Oxygen Potential Measurements Research Articles

Oxygen potential of plutonium and plutonium−americium dioxides

The oxygen potential measurements of PuO2−x and Pu1−yAmyO2−x, with 0 ≤ y ≤ 1 and 0 ≤ x ≤ 0.4, reported in the open literature were reviewed. The selected variations in their deviation from stoichiometry “x” as a function of the equilibrium oxygen partial pressure “pO2” were re-analyzed using a point defect chemistry model for various temperatures and Am contents. In plutonia, the log(x) vs. log(pO2) plots revealed that the VO••PuPu′• clusters prevail for hypo-stoichiometry values of x ≥ ∼10−3 and doubly-charged oxygen vacancies VO•• dominate when x ≤ ∼10−3. A relationship connecting the Oxygen/Plutonium ratio (1.60 ≤ O/Pu ≤ 2.00), the equilibrium oxygen potential, and the temperature (953 K ≤ T ≤ 2100 K) was deduced from the re-evaluation of the defect chemistry reactions.

Oxygen potential measurement of U0.85Am0.15O2 at 1473, 1573, and 1673 K

Oxygen potential data of U0.85Am0.15O2−x were measured at 1473, 1573, and 1673 K by thermogravimetry. In U1−yAnyO2−x, where An stands for Pu or Am, and for a given value of “y” and Oxygen/Metal ratio, the oxygen potential of U1−yAmyO2−x is higher than that of U1−yPuyO2−x. The valence of cations in the hypostoichiometric region is similar to that of Nd-doped UO2. At the stoichiometric composition, it is estimated to be Am3+, U4+, and U5+ (for charge compensation of Am3+) in the stoichiometric composition. The experimental data were analyzed using a defect chemistry model, and a relationship connecting the oxygen-to-metal ratio, the temperature, and the equilibrium oxygen partial pressure was proposed.

Relative oxygen potential measurements of (U,Pu)O2 with Pu = 0.45 and 0.68 and related defect formation energy

Using a thermogravimetric analyzer and an oxygen sensor, relative oxygen potentials of mixed oxide (MOX) fuels with Pu contents of 45% and 68% were measured at 1473 K – 1873 K; the relative oxygen potentials provide the relation among the equilibrium oxygen-to-metal (O/M) ratio, temperature, and oxygen partial pressure of the flowing gas. To evaluate the effect of the Pu content on the relative oxygen potential of MOX and related defect formation energy, this work aims to bridge the gap in the results available in existing reports, where Pu contents of 0%-46% and 100% have been studied [1–11]. The measured data indicate that the trend of increasing relative oxygen potential with increasing Pu content, which can occur as a consequence of more Pu cations changing from being tetravalent to trivalent, reaches a saturation at a Pu content of 68%, in particular in the region of O/M < 1.96. As the formation of defects related to oxygen results in oxidation and reduction of nonstoichiometric MOX and the intrinsic ionization is the dominant defect formation process, the relation between the equilibrium O/M ratio and the oxygen partial pressure was well reproduced by introducing the equilibrium constants of the defect formation process. The highest concentration of electron and hole defects caused by the intrinsic ionization was observed in MOX with a Pu content of 68%, and the related properties, such as electrical conductivity and specific heat, also exhibited the highest values upon increasing the defect concentration at high temperature.

Oxygen potential measurement of (U,Pu,Am)O2±x and (U,Pu,Am,Np)O2±x

The measurement of oxygen potential was conducted at 1673, 1773, and 1873 K for (U0.623Pu0.350Am0.027)O2 and at 1873 and 1923 K for (U0.553Pu0.285Am0.015Np0.147)O2 by using a thermo-gravimeter and an oxygen sensor. Am inclusion in terms of substituting the U significantly increased the oxygen potential. Similarly, the inclusion of Np as a substitute for U increased the oxygen potential; however, the effect was not as large as that with the Pu addition at the same rate. The results were analyzed via defect chemistry and certain defect formations were suggested in the reducing region and the near-stoichiometric region by plotting the relationship between PO2 and the deviation from the stoichiometry. The equilibrium constants of the defect reactions were arranged to reproduce the experiment such that Am/Np contents were included in the entropy with coefficients fitting the experimental data.

Evaluating Thermodynamic Parameters of Alloy Elements in Molten Iron-based Metal by EMF Method Using Oxygen Sensor

EMF (Electro-Motive Force) method using oxygen sensor has been widely applied for determining activity coefficient and interaction parameters between alloying elements through soluble oxygen activity in molten solute metal. This method is effective particularly for evaluating activity coefficient of a deoxidizing element in molten metal through oxygen potential measurement, where chemical equilibrium can be achieved between solid oxide consisting of the deoxidizing element, the deoxidizing element and oxygen soluble in molten metal. In this paper, evaluating procedure and several key points are reviewed in terms of EMF method to determine thermodynamic parameters, particularly activity coefficient and interaction parameters, of alloying elements in molten iron-based metal.

