全文获取类型
收费全文 | 3175篇 |
免费 | 371篇 |
国内免费 | 395篇 |
专业分类
化学 | 3791篇 |
晶体学 | 3篇 |
力学 | 2篇 |
综合类 | 14篇 |
数学 | 2篇 |
物理学 | 129篇 |
出版年
2024年 | 19篇 |
2023年 | 36篇 |
2022年 | 81篇 |
2021年 | 123篇 |
2020年 | 177篇 |
2019年 | 106篇 |
2018年 | 103篇 |
2017年 | 105篇 |
2016年 | 147篇 |
2015年 | 123篇 |
2014年 | 131篇 |
2013年 | 187篇 |
2012年 | 164篇 |
2011年 | 153篇 |
2010年 | 108篇 |
2009年 | 161篇 |
2008年 | 212篇 |
2007年 | 208篇 |
2006年 | 183篇 |
2005年 | 183篇 |
2004年 | 171篇 |
2003年 | 156篇 |
2002年 | 172篇 |
2001年 | 118篇 |
2000年 | 96篇 |
1999年 | 73篇 |
1998年 | 71篇 |
1997年 | 68篇 |
1996年 | 56篇 |
1995年 | 44篇 |
1994年 | 48篇 |
1993年 | 39篇 |
1992年 | 45篇 |
1991年 | 16篇 |
1990年 | 16篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 7篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有3941条查询结果,搜索用时 15 毫秒
91.
The effect of the addition of a second metals such as Zn and Ni on the calcinaton and reduction of alumina, magnesia and silicasupported iron catalysis with total iron loading of 5wt% is investigated by Mössbauer spectroscopy. It is shown that the reducibility of supported α-Fe2O3 is gradually increased by adding the second metal. The values of the magnetic hyperfine field obtained from Mössbauer spectra for the Zn or Ni-added α-Fe2O3 or Fe catalysts decreased with increasing second metal loading. 相似文献
92.
对500℃和800℃焙烧制得的氧化态K-MoO3/γ-AlO3,K-MoO3/SiO2及非负载K-Mo催化剂进行硫化后,测试其合成醇活性.结果表明500℃焙烧制得的负载型催化剂显示较高的合成低碳烃活性和较低的合成低磷醇选择性,经800℃焙烧后,合成低磷醇的选择性大幅度提高.500℃焙烧的非负载K-Mo催化剂显示较高的合成醇选择性,经800℃焙烧后,促碳醇的选择性保持不变.用氨的吸附及TPD方法测定了各样品的酸性,并与催化剂活性对照,发现催化剂酸性越强,酸量越大,则其合成醇选择性越低.催化剂上的乙醇分解实验证实,催化剂的酸量大小与它的醇分解活性成正变关系,这些结果说明催化剂酸性对其合成醇性能有直接的影响. 相似文献
93.
T. S. Ulbaev Yu. S. Mardashev M. P. Koroteev V. N. Khrustalev M. Yu. Antipin 《Journal of Structural Chemistry》2005,46(5):891-894
Metallocomplexes with optically active ligands are of considerable interest as catalysts of stereoselective processes. A complex of 1,2-isopropylidene-α-D-glucofuranose 3,5,6-bis-cyclophosphite with tungsten(0) pentacarbonyl has been synthesized for the first time and studied by X-ray crystallography. 相似文献
94.
Colloidal palladium nanocatalysts prepared by in situ reductions of palladium chloride were immobilized and protected by either of two water-soluble polymers. The particle sizes and size distributions of the palladium colloids were determined by transmission electron microscopy. Their selectivities as catalysts were determined by comparing the extents of hydrogenation of carefully chosen pairs of small-molecule olefins. There was found to be high hydrogenation selectivity with regard to cyclic vs. noncyclic olefins. Selectivity was relatively low, however, among olefins that differed only in size (such as hexene vs. octene), or olefins differing only with regard to the positions of the double bonds (such as 1-octene vs. 3-octene). Selectivity could be improved by careful choice of the immobilizing polymer, and by its use at relatively high concentrations. 相似文献
95.
