全文获取类型
收费全文 | 8368篇 |
免费 | 389篇 |
国内免费 | 48篇 |
专业分类
化学 | 5873篇 |
晶体学 | 97篇 |
力学 | 255篇 |
数学 | 572篇 |
物理学 | 2008篇 |
出版年
2024年 | 9篇 |
2023年 | 53篇 |
2022年 | 182篇 |
2021年 | 210篇 |
2020年 | 142篇 |
2019年 | 172篇 |
2018年 | 125篇 |
2017年 | 136篇 |
2016年 | 237篇 |
2015年 | 260篇 |
2014年 | 331篇 |
2013年 | 531篇 |
2012年 | 682篇 |
2011年 | 700篇 |
2010年 | 434篇 |
2009年 | 376篇 |
2008年 | 596篇 |
2007年 | 515篇 |
2006年 | 514篇 |
2005年 | 427篇 |
2004年 | 393篇 |
2003年 | 331篇 |
2002年 | 328篇 |
2001年 | 207篇 |
2000年 | 147篇 |
1999年 | 124篇 |
1998年 | 64篇 |
1997年 | 39篇 |
1996年 | 57篇 |
1995年 | 67篇 |
1994年 | 40篇 |
1993年 | 51篇 |
1992年 | 35篇 |
1991年 | 26篇 |
1990年 | 34篇 |
1989年 | 37篇 |
1988年 | 15篇 |
1987年 | 17篇 |
1986年 | 8篇 |
1985年 | 24篇 |
1984年 | 13篇 |
1983年 | 16篇 |
1982年 | 14篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1974年 | 5篇 |
1970年 | 6篇 |
排序方式: 共有8805条查询结果,搜索用时 15 毫秒
31.
The effects of instanton-anti-instanton interactions in high-temperature QCD with colour SU(2) are studied with and without quarks. The instanton potential proves to have no repulsive core, thus creating troubles with the definition of nonperturbative contributions to physical quantities. Also the chiral condensate is calculated, and it is shown to vanish if number of quark flavoursN f≧2, whereas forN f=1 it is nonzero for the arbitrary high temperature. 相似文献
32.
C.X. Gao F.C. Yu S.Y. Jeong A.R. Choi P. Parchinskiy D.J. Kim H.J. Kim Y.E. Ihm C.G. Kim C.S. Kim 《Journal of magnetism and magnetic materials》2006
GaMnN and Be-codoped GaMnN were grown via molecular beam epitaxy using a single GaN precursor and their structural and magnetic properties were examined. X-ray diffraction and superconducting quantum interference device (SQUID) measurements revealed that the grown layers are homogeneous without precipitates. The saturation magnetization of GaMnN has increased from ∼4 to ∼16 emu/cm3 via codoping of Be. The d–d exchange interaction between Mn atoms was discussed for the ferromagnetism of GaMnN. 相似文献
33.
Oil migration in filled chocolates limits shelf life due to texture changes and loss of visual appeal. Spatial and temporal
oil concentration changes of a two-layer chocolate and peanut butter filling system, which models a filled chocolate, were
monitored by magnetic resonance imaging. Formulations of chocolate varied with respect to particle size, milk fat content
and emulsifier level. Sorption-time experimental data was used to establish the best fit diffusion coefficient for a Fickian
diffusion model. On the basis of the comparison of spatial experimental data to the model, Fickian diffusion alone does not
account for oil migration.
Authors' address: Michael J. McCarthy, Department of Food Science and Technology, University of California, One Shields Avenue,
Davis, CA 95616, USA 相似文献
34.
We study the large-sample properties of a class of parametric mixture models with covariates for competing risks. The models allow general distributions for the survival times and incorporate the idea of long-term survivors. Asymptotic results are obtained under a commonly assumed independent censoring mechanism and some modest regularity conditions on the survival distributions. The existence, consistency, and asymptotic normality of maximum likelihood estimators for the parameters of the model are rigorously derived under general sufficient conditions. Specific conditions for particular models can be derived from the general conditions for ready check. In addition, a likelihood-ratio statistic is proposed to test various hypotheses of practical interest, and its asymptotic distribution is provided. 相似文献
35.
This paper investigates second best policies to regulate nitrogen leaching. When the policy maker controls the total nitrogen use, an overall tax is superior to an action equivalent overall quota. When the goal is to regulate the expected level of nitrogen leaching, expected profit of a risk neutral farmer is greater under an overall tax than under an overall quota under a reasonable condition. The welfare ranking of action equivalent taxes and quotas for a risk averse farmer is generally ambiguous. In the numerical analyses, overall taxes turned out to be superior to overall quotas with both targets, despite a very high value of absolute risk aversion, 0.01. These results suggest that an overall tax is probably a superior policy to an overall quota to regulate expected total nitrogen use or expected nitrogen leaching for risk neutral and most of risk averse farmers. 相似文献
36.
Ha Yong Lee Se Hun Jung Soo Yong Lee Young Ho You Kyung Hyun Ko 《Applied Surface Science》2005,252(5):1891-1898
Al-Al2O3 composite coatings with different Al2O3 particle shapes were prepared on Si and Al substrate by cold spray. The powder compositions of metal (Al) and ceramic (Al2O3) having different sizes and agglomerations were varied into ratios of 10:1 wt% and 1:1 wt%. Al2O3 particles were successfully incorporated into the soft metal matrix of Al. It was found that crater formation between the coatings and substrate, which is typical characteristic signature of cold spray could be affected by initial starting Al2O3 particles. In addition, when the large hard particles of fused Al2O3 were employed, the deep and big craters were generated at the interface between coatings and hard substrates. In the case of pure soft metal coating such as Al on hard substrate, it is very hard to get proper adhesion due to lack of crater formation. Therefore, the composite coating would have certain advantages. 相似文献
37.
J. Pollanen K.R. Shirer S. Blinstein J.P. Davis H. Choi T.M. Lippman W.P. Halperin L.B. Lurio 《Journal of Non》2008,354(40-41):4668-4674
We discuss two methods by which high porosity silica aerogels can be engineered to exhibit global anisotropy. First, anisotropy can be introduced with axial strain (i.e. axial compression). In addition, intrinsic anisotropy can result during growth and drying stages and, suitably controlled, it can be correlated with preferential radial shrinkage in cylindrical samples. We have performed small angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We show that global anisotropy originating from either strain or shrinkage leads to optical birefringence and that optical cross-polarization studies are a useful characterization of the uniformity of the imposed global anisotropy. 相似文献
38.
39.
40.
Giovanni Bruno Maria M. Giangregorio April S. Brown Soojeong Choi 《Applied Surface Science》2006,253(1):219-223
GaN is grown on Si-face 4H-SiC(0 0 0 1) substrates using remote plasma-assisted methods including metalorganic chemical vapour deposition (RP-MOCVD) and molecular beam epitaxy (MBE). Real time spectroscopic ellipsometry is used for monitoring all the steps of substrate pre-treatments and the heteroepitaxial growth of GaN on SiC. Our characterization emphasis is on understanding the nucleation mechanism and the GaN growth mode, which depend on the SiC surface preparation. 相似文献