全文获取类型
收费全文 | 108778篇 |
免费 | 6765篇 |
国内免费 | 6503篇 |
专业分类
化学 | 24137篇 |
晶体学 | 1808篇 |
力学 | 3501篇 |
综合类 | 313篇 |
数学 | 17461篇 |
物理学 | 43217篇 |
综合类 | 31609篇 |
出版年
2022年 | 384篇 |
2021年 | 386篇 |
2020年 | 701篇 |
2019年 | 1022篇 |
2018年 | 952篇 |
2017年 | 700篇 |
2016年 | 528篇 |
2015年 | 535篇 |
2014年 | 1529篇 |
2013年 | 1969篇 |
2012年 | 1753篇 |
2011年 | 2225篇 |
2010年 | 2725篇 |
2009年 | 7460篇 |
2008年 | 8592篇 |
2007年 | 7817篇 |
2006年 | 7382篇 |
2005年 | 5431篇 |
2004年 | 5285篇 |
2003年 | 5514篇 |
2002年 | 6255篇 |
2001年 | 4900篇 |
2000年 | 4442篇 |
1999年 | 4513篇 |
1998年 | 3572篇 |
1997年 | 2967篇 |
1996年 | 3026篇 |
1995年 | 3534篇 |
1994年 | 3590篇 |
1993年 | 2676篇 |
1992年 | 1988篇 |
1991年 | 2152篇 |
1990年 | 2166篇 |
1989年 | 1843篇 |
1988年 | 1709篇 |
1987年 | 1042篇 |
1986年 | 559篇 |
1985年 | 1136篇 |
1984年 | 654篇 |
1983年 | 487篇 |
1982年 | 638篇 |
1981年 | 793篇 |
1980年 | 720篇 |
1979年 | 558篇 |
1978年 | 581篇 |
1977年 | 538篇 |
1976年 | 540篇 |
1975年 | 319篇 |
1974年 | 355篇 |
1973年 | 462篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Wei Gengping~ Shen Jianhua~ 《高校应用数学学报(英文版)》2006,21(3):320-326
This paper studies the nonautonomous nonlinear system of difference equationsΔx(n)=A(n)x(n)+f(n,x(n)),n∈Z,(*) where x(n)∈R~N,A(n)=(a_(ij)(n))N×N is an N×N matrix,with a-(ij)∈C(R,R) for i,j= 1,2,3,...,N,and f=(f_1,f_2,...,f_N)~T∈C(R×R~N,R~N),satisfying A(t+ω)=A(t),f(t+ω,z)=f(t,z) for any t∈R,(t,z)∈R×R~N andωis a positive integer.Sufficient conditions for the existence ofω-periodic solutions to equations (*) are obtained. 相似文献
62.
The interferometric intensity patterns from a 2D shearing interferometer are shown and discussed. The intensity patterns can be obtained in two different approaches incorporating differential and extended wavefront controlled displacements. The reliable directional sensitivity of this interferometer allows the optimization of the measurement parameters to estimate the wavefront of the intensity patterns by regularization techniques. 相似文献
63.
P. Nithiananthi 《Solid State Communications》2006,137(8):427-430
The binding energies of a hydrogenic donor both in the parabolic and non-parabolic conduction band model within the effective mass approximation have been computed for the low-dimensional semiconducting systems (LDSS) like quantum well, quantum well wire and quantum dot taking GaAs/AlxGa1−xAs systems as an example. It is observed that the effect of non-parabolicity is not effective when the system goes to lower dimensionality. The diamagnetic susceptibility of a hydrogenic donor impurity has also been computed in these LDSS in the infinite barrier model. Since no theoretical or experimental works on the diamagnetic susceptibility of LDSS are available in the literature, as a realistic case the diamagnetic susceptibility has been computed in the finite barrier model (x=0.3) for a quantum well and the results are discussed in the light of semiconductor-metal transition. 相似文献
64.
Polymethyl methacrylate (PMMA) is one of the most commonly used optical materials. However, the application of it in the area of optical communication is strongly limited by the intrinsic absorption loss of carbon-hydrogen stretching vibration. In this paper, we present a method to solve the problem by adopting the hollow-core fibers with cobweb cladding structure. The fibers use a single dielectric material and may solve the problem of structural support. Thus the feasibility of the “OmniGuide” fibers is improved, while a series of advantages of the “OmniGuide” hollow-core fiber are retained. It is promising that a fiber with low transmission loss, high bandwidth, large-core, and low costs can be designed and fabricated using PMMA. At the same time, a very broad range of the wavelengths (from visible to near infrared region, for instance, wavelengths at 0.65-1.12 μm, and even 1.30 μm, 1.54 μm and their neighbors) may be adopted for signal wavelength. 相似文献
65.
66.
本文定义了二阶微分方程的弱 Carathéodory解 ,在不涉及紧型条件的情形下 ,直接用迭代法证明了 Banach空间二阶非线性常微分方程两点边值问题存在唯一解 ,并给出逼近解迭代序列的误差估计 ,对周期边值问题得到类似的结果 相似文献
67.
天然橡胶胶乳常压氢化的研究 总被引:2,自引:0,他引:2
用N2H4/H2O2/Cu^2 体系对天然橡胶胶乳进行了常压氢化试验。红外光谱表明,氢化过程中对应的双键的伸缩振动谱带的峰面积减少.天然橡胶产生了氢化.由红外光谱峰面积推算得到产物的最高氢化度为38%.产物的凝肢合量为1%。分析结果表明:高分子链双键被饱和后.形成聚乙烯链段而结晶,结晶起到了物理交联作用。 相似文献
68.
69.
Carlos Guerrero-Sanchez 《Applied Surface Science》2006,252(7):2555-2561
The application of combinatorial and high-throughput approaches in polymer research is described. An overview of the utilized synthesis robots is given, including different parallel synthesizers and a process development robot. In addition, the application of the parallel synthesis robots to reversible addition fragmentation termination (RAFT) radical polymerizations and ionic copolymerizations is overviewed. Moreover, first results concerning the process development of semi-batch free radical polymerizations are described. 相似文献
70.
Er-Si-O crystalline compounds, which exhibit superlattice structures and sharp and strong Er-related 1.54 μm photoluminescence (PL) spectra at room temperature have been formed by self-assembling growth mechanism. Oxidation of the starting materials which have Si and Er at an atomic ratio of 2:1 are prepared and then oxidation and succeeding high-temperature annealing in Ar above 1250 °C cause a self-assembled superlattice-structured Er-Si-O crystalline compounds. The control of the ratio of Si and Er, as well as the following oxidation and annealing processes, is found to be sensitive to the crystalline properties, PL spectra and electrical properties. In this study, Er-Si-O crystalline thin films are formed on Si substrates by sol-gel and MOMBE methods, and their crystalline properties such as crystalline orientation and concentration ratio of Er, Si and O are investigated. Crystalline Er-Si-O films of high orientation are successfully grown on Si(1 0 0) and its inclined surface. The PL and excitation spectra, fluorescence decay and the electrical properties are found to be strongly related to the crystalline properties. Excess O causes a broader 1.54 μm PL spectra, slower fluorescence decay, lower carrier-mediated excitation and higher resistivity. A precise control of O is found to be necessary to grow superlattice-structured Er-Si-O compounds, which are semiconducting and are excitable via carrier-mediated excitation mechanism. 相似文献