全文获取类型
收费全文 | 33833篇 |
免费 | 3645篇 |
国内免费 | 1778篇 |
专业分类
电工技术 | 1815篇 |
综合类 | 3484篇 |
化学工业 | 5433篇 |
金属工艺 | 1319篇 |
机械仪表 | 1795篇 |
建筑科学 | 6119篇 |
矿业工程 | 1326篇 |
能源动力 | 1774篇 |
轻工业 | 1731篇 |
水利工程 | 1186篇 |
石油天然气 | 2141篇 |
武器工业 | 324篇 |
无线电 | 3191篇 |
一般工业技术 | 2760篇 |
冶金工业 | 1867篇 |
原子能技术 | 660篇 |
自动化技术 | 2331篇 |
出版年
2024年 | 49篇 |
2023年 | 414篇 |
2022年 | 815篇 |
2021年 | 958篇 |
2020年 | 1077篇 |
2019年 | 973篇 |
2018年 | 912篇 |
2017年 | 1153篇 |
2016年 | 1329篇 |
2015年 | 1372篇 |
2014年 | 1994篇 |
2013年 | 2124篇 |
2012年 | 2543篇 |
2011年 | 2677篇 |
2010年 | 2055篇 |
2009年 | 1955篇 |
2008年 | 1933篇 |
2007年 | 2339篇 |
2006年 | 2083篇 |
2005年 | 1682篇 |
2004年 | 1408篇 |
2003年 | 1254篇 |
2002年 | 1055篇 |
2001年 | 895篇 |
2000年 | 732篇 |
1999年 | 610篇 |
1998年 | 449篇 |
1997年 | 434篇 |
1996年 | 336篇 |
1995年 | 322篇 |
1994年 | 296篇 |
1993年 | 198篇 |
1992年 | 201篇 |
1991年 | 121篇 |
1990年 | 101篇 |
1989年 | 105篇 |
1988年 | 70篇 |
1987年 | 48篇 |
1986年 | 34篇 |
1985年 | 38篇 |
1984年 | 24篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 15篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1978年 | 4篇 |
1975年 | 3篇 |
1959年 | 2篇 |
1951年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
目的 以圆锥台成形件为例,基于粗糙表面的区域支承率曲线,进行了用功能参数表征表面几何形貌的研究,以全面、准确、定量地表征渐进成形件的表面质量,预测表面性能。方法 用渐进成形工艺加工成形角分别为35°、45°和55°的圆锥台件,将各圆锥台的成形面分为顶部、中部和底部三个区域,并使用三维表面轮廓仪,在上述三个区域中随机测量各4个样本。根据ISO 25178-2:2012,将各样本表面的区域支承率曲线分为峰区、核心区和谷区,选用部分功能参数——核心区高度Sk、峰区材料体积Vmp、核心区材料体积Vmc和谷区空隙体积Vvv表征表面,并与常用的高度表征参数——Sa、Sq、Ssk和Sku进行比较。结果 成形件表面的Ssk和Sku的计算值极不稳定,变异系数(标准偏差与均值之比)最高值分别可达127.67%和39.11%,Sa、Sq的最大变异系数分别为4.41%和4.26%,虽然它们的参数计算值较为稳定,但常无法体现表面形貌的实际差异。功能参数独立表征不同功能区,Sk和Vmc的最大变异系数分别为9.32%和7.73%,说明同一表面各处,在长期工作阶段具有较为一致的表面性能,Vmp和Vvv的最大变异系数分别为60.53%和31.25%,说明表面各处峰、谷区的形貌有所不同,因而在磨合时间、磨粒存储等方面会有不同的表现。结论 粗糙表面的三维本质要求用三维表征参数才能全面表征其几何形貌。高度参数虽然计算简单,但常难以解读它们的物理意义,无法定量评价表面性能;功能参数具有明确的物理意义,可为表征表面质量、预测表面性能提供定量、实用的方法。 相似文献
102.
103.
104.
利用织构增强原理,开发出了TA18(Ti-3Al-2.5V)钛合金860 MPa级高强度管材,保证了管材的工艺性能,同时又具有优异的力学服役性能。标准规定反映径向织构的管材收缩应变比(CSR)大于1.3,但对上限值未作规定。本研究设计出小管径的环向拉伸夹具,测试了不同CSR管材的环向拉伸力学性能,分析织构对环向抗拉强度的影响,发现存在一个最有利的CSR值为1.75,大于或小于该CSR值时都会导致管材环向屈服强度下降。进一步测定了对应管材的极图和ODF图,从微观结构上可以解释这一规律和本质。环向抗拉强度下降会引起液压系统工作时疲劳强度降低,CSR值的上限在工艺设计时应当加以控制。本研究对发展安全可靠的航空液压系统具有十分重要的理论意义和工程应用价值。 相似文献
105.
The porosity dependence of Poisson's ratio of materials with random microstructure is investigated via analytical and numerical modeling. It is shown that all analytical models predict porosity independence if the solid Poisson ratio is 0.2 and for low porosities a converging trend toward this value with increasing porosity. From all theory-based relations, only power-law and exponential relations allow for auxetic behavior. Numerical calculations on computer-generated digital microstructures (overlapping and isolated spherical pores, pores between overlapping spherical grains, wall-based cellular materials/closed-cell foams, and strut-based cellular materials/open-cell foams) confirm the general qualitative trends of the analytical models, although a closer look reveals significant quantitative differences. Cellular materials and foams exhibit similar features as porous materials in general, but lack their converging trend toward values around 0.2. Comparison of our results with the classical Roberts-Garboczi results shows good agreement, with subtle differences due to the different microstructures generated. 相似文献
106.
