全文获取类型
收费全文 | 3899篇 |
免费 | 43篇 |
国内免费 | 3篇 |
专业分类
电工技术 | 27篇 |
综合类 | 2篇 |
化学工业 | 345篇 |
金属工艺 | 28篇 |
机械仪表 | 57篇 |
建筑科学 | 60篇 |
矿业工程 | 2篇 |
能源动力 | 38篇 |
轻工业 | 305篇 |
水利工程 | 10篇 |
石油天然气 | 5篇 |
无线电 | 222篇 |
一般工业技术 | 398篇 |
冶金工业 | 2190篇 |
原子能技术 | 25篇 |
自动化技术 | 231篇 |
出版年
2021年 | 13篇 |
2018年 | 22篇 |
2017年 | 25篇 |
2016年 | 27篇 |
2015年 | 18篇 |
2014年 | 20篇 |
2013年 | 88篇 |
2012年 | 51篇 |
2011年 | 96篇 |
2010年 | 59篇 |
2009年 | 56篇 |
2008年 | 92篇 |
2007年 | 79篇 |
2006年 | 61篇 |
2005年 | 61篇 |
2004年 | 66篇 |
2003年 | 57篇 |
2002年 | 58篇 |
2001年 | 54篇 |
2000年 | 49篇 |
1999年 | 111篇 |
1998年 | 601篇 |
1997年 | 364篇 |
1996年 | 275篇 |
1995年 | 173篇 |
1994年 | 154篇 |
1993年 | 141篇 |
1992年 | 46篇 |
1991年 | 51篇 |
1990年 | 54篇 |
1989年 | 64篇 |
1988年 | 57篇 |
1987年 | 56篇 |
1986年 | 36篇 |
1985年 | 67篇 |
1984年 | 37篇 |
1983年 | 28篇 |
1982年 | 28篇 |
1981年 | 35篇 |
1980年 | 28篇 |
1979年 | 34篇 |
1978年 | 40篇 |
1977年 | 72篇 |
1976年 | 127篇 |
1975年 | 26篇 |
1974年 | 24篇 |
1973年 | 24篇 |
1972年 | 22篇 |
1971年 | 18篇 |
1969年 | 11篇 |
排序方式: 共有3945条查询结果,搜索用时 203 毫秒
1.
2.
3.
4.
5.
T.L. Phillips 《Polymer》2005,46(24):11035-11050
Results are presented from atomistic molecular dynamics simulations of the mobile pseudo-hexagonal phase of polyethylene, which occurs under conditions of elevated pressure and temperature. Three different types of model are considered, all of which employ periodic boundary conditions. The first model consists of n-alkane sequences (48×-C24H48-) that are bonded across the simulation box boundaries to produce chains that are effectively infinite in extent. On heating, at high pressure, this system displays a rotator phase, in which the chains retain an all-trans conformation, and rotate as semi-rigid units. A second model, consisting of finite n-alkanes (48×C24H50) displays the same behaviour at low temperatures, but at high temperature and pressure forms a conformationally disordered rotator phase, characterised by a large proportion of gauche defects and a significant lattice expansion. The final model considered contains long n-alkanes (24×C102H206) which contain jog defects and each pass twice through the simulation box. This model forms a conformationally disordered rotator phase at high temperature and ambient pressure. The behaviour of the three models, in terms of the variations in chain conformation and rotational and translational dynamics, are compared. The conformationally disordered phases provide useful representations of the experimentally observed mobile phase. 相似文献
6.
J. M. Richards M. I. Darby R. Baggott G. H. Wosteim Holm B. Yates G. Dorey L. N. Phillips 《Journal of Materials Science》1989,24(2):584-589
The effects of defects on the mechanical properties of carbon fibre reinforced polyethersulphone laminates have been measured. The defects studied were cut fibre plies, omission of polymer films and local delamination produced by the inclusion of foreign matter. Of these it was found that only cut plies had a significant detrimental effect on the strength of a laminate. For specimens with two cut plies, the failure stress, tensile, flexural and compressive in the remaining continuous plies was the same as in the defect-free material, provided that the cut plies were widely separated. However the failure stresses were 15–18% lower in the continuous plies in the specimens containing two cut plies which were more closely spaced and in specimens containing four cut plies. 相似文献
7.
Wood A.C.G. O'Neill A.G. Phillips P.J. Biswas R.G. Whall T.E. Parker E.H.C. 《Electron Devices, IEEE Transactions on》1993,40(1):157-162
Delta-doped MOSFETs have been fabricated in MBE-grown silicon using for the first time boron as the dopant within the delta layer. Current-voltage characteristics have been measured, and secondary ion mass spectrometry (SIMS) is used to confirm the location of the delta layer and the extent of layer broadening by diffusion during processing. Precise threshold voltages of the devices are difficult to determine since the devices (which all operate in depletion mode) take several volts to switch off. Transconductances of the devices have been measured, and it is shown how analysis of these results can yield estimates of the carrier mobility for transport along the delta layers despite the uncertainty in the threshold voltage. A clear transition is observed in the results which is attributed to the formation of a parasitic surface-channel field-effect transistor, providing conclusive evidence that the devices are conducting along a delta channel for part of the measured range of applied gate biases 相似文献
8.