Quasi‐Steady‐State Polarization Reveals the Interplay of Capacitive and Faradaic Processes in Capacitive Deionization

AbstractWe employ slow cyclic voltammetry to study the quasi‐steady‐state capacitive deionization (CDI) of aerated 10 mM NaCl from Vcell=−0.2 to 2 V. The method allows for the deconvolution of capacitive and faradaic processes, which are significant even at low voltage, in conjunction with pH, conductivity, dissolved oxygen (DO), and electrode potential measurements. The pH at three locations and effluent DO data identify asymmetric electrosorption in the cell configured with pristine activated carbon electrodes, where charge is irreversibly consumed to produce H+/OH− instead of reversibly adsorbing Cl−/Na+. Implementing a reference electrode resolves ion adsorption as a function of the individual potential at each electrode. Near‐electrode pH probes reveal pH environments that diverge by up to 8 units during cell polarization and allow more accurate calculation of faradaic redox potentials in a flow‐by CDI cell. In the aerated NaCl solution that will be relevant to industrial‐scale water treatment, we find that DO reduction occurs to a greater degree than Na+ adsorption at the cathode until DO removal becomes mass transfer limited at &lt;−0.2 V vs. SHE. Two cell architectures – flow‐by and flow‐through – corroborate our findings. Finally, we reconcile all measurements to generate a map displaying how the potential is partitioned amongst the capacitive and faradaic processes occurring during CDI operation over a 2.2 V window.

Insights from “The Hidden Half”: The impact of root-zone oxygen and redox dynamics on the response of avocado to long-term irrigation with treated wastewater in clayey soil

Treated wastewater (TWW) is a major source of water for agriculture in Israel; however, recent reports indicate a marked yield loss in TWW-irrigated avocado and citrus orchards planted in clayey soils. The association of the yield loss with clayey soils rather than sandy soils suggests that it is associated with conditions in the root zone, and specifically poor aeration. A three-year study (2012–2015) was conducted in an avocado orchard planted in clayey soil, comparing the oxygen and redox conditions in the root zone of TWW-irrigated plots with fresh water (FW)-irrigated plots, together with the physiological status of the trees. Soil parameters included: continuous in-situ measurement of soil-water tension (SWT), soil oxygen, and soil redox potential, and periodic measurements of soil solution composition. Physiological parameters included: mineral composition of plant tissue from the leaves, trunk xylem and roots, root growth, yield, fruit setting, plant volume, and yield. TWW-irrigated plots were found to endure longer periods of low SWT indicating higher water content, accompanied by lower oxygen levels and more reduced conditions in comparison to FW-irrigated plots. The differences in these soil parameters between treatments were greater during the irrigation season than during the rainy period. The more reduced conditions in the TWW plots did not lead to significant differences in Fe or Mn concentrations in the soil solution or in plant leaves. TWW soil solution had significantly higher Na levels compared with FW. This did not affect the leaf Na content, but was expressed in substantially higher Na content in the root and trunk xylem, with up to seven times more trunk xylem Na in TWW-irrigated plants compared with FW-irrigated plants. Root growth was significantly hindered in TWW-irrigated plots compared with FW-irrigated plots. A negative correlation was found between root growth and the duration of hypoxic conditions, and similarly between root growth and the Na levels in the roots. TWW-irrigated plants had greater fruitlet numbers at the initial fruit-setting stage, but had a smaller number of fruit and a lower yield at harvest. The yield (kg/tree) negatively correlated with the duration of hypoxic conditions in the root zone but not with the Na levels in the roots or xylem. Our findings point towards a substantial role of oxygen deprivation as a major factor leading to the damage to TWW-irrigated orchards in clayey soils. Based on the assimilation of data, we suggest that a downward cascade is instigated in the TWW-irrigated orchards by increased input of Na into the soil, leading to degradation of soil hydraulic properties and reduced aeration. Impaired physiological functioning of the roots due to limited oxygen supply results in less roots growth, lower water uptake and impaired selectivity against Na uptake, thus imposing a negative feedback to increase soil water content, reduce aeration and root-zone oxygen availability for the roots, and further impair plant resistance to the high Na levels.

