【作者】 李敏；

【导师】 韦建军；

【作者基本信息】 四川大学 ， 凝聚态物理， 2021， 硕士

【摘要】 高质量铌薄膜的超导转变温度（Tc）可达9.3 K,能够承受多次高低温循环保持超导性能不降低。具有机械强度高,化学性质稳定,耐腐蚀等优点。因此,被广泛应用于能源、国防等领域。本文主要讨论铌薄膜在加速器方面的应用。基于铌材料的超导射频（SRF）腔因其优异的超导性能而被粒子加速器采用。薄壁SRF腔虽然可以提高传热效率,但机械振动和氦压扰动引起的结构变形会对其性能和寿命造成严重威胁。高纯度铌材要达到理想的导热性能,其过高的制造成本也限制了加速器的发展。由于铜具有高导热性,射频场衰减到腔体表面的1/e时其穿透深度小于1μm,厚壁铜腔内壁的铌膜结构与纯铌的结构相比具有一致的超导性能,并且系统的热稳定性和力学稳定性也会显著提高。因此本论文基于化学气相沉积法（CVD）在铜基底上开展了超导铌金属薄膜的制备工艺的研究,采用等离子体增强沉积法（PECVD）进行了初步探索,主要研究成果如下:首先采用化学气相沉积（CVD）的方法,在纯氩气氛下,以氢和五氯化铌（Nb Cl₅）为原料在平面铜基底上沉积铌膜,结合X射线衍射仪（XRD）,扫描电子显微镜（SEM）、能谱仪（EDS）、光学轮廓仪（optical profiler）、物理性能测量系统（PPMS-9）等多种表征手段系统地研究了沉积温度、沉积区域（边界层区域、平流层区域）、恒温区位置、退火处理等工艺参数对铌膜质量的影响,并利用沉积动力学理论详细探讨了铌薄膜的形成机理。研究发现:Nb Cl₅裂解生成铌的最低温度为550°C。制备的铌薄膜为多晶薄膜,晶体结构为体心立方结构,铌薄膜的择优生长取向为（110）晶面。提高沉积温度、适当增加反应距离、以及原位退火处理可以提高铌薄膜的结晶质量,减少铌膜的内部缺陷。由于气体流速和反应物浓度的不同,薄膜的沉积区域也会显著影响薄膜的沉积质量,相同温度条件下在边界层区域制备的铌薄膜显示出更高的结晶度和较少的缺陷。CVD技术成功地制备出了具有优良结晶质量的铌薄膜。我们对700℃条件下制备的铌薄膜磁矩与温度的关系进行了测量,相比于平流层区域,边界层沉积的薄膜由于具有高的结晶度和较少的缺陷,超导转变温度几乎与纯铌块材一样为9.2 K。其次为了模拟超导射频腔的复杂形状,设计了直角状和曲面状的铜基底,通过前一章的参数探索,实现了在异形形状的铜基底上沉积铌薄膜。结合不同位点上XRD、SEM的分析,表明在三种形状上制备出了结晶度高,附着性好,分布均匀的金属铌薄膜,其晶粒尺寸均在15 nm左右。最后,通过在管式炉的进气端安装射频线圈以产生等离子体,尝试在等离子体驱动下将Nb Cl₅裂解从而在平面铜基底上制备铌薄膜。相同温度条件下,薄膜似乎显示出较高的结晶度,但与化学气相沉积相比差别不大。从表面形貌来看,薄膜显示出较少的缺陷,较高的致密性与均匀性,但沉积温度没有得到较低。更多还原

【Abstract】 The superconducting transition temperature（Tc）of high quality niobium films can reach 9.3 K,it can withstand several cycles of high and low temperature to maintain the superconducting properties.It has the advantages of high mechanical strength,stable chemical properties and corrosion resistance.Therefore,it is widely used in energy,national defense and other fields.This paper mainly discusses the application of Nb films in accelerators.The superconducting radio frequency（SRF）cavity based on Nb material is adopted by the particle accelerator due to its unique superconductivity.Although the heat transfer efficiency can be increased by utilizing a thin-walled SRF cavity,however,its performance and lifetime would be susceptible to structural deformation caused by mechanical vibration and helium pressure disturbance.To reach satisfied thermal conductivity using high-purity Nb bulk material,its excessive-high manufacturing cost also restricts the development of accelerator[4,5].Benefiting from the high thermal conductivity of copper and the low penetration depth（<1μm）of the RF field attenuation to 1/e of the cavity surface,the structure of Nb film on the inner wall of the thick-walled copper cavity can achieve consistent superconducting performance in contrast to that of pure Nb.In addition,the thermal and mechanical stability of the SRF system would be also enhanced remarkably.Therefore,in this paper,the preparation technology of superconducting Nb metal film on copper substrate was studied based on CVD method,and preliminary exploration was made by using PECVD.The main research results are as follows:Firstly,niobium films were deposited on copper substrate by chemical vapor deposition（CVD）in pure argon atmosphere with hydrogen and niobium pentachloride（Nb Cl₅）as raw materials.X-ray diffraction（XRD）,scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）,optical profiler,physical property measurement system（PPMS-9）and other characterization methods were combined,the effects of deposition temperature,deposition region（boundary layer region,stratospheric region）,location of constant temperature region and annealing treatment on the quality of niobium films were systematically studied,and the formation mechanism of niobium films was discussed in detail by using deposition kinetics theory.It is found that the Nb thin film is polycrystalline and its crystal structure is body-centered cubic.The preferred growth orientation of the Nb thin film is（110）crystal plane.Through the study of the film forming quality of Nb Cl₅films under different technological parameters,it is found that the lowest temperature of Nb Cl₅cracking to produce Nb is about 550℃.Increasing the deposition temperature,Increasing the deposition temperature,increasing the reaction distance and in-situ annealing treatment can improve the crystallization quality and reduce the internal defects of Nb films.Due to the difference of gas flow rate and reactant concentration,the deposition area of the films will significantly affect the deposition quality of the films.The Nb films prepared in the boundary layer region at the same temperature show higher crystallinity and fewer defects.Nb films with excellent crystal quality have been successfully prepared by CVD technology.The relationship between the magnetic moment and temperature of the Nb thin film prepared at 700℃was measured.Compared with the stratospheric region,the superconducting transition temperature of the thin film deposited in the boundary layer was almost the same as that of the pure Nb block,which was 9.2 K due to its higher crystallinity and fewer defects.Secondly,in order to simulate the complex shape of the SRF cavity,rectangular and curved copper substrates were designed.Through the parameter exploration in the previous chapter,Nb films were deposited on the special-shaped copper substrates.Combined with the analysis of XRD and SEM on different sites,the results show that niobium films with high crystallinity,good adhesion and uniform distribution have been prepared on three kinds of shapes,the grain size of the films is about 15 nm.Finally,plasma is produced by installing radio frequency coils at the inlet end of the tubular furnace,Nb Cl₅was cracked by plasma to prepare Nb thin films on flat copper substrates.Under the same temperature conditions,the film seemed to show a higher crystallinity,but the difference was not significant compared with that of CVD.In terms of surface morphology,the films show fewer defects,higher compactness and uniformity,but the deposition temperature is not lower.更多还原