Li/Mn co-doped K_(0.5)Bi_(4.5)Ti_(4)O_(15) high-temperature piezoelectric ceramics:Structures,properties,and ultrasonic transducer applicationsOA

K_(0.5)Bi_(4.5)Ti_(4)O_(15)(KBT,Curie temperature T_(C)=558℃),a representative Aurivillius-type ferroelectric material,is considered a promising candidate for high-temperature piezoelectric devices.In this study,the co-doping strategy was adopted by introducing Li/Mn ions into the A-site of KBT,and a series of polycrystalline ceramics with the formula(KBi)_(0.5-x)(LiMn)_(x)Bi_(4)Ti_(4)O_(15)(x=0-0.065,abbreviated as KBT-LM1000x)were synthesized via the conventional solid-state reaction method.The substitution mechanisms of Li/Mn ions and their doping concentration effects on the crystalline structure,defect chemistry,and dielectric/ferroelectric properties were investigated.The synergistic substitution of Li/Mn for K/Bi induced a transition of KBT in its dielectric behavior from a sharp λ-type anomaly to a diffuse or relaxor-like response,with both diffuseness parameter γ and T_(C) values gradually increasing with increasing x.The Li/Mn co-doping strategy effectively suppressed the leakage conduction of KBT,achieving balanced control of conduction suppression and domain-wall mobility.Among the systems,KBM55 achieved the highest piezoelectric coefficient(d_(33)=30 pC/N)and planar electromechanical coupling factor(k_(p)=6.4%),as well as a higher Curie temperature(T_(C)=590℃).Furthermore,an ultrasonic transducer was fabricated by using this sample as conversion elements,which demonstrated a center frequency(f_(c))of 2.24 MHz,a-6 dB bandwidth(BW_(-6 dB))of 33.95%,and stable pulse-echo signals up to 250℃.