JPS6363506B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention mainly uses polycrystalline lithium fluoride (LiF), calcium fluoride (CaF ₂ ) for optical applications.
and a method for producing magnesium fluoride (MgF ₂ ).

LiF, CaF ₂ and MgF ₂ are known as excellent optical materials that can be used as prisms, lenses, filters or other light transmitting windows for visible light, infrared, ultraviolet and X-ray spectroscopy.

However, it is not easy to obtain them as transparent homogeneous solids of a size that can be used as optical materials, and conventional methods have relied on growing single crystals over a very long period of time. For this reason, larger crystals inevitably become progressively more expensive, and since single crystals generally have open wall planes, they have the disadvantage of being easily damaged.

On the other hand, polycrystalline solids obtained by sintering powders of LiF, CaF ₂ and MgF ₂ are
Compared to growing single crystals, they can be mass-produced in an extremely short time, and even if stress is applied, such polycrystals have the effect of dispersing stress, making them less mechanically resistant than single crystals. It has the advantage that there is no extremely weak axis.

Therefore, for example, Japanese Patent Publication No. 42-530 proposes a method and apparatus for producing transparent polycrystalline calcium fluoride solid by hot pressing and sintering calcium fluoride powder in a vacuum. . However, in this type of method, the calcium fluoride powder is press-molded while being heated exclusively from the outside, that is, the mold filled with the powder is heated by a heater or high-frequency induction heating, so the heating efficiency is low. It is difficult to obtain a uniform and distortion-free sintered body because the heating temperature and temperature control are limited, and the temperature distribution is not uniform.
Furthermore, in order to prevent bubbles from forming in the sintered body, sintering must be performed under a high vacuum, which poses a problem in that the entire apparatus becomes complicated.

The present invention has been made based on the above-mentioned viewpoints, and the gist thereof is to prepare, for example, about 100 kg of LiF, CaF ₂ or MgF ₂ powder raw material filled into a mold made of an electrically insulating high-resistance material. It is kept under relatively light pressure (on the order of around / ^cm2 or less), preheated to a predetermined temperature using an external heat source, and heated to at least
Around 700Kg/cm ² or more, usually about 2000-3000Kg/cm ² ,
Or, by sintering with a high voltage power source on the order of several KV or more under heavy pressure,
The object of the present invention is to obtain a polycrystalline LiF, CaF ₂ or MgF ₂ solid suitable as an optical material, that is, with few micropores and microstrains.

When this type of current sintering (including discharge sintering and resistance sintering) is used, micro discharges and Joule heat are generated between pressurized powder particles, unlike when using simple external heating. Since sintering is performed by electric field diffusion and thermal diffusion of ions of powder molecules, compared to the case of mere hot pressing as described in the above-mentioned Japanese Patent Publication No. 42-530,
A much more uniform and better quality sintered body can be obtained. In addition, gas molecules attached to the particle surface are also removed by the micro-discharge, so the generation of micropores is reduced, and this current sintering does not necessarily have to be carried out in a strict vacuum atmosphere. can be carried out efficiently with relatively simple equipment.

Therefore, in order to conduct current sintering, it is necessary for the raw material powder to have a certain degree of conductivity, but LiF, CaF ₂ and MgF ₂ powders have high conductivity close to that of insulators at room temperature. It is a resistor and cannot be sintered using a normal method. However, these substances are compounds made up of molecules formed by ionic bonds, and therefore, when they are heated to loosen their bonds with thermal energy, they become conductive due to the vibrational fluctuations of the ionic particles. It comes to have. The conductivity ρ due to fluctuation is given by the following equation (1) using Maxwell-Boltzmann's probability distribution law.

ρ=Cexp(-Q/kT)...(1) (where, C: Constant specific to the substance Q: Activation energy specific to the substance k: Boltzmann constant T: Heating temperature) Therefore, the specific resistance r is Since it is the reciprocal of the conductivity ρ, it is expressed as r=C'exp(Q/kT)...(2), and it is understood that as the heating temperature T increases, it converges to a constant C'. Ru.

Figure 1 is a graph showing the results of actual measurement of changes in specific resistance with respect to temperature of semi-sintered powder compacts of LiF, CaF ₂ and MgF ₂ , and it is also affected by contact resistance between compact powder particles. Although it does not completely match equation (2), it shows almost the same tendency. As you can see from this graph, all of these compounds have extremely high resistance at room temperature or below 100℃, but as the temperature rises, this value decreases rapidly and reaches a certain value unique to the substance. Therefore, in the case of LiF, about
If heated to around 200℃ or higher, in the case of CaF ₂ to around 40℃ or higher, and in the case of MgF ₂ to around 500℃ or higher, it has sufficient conductivity to perform current sintering. It turns out that it will be like this. Comparing this to a good conductor such as a metal material or graphite, the resistance value is still several orders of magnitude higher, but the electrical resistance of an object with a specific shape is proportional to the specific resistance and thickness of the material, and Since it is inversely proportional to the area, the optical materials related to the present invention generally have a large area and a small thickness, so that the resistance can be reduced by two to three orders of magnitude in terms of shape. Therefore, during sintering, a voltage as high as estimated from the resistivity is not required.

