全文获取类型
收费全文 | 7798篇 |
免费 | 326篇 |
国内免费 | 323篇 |
专业分类
电工技术 | 1673篇 |
综合类 | 325篇 |
化学工业 | 905篇 |
金属工艺 | 559篇 |
机械仪表 | 597篇 |
建筑科学 | 341篇 |
矿业工程 | 167篇 |
能源动力 | 560篇 |
轻工业 | 257篇 |
水利工程 | 60篇 |
石油天然气 | 162篇 |
武器工业 | 146篇 |
无线电 | 682篇 |
一般工业技术 | 828篇 |
冶金工业 | 298篇 |
原子能技术 | 292篇 |
自动化技术 | 595篇 |
出版年
2024年 | 37篇 |
2023年 | 135篇 |
2022年 | 184篇 |
2021年 | 232篇 |
2020年 | 233篇 |
2019年 | 177篇 |
2018年 | 150篇 |
2017年 | 210篇 |
2016年 | 212篇 |
2015年 | 221篇 |
2014年 | 344篇 |
2013年 | 461篇 |
2012年 | 418篇 |
2011年 | 566篇 |
2010年 | 439篇 |
2009年 | 394篇 |
2008年 | 372篇 |
2007年 | 451篇 |
2006年 | 460篇 |
2005年 | 457篇 |
2004年 | 441篇 |
2003年 | 265篇 |
2002年 | 214篇 |
2001年 | 185篇 |
2000年 | 165篇 |
1999年 | 174篇 |
1998年 | 204篇 |
1997年 | 113篇 |
1996年 | 119篇 |
1995年 | 108篇 |
1994年 | 93篇 |
1993年 | 43篇 |
1992年 | 37篇 |
1991年 | 21篇 |
1990年 | 23篇 |
1989年 | 27篇 |
1988年 | 12篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1959年 | 4篇 |
1954年 | 1篇 |
排序方式: 共有8447条查询结果,搜索用时 46 毫秒
1.
Pu Mao Jiping Wang Peng Xiao Lixue Zhang Fang Kang Hao Gong 《Ceramics International》2021,47(1):111-120
In this work, we developed a novel system of isovalent Zr4+ and donor Nb5+ co-doped CaCu3Ti4O12 (CCTO) ceramics to enhance dielectric response. The influences of Zr4+ and Nb5+ co-substituting on the colossal dielectric response and relaxation behavior of the CCTO ceramics fabricated by a conventional solid-phase synthesis method were investigated methodically. Co-doping of Zr4+ and Nb5+ ions leads to a significant reduction in grain size for the CCTO ceramics sintered at 1060 °C for 10 h. XRD and Raman results of the CaCu3Ti3.8-xZrxNb0.2O12 (CCTZNO) ceramics show a cubic perovskite structure with space group Im-3. The first principle calculation result exhibits a better thermodynamic stability of the CCTO structure co-doped with Zr4+ and Nb5+ ions than that of single-doped with Zr4+ or Nb5+ ion. Interestingly, the CCTZNO ceramics exhibit greatly improved dielectric constant (~105) at a frequency range of 102–105 Hz and at a temperature range of 20–210 °C, indicating a giant dielectric response within broader frequency and temperature ranges. The dielectric properties of CCTZNO ceramics were analyzed from the viewpoints of defect-dipole effect and internal barrier layer capacitance (IBLC) model. Accordingly, the immensely enhanced dielectric response is primarily ascribed to the complex defect dipoles associated with oxygen vacancies by co-doping Zr4+ and Nb5+ ions into CCTO structure. In addition, the obvious dielectric relaxation behavior has been found in CCTZNO ceramics, and the relaxation process in middle frequency regions is attributed to the grain boundary response confirmed by complex impedance spectroscopy and electric modulus. 相似文献
2.
