全文获取类型
收费全文 | 159608篇 |
免费 | 9930篇 |
国内免费 | 18232篇 |
专业分类
化学 | 99617篇 |
晶体学 | 1423篇 |
力学 | 8613篇 |
综合类 | 2097篇 |
数学 | 36727篇 |
物理学 | 39293篇 |
出版年
2024年 | 211篇 |
2023年 | 1315篇 |
2022年 | 3505篇 |
2021年 | 3459篇 |
2020年 | 3843篇 |
2019年 | 3881篇 |
2018年 | 3231篇 |
2017年 | 4292篇 |
2016年 | 4767篇 |
2015年 | 4128篇 |
2014年 | 5751篇 |
2013年 | 10988篇 |
2012年 | 8946篇 |
2011年 | 8008篇 |
2010年 | 6862篇 |
2009年 | 9508篇 |
2008年 | 10058篇 |
2007年 | 10453篇 |
2006年 | 9620篇 |
2005年 | 8170篇 |
2004年 | 7539篇 |
2003年 | 6781篇 |
2002年 | 7670篇 |
2001年 | 5165篇 |
2000年 | 4943篇 |
1999年 | 4552篇 |
1998年 | 4084篇 |
1997年 | 3321篇 |
1996年 | 2857篇 |
1995年 | 2617篇 |
1994年 | 2372篇 |
1993年 | 1985篇 |
1992年 | 1886篇 |
1991年 | 1400篇 |
1990年 | 1173篇 |
1989年 | 1054篇 |
1988年 | 876篇 |
1987年 | 683篇 |
1986年 | 621篇 |
1985年 | 632篇 |
1984年 | 640篇 |
1983年 | 337篇 |
1982年 | 528篇 |
1981年 | 555篇 |
1980年 | 479篇 |
1979年 | 449篇 |
1978年 | 377篇 |
1977年 | 315篇 |
1976年 | 253篇 |
1973年 | 167篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
《Physics letters. A》2019,383(17):2090-2092
In this paper, we have used Monte Carlo (MC) method to simulate and study the temperature and doping effects on the electric conductivity of fullerene (C60). The results show that the band gap has reduced by the doping and the charge carrier transport is facilitated from valence band to conduction band by the temperature where is touched a 300 K. In this case, the conductivity reached a value of . The electric conductivity of C60 can increase by the triphenylmethane dye crystal violet (CV) alkali metal to reach at 303 K. Our results of MC simulation have a good agreement with those extracted from literature [10], [33]. 相似文献
3.
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
4.
The finiteness is proved of the set of isomorphism classes of potentially abelian geometric Galois representations with a given set of data. This is a special case of the finiteness conjecture of Fontaine and Mazur. 相似文献
5.
文[1]提出了两个DEA的逆问题,并用搜索法来解.而本文根据所证的定理,对每个问题一般只要解二、三个线性规划问题就能得到答案. 相似文献
6.
Pavan Mathur 《Journal of Chemical Sciences》2006,118(6):553-568
New tetradentatebis-benzimidazole ligands have been synthesized and utilized to prepare copper (II) complexes. Some of these copper (II) complexes
have been characterized structurally. The copper (II) in these complexes is found to possess varying geometries. A distorted
octahedral geometry is found with a highly unsymmetrical bidentate nitrate group. An unusual polymeric one-dimensional structure
is observed where copper (II) is in a distorted square pyramidal geometry with a monodentate nitrate ion, having long Cu-O
bond, while a distorted triagonal bipyramidal geometry is found with two carbonyl O atoms and a Cl atom in the equatorial
plane, and two benzimidazole imine N atoms occupy the axial position. These compounds are found to activate the cumylperoxyl
group, and this has been utilized in the facile oxidation of aromatic alcohols to aldehydes, where they act as catalysts with
large turnovers. The yields of the respective products vary from 32 to 65%. The role of molecular oxygen has been studied
and an attempt has been made to identify the “active copper species”. Activation of molecular oxygen has also been observed
and has been used for oxidative dealkylation of a hindered phenol, producing di-butyl quinones with yields of 20–25% and 10–12
fold catalytic turnover. Dihydroxybenzenes and substituted catechols are also readily oxidized to the corresponding quinones,
in oxygen-saturated solvents. Yields of 84% have been observed with 34-fold catalyst turnover, with di-t-butylcatechol. The activity of these copper (II) —bis-benzmidazolediamide compounds is reminiscent of the functioning of copper centres in galactose oxidase, tyrosinase and catechol
oxidase. 相似文献
7.
8.
M. Georgelin A. Pocheau 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):169-174
The polycrystalline perovskitelike manganese oxides La1-xAxMnO (A = Na, and K, ) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation
state is practically constant, at for A = Na, and for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (Tc) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates
for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie
temperature.
Received: 12 September 1997 / Revised: 18 December 1997 / Accepted: 21 January 1998 相似文献
9.
10.
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