Study Of Potassium Sodium Niobate-Tantalate Ceramics Based Piezoelectric Transformer

Piezoelectric ceramics as one of the most important functional ceramic material is used in many practical applications, such as: piezoelectric ultrasonic transducer, piezoelectric ceramic filter, piezoelectric buzzer, different kinds of sensors, actuators, piezoelectric high voltage generators, etc. Recently piezoelectric needle selectors have been used for Jacquard knitting machine. Most piezoelectric ceramic materials in practical use are lead zirconate-titanate based systems. These materials contain more than 70% PbO by weight, so they cause environmental pollution during manufacture and after usage. It is well known, the environmental protection has been becoming an important issue for the governments and the public. Therefore, it is very essential to develop lead-free or lead-less content piezoelectric materials to solve the lead pollution in piezoelectric industry.After intensive and extensive survey, potassium sodium niobate-tantalate (KNNT) ceramics has been selected as materials for piezoelectric transformer. In this work, practicality, KNNT ceramics and piezoelectric transformer based on KNNT have been studied. From the start of designing piezoelectric materials, physical properties of KNNT ceramics was studied which aimed to find high performance KNNT materials for piezoelectric transformer. Reasonable explanation to the phenomena was given. Secondly, application-oriented research approach was chosen and disk-type piezoelectric transformer was fabricated using the modified KNNT ceramics.Ta-modified KNNT ceramics has been prepared using conventional solid-state reaction technique. Preparation conditions, microstructures, dielectric properties and piezoelectric properties were analyzed. The effects of the Ta content and K, Na ratio on the microstructures, phase transition and piezoelectric properties were studied. The doping of KCN and proper amount of MnO₂ was effective in promoting the mechanical quality factor Q_m and reducing the dielectric loss tanδ. It made KNNT ceramics possess "harder" properties. This material is a promising candidate for the lead-free piezoelectric transformer applications. The high performance KNNT piezoelectric materials was obtained, high mechanical quality of Q_m=1563, dielectric loss about tanδ=0.4% at 1kHz, electromechanical coupling coefficient of k_p=42.2% and piezoelectric constant d₃₃=96pC/N.The equivalent circuit model of Rosen-type piezoelectric transformer was analyzed using electrical and mechanical equivalent network method. With reference to its voltage gain formula, optimal KNNT ceramics was chosen for transformer. Disk-type piezoelectric transformers with different input and output area ratio were fabricated using modified (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃ ceramics. Impedance and voltage gain characteristics were investigated of the transformer and preliminary research on the driver circuit was developed. The maximum step-up ratio more than 42 with no load condition has been obtained. It suggests that this system is a promising candidate for the lead-free piezoelectric transformer applications.