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Varved sediments of Lake Yoa (Ounianga Kebir,Chad) reveal progressive drying of the Sahara during the last 6100 years 总被引:1,自引:0,他引:1
Pierre Francus Hans von Suchodoletz Michael Dietze Reik V. Donner Frédéric Bouchard Ann‐Julie Roy Maureen Fagot Stefan Kröpelin 《Sedimentology》2013,60(4):911-934
The sedimentological and geochemical properties of a 7·47 m long laminated sequence from hypersaline Lake Yoa in northern Chad have been investigated, representing a unique, continuous 6100 year long continental record of climate and environmental change in the eastern Central Sahara. These data were used to reconstruct the Mid to Late Holocene history of this currently hyper‐arid region, in order to address the question of whether the Mid Holocene environmental transition from a humid to a dry Sahara was progressive or abrupt. This study involved a suite of analyses, including petrographic and scanning electron microscope examination of thin sections, X‐ray diffraction, X‐radiography, granulometry, loss on ignition and magnetic susceptibility. The potential of micro‐X‐ray fluorescence core scanning was tested at very high resolution. Detailed microscopic investigation revealed the sedimentary processes responsible for the formation of the fine laminations, identified the season during which they were formed, and confirmed their annually rhythmic nature. High‐resolution X‐ray fluorescence core scanning allowed the distinction of each individual lamination over the entire record, opening new perspectives for the study of finely laminated sediment sequences. Geochemical and mineralogical data reveal that, due to decreasing monsoon rainfall combined with continuous and strong evaporation, the hydrologically open and fresh Mid Holocene Lake Yoa slowly evolved into the present‐day hypersaline brine depleted in calcium, which has existed for about the past 1050 years. During the oldest part of the investigated period, Lake Yoa probably contained a permanently stratified lower water column that was nevertheless disrupted relatively frequently by mixing events. Deep‐water anoxia became more stable because of increased salinity‐driven density stratification. In parallel, the sediment grain‐size proxies record a progressive increase of aeolian input in the course of the last 6100 years. Altogether, all geochemical and sedimentological indicators point to a progressive drying of the eastern Central Sahara, strengthening previous conclusions based on palaeoecological indicators. 相似文献
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利用红外光谱仪及荧光分光光度计对象牙和猛犸象牙样品进行测试,通过对测试结果的比对、分析,总结二者的谱图特点,旨在探索更准确的象牙与猛犸象牙的检测方法.猛犸象牙内部由于受石化作用小,其红外光谱与象牙的没有本质区别,因此红外光谱仪在象牙饰品与猛犸象牙饰品的检测上难以提供准确信息.荧光光谱的测试结果显示,在280 nm波长光的激发下,由于象牙和猛犸象牙中的氨基酸在微环境及其质量分数不同,象牙的发射光谱峰值在307 nm处,猛犸象牙的发射光谱峰值在315 nm处,且象牙的荧光强度高于猛犸象牙.同步荧光光谱测试中△λ分别取15,60 nm,分别测试样品中的酪氨酸、色氨酸,测试结果显示两类牙质测试图谱谱形一致但强度差异明显,即荧光分光光度计在象牙和猛犸象牙的检测上可提供有效的信息. 相似文献
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最近NGTC实验室检测出多批次CVD合成钻石,这些CVD合成钻石均具有明显的鉴定特征,使用钻石观测仪(DiamondViewTM)观察可见蓝色、橙红色荧光和蓝色磷光以及细密层状生长纹理,光致发光光谱中可见737 nm处的特征发光线.这些特征均与其生长结构及生长过程中进入钻石晶体中的杂质有关.笔者以最近检测过程中发现的一颗CVD合成钻石为例,使用高倍率显微技术对其层状结构的微细特征及其在钻石观测仪下的荧光特征的关系进行了详细观察和对比.结果显示,此样品与之前检测到的CVD合成钻石略有差异.此次检测到的样品在钻石观测仪(DiamondViewTM)下观察具有较宽的层状生长分区,层与层之间的分界线十分明显且在高倍显微镜下也可见到细微的生长特征.通过对样品特征的分析,了解了晶体生长过程中多阶段生长条件的变化,更直观的展现了CVD合成钻石的生长过程. 相似文献
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悬浮液进样氢化物发生原子荧光法测定海洋沉积物样品中痕量砷 总被引:6,自引:0,他引:6
人类对砷的认识已有很长的历史,近几十年来关于砷对环境的污染以及对人体健康的影响日益受到人们的关注.砷是海洋沉积物中常见的主要环境污染元素之一,目前测定它的常用方法有砷钼酸-结晶紫分光光度法[1]、氢化物-原子吸收分光光度法[1]、催化极谱法[1]、氢化物-原子荧光光谱法[2],但是这些方法需将样品制备成溶液,处理过程冗长,易引起操作沾污和待测成分损失,而且试剂消耗量大,同时产生的酸气对操作人员的身体健康非常有害,为此分析工作者一直在寻求一些新的前处理技术. 相似文献
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N. Schartel 《Astronomische Nachrichten》2006,327(10):941-942
“Variable and Broad Iron Lines around Black Holes” was the topic of the first scientific workshop organized by the XMMNewton Science Operations Centre. This foreword gives a brief introduction to the history of black hole research, which motivated the workshop's scientific topic. The organizing committees are listed and the meeting is shortly outlined. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
160.
A preliminary study was carried out toexamine the feasibility of measuring tropospherichydroxyl radicals (OH) by liquidphase scrubbing andhigh performance liquid chromatography (HPLC). Thepotential advantages of this approach are itssimplicity, portability, and low expense. Thesampling system employs glass bubblers to trapatmospheric OH into a buffered solution of salicylicacid (o-hydroxybenzoic acid, OHBA). Rapidreaction of OH with OHBA produces a stable fluorescentproduct, 2,5-dihydroxybenzoic acid (2,5-DHBA), whichis determined by reverse-phase HPLC and fluorescencedetection. Our preliminary field results indicatethat this method is most suitable for OH measurementsin clean tropospheric air, where interferences fromother atmospheric species appear to be negligible orminor relative to polluted air. In clean air, thesampling period is about 45–90 minutes, which yieldsa detection limit of approximately 3–6 ×105 radicalscm-3. During an OHintercomparison experiment at the Caribou samplingsite in Colorado, our liquidphase scrubber method wascompared with the ion-assisted mass spectrometry (MS)method. Our results were within the same range asthose of the ion-assisted MS method (1–5 ×106 radicals cm-3) within our precision atthat time (about ±30–50%). Preliminary testsin Pullman, WA indicated that the method might alsofunction in moderately polluted air by acidifying thescrubbing solution or by adding a scavenger tosuppress interferences. In Pullman, mid-day OHconcentrations were usually in the range of 2–20 ×106 radicals cm-3. Nighttime OHconcentrations were always low, either at or slightlyabove the detection limit. 相似文献