全文获取类型
收费全文 | 54010篇 |
免费 | 3607篇 |
国内免费 | 3105篇 |
专业分类
电工技术 | 2092篇 |
技术理论 | 2篇 |
综合类 | 4137篇 |
化学工业 | 13909篇 |
金属工艺 | 2209篇 |
机械仪表 | 1996篇 |
建筑科学 | 2698篇 |
矿业工程 | 765篇 |
能源动力 | 1299篇 |
轻工业 | 5318篇 |
水利工程 | 848篇 |
石油天然气 | 2371篇 |
武器工业 | 313篇 |
无线电 | 3978篇 |
一般工业技术 | 5188篇 |
冶金工业 | 1729篇 |
原子能技术 | 663篇 |
自动化技术 | 11207篇 |
出版年
2024年 | 125篇 |
2023年 | 661篇 |
2022年 | 2474篇 |
2021年 | 2542篇 |
2020年 | 1278篇 |
2019年 | 1235篇 |
2018年 | 1176篇 |
2017年 | 1531篇 |
2016年 | 1797篇 |
2015年 | 1894篇 |
2014年 | 2923篇 |
2013年 | 3189篇 |
2012年 | 3472篇 |
2011年 | 3853篇 |
2010年 | 2864篇 |
2009年 | 3265篇 |
2008年 | 3155篇 |
2007年 | 3342篇 |
2006年 | 3020篇 |
2005年 | 2578篇 |
2004年 | 2074篇 |
2003年 | 1846篇 |
2002年 | 1513篇 |
2001年 | 1353篇 |
2000年 | 1232篇 |
1999年 | 975篇 |
1998年 | 741篇 |
1997年 | 652篇 |
1996年 | 589篇 |
1995年 | 568篇 |
1994年 | 506篇 |
1993年 | 371篇 |
1992年 | 314篇 |
1991年 | 258篇 |
1990年 | 239篇 |
1989年 | 206篇 |
1988年 | 159篇 |
1987年 | 97篇 |
1986年 | 113篇 |
1985年 | 78篇 |
1984年 | 67篇 |
1983年 | 46篇 |
1982年 | 54篇 |
1981年 | 50篇 |
1980年 | 40篇 |
1979年 | 43篇 |
1978年 | 18篇 |
1977年 | 15篇 |
1976年 | 13篇 |
1975年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
81.
The effect of recycle on heat (and mass) transfer in concentric circular tubes has been investigated by finite difference methods. Theoretical results show that recycle can enhance the heat transfer rate for large Graetz numbers compared with that in an open tube (without an inner tube inserted and without recycle). Competition between a preheating effect and a residence-time effect can be used to explain the heat transfer behavior. The heat transfer rate can be further augmented, with nearly no increase in total pressure drop, by employing flow pattern B instead of flow pattern A. 相似文献
82.
对二聚炔醇化合物DMH的合成、分子结构及缓蚀作用机理进行了论述。因DMH化合物具有独特的分子结构,形成了稳定的络合体系及基团的屏蔽效应,增加了缓蚀被膜厚度及缓蚀体系的化学稳定性,实验证实DMH是酸性介质中的高效缓蚀剂。 相似文献
83.
高霞 《网络安全技术与应用》2014,(11):172-173
在教学中,创设问题情境是一种科学有效的教学手段。通过对一堂计算机基础课《PPT动画路径》的分析,探究如何在中职计算机课程中创设问题情境,提高课堂教学实效性,并对容易出现的问题进行简单的阐述。 相似文献
84.
Advanced oxidation processes are defined as those which involve the generation of hydroxyl radicals in sufficient quantity to affect water purification. The theoretical and (practical yield of OH from O3 at high pH, 03/H202, O3/UV and H2O2/UV systems is reviewed. New data is presented which illustrates the importance of direct photolysis in the O3/UV process, the effect of the H202:03 ratio in the O3/H2O2 process, and the impact of the low extinction coefficient of H2O2 in the H202/UV process. 相似文献
85.
86.
