全文获取类型
收费全文 | 15429篇 |
免费 | 980篇 |
国内免费 | 1124篇 |
专业分类
化学 | 7875篇 |
晶体学 | 85篇 |
力学 | 798篇 |
综合类 | 158篇 |
数学 | 3773篇 |
物理学 | 4844篇 |
出版年
2023年 | 144篇 |
2022年 | 347篇 |
2021年 | 402篇 |
2020年 | 376篇 |
2019年 | 373篇 |
2018年 | 317篇 |
2017年 | 400篇 |
2016年 | 425篇 |
2015年 | 378篇 |
2014年 | 530篇 |
2013年 | 1067篇 |
2012年 | 624篇 |
2011年 | 637篇 |
2010年 | 567篇 |
2009年 | 867篇 |
2008年 | 894篇 |
2007年 | 981篇 |
2006年 | 851篇 |
2005年 | 611篇 |
2004年 | 582篇 |
2003年 | 560篇 |
2002年 | 566篇 |
2001年 | 494篇 |
2000年 | 462篇 |
1999年 | 416篇 |
1998年 | 381篇 |
1997年 | 264篇 |
1996年 | 193篇 |
1995年 | 192篇 |
1994年 | 173篇 |
1993年 | 173篇 |
1992年 | 252篇 |
1991年 | 143篇 |
1990年 | 101篇 |
1989年 | 89篇 |
1988年 | 98篇 |
1987年 | 98篇 |
1986年 | 77篇 |
1985年 | 84篇 |
1984年 | 78篇 |
1983年 | 50篇 |
1982年 | 95篇 |
1981年 | 171篇 |
1980年 | 184篇 |
1979年 | 184篇 |
1978年 | 164篇 |
1977年 | 102篇 |
1976年 | 108篇 |
1973年 | 78篇 |
1972年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
181.
Frank Natterer Olga Klyubina 《计算数学(英文版)》2007,25(3):368-373
We study the initial value problem of the Helmholtz equation with spatially variable wave number. We show that it can be stabilized by suppressing the evanescent waves. The stabilized Helmholtz equation can be solved numerically by a marching scheme combined with FFT. The resulting algorithm has complexity n^2 log n on a n x n grid. We demonstrate the efficacy of the method by numerical examples with caustics. For the Maxwell equation the same treatment is possible after reducing it to a second order system. We show how the method can be used for inverse problems arising in acoustic tomography and microwave imaging. 相似文献
182.
Chi-So Lee 《中国化学会会志》1987,34(3):207-210
A differential pulse polarographic method for the determination of nitrate ion has been developed. With 0.5 M CaCl2 as supporting electrolyte, NO?3 is reduced to give a peak with E1/2=–1.836 Volt vs. the Ag/AgCl electrode. The differential pulse polarographic peak height is proportional to the nitrate concentration from 20 to 60 ppm. The detection limit for nitrate is 2 ppm in pure aqueous solution. In the determination of 40 ppm nitrate a relative precision (relative standard deviation) of less than 2% was achieved. Nitrite interferes seriously and should be absent if accurate results are required. The method has been applied to the determination of nitrate in Ammonium Uranyl Tricarbonate (AUT) Solution, results obtained by this method are compared to those obtained by ion chromatography. The agreement between the two sets of results suggests that the DPP method can be used with a fair degree of confidence. 相似文献
183.
We prove analogs of the Telyakovskii-Pochuev criteria for multipliers of uniform convergence and of convergence in the integral metric for multiplicative systems with bounded generating sequence. 相似文献
184.
185.
Jon Samseth Richard J. Spontak Kell Mortensen 《Journal of Polymer Science.Polymer Physics》1993,31(4):467-474
Poly(siloxaneimide) (PSI) segmented copolymers exhibit organized microdomains if the blocks are sufficiently incompatible. As with neat diblock and triblock copolymers, the processing route employed to prepare films of PSI materials is expected to influence the dimensions and/or morphology of the resultant microstructure. In this work, small-angle neutron scattering (SANS) is utilized to characterize the disordered microstructure found in films of a series of PSI copolymers which are subjected to solvent casting and various thermal treatments. Microstructural dimensions such as the periodicity and correlation length are deduced from the Teubner-Strey (TS) model for disordered microemulsions. The scattering intensity of each copolymer up to q = 5.0 nm?1, where q is the scattering vector, is found to scale as q?2.8+?0.1. Results indicate that processing the materials as cast films or as melt-pressed films allowed to cool slowly has a small, but discernible, effect on microstructural characteristics. SANS profiles of films quenched from elevated temperatures reveal a clear transition in microdomain periodicity, which correlates well with the glass transition temperature of the imide microphase in these and other materials of similar chemical structure. © 1993 John Wiley & Sons, Inc. 相似文献
186.
We introduce a new class of experiments which provide graphic insights into the propagation of acoustic waves in anisotropic media. Simply stated, we have devised a means of observing the expanding acoustic wavefront from a point disturbance in a solid. The data may be viewed as a movie or a series of snapshots. The observed wavefronts represent the group-velocity surfaces of acoustic waves, which reflect the basic elastic anisotropy of the solid. The technique has been applied to coherent acoustic waves with frequencies in the megahertz range (at ambient temperatures) and to incoherent heat pulses in the hundred-gigahertz range (at liquid-helium temperatures). In this article, we first provide a pedagogical introduction to wave propagation in elastically anisotropic media, reviewing some early methods for visualizing acoustic waves. Next, we describe the “acoustic wavefront imaging” method and give representative results in crystals and composite materials. Finally, we show how this method relates to recent advances in phonon imaging and internal diffraction of ultrasound. 相似文献
187.
Erik Aurell 《Journal of statistical physics》1990,58(5-6):967-995
I study poles and zeros of zeta functions in one-dimensional maps. Numerical and analytical arguments are given to show that the first pole of one such zeta function is given by the first zero ofanother zeta function: this describes convergence of the calculations of the first zero, which is generally the physically interesting quantity. Some remarks on how these results should generalize to zeta functions of dynamical systems with pruned symbolic dynamics and in higher dimensions follow. 相似文献
188.
Any global minimization algorithm is made by several local searches performed sequentially. In the classical multistart algorithm, the starting point for each new local search is selected at random uniformly in the region of interest. In the tunneling algorithm, such a starting point is required to have the same function value obtained by the last local minimization. We introduce the class of acceptance-rejection based algorithms in order to investigate intermediate procedures. A particular instance is to choose at random the new point approximately according to a Boltzmann distribution, whose temperatureT is updated during the algorithm. AsT 0, such distribution peaks around the global minima of the cost function, producing a kind of random tunneling effect. The motivation for such an approach comes from recent works on the simulated annealing approach in global optimization. The resulting algorithm has been tested on several examples proposed in the literature. 相似文献
189.
本文从“同温度层不完全平行于地面,且在变化”的“任意分层”假设出发,导出了水准折光修正公式,并利用光的色散效应在10~(-6)的精度要求下,求得r=r_T=h_1/h_2=(n_(01)-1)/(n_(02)-1)=常数最后指出,研制双波长激光水准仪的必要性和可能性,并提出了研制此种仪器的技术参数. 相似文献
190.
负载型双金属簇催化剂的多相一氧化碳加氢反应性能索全伶,李晔(内蒙古工业大学化学工程系,呼和浩特010062)殷元骐,金道森(中国科学院兰州化学物理研究所,兰州730000)关键词一氧化碳加氢,多相催化,双金属簇催化剂,钾效应。1.引言异核金属羰基簇作... 相似文献