APPLICATION OF SILVER CONDUCTING GLASSES TO SOLID STATE BATTERIES AND SENSORS

Fast silver ion conducting glasses as electrochemical devices have been tested. A silver iodine battery using a silver ionic conducting glass (AgPO3-AgzS-AgI) has been studied. The interaction of some gases (O.,, CI_,, H_,S) with the electrochemical chains: Pt/Sb;S3-AgI (glass)/Ag and Pt/AgCI (thin film)/Sb,.S3AgI (glass)/Ag has been investigated. Finally, the behavior of thin films of AgzS3-AgzS-CdS glasses as sensitive membranes for Cd detection in solution has been tested on MIS structures Au/Si/SiO2/ Membrane/Cd in solution/Reference electrode.


INTRODUCTION
Silver ions are well known to possess a very high mobility in some solid phases.RbAg4Is, for example, has a conductivity at room temperature is as high as 0.3 (2-cm-.But the stability of these materials is very poor; silver rubidium iodide decomposes at 18 C and is not very stable towards moisture and iodine.New solid electrolytes have been prepared by reaction of a glass former (As2S3, Sb2S3, AgPO3 ) with AgI and Ag2S mixtures.These glassy electrolytes have conductivities reaching 10 -2 to 10-tl2-cmat room temperature2.3.Although the conductivity of these glasses is less (10 to 100 times lower) than RbAg4I, they possess some advantages: good stability, easy to shape, possibility to prepare thin films.
The present paper deals with the use of some of these glasses in applications such as solid batteries and chemical sensors.
Saito and Kashihara have proposed the use of RbAg415 in a silver iodine battery4.
Compared to lithium batteries, they possess a much lower ocv (E 0.68 V for Ag/I2 system versus E 2.6 V for Li/TiS2) but a higher discharge rate.
In the case of chemical sensors, H6tzel and Weppner have studied a chlorine gas sensor based on the following galvanic chain: Pt/AgC1 (thin film)/RbAg4Is/ Ag6.
In both cases we have tried to use the silver ion conducting glasses to replace RbAg415 in these devices.
Glasses are also well adapted to the preparation of sensitive membranes 7 for ion detection, as thin films of glassy electrolytes have good stability in solution and a fixed activity for the ions to detect.This report describes the easibility of such membranes with silver ion conducting glasses for Cd + + ion detection.

Experimental
The electrolyte was a glass of composition 0.12 Ag2S-0.44AgI-0.44 AgPO3 prepared as described elsewhere3.The galvanic chain was as follows: Ag + electrolyte/electrolyte/electrolyte + phenothiazine + 12 + C The anode was made of 70 w% glass and 30 w% silver powder.The composition of the cathode was 24 w% Carbon, 16 w% phenothiazine, 60 w% iodine.Carbon was added to the cathode to insure electronic conductivity.The charge transfer complex iodine-phenothiazine was used in place of pure iodine to disperse the iodine atoms in order to avoid the formation of an ionic and electronic insulating film of B AgI4. Anode electrolyte and cathode were pressed together under 5 t/ cm-.The surface of the obtained pellet was coated with epoxy resin and then introduced in a teflon cell (Fig. 1).A discharge curve is displayed in Fig. 2. It corresponds to a load resistance of 47 kf2 and a current density of 10/A/cm 2. In a previous study, Sun Hongwei studied a comparable galvanic chain (Ag/electrolyte/I2 + C)7, where the discharge voltage for a 2 tA/cm discharge current density was 0.66 V.In our case, the plateau of the discharge curve corresponds to a slightly lowered voltage (0.63 V) due to the use of the charge transfer complex.However, the battery yield is about 2.5 times higher.This result is also to be compared to that obtained by Saito and Kashihara4.Under comparable discharge conditions, the lifetime of the battery using the glass electrolyte is 25 % longer than with RbAg4I (500 h against 400 h).

