摘要:

The microstructure and humidity‐sensitive characteristics ofα‐Fe2O3porous ceramic were investigated. Microporousα‐Fe2O3powders were obtained by controlling the topotactic decomposition reaction ofα‐FeOOH. Water vapor adsorption thermogravimetrical experiments were carried out in the relative humidity (rh) range 0% to 95% on theα‐Fe2O3powder and the 900°C sintered compact. The microstructure was investigated by SEM, TEM, Hg intrusion, and N2adsorption porosimetry techniques. The humidity sensitivity was investigated by the impedance measurements technique in 0% to 95% rh on the compacts sintered at 50°C steps in the 850° to 1100°C range. Humidity response was found to be affected by the microstructure, i.e., the characteristics of the precursor powders and sinte

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07840.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Amorphous CeO2–ZrO2gels were prepared by coprecipitation in ammonia solutions. The onset of crystallization of the gels, from calcining in air, was 420°C, while 200° to 250°C in the presence of water and organic solvents such as methanol and ethanol. The sintering behaviors of CeO2–ZrO2powders were sensitive to the crystallizing conditions, since hard agglomerates formed when the precipitated gels were crystallized by normal calcination in air, whereas soft agglomerates formed when they were crystallized in water or organic solvents. CeO2–ZrO2powders crystallized in methanol and water at 250°C were sintered to full theoretical density at 1150° and 1400°C, respectively, whereas that crystallized by calcination in air at 450°C was sintered to only 95.2% of theoretical density, ev

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07841.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

In situ electrical resistivity measurements were employed to follow the kinetics of cation redistribution in ferrospinels,xFe3O4–(1 –x)MeFe2O4(Me = Co, Mg, Mn, or Ni) orxFe3O4–FeMe2O4(Me = Al) withx≥ 0.2. Relaxation temperatures (at 20°C/min heating rate) were established and kinetic parameters—activation energies and time constants—were determined for each cation. These parameters were insensitive to grain size, cation ratio, and oxygen nonstoichiometry. The mechanism was shown to be a local “homogeneous” point defect reaction in contrast to the “heterogeneous” nucleation and growth mechanism reported to take place in ferrispinels lacking Fe2+. Electron hopping between Fe2+and Fe3+in ferrospinels apparently screens the excess ionic charge associated with the diffusing species and enables homogeneous point defect reactions with lower attendant activation en

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07842.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The low‐temperature, Zr–Ti‐ordered, form of zirconium titanate has been investigated using high‐resolution transmission electron microscopy in order to characterize the incommensurate structure of phases with compositions ZrTiO4to near Zr5Ti7O24. Electron diffraction reveals that compositions with Zr : Ti between 5 : 7 and 1 : 1 have incommensurate superstructures, and phases close to 1 : 1 are commensurate with ana‐axis repeat 2X that of the disordered structure. High‐resolution images show that thea‐doubling in ZrTiO4corresponds to a new structure, one that consists of two Zr‐rich, distorted octahedral layers alternated with two Ti‐rich octahedral layers. The incommensurate compositions are composed of blocks of the 1 : 1 structure intercalated with blocks of the commensurate 5 : 7 structure, the latter having a tripleda‐repeat and a ZTTZTT sequence of cation layers. The intercalation can be described as an “interface‐modulated” structure resulting from the quasiperiodic insertion of (100) faults with displacement vector R =–1/3 aordin the ordered 5 : 7 phase. Although their spacing is variable, the faults are uniformly distributed in such a way as to produce incommensurate satellite reflections. The findings regarding the structure of ordered compositions in the ZrTiO4–Zr5Ti7O24system provide an improved framework for understanding the effects of ordering on the properties of zirconium

