全文获取类型
收费全文 | 223992篇 |
免费 | 2787篇 |
国内免费 | 1326篇 |
专业分类
化学 | 124472篇 |
晶体学 | 3233篇 |
力学 | 8962篇 |
综合类 | 61篇 |
数学 | 24889篇 |
物理学 | 66488篇 |
出版年
2020年 | 1791篇 |
2019年 | 1923篇 |
2018年 | 2523篇 |
2017年 | 2558篇 |
2016年 | 3707篇 |
2015年 | 2480篇 |
2014年 | 3616篇 |
2013年 | 9023篇 |
2012年 | 7847篇 |
2011年 | 9593篇 |
2010年 | 6732篇 |
2009年 | 6519篇 |
2008年 | 8962篇 |
2007年 | 9082篇 |
2006年 | 8569篇 |
2005年 | 7888篇 |
2004年 | 7007篇 |
2003年 | 6172篇 |
2002年 | 6095篇 |
2001年 | 6782篇 |
2000年 | 5120篇 |
1999年 | 3710篇 |
1998年 | 3062篇 |
1997年 | 3045篇 |
1996年 | 3034篇 |
1995年 | 2649篇 |
1994年 | 2742篇 |
1993年 | 2601篇 |
1992年 | 2856篇 |
1991年 | 2909篇 |
1990年 | 2719篇 |
1989年 | 2637篇 |
1988年 | 2577篇 |
1987年 | 2514篇 |
1986年 | 2549篇 |
1985年 | 3328篇 |
1984年 | 3402篇 |
1983年 | 2842篇 |
1982年 | 3101篇 |
1981年 | 2847篇 |
1980年 | 2644篇 |
1979年 | 2829篇 |
1978年 | 3041篇 |
1977年 | 3074篇 |
1976年 | 3089篇 |
1975年 | 2827篇 |
1974年 | 2944篇 |
1973年 | 2990篇 |
1972年 | 2321篇 |
1971年 | 1854篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
The rational and sustainable exploitation of nature resources is one of the priority objectives of our society. In this article a method is proposed to evaluate and exploit the energetic resources contained in different forest formations. This method is based on the use of a combustion bomb calorimeter to determine the calorific values of the different samples studied. These results were complemented with chemical analysis of the samples and with environmental and geomorphological studies of the zones were samples were taken. The results obtained during the last 3 years for the residual biomass originated from the forest formations existing in Galicia ensure the reliability and usefulness of the method that can be extended to any forest formation all over the world. 相似文献
92.
A method has been developed for the determination of melengestrol acetate in bovine tissues at lower levels than previously reported. Liquid-liquid extraction of tissue homogenates provided crude clean-up while final isolation, screening, and quantification was done on-line with an automated, normal-phase, coupled-column high-performance liquid chromatographic system. The chromatographic system included phenyl and silica analytical columns for the purposes of isolation and final separation, respectively. These columns provided a large difference in selectivity when operated under normal-phase conditions which allowed for the efficient isolation of melengestrol acetate from the complex tissue extracts. Mobile phases were composed of hexane and dichloromethane modified with methanol and water. Transfer and enrichment of the analyte from the primary phenyl column to the silica column was via a short (12 mm x 4 mm I.D.) silica column. Regeneration and equilibration of the phenyl column was performed after the injection of each tissue extract and was accomplished simultaneously while analytical separation occurred on the final silica column. Routing of the mobile phases and regeneration solvent was performed with automated switching valves. The total time required for each analysis was 12 min. Quantification is demonstrated using external standards with UV detection at 287 nm. The overall recovery of the method was 86% with a coefficient of variation of 9.84% at the 10 ppb [the American billion (10(9] is used in this article] level in bovine liver extracts. 相似文献
93.
L. Ya. Yukel'son I. I. Parilis G. L. Bussel' D. Kh. Khamidov 《Chemistry of Natural Compounds》1989,25(6):698-704
On the basis of a computer analysis of four classes of toxic polypeptides, generalized primary structures of each class have been obtained which can be used for planning the synthesis of biologically active compounds. 相似文献
94.