Introduction
Gas chemical sensors are becoming increasingly important in many fields; power consumption, raw material savings, industrial process improvements, pollution con- H h FIGURE 3 Gas line used to test the sensors: a) gas tank (high pressure), b) gas tank (low pressure), c) manometer, d) vacuum pump, e) mixture chamber, f) mixing gas device, g) cycling pump, h) test chamber, i) heater FIGURE 4 Schematic diagram of the investigated cell: a) sputtered Pt thin film, b) sputtered AgCI thin film, c) epoxy resin coating, d) Ag conducting glass electrolyte, e) Ag reference electrode, f) Gold leaf, g) A1203 ceramic substrate trol, etc.Among the various kinds of gas sensors in use, potentiometric ones are specially interesting due to their specific property of selectivity.However, for many applications the main drawback of potentiometric sensors is their high working temperature.A great deal of research is currently aimed at lowering this temperature6'8.. Oxygen sensor operating at room temperature or slightly above have been studied 1,12.Sensors are also being developed industrially for other gases (i.e., C12, SO2, SO3)13'14..In our case, the galvanic cell used to detect C12 gas was: Ag/Ag / conducting glass/AgC1, Pt, C12 Ag metal forms a reference electrode and the Ag / conducting glass is the elec- trolyte.AgCI acts as an auxiliary layer and Pt is the measurement electrode.The According to Nernst's law, the e.m.f, obeys the equation: E E0 + (kT/nq) In PCI2 with k Boltzmann constant, q the electron charge, T the temperature and n 2 is the number of electrons involved in the electrode reaction, E0, the standard potential, can be calculated from the Gibbs energy of AgC1.

Experimental
CIr. gas was obtained from UCAR (Research purity) and the gas line used in this study is shown in Fig. 3.The sensor device is depicted in Fig. 4. AgI-Sb2S3 glass was used as a solid electrolyte.The measurement electrode was made by a sputtered AgCI film coated with a sputtered Pt film.A pellet of silver powder was co-pressed with the electrolyte as the reference electrode.The sensor was then sealed using epoxy resin to avoid any reaction of the reference electrode with the gas atmosphere.A sputtered Pt electrode and gold leads on an A1203 ceramic substrate were used.The gas-carrier was Argon or air.The sensor e.m.f, was measured for various gas concentrations with a high-input-impedance (1014 I2) Keithley 16 multimeter.

3.
3.1 Response to 0 and H2S gases.Oxygen response was first tested in the simple galvanic cell Ag/Ag conducting glass/Pt.A variation of the e.m.f, was observed due to the formation of a Ag20 film at the surface of the electrolyte.The experimental curve is shown in Fig. 5a.For high oxygen concentration, the e.m.f.varies linearly with the logarithm of oxygen partial pressure.However, at low oxygen pressure the line slope changes.This behavior could be due to a kinetic adsorption effect.Indeed, at room temperature the response time increases.
The sensitivity to H2S has been also tested.In air, when the partial pressure of H2S is lower than 10 -2 atm. (Fig. 5b) the e.m.f, value corresponds to the response at Po 0.21 atm., which is the partial pressure of oxygen in air (Fig. 5a).When H2S is mixed with Argon (Fig. 5c) with Prs < 10-2 atm, the e.m.f, approaches a value corresponding to that of Po at the same pressure in Argon mixture (Fig. 3a)..3.2Response to chlorine.The chlorine response of the AgCI sensitive layer 800 600 400 2 0 0 0 2 4 8 10 t(mn) FIGURE 7 Voltage response to addition of chlorine at a pressure of 5.10arm, (1000/ thick) coated galvanic cell is given in Fig. 6c.In the pressure range above 10 -4 atm a linear variation is observed.At lower pressure the linear relationship is no longer obeyed due to a slow absorption kinetics.Even in the linear domain the e.m.f, is lower than the potential expected from Nernst's law, indicating an extra chemical reaction between the electrolyte and chlorine due to the permeability of the silver chloride film.Consequently, another test was undertaken with a sensor coated with a thick (about 1 am) AgC1 film.The results are shown in Fig. 6b.The experimental values approach theoretical data.In a limited case a sensor using a pure AgC1 layer as electrolyte was tested.For high C12 pressure the response is very near the theoretical value (Fig. 6a), while for chlorine partial pressure lower than 10atm, the e.m.f, deviates due to the poor ionic conductivity of silver chloride at room temperature.
These results show that the conducting glass that contains Iions may react with C12.The reaction can be as follows: 2 I-(glass) + C12 ) 2 CI-+ 12 As the sensor e.m.f, depends on the activity of the gas at the electrolyte surface (i.e., at the "triple contact" gas/electrolyte/electronic conductor), the potential is lower than that given by Nernst's law using Gibbs energy of formation of AgC1.
With a thick AgCI film, the response time is less than 3 mm for a chlorine partial pressure of 5 x 10 -4 atm (Fig. 7).At lower pressure the response time becomes longer and the e.m.f, variation is no longer linear.

ISFET SENSITIVE MEMBRANES
Chalcogenide and derived chalcohalide glasses have already been used to prepare selective electrodes7-2.Their properties are well adapted to this kind of appli- cation; low solubility in water, good ionic and very low electronic conductivities, and a wide range of composition 2-3.2 permitting the preparation of various mem- branes.They are potentially interesting as membranes for ISFET (insulator semi- conductor field effect transistors) devices as it appears possible to prepare thin films of various glasses having a constant activity for the ions detection and a sufficient ionic conductivity (Ag /, Li / or other ions...) to obtain good perform- ances.
We now present the preparation and performance of a thin film chalcogenide glass used as sensitive membrane for cadmium detection.

Membrane Preparation
A chalcogenide circular target (50 mm diameter, 4 mm thickness) was first fabri- cated by grinding and mixing ultrapure powders (Merck products) with the composition 7 57 ms2S3, 38 Ag2S, 5 CdS (moles %).The mixture was pressed under 1T/cm and then heated at 220C in a vacuum chamber (5 x 10 -7 Torr).Chal- cogenide thin films were prepared by RF magnetron sputtering of this target in an argon gas plasma at a pressure of 10 -2 Torr, which an incident powder of 25 W and a deposition rate of 20 Even heated at 420C the films appear amorphous.The composition of the films has been studied by RBS (Rutherford back scattering) using 2 MeV a particles.
A typical spectrum is given in Figure 8.The film composition appears different from the one of the target Moreover, a decrease of the sulfur and silver concen- trations accompanying some aging process of the target has been observed.Con- currently, the deposition rate decreases (Fig. 9).A glassy target would probably give a better behavior than the one made of pressed powder.
Thin film symmetrical cells Au/glass/Au have been prepared to perform ad- mittance spectroscopy study.The obtained frequency response has been used to calculate the ionic conductivity at room temperature: 0"300 K 2.5 10 -2 S.cm -'.

Membrane Characterization: C(V) Study
To determine whether the glass films could be used as ISFET membranes, we have studied the C(V) response of Au/Si/SiO/Membrane/Ions in Solution/Electrode cells.The Si/SiO wafers, provided by LETI (Grenoble, France*), were prepared with p type silicon at a doping density about 10 21 m -3 and a SiO thickness of 1000 Preliminary measurements: As a basis for our analysis we first studied the C(V) response of the structure Au/Si/SiO/Hg before the membrane deposition.The study was performed with a Boonton capacitance meter at 1 MHz.The corre- sponding curve is given in Fig. 10 and can be compared to the C(V) response of the Au/Si/SiO2/Membrane/Hg structure.It shows important voltage shifts (due in part to the in-series addition of the impedance of the membrane and in part to states and charges created by sputtering) and the need for a low frequency study (<100 Hz) in order to obtain the complete accumulation process (due to the ion response time consant in the membrane).
Studying Cadmium detection: The Cadmium detection ability of the chalcogenide membrane has been tested by studying the C(V) response at low frequencies of a Au/Si/SiO2/Membrane/solution/electrode structure.Figure 11 shows the exper- imental cell we have used.The applied ac signal frequency was in the range 2 to 200 Hz and the scanning rate from 0.0001 to 1 Vs -.The real and imaginary parts of the current were measured by using a PAR model 124 Lock-in Amplifier.Various buffer solutions have been tested with pH ranging from 1.679 to 9. The desired Cd / / concentration was obtained by adjonction of Cd(NO3)2 to the solution.The most significant and reproducible results (Fig. 12) have been obtained with the Tacussel buffer solution TRIS of pH 9. We can observe a constant slope (25 mV/ decade) of the curve giving the voltage shift 3V of the C(V) characteristics versus the log of the Cd / / concentration in the range 10 -5 to 10mole -1.This shift must be the result of an equilibrium for Cd / / ions at the surface of the membrane.

CONCLUSION
This study shows the feasibility of silver conducting glasses as electrolytes in elec- trochemical devices.Easy to prepare and to shape, and having a good chemical durability, they can replace RbAgA5 as an electrolyte in electrochemical devices, or act as sensitive membranes in chemical sensors.

FIGURE 6
FIGURE 6 Variation of the EMF versus chlorine partial pressure (T 25C) a) pure AgCI sensor, b) sensor coated with AgO thick film, c) sensor coated with AgCI thin film.

FIGURE 9
FIGURE 9  Deposition rate versus RF power for 1) new target 2) one month old target A/mn.LIU JUN et al.Study of the composition of a glass film by Rutherford Backscattering