全文获取类型
收费全文 | 5114篇 |
免费 | 599篇 |
国内免费 | 387篇 |
专业分类
化学 | 2971篇 |
晶体学 | 60篇 |
力学 | 190篇 |
综合类 | 30篇 |
数学 | 429篇 |
物理学 | 1075篇 |
无线电 | 1345篇 |
出版年
2024年 | 9篇 |
2023年 | 79篇 |
2022年 | 104篇 |
2021年 | 167篇 |
2020年 | 152篇 |
2019年 | 191篇 |
2018年 | 135篇 |
2017年 | 136篇 |
2016年 | 235篇 |
2015年 | 247篇 |
2014年 | 291篇 |
2013年 | 383篇 |
2012年 | 412篇 |
2011年 | 386篇 |
2010年 | 298篇 |
2009年 | 302篇 |
2008年 | 335篇 |
2007年 | 292篇 |
2006年 | 268篇 |
2005年 | 257篇 |
2004年 | 168篇 |
2003年 | 180篇 |
2002年 | 160篇 |
2001年 | 111篇 |
2000年 | 84篇 |
1999年 | 93篇 |
1998年 | 71篇 |
1997年 | 60篇 |
1996年 | 46篇 |
1995年 | 47篇 |
1994年 | 41篇 |
1993年 | 22篇 |
1992年 | 22篇 |
1991年 | 23篇 |
1990年 | 18篇 |
1989年 | 16篇 |
1988年 | 7篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1980年 | 25篇 |
1978年 | 25篇 |
1977年 | 48篇 |
1976年 | 18篇 |
1975年 | 18篇 |
1974年 | 11篇 |
1973年 | 6篇 |
1943年 | 10篇 |
排序方式: 共有6100条查询结果,搜索用时 17 毫秒
101.
Intermolecular nuclear relaxation studies of real liquids and the results of theoretical calculations for model potential functions have provided significant information on the role of molecular interactions in the structure of liquids. The intermolecular proton-proton paircorrelation function (pcf), obtained from the reference interaction site model (RISM) is used as the equilibrium distribution and is used to obtarn an effective force for the calculation of intermolecular proton relaxation rates in liquid benzene, 1,3,5-trideuterobenzene and ethane. For liquid ethane, better agreement with experiment is observed with the pcf obtained from the Monte Carlo simulation than with the RISM result. 相似文献
102.
The compound cesium niobate, Cs2Nb4O11, is an antiferroelectric, as demonstrated by double hysteresis loops in the electric field versus polarization plot. The crystal structure refinement by X-ray diffraction at both 100 and 297 K shows it to have a centrosymmetric structure in point group mmm and orthorhombic space group Pnna, which is consistent with its antiferroelectric behavior. The 100-K structure data is reported herein. The lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. Antiferroelectricity is produced by antiparallel displacements of niobium atoms along the c-axis at the phase transition temperature of 165 °C. The critical field for onset of ferroelectric behavior in a single-crystal sample is 9.5 kV/cm at room temperature. 相似文献
103.
Luís J. Moreno López Manuel Callejón Mochón Juán C. Jiménez Sánchez Alfonso Guiraúm Pérez 《Mikrochimica acta》1996,124(3-4):187-194
A method for the determination of salicylaldehyde (2 × 10–6–10–4
M) by differential-pulse polarography, based on the in situ formation of its Girard-P derivative in aqueous solution at pH 2.5, is proposed. The relative standard deviation was 1.5% (ten determinations of 4 × 10–5
M level). The applicability of this method was checked in synthetic samples containing salicyl alcohol,o-aminophenol, 2-methylphenol, salicylic acid and 4-aminobenzoic acid. 相似文献
104.
表面修饰二氧化锡纳米微晶的制备与表征 总被引:10,自引:0,他引:10
制备了硅烷偶联剂KH-570表面修饰的SnO2纳米微晶,通过FT-IR、XPS、TEM和TG-DTA对其结构和表面特性进行表征和研究. FT-IR和XPS分析结果确证了KH-570与SnO2表面是以化学键合或物理吸附方式相结合,粒子表面存在酯基等有机官能团的红外吸收特征;观测到KH-570中Si原子的Si2s和Si2p谱线. TEM分析表明,表面修饰反应增强了SnO2纳米微晶的疏水性和分散性.由XPS和TG的实测数据探讨了纳米粒子具有较低包覆量的可能原因. 相似文献
105.
Formalism for calculations of the singlet and triplet Auger energies within the Xα model is presented. Using this method the KVV Auger spectrum of NH4Cl is calculated and the theoretical intensities are determined within a one-center approximation. The calculated spectrum reproduces quite well all the main features observed for solid NH4Cl. A new interpretation of the spectrum is suggested. 相似文献
106.
