全文获取类型
收费全文 | 8719篇 |
免费 | 1430篇 |
国内免费 | 787篇 |
专业分类
电工技术 | 313篇 |
技术理论 | 1篇 |
综合类 | 1122篇 |
化学工业 | 931篇 |
金属工艺 | 125篇 |
机械仪表 | 654篇 |
建筑科学 | 600篇 |
矿业工程 | 206篇 |
能源动力 | 96篇 |
轻工业 | 304篇 |
水利工程 | 156篇 |
石油天然气 | 140篇 |
武器工业 | 43篇 |
无线电 | 951篇 |
一般工业技术 | 681篇 |
冶金工业 | 646篇 |
原子能技术 | 38篇 |
自动化技术 | 3929篇 |
出版年
2024年 | 19篇 |
2023年 | 114篇 |
2022年 | 190篇 |
2021年 | 240篇 |
2020年 | 309篇 |
2019年 | 201篇 |
2018年 | 232篇 |
2017年 | 274篇 |
2016年 | 373篇 |
2015年 | 455篇 |
2014年 | 595篇 |
2013年 | 624篇 |
2012年 | 582篇 |
2011年 | 679篇 |
2010年 | 609篇 |
2009年 | 587篇 |
2008年 | 629篇 |
2007年 | 630篇 |
2006年 | 572篇 |
2005年 | 464篇 |
2004年 | 422篇 |
2003年 | 382篇 |
2002年 | 252篇 |
2001年 | 216篇 |
2000年 | 187篇 |
1999年 | 143篇 |
1998年 | 138篇 |
1997年 | 102篇 |
1996年 | 122篇 |
1995年 | 96篇 |
1994年 | 76篇 |
1993年 | 61篇 |
1992年 | 61篇 |
1991年 | 37篇 |
1990年 | 37篇 |
1989年 | 34篇 |
1988年 | 23篇 |
1987年 | 23篇 |
1986年 | 19篇 |
1985年 | 21篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 5篇 |
1962年 | 4篇 |
1959年 | 8篇 |
1958年 | 6篇 |
1957年 | 8篇 |
1956年 | 7篇 |
1955年 | 4篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
U. Reimer
H. -J. Schek
《Data & Knowledge Engineering》1989,4(4):321-352The first part of this paper introduces the basic constructs of a frame representation model and gives a formal definition for them. Subsequently the nested relational model (NF2) is described as an extension of the classical relational model to support relation-valued attributes. In the second part of this paper both models are compared with each other and a mapping of frame representation structures to NF2 structures is specified. The structural similarities between frames and NF2 relations are made clear and it is shown that their main difference is due to the type polymorphism introduced by the concept hierarchy of the frame model. This causes type collisions to occur in the strictly typed NF2 model when frames are directly mapped to NF2 structures. Two solutions to this problem are suggested and compared. The paper concludes with a reformulation of query operations of the frame model in terms of NF2 algebra operations. 相似文献
22.
During the 1997 winter season, shipborne polarimetric backscatter measurements of Great Lakes (freshwater) ice types using the Jet Propulsion Laboratory C-band scatterometer, together with surface-based ice physical characterization measurements and environmental parameters, were acquired concurrently with Earth Resource Satellite 2 (ERS-2) and RADARSAT Synthetic Aperture Radar (SAR) data. This polarimetric data set, composed of over 20 variations of different ice types measured at incident angles from 0° to 60° for all polarizations, was processed to radar cross-section to establish a library of signatures (look-up table) for different ice types. The library is used in the computer classification of calibrated satellite SAR data. Computer analysis of ERS-2 and RADARSAT ScanSAR images of Great Lakes ice cover using a supervised classification technique indicates that different ice types in the ice cover can be identified and mapped, and that wind speed and direction can have an influence on the classification of water as ice based on single frequency, single polarization data. Once satellite SAR data are classified into ice types, the ice map provides important and necessary input for environmental protection and management, ice control and ice breaking operations, and ice forecasting and modeling efforts. 相似文献
23.
24.
Ramirez Juan R.; Crano William D.; Quist Ryan; Burgoon Michael; Alvaro Eusebio M.; Grandpre Joseph 《Canadian Metallurgical Quarterly》2004,18(1):3
The authors investigated relationships between marijuana and inhalant use and several cultural and demographic factors in Anglo American and Hispanic American adolescents (N=1,094). Outcome measures assessed lifetime and 30-day marijuana and inhalant use. Predictors and covariates used in logistic regression analyses were region, grade, gender, knowledge, acculturation, familism, and parental monitoring. Hispanic Americans exhibited higher usage across all measures. In this group, high acculturation was associated with low marijuana, but high inhalant, use. Across all participants, positive family relations and parental monitoring were strongly associated with attenuated marijuana use hut only among those most knowledgeable about drugs. Familism and monitoring were not associated with diminished usage among the less knowledgeable. For inhalants, monitoring combined with high knowledge or high familism was associated with diminished usage. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
25.
介绍了计算机光学零件图自动绘制软件系统的功能,给出了用该软件完成的光学零件图图样,对程序设计过程中的难点及处理方法也进行了详细阐述。 相似文献
26.
数字测图,数字制图及勘测设计一体化 总被引:1,自引:0,他引:1
简述引用瑞得软件建立数字化成图系统,以进一步实现测量与工程设计一体化,使得繁琐的作业过程变为简单,缩短作业时间,不断提高生产作业效率。 相似文献
27.
Shaw Daniel S.; Gilliom Miles; Ingoldsby Erin M.; Nagin Daniel S. 《Canadian Metallurgical Quarterly》2003,39(2):189
The present study applied a semiparametric mixture model to a sample of 284 low-income boys to model developmental trajectories of overt conduct problems from ages 2 to 8. As in research on older children, 4 developmental trajectories were identified: a persistent problem trajectory, a high-level desister trajectory, a moderate-level desister trajectory, and a persistent low trajectory. Follow-up analyses indicated that initially high and low groups were differentiated in early childhood by high child fearlessness and elevated maternal depressive symptomatology. Persistent problem and high desister trajectories were differentiated by high child fearlessness and maternal rejecting parenting. The implications of the results for early intervention research are discussed, with an emphasis on the identification of at-risk parent-child dyads. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
28.
29.
水利测绘产品质量分析与检验方法研究 总被引:1,自引:0,他引:1
分析了影响测绘产品质量的主要因素,提出了完善测绘市场、加强水电测绘产品的检查验收是保证产品质量的关键,同时提出了产品质量的等级划分和检验方法,并且说明了根据测量工作的不同要求对检验方法及检验深度合理定位的必要性。 相似文献
30.
David Arditi 《Canadian Metallurgical Quarterly》1985,111(1):1-14
Construction productivity has been on the decline in the last decade. The results are presented on a survey of the Engineering News‐Record 400 largest contractors to obtain their views on where productivity improvements would most help and to compare the trends with a similar survey carried out in 1979. Data were collected on the general company characteristics of the responding contractors, and on the contractors' opinions on potential areas for productivity improvement in the office and in the field. Findings indicate that immediate research should concentrate on improving marketing practices, planning and scheduling, labor‐management relations, site supervision, industrialized building systems, equipment policy and engineering design; and that governmental regulations have lost the immediate urgency attached to them in 1979.It is also recommended that similar surveys be conducted every 3 to 4 years to identify new trends and to steer research in the appropriate direction. 相似文献