NASIGLAS structure and properties

Journal of Non-Crystalline Solids 293±295 (2001) 709±714

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NASIGLAS structure and properties A. Niyompan *, D. Holland Physics Department, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK

Abstract ZrO2 -de®cient compositions of NASICON …Na1‡x Zr2 x=3 Six P3 x O12 2x=3 where 0 6 x 6 3) have been prepared in glassy form (sodium superionic conducting glass (NASIGLAS)) using the melt quench method. The corresponding glass-ceramics have also been produced by heat treatment at appropriate temperatures identi®ed from DTA. From 29 Si MAS NMR, it is found that [SiO4 ] tetrahedra are predominantly Q2 species for composition x ˆ 3:0, but are mixed Q2 ±Q3 for x < 3. Crystalline Na2 ZrSi2 O7 is found in all the corresponding glass-ceramic samples. The complex plane impedance method was used to determine the conductivity of NASIGLAS, which appears to be a complex function of mobility and concentration of Na‡ . 23 Na NMR indicates anomalous motion when x ˆ 2:5. Low solubility of ZrO2 is noticeable in glasses with low value of x. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction In 1981, Von Alpen and co-workers [1] reported ZrO2 -de®cient compositions …Na1‡x Zr2 x=3 Six P3 x O12 2x=3 ± NASICON) and high ionic conduction was obtained in the range 1:6 6 x 6 3:0. The amorphous forms of these ZrO2 -de®cient compositions were prepared ®rst by Susman et al. in 1983 [2] and generically termed NASIGLAS (sodium superionic conducting glass). It was proposed as an electrolyte material for battery technology since, in common with the related crystalline phase, NASIGLAS exhibited fast-ion conduction. High ionic conduction was also found in NASIGLAS compositions produced using the sol±gel technique [3,4]. Clearly, preparing superionic material in amorphous form may have some advantages such as ease of fabrication, high volume production, good mechanical

strength and low cost. Considering these advantages and its high ionic conduction, NASIGLAS is a promising electrolyte material for a variety of electrochemical devices. In this work, we have prepared NASIGLAS samples using conventional melt quenching. Four compositions were selected from the Na2 O± ZrO2 ±SiO2 ±P2 O5 glass forming region, with x ˆ 2:25, 2.50, 2.75 and 3.00. To establish greater understanding of NASIGLAS material, which could contribute to the improvement of properties, information on structure has been derived from nuclear magnetic resonance (NMR) and related to ionic conductivity.

2. Experimental 2.1. Glass preparation

*

Corresponding author. Tel.: +44-24 7652 3415; fax: +44-24 7669 2016. E-mail address: [email protected] (A. Niyompan).

The glass samples were prepared from 50 g batches containing the appropriate amounts of starting materials NaCO3 , ZrO2 , SiO2 and Na2 HPO4 ,

0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 7 8 1 - 5

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A. Niyompan, D. Holland / Journal of Non-Crystalline Solids 293±295 (2001) 709±714

accurately weighed and mixed to provide the stoichiometries Na1‡x Zr2 x=3 Six P3 x O12 2x=3 (x ˆ 2:25, 2.50, 2.75, 3.00). These glass batches were melted in a Pt/10% Rh crucible in air using two melting steps; 1600 °C for 2 h, followed by 1650 °C for 40 min. The melts were quenched in cold water, dried for several hours and glass frit produced. After remelting, two sample shapes, sheet and rod, were formed by casting onto a steel plate or quenching into a steel mould, respectively. The glasses were annealed at the appropriate temperature identi®ed from di€erential thermal analysis (DTA).

polished to a 6 lm ®nish and electrodes formed by sputtered Pt. The conductivity of the glass disks (thickness about 3 mm) was analysed from 1 Hz to 1 MHz at temperatures from 100 °C to about 300 °C. A Solartron impedance analyser was used to collect the impedance data and the complex plane impedance method was used to determine the conductivity of the glass samples [5].

