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1.
The hydration characteristics of calcium monoaluminate were studied using an effective water/aluminate ratio of 0.15 at 20° or 80°C, from a few minutes of two months. The material hydrated at 80°C shows a large shrinkage while at the lower temperature a continuous expansion occurs. The product at 80°C shows a much higher strength than that hydrated at 20°C. The main initial hydration products are 2Ca0, Aℓ 20 3, 8H 20 and alumina gel. Microcracks are developed in the products hydrated at 20°C while at the higher temperature a very compact mass results. The data indicate that it is possible to obtain a durable high alumina cement by using a low water/cement ratio and hydrating at higher temperatures, and under these conditions C 3AH 3---C 3AH 6 bond is favoured. 相似文献
2.
Rhodium catalysts, supported on six γ-Al 2O 3 supports with different crystallinities, were exposed to sequential treatments in hydrogen at 500°C, in oxygen at 760°C, in hydrogen at 500°C and at 760°C, respectively. Samples were characterized by X-ray diffraction and hydrogen chemisorption at various stages in the sequential treatment. Based on the characterization results, it is concluded that the formation of crystalline Rh 2O 3 is a function of γ-Al 2O 3 crystallinity; formation of crystalline Rh 2O 3 increased with increasing crystallinity of γ-Al 2O 3 during treatment in oxygen at 760°C. The crystalline Rh 2O 3 formed during treatment in oxygen at 760°C was reduced to Rh metal by hydrogen at 500°C, but most of the Rh did not adsorb hydrogen at room temperature. Subsequent treatment in hydrogen at 760°C increased the hydrogen adsorption capacity by as much as a factor of three. X-ray line broadening measurements showed that oxygen treatment of reduced Rh/γ-Al 2O 3 at 760°C followed by hydrogen reduction at 500°C resulted in significant increases in Rh crystal size; further treatment in hydrogen at 760°C resulted in additional sintering of Rh. 相似文献
3.
In the present study, we have examined sulfation of cerium oxide via impregnation of (NH 4) 2SO 4, followed by heating in the temperature range of 220–720°C, using Raman Spectroscopy. Based on the SO and SO stretching frequencies in the range of 900–1400 cm −1, a wide range of surface oxysulfur species and bulk cerium-oxy-sulfur species are identified. At 220°C, a mixture of (NH 4) 2SO 4 crystals, SO 2−4(aq) and HSO 1−r(aq) is found to have formed on ceria's surface, whereas complete conversion of (NH 4) 2SO 4 to SO 2−4(aq) and HSO 1−4(aq) occurs at 280°C. At 350°C, formation of a mixture of surface pyrosulfate S 2O 2−7(surf.0, consisting of two SO oscillators and a bulk type compound identified as Ce(IV)(SO 4) x(SO 3) 2−x (0 < x < 2) have been observed. Upon introduction of moisture, the former transforms to HSO 1−4(surf.), whereas the latter remains unchanged. At 400°C, only the bulk type compound can be observed. At 450°C, only Ce 2(SO 4) 3 is generated and remains stable until 650°C. Further increase in the temperature to 720°C results in the formation of CeOSO 4. The present study not only provides a more thorough understanding of the sulfation of cerium oxide at a molecular level, but also demonstrates that Raman spectroscopy is a highly effective technique to characterize sulfation of metal oxides. 相似文献
4.
The storage and release of NO 2 on alumina-supported barium oxide has been studied with particular attention to the stoichiometry of the two processes. At 400 °C the storage process is characterised by a short period of complete uptake, possibly as nitrito or nitro species, followed by a slower partial uptake in which approximately one NO is released for every three NO 2 lost. The latter reaction appears to supply the oxygen necessary to store NO 2 as nitrate ions. Molecular O 2 has little direct involvement even if in large excess. The second storage reaction also occurs, but to a much lesser extent, with Al 2O 3 alone. During temperature programmed desorption, release of NO x from Al 2O 3 peaks at 430 °C with evolution of NO 2 and some O 2. Release from BaO/Al 2O 3 exhibits an additional peak near 520 °C corresponding to formation of NO and a higher O 2 concentration. The NO may arise from NO 2 since BaO/Al 2O 3 has activity for NO 2 decomposition by 500 °C. Although CO 2 at low concentration is rapidly taken up by BaO/Al 2O 3 at 400 °C it is displaced by NO 2 and does not interfere with storage. Thermodynamic calculations show that the formation of Ba(NO 3) 2 by the reaction of NO 2 with bulk BaCO 3 under the conditions used here is more favourable above 380 °C if NO is evolved than if O 2 is consumed. 相似文献
5.
