全文获取类型
收费全文 | 3524篇 |
免费 | 60篇 |
国内免费 | 13篇 |
专业分类
化学 | 1909篇 |
晶体学 | 24篇 |
力学 | 96篇 |
数学 | 504篇 |
物理学 | 1064篇 |
出版年
2021年 | 27篇 |
2020年 | 25篇 |
2019年 | 29篇 |
2018年 | 22篇 |
2017年 | 32篇 |
2016年 | 43篇 |
2015年 | 62篇 |
2014年 | 61篇 |
2013年 | 184篇 |
2012年 | 136篇 |
2011年 | 165篇 |
2010年 | 96篇 |
2009年 | 60篇 |
2008年 | 196篇 |
2007年 | 168篇 |
2006年 | 156篇 |
2005年 | 163篇 |
2004年 | 107篇 |
2003年 | 117篇 |
2002年 | 101篇 |
2001年 | 93篇 |
2000年 | 85篇 |
1999年 | 33篇 |
1998年 | 40篇 |
1997年 | 37篇 |
1996年 | 67篇 |
1995年 | 72篇 |
1994年 | 61篇 |
1993年 | 58篇 |
1992年 | 52篇 |
1991年 | 55篇 |
1990年 | 51篇 |
1989年 | 27篇 |
1988年 | 36篇 |
1987年 | 22篇 |
1986年 | 33篇 |
1985年 | 49篇 |
1984年 | 39篇 |
1983年 | 35篇 |
1982年 | 42篇 |
1981年 | 52篇 |
1980年 | 43篇 |
1979年 | 55篇 |
1978年 | 44篇 |
1977年 | 64篇 |
1976年 | 55篇 |
1975年 | 48篇 |
1974年 | 40篇 |
1973年 | 54篇 |
1972年 | 20篇 |
排序方式: 共有3597条查询结果,搜索用时 15 毫秒
941.
Hawryluk RJ Adler H Alling P Ancher C Anderson H Anderson JL Ashcroft D Barnes CW Barnes G Batha S Bell MG Bell R Bitter M Blanchard W Bretz NL Budny R Bush CE Camp R Caorlin M Cauffman S Chang Z Cheng CZ Collins J Coward G Darrow DS DeLooper J Duong H Dudek L Durst R Efthimion PC Ernst D Fisher R Fonck RJ Fredrickson E Fromm N Fu GY Furth HP Gentile C Gorelenkov N Grek B Grisham LR Hammett G Hanson GR Heidbrink W Herrmann HW Hill KW Hosea J Hsuan H Janos A Jassby DL Jobes FC Johnson DW 《Physical review letters》1994,72(22):3530-3533
942.
E. E. Kern D. K. Anderson 《Journal of polymer science. Part A, Polymer chemistry》1968,6(10):2765-2772
Diffusion and anisotropic electrical conductivity measurements have been used to investigate the structure of poly(methacrylic acid) in aqueous solution at various degrees of neutralization. Both kinds of measurement indicate that the polyion chain opens from a coiled to an extended configuration as the chain is neutralized. In addition it was found that, at low concentrations, the opening of the chain is a two-step process, whereas at higher concentrations the mechanism involves a single step. 相似文献
943.
Protein film voltammetry of NarGH catalysing nitrate reduction under steady state conditions provides information on events occurring within the enzyme during the catalytic cycle. In this discussion we have focused on exploring the ability of two simple catalytic schemes to reproduce the voltammetric response of NarGH; electron transfer to the enzyme's active site being described either by interfacial electron exchange (Scheme 1) or intramolecular electron delivery via the operation of an electron relay centre (Scheme 2). When the two electron reduced, catalytically competent active site of the enzyme is generated from the oxidised form in 'rapid', non-rate limiting steps of the catalytic cycle, the voltammetric behaviour of NarGH cannot be reproduced. Rather under all the conditions investigated, one electron reduction of the active site from a semi-reduced to a fully-reduced state is found to be crucial to progression through the enzyme's catalytic cycle. The catalytically relevant semi- and fully-reduced oxidation states of the NarGH active site are most likely to correspond to the Mo(V) and Mo(IV) states of the Mo(MGD)2 centre, respectively, although it is not possible to rule out the possibility that they correspond to molybdopterin based oxidation states as observed in other enzymes. We suggest that the rate of either conformational rearrangement within the semi-reduced active site or intramolecular electron delivery to the active site constitutes a defining feature in the catalytic cycle of NarGH and results in the napp approximately 1 appearance of the catalytic waveform. 相似文献
944.
