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1.
The chemical compatibility of perovskite-type Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3?δ (BSCF) oxides with Cr 2O 3 has been examined between room temperature and 1,100 °C. Differential thermal analysis and thermogravimetric analysis were used to analyze the thermal behavior of BSCF–Cr 2O 3 binary mixtures in all composition ranges (0–100 mass% BSCF). The reaction products were identified by X-ray analysis after heating at 700–1,100 °C. As we expected, it was found that perovskite-type BSCF oxide had a poor chemical compatibility with the Cr 2O 3 oxide. In particular, the decomposition process of the BSCF–Cr 2O 3 binary mixture is quite complex and it starts at about 700–750 °C. The mixtures of BSCF and Cr 2O 3 oxides reacted forming mixed complex oxides based on (Ba/Sr)FeO 3, (Co/Fe)CrO 4, and (Ba/Sr)CrO 4 mixtures. 相似文献
2.
LiCr 0.2Ni 0.4Mn 1.4O 4 was synthesized by a sol–gel technique in which tartaric acid was used as oxide precursor. The synthesized powder was annealed at five different temperatures from 600 to 1,000 °C and tested as a 5-V cathode material in Li-ion batteries. The study shows that annealing at higher temperatures resulted in improved electrochemical performance, increased particle size, and a differentiated surface composition. Spinel powders synthesized at 900 °C had initial discharge capacities close to 130 mAh g ?1 at C and C/2 discharge rates. Powders synthesized at 1,000 °C showed capacity retention values higher than 85 % at C/2, C, and 2C rates at 25 °C after 50 cycles. Annealing at 600–800 °C resulted in formation of spinel particles smaller than 200 nm, while almost micron-sized particles were obtained at 900–1,000 °C. Chromium deficiency was detected at the surface of the active materials annealed at low temperatures. The XPS results indicate presence of Cr 6+ impurity when the annealing temperature was not high enough. The study revealed that increased annealing temperature is beneficial for both improved electrochemical performance of LiCr 0.2Ni 0.4Mn 1.4O 4 and for avoiding formation of Cr 6+ impurity on its surface. 相似文献
3.
Ni‐based superalloys with niobium (Nb) or/and yttrium (Y) were prepared by vacuum melting. The oxidation kinetics of these alloys was studied by thermogravimetry at 800 °C for 100 h in static air. Morphology of oxides was studied using SEM, and the composition was analyzed by X‐ray diffraction. Energy‐dispersive X‐ray spectrometer was employed to examine the linear element distribution of the cross section of the oxidation films. Results showed that the oxidation kinetics all followed a parabolic law at different stages. The oxide films were mainly comprised of Cr 2O 3, NiCr 2O 4, Al 2O 3 and TiO 2. All the oxide films exhibited layered structure owing to different oxidation stages. With the addition of Nb or Y, the high‐temperature oxidation resistance of the superalloy was improved significantly and the surface morphology of the oxidation film was ameliorated. The comprehensive effect of Nb and Y was remarkable in improving the high‐temperature oxidation resistance of Ni‐based alloys. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
4.
Manganese ferrite nanopowder was prepared by a new solvothermal method, using 1,2 propanediol as solvent and KOH as precipitant. The as-synthesized powder, by solvothermal treatment in autoclave at 195 °C, for 12 h, consisted of fine manganese ferrite nanoparticles. The further thermal treatment of the initial manganese ferrite powder to higher temperature resulted in manganese ferrite decomposition due to Mn(II) oxidation to Mn(III), as observed by X-ray diffraction. FT-IR spectroscopy has evidenced that the oxidation takes place even at 400 °C. The oxidation of Mn(II) to Mn(III) was studied by TG/DSC simultaneous thermal analysis. It was shown that Mn(II) oxidation takes place in a very small extent up to 400 °C. The main oxidation step occurs around 600 °C, when a clear mass gain is registered on TG curve, associated with a sharp exothermic effect on DSC curve. The exothermic effect is smaller in case of the powder annealed at 400 °C, confirming the superficial oxidation of Mn(II) up to 400 °C. In order to avoid Mn(II) oxidation, the powder obtained at 400 °C was further annealed at 800 °C in argon atmosphere, without degassing, when manganese ferrite MnFe 2O 4 was obtained as major crystalline phase (69 %). All manganese ferrite powders showed a superparamagnetic behavior, with maximum magnetization of 51 emu g ?1 in case of the as-synthesized powder, characteristic of magnetic ferrite nanopowders. 相似文献
5.
