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   4462篇
  免费   402篇
  国内免费   6篇
电工技术   38篇
综合类   4篇
化学工业   1064篇
金属工艺   61篇
机械仪表   101篇
建筑科学   156篇
矿业工程   6篇
能源动力   130篇
轻工业   870篇
水利工程   64篇
石油天然气   16篇
无线电   220篇
一般工业技术   673篇
冶金工业   788篇
原子能技术   5篇
自动化技术   674篇
  2024年   17篇
  2023年   51篇
  2022年   106篇
  2021年   173篇
  2020年   119篇
  2019年   148篇
  2018年   206篇
  2017年   210篇
  2016年   240篇
  2015年   175篇
  2014年   189篇
  2013年   362篇
  2012年   304篇
  2011年   362篇
  2010年   219篇
  2009年   189篇
  2008年   181篇
  2007年   204篇
  2006年   120篇
  2005年   93篇
  2004年   87篇
  2003年   80篇
  2002年   94篇
  2001年   51篇
  2000年   39篇
  1999年   50篇
  1998年   281篇
  1997年   166篇
  1996年   106篇
  1995年   50篇
  1994年   35篇
  1993年   40篇
  1992年   9篇
  1991年   15篇
  1990年   8篇
  1989年   5篇
  1988年   9篇
  1987年   8篇
  1986年   3篇
  1985年   10篇
  1984年   4篇
  1983年   2篇
  1981年   7篇
  1980年   5篇
  1978年   6篇
  1977年   7篇
  1976年   23篇
  1973年   1篇
  1969年   1篇
排序方式: 共有4870条查询结果,搜索用时 15 毫秒
971.
Development of extruded films composed of biopolymers blended with synthetic polymers aims to minimize the environmental impact of plastic waste-materials and lead to the sustainable plastic industry. To produce biodegradable polymeric blends, the weight content of biopolymers must be maximum without compromising the performance properties of the extruded films. Using a solvent-free extrusion method, films composed of polypropylene, poly(lactic-acid), and Chitosan, can be obtained with the use of polypropylene-graft-maleic anhydride and glycerol as compatibilizer and plasticizer, respectively. Extruded films with up to a 50 wt% content of biopolymers show acceptable thermal and mechanical properties, where the use of compatibilizer improves the processing characteristics and homogeneous distribution of chitosan throughout the films. Therefore, the extruded films can be considered as alternatives to conventional synthetic-polymer films, due to their acceptable mechanical and thermal properties with direct potential applications in extrusion-method mass production of biodegradable polymers.  相似文献   
972.
In a world with great challenges in food security, optimising cereal production is critical. Cereals are the most important food source for human consumption. The fourth important cereal worldwide after wheat, rice and maize, is Barley, and its production strongly depends on fertilization treatments. The adoption of suitable fertilizer management strategies often results in large economic benefits to producers. However, determining optimal fertilizer doses for a specific barley variety is complex. The collection of data and their analyses can be cost prohibitive for small farmers regarding time and money. This paper introduces an approach to support producers with automatic tools for the analysis of fertilization management of barley. The proposed methodology aims to simultaneously estimate nitrogen fertilization and barley yield, from information derived from aerial RGB images captured by a UAV. Our long term goal is to provide a low-cost and wide-are-coverage solution for the estimation of barley variables that can be leveraged to increase barley yield without increasing costs. A low-cost UAV is used to capture RGB crop field images. Then, a deep convolutional neural network is used for the automated extraction of features from the images. Extracted features are feed into predictive models that estimate the variables of interest. Experimental results reveal that the proposed methodology is able to reach an accuracy above 83% when estimating nitrogen fertilization and a high correlation and low RMSE in the estimation of yield in grams. Experimental results are promising and will pave the way for the development of deep learning methods for barley analysis from aerial imagery that can be accessed by the average farmer.  相似文献   
973.
974.
Iron-containing garnets (YIG) were used as catalysts for selective oxidation of ethylbenzene (EB) in the presence of H2O2 as oxidant. The catalysts comprising of two series of garnets e.g., Y3(Fe1–xZnx)5O12 and Y3(Fe1–xNix)5O12 had distinct Zn and Ni contents (x = 0.00 0.01, 0.03, and 0.05). XRD, Raman and FTIR spectroscopies revealed that the cubic structure of Y3Fe5O12 garnet was present for x = 0.00 and 0.01. For higher contents, the garnets had the Y3Fe5O12 phase besides hematite (α-Fe2O3). The catalytic activity was dependent on the contents of metals in the garnets with Y3Fe4·97Ni0·03O12-γ and Y3Fe4·95Zn0·05O12-γ catalysts achieving better results. The influence of the reaction conditions such as reaction time, reaction temperature and effect of the solvents as well as the substrates to H2O2 molar ratios were studied. SEM-EDS, XPS and EPR results demonstrated the affinity of the Fe2+/Fe3+ pairs with Ni2+ species for the ethylbenzene molecule, which gave an EB conversion of 77% with a good production of acetophenone over the Y3Fe4·97Ni0·03O12-γ catalyst compared to other binary and ternary solids.  相似文献   
975.
976.
Before availability of dense SNP data, genetic diversity was characterized and managed with pedigree-based information. Besides this classical approach, 2 methodologies have been proposed in recent years to characterize and manage diversity from dense SNP data: the SNP-by-SNP approach and the alternative based on runs of homozygosity (ROH). The establishment of criteria to identify ROH is a current constraint in the literature dealing with ROH. The objective of this study was, using a medium-density SNP chip, to quantify by 3 methods (pedigree, SNP-by-SNP, and ROH) the genetic diversity on 5 selected French dairy sheep subpopulations and breeds and to assess the effect of the definition of ROH on these estimates. The data set available included individuals from the breeds Basco-Béarnaise, Manech Tête Noire, Manech Tête Rousse, and 2 subpopulations of Lacaune: Lacaune Confederation and Lacaune Ovitest. Animals were genotyped with the Illumina OvineSNP50 BeadChip (Illumina Inc., San Diego, CA). After filtering, the genomic data included 38,287 autosomal SNP and 8,700 individuals, which comprised 72,803 animals in the pedigree. The results indicated that no significant differences were observed in effective population size estimates obtained from pedigree or genomic (SNP-by-SNP or ROH) information. In general, estimates of effective population size were above 200 in Lacaune Confederation and Lacaune Ovitest subpopulations and below 200 in Basco-Béarnaise, Manech Tête Noire, and Manech Tête Rousse breeds. The minimum length that constituted a ROH, the minimum number of SNP that constituted a ROH, as well as the minimum density and the maximum distance allowed between 2 homozygous SNP are ROH-defining factors with important implications in the estimation of the rate of inbreeding. The ROH-based rates of inbreeding in concordance with those obtained from pedigree information require a specific set of values. This particular set of values is different from that identified to obtain ROH-based rates of inbreeding similar to those obtained on a SNP-by-SNP basis. Factors to define ROH do not change the results much unless extreme values are considered, although further research on ROH-based inbreeding is still required.  相似文献   
977.
Caprine whey protein concentrates (WPC) were incorporated as emulsifiers in a salad dressing. The concentrates were manufactured by ultrafiltration-diafiltration with or without previous clarification by thermocalcic precipitation. Aggregates obtained in the clarification treatment and a commercial bovine WPC were also used as emulsifiers. Good emulsifying properties of caprine products were observed. Dressings made with caprine products showed higher firmness and stability than dressings made with bovine WPC. Thermocalcic precipitation improved the properties of caprine WPC. Aggregates modified the colour of dressings contributing to a more yellowish aspect. Microstructure of emulsions was affected by the type of protein.  相似文献   
978.
Leaf rust and powdery mildew are two important foliar diseases in wheat. A recombinant inbred line (RIL) population, obtained by crossing two bread wheat cultivars (‘Victo’ and ‘Spada’), was evaluated for resistance to the two pathogens at seedling stage. Upon developing a genetic map of 8726 SNP loci, linkage analysis identified three resistance Quantitative Trait Loci (QTLs), with ‘Victo’ contributing the resistant alleles to all loci. One major QTL (QPm.gb-7A) was detected in response to Blumeria graminis on chromosome 7A, which explained 90% of phenotypic variation (PV). The co-positional relationship with known powdery mildew (Pm) resistance loci suggested that a new source of resistance was identified in T. aestivum. Two QTLs were detected in response to Puccinia triticina: a major gene on chromosome 5D (QLr.gb-5D), explaining a total PV of about 59%, and a minor QTL on chromosome 2B (QLr.gb-2B). A positional relationship was observed between the QLr.gb-5D with the known Lr1 gene, but polymorphisms were found between the cloned Lr1 and the corresponding ‘Victo’ allele, suggesting that QLr.gb-5D could represent a new functional Lr1 allele. Lastly, upon anchoring the QTL on the T. aestivum reference genome, candidate genes were hypothesized on the basis of gene annotation and in silico gene expression analysis.  相似文献   
979.
980.
Two new cytotoxic antitumor diterpenoids of the cembrane class, named 14-deoxycrassin (3) and pseudoplexaurol (4), have been isolated from the Caribbean gorgonian octocoral Pseudoplexaura porosa. The structure of lactone 3, possessing the infrequently encountered alpha-methylene-delta-lactone ring, has been established from spectral and chemical data and that of alcohol 4 has been established from spectral data.  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