全文获取类型
收费全文 | 339785篇 |
免费 | 27359篇 |
国内免费 | 15864篇 |
专业分类
电工技术 | 26429篇 |
技术理论 | 59篇 |
综合类 | 47081篇 |
化学工业 | 34055篇 |
金属工艺 | 11889篇 |
机械仪表 | 18864篇 |
建筑科学 | 51552篇 |
矿业工程 | 18265篇 |
能源动力 | 10713篇 |
轻工业 | 18371篇 |
水利工程 | 18796篇 |
石油天然气 | 16389篇 |
武器工业 | 3012篇 |
无线电 | 21926篇 |
一般工业技术 | 22905篇 |
冶金工业 | 18214篇 |
原子能技术 | 3031篇 |
自动化技术 | 41457篇 |
出版年
2024年 | 454篇 |
2023年 | 2981篇 |
2022年 | 5777篇 |
2021年 | 7254篇 |
2020年 | 7686篇 |
2019年 | 6374篇 |
2018年 | 6011篇 |
2017年 | 7496篇 |
2016年 | 9047篇 |
2015年 | 10353篇 |
2014年 | 20323篇 |
2013年 | 17372篇 |
2012年 | 23286篇 |
2011年 | 24792篇 |
2010年 | 19617篇 |
2009年 | 20750篇 |
2008年 | 19276篇 |
2007年 | 25089篇 |
2006年 | 23680篇 |
2005年 | 20321篇 |
2004年 | 17155篇 |
2003年 | 15337篇 |
2002年 | 12546篇 |
2001年 | 10609篇 |
2000年 | 8844篇 |
1999年 | 7056篇 |
1998年 | 5259篇 |
1997年 | 4532篇 |
1996年 | 4189篇 |
1995年 | 3553篇 |
1994年 | 3108篇 |
1993年 | 2301篇 |
1992年 | 2008篇 |
1991年 | 1486篇 |
1990年 | 1319篇 |
1989年 | 1163篇 |
1988年 | 915篇 |
1987年 | 609篇 |
1986年 | 459篇 |
1985年 | 409篇 |
1984年 | 378篇 |
1983年 | 267篇 |
1982年 | 236篇 |
1981年 | 190篇 |
1980年 | 166篇 |
1979年 | 129篇 |
1978年 | 72篇 |
1977年 | 80篇 |
1976年 | 61篇 |
1975年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
为了研究温度分布对于管阵列结构中的声透射特性的影响,以核电站的实际工况为背景,构建了不同的温度场以及周期性变化的非均匀温度场,利用有限元方法进行数值模拟。结果表明:(1)温度分布会改变管阵列声透射频谱的“禁带”宽度以及中心频率位置。在同一介质中,温度变化对频率较高位置的影响大于频率较低的位置。(2)在同样为10℃的温度差下,当水的平均声速为1 653 m·s-1、饱和水蒸气的平均声速为522.5 m·s-1时,介质为水时的禁带宽度及中心频率位置变化较大,即声速大的介质的频谱对于温度的变化更敏感。(3)当温度差在10℃以内,在周期性变化的非均匀温度场和与均匀温度场中管阵列声透射特性在第一中心频率23 996.1 Hz之前,两频谱差别很小,在第一禁带之后会出现明显区别。该研究成果对完善核电站应用的声学检测提供了理论基础。 相似文献
22.
23.
24.
This article proposes an active balancer, which features bidirectional charge shuttling and adaptive equalization current control, to fast counterbalance the state of charge (SOC) of cells in a lithium-ion battery (LIB) string. The power circuit consists of certain bidirectional buck-boost converters to transfer energy among the different cells back and forth. Owing to the characterization of the open-circuit voltage (OCV) vs SOC in LIB being relatively smooth near the SOC middle range, the SOC-inspected balance strategy can achieve more precise and efficient equilibrium than the voltage-based control. Accordingly, a compensated OCV-based SOC estimation is put forward to take into account the discrepancy of SOC estimation. Besides, the varied-duty-cycle (VDC) and curve-fitting modulation (CFM) methods are devised herein to tackle the problems of slow equalization rate and low balance efficacy, which arise from the diminution in balancing current as the SOC difference between the cells decreases in the later duration of equalization especially. The proposed strategies have taken the battery nonlinear characteristic and circuit parameter nonideality into account and can adaptively modulate the duty cycle with the SOC difference to keep balancing current constant throughout the balancing cycle. Simulated and experimental results are given to demonstrate the feasibility and effectiveness of the same prototype constructed. Compared with the fixed duty cycle and the VDC methods, the proposed CFM has the best balancing efficiency of 81.4%, and the balance time is shortened by 27.1% and 18.6%, respectively. 相似文献
25.
26.
