全文获取类型
收费全文 | 50861篇 |
免费 | 5332篇 |
国内免费 | 3535篇 |
专业分类
电工技术 | 3907篇 |
技术理论 | 1篇 |
综合类 | 5300篇 |
化学工业 | 5562篇 |
金属工艺 | 6106篇 |
机械仪表 | 4225篇 |
建筑科学 | 6170篇 |
矿业工程 | 3208篇 |
能源动力 | 1277篇 |
轻工业 | 2603篇 |
水利工程 | 2016篇 |
石油天然气 | 2683篇 |
武器工业 | 679篇 |
无线电 | 3412篇 |
一般工业技术 | 7293篇 |
冶金工业 | 3228篇 |
原子能技术 | 469篇 |
自动化技术 | 1589篇 |
出版年
2024年 | 234篇 |
2023年 | 768篇 |
2022年 | 2034篇 |
2021年 | 2131篇 |
2020年 | 1686篇 |
2019年 | 1495篇 |
2018年 | 1359篇 |
2017年 | 1753篇 |
2016年 | 1886篇 |
2015年 | 2022篇 |
2014年 | 2940篇 |
2013年 | 3090篇 |
2012年 | 3471篇 |
2011年 | 3841篇 |
2010年 | 2917篇 |
2009年 | 3004篇 |
2008年 | 2754篇 |
2007年 | 3337篇 |
2006年 | 2928篇 |
2005年 | 2505篇 |
2004年 | 2128篇 |
2003年 | 1846篇 |
2002年 | 1591篇 |
2001年 | 1304篇 |
2000年 | 1199篇 |
1999年 | 936篇 |
1998年 | 767篇 |
1997年 | 692篇 |
1996年 | 590篇 |
1995年 | 476篇 |
1994年 | 410篇 |
1993年 | 315篇 |
1992年 | 292篇 |
1991年 | 189篇 |
1990年 | 181篇 |
1989年 | 175篇 |
1988年 | 109篇 |
1987年 | 77篇 |
1986年 | 48篇 |
1985年 | 40篇 |
1984年 | 42篇 |
1983年 | 24篇 |
1982年 | 29篇 |
1981年 | 11篇 |
1980年 | 15篇 |
1979年 | 21篇 |
1964年 | 10篇 |
1961年 | 6篇 |
1959年 | 8篇 |
1955年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
脉冲镀金在半导体激光器中的应用及工艺优化 总被引:1,自引:0,他引:1
为提高半导体激光器芯片的焊接成功率以及器件的性能寿命,采用脉冲电镀技术在半导体激光器芯片 P 面沉积了厚金层,详细研究了镀金液 pH 值和电流通断比对镀金层组织形貌、表面粗糙度、内应力、沉积均匀性以及粘附力的影响规律。结果表明,表面粗糙度随 pH 值的升高或通断比的提高而增大。沉积均匀性随 pH 的增大先降低后升高,而随通断比的增大而变差。pH 较大(>10.0)或较小(<8.5)时,镀金层粘附性均不理想。而通断比对镀金层的粘附性影响不大。不同条件下,金膜内应力均为张应力,大小为29~88 MPa。 相似文献
992.
993.
利用电化学技术和U形弯试样浸泡实验研究了16Mn钢及其模拟热影响区(HAZ)在不同pH的碱性硫化物和Cl-介质中的应力腐蚀开裂(SCC)行为与机理.结果表明:16Mn钢原始组织、粗晶组织(空冷组织)和硬化组织(淬火组织)在pH为11.8的碱性硫化物环境中均近似呈钝化状态,维钝电流密度依次降低;随着pH的降低,原始组织和粗晶组织的阳极过程逐渐由钝化态转变为活化态;HAZ中硬化组织、粗晶组织和原始组织在碱性硫化物环境下的SCC敏感性依次降低,其中硬化组织具有明显的SCC特征;随着pH的降低,SCC裂纹有从沿晶裂纹转变为穿晶和沿晶混合裂纹的趋势,并且裂纹的宽度增加. 相似文献
994.
《The IES Journal Part A: Civil & Structural Engineering》2013,6(4):296-303
This paper presents the study of a plane elasticity problem for a concentric circular ring loaded by a system of equal radial forces equally spaced around its outer ring boundary. The problem is solved by the Kolosov–Muskhelishvili complex variables method. Explicit formulas are derived for calculating components of the stress tensor and of the displacement vector. Numerical values for circumferential stress and radial displacement are given for various ratios of inner and outer ring radii and various numbers of forces. 相似文献
995.
996.
