JPH1095699A - Growth of zirconium diboride single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the growth of ZrB2 single crystal of good quality, free from crystal defect (sub-boundary structure), by growing the crystal in a He gas atmosphere and specifying the melting zone composition and the growing speed, when ZrB2 single crystal is grown through the floating zone technique. SOLUTION: A ZrB2 powder and a B powder are mixed at a specific ratio, the powder is compressed into a bar and the bar is sintered under prescribed conditions to prepare a starting sintered bar 5. The sintered bar 5 is fixed through a holder 3 to the upper shaft 2, and through the holder 30 to the lower shaft 20 thereby fixing the sintered bar 8 for forming the initial melting zone. A sintered boron (B) is inserted between the sintered bar 8 and the starting sintered bar 5 and the sintered boron and its periphery are heated by the work coil with high-frequency current to form the melting band 6 and the melding zone is allowed to move downward between the upper shaft 2 to the lower shaft 20 thereby growing the single crystal 7. This process is carried out in an atmosphere of He gas, preferably at about 3-15 atmospheric pressure and the molecular ratio of B/Zr (atomic ratio) in the melting zone composition is set to about 1.5-2.8 and the growth rate is set to 3-10 cm/hour.

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 a zirconium diboride single crystal. More specifically, the present invention relates to a method for growing a single crystal of zirconium diboride useful as a long-lived high-brightness electron-emitting material used for a scanning electron microscope, an electron drawing apparatus, and the like.

[0002]

2. Description of the Related Art Conventionally, zirconium diboride has a high melting point (3220 ° C.) and a relatively low work function (about 3.6 eV), so that it has a long life and is a high-brightness electron emitting material. The application thereof is expected to a scanning electron microscope, an electron drawing apparatus, and the like. In recent years, for applications in these fields, a single crystal of higher purity and higher quality has been demanded.

Hitherto, it has been considered that a floating zone method in which a growing temperature is high and impurities are removed by evaporation is suitable as a method of growing a higher purity zirconium diboride single crystal. However, in the conventional floating zone method, when the growth rate is increased, inclusions are included in the crystal, and further, the growth rate greatly depends on the type of the atmospheric gas.

Therefore, the present invention solves such a drawback of the conventional method, and provides a new method capable of growing a high-quality and large-sized zirconium diboride single crystal in a shorter time by using a floating zone method. It is intended to provide.

[0005]

The present invention solves the above-mentioned problems by providing a method of growing a zirconium diboride single crystal (ZrB ₂ ) by a floating zone method, wherein He gas is used as an atmosphere. Melt zone composition B / Zr
(Atomic ratio) of about 1.5 to 2.8 and 3 to 10 cm /
A method for growing a zirconium diboride single crystal, which comprises growing a crystal at a growth rate of about hr (claim 1).

[0006] Further, the present invention relates to an atmosphere gas of about 3
An embodiment such as using He gas of up to 15 atm (claim 2) is also provided.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the method of growing a single crystal of zirconium diboride by the floating zone method of the present invention,
The zirconium diboride single crystal to be grown contains impurities by using a specific melting zone composition and a growth rate of 3 cm / hr or more, which is higher than the conventional method in which the growth rate is about 2 cm / hr. However, by growing at a high speed, a high-quality crystal with few sub-grain boundaries can be obtained.

FIG. 1 of the accompanying drawings is a schematic view illustrating the configuration of an apparatus for growing a zirconium diboride single crystal of the present invention. A method for growing a single crystal by the floating zone method will be described with reference to FIG.
First, a zirconium diboride powder and a boron powder are mixed at a predetermined ratio, and a dust bar is prepared by a rubber press (2000 kg / cm ² ). This powder bar is heated in vacuum or in an inert gas to a temperature of several hundreds of degrees Celsius to produce a raw material sintered rod (5). The raw material sintering rod (5) is set on the upper shaft (2) via the holder (3), and the lower shaft (20) is provided with a sintering rod (8) for forming a seed crystal or an initial melt zone. Set via 30). Next, a boron sintered body for controlling the composition of the initial molten zone is sandwiched between the raw material sintered rod (5) and the sintered rod (8) for forming the seed crystal or the initial molten zone. Next, the boron sintered body and its surroundings are melted by applying high frequency current to the work coil (4) and heating by Joule heat generated by generating an induced current in the sample. Thereby, a fusion zone (6) is formed, and the upper axis (2) and the lower axis (2) are formed.
0) is slowly moved downward to grow a single crystal (7).