In view of the susceptibility of TiB 2 to oxidation, the thermal stability of monolithic TiB 2 and of Al 2O 3-30 vol% TiB 2 and Si 3N 4-20 vol% TiB 2 composites was investigated. The temperature at which TiB 2 ceramic starts to oxidize is about 400°C, oxidation kinetics being controlled by diffusion up to T≈900°C and in the first stage of the oxidation at 1000°C and 1100°C (up to 800 min and 500 min respectively), and by a linear law at higher temperatures and for longer periods. Weight gains in the Al 2O 3-TiB 2 composite can be detected only at temperatures above ≈700°C and the rate governing step of the oxidation reaction is characterized by a one-dimensional diffusion mechanism at T=700°C and T=800°C and by two-dimensional diffusion at higher temperatures. Concerning the Si 3N 4-TiB 2 composite, three different oxidation behaviours related to the temperature were observed, i.e. up to ≈1000°C the reaction detected regards only the second phase; at ≈1000< T<≈1200°C, the diffusion of O 2 or N 2 through an oxide layer is proposed as the rate-governing step; at T〉=1200°C, a linear kinetic indicates the formation of a non protective scale. 相似文献
6.
This paper presents a study regarding the obtaining of NiCr 2O 4 by two new unconventional synthesis methods: (i) the first method is based on the formation of Cr(III) and Ni(II) carboxylate-type precursors in the redox reaction between the nitrate ion and 1,3-propanediol. The thermal decomposition of these complex combinations, at ~300 °C, leads to an oxide mixture of Cr 2O 3+x and NiO, with advanced homogeneity, small particles and high reactivity. On heating this mixture at 500 °C, Cr 2O 3 reacts with NiO to form NiCr 2O 4, which was evidenced by FT-IR and X-ray diffractometry (XRD) analysis; (ii) the second method starts from a mechanical mixture of (NH 4) 2Cr 2O 7 and Ni(NO 3) 2·6H 2O. On heating this mixture, a violent decomposition at 240 °C with formation of an oxides mixture (Cr 2O 3 + CrO 3) and NiO takes place. On thermal treatment up to 500 °C, an intermediary phase NiCrO 4 is formed, which by decomposition at ~700 °C leads to NiCr 2O 4, evidenced by FT-IR and XRD analysis. NiCr 2O 4 is formed, in both cases, starting with a temperature higher than 400 °C, when the non-stoichiometric chromium oxide (Cr 2O 3+x ) loses the oxygen excess and turns to stoichiometric chromium oxide (Cr 2O 3), which further reacts with NiO. 相似文献
7.
In this paper, novel low-cost oxygen carriers containing Fe 2O 3 are evaluated for use in chemical looping combustion. Sewage sludge ashes and reference samples were prepared and used in cyclic reduction and oxidation experiments in a thermogravimetric analyzer (TG). A gaseous (3 % H 2) fuel and a solid fuel (hard coal) were tested. Three-cycle CLC tests were carried out in the 600–800 °C temperature range and long-term testing was performed at 950 °C. A reactivity study showed that the natural sewage sludge ash sample was stable during the cycling TG tests when hydrogen was used as a fuel at all of the temperatures investigated. Strong temperature effects on the oxygen transport capacity were observed. An one-cycle test at 900 °C showed also that the sewage sludge ash successfully reacted with coal. The oxygen released was fully used for coal combustion, with appreciable reaction rate at temperature of ~750–800 °C, that is significantly lower than that obtained for pure Fe 2O 3-based oxygen carrier. The oxidation reaction was much faster than the reduction reaction. Moreover, the sewage sludge ash showed a low tendency toward agglomeration in the cyclic test, which was superior to the behavior of synthetic materials. The sewage sludge ash exhibited also high mechanical strength, an attrition index of 1 % and a high-temperature resistance of 1,170 °C in a reducing atmosphere. We conclude that sewage sludge ash can be effectively used as a low-cost, valuable oxygen carrier in practical application in chemical looping combustion technology for power generation. 相似文献
8.
