首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   394篇
  免费   45篇
  国内免费   65篇
测绘学   33篇
大气科学   58篇
地球物理   89篇
地质学   197篇
海洋学   29篇
天文学   21篇
综合类   34篇
自然地理   43篇
  2024年   1篇
  2023年   3篇
  2022年   12篇
  2021年   23篇
  2020年   19篇
  2019年   12篇
  2018年   29篇
  2017年   28篇
  2016年   27篇
  2015年   20篇
  2014年   31篇
  2013年   29篇
  2012年   22篇
  2011年   17篇
  2010年   27篇
  2009年   27篇
  2008年   17篇
  2007年   14篇
  2006年   14篇
  2005年   9篇
  2004年   8篇
  2003年   12篇
  2002年   4篇
  2001年   5篇
  2000年   9篇
  1999年   11篇
  1998年   16篇
  1997年   10篇
  1996年   9篇
  1995年   6篇
  1994年   4篇
  1993年   7篇
  1992年   4篇
  1991年   4篇
  1990年   1篇
  1989年   2篇
  1988年   3篇
  1987年   1篇
  1986年   1篇
  1985年   1篇
  1984年   1篇
  1983年   1篇
  1982年   2篇
  1958年   1篇
排序方式: 共有504条查询结果,搜索用时 31 毫秒
91.
玄武岩—水相互作用的溶解机理研究   总被引:11,自引:0,他引:11  
党志  侯瑛 《岩石学报》1995,11(1):9-15
在常温常压排除大气干扰的封闭体系条件下,将一定粒度的玄武岩样品浸泡在不同酸度的水溶液里,研究玄武岩-水相互作用过程中元素释放和溶液酸度的变化情况。结果发现:(1)经过一段时间后,溶液中主要元素的浓度趋向平衡;(2)溶液酸度发生明显变化,即不论反应初始溶液是碱性的还是酸性的,最终都将趋于中性,我们把此现象称为溶液的中性化效应。根据溶液中溶解组分的浓度和样品表面组成的XPS测定结果,提出玄武岩在碱性和酸性溶液中不同的溶解机理。  相似文献   
92.
In the context of the sea level survey at the mm level, it is necessary all along the lifetime of the altimeter mission to survey the quality of the products from the microwave radiometer. The calibration of the brightness temperatures has been validated using reference brightness temperatures over selected continental areas as well as simulations for a wide range of oceanic and atmospheric situations. The validation of the wet path delay is performed by comparison with radiosonde measurements and pointed out that both the JMR and the TMR estimate wet path delay around 5 mm higher than the one measured by radiosondes. Furthermore, it appeared that the correction of the TMR drift degrades the product with respect to radiosonde measurements. The monitoring of the brightness temperatures since launch shows a mean drift around +0.1 K/year for the 18.7 GHz, -0.6 K/year for the 23.8 GHz channel, and around -0.4 K/year for the 34 GHz channel.  相似文献   
93.
The objective of the present study is to develop a volume of fluid (VOF)-based two-phase flow model and to discuss the applicability of the model to the simulation of wave–structure interactions. First, an overview of the development of VOF-type models for applications in the field of coastal engineering is presented. The numerical VOF-based two-phase flow model has been developed and applied to the simulations of wave interactions with a submerged breakwater as well as of wave breaking on a slope. Numerical results are then compared with laboratory experimental data in order to verify the applicability of the numerical model to the simulations of complex interactions of waves and permeable coastal structures, including the effects of wave breaking. It is concluded that the two-phase flow model with the aid of the advanced VOF technique can provide with acceptably accurate numerical results on the route to practical purposes.  相似文献   
94.
An efficient finite–discrete element method applicable for the analysis of quasi‐static nonlinear soil–structure interaction problems involving large deformations in three‐dimensional space was presented in this paper. The present method differs from previous approaches in that the use of very fine mesh and small time steps was not needed to stabilize the calculation. The domain involving the large displacement was modeled using discrete elements, whereas the rest of the domain was modeled using finite elements. Forces acting on the discrete and finite elements were related by introducing interface elements at the boundary of the two domains. To improve the stability of the developed method, we used explicit time integration with different damping schemes applied to each domain to relax the system and to reach stability condition. With appropriate damping schemes, a relatively coarse finite element mesh can be used, resulting in significant savings in the computation time. The proposed algorithm was validated using three different benchmark problems, and the numerical results were compared with existing analytical and numerical solutions. The algorithm performance in solving practical soil–structure interaction problems was also investigated by simulating a large‐scale soft ground tunneling problem involving soil loss near an existing lining. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   
95.
The Mekong floodplains, which encompasses the region from Kratie Township in Central Cambodia to the Vietnamese East Sea, is a region of globally renown agricultural productivity and biodiversity. The construction of 135 dams across the Mekong basin and the development of delta‐based flood prevention systems have caused public concern given possible threats on the stability of agricultural and ecological systems in the floodplains. Mekong dams store water upstream and regulate flow seasonality, while in situ flood prevention systems re‐distribute water retention capacity in the floodplains. The main aim of this paper is to evaluate possible impacts of the recent development of both hydropower dams and flood prevention systems on hydrological regimes in the Mekong floodplains. An analysis of measured daily and hourly water level data for key stations in the Mekong floodplains from Kratie to the river mouth in Vietnam was conducted. Hydropower dam information was obtained from the hydropower database managed by the Mekong River Commission, and the MODIS satellite imagery was used to detect changes in flooding extent related to the operation of flood prevention systems in the Vietnam Mekong Delta. Results indicate that the upper part of the floodplains, the Cambodian floodplains, may buffer upstream dam impacts to the Vietnam Mekong Delta. Flood prevention up to date has had the greatest effect on the natural hydrological regime of the Mekong floodplains, evidenced by a significant increase of water level rise and fall rates in the upper delta and causing water levels in the middle delta to increase. The development of flood prevention systems has also effected spatial distribution of flooding as indicated via a time series analysis of satellite imagery. While this development leads to increase localized agricultural productivity, our historical data analysis indicates that development of one region detrimentally affects other regions within the delta, which could increase the risk of future conflicts among regions, economic sectors and the ecological value of these important floodplains. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
96.
