全文获取类型
收费全文 | 10851篇 |
免费 | 2082篇 |
国内免费 | 2314篇 |
专业分类
化学 | 8390篇 |
晶体学 | 125篇 |
力学 | 648篇 |
综合类 | 149篇 |
数学 | 1371篇 |
物理学 | 4564篇 |
出版年
2024年 | 32篇 |
2023年 | 253篇 |
2022年 | 451篇 |
2021年 | 488篇 |
2020年 | 517篇 |
2019年 | 498篇 |
2018年 | 405篇 |
2017年 | 439篇 |
2016年 | 599篇 |
2015年 | 609篇 |
2014年 | 676篇 |
2013年 | 926篇 |
2012年 | 894篇 |
2011年 | 968篇 |
2010年 | 729篇 |
2009年 | 683篇 |
2008年 | 791篇 |
2007年 | 649篇 |
2006年 | 619篇 |
2005年 | 553篇 |
2004年 | 460篇 |
2003年 | 381篇 |
2002年 | 419篇 |
2001年 | 327篇 |
2000年 | 287篇 |
1999年 | 240篇 |
1998年 | 190篇 |
1997年 | 135篇 |
1996年 | 155篇 |
1995年 | 121篇 |
1994年 | 136篇 |
1993年 | 106篇 |
1992年 | 78篇 |
1991年 | 96篇 |
1990年 | 90篇 |
1989年 | 42篇 |
1988年 | 40篇 |
1987年 | 29篇 |
1986年 | 34篇 |
1985年 | 37篇 |
1984年 | 16篇 |
1983年 | 18篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1971年 | 1篇 |
1957年 | 3篇 |
1936年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
301.
For B2 NiAl and NiTi intermetallic compounds, the ideal stress–strain image is lack from the perspective of elastic constants. We use first-principles calculation to investigate the ideal strength and elastic behavior under the tensile and shear loads. The relation between the ideal strength and elastic constants is found. The uniaxial tension of NiAl and NiTi along <001> crystal direction leads to the change from tetragonal path to orthogonal path, which is driven by the vanishing of the shear constant C(66). The shear failure under {110}{111} shear deformation occurring in process of tension may result in a small ideal tensile strength(~ 2 GPa) for NiTi. The unlikeness in the ideal strength of Ni Al and Ni Ti alloys is discussed based on the charge density difference. 相似文献
302.
303.
The formability of the material determines the amount of available deformation before failure and thus is important for the production of various structural components in industries. The workability of materials is commonly evaluated by different forms of failure models during sheet metal forming(SMF) processes. In order to provide a whole picture about the prediction models for SMF failure, necking-related formability and ductile fracture-related formability studies in SMF processes are systematically summarized, the applicability and limitation of each model are highlighted, and the link between forming limit diagram and ductile fracture criterion is pointed out. Conclusions about some critical issues on failure in SMF are made. 相似文献
304.
Nonlinear Dynamics - Global dynamics of a flexible asymmetrical rotor resting on vibrating supports is investigated. Hamilton’s principle is used to derive the partial differential governing... 相似文献
305.
For a general quantum ensemble with Hamiltonian fluctuations, this paper proposes a sampling-based two-stage approximate time-optimal control algorithm with momentum terms and achieves a high-fidelity state transition of all member systems to a common target state within an approximate minimum time. The fidelity and the control time are respectively optimized in the two stages. In particular, the introduction of momentum terms greatly improves the convergence rate of the algorithm. Simulation experiments on a two-level quantum ensemble verify the effectiveness of the proposed algorithm. 相似文献
306.
Studying the effect of oxygen in coal oxidation is very important for understanding and controlling coal spontaneous combustion. However, the oxygen effect is not very easy to determine clearly due to the large effect of heat source on coal oxidation in temperature rising experiments. Here, focused on sub-bituminous coal, the oxygen effect was separated from coal oxidation by continuously measuring FTIR spectra of coal with respect to varying temperatures and under oxygen and nitrogen. The active groups’ real-time changes of coal oxidation, thermal treatment and oxygen effect were measured. The carboxylic ester and carboxyl units are the main functional groups that increase with temperatures increasing under oxygen and nitrogen, while the other functional groups decrease in quantity. The oxygen effect promoted the consumption of aliphatic hydrocarbons and hydroxyl groups and also promoted the formation of oxygen-containing groups (except hydroxyl). Four characteristic temperature stages involved in the oxygen effect and their key functional groups were identified. Simultaneously, the relationship of oxygen consumption and chemisorption in oxygen effect was analyzed. The starting temperature of oxygen chemisorption is between 50 and 60°C. The maximum contribution of oxygen effect was observed in methyl and methylene groups. These results are important for chemical control of coal spontaneous combustion. The oxidation of aliphatic hydrocarbon should be controlled before oxygen chemisorption. The value of oxygen consumption between 70 and 80°C can be measured accurately due to the constant chemisorption rate, which help to identify the tendency for spontaneous combustion. These results will help in better understanding of the reaction mechanism of coal oxidation, especially the oxygen effect. 相似文献
307.
308.
LB薄膜中四新戊氧基酞菁锌和四壬基酞菁铜分子聚集体的吸收光谱的温度特性 总被引:1,自引:2,他引:1
本文制备了四新戊氧基酞菁锌(Tetra-neopentoxy phthalocyanine zine)(TNPPeZn)和四壬基酞菁铜(Tetra-nonyl phthalocyanine copper)(TNPeCu)两种酞菁衍生物的Langmuir-Blodgett(LB)薄膜.通过测量10~473K温度下的吸收光谱,研究了两种薄膜的分子聚集状态.TNPPeZn的LB薄膜中,存在着分子单体和分子二聚体,在吸收光谱中分别表现为680nm和620nm的吸收峰.随着温度的升高,分子单体逐渐转变为分子二聚体,这个过程是不可逆的.TNPeCu的LB薄膜中,除了分子单体和分子二聚体以外,还有吸收为740nm的分子J聚集体存在.随着温度的变化,J聚集体发生可逆变化. 相似文献
309.
The “relative entropy” has been used as a minimization function to predict the tertiary structure of a protein backbone, and
good results have been obtained. However, in our previous work, the ensemble average of the contact potential was estimated
by an approximate calculation. In order to improve the theoretical integrity of the relative-entropy-based method, a new theoretical
calculation method of the ensemble average of the contact potential was presented in this work, which is based on the thermodynamic
perturbation theory. Tests of the improved algorithm were performed on twelve small proteins. The root mean square deviations
of the predicted versus the native structures from Protein Data Bank range from 0.40 to 0.60 nm. Compared with the previous
approximate values, the average prediction accuracy is improved by 0.04 nm.
Contributed equally to this work
Supported by the National Natural Science Foundation of China (Grant No. 30670497), the Beijing Natural Science Foundation
(Grant No. 5072002), and the Specialized Research Foundation for the Doctoral Program of Higher Education (Grant No. 200800050003) 相似文献
310.