Biomedical Engineering Reference
In-Depth Information
− +
H
H
+ −
H
+ −
H
+ −
− +
H
H
+ −
×
4
− +
H
− +
H
H
+ −
− +
H
− +
H
H
+ −
− +
H
− +
H
H
+ −
H
+ −
Figure 3.4
A schematic of silica tetrahedra forming a nanoparticle in a sol. The
nanoparticle will coalesce with other similar particles and bonds will form between
them to form a silica network. When it becomes rigid, it will be a gel.
The hydrolysis and polycondensation reactions occur simultaneously
within the solution during mixing. When sufficient interconnected
Si-O-Si bonds are formed in a region, they behave as colloidal (sub-
micrometre) particles and a sol is formed (Figure 3.4). The first small
particles that form are termed 'primary particles'.
3.4.2 Stage 2: Casting
The sol can be cast prior to gelation in air-tight moulds. The moulds
must be made of material that will not adhere to the gel (e.g. polyte-
trafluoroethylene, PTFE). The gelation process can take several days at
room temperature but can be tailored to the needs of the process.
3.4.3 Stage 3: Gelation
As mixing continues, agglomeration of the primary particles occurs and
condensation continues, causing coarsening of the particles. As more
and more particles join together, a network of O-Si-O bonds begins to
form, the viscosity of the sol increases and eventually a gel (rigid silica
network) is formed. The reaction continues until all the precursor is
used up.
3.4.4 Stage 4: Ageing
After gelation, the gel is aged, either at room temperature or at slightly
elevated temperatures, for example 60
◦
C, for several hours or days.
