全文获取类型
收费全文 | 1659篇 |
免费 | 35篇 |
国内免费 | 27篇 |
专业分类
化学 | 394篇 |
晶体学 | 1篇 |
力学 | 101篇 |
综合类 | 3篇 |
数学 | 892篇 |
物理学 | 330篇 |
出版年
2023年 | 26篇 |
2022年 | 41篇 |
2021年 | 18篇 |
2020年 | 30篇 |
2019年 | 19篇 |
2018年 | 19篇 |
2017年 | 27篇 |
2016年 | 27篇 |
2015年 | 33篇 |
2014年 | 57篇 |
2013年 | 97篇 |
2012年 | 157篇 |
2011年 | 105篇 |
2010年 | 66篇 |
2009年 | 122篇 |
2008年 | 111篇 |
2007年 | 99篇 |
2006年 | 69篇 |
2005年 | 32篇 |
2004年 | 40篇 |
2003年 | 56篇 |
2002年 | 20篇 |
2001年 | 23篇 |
2000年 | 19篇 |
1999年 | 17篇 |
1998年 | 37篇 |
1997年 | 22篇 |
1996年 | 29篇 |
1995年 | 20篇 |
1994年 | 19篇 |
1993年 | 17篇 |
1992年 | 13篇 |
1991年 | 16篇 |
1990年 | 15篇 |
1989年 | 10篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 25篇 |
1984年 | 20篇 |
1983年 | 15篇 |
1982年 | 23篇 |
1981年 | 16篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1978年 | 25篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1967年 | 1篇 |
排序方式: 共有1721条查询结果,搜索用时 31 毫秒
41.
Yu-Ping Zhang Kwang-Pill Lee Hyun-Joo Noh Ali Md Showkat Seong-Ho Choi 《Microchemical Journal》2004,78(2):99-106
An appropriate optimization strategy should be used to find a desired resolution or selectivity with a minimum number of experiments in a limited time, which could assure the baseline separation of all target compounds. It was usually realized by means of a specialized computer program. In this paper, mapping optimization method and overlapping resolution mapping were compared for the optimization of a reversed-phase high-performance liquid chromatography (HPLC) isocratic separation of neutral compounds. The calculated resolutions and separation time of 7 to 10 experiments are fitted by different equations, which were used to build a contour plot with a minimum effective resolution and maximum retention time as a function of a mobile phase composition. The balance between resolution and analysis time could be easily realized by the overlapping of the final overlapping resolution mapping and analysis time mapping. The validity of the two methods was confirmed by some typical experiments. The models are simple, visual, and common without theoretical arithmetic. 相似文献
42.
Summary After a brief introduction of the subject, the paper focusses on the first step in any optimization procedure: the delineation
of the parameter space, wherein the global optimum is to be found. For organic modifier optimization in reversed-phase liquid
chromatography it is shown that the necessary information can be derived from a single water-methanol gradient. It first yields
an estimate of the total number of solutes in the sample, which is vital to define the peak capacity needed to achieve separation
at a certain confidence level. Next, the gradient allows the prediction of suitable isocratic methanol binary solvents, and
transfer rules formulate the iso-eluotropic composition of the common binary solvents (tetrahydrofuran and acetonitrile).
Because all predictions are based on a statistical analysis of a limited data base, attention is given to the practical situation
where an actual sample deviates from the average solute behaviour. Such deviations are revealed in the first isocratic run
and can be used to arrive at a better estimate of solute retention. 相似文献
43.
The acid-catalysed esterification of myristic acid with isopropanol was studied by using near-infrared spectroscopy (NIR) in combination with soft-modeling curve resolution (MCR) methodology with a view to establishing the effect of experimental variables on the reaction kinetics. The reaction was conducted at temperatures above the boiling point of the alcohol, with continuous addition of an isopropanol/water mixture to the reactor. Spectral and concentration profiles were determined by applying soft-modeling curve resolution methodology to a column-wise augmented data matrix containing the spectra for the pure components. MCR profiles were compared with reference values and found to depart from then by less than 3% as %RSE for concentrations and to exhibit correlation above 0.999 for spectra.The reaction kinetics as estimated from the concentration profiles was found to be pseudo-first-order. Also, the pseudo-first-order rate constant was found to depend on the flow-rate of the isopropanol/water mixture and its water content; although the constant decreased with increase in the proportion of water, a content of ca. 15% could be used without important retarding effects on the kinetics. The proposed NIR-MCR method allows the rate constant and the influence of the initial water content to be determined with a view to minimizing consumption of the raw materials and optimizing the experimental conditions. 相似文献
44.
