全文获取类型
收费全文 | 208776篇 |
免费 | 33547篇 |
国内免费 | 21261篇 |
专业分类
化学 | 137265篇 |
晶体学 | 2187篇 |
力学 | 14189篇 |
综合类 | 1308篇 |
数学 | 26961篇 |
物理学 | 81674篇 |
出版年
2024年 | 645篇 |
2023年 | 3792篇 |
2022年 | 6650篇 |
2021年 | 7406篇 |
2020年 | 7864篇 |
2019年 | 7556篇 |
2018年 | 6846篇 |
2017年 | 6322篇 |
2016年 | 9740篇 |
2015年 | 9375篇 |
2014年 | 11508篇 |
2013年 | 15008篇 |
2012年 | 18054篇 |
2011年 | 18667篇 |
2010年 | 12679篇 |
2009年 | 12190篇 |
2008年 | 13228篇 |
2007年 | 11935篇 |
2006年 | 11112篇 |
2005年 | 9268篇 |
2004年 | 7093篇 |
2003年 | 5712篇 |
2002年 | 5351篇 |
2001年 | 4451篇 |
2000年 | 4064篇 |
1999年 | 4452篇 |
1998年 | 3905篇 |
1997年 | 3717篇 |
1996年 | 3904篇 |
1995年 | 3265篇 |
1994年 | 2974篇 |
1993年 | 2466篇 |
1992年 | 2279篇 |
1991年 | 2044篇 |
1990年 | 1644篇 |
1989年 | 1267篇 |
1988年 | 972篇 |
1987年 | 814篇 |
1986年 | 793篇 |
1985年 | 687篇 |
1984年 | 466篇 |
1983年 | 323篇 |
1982年 | 289篇 |
1981年 | 193篇 |
1980年 | 125篇 |
1979年 | 66篇 |
1976年 | 37篇 |
1975年 | 35篇 |
1974年 | 45篇 |
1957年 | 51篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Qipeng Guo Fei Chen Ke Wang Ling Chen 《Journal of Polymer Science.Polymer Physics》2006,44(21):3042-3052
An amphiphilic poly(ethylene oxide)‐block‐poly(dimethylsiloxane) (PEO–PDMS) diblock copolymer was used to template a bisphenol A type epoxy resin (ER); nanostructured thermoset blends of ER and PEO–PDMS were prepared with 4,4′‐methylenedianiline (MDA) as the curing agent. The phase behavior, crystallization, hydrogen‐bonding interactions, and nanoscale structures were investigated with differential scanning calorimetry, Fourier transform infrared spectroscopy, transmission electron microscopy, and small‐angle X‐ray scattering. The uncured ER was miscible with the poly(ethylene oxide) block of PEO–PDMS, and the uncured blends were not macroscopically phase‐separated. Macroscopic phase separation took place in the MDA‐cured ER/PEO–PDMS blends containing 60–80 wt % PEO–PDMS diblock copolymer. However, the composition‐dependent nanostructures were formed in the cured blends with 10–50 wt % PEO–PDMS, which did not show macroscopic phase separation. The poly(dimethylsiloxane) microdomains with sizes of 10–20 nm were dispersed in a continuous ER‐rich phase; the average distance between the neighboring microdomains was in the range of 20–50 nm. The miscibility between the cured ER and the poly(ethylene oxide) block of PEO–PDMS was ascribed to the favorable hydrogen‐bonding interaction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3042–3052, 2006 相似文献
22.
23.
24.
Incomplete LU factorization preconditioning techniques often have difficulty on indefinite sparse matrices. We present hybrid reordering strategies to deal with such matrices, which include new diagonal reorderings that are in conjunction with a symmetric nondecreasing degree algorithm. We first use the diagonal reorderings to efficiently search for entries of single element rows and columns and/or the maximum absolute value to be placed on the diagonal for computing a nonsymmetric permutation. To augment the effectiveness of the diagonal reorderings, a nondecreasing degree algorithm is applied to reduce the amount of fill-in during the ILU factorization. With the reordered matrices, we achieve a noticeable improvement in enhancing the stability of incomplete LU factorizations. Consequently, we reduce the convergence cost of the preconditioned Krylov subspace methods on solving the reordered indefinite matrices. 相似文献
25.
A two-dimensional, nonlinear, compressible, diabatic, nonhydrostatic photochemical-dynamical gravity wave model has been advanced. The model includes diabetic process produced by photochemistry and the effect of gravity wave on atmospheric chemical species. In the horizontal direction, the pseudospectral method is used. The finite difference approximations are used in vertical direction z and time t. The FICE method is used to solve the model. The model results on small amplitude fluctuation are very close to those of linear theory, which demonstrates the correctness of the model.
相似文献26.
Q. Sun Q. Wang X.G. Gong V. Kumar Y. Kawazoe 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(1):77-81
We report results of the atomic and electronic structures of Al7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is
found to be the one in which carbon atom occupies an interstitial position in Al7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al7C- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al7C- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al7N which has the same number of valence electrons as in Al7C- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al7N)2 to study interaction between magic clusters.
Received 28 July 2001 相似文献
27.
Fractional factorial split-plot (FFSP) designs have an important value of investigation for their special structures. There are two types of factors in an FFSP design: the whole-plot (WP) factors and sub-plot (SP) factors, which can form three types of two-factor interactions: WP2fi, WS2fi and SP2fi. This paper considers FFSP designs with resolution III or IV under the clear effects criterion. It derives the upper and lower bounds on the maximum numbers of clear WP2fis and WS2fis for FFSP designs, and gives some methods for constructing the desired FFSP designs. It further examines the performance of the construction methods. 相似文献
28.
科学与完美是不是一致的?科学家们是否有权使用完美这个词?至少,对于物理学家和数学家来说答案是肯定的“是”.2002年,温伯格(Steven Weinberg)写道:“近代物理学的一些重要方程式是科学知识的永久部分,甚至比它们较早出现的美丽大教堂还要经久不衰.”1960年代,狄拉克(Paul A.M.Dirac)有句名言:“完整的方程式之完美较之它们适合实验结果更为重要.”1965年诺贝尔物理学奖获得者费因曼(Richard P.Feynman)也坚信他提出的一项理论,即使该理论显得与实验数据有矛盾.1957年,他写道:“理论是优雅和完美的,重大事件是短暂的闪光.” 相似文献
29.
In this paper, the dimension of the spaces of bivariate spline with degree less that 2r and smoothness order r on the Morgan-Scott triangulation is considered. The concept of the instability degree in the dimension of spaces of bivariate spline is presented. The results in the paper make us conjecture the instability degree in the dimension of spaces of bivariate spline is infinity. 相似文献
30.
Chong-Guang Cao Zhong-Peng Yang Xian Zhang 《Journal of Applied Mathematics and Computing》2002,10(1-2):101-109
We extend two inequalities involving Hadamard products of positive definite Hermitian matrices to positive semi-definite Hermitian matrices. Simultaneously, we also show the sufficient conditions for equalities to hold. Moreover, some other matrix inequalities are also obtained. Our results and methods are different from those which are obtained by S. Liu in [J. Math. Anal. Appl. 243:458–463(2000)] and B.-Y. Wang et al. in [Lin. Alg. Appl. 302–303: 163–172(1999)]. 相似文献