中国考古出土的蚕业实物及蚕的艺术形象比较丰富,蚕的艺术形象如蚕纹、陶蚕蛹、牙雕蚕、玉石蚕、铜蚕、金蚕等,可统称为"蚕的模拟形态"。对蚕的模拟形态的功用,已有的诸多解释都有待完善。研究表明,蚕的模拟形态或艺术形象表达的功用或为饰品,或为装饰图案,或有待进一步考究。但无论哪种功用,用"蚕"这一形象都蕴含了特有的用意。通过对中国古代生命观的考察,文章认为蚕的艺术形象折射出相应的中国古代哲学生命观,即中国古人追求的死而复生、生生不息、羽化成仙、长乐无极等观念。 相似文献
27.
《International Journal of Hydrogen Energy》2022,47(1):320-338
Having accurate information about the hydrogen solubility in hydrocarbon fuels and feedstocks is very important in petroleum refineries and coal processing plants. In the present work, extreme gradient boosting (XGBoost), multi-layer perceptron (MLP) trained with Levenberg–Marquardt (LM) algorithm, adaptive boosting support vector regression (AdaBoost?SVR), and a memory-efficient gradient boosting tree system on adaptive compact distributions (LiteMORT) as four novel machine learning methods were used for estimating the hydrogen solubility in hydrocarbon fuels. To achieve this goal, a database containing 445 experimental data of hydrogen solubilities in 17 various hydrocarbon fuels/feedstocks was collected in wide-spread ranges of operating pressures and temperatures. These hydrocarbon fuels include petroleum fractions, refinery products, coal liquids, bitumen, and shale oil. Input parameters of the models are temperature and pressure along with density at 20 °C, molecular weight, and weight percentage of carbon (C) and hydrogen (H) of hydrocarbon fuels. XGBoost showed the highest accuracy compared to the other models with an overall mean absolute percent relative error of 1.41% and coefficient of determination (R2) of 0.9998. Also, seven equations of state (EOSs) were used to predict hydrogen solubilities in hydrocarbon fuels. The 2- and 3-parameter Soave-Redlich-Kwong EOS rendered the best estimates for hydrogen solubilities among the EOSs. Moreover, sensitivity analysis indicated that pressure owns the highest influence on hydrogen solubilities in hydrocarbon fuels and then temperature and hydrogen weight percent of the hydrocarbon fuels are ranked, respectively. Finally, Leverage approach results exhibited that the XGBoost model could be well trusted to estimate the hydrogen solubility in hydrocarbon fuels. 相似文献
28.
《International Journal of Hydrogen Energy》2022,47(9):5757-5765
At present, as the demand for electricity increases in all sectors, there is an urgent need to introduce alternative renewable energy sources into modern energy systems. Renewable energy sources, which consist of solar (photovoltaic, PV), wind and hydro power, are key alternative sources of “green energy’’ energies, but it can also be used to produce “green” hydrogen. Thanks to scientific and technological progress, the cost of photovoltaic solar radiation converters is constantly decreasing at a high rate, which makes it possible to build solar power plants of sufficiently large capacity. In the coming decades, solar energy will become an incentive for the economic development of countries that have the maximum “solar” resource. The Republic of Tajikistan is one of these countries with a high potential for solar energy.The article presents an analysis of the resources and potential of solar energy in the Republic of Tajikistan. The study of electromagnetic transients in networks with photovoltaic solar power plants is performed. The main equations, simulation model and calculations of transients are presented, taking into account changes in voltage on DC buses. An algorithm for controlling the system of automatic control of output parameters is proposed. The analysis of dynamic and static modes in parallel operation of a solar power plant with the grid is carried out. A block diagram and computer model is constructed in the MATLAB package together with Simulink and Power System Blockset. 相似文献
29.
30.
以H型垂直轴风力机及其内含圆柱形实体为研究对象,对NACA0018翼型的五叶片H型垂直轴风力机的气动性能进行数值模拟和实验验证。分析8种不同直径的内含圆柱体,在内含实体截面积占风轮迎风面积之比分别为21.2%、50.0%和76.9%时,风力机风能利用率的峰值分别下降8.04%、20.7%及74.3%。结果表明:随着内含实体直径的增大,风能利用率的峰值逐渐减小,开始较为缓慢,达到一定值时快速下降。小直径内含实体主要影响叶片在下风区的转矩,对风能利用率的影响较小,而大直径内含实体还会影响叶片在上风区的转矩,其风能利用率迅速减小。对于内含固定直径的实体,比如在现有建筑物外侧安装风力机时,其风轮半径的选择需综合考虑风能利用率和风力机的建造成本两方面的因素。研究结果可为建筑物与垂直轴风力机进行有效结合以提高风能的利用提供参考。 相似文献