摘要:

Complex impedance patterns of aluminum nitride (AlN) doped with various ions of different ionic sizes and valences have been examined at temperatures between 400° and 950°C. The patterns showed distinct differences between samples doped with ions of different sizes. Grain and grain boundary contributions to the resistance and capacitance were compared. It was found that the grain boundary: grain resistance ratio increases with density. Samples doped with large cations have higher effective dielectric constants at the grain boundary than samples doped with small cations or undoped samples, indicating a thinner grain‐boundary la

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04402.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Oxidation of sintered aluminum nitride at low temperatures (20°–200°C) was studied using transmission electron microscopy (TEM). Particles of α‐Al2O3, about 20–30 Å in size, were found to form within minutes on freshly cleaned surfaces of AlN at room temperature. The oxide was found to grow nearly epitaxially on AlN when the {0001}AlNplanes were exposed to the surface. Limited nonepitaxial oxidation was also observed when the basal planes were inclined to the TEM foil surface. After 10 h in air at 75°C, the particles coarsened to about 50 Å, while after 150 h at 200°C, an oxide film, about 500 Å thick, was observed o

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04403.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A mullite precursor solution was made by hydrolyzing TEOS in a diluted aqueous solution of aluminum nitrate. This solution was spray‐dried at a relatively low temperature (200°C), producing a chemically homogeneous powder crystallizing completely into mullite at 975°C. By aging this solution at 100°C, a diphasic precursor sol made of colloidal silica and aluminum nitrate was made. The resulting spraydried powder was less homogeneous, crystallizing into mullite and spinel or only spinel. The aging by thermal treatment may thus be a way to control the heterogeneity of the po

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04404.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Cyclic compaction is shown to be more efficient than single‐stroke and static compaction. The application of a bias pressure during cyclic compaction decreases the degree of powder compaction. A densification equation, based on the Cooper and Eaton compaction equation, is proposed to describe the cycle‐dependent pressure–volume fraction relation for ceramic powder compaction. Data from cyclic compaction of alumina powder are well represented by the proposed equ

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04405.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A series of MoSi2compacts were fabricated at increasing hot‐pressing temperatures to achieve different grain sizes. The materials were evaluated by Vickers indentation fracture to determine room‐temperature fracture toughness, hardness, and fracture mode. From 1500° to 1800°C, MoSi2had a constant 67% transgranular fracture and linearly increasing grain size from 14 to 21 μm. Above 1800°C, the fracture percentage increased rapidly to 97% transgranular at 1920°C (32‐μm grain size). Fracture toughness and hardness decreased slightly with increasing temperature. MoSi2processed at 1600°C had the highest fracture toughness and hardness values of 3.6 MPa.m1/2and 9.9 GPa, respectively. The effects of SiO2formation from oxygen impurities in the MoSi2starting powders and MoSi2–Mo5Si3eutectic liquid formatio

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04406.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

MgO in clinker promotes the stable growth of alite in favor of the occurrence ofM3. Mg ions go into solid solution in alite by simply replacing the Ca‐sites in the structure. While the concentration of Mg in alite increases in proportion to the amount of MgO in the clinker, the concentration of Al, Fe, and Ti in alite remains constant. It is thus the Mg ions occupying the Ca‐sites that give rise to the occurrence ofM3at ambient temperature. By contrast, SO3encourages the growth instability of alite. The concentration of impurity elements in alite tends to increase with increasing amount of SO3in clinker. This is most remarkable for Al and seems closely connected with the occurrence ofM1at ambient temperature. The presence of S was clearly recognized in al

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04407.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The solidification process of liquid garnets Ln3Al5O12(Ln = Dy, Y, Lu) was probed by differential thermal analysis. A critical temperatureTclgwas evidenced in the liquid. Cooling from temperatures underTclgleads to the crystallization of the single garnet phase. Cooling from temperatures aboveTclgleads to the solidification of a mixture of perovskite LnAlO3and aluminum oxide α‐Al2O3. For Ln = Y, the results are in agreement with stable and metastable phase diagrams previously proposed. The YAIO3melt was shown to display a critical temperatureTclpalso. Its metastable solidification from temperatures aboveTclpleads to the formation of the perovskite at around 1550

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04408.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

An experimental technique for the measurement of interfacial sliding properties in fiber‐reinforced materials is presented. The technique involves pushing abundleof fibers simultaneously through a thin section of the composite. Using this technique, sample averages are immediately available, eliminating the need for repeated single‐fiber tests. The technique is particularly useful for composites that contain small‐diameter fibers. Salient features of the technique are demonstrated through tests on a CAS/SiC comp

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04409.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Studies on crack propagation in a model glass‐matrix composite were undertaken to observe how the crack propagates around a nonfailing elastic reinforcing fiber. The model system consisted of a 1.6‐mm‐diameter ferrous alloy rod embedded in a glass that very closely matched the thermal expansion of the alloy. Cracks were driven through the glass so as to intersect the rod. The crack trajectory, spallation region, and debonding effects are described. A computational model is offered for the spallation needed to accommodate crack opening in the absence of significant matrix compl

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04410.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY