【作者】 曹伟伟；

【导师】 朱波；

【作者基本信息】 山东大学 ， 材料学， 2009， 博士

【摘要】 碳素纤维及其复合材料具有优异的热辐射性能,以其作为辐射源的碳素电热元件波长匹配性良好,加热效率较高。针对自行研制的碳素纤维及其复合材料电热元件开展电热辐射性能的应用研究,分析元件制备工艺与电热性能的相互关系,系统探讨物相结构、制备工艺与碳素纤维及其复合材料热辐射性能的变化规律,可为高性能碳素热辐射材料的制备提供帮助。本文首先进行了碳素电热元件的失效问题分析,以优化工艺为目的研制了碳素纤维电热体股线的捻制缠绕设备和碳素复合材料电热体的高温定型装备,在制备出高质量碳素电热元件的基础上测试其相关的电热性能并进行机理分析,之后系统研究碳素纤维及其复合材料电热体热辐射性能的变化规律,最后从工程应用出发,进行碳素复合材料电热元件辐射热流密度分布的数值模拟。通过报废碳素电热元件的失效原因总结,确定了股线型碳素纤维电热体的捻制缠绕以及碳素复合材料电热体的高温定型是碳素电热元件制备的关键步骤。在此基础上研制了碳素纤维电热体股线的捻制缠绕设备并建立其工艺计算模型,开发了碳素复合材料电热体的高温定型装备并优化了定型后电热体的脱芯工艺。性能测试表明,经工艺优化后碳素复合材料电热元件的功率偏差可控制在5W内,表面温度均匀性较好,使用寿命达12000小时,电阻变化率较小,高压负载条件下的安全性较好,在2～20μm具有较强的热辐射能量分布。系统研究了制备工艺与碳素电热元件电热性能(功率密度-温度关系、阻温特性和热惯性)的相互关系。结果表明,采用氮气瞬时高温法进行碳素纤维电热体表面除杂,所得的带支撑芯股线型碳素纤维电热元件在相同功率密度下的稳定工作温度较高,电阻变化率较低,热惯性小。此外,支撑芯的存在不利于元件电热性能的提高,脱离石英支撑芯的限制,采用碳素纤维多股线,选用适宜浓度的定型胶进行高温定型处理,所得的无支撑芯碳素复合材料电热元件在相同功率密度下的稳定工作温度较高,电阻变化率较低,热惯性较小,其各项电热性能均优于碳毡元件。建立碳素电热元件传热微分方程,求解得到元件稳定温升及瞬时升温时间的数学表达式,经分析,碳素电热体的热辐射性能是影响元件功率密度-温度关系和热惯性的重要因素,提高碳素电热体的总发射率可以提高元件的稳定工作温度、降低瞬时升温时间。建立元件电阻变化率与端头接点特性、碳素材料初始电阻率的关系,碳素电热体的初始电阻率越小、单丝搭接点越多、接点处接触压力越大及接触点越多,元件电阻变化率越小。采用自动分光辐射测量测试了不同纤维类型、高温石墨化以及表面除杂处理下碳素纤维的法向光谱发射率和总发射率,利用X射线衍射仪(XRD)、显微激光拉曼光谱仪(LRS)进行物相结构分析。碳素纤维物相结构与热辐射性能的变化关系表明,T700碳素纤维中石墨微晶的结构规整程度优于T300样品,载流子辐射机制与石墨微晶的晶格振动模式增强,使得T700样品在2500～5000nm和5000～6500nm波段内的光谱发射率优于T300,无定型碳等紊乱结构的减少削弱了畸变结构和杂质缺陷引起的振动模式,使得6500～13000nm波段的光谱发射率较低,T700碳素纤维的光谱发射率呈现一定波长依赖性,法向总发射率低于T300样品。高温石墨化后的T300碳素纤维和国产碳毡,石墨微晶规整程度提高,载流子辐射机制与石墨晶格振动模式的增强使2500～5000nm和5000～6500nm波段内光谱发射率明显提高,同时畸变结构和杂质缺陷的减少使相关的辐射机制削弱,6500～13000nm内的光谱发射率降低,石墨化后碳素纤维的光谱发射率表现出明显的波长依赖性,总发射率降低。与氮气瞬时高温法相比,空气瞬时高温和硝酸液相表面除杂引起碳素纤维内部碳质物相结构的刻蚀程度不同,石墨微晶尺寸的减小削弱了载流子辐射机制,2500～5000nm波段的光谱发射率降低,石墨微晶相对较小的刻蚀程度使5000～6500nm波段的光谱发射率相对稳定而没有明显下降,而更加紊乱的无定型碳等物相结构的稳定性较差,发生了严重刻蚀,相关的振动模式减少使其辐射机制削弱,引起了6500～13000nm内光谱发射率的降低,光谱发射率的波长依赖性增强,总发射率降低。载流子辐射机制、石墨晶格振动辐射机制以及畸变结构、杂质缺陷的辐射机制共同作用使碳素纤维在测试波段内光谱发射率的波长依赖性较弱,表现出灰体材料的特性。测定了碳素复合材料的法向光谱发射率和法向总发射率,利用扫描电子显微镜(SEM)进行表面形貌分析,系统研究了纤维排布、碳质组成、浸渍致密化工艺以及表面形貌与碳素复合材料法向光谱发射率和法向总发射率的变化关系。结果表明,不同的纤维排布改变了样品的结构松散程度,多股线排布的碳素纤维预制体及碳毡的法向光谱发射率和法向总发射率均优于碳素纤维布预制体,短切纤维增强碳/碳多孔坯体的法向光谱发射率和法向总发射率优于碳布增强坯体。碳质组成的不同引起了碳素复合材料热辐射性能的变化。树脂碳紊乱畸变的结构比例大于纤维碳,较多的畸变晶格振动模式及缺陷杂质局域振动模式引起相应辐射机制的增强,使得树脂碳的热辐射性能优于纤维碳,因此碳/碳多孔坯体的法向光谱发射率和总发射率均优于其相应的纤维预制体。碳布和短纤维增强碳素复合材料的法向光谱发射率和总发射率均随着浸渍致密化次数的增加出现先降低后升高的趋势,多次致密化后两组样品总发射率间的差距减小,增加树脂碳含量有利于碳素复合材料发射率的提高。表面抛光引起了样品粗糙度的显著降低,碳素复合材料的法向光谱发射率和总发射率均明显下降。建立单支碳素复合材料电热元件和平行排布的多支元件的辐射热流密度分布模型,利用MATLAB软件进行数值计算和可视化分析,结果表明,元件辐射热流密度随碳素复合材料发射率的增大而增大,热流密度的不均匀度随之提高。元件辐射热流密度随石英发射率的增加而增大,热流密度的不均匀度随之提高。随石英管外半径增加,元件辐射热流密度先有所提高,在设定初始条件下,当外径大于0.45cm时热流密度又迅速减小,热流密度不均匀度随石英管外半径的增加最初增加,之后变化不大。元件辐射热流密度随输入功率而提高,热流密度不均匀度最初急剧降低,之后变化逐渐缓和。多元件辐射热流密度的分布受元件轴线与受热面间距离、元件轴线间距以及多支元件间功率配置的影响。在设定初始条件下,元件轴线与受热面距离h=12cm、元件轴线间距s=15cm、采用1000W-600W-1000W的功率配置时,辐射热流密度场呈“平拱形”分布,平行排布多元件采用合理的交替功率布置,能够获得均匀性相对较好的辐射热流密度分布场。更多还原

【Abstract】 Carbon fiber and carbon composite have excellent thermal radiation property, and the carbon heating element used it as radiation source has favorable wavelength matching ability and heating efficiency. In order to obtain high quality carbon heating element, it is important to carry out application study on the electric heating and thermal radiation property of self developed carbon fiber and carbon composite heating elements. The researches on the correlation between preparation process and electric heating properties of element, as well as the change law of thermal radiation property with phase structure and preparation process of carbon fiber and carbon composite can provide help for the preparation of high performance carbon radiation material. In this work, the failure problem analysis was