Biomedical Engineering Reference
In-Depth Information
Table 15.1
Physical properties of MgO and BaTiO 3
Ceramic
oxide
Melting
temperature (8C)
Density
(g cm 3 )
Dielectric
constant (κ)
Mean particle
size
Mass of
particle (g)
2.34610 10
1.50
MgO
2852
3.58
9.65
5 μ m
20 nm
10 17
6
10 12
BaTiO 3
6.02
(tetragonal) k
11 = 3600
k
1
m
70 nm
9.45
6
μ
10 15
33 = 150
8.65
6
8
nanosize specimen possesses lower conductivity above 50
C but higher
conductivity below 50
C as compared to the microsize BaTiO 3 specimen.
This trend was also noted for MgO in Fig. 15.1, but to a much lower degree.
The ambient temperature conductivities of BaTiO 3 -containing specimens
are in the range 10 4 -10 5 Scm 1 .
Table 15.1 presents the physical properties of MgO and BaTiO 3 materials.
BaTiO 3 is an important ferroelectric material and its dielectric constant in
the tetragonal phase (ferroelectric state) is much higher and anisotropic. The
dielectric constant of BaTiO 3 is also known to be particle-size dependent.
The two ceramic materials, MgO and BaTiO 3 , in effect represent the upper
and lower limits of dielectric constants of available ceramic materials. The
last column of Table 15.1 shows masses corresponding to the two particle
sizes used for each material. For MgO and BaTiO 3 , the masses
corresponding to each particle size differ by approximately seven and
three orders of magnitude, respectively.
In spite of widely different physical characteristics, including density and
dielectric constant, both MgO and BaTiO 3 additives exhibit similar effects
with regard to particle size, and thus this is believed to be a dominant
parameter. Reducing the particle size from the micro- to nano-range
increases low-temperature conductivities and decreases their temperature
dependence, as shown in Fig. 15.2. The enhancements are pronounced and
associated with lower activation energies for the ionic transport.
The average length and mass of a PEO chain are 20
8
￿ ￿ ￿ ￿ ￿ ￿
10 18 g,
respectively. These length and mass values must now be compared with the
particle size and mass of the ceramic dopants to develop insight into possible
interactions. The proximity and interaction of a polymer chain and a
nanosize MgO particle (0.02
μ
6
m and 3
m) are shown schematically in Fig. 15.3. A
constant segmental chain motion near the glass transition temperature is
expected to cause displacement and distribution of MgO particles.
Furthermore, the process can be facilitated by thermal treatments and
cycling. The displacement and distribution of MgO particles continues until
the polymer chain dipole and MgO dipole interaction takes place. The
interaction leads to latching of the polymer chain and MgO particle, and
thus stabilization of the structure. When experimentally monitored, this
μ
 
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