全文获取类型
收费全文 | 378273篇 |
免费 | 7336篇 |
国内免费 | 5140篇 |
专业分类
化学 | 189676篇 |
晶体学 | 5962篇 |
力学 | 19733篇 |
综合类 | 333篇 |
数学 | 53324篇 |
物理学 | 121721篇 |
出版年
2022年 | 2714篇 |
2021年 | 3435篇 |
2020年 | 3883篇 |
2019年 | 4232篇 |
2018年 | 12492篇 |
2017年 | 13054篇 |
2016年 | 10205篇 |
2015年 | 5159篇 |
2014年 | 6908篇 |
2013年 | 13771篇 |
2012年 | 15041篇 |
2011年 | 24004篇 |
2010年 | 15844篇 |
2009年 | 15944篇 |
2008年 | 20617篇 |
2007年 | 23501篇 |
2006年 | 10315篇 |
2005年 | 14846篇 |
2004年 | 11142篇 |
2003年 | 10545篇 |
2002年 | 8753篇 |
2001年 | 8594篇 |
2000年 | 6739篇 |
1999年 | 5043篇 |
1998年 | 4205篇 |
1997年 | 3958篇 |
1996年 | 3947篇 |
1995年 | 3527篇 |
1994年 | 3493篇 |
1993年 | 3263篇 |
1992年 | 3888篇 |
1991年 | 3829篇 |
1990年 | 3759篇 |
1989年 | 3712篇 |
1988年 | 3614篇 |
1987年 | 3633篇 |
1986年 | 3452篇 |
1985年 | 4194篇 |
1984年 | 4235篇 |
1983年 | 3442篇 |
1982年 | 3502篇 |
1981年 | 3463篇 |
1980年 | 3222篇 |
1979年 | 3692篇 |
1978年 | 3698篇 |
1977年 | 3709篇 |
1976年 | 3675篇 |
1975年 | 3312篇 |
1974年 | 3300篇 |
1973年 | 3303篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
ABSTRACTQM(UB3LYP)/MM(AMBER) calculations were performed for the locations of the transition structure (TS) of the oxygen–oxygen (O–O) bond formation in the S4 state of the oxygen-evolving complex (OEC) of photosystem II (PSII). The natural orbital (NO) analysis of the broken-symmetry (BS) solutions was also performed to elucidate the nature of the chemical bonds at TS on the basis of several chemical indices defined by the occupation numbers of NO. The computational results revealed a concerted bond switching (CBS) mechanism for the oxygen–oxygen bond formation coupled with the one-electron transfer (OET) for water oxidation in OEC of PSII. The orbital interaction between the σ-HOMO of the Mn(IV)4–O(5) bond and the π*-LUMO of the Mn(V)1=O(6) bond plays an important role for the concerted O–O bond formation for water oxidation in the CaMn4O6 cluster of OEC of PSII. One electron transfer (OET) from the π-HOMO of the Mn(V)1=O(6) bond to the σ*-LUMO of the Mn(IV)4–O(5) bond occurs for the formation of electron transfer diradical, where the generated anion radical [Mn(IV)4–O(5)]-? part is relaxed to the ?Mn(III)4?…?O(5)- structure and the cation radical [O(6)=Mn(V)1]+ ? part is relaxed to the +O(6)–Mn(IV)1? structure because of the charge-spin separation for the electron-and hole-doped Mn–oxo bonds. Therefore, the local spins are responsible for the one-electron reductions of Mn(IV)4->Mn(III)4 and Mn(V)1->Mn(IV)1. On the other hand, the O(5)- and O(6)+ sites generated undergo the O–O bond formation in the CaMn4O6 cluster. The Ca(II) ion in the cubane- skeleton of the CaMn4O6 cluster assists the above orbital interactions by the lowering of the orbital energy levels of π*-LUMO of Mn(V)1=O(6) and σ*-LUMO of Mn(IV)4–O(5), indicating an important role of its Lewis acidity. Present CBS mechanism for the O–O bond formation coupled with one electron reductions of the high-valent Mn ions is different from the conventional radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems. The proton-coupled electron transfer (PC-OET) mechanism for the O–O bond formation is also touched in relation to the CBS-OET mechanism. 相似文献
3.