A Ti–45.9Al–8Nb
(at%) alloy with a lamellar structure (γ+α 2)
was oxidised in air at 700, 800, 850 and 900°C in isothermal and thermal
cycling conditions. The reaction progress was followed by thermogravimetric
measurements. In isothermal conditions the oxidation kinetics followed approximately
a parabolic rate law and the rate constants ranged from about 10 –12
kg 2 m –4 s –1
at 700°C to 10 –10 kg 2
m –4 s –1
at 900°C. The oxide scales were built of Al 2O 3
and TiO 2, the former being the main component of the
outermost layer. The oxidation behaviour of Ti–45.9Al–8Nb was
referred to a commercial titanium alloy, WT4 (Ti–6Al–1Mn), and
selected oxidation-resistant alloys. 相似文献
9.
The isothermal and constant heating rate oxidation behaviour of the alloy Fe 78Cr 22 was examined in air with 1% H 2O and in 7% H 2/93% Ar with 1% or 12% H 2O. The measurements were performed in the temperature range 700–1300°C. The effect of surface treatment prior to oxidation
was examined. A Cr 2O 3 scale developed slowly up to900°C. At 1100°C a catastrophic oxidation was observed after heat treatment for 70 h in air with
1% H 2O and in 7% H 2/93% Ar with 12% H 2O. The scale developed in these cases consisted of iron rich oxides such as Fe 2O 3 or FeCr 2O 4, in contrast to the more protective Cr 2O 3 scale seen under other test conditions. Possible causes for the catastrophic oxidation are discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
The nanocrystalline cubic Phase of zirconia was found to be thermally stabilized by the addition of 2.56 to 17.65 mol % Y 2O 3 (5.0 to 30.0 mol % Y, 95.0 to 70.0 mol % Zr cation content). The cubic phase of yttria stabilized zirconia was prepared by thermal decomposition of the hydroxides at 400°C for 1 hr. 2.56 mol % Y 2O 3‐ZrO 2 was stable up to 800°C in an argon atmosphere. The samples with 4.17 to 17.65 mol % Y 2O 3 were stable to 1200°C and higher. All samples at temperatures between 1450°C to 1700°C were cubic except the sample with 2.56 mol % Y 2O 3 which was tetragonal. The crystallite sizes observed for the cubic phase ranged from 50 to 150 Å at temperatures below 900°C and varied from 600 to 800 nm between 1450°C and 1700°C. Control of furnace atmosphere is the main factor for obtaining the cubic phase of Y‐SZ at higher temperature. Nanocrystalline cubic Fe‐SZ (Iron Stabilized Zirconia) with crystallite sizes from 70 to 137 Å was also prepared at 400°C. It transformed isothermally at temperatures above 800°C to the tetragonal Fe‐SZ and ultimately to the monoclinic phase at 900°C. The addition of up to 30 mol % Fe(III) thermally stabilized the cubic phase above 800°C in argon. Higher mol % resulted in a separation of Fe 2O 3. The nanocrystalline cubic Fe‐SZ containing a minimum 20 mol % Fe (III) was found to have the greatest thermal stability. The particle size was a primary factor in determining cubic or tetragonal formation. The oxidation state of Fe in zirconia remained Fe 3+. Fe‐SZ lattice parameters and rate of particle growth were observed to decrease with higher iron content. The thermal stability of Fe‐SZ is comparable with that of Ca‐SZ, Mg‐SZ and Mn‐SZ prepared by this method. 相似文献
11.
