全文获取类型
收费全文 | 330159篇 |
免费 | 3051篇 |
国内免费 | 933篇 |
专业分类
化学 | 164976篇 |
晶体学 | 5377篇 |
力学 | 15532篇 |
综合类 | 7篇 |
数学 | 38465篇 |
物理学 | 109786篇 |
出版年
2021年 | 3135篇 |
2020年 | 3330篇 |
2019年 | 3760篇 |
2018年 | 5198篇 |
2017年 | 5441篇 |
2016年 | 7373篇 |
2015年 | 4143篇 |
2014年 | 7031篇 |
2013年 | 15908篇 |
2012年 | 12382篇 |
2011年 | 14832篇 |
2010年 | 11016篇 |
2009年 | 10919篇 |
2008年 | 13245篇 |
2007年 | 13194篇 |
2006年 | 12141篇 |
2005年 | 10523篇 |
2004年 | 9838篇 |
2003年 | 8692篇 |
2002年 | 8677篇 |
2001年 | 10398篇 |
2000年 | 7660篇 |
1999年 | 5982篇 |
1998年 | 4977篇 |
1997年 | 4816篇 |
1996年 | 4485篇 |
1995年 | 3866篇 |
1994年 | 3829篇 |
1993年 | 3704篇 |
1992年 | 4105篇 |
1991年 | 4309篇 |
1990年 | 4106篇 |
1989年 | 4015篇 |
1988年 | 3783篇 |
1987年 | 3922篇 |
1986年 | 3686篇 |
1985年 | 4633篇 |
1984年 | 4695篇 |
1983年 | 3891篇 |
1982年 | 3979篇 |
1981年 | 3701篇 |
1980年 | 3659篇 |
1979年 | 3914篇 |
1978年 | 3905篇 |
1977年 | 3907篇 |
1976年 | 3879篇 |
1975年 | 3677篇 |
1974年 | 3615篇 |
1973年 | 3651篇 |
1972年 | 2603篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
Effect of track stiffness on vibration levels in railway tunnels 总被引:1,自引:0,他引:1
The paper discusses the difficulties in comparing and drawing conclusions from vibration measurements made in different railway tunnels. A number of new measurements are presented, which have been made and analyzed on a consistent basis. The track stiffness has been determined for each of the measurements. Graphs are presented which indicate the level of rail and tunnel floor vibration likely to occur for different track stiffnesses. The variance in the measurement results also indicates the potential error in using the data to predict vibration levels in new situations. 相似文献
995.
The nonstationary Reynolds equation of the theory of gas lubrication is considered. The existence and uniqueness of a solution to the initial-boundary value problem for this equation are established in the case of sufficiently smooth data. Estimates for the solution are obtained for large bearing numbers. Bibliography: 8 titles. 相似文献
996.
S. Anyogita 《Applied Acoustics》2004,65(2):195-201
Measurements of noise and its spectral characteristics were made inside various types of transport running on Compressed Natural Gas (CNG) fuel in Delhi. Noise indices L10, L50, L90 and Leq were estimated from the measured noise levels for vehicles in neutral gear, slow speed (speed ?20 km/h) and under free flow (speed ?30 km/h) conditions. It is found that background levels, when averaged over all speeds, are maximum in Rural Transport Vehicles (RTV) followed by Buses, Auto-rickshaws and Taxis. With increase in the speed, noise levels are appreciably enhanced except in the case of auto-rickshaws where the increase is moderate. The spectral distributions of noise inside vehicles obtained at 1-octave band frequencies, show a rather similar nature. The study reveals significantly lower noise levels inside CNG driven public modes of transport compared to those found in an earlier survey inside diesel and petrol driven vehicles. 相似文献
997.
S. K. Gorbatsevich I. A. Kaputskaya S. A. Maskevich S. Yu. Mikhnevich N. D. Strekal’ 《Optics and Spectroscopy》2004,97(2):238-243
The fluorescence of solid solutions of CdSe/ZnS quantum dots and the organic dye DODCI is investigated. It is shown that nonradiative transfer of electronic excitation energy to dye molecules, which with some probability lose their acceptor properties as a result of photoisomerization or photodegradation, is responsible for a significant increase in the fluorescence intensity of a donor. The degree of polarization of the donor fluorescence attains values exceeding 0.5, which is due to the difference in the fluorescence quantum yields of donors with different orientations of the oscillator with respect to the electric vector of an excitation light wave. A numerical simulation of the experimentally observed dependences is performed. 相似文献
998.
999.
We present a review of our recent measurements of total cross sections (Q
T's) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (Q
Ps's) for Na and K. For our total cross section measurements, a beam transmission technique has been used. For ourQ
Ps measurements, our approach involves setting upper and lower limits onQ
Ps using a combination of (1) measuring the transmission of the positron beam with the angular discrimination of the apparatus made as poor as possible, and (2) measuring the 511 keV annihilation gamma rays in coincidence produced by the decay of para-Ps formed in the scattering cell. Comparison with theoretical calculations shows that our measuredQ
T's andQ
Ps's for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in then=1 andn=2 states. In the 3–10 eV energy range, this calculation predicts a peak in theQ
T's andQ
Ps's for K which also appears in our measurements. The absence of such a peak in our measuredQ
T's andQ
Ps's (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations ofQ
T which do not take Ps-formation into account also show good agreement with our positron-Na, K, and RbQ
T measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV for K and Rb. 相似文献
1000.
E. A. Davis A. Singh S. F. J. Cox S. R. Kreitzman T. L. Estle B. Hitti R. L. Lichti R. DuVarney 《Hyperfine Interactions》1994,86(1):705-710
Studies of the muonium fractions in the amorphous oxide a-SiO have been carried out by RF resonance at TRIUMF, Canada and LF repolarization techniques at RAL, U.K. The resonance measurements confirm the presence of the interstitial Mu centre in this intermediate oxide of silicon. Analysis of the data gathered at RAL, using a recently-developed fitting technique, reveals that the Mu* state is present here as well, but with lower relative fractions than in a-Si. However, as in the latter material, but in contrast to c-Si, this bond-centre species appears to be stable up to room temperature. 相似文献