全文获取类型
收费全文 | 469998篇 |
免费 | 4633篇 |
国内免费 | 1042篇 |
专业分类
电工技术 | 8403篇 |
综合类 | 415篇 |
化学工业 | 71318篇 |
金属工艺 | 23227篇 |
机械仪表 | 16635篇 |
建筑科学 | 9768篇 |
矿业工程 | 4446篇 |
能源动力 | 9688篇 |
轻工业 | 30903篇 |
水利工程 | 6138篇 |
石油天然气 | 14912篇 |
武器工业 | 91篇 |
无线电 | 50574篇 |
一般工业技术 | 103632篇 |
冶金工业 | 73439篇 |
原子能技术 | 14673篇 |
自动化技术 | 37411篇 |
出版年
2021年 | 4199篇 |
2019年 | 4170篇 |
2018年 | 7789篇 |
2017年 | 8074篇 |
2016年 | 8526篇 |
2015年 | 4817篇 |
2014年 | 8337篇 |
2013年 | 19638篇 |
2012年 | 12514篇 |
2011年 | 16300篇 |
2010年 | 13057篇 |
2009年 | 14602篇 |
2008年 | 15449篇 |
2007年 | 15171篇 |
2006年 | 13044篇 |
2005年 | 11757篇 |
2004年 | 11411篇 |
2003年 | 11202篇 |
2002年 | 10719篇 |
2001年 | 10621篇 |
2000年 | 10339篇 |
1999年 | 9853篇 |
1998年 | 20248篇 |
1997年 | 15235篇 |
1996年 | 11924篇 |
1995年 | 9545篇 |
1994年 | 8665篇 |
1993年 | 8719篇 |
1992年 | 7274篇 |
1991年 | 7078篇 |
1990年 | 7036篇 |
1989年 | 6714篇 |
1988年 | 6448篇 |
1987年 | 6032篇 |
1986年 | 5878篇 |
1985年 | 6485篇 |
1984年 | 6218篇 |
1983年 | 5753篇 |
1982年 | 5453篇 |
1981年 | 5566篇 |
1980年 | 5403篇 |
1979年 | 5485篇 |
1978年 | 5542篇 |
1977年 | 5910篇 |
1976年 | 6958篇 |
1975年 | 5105篇 |
1974年 | 5146篇 |
1973年 | 5228篇 |
1972年 | 4467篇 |
1971年 | 4108篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
131.
132.
133.
134.
ME Sutherlin I Nishimori T Caffrey EP Bennett H Hassan U Mandel D Mack T Iwamura H Clausen MA Hollingsworth 《Canadian Metallurgical Quarterly》1997,57(21):4744-4748
The levels of mRNA expression of three UDP-N-acetyl-alpha-D-galactosamine:polypeptide GalNAc N-acetylgalactosaminyltransferases (GalNAc-transferases) were quantified for human adenocarcinoma cell lines from pancreas, colon, stomach, and breast. Two of the GalNAc-transferases, GalNAc-T1 and GalNAc-T2, were expressed constitutively and at low levels in most or all cell lines examined. A third GalNAc-transferase, GalNAc-T3, was differentially expressed. Well-differentiated adenocarcinoma cell lines expressed high levels and moderately differentiated cell lines expressed lower levels of GalNAc-T3. Cell lines classified as poorly differentiated failed to express GalNAc-T3 mRNA at levels that could be detected by Northern blot analysis. Differential expression of the GalNAc-T3 protein was confirmed in these cell lines by Western blotting. We propose that glycosylation in tumor cell lines may be regulated in part by differential expression of GalNAc-transferases, and we suggest that GalNAc-T3 gene expression may be a molecular indicator of differentiated adenocarcinoma. 相似文献
135.
136.
