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   14447篇
  免费   2219篇
  国内免费   1331篇
化学   10517篇
晶体学   179篇
力学   813篇
综合类   67篇
数学   1786篇
物理学   4635篇
  2024年   30篇
  2023年   308篇
  2022年   393篇
  2021年   526篇
  2020年   666篇
  2019年   570篇
  2018年   504篇
  2017年   424篇
  2016年   690篇
  2015年   714篇
  2014年   737篇
  2013年   1081篇
  2012年   1242篇
  2011年   1256篇
  2010年   826篇
  2009年   832篇
  2008年   972篇
  2007年   830篇
  2006年   742篇
  2005年   704篇
  2004年   513篇
  2003年   421篇
  2002年   409篇
  2001年   324篇
  2000年   292篇
  1999年   299篇
  1998年   198篇
  1997年   154篇
  1996年   197篇
  1995年   165篇
  1994年   138篇
  1993年   123篇
  1992年   102篇
  1991年   86篇
  1990年   91篇
  1989年   68篇
  1988年   63篇
  1987年   49篇
  1986年   40篇
  1985年   42篇
  1984年   37篇
  1983年   17篇
  1982年   16篇
  1981年   20篇
  1980年   11篇
  1979年   8篇
  1976年   10篇
  1975年   11篇
  1971年   7篇
  1969年   5篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Piecewise quadratic trigonometric polynomial curves   总被引:6,自引:0,他引:6  
Analogous to the quadratic B-spline curve, a piecewise quadratic trigonometric polynomial curve is presented in this paper. The quadratic trigonometric polynomial curve has continuity, while the quadratic B-spline curve has continuity. The quadratic trigonometric polynomial curve is closer to the given control polygon than the quadratic B-spline curve.

  相似文献   

992.
Controlled Wild Algebras   总被引:1,自引:0,他引:1  
The controlled wild algebra is introduced, a covering criterionfor a finite-dimensional algebra to be controlled wild is given,and this criterion is applied to the algebras with radical squarezero, algebras with zero relations, local algebras and finitep-group algebras. 2000 Mathematical Subject Classification:16G20, 16G60.  相似文献   
993.
We describe a new class of nanoscale structured metals wherein the effects of quantum confinement are combined with dispersive metallic electronic states to induce modifications to the fundamental low-energy microscopic properties of a three-dimensional metal: the density of states, the distribution of Fermi velocities, and the collective electronic response.  相似文献   
994.
We study the observability for a lepton flavor-changing decay of a Higgs boson h--> mutau at Hadron colliders. Flavor-changing couplings of a Higgs boson exist at tree level in models with multiple Higgs doublets. The hmutau coupling is particularly motivated by the favorable interpretation of nu(mu)-nu(tau) oscillation. We find that at the Tevatron run II the unique mutau signature could serve as the Higgs discovery channel, surpassing expectations for Higgs boson searches in the SM and in a large parameter region of the MSSM. The sensitivity will be greatly improved at the LHC, beyond the coverage at a muon collider Higgs factory.  相似文献   
995.
Pipe flow of blood in tubes of 1 and 7 mm inner diameter, respectively, was investigated employing two-dimensional NMR velocity imaging and PFG propagator measurements at different Reynolds numbers between 10 and 3500. The results are compared to flow of a water/glycerol mixture of matching viscosity under identical conditions. The transition from laminar to turbulent flow is observed by both a flattening of the velocity profile and a change of the propagator shape. For blood flow this transition is found to be shifted toward higher Reynolds numbers as compared to the transition of the water/glycerol mixture. This observation is in agreement with predictions from hydraulic measurements and is a consequence of the non-Newtonian flow characteristics of blood as a suspension of erythrocytes and plasma. Likewise, a deviation from the laminar flow condition is observed for blood at low Reynolds numbers between 10 and 100. This phenomenon is unknown for Newtonian liquids and is explained by the onset of a geometrical arrangement of the erythrocytes, the so-called rouleaux effect.  相似文献   
996.
