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   133篇
  免费   27篇
  2024年   1篇
  2023年   4篇
  2022年   2篇
  2021年   2篇
  2020年   7篇
  2019年   6篇
  2018年   5篇
  2017年   13篇
  2016年   16篇
  2015年   8篇
  2014年   8篇
  2013年   27篇
  2012年   5篇
  2011年   8篇
  2010年   7篇
  2009年   3篇
  2008年   11篇
  2007年   6篇
  2006年   3篇
  2005年   1篇
  2004年   3篇
  2003年   5篇
  2002年   2篇
  2001年   2篇
  2000年   1篇
  1999年   1篇
  1996年   3篇
排序方式: 共有160条查询结果,搜索用时 15 毫秒
101.
This article looks at the issues facing the regulation of commercial public service broadcasting in Norway. A document analysis of the 2010 renegotiation process to renew TV 2’s public service licence illuminates how a general marketization affects the regulation of the television sector. This process comments on Hallin and Mancini’s model of democratic corporatism in the Nordic countries regarding the power of the state to regulate private media enterprises as cultural institutions. As such, the process of commercialization should include not only the broadcasting sector, but also the political sector. As part of a solution to retain TV 2’s licence agreement, the government effectively had to re‐regulate the cable distribution sector. This case study demonstrates not only the enduring strength and value of the public service institution in a political context, but also the extent to which regulation is guided by technical and economic forces to sustain democratic considerations.  相似文献   
102.
During 2002, the extensive archaeological excavations of the ancient Constância necropolis (Centre of Portugal), dated from 14th–19th centuries, resulted in the exhumation of 151 individuals. Among the several paleopathological cases, a middle‐aged female skeleton with osteolytic lesions in her skull, axial skeleton, upper limbs and femurs was observed. These pathological findings are characterised by an asymmetric pattern with osteolytic focus of distinct size and irregular shape. Some skeletal elements display both osteolytic and osteoblastic lesions. The latter exhibit deposition of fine layers of woven bone. Lesions were observed macroscopically and radiology was used as a complementary method of scrutiny, especially in cases of unclear observation. The case was diagnosed as that of a probable metastatic carcinoma due to the multifocal distribution of the lesions in areas of intense haematopoietic activity, their morphology and some osteoblastic responses, as well as the presence of pathological fractures in the ribs. The skeleton's sex and age at death are in agreement with the proposed diagnostic, constituting the first case of malignant carcinoma detected in non‐identified Portuguese human skeletal remains. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   
103.
Eastern Sicily (southern Italy) is characterised by the presence of many natural gas emissions (mofettes, mud volcanoes). These gases are mostly carbon dioxide and methane, with minor amounts of helium, hydrogen, carbon monoxide and hydrocarbons. In this study, the extent and orientation of soil gas anomalies (He and CO2) were investigated on a wide area (approximately 110 km2) located just SW of Mt. Etna. From a structural point of view, this area lays on a typical foredeep–foreland system that marks the boundary between the southern part of the Eurasian plate and the northern part of the African plate in the central Mediterranean. No tectonic structure was revealed in this area by surface geological surveys. Very high soil emissions were found, and their spatial pattern reveals the existence of some active faults all directed about N50°E. This direction coincides with that of two major fault systems that cut eastern Sicily and are evident, respectively, NE and SW of the study area. Soil gas data suggest that these fault systems are the expression of a single continuous structural line which is probably responsible for the past and present magma uprise in eastern Sicily. Isotopic values of carbon of CO2 suggest a minor contribution of organic carbon. Moreover, in the highest degassing sites the isotopic values of He found in association with CO2 (He abundance = 11–70 p.p.m.; R/Ra between 6.0 and 6.2) suggest that both gases are mantle derived. The extent of the areas affected by high gas emissions and the amounts of deep CO2 emitted in the investigated area (several hundred tonnes per day) may provide additional supporting evidence of a mantle upwelling taking place beneath this region.  相似文献   
104.
Paul Mellars has long used cave and rockshelter ungulate faunal assemblages from southwestern France to argue that the early Upper Palaeolithic people of this region focused their hunting on reindeer (Rangifer tarandus), and that such specialized hunting distinguishes the Upper from the Middle Palaeolithic in at least this region. We examine this argument quantitatively, using a sample of 133 Mousterian, Châtelperronian, and Aurignacian ungulate assemblages. We show that only five Aurignacian assemblages, from three sites, stand out in terms of the degree to which their ungulate faunas are dominated by a single taxon. We also show that some Mousterian cave and rock shelter ungulate assemblages are more heavily dominated by large bovids than Aurignacian assemblages are dominated by reindeer, and that Mellars' argument is highly dependent on the exclusion of open sites from the analysis and on the numerical threshold he has selected to indicate hunting specialization.  相似文献   
105.
