全文获取类型
收费全文 | 425474篇 |
免费 | 4873篇 |
国内免费 | 2148篇 |
专业分类
化学 | 232311篇 |
晶体学 | 6346篇 |
力学 | 17608篇 |
综合类 | 63篇 |
数学 | 49400篇 |
物理学 | 126767篇 |
出版年
2020年 | 3295篇 |
2019年 | 3671篇 |
2018年 | 4545篇 |
2017年 | 4568篇 |
2016年 | 6932篇 |
2015年 | 4686篇 |
2014年 | 6882篇 |
2013年 | 18639篇 |
2012年 | 14484篇 |
2011年 | 17774篇 |
2010年 | 12169篇 |
2009年 | 11905篇 |
2008年 | 16234篇 |
2007年 | 16526篇 |
2006年 | 15311篇 |
2005年 | 14153篇 |
2004年 | 12632篇 |
2003年 | 11205篇 |
2002年 | 11069篇 |
2001年 | 12377篇 |
2000年 | 9312篇 |
1999年 | 7183篇 |
1998年 | 6018篇 |
1997年 | 5986篇 |
1996年 | 5784篇 |
1995年 | 5271篇 |
1994年 | 5191篇 |
1993年 | 5129篇 |
1992年 | 5575篇 |
1991年 | 5668篇 |
1990年 | 5321篇 |
1989年 | 5262篇 |
1988年 | 5247篇 |
1987年 | 5117篇 |
1986年 | 4983篇 |
1985年 | 6734篇 |
1984年 | 6924篇 |
1983年 | 5721篇 |
1982年 | 6266篇 |
1981年 | 5912篇 |
1980年 | 5670篇 |
1979年 | 5842篇 |
1978年 | 6207篇 |
1977年 | 6123篇 |
1976年 | 6312篇 |
1975年 | 5717篇 |
1974年 | 5898篇 |
1973年 | 6203篇 |
1972年 | 4306篇 |
1971年 | 3407篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
A Garcia-Quiroz David L AzevedoE.C da Silva 《Journal of Physics and Chemistry of Solids》2002,63(10):1863-1866
Properties of oxygenated carbon nitride films have attracted the attention of physics researchers due to their magnetic and physical properties, as well as for their usefulness in the industry. The free radicals were investigated using electron paramagnetic resonance applied in the study of spin concentration due to the different mechanism of preparation of carbon nitride films by RF-discharge with different kinds of plasma. Unpaired spin concentrations, in the order of 1020 per cm3, were measured and their time recombination dependency was important in those films. The films were grown by plasma enhanced chemical vapor deposition using mixtures of hydrocarbons, N2 and O2 in different proportions. 相似文献
72.
D.W. Mueller JrA.L. Crosbie 《Journal of Quantitative Spectroscopy & Radiative Transfer》2002,74(1):43-68
Three-dimensional radiative transfer in an anisotropic scattering medium exposed to spatially varying, collimated radiation is studied. The generalized reflection function for a semi-infinite medium with a very general scattering phase function is the focus of this investigation. An integral transform is used to reduce the three-dimensional transport equation to a one-dimensional form, and a modified Ambarzumian's method is applied to formulate a nonlinear integral equation for the generalized reflection function. The integration is over both the polar and azimuthal angles; hence, the integral equation is said to be in the double-integral form. The double-integral, reflection function formulation can handle a variety of anisotropic phase functions and does not require an expansion of the phase function in a Legendre polynomial series. Complicated kernel transformations of previous single-integral studies are eliminated. Single and double scattering approximations are developed. Numerical results are presented for a Rayleigh phase function to illustrate the computational characteristics of the method and are compared to results obtained with the single-integral method. Agreement between the two approaches is excellent; however, as the transform variable increases beyond five the number of quadrature points required for the double-integral method to produce accurate solutions significantly increases. A new interpolation scheme produces accurate results when the transform variable is large. 相似文献
73.
