Nasicon, an ionic conductor for solid-state Na+-selective electrode

Sensors and Actuators,

15 (1988)

NASICON, AN IONIC CONDUCTOR Na+-SELECTIVE ELECTRODE P. FABRY,

33

33 - 49

FOR SOLID-STATE

J. P. GROS, J. F. MILLION-BRODAZ

and M. KLEITZ

Laboratoire d’lonique et d%lectrochimie du Solide (CNRS - UA 1213), ENSEEG, E.P. 75, 38402 Saint-Martin-d ‘Hikes Ckdex (France) (Received March 19,1987; inrevised form October 15,1987;accepted December 14,1987)

Abstract Polycrystalline NASICON, NasZr2SizP012, has been tested 88 a Na+ ionsensitive membrane. Solid-state miniaturized selective electrodes involving a solid internal reference compatible with IC technology were developed on this membrane. The response is Nernstian down to lOa - 10e4 M. Sensitivity is probably limited to this value by the solubility of traces of unreacted sodium compounds. Comparisons with existing commercial electrodes show that NASICON offers significant selectivity advantages with respect to K+, Ca2+, Li’ and especially H,O+,

Introduction A recent paper [l] stressed the interest of three-dimensional-feworked fast ion conductors as sensitive membranes for ion analysis. An experimental investigation of NASICON was undertaken to confirm this conclusion. NASICON, which was first synthesized by Hong [2], has the general formula Na ~+tZr2-,SixP3-x012 133. It was selected for the following reasons: (a) It is framed by a threedimensional network of covalently-bonded oxygen polyhedra and exhibits a rather high unipolar ionic conductivity at room temperature, of the order of 10M3 Jz-i cm-’ [4]. (b) Its Na+ conductivity should make it a Na+ ion-sensitive membrane. Sensing this ion is of interest in many fields, for instance in biomedical analysis. Furthermore, it can be compared with existing selective electrodes, providing an easy and recognized quality test. (c) The material is well characterized in solid-state battery literature as regards its structure and conductivity (see, for instance, refs. 5 and 6). (d) Dense NASICON does not absorb water. This has been experimentally demonstrated with the compound formula corresponding to x = 2, y = 0 and z = 2 [7 - 81. However, with a higher concentration of Si02 (x > 2.2), a vitreous phase at the grain boundaries has been found to be slightly soluble (review paper, ref. 9). 0250-6874/88:$3.50

0 Elsevier Sequoia/Printed in The Netherlands

34

(e) Its stability range in composition is rather broad and many similar compounds have been synthesized [5 - 61. This allows us to vary several essential parameters (ionic conductivity, concentration of mobile Na+ ions, dimensions of the conduction pathways etc.) [lo - 113 in order to optimize sensor selectivity. (f) Sol-gel techniques have been developed [12 - 14 ] that already permit thick-layer deposition and therefore an integration of the material in an ISFET configuration. The use of NASICON as a sensitive membrane in a specific electrode with a liquid internal reference has also been proposed by Engell and Mortensen [ 153. Sample preparation and experimental set-up Cell assent bly

In this initial attempt to confirm our analysis, we worked on macroscopic cells. We did not try to miniaturize them and to incorporate them into ISFETs, although the solid-state components were selected to be compatible with this application. The cell assembly is very simple (see Fig. 1). It is supported by an alumina substrate coated with a strip of Ag or Ag-Cd alloy. A silver wire is welded to this coating as an electric lead. The NASICON pellet is 2 mm in diameter and from 0.2 to 0.5 mm thick. It is simply cemented onto the alumina substrate by a special intermediate ionic conductor called the ionic bridge. Cementing is obtained by melting this bridge and cooling it rather rapidly to limit possible chemical ion exchange with the NASICON pellet. The whole cell is then encapsulated in an insulating resin (epoxy resin, TorrSeal* or Altufix*) leaving only a window over the NASICON membrane. NASICON

I

alumina

i

substrata

pellet AyCd

/

epoxy

Ag

alumirk

tube

Fig. 1. Cell diagram.

PrepaWion of the NASICON pellets The pellets were machined with diamond tools from bigger samples about 1 cm3 in size. Generally the NASZCON powders are prepared by : ball-milling (grinding and mixing) of the component powders [7,16 1; sol-gel processes [ 17,12 - 141, which yield homogeneous fine-grain powders; or hydrothermal crystallite growth [18].

