Shear and Densification of Glass Powder Compacts

Sinter-forging types of experiments were carried out on powder compacts of glass. The shear and densification strains were measured simultaneously during the forging process. This information was analyzed to obtain the shear viscosity of the glass and the intrinsic sintering pressure, both as a function of density. The viscosity of the porous glass changed nonlinearly with density and was empirically fitted to an exponential function. The sintering pressure was found to increase with density; it was 100 kPa at ρ= 0.55 and 250 kPa at ρ= 0.90. In this range the sintering pressure could be adequately described in terms of a simple geometrical model. However, when the density approached 95%, the sintering pressure appeared to assume an uncertain value. The measurements were also used to calculate the viscous bulk modulus of the porous glass; these estimates agreed well with theoretical prediction.     

Creep and Densification During Sintering of Glass Powder Compacts

The simultaneous creep and densification of glass powder compacts was studied as a function of low applied uniaxial stress, temperature, and particle size. The creep rate can be expressed as the sum of the contribution from the applied stress that varies linearly with stress, and a contribution due to anisotropic densification that varies linearly with the densification rate. For a constant applied stress, the ratio of the creep rate to the densification rate is almost independent of both termperature and density. While these observations are consistent with the model of Scherer for the viscous sintering of glass, other observations show significant deviations from the model. Both the densification rate and the creep viscosity, which has an exponential dependence on porosity, show much stronger dependence on density compared with theoretical predictions.     

Crystallization and Densification of Nano-Size Amorphous Cordierite Powder Prepared by a PVA Solution-Polymerization Route

A homogeneous and stable amorphous-type cordierite (2MgO2Al₂O₃5SiO₂) powder was prepared by a solution-polymerization route employing a Pechini resin or a poly(vinyl alcohol) (PVA) solution as the polymeric carrier. After calcination at 800°C for 1 h under atmospheric conditions, the bulky precursor changed into a very soft and porous powder. A 30 nm size, amorphous-type cordierite powder was prepared by attrition milling the calcined powder, which was made using a PVA precursor solution. The nano-size powder, which had a high specific surface area of 181 m²/g, was obtained after milling for <1 h. The sintered cordierite grains did not show the presence of any amorphous SiO₂ phase and had a dense microstructure with a relative density of 99% and a thermal expansion coefficient of 2.1 10^-6/°C.     

Controlled Phase Transformations by Glass Particle Size in Spodumene and Cordierite Glass-Ceramics

The effects of the particle size of initial glass powders on the phase transformations in Li₂O-Al₂O₃-SiO₂ and MgO-Al₂O₃-SiO₂ glass-ceramics were investigated. Both materials undergo crystallization of glass to a metastable phase and a subsequent metastable-stable transformation. As particle size is decreased, the crystallization of glass to the metastable phase is accelerated, because of the increase in the total number of nucleating sites on the particle surfaces. The metastable-stable transformation is restricted as the particle size is decreased. This phenomenon is explained by the decrease in the crystal size of the metastable phase as a result of decreasing the particle size. The metastable-stable transformation of the fine crystals is restricted by the constraint imposed by the surrounding matrix.     

Low-Temperature Synthesis of the Cordierite Phase in Ceramic Mixtures of Natural Raw Materials

The synthesis of cordierite in mixtures of magnesia and clay raw materials recovered from deposits in Siberia is studied experimentally. For reference, a mixture based on clay from the Veselovskoe deposit (Ukraine) is used. To promote the synthesis, raw materials are mechanically activated using a vibratory centrifugal mill. The synthesis can be brought to completion at lower temperatures under mechanical activation conditions using a cordierite addition. Production of synthetic cordierite and cordierite products (blocks of catalyst carries with a cellular structure) organized on a pilot-plant basis at the G. K. Boreskov Institute of Catalysis is reported.     

Phase Transformation and Densification of an Attrition-Milled Amorphous Yttria-Partially-Stabilized Zirconia–20 mol% Alumina Powder

Phase transformations during consolidation treatments of an attrition-milled amorphous yttria-partially-stabilized zirconia (Y-PSZ: ZrO₂–3 mol% Y₂O₃)–20 mol% Al₂O₃ powder and the resulting microstructures have been investigated. A metastable cubic phase ( c -ZrO₂ solid solution) together with an α-Al₂O₃ phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase ( t -ZrO₂ solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α-Al₂O₃ particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic-to-tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition-milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.     

