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   90344篇
  免费   7932篇
  国内免费   4081篇
电工技术   5688篇
技术理论   2篇
综合类   5777篇
化学工业   14013篇
金属工艺   4956篇
机械仪表   5216篇
建筑科学   6721篇
矿业工程   1853篇
能源动力   2647篇
轻工业   7087篇
水利工程   1811篇
石油天然气   3877篇
武器工业   675篇
无线电   12040篇
一般工业技术   11651篇
冶金工业   4617篇
原子能技术   1029篇
自动化技术   12697篇
  2024年   370篇
  2023年   1371篇
  2022年   2574篇
  2021年   3563篇
  2020年   2502篇
  2019年   2105篇
  2018年   2358篇
  2017年   2801篇
  2016年   2455篇
  2015年   3396篇
  2014年   4445篇
  2013年   5665篇
  2012年   6040篇
  2011年   6751篇
  2010年   5857篇
  2009年   5649篇
  2008年   5585篇
  2007年   5347篇
  2006年   5122篇
  2005年   4178篇
  2004年   2844篇
  2003年   2349篇
  2002年   2370篇
  2001年   2025篇
  2000年   1913篇
  1999年   1939篇
  1998年   1826篇
  1997年   1576篇
  1996年   1411篇
  1995年   1185篇
  1994年   923篇
  1993年   761篇
  1992年   607篇
  1991年   462篇
  1990年   385篇
  1989年   298篇
  1988年   252篇
  1987年   182篇
  1986年   152篇
  1985年   139篇
  1984年   100篇
  1983年   75篇
  1982年   68篇
  1981年   59篇
  1980年   51篇
  1979年   35篇
  1978年   32篇
  1977年   32篇
  1976年   54篇
  1973年   21篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
231.
预测井眼稳定性的力学化学耦合方法   总被引:13,自引:8,他引:5  
在井眼力学稳定性评价的基础上,将硬脆性页岩水化后岩石强度等参数的变化规律用于页岩水化前后坍塌压力变化的对比分析,结果表明,地层坍塌压力当量密度随页岩水化程度的增加逐渐升高。给出了预测井眼稳定性的力学化学耦合方法,并将该方法应用于现场生产中,取得了较好的预测效果  相似文献   
232.
介绍了在直流电弧放电法制备富勒烯过程中,从阴电极表面收集到大量的碳纳米管及巴基葱,产生率达到50%,并用电子显微镜对其进行了观察和初步研究。  相似文献   
233.
We attempted to purify endogenous growth factors of chick embryonal retinal pigment epithelial cells from bovine neural retina. Ammonium sulfate precipitation, gel filtration, and anion exchange column chromatography were used. Though only partially purification, bovine retinal extract had two effects, growth and transformation, on chick embryonal pigment epithelial cells. We think that these purified factors may be novel from the estimation of their molecular weight by gel filtration.  相似文献   
234.
235.
A technique for SiO2 formation by liquid-phase deposition (LPD) at nearly room temperature for low-temperature processed (LTP) polysilicon thin-film transistor (poly-Si TFT) was developed. LPD SiO2 film with a lower P-etch rate shows a dense structure. LPD SiO2 also exhibits good electrical characteristics. LTP poly-Si thin-film transistors (TFTs) with LPD SiO 2 as the gate insulator have been fabricated and investigated. Their characteristics indicate performance adequate for their use as pixel transistors in liquid crystal displays (LCDs)  相似文献   
236.