35

We have chosen the first method, as it is the simplest and least expensive for eventual industrial production. The composition NajZraSizP012 was selected. It exhibits the highest conductivity of the silica-zirconia derivatives. Its specific mass is 3.27 g/cm3. Commercial powders were used: ZrOz (Merck 99.9%), Si02 (Merck Lab.) and Na3P04 (Prolabo Rectapur). They were dryed and then coground, pressed and sintered in air according to the following procedures: Samples A (a) Cogrinding in acetone for 15 min in a vibrating mortar (Prolabo). This type of grinding is rapid but yields irregular powders. (b) Pressing under 1 T/cm2 in a double-punch die. (c) Sintering at 1100 “c for 14 hours. The samples were homogeneous but the porosity was high (74% of theoretical density). Samples B (a) Same cogrinding. (b) Pre-sintering at 1150 “c for 24 hours. (c) Same grinding. (d) Pre-pressing followed by isostatic pressing under 2 T/cm*. (e) Sintering at 1240 “c for 22 hours. The density obtained is 84% of the theoretical value. The samples showed bubbles and small cracks. Samples C (a) Long milling by zirconia balls in alcohol, in jars lined with Vinatex@. (b) Drying and rapid calcination (a few minutes at 800 “c) to eliminate the traces of Vinatex. (c) Pressing as with samples B. (d) Sintering at 1200 “c for 24 hours under a layer of powdered sodium aluminate. The density was significantly higher (92% of theoretical), but bubbles and cracks were still observed. Shorter sintering times gave similar results. Samples D Same conditions as with samples C except that sintering was performed at a lower temperature (1000 “c) and over a much longer time (192 hours). Under the microscope the sample looked very homogeneous with neither bubbles nor cracks. The density was also very close to theoretical, but X-ray analysis showed that the formation of the compound was incomplete, with traces of unreacted powders. Samples E Same conditions as with samples C except the sintering: 1250 “c for 2 hours under a load of 0.1 T/cm2 in a zirconia die. The density was very close to the theoretical value.

36

Here the only unreacted species detected was Zr02, which was in fact observed in all the samples. The same observation was made by Quon et al. [ 171. This excess zirconia may come from the zirconia balls used for milling or, more likely, may result from a slight volatilization of sodium phosphate. Ionic bridge As described in ref. 1, the role of the ionic bridge is to provide a transition from the ionic conductivity of the sensitive membrane to another ionic conductivity compatible with the inner metallic conductor with which it will form an internal reference electrode. In this investigation we have selected materials that exhibit a cationic conductivity by both Na+ and Ag+ ions. In this way, the bridge ensures stable profiles for the electrostatic potential at the membrane-bridge contact and at the bridge-silver strip contact (cf. theoretical calculation). The system AgCl-NaCl is a typical candidate for this function. It exhibits a broad solid solution domain [ 191. It melts in a temperature range appropriate for the additiona cementing function and, like many silver ionic salts, it is sufficiently soft to be adequately sticky. Its main drawback is a rather low conductivity, of the order of 10V9a2-l cm-’ at room temperature. In this respect a mixture of AgI and NaI may be a better candidate because of the room-temperature conductivity of AgI of the order of 10m6 &X1 cm-‘. Unf o rtunately the relevant phase diagram is not known at room temperature and the extrapolation of the data does not predict any welldefined solid solution. These two materials were tested as mixtures containing a few per cent of the sodium salt. They were quenched from the melt. To investigate the influence of the magnitude of the bridge conductivity, measurements were also performed with the following glassy phase: (AgI)0,2doped or not with a few per cent of NaI. Its room WQ32O-B2%-P2Wo.a temperature conductivity is of the order of 10m4 K1 cm-’ [20]. This material is not wetting and its utilization as a cement is not convenient. However, this may not be a drawback with other techniques, especially with thin layers [ 2 11. Thermodynamical sensor voltage In potentiometric chemical sensors, ionic membranes convert the variations in the analysed-ion concentration into thermodynamically-defined variations of an electrode potential. The diagram of the investigated electrochemical chain is: 2 3 4 1 ref. electrode analysed solution NASICON ionic bridge Ag or Ag-Cd Na+ cond. Na’ and Ag* Na+ cond. (e) (s) (m) (b)

