ and alumina, mullite, or zirconia were found to be machinable; i.e., they can be cut and drilled using conventional tungsten carbide metal-working tools. Single-phase LaPO₄ was also machinable. Measurements of drilling rates, grinding rates, and normal forces are used to compare the ease of machining in these materials and in a conventional machinable glass-ceramic material, and to provide preliminary information on the relation between microstructure and machining properties. In Hertzian contact experiments these materials showed extensive nonlinear behavior associated with a damage zone beneath the contact site, similar to other machinable ceramics. Mechanisms of material removal are discussed.     

Novel Low-Temperature Processing Route of Dense Mullite Ceramics by Reaction Sintering of Amorphous SiO2-Coated γ-Al2O3 Particle Nanocomposites

Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO₂-coated gamma-Al₂O₃ particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO₂-coated alpha-Al₂O₃ particle microcomposites (AS-alphaA) and to other Al₂O₃-SiO₂ reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites.     

Preparation and Structure of Lithium-Ion-Conducting Mixed-Anion Glasses in the System LiBO2–LiBS2

Oxysulfide glasses were prepared in a wide range of compositions in the system LiBO₂-LiBS₂. Temperatures of glass transition ( T_g ), crystallization ( T_c ), and liquidus ( T_l ) were determined; a maximum of T_g was observed near the composition with 20 mol% LiBS₂. The electrical conductivity at 500 K ranges from 5×10⁻⁴ to 5×10⁻³ S·cm⁻¹ with the maxima in conductivity observed near the composition 55LiBO₂·45LiBS₂. This conductivity enhancement with a mixing of two components, which can be called the mixed-anion effect, is accompanied by a decrease in the degree of undercooling of glass expressed by the ratio ( T_l - T_g )/ T_l . The infrared and Raman spectra showed that the structural units with bridging oxygens B-O-B and nonbridging sulfurs B-S⁻ predominated rather than those with nonbridging oxygens B-O⁻ and bridging sulfurs B-S-B in these glasses.     

Preparation and Mechanical Properties of SiC Whisker and Nanosized Mullite Particulate Reinforced TZP Composites

The influence of the addition of nanometer mullite particulates and SiC whiskers coated with alumina on the mechanical properties of tetragonal zirconia polycrystals (TZP) was studied. With increasing mullite( p ) content the high-temperature flexural strength increased, and a maximum value of 360 MPa at 1000°C was reached at 15 vol% mullite( p . Furthermore, 10 vol% SiC( w ) reinforced 15 vol% mullite/TZP composites improved the high-temperature strength up to 490 MPa at 1000°C, 2.7 times that of pure TZP matrix. This high-temperature strengthening is attributed to load transfer from TZP matrix to SiC( w ) and mullite particulates. Significant whisker pull-out and interface debonding were also observed on the fractured surfaces when SiC( w ) was coated with Al₂O₃ film.     

Desiliconization of Mullite Felt

High-temperature evaporation from 80% porous, rigid mullite (3Al₂O₃·2SiO₂) whisker felt was studied under vacuum and at various helium pressures using gravimetry, X-ray diffractometry, SEM, and EDS. Heat treatments at 1350° to 1550°C resulted in evaporation of SiO₂ from mullite with a rate strongly dependent on temperature and pressure. A concentration gradient of SiO₂ was observed through the cross section of samples after heating under vacuum, indicating that SiO₂ preferentially evaporated from whiskers on the periphery of the samples. The SiO₂ concentration gradient was accompanied by sharp microstructural changes across the specimens. The gradient was decreased by raising the ambient helium pressure during heat treatment. A mathematical model was developed to predict the SiO₂ concentration profile. The model agrees well with experimental results and demonstrates that the diffusivity of SiO₂ in the vapor phase controls the gradient of SiO₂ through the cross section of mullite felt.     

Nanostructure and Micromechanical Properties of Silica/Silicon Oxycarbide Porous Composites

The microhardness–nanostructure correlation of a series of silica/silicon oxycarbide porous composites has been investigated, as a function of pyrolysis temperature, T _p. The pyrolyzed products have been studied by means of scanning electron microscopy, mercury porosimetry, chemical analysis, solid-state ²⁹Si-NMR, X-ray diffraction, Raman spectroscopy, and microindentation hardness. Two distinct regimes are found for the microhardness behavior with T _p. In the low-temperature regime (1000°C ≤ T _p < 1300°C), the material response to indentation seems to be dominated by the large amount of pores present in the samples. In this T _p range, low microhardness values, H , are found (<110 MPa). Above T _p= 1300°C, a conspicuous H increase is observed. In this high-temperature regime ( T _p= 1300–1500°C), microhardness values are shown to notably increase with increasing pyrolysis temperature. The H behavior at T _p= 1300–1500°C is discussed in terms of (i) the volume fraction of pores and the average pore size, (ii) the bond density of the oxycarbide network, and (iii) the occurrence of a nanocrystalline SiC phase.     

