全文获取类型
收费全文 | 4866篇 |
免费 | 601篇 |
国内免费 | 392篇 |
专业分类
电工技术 | 268篇 |
综合类 | 445篇 |
化学工业 | 729篇 |
金属工艺 | 177篇 |
机械仪表 | 355篇 |
建筑科学 | 333篇 |
矿业工程 | 195篇 |
能源动力 | 91篇 |
轻工业 | 1009篇 |
水利工程 | 150篇 |
石油天然气 | 646篇 |
武器工业 | 50篇 |
无线电 | 277篇 |
一般工业技术 | 341篇 |
冶金工业 | 181篇 |
原子能技术 | 72篇 |
自动化技术 | 540篇 |
出版年
2024年 | 27篇 |
2023年 | 100篇 |
2022年 | 211篇 |
2021年 | 261篇 |
2020年 | 224篇 |
2019年 | 206篇 |
2018年 | 190篇 |
2017年 | 204篇 |
2016年 | 255篇 |
2015年 | 231篇 |
2014年 | 297篇 |
2013年 | 302篇 |
2012年 | 371篇 |
2011年 | 372篇 |
2010年 | 243篇 |
2009年 | 270篇 |
2008年 | 219篇 |
2007年 | 266篇 |
2006年 | 256篇 |
2005年 | 204篇 |
2004年 | 194篇 |
2003年 | 170篇 |
2002年 | 134篇 |
2001年 | 86篇 |
2000年 | 94篇 |
1999年 | 98篇 |
1998年 | 64篇 |
1997年 | 50篇 |
1996年 | 39篇 |
1995年 | 43篇 |
1994年 | 42篇 |
1993年 | 24篇 |
1992年 | 18篇 |
1991年 | 13篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1977年 | 2篇 |
1972年 | 2篇 |
1964年 | 2篇 |
1959年 | 1篇 |
1956年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有5859条查询结果,搜索用时 31 毫秒
41.
42.
灞河流域气候因子对水沙变化的影响 总被引:1,自引:0,他引:1
利用灞河流域蓝田气象站和马渡王水文站1960—2012年的气象、水文实测资料,分析灞河流域气候及水沙变化规律,同时运用相关性分析、灰色关联分析、多元线性回归模型等多种方法探讨了该流域水沙变化与气候变化的关系。结果表明:灞河流域降雨量、蒸发量、径流量和输沙量皆呈显著下降趋势,而气温呈上升趋势;降雨量与水沙都有重要的相关关系,1960—1990年影响径流量的气候因子敏感度由大至小依次为降雨量、气温、蒸发量,而1991—2012年则为降雨量、蒸发量、气温,当气温和蒸发量不变时,降雨量每增加1 mm,两阶段的年径流量分别增加0.14亿m3和0.08亿m3;1960—2012年影响输沙量的气候因子敏感度由大至小依次为降雨量、气温、蒸发量,当气温和蒸发量不变时,降雨量每增加1 mm,年输沙量增加0.668万t。 相似文献
43.
