全文获取类型
收费全文 | 204294篇 |
免费 | 26745篇 |
国内免费 | 25256篇 |
专业分类
电工技术 | 20050篇 |
技术理论 | 10篇 |
综合类 | 15233篇 |
化学工业 | 40049篇 |
金属工艺 | 7903篇 |
机械仪表 | 12694篇 |
建筑科学 | 9825篇 |
矿业工程 | 2429篇 |
能源动力 | 6113篇 |
轻工业 | 14615篇 |
水利工程 | 2745篇 |
石油天然气 | 4310篇 |
武器工业 | 2061篇 |
无线电 | 33861篇 |
一般工业技术 | 21774篇 |
冶金工业 | 4318篇 |
原子能技术 | 3349篇 |
自动化技术 | 54956篇 |
出版年
2024年 | 1156篇 |
2023年 | 3764篇 |
2022年 | 6671篇 |
2021年 | 7736篇 |
2020年 | 7428篇 |
2019年 | 6757篇 |
2018年 | 6318篇 |
2017年 | 8495篇 |
2016年 | 9237篇 |
2015年 | 10499篇 |
2014年 | 10420篇 |
2013年 | 13650篇 |
2012年 | 15536篇 |
2011年 | 17474篇 |
2010年 | 12581篇 |
2009年 | 12359篇 |
2008年 | 13614篇 |
2007年 | 15106篇 |
2006年 | 14260篇 |
2005年 | 12261篇 |
2004年 | 10306篇 |
2003年 | 8093篇 |
2002年 | 6279篇 |
2001年 | 4733篇 |
2000年 | 3907篇 |
1999年 | 3269篇 |
1998年 | 2711篇 |
1997年 | 2180篇 |
1996年 | 1730篇 |
1995年 | 1428篇 |
1994年 | 1258篇 |
1993年 | 941篇 |
1992年 | 767篇 |
1991年 | 605篇 |
1990年 | 522篇 |
1989年 | 379篇 |
1988年 | 302篇 |
1987年 | 189篇 |
1986年 | 184篇 |
1985年 | 242篇 |
1984年 | 205篇 |
1983年 | 149篇 |
1982年 | 204篇 |
1981年 | 97篇 |
1980年 | 99篇 |
1979年 | 27篇 |
1978年 | 19篇 |
1977年 | 26篇 |
1976年 | 18篇 |
1959年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
Different NMR techniques were combined to obtain the structure and velocity information for a systematic investigation of fixed beds with low aspect ratio (tube diameter to particle diamter, dt/dp) in the range 1.4 to 32. The structure of the void space was determined for a variety of packed beds of glass beads or regular and irregular porous pellets by magnetic resonance imaging (MRI). Based on the images the radial distribution of the voids within the bed was obtained. Ordering effects were found even for non‐spherical and polydisperse particles, and a maximum of the fluid density near the tube wall was confirmed for all pellet geometries and sizes. By combining MRI with velocity encoding, velocity profiles and distributions of flow velocity components of a single fluid phase through packed beds have been acquired. The radial velocity distribution follows an oscillatory pattern which largely reflects the ordering of the particles, which can be accessed from the density distribution of the interparticle fluid. Maximum velocities of up to four times the average value were found to occur near the tube wall. This wall effect was observed for all but the smallest particles, where the aspect ratio was dt/dp = 32. Moreover, a visualisation of flow pattern in the presence of packed particles was achieved by using a tagging technique, and the stationary flow field could be identified for an experimental time of several hours. 相似文献
973.
