全文获取类型
收费全文 | 3079篇 |
免费 | 89篇 |
国内免费 | 18篇 |
专业分类
化学 | 2058篇 |
晶体学 | 12篇 |
力学 | 95篇 |
数学 | 471篇 |
物理学 | 550篇 |
出版年
2023年 | 22篇 |
2022年 | 21篇 |
2021年 | 38篇 |
2020年 | 61篇 |
2019年 | 53篇 |
2018年 | 18篇 |
2017年 | 31篇 |
2016年 | 87篇 |
2015年 | 76篇 |
2014年 | 84篇 |
2013年 | 138篇 |
2012年 | 138篇 |
2011年 | 197篇 |
2010年 | 111篇 |
2009年 | 127篇 |
2008年 | 193篇 |
2007年 | 176篇 |
2006年 | 178篇 |
2005年 | 135篇 |
2004年 | 146篇 |
2003年 | 112篇 |
2002年 | 112篇 |
2001年 | 51篇 |
2000年 | 56篇 |
1999年 | 45篇 |
1998年 | 37篇 |
1997年 | 32篇 |
1996年 | 27篇 |
1995年 | 28篇 |
1994年 | 21篇 |
1993年 | 25篇 |
1992年 | 29篇 |
1991年 | 26篇 |
1990年 | 28篇 |
1988年 | 15篇 |
1987年 | 27篇 |
1985年 | 26篇 |
1984年 | 39篇 |
1983年 | 31篇 |
1982年 | 24篇 |
1981年 | 34篇 |
1980年 | 39篇 |
1979年 | 25篇 |
1978年 | 23篇 |
1977年 | 16篇 |
1976年 | 16篇 |
1975年 | 21篇 |
1974年 | 18篇 |
1973年 | 21篇 |
1972年 | 15篇 |
排序方式: 共有3186条查询结果,搜索用时 156 毫秒
91.
Rates of volatilisation and chain scission have been measured in the thermal degradation, photodegradation in solution, photodegradation in thin films and photothermal degradation of poly(methyl methacrylate) and a series of copolymers of methyl methacrylate with maleic anhydride. In each case the rate of volatilisation is depressed by the maleic anhydride units. On the other hand, rates of chain scission are accelerated by maleic anhydride except in the case of photothermal degradation. These results are discussed from a mechanistic point of view. 相似文献
92.
Stephen R. Langhoff Stephen T. Elbert Ernest R. Davidson 《International journal of quantum chemistry》1973,7(5):999-1019
The electron spin dipole-dipole contribution to the zero field splitting has been evaluated for the 3A2 (n → π*) and 3A1 (π → π*) states of formaldehyde using a CI wave function constructed from contracted Gaussian-lobe functions. The values D = 0.539 cm?1 and E = 0.031 cm?1 were obtained for the 3A2(n → π*) state and D = ?0.588 cm?1 and E = 0.058 cm?1 were obtained for the 3A1 (π → π*) state using the CI wave function constructed from SCF orbitals of the respective parent configurations. An analysis of the effect of CI on the parameters is given for the 3A2 (n n → π*) state of formaldehyde and the 3B1 ground state of methylene. Numerical results are given which show that internally consistent self-consistent field orbitals (ICSCF ) are superior to canonical SCF orbitals as a starting point for a CI calculation. Our CI wave function for the 1A1 ground state gave an energy of ?114.13658 hartrees which is significantly lower than any previously reported energy calculation. This wave function gives a dipole moment of 2.22 Debye (C+O?) in good agreement with the experimental value of 2.33 ± 0.02 Debye. 相似文献
93.
Robert W. Lenz Lars C. Westfelt 《Journal of polymer science. Part A, Polymer chemistry》1976,14(9):2147-2153
The cationic polymerization of p-chloro-α-methylstyrene was investigated for the effect of initiator and solvent on polymer tacticity, molecular weight, and molecular weight distribution. The products were generally crystalline polymers of 80–90% syndiotactic content with fairly high molecular weights and broad molecular weight distributions. Tacticities and molecular weight distributions followed similar patterns suggesting that the effect of reaction conditions on ionpair end-group structures was the dominant factor. 相似文献
94.
