【作者】 王涛；

【导师】 贾志泰； 陶绪堂；

【作者基本信息】 山东大学 ， 材料学， 2021， 博士

【摘要】 人工晶体作为推动科技进步、社会发展的关键材料,受到了各国政府和科研工作者的广泛关注。进入21世纪以来,现代工业与军事国防领域的飞速发展对人工晶体材料提出了新的要求,也驱动着人工晶体材料的发展,功能晶体材料的研究也逐渐进入“百花齐放,百家争鸣”的新阶段。单晶光纤作为一种“准一维”功能晶体材料,继承了体块单晶优异的物理和化学性能同时兼具了传统光纤材料大长径比的结构优势,在高能激光、高温传感、辐射探测、信息通信等众多领域都有着广泛的应用前景。目前以美国、法国、日本为代表的西方国家在单晶光纤领域的研究处于领先地位,其中以法国Fibercryst与美国Photran为代表的企业已经实现了单晶光纤材料及器件的产业化。与此同时,美国陆军实验室于2015年开展了一项基于单晶光纤的激光武器研发项目,更加凸显出单晶光纤在军事国防以及社会民生领域的重要地位。我国单晶光纤领域的相关研究起步较晚,系统性研究相对匮乏,缺乏先进的晶体生长设备以及成熟的晶体生长工艺,为了不在关键材料领域受制于人,亟需开展相关研究。激光加热基座法是目前应用最为广泛的单晶光纤制备技术之一,其具有原料用量少、加热温度高、升温速度快、实验周期短、无坩埚生长等特点,多用于制备高熔点单晶光纤或超细直径柔性单晶光纤。美国海军实验室通过激光加热基座法成功制备出直径17μm的柔性YAG单晶光纤,这也是目前已知的直径最细的单晶光纤。本论文以单晶光纤在光电信息领域的应用需求为牵引,以激光加热基座法单晶光纤制备技术为出发点开展相关研究,解决了单晶光纤制备过程中出现的直径波动、应力集中、气泡包裹等问题,成功制备出高质量的柔性YAG单晶光纤,实现了技术突破。同时提出并完善了单晶光纤的质量表征体系,结合体块晶体与玻璃光纤的表征方法,从晶体质量、直径起伏、光损耗、光学均匀性等多个维度对YAG单晶光纤进行了质量评估。在应用研究方面,开展了单晶光纤激光器以及单晶光纤高温传感器的相关研究:探究了 Yb:YAG单晶色心缺陷产生的原因及影响,以无色心缺陷的Yb:YAG单晶光纤作为激光增益介质,实现了 1μm波段瓦级连续激光输出;首次将超声测温技术与石榴石、尖晶石结构单晶光纤相结合,研制出使役温度>1800℃的高温传感器,掌握了声波模式、晶体结构、离子掺杂、晶体取向等因素对单晶光纤声学性能的影响规律,实现了对传感器性能的调控;面向2500℃以上的超高温测温需求,设计了基于Y2O3-ZrO2单晶光纤的高温电学性能的高温传感器,对使役温度范围、测温灵敏度等关键性能进行了表征。本论文的主要研究工作和结论如下:Ⅰ.激光加热基座法柔性单晶光纤制备技术本课题组引进了商品化激光加热基座单晶生长炉,系统研究了单晶光纤生长过程中所面临的偏斜生长、直径起伏、应力集中、气泡包裹、柔性单晶光纤制备等主要问题。通过对光路的优化以及机械装置的改进,解决了单晶光纤的偏斜生长;通过对料棒均匀性的优化以及对CO2激光器功率波动的控制,降低了单晶光纤的直径起伏;通过理论模拟指导,适当降低单晶光纤的生长直径,同时辅助电阻/光学后热降低固液界面处的温度梯度,缓解了单晶光纤内部的应力集中,避免了晶体开裂;通过提高料棒致密度,调整固液界面形状为平坦/凹界面,设置负压生长环境等措施降低了熔体内气泡的含量并促进了气泡的排出,实现了对气泡缺陷的优化;通过改进激光加热基座设备,将原有的球面反射镜替换为抛物面反射镜,消除了聚焦像差,提高了聚焦精度,为超细直径柔性单晶光纤的制备创造了条件。在此基础上,首次通过激光加热基座法以36:1的拉送比实现了直径150μm,长径比>2000:1的柔性Yb:YAG激光单晶光纤制备,相关制备工艺处于国内领先水平。Ⅱ.Yb:YAG单晶光纤的优化生长、质量表征及激光性能在前期工作的基础上,通过激光加热基座法在惰性气氛下制备了无色心缺陷的高质量Yb:YAG单晶光纤,通过XRD、XPS、拉曼光谱、吸收光谱以及第一性原理计算等方法确定了 Yb:YAG晶体中色心缺陷产生的原因是氧空位和Yb2+离子的存在。Yb:YAG单晶光纤中没有色心缺陷的主要原因是单晶光纤巨大的比表面积以及激光加热基座技术敞开式的熔区环境,使得氧离子能够充分与熔体接触并传输。色心缺陷的消除大幅度提升了 Yb:YAG单晶光纤的光学和热学性能,其在近红外波段的透过率达到了 85%以上,室温热导率达到了 8 W.m-1·K-1。我们从单晶质量、直径起伏、掺杂均匀性、应力分布、光传输损耗、光学均匀性等多个维度对所得的Yb:YAG单晶光纤进行质量评估。Yb:YAG单晶光纤沿长度方向劳厄衍射斑点清晰明亮且衍射图样一致,说明整根单晶光纤具有一致的晶体取向,单晶性良好。其在808 nm波段的光损耗约为0.01 dB/cm,与美国陆军实验室的实验数据相当。ZYGO激光平面干涉仪测试结果显示直径1 mm与2 mm的Yb:YAG单晶光纤光学均匀性可达10-6次方量级,展现出了优异的光学性能。在此基础上,以掺杂浓度2 at.%的Yb:YAG单晶光纤为增益介质,实现了最大功率3.93 W的连续激光输出,最大斜效率为28.2%。Ⅲ.石榴石结构单晶光纤超声温度传感器研究将超声测温技术与YAG/LuAG单晶光纤相结合,研制出使役温度>1800℃的超声温度传感器。系统探究了超声波模式、光纤直径、晶体结构、离子掺杂以及晶体取向对单晶光纤高温声学特性以及传感器性能的影响。实验结果显示声速随温度的升高而降低且相同温度下横波声速远低于纵波声速,YAG单晶光纤在1100℃时的横波声速为4860.6 m/s,较纵波降低了近3000 m/s。并且,随着Yb3+离子掺杂浓度的提高,YAG单晶光纤的声速进一步降低,2 at.%掺杂的Yb:YAG单晶光纤在1100时的横波声速为4680.8 m/s。在各向异性的研究中发现,石榴石结构单晶光纤声学各向异性相对较弱,YAG、LuAG的各向异性因子分别为1.05与1.07。通过声波模式、离子掺杂和晶体取向的调控,10 at.%[111]-Yb:LuAG单晶光纤超声温度传感器在20-1100℃范围内的横波声速达到了 4198.26 m/s-3929.95m/s,最大单位灵敏度为34.83 ns·℃-1·m-1,最大分辨率为2.87。Ⅳ.声学各向异性MgAl2O4单晶光纤超声温度传感器研究单晶光纤声学各向异性取决于弹性各向异性,MgAl2O4晶体具有明显的弹性各向异性,其各向异性因子A=2.13,是YAG/LuAG的两倍以上。其在横波和纵波模式下表现出不同的声学各向异性,纵波模式下V[100]<V[110]<V[111],横波模式下V[110]<V[111]<V[100]。MgAl2O4单晶光纤在1200℃时的最大声速为9938.66 m/s,最小声速为3872.56 m/s,调节范围较大。在此基础上,结合前期工作经验,对MgAl2O4单晶光纤进行掺杂改性,成功制备了[110]-(Mg0.9Zn0.1)(Al0.995Cr0.005)2O4单晶光纤超声温度传感器,其在20-1200℃范围内的横波声速为4220.12-3738.04 m/s,单位灵敏度为40.38-67.50 ns·℃-1·m-1,温度分辨率为1.24-0.74℃。分辨率与灵敏度均是目前单晶光纤超声温度传感器所实现的最佳性能。此外,传感器的性能与温度呈正相关关系,展现出其在超高温探测领域巨大的应用前景。V.基于高温电学特性的Y2O3-ZrO2单晶光纤高温传感器研究Y203-ZrO2(YSZ)单晶光纤熔点极高,超过2700℃,其在室温下处于绝缘状态,由于Y3+与Zr4+离子的非等价取代,引入了大量的氧空位,使其在高温下具备了导电性。基于此,我们通过激光加热基座法制备了立方相的8YSZ单晶光纤,探究了其在20-1400℃范围内的电学性能,实验结果表明,当温度超过400℃时,YSZ单晶光纤的离子电导率开始与环境温度呈一定的函数关系。8YSZ单晶光纤1400℃时的离子电导率和测温灵敏度分别为0.531 S/cm和0.129 S·m-1·℃-1,是同组分YSZ陶瓷纤维的4倍以上,且其性能随温度升高呈明显的上升趋势,结合YSZ单晶光纤超高的熔点,其理论使役温度可以达到2500℃。更多还原

