全文获取类型
收费全文 | 8149篇 |
免费 | 1340篇 |
国内免费 | 657篇 |
专业分类
化学 | 5529篇 |
晶体学 | 83篇 |
力学 | 490篇 |
综合类 | 36篇 |
数学 | 989篇 |
物理学 | 3019篇 |
出版年
2023年 | 159篇 |
2022年 | 175篇 |
2021年 | 232篇 |
2020年 | 358篇 |
2019年 | 299篇 |
2018年 | 266篇 |
2017年 | 249篇 |
2016年 | 409篇 |
2015年 | 404篇 |
2014年 | 454篇 |
2013年 | 575篇 |
2012年 | 773篇 |
2011年 | 750篇 |
2010年 | 528篇 |
2009年 | 487篇 |
2008年 | 549篇 |
2007年 | 418篇 |
2006年 | 455篇 |
2005年 | 330篇 |
2004年 | 260篇 |
2003年 | 212篇 |
2002年 | 164篇 |
2001年 | 132篇 |
2000年 | 138篇 |
1999年 | 168篇 |
1998年 | 150篇 |
1997年 | 125篇 |
1996年 | 124篇 |
1995年 | 110篇 |
1994年 | 94篇 |
1993年 | 78篇 |
1992年 | 77篇 |
1991年 | 76篇 |
1990年 | 43篇 |
1989年 | 37篇 |
1988年 | 39篇 |
1987年 | 34篇 |
1986年 | 28篇 |
1985年 | 20篇 |
1984年 | 20篇 |
1983年 | 20篇 |
1982年 | 9篇 |
1981年 | 14篇 |
1980年 | 9篇 |
1977年 | 6篇 |
1976年 | 10篇 |
1975年 | 9篇 |
1973年 | 8篇 |
1972年 | 6篇 |
1970年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 216 毫秒
991.
Stability radii for linear time-varying differential-algebraic equations with respect to dynamic perturbations 总被引:1,自引:0,他引:1
Nguyen Huu Du 《Journal of Differential Equations》2006,230(2):579-599
This paper is concerned with the robust stability for linear time-varying differential-algebraic equations. We consider the systems under the effect of uncertain dynamic perturbations. A formula of the structured stability radius is obtained. The result is an extension of a previous result for time-varying ordinary differential equations proven by Birgit Jacob [B. Jacob, A formula for the stability radius of time-varying systems, J. Differential Equations 142 (1998) 167-187]. 相似文献
992.
In this paper, we prove a uniqueness theorem for a free boundary problem which is given in the form of a variational inequality. This free boundary problem arises as the limit of an equation that serves as a basic model in population biology. Apart from the interest in the problem itself, the techniques used in this paper, which are based on the regularity theory of variational inequalities and of harmonic functions, are of independent interest, and may have other applications.
993.
An exfoliated poly(4,4′-oxybis(benzene)disulfide) (POBDS)/vermiculite (VMT) nanocomposite was prepared by using cyclo(4,4′-oxybis(benzene)disulfide) (COBDS) oligomers and cetyltrimethyl ammonium bromide exchanged VMT. The POBDS/VMT nanocomposites were fabricated in two steps. First, the COBDS oligomers were used to swell and exfoliate organo VMT to afford COBDS-VMT nanocomposite precursor. Subsequently, the exfoliated POBDS-VMT nanocomposite can be made via in situ and instant melt ring-opening polymerization of the COBDS-VMT nanocomposite precursor. High molecular weight POBDS polymer can be formed in a few minutes. The nano scale dispersion of VMT layers within POBDS polymer was confirmed by both the X-ray diffraction patterns and TEM examinations. This methodology provides a potential approach to synthesize high performance polymer nanocomposite. 相似文献
994.
995.
