全文获取类型
收费全文 | 126962篇 |
免费 | 9543篇 |
国内免费 | 13369篇 |
专业分类
化学 | 71155篇 |
晶体学 | 1193篇 |
力学 | 5274篇 |
综合类 | 1018篇 |
数学 | 21184篇 |
物理学 | 20903篇 |
无线电 | 29147篇 |
出版年
2024年 | 239篇 |
2023年 | 1177篇 |
2022年 | 2147篇 |
2021年 | 2387篇 |
2020年 | 2995篇 |
2019年 | 2923篇 |
2018年 | 2731篇 |
2017年 | 3946篇 |
2016年 | 4023篇 |
2015年 | 3930篇 |
2014年 | 5100篇 |
2013年 | 9008篇 |
2012年 | 8371篇 |
2011年 | 7650篇 |
2010年 | 6167篇 |
2009年 | 7942篇 |
2008年 | 8350篇 |
2007年 | 8834篇 |
2006年 | 8174篇 |
2005年 | 6998篇 |
2004年 | 6366篇 |
2003年 | 5308篇 |
2002年 | 6192篇 |
2001年 | 3838篇 |
2000年 | 3524篇 |
1999年 | 3173篇 |
1998年 | 2840篇 |
1997年 | 2279篇 |
1996年 | 1936篇 |
1995年 | 1790篇 |
1994年 | 1565篇 |
1993年 | 1256篇 |
1992年 | 1178篇 |
1991年 | 853篇 |
1990年 | 680篇 |
1989年 | 600篇 |
1988年 | 432篇 |
1987年 | 335篇 |
1986年 | 314篇 |
1985年 | 272篇 |
1984年 | 305篇 |
1983年 | 174篇 |
1982年 | 258篇 |
1981年 | 208篇 |
1980年 | 218篇 |
1979年 | 201篇 |
1978年 | 185篇 |
1977年 | 129篇 |
1976年 | 114篇 |
1973年 | 75篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
901.
902.
903.
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
904.
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
905.
In this paper a uniqueness theorem for a skewperiodic boundary value problem is obtained. By using the optimal control method, we derive the best estimate for the integration mean ensuring that the results hold. 相似文献
906.
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
907.
取样光栅周期中等效啁啾的实验研究 总被引:2,自引:0,他引:2
在Moire取样Bragg光纤光栅(SBG)的理论基础上,首次通过用相位模板旋转和2次曝光的方法,得出了带有取样啁啾的SBG的各个Fourier级数的等效啁啾率。实验数据和理论值的误差小于9%。 相似文献
908.
An initial-value method is given for second-order singularly perturbed boundary-value problems with a boundary layer at one endpoint. The idea is to replace the original two-point boundary value problem by two suitable initial-value problems. The method is very easy to use and to implement. Nontrivial text problems are used to show the feasibility of the given method, its versatility, and its performance in solving linear and nonlinear singularly perturbed problems.This work was supported in part by the Consiglio Nazionale delle Ricerche, Contract No. 86.02108.01, and in part by the Ministero della Pubblica Istruzione. 相似文献
909.
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
910.
Summary At first, a necessary and sufficient condition for a K?hler-Norden manifold to be holomorphic Einstein is found. Next, it
is shown that the so-called (real) generalized Einstein conditions for K?hler-Norden manifolds are not essential since the
scalarcurvature of such manifolds is constant. In this context, we study generalized holomorphic Einstein conditions. Using
the one-to-one correspondence between K?hler-Norden structures and holomorphic Riemannian metrics, we establish necessary
and sufficient conditions for K?hler-Norden manifolds to satisfy the generalized holomorphic Einstein conditions. And a class
of new examples of such manifolds is presented. Finally, in virtue of the obtained results, we mention that Theorems 1 and
2 of H. Kim and J. Kim [10] are not true in general. 相似文献