The Thermoelectric Properties of Mixed Crystals of Mg[sub 2]Ge[sub x]Si[sub 1−x]

Abstract

The purpose of this work was to determine the thermoelectric properties of the pseudobinary system MgfSi-MgfGe. The compositions investigated were Mg2Si, Mg2Ge0.fSi0.s, MgaGe0.4Si0.6, Mg2Ge0.6Si0.4, Mg2Ge0.8Si0.2, and Mg2Ge. X-ray diffraction lattice parameter measurements and differential thermal analysis measurements established the existence of complete solid solubility between Mg~Si and Mg2Ge. Both the lattice parameter and liquidus temperature show almost linear variation with composition in this system. The melting temperature of MgfSi was found to be 1070" ___ 5"C, while that of MgfGe was found to be 1102" _ 5~Electrical resistivity and Hall effect measurements indicated that at 300*K the electron Hall mobility in the mixed crystals is essentially the same as that of the pure compounds. Maximum values obtained were slightly above 300 cm2/volt sec. The forbidden energy gap appeared to vary monotonically from about 0.78 electronvolt (ev) for MgfSi to about 0.70 ev for MgfGe. Thermal conductivity measurements on the pseudobinary system showed that the lattice thermal conductivity of the solid solutions is substantially lower than that of either of the pure compounds at 300~ At this temperature the lattice thermal conductivity of Mg2Ge0.6Sio.4 was found to be 0.0268 watt/cm *K. The maximum thermoelectric figure of merit which could be obtained with these materials is not as good as that of other materials now in use.

Thermoelectric Properties of Mg2Si Produced by New Chemical Route and SPS

“…Such loading and de-loading cycles were repeated 10 times. The system starts initially from the separated phases 2Mg + 2H 2 ⇆ 2MgH 2 (1)…”

“…Mg 2 Si-based thermoelectric (TE) materials were reported in the early 1960s [1,2] as promising devices for heat-into-electricity-conversion in the middle temperature range, because of their attractive figure-of-merit ZT = 1.3 [3][4][5]. Compared with PbTe and CoSb 3 or other TEs usable in the same temperature range, silicides have advantages as they are considered as environmental-friendly materials, because they consist of elements which are abundant in the earth's crust and they are nontoxic.…”

“…In addition, since they are composed of light metals, they have low densities in the range of 2-3.6 g/cm 3 which is a particularly favorable feature for heat recovery in automobile applications. Processing is a challenge, as the Mg 2 Si phase melts at 1358 K, which is extremely close to the boiling point of pure Mg (1363 K) [1]. This close difference prevents the easy-to-use arc-melting method which is popular for intermetallic TE processing [6,7].…”

“…This close difference prevents the easy-to-use arc-melting method which is popular for intermetallic TE processing [6,7]. Hence, as soon as new processing routes were established from the 1990s on, such as ball-milling [8][9][10][11], hydrogen synthesis [12,13], combustion methods [14], spark-plasma sintering (SPS) [15][16][17][18], Mg 2 Si came into focus again in the search for efficient TEs.…”

“…The best dopants, with which the best results for n-type Mg 2 Si have been achieved, are 0.3 at% Bi which exhibits S = −0.2 mV/K and ZT =1.4 [5,19], and doping with 3% Ag for p-type Mg 2 Si with S = 0.4 mV/K and ZT = 0.7 [19,20] The values do not change very much when industrial raw material [20] or processing conditions are used [21], 3at% Bi yield to S = −0.2 mV/K and ZT = 0.8 [20], or 0.5 [21] and 3a% Ag to S = +0.2 mV/K and ZT = 0.6 [20], or 0.3 [21]. The present study aims to confirm such results when using a new processing technique, the principle of which is briefly summarized below.…”