Suppression of secondary phase in CrN matrix to boost the high-temperature thermoelectric performance

Abstract With the escalation of energy and environmental crisis, more and more attentions have been paid to the waste heat recovery, especially in the high-temperature range (>900 K). Chromium nitride (CrN) is expected to be a promising high-temperature thermoelectric material that can be used in harsh environments because of its high thermal stability, corrosion resistance and excellent mechanical properties. However, the relatively low figure of merit zT hinders its realistic applications. In this work, we reveal that the easily formed secondary phase Cr2N is one of the key factors restricting the CrN thermoelectric performance. Acid treatment is utilized to remove the metallic Cr2N phase from the commercially available powders. Nevertheless, after the sintering process at 1373 K, Cr2N phase can emerge again due to the decomposition of CrN. Interestingly, this decomposition process can be inhibited by introducing carbon nanotubes (CNTs), and the resulted hybrid material demonstrates greatly improved Seebeck coefficient and reduced thermal conductivity. A maximum zT value of 0.33 for the CNTs/CrN can be achieved at 973 K, which is 1.8 times as high as that of the acid-pickled CrN (0.18), and 40 times higher than that of the pristine chromium nitride (0.008). Additionally, the CNTs/CrN hybrid material exhibits outstanding mechanical hardness (11.5 GPa) as well as chemical and thermal stability. These results will promote the practical applications of CrN-based materials in the high-temperature thermoelectric field.