9,247 research outputs found
Role of Metastable States in Phase Ordering Dynamics
We show that the rate of separation of two phases of different densities
(e.g. gas and solid) can be radically altered by the presence of a metastable
intermediate phase (e.g. liquid). Within a Cahn-Hilliard theory we study the
growth in one dimension of a solid droplet from a supersaturated gas. A moving
interface between solid and gas phases (say) can, for sufficient (transient)
supersaturation, unbind into two interfaces separated by a slab of metastable
liquid phase. We investigate the criteria for unbinding, and show that it may
strongly impede the growth of the solid phase.Comment: 4 pages, Latex, Revtex, epsf. Updated two reference
High Thermoelectric Figure of Merit by Resonant Dopant in Half-Heusler Alloys
Half-Heusler alloys have been one of the benchmark high temperature
thermoelectric materials owing to their thermal stability and promising figure
of merit ZT. Simonson et al. early showed that small amounts of vanadium doped
in Hf0.75Zr0.25NiSn enhanced the Seebeck coefficient and correlated the change
with the increased density of states near the Fermi level. We herein report a
systematic study on the role of vanadium (V), niobium (Nb), and tantalum (Ta)
as prospective resonant dopants in enhancing the ZT of n-type half-Heusler
alloys based on Hf0.6Zr0.4NiSn0.995Sb0.005. The V doping was found to increase
the Seebeck coefficient in the temperature range 300-1000 K, consistent with a
resonant doping scheme. In contrast, Nb and Ta act as normal n-type dopants, as
evident by the systematic decrease in electrical resistivity and Seebeck
coefficient. The combination of enhanced Seebeck coefficient due to the
presence of V resonant states and the reduced thermal conductivity has led to a
state-of-the-art ZT of 1.3 near 850 K in n-type
(Hf0.6Zr0.4)0.99V0.01NiSn0.995Sb0.005 alloys.Comment: Submitted to AIP Advance
Physical transformations between quantum states
Given two sets of quantum states {A_1, ..., A_k} and {B_1, ..., B_k},
represented as sets of density matrices, necessary and sufficient conditions
are obtained for the existence of a physical transformation T, represented as a
trace-preserving completely positive map, such that T(A_i) = B_i for i = 1,
..., k. General completely positive maps without the trace-preserving
requirement, and unital completely positive maps transforming the states are
also considered
Recent Advances in Nanostructured Thermoelectric Half-Heusler Compounds
Half-Heusler (HH) alloys have attracted considerable interest as promising
thermoelectric (TE) materials in the temperature range around 700 K and above,
which is close to the temperature range of most industrial waste heat sources.
The past few years have seen nanostructuing play an important role in
significantly enhancing the TE performance of several HH alloys. In this
article, we briefly review the recent progress and advances in these HH
nanocomposites. We begin by presenting the structure of HH alloys and the
different strategies that have been utilized for improving the TE properties of
HH alloys. Next, we review the details of HH nanocomposites as obtained by
different techniques. Finally, the review closes by highlighting several
promising strategies for further research directions in these very promising TE
materials.Comment: 34 pages, 22 figure
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