全文获取类型
收费全文 | 288485篇 |
免费 | 22417篇 |
国内免费 | 11059篇 |
专业分类
电工技术 | 15366篇 |
技术理论 | 31篇 |
综合类 | 17429篇 |
化学工业 | 50184篇 |
金属工艺 | 16859篇 |
机械仪表 | 18428篇 |
建筑科学 | 22874篇 |
矿业工程 | 9523篇 |
能源动力 | 7954篇 |
轻工业 | 16659篇 |
水利工程 | 4778篇 |
石油天然气 | 20256篇 |
武器工业 | 2234篇 |
无线电 | 31458篇 |
一般工业技术 | 34375篇 |
冶金工业 | 15267篇 |
原子能技术 | 2857篇 |
自动化技术 | 35429篇 |
出版年
2024年 | 1216篇 |
2023年 | 4675篇 |
2022年 | 7915篇 |
2021年 | 11401篇 |
2020年 | 8525篇 |
2019年 | 7266篇 |
2018年 | 8166篇 |
2017年 | 9215篇 |
2016年 | 8023篇 |
2015年 | 11167篇 |
2014年 | 13769篇 |
2013年 | 16443篇 |
2012年 | 17752篇 |
2011年 | 19400篇 |
2010年 | 16750篇 |
2009年 | 15901篇 |
2008年 | 15437篇 |
2007年 | 15084篇 |
2006年 | 15973篇 |
2005年 | 14146篇 |
2004年 | 9045篇 |
2003年 | 7903篇 |
2002年 | 7304篇 |
2001年 | 6483篇 |
2000年 | 7099篇 |
1999年 | 8499篇 |
1998年 | 6845篇 |
1997年 | 5821篇 |
1996年 | 5450篇 |
1995年 | 4507篇 |
1994年 | 3757篇 |
1993年 | 2628篇 |
1992年 | 2136篇 |
1991年 | 1601篇 |
1990年 | 1173篇 |
1989年 | 933篇 |
1988年 | 759篇 |
1987年 | 523篇 |
1986年 | 389篇 |
1985年 | 265篇 |
1984年 | 183篇 |
1983年 | 120篇 |
1982年 | 130篇 |
1981年 | 86篇 |
1980年 | 70篇 |
1979年 | 24篇 |
1978年 | 2篇 |
1965年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
921.
922.
We show that an n-vertex bipartite K3,3-free graph with n?3 has at most 2n−4 edges and that an n-vertex bipartite K5-free graph with n?5 has at most 3n−9 edges. These bounds are also tight. We then use the bound on the number of edges in a K3,3-free graph to extend two known NC algorithms for planar graphs to K3,3-free graphs. 相似文献
923.
924.
本文从TBM地区目的层的地质和地球物理特征分析出发,以三维地震资料为基础,综合利用人工和神经网络波形分类等定性储层预测方法和利用反演及敏感地震属性分析,对TBM地区低孔渗砂岩分布区开展了砂体展布特征分析和天然气高产区预测研究。根据研究结果部署了10口钻井,已完钻的4口井中有3口获得高产气流(其中DK27井日产天然气36.8×104m3)。值得指出的是,目前所用的预测方法还不能预测厚度小于6m的单砂层或砂层组构成的储层。 相似文献
925.
