全文获取类型
收费全文 | 113951篇 |
免费 | 2566篇 |
国内免费 | 440篇 |
专业分类
电工技术 | 1080篇 |
综合类 | 2369篇 |
化学工业 | 18129篇 |
金属工艺 | 5275篇 |
机械仪表 | 3570篇 |
建筑科学 | 3989篇 |
矿业工程 | 636篇 |
能源动力 | 1959篇 |
轻工业 | 6205篇 |
水利工程 | 1554篇 |
石油天然气 | 506篇 |
武器工业 | 3篇 |
无线电 | 11376篇 |
一般工业技术 | 21911篇 |
冶金工业 | 7785篇 |
原子能技术 | 456篇 |
自动化技术 | 30154篇 |
出版年
2023年 | 276篇 |
2022年 | 447篇 |
2021年 | 696篇 |
2020年 | 506篇 |
2019年 | 594篇 |
2018年 | 14981篇 |
2017年 | 13892篇 |
2016年 | 10718篇 |
2015年 | 1401篇 |
2014年 | 1243篇 |
2013年 | 2094篇 |
2012年 | 4737篇 |
2011年 | 11350篇 |
2010年 | 9632篇 |
2009年 | 6899篇 |
2008年 | 8391篇 |
2007年 | 9244篇 |
2006年 | 1546篇 |
2005年 | 2410篇 |
2004年 | 2190篇 |
2003年 | 2086篇 |
2002年 | 1433篇 |
2001年 | 635篇 |
2000年 | 695篇 |
1999年 | 586篇 |
1998年 | 535篇 |
1997年 | 476篇 |
1996年 | 502篇 |
1995年 | 447篇 |
1994年 | 435篇 |
1993年 | 436篇 |
1992年 | 395篇 |
1991年 | 251篇 |
1990年 | 319篇 |
1989年 | 321篇 |
1988年 | 279篇 |
1987年 | 296篇 |
1986年 | 286篇 |
1985年 | 306篇 |
1984年 | 267篇 |
1983年 | 256篇 |
1982年 | 239篇 |
1981年 | 209篇 |
1980年 | 175篇 |
1979年 | 182篇 |
1978年 | 165篇 |
1977年 | 136篇 |
1976年 | 122篇 |
1975年 | 134篇 |
1974年 | 109篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Chloride doped polyaniline conducting polymer films have been prepared in a protic acid medium (HCl) by potentiodynamic method
in an electrochemical cell and studied by cyclic voltammetry and FTIR techniques. The FTIR spectra confirmed Cl- ion doping in the polymers. The polymerization rate was found to increase with increasing concentration of aniline monomer.
But the films obtained at high monomer concentration were rough having a nonuniform flaky polyaniline distribution. Results
showed that the polymerization rate did not increase beyond a critical HCl concentration. Cyclic voltammetry suggested that,
the oxidation-reduction current increased with an increase in scan rate and that the undoped polyaniline films were not hygroscopic
whereas chloride doped polyaniline films were found to be highly hygroscopic. 相似文献
62.
63.
Michael Fish 《电子设计技术》2003,10(1):66-68
1940年,William Hewlett和DavidPackard两人将他们在车库中制作的一种产品投放市场,这种产品就是维氏电桥(Wien-Bridge)振荡器。它由一个单极点高通滤波器与一个单极点低通滤波串联而成。为了保持增益恒定不变,该振荡器电路采用一只白炽灯作指示灯来提供AGC(自动增益控制)。正如所有的白炽灯那样,这种指示灯具有非线性的电阻值。当你给这一振荡器电路通电时,冷白炽灯的电阻很小,从而产生很高的增益。随着增益的增大,温度不断升高的白炽灯的电阻也随之增大。因此,白炽灯具有AGC功能。这种振荡器电路一直使用了60多年,现在仍然在使用中。维氏电桥振荡器存在的唯一问题就是,增益小于1时,它不工作。我在一家电话公司供职时, 相似文献
64.
