摘要:

Transmission electron microscopy studies of calcium silicate hydrate gels in portland cement were performed. Selected area electron diffraction studies revealed diffuse rings, indicating the presence of short‐range ordering. Investigations of various regions within the gel found strong fluctuations in the diffuseness of the rings; corresponding local compositional probes showed the presence of large fluctuations in the local calcium:silicon ratio. Lattice imaging studies were then performed to determine the nature of the short‐range ordering These investigations revealed nanocrystalline regions within an amorphous matrix. The local composition within the nanocrystalline regions is believed to be homogeneous. However, strong fluctuations in the local compositional order parameter between various regions would seemingly exist. Optical diffractograms taken from individual nanocrystalline regions demonstrated the coexistence of both jennite and 1.4‐nm tobermorite structural elements, supporting the essential features of the Taylor nanophasic

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07990.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Aqueous solutions of Al3+and Fe3+nitrates were pyrolyzed to produce (Al,Fe)2O3amorphous solid solutions with up to 50 mol% Fe2O3. Following subsequent heat treatments, all compositions first crystallized to γ‐(Al,Fe)2O3and those with ≥20% Fe2O3ultimately partitioned into a mixture of A12O3and Fe2O3‐rich α phases as predicted by the equilibrium diagram. The transformation path between the γ phase and the final microstructure was dependent on composition. Materials with 10%–20% Fe2O3formed extended α solid solutions which partitioned only sluggishly at 1100°C. In contrast, materials with compositions above ∼30% Fe2O3formed an intermediate metastable orthorhombic phase (O) based on the equiatomic AlFeO3compound. TheO‐phase also decomposed into a mixture of two α phases upon further heating. It appears that the α1+α2mixture may evolve directly from γ for compositions near 30% Fe2O3, but clearly forms fromOas the Fe content increases. Resistance to partitioning also reaches a minimum at ∼30% Fe2O3. At 900°C, the size of the α grains was ≥1 μm for compositions ≤5% Fe2O3, and decreased to ≤200 nm for compositions ≥10% Fe2O3, reflecting the strong effect of Fe2O3on grain refinement relative to pure A12O3. Contrary to literature, the formation of single‐phase α in compositions ≤20% Fe2O3appears to occur without the prior nucleation of Fe2O3‐rich “seeds.” Partitioned α mixtures exhibit orientation relationships suggestive of epitaxial nucleation of one phase on the other. The present results bear on important thermodynamic and kinetic issues related to the synthesis o

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07991.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Ultrafine powders of aluminum‐rich mullite precursor phases were prepared by spraying mixed acid solutions of silicic acid and aluminum chloride into ammonium hydroxide at different pH values. The aluminum‐rich composition was chosen to avoid abnormal grain growth of the mullite grains caused by silica‐rich liquid phases. The influence of coprecipitation pH on particle size, zeta potential, and the mixing degree of aluminum and silicon in the precipitated powders and on the sintered microstructures have been described. In the basic pH region, the degree of mixing of both components increased as the pH of coprecipitation decreased. Under these conditions, direct slip casting of the coprecipitated slurries was possible, and fine and equiaxed microstructures could be obtained in the sintered b

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07992.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Transposed temperature drop calorimetry and hightemperature drop solution calorimetry in molten 2PbO·B2O3at 977 K were used to study the energetics of some manganese oxides, namely pyrolusite (MnO2), bixbyite (Mn2O3), hausmannite (Mn3O4), and manganosite (MnO). The enthalpies of oxidation at 298 K in the manganeseoxygen system, which were determined by appropriate thermodynamic cycles, were (in kJ/mol of oxygen): –441.4 ± 5.8 for the reaction 6MnO + O2→ 2Mn3O4, –201.8 ± 8.7 for the reaction 4Mn3O4+ O2→ 6Mn2O3, and –162.1 + 7.2 for the reaction 2Mn2O3+ O2→ 4MnO2. These values agreed very well with previous data that were obtained using equilibrium measurements that were reported in the literature. Thus, direct calorimetric measurements were well suited to obtain reliable enthalpy of formation data for oxides that contain manganese in the 2+, 3+, and 4+ states. Using these new values of enthalpies and reliable standard entropies, the phase‐stability diagram of the manganeseoxygen system

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07993.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Strength measurements and fractography were used to investigate the failure of alumina‐glass dental composites containing 75 vol% alumina and 25 vol% glass. Alumina compacts were prepared by slip casting and sintering at 1100°C for 2 h. Dense composites were made by infiltrating partially sintered alumina with glass at 1150°C for 8 h. Young's modulus and the hardness of the composites were 270 GPa and 12 GPa, respectively. The mean strength (460 MPa) and fracture toughness (4.0 MPa·m1/2) of the composites were insensitive to the glass thermal expansion coefficient (αglass= 5.9 × 10−6to 7.8 × 10−6°C−1). Typical flaws were pores and cracklike voids formed by poor particle packing and differential sintering near agglomerates of alumina in the composite. Crack deflection and crack bridging were observed in indentation cracks. Fracture toughness was single‐valued because the alumina particle size was small (∼3 μm). Alumina‐glass composites are promising new ceramics for dental crown and bridge applications, because their strength and fracture toughness are ∼2 times greater than those of

