全文获取类型
收费全文 | 3917篇 |
免费 | 133篇 |
国内免费 | 11篇 |
专业分类
化学 | 2488篇 |
晶体学 | 5篇 |
力学 | 108篇 |
数学 | 749篇 |
物理学 | 711篇 |
出版年
2023年 | 29篇 |
2022年 | 27篇 |
2021年 | 41篇 |
2020年 | 61篇 |
2019年 | 62篇 |
2018年 | 51篇 |
2017年 | 57篇 |
2016年 | 105篇 |
2015年 | 88篇 |
2014年 | 123篇 |
2013年 | 178篇 |
2012年 | 238篇 |
2011年 | 301篇 |
2010年 | 181篇 |
2009年 | 145篇 |
2008年 | 243篇 |
2007年 | 285篇 |
2006年 | 240篇 |
2005年 | 195篇 |
2004年 | 180篇 |
2003年 | 132篇 |
2002年 | 125篇 |
2001年 | 54篇 |
2000年 | 49篇 |
1999年 | 35篇 |
1998年 | 37篇 |
1997年 | 47篇 |
1996年 | 39篇 |
1995年 | 35篇 |
1994年 | 31篇 |
1993年 | 31篇 |
1992年 | 43篇 |
1991年 | 23篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 19篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 30篇 |
1984年 | 23篇 |
1983年 | 19篇 |
1982年 | 29篇 |
1981年 | 31篇 |
1980年 | 28篇 |
1979年 | 17篇 |
1978年 | 26篇 |
1977年 | 29篇 |
1976年 | 17篇 |
1974年 | 21篇 |
1973年 | 16篇 |
排序方式: 共有4061条查询结果,搜索用时 171 毫秒
11.
The synthesis of unsaturated unnatural amino acids according to the Heck-Jeffery protocol using a ball-milling procedure is presented. NOE data recorded on the products confirm that the Z-isomer is formed. This type of mechano-chemistry provides an efficient, “solvent free” and reliable method for the synthesis of substituted dehydroalanines. These conditions provide patterns of reactivity complementary to conventional procedures. 相似文献
12.
Olga Bori Timo J. Tolmunen Erik Kollberg Margaret A. Frerking 《International Journal of Infrared and Millimeter Waves》1992,13(6):799-814
The S-parameters of several different quantum well double barrier diodes have been measured. A technique has been developed for measuring whisker contacted diodes with and HP 8510B automatic network analyzer. Special coaxial mounts using K-connectors were designed to enable measurements up to 20 GHz. The voltage-dependent conductance and capacitance were derived from the measured reflection coefficient of each device. The C/V characteristics were observed to exhibit an anomalous increase at voltages corresponding the the negative differential resistance region (NDR). These are the first reported S-parameter measurements in the negative differential resistance region of quantum well double barrier diodes. A theory is presented that explains, in part, the observed results. 相似文献
13.
14.
An acoustic pulse propagating on a two-mode fiber can act as a beam splitter in a scanning interferometer. When this device
is employed in white-light interferometry, the effects of distributed coupling and dispersive interferometer arms need to
be considered. A theory suitable for treating acousto-optic interaction of partially coherent light in a moving interaction
region was developed. It was found that differential optical dispersion should be negligible and the acoustic pulse length
short. Also the coherence time should be short but long compared to the intermodal group delay difference over a pulse length.
Experiments with long acoustic pulses were performed, and fairly good agreement with theory was obtained. 相似文献
15.
Erik Woldt 《Journal of Physics and Chemistry of Solids》2002,63(4):699-704
The paper investigates whether a change from a homogeneous to an inhomogeneous dislocation distribution, assumed to be caused by a slight additional deformation, can lead to an increase of the recrystallization temperature of a deformed metal. In this case, the higher temperature would indicate a more stable deformation structure despite the increase of stored energy. The recrystallization temperature is related to the growth rate. Hence, the steady state velocity of a recrystallization front moving either parallel or vertically to the stripes of a simplified two-dimensional heterogeneous dislocation distribution of parallel sections of higher and lower dislocation densities is calculated. The results show that if a front growths through the high and low density sections in series an overall slower rate despite higher mean dislocation density is, indeed, possible. However, growing in the parallel arrangement always leads to a higher growth rate compared with the homogeneous case of slightly less stored energy. Since in a real structure the faster growth is likely to succeed, the recrystallization temperature observed will be lowered with additional deformation in accordance with experimental experience. 相似文献
16.
