全文获取类型
收费全文 | 2517篇 |
免费 | 141篇 |
国内免费 | 1篇 |
专业分类
化学 | 1696篇 |
晶体学 | 12篇 |
力学 | 76篇 |
数学 | 234篇 |
物理学 | 641篇 |
出版年
2023年 | 23篇 |
2022年 | 17篇 |
2021年 | 33篇 |
2020年 | 37篇 |
2019年 | 22篇 |
2018年 | 19篇 |
2017年 | 36篇 |
2016年 | 72篇 |
2015年 | 75篇 |
2014年 | 66篇 |
2013年 | 102篇 |
2012年 | 132篇 |
2011年 | 170篇 |
2010年 | 75篇 |
2009年 | 71篇 |
2008年 | 145篇 |
2007年 | 140篇 |
2006年 | 112篇 |
2005年 | 110篇 |
2004年 | 81篇 |
2003年 | 85篇 |
2002年 | 78篇 |
2001年 | 49篇 |
2000年 | 38篇 |
1999年 | 20篇 |
1998年 | 22篇 |
1997年 | 19篇 |
1996年 | 22篇 |
1995年 | 24篇 |
1994年 | 25篇 |
1993年 | 33篇 |
1992年 | 27篇 |
1991年 | 25篇 |
1990年 | 23篇 |
1989年 | 18篇 |
1988年 | 17篇 |
1987年 | 19篇 |
1986年 | 18篇 |
1985年 | 25篇 |
1984年 | 21篇 |
1983年 | 16篇 |
1982年 | 23篇 |
1981年 | 23篇 |
1980年 | 23篇 |
1979年 | 16篇 |
1978年 | 25篇 |
1977年 | 19篇 |
1976年 | 22篇 |
1974年 | 16篇 |
1973年 | 18篇 |
排序方式: 共有2659条查询结果,搜索用时 15 毫秒
51.
P. Brüesch F. Stucki Th. Baumann P. Kluge-Weiss B. Brühl L. Niemeyer R. Strümpler B. Ziegler M. Mielke 《Applied Physics A: Materials Science & Processing》1993,57(4):329-337
In this contribution we have studied the key electrical parameters of silica aerogels and of silica-aerogel-based composites, namely the dielectric constants , the dielectric losses tan (at 1 kHz), and the breakdown fields E
b (at 50 Hz). For low-density bulk silica aerogels we find =1.25 and tan =0.0005. E
b is about 500 kV/cm in quasi-homogeneous fields, and of the order of MV/cm in strongly inhomogeneous fields. The dielectric constants of partially densified aerogels increase linearly with density; their dielectric losses are relatively large and their breakdown fields are comparativiely low. The same results are found for aerogels in the form of settled materials, i.e. aerogel granules and powders in air. Acrylate-based aerogel composites with volume fractions larger than 70% have low dielectric constants but their losses are at least 10 times higher than those of low-density aerogels. These materials sustain high local fields in the MV/cm region, while in quasihomogeneous fields, breakdown occurs at about 100 kV/cm. Based on the present results and the interplay with other physical properties (low mechanical resistance, low thermal conductivity, adsorption of water, etc.), silica aerogels and silica aerogel-acrylate-based composites are predicted to have a low potential for electrical insulation. 相似文献
52.
E. Brézin E. Korutcheva Th. Jolicoeur J. Zinn-Justin 《Journal of statistical physics》1993,70(3-4):583-598
Prompted by a recent article of Chakravarty, we reexamine theO(N) vector model with twisted boundary conditions ind dimensions in the various frameworks of the =d–2 expansion, the =4–d expansion, and the large-N expansion. These continuum models describe the physics below the critical temperatureT
c and nearT
c of a latticeO(N) spin model. We determine the effect of the twisting on finite-size scaling functions, for various geometries.On leave from G. Nadjakov Institute of Solid State Physics, 1784 Sofia, Bulgaria. 相似文献
53.
54.
55.
