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   291474篇
  免费   20391篇
  国内免费   1049篇
化学   184658篇
晶体学   3055篇
力学   11842篇
综合类   20篇
数学   39609篇
物理学   73730篇
  2023年   4490篇
  2021年   3797篇
  2020年   6059篇
  2019年   3941篇
  2018年   5547篇
  2017年   4115篇
  2016年   9977篇
  2015年   8976篇
  2014年   8987篇
  2013年   16549篇
  2012年   15300篇
  2011年   14461篇
  2010年   11601篇
  2009年   11125篇
  2008年   11829篇
  2007年   11459篇
  2006年   10305篇
  2005年   13153篇
  2004年   11597篇
  2003年   8878篇
  2002年   6923篇
  2001年   7314篇
  2000年   5700篇
  1999年   4370篇
  1998年   3318篇
  1997年   3256篇
  1996年   3360篇
  1995年   2921篇
  1994年   2850篇
  1993年   2811篇
  1992年   3127篇
  1991年   2903篇
  1990年   2648篇
  1989年   2564篇
  1988年   2598篇
  1987年   2472篇
  1986年   2334篇
  1985年   3365篇
  1984年   3329篇
  1983年   2691篇
  1982年   2890篇
  1981年   2809篇
  1980年   2746篇
  1979年   2689篇
  1978年   2749篇
  1977年   2804篇
  1976年   2853篇
  1975年   2853篇
  1974年   2774篇
  1973年   2721篇
排序方式: 共有10000条查询结果,搜索用时 580 毫秒
991.
992.
The results of the experimental investigation of the radiation amplification due to the stimulated four-photon parametric process in potassium vapor are reported. Two cells with active medium (an oscillatoramplifier system) are used in the experiments. The dominating role of IR radiation 6S → 5P (3.66 µm) in the amplification of violet radiation 5P → 4S (0.4 µm) generated due to the parametric process at two-photon excitation 4S → 6S is shown. This role is conditioned by optically pumped stimulated emission (OPSE). Optical conditions for the amplification of a seed radiation are determined in accordance with their intensity and the buffer gas (He) pressure. The problem of the image conversion from the IR to a visible one with quantum efficiency above 100% is discussed.  相似文献   
993.
The article is concerned with a characterization of quasi-symmetric (QS) designs with intersection numbers 0 and y. It uses the idea of a good block. Such a block G has the property that for any block B with |G ∩ B| = y, every point is on a block containing G ∩ B. It is proved that if a QS design II with intersection numbers 0 and y has a good block, then II must (i) be affine, symmetric, a linear space or (ii) have one of two possible exceptional parameter sets. Only one example is known in case (ii). If all blocks of II are good and II is not a linear space, then it is a projective or affine geometry or it is an extension (in a more general sense than usual) of a projective plane of order y2 or y3+ y. © 1995 John Wiley & Sons, Inc.  相似文献   
994.
995.
A novel method for measuring AMP-deaminase activity in human erythrocytes is presented, based on the determination of the reaction product, IMP, using high performance liquid chromatography. IMP formation was found to be proportional both to the incubation time and the amount of haemolysate over a wide range. The minimal detectable AMP-deaminase activity was more than 1000 times lower than the mean activity found in healthy controls (1083 nmol/h/mg Hb). No marked difference of activity was found in the patients with the following inherited purine disorders: familial juvenile gouty nephropathy and deficiencies of adenosine deaminase, hypoxanthine-guanine phosphoribosyltransferase or adenine phosphoribosyltransferase. The activity in the erythrocytes of patients with chronic renal failure was also similar to controls. The existence of subjects with low erythrocyte AMP-deaminase activity in the population has been confirmed.  相似文献   
996.
Differential cross sections of nucleons excited in photonuclear reactions in medium and heavy nuclei are studied by considering all relevant reaction mechanisms leading to the excitation of protons or neutrons. We take advantage of previous microscopic studies for the absorption and scattering of photons and photoproduced pions, and implement a simulation code in order to take into account the propagation of the nucleons as well as their collisions with other nucleons in the nuclear medium, which generate secondary excited nucleons. Comparison with experimental data is done. Cross sections for nucleon emission in coincidence with one pion are also calculated, and some coincidence observables are discussed.  相似文献   
997.
We elaborate in some detail on a new phase space approach to complexity, due to Y.-C. Zhang. We show in particular that the connection between maximal complexity and power law noise or correlations can be derived from a simple variational principle. For a 1D signal we find 1/f noise, in accordance with Zhang.  相似文献   
998.
In the neutron deficient isotopes 104,105In lifetimes of high spin states in the range from 0.5 ps to 600 ps have been measured in a coincidence recoil distance Doppler shift (RDDS) experiment. By combining the Doppler shift attenuation (DSA) and RDDS methods at few μm flight distances, the problem of delayed feeding has been avoided and very short lifetimes in the range 0.5–0.8 ps have been determined. Shell model calculations with strong restrictions for the neutron orbitals reveal good agreement with experimental level energies and still fair agreement for most measured B(M1) and B(E2) values. Received: 30 June 1998  相似文献   
999.
1000.
Excess molar enthalpies hE at 25 and 35° C and atmospheric pressure, are reported for the binary mixtures formed by a 2-butanone and 2-pentanone with 1-chlorobutane, 1-chloropentane, 1-chlorohexane, or 1-chlorooctane. The hE values for all the mixtures are positive, increasing as the 1-chloroalkane length increases and as the ketone length decreases. Excess molar enthalpies depend slightly on the temperature. The experimental values together with those from the literature were used to calculate the interaction parameters for the Dang-Tassios version of the UNIFAC model.Communicated at the Festsymposium celebrating Dr. Henry V. Kehiaian's 60th birthday, Clermont-Ferrand, France, 17–18 May 1990.  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