Summary The elution behavior of alkali and earth alkaline cations has been studied on a newly synthesized weak cationexchanger (WCX). Applying this stationary phase isocratic separation of these cations is possible. The dependence of retention on competing ion concentration is described via a simple model. The influence of crown ethers as well as organic eluent additives on selectivity and peak efficiency is demonstrated. With optimized separation such cations can be determined with detection limits of about 0.5 ppm using a conductivity detector without ion suppression. 相似文献
45.
J. Zhu 《Mathematical Methods of Operations Research》1992,36(4):359-377
We present a primal-dual path following interior algorithm for a class of linearly constrained convex programming problems with non-negative decision variables. We introduce the definition of a Scaled Lipschitz Condition and show that if the objective function satisfies the Scaled Lipschitz Condition then, at each iteration, our algorithm reduces the duality gap by at least a factor of (1–/n), where is positive and depends on the curvature of the objective function, by means of solving a system of linear equations which requires no more than O(n3) arithmetic operations. The class of functions having the Scaled Lipschitz Condition includes linear, convex quadratic and entropy functions. 相似文献
46.
Summary A simple optimization method based on the well-known Rs-minimum method and on the information theory of FUMI Φ is proposed.
Resolution (Rs), peak area and height (or width) are the only parameters necessary for the calculation of the information
Φ and information flow ϑ. The most precise analysis can be selected as the chromatogram having maximal ϑ. Mobile phase composition,
column length, flow rate, detection wavelength, amount of internal standard, etc. can be optimized by this method. 相似文献
47.
Theoretical equations are given to relate resolution with development distance. Comparative experiments undertaken with quaternary ammonium compounds as test substances and using a selection of basic drugs are presented. As expected, resolution decreases with shorter development distance. However, this effect is less pronounced for compounds with small Rf values and when the spot size at the starting point is kept small. It was also shown that, for correctly assessing resolution in relation to development distance, the amounts of material spotted have to be decreased for shorter development distances. 相似文献
48.
酚醛树脂是一种广泛使用的合成树脂,包括热固性和热塑性两类,具有良好的阻燃性、耐热性和耐腐蚀性。酚醛纤维是由酚醛树脂所制成的交联纤维,传统的酚醛纤维制备方法有熔融纺丝法和湿法纺丝法,后来出现了静电纺丝法。本文根据酚醛树脂的种类分别介绍了热固性、热塑性和热塑/热固混合酚醛树脂三类材料静电纺丝的研究进展。在改善酚醛纤维特性方面,综述了四种优化措施,包括加入无机盐、微波辐射辅助固化、非匀速阶梯式加热固化、氧化石墨烯修饰的静电纺丝法等。此外,对本实验室制备酚醛纤维的研究也进行了概述,阐述了酚醛纤维当前存在的问题及未来发展方向。 相似文献
49.
《Optimization》2012,61(2):99-112
In optimization, the literature on invariant definitions of convexity has largely centered on the notion of invexity. However, the connection with Morse theory and related geometric trends has not been recognized and results do not appear to be as impressive as those in topology. Our purpose is to improve the situation in optimization, and to clarify the relationship between these two trends. The main result is that the problem can be reduced to the case of functions of one variable. 相似文献
50.
We consider the problem of recovering a two-dimensional periodic structure from scattered waves measured above the structure. Following an approach by Kirsch and Kress, this inverse problem is reformulated as a nonlinear optimization problem. We develop a theoretical basis for the reconstruction method in the case of an arbitrary Lipschitz grating profile. The convergence analysis is based on new perturbation and stability results for the forward problem. 相似文献