Thermal Conductivity of Anisotropic Solids at High Temperatures—The Thermal Conductivity of Molded and Pyrolytic Graphites

A method has been developed for the determination of the thermal conductivities of anisotropic solids under conditions of two–dimensional, steady–state heat conduction in a cylinder of finite length heated in vacuum by high–frequency induction and radiating heat to the surroundings. The method has been used to determine the radial thermal conductivity, K_r , and the axial thermal conductivity, K_z , of molded ZT-type and pyrolytic graphite in the temperature range 1260° to 2200°K. For ZT-type graphite, K_z/K_r =–0.101 + 2.002 × 10⁻⁴± T (1260°K < T < 2200°K); for pyrolytic graphite, K_z/K_r = 0.0376 at 1817°K.     

Nucleation and Crystallization of a Lithium Aluminosilicate Glass

An aluminosilicate glass of composition 61SiO₂6Al₂O₃10MgO6ZnO·12Li₂O·5TiO₂ (mol%) has been prepared by a melting process and investigated as far as crystallization is concerned. Glass-ceramic is easily obtained because glass shows a high tendency to crystallize starting from 700°C. The crystalline phases evolve with temperature, showing the aluminosilicates to be the main phase up to 1050°C, followed by metasilicates and silicates, some of which have lower melting points. The titanates of Mg and Zn develop from the phase-separated glass, soon after T _g, and grow to form nucleation centers for the other crystalline phases. The evolution from phase-separated glass to glass-ceramic has been followed by many thermal, diffractometric, spectroscopic, and microscopic techniques.     

Porous Glass-Ceramics with Bacteriostatic Properties in Silver-Containing Titanium Phosphates: Control of Release of Silver Ions from Glass-Ceramics into Aqueous Solution

Porous glass-ceramics with the surface phase consisting predominantly of AgTi₂(PO₄)₃ crystal and the interior phase of LiTi₂(PO₄)₃ crystal are prepared by exchange of Ag⁺ions for Li⁺ions. In the present work, the release of Ag⁺ ions from glass-ceramics into aqueous solutions was investigated. Exchanged Ag⁺ ions were chemically stable in water. The as-exchanged glass-ceramics released Ag⁺ ions of 3045 equiv/g into phosphate buffer solution containing Na⁺ ions. X-ray diffraction analysis showed that silver-containing titanium phosphate crystalline phase in the glass-ceramics did not deteriorate even by heating at 900°C. The amount released from the heated glass-ceramics into the buffer solution was found to decrease drastically by 1–2 μequiv/g. The heated glass-ceramics showed excellent bacteriostatic properties. Glass-ceramics are expected to be novel bacteriostatic materials which have high thermal resistance and are medically safe.     

Thermal Diffusivity/Conductivity and Specific Heat of Mullite-Zirconia-Silicon Carbide Whisker Composites

The thermal diffusivity of mullite-ZrO₂-SiC_w materials was determined from 25° to 1000°C for composites in which the ZrO₂ had different amounts of monoclinic and tetragonal phase, and varying SiC _w content. At 25°C the thermal diffusivity of the matrix materials increased with increasing amounts of monoclinic phase, and little or no anisotropy was seen. Composites with SiC _w additions showed significant increases in thermal diffusivity and anisotropy as compared with the matrix materials. With increasing temperature, thermal diffusivities of the matrices and their respective composites decreased, and at 1000°C differences were small. Specific heat was determined from 25° to 700°C and thermal conductivity values calculated. Specific heat increased with temperature and was not composition dependent, except for one sample. Thermal conductivity values as a function of temperature followed the same trends as the thermal diffusivity values.     

Thermal Expansion Behavior of Boron-Doped Cordierite Glass-Ceramics

The linear thermal expansion coefficients of cordierite glass-ceramics that have been doped with a fixed amount of P₂O₅ and 1, 2, and 3 wt% of B₂O₃ show negative expansion in the temperature range of 100°-300°C. The expansion of the undoped cordierite sample is positive. A relative decrease in the degree of negative expansion is observed as the B₂O₃ concentration increases. These negative expansion coefficients are similar to those of cordierite glass-ceramics that have been doped with the potassium cation.     

High-Temperature Characteristics of Transition Al2O3 Powder with Ultrafine Spherical Particles

Ultrafine transition Al₂O₃ powder with spherical particles was prepared by an arc-discharge method. High-temperature characteristics were found to be superior to those of commercial A1₂O₃ powders by DTA, specific surface-area measurements, XRD, and TEM. The powder studied transformed to α-phase at about 1335°C. After heat treatment at 1260°C for 1 h, the specific surface area of the powder decreased from 25.0 to 17.3 m²/g. Some particles were able to retain the transition phase even after 106 h at 1160°C.     

