全文获取类型
收费全文 | 24751篇 |
免费 | 1467篇 |
国内免费 | 22篇 |
专业分类
电工技术 | 449篇 |
综合类 | 60篇 |
化学工业 | 4983篇 |
金属工艺 | 686篇 |
机械仪表 | 579篇 |
建筑科学 | 1113篇 |
矿业工程 | 137篇 |
能源动力 | 451篇 |
轻工业 | 3222篇 |
水利工程 | 228篇 |
石油天然气 | 31篇 |
无线电 | 1794篇 |
一般工业技术 | 5895篇 |
冶金工业 | 2606篇 |
原子能技术 | 217篇 |
自动化技术 | 3789篇 |
出版年
2023年 | 264篇 |
2022年 | 199篇 |
2021年 | 506篇 |
2020年 | 398篇 |
2019年 | 405篇 |
2018年 | 760篇 |
2017年 | 760篇 |
2016年 | 836篇 |
2015年 | 692篇 |
2014年 | 885篇 |
2013年 | 1746篇 |
2012年 | 1164篇 |
2011年 | 1390篇 |
2010年 | 1154篇 |
2009年 | 1060篇 |
2008年 | 1191篇 |
2007年 | 1188篇 |
2006年 | 838篇 |
2005年 | 697篇 |
2004年 | 607篇 |
2003年 | 588篇 |
2002年 | 593篇 |
2001年 | 466篇 |
2000年 | 410篇 |
1999年 | 391篇 |
1998年 | 892篇 |
1997年 | 666篇 |
1996年 | 487篇 |
1995年 | 364篇 |
1994年 | 328篇 |
1993年 | 302篇 |
1992年 | 197篇 |
1991年 | 167篇 |
1990年 | 165篇 |
1989年 | 143篇 |
1988年 | 160篇 |
1987年 | 125篇 |
1986年 | 112篇 |
1985年 | 221篇 |
1984年 | 212篇 |
1983年 | 140篇 |
1982年 | 150篇 |
1981年 | 172篇 |
1980年 | 143篇 |
1979年 | 127篇 |
1978年 | 127篇 |
1977年 | 123篇 |
1976年 | 156篇 |
1975年 | 116篇 |
1974年 | 89篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
N. C. W. Kuijpers W. H. Kool P. T. G. Koenis K. E. Nilsen I. Todd S. van der Zwaag 《Materials Characterization》2002,49(5)
During homogenisation of AA 6xxx aluminium alloys, the platelike β-AlFeSi intermetallic phase will transform to a less Si-rich and more spheroidised α-Al(FeMn)Si phase which is more favourable for extrusion. In this study, several quantitative methods, which determine the relative volume fraction of α-Al(FeMn)Si and β-AlFeSi, are compared and an assessment of each method is made. The methods used are optical microscopy, scanning electron microscopy (SEM) in combination with electron dispersive X-ray (EDX) using polished samples, and X-ray diffraction (XRD) on intermetallics, extracted through selective dissolution of the Al matrix. The highest accuracy is obtained by using SEM/EDX analysis and applying two criteria. 相似文献
102.
103.
Biosynthesis and assembly of alcohol oxidase, a peroxisomal matrix protein in methylotrophic yeasts: a review 总被引:8,自引:0,他引:8
Alcohol oxidase (AO) catalyses the first step of methanol metabolism in yeasts. In vivo the enzyme is compartmentalized in special cell compartments, called peroxisomes. The enzyme along with the organelles are induced during growth of methylotrophic yeasts on methanol as the sole carbon source. Like all other peroxisomal matrix proteins, AO is encoded by a nuclear gene. Expression of the protein is regulated by a repression/derepression mechanism, but also by induction. Inactive monomeric precursor protein is synthesized in the cytosol and subsequently imported post-translationally into peroxisomes without further processing. Assembly into the active homo-octameric enzyme and binding of the prosthetic group flavin adenine dinucleotide occurs inside the organelle. When enhanced concentration of octameric alcohol oxidase are present in the organelles, the enzyme may form a crystalloid. Oligomerization is not dependent on translocation of AO precursors into their target organelle since octameric, active AO is detected in the cytosol and nucleus of peroxisome-deficient mutants of Hansenula polymorpha: at high expression rates large cytosolic AO crystalloids are formed, which occasionally are also encountered inside the nucleus of such mutants. This paper summarizes recent findings and views on the mechanisms involved in synthesis, import, assembly and crystallization of this important peroxisomal enzyme. 相似文献
104.
Hydrogenation of canola oil in the presence of nickel and the methyl benzoate-chrome carbonyl complex 总被引:1,自引:0,他引:1
L. J. Rubin S. S. Köseoglu L. L. Diosady W. F. Graydon 《Journal of the American Oil Chemists' Society》1986,63(12):1551-1557
Canola oil was hydrogenated using a mixture of homogeneous methyl benzoate-Cr(CO)3 and heterogeneous nickel catalysts. The effect of the methyl benzoate-Cr(CO)3_to-nickel ratio on the activity, specific isomerization index, linoleate and linolenate selectivities, and fatty acid composition
was evaluated, and the results compared with those obtained with commercial nickel catalyst and methyl benzoate-Cr(CO)3 used individually. At higher chromium-to-nickel ratios the activity of nickel was inhibited and the system behaved essentially
like the pure chrome complex, while at low chromium-to-nickel ratios the characteristics of the nickel predominated. In a
short transition zone relatively high reaction rates were obtained with significantly reducedtrans-isomer levels in the product. In a broader sense, it may be possible to combine a homogeneous and heterogeneous catalyst
while retaining the advantages of both. We may thus be able to design catalyst systems for specific applications. 相似文献
105.
Different qualities of concrete have been fire tested using different geometries of the specimens as well as different load levels and load configurations. The main objective with the study was to examine a test methodology consisting of a full‐scale test and different small scale‐tests for determining the probability of spalling and the amount of spalling of fire exposed concrete structures. A reference specimen was defined as a one‐sided fire exposed slab with the dimensions 1800 × 1200 mm2 giving an exposed area of 1500 × 1200 mm2. A number of concrete qualities with different probabilities for spalling, were tested using the reference specimen. These tests showed that the reference specimens worked well giving the expected test results. Small specimens were manufactured in different shapes with the same concrete as the one used in the reference tests. These small specimens were tested either at the same time as the reference specimens in the large furnace or afterwards on a small‐scale furnace where the fire exposed surface was 450 × 360 mm2. The test results clearly show the increased probability and the increased amount of spalling by using external compressive loading. The results also show that by using pre‐stress through bars or wires the load can be lost due to heating of the bars/wires which results in a decreased amount of spalling. The boundary of the specimen also affects the amount of spalling. The spalling around the edges was in all tests less than the spalling on the central parts of the exposed area. It could also be noted that the spalling did not pass completely through any of the specimens. The reason for this is probably that the water/vapour could migrate out from the unexposed surface of the specimen. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
106.
RM Bionta G Blewitt CB Bratton D Casper A Ciocio R Claus M Crouch ST Dye S Errede GW Foster W Gajewski KS Ganezer M Goldhaber TJ Haines TW Jones D Kielczewska WR Kropp JG Learned JM LoSecco J Matthews HS Park LR Price F Reines J Schulz S Seidel E Shumard D Sinclair HW Sobel JL Stone L Sulak R Svoboda G Thornton van der Velde JC C Wuest 《Canadian Metallurgical Quarterly》1987,36(1):30-36
107.
108.
J H J Hulskotte H A C Denier van der Gon A J H Visschedijk M Schaap 《Water science and technology》2007,56(1):223-231
In this article we show that brake wear from road traffic vehicles is an important source of atmospheric (particulate) copper concentrations in Europe. Consequently, brake wear also contributes significantly to deposition fluxes of copper to surface waters. We estimated the copper emission due to brake wear to be 2.4 kiloton per year. For comparison, the official database for Europe (without brake wear) totals 2.6 kiloton per year. In Western Europe the brake wear emissions dominate the total emission of copper. Using the spatially resolved emission data, copper distributions over Europe were calculated with the LOTOS-EUROS model. Without brake wear the model underestimates observed copper concentrations by a factor of 3, which is in accordance with other studies. Including the brake wear emissions largely removes the bias. We find that 75% of the atmospheric copper input in the North Sea may be due to brake wear. We estimate that about 25% of the total copper input in the Dutch part of the North Sea stems from brake wear. Although the estimated brake wear copper emission is associated with a large uncertainty, it significantly improves our understanding of the copper cycle in the environment. 相似文献
109.
This paper describes the spatial distribution of customer demand, supply of customer services and facility investment, as the outcome of a three-level game-like interaction between customers (e.g., shoppers), suppliers (e.g., retailers) and developers (e.g., landlords). Treating both the suppliers and developers at each centre as competitors, oligopolistic equilibria of the Nash-Stackelberg type are developed, and conditions for their existence and uniqueness established. Uncertain customer demand is specified in probabilistic terms, representing the suppliers' perception of expected customer behaviour. 相似文献
110.