全文获取类型
收费全文 | 372222篇 |
免费 | 4465篇 |
国内免费 | 959篇 |
专业分类
化学 | 191991篇 |
晶体学 | 5574篇 |
力学 | 16907篇 |
综合类 | 7篇 |
数学 | 45490篇 |
物理学 | 117677篇 |
出版年
2021年 | 3579篇 |
2020年 | 3919篇 |
2019年 | 4344篇 |
2018年 | 6084篇 |
2017年 | 6141篇 |
2016年 | 8684篇 |
2015年 | 5364篇 |
2014年 | 8228篇 |
2013年 | 18440篇 |
2012年 | 15273篇 |
2011年 | 17841篇 |
2010年 | 12786篇 |
2009年 | 12373篇 |
2008年 | 15755篇 |
2007年 | 15741篇 |
2006年 | 14413篇 |
2005年 | 12767篇 |
2004年 | 11720篇 |
2003年 | 10220篇 |
2002年 | 10008篇 |
2001年 | 11095篇 |
2000年 | 8271篇 |
1999年 | 6409篇 |
1998年 | 5309篇 |
1997年 | 5109篇 |
1996年 | 4871篇 |
1995年 | 4120篇 |
1994年 | 4125篇 |
1993年 | 4008篇 |
1992年 | 4426篇 |
1991年 | 4574篇 |
1990年 | 4352篇 |
1989年 | 4267篇 |
1988年 | 3997篇 |
1987年 | 4122篇 |
1986年 | 3857篇 |
1985年 | 4940篇 |
1984年 | 4989篇 |
1983年 | 4088篇 |
1982年 | 4267篇 |
1981年 | 3973篇 |
1980年 | 3885篇 |
1979年 | 4153篇 |
1978年 | 4133篇 |
1977年 | 4086篇 |
1976年 | 4115篇 |
1975年 | 3872篇 |
1974年 | 3837篇 |
1973年 | 3862篇 |
1972年 | 2690篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
22.
23.
In developing travel demand models it is generally assumed that the base-year data used in developing the parameters, as well as the forecasted data to be used as independent variables for the design year, are of acceptable quality. The purpose of this paper is to present the application of error propagation theory in assesing the predictive quality of one type of travel demand forecasting model (multinomial logit models) and to demonstrate how error considerations can be used as a tool for identifying the optimal model. The general conclusions of this study are that: (1) it is indeed possible to quantify errors in dependent variables in logit models as a consequence of errors in independent variables; and (2) error consideration can be used as a tool for identifying the optimal model from a set of candidate models. Further research is recommended to develop better insights into the phenomenon of error propagation so that the consideration of errors can be a factor in decisions on model selection. 相似文献
24.
H.S RagabA Shehap M.S Abo EllilW.H Osman F.H Abd El-Kader 《Journal of Physics and Chemistry of Solids》2002,63(10):1839-1847
Ternary-phase ceramic system of Li2O Al2O3 4SiO2 doped with CuO, FeO and TiO2 has been prepared and subjected to dc electrical conductivity and thermally stimulated depolarization current (TSDC) measurements as a function of temperature (30-250 °C) and field strength. The electrical conductivity results are explained by assuming both ionic and electronic conduction mechanisms coexist with different contributions over the whole temperature range of experiments. TSDC spectra have been found to be characterized by a broad intense relaxation peak, which can be attributed to an ionic charge polarization. The broad relaxation transitions are apparently a result of the nonuniform nature of this process. Activation energies are calculated for both dc electrical conductivity and TSDC according to Arrhenius equation and initial rise method, respectively. 相似文献
25.
Using Wagner's polarization technique and EMF method the joinic and silver ionic transport number measurements of various
compositions of the mixed system CdI2-Ag2O-CrO3 have been made. The presence of AgI in these materials has been inferred from the typical β → α phase transition of AgI,
which is characterized by an endothermic peak at around 420 K in the DSC traces of these specimens. While the structural analysis
performed by means of powder X-ray diffraction has revealed the formation of ionic phases involving polycrystalline compounds,
the Fourier transform infrared (FTIR) spectroscopic results have indicated the presence of ionic species thus confirming the
ionic nature of the products. The complex impedance studies carried out in the frequency range 20 Hz - 1 MHz and over the
temperature range 294 – 442 K have revealed that the best conducting composition, namely 55%(CdI2) – 45%(Ag2O·CrO3), would exhibit a silver ionic conductivity of 1.3×10−5Scm−1 at 294 K. 相似文献
26.
A. I. Magunov A. Ya. Faenov I. Yu. Skobelev T. A. Pikuz E. Biémont P. Quinet F. Blasco C. Bonte F. Dorchies T. Caillaud F. Salin C. Stenz 《Journal of Experimental and Theoretical Physics》2002,95(6):998-1005
The satellite structure of 1s2p 1,3 P 1-1s 21 S 0 lines of the He-like argon ion in plasma produced by a 45-fs laser pulse in a gas-jet cluster target is measured with a high spectral resolution. Radiation transitions 2p → 1s from autoionizing states (AISs) are detected for ions ranging from Li-like to F-like. The spectrum observed is theoretically simulated with the use of the spectroscopic data for the AISs of multicharged ions obtained within the multiconfiguration relativistic Hartree-Fock method. Good agreement with experimental data is obtained when the main population channels of these states are taken into account for typical values of cluster-target plasma parameters. 相似文献
27.
The present paper analyzes the properties of structural phase transitions under the extremal conditions of high pressures. In the context of the theory of density functional (TDF), the pressures of the B1–B2 transitions are calculated for small alkali-halide crystals as functions of the crystal size. The size effect of the B1–B2 transition – an increase in the transition pressure with the decreasing crystal grain radius – has been established for all investigated halides except lithium fluoride for which the dependence is reverse. 相似文献
28.
Sanzharov N. A. Yarovoi A. T. Bekhtereva E. S. Ulenikov O. N. 《Russian Physics Journal》2002,45(10):1024-1031
Analytical formulas are derived that describe the dependences of the transition probabilities in vibrational-rotational spectra of XY3-type molecules with C
3v
symmetry on the rotational quantum numbers. 相似文献
29.
30.