全文获取类型
收费全文 | 62493篇 |
免费 | 4703篇 |
国内免费 | 4144篇 |
专业分类
化学 | 38318篇 |
晶体学 | 1098篇 |
力学 | 2881篇 |
综合类 | 298篇 |
数学 | 6507篇 |
物理学 | 22238篇 |
出版年
2023年 | 523篇 |
2022年 | 1191篇 |
2021年 | 1257篇 |
2020年 | 1347篇 |
2019年 | 1414篇 |
2018年 | 1425篇 |
2017年 | 1406篇 |
2016年 | 1943篇 |
2015年 | 1661篇 |
2014年 | 2311篇 |
2013年 | 3576篇 |
2012年 | 3887篇 |
2011年 | 4341篇 |
2010年 | 3257篇 |
2009年 | 3146篇 |
2008年 | 3452篇 |
2007年 | 3288篇 |
2006年 | 3058篇 |
2005年 | 2527篇 |
2004年 | 2106篇 |
2003年 | 1759篇 |
2002年 | 1665篇 |
2001年 | 1979篇 |
2000年 | 1526篇 |
1999年 | 1270篇 |
1998年 | 955篇 |
1997年 | 890篇 |
1996年 | 773篇 |
1995年 | 724篇 |
1994年 | 674篇 |
1993年 | 556篇 |
1992年 | 606篇 |
1991年 | 602篇 |
1990年 | 578篇 |
1989年 | 508篇 |
1988年 | 515篇 |
1987年 | 480篇 |
1986年 | 434篇 |
1985年 | 489篇 |
1984年 | 487篇 |
1983年 | 361篇 |
1982年 | 404篇 |
1981年 | 381篇 |
1980年 | 324篇 |
1979年 | 408篇 |
1978年 | 381篇 |
1977年 | 396篇 |
1976年 | 392篇 |
1975年 | 342篇 |
1973年 | 341篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Tawarah Khalid M. Jibril Ibrahim Bani-Fwaz Mutasem Z. 《Transition Metal Chemistry》2000,25(6):659-663
The kinetics of the photolytic CO-substitution of CpFe(CO)2SCOR [Cp = C5H5, Bu
t
C5H4, 1,3-Bu2
t
C5H3; R = Me, Bu
t
, Ph, 2-(O2N)C6H4, 3-(O2N)C6H4, 4-(O2N)C6H4, 3,5-(O2N)2C6H3] with PPh3 were studied in CH2Cl2 at 0 °C by i.r. spectroscopy. The reactions yielded exclusively the mono-CO-substituted derivatives, CpFe-(CO)(PPh3) SCOR, and were found to follow second order kinetics with first order dependence on the concentration of each reactant. The differences in rates are discussed in terms of current knowledge pertaining to such reactions. An associative mechanism is proposed to account for the kinetic data of the reactions described. 相似文献
992.
Determination of trace elements in marine plankton by inductively coupled plasma mass spectrometry (ICP-MS) 总被引:3,自引:0,他引:3
Arslan Z Ertas N Tyson JF Uden PC Denoyer ER 《Fresenius' Journal of Analytical Chemistry》2000,366(3):273-282
A method has been developed for the determination of 23 elements in marine plankton in which inductively coupled plasma (ICP) source mass spectrometry (MS) was used to quantify the elements in the solution after digestion in a mixture of hydrofluoric and nitric acids in sealed PTFE vessels in a microwave field. The procedure was validated by the analysis of a standard reference soil (SRM 2709 San Joaquin Soil) and a standard reference fresh water plankton (CRM 414). The method was applied to the analysis of several marine plankton samples grown under controlled conditions including several whose growth media had been enriched with selenium. Matrix induced signal suppressions and instrumental drift were corrected by internal standardization. The suitabilities of germanium, indium, rhodium, scandium and yttrium as internal standard elements were evaluated. Neither scandium nor yttrium could be used due to the presence of these elements in the samples, germanium was used for the determination of As, Co, Cu, Fe, Ni, Se, Si and Zn, indium was used for Al, Ba, Ca, Eu, Sr, and Tl, and rhodium was used for Cd, Cr, Hg, Mg, Pb, Sb, Sn, and V. For Al, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Si, Sr, V, and Zn internal standardization did not completely compensate for the suppressive effect of the heavier elements and the solutions were diluted. However, for As, Ba, Cd, Co, Eu, Hg, Pb, Sb, Se, Sn and T1, it was possible to obtain accurate results despite the 35-40% suppression in the signals. Isobaric overlap was only a problem in the cases of 42Ca and 78Se; 44Ca and 77Se, respectively, were used. Memory effects were only observed with Hg for which a nitric acid-sodium chloride solution was the most effective wash-out solution. The marine plankton samples were able to tolerate a higher concentration of Hg as the selenium concentration increased. 相似文献
993.
994.
995.
