全文获取类型
收费全文 | 246270篇 |
免费 | 1698篇 |
国内免费 | 1117篇 |
专业分类
电工技术 | 4852篇 |
综合类 | 290篇 |
化学工业 | 35591篇 |
金属工艺 | 10886篇 |
机械仪表 | 7075篇 |
建筑科学 | 6303篇 |
矿业工程 | 483篇 |
能源动力 | 6322篇 |
轻工业 | 24476篇 |
水利工程 | 1992篇 |
石油天然气 | 1337篇 |
武器工业 | 3篇 |
无线电 | 31233篇 |
一般工业技术 | 45103篇 |
冶金工业 | 50111篇 |
原子能技术 | 3867篇 |
自动化技术 | 19161篇 |
出版年
2021年 | 1656篇 |
2019年 | 1584篇 |
2018年 | 2327篇 |
2017年 | 2231篇 |
2016年 | 2508篇 |
2015年 | 1991篇 |
2014年 | 3190篇 |
2013年 | 10807篇 |
2012年 | 5466篇 |
2011年 | 7798篇 |
2010年 | 6302篇 |
2009年 | 6970篇 |
2008年 | 7569篇 |
2007年 | 7664篇 |
2006年 | 7044篇 |
2005年 | 6530篇 |
2004年 | 6260篇 |
2003年 | 6278篇 |
2002年 | 6250篇 |
2001年 | 6375篇 |
2000年 | 5815篇 |
1999年 | 6432篇 |
1998年 | 16571篇 |
1997年 | 11605篇 |
1996年 | 8959篇 |
1995年 | 6700篇 |
1994年 | 5921篇 |
1993年 | 5739篇 |
1992年 | 4059篇 |
1991年 | 4004篇 |
1990年 | 3588篇 |
1989年 | 3511篇 |
1988年 | 3444篇 |
1987年 | 2912篇 |
1986年 | 2844篇 |
1985年 | 3414篇 |
1984年 | 3050篇 |
1983年 | 2744篇 |
1982年 | 2547篇 |
1981年 | 2568篇 |
1980年 | 2431篇 |
1979年 | 2271篇 |
1978年 | 2235篇 |
1977年 | 2808篇 |
1976年 | 4172篇 |
1975年 | 1899篇 |
1974年 | 1795篇 |
1973年 | 1783篇 |
1972年 | 1460篇 |
1971年 | 1317篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Paul R. Ehrlich Anne H. Ehrlich 《The International journal of environmental studies》2013,70(4):481-492
The development of an enormous culture gap, in which no individuals of advanced societies possess even a billionth of the non‐genetic information possessed by their entire society, has threatened a global collapse of civilisation. Critical parts of that gap must be rapidly bridged so that problems such as climate disruption, toxification of the Earth, loss of biodiversity and ecosystem services, and the decay of the epidemiological environment can be satisfactorily attacked. The essential need is to alter human behaviour to put society on a route to sustainability; one cheering development is a growing interest in the Millennium Assessment of Human Behaviour (MAHB), whose goal is to do just that. 相似文献
42.
Mohamed E. Hereher Khaled H. El-Ezaby 《The International journal of environmental studies》2013,70(1):65-77
The paper reports an assessment of the soil and water in the coastal strip between Marsa Alam and Shalateen at the southern part of the Red Sea coast of Egypt. There is limited potential for agricultural development. There is a need for water re-use and some potential for tourism. 相似文献
43.
44.
45.
46.
47.
Ahmed Jasim M. Al-Karawi Zyad H. Jawad Al-Qaisi Alaa H. Jawad Al-Qaisi Faliah Hassan A. Al-Jeboori 《Polymer Bulletin》2014,71(7):1575-1590
The graft copolymerization of methyl acrylate (MA) onto chitosan in aqueous medium was investigated using potassium persulfate (KPS) as initiator. The grafting conditions were optimized by studying the effects of the polymerization variables (the initiator concentration, the ratio of monomer to chitosan, and reaction temperature) on the percentage of grafting (PG). PG was found to depend on these variables, and the highest grafting percentage (256 %) could be obtained at chitosan = 1 g, KPS = 4.5 × 10?3 M, methyl acrylate monomer = 6 g, T = 60 °C and t = 180 min. The graft copolymer was characterized by Fourier transform infrared spectra analysis, thermogravimetry (differential thermogravimetry, differential scanning calorimetric), X-ray powder diffraction as well as CP-MAS 13C NMR spectroscopy. These analyses are highly confirmed the formation of poly(methyl acrylate) grafted chitosan (PMAGC). Furthermore, the gelation of the grafted polymers (PG 68, 122, 218 and 256 %) in distilled water has been studied, and the results revealed that the percentage of swelling number increase with increasing PG of the polymers. Controlled release of niacin (vitamin B3) from the hydrogel of the grafted polymers (PG 68, 122 and 256 %) in aqueous medium has been studied using ultraviolet absorption to follow quantities released at different times (for each experiment: PMAGC 100 mg, niacin 2.46 mg, distilled water 100 ml). The study was repeated again with same conditions except the using of 4.92 mg of niacin instead of 2.46 mg (PG of the grafted polymer is 256 %). The diffusion coefficient (D, cm2/h) of niacin from the hydrogel of the grafted polymer (PG 256 %) was calculated depending on Higuchi model (diffusion coefficient of the first load is 0.00194 cm2/h while 0.00255 cm2/h of the second load). 相似文献
48.
49.
50.
Metal Science and Heat Treatment - The influence of the temperature of heating for quenching and subsequent heating for redistribution of carbon between martensite and retained austenite... 相似文献