Fredrik Grønvold

High-temperature X-ray study of uranium oxides in the UO2U3O8 region

Abstract Uranium oxides in the UO2U3O8 range have been studied by the X-ray powder method at temperatures between 20° and 969°C after annealing at 1000°C. The following phases were found: 1. 1. UO2+x with no detectable range of homogeneity at room temperature. The homogeneity range extends to about UO2·17 at 950°C. The lattice constant of UO2 is a = 5·4704 A at 20°C, and the average linear thermal coefficient of expansion α is 10·8 . 10−6 °C−1 between 20° and 946°C. The observed density of UO2·00 is 10·793 g.cm−3 at 25°C. 2. 2. UO2·25 or U4O9 with a narrow range of homogeneity. The lattice constant decreases from a = 5·4411 A at 20°C to 5·4397 A at 86°C. From then on it increases linearly to 960°C with α = 11·6 . 10−6 °C−1. The high density observed, 11·159 g.cm−3, shows that oxygen atoms are taken up interstitially in the UO2-like structure. 3. 3. U3O8−x with a range of homogeneity between UO2·62 and UO2·667 at room temperature. The lattice constants vary between these limits; UO 2·633 : a = 6·735 A , b = 3·966 A , c = 4·144 A . UO 2·667 : a = 6·720 A , b = 3·983 A , c = 4·146 A . The observed density of UO2·633 is 8·408 g.cm−3, and of UO2·667, 8·378 g.cm−3. The homogeneity range of the U3O8−x phase extends to about UO2·55 at 500° and 750°C.

Magnetite (Fe3O4) Heat capacity and thermodynamic properties from 5 to 350 K, low-temperature transition

Abstract The heat capacity of synthetic Fe3O4 has been measured over the range from 5 to 350 K. Values of thermodynamic functions have been calculated and Cp, So, and (H o − H 0 o ) T at 298.15 K are 36.04, 34.93, and 19.85 cal mol−1 K−1, respectively. In this sample the previously known λ-type transition was found to consist of two maxima, a larger one at 118.9 K and a smaller one at 113.3 K, with entropy increments of 1.2 cal mol−1 K−1 and 0.2 cal mol−1 K−1, respectively. The mechanism of the transitions is discussed.

Thermodynamics of the UO2+x phase I. Heat capacities of UO2.017 and UO2.254 from 300 to 1000 K and electronic contributions

Abstract Heat capacities of UO 2.017 and UO 2.254 have been measured in the range 300 to 1000 K by adiabatic calorimetry. The enthalpy and entropy increments { X o (1000 K) − X o (0)} are 66.49 kJ mol −1 and 170.2 J K −1 mol −1 for UO 2.017 , and 72.91 kJ mol −1 and 185.7 J K −1 mol −1 for UO 2.254 . The increments { X o (298 K) − X o (0)} are based upon literature data. In UO 2.017 the presence of the U 4 O 9 phase causes slightly variable heat capacities in the range 300 to 360 K due to a partial suppression of the transition in U 4 O 9 in the super-saturated solid solution of U 4 O 9 in UO 2 . Augmented heat capacity values are observed in the range 500 to 800 K, due to the solution of U 4 O 9 in UO 2 . In UO 2.254 a λ-type transition is present with maximum at 348 K and an entropy increment of 2.18 J K −1 mol −1 , presumably connected with interchange of interstitial and displaced oxygen atoms. A small irregularity in the heat capacity is observed in the region 900 to 950 K with an entropy increment of 0.15 J K −1 mol −1 . It is superimposed on a general increase in the heat capacity, presumably due to the onset of a transition. Attempts to resolve the various contributions to the heat capacity of UO 2 show that the electronic contribution is large. It is interpreted in terms of a doubly degenerate level of the 5f 2 electrons about 900 cm −1 above the triply degenerate ground level followed by a triply degenerate level about 1600 cm −1 above, and a singly degenerate level about 2900 cm −1 , above the ground level. Further excited levels are also considered, but they are apparently not numerous enough to explain the enhanced heat capacity above 2000 K. In the region 2000 to 3000 K structural order-disorder processes, vacancy formation, and phase separation from the stoichiometric dioxide probably occur. The results for UO 2.254 also indicate a large electronic contribution. It is tentatively analyzed assuming the presence of U(IV) and U(V) in equal amounts, corresponding to the formula 2UO 2 ·U 2 O 5 . If the contribution from U(IV) remains unchanged above 300 K, the electronic entropy of U(V) is about 7.3 J K −1 mol −1 for 1 2 UO 2 . 1 4 U 2 O 5 at 600 K. This indicates that at least six levels of the 5f 1 configuration are of importance in the region up to 600 K. The possibility of a doubly degenerate ground level followed by a quadruply degenerate level about 500 cm −1 above it is discussed in terms of available ligand field data for U(V).

