全文获取类型
收费全文 | 80689篇 |
免费 | 9363篇 |
国内免费 | 5178篇 |
专业分类
电工技术 | 7038篇 |
技术理论 | 7篇 |
综合类 | 6044篇 |
化学工业 | 10285篇 |
金属工艺 | 7038篇 |
机械仪表 | 5373篇 |
建筑科学 | 6758篇 |
矿业工程 | 3744篇 |
能源动力 | 1853篇 |
轻工业 | 7183篇 |
水利工程 | 2108篇 |
石油天然气 | 6070篇 |
武器工业 | 1091篇 |
无线电 | 9815篇 |
一般工业技术 | 7414篇 |
冶金工业 | 5652篇 |
原子能技术 | 1490篇 |
自动化技术 | 6267篇 |
出版年
2024年 | 334篇 |
2023年 | 1082篇 |
2022年 | 2190篇 |
2021年 | 2736篇 |
2020年 | 2914篇 |
2019年 | 2497篇 |
2018年 | 2348篇 |
2017年 | 2860篇 |
2016年 | 3295篇 |
2015年 | 3354篇 |
2014年 | 5075篇 |
2013年 | 4711篇 |
2012年 | 5983篇 |
2011年 | 6128篇 |
2010年 | 4336篇 |
2009年 | 4562篇 |
2008年 | 4111篇 |
2007年 | 5339篇 |
2006年 | 4859篇 |
2005年 | 4163篇 |
2004年 | 3553篇 |
2003年 | 3240篇 |
2002年 | 2807篇 |
2001年 | 2491篇 |
2000年 | 2062篇 |
1999年 | 1727篇 |
1998年 | 1256篇 |
1997年 | 965篇 |
1996年 | 883篇 |
1995年 | 705篇 |
1994年 | 541篇 |
1993年 | 416篇 |
1992年 | 338篇 |
1991年 | 262篇 |
1990年 | 234篇 |
1989年 | 153篇 |
1988年 | 134篇 |
1987年 | 102篇 |
1986年 | 77篇 |
1985年 | 77篇 |
1984年 | 52篇 |
1983年 | 39篇 |
1982年 | 35篇 |
1981年 | 32篇 |
1980年 | 31篇 |
1979年 | 11篇 |
1964年 | 13篇 |
1963年 | 9篇 |
1962年 | 9篇 |
1959年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 31 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.
In this study, we have proposed an automated classification approach to identify meaningful patterns in wind field data. Utilizing an extensive simulated wind database, we have demonstrated that the proposed approach can identify low‐level jets, near‐uniform profiles, and other patterns in a reliable manner. We have studied the dependence of these wind profile patterns on locations (eg, offshore vs onshore), seasons, and diurnal cycles. Furthermore, we have found that the probability distributions of some of the patterns depend on the underlying planetary boundary layer schemes in a significant way. The future potential of the proposed approach in wind resource assessment and, more generally, in mesoscale model parameterization improvement is touched upon in this paper. 相似文献
33.
A heavy crude oil has been treated with deuterated alkylating reagents (CD3I and C2D5I) and directly analyzed without any prior fractionation and chromatographic separation by high-field Orbitrap Fourier Transform Mass Spectrometry (FTMS) and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) using electrospray ionization (ESI). The reaction of a polycyclic aromatic sulfur heterocycles (PASHs) dibenzothiophene (DBT), in the presence of silver tetrafluoroborate (AgBF4) with ethyl iodide (C2H5I) in anhydrous dichloroethane (DCE) was optimized as a sample reaction to study heavy crude oil mixtures, and the reaction yield was monitored and determined by proton nuclear magnetic resonance spectroscopy (1H-NMR). The obtained conditions were then applied to a mixture of standard aromatic CH-, N-, O- and S-containing compounds and then a heavy crude oil, and only sulfur-containing compounds were selectively alkylated. The deuterium labeled alkylating reagents, iodomethane-d3 (CD3I) and iodoethane-d5 (C2D5I), were employed to the alkylation of heavy crude oil to selectively differentiate the tagged sulfur species from the original crude oil. 相似文献
34.
