全文获取类型
收费全文 | 94797篇 |
免费 | 9628篇 |
国内免费 | 6329篇 |
专业分类
电工技术 | 10820篇 |
技术理论 | 8篇 |
综合类 | 8897篇 |
化学工业 | 11874篇 |
金属工艺 | 2608篇 |
机械仪表 | 3823篇 |
建筑科学 | 12224篇 |
矿业工程 | 5331篇 |
能源动力 | 3335篇 |
轻工业 | 10841篇 |
水利工程 | 5347篇 |
石油天然气 | 5432篇 |
武器工业 | 947篇 |
无线电 | 4877篇 |
一般工业技术 | 8524篇 |
冶金工业 | 3037篇 |
原子能技术 | 1879篇 |
自动化技术 | 10950篇 |
出版年
2024年 | 409篇 |
2023年 | 1476篇 |
2022年 | 2704篇 |
2021年 | 3518篇 |
2020年 | 3422篇 |
2019年 | 2920篇 |
2018年 | 2776篇 |
2017年 | 3155篇 |
2016年 | 3613篇 |
2015年 | 3788篇 |
2014年 | 6340篇 |
2013年 | 6263篇 |
2012年 | 7169篇 |
2011年 | 7447篇 |
2010年 | 5523篇 |
2009年 | 5756篇 |
2008年 | 5319篇 |
2007年 | 6245篇 |
2006年 | 5509篇 |
2005年 | 4654篇 |
2004年 | 3843篇 |
2003年 | 3216篇 |
2002年 | 2640篇 |
2001年 | 2181篇 |
2000年 | 1860篇 |
1999年 | 1655篇 |
1998年 | 1269篇 |
1997年 | 1097篇 |
1996年 | 935篇 |
1995年 | 765篇 |
1994年 | 664篇 |
1993年 | 515篇 |
1992年 | 465篇 |
1991年 | 312篇 |
1990年 | 245篇 |
1989年 | 219篇 |
1988年 | 191篇 |
1987年 | 107篇 |
1986年 | 71篇 |
1985年 | 62篇 |
1984年 | 85篇 |
1983年 | 51篇 |
1982年 | 51篇 |
1981年 | 26篇 |
1980年 | 33篇 |
1979年 | 24篇 |
1977年 | 13篇 |
1959年 | 15篇 |
1955年 | 13篇 |
1951年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Flexible conductive polymer composites were prepared using styrene–butadiene rubber (SBR) as a matrix and conductive carbon black as filler. The filler loading was varied from 10 to 60 phr. The effect of frequency, filler loading, temperature, and applied pressure on the AC conductivity, permittivity, and loss factor of the composites was studied. The AC conductivity of low and high loaded composites was found to be frequency dependent and independent respectively. The permittivity and the loss factor were continuously decreasing with increasing frequency. The increase in filler loading increased the AC conductivity, dielectric constant, and loss factor of the composites. Increase in temperature imposed increase in conductivity and permittivity of the composites. With increasing applied pressure the properties showed exponential increase. The effect of time under a constant compressive stress was studied and dielectric relaxation times were evaluated. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 986–995, 2007 相似文献
992.
993.
T.V Malleswara Rao 《Chemical engineering science》2003,58(7):1365-1372
The steady flow of non-Newtonian polymer solutions on their own and together with air transverse to a bundle of circular rods, has been studied experimentally. In particular, the frictional pressure drop has been measured as a function of non-Newtonian power-law constants, flow rates, voidage of bundles and the input fraction of air over wide ranges of conditions as: power-law flow behaviour index, 0.5 to 1; voidage values of 0.78 and 0.87-0.88; air velocity (superficial), 0.09 to 0.28 m/s and liquid velocity (superficial) 0.018 to 0.5 m/s. While the single phase flow results for power-law liquids are in excellent agreement with an existing equation, the two-phase pressure drop results are also in accordance with the form of the well-known Lockhart-Martinelli correlation. Based on these results, it is suggested that a value of 5-10 for the Reynolds number based on rod diameter marks the limit of the laminar flow in such systems. 相似文献
994.
A new series of liquid crystalline polymers containing aromatic triad ester mesogen and 1,1′‐disubstituted ferrocene as a nonmesogenic unit along with polymethylene spacer was synthesized. The polymer was synthesized by a room temperature polycondensation reaction between bis(4‐chloroformyl phenyloxy alkyl ferrocene dicarboxylate) and quinol. The alkyl groups have been varied by an even number of methylene groups with a range from two to ten groups. All the polymers were found to possess liquid crystalline properties. The identification of the mesophase is more transparent with an increase in the spacer. The thermal characteristics were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results reveal that the thermal stability of the polymers was decreased with increasing spacer length. The Tg, Tm, and Ti of the polymers decreased with increasing methylene groups. The incorporation of the ferrocene moiety also has a considerable effect on the glass transition temperature. The char yield of the polymer decreases with an increasing methylene chain length. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3494–3501, 2002 相似文献
995.
In the miniaturized designs of optical scanning systems, UV-curing adhesives have been substituted for screws and bolts. The assembly of delicate optical components enforces stringent requirements on positional accuracy and stability after temperature and humidity cycle testing. The influence of the UV-curing adhesive is mainly determined by its polymerization shrinkage (typically more than 6%). With low amounts of Aerosil® fillers (5% by weight), shrinkage values of 3% can be attained. Optimized adhesive constructions lead to close positional tolerances (0.5 μm) and angular displacements (0.1 mrad). 相似文献
996.
