We theoretically investigate the thermionic emission from nodal-ring semimetals. The thermionic emission is found to be anisotropic in the x- and y-directions. The anisotropic emission can be enhanced by increasing the radius of nodal-ring b. The main feature of nodal-ring semimetals not only results in anisotropic thermionic emission but also affects the value of thermionic emission current density (TECD). The TECD of the lower branch of the energy–momentum dispersion increases with b, while the TECD of the upper branch decreases with b. Unlike in conventional materials, the TECD in nodal-ring semimetals depends on Fermi energy that is similar to the situation in Dirac semimetals. The underlined reason is that Dirac semimetals and nodal-ring semimetals have a linear or a linear-like energy–momentum dispersion while conventional materials have a parabolic energy–momentum dispersion. The TECD of nodal-ring semimetals depends strongly on work function and temperature.

Thermionic emission in nodal-ring semimetals

Thermionic refrigeration using semiconductor heterostructures is examined theoretically and experimentally. A theory of single-barrier devices is first developed where two classes of single-barrier devices are defined and compared. So-called class 1 devices are found to always perform better. A theory of multiple-barrier devices based on class 1 barriers is then developed using a numerical solution. Experimentally, three generations of 10-barrier devices based on AlxGa1−xAsGaAs heterostructures were made and electrically characterised. This material is by no means ideal (as will be discussed) but was used due to availability and because, at the commencement of this work, had never been used for this purpose before. Thermal measurements were made to determine if any cooling occurred at room temperature. No cooling was observed but the electrical characteristics allowed for examination of the models developed. It was found that the earlier models used did not accurately model the I-V characteristics of the devices. This was attributed to the fact that the initial models did not take space-charge into account. A more robust numerical model is developed in which the I-V characteristics of devices are predicted much more accurately. This model is then used to design new generations of devices. The work concludes by recommending a next generation design in which substantially more cooling is expected compared to the samples examined here. The probability of cooling being observed in the future is therefore increased. The type of devices described here will always be hindered because of heat conduction. Other methods incorporating thermionic emission, such as an opto-thermionic system in which removed heat is given off as light, may ultimately prove to be the best solution. This aside, it is hoped that the work presented here will contribute to the understanding of the field.

/pdf/investigations-into-thermionic-cooling-for-domestic-4q0finz48v.pdf

Investigations into thermionic cooling for domestic refrigeration

We demonstrate that Weyl semimetals can exhibit intriguing thermionic properties. The emission current varies significantly in systems with different Weyl point separation, the key topological parameter of such materials. The emission is highly anisotropic along directions parallel and perpendicular to the Weyl point separation. For large separations, emission is higher along the perpendicular direction. However, for smaller separations, there exists a changeover temperature at which the dominant emission direction changes from parallel to perpendicular when increasing temperature. The optimal cooling efficiency of a single barrier device can approach 80% of the theoretical limit in the perpendicular direction, 5% greater than a conventional parabolic material. Our results suggest that this class of material has potential applications in thermionic cooling and power generation.

Anisotropic thermionic response of Weyl semimetals with application in thermionic cooling

We calculate the heat transfer from electronic devices based on three-dimensional Dirac materials without and with thermionic cooling. Without thermionic cooling, the internal temperature of the devices is at best equal to and usually higher than the temperature of the surrounding environment. However, when thermionic cooling is employed to transport heat, the internal temperature can be considerably lower than the environmental temperature. In the proposed thermionic cooling process, the energy efficiency can be as high as 75% of the Carnot efficiency.

/pdf/thermionic-enhanced-heat-transfer-in-electronic-devices-6yr315fjm9.pdf

Thermionic enhanced heat transfer in electronic devices based on 3D Dirac materials

We report our recent experimental and numerical investigation into the thermal and electrical transport in GaAs-AlGaAs semiconductor multilayer structures. Electrical and thermal conduction measurements were performed on multilayer structures to determine the temperature gradient across the sample. AuGe was used for top contact metallisation, and an InGa eutectic for bottom substrate contact. Metallisation contacts were also grown directly onto the substrate in order to compare results with and without the device included. By using a variable load resistor connected in series with the device, we can accurately determine the current-voltage characteristics of the device. Thus the power input can be obtained. The temperature distribution on the top and bottom substrate was measured with micro thermocouples. Since the cooling device is grown on an n-type semiconductor substrate the effects of joule heating in the substrate had to be considered. Treating the substrate as bulk material and calculating joule heating showed that this effect is negligible. Comparing experimental measurements of the device and of the substrate alone support this. The experimental I-V characteristics of the device differ significantly in shape from theoretical I-V characteristics. This may be due to that fact that space-charge effects are not included in the currently accepted model (Richardson's equation). Due to the small size of the devices and therefore very large electric fields, this effect may be important. Work is currently being carried out to modify the model. The devices studied so far have been made from undoped GaAs-AI/sub 0.07/Ga/sub 0.03/As heterostructures. For large cooling power it is a requirement that the conduction band of the layers be close to the Fermi level.

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1049&context=engpapers

Thermal and electrical characteristics of a multilayer thermionic device

The currently accepted model for the electronic transport in semiconductor thermionic devices does not take into account space charge effects between electrodes. We have observed that the experimental I – V characteristics of a multi-layer semiconductor thermionic device differ in shape significantly from those predicted by theory. Due to the small geometries involved in the devices, large electric fields can be present between electrodes. We have calculated the I – V characteristics of a device with and without these effects taken into account and compare them with experimental data.

Investigation into space charge effects in I–V characteristics of multi-layer semiconductor thermionic devices

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Investigation into space charge effects in I–V characteristics of multi-layer semiconductor thermionic devices

Thermal and electrical characteristics of a multilayer thermionic device