铀表面鎏锌防护层制备技术研究

【作者】 沈崇雨；

【导师】 李炬；

【作者基本信息】 中国工程物理研究院 ， 辐射防护与环境保护， 2012， 硕士

【摘要】 铀是重要的核材料,具有极强的化学活性,易受环境气氛的腐蚀,不仅会造成材料的变质,而且对环境造成污染,增大辐射防护压力。给金属铀施以保护涂层是阻止金属铀腐蚀的有效措施之一,涂层的抗腐性能受涂层材料和涂层致密度的影响,不同涂层制备方法和工艺是影响涂层致密度的主要因素。目前的涂层制备方法各具特点,又各有不足,为此,提出一种新的在铀表面制备涂层的方法：采用金属与汞制备汞齐,将汞齐涂布于金属铀表面并进行加热,使汞齐分解并将汞蒸发,由此在金属铀表面制备一定厚度的金属镀层,主要开展了锌涂层的制备工艺、结构和抗腐蚀性能研究。采用第一原理模拟计算研究了在铀上鎏镀锌涂层的机理,同时获得了锌涂层在铀表面的理论结合机理,锌以金属键方式与铀结合,Zn-U原子间距与U2Zn17中的Zn-U原子间距极为接近,为涂层制备工艺的改善提供了有效的判据。XRD分析结果证明,铀扩散至锌涂层中,形成U2Zn17金属间化合物,说明使用汞齐在铀表面鎏镀锌涂层时,铀汞齐与锌汞齐在加热分解时发生共结晶反应生成U2Zn17金属间化合物,从而在较低的温度下使锌、铀发生冶金结合。以Au和Sn作为参比,选择Zn、Au和Sn制备汞齐,在金属铀上开展了鎏镀锌涂层的工艺研究。对于熔点低的Sn,当温度达到锡的熔点附近(230℃)时,涂层与基体之间的结合紧密,涂层本身较致密；由于Zn易升华,提高制备温度不能制备结合良好的涂层,涂层本身的致密性较差；对于高熔点金属Au,涂层优先在表面形成,从而阻挡了下层汞的蒸发,即使提高温度也不能改善涂层与基体的结合情况,只能通过减少单次制备涂层的厚度,采用多次制备的方法实现较厚涂层的制备。涂层制备温度是影响涂层性能的决定性因素,通过实验,初步确定了不同金属涂层在铀表面的鎏镀工艺。大气环境下,在表面氧化的金属铀表面鎏镀了锌涂层。实验证明,铀表面氧化物在制备过程中弥散于锌涂层之内,从而减弱了其对涂层与基体的结合性能的影响。采用热重腐蚀考核了铀表面鎏镀锌涂层在150℃、含40%O2的Ar中的抗腐蚀性能,锌涂层能够对铀基体起到良好的抗腐蚀功能,锌涂层的抗腐蚀行为是通过在其表面形成致密的ZnO保护膜体现的,在空气中,由于水蒸气和二氧化碳的存在,易在涂层表面生成ZnCO3, ZnCO3的生成并不会降低涂层的抗腐蚀性能。采用激光拉曼光谱研究了铀表面锌涂层的抗腐蚀机理,实验选用铀扩散至锌涂层表面的样品,通过改变入射激光强度,在70%RH的空气中,对铀表面锌涂层的抗腐蚀性能进行了研究,实验证明锌优先于铀腐蚀,说明锌涂层对铀起到阳极保护的作用。更多还原

【Abstract】 As an important nuclear material, uranium(U) is easily corroded in many environment medium for its strong chemical activity. This not only leads to worsening material capability, but also to environment pollution and more radio protection work. One of the effective means to protect uranium from corrosion is to coat it with noncorrosive materials. The coating compaction and connection with substrate which ties up with coating technics have important effects on the corrosion resistance. So far as it goes, every technics has its only strongpoint as well as its shortcoming. Here we developed a new way to deposit coatings on uranium. In our work, we dissolved metal in mercury to form amalgam (i.e. mercury alloy). And the amalgam was rolled out on the uranium as film, then the amalgam is resolved in a higher temperature, and mercury can be evaporated for its high saturated vapor pressure, metal is left to form coating. In this thesis, the mechanics and technics of amalgam coating on uranium were studied in detail, coating structure and corrosion resistance were also tested as well.The mechanism of amalgam zinc(Zn) coating on uranium was studied using the first principle calculation which based on density functional theory (DFT). The result suggests that zinc connects with uranium in metal bond; the Zn-U bond length is comparable to the Zn-U distance in U2Zn17, this probably gives an effective criterion to evaluate the capability of zinc coatings. The metallic compound U2Zn17can be found in the XRD spectra of amalgam zinc coating on uranium, which means that zinc reacts with uranium to form compound at a much lower temperature and pressure with the effect of mercury.Comparing with aurum(Au) and tin(Sn), zinc, aurum and tin were selected to prepare amalgam separately. The technics of amalgam coatings on uranium was studied in this thesis. In our experiments, compact tin coating tightly connected with substrate can be obtained at about230℃because of its lower melting point. Compact zinc coating is not easily obtained at higher temperature, because zinc will sublime for its high saturated vapor pressure, and this could be improved at higher pressure.Although aurum has high melting point and low saturated vapor pressure, Compact thicker aurum coating is also not easily obtained in one time. Because mercury can not easily through the compact aurum film forming preferred at surface. This could be improved by attenuating the amalgam rolling out on the uranium and thickening the aurum coating by more times. Experiments suggest that temperature is the key factor to the amalgam coatings’ capability. The technics of preparing metal coatings on uranium using amalgam was developed primly.Zinc coating was amalgam coated on uranium coated with uranium dioxide in air, it was proved that the uranium dioxide was dispersed in the zinc coating in this technics, and this exactly improved the coating connection with substrate.The corrosion resistance of zinc coating on uranium was tested at150℃in argon(Ar) involving40v%oxygen(O2) through thermo gravimetric analysis(TGA). It suggested that the zinc coating can effectively resist uranium from corrosion through forming compact zinc oxide film on the surface. If the zinc coating was exposed to air for a long tine, ZnCO3can be discovered on the coating surface, and this must be the reaction between zinc and water and carbon dioxide in the air. But the ZnCO3would not worsen the corrosion resistance of zinc coating.The corrosion resistance mechanism of zinc coating was studied by Raman spectra analysis. Zinc coatings which uranium had diffused to the surface of coating were exposed to different intensity laser in70RH%air, and the different Raman spectra were obtained. It was improved that zinc is more easily corroded than uranium, and this can also be seen in XRD spectra of zinc coating exposed to air for different time. All of these phenomenons suggest that zinc is corroded preferred as anode in the primary cell formed by zinc and uranium in humid air.更多还原