全文获取类型
收费全文 | 203164篇 |
免费 | 6437篇 |
国内免费 | 3573篇 |
专业分类
化学 | 99523篇 |
晶体学 | 2308篇 |
力学 | 8165篇 |
综合类 | 116篇 |
数学 | 18167篇 |
物理学 | 57527篇 |
无线电 | 27368篇 |
出版年
2022年 | 1820篇 |
2021年 | 2317篇 |
2020年 | 2433篇 |
2019年 | 2418篇 |
2018年 | 2576篇 |
2017年 | 2491篇 |
2016年 | 4016篇 |
2015年 | 3032篇 |
2014年 | 4267篇 |
2013年 | 8814篇 |
2012年 | 7924篇 |
2011年 | 9294篇 |
2010年 | 6513篇 |
2009年 | 6716篇 |
2008年 | 8467篇 |
2007年 | 8492篇 |
2006年 | 7913篇 |
2005年 | 7373篇 |
2004年 | 6311篇 |
2003年 | 5456篇 |
2002年 | 5426篇 |
2001年 | 5697篇 |
2000年 | 4672篇 |
1999年 | 4007篇 |
1998年 | 3539篇 |
1997年 | 3439篇 |
1996年 | 3182篇 |
1995年 | 2972篇 |
1994年 | 2850篇 |
1993年 | 2548篇 |
1992年 | 2777篇 |
1991年 | 2839篇 |
1990年 | 2564篇 |
1989年 | 2501篇 |
1988年 | 2373篇 |
1987年 | 2056篇 |
1986年 | 1999篇 |
1985年 | 2472篇 |
1984年 | 2544篇 |
1983年 | 2129篇 |
1982年 | 2268篇 |
1981年 | 2140篇 |
1980年 | 2057篇 |
1979年 | 2157篇 |
1978年 | 2308篇 |
1977年 | 2185篇 |
1976年 | 2232篇 |
1975年 | 2126篇 |
1974年 | 2158篇 |
1973年 | 2156篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
Summary Two polyampholyte copolymers based on sodium-2-acrylamido-2-methylsulfonate (NaAMPS) and methacryloyloxyethyltrimethylammonium
chloride (MADQUAT) have been prepared either in homogeneous solution or by an inverse microemulsion polymerization technique.
The copolymer microstructure was shown to depend on the method of preparation. The microemulsion polymerization yields copolymers
with a monomer sequence distribution not far from random while those obtained by polymerization in solution have a strong
tendency to alternation. The aqueous-solution properties of the two samples have been investigated by viscometry. The results
show that the charge distribution along the copolymer chain affects considerably its conformation, in good agreement with
recent theoretical studies.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994 相似文献
943.
C. Saragovi F. Labenski S. M. Duhalde S. Acebal R. Venegas 《Hyperfine Interactions》1994,91(1):765-769
Clay fractions of a Mollisol sample as is, treated with ammonium oxalate (AO), with dithionite-citrate-bicarbonate (DCB) and with dithionite-ethilene-diamine-tetraacetic acid (D-EDTA) methods, were studied. Illite-montmorillonites together with hematites, goethites and maghemites, all of the AI-substituted and with a wide range of sizes, were identified. It is found that the AO attack extracts little iron, whereas the other two attacks extract the magnetic signal. Furthermore, the DCB attack facilitates the reduction of the Fe3+ ions, while the D-EDTA method does not. Instead, this attack extracts more clay mineral Fe ions. A comparison with large grain soil samples is made. 相似文献
944.
G. F. R. Ellis 《General Relativity and Gravitation》1992,24(10):1047-1068
This paper gives a covariant formalism enabling investigation of the possibility of change of signature in classical General Relativity, when the geometry is that of a Robertson-Walker universe. It is shown that such changes are compatible with the Einstein field equations, both in the case of a barotropic fluid and of a scalar field. A criterion is given for when such a change of signature should take place in the scalar field case. Some examples show the kind of resulting exact solutions of the field equations. 相似文献
945.
Data-adaptive evolutionary spectral estimation 总被引:3,自引:0,他引:3
We present a novel data-adaptive estimator for the evolutionary spectrum of nonstationary signals. We model the signal at a frequency of interest as a sinusoid with a time-varying amplitude, which is accurately represented by an orthonormal basis expansion. We then compute a minimum mean-squared error estimate of this amplitude and use it to estimate the time-varying spectrum at that frequency, all while minimizing the interference from the signal components at other frequencies. Repeating the process over all frequencies, we obtain a power distribution that is consistent with the Wold-Cramer evolutionary spectrum and reduces to Capon's (1969) method for the stationary case. Our estimator possesses desirable properties in terms of time-frequency resolution and positivity and is robust in the spectral estimation of noisy nonstationary data. We also propose a new estimator for the autocorrelation of nonstationary signals. This autocorrelation estimate is needed in the data-adaptive spectral estimation. We illustrate the performance of our estimator using simulation examples and compare it with the recently presented evolutionary periodogram and the bilinear time-frequency distribution with exponential kernels 相似文献
946.
B. F. Araujo H. T. Matsuda T. A. Kuada J. A. Araujo 《Journal of Radioanalytical and Nuclear Chemistry》1992,166(1):75-83
In the actinide separation process using TBP as extractant, hydrolytic and radiolytic reactions cause a gradual decomposition of the extractant, resulting in the formation of degradation products which can affect process performance adversely. The TBP diluent has to be cleaned from these degradation products before it is reused. This paper deals with a procedure for solvent cleanup, using a fourstage continuous contactor for increasing the scrubbing efficiency. 相似文献
947.
M F Yama 《The Journal of the Acoustical Society of America》1992,91(1):327-335
In the present study detection under diotic (NoSo) and dichotic (NoS pi) listening conditions in a forward masking paradigm was investigated. Both the level of a noise masker and the temporal separation between the masker and a 250-Hz tone burst served as independent variables. Results showed that most of the variance in the data could be accounted for by the amount of masking in the NoSo condition, independent of the value of the temporal parameter, which itself accounted for only 1.4% of the variance that remained. Once the data were corrected for NoSo masking effectiveness, the MLD was found to decrease by only 1.4 dB as temporal separation increased from 5-100 ms, which is consistent with a very long time constant for the binaural system. Consistent with this finding, it was shown that slope changes of the growth of masking functions, for simultaneous as compared to forward masking, were similar for both the NoSo and NoS pi conditions. 相似文献
948.
本文介绍作者近些年来对椭圆、矩形、扇形、任意三角形、正多边形、环形等各种非圆断面通道层流摩擦阻力与换热理论的研究成果,给出大量新的速度与温度场表达式,以及摩擦阻力与换热的新的计算公式。本文对传统的层流阻力与换热理论进行了一次系统的补充与完善,同时考虑到工程需要,对所有繁杂的理论公式都进行了化简,给出形式简单而且精度相当高的近似公式。 相似文献
949.
Summary A Modified-Melt-Powder-Melt-Growth (MMPMG) technique has been developed for the preparation of melt-textured Y-Ba−Cu−O pellets.
To test their performances, the samples were characterized with respect to their microstructure, pinning strength and interaction
force with permanent magnets. A superconducting magnetic bearing was built and integrated in a levitating flywheel system
for energy storage.
Paper presented at the “VII Congresso SATT?, Torino, 4–7 October 1994.
Under a Fondazione Confalonieri grant. 相似文献
950.