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   3724篇
  免费   290篇
  国内免费   193篇
电工技术   183篇
综合类   250篇
化学工业   379篇
金属工艺   216篇
机械仪表   184篇
建筑科学   251篇
矿业工程   115篇
能源动力   65篇
轻工业   309篇
水利工程   190篇
石油天然气   98篇
武器工业   74篇
无线电   273篇
一般工业技术   313篇
冶金工业   845篇
原子能技术   75篇
自动化技术   387篇
  2024年   13篇
  2023年   36篇
  2022年   101篇
  2021年   119篇
  2020年   84篇
  2019年   72篇
  2018年   85篇
  2017年   81篇
  2016年   87篇
  2015年   148篇
  2014年   190篇
  2013年   229篇
  2012年   262篇
  2011年   257篇
  2010年   203篇
  2009年   223篇
  2008年   207篇
  2007年   240篇
  2006年   166篇
  2005年   149篇
  2004年   112篇
  2003年   93篇
  2002年   88篇
  2001年   71篇
  2000年   50篇
  1999年   52篇
  1998年   171篇
  1997年   115篇
  1996年   85篇
  1995年   46篇
  1994年   38篇
  1993年   44篇
  1992年   12篇
  1991年   17篇
  1990年   18篇
  1989年   24篇
  1988年   16篇
  1987年   10篇
  1986年   18篇
  1985年   11篇
  1984年   8篇
  1983年   7篇
  1982年   7篇
  1981年   8篇
  1979年   11篇
  1978年   7篇
  1977年   19篇
  1976年   50篇
  1975年   8篇
  1972年   7篇
排序方式: 共有4207条查询结果,搜索用时 17 毫秒
1.
依据基准面旋回原理和方法,综合利用哈得逊地区的钻井、测井、地震以及地球化学资料,在研究区下石炭统识别出1个Ⅰ型和3个Ⅱ型层序界面,将哈得逊地区下石炭统划分为3个中期基准面旋回:SC1、SC2、SC3,对应于早石炭世海侵的3个海进-海退旋回.SC1旋回以滨岸砂坝沉积为主,而SC2、SC3旋回发育潮坪沉积体系.研究认为,SC1旋回滨岸砂岩及SC2下降半旋回的潮间带薄层砂岩为有利储集砂体,是地层-岩性型油气藏的重点勘探目标.  相似文献   
2.
The densities of three Ni-based superalloys have been measured in both liquid and mushy states by both a modified sessile drop method (MSDM) and a modified pycnometric method (MPM) for alloys CMSX-4 and CM186LC, and for CMSX-10 alloy by MSDM only. The surface tensions of liquid CMSX-4, CM186LC, and CMSX-10 superalloys were measured using the sessile drop method. All measurements were carried out in a highly purified argon atmosphere with the oxygen partial pressure of less than 10−19 MPa in the gas outlet. The densities of all superalloys in both liquid and mushy states were found to decrease with increasing temperature. The volume thermal expansion of each superalloy in the mushy state was found to be higher than that in the liquid state. The densities determined by different methods have been critically assessed and recommended values in both liquid and mushy states are given as a linear function of temperature for the three Ni-based superalloys. The surface tension of liquid CMSX-4 superalloy was found to decrease with increasing temperature, while that of liquid CMSX-10 superalloy increases with increasing temperature. The wetting behavior of liquid CM186LC on the alumina substrate was found (1) to differ significantly from that of CMSX-4 and CMSX-10 and (2) to vary with time. A HfO2-rich layer was found in the contact area of CM186LC with the alumina substrate, which could lead to some uncertainty in the value obtained for the surface tension determined for CM186LC.  相似文献   
3.
