全文获取类型
收费全文 | 3869篇 |
免费 | 79篇 |
国内免费 | 13篇 |
专业分类
电工技术 | 41篇 |
综合类 | 5篇 |
化学工业 | 386篇 |
金属工艺 | 59篇 |
机械仪表 | 54篇 |
建筑科学 | 116篇 |
矿业工程 | 10篇 |
能源动力 | 104篇 |
轻工业 | 341篇 |
水利工程 | 17篇 |
石油天然气 | 19篇 |
无线电 | 235篇 |
一般工业技术 | 345篇 |
冶金工业 | 1793篇 |
原子能技术 | 24篇 |
自动化技术 | 412篇 |
出版年
2022年 | 22篇 |
2021年 | 35篇 |
2020年 | 26篇 |
2019年 | 19篇 |
2018年 | 42篇 |
2017年 | 33篇 |
2016年 | 41篇 |
2015年 | 44篇 |
2014年 | 78篇 |
2013年 | 129篇 |
2012年 | 92篇 |
2011年 | 116篇 |
2010年 | 95篇 |
2009年 | 86篇 |
2008年 | 101篇 |
2007年 | 111篇 |
2006年 | 99篇 |
2005年 | 62篇 |
2004年 | 66篇 |
2003年 | 80篇 |
2002年 | 60篇 |
2001年 | 61篇 |
2000年 | 60篇 |
1999年 | 91篇 |
1998年 | 572篇 |
1997年 | 368篇 |
1996年 | 225篇 |
1995年 | 134篇 |
1994年 | 128篇 |
1993年 | 147篇 |
1992年 | 44篇 |
1991年 | 42篇 |
1990年 | 42篇 |
1989年 | 45篇 |
1988年 | 37篇 |
1987年 | 46篇 |
1986年 | 26篇 |
1985年 | 33篇 |
1984年 | 20篇 |
1983年 | 23篇 |
1982年 | 14篇 |
1981年 | 24篇 |
1980年 | 29篇 |
1979年 | 13篇 |
1978年 | 18篇 |
1977年 | 51篇 |
1976年 | 99篇 |
1974年 | 15篇 |
1973年 | 19篇 |
1972年 | 13篇 |
排序方式: 共有3961条查询结果,搜索用时 390 毫秒
71.
In this study we examined the lymphatic transport in rats of FA after administration of interesterified oils containing CLA, with emphasis on the location of CLA and octanoic acid in the TAG. The oils were produced by enzymatic interesterification. Eight oils with different structures or FA profiles were examined in this study: MCM, CMC, OCO, and COC, where M was expected to be octanoic acid and O oleic acid. In group 1, C was CLA as a mixture of the two CLA isomers c9, t11 or t10, c12, and in group 2, C was mainly the isomer t10, c12. Rats were subjected to cannulation of the mesenteric lymph duct, and the following day they were intragastrically administered one of the oils and lymph samples were collected for 24 h. The lymphatic transport of total FA from 0 to 8 h in group 1 was significantly (P < 0.05) higher for the OCO-1 and the COC-1 oils than for the CMC-1 oil. Similarly, in group 2 the transport was higher for the OCO-2 oil than for the CMC-2 oil. The recovery of both of the CLA isomers examined was similar (50-70%) and independent of the isomer, oil structure, and FA profile, whereas more octanoic acid was recovered from the CMC oils than from the MCM oils. The results indicated that the FA profiles and the position of octanoic acid had only a minor influence on the absorption of CLA. 相似文献
72.