CL Mesh BL Cmolik DW Van Heekeren JH Lee D Whittlesey LM Graham AS Geha SJ Bowlin 《Canadian Metallurgical Quarterly》1997,11(6):612-619
A premise of cardiac risk stratification is that the added risk of coronary artery bypass grafting (CABG) is offset by the improved safety of subsequent vascular reconstruction (VR). We questioned if elective CABG is patients with severe peripheral vascular disease (PVD) is a relatively high-risk procedure. A cohort study of 680 elective CABG patients from January 1993 to December 1994 was performed using three mutually exclusive outcomes of complication-free survival, morbidity, and mortality. Patient characteristic, operative, and outcome data were prospectively collected. Retrospective review determined that 58 patients had either a standard indication for or a history of VR. Overall CABG mortality was 2.5%, with statistically similar but relatively higher rates for PVD as compared to non-PVD patients. In contrast, major morbidity occurred at rates 3.6-fold higher in PVD patients (39.7%) than in disease-free patients (16.7%) after adjustment for the effects of patient and operative variables (odds ratio [OR] 3.67, 95% confidence interval [CI] 1.93-6.99). CABG morbidity in the PVD patient was most likely in those patients with aortoiliac (OR 9.51, CI 3.20-28.27) and aortic aneurysmal (OR 5.24, CI 1.28-21.41) disease types. CABG in PVD patients is associated with significant major morbidity. Such morbidity may preclude or alter the timing of subsequent VR. 相似文献
137.
V. I. Shtykov 《Power Technology and Engineering (formerly Hydrotechnical Construction)》1998,32(8):459-462
Conclusions 1. Use of dewatering systems based on voidless drainage is most effective when dewatering slightly permeable soils in regions
with a deep seasonal frost.
2. Two-story drainage with the upper and lower stories spaced 30 m apart accelerates the discharge of gravitational water
from the arable layer. Results of observations on two-story drainage in experimental-production sections indicated that it
discharges 2 – 3.8 times more water than systematic tubular drainage with an interdrain spacing of 10 m and a 0.2-m thick
three-dimensional filter of a sandy-gravelly mixture.
3. It is recommended to use crushed stone and porous-clay gravel with a fineness of 10 – 20 mm as a filler. In that case,
protection of the drain filler is, as a rule, required only from the intrusion of soil from above during construction in cohesive
soils with a plasticity indexW
p≥7.
4. seepage rate in a voidless drain should be lower than the scouring velocity for the soil in which the voidless drainage
is placed. In the opposite case, it is necessary to lower the grade of the drains or select a filler gradation such that the
seepage rate in the drain be lower than the scouring velocity.
Translated from Gidrotekhnicheskoe Stroitel’stvo, No. 8, pp. 41–43, August, 1998. 相似文献
138.
139.
Graefe G. Linville A. Shapiro L.D. 《Knowledge and Data Engineering, IEEE Transactions on》1994,6(6):934-944
Efficient algorithms for processing large volumes of data are very important both for relational and new object-oriented database systems. Many query-processing operations can be implemented using sort- or hash-based algorithms, e.g. intersections, joins, and duplicate elimination. In the early relational database systems, only sort-based algorithms were employed. In the last decade, hash-based algorithms have gained acceptance and popularity, and are often considered generally superior to sort-based algorithms such as merge-join. In this article, we compare the concepts behind sort- and hash-based query-processing algorithms and conclude that (1) many dualities exist between the two types of algorithms, (2) their costs differ mostly by percentages rather than by factors, (3) several special cases exist that favor one or the other choice, and (4) there is a strong reason why both hash- and sort-based algorithms should be available in a query-processing system. Our conclusions are supported by experiments performed using the Volcano query execution engine 相似文献
140.
Moser L.E. Melliar-Smith P.M. Agrawala V. 《Parallel and Distributed Systems, IEEE Transactions on》1994,5(5):459-473
Presents protocols for determining processor membership in asynchronous distributed systems that are subject to processor and communication faults. These protocols depend on the placement of a total order on broadcast messages. The types of systems for which each of these protocols is applicable are characterized by the properties of the communication mechanisms and by the availability of stable storage. In the absence of stable storage or of a mechanism for distinguishing promptly delivered messages, the authors show that no membership protocol can exist. They also discuss their experience in implementing these membership protocols 相似文献