2.2. Glass characterisation

XRD of the glass samples con®rmed the absence of any crystalline phase, within the detection limit of the equipment, for x ˆ 3:00 and 2.75. A few, very small crystal peaks were seen for x ˆ 2:50 and 2.25. Glass compositions, density, Tg , Tc and details of crystalline phases formed in the glass-ceramic samples, are summarised in Table 1. It can be seen that density increases and Tg decreases with increasing x. Figs. 1 and 2(a) and (b) show the MAS NMR spectra of 29 Si; 31 P and 23 Na, respectively. The results of Gaussian ®tting of the 29 Si and 31 P spectra are included in Table 2. The Arrhenius plots of the dc conductivities of the NASIGLAS samples are shown in Fig. 3. The formula r ˆ r0 exp… Ea =RT † was used to obtain values of Ea , the transport activation energy of Na‡ . The preexponential term r0 is assumed to be non-temperature dependent. Conductivities of NASIGLAS at around 300 °C and activation energies are reported in Table 3.

The densities (q) of the glass samples were determined using Archimedes' principle with distilled water as the displacement ¯uid. The glass transition temperatures (Tg ) and crystallisation temperatures (Tc ) were determined by DTA using a heating rate of 5 K/min. Using the observed Tc , the corresponding glass-ceramics were produced by heat treatment for 4 h at temperature. X-ray di€raction (XRD-CuKa1 ) was used to con®rm the amorphicity of glass samples and to identify crystalline phases formed in the glass-ceramic samples. 29 Si MAS NMR was carried out at 7.05 T (Bruker, MSL300) and 29 Si resonance frequency …m0 † ˆ 59:70 MHz. The single pulse experiment used a 20 ls pulse width and 30 s time delay for p=2 pulses and a spinning speed of 3.7 kHz was applied. The chemical shifts …d29 Si† were referenced to tetramethylsilane (TMS). The 23 Na spectra were acquired at 8.45 T (Bruker, MSL360) and 23 Na resonance frequency 95.26 MHz. The sample spinning rate was 10 kHz and a p=2 pulse width of 2 ls and repetition time of 1 s were employed. Chemical shifts …d23 Na† were referenced to NaCl solution. 31 P spectra were acquired at 8.45 T (m0 ˆ 145:78 MHz). A pulse width of 2 ls and repetition time of 10 s were employed and (d31 P) was referenced to NH4 H2 PO4 (d31 P  0:4±0:9 ppm with respect to H3 PO4 ). 2.3. Electrical conductivity measurement Ionic conductivity of NASIGLAS was measured on disks cut from rod. Both surfaces were

3. Results

4. Discussion 4.1. Glass samples and corresponding glass-ceramics On quenching the glass melt, transparent clear glasses were obtained but trace amounts of crystalline ZrO2 were present for x ˆ 2:50 and 2.25. The observation of ZrO2 indicates that the solubility limit of this component is being reached. Densities decrease as SiO2 content decreases. This is despite the fact that SiO2 is being replaced by heavier ZrO2 and P2 O5 , i.e., the increased mass of

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Table 1 Composition, density (q), glass transition temperature (Tg ) and crystallisation temperature (Tc ) of NASIGLAS and the crystalline phases formed in the corresponding glass-ceramic samples (P ˆ Na2 ZrSi2 O7 , NC ˆ Na4 ZrSi3 O10 , NS ˆ Na2 SiO3 , Un ˆ unidenti®ed) x

Sample

Na2 O

ZrO2

SiO2

P2 O5

q …g=cm 1 † (0.005)

Composition (mol%)

Tg (°C) (3)

Tc (°C) (3)

Phase formed in glass-ceramics

786 841 827 851 913 910

P + NS P + NC + NS P P + Un P + Un P + NC

3.00

G1

33.33

16.67

50.00

±

2.9498

675

2.75

G2

32.10

18.54

47.21

2.15

2.9107

755

2.50 2.25

G3 G4

30.92 29.54

20.49 22.73

44.17 40.91

4.42 6.82

2.9036 2.8902

818 857

1293) was found to form in all samples although Tc for this phase increased with ZrO2 content. In addition, Na4 ZrSi3 O10 also formed from G1 and G4. At high SiO2 content (G1 and G2), Na2 SiO3 was also found. 4.2.

