Bulk response and grain boundary microelectrical activity of high T_C BaTiO₃–(Bi_1/2K_1/2)TiO₃-based positive temperature coefficient of resistance ceramics

Abstract

Lead-free positive temperature coefficient of resistance (PTC) thermistors were synthesized from (1 À x/100)BaTiO 3 -(x/100)(Bi 1/2 K 1/2 )TiO 3 -based solid solutions, using a conventional mixed-oxide fabrication route, and sintered in N 2 followed by air annealing. A maximum T C of 205°C was achieved for x 5 20. An increase in x from 0 to 20 decreased the grain size by more than 92% and increased room temperature resistivity (q RT ) by 7 orders of magnitude. For x # 10, PTC ratio (q max /q min ) % 10 4.5 and temperature coefficient of resistivity (a) . 10.3%/°C were achieved using Mn and Al 2 O 3 :SiO 2 :TiO 2 (AST) additions. For x . 10, q max /q min . 10 3 and a . 8%/°C were only obtained in samples sintered in N 2 without subsequent air annealing. Complex impedance analysis revealed three relaxation processes, attributed to a semiconducting grain core, a PTC active grain boundary interface, and a grain boundary insulating layer. Local electrical activity was investigated by hot-stage conductive mode microscopy. The existence of symmetrical grain boundary electron beam-induced current and b-conductivity contrast at the grain boundaries, consistent with the presence of an electron trapping two-dimensional grain boundary plane, compensated by positive space charge layers and a low conductivity vacancy-rich layer, was revealed for the first time within this system.

Barium titanate-based thermistors: Past achievements, state of the art, and future perspectives

“…263 Zubair et al also demonstrated the presence of a third electroactive component for BaTiO 3 À (Bi 0.5 K 0.5 )TiO 3 systems, which were doped with manganese and a Al 2 O 3 :SiO 2 :TiO 2 sintering aid and sintered in N 2 and reoxidized in air. 278,279 This third relaxation process was speculated to be due to the presence of a thin shell composed of barium vacancies (V 00 Ba ), which is formed between the grain and grain boundary region, and was only present above the Curie point at 170 C. The bulk grain semiconductivity was explained due to the reduction of Ti 4þ to Ti 3þ by V •• O , generated by sintering under an N 2 atmosphere. The grain boundary resistivity was attributed to a 2D layer of chemisorbed oxygen atoms and segregated Mn acceptor ions at the grain boundary.…”

Resistance reduction effect of potassium bismuth titanate doping in yttrium–manganese co-doped medium Curie temperature barium titanate lead free ceramics through improved synthesis process

“…However, the room temperature resistance (R 25 ) and T c of BT elements generally rise together with increasing KBT concentration [12,13] for KBT's decomposition and volatilization in synthesis process [14,15]. In fact, magnitude of R 25 usually limits the application range of lead free devices and ultra high R 25 or electrical insulation will restrict further KBT addition.…”