Gilbert Rodríguez Eudes Villanueva Danco Cortez Esther Sanchez Elza Aguirre Alyssa Hidalgo 《Journal of the American Oil Chemists' Society》2020,97(7):729-735
Chia and sesame oils are important sources of essential fatty acids; however, their ω-3:ω-6 proportions do not comply with nutritional recommendation. A feasible approach to improve the ratio is to blend different oils, but only after understanding physical and chemical changes of the new matrix. Objective of the investigation was to determine the physico-chemical characteristics and the oxidative stability index (OSI), using the Rancimat method, of chia-sesame oil blends. The four ω-3:ω-6 blends tested (1:4, 1:6, 1:8, and 1:10) were exposed to temperatures of 110, 120, and 130 °C. The OSI values of the mixtures varied between 6.24–8.08, 3.07–4.00, and 1.62–2.01 hours for each temperature, respectively. In addition, their mean activation energy, enthalpy, entropy, and Q10 were 88.4 kJ/mol, 85.2 kJ/mol, −41.1 J/mol K, and 2.0. Finally, a shelf life prediction performed at 25 °C indicated stability times between 80 and 123 days. Therefore, combining chia and sesame oils produced blends with a good balance of essential fatty acids. 相似文献
107.
《International Journal of Hydrogen Energy》2020,45(48):25945-25959
In this paper, a new cascade control strategy is proposed for a higher order Proton Exchange Membrane (PEM) Fuel Cell system for improving the performance on the basis of stack voltage. In the proposed strategy, stack voltage is considered as the primary objective and maintaining oxygen excess ratio value as a secondary aim, by manipulating the air compressor voltage. A higher-order PEMFC model is reduced to lower order integer and fractional models using varied model reduction techniques, which are then used to realize the primary as well as secondary controllers. Both integer and fractional order PID controllers are designed for the reduced order models and then implemented for the higher order system. The control performance is evaluated for disturbance rejection, system model mismatch and better controlled output for continuous random disturbance on the basis of Integral Absolute Error and controller effort. The outcome of the proposed control strategy is advantageous in terms of disturbance rejection, robustness, parameter uncertainty and reduction of plant-model mismatch. 相似文献
108.
《中国有色金属学会会刊》2020,30(4):883-895
Based on orthogonal experiments, the effects of voltage, frequency, duty ratio and their interactions on the thickness and corrosion resistance of coatings prepared by plasma electrolytic oxidation (PEO) on aluminum in an alkaline silicate-containing electrolyte were investigated. The thicknesses of these coatings were obtained by measuring their cross-section using Image J software. Their corrosion resistances were evaluated in HCl and NaCl media through spot test and electrochemical test. The results show that the experimental design of this study is the key to investigate the interactions among these electrical parameters. Additionally, not only each independent factor, but also their interactions exhibit a remarkable influence on the coatings. The combination of high voltage, low frequency and large duty ratio significantly increases the coating thickness and content of the corrosion resistance phase, and thus improves the corrosion resistance of the coating in HNO3 medium. Conversely, the coating possessing the densest microstructure and best corrosion resistance in NaCl medium is obtained when low voltage and high frequency match with a small duty ratio. 相似文献
109.
《中国有色金属学会会刊》2020,30(12):3347-3355
MnO2/MnO cathode material with superior Zn2+ storage performance is prepared through a simple physical mixing method. The MnO2/MnO nanocomposite with a mixed mass ratio of 12:1 exhibits the highest specific capacity (364.2 mA·h/g at 0.2C), good cycle performance (170.4 mA·h/g after 100 cycles) and excellent rate performance (205.7 mA·h/g at 2C). Analysis of cyclic voltammetry (CV) data at various scan rates shows that both diffusion- controlled insertion behavior and surface capacitive behavior contribute to the Zn2+ storage performance of MnO2/MnO cathodes. And the capacitive behavior contributes more at high discharge rates, due to the short paths of ion diffusion and the rapid transfer of electrons. 相似文献
110.
Chunli Li Jingyu Li Dongchuan Li Shuaiming Ma Hao Li 《American Institute of Chemical Engineers》2020,66(8):e16266
The vapor split ratio (RV) adjustment plays an important role on energy efficiency during dividing wall column (DWC) operation. In order to achieve active control of RV, this aticle presents an innovative vapor splitter driven by hydraulics. The vapor flows into main tower from prefractionation section through the rectangle hole located at the end of the partition. Vapor splitting is implemented by the change of flow resistance at the rectangular hole caused by adjusting the liquid level on the bottom plate. This design makes full use of the hydraulic properties in DWC, employing simpler construction with single tunable parameter. Numerical simulations and laboratory tests were both carried out to validate its performance in the DWC with a diameter of 600 mm. The results demonstrate that the desired RV can be handled effectively in the approximate range from 0.5 to 2, basically satisfying the industrial demand for the gas distribution. 相似文献