A coupled finite-element model, CON2D, has been developed to simulate temperature, stress, and shape development during the
continuous casting of steel, both in and below the mold. The model simulates a transverse section of the strand in generalized
plane strain as it moves down at the casting speed. It includes the effects of heat conduction, solidification, nonuniform
superheat dissipation due to turbulent fluid flow, mutual dependence of the heat transfer and shrinkage on the size of the
interfacial gap, the taper of the mold wall, and the thermal distortion of the mold. The stress model features an elastic-viscoplastic
creep constitutive equation that accounts for the different responses of the liquid, semisolid, delta-ferrite, and austenite
phases. Functions depending on temperature and composition are employed for properties such as thermal linear expansion. A
contact algorithm is used to prevent penetration of the shell into the mold wall due to the internal liquid pressure. An efficient
two-step algorithm is used to integrate these highly nonlinear equations. The model is validated with an analytical solution
for both temperature and stress in a solidifying slab. It is applied to simulate continuous casting of a 120 mm billet and
compares favorably with plant measurements of mold wall temperature, total heat removal, and shell thickness, including thinning
of the corner. The model is ready to investigate issues in continuous casting such as mold taper optimization, minimum shell
thickness to avoid breakouts, and maximum casting speed to avoid hot-tear crack formation due to submold bulging. 相似文献
65.
Atsushi Narumi Kosei Kawasaki Harumi Kaga Toshifumi Satoh Naoya Sugimoto Toyoji Kakuchi 《Polymer Bulletin》2003,49(6):405-410
Summary
The potato phosphorylase-catalyzed polymerization of α-D-glucose-1-phosphate (G-1-P) onto poly[styrene-block-(4-vinylbenzyl maltohexaoside)] (1) was performed at the molar ratios of [G-l-P]0 and [maltohexaose]0 of 35, 80, and 250. The product was found to be soluble in dimethyl sulfoxide, which was a good solvent for amylose, and
showed the complex-formation with iodine, indicating that the product was assignable to poly[styrene-block-(styrene-graft-amylose)] (2). The quantitative analysis of the liberated phosphoric acid gave the average degree of polymerization o f the glucose unit
(n) as 27, 5 1, and 180 for 2-I, 2-II, and 2-III, respectively.
Received: 29 November 2002/Accepted: 22 December 2002
Correspondence to Toyoji Kakuchi 相似文献
66.
ZhaoYahong ZhangZhongpei WuWeiling 《电子科学学刊(英文版)》2003,20(3):177-182
Adaptive modulation and power allocation is introduced into the multicarrier DSCDMA system to improve the system performance and bandwidth efficiency.First,the system design appropriate for adaptive modulation and power allocation is given,then the algorithm of adaptive modulation and power allocation is applied.Simulation results demonstrate great performance improvement compared with the fixed modulated one. 相似文献
67.
As an aid towards improving the treatment of exchange and correlation effects in electronic structure calculations, it is
desirable to have a clear picture of the errors introduced by currently popular approximate exchange-correlation functionals.
We have performed ab initio density functional theory and density functional perturbation theory calculations to investigate
the thermal properties of bulk Cu, using both the local density approximation (LDA) and the generalized gradient approximation
(GGA). Thermal effects are treated within the quasiharmonic approximation. We find that the LDA and GGA errors for anharmonic
quantities are an order of magnitude smaller than for harmonic quantities; we argue that this might be a general feature.
We also obtain much closer agreement with experiment than earlier, more approximate calculations. 相似文献
68.
Effect of doping of carbon nanotubes by magnetic transition metal atoms has been considered in this paper. In the case of
semiconducting tubes, it was found that the system has zero magnetization, whereas in metallic tubes the valence electrons
of the tube screen the magnetization of the dopants: the coupling to the tube is usually antiferromagnetic (except for Cr). 相似文献
69.
Rats exposed to a footshock show conditional fear when reexposed to the shock context. Immediate presentation of shock after placement in the context significantly reduces this fear. Preexposure to the context in the absence of shock, coupled with a minimum preshock interval during training, overcomes this immediate shock deficit. Because rats learn about the context during preexposure and express that learning after being reinforced, the context preexposure effect is an aversive analogue of latent learning. The authors examined the effect of the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphovalerate (APV) on the facilitatory effect of context preexposure. Rats were preexposed to a chamber after APV administration. The next day they were placed in the same chamber without drug and received shock 35 s later. APV blocked the facilitatory effect of preexposure. Therefore NMDA receptors are important for contextual latent learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
70.
Due to the demand for real time wavelet processors in applications such as video compression [1], Internet communications compression [2], object recognition [3], and numerical analysis, many architectures for the Discrete Wavelet Transform (DWT) systems have been proposed. This paper surveys the different approaches to designing DWT architectures. The types of architectures depend on whether the application is 1-D, 2-D, or 3-D, as well as the style of architecture: systolic, semi-systolic, folded, digit-serial, etc. This paper presents an overview and evaluation of the architectures based on the criteria of latency, control, area, memory, and number of multipliers and adders. This paper will give the reader an indication of the advantages and disadvantages of each design. 相似文献