全文获取类型
收费全文 | 9387篇 |
免费 | 1197篇 |
国内免费 | 583篇 |
专业分类
化学 | 4277篇 |
晶体学 | 40篇 |
力学 | 738篇 |
综合类 | 79篇 |
数学 | 2333篇 |
物理学 | 3700篇 |
出版年
2023年 | 110篇 |
2022年 | 134篇 |
2021年 | 163篇 |
2020年 | 147篇 |
2019年 | 207篇 |
2018年 | 142篇 |
2017年 | 176篇 |
2016年 | 179篇 |
2015年 | 149篇 |
2014年 | 267篇 |
2013年 | 522篇 |
2012年 | 403篇 |
2011年 | 522篇 |
2010年 | 522篇 |
2009年 | 673篇 |
2008年 | 594篇 |
2007年 | 543篇 |
2006年 | 549篇 |
2005年 | 406篇 |
2004年 | 263篇 |
2003年 | 306篇 |
2002年 | 277篇 |
2001年 | 246篇 |
2000年 | 216篇 |
1999年 | 198篇 |
1998年 | 188篇 |
1997年 | 145篇 |
1996年 | 118篇 |
1995年 | 105篇 |
1994年 | 85篇 |
1993年 | 79篇 |
1992年 | 77篇 |
1991年 | 65篇 |
1990年 | 42篇 |
1989年 | 37篇 |
1988年 | 34篇 |
1987年 | 41篇 |
1985年 | 429篇 |
1984年 | 288篇 |
1983年 | 239篇 |
1982年 | 214篇 |
1981年 | 159篇 |
1980年 | 126篇 |
1979年 | 136篇 |
1978年 | 76篇 |
1977年 | 87篇 |
1976年 | 88篇 |
1975年 | 93篇 |
1974年 | 117篇 |
1973年 | 134篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
详细论述了管理评审的基本概念、如何开展管理评审工作以及中国实验室国家认可委员会的有关政策。 相似文献
103.
O. N. Babkina O. M. Chukanova E. E. Faingol"d N. M. Bravaya 《Russian Chemical Bulletin》2004,53(4):785-790
Quantitative characteristics of changes in the energy of charge transfer from -bonded ligands to metal are presented for the series of structurally similar ansa-metallocene complexes of IVB Group elements. The changes are caused by the solvation effect, replacement of -bonded chlorine ligands by methyl groups and of bridging ethylene groups by dimethylsilylene moieties, introduction of the methyl substituent into position 2 and phenyl substituent into position 4 of the indenyl system, variation of the transition metal in metallocene, and formation of complexes with polymethylalumoxane (MAO) at different AlMAO/Zr ratios. These effects are found to be additive. 相似文献
104.
The objective of the work described in this paper was to produce dispersions of small spherical carbon particles, having particle diameters in the region of 0.1 μm. To this end, the dehydrochlorination of poly(vinylidene chloride) (PVDC) latex particles was attempted. The PVDC latex was prepared by a dispersion polymerization route. Both chemical and thermal dehydrochlorination routes were attempted. Chemical dehydrochlorination, using a variety of base/solvent systems, led to nonporous, spherical black particles of the required size, but which contained only 60% carbon; most of the remainder was oxygen, introduced by nucleophilic substitution reactions. Thermal dehydrochlorination, at 700°C under a nitrogen atmosphere, using a fluidized bed arrangement, on the other hand, led to black particles, having 90% carbon and which retained their sphericity, but which were highly porous. Initial chemical dehydrochlorination, prior to thermal treatment, did not seem to reduce the porosity of the final carbons. Dispersions of the carbon particles in a variety of solvents were readily achieved. 相似文献
105.
Zhengping Hao Lidun An Junling Zhou Hongli Wang 《Reaction Kinetics and Catalysis Letters》1996,59(2):295-300
Supported gold catalyst for eliminating hydrogen from CO2 feed gas in the production of urea is found to be superior to other industrial catalysts (e.g. Pt/Al2O3 and PdPt/Al2O3) in catalytic activity and resistance to sulfur poisoning. 相似文献
106.
Conventional criteria and indices of aromaticity, including electronic, geometric, energetic and magnetic aspects have been applied to examine the aromaticity of five typical transition metal heterocyclic complexes, i.e. six-membered osmabezene 1 and iridabenzene 2, five-membered cobaltacyclopentadiene 3 and iridacyclopentadiene 4, and four-membered tungstacyclobutadiene 5. The results show that the cyclic, planar, conjugated and Hückel 4n+2 rule’s criteria in the transition-metal-containing heterocycles of the five complexes studied are all met. Five quantitative aromaticity indices, including Bird aromatic index (In), homodesmotic reaction aromatic stabilization energy (HASE), absolute hardness (η), diamagnetic susceptibility exaltation (Λ) and NMR chemical shift (δH), qualitatively lead to a consistent and affirmative conclusion that all of them are aromatic. However, they fail to draw a common conclusion for their relative magnitudes of aromaticity, which proves once again the multidimensional character of aromaticity. 相似文献
107.
The first evidence that α-acylnitrone is an intermediate in the base-promoted reaction of 4-nitro-N,N-diethylaniline with aceptophenome to give an enaminoketone was obtained.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1645–1647, August, 1998. 相似文献
108.
用AM1方法(采用非限制的Hartree-FockUHF计算)研究乙烯与环己-1,3-二烯的热Diels-Alder反应。结果表明反应是放热的且存在两条竞争的路径;协同反应的活化能以及双自由反应速度控制步骤的活化能分别为112.667kJ/mol和78.406kJ/mol。 相似文献
109.
甲烷部分氧化制合成气 Ⅰ.甲烷在过渡金属上的活化 总被引:1,自引:0,他引:1
文中用同位素交换(D_2─CH_4)反应来测量四种过渡金属负载型催化剂对甲烷的活化能力,并与Svensson等从头算的计算结果作了比较,表明最有效活化甲烷分子的C─H键的金属是铑(Rh).文中还测定了在甲烷部分氧化制合成气反应中,四个不同催化剂的催化活性与选择性,并与它们对甲烷的活化能力进行对比,可以看出甲烷部分氧化反应中,反应活性高的催化剂有效地活化甲烷,断裂C─H键,而且氘代甲烷产物分布以形成CD_4产物为主要成份. 相似文献
110.
Summary The novel theoretical scheme IMOMM, integrating ab initio and molecular mechanics contributions in a single geometry optimization process, is applied to the structural determination of different [OsO4(NR3)] (NR3 = bulky chiral alkaloid derivative) species closely related to active catalysts for the asymmetric dihydroxylation of olefins. Computed values compare in a satisfactory way with available X-ray data, the relationship between the Os-N distance and the nature of the NR3, ligand being properly reproduced. The computational scheme allows the separate quantification of electronic and steric effects, as well as the identification of the specific steric repulsions responsible for the difference. 相似文献