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   326366篇
  免费   5810篇
  国内免费   1370篇
电工技术   5154篇
综合类   565篇
化学工业   50991篇
金属工艺   12001篇
机械仪表   9388篇
建筑科学   7890篇
矿业工程   1682篇
能源动力   8248篇
轻工业   30686篇
水利工程   3488篇
石油天然气   5985篇
武器工业   50篇
无线电   35429篇
一般工业技术   63831篇
冶金工业   60770篇
原子能技术   7739篇
自动化技术   29649篇
  2022年   2194篇
  2021年   3469篇
  2020年   2619篇
  2019年   3234篇
  2018年   5271篇
  2017年   5306篇
  2016年   5718篇
  2015年   3903篇
  2014年   6126篇
  2013年   15566篇
  2012年   9757篇
  2011年   12572篇
  2010年   9834篇
  2009年   10913篇
  2008年   11115篇
  2007年   10923篇
  2006年   9162篇
  2005年   8420篇
  2004年   8277篇
  2003年   7961篇
  2002年   7626篇
  2001年   7482篇
  2000年   7108篇
  1999年   7448篇
  1998年   19251篇
  1997年   13574篇
  1996年   10283篇
  1995年   7638篇
  1994年   6649篇
  1993年   6785篇
  1992年   4664篇
  1991年   4459篇
  1990年   4286篇
  1989年   4300篇
  1988年   3972篇
  1987年   3591篇
  1986年   3566篇
  1985年   3959篇
  1984年   3627篇
  1983年   3358篇
  1982年   3187篇
  1981年   3234篇
  1980年   3227篇
  1979年   2934篇
  1978年   2970篇
  1977年   3503篇
  1976年   4856篇
  1975年   2491篇
  1974年   2436篇
  1973年   2481篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
Wei  LingNan  Wang  ZhenHua  Zhang  ZhiDong  Liu  Chieh-Wen  Gao  Xuan P. A. 《Nano Research》2020,13(5):1332-1338
Nano Research - We report the composition and back-gate voltage tuned transport properties of ternary compound Bi2(Te1−xSex)3 nanowires synthesized by chemical vapor deposition (CVD). It is...  相似文献   
132.
d -Glycero-d -manno-heptose-1β,7-bisphosphate (HBP) and d -glycero-d -manno-heptose-1β-phosphate (H1P) are bacterial metabolites that were recently shown to stimulate inflammatory responses in host cells through the activation of the TIFA-dependent NF-κB pathway. To better understand structure-based activity in relation to this process, a family of nonhydrolyzable phosphonate analogues of HBP and H1P was synthesized. The inflammation modulation by which these molecules induce the TIFA-NF-κB signal axis was evaluated in vivo at a low-nanomolar concentration (6 nM) and compared to that of the natural metabolites. Our data showed that three phosphonate analogues had similar stimulatory activity to HBP, whereas two phosphonates antagonized HBP-induced TIFA-NF-κB signaling. These results open new horizons for the design of pro-inflammatory and innate immune modulators that could be used as vaccine adjuvant.  相似文献   
133.
Over the last few years, the global biosurfactant market has raised due to the increasing awareness among consumers, for the use of biological or bio-based products. Because of their composition, it can be speculated that these are more biocompatible and more biodegradable than their chemical homologous. However, at the moment, no studies exist in the literature about the biodegradability of biosurfactants. In this work, a biosurfactant contained in a crude extract, obtained from a corn wet-milling industry stream that ferments spontaneously in the presence of lactic acid bacteria, was subjected to a biodegradation study, without addition of external microbial biomass, under different conditions of temperature (5–45 °C), biodegradation time (15–55 days), and pH (5–7). For that, a Box–Behnken factorial design was applied, which allowed to predict the percentage of biodegradation for the biosurfactant contained in the crude extract, between the range of the independent variables selected in the study, obtaining biodegradation values between 3 and 80%. The percentage of biodegradation for the biosurfactant was calculated based on the increase in the surface tension of samples of the crude extract. Furthermore, it was also possible to predict the variation in t1/2 for the biosurfactant (time to achieve the 50% of biodegradation) under different conditions.  相似文献   
134.
Electrochemical impedance spectroscopy (EIS), anodic polarization and scanning electron microscopy techniques were used to investigate the damage mechanism in the transpassive potential region of AISI ...  相似文献   
135.
The authors have developed a procedure and a test bench for studying evolution of various nature physical fields in modeling geomedium fracture by fluids. The test bench performs synchronous recording of macro- and micro-deformation, heat and acoustic emission induced in physical models of geomedium under loading to discontinuity. The experimental procedure has been trialed. The analysis of the synchronized test data allows a conclusion on the existence of time–space relationship between different nature physical fields induced during failure of solids.  相似文献   
136.
While qualitative studies have identified chlorogenic acids in antioxidant extracts, particularly ethyl acetate‐derived extracts, of Taraxacum officinale, quantitative analysis of these phenolic compounds remains largely unreported for this species. In this study, bioactivity‐guided fractionation of an antioxidant crude ethyl acetate extract (DPPH = 295.481 ± 0.955 mg TE g?1 extract) from T. officinale root resulted in a number of reverse‐phase fractions that demonstrated high antioxidant activity (DPPH = 1058.733–1312.136 mg TE g?1 extract), stronger than that of the synthetic antioxidant Trolox®. UPLC‐MS/MS screening of these fractions for the presence of selected mono‐ and di‐caffeoylquinic acids revealed large quantities of 1,5‐dicaffeoylquinic acid present in several fractions (853.052–907.324 μg mg?1), respectively. Due to the antioxidant potency and high levels of 1,5‐dicaffeoylquinic acid observed in these fractions, it was concluded that specifically this chlorogenic acid derivative is a major contributor to the antioxidant efficacy of dandelion root.  相似文献   
137.
138.
139.
140.
The primary goal of this study is to create and test a lecture‐capture system that can rearrange visual elements while recording is still taking place, in such a way that student performance can be positively influenced. The system we have devised is capable of integrating and rearranging multimedia sources, including learning content, the instructor and students' images, into lecture videos that are embedded in a website for students to review after school. The present study employed a two‐group experimental design, with 153 participants (145 females and 8 males) making up an experimental group in which lecture courses were recorded using the new lecture‐capture system, and 149 participants (140 females and 9 males) forming a control group whose lectures were recorded by traditional means. All participants were in the freshman college and studying Introduction to Computer and Information Science in one of six classes, and were randomly assigned to one of the two groups. The participants' midterm examination and final examination scores were collected as indicators of their academic performance, with their mathematics entrance scores used as a pre‐test. The findings obtained from analysis of covariance (ANCOVA) suggest that appropriate rearrangement of visual elements in lecture videos can significantly impact students' learning performance.  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