全文获取类型
收费全文 | 464篇 |
免费 | 23篇 |
国内免费 | 81篇 |
专业分类
化学 | 349篇 |
晶体学 | 2篇 |
力学 | 53篇 |
综合类 | 5篇 |
数学 | 7篇 |
物理学 | 152篇 |
出版年
2023年 | 1篇 |
2022年 | 6篇 |
2021年 | 18篇 |
2020年 | 13篇 |
2019年 | 11篇 |
2018年 | 16篇 |
2017年 | 11篇 |
2016年 | 12篇 |
2015年 | 16篇 |
2014年 | 20篇 |
2013年 | 27篇 |
2012年 | 35篇 |
2011年 | 25篇 |
2010年 | 28篇 |
2009年 | 28篇 |
2008年 | 22篇 |
2007年 | 39篇 |
2006年 | 35篇 |
2005年 | 30篇 |
2004年 | 27篇 |
2003年 | 22篇 |
2002年 | 13篇 |
2001年 | 13篇 |
2000年 | 12篇 |
1999年 | 7篇 |
1998年 | 13篇 |
1997年 | 16篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 3篇 |
1992年 | 7篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 1篇 |
排序方式: 共有568条查询结果,搜索用时 15 毫秒
81.
Selective production of hydrogen by oxidative steam reforming of methanol (OSRM) was studied over Cu/SiO2 catalyst using fixed bed flow reactor. Textural and structural properties of the catalyst were analyzed by various instrumental methods. TPR analysis illustrates that the reduction temperature peak was observed between 510?K and 532?K at various copper loadings and calcination temperatures and the peaks shifted to higher temperature with increasing copper loading and calcination temperature. The XRD and XPS analysis demonstrates that the copper existed in different oxidation states at different conditions: Cu2O, Cu0, CuO and Cu(OH)2 in uncalcined sample; CuO in calcined sample: Cu2O and metallic Cu after reduction at 600?K and Cu0 and CuO after catalytic test. TEM analysis reveals that at various copper loadings, the copper particle size is in the range between 3.0?nm and 3.8?nm. The Cu particle size after catalytic test increased from 3.6 to 4.8?nm, which is due to the formation of oxides of copper as evidenced from XRD and XPS analysis. The catalytic performance at various Cu loadings shows that with increasing Cu loading from 4.7 to 17.3?wt%, the activity increases and thereafter it decreases. Effect of calcination shows that the sample calcined at 673?K exhibited high activity. The O2/CH3OH and H2O/CH3OH molar ratios play important role in reaction rate and product distribution. The optimum molar ratios of O2/CH3OH and H2O/CH3OH are 0.25 and 0.1, respectively. When the reaction temperature varied from 473 to 548?K, the methanol conversion and H2 production rate are in the range of 21.9–97.5% and 1.2–300.9?mmol?kg?1?s?1, respectively. The CO selectivity is negligible at these temperatures. Under the optimum conditions (17.3?wt%, Cu/SiO2; calcination temperature 673?K; 0.25 O2/CH3OH molar ratio, 0.5 H2O/CH3OH molar ratio and reaction temperature 548?K), the maximum hydrogen yield obtained was 2.45?mol of hydrogen per mole of methanol. The time on stream stability test showed that the Cu/SiO2 catalyst is quite stable for 48?h. 相似文献
82.
Stoyan Stoyanov Georgi Yordanov Ceco Dushkin 《Reaction Kinetics and Catalysis Letters》2008,94(2):227-232
The effect of Ni loading on the catalytic activity of Ni/ZrO2 catalyst for methane steam reforming was investigated. The sample containing 15 wt.% Ni exhibited the highest activity as
well as stability at 600°C.
An erratum to this article is available at . 相似文献
83.
Reaction parameters influence on the catalytic performance of copper-silica aerogel in the methanol steam reforming 《燃料化学学报》2016,44(1):84-90
Steam reforming of methanol was carried out on the copper-silica aerogel catalyst.The effects of reaction temperature,feed rate,water to methanol molar ratio and carrier gas flowrate on the H_2 production rate and CO selectivity were investigated.M ethanol conversion was increased considerably in the range of about 240-300,after which it increased at a slightly lower rate.The used feed flowrate,steam to methanol molar ratio and carrier gas flowwere 1.2-9.0 m L/h,1.2-5.0 and 20-80 m L/min,respectively.Reducing the feed flowrate increased the H_2 production rate.It was found that an increase in the water to methanol ratio and decreasing the carrier gas flowrate slightly increases the H2production rate.Increasing the water to methanol ratio causes the lowest temperature in which CO formation was observed to rise,so that for the ratio of 5.0 no CO formation was detected in temperatures lower than 375℃.In all conditions,by approaching the complete conversion,increasing the main product concentration,increasing the temperature and contact time,and decreasing the steam to methanol ratio,the CO selectivity was increased.These results suggested that CO was formed as a secondary product through reverse water-gas shift reaction and did not participate in the methanol steam reforming reaction mechanism. 相似文献
84.