INTROGRESSING PHEROMONE QTL BETWEEN SPECIES: TOWARDS AN EVOLUTIONARY UNDERSTANDING OF DIFFERENTIATION IN SEXUAL COMMUNICATION 总被引:2,自引:0,他引:2
Groot AT Ward C Wang J Pokrzywa A O'Brien J Bennett J Kelly J Santangelo RG Schal C Gould F 《Journal of chemical ecology》2004,30(12):2495-2514
As a first step toward understanding how noctuid moths evolve species-specific pheromone communication systems, we hybridized and backcrossed two closely related moth species, Heliothis virescens (Hv) and H. subflexa (Hs), which differ qualitatively and quantitatively in their multi-component sex pheromone blends. We used amplified fragment length polymorphism (AFLP) marker-based mapping of backcross families to determine which of the 30 autosomes in these moths contained quantitative trait loci (QTL) controlling the percentages of specific chemical components in the pheromone blends. In two previous backcrosses to Hs, we found a strong depressive effect of Hv-chromosome 22 on the percentage of three acetate components in the pheromone gland. These acetates are present in Hs and absent in Hv. Here, we describe how we introgressed Hv-chromosome 22 into the genomic background of Hs. Selection for Hv-chromosome 22 started from backcross 3 (BC3) females. All females that had Hv-chromosome 22 and a low percentage of acetates (< 3% of the total amount of pheromone components present) were backcrossed to Hs males. In BC5 to BC8, we determined whether Hv-chromosome 22 was present by a) running only the primer pairs that would yield the markers for that chromosome, and/or b) determining the relative percentages of acetates in the pheromone glands. Either or both genotype and phenotype were used as a criterion to continue to backcross these females to Hs males. In BC9, we confirmed the isolation of Hv-chromosome 22 in the Hs genomic background, and backcrossed the males to Hs females to eliminate the Hv-sex chromosome as well as mitochondrial DNA. The pheromone composition was determined in BC3, BC5, and BC11 females with and without Hv-chromosome 22. All backcross females with Hv-chromosome 22 contained significantly less acetates than females without this chromosome. In addition, BC3 females with Hv-chromosome 22 contained significantly more Z11-16:OH than BC3 females without Hv-chromosome 22. However, in BC5 and BC11 females, the correlation between Z11-16:OH and Hv-chromosome 22 was lost, suggesting that there are separate QTL for the acetates and for Z11-16:OH, and that the relative amount of the alcohol component is only affected in epistasis with other (minor) QTL. Now that we have succeeded in isolating the chromosome that has a major effect on acetate production, we can test in behavioral experiments whether the presence of acetates may have been a driving force for a shift in pheromone composition. Such tests are necessary to move towards an evolutionary understanding of the differentiation in sexual communication in Heliothis spp. moths. 相似文献
44.
水杨酸双嘧啶(钠)的化学定量分析方法 总被引:2,自引:0,他引:2
本文介绍了以常规的化学分析方法替代文献报道的高压液相色谱法对稻田除草剂水杨酸双嘧啶(钠)进行定量分析的研究。 相似文献
45.
The application of combined g.c.-m.s. with automated data processing to the rapid analysis of coal solvents has been described. An analytical method, using commercially available data processing routines, has been developed for the automatic quantitative analysis of specific aromatic and hydroaromatic compounds in coal processing solvents. To demonstrate the use of this technique, raw anthracene oil was hydrogenated under different conditions and the resulting hydrogenated anthracene oils were analysed. The results showed correlations between the hydrogen content and the absolute concentrations of specific compounds in the samples. 相似文献
46.
行政监督量化考核指标虽然实施数年,对这些指标量之间的关系和作用的分析却很少涉及,为了提高行政监督考核量化指标的设置,分析指标间的相互关系,利用行政监督管信息管理系统的数据,应用主成分分析方法对不同业务的量化分级管理数据进行处理,依照不同的贡献率从数据中提炼出主信息,可去掉变量之间的多重相关性,简化考核指标项,指导优化行政监督过程,提高效率、降低成本。 相似文献
47.
48.
49.
定量~(13)C-NMR谱表征蔗渣碱木素结构 总被引:4,自引:0,他引:4
从蔗渣纸浆黑液中制出纯蔗渣碱木素,然后测定其定量13C-NMR谱.图谱分析表明,蔗渣碱木素主要由紫丁香基,愈创木基和对羟苯基三种丙烷结构单元组成,其中未绩合的紫丁香基、愈创木基和对羟羊基含量比为2.2:1.0:1.4.蔗渣碱术素结构单元间主要以β-O-4烷基芳基醚键连接,也有一定量的非环状α-醚键存在.蔗渣碱木素中,相当部分对香豆酸类结构并非通过酯键而是通过醚键或碳双键连接于芳核上. 相似文献
50.