全文获取类型
收费全文 | 3264篇 |
免费 | 505篇 |
国内免费 | 176篇 |
专业分类
化学 | 486篇 |
晶体学 | 15篇 |
力学 | 214篇 |
综合类 | 58篇 |
数学 | 1552篇 |
物理学 | 1620篇 |
出版年
2024年 | 2篇 |
2023年 | 29篇 |
2022年 | 86篇 |
2021年 | 102篇 |
2020年 | 117篇 |
2019年 | 93篇 |
2018年 | 84篇 |
2017年 | 98篇 |
2016年 | 154篇 |
2015年 | 96篇 |
2014年 | 211篇 |
2013年 | 218篇 |
2012年 | 185篇 |
2011年 | 216篇 |
2010年 | 171篇 |
2009年 | 213篇 |
2008年 | 206篇 |
2007年 | 223篇 |
2006年 | 189篇 |
2005年 | 157篇 |
2004年 | 123篇 |
2003年 | 133篇 |
2002年 | 118篇 |
2001年 | 103篇 |
2000年 | 73篇 |
1999年 | 73篇 |
1998年 | 107篇 |
1997年 | 47篇 |
1996年 | 50篇 |
1995年 | 46篇 |
1994年 | 36篇 |
1993年 | 28篇 |
1992年 | 29篇 |
1991年 | 19篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 17篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1973年 | 1篇 |
排序方式: 共有3945条查询结果,搜索用时 62 毫秒
81.
M.J. CapdevilleH. Budzinski 《Trends in analytical chemistry : TRAC》2011,30(4):586-606
We expect drinking water and groundwater samples to be contaminated very little, so they are subject to trace-level analysis. Due to the very low levels of contamination, this sort of analysis requires not only powerful analytical technologies to reach limits around the ng/L level, but also quality-control parameters (e.g., blank and spike samples) to monitor potential contamination or losses during sample treatment. Based on a literature review and laboratory experience, we discuss the problems linked to the difficulties of calculating limits of detection, distinguishing instrumental from methodological limits and preventing false-positive results in cases of sample contamination, or false-negative results in cases of compound losses. When possible, we suggest solutions to compensate for, or to prevent, these problems. 相似文献
82.
Corey T. Love Wojtek Dmowski Karen E. Swider-Lyons 《Journal of solid state chemistry》2011,184(9):2412-2419
We use electrochemistry, high-energy X-ray diffraction (XRD) with pair-distribution function analysis (PDF), and density functional theory (DFT) to study the instabilities of Li2CuO2 at varying state of charge. Rietveld refinement of XRD patterns revealed phase evolution from pure Li2CuO2 body-centered orthorhombic (Immm) space group to multiphase compositions after cycling. The PDF showed CuO4 square chains with varying packing during electrochemical cycling. Peaks in the G(r) at the Cu-O distance for delithiated, LiCuO2, showed CuO4 square chains with reduced ionic radius for Cu in the 3+ state. At full depth of discharge to 1.5 V, CuO was observed in fractions greater than the initial impurity level which strongly affects the reversibility of the lithiation reactions contributing to capacity loss. DFT calculations showed electron removal from Cu and O during delithiation of Li2CuO2. 相似文献
83.
A silane‐modified mono‐lacunary Keggin‐type polyoxometalate (POM), (Bu4N)4[SiW11O39{(CH2?CH? Si)2O}] (SiW11? CH?CH2), was obtained by reaction of vinyltrimethoxysilane with K8(SiW11O39) in acidic MeCN/H2O mixed solutions. Then, the modified polyoxometalate was physically blended with the pyromellitic dianhydride (PMDA)‐4,4′‐oxydianiline (ODA) poly(amic acid) and the blends were thermally imidized to form polyimide/ polyoxometalate composites. The X‐ray diffraction (XRD) analysis indicates that the polyoxometalate clusters cannot form crystalline structures in the composite, suggesting that the blending leads to improved compatibility between the polymer matrix and the modified polyoxometalate. The EDS (W‐mapping) studies on the composite films reveal that the polyoxometalate clusters are well dispersed in the polyimide matrix. The physical incorporation of modified POM into polyimide remarkably reduced the dielectric constant of the latter from 3.29 to 2.05 when 20 wt% of SiW11? CH?CH2 was used. Besides, the addition of the modified POM into polyimide increased the storage modulus of polyimide without severely affecting its thermal properties. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
84.
85.
The integration technology of hydrogen preparation–hydrogen storage not only can utilize hydrogen energy efficiently but also can improve the selectivity of the electrode maximally. In the present work, the structure and composition of the PtNi catalyst was characterized by X-ray diffraction (XRD); and its electrochemical properties, morphology, and surface binding energy were analyzed by cyclic voltammetry (CV) and linear scanning voltammetry (LSV), scanning electron microscopy equipped with energy-dispersive spectrometry (SEM-EDS), and X-ray photoelectron spectroscopy (XPS), respectively. The effects of different acid etching treatments (e.g., etching time, etchant concentration, and etching temperature) on the structure and surface active sites were investigated by the orthogonal experiment. The experimental results reveal that after etching with 0.5 mol/L of perchloric acid for 0.5 h at 60°C, the electrode weight loss of the PtNi catalyst is mainly attributed to the large loss of Ni atoms in film layer. This results in the reduced alloy phase in film layer and the appearance of Pt characteristic diffraction peak. The relative content of Pt on the surface of the film electrode increases significantly, and the total number of active sites also increases correspondingly. The binding energy of Pt4f7/2 decreases by 0.19 eV, and the number of active sites involved in hydrogen release decreases, indicative of the reduced promotion effect of the PtNi catalyst on hydrogen release. 相似文献
86.
In the process of industrial corporation activities a lot of waste, which pollutes the atmosphere, is generated, for example ash and slag. In Tomsk region, by estimates, ash stores occupy about 600 hectares, which contain about 25 million tons by weight. In Russian thermal power-stations ash disposal areas there are about 1.3 billion tons of ash, and only 10% of it is used. That is why this problem is topical enough. In this paper the scheme of producing ash ceramic bricks and complex ash and slag waste processing is shown. Besides, profitability of the project is presented. 相似文献
87.
88.
89.
研究了保险公司的最优投资和再保险问题.保险公司的盈余通过跳-扩散风险模型来模拟,可以把盈余的一部分投资到金融市场,金融市场由一个无风险资产和n个风险资产组成,并且保险公司还可以购买比例再保险;在买卖风险资产时,考虑了交易费用.通过随机控制的理论,获得了最优策略和值函数的显示解. 相似文献
90.
This paper concerns optimal investment problem with proportional transaction costs and finite time horizon based on exponential
utility function. Using a partial differential equation approach, we reveal that the problem is equivalent to a parabolic
double obstacle problem involving two free boundaries that correspond to the optimal buying and selling policies. Numerical
examples are obtained by the binomial method. 相似文献