【Abstract】 As a key material to promote scientific and technological progress and social development,artificial crystals have received extensive attention from governments and scientific researchers from all over the world.Since the beginning of the 21st century,the rapid development of modem industry and military defense has put forward new requirements for artificial crystal materials and promoted the development of crystal materials.Therefore,the research of crystal materials has gradually entered a diversified and brilliant new stage.Single crystal fiber(SCF),as a kind of quasi-one-dimensional functional crystal material,inherits the excellent physical and chemical properties of bulk single crystal and possesses the structural advantages of large aspect ratio of traditional fiber materials.It has been widely used in many fields,such as high-power laser,high temperature sensing,radiation detection,information communication and so on.At present,United States,France and Japan are the leading countries in the research of SCF.Among them,companies represented by Fibercryst and Photran have realized the industrialization of SCF materials and SCF devices.Meanwhile,the US Army laboratory launched a laser weapon research project based on SCF in 2015,which highlights the importance of SCF in military defense and social livelihood.In domestic,the related research is relatively backward lacking of systematic research,advanced crystal growth equipment and mature crystal growth technology.In order not to be limited in the field of key materials,it is urgent to carry out relevant research.Laser heating pedestal growth(LHPG)is one of the most widely used tecnique for the growth of SCF with the advantages of saving raw materials,high heating temperature,fast heating speed,short experiment period and no crucible growth,which is mainly used to prepare high melting point SCF or flexible SCF.The US Naval Laboratory has fabricated a flexible YAG SCF with a diameter of 17 μm through the LHPG method,which is also the smallest diameter single crystal fiber known,which is also the thinnest single crystal fiber so far.Guided by the application requirements of SCF in the field of optoelectronic information,this thesis focuses on the LHPG technique and solve the problems of diameter fluctuation,stress concentration and bubble wrapping during the growth of SCF.On this basis,the flexible YAG SCF was successfully fabricated,demonstrating a technological breakthrough.Besides,the characterization system of SCF is proposed and improved.Combined with the characterization methods of bulk crystal and glass fiber,the quality of YAG SCF is evaluated from crystal quality,diameter fluctuation,optical loss,optical uniformity and other dimensions.In terms of application research,related researches on SCF lasers and SCF temperature sensors have been carried out.Firstly,the causes and effects of the color centers in Yb:YAG single crystals are investigated and the watt-class CW laser output in the 1μm band was realized by using Yb:YAG without color center defectas as the laser gain medium.Secondly,the ultrasonic temperature measurement technology is combined with garnet and spinel SCF for the first time to develop a high temperature sensor with service temperature over 1800℃.The