carried out based on self developed carbon fiber and carbon composite heating elements, with the purpose of process optimization, the twisting and winding equipment for ply yarn carbon fiber heating core and the high temperature holding equipment for carbon composite heating core were developed, on the basis of preparing high quality carbon heating element, the related electric heating properties and mechanism analysis of which were measured, then, the change law of thermal radiation properties of carbon fiber and carbon composite was systematically and deeply studied, at last, with the view of engineering application, the numerical simulation of radiative heat flux density distribution of carbon composite heating element was carried out.According to the failure cause summary of abandoned elements, it is determined the twisting and winding process of carbon fiber ply yarn and high temperature holding process of carbon composite were key steps during the preparation of carbon heating element. On the basis of which, the twisting and winding equipment for ply yarn carbon fiber heating core was manufactured and the process calculation model was established, meanwhile, the high temperature holding equipment for carbon composite heating core was developed and the removal process of holding heating core was optimized. The performance tests show that the power deviation of carbon composite heating elements after process optimization can be controlled in the range of 5W, the uniformity of surface temperature was better, the service life can reach 12000h, the resistance variability rate was less, safety performance at high voltage load was better, the distribution of thermal radiation energy was stronger in the range of 2～20μm.The correlation between preparation process and related electric heating properties of carbon heating elements (relationship between power density and temperature, resistance temperature characteristic and thermal inertia) were systematically studied, the results show that the sizing agent on the surface of carbon fiber electric heating body can be removed by means of the instant high temperature method in nitrogen, the ply yarn carbon fiber heating element with supporting core from which has relatively higher stable working temperature under the same power density, the resistance variability rate was lower, and the thermal inertia was smaller. In addition, the existence of supporting core goes against the improvement of electric heating property of element, by removing supporting core, using carbon fiber ply yarn and combining with the reasonable holding process, the carbon composite heating element without supporting core can have relatively higher stable working temperature under the same power density, lower resistance variability rate and smaller thermal inertia, and all the electric heating properties of which is better than that of the carbon felt element.The heat transfer differential equation of carbon heating element was established, and the mathematical expressions for stable temperature rise and instant temperature increasing time can be obtained from solving above transfer equation. The analysis shows that the thermal radiation properties of carbon heating core were key factors, which affected the relationship between power density and temperature and the thermal inertia, the stable working temperature can be improved and the instant temperature increasing time can be reduced by increasing total emissivity of carbon heating core. The relationship between resistance variability rate