全文获取类型
收费全文 | 27919篇 |
免费 | 2670篇 |
国内免费 | 2016篇 |
专业分类
电工技术 | 2276篇 |
综合类 | 2452篇 |
化学工业 | 3871篇 |
金属工艺 | 1622篇 |
机械仪表 | 1857篇 |
建筑科学 | 2106篇 |
矿业工程 | 732篇 |
能源动力 | 775篇 |
轻工业 | 3199篇 |
水利工程 | 716篇 |
石油天然气 | 958篇 |
武器工业 | 353篇 |
无线电 | 3162篇 |
一般工业技术 | 2792篇 |
冶金工业 | 1173篇 |
原子能技术 | 438篇 |
自动化技术 | 4123篇 |
出版年
2024年 | 90篇 |
2023年 | 417篇 |
2022年 | 799篇 |
2021年 | 1035篇 |
2020年 | 826篇 |
2019年 | 689篇 |
2018年 | 677篇 |
2017年 | 837篇 |
2016年 | 702篇 |
2015年 | 1125篇 |
2014年 | 1504篇 |
2013年 | 1778篇 |
2012年 | 2045篇 |
2011年 | 2144篇 |
2010年 | 2066篇 |
2009年 | 2004篇 |
2008年 | 2044篇 |
2007年 | 1998篇 |
2006年 | 1813篇 |
2005年 | 1565篇 |
2004年 | 1202篇 |
2003年 | 970篇 |
2002年 | 1001篇 |
2001年 | 904篇 |
2000年 | 683篇 |
1999年 | 424篇 |
1998年 | 272篇 |
1997年 | 196篇 |
1996年 | 172篇 |
1995年 | 141篇 |
1994年 | 129篇 |
1993年 | 69篇 |
1992年 | 75篇 |
1991年 | 43篇 |
1990年 | 46篇 |
1989年 | 37篇 |
1988年 | 20篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 6篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1967年 | 1篇 |
1959年 | 1篇 |
1951年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
101.
容跃堂 《纺织高校基础科学学报》1992,(1)
讨论牛曼边界条件下具有常数收获率的竞争—扩散问题解的性态,得到当收获率满足当条件时,种群u将在有限时刻被消灭。 相似文献
102.
Ao Sheng Rong Zhong Liang Sun 《Antennas and Propagation, IEEE Transactions on》1991,39(9):1366-1371
The radiation of millimeter waves from the nonradiative dielectric (NRD) leaky wave antennas with the tapered transition and the tuning aperture is investigated. An accurate theory for the leakage constant and the phase constant of the antennas is presented, which is based on the generalized scattering parameters considering the higher-order mode interactions at the discontinuities. The associated transverse equivalent network involves the characterization of the mode couplings at two air-to-dielectric interfaces, the coupled mode analysis of the tapered transition, and the calculation of the radiation fields at the tuning aperture via the extended spectral domain approach. The interesting behavior of the present modified configuration of NRD leaky wave antennas is illustrated by typical examples. The validity of the theory is confirmed by the comparison of the theoretical results with experimental data for the limiting cases 相似文献
103.
104.
105.
106.
107.
A series of new o‐phenylenediamine (OPD)/o‐phenetidine (PHT) copolymers with partly phenazine‐like structures has been successfully synthesized at three polymerization temperatures by chemically oxidative polymerization in four different polymerization media. The molecular structures and properties of the resulting OPD/PHT polymers were investigated by IR, UV–vis and high‐resolution 1H NMR spectroscopies, and DSC, in order to ascertain the effect of reaction temperature, comonomer ratio and acid medium. The copolymerization mechanism of OPD with PHT monomers has been proposed. It is found that the statistical OPD/PHT copolymer obtained at a temperature of 118 °C has a higher degree of polymerization than that obtained at 12–17 °C. The OPD content in the copolymers calculated from NMR spectroscopic analysis is higher than that in the feed OPD content, whereas the OPD content calculated from element analysis is slightly lower than the feed OPD content. It can be predicted that denitrogenation takes place in the OPD units during the polymerization process at OPD/PHT molar ratios of 90/10 and 100/0. These OPD/PHT copolymers exhibit a much better solubility than the OPD homopolymer, hence suggesting an incorporation of PHT units into the phenazine structure of the homopolymer. The thermal behavior of the copolymers was also studied. Copyright © 2004 Society of Chemical Industry 相似文献
108.
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
109.
Guanghui Wang Lei Huang Rong Yu Nansheng Deng 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》2008,83(5):601-608
BACKGROUND: Endocrine disruptors in the aquatic environment and their potential adverse effects are currently issues of concern. One of these endocrine disruptors is 2,2‐bis(4‐hydroxy‐3‐methylphenyl)propane (BPP). In this work the molecular recognition interaction of BPP with β‐cyclodextrin (β‐CD) was studied using IR spectroscopy and steady state fluorescence spectroscopy, and the photocatalytic degradation behaviour of BPP based on molecular recognition interaction was investigated in a TiO2/UV–visible (λmax = 365 nm) system. This might provide a new method for the treatment of some organic pollutants in wastewater. RESULTS: β‐CD reacts with BPP to form a 1:1 inclusion complex, the formation constant of which is 4.94 × 103 L mol?1. The photodegradation rate constant of BPP after molecular recognition by β‐CD showed a 1.42‐fold increase in the TiO2/UV–visible (λmax = 365 nm) system. The photodegradation of BPP depended on the concentration of β‐CD, the pH value, the gaseous medium and the initial concentration of BPP. The photodegradation efficiency of BPP with molecular recognition was higher than that without molecular recognition. After 100 min of irradiation the mineralisation efficiency of BPP after molecular recognition by β‐CD reached 94.8%, whereas the mineralisation efficiency of BPP before molecular recognition by β‐CD was only 40.6%. CONCLUSION: The photocatalytic degradation of BPP after molecular recognition by β‐CD can be enhanced in the TiO2/UV‐visible (λmax = 365 nm) system. This enhancement is dependent on the enhancement of the adsorption of BPP, the moderate inclusion depth of BPP in the β‐CD cavity and the increase in the frontier electron density of BPP after molecular recognition. Copyright © 2008 Society of Chemical Industry 相似文献
110.