The Chemical Interaction of Silver–Palladium Alloy Electrodes with Bismuth‐Based Piezomaterials

Abstract

Multilayer technology relies heavily on the chemical compatibility of metal and ceramic. This work focuses on the ceramic–electrode interaction between 92Bi0.5Na0.5TiO3–6 BaTiO3–2K0.5Na0.5NbO3 [(Bi0.46Na0.47Ba0.06K0.01)(Nb0.02Ti0.98)O3], a promising actuator material and forerunner to an emerging class of lead‐free actuator materials, and a silver–palladium alloy for inner electrodes, the only currently viable material for the firing temperatures necessary (1100°C). Of special concern was the high content of bismuth in the ceramic since prior investigations suggest that Bi2O3 (as well as various bismuth titanates) used as a fluxor in electroceramics are prone to forming the intermediate‐phase bismuth palladate (Bi2PdO4), which can lead to poor contacting and delamination of multilayer stacks. Remarkably, no evidence of bismuth palladate formation could be found. However, the phase relations of the bulk ceramic have proven to be quite complex. Potassium was being drained out of the bulk ceramic either constituting the secondary phase K4Na2(TiO3)3 in unmodified experiments or evaporating and being replaced by silver in samples in contact with Ag. Mechanisms for the formation of these phases or the lack thereof are proposed. These findings were obtained by XRD, TG‐DSC, and SEM with EDX, and LA‐ICPMS.

Bismuth Sodium Titanate Based Materials for Piezoelectric Actuators

“…Thus, the use of silver-palladium alloy together with Bismuth-based ceramics was generally excluded and the first multilayer sample described by Nagata, et al [156] contained platinum electrodes. In a study by Schütz, et al [157] it was shown that the bismuth-palladium reaction does not take place if the presence of free Bi 2 O 3 in the ceramic is avoided. This encouraged the use of silver palladium alloys for BNT-based multilayer devices.…”

Investigation of the Pastes for Multilayer Ceramic Capacitors Termination

“…The interaction of palladium with bismuth oxide occurs in the process of burning paste. In the temperature range of 250-750 o C Pd, palladium is oxidized, and at temperatures of 350-835 o C, palladium oxide reacts with bismuth oxide [16,17,18]. Thus, two reactions occur in the paste during the burning process, which lead to the discontinuities of the contact pad (see Eq.…”

BNT-based multilayer device with large and temperature independent strain made by a water-based preparation process

“…The first BNT-based multilayer actuator was presented by Takenaka et al, 10 who used Pt-electrodes because of the concern of a bismuth-palladium reaction. 11,12 Schütz et al 13 showed that BNT based ceramics do not interact with palladium during the sintering process as long as free bismuth oxide is not present. Hence the use of Ag/Pd-alloys for the co-fired inner-electrodes should be possible.…”