摘要:

The microstructure and dielectric properties of lead magnesium niobate (PMN) ceramics containing the determental second phase, pyrochlore, have been studied. The pyrochlore phase within PMN was found to be present as isolated grains in the microstructure with a composition of Pb2(Mg0.25Nb1.75)O6.625and a preferred octahedral morphology. The amount of pyrochlore second phase can be easily controlled by changing the Mg/Nb ratio on the B‐site of the perovskite structure, and a series of diphasic mixtures from near‐pure PMN to near‐pure pyrochlore were obtained. Its effect on the permittivity follows the isolated Wiener's mixture rule up to a pyrochlore content of 18

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05092.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The hot isostatic pressing (HIP) of four alumina powders is studied in the temperature range 1100° to 1400°C, at 5‐ to 200‐MPa applied pressure, and for times ranging from 0.5 to 4 h. Density increases with increasing HIP temperature, pressure, and time; decreasing grain size results in increased density after HIP. An empirical relation is derived for grain growth during HIP, and the HIP map proposed by Helleet al.is found applicable to the present results. Densification is governed by the grain‐boundary diffusion of aluminum ions; with the transport coefficient and the grain‐growth values found in the present study, the map can be used to express experimental results to within a factor of 4 for all densification stages except near ful

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05093.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Measurements of surface area reduction of TiO2anatase powder (of initially 100 m2· g−1) were carried out for various partial pressures of water and oxygen at 823 K. The kinetic equation obtained for the experimental rates has the formulationr=k[PH2O]1/α[PO2]1/βwhere α and β are equal to 2 and ‐12, respectively. A mechanistic model involving six consecutive elementary steps was developed, in which hydroxyl species play a dominant role. No geometrical assumption is required. The general expression of the deduced theoretical rates has the same form as previously given. The comparison between the experimental and the predicted rates points out that the rate‐limiting step is the surface diffusion of hydroxyl species. This model can easily be used for any other compounds, for particle growth, and for porosity e

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05094.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

A novel process for fabricating lightweight, cellular ceramics from sols is presented. The process utilizes the rapid viscosity change during gelation to stabilize the structure of a foamed silica sol. Manipulation of gel viscosity and foaming agent concentration resulted in a minimum cell size of 90 μm at 31% density and minimum density of 17% with average cell size of 400 μm. The flexural strength compared favorably with space shuttle tiles and sintered hollow glass spheres. From −50° to 150°C, the dielectric constant ranged from 1.51 to 1.55 for a 20% dense foam and was slightly dispersive, whereas the dielectric loss was comparable with fused s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05095.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Alumina composites reinforced with 20 vol% SiC whiskers were exposed to applied stresses in four‐point flexure at temperatures of 1000°, 1100°, and 1200°C in air for periods of up to 14 weeks. At 1000° and 1100°C, an “apparent” fatigue limit was established at stresses of ∼ 75% of the fast fracture strength. However, after long‐term (>6 weeks) tests at 1100°C, some evidence of crack generation as a result of creep cavitation was detected. At 1200°C applied stresses as low as 38% of the 1200°C fracture strength were sufficient to promote creep deformation and accompanying cavitation and crack generation and growth resulting in failures

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05096.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

A new ceramic‐metal composite with continuous interconnected ceramic and metal phases has been fabricated from sintered aluminum nitride infiltrated with aluminum metal. A dense, lightweight, hard material with high strength has been obtained. Advance of the infiltration front was linear with time. The activation energy of the process was very high (330 kJ/mol), suggesting that a chemical interaction rather than viscous flow was rate‐controlling. The infiltration rate was observed to be inversely proportional to the pore size. Thermal and mechanical properties of the composite were promis

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05097.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The physical, chemical, and structural properties of a commercially available β‐Si3N4whisker were characterized. Bulk chemical analysis indicated that the whiskers were close to stoichiometric silicon nitride, with oxygen and yttrium as the major impurities. Surface chemistry analysis by XPS analysis revealed that the surfaces consisted primarily of silicon nitride, with the oxygen and yttrium impurities concentrated at the surfaces. SEM and STEM studies indicated that the whiskers were dimensionally straight with relatively featureless surfaces, although some whiskers had Y‐rich particles attached. The whiskers were also found to be of extreme crystallographic perfection, as determined by TEM anal

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05098.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

Grain growth in a high‐purity ZnO and for the same ZnO with Bi2O3additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression:Gn—Gn0=K0texp(—Q/RT). For the pure ZnO, the grain growth exponent ornvalue was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn2+lattice diffusion mechanism. Additions of Bi2O3to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi2O3content. The preexponential termK0was also independent of Bi2O3content. It is concluded that the grain growth of ZnO in liquid‐phase‐sintered ZnO‐Bi2O3ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi2O3‐r

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05099.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

The addition of Ta2O5, Nb2O5, and HfO2enhanced the transformability of Y2O3‐stabilized tetragonal ZrO2polycrystal (Y‐TZP), which was indicated by an increase in phase transformation temperatures and fracture toughness of Y‐TZP. Comparison of the alloying effects of these oxides on the transformability and crystal structure of Y‐TZP suggested that an alloying oxide which increases thec/aaxial ratio (tetragonality) of TZP also increases the transformability. Empirical equations to predict the tetragonality are proposed. Calculated tetragonalities showed good agreement with measured values in the systems ZrO2‐Y2O3‐Ta2O5, ‐Nb2

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05100.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY

摘要:

X‐ray diffraction, infrared spectroscopy, transmission electron microscopy, and thermogravimetry were used to study the solid phase obtained from sulfuric acid solutions of Fe(III) hydrolyzed with urea. The influence of solution factors (temperature, pH, hydrolysis rate, urea and iron concentration, and aging time) on the solid phase was discussed. Hematite (α‐Fe2O3), with impurities of goethite (α‐FeOOH), was obtained at a very rapid hydrolysis rate, whereas goethite was formed at a slow hydrolysis rate. As the pH and aging time increased, so did the crystallinity of goethite. This goethite contained adsorbed SO2‐4ions which ranged from 1.3% to 9.6% in weight, depending on its crystallinity. In no cases were sulfated compounds

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1990.tb05101.x

出版商:Blackwell Publishing Ltd    

年代:1990

数据来源: WILEY