Fig. 1. 29 Si MAS spectra of NASIGLAS acquired at 7.05 T and sample spinning rate ˆ 3.7 kHz. A small chemical shift range has been used to emphasise the di€erences in the spectra and the spinning sidebands are therefore not shown.

the components is o€set by an increasing number of network units. From XRD of heat-treated samples, crystalline Na2 ZrSi3 O7 (PDF No. 29-

29

Si,

31

P and

23

Na spectra

The results of peak ®tting the 29 Si spectra are summarised in Table 2. By referring to the shift ranges observed in binary alkali±silicate glass systems [6], it can be deduced that the [SiO4 ] tetrahedral unit is predominantly Q2 in sample G1 but mixed Q2 ±Q3 in other samples. The intensity ratio for Q3 =Q2 for G1, G2, G3 and G4 was observed to be 0, 0.16, 0.60 and 1.55, respectively. Additionally, line widths of 29 Si spectra increase slightly when x decreases. In crystalline NASICON, the line width has maximum value at x ˆ 1:5 [7]. 31 P resonances with chemical shifts around 15, 6 and 0 ppm represent the contributions from P0 , P1 , and P2 , respectively. These are summarised in Table 2. At low mol% of P2 O5 , most [PO4 ] tetrahedra are orthophosphate (P0 ) species but when more [PO4 ] is added, pyrophosphate …P1 † and metaphosphate P2 increase in concentration [8]. Therefore, as the concentration of P2 O5 is increased, there is increasing polymerisation of the phosphate units P0 ! P1 ! P2 . This means that less Na‡ is being removed from the silicate network to associate with phosphate units. Fig. 3 shows the 23 Na MAS spectra of NASIGLAS. The spectra of G1, G2 and G4 are very

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A. Niyompan, D. Holland / Journal of Non-Crystalline Solids 293±295 (2001) 709±714

Fig. 2. (a) 31 P MAS, (b) 23 Na MAS spectra of NASIGLAS, acquired at 8.45 T and sample spinning rate ˆ 10 kHz. Note that the 31 P chemical shift is relative to NH4 H2 PO4 (d31 P  0:4±0.9 ppm w.r.t. H3 PO4 ). Table 2 Results from Gaussian ®tting of 29 Si, 31 P; d is chemical shift, W is line width (FWHM), Q2 and Q3 are the [SiO4 ] units, P0 , P1 and P3 are the [PO4 ] units, PPNa and WNa are the peak position and the line width of 23 Na spectra, respectively Sample

G1 G2 G3 G4

Q2

Q3

P0

P1

P2

PPNa

WNa

(ppm) (0.5)

(ppm) (0.5)

d (ppm) (0.5)

W (ppm) (0.5)

d (ppm) (0.5)

W (ppm) (0.5)

d (ppm) (0.5)

d (ppm) (0.5)

d (ppm) (0.5)

)83.4 )84.7 )86.1 )87.0

12.0 11.4 10.7 9.7

± )87.3 )88.8 )90.6

± 13.5 13.6 13.9

± 15.3 14.0 14.5

± 7.6 6.8 6.9

± 0.3 0.5 )0.1

similar, but the line width of the 23 Na spectrum from G3 is twice that of the others and the chemical shift is about )5 ppm, very di€erent from )19 to )16 ppm observed for the other three samples. This spectrum is reproducible for di€erent samples of G3 from both the same and different melt preparations. The Lorentzian lineshape of the 23 Na resonance from this particular sample indicates that the width is dominated by T2 (the spin±spin relaxation time, which is inversely proportional to spectrum linewidth) rather than dipolar interactions [11]. The implication of this is

)16.4 )19.1 )5.8 )16.5

43.1 40.6 88.4 35.9

that the Na‡ ions are undergoing very di€erent motion in sample G3 than in the other compositions. The environment of Zr atoms in the glass framework was not investigated in this study but has been reported to take the form of ZrO6 octahedral units [10]. 4.3. Ionic conductivity Conductivity is a combination of concentration of mobile species and mobility of these

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713

5. Conclusion

Fig. 3. Arrhenius plots of NASIGLAS samples, the conductivities (r) were determined from complex plane impedance and the errors of log r are approximately 1%. The value of log r for crystalline Na4 ZrSi3 O10 is included for comparison.

The glassy form of ZrO2 -de®cient NASIGLAS has been successfully prepared using convention melt-quench. The glasses obtained have Tg values in the range 675±857 °C and Tc in the range 786± 910 °C. All corresponding glass-ceramic samples, prepared by heat treatment, contain crystalline Na2 ZrSi2 O7 as a major phase. The short-range structure of NASIGLAS is controlled by silicate and phosphate networks combined with intermediate ZrO2 and modi®er Na2 O. Anomalous Na‡ motion is detected when x ˆ 2:5. In further work, more investigations will be carried out on G3 and some adjacent compositions and high temperature NMR will be employed to study Na‡ motion in all samples. Acknowledgements

Table 3 Ionic conductivity of Na‡ in NASIGLAS and associated activation energies (Ea ) Sample

Conductivity (r) at indicated temperature …10 5 S=cm†

Ea (kJ/mol)

G1 G2 G3 G4

41.6  0.1 1.3  0.4 5.1  0.2 3.0  0.1

43  2:6 52  3:4 79  3:1 50  2:8

(300 (300 (300 (310

°C) °C) °C) °C)

species. The concentration of Na‡ increases sharply with x but the activation energy for conduction goes through a maximum at G3. This is surprising in view of the 23 Na NMR result which implies greater Na‡ motion but it must be remembered that NMR only measures local motion. However, NASIGLAS samples have shown high conduction behaviour and the conductivity of G1 at 300 °C is just one order of magnitude lower than that of crystalline Na4 ZrSi3 O10 (i.e., 4  10 3 S/cm at 300 °C) reported by Von Alpen [9]. The lower average coordination number of Na‡ in the glassy phase compared to that in the corresponding crystalline phases of this composition may be causing the lower conductivity [10].

The authors would like to thank Dr M.E. Smith and Dr I.A. Poplett for assistance with NMR experiments and Professor M.D. Ingram and colleagues at University of Aberdeen (UK) who provided both conductivity measurement facility and valuable advice. EPSRC are thanked for the provision of the NMR facilities at Warwick. References [1] U. Von Alpen, M.F. Bell, H.H. H ofer, Solid State Ionics 3&4 (1981) 215. [2] S. Susman, C.J. Delbecq, J.A. McMillan, M.F. Roche, Solid State Ionics 9&10 (1983) 667. [3] J.P. Boilot, Ph. Colomban, J. Mater. Sci. Lett. 4 (1985) 22. [4] J.P. Boilot, Ph. Colomban, Solid State Ionics 18&19 (1986) 974. [5] K.C. Sabha, K.J. Rao, Solid State Ionics 81 (1995) 145. [6] R. Dupree, D. Holland, P.W. Mcmillan, R.F. Pettifer, J. Non-Cryst. Solids 68 (1984) 399. [7] C. Jager, G. Scheler, U. Sternberg, S. Barth, A. Feltz, Chem. Phys. Lett. 147 (1) (1988) 49. [8] R. Dupree, in: G.A. Maciel (Ed.), NMR Studies of Glasses and Ceramics: NMR in Modern Technology, Kluwer Academic, Netherlands, 1994. [9] U. Von Alpen, M.F. Bell, H.H. H ofer, Solid State Ionics 7 (1982) 345.

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[10] G. Ennas, A. Musinu, G. Piccaluga, G. Pinna, M. Magini, Chem. Phys. Lett. 141 (1,2) (1987) 143.

[11] M. Forsyth, M.E. Smith, P. Meakin, D.R. MacFarlane, J. Polym. Sci. B 32 (1994) 2077.