全文获取类型
收费全文 | 21421篇 |
免费 | 1760篇 |
国内免费 | 704篇 |
专业分类
化学 | 12536篇 |
晶体学 | 158篇 |
力学 | 820篇 |
综合类 | 52篇 |
数学 | 3952篇 |
物理学 | 6367篇 |
出版年
2023年 | 232篇 |
2022年 | 281篇 |
2021年 | 386篇 |
2020年 | 533篇 |
2019年 | 455篇 |
2018年 | 547篇 |
2017年 | 526篇 |
2016年 | 967篇 |
2015年 | 799篇 |
2014年 | 933篇 |
2013年 | 1679篇 |
2012年 | 1404篇 |
2011年 | 1556篇 |
2010年 | 1018篇 |
2009年 | 918篇 |
2008年 | 1124篇 |
2007年 | 938篇 |
2006年 | 916篇 |
2005年 | 661篇 |
2004年 | 573篇 |
2003年 | 465篇 |
2002年 | 462篇 |
2001年 | 372篇 |
2000年 | 331篇 |
1999年 | 330篇 |
1998年 | 305篇 |
1997年 | 274篇 |
1996年 | 272篇 |
1995年 | 237篇 |
1994年 | 200篇 |
1993年 | 192篇 |
1992年 | 198篇 |
1991年 | 178篇 |
1990年 | 144篇 |
1989年 | 147篇 |
1988年 | 147篇 |
1987年 | 142篇 |
1986年 | 134篇 |
1985年 | 165篇 |
1984年 | 160篇 |
1983年 | 150篇 |
1982年 | 134篇 |
1981年 | 126篇 |
1980年 | 133篇 |
1979年 | 117篇 |
1978年 | 148篇 |
1977年 | 129篇 |
1976年 | 122篇 |
1974年 | 118篇 |
1973年 | 120篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
21.
22.
In this paper, we consider a viscoelastic wave equation of variable coefficients in the presence of past history with nonlinear damping and delay in the internal feedback and dynamic boundary conditions. Under suitable assumptions, we establish an explicit and general decay rate result without imposing restrictive assumption on the behavior of the relaxation function at infinity by Riemannian geometry method and Lyapunov functional method. 相似文献
23.
In this study, stoichiometric protonation constants of L-tyrosine, L-cysteine, L-tryptophane, L-lysine, and L-histidine, and
their methyl and ethyl esters in water and ethanol–water mixtures of 30, 50, and 70% ethanol (v/v), were determined potentiometrically
using a combined pH electrode system calibrated as the concentration of hydrogen ion. Titrations were performed at 25∘C and the ionic strength of the medium was maintained at 0.10 mol⋅L−1 using sodium chloride. Protonation constants were calculated by using the BEST computer program. The effect of solvent composition
on the protonation constants is discussed. The log10 K2 values of esters generally decreased with increasing ethanol content. However, the log10 K1 values of the esters of L-tyrosine, L-cysteine, and L-tryptophane were found to increase with increasing ethanol content
in contrast those of L-lysine and L-histidine esters. 相似文献
24.
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
25.
26.
Chromatographia - A theoretical model is used to predict retention times of divalent metal cations in single-column ion chromatography using one or more ligands in the eluent. Stability constants... 相似文献
27.
以ZnO和HGaO2为原料,用不同配比合成出系列ZnGa2O4,并对其晶体结构和发光性能进行了研究。用荧光分光光度计检测了ZnGa2O4的激发和发射光谱,用X射线衍射仪检测了ZnGa2O4的衍射图谱,用热重差热仪绘制了TGA-DAT曲线。对检测结果分析认为:1.ZnGa2O4属于尖晶石结构,稍过量的Zn或Ga能进入ZnGa2O4结构中,并对ZnGa2O4的晶格常数产生一定影响。2.ZnGa2O4存在两个自激发光中心,当Ga稍过量时,自激发光中心是四面体镓氧键[Td(Ga-O)],最大激发波长约248nm,最大发射波长约367nm;当Zn稍过量时,自激发光中心是八面体镓氧键[Oh(Ga-O)],最大激发波长约270nm,最大发射波长约441nm。当n(Zn):n(Ga)在理论值附近,激发和发射光强度最大,而且光谱峰位发生了红移。3.ZnGa2O4的热稳定性能非常好。上述结论对研究ZnGa2O4基质或掺杂的发光材料具有一定意义。 相似文献
28.
29.
Boban Marinković 《Mathematical Methods of Operations Research》2006,63(3):513-524
We present local sensitivity analysis for discrete optimal control problems with varying endpoints in the case when the customary regularity of boundary conditions can be violated. We study the behavior of the optimal solutions subject to parametric perturbations of the problem. 相似文献
30.
Chunguang Du Hongyi Chen Shiqun Li 《量子光学学报》2006,12(B08):70-71
Recently, research on left-handed materials (LHMs) has attracted considerable attentions. The LHMs are a kind of metameterial which have negative permittivity and negative permeability, which lead to negative refractive index in a frequency range. The LHMs that have been available so far are in the microwave range and are usually composed of classical particles, such as split-ring resonators. Some quantum phenomena such as spontaneous emission of atoms in the LHMs which are considered to be 'classical background' have also been investigated. Many potential applications of LHMs have been proposed, such as superlenses which, in principle, can achieve arbitrary subwavelength resolution. 相似文献