JPS5814399B2 - Method for producing titanium carbide crystals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing titanium carbide crystals having a uniform composition.

Titanium carbide has an extremely high melting point (3100℃) and high hardness (Vickers hardness 3000KP/mN).
”), it is widely used as a variety of carbide tools and as a surface protection material.

It also has considerable toughness at high temperatures and relatively good oxidation resistance, so it is expected to be used as a high-temperature structural material.

Recently, applications to electronic materials have also been considered, and the work function (3.
25ev), less evaporation at high temperatures, and is chemically stable, its use as an electron emitter material, especially a field emitter material, is being considered.

Single crystal is used as the field emitter material,
Sintered bodies are usually used for applications such as carbide materials and high-temperature structural materials, but in order to accurately understand the characteristics of the material, it is important to use crystals with sufficient characteristics.

The following methods are known as conventional methods for producing titanium carbide.

(a) Flux method: Use metals such as Fe and AJ as flux, add Ti or TiOt and graphite powder, mix and dissolve, keep at a constant temperature, cool to room temperature, and dissolve the flux metal with hydrochloric acid, etc. and how to obtain titanium carbide crystals.

(Note) Gas phase method: A method in which T+Cl4 and hydrocarbon or CO gas are transported using hydrogen or the like as a carrier gas, and thermally decomposed on a high-temperature substrate to precipitate titanium carbide.

(c) Arc Bernoulli method In the so-called Bernoulli method, in which raw material powder is supplied from the upper part and crystals are grown while being heated and melted in an oxyhydrogen flame, J high frequency plasma is used instead of the oxyhydrogen flame as the heating source.

(d) Floating zone method (hereinafter referred to as FZ method)
(This will be explained in detail later.) However, the flux method (a) generally yields only microcrystals, and it is not possible to obtain highly pure crystals because the contamination of impurities from the Franks metal crucible is unavoidable. There are drawbacks.

The vapor phase method described above has the disadvantage that only acicular or columnar microcrystals can be obtained.

In (c), the Arc Bernoulli method, the crystals obtained are relatively small, and when the crystal structure is examined, it is found that precipitation of free carbon is observed, as well as a twin structure, low-angle grain boundaries, etc., and the crystals are of good quality. There is a drawback that you cannot get something like that.

The FZ method (d) is said to yield comparatively large crystals, with no precipitation of free carbon, and is said to yield crystals of the highest quality among the above-mentioned methods.

The present inventors attempted to grow titanium carbide crystals using the FZ method in order to obtain large, high-quality crystals. Although the crystals produced using the FZ method are certainly excellent in purity and crystallinity, It was found that there are significant problems associated with variations in chemical composition at different sites.

That is, by chemical analysis and physical property measurements of the obtained crystals,
It was found that the composition of the grown crystal changes continuously from the beginning to the end.

Next, the FZ method will be described.

Figure 1 is a conceptual diagram of the FZ method, and the device is, for example, A
A high-pressure type crystal growth stream manufactured by DL Corporation is used.

One end of two sintered bodies having a length of about 1 to 20 cr is brought into contact, the other end is supported by a holder, and the contact portion is heated, for example, by high frequency to generate a melt zone.

Thereafter, when both sintered bodies are moved upward or downward together, crystals gradually continue to grow.

The appropriate moving speed at this time is 0.5 to 5011/hr, and the moving direction can be either upward or downward.

An inert gas or Hφ3 is used as the atmosphere, but it is usually Ar, He, or a mixed gas thereof.

The atmospheric gas is used under pressure in order to suppress the evaporation of the components of the sintered body, and the appropriate pressure is usually 2 to 30 atmospheres, preferably 3 to 25 atmospheres.

If the pressure is lower than this, the evaporation suppressing effect is almost lost, and if the pressure is higher than this, heat loss due to convection becomes large, which is not preferable.

Furthermore, He has a better effect of suppressing discharge between the coils and between the coil and the sintered body than Ar.

This is considered to be because the ionization potential of He is higher than that of Ar.

The above are the conditions for manufacturing titanium carbide crystals using the normal FZ method. However, the range of 30+s from the starting end (starting part of FZ) of a crystal grown under these conditions is usually a collection of several grains. It is a polycrystalline body consisting of a body, but after that the center grows into a single grain.

