全文获取类型
收费全文 | 444623篇 |
免费 | 5442篇 |
国内免费 | 2095篇 |
专业分类
化学 | 206740篇 |
晶体学 | 5919篇 |
力学 | 17124篇 |
综合类 | 64篇 |
数学 | 45113篇 |
物理学 | 120600篇 |
无线电 | 56600篇 |
出版年
2020年 | 3580篇 |
2019年 | 4005篇 |
2018年 | 5191篇 |
2017年 | 5085篇 |
2016年 | 7394篇 |
2015年 | 4636篇 |
2014年 | 7264篇 |
2013年 | 17729篇 |
2012年 | 14434篇 |
2011年 | 17428篇 |
2010年 | 12621篇 |
2009年 | 12963篇 |
2008年 | 17315篇 |
2007年 | 18086篇 |
2006年 | 17296篇 |
2005年 | 15680篇 |
2004年 | 14336篇 |
2003年 | 12905篇 |
2002年 | 12457篇 |
2001年 | 13857篇 |
2000年 | 11059篇 |
1999年 | 8680篇 |
1998年 | 7336篇 |
1997年 | 7314篇 |
1996年 | 6938篇 |
1995年 | 6390篇 |
1994年 | 6401篇 |
1993年 | 6143篇 |
1992年 | 6606篇 |
1991年 | 6525篇 |
1990年 | 6229篇 |
1989年 | 5894篇 |
1988年 | 5671篇 |
1987年 | 5186篇 |
1986年 | 4964篇 |
1985年 | 6414篇 |
1984年 | 6646篇 |
1983年 | 5519篇 |
1982年 | 5861篇 |
1981年 | 5600篇 |
1980年 | 5286篇 |
1979年 | 5461篇 |
1978年 | 5793篇 |
1977年 | 5772篇 |
1976年 | 5760篇 |
1975年 | 5374篇 |
1974年 | 5444篇 |
1973年 | 5653篇 |
1972年 | 4055篇 |
1971年 | 3245篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
An ultrasonic Doppler velocimeter (UDV) is used for an experimental investigation of turbulent pipe flow at eight different Reynolds numbers (22,300–854,900). The UDV is a multipoint probe in the sense that it takes instantaneous measurements of fluid velocities at different locations simultaneously, with remarkable resolutions in space and time. The performances of the instrument with respect to the properties of the overlap layer of the turbulent flow field are investigated; the experimental results are compared with both already existing and more recently proposed scaling laws, and with other data of experimental nature. 相似文献
942.
V. A. Popov S. I. Lopatin L. Sh. Tsemekhman L. A. Pavlinova S. P. Kormilitsyn V. V. Barsegyan 《Russian Journal of Applied Chemistry》2003,76(10):1564-1567
Evaporation of chromium ore and concentrates was studied using high-temperature differential mass-spectrometry. The data obtained, concerning the component vapor pressures in evaporation of complex oxide systems in the range 1700-2100 K, can be used to calculate the evaporation loss in melting and to assess the possibility of utilization of the sublimates formed. 相似文献
943.
944.
The ρ-T curves in our single phase HgBa2Ca2Cu3O8+δ superconductor were measured as a function of temperature and magnetic field, ρ=ρ0exp(−Ueff/κBT). It can be transformed to another form d(lnρ)/d(1/T)=−Ueff+TdUeff/dT, then this becomes a plot of the activation energy Ueff as a function of temperature. Our data plotted in these ways show a clear crossover from high-temperature two-dimensional vortex-liquid to a critical region associated with the low-temperature three-dimensional vortex-glass phase transition. The critical exponents v(z−1)=3.9±1.9 in this system are little different with previous measurements in BSCCO and YBCO systems. 相似文献
945.
V. A. Burov I. M. Grishina O. I. Lapshenkina S. A. Morozov O. D. Rumyantseva E. G. Sukhov 《Acoustical Physics》2003,49(6):627-637
In the ultrasonic diagnostics of small-size neoplasms of biological tissues at the earliest stage of their development, an efficient way to eliminate the distorting influence of high-contrast or large inhomogeneities of the biological medium is to apply the iterative technique. A simple approach is proposed, which makes it possible with only two iteration steps to achieve an efficient focusing of the tomograph array. At the first step, the unknown distribution of the large-scale inhomogeneities of sound velocity and absorption over the scatterer is reconstructed, where the large-scale inhomogeneities are those whose size exceeds several wavelengths. At the second step, the fine structure of the scatterer is reconstructed against the large-scale background, which can be performed with a high accuracy owing to the evaluation of the background at the first step. The possibility of simultaneous reconstruction of the large-scale and fine structures by the noniterative Grinevich-Novikov algorithm is considered as an alternative. This algorithm reconstructs in an explicit form two-dimensional refractive-absorbing acoustic scatterers of almost arbitrary shape and strength. Taking into account the effects of multiple scattering, this algorithm provides resolution of the fine structure almost as good as that achieved in reconstructing the same structure against an undistorting homogeneous background. The results of numerical simulations of both algorithms are presented. 相似文献
946.
J. T. Hoeft M. Polcik D. I. Sayago M. Kittel R. Terborg R. L. Toomes J. Robinson D. P. Woodruff M. Pascal G. Nisbet C. L. A. Lamont 《Surface science》2003,540(2-3):441-456
The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) precovered with atomic hydrogen has been determined by C 1s (and O 1s) scanned-energy mode photoelectron diffraction, using the photoelectron binding energy changes to characterise the different states. The results confirm previous spectroscopic assignments of local atop and bridge sites both with and without coadsorbed hydrogen. The measured Ni–C bondlengths for the Ni(1 0 0)/CO states show an increase of 0.16 ± 0.04 Å in going from atop to bridge sites, while comparison with similar results for Ni(1 1 1)/CO for threefold coordinated adsorption sites show a further lengthening of the bond by 0.05 ± 0.04 Å. These changes in the Ni–CO chemisorption bondlength with bond order (for approximately constant adsorption energy) are consistent with the standard Pauling rules. However, comparison of CO adsorbed in the atop geometry with and without coadsorbed hydrogen shows that the coadsorption increases the Ni–C bondlength by only 0.06 ± 0.04 Å, despite the decrease in adsorption energy of a factor of 2 or more. This result is also reproduced by density functional theory slab calculations. The results of both the experiments and the density functional theory calculations show that CO adsorption onto the Ni(1 0 0)/H surface is accompanied by significant structural modification; the low desorption energy may then be attributed to the energy cost of this restructuring rather than weak local bonding. 相似文献
947.
Chadwin D. Young Gennadi Bersuker Yuegang Zhao Jeff J. Peterson Joel Barnett George A. Brown Jang H. Sim Rino Choi Byoung Hun Lee Peter Zeitzoff 《Microelectronics Reliability》2005,45(5-6):806
Effects of constant voltage stress (CVS) on gate stacks consisting of an ALD HfO2 dielectric with various interfacial layers were studied with time dependent sensing measurements: DC I–V, pulse I–V, and charge pumping (CP) at different frequencies. The process of injected electron trapping/de-trapping on pre-existing defects in the bulk of the high-κ film was found to constitute the major contribution to the time dependence of the threshold voltage (Vt) shift during stress. The trap generation observed with the low frequency CP measurements is suggested to occur within the interfacial oxide layer or the interfacial layer/high-κ interface, with only a minor effect on Vt. 相似文献
948.
The results of experimental investigation into the mechanical properties of blends of low-density polyethylene (LDPE) with chlorinated polyethylene (CPE) in tension are presented. The specimens of pure LDPE, CPE, and nine types of LDPE/CPE blends, with different content of components at 10 wt.% intervals, were examined. Data on the influence of blend composition on the tensile stress-strain diagram, elastic modulus, yield stress, breaking stress, and ultimate elongation are obtained. The results of investigations of creep are also reported. It is found that the creep compliance (the total current compliance minus the elastic compliance) obeys the power law of creep.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 3, pp. 391–404, May–June, 2005. 相似文献
949.
950.
The function lattice, or generalized Boolean algebra, is the set of ℓ-tuples with the ith coordinate an integer between 0 and a bound ni. Two ℓ-tuples t-intersect if they have at least t common nonzero coordinates. We prove a Hilton–Milner type theorem for systems of t-intersecting ℓ-tuples.Received September 29, 2004 相似文献