共查询到20条相似文献,搜索用时 46 毫秒
1.
F. G. BUTTLER L. S. DENT GLASSER H. F. W. TAYLOR 《Journal of the American Ceramic Society》1959,42(3):121-126
Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al2 O3 . 13H2 O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca2 (OH)7 - 3H2 O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A12 O3 .13H2 O, from which it is derived by substitution of CO3 2- for 20H- + 3H2 O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al2 O3 .Ca Y 2 - xH2 O and 3CaO Al2 O3 Ca Y xH2 O. On dehydration, 4CaO.Al2 O3 .13H2 O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)2 and 4CaO.3Al2 O3 .3H2 O are formed and, by 1000°C., CaO and 12CaO.7Al2 O8 . The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al2 O3 .3H2 O is formed. 相似文献
2.
Nathan S. Jacobson rew J. Eckel Ajay K. Misra Donald L. Humphrey 《Journal of the American Ceramic Society》1990,73(8):2330-2332
The reactions of a sintered α-SiC with 5% H2 /H2 O/Ar at 1300°C were studied. Thermomchemical modeling indicates that three reaction regions are expected, depending on the initial water vapor or equivalently oxygen content of the gas stream. A high oxygen content ( P (O2 ) > 10−22 atm) leads to a SiO2 formation. This generally forms as a protective film and limits consumption of the SiC (passive oxidation). An intermediate oxygen content (10−22 atm > P (O2 ) > 10−26 atm) leads to SiO and CO formation. These gaseous products can lead to rapid consumption of the SiC (active oxidation). Thermogravimetric studies in this intermediate region gave reaction rates which appear to be controlled by H2 O gas-phase transport to the sample and reacted microstructures showed extensive grain-boundary attack in this region. Finally, a very low oxygen content ( P (O2 ) < 10−26 atm) is thermochemically predicted to lead to selective removal of carbon and formation of free silicon. Experimentally low weight losses and iron silicides are observed in this region. The iron silicides are attributed to reaction of free silicon and iron impurities in the system. 相似文献
3.
Sun-Jae Kim Soon-Dong Park Yong Hwan Jeong Sung Park 《Journal of the American Ceramic Society》1999,82(4):927-932
Crystalline TiO2 powders were prepared by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl2 solution with a Ti4+ concentration of 0.5 M at room temperature to 100°C under a pressure of 1 atm. TiO2 precipitates with pure rutile phase having spherical shapes 200-400 nm in diameter formed between room temperature and 65°C, whereas TiO2 precipitates with anatase phase started to form at temperatures >65°C. Precipitates with pure anatase phase having irregular shapes 2-5 µm in size formed at 100°C. Possibly because of the crystallization of an unstable intermediate product, TiO(OH)2 , to TiO2 x H2 O during precipitation, crystalline and ultrafine TiO2 precipitates were formed in aqueous TiOCl2 solution without hydrolyzing directly to Ti(OH)4 . Also, formation of a stable TiO2 rutile phase between room temperature and 65°C was likely to occur slowly under these conditions, although TiO2 with rutile phase formed thermodynamically at higher temperatures. 相似文献
4.
Glasses containing up to several percent of Ar, N2 , and CO2 were prepared at 2000 to 150,000 psi and up to 950°C and retained under ambient conditions. The solubilities are presented as a function of pressure, temperature, and composition of glass. Solubilities of O, He, and H2 O were also investigated in various glass compositions, especially K2 O–4SiO2 and B2 O3 . The evolution of the gases at atmospheric pressure was followed by electron microscopy and density measurements. 相似文献
5.
Compatible phases in the system Li2 O-Al2 O3 -TiO2 at various temperature levels were determined mainly by solid-state reactions for the portion of the ternary system bounded by Li2 O Al2 O2 , Li2 O.TiO2 , Al2 O, and TiO2 . The existence of a ternary compound, Li2 O.Al2 O3 .4TiO2 , and nine joins was established. The ternary compound has a lower limit of stability at 1090°± 15°C. and dissociates and recombines rapidly at 1380°± 15°C. 相似文献
6.