On the other hand, the periphery is usually covered with a polycrystalline skin of about 1 m or less.

It can be confirmed from the X-ray Laue method and observation of etching images that the central portion is a single crystal with good crystallinity.

However, chemical analysis of the starting and ending parts revealed a clear difference in carbon content.

For example, as shown in Comparative Example 1, composition x=0.932 (
The carbon content at the beginning and end of a crystal with a length of approximately 6 cm grown using a sintered body Rondo (X = C/Ti) as a raw material was 18.25% by weight of total carbon (in which free carbon was 0).
．． 01 weight ratio) and 19.01 weight ratio (free carbon 0
．． 00 weight ratio), a clear difference in carbon content is recognized.

It is known that the amount of carbon vacancies in titanium carbide strongly influences the electrical resistance of titanium carbide.

Therefore, measuring the electrical resistance of a crystal can reveal relative changes in titanium carbide composition.

The electrical resistance of the above crystal was measured at 511 intervals, and the results are shown in FIG.

As is clear from this, the resistance decreases almost continuously from the start end toward the end, and X increases toward the end.

This agrees well with the carbon content analysis results, and it is clear that there is variation in the crystal composition.

The cause of this will be explained with reference to FIG. 4. In the normal FZ method, a sintered body having a composition of Xs is melted at a temperature T2 to form a melt zone.

The composition of the crystal obtained at this time is X8'.

As the fusion zone continues to move, the composition of the fusion zone changes to Xs along the liquidus line.
At the same time, the crystal composition shifts from Xs' to Xs along the solidus line, and the crystal composition changes.

The present inventors conducted studies on various compositions, and found that titanium carbide has compositions other than around x=0.82.
Anharmonic melting (the coexisting solid and liquid phases have different compositions).

It was also concluded that for anharmonic melting compounds, compositional fluctuations are an essentially unavoidable problem when using the normal FZ method.

The present invention aims to solve this problem.

The present inventors investigated various methods for obtaining crystals without compositional fluctuations, and as a result, the problem was solved by method T shown below.

First, using a sintered rondo with a well-defined composition, a crystal rondo with a length of several boxes is produced by the FZ method.

The carbon content at the beginning, end, and solidified melt zone of this crystalline rond is analyzed.

From this, we will find the solid phase composition of titanium carbide and the liquid phase composition that coexists in equilibrium with it.

Next, if a similar experiment is performed by changing the composition of the raw material Rondo, the correspondence between the liquidus line and the solidus line can be clarified.

FIG. 3 shows the liquidus line and solidus line obtained in this manner.

(The temperature axis has an arbitrary numerical value.) FIG. 3 is a Ti-C system phase diagram, and an arbitrary point (XQrO) indicates the state of a composition c/Ti=XQ at a temperature To.

As shown in the figure, when (XOTO) is in the region surrounded by solid line B, it indicates that it consists only of the crystalline phase of titanium carbide, and in the region surrounded by liquidus line A and solidus line B (TiC
The symbol X+L indicates that the crystalline phase of titanium carbide and the liquid phase coexist, and the region above the liquidus line A (indicated by L) consists only of the liquid phase.

When the current point (xoTo) is on the solidus line B, the liquid phase composition or temperature T can coexist in equilibrium with the solid phase composition xO.

The value of X at the point where the horizontal line crosses the liquidus line A outside the solidus line B will be shown.

This chart shows how much the liquid phase composition should be in order to obtain a crystal with a certain composition.

For example, if you want to obtain a crystalline Rondo with a composition x = 0.9, if you perform the FZ method with the composition of the sintered Rondo as x = 0.90 and the composition of the fusion zone as x = 0.99, x = A crystalline rond with a uniform composition of 0.90 is obtained.

Examples of methods for changing the structure of the sintered body Rondo to the melt zone composition include the following methods.

(1) Of the two sintered body ronds, the purpose is the lower sintered body rond, which has a titanium carbide solid phase composition;
A method in which the composition of the upper sintered body Rondo is a liquid phase composition that coexists in equilibrium with the solid phase composition, the upper sintered body Rondo is melted to form a melt zone, and both Rondos are moved upward as a unit.

