全文获取类型
收费全文 | 1186篇 |
免费 | 124篇 |
国内免费 | 98篇 |
专业分类
化学 | 778篇 |
晶体学 | 22篇 |
力学 | 61篇 |
综合类 | 4篇 |
数学 | 171篇 |
物理学 | 372篇 |
出版年
2023年 | 17篇 |
2022年 | 21篇 |
2021年 | 32篇 |
2020年 | 28篇 |
2019年 | 34篇 |
2018年 | 36篇 |
2017年 | 30篇 |
2016年 | 39篇 |
2015年 | 48篇 |
2014年 | 70篇 |
2013年 | 92篇 |
2012年 | 92篇 |
2011年 | 100篇 |
2010年 | 65篇 |
2009年 | 48篇 |
2008年 | 58篇 |
2007年 | 61篇 |
2006年 | 63篇 |
2005年 | 45篇 |
2004年 | 50篇 |
2003年 | 29篇 |
2002年 | 27篇 |
2001年 | 25篇 |
2000年 | 25篇 |
1999年 | 18篇 |
1998年 | 11篇 |
1997年 | 18篇 |
1996年 | 15篇 |
1995年 | 17篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1980年 | 21篇 |
1979年 | 6篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1972年 | 4篇 |
1969年 | 4篇 |
排序方式: 共有1408条查询结果,搜索用时 15 毫秒
991.
In order to evaluate the bond strength between the reinforcement and concrete after fire damage, a combination of acoustic through-transmission and pull-out tests were used. Previous studies have shown a 25% decrease in the ultrasonic pulse velocity at 90% of the maximum load at room temperature. The specimens were kept in the oven at an elevated temperature for 1, 2, or 3 h. They were then removed and cooled to room temperature. Inspection was conducted using a high-power ultrasonic pulse velocity system while a pull-out load was applied. The correlation between preheated temperature, acoustic wave velocity, and the applied load was analyzed. Initial results show that bond strength and pulse velocity decreased substantially as the temperature or the heating time increased. 相似文献
992.
993.
994.
995.
Zhixiong?GuoEmail author Slew?Kan?Wan Kyunghan?Kim Chakravarthi?Kosaraju 《Optical Review》2003,10(5):415-421
The diffusion model that is an approximation of the equation of radiation transfer is typically used to describe photon migration in scattering-dominant media. In general biological tissue is highly scattering and very weakly absorbing against near-infrared light, yet it is heterogeneous and may contain relatively highly absorbing or low-scattering regions. Here applicability of the diffusion approximation over the radiative transfer theory for describing ultrafast laser transport in biological tissues is numerically studied and investigated over different kinds of tissue conditions and geometries. Tissues having tumors of different sizes, locations and nature as well as dual-tumor and low-scattering conditions are considered. Radiation transfer analysis is taken as a comparison objective and it is initially proved to be accurate in benchmark comparisons with Monte Carlo simulation. The results predict systematically about the compatible conditions where and when we can use the diffusion approximation and the conditions in which the diffusion approximation may provide misleading results. 相似文献
996.
The transverse magnetoresistance in films of nickel, iron, and several alloys is investigated when there is a current along the [100] and [110] directions as a function of the angle between the direction of the field (16 kG) and the plane of the film. The results obtained are compared with experimental data and with theory for bulk single crystals. An explanation of the anomalies observed in the film is given.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 7–10, December, 1973. 相似文献
997.
Two series of 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl trans-4-n-alkylcyclohexylcarboxylates and 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl 4-(trans-4-n-alkylcyclohexyl)benzoates have been prepared. Their mesmorphic properties were observed and measured by polarizing optical microscopy and differential scanning calorimetry. The relationship between structures and properties is discussed, showing that the ferroelectric phase can be observed when chiral terminal chains exist. 相似文献
998.
999.
The structure of guettardine 1, an indole alkaloid isolated from the bark of (Rubiaceae), was determined from an analysis of its MS, 1H and 13C NMR spectral data, and by a chemical correlation with dihydrocorynantheol 7. Guettardine 1 can be considered to be an intermediate between the Corynanthé and the cinchonamine groups of alkaloids which are precursors of quinine and its derivatives. 相似文献
1000.
Methylperoxy radicals were generated by the flash photolysis of azomethane–oxygen mixtures. The observed broadband spectrum of the CH3O2 radical is similar, but not identical to those reported previously. The CH3O2 decay followed second-order kinetics at high CH3O2 concentrations with k4' = (2.5 ± 0.3) × 108 liter/mol·sec (23 ± 2°C); 2CH3O2 → products (4). Because of the potential loss of CH3O2 through the reactions with HO2 and CH3O radicals subsequently formed in this system, simulations suggest that the true k4 is in the range: 2.5 × 108 ≥ k4 ≥ 2.3 × 108 liter/mol·sec. Deviations from linearity of the plot of the reciprocal of the CH3O2 absorbance versus time were seen at long times and were attributed to the reaction (5) with an apparent rate constant k5' ? (1.6 ± 0.4) × 105 liter/mol·sec; CH3O2 + Me2N2 → product (5). The CH3O2–SO2 reaction, CH3O2 + SO2 → products (16), was studied by observing CH3O2 decay in flashed mixtures of Me2N2, O2, and SO2. The results gave the apparent second-order rate constant k16' ? (6.4 ± 1.4) × 106 liter/mol·sec. It appears likely that each occurrence of reaction (5) and (16) is followed by the loss of an additional CH3O2 radical and that k5 ? k5'/2 and k16 ? k16'/2. Our findings suggest that a significant fraction of the SO2 oxidation in a sunlight-irradiated NOx?RH-polluted atmosphere, may occur by reaction with CH3O2 as well as from the HO and HO2 reactions. 相似文献