Sintering Mechanisms in Thoria Gel

Isothermal sintering experiments were combined with measurements of crystallite size distribution and BET surface area in a study of the sintering mechanisms in thoria gel. The results suggest that crystallites in the gel have a coordination number of ∼5 and sinter together by a grain-boundary diffusion mechanism.     

Effect of Calcination on the Sintering of Gel-Derived, Zirconia-Toughened Alumina

The densification behavior of ZrO₂ (+ 3 mol% Y₂O₃)/85 wt% Al₂O₃ powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.     

Effects of Two Step Calcination Process on Reaction Sintering Properties of Cordierite Materials

Cordierite materials were synthesized using natural mineral materials via the two step calcination process.The phase composition and microstructure of the specimens were analysized by X-ray diffraction( XRD) and scanning electron microscopy( SEM),respectively. The sintering properties of cordierite materials were also characterized. The effect of two step calcination process on the reaction sintering properties of cordierite materials was investigated. The results show that the two step calcination process can significantly increase the reaction sintering properties of the cordierite materials. When the weight ratio of the presynthesized cordierite to the premixed raw mineral materials is 2 1,the cordierite materials obtained by the two step calcination process have the higher bulk density and the lower apparent porosity( 2. 18 g·cm-3and 13. 85%,respectively). Furthermore,the addition of presynthesized cordierite clinker in the process of synthesizing cordierite materials can promote the growth of cordierite crystalline size.     

Pore Growth in Sintered Thoria

Sintered ThO₂ will sometimes undergo a decrease in density, i.e. become more porous, if held at high temperature for a sufficient time. The bulk density of sol-gel ThO₂ sintered to 99% of theoretical density at 1200°C will decrease steadily at 1800°C in air to <90% of theoretical after ∼ 15 h. Gas-release measurements from such porous ThO₂ specimens annealed in air or in vacuum indicate that some gases move through the material freely at 1800°C. The only gases that could contribute to the decrease in density are the oxides of sulfur and carbon, which, if they exist, could be produced or eliminated rapidly by controlling the annealing environment. However, the density decrease appears to depend on factors that affect grain growth, rather than on the generation or elimination of postulated gases.     

Crystallite Size Measurements on Thoria Gel

As part of a detailed study of the sintering behavior of thoria gel, the crystallite size distribution in the gel was measured by electron microscopy. The distribution is in quantitative agreement with BET surface area and X-ray crystallite size measurement, and X-ray diffraction line profiles calculated using the measured size distribution agree with experiment. The relative accuracies of the integral and half-maximum intensity breadth techniques, and the relation between the size distribution and the crystallite size measured by each technique are discussed. Some implications of this work to the sintering study are mentioned.     

Color Centers and Point Defects in Irradiated Thoria

The optical absorbance of commercially pure thoria single crystals was measured after oxidizing and reducing treatments, irradiation by 18-MeV protons, 48-MeV ¹⁶O ions, and ⁶⁰Co gamma photons, and thermal annealing at temperatures up to 950° C. For unirradiated specimens, absorption bands previously observed at 2.80, 3.03, and 4.03 eV were confirmed, and additional bands were identified at 1.67, 2.04, 2.45, 3.61, and 4.52 eV. Reduction and irradiation enhanced the 3.03 and 3.61 eV bands but diminished the 2.80, 4.03, and 4.52 eV bands. The centers responsible for the two groups of bands are thus presumed to be electron-type and hole-type, respectively. Impurity atoms, also, are probably involved in the centers. Ion irradiation produced an intense blue color in the exposed volume of the specimens, and additional absorption bands appeared at 1.49, 1.80, 2.03, and 2.32 eV. The behavior of these bands on annealing and on subsequent low-exposure gamma irradiation indicated that the corresponding centers are combinations of impurity atoms and oxygen interstitials.     

Phase Equilibria and Physical Properties of Oxygen-Deficient Zirconia and Thoria

The compositions of anion-deficient zirconia and thoria in equilibrium with O₂ were measured from 1 to 10⁻⁶ atm and 1400° to 1900°C; for ZrO_{2- x} (po₂ in atm, and T in °K), log x∼−0.890-[(0.400×10⁴)/ T ]-[(log p )/6]; for ThO_{2- x} , log x∼−1.870-[(0.340×10⁴)/ T ]-[(log p )/6]. The ZrO_{2- x} -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O₂/inert gas mixtures were used, but use of H₂/H₂O and CO/CO₂ at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO₂ and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO₂ and ThO₂ bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry.     

