全文获取类型
收费全文 | 30492篇 |
免费 | 1537篇 |
国内免费 | 135篇 |
专业分类
化学 | 15486篇 |
晶体学 | 243篇 |
力学 | 695篇 |
数学 | 2180篇 |
物理学 | 5750篇 |
无线电 | 7810篇 |
出版年
2023年 | 313篇 |
2022年 | 331篇 |
2021年 | 786篇 |
2020年 | 606篇 |
2019年 | 647篇 |
2018年 | 574篇 |
2017年 | 550篇 |
2016年 | 974篇 |
2015年 | 756篇 |
2014年 | 1132篇 |
2013年 | 1764篇 |
2012年 | 1985篇 |
2011年 | 2175篇 |
2010年 | 1452篇 |
2009年 | 1455篇 |
2008年 | 2041篇 |
2007年 | 1787篇 |
2006年 | 1807篇 |
2005年 | 1594篇 |
2004年 | 1431篇 |
2003年 | 1257篇 |
2002年 | 1124篇 |
2001年 | 752篇 |
2000年 | 688篇 |
1999年 | 539篇 |
1998年 | 393篇 |
1997年 | 359篇 |
1996年 | 385篇 |
1995年 | 265篇 |
1994年 | 257篇 |
1993年 | 266篇 |
1992年 | 211篇 |
1991年 | 179篇 |
1990年 | 168篇 |
1989年 | 131篇 |
1988年 | 104篇 |
1987年 | 90篇 |
1986年 | 57篇 |
1985年 | 89篇 |
1984年 | 62篇 |
1983年 | 56篇 |
1982年 | 70篇 |
1981年 | 71篇 |
1980年 | 39篇 |
1979年 | 49篇 |
1978年 | 49篇 |
1976年 | 41篇 |
1975年 | 38篇 |
1974年 | 38篇 |
1973年 | 36篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
201.
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
202.
Laser cleaning of a photoresist (PR) on a glass substrate using ns-pulsed Nd:YAG laser was studied. The direction of the substrate facing the laser beam was varied as a main parameter as well as the power of the laser beam. The backward irradiation (BWI) of the third harmonic beam (355 nm) completely removed 1.2 μm thick PR layer with three pulses at 1.5 J/cm2 leaving no residues behind; while the forward irradiation (FWI) at the same condition just partially cleaned it. To investigate the difference of removal mechanisms between irradiation directions, the size distributions of particulates generated during laser cleaning were observed using an optical particle counter. The concentration of micron-sized particulates increased with increasing laser fluence up to 1 J/cm2 for FWI and 0.5 J/cm2 for BWI and then decreased at higher fluences because the target was a very thin film. The concentration of larger particulates for BWI was much higher than that for FWI implying the difference in removal mechanisms. In consideration of the size characteristics of the particulates and the temperature profiles of the PR layer, the most probable distinct mechanism for the BWI would be a blasting due to high temperature at the PR/glass interface. The particulate number concentration decreased rapidly after the completion of cleaning, suggesting that the measurement of the particulate concentration could detect the progress of the cleaning. Our results demonstrated that the backward irradiation will be useful for the laser cleaning of film-type contaminants on an optically transparent substrate. 相似文献
203.
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
204.
205.
This paper proposes a new method to describe and identify a 3-D curved object for the purpose of validating a fabricated object to the design specification. Curved 3-D objects are, in general, difficult to represent and identify because they lack distinct properties such as edges, planes, or cylindrical surfaces which are the building blocks in representing objects. In this paper, the authors propose to use principal axes of a 3-D object to establish a reference for the representation. A method of obtaining an inertia matrix from a 3-D range image is developed. The unique set of principal axes is obtained from the inertia matrix of an object with an arbitrary 3-D position and orientation, and the object can be described uniquely on these principal axes. On the principal axes, an object is described by a set of features describing the shape of the object such as spine, section size, section orientation, and section contraction. The features are used for comparing two objects for the validation purpose. The authors also propose a direct measure of similarity between two objects as a mean-squared difference of radii. As an experiment, two 3-D object models are designed through a CAD package, and fabricated objects are compared with the designed models for validation purposes 相似文献
206.
A new controller for a digital audio amplifier with bit stream input is proposed. The proposed controller has excellent features such as wide error correction range and no limitation on the modulation index. The controller is implemented in the half-bridge class-D amplifier and performance is verified through experiments. 相似文献
207.
208.
209.
Ranked-set sampling is useful when measurements are destructive or costly to obtain but ranking of the observations is relatively easy. The Wilcoxon signed rank test statistic based on the ranked-set sample is considered. We compared the asymptotic relative efficiencies of the RSS Wilcoxon signed rank test statistic with respect to the SRS Wilcoxon signed rank test statistic and the RSS sign test statistic. Throughout the ARE’s, the proposed test statistic is superior to the SRS Wilcoxon signed rank test statistic and the RSS sign test statistic. 相似文献
210.
This paper presents a simulated annealing algorithm for resource constrained project scheduling problems with the objective of minimising makespan. In the search algorithm, a solution is represented with a priority list, a vector of numbers each of which denotes the priority of each activity. In the algorithm, a priority scheduling method is used for making a complete schedule from a given priority list (and hence a project schedule is defined by a priority list). The search algorithm is applied to find a priority list which corresponds to a good project schedule. Unlike most of priority scheduling methods, in the suggested algorithm some activities are delayed on purpose so as to extend search space. Solutions can be further improved by delaying certain activities, since non-delay schedules are not dominant in the problem (the set of non-delay schedules does not always include an optimal solution). The suggested algorithm is flexible in that it can be easily applied to problems with an objective function of a general form and/or complex constraints. The performance of the simulated annealing algorithm is compared with existing heuristics on problems prepared by Patterson and randomly generated test problems. Computational results showed that the suggested algorithm outperformed existing ones. 相似文献