In this article, the silver-plated polyamide fabrics (SPPAFs) with high electroconductibility and shielding effectiveness were fabricated by using in situ reduction of polydopamine and chemical silvering. The effects of SPPAFs dopamine (C8H11O2N) and silver nitrate (AgNO3) concentration on surface resistivity and electromagnetic interference shielding effectiveness were studied. The results showed that the surface resistivity of SPPAFs can reach a minimum value of 0.06 ± 0.014 Ω cm−1, when C8H11O2N concentration is 4 g L−1 and the AgNO3 concentration is 120 g L−1. The shielding effectiveness of SPPAFs in the wide frequency range of 10–3000 MHz increases with the increase in the concentration of AgNO3, and increases first and stabilizes afterward with increasing C8H11O2N concentration. When the concentration of C8H11O2N and AgNO3 is 3 and 120 g L−1, respectively, mean shielding effectiveness values in the low-, medium-, and high-frequency bands are 71.3, 73.8, and 76.1 dB, respectively. Moreover, the mean shielding effectiveness values is 83.79 dB in the frequency range of 1.2–2.3 GHz. The dominant shielding mechanism of SPPAFs is the reflected electromagnetic waves and the absorption shielding effectiveness is less than 2 dB. The average electromagnetic shielding values of SPPAFs are above 67 dB after 16 weeks of storage, when C8H11O2N concentration is 4 g L−1 and the AgNO3 concentration is 80 and 100 g L−1. The prepared SPPAFs show promising applications in military textiles and smart wearable clothing. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48227. 相似文献
3.
Numerous design choices need to be made at several levels when designing high-tech products: technology, processes, architecture, components, materials… and these choices need to be made in relation to the product life cycle with the corresponding experts for each stage of the life cycle. At the same time, to ensure product sustainability, a specific focus on the future potential environmental impacts is highly recommended. In this research, an agile model is proposed to help designers make decisions while monitoring environmental performance indicators of high-tech solutions. The concept of Critical Product Life Cycle Parameters had to be introduced to facilitate the eco-design of the final product. The approach is illustrated by the Electric Vehicle Li-Ion Batteries case study. 相似文献
4.
5.
《Energy Policy》2015
Electric vehicles (EVs) are considered alternatives to internal combustion engines due to their energy efficiency and contribution to CO2 mitigation. The adoption of EVs depends on consumer preferences, including cost, social status and driving habits, although it is agreed that current and expected costs play a major role. We use a partial equilibrium model that minimizes total energy system costs to assess whether EVs can be a cost-effective option for the consumers of each EU27 member state up to 2050, focusing on the impact of different vehicle investment costs and CO2 mitigation targets. We found that for an EU-wide greenhouse gas emission reduction cap of 40% and 70% by 2050 vis-à-vis 1990 emissions, battery electric vehicles (BEVs) are cost-effective in the EU only by 2030 and only if their costs are 30% lower than currently expected. At the EU level, vehicle costs and the capability to deliver both short- and long-distance mobility are the main drivers of BEV deployment. Other drivers include each state’s national mobility patterns and the cost-effectiveness of alternative mitigation options, both in the transport sector, such as plug-in hybrid electric vehicles (PHEVs) or biofuels, and in other sectors, such as renewable electricity. 相似文献
6.
智能型电子式变频电动执行器及其应用 总被引:2,自引:0,他引:2
针对原有电动执行器只有以低速运行,通过极强的微调才能修正微小偏差这一缺点,提出智能型电动执行器与各种阀体譬合,利用数字化变频技术,改造目前采用模拟技术的电动执行器,使之具有变速运行、动态响应块,调节定位精度高、稳定性好,故障率低、使用寿命长和应用场合广泛的特点。 相似文献
7.
8.
Hopes have again been raised about developing a “hydrogen economy”, in which hydrogen could be expected to replace oil and natural gas for most uses, including transportation and heating. It is again being claimed that hydrogen will be a widely available, clean, safe fuel. This article argues that such expectations are almost certainly illusory. Hydrogen, like electricity, is not an energy resource but an energy carrier. It takes more energy to extract hydrogen from water than burning the hydrogen can ever provide. There are also inevitable losses in storage, transmission, and final mechanical or heating applications. The question then turns on the efficiency—and safety—of the entire chain of conversion, from the energy source (fossil, solar, or other) to the final use. Moreover, energy sources (preferably renewable, for the long term) can be used for the direct creation of electricity, which can be introduced into the existing grid without requiring a vast investment in a new hydrogen distribution system. In addition, a hydrogen-based system would be unacceptably dangerous. This report will present a detailed technical and economic analysis of the problems with the proposed hydrogen economy and the advantages of some alternatives, principally electricity-based. A hypothetical case of what would be required for a hydrogen filling station serving the general public is closely examined. 相似文献
9.
微波体声波延迟线的电磁屏蔽 总被引:1,自引:1,他引:0
本文分析了微波体声波延迟线多层介质的电磁屏蔽效应,结果表明,全屏蔽方式可以有效地解决微波延迟线研制中的直通抑制问题。实验结果证实了理论分析。 相似文献
10.