Simulating the psychological experience of human vision,a road extraction model based on the format tower is proposed to extract the road in the high resolution remote sensing image from the perspective of morphology.Firstly,based on the spectral and texture information,the suspected road targets are extracted by using segmentation technology.Then these targets are classified according to their reliability and extract the road targets for each category.Finally,three types of identified road information are verified and merged,and the continuous smooth road extraction results are obtained.Experiments on real high resolution images show that the results are consistent with the visual perception of the human eye,and the overall classification accuracy is higher,indicating that the algorithm is effective and feasible and has good use value. 相似文献
35.
为了实现对微小环片零件的自动化装配,搭建了自动装配系统.通过4根直线导向轴与4个直线轴承来提高系统的导向精度和刚度.采用直线导轨进行各装配作业模块之间的切换,保证了微小环片零件的自动装配与取出.在环片的装配方向上,螺旋升降机和光栅尺实现环片的位置精度控制.在Lab VIEW编程环境中,采用分层软件架构和模块化控制思想,避免了不必要的数据循环检测与丢失,能够达到环片组件的装配精度要求.控制系统分为系统初始化模块、参数设置模块、装配模块和取出模块,自动装配系统通过各个模块间的相互交流配合完成装配任务.采用本文中自动装配系统装配环片的实验结果表明,环片零件装配的最大位置误差为26μm,垂直度误差为17μm,平均装配时间为75 s/片,可满足环片组件所需的精度要求. 相似文献
36.
37.
Porous polyimide (PI) films with low dielectric constants and excellent thermal properties have been a pressing demand for the next generation of high-performance, miniature, and ultrathin microelectronic devices. A series of novel porous PI films containing fluorenyl-adamantane groups were prepared successfully via thermolysis of poly(ethylene glycol) (PEG) added in the PI matrix. The cross-sectional morphologies of porous PI films showed closed pores with diameters ranging from 135 to 158 nm, which were uniform and regular in shape without interconnectivity. These porous PI films exhibited excellent thermal properties with a glass-transition temperature at 376 °C whereas the 5% weight loss temperature in air excess of 405 °C due to enhanced rigidity afforded by fluorenyl-adamantane groups. Accompanied by thermolysis content of PEG increasing from 0 to 20 wt %, the density of porous PI films decreased, and the corresponding porosity grew significantly from 0 to 11.48%. Depending on porosity, the dielectric constant and dielectric loss of porous PI films significantly declined from 2.89 to 2.37 and from 0.050 to 0.021, respectively. These excellent properties benefit the as-prepared porous PI films for application as interlayer dielectrics, integrated circuit chips, or multichip modules in microelectronic fields. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47313. 相似文献
38.
高浓度聚合物驱提高采收率方法实验研究 总被引:3,自引:0,他引:3
为探索一种新的提高原油采收率的方法,在外形尺寸为4.5 cm×4.5 cm×30 cm、气测渗透率0.9~1.0μm2、变异系数为0.72的二维纵向非均质人造岩心上,模拟大庆油田油藏流体性质及温度条件,研究了高浓度聚合物(HPAM)驱注入时机、聚合物相对分子质量、聚合物段塞体积及段塞组合对驱油效果的影响。通过注入大分子量、高浓度聚合物,结合合理的注入方式,在化学剂成本与三元复合驱相当的情况下,采收率比水驱提高20个百分点以上,接近或超过三元复合驱的水平。实验表明,采用高浓度聚合物驱油是一种较好的提高原油采收率的方法。 相似文献
39.
Defects are often present in rolled products, such as wire rod. The market demand for wire rod without any defects has increased. In the final wire rod products, defects originating from steel making, casting, pre‐rolling of billets and during wire rod rolling can appear. In this work, artificial V‐shaped longitudinal surface cracks have been analysed experimentally and by means of FEM. The results indicate that the experiments and FEM calculations show the same tendency except in two cases, where instability due to fairly “round” false round bars disturbed the experiment. FE studies in combination with practical experiments are necessary in order to understand the behaviour of the material flows in the groove and to explain whether the crack will open up as a V‐shape or if it will be closed as an I‐shape. 相似文献
40.