全文获取类型
收费全文 | 652篇 |
免费 | 98篇 |
国内免费 | 110篇 |
专业分类
大气科学 | 24篇 |
地球物理 | 241篇 |
地质学 | 430篇 |
海洋学 | 100篇 |
天文学 | 6篇 |
综合类 | 18篇 |
自然地理 | 41篇 |
出版年
2023年 | 6篇 |
2022年 | 6篇 |
2021年 | 5篇 |
2020年 | 11篇 |
2019年 | 14篇 |
2018年 | 13篇 |
2017年 | 17篇 |
2016年 | 26篇 |
2015年 | 27篇 |
2014年 | 28篇 |
2013年 | 42篇 |
2012年 | 45篇 |
2011年 | 48篇 |
2010年 | 35篇 |
2009年 | 44篇 |
2008年 | 48篇 |
2007年 | 39篇 |
2006年 | 61篇 |
2005年 | 31篇 |
2004年 | 24篇 |
2003年 | 29篇 |
2002年 | 33篇 |
2001年 | 25篇 |
2000年 | 30篇 |
1999年 | 22篇 |
1998年 | 20篇 |
1997年 | 16篇 |
1996年 | 17篇 |
1995年 | 24篇 |
1994年 | 14篇 |
1993年 | 18篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有860条查询结果,搜索用时 15 毫秒
851.
We examine how the stable isotope composition of meteoric water is transmitted through soil and epikarst to dripwaters in a cave in western Romania. δ2H and δ18O in precipitation at this site are influenced by temperature and moisture sources (Atlantic and Mediterranean), with lower δ18O in winter and higher in summer. The stable isotope composition of cave dripwaters mimics this seasonal pattern of low and high δ18O, but the onset and end of freezing conditions in the winter season are marked by sharp transitions in the isotopic signature of cave dripwaters of approximately 1 ‰. We interpret these shifts as the result of kinetic isotopic fractionation during the transition phase from water to ice at the onset of freezing conditions and the input of meltwater to the cave at the beginning of the spring season. This process is captured in dripwaters and therefore speleothems from Ur?ilor Cave, which grew under such dripping points, may have the potential to record past changes in the severity of winters. Similar isotopic changes in dripwaters driven by freeze–thaw processes can affect other caves in areas with winter snow cover, and cave monitoring during such changes is essential in linking the isotopic variability in dripwaters and speleothems to surface climate. 相似文献
852.
853.
Rúbia Martins Bernardes Ramos Ana Paula Biz Deolinda Alvarez Tavares Mnica Beatriz Kolicheski Tirzh Lins Porto Dantas 《洁净——土壤、空气、水》2020,48(7-8)
Given the issue of lipids in effluent treatment systems and their negative impact on the environment, this study aimed to examine lipid degradation by homogenous catalytic ozonation with the aid of iron and manganese ions. This technology presents the possibility of completely mineralizing pollutants using hydroxyl radicals. Milk is chosen as the lipid source because of the high concentration of triglycerides in its matrix, this kind of lipid being the one found most frequently in food and, consequently, in effluent treatment systems. The milk pH value is controlled, and acidic, neutral, and basic conditions are evaluated. The rates of pseudo‐first‐order reactions and the effective value are estimated. It is shown that under acidic conditions low catalyst dosages are enough to cause the complete degradation of lipids. Under neutral conditions, a similar behavior is observed. Under basic conditions, higher catalyst dosages give higher reaction rates. The order of effectiveness of the catalysts under acidic and basic conditions is Fe2+ > Mn2+, with Mn2+ > Fe2+ under neutral conditions. Homogeneous catalytic ozonation is therefore efficient at lipid degradation. This technique is viable economically, since the lipid removal occurred at low ozone levels. In addition, the ions used as catalysts are naturally abundant. 相似文献
854.
855.
856.