influences of acoustic mode,crystal structure,ions doping and crystal orientation on the acoustic performance of SCF were mastered,and sensor performance is well improved.In addition,to meet the needs of ultra-high temperature temperature measurement above 2500℃,the temperature sensor based on Y2O3-ZrO2 SCF with temperature-dependent electrical properties was designed,and the key properties such as service temperature range and temperature measurement sensitivity were characterized.The main research outline and conclusions of this thesis are as follows:Ⅰ.Fabrication of high-qualty flexible SCF by LHPG methodWe have and systematically studied the main problems of skew growth,diameter fluctuations,stress concentration,bubble wrapping,and flexible crystal fabrication.The skew growth of SCF is solved by optimizing the optical path and improving the mechanical device.The diameter fluctuation of SCF is improved by optimizing the uniformity of pedestals and controlling the fluctuation of CO2 laser.Stress concentration inside the SCF has been well alleviated by reducing the diameter of the SCF and install resistance or optical after-heater.the bubble defects have been well avoided by increasing the density of the pedestals,adjusting the shape of the solid-liquid interface to flat or concave interface and setting the negative pressure growth environment.By replacing the spherical mirror with parabolic mirror,the focusing aberration is eliminated,which Lays the foundation for the growth of ultra-fine diameter flexible SCF.On this basis,for the first time,the flexible Yb:YAG laser SCF with a diameter of 150 μm and a length-to-diameter ratio over2000:1 was growth through the LHPG method with a draw ratio of 36:1.Ⅱ.Optimal growth,quality characterization and laser performance of Yb:YAG SCFOn the basis of the previous work,a high-quality Yb:YAG SCF without color center defects was growth by LHPG method in an inert atmosphere.It is found that the color center exists in Yb:YAG crystal are caused by oxygen vacancy and Yb2+ ions.The huge specific surface area of the SCF and the open melting zone of the LHPG technique enable oxygen ions to fully contact the melt and transmit,avoiding the occurrence of color center defects.The optical and thermal properties of Yb:YAG SCF are greatly improved by eliminating color center defects.The transmittance of Yb:YAG SCF in the near infrared band is more than 85%,and the thermal conductivity at room temperature is about 8 W·m-1·K-1.The quality of Yb:YAG SCF was evaluated from the aspects of crystallinity diameter fluctuation,doping uniformity,optical transmission loss and optical uniformity.The Laue diffraction spots of Yb:YAG SCF along the longitudinal direction are clear and bright and the diffraction pattern is consistent,indicating that the entire SCF has consistent orientation and good crystallinity.The optical loss of the SCF at 808 nm is about 0.01 dB/cm,which is equivalent to the experimental data of the US Army Laboratory.The results of the ZYGO laser plane interferometer show that the optical uniformity of the Yb:YAG SCF with the diameters of 1 mm and 2 mm can reach the order of 10-6,indicating excellent optical performance.On this basis,Yb:YAG SCF with the doping concentration of 2 at.%was used as gain medium,and the CW laser output with maximum power of 3.93 W and maximum slope efficiency of 28.2%was achieved.