全文获取类型
收费全文 | 15540篇 |
免费 | 1233篇 |
国内免费 | 1878篇 |
专业分类
化学 | 7069篇 |
晶体学 | 76篇 |
力学 | 935篇 |
综合类 | 234篇 |
数学 | 6215篇 |
物理学 | 4122篇 |
出版年
2024年 | 41篇 |
2023年 | 277篇 |
2022年 | 334篇 |
2021年 | 407篇 |
2020年 | 501篇 |
2019年 | 450篇 |
2018年 | 401篇 |
2017年 | 503篇 |
2016年 | 635篇 |
2015年 | 461篇 |
2014年 | 738篇 |
2013年 | 1300篇 |
2012年 | 766篇 |
2011年 | 1043篇 |
2010年 | 888篇 |
2009年 | 1065篇 |
2008年 | 1022篇 |
2007年 | 1132篇 |
2006年 | 898篇 |
2005年 | 803篇 |
2004年 | 688篇 |
2003年 | 655篇 |
2002年 | 513篇 |
2001年 | 440篇 |
2000年 | 391篇 |
1999年 | 323篇 |
1998年 | 340篇 |
1997年 | 286篇 |
1996年 | 230篇 |
1995年 | 188篇 |
1994年 | 141篇 |
1993年 | 124篇 |
1992年 | 117篇 |
1991年 | 98篇 |
1990年 | 55篇 |
1989年 | 63篇 |
1988年 | 46篇 |
1987年 | 28篇 |
1986年 | 41篇 |
1985年 | 40篇 |
1984年 | 26篇 |
1983年 | 15篇 |
1982年 | 23篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 19篇 |
1978年 | 12篇 |
1977年 | 15篇 |
1976年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
In this study, we investigated an alternative method for the chemical CO2 reduction reaction in which power ultrasound (488 kHz ultrasonic plate transducer) was applied to CO2-saturated (up to 3%) pure water, NaCl and synthetic seawater solutions. Under ultrasonic conditions, the converted CO2 products were found to be mainly CH4, C2H4 and C2H6 including large amount of CO which was subsequently converted into CH4. We have found that introducing molecular H2 plays a crucial role in the CO2 conversion process and that increasing hydrogen concentration increased the yields of hydrocarbons. However, it was observed that at higher hydrogen concentrations, the overall conversion decreased since hydrogen, a diatomic gas, is known to decrease cavitational activity in liquids. It was also found that 1.0 M NaCl solutions saturated with 2% CO2 + 98% H2 led to maximum hydrocarbon yields (close to 5%) and increasing the salt concentrations further decreased the yield of hydrocarbons due to the combined physical and chemical effects of ultrasound. It was shown that CO2 present in a synthetic industrial flue gas (86.74% N2, 13% CO2, 0.2% O2 and 600 ppm of CO) could be converted into hydrocarbons through this method by diluting the flue gas with hydrogen. Moreover, it was observed that in addition to pure water, synthetic seawater can also be used as an ultrasonicating media for the sonochemical process where the presence of NaCl improves the yields of hydrocarbons by ca. 40%. We have also shown that by using low frequency high-power ultrasound in the absence of catalysts, it is possible to carry out the conversion process at ambient conditions i.e., at room temperature and pressure. We are postulating that each cavitation bubble formed during ultrasonication act as a “micro-reactor” where the so-called Sabatier reaction - - takes place upon collapse of the bubble. We are naming this novel approach as the “Islam-Pollet-Hihn process”. 相似文献
3.
The behaviors of ferromagnetic transition metals of the first period: Fe, Co and Ni are examined within density functional theory calculations in two dimensional carbon extended networks using model structure LiC6. Around geometry optimized structures, the energy-volume equations of states considering non magnetic and spin polarized configurations established ferromagnetic ground states with magnetizations –reduced with respect to the metals’– of 2 μB for FeC6 and 1 μB for CoC6 while no magnetic solution could be identified for NiC6. In the D6h point group of the P6/mmm space group lm decomposition of the d states results with increasing energy into doublet state E1g with d(x2-y2) and d(xy); singlet state A1g d(z2) and doublet state E2g d(xz) and d(yz) lying on EF and responsible of the onset of magnetic moments. This was mirrored via molecular orbital approach with a construct of Fe embedded between two extended carbon networks thus validating the model structure proposed for TC6 compounds. The 100% polarization in one spin channel allows proposing potential uses in spintronics applications. 相似文献
4.
In this paper, we study the existence of the uniformly minimum risk equivariant (UMRE) estimators of parameters in a class
of normal linear models, which include the normal variance components model, the growth curve model, the extended growth curve
model, and the seemingly unrelated regression equations model, and so on. The necessary and sufficient conditions are given
for the existence of UMRE estimators of the estimable linear functions of regression coefficients, the covariance matrixV and (trV)α, where α > 0 is known, in the models under an affine group of transformations for quadratic losses and matrix losses, respectively.
Under the (extended) growth curve model and the seemingly unrelated regression equations model, the conclusions given in literature
for estimating regression coefficients can be derived by applying the general results in this paper, and the sufficient conditions
for non-existence of UMRE estimators ofV and tr(V) are expanded to be necessary and sufficient conditions. In addition, the necessary and sufficient conditions that there
exist UMRE estimators of parameters in the variance components model are obtained for the first time. 相似文献
5.
6.
Mathematical Notes - 相似文献
7.
Lubomír Kubáček 《Applications of Mathematics》2006,51(6):565-582
Some remarks to problems of point and interval estimation, testing and problems of outliers are presented in the case of multivariate
regression model.
This work was supported by the Council of Czech Government J14/98:153100011. 相似文献
8.
9.
Several new families of c‐Bhaskar Rao designs with block size 4 are constructed. The necessary conditions for the existence of a c‐BRD (υ,4,λ) are that: (1)λmin=?λ/3 ≤ c ≤ λ and (2a) c≡λ (mod 2), if υ > 4 or (2b) c≡ λ (mod 4), if υ = 4 or (2c) c≠ λ ? 2, if υ = 5. It is proved that these conditions are necessary, and are sufficient for most pairs of c and λ; in particular, they are sufficient whenever λ?c ≠ 2 for c > 0 and whenever c ? λmin≠ 2 for c < 0. For c < 0, the necessary conditions are sufficient for υ> 101; for the classic Bhaskar Rao designs, i.e., c = 0, we show the necessary conditions are sufficient with the possible exception of 0‐BRD (υ,4,2)'s for υ≡ 4 (mod 6). © 2002 Wiley Periodicals, Inc. J Combin Designs 10: 361–386, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jcd.10009 相似文献
10.