促性腺激素诱导休情期母貂发情的结果表明,激素处理后,母貂均表现出不同程度的发情征状外阴肿胀、卵巢和子宫增重、血液雌二醇和孕酮水平升高;发情高潮时,卵巢和子宫质量达最大值,孕酮水平上升,雌二醇水平下降,这是母貂发情旺期的特征.对各组激素作用的比较,确认HCG是诱导休情期母貂发情的最佳激素.  相似文献   
997.
Time-domain transillumination of biological tissues with terahertz pulses   总被引:6,自引:0,他引:6  
Han PY  Cho GC  Zhang XC 《Optics letters》2000,25(4):242-244
We present time-domain transmission imaging of an opaque structure in pork-fat tissue obtained with a terahertz (THz) field sampling technique. Compared with imaging with near-infrared pulses, the terahertz sampling technique shows significantly enhanced contrast, as a result of low scattering. For enhanced spatial resolution, we show mid-infrared THz imaging of onion cells. Water absorption of THz pulse in muscle tissues is discussed.  相似文献   
998.
Xue B  Han J  Dai HL 《Physical review letters》2000,84(12):2606-2609
Through quantum-beat spectroscopy collision relaxation of a high vibrational level of SO2 at 44 877.52 cm(-1) is characterized. This is a first measurement of collision relaxation for a single, highly excited vibrational level. The deduced relaxation cross section of this excited level by Ar is 216 A(2), 5 times the area of the hard sphere, and by an ambient temperature SO2 molecule is 969 A(2), almost 20 times the hard sphere. These cross sections indicate that relaxation collisions of highly vibrationally excited molecules have effective distances much longer than van der Waals radii and involve mechanisms qualitatively different from lower excitations.  相似文献   
999.
Use of the organic crystal DAST for terahertz beam applications   总被引:2,自引:0,他引:2  
Han PY  Tani M  Pan F  Zhang XC 《Optics letters》2000,25(9):675-677
We report the use of the organic crystal DAST as a freely propagating electromagnetic-wave sensor at terahertz (THz) frequency. We also report the result of using a DAST crystal as a mid-IR THz emitter. Compared with the optical rectification from our best ZnTe emitter, that obtained by use of DAST demonstrates a sixfold enhancement of radiation at 7-20 THz.  相似文献   
1000.
Water diffusion measurements were performed on rabbit Achilles tendons during static tensile loading and tendons in an unloaded state. The apparent diffusion coefficient (ADC) was measured along two directions: parallel and perpendicular to the long axis of the tendon. Tendons were studied after being prepared in two ways: (a) after being stored frozen in phosphate-buffered saline (PBS) and (b) freshly isolated. Statistically significant directional anisotropy was observed in the ADC in all tendons. The ADC was significantly greater in the direction parallel to the long axis of the tendon than in the perpendicular direction. The anisotropy is attributed to the greater restrictions seen by the water molecules in the perpendicular direction and is consistent with the known geometry of the tendon. Storage in PBS caused tendons to swell. This increased the ADC measured along both directions and reduced the anisotropy. The existence of anisotropy in the ADC was not related to the orientation of the specimen in the magnet. The ADC increased along both directions following the application of a 5-N tensile load; the increase was greatest along the perpendicular axis of the tendon. In order to determine whether load-related changes in the ADC reflected changes in interfibrilar spacing, we used electron microscopy to measure load-related changes in fibril spacing. Load-related changes in fiber spacing could not account for the observed changes in the ADC. The increase in ADC caused by loading was attributed to the extrusion of tendon water into a bulk phase along the outside surface of the tendon. In PBS-stored samples, enough fluid was extruded that it could be visualized. The transient response of the ADC to a 5-N tensile load was also studied. The absolute ADC in both directions increased with loading and recovered to baseline upon unloading. The transient changes in ADC, for both loading and unloading, had a mean time constant of approximately 15 min. The magnitude of the load-induced transient ADC changes was comparable to that seen in the static-loading experiments.  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