堇青石粉末合成及其烧结特性的研究

以热喷雾法制备的MgAl2.6(OH)x粉末、水玻璃和MgCb为原料,用沉淀包裹法制备堇青石前躯体粉末。通过控制沉淀过程中体系的pH值,在MgAl2.6(OH),粉末粒子上形成无定形连续包裹层。烧结过程中,在700℃之前,包裹层与被包裹粒子反应形成无定形相;在850℃,从无定形体系中析出的纳米尖晶石对致密化烧结过程没有影响,相反,方石英的析出将阻碍烧结。堇青石的烧结属液相烧结,其致密化过程发生在堇青石合成之前。700℃煅烧的堇青石前躯体在1250℃烧结2h,其相对密度可达96．4％。     

Shear Deformation and Densification of Powder Compacts

Kinetics of densification and deformation can be studied simultaneously by measuring the time-dependent axial and radial strains in cylindrical specimens of powder compacts when they are sintered under a superimposed uniaxial load. Such experiments, carried out on powder compacts of magnesia-doped alumina, are reported. Several results have been obtained. The experiments have provided a direct measure of the intrinsic sintering pressure, which we find to lie in the range 0.4 to 0.8 MPa. The deformation rate, or shear rate of the porous body, is found to follow Coble creep behavior except for a stress enhancement factor arising from the reduced effective cross-sectional area from the presence of voids. The rate of densification follows a linear dependence on the total mean pressure and a cubic dependence on the average grain size.     

Influences of the Glass Phase on Densification, Microstructure, and Properties of Low-Temperature Co-Fired Ceramics

Multilayer ceramic devices based on low-temperature co-fired ceramics (LTCC) materials provide a very promising technology. Most LTCC tapes available today contain considerable fractions of glass powders to lower the sintering temperature. However, the glassy phases offer more possibilities to set a proper sintering behavior, on the one hand, and to tailor the desired properties of the final LTCC substrate, on the other. The exploitation of demixing and subsequent crystallizing glass compositions was shown on an example of a low-permittivity (4.4)—low-loss (1.5 × 10⁻³) LTCC with a high quartz content. In another LTCC material, undesired demixing could be restricted and the crystal phase anorthite could be triggered by partial dissolution of alumina in the liquid phase during sintering. To estimate the effect of silver diffusion in the latter material, the surroundings of a pure silver via were studied. A silver-contaminated range of 50 μm was detected. Using model glasses containing silver oxide, a strong influence of dissolved silver on viscosity and crystallization behavior of the liquid phase was demonstrated. The dielectric properties of the sintered substrates were not degraded.     

Triboelectric Charging and Separation of Fine Powder Mixtures

For increasingly finer powders, the material-specific separation at high loadings is a challenging task, for instance in recycling processes. Here, a combination of triboelectric charging and electrostatic separation was investigated for powder mixtures of talcum and calcite. The dependencies of the triboelectric charge on the mass loading, the gas velocity, and the mixture ratio were investigated. While higher charge levels were achieved with increasing gas velocity, the mass loading had an opposite effect on the net charge. Although bipolar charge distributions were observed within pure materials and mixtures, electrical neutralization did not occur in the mixtures. Therefore, already in a non-optimized setup, a decent degree of material enrichment (of up to 53 %) was found on the separating electrodes.     

玻璃防护材料——玻璃防霉粉的绿色发展

玻璃的霉变极大地影响了玻璃的应用质量,需采用防护材料为玻璃应用质量保驾护航。浮法玻璃在发展应用中壮大,以玻璃防霉粉为主角的防护材料也在创新中发展。1990年中国版的玻璃防霉粉诞生后,防霉粉不断更新换代,使玻璃防霉粉的组成发生了质的变化,从窄谱防霉域的第一代,到广谱防霉域的第二代,再到环保型广谱型防霉粉的第三代,创新引领了玻璃防霉粉的绿色发展。     

混合烧结法在玻璃陶瓷制备中的应用

概述了混合烧结法的分类、致密化机理、影响因素和在玻璃陶瓷制备中的应用.和传统玻璃陶瓷制备工艺相比,混合烧结法的特点在于玻璃陶瓷中的晶体是直接加入或是通过外加晶体和玻璃反应析出而不是从母相玻璃中直接析出,因而对母相玻璃组分要求不十分严格,在直接利用废玻璃制备玻璃陶瓷和复相玻璃陶瓷方面有着独特的优势.本文重点介绍了近年来该工艺在这方面的应用和进展.     