Prospects of Energy and Nitrogen Balances in Rapeseed Cropping Comparing the performance of different agricultural energy crops needs a suitable energetic scale considering different chemical composition of storage substances. Among different field crops (wheat, barley, rapeseed) wheat and rapeseed generated the highest energy yields of above-ground biomass. On an energetic basis, the ratio between reproductive organs and total above-ground biomass (energy harvest index) ranged equally for both crops. The specific energy concentration of plant material was merely influenced by nitrogen (N) fertilization. Generally, energy use efficiency, i. e. solar energy income as related to bound energy in biomass, usually exhibited a low level. Highest values were found in rapeseed stands (3.4%). Nitrogen fertilization enhanced the energy output of the cropping system. However, as N-fertilizers represent the main source of input, reduced N-application as combined with yield-stabilizing fungicide treatment resulted in highest net energy yields. Due to these interrelation, replacement of mineral N-fertilizers by organic fertilizers should be further investigated. 相似文献
73.
Mercaptoacetyltriglycine forms complexes with (186/188)Re and (99m)Tc radionuclides that are useful in nuclear medicine because they are substrates of the renal anion transport system. However, the renal clearance of [MO(MAG(3))](2-)(MAG(3) = penta-anionic form of mercaptoacetyltriglycine, M = Re, Tc) complexes are less than ideal. Organic sulphonates are also transported by the renal anion transport system and phosphonates are similar to sulphonates in size and shape. In an effort to develop new ligands that form Re and Tc complexes and have improved renal clearances compared to [MO(MAG(3))](2-) complexes, the sulphonate and phosphonate derivatives of mercaptoacetyltriglycine were synthesized. The dianion [ReO(MAG(2)-AMS)](2-) (MAG(2)-AMS = penta-anionic form of mercaptoacetylglycylglycylaminomethanesulphonic acid) was prepared for characterization by exchange reaction of ReOCl(3)(Me(2)S)(OPPh(3)) and isolated as the disodium salt. The structure of Na(2)[ReO(MAG(2)-AMS)].3H(2)O (6) was determined by X-ray diffraction. The coordination geometry is pseudo square pyramidal, with the nitrogen and sulfur donor atoms forming a square base and the oxo ligand at the apex. The deprotonated sulphonate group has a syn conformation with respect to the oxo ligand. The renal clearances of [(99m)TcO(MAG(2)-AMS)](2-) and [(99m)TcO(MAG(2)-AMP)](3-) were similar in rats and suggest that the difference in total charge between the SO(3) (-) and PO(3) (2-) groups is not important to renal clearance. However, their renal clearances were 40-50% less than that of [(99m)TcO(MAG(3))](2-) suggesting that the size and shape of the large tetrahedral SO(3) (-) and PO(3) (2-) groups of [(99m)TcO(MAG(2)-AMS)](2-) and [(99m)TcO(MAG(2)-AMP)](3-) inhibit recognition by the renal transport system compared to the small planar CO(2) (-) group of [(99m)TcO(MAG(3))](2-). 相似文献
74.
Ammoxidation of toluene over the perovskites YBa2Cu3O6.1, YBa2Cu2CoO6.7 and YBaCuCoO4.9 was investigated at 400 °C. At low partial pressures of O2 benzonitrile was selectively formed, while CO2 was the main product at high pressures of O2. Systematic differences in activity were observed for the three phases and are related to the crystal contents of Cu and Co. At low O2 pressures, Cu-sites are active for nitrile formation, while Co-sites give CO2. At high O2 pressures, the activity for CO2 of Cu-sites increases more than that of Co-sites due to filling of near-surface oxygen vacancies. 相似文献
75.
Nielsen Per F.; Roepstorff Peter; Clausen Ib G.; Jensen Ejner B.; Jonassen Ib; Svendsen Allan; Balschmidt Per; Hansen Finn B. 《Protein engineering, design & selection : PEDS》1989,2(6):449-457
Californium-252 plasma desorption mass spectrometry (PDMS) hasbeen employed for the characterization of a series of humaninsulin derivatives in order to evaluate the performance ofthis technique as an analytical tool in protein engineering.Several of the characterized modifications result in a 1 a.m.u.mass change. The precision in mass determination obtainableby PDMS analysis is not sufficient for unambiguous verificationof such modifications based on the molecular weight alone. Itis, however, possible to carry out in situ enzymatic digestionof the sample. Subsequent PDMS analysis will in most cases revealif the modification has been introduced as intended. 相似文献
76.