A.G. Gonzalez J.M. Arteaga J.J. Fernandez J.D. Martin M. Norte J.Z. Ruano 《Tetrahedron》1984,40(14):2751-2755
The structures of three brominated terpenoids which are natural products from the red alga Laurencia pinnatifida (Gmal. Lamour) are described. The structures of the sesquiterpenes and were determined by spectral comparison and chemical interconversion. The structure of the squalene-derived terpenoid was secured by chemical transformation into thyrsiferol, a brominated triterpene previously isolated from the red alga Laurencia thyrsifera. 相似文献
996.
A. Dávid G. Horváth Z. Mészáros T. Meisel Z. Halmos 《Journal of Thermal Analysis and Calorimetry》1974,6(1-2):59-66
The thermal behaviour of tetrahydroperparine (THP) and its derivatives with various acids has been investigated. The acid is liberated from the formate derivative basically in two steps, while the other aliphatic acids are released quantitatively before the thermal decomposition of the THP molecule. The thermoanalytical curves, electrical conductivity data measured in the molten phase, and infrared spectra prove that while part of the formic acid is hydrogen-bonded the other part is bound ionically in the molecule. It may be assumed that the marked biological activity of the compound can be explained by this difference in the nature of the bonds. 相似文献
997.
Pyrophyllite introduced into a ceramic batch prepared from the gravity tailings of zircon-ilmenite ore raises the mullite formation temperature by 50–80°C and increases the mullite content of the resulting material. The first stage of the synthesis yields non-acid-resistant mullite. The early formation and rapid buildup of mullite are due to the formation of small amounts of a liquid phase. Large amounts of the liquid phase (>50%) do not favor mullite buildup because they heal lattice defects and thus hamper sintering. 相似文献
998.
Proton transfer along a single-file hydrogen-bonded water chain is elucidated with a special emphasis on the investigation of chain length, side water, and solvent effects, as well as the temperature and pressure dependences. The number of water molecules in the chain varies from one to nine. The proton can be transported to the acceptor fragment through the single-file hydrogen-bonded water wire which contains at most five water molecules. If the number of water molecule is more than five, the proton is trapped by the chain in the hydroxyl-centered H(7)O(3) (+) state. The farthest water molecule involved in the formation of H(7)O(3) (+) is the fifth one away from the donor fragment. These phenomena reappear in the molecular dynamics simulations. The energy of the system is reduced along with the proton conduction. The proton transfer mechanism can be altered by excess proton. The augmentation of the solvent dielectric constant weakens the stability of the system, but favors the proton transfer. NMR spin-spin coupling constants can be used as a criterion in judging whether the proton is transferred or not. The enhancement of temperature increases the thermal motion of the molecule, augments the internal energy of the system, and favors the proton transfer. The lengthening of the water wire increases the entropy of the system, concomitantly, the temperature dependence of the Gibbs free energy increases. The most favorable condition for the proton transfer along the H-bonded water wire is the four-water contained chain with side water attached near to the acceptor fragment in polar solvent under higher temperature. 相似文献
999.
Summary The photodegradation behaviour of 12 nitrogen-containing herbicides (atrazine, cyanazine, terbuthylazine, terbutryn, EPTC,
buthylate, molinate, cycloate, vernolate, fenuron, chloroxuron, and methabenzthiazuron) has been examined. The compounds were
degraded completely when exposed to a mercury-vapour lamp; the degradation process was followed by consecutive GC measurements.
All the compounds studied had measurable photochemical activity, although actual and average degradation rates varied significantly.
All the compounds except terbutryn furnished more than one major degradation product, in different ratios.
Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001 相似文献
1000.
It has been suggested [F. H. Stillinger, J. Chem. Phys. 112, 9711 (2000)] that the convergence or divergence of M?ller-Plesset perturbation theory is determined by a critical point at a negative value of the perturbation parameter z at which an electron cluster dissociates from the nuclei. This conjecture is examined using configuration-interaction computations as a function of z and using a quadratic approximant analysis of the high-order perturbation series. Results are presented for the He, Ne, and Ar atoms and the hydrogen fluoride molecule. The original theoretical analysis used the true Hamiltonian without the approximation of a finite basis set. In practice, the singularity structure depends strongly on the choice of basis set. Standard basis sets cannot model dissociation to an electron cluster, but if the basis includes diffuse functions then it can model another critical point corresponding to complete dissociation of all the valence electrons. This point is farther from the origin of the z plane than is the critical point for the electron cluster, but it is still close enough to cause divergence of the perturbation series. For the hydrogen fluoride molecule a critical point is present even without diffuse functions. The basis functions centered on the H atom are far enough from the F atom to model the escape of electrons away from the fluorine end of the molecule. For the Ar atom a critical point for a one-electron ionization, which was not previously predicted, seems to be present at a positive value of the perturbation parameter. Implications of the existence of critical points for quantum-chemical applications are discussed. 相似文献