37

At the solution (s)/NASICON (m) .interface, the following exchange reaction prevails: N%+=

Nb+

It fixes an interface electrostatic potential step A@,,: A#,

= A@,’

+ RT/F

h@Na+,

(11

,)

s is the chemical activity of Naf ions in the solution and A@,* is given by the following expression:

aNa+,

A&O = A - RT/F h&a+, ,)

(2)

where A is a constant and aNa+,m the chemical activity of Na+ ions in the membrane (m). At the NASICON (m)/ionic bridge (b) interface, the potential step A@,, is fixed by another sodium exchange reaction: Nh+ s

Nab+

(3) A$‘,, = k&b* - RTIF InhNa+, b/aNa+, rn) It is constant when the local concentrations (aNp+,b and aNa+,& of Na+ ions are themselves constant. The process at the ionic bridge (b)/metallic connection (e) interface can be described as an electrode reaction: Agb++ e,- +k

Ag,

Here too, the relevant elecbrode potential A&,, is constant if the chemical composition of the phases in contact is stable. All these conditions can also be written in terms of equations involving the appropriate electrochemical potentials @ (the + signs of the ions will be omitted for simplicity). Under correct measuring conditions, no current flows through the electrochemical chain, thus the electrochemical potentials of the electroactive species are constant over the NASICON and over the bridge. The conditions and relevant equations to be taken into account are: * solution-membrane interface: fi&, (I= &,. m

(4)

(For the meanings of the superscripts, see the diagram of the electrochemical chain on the previous page.) *membrane bulk: j&, m = &,, m * membrane-bridge interface : *bridge

&,

(5) m = &,,

b

bulk: &,, b = j&,, b

(431

(7)

or: *bridge-electrode interface: &, b + j&-, e = ~~0, e

(8)

38

Summing all these equations results in:

(9) Formally, this can be arranged by adding j&b equations. Taking into account that:

tie-, e

=

to both sides of the

Ct - Fg5

(10)

where F is the Faraday constant and $I the electrostatic potential of the metallic conductor, one eventually obtains: $’ = Ct +

WW~ga, b + &a, s - I-Lisa, b

-

pAgo,

e) +

RT/F

IntaNa+.

5)

In eqns. (IO) and (ll), Ct stands for different constant parameters (whose meanings are not essential for the derivation). This equation shows more accurately which parameters are likely to influence the sensor voltage. It can be written as where 4’ is a constant related to the composition of the ionic bridge. Equation (12) shows again that the metallic conductor potential is thermodynamically defined as a function of the chemical activity of the analysed ions.

Experimental results Stability: influence of NASICON porosity

Sensors were assembled as shown in Fig. 1 with samples prepared according to the procedures A, B, C, D and E. They were tested in NaCl solutions of controlled concentrations and ionic activities. This solute was found to provide a high sensitivity to porosity faults. All the measurements were performed at room temperature. When the pellet exhibits visible cracks or large pores, the sensor responds faster to the Cl- ions than to the Na’ ions. Its voltage varies in a direction opposite to that expected. This is due to the components of the internal reference electrode, essentially AgCl or Ag, coming into contact with the solution and forming a parallel Cl- ion-sensitive electrode. A typical experiment simply involves recording the sensor response immediately after its immersion into the solution. Increasing and decreasing concentrations of N/1000, N/100, N/10 and N were analysed successively with intermediate washing of the electrode with distilled water. Figure 2 shows a typical result obtained with a high-porosity membrane (pellet A: 74% theoretical density). After a peak demonstrating the adequate Na+ sensitivity, the sensor voltage drifts towards values characteristic of Clsensitivity.

t Fig. 2. Results obtained with a porous NASICON solutions.

lo2

aNEi+

membrane (pellet A) in NaCl aqueous

Fig. 3. Typical sensitivity plots of averagequality NASICON pellet after various times of immersion in aqueous solution.

With samples B (84% theoretical density) the response is qualitatively the same, but over a significantljl longer time scale. During the first hours the response is good and drift is observed only after about 10 hours. With samples C, of higher density (92% theoretical density), the response is initially correct, but after long immersion (100 to 400 hours, depending on the sample) degradation sets in. This is reflected by two features: the drift becomes much faster, comparable to that observed with samples B, and the constant parameter in the sensor voltage equation (eqns. (11) - (12)) changes toward smaller values. Figure 3 shows the typical performance of an average-quality sample of this category. With samples D, of density very close to theoretical, but with traces of unreacted raw components, the behaviour is similar, with lifetimes of the order of 100 hours. Figure 4 shows response curves recorded after a 100 hour immersion. The response was still Nernstian. With samples E, sintered under loading, a lifetime of about 1000 hours is easily obtained. It is limited by the failure of the encapsulation resin, with leakages occurring along the interface with the NASICON membrane. Sensitivity

The tests were carried out with standard solutions of concentrations varying from N to 10m5 N. The supporting electrolyte was a pH 8 buffer solution with a constant ionic activity (Tris TS THC@ from Tacussel). This ensures a constant activity coefficient. A saturated calomel reference

40 3 ::

mmde 0 A&I-Maa

2

brmea

1

W

-100

-

-50

0

8 c a

50

85

s

f

88

87

t hour)

Fig. 4. Responses of sample D electrode after 100 hours’ immersion.

electrode (SCE) was used with a microcapillary junction (C5 from Tacussel). To avoid any contamination of the analysed solution by K* coming from the reference electrode, an additional ionic junction (PCLl from Tacussel) was inserted between the solution and the reference. It was always filled with the same sol&ion as the analyte. Results obtained with an E sample are reported in Fig. 5, The sensitivity limit is between 10B3 and 10B4 M. With samples C and D the limits were about the same. The commercial glass electrode (PNAV@‘, Tacussel) used for comparison was found to work down to 1O-5 M, as specified by the manufacturer. The observed limit is probably fixed by the dissolution of traces of a sodium-containing second phase. The selected composition (x = 2) and the preparation procedures are therefore not appropriate for the analysis of low sodium concentrations. These parameters should be adjusted for this application. For biomedical analyses the investigated membrane is, however, perfectly adequate.

Influence of the internal ionic bridge The calculation developed above shows that the ionic bridge is only involved in the constant 9’ of the sensor voltage (eqns. (11) - (12)). This calculation assumes that equilibrium is established. This supposes that: (a) The solid solution forming the bridge is stable and does not suffer any composition alteration, which would result in a voltage drift.

41

Fig. 5. Sensitivities of dense NASICON NaCl aqueous solutions (pH = 8).

pellet (0) and commercial glass (0)

in buffered

(b) The material is sufficiently conductive for the electrochemical potential of the predominant mobile species (e.g., Ag+) to reach equilibrium rapidly after any change in the sensor voltage (eqn. (7)). If this condition is not met, an additional time constant may be added to the response time. In order to compare the AgCl-NaCl and AgI-NaI systems, ionic bridges were formed by quenching molten solutions containing a few per cent of the sodium salt. C samples; from the same batch, were used as sensitive membranes. With the chloride bridge, stability is reasonably good. On the other hand, with the iodide bridge the voltage drifts significantly (virtually linearly) (Fig. 6). This seems to confirm the non-existence of a AgI solid solution containing a few per cent of NaI and the instability of the quenched phase. With the chloride system, the voltages measured with different sensors are relatively reproducible. Systematic measurements were performed with a reference solution containing 0.1 M NaCl and D samples. The results reported in Fig. 7 show that the maximum deviation is 20 mV. In Fig. 8, response times are compared as a function of the nature of the internal ionic bridge. C samples were used. As a rule, iodide bridges give faster responses than the chloride bridges. This new observation suggests that the ionic conductivity of the bridge may influence the response time of the sensor. With the silver conducting glass as the internal ionic bridge material, the observed response times are close to that of the iodide bridge. The stability

42

.’ 0

”

20

1

‘1

40

60

”

80

t pour)

0

20

40

60

Fig. 6. Electrode voltage drift associated with non-equilibrated aqueous solution (M/10).

80

4

t (hour)

ionic bridge in NaCl

Fig. 7. Comparison of several sensors .with sample D and AgCl-NaCI NaCl aqueous solution (M/10).