A.L. Kustov T.W. Hansen M. Kustova C.H. Christensen 《Applied catalysis. B, Environmental》2007,76(3-4):311-319
Mesoporous and conventional Fe-containing ZSM-5 and ZSM-12 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in the selective catalytic reduction (SCR) of NO with NH3. It was found that for both Fe/HZSM-5 and Fe/HZSM-12 catalysts with similar Fe contents, the activity of the mesoporous samples in NO SCR with NH3 is significantly higher than for conventional samples. Such a difference in the activity is probably related with the better diffusion of reactants and products in the mesopores and better dispersion of the iron particles in the mesoporous zeolite as was confirmed by SEM analysis. Moreover, the maximum activity for the mesoporous zeolites is found at higher Fe concentrations than for the conventional zeolites. This also illustrates that the mesoporous zeolites allow a better dispersion of the metal component than the conventional zeolites. Finally, the influence of different pretreatment conditions on the catalytic activity was studied and interestingly, it was found that it is possible to increase the SCR performance significantly by preactivation of the catalysts in a 1% NH3/N2 mixture at 500 °C for 5 h. After preactivation, the activity of mesoporous 6 wt% Fe/HZSM-5 and 6 wt% Fe/HZSM-12 catalyst is comparable with that of traditional 3 wt% V2O5/TiO2 catalyst used as a reference at temperatures below 400 °C and even more active at higher temperatures. 相似文献
77.
Mixtures of 90, 80, and 70 percent by weight bisphenol-A-polycarbonate (PC) and 10, 20, and 30 percent by weight styrene maleic anhydride (SMA) copolymer were melt-blended in a single screw extruder. Differential scanning calorimetry (DCS) and scanning electron microscopy (SEM) were used to determine the miscibility of the blends. The viscosity, as a function of shear rate and temperature, was measured by an Instron capillary viscometer. The notched impact strength as a function of temperature was measured by an Izod impact tester. The results of DSC showed two glass transition temperatures which merged slightly towards each other, indicating marginal miscibility of these blends. There was a decrease in viscosity as the fraction of SMA copolymer was increased. The most significant decrease occurred with the initial addition of SMA copolymer. The viscosity also decreased with increases in temperature. The impact strength of the blends was also dependent on SMA copolymer content. The blends showed six to ten times lower impact strengths at room temperature than the 100 percent polycarbonate. SEM analysis helped to determine the reason why the impact strength was lower for the blends. High magnification showed the presence of SMA copolymer inclusions dispersed throughout the PC matrix. These inclusions, which increased in size as SMA copolymer content was increased, acted as defects in the system. 相似文献
78.
Arkadev Chattopadhyay Ricard Gavaldà Kristoffer Arnsfelt Hansen Denis Thérien 《Theory of Computing Systems》2014,55(2):404-420
Boolean functions that have constant degree polynomial representation over a fixed finite ring form a natural and strict subclass of the complexity class ACC0. They are also precisely the functions computable efficiently by programs over fixed and finite nilpotent groups. This class is not known to be learnable in any reasonable learning model. In this paper, we provide a deterministic polynomial time algorithm for learning Boolean functions represented by polynomials of constant degree over arbitrary finite rings from membership queries, with the additional constraint that each variable in the target polynomial appears in a constant number of monomials. Our algorithm extends to superconstant but low degree polynomials and still runs in quasipolynomial time. 相似文献
79.
Some earlier observations on the influence of molecular weight, plasticizer, and irradiation on the thermal conductivity of polymers is reviewed. Also some new data on polystyrene and some new calculations based on the theory of Hansen and Ho are presented. These data show almost a two fold increase in the thermal conductivity of polystyrene as the molecular weight goes from 900 to 100,000. Data on the moleculer weight effect on thermal conductivity appear to be consistent with the theoretical calculations. 相似文献
80.