This paper describes the design features and capabilities of a portable automated in‐situ closed chamber (ISCC) for the quantification of CO2 fluxes in dryland soils where both photosynthetic and respiratory components may be associated with a cyanobacterial crust. The processes of CO2 flux in dryland soils are briefly described in order to clarify the conditions that make quantification of these fluxes problematic. The instrumentation currently available for in‐situ soil CO2 flux measurements is then reviewed demonstrating their inadequacies for the dryland environment. The ISCC described here is a member of the closed or enrichment class of soil respiration chambers. The ISCC, however, features an optical window possessing high (>90%) transmission in the photosynthetic active region (PAR) of the solar irradiance spectrum, permitting observations of photosynthesis. The ISCC possesses automatic venting and purging so that gaseous concentrations inside the chamber do not change from ambient sufficiently to significantly affect diffusion. The ISCC features both active and passive cooling employing internal solid‐state Peltier coolers and external aluminised Mylar respectively. This avoids severe disturbance of the microclimate within the chamber due to admission of high fluxes of PAR and permits in‐situ operation under a wide range of ambient field temperatures (~ ?5 to 40°C). Sensors internal to the chamber monitor temperature, relative humidity, irradiance and pressure. In this implementation the ISCC is coupled to a portable gas chromatograph (Agilent GC‐3000) to sample the chamber atmosphere. Indicative data for Kalahari Sand soils of Botswana are presented as an illustration of the general performance characteristics.  相似文献   
106.
The aim of this study was to determine the process–structure–property relationships between the pre‐ and post‐CO2 injection pore network geometry and the intrinsic permeability tensor for samples of core from low‐permeability Lower Triassic Sherwood Sandstone, UK. Samples were characterised using SEM‐EDS, XRD, MIP, XRCT and a triaxial permeability cell both before and after a three‐month continuous‐flow experiment using acidic CO2‐rich saline fluid. The change in flow properties was compared to those predicted by pore‐scale numerical modelling using an implicit finite volume solution to the Navier–Stokes equations. Mass loss and increased secondary porosity appeared to occur primarily due to dissolution of intergranular cements and K‐feldspar grains, with some associated loss of clay, carbonate and mudstone clasts. This resulted in a bulk porosity increase from 18 to 25% and caused a reduction in mean diameter of mineral grains with an increase in apparent pore wall roughness, where the fractal dimension, Df, increased from 1.68 to 1.84. All significant dissolution mass loss occurred in pores above c. 100 μm mean diameter. Relative dilation of post‐treatment pore area appeared to increase in relation to initial pore area, suggesting that the rate of dissolution mass loss had a positive relationship with fluid flow velocity; that is, critical flow pathways are preferentially widened. Variation in packing density within sedimentary planes (occurring at cm‐scale along the ‐z plane) caused the intrinsic permeability tensor to vary by more than a factor of ten. The bulk permeability tensor is anisotropic having almost equal value in ‐z and ‐y planes but with a 68% higher value in the ‐x plane (parallel to sedimentary bedding planes) for the pretreated sample, reducing to only 30% higher for the post‐treated sample. The intrinsic permeability of the post‐treatment sample increased by one order of magnitude and showed very close agreement between the modelled and experimental results.  相似文献   
107.
Various thermodynamic properties of H2O that are defined as pressure or temperature derivatives of some other variable, such as isothermal compressibility (β, pressure derivative of density), isobaric thermal expansion (α, temperature derivative of density), and specific isobaric heat capacity (cf, temperature derivative of enthalpy), all show large magnitudes near the critical point, reflecting large variations in fluid density and specific enthalpy with small changes in temperature and pressure. As a result, mass (related to fluid density) and energy (related to fluid enthalpy) transport in this PT region are sensitive to changing PT conditions. Addition of NaCl to H2O causes the region of anomalous behavior, here defined as the critical region, to migrate to higher temperatures and pressures. The critical region is defined as that region of PT space in which the dimensionless reduced susceptibility  ≥ 0.5. When NaCl is added to H2O, the critical region migrates to higher temperature and pressure. However, the absolute magnitudes of thermodynamic properties that are defined as temperature and/or pressure derivatives (α, β, and cf) all decrease with increasing salinity. Thus, the mass and energy transporting capacities of hydrothermal fluids in the critical region become less sensitive to changing PT conditions as the salinity increases. For example, quartz solubility can be described as a function of fluid density, and because density becomes less sensitive to changing PT conditions as salinity increases, quartz solubility also becomes less sensitive to changing PT conditions as fluid salinity increases. Similarly, fluxibility describes the ability of a fluid to transport heat by buoyancy‐driven convection, and fluxibility decreases with increasing salinity. Results of this study show that the mass and energy transport capacity of fluids in the Earth's crust are maximized in the critical region and that the sensitivity to changing PT conditions decreases with increasing salinity.  相似文献   
108.
