全文获取类型
收费全文 | 5913篇 |
免费 | 435篇 |
国内免费 | 165篇 |
专业分类
电工技术 | 282篇 |
综合类 | 235篇 |
化学工业 | 808篇 |
金属工艺 | 175篇 |
机械仪表 | 216篇 |
建筑科学 | 357篇 |
矿业工程 | 81篇 |
能源动力 | 108篇 |
轻工业 | 1967篇 |
水利工程 | 91篇 |
石油天然气 | 221篇 |
武器工业 | 27篇 |
无线电 | 476篇 |
一般工业技术 | 801篇 |
冶金工业 | 165篇 |
原子能技术 | 66篇 |
自动化技术 | 437篇 |
出版年
2023年 | 83篇 |
2022年 | 146篇 |
2021年 | 241篇 |
2020年 | 182篇 |
2019年 | 119篇 |
2018年 | 131篇 |
2017年 | 165篇 |
2016年 | 147篇 |
2015年 | 173篇 |
2014年 | 213篇 |
2013年 | 262篇 |
2012年 | 271篇 |
2011年 | 295篇 |
2010年 | 256篇 |
2009年 | 191篇 |
2008年 | 221篇 |
2007年 | 174篇 |
2006年 | 189篇 |
2005年 | 137篇 |
2004年 | 118篇 |
2003年 | 133篇 |
2002年 | 115篇 |
2001年 | 92篇 |
2000年 | 75篇 |
1999年 | 83篇 |
1998年 | 84篇 |
1997年 | 50篇 |
1996年 | 54篇 |
1995年 | 60篇 |
1994年 | 68篇 |
1993年 | 43篇 |
1921年 | 34篇 |
1919年 | 35篇 |
1917年 | 48篇 |
1916年 | 78篇 |
1915年 | 88篇 |
1914年 | 54篇 |
1913年 | 61篇 |
1912年 | 82篇 |
1911年 | 84篇 |
1910年 | 94篇 |
1909年 | 97篇 |
1908年 | 102篇 |
1907年 | 101篇 |
1906年 | 94篇 |
1905年 | 118篇 |
1904年 | 150篇 |
1903年 | 94篇 |
1902年 | 65篇 |
1901年 | 74篇 |
排序方式: 共有6513条查询结果,搜索用时 484 毫秒
991.
Sonia Gnanapragasam Eberhard Richter Frank Brunner Andrea Denker Richard Lossy Michael Mai Friedrich Lenk Jörg Opitz-Coutureau Gerhard Pensl Jens Schmidt Ute Zeimer Liun Wang Baskar Krishnan Markus Weyers Jaochim Würfl Günther Tränkle 《Journal of Materials Science: Materials in Electronics》2008,19(1):64-67
AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with protons as well as carbon, oxygen, iron and krypton ions of high (68 and 120 MeV) and low (2 MeV) energy with fluences in the range from 1 × 107 to 1 × 1013 cm?2. High energy irradiation with protons, carbon and oxygen produced no degradation in devices while krypton irradiation at the fluence of 1 × 1010 cm?2 resulted in a small reduction of 2% in the transconductance. Similarly, for GaN samples irradiated with protons, carbon and oxygen at high energy no changes were seen by XRD, PL and Hall effect, while changes in lattice constant and a reduction in PL intensity were observed after irradiation with high energy krypton. Low energy irradiation with carbon and oxygen at a fluence of 5 × 1010 cm?2 results in small change in the device performance while remarkable changes in device characteristics are seen at a fluence of 1 × 1012 cm?2 for carbon, oxygen, iron and krypton irradiation. Similarly changes are also observed by XRD, PL and Hall effect for the thick GaN layer irradiated at the fluence of 1 × 1012 cm?2. The device results and GaN layer properties are strongly correlated. 相似文献
992.
Here, we report on a new zeolite-based silicalite nanoparticle that can enhance the transfection efficiencies generated by poly ethylene imine-plasmid DNA (PEI-pDNA) complexes via a sedimentation mechanism and can enhance the transfection efficiencies of pDNA alone when surface functionalized with amine groups. The silicalite nanoparticles have a mean size of 55?nm. Functionalizing the silicalite nanoparticles with amine groups results in a clear transition in zeta potential from -25.9 ± 2.3?mV (pH 7.4) for unfunctionalized silicalite nanoparticles to 4.9 ± 0.7?mV (pH 7.4) for amine functionalized silicalite nanoparticles. We identify that silicalite nanoparticles used to promote non-viral vector acceleration to the cell surface are found in acidic vesicles or the cytoplasm but not the nucleus. An MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) assay showed that the silicalite nanoparticles were non-toxic at the concentrations tested for transfection. We show that surface functionalization of silicalite nanoparticles with amine groups results in a significant (230%) increase in transfection efficiency of pDNA when compared to unfunctionalized silicalite nanoparticles. Silicalite nanoparticles enhanced pDNA-PEI induced transfection of human embryonic kidney (HEK-293) cells by over 150%. 相似文献
993.
得到性能良好的SA/PU共混微球以用于药物缓控释;方法:利用预聚-扩链-中和-分散法合成阴离子型PU水溶液;将SA水溶液与PU水溶液按质量比为1∶1,1∶2,1∶3,1∶4,1∶5混合,用滴制法制备共混微球;测定了微球的凝胶化、圆整性及溶胀性能.结果表明:用TDI与PEG-6000和PEG-4000反应才能制备出PU水溶液,而只有水溶液才能和海藻酸钠水溶液混合完全得到理想的共混溶液;SA/PU共混微球28h后凝胶化完全,SA微球48h后可凝胶化完全.PEG-6000合成的PU与海藻酸钠共混得到的微球圆整性要好于PEG-4000合成的PU与海藻酸钠共混得到的微球.且随着复合微球中PU的含量增大,微球的圆整性变好;SA/PU复合微球在蒸馏水(pH7)和盐酸(pH1)中均不溶胀,在磷酸缓冲溶液(pH6.86)中快速溶胀,在4~5h后开始崩解.结论:SA/PU复合微球在胃液中保持原状,在肠液中溶胀,可作为药物的缓释载体. 相似文献
994.
