全文获取类型
收费全文 | 4422篇 |
免费 | 667篇 |
国内免费 | 441篇 |
专业分类
电工技术 | 322篇 |
技术理论 | 2篇 |
综合类 | 690篇 |
化学工业 | 121篇 |
金属工艺 | 33篇 |
机械仪表 | 181篇 |
建筑科学 | 225篇 |
矿业工程 | 44篇 |
能源动力 | 39篇 |
轻工业 | 65篇 |
水利工程 | 43篇 |
石油天然气 | 19篇 |
武器工业 | 104篇 |
无线电 | 1095篇 |
一般工业技术 | 268篇 |
冶金工业 | 243篇 |
原子能技术 | 8篇 |
自动化技术 | 2028篇 |
出版年
2024年 | 47篇 |
2023年 | 99篇 |
2022年 | 157篇 |
2021年 | 195篇 |
2020年 | 186篇 |
2019年 | 191篇 |
2018年 | 137篇 |
2017年 | 195篇 |
2016年 | 219篇 |
2015年 | 229篇 |
2014年 | 376篇 |
2013年 | 338篇 |
2012年 | 374篇 |
2011年 | 429篇 |
2010年 | 322篇 |
2009年 | 302篇 |
2008年 | 261篇 |
2007年 | 253篇 |
2006年 | 203篇 |
2005年 | 193篇 |
2004年 | 164篇 |
2003年 | 125篇 |
2002年 | 116篇 |
2001年 | 89篇 |
2000年 | 65篇 |
1999年 | 63篇 |
1998年 | 36篇 |
1997年 | 28篇 |
1996年 | 27篇 |
1995年 | 12篇 |
1994年 | 16篇 |
1993年 | 10篇 |
1992年 | 12篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1982年 | 5篇 |
1979年 | 2篇 |
1965年 | 1篇 |
1964年 | 1篇 |
1963年 | 6篇 |
1962年 | 3篇 |
1961年 | 4篇 |
1960年 | 2篇 |
1959年 | 4篇 |
1958年 | 2篇 |
1957年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有5530条查询结果,搜索用时 31 毫秒
1.
Kamalesh Pal Amitava Mukherjee Md. Motin Seikh Parthasarathi Bera Arup Gayen 《Ceramics International》2021,47(10):14798-14808
Synthesis of nanocrystalline pristine and Mn-doped calcium copper titanate quadruple perovskites, CaCu3?xMnxTi4?xMnxO12 (x = 0, 0.5, and 1.0) by modified citrate solution combustion method has been reported. Powder X-ray diffraction patterns attest the phase purity of the perovskite materials. Average particle sizes of all the materials obtained from the Scherrer's formula are in the range of 55–70 nm. The specific surface areas for all the perovskites obtained from BET isotherms are found to be low as expected for the condensed oxide systems and fall in the range of 13–17 m2 g?1. Transmission electron microscopy studies show a reduction in particle size of CaCu3Ti4O12 with increase in Mn doping. Ca and Ti are present in +2 and +4 oxidation states in all the materials as demonstrated by X-ray photoelectron spectroscopy analyses. Cu2+ gets reduced in CaCu3Ti4O12 with higher Mn content. Mn is observed to be present only in +3 oxidation state. All the materials have been examined to be active in CO oxidation as well as H2 production from methanol steam reforming. CaCu3Ti4O12 with ~14 at.% Mn is found to show best catalytic activities among these materials. A comprehensive analysis of the catalytic activities of these perovskites toward CO oxidation and H2 production from MSR reveal the cooperative activity of copper-manganese in the doped perovskites and it is more effective at lower manganese content. 相似文献
2.
3.
No authorship indicated 《Canadian Metallurgical Quarterly》2003,58(11):931
Provides the biography of David W. Johnson and announces that he has won the Award for Distinguished Contributions of Applications of Psychology to Education and Training for programmatic research, comprehensive research reviews, and revision of theory over four decades of persistence that have provided the knowledge foundation for cooperative learning, constructive controversy, conflict resolution, and peer mediation training. A selected bibliography is also provided. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
4.
Business, not technology leaders should manage the change of Information Technology (IT) from a self-contained business support unit of the sixties to a distributed strategic business function of the nineties. Top management should recognize that a good business plan can only be developed and executed through the partnership between the business unit managers and the managers of the financial, the personnel, and the IT resource units. The technical problems associated with the management of IT may again be the easier part of the total management challenge. 相似文献
5.
The main purpose of the study reported here was to investigate the relative importance of complexity of syllable structure and task differences in measuring phonological awareness in low-literacy adults. This study is a replication of a study with children conducted by S. A. Stahl and B. A. Murray (1994). Results indicated that the complexity of syllable structure did indeed better describe the construct of phonological awareness in low-literacy adults. At the same time, the authors also found some differences in the pattern of literacy acquisition for their Spanish adults in comparison with child literacy acquisition in English. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
6.
The State of the Art in Text Filtering 总被引:1,自引:0,他引:1
This paper develops a conceptual framework for text filtering practice and research, and reviews present practice in the field.
Text filtering is an information seeking process in which documents are selected from a dynamic text stream to satisfy a relatively
stable and specific information need. A model of the information seeking process is introduced and specialized to define text
filtering. The historical development of text filtering is then reviewed and case studies of recent work are used to highlight
important design characteristics of modern text filtering systems. User modeling techniques drawn from information retrieval,
recommender systems, machine learning and other fields are described. The paper concludes with observations on the present
state of the art and implications for future research on text filtering.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
The authors examined whether participants can shift their criterion for recognition decisions in response to the probability that an item was previously studied. Participants in 3 experiments were given recognition tests in which the probability that an item was studied was correlated with its location during the test. Results from all 3 experiments indicated that participants' response criteria were sensitive to the probability that an item was previously studied and that shifts in criterion were robust. In addition, awareness of the bases for criterion shifts and feedback on performance were key factors contributing to the observed shifts in decision criteria. These data suggest that decision processes can operate in a dynamic fashion, shifting from item to item. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
8.
Tek Bahadur Gurung 《Lakes & Reservoirs: Research and Management》2007,12(4):237-246
Lake Rupa is a small, subtropical, shallow lake with a surface area of 100 ha situated 600 m a.s.l. in Central Himalaya, Nepal. This degraded lake was studied between 2000 and 2006, with the goal of determining whether or not it could be restored by a community‐based cooperative of local people living in its catchment. Main threats to the lake, its aquatic life and its very existence include encroachment by excessive aquatic vegetation, sedimentation, and low in‐lake concentrations of dissolved oxygen (DO). Small lakes (≤ 500 ha) are relatively prone to the process of ‘succession and climax’, although they also can be of social, environmental and ecological importance. Thus, their disappearance could spark social chaos and disorder in areas already having to cope with other rapid environmental changes. This means that credible mechanisms for revitalizing or protecting small lakes are an important goal. Accordingly, a lake cooperative of 329 households living in close vicinity to Lake Rupa was formed in 2002, by prioritizing traditional fishers, women and other deprived community members with the goals of respecting the citizenry and equity. Following the cooperative's campaigns directed at weed removal and fish stocking, encroachment of vegetation on the lake margins was halted, its aquatic weeds became under control, and its fisheries improved. In addition to the cooperative's restoration activities, several water quality parameters, including water transparency and DO and chlorophyll‐a concentrations, were monitored on a monthly basis. The measured water transparency was inconsistent, exhibiting large variations between 2000 and 2003. Relatively low, but consistent, values, however, were measured after 2004. The results imply that the removal of weeds, and sufficient nutrients and solar radiation are subsequently available to the lake's phytoplankton communities. Supporting this notion is that the chlorophyll‐a concentration spiked to 205 µg L−1 in November 2006, the water transparency became consistent, and the DO concentration increased to >3.8 mg L−1 during the critical months (March–May) after 2004. These water quality indicators indicated improvement in the degraded Lake Rupa, suggesting that the establishment of cooperatives such as that highlighted in this study could be a powerful and sustainable mechanism for restoring degraded lakes in similar socioeconomic settings by maintaining equity, by connecting communities with their resources, and by facilitating integrity, equity, citizenry and social justice. 相似文献
9.
指出当今建筑业良好的市场给建筑师们提供了展示自己作品的广大空间 ,但其精品却为之甚少。为此 ,对建筑业存在的一些问题及制约建筑师出精品的主、客观因素进行了分析 ,指出建筑师应提高自身素质 ,树立精品意识 ,增强市场竞争能力 相似文献
10.