探讨了直角域附近一圆形衬砌对压电材料的反平面动力学行为的影响.通过利用镜像法对直角域情况进行等效处理到全空间中,在复数域中利用衬砌周围的边界条件构造方程求解未知系数,并通过编程进行最终的数值计算得出压电材料在衬砌周围的动应力和电场集中系数.在数值计算中,重点讨论了衬砌和基体为不同材料、衬砌和基体电极化方向相同和不同时,它们随压电材料物理系数、结构几何参数,以及入射波的频率变化的计算结果.结果表明,在SH波的作用下,压电材料在衬砌周围的动应力集中系数和电场集中系数都有一定的相似性,并且沿衬砌周围有规律的分布,然而在入射波的频率较低时二者的值相对较大,同时计算结果还与全空间衬砌下的计算结果有一定的相似性. 相似文献
997.
998.
The laser soldering process was introduced in Micro‐USB electronic packaging in this paper. Numerical modeling and analysis was used for laser process parameter design, because experimental measurements of peak temperature and soldering stress are expensive and often intractable. Accurate knowledge of the residual stress and distortion is a prerequisite for the reliability of the electric connector. The present work proposes a three‐dimensional transient heat transfer model of Micro‐USB package laser soldering process. The calculated thermal cycles and distribution are subsequently used to calculate the distortion and thermal stresses. Meanwhile, the effect of process parameters on residual stress and pin distortion was discussed, and the tensile strength, microstructure, and residual stresses of laser soldering were tested. It can be concluded that the proper laser soldering parameters and power can improve the mechanical property of solder joints on Micro‐USB connector. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
999.
Ifechukwu E. Adieze Justina C. Orji Rose N. Nwabueze G.O.C. Onyeze 《The International journal of environmental studies》2013,70(3):490-500
Responses to hydrocarbon stress of four tropical plants Panicum maximum, Zea mays, Centrosema sp. and Pueraria sp. grown in crude oil contaminated soils (1%, 5% and 10% w/w) were evaluated in a green house. Plants’ percentage survival, shoot heights, biomass development, and phytotoxicity susceptibility were used as indicators of growth, stress response and hydrocarbon tolerance. Relative to control, shoot heights and biomass of plants reduced with increasing hydrocarbon concentration, but 1% w/w oil-in-soil, stimulated shoot heights (5.9% and 6.4%) and weights (21.9% and 2.3%) in P. maximum and Centrosema sp. respectively. P. maximum tolerated the contaminant stress with biomass yields of 113% and 57% of control respectively in 1% and 10% w/w oil-in-soil. All the plants had 100% survival in 1% w/w, but considerably reduced survival in 10% w/w oil-in-soil. These results show that P. maximum has great potential for phytoremediation of petroleum contaminated soil. 相似文献
1000.
Maria José Farré Sophie Day Peta A. Neale Daniel Stalter Janet Y.M. Tang Beate I. Escher 《Water research》2013
Disinfection by-products (DBP) formed from natural organic matter and disinfectants like chlorine and chloramine may cause adverse health effects. Here, we evaluate how the quantity and quality of natural organic matter and other precursors influence the formation of DBPs during chlorination and chloramination using a comprehensive approach including chemical analysis of regulated and emerging DBPs, total organic halogen quantification, organic matter characterisation and bioanalytical tools. In vitro bioassays allow us to assess the hazard potential of DBPs early in the chain of cellular events, when the DBPs react with their molecular target(s) and activate stress response and defence mechanisms. Given the reactive properties of known DBPs, a suite of bioassays targeting reactive modes of toxic action including genotoxicity and sensitive early warning endpoints such as protein damage and oxidative stress were evaluated in addition to cytotoxicity. Coagulated surface water was collected from three different drinking water treatment plants, along with reverse osmosis permeate from a desalination plant, and DBP formation potential was assessed after chlorination and chloramination. While effects were low or below the limit of detection before disinfection, the observed effects and DBP levels increased after disinfection and were generally higher after chlorination than after chloramination, indicating that chlorination forms higher concentrations of DBPs or more potent DBPs in the studied waters. Bacterial cytotoxicity, assessed using the bioluminescence inhibition assay, and induction of the oxidative stress response were the most sensitive endpoints, followed by genotoxicity. Source waters with higher dissolved organic carbon levels induced increased DBP formation and caused greater effects in the endpoints related to DNA damage repair, glutathione conjugation/protein damage and the Nrf2 oxidative stress response pathway after disinfection. Fractionation studies indicated that all molecular weight fractions of organic carbon contributed to the DBP formation potential, with the humic rich fractions forming the greatest amount of DBPs, while the low molecular weight fractions formed more brominated DBPs due to the high bromide to organic carbon ratio. The presence of higher bromide concentrations also led to a higher fraction of brominated DBPs as well as proportionally higher effects. This study demonstrates how a suite of analytical and bioanalytical tools can be used to effectively characterise the precursors and formation potential of DBPs. 相似文献