The 18-electron ternary intermetallic systems TiNiSn
and TiCoSb
are promising for applications as high-temperature thermoelectrics
and comprise earth-abundant, and relatively nontoxic elements. Heusler
and half-Heusler compounds are usually prepared by conventional solid
state methods involving arc-melting and annealing at high temperatures
for an extended period of time. Here, we report an energy-saving preparation
route using a domestic microwave oven, reducing the reaction time
significantly from more than a week to one minute. A microwave susceptor
material rapidly heats the elemental starting materials inside an
evacuated quartz tube resulting in near single phase compounds. The
initial preparation is followed by a densification step involving
hot-pressing, which reduces the amount of secondary phases, as verified
by synchrotron X-ray diffraction, leading to the desired half-Heusler
compounds, demonstrating that hot-pressing should be treated as part
of the preparative process. For TiNiSn, high thermoelectric power
factors of 2 mW/mK2 at temperatures in the 700 to 800 K
range, and zT values of around 0.4 are found, with
the microwave-prepared sample displaying somewhat superior properties
to conventionally prepared half-Heuslers due to lower thermal conductivity.
The TiCoSb sample shows a lower thermoelectric figure of merit when
prepared using microwave methods because of a metallic second phase.
Ce3+-substituted aluminum garnet compounds
of yttrium
(Y3Al5O12) and lutetium (Lu3Al5O12)both important compounds in
the generation of (In,Ga)N-based solid state white lightinghave
been prepared using a simple microwave heating technique involving
the use of a microwave susceptor to provide the initial heat source.
Carbon used as the susceptor additionally creates a reducing atmosphere
around the sample that helps stabilize the desired luminescent compound.
High quality, phase-pure materials are prepared within a fraction
of the time and using a fraction of the energy required in a conventional
ceramic preparation; the microwave technique allows for a reduction
of about 95% in preparation time, making it possible to obtain phase
pure, Ce3+-substituted garnet compounds in under 20 min
of reaction time. It is estimated that the overall reduction in energy
compared with ceramic routes as practised in the lab is close to 99%.
Conventionally prepared material is compared with material prepared
using microwave heating in terms of structure, morphology, and optical
properties, including quantum yield and thermal quenching of luminescence.
Finally, the microwave-prepared compounds have been incorporated into
light-emitting diode “caps” to test their performance
characteristics in a real device, in terms of their photon efficiency
and color coordinates.
Structural intricacies of the orange-red nitride phosphor system La(3-x)Ce(x)Si6N11 (0 < x ≤ 3) have been elucidated using a combination of state-of-the art tools, in order to understand the origins of the exceptional optical properties of this important solid-state lighting material. In addition, the optical properties of the end-member (x = 3) compound, Ce3Si6N11, are described for the first time. A combination of synchrotron powder X-ray diffraction and neutron scattering is employed to establish site preferences and the rigid nature of the structure, which is characterized by a high Debye temperature. The high Debye temperature is also corroborated from ab initio electronic structure calculations. Solid-state (29)Si nuclear magnetic resonance, including paramagnetic shifts of (29)Si spectra, are employed in conjunction with low-temperature electron spin resonance studies to probes of the local environments of Ce ions. Detailed wavelength-, time-, and temperature-dependent luminescence properties of the solid solution are presented. Temperature-dependent quantum yield measurements demonstrate the remarkable thermal robustness of luminescence of La2.82Ce0.18Si6N11, which shows little sign of thermal quenching, even at temperatures as high as 500 K. This robustness is attributed to the highly rigid lattice. Luminescence decay measurements indicate very short decay times (close to 40 ns). The fast decay is suggested to prevent strong self-quenching of luminescence, allowing even the end-member compound Ce3Si6N11 to display bright luminescence.
Reaction pathways to SnO(2) nanomaterials through the hydrolysis of hydrated tin tetrachloride precursors were investigated. The products were prepared solvothermally starting from hydrated tin tetrachloride and various (e.g., alkali) hydroxides. The influence of the precursor base on the final morphology of the nanomaterials was studied. X-ray powder diffraction (XRD) data indicated the formation of rutile-type SnO(2). Transmission electron microscopy (TEM) studies revealed different morphologies that were formed with different precursor base cations. Data from molecular dynamics (MD) simulations provide theoretical evidence that the adsorption of the cations of the precursor base to the faces of the growing SnO(2) nanocrystals is crucial for the morphology of the nanostructures.
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