Thermodynamic and kinetic study on the phase transformation of the glass-ceramic Na+ superionic conductors Na3+3x−yRe1−xPySi3−yO9

SOLID -STATE IOWICS

Solid State Ionics 58 (1992) 231-236 North-Holland

Thermodynamic and kinetic study on the phase transformation of the glass-ceramic Na + superionic conductors Na 3+sx-,Rel-,P,Si3-,09* Kimihiro

Yamashita,

Masashi Tanaka and Takao Umegaki

Department oflndustrial Chemistry, Faculty of Technology, Tokyo Metropolitan University, Minami Osawa l-1. Hachioji, Tokyo 192-03, Japan

Received 20 October 199 1; accepted for publication 3 1 August 1992

Based on the composition formula, Nas+,_,Re,_xP,Sis_-y09 (R e = rare-earth element), we have successfully developed Na+superionic conducting glass-ceramics (named Narpsio-V) with the structure analogous to Na5YSi40i2. In the crystallization of Narpsio-V, its relatives (named Narpsio-III and -1X) structurally belonging to the family of Naz4-sxYr Si,zO36 were found to crystallize as the precursor phase at low temperatures. In order to produce single phase Narpsio-V glass-ceramics, the concentration of both phosphorus and rare-earth was found important. The meaning of the composition was evaluated by thermodynamic and kinetic studies on the crystallization of each Narpsio and phase transformation of metastable Narpsio-III or -1X to stable phase Narpsio-V.

1. Introduction We have found a new family of Na+-superionic conductors with the composition formula, Na 3+3x_-yRel_-xPySi3_-y09(Re = rare-earth elements, mainly Y, Gd, and Sm) [ 1,2]. Their conductivities and activation energies are of the order of 0.1 Scm-’ at 300°C and of 22 to 28 kJ.mol-‘, respectively. On the basis of these materials, we have also developed glass-ceramic Na+-superionic conductors [ 3-5 ] with an aim of overcoming the difficulty in the fabrication of their various shapes. It has also been the reason for the development that a glass-ceramic Na+superionic conductor with fine-grained microstructure can be expected to have the advantages over the conventional ceramic Na+-conductors such as Na3Zr2PSi20i2, j3- and p”-aluminas in chemical durability and mechanical strength. In the course of the fundamental studies on glassceramic Na 3+Jx_-yRel_-xPySi3_-y09, we have interestingly found the crystallization of those NaJYSi909and Na9YSi60,,-type phases as the precursors in the * Paper presented at SSI-8, Lake Louise, Canada, October 2026, 1991.

glasses [ 5 1. These are the analogues to the silicates Na3YSi309 and Na9YSieOls [ 1,2], and therefore are the same members of the family of Na24-3xYxSi12036 [6] as Na5YSi40i2. Although we had also successfully synthesized those materials by the solid-state reactions of powders with the above composition of various sets of the parameters x and y [ 1,2], the metastability of those precursor phases had not been noticed in the synthesis. It has been observed that such precursor phases were transformed to the Na+superionic conducting phase on specimens with appropriate sets of x and y. The present paper will deal with the thermodynamic and kinetic study on the phase transformation of metastable phases to the stable phase with Na+ superionic conductivity. The superiority of our present materials to the other silicate NaSYSi40i2 will also be detailed based on the kinetic results. For convenience, we named our silicophosphates “Narpsio” materials after their composition, and the Narpsio compounds corresponding to Na3YSi309, NasYSi,O,, and NagYSi,O,ll will designate as Narpsio-III, Narpsio-V, and Narpsio-IX, respectively. The terms “Narpsio-J (J=III, V, IX)” will also be used for the description of the corresponding phases and

0167-2738/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

232

structures. A glass Na 3+3x-yRel_xPySi3_y09 “Narpsio glass”.