《Comptes Rendus Chimie》2016,19(4):433-440
In this study, an experimental plasma-chemical reactor equipped with an arc discharge water steam plasma torch was used for biomass conversion to hydrogen-rich synthesis fuels. Glycerol and crushed wood were used as biomass sources. The effects of different conversion parameters including the water steam flow rate, treated material flow rate, and plasma torch power were studied. The experimentally obtained results were compared with the model based on the thermodynamic equilibrium. Additionally, the quantification of the plasma conversion system in terms of energy efficiency and specific energy requirement was performed. It has been found that the synthesis gas can be effectively produced from the biomass using water steam plasma. 相似文献
85.
透平转子的可靠性设计 总被引:3,自引:0,他引:3
介绍透平转子的可靠性设计方法。该方法以传统的转子强度和振动的设计方法为基础,把透平转子的静应力、材料静强度、应力幅、尺寸系数、表面加工系数、应力集中系数、材料疲劳极限、热疲劳累积损伤程度、材料低周疲劳程度、蠕变寿命、固有频率、激振力频率、不平衡响应等处理为随机变量,使用机械概率设计法确定转子设计的可靠度。文中给出了透平转子静强度、疲劳强度、热疲劳、蠕变寿命、避开弯曲共振、避开扭转共振和限制不平稳响 相似文献
86.
87.
88.
Hongsheng Wang Bingzheng Wang Sean-Thomas B. Lundin Hui Kong Bosheng Su Jian Wang 《Molecules (Basel, Switzerland)》2021,26(22)
To efficiently convert and utilize intermittent solar energy, a novel solar-driven ethanol steam reforming (ESR) system integrated with a membrane reactor is proposed. It has the potential to convert low-grade solar thermal energy into high energy level chemical energy. Driven by chemical potential, hydrogen permeation membranes (HPM) can separate the generated hydrogen and shift the ESR equilibrium forward to increase conversion and thermodynamic efficiency. The thermodynamic and environmental performances are analyzed via numerical simulation under a reaction temperature range of 100–400 °C with permeate pressures of 0.01–0.75 bar. The highest theoretical conversion rate is 98.3% at 100 °C and 0.01 bar, while the highest first-law efficiency, solar-to-fuel efficiency, and exergy efficiency are 82.3%, 45.3%, and 70.4% at 215 °C and 0.20 bar. The standard coal saving rate (SCSR) and carbon dioxide reduction rate (CDRR) are maximums of 101 g·m−2·h−1 and 247 g·m−2·h−1 at 200 °C and 0.20 bar with a hydrogen generation rate of 22.4 mol·m−2·h−1. This study illustrates the feasibility of solar-driven ESR integrated with a membrane reactor and distinguishes a novel approach for distributed hydrogen generation and solar energy utilization and upgradation. 相似文献
89.
湿混法制备甲醇氧化重整制氢CuZnAlZr催化剂 总被引:4,自引:1,他引:3
用简易湿混法制备了用于甲醇氧化重整制氢的CuZnAlZr催化剂,与共沉淀法制备的催化剂比较,结果表明,湿混法制备的催化剂具有相当的中高温活性和略低的低温活性,有较高的CO2选择性。XRD、TPR、TG-DSC等表征结果显示,湿混法制备的催化剂中铜组分易于向表面迁移和富集,并可能与氧化铝作用生成铜铝复合氧化物,具有了更高的Cu分散度和Cu0比表面浓度。湿混法制备的催化剂对甲醇氧化重整反应有较好的稳定性,经100 h的连续反应,在275 ℃时甲醇转化率在90%以上,重整气中氢气体积分数大于60%,CO2选择性接近99%。 相似文献
90.
蜂窝催化剂上甲醇自热重整制氢的动力学研究 总被引:7,自引:2,他引:5
400 ℃~460 ℃,400 h-1~1 600 h-1,O2/CH3OH(mol ratio)=0.10~0.25,H2O/CH3OH (mol ratio) =1.0~1.8下,通过正交实验设计方法,用BSD-2A型内循环无梯度反应器研究了Zn-Cr/CeO2-ZrO2蜂窝催化剂上甲醇自热重整制氢反应的宏观动力学。以甲醇水蒸气重整反应、甲醇分解反应和甲醇完全氧化反应为独立反应进行了研究,得出了幂指数型反应速率表达式,并根据实验结果,利用最小二乘法求解了动力学参数值。F检验表明该动力学模型是高度显著的。该多重反应动力学方程的得出为蜂窝反应器的进一步模拟、优化和设计提供了数据。 相似文献