Preparation And Thermoelectric Properties Of P-type Doped Higher Manganese Silicide

Thermoelectric materials can be used in many fields, such as the recovery of exhausting heat of automobile and industrial waste heat, small temperature difference power generation, solar photovoltaic-thermoelectric hybrid power generation, RTG power of deep space. But the most constituent elements of thermoelectric materials are scare resource, and some of them are toxic,resulting in environmental pollution. In new century, under the background of energy-saving, sustainable development, higher manganese silicide (HMS) has been widespread concerned for its elemental abundance, environmental friendliness, high temperature oxidation resistance and good workability. Because of its anisotropic, and high energy consumption in traditional single crystal preparation, polycrystalline HMS is of great significance in practical applications. The main reason for the low thermoelectric properties of polycrystalline HMS is its low electrical conductivity and high thermal conductivity. It is difficult to control carrier mobility for its complex structure. Hence, increasing carrier concentration and optimizing Fermi level by doping are direct and efficient ways to improve electrical properties.In this research p-type HMS compounds are prepared by induction melting, annealing followed by SPS method, The samples are of high relative density(above96%),and mainly HMS phase.Besides the main HMS phase,a small amount of Si and MnSi phases are found.The doped elements and the main phase elements are distributed evenly,and all the doped elements successfully come into the cell lattice.This study mainly focuses on the doping (doping elements, doping quantity) influence on phase composition, microstructure,and TE properties of HMS compounds:B, Ge as accepter are chosen as Si site dopents, Ge can change the atomic arrangement and positional modulation of the Si atoms, in which stacking faults are created in the structures, Mo, Cu as accepter are chosen as Mn site dopents.The solid solution limit of B element is about1%in HMS. The electrical conductivity of the B-doped samples increases notably compared with the undoped one.Among the sample,the0.8％B doping amount sample shows the highest electrical conductivity(7.22×104S.m-1)at room temperature,this is attributed to high carrier concentration(1.61×1021cm-3),while the Seebeck coefficient doesn't decrease obviously.Therefore,the sample shows the good TE performance,the PFmax reaches1.62mWm-1K-2at800K and ZTmax is0.53at850K,which improve about30％and20％respectively compared to the data of the undoped one.For B and Ge co-doping samples,when B doping amount is0.8％,the solid solution limit of Ge is about2.5％in HMS.The carrier concentration and electrical conductivity are improved markedly by B and Ge co-doping which simultaneously suppress the revearse of Seebeck coefficient at high temperature.Hence,the power factor increases notably.Moreover,the B and Ge co-doping introduce alloy-scatter effect,thereby the lattice thermal conductiVity decreases efficiently.For the samples, the Mn(Si0.967B0.008Ge0.025)1.8sample possesses the optimal thermoelectric properties, ZTmax value reaches and0.68at900K,which is approaching the reported highest value(0.7)so far.For Mo and Ge co-doping samples,while Mo doping amount is0.3％,the solid solution limit of Ge is about2.0％in HMS.The carrier concentration and effective mass of the co-doping samples have been increased,hence make the electrical transport properties better.Among the samples,the Mo0.003Mn0.997(Si1-yGey)1.8sample shows the better thermroelectricities,the PFmax reaches2.15mWm-1K-2at800K and ZTmax is0.67at850K,which increse65％and50％compared with the undoped one, respectively.For Cu and Ge coodoping samples,while Ge doping amount is2.5％,the solid solution limit of Cu element is about0.6％in HMS.Among the co-doping samples samples, ZTmax of Cu0.006Mn0.994(Si0.975Ge0.025)1.8is0.61at900K. The thermoelectricities of Cu and Ge co-doping samples are lower than the Mo and Ge co-doping samples owing that me atomic orbit of Cu differs to Mn largerly than Mo, which doesn't optimize the HMS thermoelectricities.