Dielectrics for High Temperature Capacitors Applications

Most of the dielectric materials developed in this thesis were relaxor dielectrics with very broad, frequency dependent peaks in relative permittivity-temperature, εr – T, plots. In some cases, plateau-like permittivity plots with very wide temperature ranges of stable permittivity varying by no more than ±15 % were obtained through compositional engineering of the relaxor base material using high levels of lattice substitution of cations of different charge and size to the host lattice. The solid solution series (1–x)Na0.5Bi0.5TiO3 – xBa0.8Ca0.2TiO3 [(1–x)NBT–xBCT] was studied initially: it showed little evidence of temperature-stable relative permittivity. However, 0.85NBT – 0.15BCT ceramics modified by BiMg0.5Ti0.5O3, [BMT], were more promising. The series, (1–x)[0.85Na0.5Bi0.5TiO3 – 0.15Ba0.8Ca0.2TiO3] – xBiMg0.5Ti0.5O3, x = 0.3, gave εr ~ 1720 ± 15 %, in the temperature range 120 to 450 ºC, and low dielectric loss tangent, tanδ ≤ 0.02, over the slightly narrower temperature range, 150 to 360 ºC. Thus an upper operating temperature > 300°C was demonstrated but without also achieving sub-zero temperature capability. Similarly, the system (1–x)[0.85Na0.5Bi0.5TiO3 – 0.15Ba0.8Ca0.2TiO3] – xLiNbO3 abbreviated [(1–x)[NBT – BCT] – xLN] gave εr ~ 2630 ± 15 % from 50 ºC to 410 ºC, and tanδ ≤ 0.02 from 120 ºC to 400 ºC, at composition x = 0.06. The incorporation of NaNbO3 [NN] in place of LiNbO3 achieved the desired sub-zero temperature capability in relative permittivity, and with an upper temperature limit slightly above 200 ºC. Examples include (1–x)[NBT – BCT] – xNN composition x = 0.3 with εr = 1400 ± 15 % from -50 to 240 °C and tanδ ≤ 0.02 from 20 ºC to 240 v ºC; for x = 0.4, εr = 1300 ± 15 % from -70 to 210 °C and tanδ ≤ 0.02 from -10 ºC to 210 ºC. In terms of the project goal of achieving a temperature range of stable permittivity combined with a low loss extending from temperatures of -55 °C (to meet the Electronic Industry Alliance military specification) up to 300 °C, the binary solid solution system (1–x)Ba0.6Sr0.4Zr0.2Ti0.8O3 – xBiMg0.5Ti0.5O3 [(1–x)BSZT – xBMT] was most promising. However there was a trade-off in that permittivity values were lower than the foregoing systems. The (1–x)BSZT – xBMT sample composition x = 0.2 gave εr ~ 500 ± 15%, in the temperature range -70 to 300 ºC and tanδ ≤ 0.02, in the range -60 to 300 ºC. This system closely matches the target temperature-range of the project, but the volumetric efficiency of a capacitor made from this dielectric would be compromised due its moderate relative permittivity values. Slightly higher values of relative permittivity were obtained for x = 0.3 in the (1–x)BSZT – xBMT series, with εr ~ 590 ± 15%, across the temperature range -60 to 340 ºC, but in this case the temperature range of low dielectric losses tanδ ≤ 0.02 was restricted to -10 to 280 ºC. Piezoelectric properties of selected samples have also been investigated in the thesis.