全文获取类型
收费全文 | 8092篇 |
免费 | 246篇 |
国内免费 | 16篇 |
专业分类
化学 | 5566篇 |
晶体学 | 58篇 |
力学 | 190篇 |
综合类 | 5篇 |
数学 | 929篇 |
物理学 | 1606篇 |
出版年
2023年 | 58篇 |
2022年 | 182篇 |
2021年 | 278篇 |
2020年 | 223篇 |
2019年 | 215篇 |
2018年 | 221篇 |
2017年 | 158篇 |
2016年 | 329篇 |
2015年 | 269篇 |
2014年 | 327篇 |
2013年 | 514篇 |
2012年 | 546篇 |
2011年 | 611篇 |
2010年 | 415篇 |
2009年 | 416篇 |
2008年 | 516篇 |
2007年 | 454篇 |
2006年 | 376篇 |
2005年 | 312篇 |
2004年 | 284篇 |
2003年 | 214篇 |
2002年 | 190篇 |
2001年 | 142篇 |
2000年 | 128篇 |
1999年 | 75篇 |
1998年 | 69篇 |
1997年 | 70篇 |
1996年 | 54篇 |
1995年 | 46篇 |
1994年 | 69篇 |
1993年 | 61篇 |
1992年 | 65篇 |
1991年 | 35篇 |
1990年 | 31篇 |
1989年 | 26篇 |
1988年 | 27篇 |
1987年 | 20篇 |
1986年 | 23篇 |
1985年 | 25篇 |
1984年 | 25篇 |
1983年 | 28篇 |
1982年 | 16篇 |
1981年 | 27篇 |
1980年 | 18篇 |
1979年 | 28篇 |
1978年 | 22篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 12篇 |
1973年 | 11篇 |
排序方式: 共有8354条查询结果,搜索用时 531 毫秒
991.
Gabriela R. Argiroffo Silvia M. Bianchi Graciela L. Nasini 《Mathematical Methods of Operations Research》2008,67(2):245-256
Lehman (Polyhedral combinatorics 1 of DIMACS series in discrete math. and theoretical computer science, pp 101–105, 1990) described some conditions regular
minimally nonideal (mni) matrices must satisfy. Although, there are few results on sufficient conditions for mni matrices.
In most of these results, the covering polyhedron must have a unique fractional extreme point. This condition corresponds
to ask the matrix to be the blocker of a near-ideal matrix, defined by the authors in a previous work (2006). In this paper
we prove that, having the blocker of a near-ideal matrix, only a few very easy conditions have to be checked in order to decide
if the matrix is regular mni. In doing so, we define the class of quasi mni matrices, containing regular mni matrices, and
we find a generalization on the number of integer extreme points adjacent to the fractional extreme point in the covering
polyhedron. We also give a relationship between the covering and stability number of regular mni matrices which allows to
prove when a regular mni matrix can be a proper minor of a quasi mni.
Partially supported by CONICET Grant PIP 2807/2000 (Argentina) and by CNPq/PROSUL Grant 490333/2004-4 (Brasil). 相似文献
992.
I. Pop T. Petrişor A. Giurgiu A. Néda 《Journal of Physics and Chemistry of Solids》1985,46(9):1077-1081
The present study reports magnetic susceptibility, electrical resistivity and specific heat measurements on α-titanium. The experimental results indicate the existence of an anomaly in the temperature dependence of these parameters. The temperature dependence of the specific heat indicates that the anomaly observed in these parameters is a second order phase transition. The similarity between the temperature dependence of the magnetic susceptibility of α-titanium and of chromium suggests that α-titanium is an itinerant-electron antiferromagnet, such as chromium, with the Néel temperature at 276 ± 4 K. 相似文献
993.
994.
An automatic stopped-flow method is proposed for the routine determination of microgram amounts of l-cysteine, based on its oxidation by 2,6-dichlorophenolindophenol (DPIP) in a weakly basic medium. The oxidation reaction is monitored by measuring the rate of the absorbance decrease at 615 nm, the wavelength of the maximum absorption for DPIP. Under optimal conditions, the calibration graph is linear over the range 2-32 mug/ml and the detection limit is 300 ng/ml. Most of the amino-acids tested have no effect on the rate of the oxidation reaction and are tolerated at high concentration levels. The sample throughput achieved, 80 samples per hr, allows the proposed method to be used for the routine determination of l-cysteine. 相似文献
995.
