全文获取类型
收费全文 | 5153篇 |
免费 | 108篇 |
国内免费 | 14篇 |
专业分类
化学 | 2642篇 |
晶体学 | 134篇 |
力学 | 231篇 |
数学 | 683篇 |
物理学 | 1585篇 |
出版年
2022年 | 71篇 |
2021年 | 75篇 |
2020年 | 79篇 |
2019年 | 97篇 |
2018年 | 68篇 |
2017年 | 89篇 |
2016年 | 135篇 |
2015年 | 105篇 |
2014年 | 138篇 |
2013年 | 376篇 |
2012年 | 249篇 |
2011年 | 287篇 |
2010年 | 185篇 |
2009年 | 165篇 |
2008年 | 208篇 |
2007年 | 231篇 |
2006年 | 179篇 |
2005年 | 181篇 |
2004年 | 151篇 |
2003年 | 106篇 |
2002年 | 100篇 |
2001年 | 71篇 |
2000年 | 54篇 |
1999年 | 52篇 |
1998年 | 54篇 |
1997年 | 31篇 |
1996年 | 61篇 |
1995年 | 48篇 |
1994年 | 57篇 |
1993年 | 71篇 |
1992年 | 63篇 |
1991年 | 48篇 |
1990年 | 61篇 |
1989年 | 53篇 |
1988年 | 62篇 |
1987年 | 52篇 |
1985年 | 73篇 |
1984年 | 99篇 |
1983年 | 57篇 |
1982年 | 64篇 |
1981年 | 71篇 |
1980年 | 72篇 |
1979年 | 73篇 |
1978年 | 68篇 |
1977年 | 63篇 |
1976年 | 57篇 |
1975年 | 44篇 |
1974年 | 47篇 |
1973年 | 55篇 |
1972年 | 34篇 |
排序方式: 共有5275条查询结果,搜索用时 62 毫秒
11.
12.
Europium (Eu+) ions were confined in a Paul trap and detected by non-destructive method. Storage time of Eu+ ions achieved in vacuum was improved by orders of magnitude employing buffer gas cooling. The experimentally detected signal
was fitted to the ion response signal and the total number of ions trapped was estimated. It is found that the peak signal
amplitude as well as the product of FWHM and the peak signal amplitude is proportional to the total number of trapped ions.
The trapped ion secular frequency was swept at different rates and its effect on the absorption line profile was studied both
experimentally and theoretically. 相似文献
13.
A Monte Carlo simulation method is used to study thermal desorption of gas molecules from mixed adlayers containing two species. The effects of lateral interactions among adatoms and initial surface coverage on the desorption spectra are examined. It is shown that attractive lateral interactions lead to sharper peaks and that desorption occurs at high temperatures, whereas repulsive interactions lead to multiple peak spectra. It is found that interactions between unlike molecules affect the spectra for species with lower desorption energy only, and that the two species desorb together only in certain cases. Lateral interactions also affect the desorption kinetics significantly and very different behavior for the two species may be obtained. 相似文献
14.
S K Samanta U C Gupta R K Sharma 《The Journal of the Operational Research Society》2007,58(3):368-377
This paper investigates a discrete-time single-server finite-buffer queueing system with multiple vacations in which arrivals occur according to a discrete-time renewal process. Service and vacation times are mutually independent and geometrically distributed. We obtain steady-state system length distributions at prearrival, arbitrary and outside observer's observation epochs under the late arrival system with delayed access and early arrival system. The analysis of actual waiting-time for both the systems has also been carried out. The model has potential application in high-speed computer network, digital communication systems and other related areas. 相似文献
15.
M.K. Gupta Vijay Gupta Manoj Kumar 《Journal of Mathematical Analysis and Applications》2007,330(2):799-816
In the present paper we introduce a new family of linear positive operators and study some direct and inverse results in simultaneous approximation. 相似文献
16.