S Modi DE Gilham MJ Sutcliffe LY Lian WU Primrose CR Wolf GC Roberts 《Canadian Metallurgical Quarterly》1997,36(15):4461-4470
1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin that produces Parkinsonism symptoms in man, has been examined as a substrate of recombinant human cytochrome P450 2D6. When cumene hydroperoxide is used as an oxygen and electron donor, a single product is formed, identified as 4-phenyl-1,2,3,6-tetrahydropyridine. The K(m) for formation of this product (130 microM) is in agreement with the dissociation constants for MPTP binding to the enzyme determined by optical and nuclear magnetic resonance (NMR) spectroscopy. When the reaction is carried out with nicotinamide adenine dinucleotide phosphate (reduced) (NADPH) and recombinant human NADPH-cytochrome P450 reductase, a second product, identified as 1-methyl-4-(4'-hydroxyphenyl)-1,2,3,6-tetrahydropyridine, is formed in addition to 4-phenyl-1,2,3,6-tetrahydropyridine. The K(m) values for formation of these two products are 19 microM and 120 microM, respectively. Paramagnetic relaxation experiments have been used to measure distances between the protons of bound MPTP and the heme iron, and these have been used to construct models for the position and orientation of MPTP in the active site. For the cytochrome alone, a single mode of binding was observed, with the N-methyl close to the heme iron in a position appropriate for the observed N-demethylation reaction. In the presence of the reductase, the data were not consistent with a single mode of binding but could be explained by the existence of two alternative orientations of MPTP in the active site. One of these, characterized by a dissociation constant of 150 microM, is essentially identical to that observed in the absence of the reductase. In the second, which has a K(d) of 25 microM, the MPTP is oriented so that the aromatic ring is close to the heme iron, in a position appropriate for p-hydroxylation leading to the formation of the product seen only in the presence of the reductase. In the case of codeine, another substrate for cytochrome P450 2D6, the addition of reductase had no effect on the nature of the product formed, the dissociation constant, or the orientation in the binding site. These observations show that NADPH-cytochrome P450 reductase has an allosteric effect on the active site of cytochrome P450 2D6 that affects the binding of some substrates but not others. 相似文献
9.
Previous results, using a passive transfer assay, have shown that recrudescences of Plasmodium chabaudi chabaudi AS strain are antigenically different from the infecting parental population and also that the recrudescence appears to be a mix of antigenic types. This present study examines further these recrudescent populations using an indirect fluorescent antibody test on live, schizont-infected red blood cells. This analysis shows that ten clones derived from a recrudescence are all antigenically different from the parent population and that some are different from each other. The use of this method to examine the antigenic types of recrudescent clones after transmission through mosquitoes also demonstrates a resulting change in antigenicity. Such results showing a link between mosquito transmission and varying antigenicity may have important implications in terms of immunity and vaccine development. 相似文献
10.
MW Beatty ML Swartz BK Moore RW Phillips TA Roberts 《Canadian Metallurgical Quarterly》1993,27(3):403-413
Seven mechanical/physical properties were used to evaluate 10 unfilled resins: eight aromatic dimethacrylates and two urethane dimethacrylates. Physical property tests included compressive strength, Young's modulus in compression, uniaxial tensile strength, intrinsic yield point, toothbrush abrasion, Knoop hardness, and water sorption. Controlled changes were made in the following four material parameters: amount of crosslinking diluent present in the uncured monomer, functionality of the monomer, repeat unit chemistry of the monomer (urethane vs. aromatic structure) and mode of activation (chemical vs. visible light). Polymers containing a high concentration of crosslinking agent (50 wt%) were found to be tougher and to possess lower hardness than materials containing lesser amounts of crosslinking agent. This was attributed to the flexible nature of the triethylene glycol dimethacrylate crosslinking molecule. Exposure to water plasticized the highly crosslinked materials to the degree that the yield point and elastic modulus were effectively lowered. Most of the tested properties were unaffected by differences in functionality except resistance to toothbrush abrasion, which was enhanced for polymers derived from high functionality monomers. The urethane-based polymers sorbed substantially more water than the aromatic-based materials, which presumably resulted in lower values for surface hardness. However, the urethane resins were very tough, and excellent resistance to toothbrush abrasion was observed. Property differences caused by differences in activation mode were small, although the visible light materials did sorb more water. 相似文献