Synthesis and Characterization of NiO–YSZ Anode Materials: Precipitation, Calcination, and the Effects on Sintering

Nickel oxide yttria-stabilized zirconia (NiO–YSZ) anode materials were synthesized via hydrolysis of the corresponding chloride solutions with NH₃, NH₃+NaOH, and NaOH as precipitation agents. Powder properties such as crystallite size, morphology, and sintering behavior of the final NiO–YSZ materials were also studied. The mechanism of the formation of NiO–YSZ was established for the different co-precipitation techniques by the direct observation of Ni(NH₃) _n ⁺² complexes, Ni(OH)₂ and NiO at different stages of the synthesis process. A direct relationship between the precipitation agent, the order of calcination from dry sample to final product, the final composition, the crystallite sizes and particle sizes of NiO, and the sinterability of the final products was established. A comparison of the powder and individual component properties indicate that the choice of precipitation agent greatly influences the final characteristics. Ni/YSZ materials prepared by NH₃+NaOH precipitation offer higher Ni dispersion and nanocrystallinity of both the Ni and YSZ phases. The conductivity of both prepared materials compares well with mixed-oxide materials of higher Ni content.     

Permeability of Zirconia, Hafnia, and Thoria to Oxygen

The permeability to oxygen of 4 wt% lime-stabilized zirconia and hafnia and of pure thoria was determined between 1100° and 2050°C. The permeability constant (permeability multiplied by the thickness) at 25 torr oxygen pressure is given by:
A 1/4-power oxygen pressure dependence of the permeability was found for zirconia and hafnia and a variable power dependence was found for thoria. The diffusivities for zirconia and hafnia were determined from the time dependence of permeation. These are given by:
A transport mechanism suggested for zirconia and hafnia involves migration of oxygen ion vacancies and electron holes.     

Phase Relationships in the System Zirconia—Thoria

The phase relationships in the system zirconia-thoria were investigated by X-ray diffraction and dilatometric methods. The specimens were melted in air at the focus of a solar furnace. The existence of a cubic calcium fluoride-type solid solution was established for thoria concentrations above 17.5 mole %. However, this solid solution is stable only at high temperature, and, after prolonged heating at 2000°C. and at 1350°C., it decomposes into tetragonal zirconia (which during subsequent cooling undergoes the unavoidable tetragonal-monoclinic inversion) and a thoria-rich phase containing at least 99 mole % thoria.     

Some Physical Properties of Thoria Reinforced by Metal Fibers

Values are presented for several physical properties of thoria reinforced by metal fibers. Properties determined at room temperature include compressive strength, modulus of rupture, and Young's modulus (dynamic method); those determined at elevated temperatures include modulus of rupture, Young's modulus, thermal conductivity, thermal expansion, and oxidation resistance. The widespread presence of fine cracks in reinforced thoria results in significantly lower strengths compared with thoria alone. The thermal conductivity of reinforced thoria is slightly greater than that of thoria at room temperature and the difference becomes more pronounced at elevated temperatures.     

煤系高岭土煅烧与微生物提质试验研究

以辽宁凌源煤系高岭土为原料,进行提质试验研究.通过矿物组成、扫描电镜分析得出,主要杂质为碳和铁,先通过煅烧的方法去除碳质,试验结果表明,当煅烧温度950℃、恒温时间2.5h、升温时间3h时煅烧白度可达86.66％,再利用生物产出的混合酸设计正交试验去除铁杂质,在最佳试验条件下白度达到89.25％,符合高级煅烧高岭土的质量要求.     

Further Study of Sintering Phenomena

The sintering mechanism of aluminum oxide was studied with attention to the influence of time, temperature, and particle size. Mechanisms proposed by other investigators are reviewed as they pertain to the information obtained from the study. Shrinkage, density, and grain growth are considered; they indicate that rate and degree of sintering increase with decreasing particle size. Loose powders are shown to exhibit sintering phenomena when the particle size is small.     

Sintering of calcium phosphate hydroxyapatite Ca10(PO4)6(OH)2 I. Calcination and particle growth

The influence of the temperature and the partial pressure of water vapour on the kinetic of surface reduction of hydroxyapatite during calcination treatment was studied. The water vapour catalysed the growth of HAP grains. A theoretical model was proposed and compared with the experimental data. It allowed the identification of the mechanisms of surface reduction and the determination of the kinetic laws. At low temperature (T<850 °C), grain coalescence occurs without densification. The superficial diffusion is the predominant mechanism and it is controlled by the adsorption–desorption of water vapour on the surface of the HAP grains. At higher temperature, the densification mechanisms are effective.     

水泥熟料流态化煅烧工艺研究进展

综合介绍了水泥熟料流态化煅烧工艺的研究进展，对三种典型的流化烧成系统的原理、工艺流程和试验结果进行了简要的总结和分析，分析了水泥熟料流态化煅烧工艺的优点和目前存在的问题，并对其在未来水泥工业中的地位给予了应有的评价。