JP2730674B2 - Growth method of rare earth hexaboride single crystals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for growing rare earth hexaboride single crystals. For more information,
The present invention relates to a method for growing a rare earth hexaboride single crystal useful as a high-brightness thermionic emission material used in a scanning electron microscope, an electron drawing apparatus, and the like.

[0002]

[Prior art and its problems]
Single crystals of LaB ₆ and CeB ₆ are grown by the zone method, and these rare earth hexaboride single crystals are used as high-brightness thermionic emission materials in scanning electron microscopes, electron drawing apparatuses, and the like. These solid solutions (La,
It is known that the Ce) B ₆ single crystal is also an excellent electron-emitting material like LaB ₆ and CeB ₆ .

[0003] However, even with such a high-performance thermoelectron-emitting material, further improvement in its performance has been demanded in recent years. The realization of high quality single crystals is desired. Therefore, when the composition of the LaB ₆ , (La, Ce) B ₆ , and CeB ₆ hot cathode materials is controlled to be excessively boron, it is expected that the vapor pressure is reduced and the life of the hot cathode materials is prolonged. From the above, in order to grow a single crystal having a high boron content instead of the stoichiometric composition in growing these hexaboride single crystals in the conventional floating zone method, it is conceivable to increase the boron component in the melt zone (liquid). Can be

[0004] However, in practice, when the boron component is excessive, there is a problem that the melt zone invades the raw material, the melt zone cannot be maintained, and crystal growth becomes impossible. The present invention has been made in view of the circumstances as described above, and overcomes the limitations of the conventional technology, and is useful as a long-life, high-brightness thermionic emission material using a floating zone method. It is an object of the present invention to provide an improved method of growing a boron-excess rare earth hexaboride single crystal.

[0005]

The present invention solves the above-mentioned problems by providing Re _{6 + x} (where Re is La, Ce or (La, Ce)) by the floating zone method, and 0.05 ≦ x ≦ 0.2) is a method for growing a rare earth hexaboride single crystal represented by the formula:
A method for growing a rare earth hexaboride single crystal characterized by obtaining a high-quality boron-excess rare earth hexaboride single crystal containing no crystal defects (sub-grain boundaries).

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, as described above, in a method for growing a rare earth hexaboride single crystal by a floating method, a raw material rod having a boron component larger than a stoichiometric composition is used to provide a long life. Thus, it is based on the knowledge that a high-quality boron-excess rare earth hexaboride single crystal containing no crystal defects (sub-grain boundaries) can be obtained.

The method of growing a boron-excess rare earth hexaboride single crystal of the present invention by the floating zone method will be described. First, it is necessary to prepare a raw material rod. Therefore,
For example, a rare earth hexaboride powder and a powder containing a large amount of a boron component are each filled in a rubber bag, and the rubber press (2)
2,000 kg / cm ² ) to prepare a dust bar. This powder bar is heated in a vacuum or an inert gas to a few hundred degrees centigrade,
A boron-containing raw material sintered rod containing a large amount of a boron component is manufactured.

In this case, the raw material rod is usually
With respect to the stoichiometric composition of B ₆ , boron has an atomic ratio of B / Re
> 20 excess. More preferably B /
Re> about 30. Next, as illustrated in FIG. 1, the obtained boron-containing raw material sintered rod (6) is
And a rare-earth boride sintered rod (5) is set on the lower shaft (20) via the holder (30). Next, Joule heat generated by causing a high-frequency current to flow through the work coil (4) to generate an induced current in the sample is generated between the boron-containing raw material sintered rod (6), the rare earth boride sintered rod (5), and the periphery thereof. To melt and fix.

Thereafter, the boron-containing raw material sintering rod (6) is slightly moved to form an initial molten zone (7). afterwards,
The single crystal (8) is grown by slowly moving the upper axis (2) and the lower axis (20) downward. At this time, the moving speed of the lower shaft (20), that is, the crystal growth speed is always kept constant during the growth. The range is usually 0.2-3.0c
m / hr. More preferably, 1.0 cm / h
It is desirably about r. Further, the moving speed of the upper shaft (2), that is, the feeding speed of the boron-containing raw material sintering rod (6) to the melt zone is usually set so as to compensate for the low density of the boron-containing raw material sintering rod (6). Is set about 10 to 30% higher.

[0010] As a growing atmosphere, A
It is preferable to use an inert gas such as r or He, in order to prevent discharge generated in the high-frequency work coil (4). The initial 3 ~
At the stage of moving the molten zone on the boron-containing raw material sintered rod (6) of about 4 cm, the heating power is kept substantially constant, and the crystal grain boundaries introduced at the start of growth are removed. In the subsequent movement of the melt zone on the boron-containing raw material sintering rod (6) containing a large amount of the boron component, the boron component increases in the melt zone and the growth temperature decreases. Regarding the decrease in the growth temperature, assuming that the heating power at the time of growth in the case of the stoichiometric composition of ReB ₆ is 100, the rare-earth borane which the present invention aims at has a boron component larger than the stoichiometric composition in about 40 to 90. Is obtained. The state in which coexistence with boron and a single crystal cannot be obtained is such that the composition immediately before starting to precipitate boron has a composition in which the atomic ratio of boron to the rare earth metal exceeds B / Re = 6.2. I have. This state can be avoided by controlling conditions such as the moving speed of the zone.

In the growing method of the present invention as described above, a high-quality single crystal having no grain boundaries can be obtained with good reproducibility without intrusion of the boron-excess melt into the raw material rod. Further, in the growing method of the present invention, a heating method other than high-frequency heating, for example, an infrared concentrated heating method or the like may be used.
Hereinafter, the method for growing a boron-excess rare earth hexaboride single crystal of the present invention will be described in more detail with reference to examples.

[0012]

[Example] LaB ₆ powder, CeB together with boron powder
₆ powder, and LaB ₆ and mixed powder of CeB ₆ (molar ratio 7: 3) to each, and the cylindrical packed into a rubber bag diameter 10 mm. This is pressed with a rubber press (2000kg /
cm ² ) to obtain a green compact. This green compact is placed in a vacuum for 1
It was heated at 800 ° C. to obtain a sintered rod having a diameter of 9 mm and a length of about 12 cm.

The composition of the boron-containing raw material sintered rod is B / Re
$ 30. The obtained boron-containing raw material sintered rod (6) is set on an upper shaft (2) via a holder (3), and a rare earth boride sintered rod (5) is mounted on a lower shaft (20) with a holder (30). Set through. Next, the boron-containing raw material sintered rod (6) and the rare earth boride sintered rod (5) are heated and fixed by melting.
cm to form an initial melt zone (7). Thereafter, the upper axis (2) and the lower axis (20) were slowly moved downward to grow a single crystal (8).

Specifically, after filling the growth furnace with 7 atm of Ar, the high-frequency work coil (4) (inner diameter 16 mm, 3 mm
The lower end of the rare-earth boride sintered rod (5) was melted by a two-step winding to form an initial melt zone, and was moved downward at a speed of 1 cm / hr for 6 hours. FIG. 2 of the accompanying drawings shows that LaB
_It shows the heating power and the resulting crystal composition for _{6 + x} as a function of the zone travel distance.

In the conventional method, a 1% or more boron-excess crystal, which is difficult to grow, is obtained at a portion where the heating power is reduced by about 17% or more, and from a portion where the heating power is reduced by 40% or more. , Atomic ratio of boron to lanthanum B / R
Boron began to precipitate from the portion where e exceeded 6.2. The length of the crystal obtained during that time was about 5 mm. Ce
In the case of B _{6 + X} and (La _0.7 Ce _0.8 ) B _{6 + x} , LaB
Similar results were obtained with _{6 + x} . However, heating power is about 30%
When decreasing start to precipitate boron, width single crystals having a boron excess composition 1% or more as compared to the LaB _6, was in the slightly narrower trend.

[0016]

According to the present invention, as described in detail above, a high-quality boron-excess rare earth hexaboride single crystal containing no crystal defects (sub-grain boundaries) can be grown and obtained. This makes it possible to use the rare earth hexaboride single crystal as a long-lived high-brightness thermionic emission material.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an apparatus for growing a single crystal.

FIG. 2 is a diagram showing the heating power and the composition of the obtained crystal in the case of LaB _{6 + x} as a function of the fusion zone movement distance.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Upper shaft drive part 10 Lower shaft drive part 2 Upper shaft 20 Lower shaft 3 Holder 30 Holder 4 High frequency work coil 5 Rare earth boride sintered rod 6 Boron-containing raw material sintered rod 7 Fusion zone

Claims (5)

(57) [Claims] 1. ReB by the floating zone method
_{6 + x} (Re indicates La, Ce or (La, Ce),
A rare earth hexaboride single crystal represented by the following formula: 0.05 ≦ x ≦ 0.2), characterized in that the single crystal is grown using a raw material rod containing a larger amount of a boron component than a stoichiometric composition. To grow a boron-excess rare earth hexaboride single crystal. 2. The method according to claim 1, wherein the boron content of the raw material rod is B / Re> 20 in atomic ratio with respect to the stoichiometric composition of ReB _6.
Nurturing method. 3. A crystal growth rate of 0.2 to 3.0 cm / h.
3. The method according to claim 1, wherein r is r. 4. The growth method according to claim 1, wherein the feed rate of the raw material rods to the melt zone is higher than the crystal growth rate by 10 to 30%. 5. The growth method according to claim 1, wherein the growth is carried out at a level of about 40 to 90% lower than the heating power during the growth in the stoichiometric composition of ReB ₆ .