全文获取类型
收费全文 | 677篇 |
免费 | 18篇 |
国内免费 | 1篇 |
专业分类
化学 | 471篇 |
晶体学 | 11篇 |
力学 | 48篇 |
数学 | 38篇 |
物理学 | 128篇 |
出版年
2024年 | 8篇 |
2023年 | 9篇 |
2022年 | 17篇 |
2021年 | 26篇 |
2020年 | 35篇 |
2019年 | 32篇 |
2018年 | 28篇 |
2017年 | 25篇 |
2016年 | 28篇 |
2015年 | 24篇 |
2014年 | 21篇 |
2013年 | 63篇 |
2012年 | 55篇 |
2011年 | 55篇 |
2010年 | 26篇 |
2009年 | 26篇 |
2008年 | 27篇 |
2007年 | 20篇 |
2006年 | 16篇 |
2005年 | 12篇 |
2004年 | 15篇 |
2003年 | 8篇 |
2002年 | 9篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 9篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1981年 | 4篇 |
1978年 | 4篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 4篇 |
1969年 | 3篇 |
1959年 | 2篇 |
1948年 | 2篇 |
1943年 | 2篇 |
排序方式: 共有696条查询结果,搜索用时 31 毫秒
1.
Nonlinear Dynamics - In the present work, we have studied a diffusive tritrophic food chain model in which the species at each trophic level interact in accordance with Crowley–Martin... 相似文献
2.
The instability of the plane interface between two uniform, superposed, electrically conducting and counter-streaming fluids through a porous medium is considered in the presence of a horizontal magnetic field. In the absence of surface tension, perturbations transverse to the direction of streaming are found to be unaffected by the presence of streaming if perturbations in the direction of streaming are ignored. For perturbations in all other directions there exists instability for a certain wavenumber range. The instability of this system is postponed by the presence of magnetic field. The magnetic field and surface tension are able to suppress this Kelvin-Helmholtz instability for small wavelength perturbations and the medium porosity reduces the stability range given in terms of a difference between the streaming velocities and the Alfvén velocity.This research forms a part of the research project awarded to the first author (R.C.S.) by the University Grants Commission. 相似文献
3.
Rashmi 《Solid State Communications》2005,134(3):217-222
A detailed analysis of nonlinear effects-electron switching and rectification, in InAlAs/InGaAs based Y-branch junction (YBJ) devices is presented to investigate the potential of YBJ for high frequency applications at 300 K. Results based on semi-classical simulations yield good qualitative agreement with measurements and previously reported theoretical and experimental results. The nonlinear parabolic behaviour of our device is attributed to device geometry and space charge effects. RF analysis shows that the YBJ has tremendous intrinsic potential to function as a frequency doubler and microwave rectifier when operated in the parabolic regime. The present analysis serves as a tool to optimize the bias conditions for RF measurements and to estimate the effect of interconnects and parasitic elements on the RF performance of real devices. 相似文献
4.
The carrier technique has been used to concentrate Cr(VI) radiometrically by studying the carrying efficiency as a function of pH of aqueous medium, added ions, carrying capacity, amount of carrier and subsequent desorption of the activity adsorbed under optimum experimental conditions. The study shows that under optimum experimental conditions, 0.15 g of ZrO2 and 0.25 g of TiO2 carry 0.01 mg and 0.001 mg of chromium respectively. Further, the activity carried under the specified conditions can be recovered more or less quantitatively by leaching the carrier. 相似文献
5.
Triorganotin(IV) and triorganolead(IV) derivatives of the types Me3Sn(SCZ) and Ph3Pb(SCZ) (where SCZ? is the anion of a semicarbanzone ligand) have been synthesized by substitution reactions of trimethyltin chloride and triphenyl-lead chloride with semicarbazones derived from heterocyclic ketones. The resulting complexes have been characterized by elemental analyses, molecualr weight determinations and conductivity measurements. The mode of bonding has been established on the basis of infrared and 1H, 13C and 119Sn NMR spectroscopic studies. Some respresentative complexes have also been evaluated for their antimicrobial effects on different species of pathogenic fungi and bacteria; the results of these investigations have been reported in the present paper. 相似文献
6.
