Thermoelectric conversion material, thermoelectric conversion module using the same, and manufacturing method of the same
According to an embodiment, a thermoelectric conversion material is made of a polycrystalline material which is represented by a composition formula (1) shown below and has a MgAgAs type crystal structure. The polycrystalline material includes a MgAgAs type crystal grain having regions of different Ti concentrations. (A a Ti b ) c D d X e Composition formula (1) wherein 0.2≦a≦0.7, 0.3≦b≦0.8, a+b=1, 0.93≦c≦1.08, and 0.93≦e≦1.08 hold when d=1; 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.
1. A thermoelectric conversion material made of a polycrystalline material which is represented by a composition formula (1) shown below and has a MgAgAs type crystal structure, the polycrystalline material comprising: MgAgAs type crystal grains having regions of different Ti concentrations,
(A a Ti b ) c D d X e Composition formula (1)
wherein 0.2≦a≦0.7, 0.3≦b≦0.8, a+b=1, 0.93≦c≦1.08, 0.93≦e≦1.08, and d=1; 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.
2. The thermoelectric conversion material according to claim 1 , which comprises MgAgAs type crystal grains for which two or more peaks are present in a frequency graph in which a strength ratio of characteristic X-rays of Ti is plotted on a horizontal axis and a frequency for each strength of the characteristic X-rays of Ti is plotted on a vertical axis when the characteristic X-rays of Ti are measured at intervals of 0.2 μm in a MgAgAs type crystal grain on any cross section of the thermoelectric conversion material by EBSD (backscattering electron beam diffraction).
3. The thermoelectric conversion material according to claim 1 , which comprises MgAgAs type crystal grains for which two or more peaks are present in a frequency graph in which a strength ratio of characteristic X-rays of Ti is plotted on a horizontal axis and a frequency for each strength of the characteristic X-rays of Ti is plotted on a vertical axis when the characteristic X-rays of Ti are measured at intervals of 0.2 μm in a MgAgAs type crystal grain on any cross section of the thermoelectric conversion material by EBSD (backscattering electron beam diffraction), and a ratio (K1/K2) of a frequency K1 of a highest peak to a frequency K2 of a lowest peak is 1.2 or greater.
4. The thermoelectric conversion material according to claim 1 , which comprises MgAgAs type crystal grains for which two or more peaks are present in a frequency graph in which a strength ratio of characteristic X-rays of Ti is plotted on a horizontal axis and a frequency for each strength of the characteristic X-rays of Ti is plotted on a vertical axis when the characteristic X-rays of Ti are measured at intervals of 0.2 μm in a MgAgAs type crystal grain in a unit area of 240 μm×80 μm on any cross section of the thermoelectric conversion material by EBSD (backscattering electron beam diffraction), and the MgAgAs type crystal grains have a number ratio of crystal grains of 30% or more and 100% or less.
5. The thermoelectric conversion material according to claim 2 , wherein a value obtained by subtracting a lower limit from an upper limit of the frequency graph is 10 or more.
6. The thermoelectric conversion material according to claim 1 , wherein when characteristic X-rays of Ti are measured at intervals of 0.2 μm in the MgAgAs type crystal grain on any cross section of the thermoelectric conversion material by EBSD (backscattering electron beam diffraction) to create a frequency graph in which a strength ratio of the characteristic X-rays of Ti is plotted on a horizontal axis and a frequency for each strength of the characteristic X-rays of Ti is plotted on a vertical axis, the frequency graph has one peak and a lower limit of the frequency graph is smaller than a frequency indicating a peak by 5 or more and an upper limit of the frequency graph is larger than the frequency indicating the peak by 5 or more.
7. The thermoelectric conversion material according to claim 6 , wherein a value obtained by subtracting the lower limit from the upper limit of the frequency graph is 15 or more.
8. The thermoelectric conversion material according to claim 1 , wherein when an area of 240 μm×80 μm on any sintered surface of the thermoelectric conversion material is analyzed by EBSD (backscattering electron beam diffraction) and a total number of crystal grains per unit area is set to 1, a number ratio of crystal grains comprising any crystal orientation of a (001) plane, a (101) plane, and a (111) plane is less than 0.5.
9. The thermoelectric conversion material according to claim 1 , wherein an average crystal grain size is 2 to 40 μm.
10. The thermoelectric conversion material according to claim 1 , wherein the polycrystalline material is a sintered body.
11. The thermoelectric conversion material according to claim 1 , wherein MgAgAs type crystal grains have an area ratio of 92% or more in an area of 240 μm×80 μm.
12. 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.
13. A thermoelectric conversion module, which comprises the thermoelectric conversion material according to claim 1 .
14. A manufacturing method of the thermoelectric conversion material according to claim 1 , comprising: preparing a raw material molten metal satisfying the composition formula (1); preparing a raw material powder by cooling the raw material molten metal at a cooling rate of 100° C./s or slower; molding the obtained raw material powder; and sintering the obtained molded body.
15. The manufacturing method of the thermoelectric conversion material according to claim 14 , wherein the preparing the raw material powder comprises producing an ingot by the cooling and pulverizing the ingot.
16. A manufacturing method of the thermoelectric conversion material according to claim 1 , comprising: preparing the raw material molten metal satisfying the composition formula (1); preparing the raw material powder by rapidly cooling the raw material molten metal at the cooling rate exceeding 100° C./s; heat-treating the obtained raw material powder in order to form the MgAgAs type crystal grains having the regions of different Ti concentrations; molding the heat-treated raw material powder; and sintering the obtained molded body.
17. A manufacturing method of the thermoelectric conversion material according to claim 1 , comprising: preparing the raw material molten metal satisfying the composition formula (1); preparing the raw material powder by rapidly cooling the raw material molten metal at the cooling rate exceeding 100° C./s; molding the obtained raw material powder; sintering the obtained molded body; and heat-treating the obtained sintered body in order to form the MgAgAs type crystal grains having the regions of different Ti concentrations.
18. The manufacturing method of the thermoelectric conversion material according to claim 16 , wherein the preparing the raw material powder by rapidly cooling the raw material molten metal at the cooling rate exceeding 100° C./s is an atomizing method.