全文获取类型
收费全文 | 42079篇 |
免费 | 5908篇 |
国内免费 | 4415篇 |
专业分类
化学 | 29165篇 |
晶体学 | 442篇 |
力学 | 2683篇 |
综合类 | 393篇 |
数学 | 4842篇 |
物理学 | 14877篇 |
出版年
2024年 | 146篇 |
2023年 | 754篇 |
2022年 | 1260篇 |
2021年 | 1409篇 |
2020年 | 1512篇 |
2019年 | 1483篇 |
2018年 | 1290篇 |
2017年 | 1185篇 |
2016年 | 1801篇 |
2015年 | 1815篇 |
2014年 | 2218篇 |
2013年 | 2915篇 |
2012年 | 3630篇 |
2011年 | 3790篇 |
2010年 | 2455篇 |
2009年 | 2480篇 |
2008年 | 2694篇 |
2007年 | 2475篇 |
2006年 | 2227篇 |
2005年 | 1812篇 |
2004年 | 1369篇 |
2003年 | 1135篇 |
2002年 | 1134篇 |
2001年 | 897篇 |
2000年 | 858篇 |
1999年 | 903篇 |
1998年 | 767篇 |
1997年 | 740篇 |
1996年 | 850篇 |
1995年 | 673篇 |
1994年 | 568篇 |
1993年 | 488篇 |
1992年 | 454篇 |
1991年 | 406篇 |
1990年 | 319篇 |
1989年 | 266篇 |
1988年 | 195篇 |
1987年 | 168篇 |
1986年 | 162篇 |
1985年 | 142篇 |
1984年 | 106篇 |
1983年 | 83篇 |
1982年 | 55篇 |
1981年 | 46篇 |
1980年 | 22篇 |
1978年 | 21篇 |
1977年 | 24篇 |
1976年 | 21篇 |
1975年 | 29篇 |
1974年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
透射式跑道能见度激光测量仪的研究 总被引:10,自引:0,他引:10
介绍了一种利用激光测量跑道水平及斜视能见度的智能化仪器。该仪器主要由光学系统、信号放大与处理系统及显示系统三部分组成。简要介绍了该仪器的基本结构、工作原理和主要技术指标,讨论了其中的技术难点及其相应的解决办法。 相似文献
122.
J. Telser H. -I. Lee E. T. Smith H. Huang P. Brereton M. W. W. Adams R. C. Conover M. K. Johnson B. M. Hoffman 《Applied magnetic resonance》1998,14(2-3):305-321
The hyperthermophilic archaeonPyrococcus furiosus contains a four-Fe ferredoxin (Pf- Fd) that differs from most other 4Fe-Fd’s in that its [Fe4S4] cluster is anchored to protein by only three cysteinyl residues.Pf- Fd also is of interest because in its reduced form, [Fe4S4]+, the cluster exhibits bothS = 1/2 andS = 3/2 spin states. Addition of excess cyanide ion converts the cluster exclusively to anS = 1/2 state (g1 = 2.09, g2 = 1.95, g3 = 1.92), however dialysis restores the EPR signal of native reduced protein indicating that the cluster is not irreversibly altered by cyanide. Both the native protein and protein in the presence of excess cyanide ion (Pf- Fd 4Fe-CN) were investigated here using the techniques of electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy. In particular,Pf- Fd 4Fe-CN was investigated using13CN? and C15N? ligands.13C and15N ENDOR indicated that a single cyanide ion bound directly, with the cluster showing an unusually small contact interaction (aiso(13C)~ ?3 MHz, aiso(15N) ~ 0). This is in contrast to cyanide bound to monomeric low-spin Fe(III)-containing proteins such as transferrin and myoglobin, for which the13C hyperfine coupling has a large isotropic component (aiso(13C) ≈ ?30 MHz). This small contact interaction is not due to low spin density of Fe, as57Fe ENDOR of the singly and triply labeledPf- Fd 4FeCN isotopologs, [57FeFe3S4]+ and [Fe57Fe3S4]+, show hyperfine coupling characteristic for [Fe4S4]+ clusters, particularly for the Fe to which cyanide binds. Thus, the low spin density on13C is not due to low spin density on the Fe ion to which it binds. Further theoretical work is needed to explain the contrast between the strong electronic effect of cyanide ion binding with the low spin density on the ligand. 相似文献
123.
124.
报道了采用真空蒸镀的方法制备的MnBiDy(Sm)永磁膜的结构和磁性.Mn:Bi:Dy(Sm)的投料比为2:1:0.10.薄膜经过退出处理(350—425℃,保温4h)后,具有NiAs型hcp结构,C轴垂直膜面,剩余磁感应强度B=3.4-6.5kG,内禀矫顽力_MH_c≈3-8kOe,最大磁能积(BH)_(max)=13.39MG·Oe.实验发现,薄膜厚度对MnBiDy(Sm)永磁膜的结构和磁性具有很大的影响.
关键词: 相似文献
125.
A review of the advance in the theory of wavelet analysis in recent years is given. 相似文献
126.
127.
128.
129.
In this paper we study the existence of limit cycle for cubic system ()3, of Kolmogorov type with a conic algebraic trajectory
相似文献
130.
Quadratically constrained minimum cross-entropy analysis 总被引:3,自引:0,他引:3
Quadratically constrained minimum cross-entropy problem has recently been studied by Zhang and Brockett through an elaborately constructed dual. In this paper, we take a geometric programming approach to analyze this problem. Unlike Zhang and Brockett, we separate the probability constraint from general quadratic constraints and use two simple geometric inequalities to derive its dual problem. Furthermore, by using the dual perturbation method, we directly prove the strong duality theorem and derive a dual-to-primal conversion formula. As a by-product, the perturbation proof gives us insights to develop a computation procedure that avoids dual non-differentiability and allows us to use a general purpose optimizer to find an-optimal solution for the quadratically constrained minimum cross-entropy analysis. 相似文献
|