Preparation and characterization of sodium potassium niobate-based electro-optic ceramics

Abstract

The main objective of the present work is to develop new lead-free KNN-based electro-optic ceramics. The demand on the electro-optic (EO) materials increases with the rapid development of optic communication technology. The most widely used EO ceramics such as (Pb₁-xLax)(Zr₁-yTiy)1-x/4O₃ (PLZT) and Pb(Mg₁/₃Nb₂/₃)O₃-PbTiO₃ (PMN-PT) are contain a large amount ( > 65%) of lead which is highly toxic. For environmental protection reasons, lead-free materials have been extensively studied in recent years as replacements for lead-containing materials. In this work, new lead-free, (K₀.₅Na₀.₅)₀.₉₄₇₅La₀.₀₁₇₅NbO₃ + x mol% Bi₂O₃ (abbreviated as KNN-La-xBi) electro-optic ceramics exhibiting good optical transparency and strong linear electro-optic responses have been successfully fabricated. The two-step sintering technique has been used to fabricate the K₀.₅Na₀.₅NbO₃ (KNN) and (K₀.₅Na₀.₅)₀.₉₄₇₅La₀.₀₁₇₅NbO₃ (KNN-La) ceramics. The ceramics were first sintered at a higher temperature T₁ for a period t1. Then, the temperature was decreased rapidly to T₂ and held for t₂. The effects of the sintering conditions on the the grain growth, dielectric, piezoelectric and optical properties of the ceramics have been studied. Our results reveal that the technique is capable of suppressing the grain growth of the ceramics. The grain growth of the ceramics is mainly affected by the first-step sintering temperature T₁ and sintering time t₁ while the densification is controlled by the second-step sintering temperature T₂ and the dwelling time t₂. The grain sizes of the KNN ceramics are reduced significantly from ~16 to ~3 um after applying the two-step sintering technique. The grain sizes of the KNN-La ceramics are slightly reduced from 0.8 to 0.5 um by the two-step sintering technique. Although the grain size of the KNN-La ceramics is reduced to ~ 0.5 um, the ceramics only become translucent. Our results also show that the dielectric and piezoelectric properties of the ceramics are not deteriorated by the reduction of the grain size. Excess Bi₂O₃ has been added as a sintering aid to lower the sintering temperature and suppress the grain growth of the KNN-La ceramics. Our results show that excess Bi₂O₃ is effective in promoting uniform densification as well as suppressing the grain growth of the ceramics. The KNN-La-xBi ceramics possess smaller and more uniform grains, and hence their optical properties are improved significantly. For the KNN-La-2Bi ceramic, the transmittance reaches a high value of 50% in the near-IR region. Our results also reveal that the ceramics possess a cubic-like crystal structure and hence minimal optical anisotropy. The ceramics also exhibit a diffuse phase transition, suggesting that they have become more relaxor-like and contain more polar nano-regions. As a result, the optical properties of the ceramics are improved by the significant reduction of the light scattering at the grain boundaries as well as the domain walls. Although a relaxor-like behaviour is induced, the ceramics exhibit strong linear electro-optic response, giving a high effective linear EO coefficient of 120-145 pm/V. Transparent KNN-La and KNN-La-xBi ceramics have also been successfully fabricated by hot-press sintering technique. The pressure applied during the hot-press sintering technique is effective in promoting uniform densification and hence reducing the pores in the ceramics. The light scattering by the pores is reduced, and thus resulting in the significant improvement of the optical properties of the ceramic. For the KNN-La-1Bi ceramic prepared by hot-press sintering technique, the observed transmittance is increased by more than 100%, reaching a high value of 55% in the near-IR region. However, probably due to the non-uniform shrinkage of the ceramics, the ceramics become easily broken during the sintering process.