2

100

as ionic bridge in

t(h) @Jo

Fig. 8. Response times of sensors with sample D and chloride (a) or iodide (b) as ionic bridge.

of the sensor voltage is quite good and its reproducibility, te&x!l with the reference solution, is within 40 mV. However, this material does not wet the ceramic substrate and the sensitive membrane and it is not an adequate cementing agent for the technology we have developed, as mentioned above. One possible difficulty with these ionic bridges is the exchange of silver ions with the sensitive membrane. Ag+ has an ionic radius (1.26 A) relatively close to that of Na+ (0.95 A). This contamination may especially occur during the cementing process at high temperature. To check this point, we intentionally contamined the sensitive surface by immersing the membrane into molten AgCl for 10 min. After cleaning the surface by abrasion, this membrane was mounted in the normal manner. Silver contamination was shown by a slight blackening under light and by a sub-Nemstian response.

43

These observations were not made with regular membranes, indicating only slight contamination if any. To further reduce this risk of contamination, bridges based on PEO polymers were also investigated. They are excellent cementing agents at room temperature and do not require any high-temperature processing. Ftihermore, the silver ions are usually complexed in these materials, forming heavy species less easily exchanged with NASICON. Copper salts used instead of silver salts were also successfully tested. The detailed results have been published elsewhere [22]. Selectivity

The basic reason for this investigation of a NASICON-type material is an expected improved selectivity [l, 16,231. To verify this assumption, we systematically investigated the interferences with the cations most likely to exchange for Na+ and the most probable in vivo interferents: 1130’, K+, Li+ and Ca2+ The performances of our NASICON electrodes were also compared with those of a commercial Na+-specific electrode (PNAVa, Tacussel) based on a glassy material. This investigation was carried out on electrodes made out of C samples (density >90% of theoretical) and E samples (sintered under loading). The results are expressed according to the Nickolskii equation [24 - 271 for a single interferent i, in terms of a selectivity coefficient kN,, I:

E = E” + RT/F ln(aNa+kNa, j(ui)l’*i

(13)

where zi is the electric charge of the interferent. Selectivity coefficients can be determined in various ways [32 1. The results reported here were obtained by plotting E a(Na) =

f(%)

i.e., the electrode voltage, at constant Na+ activity, as a function of the interferent activity Ui. A typical measurement was performed at [Na*] = 10m2M. when uNa s kNa, ibi) “‘f, the sensor voltage is simply given by the Nernst equation: E = E” + RT/F ln cNa

(14)

So, the plot E = f(ui) is a straight horizontal line. Its ordinate gives the value of E”. eqn. (13) shows that E = f(uf) can be approxiWhen UNa4 kNa, &Jpf, mated as E=E”+RT/FlnkNp,f+RT/ziFlnuf

(15)

By extrapolating to ai = 1, one easily gets: E* = E” + RTIF

In kNa, i

and then the selectivity coefficient kN,, i.

(16)

44 pH

interference

Figure 9 shows results obtained under identical conditions with a NASICON-based electrode and with the commercial PNAV* electrode. The pH variations were produced by additions of an acidic solution containing 10B2M NaCl in an analyte also containing 10d2 M NaCl. Values of kNp,n determined in this way lie between 1.5 and 1.7 for E samples and close to 8 for the PNAV electrode. Million-Brodaz [ 321 found 3.5 with C samples. With higher concentrations in NaCl, kN,, n slightly increases. At 1 M concentration it is equal to 3.7 (C sample) [ 321. These results show that NASICON is indeed more selective than the glass used in commercial electrodes. The voltage of a NASICON electrode is very little altered by pH in the range 3 to 9. With glass the interference is already significant at pH 7. At higher NaCl concentrations, NASICON compares even better, glass becoming more sensitive to the pH interference (c.f. Fig. 7 in ref. 1).

(a)

O

*

4

’

’

pH

(b? *

4

=

=

”

Fig. 9. Comparison of pH interferences on: (a) NASICON-based electrode (o), (b) commercialglasselectrode(O), in NaCl aqueoussolution (M/100). The dashed lines correspond to the pH response (see text).

