3.1. Establishment of process parameters to obtain optimum foam
3D geometry
The three-dimensional porous network of the foam can be tailored to
specific applications by adjusting some intrinsic parameters such as porosity
andmean pore size. This can bemainly defined by two parameters,
the volume percentage between the parent material powder and the
space holder particles, and the particle size of the space holder material
used during the fabrication of the foam. The aim is to understand the
governing relation between the process parameters and the final structural
properties which in turn define the foam's functional properties.
Another important factor affecting the final pore morphological characteristics
and the porosity percentage are the sintering conditions (temperature,
holding time and environment) which govern the shrinkage
mechanism during the heating cycle. In this study, the sintering time
and temperature were kept constant while the effect of the volume percentage
and particle size of sugar in the final macrostructure of the Cufoam
was examined. Sintering at temperatures below 800 °C resulted in
insufficient bonding and required a prolonged time period to establish
bonding between the Cu particles. Sintering above 880 °C should be
avoided because it caused intense radial deformation of the foamresulting
in the non-uniform distribution of cell size and density as well as shape
distortion. In this context, after several trials the optimum sintering temperature
was set to 840 °C, while the holding time was fixed to 4 h.
Preliminary screening experiments were used to determine the
range of values of volume percentage and particle size in the space
holder material. Values in the range of 65–90% for volume percentage
and 0.35–1.3 mm for mean sugar particle size were examined. For the
given sintering conditions, it was noted that foam shrinkage was the
prevailing mechanism influencing the structure and porosity of the
produced foam. In all the examined samples a volume shrinkage of
about 25% was observed leading to an actual porosity about 7% lower
than the initial volume percentage of sugar used.
On one hand, when using sugar volume percentages higher than
90%, severe spalling of Cu powderswas observed following space holder
leaching in water, which in some cases caused a total collapse of the
green product. On the other hand, when sugar volume percentage was
lower than 65%, the foam structure obtained had reduced interconnectivity.
Consequently, these values were excluded from further study.
When the sugar's volume percentage was set to 80%, a change
in pore morphology was noticed. From an initially ellipsoid shape
(resembling the sugar particle morphology) the pores became elongated
due to the pronounced coalescence of sugar particles during the
compaction stage. This pore agglomeration was observed for all mean
particle sizes of sugar used in this study due to the specific volume
space of the die used. Agglomeration of the space holder particles occurs
at higher porosities and leads to inhomogeneity in the density distribution
of copper powder, which can cause reduced foam efficiency in the
target application. Taking all the above into consideration, a volume
percentage of 72% was chosen for the sugar particles, yielding to a
final foam porosity of 65%.
Concerning pore size, a decrement of this value may increase the
density of stronger ligaments, i.e. the number of struts in a definite
volume, given that the pore density, known as pores per linear inch
(ppi), rises when average pore size decreases. This also results in more
homogeneous structures, larger external surfaces and enhanced interconnectivity
of the macropores, thereupon leading to a possible
improvement in foam efficiency. For these reasons, sugar particles
with a mean size of above 0.8 mm were not used in this study. Consequently,
only sugar particles with mean sizes of 0.7 and 0.35 mm were
examined. Similar values for mean pore size and porosity have been
reported in literature [1–3,17–21,24] for copper foams manufactured
employing other space holder materials.