Dang  Xiaohu  Zhang  Miao  Xia  Zidun  Fan  Liangxin  Liu  Guobin  Zhao  Gaochang  Tao  Rui  Wei  Xuexiao 《GeoJournal》2021,86(4):1639-1655
GeoJournal - As the largest conservation program and payments for ecosystem services in China, the grain-for-green program (GGP) has substantially changed land use and cover, and reduced soil...  相似文献   
97.
以17个气象站1960—2012年的观测资料,利用趋势分析, Mann Kendall检验,Morlet小波分析等方法,分析了石家庄地区气温稳定通过10 ℃的农业气候资源变化特征,结果表明:近53年气温稳定通过10 ℃的初日以14 d/10a的趋势提前,终日以1.1 d/10a的趋势后延,持续日数以2.5 d/10a的趋势增加,积温以64.5 ℃〖DK〗·d/10a的趋势增加,降水量无明显变化,日照时数以53.4 h/10a的趋势减少;各要素的时间序列除降水外均存在不同程度的突变,终日、持续日数、积温以及日照时数的突变较为显著,终日在2007年后后延显著,持续日数在1998年后显著增多,积温在2000年后显著增加,日照时数在1993年后显著减少;通过Morlet小波分析发现,各要素在近53年中存在周期性变化规律,初日和期间积温存在准2~4年的振荡周期,终日存在准2~3年的振荡周期,持续日数存在准2年的振荡周期。降水量存在准3年和准3~7年的振荡周期,日照时数存在准2~3年的振荡周期。石家庄地区气温稳定通过10 ℃的农业气候资源变化使作物的生长发育提前,生长周期变短,日照时数减少对作物生长有一定的不利影响。  相似文献   
98.
Oxytetracycline-resistant bacteria were isolated from a mariculture farm in China, and accounted for 32.23% and 5.63% of the total culturable microbes of the sea cucumber and the sea urchin rearing waters respectively. Marine vibrios, especially strains related to Vibrio splendidus or V. tasmaniensis, were the most abundant resistant isolates. For oxytetracycline resistance, tet(A), tet(B) and tet(D) genes were detected in both sea cucumber and sea urchin rearing ponds. The dominant resistance type for V. tasmaniensis-like strains was the combination of both tet(A) and tet(B) genes, while the major resistance type for V. splendidus-like strains was a single tet(D) gene. Most of the sea cucumber tet-positive isolates harbored a chloramphenicol-resistance gene, either cat IV or cat II, while only a few sea urchin tet-positive isolates harbored a cat gene, actually cat IV. The coexistence of tet and cat genes in the strains isolated from the mariculture farm studied was helpful in explaining some of the multi-resistance mechanisms.  相似文献   
99.
The spectrometers of the Cassini mission to the Saturn system have detected haze layers reaching up to 800 km in Titan’s atmosphere. Knowledge of the complex refractive index (k) of the haze is important for modeling the surface and atmosphere of Titan and retrieving some information about the functional groups present in the aerosols. Plasma discharges or ultraviolet radiation are commonly used to drive the formation of solid organics assumed to be good analogs of the Titan aerosols. [Tran, B.N., Ferris, J.P., Chera, J.J., 2003a. The photochemical formation of a Titan haze analog. Structural analysis by X-ray photoelectron and infrared spectroscopy. Icarus 162, 114-124; Tran, B.N., Force, M., Briggs, R., Ferris J.P., Persans, P., Chera, J.J., 2008. Photochemical processes on Titan: Irradiation of mixtures of gases that simulate Titan’s atmosphere. Icarus 177, 106-115] reported the index of refraction of analogs synthesized by far ultraviolet irradiation of various gas mixtures. k was determined in the 200-800 nm wavelength range from transmission and reflection spectroscopy. However, this technique is limited by (i) uncertainties in the absorption values because of the small amounts of organics available, (ii) light scattering by the surface roughness and particulates in the sample. These limitations prompted us to perform new measurements using photothermal deflection spectroscopy (PDS), a technique based on the conversion of absorbed light into heat in the material of interest. By combining traditional spectroscopy (λ < 500 nm) and PDS (λ > 500 nm), we determined values of k over the 375-1550 nm range. k values as low as 10−4 above 1000 nm were determined. This is one order of magnitude lower than the measurements generally used as a reference for Titan’s aerosols analogs [Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callicott, T.A., Williams, M.W., 1984. Optical-constants of organic Tholins produced in a simulated Titanian atmosphere—from soft-X-ray to microwave-frequencies. Icarus 60(1), 127-137]. We recommend that these results were used in models to describe the optical properties of the aerosols produced in Titan’s stratosphere.  相似文献   
100.
A comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC/TOFMS) method has been developed for separation of a series of petroleum samples using a polar/non-polar column set configuration. Groups of oils were selected to provide samples from the same oil family to assist in comparison of compositional changes during biodegradation. The groups also represented different sources and ages to allow an assessment of the application of GC × GC for the differentiation of oil source, as well as fluid history. The increased resolution and separation afforded by the GC × GC technique provides more complete compositional information on complex biodegraded oil samples than one-dimensional GC, and improves the ability to study biodegradation trends. Among the components identified, it is proposed that alkyl-decahydronaphthalenes constitute a significant contribution to the UCM.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号