SOLION ELECTROCHEMICAL DEVICES 



J. L. COLLINS 

 Defense Research Laboratory 

 University of Texas 

 Austin, Texas 



ABSTRACT 



The solion is a new class of electrochemical 

 devices which act as current limiting diodes, 

 signal processing elements, pressure or flow 

 detectors, etc. Of particular interest is the 

 solion linear pressure transducer or linear flow 

 detector, with its wide range of possible appli- 

 cation at the lower frequencies normally associ- 

 ated with oceanography and geophysics. A discus- 

 sion of the principles of solion operation will 

 be presented along with some of the practical 

 applications of solions in the field of oceano- 

 graphic instrumentation. 



INTRODUCTION 



The science of electrochemistry began around 

 1800 when Galvani noticed that if two dissimilar 

 conducting materials were placed in contact with 

 a freshly prepared frog's leg, the leg twitched 

 as if alive. Although the solion does not util- 

 ize a freshly prepared frog's leg, it does util- 

 ize some of the principles noticed in Galvani 's 

 experiment. 



The solion is a very low power consumption 

 electrochemical device, commonly known as a redox 

 system. With a redox electrode the reaction 

 occurring at the electrode is completely rever- 

 sible. Furthermore, the electrodes consist of 

 an unattackable metal, usually platinum, which 

 will not enter into the reaction itself. The 

 electrochemical system consists of an electrode 

 set immersed in a solution containing soluble 

 forms of the same chemical in two different oxi- 

 dation states . For the case of the solion this 

 is generally an iodine/iodide electrolyte system. 



The name solion is derived from the phrase 

 "ions in solution." Electric current is trans- 

 ferred via ion flow in a solution as opposed to 

 ion flow in a gas or solid . The redox electro- 

 chemical system is discussed in this paper, but 

 other solions utilize different electrochemical 

 principles such as electrokinetic transduction. 



THE SOLION ELECTROCHEMICAL DIODE 



Consider the case of a solion diode. This 

 will illustrate the basic characteristics of the 



DC MILLIAMP 

 METER 



ELECTROLYTE 

 SOLUTION 



MOLDED AND SEALED 

 PLASTIC HOUSING 



Fig. 1. The basic electrochemical diode system. 



solion, not including those effects due to hydro- 

 acoustic flow. Fig. 1 is a schematic of the basic 

 electrochemical-diode system. The diode consists 

 of a pair of platinum electrodes sealed into an 

 airtight chamber that has been filled with an 

 iodine/potassium iodide/water solution. One elec- 

 trode, the anode, is chosen with an effective sur- 

 face 10 times that of the other electrode, the 

 cathode. The chamber is constructed of a chemi- 

 cally inert plastic material such as Kel-F. The 

 external electrical circuit consists of a variable 

 low voltage, DC supply, a DC milliamp meter and 

 a high impedance DC voltmeter. 



With the circuit connected as in Fig. 1, the 

 voltage between the electrodes is increased and 

 the current through the cell is monitored. The 

 voltage-current relationship is shown in the con- 

 centration polarization curve of Fig. 2. Maximum 

 voltage is about 0.9 volts DC; any substantial 

 increase above this value will tend to cause 

 hydrogen evolution at the cathode. This curve 

 illustrates one of the basic features of the 

 solion--the output current is independent of the 

 applied voltage over the range 0.1 to 0.9 volts. 

 Typical value of the slope of the plateau is 

 approximately 1 megohm. Over the region of the 

 plateau, the limiting current is given by the 

 following relationship. ' ^ 



Superior numbers refer to similarly numbered references at the end of this paper. 



163 



