全文获取类型
收费全文 | 242404篇 |
免费 | 2758篇 |
国内免费 | 868篇 |
专业分类
化学 | 130201篇 |
晶体学 | 3688篇 |
力学 | 9892篇 |
综合类 | 2篇 |
数学 | 27060篇 |
物理学 | 75187篇 |
出版年
2020年 | 1764篇 |
2019年 | 2035篇 |
2018年 | 2470篇 |
2017年 | 2401篇 |
2016年 | 3817篇 |
2015年 | 2588篇 |
2014年 | 3890篇 |
2013年 | 11145篇 |
2012年 | 8069篇 |
2011年 | 9992篇 |
2010年 | 6564篇 |
2009年 | 6461篇 |
2008年 | 8975篇 |
2007年 | 9123篇 |
2006年 | 8367篇 |
2005年 | 7826篇 |
2004年 | 6978篇 |
2003年 | 6199篇 |
2002年 | 6135篇 |
2001年 | 7198篇 |
2000年 | 5357篇 |
1999年 | 4217篇 |
1998年 | 3549篇 |
1997年 | 3577篇 |
1996年 | 3291篇 |
1995年 | 3130篇 |
1994年 | 2946篇 |
1993年 | 3006篇 |
1992年 | 3297篇 |
1991年 | 3329篇 |
1990年 | 3148篇 |
1989年 | 3098篇 |
1988年 | 3160篇 |
1987年 | 3031篇 |
1986年 | 2924篇 |
1985年 | 3987篇 |
1984年 | 4146篇 |
1983年 | 3400篇 |
1982年 | 3749篇 |
1981年 | 3645篇 |
1980年 | 3519篇 |
1979年 | 3551篇 |
1978年 | 3779篇 |
1977年 | 3612篇 |
1976年 | 3815篇 |
1975年 | 3390篇 |
1974年 | 3516篇 |
1973年 | 3819篇 |
1972年 | 2328篇 |
1971年 | 1786篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A. Cerdán Vidal A. R. Maurí Aucejo M. Llobat Estellés C. Pascual Martí J. Simeón Martí 《Fresenius' Journal of Analytical Chemistry》1994,350(12):706-711
The possibility of identification of substances which have similar spectral behaviour by means of Apparent Content Curves has been studied. This study is carried out with absorption, excitation and emission spectra of several amphetamines of widespread pharmaceutical use. Results obtained show that amphetamine, phenylpropanolamine, pseudoephedrine, phenylephrine, epinephrine, dopamine and methoxyamphetamine can be identified with a probability of >95%. 相似文献
992.
J.-J. Sauvain L. F. de Alencastro J. Tarradellas R. Camenzind G. Karlaganis C. Vuilleumier 《Fresenius' Journal of Analytical Chemistry》1994,350(7-9):555-562
The analysis of PCB in transformer oils has been achieved with three HRGC/ECD based methods, proposed by the International Electrotechnical Commission (IEC/TC 10), by the Comité Européen de Normalisation (CEN/TC 19/WG 22) and by the Deutsches Institut für Normung (DIN 51 527). The same clean-up, described in the CEN/TC 19/WG 22, has been used for all the samples, allowing a comparison of the quantification procedure only. The total chlorine content has also been determined with X-ray fluorescence. These different methods of quantification are compared and their application for legislative purposes is discussed.Dedicated to Professor Dr Dieter Klockow on the occasion of his 60th birthday 相似文献
993.
Classical methods of structural analysis cannot be applied to liquid crystals because higher order reflections disappear during the transition from crystal to liquid crystal due to the reduction in long range orientational and translational correlations. However, in order to relate physical properties to the molecular architecture, it is essential to have information about molecular positions and orientations in the crystalline state as well as in the liquid crystalline state. In this work, the transition from crystalline to liquid crystalline phase is carefully monitored and the relationship between the original lattice and the new molecular positions found using electron diffraction. In addition to this, a new high resolution electron-microscopic technique is described in which the positions of molecules in the crystalline and the quenched discotic phase are directly imaged and the defects observed in the crystalline and LC phase compared and quantitatively analysed. 相似文献
994.
Di[2-(4-substituted-phenyl)-3-mercapto-propenato]-nickel(II) complexes have been synthesized and investigated. All of them are mesomorphic; they have nematic phases with wide ranges and typical Schlieren textures. 相似文献
995.
