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   2658篇
  免费   121篇
  2020年   34篇
  2019年   42篇
  2018年   72篇
  2017年   69篇
  2016年   73篇
  2015年   50篇
  2014年   53篇
  2013年   929篇
  2012年   73篇
  2011年   67篇
  2010年   66篇
  2009年   62篇
  2008年   59篇
  2007年   49篇
  2006年   48篇
  2005年   30篇
  2004年   43篇
  2003年   41篇
  2002年   26篇
  2001年   26篇
  2000年   21篇
  1999年   24篇
  1998年   18篇
  1997年   21篇
  1995年   24篇
  1994年   24篇
  1993年   18篇
  1992年   22篇
  1990年   17篇
  1989年   21篇
  1988年   18篇
  1987年   19篇
  1986年   20篇
  1985年   23篇
  1984年   29篇
  1983年   31篇
  1982年   23篇
  1981年   23篇
  1980年   25篇
  1979年   29篇
  1978年   28篇
  1977年   28篇
  1976年   23篇
  1975年   24篇
  1974年   18篇
  1973年   25篇
  1972年   18篇
  1971年   22篇
  1970年   17篇
  1969年   23篇
排序方式: 共有2779条查询结果,搜索用时 31 毫秒
991.
In 2000 several bronze and lead objects were discovered at the Venetic‐Roman site (200 bc –ad 200) of Monte Calvario di Auronzo, in the Italian eastern Alps. They were mostly finished artefacts plus semi‐worked products. Analysis included SEM–EDS, EMPA, XRPD, TMS and AMS spectrometry. The compositional results of the finished artefacts suggest that the choice of the alloy had been made according to the decorative techniques to be used. A semi‐worked high‐leaded bronze object indicates the existence of a metallurgical production of cast artefacts at the site. TMS analyses of a lead ingot exclude local provenance, proving the existence of important trade routes.  相似文献   
992.
The majority of prehistoric lithic artefacts were fashioned from rocks and minerals no harder than quartz, and there is no prehistoric evidence for the working of harder materials, such as corundum and diamond. The earliest physical evidence for the use of corundum (ruby, sapphire) is thought to be the abrasive grit recovered from Bronze Age Minoan quartz beads (c. 1700–1500 bc ), while diamond is thought to have been used no earlier than 500 bc , in India. Here we show that corundum was worked c. 4000–3500 bc during the Neolithic period in China, in the form of polished axes from the Liangzhu and Sanxingcun cultures. We also present physical evidence that later Liangzhu axes (c. 2500 bc ), made from the same previously undescribed rock whose most abundant component is corundum, were polished to a mirror‐like finish with a diamond abrasive. Our findings, which are the first to support the use of corundum and diamond in a prehistoric context, may also help to explain the trademark feature of the Neolithic in China, vast quantities of finely polished nephrite jade artefacts.  相似文献   
993.
American Colossus . Blakeway Productions for Channel 4 Television , United Kingdom . 102 minutes; broadcast 5 June 2004 .  相似文献   
994.
995.
Uranium/thorium (U/Th)‐rich bitumen has been discovered within both Palaeoproterozoic black pelites and the Archean granitic basement of the Rum Jungle Mineral Field, Northern Territory, Australia. Granite‐hosted bitumen occurs as small (up to 400‐µm diameter) discrete individual nodules, which exhibit many morphological similarities to those observed in Phanerozoic siliciclastic rocks. Thorium, the dominant radioelement, occurs primarily as a hydrated Th–Y–Si–P phase. Uranium‐rich inclusions are rare, and correspond to a hydrated U–Th–Y–Si–P phase, identified as coffinite–thorogummite. Metasediment‐hosted bitumen is more variable in morphology, occurring as massive (<2 cm in width) veins that cross‐cut all foliations, as discrete individual nodules or as elongate seams (up to 500 µm in length), interpreted to represent a series of coalesced individual nodules. In all examples, uranium, the dominant radioelement in the metasediment‐hosted bitumen, is present as Th‐poor uraninite, with variable Y2O3 contents (up to 3.21 wt.%). Raman investigation of all types of bitumen indicates that it is a poorly organized carbonaceous matter, which has not been subjected to metamorphism. Consequently, a post‐metamorphic timing for hydrocarbon emplacement can be inferred and a magmatic origin can be precluded. Potential source rocks for the bitumen are black shales of the Whites Formation (up to 8 wt.% total organic carbon (TOC)) and the Koolpin Formation (approximately 13 wt.% TOC). Post‐metamorphic sericitization of rocks within the Whites Formation is accompanied by a near‐complete removal of organic matter. Alteration was possibly the catalyst for hydrocarbon generation. The Th–Y–Si–P phase within the granite‐hosted nodules is interpreted to be the result of the alteration of antecedent monazite. During this alteration, U, LREE and P were fractionated and removed, while Th, Y and Si remained immobile, and recombined to form a hydrated Th–Y–Si phase. This pervasive alteration within the basement U/Th‐rich granites is proposed as a genetic model for the formation of uranium deposits in the Rum Jungle Mineral Field and possibly unconformity associated uranium deposits on a global scale.  相似文献   
996.
