DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

5.3.6. Fabrication of BT-Ni composite Structures
Two types of BT-Ni composite structures are fabricated by serial printing of BT
and Ni(m) inks: 1) 2D NPR bowtie network and 2) MLCC lattices. Figure 5.9 shows a
sintered complex graded NPR structure resembling interconnected alternating layers of
bowtie shaped stripes. This NPR structure is a 2D simplification of the proposed 3D NPR
composite structure. The white colored stripes are BT before re-oxidation, and the dark
colored stripes are Ni(m) with a BT:Ni ratio of 4:1. The interface between the two
compositions is at the mid-point of a thin wall of stacked filaments at around 160 µm in
thickness. These filaments are printed with Ø200 µm nozzles. Each unit of the bowtie
stripes is slightly distorted as the result of the printing process and supposedly the internal
stress generated during sintering process. Both ends of the bowtie stripes are deformed
during drying process because these free-standing thin walls are then not structurally
stable; gravitation and unbalanced surface tension from the lamp oil cause these thin
walls to skew sideways. However, the BT:Ni=4:1 Ni(m) bowtie stripes are brittle due to
the high BT inclusion. To achieve the proposed non-linear load deformation
characteristics of a NPR composite structure, a much higher Ni solid volume fraction is
preferred to use in the Ni(m) component to allow sufficient ductility. But this is in
conflict with the requirement of successful co-sintering in the solid state sintering
paradigm.
A composite lattice not shown here is a structure resembling a multilayer ceramic
capacitor, assembled with pure BT and Ni(m) (BT:Ni=4:1) inks. As the sintered Ni(m)
(BT:Ni=4:1) has a low electrical conductivity due to high ceramic inclusion. To achieve
this specific structure and function with a lower BT inclusion in the Ni(m) electrodes (for
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