Decomposition and Crystallization of a Sol–Gel-Derived PbTiO3 Precursor

A lead titanate (PbTiO₃) precursor, prepared by the Pechini method, has been heat treated to study the transformation from amorphous to crystalline PbTiO₃. Nucleation of PbTiO₃ in the temperature interval 400°–475°C occurred before completion of the thermal decomposition of the polymeric precursor, resulting in nanocrystalline PbTiO₃ with an unexpectedly high tetragonality ( c/a ratio). Annealing and crystallite growth at 600°C resulted in an increasing c/a ratio with annealing time in line with the expected finite size effect of PbTiO₃. The unusually high c/a ratios observed in PbTiO₃ nucleated at 400°–475°C are discussed in relation to partial reduction and point defects in PbTiO₃.     

Formation and Stability of β-Quartz Solid-Solution Phase in the Li-Si-Al-O-N System

The development of crystalline phases in lithium oxynitride glass-ceramics was examined, with particular emphasis placed on the effect of the nitrogen source (AlN or Si₃N₄) on the formation and stability of a β-quartz solid-solution ( ss ) phase. Oxynitride glasses derived from the Li-Si-Al-O-N system were heat-treated at temperatures up to 1200°C to yield glass-ceramics in which β-quartz( ss ) and β-spodumene( ss ) of approximate composition Li₂OAl₂O₃4SiO₂ formed as major phases and in which X-phase (Si₃Al₆O₁₂N₂) and silicon oxynitride (Si₂N₂O) were present as minor phases. The nitrogen-containing β-quartz( ss ) phase that was prepared with AlN was stable at 1200°C; however, the use of Si₃N₄ as the nitrogen source was significantly less effective in promoting such thermal stabilization. Lattice parameter measurements revealed that AlN and Si₃N₄ had different effects on the crystalline structures, and it was proposed that the enhanced thermal stability of the β-quartz( ss ) phase that was prepared with AlN was due to both the replacement of oxygen by nitrogen and the positioning of excess Al³⁺ ions into interstitial sites within the β-quartz( ss ) crystal lattice.     

Mechanical Properties of Mica Glass-Ceramics

The mechanical properties of fluorophlogopite mica-based glass-ceramics, such as fracture toughness, flexural strength, and machinability, were investigated. It has been shown that toughening increments in the glass-ceramics occurred by crack deflection and branching by crystals with a high aspect ratio. All the glass-ceramics heat treated at 1000°–1150°C exhibited a higher fracture toughness of 1.2–2.2 MPa-m^1/2 as compared to 0.8 MPa-m^1/2 for the parent glass, and showed average flexural strengths of 140–160 MPa. It has been suggested that ( H/K_Ic )² of the mica glass-ceramic be used to estimate the machinability, because it decreases linearly with machinability.     

Suitable Glass-Ceramic Sealant for Planar Solid-Oxide Fuel Cells

Various glass-ceramics were prepared based on the BaO–Al₂O₃–B₂O₃–SiO₂ system with the addition of La₂O₃, ZrO₂, or NiO in an attempt to develop a suitable sealant for planar solid-oxide fuel cells operating at 800°C. To estimate the applicability of these glasses as suitable sealants, their thermal and chemical stabilities as well as the crystallization behavior and bonding characteristics of their parent glasses were investigated. The thermal properties and crystallization behavior of the parent glasses were dependent on the type of additive used, and the bonding characteristics were strongly influenced by the B₂O₃/SiO₂ value in the glass composition. Observation of microstructural change and chemical reaction at the glass-ceramic/electrolyte interface confirmed that the prepared glass-ceramics possessed long-term stability during heat treatment at the operation temperature for up to 1000 h.     

Thermal Conductivity of Li2O·Al2O3·nSiO2 Glass-Ceramics between 5 and 100 K

The thermal conductivity of Li₂O·Al₂O₃· n SiO₂ glass-ceramics is studied for n = 4, 6, 8, 10 between 5 and 100 K. A monotonic increase in conductivity is observed for all samples. This behavior is different from that of glassy counterparts which exhibit a plateau in thermal conductivity between 10 and 20 K. It is also observed that while the conductivity of glass-ceramics is lower than that of glasses at low temperatures, the situation is reversed at higher temperatures. A crossover occurs around 40 K for all studied samples. The glass-ceramic behavior is interpreted in the light of the acoustic mismatch theory of Little. At low temperatures, the thermal boundary resistance that exists at the crystalline-amorphous mismatch is high and the thermal conductivity is low. At higher temperatures, the boundary resistance is very small and the high conductivity is mainly due to the crystalline region within the amorphous structure.     

