全文获取类型
收费全文 | 6283篇 |
免费 | 627篇 |
国内免费 | 469篇 |
专业分类
电工技术 | 544篇 |
综合类 | 582篇 |
化学工业 | 662篇 |
金属工艺 | 379篇 |
机械仪表 | 518篇 |
建筑科学 | 443篇 |
矿业工程 | 163篇 |
能源动力 | 112篇 |
轻工业 | 834篇 |
水利工程 | 256篇 |
石油天然气 | 121篇 |
武器工业 | 62篇 |
无线电 | 570篇 |
一般工业技术 | 474篇 |
冶金工业 | 704篇 |
原子能技术 | 86篇 |
自动化技术 | 869篇 |
出版年
2024年 | 44篇 |
2023年 | 108篇 |
2022年 | 199篇 |
2021年 | 336篇 |
2020年 | 210篇 |
2019年 | 125篇 |
2018年 | 150篇 |
2017年 | 166篇 |
2016年 | 149篇 |
2015年 | 280篇 |
2014年 | 348篇 |
2013年 | 407篇 |
2012年 | 458篇 |
2011年 | 463篇 |
2010年 | 484篇 |
2009年 | 475篇 |
2008年 | 446篇 |
2007年 | 448篇 |
2006年 | 408篇 |
2005年 | 315篇 |
2004年 | 218篇 |
2003年 | 134篇 |
2002年 | 123篇 |
2001年 | 138篇 |
2000年 | 110篇 |
1999年 | 51篇 |
1998年 | 153篇 |
1997年 | 102篇 |
1996年 | 58篇 |
1995年 | 37篇 |
1994年 | 25篇 |
1993年 | 34篇 |
1992年 | 13篇 |
1991年 | 23篇 |
1990年 | 22篇 |
1989年 | 18篇 |
1988年 | 16篇 |
1987年 | 13篇 |
1986年 | 12篇 |
1985年 | 6篇 |
1982年 | 2篇 |
1980年 | 6篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 10篇 |
1976年 | 26篇 |
1975年 | 2篇 |
1965年 | 2篇 |
1959年 | 1篇 |
1951年 | 1篇 |
排序方式: 共有7379条查询结果,搜索用时 8 毫秒
21.
We examined the effects of different cytokine combinations and culture conditions on the expansion and modulation of cell surface antigens of CD34+ derived dendritic cells (DCs), the most efficient antigen-presenting cells capable of stimulating resting T cells in the primary immune response. Cells with a dendritic morphology and expressing HLA-DR, CD1a, S100 and CD83 were maximally expanded under serum-free conditions with the addition of SCF, GM-CSF, TNF-alpha, TGF-beta and Flt-3 ligand (fold increase of CD1a+ cells = 102 +/- 32 after 2 weeks of culture). CD34+ cells were also grown under continuous flow conditions in an artificial capillary system: after 14d of culture, the expansion in the total cell number was lower than that of the static cultures (3.3 +/- 2 v 18.9 +/- 4) but the percentage of CD1a+/CD83+/ CD80+ cells was considerably higher, whereas the CD14+ cells were significantly reduced (8.9 +/- 2 v 26 +/- 13). In continuous perfusion cultures, low levels of DC precursors and of LTC-IC were still present up to day 14. The DCs generated under flow conditions stimulated the mixed leucocyte reaction (MLR) more than the cells grown in static cultures. By electron microscopy, cells grown in the continuous flow system showed an increased number of large cells with numerous dendritic processes and abundant multilamellar complexes. The cells expanded under these conditions were sorted on the basis of their light-scatter properties into two fractions: one containing a predominance of CD1a+/S100+/ CD8 3+/CD80+/CD14- 'large cells' with great internal complexity (mature DCs); the second including 'small cells' either CD33+/CD14+, CD33+/CD15+ or CD33+/CD13-/CD14. The DCs generated and selected with this method are therefore particularly well suited for immunotherapeutic protocols. 相似文献
22.
