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   216362篇
  免费   763篇
  国内免费   435篇
化学   103009篇
晶体学   5209篇
力学   9744篇
综合类   6篇
数学   22565篇
物理学   77027篇
  2021年   1834篇
  2020年   2081篇
  2019年   2280篇
  2018年   2799篇
  2017年   2729篇
  2016年   3935篇
  2015年   2481篇
  2014年   3808篇
  2013年   9255篇
  2012年   7453篇
  2011年   8989篇
  2010年   6250篇
  2009年   6111篇
  2008年   8183篇
  2007年   8240篇
  2006年   7911篇
  2005年   7040篇
  2004年   6642篇
  2003年   5840篇
  2002年   5809篇
  2001年   6728篇
  2000年   5156篇
  1999年   4037篇
  1998年   3292篇
  1997年   3287篇
  1996年   2986篇
  1995年   2814篇
  1994年   2647篇
  1993年   2523篇
  1992年   2879篇
  1991年   2845篇
  1990年   2764篇
  1989年   2616篇
  1988年   2651篇
  1987年   2618篇
  1986年   2456篇
  1985年   3150篇
  1984年   3416篇
  1983年   2824篇
  1982年   3002篇
  1981年   2988篇
  1980年   2843篇
  1979年   2961篇
  1978年   3019篇
  1977年   3011篇
  1976年   3070篇
  1975年   2856篇
  1974年   2823篇
  1973年   2971篇
  1972年   1942篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
We are defining the trigonometric Lie subalgebras in which are the natural generalization of the well known Sin-Lie algebra. The embedding formulas into are introduced. These algebras can be considered as some Lie algebras of quantum tori. An irreducible representation ofA, B series of trigonometric Lie algebras is constructed. Special cases of the trigonometric Lie factor algebras, which can be considered as a quantum (preserving Lie algebra structure) deformation of the Kac-Moody algebras are considered.  相似文献   
982.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 56, No. 1, pp. 116–121, January, 1992.  相似文献   
983.
984.
Black holes have piqued much curiosity. But thus far they have been important only in remote subjects like astrophysics and quantum gravity. We show that the situation can be improved. By a judicious application of black hole physics, one can obtain new results in everyday physics. For example, black holes yield a quantum universal upper bound on the entropy-to-energy ratio for ordinary thermodynamical systems which was unknown earlier. It can be checked, albeit with much labor, by ordinary statistical methods. Black holes set a limitation on the number of species of elementary particles-quarks, leptons, neutrinos-which may exist. And black holes lead to a fundamental limitation on the rate at which information can be transferred for given message energy by any communication system.This Essay received the first award from the Gravity Research Foundation for the year 1981-Ed.  相似文献   
985.
In this paper we describe a propagation experiment to measure the one-way velocity of electromagnetic radiation. The experiment utilizes the rotation of the earth to interchange the positions of two rubidium vapor frequency standards over12 h, thereby canceling initial clock phase differences. It is demonstrated that although the drift characteristics of modern rubidium atomic clocks may be large for long-term absolute timing requirements, the short-term random fluctuations are small. It is found that over a24-h period, the long-term drift can be accurately parameterized in retrospect and removed, thereby permitting the detection of temporal variations less than1 nsec in magnitude. With coherent summing techniques this value may be significantly reduced, and it becomes realistic to consider an experiment where the clocks are separated by distances of the order of several hundreds of meters in order to detect velocities of the order of that of the solar system with respect to the center of the galaxy(10 5 m sec –1 ), thus ensuring that the rotational motion of the earth has a negligible effect in altering the relative inertial characteristics of the reference frames of each clock. It is demonstrated that under such conditions the measurement of the one-way speed of propagation of electromagnetic radiation is not only meaningful, but can be simply implemented with commercially available instrumentation.This study was supported at Utah State University by a grant from the USU Vice President for Research, and by NASA Grant NAGW-119.  相似文献   
986.
Microwave transitions up to J = 53 in the ground vibrational state of deuterothioformaldehyde, CD2S, were studied between 8 and 40 GHz. A detailed centrifugal distortion analysis yields accurate constants for comparison with force field values. The isotopic species 13CH2S, CH234S, CH233S, 13CD2S, CD234S, and CD233S were studied in natural abundance. Accurate average zero-point structures were determined for both CD2S and CH2S:
CH2S CS=1.6138(4) CH = 1.0962(6) A?HCH =116° 16(6)′, CD2S CS=1.6136(4) CD = 1.0931(4) A?DCD =116° 25(5)′
Changes in the zero-point geometry for deuterium substitution were established. Quadrupole fine structure arising from the 33S nucleus has been measured in CH233S and CD233S. Analysis gives the following coupling constants (for both molecules) as χaa = ?11.7 and χbb - χcc = 88.1 MHz. The dipole moment of CD2S was measured to be 1.6588(8)D and an accurate comparison with CH2S was made; the ratio of dipole moments CD2SCH2S was found to be 1.0062(4). The spectroscopic and bonding properties of CH2S will be compared with formaldehyde and other molecules.  相似文献   
987.
Flow birefringence (FB) in solutions has been investigated for a number of samples of polyamide-hydrazide (PPAH) in dimethylsulphoxide; intrinsic viscosities [η] of the same solutions have been measured. Characteristic values of FB, [η], and orientation angles [χ/g] of the solutions were determined. Molecular weights of the PPAH samples were calculated from values of [χ/g] and [η]. The use of characteristic values of [n] and [η] and the theory of optical anisotropy of persistent chains gives quantitative data on the equilibrium rigidity of PPAH molecules. It was shown that the length of the statistical segment of the PPAH chain is 800 Å. The possible reasons for the fact that the equilibrium flexibility of PPAH molecules is greater than that of poly-p-phenylene terephthalamide molecules have been discussed.  相似文献   
988.
When dissolved in SbF5, polyfluorinated trifluoromethylbenzenes produce polyfluorinated α,α-difluorobenzyl ions. The 19F chemicals shifts and spin-spin coupling constants J(FF) have been analysed in terms of the resonance structures which contribute to the overall positive charge distribution. The influence of halogen atoms situated at position 4 on the downfield signal shift of α-fluorines follows the sequence F > Cl > Br which is opposite to that observed for 4-XC6H4CF2+ ions.  相似文献   
989.
Iodide ions react with thallic ions at pH 2–8 to form a complex iodide, which is suitable for the spectrophotometric determination of thallium. The reaction has a sensitivity of 0.05 μg Tl per cm2 for log I0I = 0.001 and obeys Beer's law up to 40 p.p.m. Optimum conditions for the reaction have been established. The standard deviation is 0.6%. The effects of temperature and pH, the ratio of thallium to reagent, stability of the complex, its conformity to Beer's law, and the rate of color formation were studied. The effect of many diverse ions was examined.  相似文献   
990.
The free-radical polymerization of N-vinylpyrrolidone and its copolymerization with methyl methacrylate have been studied at 60°C in the presence of C-phenyl-N-tert-butylnitrone as a chain-growth regulator. It has been shown that the above additive makes it possible to synthesize poly(N-vinylpyrrolidone) and its copolymers without any gel effect and to control molecular-mass characteristics of homo-and (co)polymers.  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