-
[show abstract]
[hide abstract]
ABSTRACT: The performance of vacuum thermionic refrigerators with emitter–collector separations of the order of a few nanometres is examined. The importance of the spectrum of transmitted electrons on device behaviour is highlighted. We find that for room temperature refrigeration applications, radiation losses are not negligible when the device is designed for high efficiency. A trade off between currents below and above the Fermi level is found to occur, with the optimal result not necessarily being achieved with minimum emitter–collector separation.
Journal of Physics D Applied Physics 01/2009; 42(3):035417. · 2.54 Impact Factor
-
Microelectronics Journal. 01/2008; 39:656-659.
-
[show abstract]
[hide abstract]
ABSTRACT: A new solid-state thermionic device structure is proposed, which employs short-period superlattices to reduce heat backflow in the device while maintaining good electrical transport. Short-period superlattices have been shown to have thermal conductivities significantly lower than their bulk constituents or the relevant alloy. Here we discuss how this might be utilized to achieve higher efficiencies in thermionic devices. The barrier in a conventional device is replaced by a short-period superlattice with periodicity selected based on experiments reporting their low thermal conductivity. Calculations are performed showing how the nature of this structure affects the cooling current due to electrons flowing in the device and how it can be optimized. Such a device could significantly out-perform a conventional solid-state thermionic device. Finally, we discuss how this device structure provides a logical progression in design methodology from conventional multi-barrier thermionics to the current state-of-the-art thermoelectric devices.
Journal of Physics D Applied Physics 09/2006; 39(19):4153. · 2.54 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In systems with broken spatial symmetry, directed transport of particles can be observed in the absence of any time-averaged forces or gradients, if the system is kept away from thermal equilibrium. The underlying principle of such "ratchets" has found applications in particle separation and in the modelling of molecular motors in living systems, and has also attracted much interest from a fundamental point of view. In particular, chaos in classical ratchets, and quantum effects such as tunnelling, have each been found to have a strong effect on direction and magnitude of the particle current. Here we are interested in the question how classically chaotic ratchets would behave in the quantum regime. Can chaotic behaviour be observed in quantum ratchets, or, alternatively, could ratchet effects be generated from quantum chaotic behaviour? From the point of view of experimentalists, we discuss these questions with a focus on mesoscopic electron devices, and point to possible topics for theoretical investigations.
Physica Scripta 09/2006; 2001(T90):54. · 1.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Materials capable of highly efficient, direct thermal-to-electric energy conversion would have substantial economic potential. Theory predicts that thermoelectric efficiencies approaching the Carnot limit can be achieved at low temperatures in one-dimensional conductors that contain an energy filter such as a double-barrier resonant tunneling structure. The recent advances in growth techniques suggest that such devices can now be realized in heterostructured, semiconductor nanowires. Here we propose specific structural parameters for InAs/InP nanowires that may allow the experimental observation of near-Carnot efficient thermoelectric energy conversion in a single nanowire at low temperature.
01/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: We show that the expressions for current and heat current calculated via (the non-linearized) ballistic and diffusive transport formalisms reduce to the same form for solid-state devices one electron mean free path in length. The materials parameters for thermionic and thermoelectric devices are also shown to be equal, rather than differing by a multiplicative constant. We derive a simple transport equation that includes both ballistic and diffusive contributions to the current, and, as an example, use this to calculate the maximum temperature difference obtainable for a piece of Bi<sub>2</sub>Te<sub>3</sub> as a function of its length, from less than an electron mean-free path to much greater than a mean-free path. Finally we briefly discuss similarities and differences between thermionic and thermoelectric devices in the regime where device length is of the order of a mean-free path length.
Thermoelectrics, 2005. ICT 2005. 24th International Conference on; 07/2005
-
[show abstract]
[hide abstract]
ABSTRACT: We show that the details of the energy spectrum of transmitted electrons in thermionic and thermoelectric devices have a significant impact on their performance. We distinguish between traditional thermionic devices where electron momentum is filtered in the direction of transport only and a second type, in which the electron filtering occurs according to total electron momentum. Our main result is that the electronic efficiency of a device is not only improved by reducing the width of the transmission filter, but also strongly depends on whether the transmission probability rises sharply from zero to full transmission. Finally, we comment on the implications of the effect the shape of the electron energy spectrum has on the efficiency of thermoelectric devices and suggest an experimental measure for providing insight into the nature of the electron energy spectrum.