K. Yamashita et al./Glass-ceramic Na+ superionic conductors

with will

the composition be referred to as

2. Experimental A variety of Narpsio glasses were made by melting the mixed powders of the reagent-grade anhydrous Na2C03, Re203, SiOl, and NH4H2P04, at 1200 to 1350” C. The contents of the oxides of sodium, rareearth, silicon, and phosphorus were as follows; 33.7 mol%s [Na,O] $42.2 mol%, 3.8 mol%s [Rez03] 58.2 mol%, and 53.8 mol%s [SiOZ]+ [P,O,] 558.2 mol%, or 01xsO.6 and OsysO.45 in terms of composition parameter. The precursor phases were determined on various kinds of glass specimens annealed at the temperature of 700 to 750°C at which a specimen was evaluated to be fully crystallized by XRD and DTA. Phase analysis was carried out on bulk and crushed powder glass-ceramic Narpsio by X-ray diffraction technique. Glass-ceramics of Narpsio-V were made by annealing of Narpsio glass specimens at 900 to 1100°C for various times. The stable phases were determined by XRD on specimens annealed at 1000°C for 36 h. The transformation rate (a,) of a precursor phase to the stable Narpsio-V phase was determined as the weight ratio of Narpsio-V in a glass-ceramic specimen. The value of (Y, was experimentally obtained from the relationship of weight ratio to XRD intensity ratio, which relationship had been made previously by XRD intensity measurement on specimens with given weight ratio of Narpsio-V to metastable phases.

3. Results and discussion Fig. 1 shows the composition dependence of both the precursor phases and the high-temperature stable phases of glass-ceramic Narpsio on the maps of phosphorus-yttrium (P-Y, fig. 1a), yttrium-sodium (Y-Na, fig. lb), and phosphorus-sodium (PNa, fig. lc), where the variables on the abscissas and ordinals are expressed with the composition parameters 1 -x, y, and 3 + 3x-y for yttrium, phosphorus and sodium, respectively. As reported before [ 2,5 1,

a3

,

0

0i

,

, 0.2 a3

, I 0.4 a5

Aa) 0.6

P Fig. 1. Composition dependence of precursor (pp) and high temperature-stable phases (sp) of glass-ceramic Narpsio on P-Y (a), Y-Na (b) and P-Na (c) maps, where precursor phases NarpsioIII and Narpsio-IX are shown with triangles and squares, respectively. High temperature-stable phases are shown in such a way that solid marks means that Narpsio-V is the stable, while open marks indicate that the precursor phases are also stable even at high temperatures. Mixed phases are also shown: (A ) pp=sp=III; (A) pp=III,sp=v; (0) pp=sp=IX; (W) pp=IX, Sp=v;(e)pp=Sp=v;

(A)

pp=III,SP=III+V;(m)PP=IX,

sp=IX+V.

Narpsio-III and Narpsio-IX can be crystallized as the high-temperature stable phases at the regions of higher [Y] (1 -xhca. 0.8) and rather lower [Y] (l-xsca. 0.55), respectively, in the [Y]-[P] relation. Concerning the precursor phases, only either Narpsio-III or -1X was found in any composition, while Narpsio-V was difficult to crystallize from glasses at low temperatures. It is also seen in the [P ] [Y] map (fig. la) that, under a given phosphorus content ( [P] ~0.6) a composition with higher content of yttrium ( [Y ] ) gives Narpsio-III ( A ) as the precursor phase, while lower [Y ] content results in Narpsio-IX phase ( 0 ). The values of [Y ] dividing the regions allowed for Narpsio-III and Narpsio-IX decreased with increasing [P 1, and the boundary

233

K. Yamashita et al./Glass-ceramic Na+ superionic conductors

seems to locate slightly apart from the deduced line of [Y] ~0.75-0.5 [P] [2] shown with the solid line. Around the boundary region Narpsio-V can be obtained as the stable phase at high temperatures (solid marks of (A) or (m)). In the [Y]-[Na] or [PI[ Na] relations (figs. lb and lc), the region where Narpsio-V can be found as the high-temperature stable phase is found under ca. 3.6~ [Na] ~4.3. The effect of sodium content seems insignificant, because the value of [ Na] is subordinately determined as [Na]=6-3[Y]-[P] (=3+3x-y) dependingon the contents of both yttrium and phosphorus. The above results may suggest that the [PI- [Y] relation dominates the region which is allowed for each Narpsio at high temperatures. Considering this inference, we calculated the products of [P] x [Y] for all of the specimens. The values of [P] x [Y] were as follows (shown in fig. 2); 0.15-0.24 for single phase Narpsio-III, 0.14 for mixed phases of NarpsioIII and Narpsio-V, 0.12-0.20 for single phase Narpsio-V, O-O. 15 for the mixed phases of Narpsio-V and Narpsio-IX, and O-O. 18 for single phase Narpsio-IX, respectively. It was therefore deduced (fig. 2) that the free energy of formation (AG,) of Narpsio-IX would be the lowest in a lower region of [P ] x [Y 1, Narpsio-V may have the lowest AGf in a medium [P] X [Y ] region, and higher [P ] x [Y ] would lower AGr of Narpsio-III. Although the detailed explanation for the inference of fig. 2 cannot be fully given at present, a struc-