4.
The structure, chemical composition, and magnetic properties of electrochemically deposited nanocrystalline Co-Ni-Fe films
were investigated using a number of techniques. A high saturation magnetic induction up to B
s
= 21 kG was attained. An enhancement of the saturation magnetization compared to the ideal anticipated one was revealed,
which correlated with the nonlinear behavior of the structural phase composition and lattice parameters with the change of
the composition.
The text was submitted by the authors in English. 相似文献
5.
The γcmc values of CTAB-SDS decrease from 63.67 mN/m at 10‡C to 36.38 mN/m at 90‡C, slightly lower than those of either CTAB or SDS.
Correspondingly, the CMC of CTAB-SDS decreases almost by half. The increase of surface activity of CTAB-SDS can be attributed
to the relatively weak electrostatic interaction at high temperature, which is supported by the increase of solubility of
CTAB-SDS with rise in temperature. Catalytic effect on oxidation of toluene derivatives with potassium permanganate follows
the order CTAB-SDS > SDS > CTAB. This is not caused by the dissociative effect of CTAB-SDS with low surface activity at low
temperature, as seen from the fact that almost all oxidative products can be retrieved for different toluene derivatives and
surfactants by mimicking the conditions of reaction. In the emulsifications of toluene derivatives at 90‡C, the time that
turbid water layers of surfactant solutions take to become clear is the same as that of the catalytic effect on oxidation
of toluene derivatives. Thus, it can be inferred that surfactants can improve the oxidation yields of toluene derivatives
by increasing the contact between two reacting phases. 相似文献
6.
Experimental data on the effect of thallium and sodium impurities on the lattice heat conductivity of PbTe at room temperature
are reported. Because the lattice of lead chalcogenides is strongly polarized near charged impurities, the effect of impurities
on the lattice heat conductivity depends substantially on their charge state. This property of the material has been used
to determine the charge state of the thallium impurity in PbTe. The results obtained argue for a model of quasi-local thallium-impurity
states which assumes low electron-correlation energy at an impurity center.
Fiz. Tverd. Tela (St. Petersburg) 40, 1206–1208 (July 1998) 相似文献
7.
Yu. A. Sharin 《Russian Physics Journal》1998,41(5):447-450
Ideas previously enunciated by the author about the physical interpretation of curvilinear space with torsion are developed.
With the new equations describing the eigenstates of such a space, the static centrosymmetric solution for gravitational and
electric fields can be made consistent with the Newton and Coulomb laws. The asymptotic behavior of the axisymmetric solution
at infinity is studied. The gyromagnetic ratio obtained is characteristic of the electron.
Ural State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, p. 56–60, May, 1998. 相似文献
8.
E. Jackiewicz M. Lukasiak M. Kopcewicz K. Szpila N. Bakun-Czubarow 《Hyperfine Interactions》1992,70(1-4):993-996
Terrestrial basalt rocks considered to be a good analogues of the Martian regolith were studied by using the Mössbauer spectroscopy and X-ray diffraction. The weathering of basalts was followed by the changes of the Fe2+/Fe3+ ratio. The basalt rock undergoes alteration from primary mineral composition (olivine+pyroxene+plagioclase) through smectite to kaolinite clay with oxides and hydroxides with the increase of the Fe2+/Fe3+ ratio. 相似文献
9.
在氢化丁苯共聚物13C-NMR谱脂肪碳部分谱带归属的基础上,根据各谱带的主要来源,推导出六个二单元浓度的计算公式。计算出二单元、一单元的相对含量,各结构单元的数均序列长度、嵌段含量和其它结构参数。加氢前后的1H-和13C-NMR谱的组成计算结果基本一致。初步探讨了作为粘度指数改进剂的氢化丁苯共聚物微观结构与性能的关系,为合成提供了依据。 相似文献
10.