Upadhyay R. K. Bansal Rashmi R. Vajpai Usha 《Monatshefte für Chemie / Chemical Monthly》1976,107(5):1221-1225
Thin layer chromatographic behaviour of ten anils was studied on silica gel adsorbent mixed with starch binder with ten solvent systems. Simultaneous separation, identification and determination of ternary mixtures of isomers was stressed. TheR
F
increasing order was found to be dependent on the solvent and independent of the degree of saturation of the chamber. The correlations ofR
F
with max and (C=O) of isomers were established and used in their identification. Six anils were separated in methanol—benzene (11). 相似文献
7.
Kalagouda?B.?GudasiEmail author Ramesh?S.?Vadavi Rashmi?V.?Shenoy Manjula?S.?Patil Siddappa?A.?Patil Munirathinam?Nethaji 《Transition Metal Chemistry》2005,30(6):661-668
The synthesis of cobalt(II), nickel(II) and zinc(II) complexes of 2-pyridine-2-yl-3(pyridine-2-carboxylideneamino)-1,2-dihydroquinazolin-4(3H)-one is described. The ligand and metal complexes were characterized by elemental analysis, conductivity measurements, spectral (u.v.–vis., i.r., 1D n.m.r., 2D hetcor and mass) and thermal studies. The cobalt(II) complex crystallizes as pink crystals in the monoclinic crystal system, space group P21/n with a = 10.066(6) Å, b = 15.929(9) Å, c = 12.624(7) Å, α = 90.00(9)°, β = 110.850 (8)°, γ = 90.00, V = 1891.5 (18) Å3 and Z = 4. The geometry around the cobalt atom is distorted trigonal bipyramidal with τ = 0.83 [structural parameter, τ = (β − α)/60; where α and β are the two basal angles in a five coordinate complex]. 相似文献
8.
Navjeet Kaur Yamini Verma Pooja Grewal Neha Ahlawat Pranshu Bhardwaj Nirmala Kumari Jangid 《合成通讯》2020,50(9):1286-1334
AbstractFew conversions cannot take place with ground-state reactions even with the help of a catalyst, therefore they are made to occur under photochemical conditions. The transfer of electrons took place even with the photochemical excitement of one molecule where redox reaction cannot occur at the ground state. The ground-state reactions resulted in the formation of side products. The substrates did not require any sort of chemical activation for C–N bond construction in the course of photochemical reactions. The source of energy; light has always been the interest of researchers in order to induce chemical reactions ever since the starting of scientific chemistry. The present review encloses the chemistry of photochemical transformations with a focus on their synthetic uses. The organic photochemical reactions prevent the polluting or harmful reagents thus, provides a possibility for sustainable procedures as well as green chemistry. This review article displays the formation of numerous of five-membered fused nitrogen-heterocyclic compounds. 相似文献
9.
Density functional calculations have been carried out on a series of linearly annelated acenes and their BN analogues. Even though borazine shows aromatic and reactivity behavior parallel with that of benzene, its condensed derivatives show patterns different from those of their hydrocarbon analogues. Nucleus independent chemical shift (NICS) values in acenes suggest that the aromaticity of the inner rings is more than that of benzene, whereas in BN-acenes there is no substantial change in the aromaticity of the individual rings. Molecular electrostatic potential (MESP) is employed to obtain further insights into the bonding and reactivity trends for these systems. The MESP topography patterns of acenes and BN-acenes are substantially different, with BN-acenes showing more localized pi electron features compared to those of acenes. The MESP values at the critical points (CPs) indicate overall lowering of aromaticity in these annelated systems. However, this change is gradual among the BN-acenes. 相似文献
10.
Summary The kinetics of the RuIII-catalyzed oxidation of the hydroxy acids; lactic, tartaric, malic and citric acids byN-bromosuccinimide in HClO4 and in the presence of Hg(OAc)2 have been studied. The reactions exhibit a first order rate dependence with respect to the oxidant and zeroth order rate dependence with respect to substrate. The rate is retarded by [H+], accelerated at law RuIII concentrations but independent of [RuIII] at higher RuIII concentrations. A mechanism consistent with the observed kinetic data is proposed. 相似文献