不同水生植物对富营养化水体无机氮吸收动力学特征 总被引:3,自引:2,他引:1
采用浓度梯度法,研究了鸢尾(Iris louisiana)、狐尾藻(Myriophyllum verticillatum)、茭白(Zizania latifolia)和水芹(Oenanthe clecumbens)对硝态氮(NO3--N)和铵态氮(NH4+-N)吸收动力学特征.结果表明:4种水生植物对水体NO3--N和NH4+-N吸收可用Michaelis-Menten酶动力学方程描述,随溶液NO3--N和NH4+-N浓度增加,植物吸收NO3--N和NH4+-N速率增加,当溶液NO3--N和NH4+-N浓度接近于2.0 mmol/L时,吸收速率增加趋缓;4种水生植物对NO3--N和NH4+-N的Vmax值大小为水芹 >茭白 >鸢尾 >狐尾藻,对NO3--N的Km值大小为水芹 >鸢尾=茭白=狐尾藻,对NH4+-N的Km值大小为水芹 >狐尾藻 >茭白=鸢尾.根据吸收动力学参数(Vmax,Km)判断水芹适宜于净化NO3--N和NH4+-N浓度较高的水体,茭白、鸢尾和狐尾藻适合净化NO3--N和NH4+-N浓度较低水体;4种水生植物对NO3--N、NH4+-N表现出不同的吸收偏好性,鸢尾吸收NO3--N的潜力大于吸收NH4+-N的,但对NH4+-N的亲和力大于NO3--N,表明能在NO3--N浓度较高环境中优先吸收NH4+-N.狐尾藻和水芹对NO3--N和NH4+-N能均衡吸收.茭白对NH4+-N具有较高的吸收潜力与亲和力. 相似文献
857.
针对电化学作用能够显著增强煤中甲烷解吸渗流特性,但对其电极材料的选择缺乏依据的问题,采用实验的方法研究了铝、铁、铜和石墨四种电极材料对电化学改性无烟煤甲烷吸附解吸特性的影响.结果表明:(1)未改性自然煤样的甲烷饱和吸附量a为39.92mL·g~(-1),由铝、铁、铜和石墨四种电极材料进行电化学改性后阳极区域煤样的甲烷饱和吸附量a分别降低了5.22%、8.48%、9.24%和11.33%,阴极区域煤样的甲烷饱和吸附量a分别降低了9.53%、4.97%、6.25%和4.97%;(2)未改性自然煤样在300min的甲烷解吸率为83.17%,经铝、铁、铜和石墨四种电极材料改性后阳极区域煤样的甲烷解吸率分别提高了9.50%、7.10%、8.36%和15.75%,阴极区域煤样的甲烷解吸率分别提高了1.76%、1.12%、6.10%和16.23%;(3)采用石墨作为电极的电化学对无烟煤甲烷解吸的影响效果最为明显,原因在于石墨阳极处电解反应生成较多的H~+离子,一方面对煤表面进行酸化,抑制了煤甲烷的吸附,另一方面更多地溶蚀煤中的碳酸盐和硫酸盐矿物,增加了煤中的孔隙,使得甲烷最终解吸率有所升高.本文的研究结果可为电化学强化煤甲烷解吸电极材料的优选提供基础依据. 相似文献
858.
Dodecylammonium bentonite (DB) and dodecylammonium sepiolite (DS) were used as sorbents for phenoxyalkanoic acid herbicides 2,4‐D ((2,4‐dichlorophenoxy)acetic acid), 2,4‐DP ((RS)‐2‐(2,4‐dichlorophenoxy)propionic acid), 2,4‐DB (4‐(2,4‐dichlorophenoxy)butyric acid), 2,4,5‐T ((2,4,5‐trichlorophenoxy)acetic acid), and MCPA ((4‐chloro‐2‐methylphenoxy)acetic acid). Langmuir, Freundlich, and the linear Henry’s Law isotherm adsorption parameters were calculated from the adsorption isotherms. Langmuir equation showed poor fit for both adsorbents. According to the evaluation using the Freundlich equation, the DS sample showed much higher and stronger sorption capacity than DB. Similar behaviour was also observed in the case of the linear Henry’s Law isotherm. The adsorption of the herbicides on both DB and DS decreased in the order of 2,4‐DB > 2,4,5‐T > 2,4‐DP > 2,4‐D > MCPA. 相似文献
859.
A characterization method for AOX in surface water samples was developed and tested. The method involves fractionation using a hydrophobic C18 resin and a weak anionic exchange resin and allows the fractionation of the AOX pool of surface water samples into four fractions: (1) hydrophilic acidic, (2) hydrophilic non‐acidic, (3) hydrophobic acidic, and (4) hydrophobic non‐acidic. The adsorption analysis was verified with AOX‐relevant model compounds and was applied to characterize the AOX pool of a stream sample from the Moskva river (Russia). In addition to the fractionation analysis, size‐exclusion chromatography was used to characterize the AOX pool of the sample studied. Hydrophilic acids made up the major fraction of the AOX pool (55 %). Among this fraction chlorinated high‐molecular acids (humic substances) made up the main fraction (35 %). 相似文献
860.