Electrochemical Oxygen Meter Research Articles

Gibbs energy of formation of Na 2WO 4 and Na 3WO 4 in liquid sodium

Oxygen potentials of the equilibrated mixtures of Na+WO 3 and Na+WO 2 at temperatures in the range of 623 to 823 K, measured with electrochemical oxygen meters using ThO 2–Y 2O 3 solid electrolyte, showed two linear oxygen potential ranges corresponding to the two three-phase fields, [Na(l)–W(s)–Na 2WO 4(s)] at 623 to 695 K and [Na(l)–W(s)–Na 3WO 4(s)] between 695 and 823 K. The reaction products of WO 3 and WO 2 with an excess of sodium were identical. Standard molar Gibbs energy of formation data for the two ternary oxides were derived from the oxygen potentials: Δ f G°〈Na 2WO 4〉±11.2/(kJ mol −1)=−1516.78+0.2088( T/K) (623–695 K), Δ f G°〈Na 3WO 4〉±1.7/(kJ mol −1)=−1680.97+0.4465( T/K) (695–823 K)

Oxygen in the Liquid-Metal Fusion Reactor Blanket and Its Possible Influence on the Compatibility with Materials

Electrochemical oxygen meters with yttrium-doped thoria, yttrium-stabilized zirconia, and calcium-stabilized zirconia solid electrolytes are tested for the measurement of oxygen activity in Pb-17Li eutectic alloy. The proposed breeder blanket fluid for fusion reactors. In this paper the oxygen potentials of the alloy measured by these three meters are compared with the oxygen potentials of the alloy after the addition of lithium oxide and with theoretical values. Zirconium and yttrium metal foils are tested for gettering oxygen from the alloy. The compatibility of the three ceramic materials in the liquid alloy and the stability of the meters for prolonged use are discussed. The influence of oxygen on the corrosion of steels in Pb-17Li is reviewed in light of the oxygen potentials of the alloy compared to thermodynamic data of the ternary oxides of the metal constituents of steels. The possibility of using the oxygen meters to measure the lithium activity of the alloy is also indicated.

The formation of ternary oxides in KMeO (Me ≡ Cr, Fe) systems and their free energies of formation

The formation of ternary compounds in the systems K-Cr-O and K-Fe-O was studied by reacting liquid potassium and/or potassium oxide with oxides of chromium and of iron at 500 °C for 336 h. The resulting compounds were isolated by distilling off the excess potassium metal under high vacuum and characterized by chemical and X-ray powder diffraction methods. Measurements of the equilibrium oxygen potential of these compounds in liquid potassium were carried out using an electrochemical oxygen meter utilizing thoria-7.5wt.%yttria (YDT) as solid electrolyte, from which the standard free energies of formation of the various ternary compounds and the threshold oxygen concentrations to form these compounds in liquid potassium were calculated.

Development of electrochemical oxygen meter for liquid sodium

The development of an electrochemical oxygen meter for liquid sodium is briefly accounted. The current design, based on a solid electrolyte crucible machined out of a sintered pellet is described along with some technological details. Present experience with testing in sodium loops and post-exposure analysis has shown that the chosen assembly is capable of achieving approximately the Nernst voltage. The service period was limited mainly by corrosion and by some incompatibilities in reference electrode compartment materials.

Compound formation in the rubidium-chromium-oxygen system

Results from a study of the Rb-Cr-O system in the presence of liquid rubidium metal are presented. Compound formation in the system was studied by means of bulk reactions between various chromium oxides and rubidium metal at 500° C followed by vacuum distillation of the excess metal and X-ray powder diffraction analysis of the residues. Equilibrium oxygen potential measurements of Rb-O solutions in contact with chromium metal or chromium compounds using an electrochemical oxygen meter (ThO 2/7.5 wt%Y 2O 3 electrolyte, Sn/SnO 2 reference electrode) gave information on the stability and conditions of formation of these compounds in the liquid. The investigations showed that Cr metal getters oxygen from rubidium to form a compound (possibly RbCrO 2) in equilibrium with low oxygen activities ( a 0 = 5.43 × 10 −7, 500°C ). At higher oxygen levels, however, the formation of Rb 4CrO 4 takes place. A metastable phase Rb 3CrO 4 was also observed in some bulk reactions.

Testing and performance of electrolytic oxygen meters for use in liquid sodium

The performance of yttria-doped thoria ceramic electrochemical oxygen meters in liquid sodium is described. Tests were carried out using laboratory loops. Temperature coefficients of the oxygen meters have been measured between 380°C and 480°C, and the response to changes in oxygen level using cold-trap temperatures from 125°C to 250°C was determined. The ceramic has been shown to give good performance over lifetimes exceeding 400 days in some cases. The temperature coefficients and response to oxygen level changes are in good agreement with thermodynamic predictions. The effect of running the meters in high-oxygen sodium has been studied and a general mode of failure has been shown to be grain-boundary attack by oxygen/sodium solutions. The effect of γ-radiation on the meters has been studied. The meters with a metal/metal oxide reference electrode were unaffected by dose rates up to 52860 mGy h −1. Meters with an air reference electrode do show an effect as a voltage reduction at levels down to 2420 mGy h −1. This effect was temperature-dependent and was insignificant at 500°C.

Developments in Electrochemical Oxygen Meter Probes for Use in Liquid Sodium

A program was carried out to develop small less fragile ceramic electrodes for use in measuring oxygen in sodium. To use the electrodes as inserts in existing sampler hardware, composite metal-ceramic tubes were required. Four configurations of composite metal-ceramic probes employing yttria-doped thoria as the solid electrolyte were fabricated and tested. These incorporated both mechanical and brazed seals between the metal and ceramic. The most useful for fast flux test facility applications utilized a brazed seal between the ceramic and an alloy of matching coefficient of thermal expansion. A number of alternate reference systems were also investigated. Reference electrodes tested included Na-Na/sub 2/O, In-In/sub 2/O/sub 3/, Ni-NiO, and Ag-air.