Therefore, when sintering a material having such a relatively high resistance value by applying electricity, a method of preheating the raw material powder is known, for example, as described in Japanese Patent Publication No. 53-4239. As shown in the figure, raw material powder is filled into a conductive mold such as graphite material,
Electricity is applied through the counter electrode punch that fits into this mold, and in such a case, a large amount of current first flows toward the conductive mold with a low resistance value.
As a result, the mold is first heated, and the heat acts on the filling powder to reduce the resistance of the filling powder, and when the resistance value becomes equal to or lower than the resistance of the mold, a large amount of current flows toward the filling powder. Thereafter, energization sintering proceeded rapidly. Therefore, in this case, the conductive mold filled with the raw material powder itself becomes the heating means, but in such a case, the current is divided between the mold and the raw material powder, so the sintering process is It has been difficult to adjust the current to keep the power flowing through the raw material powder constant as the injection energy for this purpose, and it has been difficult to obtain a sintered compact with uniform characteristics.

In order to solve the problems mentioned above in the conventional preheating of raw material powder, in the present invention,
A mold to be filled with the raw material powder is made of at least an electrically insulating high-resistance material, and the raw material powder is preheated by a separate heating means provided outside the electrically insulating part of this mold. When the raw material powder reaches a predetermined temperature, a voltage is applied to the raw material powder to perform energization sintering. As the electrically insulating, high-resistance material for the mold, a material that has a resistance higher than that of the raw material powder to some extent or sufficiently higher even at high temperatures, such as BN, Si ₃ N ₄ , AlN, etc., is used. I'll do what I do. In this way, energy efficiency is high because at an appropriate stage, instead of external heating, which has poor heating efficiency, it can be switched to energized heating that generates heat by energizing itself. Since it is completely separated from the power supply, it is easy to keep the power for energization sintering constant, and it is possible to obtain a uniform and high quality sintered body.

Here, the method of the present invention will be specifically explained with reference to FIG. 2, which shows an embodiment of an apparatus for carrying out the method of the present invention.

In the method of the present invention, first, raw material powder 1 of LiF, CaF ₂ or MgF ₂ is produced by known chemical means and adjusted or pulverized to a predetermined particle size.
, heat-resistant electrically insulating high-resistance materials (e.g.
A heat-resistant and pressure-resistant metal alloy made of Al ₂ O ₃ , Si ₃ N ₄ , BN, etc.) was filled in the mold 2 and inserted from both ends of the mold.
Light pressure is applied using conductive electrode punches 3, 3 made of a carbon material or the like that has been subjected to high pressure resistance or high pressure resistance treatment or processing.
Next, this insulating mold 2 is heated slowly from the outside to the extent that it will not break due to thermal shock. As the heating means, any known means can be used, such as heating with an electric wire heater wrapped around the mold 2, or fitting an outer mold 4 made of graphite around the mold 2 as shown in the figure. This outer mold 4 is fitted with a heating electric 6.
Then, the outer mold 4 is heated by induction heating by supplying an appropriate current such as direct current, or by induction heating by supplying a high frequency current from the heating power source 6 to the induction coil 5 provided around the outer mold 4. may be transmitted to the raw material powder through the mold 2. While detecting this temperature rise with, for example, a thermocouple 7 embedded in the mold 2, the temperature at which the resistivity of the raw material powder becomes approximately 1 to 3 MΩ, that is, as shown in FIG. Approximately 200 to 700, depending on the type
The temperature is maintained at a predetermined temperature within the range of 0.degree. C. for several minutes, determined according to the type of powder, to ensure complete preheating. After that, a constant power voltage of about several KV or more is applied between the electrode punches 3 and 3 by the sintering power source 8, and at the same time, the sintering power source 8 applies a constant power voltage of about several KV or more, for example, about 2000 to 3000 Kg/
A heavy pressure P of cm ² is applied to carry out electrical sintering of the raw material powder. The temperature of the preheating described above may be around the above temperature or lower if a higher voltage power source 8 is available for use as the sintering power source 8. Conversely, if the power source 8 is not a high voltage power source, a higher temperature preheating is required. The preheating temperature and other conditions are selected as necessary, taking into consideration the type of target sintering raw material powder, heating efficiency, and various other economical factors. Power source 8 for energizing sintering above
may be a direct current, but if a medium-frequency alternating current of around 2KHz or less is superimposed on the direct current, micro-discharges will be induced between the powder particles, and the current density will be approximately uniform in each part. A high quality sintered body can be obtained from this location. In the conventional electric sintering method, the molding pressure is generally around 300 to 500 kg/cm ³ , but since the purpose of the present invention is to manufacture optical materials, there are no pinholes or pinholes that do not affect the density. Internal scattering occurs even in the gaps between powder particles on the order of microns, reducing light transmittance. Therefore, in order to plastically deform the powder particles themselves and remove even the slightest gaps between particles, electricity is supplied using a power source with a not very high voltage. The above-mentioned approximately 700 to 3000
It uses extremely high pressurization of Kg/cm ³ or more and electricity from a high voltage power source of around several KV or more. Due to this high pressure, the particles come close enough to each other and come into contact with each other, and the ions in the particles move between particles due to the effects of electric field diffusion and thermal diffusion, and gas is exhausted to form a homogeneous and strong sintered compact. form. In addition, gas molecules attached to the particle surface are ionized due to the micro-discharge between the particles, diffused between the raw material molecules, and then discharged, so even if the electrical sintering described above is not necessarily performed in a vacuum, pinholes caused by air bubbles will be eliminated. Does not occur. After sintering, the sintered body may be allowed to cool naturally by turning off the current from the normal power supply and pressurization at the same time, or the heating means for preheating may be used if necessary to prevent thermal shock, etc. , slow cooling by other conventional methods,
Alternatively, it is taken out from the mold 2 while slowly cooling and gradually reducing the pressure.