K. Južnič 《Journal of Radioanalytical and Nuclear Chemistry》1988,126(4):315-322
Some properties of natural zeolite from Zaloka gorica, Slovenia, Yugoslavia were investigated to establish its applicability in solidification and for storage purposes of radioactive waste. It was found that this material is a rather good sorber for cesium, with a capacity of about 0.6 meq g–1. The migration rate of Cs+ in a system ion exchanger-aqueous solution was investigated and correlated with the sorption behaviour of cesium in a system. The leaching rate of cesium from concrete containing zeolite was measured. 相似文献
107.
K. Južnič 《Journal of Radioanalytical and Nuclear Chemistry》1977,40(1-2):217-220
The equilibria between the solution of TOA in toluene and mixtures of sulphuric and hydrochloric acid been studied. They were
characterized by ratios of stability constants of amine salts formed in the organic phase. 相似文献
108.
Thermal degradation behavior of poly(4-hydroxybutyric acid) (P(4HB)) was investigated by thermogravimetric and pyrolysis-gas chromatography mass spectrometric analyses under both isothermal and non-isothermal conditions. Based on the thermogravimetric analysis, it was found that two distinct processes occurred at temperatures below and above 350 °C during the non-isothermal degradation of P(4HB) samples depending on both the molecular weight and the heating rate. From 1H NMR analysis of the residual P(4HB) molecules after isothermal degradations at different temperatures, it was confirmed that the ω-hydroxyl chain-end was remained unchanged in the residual P(4HB) molecules at temperatures below 300 °C, while the ω-chain-end of P(4HB) molecules was converted to 3-butenoyl units at temperatures above 300 °C. In contrast, the majority of the volatile products evolved during thermal degradation of P(4HB) was γ-butyrolactone regardless of the degradation temperature. From these results, it is concluded that during the thermal degradation of P(4HB), the selective formation of γ-butyrolactone via unzipping reaction from the ω-hydroxyl chain-end predominantly occurs at temperatures below 300 °C. At temperatures above 300 °C, both the cis-elimination reaction of 4HB unit and the formation of cyclic macromolecules of P(4HB) via intramolecular transesterification take place in addition to unzipping reaction from the ω-hydroxyl chain-end. Finally, the primary reaction of thermal degradation of P(4HB) at temperatures above 350 °C progresses by the cyclic rupture via intramolecular transesterification of P(4HB) molecules with a release of γ-butyrolactone as volatile product. Moreover, we carried out the thermal degradation tests for copolymer of 93 mol% of 4HB with 7 mol% of 3-hydroxybutyric acid (3HB) to examine the effect of 3HB units on thermal stability of P(4HB). 相似文献
109.
Chi Chen Chang‐Wei Su Chun‐Wei Yeh Hui‐Ling Hu Jhy‐Der Chen Ju‐Chun Wang 《中国化学会会志》2006,53(2):299-303
The reaction of N,N′‐di(2‐pyridyl)formamidine (HDpyF) with MnCl2‐4H2O afforded the complex MnCl2(HDpyF), which was characterized by X‐ray crystallography. The HDpyF ligand chelates to the Mn(II) center through the first and the third nitrogen atoms to form a six‐membered ring, leaving the second and the fourth nitrogen atoms uncoordinated. The HDpyF ligand is crystallographically disordered such that two different molecules can be solved. The neutral HDpyF ligand adopts the new s‐cis‐syn‐s‐trans conformation. 相似文献
110.
The closo‐dodecaborate [B12H12]2? is degraded at room temperature by oxygen in an acidic aqueous solution in the course of several weeks to give B(OH)3. The degradation is induced by Ag2+ ions, generated from Ag+ by the action of H2S2O8. Oxa‐nido‐dodecaborate(1?) is an intermediate anion, that can be separated from the reaction mixture as [NBzlEt3][OB11H12] after five days in a yield of 18 %. The action of FeCl3 on the closo‐undecaborate [B11H11]2? in an aqueous solution gives either [B22H22]2? (by fusion) or nido‐B11H13(OH)? (by protonation and hydration), depending on the concentration of FeCl3. In acetonitrile, however, [B11H11]2? is transformed into [OB11H12]? by Fe3+ and oxygen. The radical anions [B12H12] ˙ ? and [B11H11] ˙ ? are assumed to be the primary products of the oxidation with the one‐electron oxidants Ag2+ and Fe3+, respectively. These radical anions are subsequently transformed into [OB11H12]? by oxygen. The crystal structure analysis shows that the structure of [OB11H12]? is derived from the hypothetical closo‐oxaborane OB12H12 by removal of the B3 vertex, leaving a non‐planar pentagonal aperture with a three‐coordinate O vertex, as predicted by NMR spectra and theory. 相似文献