Masayasu Akasaka

The thermoelectric properties of bulk crystalline n- and p-type Mg2Si prepared by the vertical Bridgman method

Bulk Mg2Si crystals were grown using the vertical Bridgman melt growth method. The n-type and p-type dopants, bismuth (Bi) and silver (Ag), respectively, were incorporated during the growth. X-ray powder diffraction analysis revealed clear peaks of Mg2Si with no peaks associated with the metallic Mg and Si phases. Residual impurities and process induced contaminants were investigated by using glow discharge mass spectrometry (GDMS). A comparison between the results of GDMS and Hall effect measurements indicated that electrical activation of the Bi doping in the Mg2Si was sufficient, while activation of the Ag doping was relatively smaller. It was shown that an undoped n-type specimen contained a certain amount of aluminum (Al), which was due either to residual impurities in the Mg source or the incorporation of process-induced impurities. Thermoelectric properties such as the Seebeck coefficient and the electrical and thermal conductivities were measured as a function of temperature up to 850 K. The dimen...

Crystal Growth of Mg2Si by the Vertical Bridgman Method and the Doping Effect of Bi and Al on Thermoelectric Characteristics

Magnesium silicide (Mg 2 Si) has been regarded as a candidate for advanced thermoelectric materials which is used in the temperature ranging from 500 to 800 K correspond to that of vehicle exhaust emission. Besides, Mg 2 Si has benefits such as abundance of constituent element of Mg 2 Si in the earths crust and its non-toxicity substances compared with other thermoelectric materials that operate in the conversion temperature range such as PbTe and CoSb 3 . The efficiency of a thermoelectric device is characterized by the dimensionless figure of merit, ZT=S 2 aT/U, of its constituent thermoelectric material where S is the Seebeck coefficient, a is the electrical conductivity, U is the thermal conductivity, and T is the absolute temperature. For thermoelectric device operation, the use of a material with ZT more than unity is needed to realize a conversion efficiency of ∼10 %. The optimization of doping careers in Mg 2 Si is required in order to realize unity of ZT. In that way, we have grown Mg 2 Si crystals along with doping elements of Bi and Al using vertical Bridgman method. Mg (99.99 %) and Si (99.99999 %) with a stoichiometric Mg : Si ratio of 67 : 33 were mixed congruently and melt into Mg 2 Si. Prior to the growth, Bi (99.999 %) powder at the ratio from 0.5 to 3 at % for Mg 2 Si and the pre-synthesized polycrystalline Mg 2 Si powder were mixed, and Mg 2 Si crystals were grown at a rate of 3 mm/h by vertical Bridgman method. Grown samples were characterized by x-ray diffraction (XRD) patterns and electron-prove microanalysis (EPMA), and the results indicated that Mg 2 Si crystals were reproductively grown due to use of polycrystalline Mg 2 Si as a source material of growth. Hall carrier concentrations were evaluated at room temperature. The electrical conductivity, the Seebeck coefficient, and the thermal conductivity were estimated in the temperature range from RT to 850 K. The grown crystals exhibited n-type conductivity in undoped and all Bi doped conditions. All the Bi doped crystals showed high electrical conductivity and high carrier concentration compared with that of the undoped crystal. On the other hand, the thermal conductivity was lowered in the proportion of the amount of Bi. Consequently, the thermal conductivity for the crystal that was Bi doped at 3 at % was 0.021 W/cmK at 842K, and its ZT reached 0.99 at 842 K, which is near the unity of ZT that is regarded as a standard of practical use for thermoelectric materials. The solid solubility limit of Bi to Mg 2 Si was assumed to be around 3 at % from our findings, and thus Al was codoped besides Bi in order further to improve the thermoelectric properties. We will discuss the results, additionally.

Sticking-free growth of bulk Mg/sub 2/Si crystals by the vertical bridgman method and their thermoelectric properties

Melt growth of Mg2Si was conducted using the vertical Bridgman growth method. Because magnesium is highly reactive at the growth temperature (1358 K), crucible materials were chosen to avoid chemical reactions and sticking between the crucibles, the molten Mg2Si and magnesium vapor. Crucibles made of alumina, pyrolytic boron nitride (pBN), and hexagonal boron nitride (hBN) were examined. The use of a pBN crucible enabled in the suppression of chemical reactions involving molten Mg2Si and vaporized magnesium at elevated growth temperatures, resulting in no sticking of the grown crystal to the inner wall of the crucible. While the measured conductivity for samples grown using an alumina crucible was n-type for all of the temperatures that were investigated, the samples that were grown using a pBN crucible exhibited p-type conductivity. This correlates closely with unintentional boron incorporation during the growth process.

Thermoelectric properties of polycrystalline Si1-xGex grown by die-casting vertical Bridgman growth technique

The die-casting growth process combined with an advanced version of the Bridgman method was employed for manufacturing the multicrystalline bulk crystal of Si 1−x Ge x . This process provides a form of phase transformation which is completely different from that predicted by the Si-Ge phase diagram. By combining this growth with subsequent heat treatment of the precipitated sample, the variation in the germanium content obtained was within ± 4 % for Si 0.65 Ge 0.35 sample with a carrier concentration in the mid-10 18 cm −3 . The power factor obtained exhibited a quite flat characteristic over the temperature range of room temperature to 800 K. However, there was a drop in the Seebeck coefficient at about 800 K, which corresponded to a rise in the electrical conductivity. The value of the thermal conductivity was about 0.04 W/cmK at temperatures ranging from 600 to 900 K. The maximum value of the figure of merit obtained for the grown Si 0.65 Ge 0.35 sample was 0.19 at 773 K.

Doping Characteristics of Silver in Mg 2 Si 1-x Ge x Prepared by Plasma Activated Sintering

Silver (Ag) doped Mg 2 Si 1-x Ge x ( x =0.1 to 0.4) samples were fabricated using a plasma activated sintering (PAS) method. The doping concentration of Ag was varied from 1 to 5 at.%. Undoped Mg 2 Si 1-x Ge x exhibits n -type conductivity due to residual impurities in the Mg source material used and unintentionally process-induced impurities. The observed unstable behavior of the Seebeck coefficient of Ag-doped p-type Mg 2 Si 1-x Ge x ( x ≤ 0.3) in the region of 550 to 650 K, exhibiting a considerable drop in the value and occasional conduction type conversion, was correlated with the specific contaminants. For x ∼0.4, the observed Seebeck coefficient varied from 0.2 mV/K at 823 K to 0.4 mV/K at room temperature, with no remarkable drop in the value with increasing temperature. An estimated ZT value of 5 at.% Ag doped Mg 2 Si 0.6 Ge 0.4 was 0.18 at 844 K. It was found that both specific residual impurities and process-induced impurities affected the characteristics of the Seebeck coefficient of Mg 2 Si 1-x Ge x .

Thermoelectric properties of p-type sponge-structure CoSb/sub 3/ prepared by melt growth and post annealing

Crystals of a peritectic compound of CoSb3 were grown by the vertical Bridgman method. Changes in the growth parameters resulted in the formation of polycrystalline CoSb3 grains surrounded by Sb metal. In order to remove the excess Sb as-grown samples were annealed. Measurements of theses samples were made from room temperature to 873 K. Removing the residual Sb by post annealing brought about the formation of voids in the crystal (sponge-like structure), resulting in a increase in the Seebeck coefficient from 250 to 500 µV/K. Since the carrier concentration of the grown CoSb3 was of the order of 1E16 cm -3 , the observed values of the Seebeck coefficient were higher than that of single crystalline CoSb3 of a corresponding carrier concentration. However, the samples with much higher Seebeck coefficients had lower electrical conductivity. To improve this, metal was deposited on the surface of the post annealed samples. The temperature dependent thermoelectric properties for the postannealed samples showed that a slight decrease in Seebeck coefficient and a considerable increase in electrical conductivity were observed with increasing temperature over the temperature range from RT to 773K. The thermal conductivity obtained for the CoSb3 with a sponge-like structure was lower than that of bulk single crystal. The calculated dimensionless figure of merit, ZT, was 0.84 at 600 K for the metal-deposited sponge-like p-CoSb3.