全文获取类型
收费全文 | 206250篇 |
免费 | 2157篇 |
国内免费 | 522篇 |
专业分类
化学 | 113578篇 |
晶体学 | 3662篇 |
力学 | 8072篇 |
综合类 | 11篇 |
数学 | 19722篇 |
物理学 | 63884篇 |
出版年
2020年 | 1858篇 |
2019年 | 2024篇 |
2018年 | 2296篇 |
2017年 | 2430篇 |
2016年 | 3680篇 |
2015年 | 2329篇 |
2014年 | 3716篇 |
2013年 | 9269篇 |
2012年 | 6786篇 |
2011年 | 8252篇 |
2010年 | 5833篇 |
2009年 | 5790篇 |
2008年 | 7370篇 |
2007年 | 7273篇 |
2006年 | 6865篇 |
2005年 | 6242篇 |
2004年 | 5695篇 |
2003年 | 5215篇 |
2002年 | 5018篇 |
2001年 | 6206篇 |
2000年 | 4682篇 |
1999年 | 3567篇 |
1998年 | 2803篇 |
1997年 | 2752篇 |
1996年 | 2645篇 |
1995年 | 2478篇 |
1994年 | 2361篇 |
1993年 | 2196篇 |
1992年 | 2865篇 |
1991年 | 2751篇 |
1990年 | 2699篇 |
1989年 | 2723篇 |
1988年 | 2746篇 |
1987年 | 2751篇 |
1986年 | 2614篇 |
1985年 | 3330篇 |
1984年 | 3344篇 |
1983年 | 2623篇 |
1982年 | 2730篇 |
1981年 | 2800篇 |
1980年 | 2556篇 |
1979年 | 2894篇 |
1978年 | 2876篇 |
1977年 | 3007篇 |
1976年 | 2826篇 |
1975年 | 2565篇 |
1974年 | 2517篇 |
1973年 | 2474篇 |
1972年 | 1706篇 |
1968年 | 1707篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
An exact transcendental equation was obtained determining the energy levels of a particle bound by a short-range force and placed in uniform electric field. At high intensities there is a considerable difference between the values of decay probabilities calculated by previous theory and by this one. The results obtained were used to determine the electron binding energy and the polarizability of negative ions by employing experimental data on ion decay during a flight through the field region. Concrete calculations were carried out for the C? ion.The calculation of the resonance energy Er and the decay probability 2Γ of the quasistationary state n = S, n1 = 3, n2 = 0, and m = 1 of the hydrogen atom in a uniform electric field was performed by means of the direct integration of the time-independent Schrödinger equation for field intensities ranging from 0.8 × 106V/cm to 1.7 × 106V/cm. The calculated values fo Er perfectly agree with those of the Rayleigh—Schrödinger perturbation energies calculated up to the fourth power in the field strength and differ substantially from the resonance energies obtained earlier by Hirschfelder and Curtiss. Our results concerning the rate of spontaneous ionization also differ substantially from the results of Hirschfelder and Curtiss and agree rather well with the results of Bailey, Hiskes and Riviere who used the general WKB method of Rice and Good. 相似文献
942.
U. Wannagat F. Vielberg H. Voß K. Hensen W. Sarholz 《Monatshefte für Chemie / Chemical Monthly》1969,100(4):1127-1132
The enthalpies of formation of the addition compounds F4Si(py)2, Cl4Si(py)2, Br4Si(py)2 and I4Si(py)4 have been measured calorimetrically by two different ways (see table 1). It was not possible to confirm enthalpy data published in7, 8. The sequence SiF44~SiBr4>SiI4(?) is proposed for the acceptor power of the silicon tetrahalides towards pyridine. 相似文献
943.
944.
Fischer indolization of ethyl pyruvate 2-methoxyphenylhydrazone (1) with p-toluene-sulfonic acid in benzene in the presence of an enolizable dicarbonyl or an indolic compound gave either an indole product having an active methine group at C6 or a novel type of 3,6′-biindole compound. Structures of the products were established by NMR spectra and chemical evidence. 相似文献
945.
Using already published methods, the authors have determined in eight brains of premature and seven brains of new-born children: dry matter, total lipids, pure lipids, and acid and neutral lipids. The latter have been separated into cerebrosides, sphingomyelines, colamine cephalines, and lecithins. 相似文献
946.
