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   9135篇
  免费   1665篇
  国内免费   2199篇
测绘学   464篇
大气科学   1897篇
地球物理   2493篇
地质学   4585篇
海洋学   910篇
天文学   513篇
综合类   934篇
自然地理   1203篇
  2024年   40篇
  2023年   130篇
  2022年   402篇
  2021年   446篇
  2020年   379篇
  2019年   490篇
  2018年   539篇
  2017年   470篇
  2016年   556篇
  2015年   456篇
  2014年   562篇
  2013年   515篇
  2012年   486篇
  2011年   569篇
  2010年   532篇
  2009年   499篇
  2008年   460篇
  2007年   473篇
  2006年   356篇
  2005年   293篇
  2004年   284篇
  2003年   277篇
  2002年   284篇
  2001年   258篇
  2000年   304篇
  1999年   389篇
  1998年   328篇
  1997年   336篇
  1996年   275篇
  1995年   241篇
  1994年   287篇
  1993年   215篇
  1992年   166篇
  1991年   128篇
  1990年   103篇
  1989年   76篇
  1988年   98篇
  1987年   50篇
  1986年   44篇
  1985年   40篇
  1984年   35篇
  1983年   32篇
  1982年   35篇
  1981年   27篇
  1980年   7篇
  1979年   10篇
  1978年   5篇
  1975年   2篇
  1958年   7篇
  1954年   2篇
排序方式: 共有10000条查询结果,搜索用时 843 毫秒
471.
上组煤采空区对于下组煤的成像和安全回采有着重要影响,有必要研究矿区上组煤采空区精准成像技术.对于实际采空区及其下组煤的地震响应特征预测来说,建立和实际地质情况相吻合的正演模型是基础.根据搜集到的研究矿区钻孔测井资料,获取建模所需的等效地层物性参数、采空区垮落带等效模量、裂隙带等效模量等数据,建立与研究矿区地质情况比较接...  相似文献   
472.
Urban planning construction land standard is the technical specification for scientifically allocating various types of urban construction land, and it is the basis for drawing up and revising the overall urban planning scheme. Considering China's current urban planning construction land standard, many problems exist, such as the gap in the land use control threshold, the lack of regional differences in the climate revision, and failing to consider the topographic factors. To resolve these problems, this study proposed a step-by-step process framework and quantitative calculation method for the establishment and revision of standards in accordance with the principle of Total-Structure control. By setting the conditions, a universal basic standard for construction land was established. Quantitative analysis was then conducted on the relationship between the basic standard and the selected key indicators, such as urban population size, sunshine spacing coefficient, the width of river valleys or inter-montane basins, and terrain slope, among others. Finally, revised standards were formed for climate conditions, topography, and geomorphologic conditions, which were matched with the basic standards. The key results are three-fold:(1) The per capita construction land standard of 95 m~2/person can be used as the total indicator of China's urban planning basic standard, and the corresponding per capita single construction land comprises 32.50% of residential land, 7.42% of public management and public service land, 22.50% of industrial land, 17.50% of transportation facilities, 12.50% of green space, and 7.58% of other land-use types. The results of the revision of the urban population size indicate that the difference in population size has little effect on the total amount of per capita construction land.(2) The climate revision results of per capita residential land and per capita construction land in major cities reveal that the revised climate value varies greatly between north and south China. The revised climate values of the per capita area of construction land vary by latitude as follows: the value at 20°N is 93 m~2/person, the value at 30°N is 97 m~2/person, the value at 40°N is 103 m~2/person, and the value at 50°N is 115 m~2/person. The basic standard land value of 95 m~2/person is generally distributed across the Xiamen-Guilin-Kunming line.(3) The cities located in mountainous areas, hilly valleys, or inter-montane basins can reduce the allocation of community parks and comprehensive parks when the average width of an existing river valley or inter-montane basin is less than 2 km. When the average width of the valley or inter-montane basin is between 2 km to 4 km, the allocation of the comprehensive parks can be reduced. The revised results of per capita sloping construction land reveal that the terrain slope greatly affects the revised value of per capita construction land. Specifically, the revised value at 3° is 3.68% higher than the basic standard value, and the increase rates at 8°, 15°, and 25° are 11.25%, 26.49%, and 68.47%, respectively.  相似文献   
473.
李芳  吴凤平  陈柳鑫  许霞  赵越 《地理科学》2021,41(4):728-736
基于公平合理利用视角,将多准则决策模型与破产理论相结合探讨跨境水资源分配问题.通过梳理国际水法,设计了跨境流域水资源公平合理分配的指标体系,采用投影法解决这一多准则决策问题,并依此加权调整流域国用水需求.在加权调整用水需求和考虑议价能力的基础上,基于破产博弈模型探讨跨境流域水资源分配问题,以提高分配方案的公平合理性和可...  相似文献   
474.
中国金红石矿床成矿地质条件及成矿远景   总被引:1,自引:0,他引:1  
中国金红石矿床成因类型不一,控矿因素各异。通过对地层、构造(包括大地构造环境及褶皱)等控矿因素,区域变质作用、岩浆作用、热液作用、风化及沉积作用等成矿地质条件及矿床分布等规律的归纳与研究,提出了陕西石泉-镇坪、陕西丹凤-西峡、河南南召-方城、湖北枣阳-河南新县、湖北英山-潜山、江苏徐州-东海、山东栖霞-荣城、山西代县-河北涞水、四川会东9个金红石成矿远景区。并圈定出山东省荣城曲家、安徽凤阳板桥、湖北郧西土门—涧池、湖北七里坪大悟仙山、山西代县义成沟—羊廷寺、河北涞源司各庄、河北丰宁黄土梁等7个找矿靶区。  相似文献   
475.
