全文获取类型
收费全文 | 6652篇 |
免费 | 243篇 |
国内免费 | 7篇 |
专业分类
电工技术 | 82篇 |
综合类 | 5篇 |
化学工业 | 1323篇 |
金属工艺 | 137篇 |
机械仪表 | 143篇 |
建筑科学 | 396篇 |
矿业工程 | 13篇 |
能源动力 | 216篇 |
轻工业 | 678篇 |
水利工程 | 109篇 |
石油天然气 | 30篇 |
无线电 | 453篇 |
一般工业技术 | 1182篇 |
冶金工业 | 1087篇 |
原子能技术 | 48篇 |
自动化技术 | 1000篇 |
出版年
2023年 | 35篇 |
2022年 | 81篇 |
2021年 | 103篇 |
2020年 | 92篇 |
2019年 | 98篇 |
2018年 | 113篇 |
2017年 | 127篇 |
2016年 | 120篇 |
2015年 | 138篇 |
2014年 | 186篇 |
2013年 | 416篇 |
2012年 | 343篇 |
2011年 | 421篇 |
2010年 | 291篇 |
2009年 | 335篇 |
2008年 | 443篇 |
2007年 | 362篇 |
2006年 | 304篇 |
2005年 | 317篇 |
2004年 | 255篇 |
2003年 | 232篇 |
2002年 | 217篇 |
2001年 | 117篇 |
2000年 | 90篇 |
1999年 | 113篇 |
1998年 | 129篇 |
1997年 | 103篇 |
1996年 | 123篇 |
1995年 | 87篇 |
1994年 | 89篇 |
1993年 | 83篇 |
1992年 | 68篇 |
1991年 | 52篇 |
1990年 | 58篇 |
1989年 | 79篇 |
1988年 | 40篇 |
1987年 | 54篇 |
1986年 | 46篇 |
1985年 | 63篇 |
1984年 | 62篇 |
1983年 | 62篇 |
1982年 | 48篇 |
1981年 | 48篇 |
1980年 | 29篇 |
1979年 | 38篇 |
1978年 | 32篇 |
1977年 | 24篇 |
1976年 | 24篇 |
1975年 | 30篇 |
1974年 | 21篇 |
排序方式: 共有6902条查询结果,搜索用时 15 毫秒
101.
Summary CP/DD/MASS 13C-NMR spectra have been obtained for regenerated cellulose samples with different crystallinities as well as for cotton, -D-glucose, -D-cellobiose, and cellopentaose. The spectra of the regenerated cellulose samples exhibit broad multiplicities of the C-4 and C-6 resonance lines in a similar manner as those of native cellulose samples such as cotton and ramie, and, in addition, another broad tailing of the C-1 resonance. Since these multiplicities change linearly with crystallinity, it is concluded that they are ascribed to the contributions from the crystalline and noncrystalline components. Effects of hydrogen bonds and conformations of the -1,4-glycosidic linkage on the chemical shifts are also discussed. 相似文献
102.
Richard V. Flor Le Tiet Hecking Brian David Martin 《Journal of the American Oil Chemists' Society》1993,70(2):199-203
Criteria for authentic olive oils were developed from isocratic high-performance liquid chromatography analyses of 99 olive
oils from the major Mediterranean producers in the 1983–1986 crop years. Authentic olive oils include extra virgin, virgin
and pure or refined oils, but exclude all reesterified and adulterated oils. The extra virgin through pure grades will have
a combined area for the LOO (C18:2C18:1C18:1), LOP (C18:2C18:1C16:0), OOO (C18:1C18:1C18:1), POO (C16:0C18:1C18:1), POP (C16:0C18:1C18:1), and SOO (C18:0C18:1C18:1) peaks between 82.0 and 92.6% of the total area (L, linoleic; O, oleic; P, palmitic; S, stearic). Authentic oils will have
ratios of LOO/LOP and OOO/POO that coincide with a line defined by OOO/POO=0.7844(LOO/LOP)+0.0968; correlation coefficient
is 0.885. Authentic oils will not have a trilinolein (LLL) peak over 0.5% in area. Neither triolein (OOO) nor any other single
peak suffices to characterize an olive oil sample as one of the authentic grades. 相似文献
103.
