全文获取类型
收费全文 | 6521篇 |
免费 | 646篇 |
国内免费 | 365篇 |
专业分类
电工技术 | 644篇 |
综合类 | 500篇 |
化学工业 | 670篇 |
金属工艺 | 418篇 |
机械仪表 | 369篇 |
建筑科学 | 358篇 |
矿业工程 | 236篇 |
能源动力 | 152篇 |
轻工业 | 662篇 |
水利工程 | 170篇 |
石油天然气 | 258篇 |
武器工业 | 43篇 |
无线电 | 543篇 |
一般工业技术 | 420篇 |
冶金工业 | 1388篇 |
原子能技术 | 74篇 |
自动化技术 | 627篇 |
出版年
2024年 | 34篇 |
2023年 | 102篇 |
2022年 | 259篇 |
2021年 | 288篇 |
2020年 | 222篇 |
2019年 | 150篇 |
2018年 | 155篇 |
2017年 | 189篇 |
2016年 | 163篇 |
2015年 | 258篇 |
2014年 | 327篇 |
2013年 | 347篇 |
2012年 | 451篇 |
2011年 | 403篇 |
2010年 | 410篇 |
2009年 | 369篇 |
2008年 | 429篇 |
2007年 | 377篇 |
2006年 | 332篇 |
2005年 | 251篇 |
2004年 | 215篇 |
2003年 | 143篇 |
2002年 | 121篇 |
2001年 | 121篇 |
2000年 | 87篇 |
1999年 | 64篇 |
1998年 | 307篇 |
1997年 | 214篇 |
1996年 | 160篇 |
1995年 | 88篇 |
1994年 | 62篇 |
1993年 | 76篇 |
1992年 | 20篇 |
1991年 | 13篇 |
1990年 | 11篇 |
1989年 | 14篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1982年 | 7篇 |
1981年 | 14篇 |
1980年 | 20篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 63篇 |
1976年 | 136篇 |
1975年 | 3篇 |
1965年 | 2篇 |
1951年 | 2篇 |
排序方式: 共有7532条查询结果,搜索用时 0 毫秒
51.
52.
对四种植被类型下土壤微生物量碳、氮、土壤呼吸速率以及代谢熵的研究,结果表明,土壤微生物量碳、氮以及土壤呼吸在四种植被类型间差异显著,而土壤微生物代谢熵之间差异不明显.且不同植被演替阶段土壤微生物量碳、氮从初级阶段到次生林阶段逐步增大,从次生林到成熟林阶段明显减小;土壤呼吸随着植被的正向演替,呼吸速率逐渐增强;四种植被类型间土壤微生物代谢熵在次生林中最低,为0.44 mgg-1 h-1,万熟林地中最高,为1.01 mgg-1 h-1,表明土壤微生物对土壤碳的利用效率次生林较高,成熟林地较低. 相似文献
53.
54.
55.
面向毫米波相控阵雷达系统应用,该文基于55?nm?CMOS工艺设计了一款工作于130?GHz的有源矢量(VM)合成移相器.该电路包含宽带正交发生器、可变增益放大和矢量合成模块.为提升移相器相位分辨率和移相精度,该电路可变增益放大采用了具有高频宽带属性的共栅放大结构和具有高增益属性的含中和电容的共源共栅放大结构多级级联的形式.为避免移相器在矢量合成时由自身结构特点产生相位断裂而导致移相范围下降,该设计电路在矢量合成模块中融入了数控人工介质(DiCAD)结构.通过全波电磁仿真对所设计毫米波移相器进行验证,在125~135?GHz频率范围内,所设计移相器平均增益大于1?dB,移相器可由控制电压控制实现全360°范围内5.625°的相位步进,RMS相位误差小于4°,电路面积为1100?mm×600?mm,功耗33?mW. 相似文献
56.