Raju S. Parikh Annamarie Lightfoot John S. Haggerty Brian W. Sheldon 《Journal of the American Ceramic Society》1999,82(10):2626-2632
The effects of trace O2 levels on the nitridation of compacts made from silane-derived Si powders were studied in N2 atmospheres, with oxygen levels of either 5 ppm or 10 ppb (approximately). The nitriding kinetics were studied by thermogravimetric analysis as a function of temperature (1100–1200°C) and heating rate (5°C/min and 100°C/min). Reducing the O2 level in the nitriding gas enhanced conversion to Si3 N4 at lower temperatures, reduced the composition variations within the samples, and decreased the α/β ratios. The results suggest that nucleation and rapid growth of Si3 N4 at relatively low temperatures are possible only when the oxygen partial pressure in the system is below the threshold value for passive oxidation. 相似文献
7.
Xinhua Liang David M. King Peng Li Alan W. Weimer 《Journal of the American Ceramic Society》2009,92(3):649-654
Atomic layer deposition (ALD) has been successfully utilized for the conformal and uniform deposition of ultrathin titanium dioxide (TiO2 ) films on high-density polyethylene (HDPE) particles. The deposition was carried out by alternating reactions of titanium tetraisopropoxide and H2 O2 (50 wt% in H2 O) at 77°C in a fluidized bed reactor. X-ray photoelectron spectroscopy confirmed the deposition of TiO2 and scanning transmission electron microscopy showed the conformal TiO2 films deposited on polymer particle surfaces. The TiO2 ALD process yielded a growth rate of 0.15 nm/cycle at 77°C. The results of inductively coupled plasma atomic emission spectroscopy suggested that there was a nucleation period, which showed the reaction mechanism of TiO2 ALD on HDPE particles without chemical functional groups. TiO2 ALD films deposited at such a low temperature had an amorphous structure and showed a much weaker photoactivity intensity than common pigment-grade anatase TiO2 particles. 相似文献
8.
Masamichi Tsuji Masahiro Tabata Yutaka Tamaura 《Journal of the American Ceramic Society》1994,77(1):161-164
Iron oxide polymers intercalated and/or loaded within täniolite have been studied as a CO2 decomposition medium. Fe2+ was exchanged for Li+ in täniolite, oxidized by air-bubbling at 60°–70°C. The basic d -spacing (13.75 Å in the Li+ form) was expanded to give 14.86 Å in the Fe2+ form. Oxidation by air in the form of suspension gave a 15.3-Å phase, which was ascribed to formation of magnetite within the interlayer. The interlayer distance of the intercalated phase remained the same upon heating at 300°C. The magnetite–intercalated täniolite was heated to activate in a H2 and/or H2 O steam. CO2 decomposition reactivity at 300°C has been evidenced by evolution of CO gas. The high reactivity for CO2 decomposition is ascribed to the highly dispersed iron oxide ceramics within the interlayer of täniolite Li[(Mg2 Li)(Si4 O10 )]F2 n H2 O. 相似文献
9.
Solid-state reactions between Li2 O and Al2 O3 were studied in the region between Li2 O.Al2 O 3 and Al2 O 3 . The compound Li2 O Al2 O 3 melts at 1610°± 15°C. and undergoes a rapid reversible inversion between 1200° and 1300°C. Vaporization of Li2 O from compositions in the system proceeds at an appreciable rate at 1400°C, as shown by fluorescence. Lithium spinel, Li2 O -5Al2 O3 , was the only other compound observed. The effect of Li2 O on the sintering of alumina was investigated. 相似文献
10.
The effects of heat treatment in Ar-O2 and H2 -H2 O atmospheres on the flexural strength of hot isostatically pressed Si3 N4 were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si3 N4 was exposed at 1400°C to (1) H2 with water vapor pressure ( P H2O ) greater than 1 × 10−4 MPa or (2) Ar with oxygen partial pressure ( P O2 ) of between 7 × 10−6 and 1.5 × 10−5 MPa. However, the strength of the material was degraded when the P H2O in H2 was lower than 1 × 10−4 MPa, and essentially unaffected when the P O2 in Ar was higher than 1.5 × 10−5 MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y2 Si2 O7 layer formed during exposure. 相似文献
11.
