Single crystal growth of oxides using additive laser material deposition

激光材料沉积（LMD）涉及将粉末馈送到通过激光辐射完全熔化的基底上。可以通过逐层沉积来建立体体积。通过LMD的单晶生长在可从顶部接近的炉中进行。该炉允许关于加热和冷却速率的温度管理以及熔体和晶体中的小温度梯度的设置。与传统的单晶生长方法相比，LMD避免了热源（Verneuil法中的气体火焰）或坩埚（Czochralski法）对熔体的污染。激光生长大块单晶体的技术尚未得到发展，本项目的主要目标是以蓝宝石为实验材料，对激光材料沉积法生长氧化物单晶体有基本的认识。为了实现这一目标，必须了解一方面的工艺参数与另一方面的晶体生长性能和性质之间的相关性以及加工技术。应实现以下子目标：使用空间和时间分辨的温度场的有限元模拟的生长过程的建模。基于这些场，可以研究液态和固态的温度梯度。通过调整光束强度分布和激光功率，为蓝宝石单晶生长创造最佳条件。工艺参数对晶体生长和性能的影响。主要参数是激光功率、强度分布、加热和冷却速率、送粉、粉末和基底的性质、熔炉温度和晶体的旋转。调整工艺和系统工程以实现最佳生长条件。这涉及到一种用于CO2激光辐射的光束均化器的开发，其最大功率为3 kW。此外，粉末进料必须适应于实现粉末在基底上的均匀分布。增材激光材料沉积作为一种用于生长单晶的灵活方法为未来的应用开辟了许多机会，例如不同几何形状的自由成形制造（圆柱体、立方体、圆盘），或者通过在逐层堆积期间改变粉末组成来制造梯度晶体。

Laser material deposition (LMD) involves the feeding of a powder onto a substrate which is completely melted by laser radiation. Bulk volumes can be built up through layer-by-layer deposition. Single crystal growth by LMD is performed in a furnace which is accessible form the top. The furnace allows the temperature management regarding heating and cooling rates and setting of small temperature gradients in melt and crystal. Compared to conventional single crystal growth methods LMD avoids contamination of the melt from the heat source (gas flame in Verneuil method) or from the crucible (Czochralski method). Laser based growth of bulk single crystals has not been developed so far.The main goal of the project is the fundamental understanding of single crystal growth of oxides by additive laser material deposition using sapphire as demonstrator material. To achieve this goal knowledge has to be gained in the correlation between process parameters one side and growth performance and properties of the crystals on the other side as well as in processing technology. The following sub-goals should be achieved:Modeling of the growth process using FEM simulation of space and time resolved temperature fields. Based on these fields temperature gradients in the liquid and solid state can be studied. Beam intensity distribution and laser power will be adapted to create optimum conditions for single crystal growth of sapphire.Influence of the process parameters on the growth and properties of the crystal. The main parameters are laser power, intensity distribution, heating and cooling rates, powder feeding, properties of the powder and the substrate, furnace temperature and rotation of the crystal. Adaptation of processing and system engineering to achieve optimum growth conditions. This involves the development of a beam homogenizer for CO2 laser radiation for a maximum power of 3 kW. Furthermore the powder feeding has to be adapted to achieve a homogeneous distribution of the powder on the substrate.Additive laser material deposition as a flexible method for the growth of single crystals opens many opportunities for future applications, e.g. the free form manufacturing of different geometries (cylinder, cube, disc) by beam shaping or scanning or the manufacturing of gradient crystals by variation of powder composition during layer wise build-up.