Well crystallised aluminium borate Al 18B 4O 33 has been synthesised from alumina and boric acid with a BET area of 18 m 2/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al 18B 4O 33 was prepared by conventional impregnation of Pd(NO 3) 2 aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al 18B 4O 33 in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al 2O 3. Pd/Al 18B 4O 33 exhibited a much lower activity than Pd/Al 2O 3 when treated in hydrogen at 500 °C or aged in O 2/H 2O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al 18B 4O 33 which became as active as Pd/Al 2O 3. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al 2O 3 and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al 18B 4O 33. The existence of two distinct types of PdO species formed on Al 18B 4O 33 and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O 2. 相似文献
6.
The system Al 2O 3–ZrO 2 was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO 2. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al 2O 3. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO 2 transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO 2, with 0.7±0.2% mass Al 2O 3 at this temperature, transforms at 1085±5°C. Inverse transitions occur with hysteresis correspondingly at 1055±5 and 995±5°C. Enthalpy of transformation of pure ZrO 2 from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO 2 saturated solid solution. 相似文献
7.
The influence of sintering and poling conditions on dielectric properties and microstructures of the system 0·125Pb(Mg 1/3Nb 2/3)O 3−0·875Pb (Zr 0·5Ti 0·5)O 3 was investigated. Specimens were prepared by the conventional mixed-oxide technique. On account of eliminating the pyrochlore phase and lowering the sintering temperature, the calcined 0·125PZT−0·875PMN ceramic was doped with 4PbO.B 2O 3 glass powder. The 4PbO.B 2O 3 glass frit not only has a low flow temperature, but also a high polarizability. Additions of 4PbO.B 2O 3 to the perovskite 0·125PMN–0·875PZT solid solution will form a liquid phase, which served as a densification aid for the ceramics. With additions of 0·2 wt% glass frit, densities in excess of 98% of theoretical were obtained after sintering at 115°C. By variation of the fabrication processes, the influence of sintering and poling conditions on the properties of the ceramics was studied. 相似文献
8.
The required sintering temperature of Pb(Zr 0·52Ti 0·48)O 3 ceramics (abbreviated as PZT 52/48) can be lowered to about 1000°C by incorporating Li 2CO 3, Na 2CO 3 or Bi 2O 3. A dielectric constant of about 1000 and a planar coupling factor of between 45% and 65% are obtained in PZT 52/48 ceramics sintered at 1025°C, with added Li 2CO 3 and Bi 2O 3. The optimal amount of the additives, which can be deduced from the densification, the dielectric and piezoelectric properties of the sintered PZT 52/48 ceramics, is 0·375 wt% of Li 2CO 3 together with an equal mole fraction of Bi 2O 3. A planar coupling factor of 65% is obtained. This is explained, with the aid of X-ray diffraction (XRD) analysis, by a maximum c/ a ratio and consequently by a large spontaneous polarization. The PZT 52/48 ceramics sintered with Li 2CO 3 and Bi 2O 3 under the optimal conditions can have ε 33T of about 1000, kp higher than 60%, Qm around 100 and tan δ less than 2·0%. 相似文献
9.
The durability of plasma-sprayed metals bonded with a polyimide adhesive has been studied. Metal adherend surfaces were prepared for adhesive bonding by plasma-spraying inorganic powders on aluminum and titanium. The plasma-sprayed materials included Al 2O 3, AlPO 4. MgO, and SiO 2 on aluminum, and TiO 2, TiSi 2, MgO, and SiO 2 on titanium. The coatings were sprayed at two different thicknesses. Durability studies of samples prepared in a wedge-type geometry were carried out. Bonded specimens were maintained in an environmental cycle that included exposure to the conditions; low temperature, - 20°C; relative humidity at elevated temperature, 70% RH at 66°C; elevated temperature (160°C) in air, high temperature (160°C) in vacuum (130 torr, 0.2 atm.), and room temperature. Crack growth rate and mode of failure were determined. The results of the durability tests indicate that thin coatings (25 μm) of plasma-sprayed materials perform better than thicker (150 μm) coatings. The crack growth rate for thin coatings (25 μm) of Al 2O 3, AlPO 4, SiO 2, and MgO plasma-sprayed on aluminum was equivalent to that for phosphoric acid anodized aluminum. Similarly, the durability performance for titanium samples prepared with a 25 μm-thick TiO 2, TiSi 2, and SiO 2 plasma-sprayed coatings was equivalent to that for a Turco®-prepared titanium surface. Although the evaluation of durability as a function of surface chemistry was an objective of the study, it was not possible to evaluate the effect, since most failures occurred within the adhesive (cohesive failure) during the environmental tests. That failure occurred in the adhesive indicates that the coating-adherend and the coating-adhesive interactions are sufficiently robust to prevent interfacial failure under the experimental conditions investigated. 相似文献
10.