Oxygen potential measurement of (Pu0.928Am0.072)O2–x at high temperatures

The oxygen potentials of (Pu0.928Am0.072)O2–x were measured as a function of oxygen to metal ratio (O/M) in the temperature range from 1473 to 1873 K by the gas equilibrium method using thermogravimetry. The measured data were about 200 kJ/mol higher in the O/M ratio range from 2.00 to 1.96 and about 50 kJ/mol higher in the reducing region (below O/M ratio = 1.94) compared with those of PuO2–x. The measured oxygen potentials were analyzed by the point defect chemistry method. From this analysis, the oxygen potential of (Pu0.928Am0.072)O2–x was represented as a function of O/M ratio and temperature and calculated results were consistent with the measured data.

Measurement of non-isothermal oxygen potentials in a Cu–Fe–Ca–S–O liquid during reduction of chalcopyrite in lime and carbon mixtures

The reduction of Indian chalcopyrite concentrates from the Ghatshila mines with carbon was studied in the presence of lime in the temperature range of 1123–1573 K. An attempt was made to determine the oxygen potential corresponding to the selective separation of two metals, copper and iron, during the reduction of chalcopyrite by carrying out electro-motive force (emf) measurements under isochronal heating rate conditions using an yttria-stabilised zirconia solid electrolyte cell with Cu/Cu2O as the reference electrode. The isochronal rate data for oxygen potential measurements were analysed by comparing them with the data from isothermal kinetics studies on the reduction of natural and synthetic chalcopyrite minerals in the above temperature range. The analyses of phases formed were carried out using the X-ray powder diffraction and scanning electron microscopic techniques, which showed the presence of a new quinary solid solution CIII phase. The solid solution phase CIII [Cu2S-CII(FeS⋅CaO)] forms as a result of mixing between the two rhombohedral lattices: high-temperature form of Cu2S and CII (FeO⋅CaS) compounds. The mechanism of reduction of chalcopyrite with carbon in the presence of lime is illustrated by the explanation of the role of the intermediate phases, CII, CIII and matte phases.

Newly developed EMF cell with zirconia solid electrolyte for measurement of low oxygen potentials in liquid Cu-Cr and Cu-Zr alloys

In order to measure the very low oxygen potential by use of stabilized zirconia solid electrolyte emf method, a new cell construction was devised. The idea was based on Janke but a zirconia rod was used instead of the zirconia crucible which contacts liquid alloy electrode. The cell was used for determination of the oxygen potentials in liquid dilute Cu-Cr and Cu-Zr alloys. The reference electrode was Cr,Cr2O3. Emf measurements were performed in the temperature range of 1400-1580K and composition range of 0.198-3.10at%Cr-Cu alloys, and 1380-1465K, 0.085-0.761at%Zr-Cu alloys. The composition of liquid alloys were determined by picking up from the liquid alloys and ICP analysis. By use of the newly devised cell construction in this study, stable emf values were obtained at each temperature and alloy composition. Emf values were corrected by using the parameter for electronic contribution of the YSZ. Activity of Cr obeys Henry?s law and activity coefficient at infinitely dilute alloys of Cr in Cu-Cr alloys are: lng0 Cr =(3.80 at 1423K), (3.57 at 1473K), (3.38 at 1523K) and (3.20 at 1573K). At 1423 K activity coefficient of Zr at infinitely diluted alloy is ln?o Zr = -4.0.

Thermochemical studies on the system Na–U–O

Measurement of sodium vapor pressures and oxygen potentials over the three-phase fields 〈NaUO 3〉–〈Na 2U 2O 7〉–〈Na 2UO 4〉, 〈Na 2U 2O 7〉–〈Na 6U 7O 24〉–〈NaUO 3〉 and 〈Na 6U 7O 24〉–〈NaUO 3〉–UO 2 are made in the present study. Vapor pressure of sodium was measured by using a transpiration technique and the oxygen potential was measured by using emf (electro motive force) measurements with oxide and fluoride solid electrolytes. From the vapor pressure measurements a mean value of −1496.7 kJ mol −1 was obtained for the Δ f H 〈 NaUO 3 〉 ° at 298 K. The Δ f G 〈 Na 6 U 7 O 24 〉 ° was determined by measuring the sodium potential and the oxygen potential over the phase field 〈Na 2U 2O 7〉–〈NaUO 3〉–〈Na 6U 7O 24〉. The Δ f G 〈 Na 6 U 7 O 24 〉 ° in the temperature range 774–1474 K was determined for the first time.