Young C. Kim Mikhail A. Anisimov Jan V. Sengers Erik Luijten 《Journal of statistical physics》2003,110(3-6):591-609
The character of critical behavior in physical systems depends on the range of interactions. In the limit of infinite range of the interactions, systems will exhibit mean-field critical behavior, i.e., critical behavior not affected by fluctuations of the order parameter. If the interaction range is finite, the critical behavior asymptotically close to the critical point is determined by fluctuations and the actual critical behavior depends on the particular universality class. A variety of systems, including fluids and anisotropic ferromagnets, belongs to the three-dimensional Ising universality class. Recent numerical studies of Ising models with different interaction ranges have revealed a spectacular crossover between the asymptotic fluctuation-induced critical behavior and mean-field-type critical behavior. In this work, we compare these numerical results with a crossover Landau model based on renormalization-group matching. For this purpose we consider an application of the crossover Landau model to the three-dimensional Ising model without fitting to any adjustable parameters. The crossover behavior of the critical susceptibility and of the order parameter is analyzed over a broad range (ten orders) of the scaled distance to the critical temperature. The dependence of the coupling constant on the interaction range, governing the crossover critical behavior, is discussed. 相似文献
17.
18.
A q × n array with entries from 0, 1,…,q − 1 is said to form a difference matrix if the vector difference (modulo q) of each pair of columns consists of a permutation of [0, 1,… q − 1]; this definition is inverted from the more standard one to be found, e.g., in Colbourn and de Launey (1996). The following idea generalizes this notion: Given an appropriate δ (-[−1, 1]t, a λq × n array will be said to form a (t, q, λ, Δ) sign-balanced matrix if for each choice C1, C2,…, Ct of t columns and for each choice = (1,…,t) Δ of signs, the linear combination ∑j=1t jCj contains (mod q) each entry of [0, 1,…, q − 1] exactly λ times. We consider the following extremal problem in this paper: How large does the number k = k(n, t, q, λ, δ) of rows have to be so that for each choice of t columns and for each choice (1, …, t) of signs in δ, the linear combination ∑j=1t jCj contains each entry of [0, 1,…, q t- 1] at least λ times? We use probabilistic methods, in particular the Lovász local lemma and the Stein-Chen method of Poisson approximation to obtain general (logarithmic) upper bounds on the numbers k(n, t, q, λ, δ), and to provide Poisson approximations for the probability distribution of the number W of deficient sets of t columns, given a random array. It is proved, in addition, that arithmetic modulo q yields the smallest array - in a sense to be described. 相似文献
19.
Given a simplicial complex δ on vertices {1, …,n} and a fieldF we consider the subvariety of projective (n−1)-space overF consisting of points whose homogeneous coordinates have support in δ. We give a simple rational expression for the zeta function
of this singular projective variety overF
q and show a close connection with the Betti numbers of the corresponding variety over ℂ. This connection is particularly simple
in the case when Δ is Cohen-Macaulay. 相似文献
20.
Let R be a local ring and let (x
1, …, x
r) be part of a system of parameters of a finitely generated R-module M, where r < dimR
M. We will show that if (y
1, …, y
r) is part of a reducing system of parameters of M with (y
1, …, y
r) M = (x
1, …, x
r) M then (x
1, …, x
r) is already reducing. Moreover, there is such a part of a reducing system of parameters of M iff for all primes P ε Supp M ∩ V
R(x
1, …, x
r) with dimR
R/P = dimR
M − r the localization M
P of M at P is an r-dimensional Cohen-Macaulay module over R
P.
Furthermore, we will show that M is a Cohen-Macaulay module iff y
d is a non zero divisor on M/(y
1, …, y
d−1) M, where (y
1, …, y
d) is a reducing system of parameters of M (d:= dimR
M). 相似文献