Effect of routine screening for Down's syndrome on the significance of isolated fetal hydronephrosis
OBJECTIVE: To determine the risk of Down's syndrome in fetuses with isolated hydronephrosis at 18-23 weeks in an unselected general population after routine screening for Down's syndrome, using first trimester nuchal translucency measurement and second trimester maternal serum biochemistry. POPULATION: All pregnant women undergoing a routine 18-23 week ultrasound scan, from a population who had been offered screening for Down's syndrome. SETTING: A district general hospital serving a low risk obstetric population. METHODS: Prospective study of all routine 18-23 weeks ultrasound scans. The prevalence of isolated hydronephrosis and Down's syndrome was determined and the relative risk for Down's syndrome was calculated for different ultrasound findings. RESULTS: 10,971 women were scanned at 18-23 weeks during the study period. Down's syndrome was diagnosed in 14 of 20 cases before this stage using first trimester nuchal translucency measurement and second trimester maternal serum biochemistry. Isolated fetal hydronephrosis was diagnosed in 423 pregnancies (3.9%); none of these pregnancies were affected by Down's syndrome. The relative risk for Down's syndrome was 0.18 (95% CI 0.06-0.53) for women with a normal scan (n = 9983). When multiple ultrasound markers were found (n = 565), the relative risk for Down's syndrome was 2.00 (95% CI 0.18-22.10) and 9.00 (95% CI 1.14-71.30) for all other aneuploidies. CONCLUSION: The finding of isolated fetal hydronephrosis does not significantly increase the age-related risk for Down's syndrome. The presence of multiple ultrasound markers is associated with an increased risk of aneuploidies other than Down's syndiome. These findings are explained by the reduced prevalence of Down's syndrome as a result of prior screening and diagnosis of this condition. 相似文献
23.
24.
为提升雄安新区应对气候变化和急性冲击灾害的适应能力,建设高韧性高弹性的城市水系统,实现新区水系统的安全运行与可持续发展,构建了基于不确定性的雄安新区城市水系统安全保障技术.该技术包括水系统的风险源识别、风险综合评价和风险管理策略3部分.在应对气候变化方面,提出了"气候变化模拟预估—气候变化增量分析—水系统影响风险分析"的技术框架、不同精度的气候变化增量分析方法,以及分层级的城市水系统气候变化影响与风险分析方法.在应对急性冲击方面,提出了有效识别城市水系统面临的急性冲击风险源识别方法,以及急性冲击灾害多情景多灾种风险分析方法.在安全保障机制方面,提出了建立雄安新区水系统应急预案体系的要求,以及区域协同、综合应对、专业负责和网格化管理的"城市-组团-社区"三级风险管控机制. 相似文献
25.
26.
27.
28.
29.
为完整刻画柔性直流配电网在满足N-1安全准则前提下的最大允许运行范围,提出了柔性直流配电网的N-1安全域模型与解析化求解方法.首先讨论了柔性直流配电网的典型接线模式与N-1事件前后的稳态特性;在此基础上建立了柔性直流配电网的N-1安全域模型,分析了其与交流配电网安全域模型的区别及其能适用于柔性直流配电网的原因;然后通过DistFlow支路潮流线性化并配合下垂节点的处理,实现了模型的解析化求解;最后通过算例验证了模型与求解方法的有效性.结果 表明:所提模型与求解方法可用于获得柔性直流配电网在主从控制、电压裕度控制、下垂控制下的安全域解析表达式;不同控制策略下安全域大小的规律是电压裕度控制下小于主从控制下,下垂控制下小于等于主从控制下. 相似文献
30.
论述1,4-丁烯二醇发展现状,简述1,4-丁烯二醇BYD加氢法、3,4-环氧-1-丁烯水解法和1,3-丁二烯法3种生产工艺特点.其中,BYD催化加氢法是生产BED技术最为成熟、应用最为广泛的生产工艺.BYD加氢反应是一个多步复杂反应.论述了BYD加氢催化反应过程以及3种催化剂(贵金属催化剂、过渡金属催化剂、骨架型Raney-Ni催化剂)在BYD加氢反应过程中的应用特点,指出Raney-Ni催化剂具有价格低、活性高的优点,开发中压下BYD加氢制备BED的Raney-Ni催化剂具有重要意义. 相似文献