K+ interference

Selectivity with respect to K+ is an important criterion for biomedical applications. In typical situations the activity ratio is about 10 between Na+ and K+ [28]. Figure 10 compares the performance of a NASICON electrode and that of the PNAV@ electrode. The supporting electrolyte of the analyte was buffered at pH 8 and was also maintained at constant ionic strength (Tris T8 THC* Buffer, Tacussel). At [Na+] = 10m2 M, the selectivity coefficient kNa,K was calculated to be equal to 1.5 X lo-’ with E samples. Other measurements with E samples gave values between 1.3 X 10s2 and 2 X 10m2. With glass we obtained 2.7 x 10-2.

45

PNAV ekctmb

\

-780-

I 0

-1

-2

I

0

kg ‘K

(a)

-1

-2

bs’K

(b)

Fig. 10. Comparison of K+ interference6 on: (a) NASICON-based mercial glass electrode (0) in buffered NaCl solution (M/100).

electrode (o), (b) com-

With less dense NASICON samples and in a non-buffered analyte, a value of 4 X 10-l was obtained for kN,, K [32]. Li+ interference

Measurements were performed in the same buffer solution with a sodium concentration of lo-’ M. Typical results are shown in Fig. 11. kNa,Li was found to be equal to 1.6 X 10s2 for E samples and to 4 X 10m2for the PNAV electrode. I

\ PNAV

\

r

l lulmd9

120-

loo-

so-

0

-t

(a)

1 -2

1

@4 ‘Li

f

I

0

-1

-2

loq qj

I

(b)

Fig. 11. Comparison of Li+ interferences on: (a) NASICON-baaed mercial glass electrode (0) in buffered NaCl solution (M/100).

Ca2+ interference

electrode (a), (b) com-

The same conditions were maintained. Results are reported in Fig. 12. Various measurements gave k NP a values equal to 2.3 X 10m2and 2.7 X low2 for the NASICON and 1.7 X 10d2 for the glass. With a less dense sample (92% of theoretical) and in a non-buffered solution, a value of 5 X 10B3 was obtained for k,,, cp [ 321.

46

(a)

WI

Fig. 12, Comparison of Ca2+ interferences on: (a) NASICON-based mercial glass electrode (0) in buffered NaCl solution (M/100), TABLE

electrode I*), (b) eom-

1

Comparison of seiectitity coefficients for [I%+] = low2 M Interferent ion

H,O+

K+

Li+

ca2+

NASICON electrode {sample E)

1.5 to I..?

1.5 x 10-2

1.6 x lO-a

2.2 x 10-2 2.7 x 10-2

PNAV@ commercial electrode

6 to 8

2.7 x 1O-2

4 x 10-2

1.7 x 10-2

Table 1 summarizes the results obtained with high-density samples (E samples) and with the commercial glass electrode. The NASICON values are better in every case. The most spectacular improvement is for pH interference. Engell and Mortensen El51 have also observed lower pH interference. As regards Ca2+ , the adv~t~e is not so obvious. It should be noted that these results have been obtained with a composition that has not been optimized. NASICUN compositions other than Na3ZrzSizP012 may give better performance, at least for specific characteristics. Conclusion The values of the selectivity coefficients determined in this investigation confirm that covalent-framework ionic conductors are appropriate for ion analysis. The density of the sensitive membrane is a crucial parameter, which determines the stability of the measurement and the life of the electrode; It also determines the selectivity coefficients. Regarding NASICON more specifically, traces of a second phase precipitated at the grain boundaries are likely to play a detr~e~~l role. By di~lution, they may open porosity

47

channels. They may also alter the selectivity coefficients (compare results obtained with E and C samples in the K* analyses). A rather surprising limitation observed with NASICON is a rather high sensitivity limit around 1W4 M of Na*. This might also be associated with traces of a soluble second phase. This study also demonstrates that solid-state internal references are appropriate for room-temperature analysis, in contact with framework ion conductors. It shows that the conductivities of the materials forming this internal reference are also crucial parameters, si@ficantly influencing the electrode response time. Co~uct~ties lower than 10e4 W1 cm-l may slow down the electrode responses. Acknowledgemen& This work has been supported by SNEA-ELF 1291 and the French Research Ministry. It was initially developed in cooperation with SFERNICE [ 301. It has been the subject of two French ‘Diplbmes d’Etudes Approfondies’ by J.P. Gros [ 311 and J.F. Million-Brodaz [321.