Clarissa O. da Silva André Gustavo H. Barbosa Emerson T. da Silva Edson Luiz L. da Silva Marco Antonio C. Nascimento 《Theoretical chemistry accounts》2004,111(2-6):231-236
In order to better understand the performance of 1,2-dimethyl-5-acetyl barbituric acid (DMB) as an amino protecting group relative to 5,5-dimethylcyclohexane-1,3-dione (DMD), ab initio calculations were performed. pKa calculations using the PCM model indicated that both molecules are more acidic in the enol form. Therefore, the protecting reaction of these molecules should involve the anions formed from the loss of a proton from the enol compounds. Contrary to what would be expected, the larger efficiency exhibited by the DMB molecule cannot be attributed to an extension of the electronic conjugation effect. In the absence of any other noticeable effect that could be responsible for the greater efficiency of the DMB molecule, we are inclined to believe that the difference could be accounted for by the presence of two independent centers of conjugation.This paper is dedicated to Jacopo Tomasi in recognition of his outstanding contribution to the field of computational chemistry in solution. The authors are honored to contribute to this volume; especially so for two of them (COS and MACN) who have the privilege of his friendship.Acknowledgements The authors would like to thank the Brazilian research agencies CNPq, CAPES and FAPERJ for the financial support. C. O. da Silva thanks the Dipartimento di Chimica e Chimica Industriale, University of Pisa, where the MCSCF calculations were performed. 相似文献
996.
Valfredo Azevedo Lemos Moacy Selis Santos Márcio José Silva dos Santos Daniel Rodrigues Vieira Cleber Galvão Novaes 《Mikrochimica acta》2007,157(3-4):215-222
Cloud point extraction employing the new reagent 6-[2′-(6′-methyl-benzothiazolylazo)]-1,2-dihydroxy-3,5-benzenedisulfonic
acid as complexing agent and Triton X-114 as the surfactant is proposed for copper determination. A sample volume of 10 mL
was used. Dilution of the surfactant-rich phase with acidified methanol was performed after phase separation, and the copper
contents were measured by flame atomic absorption spectrometry. Variables affecting the system were optimized using factorial
design and Doehlert matrix. Signals were measured as peak height using an instrument software. Using the experimental conditions
defined in the optimization, the method allowed copper determination with a detection limit of 1.5 μg L−1. The calculated enrichment factor is 14. The effects of foreign ions are reported. The accuracy of the procedure was tested
by analyzing certified reference material. The method was successfully applied to copper determination in natural and drinking
water samples. 相似文献
997.
The electronic structure and magnetic properties of Fe4C are investigated with the first-principles discrete variational method (DV) in spin-polarized case. It is used for the local
spin density approximation of the density functional theory. Clusters of 21 and 27 atoms were adopted to represent the two
non-equivalent iron sites in this compound. The calculated magnetic moments for the corner and face centered iron sites are
2.63\muB and 1.77\muB, respectively, which are in very good agreement with the experimental results. A collapse of the magnetic moment for both
inequivalent iron sites was verified above 12% of contraction of the lattice spacing. The C atom acts as an acceptor for electrons
in this host.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
998.
Da Silva L.B. Cauble R. Barbee T.W. Jr. Celliers P. Ciarlo D. Libby S. London R.A. Matthews D. Mrowka S. Moreno J.C. Ress D. Trebes J.E. Wan A.S. Weber F. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1996,24(1):31-32
We have developed a soft X-ray interferometer capable of probing large high-density plasmas with micron spatial resolutions. A neon-like yttrium X-ray laser operating at 155 Å was combined with the Mach-Zehnder interferometer to obtain electron density profiles of a laser-produced exploding selenium foil plasma. The plasma was produced with one Nova laser beam using a 120-μm line focus, the same conditions used to create a selenium X-ray laser. The interferogram of the selenium plasma was obtained from an end-on perspective 相似文献
999.
M. Behar L. G. Brunnet J. A. H. da Jornada R. P. Livi F. C. Zawislak 《Hyperfine Interactions》1983,15(1-4):261-264
The temperature dependence of the electric field gradient in the AgIn2 intermetallic compound was measured using the TDPA correlation technique. From 80 K up to 415 K the electric field gradient follows a linear temperature dependence. Below 80 K a change in this behaviour is observed.Work supported in part by FINEP and CNPq. 相似文献
1000.
M.H.P. Corrêa A. Vasquez M.I. da Costa P.J. Viccaro C.E.T. Gonçalves da Silva 《Solid State Communications》1981,40(3):211-213
Ordered alloys of Pd3Fe are shown to readily absorbe hydrogen through electrolytic loading. The resultant ternary hydride phase is observed to retain the fcc structure of Pd3Fe with approximately the same lattice constant. The 57Fe hyperfine field determined by Mössbauer spectroscopy is found to be 30% smaller in the hydride compared to Pd3Fe. The reduction appears to be associated with a perturbation of the Pd moment by hydrogen. The results suggest the occupation of one type of interstitial site in the structure. The absence of the site in disordered Pd3Fe would explain the much smaller hydrogen capacity observed for this alloy. 相似文献