全文获取类型
收费全文 | 5455篇 |
免费 | 601篇 |
国内免费 | 394篇 |
专业分类
化学 | 1337篇 |
晶体学 | 56篇 |
力学 | 762篇 |
综合类 | 63篇 |
数学 | 2525篇 |
物理学 | 1707篇 |
出版年
2024年 | 8篇 |
2023年 | 63篇 |
2022年 | 124篇 |
2021年 | 138篇 |
2020年 | 140篇 |
2019年 | 132篇 |
2018年 | 135篇 |
2017年 | 187篇 |
2016年 | 220篇 |
2015年 | 176篇 |
2014年 | 281篇 |
2013年 | 382篇 |
2012年 | 308篇 |
2011年 | 313篇 |
2010年 | 290篇 |
2009年 | 352篇 |
2008年 | 328篇 |
2007年 | 329篇 |
2006年 | 288篇 |
2005年 | 259篇 |
2004年 | 239篇 |
2003年 | 228篇 |
2002年 | 228篇 |
2001年 | 155篇 |
2000年 | 151篇 |
1999年 | 124篇 |
1998年 | 138篇 |
1997年 | 114篇 |
1996年 | 100篇 |
1995年 | 59篇 |
1994年 | 58篇 |
1993年 | 53篇 |
1992年 | 51篇 |
1991年 | 34篇 |
1990年 | 36篇 |
1989年 | 17篇 |
1988年 | 25篇 |
1987年 | 35篇 |
1986年 | 19篇 |
1985年 | 20篇 |
1984年 | 21篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 8篇 |
1978年 | 18篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 4篇 |
排序方式: 共有6450条查询结果,搜索用时 15 毫秒
51.
A. Y. Lee 《Journal of Optimization Theory and Applications》1988,56(1):157-166
In this paper, we consider a particular approximation scheme which can be used to solve hereditary optimal control problems. These problems are characterized by variables with a time-delayed argumentx(t – ). In our approximation scheme, we first replace the variable with an augmented statey(t) x(t - ). The two-sided Laplace transform ofy(t) is a product of the Laplace transform ofx(t) and an exponential factor. This factor is approximated by a first-order Padé approximation, and a differential relation fory(t) can be found. The transformed problem, without any time-delayed argument, can then be solved using a gradient algorithm in the usual way. Four problems are solved to illustrate the validity and usefulness of this technique.This research was supported in part by the National Aeronautics and Space Administration under NASA Grant NCC-2-106. 相似文献
52.
53.
54.
用转换器和IBMPC/XT微机与Dionex2020i型离子色谱仪连接组成自动控制与数据处理系统,转换器由8031及8255等芯片组成,其功能是采集数据和转送数据,软件由汇编及C语言编制。这套新系统增加了注入/记录同步,分级梯度淋洗,自动连续进样等功能以及很强的数据处理能力。 相似文献
55.
There are many organic pollutants in the environment, such as polychlorinated biphenyl, polycyclic aromatic hydrocarbons, dichlorodiphenyl-trichloroethane (DDT), and polychlorinated naphthalene. These organic pollutants are persistent,liposoluble and easily cumulated in organism; consequently, the potential toxicity will be high. Risk assessment of industrial chemicals is currently carried out using scanty experimental data, because many of these chemicals have very little or no test data. S… 相似文献
56.
We present a systematic study of numerical accuracy of various forms of molecular caps that are employed in a recently developed molecular fractionation scheme for full quantum mechanical computation of protein-molecule interaction energy. A previously studied pentapeptide (Gly-Ser-Ala-Asp-Val) or P5 interacting with a water molecule is used as a benchmark system for numerical testing. One-dimensional potential energy curves are generated for a number of peptide-water interaction pathways. Our study shows that various forms of caps all give consistently accurate energies compared to the corresponding full system calculation with only small deviations. We also tested the accuracy of cutting peptide backbone at different positions and comparisons of results are presented. 相似文献
57.
Josef Janča 《Mikrochimica acta》1994,112(5-6):197-215
The primary field forces can generate spatially oriented gradient of the effective property of a continuum or pseudo-continuum fluid (carrier liquid). When this gradient is coupled with the action of a secondary field of identical or different nature the isoperichoric focused zones of the dispersed species can appear. Consequently, they can be separated according to differences responding to the property gradient of the carrier liquid. This concept can be applied under static (non-flow) conditions in thin layer focusing as well as under dynamic conditions with the elution due to the carrier liquid flow in focusing field-flow fractionation. The gradient established by the action of the primary field and the concentration distribution of the isoperichoric focused zone formed by the coupled effect of the gradient and of the primary or secondary field are described theoretically. The rigorous relationship describing the shape of the focused zone is compared with the approximate solutions. The performances of the proposed principle were evaluated by model calculations. Potential experimental configurations considering the implementation of the static and dynamic conditions are discussed. The generalized isoperichoric focusing theory can be applied to describe the particular processes operating in analytical and preparative focusing separations of the particles of various, but especially of biological origin. 相似文献
58.
One of the major techniques used for the method development of ternary and quaternary high performance liquid chromatography (HPLC) systems has been to use mixture designs, often referred to as "Glajch's Triangle". This technique does not allow for the systematic and simultaneous optimization of other factors such as gradient time, pH and temperature that affect the quality of separations. An alternative approach is to use experimental designs. The condition, however, that the composition of all components of the mobile phase must total 100% presents a problem when trying to mathematically represent ranges of each mobile phase constituent of a ternary or quaternary system. A method is described here, based on spherical coordinate representations, that adheres to the constraints of the mobile phase composition and allows experimental designs, such as central composite and factorial designs, to be applied to the simultaneous optimization of the mobile phase composition. Other factors, in particular temperature and gradient time, can then be included in the design. As a result of applying these designs to the HPLC separation of phenols and corticosteroids, it was found necessary to include three-way interactions between experimental factors in the model. The significance of these interactions shows that they need to be considered in HPLC method development. 相似文献
59.
《Journal of separation science》2003,26(17):1541-1546
A novel approach is proposed for the simultaneous optimization of mobile phase pH and gradient steepness in RP‐HPLC using artificial neural networks. By presetting the initial and final concentration of the organic solvent, a limited number of experiments with different gradient time and pH value of mobile phase are arranged in the two‐dimensional space of mobile phase parameters. The retention behavior of each solute is modeled using an individual artificial neural network. An “early stopping” strategy is adopted to ensure the predicting capability of neural networks. The trained neural networks can be used to predict the retention time of solutes under arbitrary mobile phase conditions in the optimization region. Finally, the optimal separation conditions can be found according to a global resolution function. The effectiveness of this method is validated by optimization of separation conditions for amino acids derivatised by a new fluorescent reagent. 相似文献
60.
一种基于流动注射梯度技术识别异常峰及校正的方法 总被引:1,自引:0,他引:1
基于流动注射梯度信息提出了一种利用梯度比均值进行定量的校正方法。方法具有在线自动判别和修复异常峰并自行校正的功能,和通用的标准系列定量方法相比,两者测定精度相当。但本文提出的方法抗干扰能力明显优于后者,可适用于在线过程监测。 相似文献