Chemistry of inorganic compounds – Nitrogen or compound thereof – Binary compound
Reexamination Certificate
1999-01-19
2001-11-20
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Nitrogen or compound thereof
Binary compound
C075S230000, C148S306000, C423S143000, C423S151000
Reexamination Certificate
active
06319485
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic material having a high saturation magnetic flux density obtained by chemically synthesizing, as an isolated crystal, &agr;″-Fe
16
N
2
of a metastable phase, which is originally crystallized in the case of annealing a martensite having nitrogen dissolved therein, and to a production method of the magnetic material.
BACKGROUND OF THE INVENTION
Of Fe-N-based compounds, &agr;″-Fe
16
N
2
is known as a metastable compound crystallized in the case of annealing a martensite having nitrogen dissolved therein for a long time. Though the crystal of &agr;″-Fe
16
N
2
is a bct structure (body-centered tetragonal system) and is expected as a giant magnetic substance having a large saturation magnetization, there has hitherto been no example to report that the compound is chemically synthesized in an isolated state.
Hitherto, to prepare &agr;″-Fe
16
N
2
, various methods such as a vapor-deposition method, an MBE (molecular beam epitaxy) method, an ion implantation method, a sputtering method, an ammonia nitriding method, etc., have been tried. However, since &agr;″-Fe
16
N
2
is in a metastable phase, and &ggr;′-Fe
4
N, &egr;-Fe
2-3
N, etc., which are more stable than &agr;″-Fe
16
N
2
, are liable to form, it is difficult to produce &agr;″-Fe
16
N
2
as a single compound by isolating it. Also, even when the &agr;″-Fe
16
N
2
crystal is obtained as a thin film, there is a limitation in the application of such a thin film to a magnetic material.
Accordingly, various attempts for producing the powders of &agr;″-Fe
16
N
2
have been made. For example, JP-A-8-165502 proposes a method wherein powders of Fe(N) rich in &agr;′ (martensite phase) are formed by quenching powders of Fe(N) from a &ggr; phase (austenite phase) and further grinding the powders to form a strain induced martensite, and the powders of the martensite phase are subjected to an annealing treatment to crystallize &agr;″-Fe
16
N
2
However, even by this method, it cannot be avoided that &agr;-Fe remains. In fact, in the working examples described in the above-cited patent publication, the content of &agr;″-Fe
16
N
2
is less than 80% by weight, and thus, the product obtained by the method cannot be a powder of the &agr;″-Fe
16
N
2
single phase.
Also, JP-A-7-118702 proposes a method wherein a pure iron powder is reacted with a mixed gas of ammonia and hydrogen and retained at high temperatures for a long time to form a powder of an austenite single phase, the powder is quenched from an austenite single phase of a high temperature to form a powder of a martensite main phase, and the powder is then subjected to an aging treatment, whereby a powder having a trace amount of the &agr;″-Fe
16
N
2
phase is deposited in a martensite having nitrogen dissolved therein is formed, followed by further subjecting to a grinding treatment in a nitrogen atmosphere to accelerate the deposition of &agr;″-Fe
16
N
2
. However, even by this method, the content of the &agr;″-Fe
16
N
2
phase is at most about 70% by volume.
Because in the method of crystallizing &agr;″-Fe
16
N
2
from a martensite phase [&agr;′-Fe(N)], &agr;″-Fe
16
N
2
exists as a deposition phase in a metal texture, it is difficult to separate the crystal compound of &agr;″-Fe
16
N
2
as a single phase. Accordingly, by the crystallization method of &agr;″-Fe
16
N
2
accompanied by such phase transformation, a magnetic material of the &agr;″-Fe
16
N
2
single phase cannot be obtained. This is true when the material is in a powder form, and the above-described matter is also found in the method of crystallizing &agr;″-Fe
16
N
2
in a powder as shown in the above-cited patent publications.
Also, in the thin-film forming method, even when a thin film of &agr;′-Fe
16
N
2
is obtained, use of such a thin film as a general-purpose magnetic material is problematic in productivity and economy.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to obtain a crystal compound of &agr;″-Fe
16
N
2
in a separated form from a mother phase, different from the crystallization by a heat treatment accompanied by phase transformation as described above and to provide a magnetic material substantially consisting of an &agr;″-Fe
16
N
2
crystal compound.
As a result of making various investigations about chemical synthesis methods of &agr;″-Fe
16
N
2
for attaining the above-described object, the inventors have succeeded in synthesizing &agr;″-Fe
16
N
2
as a bulk substance in an &agr;″-Fe
16
N
2
crystal single phase. That is, it becomes possible to chemically synthesize a crystal itself of &agr;″-Fe
16
N
2
which is a metastable compound without using the conventional crystallization by a phase transformation process. Therefore, according to the present invention, there is provided a magnetic material made up of an assembly of particles each substantially consisting of an &agr;″-Fe
16
N
2
crystal having a body-centered tetragonal system (bct). That is, the particles are a single phase of the &agr;″-Fe
16
N
2
crystal and the particle powder has a high saturation magnetic flux density (saturation magnetization).
Such particle powders of the single phase of the &agr;″-Fe
16
N
2
crystal can be directly synthesized by reacting fine particles of &agr;-Fe and a nitrogen-containing gas such as an ammonia gas at a temperature of 200° C. or lower, preferably 150° C. or lower, and more preferably 120° C. or lower.
REFERENCES:
patent: 5330554 (1994-07-01), Koyano et al.
patent: 359045911A (1984-03-01), None
Hawley's Condensed Chemical Dictionay, 13th edition, p. 924, 1999.*
The 36thSymposium of Ceramics Basic Science, A Collection of the Gist of Lectures, Jan. 20, 1998, pp. 60-61.
Kanamaru Fumikazu
Kikkawa Shinichi
Nagatomi Akira
Dowa Mining Co. Ltd.
Griffin Steven P.
McDermott & Will & Emery
Medina Maribel
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