Converting cysteine 543 to tyrosine in the influenza virus hemagglutinin (HA) introduces both a basolateral sorting signal and an internalization signal into the HA cytoplasmic domain. Another HA mutant, HA+8, contains eight additional amino acids at the end of the cytoplasmic domain that include a powerful internalization signal. HA+8 was also sorted efficiently to the basolateral surface of Madin-Darby canine kidney cells. The simplest explanation for the observation that multiple sorting phenotypes depend upon the same small amino acid sequence is that certain tyrosine-based internalization signals might also function as basolateral sorting signals. To test this hypothesis, second-site mutations were introduced into HA C543Y or HA+8 to determine if the internalization and basolateral sorting functions can be separated. For HA C543Y, the same sequence positions were important for both basolateral sorting and internalization, but the two functions responded differently to individual amino acid replacements, indicating that they were distinct. For HA+8, the basolateral sorting signal required the same tyrosine as the internalization signal, but did not share any other characteristics. Thus, even when basolateral sorting signals that depend on tyrosine overlap or are co-linear with internalizations signals, the two sorting processes are sensitive to different characteristics of the sequence.  相似文献   
237.
Although stereotyped behavior in adult rats is partly regulated by dopamine (DA) D2 autoreceptors, previous attempts to demonstrate D2 autoreceptor regulation of stereotypy in developing rats have been unsuccessful. In the present study, two highly selective D2 autoreceptor agonists were used to demonstrate D2 autoreceptor regulation of spontaneous stereotyped behavior in developing rats. Both SND 919 and PD 128483 produced significant dose-dependent decreases in the stereotypy counts of 21-day-old, 35-day-old, and adult rats. There was a 51% decrease in the stereotypy counts of 21-day-old rats injected with SND 919, 0.05 mg/kg, compared to a 36% decrease in the counts of rats pretreated with haloperidol. Similarly, PD 128483 significantly decreased the stereotypy counts of 21-, 35-day-old, and adult rats in a dose-dependent fashion. There was a 58% decrease in the stereotypy counts of 21-day-old rats injected with PD 128483, 0.1 mg/kg, compared to a 17% decrease in counts when the rats were first treated with haloperidol. The effect of haloperidol plus PD 128483 was significantly different from the effect of PD 128483 alone. Injection of SND 919 or PD 128483 had no significant effects on the stereotypy counts of 10-day-old rats. The results suggest that DA D2 autoreceptor-mediated regulation of spontaneous stereotyped behavior is functional at 21, but not 10, days of age.  相似文献   
238.
239.
Mental health supplement to the Ontario Health Survey: methodology   总被引:1,自引:0,他引:1  
OBJECTIVE: To describe the methodology of a province-wide, cross-sectional, epidemiologic study of psychiatric disorder among those aged 15 years and over living in household dwellings in Ontario. METHOD: Respondents for the survey were drawn from households (N = 13002) participating in a province-wide health survey. One person per household was selected, and 9953 (76.5%) participated. RESULTS: Participants and nonparticipants were similar to each other. An extensive array of data, including measures of psychiatric disorder classified using a revised version of the Composite International Diagnostic Interview (CIDI), are available for all respondents. CONCLUSIONS: The Ontario Health Supplement is contained in a public-use data file at the Ontario Ministry of Health and is available to investigators for study. A strong survey design, careful measurement, and acceptable levels of response provide the rationale for our inviting researchers to access and use the Ontario Health Supplement data base.  相似文献   
240.
The sub-rating channel assignment strategy for PCS hand-offs   总被引:2,自引:0,他引:2  
A new personal communications services (PCS) hand-off scheme is proposed. This scheme provides for hand-off to radio ports on which there is no free channel by “sub-rating” an existing connection. With sub-rating, an occupied full-rate channel is temporarily divided into two half-rate channels: one to serve the existing call and the other to serve the hand-off request. The blocking probabilities (combined forced terminations of existing calls and blocking of new call attempts) of this new scheme compare favorably with the standard scheme (nonprioritizing) and the previously proposed prioritizing schemes. The costs for this scheme are presented and discussed, as well as the additional procedural complexity of implementing on-the-fly sub-rating and the impact of continuing the conversation on a lower rate channel (which may lower speech quality of increase battery drain). Analytical models and simulations investigating the traffic impacts are presented, as are the results that show that even in the highest offered load considered a 3-min conversation in the busy hour experiences less than half a second of sub-rated conversation on average and only about 3% of the calls experience more than 5.12 s of sub-rated conversation. This scheme can increase capacity by 8-35% for systems with 1% call incompletion probability  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