摘要:

Phase relations and phase stabilities have been derived for the ternary systems RE─B─N (RE = Nd, Sm, or Gd) at elevated temperatures (1400°C and above) by means of X‐ray powder analysis. Under the experimental conditions selected, various ternary compounds are found to be stable: Nd3B2N4with the Ce3B2N4type and (Nd,Sm,Gd)BN2with the PrBN2type. Phase equilibria at 1400°C and under 105Pa of argon are mainly characterized by the incompatibility of the RE metals Nd, Sm, and Gd with BN due to the competing equilibria between the RE tetraborides and the RE mononitrides. Each of the ternary compounds, however, is found to be in a two‐phase equilibrium withhex‐BN. Because of the different thermodynamic stabilities within the various structure series of ternary rare‐earth boron nitrides RE3B2N4and REBN2, the compound Nd3B2N4is observed only at temperatures below 1800°C and under 105Pa of Ar, whereas GdBN2is found to be stable only at temperatures above 1400°C under a partial pressure

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05511.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Ceria‐stabilized tetragonal zirconia polycrystals were obtained by thermal treatment of amorphous powder prepared by the sol–gel method. Detailed XRD profile analysis was employed to study microstructural disorder and crystallite size and shape; in particular, no fluctuation of stoichiometry was found, the main cause of disorder being attributable to dislocations. Thermal expansion measurements were carried out by high‐temperature XRD at 294, 473, 673, 873, and 1073 K using silicon as an internal standard. Thermal expansion coefficients are anisotropic and changes in the stabilizer content have little effect on them. A mean value, αa= 10.6 × 10−6(K−1) and αc= 13.5 × 10−6(K−1), can be assumed for Zr1−xCexO2withxin

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05512.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Phase equilibria in the SrO─CuO─Cu system were determined at 1173 K from the results of X‐ray diffraction measurements using specimens annealed under low oxygen partial pressure (po2= 0.21–10−8atm). Electromotive force (EMF) measurements using ZrO2solid electrolyte cells were carried out in the ternary phase equilibria. Gibbs free energies for five reactions in the cell electrodes were summarized by equations with linear temperature dependence. The standard free energies of formation for Sr2CuO3, SrCuO2, Sr14Cu24O41, and SrCu2O2were derived. The stability conditions of all oxides are displayed in thep‐T‐xdiagram using

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05513.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The phase separation in 12 mol% CeO2─ZrO2ceramic heattreated in a mixture of H2and Ar was investigated by X‐ray diffractometry (XRD), X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and Raman scattering. After heat treatment at temperatures above 1200°C, the tetragonal solid‐solution phase separated into Zr2Ce2O7and the monoclinic phase. Raman scattering measurements also provided supplementary evidence for the phase separation. XPS showed that the valence change from Ce4+to Ce3+predominantly occurred, whereas the reduction from Zr4+to Zr3+took place above 1200°C. It is concluded, that in the highly reduced sample, where the valence changes from Ce4+(Zr4+) to Ce3+(Zr3+), the phase separation is noticeably promoted. Below 1000°C the phase separation was suppressed because of no appreciable valence change to trigger the phase separation, and the single tetragonal phase wa

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05514.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The binding energies of core electrons ind‐type stoichiometric diborides of several transition metals were measured by X‐ray photoelectron spectroscopy. Metal core lines in the diborides shifted either to higher or to lower binding energies compared with those appertaining to corresponding elemental states and varied with the metal. The shifts were<0.6 eV. Carbides, oxides, and oxycarbides of variable stoichiometries were the major impurities detected in the diborides. The binding energy shifts showed a progressive change as the electronegativity difference between the and the metalloid (boron, carbon, nitrogen, oxygen) increased. This indicated that the bond in boro‐oxycarbides may be partially

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05515.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Refractory bodies of 65 wt% Al2O3were prepared from a mixture of calcined alumina and raw kaolin with the addition of Cr2O3up to 15 wt%. The Cr2O3addition effectively enhances slag resistance and reduces mullite formation. Petrographic analysis of the refractories after the slag test suggests that Cr2O3increases the viscosity of both the glassy phase in the refractory as well as the slag, thereby retarding slag penetration and reaction at elevated temperature.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05516.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Porous glass‐ceramics with a skeleton of the fast‐lithium‐conducting crystal Li1+xTi2−xAlx(PO4)3(wherex= 0.3–0.5) were prepared by crystallization of glasses in the Li2O─CaO─TiO2─Al2O3–P2O5system and subsequent acid leaching of the resulting dense glass‐ceramics composed of the interlocking of Li1+xTi2−xAlx(PO4)3and β‐Ca3(PO4)2phases. The median pore diameter and surface area of the resulting porous Li1+xTi2−xAlx(PO4)3glass‐ceramics were approximately 0.2 μm and 50 m2/g, respectively. The electrical conductivity of the porous glass‐ceramics after heating in LiNO3aqueous solution was 8 × 10−5S/cm

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05517.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

We reported the first preparation of C70‐doped solid gel. Its UV‐visible spectrum was recor

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05518.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The dynamic stress intensity factors, which were determined with newly developed bar impact facilities and a new data reduction procedure, for an Al2O3ceramic and 29 vol% SiCw/Al2O3composite were virtually identical, thus indicating that the short SiC whiskers were ineffective under dynamic fracture. SEM studies revealed five distinct fracture morphologies with increased percentage area of transgranular fracture in both materials with rapid crack propagation. Also, the high dynamic stress intensity factor caused multiple microscopic crack planes to form and then join as the crack advanced.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05519.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The coefficient of thermal expansion (CTE) of a 140‐μm‐diameter SiC monofilament was determined to be 6.5 ± 0.5 × 10−60C−1in the temperature range 25°–900°C. Heat treatment of the fibers at 1400°C for 90 min resulted in both an increase in the intensities of the SiC peaks and a reduction of their width, indicating some grain growth. Furthermore, heat treatment of the fibers for 2 h at 1600°C in vacuum reduced their CTE to 5.7 × 10−60C−1. The anomalously high CTE and its decrease upon heat treatment were attributed to the presence of a large fraction of the atoms in the boundaries between the nanometer β‐SiC grains. The thermal expansion coefficients of a series of SiC/borosilicate composites were measured as a function of fiber volume fraction in the temperature range 25°–500°C and found to

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05520.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY