全文获取类型
收费全文 | 1080篇 |
免费 | 22篇 |
国内免费 | 187篇 |
专业分类
化学 | 776篇 |
晶体学 | 7篇 |
力学 | 31篇 |
数学 | 22篇 |
物理学 | 453篇 |
出版年
2023年 | 12篇 |
2022年 | 8篇 |
2021年 | 10篇 |
2020年 | 11篇 |
2019年 | 12篇 |
2018年 | 18篇 |
2017年 | 23篇 |
2016年 | 26篇 |
2015年 | 59篇 |
2014年 | 45篇 |
2013年 | 74篇 |
2012年 | 55篇 |
2011年 | 106篇 |
2010年 | 69篇 |
2009年 | 102篇 |
2008年 | 86篇 |
2007年 | 67篇 |
2006年 | 78篇 |
2005年 | 64篇 |
2004年 | 56篇 |
2003年 | 52篇 |
2002年 | 31篇 |
2001年 | 23篇 |
2000年 | 27篇 |
1999年 | 23篇 |
1998年 | 20篇 |
1997年 | 19篇 |
1996年 | 14篇 |
1995年 | 7篇 |
1994年 | 12篇 |
1993年 | 7篇 |
1992年 | 12篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 2篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 3篇 |
1971年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有1289条查询结果,搜索用时 0 毫秒
141.
A finite element study is conducted to determine the thermal response of a widely used glass reinforced plastic panel exposed to fire. This study is performed based on a formulation developed previously by the authors and improved by including the moisture and temperature-dependent thermal properties and a newly developed time-dependent non-linear mixed boundary condition at the unexposed surface of the panel. In addition, the influence of non-zero final resin mass is considered according to a recently performed thermal gravimetric analysis. In order to derive the appropriate element equations, a mixed explicit–implicit Bubnov–Galerkin finite element approach is adopted. Results of this study are presented for a standard, 10.9 mm, thickness of single-skinned polyester-based glass reinforced plastic panel and comprise temperature profiles, density distributions and moisture profiles. Comparisons are made between the predicted results and those obtained experimentally. The predicted temperatures agreed with the experimental results with an average difference of 21.41°C. A simple comparison of the present value with that of the authors’ previous model, 29.66°C, indicates a considerable improvement of 38.53% in the fire-performance prediction of the material. 相似文献
142.
Equilibrium calculations of Si-doping in GaN are investigated using the Gemini code. The method of the calculation is based on the minimisation of the Gibbs free energy. Experimental growth conditions are used for the calculation. The variables are the amount of the dopant and the temperature. The results show the formation of a solid Si3N4 compound with a certain quantity of the input SiH4, that is the silicon precursor in our MOVPE system. Si3N4 formation can explain the limitation of Si incorporation and the surface roughening as revealed by MOVPE Si doped layers. 相似文献
143.
Irina V. Glinkina Vladimir A. Durov Galina A. Mel''nitchenko 《Journal of Molecular Liquids》2004,110(1-3):63-67
Modelling of the biologic solutions has a great importance for basic physicochemical backgrounds of the living processes, mechanism of the diseases and drugs action, etc. The modelling of the chemical equilibria in solution that served as a prototype of the blood plasma with application to calcification of the tissues is performed. The concentrations of molecular–ionic forms containing calcium and hydrogen cations and phosphate anions in the range of ionized-calcium and total phosphorus concentrations from 0.5 to 3.0 mM and at the solution pH of 7.1–7.8 were calculated. The activities of the ionized species were described in approach to Debye–Hückel's theory. The full set of the equilibria taking into consideration dissociation of the water, phosphoric acid, formation of both inert and ionic calcium phosphates was considered. The states of calcium hydrophosphate dihydrate CaHPO4·2H2O (CHPD), calcium phosphate Ca3(PO4)2 (CP), calcium hydrophosphate-phosphate dihydrate Ca4H(PO4)3·2H2O (CHPPD) and hydroxyapatite Ca3OH(PO4)3 (HA) with respect to the boundary of the region in which they crystallise were determined. A criterion has been introduced to characterize the degree of salt supersaturation with respect to crystallisation, which is based on the concentration distance between the states of a salt in solution and at the boundary of its crystallisation. This criterion is used to provide a quantitative characteristic of the supersaturation of the phosphates and their tendency to crystallise in blood's plasma. It was established that the most soluble of the phosphates, CaHPO4·2H2O, is undersaturated and the other phosphates are supersaturated with respect to crystallisation. Thus, this phosphate does not take part in the calcification, and this is the source for ionized calcium in a blood plasma from the soft tissues. The role of the other phosphates in calcification of the soft tissues is decreased in the series HA>CP>CHPPD. The dependencies of the supersaturation of the solution on the pH and on the concentrations of calcium and phosphorus in a mixture are discussed. 相似文献
144.
E. ?ernošková T. Qu Z. ?ernošek P. Boolchand 《Journal of Physics and Chemistry of Solids》2005,66(1):185-189
Thermoanalytical characteristics and Raman scattering of high purity sulfur and ternary bulk glasses GexAsxS(100−2x) for x=4-22 at. % were studied. The intermediate phase characterized by vanishing of non-reversing heat flow ΔHnr, i.e. so-called the thermally reversing window was found between mean coordination number 〈r〉∼2.28-2.47. Separated phase of non-crystalline cycloocta-S, manifesting itself by λ-transition at ∼155 °C, was found for glasses with sulfur content higher than ∼80 at.%. Raman spectra of studied Ge-As-S glasses showed different shapes in three different areas according to three distinct phases of network glasses-floppy, intermediate, rigid. 相似文献
145.
In this paper, the azeotropic behaviour of the (benzene + cyclohexane + chlorobenzene) ternary mixture was experimentally investigated with the aim of enhancing the knowledge for the feasible use of chlorobenzene as an entrainer for the azeotropic distillation of the binary azeotrope. Such a study has not been reported in the literature to the best of the authors’ knowledge. (Vapour + liquid) equilibria data for (benzene + cyclohexane + chlorobenzene) at 101.3 kPa were obtained with a Othmer-type ebulliometer. Data were tested and considered thermodynamically consistent. The experimental results showed that this ternary mixture is completely miscible and exhibits an unique binary homogeneous azeotrope, an unstable node at the conditions studied, and the propitious topological characteristics (residual curve map and relative volatility) to be separated. Satisfactory results were obtained for the correlation of equilibrium compositions with the UNIQUAC activity coefficients model and also for prediction with the UNIFAC method. In both cases, low root mean square deviations of the vapour mole fraction and temperature were calculated. The capability of chlorobenzene as a modified distillation agent at atmospheric condition is discussed in terms of the thermodynamic topological analysis. A conceptual distillation scheme with reversed volatility is proposed to separate the azeotropic mixture. In order to reduce the operational cost requirements of the sequence of columns proposed, the range for optimal reflux and the ratio for feed flow conditions were studied. 相似文献
146.
Thermodynamic properties of complex oxides in the La-Ni-O system 总被引:1,自引:0,他引:1
Complex oxides La2NiO4+δ, La3Ni2O7−δ, La4Ni3O10−δ and LaNiO3−δ, the members of Ruddlesden-Popper series Lan+1NinO3n+1, were prepared using citrate precursors. The stability range of LaNiO3−δ in air as well as the oxygen nonstoichiometry of La3Ni2O7−δ and La4Ni3O10−δ as a function of temperature and oxygen partial pressure was determined by means of thermogravimetric technique. Decomposition temperatures of La3Ni2O7−δ, La4Ni3O10−δ and LaNiO3−δ in air were determined by conductivity measurement method. The boundary of stability for La4Ni3O10−δ was determined by EMF measurements of galvanic cell with oxygen conducting solid electrolyte. The isothermal (1400 K) projection of La-Ni-O system phase diagram to the plane “log(PO2)-relative mole fraction of metal components” was suggested. 相似文献
147.
A. L. Rebenko 《Journal of statistical physics》1998,91(3-4):815-826
For a slightly stronger assumptions on the interaction we give a very transparent proof of Ruelle's result in the language of Poisson integral measure representation for the correlation functions on the configuration space using some kind of cluster expansion in the densities of configurations. 相似文献
148.
烷基苯的热力学性质与分子拓扑指数的关系研究 总被引:6,自引:0,他引:6
倪才华 《原子与分子物理学报》1995,12(2):213-218
此文分析了烷基苯的结构特点,以分子的拓扑指数为结构参数,研究了苯、烷基系列的燃烧热,生成自由能等八种热力学性质与结构参数的关系,提出计算热力学性质的一个通用关系式Y=a·N+b·R+c·NR+d,用计算机处理,得到一系列经验公式,计算结果表明:烷基苯系列的热力学性质与拓扑指数相关性能很好。 相似文献
149.
Two acyl and three fluoroacyl derivatives of 32 chiral alcohols have been chromatographed on a GC column coated with octakis(2,6-di-O-n-pentyl-3-O-trifluoroacetyl)- γ-cyclodextrin. Significant differences were observed between the stereoselectivity obtained for the derivatives and that for the underivatized alcohols. Of the derivatives, only the fluoroacylated compounds were separated into enantiomers. Derivatization with fluoroacyl groups reversed the elution order for at least some of the analytes. Stereoselectivity towards simple 2- and 3-hydroxy alkanes and their fluoroacyl derivatives was highest for those alcohols with a four-carbon chain attached to the stereogenic center. For longer-chain fluoroacyl derivative groups stereoselectivity was higher for the 2- and 3-hydroxy alkanes. Differences in stereoselectivity towards alcohols with a methyl-branched alkane chain and their fluoroacyl derivatives was related to the distance between the methyl group and the hydroxyl or fluoroacyl groups. Different degrees of saturation in the carbon chain resulted in differences in stereoselectivity. Thermodynamic data calculated for a number of analytes suggest that the alcohols and trifluoroacetate derivatives are interacting with the stationary phase by similar mechanisms. The stereospecific interaction appears to have a hydrogen bonding or dipole–dipole contribution and some form of steric component, depending upon the shape and/or size of the solute. 相似文献
150.
Structure, oxygen stoichiometry and electrical conductivity in the system Sr-Ce-Co-O 总被引:1,自引:0,他引:1
Mixed oxides in the system S-Ce-Co-O were prepared by solid state reaction and by freeze-drying of precursor compounds followed by thermal treatment. Two types of perovskite oxides exist in the system: Solid solutions of the type Sr1 − yCeyCoO3 − x and mixed oxides of the type (1 − y)SrCeO3 − ySrCoO3 − x. Microstructures and phase compositions were determined by electron microscopy and X-ray diffraction. SrCoO3 − x forms a solid solution of ceria on the A-site in the strontium cobaltite lattice up to 0.15 mol Ce. This solid solution corresponds to the high-temperature structure of pure SrCoO3 − x and is characterized by high oxygen exchange and electrical conductivity. The oxygen deficiency x was measured by solid electrolyte coulometry. The oxygen deficiency of solid solutions Sr1 − yCeyCoO3 − x increases with temperature and decreases with pO2 in the ambient atmosphere and with increasing Ce dopant concentration. The pO2-T-x diagram of the solid solution was determined. The T, pO2 and dopant concentration dependencies of electrical conductivity were measured by a four-point d.c. technique. By Ce doping strontium cobaltite becomes a stabilized high-conductive material (maximum conductivity: 500 S cm−1 at 400 °C, Ea = 0.025 eV, p-type). Above this temperature the T-coefficient of the conductivity changes from positive (semiconducting) to negative values. 相似文献