Ⅲ.Garnet crystal fiber ultrasonic temperature sensorAn ultrasonic temperature sensor(UTS)with the service temperature over 1800℃was developed by combining ultrasonic temperature measurement technology with YAG and LuAG SCF.The influences of ultrasonic mode,fiber diameter,crystal structure,ion doping and crystal orientation on the high-temperature acoustic properties of SCF and sensor performance were systematically explored.The experimental results show that the ultrasonic velocity decreases with the increase of temperature and the velocity of S-wave is far lower than that of P-wave at the same temperature.The S-wave velocity of YAG SCF is 4860.6 m/s at 1100℃,which is about 3000 m/s lower than that of P-wave.Moreover,with the increase of Yb3+concentration,the ultrasonic velocity of YAG SCF further decreases,and the P-wave velocity of 2 at.%Yb:YAG SCF at 1100℃ is 4680.8 m/s.It is found that the acoustic anisotropy of garnet crystal fiber is relatively weak,and the anisotropy factors of YAG and LuAG are 1.05 and 1.07 respectively.Through the adjustment of acoustic wave mode,doped ions and orientation,the S-wave velocity of 10 at.%[111]-Yb:LuAG SCF-UTS in the range of 20-1100℃ reached 4198.26 m/s-3929.95 m/s,leading to the maximum unit sensitivity of 34.83 ns·℃-1·m-1,and the maximum resolution of 2.87℃.Ⅳ.Anisotropic MgAl2O4 ultrasonic temperature sensorThe acoustic anisotropy of SCF depends on the elastic anisotropy.MgAl2O4 crystal exhibits significant elastic anisotropy with an anisotropy factor of 2.13,which is more than twice that of YAG/LuAG.MgAl2O4 shows different acoustic anisotropy behaviors in P-wave and S-wave mode,which are V[100]<V[110]<V[111]and V[110]<V[111]<V[100]respectively.The maximum and minimum ultrasonic velocities of MgAl2O4 SCF at 1200℃ are measured to be 9936.66 m/s and 3872.56 m/s,respectively,indicating a huge adjustment range.Combined with the previous work,[110]-(Mg0.9Zn0.1)(Al0.995Cr0.005)2O4 SCF-UTS was prepared by doping modification with the ultrasonic velocity of 4220.12-3738.04 m/s,the unit sensitivity of 40.38-67.50 ns·℃-1·m-1 and the resolution of 1.24-0.74℃.Both resolution and sensitivity are the best performance achieved by SCF-UTS so far.Furthermore,the performance of the sensor is positively correlated with ambient temperature,demonstrating huge application prospects in the field of ultra-high temperature sensing.Ⅴ.Y2O3-ZrO2 SCF temperature sensor based on temperature-dependent conductivityThe melting point of Y2O3-ZrO2(YSZ)SCF exceeds 2700℃,and it is insulated at room temperature.A large number of oxygen vacancies are introduced due to the non-equivalent substitution between Y3+and Zr4+ ions,making it conductive at high temperatures.Cubic-phase 8YSZ SCF were grown by the LHPG method and the electrical properties in the range of 20-1400℃ were explored in detail.Further research found that the ionic conductivity of YSZ single crystal fiber starts to be a certain function of ambient temperature when the temperature over 400℃.The ionic conductivity and measurement sensitivity of 8YSZ SCF at 1400℃ are 0.531 S/cm and 0.129 S·m-1·℃-1,which are more than 4 times that of 8YSZ ceramic fiber.Furthermore,the performance exhibits an upward trend with the increase of temperature,accompanied with the high melting temperature,resulting in a service temperature exceeds 2500℃.更多还原