The Luminescence Properties Of Rare-Earth Doped Potassium Sodium Niobate Based Lead-Free Piezoelectric Ceramics

Recently, lead-free piezoelectric materials, such as K0.5Na0.5NbO3 (piezoelectric coefficient d33 ～80 pC/N, Curie temperature Tc ～420 ℃) have been extensively studied due to the environmental protection and human health reasons. Compared with the traditional lead based piezoelectric materials such as PZT-5H (d33～593 pC/N), the low d33 makes the lead-free piezoelectric materials quite difficult to replace PZT-5H in the practical applications. Many efforts have been made to improve the ferro-/piezoelectric performance of lead-free materials and rare earth doping is a simple and effective way to further improve the ferro-/piezoelectric performance of lead free piezoelectric ceramics. Furthermore, the rare earth doped perovskite oxide ceramics can also possess the excellent photoluminescence (PL) effect. Thus, the electrical and PL properties of rare-earth doped K0.5Na0.5NbO3-based lead-free piezoelectric ceramics are studied in this article, as follows:(1) The influences of Pr3+ doping on the ferro-/piezoelectric properties of K0.5Na0.5NbO3 lead free ceramics.The K0.5Na0.5NbO3:xPr lead-free ceramics with x= 0%,0.1%,0.2%,0.3%,0.4%, and 0.5% were fabricated by the conventional solid-state reaction method. After Pr3+ doping, the K0.5Na0.5NbO3 ceramics exhibit the maximum enhancement of-1.2 times in the ferroelectric remanent polarization strength (increased from 11.46 μC/cm2 to 13.75 μC/cm2) and ～1.25 times in the piezoelectric coefficient d33 (increased from 86.6 pC/N to 108.4 pC/N), respectively. After the trivalent Pr3+ unequivalently substituting the univalent (K0.5Na0.5)+, A-sites ionic vacancies will occur to maintain charge neutrality, which may reduce the inner stress and ease the domain wall motions, yielding to the enhancement in ferro-/piezoelectric performance.(2) The influences of ferro-/piezoelectric polarization treatment on the PL intensity of K0.5Na0.5NbO3:rxPr lead free ceramics.After undergoing a ferro-/piezoelectric polarization treatment, it was found that the PL intensity of the K0.5Na0.5NbO3:xPr lead free ceramics can be enhanced remarkablely. The maximum enhancement of-1.3 times in PL was observed in the poled 0.3% Pr3+ doped sample. The polarization-induced enhancement in PL is attributed to the decrease of crystal symmetry abound the Pr3+ions after polarization.(3) Effects of compositional changes on luminescence of rare earth doped potassium sodium niobate based piezoelectric ceramicsⅠ. The 0.4% Eu3+-doped K1-yNayNbO3 lead-free piezoelectric ceramics were synthesized. The structure transition related to the monoclinic phase to orthorhombic phase is located at y= 0.50 by measuring the dielectric and piezoelectric constants of the ceramics. Increasing y from 0.42 to 0.56, the PL peak intensity of red emission at 615 nm decreased firstly and then increases, giving to the extreme point at y= 0.50. The strong PL dependence on the structure transition was attributed to the high hypersensitivity of Eu3+ to the changes of the lattice environment.Ⅱ. Lead-free (1-z)K0.5Na0.5NbO3-zLiNbO3:Pr (z= 0-0.07) were synthesized. The influences of LiNbO3 content on the XRD, ferroelectric and PL performance was investigated. It can be seen from the XRD patterns that there is a structure transition related to the orthorhombic phase to tetragonal phase at z= 0.50. All the samples exhibit strong red emission at 607 nm and 650 nm, and the K0.5Na0.5NbO3-zLiNbO3: Pr ceramics show the optimal PL property at z= 0.05.Based on the above results, it is potential to develop a non-contact, quick response and simple operation technology to monitor the phase transitions near the morphotropic phase boundary of piezoelectric materials via measuring the PL spectrum.