Method of producing single crystal and piezoelectric element

Details Method of producing single crystal and piezoelectric element Method of producing single crystal and piezoelectric element

2001-05-21

2002-07-02

Hiteshew, Felisa (Department: 1765)

Compositions

Magnetic

Iron-oxygen compound containing

C117S001000, C117S013000, C117S937000, C252S062620, C423S328200

Reexamination Certificate

active

06413442

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing a single crystal of a composition M
3
NbGa
3
Si
2
O
14
useful as a piezoelectric material and a piezoelectric element made using the same.
2. Description of the Related Art
Up until now, the only reports regarding a single crystal of the composition M
3
NbGa
3
Si
2
O
14
(where M in the composition is an alkaline earth metal) having a Ca
3
Ga
2
Ge
4
O
14
structure (space group P321) have been findings on powder X-ray diffraction and structural analysis of Ca
3
NbGa
3
Si
2
O
14
in Japanese Unexamined Patent Publication (Kokai) No. 11-171696 and the report of B. V. Mill et al. (Zh. Neorg. Khim., 1998, vol. 43, no. 8). There have not been any reports on growth of large, good quality single crystals.
Attempts have however been made to grow a single crystal by the Czochralski method (CZ method) of dipping a seed crystal in a melt in a crucible, pulling it up while rotating it, and thereby growing a single crystal at the lower end of the seed crystal for numerous compositions having a Ca
3
Ga
2
Ge
4
O
14
structure.
When trying to grow either Ca
3
NbGa
3
Si
2
O
14
or Sr
3
NbGa
3
Si
2
O
14
using a seed crystal of a [001] orientation as found in many examples of growth of other compositions, as shown in FIG.
1
and
FIG. 2
, polycrystallization occurred immediately after the start of growth and a good single crystal could not be obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of producing a single crystal of a composition M
3
NbGa
3
Si
2
O
14
(where M is an alkaline earth metal) having sufficient quality as a piezoelectric material and a piezoelectric element made using the same.
The present inventors engaged in in-depth studies on in what crystal orientation the crystal should be grown in order to achieve the object of the present invention taking into consideration the fact that a single crystal having a lattice direction identical to the lattice direction of the seed crystal used for pullup grows at the lower end of the seed crystal, for example, the Czochralski method, and as a result discovered that by growing the crystal in an orientation inclined by a predetermined angle from the [001] axis, more specifically by pulling up the crystal while bringing the melt into contact with the face inclined by a predetermined angle from the [001] axis of the seed crystal, a single crystal of a specific composition having sufficient quality as a piezoelectric material can be obtained and thereby completed the present invention.
According to a first aspect of the present invention, there is provided a method of producing a single crystal of a composition M
3
NbGa
3
Si
2
O
14
(where M is an alkaline earth metal) comprising growing the crystal in a lattice direction inclined by an angle of 50.8 to 90 degrees from a [001] axis, preferably 51.4 to 90 degrees, particularly preferably 90 degrees. The closer the angle of inclination from the [001] axis to 90 degrees, the greater the size of the single crystal and the better the productivity of the single crystal.
According to a second aspect of the present invention, there is provided a method of producing a single crystal comprising pulling a seed crystal up while bringing a face, inclined at an angle of 50.8 to 90 degrees from a [001] axis of the seed crystal, into contact with a melt in a crucible, so that a single crystal of a composition M
3
NbGa
3
Si
2
O
14
(where M is an alkaline earth metal) is grown at a lower end of the seed crystal.
The first and second aspects of the invention preferably the single crystal is grown at an angle (&psgr;) of an enlarged crystal part of the crystal so as to satisfy the following formula when a pullup direction of the single crystal of said composition is made a direction “v” rotated by &phgr; (50.8≦&phgr;≦90) from the [001] axis in a plane including the [001] axis toward a vector “v
a1
” extending in a direction vertical to the [001] axis and rotated &thgr; (0≦&thgr;≦30) from a [100] axis, “a” and “c” are lattice constants, and &psgr; is an angle of the enlarged crystal part:
&psgr;≦2 cos
−1
((2 sin &phgr;cos &thgr;/
a
+cos &phgr;/
c
)/((2
/a
)
2
+(1
/c
)
2
)
½
)
In the first and second aspects of the invention and their embodiments, preferably said alkaline earth metal is any one of Ca and Sr.
The first and second aspects of the invention preferably the crystal is grown at the angle of the enlarged crystal part of less than 78.4 degrees when pulling up the crystal in the [100] direction.
The first and second aspects of the invention preferably the crystal is grown at the angle of the enlarged crystal part of less than 95.6 degrees when pulling up the crystal in a [120] direction.
According to third aspect of the present invention, there is provided a seed crystal comprised of a composition M
3
NbGa
3
Si
2
O
14
(where M is an alkaline earth metal) and having a face of which a crystal orientation inclined at an angle of 50.8 to 90 degrees from a [001] axis, preferably 51.4 to 90 degrees, particularly preferably 90 degrees.
According to fourth aspect of the present invention, there is provided a single crystal comprised of a composition M
3
NbGa
3
Si
2
O
14
(where M is an alkaline earth metal) and having a face of which a crystal orientation inclined at an angle of 50.8 to 90 degrees from a [001] axis, preferably 51.4 to 90 degrees, particularly preferably 90 degrees.
The single crystal according to the present invention can be suitably used as a component of a resonator, filter, or other various types of piezoelectric elements.
According to fifth aspect of the present invention, there is provided a piezoelectric element comprised of a single crystal including a composition M
3
NbGa
3
Si
2
O
14
(where M is an alkaline earth metal) and having a face of which a crystal orientation inclined at an angle of 50.8 to 90 degrees from a [001] axis, preferably 51.4 to 90 degrees, particularly preferably 90 degrees.
Note that in the present specification, the “enlarged crystal part” means the shoulder enlarged to a predetermined size in the single crystal grown at the lower end of the seed crystal (corresponding to reference numeral
30
in FIG.
6
B). The angle of the enlarged crystal part is the angle of the enlarged crystal part with respect to the pullup direction (corresponding to 2&ohgr; in FIG.
6
B).
Further, in the present invention, for example, when expressing the “(hk1) plane”, this is indicated as “(hk*1)”.
Ca
3
NbGa
3
Si
2
O
4
Composition
The single crystal having a Ca
3
Ga
2
Ge
4
O
14
structure in previous reports on crystal growth, like the many oxide single crystals currently industrialized, are grown by pulling the crystals up by the Czochralski method in the [001] direction (=z-axis=c-axis, see FIG.
13
). No problem of polycrystallization occurred so long as the temperature gradient at the time of growth was held in a suitable range.
FIG. 13
is a schematic view for explaining the pullup direction of the crystal.
For a Ca
3
NbGa
3
Si
2
O
14
composition, however, when actually growing a crystal by pulling it up in the [001] direction, as shown in
FIG. 1
, polycrystallization occurs immediately after the start of the growth, then a transparent crystal portion was obtained in a growth orientation vertical to the [120] direction (=y-axis, see
FIG. 13
) and inclined in the [100] direction (=x-axis =a1 axis, see FIG.
13
). The obtained transparent crystal portion was cut out and used as a seed crystal for growth of a single crystal, whereupon, as shown in
FIG. 3
, a crystal having a twinning plane was obtained. FIG.
1
and
FIG. 3
are both views of examples of growth of Ca
3
NbGa
3
Si
2
O
14
by methods of the related art.
The twinning plane of the obtained crystal was examine

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