全文获取类型
收费全文 | 130282篇 |
免费 | 11584篇 |
国内免费 | 6370篇 |
专业分类
电工技术 | 8447篇 |
技术理论 | 2篇 |
综合类 | 8671篇 |
化学工业 | 23946篇 |
金属工艺 | 9357篇 |
机械仪表 | 6413篇 |
建筑科学 | 9795篇 |
矿业工程 | 7123篇 |
能源动力 | 9688篇 |
轻工业 | 8498篇 |
水利工程 | 2051篇 |
石油天然气 | 20783篇 |
武器工业 | 921篇 |
无线电 | 7011篇 |
一般工业技术 | 10742篇 |
冶金工业 | 7750篇 |
原子能技术 | 1873篇 |
自动化技术 | 5165篇 |
出版年
2024年 | 423篇 |
2023年 | 1659篇 |
2022年 | 3395篇 |
2021年 | 4286篇 |
2020年 | 4443篇 |
2019年 | 3642篇 |
2018年 | 3292篇 |
2017年 | 4012篇 |
2016年 | 4705篇 |
2015年 | 4711篇 |
2014年 | 7934篇 |
2013年 | 7913篇 |
2012年 | 9871篇 |
2011年 | 10397篇 |
2010年 | 7114篇 |
2009年 | 7187篇 |
2008年 | 6289篇 |
2007年 | 8018篇 |
2006年 | 7922篇 |
2005年 | 6505篇 |
2004年 | 5657篇 |
2003年 | 4938篇 |
2002年 | 4147篇 |
2001年 | 3713篇 |
2000年 | 3103篇 |
1999年 | 2455篇 |
1998年 | 1926篇 |
1997年 | 1674篇 |
1996年 | 1421篇 |
1995年 | 1122篇 |
1994年 | 942篇 |
1993年 | 671篇 |
1992年 | 595篇 |
1991年 | 499篇 |
1990年 | 397篇 |
1989年 | 312篇 |
1988年 | 183篇 |
1987年 | 119篇 |
1986年 | 113篇 |
1985年 | 96篇 |
1984年 | 87篇 |
1983年 | 64篇 |
1982年 | 47篇 |
1981年 | 77篇 |
1980年 | 58篇 |
1979年 | 24篇 |
1977年 | 12篇 |
1973年 | 6篇 |
1959年 | 16篇 |
1951年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
In this study, 30 subjects were exposed to different combinations of air temperature (Ta: 24, 27, and 30°C) and CO2 level (8000, 10 000, and 12 000 ppm) in a high-humidity (RH: 85%) underground climate chamber. Subjective assessments, physiological responses, and cognitive performance were investigated. The results showed that as compared with exposure to Ta = 24°C, exposure to 30°C at all CO2 levels caused subjects to feel uncomfortably warm and experience stronger odor intensity, while increased mental effort and greater intensity of acute health symptoms were reported. However, no significant effects of Ta on task performance or physiological responses were found. This indicated that subjects had to exert more effort to maintain their performance in an uncomfortably warm environment. Increasing CO2 from 8000 to 12 000 ppm at all Ta caused subjects to report higher rates of headache, fatigue, agitation, and feeling depressed, although the results were statistically significant only at 24 and 27°C. The text typing performance and systolic blood pressure (SBP) decreased significantly at this exposure, whereas diastolic blood pressure (DBP) and thermal discomfort increased significantly. These effects suggest higher arousal/stress. No significant interaction effect of Ta and CO2 concentration on human responses was identified. 相似文献
42.
How to improve the sensitivity of the temperature-sensing luminescent materials is one of the most important objects currently. In this work, to obtain high sensitivity and learn the corresponding mechanism, the rare earth (RE) ions doped Y4.67Si3O13 (YS) phosphors were developed by solid-state reaction. The phase purity, structure, morphology and luminescence characteristics were evaluated by XRD, TEM, emission spectra, etc. The change of the optical bandgaps between the host and RE-doped phosphors was found, agreeing with the calculation results based on density-functional theory. The temperature-dependence of the upconversion (UC) luminescence revealed that a linear relationship exists between the fluorescence intensity ratio of Ho3+ and temperature. The theoretical resolution was evaluated. High absolute (0.083 K−1) and relative (3.53% K−1 at 293 K) sensitivities have been gained in the YS:1%Ho3+, 10%Yb3+. The effect of the Yb3+ doping concentration and pump power on the sensitivities was discussed. The pump-power–dependence of the UC luminescence indicated the main mechanism for high sensitivities in the YS:1%Ho3+, 10%Yb3+. Moreover, the decay-lifetime based temperature sensing was also evaluated. The above results imply that the present phosphors could be promising candidates for temperature sensors, and the proposed strategies are instructive in exploring other new temperature sensing luminescent materials. 相似文献
43.
