全文获取类型
收费全文 | 146779篇 |
免费 | 1457篇 |
国内免费 | 394篇 |
专业分类
化学 | 80142篇 |
晶体学 | 2387篇 |
力学 | 6063篇 |
综合类 | 3篇 |
数学 | 13566篇 |
物理学 | 46469篇 |
出版年
2021年 | 1030篇 |
2020年 | 1182篇 |
2019年 | 1250篇 |
2018年 | 1638篇 |
2017年 | 1689篇 |
2016年 | 2540篇 |
2015年 | 1559篇 |
2014年 | 2348篇 |
2013年 | 6031篇 |
2012年 | 4840篇 |
2011年 | 5932篇 |
2010年 | 4180篇 |
2009年 | 4206篇 |
2008年 | 5722篇 |
2007年 | 5504篇 |
2006年 | 5376篇 |
2005年 | 4941篇 |
2004年 | 4447篇 |
2003年 | 4114篇 |
2002年 | 4005篇 |
2001年 | 5541篇 |
2000年 | 3970篇 |
1999年 | 3072篇 |
1998年 | 2109篇 |
1997年 | 2207篇 |
1996年 | 2069篇 |
1995年 | 1915篇 |
1994年 | 1912篇 |
1993年 | 1725篇 |
1992年 | 2213篇 |
1991年 | 2298篇 |
1990年 | 2134篇 |
1989年 | 2099篇 |
1988年 | 2076篇 |
1987年 | 2085篇 |
1986年 | 1909篇 |
1985年 | 2370篇 |
1984年 | 2391篇 |
1983年 | 1909篇 |
1982年 | 1913篇 |
1981年 | 1751篇 |
1980年 | 1737篇 |
1979年 | 2026篇 |
1978年 | 2100篇 |
1977年 | 2046篇 |
1976年 | 1975篇 |
1975年 | 1859篇 |
1974年 | 1842篇 |
1973年 | 1874篇 |
1972年 | 1235篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
高分辨电感耦合等离子质谱测定黄连中的微量元素 总被引:8,自引:2,他引:6
文章采用高分辨电感耦合等离子体质谱(HR-ICP-MS)的方法,测定了中草药中四种黄连——味连、雅连、马尾连和胡黄连中的微量元素。结果表明:在所有16种稀土元素中,La,Ce,Nd,Gd,Sm,Sc含量较高,胡黄连的稀土总量为5.18μg·g~(-1),是所有四种黄连的最高者,其次是雅连和马尾连,最低的是味连(1.26μg·g~(-1))。而对于与清热解毒有关的金属无素如Mg,Mn,Zn,Ca和Fe元素,雅连中Mg,Mn,Zn的含量最高。高Fe和Ca的含量存在于马尾连和胡连之中。在测定不同提取方法的提取物中发现,只有部分的稀土元素能被醇和水所提取,它们分别是31.6%和4.1%。清热解毒元素的提取率明显高于稀土元素。 相似文献
922.
The effect of the lattice deformation on the electronic spectra of TlGaS2, TlGaSe2, and TlInS2 layered semiconductor crystals is analyzed. It is shown that changes in the band gap of these semiconductors due to thermal expansion and a change in the composition under hydrostatic or uniaxial pressure can be described within a unified model of the deformation potential. The main feature of this model is the inclusion of deformation potentials with different signs, which is characteristic of other semiconductors with a layered structure. An analysis of the lattice deformation of the studied semiconductors in terms of the proposed model of the deformation potential has revealed that, in the immediate vicinity of the phase transitions, the crystal lattice under pressure undergo an unusual deformation. 相似文献
923.
The existing model for multivariate skew normal data does not cohere with the joint distribution of a random sample from a
univariate skew normal distribution. This incoherence causes awkward interpretation for data analysis in practice, especially
in the development of the sampling distribution theory. In this paper, we propose a refined model that is coherent with the
joint distribution of the univariate skew normal random sample, for multivariate skew normal data. The proposed model extends
and strengthens the multivariate skew model described in Azzalini (1985,Scandinavian Journal of Statistics,12, 171–178). We present a stochastic representation for the newly proposed model, and discuss a bivariate setting, which confirms
that the newly proposed model is more plausible than the one given by Azzalini and Dalla Valle (1996,Biometrika,83, 715–726). 相似文献
924.
Emerson T da SilvaEdson L.S Lima 《Tetrahedron letters》2003,44(18):3621-3624
The ability of 1,3-dimethyl-5-acetyl-barbituric acid (DAB) to react with primary amines in a chemoselective fashion was applied to the synthesis of a selectively protected spermidine. 相似文献
925.
926.
A new hydrophilic copolymer having three kinds of hydroxyl groups, i.e. primary OH on -hydroxy allyl alcohol component, secondary OH on cyclic hemiacetal component and tertiary OH on -hydroxy acrylate component, was prepared by reducing a ‘polylactone’, poly(1-oxa-2-oxobutane-1,4:3,3-tetrayl), with NaBH4 in mixed solvents containing an alcohol. The structural analyses were performed by the solution 1H NMR and the solid state 13C NMR spectroscopies of the reduced samples. The composition of the three components turned out to be variable to the alcohol species used for the mixed solvent. 相似文献
927.
J. Haslinger T. Kozubek K. Kunisch G. Peichl 《Journal of Mathematical Analysis and Applications》2004,290(2):665-685
This contribution combines a shape optimization approach to free boundary value problems of Bernoulli type with an embedding domain technique. A theoretical framework is developed which allows to prove continuous dependence of the primal and dual variables in the resulting saddle point problems with respect to the domain. This ensures the existence of a solution of a related shape optimization problem in a sufficiently large class of admissible domains. 相似文献
928.
A. Ikegami Y. Kimura H. Suzuki T. Sato T. Tanigaki O. Kido M. Kurumada Y. Saito C. Kaito 《Surface science》2003,540(2-3):395-400
Titanium carbide formation by the solid–solid reaction on the surface of Ti nanoparticles was studied in situ using a high-resolution transmission electron microscope with a heating stage. The cross-sectional image of the Ti surface was clearly observed. Vacuum-deposited carbon covered the whole the surface of Ti nanoparticles in spite of the partly evaporation on the nanoparticle surface. The diffusion of the carbon atoms inside the Ti nanoparticles depended on the size of the nanoparticles. When the Ti nanoparticle diameter was less than 30 nm, carbon atoms diffused into the Ti nanoparticle and formed TiC. The superstructure of the Ti nanoparticles was observed, which revealed the growth process of TiC to be the diffusion of carbon atoms. For Ti nanoparticles with diameter larger than 30 nm it was observed that diffusion of Ti atoms into the carbon layer was dominant, which resulted in formation of TiC in the carbon layer at the surface of Ti nanoparticles. 相似文献
929.
A central problem that arises in evolutionary biology is that of displaying partitions of subsets of a finite set X on a tree whose vertices are partially labelled with the elements of X. Such a tree is called an X-tree and, for a collection of partitions of subsets of X, characterisations for the existence and uniqueness of an X-tree that displays have been previously given in terms of chordal graphs. In this paper, we obtain two closely related characterisations also in terms of chordal graphs. The first describes when identifies an X-tree, and the second describes when a compatible subset of is of maximum size. 相似文献
930.
A polarization-insensitive 40-Gb/s time-division demultiplexer was demonstrated in a polarization-diversity loop configuration. The power penalty was measured to be 2.3 dB at a bit error rates of 10-9 for 10 Gb/s output signals. 相似文献