全文获取类型
收费全文 | 1640篇 |
免费 | 55篇 |
国内免费 | 30篇 |
专业分类
化学 | 839篇 |
晶体学 | 1篇 |
力学 | 489篇 |
综合类 | 71篇 |
数学 | 56篇 |
物理学 | 269篇 |
出版年
2024年 | 7篇 |
2023年 | 40篇 |
2022年 | 66篇 |
2021年 | 62篇 |
2020年 | 49篇 |
2019年 | 44篇 |
2018年 | 47篇 |
2017年 | 75篇 |
2016年 | 55篇 |
2015年 | 45篇 |
2014年 | 56篇 |
2013年 | 189篇 |
2012年 | 78篇 |
2011年 | 58篇 |
2010年 | 57篇 |
2009年 | 70篇 |
2008年 | 72篇 |
2007年 | 70篇 |
2006年 | 94篇 |
2005年 | 58篇 |
2004年 | 61篇 |
2003年 | 45篇 |
2002年 | 53篇 |
2001年 | 37篇 |
2000年 | 31篇 |
1999年 | 26篇 |
1998年 | 19篇 |
1997年 | 23篇 |
1996年 | 22篇 |
1995年 | 19篇 |
1994年 | 22篇 |
1993年 | 17篇 |
1992年 | 10篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有1725条查询结果,搜索用时 10 毫秒
111.
长白山土壤腹足类物种多样性初步研究 总被引:1,自引:0,他引:1
自1993年5月至10月对吉林长白山森林生态系统定位站的土壤腹足类进行了多样性调查,据初步调查的结果显示,长白山地区土壤腹足类种类少而单调,采样区仅发现15种,隶属于腹足纲9科13属,采样区土壤腹足类物种多样性指数和丰富度指数都是5月份最低,8月份达最大值,之后逐渐减小;均匀度指数5月份最高,7月份最低;单纯度指数也是5月份最高,最低值出现在10月份,土壤表层的陆生贝类物种多样性指数高于上层。 相似文献
112.
113.
乌鲁木齐市安宁渠区蔬菜中重金属的分布特征研究 总被引:7,自引:0,他引:7
对乌鲁木齐市郊安宁渠区在土壤调查的基础上,又进行了蔬菜中的重金属调查.并对蔬菜进行了污染评价.结果标明:土壤的环境元素组合与蔬菜的不同.土壤和蔬菜的重金属聚类分析结果也不同.按食品卫生标准进行评价,蔬菜中Cd、Pb的超标率最高,大白菜的超标率比四季豆和长豇豆更高.长豇豆对重金属的富集系数相对较小. 相似文献
114.
Soilbentonite slurry walls are designed to inhibit the subsurface movement of contaminants from hazardous waste sites. Although it is generally accepted that high concentrations of organic compounds will adversely affect soilbentonite slurry walls and clay liners, previous research investigating the effects of NAPLs on the conductivity of clay wall materials has been inconclusive. In this study the effects of various organics (benzene, aniline, trichloroethylene, ethylene dichloride, methylene chloride) on the effective conductivity of a typical soilbentonite slurry wall material were studied under two effective stress conditions, 200 and 52kPa. The hydraulic conductivity for the soilbentonite material permeated with water averaged 1.52×10-8cms-1. Compared to water, there was little change in conductivity when the sample was permeated with a solution containing a NAPL compound at its solubility limit, except for aniline. However, there was a one to two order of magnitude decrease in conductivity when the sample was permeated with a pure NAPL for all NAPLs tested. When the soilbentonite material was permeated with a water/NAPL/water/NAPL sequence, the conductivity decreased one to two orders of magnitude when a NAPL was introduced following water; however, when water was reintroduced after the NAPL, the conductivity increased to the initial hydraulic conductivity. The conductivity again decreased one to two orders of magnitude when the NAPL was reintroduced. This trend occurred for all NAPLs tested, and the fluid properties of the NAPL compounds alone did not account for the decrease in conductivity compared to water. 相似文献
115.
Hydraulics of Peat Filters Treating Septic Tank Effluent 总被引:3,自引:0,他引:3
Peat is an alternative filter medium for the treatment of various waste streams including septic tank effluent. The water holding capacity and adsorption capacity of peat make it a favorable filter medium over sand or gravel which are commonly used as the filter medium for the drainage field of septic systems. This paper presents the results of a field study to evaluate the hydraulics of a peat filter used to treat the septic tank effluent from a public school west of Ottawa, Canada. An experimental box was placed within the filter during its construction to provide access to the vertical profile of the peat layer. The filter is periodically pulsed with septic tank effluent, which is distributed over the top of the filter and flows vertically through the peat. The filter was instrumented with tensiometers and transducers to monitor the pore water pressures in response to a pulse of septic tank effluent. An in-depth study of the hydraulics of the system was completed. The soil moisture retention curve and the hydraulic conductivity as a function of density were determined in laboratory experiments. A one-dimensional unsaturated flow model was used to predict the pressure response due to a pulse. A comparison of the field and model results illustrates the impact of the density variations, and the corresponding hydraulic conductivity variations, on the model predictions. The compaction of the peat is an important design consideration for the filter since it directly impacts the flow characteristics and the hydraulic retention time within the filter. 相似文献
116.
