Lead-free Dielectric and Piezoelectric Ceramics

A goal in the search for lead-free piezoelectrics is to discover solid solutions with temperature insensitive morphotropic phase boundaries, as this is likely to enhance piezoelectric properties and promote temperature-stability. Furthermore, there is a high drive for developments of temperature stable dielectric ceramics which can operate at temperatures > 200 oC, well above the limit of existing high volumetric efficiency capacitor materials. A new family of novel lead-free piezoelectric perovskite solid solutions in the binary systems (1-x)K0.5Bi0.5TiO3-xBi(Mg0.5Ti0.5)O3, (1-x)KBT-xBMT and (1-x)K0.5Bi0.5TiO3-xBa(Zr0.2Ti0.8)O3, (1-x)KBT-xBZT were fabricated. In the examination of (1-x)KBT-xBMT ceramic system, a phase boundary (MPB) between tetragonal and mixed phase tetragonal+cubic (pseudocubic) was identified at 0.025 < x < 0.03. Compositions 0.03 ≤ x < 0.08 were mixed, tetragonal and cubic phase. Compositions close to MPB exhibited favourable piezoelectric properties, for example, the piezoelectric charge coefficient, d33, was 150 pC/N for composition x = 0.03, and 133 pC/N for x = 0.04. A high bipolar electric field-strain was exhibited by MPB compositions with strains of 0.25%-0.35%. Values of temperature dependent unipolar strain for the (1-x)KBT-xBMT (x = 0.03 and 0.04) were retained ~ 0.18% at a temperature ≥ 185 oC. Thermally stimulated charge decay and kp-T measurements revealed full depolarisation at Td ~ 220 oC. The overall properties are very promising for electromechanical actuator applications. In the binary (1-x)KBT-xBZT system, the mixed phase (tetragonal+cubic) composition x = 0.1, demonstrated a piezoelectric charge coefficient, d33 = 130 pC/N, bipolar strain ~ 0.13% (60 kV/cm) and high depolarisation-temperature ~ 220 oC. Temperature stable dielectric systems; (1-x)Ba0.8Ca0.2TiO3-xBi(Mg0.5Ti0.5)O3, (1-x)BCT-xBMT), 0.45Ba0.8Ca0.2TiO3-(0.55-x)Bi(Mg0.5Ti0.5)O3-xNaNbO3, 0.45BCT-(0.55-x)BMT-xNN, and (1-x)[0.5K0.5Bi0.5TiO3-0.5Ba(Zr0.2Ti0.8)O3]-xBi(Zn2/3Nb1/3)O3, (1-x)[0.5KBT-0.5BZT]-xBZN were synthesised with near plateau in relative permittivity-temperature response (er-T), giving a ±15%, or better, consistency in er across a wide temperature range, coupled with optimum dc resistivities. The composition: 0.5BCT-0.5BMT indicated a temperature stability, ɛr = 800±15% from 40-550 oC, with tanδ ≤ 0.02 over the temperature range 100-400 oC. For a slightly higher BMT content, the dielectric properties were superior to 0.5BMT, with ɛr = 950±15% from 70 to 600 oC and tanδ ≤ 0.02 from 160-550 oC. Achieving temperature-stability down to -55 oC and below was accomplished in the 0.45BCT-0.55BMT ceramic materials by the incorporation of NaNbO3 at a level x ≥ 0.2. Modification with x = 0.3, led to the temperature stability in relative permittivity, with ɛr = 550±15% across the temperature range -70 oC-300 oC and tanδ ≤ 0.02 from -60 oC to 300 oC, thus achieving the goal of producing a temperature-stable relaxor dielectric to operate in a range of harsh environments down to < -55 oC. Similarly, a near flat dielectric response was exhibited by the ceramic system (1-x)[0.5KBT-0.5BZT]-xBZN ceramic system (x = 0.2BZN) with er = 805±15% across a wide temperature range, from -20 oC to 600 oC; with tanδ ≤ 0.02 across from 50 oC to 450 oC. These temperature stable dielectric materials were comparable to the best temperature stable dielectric materials for example; 50BaTiO3-25Bi(Zn0.5Ti0.5)O3-25BiScO3, er = 1100±15% (80-500 oC), 0.85[0.6Na0. 5Bi0.5TiO3-0.4K0.5Bi0.5TiO3]-0.15K0. 5Na0.5NbO3, er = 2167±10% (54-400 oC) and highly attractive for the high temperature capacitor applications.