Effects of M3+ Ions on the Conductivity of Glasses and Glass-Ceramics in the System Li2O—M2O3—GeO2—P2O5 (M = Al, Ga, Y, Dy, Gd, and La)

Glasses with compositions Li_1.2M_0.2Ge_1.8(PO₄)₃ (M = Al, Ga, Y, Gd, Dy, and La) were prepared and converted to glass-ceramics by heat treatment. The effects of the M³⁺ ions on the conductivity of the glasses and glass-ceramics were studied. The main phase present in the glass-ceramics was the conductive phase LiGe₂(PO₄)₃. Al³⁺ and Ga³⁺ ions entered the LiGe₂(PO₄)₃ structure by replacing Ge⁴⁺ ions, but lanthanide ions did not. The glass-ceramics exhibited much higher conductivity than the glasses. With increased ionic radius of the M³⁺ ions, the conductivity remained almost unchanged at ∼3 × 10⁻¹² S/cm for the glasses, but it decreased from 1.5 × 10⁻⁵ to 8 × 10⁻⁹ S/cm for the glass-ceramics at room temperature. The higher conductivity for Al³⁺- and Ga³⁺-containing glass-ceramics was suggested to result from the substitutions of Al³⁺ and Ga³⁺ ions for Ge⁴⁺ ions in the LiGe₂(PO₄)₃ structure.     

Phase Equilibria of Nuclear Waste Ceramics: The Effect of Oxygen Fugacity

Wasteform phase assemblages and radionuclide immobilization are described for a Synroc-D bulk composition processed at 1200°C under oxygen fugacities ( f O₂) ranging from air to those approximating graphite-CO-CO₂ buffer. All processing conditions produce assemblages of titanate and aluminate crystalline phases plus alkali aluminosilicate glass. In all cases the important radionuclides are immobilized in crystalline phases which have been shown to be highly leach-resistant.     

Crystallization of Anorthite-Seeded Albite Glass by Solid-State Epitaxy

Stoichiometric albite glass (NaAlSi₃O₈) was seeded with 5 wt% crystalline anorthite (CaAl₂Si₂O₈) to make albite glass-ceramics. The epitaxial crystallization of the albite glass to the glass-ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). High albite was observed as the major crystallization product over the temperature range of 800–1200°C. No crystalline albite could be crystallized from pure albite glass without seeds. Small amounts of nepheline (NaAlSiO₄), however, crystallized along with albite after heat treatments at temperatures lower than 1000°C. The platelike microstructure of albite crystals was revealed in the seeded glasses. The albite blades grew epitaxially from the anorthite seeds, and the Ca content decreased in the direction away from the seeds. The degree of crystallization and the grain size were dependent upon the heat treatment conditions. By increasing the particle size of the seed, the crystallization process was retarded and the resultant microstructure was degraded. The seeding efficiency was also lowered by adding nonisostructural hexagonal anorthite seeds which produced less albite but more nepheline crystals. Crystallization of albite glass by seeding with 5 wt% anorthite is much greater than with the surface nucleation which takes place in a homogeneous 95 wt% albite + 5 wt% anorthite glass.     

Electronic Conductivity and Related Properties of Amorphous and Crystallized V2O5-Based Glasses

Crystallization of V₂O₃ from V₂O₃P₂O₃, glasses containing 0 to 9 mol% B₂O₃, during heat treatment in the range 220° to 410°C, caused progressive micro structural changes which dramatically affected the electronic conductivity (γ), the activation energy for conduction ( W ), and the resistance to chemical attack. All compositions were ≊83% crystalline after heating to 410°C. As a result, the values of γ and W were almost identical to those observed for pure polycrystalline V₂O₅.     

