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   2386篇
  免费   79篇
  国内免费   110篇
系统科学   90篇
丛书文集   83篇
教育与普及   111篇
理论与方法论   5篇
现状及发展   14篇
研究方法   6篇
综合类   2264篇
自然研究   2篇
  2024年   13篇
  2023年   46篇
  2022年   61篇
  2021年   71篇
  2020年   44篇
  2019年   11篇
  2018年   26篇
  2017年   33篇
  2016年   36篇
  2015年   49篇
  2014年   78篇
  2013年   57篇
  2012年   74篇
  2011年   99篇
  2010年   75篇
  2009年   109篇
  2008年   109篇
  2007年   102篇
  2006年   87篇
  2005年   85篇
  2004年   55篇
  2003年   50篇
  2002年   74篇
  2001年   84篇
  2000年   101篇
  1999年   139篇
  1998年   106篇
  1997年   126篇
  1996年   109篇
  1995年   57篇
  1994年   76篇
  1993年   67篇
  1992年   44篇
  1991年   49篇
  1990年   45篇
  1989年   50篇
  1988年   39篇
  1987年   20篇
  1986年   6篇
  1985年   6篇
  1984年   1篇
  1983年   1篇
  1978年   1篇
  1969年   2篇
  1967年   2篇
排序方式: 共有2575条查询结果,搜索用时 31 毫秒
861.
合成了N,N'-双(2-羟基-1-萘酚醛)-2,6-二亚胺吡啶配体(L)及其Co(Ⅱ)配合物.通过元素分析、摩尔电导率、红外吸收光谱、紫外吸收光谱及差热-热重分析,确定了配合物的组成为Co(NO3)2L·2H2O.在PH值为7.25的Tris-HCl缓冲溶液中,以溴化乙锭为荧光探针,用荧光光谱、黏度实验测定研究了Co(NO3)2L·2H2O与ct-DNA的相互作用,结果表明,Co(NO3)2L·2H2O与Ct-DNA客易发生插入作用,结合常数Ka=2.34×105 L/mol,结合位点为1.  相似文献   
862.
Lowest temperature and snow accumulation rate are preconditions for retrieving the oldest ice core from the polar ice sheets. The 10-m depth firn temperature at Dome A, the summit of the Antarctic Ice Sheet, recorded by an automatic weather station (AWS) was -58.3℃ in 2005 and -58.2℃ in 2006, respectively. The 10-m firn temperature is an approximation of the annual mean air temperature (AMAT), and this is the lowest AMAT that has been recorded on the surface of the Earth. The stable isotopic ratios (δ^18O and δD) of surface snow at Dome A are also lower than at other ice sheet domes along the East Antarctic Ice Divide such as Dome C, Dome F, Dome B and Vostok. These facts indicate that Dome A is the "pole of cold" on the Earth. The total amount of snow accumulation rate in 2005 and 2006 was only 0.16 cm, equaling 0.016 m water equivalent per year, the lowest precipitation ever recorded from Antarctica. Preliminary evidences indicate that Dome A is a candidate site for recovering the oldest ice core.  相似文献   
863.
研制出一种低频振动传感器作为谐振器,利用谐波小波在频域连续分布且具有严格的盒形谱特性,通过适当调整参数m和n,构造出不同频段的带通滤波器对信号进行广义谐波小波变换.通过广义谐波小波变换实现低速重载轴承振动信号的共振解调,从而提取出故障轴承的低频冲击信息,为低速重载轴承的故障诊断提供了一种新的有效方法.运用该方法准确判断出炼钢转炉悬挂齿轮箱耳轴轴承的故障.  相似文献   
864.
介绍微型扑翼飞行器虚拟设计与仿真系统的总体结构和各功能模块的实现.根据拟合的仿生公式进行微扑翼飞行器的仿生学初步设计,进而建立了飞行器的三维结构模型,并根据飞行器的结构与扑翼运动规律进行了运动仿真.由飞行器的几种典型任务确定飞行模式,在建立飞行器动力学模型的基础上进行飞行姿态控制.采用随机分形技术生成具有真实感的地形,由此讨论了飞行器虚拟飞行过程中的静态与动态模拟方法.  相似文献   
865.
文章利用扩展线性支出系统,对西藏城镇居民的消费结构及各类消费品需求变化趋势进行分析与探讨,并给出了合理的政策建议。文章旨在揭示消费品市场的规律,帮助预测各类消费品在西藏的未来需求,这对于引导居民消费,优化消费结构乃至建立合理的产业结构和产品结构均具有现实的指导意义。  相似文献   
866.