Measurement of oxygen potential of [formula omitted] at 1773 and 1873 K, and its analysis based on point defect chemistry

Oxygen potential, G¯O2, of (U0.8Pu0.2) O2±x was measured at 1773 and 1873 K by a gas equilibrium method using a thermogravimeter. The measurement data were analyzed based on point defect chemistry, and the partial Gibbs energies at stoichiometric composition were determined from the analysis results which were obtained by using the relationship between deviation of stoichiometric composition x and oxygen partial pressure PO2, x∝PO2±1/2, in the near stoichiometric region. The G¯O2 was represented as functions of the deviation x and temperature, being the calculation results in good agreement with experimental data. The G¯O2 at stoichiometric composition was described by the equation: G¯O2(J/mol)=−517.5×103+116.5⋅T.

Oxygen potential measurements of Cm0.09Pu0.91O2−x by EMF method

The dependence of the oxygen potentials on oxygen nonstoichiometry (x) and temperature of Cm0.09Pu0.91O2−x was measured by electromotive force (EMF) measurements. The galvanic cell type is shown: (Pt) air | Zr(Ca)O2−x | Cm0.09Pu0.91O2−x (Pt). The coulomb titration has been made for the sample at the intervals of 40 K between 1173 and 1333 K over the x range of 0.018 ≤ x ≤ 0.053. The oxygen potentials decreased gradually from −31.02 to −117.48 kJmol−1 with increasing x from 0.018 to 0.045, and more rapidly decreased from −117.48 to −283.74 kJmol−1 up to 0.053. The temperature dependence of the oxygen potentials was also measured between 1173 and 1333 K for several constant x values. The temperature dependences exhibited the smooth curves over the x and temperature ranges concerned. Systematic comparison of the oxygen potentials between Cm0.09Pu0.91O2−x and those published for CmO2 −x and Am-containing oxides was also made. The oxygen potential of Cm09Pu0.91O2−x at the equimolar ratio of Cm3+/Cm4+ was higher than those of AmyPu1−yO2−x at the equimolar ratio of Am3+/Am4+ by approximately 150 kJmol−1 when the Pu valence was assumed to be +4 in both oxides.

Oxygen potential measurements of Am 0.5Pu 0.5O 2−x by EMF method

The dependence of the oxygen potentials on oxygen non-stoichiometry and temperature of Am 0.5Pu 0.5O 2− x has been obtained by the electromotive force (EMF) method with the cell: (Pt) air |Zr(Ca)O 2− x | Am 0.5Pu 0.5O 2− x (Pt). The x value of Am 0.5Pu 0.5O 2− x was changed at 1333 K over 0.02 < x ⩽ 0.25 by the coulomb titration method. The temperature dependence of the oxygen potential was also measured over the range of 1173–1333 K. It was found that the oxygen potential decreased from −80 to −360 kJ mol −1 with increasing x from 0.021 to 0.22 at 1333 K and that it remained almost constant at −360 kJmol −1 around x = 0.23. It was concluded that Am 0.5Pu 0.5O 2− x should be composed of the single fluorite-type phase over 0.02 < x ⩽ 0.22 and the mixed phases of fluorite-type and (Am, Pu) 9O 16 at around x = 0.23.

Electrochemical Description of Redox Equilibria in Nuclear Glass

AbstractGlass matrices were selected to immobilize fission products because glass is capable of chemically incorporating a wide spectrum of elements within a single matrix. Some of these elements can be found at different oxidation states. The redox equilibrium constants of multivalent species can be used to develop thermodynamic models for a better description of nuclear glasses. Some of the multivalent species loaded in nuclear glass, such as iron and sulfur, have already been a subject of investigation by conventional glassmakers or geochemists in the earth sciences. Other redox species more specifically related to nuclear glass, including cerium and ruthenium, have also been investigated. These studies have demonstrated the advantages of using electrochemical techniques, voltammetry and potentiometry, to determine the equilibrium constants. Oxygen potential measurements are also particularly suitable for characterizing the redox state of the multivalent dissolved species in molten glass.

Oxygen Potential Measurement of Americium Oxide by Electromotive Force Method

The oxygen potentials of AmO2−x were measured in the x range of 0.01–0.5 and the temperature range of 1000–1333 K by the electromotive force method. The oxygen potentials at 1333 K were −19.83 kJ/mol for x=0.019 and −319.1 kJ/mol for x=0.485, which were higher than those of CeO2−x by approximately 200 kJ/mol for the corresponding x values. From the dependence of the oxygen potentials on x and temperature, a tentative phase diagram of Am–O system was proposed, which suggested the presence of the intermediate phases of Am7O12 and Am9O16 in the Am–O system.

