全文获取类型
收费全文 | 8853篇 |
免费 | 874篇 |
国内免费 | 1349篇 |
专业分类
化学 | 7293篇 |
晶体学 | 47篇 |
力学 | 946篇 |
综合类 | 86篇 |
数学 | 1126篇 |
物理学 | 1578篇 |
出版年
2024年 | 6篇 |
2023年 | 62篇 |
2022年 | 145篇 |
2021年 | 217篇 |
2020年 | 255篇 |
2019年 | 253篇 |
2018年 | 291篇 |
2017年 | 357篇 |
2016年 | 349篇 |
2015年 | 299篇 |
2014年 | 410篇 |
2013年 | 1026篇 |
2012年 | 480篇 |
2011年 | 502篇 |
2010年 | 389篇 |
2009年 | 461篇 |
2008年 | 485篇 |
2007年 | 503篇 |
2006年 | 497篇 |
2005年 | 479篇 |
2004年 | 429篇 |
2003年 | 414篇 |
2002年 | 336篇 |
2001年 | 285篇 |
2000年 | 267篇 |
1999年 | 245篇 |
1998年 | 253篇 |
1997年 | 215篇 |
1996年 | 208篇 |
1995年 | 165篇 |
1994年 | 141篇 |
1993年 | 105篇 |
1992年 | 106篇 |
1991年 | 86篇 |
1990年 | 63篇 |
1989年 | 35篇 |
1988年 | 46篇 |
1987年 | 36篇 |
1986年 | 31篇 |
1985年 | 30篇 |
1984年 | 17篇 |
1983年 | 7篇 |
1982年 | 21篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1972年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 748 毫秒
881.
Oxidation of Aromatic Amines to Quinones by Iodic Acid under Microwave Irradiation in the Presence of Montmorillonite K10 and Silica Gel 总被引:1,自引:0,他引:1
Mohammed M.?HashemiEmail author Maryam?Akhbari 《Monatshefte für Chemie / Chemical Monthly》2003,134(12):1561-1563
Summary. A variety of aromatic amines are oxidized to their corresponding quinones in excellent yields by supported iodic acid under microwave irradiation and solvent free conditions. 相似文献
882.
We describe the implementation of an adaptive umbrella sampling method, making use of the weighted histogram analysis method, for computing multidimensional potential of mean force for chemical reaction in solution. The approach is illustrated by investigating the effect of aqueous solution on the free energy surface for the proton transfer reaction of [H(3)N-H-NH(3)](+) using a combined quantum mechanical and molecular mechanical AM1/TIP3P potential. 相似文献
883.
Akio?OhtaEmail author Satoru?Nakashima Hiroki?Matsuyanagi Tsuyoshi?Asakawa Shigeyoshi?Miyagishi 《Colloid and polymer science》2003,282(2):162-169
The Krafft temperatures and enthalpies of solution of N-hexadecanoyl alaninate and valinate, and N-tetradecanoyl phenylalaninate were obtained from differential scanning calorimetry. The Krafft temperature of N-acyl amino acid surfactant increased with decreasing size of the counter ion, with some exceptions. The enthalpy of solution was endothermic and increased with decreasing size of the counter ion except for the cases of lithium salt. The results showed that the L-L interaction in the solid state of N-hexadecanoyl amino acid surfactant salt was superior to the D-L interaction for both the alanine and valine systems when the counter ion size increased. However, the D-L interaction was still advantageous for the phenylalanine system with Cs+ as a counter ion. Both Fourier transform infrared spectroscopy studies and theoretical calculations suggested that the difference in magnitudes of the interactions between peptide and counter ion was a dominant factor for the chiral effect. 相似文献
884.
