全文获取类型
收费全文 | 60804篇 |
免费 | 6806篇 |
国内免费 | 4084篇 |
专业分类
电工技术 | 3660篇 |
技术理论 | 7篇 |
综合类 | 6537篇 |
化学工业 | 10447篇 |
金属工艺 | 4964篇 |
机械仪表 | 2826篇 |
建筑科学 | 13508篇 |
矿业工程 | 1155篇 |
能源动力 | 1200篇 |
轻工业 | 2141篇 |
水利工程 | 656篇 |
石油天然气 | 1305篇 |
武器工业 | 859篇 |
无线电 | 3762篇 |
一般工业技术 | 10480篇 |
冶金工业 | 1437篇 |
原子能技术 | 266篇 |
自动化技术 | 6484篇 |
出版年
2024年 | 264篇 |
2023年 | 826篇 |
2022年 | 1558篇 |
2021年 | 1859篇 |
2020年 | 1960篇 |
2019年 | 1671篇 |
2018年 | 1610篇 |
2017年 | 2102篇 |
2016年 | 2315篇 |
2015年 | 2375篇 |
2014年 | 3636篇 |
2013年 | 3341篇 |
2012年 | 4476篇 |
2011年 | 4722篇 |
2010年 | 3877篇 |
2009年 | 4025篇 |
2008年 | 3511篇 |
2007年 | 4409篇 |
2006年 | 4016篇 |
2005年 | 3251篇 |
2004年 | 2587篇 |
2003年 | 2293篇 |
2002年 | 1900篇 |
2001年 | 1697篇 |
2000年 | 1488篇 |
1999年 | 1195篇 |
1998年 | 918篇 |
1997年 | 769篇 |
1996年 | 642篇 |
1995年 | 548篇 |
1994年 | 443篇 |
1993年 | 374篇 |
1992年 | 282篇 |
1991年 | 189篇 |
1990年 | 127篇 |
1989年 | 98篇 |
1988年 | 83篇 |
1987年 | 50篇 |
1986年 | 27篇 |
1985年 | 27篇 |
1984年 | 22篇 |
1983年 | 14篇 |
1982年 | 20篇 |
1981年 | 14篇 |
1980年 | 23篇 |
1979年 | 15篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1959年 | 7篇 |
1951年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Electrical Breakdown Properties and Space Charge Formation in High Temperature Region in Ultraviolet Ray Irradiated PVC 下载免费PDF全文
Polyvinyl chloride (PVC) is the most popular insulating material for electric wiring instruments. However, an exothermic reaction above 150 °C may cause deterioration of the insulating properties of PVC. Therefore, it is important to clarify the heat degradation in PVC, not only to investigate the ignition of electrical wiring products but also to use electrical products safely. It is known that ultraviolet (UV) irradiation causes chemical deterioration of PVC and an increase in its conductivity. Generally, it has been thought that the electrical breakdown properties, electrical conduction, and insulating performance are affected by space charge accumulation in an insulating material. A high temperature pulsed electroacoustic (PEA) system usable up to 250 °C has been developed, and the PEA system can measure the space charge distribution and conduction current in the high temperature range simultaneously. In this investigation, the space charge distribution and conduction current were measured up to electrical breakdown in a non‐UV irradiated sample (normal PVC) and in 353 nm and 253 nm UV‐irradiated PVC samples in the range from room temperature to 200 °C in a DC electric field. In the short wavelength UV irradiated PVC sample (253 nm, 300 h), a deterioration of breakdown strength at 90 °C to 150 °C and negative packet‐like charges were observed at 60 °C and 100 °C, a positive charge accumulated in front of both the anode and cathode above 90 °C, and a higher electric field near the cathode side because the positive charge of the cathode side was greater. 相似文献
32.
33.
如今我国信息化技术全面发展,尤其对于工程测量工作领域来讲,不管是在工程建设和管理方面都产生不小的支撑引导效用。由此,笔者具体结合如今工程建设信息化测绘核心任何以及设备布置细节,进行结构整体安全管理周期和周边地理空间信息技术发展能效整理解析,试图将工程测量最新发展机遇和技术挑战问题处理完全。希望能够借此为日后一定时期范围内相关工程测量规划主体提供合理指导性建议内容,最终为我国各类工程事业可持续发展前景绽放奠定深刻适应基础。 相似文献
34.
