全文获取类型
收费全文 | 239772篇 |
免费 | 2504篇 |
国内免费 | 803篇 |
专业分类
化学 | 121128篇 |
晶体学 | 4148篇 |
力学 | 10909篇 |
综合类 | 1篇 |
数学 | 26929篇 |
物理学 | 79964篇 |
出版年
2020年 | 1910篇 |
2019年 | 2206篇 |
2018年 | 2896篇 |
2017年 | 2864篇 |
2016年 | 4310篇 |
2015年 | 2689篇 |
2014年 | 4322篇 |
2013年 | 10747篇 |
2012年 | 7916篇 |
2011年 | 9795篇 |
2010年 | 6989篇 |
2009年 | 6887篇 |
2008年 | 9123篇 |
2007年 | 9102篇 |
2006年 | 8571篇 |
2005年 | 7723篇 |
2004年 | 7118篇 |
2003年 | 6261篇 |
2002年 | 6186篇 |
2001年 | 7183篇 |
2000年 | 5459篇 |
1999年 | 4289篇 |
1998年 | 3622篇 |
1997年 | 3515篇 |
1996年 | 3350篇 |
1995年 | 3053篇 |
1994年 | 2987篇 |
1993年 | 3065篇 |
1992年 | 3181篇 |
1991年 | 3380篇 |
1990年 | 3124篇 |
1989年 | 3076篇 |
1988年 | 3117篇 |
1987年 | 3001篇 |
1986年 | 2845篇 |
1985年 | 3903篇 |
1984年 | 4093篇 |
1983年 | 3261篇 |
1982年 | 3464篇 |
1981年 | 3393篇 |
1980年 | 3287篇 |
1979年 | 3382篇 |
1978年 | 3458篇 |
1977年 | 3381篇 |
1976年 | 3487篇 |
1975年 | 3175篇 |
1974年 | 3180篇 |
1973年 | 3217篇 |
1972年 | 2248篇 |
1971年 | 1856篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
P. Klimo 《International Journal of Theoretical Physics》1997,36(1):23-39
Invariant path integrals on symmetric and group spaces are defined in terms of a sum over the paths formed by broken geodesic segments. Their evaluation proceeds by using the mean value properties of functions over the geodesic and complex radius spheres. It is shown that on symmetric spaces the invariant path integral gives a kernel of the Schrödinger equation in terms of the spectral resolution of the zonal functions of the space. On compact group spaces the invariant path integral reduces to a sum over powers of Gaussian-type integrals which, for a free particle, yields the standard Van Vleck-Pauli propagator. Explicit calculations are performed for the case ofSU(2) andU(N) group spaces. 相似文献
962.
R. P. Sundarraj P. S. Sundararaghavan Dale R. Fox 《The Journal of the Operational Research Society》1994,45(5):549-558
Several production and flexible manufacturing systems can naturally be modelled using queueing networks. In this paper, we consider the problem of acquiring servers for the nodes of an open queueing network, so as to optimize the steady-state mean virtual system parameters subject to a budget constraint. A partial enumeration scheme and a heuristic method have been proposed to solve this problem. Empirical results based on randomly generated test problems are used to identify a class of problems for which the heuristic performs well. 相似文献
963.
P. Cifra F. E. Karasz W. J. Macknight 《Journal of Polymer Science.Polymer Physics》1991,29(11):1389-1395
Local ordering in miscible binary polymer blends with strong attractive interactions between the two types of chains was investigated by computer simulation of allowable conformations on an incompressible cubic lattice. A tendency toward maximum ordering (in which chains pack in alternate parallel rows) is shown by the calculation of pair correlations and in “snapshots” of the mixtures. A specific directional interaction is not necessarily required for ordering. A comparison of these results with those recently obtained using mean-field theory is presented. The heat capacity of mixing was also calculated and found to be positive, with a value close to that reported experimentally. 相似文献
964.
