全文获取类型
收费全文 | 83700篇 |
免费 | 7517篇 |
国内免费 | 13425篇 |
专业分类
化学 | 73204篇 |
晶体学 | 2271篇 |
力学 | 2223篇 |
综合类 | 617篇 |
数学 | 6531篇 |
物理学 | 19796篇 |
出版年
2024年 | 215篇 |
2023年 | 1269篇 |
2022年 | 2712篇 |
2021年 | 2588篇 |
2020年 | 2828篇 |
2019年 | 2957篇 |
2018年 | 2346篇 |
2017年 | 3097篇 |
2016年 | 3094篇 |
2015年 | 2795篇 |
2014年 | 3560篇 |
2013年 | 6849篇 |
2012年 | 5037篇 |
2011年 | 5476篇 |
2010年 | 4543篇 |
2009年 | 5578篇 |
2008年 | 5312篇 |
2007年 | 5341篇 |
2006年 | 5019篇 |
2005年 | 4409篇 |
2004年 | 4355篇 |
2003年 | 3499篇 |
2002年 | 3409篇 |
2001年 | 2456篇 |
2000年 | 2207篇 |
1999年 | 1756篇 |
1998年 | 1532篇 |
1997年 | 1341篇 |
1996年 | 1271篇 |
1995年 | 1256篇 |
1994年 | 1101篇 |
1993年 | 831篇 |
1992年 | 835篇 |
1991年 | 563篇 |
1990年 | 412篇 |
1989年 | 362篇 |
1988年 | 347篇 |
1987年 | 256篇 |
1986年 | 190篇 |
1985年 | 190篇 |
1984年 | 181篇 |
1983年 | 66篇 |
1982年 | 138篇 |
1981年 | 194篇 |
1980年 | 171篇 |
1979年 | 177篇 |
1978年 | 136篇 |
1977年 | 113篇 |
1976年 | 92篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
The Li oxides species formed on Li over-deposited V2O5 thin film surfaces have been studied by using X-ray and UV induced photoelectron spectroscopy (XPS and UPS). The photoelectron spectroscopic data show that the Li over-deposited V2O5 system itself is not stable. Further chemical decomposition reactions are taken place even under UHV conditions and lead to form Li2O and Li2O2 compounds on the surface. The formation of Li2O2 causes to arise an emission line at about 11.3 eV in the valence band spectra. 相似文献
102.
Lin and Zhang (J. Roy. Statist. Soc. Ser. B 61 (1999) 381) proposed the generalized additive mixed model (GAMM) as a framework for analysis of correlated data, where normally distributed random effects are used to account for correlation in the data, and proposed to use double penalized quasi-likelihood (DPQL) to estimate the nonparametric functions in the model and marginal likelihood to estimate the smoothing parameters and variance components simultaneously. However, the normal distributional assumption for the random effects may not be realistic in many applications, and it is unclear how violation of this assumption affects ensuing inferences for GAMMs. For a particular class of GAMMs, we propose a conditional estimation procedure built on a conditional likelihood for the response given a sufficient statistic for the random effect, treating the random effect as a nuisance parameter, which thus should be robust to its distribution. In extensive simulation studies, we assess performance of this estimator under a range of conditions and use it as a basis for comparison to DPQL to evaluate the impact of violation of the normality assumption. The procedure is illustrated with application to data from the Multicenter AIDS Cohort Study (MACS). 相似文献
103.
Brandon Howe 《Journal of luminescence》2004,109(1):51-59
Host-lattice emission, energy transfer and degradation processes are characterized in undoped and Eu-doped BaMgAl10O17. Undoped BaMgAl10O17 exhibits a broad emission centered at 265 nm when excited at wavelengths shorter than 190 nm. This emission is assigned to exciton recombination at Ba-O groups in the cation layer of the lattice. The emission exhibits excellent overlap with the excitation band of Eu2+ in this host, providing a means of host-to-activator energy transfer in the doped phosphor. The exciton emission is relatively stable to thermal damage, but undergoes a peak shift and significant decrease in intensity after exposure to VUV radiation. Heating of VUV-damaged materials in air leads to some repair of the spectral properties. 相似文献
104.
A. Y. Miguelote N. A. Tomimura Anzhong Wang 《General Relativity and Gravitation》2004,36(8):1883-1918
Perfect fluid with kinematic self-similarity is studied in 2+1 dimensional spacetimes with circular symmetry, and various exact solutions to the Einstein field equations are given. These include all the solutions of dust and stiff perfect fluid with self-similarity of the first kind, and all the solutions of perfect fluid with a linear equation of state and self-similarity of the zeroth and second kinds. It is found that some of these solutions represent gravitational collapse, and the final state of the collapse can be either a black hole or a null singularity. It is also shown that one solution can have two different kinds of kinematic self-similarity. 相似文献
105.
A detailed study of the generalized M2 factor of hard-edged diffracted beams based on the truncated second-order moments method, asymptotic analysis and self-convergent beam width approach is performed. The dependence of the generalized M2 factor on the parameters characterizing the spatial profile, and beam truncation, etc. is analyzed. 相似文献
106.
107.
An m‐covering of a graph G is a spanning subgraph of G with maximum degree at most m. In this paper, we shall show that every 3‐connected graph on a surface with Euler genus k ≥ 2 with sufficiently large representativity has a 2‐connected 7‐covering with at most 6k ? 12 vertices of degree 7. We also construct, for every surface F2 with Euler genus k ≥ 2, a 3‐connected graph G on F2 with arbitrarily large representativity each of whose 2‐connected 7‐coverings contains at least 6k ? 12 vertices of degree 7. © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 26–36, 2003 相似文献
108.
Gel polymer electrolyte (GPE) films comprising of poly(vinylidenefluoride), propylene carbonate, ethylene carbonate and zinc trifluoromethane sulfonate are prepared and characterized. The composition of GPE is optimized to contain minimum liquid components with a maximum specific conductivity of 3.94×10−3 S cm−1 at (25±1) °C. A detailed investigation on the properties such as ionic conductivity, transport number, electrochemical stability window, reversibility of Zn/Zn2+ couple and Zn/gel electrolyte interfacial stability have been carried out. The ionic conductivity follows a VTF behaviour with an activation energy of about 0.0014 eV. Cationic transport number varies from 0.51 at 25 °C to 0.18 at 70 °C. Several cells have been assembled with GPE as the electrolyte, zinc as the anode, γ-MnO2 as the cathode and their charge–discharge behaviour followed. Capacity values of 105, 82, 64 and 37 mAh/g of MnO2 have been achieved at 10, 50, 100 and 200 μA/cm2 discharge current densities, respectively. The discharge capacity values are almost constant for about 55 cycles for all values of current densities. Cyclic voltammetric study of MnO2 electrode in Zn/GPE/MnO2 cell clearly shows intercalation/deintercalation of Zn2+. 相似文献
109.
Maculalactones A, B and C from the marine cyanobacterium Kyrtuthrix maculans are amongst the only compounds based on the tribenzylbutyrolactone skeleton known in nature and (+) maculalactone A from the natural source possesses significant biological activity against various marine herbivores and marine settlers. We now report a concise synthesis of racemic maculalactone A in five steps from inexpensive starting materials. Maculalactones B and C were synthesized by a minor modification to this procedure, and the synthetic design also permitted an asymmetric synthesis of maculalactone A to be achieved in around 85% ee. The (+) and (−) enantiomers of maculalactone A were assigned, respectively, to the S and R configurations on the basis of the chiral selectivity expected for catecholborane reduction of an unsymmetrical ketone in the presence of Corey's oxazoborolidine catalyst. Surprisingly, it appeared that natural (+) maculalactone A was biosynthesized in K. maculans in a partially racemic form, comprising ca. 90-95% of the (S) enantiomer and 5-10% of its (R) enantiomer. Coincidentally therefore, the percentage enantiomeric excess of the product obtained from asymmetric synthesis almost exactly matched that found in nature. 相似文献
110.