The present study focuses on experimental investigation of through the thickness displacement and strain field in thin adhesive layer in single sided (unsymmetrical) patch repaired CFRP (carbon fiber reinforced polymer) panel under tensile load. Digital image correlation (DIC) technique is employed to acquire the displacement and strain (longitudinal, peel and shear) field. Experimental determination of shear transfer length based on shear strain field obtained from DIC is introduced to estimate the optimum overlap length which is an essential parameter in patch design for the repair of CFRP structures. Further, DIC experiment with magnified optics is performed to get an insight into complex and localized strain field over thin adhesive layer especially at critical zones leading to damage initiation. The failure mechanism, load displacement behavior, damage initiation and propagation are closely monitored using DIC. The influence of patch edge tapering on strain distribution in adhesive layer is also investigated. The DIC successfully captures the global and localized strain field at critical zones over thin adhesive layer and further helps in monitoring the damage based on strain anomalies. Strains are found to have maximum magnitude at the patch overlap edge and the shear strain level in adhesive layer is higher than the peel strain. Normal tapering increases the peel strain and has negligible influence on shear strain level in adhesive layer. The recommended overlap length is found to be consistent with the recommendation in the literature. Whole field strain pattern and the overlap length obtained from experiment are further compared with the finite element analysis results and they appear to be in good coherence. 相似文献
35.
目前,基于因素空间理论的背景基提取算法计算过程复杂,初始化必须依赖各因素极值,基点数量提取冗余等原因,未能在应用中取得很好效果。为此,结合内点判别法和知识可继承、可扩展的思想,提出一种计算简单、初始化独立、基点数量小的改进的背景基提取算法。然后,利用改进的背景基提取算法构造出一种全新的数据分类算法-基点分类算法,基点分类算法以提取每一类样本的背景基为预测模型,再通过新定义的λ-背景基,优化预测模型。数值实验表明:基点分类算法原理简单、构造难度小、分类模型泛化能力强,预测能力准确率高,同时严格的模型限定区域又能为识别新类别提供新方法。 相似文献
36.
Ponnusamy Nandha Kumar Shangavi Subramanian Uthirapathy Vijayalakshmi Sanjeevi Kannan 《Journal of the American Ceramic Society》2020,103(6):3528-3540
Structurally stable β-Ca3(PO4)2/t-ZrO2 composite mixtures with the aid of Dy3+ stabilizer were accomplished at 1500°C. The precursors comprising Ca2+, P5+, Zr4+, and Dy3+ have been varied to obtain five different combinations. The results revealed the fact that complete phase transformation of calcium-deficient apatite to β-Ca3(PO4)2 occurred only at 1300°C, whereas the evidence of t-ZrO2 crystallization is obvious at 900°C. The dual occupancy of Dy3+ at β-Ca3(PO4)2 and t-ZrO2 structures was evident; however, Dy3+ initially prefers to occupy β-Ca3(PO4)2 lattice until its saturation limit and thereafter accommodates at the lattice site of ZrO2. The typical absorption and emission behavior of Dy3+ were noticed in all the systems and, moreover, the surrounding symmetry of Dy3+ domains has been determined from the luminescence study. All the systems ensured paramagnetic response that is generally contributed by the presence of Dy3+. A gradual increment in the phase content of t-ZrO2 in the composite mixtures ensured a significant improvement in the hardness and Young's modulus of the investigated compositions. 相似文献
37.
垦71井区三维VSP资料波场分离方法应用研究 总被引:8,自引:5,他引:3
三维VSP资料是多偏移距VSP资料,资料中多种类型的波叠合在一起形成复杂波场。从复杂波场中分离出单一的保幅反射波波场是三维VSP波场分离的重要工作。常规二维VSP波场处理方法单一,难以适用于复杂的三维波场处理。针对三维VSP资料的波场特点,以分离上行反射P波为例,将单一波场分离方法加以适当组合,对垦71井区三维VSP实际资料进行了应用研究。结果表明,波场分离处理中叠加消去法和中值滤波相结合以及F—K滤波和中值滤波相结合的方法克服了单一方法的缺陷,波场处理后获得了波组特征明显、波场清晰单一的上行反射P波保幅波场,取得了较好的波场分离效果。 相似文献
38.
39.
40.
复合材料的最新进展与应用 总被引:2,自引:0,他引:2
叙述了复合材料的发展概况 ,详细介绍了增强材料的种类、性能、形式及加工技术 ,指出了复合材料具有广阔的应用前景 相似文献