全文获取类型
收费全文 | 202538篇 |
免费 | 2985篇 |
国内免费 | 224篇 |
专业分类
电工技术 | 2813篇 |
综合类 | 146篇 |
化学工业 | 33753篇 |
金属工艺 | 10086篇 |
机械仪表 | 6454篇 |
建筑科学 | 3990篇 |
矿业工程 | 1961篇 |
能源动力 | 3428篇 |
轻工业 | 14154篇 |
水利工程 | 3032篇 |
石油天然气 | 8811篇 |
武器工业 | 18篇 |
无线电 | 15838篇 |
一般工业技术 | 43948篇 |
冶金工业 | 35182篇 |
原子能技术 | 7084篇 |
自动化技术 | 15049篇 |
出版年
2021年 | 2380篇 |
2019年 | 2334篇 |
2018年 | 4404篇 |
2017年 | 4463篇 |
2016年 | 4771篇 |
2015年 | 2773篇 |
2014年 | 4244篇 |
2013年 | 9030篇 |
2012年 | 6326篇 |
2011年 | 7270篇 |
2010年 | 5943篇 |
2009年 | 6310篇 |
2008年 | 6267篇 |
2007年 | 6107篇 |
2006年 | 5021篇 |
2005年 | 4549篇 |
2004年 | 4303篇 |
2003年 | 4124篇 |
2002年 | 4021篇 |
2001年 | 3870篇 |
2000年 | 3734篇 |
1999年 | 3620篇 |
1998年 | 9150篇 |
1997年 | 6605篇 |
1996年 | 4768篇 |
1995年 | 3503篇 |
1994年 | 3058篇 |
1993年 | 3240篇 |
1992年 | 2452篇 |
1991年 | 2506篇 |
1990年 | 2561篇 |
1989年 | 2469篇 |
1988年 | 2467篇 |
1987年 | 2327篇 |
1986年 | 2363篇 |
1985年 | 2429篇 |
1984年 | 2329篇 |
1983年 | 2231篇 |
1982年 | 2107篇 |
1981年 | 2338篇 |
1980年 | 2202篇 |
1979年 | 2357篇 |
1978年 | 2508篇 |
1977年 | 2689篇 |
1976年 | 3685篇 |
1975年 | 2305篇 |
1974年 | 2356篇 |
1973年 | 2416篇 |
1972年 | 2182篇 |
1971年 | 1954篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
It was found that vanadyl porphyrin complexes synthesized from petroleum metal porphyrin concentrates stimulated epoxidation during the olefin oxygenation process. The yields of obtained oxiranes turned out to be 38–75%, depending on the olefin structure. An epoxidation mechanism that suggests the formation of a protonated dioxygen adduct as an intermediate during oxygenation of olefins in the presence of vanadyl porphyrin complexes was proposed. An analogy is drawn between the epoxide formation reaction upon the catalytic oxygenation of olefins and the Prilezhaev reaction. 相似文献
12.
The Magnitogorsk Metallurgical Combine has conducted a study of the effect of technological factors on the hydrogen content
of chromium-nickel-molybdenum steel after vacuum degassing. It was established that the most important factor is the hydrogen
content of the steel before the degassing operation. The study also determined the effects of the circulation coefficient,
the duration of the degassing operation, and the vacuum used in the treatment.
__________
Translated from Metallurg, No. 7, pp. 68–69, July, 2006. 相似文献
13.
Yu. I. Golovin D. V. Lopatin R. K. Nikolaev A. V. Umrikhin 《Technical Physics Letters》2004,30(5):426-428
The effect of crystal orientation on the photogeneration of free charge carriers was studied for C60 single crystals in a weak magnetic field. The photoconductivity sharply depends on the orientation of magnetic field with
respect to the crystallographic directions, showing a 5–8% increase for seven axes of the C60 crystal. 相似文献
14.
Korolev I. A. Alekseenko N. N. Porodnov B. T. Sapunov V. A. Savel'ev D. V. 《Measurement Techniques》2003,46(9):865-871
The design of a sylphon bellows sensor and the basic circuits of an LC-generator and of a microprocessor unit are presented. An analytical pressure–frequency conversion function and a special method of adjusting the sensor ensure an error of less than 0.05%. The dynamic range is up to 105. The instruments developed cover the ranges 103, 104, and 105 Pa. 相似文献
15.
