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Low‐thermal conductivity ceramics in monazite‐type REPO4 (RE=La, Ce, Nd, Sm, Eu, Gd) ceramics are expected to have potential in structural (refractories, thermal insulator) and nuclear applications. To this end, the present study determines their thermal conductivities and examines how differences of the rare earth ions change their thermal conductivity at different temperatures. The results show that their conductivities are remarkably low from 25° to 1000°C. In addition, different conductivity variation mechanisms exist that change gradually upon altering from LaPO4 to GdPO4 at low and high temperatures. At relatively lower temperatures (≤400°C), the thermal conductivities of all the REPO4 ceramics decrease nearly at first, reach a minimum value, and then rise with gradual altering from LaPO4 to GdPO4. It may be due to the combined effects of the increase of both the anharmonicities in lattice vibrations and the bond strength. As the temperature increases, the conductivity trends become obscure, and the conductivities of the monazite‐type REPO4 approach their minimum thermal conductivities when the temperature is above 800°C.

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Low‐thermal‐conductivity rare earth zirconates (Re2Zr2O7) have recently been identified as promising thermal barrier coating materials. We observed an order–disorder transition in the (Sm1−xYbx)2Zr2O7 series with the changing x value and investigated the thermal conductivity variation. Structural analysis by X‐ray diffraction and Raman spectroscopy shows that the (Sm1−xYbx)2Zr2O7 series undergo a discontinuous phase transition from an ordered pyrochlore phase to a disordered fluorite one between the x=1/6 and x=1/3 compositions. Meanwhile, both of the sound velocity and Young's modulus reveal a dramatic reduction, indicating the lattice softening accompanying the order–disorder transition. The thermal conductivities of the (Sm1−xYbx)2Zr2O7 series are different from the conventional behavior of a simple alloying system and show a minimum thermal conductivity value at the transition composition (Sm2/3Yb1/3)2Zr2O7, which possibly arises from the enhanced phonon scattering due to the lattice softening.

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LaPO4 monazite, as an excellent structural ceramic, is expected to have thermal conductivity anisotropy because of its asymmetric crystal structure (monoclinic). For this motivation, the LaPO4 samples with different degrees of texture are fabricated via varying the powder synthesis processes and using hot‐pressing sintering and spark plasma sintering technique. The effect of the texture on the thermal conductivity of the LaPO4 samples is subsequently explored in this study. The results reveal that the thermal conductivities of the LaPO4 samples decrease firstly with the increasing degree of preferred orientation of the (200) planes along the direction perpendicular to the pressing direction, and approach a particular value, which is possibly the conductivity of the LaPO4 single crystal along the direction perpendicular to the (200) planes (a‐axis direction), when the orientation achieves a certain degree. The longitudinal or transverse speeds of all the textured LaPO4 samples are also measured and used to clarify the thermal conductivity anisotropy.

LaPO 4 /Al 2 O 3 composites were fabricated by spark plasma sintering. The effects of LaPO 4 contents on the mechanical properties of the composites were investigated. The bending strength and fracture toughness can reach the maximum value of 568.2730 MPa and 4.870.5 MPa Á m 1/2 for the composite with 16.4 vol% LaPO 4 addition, respectively. The elastic moduli and hardness of the composites decreased with increasing LaPO 4 content. Furthermore, the experimental results show that the composites can be machined by a tungsten carbide drill as the LaPO 4 volume fraction is higher than 34.4 vol%.

Due to the complex products and irradiation-induced defects, it is hard to understand and even predict the thermal conductivity variation of materials under fast neutron irradiation, such as the abrupt degradation of thermal conductivity of boron carbide (B4C) at the very beginning of the irradiation process. In this work, the contributions of various irradiation-induced defects in B4C primarily consisting of the substitutional defects, Frenkel defect pairs, and helium bubbles were re-evaluated separately and quantitatively in terms of the phonon scattering theory. A theoretical model with an overall consideration of the contributions of all these irradiation-induced defects was proposed without any adjustable parameters, and validated to predict the thermal conductivity variation under irradiation based on the experimental data of the unirradiated, irradiated, and annealed B4C samples. The predicted thermal conductivities by this model show a good agreement with the experimental data after irradiation. The calculation results and theoretical analysis in light of the experimental data demonstrate that the substitutional defects of boron atoms by lithium atoms, and the Frenkel defect pairs due to the collisions with the fast neutrons, rather than the helium bubbles with strain fields surrounding them, play determining roles in the abrupt degradation of thermal conductivity with burnup.