全文获取类型
收费全文 | 1831篇 |
免费 | 24篇 |
国内免费 | 45篇 |
专业分类
化学 | 829篇 |
晶体学 | 5篇 |
力学 | 55篇 |
综合类 | 14篇 |
数学 | 450篇 |
物理学 | 547篇 |
出版年
2024年 | 4篇 |
2023年 | 48篇 |
2022年 | 23篇 |
2021年 | 19篇 |
2020年 | 24篇 |
2019年 | 19篇 |
2018年 | 22篇 |
2017年 | 17篇 |
2016年 | 34篇 |
2015年 | 19篇 |
2014年 | 46篇 |
2013年 | 204篇 |
2012年 | 206篇 |
2011年 | 93篇 |
2010年 | 57篇 |
2009年 | 97篇 |
2008年 | 52篇 |
2007年 | 84篇 |
2006年 | 90篇 |
2005年 | 87篇 |
2004年 | 71篇 |
2003年 | 46篇 |
2002年 | 45篇 |
2001年 | 35篇 |
2000年 | 44篇 |
1999年 | 40篇 |
1998年 | 39篇 |
1997年 | 43篇 |
1996年 | 46篇 |
1995年 | 46篇 |
1994年 | 23篇 |
1993年 | 28篇 |
1992年 | 20篇 |
1991年 | 23篇 |
1990年 | 11篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 14篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有1900条查询结果,搜索用时 0 毫秒
11.
Near-edge X-ray absorption spectroscopy (NEXAFS) is widely used to probe the chemistry and structure of surface layers. Moreover, using ultra-high brilliance polarised synchrotron light sources, it is possible to determine the molecular alignment of ultra-thin surface films. However, the quantitative analysis of NEXAFS data is complicated by many experimental factors and, historically, the essential methods of calibration, normalisation and artefact removal are presented in the literature in a somewhat fragmented manner, thus hindering their integrated implementation as well as their further development. This paper outlines a unified, systematic approach to the collection and quantitative analysis of NEXAFS data with a particular focus upon carbon K-edge spectra. As a consequence, we show that current methods neglect several important aspects of the data analysis process, which we address with a combination of novel and adapted techniques. We discuss multiple approaches in solving the issues commonly encountered in the analysis of NEXAFS data, revealing the inherent assumptions of each approach and providing guidelines for assessing their appropriateness in a broad range of experimental situations. 相似文献
12.
A review of the advance in the theory of wavelet analysis in recent years is given. 相似文献
13.
R. Matissek 《Chromatographia》1989,28(1-2):34-38
Summary Antimicrobially active formulations based on 2-methyl-3(2H)-isothiazolone and 5-chloro-2-methyl-3(2H)-isothiazolone (Kathon CG, Euxyl K 100) are incorporated into manufactured cosmetics as preservatives. In this paper we report on the analysis of these active components by means of liquid chromatography. It is conventional for the original methylisothiazolone components in cosmetics to be analyzed by reversed phase high-performance liquid chromatography after column chromatographic separation and flash chromatographic purification. In our new analytical method the methylisothiazolones are converted into the respective -thiosubstituted acrylamide derivatives, which are ionic substances, with the help of the nucleophilic reagent hydrogensulfite. An ion pair high-performance liquid chromatographic method has been developed for the separation and quantification of these derivatives. 相似文献
14.
采用Hartree-Fock和MP2方法在多种水平下优化了(H2O)^+n(n =1,2)的几何结构,并进行了振动光谱分析。结果表明:对(H2O)^+采用MP2/6-311++G(d,p)可得出最满意的结论。 相似文献
15.
This article discusses the development of various analytical methods for identification and quantification of residues of antibiotics in meeting international regulatory requirements, and, as an example, overviews the screening, purification, fractionation and analytical assays of bacitracin, a commonly applied polypeptide antibiotic mixture. Attention is focused on the development of chemical methods, as they provide extraordinary performance, in terms of sensitivity and selectivity, for measuring residual levels of the various components of bacitracin in food and other matrices; the non-compound specific biological assay is discussed only briefly. Because of its advantages, state-of-the-art high-performance liquid chromatography (HPLC) coupled with various detection techniques, in particular mass spectrometry (MS), has recently gained in popularity in confirming and quantifying drug residues. In this article, the authors also outline the latest results on the application of highly sensitive and selective LC-MS in analysis of bacitracin residues in milk and animal tissues. LC-MS is suggested as the technique of choice for confirmation of polar polypeptide antibiotics and the like at trace concentrations. 相似文献
16.
