全文获取类型
收费全文 | 4181篇 |
免费 | 971篇 |
国内免费 | 685篇 |
专业分类
化学 | 1812篇 |
晶体学 | 91篇 |
力学 | 205篇 |
综合类 | 60篇 |
数学 | 908篇 |
物理学 | 2761篇 |
出版年
2024年 | 11篇 |
2023年 | 61篇 |
2022年 | 222篇 |
2021年 | 171篇 |
2020年 | 182篇 |
2019年 | 138篇 |
2018年 | 149篇 |
2017年 | 226篇 |
2016年 | 240篇 |
2015年 | 211篇 |
2014年 | 352篇 |
2013年 | 411篇 |
2012年 | 298篇 |
2011年 | 348篇 |
2010年 | 261篇 |
2009年 | 293篇 |
2008年 | 272篇 |
2007年 | 241篇 |
2006年 | 200篇 |
2005年 | 220篇 |
2004年 | 167篇 |
2003年 | 155篇 |
2002年 | 136篇 |
2001年 | 118篇 |
2000年 | 107篇 |
1999年 | 91篇 |
1998年 | 76篇 |
1997年 | 63篇 |
1996年 | 75篇 |
1995年 | 66篇 |
1994年 | 43篇 |
1993年 | 32篇 |
1992年 | 27篇 |
1991年 | 24篇 |
1990年 | 22篇 |
1989年 | 23篇 |
1988年 | 14篇 |
1987年 | 13篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1974年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有5837条查询结果,搜索用时 15 毫秒
11.
Some previous works show that symmetric fixed- and variable-stepsize linear multistep methods for second-order systems which do not have any parasitic root in their first characteristic polynomial give rise to a slow error growth with time when integrating reversible systems. In this paper, we give a technique to construct variable-stepsize symmetric methods from their fixed-stepsize counterparts, in such a way that the former have the same order as the latter. The order and symmetry of the integrators obtained is proved independently of the order of the underlying fixed-stepsize integrators. As this technique looks for efficiency, we concentrate on explicit linear multistep methods, which just make one function evaluation per step, and we offer some numerical comparisons with other one-step adaptive methods which also show a good long-term behaviour.
12.
Benson真有效意义下集值优化的广义最优性条件 总被引:12,自引:0,他引:12
本文引入了关于集值映射的α-阶Clarke切导数、α-阶邻接切导数及α-阶 伴随切导数的概念,借此建立了约束向量集值优化Benson真有效解导数型的Kuhn- Tucker条件. 相似文献
13.
The article concerns heterojunction resonant cavity-enhanced (RCE) Schottky photodiodes with GaAs in the absorption layer. The quantum efficiency and linear pulse response have thoroughly been analysed. For the first time, the response of a heterojunction photodiode has been modelled by the phenomenological model for a two-valley semiconductor. The results obtained have shown that the satellite valleys, as well as the parasitic time constant, significantly influence the response and, accordingly, have to be taken into account when analysing and optimizing RCE photodetectors. 相似文献
14.
15.
A method for the deposition of BN onto graphite and other substrates is described. Boron trichloride (BCl3) and ammonia (NH3) diluted with Ar were used as reacting gases. The deposition process was carried out at 1300 K as well as lower temperatures in an open system at pressures of 1 atm. The consequences of the introduction of hydrogen to the system were considered. It was demonstrated that the replacement of argon with hydrogen increases the efficiency of the process as well as the theoretical rate of BN deposition. However, the acceleration of the deposition seems to be unprofitable, because the resulting supersaturation leads to the formation of an amorphous phase. The modification of the experimental conditions were proposed.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday 相似文献
16.
本文通过对有效需求一物价指数统计模型的研究 ,估计出了有效需求方程 ,并且通过对模型的估计结果分析得出如下结论 :物价指数对有效需求的影响是显著的 :通货紧缩、物价指数持续走低是有效需求不足的主要原因 ,治理通货紧缩应该是解决我国有效需求不足的主要任务。 相似文献
17.
Xiangdong Liu Hongwei Liu Kexu Yuan Huiping Du 《International Journal of Infrared and Millimeter Waves》1991,12(9):1023-1034
According to the angular-spectrum method, a radiation beam of an antenna horn in the Fresnal region is decomposed into the plane waves with the fast Fourier transform algorithm and the waves diffracted from a reflection grating are superposed as a diffracted beam. Compared with the reflected beam from the same size mirror, the radiation half width of the diffracted beam from a grating is narrower and its lateral shape is shift. These performances have been experimentally verified at Ka-band by: (1). The relative diffraction efficiencies in the first order with two triangular gratings which is put along propagating direction of a beam produced by a conical lens-horn. (2). H-plane lateral width with power—3dB lapsed of the focus beam from a grating and a mirror in a grating spectrometer for millimeter waves (25–100 GHz). 相似文献
18.
T. Rietmann S. Sohn M. Schröder D. Lipinsky H.F. Arlinghaus 《Applied Surface Science》2006,252(19):6640-6643
In order to improve quantification of high mass ions, the influence of cluster composition on detection efficiencies has been studied using a TOF-SIMS IV with the extended capability of postaccelerating ions up to 20 keV. In this experimental study, we focus on the comparison of detection efficiencies for three types of negatively charged secondary cluster ions: gold-alkanethiolate-clusters (AuxMy), gold-sulfur-clusters (AuxSy) and gold-clusters (Aux). The clusters were sputtered from self-assembled monolayers of hexadecanethiols on gold substrates using 10 keV Ar+ primary ions. The detection efficiencies were derived on the basis of a function for the secondary electron yield and a fourth-order approximated Poisson probability distribution for electron propagation and amplification within the microchannel plate.In addition to the well-known dependence of detection efficiencies on ion mass and energy, which has already been studied for positively charged ions, we were able to show a similar behaviour for the investigated negatively charged secondary ions. We have observed major variations among the three types of clusters at similar mass and energy as predicted in a theoretical approach. The observed differences are due to the different composition of the investigated clusters which has a major influence on the kinetic ion induced electron emission within the microchannel plate. For the first time it was possible to experimentally verify these predictions for detection efficiencies. 相似文献
19.
在常温常压下采用新型旋转电极等离子体反应器,对辉光等离子体作用下的甲烷偶联反应制C2烃进行了研究。结果表明,甲烷偶联反应的主要产物为C2H2,占C2烃的80%以上,能量效率在5.6%~11.2%之间;增加H2含量可以提高CH4转化率和C2烃收率;在500~2200kJ•mol−1的能量密度范围内,CH4转化率随能量密度的增大而线性增加,C2烃收率随着能量密度的增加呈峰形变化趋势。 相似文献
20.
WANG Jiang-yun GU Wen WANG Wen-zhen LIU Xin LIAO Dai-zheng 《高等学校化学研究》2006,22(3):283-286
Introduction Itiswell knownthatcyanogroupsincyanometa latessuchas[Ag(CN)2]-unitscanbeusedasbridg ingligandsandapolymericstructurecanbeformed throughsilver silver(argentophilic)interactions.This propertyhasbeenexploredintheconstructionofmany oligomericandp… 相似文献