The thermal decomposition up to 400 °C of ammonium ferric citrate hydrate, of unknown structure and formula weight, was studied by thermogravimetry, differential thermal analysis, infrared (IR) spectroscopy and X-ray diffractometry. The possible identities of the formula weight and the intermediate products of calcination are discussed. The results revealed that the parent material is amorphous and contains two moles of water and two moles of ammonia. Decomposition takes place via six weight-loss processes, three endothermic (90–230 °C) and three exothermic (240–298 °C), leading eventually to the formation of Fe 2O 3. The intermediate solid products are mainly unstable amorphous oxycarbonates, as indicated by X-ray and IR spectroscopies. The gas-phase decomposition products identified by IR spectroscopy are NH 3, CO 2, CO, CH 3COCH 3, CH 4 and NH 4OH. Surface area measurement and scanning electron microscopy showed that Fe 2O 3, the final product at 400 °C, hada surface area of 40 m 2/g and good crystalline and porous character. 相似文献
11.
Dispersing La 2O 3 on δ- or γ-Al 2O 3 significantly enhances the rate of NO reduction by CH 4 in 1% O 2, compared to unsupported La 2O 3. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La 2O 3 precursor used, the pretreatment, and the La 2O 3 loading. The most active family of catalysts consisted of La 2O 3 on γ-Al 2O 3 prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m 2) occurred between the addition of one and two theoretical monolayers of La 2O 3 on the γ-Al 2O 3 surface. The best catalyst, 40% La 2O 3/γ-Al 2O 3, had a turnover frequency at 700°C of 0.05 s −1, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La 2O 3/Al 2O 3 catalysts exhibited stable activity under high conversion conditions as well as high CH 4 selectivity (CH 4 + NO vs. CH 4 + O 2). The addition of Sr to a 20% La 2O 3/γ-Al 2O 3 sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO =4 to the latter Sr-promoted La 2O 3/Al 2O 3 catalyst decreased activity although sulfate increased the activity of Sr-promoted La 2O 3. Dispersing La 2O 3 on SiO 2 produced catalysts with extremely low specific activities, and rates were even lower than with pure La 2O 3. This is presumably due to water sensitivity and silicate formation. The La 2O 3/Al 2O 3 catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications. 相似文献
12.
A 1% Pd catalyst (38% dispersion) was prepared by impregnating a γ-alumina with palladium acetylacetonate dissolved in acetone. The behaviour of this catalyst in oxidation and steam reforming (SR) of propane was investigated. Temperature-programmed reactions of C 3H 8 with O 2 or with O 2 + H 2O were carried out with different stoichiometric ratios S( S =[O 2]/5[C 3H 8]). The conversion profiles of C 3H 8 for the reaction carried out in substoichiometry of O 2 ( S < 1) showed two discrete domains of conversion: oxidation at temperatures below 350°C and SR at temperatures above 350°C. The presence of steam in the inlet gases is not necessary for SR to occur: there is sufficient water produced in the oxidation to form H 2 and carbon oxides by this reaction. Contrary to what was observed with Pt, an apparent deactivation between 310 and 385°C could be observed with Pd in oxidation. This is due to a reduction of PdO x into Pd 0, which is much less active than the oxide in propane oxidation. Steam added to the reactants inhibits oxidation while it prevents the reduction of PdO x into Pd 0. Compared to Pt and to Rh, Pd has a higher thermal resistance: no deactivation occurred after treatment up to 700°C and limited deactivation after treatment up to 900°C, provided that the catalyst is maintained in an oxygen-rich atmosphere during the cooling. 相似文献
13.
