Effect Of Rare-earth Doping On The Thermoelectric Properties Of The Half-Heusler Alloys

Thermoelectric materials are semiconducting functional materials, which can interconvert heat and electricity directly. It is of great interest which offers a reliable, fully solid-state means of power generation and cooling. Over the past decade due to the need for environmental protection and military applications, thermoelectric materials attracted renewed attention. The half-Heusler based semiconductors are one of the most promising novel materials because they are nontoxic, and cheaper than the traditional materials.In this paper, we firstly described the background and basic theory of the thermoelectrics. Then, the traditional preparing methods and the results of current studies on half-Heusler based compounds are reviewed in detail. According to the theories of Crystal Growth and Nucleation, we prepared and studied some compounds which have half-Heusler MgAgAs-type structure. The main fruitful results are given below:(1) Compound NiMnSi or NiMnGe, which has half-Heusler MgAgAs-type structure, can not be directly synthesized by arc melting under argon atmosphere, while compound half-Heusler ZrNiSn can be synthesized by arc melting under argon atmosphere easily.(2) On the studying of the samples of Zr_1-xLa_xNiSn (x=0.05,0.1,0.15,0.2,0.3,0.4,0.6), a small substitution for Zr by La in ZrNiSn did not change the structure of half-Heusler MgAgAs-type. Some Heusler phase occurs in the compound with increasing x, and for x=0.6, the Heusler compound becomes the main phase. The absolute value of the seebeck coefficient of Zr_1-xLa_xNiSn decreases comparing with ZrNiSn.(3) On the studying of the samples of Zr_0.98R_0.02NiSn_0.98X_0.02(R=La,Ce;X=Sb,Bi)a small substitution for Zr by La (or Ce) and Sb by Sb (or Bi) in ZrNiSn can keep the structure of half-Heusler MgAgAs-type.(4) According to the value of the electrical conductivity of Zr_0.98R_0.02NiSn_0.98X_0.02 (R=La,Ce;X=Sb,Bi)in the temperature range of 300-925 K, we therefore conclude that compared with a small substitution for Zr by La and Zr by Ce in ZrNiSn, they have nearly the same influence on the value of the electrical conductivity. A small substitution for Sn by Sb in ZrNiSn can increase the value of the electrical conductivity. A small substitution for Sn by Bi in ZrNiSn can increase the value of the thermal diffusivity. The low density of sample can decrease the value of the electrical conductivity.(5) According to the absolute value of the seebeck coefficient of Zr_0.98R_0.02NiSn_0.98X_0.02 (R=La,Ce;X=Sb,Bi)in the temperature range of 305-605 K, we therefore conclude that compared with a small substitution for Zr by La and Zr by Ce in ZrNiSn, they have nearly the same influence on the absolute value of the seebeck coefficient, and they both decrease the absolute value. A small substitution for Sn by Bi in ZrNiSn can increase the absolute value of the seebeck coefficient. The low density of sample has some influence on the absolute value of the seebeck coefficient. Zr_0.98La_0.02NiSn_0.98Bi_0.02 shows a maximum seebeck coefficient of -203 _μV/K at 430 K.(6) According to the value of the thermal diffusivity of Zr_0.98R_0.02NiSn_0.98X_0.02 (R=La,Ce;X=Sb,Bi)in the temperature range of 300-873 K, we therefore conclude that compared with a small substitution for Zr by La and Zr by Ce in ZrNiSn, they have nearly the same influence on the value of the thermal diffusivity. A small substitution for Sn by Sb in ZrNiSn can increase the value of the thermal diffusivity. A small substitution for Sn by Bi in ZrNiSn can decrease the value of the thermal diffusivity. The low density of sample can decrease the value of the thermal diffusivity. Zr_0.98La_0.02NiSn_0.98Bi_0.02 shows a minimum thermal diffusivity of 2.085mm2/s at 773 K.(7) According to the value of the thermal conductivity of Zr_0.98R_0.02NiSn_0.98X_0.02 (R=La,Ce;X=Sb,Bi)in the temperature range of 300-873 K, we therefore conclude that compared with a small substitution for Zr by La and Zr by Ce in ZrNiSn, they have nearly the same influence on the value of the thermal conductivity. A small substitution for Sn by Sb in ZrNiSn can increase the value of the thermal conductivity. A small substitution for Sn by Bi in ZrNiSn can decrease the value of the thermal conductivity. Zr0.98La0.02NiSn0.98Sb0.02 shows a minimum thermal conductivity of 4.253 W/ zm*K at 773 K.(8) According to the value of ZT of Zr_0.98R_0.02NiSn_0.98X_0.02(R=La,Ce;X=Sb,Bi)in the temperature range of 375-573 K, Zr0.98La0.02NiSn0.98Sb0.02 shows a minimum ZT of 0.5 at 773 K.