Skeletal development of transgenic mice with a type II collagen mutation was analyzed and compared with wild-type littermates. The single base substitution in Col2a1 resulted in a glycine to serine mutation within the helical domain and corresponded to one previously identified in a patient with the lethal human chondrodysplasia, hypochondrogenesis (Horton et al. [1992] Proc. Natl. Acad. Sci. U.S.A. 89:4583-4587). Skeletal staining of embryos from 14.5 through 18.5 days of gestation demonstrated a dwarf phenotype in the transgenic embryos, most notably short limb bones and vertebral column that was first detected at 15.5 days post-coitus. In addition to the reduced length, the extent of ossification was less in the transgenic mice. The architecture of the long bone growth plate was abnormal in the transgenic tissue, in particular there was no discernible proliferative zone. There were few stacks of characteristically flattened cells and the overall length of the growth plate in the mutant embryos was reduced. At the ultrastructural level, there were fewer collagen fibrils present in the transgenic mouse cartilage compared to that of wild-type littermates. Ultrastructural localization of collagen types II, IX and XI revealed a similar pattern between the transgenic and wild-type pups, suggesting that the collagen fibrils present in the matrix of littermates with both phenotypes had a similar composition. Skeletal analysis and cartilage histochemistry indicated that effect of the type II collagen mutation was to reduce the density of the collagen fibrils within the cartilage matrix which was associated with delayed bone formation and resulted in a short-limbed phenotype.  相似文献   
4.
According to FDA regulations, a combination drug is not efficacious unless each component contributes to the claimed effects. For a univariate endpoint, this implies that the combination at specific doses must be superior to each of its components at the same doses. More demanding is the property of synergy, in which the effect of the combination must be superior to the effect expected based on those of its components. If it is equal to those effects, it is additive, and if it is inferior, it is antagonistic. We give regions in the combination dose plane where these concepts are well defined. If the effect of the combination is greater than the greatest effect achievable by any of its components it is therapeutically synergistic. A combination can be antagonistic, yet its components can still contribute to the claimed effects. If it is additive, synergistic or therapeutically synergistic, its components must contribute to the claimed effects. We relate these concepts and provide designs and sequential procedures for determining whether a combination is therapeutically synergistic, synergistic, additive, antagonistic and contributing or antagonistic and non-contributing.  相似文献   
5.
Based on generalization and analysis of instructions and guidelines for therapeutical and prophylactic diets of workers, ways of its optimization were proposed, by using bifide-containing acid dairy products as a preventive agent against possible occupational diseases caused by occupational factors.  相似文献   
6.
7.
BACKGROUND: Primary care has an important role to play in the prevention and management of the human immunodeficiency virus (HIV). It has been suggested that homosexual men experience a variety of problems in relation to primary care. AIM: As part of a larger study, it was decided to examine the extent to which a sample of homosexually active men experienced difficulties in general practice and whether they consulted their general practitioner for problems related to HIV or the acquired immune deficiency syndrome (AIDS). METHOD: Homosexual men were recruited for interview in 1991-92 from a variety of sources including genitourinary clinics and homosexual organizations. RESULTS: Of 623 men registered with a general practitioner 44% had not informed their general practitioner of their sexual orientation and 44% of the 77 men who were HIV antibody positive, as confirmed by the study, had not informed their general practitioner of this fact. Men who viewed their practice as unsympathetic towards homosexual men were less likely to have informed their general practitioner of their sexual orientation or HIV status. The majority of men (87%) nevertheless viewed primary care as an appropriate source of HIV/AIDS advice. CONCLUSION: There is considerable scope for improvement in the acceptability of general practice to homosexual men.  相似文献   
8.
E.Wei  Y-D.Yang  C.Feng  I.D.Sommerville  A.McLean 《钢铁》2003,38(Z1):185-188
在钢的连铸过程中,结晶器保护渣的性质强烈地影响着铸造的行为、钢的质量、以及连铸工艺的环境.本研究采用高温显微技术来研究保护渣的熔化行为,采用液滴测试方法来测定保护渣的熔化速率.结果表明:自由碳是控制保护渣熔化行为的主要因素.熔化速率随碳的反应能力的增加而增加,随着碳含量的减少而增加.  相似文献   
9.
Electroabsorption optical modulators have been fabricated on GaInAs/GaInAsP multiquantum well structures whose bandgap had been increased by laser photoabsorption-induced disordering. Modulation depths of 20 dB have been obtained in material which has been bandgap blue shifted by as much as 120 nm, while samples shifted by 80 nm gave depths as high as 27 dB  相似文献   
10.
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