arXiv subjects

Renkun Chen

Publications and source records attributed to Renkun Chen.

22 records · Page 2Linked to original sources

arXiv2020

From High-Entropy Ceramics to Compositionally-Complex Ceramics: A Case Study of Fluorite Oxides

Using fluorite oxides as an example, this study broadens high-entropy ceramics (HECs) to compositionally-complex ceramics (CCCs) or multi-principal cation ceramics (MPCCs) to include medium-entropy and/or non-equimolar compositions. Nine compositions of compositionally-complex fluorite oxides (CCFOs) with the general formula of (Hf1/3Zr1/3Ce1/3)1-x(Y1/2X1/2)xO2-delta (X = Yb, Ca, and Gd; x = 0.4, 0.148, and 0.058) are fabricated. The phase stability, mechanical properties, and thermal conductivities are measured. Compared with yttria-stabilized zirconia, these CCFOs exhibit increased cubic phase stability and reduced thermal conductivity, while retaining high Young's modulus (~210 GPa) and nanohardness (~18 GPa). Moreover, the temperature-dependent thermal conductivity in the non-equimolar CCFOs shows an amorphous-like behavior. In comparison with their equimolar high-entropy counterparts, the medium-entropy non-equimolar CCFOs exhibit even lower thermal conductivity (k) while maintaining high modulus (E), thereby achieving higher E/k ratios. These results suggest a new direction to achieve thermally-insulative yet stiff CCCs (MPCCs) via exploring non-equimolar and/or medium-entropy compositions.

cond-mat.mtrl-sci↗

arXiv2019

Size Disorder as a Descriptor for Predicting Reduced Thermal Conductivity in Medium- and High-Entropy Pyrochlores

High-entropy ceramics generally exhibit reduced thermal conductivity, but little is known about what controls this suppression and which descriptor can predict it. Herein, 18 medium- and high-entropy pyrochlores were synthesized to measure their thermal conductivity and Young's modulus. Up to 35% reductions in thermal conductivity were achieved with retained moduli, thereby attaining insulative yet stiff properties for potential thermal barrier coating applications. Notably, the measured thermal conductivity correlates well with a modified size disorder parameter. Thus, this modified size disorder parameter is suggested as a useful descriptor for designing thermally-insulative medium- and high-entropy ceramics (broadly defined as "compositionally-complex ceramics").

cond-mat.mtrl-sci↗

arXiv2016

Unusually High and Anisotropic Thermal Conductivity in Amorphous Silicon Nanostructures

Amorphous Si (a-Si) nanostructures are ubiquitous in numerous electronic and optoelectronic devices. Amorphous materials are considered to possess the lower limit to the thermal conductivity (k), which is ~1 W/m-K for a-Si. However, recent work suggested that k of micro-thick a-Si films can be greater than 3 W/m-K, which is contributed by propagating vibrational modes, referred to as "propagons". However, precise determination of k in a-Si has been elusive. Here, we used novel structures of a-Si nanotubes and suspended a-Si films that enabled precise in-plane k measurement within a wide thickness range of 5 nm to 1.7 um. We showed unexpectedly high in plane k in a-Si nanostructures, reaching ~3.0 and 5.3 W/m-K at 100 nm and 1.7 um, respectively. Furthermore, the measured in plane k is significantly higher than the cross-plane k on the same films. This usually high and anisotropic k in the amorphous Si nanostructures manifests the surprising broad propaganda mean free path distribution, which is found to range from 10 nm to 10 um, in the disordered and atomically isotropic structure. This result provides an unambiguous answer to the century-old problem regarding the mean free path distribution of propagons and also shed light on the design and performance of numerous a-Si based electronic and optoelectronics devices.

cond-mat.mtrl-sci↗

arXiv2016

Fluid-like Surface Layer and Its Flow Characteristics in Glassy Nanotubes

We observed strongly size-dependent viscoelasticity in amorphous SiO2 and Si nanotubes with shell thickness down to ~8 nm. A core-shell model shows that a ~1 nm thick fluid-like surface layer has a significant effect on the mechanical behavior of nanotubes and matches well with our experimental results. Surprising, the surface layer exhibits a room temperature viscosity equivalent to that of bulk glass above 1000 C. Additionally, a low activation energy extracted from temperature dependent creep tests indicates that the viscous flow in the surface layer is due to bond motion/switching, instead of bond breaking. These findings unambiguously show the presence of a fluid-like surface layer and elucidate its role on dynamic mechanical behavior in nanoscale inorganic glass.

cond-mat.mtrl-sci↗