L. J. Qin X. L. Meng Ch. L. Du L. Zhu B. Ch. Xu H. Zh. Xu F. Y. Jiang Z. Sh. Shao 《Optics & Laser Technology》2004,36(1):47-50
We have demonstrated a passively Q-switched operation of Nd:GdVO4 laser in which a GaAs crystal is used as the saturable absorber for the first time as far as we know. A maximum average output power of 1.64 W was obtained at an incident pump power of 12 W, the corresponding optical conversion efficiency and peak power were 13.7% and 116.8 W, respectively. The maximum peak energy obtained in the experiment by 50% transmission couple was 19 μJ. 相似文献
996.
In this paper, we aim to investigate the difference equation
\begin{align*}
\Delta^{2}y(t-1)+|y(t)|=0, \ \ \ \ \ t\in[1,T]_{\mathbb{Z}}
\end{align*}
with different boundary conditions $y(0)=0$ or $\Delta y(0)=0$ and $y(T+1)=B$ or $\Delta y(T)=B$,\ where\ $T\geq 1$ is an integer and $B\in\mathbb{R}$. We will show that how the values of $T$ and $B$ influence the existence and uniqueness of the solutions to the about problem. In details, for the different problems, the $TB$-plane explicitly divided into different parts according to the number of the solutions to the above problems. These parts of $TB$-plane for the value of $T$ and $B$ guarantee the uniqueness, the existence and the nonexistence of solutions respectively. 相似文献
997.
Lamination Design Optimization for Continuous Fiber Reinforced Composites of Variable Thicknesses北大核心CSCD 下载免费PDF全文
由于具备高的比强度、比刚度,利用连续纤维增强复合材料代替传统金属材料以实现结构轻量化正受到设计者们的广泛关注。然而,结构的复杂性给复合材料的铺层设计与优化带来了很大的挑战。针对航空用复合材料铺层设计约束多的问题,通过逐步构建设计变量准确表达结构的铺层信息。基于经典遗传算法框架,结合各设计变量特点,定义了铺层优化算法中的遗传算子,通过引入“修复”策略保证了每一代解都能满足设计约束,分布在可行域区间内。最后利用精英保留策略提高了算法的局部寻优能力,可以降低复杂复合材料结构铺层设计的计算成本。通过解决经典benchmark问题并与已有优化结果的比较,验证了前述铺层优化算法的全局、局部寻优能力,为工程实际中的复合材料铺层设计优化提供了理论支撑。 相似文献
998.
We investigate a diffusive, stage-structured epidemic model with the maturation delay and freelymoving delay. Choosing delays and diffusive rates as bifurcation parameters, the only possible way to destabilize the endemic equilibrium is through Hopf bifurcation. The normal forms of Hopf bifurcations on the center manifold are calculated, and explicit formulae determining the criticality of bifurcations are derived. There are two different kinds of stable oscillations near the first bifurcation: ... 相似文献
999.
Czechoslovak Mathematical Journal - A nonincreasing sequence π = (d1,…, dn) of nonnegative integers is a graphic sequence if it is realizable by a simple graph G on n vertices. In this... 相似文献
1000.
A 1:2 inclusion compound of tris(2-benzimidazylmethyl)amine with 4-nitrobenzoic acid [(C24H21N7) · (C7H5NO2)2] has been prepared and the molecular assembly in solid state has been determined by X-ray crystal structural analysis. The crystal structure (C 2/c, a = 25.488(5), b = 13.304(3), c = 21.479(4) Å, = 93.35 (3)°, Z = 8, R = 0.065) consists of discrete dimeric inclusion compounds joined together by N(5)...N(6) intermolecular hydrogen bonds between pairs of centrosymmetrically related benzimidazole rings of the hosts. There are two types of guest molecules in the crystal, one is incorporated within the cavity of the host via three intramolecular N—H···O hydrogen bonds while the other is situated outside the cavity and connected with the host through one O—H···N hydrogen bond. The – stacking interactions between guests and benzimidazole rings result in one-dimensional columns. The aromatic face–face interactions between neighbouring columns facilitate the assembly of those columns into two-dimensional layers. 相似文献