全文获取类型
收费全文 | 255444篇 |
免费 | 4294篇 |
国内免费 | 2379篇 |
专业分类
化学 | 120121篇 |
晶体学 | 3805篇 |
力学 | 13568篇 |
综合类 | 104篇 |
数学 | 39661篇 |
物理学 | 84858篇 |
出版年
2020年 | 1329篇 |
2019年 | 1320篇 |
2018年 | 10731篇 |
2017年 | 11558篇 |
2016年 | 6523篇 |
2015年 | 2789篇 |
2014年 | 2537篇 |
2013年 | 6607篇 |
2012年 | 9885篇 |
2011年 | 19608篇 |
2010年 | 11888篇 |
2009年 | 11793篇 |
2008年 | 16836篇 |
2007年 | 20530篇 |
2006年 | 5775篇 |
2005年 | 12652篇 |
2004年 | 8268篇 |
2003年 | 7645篇 |
2002年 | 5480篇 |
2001年 | 4849篇 |
2000年 | 3760篇 |
1999年 | 2949篇 |
1998年 | 2444篇 |
1997年 | 2379篇 |
1996年 | 2396篇 |
1995年 | 2228篇 |
1994年 | 1950篇 |
1993年 | 1864篇 |
1992年 | 2074篇 |
1991年 | 2046篇 |
1990年 | 1940篇 |
1989年 | 1914篇 |
1988年 | 1902篇 |
1987年 | 1867篇 |
1986年 | 1761篇 |
1985年 | 2332篇 |
1984年 | 2430篇 |
1983年 | 2023篇 |
1982年 | 2328篇 |
1981年 | 2106篇 |
1980年 | 2129篇 |
1979年 | 2150篇 |
1978年 | 2276篇 |
1977年 | 2092篇 |
1976年 | 2169篇 |
1975年 | 2101篇 |
1974年 | 1931篇 |
1973年 | 2188篇 |
1972年 | 1316篇 |
1968年 | 1155篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
951.
X.X. Guo 《Surface science》2004,549(3):211-216
We studied parallel conductivities of pure BaF2 films with thicknesses ranging from 35 to 300 nm, epitaxially grown on Al2O3(0 1 2) substrates by molecular beam epitaxy technique. The overall conductivities of the films are found to increase with decreasing thickness. The detailed investigation of the overall conductance as a function of the thickness permits the deconvolution of bulk and boundary effects, the latter being attributed to distinct space charge effects in the interface between BaF2 film and Al2O3 substrate. The (extrinsic) Debye length (λ) is estimated to be about 8 nm at T=593 K, which corresponds to an impurity content of 1018/cm3 (singly ionized dopant assumed). This is consistent with the fact that we observed a constant boundary contribution for all investigated films (film thickness >4λ). It is also consistent with the Debye length observed in a previous report on CaF2/BaF2 heterolayers fabricated by the same technique, in which the low temperature enhancement was also attributed to space charges in BaF2 [Nature 408 (2000) 946]. Only at low temperatures (below 370 °C), the conductance seems to be influenced by strain effect. 相似文献
952.
The use of liquid fuels such as kerosene is of interest for the pulse detonation engine (PDE). Within this context, the aim
of this work, which is a preliminary study, was to show the feasibility to initiate a detonation in air with liquid-fuel pyrolysis
products, using energies and dimensions of test facility similars to those of PDEs. Therefore, two liquids fuels have been
compared, JP10, which is a synthesis fuel generally used in the field of missile applications, and decane, which is one of
the major components of standard kerosenes (F-34, Jet A1, ...). The thermal degradation of these fuels was studied with two
pyrolysis processes, a batch reactor and a flow reactor. The temperatures varied from 600°C to 1,000°C and residence times
for the batch reactor and the flow reactor were, respectively, between 10–30 s and 0.1–2 s. Subsequently, the detonability
of synthetic gaseous mixtures, which was a schematisation of the decomposition state after the pyrolysis process, has been
studied. The detonability study, regarding nitrogen dilution and equivalence ratio, was investigated in a 50 mm-diameter,
2.5 m-long detonation tube. These dimensions are compatible with applications in the aircraft industry and, more particularly,
in PDEs. Therefore, JP10 and decane were compared to choose the best candidate for liquid-fuel PDE studies.
