Ferrite core for RFID application, method of manufacturing the same, and ferrite coil using the same
A ferrite core comprising a sintered oxide containing at least 48.6 to 53.9 mol % of Fe on Fe 2 O 3 basis, 12.3 to 35.2 mol % of Ni on NiO basis and 16.4 to 37.0 mol % of Zn on ZnO basis as metal elements, and contains a crystal phase comprising two or more kinds of solid solutions selected from NiFe 2 O 4 , ZnFe 2 O 4 and FeFe 2 O 4 , wherein full width at half maximum of a diffraction peak, of crystal phase of which diffraction angle 2θ is in a range from 34.6 to 36.4° as measured by X-ray diffraction analysis using Cu—Kα beam, is 0.4° or less.
1. The A ferrite core for RFID application comprising a sintered oxide containing at least 48.6 to 53.9 mol % of Fe on Fe 2 O 3 basis, 12.3 to 35.2 mol % of Ni on NiO basis and 16.4 to 37.0 mol % of Zn on ZnO basis as metal elements, and contains a crystal phase comprising two or more kinds of solid solutions selected from NiFe 2 O 4 , ZnFe 2 O 4 and FeFe 2 O 4 , wherein full width at half maximum of a diffraction peak, of said crystal phase of which diffraction angle 2θ is in a range from 34.6 to 36.4° as measured by X-ray diffraction analysis using Cu—Kα beam, is 0.4 or less, wherein 9 parts by weight or less of Cu on CuO basis is contained for 100 parts by weight of said sintered oxide, wherein 0.001 to 0.6 parts by weight of Zr on ZrO 2 basis is contained for 100 parts by weight of said sintered oxide, Zr is uniformly distributed in said sintered oxide and thickness of the grain boundary phase in said sintered oxide is 20 nm or less.
2. The ferrite core for RFID application according to claim 1 , wherein the mean crystal grain size D is in a range from 2 to 15 μm and the number of crystal grains having sizes in a range from 0.5D to 3D is 60% or more of the total number of crystal grains.
3. The ferrite core for RFID application according to claim 1 , wherein the difference between the mean crystal grain size in the surface and the mean crystal grain size in the inside is 10 μm or less.
4. The ferrite core for RFID application according to claim 1 , wherein the magnetic permeability is 400 or higher.
5. The ferrite core for RFID application according to claim 1 , wherein the relative temperature coefficient of the magnetic permeability over a range from −50 to 150° C. is from −2×10 −6 to 2×10 31 6 /° C.
6. The ferrite core for RFID application according to claim 1 , wherein the density is 5.0 g/cm 3 or higher.
7. The ferrite core for RFID application according to claim 1 , wherein the strength is 140 MPa or higher as measured by 3-point bending test.
8. The ferrite core for RFID application according to claim 1 , wherein changes in inductance under compressive pressure of 50 MPa per 1 mm 2 are within ±10%.
9. The ferrite core for RFID application according to claim 1 , wherein the fracture toughness measured by IF method is 0.8 MPa·m 1/2 or higher.
10. A method of manufacturing the ferrite core for RFID application of claim 4 which comprises
a primary crushing step of making a primary crushed powder having a specific surface area exceeding 5 m 2/ g by mixing and crushing a powder containing at least a powder of Fe oxide, a powder of Ni oxide, a powder of Zn oxide, a powder of Cu oxide and a powder of Zr oxide, and containing 48.6 to 53.9 mol % of Fe 2 O 3 oxide on Fe 2 O 3 basis, 12.3 to 35.2 mol % of Ni oxide on NiO basis, and 16.4 to 37.0 mol % of Zn oxide on ZnO, 9 parts by weight of less of Cu on CuO basis and 0.001 to 0.6 parts by weight of ZrO 2 basis, and at least one of Fe oxide, Ni oxide, Zn oxide, Cu oxide and Zr oxide has specific surface area exceeding 5 m 2 /g;
a calcination step of calcinating said primary crushed powder so as to make a calcined powder;
a secondary crushing step of crushing said calcined powder to make a secondary crushed powder having a specific surface area exceeding 5 m 2 /g;
a forming step of forming a green body by mixing said secondary crushed powder and an organic binder, granulating the mixture and molding the granulated material;
a debinding step of removing the organic binder from said green body to make a debound green body having carbon content of 300 ppm or less; and
a firing step of firing said debound green body in a firing furnace at a temperature from 1050 to 1300° C. to make the ferrite core for RFID application made of a sintered body.
11. A ferrite coil comprising a ferrite core for RFID application comprising a sintered oxide containing at least 48.6 to 53.9 mol % of Fe on Fe 2 O 3 basis, 12.3 to 35.2 mol % of Ni on NiO basis and 16.4 to 37.0 mol % of Zn on ZnO basis as metal elements, and contains a crystal phase comprising two or more kinds of solid solutions selected from NiFe 2 O 4 , ZnFe 2 O 4 and FeFe 2 O 4 , wherein full width at half maximum of a diffraction peak, of said crystal phase of which diffraction angle 2θ is in a range from 34.6 to 36.4° as measured by X-ray diffraction analysis using Cu—Kα beam, is 0.4 or less, wherein said ferrite core far RFID application contains 9 parts by weight or less of Cu on CuO basis for 100 parts by weight of said sintered oxide, wherein said ferrite core for RFID application contains 0.001 to 0.6 parts by weight of Zr on ZrO 2 basis for 100 parts by weight of said sintered oxide, Zr is uniformly distributed in said sintered oxide and thickness of the grain boundary phase in said sintered oxide is 20 nm or less.
12. The ferrite coil according to claim 1 , wherein said ferrite core for RFID application has a mean crystal grain size D in a range from 2 to 15 μm and the number of crystal grains having sizes in a range from 0.5D to 3D is 60% or more of the total number of crystal grains.
13. The ferrite coil according to claim 11 , wherein said ferrite core for RFID application has the difference between the mean crystal grain size in the surface thereof and the mean crystal grain size in the inside thereof being 10 μm or less.
14. The ferrite coil according to claim 11 , wherein said ferrite core for RFID application has magnetic permeability of 400 or higher.
15. The ferrite coil according to claim 11 , wherein said ferrite core for RFID application has a relative temperature coefficient of the magnetic permeability in a range from −2×10 −6 to 2×10 −6 /° C./over a range from −50 to 150° C.
16. The ferrite coil according to claim 11 , wherein said ferrite core for RFID application has the density of 5.0 gym 3 or higher.
17. The ferrite coil according to claim 11 , wherein said ferrite core for RFID application has the strength of 140 MPa or higher as measured by a 3-point bending test.
18. The ferrite coil according to claim 11 , wherein said ferrite core for RFID application has changes in inductance under compressive pressure of 50 MPa per 1 mm 2 within ±10%.
19. The ferrite coil according to claim 11 , wherein said ferrite core for RFID application has fracture toughness measured by an IF method being 0.8 MPa·m 1/2 or higher.