全文获取类型
收费全文 | 1342篇 |
免费 | 65篇 |
国内免费 | 9篇 |
专业分类
电工技术 | 17篇 |
综合类 | 5篇 |
化学工业 | 298篇 |
金属工艺 | 39篇 |
机械仪表 | 39篇 |
建筑科学 | 38篇 |
矿业工程 | 7篇 |
能源动力 | 70篇 |
轻工业 | 54篇 |
水利工程 | 7篇 |
石油天然气 | 2篇 |
无线电 | 193篇 |
一般工业技术 | 248篇 |
冶金工业 | 163篇 |
原子能技术 | 15篇 |
自动化技术 | 221篇 |
出版年
2023年 | 15篇 |
2022年 | 36篇 |
2021年 | 37篇 |
2020年 | 41篇 |
2019年 | 40篇 |
2018年 | 49篇 |
2017年 | 35篇 |
2016年 | 51篇 |
2015年 | 35篇 |
2014年 | 39篇 |
2013年 | 95篇 |
2012年 | 64篇 |
2011年 | 73篇 |
2010年 | 36篇 |
2009年 | 55篇 |
2008年 | 56篇 |
2007年 | 47篇 |
2006年 | 20篇 |
2005年 | 35篇 |
2004年 | 31篇 |
2003年 | 26篇 |
2002年 | 19篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 12篇 |
1998年 | 44篇 |
1997年 | 27篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 18篇 |
1993年 | 22篇 |
1992年 | 21篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 11篇 |
1988年 | 19篇 |
1987年 | 14篇 |
1986年 | 17篇 |
1985年 | 22篇 |
1984年 | 21篇 |
1983年 | 11篇 |
1982年 | 17篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1974年 | 6篇 |
1973年 | 9篇 |
1971年 | 7篇 |
排序方式: 共有1416条查询结果,搜索用时 8 毫秒
1.
2.
Upconversion phosphors are known as a material system that can convert near-infrared light into visible/ultraviolet emissions by sequentially absorbing multiple photons. The studies on upconversion materials often use two rare earth (RE) ions as a sensitizer-activator pair. We investigated the influences on luminescence intensity depending on Cr-doping content (x) of hexagonal NaLu0.98–xCrxF4Er0.02 (x = 0–0.9) upconversion material by substituting Lu3+ ions with Cr3+in the absence of Gd3+. The change in upconversion luminescence intensity appears with saddle-like shape. We suggest that Cr3+ ions play the dual role as a constituent in host lattice and a sensitizer in the upconversion process. Optimal conditions for gaining the strongest upconversion emission correspond to x = 0.3–0.5, where there are effective energy transfers between Cr3+ and Er3+ ions and CrEr dimers. Apart from these values, the emission intensity decreases rapidly which can be ascribed to the absence of multiple-photon absorption for the case of low Cr3+ contents, and to the coupling between Cr3+ and/or Er3+ ions for the case of high Cr3+ contents. Magnetization and electron-spin-resonant measurements were performed to understand the correlation between the optical and magnetic properties. 相似文献
3.
In summary, recent literature indicates that the complications of chronic otitis media have been decreasing. However, even with the advent of modern and more powerful antimicrobials and aggressive surgical eradication of disease, the morbidity and mortality are still high. Some complications may initially be quite obvious and some complications may be quite subtle. Therefore, the most important tools in making early diagnosis are careful history and physical examination, and a high index of suspicion for impending complications. 相似文献
4.
Nguyen An Tien I. Ya. Mittova O. V. Almjasheva S. A. Kirillova V. V. Gusarov 《Glass Physics and Chemistry》2008,34(6):756-761
Nanocrystalline LaFeO3 is prepared by the dehydration of coprecipitated lanthanum and iron(III) hydroxides. It is shown that the behavior of the samples during heating and the size distribution of LaFeO3 nanocrystals can be considerably different depending on the scheme used for coprecipitation of lanthanum and iron hydroxides; independently of the method employed for coprecipitation of the initial compounds, sintering of the samples at 950°C leads to the formation of lanthanum orthoferrite crystals up to 100 nm in size. 相似文献
5.
HBM ESD tests on two types of 0.6 μm DRAM devices showed that internal circuit or output driver failures would occur after the input or I/O pins were ESD stressed negative with respect to Vcc at ground. These failures occurred at lower than expected ESD stress voltages due to power-up circuit interactions that either turned-on unique internal parasitic ESD current paths or disrupted the normal operation of the output pin’s ESD protection circuit. ESD analysis found there exists a set of power-up sensitive circuits and if placed near a Vcc bond pad can result in low voltage ESD failures. 相似文献
6.
7.
The electrical conductivity of hot-pressed polycrystalline aluminium nitride doped with oxygen and beryllium was measured as a function of temperature from 800 to 1200° C and over a range of nitrogen partial pressure from 102 to 105 Pa. The effect of beryllium dopant, the independence of conductivity from nitrogen partial pressure, and the observed activation energy suggested extrinsic electronic species or aluminium vacancies as charge carriers. Polarization measurements made with one electrode blocking to ionic species indicated that the aluminium nitride with oxygen impurity was an extrinsic electronic conductor. 相似文献
8.
The cost distributions of both the parallel hybrid-hash join and the parallel join-index join algorithms proposed in the above-named work (ibid., vol.1, p.329-43, Sept. 1989) are presented in more detail. The result shows that almost the entire relation may need to be retrieved from disk, though the join selectivity is low. A table of semi-join selectives and cube sizes is given to show the condition that the join-index method performs better than the hybrid-hash method, i.e., the really low selectivity for the join-index method. An error in one of the cost formulas is corrected, and a more efficient method on the final join in the join-index method is proposed 相似文献
9.
Streit D.C. Oki A.K. Umemoto D.K. Velebir J.R. Stolt K.S. Yamada F.M. Saito Y. Hafizi M.E. Bui S. Tran L.T. 《Electron Device Letters, IEEE》1991,12(9):471-473
The authors have developed a modified MBE growth process to produce high-gain n-p-n GaAs-AlGaAs heterojunction bipolar transistors (HBTs) with a mean time to failure (MTTF) of 1.5×108 h at 125°C. Beryllium incorporation and diffusion are controlled through a combination of reduced substrate temperature and increased As/Ga flux ratio during MBE growth, resulting in extremely stable HBT profiles. The authors also demonstrate graded InGaAs surface layers with nonalloyed refractory metal contacts that significantly improve ohmic reliability compared to alloyed AuGe contacts. The ability to produce robust HBTs by MBE is critically important to this technology 相似文献
10.
The continuous production of hydrogen from cyclohexanes is achieved effectively using Pt/ACF (ACF = activated carbon fiber) catalysts in a fixed-bed flow reactor. The Pt catalysts are more effective than a Pd/ACF catalyst for the reaction. Besides cyclohexane, methylcyclohexane, 1,4-dimethylcyclohexane, and p-menthane can also be employed as hydrogen source in the reaction system. 相似文献