The rate constant of the reaction OH + HCl → H2O + Cl was measured in a flow tube over the temperature range 224 to 460°K using resonance fluorescence detection of OH. An Arrhenius expression k1 = (2.0 ± 0.1) × 10?12 exp [?(620 ± 20 cal/mole)/RT] was obtained. Stratospheric and reaction kinetic implications are discussed briefly. 相似文献
945.
[reaction: see text] Stereo-controlled syntheses of two possible C1-C25 diastereomers of spirastrellolide A containing the cis-disubstituted tetrahydropyran and [6,6]-spiroacetal are reported, exploiting boron-mediated asymmetric aldol and allylation methodology. 相似文献
946.
Z. B. Güvenç M. A. Anderson B. H. Choi 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1995,35(1):51-55
A detailed evaluation of the structural and dynamical properties of isolated water dimers and tetramers using the Lemberg, Stillinger and Rahman potential energy surface and microcanonical molecular dynamics simulations are reported. The lowest-energy configurations of these clusters were obtained by simulated thermal quenching. Relative root-mean-square bond length fluctuations were used to characterize phase transitions. Solid-like, pre-melting, liquid-like, and dissociated states were identified. Global-local minimum transitions and the minimum energy path to dissociation of the dimer were also characterized. 相似文献
947.
A group of dissolved-bioavailable organochlorine (OC) pesticides and inorganic anions in water and total OC pesticides in sediments were measured in the Malheur Watershed, a semi-arid region in the western United States, over a 2-year period. OC pesticide levels were compared with those from a 1990 study of the lower section of the river, the most recent data available. After calculating the dissolved fraction from the 1990, study it seems that DDD and dieldrin levels have decreased in the water by 50-70%, while DDE and DDT have changed little. Although banned nearly 30 years ago, DDT is still persistent throughout the Malheur River basin/watershed because it was found in all water samples tested. All of the OC pesticides tested during the 2-year study are well below the criterion continuous concentration for aquatic community exposure as defined by the U.S. Environmental Protection Agency (EPA). OC pesticides appear to be decreasing, however, at lower Ontario there remains a human health risk (EPA Human Health Risk Water Quality Criteria) for DDT, because this criteria includes daily consumption of water and fish from the river. Overall, although the upper forest watershed sites have lower OC pesticide concentrations, they represent an important contribution to the total DDT load to this watershed, a source not previously acknowledged. The large increase in DDT and sigmaDDT between the Ontario sites may indicate a possible historical point source of contamination or historical preferential deposition of contamination. Normalized sediment (sigmaDDT/organic carbon) strongly correlates with dissolved water sigmaDDT. 相似文献
948.
James L. Anderson 《General Relativity and Gravitation》1984,16(6):595-601
The method of calculating radiation damping effects by means of energy balance is applied to a simple model problem. The balance equation is derived from the conservation law for energy that is a consequence of the equations of motion for the system by integrating over a four-dimensional space-time region bounded in the wave zone by two future-directed null cones and in the near zone by twot=const surfaces. It is assumed that the sources are in slow motion and the fields are calculated by the method of matched asymptotic expansions. Our principle conclusion is that even though the damping isO(
3) where is an expansion parameter the fields need only be determined toO(1) to calculate the effects of damping by this method.Work supported in part by NSF Grant No. 79-11664-01. 相似文献
949.
The rate constant of the reaction OH (v = 0) + O3 HO2 + O2 was measured over the temperature range from 220 to 450°K at total pressures between 2 and 5 torr using ultraviolet fluorescent scattering for the detection of OH radicals. An Arrhenius expression, k1 = 1.3 × 10?12 exp(?1900/RT) cm3/sec was obtained and the rate constant for the reaction HO2 + O3OH + 2O2 was inferred to be less than 0.1 k1 over the entire temperature interval. 相似文献
950.
The Galileo solar redshift experiment 总被引:2,自引:0,他引:2