The antioxidant vitamin E, -tocopherol, was tested as a candidate elicitor of alterable antiherbivory in soybean plants against cabbage looper larvae. Although a nonspecific antioxidant, vitamin E proved elicitory to the involved sulfhydryl-dependent receptor-energy transducer protein in soybean plasma membrane. Effects of -tocopherol were dependent on dosage, time, and space in the plant. The observed elicited effects were all decreases in herbivory. The best negative phytochemical correlate of looper feeding was the percentage of increased total HPLC peak area of extractables from elicited as compared to nonelicited leaves. Some specific compounds, e.g., glyceollins, were quantitatively major components of the total profile of secondary metabolites. 相似文献
87.
本文对生产高纯度丁烯-1的Alphabntol工艺进行了技术经济评价,认为该工艺条件简单、反应条件温和,而且投资少,所以很适合事在发展中国家应用。 相似文献
88.
Martel Zeldin Eric Granger Wilmer K. Fife 《Journal of Inorganic and Organometallic Polymers》1993,3(2):141-154
New silane monomers with the pendant 4-(3-pyridine)butyl group have been synthesized by hydrosilation of 3-(3-butenyl)pyridine with Me
n
Si(OEt)3-n
H (n=0, 1) using a platinum catalyst. Only -addition products were observed. The products were characterized by elemental analysis, infrared,1H- and13C-NMR spectroscopy, and gas chromatography-mass spectrometry. Hydrolysis-polycondensation of the difunctional monomer with a basic catalyst, Me4NOH, gave a mixture of cyclic oligomers, principally cyclic tetramer, and linear homopolymer. Under similar reaction conditions, the trifunctional monomer gave crosslinked material which was soluble in common organic solvents. The linear homopolymer and crosslinked polymer were trimethylsilyl end-blocked with hexamethyldisilazane. The cyclic and end-blocked polymers were characterized by elemental analysis and spectroscopic methods. Molecular weights of the polymers were obtained by end-group analysis using1H-NMR spectral data, size exclusion chromatography, and direct insertion-probe mass spectrometry. The cyclic, linear, and crosslinked materials were N-oxidized withm-chloroperoxybenzoic acid and characterized by spectroscopic methods. The polymeric N-oxide derivatives were shown to be effective transacylation catalysts in the synthesis of mixed carboxylic acid anhydrides in immiscible solvents (H2O/CH2Cl2) under phase-transfer conditions. The implications of the results on the mechanism of catalysis are discussed. 相似文献
89.
Santosh D. Wanjale 《Polymer》2006,47(18):6414-6421
Poly(1-butene)/MWCNT nanocomposites were prepared by simple melt processing technique. Crystallization, crystal-to-crystal phase transformation and spherulitic morphology were studied using differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and optical microscopy (OM). The non-isothermal crystallization exhibited higher values of Zt derived from Avrami theory and lower values of F(T) obtained from Avrami-Ozawa analysis, while the isothermal crystallization revealed a significant increase in crystallization temperatures and lower crystallization half times compared to pristine PB. The observed changes in the crystallization kinetics were ascribed to the enhanced nucleation of PB in the presence of MWCNT. The nucleating activity calculated from the non-isothermal crystallization data revealed that the MWCNTs provide an active surface for the nucleation of PB. The optical micrographs exhibited significantly smaller crystallites with disordered morphology for the nanocomposites compared to the well defined spherulitic morphology for pristine PB. The rate of phase transformation from kinetically favored tetragonal to thermodynamically stable hexagonal form was noticeably enhanced as evidenced by the reduction in the half time for phase transformation from 58 h to 25 h for PB reinforced with 7% MWCNT. 相似文献
90.
求解背包问题的病毒协同进化粒子群算法 总被引:2,自引:1,他引:1
为提高粒子群算法的搜索性能,提出一种基于病毒进化理论的改进离散粒子群算法:病毒协同进化粒子群算法.在粒子群中引入生物病毒机制和宿主与病毒基于感染操作的思想,病毒采用与粒子等长的编码方式,执行反向代换、结合等操作,利用病毒的水平感染和垂直传播能力较好地维持个体的多样性和对解空间的局部搜索能力.通过解决背包问题对算法进行验证,仿真表明所提算法搜索性能优于遗传算法、模拟退火及标准粒子群等其他算法.该算法能有效求解背包问题等NP难题. 相似文献