Rogin J. Kouchev I. Brenna G. Tschopp D. Qiuting Huang 《Solid-State Circuits, IEEE Journal of》2003,38(12):2239-2248
A 2-GHz direct-conversion receiver for wide-band code division multiple access (WCDMA) is presented. It includes two low-noise amplifiers (LNAs), an I/Q demodulator, and two sixth-order baseband channel select filters with programmable gain. Quadrature local oscillator (LO) signals are generated on chip in a frequency divider flip-flop. An external interstage filter between the LNAs rejects transmitter leakage to relax demodulator linearity requirements. A low-voltage demodulator topology improves linearity as well as demodulator output pole accuracy. The active-RC baseband filter uses a programmable servo loop for offset compensation and provides an adjacent channel rejection of 39 dB. Programmable gain over 71-dB range in 1-dB steps is merged with the filter to maximize dynamic range. An automatic on-chip frequency calibration scheme provides better than 1.5% corner frequency accuracy. The receiver is integrated in a 0.13-/spl mu/m CMOS process with metal-insulator-metal (MIM) capacitors. Measured receiver performance includes a 6.5-dB noise figure, IIP2 of +27 dBm, and IIP3 of -8.6 dBm. Power consumption is 45 mW. 相似文献
16.
The phenomenon of persistent tunneling photoconductivity was studied using the tunneling spectroscopy technique at liquid-helium temperature: the separation between the unoccupied levels in a δ-doped layer at the GaAs surface decreased after illumination. This decrease was due to an increase in the width of the quantum well of the δ-doped layer. For photon energies hv exceeding the GaAs band gap E g , this increase in the width of the quantum well was related to the accumulation of positive charge in the depth of GaAs induced by the generation of the electron-hole pairs and photoionization of deep centers. For hv < E g (including the case of CO2 laser), only photoionization is important. The experimental data agree with the self-consistent calculations. The critical temperature for the effect has been determined (T c = 45 K); at higher temperatures, the effect disappears. 相似文献
17.
Kruk Menno R.; Halász József; Meelis Wout; Haller József 《Canadian Metallurgical Quarterly》2004,118(5):1062
Aggressive behavior induces an adrenocortical stress response, and sudden stressors often precipitate violent behavior. Experiments in rats revealed a fast, mutual, positive feedback between the adrenocortical stress response and a brain mechanism controlling aggression. Stimulation of the aggressive area in the hypothalamus rapidly activated the adrenocortical response, even in the absence of an opponent and fighting. Hypothalamic aggression, in turn, was rapidly facilitated by a corticosterone injection in rats in which the natural adrenocortical stress response was prevented by adrenalectomy. The rapidity of both effects points to a fast, mutual, positive feedback of the controlling mechanisms within the time frame of a single conflict. Such a mutual facilitation may contribute to the precipitation and escalation of violent behavior under stressful conditions. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
18.
19.
V. N. Tsygankov V. V. Safonov A. I. Kozlov V. P. Gavrilov 《Inorganic Materials》2003,39(10):1076-1078
The GeO2–NiO system is studied in the stability range of Ni2GeO4 by temperature-dependent resistivity measurements, in combination with x-ray diffraction, differential thermal analysis, and thermogravimetry. The resistivity of the starting mixtures and heat-treated samples is measured in the composition range 55–90 mol % NiO. The effect of air humidity on the resistivity of the samples is examined. 相似文献
20.
The catalytic activities of alumina prepared from an Al alkoxide-amine adduct monomer for the reaction of cyclopentene oxide with piperidine was determined after various pretreatments, including calcination and exposure to moisture. They were compared with the activity of alumina prepared by the conventional hydrolysis method. It was found that the as-prepared sample from the alkoxide-amine monomer preparation was five times more active than a conventional preparation, suggesting that it has a higher density of surface Lewis acid sites. However, its activity was much more severely suppressed by exposure to moisture. 相似文献