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   820篇
  免费   45篇
  国内免费   2篇
电工技术   5篇
化学工业   196篇
金属工艺   8篇
机械仪表   17篇
建筑科学   64篇
矿业工程   2篇
能源动力   23篇
轻工业   165篇
水利工程   12篇
石油天然气   2篇
无线电   28篇
一般工业技术   141篇
冶金工业   91篇
原子能技术   5篇
自动化技术   108篇
  2023年   13篇
  2022年   21篇
  2021年   47篇
  2020年   22篇
  2019年   28篇
  2018年   28篇
  2017年   18篇
  2016年   26篇
  2015年   25篇
  2014年   33篇
  2013年   70篇
  2012年   41篇
  2011年   66篇
  2010年   49篇
  2009年   50篇
  2008年   31篇
  2007年   40篇
  2006年   33篇
  2005年   19篇
  2004年   20篇
  2003年   22篇
  2002年   15篇
  2001年   13篇
  2000年   16篇
  1999年   6篇
  1998年   16篇
  1997年   14篇
  1996年   9篇
  1995年   6篇
  1994年   5篇
  1993年   7篇
  1992年   9篇
  1991年   3篇
  1990年   4篇
  1989年   8篇
  1987年   7篇
  1983年   2篇
  1981年   1篇
  1979年   1篇
  1978年   1篇
  1977年   2篇
  1976年   6篇
  1974年   1篇
  1958年   1篇
  1957年   1篇
  1956年   1篇
  1954年   1篇
  1926年   1篇
  1924年   2篇
  1922年   2篇
排序方式: 共有867条查询结果,搜索用时 17 毫秒
1.
In eukaryotes, microRNAs (miRNAs) have roles in development, homeostasis, disease and the immune response. Recent work has shown that plant and mammalian miRNAs also mediate cross-kingdom and cross-domain communications. However, these studies remain controversial and are lacking critical mechanistic explanations. Bacteria do not produce miRNAs themselves, and therefore it is unclear how these eukaryotic RNA molecules could function in the bacterial recipient. In this review, we compare and contrast the biogenesis and functions of regulatory RNAs in eukaryotes and bacteria. As a result, we discovered several conserved features and homologous components in these distinct pathways. These findings enabled us to propose novel mechanisms to explain how eukaryotic miRNAs could function in bacteria. Further understanding in this area is necessary to validate the findings of existing studies and could facilitate the use of miRNAs as novel tools for the directed remodelling of the human microbiota.  相似文献   
2.
The vast chemical and structural tunability of metal–organic frameworks (MOFs) are beginning to be harnessed as functional supports for catalytic nanoparticles spanning a range of applications. However, a lack of straightforward methods for producing nanoparticle-encapsulated MOFs as efficient heterogeneous catalysts limits their usage. Herein, a mixed-metal MOF, NiMg-MOF-74, is utilized as a template to disperse small Ni nanoclusters throughout the parent MOF. By exploiting the difference in Ni O and Mg O coordination bond strength, Ni2+ is selectively reduced to form highly dispersed Ni nanoclusters constrained by the parent MOF pore diameter, while Mg2+ remains coordinated in the framework. By varying the ratio of Ni to Mg in the parent MOF, accessible surface area and crystallinity can be tuned upon thermal treatment, influencing CO2 adsorption capacity and hydrogenation selectivity. The resulting Ni nanoclusters prove to be an active catalyst for CO2 methanation and are examined using extended X-ray absorption fine structure and X-ray photoelectron spectroscopy. By preserving a segment of the Mg2+-containing MOF framework, the composite system retains a portion of its CO2 adsorption capacity while continuing to deliver catalytic activity. The approach is thus critical for designing materials that can bridge the gap between carbon capture and CO2 utilization.  相似文献   
3.
Nonnative plant species are causing enormous ecological and environmental impacts from local to global scale. Remote sensing images have had mixed success in providing spatial information on land cover characteristics to land managers that increase effective management of invasions into native habitats. However, there has been limited evaluation of the use of hyperspectral data and processing techniques for mapping specific invasive species based on their spectral characteristics. This research evaluated three different methods of processing hyperspectral imagery: minimum noise fraction (MNF), continuum removal, and band ratio indices for mapping iceplant (Carpobrotus edulis) and jubata grass (Cortaderia jubata) in California's coastal habitat. Validation with field sampling data showed high mapping accuracies for all methods for identifying presence or absence of iceplant (97%), with the MNF procedure producing the highest accuracy (55%) when the classes were divided into four different densities of iceplant.  相似文献   
4.