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07843.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Compact swelling in Pb‐doped Bi‐Sr‐Ca‐Cu‐O superconductor has been studied by observing the effects of the size of calcined powders, volatilization of materials, and sintering of high‐Tc(2223) powders. The bulk density increases at the early stage of sintering, for about 20 h, and then decreases. Densification occurs when the low‐Tc(2212) phase and a liquid phase exist, whereas dedensification occurs with the formation of the 2223 phase regardless of the presence of the liquid. Gas evolution from specimens does not appear to be responsible for compact swelling. Compact swelling is explained by anisotropic growth of thin, platelike 2223 grains in random orientation. When 2223 grains grow in a preferred direction, compact swelling

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07844.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A water‐based binder system consisting of colloidal boehmite (AlO(OH)) and 0.3 wt% poly(vinyl alcohol) was developed for low‐pressure piston‐type extrusion of alumina. Batches using this binder system and two different types of commerical alumina powder were characterized with regard to extrusion pressure, densification, and physical properties in the green and fired s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07845.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The interface of single‐crystal SiC and Al brazed at 1273 K is investigated by high‐resolution electron microscopy. The orientation relationship of SiC to the Al4C3reaction layer that forms between the SiC and the Al can be expressed as (0001)SiC∥(0001)Al4C3and [11∼00]SiC∥[11∼00]Al4C3. Furthermore, a very thin (two tetrahedral layers thick) transition phase and misfit dislocations are observed between the SiC and Al4C3lattices. The structure of the transition phase is discussed based on the high‐resolution electron microscopy, the stacking of the (Al,Si)C4tetrahedral layers, and the charge balance. The same reaction product, with the same orientation relationship, is observed at the interface of a polycrystalline SiC and Al

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07846.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effect of the cooling rate on the PTCR (positive temperature coefficient of resistivity) characteristics of 0.1 mol% Sb2O3‐doped BaTiO3ceramics has been investigated. Resistances both below and above the Curie temperature were increased by slow cooling, which indicated that the resistive layer width at the grain boundary increased as the cooling rate decreased. Concentration profiles of the Ba vacancies as a function of distance from the grain boundary have been simulated by the finite difference method. The inversion temperature of the 0.1 mol% Sb2O3‐doped BaTiO3system was determined to be 1160°C from the measured electrical properties and computed concentration prof

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07847.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A novel process for the manufacture of dense titanium carbide is described. Titanium carbide is produced by the reaction synthesis method, while densification and near‐net shaping is accomplished by a high‐velocity forging step. Disks with 10‐cm diameter were produced with densities over 96% of the theoretical density. The major problem encountered in this study has been thermal shock. Use of insulation and furnace cooling has decreased the severity of this problem. Optical and scanning electron microscopy observations of the resulting microstructure reveal equiaxed grains with an average size of 44μm. Quasi‐static and high‐strain‐rate compressive strength measurements yield values greater than 1.7 and 2.2 GPa, respectively. The morphologies of thermally induced (slow) and rapidly propagating cracks were characterized and the fracture modes were found to be intergranular and transgranular, respectively. The addition of Ni (5 and 25 wt%) yielded a ceramic–metal composite with a favorable

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07848.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The growth threshold,Kth, of large creep cracks in a glass‐ceramic has been investigated to determine the causes of the arrest and nonpropagating behaviors below a characteristic stress intensity,K, value. Using replication techniques, the dominant creep demage mechanisms during creep‐crack growth, which included grain‐boundary cavity and microcrack formation, were identified as a function ofKlevels and crack extension rates. Quantitative measurements of cavity density,D(r), and microcrack density, Ψ(r), revealed that the values ofD(r) and Ψ(r) both decrease with theKlevel and with increasing distance,r, from the crack tip at a givenKlevel. AtKvalues belowKth, the cavity density became zero as preexisting grain‐boundary cavities were sintered, suggesting that the growth threshold originated from sintering of creep cavities. AtKlevels aboveKth, microcracks located in the wake of the main crack were observed to be sintered also. Possible mechanisms responsible for the apparent self‐healing of microcracks and creep cavities in the glass‐ceramic are provided, together with theoretical and experim

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07849.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY