首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2964篇
  免费   27篇
  国内免费   1篇
电工技术   4篇
综合类   5篇
化学工业   63篇
金属工艺   8篇
机械仪表   6篇
建筑科学   5篇
能源动力   28篇
轻工业   56篇
水利工程   1篇
石油天然气   1篇
无线电   31篇
一般工业技术   51篇
冶金工业   2705篇
原子能技术   3篇
自动化技术   25篇
  2023年   9篇
  2022年   4篇
  2021年   14篇
  2020年   8篇
  2019年   10篇
  2018年   11篇
  2017年   8篇
  2016年   10篇
  2015年   6篇
  2014年   13篇
  2013年   17篇
  2012年   11篇
  2011年   10篇
  2010年   11篇
  2009年   16篇
  2008年   10篇
  2007年   13篇
  2006年   11篇
  2005年   8篇
  2004年   14篇
  2003年   14篇
  2002年   10篇
  2001年   4篇
  2000年   11篇
  1999年   110篇
  1998年   890篇
  1997年   509篇
  1996年   335篇
  1995年   173篇
  1994年   140篇
  1993年   180篇
  1992年   13篇
  1991年   24篇
  1990年   28篇
  1989年   32篇
  1988年   17篇
  1987年   21篇
  1986年   20篇
  1985年   28篇
  1983年   3篇
  1982年   14篇
  1981年   8篇
  1980年   30篇
  1979年   1篇
  1978年   5篇
  1977年   51篇
  1976年   100篇
  1975年   2篇
  1973年   2篇
  1972年   1篇
排序方式: 共有2992条查询结果,搜索用时 609 毫秒
111.
Field and pot trials were conducted to determine optimum management practices for usingLeucaena leucocephala (Lam. de Wit) leaves as a N source for crop production. Field trials with maize showed no benefit from split application of leucaena leaves or from application of fresh as opposed to dried material. Field trials also failed to show any difference between incorporation as opposed to surface application of leucaena leaves. This may be attributed to the low nitrogen response observed. Pot trials however, showed that soil incorporation of leucaena leaves was more effective than surface application in increasing plant dry weight.Leucaena leaves were not as effective as inorganic N in increasing maize grain yield (field trials) or dry matter production (pot trial). Unlike inorganic N, leucaena leaves had a significant residual effect on the succeeding maize crop. In decomposition studies, buried leucaena leaves decomposed more quickly than surface-applied leaves, and fresh leaves decomposed more rapidly than dried leaves.  相似文献   
112.
113.
The authors examined the role of family factors and the course of schizophrenia by carrying out additional assessments and analyses in 2 previously published studies of Mexican American and Anglo American patients and families. The authors found partial support for an attributional model of relapse for families who are low in emotional overinvolvement. Attributions of control, criticism, and warmth together marginally predicted relapse. The data also indicated that for Mexican Americans, family warmth is a significant protective factor, whereas for Anglo Americans, family criticism is a significant risk factor. These findings suggest that the sociocultural context shapes the pathways by which family processes are related to the course of illness. Moreover, the warmth findings suggest that families may contribute to preventing relapse. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
114.
There is an increasing demand for development of new recreation areas and more intensive management of existing areas. With an eye to the design and implementation of comprehensive zoning plans, satellite remote sensing should provide an ideal tool for terrain analysis, vegetation, and cover type mapping, which are vital to intensive recreation planning. The study undertaken was aimed at examining the applicability of satellite remote sensing for providing necessary information to be used in forest recreation planning. A Landsat TM scene (path row 128/ 56) taken on 30 January 1992 was processed digitally on a Meridian PC image processing system by selecting a representative subsection of the scene that covered the study area. Existing land use, topographical maps, and other related ground information as well as contrast stretching and a maximum likelihood classifier ( MLC) technique were used to assist in the classification. The selection of potential recreation sites was based from potential surface analysis ( PSA ). The results showed that most of the undeveloped forest area located in the north-eastern part of Langkawi Island, Malaysia, was the most potential sites for recreational development, while the moderate recreation potential zones lay on the western part of the island. The mean overall classification accuracy obtained was 82% Therefore the study implies that it is possible to select potential recreation sites ranging from most potential to least potential in Langkawi Island using Landsat TM.  相似文献   
115.
Human dental pulp stem cells (hDPSCs) are some of the most promising stem cell types for regenerative therapies given their ability to grow in the absence of serum and their realistic possibility to be used in autologous grafts. In this review, we describe the particular advantages of hDPSCs for neuroregenerative cell therapies. We thoroughly discuss the knowledge about their embryonic origin and characteristics of their postnatal niche, as well as the current status of cell culture protocols to maximize their multilineage differentiation potential, highlighting some common issues when assessing neuronal differentiation fates of hDPSCs. We also review the recent progress on neuroprotective and immunomodulatory capacity of hDPSCs and their secreted extracellular vesicles, as well as their combination with scaffold materials to improve their functional integration on the injured central nervous system (CNS) and peripheral nervous system (PNS). Finally, we offer some perspectives on the current and possible future applications of hDPSCs in neuroregenerative cell therapies.  相似文献   
116.
117.
118.
J. Polo  L. Martín  M. Cony 《Solar Energy》2012,86(1):275-282
Methodologies for computing ground albedo from geostationary satellite images are investigated within the framework of models for deriving solar radiation components from satellite information. A revision of Heliosat-3 approach for determining ground albedo and its implication in the cloud index estimation is done using two close pixels of quite different reflectivity in the Nile delta surroundings in Egypt. The analysis done has revealed that the way in which Heliosat-3 approaches the ground albedo and especially its dependence with the viewing angle is not very precise when dealing with high reflective sites. A simple method for fitting, individually for every pixel, the angular dependence and the ground albedo estimations is proposed here and applied to different points in the Mediterranean area. The results of the revised method show rather good estimations of ground albedo in every kind of terrain in terms of reflectivity. These results are translated to cloud index estimations and finally to surface solar irradiance and they show improvements in solar radiation estimation over high reflective pixels.  相似文献   
119.
Nowadays, the design of magneto‐impedance (MI) sensors requires the development of lumped circuit models that can be simulated through equivalent impedance circuits relied on Bessel functions. A new impedance model based on Senani's equivalent using the zeros of Bessel functions is developed in this paper. The model allows to describe the impedance as a transfer function that can be easily synthesized by means of current conveyor circuits and passive elements. The mathematical representation was verified under simulation of transfer functions involving different number of poles and zeros. Moreover, the model has been verified using SPICE simulations and measurement results from a fabricated prototype demonstrating its scope and validity. Finally, a study of finite tracking errors of CCIIs used in the implementation of magneto‐impedance sensor has been realized. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
120.
Armed conflict affects the biological diversity and biological research regime of a country. This paper takes a low-intensity armed conflict in the biologically and ethnically diverse Chittagong Hill Tracts (CHT) of Bangladesh, as an example, and assesses the response of biologists to this conflict. The study spans the last 50 years dividing it into three periods: pre-conflict (1961?1974), conflict (1975–1997) and post-conflict (1998–2011). Interactions with selected researchers and a literature survey showed that sporadic, intermittent biological research was conducted in this region even during the conflict. The number of research initiatives substantially increased and became more diverse over the last decade. Research opportunity has increased mainly because of improved security and accessibility. Nevertheless, despite the research capacity and a positive attitude among the researchers, some fundamental issues still limit long-term research in this area. The continuing volatile situation due to slow implementation of the 1997 Peace Accord and the tension between rebel factions and indigenous and settler communities in some areas continue to be serious concerns for studying and conserving the biodiversity of this region. Innovative research approaches and the full implementation of the peace agreement are vital to improving the situation for biological research in the CHT. The paper underlines the importance of a biologist’s self-motivation to respond to low-intensity armed conflicts.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号