of element and the characteristics of ends’ connections, the initial resistivity of carbon material, the analysis shows that the smaller the initial resistivity of carbon material, the more the overlapping contact of filament, the higher the contact pressure in the ends, and the more the contact point, the less the resistance variability rate of element.The normal spectral emissivity and normal total emissivity of carbon fiber heating core with different fibrous type, high temperature graphitization and surface desizing treatment were measured by using automatic spectrum radiation testing system, and the phase structure analysis was carried out by using XRD and LRS. The change relation between phase structure and thermal radiation property of carbon fiber shows that the turbostratic graphite structure of T700 carbon fiber was relatively more regular than T300, the enhancement of carriers radiation mechanism and graphite crystallite lattice vibration modes made the normal spectral emissivity of T700 carbon fiber in the ranges of 2500～5000nm and 5000～6500nm relatively higher than T300 carbon fiber, the decrease of disturbance structure such as amorphous carbon weakened corresponding vibration modes caused by distortion structures and impurities and defects, which made the normal spectral emissivity in the ranges of 6500～13000nm lower, the wavelength dependence of spectral emissivity for T700 carbon fiber was obvious, and the normal total emissivity was relatively lower. After being graphitized, the degree of regular structure of T300 carbon fiber and domestic carbon felt were improved, the enhancement of carriers radiation mechanism and graphite crystallite lattice vibration modes made the normal spectral emissivity in the ranges of 2500～5000nm and 5000～6500nm obviously higher, the radiation mechanism caused by distortion structures and impurities and defects were weakened, the spectral emissivity in the range of 6500～13000nm decreased, spectral emissivity demonstrated obvious wavelength dependence, and the total emissivity decreased. Compared with the instant high temperature method in nitrogen, the instant high temperature method in oxygen and nitric acid liquid phase treatment caused different etching degree of carbon phase structures, the carriers radiation mechanism was weakened, which caused the decrease of spectral emissivity in the range of 2500～5000nm, the structure of the turbostratic graphite was relatively stable, the degree of etching during treatment was relatively lower, which made the spectral emissivity in the range of 5000～6500nm relatively stable, but the disturbance structure such as amorphous carbon had comparatively poor regularity, the level of etching was greater, the corresponding radiation mechanism was weakened, which caused the decrease of spectral emissivity in the range of 6500～13000nm, the wavelength dependence of spectral emissivity was improved, and the total emissivity was reduced. Under the combined action of carrier radiation mechanism and the graphite lattice vibration radiation mechanism together with the radiation mechanism caused by distortion structures and defects, the carbon fiber showed a poor wavelength dependence of normal spectral emissivity, which had a greybody characteristic.The normal spectral emissivity and normal total emissivity of carbon composites were measured, and the surface morphology of samples were characterized by scanning electron microscope (SEM), the change relation between normal spectral emissivity and normal total emissivity of carbon