Phonon Confinement Effect in TiO₂ Nanoparticles as Thermosensor Materials
Thermal sensors have the unique ability to forensically retain the complete thermal history (spatial and temporal variation) of an event under extreme conditions.
TiO2 or ZnO nanoparticles (NPs) have a very strong finite-size dependency in their Raman spectra or photoluminescence (PL) spectra due to the phonon confinement effect or the quantum confinement effect. Together with a fast grain growth kinetics and a high stability under high temperature and pressure, they can forensically retain the complete thermal history of an event. By spatially distributing these NPs during thermal events such as blasts or weapon tests, a spatially and temporally non-uniform thermal environment can be determined by a direct read off their Raman or PL spectra at various locations.
These thermosensors can also be used in non-defense applications such as for detecting the transient heating in electronics and measuring the rapid energy release during catastrophic fractures. The protocols developed in this research can be easily extended to the design of other thermosensors where a grain growth or phase transition at lower temperatures is needed to characterize the thermal environment on the biological or cellular level.
Therefore, the objective of this research was to utilize the phonon/quantum confinement effect in Raman/PL spectra and grain growth kinetics in NPs to create thermosensor materials with the ability to forensically retain the complete thermal history (spatial and temporal variation) of a thermal event under extreme conditions.
NPs and substrates chosen for use as thermosensor materials had to meet the following requirements:
Strong phonon/quantum confinement effect;
Fast growth kinetics;
Easy synthesis to get spherical NPs;
High thermal and mechanical stability.
Substrates as NPs’ carrier and protector:
High thermal and mechanical stability;
High thermal conductivity.
Small and monodisperse TiO2 and ZnO NPs of 5-6 nm in size were synthesized and loaded onto SBA-15 or graphite nanoplatelets (GNPs) substrates. Raman and PL spectrometers were used to establish the particle size versus the Raman/PL peak position master curves. Systematic isothermal and temperature-dependent heat treatments of NPs using a ribbon pyroprobe microheater (see figures) were carried out to study their grain growth kinetics.
This work was done by Liping Huang, Rensselaer Polytechnic Institute, for the Defense Threat Reduction Agency. DTRA-0008
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Phonon Confinement Effect in TiO2 Nanoparticles as Thermosensor Materials
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