全文获取类型
收费全文 | 3701篇 |
免费 | 25篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 40篇 |
综合类 | 4篇 |
化学工业 | 415篇 |
金属工艺 | 23篇 |
机械仪表 | 33篇 |
建筑科学 | 52篇 |
矿业工程 | 5篇 |
能源动力 | 48篇 |
轻工业 | 215篇 |
水利工程 | 22篇 |
石油天然气 | 6篇 |
无线电 | 166篇 |
一般工业技术 | 297篇 |
冶金工业 | 2219篇 |
原子能技术 | 36篇 |
自动化技术 | 149篇 |
出版年
2023年 | 9篇 |
2022年 | 11篇 |
2021年 | 12篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 25篇 |
2017年 | 12篇 |
2016年 | 22篇 |
2015年 | 17篇 |
2014年 | 34篇 |
2013年 | 79篇 |
2012年 | 51篇 |
2011年 | 70篇 |
2010年 | 58篇 |
2009年 | 48篇 |
2008年 | 66篇 |
2007年 | 80篇 |
2006年 | 58篇 |
2005年 | 51篇 |
2004年 | 49篇 |
2003年 | 45篇 |
2002年 | 43篇 |
2001年 | 36篇 |
2000年 | 36篇 |
1999年 | 104篇 |
1998年 | 762篇 |
1997年 | 380篇 |
1996年 | 247篇 |
1995年 | 144篇 |
1994年 | 122篇 |
1993年 | 160篇 |
1992年 | 59篇 |
1991年 | 66篇 |
1990年 | 47篇 |
1989年 | 70篇 |
1988年 | 57篇 |
1987年 | 48篇 |
1986年 | 57篇 |
1985年 | 39篇 |
1984年 | 29篇 |
1983年 | 35篇 |
1982年 | 30篇 |
1981年 | 45篇 |
1980年 | 32篇 |
1979年 | 14篇 |
1978年 | 19篇 |
1977年 | 45篇 |
1976年 | 108篇 |
1974年 | 13篇 |
1971年 | 8篇 |
排序方式: 共有3730条查询结果,搜索用时 15 毫秒
61.
The living carbocationic polymerization (LC Pzn) of p-methylstyrene (pMeSt) was achieved by the use of the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4 initiating system in the presence of triethylamine (Et3N) electron donor in CH3Cl/nC6H14 mixed solvent at-80°C. Next, the synthesis, characterization, and some physical-mechanical characteristics of a series of poly (p-methylstyrene-b-isobutylene-b-p-methylstyrene) (PpMeSt-PIB-PpMeSt) triblocks is described. The latter synthesis involved two steps in one reactor: 1) The generation of diliving polyisobutylene dications (PIB) by the dicumyl chloride (DiCumCl)/TiCl4 system under similar conditions followed by 2) The addition of pMeSt and growth of living PpMeSt cations. The PpMeSt-PIB-PpMeSt triblocks are soft thermoplastic elastomers (TPEs).Paper L in the series Living Carbocationic Polymerization 相似文献
62.
Hydrogeologic parameters evaluated for rocks investigated in deep well testing projects for nuclear waste repository feasibility are transmissivity, hydraulic conductivity and storativity. Such studies have been carried out in rock formations of different geologic types, origins and ages for nuclear waste management agencies in North America and Europe. From transient pressure testing, an assessment of static pressure in selected fracture zones was measured from which a profile with depth was developed using modelling techniques. As well, temperatures of formation fluids and hence thermal gradients have also been provided. Hydrogeologic parameters are commonly used in risk analysis for repositories with respect to the possible escape of contaminants and worst case scenarios. The parameters are related to the rate of possible radionuclide migration into the biosphere. The techniques of the investigation and analysis methods of this work have been fully described in the literature. This paper is the first to date, however, to compare results from testing in differen geologic environments. High transmissivity zones have been observed in the upper few hundred metres of granite formations. Below that depth, the rock is more competent and discrete zones of higher permeability occur less frequently. Our studies show, however, that in these rocks, zones of high hydraulic conductivity do not always correspond to zones of high fracture frequency. Most formation pressure profiles have a vertical gradient of about 10 kPa/m. In discrete zones, deviations from this trend can occur due to vertical inhomogeneity in the rock. Thermal gradients vary considerably from one investigation site to another. For example, in the Canadian Shield, gradients are of the order of 1.0 degree C per 100 m, whereas in similar rock types in Europe, the gradient is up to 4.0 degrees C per 100 m. 相似文献
63.
The rheological behavior of sulfonated polyisobutylene based elastomeric ionomers has been studied. The effects of molecular architecture, type of cation, and addition of excess neutralization agent were investigated. The effect of temperature was studied to a limited extent. In a specific case, the influence of an ionic plasticizer, zinc stearate was also examined. It was found that in these telechelic ionomers where the ionic groups are located exclusively at the chain ends, significant Ionic interactions may persist even at 180°C. The zinc-neutralized ionomers had the lowest viscosity as compared to the corresponding potassium- or calcium-neutralized ionomers. The covalent character of zinc is believed responsible for this behavior. Other factors being constant, the triarm based ionomers are more viscous than the monofunctional ionomers. A mixture of monofunctional ionomers with the triarm, species is a model for dangling chain ends, and results in a slight lowering of the viscosity under the conditions studied. Zinc stearate acts as an ionic plasticizer. Upon the addition of 15 percent by weight of zinc-stearate to the ionomer, the low shear rate viscosity drops by several orders of magnitude and renders the ionomer thermally processable at moderate temperatures. 相似文献
64.
