摘要:

To achieve low‐temperature superplasticity in alumina, we have introduced charge‐compensating dopants, Ti4+and Mn2+, which jointly have a high solubility and significantly enhance the diffusion and deformation processes during sintering and forming. Zirconia as a second‐phase pinning agent has also been incorporated to impart microstructural stability against static and dynamic grain growth. The superplastic alumina obtained can be shape‐formed under biaxial tension to 100% engineering strain at temperatures below 1300°C. Deformation characteristics of this alumina at temperatures from 1200° to 1400°C and at strain rates from 4 × 10‐6to 3 × 10‐3/s are described. The origin of enhanced kinetics is attributed to the formation and dissociation of dopant

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07903.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Thin films of cubic BaTiO3were processed hydrothermally at 40°–80°C by reacting thin layers of titanium organo metallic liquid precursors in aqueous solutions of either Ba(OH)2or a mixture of NaOH and BaCl2. All films (thickness ∼1 μm) were polycrystalline with grain sizes ranging from nano‐ to micrometer dimensions. BaTiO3formation was facilitated by increasing [OH‐], [Ba2+], and the temperature. The film structure was related to the nucleation and growth behavior of the BaTiO3particles. Films processed at relatively low [OH‐], [Ba2+], and temperatures were coarse grain and opaque, but increasing [OH‐], [Ba2+], and temperature caused the grain size to decrease, resulting in tra

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07904.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The feasibility of fabricating dense, low‐shrinkage, mullite/ ZrO2composites based on the reaction bonding of alumina (RBAO) process and the reaction sintering of zircon is examined. Compacts pressed from an attrition‐milled powder mixture of Al, A12O3and zircon were heated in air according to a two‐step heating cycle. The phase evolution and microstructural development during reaction bonding were traced by X‐ray diffraction, nuclear magnetic resonance, and scanning electron microscopy on samples extracted from various points along the heating cycle. It is seen that, as in conventional RBAO, AI oxidizes to γ‐Al2O3which then transforms to α‐AI2O3between 1100° and 1200°C. The zircon dissociation commences at ∼1400°C and is practically complete by 1500°C. Mullite enriched in Al2O3forms initially, but 3:2 stoichiometry is attained in the final product which consists of mullite,t‐ and m‐ZrO2, and residual α‐AI2O3. The flexure strength of the composite is superior to that of pure mullite, and ∼80% of the strength is retained up to 1200°C. Although there was no toughness enhancement relative to mullite, this should be achievable by optimiz

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07905.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Phase formation and dielectric properties of the compositions in the system [Pb(Fe1/2Nb1/2)O3]1_x–[Pb(Zn1/3Nb2/3)O3]xwere investigated as possible materials for multilayer ceramic capacitors. The formation of the phase with perovskite structure and dielectric properties of ceramics at room temperature in the entire composition range are presented. The undesirable pyrochlore phase can be suppressed up tox= 0.6 by adopting calcination of B‐site oxides, followed by reaction with PbO. Compositions in the single‐phase range can be sintered at less than 1

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07906.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The doping of silver and palladium into a 0.9Pb‐(Mg1/3Nb2/3)O3–0.1PbTiO3(PMN–PT) ceramic has been investigated. It was found that Ag could be incorporated into the PMN–PT lattice, though this was almost impossible with Pd. Doping at up to 0.2 mol% of Ag and Pd reduced the maximum dielectric constant (Kmax) from 21000 to 14000 without any conspicuous change in physical properties. Order–disorder transitions in the perovskite structure contributed to the decrease inKmax, since the diffuseness parameter, δ, which represents the degree of the order‐disorder relationship, increased with Ag doping. However, addition of more than 0.2 mol% increasedKmax. Increases in grain size and in the perovskite ratio contributed to this increase. The dielectric properties of Ag/Pd‐doped samples were intermediate between the Ag‐ and

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07907.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The existence of oxidation embrittlement in previously developed ceramic‐matrix composite (CMC) systems at intermediate temperatures (500–900°C) has motivated the development of all‐oxide CMCs. Studies of the damage tolerance of an all‐oxide CMC suggest that it relies on distributing fiber bundles heterogeneously in a porous matrix with a thin matrix‐only region surrounding each bundle. For such composites, loading in the direction of the fibers causes cracks to first commence in the fiber bundles, breaching both the matrix and fibers within each bundle. These fiber‐bundle cracks then branch in the matrix‐rich region, parallel to the load, forming stable H‐shaped cracks. The mechanics behind one mechanism controlling the competition between branching and continued extension of a preexistent fiber‐bundle crack in a unidirectional all‐oxide CMC are provided in this paper. Both plane‐strain and axisymmetric cases are studied using an integral equation method as well as a numerical method based on finite elements. With no elastic mismatch assumed between the fiber and matrix, a single nondimensional combination,EΩ/σ∞, emerges as the key parameter that governs the tendency of the crack to branch (whereEis Young's modulus, Ω is the thermal misfit strain between the fiber bundle and matrix, and σ∞is the stress applied parallel to the fiber axis). Quantitative estimates of the conditions under which the branching of the bundle

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07908.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The conditions for preparing α‐aluminum silicon carbide (α‐Al4SiC4) were examined by heating stoichiometric mixtures of ultrafine A14C3and SiC powders with sizes of6MDS. The reaction of 3ES‐derived SiC with A14C3produced α‐Al4SiC4at temperatures as low as 1400°C for 240 min, regardless of the presence of A14C3(trace). Only α‐Al4SiC4was formed at and above 1500°C for 60 min; the crystal gr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07909.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Crack‐free titania composite membranes have been synthesized from colloidal titania sols (average particle size of 7–16 nm) by the pressurized sol‐gel coating technique. The Knudsen permeability of N2through the titania membrane layer heat‐treated at 300°C is about 21 × 10‐7–35 × 10‐7mol/m2·s·Pa at room temperature. The permselectivity and the separation factor of He to O2in the temperature range of 25°‐245°C are 2.49–2.74 and 1.66–1.84, respectively. The average pore diameters of these membranes estimated from the molecular weight cutoff data (6000–9000) using poly(ethylene glycol) are 3.0–4.0 nm. By doping zirconia into titania membranes, their thermal stability

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07910.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Hexagonal boron nitride powder was hot pressed in an environment of metallic copper. When compared with a copper‐free system, the sintered body exposed to the copper was composed of substantially thicker grains having an unusual arrangement, consisting of preferred alignment of (0001) basal planes parallel to the pressure axis. The character of the orientation of the boron nitride grains substantially influenced the mechanical strength of the ceramics. The difference in the orientation of the grains is explained by the interaction between copper atoms and boron nitride crystals selectively occurring on the (0001) basal planes of the latte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07911.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Infrared OH absorption spectra were measured for a series of sodium aluminophosphate glasses. The absorption spectrum appears to be dependent on both alumina content and overall O/P ratio. The latter dependency cannot be explained by the conventional interpretation of the structural role of Al ions. Based on the latest studies on the role of Al ions in the phosphate glass, the influence of Al ions on the OH infared absorption spectrum is examined and explained.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07912.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY