摘要:

The reactions were investigated that occur when lime, cement, and quartz sand are mixed together and molded, then treated at 180°C under saturated steam pressures to produce autoclaved aerated concrete. The hydrothermal treatment of mixtures gives Ca‐rich C‐S‐H with varying Ca/Si ratios as an initial product, which reacts further with silica dissolved from quartz to form 1.1‐nm tobermorite with increase of curing time. During autoclaving, the composition of C‐S‐H and tobermorite as a binder continues to change until after 8 h, when the Ca/(Al + Si) ratio becomes constant at 0.8. As the reaction proceeds, the number of micropores increases, and the strength also increases due to the binder effect of the tobermorite. However, the total pore volume does not change, remaining con

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07208.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The Ba‐doped superconducting (Bi,Pb)2Sr2‐xBaxCa2Cu3Oyand (Bi,Pb)2Sr2Ca2‐xBaxCu3Oy(0 ≦x≦ 1.0) were prepared by using a melt‐quenching method, and the effect of Ba additions on the glass‐forming ability and the crystalline phase was examined. The glass‐forming ability was not improved by substitution of Ba for Sr or Ca, and particularly BaPbO3as well as CaO was observed in the melt‐quenched sample of (Bi,Pb)2SrBaCa2Cu3Oy. BaPbO3crystals were precipitated in all glass‐ceramics with Ba substituted for Sr or Ca. The partial substitution of Ba substituted for Sr was effective for the formation of the high‐Tcphase, and (Bi,Pb)2Sr1.4Ba0.6Ca2Cu3Oyglass‐ceramics obtained by annealing at 830°C for 100 h exhibited superconductivity with aTcof 103 K, although BaPbO3and the low‐Tcphase

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07209.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The intergranular phase in an alumina containing 1 vol% anorthite glass was crystallized in order to enhance internal residual stresses within the microstructure. The influence of crystallization on the mechanical behavior was investigated by the indentation–strength method. Such crystallization was found to result in a marked improvement in the flaw tolerance of this alumina, indicative of strongR‐OrT‐curve behavior. These results are discussed in the light of a theoretical model which assumes grain‐localized crack bridging to be the predominant toughening mechanism. Particular reference is made to the influence of residual stresses and interfacial properties on grain pullout across the crack walls in the wake of the cr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07210.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Calculated X‐ray diffraction pattern data for diamond polytypes are presented, which provides the required parameters for the characterization of the proposed diamond polytypes. The literature on diamond polytypes contains some small but significant errors with respect to the details of the crystal structure, including space groups, the atom positions in the unit cell, and the atomic layer stacking. In this paper, the crystal structures of four different hexagonal diamond and two different rhombohedral diamond polytypes are theoretically calculated to provide correct crystallographic information with which the diamond polytypes in a given sample can be characterize

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07211.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Bioactivity of zirconia‐toughened glass‐ceramic composites was evaluated by their surface reaction in simulated body fluid and the bonding strength to living bone. The composite containing 30 vol% zirconia showed high bioactivity, whereas that containing 50 vol% zirconia, extremely low. TEM observation indicated that Ca in the glass‐ceramic particles reacted with the zirconia during sintering. It was found that the decrease in Ca in the particles degraded the bioactivity of the composite. In this study, the optimum composition was determined for high‐strength and bioactive

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07212.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Thermochemical calculations are performed to evaluate the chemical compatibility of oxides with carbon at temperatures of the order of 1650°C and to evaluate the implications of carbon oxidation. These calculations indicate that Al2O3, BeO, Y2O3, and Ce2O3are stable with carbon provided there are no cracks or pores for CO gas to escape. In the presence of cracks or interconnected pores, both the direct reduction of oxides with carbon as well as the oxidation of carbon can proceed rapidly. The calculations also indicate that even in the absence of such cracks the oxidation of carbon can generate high enough CO partial pressures to degrade the composite properties by formation of CO gas bubbles or of cracks in the oxide. The results of these calculations are confirmed by conducting experiments on samples of pyrolytic and vitreous carbon disks hot‐pressed within Al2O3and Y2O3disks and exposing them in argon and oxygen environments at 1650–1800°C. An oxidation model is then proposed that includes thermodynamic and kinetic considerations as well as creep behavior of oxides. The model quantitatively predicts the times required to initiate the formation of gas bubbles and qualitatively considers the factors influencing the growth rate of bubbles and formation of c

