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   287803篇
  免费   23897篇
  国内免费   14814篇
电工技术   20977篇
技术理论   51篇
综合类   38644篇
化学工业   29187篇
金属工艺   10647篇
机械仪表   18917篇
建筑科学   38911篇
矿业工程   15561篇
能源动力   8502篇
轻工业   14616篇
水利工程   14459篇
石油天然气   11543篇
武器工业   3066篇
无线电   21563篇
一般工业技术   21723篇
冶金工业   16164篇
原子能技术   3350篇
自动化技术   38633篇
  2024年   433篇
  2023年   2596篇
  2022年   4901篇
  2021年   6253篇
  2020年   6646篇
  2019年   5471篇
  2018年   5202篇
  2017年   6565篇
  2016年   7934篇
  2015年   8985篇
  2014年   16254篇
  2013年   14623篇
  2012年   19414篇
  2011年   20613篇
  2010年   16252篇
  2009年   16945篇
  2008年   16107篇
  2007年   20895篇
  2006年   19750篇
  2005年   17198篇
  2004年   14717篇
  2003年   13201篇
  2002年   10862篇
  2001年   9228篇
  2000年   7794篇
  1999年   6233篇
  1998年   4693篇
  1997年   4277篇
  1996年   3886篇
  1995年   3291篇
  1994年   2964篇
  1993年   2229篇
  1992年   1924篇
  1991年   1422篇
  1990年   1258篇
  1989年   1086篇
  1988年   871篇
  1987年   603篇
  1986年   436篇
  1985年   377篇
  1984年   366篇
  1983年   261篇
  1982年   239篇
  1981年   176篇
  1980年   137篇
  1979年   114篇
  1978年   62篇
  1977年   80篇
  1976年   61篇
  1975年   59篇
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
101.
Metal silos can reduce maize losses during storage in smallholder farm systems, contributing towards international goals of reducing food waste and moving towards Sustainable Development Goal 2, Zero Hunger. However, technologies are introduced into farming systems with complex sets of relationships. These may differentially affect the ability of women and men to secure the expected benefits of metal silos. This, in turn, may affect adoption rates and expected outcomes. To better understand these relationships, a mixed methods study was conducted in Kenya, Malawi, Zimbabwe and Zambia where metal silos were introduced through the Effective Grain Storage Project (EGSP). EGSP distributed 1717 metal silos across the four countries.Quantitative surveys using random sampling were conducted in Kenya (124 respondents) and Malawi (100 respondents) with metal silo owners. Qualitative surveys, using purposive sampling, were conducted in all four countries covering 14 ethnic groups using focus group discussions (360 respondents), key informant interviews (62 respondents), and household case studies (62 respondents). The aim was to understand gendered post-harvest management and storage strategies in traditional systems and to map changes when metal silos were introduced.The findings suggest roles and responsibilities regarding the ownership and management of storage structures are strongly gendered with some differences between ethnic groups and countries. Men benefit more than women from the introduction of metal silos. Ownership of the grain storage facility, and the benefits attached to ownership, can switch from women to men with women having less scope for bargaining over their rights to use the stored grain for their own needs and the benefit of all household members. Women and men fail to fully follow guidelines for effective use of metal silos. They differ in their preferred attributes for their ideal storage system. Whilst some attributes are shared, women’s preferences were sharply differentiated from men in others.  相似文献   
102.
Colostrum is essential for good neonate health; however, it is not known whether different calves absorb the nutrients from colostrum equally well. In this study, the absorption of protein, IgG, and γ-glutamyl transferase was compared in newborn dairy bull calves for 1 wk after feeding colostrum from different sources. Thirty-five Holstein-Friesian bull calves were randomly allocated into 3 groups and fed colostrum within 4 h after birth. Group A calves (n = 12) were bottle fed colostrum from their own dam for 3 d. Colostrum from these group A cows was also used as foster cow colostrum for the group B calves (n = 12), such that each group A and B calf pair received identical colostrum from each milking of the respective group A dam (10% of birth weight per day). The group C calves (n = 11) were fed 1 bottle (2 L) of pooled colostrum and transition milk (referred to as pooled colostrum), as was the standard practice on the dairy farm. The pooled colostrum was collected from the other dairy cows on the farm 0 to 4 d postpartum and stored at 4°C for less than 12 h. Blood was sampled from calves before the first feeding and at 1, 2, 3, and 7 d after birth. Levels of total solids, total protein, and IgG were higher in the dam colostrum than in the pooled colostrum. At birth, there were no differences between the calf groups for any measurements, and all calves had very low IgG levels. After receiving colostrum, the glucose, plasma γ-glutamyl transferase, serum total protein, and IgG concentrations increased significantly in all calves. There were no differences in any blood measurements at any time point between the pairs of group A and group B calves that received colostrum from the same cow except for the IgG concentration 2 d after birth. However, the group A calves had a higher total serum protein level and IgG concentration than the group C calves for all the time points after the first feeding. The group B calves had a higher IgG concentration than the group C calves on d 1, 2, and 7 after birth. Compared with groups A and B, there was no difference in the proportion of calves in group C that failed to have passive immunity transferred adequately based on the IgG threshold (<10 g/L). Thus, the calves receiving identical colostrum from the same cow had the same levels of IgG, and even the pooled colostrum provided sufficient transfer of IgG as the calves were fed within 4 h after birth.  相似文献   
103.
在城市化快速进程的背景下,城市 街区PM2.5污染日益严重,本文选取哈尔滨 在不同季节的典型街谷空间,对以叶面积密 度(LA D)、叶面积指数(LAI)为实测要素 的绿色界面指数以及PM2.5浓度进行实测对 比研究。通过对实测数据的分析和挖掘,最 终得出如下结论:首先,典型街谷空间PM2.5 时段浓度呈现上午比下午平均高37.75%,冬 季比夏季高4.7倍的特征;其次,街谷空间的 灌木界面对PM2.5浓度场平均积极贡献率为 18.62%;最后,对PM2.5的衰减率与实测街 谷绿色界面的叶面积密度(LAD)与叶面积 指数(LAI)进行相关性分析,结果显示街谷 绿色界面对PM2.5浓度的衰减作用与叶面积 密度(LAD)呈显著负相关关系,与叶面积指 数(LAI)的相关性程度较弱。  相似文献   
104.
In continuation to my previous work (Guha S. AIChE J. 2013;59(4):1390-1399), in this work, effects of ionic migration are evaluated for disk region of a rotating ring disk electrode system by numerically solving complex differential equations, developed for mass transfer along with kinetic complication in presence of ionic migration under limiting current condition. The system for simulation is 0.01 M Fe2(SO4)3 solution with H2SO4 as supporting electrolyte. Simulation cases are presence and absence of ionic migration with kinetic complication (oxidation of Fe2+ to Fe3+ under O2 pressure). Results show that concentration boundary layer thickness of reactant Fe3+ reduces appreciably and steady-state disk current reduces substantially in presence of migration. Simulated steady-state disk current in absence of migration case agrees well with published data. Results indicate higher Fe2+ concentration in presence of migration and thereby higher rate of oxidation of Fe2+ to Fe3+ at all rate constant values.  相似文献   
105.
Graphite–SiC micro-composites have been prepared in–house by carbothermal reduction process. Controlling the process parameters including the weight ratio of SiO2 to graphite as well as carbothermal reduction temperature during the micro-composite preparation favors the homogeneous formation of SiC with preferred morphologies like ribbons and whiskers/fibers. The micro-composite modified low carbon MgO-C refractories have exhibited significantly improved bulk properties over the standard composition. To understand the beneficial role of SiC reinforcement on hot strength performance under air oxidizing conditions, we propose a scaling parameter known as strength factor (fs) based on the ratio of hot strength (HMOR) to cold strength (CCS). Correlating the strength factor data (fs) with oxidative damage provides new insights into the reinforcing effects of distinct SiC morphologies in this new class of micro-composite fortified refractory systems over the standard compositions.  相似文献   
106.