Preparation and Optical Properties of Transparent Glass-Ceramics Containing Cobalt(ll) Ions

Transparent glass-ceramics containing ZnAl₂O₄:Co²⁺ and LiGa₅O₈:Co²⁺ crystallites have been prepared by heat treatment of glasses in the zinc aluminosilicate and lithium gallate silicate systems, respectively. Crystalline LiGa₅O₈ was already precipitated in an as-prepared specimen, while ZnAl₂O₄:Co²⁺ precipitated from the glass upon heat treatment. The crystallite size varies from about 5 to 20 nm with increasing heat treatment temperature for both systems, and glass-ceramics containing crystallites of less than about 10 nm are transparent. The low-temperature optical absorption and emission spectra are compared with those of single crystals, indicating that almost all of the Co²⁺ ions replace Zn²⁺ ions in the ZnAl₂O₄ system, while some of the Co²⁺ ions are incorporated into the LiGa₃O₈ system, although the amount of Co²⁺ which remains in the glass matrix is rather large in the latter system.     

Effect of Heat Treatment on the Microstructure and Properties of Dense AIN Sintered with Y2O3 Additions

The secondary phase constitution in two sintered AIN ceramics (1.8% and 4.2% Y₂O₃ additions) was studied as a function of heat treatment temperatures between 1750° and 1900°C under pure nitrogen atmosphere. The effect of the phase constitution on the physical properties, such as density, thermal conductivity ( K ), and lattice constants, and on the mechanical properties in three-point bending, was also investigated. Y₃Al₅O₁₂ was found to getter dissolved oxygen from the AIN lattice below 1850°C, but evaporated at 1850°C and above. Y₄Al₂O₉ appeared to sublimate below 1850°C in the atmosphere used in this study. Depending on the secondary phase constitution, heat treatment affected thermal conductivity favorably or adversely. Occasionally, samples with similar lattice oxygen contents were found to have different thermal conductivities, suggesting that factors besides dissolved oxygen can also influence K . Lattice parameter measurements indicated that, within the small range of lattice oxygen concentrations in the AIN samples studied, the c-axis was more sensitive than the a -axis to oxygen content.     

Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride: (II) Effects of Magnesium Compound and Yttria Additives

The effects of the magnesium compound and yttria additives on the processing, microstructure, and thermal conductivity of sintered reaction-bonded silicon (Si) nitride (SRBSN) were investigated using two additive compositions of Y₂O₃–MgO and Y₂O₃–MgSiN₂, and a high-purity coarse Si powder as the starting powder. The replacement of MgO by MgSiN₂ leads to the different characteristics in RBSN after complete nitridation at 1400°C for 8 h, such as a higher β-Si₃N₄ content but finer β-Si₃N₄ grains with a rod-like shape, different crystalline secondary phases, lower nitrided density, and coarser porous structure. The densification, α→β phase transformation, crystalline secondary phase, and microstructure during the post-sintering were investigated in detail. For both cases, the similar microstructure observed suggests that the β-Si₃N₄ nuclei in RBSN may play a dominant role in the microstructural evolution of SRBSN rather than the intergranular glassy chemistry during post-sintering. It is found that the SRBSN materials exhibit an increase in the thermal conductivity from ∼110 to ∼133 (Wm·K)⁻¹ for both cases with the increased time from 6 to 24 h at 1900°C, but there is almost no difference in the thermal conductivity between them, which can be explained by the similar microstructure. The present investigation reveals that as second additives, the MgO is as effective as the MgSiN₂ for enhancing the thermal conductivity of SRBSN.     

Low-Expansion Cordierite Porcelains

The crystalline nature and expansion characteristics of compositions in the region surrounding the cordierite area of the system MgO–Al₂O₃–SO₂ were investigated. The selected compositions were fired to temperatures of 1250°, 1300°, 1350°, and 1400°C., and the thermal expansion characteristics were determined with the interferometer. Coefficients of linear thermal expansion were determined in the temperature range of 20° to 300°C. and were found to lie between 9.8 ± 10^–7 and 50.5 ± 10^–7. The qualitative, as well as the quantitative, determination of crystalline constituents was made utilizing strontium fluoride as an internal standard in the X-ray spectrometer. In the compositions studied crystalline cordierite ranged from 38 to 97%. There was a close correlation between the linear thermal expansion coefficient and the crystalline cordierite content of the bodies.     

Migration Losses in Single-Crystal Ionic Conductors: Sodium Beta Alumina and

Dispersions have been observed in the ac conductivity of 2 single-crystal ionic conductors, Na β -alumina and LiGaO₂. The results have been analyzed in the complex modulus formalism, M °, where M °= (ε°)⁻¹. It is suggested that these dispersions represent conductivity losses and are similar to those observed in glasses. The losses are related to the mechanism of ion migration and may be a characteristic feature of many ionically conducting crystalline materials.