全文获取类型
收费全文 | 4096篇 |
免费 | 1068篇 |
国内免费 | 577篇 |
专业分类
化学 | 2690篇 |
晶体学 | 8篇 |
力学 | 258篇 |
综合类 | 42篇 |
数学 | 209篇 |
物理学 | 2534篇 |
出版年
2024年 | 4篇 |
2023年 | 22篇 |
2022年 | 80篇 |
2021年 | 64篇 |
2020年 | 92篇 |
2019年 | 110篇 |
2018年 | 101篇 |
2017年 | 167篇 |
2016年 | 198篇 |
2015年 | 307篇 |
2014年 | 347篇 |
2013年 | 454篇 |
2012年 | 418篇 |
2011年 | 392篇 |
2010年 | 336篇 |
2009年 | 321篇 |
2008年 | 336篇 |
2007年 | 278篇 |
2006年 | 232篇 |
2005年 | 203篇 |
2004年 | 198篇 |
2003年 | 178篇 |
2002年 | 127篇 |
2001年 | 124篇 |
2000年 | 107篇 |
1999年 | 72篇 |
1998年 | 82篇 |
1997年 | 71篇 |
1996年 | 61篇 |
1995年 | 46篇 |
1994年 | 48篇 |
1993年 | 45篇 |
1992年 | 27篇 |
1991年 | 16篇 |
1990年 | 18篇 |
1989年 | 9篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1969年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有5741条查询结果,搜索用时 31 毫秒
51.
利用交替三线性分解算法(ATLD)与高效液相色谱法相结合,在色谱保留时间为0.662 8~0.949 5min(间隔1/150 min)、紫外吸收波长为268~332 nm(间隔2 nm)且在未知干扰物2,3-二羟基苯甲酸存在下,同时测定了水溶液中光谱及色谱严重重叠的水杨酸(SA)和龙胆酸(GA)的含量,回收率分别为(102.2±6.7)%,(102.1±4.1)%,分辨结果与实际结果一致。研究结果表明:该方法定量快速准确、实验操作步骤简单,说明ATLD算法收敛快速稳定,对复杂体系中组分数估计不敏感,能有效解决色谱中的二阶校正问题。 相似文献
52.
聚丙烯酰胺/醋酸铬与聚丙烯酰胺/酚醛胶态分散凝胶的纳米颗粒自组织分形结构 总被引:5,自引:0,他引:5
采用原子力显微镜 ,分别对无机交联体系聚丙烯酰胺 Cr3+ 和有机交联体系聚丙烯酰胺 酚醛胶态分散凝胶的微观结构进行了显微图像分析 .发现无论是在有机还是无机交联体系中 ,也无论聚丙烯酰胺和交联剂浓度如何变化 ,在微米尺度上最终形成的都是具有自相似性的树枝状分形图像 ,在更小尺度上则发现单个小树杈分形体都是由纳米级的颗粒紧密堆积而成 .在所研究的胶态分散凝胶体系中 ,树枝状分形结构的形成及其具体的形态取决于聚丙烯酰胺的浓度 ,而交联剂的有无及其多少只对树枝状凝胶分形的几何形态产生一定影响 .实验结果还表明纳米级 (≤ 10 0nm)的胶体颗粒构成的分形结构的凝胶其弹性模量G′比微米级的高出一个数量级 .且粒子尺度越小 ,则凝胶的力学稳定性越强 相似文献
53.
人工神经网络及其在分析化学中的应用 总被引:31,自引:1,他引:31
人工神经网络是一种新兴的计算方法,有着广阔的发展前途,目前在分析化学领域已经有了多方面的应用。本文简要介绍了人工神经网络的原理及其在分析化学中的应用。 相似文献
54.
I. Esteban-Díez 《Analytica chimica acta》2006,555(1):84-95
Orthogonal WAVElet correction (OWAVEC) is a pre-processing method aimed at simultaneously accomplishing two essential needs in multivariate calibration, signal correction and data compression, by combining the application of an orthogonal signal correction algorithm to remove information unrelated to a certain response with the great potential that wavelet analysis has shown for signal processing. In the previous version of the OWAVEC method, once the wavelet coefficients matrix had been computed from NIR spectra and deflated from irrelevant information in the orthogonalization step, effective data compression was achieved by selecting those largest correlation/variance wavelet coefficients serving as the basis for the development of a reliable regression model. This paper presents an evolution of the OWAVEC method, maintaining the first two stages in its application procedure (wavelet signal decomposition and direct orthogonalization) intact but incorporating genetic algorithms as a wavelet coefficients selection method to perform data compression and to improve the quality of the regression models developed later. Several specific applications dealing with diverse NIR regression problems are analyzed to evaluate the actual performance of the new OWAVEC method. Results provided by OWAVEC are also compared with those obtained with original data and with other orthogonal signal correction methods. 相似文献
55.