Examples of the present invention are shown below.

Example 1 6g of LiF powder of −400 mesh was placed in a 30mm inner diameter
Place it in an Al ₂ O ₃ mold and apply light pressure of about 10 kg/cm ² from the top and bottom using electrode punches made of heat-resistant steel (SK material or SUS304 material). The outside of the mold is reinforced with a heat-resistant steel ring. In this state, the temperature of the mold and the LiF powder inside the mold is raised at a heating rate of approximately 5° C./min using an electric heater provided around the outer periphery of the mold, and the temperature is maintained at approximately 300° C. for 4 minutes. Next, the pressure is increased to about 2000 Kg/cm ² and at the same time, a DC voltage of 5 KV is applied between the upper and lower electrode punches to generate electric discharge and Joule heat between the powder particles. The maximum output of the above DC power supply is
Set it to 800W. At the beginning of energization, the temperature of the object to be sintered is low and the specific resistance is still high, so the output is only about 400W, but the input power gradually increases due to internal heat generation of the raw material powder and ionization due to micro discharge. The set value is reached. After reaching the set value, maintain that state for about 10 minutes, and then turn off the sintering power and pressure. All of the above steps were carried out in the atmosphere, and the infrared transmittance (%) of the obtained LiF sintered body, which was polished and finished, showed a satisfactory value as shown in FIG.

Example 2 7g of CaF ₂ powder of -325 mesh was placed in a 30mm inner diameter
Place in a mold made of Si ₃ N ₄ . The outside of this mold is covered with a graphite outer mold, and this outer mold is directly energized.
The temperature is raised to approximately 400°C over a period of minutes and held for 4 minutes. After that, a pressure of 2000Kg/cm ² was applied and at the same time it became a direct current.
Apply a voltage with an effective value of 5KV, which is 1KHz superimposed with alternating current. The electricity that enters the raw material powder when electricity is turned on is
It was just under 150W, but this gradually increased. The maximum power was set at 1.3 KW, and after reaching this value, it was held for about 15 minutes, then the pressure and power were turned off at the same time, and the sintered compact was taken out. The above process was performed in a reduced pressure of 1Pa (Pascal), and the obtained
The infrared transmittance of the CaF ₂ sintered body showed a satisfactory value as shown in FIG.

Example 3 7g of MgF ₂ powder of -325 mesh was placed in a 30mm inner diameter
Place in a BN mold. The outside of this mold was covered with a graphite outer mold, and a 400 KHz high-frequency current was passed through an induction coil installed outside the outer mold, causing induction heating to slowly heat the outer mold over a period of about 30 minutes. After the raw material powder reaches approximately 500℃, hold it for 10 minutes, and then
A pressure of 2000Kg/cm ² and an effective value of 7KV of direct current, alternating current, and superimposed voltage were applied simultaneously. The maximum power was set to 1.56KW and held for about 30 minutes after reaching this set point, then the pressure and power were turned off. The atmosphere was a nitrogen flow of approximately 1 Pa throughout the preheating period to approximately 500°C and the subsequent electrical sintering period.
The infrared transmittance (%) of the MgF ₂ sintered body showed a satisfactory value as shown in FIG.

