全文获取类型
收费全文 | 14631篇 |
免费 | 1488篇 |
国内免费 | 1289篇 |
专业分类
化学 | 8357篇 |
晶体学 | 281篇 |
力学 | 338篇 |
综合类 | 74篇 |
数学 | 157篇 |
物理学 | 3424篇 |
综合类 | 4777篇 |
出版年
2024年 | 37篇 |
2023年 | 160篇 |
2022年 | 523篇 |
2021年 | 478篇 |
2020年 | 535篇 |
2019年 | 447篇 |
2018年 | 349篇 |
2017年 | 385篇 |
2016年 | 577篇 |
2015年 | 576篇 |
2014年 | 624篇 |
2013年 | 972篇 |
2012年 | 712篇 |
2011年 | 814篇 |
2010年 | 626篇 |
2009年 | 683篇 |
2008年 | 715篇 |
2007年 | 932篇 |
2006年 | 902篇 |
2005年 | 757篇 |
2004年 | 734篇 |
2003年 | 606篇 |
2002年 | 539篇 |
2001年 | 464篇 |
2000年 | 464篇 |
1999年 | 374篇 |
1998年 | 304篇 |
1997年 | 308篇 |
1996年 | 255篇 |
1995年 | 238篇 |
1994年 | 217篇 |
1993年 | 167篇 |
1992年 | 193篇 |
1991年 | 125篇 |
1990年 | 138篇 |
1989年 | 86篇 |
1988年 | 76篇 |
1987年 | 87篇 |
1986年 | 45篇 |
1985年 | 37篇 |
1984年 | 30篇 |
1983年 | 10篇 |
1982年 | 11篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 11篇 |
1977年 | 9篇 |
1976年 | 11篇 |
1972年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 18 毫秒
31.
32.
L. A. Utracki 《Journal of Polymer Science.Polymer Physics》2004,42(15):2909-2915
The Simha and Somcynsky (S–S) statistical thermodynamics theory was used to compute the solubility parameters as a function of temperature and pressure [δ = δ(T, P)], for a series of polymer melts. The characteristic scaling parameters required for this task, P*, T*, and V*, were extracted from the pressure–temperature–volume (PVT) data. To determine the potential polymer–polymer miscibility, the dependence of δ versus T (at ambient pressure) was computed for 17 polymers. Close proximity of the δ versus T curves for four miscible polymer pairs: PPE/PS, PS/PVME, and PC/PMMA signaled the usefulness of this approach. It is noteworthy, that the tabulated solubility parameters (derived from the solution data under ambient conditions) propounded the immiscibility of the PVC/PVAc pair. The computed values of δ also suggested miscibility for polymer pairs of unknown miscibility, namely PPE/PVC, PPE/PVAc, and PET/PSF. In recognizing the limitations of the solubility parameter approach (the omission of several thermodynamic contributions), these preliminary results are auspicious because they indicate a new route for estimating the miscibility of any polymeric material at a given temperature and pressure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2909–2915, 2004 相似文献
33.
The electronic structure and chemical bonding in a recently synthesized inorganic fullerene-like molecule, [CuCl]20[Cp*FeP5]12[Cu-(CH3CN) +
2Cl−]5 has been studied by a density functional approach. Geometrical optimization of the three basic structural units of the molecule
is performed with Amsterdam Density Functional Program. The results are in agreement with the experiment. Localized MO’s obtained
by Boys-Foster method give a clear picture of the chemical bonding in this molecule. The reason why CuCl can react with Cp*FeP5 in solvent CH3CN to form the fullerene-like molecule is explained in terms of the soft-hard Lewis acid base theory and a new concept of
covalence. 相似文献
34.
WANG Bingwu XU Guangxian & CHEN Zhida State Key Laboratory of Rare Earth Materials Chemistry Applications College of Chemistry Molecular Engineering Peking University Beijing China 《中国科学B辑(英文版)》2004,47(2)
The titled inorganic fullerene-like molecule (hereafter abbreviated as IFM) was recently synthe-sized by Bai et al.[1], which attracts a lot of interests from inorganic and organometallic chemists, and questions are raised for this smart molecule: (ⅰ) Why CuCl can react with Cp*FeP5 in solvent CH3CN to form IFM? (ⅱ) What is the nature of chemical bond-ing? (ⅲ) What is the covalence of Cu in this mole-cule? In this paper we intend to answer these questions in terms of the soft-hard … 相似文献
35.