composites and fiber distribution, carbon composition, impregnation densification process and surface morphology was investigated. The results show that the extent of loose structure can be changed by different fiber distribution, the normal spectral emissivity and total emissivity of carbon fiber perform by adopting ply yarn process and carbon felt were higher than carbon cloth perform, and the normal spectral emissivity and total emissivity of porous C/C performs reinforced by short-cut fiber were also relatively superior to that reinforced by carbon fiber cloth. The carbon composition have great effect on the change of thermal radiation property, the proportion of disturbance and distortion structure in resin carbon matrix was relatively larger than fiber carbon, the thermal radiation property of resin carbon was better than that of fiber carbon because of more distortion lattice vibration modes and local vibration modes caused by impurities and defects, this is the reason why thermal radiation properties of C/C performs reinforced by carbon fiber cloth and short-cut fiber were superior to that of its corresponding fiber performs. With the increase of impregnation densification time, the normal spectral emissivity and total emissivity of carbon composites firstly decreased and then increased, the difference of total emissivity between two samples gradually reduced, the increase of resin carbon content was favorable to improve emissivity. After being polished, the surface roughness of carbon composites decreased significantly, the normal spectral emissivity and total emissivity of samples obviously decreased .The mathematical models of radiative heat flux density distribution for single carbon composite heating element and multiple parallel-arranged carbon composite heating elements were established, the numerical calculation and visual analysis of which were carried out by software of MATLAB, the results show that the radiative heat flux density of single element increased with the increase of emissivity of carbon composite, the non-uniformity of heat flux density also increased. The radiative heat flux density of single element increased with the increase of emissivity of quartz, also the non-uniformity of heat flux density also increased. With the increase of outside radius of quartz tube, the radiative heat flux density of single element firstly increased, under the setting initial condition, the heat flux density decreased rapidly when the outside radius of quartz tube was equal to 0.45cm, the non-uniformity of heat flux density slowly increased with the increase of outside radius of quartz tube firstly, and then had little change. The radiative heat flux density of single element improved with input power, the non-uniformity of heat flux density firstly decreased rapidly and then mildly. The radiative heat flux density distribution of multiple elements was affected by the distance between axes of element and the heated surface, the distance between axes of two adjacent elements and the input power distribution of different elements. Under the setting initial condition, when the distance between axes of element and heated surface h was equal to 12cm, the distance between axes of two adjacent elements s was equal to 15cm, the power distribution of three elements was 1000W-600W-1000W, the radiative heat flux density field presented flat arched distribution, and the relative uniform radiative heat flux density field can be achieved by alternate power distribution of multiple parallel-arranged elements.更多还原