La 4Ni 3O 10 oxide was synthesized as a cathode material for intermediate-temperature solid oxide fuel cells by a facile sol–gel method using a nonionic surfactant (EO)106(PO)70(EO)106 tri-block copolymer (F127) as the chelating agent. The crystal structure, electrical conductivity, and electrochemical properties of La 4Ni 3O 10 were investigated by X-ray diffraction, DC four-probe method, electrochemical impedance spectra, and I–V measurements. The La 4Ni 3O 10 cathode showed a significantly low polarization resistance (0.26 Ω cm 2) and cathodic overpotential value (0.037 V at the current density of 0.1 A cm ?2) at 750 °C. The results measured suggest that the diffusion process was the rate-limiting step for the oxygen reduction reaction. The La 4Ni 3O 10 cathode revealed a high exchange current density value of 62.4 mA cm ?2 at 750 °C. Furthermore, an anode-supported single cell with La 4Ni 3O 10 cathode was fabricated and tested from 650 to 800 °C with humidified hydrogen (~3 vol% H 2O) as the fuel and the static air as the oxidant. The maximum power density of 900 mW cm ?2 was achieved at 750 °C. 相似文献
12.
The composite/nanocomposite powders of Mn 0.5Ni 0.5Fe 2O 4/Fe type were synthesized starting from nanocrystalline Mn 0.5Ni 0.5Fe 2O 4 ( D = 7 nm) (obtained by ceramic method and mechanical milling) and commercial Fe powders. The composites, Mn 0.5Ni 0.5Fe 2O 4/Fe, were milled for up to 120 min and subjected to heat treatment at 600 °C and 800 °C for 2 h. The manganese-nickel ferrite/iron composite samples were subjected to differential scanning calorimetry (DSC) up to 900 °C for thermal stability investigations. The composite component phases evolution during mechanical milling and heat treatments were investigated by X-ray diffraction technique. The present phases in Mn 0.5Ni 0.5Fe 2O 4/Fe composite are stable up to 400–450 °C. In the temperature range of 450-600 °C, the interdiffusion phenomena occurs leading to the formation of Fe 1?xMn xFe 2O 4/Ni–Fe composite type. The new formed ferrite of Fe 1?xMn xFe 2O 4 type presents an increased lattice parameter as a result of the substitution of nickel cations into the spinel structure by iron ones. Further increases of the temperature lead to the ferrite phase partial reduction and the formation of wustite-FeO type phase. The spinel structure presents incipient recrystallization phenomena after both heat treatments (600 °C and 800 °C). The mean crystallites size of the ferrite after heat treatment at 800 °C is about 75 nm. After DSC treatment at 900 °C, the composite material consists in Fe 1?xMn xFe 2O 4, Ni structure, FeO, and (NiO) 0.25(MnO) 0.75 phases. 相似文献
13.
CuCr 2O 4 spinel powders were synthesized starting from different chromium sources, namely (i) chromium oxide (α-Cr 2O 3) and (ii) ammonium dichromate ((NH 4) 2Cr 2O 7). The copper source was a Cu(II) carboxylate-type complex. The Cu(II) carboxylate complex was obtained by the redox reaction between Cu(NO 3) 2·3H 2O and 1,3-propanediol (1,3PG) at 130 °C. In the first case (i), we have started from a mixture of α-Cr 2O 3, Cu(NO 3) 2·3H 2O and 1,3PG that upon heating formed the copper malonate complex, which decomposed around 220 °C forming an oxide mixture (CuO + α-Cr 2O 3). In the second case (ii), (NH 4) 2Cr 2O 7, Cu(NO 3) 2·3H 2O and 1,3PG were homogenously mixed. Heating this mixture at 130 °C resulted, in situ, in the Cu(II) complex. On controlled temperature increase, the violent decomposition of (NH 4) 2Cr 2O 7 took place at 180 °C along with the decomposition of the Cu(II) complex, leading to an amorphous oxide mixture of Cr 2O 3+x and CuO. By annealing the samples in the temperature range 400–1000 °C, the spinel phase (CuCr 2O 4) was obtained in both cases: (i) at 800 °C and (ii) at 600 °C as a result of the interactions between the precursors used, when the oxide system was amorphous and highly reactive. The presence of CuCr 2O 4 was highlighted by XRD and FTIR analyses. 相似文献
14.