董诠键 《陕西师范大学学报(自然科学版)》1996,(1)
利用把系统外参量和内参量分开的方法,给出了热力学基本方程的表示式;分析了各个热力学基本不等式包含的不可逆过程;讨论了热力学基本不等式的适用条件.为热力学理论的广泛应用提供了理论依据 相似文献
36.
A kinetic investigation on the monoesterification reaction of the maleic anhydride residue (MA) in styrene-maleic anhydride copolymers with aliphatic alcohols was carried out in ethyl benzene solution. By comparison to classic catalysts such as tributylamine (TBA) and pyridine, 4-dimethylaminopyridine (4DMAP) is by far the most effective catalyst for this reaction. While both general base and nucleophilic mechanisms contribute to the reaction catalyzed by TBA or pyridine, a nucleophilic mechanism prevails with 4DMAP. This reaction is reversible, and its chemical equilibrium constant decreases significantly with increasing temperature. Both kinetic and thermodynamic results showed that in the presence of 4DMAP, the forward and reverse reactions are second and first order, respectively. The existence of side reactions, reactivity of two styrene-maleic anhydride copolymers of different MA contents as well as two aliphatic alcohols of different lengths are also addressed. © 1993 John Wiley & Sons, Inc. 相似文献
37.
L.-H. Tang P. Smilauer D.D. Vvedensky 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(3):409-412
Two types of mechanisms are proposed for mound coarsening during unstable epitaxial growth: stochastic, due to deposition
noise, and deterministic, due to mass currents driven by surface energy differences. Both yield the relation H=(RWL)2 between the typical mound height W, mound size L, and the film thickness H. An analysis of simulations and experimental data shows that the parameter R saturates to a value which discriminates sharply between stochastic () and deterministic () coarsening. We derive a scaling relation between the coarsening exponent 1/z and the mound-height exponent which, for a saturated mound slope, yields .
Received: 11 November 1997 / Revised in final form: 28 November 1997 /
Accepted: 28 November 1997 相似文献
38.
以MnCl_2和NaOH为原料,采用胶溶法,制备了超微粒MnO_2制得的MnO_2超微粒为非晶态,直径6.0~10.0 nm,呈球状,在有机溶剂中具有良好的分散性和透明性,考察了影响氧化锰溶胶调制的因素,获得了最佳制胶条件,研究了表面活性剂用量与Mn迁移率的关系;通过X-光粉末衍射、红外光谱、热重及差热分析,研究了产品的结构性能。 相似文献
39.
本文根据一些立窑水泥厂的热工测定数据,推导出了化学不完全燃烧热损失Q_(hb),废气带走热损失Q_f与窑面空气过剩系数之间的关系,阐述了产生化学不完全燃烧热损失的原因,并就如何降低热损失,节约能源提出了设想。 相似文献
40.
Jing Zhang Dan Shan Shaolin Mu 《Journal of polymer science. Part A, Polymer chemistry》2007,45(23):5573-5582
A copolymer, poly(aniline‐co‐o‐aminophenol), was prepared chemically by using ammonium peroxydisulfate as an oxidant. The monomer concentration ratio of o‐aminophenol to aniline strongly influences the copolymerization rate and properties of the copolymer. The optimum composition of a mixture for the chemical copolymerization consisted of 0.3 M aniline, 0.021 M o‐aminophenol, 0.42 M ammonium peroxydisulfate, and 2 M H2SO4. The result of cyclic voltammograms in a potential region of ?0.20 to 0.80 V (vs.SCE) indicates that the electrochemical activity of the copolymer prepared under the optimum condition is similar to that of polyaniline in more acid solutions. However, the copolymer still holds the good electrochemical activity until pH 11.0. Therefore, the pH dependence of the electrochemical property of the copolymer is improved, compared with poly(aniline‐co‐o‐aminophenol) prepared electrochemically, and is much better than that of polyaniline. The spectra of IR and 1H NMR confirm that o‐aminophenol units are included in the copolymer chain, which play a key role in extending the usable pH region of the copolymer. The visible spectra of the copolymers show that a high concentration ratio of o‐aminophenol to aniline in a mixture inhibits the chain growth. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5573–5582, 2007 相似文献