Hamid Javaherian Naghash Ahmad Reza Momeni Hamid Alian Ahmad Reza Massah Shahla Ataie 《Polymer International》2005,54(11):1564-1571
Poly(methyl methacrylate) (PMMA) gels with varying amounts of silicone and solvent and constant amounts of crosslinker were prepared by solution free radical crosslinking copolymerization of methyl methacrylate (MMA), ethylene glycol dimethacrylate (EGDM), tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTES) comonomer systems. They were then studied in benzene at a total monomer concentration of 3.5 mol L?1 and 70 °C. The conversion of monomer, volume swelling ratio, weight fraction and gel point were measured as a function of the reaction time, silicone concentration and benzene content up to the onset of macrogelation. Structural characteristics of the gels were examined by using equilibrium swelling in benzene, gel fraction and Fourier‐transform infrared (FTIR) analysis. The morphology of the copolymers was also investigated by SEM. Based on the obtained results, it was concluded that the FTIR data did not have the capacity to show the presence of the VTES or TEOS moiety in these kinds of copolymers. On the other hand, the variation of weight fraction of gel, Wg, and its equilibrium volume swelling ratio in benzene, qv, exhibited the same behaviour as that of MMA/EGDM copolymers. Also, the dilution of the monomer mixture resulted in an increase in the gel point and swelling degree and a decrease in the percent of conversion and gel fraction. Finally, TEOS is not an ideal silicone compound for reaction in the MMA/EGDM copolymerization system, whereas VTES is a suitable silicone comonomer for this system and it has been proved useful. Copyright © 2005 Society of Chemical Industry 相似文献
974.
本文提出了一种适合于特殊环境使用的光纤局域网的拓扑结构——类星型结构。这种结构解决了多通道的同时传输问题。此外,文中还对 OLAN 网的层次划分和各层的协议进行了分析和讨论。 相似文献
975.
S. H. Ahmad 《Polymer International》1992,28(4):291-294
Measurement of the flow stress of high density polyethylene (HDPE) and nylon 66 at strain rates of 103 s?1 using a split Hopkinson pressure bar technique is discussed. The flow stress at a strain of 10% has been determined for both polymers at 20°C. The intrinsic errors involved in this technique are briefly reviewed. The results indicate that the flow stress of HDPE and nylon 66 were 50MPa and 150MPa, respectively, at strain rates of about 103s?1. 相似文献
976.
977.
978.
本文简要介绍了ER流体现象及其力学和电学性能,给出了描述ER流体行为的一般理论,提出了ER流体应用中目前存在的主要问题。 相似文献
979.
Jun Rong Li Jun Wang Jia Rui Xu Ming Qiu Zhang Min Zhi Rong Qiang Zheng 《Composites Science and Technology》2006,66(16):3126-3131
The authors of this paper synthesized a series of amphiphilic triblock copolymers of polystyrene-b-poly(ethylene glycol)-b-polystyrene (PS-PEG-PS) having different PEG/PS ratios with nearly identical molecular weights of the entire copolymers. The interfacial interactions in the composites consisting of carbon black and the copolymers can thus be tailored. When these conducting composites are exposed to certain solvent vapors, their electrical resistances greatly increase, showing the gas sensitivity. The present work indicated that this switching behavior is controlled by the structural relaxation of the composites because matrix swelling acts as the main mechanism. The response time has been correlated with absolute temperature by Arrhenius equation, and the estimated activation energy reflects mobility of the fillers involved in the solvent induced expansion of the surrounding polymer. Therefore, by using the gas sensibility of the conductive composites, the structure evolution of the composite materials in solid state and the effect of filler/matrix interfacial interaction on the relaxation property of the matrix polymer has been inspected. It was found that lower activation energy represents stronger interfacial interaction in case good solvent of the matrix was used for the test. 相似文献
980.
Ariosto B. Jorge Gabriel O. Ribeiro Timothy S. Fisher 《International journal for numerical methods in engineering》2003,56(1):117-144
This work presents two new error estimation approaches for the BEM applied to 2D potential problems. The first approach involves a local error estimator based on a gradient recovery procedure in which the error function is generated from differences between smoothed and non‐smoothed rates of change of boundary variables in the local tangential direction. The second approach involves the external problem formulation and gives both local and global measures of error, depending on a choice of the external evaluation point. These approaches are post‐processing procedures. Both estimators show consistency with mesh refinement and give similar qualitative results. The error estimator using the gradient recovery approach is more general, as this formulation does not rely on an ‘optimal’ choice of an external parameter. This work presents also the use of a local error estimator in an adaptive mesh refinement procedure. This r‐refinement approach is based on the minimization of the standard deviation of the local error estimate. A non‐linear programming procedure using a feasible‐point method is employed using Lagrange multipliers and a set of active constraints. The optimization procedure produces finer meshes close to a singularity and results that are consistent with the problem physics. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献