Prediction of the biologically active sites in eclosion hormone from the silkworm, Bombyx mori 总被引:1,自引:0,他引:1
Kikuchi T; Okamoto M; Geiser M; Schmitz A; Gohda K; Takai M; Morita T; Horii K; Fujita N 《Protein engineering, design & selection : PEDS》1997,10(3):217-222
The structure-activity relationship of eclosion hormone from the silkworm,
Bombyx mori, was analyzed. First, the probable active residues in silkworm
eclosion hormone and also tobacco hornworm eclosion hormone were predicted
by the average distance map method. To examine the contributions of those
residues to the activity of silkworm eclosion hormone, Gly-substituted
mutants for those predicted residues were produced by site-directed
mutagenesis and their activities were evaluated by a bioassay. Finally,
Glu12, Met24 and Phe25 were estimated to be the crucial residues for the
eclosion hormone activity. The possibility of the development of a blocker
of an eclosion hormone receptor on the basis of the present work is also
discussed.
相似文献
104.
The surface and mechanical properties of copolymers of hydroxyalkyl acrylates and methacrylates have been examined by a variety of techniques. This work is complementary to earlier parts of this series which describe the effect of copolymer structure on water binding properties. Water structure has been demonstrated to exert a profound effect upon mechanical properties whether measured in compression or in tension. In particular, water that is characterized by differential scanning calorimetry as ‘freezing’ water is observed to have a marked plasticizing effect upon the gel, whereas ‘non-freezing’ water has little such effect. Similarly, the ‘freezing’ water produces a more marked effect on thermally induced transitions. Two distinct transition points are observed as a result of its presence. One corresponds to the freezing point of water and the other to a glass transition temperature, whose value depends upon the proportion of ‘freezing’ or ‘plasticizing’ water in the gel. Several predictive and direct measurement techniques have been used to study the surface properties of the copolymers in both hydrated and dehydrated states. Taken together they have established a sound understanding of the way in which polar and dispersive components of surface free energy vary as a function of copolymer composition and water content. Use of protein adsorption and fibroblast cell interaction techniques demonstrate that biological phenomena respond to changes at a molecular level which current macroscopic surface energy techniques are unable to discern. 相似文献
105.
The use of isopropanol (IPA) as the reaction solvent for the preparation of high molecular weight segmented polyether-urea copolymers based on cycloaliphatic diisocyanates was investigated. Reactivity of IPA with bis(4-isocyanatohexyl)methane (HMDI) and isophorone diisocyanate (IPDI) was studied between 0 and 40 °C using in-situ FTIR spectroscopy. HMDI, which has secondary isocyanate groups, shows a very slow reaction with a large excess of IPA at 0 and 23 °C. Analysis of the kinetic data indicates an activation energy of 51 kJ/mol for the reaction between HMDI and IPA. As expected, IPDI, which has both a primary and a secondary isocyanate (NCO) group, reacts faster with IPA compared with HMDI, which only has secondary NCO groups. However, the rate of reaction of IPDI with IPA at 0 °C is extremely slow (approximately 1% consumption of isocyanate in 60 min) thus allowing the use of IPA as the reaction solvent for polyether-urea synthesis. Preparation of high molecular weight, high-strength HMDI and IPDI based polyether-urea segmented copolymers in IPA has been demonstrated. Thermal analysis and stress-strain analyses were used to characterize the products. 相似文献
106.
A method has been developed on the Rheometrics mechanical spectrometer using the eccentric rotating disks mode to study the crystallization kinetics of different semi-crystalline polymers (polyethylene, polypropylene, poly(butylene terephthalate) and Nylon 11) under oscillatory shear. Dynamic shear moduli (storage G′ and loss G″), loss tangent (tan δ), and dynamic viscosity (η′) were simultaneously, monitored during the crystallization process. The onset and completion of crystallization were characterized by the initial rise and final levelling off of G′, while the peak time, of crystallization (tp) is calculated from the time elapsed between the onset and peak of crystallization which is indicated by the G″ or η′ maximum. In the case of polypropylene, going from a low frequency of ?0.1 rad/s, to higher frequencies of up to 10 rad/s, there is a monotonic decrease in peak time of crystallization (tp) together with a progressive decrease in spherulitic morphology. The observed acceleration in crystallization is due predominantly to the increase in nucleation rate and orientation of chains in melt crystalline aggregate. The progressive disappearance of the spherulitic morphology is attributed to the disruption of the spherulite superstructure at higher frequencies of shear. 相似文献
107.
