【作者】 王丽；

【导师】 吉元； 徐学东；

【作者基本信息】 北京工业大学 ， 材料学， 2008， 硕士

【摘要】 本文研究高温扫描电子显微分析方法、实验技术和应用。本文构建了一个以环境扫描电镜(ESEM)为基础的原位加热和原位操纵的综合分析测试系统。利用该系统,研究了加热补偿荷电效应的方法、机理及应用。测试了LiFePO4微粒的温度-电导性能。实验样品包括非导电陶瓷: Al2O3,AlN,硅酸镁,液晶;半导体微粒:LiFePO4。本文研究了通过原位加热方法消除非导电样品在电子束辐照下产生的荷电效应。Al2O3和硅酸镁加热至～360℃,荷电效应消除;AlN加热至～190℃,荷电效应消除;液晶的荷电补偿温度低于95℃。与通常在变压力SEM和ESEM中的荷电补偿方法相比,二次电子(SE)像的衬度和信噪比优于低真空环境中得到的效果,因为加热过程中绝缘样品可以在高真空环境中、采用ETD-SE探头直接被观察。原位监测了Al2O3, AlN和2MgO·SiO2陶瓷样品电流(Isc)和AlN表面电势(Vs)的变化。升温过程中Isc逐渐增加且由负值变正值,Al2O3样品的Isc由室温的～10-13 A增加到360℃时的10-7 A;AlN的Vs从25℃时的10.9 keV降低到80℃时的1.8 keV。计算了总电子发射产额(η+δ)。室温下为0.99～0.88,继续升温,减少到室温下导体的范围0.2～0.3,加热到360℃,(η+δ)大量增加到2000～3000。还测试了Al2O3, AlN和2MgO·SiO2陶瓷样品表面电导σ随温度的变化。Al2O3和2MgO·SiO2的σ值在360℃比90℃提高了2个数量级,达到10-11S。AlN在190℃时σ值增加到10-10 S,显示出半导体特性。本文研究了加热荷电补偿的机理。加热补偿的基本概念是:加热提高了宽禁带绝缘体的导电率。机理涉及:(1)加热加速了绝缘体表面的被捕获电子从势阱中释放的过程,从而减小表面电势,消除了荷电效应。(2)加热减小了绝缘材料的禁带宽度,使更多的价带电子跃迁到导带,提高了总电子发射产额(η+δ),即提高了SE像衬度和信噪比。(3)材料的能带结构,介电/导电/导热性能存在的差异,使得荷电补偿的温度有所差异。(4)材料的晶体结构、缺陷、杂质及成分直接影响着荷电补偿效果。多晶样品中形成的缺陷/杂质能带,减小了禁带宽度和电离能,使得加热对多晶的补偿效果优于单晶的效果。本文研究了荷电现象在评价绝缘材料性能和微观结构方面的应用。(1)Al2O3和AlN表面在降温时出现的树枝状等离子体放电和击穿,可对表面造成机械损伤,反映出加热引起介电性能和机械性能改变。(2)温度变化导致晶界和缺陷衬度变化,反映出材料微区(晶粒-晶粒,晶粒-晶界,基体-夹杂/第二相/缺陷)在介电、导电和导热性能的差异。本文还研究了半导体LiFePO4微粒的温度-电导性能。电导随温度的升高而增大;掺杂Mo和Fe离子的LiFePO4的导电率提高的更快;当温度小于130℃时,Mo离子的LiFePO4的电导率较高;当温度大于130℃时,搀杂Fe离子的LixFePO4的电导率较高。原位加热扫描电子显微分析方法,为补偿非导电样品的荷电效应,以及研究绝缘材料和半导体材料的介电、导电和导热性能提供了一种有效而简便的方法。更多还原

【Abstract】 This thesis is aim to study the scanning Electron Microscope (SEM) analysis, experiment technology and application at high temperature.Integrated experimental system for in-situ heating and manipulating based on environmental SEM (ESEM) was set up. The method, the mechanism and the application of eliminating charge effects were studied and the temperature-conductivity characteristic of LiFePO4 particles was measured by using this system.The experiment samples include insulating ceramic: Al2O3, AlN, 2MgO·SiO2, liquid crystal,and semiconductor particles:LiFePO4.Eliminating charge effect on insulators surface by electron irradiation through in-situ heating was studied in this thesis. When the temperature increased up to 360℃the charging was entirely eliminated for Al2O3 and 2MgO·SiO2.The temperature is 190℃for AlN, however, less than 95℃for liquid crystal. Contrast and sign noise ratio of SE images obtained via heating approach was better than that obtained via the approach of electron-ion neutralization in the normal low vacuum region, because the insulating samples can be observed by Everhart-Thornley Detector-Secondary Electron (ETD-SE) in high vacuum condition in the course of heating.Sample current (Isc) of Al2O3, AlN and 2MgO·SiO2 and surface potential (Vs) of AlN were in-situ measured. The Isc increase gradually varying from negative to positive with the increasing temperature. The Isc of Al2O3 varied from～10-13 A at room temperature to 10-7 A at 360℃. The Vs of AlN decreased from 10.9 keV at 25℃to 1.8 keV at 80℃.The changes in conductivity (σ) of Al2O3, AlN and 2MgO·SiO2 was also measured with the increasing temperature. Theσvalues of Al2O3 and 2MgO·SiO2 were up to 10-11S at 360℃, which is two order magnitude larger than that at 90℃. Theσvalue of AlN increased up to 10-10 S at 190℃, which indicates the characteristic of semiconductor.The mechanism of eliminating charging by heating was studied in this thesis. Basic conception of the charging compensation is theσof width band gap materials increases with the increasing temperature. The mechanism involve in: (1) The detrapping process of electron trapped on the trap site was accelerated by heating, so the surface potential was reduced and charge effect was gradually eliminated. (2) The forbidden band width of insulators was reduced by heating, which is responsible for transitions of electron from valence band to conductive band, the increase of total electron yield (η+δ) , the increase of the contrast of SE image and sign noise ratio. (3) Charging was compensated at different temperature, which is due to difference of energy band structure, dielectric, conductivity and thermo-conductivity characteristic. (4) The effect of charging compensation was affected by crystal structure, defect, impurity and composition. The defect and impurity energy band in polycrystal reduces the forbidden band width and ionization energy, which is responsible for the better compensation effect obtained for the polycrystal than that obtained for single crystal.The application of charge effect in evaluating the characteristic and microstructure of insulators was investigated. (1) The plasma flashover and breakdown of treeing on polycrystal Al2O3 and AlN samples surface during the decreasing temperature could damage the sample surface, which indicate the change of dielectric and mechanical characteristic. (2) Changes in temperatures lead to the variation of grain boundary contrast and the abnormal contrast in defect site, which indicate the difference of dielectric, conductivity and thermo-conductivity on micro-region (grain-grain, grain boundary-grain boundary, matrix-inclusions, secondary phase and defect).Moreover, the temperature-conductivity characteristic of semi-conducting LiFePO4 particles was studied. The conductivity gradually increased with the rising temperature. There is higher rising rate for LiFePO4 doped into Mo and Fe ion. When the temperature is less than 130℃, the conductivity of LiFePO4 doped Mo is higher than others and more than 130℃, that of LixFePO4 doped Fe is higher. The in-situ heating electron microscopy is an effective and convenient method to the study of charging compensation on non-conductors and the dielectric, conductivity and thermo-conductivity property of insulators and semiconductors.更多还原