6 research outputs found
Improved Dehydrogenation Properties of Ti-Doped LiAlH<sub>4</sub>: Role of Ti Precursors
The dehydrogenation properties of LiAlH<sub>4</sub> doped
with
different Ti precursors (Ti, TiO<sub>2</sub>, and TiCl<sub>3</sub>) via ball milling are investigated. The results not only show significant
decreases in the decomposition temperatures (<i>T</i><sub>dec</sub>) and activation energies (<i>E</i><sub>A</sub>) of the first two dehydrogenation reaction steps of LiAlH<sub>4</sub> by doping with TiO<sub>2</sub> or TiCl<sub>3</sub>, but also reveal
how each Ti precursor affects the dehydrogenation process. Although
doping LiAlH<sub>4</sub> with TiCl<sub>3</sub> induced the largest
decrease in <i>T</i><sub>dec</sub> and <i>E</i><sub>A</sub>, TiO<sub>2</sub>-doped LiAlH<sub>4</sub> produced a
decrease in <i>T</i><sub>dec</sub> and <i>E</i><sub>A</sub> that is quite close to the TiCl<sub>3</sub>-doped sample
as well as superior short-term stability, suggesting that doping with
TiO<sub>2</sub> has certain advantages over doping with TiCl<sub>3</sub>. Further, the underlying mechanisms associated with the Ti precursors
during the dehydrogenation reaction of LiAlH<sub>4</sub> have been
studied using quasi in situ X-ray photoelectron spectroscopy. The
results reveal that the Ti<sup>4+</sup> and Ti<sup>3+</sup> reduction
processes and the segregation of Li cations to the surface of LiAlH<sub>4</sub> during ball milling play critical roles in the improved dehydrogenation
properties observed
Effects of Titanium-Containing Additives on the Dehydrogenation Properties of LiAlH<sub>4</sub>: A Computational and Experimental Study
Metal hydrides are attractive materials for use in thermal
storage
systems to manage excessive transient heat loads and for hydrogen
storage applications. This paper presents a combined computational
and experimental investigation of the influence of Ti, TiO<sub>2</sub>, and TiCl<sub>3</sub> additives on the dehydrogenation properties
of milled LiAlH<sub>4</sub>. Density functional theory (DFT) is used
to predict the effect of Ti-containing additives on the electronic
structure of the region surrounding the additive after its adsorption
on the LiAlH<sub>4</sub> (010) surface. The electron distribution
and charge transfer within the LiAlH<sub>4</sub>/additive system is
evaluated. Electronic structure calculations predict covalent-like
bonding between the Ti atom of the adsorbate and surrounding H atoms.
The hydrogen (H) binding energy associated with the removal of the
first H from the modified LiAlH<sub>4</sub> surface is calculated
and compared with experimental dehydration activation energies. It
is seen that the weaker H binding corresponds to the larger amount
of charge transferred from the Ti atom to adjacent H atoms. A reduction
in charge transfer between the Al atom and surrounding H atoms is
also observed when compared to charge transfer in the unmodified LiAlH<sub>4</sub> surface. This reduction in charge transfer between AlāH
weakens the covalent bond within the [AlH<sub>4</sub>]<sup>ā</sup> tetrahedron, which in turn reduces the dehydrogenation temperature
exhibited by LiAlH<sub>4</sub> when Ti-containing additives are used
Nanobrick Wall Multilayer Thin Films with High Dielectric Breakdown Strength
Current thermally conductive and electrically insulating
insulation
systems are struggling to meet the needs of modern electronics due
to increasing heat generation and power densities. Little research
has focused on creating insulation systems that excel at both dissipating
heat and withstanding high voltages (i.e., have both high thermal
conductivity and a high breakdown strength). Herein, a polyelectrolyte-based
multilayer nanocomposite is demonstrated to be a thermally conductive
high-voltage insulation. Through inclusion of both boehmite and vermiculite
clay, the breakdown strength of the nanocomposite was increased by
ā115%. It was also found that this unique nanocomposite has
an increase in its breakdown strength, modulus, and hydrophobicity
when exposed to elevated temperatures. This readily scalable insulation
exhibits a remarkable combination of breakdown strength (250 kV/mm)
and thermal conductivity (0.16 W mā1 Kā1) for a polyelectrolyte-based nanocomposite. This dual clay insulation
is a step toward meeting the needs of the next generation of high-performance
insulation systems
Nanobrick Wall Multilayer Thin Films with High Dielectric Breakdown Strength
Current thermally conductive and electrically insulating
insulation
systems are struggling to meet the needs of modern electronics due
to increasing heat generation and power densities. Little research