《Surface and interface analysis : SIA》2005,37(7):589-607
The accuracy of quantitative XPS analysis can be improved using predetermined transmission functions. Two different calibration methods are used for estimating the transmission function T(E) of a photoelectron spectrometer, applying a survey spectra approach (SSA) and a quantified peak‐area approach (QPA) to minimize the quantification error. For the SSA method, Au, Ag and Cu spectra measured with the Metrology Spectrometer II have been used. The new QPA method was built up from Au 4f, Au 4d, Au 4p3/2, Ag 3d, Ag 3p3/2, Cu 3p, Cu 2p3/2, Ge 3p and Ge 2p3/2 standard peak areas, applying adequate ionization cross‐sections and mean free path lengths for different pass energies (10 and 50 eV), lens modes (large area, large area XL, small area 150) and x‐ray sources (Al/Mg Twin and Al Mono). In the energy range 200–1500 eV a transmission function T(E) = a0 + b1E (where a0, b1 and b2 are variable parameters) was found to give an appropriate approximation for eight tested spectrometer settings, implementing the largest changes in the case of pass energy variations. Determination and application of the transmission functions were integrated in the XPS analysis software (UNIFIT 2004) and tested by means of an Ni90Cr10 alloy. The results demonstrate the practicability of the SSA and QPA methods, giving decreased errors of <8% in comparison with errors up to 38% obtained using Wagner's sensitivity factors. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
56.
Two different spectrofluorimetric methods for the determination of piroxicam (PX) in serum are presented and discussed. One of them is based on the use of three-way fluorescence data and multivariate calibration performed with parallel factor analysis (PARAFAC) and self-weighted alternating trilinear decomposition (SWATLD). This methodology exploits the so-called second-order advantage of the three-way data, allowing to obtain the concentration of the studied analyte in the presence of any number of uncalibrated (serum) components. The method was developed following two different procedures: internal standard addition and external calibration with standard solutions, which were compared and discussed. The second approach investigated is based on the combination of solid-phase extraction (SPE) and room temperature fluorimetry. Both methods here presented yield satisfactory results. The concentration range in which PX could be determined in serum was 1–10 μg ml−1. The limits of quantification for the experimental solutions using the chemometric approach were 0.09 μg ml−1 for the standard addition mode and 0.12 μg ml−1 using external calibration (both for PARAFAC and SWATLD algorithms). In the solid-surface fluorimetric method, the calibration graph was linear up to 0.22 μg ml−1 and the limit of quantification was 0.02 μg ml−1. 相似文献
57.
58.
59.
Martí MP Pino J Boqué R Busto O Guasch J 《Analytical and bioanalytical chemistry》2005,382(2):440-443
The aromatic composition of sugar cane spirits and, in general, of alcoholic beverages, is mainly influenced by the ageing process in wood barrels. There are several factors that affect the quality of the final aged product, but the time of the storage in the barrel is perhaps the most important one. Ageing time must therefore be controlled in order to detect counterfeits; however, this parameter is very difficult to control and, at present, there is no analytical method available to determine it. We propose a quantitative method for determining the ageing time of sugar cane spirits in oak barrels by using an electronic nose based on coupling directly a headspace sampler to a mass spectrometer (HS-MS), and multivariate calibration. The method developed is simple and provides, in 5 min, the ageing time of spirits with an accuracy of about 1 month. 相似文献
60.
A three-dimensional pharmacophore model was developed from a series of inhibitors of Aurora A kinase to discover new potent anti-cancer agents using the HypoGen module in the Catalyst software. The pharmacophore model was developed based on the structure of 20 currently available inhibitors, which were carefully selected from the literature. The best hypothesis (Hypo 1) was defined by four features: one hydrogen-bond donor and three hy- drophobic points, with the best correlation coefficient of 0.909, the lowest rms deviation of 1.563, and the highest cost difference of 99.075. The Hypo 1 was then validated by a test set consisting of 24 compounds and by a cross-validation of 95% confidence level through randomizing the data using the CatScramble program, which suggested that a predictive pharmacophore model had been successfully obtained. 相似文献