398
T. Okura and K. Yamashita
Na 5 RSi 4 O 12 [4]: σ increased with increasing r R . Previous works have proposed a
mechanism whereby rare earth ions, octahedrally coordinated with the non-bridging
oxide ions of the 12-membered rings of silica tetrahedra, work to expand the
conduction paths for Na + ions along the c-axis [4, 33], which could explain the
observed dependence of E a on r R in this work.
10.4.3 Structure and Conduction Properties of Grain
Boundaries
As R GB decreases rapidly with increasing temperature because of high (E a ) GB to
a comparable value with R G at 300 ◦ C (Fig. 10.10), the total conductivities (R G +
R GB ) are dominated by grain boundary conductivity. The grain size-dependence of
σ 300 is therefore explained by the decrease in the number of poorly conductive grain
boundaries with increasing grain size.
The conduction properties of grain boundaries were strongly dependent on
the annealing conditions, although those of the grains were little changed by the
annealing temperature and time. Glass-ceramics are generally composites consisting
of crystallized grains and small amounts of residual glass (< 1%) [23]. To compare
the properties of grain boundaries with those of glasses, the conduction properties of
sodium-yttrium silicophosphate glasses with various compositions were measured.
Unlike glass-ceramics, the impedance loci of glasses were comprised of one arc,
which indicates that there is no polarization arising from microstructural inhomogeneity. Based on the intercepting points on the horizontal axis, the composition
dependence of the conduction properties of σ 300 and E a was evaluated. The value
of σ 300 ranged from 1 × 10 −4 to 5 × 10 −3 S/cm, and E a increased from 53 to
67 kJ/mol with [Na] or [Na]/[Y]. These results are also in good agreement with
those reported for the glasses in the Na 2 O-Y 2 O 3 -SiO 2 system [36]. The values of
(E a ) GB of the specimens annealed below 950 ◦ C for shorter times correspond to
those in the range of glasses, strongly suggesting that their grain boundaries are a
glassy matrix. The abovementioned dependence of (E a ) GB on sodium oxide content
[Na 2 O] is explained by the well-known tendency that the conduction properties of
glasses are improved by increasing [Na 2 O], which provides the increase in carrier
Na + ions. The ratio of [Na]/[Y] is also an important parameter for the conduction
properties [36], showing an effect on the conduction properties similar to [Na 2 O].
In order to identify the structure of the grain boundaries of the specimen
(Na 3.9 Y 0.6 P 0.3 Si 2.7 O 9 ) annealed at 800 ◦ C for 0.5 h, TEM analysis was performed
both on grains and grain boundaries. The results in Fig. 10.11 show clear electron
diffraction on grains, but not on grain boundaries. This confirms that the grain
boundaries are amorphous. Compositional analyses were also performed; however,
[Na] was difficult to determine because of the evaporation by electron ablation. It
was also observed that the glassy phase was condensed at triple points enclosed by
grains and that the neck growth among the grains was well developed. Thus, it is
T. Okura and K. Yamashita
Na 5 RSi 4 O 12 [4]: σ increased with increasing r R . Previous works have proposed a
mechanism whereby rare earth ions, octahedrally coordinated with the non-bridging
oxide ions of the 12-membered rings of silica tetrahedra, work to expand the
conduction paths for Na + ions along the c-axis [4, 33], which could explain the
observed dependence of E a on r R in this work.
10.4.3 Structure and Conduction Properties of Grain
Boundaries
As R GB decreases rapidly with increasing temperature because of high (E a ) GB to
a comparable value with R G at 300 ◦ C (Fig. 10.10), the total conductivities (R G +
R GB ) are dominated by grain boundary conductivity. The grain size-dependence of
σ 300 is therefore explained by the decrease in the number of poorly conductive grain
boundaries with increasing grain size.
The conduction properties of grain boundaries were strongly dependent on
the annealing conditions, although those of the grains were little changed by the
annealing temperature and time. Glass-ceramics are generally composites consisting
of crystallized grains and small amounts of residual glass (< 1%) [23]. To compare
the properties of grain boundaries with those of glasses, the conduction properties of
sodium-yttrium silicophosphate glasses with various compositions were measured.
Unlike glass-ceramics, the impedance loci of glasses were comprised of one arc,
which indicates that there is no polarization arising from microstructural inhomogeneity. Based on the intercepting points on the horizontal axis, the composition
dependence of the conduction properties of σ 300 and E a was evaluated. The value
of σ 300 ranged from 1 × 10 −4 to 5 × 10 −3 S/cm, and E a increased from 53 to
67 kJ/mol with [Na] or [Na]/[Y]. These results are also in good agreement with
those reported for the glasses in the Na 2 O-Y 2 O 3 -SiO 2 system [36]. The values of
(E a ) GB of the specimens annealed below 950 ◦ C for shorter times correspond to
those in the range of glasses, strongly suggesting that their grain boundaries are a
glassy matrix. The abovementioned dependence of (E a ) GB on sodium oxide content
[Na 2 O] is explained by the well-known tendency that the conduction properties of
glasses are improved by increasing [Na 2 O], which provides the increase in carrier
Na + ions. The ratio of [Na]/[Y] is also an important parameter for the conduction
properties [36], showing an effect on the conduction properties similar to [Na 2 O].
In order to identify the structure of the grain boundaries of the specimen
(Na 3.9 Y 0.6 P 0.3 Si 2.7 O 9 ) annealed at 800 ◦ C for 0.5 h, TEM analysis was performed
both on grains and grain boundaries. The results in Fig. 10.11 show clear electron
diffraction on grains, but not on grain boundaries. This confirms that the grain
boundaries are amorphous. Compositional analyses were also performed; however,
[Na] was difficult to determine because of the evaporation by electron ablation. It
was also observed that the glassy phase was condensed at triple points enclosed by
grains and that the neck growth among the grains was well developed. Thus, it is