Summary Novel oxycarbonyl isocyanate telechelic polyisobutylenes have been prepared by conversion of hydroxylterminated polyisobutylenes with N-chlorocarbonyl isocyanate. This reaction is free from side-reactions and provides highly reactive isocyanate-terminated polyisobutylene prepolymers. Conversion of these prepolymers with glycol, urea, pentaerythritol, and polyethylene glycol led to linear chain extension, crosslinking, and multiblock copolymer formation, respectively. 相似文献
65.
Summary The living synthesis of ,-di-tert.-chloropolyisobutylene (
t
-Cl-telechelic PIB) has been accomplished by the use of the sterically hindered bifunctional initiator 1,3-di-(2-methoxy-2-propyl)-5-tert.-butylbenzene (tBu-m-DiCuOMe) in conjunction with BC13 coinitiator in CH3Cl or CH2Cl2 diluents at –30°C and –10°C. The living nature of the polymerizations was demonstrated by linear ¯Mn versus WPIB (g of PIB formed) plots starting at the origin and horizontal N (number of PIB moles) versus WPIB plots. The molecular weight distributions are narrow (¯MW/¯Mn < 2.0) and tend to decrease with increasing molecular weights. Number average end functionalities have been quantitated and found to be ¯Fn = 2.0±0.1. 相似文献
66.
Monofilament fibers were spun continuously from the free surface of a pool of molten poly(ethylene terephthalate) without the aid of a spinneret. For take-up velocities in the range of 12 to 400 ft/min, the denier of the filaments produced was an inverse power function of take-up velocity, and the birefringence was an inverse power function of the filament diameter. Production rate and product uniformity were strongly dependent on take-up velocity and surface temperature of the melt pool. 相似文献
67.
The study of the movement of solids in multiphase reactors using radioactive particle tracking is currently limited to fairly modest particle velocities because of count‐rate limitations of the detection system. In this work, this restriction was overcome by increasing the activity of the radioactive tracer, by decreasing the sampling time interval and by modifying the particle tracking software to recognize which detectors were saturated and to use only the data from the remaining unsaturated detectors. Higher tracer activity resulted in lower standard deviation of the calculated tracer coordinates. 相似文献
68.
Summary Living carbocationic polymerization of styrene (St) has been achieved by the use of the 1(p-methylphenyl)ethyl acetate (pMePhEtOAc)·BCl3 complex in CH3Cl solvent at –30°C using both the IMA and AMI techniques. The living nature of the polymerizations has been demonstrated by linear Mn versus WPSt (weight of polystyrene) plots passing through the origin and horizontal N (moles of PSt) versus WPSt plots starting at N=Io (the number of moles of initiator used). The molecular weight distribution MWD of the PSts is broad (Mw/Mn=~5–6) due to slow initiation and/or slow exchange between dormant and active species relative to propagation. The structure of the ester initiator strongly affects the rate and outcome of living polymerization, e.g., cumyl acetate (CumOAc), cumyl propionate (CumOPr), and 2,4,6-trimethylphenylethyl acetate (TMePhEtOAc), do not lead to truly living St polymerizations. The findings with the latter esters is explained in terms of a two path process comprising a slow living polymerization and a faster conventional cationic chain reaction. With phenylethyl acetate (PhEtOAc) living polymerization is achieved, however, initiation (cation generation) is slow. Forced termination by pyridine or methanol, or heating to ambient temperature leads to-CH2CH(C6H5)Cl end groups. 相似文献
69.
Conventional cationic polymerization can be converted into living carbocationic polymerization (LC⊕Pzn) by the introduction of common anion into the charge. Thus the conventional polymerization of isobutylene (IB) induced by the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4 combination yields LC⊕Pzn of IB upon the addition of tetrabutyl ammonium chloride (n-Bu4NCl), most likely due to then-Bu4NCl+TiCl4 ?n-Bu4N⊕+TiCl 5 ⊕ equilibrium. A kinetic model has been developed and tested which corroborates these propositions. By this model we have gained for the first time quantitative insight into the quasiliving equilibrium. 相似文献
70.
We discuss a new interpretation of mirror electron microscopy (MEM) images, whereby electric field distortions caused by surface topography and/or potential variations are sufficiently large to create caustics in the image contrast. Using a ray-based trajectory method, we consider how a family of rays overlaps to create caustics in the vicinity of the imaging plane of the magnetic objective lens. Such image caustics contain useful information on the surface topography and/or potential, and can be directly related to surface features. Specifically we show how a through-focus series of MEM images can be used to extract the contact angle of a Ga droplet on a GaAs (001) surface. 相似文献