全文获取类型
收费全文 | 2601篇 |
免费 | 91篇 |
国内免费 | 6篇 |
专业分类
化学 | 1595篇 |
晶体学 | 6篇 |
力学 | 74篇 |
数学 | 565篇 |
物理学 | 458篇 |
出版年
2023年 | 29篇 |
2022年 | 45篇 |
2021年 | 66篇 |
2020年 | 57篇 |
2019年 | 69篇 |
2018年 | 43篇 |
2017年 | 35篇 |
2016年 | 108篇 |
2015年 | 91篇 |
2014年 | 92篇 |
2013年 | 152篇 |
2012年 | 205篇 |
2011年 | 188篇 |
2010年 | 98篇 |
2009年 | 115篇 |
2008年 | 145篇 |
2007年 | 161篇 |
2006年 | 144篇 |
2005年 | 119篇 |
2004年 | 127篇 |
2003年 | 100篇 |
2002年 | 101篇 |
2001年 | 33篇 |
2000年 | 20篇 |
1999年 | 24篇 |
1998年 | 26篇 |
1997年 | 22篇 |
1996年 | 26篇 |
1995年 | 18篇 |
1994年 | 28篇 |
1993年 | 20篇 |
1992年 | 10篇 |
1991年 | 21篇 |
1990年 | 13篇 |
1989年 | 17篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 14篇 |
1984年 | 20篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 5篇 |
1966年 | 3篇 |
排序方式: 共有2698条查询结果,搜索用时 62 毫秒
221.
The salts [S(NMe2)3][MF6] (M = Nb, 2a; M = Ta, 2b) and [S(NMe2)3][M2F11] (M = Nb, 2c; M = Ta, 2d) have been prepared by reacting MF5 (M = Nb, 1a; M = Ta, 1b) with [S(NMe2)3][SiMe3F2] (TASF reagent) in the appropriate molar ratio. The solid state structure of 2b has been ascertained by X-ray diffraction. The 1:1 molar ratio reactions of 1a with a variety of organic compounds (L) give the neutral adducts NbF5L [L = Me2CO, 3a; L = MeCHO, 3b; L = Ph2CO, 3c; L = tetrahydrofuran (thf), 3d; L = MeOH, 3e; L = EtOH, 3f; L = HOCH2CH2OMe, 3g; L = Ph3PO, 3h; L = NCMe, 3i] in good yields. The complexes MF5L [M = Nb, L = HCONMe2, 3j; M = Nb, L = (NMe2)2CO, 3k; M = Ta, L = (NMe2)2CO, 3l; M = Nb, L = OC(Me)CHCMe2, 3m] have been detected in solution in admixture with other unidentified products, upon 2:1 molar reaction of 1 with the appropriate reagent L. The ionic complexes [NbF4(tht)2][NbF6], 4a, and [NbF4(tht)2][Nb2F11], 4b, have been obtained by combination of tetrahydrothiophene (tht) and 1a, in 1:1 and 2:3 molar ratios, respectively. The treatment of 1 with a two-fold excess of L leads to the species [MF4L4][MF6] [M = Nb, L = HCONMe2, 5a; M = Ta, L = HCONMe2, 5b; M = Nb, L = thf, 5c; M = Ta, L = thf, 5d; M = Nb, L = OEt2, 5e]. The new complexes have been fully characterised by NMR spectroscopy. Moreover, the revised 19F NMR features of the known compounds MF5L [M = Ta, L = Me2CO, 3n; M = Ta, L = Ph2CO, 3o; M = Ta, L = MePhCO, 3p; M = Ta, L = thf, 3q; M = Nb, L = CH3CO2H, 3r; M = Nb, L = CH2ClCO2H, 3s; M = Ta, L = CH2ClCO2H, 3t], TaF4(acac), TaF4(Me-acac) and [TaF(Me-acac)3][TaF6] (Me-acac = methylacetylacetonato anion) are reported. 相似文献
222.
Gerbaud G Caldarelli S Ziarelli F Gastaldi S 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,210(1):75-81
The intensity of the carbon signal in a CPMAS experiment has been measured for two CH and three CH(2) moieties in four test molecules under different phase-modulated proton decoupling conditions and as a function of the spinning rate. The proton decoupling schemes investigated were the golden standard TPPM and three of the GTn family. Aim of this analysis was to better describe experimentally the impact and limitations of phase-modulated decoupling. Sizeable differences in the response to decoupling were observed in otherwise chemically identical molecular fragments, such as the CHCH(2) found in tyrosine, phenyl-succinic acid or 9-Anthrylmethyl-malonate, probably due to differences in spin-diffusion rates. In keeping with known facts, the efficiency of the decoupling was observed to decrease with the MAS rate, but with somewhat different trends for the tested systems. 相似文献
223.