《Ceramics International》2022,48(5):6758-6766
La2Ce2O7 (LCO) based materials are of a paramount importance since they can be utilized for ammonium production, thermal barrier application, catalysts, hydrogen production and solid oxide fuel cells (SOFCs). In this work, a nano crystalline LCO powder was prepared using glycine-nitrate combustion method and then its properties were comprehensively characterized. The structural analysis of the synthesized LCO was carried out using conventional X-ray diffraction (XRD) and Raman spectroscopy. In a disordered phase, LCO is a biphasic mixture composed of C- and F-type phases. Densification studies were performed by sintering LCO pellets at different sintering temperatures. A densification of ≥95% was observed in all the samples with a very little variation. Sintering temperature had a marked effect on the electrical conductivity of LCO. The LCO sintered at 1100 °C showed the highest conductivity (3.68 mS/cm at 700 °C in air). The electrical conductivity was found to be decreasing with an increase in sintering temperature from 1100 to 1400 °C. To understand the behavior, the analysis of distribution function of relaxation times (DFRTs) utilized for correct separation of grain and grain boundary resistances. The presence of C- and F- type phases calculated from Raman spectra plays a crucial role in deciding conduction behavior of LCO. The results suggest a strong relationship between history of the ceramics preparation and their electrical properties.  相似文献   
107.
Self-adaptive surface measurements that can reduce data redundancy and improve time efficiency are in high demand in many fields of science and technology. For this purpose, a system implemented with Gaussian process (GP) adaptive sampling is developed. The non-parametric GP model is applied to reconstruct the topography and guide the subsequent sampling position, which is determined from the inference uncertainty estimation. A criterion is proposed to terminate the GP adaptive measurement automatically without any prior model or data of the topography. Experiments on typical surfaces validate the intelligence, adaptability, and high accuracy of the GP method along with the stabilization of the automatic iteration termination. Compared with traditional raster sampling, data redundancy is reduced and the time efficiency is improved without sacrificing the surface reconstruction accuracy. The proposed method can be implemented in other systems with similar measurement principles, thus benefitting surface characterizations.  相似文献   
108.
Demands on production, distribution and consumption of electrical energy change fundamentally with the energy revolution. Energy purchasing costs for inflexible consumers are rising and proceeds of inflexible producers are sinking. Companies are able to reduce costs by marketing operational flexibility options. This article sets out the need of flexibility in the energy system and the new marketing options. Flexibility potentials within infrastructure plants of a chemical park are analyzed, evaluated regarding marketing and activated in the presented research project FlexChemistry.  相似文献   
109.
The fashionable Parr–Pearson (PP) atoms-in-molecule/bonding (AIM/AIB) approach for determining the exchanged charge necessary for acquiring an equalized electronegativity within a chemical bond is refined and generalized here by introducing the concepts of chemical power within the chemical orthogonal space (COS) in terms of electronegativity and chemical hardness. Electronegativity and chemical hardness are conceptually orthogonal, since there are opposite tendencies in bonding, i.e., reactivity vs. stability or the HOMO-LUMO middy level vs. the HOMO-LUMO interval (gap). Thus, atoms-in-molecule/bond electronegativity and chemical hardness are provided for in orthogonal space (COS), along with a generalized analytical expression of the exchanged electrons in bonding. Moreover, the present formalism surpasses the earlier Parr–Pearson limitation to the context of hetero-bonding molecules so as to also include the important case of covalent homo-bonding. The connections of the present COS analysis with PP formalism is analytically revealed, while a numerical illustration regarding the patterning and fragmentation of chemical benchmarking bondings is also presented and fundamental open questions are critically discussed.  相似文献   
110.
This paper discusses the capability of Guo et al.'s (2021) equations to determine the discharge of radial gates under submerged flow conditions. It was concluded that Guo et al.'s (2021) equations are associated with error reduction compared to the Incomplete Self-Similarity (ISS) theory and the calibration method. However, it does not have a significant advantage over Energy-Momentum (E-M) approach. Employing E-M principles, new equations were proposed to determine the discharge of radial gates, which has some advantages compared to Guo et al. (2021), such as (1) error reduction under partially and fully submerged flow conditions, (2) least dependence on the empirical constants, (3) uniformity of form over the entire submerged condition, and (4) no need to classify the submerged flow. Field calibration showed that the proposed equations in the present study for a single gate predict the discharge of parallel radial gates with a mean absolute error of less than 4.5% subject to the submerged operation of all open gates.  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