全文获取类型
收费全文 | 4579篇 |
免费 | 160篇 |
国内免费 | 239篇 |
专业分类
化学 | 485篇 |
晶体学 | 3篇 |
力学 | 162篇 |
综合类 | 30篇 |
数学 | 3236篇 |
物理学 | 1062篇 |
出版年
2024年 | 5篇 |
2023年 | 46篇 |
2022年 | 69篇 |
2021年 | 50篇 |
2020年 | 107篇 |
2019年 | 134篇 |
2018年 | 137篇 |
2017年 | 111篇 |
2016年 | 112篇 |
2015年 | 62篇 |
2014年 | 196篇 |
2013年 | 458篇 |
2012年 | 145篇 |
2011年 | 247篇 |
2010年 | 189篇 |
2009年 | 300篇 |
2008年 | 328篇 |
2007年 | 283篇 |
2006年 | 238篇 |
2005年 | 186篇 |
2004年 | 158篇 |
2003年 | 151篇 |
2002年 | 138篇 |
2001年 | 93篇 |
2000年 | 115篇 |
1999年 | 92篇 |
1998年 | 102篇 |
1997年 | 88篇 |
1996年 | 80篇 |
1995年 | 55篇 |
1994年 | 50篇 |
1993年 | 51篇 |
1992年 | 49篇 |
1991年 | 29篇 |
1990年 | 25篇 |
1989年 | 35篇 |
1988年 | 29篇 |
1987年 | 30篇 |
1986年 | 22篇 |
1985年 | 23篇 |
1984年 | 31篇 |
1983年 | 13篇 |
1982年 | 19篇 |
1981年 | 16篇 |
1980年 | 15篇 |
1979年 | 14篇 |
1978年 | 14篇 |
1977年 | 14篇 |
1976年 | 10篇 |
1974年 | 5篇 |
排序方式: 共有4978条查询结果,搜索用时 31 毫秒
91.
Richard C. Bradley 《Journal of Theoretical Probability》1996,9(3):659-678
Schmidt(10) showed that if the family of distributions of the partial sums of a strictly stationary random sequence is tight, then that random sequence is a coboundary. Here an analog of that result is proved for some sequences of random matrices, with partial sums replaced by matrix products. 相似文献
92.
Ron M. Roth 《Designs, Codes and Cryptography》1996,9(2):177-191
Codes
of length 2
m
over {1, -1} are defined as null spaces of certain submatrices of Hadamard matrices. It is shown that the codewords of
all have an rth order spectral null at zero frequency. Establishing the connection between
and the parity-check matrix of Reed-Muller codes, the minimum distance of
is obtained along with upper bounds on the redundancy of
. An efficient algorithm is presented for encoding unconstrained binary sequences into
. 相似文献
93.
Summary A popular first step in the problem of structure-based, de novo molecule design is to identify regions where specific functional groups or chemical entities would be expected to interact strongly. When the three-dimensional structure of the receptor is not available, it may be possible to derive a pharmacophore giving the three-dimensional relationships between such chemical groups. The task then is to design synthetically feasible molecules which not only contain the required groups, but which can also position them in the desired relative orientation. One way to do this is to first link the groups using an acyclic chain. We have investigated the application of the tweak algorithm [Shenkin, P.S. et al., Biopolymers, 26 (1987) 2053] for generating families of acyclic linkers. These linking structures can subsequently be braced using a ring-joining algorithm [Leach, A.R. and Lewis, R.A., J. Comput. Chem., 15 (1994) 233], giving rise to an even wider variety of molecular skeletons for further studies. 相似文献
94.
Rahul Kumar Balakrishnan 《Polymer Degradation and Stability》2007,92(8):1583-1591
For the first time, low temperature degradation (170-240 °C) of polystyrene in benzene is carried out in the presence of hydrogen using iron(III) oxide catalyst. The effect of temperature, catalyst loading and polymer loading on degradation are studied in hydrogen atmosphere. Degradation is also carried out at different initial hydrogen partial pressure. The time dependent molecular weight is calculated using viscosity average method. It is found that the degradation is enhanced considerably in the presence of hydrogen and followed random degradation chain scission. A random degradation kinetic model of Kelen [Kelen T. Polymer degradation. New York: Van Nostrand Reinhold Company; 1983.] is used to estimate the degradation rate constants. Empirical correlations are proposed to account for the effect of catalyst loading and initial hydrogen partial pressure on degradation. The true thermal degradation rate constants are calculated using these proposed correlations at given catalyst loading and initial hydrogen partial pressure with varying temperature. The frequency factor and activation energy are also determined using Arrhenius equation considering the true thermal degradation rate constants. 相似文献
95.
Y. Yavin 《Journal of Optimization Theory and Applications》1984,44(1):159-179
Consider the random motion in the plane of a pointM, whose velocityv=(v
1,v
2) is perturbed by an 2-valued Gaussian white noise. Only noisy nonlinear observations taken on the point location (state) are available toM. The velocityv is of the formv(y)=
u
(u
1,u
2)
y
(du), wherey denotes the value of the observed signal,U is the range of the velocity, and, for eachy,
y
is a probability measure on (U). Using the available observations, the pointM wishes to steer itself into a given target set by choosing a randomized strategy ={
y
:y 2}. Sufficient conditions on weak optimal randomized strategies are derived. An algorithm for computing weak suboptimal randomized strategies is suggested, and the strategies are computed for a variety of cases.This work was partially supported by a grant from Control Data. 相似文献
96.
