Biomedical Engineering Reference
In-Depth Information
Table 15.2
Bulk conductivity (
σ
b
), number of charge carriers (
n
i
), and mobility
(
μ
i
) in yttria stabilized zirconia (YSZ) and YSZ-Al
2
O
3
specimens at 500, 600, and
700
8
C
YSZ
YSZ-Al
2
O
3
5008C
6008C
7008C
5008C
6008C
7008C
1.0963610
5
2.18610
5
2.913610
3
5.127610
5
2.02610
4
4.03610
3
σ
b
4.40610
14
8.71610
15
1.10610
17
4.16610
14
1.79610
15
1.810610
16
n
i
7.77610
2
7.82610
2
8.24610
2
28.513610
2
35.31610
2
69.4210
2
μ
i
n
YSZ-Al
2
O
3
n
YSZ
—
—
—
0.95
0.21
0.16
μ
YSZ-Al
2
O
3
μ
YSZ
—
—
—
3.67
4.52
8.42
six at 700
C suggests that there is a temperature-dependent interaction
between Al
2
O
3
and oxygen vacancies. Higher temperatures lead to more
interaction and a reduction in the number of charge carriers. The mobility,
conductivity, and number of charge carriers are related by equation 15.5.
The calculated mobility shows an increase for the YSZ-Al
2
O
3
specimens.
The mobility ratio,
8
μ
(YSZ-Al2O3)
/
μ
YSZ
, also shows an increase from 3.67 at
500
C.
Analysis of Table 15.2 suggests that a significant fraction of oxygen
vacancies in the YSZ-Al
2
O
3
specimens are immobilized at interfaces that
serve as a source of local field and influence the transport of the remaining
conducting vacancies. The interaction of oxygen vacancies and Al
2
O
3
is
illustrated by:
8
C to 8.42 at 700
8
Y
2
O
3
!
2Y
Zr
þ
3O
x
þ
Vo
::
½
:
15
18
Al
2
O
3
þ
Vo
::
!
Al
2
O
3
:
Vo
::
½
15
:
19
Equation 15.18 expresses a well-recognized structural disorder that shows a
generation of oxygen vacancies (Vo
..
) in ZrO
2
after doping with Y
2
O
3
. The
oxygen vacancies interact with Al
2
O
3
particles and form a thermally stable
complex (Al
2
O
3
:Vo
..
) after the interaction as shown in equation 15.19. The
equation is expected to be reversible if enough thermal energy becomes
available to dissociate the A
2
O
3
:Vo
..
complex; however, in the temperature
range under investigation (500-800
8
C), such a phenomenon was not
observed.
The number of conducting oxygen vacancies (Vo
..
) is reduced and
mobility is increased after interaction of the vacancies with Al
2
O
3
. The
proposed explanation is schematically illustrated in Fig. 15.7. The
conducting vacancies are represented by arrows and Al
2
O
3
particles are
depicted by solid circles. The oxygen vacancies that are immobilized by the