The toxicity of inorganic trivalent arsenic for living organisms is reduced by in vivo methylation of the element. In man, this biotransformation leads to the synthesis of monomethylarsonic (MMA) and dimethylarsinic (DMA) acids, which are efficiently eliminated in urine along with the unchanged form (Asi). In order to document the methylation process in humans, the kinetics of Asi, MMA and DMA elimination were studied in volunteers given a single dose of one of these three arsenicals or repeated doses of Asi. The arsenic methylation efficiency was also assessed in subjects acutely intoxicated with arsenic trioxide (As2O3) and in patients with liver diseases. Several observations in humans can be explained by the properties of the enzymic systems involved in the methylation process which we have characterized in vitro and in vivo in rats as follows: (1) production of Asi metabolites is catalyzed by an enzymic system whose activity is highest in liver cytosol; (2) different enzymic activities, using the same methyl group donor (S-adenosylmethionine), lead to the production of mono- and di-methylated derivatives which are excreted in urine as MMA and DMA; (3) dimethylating activity is highly sensitive to inhibition by excess of inorganic arsenic; (4) reduced glutathione concentration in liver moderates the arsenic methylation process through several mechanisms, e.g. stimulation of the first methylation reaction leading to MMA, facilitation of Asi uptake by hepatocytes, stimulation of the biliary excretion of the element, reduction of pentavalent forms before methylation, and protection of a reducing environment in the cells necessary to maintain the activity of the enzymic systems. 相似文献
996.
Polyphysa peniculus was grown in artificial seawater in the presence of arsenate, arsenite, monomethylarsonate and dimethylarsinic acid. The separation and identification of some of the arsenic species produced in the cells as well as in the growth medium were achieved by using hydride generation–gas chromatography–atomic absorption spectrometry methodology. Arsenite and dimethylarsinate were detected following incubation with arsenate. When the alga was treated with arsenite, dimethylarsinate was the major metabolite in the cells and in the growth medium; trace amounts of monomethylarsonate were also detected in the cells. With monomethylarsonate as a substrate, the metabolite is dimethylarsinate. Polyphysa peniculus did not metabolize dimethylarsinic acid when it was used as a substrate. Significant amounts of more complex arsenic species, such as arsenosungars, were not observed in the cells or medium on the evidence of flow injection–microwave digestion–hydride generation–atomic absorption spectrometry methodology. Transfer of the exposed cells to fresh medium caused release of most cell–associated arsenicals to the surrounding environment. 相似文献
997.
In order to obtain a continuous source of mitotic metaphases, gill tissue of Aphaius fasciatus (Pisces, Cyprinodontiformes) has been successfully employed. Results gathered after exposure of fish to R2SnClpenG, R3SnClpenGNa, to the parents R2SnCl2, R3SnCl and to penGNa (penGNa = penicillinGNa; R = methyl, butyl and phenyl) suggest that both the parent organotin (IV) chloride and organotin (IV) chloropenG derivatives are toxic while penGNa exerts no significant toxic activity. Essentially, all of the chromosome abnormalities are classifiable as irregularly staining of chromosomes, breakages, side-arm bridges or pseudochiasmata. 相似文献
998.
Theories and models are presented for gas sorption in polymers above and below the glass transition temperature. With the exception of predictive theories that do not represent the data well, the models are fit to data for the carbon dioxide/silicone rubber and carbon dioxide/polycarbonate systems for the purposes of comparison. During the past decade, a number of new models and theories have been proposed specifically for gas sorption in glassy polymers. Each new model attempts to incorporate aspects of the gas sorption process that are unique to polymers below the glass transition temperature. This review discusses these recent advances, the assumptions used in their development and their advantages and disadvantages. 相似文献
999.
Glow Discharge Yuan Mei Douglas C. Duckworth Paula R. Cable R. Kenneth Marcus 《Journal of the American Society for Mass Spectrometry》1994,5(9):845-851
Collision-induced dissociation (CID) of polyatomic ions sampled from an rf-powered glow discharge is examined by using three target gases including atomic (Ar and Xe) and molecular species (N2). Collisions with these targets in the first quadrupole of the double quadrupole system result in the loss of discharge species by dissociation, symmetric and asymmetric charge exchange, and scattering, each to varying degrees. These processes are seen to be a function of the relative mass, size... and ionization potentials of the target species, as well as the collision center-of-mass energies. In light of the comparisons, xenon appears to be the best collision target for both CID and charge exchange because of its relatively low ionization potential and high dissociation efficiency of polyatornic species. Evidence for both symmetric and asymmetric charge exchange is presented for Ar and Xe target gases. 相似文献
1000.
V. N. Odinoko V. R. Akhmetova R. G. Savchenko A. A. Fatykhov A. Ya. Zapevalov 《Russian Chemical Bulletin》1994,43(6):1084-1085
Perfluoro-1-octene was used as a model for developing a method for the preparative ozonolysis of perfluoro-1-alkenes to the corresponding perfluoro-nor-alkenals. Perfluoroheptanal was synthesized.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1146–1147, June, 1994. 相似文献