全文获取类型
收费全文 | 5689篇 |
免费 | 463篇 |
国内免费 | 111篇 |
专业分类
电工技术 | 247篇 |
综合类 | 202篇 |
化学工业 | 987篇 |
金属工艺 | 198篇 |
机械仪表 | 185篇 |
建筑科学 | 357篇 |
矿业工程 | 49篇 |
能源动力 | 315篇 |
轻工业 | 463篇 |
水利工程 | 96篇 |
石油天然气 | 183篇 |
武器工业 | 4篇 |
无线电 | 659篇 |
一般工业技术 | 846篇 |
冶金工业 | 366篇 |
原子能技术 | 20篇 |
自动化技术 | 1086篇 |
出版年
2024年 | 11篇 |
2023年 | 94篇 |
2022年 | 173篇 |
2021年 | 225篇 |
2020年 | 176篇 |
2019年 | 172篇 |
2018年 | 217篇 |
2017年 | 203篇 |
2016年 | 249篇 |
2015年 | 216篇 |
2014年 | 241篇 |
2013年 | 442篇 |
2012年 | 338篇 |
2011年 | 420篇 |
2010年 | 349篇 |
2009年 | 348篇 |
2008年 | 286篇 |
2007年 | 242篇 |
2006年 | 241篇 |
2005年 | 191篇 |
2004年 | 128篇 |
2003年 | 152篇 |
2002年 | 196篇 |
2001年 | 142篇 |
2000年 | 105篇 |
1999年 | 61篇 |
1998年 | 94篇 |
1997年 | 69篇 |
1996年 | 56篇 |
1995年 | 59篇 |
1994年 | 50篇 |
1993年 | 40篇 |
1992年 | 22篇 |
1991年 | 10篇 |
1990年 | 19篇 |
1989年 | 11篇 |
1988年 | 17篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 23篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 6篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 13篇 |
1976年 | 18篇 |
1975年 | 8篇 |
排序方式: 共有6263条查询结果,搜索用时 15 毫秒
71.
聚乙二醇-400催化合成橙花醚的研究 总被引:3,自引:0,他引:3
研究了利用多种相转移催化剂在不同条件下合成橙花醚。结果表明:采用聚乙二醇 400(PEG 400)为催化剂,当β 萘酚:溴乙烷:催化剂=1:12:007(摩尔比)、温度为76℃、反应7h时,产率可高达981%。 相似文献
72.
Ben Huybrechts Kozo Ishizaiki Masasuke Takata 《Journal of the American Ceramic Society》1994,77(1):286-288
The positive temperature coefficient of resistivity (PTCR) behavior of semiconductive BaTiO3 is well explained by the Heywang model, which predicts the resistivity behavior above the Curie point based on the acceptor state density at the grain boundaries, the charge carrier density, and the energy gap, E s , between the conduction band and the acceptor levels. However, the relationship between these parameters and the production parameters (sintering time, composition, and cooling rate) is not well understood. Recently, the present authors have found that E s can be increased by thorough oxidation. This increase is attributed to a change in the oxidation state of the acceptor. Based on this finding and results from the literature, a phenomenological PTCR model and an accompanying PTCR chart for acceptor–donor-codoped BaTiO3 are proposed to clarify this relationship. The PTCR chart clarifies that acceptor dopant concentrations, oxidation time, and oxygen partial pressure during oxidation or cooling can be optimized simultaneously to obtain optical PTCR properties. 相似文献
73.
Ben Huybrechts Kozo Ishizaki Masasuke Takata 《Journal of the American Ceramic Society》1992,75(3):722-724
Experimental evidence shows that the acceptor-state levels in Sb-doped positive-temperature-coefficient-type BaTiO3 are compensated up to a critical acceptor-state density. Using the slope of the natural logarithm of the resistivity with respect to 1/ T , instead of maximum resistivity as a measure for the acceptor-state density, it is possible to estimate this critical value. The value obtained (4.2 × 1017 m−2 ) is believed to be the first reported estimate based on experimental data. It is in good agreement with the estimate of 6 × 1017 m−2 (first reported by Jonker) obtained from the spontaneous polarization of BaTiO3 . This shows that the ferroelectric behavior of BaTiO3 is indeed a feasible explanation for the low resistivity below the Curie point, as proposed by Jonker. 相似文献
74.
Guimes Rodrigues Filho Leandra Cardoso Toledo Daniel Alves Cerqueira Rosana Maria Nascimento de Assunção Carla da Silva Meireles Harumi Otaguro Sizue Ota Rogero Ademar Benévolo Lugão 《Polymer Bulletin》2007,59(1):73-81
Summary In this article, cellulose acetate produced through the homogeneous acetylation of sugar cane bagasse cellulose was used to
produce membranes, using poly(ethyleneglycol) 600 (PEG 600) as an admixture. The membranes were characterized using water
flux measurements (Payne’s cup), differential scanning calorimetry (DSC) and neutral red uptake (cytotoxicity). The results
showed that PEG 600 acts as a crystallinity inductor and/or pore former in the cellulose acetate matrix. The induction of
crystallinity is important for this system since it had not been reported on the literature yet. The results also demonstrated
that the studied membranes present a nontoxic behavior. 相似文献
75.
76.
77.
Alper Sarıoğlan Ömer Tunç Savaşçı Ayşe Erdem-Şenatalar Vu Thu Ha Gilbert Sapaly Younès Ben Taârit 《Catalysis Letters》2007,118(1-2):123-128
MFI type inorganic carrier was used in two different cationic forms, hydrogen and calcium respectively. MFI-supported molybdenum
and rhenium catalysts were prepared. The activities of the catalysts were compared for the aromatization reaction of methane.
Higher activity values were attained with the catalysts supported on HZSM-5. Aromatics were also observed with the catalysts
supported on CaZSM-5, despite their deficiency in acid sites. Highly dispersed rhenium is expected to be formed with the use
of the inorganic carrier in calcium form. On the other hand, lower reaction rates were observed with rhenium supported on
CaZSM-5, in spite of the improved dispersion of the active rhenium species on this catalyst. This was interpreted in terms
of the critical role of the acid sites in the conversion of methane to aromatics, compared to the improved dispersion of the
active metal. 相似文献
78.
合成丁酸异戊酯的新方法 总被引:3,自引:0,他引:3
采用超强酸代替浓硫酸作催化剂,对丁酸异戊酯合成反应进行了研究,讨论了影响收率的多种因素,在最佳条件下,酯化反应时间短,催化剂可以重复使用,丁酸异戊酯收率达到98%。 相似文献
79.
80.