摘要:

The quasi‐static consolidation in uniaxial compression of combustion‐synthesized TiB2‐based materials was investigated. Consolidation was carried out in insulated containers upon completion of the combustion reaction, while the porous reaction products were ductile. Since the consolidation is not an isothermal process, the temperature change during consolidation was monitored and recorded. The effect of the addition of metallic elements to the elemental powders was established, and it was found that nickel and chromium provide the best compact integrity. The partial densification is sufficient to show significant differences between the effects of metallic additives. A phenomenological (not based on the micromechanisms of densification) constitutive model was applied to the hot and porous reaction products incorporating the temperature dependence of flow stress. The activation energy for the temperature dependence of the flow stress is established and indicates that, in addition to diffusion‐induced plastic deformation, other processes occur, such as fracturing of li

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08797.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The stresses near the free edge of the interface in joints of dissimilar materials with an interlayer were analyzed after a change in temperature. The effects of the thickness of the interlayer and of the material properties (elastic constants, thermal expansion coefficients) on the singular stress field are discussed.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08798.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

n alumina‐based composites containing ceria‐stabilized tetragonal zirconia, the martensite start temperature (Ms) of the tetragonal‐to‐monoclinic zirconia phase transformation exhibits a grain size dependence that becomes increasingly pronounced as the zirconia content decreases. Neutron diffraction experiments confirm earlier dilatometry measurements ofMsin composites containing ≥20 vol% ZrO2and were instrumental in obtainingMsvalues in lower zirconia content (i.e., 10 vol%) composites. The dependence ofMson zirconia content is related to the internal stresses that arise from differences in thermal expansion coefficients between the two phases. Neutron diffraction measurements show that the internal tensile stresses in the zirconia grains increase with decreasing zirconia content. The measured internal stresses are in quantitative agreement with predictions based on models assuming isolated ZrO2particles at low zirconia contents and a continuous ZrO2“matrix” phase at higher zirconia contents. This assumption is consistent with the observed microstructural development in which the low zirconia contents result in isolated zirconia grains, whereas higher zirconia contents result in more interconnected zi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08799.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The mechanism of formation of the perovskite phase and the dielectric properties of Pb(Zn,Mg)1/3Nb2/3O3(PZMN) ceramics were examined using two different types of columbite precursors, (Mg,Zn)Nb2O6(MZN) and MgNb2O6+ ZnNb2O6(MN + ZN). The formation of perovskite phase in the PbO + MN + ZN system is characterized by an initial rapid formation of Mg‐rich perovskite phase, followed by a sluggish formation of Zn‐rich perovskite phase. On the other hand, due to the formation of pyrochlore phase of mixed divalent cations Pb2–x(Zn,Mg)yNb2−yO7−x−3y/2, the pyrochlore/perovskite transformation in the PbO + MZN system proceeded uniformly with a spatial homogeneity. Further analysis suggested that the formation of perovskite phase is a diffusion‐controlled process. The degree of diffuseness of the rhombohedral/cubic phase transition (DPT) is higher in the PbO + MN + ZN system than in the PbO + MZN specimen forT>Tmax(temperature of the dielectric permittivity maximum), indicating a broadened compositional distribution of the B‐site cations in the PbO +

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08800.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

B4C/A1 offers a family of engineering materials in which a range of properties can be developed by postdensiflcation heat treatment. In applications where hardness and high modulus are required, heat treatment above 600°C provides a multiphase ceramic material containing only a small amount of residual metal. Heat treatment between 600° and 700°C produces mainly A1B2; 700° and 900°C results in a mixture of A1B2and A14BC; 900° and 980°C produces primarily A14BC; and 1000° to 1050°C results in A1B24C4with small amounts of A14C3if the heating does not exceed 5 h. Deleterious A14C3is avoided by processing below 1000°C. All of these phases tend to form large clusters of grains and result in lower strength regardless of which phase forms. Toughness is also reduced; the least determinal phase is A1B2. The highest hardness (88 Rockwell A) and Young's modulus (310 GPa) are obtained in Al4BC‐rich samples. AlB2‐containing samples exhibit lower hardness and Young's modulus but higher fracture toughness. While the modulus, Poisson's ratio, and hardness of multiphase B4C/A1 composites containing 5–10 vol% free metal are comparable to ceramics, the unique advantage of this family of materials is low density (>2.7 g/cm3) and higher than 7 MPa‐m1/2f