为了提高微波链路雨衰特征的描述精度,拓展微波链路信号的可用参数,该文利用部署于江苏江阴地区的15 GHz,18 GHz和23 GHz微波链路和雨量计开展同步对比观测,拟合得到3种频段的微波链路雨衰关系.提取并分析了接收信号电平的平均值、中位数、25%分位数、75%分位数、标准差、极大值和极小值等13个特征量与晴雨时刻、降雨强度之间的关系,得出结论:微波链路的信号变化和降雨强度的变化存在明显的负相关关系.实际拟合的雨衰关系与ITU-R的经验雨衰关系具有较好的一致性,但是在不同频段上均有差异;所有13个参量在有雨时刻和无雨时刻均存在一定概率的重叠,这是造成晴雨区分困难的主要原因;频率越高,信号变化受降雨的影响越显著,越有利于微波链路反演降雨.所得出的结论为提高微波链路测雨方法中的晴雨区分、参考值确定以及雨强反演的精度提供重要依据. 相似文献
57.
58.
Cigarette smoking and the colorectal adenoma-carcinoma sequence: a hypothesis to explain the paradox
As recognized precursor lesions to colorectal cancer, colorectal adenomatous polyps have been studied to enhance knowledge of colorectal cancer etiology. Although most of the known risk factors for colorectal cancer are also associated with the occurrence of colorectal adenomas, cigarette smoking has had a strong, consistent relationship with colorectal adenomas but is generally not associated with colorectal cancer. The explanation for this paradox is unknown. With data collected in 1986-1988 during a large case-control study based on colonoscopy results in New York City, New York, the authors investigated the possibility that the paradox may arise because subjects with colorectal adenomas were included in the control group of cancer case-control studies. The authors found a statistically significant increased risk between heavy cigarette smoking (smokers with > or = 40 pack-years of smoking) and risk of adenoma (odds ratio (OR) = 1.61, 95% confidence interval (CI) 1.06-2.44). They saw no increased colorectal cancer risk from heavy cigarette smoking (OR = 1.02, 95% CI 0.52-1.99) using a "manufactured" control group to simulate a typical unscreened, population-based control group. When the authors compared these colorectal cancer cases with an adenoma-free control group examined by colonoscopy in a polytomous model with several case groups (newly diagnosed adenomas, carcinoma in situ, intramucosal carcinoma, and colorectal cancer), they found that the risk for 20-39 pack-years of smoking was elevated, although not statistically significant, and was similar for all four case groups. The risk for the highest smoking category (> or = 40 pack-years) was more strongly elevated in all four case groups, although it was statistically significant for only the newly diagnosed adenoma and the carcinoma in situ cases (adenomas, OR = 1.59, 95% CI 1.05-2.42; carcinoma in situ, OR = 2.05, 95% CI 1.01-4.15; intramucosal carcinoma, OR = 1.30, 95% CI 0.61-2.77; and colorectal cancer, OR = 1.30, 95% CI 0.64-2.65). While the authors' study is weakened by the lack of statistical significance concerning risk for colorectal cancer, these data offer some support for the hypothesis that the association between cigarette smoking and risk of colorectal cancer may have been masked by inclusion in the control group of subjects with adenomas. They also suggest that the major effect of smoking on the colorectal adenoma-carcinoma sequence occurs in the earlier stages of the formation of adenoma and the development of carcinoma in situ. 相似文献
59.
60.
Xingyu Chen Xuguang Lan Guoqiang Liang Jianyi Liu Nanning Zheng 《Multimedia Tools and Applications》2017,76(21):22043-22058
Face recognition under variable pose and illumination is a challenging problem in computer vision tasks. In this paper, we solve this problem by proposing a new residual based deep face reconstruction neural network to extract discriminative pose-and-illumination-invariant (PII) features. Our deep model can change arbitrary pose and illumination face images to the frontal view with standard illumination. We propose a new triplet-loss training method instead of Euclidean loss to optimize our model, which has two advantages: a) The training triplets can be easily augmented by freely choosing combinations of labeled face images, in this way, overfitting can be avoided; b) The triplet-loss training makes the PII features more discriminative even when training samples have similar appearance. By using our PII features, we achieve 83.8% average recognition accuracy on MultiPIE face dataset which is competitive to the state-of-the-art face recognition methods. 相似文献