全文获取类型
收费全文 | 46565篇 |
免费 | 1420篇 |
国内免费 | 124篇 |
专业分类
化学 | 27083篇 |
晶体学 | 270篇 |
力学 | 1134篇 |
综合类 | 9篇 |
数学 | 5948篇 |
物理学 | 10312篇 |
无线电 | 3353篇 |
出版年
2023年 | 272篇 |
2022年 | 413篇 |
2021年 | 707篇 |
2020年 | 697篇 |
2019年 | 707篇 |
2018年 | 741篇 |
2017年 | 725篇 |
2016年 | 1313篇 |
2015年 | 1120篇 |
2014年 | 1342篇 |
2013年 | 2287篇 |
2012年 | 2784篇 |
2011年 | 3046篇 |
2010年 | 1762篇 |
2009年 | 1649篇 |
2008年 | 2693篇 |
2007年 | 2623篇 |
2006年 | 2344篇 |
2005年 | 2691篇 |
2004年 | 2523篇 |
2003年 | 1902篇 |
2002年 | 1455篇 |
2001年 | 1007篇 |
2000年 | 905篇 |
1999年 | 594篇 |
1998年 | 479篇 |
1997年 | 476篇 |
1996年 | 585篇 |
1995年 | 483篇 |
1994年 | 465篇 |
1993年 | 508篇 |
1992年 | 486篇 |
1991年 | 387篇 |
1990年 | 330篇 |
1989年 | 282篇 |
1988年 | 276篇 |
1987年 | 247篇 |
1986年 | 262篇 |
1985年 | 337篇 |
1984年 | 327篇 |
1983年 | 261篇 |
1982年 | 288篇 |
1981年 | 294篇 |
1980年 | 267篇 |
1979年 | 232篇 |
1978年 | 245篇 |
1977年 | 204篇 |
1976年 | 210篇 |
1974年 | 178篇 |
1973年 | 170篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
251.
F. Silva X. Bonnin J. Achard O. Brinza A. Michau A. Gicquel 《Journal of Crystal Growth》2008,310(1):187-203
Plasma-assisted CVD homoepitaxial diamond growth is a process that must satisfy many stringent requirements to meet industrial applications, particularly in high-power electronics. Purity control and crystalline quality of the obtained samples are of paramount importance and their optimization is a subject of active research. In the process of such studies, we have obtained high purity CVD diamond monocrystals with unusual morphologies, namely with apparent {1 1 3} stable faces. This phenomenon has led us to examine the process of CVD diamond growth and build up a 3D geometrical model, presented here, describing the film growth as a function of time. The model has been able to successfully describe the morphology of our obtained crystals and can be used as a predictive tool to predetermine the shape and size of a diamond crystal grown in a given process configuration. This renders accessible control of desirable properties such as largest usable diamond surface area and/or film thickness, before the cutting and polishing manufacture steps take place. The two latter steps are more sensitive to the geometry of the growth sectors, which will be addressed in a companion paper.Our model, applicable to the growth of any cubic lattice material, establishes a complete mapping of the final morphology state of growing diamond, as a function of the growth rates of the crystalline planes considered, namely {1 0 0}, {1 1 1}, {1 1 0}, and {1 1 3} planes, all of which have been observed experimentally in diamond films. The model makes no claim as to the stability of the obtained faces, such as the occurrence of non-epitaxial crystallites or twinning. It is also possible to deduce transient behavior of the crystal morphology as growth time is increased. The model conclusions are presented in the form of a series of diagrams, which trace the existence (and dominance) boundaries of each face type, in presence of the others, and where each boundary crossing represent a topology change in terms of number of faces, edges and vertices. We validate the model by matching it against crystals published in the literature and illustrate its predictive value by suggesting ways to increase usable surface area of the diamond film. 相似文献
252.
This study demonstrates the separation of active ingredients in acne formulations (salicylic acid, cloramphenicol and resorcinol in presence of azulene) by capillary zone electrophoresis. Factors affecting their separations were the buffer pH and concentration, applied voltage, sample preparation, and presence of additives. Optimun results were obtained with a 50 mM sodium tetraborate-50 mM sodium phosphate, pH 9.0. The carrier electrolyte gave baseline separation with good resolution, short migration times (<6 min), great reproducibility and accuracy. Calibration plots were linear over at least three orders of magnitude of analyte concentrations, the lower limits of detection being within the range 0.39-1.25 μg ml−1. The procedure was fast and reliable and commercial pharmaceuticals could be analysed without prior sample clean-up procedure. 相似文献
253.
Jan de Vries 《Monatshefte für Mathematik》1901,12(1):45-48
Ohne Zusammenfassung 相似文献
254.
A systematic study of the efficiency of protons, Ni, Pd and Th as chemical modifiers for the determination of cadmium by electrothermal atomic absorption spectrometry (ETAAS) using fast temperature programs was made for platform atomization. A comparison was made in terms of the salt type, absorbance-time profiles and elimination of the sodium chloride interference. The results were adapted to develop a method for the ETAAS determination of cadmium in biological and environmental samples. The highest sensitivity to determine cadmium in biological and environmental samples was obtained using nickel (together with protons) as a chemical modifier. The accuracy of the method was tested by the determination of cadmium in different certified reference materials. The best detection limit and the characteristic mass of Cd were found to be 0.03 ng mL(-1) and 0.35 pg, respectively. 相似文献
255.
José M. Arrieta Neus Cónsul Anibal Rodríguez-Bernal 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》2004,23(2):1-14
We prove the existence of nonconstant stable stationary
solutions of
an evolution problem with a nonlinear reaction acting on the
boundary. These solutions present layers at certain points of the
boundary. We also study the behavior of these solutions as the small
parameter appearing in the equation approaches zero and show some
stability properties of the profiles given by these equilibrium
solutions. 相似文献
256.
257.
258.
Andreia PeixotoMariette M. Pereira M.Graça P.M.S. NevesArtur M.S. Silva José A.S. Cavaleiro 《Tetrahedron letters》2003,44(30):5593-5595
Formyl derivatives of protoporphyrin-IX dimethyl ester metal complexes were obtained via hydroformylation reactions, catalysed by rhodium-triphenylphosphine complexes. The regioselectivity of the reaction is remarkably dependent on the metal centre of the porphyrin, yielding 100% of the branched aldehyde with zinc(II) complexes and 75% with the nickel(II). The NMR characterisation of the new compounds was carried out after their derivatisation into acetals. 相似文献
259.
Isabel Brandariz José L. Barriada Teresa Vilari?o Manuel E. Sastre de Vicente 《Monatshefte für Chemie / Chemical Monthly》2004,343(6):1475-1488
Several reported procedures for calibrating glass electrodes in proton concentration are compared. Some recommendations for non-experts are also given. The examined procedures can be classified into two broad categories, namely: those based on direct potential difference measurements of solutions of known proton concentration and those that use one or several pH standards to calibrate the electrode and subsequently measure the pH of solutions containing known proton concentrations. With a single buffer, the two types of procedures lead to equivalent results. However, if two pH buffers are used, the slope of the calibration graph in proton concentration will differ from the real electrode slope to an extent proportional to the difference between the liquid junction potentials of the two buffers. Therefore, any other method is preferable under these circumstances. 相似文献
260.