全文获取类型
收费全文 | 13038篇 |
免费 | 1532篇 |
国内免费 | 970篇 |
专业分类
电工技术 | 570篇 |
综合类 | 1245篇 |
化学工业 | 1086篇 |
金属工艺 | 1567篇 |
机械仪表 | 1195篇 |
建筑科学 | 1308篇 |
矿业工程 | 300篇 |
能源动力 | 356篇 |
轻工业 | 990篇 |
水利工程 | 569篇 |
石油天然气 | 310篇 |
武器工业 | 138篇 |
无线电 | 702篇 |
一般工业技术 | 2405篇 |
冶金工业 | 739篇 |
原子能技术 | 105篇 |
自动化技术 | 1955篇 |
出版年
2024年 | 55篇 |
2023年 | 206篇 |
2022年 | 297篇 |
2021年 | 413篇 |
2020年 | 434篇 |
2019年 | 404篇 |
2018年 | 402篇 |
2017年 | 420篇 |
2016年 | 443篇 |
2015年 | 426篇 |
2014年 | 686篇 |
2013年 | 777篇 |
2012年 | 826篇 |
2011年 | 964篇 |
2010年 | 759篇 |
2009年 | 796篇 |
2008年 | 737篇 |
2007年 | 933篇 |
2006年 | 815篇 |
2005年 | 707篇 |
2004年 | 641篇 |
2003年 | 549篇 |
2002年 | 500篇 |
2001年 | 395篇 |
2000年 | 338篇 |
1999年 | 309篇 |
1998年 | 242篇 |
1997年 | 214篇 |
1996年 | 173篇 |
1995年 | 141篇 |
1994年 | 116篇 |
1993年 | 83篇 |
1992年 | 63篇 |
1991年 | 49篇 |
1990年 | 44篇 |
1989年 | 48篇 |
1988年 | 23篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 16篇 |
1984年 | 18篇 |
1983年 | 10篇 |
1982年 | 15篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1964年 | 2篇 |
1959年 | 1篇 |
1956年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
51.
Large woody debris (LWD) can increase stream habitat heterogeneity by providing structure, altering flow patterns, enhancing sediment deposition, forming pools and retaining organic matter. In North America, the role of LWD has been studied extensively in streams of mature forests (e.g. Pacific Northwest), but few studies have assessed LWD in streams of younger forests (e.g. Midwestern USA). Our objectives were to: (1) quantify the volume and abundance of LWD in a set of Midwestern streams; (2) evaluate possible factors influencing LWD quantity; (3) identify the functional roles of LWD; and (4) compare LWD levels in the upper Midwest to those elsewhere in North America. In 2002 and 2003, we measured LWD and geomorphological variables in 15 low‐gradient streams draining previously logged watersheds in the Upper Peninsula of Michigan. Mean (±SE) LWD volume (0.77 ± 0.12 m3 100 m−2) and abundance (33 ± 3 pieces 100 m−1) were 71% and 10% lesser, respectively, than in streams of similar gradient elsewhere in North America. Channel shape (width:depth ratio) explained 30% of the variation in LWD volume (multiple stepwise regression, P = 0.015) while LWD length and length:channel width combined, explained 72% of the variation in LWD density (multiple stepwise regression, P < 0.0001). About 50% of the LWD either stored sediment or stabilized banks and 14% of the LWD formed pools, although pool density was not significantly related to LWD volume or density. LWD levels, overall, were low in upper Midwestern streams, but the relative importance of that LWD to ecosystem function may be magnified in these wood‐poor systems. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
52.
异形柱框架结构设计方法分析与参数控制 总被引:1,自引:0,他引:1
近年来,异形柱框架结构(框轻结构体系),因其能较好地满足建筑使用上和结构性能上的要求,而逐步得到了推广应用,并被列为建设部首批住宅建设新技术推荐目录。但异形柱框架结构设计目前还没有可依据使用的国家标准和行业标准。本文根据对江苏省地方标准《钢筋混凝土异形柱框架结构技术规程》(DB32/512-2002)的理解,结合笔者设计实践经验,提出个人看法,供同行参考。 相似文献
53.
54.
Arjunan Arulchakkaravarthi Rakesh Kumar Parthasarathy Santhanaraghavan Sivaramakrishnan Muralithar Rengasamy Gopalakrishanan Perumalsamy Ramasamy 《Materials Research Innovations》2002,6(5-6):273-276
Organic molecular scintillating crystals are noted for their good timing and particle discrimination process. Trans-stilbene
is one such candidate noted for its good particle detection characteristics for the past five decades. Progressive strengthening
of detection characteristics of trans-stilbene has been attempted by improving crystal perfection. A series of timing resolution
studies have been carried out for the Bridgman grown trans-stilbene crystals under different experimental conditions. The
results were compared with the previously reported values. Pulse shape discrimination process has been carried out for 241Am and 252Cf sources and good discrimination has been obtained for gamma-alpha and gamma-neutron sources from the grown organic phosphor
crystal.
Electronic Publication 相似文献
55.
精密陶瓷部件近净成型技术的发展 总被引:5,自引:0,他引:5
重点介绍了近十年发展起来的陶瓷部件的近净成型技术 ,主要包括直接凝固注模成型、凝胶注模成型、陶瓷注射成型、陶瓷无模成型和微注入成型。对这些新的成型方法的原理、工艺过程及应用特点进行了较全面的分析与比较。 相似文献
56.
Combined shape and sizing optimization of truss structures 总被引:1,自引:0,他引:1
In this paper, an evolutionary optimization method is presented for weight minimum problem of a 3-dimensional truss structure
in terms of nodal coordinates and element cross-sectional areas. The structure is subject to stress, local buckling and displacement
constraints. Two types of design variables with different natures are optimized separately: (1) a fully stressed design (FSD)
and scaling techniques are applied to sizing variables and (2) the evolutionary node shift method is applied to shape variables.
Alternating procedure is utilized to couple the two types of variables and to combine the results. The optimum solution is
achieved gradually from the initial configuration design. Two typical truss structures are examined to illustrate the validity
of the method.
Received: 22 October 2001 / Accepted: 04 June 2002
This research work is supported by the National Natural Science Foundation of China under the guarantees no. 10072050 and
10172072, respectively. 相似文献
57.
58.
A. Moran C. Madden R. Rebis R. Payne M. A. Matteson 《Journal of Thermal Spray Technology》1994,3(2):197-198
Over the past five years, the Annapolis Detachment, Carderock Division, Naval Surface Warfare Center (formerly David Taylor
Research Center) has established a state- of- the- art spray forming facility for the study and exploitation of this near
net shape manufacturing process. Spray forming is a single step gas atomization/deposition process which yields ferrous and
nonferrous, near final shape, near fully dense preforms that has proven to be a viable and cost effective alternative to conventional
metalworking technology for the production of material preforms with properties surpassing those of their cast and wrought
counterparts. Current programs at Carderock Divison Naval Surface Warfare Center are aimed at optimization of the process,
certification of the spray formed products and industrialization of the technology. 相似文献
59.
A new method for characterization of stratified thermal energy stores (TES) that integrates both the first law and the second law concerns is presented here. The first law concern is incorporated into a quantity called energy response factor and the second law concern into an entropy generation ratio. A product of these two quantities is at the heart of the TES efficiency definitions. This approach removes the overemphasis of the existing methods either on the first or the second law of thermodynamics which often biases the characterization results. The information about the evolution of the temperature field of the system in time is the prerequisite of the new method. It may be obtained from experiments or from suitable numerical simulations. The current method can be easily integrated into computational fluid dynamic (CFD) simulations and thus facilitate CFD-based design analysis. As an example of such CFD-integrated analysis, a large-scale hot water seasonal heat store is numerically studied to identify the effects of aspect ratio, containment shape, internal structures, and containment size on their efficiency. The results suggest the effectiveness of the new method in deriving useful design insights. 相似文献
60.