全文获取类型
收费全文 | 486613篇 |
免费 | 4938篇 |
国内免费 | 1654篇 |
专业分类
化学 | 264819篇 |
晶体学 | 7147篇 |
力学 | 20100篇 |
综合类 | 12篇 |
数学 | 57009篇 |
物理学 | 144118篇 |
出版年
2020年 | 3736篇 |
2019年 | 4008篇 |
2018年 | 4932篇 |
2017年 | 4752篇 |
2016年 | 7654篇 |
2015年 | 5239篇 |
2014年 | 7725篇 |
2013年 | 21655篇 |
2012年 | 16244篇 |
2011年 | 20017篇 |
2010年 | 13371篇 |
2009年 | 13237篇 |
2008年 | 18749篇 |
2007年 | 19045篇 |
2006年 | 18091篇 |
2005年 | 16405篇 |
2004年 | 14738篇 |
2003年 | 13249篇 |
2002年 | 13011篇 |
2001年 | 14386篇 |
2000年 | 11167篇 |
1999年 | 8682篇 |
1998年 | 7227篇 |
1997年 | 7102篇 |
1996年 | 6951篇 |
1995年 | 6160篇 |
1994年 | 5995篇 |
1993年 | 5839篇 |
1992年 | 6380篇 |
1991年 | 6337篇 |
1990年 | 6018篇 |
1989年 | 5783篇 |
1988年 | 6012篇 |
1987年 | 5706篇 |
1986年 | 5462篇 |
1985年 | 7622篇 |
1984年 | 7828篇 |
1983年 | 6379篇 |
1982年 | 6936篇 |
1981年 | 6879篇 |
1980年 | 6506篇 |
1979年 | 6771篇 |
1978年 | 6895篇 |
1977年 | 6876篇 |
1976年 | 6846篇 |
1975年 | 6589篇 |
1974年 | 6461篇 |
1973年 | 6746篇 |
1972年 | 4136篇 |
1971年 | 3333篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Studies show that branching the side chains of hexakis(acoyloxy)-benzenes (HAB) and -cyclohexanes (HAChx) does not modify the structure of the discotic mesophase formed. Shear fields appear to act on entire columns, not on individual discs, thereby causing alignment of columns in the shear direction. X-ray diffraction patterns of samples at temperatures between 30°C and 40°C above the clearing temperature provide evidence for the presence of mesophase order. 相似文献
962.
G. N. Stratopoulos W. J. Zakrewski 《Zeitschrift fur Physik C Particles and Fields》1993,59(2):307-312
We look at the recently proposed scheme of approximating a sine-Gordon soliton by an expression derived from two dimensional instantons. We point out that the scheme of Sutcliffe in which he uses two dimensional instantons can be generalised to higher dimensions and that these generalisations produce even better approximations than the original approximation. We also comment on generalisations to other models. 相似文献
963.
We derive and analyse four algorithms for computing the current induced on a thin straight wire by a transient electric field.
They all involve solving the thin wire electric field integral equations (EFIEs) and consist of a very accurate differential
equations solver together with various schemes to approximate the vector potential integral equation. We carry out a rigorous
numerical stability analysis of each of these methods. This has not previously been done for solution schemes for the thin
wire EFIEs. Each scheme is shown to be stable and convergent provided the radius of the wire is small enough for the thin
wire equations to be a valid model. 相似文献
964.
Klaus Anderle Richard Birenheide Martin J. A. Werner Joachim H. Wendorff 《Liquid crystals》1991,9(5):691-699
Polarized light leads to an effective reorientation of the optic axis in the glassy state of liquid-crystalline side chain polymers containing azobenzene mesogenic groups, via a trans-cis and cis-trans isomerization. Using a combination of U V and IR dichroitic studies it is shown for copolymers consisting of chromophores (azobenzene) and non-chromophores (phenylbenzoate) that only the chromophores are reoriented by light as far as the glassy state is concerned. Individual chromophores are thus addressed by photoselection. Photoselection in the fluid nematic state, on the other hand, leads also to a reorientation of the non-chromophores. 相似文献
965.
The reaction of CH3 with OH has been studied near 1200 K and 1 atmosphere pressure in shock tube experiments in which UV absorption was used to monitor [OH]. A rate coefficient of (1.1 ± 0.3) × 1013 cm3/mol-s was measured for removal of OH by CH3. This measured value is compared with previous experimental data and calculations. Several possible reaction channels are discussed, and although products were not monitored, it seems probable, on the basis of other work and theoretical estimates, that the primary mechanism (?75%) for the removal of OH by CH3 at these conditions is their combination to form CH3OH. Rate coefficients of (5.3 ± 0.8) × 1012 and (9.0 ± 1.4) × 1012 cm3/mol-s were measured for the reactions of OH with acetone and ethane, respectively, at the same temperature and pressure. 相似文献
966.
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
967.
The many facets of linear programming 总被引:1,自引:0,他引:1
Michael J. Todd 《Mathematical Programming》2002,91(3):417-436
We examine the history of linear programming from computational, geometric, and complexity points of view, looking at simplex,
ellipsoid, interior-point, and other methods.
Received: June 22, 2000 / Accepted: April 4, 2001?Published online October 2, 2001 相似文献
968.
M. Ferro-Luzzi Z.-L. Zhou J.F.J. van den Brand H.J. Bulten J. Lang H.R. Poolman 《Hyperfine Interactions》1997,110(3-4):239-257
We present data showing hyperfine transitions in an atomic deuterium beam induced by the (476 MHz) radio-frequency field of
a 704 MeV electron beam in a storage ring. A polarized deuterium beam, produced in an atomic beam source, was crossed with
a stored electron beam and analyzed with a Breit--Rabi polarimeter. Electron-beam induced transitions were singled out by
injecting different combinations of hyperfine states. Transition probabilities as high as 70% were measured at large currents
(~ 100 mA). All possible deuterium transitions for a radio-frequency of 476 MHz were observed. In addition, a 1--6 transition
resulting from the first harmonic (952 MHz) was observed.
The effects of these transitions are of general importance for the polarized internal target technique applied in nuclear
and particle physics experiments. The data are reasonably described by numerical estimates. The observed mechanism can be
exploited to create nuclear polarized atoms when injecting electron polarized atoms with no net nuclear polarization into
a storage cell. However, when nuclear polarized atoms are injected, care should be taken to avoid this mechanism, since it
would result in depolarization of the atoms. The studies enabled us to choose the magnetic guide field during our spin-dependent
electron--deuteron scattering experiments, such that electron-beam induced depolarizing effects were avoided.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
969.
We present the analytical calculation of the four-loop QCD β-function within the minimal subtraction scheme. 相似文献
970.