全文获取类型
收费全文 | 2047篇 |
免费 | 310篇 |
专业分类
2357篇 |
出版年
2023年 | 13篇 |
2022年 | 2篇 |
2021年 | 21篇 |
2020年 | 34篇 |
2019年 | 132篇 |
2018年 | 111篇 |
2017年 | 111篇 |
2016年 | 151篇 |
2015年 | 107篇 |
2014年 | 107篇 |
2013年 | 453篇 |
2012年 | 112篇 |
2011年 | 124篇 |
2010年 | 103篇 |
2009年 | 76篇 |
2008年 | 93篇 |
2007年 | 92篇 |
2006年 | 63篇 |
2005年 | 37篇 |
2004年 | 51篇 |
2003年 | 46篇 |
2002年 | 30篇 |
2001年 | 29篇 |
2000年 | 39篇 |
1999年 | 11篇 |
1998年 | 19篇 |
1997年 | 30篇 |
1996年 | 23篇 |
1995年 | 18篇 |
1994年 | 16篇 |
1993年 | 7篇 |
1992年 | 10篇 |
1991年 | 2篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 9篇 |
1980年 | 6篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1976年 | 2篇 |
1975年 | 5篇 |
1974年 | 4篇 |
1971年 | 2篇 |
1968年 | 2篇 |
排序方式: 共有2357条查询结果,搜索用时 0 毫秒
31.
K. Su J.‐P. Latham D. Pavlidis J. Xiang F. Fang P. Mostaghimi J. R. Percival C. C. Pain M. D. Jackson 《Geofluids》2015,15(4):592-607
Accurate simulation of multiphase flow in fractured porous media remains a challenge. An important problem is the representation of the discontinuous or near discontinuous behaviour of saturation in real geological formations. In the classical continuum approach, a refined mesh is required at the interface between fracture and porous media to capture the steep gradients in saturation and saturation‐dependent transport properties. This dramatically increases the computational load when large numbers of fractures are present in the numerical model. A discontinuous finite element method is reported here to model flow in fractured porous media. The governing multiphase porous media flow equations are solved in the adaptive mesh computational fluid dynamics code IC‐FERST on unstructured meshes. The method is based on a mixed control volume – discontinuous finite element formulation. This is combined with the PN+1DG‐PNDG element pair, which has discontinuous (order N+1) representation for velocity and discontinuous (order N) representation for pressure. A number of test cases are used to evaluate the method's ability to model fracture flow. The first is used to verify the performance of the element pair on structured and unstructured meshes of different resolution. Multiphase flow is then modelled in a range of idealised and simple fracture patterns. Solutions with sharp saturation fronts and computational economy in terms of mesh size are illustrated. 相似文献
32.
This article explores everyday life among Guji children in southern Ethiopia and the place of children in an intergenerational social order. Based on data generated through ethnographic fieldwork among the Guji, we show that work, school and play are significant and intertwined social practices. Local knowledge and skills of importance for sustainable livelihood are acquired through children's participation in these different social practices. Oral tradition represents a key element of local knowledge and social practices in everyday life. However, political and social changes, such as settlement policies and the introduction of schools, affect the dynamic interconnectedness of these practices, as well as relations between different generations. These changes also have implications for local knowledge and local livelihoods. 相似文献
33.
Home range is commonly understood to be the distance from home that children are allowed to go in the outdoor environment with the term being used within various academic disciplines. Different factors influence children's home range including traffic, age, parental fears and understandings of what it means to be a good parent. Research addressing home range over different generations has identified a context of changes in the built environment, demography and technology. This paper reports results from three generations of two families in Sheffield in the north of England and confirms a reduction in four major domains: home range, variety of outdoor spaces visited, range of activities undertaken and the number of companions. 相似文献
34.
35.
