Dense KNN Polycrystals Doped by Er₂O₃ Obtained by Hot Pressing with Hexagonal Boron Nitride Protective Layer

Analysis of dense Potassium Sodium Niobate (KNN) ceramic obtained by hot pressing (HP) method at 1100 °C are presented in this paper. The synthesis of KNN-based piezoelectrics meets the following challenges-low density of material, uncontrolled K/Na ratio, multiphase composition and formation of different KNN structures. The classical hot pressing approach results in contamination by carbon originating from graphite molds. The proposed hexagonal Boron Carbide (h-BN) layer between green sample and graphite mold could protect samples from carbon contamination. Additionally, the presence of h-BN may decrease the formation of oxygen vacancies, which allows us to maintain the semiconductor features of the KNN structure. Remaining issues were addressed with the addition of excess Na and Er₂O₃ doping. The results showed that excess Na addition allowed us to compensate evaporation of sodium during the synthesis and sintering. Er₂O₃ was added as sintering aid to limit abnormal grain growth caused by h-BN addition. The modification of amount of Na and Er₂O₃ addition resulted in high purity KNN samples with tetragonal structure and apparent density higher than 97%. Finally, piezoelectric features of prepared dense samples were measured and presented.