Lead-free Ceramics for High Temperature Actuator Applications

In the quest of lead-free ceramics with stable electromechanical strains at higher
temperatures, the ternary system 1-x-yBiFeO3-xREFeO3-yRE2/3TiO3 (RE = La, Nd) B(RE)FT and pseudo-binary solid solutions (1-x)K1/2Bi1/2TiO3-x(0.80BiFeO3-
0.15LaFeO3-0.05La2/3TiO3), KBT-BLFT, (1-x)K1/2Bi1/2TiO3-x(0.82BiFeO3-0.15NdFeO3-0.03Nd2/3TiO3, KBT-BNFT and (1-x)K0.4Na0.6NbO3- x(BiFeO3) KNN-BF, were studied. Unlike undoped BiFeO3, B(RE)FT, ceramics in the ternary series can sustain large electricfields up to 8 kV/mm but no ferroelectric/antiferroelectric switching is observed indicating that the coercive fields are very high and therefore these ceramics are not likely to be used
for actuator applications.
The pseudo-binary solid solutions KBT-BLFT and KBT-BNFT offer opportunities of
fabricating ceramics ranging from conventional ferroelectrics to pure electrostrictors. With increase in BLFT/BNFT concentrations, ferroelectric order is disrupted and relaxorlike behaviour is promoted. Promotion of relaxor-like behaviour is accompanied by an increase in positive strain and decrease in hysteresis of these ceramics. Electromechanical strain up to 0.16 % at 6 kV/mm can be achieved. The optimum strains are predominantly electrostrictive and exhibit fatigue resistant behaviour. More importantly the optimised
electromechanical strains are stable up to a temperature of 300 °C making them superior to PZT-based ceramics in terms of temperature stability. The ceramics can be fabricated into a multilayer structure by tape casting technique as demonstrated by a prototype multilayer actuator.
The binary solid solution KNN-BF is very limited and dense ceramics could not be
fabricated with x > 0.01. Within the limited range studied, ceramics yield a strain of 0.05 % (2kV/mm) but the operating window is strongly hampered by the ferroelectric to ferroelectric phase transition which is close to ~100 °C. With Li doping the strain values could be enhanced to 0.12 % (3 kV/mm), however the room temperature phase is strongly polymorphic in nature and a monotonic decay in properties is observed with increase in temperature making these ceramics unsuitable for high temperature applications.