摘要:

A review of inorganic fibers other than asbestos, glass, and carbon fibers and optical and hollow fibers. Special emphasis is placed on high‐performance fibers. The manufacture, structure, properties, and uses of the selected fibers are discusse

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04289.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Laminar composites, containing layers of Ce‐ZrO2and either Al2O3or a mixture of Al2O3and Ce‐ZrO2, have been fabricated using a colloidal method that allowed formation of layers with thicknesses as small as 10 μm. Strong interactions between these layers and the martensitic transformation zones surrounding cracks and indentations have been observed. In both cases, the transformation zones spread along the region adjacent to the layer, resulting in an increased fracture toughness. The enhanced fracture toughness was observed for cracks growing parallel to the layers as well as for those that were oriented normal to the la

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04290.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

A series of silica xerogels were synthesized in a closed system by hydrolyzing tetramethyl orthosilicate (TMOS) with various concentrations of hydrochloric acid solution but without the addition of alcohol. The molar ratio of H2O to TMOS was fixed at 4.96. The gels were characterized by water and nitrogen sorption measurements. The gelation time peaked at 50 to 80 h with 5 × 10−2to 10−1MHCl, apparently a consequence of the isoelectric point of silica. Surface area, pore size, and sorption capacity were, however, at a minimum for samples prepared with 5 × 10−2to 10−1MHCl. The BETcconstant and hence the net heats of sorption were at a maximum for these samples. Although water sorption of all the samples prepared with HCl exhibited moderate Type I isotherms, the concavity of the isotherms to theP/P0axis increased with increasing concentrations of HCl up to 10−1Min accordance with the decreased pore size and increased heat of sorption. Addition of F−ions (HF or NaF) in the presence of 10−1MHCl steadily decreased the gelation time. Addition of F−in the absence of HCl, however, caused rapid gelation and an increase in pore size, nitrogen BET surface area, and hydrophobicity. Consequently, the shape of water isotherms gradually converted to Type IV and/or Type V. Adsorption and desorption measurements on differently activated gels exhibited various types of hystereses as a result of development of hydrophobie sites at as low as 200°C, inelastic distortion induced by adsorption, and chemical reaction and aging of gels when expo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04291.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

A solution sol‐gel method has been developed to prepare 0.9Pb(Mg1/3Nb2/3)O3‐0.1PbTiO3(0.9PMN‐0.1PT) ceramics. During the processing the gel first converted to cubic pyrochlore phase at a calcination temperature of 600°C followed by the formation of pure perovskite phase at 775°C. The ceramics sintered at 1250°C for 4 h showed ≈98% of the theoretical density. The room‐temperature dielectric constant of the pellets sintered at 1250°C showed a maximum value of 25035 at 1 kHz. Sintering studies at different temperatures revealed that the dielectric constant increased with increasing grain size in t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04292.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Thin layers of Pb[(Mg1/3Nb2/3)1–xTix]O3(PMNT) were prepared by spin casting alkoxide‐based solutions on platinized Si. The effects of additives, heat treatment, and composition (x= 0 to 0.9) on perovskite phase development, ceramic microstructure, and dielectric properties are reported. Depending upon the processing conditions, ceramic thin layers could be formed in a nonferroelectric pyrochlore phase (A2B2O6) or in a ferroelectric perovskite phase (ABO3). The dimensions of the pyrochlore and perovskite units cells were related and increased with Mg and Nb contents. To minmize pyrochlore formation, the most effective processing method involved rapid heat treatment between successive solution depositions. Phase development and microstructure were also affected by solution additives. Additions of benzoic acid were found to affect the structure in solution and the later organic pyrolysis behavior from thin layers. The effect of composition on the dielectric and ferroelectric properties is also repor