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08801.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Reactions between Ti and SiO2were studied at 1373 K (1100°C) under vacuum conditions using planar diffusion couples. A method to correct for the presence of surface oxide was developed which led to improved oxygen measurements with the electron probe microanalyzer. An isothermal section through the Ti‐Si‐O phase diagram at 1373 K was determined using measured diffusion paths and phase compositions in equilibrated alloys. The experimentally determined isothermal section was compared to isothermal sections calculated using thermodynamic data. In addition the sequence of reaction layers formed in the diffusion couples is discussed in terms of thermodynamic activity diag

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08802.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

A detailed analysis of the microstructure of zinc oxide produced by thermal decomposition of zinc oxalate at temperatures in the range 328° to 650°C is described. Parameters describing the breadth and shape of X‐ray diffraction line profiles were obtained by means of pattern decomposition, and a method based on the Voigt function was used to obtain information on the temperature dependence of the microstructure. It was found that the crystallites are prismatic, the dimensions increasing significantly with formation temperature, and that each contains one stacking fault, on average. The results of size determinations are in broad agreement with TEM and SEM measurements. The specific surface area of the crystallites, obtained by the BET method, is compared with the results of X‐ray diffraction line‐profile analysis, leading to a determination of the variation of “packing factor” with temperature. Results suggest a homogeneous crystallite growth in the initial sintering of the loosely packed zinc o

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08803.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Lead zirconate titanate (PZT) thin films were deposited by metal‐organic chemical vapor deposition (MOCVD) using β‐diketonate precursors and 02at temperatures below 500°C on variously passivated Si substrates. PZT thin films could not be deposited on bare Si substrates, owing to a serious diffusion of Pb into the Si substrate during deposition. Pt/SiO2/Si substrates could partially block the diffusion of Pb, but a direct deposition of PZT thin films on the Pt/SiO2/Si substrates resulted in a very inhomogeneous deposition. A TiO2buffer layer deposited on Pt/SiO2/Si substrates could partially suppress the diffusion of Pb and produce homogeneous thin films. However, the crystallinity of PZT thin films deposited on the TiO2‐buffered Pt/SiO2/Si substrate was not good enough, and the films showed random growth direction. PZT thin films deposited on the PbTiO3‐buffered Pt/SiO2/Si substrates had good crystallinity anda‐and c‐axis oriented growth direction. However, the PZT thin film deposited at 350°C showed fine amorphous phases at the grain boundaries, owing to the low chemical reactivities of the constituent elements at that temperature, but they could be crystallized by rapid thermal anneaiing (RTA) at 700°C. PZT thin film deposited on a 1000‐å PbTiO3,‐thin‐film‐buffered Pt/SiO2/Si substrate at 350°C and rapid thermally annealed at 700°C for 6 min showed a single‐phase perovskite structure with a composition near the morph

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08804.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The interfaces between metal organic chemical vapor deposited PbTiO3thin films and various diffusion barrier layers deposited on Si substrates were investigated by transmission electron microscopy. Several diffusion barrier thin films such as polycrystalline TiO2, amorphous TiO2, ZrO2, and TiN were deposited between the PbTiO3thin film and Si substrate, because the deposition of PbTiO3thin films on bare Si substrates produced Pb silicate layers at the interface irrespective of the deposition conditions. The TiO2films were converted to PbTiO3by their reaction with diffused Pb and O ions during PbTiO3deposition at a gubstrate temperature of 410°C. Further diffusion of Pb and O induces formation of a Pb silicate layer at the interface. ZrO2did not seem to react with Pb and O during PbTiO3deposition at the same temperature, but the Pb and O ions that diffused through the ZrO2layer formed a Pb silicate layer between the ZrO2and Si substrate. The TiN films did not seem to react with Pb and O ions during the deposition of PbTiO3at 410°C, but reacted with PbTiO3to form a lead‐deficient pyrochlore during postdeposition rapid thermal annealing at 700°C. However, TiN could effectively block the diffusion of Pb and O ions into the Si substrate and the formation of Pb silicate at the inter

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08805.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

This paper presents29Si MAS‐NMR measurements that trace the hydration process in both cement paste and mortar specimens made from ordinary portland cement, Type I, when the cement content is replaced by 0, 10, 15, and 20 wt% of silica fume. The specimens were moist‐cured for 3, 7, 14, 28, 90, and 180 days at a laboratory temperature of 21°C (69.8°F). Compressive strength for all tested specimens was also determined. The results show that the degree of hydration (Q1+ Q2)/(Q°+ Q1+ Q2) increased with increasing content of silica fume, especially at the early ages of 3 to 28 days. In the same manner, compressive strength results were markedly increased up to 14 days and were lowered at later ages, compared to the control mix (0 wt% silica

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1995.tb08806.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY