Semiconducting Properties of Mg 2 Ge Single Crystals

Chen

2009

The temperature dependence of the thermal conductivity j(T), electrical resistivity q(T), and Seebeck coefficient S(T) of Mg 2 Sn:Ag crystals with 0 at.% to 1 at.% Ag content were measured at T = 2 K to 400 K. The crystals were cut from ingots that were prepared by the vertical Bridgman method. Undoped samples show a dramatic j µ T 3 rise at low temperatures to a peak value j 15K = 477 W m À1 K À1 . This leads to exceptionally large phonon drag effects causing giant thermopower with S rising sharply to a peak value S 20K = 3000 lV K À1 . At higher temperatures S decreases and changes sign to intrinsic values S % À60 lV K À1 . The addition of Ag changes the transport properties as follows: (a) j decreases systematically, the peak shifts to 30 K and falls to 7 W m À1 K À1 ; (b) q changes from high to low values; (c) S(T) changes to a linear dependence with S 300K % 150 lV K À1 to 200 lV K À1 .

Section: Resultsmentioning

confidence: 76%

Thermal Conductivity and Other Transport Properties of Mg2Sn:Ag Crystals

Chen

2009

“…Interestingly, Mg 2 Pb is a normal metal with a resistivity of about 200 lX cm at 300 K. These semiconducting compounds were first synthesized about 50 years ago and were shown to have excellent thermoelectric properties. [1][2][3][4] Intense interest during the last decade in thermoelectric materials, particularly the search for high figure of merit and environmentally friendly materials for medium-to high-temperature applications, has led to renewed interest in these compounds. Notably, the group 5,6 at Ioffe Physico-technical Institute has studied Mg 2 Si x Ge 1Àx solid solutions and showed that bulk polycrystalline samples possess a high dimensionless figure of merit ZT = (a 2 r/j)T % 1.1 at temperatures T = 600 K to 800 K, where a is the Seebeck coefficient, r is the electrical conductivity, j is the thermal conductivity of the material, and T is absolute temperature.…”

Section: Introductionmentioning

confidence: 99%

Magnetron Deposition of In Situ Thermoelectric Mg2Ge Thin Films

Chuang

2009

Thin films of the semiconducting compound Mg 2 Ge were deposited by magnetron cosputtering from source targets of high-purity Mg and Ge onto glass substrates at temperatures T s = 300°C to 700°C. X-ray diffraction shows that the Mg 2 Ge compound begins to form at a substrate temperature T s % 300°C. Films deposited at T s = 400°C to 600°C are single-phase Mg 2 Ge and have strong x-ray peaks. At higher T s the films tend to be dominated by a Ge-rich phase primarily due to the loss of magnesium vapor from the condensing film. At optimum deposition temperatures, 550°C to 600°C, films have an electrical conductivity r 600 K = 20 X À1 cm À1 to 40 X À1 cm À1 and a Seebeck coefficient a = 300 lV K À1 to 450 lV K À1 over a broad temperature range of 200 K to 600 K.

“…[1][2][3][4][5][6] Recent work 7 has shown that ZT % 1.1 can be achieved in Mg 2 Si 1Àx Sn x solid solutions prepared by direct co-melting. Other preparation methods such as mechanical alloying, 8 solid-state reaction, 9 spark plasma sintering, 10 and hot pressing 11,12 have also been used.…”

Section: Introductionmentioning

confidence: 99%

Eutectic Microstructure and Thermoelectric Properties of Mg2Sn

Chen

2010

Ingots of undoped and Ag-doped Mg 2 Sn were prepared from the melt using a rocking Bridgman furnace at different cooling rates: slow cooling (0.1 K/min), moderate cooling (1 K/min), and rapid quenching. The ingots show very different microstructure and thermoelectric properties. Slow-cooled ingots consist of large Mg 2 Sn crystals with minor inclusions. Moderate-cooled ingots show significant variation in composition and microstructure, with Mg-rich material at the topmost section of the ingot and Sn-rich material at the bottom surface of the ingot. Rapid quenching results in ingots with finely dispersed Mg + Mg 2 Sn eutectic microstructure in the form of lamellae 200 nm to 500 nm in thickness. Measurements of the Seebeck coefficient and electrical conductivity in the temperature range of T = 80 K to 700 K were carried out to establish correlations between the microstructure and the thermoelectric properties.