R. De
Lisi G. Lazzara S. Milioto N. Muratore 《Journal of Thermal Analysis and Calorimetry》2007,87(1):61-67
Macromolecule/laponite nanomaterials were
studied by DSC and X-ray diffraction techniques. The matrices are poly(ethylene)
glycols at various molecular masses and poly(ethylene oxides)-poly(propylene
oxides)-poly(ethylene oxides) tri-block copolymers. The latter were tuned
by modulating the molecular masses, at constant hydrophilic/hydrophobic ratio,
and the hydrophilicity. For all the investigated systems, the enthalpy of
melting (ΔH
m)
is nearly constant up to a given composition thereafter it increases monotonically
reaching the value of the pure macromolecule. We proposed a model to interpret
the DSC data.
Briefly, it was invoked a mechanism of interaction
following which some segments of the adsorbed macromolecule are anchored to
the laponite (RD) particles and the remaining segments are radiating away
from the surface. The portion of the macromolecule in contact with RD does
not contribute to ΔH
m
whereas that radiating away from the clay does. Once that the RD surface is
saturated, the excess of the macromolecule behaves like the pure one. The
proposed model allowed to compute successfully the ΔH
m
values. The X-ray diffraction experiments ruled out the polymer intercalation
between the silicate sheets. 相似文献
97.
Several features of the trapping of random walks on a one-dimensional lattice are analyzed. The results of this investigation are as follows: (1) The correction term to the known asymptotic form for the survival probability ton steps is O((
2n)–1/3), where =–ln(1–c), andc is the trap concentration. (2) The short time form for the survival probability is found to be exp[–a(c)n
1/2], wherea(c) is given in Eq. (21). (3) The mean-square displacement of a surviving random walker is found to go liken
2/3for largen. (4) When the distribution of trap-free regions is changed so that very large regions are much rarer than for ideally random trap placement the asymptotic survival probability changes its dependence onn. One such model is studied. 相似文献
98.
Olle Nilsson 《Journal of separation science》1982,5(1):38-44
The total length-based second moment contribution from longitudinal sample diffusion in both phases on a column, σD , is derived by adding individual partial differential contributions to a partial differential equation accounting for the longitudinal diffusion processes only. Although each diffusion-dispersed sample part is equilibrated between two phases, the resulting σ,D (= 2D mt m + 2D st s) can be interpreted as the sum of two independent contributions in accordance with the variance addition rule. (D m and D s are the mean diffusion coefficients and t mand t s the mean residence times of the sample in the mobile and stationary phases, respectively.) The same σD expression is derived from the random walk model of Giddings by treating the diffusional process in each phase as statistically independent of the other processes. Under these conditions the broadening contribution from longitudinal diffusion in the mobile phase is shown to be independent of the velocity profile. 相似文献
99.
Quantitative separation of zinc traces from cadmium matrices by solid-phase extraction with polyurethane foam 总被引:1,自引:0,他引:1
Santiago de Jesus D Souza de Carvalho M Spínola Costa AC Costa Ferreira SL 《Talanta》1998,46(6):1525-1530
A system for separation of zinc traces from large amounts of cadmium is proposed in this paper. It is based on the solid-phase extraction of the zinc in the form of thiocyanate complexes by the polyurethane foam. The following parameters were studied: effect of pH and of the thiocyanate concentration on the zinc extraction, shaking time required for quantitative extraction, amount of PU foam necessary for complete extraction, conditions for the separation of zinc from cadmium, influence of other cations and anions on the zinc sorption by PU foam, and required conditions for back extraction of zinc from the PU foam. The results show that zinc traces can be separated from large amounts of cadmium at pH 3.0±0.50, with the range of thiocyanate concentration from 0.15 to 0.20 mol l−1, and the shaking time of 5 min. The back extraction of zinc can be done by shaking it with water for 10 min. Calcium, barium, strontium, magnesium, aluminum, nickel and iron(II) are efficiently separated. Iron(III), copper(II) and cobalt(II) are extracted simultaneously with zinc, but the iron reduction with ascorbic acid and the use of citrate to mask copper(II) and cobalt(II) increase the selectivity of the zinc extraction. The anions nitrate, chloride, sulfate, acetate, thiosulphate, tartarate, oxalate, fluoride, citrate, and carbonate do not affect the zinc extraction. Phosphate and EDTA must be absent. The method proposed was applied to determine zinc in cadmium salts using 4-(2-pyridylazo)-resorcinol (PAR) as a spectrophotometric reagent. The result achieved did not show significant difference in the accuracy and precision (95% confidence level) with those obtained by ICP–AES analysis. 相似文献
100.
For lattices with two kinds of points (black and white), distributed according to a translation-invariant joint probability distribution, we study statistical properties of the sequence of consecutive colors encountered by a random walker moving through the lattice. The probability distribution for the single steps of the walk is considered to be independent of the colors of the points. Several exact results are presented which are valid in any number of dimensions and for arbitrary probability distributions for the coloring of the points and the steps of the walk. They are used to derive a few general properties of random walks on lattices containing traps.Presented at the Symposium on Random Walks, Gaithersburg, MD, June 1982. 相似文献