全文获取类型
收费全文 | 4302篇 |
免费 | 635篇 |
国内免费 | 487篇 |
专业分类
化学 | 1745篇 |
晶体学 | 42篇 |
力学 | 504篇 |
综合类 | 68篇 |
数学 | 834篇 |
物理学 | 2231篇 |
出版年
2023年 | 52篇 |
2022年 | 101篇 |
2021年 | 115篇 |
2020年 | 113篇 |
2019年 | 96篇 |
2018年 | 96篇 |
2017年 | 177篇 |
2016年 | 191篇 |
2015年 | 156篇 |
2014年 | 219篇 |
2013年 | 426篇 |
2012年 | 284篇 |
2011年 | 323篇 |
2010年 | 247篇 |
2009年 | 285篇 |
2008年 | 275篇 |
2007年 | 297篇 |
2006年 | 256篇 |
2005年 | 214篇 |
2004年 | 226篇 |
2003年 | 180篇 |
2002年 | 135篇 |
2001年 | 108篇 |
2000年 | 84篇 |
1999年 | 96篇 |
1998年 | 78篇 |
1997年 | 95篇 |
1996年 | 81篇 |
1995年 | 54篇 |
1994年 | 52篇 |
1993年 | 41篇 |
1992年 | 39篇 |
1991年 | 37篇 |
1990年 | 20篇 |
1989年 | 26篇 |
1988年 | 21篇 |
1987年 | 12篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 13篇 |
1983年 | 5篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1973年 | 3篇 |
排序方式: 共有5424条查询结果,搜索用时 15 毫秒
51.
调制不稳定性对级联放大光纤传输系统信噪比的影响 总被引:1,自引:0,他引:1
在考虑光纤损耗和级联放大器的情况下,推导了调制不稳定性的产生条件和增益的普适解析表达式,分析了调制不稳定性对信噪比的影响,给出了一个新的计算信噪比的表达式。 相似文献
52.
本文讨论嵌套病例对照研究中相对危险率的估计问题,引入了相对危险率的两步估计,并在一般嵌套病例对照抽样的假设下讨论了相对危险率的两步估计的相合性问题,最后给出了几个例子。 相似文献
53.
The behavior of the specific heat cp, effective mass M*, and the thermal expansion coefficient of a Fermi system located near the fermion condensation quantum phase transition (FCQPT) is considered. We observe the first type behavior if the system is close to FCQPT: the specific heat
,
, while the thermal expansion coefficient
. Thus, the Grüneisen ratio Γ(T)=/cp does not diverges. At the transition region, where the system passes over from the non-Fermi liquid to the Landau Fermi liquid, the ratio diverges as
. When the system becomes the Landau Fermi liquid, Γ(T,r)∝1/r, with r being a distance from the quantum critical point. Provided the system has undergone FCQPT, the second type takes place: the specific heat behaves as
, M*∝1/T, and =a+bT with a,b being constants. Again, the Grüneisen ratio diverges as
. 相似文献
54.
55.
56.
Optimization of plate buckling resistance is very complicated, because the in-plane stress resultants in the prebuckled state of a plate are functions of thickness distribution. This paper discusses the problem of finding the optimum thickness distribution of isotropic plate structures, with a given volume and layout, that would maximise the buckling load. A simple numerical method using the finite-element analysis is presented to obtain the optimum thickness distribution. Optimum designs of compression-loaded rectangular plates with different boundary conditions and plate aspect ratios are obtained by using the proposed method. Optimum designs from earlier studies and the methods of buckling analysis used to attain these results are discussed and compared with the designs from the proposed method. This paper also examines the reliability of the optimality criterion generally used for plate buckling optimization, which is based on the uniform strain energy density. 相似文献
57.
同位素稀释-火花源质谱法分析人参样品中微量元素 总被引:2,自引:0,他引:2
本文采用同位素稀释-火花源质谱法定量分析了吉林野山参,园参及高丽参中30多种微量元素的含量。测定下限2μg/g,相对标准偏差在5~28%以内。 相似文献
58.
苯乙烯和丙烯酸 β 羟乙酯不同温度的竞聚率测定吴平平吴玉芳陆琛杨全兴韩哲文(华东理工大学材料科学研究所上海200237)陈顺喜朱清仁(中科院中国科技大学结构分析开放实验室合肥230026)关键词苯乙烯,丙烯酸 β 羟乙酯,共聚合,竞聚率丙烯酸 β?.. 相似文献
59.
60.
The precision of isotopic measurements of Pb by thermal ionization mass spectrometry (TIMS) is limited by the fact that this element does not possess an invariant isotope ratio that can be used for the correction of mass fractionation by internal normalization. Multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) can overcome this limitation, because with plasma ionization, elements with overlapping mass ranges are thought to display identical mass discrimination. With respect to Pb, this can be exploited by the addition of Tl to the sample solutions; the mass discrimination factor obtained for Tl can then be used for the correction of the measured Pb isotope ratios. In this article we present the results of a detailed study that investigates the accuracy and precision of such an external correction technique for mass discrimination based upon the results of multiple analyses of a mixed standard solution of NIST SRM-981 Pb and SRM-997 Tl. Our data indicate that normalization of the Pb isotope ratios to the certified isotopic composition of SRM-997 Tl produces Pb isotopic results that are significantly lower than recently published reference values by TIMS. This systematic offset can be eliminated by renormalization of the Pb data to a different Tl isotopic composition to obtain an empirically determined mass discrimination factor for Pb that generates accurate results. It is furthermore shown that a linear law is least suited for the correction of mass discrimination, whereas a power or exponential law function provide significantly more accurate and precise results. In detail, it appears that a power law may provide the most appropriate correction procedure, because the corrected Pb isotope ratios display less residual correlations with mass discrimination compared to the exponentially corrected data. Using an exponential or power law correction our results, obtained over a period of over seven months, display a precision (2σ) of better than 60 parts per million (ppm) for 208Pb/206Pb and 207Pb/206Pb and of better than 350 ppm for 206Pb/204Pb, 207Pb/204Pb/204Pb, and 208Pb/204Pb. This represents a significant improvement compared to conventional TIMS techniques and demonstrates the potential of MC-ICPMS for routine, high-precision measurements of Pb isotopic compositions. 相似文献