全文获取类型
收费全文 | 215786篇 |
免费 | 19516篇 |
国内免费 | 13590篇 |
专业分类
电工技术 | 11300篇 |
技术理论 | 3篇 |
综合类 | 13610篇 |
化学工业 | 54379篇 |
金属工艺 | 19116篇 |
机械仪表 | 12805篇 |
建筑科学 | 8338篇 |
矿业工程 | 4028篇 |
能源动力 | 6435篇 |
轻工业 | 12721篇 |
水利工程 | 1871篇 |
石油天然气 | 7940篇 |
武器工业 | 1771篇 |
无线电 | 24517篇 |
一般工业技术 | 31222篇 |
冶金工业 | 7183篇 |
原子能技术 | 2922篇 |
自动化技术 | 28731篇 |
出版年
2024年 | 505篇 |
2023年 | 3196篇 |
2022年 | 4124篇 |
2021年 | 7102篇 |
2020年 | 5962篇 |
2019年 | 5766篇 |
2018年 | 5337篇 |
2017年 | 6732篇 |
2016年 | 7669篇 |
2015年 | 8060篇 |
2014年 | 10845篇 |
2013年 | 12308篇 |
2012年 | 13195篇 |
2011年 | 16112篇 |
2010年 | 12849篇 |
2009年 | 14548篇 |
2008年 | 12964篇 |
2007年 | 14739篇 |
2006年 | 13869篇 |
2005年 | 11495篇 |
2004年 | 9636篇 |
2003年 | 9093篇 |
2002年 | 7474篇 |
2001年 | 5937篇 |
2000年 | 5470篇 |
1999年 | 4314篇 |
1998年 | 3226篇 |
1997年 | 2601篇 |
1996年 | 2338篇 |
1995年 | 2073篇 |
1994年 | 1846篇 |
1993年 | 1499篇 |
1992年 | 1279篇 |
1991年 | 909篇 |
1990年 | 713篇 |
1989年 | 594篇 |
1988年 | 385篇 |
1987年 | 294篇 |
1986年 | 307篇 |
1985年 | 217篇 |
1984年 | 180篇 |
1983年 | 142篇 |
1982年 | 140篇 |
1981年 | 140篇 |
1980年 | 106篇 |
1979年 | 93篇 |
1977年 | 84篇 |
1976年 | 89篇 |
1975年 | 90篇 |
1974年 | 88篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
81.
This paper presents the Kriging model approach for stochastic free vibration analysis of composite shallow doubly curved shells. The finite element formulation is carried out considering rotary inertia and transverse shear deformation based on Mindlin’s theory. The stochastic natural frequencies are expressed in terms of Kriging surrogate models. The influence of random variation of different input parameters on the output natural frequencies is addressed. The sampling size and computational cost is reduced by employing the present method compared to direct Monte Carlo simulation. The convergence studies and error analysis are carried out to ensure the accuracy of present approach. The stochastic mode shapes and frequency response function are also depicted for a typical laminate configuration. Statistical analysis is presented to illustrate the results using Kriging model and its performance. 相似文献
82.
83.
This short communication presents a generic mathematical programming formulation for computer-aided molecular design (CAMD). A given CAMD problem, based on target properties, is formulated as a mixed integer linear/non-linear program (MILP/MINLP). The mathematical programming model presented here, which is formulated as an MILP/MINLP problem, considers first-order and second-order molecular groups for molecular structure representation and property estimation. It is shown that various CAMD problems can be formulated and solved through this model. 相似文献
84.
Microwave irradiation has been proven to be an effective heating source in synthetic chemistry, and can accelerate the reaction rate, provide more uniform heating and help in developing better synthetic routes for the fabrication of bone-grafting implant materials. In this study, a new technique, which comprises microwave heating and powder metallurgy for in situ synthesis of Ti/CaP composites by using Ti powders, calcium carbonate (CaCO3) powders and dicalcium phosphate dihydrate (CaHPO4·2H2O) powders, has been developed. Three different compositions of Ti:CaCO3:CaHPO4·2H2O powdered mixture were employed to investigate the effect of the starting atomic ratio of the CaCO3 to CaHPO4·2H2O on the phase, microstructural formation and compressive properties of the microwave synthesized composites. When the starting atomic ratio reaches 1.67, composites containing mainly alpha-titanium (α-Ti), hydroxyapatite (HA), beta-tricalcium phosphate (β-TCP) and calcium titanate (CaTiO3) with porosity of 26%, pore size up to 152 μm, compressive strength of 212 MPa and compressive modulus of 12 GPa were formed. The in vitro apatite-forming capability of the composite was evaluated by immersing the composite into a simulated body fluid (SBF) for up to 14 days. The results showed that biodissolution occurred, followed by apatite precipitation after immersion in the SBF, suggesting that the composites are suitable for bone implant applications as apatite is an essential intermediate layer for bone cells attachment. The quantity and size of the apatite globules increased over the immersion time. After 14 days of immersion, the composite surface was fully covered by an apatite layer with a Ca/P atomic ratio approximately of 1.68, which is similar to the bone-like apatite appearing in human hard tissue. The results suggested that the microwave assisted-in situ synthesis technique can be used as an alternative to traditional powder metallurgy for the fabrication of Ti/CaP biocomposites. 相似文献
85.