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07994.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Factors affecting stacking fault formation during the synthetic fabrication of β‐SiC were investigated in the present study. Two main reaction routes in the carbothermal reduction of SiO2—solid–solid and solid‐gas reactions—led to the formation of β‐SiC, depending on the vapor pressure of the SiO gas. The β‐SiC formed by the solid‐gas (carbon‐SiO gas) reaction showed a whisker morphology with a high stacking fault density (SFD), whereas that formed by the solid‐solid (Si‐carbon) reaction exhibited spherical particles with a low SFD. The average size of the synthesized particles decreased with decreasing reaction temperature and time, and their stacking fault content, measured by X‐ray diffraction, was higher than the true value, possibly because of a size effect. The stacking fault density increased with increase in the heating rate because of

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07995.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The thermal shock behavior of three types of two‐dimensional woven, continuous fiber‐reinforced (NextelTM312 (3M Co., St. Paul, MN) or NicalonTM(Nippon Carbon, Tokyo, Japan)) ceramic matrix (silicon carbide matrix that had been processed by chemical vapor infiltration or polymer impregnation and pyrolysis) composites was studied using the water‐quench technique. Thermal‐shock‐induced damage was characterized by a destructive technique of four‐point flexure and a nondestructive technique of Young's modulus measurement by the dynamic resonance method. Compared with monolithic ceramics, the continuous fiber‐reinforced ceramic composites were capable of preventing catastrophic failure that was caused by thermal shock. Analysis of the results that were based on the stresses that were generated by thermal shock and the mismatch of thermal expansion between fibers and matrices suggested possible mechanisms of the thermal shock damage. Preliminary results showed evidence of matrix cracking and delamination because of the thermal shock damage. The feasibility of using the nondestructive technique to detect thermal shock damage also was

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07996.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The effects of the dopants, Mg2+, Ca2+, Sr2+, Sc3+, Yb3+, Y3+, Gd3+, La3+, Ti4+, Zr4+, and Nb5+, on the grain boundary mobility of dense CeO2have been investigated from 1270° to 1420°C. Parabolic grain growth has been observed in all instances. Together with atmospheric effects, the results support the mechanism of cation interstitial transport being the rate‐limiting step. A strong solute drag effect has been demonstrated for diffusion‐enhancing dopants such as Mg2+ and Ca2+, which, at high concentrations, can nevertheless suppress grain boundary mobility. Severely undersized dopants (Mg, Sc, Ti, and Nb) have a tendency to markedly enhance grain boundary mobility, probably due to the large distortion of the surrounding lattice that apparently facilitates defect migration. Overall, the most effective grain growth inhibitor at 1.0 % doping is Y3+, while the most potent grain growth promoter is either Mg2+(e.g., 0.1%) or Sc3+at high concentration (greater than

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07997.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The effects of the dopants, Mg2+, Sr2+, Sc3+, Yb3+, Gd3+, La3+, Ti4+, Zr4+, Ce4+, and Nb5+, on the grain boundary mobility of dense Y2O3have been investigated from 1500° to 1650°C. Parabolic grain growth has been observed in all cases over a grain size from 0.31 to 12.5 μm. Together with atmospheric effects, the results suggest that interstitial transport is the rate‐limiting step for diffusive processes in Y2O3, which is also the case in CeO2. The effect of solute drag cannot be ascertained but the anomalous effect of undersized dopants (Ti and Nb) on diffusion enhancement, previously reported in CeO2, is again confirmed. Indications of very large binding energies between aliovalent dopants and oxygen defects are also observed. Overall, the most effective grain growth inhibitor is Zr4+, while the most potent grain growth promoter is Sr2+, both at 1.0% concentra

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07998.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Alumina powder compacts, partially densified with a lowtemperature heat treatment and then cut into bars, were infiltrated with liquid precursors that decomposed to either mullite (3Al2O3·2SiO2), fully stabilized zirconia (cubic Zr(8Y)O2), or partially stabilized zirconia (tetragonal Zr(4Y)O2). The specimens were repeatedly infiltrated and pyrolyzed to achieve a higher concentration of the precursor near the surface. The infiltrated bodies were then densified at 1500°C/2 h. Residual stresses developed during cooling from the densification temperature because of the higher concentration of the second phase near the surface and their differential thermal expansion relative to the matrix material. At least 10 bars of each two‐phase material were fractured in four‐point bending to determine the effect of the second phase on strength. The alumina bars without a second phase had a larger grain size (∼7 μm) and a mean strength of 253 MPa. The intruded phases significantly reduced the Al2O3grain size to ∼1 1μm. Despite their higher concentration near the surface and apparent surface tensile stress, both of the Zr(Y)O2phases increased the mean strength to 413 MPa (c‐Zr(8Y)O2) and 582 MPa (t‐Zr(4Y)O2, an apparent toughening agent). The mullite second phase produced a high mean strength of 588 MPa, apparently due to its concentration gradient creating a compressive

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07999.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY