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   3843篇
  免费   279篇
  国内免费   2篇
电工技术   18篇
综合类   3篇
化学工业   946篇
金属工艺   48篇
机械仪表   55篇
建筑科学   191篇
矿业工程   87篇
能源动力   59篇
轻工业   714篇
水利工程   29篇
石油天然气   9篇
无线电   231篇
一般工业技术   519篇
冶金工业   755篇
原子能技术   15篇
自动化技术   445篇
  2023年   37篇
  2022年   95篇
  2021年   128篇
  2020年   74篇
  2019年   77篇
  2018年   90篇
  2017年   122篇
  2016年   155篇
  2015年   132篇
  2014年   149篇
  2013年   298篇
  2012年   197篇
  2011年   230篇
  2010年   205篇
  2009年   166篇
  2008年   201篇
  2007年   197篇
  2006年   166篇
  2005年   145篇
  2004年   111篇
  2003年   93篇
  2002年   95篇
  2001年   71篇
  2000年   57篇
  1999年   59篇
  1998年   74篇
  1997年   68篇
  1996年   48篇
  1995年   51篇
  1994年   57篇
  1993年   37篇
  1992年   26篇
  1991年   18篇
  1990年   21篇
  1989年   22篇
  1988年   34篇
  1987年   33篇
  1986年   22篇
  1985年   31篇
  1984年   28篇
  1983年   28篇
  1982年   17篇
  1981年   23篇
  1980年   13篇
  1979年   19篇
  1978年   7篇
  1977年   11篇
  1975年   13篇
  1974年   8篇
  1970年   7篇
排序方式: 共有4124条查询结果,搜索用时 15 毫秒
111.
112.

Background

Despite limited empirical investigation, existing scientific literature suggests that individuals with a history or current diagnosis of conduct disorder (CD) may be more likely to demonstrate reckless and aggressive driving. Much of the limited research in this field examines the impact of childhood CD on driver behaviour and collision risk in young adults. Few if any, studies assess the impact of this disorder on driver behaviour beyond age 21 years. The current research is a population-based study of the impact of CD symptoms during childhood on the risk of engaging in driver aggression during adulthood.

Methods

Data are based on telephone interviews with 5230 respondents who reported having driven in the past year. Data are derived from the 2011–2013 cycles of the CAMH Monitor, an ongoing cross-sectional survey of adults in Ontario, Canada aged 18 years and older. A binary logistic regression analysis of self-reported driver aggression in the previous 12 months was conducted, consisting of measures of demographic characteristics, driving exposure, problem substance use, alcohol- and drug-impaired driving, symptoms of attention deficit hyperactivity disorder, and childhood (before age 15) symptoms of CD.

Results

When entered with demographic characteristics, driving exposure, and other potential confounders, childhood symptoms of CD increased the odds of reporting driver aggression more than two-fold (adjusted OR = 2.12). Exploratory analyses of the interaction between childhood symptoms of CD and age was not a significant predictor of driver aggression.

