Joshua White

Nickel ferrite (NiFe₂O₄) is a major constituent of the corrosion deposits formed on the exterior of nuclear fuel cladding tubes during operation. NiFe₂O₄ has attracted much recent interest, mainly due to the impact of these deposits, known as CRUD, on the operation of commercial nuclear reactors. Although advances have been made in modeling CRUD nucleation and growth under a wide range of conditions, the thermophysical properties of NiFe₂O₄ at high temperatures have only been approximated, thereby limiting the accuracy of such models. In this study, samples of NiFe₂O₄ were synthesized to provide the thermal diffusivity, specific heat capacity, and thermal expansion data from room temperature to 1300 K. These results were then used to determine thermal conductivity. Numerical fits are provided to facilitate ongoing modeling efforts. The Curie temperature determined through these measurements was in slight disagreement with literature values. Transmission electron microscopy investigation of multiple NiFe₂O₄ samples revealed that minor nonstoichiometry was likely responsible for variations in the Curie temperature. However, these small changes in composition did not impact the thermal conductivity of NiFe₂O₄, and thus are not expected to play a large role in governing reactor performance.

The internal reduction of Ni²⁺ ions dissolved in 10YSZ (10 percent by mole Y₂O₃‐ stabilized ZrO₂) was studied using magnetometry. Chemical methods were used to produce specimens with varying amounts of NiO (0.01–11.8% by mol), below and above the solubility limit of NiO in 10YSZ. The solubility limit was determined using x‐ray diffraction measurements in which the change in lattice parameter was correlated to the amount of Ni²⁺ ions in solution. Vibrating sample magnetometry (VSM) was used to unequivocally establish that NiO does not exist as a second phase for specimen compositions less than or equal to 0.5 m/o NiO. Specimens with compositions 0.5, 0.1, and 0.01 m/o NiO in YSZ were then prepared and heat treated in Ar – 2% H₂ at 1273 K for times ranging from 1 to 20 h. The ferromagnetic response of the reduced specimens was measured using SQUID magnetometry at 100 K to quantify the amount of Ni metal formed as a function of time. SQUID magnetometry measurements of the same specimens were performed at 5 K where the paramagnetic response may be used to quantify the decrease of Ni²⁺ ions during reduction. The two SQUID measurements agree well and revealed parabolic growth law kinetics. Transmission electron microscopy revealed that internal reduction in these specimens proceeds by the formation of Ni⁰ along the YSZ grain boundaries. The results indicate that there are two stages of internal reduction in polycrystalline NiO‐containing YSZ. For short times, precipitation of Ni occurs at grain‐boundary regions, and for longer times, it occurs inside the grains.

The influence of NiO on the stabilization and microstructure of yittria (Y₂O₃)‐stabilized zirconia (ZrO₂) (YSZ) is examined. Cold‐pressed powders comprising varying amounts of NiO, 8 mol%Y₂O₃, and ZrO₂ were sintered at 1500°C for 4 h. Specimens were subsequently given a 100 h heat treatment at 1500°C. Phase analysis by X‐ray diffraction revealed that the presence of NiO leads to a greater amount of cubic phase ZrO₂ for the sintered specimens compared with the control specimens. The cubic ZrO₂ lattice parameter was significantly smaller for specimens containing NiO, revealing that Ni²⁺ ions likely enter the cubic ZrO₂ lattice and play a role in decreasing the time and temperature required for stabilization of the cubic phase. A spherical diffusion model was used to estimate the diffusion of Y³⁺ and Ni²⁺ into ZrO₂. These results are discussed in the context of the role of NiO in the synthesis of YSZ.