全文获取类型
收费全文 | 86149篇 |
免费 | 8083篇 |
国内免费 | 14460篇 |
专业分类
化学 | 65376篇 |
晶体学 | 2673篇 |
力学 | 1452篇 |
综合类 | 632篇 |
数学 | 17356篇 |
物理学 | 21203篇 |
出版年
2024年 | 150篇 |
2023年 | 1189篇 |
2022年 | 1588篇 |
2021年 | 2412篇 |
2020年 | 2674篇 |
2019年 | 2753篇 |
2018年 | 2357篇 |
2017年 | 2826篇 |
2016年 | 2868篇 |
2015年 | 2562篇 |
2014年 | 3571篇 |
2013年 | 7387篇 |
2012年 | 5015篇 |
2011年 | 5826篇 |
2010年 | 5009篇 |
2009年 | 6091篇 |
2008年 | 6024篇 |
2007年 | 6043篇 |
2006年 | 5521篇 |
2005年 | 4722篇 |
2004年 | 4406篇 |
2003年 | 3609篇 |
2002年 | 3154篇 |
2001年 | 2554篇 |
2000年 | 2431篇 |
1999年 | 2062篇 |
1998年 | 1773篇 |
1997年 | 1521篇 |
1996年 | 1417篇 |
1995年 | 1348篇 |
1994年 | 1278篇 |
1993年 | 1027篇 |
1992年 | 884篇 |
1991年 | 667篇 |
1990年 | 511篇 |
1989年 | 479篇 |
1988年 | 378篇 |
1987年 | 285篇 |
1986年 | 269篇 |
1985年 | 322篇 |
1984年 | 251篇 |
1983年 | 124篇 |
1982年 | 207篇 |
1981年 | 229篇 |
1980年 | 197篇 |
1979年 | 172篇 |
1978年 | 141篇 |
1977年 | 120篇 |
1976年 | 88篇 |
1973年 | 61篇 |
排序方式: 共有10000条查询结果,搜索用时 453 毫秒
101.
非傍轴平顶高斯光束M2因子两种定义的比较研究 总被引:1,自引:1,他引:0
基于功率密度的二阶矩方法,推导出了非傍轴平顶高斯(FG)光束束宽和远场发散角的解析表达式.研究表明,当w0/λ→0时,远场发散角趋于渐近值θmax=63.435°,与阶数无关.使用非傍轴高斯光束代替傍轴高斯光束作为理想光束,研究了非傍轴FG光束的M2因子,并与传统定义的M2因子作了比较.在非傍轴范畴,非傍轴FG光束的M2因子不仅与阶数N有关,而且与w0/λ有关.按照定义,当w0/λ→0时,非傍轴FG光束的M2因子不等于0,对阶数N=1, 2, 3时,M2因子分别趋于0.913,0.882和0.886.当N→∞时,M2因子取最小值M2min=0.816. 相似文献
102.
This paper reports that the m-plane GaN layer is grown on (200)-plane LiAlO2 substrate by metal-organic chemical wpour deposition (MOCVD) method. Tetragonal-shaped crystallites appear at the smooth surface. Raman measurement illuminates the compressive stress in the layer which is released with increasing the layer's thickness. The high transmittance (80%), sharp band edge and excitonic absorption peak show that the GaN layer has good optical quality. The donor acceptor pair emission peak located at -3.41 eV with full-width at half maximum of 120 meV and no yellow peaks in the photoluminescence spectra partially show that no Li incorporated into GaN layer from the LiAlO2 substrate. 相似文献
103.
104.
主要介绍了一个引理,这个引理奠定了K4-同胚图K4(α,1,1,δ,ε,η)色性研究的基础。 相似文献
105.
106.
The paper provides significant simplifications and extensions of results obtained by Gorsich, Genton, and Strang (J. Multivariate Anal. 80 (2002) 138) on the structure of spatial design matrices. These are the matrices implicitly defined by quadratic forms that arise naturally in modelling intrinsically stationary and isotropic spatial processes. We give concise structural formulae for these matrices, and simple generating functions for them. The generating functions provide formulae for the cumulants of the quadratic forms of interest when the process is Gaussian, second-order stationary and isotropic. We use these to study the statistical properties of the associated quadratic forms, in particular those of the classical variogram estimator, under several assumptions about the actual variogram. 相似文献
107.
Parallel to Cox's [JRSS B34 (1972) 187-230] proportional hazards model, generalized logistic models have been discussed by Anderson [Bull. Int. Statist. Inst. 48 (1979) 35-53] and others. The essential assumption is that the two densities ratio has a known parametric form. A nice property of this model is that it naturally relates to the logistic regression model for categorical data. In astronomic, demographic, epidemiological, and other studies the variable of interest is often truncated by an associated variable. This paper studies generalized logistic models for the two-sample truncated data problem, where the two lifetime densities ratio is assumed to have the form exp{α+φ(x;β)}. Here φ is a known function of x and β, and the baseline density is unspecified. We develop a semiparametric maximum likelihood method for the case where the two samples have a common truncation distribution. It is shown that inferences for β do not depend the nonparametric components. We also derive an iterative algorithm to maximize the semiparametric likelihood for the general case where different truncation distributions are allowed. We further discuss how to check goodness of fit of the generalized logistic model. The developed methods are illustrated and evaluated using both simulated and real data. 相似文献
108.
陈建军 《宁波大学学报(理工版)》2002,15(3):83-86
提出了一种能用凸四边形表示并可作平面展开的三维实体造型的设计方法。该方法用旋转描述表把二维平面展开图和三维实体联系起来,模拟人工折叠过程,由二维平面展开图逐次旋转变换完成三维造型重建。造型重建后,再通过纹理映射将平面设计图案映射到实体的各个面上,完成对实体着色渲染,生成具有真实感的三维实体。 相似文献
109.
本文首先将文[1]中的BLD映射推广为弱(L1,L2)-BLD映射,并证明了如下正则性结果:存在两个可积指数 P1=P1(n,L1,L2)<n<q1=q1(n,L1,L2),使得对任意弱(L1,L2)-BLD映射f∈(Ω,Rn),都有f∈(Ω,Rn),即f为(L1,L2)-BLD映射. 相似文献
110.
Chemical bonding in isolated molecules and crystals of zwitterionic pentacoordinate silicon chelates
The electronic structures of a number of zwitterionic pentacoordinate silicon chelates were investigated using the results of X-ray diffraction studies and quantum-chemical calculatoins by the MPW1PW91/6-311G(d) method. The topological analysis of the electron density distribution function and the study in the framework of the natural bond orbital partitioning scheme showed that the character of chemical bonding in the axial fragments of the molecules under consideration changes from dative to three-center, four-electron as the silicon atom assumes a trigonal-bipyramidal coordination. 相似文献