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   5611篇
  免费   560篇
  国内免费   563篇
化学   2429篇
晶体学   34篇
力学   979篇
综合类   7篇
数学   1058篇
物理学   2227篇
  2024年   17篇
  2023年   185篇
  2022年   203篇
  2021年   225篇
  2020年   269篇
  2019年   163篇
  2018年   107篇
  2017年   212篇
  2016年   173篇
  2015年   251篇
  2014年   360篇
  2013年   395篇
  2012年   510篇
  2011年   535篇
  2010年   455篇
  2009年   462篇
  2008年   400篇
  2007年   352篇
  2006年   278篇
  2005年   165篇
  2004年   89篇
  2003年   87篇
  2002年   86篇
  2001年   87篇
  2000年   57篇
  1999年   76篇
  1998年   45篇
  1997年   35篇
  1996年   21篇
  1995年   11篇
  1994年   7篇
  1993年   4篇
  1992年   8篇
  1991年   7篇
  1990年   6篇
  1989年   10篇
  1988年   72篇
  1987年   124篇
  1986年   127篇
  1985年   13篇
  1984年   6篇
  1982年   2篇
  1981年   8篇
  1980年   5篇
  1979年   5篇
  1978年   5篇
  1977年   5篇
  1976年   2篇
  1971年   7篇
排序方式: 共有6734条查询结果,搜索用时 15 毫秒
1.
We have solved the equation of equilibrium for torsional vibrations of a sphere in which the density and rigidity are functions of the radial distance and co-latitude. Making use of the particular forms of heterogeneity the exact frequency equations are derived. The work is motivated by the recent research activity on functionally graded materials (FGM), i.e. materials with spatially varying properties tailored to satisfy particular engineering applications.  相似文献   
2.
In this paper, we combine the monotone iterative technique with fixed point theory to investigate the extremal solutions of nonlinear integro-differential equations on time scales. Several important lemmas and existence criteria are obtained.  相似文献   
3.
Activated carbon fiber (ACF) as extraction fiber for solid-phase microextraction (SPME) and its application for the analysis of banned organochlorine pesticides (OCPs) were investigated. Firstly, ACF was activated by different concentration of zinc chloride, which indicated that ACF activated by 60% zinc chloride had a reasonable specific surface area, pore volume and pore distribution. Secondly, the parameters for the ACF-SPME procedure, the adsorption and desorption conditions, were also optimized when coupled with gas chromatography-mass spectrometry (GC-MS). Thirdly, the ACF-SPME was used to analyze 17 kinds of OCPs in water. The linearity of most pesticides was found to be between 0.2 and 50 microg/l with GC-MS under the selected ion monitoring (SIM) acquisition mode. The limits of detection (LOD) at the sub microg/l were obtained. The work demonstrated here shows that ACF is a promising alternative for the SPME procedure.  相似文献   
4.
《Physics letters. A》2006,349(6):484-487
Quasi-phase-matching (QPM) is an alterative approach for nonlinear optical frequency conversion compared to conventional birefringent method in the bulk crystal. In this Letter, two-dimensional aperiodic optical superlattice (2DAOS) for achieving multi-QPM is proposed. A special optical superlattice structure containing pre-designed values of the reciprocal lattice vector G that can accomplish multi-QPM process is given by using stimulated annealing (SA) algorithm.  相似文献   
5.
Three kinds of nylon 10 14 crystals with different perfections were prepared under various crystallization conditions. The Brill transition behavior of these nylon 10 14 crystals was investigated by variable-temperature X-ray diffraction. It was found that the crystallization conditions influence the Brill transition temperature greatly. The Brill transition temperature of the lamellar crystals grown from dilute solution is so high that no Brill transition temperature can be observed before melting. However, for crystals postannealed at 125 °C, the Brill transition temperature is as low as 130 °C. The results show that the Brill transition behavior of nylons is strongly dependent on the crystallization conditions, for example, the perfections of the crystals.  相似文献   
6.
In this paper, we first presented a novel method for quantification of circulating DNA in human serum based on capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF). The serum was digested by proteinase to release free DNA, and then CZE-LIF system was used for the quantification of total circulating DNA. This method was successfully used to quantify the circulating DNA levels in sera from healthy individuals and certain cancer patients.We found the significantly elevated circulating DNA levels in certain prostate cancer patients. Our results demonstrated that CZE-LIF system has good linearity, excellent sensitivity (0.5 ng/mL DNA),satisfactory reproducibility (RSDs in one day and between days were both less than 5%) and reliability, and is well suitable to the quantification of the circulating DNA in human serum or plasma.  相似文献   
7.
Ag2S nanoparticles in hyperbranched polyurethane matrix were prepared through the in situ reaction with thioacetamide as the sulfur source at room temperature. Transmission electron microscopic analysis revealed a uniform spherical shape for Ag2S nanoparticles, with an average size of about 4-10 nm and a narrow size distribution. X-ray powder diffraction and UV-vis spectroscopy were also used to characterize the obtained nanoparticles  相似文献   
8.
Inverse relaxation is studied for hard elastic polypropylene (HEPP), rubber and non-elastic polypropylene. The results show that contractive stress, stress, and internal friction are three essential factors related to the phenomenon. A three-element model in which each element has a definite meaning is proposed to describe this phenomenon. The results also show that, in the first cyclic deformation, relaxation time increases with the increase of recovery for all the materials, which indicates that recovery viscosity increases with the increase of recovery, but the stress rising amplitude (SRA) of inverse relaxation has a maximum in the recovery range. Analysis indicates that SRA equals recovery internal friction (RIF) for ideal material in which stress is solely a function of strain, independent of paths, and approximately equals RIF for non-ideal material at a given strain. From this principle it is found that the order of the work counteracted by RIF for the four materials is the same as that of their second hysteresis loop, and the RIF of HEPP has a sudden increase at the later recovery range.  相似文献   
9.
Membrane fusion is very important for the formation of many complex organs in metazoans throughout evolution, such as muscles, bones, and placentae. Lipid vesicles (liposomes) are frequently used as model membranes to study the fusion process. This work demonstrates for the first time the real-time membrane fusion of giant polymer vesicles by directly displaying a series of high-resolution and real-time transformation images of individual vesicles. The fusion process includes the sequential steps of membrane contact, forming the center wall, symmetric expansion of fusion pore and complete fusion, undergoing the intermediates of "8" shape with a protruding rim at the contact site, peanut (pear) shape, and oblate sphere. The vesicle swells during fusion, and the fusing vesicle only deforms in the neck domain around the fusion pore in the lateral direction, which verifies the importance of the lateral tension on the fusion pore at the vesicle deformation level. The successful fusion of the synthetic and protein-free polymer vesicles reported here also supports that vesicle proximity combined with membrane perturbation suffices to induce membrane fusion, and that the protein is not necessary for the fusion process.  相似文献   
10.
Resonance-enhanced photoionization has been used to follow S(3P2) in the photodissociation of CS2 at 193 nm. The contributions from initial photodissociation and from S(1D) relaxation have been resolved and give a (15±5)% yield of S(1D). The possibility of secondary production of S(3Pj) by CS photodissociation with a second 193 nm photon is discussed. Although this might raise the S(1D) yield to (26±8)%, production of S(3PJ) is still the dominant photodissociation channel.  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