Processing-thermal conductivity relationships in MGO-pyrochlore composite inert matrix materials

Inert matrix (IM) materials are proposed to act as non-fertile matrices to burn excess plutonium and minor actinides in nuclear reactors. MgO is a good IM candidate because of its high thermal conductivity, good radiation resistance, and high temperature stability, but its hot water corrosion resistance is poor limiting its use in light water reactors. A composite approach has been suggested to improve the hydration resistance of the MgO by adding a pyrochlore phase to act as a hydration barrier while maximizing the effective thermal conductivity of the composite. In this work, MgO-Nd 2Zr2O7 composites are fabricated using four different processing methods to deliberately vary the microstructure thus enabling the investigation of processing-microstructure-thermal conductivity relationships in the composites.;The first processing-microstructure-property relationship that is developed is the effect of the composite processing method on the sample-to-sample variation in the thermal diffusivity. The processing method affects the formation of agglomerates in the mixed composite powders, and these agglomerates are the source of MgO and Nd2Zr2O7 heterogeneities in the sintered composites. Differential sintering occurs in some of the agglomerates, resulting in the formation of circumferential cracks between the heterogeneity and the matrix. The presence of the circumferential cracks cause sample-to-sample variations of up to +/- 2 Wm-1K-1 in the thermal conductivity between composites fabricated from the same batch of mixed composite powder. This variation makes it more difficult to accurately and reliably predict the thermal conductivity of the composites.;The second processing-microstructure-property relationship developed describes the effect of the contiguity of the MgO on the average thermal conductivity of the composites. The processing method is found to affect the contiguity of the MgO in the composites. Lower MgO contiguity values cause the average thermal conductivity to decrease from 23 Wm-1K-1 to 20 Wm-1K-1 at 373 K. However, the thermal conductivity of the composites is found to be independent of microstructure after ∼1000 K. The thermal conductivity of the 70 vol% MgO-30 vol% Nd 2Zr2O7 composites is ∼7 Wm-1K -1 at 1273 K, which is virtually the same thermal conductivity as pure MgO at 1273 K.;Quantification of the amount of batch-to-batch variation in the thermal conductivity is investigated on one of the four composite processing methods since the processing method is found to affect the sample-to-sample variation between composites from the same batch of powder. Ball milling produces the most consistent microstructures with the highest average thermal conductivity, therefore two additional batches of ball milled composite powder are synthesized and composites are fabricated. Composite characterization shows that there is little variation between the microstructures of the composites fabricated from each batch of the ball milled composite powder, resulting in a combined average thermal conductivity of 24.0 +/- 0.6 Wm-1K -1 at 373 K and 6.8 +/- 0.2 Wm-1K-1 at 1273 K. Thus, ball milling is shown to produce IM composites with a consistent and predictable thermal conductivity.