Longitudinal analyses on normal versus impaired functioning across 7 domains were conducted in children who had experienced profound institutional deprivation up to the age of 42 months and were adopted from Romania into U.K. families. Comparisons were made with noninstitutionalized children adopted from Romania and with nondeprived within-U.K. adoptees placed before the age of 6 months. Specifically, the validity of the assessment, the degree of continuity and change in levels of functioning from 6 to 11 years, and the factors in the pre- and postadoption environment accounting for heterogeneity in outcome were examined. Pervasive impairment was significantly raised in children experiencing institutional deprivation for ≥6 months of life, with a minority within this group showing no impairment. There was no additional significant effect of duration of deprivation beyond the 6-month cutoff, and few other predictors explained outcome. The pattern of normality/impairment was mainly established by 6 years of age, with considerable continuity at the individual level between 6 and 11 years. The findings are discussed in terms of the possibility of a sensitive period for development. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
5.
6.
Abstract— The development of singular fields for defects in steadily loaded creeping structures is examined as a function of geometry and crack size using finite-element analyses and results from the literature. Approximations for estimating the time-development of the amplitude of the fields are examined and it is shown that simple approximations are expected to apply for a wide range of geometries. It is shown both theoretically and numerically that plasticity on initial loading produces crack tip fields closer to the steady state condition than those produced elastically.  相似文献   
7.
Previous studies have found that in American culture high-arousal positive states (HAP) such as excitement are valued more and low-arousal positive states (LAP) such as calm are valued less than they are in Chinese culture. What specific factors account for these differences? The authors predicted that when people and cultures aimed to influence others (i.e., assert personal needs and change others' behaviors to meet those needs), they would value HAP more and LAP less than when they aimed to adjust to others (i.e., suppress personal needs and change their own behaviors to meet others' needs). They test these predictions in 1 survey and 3 experimental studies. The findings suggest that within and across American and Chinese contexts, differences in ideal affect are due to specific interpersonal goals. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
8.
In principle, questions about the behaviour of high-temperature structures can be answered by analysis in conjunction with material creep properties. However, because of the wide range of geometries in use, simplified methods have been developed which avoid lengthy computations yet isolate the important factors controlling component behaviour. This paper describes one approach which is to define a reference stress such that the component life is equal to the life of a simple specimen tested at the reference stress. A substantial body of work has shown that the reference stress can often be established quite simply even for complex components containing cracks. The approach then provides a simple framework for assessing structures which operate at high temperatures.  相似文献   
9.
Marks the death of John Bowlby, whose Attachment Theory (AT) has had greater impact on American psychology than any theory of personality development since Freud's. Bowlby believed in the importance of real-life experiences in the etiology of mental disorders, rather than in the fantasies that then-current psychoanalytic theory emphasized. He was convinced of the importance to a child's personality development of interaction with its parents. In 1948, he established a research unit at the Tavistock Clinic to continue his work on the effects of early separation and identified 3 phases of response: protest, despair, and detachment. Inspired by the work of Konrad Lorenz, Bowlby seized on ethology and Darwinian evolutionary theory as the foundation of AT. First applied to social development in infancy, AT is now leading to research in later childhood and adulthood. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
10.
In this work a novel amperometric biosensor for fructose determination in solutions was developed. The device was constructed by the incorporation of a tetrathiofulvalene-tetracyanoquinodimethane organic conducting salt and fructose dehydrogenase enzyme, include in a polymeric matrix of epoxy resin and graphite powder. Because of the electrocatalytic function of the salt, the direct transfer of the electron between the reduced prosthetic group (PQQH2) of the enzyme and the transducing material, was verified at a low working potential (150 mV vs. Ag/AgCl), where the interfering reactions were minimized. The response time at 90% of the steady state value was less than 20 s. The current response was directly proportional to the D-fructose concentration from 0.01 to 0.3 mmol/l with a detection limit of 0.005 mmol/l (signal/noise of 3) and a sensitivity of 1.9985 μA/mmol. The biosensor sensitivity diminishes when its surface is not polished between successive determinations, and remains constant (rsd=1.85, n=10) when the surface is polished between determinations. The effects of temperature and pH on the biosensor response were studied and analyzed; also the properties of the enzyme (Km ap, I max, Q10) were determinate in this work. The biosensor was used to determine fructose in high fructose syrups and there were not significant differences between these results and those obtained by HPLC (p≤0.05). During 4 months, in intermittent determinations the biosensor kept 100% of its original sensitivity and after 18 months stored at 4°C, it only lost 32% of its sensitivity. The simplicity, low working potential, high stability and good performance of this biosensor shows a great potential for its use in the fructose determination.  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