In this paper, we suggest and analyze a new two-step predictor–corrector type iterative method free from second derivatives for solving nonlinear equations of the type f(x)=0. This new method includes the two-step Newton method as a special case. We prove that the new iterative method is of fourth-order. Several examples are given to illustrate the efficiency of this new method and its comparison with other iterative methods. This method can be considered as a significant improvement of the Newton method and its variant forms. 相似文献
17.
The paper presents an exact analysis of the dispersion of an immiscible solute in a non-Newtonian fluid (known as an incompressible second-order fluid which shows viscoelastic behaviour) flowing slowly in a parallel plate channel in the presence of a periodic pressure gradient. Using a generalized dispersion model which is valid for all times after the solute injection, the diffusion coefficients K i (τ)(i=1,2,3,…) are obtained as functions of time τ in the case when the initial solute distribution is in the form of a slug of finite extent. The analysis leads to the novel result that K 2(τ) (which is a measure of the longitudinal dispersion coefficient of the solute) has a steady part S in addition to a fluctuating part D 2(τ) due to the pulsatility of the flow. It is found that S decreases with increase in the viscoelastic parameter M for given values of the amplitude λ and frequency ω of the pressure pulsation. On the other hand, it is found that at a fixed instant τ, the amplitude of D 2(τ) increases with increase in M for given values of λ and ω. Further it is shown that at a given instant τ, the amplitude of D 2(τ) decreases with increase in ω for given λ and M and the profile for D 2(τ) becomes progressively flatter with increase in ω. Finally the axial distribution of the average concentration θ m of the solute over the channel cross-section is determined at different instants after the solute injection for several values of M, λ and ω. The present study is likely to have important bearing on the problem of dispersion of tracers in blood flow through arteries. 相似文献
18.
G. Principi A. Maddalena Ajay Gupta R. Dal Maschio S. Diré 《Hyperfine Interactions》1992,69(1-4):619-622
Conversion electron Mössbauer spectroscopy (CEMS) has been used to study tin oxide films prepared by sol-gel dipping and sputtering. The spectra of films prepared by sol-gel route result close to that of crystalline SnO2 after heat treatment at a temperature as low as 150°C. The Mössbauer parameters of as sputter deposited films indicate that the structure of the deposited stannic oxide has an amorphous character more pronounced for thinner samples. The structure becomes predominantly that of crystalline SnO2 by heating at 550°C for 30 min provided the film thickness is higher than 10 nm. 相似文献
19.
To improve aircraft crash safety, conditions critical to occupants survival during a crash must be known. In view of the importance of this problem, studies of post-crash dynamic behavior of victims are necessary in order to reduce severe injuries. In this study, crash dynamics program SOM-LA/TA, incorporating a dynamic model of the human body with a finite element model of the seat structure was used. Modifications were performed in the program for reconstruction of an occupant's head impact with the interior walls or bulkhead. A viscoelastic-type contact force model was used to represent the compliance characteristics of the bulkhead. Correlated studies of analytical simulations with impact sled test results were accomplished. A parametric study of the coefficients in the contact force model was then performed in order to obtain the correlations between the coefficients and the Head Injury Criteria. A measure of optimal values for the bulkhead compliance and displacement requirements was thus achieved in order to keep the possibility of a head injury as little as possible. This information could in turn be used in the selection of suitable materials for the bulkhead, instrument panel, or interior walls of an aircraft. 相似文献
20.
In the present paper we derive three interesting expressions for the composition of two most general fractional integral oprators whose kernels involve the product of a general class of polynomials and a multivariableH-function. By suitably specializing the coefficients and the parameters in these functions we can get a large number of (new and known) interesting expressions for the composition of fractional integral operators involving classical orthogonal polynomials and simpler special functions (involving one or more variables) which occur rather frequently in problems of mathematical physics. We have mentioned here two special cases of the first composition formula. The first involves product of a general class of polynomials and the Fox’sH-functions and is of interest in itself. The findings of Buschman [1] and Erdélyi [4] follow as simple special cases of this composition formula. The second special case involves product of the Jacobi polynomials, the Hermite polynomials and the product of two multivariableH-functions. The present study unifies and extends a large number of results lying scattered in the lierature. Its findings are general and deep. 相似文献