The adsorption of hydrogen on a clean Cu10%/Ni90% (110) alloy single crystal was studied using flash desorption spectroscopy (FDS), Auger electron spectroscopy (AES), and work function measurements. Surface compositions were varied from 100% Ni to 35% Ni. The hydrogen chemisorption on a-surface of 100% nickel revealed strong attractive interactions between the hydrogen atoms in accordance with previous work on Ni(100). Three desorption states (β1, β2 and α) appeared in the desorption spectra. The highest temperature (α) state was occupied only after the initial population of the β2-state. As the amount of copper was increased in the nickel substrate, desorption from the higher energy binding α-state was reduced, indicating a decrease in the attractive interactions among hydrogen atoms. The hydrogen coverage at saturation was not affected by the addition of copper to the nickel substrate until the copper concentration was greater than 25% at which a sharp reduction in saturation coverage occurred. This phenomenon was apparently due to the adsorption of hydrogen on Ni atoms followed by occupation of NiNi and CuNi bridged adsorption sites, while occupation of CuCu sites was restricted due to an energy barrier to migration. 相似文献
74.
A fast-frequency modulated (FM) diode laser has been optically narrowed using the technique of resonant optical feedback, to provide linewidths in each FM mode of ≈ 200 kHz peak-to-peak. With a drive frequency of 50 MHz and modulation index of 0.2, the FM laser has been used for the first time to obtain Doppler-free FM spectra of the Rb D1 line at 795 nm. The potential use of this system for laser frequency stabilisation is discussed. 相似文献
75.
Summary Considered here are model equations for weakly nonlinear and dispersive long waves, which feature general forms of dispersion and pure power nonlinearity. Two variants of such equations are introduced, one of Korteweg-de Vries type and one of regularized long-wave type. It is proven that solutions of the pure initial-value problem for these two types of model equations are the same, to within the order of accuracy attributable to either, on the long time scale during which nonlinear and dispersive effects may accumulate to make an order-one relative difference to the wave profiles.This research was supported in part by the National Science Foundation. A considerable portion of the project was completed while the first author was resident at the Institute for Mathematics and Its Applications, University of Minnesota. 相似文献
76.
R. F. Bikbaev 《Journal of Mathematical Sciences》1991,57(3):3078-3083
Diffraction by a semitransparent screen is considered in the framework of the nonlinear Schrödinger equation. The case of a defocusing medium is investigated. The method used represents a synthesis of the method of Riemann's matrix problem and the technique of Whitham's deformations of spectral curves.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova, Vol. 179, pp. 23–31, 1989. 相似文献
77.
78.
79.
C. Ghezzi E. Gombia R. Mosca M. Pillan 《Applied Physics A: Materials Science & Processing》1989,48(5):457-463
Capacitance DLTS measurements have been performed in VPE GaAs MESFETs prepared on Bridgman Cr-doped and LEC undoped semi-insulating substrates. A band of electron traps not intrinsically related to the VPE growth process and accumulating near the metal (gate) — semiconductor interface was detected in all the samples. Deeper regions into the channel were free from any detectable trap. Near pinch-off conditions, a positive capacitance signal was found to dominate the DLTS spectra only in the case of samples prepared on Cr-doped substrates. The hypothesis of this positive transient being related to changes in the occupation of surface states in the ungated surface access regions has been checked by comparing experimental and calculated dependencies of the signal amplitude on reverse gate voltage. Unexplained discrepancies, together with the absence of positive signal in MESFETs prepared on LEC undoped substrates, suggest the possibility of hole emission from hole traps within the bulk of the device. 相似文献
80.
When solving large complex optimization problems, the user is faced with three major problems. These are (i) the cost in human time in obtaining accurate expressions for the derivatives involved; (ii) the need to store second derivative information; and (iii), of lessening importance, the time taken to solve the problem on the computer. For many problems, a significant part of the latter can be attributed to solving Newton-like equations. In the algorithm described, the equations are solved using a conjugate direction method that only needs the Hessian at the current point when it is multiplied by a trial vector. In this paper, we present a method that finds this product using automatic differentiation while only requiring vector storage. The method takes advantage of any sparsity in the Hessian matrix and computes exact derivatives. It avoids the complexity of symbolic differentiation, the inaccuracy of numerical differentiation, the labor of finding analytic derivatives, and the need for matrix store. When far from a minimum, an accurate solution to the Newton equations is not justified, so an approximate solution is obtained by using a version of Dembo and Steihaug's truncated Newton algorithm (Ref. 1).This paper was presented at the SIAM National Meeting, Boston, Massachusetts, 1986. 相似文献