95.
L. Brian Lawrence 《Transactions of the American Mathematical Society》2005,357(7):2535-2556
Working in ZFC, we give an example as indicated in the title.
96.
97.
In this paper the performances of inhomogeneous dielectric slabs as solar light collectors for both illumination and solar energy applications are analysed. 相似文献
98.
We present here a methodology for searching a robust pore size distribution (PSD) for adsorbent materials. The method is based on a combination of individual adsorption isotherms, obtained from Grand Canonical Monte Carlo simulations, a regularization procedure to invert the adsorption integral equation (Tikhonov regularization solved by singular value decomposition), and the needed experimental adsorption isotherm. The selection of several parameters from the available choices to start the procedure are discussed here: the size of the kernel (number of individual pores and number of experimental adsorption points to be included), the fulfillment of the Discrete Picard condition, and the L-curve criteria, all leading to find a reliable and robust PSD. The procedure is applied to plugged hexagonal templated silicas (PHTS), synthesized, and characterized in our laboratory. 相似文献
99.
Molecular electroactive monolayers have been produced from vinylferrocene (VFC) via light-assisted surface anchoring to H-terminated n- and p-Si(1 0 0) wafers prepared via wet chemistry, in a controlled atmosphere. The resulting Si-C bound hybrids have been characterized by means of XPS and AFM. Their performance as semiconductor functionalized electrodes and their surface composition have been followed by combining electrochemical and XPS measurements on the same samples, before and after use in an electrochemical cell. White-light photoactivated anchoring at short (1 h) exposure times has resulted in a mild route, with a very limited impact on the initial quality of the silicon substrate. In fact, the functionalized Si surface results negligibly oxidized, and the C/Fe atomic ratio is close to the value expected for the pure molecular species. The VFC/Si hybrids can be described as (η5-C5H5)Fe2+(η5-C5H4)-CH2-CH2-Si species, on the basis of XPS results. Electrochemical methods have been applied in order to investigate the role played by a robust, covalent Si-C anchoring mode towards substrate-molecule electronic communication, a crucial issue for a perspective development of molecular electronics devices. The response found from cyclic voltammograms for p-Si(1 0 0) functionalized electrodes, run in the dark and under illumination, has shown that the electron transfer is not limited by the number of charge carriers, confirming the occurrence of electron transfer via the Si valence band. The hybrids have shown a noticeable electrochemical stability and reversibility under cyclic voltammetry (cv), and the trend in peak current intensity vs. the scan rate was linear. The molecule-Si bond is preserved even after thousands of voltammetric cycles, although the surface coverage, evaluated from cv and XPS, decreases in the same sequence. An increasingly larger surface concentration of Fe3+ at the expenses of Fe2+ redox centers has been found at increasing number of cv’s, experimentally associated with the growth of silicon oxide. Surface SiO− groups from deprotonated silanol termination, induced by the electrochemical treatments, are proposed as the associated counterions for the Fe3+ species. They could be responsible for the observed decrease in the electron transfer rate constant with electrode ageing. 相似文献
100.
Stress-Strain State in the Zone of Load Transfer in a Composite Specimen under Uniaxial Tension 总被引:2,自引:2,他引:0
Kulakov V. L. Tarnopol'skii Yu. M. Arnautov A. K. Rytter J. 《Mechanics of Composite Materials》2004,40(2):91-100
The stress-strain state in the zone of load transfer in a uniaxially stretched specimen made of a unidirectional epoxy carbon-fiber-reinforced plastic (CFRP) is investigated. A parametric analysis of the influence of geometric and mechanical characteristics of the specimen on its stress-strain state is performed by means of finite-element modeling. The parameters allowing us to significantly reduce the dangerous concentration of transverse and tangential stresses are revealed. The mechanical tensile characteristics of a high-strength pultruded unidirectional CFRP are determined experimentally, and the size effect of its strength is estimated. 相似文献