全文获取类型
收费全文 | 33678篇 |
免费 | 1019篇 |
国内免费 | 66篇 |
专业分类
电工技术 | 378篇 |
综合类 | 37篇 |
化学工业 | 6904篇 |
金属工艺 | 864篇 |
机械仪表 | 628篇 |
建筑科学 | 1869篇 |
矿业工程 | 247篇 |
能源动力 | 1039篇 |
轻工业 | 2812篇 |
水利工程 | 336篇 |
石油天然气 | 207篇 |
武器工业 | 2篇 |
无线电 | 2201篇 |
一般工业技术 | 5350篇 |
冶金工业 | 6884篇 |
原子能技术 | 334篇 |
自动化技术 | 4671篇 |
出版年
2022年 | 202篇 |
2021年 | 394篇 |
2020年 | 337篇 |
2019年 | 406篇 |
2018年 | 494篇 |
2017年 | 517篇 |
2016年 | 533篇 |
2015年 | 472篇 |
2014年 | 750篇 |
2013年 | 2157篇 |
2012年 | 1257篇 |
2011年 | 1624篇 |
2010年 | 1190篇 |
2009年 | 1280篇 |
2008年 | 1468篇 |
2007年 | 1486篇 |
2006年 | 1273篇 |
2005年 | 1196篇 |
2004年 | 1063篇 |
2003年 | 1032篇 |
2002年 | 1048篇 |
2001年 | 652篇 |
2000年 | 623篇 |
1999年 | 595篇 |
1998年 | 668篇 |
1997年 | 549篇 |
1996年 | 635篇 |
1995年 | 565篇 |
1994年 | 544篇 |
1993年 | 557篇 |
1992年 | 495篇 |
1991年 | 331篇 |
1990年 | 451篇 |
1989年 | 436篇 |
1988年 | 385篇 |
1987年 | 434篇 |
1986年 | 386篇 |
1985年 | 527篇 |
1984年 | 496篇 |
1983年 | 450篇 |
1982年 | 471篇 |
1981年 | 451篇 |
1980年 | 361篇 |
1979年 | 369篇 |
1978年 | 337篇 |
1977年 | 316篇 |
1976年 | 284篇 |
1975年 | 325篇 |
1974年 | 250篇 |
1973年 | 286篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
Alexander Chroneos Nicholas J. Ashley Kaajal H. Desai John F. Maguire R. W. Grimes 《Journal of Materials Science》2007,42(6):2024-2029
The structures of a number of hydroxide and oxyhydroxide minerals have previously been reported without the hydrogen positions
explicitly defined. Here we use two atomic scale computer simulation techniques, one based on classical ionic potentials,
the other on density functional theory (DFT), to predict these positions. The aim is not only to provide data that can be
used as the basis for future experimental structure optimizations but also model parameters that can be used to predict complex
hydroxide structures. The efficacy of the approach is demonstrated through the comparison of predicted and experimental data
for minerals whose hydrogen positions are known. 相似文献
62.
RobetKrause KhoiTran JohnConstantino 《世界电子元器件》2004,(4):31-32
电动汽车常被喻作“归来的孩子”,因为这项技术在多年前进入市场后便一直沉寂,直至最近才再次活跃起来。其实,电动汽车早于1835年首次于美国出现,到1899年,纽约市接近90%的出租车都是由电力驱动。然而,当电力启动器和内燃机变得完善时,电动汽车的使用就迅速衰退。随着如今对环境保护的关注,由燃烧碳氢化合物造成的污染,加之汽油成本高升的影响,再次 相似文献
63.
Jianxin Wei Mark R Pickering Michael R Frater John F Arnold John A Boman Wenjun Zeng 《IEEE transactions on image processing》2005,14(8):1033-1042
It is well known that tile-boundary artifacts occur in wavelet-based lossy image coding. However, until now, their cause has not been understood well. In this paper, we show that boundary artifacts are an inescapable consequence of the usual methods used to choose tile size and the type of symmetric extension employed in a wavelet-based image decomposition system. This paper presents a novel method for reducing these tile-boundary artifacts. The method employs odd tile sizes (2N + 1 samples) rather than the conventional even tile sizes (2N samples). It is shown that, for the same bit rate, an image compressed using an odd tile length low-pass first (OTLPF) convention has significantly less boundary artifacts than an image compressed using even tile sizes. The OTLPF convention can also be incorporated into the JPEG 2000 image compression algorithm using extensions defined in Part 2 of this standard. 相似文献
64.
A corresponding states correlation is developed for the p - v - T properties of saturated liquids. It is similar in form to a previous correlation; however it is more accurate in the low pressure, low Pitzer factor range, owing to the incorporation of new information about low Pitzer factor behavior. The problem of creating corresponding states correlations of the spinodal lines and the metastable states is also discussed in the light of new equation of state information. A partial preliminary correlation works particularly well for non-polar molecules in the range of fairly small positive Pitzer factors. 相似文献
65.
The Florida Department of Transportation (FDOT) is in the process of implementing Pontis, a Bridge Management System, to provide decision support to engineers in the headquarters and district offices as they make routine policy, programming, and budgeting decisions regarding the preservation and improvement of the state’s bridges. As part of this effort, an ongoing research program is underway to adapt the system to FDOT needs as well as to advance the state of the art in several areas important to the Department. Most of the research results are organized around a new project-level decision support framework that complements and builds on Pontis’ existing network-level analysis. Specific new models include accident risk and user cost due to roadway width and alignment deficiencies; user cost of load capacity and vertical clearance restrictions, and moveable bridge openings; project-level prediction models for bridge element condition and costs; and prediction of economics of scale and scoping possibilities. The new models are built into a highly graphical spreadsheet model for decision support use. 相似文献
66.
Probabilistic analysis of a static frame model 总被引:1,自引:0,他引:1
This paper describes our efforts during our participation in the Sandia Validation Workshop. The focus of the paper is the calibration of material models and simulation of random fields to characterize the variations of material properties across spatial field. Both parametric and non-parametric methods were used to represent uncertainty. Part of the challenge of this problem is the small amount of data that is available for the necessary probabilistic analyses in support of calibration, validation, accreditation and prediction activities. The analysis methods and corresponding results are described. 相似文献
67.
68.
Alan A. Luo Robert C. Kubic John M. Tartaglia 《Metallurgical and Materials Transactions A》2003,34(11):2549-2557
This study investigated the microstructure and fatigue properties of hydroformed sections of the 5754 and 6063 aluminum alloys.
The second-phase particles in 6063-T7 are identified as a mixture of Al12Fe3Si and Al9Fe2Si2, with a slightly higher fraction of the former. The constituent particles in the 5754 alloy are Al4Mn-type hexagonal compounds, where Mn is partially substituted by various other elements, resulting in Al4(Fe,Mn,Si,Cr). The results show that despite its lower yield strength, the hydroformed 5754 alloy has higher ultimate tensile
strength, ductility, and, more importantly, higher fatigue resistance than the 6063 material. Both crystallographic stage
I and noncrystallographic stage II cracking are found in the 6063-T7 samples, but only stage II cracking is observed in the
5754 alloy. This implies that the low fatigue strength of 6063-T7 is related to its relatively large grain size, resulting
in rapid stage I crack propagation. The higher fatigue lives of the 5754 alloy compared to the 6063 alloy in both the low-
and high-cycle life regimes are due to the increased fatigue-crack-initiation and propagation resistance of the 5754 alloy
and its probable cyclic strain-hardening behavior. 相似文献
69.
- In uncatalyzed reactions the esterification of stearic, oleic, and linolenic acid proceeded at the same general rate though the speed was somewhat enhanced as the degree of unsaturation of the fatty acid increased. None of the reactions went to completion, after 15 hours being 72%, 83%, and 84%, respectively, completed. Only a small fraction of the distearate was converted to the triglyceride; a greater amount of the dioleate was esterified to the trioleate and about half the dilinolenate was changed to the trilinolenate.
- Of 15 metallic fluorides studied, all afforded some catalysis, but only antimony trifluoride and zinc and cobalt fluorides were outstanding. Antimony trifluoride was the most active although in large amount it caused severe decomposition and charring of the products. Zinc fluoride had the best general properties, approaching antimony trifluoride yet causing practically no decomposition even in high concentration. Its activity was enhanced with an increase in amount, the best concentration apparently being 0.1 mole per 100 grams fatty acid. Antimony trifluoride was also better in larger amount within limits, but cobalt fluoride did not exhibit this property.
- With zinc fluoride as catalyst the reactions of stearic, oleic and linolenic acids after 15 hours were 95%, 94%, and 92%, respectively, completed. At least three-fourths of the diglycerides were converted to triglycerides. From a study of reaction velocities, zinc fluoride caused the monoglyceride to form at a rate approximately four times that of the uncatalyzed reaction and the diglyceride at a six-fold rate. The triglyceride was formed at an almost undiminished rate whereas in the uncatalyzed reaction the formation of triglyceride was diminished rapidly.
- Unsaturation and viscosity were not affected seriously in the presence of the fluorides except antimony trifluoride. Certain other fluorides caused severe darkening but without accompanying serious decomposition.
70.