收费全文 | 199246篇 |
免费 | 17363篇 |
国内免费 | 10574篇 |
耳鼻咽喉 | 1876篇 |
儿科学 | 2881篇 |
妇产科学 | 3168篇 |
基础医学 | 25750篇 |
口腔科学 | 3690篇 |
临床医学 | 24199篇 |
内科学 | 32929篇 |
皮肤病学 | 2593篇 |
神经病学 | 13332篇 |
特种医学 | 6456篇 |
外国民族医学 | 88篇 |
外科学 | 21836篇 |
综合类 | 24669篇 |
现状与发展 | 41篇 |
一般理论 | 56篇 |
预防医学 | 15273篇 |
眼科学 | 5131篇 |
药学 | 18808篇 |
95篇 | |
中国医学 | 7797篇 |
肿瘤学 | 16515篇 |
2024年 | 482篇 |
2023年 | 2454篇 |
2022年 | 5810篇 |
2021年 | 7997篇 |
2020年 | 5751篇 |
2019年 | 5619篇 |
2018年 | 6340篇 |
2017年 | 5323篇 |
2016年 | 5308篇 |
2015年 | 7312篇 |
2014年 | 9132篇 |
2013年 | 9150篇 |
2012年 | 13229篇 |
2011年 | 14269篇 |
2010年 | 8608篇 |
2009年 | 7105篇 |
2008年 | 10087篇 |
2007年 | 10143篇 |
2006年 | 9656篇 |
2005年 | 9311篇 |
2004年 | 6906篇 |
2003年 | 6332篇 |
2002年 | 5799篇 |
2001年 | 7237篇 |
2000年 | 7253篇 |
1999年 | 6880篇 |
1998年 | 3600篇 |
1997年 | 3200篇 |
1996年 | 2308篇 |
1995年 | 2131篇 |
1994年 | 1834篇 |
1993年 | 1311篇 |
1992年 | 2894篇 |
1991年 | 2426篇 |
1990年 | 2177篇 |
1989年 | 2079篇 |
1988年 | 1798篇 |
1987年 | 1605篇 |
1986年 | 1434篇 |
1985年 | 1162篇 |
1984年 | 710篇 |
1983年 | 587篇 |
1982年 | 322篇 |
1981年 | 261篇 |
1980年 | 188篇 |
1979年 | 328篇 |
1978年 | 81篇 |
1974年 | 79篇 |
1973年 | 99篇 |
1972年 | 74篇 |
The relationship between the migration process and speciation distribution of Cr is important for the risk assessment in the underground environment. In this work, soil columns were collected from the chromate production site, with a 40-year operation, in the groundwater depression cone area of North China plain. The relationship between chromium pollution features and the geochemical properties of soil was established, and the migration risk of Cr(VI) was assessed based on the Nemerow composite index and Hydrus-1D model. The maximum total Cr concentration in the chromium slag dumping site reached 907 mg/kg, and that in the chromate production workshop was more than 200 mg/kg across the depth. The migration of Cr might be accelerated in the soil with abundant Mn (236–1461 mg/kg) but scarce organic matters (<?0.45%). The Hydrus simulation indicated that Cr(VI) would reach a cumulative flux of 300–729 mg/cm2 after 50 years.
相似文献Air samples were collected around industrial parks in Jiangsu, China, to allow the concentrations, profiles, and risk assessment of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs), polychlorinated naphthalenes (PCNs), and metals to be investigated. The concentrations of ΣPBDD/Fs and ΣPCNs were 1324.26–2080.98 fg/m3 (11.35–42.57 fg I-TEQ/m3) and 10,404.9–29,322.9 fg/m3 (1.32–7.19 fg I-TEQ/ m3), respectively. The highest concentration of ΣPBDD/Fs and ΣPCNs were observed at site C. PBDD/Fs were mainly dominated by PBDFs. The main contributor to the ΣPBDD/Fs in all samples was 1,2,3,4,6,7,8-HpBDF, which accounted for 25.75%–39.4%. For PCNs, the predominating homologues were tetra-, tri- and penta-CNs, which contributed 30.7%–43.3%, 24.7%–31.0%, and 10.6%–21.6%, respectively. As for metals, the pollution of As, Mn, Cr, and Ni in most samples exceeded National Ambient Air Quality Standards of China. Assessing the risk of inhalation exposure showed that there were potential carcinogenic risks to local residents.
相似文献Based on a typical residential area, this paper studies the characteristics of pollutant concentration changes in two rainfall runoffs and the first flush effect of rainfall. In rainfall runoff, the concentrations of seven pollutants (CODMn, TN, DTN, NH3-N, TP, DTP, and PO43−) increased during the initial rainfall period and decreased in the later period. Rainfall causes the erosion of pollutants on the underlying surface so that water pollution begins when rainfall runoff occurs, and the pollution level drops over time. The seven pollutants all experience this first flush effect, of which, rainfall has the strongest scouring effect on NH3-N produced by domestic sewage. The significant excess of pollutants in rainfall runoff should be considered by management departments. In addition, the existence of the first flush effect makes it possible in theory to partially intercept rainfall runoff to control water pollution, thereby reducing the cost of pollution control.
相似文献