Numerical Simulation of Nitric Oxide (NO) Formation in Methane, Methanol and Methyl Formate in a Homogeneous System Fabrication and Characterization of Zn4Sb3 Bulk Thermoelectric Materials Reinforced with SiC Nanosized Particles

Abstract

Zinc antimonide, Zn4Sb3, has been found as a promising thermoelectric material utilized in a temperature range of 200 ~ 500 ˚C, in which there exist vast waste-heat resources exhausted from many factories and vehicles. However, the compound intrinsically shows an extremely brittle feature being an impediment for practical applications. Thus, enhancement of the mechanical properties is highly crucial to prevent unexpected fractures during manufacturing and service processes of modules. We have focused on incorporating nanosized SiC particles into Zn4Sb3 matrix. The bulk samples were prepared by mechanochemical mixing of the starting powders and subsequent hot-extrusion process. The extrudates containing SiC particles up to 5 vol% exhibited sound appearances, high density, and fine-grained microstructures with single phase of Zn4Sb3. The mechanical properties such as hardness and compressive strength are remarkably improved by the addition of SiC particles, as a result of dispersion strengthening of SiC particles and microstructural refinement induced by a pinning effect of the particles. Meanwhile, the thermoelectric properties are retained comparable to the pristine compound, in contrast to a conventional behavior where the reinforcements in a semiconductor should usually role-play as an impurity.

Keywords Hot extrusion, Reinforcement, SiC particles, Thermoelectric materials.

Abstract In this study, methane/air, methanol/air, and methyl formate/air stoichiometric mixtures have been numerically simulated at constant volume, low pressure of 2.7 atm, and temperature ranging from 1000 K to 1950 K with an aim to establish the impact of fuel oxygenation on NO formation. These conditions represent those behind a reflected shock in a shock tube, which is modeled as adiabatic homogeneous mixture with constant internal energy and constant volume. Various chemical kinetic mechanisms have been employed and extensively tested so as to ensure validity of the results. A comparison of NO profiles and other radicals- CH, HCN, N, and N2- that are dominant in its formation have been done. Since the initial temperatures are high, the flame temperatures attained by all the mixtures are also high; from approximately 2800 to 3100 K for initial temperatures of 1000 and 1950 K respectively. Therefore, NO are formed mostly through thermal NO mechanism with prompt NO being less significant. It has been observed that at very high temperatures the difference in N and NO formation in the three fuels is not very significant (same order of magnitude) as compared to that observed in relatively low temperatures attained by freely propagating and diffusion flames. At high temperatures the major rate-limiting steps for NO formation, involving high activation energy are N2 + O ! NO + N (318.4 KJ/mol), CH2 + N2 ! HCN + NH (309.69 KJ/mol) and N2 + C ! CN + N (187.90 KJ/mol).