Processing and Properties of High Performance Lead Free Electro-Optic Ceramics

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, August 2016
Dr. Javier Garay Chairperson

Electro-optic  (EO)  materials  allow  for  the  precise  control  of  light  using  electrical signals, which has allowed for the advancement of an incredible array of photonic technologies such as laser systems and optical telecommunications. Most EO devices currently utilize single crystals, but high performance EO single  crystals often have  composition limitations since dopants  can  segregate  and  not  all  compositions  can  be  grown  using  equilibrium  restricted techniques. Bulk polycrystalline ceramic materials can potentially overcome such limitations and allow for the exploration of new EO systems. Due to the specific microstructures required for  transparency,  conventional  processing  techniques  have  difficulty  in  producing  bulk polycrystalline EO ceramics. Reported here for the first time are the optical and EO properties of a new class of transparent lead free ceramic that outperforms EO materials in use today. This material is a barium titanate (BaTiO3) based solid solution, (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 referred to here as BXT. The EO material was successfully processed using the Current  Activated  Pressure  Assisted  Densification  (CAPAD)  technique,  commonly  called Spark Plasma Sintering (SPS), which has been shown to be effective at consolidating optical materials. Using this technique along with a new powder synthesis method, it was possible to produce  a  transparent  EO  BXT  ceramic  with  a  highly  dense  and  homogeneously  reacted microstructure. Densified BXT shows a remarkable EO coefficient of 530 pm/V, which is superior  not  only  to  state  of  the  art  LiNbO3  crystals  but  also  top-quality  lead  containing ferroelectric ceramics such as PLZT. This exceptional coefficient will allow for miniaturized EO systems with reduced operating voltages. The mechanisms behind the high EO performance in  BXT  were  determined  using  additional  EO  and  ferroelectric  measurements.  These measurements indicate that BXT undergoes a field induced structural evolution which heavily contributes to the EO effect. Along with efficient domain motion, this structural evolution includes a field induced phase change to a lower symmetry crystal structure. This work shows that lead free EO polycrystalline ceramics can have properties that are competitive or superior to state of the art EO materials. Due to its exceptional EO properties, BXT in particular has a promising future as an EO material.