Calibration of Electrochemical Oxygen Meters for Sodium in Static Tests and in the Sodium Loop

Electrochemical oxygen meters at Kernforschungszentrum Karlsruhe, Westinghouse Electric Company, and General Electric Company, with different reference systems (gold-air; platinum-air; and indium-In2O3) were tested and calibrated in static tests as well as in a sodium loop. The calibration in static tests was based on oxygen activities of the sodium-chromium-NaCrO2 system. In the loop the meters were calibrated on cold-trap temperatures and checked by chemical analyses of the residue of distilled sodium samples.Fair agreement has been found between the calibration in sodium, the oxygen activity of which was fixed by the chemical equilibrium of chromite formation, and the calibration based on the cold-trap temperature measured in the sodium loop. The meters with indium-In2O3 reference gave results in better agreement to theory at 550 and 500°C than the meters with air reference systems.The oxygen activity in sodium in equilibrium with NaCrO2 is very low; the calibration of oxygen meters based on this reaction gives a chance to apply the probes in the range of their highest sensitivity in highly purified sodium.

Electrochemical Oxygen Sensor for Measurement of Oxygen in Liquid Sodium

The results obtained from approximately 4000 h of operation of an electrochemical oxygen meter developed at General Electric Company for measuring oxygen activities in liquid sodium are described. In the present design, a small ceramic cup (ThO/sub 2/ with 7 to 8 wt % Y/sub 2/O/sub 3/) has been brazed to a metal tube that is inserted into the liquid sodium via a standard sodium flange. The reference electrode used in the present design consists of In/In/sub 2/O/sub 3/. Several meters were tested in a flowing sodium system, where the oxygen concentrations were controlled by a cold trap. At 500/sup 0/C, the oxygen electromotive force (emf) output can be expressed as emf/sub (V)/ = 0.269 + (160/T) (+-0.003 V), where T is the cold-trap temperature in K. The temperature coefficient of the meters can be expressed as (dE/dT) = 0.51 +- (0.02) mV//sup 0/C.

On the accurate calibration of an electrochemical oxygen meter in the 10?7 bar range

A method for the accurate standardization of an electrochemical oxygen meter, in the 10−6-10−7 bar range is described. The method is based on the use of an electrochemical oxygen leak and a chemical getter pump. In this manner problems connected with traces of hydrogen, originating from the pyrolysis of water, and traces of oxygen of unknown origin are overcome.

Hydrogen and Oxygen Behavior in Liquid Sodium (Experimental)

Hydrogen and oxygen behavior in a sodium test loop similar to the sodium-cooled fast breeder reactor were examined. Hydrogen partial pressure equilibrated to the sodium was measured by a membrane-permeation-type hydrogen-meter. An electrochemical oxygen-meter was used to determine the oxygen activity. The measured hydrogen pressure was converted to the hydrogen concentration through the Sieverts' constant, and the oxygen activity was also converted to the oxygen concentration through the characteristics of the oxygen-meter cells. Hydrogen and oxygen solubilities in liquid sodium were obtained as functions of the cold-trap temperature from 115 to 208/sup 0/C. Both data have a similar temperature dependence and were anomalously larger than the extrapolated values in the temperature region lower than 180/sup 0/C. At 300/sup 0/C, the addition of hydrogen to the liquid sodium, valved off from the purification apparatus, lowered the oxygen activity in the liquid sodium. The experimental results suggest a significant interaction between hydrogen and oxygen and the existence of (OH) species in liquid sodium. Using the solubility data of oxygen, hydrogen, and (OH) species, the free energy change of the reaction, (O)/sub in Na/ + (H)/sub in Na/ = (OH)/sub in Na/, was obtained and was ..delta..G/sup 0/(kcal/mole) = -28 + 0.043 T.more » From the temperature dependence of the equilibrium hydrogen pressure, the apparent heat of a solution of hydrogen in liquid sodium was obtained as 1.6 kcal/mole.« less

Deterioration of impermeable alumina tubes in inert atmospheres at elevated temperatures

The deterioration of commercial impermeable alumina tubes when exposed to inert atmospheres at elevated temperatures (∼1300‡ C) has been monitored using solid state electrochemical oxygen meters. The inability to maintain low oxygen partial pressures within the tubes was attributed to the development of porosity at elevated temperatures in the surface skin of the inner wall. Examination of samples by light and scanning electron microscopy, together with chemical analysis, indicated that the porosity was associated with a decrease in the impurity content. Thermodynamic evaluation confirmed that the relevant quantities of SiO2, CaO, MgO could be removed as volatile species generated by the low oxygen partial pressures. The calculations also suggested that the formation of volatile species could result in significant contamination of samples undergoing heat-treatment in these alumina tubes.

Calibration of Electrochemical Oxygen Meters in Sodium Using Uranium

Methods using uranium to remove or getter oxygen quantitatively from sodium have been used to calibrate the potential of the meter as a function of the oxygen concentration in sodium. The oxygen meters were constructed with sodium‐sodium oxide reference electrodes and electrolytes. The calibration methods depended on the deviation of these meters from ideal behavior. The methods involved the summation of the weight gains of uranium, i.e., the weights of oxygen removed from the sodium, the derivative of the weight gains for a given potential change as a function of the number of repetitive oxygen removal steps, and the time dependence of the potential during the removal of oxygen from sodium by uranium. The behavior of a meter depended on the purity of the electrolyte used. High‐purity electrolytes gave the theoretical dependence of potential on concentration but impure tubes showed marked deviations from theoretical behavior at low concentrations at temperatures of 500°C.