The primary focus of this work is to elucidate the location and extent of exfoliation of clay on fracture (under both static and dynamic loading conditions) of melt-compounded nylon 66/clay/SEBS-g-MA ternary nanocomposites fabricated by different blending sequences. Distinct microstructures are obtained depending on the blending protocol employed. The state of exfoliation and dispersion of clay in nylon 66 matrix and SEBS-g-MA phase are quantified and the presence of clay in rubber is shown to have a negative effect on the toughness of the nanocomposites. The level of toughness enhancement of ternary nanocomposites depends on the blending protocol and the capability of different fillers to activate the plastic deformation mechanisms in the matrix. These mechanisms include: cavitation of SEBS-g-MA phase, stretching of voided matrix material, interfacial debonding of SEBS-g-MA particles, debonding of intercalated clay embedded inside the SEBS-g-MA phase, and delamination of intercalated clay platelets. Based on these results, new insights and approaches for the processing of better toughened polymer ternary nanocomposites are discussed. 相似文献
995.
实际边坡动力稳定性受地震竖向与水平方向效应共同作用,传统边坡地震永久位移计算方法较少考虑竖向地震波影响,采用实际地震的竖向与水平方向加速度时程曲线共同效应更符合工程实际。基于极限分析上限法和Newmark刚塑性滑块模型,提出一种基于实际水平向与竖向地震加速度时程曲线的边坡永久位移计算改进方法,以3个工程边坡为例,探讨了两组具有代表性实测典型水平和竖向地震地面运动记录对边坡地震永久位移计算的影响。研究结果表明:不考虑竖向地震加速度时程曲线时,本文方法可蜕化为与前人方法兼容;不同地震波的竖向与水平地震动时程曲线的叠加效应不同,竖向地震对边坡永久位移的影响不可忽略。 相似文献
996.
为了研究高湿度地区工频电场强度测量值偏大的问题,本文从环境湿度对测量系统的绝缘木支架电气特性参数的影响入手,用复介电常数描述木支架的电气特性,建立了准静态场的求解模型,应用有限元法求解了输电线路下方的空间电场分布,以数值模拟的方法获得了环境湿度对空间电场测量结果的影响规律。研究表明,在实际工况中环境湿度通过改变木支架含水率影响了支架的导电特性,导致支架附近的空间电场发生畸变,是引起电场强度增大的主要因素。提出了为高湿度地区的紧急工频电场强度测试时利用金属支架测量并修正结果的方法。 相似文献
997.
998.
智能电网调度控制系统中历史数据服务优化方案 总被引:1,自引:1,他引:0
在智能电网调度控制系统中,历史数据存储规模呈爆发式增长态势,海量历史数据的冲击,影响了数据访问效率,为满足历史数据高效访问的需求,针对历史数据服务提出了全新的解决方案。文中提出增加历史数据缓冲区,缓存管理电网热点数据,提高数据处理效率,减轻历史库负载;优化任务分配器,协调数据缓存区与关系库分工作业,对历史数据服务进行有效的管理和调度;设计历史数据会话监听机制,实时监视数据库会话状态,保障历史数据服务的安全性和稳定性。实践证明,基于缓存的历史数据服务能够满足爆发式海量数据场景下的数据访问需求,保障电网调度控制系统的安全稳定运行。 相似文献
999.
Ding X Wang XM Xie ZQ Xiang CH Mai BX Sun LG Zheng M Sheng GY Fu JM 《Environmental science & technology》2007,41(15):5204-5209
During the 2003 Chinese Arctic Research Expedition (CHINARE2003) from the Bohai Sea to the high Arctic (37 degrees N to 80 degrees N) aboard the icebreaker Xuelong (Snow Dragon), air samples were collected for the analysis of hexachlorocyclohexanes (HCHs) in the North Pacific Ocean and adjacent Arctic region. The sigma HCHs (alpha-HCH + gamma-HCH) ranged from 2.3 to 95.1 pg/m3 with the highest levels observed in Far East Asia (32.5 pg/m3), followed by the North Pacific Ocean (17.0 pg/m3) and the Arctic (7.3 pg/ m3). Compared to previous studies in the same areas in 1990s, our measurements were approximately 1 order of magnitude lower. Because of disproportionate chemical reduction and physical fractioning during long-range transport, the ratios of alpha-HCH to gamma-HCH (alpha/gamma-HCH) showed a significant increasing trend from low to high latitudes, suggesting thatthe alpha/gamma-HCH range of 4-7 could not be used to identify sources of technical HCHs especially in remote areas. The ratios of (+)-alpha-HCH to the sum of (+)-alpha-HCH and (-)-alpha-HCH were on average much more biased from 0.5 compared to previous observations in mid-1990s, indicating the exchange of atmospheric alpha-HCH with those in the oceans, where (+)-alpha-HCH was selectively depleted in biological degradation processes. Estimated fugacity ratios based on available data for both alpha-HCH and gamma-HCH further implied their net volatilization from seawater to air in the Arctic Ocean. 相似文献
1000.