I. I. Kapustina L. P. Ponomarenko O. P. Moiseenko V. A. Stonik 《Chemistry of Natural Compounds》2001,37(6):515-519
NMR spectroscopy, capillary GLC, and GLC-MS are used to study the composition of free and sulfated sterols from the far-east starfishLeptasterias alaskensis asiatica(Fischer) andL. Fisheri(Djakonov). The total free sterols of both species are shown to have similar qualitative and quantitative compositions and contain mainly7
-sterols. Sterol sulfate fractions contain cholesterol sulfate as the main component but differ in the ratios of5
:0
:7
-sterol derivatives. Possible reasons for these differences are discussed. A new steroid, 3-hydroxycholest-5-en-7-one sulfate, was isolated. 相似文献
885.
Experimental geometries of the HCO· and H2CO·+ -electron radicals were compared with those obtained from calculations with the total energy optimization carried out in the framework of widely used ab initio and semiempirical computational procedures. For each structural form of the radicals, the magnetic resonance parameters calculated in the MNDO approximation were correlated with experimental values determined by ESR spectroscopy. Comparative analysis of the results obtained indicates the possibility of systematic correction of the optimized geometric parameters of free radicals using the results of ESR measurements. A simple computational procedure for automatic geometry correction in the MNDO approximation is developed and evaluated. 相似文献
886.
Two flame spray methods, emulsion combustion method (ECM) and flame spray pyrolysis (FSP), were compared for synthesis of pure and mixed SiO2 and ZnO nanoparticles. The effect of silicon precursor was investigated using liquid hexamethyldisiloxane (HMDSO) or SiO2 sol, while for ZnO zinc acetate (ZA) was used. Gas phase reaction took place when using HMDSO as Si precursor, forming nanoparticles, whereas the SiO2 sol used as Si source was not evaporated in the flame, creating large aggregates of the sol particles (e.g. 1 m). The FSP of ZA produced ZnO homogeneous nanoparticles. Lower flame temperatures in ECM than in FSP resulted in mixed gas and liquid phase reaction, forming ZnO particles with inhomogeneous sizes. The FSP of HMDSO and ZA led to intimate gas-phase mixing of Zn and Si, suppressing each other's particle growth, forming nanoparticles of 19 nm in BET-equivalent average primary particle diameter. Nucleation of ZnO and SiO2 occurred independently by ECM of HMDSO and ZA as well as by FSP of the SiO2 sol and ZA, creating a ZnO and SiO2 mixture. The reaction of ZnO with SiO2 was likely to be enhanced by ECM of the SiO2 sol and ZA where both Zn and Si species were not evaporated completely, resulting in ZnO, -willemite and Zn1.7SiO4 mixed phase. 相似文献
887.
O. V. Shvartsman 《Functional Analysis and Its Applications》2003,37(2):147-154
We find all pairs (,a) consisting of a cocompact Fuchsian group of genus zero and an automorphy factor a of for which the graded algebra of a--automorphic forms is free. 相似文献
888.
A Global Solution to a
Two-dimensional Riemann Problem Involving Shocks as Free
Boundaries 总被引:4,自引:0,他引:4
YuxiZheng 《应用数学学报(英文版)》2003,19(4):559-572
We present a global solution to a Riemann problem for the pressure gradient system of equations.The Riemann problem has initially two shock waves and two contact discontinuities. The angle between the two shock waves is set initially to be close to 180 degrees. The solution has a shock wave that is usually regarded as a free boundary in the self-similar variable plane. Our main contribution in methodology is handling the tangential oblique derivative boundary values. 相似文献
889.
Let S=G/K be a strongly irreducible, simply connected, compact symmetric space and let
be its group of isometries. We classify the symmetric spaces among these that admit free, isometric circle actions. The existence of such actions is important in constructing examples of manifolds with positive sectional curvature. 相似文献
890.
We have obtained theoretically an expression for the nuclear free precession signal in an inhomogeneously broadened spin system with a complex structure of the NMR spectrum. It is shown that in this case in the nuclear precession signal there appear maxima the moments of whose formation depend on the detuning of the pulse carrier frequency from the central frequencies of the NMR lines, their widths, and the Rabi frequency. The theoretical results are in good agreement with the experimental data on the observation of proton free precession in toluene. We have constructed a physical model explaining the appearance of maxima in the free precession signal of the spin system. 相似文献