Cr‐Mn‐O spinel coating was prepared on the surface of cobalt‐based superalloy GH605 via an in‐situ oxidation method in H 2O‐H 2 environment. The composition, morphology, and chemical value state of the oxide spinel coatings were investigated by SEM, EDS, XRD, Raman spectra, and XPS. It indicated that the morphology of coating varied with oxidation temperature, and granular surface appeared when oxidation temperature increased to 1100°C. The formed Cr‐Mn‐O spinel coating was composed of Cr 2O 3 and MnCr 2O 4, and the thickness increased significantly with oxidation temperature. In the coating, Cr element existed in the state of Cr 3+ ions and Cr 6+ ions, while Mn element only existed in the form of Mn 2+ ions. 相似文献
15.
The oxidation behaviour of a B 4C based material was investigated in a dry atmosphere O 2(20 vol.%)-CO 2(5 vol.%)-He and also in the presence of moisture H 2O (2.3 vol%) as boron oxide is very sensitive to water vapour. The mass changes of samples consisting of a chemical vapour
deposit of B 4C on silicon nitride substrates were continuously monitored in the range 500–1000°C during isothermal experiments of 20 h.
The stability of boron oxide formed by oxidation of B 4C was also studied in dry and wet atmospheres to explain the kinetic curves. In both atmospheres, oxidation is diffusion controlled
at 700 and 800°C and enhanced by water vapour. At 900°C and higher temperatures, boron oxide volatilisation and consumption
by reaction with water vapour modifies the properties of the oxide film and the material is no more protected. At 600°C, B 4C oxidation is weak but the process remains diffusion controlled in dry conditions as boron oxide volatilisation is negligible.
However, in the presence of water vapour, B 2O 3 consumption rate is significant and mass losses corresponding to this consumption and to the combustion of the excess carbon
are observed.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
16.
The effect of the oxidation temperature of sintered UO 2 pellets on the powder properties of U 3O 8 was studied in the temperature range 250–900 °C in air. The U 3O 8 was obtained at 450 °C after 180 min and its particle size and surface area are respectively, 35 µm and 0.7 m 2/g. The reduction of the U 3O 8 powder resulted in UO 2 after 30 min with a surface area of 0.8 m 2/g. This value was improved more than 3.5 times by applying five alternating oxidation–reduction cycles. 相似文献
17.
This paper reports an investigation regarding the influence of the cation M(II) ( M = Zn, Ni, Mg) on the formation of MCr 2O 4 by thermal decomposition of the corresponding M(II),Cr(III)-carboxylates (precursors) obtained by redox reaction between the corresponding metal nitrates and 1,3-propanediol. The decomposition products at different temperatures have been characterized by FT-IR spectroscopy and thermal analysis. Thus, we have evidenced that by thermal decomposition of the studied precursors in the range 250–300 °C, different amorphous oxidic phases mixtures form depending on the nature of metalic cation: (Cr 2O 3+x + ZnO) (Cr 2O 3+x + Ni/NiO) and (Cr 2O 3+x+MgO). In case of M = Zn, around 400 °C when the transition Cr 2O 3+x to Cr 2O 3 takes place, zinc chromite nuclei form by the interaction ZnO with Cr 2O 3. In case of M = Ni, due to the partial reduction of Ni(II) at Ni(0) during the thermal decomposition of the precursor the formation of nickel chromite by the reaction NiO + Cr 2O 3 is shifted toward 500 °C, when Ni is oxidized at NiO. The thermal evolution of the mixture (MgO + CrO 3) is different due to the formation as intermediary phase of MgCrO 4, which decomposes to MgCr 2O 4 around 560 °C. In order to investigate the chromites formation mechanism, we have studied the mechanical mixtures of single oxides obtained from the corresponding carboxylates. These mixtures (MO + Cr 2O 3) have been annealed at 400, 500, and 600 °C to study the evolution of the crystalline phases. It results in the prepared mixture behaving different from the mixtures obtained by thermal decomposition of the binary M(II),Cr(III)-carboxylates, recommending our synthesis method for obtaining binary oxides. 相似文献
18.