As described above, according to the present invention, a polycrystalline solid is obtained by sintering the raw material powder with electricity under heavier pressure than usual, so heating energy efficiency is higher than in the past, and micropores and It is possible to efficiently produce a more homogeneous and high quality optical material with less micro-distortion.

Note that the configuration of the present invention is not limited to the above-mentioned embodiments; for example, as a means for detecting the progress of preheating, instead of a thermocouple thermometer, the resistance value of the raw material powder may be detected between both electrode punches. It is also possible to pass a weak electric current and determine the timing of the start of sintering based on the change in the electric current. A heating means may be used to gradually cool the powder at a desired cooling rate over a long period of time.In short, the present invention is capable of cooling raw material powder filled in a mold made of an electrically insulating high resistance material to an appropriate external source. It is preheated by a heat source, and then energized by a high-voltage power source through an electrode punch to generate micro-discharge and Joule heat between the raw powder particles, internally generating heat in the raw material powder, and at the same time, at least 700 kg/cm ² Heavy pressure is applied around 2,000 to 3,000 Kg/cm ² or more, usually around 2,000 to 3,000 Kg/cm 2 or more, and energization sintering is performed by applying a small current compared to conventional energization sintering using a high voltage power supply of several KV or more. As a result, a polycrystalline LiF, CaF ₂ or MgF ₂ solid is obtained, and it is intended to cover all modifications within the scope of its basic configuration.

[Brief explanation of drawings]

Figure 1 is a graph for explaining the temperature-resistivity characteristics of powder compacts of LiF, CaF ₂ and MgF ₂ ;
The figure is an explanatory diagram showing an overview of the apparatus for carrying out the method of the present invention, and Figures 3 to 5 show infrared transmission of sintered bodies of LiF, CaF ₂ and MgF ₂ produced by the method of the present invention, respectively. It is a graph showing rate characteristics. 1...Raw material powder, 2...Electric insulation type, 3...
Electrode punch, 4... Outer mold, 5... Heating coil, 6
... Power source for heating, 7... Thermocouple thermometer, 8... Power source for energizing sintering.

Claims (1)

[Scope of Claims] 1. A method for producing polycrystalline lithium fluoride, calcium fluoride, and magnesium fluoride, which comprises the steps described in items a to c below. (a) A mold made of a high-resistance material with at least an electrically insulating inner surface is filled with lithium fluoride, calcium fluoride, or magnesium fluoride powder as a raw material, and the raw material powder is punched with electrode punches facing each other. The process of applying light pressure to the (b) The raw material powder is gradually heated from the outside of the mold to a predetermined temperature of at least 200°C and held at the predetermined temperature for a desired time of several minutes or more. Preheating process. (c) A step of applying a heavy pressure of at least around 700 kg/cm ² or more to the raw material powder after the above preheating via the electrode punch and energizing it with a high voltage power source of around several KV or more to carry out energization sintering. . 2. The method for producing polycrystalline lithium fluoride, calcium fluoride, and magnesium fluoride according to claim 1, wherein the preheating and electrical sintering are performed in the atmosphere. 3. Claim 1, characterized in that the above preheating and current sintering are performed in an inert gas atmosphere.
A method for producing polycrystalline lithium fluoride, calcium fluoride, and magnesium fluoride as described in 2. 4. The method for producing polycrystalline lithium fluoride, calcium fluoride, and magnesium fluoride according to claim 1, wherein the preheating and electrical sintering are performed in a vacuum. 5. The polycrystalline crystal according to any one of claims 1 to 4, wherein the preheating is performed by an electric heater provided outside the electrically insulating high-resistance material portion of the type. A method for producing lithium fluoride, calcium fluoride and magnesium fluoride. 6. Any one of claims 1 to 4, wherein the preheating is performed by energizing an outer mold made of graphite material provided outside the electrically insulating high-resistance material portion of the mold. 1. The method for producing polycrystalline lithium fluoride, calcium fluoride, and magnesium fluoride according to 1. 7. Claims 1 to 4, characterized in that the preheating is performed by high-frequency induction heating of an outer mold made of graphite material provided outside the electrically insulating high-resistance material portion of the mold. A method for producing polycrystalline lithium fluoride, calcium fluoride, and magnesium fluoride according to any one of the above. 8. Production of polycrystalline lithium fluoride, calcium fluoride, and magnesium fluoride according to any one of claims 1 to 7, characterized in that a DC power source is used as a power source for the energization sintering. Method. 9. Polycrystalline lithium fluoride according to any one of claims 1 to 7, characterized in that a power source in which an alternating current of several KHz or less is superimposed on a direct current is used as a power source for the energization sintering. Method for producing calcium fluoride and magnesium fluoride.