At this time, the moving speed of the lower shaft (20), that is, the crystal growing speed is always kept constant during the growing.
The range is about 3 to 10 cm / hr. As for the growth rate, if the growth rate is lower than 3 cm / hr, the growth time will be longer if the growth rate is slow, and a large amount of evaporate from the fusible zone will adhere to the work coil. Become. For this reason, the quality as a single crystal does not become predetermined. On the other hand, if it exceeds 10 cm / hr,
In particular, even when the speed exceeds 12 cm / hr, the quality of the single crystal does not become the predetermined quality.

[0010] For this reason, in the present invention, the growth rate is about 3 to 10 cm / hr. Actually, as will be described later, the growth rate is about 3 to 10 cm / hr, and further about 4 to 10 cm / hr, although it differs depending on the melt zone composition B / Zr (atomic ratio), He gas pressure, and the like. Is preferred. In the present invention, the melt zone composition B / Zr (atomic ratio) is set to approximately 1.5 to 2.8 in order to enable high-quality single crystal to be grown at a high speed.

The moving speed of the upper shaft (2), that is,
The feed rate of the raw material sintering rod (5) to the fusible zone (6) is compensated for by the low density of the raw material sintering rod (5), and the feed rate of the raw material sintering rod (5) is almost the same as the raw material sintering rod (5). It is set so that a crystal is grown. In addition, He gas is used as the atmosphere gas, which prevents discharge generated in the work coil (4) and suppresses evaporation from the molten zone during growth.

In order to investigate the effect of the type of the atmosphere gas on the growth rate, the relationship between the growth rate and the crystallinity in an inert gas atmosphere of He and an inert gas was measured using a raw material sintering rod (5) having a stoichiometric composition. Looking at the melt zone composition (B / Zr atomic ratio)
Was 1.5, and a high-quality single crystal containing no inclusions was obtained even at a growth rate of 3 cm / hr in an atmosphere of He gas. Inclusions are observed.

From these results, it is apparent that the growth rate is increased by 50% or more when He gas is used as the atmospheric gas. This is presumably because the He gas has a large cooling effect, so that the temperature distribution in the melt zone was made uneven and the convection was increased, so that the melt was vigorously stirred. When examining the influence of the melt zone composition on the crystal growth rate in a He gas atmosphere, for example, in a He gas atmosphere, growth conditions in which inclusions are included in the crystal are illustrated as in FIG. X in the figure indicates the case including inclusions, and ○
Is the case without inclusions.

As shown in FIG. 2, when the melt zone composition is near the stoichiometric composition (B / Zr = 2), the growth rate is 1
Even at 0 cm / hr or more, it is possible to grow a crystal containing no inclusions. However, 10cm / h
If the speed is higher than r, the obtained crystals may contain sub-grain boundaries, and the reproducibility of growing good quality crystals is reduced. Therefore, it is preferably 10 cm / hr or less.

When the melt zone composition shifts toward the excess boron side (ie, B / Zr> 2), the composition shifts toward the zirconium excess side (ie, B / Zr <2). It can also be seen that since boron having a small atomic weight becomes a flux (solvent) when it shifts to, diffusion in the boundary layer immediately before the growth interface is easy, and it is not necessary to reduce the growth rate so rapidly.

Further, when the melt zone composition (B / Zr atomic ratio) is in the range of 1.5 to 2.8, the growth rate (3 cm / hr) is higher than the growth rate by the conventional method (up to about 3 cm / hr). hr or more) is possible. When growing compound crystals, the growth rate is generally 2 cm / h or less, but ZrB ₂ crystals can be grown at a high speed of 10 cm / hr. This is because the growth temperature is 320
Since the temperature is as high as 0 ° C., it is considered that the active movement of atoms in the melt also has an effect.