Thermoelectrics, 2005. ICT 2005. 24th International Conference on; 07/2005
-
[show abstract]
[hide abstract]
ABSTRACT: Advances in solid-state device design now allow the spectrum of transmitted electrons in thermionic and thermoelectric devices to be engineered in ways that were not previously possible. Here we show that the shape of the electron energy spectrum in these devices has a significant impact on their performance. We distinguish between traditional thermionic devices where electron momentum is filtered in the direction of transport only and a second type, in which the electron filtering occurs according to total electron momentum. Such 'total momentum filtered' kr thermionic devices could potentially be implemented in, for example, quantum dot superlattices. It is shown that whilst total momentum filtered thermionic devices may achieve efficiency equal to the Carnot value, traditional thermionic devices are limited to efficiency below this. Our second main result is that the electronic efficiency of a device is not only improved by reducing the width of the transmission filter as has previously been shown, but also strongly depends on whether the transmission probability rises sharply from zero to full transmission. The benefit of increasing efficiency through a sharply rising transmission probability is that it can be achieved without sacrificing device power, in contrast to the use of a narrow transmission filter which can greatly reduce power. We show that devices which have a sharply-rising transmission probability significantly outperform those which do not and it is shown such transmission probabilities may be achieved with practical single and multibarrier devices. Finally, we comment on the implications of the effect the shape of the electron energy spectrum on the efficiency of thermoelectric devices. Comment: 11 pages, 15 figures
06/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: Conventional thermionic power generators and refrigerators utilize a barrier in the direction of transport to selectively transmit high-energy electrons, resulting in an energy spectrum of electrons that is not optimal for high efficiency or high power. Here, we derive the ideal energy spectrum for achieving maximum power in thermionic refrigerators and power generators. By using energy barriers that block or transmit electrons according to their total momentum rather than their momentum in the direction of transport, the power of thermionic devices can, in principle, be doubled and the electronic efficiency improved by 25%.
Journal of Physics D Applied Physics 06/2005; 38(12):2051. · 2.54 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Irreversible effects in thermoelectric materials limit their efficiency and economy for applications in power generation and refrigeration. While electron transport is unavoidably irreversible in bulk materials, here we derive conditions under which reversible diffusive electron transport can be achieved in nanostructured thermoelectric materials. We provide a fundamental thermodynamic explanation for why the optimum density of states in a thermoelectric material is a delta function and for why inhomogeneous doping and segmentation improve the thermoelectric figure of merit.
Physical Review Letters 04/2005; 94(9):096601. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: It is shown that equations for electrical current in solid-state thermionic and thermoelectric devices converge for devices with a width equal to the mean free path of electrons, yielding a common expression for intensive electronic efficiency in the two types of devices. This result is used to demonstrate that the materials parameters for thermionic and thermoelectric devices are equal, rather than differing by a multiplicative factor as previously thought.
03/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: The influence of the electron energy spectrum of solid-state thermionic devices on electronic efficiency is analyzed. Calculations are performed on both single and multibarrier GaAs/AlGaAs and InGaAs/InAlAs systems. Analysis reveals a wide barrier is desirable for single-barrier thermionic devices due to the associated sharpness in the electron energy spectrum. It is also shown that high electronic efficiency may be achieved in multibarrier thermionic devices consisting of thin barriers, which would separately give low efficiency, but together can be arranged to produce a desirable electron energy spectrum
Optoelectronic and Microelectronic Materials and Devices, 2004 Conference on; 01/2005
-
[show abstract]
[hide abstract]
ABSTRACT: Inhomogeneously doped thermoelectric nanomaterials with a delta-function electronic density of states can operate with Carnot efficiency in the absence of phonon heat leaks. Here we self-consistently calculate the efficiency and power from open-circuit to short-circuit of a simple model of a thermoelectric nanomaterial with a narrow peak in the electronic density of states and finite lattice thermal conductivity, comparing the results for inhomogeneous and homogeneous doping. For power generation between 800K and 300K, we find that not only does inhomogeneous doping increase the maximum efficiency by 10%, but it also increases the maximum power by up to 60%.