The electronic structures of the 4-4 SBU, the β-cage, and the β-cage with two 4-4 SBU's attached to it have been studied by means of EH-MO calculations. No indication of the formation of a band structure has been found. The HOMO region consists of many closely spaced, localized states, 98.6% of them concentrated on the O-atoms. Reversible color changes of Cu+1 and Ag+1 zeolites observed upon hydration-dehydration experiments can be understood as charge-transfer transitions from the HOMO concentrated on the zeolite O-atoms to the metal cations. As soon as the Cu+1 or Ag+1 are partially hydrated, the ns* and np* states are shifted to higher energies. The luminescence observed with dehydrated Cu+1-zeolites X is caused by a 4p*←HOMO absorption, followed by spontaneous 4s*←4p* emission. After a detailed study of a Cu+1 in the 6-6 SBU, we discuss the electronic structure of a β-cage filled with 1,2,4,8, and 9 Cu+1. In each case, the β-cage is found to be too small to allow the formation of a band structure. The levels caused by the added copper are distinctly quantized. Calculations on [Ag3(H2O)3]3+ in a β-cage are reported. The direct interaction between the Ag-atoms is significant. As a consequence, the states formed by Ag 5s and 5p atomic orbitals are delocalized over the three Ag-centers. In both the Cu+1 and the Ag+1 zeolites, the ligand-field picture is found to be insufficient to explain the electronic structure, when the metal is coordinated to the zeolite oxygen framework. 相似文献
95.
Earlier studies of electric field assisted LC (EF-LC) have shown that the effect on charged analytes of the application of an electric field over a capillary LC column is relatively small. Charged analytes can only be affected by the electric field while present in the mobile phase, which makes the effective time for influence of the electric field t(0) independent of retention time. Because the charged analytes only can be affected for a short time the electric field strength ought to be high in order to increase the impact of the electric field on the separation. We have, however, found that only a relatively low electric field strength can be used in EF-LC when pressure is used as main driving force. The useful field strength was limited by a dramatic increase in the current. This increase in current was found to origin from an increased concentration of buffer ions that have an electrophoretic mobility towards the pumped flow. 相似文献
96.
97.
A numerical model of the negative DC corona plasma along a thin wire in dry air is presented. The electron number density and electric field are determined from solution of the one-dimensional coupled continuity equations of charge carriers and Maxwell's equation. The electron kinetic energy distribution is determined from the spatially homogeneous Boltzmann equation. A parametric study is conducted to examine the effects of linear current density (0.1–100 A per cm of wire length), wire radius (10–1000 m), and air temperature (293–800 K) on the distribution of electrons and the Townsend second ionization coefficient. The results are compared to those previously determined for the positive corona discharge. In the negative corona, energetic electrons are present beyond the ionization boundary and the number of electrons is an order of magnitude greater than in the positive corona. The number of electrons increases with increasing gas temperature. The electron energy distribution does not depend on discharge polarity. 相似文献
98.
99.
W. Davidson 《General Relativity and Gravitation》1992,24(2):179-185
Non-stationary cylindrically symmetric one-parameter solutions to Einstein's equations are given for a perfect fluid. There is a time singularity (t=0) at which the pressurep and density are equal to + throughout the radial coordinate range 0 r < , but the solutions are well behaved fort > 0,p and decreasing steadily to zero asr increases through the range 0r<, or as t increases through the range 0<t<. The motion is irrotational with shear, expansion and acceleration. The family of solutions, of Petrov type I, are generally spatially inhomogeneous, of class B(ii), having two spacelike Killing vectors which are mutually orthogonal and hypersurface orthogonal, associated with an orthogonally transitive groupG
2. The particular members for which there are equations of statep=/3 andp= are specially considered. 相似文献
100.
A number of nucleophilic ring openings of 3-substituted pyridinium salts have been reinvestigated and summarized. The structure of the resulting stable glutaconaldehyde derivatives was investigated in detail by 1H NMR. It has been concluded that in general nucleophilic pyridinium ring openings are highly regiospecific. In each case investigated to date a single product was isolated, as a result of attack by the nucleophile at only one of the pyridine α-positions. With the OH ion as the only nucleophile, attack occurs at the pyridine C-2, while larger nucleophiles such as amines and carbanions attack at the pyridine C-6. This was found to be the case for a variety of 3-substituted pyridines such as 3-methyl, 3-methoxy-, 3-cyano-, 3 chloro-pyridine. 相似文献