New Generation of Plasma-Sprayed Mullite Coatings on Silicon Carbide

Mullite is promising as a protective coating for silicon-based ceramics in aggressive high-temperature environments. Conventionally plasma-sprayed mullite on SiC tends to crack and debond on thermal cycling. It is shown that this behavior is due to the presence of amorphous mullite in the conventionally sprayed mullite. Heating the SiC substrate during the plasma spraying eliminated the amorphous phase and produced coatings with dramatically improved properties. The new coating exhibits excellent adherence and crack resistance under thermal cycling between room temperature and 1000–1400°C. Preliminary tests showed good resistance to Na₂CO₃-induced hot corrosion.     

Infrared-Transparent Mullite Ceramic

Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al₂O₃ yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A1₂O₃ formed precipitates of α-Al₂O₃ at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO₂-Al₂O₃ phase diagram.     

Mullite Joining by the Oxidation of Malleable, Alkaline-Earth-Metal-Bearing Bonding Agents

Metallic Ba-Al-Si bonding agents have been used to produce all-ceramic, BaO-Al₂O₃-SiO₂ bonds between plates of mullite (Al₆Si₂O₁₃). Ba-Al-Si tapes (200 (μm thick) were fabricated by compaction and rolling of mechanically alloyed powder. The Ba-Al-Si tapes were placed between mullite plates and then oxidized by heating to a peak temperature of 1230°C in air. The oxidized tapes strongly adhered to the mullite plates at 25° and 1000°C, as indicated by the fracture morphologies obtained from compressive shear tests. Electron microscopy (EPMA, TEM) revealed that the bulk of the oxidized Ba-Al-Si tapes (away from the interfaces with mullite) consisted largely of the compound BaAl₂Si₂O₈, along with some BaSiO₃ and an amorphous, barium-rich aluminosilicate. The interface between the oxidized bonding agent and bulk mullite consisted of a mixture of BaAl₂Si₂O₈, Al₆Si₂O₁₃, A1₂O₃, BaAl₂O₄, and an amorphous, barium-bearing aluminosilicate.     

Rapid Crystallization of SiO2-Al2O3 Glasses

The crystallization of Al₂O₃-rich glasses in the system SiO₂-Al₂O₃ which were prepared by flame-spraying and/or splat-cooling was studied by DTA, electron microscopy, and X-ray diffraction. Over a wide range of compositions, the crystallization temperature ( T_x ) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X-ray diffraction. In the low-Al₂O₃ region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high-Al₂O₃ region mullite and γ-Al₂O₃ cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective T_x 's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al₂O₃ content. In light of the T_x data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO₂-Al₂O₃ system is questioned.     

Decomposition of Mullite by Silica Volatilization

Thermal decomposition of mullite into corundum was investigated using a high-temperature X-ray single-crystal camera equipped with a gas-flame furnace and by scanning electron microscopy and electron probe microanalysis (EPMA). When heated to ∼1750°C, mullite decomposed to corundum by the liberation of the SiO₂ component with topotaxial relations of:

• (1) 

  (310)_mull∥(001)_cor; [001]_mull∥[110]_cor
• (2) 

  (130)_mull∥(001)_cor; [001]_mull∥[110]_cor
• (3) 

  (110)_mull∥(001)_cor; [001]_mull∥[110]_cor

Thus, it was considered that, when mullite decomposed into corundum, their oxygen close-packed planes were almost preserved. The SEM photographs showed that the crystals of the developed corundum are prismatic and ∼5 μm wide. The EPMA showed that the phase boundary between mullite and developed corundum is discontinuous.     

Pyroelectric Properties of Diphasic Polar Glass-Ceramics at Low Temperatures

The pyroelectric properties of the diphasic polar glassceramic (Sr₂TiSi₂O₈ and SrTiO₃) are investigated in the temperature range from 10 to 300 K. The pyroelectric coefficient of the diphasic polar glass-ceramic is −1.8 × 10⁻⁷ C/ (m^2.K) at room temperature. It is interesting that the absolute value of p₃ decreases quickly below 100 K and eventually becomes positive below 40 K, corresponding to the structural phase transition of the crystalline SrTiO₃ in the glass-ceramic. XRD, DTA, and SEM were used to investigate the coupling patterns of Sr₂TiSi₂O₈ and SrTiO₃ crystallites in the glass-ceramic and to explain the special coupling effects