Heat capacity and thermodynamic properties of synthetic magnetite (Fe3O4) from 300 to 1050 K. Ferrimagnetic transition and zero-point entropy

The heat capacity of synthetic Fe3O4 has been measured in the range 300 to 1050 K. The λ-type transition, related to the change from ferri- to paramagnetism in the compound, is delineated and a maximum heat capacity of about 340 J K−1 mol−1 is observed at 848.5 K. Values of various thermodynamic functions have been derived; Cp(1000 K), {Ho(1000 K)−Ho(298.15 K)}, and {So(1000 K)−So(298.15 K)} are 212.4 J K−1 mol−1, 149.80 kJ mol−1, and 246.2 J K−1 mol−1, respectively. The non-cooperative heat capacity has been estimated and the excess thermodynamic properties have been evaluated. Combination of available data for the reaction: 3Fe(s)+4H2O(g) = Fe3O4(s)+4H2(g), indicates that zero-point entropy of about 4 J K−1 mol−1 might be present in Fe3O4. This is in agreement with theoretical predictions of 3.4 J K−1 mol−1.

Critical assessment of the enthalpy of fusion of metals used as enthalpy standards at moderate to high temperatures

Enthalpy of fusion values for metals used as enthalpy standards are critically assessed. Recent developments of high temperature DSCs imply that potential standards for higher temperature use must also be considered. The fusion values for Ga, In, Sn, Cd, Bi, Pb, Zn, Sb, Al, Ag, Au, Cu, Ni and Co are treated here. We are hence covering materials for use from ambient temperature and up to 1768 K. A detailed review of the enthalpy of fusion determinations reported for each individual metal is presented in terms of sample quality and the methodology used. The accompanying evaluation leads to recommended values for the enthalpy of fusion of the metals and estimates of the uncertainty of the mean. The statistical method used is discussed in some detail. In reaching the recommended values for the enthalpy of fusion of the metals, the uncertainty of each individual determination has been estimated. Arguments regarding the effects of sample quality, intrinsic defects, and thermal treatment to aid in the assignment of uncertainties as well as a short review of the main calorimetric techniques used are presented.

Thermodynamic properties and phase transitions of salt hydrates between 270 and 400 K I. NH4Al(SO4)2 · 12H2O, KAl(SO4)2 · 12H2O, Al2(SO4)3 · 17H2O, ZnSO4 · 7H2O, Na2SO4 · 10H2O, and Na2S2O3 · 5H2O☆

Abstract Heat capacities and phase-transition enthalpies have been measured for five sulphate hydrates and sodium thiosulphate pentahydrate by adiabatic-shield calorimetry between 270 and 400 K. Enthalpies and entropies have been derived for the exact stoichiometries and tabulated for selected temperatures. The enthalpy of fusion of NH4Al(SO4)2 · 12H2O at 367.13 K is (122.0 ± 0.9) kJ · mol−1. The enthalpy of transition of KAl(SO4)2 · 12H2O to KAl(SO4)2 · 3H2O plus aqueous solution at 358.99 K is (95.2 ± 1.2) kJ · mol−1. Including the enthalpy of dissolution of 3-hydrate, the total enthalpy of transition of KAl(SO4)2 · 12H2O to an aqueous solution in the range 358.99 to 386 K is (112.7 ± 1.5) kJ · mol−1. Al2(SO4)3 · 17H2O melted in the range 350 to 381 K with enthalpy of fusion (120.5 ± 1.0) kJ · mol−1. The gradual onset of the phase transition is probably due to release of zeolitically bound water. The enthalpy of transition of ZnSO4 · 7H2O to ZnSO4 · 6H2O plus aqueous solution at 311.3 K is (16.5 ± 0.5) kJ · mol−1. The temperature of the peritectic transition of Na2SO4 · 10H2O to Na2SO4 plus aqueous solution is (305.533 ± 0.002) K, and the phase-transition enthalpy (78.04 ± 0.3) kJ · mol−1. The enthalpy of transition of Na2S2O3 · 5H2O to Na2S2O3 · 2H2O plus aqueous solution at 321.31 K is (48.8 ± 0.6) kJ · mol−1. Including the enthalpy of dissolution of 2-hydrate, the total transition enthalpy of Na2S2O3 · 5H2O to an aqueous solution in the range 321 to 335 K is (51.8 ± 0.4) kJ · mol−1. Sulphate and thiosulphate hydrates may be of interest as energy-storage materials because of their high enthalpies of fusion, if supercooling and segregation of unwanted solid phases can be prevented.

