摘要:

The glass‐formation region of the soda–yttria–silica system was determined. The glasses within this region were measured to have a density of 2.4 to 3.1 g/cm3, a refractive index of 1.50 to 1.60, Vickers hardness values of 3.7 to 5.8 GPa, softening temperature between 500° and 780°C, and a coefficient of thermal expansion of 7 × 10‐6/oC to 19 × 10‐6/oC. Aqueous chemical durability measurements were made on select glass compositions while infrared transmission spectra were used to investigate the glass structure and its effect on glass properties. A compositional region was identified which exhibited high thermal expansion, high softening temperatures, and good chemic

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04421.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Compaction of ultrafine silicon nitride (Si3N4) powder at high pressures and various temperatures followed by pressureless sintering was investigated. The powder, consisting of nearly spherical particles (16 nm in diameter) of amorphous stoichiometric Si3N4, was pressed in a diamond anvil cell under pressures up to 5 GPa and temperatures ranging from liquid nitrogen to 500°C. Quality of compaction, evaluated by visual transparency and hardness of the produced compacts, depended on the amount of adsorbed gases on the surface of the particles and on the temperature of compaction. Visually transparent compacts were produced by pressing the starting powder without outgassing in liquid nitrogen under 5 GPa. The transparent compacts exhibited a hardness of 1200 kg/mm2after pressing in the diamond anvil cell at 500°C for 3 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 5 GPa. After subsequent pressureless sintering conducted for 1 h at 1400°C in a tube furnace under nitrogen, the hardness of these samples increased to over 2000 kg/mm2and the visual transparency was maintained. The results demonstrated that transparency was maintained. The results demonstrated that transparent compacts of nanosize amorphous Si3N4particles could be sintered to high hardness at relatively low temperatures without using sintering aids or applying pressure during sinteri

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04422.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The oxidation behavior of the silicon yttrium oxynitride Y10Si7O23N4, so‐called H‐phase, in the temperature range 700–1400°C has been investigated. A nitrogen retention phenomenon in the oxidation product Y4.67(SiO4)3O (O‐apatite) is discussed. The H‐phase is one of the four quaternary compounds identified in hot‐pressed Si3N4materials fabricated within the Si3N4–SiO2–Y2O3pseu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04423.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

An aluminum (Al) target was laser ablated in an oxygen (O2) atmosphere, producing nanosize alumina (Al2O3) powder. The powder surface area decreased (and the particle size increased) with both increasing oxygen pressure and laser fluence. All powders produced had surface areas between 135 and 250 m2/g, corresponding to primary particle sizes ranging from 7 to 3 nm in radius. Phase evolution with temperature was studied via X‐ray diffraction. These powders showed a direct transformation from γ‐ to α‐alumina at approximately 1200°C, bypassing other transition alumina phases, while still maintaining small particle size ( 30 nm). Despite the nanosize particles, green densities equal to 54% of the skeletal density (i.e., true density of the solid phase) were obtained by uniaxial pressing a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04424.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Measurements of the load dependence of the radial crack size with Vickers and Berkovich indenters are compared for a range of materials. It is found that the extent of radial cracks was slightly larger for the Berkovich than for the Vickers indenter. The observations reveal that cracks from a Berkovich indenter are best described by an expression developed by Laugier combined with a modification proposed by Ouchterlony to account for the number of radial cracks. It was also found that the Berkovich indenter, which offers advantages for ultramicroindentation, gave more consistent toughness values at lower loads than a Vickers indenter.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04425.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The consolidation behavior of flocculated alumina suspensions has been analyzed as a function of the interparticle energy. Consolidation was performed by a centrifugal force field or by gravity, and both the time‐dependent and equilibrium density profiles were measured by a gamma‐ray absorption technique. The interparicle energy at contact was controlled by adsorbing fatty acids of varying molecular weight at the alumina/decalin interface. We found that strongly attractive interactions result in a particle network which resists consolidation and shows compressible behavior over a large stress range. The most weakly flocculated suspension showed an essentially incompressible, homogeneous density profile after consolidation at different centrifugal speeds. We also found a significant variation in the maximum volume fraction, φm, obtained, with φm∼ 0.54 for the most strongly flocculated suspension to φm∼ 0.63 for the most weakly flocculated suspension. The compresive yield stresses show a behavior which can be fitted to a modified power law. In this paper, we discuss possible correlations between the fitting parameters and physical properties of the flocculated s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04426.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Bulk nucleation has been studied for glasses within the Y2O3–Al2O3–SiO2system, using a two‐stage nucleation and growth heat treatment. The crystalline phases formed have been identified. Annealing in an inert atmosphere is required in order to prevent surface nucleation from dominating the results. Despite this, the phases observed are in general agreement with those observed in previous studies done in air. The nucleation kinetics of they‐Y2Si2O7phase have been measured and the optimum nucleation temperatures have been identified. The kinetic data have been analyzed using two existing models based on classical nucleation theory. The results of this study have been compared with crystallization results of a previous study of glass of similar composition within sintered silicon nitride samples. A two‐stage heat treatment is suggested as a process that may lead to improved devitrification of grain boundary glass in such

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04427.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The tensile properties of a 0/90 laminated CAS matrix composite reinforced with Nicalon fibers have been measured. Some effects of notches have also been explored. Changes in modulus and permanent strain caused by matrix cracking have been measured and compared with available models. For this comparison, independent measurements have been made of the constituent properties and the residual stress. The ultimate tensile strength has also been measured and compared with a global load‐sharing model. It is concluded that lower‐bound matrix cracking models provide good predictability of the stresses at which various matrix cracking mechanisms first operate. Also, the ultimate tensile strength is found to be consistent with a global load‐sharing model, based on the in situ strength properties of the fibers. Conversely, the evolution of matrix cracks at stresses above the lower bound has yet to be adequately modeled. In addition, a need is identified for improved models relating elastic properties and permanent strains to matrix crack sp

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04428.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Limited past studies have indicated that Si3N4doped with Sc2O3may exhibit high‐temperature mechanical properties superior to Si3N4systems with various other oxide sintering additives. High‐temperature deformation of this system was studied by characterizing the microstructures before and after deformation. It was found that elements of the additive, Sc and O, exist in small amounts at thin grain boundary layers and within secondary phases at triple and multiple grain boundary junctions. The secondary phase is devitrified as crystalline Sc2Si2O7. Deformation of the samples was dominated by cavitation rather than dislocation processes. Thus, the excellent deformation resistance of the samples at high temperature can be attributed to the high refractories and enhanced crystallization of a secondary ph

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04429.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The sol–gel method has been developed for the preparation of pure Ba(Mg1/3Ta2/3)O3ceramics. This involves the reaction of the heterometallic alkoxide Ta2Mg(OEt)12with hydrated barium hydroxide Ba(OH)2·8H2O. Complete crystallization of the sol–gel‐derived powder is achieved at 600°C, leading to a cubic perovskite type phase. After sintering at 1400°C (2–5 h), a trigonal cell arises from Mg–Ta ordering (the degree of order is greater than 0.9), and about 98.5% of the theoretical density is obtained. Preliminary microwave dielectric measurements show that the dielectric constant and the unloadedQuof the ceramics are 24.2 and 6750, respectively

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04430.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY