摘要:

The low‐temperature formation of Fluorapatite by the reaction of particulate calcium phosphate precursors in aqueous solution was investigated:Variations in the aqueous phase were measured and these corelated with the consumption of reations and the formation of apatitic phases. The variations in solution pH and fluoide are in accord with a mechanism by which CaF2precipitates relatively early in the reaction. After this, minimal fluoride is consumed until the formation of an apatitic phase reaches substantial for hydroxl and the pH increase. The extent of aptite formation exhibits a storong dependence on the concetration of the NaF solution in which reation is occuring. Analysis of reaction is saturated CaF2solution indicates that apatite formation precedes the uptake of fluoride from solution. For reaction in dilute suspension, a low concentration of NaF retards apatite formation whereas a higher one accelerates it. Alternatively, when reaction is carried outa low liquid‐to‐solids rario, apatite formation is accelerated in NaF solution regardless of concetr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04441.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Gas‐phase synthesis of titania by TiCl4oxidation in the presence of dopants (SiCl4, POCl3, and BCl3) was investigated in an aerosol reactor as a function of temperature (1300–1700 K) and dopant concentration (0–25 mol% of TiCl4). The addition of dopants, most notably silicon and phosphorus, drastically altered the morphology of titania particles from polyhedral to spheroidal, increased the extent of aggregation, increased the specific surface area, reduced the primary particle size, and decreased the rutile content. The observed morphology/crystallinity changes were explained in terms of ionic radii and valence of the dopant el

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04442.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Amorphous films in the system SiO2–AlPO4were prepared by means of the rf‐sputtering method, and their physical properties, such as density, refractive index, and temperature coefficient of Young's modulus, and infrared spectra were measured. Also, theKα X‐ray emission spectra of silicon and aluminum were measured in order to investigate the coordination state of these cations in the amorphous films. The density and the refractive index were close to those of amorphous SiO2and AlPO4and the compositional dependence showed a small deviation from linearity. The temperature coefficients of Young's modulus were positive for all of the samples. The infrared absorption spectra of all of the samples were similar to those of SiO2glass and amorphous AlPO4film, and there was no evidence of the presence of P═O bonds. The coordination states of silicon and aluminum ions in the present amorphous films were the same as those in fused silica and AlPO4crystal, respectively. The results of the properties, infrared absorption spectra, and X‐ray emission spectra suggest that SiO4tetrahedrons and AlO4–PO4connecting tetrahedral dimers constitute the network of the present amorphous films. A small deviation of the physical properties from an additive rule was thought to result from the difference in the bond character between the newly formed Si–O–Al and Si–O–P bonds and the bonds in the end members

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04443.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The compressive creep behavior of single‐crystal 6H α‐SiC was measured for orientations parallel to and at 45° to [0001]. Deformation of the 45° orientation was dominated by basal slip. Steady‐state creep rates above 10‐7/s were measured at temperatures as low as 800°C. An activation energy of 277 kJ/mol and a stress exponent of 3.32 were determined. Creep testing with applied stresses parallel to [0001] was performed at 1650°C to 1850°C, yielding a stress exponent and activation energy of 4.93 and 180 kJ/mol, respectively. The occurrence of basal slip in the [0001]specimens suggested that significant off‐axis stresses were present

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04444.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A review of experimental results and theoretical models for thermal conductivities of ceramic materials with porosity less than 30% is given. It is shown that the abnormal non‐monotonic pressure and temperature dependences of thermal conductivity arise from the effects of microcracks and porous grain boundaries, characterizing many industrial refractories, and from the competitive influences of classical and novel mechanisms of heat transfer in composite multiphase materials. The latter mechanisms include segregation and surface diffusion of impurities and defects in crystal structure, and the mechanism arising from chemical conversion and gas emission, occurring within pores of ceramic materials. A fractal model of porous materials' structure is proposed and used for analysis, explanation, and classification of thermophysical properties of ceramic materials measured in various thermodynamic condition

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04445.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The fracture of a transparent, large‐grain‐sized MgAl2O4spinel has been studied through temperatures of 1400°C. Fracture toughness values, falling between about 1.3 and 2.3 MPa.m12;1/2, behaved sigmoidally with temperature, with a lower shelf transition appearing near 800°C. RisingR‐curves, displaying a run‐arrest character, were found at both the lowest and the highest temperatures, while only minimal values of dKR/dΔawere produced at intermediate temperatures of 800°C. This fracture character is ascribed largely to finite concentrations of a residual LiF pressing aid identified on the fracture surface, while additional influences associated with the lower shelf region at the highest temperatures may include the increased fraction of intergranular crack path and the onset of plasticity. This cubic monolithic ceramic displayed strong nonlinear fracture behavior in both temperature regimes. In both cases the fracture character is linked directly to an active toughening mechanism in the wake region, which depends upon crack fa

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04446.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The strengths of reaction‐bonded silicon carbide radiant tubes were examined after a 14‐month campaign at 800°–950°C in an industrial heat‐treating furnace. The inner tube of the single‐ended radiant tube (SER) burner system was exposed to a combustion gas environment, and the outer tube was exposed to both a combustion gas (inner surface) and an endothermic gas (outer surface) environment. Diametral compression testing of C‐rings and O‐rings was employed to assess the effect of exposure on the strengths where failure initiated from the outer and inner surfaces of the tubes, respectively. Comparisons of strengths obtained on exposed versus as‐received tubes revealed that exposure to the combustion ambient yields an apparent moderate strengthening, with a concurrent increase in strength variability. No significant difference in strength distribution was observed for tubes exposed to combustion versus endothermic gas ambients. Failure most often originated at silicon‐rich regions remaining from silicon infiltration of macroporosity in the slip cast tubes during reaction sintering. Specimens sectioned from the outer tube exhibited unexpectedly high strengths (based on predictions from Weibull statistics) compared to those sectioned from the smaller inner tube after the same exposure. These results indicate that the larger tubes have flaws with characteristics different from those present in smaller tubes, and underscore the need to use data from the same material, with the same flaw populations, in making predictions of the effect of component size on strength distributions and rel

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04447.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A boric acid‐stabilized aluminum acetate powder was decomposed to aluminum borate (Al18B4O33) by calcining at 1000°C. The powder was then ball‐milled, compacted, and fired to temperatures of 1500°, 1600°, and 1700°C for a period of 1 h. The resulting aluminum borate ceramic had a whiskerlike grain morphology, with the whisker length approaching 20 μm and a diameter of 2–3 μm. The sintered compact showed no shrinkage upon firing, had a porosity of ∼50%, a narrow pore size distribution, and an average pore s

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04448.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The effects of green density as an experimental parameter in influencing the crystallization of an amorphous silica layer coated on the surface of α‐alumina powder particles and the subsequent reaction of crystalline cristobalite and alumina to form mullite have been investigated. Green compacts consisting of alumina powder particles coated with an amorphous silica layer were fabricated using both a conventional uniaxial die pressing and rotary forging at room temperature, resulting in various green densities from 15% to 68% theoretical density. It has been shown that both the crystallization temperature of the amorphous silica layer coated on the surface of alumina powder particles and the subsequent mullitization temperature are lowered in the highly compacted materia

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04449.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A method is discussed for etching stabilized zirconia using microwave heating. Microstructural features of microwave‐sintered stabilized zirconia are revealed by this technique. This approach requires less time at temperature than conventional thermal etching to achieve comparable result

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04450.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY