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排序方式: 共有992条查询结果,搜索用时 15 毫秒
21.
D. Berger C. Matei F. Papa G. Voicu V. Fruth 《Progress in Solid State Chemistry》2007,35(2-4):183-191
We report the synthesis of La1−xSrxCoO3 nanopowders by solution combustion method using metal nitrates and -alanine (alanine method) or urea (urea method) as fuel. The influence of metal nitrates/organic substance molar ratio and the type of fuel was investigated. The isolated complex precursors were characterized by atomic absorption spectroscopy (AAS), FT-IR spectra and DTA–TG analysis. The La1−xSrxCoO3 (x = 0–0.3) powders were characterized by X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive X-ray analysis (SEM–EDX), as well as by specific surface area measurements. XRD patterns indicate the formation of single-phase LaCoO3 (rhombohedral) when as-synthesized powders were calcined at 873 K, 3 h in the case of the alanine method and at 1073 K, 3 h for urea-based system. Also, strontium doped lanthanum cobaltites obtained by both methods at 1273 K are single phase with rhombohedral perovskite-like structure as XRD data have proved. SEM investigation of pure and doped lanthanum cobaltites reveal that the samples prepared by both methods have fine particles with tendency of agglomerates formation with different shapes, spongy aspect and high porosity. La1−xSrxCoO3 nanopowders obtained by alanine method have larger specific surface area values than those prepared by urea method. 相似文献
22.
TiC/FeTi composites have been obtained in situ by Self-propagating High-temperature Synthesis (SHS) of an intimate mixture of compacted powders of elemental carbon, titanium and iron. The reaction has been followed in real time by X-ray diffraction at the ESRF. The mechanism of the reaction is discussed in terms of the formation of a liquid phase corresponding to the eutectic of the Fe/Ti system prior to the TiC synthesis. Temperatures of reaction have been estimated by correlating thermal expansion coefficients with diffraction peaks shifts. The microstructures obtained by this method, suitable for cutting tools and wear resistant applications, are presented. 相似文献
23.
A study on the synthesis of La1−xAgxMnO3+δ (x = 0, 0.2) using a microwave process (MWhyd) has been carried out by comparing the heating time and reaction temperature with the same factor under conventional thermal process (CHhyd). Experiments have been conducted using the hydrothermal method at medium pressure (T = 200 °C, P = 20 atm) followed by a thermal treatment of the precursor at 700 °C (10 h).Structural and physico-chemical properties of the catalysts were investigated using X-ray diffraction (XRD), BET sorption, temperature-programmed reduction or desorption, mass spectrometry (TPR-MS and TPD-MS), and X-ray photoelectron spectroscopy (XPS). While CHhyd and MWhyd powder catalysts exhibited the same XRD patterns indexed as pure perovskite structure, their surface physico-chemical properties were found to be strongly influenced by the preparation method. The effect of the nature of oxygen species, their amount and mobility, evidenced by temperature programmed experiments, on the catalytic properties in methane combustion in the presence and in the absence of hydrogen sulphide has been studied. MWhyd-La0.8Ag0.2MnO3+δ catalysts were found to exhibit a much better performance in methane combustion together with higher resistance to sulphur poisoning than CHhyd catalysts. 相似文献
24.
本用提高样品蒸发速率的方法提高氧化钪中难熔杂质光谱分析灵敏度。采用一种特殊的阴极设计,配合使用N2+Ar气氛控制,在旋流气室中放电激发,取样仅5mg的条件下,光谱测定氧化钪中难烷质Ti,Zr,Mo,W,V灵敏度可达(1-3)×10^-4%。测定的相对标准偏差为9%-27%。 相似文献
25.
The heats of combustion of trans-9,10-bis-hydroxymethyl-9,10-dihydrophenanthrene, trans-5-hydroxymethyl-5,6-dihydro-7H-dibenzo[a,c]cyclohepten-6-ol, 5-hydroxymethyl-5,6-dihydro-7H-di-benzo[a,c]cycloheptene, 6-hydroxymethyl-5,6-dihydro-7H-dibenzo[a,c]cycloheptene, 5H-dibenzo-[a,d]cyclohepten-5-ol and 5H-10,11-dihydrodibenzo[a,d]cyclohepten-5-ol were measured by means of a Gallenkamp adiabatic bomb calorimeter. Uncertainties in the determination of the heats of combustion ranged between 0.2 and 0.3%. The enthalpies of formation and atomization for the six compounds were derived. The experimental values of the heats of atomization were compared with those calculated using the Allen–Skinner bond energy scheme. Conclusions about energetic contributions which stabilize the structure of the investigated compounds were drawn. 相似文献
26.
Twenty-two isomers/conformers of C3H6S+√ radical cations have been identified and their heats of formation (ΔHf) at 0 and 298 K have been calculated using the Gaussian-3 (G3) method. Seven of these isomers are known and their ΔHf data are available in the literature for comparison. The least energy isomer is found to be the thioacetone radical cation (4+) with C2v symmetry. In contrast, the least energy C3H6O+√ isomer is the 1-propen-2-ol radical cation. The G3 ΔHf298 of 4+ is calculated to be 859.4 kJ mol−1, ca. 38 kJ mol−1 higher than the literature value, ≤821 kJ mol−1. For allyl mercaptan radical cation (7+), the G3 ΔHf298 is calculated to be 927.8 kJ mol−1, also not in good agreement with the experimental estimate, 956 kJ mol−1. Upon examining the experimental data and carrying out further calculations, it is shown that the G3 ΔHf298 values for 4+ and 7+ should be more reliable than the compiled values. For the five remaining cations with available experimental thermal data, the agreement between the experimental and G3 results ranges from fair to excellent.
Cation CH3CHSCH2+√ (10+) has the least energy among the eleven distonic radical cations identified. Their ΔHf298 values range from 918 to 1151 kJ mol−1. Nevertheless, only one of them, CH2=SCH2CH2+√ (12+), has been observed. Its G3 ΔHf298 value is 980.9 kJ mol−1, in fair agreement with the experimental result, 990 kJ mol−1.
A couple of reactions involving C3H6S+√ isomers CH2=SCH2CH2+√ (12+) and trimethylene sulfide radical cation (13+) have also been studied with the G3 method and the results are consistent with experimental findings. 相似文献
27.
The gas dynamics in an exhaust pipe is studied. In particular we focus on the warm up of the catalytic converter in very short times after the engine start. This is done by combustion a small unburnt part of the exhaust gas. This process is classically modelled by gas dynamic equations. Compared to the existing literature we improve the (one-dimensional) modelling approach using a small Mach number technique and a network ansatz for the full exhaust pipe. The final simplified model on one hand still describes the main features and on the other hand it is computationally a few orders of magnitude faster than the original model. Performing numerical simulations we compare the new model to the (classical) full model and to experimental results in the literature. 相似文献
28.
Wilson Merchan-Merchan Alexei V. Saveliev Walmy Cuello Jimenez 《Proceedings of the Combustion Institute》2011,33(2):1899-1908
In this paper we report the growth of 1-D and 3-D tungsten-oxide nanostructures on tungsten wire probes inserted in an opposed-flow oxy-fuel flame. The probe diameter and oxygen content in the oxidizer were varied to study their influence on the growth of tungsten-oxide nanostructures. The introduction of a 1-mm diameter W probe into the flame environment with an oxidizer composition of 50%O2 + 50%N2, resulted in the formation of 1-D nanorods on the upper surface of the probe. The formation of triangular, rectangular, square, and cylindrical 3-D channels with completely hollow or semi-hollow morphology was achieved by reducing the probe diameter to 0.5 mm. Whereas, the increase of the O2 content to 100% and the employment of a 1-mm probe resulted in the growth of ribbon-like micron-sized structures. The lattice spacing of ∼0.38 nm measured for the 1-D W-oxides closely matches a monoclinic WO3 structure. X-ray photoelectron spectroscopy analysis revealed that the larger 3-D structures also consist of WO3 confirming that the chemical composition of the structures remains the same while varying the probe and flame parameters. The proposed growth mechanism states that the 3-D WO3 structures are formed through the lateral coalescence of 1-D W-oxide nanorods. 相似文献
29.
为分析尖劈形材料的吸波性能,建立了半圆镜像模型将辐射波在尖劈空间的传播等效为射线对等分半圆的切割,确定了反射波线和吸波材料几何参数之间的解析关系.对于微波暗室问题,假设在余弦辐射体上随机选取一个方向的能量波束集中了其所有能量,分别建立了加权能量分布模型和Monte Carlo模型,仿真验证了两模型结论的一致性. 相似文献
30.