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with amorphous Ge will increase the band gap and potentially lower solar absorp- tance, the altered and possibility spatially varying refractive index can allow for enhanced anti-reflection. Further enhancement to temperature stability, could also be realized be depositing crystalline layers of Ge under optimal conditions [140].
Such improvements would extend semiconductor materials for use in photothermal conversion of non-concentrated sunlight for small-scale process heat.
To better compare selective solar thermal technologies to one another in the long run, standard evaluation procedures like those in place for solar cells, should be implemented. Although off-angle FTIR spectra like those used in this thesis are necessary for visualizing optical properties, measuring the temperature dependent solar absorptance and hemispherically averaged thermal emittance offers complete sample characterization. Temperature stability of these two values will ultimately determine implementation flat panel solar thermal absorbers for industrial process heat. As the optical properties for selective surfaces improve, continued thermal testing will be necessary in order to predict projected lifetime.
In the active case of controlling thermal radiation, we demonstrated for the first time the modulation of near-field radiative heat flow by electrostatic biasing of a graphene field effect device over multiple voltage cycles. We observed a reversible change in heat flux over multiple ramping cycles as well as the heat flux modulation over multiple temperatures. The maximum measured modulation was 4 ± 3% and the maximum measured modulation rate was 24±7 mWm−2per V bias. Modern heat switches often require mechanical components that are slow and prone to failure.
These results can potentially lead to high-speed, solid-state thermal switches.
The modulation depth from these experiments was around 4 Wm−2, limited by the large gap spacing between the top and bottom samples due to wafer warp and resist residue. These obstacles can be surpassed with different substrate materials and an alternative sample geometry. To continue using large scale graphene samples for large signal response, a geometry like that used in Ghashami et al. with fused silica wafers would work well [134]. As the top and bottom substrates are independently suspended, silica post fabrication would not be necessary. Moreover, if the metallic leads were pre-fabricated on the substrate before graphene transfer, then no lithog- raphy on the graphene would be necessary. More aggressive resist developers could be used, which would reduce the amount surface residue. As the top and bottom substrates would also be independently actuated, the graphene-to-graphene contact resistance could be a metric to determine sample contact, ensuring zero parasitic
conductive heat flow through the heterostructure.
To use a silica wafer as the substrate, however, a back electrode and a gate dielectric would need to deposited. Deposited SiO2alone does not function well on its own as a gate dielectric as there are too many pinholes and impurities. Additional captivation layers grown by ALD of high dielectric materials like HfO2 and Al2O3 are likely necessary. Deposited films tend to be rougher than the underlying polished substrate.
As the graphene mobility drops when transferred to a rough surface, continual care for the surface roughness will need to be maintained.
There are considerable advantages in going to smaller scale. The likelihood of encountering a pinhole in the dielectric goes down with the graphene area. Also, the wafer bow and warp is less pronounced with a smaller substrate. As a result, miniature versions of the heterostructures studied in this thesis with active area on the order of 1 mm2 would be well suited for future thermal modulation studies.
Additional care should be given to the sample holder design, as the spring-loaded resistive heater used in this thesis is likely too bulky for smaller, more fragile heterostructures. Custom resisitive temperature sensors for measuring the top and bottom temperatures will also provide flexibility in sample design. In this thesis, we found it prohibitively difficult to find a commercial sensor that is both small operates near liquid nitrogen temperatures.
Nanofabricated samples and an experimental setup like those in Song et al. which were used to measure near field radiative heat transfer between a film and a coated mesa would also work well [110]. Although these structures are not scalable to much greater than 100’s of µm on a side, the reported gap distances are so low that enhanced heat flux modulation is likely. In these systems, the mesa is already coated with deposited layers of Au or SiO2. Graphene could potentially be transferred to such a mesa with pre-fabricated metallic leads.
In cases where the dielectric would need to be fabricated, atomic layer deposition of oxides will be necessary for fabrication of a high quality gate dielectric layer.
Distributed layers of alternating dielectrics can produce additional hyperbolic bands that contribute to heat flux [141]. Theoretical work similar to that in Chapter 3 in this thesis will be necessary to determine optimal layer thicknesses.
An additional area to pursue would be switching speed. Due to the large thermal capacitance and the delicate gate-dielectrics in this thesis, the heat flux change and voltage ramp rate were quite slow. One advantage radiative thermal switches that
function by external bias have over other technologies is their potential to operate very quickly. Graphene based optical modulators have been shown to function in the GHz regime [142, 143]. However, to measure heat flux variation that fast will require temperature measurement devices with low thermal load. Lock-in techniques to isolate the signal from noise could also prove useful.
Ultimately, the switching ratio will be the biggest factor in implementing radiative thermal modulators. Using patterned resonators could provide enhanced modulation depths beyond what is possible with simple planar structures [118]. Although difficult, developing new dielectrics and patterned structures could greatly enhance radiative thermal switching.
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