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   5844篇
  免费   903篇
  国内免费   1214篇
测绘学   220篇
大气科学   546篇
地球物理   701篇
地质学   2375篇
海洋学   2194篇
天文学   34篇
综合类   431篇
自然地理   1460篇
  2024年   30篇
  2023年   93篇
  2022年   261篇
  2021年   238篇
  2020年   239篇
  2019年   315篇
  2018年   248篇
  2017年   242篇
  2016年   225篇
  2015年   228篇
  2014年   336篇
  2013年   382篇
  2012年   333篇
  2011年   294篇
  2010年   267篇
  2009年   311篇
  2008年   351篇
  2007年   358篇
  2006年   409篇
  2005年   384篇
  2004年   329篇
  2003年   295篇
  2002年   295篇
  2001年   288篇
  2000年   228篇
  1999年   187篇
  1998年   166篇
  1997年   114篇
  1996年   100篇
  1995年   86篇
  1994年   62篇
  1993年   60篇
  1992年   49篇
  1991年   38篇
  1990年   21篇
  1989年   24篇
  1988年   15篇
  1987年   11篇
  1986年   12篇
  1985年   12篇
  1984年   13篇
  1983年   6篇
  1982年   3篇
  1979年   1篇
  1977年   2篇
排序方式: 共有7961条查询结果,搜索用时 15 毫秒
131.
国内外旅游解说研究进展综述   总被引:14,自引:0,他引:14  
本文对国外旅游解说系统研究从旅游解说的起源、原理、内涵、框架、功能、受众类型、解说方式、解说规划等方面进行了全面的总结,综合了国内旅游研究的研究现状与趋势。寻找出国内解说研究与国外解说研究的差距,借此推动国内的旅游解说系统研究,从而更好地为旅游规划、旅游业的可持续发展和游客管理服务。  相似文献   
132.
A simple and rapid procedure to extract organochlorine pesticides (OCPs) from sediments by means of microwave energy is proposed. Sediment samples were irradiated with microwaves in a closed vessel system while immersed in hexane-acetone (1 : 1, v/v). The sample extracts were cleaned up using solid phase extraction with Florisil as adsorbent. Pesticides were eluted with hexane-ethyl acetate (80 : 20, v/v) and determined by gas chromatographic separation with electron capture detection. Three oven programs were assayed with two different solvent mixtures in order to achieve adequate experimental conditions for the complete extraction of organochlorine pesticides from the matrix. Different variables such as the composition of extraction solvent,  相似文献   
133.
The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (XJd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial 40Ar / 36Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic 40Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.  相似文献   

134.
本文对影响南极细菌S-15-13生长和胞外多糖产量的主要环境因子进行了研究,同时采用16S rDNA序列分析及系统发育分析方法对其进行了分子鉴定。结果表明:菌株S-15-13最佳产糖条件为:培养时间,56h;培养温度,8℃;碳源,1.0%葡萄糖;NaCl浓度,3.0%;pH,6.0-7.0。16S rDNA序列分析和系统发育分析表明,菌株S-15-13属于假交替单胞菌属(Pseudoaltero- monas)。  相似文献   
135.
136.
137.
ABSTRACT. This study examines altitudinal residential segregation by race in 146 cities in the U.S. South. It begins by embedding the topic in recent theorizations of the social construction of nature, the geography of race, and environmental justice. Second, it focuses on how housing markets, particularly in the South, tend to segregate minorities in low‐lying, flood‐prone, and amenity‐poor segments of urban areas. It tests empirically the hypothesis that blacks are disproportionately concentrated in lower‐altitude areas using gis to correlate race and elevation by digital elevation‐model block group within each city in 1990 and 2000. The statistical results confirm the suspected trend. A map of coefficients indicates strong positive associations in cities in the interior South‐where the hypothesis is confirmed‐and an inverse relationship near the coast, where whites dominate higher‐valued coastal properties. Selected city case studies demonstrate these relationships connecting the broad dynamics of racial segregation to the particularities of individual places.  相似文献   
138.
Gavin Hilson  Frank Nyame 《Area》2006,38(2):175-185
This paper examines the debate surrounding a recent decision made by the Ghanaian government to permit gold exploration – and potentially, mining – in 'protected' forest reserves. In 2001, four mining companies were awarded mineral exploration concessions in forested regions of the country, and have since put forward applications to mine for gold. Notwithstanding the sharp divide in opinion on the issue, the continued uncertainty surrounding the implications of the proposed activities makes further research on the ground imperative in the short term. Work aiming to elicit indigenous perspectives on the projects, as well as research that facilitates dialogue between and/or among stakeholder parties, should be prioritized.  相似文献   
139.
Evaluation of Ontario's Environmental Farm Plan (EFP) scheme, launched in 1993, provides an opportunity for comparisons with agri-environmental measures instituted in the European Union and other parts of North America. The EFP has a strong 'bottom-up' dimension in that it is farmers' organizations that have been central both to the scheme's instigation and to its ongoing management. This has affected the nature of the actions taken by individual farmers participating in the scheme. These actions are reviewed, especially in terms of the participants' attitudes towards stewardship of the land, environmental outcomes, cross-compliance measures, barriers to participation and the role of statutory regulation. Some contrasts are drawn with the greater 'top-down' controls exerted in several EU agri-environment schemes, with the latter's promotion of extensification and the changing role of farmers as 'producers of countryside' in a multi-functional agricultural system. The diffusion of EFP schemes throughout Canada is noted and is cited as confirming the maintenance of fundamentally different attitudes to the development of farm-based environmental actions compared with those adopted in the EU.  相似文献   
140.
油页岩综合利用对周围环境的影响——以抚顺矿区为例   总被引:3,自引:0,他引:3  
世界能源日益减少的局面为油页岩的开发应用带来广阔的前景,但油页岩工业所带来的环境影响亦不容忽视。以抚顺油页岩为例,从油页岩综合利用和油页岩工业对环境的影响等方面系统讨论了在我国发展油页岩工业时,如何本着节约能源、保护环境、实现可持续发展的观念,利用现有先进技术,科学发展油页岩工业,降低生产过程中的固、液、气污染物对环境的危害。  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