Oxygen potential measurements of fluorite-type Zr 0.5Pu 0.5O 2− y by EMF method

The dependence of the oxygen potentials ( g(O 2)) on oxygen non-stoichiometry ( y) and temperature ( T) of oxygen-deficient fluorite-type (F-type) plutonium zirconate Zr 0.5Pu 0.5O 2− y has been elucidated by electromotive force (EMF) measurements: (Pt) air ∣Zr(Ca)O 2− x ∣ Zr 0.5Pu 0.5O 2− y (Pt). The y value of Zr 0.5Pu 0.5O 2− y was varied at 1078 K over 0 ⩽ y < 0.10 and the isothermal g(O 2) were measured by applying the coulomb titration method. The temperature dependence of the g(O 2) was also measured for each y by performing the temperature scan down to 793 K (793 ⩽ T ⩽ 1078 K). It was found that the g(O 2) decreased from ∼0 to −480 kJ/mol with increasing y from ∼0 to 0.1 at 1078 K and that the isothermal g(O 2) as a function of y exhibited a simple concave curve. The powder XRD results showed that Zr 0.5Pu 0.5O 2− y maintained its single F-type structure over 0 ⩽ y < 0.10 at 1078 K. The lattice parameters ( a 0) in this system were found to increase almost linearly with y over 0 ⩽ y < 0.10.

Oxygen potential measurements of pyrochlore-type Zr2M2O7+x (M= Pu, Ce) by EMF method

Pyrochlore-type (P-type) zirconia has been widely studied as a disposable form of the high-level radioactive nuclear waste. We have developed the EMF (electromotive force) measurement system using the cubic zirconia oxygen sensor, and investigated the relation between oxygen potential (g(O2)), oxygen composition (x) and temperature (T) for oxygen-excess pyrochlore Zr2Pu2O7+x between 798 K and 1078 K over 0.42<x<0.95. Similar experiments were also carried out for P-type Zr2Ce2O7+x. It was found that g(O2) of P-type Zr2Pu2O7+x. are significantly higher than those of oxygen deficient fluorite PuO2-x at the same values of the oxygen to metal molar ratio (O/M) or the molar ratio of Pu(Ce)3+ ions to the total Pu(Ce)3++Pu(Ce)4+ions (y).g(O2) of P-type Zr2Pu2O7+x. are about -150 kJ/mol lower than those of P-type Zr2Ce2O7+x at the same values of O/M or y, though the shape of g(O2) versus x curves resembles each other over the range of x investigated in this study. These results are consistent with the fact that g(O2) of PuO2-x are about -150 kJ/mol lower than those of CeO2-x at the same values of O/M or y, and their g(O2) curves resemble each other.

Measurement of oxygen potentials in Ag-Pb system employing oxygen sensor

Accurate and instantaneous analysis of dissolved nonmetallic or metallic species in molten metals at elevated temperatures using an online electrochemical sensor is important for continuous process control during metal refining and alloying operations in metallurgical industries. In this article, the application of long-life, solid-state electrochemical sensors for oxygen has been demonstrated to measure the oxygen potential as a function of the lead concentration in a molten Ag-Pb system at 1323 K. Yttria-stabilized zirconia (YSZ) in the form of one end-closed tube 20 mm long, 3 mm inner diameter, and 1 mm wall thickness has been used as a solid electrolyte in the oxygen sensor. Electromotive force (EMF) of the solid-state electrochemical cell (−)Pt, Ni-NiO//YSZ//OAg-Pb, Mo(+) has been measured at 1323 K. The reference electrode consisted of Ni + NiO biphasic mixture; the working electrode was composed of a molten Ag-Pb alloy of varying concentrations of lead. The concentrations of lead in silver ranged from 0.02 to 10.20 wt pct. Samples of the molten alloy were drawn after each addition of Pb to Ag and analyzed by induction coupled plasma (ICP).

Oxygen Potential in Molten Tin and Gibbs Energy of Formation of SnO2 Employing an Oxygen Sensor

On-line monitoring of the dissolved impurities in molten metals is important for better process control during alloying and metal refining operations. Disposable-type oxygen sensors are commercially available and are used in the iron and steel industry worldwide. However, there is a need for developing low-cost, reliable, long-life oxygen sensors for continuous monitoring of dissolved oxygen in molten metals. In this paper we present the results of the physical and electrical characterization of the solid electrolyte material ZrO2 (8 mol%Y2O3). The experimental results of the application of long-life solid-state electrochemical sensors designed using yttria-stabilized zirconia solid electrolyte for the measurement of oxygen potential in molten tin between 823 to 1273 K and standard Gibbs energy of formation of SnO2 from its elements are also reported.