This paper was based on work that was presented at the 20th International Colloquium on the Dynamics of Explosions and Reactive
Systems, Montreal, Canada, July 31 – August 5, 2005. 相似文献
953.
Based on the results of fracture in polymethyl methacrylate and a spheroplastic using a magnetic-pulse setup, the specific work of the formation of a new surface is estimated, which is similar to Griffith’s surface energy for quasi-static tests. The value obtained is greater than the corresponding value determined from the quasi-static tests by an order of magnitude and tends to increase as the loading time decreases. 相似文献
954.
C. Adamuscin A.-Z. Dubnicková S. Dubnicka R. Pekárik P. Weisenpacher 《The European Physical Journal C - Particles and Fields》2003,28(1):115-118
A system of linear homogeneous algebraic equations for the coupling constant ratios of vector mesons to hadrons is derived
by imposing the assumed asymptotic behavior upon the VMD pole parameterization of an hadron electromagnetic form factor. A
similar system of equations with a simpler structure of the coefficients, taken as even powers of the vector-meson masses,
is derived by means of integral superconvergent sum rules for the imaginary part of the considered form factor using its
appropriate -function approximation. Although both systems have been derived starting from different properties of the electromagnetic
form factor and they each have their own appearances, it is shown explicitly that they are fully equivalent.
Received: 20 March 2002 / Revised version: 3 December 2002 / Published online: 7 March 2003 相似文献
955.
956.
A. B. Vasil’eva 《Ukrainian Mathematical Journal》2007,59(3):396-408
We present results from the theory of singular perturbations and, in particular, from a new branch of this theory (contrast
alternating-type structures).
__________
Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 59, No. 3, pp. 359–369, March, 2007. 相似文献
957.
A. Yu. Kolesov E. F. Mishchenko N. Kh. Rozov 《Proceedings of the Steklov Institute of Mathematics》2007,259(1):101-127
We carry out a detailed analysis of the existence, asymptotics, and stability problems for periodic solutions that bifurcate
from the zero equilibrium state in systems with large delay. The account is based on a specific meaningful example given by
a certain scalar nonlinear second-order differential-difference equation that is a mathematical model of a single-circuit
RCL oscillator with delay in a feedback loop. 相似文献
958.
Athanassios G. Bratsos 《Numerical Algorithms》2007,46(1):45-58
A predictor–corrector (P-C) scheme is applied successfully to a nonlinear method arising from the use of rational approximants
to the matrix-exponential term in a three-time level recurrence relation. The resulting nonlinear finite-difference scheme,
which is analyzed for local truncation error and stability, is solved using a P-C scheme, in which the predictor and the corrector
are explicit schemes of order 2. This scheme is accelerated by using a modification (MPC) in which the already evaluated values
are used for the corrector. The behaviour of the P-C/MPC schemes is tested numerically on the Boussinesq equation already
known from the bibliography free of boundary conditions. The numerical results are derived for both the bad and the good Boussinesq
equation and conclusions from the relevant known results are derived.
相似文献
959.
960.
Bernhard G. Bodmann Vern I. Paulsen Soha A. Abdulbaki 《Journal of Fourier Analysis and Applications》2007,13(3):285-307
We study the performance of finite frames for the encoding of vectors by applying standard higher-order sigma-delta quantization
to the frame coefficients. Our results are valid for any quantizer with accuracy ε > 0 operating in the no-overload regime.
The frames under consideration are obtained from regular sampling of a path in a Hilbert space. In order to achieve error
bounds that are comparable to results on higher-order sigma-delta for the quantization of oversampled bandlimited functions,
we construct frame paths that terminate smoothly in the zero vector, that is, with an appropriate number of vanishing derivatives
at the endpoint. 相似文献