Biomedical Engineering Reference
In-Depth Information
(a)
Micropore
Grain
boundaries
1
m
m
(b)
5.4
SEM micrographs of hydroxyapatite. (a) Macroporosity in a
scaffold designed for bone ingrowth. Dip-coated hydroxyapatite
scaffold made using the reticulated foam method. Courtesy J.H.
Robinson, University of Cambridge. (b) Microporosity in sintered
(densified) polycrystalline hydroxyapatite. Grains and grain
boundaries are also evident.
surface areas of a material which may be riddled with micropores owing to
incomplete densification during sintering. One of the standard techniques
for doing this, as just mentioned, is mercury porosimetry. Another option,
for more delicate materials, is nitrogen adsorption, in which gas molecules
are physically adsorbed onto a solid surface. Using Bet theory, one can
use the data from nitrogen adsorption to calculate the specific surface area
(Braunauer
et al.
, 1938), measured in units of m
2
g
-1
. A green (unsintered)
ceramic powder may have a surface area of hundreds of square metres per
gram, while a sintered powder may be in the low tens of square metres per
gram. Pore size distributions can also be calculated from nitrogen adsorption
data with BJh theory (Barrett
et al.
, 1951). Adsorption mechanisms are
described in more detail in (Lowell
et al.
, 2006a).