References 1 M. Kleitz, J. F. Million-Brodaz and P. Fabry, New compounds for ISFETS, Solid State fonics, 22 (1987) 295 - 303. 2 If. Y. P. Hong, Crystal structures and crystal chemistry in the system Nal + XZrzSi,Ps-,O1*, Mater. Res. Bull., Ii (1976) 173 - 182. 3 H. Kohler and H. Schulz, Single crystal investigations on NASICON, Nal + zZr2 _$irP3--x012, 0 6 x c 3, 0 < y < 314, 0 < z < 3. Comparison of the compounds x = 1.24 and x = 3, Solid Sfate Ionics, S - 10 (1983) 795 - 798. 4 M. L. Bayard and G. G. Barna, A complex impedance analysis of the ionic conductivity of Na~,,Zr2SiXP3--x012 ceramics, J. ~lect~unul. Ckem., $1 (1978) 201 - 209. 5 M. Kleitz, B. Sapoval and D. Ravaine (eds.), Solid State Ionics - 83, Part II, NorthHolland, Amsterdam, 1983. 6 J. B. Boyce, L. C. De Jonghe and R. A. Huggina (eds.), Solid St&e Ionics - 85, NorthHolland, Amsterdam, 1986. 7 G. R. ,Miller, B. J. MeEntire, T. D. Hadnagy, J, R, Rasmussen, G. S. Gordon and A. V. Viikar, Processing and properties of aodium @“-alumina and NASICON ceramic electrolytes, in P. Vashiita, J, N. Mundy and G. K. Shenoy (eds.), Fast Ion Transport in Solids, North-Holland, Amsterdam, 1979, pp. 83 - 86. 8 J. J. Auborn and D. W. Johnson, Jr., Conductivity of NASICON ceramic membranes in aqueous solutions, Solid State lonics. 5 (1981) 315 - 316. 9 A. K. Kuriakose, A. Ahmad and T. A. Wheat, Influence of processing on NASICON, in T. A. Wheat, A. Ahmad and A. K. Kuriakose feds.), Profleas in Solid ~lect~~ytes, Energy, Mines and Resources, Ottawa, Canada, ERPlMSL 83-94TR. 1983, pp. 141 167. 10 D. Tran Qui, J. J. Capponi, M. Gondrand, M. Saib and J. C. Joubert, Thermal expansion of the framework in NASICON-type structure and its relation to Nat mobility, Solid State Ionics, 3 - 4 (1981) 219 - 222. 11 H. Kohler and H. Schulz, NASICON solid electrolytes. Part I: the Na+ diffusion path and its relation to the structure, Mater. Res. Bull., 20 (1985) 1461 - 1471.

48 12 R. S. Gordon, G. R. Miller, B. J. MeEntire, E. D. Beck and J, R. Rasmussen, Fabrication and characterization of NASICON electrolytes, Solid State Ionice, 3 - 4 (1981) 243 - 248. 13 J. P. Boilot, P. Colomban and N. Blanchard, Formation of superionic gels and glasses by low temperature chemical polymerisation, Solid St&e Ionic& 9 - 10 (1983) 639 644. 14 J. Engell, S. Mortensen and L. Moller, Fabrication of NASICON electrolytes from metal alkoxide derived gels, Solid State lonics, 9 - 10 (1983) 877 - 884. 15 J. Engell and S. Mortensen, Ion sensitive measuring device, Radiometer Int. Patent WO 84101829 (1984). 16 1. K. Lloyd, T. K. Gupta and B. 0. Hall, Sintering and characterization of alkaline earth-doped and zirconium deficient Na3ZrzSizP012, Solid State Zonics, 11 (1983) 39 - 44. 17 D. H. H. Quon, T. A. Wheat and W. Nesbitt, Synthesis, characterization and fabrication of NASICON ( Nal +XZrzSi,P3-,012), Mater. Res. Bull., 15 (1980) 1533 - 1539. 18 A. Clearfield, M. A. Subramanian, W. Wang and P. Jerus, The use of hydrothermal procedures to synthesize NASICON and some comments on the stoichiometry of NASICON phases, Solid State Ionics 9 - 10 (1983) 895 - 902. 19 E. M. Levin, C. R. Robbins and M. F. McMurdie (ede.), Phase Diagmms for Cemmists. American Ceram. SOC., 1964,1969,1975. 20 A. Magistris, G. Chiodelli and M. Duclot, Silver borophosphate glasses: ion transport, thermal stability and electrochemical behaviour, Solid State lonics, 9 - 10 (1983) 611. 616. 21 L. Jourdaine, Les couches minces amorphes conductrices: application au microstockage, Thesis, INP Grenoble, France, 1986. 22 P. Fabry, C. Montero-Ocampo and M. Armand, Internal ionic bridge for ion solid state sensors, in J. L. Aucouturier, J. S. Cauhade, M. Destriau, P. Hagenmuller, C. Lucat, F. Menil, J. Portier and J. Salardenne (eds.), Proc. 2nd Int. Meet. on Chemical Sensors, Bordeaux, France, July 7 - 10, 1986, pp. 473 - 476. 23 P. Fabry, J. P. Gros and M. Kleitz, Solid state ionics for ISFETS, Symp. Electrochemical Sensors, Rome, June I2 - 14, 1984. 24 B. P. Nickolskii, Theory of the glass electrode, Actu Physicochimi U.S.S.R., 7 (1937) 597 - 610. 25 R. P. Buck, Electrochemistry of ion-selective electrodes, Sensors and Actuators. 1 (1981) 197 - 260. 26 P. Livrozet and J. Tacussel, Capteurs glectrochimiques, in G. Asch (ed.), Les Cupteurs en Instrumentation Industrielle, Dunod, Paris, 1982, pp. 733 - 755. 27 B. P. Nickolskii and E. A. Materova, Solid contact in membrane ion-selective electrodes, Ion Selective Electrode Rev., 7 (1985) 3 - 39. 28 F. Gremy and F. Leterrier (eds.), Riophysique Gknkrale et Me’dicale, Tome I, Flammarion MUecine Sciences, Paris, 1975. 29 P. Fabry and M. Kleitz, Rapport d’activitf! du contrat ISFET, SNEA-LEE (ENSEEG), No. 510-81-101, SNEA No. 3905, February 1983. 30 P. Fabry, J. P. Gros, J. F. Million-Brodaz and M. Kleitz, Capteurs ionosenaibles aux alcalins pour structure ISFET-hybride, Rapport final d’uctiuitd SFERNICE-ADR, No. 510-83-109, April 1984. 31 J. P. Gros, Capteur & &ctrolyte solide sensible aux ions Na+ en solution aqueuse, DiplSme d’Etudes Approfondies, INP Grenoble (France), September 1983. 32 J. F. Million-Brodaz, Etude d’un capteur tout solid@ spgcifique aux ions Na+, Dipibme d%tudes Approfondies, INP Grenoble (France), September 1984.

Biographies Pierre Fabry was born in January 1943 and studied physics in Grenoble (Master of Sciences). He received the degree of Doctor in Physics in 1976

49

and he is presently assistant professor in the Scientific Technical and Medical University of Grenoble. He specializes in the electrochemistry of solids (point defects and electrochemical coloration) and, more recently, in the use of solid electrolytes for ion analysis.

Jean Pierre Gras was born in February 1959 and obtained a diploma in engineering from the Ecole Suptkieure d’Electrochimie et d’Electrom&allurgie de Grenoble (ENSEEG) in 1983. His subject in the Solid Electrochemistry Group was the use of NASICON as a sensitive membrane in aqueous solutions.

Jean Francois Million-Brodaz was born in November 1960. He obtained a diploma in engineering from ENSEEG in 1984. His subject in the Electrochemistry Group concerned the study of the selectivity of NASICON as an ion-sensitive material. Michel Kleitz was born in August 1937. From 1958 to 1968 he obtained a diploma in engineering from ENSEEG, Masters Degrees in chemistry and mathematics and a Ph.D. in physics on electrode reactions on solid oxide electrolytes. He spent several periods abroad in University and Industrial laboratories, organized international conferences, co-founded and edited SoZid State Ionics and is presently Research Director at the CNRS. His fields of expertise are: electrochemistry of solids, electronic properties of the ionic conductors, solid-state electrode reactions, impedance spectroscopy and solid-state potentiometric sensors.