M. Alves Da Silva M. Helena Gil A. P. Piedade J. S. Redinha A. M. Oliveira Brett J. M. Caridade Costa 《Journal of polymer science. Part A, Polymer chemistry》1991,29(2):269-274
The immobilization of catalase on grafted membranes of poly(ethylene)-g-co-acrylic acid and poly(tetrafluoroethylene)-g-co-acrylic acid and their application in hydrogen peroxide electrochemical sensors is described. The introduction of carboxylic acid groups onto a hydrophobic support provides a good environment for subsequent enzyme immobilization. This single membrane, hydrogen peroxide sensor showed significant improvement with respect to the double membrane versions. The response is very rapid, the linear range being from 10 μM up to 6 mM, with a detection limit of 4.7 μM, and a lifetime of more than 4 months. 相似文献
965.
966.
P. Ila 《Journal of Radioanalytical and Nuclear Chemistry》1991,148(2):415-422
Total oxygen, in fourteen rock standards and three ores, was determined with an experimental set up employing cyclic activation analysis using a 14 MeV neutron generator gamma-spectroscopy and NaI(T1) detectors. 相似文献
967.
P. G. Kosky 《Journal of polymer science. Part A, Polymer chemistry》1991,29(1):101-111
Cyclic oligocarbonates are synthesized by two-phase interfacial reactions of aryl bischloroformates in the presence of an amine phase transfer catalyst and an alkali: nClOCOArOCOCl + 4nNaOH → (OArOCO)n + 2nNaCl + nNa2CO3 + 2nH2O. The organocarbonate product may be a macrocyclic oligomer or a linear chain polymer. A qualitative mechanism for this behavior has been proposed by Brunelle, Boden, and co-workers. Four steps are identifiable: activation of the aryl bischloroformate by the amine catalyst, hydrolysis of a portion of this intermediate at the aqueous/organic phase interface, oligomerization between activated and hydrolyzed moieties also at the interface, and chain terminating carbamate formation that leads to polymer. An important modification is made within the framework of the Brunelle and Boden mechanism. While the intramolecular cyclization reaction is formally second-order overall, it behaves as a first-order process. The kinetic constants for both this pseudo-first-order cyclization step and the corresponding second-order linearization step are simply related. It is speculated that the above relationship can be generalized for a whole class of pseudo-first- and second-order rate constants for similar macrocyclic reactions. 相似文献
968.
A E Abdel-Megied P Hansen E B Pedersen C M Nielsen 《Acta chemica Scandinavica (Copenhagen, Denmark : 1989)》1991,45(10):1060-1063
The 3'-iodonucleoside 4 and the 3'-O-methylsulfonylthymidine 9 have been synthesized by condensation of silylated uracils 2 with methyl 5-O-tert-butyldiphensilyl-2,3-dideoxy-3-iodo-D-threo-pentofuran oside (3) and methyl 5-O-tert-butyldiphenylsilyl-2-deoxy-3-methylsulfonyl-D-erythro- pentofuranoside (8), respectively. The nucleoside 4 and 9 produced the corresponding 2',3'-didehydro-2',3'-dideoxynucleosides 5 in an elimination reaction on treatment with sodium methoxide. The compounds 5b showed no antiviral activity against HIV-1. 相似文献
969.
A three-phase model of a hybrid chiral quark bag is proposed. In addition to the phases of asymptotically free current quarks
and of totally colorless mesons, the model contains the intermediate phase of massive constituent quarks. A self-consistent
solution with an account for the “Dirac sea” contribution is found in the (1+1)-dimensional case. The dependence of the bag
parameters on the parameters of the theory is investigated analytically and numerically.
Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 117, No. 2, pp. 263–299, November, 1998. 相似文献
970.
Summary An attempt is made to explain the masses of Z and W± bosons by means of a screening mechanism analogous to charge screening in metals. It considers the effect of the electroweak
force on the distribution of negativekinetic-energy neutrinos, which fills the whole universe (neutrino Fermi sea). The large
weak-boson masses appear to be a direct consequence of the vanishing neutrino mass. 相似文献