首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   21767篇
  免费   2286篇
  国内免费   5307篇
电工技术   470篇
综合类   2481篇
化学工业   2344篇
金属工艺   886篇
机械仪表   1583篇
建筑科学   7716篇
矿业工程   1150篇
能源动力   376篇
轻工业   783篇
水利工程   884篇
石油天然气   1939篇
武器工业   1462篇
无线电   859篇
一般工业技术   4799篇
冶金工业   561篇
原子能技术   451篇
自动化技术   616篇
  2024年   94篇
  2023年   266篇
  2022年   620篇
  2021年   784篇
  2020年   696篇
  2019年   616篇
  2018年   652篇
  2017年   888篇
  2016年   961篇
  2015年   972篇
  2014年   1263篇
  2013年   1545篇
  2012年   1477篇
  2011年   1947篇
  2010年   1485篇
  2009年   1570篇
  2008年   1519篇
  2007年   1659篇
  2006年   1720篇
  2005年   1680篇
  2004年   1426篇
  2003年   1021篇
  2002年   763篇
  2001年   604篇
  2000年   520篇
  1999年   463篇
  1998年   388篇
  1997年   312篇
  1996年   264篇
  1995年   227篇
  1994年   154篇
  1993年   146篇
  1992年   114篇
  1991年   117篇
  1990年   89篇
  1989年   66篇
  1988年   55篇
  1987年   42篇
  1986年   38篇
  1985年   22篇
  1984年   18篇
  1983年   11篇
  1982年   19篇
  1981年   4篇
  1980年   19篇
  1979年   12篇
  1966年   4篇
  1965年   3篇
  1964年   3篇
  1955年   5篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
东海西湖凹陷探井钻井完井液技术   总被引:2,自引:2,他引:0  
介绍了东海西湖凹陷的储层特征。在分析探井储层损害原因的基础上,提出了优化的海水钻井完井液体系。两口井应用表明,储层保护效果明显。  相似文献   
132.
通过密度、可见光光谱、红外吸收光谱、Co-60辐照损伤试验及荧光光谱的测试,研究了PbO-Bi2O3-B2O3-SiO2玻璃系的光学性能与结构.密度最高可达8.464g/cm3其紫外吸收达截止波长随Pb2+及Bi3+含量升高而红移.玻璃熔化温度低达850℃.在PbO-Bi2O3-B2O3系玻璃中加人SiO2可使玻璃结构更致密.室温下该系统玻璃在360nm有一个宽的激发峰,能产生418um及438um两个弱的发射峰.该系统玻璃的结构是由[SiO4]4-、[BO3]3-、[BO4]5-、[PbO4]6-及[BiO6]9-构成.其中部分Pb2+及Bi3+以网络外体进入玻璃.  相似文献   
133.
A fundamental research of structural defects induced upon post-growth processing of ZnSe/GaAs epilayers grown on (100) GaAs was done by identifying defect-related reflections in the transmission electron diffraction (TED) patterns of ZnSe. Structural artifacts, other than the as-grown defects, on this material system could be excluded according to our results. Four types of abnormal reflections have been observed in addition to primary reflections. These extra reflections are sensitive to the post-growth processing of ZnSe epilayers and may arise from various external effects, rather than epitaxy growth, such as irradiation damage, surface oxidation, and surface contamination. By mapping these reflections at several major zone axes using TED patterns, we found that the reciprocal lattice for a ZnSe crystal with structural defects consists of two distinct types of extra reflections associated with irradiation damage. The first type of extra reflections is ±1/3{111} and the other is ±1/2{111} corresponding to pure-edge and non-edge dislocation loops, respectively. For (100) oriented wafers, the ±1/3{111} and ±1/2{111} reflections were observed only on two of the four possible 〈111〉 variants (i.e. [111]Zn and [111]Zn)and this phenomenon was attributed to the anisotropy of defect distribution. Extra reflections associated with surface oxidation and contamination are also observed. The orientation relationships between a surface hexagonal ZnO and a cubic ZnSe film are [0001]ZnO//[−111]ZnSe, and [01−11]ZnO//[011]ZnSe. The origin, characterization, and elimination of these induced reflections are discussed. With the knowledge about these extra effects on structural defect formation, we have shown the real microstructure of ZnSe epilayers.  相似文献   
134.
本文对激光光束通过望远系统后光束的变化进行了分析,并对6倍望远镜进行了激光损伤和破坏的实验研究,实验结果表明,激光对望远系统的损伤破坏主要是热效应所致,损伤破坏的情况与激光的能量,波长,脉冲宽度和望远系统的结构有关。  相似文献   
135.