A series of polyimides were synthesized from new diamine, Bis [4,4′‐amino‐5,5′ trifluoromethyl phenoxy‐(hexafluoro isopropylidine) phenoxy phenyl] benzonitrile [BATFB] and various aromatic tetracarboxylic anhydrides by thermal and chemical imidization routes. The BATFB was synthesized in two steps by nucleophilic displacement reaction of 2,6‐dichloro benzonitrile, 4,4′‐(hexafluoro isopropylidine) diphenol and 2‐amino‐5‐fluoro benzotrifluoride in the presence of anhydrous potassium carbonate in N,N′‐dimethyl acetamide (DMAc) and the structure was confirmed by FTIR spectroscopy and CHNSO analyzer. The polymers were characterized by FTIR spectroscopy and thermal analysis were performed by differential scanning calorimetry and thermogravimetric analysis methods. The prepared polyimides had glass transition temperatures between 230 and 290°C and their 10% weight loss were recorded in the range 550–590°C in N2 atmosphere. Majority of polymers are found to be soluble in most of the organic solvents such as DMSO, DMF, DMAc, m‐cresol, and THF even at room temperature and few becomes soluble on heating. The prepared polyimides showed water uptake values 0.34–0.54 wt % at room temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3455–3461, 2006 相似文献
997.
New fluorinated poly(1,3,4-oxadiazole-ether-imide)s have been prepared by solution polycondensation reaction of different aromatic diamines having preformed 1,3,4-oxadiazole ring, such as 2,5-bis(p-aminophenyl)-1,3,4-oxadiazole, 2,5-bis[p-(4-aminophenoxy)phenyl]-1,3,4-oxadiazole, 2,5-bis[p-(3-aminophenoxy)phenyl]-1,3,4-oxadiazole, 2-(4-dimethylaminophenyl)-5-(3,5-diaminophenyl)-1,3,4-oxadiazole and 2-(4-fluorophenyl)-5-(3,5-diaminophenyl)-1,3,4-oxadiazole, with an aromatic dianhydride incorporating ether linkages and hexafluoroisopropylidene group, namely 1,1,1,3,3,3-hexafluoro-2,2-bis-[(3,4-dicarboxyphenoxy)phenyl]-propane dianhydride. The polymers were easily soluble in polar organic solvents, such as N-methylpyrrolidinone, N,N-dimethylformamide, and pyridine, as well as in certain low boiling-point organic solvents, such as tetrahydrofuran and chloroform. Very thin coatings deposited onto silicon wafers exhibited smooth, pinhole-free surface in atomic force microscopy. The polymers showed high thermal stability with decomposition temperature being above 410 °C. They exhibited a glass transition in the temperature range of 183-217 °C, with reasonable interval between glass transition and decomposition temperature. Solutions of some polymers in N,N-dimethylformamide exhibited blue fluorescence, having maximum emission wavelength in the range of 411-424 nm. 相似文献
998.
Eijsink Vincent G.H.; Vriend Gerrit; van den Burg Bertus; Venema Gerard; Stulp Ben K. 《Protein engineering, design & selection : PEDS》1990,4(1):99-104
The role of the C-terminal Leu300 in maintaining thermal stabilityof the neutral protease of Bacillus subtilis was investigated.From model building studies based on the three dimensional structureof thermolysin, the neutral protease of B.thermoproteolyticus,it was conduded that this residue is located in a hydrophobicpocket composed of residues located in the C-terminal and themiddle domain. To test the hypothesis that Leu300, by contributingto a stabilizing interaction between these domains, is importantfor enzyme stability, several neutral protease mutants wereconstructed and characterized. The thermostability of the enzymewas lowered by deleting Leu300 or by replacing this residueby a smaller (Ala), a polar (Asn) or a sterically unfavourable(He) amino acid. Thermostabiity was increased upon replacingLeu300 by Phe. These results are in agreement with model-buildingstudies. The effects on thermostability observed after mutatingthe corresponding Val318 in the thermostable neutral proteaseof B.stearothermophilus were less pronounced. 相似文献
999.
The oxidative stabilities of one canola oil and six soybean oils of various fatty acid compositions were compared in terms
of peroxide values, conjugated dienoic acid values and sensory evaluations. Two of the soybean oils (Hardin and BSR 101) were
from common commercial varieties. The other four soybean oils were from experimental lines developed in a mutation breeding
program at Iowa State University that included A17 with 1.5% linolenate and 15.2% palmitate; A16 with 2% linolenate and 10.8%
palmitate; A87-191039 with 2% linolenate and 29.6% oleate; and A6 with 27.5% stearate. Seed from the soybean genotypes was
cold pressed. Crude canola oil was obtained without additives. All oils were refined, bleached and deodorized under laboratory
conditions with no additives and stored at 60°C for 15 days. The A17, A16, A87-191039 and A6 oils were generally more stable
to oxidation than the commercial soybean varieties and canola oil as evaluated by chemical and sensory tests. Canola oil was
much less stable than Hardin and BSR 101 oils by both chemical and sensory tests. The peroxide values and flavor scores of
oils were highly correlated with the initial amounts of linolenate (r=0.95, P=0.001). Flavor quality and flavor intensity
had negative correlations with linolenate, (r=−0.89, P=0.007) and (r=−0.86, P=0.013), respectively. 相似文献
1000.