Fully dense YMoO/Al composites were prepared by squeeze-casting. Relatively mild conditions of 750 °C/20 min/50 MPa were used in order to avoid reaction of the components. SEM, Raman spectroscopy, XRD and dilatometry were used to characterize the microstructures and morphologies of the composites. Zero thermal expansion was achieved in the temperature range where the thermal mismatch strain was zero. We show that the CTE mismatch of Al and YMoO results in compressive and tensile strains that distort the YMoO lattice. We establish a novel method to measure the negative thermal expansion (NTE) materials' CTE under strain by measuring the composites' CTE and calculating the thermal mismatch strain between the NTE ceramic and the metal matrix. The relationship between thermal strain and Raman shift is established and measured and the simulated results are in good agreement. We also find YMoO to have a positive CTE when the surface strain is ≥0.80 × 10%.

The urgent need for high-performance solid electrolytes has aroused considerable focus on NASICON ceramics. Optimization of processing routes to dense, defect-free materials has yet to receive sufficient attention to date. Although traditional solid-state reaction methods followed by repetitive ball milling and sintering up to 10 h above 1200 °C are common place, the resulting average particle sizes are usually too large to produce dense, robust structures because of excessive grain growth. In this study, nanopowders (NPs) are produced, which offer a superior opportunity to make dense, high-phasepurity sintered bodies. Here, we report on the effect of sintering conditions on the microstructures and phase of Ce 4+ -substituted NASICON samples, Na 3 Ce x Zr 2−x Si 2 PO 12 (x = 0, 0.1, 0.2, 0.3). NPs permit processing fine-grained solid-state electrolytes with 98% relative density at 1100 °C/5 h. In addition, Rietveld refinement was applied to evaluate 3-D Na-ion diffusion channels among different NASICON samples. Also, it is found that adding 5 at % Ce 4+ does not change the phase structure but dramatically enlarges the Na + diffusion "bottleneck" from 5.4 to 5.6 Å 2 . This may be one reason for these samples to exhibit conductivities of 2.4 × 10 −2 S cm −1 at 140 °C.

No abstract