(2) The upper sintered body Rondo has the desired titanium-carbide solid phase composition, and only its tip has a liquid phase composition that coexists in equilibrium with the solid phase composition, and the tip of the lower sintered body Rondo has crystal seeds. A method in which the tip of the upper sintered body rond is melted to form a fusion zone, and both ronds are moved downward.

The moving speed of the sintered body Rondo is preferably slower than that of the normal FZ method, and is 0.5 to 20 mx/hr, preferably 1
~10 Il/hr is suitable.

If the movement is faster than this, diffusion in the fusion zone will be insufficient,
Compositional fluctuations occur within the melting zone, and if the moving speed is made slower than this, the melting zone cannot be kept stable and is not economical.

In order to keep the composition of the melt zone uniform, it is effective to add a rotation operation.

That is, the shafts to which the upper and lower samples are fixed are rotated in opposite directions to thoroughly mix the melt zone.

The appropriate rotational speed is usually about 10 to 60 r)1.

Further, the atmosphere for growing the crystal is the same as in the case of the normal FZ method.

The raw material sintered body Rondo used in the present invention is titanium.
Data such as melting point, hardness, work function, and mechanical strength of carbide all strongly depend on the chemical composition of titanium carbide, so unless a material with stable physical properties is provided, crystalline rondite with a uniform composition will not be produced. cannot be made.

Titanium carbide has a wide composition range (0.5
<x<1.

It is a non-stoichiometric compound that exhibits an NaC# type structure (which is assumed to be 0) and has stable vacancies at carbon lattice positions.

Therefore, in order to obtain crystals of various compositions, raw material preparation is first necessary.

The raw material powder can be produced by (1) reducing Ti02 powder with carbon, (2) reacting Ti metal or TiH2 with carbon, and (3) reacting Ti with carbon using Fe, A7, etc. as a flux. There are known methods to do this.

In either method, the desired composition can be obtained by appropriately blending the amounts of Ti and carbon.

The higher the purity of the raw material, the better, usually 98 coefficients or higher, preferably 99 coefficients or higher.

The finer the particle size of the raw powder, the more convenient it is for molding.

Generally, the average particle size is 10 μm or less, preferably 5 μm or less.

If it exceeds 10 μm, the density of the molded product will not increase, making the molded product weak in strength and difficult to handle.

The shape of the molded body is a prism (for example, 10X10X20
0i+1,15X15X10011) or a cylinder (for example l
OψX1501m) is commonly used.

As the molding method, it is preferable to use a rubber press in order to obtain a molded product with as uniform a density as possible.

The molding pressure is usually about 1 t/cm'. The compact is sintered by a suitable method.

Sintering is usually carried out at 1700-2300°C, preferably at 1750°C.
~2100°C.

If the temperature is lower than 1700°C, the density of the sintered body is low, which not only makes it impossible to obtain sufficient strength, but also causes various problems such as difficulty in forming a stable melt zone in the subsequent crystal growth process.

If the temperature exceeds 2300°C, the sintered body becomes too dense and difficult to process, and is also unfavorable from the viewpoint of energy consumption.

The sintering time is usually 0.3 to 5 hours, preferably 0.5 to 3 hours at the maximum temperature.

The sintering atmosphere is vacuum, inert gas, H2, or C.
The sintering furnace in which O is used may be any type of furnace as long as it satisfies the above conditions, but it is convenient to use a high frequency induction heating furnace, for example.

The sintered body obtained under these conditions usually has a theoretical density of 7.
Although the density is in the range of 0 to 95, it is preferable to use a sintered body Rondo having a density of this level as a raw material in the subsequent crystal growth step using the FZ method.

Note that in the sintering process, evaporation of carbon and TiO occurs, so the chemical composition of the sintered body usually deviates somewhat from the composition at the time of raw material preparation.

Therefore, to strictly control the composition of the sintered body,
It is preferable to keep in mind the correspondence between the blending composition of the raw materials and the composition of the sintered body.

When the method of the present invention is used, it has an excellent effect that high-quality, large-sized crystals of various compositions having a substantially uniform composition can be obtained very easily. 19.13 weight factor, free carbon 0.08 weight factor, average particle size 1.5 μm), a cylinder with a diameter of about 10 mm and a length of 150 mm was formed using a rubber press, and this was heated in a vacuum for 1800 mm. Sintered at ℃ for 30 minutes.