In this study, the initial oxidation behaviour of laser‐treated Al/NiCrAlY bond‐coat is investigated. Two approaches, (i) Al film sputtering on the surface of bond‐coat and (ii) laser treatment, have been taken to enhance the oxidation resistance of NiCrAlY bond‐coat. Experimental results showed that after laser treatment, the Al/NiCrAlY bond‐coat exhibited a columnar dendritic microstructure without cracks and voids. A dense and continuous α‐Al 2O 3/Cr 2O 3 multilayer was found to form on the bond‐coat surface. Results on the cyclic oxidation at 1200 °C (for time ≤ 204 h) revealed that the laser‐treated Al/NiCrAlY bond‐coat exhibited better oxidation resistance compared to as‐sprayed NiCrAlY, Al/NiCrAlY and laser‐remelted NiCrAlY bond‐coat. The formation of θ‐Al 2O 3, NiO, Cr 2O 3 and NiCr 2O 4 spinel oxides was observed to be suppressed due to the preformed α‐Al 2O 3 scale during initial oxidation on the surface of laser pre‐oxidized Al/NiCrAlY bond‐coat. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
19.
Thermal oxidation of commercial ilmenite concentrate from Kahnouj titanium mines, Iran, at 500–950 °C was investigated for the first time. Fractional conversion was calculated from mass change of the samples during oxidation. Maximum FeO to Fe 2O 3 conversion of 98.63 % occurred at 900 °C after 120 min. Curve fit trials together with SEM line scan results indicated constant-size shrinking core model as the closest kinetic mechanism of the oxidation process. Below 750 °C, chemical reaction with activation energy of 80.65 kJ mol ?1 and between 775 and 950 °C, ash diffusion with activation energy of 53.50 kJ mol ?1 were the prevailing mechanisms. X-ray diffraction patterns approved presence of pseudobrookite, rutile, hematite, and Fe 2O 3·2TiO 2 phases after oxidation of ilmenite concentrate at 950 °C. 相似文献
20.
A series of La1?x
Sr
x
Cr1?y
Ru
y
O3?δ
(0.1 ≤ x ≤ 0.5, 0.05 ≤ y ≤ 0.15) materials was prepared by the sol–gel method to develop alternative catalysts for propane steam reforming. Catalyst characteristics were evaluated using physicochemical methods including X-ray diffraction, Brunauer–Emmett–Teller methods, H2 temperature-programmed reduction, and thermogravimetry analysis (TGA). Effects of the amount of ruthenium (Ru) and strontium and the steam-to-carbon ratio (S/C) were investigated. An increase in Ru content led to increased propane conversion and H2 yield, especially below 700 °C. Dramatic enhancement of catalytic activity was observed with La0.8Sr0.2Cr0.85Ru0.15O3 under 600 °C, achieving propane conversion over 79% between 600 and 800 °C with maximum propane conversion and H2 yield of 98.3% and 63.3%, respectively. Also, good resistance to carbon formation for the La0.8Sr0.2Cr0.85Ru0.15O3 catalyst was confirmed by long-term testing and TGA analysis. 相似文献
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