Khazaei M Liang Y Bahramy MS Pichierri F Esfarjani K Kawazoe Y 《J Phys Condens Matter》2011,23(40):405403
We have performed a set of first-principles simulations to consider the possible phase transitions in molecular crystals of HCN under high pressure. Our calculations reveal several transition paths from the orthorhombic phase to tetragonal and then to triclinic phases. The transitions from the orthorhombic to the tetragonal phases are of the second order, whereas those from the tetragonal to the triclinic phases turn out to be of the first-order type and characterized by an abrupt decrease in volume. Our calculations show that, by adjustment of the temperature and pressure of the HCN molecular crystal, novel layered and polymeric crystals with insulating, semiconducting or metallic properties can be found. Based on our simulation results, two different crystal formation mechanisms are deduced. The stabilities of the predicted structures at ambient pressure are further assessed by performing phonon or MD simulations. In addition, the electron transport properties of the predicted polymers are obtained using the non-equilibrium Green's function technique combined with density functional theory. The results show that the polymers have metallic-like I-V characteristics. 相似文献
224.
Florian Herzog Fabio Finocchi Laetitia Soukiassian Olivier Pluchery 《Surface science》2011,605(1-2):166-173
The adsorption of 3-ethynyl-trifluoro-toluene (ETFT) on Si(100)-2 × 1 surface in ultra high vacuum is studied in the low coverage regime, through a joint experimental and theoretical approach. The STM images of both filled and empty states revealed few distinct adsorption configurations. On the basis of Density Functional Theory (DFT) calculations the STM images were simulated and three main adsorption configurations were identified, with a predominance of di-sigma bonded species that leave the benzene ring unreacted. A discussion of the reactivity of the reconstructed silicon surface towards benzene derivatives is proposed by comparing the adsorption of ETFT close related molecules, like styrene and phenylacetylene. 相似文献
225.
The acquisition of high-resolution images in three dimensions is of utmost importance for the morphological and functional investigation of biological tissues. Here, we present a laser scanning two-photon microscope with remote and motionless control of the focus position. The movement of the excitation spot along the propagation direction is achieved by shaping the laser wavefront with a spatial light modulator. Depending on the optical properties of the objective in use, this approach allows z movements in a range of tens to hundreds of micrometers with small changes of the point spread function. We applied this technique for the three-dimensional (3D) imaging of fluorescent cells in the mouse neocortex in vivo. The presented system bypasses the limitations of microscopes based on moving objectives, enabling high-resolution inertia-free 3D imaging. 相似文献
226.
Riccardo Ruffo Fabio La MantiaColin Wessells Robert A. HugginsYi Cui 《Solid State Ionics》2011,192(1):289-292
The development of lithium ion aqueous batteries is getting renewed interest due to their safety and low cost. We have demonstrated that the layer-structure LiCoO2 phase, the most commonly used electrode material in organic systems, can be successful delithiated and lithiated again in a water-based electrolyte at currents up to 2.70 A/g. The capacity is about 100 mAh/g at 0.135 A/g and can be tuned by cycling the electrode in different potential ranges. In fact, increasing the high cut-off voltage leads to higher specific capacity (up to 135 mAh/g) but the Coulomb efficiency is reduced (from 99.9% to 98.5%). The very good electrode kinetic is probably due to the high conductivity of the electrolyte solution (0.17 Scm− 1 at 25 °C) but this behavior is affected by the electrode load. 相似文献
227.
228.
In this paper we discuss convex envelopes for bivariate functions, satisfying suitable assumptions, over polytopes. We first propose a technique to compute the value and a supporting hyperplane of the convex envelope over a general two-dimensional polytope through the solution of a three-dimensional convex subproblem with continuously differentiable constraint functions. Then, for quadratic functions as well as for some polynomial and rational ones, again satisfying suitable assumptions, we show how the same computations can be carried out through the solution of a single semidefinite problem. 相似文献
229.
E. P. Serrano M. I. Troparevsky M. A. Fabio 《Journal of Contemporary Mathematical Analysis (Armenian Academy of Sciences)》2014,49(2):91-97
The paper considers an inverse problem associated with equations of the form Kf = g, where K is a convolution-type operator. The aim is to find a solution f for given function g. We construct approximate solutions by applying a wavelet basis that is well adapted to this problem. For this basis we calculate the elementary solutions that are the approximate preimages of the wavelets. The solution for the inverse problem is then constructed as an appropriate finite linear combination of the elementary solutions. Under certain assumptions we estimate the approximation error and discuss the advantages of the proposed scheme. 相似文献
230.