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07213.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Specimens of heated, open‐cell ceramics were thermally shocked by immersion in water or oil. It was found the strength retained after thermal shock underwent a gradual decrease with increasing quench temperature, indicative of a cumulative damage mechanism which manifests itself with increasing thermal stress. This damage could also be monitored using measurements of the elastic constants before and after quenching. The thermal shock resistance of the open‐cell materials was found to be strongly dependent on cell size (increased with increasing cell size) and weakly dependent on density (increased with increasing density). Two possible sources of thermal stress were considered; one was associated with the temperature gradient across the microscopic struts and the other with the heating of the quenching medium as it infiltrates the cellular structure. Such heating was confirmed and it was concluded that this was the dominant source of thermal stress in this particular study, controlling the thermal stress in this particular study, controlling the thermal shock resistance of the open‐cell cer

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07214.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Computer simulation was carried out for the kinetics of spinodal decomposition in the tetragonal TiO2–SnO2system on the basis of a nonlinear diffusion equation. A time evolution of the microstructure and the effect of coherent strain on the separated two phases were investigated by Langer's approximate method and the finite difference method. It was shown that the composition fluctuations develop in the first stage of the spinodal decomposition, and the formation of interface and the grain growth appear in the second stage. The local stress field and the local strain field with the coherence of the lattice were calculated. Subsequently the appearance of the interface dislocations in the (100) and (010) planes was demonstrated to occur in the third stage. Physical interpretation was given to the experimental observations for the tetragonal TiO2–SnO2system on the basis of those calculati

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07215.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

An apparatus for measurement of the fiber/matrix interfacial shear stress at temperatures up to 1100° is described. Equipment was used to measure interfacial properties as a function of temperature in two ceramic‐matrix composites and one metal‐matrix composite. In the composites where the thermal expansion of the matrix was higher than that of the fiber, the interfacial shear stress decreased with temperature. The opposite trend was observed in a system with low matrix thermal expansion. The change of the interfacial shear stress with temperature of all the composites studied can be fully explained by considering the fiber/matrix expansion differe

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07216.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The oxide distribution in a bed of thermally decomposed zirconium fluoride monohydrate (ZrF4· H2O) is determined via inert gas carbon fusion analysis (LECO). It is shown that the oxide concentration in the bed of thermally decomposed ZrF4· H2O is a function of position in the crucible. The oxide concentration in the crucible decreases in the order, from greatest to least, surface ≫ bottom>sides>middle. XRD analysis and thermogravimetric analysis indicate that the predominant phase in the bottom, side, and middle regions of the bed is ZrF4, while in the surface region Zr7O9F10is the predominant phase with traces of ZrO2and ZrF4present. It is proposed that the production of HF(g)from the thermal decomposition of the bottom and side regions of the bed of ZrF4· H2O causes the central region of the bed to contain a higher concentration of HF(g)than any other region in the bed. The higher concentration of HF(g)in the central region of the bed of decomposing ZrF4· H2O is expected to shift the equilibrium from formation of zirconium oxyfluoride and elimination of HF(g)toward the formation of ZrF4and elimination of water. Thermal decomposition of different masses of ZrF4· H2O demonstrates that the oxide constration increases in the middle of the bed as the initial mass decreases. This result is consistent with the proposed mechanism for production of different phases in the bed of material. It is also shown that thermally decomposed ZrF4· H2O can be used as feedstock for preparation of low‐oxide zirconium fluoride via sublimation. The best results for sublimation from this feedstock material are obtained when the surface, which contains the highest oxide concentration, is removed before su

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07217.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY