DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

None of the compositions reached full density. As all the cylindrical rods started
with φsolids=0.43, complete elimination of porosity would result in a sintering strain ε of
0.245. The largest ε observed is in the range from 0.18 to 0.22 at 1350 °C for these
samples, which corresponds to residual porosity in the range of 9%-22% by volume. The
explanation to these observations includes the following: i) subjected to the same
temperature profile, composition that has a high solid volume fraction of Ni will have
larger shrinkage as a collective effect of more local Ni-Ni densification. ii) BT particles
acts as a refractory phase in the BT-Ni composite at temperatures lower than 1350 °C,
preventing the sintering of the Ni matrix, especially for composition with high BT
volume fractions. This makes complete exclusion of porosity difficult for these
composites without the assist of an external pressure. iii) Pure BT is usually sintered at
1350 °C for 1 hour to reach a high degree of densification; a lack of isothermal hold at
1350 °C leads to its high porosity.
5.3.4. Co-Sintering and Re-oxidation
After binder burnout at 260 °C, only the Ni(m) ink with BT:Ni=4:1 composition
successfully co-sinters with pure BT at 1350 °C without cracking. Unfortunately, such
high ceramic content leads to brittle mechanical behavior and the Ni(m) has low
conductivity due to the minimal metallic phase. This lack of metallic properties in the
BT-Ni composite phase must be remedied for uses where metallic properties provide
crucial functions. Nevertheless, mechanical brittleness may not be problematic for many
electronic applications and the dielectric and ferroelectric behavior of the BT phase may
be more important.
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