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Title: Development of “Open-Short Circuit” dimensionless figure-of-merit (ZT) measurement technique for investigation of thermoelements and segmented thermoelectric structures
Author: Md Yatim, Nadhrah
ISNI:       0000 0004 2734 1016
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2012
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The thermoelectric dimensionless figure-of-merit, ZT, which consists of the Seebeck coefficient, , electrical resistivity,  and thermal conductivity, , is an important parameter that characterizes the energy conversion performance of thermoelectric materials and devices. Larger ZT indicates higher performance of thermoelectric device. Current techniques for determining ZT involve measurements of ,  and  individually or ZT directly, but all techniques are carried out under a small temperature difference (T). In reality, a thermoelectric device generally operates under a much larger T and with an electrical current flowing through the thermoelectric materials. Clearly, ZT values are conventionally evaluated under a condition which differs significantly from the real operating conditions of thermoelectric devices. Recently, a novel principle for ZT measurement has been proposed, which has the capability of measuring ZT values under a large T and with an electrical current flowing through the samples. The main objective of the research embodied in this thesis is to investigate experimentally the feasibility of the proposed technique and subsequently to develop a laboratory measurement system for thermoelectric materials research. The feasibility of the proposed technique was investigated initially using thermoelectric modules. The results show a reasonable agreement with conventional techniques when it is used to measure ZT under a small T. Furthermore, the investigation reveals that ZT obtained under a large T differ significantly from those obtained under a small T. This confirms the unique capability of the proposed technique. The implementation of this technique for measuring the ZT of thermoelectric materials has proved to be very challenging due to the low electrical resistance (< 0.01 ) of the material samples. Following an in-depth experimental and theoretical investigation, a new design with a modified operating principle was proposed and carried out. The measurement system based on this new design was successfully developed, which has the capability of measuring single materials with different dimensions and under a larger T. The performance of this system was investigated using a standard Bi2Te3 sample as the reference for calibration. The results show that the system has a repeatability of <10% and an accuracy of 13-32%. Investigation on single materials and segmented structures showed that there were noticeable differences between a small and a large T, which can be attributed to the Thomson effect and changes in  values. This finding contributes to an improved understanding and new knowledge of thermoelectric behaviour under a large temperature difference. The measurement technique developed in this work will provide a useful tool for investigation and for the optimization of advanced thermoelectric structures.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK Electrical engineering. Electronics Nuclear engineering