476.
Urban expansion is a phenomenon of urban space increase, and an important measuring index of the process of urbanization. Taking Shanghai as an example, the changes of urban average height and built-up area were studied to represent city’s vertical and horizontal increases respectively, and statistical methods were used to analyze the driving forces of urban expansion. The research drew following conclusions: 1) The urban expansion process of Shanghai from 1985 to 2006 had a clear periodic feature, and could be divided into three stages: vertical expansion in dominance, coordinated vertical and horizontal expansion, and horizontal expansion in dominance. 2) The average height and quantity of buildings in core city were significantly bigger than those in suburbs, but the changing speed of the latter was faster. And 3) urbanization process was the major driving force for the city’s horizontal expansion, while industrial structure improvement was the key driving factor for the vertical expansion. Those two driving forces were simultaneously affected by city’s political factors.  相似文献   
477.
This paper explores the method of comprehensive evaluation of water resources carrying capacity and sets up an evaluation model applying the fuzzy comprehensive evaluation method. Based on the data of nature, society, economics and water resources of the Tarim River Basin in 2002, we evaluated the water resources carrying capacity of the basin by means of the model. The results show that the comprehensive grades are 0.438 and 0.454 for Aksu and Kashi prefectures respectively, where the current water resources exploitation and utilization has reached a relative high degree and there is only a very limited water carrying capacity, 0.620 for Kizilsu Kirgiz Autonomous Prefecture, where water resources carrying capacity is much higher, and in between for Hotan Prefecture and Bayingolin Mongolian Autonomous Prefecture. As a whole, the comprehensive grade of the Tarim River Basin is 0.508 and the current water resources exploitation and utilization has reached a relative high degree. Thus, we suggest that the integrated management of the water resources in the basin should be strengthened in order to utilize water resources scientifically and sustainably.  相似文献   
478.
The source rock from which the sillimanite gneisses derive mainly was the biotite plagioclase gneiss in the Larsemann Hills. It is the deformation-metamorphism process under special pressure and temperature condition, not the original rock compositions, that controls the presence of sillimanite. To a great degree, the sillimanite gneiss was the mixture of the detaining materials of the migrating felsic melt from the bt-plagioclase gneiss that underwent partial melting and the relics when the melt was removed. In sillimanitization the original rock had been changed substantially in chemical composition. The related metamorphism process severely deviated from the isochemical series, the process was of, therefore, an open system. In addition, the Al2O3 contents of the original rock was an important, but not critical factor for the formation of sillimanite, i.e., the sillimanite-bearing rock need not be of aluminum rich in composition, and vise contrarily, the aluminum rock may not produce sillimanite. The authors of the present paper postulate that the source rock from which the aluminum rich rock derives need not be of aluminum rich, but sillimanitization is generally the Al2O3 increasing process. The aluminum rich sediments such as clay or shale need not correspond directly to sillimanite-rich gneisses. No argillaceous rock present equals to sillimanite-rich gneiss in chemical composition. The protoliths to the sillimanite gneisses from the Larsemann Hills, east Antarctica, and their adjacent area may be pelite, shale greywacke, sub-greywacke, quartz sandstone and quartz-tourmalinite. If correct, the conclusion will be of significant implication for the determination of the sillimanite gneiss formation process and the reconstruction of the protolith setting.  相似文献   
479.
In order to select an optimum extraction method for the target glycoprotein (TGP) from jellyfish (Rhopilema esculentum) oral-arms, a high performance liquid chromatography (HPLC)-assay for the determination of the TGP was developed. Purified target glycoprotein was taken as a standard glycoprotein. The results showed that the calibration curves for peak area plotted against concentration for TGP were linear (r = 0.9984, y = 4.5895x+47.601) over concentrations ranging from 50 to 400 mgL-1. The mean extraction recovery was 97.84% (CV2.60%). The fractions containing TGP were isolated from jellyfish (R. esculentum) oral-arms by four extraction methods: 1) water extraction (WE), 2) phosphate buffer solution (PBS) extraction (PE), 3) ultrasound-assisted water extraction (UA-WE), 4) ultrasound-assisted PBS extraction (UA-PE). The lyophilized extract was dissolved in Milli-Q water and analyzed directly on a short TSK-GEL G4000PWXL (7.8 mm×300 mm) column. Our results indicated that the UA-PE method was the optimum extraction method selected by HPLC.  相似文献   
480.
Fish skin collagen hydrolysates (FSCH) were prepared from walleye pollock (Theragra chalcogramma) using a mixture of enzymes, namely trypsin and flavourzyme. The degree of hydrolysis of the skin collagen was 27.3%. FSCH was mainly composed of low-molecular-weight peptides and the relative proportion of <1000Da fraction was 70.6%. Free radical and oxygen species scavenging activities of FSCH were investigated in four model systems, including diphenylpicrylhy-drazyl radical (DPPH), superoxide anion radical, hydroxyl radical and hydrogen peroxide model, and compared with that of a native antioxidant, reduced glutathione (GSH). FSCH was also evaluated by water-absorbing and water-holding capacity. The results showed that FSCH was able to scavenge free radical and oxygen species significantly and to enhance water-absorbing and water-holding capacity remarkably. Therefore, FSCH may have potential applications in the medicine and food industries.  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