Although the pressure of the He gas is not strict, it is usually considered that the pressure is not more than 15 atm, more preferably about 3 to 15 atm. Atmospheric gas has a function of suppressing evaporation from the melt zone. If the atmospheric pressure is low, evaporation cannot be sufficiently suppressed, and a large amount of evaporant adheres to the work coil, making it difficult to grow for a long time. In the case of He gas, it is difficult to maintain the melt zone for 30 minutes under less than 3 atm, and it is not suitable for actual crystal growth. In addition, increasing the atmospheric pressure beyond 15 atm has no meaning in crystal growth, and does not change the effect.

In place of He gas, Ar, Ne, X
The use of another inert gas such as e may be considered, but in the case of Ar, the growth rate is slow, as described above, and is not optimal as an atmosphere gas. In addition, the effect of other gases is not sufficient, and it is not feasible considering the availability and cost.

Hereinafter, the method for growing a single crystal of zirconium diboride of the present invention will be described in more detail with reference to examples.

[0020]

EXAMPLE A zirconium diboride single crystal was grown as follows using the growing apparatus shown in FIG. After adding and mixing boron powder to zirconium diboride powder so that B / Zr = 2.1, it was packed in a rubber bag having a diameter of 10 mm and formed into a cylindrical shape. This was subjected to a rubber press of 2000 kg / cm ² to obtain a green compact. This green compact is vacuumed to 1700
C. to obtain a raw material sintered body (5) having a diameter of 9 mm and a length of about 150 mm. The density of the raw material sintered body (5) is about 6
It was 0%.

As shown in FIG. 1, the raw material sintering rod (5) is fixed to the upper shaft (2) of the growing furnace via a holder (3).
A zirconium diboride sintered body was fixed to the lower shaft (20) via a holder (3). After filling the growth furnace with 6 atm of He gas, the work coil (4) (inner diameter 16 mm, 3
The boron sintered body and its peripheral portion were melted by a two-step winding to form an initial melt zone, and were moved downward at a speed of 9 cm / h to grow a single crystal having a total length of 6 cm and a diameter of 0.9 cm. The crystal composition B / Zr was 2.0. At that time, the melt zone composition is B
/Zr=2.0.

With respect to the single crystal inclusion, the (1120) plane is cut out from the crystal end portion, mirror-polished, and then etched (for about several minutes with a liquid of nitric acid: hydrofluoric acid: water = 1: 1: 2). Microscopic observation was performed. The above crystal was covered with a 1 mm-thick polycrystalline skin, but it was confirmed that the crystal was a good quality crystal without any inclusion at the center and no subgrain boundaries.

[0023]

According to the present invention, as described in detail above, a high-quality zirconium diboride single crystal containing no crystal defects (sub-grain boundaries) can be grown at a high speed. This makes it possible to use the zirconium diboride single crystal as a long-lived high-brightness electron-emitting material.

[Brief description of the drawings]

FIG. 1 is a schematic view illustrating an apparatus for growing a zirconium diboride single crystal of the present invention.

FIG. 2 is a view showing growth conditions in which inclusions are contained in crystals in a He gas atmosphere. X indicates a case including inclusions, and o indicates a case not including inclusions.

[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 Work coil 5 Raw material sintering rod 6 Fusion zone 7 Single crystal 8 Seed crystal or sintering rod for initial fusion zone formation

Claims (2)

[Claims] 1. A method for growing a single crystal of zirconium diboride (ZrB ₂ ) by a floating zone method,
Melt zone composition B / Zr (atomic ratio) with He gas as atmosphere
And growing the crystal at a growth rate of about 3 to 10 cm / hr. 2. A method for growing a zirconium diboride single crystal, comprising: 2. The method of growing a zirconium diboride single crystal according to claim 1, wherein a He gas at about 3 to 15 atm is used as an atmosphere.