全文获取类型
收费全文 | 6537篇 |
免费 | 503篇 |
国内免费 | 359篇 |
专业分类
化学 | 4029篇 |
晶体学 | 58篇 |
力学 | 310篇 |
综合类 | 40篇 |
数学 | 827篇 |
物理学 | 2135篇 |
出版年
2024年 | 17篇 |
2023年 | 77篇 |
2022年 | 240篇 |
2021年 | 216篇 |
2020年 | 258篇 |
2019年 | 207篇 |
2018年 | 175篇 |
2017年 | 167篇 |
2016年 | 261篇 |
2015年 | 274篇 |
2014年 | 257篇 |
2013年 | 437篇 |
2012年 | 434篇 |
2011年 | 511篇 |
2010年 | 326篇 |
2009年 | 320篇 |
2008年 | 378篇 |
2007年 | 364篇 |
2006年 | 329篇 |
2005年 | 283篇 |
2004年 | 229篇 |
2003年 | 195篇 |
2002年 | 166篇 |
2001年 | 141篇 |
2000年 | 89篇 |
1999年 | 99篇 |
1998年 | 61篇 |
1997年 | 53篇 |
1996年 | 70篇 |
1995年 | 57篇 |
1994年 | 71篇 |
1993年 | 56篇 |
1992年 | 49篇 |
1991年 | 39篇 |
1990年 | 41篇 |
1989年 | 30篇 |
1988年 | 22篇 |
1987年 | 24篇 |
1986年 | 18篇 |
1985年 | 30篇 |
1984年 | 40篇 |
1983年 | 16篇 |
1982年 | 35篇 |
1981年 | 19篇 |
1980年 | 22篇 |
1979年 | 37篇 |
1978年 | 26篇 |
1976年 | 23篇 |
1975年 | 15篇 |
1974年 | 20篇 |
排序方式: 共有7399条查询结果,搜索用时 15 毫秒
981.
Carbon capture and sequestration are the major applied techniques for mitigating emission. The marked affinity of carbon dioxide to react with amino groups is well known, and the amine scrubbing process is the most widespread technology. Among various compounds and solutions containing amine groups, in biodegradability and biocompatibility perspectives, amino acid ionic liquids (AAILs) are a very promising class of materials having good absorption capacity. The reaction of amines with follows a multi-step mechanism where the initial pathway is the formation of the bond between the group and . The added product has a zwitterionic character and can rearrange to give a carbamic derivative. These steps of the mechanism have been investigated in the present study by quantum mechanical methods by considering three ILs where amino acid anions are coupled with choline cations. Glycinate, L-phenylalanilate and L-prolinate anions have been compared with the aim of examining if different local structural properties of the amine group can affect some fundamental steps of the absorption mechanism. All reaction pathways have been studied by DFT methods considering, first, isolated anions in a vacuum as well as in a liquid continuum environment. Subsequently, the role of specific interactions of the anion with a choline cation has been investigated, analyzing the mechanism of the amine– reaction, including different coupling anion–cation structures. The overall reaction is exothermic for the three anions in all models adopted; however, the presence of the solvent, described by a continuum medium as well as by models, including specific cation- -anion interactions, modifies the values of the reaction energies of each step. In particular, both reaction steps, the addition of to form the zwitterionic complex and its subsequent rearrangement, are affected by the presence of the solvent. The reaction enthalpies for the three systems are indeed found comparable in the models, including solvent effects. 相似文献
982.
983.
用体视学方法分析大鼠脑缺血再灌注后大脑运动皮质细胞的凋亡情况。随机将24只SD大鼠分为缺血再灌注后6、12、24 h和对照组。采用结扎大鼠左侧颈总动脉和夹闭右侧颈总动脉15m in后放开的方法制备大鼠脑缺血再灌注模型。应用Hoechst33258荧光染色及免疫组化Bax蛋白表达来检测细 相似文献
984.
An existence result of multiple solutions for a fourth-order Sturm-Liouville boundary value problem with variable parameters is established. As a consequence, three solutions for a boundary value problem with a fourth-order equation in a complete form are obtained. Our approach is based on variational methods. 相似文献
985.
986.
987.
1 IntroductionIn this paper we study the blow--up of positive solutions as A - 0+ fOr thefOllowing boundary value problem of the weighted pLaplacian on the unit ballcenter at the origin B = B1(0) C Rn:where cr5 u > fi --p > --n and p* = p(n + a)/(fi +n --p) is the critical exponentof the embedding Wl"(fl, lxtpdx) - Lp*(fl, lxl"dx) (see, for example, [6]).When u is radially symmetric, y(r) = u(x), fOr r = lxl, satisfieswhere the new exponents cr, g, p and p* satisfyThe existence and non-exi… 相似文献
988.
Rui Yan Li Zhi Ru Li Di Wu Xi Yun Hao Ru Jiao Li Chia Chung Sun 《International journal of quantum chemistry》2005,103(2):157-166
Using four basis bets, (6‐311G(d,p), 6‐31+G(d,p), 6‐31++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for the dimers CH2O? HF, CH2O? H2O, CH2O? NH3, and CH2O? CH4. The structures of CH2O? HF, CH2O? H2O, and CH2O? NH3 are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the H atoms of CH2O and lone pair(s) on the F atom in HF, the O atom in H2O, or the N atom in NH3. In contrast with the above three dimers, for CH2O? CH4, because there is not a π‐type hydrogen bond to bend its linear hydrogen bond, the structure of CH2O? CH4 is noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD (T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
989.
为了探讨乌鳢、草鱼、鲫鱼脑垂体的组织学结构,选用H.E.染色法、Mallory—Heidenhain—Azan染色法、组织学和体视学方法研究乌鳢、草鱼、鲫鱼脑垂体的形态结构和分泌细胞的分布。结果表明:(1)乌鳢脑垂体的形态结构为前后型、草鱼和鲫鱼脑垂体的形态结构为背腹型。(2)经体视学测算,每种鱼在垂体前外侧部、垂体中外侧部和垂体中间部中的嗜酸(碱)性细胞的体积密度和数密度均有极显著差异,表明这三种鱼的脑垂体在垂体前外侧部、垂体中外侧部和垂体中间部中嗜酸(碱)性细胞的分布不均匀。另外在不同鱼种之间,垂体前外侧部、垂体中外侧部或垂体中间部中的嗜酸(碱)性细胞的体积密度和数密度也有极显著差异,表明由于鱼种不同,在垂体前外侧部、垂体中外侧部或垂体中间部的嗜酸(碱)性细胞的分布有较大差异。 相似文献
990.
Florio M. Ciaglia Fabio Di Cosmo Alberto Ibort Giuseppe Marmo 《Entropy (Basel, Switzerland)》2020,22(11)
The evolution of states of the composition of classical and quantum systems in the groupoid formalism for physical theories introduced recently is discussed. It is shown that the notion of a classical system, in the sense of Birkhoff and von Neumann, is equivalent, in the case of systems with a countable number of outputs, to a totally disconnected groupoid with Abelian von Neumann algebra. The impossibility of evolving a separable state of a composite system made up of a classical and a quantum one into an entangled state by means of a unitary evolution is proven in accordance with Raggio’s theorem, which is extended to include a new family of separable states corresponding to the composition of a system with a totally disconnected space of outcomes and a quantum one. 相似文献