10 New Na + Superionic Conductor Narpsio Glass-Ceramics
411
2.38 × 10 −2 S/cm to 7.15 × 10 −2 S/cm, respectively, upon implantation of 200 keV
Na + ions with a flux density of 10 15 ions/cm 2 .
10.5.4 Structure and Conduction Properties
of Na 5 YSi 4 O 12 -Type Glass-Ceramics Synthesized
by Bias Crystallization of Glass [27]
Glass-ceramics of the phosphorus-containing N5-type Na + superionic conductors were prepared by bias crystallization of glasses with the composition
Na 4.05 Y 0.55 P 0.3 Si 2.7 O 9 in an electric field. The conditions for bias crystallization
are discussed with respect to the microstructure and the conduction properties.
The precursor glasses were made by melting stoichiometric mixtures of reagentgrade powders of anhydrous Na 2 CO 3 , Y 2 O 3 , SiO 2 , and NH 4 H 2 PO 4 at 1350 ◦ C
for 1 h, followed by annealing for several hours at an optimum temperature.
The annealed specimens were heated to 900 ◦ C in an electric field for the bias
crystallization. The thermostable heating holder was produced in order to do the
crystallization in a direct current electric field. This holder is made of alumina and
platinum. Glass samples (5 mm × 5 mm × 8 mm) were held between the platinum
plates and crystallized in an electrical field of 1 V/mm. The thermal treatment was
the same as that used in conventional crystallization without the electric field.
The microstructure was investigated with SEM. The grain length of the cross
section parallel with the electric field direction was 10–15 nm, and it was proven
to be smaller than the 15–30 nm grain length of the cross section perpendicular
to the direction and the specimen crystallized by the conventional method. It was
possible to control shape and orientation of crystal grain by the crystallization in the
electrical field.
Owing to the bias field, an electric current related to temperature was measured
during the crystallization process. Figure 10.23 shows a current profile with
respect to temperature during the crystallization process in the electric field. The
largest observed current was 250 μA. The current profile exhibits three peaks at
approximately 600 ◦ C, 700 ◦ C, and 850 ◦ C. These temperatures correspond to those
of nucleation, phase transition from N3 phase to N5 phase, and crystallization of
glass specimens determined by DTA analysis, respectively. An electric current with
respect to temperature was measured newly by applying the bias voltage only in two
limited temperature ranges, because two main peaks were observed in Fig. 10.23.
One range is from right before of the first main peak (511–652 ◦ C), and another
range is from right before of the second main peak (790–865 ◦ C). The resulting
current profile is shown in Fig. 10.24. It was found that the mass transfer in the
specimen is generated even in the condition of no applied voltage.
Crystalline phases were identified on the sample after the crystallization in the
electric field by XRD in order to consider the possibility of structural changes by the
movement of Na + ion, which is a carrier. In the several cut sections, no difference in
411
2.38 × 10 −2 S/cm to 7.15 × 10 −2 S/cm, respectively, upon implantation of 200 keV
Na + ions with a flux density of 10 15 ions/cm 2 .
10.5.4 Structure and Conduction Properties
of Na 5 YSi 4 O 12 -Type Glass-Ceramics Synthesized
by Bias Crystallization of Glass [27]
Glass-ceramics of the phosphorus-containing N5-type Na + superionic conductors were prepared by bias crystallization of glasses with the composition
Na 4.05 Y 0.55 P 0.3 Si 2.7 O 9 in an electric field. The conditions for bias crystallization
are discussed with respect to the microstructure and the conduction properties.
The precursor glasses were made by melting stoichiometric mixtures of reagentgrade powders of anhydrous Na 2 CO 3 , Y 2 O 3 , SiO 2 , and NH 4 H 2 PO 4 at 1350 ◦ C
for 1 h, followed by annealing for several hours at an optimum temperature.
The annealed specimens were heated to 900 ◦ C in an electric field for the bias
crystallization. The thermostable heating holder was produced in order to do the
crystallization in a direct current electric field. This holder is made of alumina and
platinum. Glass samples (5 mm × 5 mm × 8 mm) were held between the platinum
plates and crystallized in an electrical field of 1 V/mm. The thermal treatment was
the same as that used in conventional crystallization without the electric field.
The microstructure was investigated with SEM. The grain length of the cross
section parallel with the electric field direction was 10–15 nm, and it was proven
to be smaller than the 15–30 nm grain length of the cross section perpendicular
to the direction and the specimen crystallized by the conventional method. It was
possible to control shape and orientation of crystal grain by the crystallization in the
electrical field.
Owing to the bias field, an electric current related to temperature was measured
during the crystallization process. Figure 10.23 shows a current profile with
respect to temperature during the crystallization process in the electric field. The
largest observed current was 250 μA. The current profile exhibits three peaks at
approximately 600 ◦ C, 700 ◦ C, and 850 ◦ C. These temperatures correspond to those
of nucleation, phase transition from N3 phase to N5 phase, and crystallization of
glass specimens determined by DTA analysis, respectively. An electric current with
respect to temperature was measured newly by applying the bias voltage only in two
limited temperature ranges, because two main peaks were observed in Fig. 10.23.
One range is from right before of the first main peak (511–652 ◦ C), and another
range is from right before of the second main peak (790–865 ◦ C). The resulting
current profile is shown in Fig. 10.24. It was found that the mass transfer in the
specimen is generated even in the condition of no applied voltage.
Crystalline phases were identified on the sample after the crystallization in the
electric field by XRD in order to consider the possibility of structural changes by the
movement of Na + ion, which is a carrier. In the several cut sections, no difference in