The ‘Movius–Schick hypothesis’ claims that an absence of Levallois (Mode 3) technologies in East Asia is due to the lack of a strong ancestral Acheulean (Mode 2) tradition in that region. Hence, this hypothesis is based on the assumption that similarities between Acheulean handaxes and Levallois cores can be interpreted as being phylogenetically homologous (i.e. due to common technological ancestry) as opposed to being homoplasic (i.e. due to convergent technological evolution). Here, the phylogenetic basis of this hypothesis is tested using a formal cladistic procedure. Under the framework of an ‘iterative’ approach to phylogenetic analysis, a series of post-hoc tests and re-evaluations of results follow the initial cladistic analysis. Results of these combined analyses indicate that morphological similarities between Mode 2 Acheulean handaxes and Mode 3 Levallois cores can, most parsimoniously, be seen as phylogenetically homologous. Hence, these results support the tenets of the ‘Movius–Schick’ hypothesis in suggesting that a lack of Levallois industries in East Asia may be due to the paucity of an ancestral Acheulean tradition in that region. The implications of these phylogenetic analyses for the concept of Palaeolithic ‘Modes’ are discussed. It is further suggested that low demographic levels (i.e. small effective population sizes) in East Asia may have constrained the technological phylogenetic trajectory of East Asia compared with that seen in other regions of the Old World during the Lower Palaeolithic. In addition, it is hoped that several methodological issues discussed here will contribute to the growing field of cultural phylogenetic analysis.  相似文献   
109.
One of the critical factors that control the efficiency of CO2 geological storage process in aquifers and hydrocarbon reservoirs is the capillary‐sealing potential of the caprock. This potential can be expressed in terms of the maximum reservoir overpressure that the brine‐saturated caprock can sustain, i.e. of the CO2 capillary entry pressure. It is controlled by the brine/CO2 interfacial tension, the water‐wettability of caprock minerals, and the pore size distribution within the caprock. By means of contact angle measurements, experimental evidence was obtained showing that the water‐wettability of mica and quartz is altered in the presence of CO2 under pressures typical of geological storage conditions. The alteration is more pronounced in the case of mica. Both minerals are representative of shaly caprocks and are strongly water‐wet in the presence of hydrocarbons. A careful analysis of the available literature data on breakthrough pressure measurements in caprock samples confirms the existence of a wettability alteration by dense CO2, both in shaly and in evaporitic caprocks. The consequences of this effect on the maximum CO2 storage pressure and on CO2 storage capacity in the underground reservoir are discussed. For hydrocarbon reservoirs that were initially close to capillary leakage, the maximum allowable CO2 storage pressure is only a fraction of the initial reservoir pressure.  相似文献   
110.
This paper identifies external factors affecting the capacity of Australia's now‐formalised 56 regional natural resource management (NRM) bodies and their community‐based Boards to meet planning and management responsibilities. It demonstrates that little is known about the basic capacity‐related characteristics of NRM regions, despite the lengthy and elaborate process of regionalism that Australia has embarked upon, with its associated and substantial devolution of responsibilities and resources. A suite of indicators is used to develop an ‘exploratory’ capacity typology of NRM regions. The ten regional ‘types’ identified are found to attract varying budget allocations under the Natural Heritage Trust Extension and National Action Plan for Salinity and Water Quality. There are indications that State and regional interests within and outside NRM can significantly influence the distribution of resources. An examination of resources allocated to capacity‐building activities shows significant differences between regions in the scale of resources allocated (0–96% of total budget). The paper argues the case for intervention to reduce the gap between ‘have’ and ‘have not’ regions, and for further exploration of disparities in the allocation of resources to capacity‐building activities. Clarification is needed of the extent to which capacity‐building activities adequately target regional NRM bodies and their Boards.  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