•Methods for producing the highest reported dopant concentration of Sr to date are discussed•The highest Sr dopant concentration in LaPO4 was achieved•The solid solution stability and the ionic conductivity as a function of temperature and atmosphere are establishedMonazite-type LnPO4 is a stable phase for many of the larger rare earths. The unusually asymmetric 9-fold coordinated La3 + sites can be substituted by other large ions including aliovalents such as Sr2 +. In the case of divalent ions, “charge balance” can be maintained by substituted monovalent anionic units such as (OH)−. The solid solution series has the chemical formula La1 − xSrxPO4 − x(OH)x, which may exist without defects such as vacancies as long as sufficient water is present. X up to as high as 0.3 is found, much higher than previously reported, when using a direct precipitation process in hot, strong phosphoric acid. Physical properties of Sr-doped LaPO4 up to that level, including proton transport, have been measured. At high temperatures, (> 400 °C) proton ionic conductivity in the bulk is expected to be high, but the structure becomes unstable. As (OH)− is given off, Sr also leaves the structure and forms an intergranular phase with phosphorus, a process that detrimentally affects the ionic conductivity and cannot be suppressed even when conducting measurements in water vapor that should encourage retaining (OH)− in the structure.

Improved thermal shock resistance for cubic 8 mol% yttria-stabilized zirconia (8YSZ) used in fuel cells and oxygen sensors can be achieved by the addition of higher thermal conductivity second phases. This work compares 10–20 vol% alumina (α-Al2O3) and mullite (3Al2O3·2SiO2) additions that increase thermal conductivity, reduce grain size, and increase strength and fracture toughness of 8YSZ. Improvements in thermal shock behavior correlate best with increased thermal conductivity. Second phase additions result in a smaller grain size that reduces the ionic conductivity, measured by electrochemical impedance spectroscopy, primarily through the creation of a higher density of blocking grain boundaries. The blocking effect correlates with decreasing grain size in 8YSZ but also is strongly influenced by the wetting behavior and distribution of intergranular phases. The addition of an appropriate dilute second phase of higher thermal conductivity, however, may compensate for a slightly lower ionic conductivity in certain applications such as oxygen sensors.

•Two multiphase ceramic composites were fabricated using UO2 surrogates.•Thermal behavior of composites were studied from 373 to 1273 K (100–1000 °C).•Simulations were performed using microstructural and single-phase information.•Incorporation of UO2 into simulations provided information on theoretical fuels.•Computational approach has potential for evaluation of theoretical fuel systems.This study investigates the temperature dependent thermal conductivity of multiphase ceramic composites for simulated inert matrix nuclear fuel. Fine grained composites were made of CeO2–MgAl2O4–CeMgAl11O19 or 3Y-TZP–Al2O3–MgAl2O4–LaPO4. CeO2 and 3Y-TZP are used as UO2 surrogates due to their similar structures and low thermal conductivities. Laser flash analysis from room temperature to 1273 K (1000 °C) was used to determine the temperature dependent thermal conductivity. A computational approach using Object Oriented Finite Element Analysis Version 2 (OOF2) was employed to simulate the composite thermal conductivity based on the microstructure. Observed discrepancies between experimental and simulated thermal conductivities at low temperature may be due to Kapitza resistance; however, there is less than 3% deviation between models and experiments above 673 K (400 °C) for both compositions. When the surrogate phase was replaced with UO2 in the computational model for the four-phase composite, a 12–16% increase in thermal conductivity resulted compared to single phase UO2, in the range of 673–1273 K (400–1000 °C). This computational approach may be potentially viable for the high-throughput evaluation of composite systems and the strategic selection of inert phases without extensive sample fabrication during the initial development stages of composite nuclear fuel design.Graphical abstract

Four-phase ceramic composites containing 3 mol% Y2O3 stabilized ZrO2 (3Y-TZP), Al2O3, MgAl2O4, and LaPO4 were synthesized as model materials representing inert matrix fuel with enhanced thermal conductivity and decreased radiation-induced microstructural damage with respect to single-phase UO2. This multi-phase concept, if successful, could be applied to design advanced nuclear fuels which could then be irradiated to higher burn-ups. 3Y-TZP in the composite represents a host (fuel) phase with the lowest thermal conductivity and Al2O3 is the high thermal conductivity phase. The role of MgAl2O4 and LaPO4 was to stabilize the structure under irradiation. The radiation response was evaluated by ion irradiation at 500 °C with 10 MeV Au ions and at 800 °C with 92 MeV Xe ions, to simulate damage due to primary knock-on atoms and fission fragments, respectively. Radiation damage and microstructural changes were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy and computational modeling. Al2O3, Y2O3 stabilized ZrO2 and MgAl2O4 phases exhibit high amorphization resistance and remain stable when irradiated with both Au and Xe ions. A monoclinic-to-tetragonal phase transformation, however, is promoted by Xe and Au ion irradiation in 3Y-TZP. The LaPO4 monazite phase appears to melt, dewet the other phases, and recrystallize under Au irradiation, but does not change under Xe irradiation.Graphical abstract

This paper compares the accuracy of conventional dynamic light scattering (DLS) and atomic force microscopy (AFM) for characterizing size distributions of polystyrene nanoparticles in the size range of 20–100 nm. Average DLS values for monosize dispersed particles are slightly higher than the nominal values whereas AFM values were slightly lower than nominal values. Bimodal distributions were easily identified with AFM, but DLS results were skewed toward larger particles. AFM characterization of nanoparticles using automated analysis software provides an accurate and rapid analysis for nanoparticle characterization and has advantages over DLS for non-monodispersed solutions.