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 46 毫秒
951.
非均相成核中间相炭微球的形成过程及其结构演变 总被引:15,自引:3,他引:12
以含有喹啉不溶物的煤焦油沥青为原料,在410℃、不同反应时间下获得一系列中间相炭微球(MCMB)和小于0.5μm的碳质颗粒(SCP)。通过对中间相沥青的软化点、族组成和MCMB、SCP的形貌及MCMB断面结构的分析,认为非均相成核MCMB的形成和生长过程不同于均相成核的融并生长过程,而是SCP碳质颗粒的构筑过程,我们称之为“球形基本单元构筑”过程。在MCMB生长过程中,体系的黏度对SCP的形成和MCMB的生长起着重要的作用。非均相成核MCMB的内部结构不是三种模型结构(“地球仪”型、“洋葱”型和“同心圆”型)中的任一种,而是炭层具有收缩点的复杂结构,并且球体的结构不具有“继承性”,即随着反应时间的延长而不断发生变化和调整。MCMB内部分子在球体生长过程中的主要变化趋势是分子的不断缩合长大,而不是分子片层的平行趋向。 相似文献
952.
不用专门相移装置的三维面形测量 总被引:1,自引:1,他引:0
提出一种不需要专用相移装置实现相移,主要利用数字图像处理技术实现三维面形测量的方法,研究结果表明,用原干涉图像及1幅以上的移值条纹图主可能实现物体三维面形的测量。 相似文献
953.
一种新的PCC-OFDM定时时偏盲估计 总被引:4,自引:2,他引:2
多项式相消编码PCC是一种编码技术,在PCC—OFDM调制中,它把将发送的信息调制到一组加权的子载波而不是单个子载波中去。PCC能够显著地降低OFDM对载波频偏的敏感性。本文对PCC—OFDM系统提出了一种新的定时盲估计,估计器是在接收机中的FFT之前进行的。基于PCC—OFDM的符号波形特征,构建定时时偏估计度量函数。最大化度量来估计定时时偏。本估计器不需要训练序列、导频信号或周期前缀。仿真表明,估计器在一个OFDM符号内收敛,且误差较小。 相似文献
954.
In this work, five branched polyethylenes with different branching units were synthesized using bidentate nickel (II) catalyst containing -diimine ligands. For comparison, one linear polyethylene was also prepared using tridentate iron (II) catalyst containing -diimine ligand. The crystalline structure of the polyethylenes was investigated using X-ray diffraction (XRD) and polarized optical microscope. The crystalline properties were also measured by differential scanning calorimeter (DSC). Viscoelastic properties of the polyethylenes were investigated using rheometric dynamic analyzer. The DSC and XRD results showed that highly branched polyethylenes exhibit no melting points and no predominating crystalline forms, while the linear polyethylene exhibits clear orthorhombic (110) and (200) reflections on XRD pattern and a clear melting point at 118 °C. The viscoelastic properties of the branched polyethylenes were very complicated due to the combined effect of the molecular weight difference and the degree of chain branching as well as the branching structure. 相似文献
955.
1 IntroductionWiththerapiddevelopmentofInternetandmobilecommunications,theresearchonwirelessmobilenetworkshasbecomethenewhotspotinin formationtechnologyfield .Asweknow ,mobileterminals (mobilephone,forexample)arethetooltohelppeopletoaccessanyinformationtheyneedbyanymediaatanytimeandanywhere .However,itissometimesdifficultformobileterminalstoplaysucharolebecauseofthelimitationssuchaslimitedca pacity ,lowpower,andinconvenienceinoperating .So,peoplehopetofindanotherwaytoaccessnet workinformatio… 相似文献
956.
Stanislav I. StoliarovPhillip R. Westmoreland Marc R. Nyden Glenn P. Forney 《Polymer》2003,44(3):883-894
The theory and implementation of reactive molecular dynamics (RMD) are presented. The capabilities of RMD and its potential use as a tool for investigating the mechanisms of thermal transformations in materials are demonstrated by presenting results from simulations of the thermal degradation of poly(methyl methacrylate) (PMMA). While it is known that depolymerization must be the major decomposition channel for PMMA, there are unanswered questions about the nature of the initiation reaction and the relative reactivities of the tertiary and primary radicals formed in the degradation process. The results of our RMD simulations, performed directly in the condensed phase, are consistent with available experimental information. They also provide new insights into the mechanism of the thermally induced conversion of this polymer into its constituent monomers. 相似文献
957.
Novel diols containing imide groups were prepared via condensation of aromatic dianhydrides with 5‐amino‐1‐naphthol. The diimide–dinaphthols prepared were characterized by conventional methods and used to synthesize new poly(urethane–imide)s (PUIs). All the polymers were characterized and their physical properties, such as solubility, solution viscosity, thermal stability, and thermal behaviour were studied. The polymers obtained showed more thermal stability than typical polyurethanes because of the presence of the imide groups. Copyright © 2003 Society of Chemical Industry 相似文献
958.
A new interaction integral formulation is developed for evaluating the elastic T-stress for mixed-mode crack problems with
arbitrarily oriented straight or curved cracks in orthotropic nonhomogeneous materials. The development includes both the
Lekhnitskii and Stroh formalisms. The former is physical and relatively simple, and the latter is mathematically elegant.
The gradation of orthotropic material properties is integrated into the element stiffness matrix using a “generalized isoparametric
formulation” and (special) graded elements. The specific types of material gradation considered include exponential and hyperbolic-tangent
functions, but micromechanics models can also be considered within the scope of the present formulation. This paper investigates
several fracture problems to validate the proposed method and also provides numerical solutions, which can be used as benchmark
results (e.g. investigation of fracture specimens). The accuracy of results is verified by comparison with analytical solutions. 相似文献
959.
960.
Seong Hyuk Lee Junghee Lee Kwan-Gu Kang Joon Sik Lee 《Journal of Mechanical Science and Technology》2006,20(8):1292-1301
This article investigates numerically the carrier-phonon interactions in thin gallium arsenide (GaAs) film structures irradiated
by subpicosecond laser pulses to figure out the role of several recombination processes on the energy transport during laser
pulses and to examine the effects of laser fluences and pulses on non-equilibrium energy transfer characteristics in thin
film structures. The self-consistent hydrodynamic equations derived from the Boltzmann transport equations are established
for carriers and two different types of phonons, i.e., acoustic phonons and longitudinal optical (LO) phonons. From the results,
it is found that the two-peak structure of carrier temperatures depends mainly on the pulse durations, laser fluences, and
nonradiative recombination processes, two different phonons are in nonequilibrium state within such lagging times, and this
lagging effect can be neglected for longer pulses. Finally, at the initial stage of laser irradiation, SRH recombination rates
increases sufficiently because the abrupt increase in carrier number density no longer permits Auger recombination to be activated.
For thin GaAs film structures, it is thus seen that Auger recombination is negligible even at high temperature during laser
irradiation. 相似文献