Biomedical Engineering Reference
In-Depth Information
soft and hardwoods, such as beech (
Fagus sylvatica
), eucalyptus
(
Eucalyptus globulus
), sapelli (
Entandrophragma cylindricum
), oak
(
Quercus robur
), etc., has been reported.
9
,
12
−
14
Basically the bioceramization process consists of the infiltra-
tion of molten Si in carbonaceous scaffolds derived from vegetable
precursors.
9
,
12
,
14
,
17
ThewholefabricationprocessofbioinspiredSiC
ceramics can bedescribed in three main steps:
(a) Precursors selection: In this first stage, the appropriate veg-
etable source is selected according to the desired properties of
the final product, using a criteria of density, size, and intercon-
nection of the pores. Depending on the nature of the vegetable,
the natural samples are freeze-dried or dried with warm air in
order to avoid the collapse of the vegetable architecture and to
keep the originalmicrostructure as itis.
(b) Pyrolyzation process: The natural vegetable samples are
exposed to the pyrolysis process in a high-temperature furnace
under inert conditions with a well-controlled ramp of 2
◦
C/min
up to 800
◦
C, keeping the sample at this temperature for one
hour.Aftercoolingattherateof20
◦
C/min,acarbonaceousscaf-
foldthatkeepsthebasicmicrostructuralfeaturesofthestarting
natural material is obtained.
12
,
13
,
17
In this step, about 75
±
5%
ofthestartingweightofthevegetableislost,mainlyintheform
of water vapor and other volatiles products. Volume is reduced
atthesametimebyabout60
±
5%. These reductions appear to
beindependentofthetypeofvegetable,whichisveryusefulfor
precursor selection aimed at a specific final density.
13
(c) Infiltration process: The finalstep isthemolten Si infiltrationof
the carbonaceous scaffold in vacuum conditions. For this pur-
pose, the carbon sample is introduced in a controlled heat-
ing furnace where the temperature is increased at 5-10
◦
C/min
until 1,550
◦
C, exceeding the Si melting point (1,410
◦
C), with 30
minutes of permanence at this last temperature. After the cool-
ing of the sample with a well-controlled ramp of 20
◦
C/min, the
finalSiCceramicsisobtained.
12
,
13
,
17
TheSiinfiltratesthecarbon
preform rapidly by capillarity. Through the connected poros-
ity the molten Si reaches all pores and channels within a few
seconds. The subsequent SiC formation reaction is spontaneous
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