The influence of catalyst pre-treatment temperature (650 and 750 °C) and oxygen concentration ( λ = 8 and 1) on the light-off temperature of methane combustion has been investigated over two composite oxides, Co 3O 4/CeO 2 and Co 3O 4/CeO 2–ZrO 2 containing 30 wt.% of Co 3O 4. The catalytic materials prepared by the co-precipitation method were calcined at 650 °C for 5 h (fresh samples); a portion of them was further treated at 750 °C for 7 h, in a furnace in static air (aged samples). Tests of methane combustion were carried out on fresh and aged catalysts at two different WHSV values (12 000 and 60 000 mL g−1 h−1). The catalytic performance of Co3O4/CeO2 and Co3O4/CeO2–ZrO2 were compared with those of two pure Co3O4 oxides, a sample obtained by the precipitation method and a commercial reference. Characterization studies by X-ray diffraction (XRD), BET and temperature-programmed reduction (TPR) show that the catalytic activity is related to the dispersion of crystalline phases, Co3O4/CeO2 and Co3O4/CeO2–ZrO2 as well as to their reducibility. Particular attention was paid to the thermal stability of the Co3O4 phase in the temperature range of 750–800 °C, in both static (in a furnace) and dynamic conditions (continuous flow). The results indicate that the thermal stability of the phase Co3O4 heated up to 800 °C depends on the size of the cobalt oxide crystallites (fresh or aged samples) and on the oxygen content (excess λ = 8, stoichiometric λ = 1) in the reaction mixture. A stabilizing effect due to the presence of ceria or ceria–zirconia against Co3O4 decomposition into CoO was observed. Moreover, the role of ceria and ceria–zirconia is to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co3O4 and by promoting the reduction at low temperature. 相似文献
14.
The coupled substitution of Bi3+ for Pb2+ and B3+ for Ge4+ was successfully achieved in ferroelectric Pb 5Ge 3O 11. Large single crystals of optical quality were grown from the melt. Deterioration of the crystal quality could not be observed within a period of two years under ambient conditions. The dielectric permittivity ε r, electric conductivity p and pyroelectric coefficient γ are investigated as a function of temperature. The room temperature data (εr = 43, γ = 2.4·10 -4Cm-2 K-1, p = 7·107 Ω, volume specific heat s = 2·10 6 Jm-3K-1) show the usefulness of Pb 5-xBi xGe 3-xB xO 11 around x = 0.1 as a material for pyroelectric thermal detectors operating at room temperature. Also the difficulty of producing lead germanate silicate mixed crystals in the size and quality needed for vidicon applications is overcome by additional replacement of some Pb and Ge by Bi and B. 相似文献
15.
Powders of pure and 5% ytterbium substituted strontium cerate (SrCeO 3/SrCe 0.95Yb 0.05O 3−δ) were prepared by spray pyrolysis of nitrate salt solutions. The powders were single phase after calcination in nitrogen atmosphere at 1100 °C (SrCeO 3) and 1200 °C (SrCe 0.95Yb 0.05O 3−δ). Dense SrCeO 3 and SrCe 0.95Yb 0.05O 3−δ materials were obtained by sintering at 1350–1400 °C in air. Heat treatment at 850 and 1000 °C, respectively, was necessary prior to sintering to obtain high density. The dense materials had homogenous microstructures with grain size in the range 6–10 μm for SrCeO 3 and 1–2 μm for SrCe 0.95Yb 0.05O 3−δ. The electrical conductivity of SrCe 0.95Yb 0.05O 3−δ was in good agreement with reported data, showing mixed ionic–electronic conduction. The ionic contribution was dominated by protons below 1000 °C and the proton conductivity reached a maximum of 0.005 S/cm above 900 °C. In oxidizing atmosphere the p-type electronic conduction was dominating above 700 °C, while the contribution from n-type electronic conduction only was significant above 1000 °C in reducing atmosphere. 相似文献
16.
In a paste CA 2 hydrates very slowly at 20°C but the reaction is more rapid at 40°C. At both temperatures the products of hydration comprise C 2AH 8, gibbsite and alumina gel after 7 days. For mixtures of CA 2 + CA the initial hydration at 20°C is dominated by CA. In the presence of ground granulated blastfurnace slag CA 2 or a mixture of CA 2 + CA will form strätlingite on hydration at 40°C within 3 days. Slag addition reduces the amount of heat evolved during the first 24 hours of hydration at both 20° and 40°C. 相似文献
17.