has focused on creating insulation systems that excel at both dissipating
heat and withstanding high voltages (i.e., have both high thermal
conductivity and a high breakdown strength). Herein, a polyelectrolyte-based
multilayer nanocomposite is demonstrated to be a thermally conductive
high-voltage insulation. Through inclusion of both boehmite and vermiculite
clay, the breakdown strength of the nanocomposite was increased by
ā115%. It was also found that this unique nanocomposite has
an increase in its breakdown strength, modulus, and hydrophobicity
when exposed to elevated temperatures. This readily scalable insulation
exhibits a remarkable combination of breakdown strength (250 kV/mm)
and thermal conductivity (0.16 W mā1 Kā1) for a polyelectrolyte-based nanocomposite. This dual clay insulation
is a step toward meeting the needs of the next generation of high-performance
insulation systems
Modulating the Hysteresis of an Electronic Transition: Launching Alternative Transformation Pathways in the MetalāInsulator Transition of Vanadium(IV) Oxide
Materials
exhibiting pronounced metalāinsulator transitions
such as VO<sub>2</sub> have acquired great importance as potential
computing vectors and electromagnetic cloaking elements given the
large accompanying reversible modulation of properties such as electrical
conductance and optical transmittance. As a first-order phase transition,
considerable phase coexistence and hysteresis is typically observed
between the heating insulator ā metal and cooling metal ā
insulator transformations of VO<sub>2</sub>. Here, we illustrate that
substitutional incorporation of tungsten greatly modifies the hysteresis
of VO<sub>2</sub>, both increasing the hysteresis as well as introducing
a distinctive kinetic asymmetry wherein the heating symmetry-raising
transition is observed to happen much faster as compared to the cooling
symmetry-lowering transition, which shows a pronounced rate dependence
of the transition temperature. This observed kinetic asymmetry upon
tungsten doping is attributed to the introduction of phase boundaries
resulting from stabilization of nanoscopic M<sub>2</sub> domains at
the interface of the monoclinic M<sub>1</sub> and tetragonal phases.
In contrast, the reverse cooling transition is mediated by point defects,
giving rise to a pronounced size dependence of the hysteresis. Mechanistic
elucidation of the influence of dopant incorporation on hysteresis
provides a means to rationally modulate the hysteretic width and kinetic
asymmetry, suggesting a remarkable programmable means of altering
hysteretic widths of an electronic phase transition
Postsynthetic Route for Modifying the MetalīøInsulator Transition of VO<sub>2</sub> by Interstitial Dopant Incorporation
The
thermally driven orders-of-magnitude modulation of resistance
and optical transmittance observed in VO<sub>2</sub> makes it an archetypal
first-order phase transition material and underpins functional applications
in logic and memory circuitry, electromagnetic cloaking, ballistic
modulation, and thermochromic glazing to provide just a few representative
examples. VO<sub>2</sub> can be reversibly switched from an insulating
to a metallic state at an equilibrium transition temperature of 67
Ā°C. Tuning the phase diagram of VO<sub>2</sub> to bring the transition
temperature closer to room temperature has been a longstanding objective
and one that has tremendous practical relevance. Substitutional incorporation
of dopants has been the most common strategy for modulating the metalīøinsulator
transition temperature but requires that the dopants be incorporated
during synthesis. Here we demonstrate a novel postsynthetic diffusive
annealing approach for incorporating interstitial B dopants within
VO<sub>2</sub>. The postsynthetic method allows for the transition
temperature to be programmed after synthesis and furthermore represents
an entirely distinctive mode of modulating the phase diagram of VO<sub>2</sub>. Local structure studies in conjunction with density functional
theory calculations point to the strong preference of B atoms for
tetrahedral coordination within interstitial sites of VO<sub>2</sub>; these tetrahedrally coordinated dopant atoms hinder the rutile
ā monoclinic transition by impeding the dimerization of VāV
chains and decreasing the covalency of the lattice. The results suggest
that interstitial dopant incorporation is a powerful method for modulating
the transition temperature and electronic instabilities of VO<sub>2</sub> and provides a facile approach for postsynthetic dopant incorporation
to reach a switching temperature required for a specific application