Effects of Complex Precursors on Alkoxide-Derived Cordierite Powder

Preparation of chemically homogeneous cordierite powders of submicrometer size was investigated using alkoxide precursors, synthesized from Mg metal, Al(OBu^s)₃, and partially prehydrolyzed Si(OEt)₄. The amount of partial prehydrolysis water in precursor synthesis was changed to study the effects on the hydrolysis products. In the hydrolysis of the diluted solution of the precursor, product morphology was quite sensitive to the precursor, and different morphologies such as gelation, gel precipitation, and powder precipitation were observed. A redispersable powder of submicrometer size was obtained only from the precursor synthesized at a H₂O/Si(OEt)₄ ratio of 1.2. Differences were also observed in minor crystalline phases among calcined hydrolysis products. The alkoxide precursors were characterized using Raman spectroscopy, gas chromatography, and NMR spectroscopy. The differences in hydrolysis products are most likely due to differences in hydrolysis and condensation of different chemical species in precursors.     

溶胶-凝胶法制备堇青石纳米晶粉体的研究

以正硅酸乙酯(TEOS)、硝酸镁(Mg(NO3)2*6H2O)、硝酸铝(Al(NO3)3*9H2O)为原料,采用溶胶-凝胶法制备了堇青石纳米晶粉体.利用DTA,XRD,SEM等测试手段对堇青石晶粉体进行了表征,并分析讨论了不同的热处理温度对堇青石烧结性能的影响.     

Densification Energy during Nanoindentation of Silica Glass

Load/unload displacement curves at room temperature (humidity 49%) for silica glass have been measured in the penetration range of 0.5–1.2 μm using a Vickers nanoindentation technique (load/unload speed 50 mN/s). Deformation energies have been estimated for the first time. The universal (dynamic) hardness, H _u, and elastic recovery, E _R, at the penetration depth, h _t, of 1.0 μm are H _u= 4.1 GPa and E _R= 0.7. The following energies for total deformation, U _t, elastic deformation, U _e, and plastic deformation (i.e., densification during loading), U _p, are obtained: U _t=190, U _e=135 and U _p= 55 kJ/mol at h _t= 0.5 μm and U _t= 139, U _e= 96 and U _p= 43 kJ/mol at h _t=1.0 μm. All these deformation energies increase with decreasing penetration depth. It is found that plastic deformation energies of 38–55 kJ/mol for 0.5 < h _t < 1.2 μm are very close to the activation energy (46–54 kJ/mol) for the recovery of densification in silica glass, but are very small compared with the single bond strength (443 kJ/mol for Si—O bond) of SiO₂.     

Densification of Alumina-Silicon Carbide Powder Composites: II, Microstructural Evolution and Densification

Microstructural evolution adn densification kinetics of Al₂O₃-SiC powder composites were studied using two different SiC powders. Examination of the microstructural evolution of Al₂O₃-fine SiC powder composites showed three well-defined stages of densification: the first was characterized by constant pore size and no grain growth; the second involved rapid pore coarsening and grain growth; the third was characterized by pore shrinkage and slow grain growth. Studies of the densification kinetics of Al₂O₃-coarse SiC powder composites exhibited two stages of densification: in the first stage there were no significant differences in densification rate between pure Al₂O₃ compacts and composites; in the second stage, however, differences in densification behavior between pure Al₂O₃ compacts and composites became pronounced.     

Tribocharging of Three-Component Powder Mixtures

Adhesion of charged toner particles used in electrophotography is dominated by electrostatic forces. In this paper we discuss a model which describes the process by which toner particles acquire their electrostatic charge, tribocharging.

In previous papers, we have presented a model of tribocharging of two-component mixtures of powders based on the assumptions that:

1. The surface of each powder is populated with electron accepting and donating sites;
2. The density of states of the donating and accepting sites can be represented by narrow bands, all of which have the same energy; and
3. Charge is exchanged between donor and acceptor sites until thermodynamic equilibrium is established.

In this paper, we show how to extend this model to multi-component mixtures. The extended model can be used to calculate the charging behavior of three-component mixtures of electrophotographic toners and carriers based on measurements with two component mixtures. Experimentally-measured charging behavior agrees with the model predictions.

These results confirm it is possible to assign charging site densities to individual materials empirically. The site densities can then be used to predict charging behavior of the materials in mixtures which have not been studied experimentally. The success of the model also implies that toner particles migrate freely from carrier particle to carrier particle and that triboelectric interactions take place between toner particles of different compositions in mixtures.