程硕  王伟  谈明光  陈建敏  张桂林  李燕 《核技术》2006,29(3):182-188
通过ICP-MS测定了上海市吴淞地区大气PM2.5水溶成分金属元素含量,通过四唑盐(MTT)测定细胞存活率,通过超氧化歧化酶(SOD)和丙二醛(MDA)两个指标测定细胞氧化损伤,琼脂糖凝胶电泳测定细胞内DNA损伤,流式细胞仪(Flow cytometry)检测细胞凋亡和周期等实验研究了大气PM2.5水溶成分的细胞毒性.实验结果表明:PM2.5可溶成分中Zn元素含量最高,PM2.5在一定浓度范围内能导致细胞死亡,引起细胞的氧化损伤,影响DNA的复制,阻碍细胞增殖,但没有诱导细胞凋亡.PM2.5中可溶金属离子对细胞毒性表现为联合作用.  相似文献   
136.
A new hydrocyclone was designed with a winged core fixed below the vortex finder in this study. With the winged core, the turbulence structure characteristics inside the hydrocyclone, including time-averaged pressure, pressure fluctuation, relative pressure fluctuation characteristics, and distribution characteristics of the probability density of the turbulence pressure, were all positively controlled. By controlling the turbulence structure, the performance of the new hydrocyclone was improved effectively. Compared with the common hydrocyclone, the new hydrocyclone was featured with lower energy loss coefficient, higher reduced separation efficiency, higher separation sharpness and larger capacity.  相似文献   
137.
大庆油田深层岩石力学特性参数的试验研究   总被引:2,自引:1,他引:1  
通过对岩石可钻性、硬度、抗压强度等参数的测量 ,对大庆油田深井不同深度、不同层位、不同岩性的具有广泛代表性的岩心进行试验。并通过线性回归分析 ,建立了适用性较广的钻井岩石力学特性参数预测模型 ,为深井钻头选型提供了依据。  相似文献   
138.
The surface-crack-in-flexure (SCF) method uses a Knoop indenter to create small, semielliptical surface precracks in beam specimens. Lateral cracks may interfere with the primary median crack and cause errors of up to 10% in determination of fracture toughness, particularly for materials for which the fracture toughness is ∼3 MPa·m1/2 or less. Although the residual-stress-damage zone is ground or polished away by hand by removing 4.5–5 times the indentation depth, this amount may not be sufficient to completely remove the lateral cracks in low-fracture-toughness materials. A series of tests were conducted on sintered alpha silicon carbide with different amounts of material removed after indentation. Once the lateral cracks were fully removed, the SCF results concurred with single-edged-precracked-beam and chevron-notched-beam data collected in accordance with ASTM Designation C1421. A simple remedy for the SCF method is to examine the outer ground surface for remnants of lateral cracks before fracture and to remove more material if necessary.  相似文献   
139.
混凝土破裂过程三维数值模拟及CT验证   总被引:10,自引:1,他引:9  
将混凝土简化成由骨料、水泥砂浆及其二者之间的界面层所组成的三相复合材料,建立了混凝土骨料随机分布三维模型,用界面单元多次细分技术进行有限元网格划分,基于弹性损伤本构关系采用双折线损伤演化模型描述混凝土细观各相弹性损伤退化,用弹性模量的折减程度来反映混凝土试件在加载过程中的损伤程度。对混凝土圆柱体试件进行了数值模拟计算,并从混凝土破坏过程图和荷载-位移曲线图两方面比较了数值模拟的破坏过程与CT试验结果,发现试件破坏时裂纹的萌生、扩展过程与CT观测到的过程具有相似性,说明用本文建立的三维数值模型模拟混凝土材料的损伤破坏过程是基本可行的。  相似文献   
140.
An investigation has been undertaken to determine the damage mechanisms and the associated mechanical response of a 2D reinforced composite of carbon fibers in an SiC CVI-processed matrix subjected to uniaxial tensile and compressive loadings at room temperature. Under tension loading, an extended non-linear stress/strain response was evidenced and related to a multi-stage development of damage involving transverse matrix microcracking, bundle/matrix and inter-bundle debonding as well as thermal residual stress release. This tensile behavior proved to be damageable-elastic with respect to a fictitious thermalstress-free origin of the stress/strain axis lying in the compression domain. In compression, after an initial stage involving closure of the thermal microcracks present from processing, the composite displayed a linear-elastic behavior until failure. The extent of damage over the material was characterized quantitatively at the microscale by the decrease of the average transverse microcrack spacing and at the macroscale by the decrease of both the longitudinal Young's modulus and the in-plane Poisson's ratio.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号