Thermosetting materials are widely used as encapsulation in the electrical packaging to protect the core electronic components from external force, moisture, dust, and other factors. However, the spreading and curing behaviors of such kind of fluid on a heated surface have been rarely explored. In this study, we experimentally and numerically investigated the spreading and curing behaviors of the silicone(OE6550 A/B, which is widely used in the light-emitting diode packaging) droplet with diameter of ~2.2 mm on a heated surface with temperature ranging from 25 ℃ to 250 ℃. For the experiments, we established a setup with high-speed camera and heating unit to capture the fast spreading process of the silicone droplet on the heated surface. For the numerical simulation, we built a viscosity model of the silicone by using the Kiuna's model and combined the viscosity model with the Volume of Fluid(VOF) model by the User Defined Function(UDF) method. The results show that the surface temperature significantly affected the spreading behaviors of the silicone droplet since it determines the temperature and viscosity distribution inside the droplet. For surface temperature varied from 25 ℃ to 250 ℃, the final contact radius changed from ~2.95 mm to ~1.78 mm and the total spreading time changed from ~511 s to ~0.15 s. By further analyzing the viscosity evolution of the droplet, we found that the decreasing of the total spreading time was caused by the decrease of the viscosity under high surface temperature at initial spreading stage, while the reduction of the final contact radius was caused by the curing of the precursor film. This study supplies a strategy to tuning the spreading and curing behavior of silicone by imposing high surface temperature, which is of great importance to the electronic packaging. 相似文献
44.
45.
仪器仪表产品的脉冲耐压试验是产品型式试验、例行试验中的基本内容。通过对GB4793.1标准的整理,归纳了仪器仪表产品的脉冲耐压试验要求。通过对试验方法和设备特性的梳理,总结了特性参数,并介绍了应对脉冲耐压试验可采取的保护器件的类别及选用方法。 相似文献
46.
《Oil and Energy Trends》2019,44(1):26-28
Current data on natural gas production, as well as a breakdown of production by country. Updated on a monthly basis. 相似文献
47.
《Ceramics International》2022,48(2):1889-1897
SiC fiber reinforced ceramic matrix composites (SiCf-CMCs) are considered to be one of the most promising materials in the electromagnetic (EM) stealth of aero-engines, which is expected to achieve strong absorption and broad-band performance. Multiscale structural design was applied to SiCf/Si3N4–SiOC composites by construction of micro/nanoscale heterogeneous interfaces and macro double-layer impedance matching structure. SiCf/Si3N4–SiOC composites were fabricated by using SiC fibers with different conductivities and SiOC–Si3N4 matrices with gradient impedance structures to improve impedance matching effectively. Owing to its unique structure, SiCf/Si3N4–SiOC composites (A3-composites) achieved excellent EM wave absorption performance with a minimum reflection coefficient (RCmin) of ?25.1 dB at 2.45 mm and an effective absorption bandwidth (EAB) of 4.0 GHz at 2.85 mm in X-band. Moreover, double-layer SiCf/Si3N4–SiOC with an improved impedance matching structure obtained an RCmin of ?56.9 dB and an EAB of 4.2 GHz at 3.00 mm, which means it can absorb more than 90% of the EM waves in the whole X-band. The RC is less than ?8 dB at 2.6–2.8 mm from RT to 600 °C in the whole X-band, displaying excellent high-temperature absorption performance. The results provide a new design opinion for broad-band EM absorbing SiCf-CMCs at high temperatures. 相似文献
48.
49.
The increase in the production of acid gas consisting of H2S, CO2, and associated impurities such as ammonia and hydrocarbons from oil and gas plants and gasification facilities has stimulated the interest in the development of alternative means of acid gas utilization to produce hydrogen and sulfur, simultaneously. The present literature lacks a detailed reaction mechanism that can reliably predict the thermal destruction of NH3 and its blend with H2S and CO2 to facilitate process optimization and commercialization. In this paper, a detailed mechanism of NH3 pyrolysis is developed and is merged with the reactions of NH3 oxidation and H2S/CO2 thermal decomposition from our previous works. The mechanism is validated successfully using different sets of experimental data on the pyrolysis and oxidation of NH3, H2S, and CO2. The proposed mechanism predicts the experimental data on NH3 pyrolysis remarkably better than the existing mechanisms in the literature. The mechanism is used to investigate the effects of NH3 concentration (0–20%) and reactor temperature (1000–1800 K) on the thermal decomposition of H2S and CO2. A synergistic effect is observed in the simultaneous decomposition of NH3 and CO2, i.e., NH3 conversion is improved in the presence of CO2 and the decomposition CO2 to CO is enhanced in the presence of NH3. The presence of H2S suppressed NH3 conversion, while the conversion of H2S remained unchanged with increasing NH3 concentration at temperature below 1400 K due to the low conversion of NH3 (up to 18%). At temperature above 1400 K, NH3 conversion increased rapidly and it triggered a decrease in H2S conversion as well as the yields of H2 and S2. The major reactions involved in the decomposition of H2S, CO2, and NH3 and the production of major products such as H2, S2, and CO are identified. The detailed reaction mechanism can facilitate the design and optimization of acid gas thermal decomposition to produce hydrogen and sulfur, simultaneously. 相似文献
50.
随着我国石油需求量不断增加,致密油藏的开发也愈发重要。如何高效开发致密油藏是一项重点难点,其中致密油藏注天然气提高采收率是一个极具潜力的研究方向。因此,本文着重介绍以天然气作为能量补充介质在国内外的研究现状和应用现状,并且从两相特征等方面总结了理论研究中的一些机理,对致密油藏注天然气提高采收率的发展前景进行了一定的展望。 相似文献