The composition of the sintered body was x=0.932 from carbon analysis.

From Figure 3, it can be seen that the liquid phase composition corresponding to the above composition is x = 1.3, so 7g
of carbon for emission analysis was added, and after molding (about 10 mm in diameter)
(length: 5 Qim) sintered in vacuum at 1800° C. for 1.5 hours.

The composition of the sintered body was x=1.29 based on carbon analysis.

This was used to form a melting zone, and the sintered body Rondo (x=0.9
FZ was performed using 32) as the lower rond.

That is, after a part of the melt zone forming field is melted to generate a melt, the sample is slowly moved to the upper part.

Gradually, the lower rondo dissolves into the fusion zone, while crystals grow above the fusion zone.

The sample was moved at a speed of 5 mW/h1 for about 12 hours.

Incidentally, the upper and lower rotations were kept at 20 rpm, and 15 atmospheres of He was used as the atmospheric gas.

Chemical analysis of the starting and terminal ends of the obtained crystal revealed that the carbon content (total carbon) was 18.90% by weight and 18.97% by weight, respectively, and within the analysis error range, the values were almost identical. It was returned.

Example 2 Same as Example 1, 4.7 g of T per 100 g of Tic raw material powder
Prepare a powder mixed with iO2 powder.

On the other hand, apart from this, x per 100g of the same raw material powder. A powder containing 25 g of carbon for emission analysis is prepared.

Next, a molded body (diameter: approximately 10 mm, length: approximately 150 mm) is made using a rubber press, with only the tip of the upper mouth made of the former composition changed to the latter composition. Sintered for half an hour.

When a part of the sintered body was cut out and analyzed for carbon, the composition of the sintered body calculated from the carbon content was x=0.89, except for the tip part, where x=0.99.

Using this rond as an upper rond, crystals were grown by the FZ method.

That is, first, the tip of the upper sintered body (composition x = 0.9
Melt only part 9).

Thereafter, the entire sintered body Rondo is slowly moved downward.

The upper sintered body Rondo gradually dissolves into the melting zone, while crystals grow in the lower part of the melting zone.

The sample was moved at a speed of 3.5 ti/hr for about 14 hours.

The rotation was 30 rll for both the top and bottom, and 7 atmospheres of He was used as the atmospheric gas.

Chemical analysis of the starting and terminal ends of the obtained crystals revealed that the carbon content (total carbon) was 18.19% by weight and 1% by weight, respectively.
The result was 8.26 weight, and the value was changed within the analytical error range.

Comparative Example 1 Using the same commercially available TiC raw material powder as in Example 1, Example 1
Shape and sinter under similar conditions.

Using this sintered body rod as a raw material, the contact portion was melted and crystals were grown by the usual FZ method.

The crystal production conditions are as follows.

That is, the moving speed is 5 mt/hr, the moving direction is 20 rfl on rotation, and the atmospheric gas is Hel 5 atm.

A crystal with a length of about 6 cr was grown under the above conditions, and chemical analysis of the starting and ending parts revealed that the carbon content (total carbon)
were 18.25 weight ratio and 19.01 weight ratio, respectively.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram of the floating zone (FZ) method.
Figure 2 shows the electrical resistance of titanium carbide, Figure 3 shows the electrical resistance of Ti
Phase diagram of C-C system, Figure 4 shows solid phase composition Xs and liquid phase composition xL
The phase diagram with 1: Shaft, 2: Holder, 3: Sintered rod, 4:
TiC crystal rod, 5: melt zone, V6: high frequency induction heating coil, A: liquidus line, B: solidus line, C: eutectic point.

Claims (1)

[Claims] 1 One end of the two sintered bodies is brought into contact, the other end is supported by a holder, and both sintered bodies are heated in a pressurized inert gas or hydrogen atmosphere to form a melting zone. In the method of manufacturing titanium carbide crystals by moving the rond upward or downward, the purpose is to grow crystals with a liquid phase composition that coexists in equilibrium with the solid phase composition of titanium carbide, with the aim of achieving a melt zone composition. A method for producing a titanium carbide crystal characterized by: