Pierre Fabry

Comparative study of lithium ion conductors in the system Li1+xAlxA2−xIV (PO4)3 with AIV=Ti or Ge and 0≤x≤0·7 for use as Li+ sensitive membranes

Abstract Preparations and physico-chemical characterizations of NASICON-type compounds in the system Li1+xAlxA2−xIV(PO4)3 (AIV=Ti or Ge) are described. Ceramics have been fabricated by sol-gel and co-grinding processes for use as ionosensitive membrane for Li+ selective electrodes. The structural and electrical characteristics of the pellets have been examined. Solid solutions are obtained with Al/Ti and Al/Ge substitutions in the range 0≤x≤0·6. A minimum of the rhombohedral c parameter appears for x about 0·1 for both solutions. The grain ionic conductivity has been characterized only in the case of Ge-based compounds. It is related to the carrier concentration and the structural properties of the NASICON covalent skeleton. The results confirm that the Ti-based framework is more calibrated to Li+ migration than the Ge-based one. A grain conductivity of 10−3 S cm−1 is obtained at 25°C in the case of Li1·3Al0·3Ti1·7(PO4)3. A total conductivity of about 6×10−5 S cm−1 is measured on sintered pellets because of grain boundary effects. The use of such ceramics in ISE devices has shown that the most confined unit cell (i.e. in Ge-based materials) is more appropriate for selectivity effect, although it is less conductive.©

Study of Li1 + xAlxTi2 − x(PO4)3 for Li+ potentiometric sensors

Mineral compounds Li1 + xAlxTi2 − x(PO4)3 (x = 0 and x = 0.3) have been made by cogrinding and soil-gel processes. Structural characterizations by X-ray diffraction and Raman spectroscopy indicate that alumina substitution (x = 0.3) does not modify the crystallographic structure, whatever the synthesis process: compounds crystallize in the rhombohedral system with an R-3C space group. The use of the sol-gel route makes low-temperature sintering (950 °C) easier and, moreover, leads to ceramics with a high water stability. Li1.3Al0.3Ti1.7(PO4)3 compounds are fast ionic conductors: σ25 °C varies from 15−5 to 10−4 S cm−1, depending on the synthesis process. They have been used as ionic membranes for lithium-selective electrodes. Sensors prepared with sol-gel membranes have the best performance: the detection limit is 1.4 × 10−4 mol dm−3. The potassium and the protonic selectivity properties are attractive for such electrodes. For sodium, they need to be improved for biomedical applications.

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

Abstract Polycrystalline NASICON, Na 3 Zr 2 Si 2 PO 12 , has been tested as a Na + ion-sensitive 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 10 −3 – 10 −4 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 + , Ca 2+ , Li + , and especially H 3 O + .

Optimization of NASICON composition for Na+ recognition

Abstract Na 1+ x Zr 2 Si x P 3−x O 12 are ceramic materials which are fast ionic conductors by Na + . The present work concerns the characterizations of such materials in the range x = 1.4–3.0 for their use as ion sensitive membranes. Samples were prepared by sol-gel route to obtain pellets with a high density. Characteristics, such as lattice parameters and bulk conductivity are given. Results on detection limit and selectivity of such membranes used in ISE devices are presented. The effect of the stoichiometry on electrochemical characteristics is discussed. The best performances are obtained for x = 2.0–2.2, with samples sintered at 1200°C. No influence of sintering temperature is noticeable for the selectivity, excepted for proton which is less interfering after sintering at 1200°C.

New compounds for ISFETS

Abstract ChemFet literature is firstly reviewed. It is shown that some 3-D framework fast ion conductors are good candidates for ion-selective membranes. This statement is supported by experimental results on NASICON. Ion-selective electrodes are less sensitive to the additional electronic conductivity of the membrane than other potentiometric sensors. Mixed conductors can be used. Ionic bridges enable utilisation of various internal reference electrodes. Mixed conductors do not require such electrodes.

Utilization of a dilute solid electrolyte in an oxygen gauge

Abstract An electrochemical cell involving a SrCl 2 -based solid electrolyte either doped or undoped with SrO was investigated. The measuring electrode was of graphite or vitreous carbon and the reference electrode was formed by dissolving Ag + ions and embedding a silver wire. A simple fabrication technique is described. The reported results indicate that an electrode reaction involving dissolved oxide ions: O 2 (gas)+4 e − =2 O 2− (SrCl 2 ) can take place and be utilized in an oxygen gauge down to 200°C. The same cell also functions as a chlorinesensor. At least down to 10 −3 atm, interference with oxygen is negligible.

Polymer electrolyte as internal ionic bridge for ion solid-state sensors☆

A solid-state material is proposed as a substitute for the internal liquid ionic phase in electrochemical sensors. Poly(ethylene oxide) is used for an Na+ sensor with NASICON as the sensitive membrane. The polymer is doped with Na+ and M+ (Cu+, Ag+) (metal of the electrical lead) iodides. The iodide ions tend to form complexes with 1b ions and contamination of the NASICON membrane is thus minimized. The conductivities and the kinetics of the redox reactions have been compared in detail.

NASICON structure for alkaline ion recognition

Abstract Crystallized fast-alkaline conductors have promising properties for ion electrochemical sensors. Their conductivity at room temperature is very high (about 10−4–10−3 S cm−1). The selectivity effect of such membranes is based on the calibration of conductive sites in the structure. NASICON materials are good examples from this point of view. Suitable cationic substitutions allow to adjust the size of the conduction sites and then to improve the selectivity effect. Recent experimental results on Na + and Li+ membranes are shown in this field.

Study of CO2 electrodes in open devices of potentiometric sensors

Abstract Potentiometric CO2 sensors in open devices with NASICON as solid electrolyte were tested in the temperature range 150–500 °C and at partial pressures between 10−6 and 0.1 atm in air. The sensors were built with sodium titanate and La1−xSrXMnO3±δ (LSM) mixtures as oxygen reference electrode and Na2CO3–BaCO3 or Na2CO3–SrCO3 mixed with gold as measuring electrodes. The CO2 electrode reaction is based on an electrochemical mechanism with a fast electron transfer and an adsorption–diffusion phenomenon. In sensor devices, the system with Na2CO3–BaCO3 deviates from Nernst law in all the pressure ranges whatever the device (pellet or coplanar devices) is. The system with Na2CO3–SrCO3 obeys a Nernst behaviour for the highest CO2 pressures. The observed responses are very fast. A discussion is engaged to explain the subnernstian behaviour from the thermodynamic stability of Na2CO3 and a mixed voltage phenomenon.

Ionic exchange and selectivity of NASICON sensitive membranes

Abstract The complex impedance spectra of NASICON Na + ion-sensitive membrane contacted with solutions of different concentrations of the primary ion and of the main interfering ions (Li + , K + , Ca 2+ and H 3 O + ) were measured over the range 65 kHz–0.01 Hz. The impedance data can be correlated to the potentiometric selectivity coefficients determined by the separate solution method for 0.1 M solution.