STUDY OF SINTERING BEHAVIOURS AND MECHANICAL PROPERTIES OF BARIUM STRONTIUM COBALT IRON OXIDE CERAMICS

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Abstract

The thesis studies the sintering behaviours and mechanical properties of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) ceramics. The sintering behaviours of BSCF are studied by sintering BSCF powder using a series of sintering temperatures and dwell times. Under all circumstances, only a cubic perovskite structure is identified in as-sintered samples. The relative density of BSCF increases with increasing sintering temperature and dwell time, but shows a more significant increase with increasing temperature. While the grain size increases with increasing sintering temperature and dwell time, it is found that the increasing temperature contributes much more significantly than increasing dwell time in grain growth. The shape of grain size distribution profile is independent of sintering temperature and dwell time, but the profile shifts with different sintering conditions. The grain maintains an aspect ratio of 1.8 irrespective of sintering conditions. Similar findings are also made on the Ni-doped BSCF, but it is found that Ni doping inhibits the grain growth and retards the densification of BSCF while it has little influence on the grain size distributions and grain aspect ratio distributions. The grain growth exponent (n) and apparent activation energy (Q) are also systematically studied. It is found that grain boundary diffusion is the dominant controlling mechanism for BSCF while both grain boundary and lattice diffusions are the equally dominant controlling mechanisms for BSCF-Ni8. The fracture stress of BSCF is measured by both three-point and ring-on-ring bending tests at room and high temperatures. The fracture stress determined by three-point bending tests is consistently higher than that value measured by ring-on-ring tests for a given temperature. By utilising Weibull statistics a close prediction is made of the three-point values from the ring-on-ring values. Compared with the Young’s modulus of BSCF obtained from three-point bending tests between RT and 800 °C, the values determined from ring-on-ring tests shows a fairly good agreement. However, the Young’s modulus measured by both bending tests is lower than that value determined by micro-indentation tests. Hardness and fracture toughness are independent of grain size and grain orientation. Porosity is the dominant factor in Young’s modulus, hardness and fracture toughness of BSCF. The intrinsic hardness, intrinsic Young’s modulus and intrinsic fracture toughness of BSCF are also determined. The subcritical crack growth (SCG) of BSCF is also studied using constant load method at RT and constant stress rate method at 800 °C. It is found that that BSCF is not susceptible to SCG at RT but becomes relatively sensitive to SCG at 800 °C. The results are subsequently used as a basis for a strength–probability–time (SPT) lifetime prediction. Ni doping increases the Young’s modulus, hardness and fracture toughness of BSCF determined micro-indentation tests at RT. Both hardness and Young’s modulus show a non-monotonic trend with Ni doping content, which is attributed to the porosity and secondary phase. The intrinsic hardness, intrinsic Young’s modulus and intrinsic fracture toughness of 8 mol% Ni-doped BSCF are determined. Dopants have little influence on grain orientation and the distribution of grain boundary misorientation angles of BSCF.

Keyword(s)

BSCF; DOPING; MECHANICAL PROPERTY; SINTERING BEHAVIOUR

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