土壤和沉积物石油污染现状 总被引:4,自引:0,他引:4
针对全球石油污染的现状从原油的组成、分类与对环境的潜在影响、原油中有毒有害物质的成分、石油的工业工艺流程等方面进行了论述,并着重分析了中国土壤和沉积物原油污染,对松辽盆地、济阳坳陷、塔里木盆地、渤海油区等区域污染现状作了详细介绍,对石油勘探开发中重要污染源之一——化学助剂造成的污染也作了总括. 相似文献
117.
Watson S.J. Barry D.A. Schotting R.J. Hassanizadeh S.M. 《Transport in Porous Media》2002,47(2):149-167
Recently, it has been suggested that Darcy's Law might not be applicable for modelling miscible, density-dependent flow in porous media. To investigate this, three sets of careful laboratory column experiments were performed on coarse and medium sands, consisting of upward displacement of water by sodium chloride solutions with concentrations ranging from 5 to 200g/l. Data on salt concentrations and water pressures were collected in horizontal transects along the flow direction. Salt concentration data were also collected in the influent and exit lines. The experimental data were analysed using a simplified approach based on Darcy's Law alone, applied with the assumption of a sharp interface. Darcy's Law was used to estimate porous medium permeability by fitting predictions to experimental data. Consistent estimates of permeability were obtained for each set of experiments. The results indicate that Darcy's Law adequately describes high concentration displacements through saturated coarse- and medium-grained porous media. 相似文献
118.
本文根据桩在轴向谐振力作用下,探讨了非均质层下基桩的动力稳定性问题,分析了各种地基土横向抗力系数K以及桩的几何特征,激振频率,桩土阻尼对桩动力稳定性的影响,结果表明,随地基土层的不同,基桩的动力不稳定区域将发生不同的变化,随桩径,桩长的增大,基桩的动力不稳定区域将逐渐减小,表明地基土的横向抗力对桩的动力稳定性起加强作用,桩土阻尼对基桩的动力稳定性影响较大,得出了一些有价值的结论,可为工程设计提供参考依据。 相似文献
119.
The stress vector-based constitutive model for cohesionless soil, proposed by SHI Hong-yan et al., was applied to analyze
the deformation behaviors of materials subjected to various stress paths. The result of analysis shows that the constitutive
model can capture well the main deformation behavior of cohesionless soil, such as stress-strain nonlinearity, hardening property,
dilatancy, stress path dependency, non-coaxiality between the principal stress and the principal strain increment directions,
and the coupling of mean effective and deviatoric stress with deformation. In addition, the model can also take into account
the rotation of principal stress axes and the influence of intermediate principal stress on deformation and strength of soil
simultaneously. The excellent agreement between the predicted and measured behavior indicates the comprehensive applicability
of the model.
Biography: SHI Hong-yan (1962-), Associate Professor, Doctor 相似文献
120.
Masayuki Koike Toru Kaji Prathuang Usaborisut Tomohiro Takigawa Akira Yoda Shozo Takahashi 《Journal of Terramechanics》2002,39(3):127-141
To investigate the soil compactibility during the cyclic loading, three different kinds of paddy field soils were prepared so as to clarify interrelationships among stress amplitude ratio, bulk density, soil water content and pore water pressure. The presetting values of specimen include the soil water content(percent dry basis) and bulk density of 25% d.b. and 1.1 Mg/m3 respectively. The relation between the number of cyclic loading and axial strain exhibited an asymptotically increasing trend, converging toward a specific value for each experimental condition. Possible effect caused by elastic–plastic characteristics could be recognized, when axial strain for 0.5 Hz excitation becomes greater than the one of 1.0 Hz under same stress amplitude ratio. When the stress amplitude ratio took 0.1, the absolute value of axial strain of 0.5 Hz was greater than the one of 1.0 Hz, whereas its decreasing trend was recognized in the sequence of silt, clay and silty sand. The qualitative relations between pore water pressure and number of cyclic loading were also examined to scrutinize the effect of effective water pressure to the soil compactibility. 相似文献