K. C. Stueben 《Journal of polymer science. Part A, Polymer chemistry》1966,4(4):829-846
A wide variety of polyesters and polyamides was prepared from 1,2- and 1,3-disubstituted cyclobutane derivatives. In general, polyamides containing the trans-disubstituted cyclobutane ring were crystalline. Polyesters containing predominantly the trans-isomer tended to be crystalline but those containing mixtures of cis-trans isomers were amorphous. Mechanical properties have been determined wherever possible. Appreciable yields of cyclic dimers were isolated during the preparation of certain polyesters. These cyclic dimers readily polymerized when treated with dibutylzinc. Examination of acrylonitrile dimer (1,2-dicyanocyclobutane) by gas chromatography failed to reveal the presence of any 1,3-dicyanocyclobutanes. 相似文献
947.
B. A. Tashpulatova K. D. Dravranov L. G. Mezhlum'yan 《Chemistry of Natural Compounds》1991,27(6):735-739
The molecular mass (Mr) of highly purified glucose isomerase fromStreptomyces atratus has been determined. The native glucose isomerase, which has affinity for xylose, consists of an oligomeric protein containing 1552 amino acid residues and formed of four subunits. The enzyme is stable in the range of temperatures from 40 to 80°C and at pH values of from 6.0 to 11.0. 相似文献
948.
Socher G Nussbaum R Rissler K Lankmayr E 《Fresenius' Journal of Analytical Chemistry》2001,371(3):369-375
Unknown fatty acid ethoxylate samples have been transesterified in supercritical methanol, using a loop made of a stainless-steel tubing as the reactor vessel. The initiator acids, now present as the corresponding methyl esters, were determined by gas chromatography-mass spectrometry (GC-MS). Quantitative transesterification is achieved by heating a solution of the ethoxylate sample in methanol at 280 degrees C for 10 min under pressure. The influence of reaction time, temperature, and presence of a Lewis acid catalyst has been investigated. The method of transesterification was optimized in respect of low cost, short reaction time, and availability to laboratories with standard analytical equipment. 相似文献
949.
van Bokhoven JA Roelofs JC de Jong KP Koningsberger DC 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(6):1258-1265
The changes in the layered structure of Mg-Al hydrotalcite (Mg/ Al = 2) during heat treatment have been investigated by using in situ XAFS simultaneously at the Mg and Al K-edges. The development of unique in situ instrumentation allowed the coordination environments at both the Mg and Al centers to be monitored as a function of the temperature and heat treatment. The results of this study show that the hydrotalcite structure is highly flexible, and should lead to the further development of hydrotalcites as new solid basic catalysts. Moreover, the Mg and Al cations in the cation layers show different behavior as a function of temperature. The coordination of some octahedral Al ions decreases already at a temperature of 425 K, whereas the coordination about Mg does not show any modification at this temperature. However, hydrotalcite treated at 425 K, followed by cooling down to room temperature resulted in a complete reversal to the original octahedral Al coordination. It is proposed that Al-OH bond breakage occurs at 425 K, without the evolution of H2O. This bond is restored after cooling to room temperature. The actual dehydroxylation of hydrotalcite commences between 425 and 475 K, as indicated by a change in coordination of both the Mg and Al centers. This is accompanied by the evolution of H2O molecules and the changes are hence irreversible without the presence of excess water. Heat treatment at 725 K leads to the development of an MgO-like phase (octahedral Mg) and a mixed octahedral/tetrahedral Al phase. A subsequent rehydration at room temperature entirely restores the original coordination about the Al and Mg centers of hydrotalcite to a distance of 15 A, to which XAFS spectroscopy is sensitive. 相似文献
950.
The thermochemical decomposition of waste products from a coconut (Cocos nucifera L.), a nut (Juglans regia L.) an apricot (Armeniaca vulgaris L.), an almond (Amygdalus communis L.var amara DC.), a grape (Vitis vinifera L.) and a cherry (Prunus avium L.), as raw materials for producing carbon adsorbents, was investigated by DTA and TG. The thermo-oxidative and decomposition processes resulting in increased number of oxygen-containing functional groups of phenolic and carbonyl type are responsible for the high adsorption capacities of the carbon adsorbents. The thermochemical changes in adsorbents based on agricultural wastes are similar to those in commercial activated carbons. The adsorbents derived from apricot stones have the highest overall thermal stability. 相似文献