Triuranium heptaoxides: Heat capacities and thermodynamic properties of α- and β-U3O7 from 5 to 350°K☆

Abstract Low temperature heat capacities have been measured by adiabatic calorimetry on two phases with composition UO2.33 designated α- and β-U3O7. They were obtained by oxidation of UO2 at 135 and 165°C, respectively. β-U3O7 was subsequently heat treated at 225°C. Both substances possess UO2-like structures, apparently tetragonally deformed, with c/a = 0.986 for the face-centered uranium lattice of α-U3O7 and c/a = 1.031 for that of β-U3O7. Both have normal and almost equal heat capacities over the measured range, except for a small lambda-type anomaly at 30.5°K in α-U3O7. At 298.15°K the values of the practical entropy, S0, and the free energy function, −(F0−H00)/T, are 19.73 and 9.66 cal gfw−1°K−1 for α-UO2.333, and 19.96 and 9.77 cal gfw1°K−1 for β-UO2.333, respectively. These new data are correlated with structural and magnetic properties and thermodynamic data for other uranium oxides.

Heat capacity and thermodynamic properties of nearly stoichiometric wustite from 13 to 450 K

Abstract The heat capacity of a three-phase sample of Fe0.990±0.005O, Fe3O4, and Fe (mole fractions 0.915, 0.078, and 0.007, respectively) has been measured by adiabatic shield calorimetry at temperatures from 13 to 450 K. Fe0.99O and magnetite are formed as a metastable intermediate on heating of a quenched nonstoichiometric wüstite with composition Fe0.9374O. The small amount of Fe present stems from the second disproportionation reaction, in which the stable two-phase mixture of Fe and magnetite is formed from Fe0.99O. The value of ΔSm (Fe0.99O, 298.15 K), 60.45 J/(K·mol), is derived from the entropy of the three- phase sample and recommended standard entropies of Fe and magnetite. The character of the magnetic order-disorder transition changes with composition and is strongly cooperative in Fe0.99O, with Tn ≈ 191 K. A minor, seemingly higher order transition is observed at ~ 124 K. It is caused by the Verwey transition in the magnetite. This magnetite, formed in metastable equilibrium with Fe0.99O, is presumably more Fe-rich than magnetite in stable equilibrium with Fe.

Heat capacities and thermodynamic properties of hexagonal and liquid selenium in the range 298 to 1000 K. Enthalpy and temperature of fusion

The heat capacity of selenium has been determined by adiabatic-shield calorimetry in the range 298 to 1000 K. Values of thermodynamic functions have been calculated and Cp (1000 K), {H°(1000 K) − H°(298.15 K)}, and {S°(1000 K) − S°(298.15 K)} are 35.62 JK−1 mol−1, 28.593 kJ mol−1, and 49.81 JK−1 mol−1, respectively. Hexagonal selenium melts at (494.33 ± 0.02) K (IPTS-68) and the enthalpy of fusion is (6159±4) J mol−1.

Nearly stoichiometric iron monoxide formed as a metastable intermediate in a two-stage disproportionation of quenched wüstite. Thermodynamic and kinetic aspects☆

Quenched metastable wustites are shown to undergo a two-stage disproportionation reaction on heating. A mixture of nearly stoichiometric iron monoxide and magnetite is formed during the first stage, which takes place at ≈ 470 K. The resulting nearly stoichiometric Fe1−yO remains metastable up to ≈ 530 K. Above this temperature the stable two-phase mixture of iron and magnetite is slowly formed. The thermodynamic and kinetic aspects of this two-stage disproportionation reaction have been studied in a step-wise heated adiabatic calorimeter. The decomposition behaviour is rationalized in a Gibbs energy of formation representation of stable and metastable phases in the iron-oxygen system. The antiferromagnetic to paramagnetic order-disorder transition which takes place in Fe1−yO at ≈ 196 K is found to be greatly influenced by the oxygen content; it becomes much more cooperative as the exact 1:1 stoichiometry is approached.