A study of the solid solution of TiO2 , Fe2 O3 , and Cr2 03 in mullite was made by measuring the changes in lattice parameters and unit-cell volume. Synthetic mullite (3O3 -2SiO2 ) was reacted with up to 12 weight % of the oxides at temperatures ranging from 1000° to 17000C. The approximate minimum temperature required for the formation of solid solution was 12000C. for Fe2 03 and 1400°C. for Cr2 O3 and TiO3 . The maximum amount of solid solution found was 2 to 4% TiO2 at 1600°C., 10 to 12% Fe2 Os at 1300°C., and 8 to 10% CrZ O3 at 1600OC. Lattice parameters and unit-cell volumes for each solid solution series increased with increasing amounts of foreign oxide. There was good agreement between the calculated and observed increase in cell dimensions for the iron oxide series. Except in the case of titania, there was good agreement between X-ray data and petrographic observations. 相似文献
12.
The phase relations in the system U02-U03-Yz03, particularly in the Y203-rich region, were examined by X-ray and chemical analyses of reacted powders heated at temperatures up to 1700°C in H2 , CO2 -CO2 and air. Four phases were identified in the system at temperatures between 1000° and 1700°C: U308, face-centered cubic solid solution, body-centered cubic solid solution, and a rhombohedral phase of composition (U,Y)7 O2 ranging from 52.5 to 75 mole % Y2 O3 . The rhombohedral phase oxidized to a second rhombohedral phase with a nominal composition (U,Y), at temperatures below 1000°C. This phase transformed to a face-centered cubic phase after heating in air above 1000° C. The solubility of UO, in the body-centered cubic phase is about 14 mole % between 1000° and 1700°C but decreases to zero as the uranium approaches the hexavalent oxidation state. The solubility of Yz03 in the face-centered cubic solid solution ranges from 0 to 50 mole % Y2O3 under reducing conditions and from 33 to 60 mole % Y2O3 under oxidizing conditions at 1000°C. At temperatures above 1000° C, the face-centered cubic solid solution is limited by a filled fluorite lattice of composition (U,Y)O2 . For low-yttria content, oxidation at low temperatures (<300°C) permits additional oxygen to be retained in the structure to a composition approaching (U,Y)O2.25 A tentative ternary phase diagram for the system UO2 -UO3 -Y2 O3 is presented and the change in lattice parameter and in cell volume for the solid-solution phases is correlated with the composition. 相似文献
13.
A. Cüneyt Tas 《Journal of the American Ceramic Society》1999,82(6):1582-1584
Preparation of phase-pure PZT (Pb(Zr0.52 Ti0.48 )O3 ) powders was achieved, in the presence of urea (CH4 N2 O), by homogeneous precipitation. Aqueous solutions of PbCl2 , ZrOCl2 ·8H2 O, and TiCl4 were used as the starting materials in the synthesis of phase-pure PZT powders. Phase evolution behavior of precursor powders was studied by powder X-ray diffraction (XRD) in air, over the temperature range of 90° to 750°C. The morphology of the formed powders was studied by scanning electron microscopy (SEM). Semiquantitative chemical analyses of the samples were performed by energy-dispersive X-ray spectroscopy (EDXS). 相似文献
14.
The quenching technique was used to study subliquidus and subsolidus phase relations in the pseudobinary system Na2 Ti2 Si2 O11 -Na2 Ti2 Si2 O9 . Both narsarukite (Na2 TiSi4 O11 ) and lorenzenite (Na2 Ti2 Si2 O9 ) melt incongruently. Narsarsukite melts at 911°±°C to SiO2 +liquid, with the liquidus at 1016°C. Lorenzenite melts at 910°±5°C to Na2 Ti6 O13 +liquid; Na2 Ti6 O13 reacts with liquid to form TiO2 and is thus consumed by 985°±5°C. The liquidus occurs at 1252°C. 相似文献
15.
Phase equilibrium relations in the system PbO–TiO2 –ZrO2 were studied by quenching in the range where the PbO content is 50 mole % and more. Isotherms were examined at 1100°, 1200°, and 1300°C and tie lines were determined between the liquid and solid solution in equilibrium. The incongruent melting point of PbZrO3 was 1570°C and the equilibrium between liquid, PbO-type solid, and PbZrO3 is peritectic. Pb(Zr,Ti)O3 solid solutions containing more than 14 mole % PbZrO3 decomposed to liquid, ZrO2 , and Pb(Zr,Ti)O3 and the decomposition temperature rises from 1340° to 1570°C with increasing PbZrO3 content. The system PbTiO3 –PbZrO3 should not be treated as a binary, but as a section of the ternary system. 相似文献
16.