D. Yadav 《Materials Science & Technology》2015,31(4):494-500
Copper particles were incorporated and retained in elemental state in an aluminium matrix by friction stir processing thereby producing a non-equilibrium particulate composite. The processed Al–Cup composite exhibited improved strength with significantly high ductility. The composite was stable up to a temperature of more than 300°C. Thermal exposure at 350°C for more than 10 min led to diffusion of Cu atoms into the Al matrix forming a core-shell type structure in the Cu particles and thus producing an Al–Cu core-shell composite. The shell consists of multiple layers, the thickness of which was controllable. 相似文献
86.
87.
As a solid state joining process, ultrasonic spot welding has been proven to be a promising technique for joining copper alloys. However, challenges still remain in employing ultrasonic spot welding to join copper alloys. This article comprehensively reviews the current state of ultrasonic spot welding of copper alloys with a number of critical issues including materials flow, plastic deformation, temperature distribution, vibration, relative motion, vertical displacement, interface friction coefficient, online monitoring technique, coupled with the macrostructure and microstructure, the mechanical properties and electrical conductivity. In addition, the future trends in this field are provided. 相似文献
88.
某出口型号交流电力机车牵引控制系统要求变流器控制逻辑具备接触器控制、牵引系统数据交互、变流器故障保护、变流器启动自检等功能。针对以上功能,基于MATLAB中的Simulink/Stateflow可视化编程工具进行牵引控制模型的搭建,遵循模块化编程理念,采用较少的程序代码编写实现复杂的逻辑控制模型,生成逻辑清晰的控制功能流程图,进行半实物仿真测试,增强控制模型的可靠性。通过该变流器产品型式试验检验,验证变流器控制逻辑的功能可以满足控制系统的需求。 相似文献
89.
Rui ZHAO Weikai LI Tian WANG Ke ZHAN Zheng YANG Ya YAN Bin ZHAO Junhe YANG 《材料科学前沿(英文版)》2020,14(2):188
Effective thermal management of electronic integrated devices with high powder density has become a serious issue, which requires materials with high thermal conductivity (TC). In order to solve the problem of weak bonding between graphite and Cu, a novel Cu/graphite film/Cu sandwich composite (Cu/GF/Cu composite) with ultrahigh TC was fabricated by electro-deposition. The micro-riveting structure was introduced to enhance the bonding strength between graphite film and deposited Cu layers by preparing a rectangular array of micro-holes on the graphite film before electro-deposition. TC and mechanical properties of the composites with different graphite volume fractions and current densities were investigated. The results showed that the TC enhancement generated by the micro-riveting structure for Cu/GF/Cu composites at low graphite content was more effective than that at high graphite content, and the strong texture orientation of deposited Cu resulted in high TC. Under the optimizing preparing condition, the highest in-plane TC reached 824.3 W·m−1·K−1, while the ultimate tensile strength of this composite was about four times higher than that of the graphite film. 相似文献
90.
Biosolids reduction model by return activated sludge ozonation was validated by simulating nitrification data compiled from our pilot-scale and the literature studies. Then, a global sensitivity analysis (GSA) was performed to identify influential and non-influential parameters for biosolids reduction efficiency, change in specific nitrification activity (SNA), and alteration to expected nitrification stability. In general, the model outputs were sensitive to operational and ozone reaction parameters, but not to biochemical parameters. For operational parameters, mainly temperature and initial solids retention time (SRT) influenced all model outputs. For biosolids reduction, increase in the degradability of the influent COD decreased the reduction efficiency. For SNA, the changes were highly dependent on the influent TKN/COD ratio. Our findings also imply that the stability of the nitrification process in ozonated systems should be enhanced at constant MLVSS for warm temperatures, but could be reduced at temperatures below 12 °C and aerated SRTs below 10 days. 相似文献