Spectral slope (S), describing the exponential decrease of the absorption spectrum over a given wavelength range, is an important parameter in the study of of chromophoric dissolved organic matter (CDOM) dynamics, and also an essential input parameter in remote sensing models. Furthermore, S is often used as a proxy for CDOM composition, including the ratio of fulvic to humic acids and molecular weight. The relative broad range in S values reported in the literature can be explained by the different spectral ranges and fitting methods used. A single exponential model is used to fit the S values for 17 investigations involving 458 samples in Lake Taihu from January to October in 2004. The average S value was 15.18 ± 1.39 μm−1 for the range of 280–500 nm, which fell within the range reported in the literature. The frequency distribution of S value basically obeyed a normal distribution. Significant differences in S values between summer and other seasons showed that phytoplankton degradation was one of the important sources of CDOM in summer, whereas CDOM mainly came from the river input in other seasons. Furthermore, the estimated S value decreased with increasing wavelength range used in regression. The maximum and minimum values derived from the regression were 17.89 ± 1.25 μm−1 and 13.62 ± 2.11 μm−1 for the wavelength ranges of 280–380 nm and 400–500 nm, respectively, a decrease of 23.9%. S values significantly decreased with the increase of CDOM absorption coefficients. CDOM absorption coefficients could be more appropriately estimated from exponential model introducing the variation of S with absorption coefficients, making them useful for a remote sensing bio-optical model of Lake Taihu. DOC-specific absorption coefficient a*(λ) and the parameter M describing molecular size of the humic molecules could also be used as a proxy for the sources and types of CDOM. A general relationship was found between S and a*(λ), and M values. S increased with the decrease of DOC-specific absorption coefficient and the increase of M corresponding to the decrease of molecular weight. 相似文献
926.
张茜 《四川烹饪高等专科学校学报》2007,(2):11-13
川菜菜系不是短时间内突然出现的,它有一个萌芽、雏形(开始形成)、成熟和发展的过程。本文尝试通过解析川菜经典食谱之一的曾懿《中馈录》,来探讨川菜菜系雏形的形成。 相似文献
927.
《建筑创作》出刊已100期,它不仅发表了许多令人赞赏的建筑作品,许多发人深思的文章,而且组织茶室、考察、访旧等活动,已经成为我国建筑文化研究和开拓的一个中心.希望它越办越好.…… 相似文献
928.
Zhihui Zhang Bing Q. Han Kyung H. Chung Enrique J. Lavernia 《Metallurgical and Materials Transactions A》2006,37(7):2265-2273
The current study aims to provide fundamental insight into the behavior of microstructures containing grain sizes that span
multiple length scales. A commercial 5083 Al alloy was selected as the material of interest to facilitate comparison with
recently published data. The materials studied here were prepared via the thermal consolidation of powders that were cryomilled
for different times (i.e., 0, 2, 4, and 8 hours). Following consolidation, the resultant microstructure was characterized by an equiaxed grain morphology
with a size distribution centered around 200∼300 nm. Dispersed among the 200- to 300-nm grains were coarse-grained regions
or ligaments with a grain size ranging from 600 nm to 2 μm. The occurrence of coarse-grained regions is rationalized on the
basis of recrystallization or subgrain coarsening, whereas the occurrence of equiaxed fine regions is proposed to be a result
of continuous grain growth. Two types of microstructures were selected for study, containing coarse-grained volumes of approximately
28 pct and 43 pct that corresponded to an ultimate tensile strength (UTS) of 566 MPa and 535 MPa, and a fracture strain of
3.2 pct and 3.5 pct, respectively. The observed ductility and the relevant toughening mechanisms were discussed in light of
the presence of multiple length scales. 相似文献
929.
基于项目的大学英语在线合作学习研究 总被引:2,自引:0,他引:2
本文根据教育部外语教学改革的总体要求和建构主义教学理论,总结了两年来作者将计算机技术和网络资源整合于大学英语教学,开展基于项目的大学英语在线合作学习研究的探索和实践。 相似文献
930.
利用高酸值餐饮废油脂制备生物柴油 总被引:8,自引:0,他引:8
以高酸值餐饮废油脂为原料,在酯化反应后再经两步酯交换反应制备生物柴油。在酯化反应中添加吸水剂可降低油脂的酸值,经一次酯化反应即可将油脂的酸值降至2m g/g左右,满足酯交换反应的要求。考察了酯化反应中吸水剂的添加方式、种类及其用量对酯化反应的影响,以及酯交换催化剂的种类及用量对脂肪酸甲酯(FAME)收率和产物组成的影响。实验结果表明,以凹凸棒土为吸水剂(用量为餐饮废油脂中游离脂肪酸质量的3%),且在酯化反应30m in时加入,酯化效果较好;以质量比为1∶1的NaOH和KOH混合物为催化剂进行两步酯交换反应时,催化剂的最佳用量依次为1.00%和0.75%(质量分数,基于油脂的质量),FAME收率最高达到96.33%。 相似文献