STUDY ON 1-HEXENE POLYMERIZATION BASED ON ZIEGLER-NATTA CATALYSTS WITH DOPED SUPPORT 总被引:1,自引:0,他引:1
XueJiang Zhi-qiangFan 《高分子科学》2004,(4):305-308
A series of Ti/Mg supported catalysts are prepared by using ball-milled mixtures of MgCl2-ethanol adducts and NaCl as supports, and 1-hexene polymerizations catalyzed by the novel catalysts are studied. It is found that the molecular weight distribution of poly(1-hexene) becomes apparently narrower when catalysts with doped supports are used, indicating that changing the structure of the support is an effective way to regulate the active center distribution of heterogeneous Ziegler-Natta catalyst. 相似文献
96.
Polypropylene (PP) has become an indispensable material in our daily lives. Annual worldwide production of PP is now more than 30000000 tons and is predicted to grow at an annual rate of about 6% during the first decade of the 21st century. Commercial production of PP began in 1957 with the use of TiCl(3) catalysts established by Ziegler and Natta. However, the low activities and low stereospecificities of the catalysts resulted in large amounts of catalyst residue and atactic PP in the product, necessitating steps for their removal in commercial production. As a means of finding appropriate catalysts, we developed MgCl(2)-supported TiCl(4) catalysts, which basic concept was introduction of organic compounds onto the inorganic crystal catalyst surface. This addition led to remarkable enhancements in stereospecificity with extremely high activity. Use of the new catalysts enlarged and simplified the PP production process by eliminating the steps previously required for removal of catalyst residue and atactic PP. In addition, it greatly improved the properties of the PP, enabling a much wider range of PP applications by replacing metal and engineering plastics with the highly stereoregular PP. Therefore, these catalysts helped the rapid establishment of the current PP industry and now play a major role in production. The latest MgCl(2)-supported TiCl(4) catalyst is providing precise control of the isotactic PP structure. Future expectations for this type of catalyst are to acquire a single-site nature and to contribute to the creation of a new class of hybrid materials. 相似文献
97.
98.
A. I. Bostan Yu. I. Pyatnitskii L. N. Raevskaya V. G. Pryanikova S. A. Nedil’ko A. G. Dzyaz’ko E. G. Zen’kovich 《Theoretical and Experimental Chemistry》2005,41(1):32-36
The oxidative coupling of methane (OCM) in a periodic regime over the SrMnO3 and its derivatives has been investigated. It has been established that partial replacement of the strontium ion by alkali metal ions leads to an increase in both catalytic activity and in selectivity with respect to higher hydrocarbons. Comparison of the results obtained in the present work with those obtained earlier for catalysts based on SrCoO3 led to the conclusion that the Co-containing catalysts were preferable to the Mn-containing catalysts for carrying out oxidative coupling of methane in a periodic regime.__________Translated from Teoreticheskaya i Éksperimentalnaya Khimiya, Vol. 41, No. 1, pp. 30–34, January–February, 2005. 相似文献
99.
Soot deep oxidation catalyzed by molybdena and molybdates: a thermogravimetric investigation 总被引:2,自引:0,他引:2
Molybdena (MoO3) and molybdates of bismuth (Bi2Mo3O12), chromium (Cr2Mo3O12), barium (BaMoO4), manganese (MnMoO4) and copper (Cu3Mo2O9) were synthesized and characterized by X-ray powder diffractometry and infrared spectroscopy. They were then assessed as ‘loose contact' catalysts for soot deep oxidation (combustion) in air by thermogravimetry. A similar assessment was carried out on commercial chromia (Cr2O3) and tungsta (WO3). Observed high oxidation activity of MoO3, as compared to both Cr2O3 and WO3, is attributed to the higher volatility (mobility) of MoOx species. On similar grounds, observed high activity of MoO3 and Cu3Mo2O9, as compared to the other test molybdates, is explained. Relatively speaking, however, a higher activity was observed for Cu3Mo2O9 than MoO3, whereby soot ignition temperature decreased from 571°C (uncatalyzed oxidation) to 430°C, to occur within the temperature range of diesel exhaust (200–450°C). This observation is ascribed to copper-promoted redox conduct of Mo(VI) in the oxidation reaction of soot. Kinetics of the reaction was studied non-isothermally, and the kinetic parameters (A, k, ΔE and the reaction order) were calculated. 相似文献
100.