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   1837篇
  免费   82篇
  2024年   3篇
  2023年   21篇
  2022年   21篇
  2021年   41篇
  2020年   80篇
  2019年   68篇
  2018年   78篇
  2017年   109篇
  2016年   83篇
  2015年   51篇
  2014年   61篇
  2013年   454篇
  2012年   123篇
  2011年   81篇
  2010年   54篇
  2009年   74篇
  2008年   77篇
  2007年   65篇
  2006年   57篇
  2005年   38篇
  2004年   46篇
  2003年   47篇
  2002年   25篇
  2001年   35篇
  2000年   18篇
  1999年   12篇
  1997年   6篇
  1996年   7篇
  1995年   9篇
  1994年   8篇
  1993年   5篇
  1992年   13篇
  1991年   8篇
  1990年   1篇
  1989年   5篇
  1988年   5篇
  1987年   7篇
  1986年   5篇
  1985年   5篇
  1984年   10篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
排序方式: 共有1919条查询结果,搜索用时 15 毫秒
41.
G. Etiope  A. Ionescu 《Geofluids》2015,15(3):438-452
Metal‐catalysed CO2 hydrogenation is considered a source of methane in serpentinized (hydrated) igneous rocks and a fundamental abiotic process germane to the origin of life. Iron, nickel, chromium and cobalt are the catalysts typically employed in hydrothermal simulation experiments to obtain methane at temperatures >200°C. However, land‐based present‐day serpentinization and abiotic gas apparently develop below 100°C, down to approximately 40–50°C. Here, we document considerable methane production in thirteen CO2 hydrogenation experiments performed in a closed dry system, from 20 to 90°C and atmospheric pressure, over 0.9–122 days, using concentrations of non‐pretreated ruthenium equivalent to those occurring in chromitites in ophiolites or igneous complexes (from 0.4 to 76 mg of Ru, equivalent to the amount occurring approximately in 0.4–760 kg of chromitite). Methane production increased with time and temperature, reaching approximately 87 mg CH4 per gram of Ru after 30 days (2.9 mgCH4/gru/day) at 90°C. At room temperature, CH4 production rate was approximately three orders of magnitude lower (0.003 mgCH4/gru/day). We report the first stable carbon and hydrogen isotope ratios of abiotic CH4 generated below 100°C. Using initial δ13CCO2 of ‐40‰, we obtained room temperature δ13CCH4 values as 13C depleted as ?142‰. With time and temperature, the C‐isotope separation between CO2 and CH4 decreased significantly and the final δ13CCH4 values approached that of initial δ13CCO2. The presence of minor amounts of C2‐C6 hydrocarbons is consistent with observations in natural settings. Comparative experiments at the same temperatures with iron and nichel catalysts did not generate CH4. Ru‐enriched chromitites could potentially generate methane at low temperatures on Earth and on other planets.  相似文献   
42.
43.
In the Iberian Peninsula, the copper metallurgy from the Chalcolithic to the Middle Bronze Age (MBA) was mostly characterized by low arsenic contents. A collection of 53 MBA artefacts from southern Portugal was analysed by micro‐EDXRF, optical microscopy, SEM–EDS and Vickers to investigate the metal composition and manufacture. No technological distinction was found between artefacts from domestic and funerary contexts, which were radiocarbon‐dated to 2000–1500 cal bc . The arsenic contents of almost 100 MBA artefacts from this region, including the above‐mentioned set, have a Gaussian distribution with a high average (3.9 wt% As). Possible explanations are discussed for this distinctive metallurgy at the south‐western end of the Iberian Peninsula.  相似文献   
44.
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   
45.
While Sunnism is the largest sect of Islam, Sunnism itself is divided into many strands. The two main strands of Sunnism for the last few centuries are the Atharī (or Salafī) strand, and the Ash?arī strand. This papers seeks to examine the earliest recorded controversies that signal this intra‐Sunnī divide. While each of these two groups has historically attempted to claim true Sunnī ‘orthodoxy’ by denying the legitimacy, and at times even the early existence, of the other, it will be shown that the genesis of this intra‐Sunnī divide dates back to the first century of Sunnism itself, viz., the 3rd AH/8th CE centuries, and that there are specific historic incidents that indicate theological tensions within the nascent Sunnī movement that would later be manifested in the two streams of Ash?arism and Salafism. Hence, it can be argued that Sunnism was never truly unified to begin with, and that these incidents demonstrate theological tensions that existed from its very origins.  相似文献   
46.
