全文获取类型
收费全文 | 35789篇 |
免费 | 923篇 |
国内免费 | 691篇 |
专业分类
化学 | 12692篇 |
晶体学 | 313篇 |
力学 | 1665篇 |
综合类 | 29篇 |
数学 | 9320篇 |
物理学 | 9673篇 |
综合类 | 3711篇 |
出版年
2023年 | 168篇 |
2022年 | 188篇 |
2021年 | 238篇 |
2020年 | 205篇 |
2019年 | 172篇 |
2018年 | 1345篇 |
2017年 | 1589篇 |
2016年 | 935篇 |
2015年 | 795篇 |
2014年 | 775篇 |
2013年 | 1003篇 |
2012年 | 3550篇 |
2011年 | 2688篇 |
2010年 | 2079篇 |
2009年 | 1787篇 |
2008年 | 812篇 |
2007年 | 865篇 |
2006年 | 845篇 |
2005年 | 4795篇 |
2004年 | 4514篇 |
2003年 | 2855篇 |
2002年 | 693篇 |
2001年 | 427篇 |
2000年 | 345篇 |
1999年 | 382篇 |
1998年 | 258篇 |
1997年 | 202篇 |
1996年 | 170篇 |
1995年 | 145篇 |
1994年 | 134篇 |
1993年 | 114篇 |
1992年 | 280篇 |
1991年 | 231篇 |
1990年 | 223篇 |
1989年 | 178篇 |
1988年 | 146篇 |
1987年 | 82篇 |
1986年 | 69篇 |
1985年 | 64篇 |
1984年 | 46篇 |
1979年 | 49篇 |
1976年 | 89篇 |
1975年 | 50篇 |
1974年 | 51篇 |
1973年 | 51篇 |
1970年 | 49篇 |
1969年 | 49篇 |
1968年 | 52篇 |
1967年 | 47篇 |
1966年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
以应用ProCAST和MAGMAsoft 2款软件为例,针对铸造工艺仿真设计前处理和过程处理每个环节的主要内容与使用方法进行了较为详细地归纳、分析和总结。提出了处理过程的技术路线,并系统地介绍了每项应用内容的操作平台与使用步骤,从而为高效地应用铸造工艺仿真设计提供理论和技术支持。 相似文献
2.
Pu Duan Bo Zhi Luke Coburn Christy L. Haynes Klaus Schmidt-Rohr 《Magnetic resonance in chemistry : MRC》2020,58(11):1130-1138
The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave-assisted reaction of citric acid and ethylenediamine was investigated by 13C, 13C{1H}, 1H─13C, 13C{14N}, and 15N solid-state nuclear magnetic resonance (NMR) experiments. 13C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13C NMR peaks, including an unusual ═CH signal at 84 ppm (1H chemical shift of 5.8 ppm) and ═CN2 at 155 ppm, and two distinctive 15N NMR resonances near 80 and 160 ppm proved the presence of 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross-peaks in a 1H─13C HETCOR spectrum with brief 1H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13C and 15N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13C chemical-shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation. 相似文献
3.
本文给出了ρ~0→π~+π~-衰变的么正计算及其衰变率和ρ~2极化插入的关系.ρ~0极化插入的虚部是用维度积分的方法来计算.极化插入虚部的计算方法是普适的. 相似文献
4.
对非线性叠加效应进行了研究,在Toda晶格中,当两孤立子运动方向相同时,其速度波的振幅的极大值是原来两波振幅之差,两波叠加的最大振幅是原来两波振幅之和。这些结果跟线性波叠加情况是完全不同的。 相似文献
5.
Julia K Pruns Jens-Peter Vietzke Manfred Strassner Claudius Rapp Ulrich Hintze Wilfried A K?nig 《Rapid communications in mass spectrometry : RCM》2002,16(3):208-211
A new solvent-free sample preparation method using silver trifluoroacetate (AgTFA) was developed for the analysis of low molecular weight paraffins and microcrystalline waxes by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Experiments show that spectral quality can be enhanced by dispersing AgTFA directly in liquid paraffins without the use of additional solvents. This preparation mixture is applied directly to the MALDI probe. Solid waxes could be examined by melting prior to analysis. The method also provides sufficiently reproducible spectra that peak area ratios between mono- and bicyclic alkane peaks indicated variations in the cycloalkane content of paraffin samples. Dehydrogenation of hydrocarbons observed during the desorption/ionization process was studied by analysis of alkane standards. 相似文献
6.
The versatile behavior of a trihydrazidophosphoradamantane allowing the synthesis of a variety of neutral, mono- or di-cationic water-soluble molecules of potential interest for biphasic catalysis is reported. 相似文献
7.
Based on the vectorial Rayleigh–Sommerfeld diffraction integrals, analytical expressions for the transversal and axial field distribution of plane waves propagating through a thin lens followed by a small circular aperture are derived and used to study the focusing and diffraction properties of plane waves. Some special cases of our general result are discussed, and illustrative numerical calculation results are given. It is found that the vectorial nonparaxial approach should be applied if the aperture dimension is comparable with the wavelength or the focusing is strong. 相似文献
8.
In this paper we construct an example of a word metric on an infinite cyclic subgroup. This example shows that subexponential
distortion does not obstruct non-trivial growth of connected radii. This answers a question of Gromov [6]. The constructed
metric has other pathological properies. Specifically, its asymptotic cone depends on the choice of ultrafilter and scaling
sequence.
The work has been partially supported by the Swiss National Science Foundation. 相似文献
9.
We build a metric space which is homeomorphic to a Cantor set but cannot be realized as the attractor of an iterated function system. We give also an example of a Cantor set K in R3 such that every homeomorphism f of R3 which preserves K coincides with the identity on K. 相似文献
10.
综述了关于过度训练状态下机体神经内分泌方面的变化,并对其中一些矛盾的结果提出了可能的解释。 相似文献