Latent trajectory models for space‐time analysis: An application in deciphering spatial panel data 下载免费PDF全文
Li An Ming‐Hsiang Tsou Brian H. Spitzberg Dipak K. Gupta J. Mark Gawron 《Geographical analysis》2016,48(3):314-336
This article introduces latent trajectory models (LTMs), an approach often employed in social sciences to handle longitudinal data, to the arena of GIScience, particularly space‐time analysis. Using the space‐time data collected at county level for the whole United States through webpage search on the keyword “climate change,” we show that LTMs, when combined with eigenvector filtering of spatial dependence in data, are very useful in unveiling temporal trends hidden in such data: the webpage‐data derived popularity measure for climate change has been increasing from December 2011 to March 2013, but the increase rate has been slowing down. In addition, LTMs help reveal potential mechanisms behind observed space‐time trajectories through linking the webpage‐data derived popularity measure about climate change to a set of socio‐demographic covariates. Our analysis shows that controlling for population density, greater drought exposure, higher percent of people who are 16 years old or above, and higher household income are positively predictive of the trajectory slopes. Higher percentages of Republicans and number of hot days in summer are negatively related to the trajectory slopes. Implications of these results are examined, concluding with consideration of the potential utility of LTMs in space‐time analysis and more generally in GIScience. 相似文献
36.
Characterization of a Historical Cannonball from the Fortress of San Juan De Ulúa Exposed to a Marine Environment 下载免费PDF全文
M. Hernandez M. Hernandez‐Escampa C. Abreu J. Uruchurtu M. Bethencourt A. Covelo 《Archaeometry》2016,58(4):610-623
Metallurgical analyses and chemical characterizations were carried out on historical cannonballs from the Fortress of San Juan de Ulúa, Veracruz, México. Cannonballs dating from the 18th and 19th centuries share metallurgical characteristics similar to those of material coming from a shipment of ammunition found in the wreck of a sunken French ship from the battle of Trafalgar. The analyses show that the base material is grey cast iron with a carbon equivalent of 4.94 and a ferritic–perlitic matrix, in which the high phosphorus content has led to the formation of iron phosphide compounds in conjunction with a homogeneous distribution of carbon graphite flakes of Type C. In addition, corrosion products from samples revealed the presence of various crystalline iron compounds (X‐ray diffraction), mostly highly chlorinated iron compounds identified as akaganeite. X‐ray fluorescence identified various characteristics of the corrosion products as a function of the sampling depth. FT–IR spectroscopy revealed that the main difference between the corrosion products (internal and external) is determined by the number of organic species. Differential scanning calorimetry corroborated that these corrosion products are thermally stable compounds at elevated temperatures. 相似文献
37.
A Box Containing Carpenter's Accessories from The Akko 1 Shipwreck,Israel: Archaeometallurgical Analysis of Surviving Ironwork 下载免费PDF全文
The Akko 1 shipwreck was an Egyptian armed vessel, built at the beginning of the 19th century. A wooden saw handle and a box containing iron nails and two split pins were discovered towards the stern. Given their function, location and context, these were part of the ship's carpenter's tools and accessories. A methodology was developed for conducting systematic metallurgical analysis in order to understand the manufacturing process of the surviving ironwork items, as well as to enlarge our knowledge regarding ironworking technologies during the early 19th century. Such methodology may assist in the future understanding of the technological evolution of similar wrought‐iron objects. The results demonstrated that the artefacts have a wrought‐iron heterogeneous microstructure and were manufactured by hot‐working prior to surface hardening by pack carburization. 相似文献
38.
SENIOR MIGRATION: SPATIAL CONSIDERATIONS OF AMENITY AND HEALTH ACCESS DRIVERS* 总被引:1,自引:0,他引:1 下载免费PDF全文
This paper extends the literature on amenity migration by focusing on healthcare access for later‐life migrants. Previous studies have strongly suggested that natural amenities are strong pull factors for later‐life migrants, but high natural amenity counties rarely possess the quality healthcare access optimal for elderly migrants. Utilizing a spatial Bayesian estimation strategy, we explicitly consider numerous drivers of later‐life migration to examine the extent to which health access is a driver in location decisions. We find healthcare access measures, including hospital expenditures, hospital beds, and number of doctors, are positively associated with later‐life migration decisions. 相似文献
39.
We examine the relationship between individual commuting behavior and household responsibilities, with a focus on gender differences in that relationship. Using the Dutch Time Use Survey for the years 2000 and 2005, we analyze the relationship between commuting time, home production, and childcare. To deal with reverse causality, we use Propenstity Score Matching techniques to obtain imputed data for individuals. We find that the effect of home production on commuting time for women is more than double that for men, while childcare time has an effect on women's commuting behavior only. Our results shedding light on the Household Responsibility Hypothesis. 相似文献
40.