Conclusions

Results suggest that symptoms of CD during childhood are associated with significantly increased odds of self-reported driver aggression during adulthood. Limitations and future directions of the research are discussed.  相似文献   
113.
The causes of environmental changes, including climate, are neither fully understood nor identified, nor do scientists agree about their nature. Yet, their impact is such that action cannot be delayed. This involves spreading public awareness through environmental education and sensitization. Hence a need to reach, to educate the poorly informed public at large. With the classroom as the base, “town meetings” and eco‐tourism may constitute potentially useful approaches towards that goal.  相似文献   
114.
The brief is a central document in decision-making during the early stages of the planning and construction (briefing/programming) of healthcare environments. Thus, the demand on the content and quality of these documents is high. The content and quality of initial briefs developed in Sweden were assessed to obtain an up-to-date picture of current practices of writing project statements in the early stages of designing healthcare environments. A new instrument called the Content and Quality of Brief Instrument (CQB-I) was used to study 29 initial briefs. In many cases, the documentation in the briefs was incomplete and the information did not adequately address users’ needs. In addition, the briefs often did not include any measurable outcomes or explicitly evidence-based information. These findings have implications for identifying areas for improvement in design teams’ documentation practices. Assessment can have a real impact on the quality of briefs and can raise awareness of the need to improve the briefing process itself. This is one of the first studies to assess the content and quality of briefs and it provides a starting point for both practitioners and future researchers to explore the role of briefs in briefing process quality improvement.  相似文献   
115.
Cellulose nanocrystals (CNC) were successfully grafted with a low molecular weight poly(butylene glutarate) through an in situ polymerization procedure. The grafting treatment decreased the CNC hydrophilic character and increased the onset of their thermal degradation by approximately 20°C, thus increasing the possibilities of CNC application. Composites of grafted and nongrafted CNC with a poly(butylene‐adipate‐co‐terephthalate) (PBAT) matrix were prepared by melt extrusion. The CNC addition led to an increase of 50% of the tensile elastic modulus of the PBAT. In addition, dynamic mechanical thermal analysis showed that the composite with CNC retained its high modulus even at temperatures far above the glass transition temperature of PBAT. At 60°C the storage modulus of the composite with CNC was approximately 200% higher than that of the pure PBAT. Thus, in this work, nanocomposites of improved properties were obtained through a combination of in situ polymerization and melt extrusion. POLYM. ENG. SCI., 56:1339–1348, 2016. © 2016 Society of Plastics Engineers  相似文献   
116.
Dense symmetric membranes of CaTi0.85?xFe0.15MnxO3?δ (x = 0.1, 0.15, 0.25, 0.4) are investigated in order to determine the optimal Mn dopant content with respect to highest O2 flux. O2 permeation measurements are performed as function of temperature between 700°C–1000°C and as function of the feed side ranging between 0.01 and 1 bar. X‐ray photoelectron spectroscopy is utilized to elucidate the charge state of Mn, and synchrotron radiation X‐ray powder diffraction (SR‐XPD) is employed to investigate the structure symmetry and cell volume of the perovskite phase at temperatures up to 800°C. The highest O2 permeability is found for x = 0.25 over the whole temperature and ranges, followed by x = 0.4 above 850°C. The O2 permeability for x = 0.25 reaches 0.01 mL(STP) min?1 cm?1 at 925°C with 0.21 bar feed side and Ar sweep gas. X‐ray photoelectron spectroscopy indicates that the charge state of Mn changes from approx. +3 to +4 when x > 0.1, which implies that Mn mainly improves electronic conductivity for x > 0.1. The cell volume is found to decrease linearly with Mn content, which coincides with an increase in the activation energy of O2 permeability. These results are consistent with the interpretation of the temperature and dependency of O2 permeation. The sintering behavior and thermal expansion properties are investigated by dilatometry, which show improved sinterability with increasing Mn content and that the thermal expansion coefficient decreases from 12.4 to 11.9 × 10?6 K?1 for x = 0 and x = 0.25, respectively.  相似文献   
117.
As part of a continued push for high permittivity dielectrics suitable for use at elevated operating temperatures and/or large electric fields, modifications of BaTiO3 with Bi(M)O3, where M represents a net‐trivalent B‐site occupied by one or more species, have received a great deal of recent attention. Materials in this composition family exhibit weakly coupled relaxor behavior that is not only remarkably stable at high temperatures and under large electric fields, but is also quite similar across various identities of M. Moderate levels of Bi content (as much as 50 mol%) appear to be crucial to the stability of the dielectric response. In addition, the presence of significant Bi reduces the processing temperatures required for densification and increases the required oxygen content in processing atmospheres relative to traditional X7R‐type BaTiO3‐based dielectrics. Although detailed understanding of the structure–processing–property relationships in this class of materials is still in its infancy, this article reviews the current state of understanding of the mechanisms underlying the high and stable values of both relative permittivity and resistivity that are characteristic of BaTiO3‐Bi(M)O3 dielectrics as well as the processing challenges and opportunities associated with these materials.  相似文献   
118.
Diffraction peaks of nanoscale particles of 3 mol% yttria‐stabilized zirconia become sharper as the powder sinters. The reduction in the peak width is correlated with the increase in density. The sharpening of the peak agrees reasonably well with the remaining free surface area as the sample sinters. Therefore, high curvature of the free surface of the pores is assumed to lead to peak broadening (the grain boundaries that grow at the expense of the free surfaces of the pores do not have this curvature). The change in the grain size during sintering does not make a significant contribution to peak width.  相似文献   
119.
Cathepsin D (CathD) is overexpressed and secreted by several solid tumors and stimulates their growth, the mechanism of which is still not understood. In this context, the pepstatin bioconjugate JMV4463 [Ac‐arg‐O2Oc‐(Val)3‐Sta‐Ala‐Sta‐(AMPA)4‐NH2; O2Oc=8‐amino‐3,6‐dioxaoctanoyl, Sta=statine, AMPA=ortho‐aminomethylphenylacetyl], containing a new kind of cell‐penetrating vector, was previously shown to exhibit potent antiproliferative effects in vitro and to delay the onset of tumors in vivo. In this study, we performed a structure–activity relationship analysis to evaluate the significance of the inhibitor and vector moieties of JMV4463. By modifying both statine residues of pepstatin we found that the antiproliferative activity is correlated with CathD inhibition, supporting a major role of the catalytic activity of intracellular CathD in cancer cell proliferation. Replacing the vector composed of four AMPA units with other vectors was found to abolish cytotoxicity, although all of the conjugates enabled pepstatin transport into cells. In addition, the AMPA4 vector must be localized at the C terminus of the bioconjugate. The unexpected importance of the vector structure and position for cytotoxic action suggests that AMPA4 enables pepstatin to inhibit the proteolysis of critical CathD substrates involved in cell proliferation via a unique mechanism of action.  相似文献   
120.
Neglected tropical diseases caused by parasitic infections are an ongoing and increasing concern. They are a burden to human and animal health, having the most devastating effect on the world′s poorest countries. Building upon our previously reported triazole analogues, in this study we describe the synthesis and biological testing of other novel heterocyclic acetogenin‐inspired derivatives, namely 3,5‐isoxazoles, furoxans, and furazans. Several of these compounds maintain low‐micromolar levels of inhibition against Trypanosoma brucei, whilst having no observable inhibitory effect on mammalian cells, leading to the possibility of novel lead compounds for selective treatment.  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