民国《贵州通志·艺文志》著录黄彭年撰《尔雅集韵》一书。而追索《艺文志》著录的史源,并取其他史料相印证,推寻其致误的原因,证明黄彭年曾批校《尔雅》《集韵》二书,但并未撰《尔雅集韵》一书。此外,又据各家书目题跋,梳理了黄彭年批校《集韵》的经过及其批校本流传情况。  相似文献   
47.
Since the 1980s there is an overall agreement that German academic and applied geography between 1933 and 1945 were closely linked to the ideology and practice of National Socialism. There is very little historical work, however, on how geography was reestablished in Germany after the end of National Socialism. This paper deals with West German geography after 1945 and the attempts to reestablish geography as a legitimate discipline within academia. Taking the influential paper by German geographer Carl Troll as a starting point, this paper deals, on the one hand, with the way geographers positioned geography in relation to National Socialism, and how they told the history of their recent past. It then asks what the defeat of Germany and the experiences of the war in general meant for how geographers in Germany thought about the relation between the discipline and politics. It is argued that a number of cleansing and legitimating strategies that freed geography from direct involvement with National Socialism, went hand in hand with a very quick adaption to the new world order and a rebranding of geography as a science of peace.  相似文献   
48.
According to the Hebrew version of the transport of the ark to Jerusalem in 2 Samuel 6, King David is so scantily dressed that he publicly exposes himself while dancing before G*d (????). David?s wild, gay and possibly sexual conduct can evoke associations with the behavior of gay persons of today. Queer readers may identify with David and like him turn their backs on dominant rulers—like the members of Saul?s dynasty—if they are not respected because of their queer way of life, but persecuted—as David was persecuted by King Saul. Such an interpretation implies that G*d (????) is on the side of persons like King David, who—from the point of view of other people as well as of David?s wife Michal—behave in a strange fashion, thus act queerly.  相似文献   
49.
This paper explores the effectiveness of policies ‘in’ attracting the foreign research and development (R&D) of multinational enterprises (MNEs) to specific countries in Europe. We develop a macroeconomic investigation covering 29 European countries during the period between 1990 and 2012 in order to address: (a) whether the provision of direct financial support for business R&D is effective for the attraction of foreign R&D; (b) whether direct support is more effective than indirect support for this purpose and (c) whether the link between direct financial support for business R&D and the foreign R&D of MNEs is conditioned by the context within which the support is provided. The results of the analysis show that, first, the provision of direct financial support is generally effective for the attraction of foreign R&D by MNEs. Second, direct support for business R&D is more effective for this purpose than indirect support. Third, the provision of direct financial support for business R&D yields greater returns in contexts that are more socio-economically suitable for knowledge-intensive, innovative activity.  相似文献   
50.
In 2004, a controversy arose in Aotearoa New Zealand's coal mining sector that involved Powelliphanta augusta, a species of land snail. This paper considers that controversy in terms of emergent assemblages involving the snail, the mining industry, the Department of Conservation, political action by environmental non‐government organisations, and public expressions of environmental and other values. Three sets of relations embedded in those assemblages are examined—those related to positioning and organising nature; values, politics, and power relations; and human and non‐human materialities. Those relations illuminate how a protracted controversy and its objects form a “social test bed” for contested views in legislation and guidelines dealing with economy, conservation, and restoration. To assess the dynamics, complexities, and contingencies of such relations, attention is paid to the controversy's detailed time line and legal framing, as well as to ontological shifts of snail and habitat and snail metrics. This analytical focus highlights the shifting quality of ideas about how the snail and its habitat are understood as entangled in fraught and mutating strategies of translocation that are played out in a particular regulatory setting. It also provides insights into international geographical efforts to re‐shape environmental governance and management.  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