全文获取类型
收费全文 | 440994篇 |
免费 | 18874篇 |
国内免费 | 8787篇 |
专业分类
电工技术 | 15892篇 |
技术理论 | 14篇 |
综合类 | 11404篇 |
化学工业 | 69855篇 |
金属工艺 | 20894篇 |
机械仪表 | 18860篇 |
建筑科学 | 22365篇 |
矿业工程 | 5337篇 |
能源动力 | 11926篇 |
轻工业 | 36508篇 |
水利工程 | 4975篇 |
石油天然气 | 12205篇 |
武器工业 | 1348篇 |
无线电 | 55358篇 |
一般工业技术 | 69853篇 |
冶金工业 | 60196篇 |
原子能技术 | 5935篇 |
自动化技术 | 45730篇 |
出版年
2023年 | 3437篇 |
2022年 | 6022篇 |
2021年 | 9266篇 |
2020年 | 6658篇 |
2019年 | 6250篇 |
2018年 | 7554篇 |
2017年 | 8201篇 |
2016年 | 7937篇 |
2015年 | 9057篇 |
2014年 | 12492篇 |
2013年 | 22702篇 |
2012年 | 17455篇 |
2011年 | 21292篇 |
2010年 | 17676篇 |
2009年 | 18156篇 |
2008年 | 18092篇 |
2007年 | 17807篇 |
2006年 | 17639篇 |
2005年 | 15947篇 |
2004年 | 12603篇 |
2003年 | 11845篇 |
2002年 | 11284篇 |
2001年 | 10928篇 |
2000年 | 10536篇 |
1999年 | 11891篇 |
1998年 | 21598篇 |
1997年 | 15734篇 |
1996年 | 12702篇 |
1995年 | 9883篇 |
1994年 | 8547篇 |
1993年 | 7819篇 |
1992年 | 5663篇 |
1991年 | 5194篇 |
1990年 | 4530篇 |
1989年 | 4316篇 |
1988年 | 4073篇 |
1987年 | 3361篇 |
1986年 | 3195篇 |
1985年 | 3700篇 |
1984年 | 3229篇 |
1983年 | 2900篇 |
1982年 | 2689篇 |
1981年 | 2695篇 |
1980年 | 2542篇 |
1979年 | 2360篇 |
1978年 | 2288篇 |
1977年 | 2865篇 |
1976年 | 4234篇 |
1975年 | 1927篇 |
1974年 | 1817篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
251.
Design and Fabrication of Concentration‐Gradient Generators with Two and Three Inlets in Microfluidic Chips 下载免费PDF全文
A simple and low‐cost method for designing and fabricating concentration‐gradient generators with two and three inlets is proposed which can generate different concentration gradients at varying flow velocities. The microchannel structure was designed in S‐shape and left‐right symmetry. The concentration‐gradient generator was simulated based on the finite element method. The microchannels were processed on a computer numerical control (CNC) engraving and milling machine on poly(methylmethacrylate) substrate, and then two concentration‐gradient generators were fabricated by hot bonding technology. The results of experiment and simulation were compared to prove the feasibility of the method. Flow velocity was an important factor for generating different concentration gradients. The concentration‐gradient profiles of the generators with two and three inlets present approximately linear and quadratic curves. 相似文献
252.
253.
In this paper, the development of the models for the prediction of rock mass P wave velocity is presented. For model development, the database of 53 cases including widely used and recorded drilling parameters and P wave velocity was constructed from the field studies conducted in 13 open pit lignite mines. Both conventional linear, non-linear multiple regression and Adaptive Neuro Fuzzy Inference System (ANFIS) were used for model development. Prediction performance indicators showed that ANFIS model presented the best performance and it can successfully be used for the preliminary prediction of P wave velocities of rock masses. 相似文献
254.
255.
Semiconductors - The performance of InGaN/GaN multiple quantum well (MQW) solar cells with five different Si-doping concentrations, namely 0, 4 × 1017 cm–3, 1 × 1018 cm–3, 3... 相似文献
256.
