全文获取类型
收费全文 | 127975篇 |
免费 | 16701篇 |
国内免费 | 12007篇 |
专业分类
化学 | 80630篇 |
晶体学 | 1331篇 |
力学 | 7050篇 |
综合类 | 680篇 |
数学 | 21419篇 |
物理学 | 45573篇 |
出版年
2024年 | 223篇 |
2023年 | 2002篇 |
2022年 | 2297篇 |
2021年 | 3405篇 |
2020年 | 4124篇 |
2019年 | 3879篇 |
2018年 | 4679篇 |
2017年 | 4577篇 |
2016年 | 5498篇 |
2015年 | 5197篇 |
2014年 | 6227篇 |
2013年 | 8145篇 |
2012年 | 12154篇 |
2011年 | 11706篇 |
2010年 | 8381篇 |
2009年 | 7850篇 |
2008年 | 7182篇 |
2007年 | 6527篇 |
2006年 | 6139篇 |
2005年 | 8927篇 |
2004年 | 7439篇 |
2003年 | 5137篇 |
2002年 | 3103篇 |
2001年 | 2505篇 |
2000年 | 2032篇 |
1999年 | 2217篇 |
1998年 | 1844篇 |
1997年 | 1626篇 |
1996年 | 1529篇 |
1995年 | 1386篇 |
1994年 | 1271篇 |
1993年 | 1010篇 |
1992年 | 1060篇 |
1991年 | 925篇 |
1990年 | 780篇 |
1989年 | 606篇 |
1988年 | 470篇 |
1987年 | 390篇 |
1986年 | 372篇 |
1985年 | 297篇 |
1984年 | 215篇 |
1983年 | 178篇 |
1982年 | 143篇 |
1981年 | 96篇 |
1980年 | 71篇 |
1979年 | 53篇 |
1976年 | 101篇 |
1975年 | 59篇 |
1974年 | 48篇 |
1973年 | 65篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
951.
本文考察了自体移植静脉的纵向应力应变关系。将狗股静脉移植于股动脉,在术后不同时间切取移植静脉进行纵向单轴拉伸实验。结果表明,移植静脉的应力-应变曲线比正常静脉靠近应力坐标轴并在术后逐渐向左移动。这显示了移植血管管壁组织在术后的逐渐硬化。 相似文献
952.
闪光灯泵浦的Nd:KGd(WO4)2激光器 总被引:1,自引:0,他引:1
报道了用闪光灯泵浦的掺钕的钨酸钆镓(Nd3+:KGd(WO4)2激光器.在重复频率1Hz,脉宽120μs的灯泵下,测量了不同透过率下激光输出能量,获得最大输出能量384mJ,斜率效率1.0%,外推阈值0.54J.在调Q运行时,Nd:KGW晶体的斜率效率为0.16%,最大输出能量46mJ. 相似文献
953.
954.
Based on the anisotropic Seebeck tensor coefficients, a light-thermo-radiation detector made of high-Tc superconductor (YBa2Cu3O7-δ) was fabricated, which can function at room temperature. The induced voltaic signals at various modulation frequencies for radiation from He-Ne laser and from a 500 K black body were measured. The noise ratio for detecting He-Ne laser radiation and the D* for black body were evaluated. Compared with bolometer made of high-Tc superconductor which functions at liquid nitrogen, and with pyroelectric detector, it was found that the advantages of this new device are that the device can work at room temperature with very low noise and very fast response. Although having a lower D* value than that of bolometer for the present device construction, we point out the possibility and the direction for improving the D* value, hence the promising prospect of this kind of device. 相似文献
955.
Hui-Zhen Li Cheng-tian Xue Xiang-wen Chen 《International Journal of Infrared and Millimeter Waves》1991,12(12):1425-1436
This paper describes a global optimization design for quasi-optics power combiner of solid-state millimeter-wave sources. The operating frequency is about 100 GHz and the oscillating sources are GaAs Gunn diodes. The source-array plane is a matrix consisting of NxN (N=3,5, ..., 2k+1, where k is positive integer) diodes. The radio frequency power of the combiner is from threefold to sevenfold larger than the sum total of the single diodes power. The global optimums of the array and the location in the cuvity of the suorce-array plane will be given by a global optimizition method. 相似文献
956.
The interaction of the synthetic oligonucleotide d(C-G-C-G-A-A-T-T-C-G-C-G)2 with two different transition-metal ions has been investigated in aqueous solution by means of 1H NMR spectroscopy. The effects on the DNA due to the presence of manganese(II) or zinc(II) have been monitored by observing the paramagnetic broadening and diamagnetic shifts of the non-exchangeable proton resonance lines, respectively. The 1H NMR spectra acquired during the course of the manganese(II) titration show very distinct broadening effects on certain DNA resonance lines. Primarily, the H8 resonance of G4 is affected, but also the H5 and H6 resonances of C3 are clearly affected by the metal. The results imply that the binding of manganese(II) to DNA is sequence specific. The 1H spectra obtained during the zinc(II) titration reveal diamagnetic shift effects which largely conform with the paramagnetic broadening effects due to the presence of manganese(II), although this picture is somewhat more complex. The H8 resonance of G4 displays a clearly visible high-field shift, while for the other guanosine H8 protons this effect is absent. The H1' and H2' protons of C3 show an effect of similar strength, although in the opposite direction, while H5 and H6 of C3 are only slightly affected. Local differences in the structure of the DNA and the basicities of potential binding sites on different base steps in the sequence might account for the observed sequence selectivity. 相似文献
957.
Three New Diarylpropanes from Dioscorea composita 总被引:1,自引:0,他引:1
ShunLiYANG XiKuiLIU 《中国化学快报》2005,16(1):57-60
Three new diarylpropanes were isolated from the dried rhizomes of Dioscorea composita Hemsl., and their structures were determined as 1, 3-bis-(2-hydroxy-4-methoxyphenyl) propane (1), 1, 3-bis-(2, 4-dihydroxyphenyl)propane (2), 1-(2′-hydroxy-4′-O-β-D- glucopymnosy-phenyl)-3-(2“, 4“-dihydroxyphenyl)propane (3), by spectroscopic analysis, respectively. 相似文献
958.
959.
960.
一个包含Smarandache函数的方程 总被引:2,自引:0,他引:2
对于任意正整数n,我们用S(n)表示Smarandache函数,即S(n)=min{m:n|m!}.本文的主要目的是运用初等方法研究方程∑_(d|n)S(d)=n的可解性,并给出它的所有正整数解. 相似文献