Accelerated thermal reaction kinetics by indirect microwave heating of a microwave-transparent substrate
Macroscopically homogeneous mixtures of p -nitroanisole ( p NA) and mesitylene (MES) can be selectively heated using microwave (MW) energy. The p NA solutes agglomerate into distinct phase domains on the attoliter-scale (1 aL = 10 −18 L), and these agglomerates can be MW-heated selectively to temperatures that far exceed the boiling point of the surrounding MES solvent. Here, a 1 : 20 mixture of p NA : MES is used as a mixed solvent for aryl Claisen rearrangement of allyl naphthyl ether (ANE). ANE itself does not heat effectively in the MW, but selective MW heating of p NA allows for transfer of thermal energy to ANE to accelerate rearrangement kinetics above what would be expected based on Arrhenius kinetics and the measured bulk solution temperature. This focused study builds on prior work and highlights 1 : 20 p NA : MES as a mixed solvent system to consider for strategically exploiting MW-specific thermal effects.
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Publication Date:
NSF-PAR ID:
10380229
Journal Name:
Physical Chemistry Chemical Physics
Volume:
24
Issue:
5
Page Range or eLocation-ID:
2794 to 2799
ISSN:
1463-9076
Thin film evaporation is a widely-used thermal management solution for micro/nano-devices with high energy densities. Local measurements of the evaporation rate at a liquid-vapor interface, however, are limited. We present a continuous profile of the evaporation heat transfer coefficient ($$h_{\text {evap}}$$${h}_{\text{evap}}$) in the submicron thin film region of a water meniscus obtained through local measurements interpreted by a machine learned surrogate of the physical system. Frequency domain thermoreflectance (FDTR), a non-contact laser-based method with micrometer lateral resolution, is used to induce and measure the meniscus evaporation. A neural network is then trained using finite element simulations to extract the$$h_{\text {evap}}$$${h}_{\text{evap}}$profile from the FDTR data. For a substrate superheat of 20 K, the maximum$$h_{\text {evap}}$$${h}_{\text{evap}}$is$$1.0_{-0.3}^{+0.5}$$$1.{0}_{-0.3}^{+0.5}$ MW/$$\text {m}^2$$${\text{m}}^{2}$-K at a film thickness of$$15_{-3}^{+29}$$${15}_{-3}^{+29}$ nm. This ultrahigh$$h_{\text {evap}}$$${h}_{\text{evap}}$value is two orders of magnitude larger than the heat transfer coefficient for single-phase forced convection or evaporation from a bulk liquid. Under the assumption of constant wall temperature, our profiles of$$h_{\text {evap}}$$${h}_{\text{evap}}$and meniscus thickness suggest that 62% of the heat transfer comes from the region lying 0.1–1 μm from the meniscus edge, whereas just 29% comes from the next 100 μm.