Mahyar S. Dadkhah David B. Marshall Winfred L. Morris Brian N. Cox 《Journal of the American Ceramic Society》1991,74(3):584-592
Residual strains responsible for crack tip shielding have been measured within transformation zones surrounding cracks in Mg-PSZ. Two techniques were used for strain measurement: moiré interferometry and high-resolution image matching. Both methods provide maps of differential in-plane displacements within the specimen surface intersected by the crack, the latter method with the higher resolution. The results are compared with finite-element analysis to assess surface relaxation effects, and the measured strains are used to evaluate the crack tip shielding stress intensity factor. Calculations based on the assumption that the unconstrained transformation strain is hydrostatic dilatation yielded results that were significantly smaller than the measured toughness increases. 相似文献
108.
Brian Holland 《Journal of Porous Materials》2003,10(1):17-22
Three-dimensionally ordered silica structures containing both mesopores and macropores are created using polystyrene coacervate spheres with a diameter of ca. 146 nm. The close-packed polystyrene coacervate spheres are intercalated with tetraethyl orthosilicate. The spheres are removed by calcination leaving an inverse silica replica with a spherical macropore cavity diameter of 110 nm. Due to the nature of these porous structures, pores leading into the macropore cavity are in the mesopore regime, 40 nm in diameter. The nitrogen adsorption data described in the following paper gives a pore size for both the macropore cavity and the mesopore openings leading into the cavity. The pore sizes as determined by nitrogen sorption are in good agreement with the pore sizes observed by scanning electron microscopy. Mercury intrusion porosimetry results confirm the size of the mesopore openings leading into the macropore cavity, however due to destruction of the sample upon intrusion, extrusion results can not be obtained to determine main cavity diameters. As a result, nitrogen sorption may be a viable option for determining pore sizes with these three-dimensionally ordered materials containing both mesopores and macropores. 相似文献
109.
Bottomley Stephen P.; Popplewell Andrew G.; Scawen Michael; Wan Tommy; Sutton Brian J.; Gore Michael G. 《Protein engineering, design & selection : PEDS》1994,7(12):1463-1470
The stability and unfolding of an immunoglobulin (Ig) G bindingprotein based upon the B domain of protein A (SpAB) from Staphylococcusaureus were studied by substituting tryptophan residues at strategiclocations within each of the three a-helical regions (al-a3)of the domain. The role of the C-terminal helix, a3, was investigatedby generating two protein constructs, one corresponding to thecomplete SpAB, the other lacking a part of ct3; the Trp substitutionswere made in both one-and two-domain versions of each of theseconstructs. The fluorescence properties of each of the single-tryptophanmutants were studied in the native state and as a function ofguanidine-HCl-mediated unfolding, and their IgG binding activitieswere determined by a competitive enzyme-linked immunosorbentassay. The free energies of folding and of binding to IgG foreach mutant were compared with those for the native domains.The effect of each substitution upon the overall structure andupon the IgG binding interface was modelled by molecular graphicsand energy minimization. These studies indicate that (i) 3 contributesto the overall stability of the domain and to the formationof the IgG binding site in l and 2, and (ii) al unfolds first,followed by 2 and 3 together. 相似文献
110.
The diffusion coefficient at zero penetrant concentration D0 of dichloromethane, chloroform, carbon tetrachloride, cyclohexane, benzene, o-xylene, m-xylene, and p-xylene, and n-hexane in LDPE were measured at 25°C, using the desorption method. The D0 values obtained in this way are correlated with the size, shape, and chemical nature of the penetrant molecules. The temperature dependence of the diffusion coefficients of toluene and n-hexane in LDPE are also reported in the limited temperature range of 25–45°C. It indicates that, in spite of a size larger than that of toluene, n-hexane has a lower activation energy of diffusion. 相似文献