08/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: Differences between the thermodynamic behavior of the three-level amplifier (a quantum heat engine based on a thermally pumped laser) and the classical Carnot cycle are usually attributed to the essentially quantum or discrete nature of the former. Here we provide examples of a number of classical and semiclassical heat engines, such as thermionic, thermoelectric and photovoltaic devices, which all utilize the same thermodynamic mechanism for achieving reversibility as the three-level amplifier, namely isentropic (but non-isothermal) particle transfer between hot and cold reservoirs. This mechanism is distinct from the isothermal heat transfer required to achieve reversibility in cyclic engines such as the Carnot, Otto or Brayton cycles. We point out that some of the qualitative differences previously uncovered between the three-level amplifier and the Carnot cycle may be attributed to the fact that they are not the same 'type' of heat engine, rather than to the quantum nature of the three-level amplifier per se. Comment: 9 pages. Proceedings of 'Frontiers of Quantum and Mesoscopic Thermodynamics', Prague 2004
07/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: Conventional thermionic power generators and refrigerators utilize a barrier in the direction of transport to selectively transmit high-energy electrons. Here we show that the energy spectrum of electrons transmitted in this way is not optimal, and we derive the ideal energy spectrum for operation in the maximum power regime. By using suitable energy filters, such as resonances in quantum dots, the power of thermionic devices can, in principle, be improved by an order of magnitude. Comment: 3 pages, 2 figures
01/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: Brownian heat engines use local temperature gradients in asymmetric, periodic potentials to move particles against an external force. The efficiency of such machines is generally limited by irreversible heat flow carried by particles that make contact with different heat baths. Here we show that by using a suitably chosen energy filter, particles can be transferred reversibly between reservoirs that have different temperatures and electrochemical potentials. We use an idealized mesoscopic electron system as our paradigm. Electrons are transferred elastically through an energy filter between two otherwise thermally isolated, identical, two‐dimensional (2D) electron reservoirs. If the transmission energy of the ideal filter is tuned to the energy at which the Fermi occupation functions in the two reservoirs are equal, then no spontaneous energy or particle flow will occur between the reservoirs, even though the reservoirs are not in equilibrium. Operation as a heat engine or refrigerator respectively is obtained for filter energies above and below the energy at which the occupation function of the reservoirs are equal. By choosing the width and position of the energy filter, the machine can operate arbitrarily closely to Carnot efficiency, in full agreement with the Second Law of thermodynamics. It is interesting to note that, to our knowledge, the required ideal energy filters can be achieved only using the quantum wave properties of particles, for instance using resonant tunneling via a 0D quantum dot. © 2002 American Institute of Physics
AIP Conference Proceedings. 11/2002; 643(1):320-325.
-
[show abstract]
[hide abstract]
ABSTRACT: Brownian heat engines use local temperature gradients in asymmetric potentials to move particles against an external force. The energy efficiency of such machines is generally limited by irreversible heat flow carried by particles that make contact with different heat baths. Here we show that, by using a suitably chosen energy filter, electrons can be transferred reversibly between reservoirs that have different temperatures and electrochemical potentials. We apply this result to propose heat engines based on mesoscopic semiconductor ratchets, which can quasistatically operate arbitrarily close to Carnot efficiency.
Physical Review Letters 10/2002; 89(11):116801. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Quantum ratchets are Brownian motors in which the quantum dynamics of particles induces qualitatively new behavior. We review
a series of experiments in which asymmetric semiconductor devices of sub-micron dimensions are used to study quantum ratchets
for electrons. In rocked quantum-dot ratchets electron-wave interference is used to create a non-linear voltage response, leading to a ratchet effect. The direction of
the net ratchet current in this type of device can be sensitively controlled by changing one of the following experimental
variables: a small external magnetic field, the amplitude of the rocking force, or the Fermi energy. We also describe a tunneling
ratchet in which the current direction depends on temperature. In our discussion of the tunneling ratchet we distinguish between
three contributions to the non-linear current–voltage characteristics that lead to the ratchet effect: thermal excitation
over energy barriers, tunneling through barriers, and wave reflection from barriers. Finally, we discuss the operation of
adiabatically rocked tunneling ratchets as heat pumps.
Applied Physics A 01/2002; 75(2):237-246. · 1.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We describe how adiabatically rocked quantum electron ratchets can act as heat pumps. In general, ratchets may be described as non-equilibrium systems in which directed particle motion is generated using spatial or temporal asymmetry. In a rocked ratchet, which may also be described as a non-linear rectifier, an asymmetric potential is tilted symmetrically and periodically. The potential deforms differently during each half-cycle, producing a net current of particles when averaged over a full period of rocking. Recently it was found that in the quantum regime, where tunnelling contributes to transport, the net current may change sign with temperature. Here we show that a Landauer model of an experimental tunnelling ratchet [Linke et. al., Science 286, 2314 (1999)] predicts the existence of a net heat current even when the net particle current goes through zero. We quantify this heat current and define a coefficient of performance for the ratchet as a heat pump, finding that more heat is deposited in each of the two electron reservoirs due to the process of rocking than is pumped from one reservoir to the other by the ratchet. Comment: 7 pages, 4 figures, minor text changes and additional author
03/2001;
-
[show abstract]
[hide abstract]
ABSTRACT: Ratchets are systems that combine local asymmetry with non-equilibrium processes to generate directed particle flow. Recently it was found that in quantum ratchets, where an asymmetric tunnelling barrier is tilted symmetrically and periodically, the direction of the generated current depends on temperature—that is, on the energy distribution of particles. Here we highlight the fact that ratchets that have this property can be operated as heat pumps. We analyse a specific mesoscopic ratchet for electrons in terms of a heat pump, and discuss limits to the coefficient of performance.
Physica B: Condensed Matter.