Ingeborg Kaus Kjell Wiik Bente Krogh Marit Dahle Karina H. Hofstad Siv Aasland 《Journal of the American Ceramic Society》2007,90(7):2226-2230
The chemical stability of SrFeO3 -based perovskites in H2 O- and CO2 -containing atmospheres at high temperatures and pressures has been examined. The extent of reaction as a function of p CO2 , p H2 O, temperature, and time has been determined. Either strontium carbonate or Sr(OH)2 ·H2 O was observed on sample surfaces after exposure. Observation of two different reaction-rate behaviors could be explained by the formation of different products. The stability of the perovskite has been found to increase when the activity of Sr is decreased. Chemical stability in H2 O/CO2 is important to understand in order to use these membrane materials for syngas production. 相似文献
17.
The formation of BaTiO3 from equimolar BaCO3 and TiO2 (rutile) mixtures was studied in air and in CO2 . A small amount of BaTiO3 is formed first directly from BaCO3 and TiO2 at the surface of contact. From then on it is a diffusion-controlled reaction, and both BaTiO3 and Ba2 TiO4 are produced, with Ba2 TiO4 being formed in much larger amounts. In 1 atmosphere of CO2 , the intermediate Ba2 TiO4 was suppressed up to a temperature of about 1100°C. in agreement with thermodynamic calculations. Ba2 TiO4 reacts fast with 1 atmosphere of CO2 below about 1100°C. to produce BaTiO3 and BaCO3 相似文献
18.
Polycrystalline Al2 O3 was chemically vapor-deposited onto sintered Al2 O3 substrates by reaction of AlCl3 with (1) H2 O, (2) CO:H2 , and (3) O2 at 1000° and 1500°C and 0.5 and 5.0 torr. Although the thermodynamics of all these reactions predict the formation of solid Al2 O3 , the deposition rate of the first reaction was considerably greater than that of the second. The third reaction was so slow that no measurable deposit was formed in 6 h at 1500°C. Formation of dense deposits of α-Al2 O3 was favored by increasing temperature and decreasing pressure. Microstructural examination of the dense deposits showed long columnar grains, the largest of which extended through the deposit from the substrate to the surface. 相似文献
19.
The rate of permeation of CaO-stabilized ZrO2 (CSZ) by O2 gas was measured from 640° to 1200°C with the CSZ tubing used simultaneously as the sample and the O2 pressure detector. The apparent permeation rate depended significantly on the O2 pressure at the low-pressure side. The rate measured by this method was orders of magnitude smaller than that measured under steady-state conditions, except when the O2 partial pressure was high (>10−4 atm), in which case the agreement was good. The difference between steady-state permeability and non-steady-state permeability is related to the deviation in stoichiometry in a sample or detector. The transient response (measured under variable pressure difference) may be very different from steady-state permeation (measured under constant differential pressures across the membranes). To apply CSZ to typical O2 gas permeability measurements, the O2 pressure must be kept above ∼ 10−3.5 atm. In this range, the permeability of CSZ may be regarded as a temperature-dependent material property which is governed by the electron-hole mobility. At lower O2 pressures the permeation rate is a more complex function of the pressure difference and level. 相似文献
20.
DELLA M. ROY 《Journal of the American Ceramic Society》1958,41(8):293-299
The nature of the low-temperature inversions γ-α' and α'-β was investigated by various techniques: hydrothermal and "dry" quenching runs, differential thermal analysis at atmospheric and elevated nitrogen pressures, X-ray diffractometer patterns obtained at elevated temperatures, "static" pressure techniques, and infrared absorption spectrometry. A revised energy-temperature diagram is presented for Ca2 SiO4 , with the transition γ' to α' taking place at about 725°C. and the α'-β transition, although not reversible at an exact temperature, taking place at about 670° C. At low water pressures (2000 lb. per sq. in.) the inversion γ-α' was placed at 675°C. Attempts to extrapolate the value obtained at 2000 lb. per sq. in. to obtain a more accurate reversible inversion temperature at atmospheric pressure, although limited in accuracy by the reliability of heat-of-transition data, would indicate a temperature of about 725° C. at atmospheric pressure. Three new compounds, 8CaO.3SiO2 -3H2 O (X), 6CaO 3SiO2 .H2 O (Y), and 9CaO-6SiO2 H2 O (Z), were found to be stable above 700°C. at H2 O pressures greater than 7500 lb. per sq. in. 相似文献