Method for preparing electroconductive mayenite type compound

1. A method for preparing an electroconductive mayenite type compound, comprising subjecting a precursor to a heat treatment,

wherein the precursor is a vitreous or crystalline material, which comprises Ca and Al, in which a molar ratio of CaO:7Al 2 O 3 is from 12.6:6.4 to 11.7:7.3 calculated as oxides, and in which a total amount of CaO and Al 2 O 3 is at least 50 mol %, and

wherein the heat treatment is a heat treatment comprising holding the precursor at a heat treatment temperature T of from 600 to 1,415° C. in an inert gas or vacuum atmosphere with an oxygen partial pressure P O2 in a range of

P O2 ≦10 5 ×exp[{−7.9×10 4 /(T+273)}+14.4] in the unit of Pa.

2. The method for preparing an electroconductive mayenite type compound according to claim 1 , wherein the crystalline precursor is a mayenite type compound having a composition of 12CaO.7Al 2 O 3 and having a crystal structure composed of three-dimensionally linked cages, or an isomorphous compound obtained by replacing at least a part of Ca and Al in the mayenite type compound by another element.

3. The method for preparing an electroconductive mayenite type compound according to claim 1 , wherein a part or all of Ca in the precursor is replaced by the same number of Sr atoms.

4. The method for preparing an electroconductive mayenite type compound according to claim 1 , wherein at least a part of Al in the precursor is replaced by the same number of Si atoms or Ge atoms.

5. The method for preparing an electroconductive mayenite type compound according to claim 1 , wherein the precursor comprises at least one component selected from the group consisting of (a), (b) and (c), wherein (a) is at least one member selected from the group consisting of Si, Ge and B, and an amount of the component (a) is from 0 to 17 mol % calculated as oxides; (b) is at least one member selected from the group consisting of Li, Na and K, and an amount of the compound (b) is from 0 to 5 mol % calculated as oxides; and (c) is at least one member selected from the group consisting of Mg and Ba, and an amount of the compound (c) is from 0 to 10 mol % calculated as oxides; and

wherein the precursor further comprises from 0 to 8 mol % in total of (i) at least one rare earth element selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm and Yb, (ii) at least one transition metal element or typical metal element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Cu, or a combination of(i) and (ii) calculated as oxides.

6. The method for preparing an electroconductive mayenite type compound according to claim 1 , the method further comprising (i) heating the precursor at 950-1,415° C. prior to the heat treatment, or (ii) the heat treatment at a temperature T from 950 to 1,415° C., wherein the precursor is a vitreous material.

7. The method for preparing an electroconductive mayenite type compound according to claim 1 , the method further comprising (i) heating the precursor at 1,000-1,415° C. prior to the heat treatment, or (ii) the heat treatment at a temperature T from 1,000 to 1,415° C., wherein the precursor is a crystalline material with a crystal structure other than that of the mayenite type compound.

8. The method for preparing an electroconductive mayenite type compound according to claim 1 , wherein the precursor is a powder, a press-molded product obtained by press-molding the powder, or a sintered product obtained by sintering a press-molded product resulting from molding of the powder.

9. The method for preparing an electroconductive mayenite type compound according to claim 1 , wherein the precursor is in a form of a plate.

10. The method for preparing an electroconductive mayenite type compound according to claim 1 , wherein the heat treatment is carried out in such an atmosphere that the precursor is sealed in a container with a reducing agent selected from the group consisting of carbon, Al and Ti.