Thermoelectronic energy conversion: Concepts and materials
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Introduction The efficient and clean conversion of heat into electric power is a crucial challenge today. Recognizing that thermionic energy conversion1 may achieve high efficiencies, numerous analyses have suggested that the thermionic conversion process can meet this challenge.2,3 The so-called space-charge problem—electrons between the cathodes will generate a potential barrier—reduces the effectiveness of standard thermionic energy conversion to unacceptably small values.1 However, thermoelectronic energy conversion3 solves the space-charge problem by removing the space charge with electric-gate fields. In this article, we present this conversion process, summarize the studies conducted to analyze its potential, and critically assess its possibilities and challenges. The success of thermoelectronic energy conversion hinges on developing materials with designed and small work functions as well as high electron emissivity that are stable under hightemperature operating conditions. Materials with engineered work functions are of prime interest for thermionic and thermoelectronic energy conversion as well as for numerous other applications such as electron sources,4 electronic devices,5 and catalysis6 (Figure 1).
Principle of thermoelectronic energy conversion A conducting electrode heated to high temperatures emits electrons.7–9 These electrons overcome the electrode’s work function and generate a thermionic emission current. Simon and Schlichter10 were the first to recognize that thermionic electron emission could be an efficient way to convert energy, if a second, colder electrode, a collector, is placed next to the emitter. The emitted electrons condense on the collector and generate an imbalance of the electrochemical potential of both electrodes. The maximum output voltage of such a device is determined by the difference in the work functions of the two electrode materials. Such a system essentially requires only the two contacted electrodes mounted in vacuum. It is therefore feasible to operate the two electrodes with an exceptionally large temperature difference, yielding high Carnot efficiencies. Because thermoelectronic energy converters obviously are thermal machines, the Carnot efficiency represents the ultimate efficiency limit. However, the thermionic energy-conversion process faces severe challenges, the most important is known as the space-charge problem—the electrons traveling in vacuum generate a negative electrostatic potential that suppresses
R. Wanke, Department of Solid State Quantum Electronics, Max Planck Institute for Solid State Research, Germany; [email protected] W. Voesch, Department of Solid State Quantum Electronics, Max Planck Institute for Solid State Research, Germany; [email protected] I. Rastegar, Department of Solid State Quantum Electronics, Max Planck Institute for Solid State Research, Germany; [email protected] A. Kyriazis, Department of Solid State Quantum Electronics, Max Planck Institute for Solid State Research, Germany; [email protected] W. Braun, Depart
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