Thermoelectric conversion material, thermoelectric conversion module using the same, and manufacturing method of the same

1. A thermoelectric conversion material made of a polycrystalline material represented by a composition formula (1) shown below and having an MgAgAs type crystal structure,

(A a1 Ti b1 ) x D y X 100-x-y ,  compositon formula (1):

wherein 0.2≦a1≦0.7, 0.3≦b1≦0.8, a1+b1=1, 30≦x≦35, and 30≦y≦35 hold, wherein A is at least one element selected from the group consisting of Zr and Hf, D is at least one element selected from the group consisting of Ni, Co, and Fe, and X is at least one element selected from the group consisting of Sn and Sb;

wherein an insulating coat is provided on surfaces other than an electrode joint surface of the polycrystalline material, the insulating coat including a metallic oxide as a main component thereof in an amount of 50 mass % or more as a mass ratio in the insulating coat, wherein the main component of the insulating coat includes at least one component selected from the group consisting of iron oxide and chromium oxide, and wherein the insulating coat has a thermal expansion coefficient in a range of 7×10 −6 to 12×10 −6 /° C.;

wherein the electrode joint surface of the polycrystalline material has a metal film formed thereon; and

wherein a surface roughness Ra of the surfaces other than the electrode joint surface of the polycrystalline material is 0.2 μm or more and 5 μm or less, and wherein an average thickness of the insulating coat is 3 μm or more and 1 mm or less.

2. The thermoelectric conversion material according to claim 1 , wherein the D is at least one element selected from the group consisting of Ni and Co.

3. A thermoelectric conversion module comprising the thermoelectric conversion material according to claim 1 .

4. A method of manufacturing a thermoelectric conversion material, comprising:

preparing a thermoelectric conversion material made of a polycrystalline material represented by a composition formula (1) shown below and having an MgAgAs type crystal structure,

(A a1 Ti b1 ) x D y X 100-x-y ,  composition formula (1):

wherein 0.2≦a1≦0.7, 0.3≦b1≦0.8, a1+b1=1, 30≦x≦35, and 30≦y≦35 hold, wherein A is at least one element selected from the group consisting of Zr and Hf, D is at least one element selected from the group consisting of Ni, Co, and Fe, and X is at least one element selected from the group consisting of Sn and Sb; and

forming an insulating coat on at least one surface of the thermoelectric conversion material, wherein the insulating coat includes a metallic oxide as a main component thereof in an amount of 50 mass % or more as a mass ratio in the insulating coat, wherein the main component of the insulating coat includes at least one component selected from the group consisting of iron oxide and chromium oxide, and wherein the insulating coat has a thermal expansion coefficient in a range of 7×10 −6 to 12×10 −6 /° C.;

wherein the step of forming the insulating coat comprises providing the insulating coat on surfaces other than an electrode joint surface; and

wherein a surface roughness Ra of the surfaces other than the electrode joint surface of the polycrystalline material is 0.2 μm or more and 5 μm or less, and wherein an average thickness of the insulating coat is 3 μm or more and 1 mm or less.

5. The method of manufacturing a thermoelectric conversion material according to claim 4 , wherein after the insulating coat is formed, the electrode joint surface is plated with a metal film comprising at least one of Ni and Au and having a thickness of 1 μm or more, or an evaporated film comprising at least one of Ni and Au and having a thickness of 1 μm or more is formed on the electrode joint surface.