Flögel O Dash J Brüdgam I Hartl H Reissig HU 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(17):4283-4290
Addition of lithiated methoxyallene to pivalonitrile afforded after aqueous workup the expected iminoallene 1 in excellent yield. Treatment of this intermediate with silver nitrate accomplished the desired cyclization to the electron-rich pyrrole derivative 2 in moderate yield. Surprisingly, trifluoroacetic acid converted iminoallene 1 to a mixture of enamide 3 and trifluoromethyl-substituted pyridinol 4 (together with its tautomer 5). A plausible mechanism proposed for this intriguing transformation involves addition of trifluoroacetate to the central allene carbon atom of an allenyl iminium intermediate as crucial step. Enamide 3 is converted to pyridinol 4 by an intramolecular aldol-type process. A practical direct synthesis of trifluoromethyl-substituted pyridinols 4, 10, 11, and 12 starting from typical nitriles and methoxyallene was established. Pyridinol 10 shows an interesting crystal packing with three molecules in the elementary cell and a remarkable helical supramolecular arrangement. Trifluoromethyl-substituted pyridinol 4 was converted to the corresponding pyridyl nonaflate 13, which is an excellent precursor for palladium-catalyzed reactions leading to pyridine derivatives 14-16 in good to excellent yields. The new synthesis of trifluoromethyl-substituted pyridines disclosed here demonstrates a novel reactivity pattern of lithiated methoxyallene which is incorporated into the products as the unusual tripolar synthon B. 相似文献
56.
For the determination of trace elements in neutron irradiated, selenium by gamma-ray spectrometry the separation of the matrix activity is often necessary. In model experiments the decontamination of cation and anion impurities from the matrix solution was investigated by the counter current ion migration. After a processing of 3 hrs the trace activities of Co, Cr, Ga, Na and Zn were decontaminated from Se with a factor of >103. For the trace elements As and Te representing the anionic constituents, a decontamination factor of 3·102 was obtained. 相似文献
57.
Armbruster P Agarwal YK Brüchle W Brügger M Dufour JP Gaggeler H Hessberger FP Hofmann S Lemmertz P Münzenberg G Poppensieker K Reisdorf W Schadel M Schmidt K Schneider JH Schneider WF Sümmerer K Vermeulen D Wirth aG Ghiorso A Gregorich KE Lee D Leino M Moody KJ Seaborg GT Welch RB Wilmarth P Yashita S Frink C Greulich N Herrmann G Hickmann U Hildebrand N Kratz JV Trautman N Fowler MM Hoffman DC Daniels WR von Gunten HR Dornhöfer H 《Physical review letters》1985,54(5):406-409
58.
Bissessur R Haines RI Hutchings DR Brüning R 《Chemical communications (Cambridge, England)》2001,(17):1598-1599
A novel macrocycle-MoS2 nanocomposite has been synthesized and characterized using the exfoliation/restacking properties of LiMoS2, providing the first of a new family of intercalation compounds. 相似文献
59.
F. Chisela D. Gawlik P. Brätter 《Journal of Radioanalytical and Nuclear Chemistry》1987,112(2):293-308
Boron carbide (B4C) and boron nitride (BN) irradiation filters have been constructed and used for epithermal neutron activation analysis of biological materials. The major advantage offered by these filters in this application is the substantial reduction of interfering matrix activities. Consequently some trace elements of interest can be determined instrumentally with minimum delay. The filters are particulary beneficial if utilized in short reactor irradiations for rapid analysis of biological materials. 相似文献
60.
In the EURACHEM/CITAC draft ”Quantifying uncertainty in analytical measurement” estimations of measurement uncertainty in
analytical results for linear calibration are given. In this work these estimations are compared, i.e. the uncertainty deduced
from repeated observations of the sample vs. the uncertainty deduced from the standard residual deviation of the regression.
As a result of this study it is shown that an uncertainty estimation based on repeated observations can give more realistic
values if the condition of variance homogeneity is not correctly fulfilled in the calibration range. The complete calculation
of measurement uncertainty including assessment of trueness is represented by an example concerning the determination of zinc
in sediment samples using ICP-atomic emission spectrometry.
Received: 9 February 2002 Accepted: 17 April 2002 相似文献