Spontaneous precipitation in the aqueous system Mg +2-Na +-SO 3-2-SO 4-2 affected by mixing solutions of MgSO 4 and Na 2SO 3, together was studied for temperature varying from 40 to 80°C and for pH from 5,5 to 9. The initial composition of a precipitating system was 0·67 and 1·17 mol of MgSO 3 and Na 2SO 4 per liter and 0·82 mol of MgSO 3 and Na 2SO 4 together with 0·83 mol of MgSO 4 per liter, respectively. Depending on the prevailing reaction conditions, solid phase consisting of MgSO 3 6H 2O, MgSO 3 3H 2O, Mg 2NaOH(SO 3) 2 H 2O or their mixture is formed. Each solid phase forms crystals of typical size and shape. The precipitation diagrams drawn in the temperature and pH coordinates for three different initial composition of the studied system are presented. 相似文献
18.
The nucleation and growth of Pd clusters in mordenite were investigated using in situ extended X-ray absorption fine structure (EXAFS) spectroscopy and Fourier transform infrared (FTIR) spectroscopy of absorbed CO. Calcination of [Pd(NH 3) 4] 2+-exchanged mordenite at 350°C in O 2 results in decomposition of the amine complex and formation of square-planar Pd 2+ oxo species within the mordenite pores. Reduction of these species at 150°C in H 2 yields Pd clusters with an average nuclearity of 3. On an average two 2.22 Å Pd-O bonds are associated with each Pd 3 cluster; we infer that this interaction serves to anchor the clusters within the pores. After reduction at 150°C, the FTIR spectrum of irreversibly adsorbed CO is indicative of a mixture of Pd +, Pd δ+, and Pd 0 carbonyl species. Reduction at 350°C produces larger intrazeolitic Pd clusters (average nuclearity of 6) that exhibit only a weak interaction with the mordenite, as evidenced by their facile aggregation in the presence of CO at 30°C. Reduction at 450°C yields large 20 Å Pd clusters that we infer are located on external mordenite surfaces or locally disrupt the intracrystalline structure. 相似文献
19.
Iron (III) adsorption from aqueous solutions onto raw and pretreated clinoptilolite was investigated here. Various parameters for iron removal; initial solution pH, contact time and metal ion concentration were optimized. The equilibrium data were modeled by both the Langmuir and Freundlich adsorption isotherms at optimal conditions. Adsorption capacities of raw samples and those pretreated with Na 2S 2O 8 at 20 °C , 70 °C and with HNO 3 at 20 °C were all similar but samples pretreated with HNO 3 at 70 °C were significantly different; iron (III) removal from samples pretreated with HNO 3 decreased with increasing pretreatment temperature. Tests with Fe +3 solutions containing phenol, CsCl or KCl, indicated the continued presence of these ions in zeolite which either promoted or retarded the adsorption of iron. The Fe +3 adsorption capacity of clinoptilolite pretreated with HNO 3 at 70 °C was about two times greater with, than without, CsCl and KCl. The kinetics of iron adsorption from aqueous solution were also investigated using the first-order Lagergren equation and a pseudo-second-order model. 相似文献
20.
(CH 3)NH gBrI 2 is an electro-optic material which shows a structural phase transition with a symmetry change from orthorhombic to monoclinic at T = 108°C upon heating. In this contribution we report first experimental data on the optical properties (refractive indices, absorption coefficients), on the elastic and electro-optical properties. It is shown, that the dispersion of the refractive indices is adequately described by a single term Sellmeier oscillator model. Measurements of the temperature dependence of the birefringence and of the elastic constants show pronounced changes at the structural phase transition. The biaxial crystal with the three main indices n a = 1.761±0.001, n b = 1.802±0.001 and n c = 1.755±0.001 at room temperature and a wavelength λ=633nm becomes optically uniaxial at T = (50±0.5)°C with n a=n c=1.750, due to different temperature dependences n a(T) and n b(T). Room temperature electro-optic coefficients and the corresponding half-wave voltages have been determined, and indicate that the electro-optic activity of (CH 3) 4NH gBrI 2 is about two orders of magnitude smaller than in KH 2PO 4. 相似文献
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