全文获取类型
收费全文 | 141757篇 |
免费 | 9585篇 |
国内免费 | 18447篇 |
专业分类
化学 | 108862篇 |
晶体学 | 2361篇 |
力学 | 3646篇 |
综合类 | 1589篇 |
数学 | 19471篇 |
物理学 | 33860篇 |
出版年
2023年 | 1222篇 |
2022年 | 1939篇 |
2021年 | 3712篇 |
2020年 | 3651篇 |
2019年 | 3748篇 |
2018年 | 3109篇 |
2017年 | 4254篇 |
2016年 | 4481篇 |
2015年 | 4008篇 |
2014年 | 5513篇 |
2013年 | 10326篇 |
2012年 | 8920篇 |
2011年 | 8342篇 |
2010年 | 7034篇 |
2009年 | 9171篇 |
2008年 | 9417篇 |
2007年 | 9756篇 |
2006年 | 8550篇 |
2005年 | 7033篇 |
2004年 | 6546篇 |
2003年 | 5508篇 |
2002年 | 7777篇 |
2001年 | 4421篇 |
2000年 | 3914篇 |
1999年 | 3503篇 |
1998年 | 3064篇 |
1997年 | 2430篇 |
1996年 | 1865篇 |
1995年 | 2033篇 |
1994年 | 1834篇 |
1993年 | 1614篇 |
1992年 | 1496篇 |
1991年 | 1026篇 |
1990年 | 822篇 |
1989年 | 762篇 |
1988年 | 641篇 |
1987年 | 504篇 |
1986年 | 493篇 |
1985年 | 533篇 |
1984年 | 460篇 |
1983年 | 275篇 |
1982年 | 488篇 |
1981年 | 626篇 |
1980年 | 578篇 |
1979年 | 595篇 |
1978年 | 488篇 |
1977年 | 361篇 |
1976年 | 301篇 |
1974年 | 115篇 |
1973年 | 185篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
We present a design technique of retarders suitable for nonlinear optical systems. A thin sample of LiNbO3 crystal is utilized as a retarder for wide-ranging applications. Analysis shows that the same crystal can be used for single-wave or two-wave retardations, changing the orientation of the crystal only. As an application, the retarder is used to polarize two waves orthogonal, as necessary for difference frequency mixing in an AgGaSe2 crystal. 相似文献
943.
Materials from the Mn(0.5−x)CaxTi2(PO4)3 (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, 31P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R
space group. Mn2+/Ca2+ ions occupy only the M1 sites in the Ti2(PO4)3 framework. The divalent cations are ordered in one of two M1 sites, except for the Mn0.50Ti2(PO4)3 phase, where a small departure from the ideal order is observed by XRD and 31P MAS NMR. The electrochemical behaviour of Mn0.50Ti2(PO4)3 and Mn(0.5−x)CaxTi2(PO4)3 phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti2(PO4)3 framework. In the 0≤y≤2 range, the OCV curves of Li//LiyMn0.50Ti2(PO4)3 cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li(1+y)Ti2(PO4)3 and Li//LiyMn0.50Ti2(PO4)3 shows that the intercalation occurs first in the unoccupied M1 site of Mn0.50Ti2(PO4)3 at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn0.50Ti2(PO4)3 on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn0.50Ti2(PO4)3, the stability of Mn2+ either in an octahedral or tetrahedral environment facilitates cationic migration. 相似文献
944.
研究了充有不同温度退火Fe基纳米晶粉芯LC回路的磁致频移特性(MFS),发现充有不同温度退火Fe基纳米晶粉芯LC回路的磁致频移不同,充有经600℃退火Fe基纳米晶粉芯LC回路的磁致频移最灵敏。 相似文献
945.
Maksimov R. D. Zicans J. Ivanova T. Negreeva S. N. Plume E. 《Mechanics of Composite Materials》2002,38(2):141-148
The results of experimental and theoretical investigations of elastic and thermophysical properties of poly(vinyl chloride) (PVC) and chlorinated polyethylene (CPE) blends are presented. Eight types of specimens with different ratios of weight contents of PVC and CPE (PVC/CPE = 100/0, 90/10, 80/20, 60/40, 40/60, 20/80, 10/90, and 0/100) were tested. The effect of blend composition on the elastic constants (tensile and shear modulus) is discussed. The data on the thermal conductivity, thermal diffusivity, and heat capacity of the blends investigated are also presented. 相似文献
946.
A posteriori error estimators for the Stokes equations 总被引:5,自引:0,他引:5
Summary We present two a posteriori error estimators for the mini-element discretization of the Stokes equations. One is based on a suitable evaluation of the residual of the finite element solution. The other one is based on the solution of suitable local Stokes problems involving the residual of the finite element solution. Both estimators are globally upper and locally lower bounds for the error of the finite element discretization. Numerical examples show their efficiency both in estimating the error and in controlling an automatic, self-adaptive mesh-refinement process. The methods presented here can easily be generalized to the Navier-Stokes equations and to other discretization schemes.This work was accomplished at the Universität Heidelberg with the support of the Deutsche Forschungsgemeinschaft 相似文献
947.
The von Neumann cellular automaton appears in many different settings in Operations Research varying from applications in Formal Languages to Biology. One of the major questions related to it is to find a general condition for irreversibility of a class of two-dimensional cellular automata on square grids (σ+-automata). This question is partially answered here with the proposal of a sufficient condition for the irreversibility of σ+-automata. 相似文献
948.
Annalisa Buffa 《Numerische Mathematik》2002,90(4):617-640
Summary. In this paper, we analyse a stabilisation technique for the so-called three-field formulation for nonoverlapping domain decomposition
methods. The stabilisation is based on boundary bubble functions in each subdomain which are then eliminated by static condensation.
The discretisation grids in the subdomains can be chosen independently as well as the grid for the final interface problem.
We present the analysis of the method and we construct a set of bubble functions which guarantees the optimal rate of convergence.
Received May 12, 1998 / Revised version received November 21, 2000 / Published online June 7, 2001 相似文献
949.
考虑一类具时滞的 Liénard型方程x+f[x(t) ]x(t) +g[t,x(t-τ(t) ) ]=p(t) =p(t+T) ,利用重合度理论 ,获得了此方程至少存在一个 T周期解的充分条件 相似文献
950.
梅家骝 《高校应用数学学报(A辑)》1989,4(4):541-549
本文讨论了笔者在[1]中提出的伪凸集,拟凸集的支撑函数与障碍锥的性质,并通过这些性质得出了二个闭性准则。 相似文献