We present the results of simple numerical experiments in which we study the evolution with time of fluid flow around and within a permeable fault embedded in a less permeable porous medium. Fluid movement is driven by an imposed vertical pressure gradient. The results show that fluid flow is controlled by two timescales: τf = Sl2/κF and τF = Sl2/κM, where S is the specific storage of the porous material, l the length of the fault, and κM and κF are the hydraulic conductivities of the porous material and the fault, respectively. Fluid flow and the associated fluid pressure field evolve through three temporal stages: an early phase [t < τf] during which the initial fluid pressure gradient within the fault is relaxed; a second transient stage [τf < t < τF] when fluid is rapidly expelled at one end of the fault and extracted from the surrounding rocks at the other end leading to a reduction in the pressure gradient in the intact rock; a third phase [t < τF] characterized by a steady‐state flow. From the numerical experiments we derive an expression for the steady‐state maximum fluid velocity in the fault and the values of the two timescales, τf and τF. A comparison indicates excellent agreement of our results with existing asymptotic solutions. For km‐scale faults, the model results suggest that steady‐state is unlikely to be reached over geological timescales. Thus, the current use of parameters such as the focusing ratio defined under the assumption of steady‐state conditions should be reconsidered.  相似文献   
997.
It is now common practice, by users of geographic information, to link data held at the postcode level to that obtained from the national census. This paper examines the relationship between Australia Post (AP) postcodes and Australian Bureau of Statistics (ABS) derived postal areas — which are an approximation of the former based on aggregated census collection districts (CDs). A group of adjacent ABS postal areas in northwest Melbourne was compared with the true AP postcode areas they purported to represent and the discrepancies were investigated. Firstly, shape mismatches were studied and their potential impacts upon resource allocation decisions were assessed. Next, comparisons of areas were undertaken. It was found that, in established inner city urban areas, the two sets of boundaries were highly correlated. However, outer suburban neighborhoods were identified as being particularly prone to major areal discrepancies. The implications of mismatches between these two key boundary data sources may be severe, given that management decisions and the allocation of public and private resources are often based on spatial statistical analyses which use these data sets. The authors acknowledge ABS efforts in providing information at the levels of aggregation that society demands. The introduction of ABS postal areas data has undoubtedly facilitated the use of demographic data in many sectors; it has, however, also caused some problems, for instance, when users assume that ABS postal areas are identical to AP postcodes. These issues could easily be avoided with the inclusion of more comprehensive metadata documentation accompanying ABS data. Research is continuing to develop a method by which agencies may derive common boundaries for their administrative units, yet still meet their own individual data and sampling requirements.  相似文献   
998.
Thermal springs are poorly known in the sedimentary sites of the Pyrenees. In this paper, we describe the ‘Bagnères‐de‐Bigorre’ springs which occur in a remarkably active seismotectonic context. A chemical and isotopic study of 15 spring waters (both cold and thermal, ranging in temperature from 7.0 to 49.9°C), and continuous monitoring of a single spring allow us to characterise water–rock interactions, fluids paths and mixing processes. Three groups of waters are distinguished: (I) SO42+–Ca2+–Cl thermal waters (II) SO42+–Cl–Ca2+ thermal waters and (III) HCO3–Ca2+ cold shallow waters. Their characteristics suggest interactions with Mesozoic evaporite and carbonate formations. O and D isotopes from thermal waters indicate a local meteoric origin of Atlantic signature and a recharge elevation of 800 to 1000 m, which corresponds to a single feeding area. Their δ13C values (?2.8 to ?9.6‰) are consistent with carbonate dissolution, slight fractionation and a surficial organic input leading to δ13C depletion. Sr isotopes (0.70751 to 0.70777), Na+/Cl and (Ca2+ + Mg2+)/SO42– ratios as well as thermodynamic calculations show that the dissolution of anhydrite and halite‐bearing Triassic layers control the chemical composition of group‐I and ‐II waters. The contrasting trends of cation/Cl ratios and TDS of waters from groups I and II suggest the existence of two different circulation paths at depth as well as dilution with surficial waters similar to group III. Calculated mixing proportions show that three waters from group I are diluted from 17 to 66%, whereas all waters from group II are mixed. The aquifer temperature is estimated to be in the range 55–64°C using the retrograde and prograde solubilities of anhydrite and chalcedony, respectively. Accordingly, the mean depth of the reservoir is around 1.7 km, which allows us to constrain the depth of the Triassic layer.  相似文献   
999.
This paper probes the explanatory value of mentality as a social emergent in general and of the Zeitgeist in particular. Durkheim's contention that social facts have emergent properties is open to the charge that it implies logically inconsistent “downward causation.” on the basis of an analogy with the brain‐mind dilemma and mental emergentism, the first part of the essay discusses and dismisses the notion of social emergent properties that cannot be reduced to the properties of their component parts—individuals—and their internal relations. However, ontological individualism need not compel us to methodological individualism. The second part introduces two challenges to methodological individualism. The most radical is Rajeev Bhargava's assertion that the meaning of a belief is determined not by the individual holding the belief but by the entire linguistic community. Bhargava's “contextualism” is closely related to the (post)structural demand that we focus on discourse as a communal entity instead of continuing a delusive quest for the intentions of individual speakers. a more modest alternative is Margaret Gilbert's plea for using “plural subjects”—social groups in which “participant agents” act jointly or have a jointly accepted view—in the practical syllogisms that are central to rationalizing action explanation. The notion of plural subjects lends credence to, and is reinforced by, “situationist” social psychology, which shows how people conform to peer groups, authorities, and roles. building on Wesley Salmon's and Peter Railton's ecumenical accounts of explanation, the essay argues that both individual rationalizing action explanations and explanations based on plural agents can give explanatory information: we need not choose one or the other. The third part discusses how the Zeitgeist can provide added explanatory value in an analysis of the New left. This is possible if the “spirit of the sixties” is seen as representing the values and worldviews of the “sixties generation” as a social group in Gilbert's terms. Radical youth would suspend judgment and pool their wills to conform to what they perceived were the views of the imagined “sixties community” or—rendering more explanatory force—to smaller parts of it in the guise of peer groups and organizations.  相似文献   
1000.
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