V. Balek H. Hamerská L. Montanaro A. Negro 《Journal of Thermal Analysis and Calorimetry》1992,38(4):567-573
The results of the investigations of thermal behaviour of Li2ZrO3, prepared in the amorphous state by means of sol-gel technique are demonstrated. The thermal treatment was carried out in air under constant heating rate of 5 deg·min–1 and cooling rate of 2.5 deg·min–1. The methods of DTA, TG, Emanation Thermal Analysis (ETA) and dilatometry were used, for characterization of the thermal behaviour in dynamic conditions. The X-ray diffraction patterns were used for characterization of the phase changes observed by TA Methods.
Zusammenfassung Ergebnisse aus Untersuchungen des thermischen Verhaltens von Li2ZrO3, hergestellt mittels einer Sol-Gel-Technik im amorphen Zustand werden dargelegt. Die thermische Behandlung wurde in Luft bei einer konstanten Aufheizgeschwindigkeit von 5 Grad·min–1 und einer Abkühlgeschwindigkeit von 2,5Grad·min–1 durchgeführt. Zur Beschreibung des thermischen Verhaltens unter dynamischen Verhältnissen wurden DTA, TG, Emanationsthermoanalyse und Dilatometrie angewendet. Zur näheren Charakterisierung der bei TG beobachteten Phasenumwandlungen wurde Röntgendiffraktion verwendet.相似文献
17.
18.
I. Kuselman 《Accreditation and quality assurance》1998,3(3):131-133
It is argued that results of uncertainty calculations in chemical analysis should be taken into consideration with some caution
owing to their limited generality. The issue of the uncertainty in uncertainty estimation is discussed in two aspects. The
first is due to the differences between procedure-oriented and result-oriented uncertainty assessments, and the second is
due to the differences between the theoretical calculation of uncertainty and its quantication using the validation (experimental)
data. It is shown that the uncertainty calculation for instrumental analytical methods using a regression calibration curve
is result-oriented and meaningful only until the next calibration. A scheme for evaluation of the uncertainty in uncertainty
calculation by statistical analysis of experimental data is given and illustrated with examples from the author's practice.
Some recommendations for the design of corresponding experiments are formulated. 相似文献
19.
Using statistically designed experiments, 12,500 observations are generated from a 4-pieced Cobb-Douglas function exhibiting increasing and decreasing returns to scale in its different pieces. Performances of DEA and frontier regressions represented by COLS (Corrected Ordinary Least Squares) are compared at sample sizes ofn=50, 100, 150 and 200. Statistical consistency is exhibited, with performances improving as sample sizes increase. Both DEA and COLS generally give good results at all sample sizes. In evaluating efficiency, DEA generally shows superior performance, with BCC models being best (except at corner points), followed by the CCR model and then by COLS, with log-linear regressions performing better than their translog counterparts at almost all sample sizes. Because of the need to consider locally varying behavior, only the CCR and translog models are used for returns to scale, with CCR being the better performer. An additional set of 7,500 observations were generated under conditions that made it possible to compare efficiency evaluations in the presence of collinearity and with model misspecification in the form of added and omitted variables. Results were similar to the larger experiment: the BCC model is the best performer. However, COLS exhibited surprisingly good performances — which suggests that COLS may have previously unidentified robustness properties — while the CCR model is the poorest performer when one of the variables used to generate the observations is omitted. 相似文献
20.
Syhi-Hwan Chen 《中国化学会会志》1997,44(2):115-121
The purpose of this investigation is to try and understand the degree of arsenic poisoning in the waste water from agricultural chemical factories which release effluents into the waterways in their vicinity. The method of determination is to use the water samples, soil, fish, clams, shell-fish, etc. found in the areas along the canals and aqueducts and those which lie upstream, midstream, and downstream and also the hair samples of people living in the contiguous areas, to check for the levels of arsenic in them by emission spectroscopy. From the analysis during a one year investigation, we find that the level of As in the water creatures of Chungkang Stream and Lu-Er-Men Stream is much higher than in the other water areas of Taiwan such as along beaches and the other rivers; the levels in the hair of the people who live nearby these water courses have a certain degree of correspondence with their eating habits and the quantity of food eaten; however, even those people who eat relatively little have almost 3% As, beyond the 2 mg/kg limit allowed for human beings. We are fully aware of the high level of food contamination on Taiwan, and this fact has been taken into consideration. The effect of As contamination in the soil on agricultural produce does not seem to have any apparent relationship. By the way, the influence of the monthly average rainfall on As contamination in the soil is not obvious. 相似文献