Flavonols are plant pigments that are ubiquitous in nature. Morin and other related plant flavonols have come into recent prominence because of their usefulness as anticancer, anti-tumor, anti-AIDS, and other important therapeutic activities of significant potency and low systemic toxicity. The heat of combustion of morin (molecular formula, C15H10O7·H2O) in oxygen was measured by a rotating-bomb type combustion calorimeter, the standard molar enthalpy of combustion of morin at T = 298.15 K was determined to be Δc H m (C15H10O7·H2O, s) = −(5 937.99 ± 2.99) kJ · mol−1. The derived standard molar enthalpy of the formation of morin in solid powder state at T = 298.15 K, Δf H m (C15H10O7·H2O, s), was −(1 682.12 ± 3.58) kJ · mol−1, which provide an accurate data of the stability of morin to the pharmacy and pharmacology. Biography: HOU Hanna(1956–), female, Visiting researcher, Associate professor of Hubei University of Education, research direction: thermochemistry.  相似文献   
867.
基于三维信息的人脸识别建立在二维人脸识别和人脸真实三维模型重建基础上,通过利用人脸三维模型的不变性,克服目前常用的二维人脸识别过程中存在的成像限制大(必须是正面像)、易于被欺骗(如化装、带眼镜等),从而进一步提高识别的正确率和可靠性.  相似文献   
868.
"土地利用/土地覆被"(简称LUCC)是全球环境变化研究的热点问题之一,遥感技术是LUCC研究的重要手段,遥感影像LUCC信息的高精度自动提取成为众多专家学者关注的重要研究领域.Landsat系列卫星影像是最早且最为广泛使用的中分辨率陆地遥感影像.文章以2013年2月发射的Landsat8卫星的OLI影像为研究对象,利用ENVI软件,对太原市区的土地利用/土地覆被信息分别用最大似然法和决策树分类法实现自动提取,并进行精度比较.结果表明:最大似然法提取的总体精度为69.33%,Kappa系数为0.605 5,主要是耕地与林地、草地、建设用地以及建设用地与裸地的混分、错分现象较为严重,造成分类精度较低;而决策树分类法的自动提取通过确定适当的判别规则,使地类间的混分、错分现象明显改善,分类总体精度提高到91.33%,Kappa系数达到0.892 3,各土地利用类型的分类精度均有一定提高.  相似文献   
869.
The dissolution of TiO2 in the CaO-Al2O3-SiO2 slag under static conditions was studied in the temperature range from 1643 K to 1703 K. After TiO2 dissolved, the microstructure of the interface between TiO2 and the slag was observed by scanning electron microscopy, and the concentration profiles of Ti4+and other ions across the TiO2/slag interfaces were analyzed by energy-dispersive X-ray spectroscopy. On the basis of these results, the dissolution behavior of TiO2 was evaluated, and the diffusivity of Ti4+in the bulk slag was estimated. Ac-cording to the Stokes-Einstein relation, the viscosity calculated by a previously reported model gave a diffusivity of Ti4+ions greater than that estimated by the concentration profiles of Ti4+ions. The mechanism of TiO2 dissolution in the CaO-Al2O3-SiO2 slag is discussed in de-tail.  相似文献   
870.
To use the potential heat of molten blast furnace slag completely, a CaO-Al2O3-SiO2 system glass (MSG) was prepared from the molten industrial slag. The corresponding method proposed in this study utilized both slag and its potential heat, improving the production rate and avoiding the environmental pollution. Using appropriate techniques, an MSG with uniform color and superior performances was produced. Based on the experimental results and phase diagram, the chemical composition of MSG by mass is obtained as follows: CaO 27%–33%, SiO2 42%–51%, Al2O3 11%–14%, MgO 6%–8%, and Na2O+K2O 1%–4%. Thermodynamic processes of MSG preparation were analyzed, and the phases and microstructures of MSG were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that alkali metal oxides serve as the fluxes, calcium oxide serves as the stabilizer, and alumina reinforces the Si-O network. XRD and SEM analyses show that, the prepared MSG displays the glass-feature patterns, the melting process is more complete, and the melt viscosity is lowered with an increase in calcium oxide content; however, a continuous increase in slag content induces the crystallization of glass, leading to the formation of glass subphase. The optimum content of molten slag in MSG is 67.37wt%. With respect to bending strength and acid/alkali resistance, the performance of MSG is better than that of ordinary marble.  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