257.
Hai Huang Haichao An Haibo Ma Shenyan Chen 《International journal for numerical methods in engineering》2019,117(3):291-315
This work presents an engineering method for optimizing structures made of bars, beams, plates, or a combination of those components. Corresponding problems involve both continuous (size) and discrete (topology) variables. Using a branched multipoint approximate function, which involves such mixed variables, a series of sequential approximate problems are constructed to make the primal problem explicit. To solve the approximate problems, genetic algorithm (GA) is utilized to optimize discrete variables, and when calculating individual fitness values in GA, a second-level approximate problem only involving retained continuous variables is built to optimize continuous variables. The solution to the second-level approximate problem can be easily obtained with dual methods. Structural analyses are only needed before improving the branched approximate functions in the iteration cycles. The method aims at optimal design of discrete structures consisting of bars, beams, plates, or other components. Numerical examples are given to illustrate its effectiveness, including frame topology optimization, layout optimization of stiffeners modeled with beams or shells, concurrent layout optimization of beam and shell components, and an application in a microsatellite structure. Optimization results show that the number of structural analyses is dramatically decreased when compared with pure GA while even comparable to pure sizing optimization. 相似文献
258.
259.
Multicellular tumor spheroid models (MCTS) are often coined as 3D in vitro models that can mimic the microenvironment of tissues. MCTS have gained increasing interest in the nano‐biotechnology field as they can provide easily accessible information on the performance of nanoparticles without using animal models. Considering that many countries have put restrictions on animals testing, which will only tighten in the future as seen by the recent developments in the Netherlands, 3D models will become an even more valuable tool. Here, an overview on MCTS is provided, focusing on their use in cancer research as most nanoparticles are tested in MCTS for treatment of primary tumors. Thereafter, various types of nanoparticles—from self‐assembled block copolymers to inorganic nanoparticles, are discussed. A range of physicochemical parameters including the size, shape, surface chemistry, ligands attachment, stability, and stiffness are found to influence nanoparticles in MCTS. Some of these studies are complemented by animal studies confirming that lessons from MCTS can in part predict the behaviour in vivo. In summary, MCTS are suitable models to gain additional information on nanoparticles. While not being able to replace in vivo studies, they can bridge the gap between traditional 2D in vitro studies and in vivo models. 相似文献
260.
The relatively low capacitance of negative electrodes, as compared to the capacitance of advanced positive electrodes, poses a serious problem, since this limits the development of asymmetric supercapacitor (SC) devices with a large voltage window and enhanced power-energy characteristics. We fabricate negative SC electrodes with a high capacitance that match the capacitance of advanced positive electrodes at similar active mass loadings, as high as 37?mg?cm?2. Cyclic voltammetry, impedance spectroscopy, galvanostatic charge-discharge data and the power-energy characteristics of the asymmetric SC device exhibit good electrochemical performance for a voltage window of 1.6?V. Our approach involves the development and application of particle extraction through liquid-liquid interface (PELLI) methods, new extraction mechanisms and efficient extractors to synthesize α-FeOOH and β-FeOOH electrode materials. The use of PELLI allows agglomerate-free processing of powders, which facilitates their efficient mixing with multiwalled carbon nanotubes (MWCNT) and allows improved electrolyte access to the particle surface. Experiments to determine the properties of FeOOH-MWCNT composites provided insight into the influence of the electrode material and the structure of extractor molecules on the composite properties. The highest capacitance of 5.86?F?cm?2 for negative electrodes and low impedance were achieved using α-FeOOH-MWCNT composites and a 16-phosphonohexadecanoic acid (PHDA) extractor. This extractor allows adsorption on particles, not only at the liquid-liquid interface, but also in the bulk aqueous phase and can potentially be used as a capping agent for particle synthesis and as an extractor in the PELLI method. 相似文献