56 research outputs found
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.Comment: 18 pages, 4 figure
Preface - focus issue on thermoelectric materials & devices: Phonon engineering, advanced materials and thermal transport
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.Comment: 39 pages; 8 + 5 figures; Accepted for publications in Journal of the
European Ceramic Society (1/7/2020
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