tural consideration may give a reason for it. It is assumed at present that the skeleton structures of Narpsio-III and -1X would consist of 6- ( Si04, Pod)tetrahedron-membered rings, while Narpsio-V has 12-tetrahedron-membered skeleton structure [ 1,2]. Considering the case of the composition Na,,,,+,,Y, _-xPo,25Si2,7509, in which 1 of 12 SiO, tetrahedra are replaced by P04, Narpsio-III or -1X would consist of two kinds of rings such as 6-SiO,tetrahedron-membered rings and ( 5-Si04, l-Pod)tetrahedron-membered rings (fig. 3a). The skeleton of Narpsio-V, on the other hand, might consist of only one kind of ( 11-SiO,-tetrahedron, I-PO,)-tetrahedron-membered rings (fig. 3b). The inhomogeneous former structure would be thought more unstable in comparison with the latter. Presently, we assume that the effect of the substitution of Si with P should be to bring about the difference of homogeneity in the ring structure between Narpsio-V and -111 or -1X. The effect of composition on the temperature at which Narpsio-V is transformed from the precursor Narpsio-III or -1X may be recognized in the phasetemperature maps (fig. 4a-d) illustrated for the 8 specimens, in which Narpsio-V was the high temperature-stable phase (see fig. 1). The compositions of those specimens are summarized in table 1. Specimens a-2, b-l and -2 demonstrated the examples of the phase transformation at lower temperature. Thus the phase transformation of metastable Narpsio-III of -1X to Narpsio-V takes place at the temperature which strongly depends on both [P] and [Y 1. The characteristics of such phase transformation can be explained by considering the free energy change (AG)

b 0

0.05

0.1

0.15

0.2

0.25

YXP Fig. 2. Schematic figure of composition ( [Y] dence of free energy of Narpsio-V, -III and -1X.

X

[P] ) depen-

Fig. 3. Schematic pictures of 6-tetrahedron-membered rings of Narpsio-III or IX (a) and 12-tetrahedron membered rings of Narpsio-V (b ) , where open and solid circles indicate silicon and phosphorus, respectively.

K. Yamashita et al./Glass-ceramic Na+ superionic conductors

234

a-2

a-l

c-1

a-3

c-2

b-l

b-2

d

Fig. 4. (a)-(d). Phase change of glass-ceramic specimens with temperature, where the notation of specimens correspond to those of table 1. Phase notation: Narpsio-III: III, Narpsio-V: V, Narpsio-IX: IX.

Table 1 Composition of Narpsio-V glass-ceramic specimens. Specimen

Composition

a-l a-2 a-3 b-l b-2 c-l c-2 d

Na 3.70 Y 0.70 P 0.20Si2.80 09 Na 3.85 Y0.65P 0.20Si2.80 09 Na 4.WY 0.60 P 0.20SiZ.80 09 Na 3.90 Y 0.60 P 0.30 Si2.70 09 Na 4.05 Y 0.55 P 0.30Si2.70 09 Na 4.10 Y 0.50 P 0.40Si2.60 09 Na 4.25 Y 0.45 P 0.40Si2.60 09 Na 4.20 Y 0.45P 0.45Si2.55 09

which is assumed for the crystallization of each phase from a glass. For a specimen in which Narpsio-V is the stable phase at high temperatures, the aspect such as fig. 5a would be illustrated in that AG of NarpsioIII (or IX) would be much smaller than that of Narpsio-V near the crystallization temperature ( T,), and the value of Narpsio-V would be lowered much less than that of the other two. Fig. 5b indicates the aspect that AG or Narpsio-III (or IX) would be much smaller than that of Narpsio-V at any temperature. This case corresponds to the composition which makes Narpsio-III (or IX) stable. The kinetic effects of composition on the phase transformation is shown in fig. 6, which compares

235

K. Yamashita et al./Gla.w-ceramic Na+ superionic conductors

Fig. 5. Schematic figures of temperature dependence of free energy change of Narpsio-V and -III or -IX in the cases assuming Narpsio-V (a) and -111or -1X (b) as the high temperature-stable phase, where r, is the crystallization temperature.

0

0.5

1.0

1.5

2.0

Annealing time

2.5

3.0

h

Fig. 7. Phase transformation rate (a,) of Narpsio-III to NarpsioV on the specimen b-l (Na3.90Yo.soPag~Siz.7009).

Table 2 Kinetic parameters of phase-transformation

of Narpsio-III to

Narpsio-VofNa9.9oYo.60Po.~oSi2.7009.

0.

800

900

1100

1000

Annealing temperature

c

Fig. 6. Comparison of phase transformation rate (a,) between specimens Na3.90Y~.60Po.soSiz.,~O~ ( 1 h-annealing: (0 ); 3 h-annealing: (0 ) and Na9.,~Yo.ssPo.soSi2.,~O~( 1 h-annealing: (0 ); 3 h-annealing: ( n ).

the phase

transformation

rates

of specimens

b-l

(Na3.9Y0.6P0.3Si2.709) and Na3.75YO.65PO.3Si2.709.It is seen that the composition b-l is superior to the other, for the single phase Narpsio-V was difficult to obtain in the latter specimen. In specimen b-l a glassceramic of single phase Narpsio-V was easily obtained at a temperature higher than 900°C for only three hours. The composition Na3.75Y0,75Si309 (or Na5YSi40,2) was inferior in the same meaning. Fig. 7 shows the kinetic characteristics of phase transformation of the metastable phase of Narpsio-

Annealing temp. (K)

Avrami modulus n

In k

1073 1123 1173 1223

2.61 1.94 1.39 0.75

-20.7 - 14.6 - 9.54 - 4.41

III to Narpsio-V of specimen b-l at various temperatures. The transition rates, (Y,, of the silicophosphate Narpsio were much higher than those of the Na3.75Y0.7SSi309 silicate material. The results shown were analyzed with the Avrami empirical equation, (Y,= 1 -exp( - kt”), where k is the rate constant, and n is a constant. The data on CX,obtained at the initial and intermediate stages gave a linear relationship between ln( ln( 1 -a,) -’ ) and ln( t) with a correlation coefficient of more than 0.99. The Avrami parameter and rate constants obtained are summarized in table 2. Based on the Arrhenius relationship (fig. 8 ), k=A exp ( - EJRT) with E, as the activation energy and constants A and R, on those k values which increased with increasing temperature, we obtained an activation energy of 1.2x 1O3 kJ/mol, suggesting that the phase transformation can be rather difficult to take place. An addition of phosphorus and the excess sodium seem effective to the

236

K. Yamashita

et al./Glass-ceramic

Na+ superionic conductors

tent product as [P ] x [Re ] is important tallization of single phase Narpsio-V.

-5

in the crys-

Acknowledgements -10l\;;;

Y

c -15-

-20; 1180

,; 0.85

0.90

0.95 K-'

lO?i’ Fig. 8. Arrhenius-type

plot

of In k with

103/T

of specimen

This work was partly supported by Grant-in-Aid for Scientific Research (No. 03205103) from the Ministry of Education, Culture and Science of Japan. One of the authors (K.Y.) has performed his study under the Research Grant-in-Aids of the Special Research Grant of Tokyo Metropolitan University and Iketani Science Research Grant. K.Y. would also like to express his special thanks to the Nippon Sheet Glass Foundation for Research and Development of Materials Technology, and also SEINAN Auto Industry Ltd. for the financial support to his attendance at the SSI-8 Conference.

Na~.~~Y0.~0Si2.7009.

promotion

of the phase transformation.

4. Summary formula, meaning of composition The can be signified in the Na 3+3x_yRel_xPySi3_y09, thermodynamic and kinetic study of crystallization and phase transformation of metastable to stable phase in the production of glass-ceramic Narpsio-V. It was demonstrated that the medium value of con-

References [ 1] K. Yamashita, S. Ohkura, T. Umegaki and T. Kanazawa, J. Ceram. Sot. Japan. 96 (1988) 967. [2] K Yamashita, T. Nojiri, T. Umegaki and T. Kanazawa, Solid State Ionics 35 (1989) 299. [ 31 K. Yamashita, T. Nojiri, T. Umegaki and T. Kanazawa, Chem. Lett. (1988) 375. [4] T. Nojiri, K. Yamashita and T. Kanazawa, J. Ceram. Sot. Japan. 97 (1989) 1087 [in Japanese]. [ 51 K. Yamashita, T. Nojiri, T. Umegaki and T. Kanazawa, Solid State Ionics 40/41 (1990) 48. [6] F. Cervantes, L.J. Marr and F.P. Glasser, Ceram. Intern. 7 (1981) 43.