全文获取类型
收费全文 | 159281篇 |
免费 | 9438篇 |
国内免费 | 5063篇 |
专业分类
电工技术 | 6468篇 |
技术理论 | 7篇 |
综合类 | 8364篇 |
化学工业 | 23348篇 |
金属工艺 | 8950篇 |
机械仪表 | 7879篇 |
建筑科学 | 8191篇 |
矿业工程 | 2916篇 |
能源动力 | 3367篇 |
轻工业 | 10612篇 |
水利工程 | 2935篇 |
石油天然气 | 3945篇 |
武器工业 | 832篇 |
无线电 | 18562篇 |
一般工业技术 | 24160篇 |
冶金工业 | 5623篇 |
原子能技术 | 958篇 |
自动化技术 | 36665篇 |
出版年
2024年 | 513篇 |
2023年 | 1724篇 |
2022年 | 3343篇 |
2021年 | 4328篇 |
2020年 | 3037篇 |
2019年 | 2442篇 |
2018年 | 16818篇 |
2017年 | 15921篇 |
2016年 | 12223篇 |
2015年 | 4144篇 |
2014年 | 4520篇 |
2013年 | 5044篇 |
2012年 | 8736篇 |
2011年 | 15167篇 |
2010年 | 13132篇 |
2009年 | 10329篇 |
2008年 | 11345篇 |
2007年 | 11980篇 |
2006年 | 4028篇 |
2005年 | 4469篇 |
2004年 | 3489篇 |
2003年 | 3379篇 |
2002年 | 2857篇 |
2001年 | 2016篇 |
2000年 | 1653篇 |
1999年 | 1293篇 |
1998年 | 945篇 |
1997年 | 799篇 |
1996年 | 727篇 |
1995年 | 541篇 |
1994年 | 487篇 |
1993年 | 360篇 |
1992年 | 280篇 |
1991年 | 234篇 |
1990年 | 178篇 |
1989年 | 130篇 |
1988年 | 121篇 |
1987年 | 80篇 |
1986年 | 68篇 |
1985年 | 47篇 |
1982年 | 48篇 |
1980年 | 37篇 |
1968年 | 44篇 |
1966年 | 43篇 |
1965年 | 45篇 |
1959年 | 37篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 29 毫秒
91.
WC/Cu复合材料制备及其高温性能 总被引:11,自引:0,他引:11
用机械合金化法结合冷变形,制备了WC/Cu复合材料,研究了冷变形后复合材料的组织特征和高温退火时韵性能变化。结果表明:烧结后的材料经冷变形,组织呈显著纤维状,WC颗粒弥散分布,密度明显提高,达到理论密度的99.2%;复合材料经600~900℃高温退火,强度和硬度略有下降,塑性则有大幅提高;900℃退火时未发生明显的再结晶,界面结合良好;所制备的WC/Cu复合材料有优良的综合性能。 相似文献
92.
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
93.
采用沉淀硫酸化法制备了复合固体超强酸催化剂SO2-4/Fe2O3-γ-Al2O3,确定了其最佳制备工艺条件:硫酸浸渍浓度为0.6 mol/L,浸渍时间4 h;焙烧温度550℃,焙烧时间3 h.并采用该催化剂合成丁酸丁酯,考察了物料配比、催化剂用量和反应时间对酯化反应的影响,确定了丁酸丁酯的最佳合成条件:丁醇与丁酸摩尔比为1.4:1,催化剂用量0.9%(以反应物质量计),回流条件下反应3.0 h,在此最佳合成条件下,酯化率可达95.6%. 相似文献
94.
95.
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
96.
超级电容器在汽车启动中的应用 总被引:2,自引:0,他引:2
根据超级电容器的结构特性,介绍在汽车启动过程中如何利用超级电容器减小对车内其他电子设备的干扰,改善汽车的启动性能,延长蓄电池使用寿命. 相似文献
97.
R. Murgan F. Razak D. R. Tilley T. Y. Tan J. Osman M. N. A. Halif 《Computational Materials Science》2004,30(3-4):468-473
We derive an expression for transmittivity (TSHG) of second harmonic generation (SHG) signals from a ferroelectric (FE) film. Intensities of up and down fields in the medium are investigated in relation to TSHG. The derivations are made based on undepletion of input fields and nonlinear wave equation derived from the Maxwell equations. We present two cases: film without mirrors and with partial mirrors. Expressions for the newly derived nonlinear susceptibility coefficients of SHG for real crystal symmetry [J. Opt. Soc. Am. B 19 (2002) 2007] are used to get more realistic results. Variations in TSHG with respect to film thickness are illustrated. 相似文献
98.
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
99.
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
100.
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