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04293.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The phase transformation from metastable tetragonal (t) into monoclinic (m) phase must be studied using samples that are homogeneous in their compositions and microstructures. The present study investigated the transformability by grinding for compositionally homogeneous samples prepared by rapid quenching of melts in the system ZrO2–RO1.5(R: Nd, Sm, Y, Er, Yb). The volume fraction ofm‐phaseVm(tg) against grinding timetgwas investigated using powder X‐ray diffraction for the rapidly quenched sample. TheVm(tg) value increased with grinding timetg, then saturated to a certain valueVm(∞) of between 1 and 0 when it was ground and crushed for a long time. Both the transformation rate (dVm/dtg)tg = 0and the saturated valueVm(∞) decreased with dopant contentX.The species of dopants had less influence on the transformability. The experimentalVm(∞) andVm(0) data could be explained using a model which has a normal distribution of effective free energy change betweentandmphases, ΔGt‐m, among hypothetical parts of the sample. This distribution occurred because of nonchemical energies such as strain, interfacial, and surface energies, which associated with the nucleation ofmphase. The average effective free energy change ΔG¯t‐m/σ was found to be a linear function of ErO1.5contentXas 0.65X+ 3.00, where σ is a standard deviation of the normal distribution. The transformation rate dVm/dtgattg= 0 decreased with RO1.5content. The axial ratioc/aof thetphase was an important factor in the transformability, theVm(0),Vm(∞), and the dVm/dtg. The validity and limitation that thec/ais a factor of the transform

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04294.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The microstructure of mullite ceramics hot‐pressed and sintered at different temperatures was studied using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and electron probe microanalysis (EPMA). The specimens, consisting of stoichiometric mullite grains without glassy phase, are obtained by hot‐pressing stoichiometric mullite powder at 1575°C for 1 h. Silica‐rich glassy phases are observed using TEM at three‐grain junctions of mullite grains in specimens heated at and above 1600°C. However, high‐resolution transmission electron micrographs show no glassy phase at two‐grain boundaries in all specimens. SEM with EDS analyses show that the average value of Al2O3contents of mullite grains increases slightly with increasing temperature. These results are consistent with a published Al2O3–SiO2phase diagram. The flexural strength of mullite specimens at room temperature depends on their microstructure, such as the grain size and grain size distribution of mullite grains. The strength is high at room temperature and up to 1200°C, and it decreases at and above 1350°C, irrespective of the presence of

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04295.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Lanthanum‐doped lead titanate thin films have been prepared by sol–gel processing. These studies reveal the structural and electrical properties of 18 mol% La‐doped PbTiO3thin films to be dependent on the annealing conditions. With appropriate annealing conditions, a relative dielectric constant of about 900 was obtained, comparable to the bulk compos

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04296.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

Crack resistance characteristics and fatigue properties have been investigated in Ce‐TZP ceramics with different grain sizes. The relatively low critical transformation stress allows the development of larger transformation zones (≦200 μm), leading to flaw‐tolerant behavior. However, autocatalytic transformation processes are found to be bound to grain sizes beyond a critical value; transformation is then very limited in finer microstructures. Fatigue as a specific cyclic effect is more pronounced in microstructures with larger grains. Thus, damaging processes in the course of extensivet–mtransformation are suspected to be intensified during cyclic

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04297.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

The thermal conductivity of Li2O·Al2O3·nSiO2glass‐ceramics is studied forn= 4, 6, 8, 10 between 5 and 100 K. A monotonic increase in conductivity is observed for all samples. This behavior is different from that of glassy counterparts which exhibit a plateau in thermal conductivity between 10 and 20 K. It is also observed that while the conductivity of glass‐ceramics is lower than that of glasses at low temperatures, the situation is reversed at higher temperatures. A crossover occurs around 40 K for all studied samples. The glass‐ceramic behavior is interpreted in the light of the acoustic mismatch theory of Little. At low temperatures, the thermal boundary resistance that exists at the crystalline‐amorphous mismatch is high and the thermal conductivity is low. At higher temperatures, the boundary resistance is very small and the high conductivity is mainly due to the crystalline region within the amorphous

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1991.tb04298.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY