全文获取类型
收费全文 | 217633篇 |
免费 | 3350篇 |
国内免费 | 735篇 |
专业分类
化学 | 120250篇 |
晶体学 | 2708篇 |
力学 | 8403篇 |
综合类 | 8篇 |
数学 | 26748篇 |
物理学 | 63601篇 |
出版年
2021年 | 1473篇 |
2020年 | 1811篇 |
2019年 | 1763篇 |
2018年 | 2102篇 |
2017年 | 2052篇 |
2016年 | 4036篇 |
2015年 | 3307篇 |
2014年 | 3875篇 |
2013年 | 10743篇 |
2012年 | 9080篇 |
2011年 | 10623篇 |
2010年 | 6642篇 |
2009年 | 6545篇 |
2008年 | 9690篇 |
2007年 | 9482篇 |
2006年 | 9006篇 |
2005年 | 8533篇 |
2004年 | 7550篇 |
2003年 | 6477篇 |
2002年 | 6224篇 |
2001年 | 6886篇 |
2000年 | 5447篇 |
1999年 | 4097篇 |
1998年 | 3132篇 |
1997年 | 3063篇 |
1996年 | 3144篇 |
1995年 | 2689篇 |
1994年 | 2658篇 |
1993年 | 2511篇 |
1992年 | 2826篇 |
1991年 | 2696篇 |
1990年 | 2462篇 |
1989年 | 2376篇 |
1988年 | 2392篇 |
1987年 | 2291篇 |
1986年 | 2191篇 |
1985年 | 3205篇 |
1984年 | 3169篇 |
1983年 | 2541篇 |
1982年 | 2718篇 |
1981年 | 2617篇 |
1980年 | 2512篇 |
1979年 | 2444篇 |
1978年 | 2520篇 |
1977年 | 2458篇 |
1976年 | 2411篇 |
1975年 | 2356篇 |
1974年 | 2275篇 |
1973年 | 2363篇 |
1972年 | 1341篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Abull is the (self-complementary) graph with verticesa, b, c, d, e and edgesab, ac, bc, bd, ce; a graphG is calledBerge if neitherG not its complement contains a chordless cycle whose length is odd and at least five. We prove that bull-free Berge graphs are perfect; a part of our argument relies on a new property of minimal imperfect graphs.This work was done while both authors were at the School of Computer Science, McGill University; support by NSERC is gratefully acknowledged. 相似文献
962.
The performance of high power transistor devices is intimately connected to the substrate thermal conductivity. In this study, the relationship between thermal conductivity and dislocation density is examined using the 3 omega technique and free standing HVPE GaN substrates. Dislocation density is measured using imaging cathodoluminescence. In a low dislocation density regime below 105 cm−2, the thermal conductivity appears to plateau out near 230 W/K m and can be altered by the presence of isotopic defects and point defects. For high dislocation densities the thermal conductivity is severely degraded due to phonon scattering from dislocations. These results are applied to the design of homoepitaxially and heteroepitaxially grown HEMT devices and the efficiency of heat extraction and the influence of lateral heat spreading on device performance are compared. 相似文献
963.
964.
We discuss equivalent definitions of holomorphic second-order cusp forms and prove bounds on their Fourier coefficients. We
also introduce their associated L-functions, prove functional equations for twisted versions of these L-functions and establish a criterion for a Dirichlet series to originate from a second order form. In the last section we
investigate the effect of adding an assumption of periodicity to this criterion.
2000 Mathematics Subject Classification Primary—11F12, 11F66
G. Mason: Research supported in part by NSF Grant DMS 0245225.
C. O’Sullivan: Research supported in part by PSC CUNY Research Award No. 65453-00 34. 相似文献
965.
M.C. Robinson D.J. Morris P.D. Hayenga J.H. Cho C.D. Richards R.F. Richards D.F. Bahr 《Applied Physics A: Materials Science & Processing》2006,85(2):135-140
Piezoelectric membranes have been fabricated that incorporate a gold bottom electrode with an adhesion layer of titanium–tungsten (10:90 wt. %). For solution-deposited acetic acid based lead zirconate titanate (HoAc-PZT) with a Zr:Ti ratio of 40:60, the film’s average piezoelectric coefficient, e31, is -5.31 C/m2, with a dielectric constant of 814 at 200 Hz, which is similar to values for platinum bottom electrodes. The PZT structure remains columnar on both types of bottom electrodes. Initial fabrication attempts resulted in cracking that initiated in the PZT layer of the structure. X-ray photoelectron spectroscopy was utilized to establish how processing affects diffusion throughout the composite membrane structure. Crack-free membranes were fabricated and tested. This paper discusses the performance properties and piezoelectric fatigue results for these membranes. PACS 77.84.-s; 77.84.Lf 相似文献
966.
967.
I. Kandarakis D. Cavouras P. Prassopoulos E. Kanellopoulos C.D. Nomicos G.S. Panayiotakis 《Applied Physics A: Materials Science & Processing》1998,66(5):521-525
2 SiO4:Mn phosphor was evaluated for use in radiation detectors of medical imaging systems. Zn2SiO4:Mn was used in the form of laboratory-prepared fluorescent layers (screens) with coating weights from 18 to 150 mg/cm2. The phosphor was excited to luminescence by low-energy X-raysusing X-raytube voltages ranging from 15 to 50 kVp. The number
of emitted optical photons per incident X-rayquantum was thus determined for various X-rayenergies and phosphor coating weights.
The optical emission spectrum was also measured and it was used to evaluate the spectral compatibility of Zn2SiO4:Mn with radiographic films, photocathodes and the Si photodiode. Finally, phosphor optical properties were estimated by fitting
a theoretical model to experimental data. Results showed that Zn2SiO4:Mn is more efficient for low-energy X-rays. Its intrinsic conversion efficiency was found equal to 0.08, which is comparable
to that of actually used phosphors. Zn2SiO4:Mn was also adequately compatible with orthochromatic films and the ES-20 photocathode, thus being appropriate for low-voltage
radiography and fluoroscopy.
Received: 31 July 1998/Accepted: 3 August 1998 相似文献
968.
We study Willmore immersed submanifoldsf: M m →S n into then-Möbius space, withm≥2, as critical points of a conformally invariant functionalW. We compute the Euler-Lagrange equation and relate this functional with another one applied to the conformal Gauss map of immersions intoS n . We solve a Bernestein-type problem for compact Willmore hypersurfaces ofS n , namely, if ?a ∈? n+2 such that <γf, a > ≠ 0 onM, whereγ f is the hyperbolic conformal Gauss map and <, > is the Lorentz inner product of? n+2, and iff satisfies an additional condition, thenf(M) is an (n?1)-sphere. 相似文献
969.
D. Abbaneo P. Antilogus T. Behnke S.C. Blyth M. Elsing R. Faccini R.W.L. Jones K. Mönig S. Petzold R. Tenchini 《The European Physical Journal C - Particles and Fields》1998,4(2):185-191
Measurements of the forward-backward production asymmetry of heavy quarks in Z decays provide a precise determination of
. The asymmetries are sensitive to QCD effects, in particular hard gluon radiation. In this paper QCD corrections for and are discussed. The interplay between the experimental techniques used to measure the asymmetries and the QCD effects is
investigated using simulated events. A procedure to estimate the correction needed for experimental measurements is proposed,
and some specific examples are given.
Received: 26 February 1998 / Published online: 2 June 1998 相似文献
970.
Orbital magnetism in an integrable model of a multichannel ring with long-ranged electron-electron interactions is investigated. In a noninteracting multichannel system, the response to an external magnetic flux is the sum of many diamagnetic and paramagnetic contributions, but we find that for sufficiently strong correlations, the contributions of all channels add constructively, leading to a parity (diamagnetic or paramagnetic) which depends only on the total number of electrons. Numerical results confirm that this parity-locking effect is robust with respect to subband mixing due to disorder. 相似文献