首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3306篇
  免费   112篇
  国内免费   7篇
化学   2218篇
晶体学   49篇
力学   123篇
数学   340篇
物理学   695篇
  2023年   24篇
  2022年   63篇
  2021年   71篇
  2020年   72篇
  2019年   60篇
  2018年   58篇
  2017年   56篇
  2016年   119篇
  2015年   93篇
  2014年   134篇
  2013年   202篇
  2012年   287篇
  2011年   270篇
  2010年   178篇
  2009年   149篇
  2008年   218篇
  2007年   212篇
  2006年   192篇
  2005年   142篇
  2004年   135篇
  2003年   105篇
  2002年   110篇
  2001年   57篇
  2000年   56篇
  1999年   24篇
  1998年   19篇
  1997年   14篇
  1996年   17篇
  1995年   13篇
  1994年   15篇
  1993年   21篇
  1992年   12篇
  1991年   15篇
  1990年   18篇
  1989年   19篇
  1988年   12篇
  1987年   14篇
  1986年   10篇
  1985年   7篇
  1984年   11篇
  1983年   7篇
  1982年   7篇
  1981年   8篇
  1980年   16篇
  1979年   12篇
  1978年   16篇
  1977年   10篇
  1976年   9篇
  1974年   6篇
  1973年   6篇
排序方式: 共有3425条查询结果,搜索用时 15 毫秒
991.

Background  

Adjuvants are known to significantly enhance vaccine efficacy. However, commercial adjuvants often have limited use because of toxicity in humans. The objective of this study was to determine the comparative effectiveness of a diterpene alcohol, phyto l and its hydrogenated derivative PHIS-01, relative to incomplete Freund's adjuvant (IFA), a commonly used adjuvant in augmenting protective immunity in mice against E. coli and S. aureus, and in terms of inflammatory cytokines.  相似文献   
992.
Thin ZnSe layers were deposited on ZnO nanowires by a novel successive ionic layer adsorption and reaction technique in order to solve recombination problems in ZnO nanowire-based dye-sensitized solar cells (DSSCs). Cell efficiency increased from 0.1 to 1.3–1.4% with the deposition of a 9- to13-nm-thick ZnSe shell on ZnO nanowires due to a large increase in JSC. The dramatic increase in JSC and cell efficiency is due to the facilitation of electron transfer related to ambipolar diffusion by the formation of a type II band alignment and the suppression of recombination in the presence of the ZnSe shell.  相似文献   
993.
An analytically nonlocal Euler–Bernoulli beam model for the wave propagation in fluid-filled single-walled carbon nanotube (SWCNT) is established. The governing equations with the nonlocal effects are derived on the variational principle, and used in the wave propagation analysis of the SWCNT beam. Compared with the partially nonlocal Euler–Bernoulli beam models used previously, the analytically nonlocal model presented in the present study predicts well the effects of the stiffness enhancement and the wave damping at the high wavenumber or the strong nonlocal effects area for the fluid-filled SWCNT beam. Though the analytical model is less sensitive than the partially nonlocal model when the moving velocity of the internal fluid is high enough, it simulates more of the high-order nonlocal effecting information than the partially nonlocal model does in many cases.  相似文献   
994.
Microfiber Mach Zehnder Interferometer (MMZI) is demonstrated by micromanipulating an optical microfiber drawn from a single mode fiber (SMF) using a flame brushing technique. The MMZI shows good interference fringes with an extinction ratio of 13 dB and a free spectral range (FSR) of 0.52 nm at 1530 nm. The MMZI is then embedded in a polymer with the refractive index of 1.36 to increase the stability and robustnes of the device. It is found that the transmission spectrum of the packaged MMZI is changed by the polymer, which increases the FSR to 0.83 nm. The degradation in transmission loss and extinction ratio are attributed to the disturbance at the coupling area during the packaging. Compared with waveguide based mach zehnder interferometer, the proposed MMZI is favoured due to easy fabrication, compact size, and easy integration with the fiber system.  相似文献   
995.
Ahmed Galib Reza  Hyotaek Lim 《Optik》2011,122(7):591-593
Packet contention is a major issue in optical packet switching network and it is not a trivial task to resolve due to lack of optical RAM technology. In order to resolve contention optical buffering approach is used using fiber delay lines (FDLs). Yet there is a heavy packet loss rate due to unavailability of output port and free FDLs. This paper proposes a hybrid buffering architecture using feed-forward and feedback shared FLDs to resolve packet contention resolution of an optical packet switch. Feed-forward FDLs are used as primary buffer and feedback FDLs are implemented as supplementary buffer. Simulation result shows that proposed hybrid buffering switch achieves packet loss rate between 10−1 and 10−2 at heavy traffic load (ρ = 0.9) for a 32 × 32 switch using different FDL length.  相似文献   
996.
We investigate the effects induced by ferromagnetic contacts attached to a serial double quantum dot. Spin polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The superexchange interaction J(AFM), tuned by the interdot tunneling rate t, can be used to compensate the effective fields and restore the Kondo resonance when the contact polarizations are aligned. As a consequence, the direction of the spin conductance can be controlled and even reversed using electrostatic gates alone. Our results demonstrate a new approach for controlling spin-dependent transport in carbon nanotube double dot devices.  相似文献   
997.
In this paper we give a survey of results concerning linear mappings on symmetry classes of tensors that preserve decomposable elements and its related topic about linear mappings on spaces of matrices that preserve a fixed rank.  相似文献   
998.
999.
In this paper, we examine the two-dimensional variable-sized bin packing problem (2DVSBPP), where the task is to pack all given rectangles into bins of various sizes such that the total area of the used bins is minimized. We partition the search space of the 2DVSBPP into sets and impose an order on the sets, and then use a goal-driven approach to take advantage of the special structure of this partitioned solution space. Since the 2DVSBPP is a generalization of the two-dimensional bin packing problem (2DBPP), our approach can be adapted to the 2DBPP with minimal changes. Computational experiments on the standard benchmark data for both the 2DVSBPP and 2DBPP shows that our approach is more effective than existing approaches in literature.  相似文献   
1000.
Data envelopment analysis (DEA) is a technique for evaluating relative efficiencies of peer decision making units (DMUs) which have multiple performance measures. These performance measures have to be classified as either inputs or outputs in DEA. DEA assumes that higher output levels and/or lower input levels indicate better performance. This study is motivated by the fact that there are performance measures (or factors) that cannot be classified as an input or output, because they have target levels with which all DMUs strive to achieve in order to attain the best practice, and any deviations from the target levels are not desirable and may indicate inefficiency. We show how such performance measures with target levels can be incorporated in DEA. We formulate a new production possibility set by extending the standard DEA production possibility set under variable returns-to-scale assumption based on a set of axiomatic properties postulated to suit the case of targeted factors. We develop three efficiency measures by extending the standard radial, slacks-based, and Nerlove–Luenberger measures. We illustrate the proposed model and efficiency measures by applying them to the efficiency evaluation of 36 US universities.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号