JPH0127017B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a magnesium fluoride (MgF ₂ ) polycrystal and a method for producing the same.

MgF ₂ has good transmission performance for visible, near-infrared, and infrared light with a wavelength of 9 μm or less, so it is used in window materials for infrared equipment, infrared light filters, etc. In many cases, it is produced and used in the form of a single crystal, but in cases where large dimensions are required and for applications where high mechanical strength is required, it is relatively easy to produce large materials, and A polycrystalline material having a fine crystal structure is more suitable.

(Prior art and its problems) Conventionally known methods for producing MgF ₂ polycrystals include using MgF ₂ powder as a raw material, hot pressing using a mold, hot isostatic pressing, hot pressing and hot pressing. This is a method of increasing the density and obtaining infrared transparency using a method that combines isostatic pressing. According to these methods, it is possible to obtain a polycrystalline material of very good quality, and it is possible to manufacture a polycrystalline material having an infrared light transmittance of 70% or more (thickness of 3 mm) in the wavelength range of 2 to 8 μm.

However, the light transmission performance in the range from so-called visible light with a wavelength of 0.4 to 2 μm to the near-infrared region is not so good, and as shown by the solid line in FIG. 1, the transmission performance decreases rapidly as the wavelength becomes shorter. For optical systems that use the 0.4-2 μm band in addition to the 2-8 μm band, it is necessary to improve the transmission performance in the 0.4-2 μm band as much as possible.

(Objective of the Invention) In view of the above facts, the present inventors have improved the translucency in the wavelength range of 0.4 to 2 μm, and created a MgF ₂ polycrystalline material with good translucency in the wide wavelength range of 0.4 to 8 μm. With the aim of obtaining this, various manufacturing methods were investigated.

(Contents of the invention) As a method for achieving the above object, the present inventors investigated adding various additives to _MgF2 raw material powder in various proportions, and determined the optimum additive elements and amounts. I was able to find out. That is, a raw material powder containing at least one of V, Fe, Cu, and Ag as an additive element of 10 ppm or more and 500 ppm or less is used, and this is hot pressed to a density of 98% of the theoretical density.
By increasing the density, wavelengths of 0.4 to 8 μm can be achieved.
It has become clear that MgF ₂ polycrystals with improved light transmission performance can be obtained.

As a result of the study, the types of additive elements are V, Fe,
Four types were adopted: Cu and Ag. These elements are
Although each of them was effective when added alone, effects were also observed when two or a combination of three to four types were added.

As for the method of addition, it is desirable that the raw material powder be mixed as uniformly as possible. The present inventors adopted a method of wet-adding these additives in a solution state during the raw material powder synthesis process. However, it is thought that similar effects can be observed even by dry mixing in a powdered state. Non-uniformity in the mixed state appears as segregation of these elements in the hot-pressed material, and is visually observed as black dot-like inclusions.

The amount of additional elements added is limited to 10 ppm or more and 500 ppm or less. The reason for this is that if it is less than 10 ppm, no improvement in transmission properties is observed, and if it exceeds 500 ppm, the segregation of the added element in the hot-pressed material becomes significant in both quantity and size, and has a negative impact on the transmission properties. .

Figure 2 A, B, and C show the improvement in transmittance at a wavelength of 1.5 μm when the added amount of each element was changed and hot-pressed under exactly the same conditions.
In the case of Cu or Ag, it has a gentle peak around 500 ppm, and in the case of V shown in B, it is several hundred ppm.
is the optimum range, and C indicates the case of Fe. When a mixture of several elements was added, an average effect was observed.

In order to examine the cause of the effect of improving translucency due to the addition of these elements, we observed the fractured surfaces of each hot-pressed sample using a scanning electron microscope (SEM). A tendency for the average grain size to become finer was observed. That is, the average crystal grain size of the non-additive material is about 2 to 4 μm, while that of the additive material is about 0.5 to 2 μm, and this change is considered to have an effect on the improvement of visible to near-infrared light transmittance.

In addition, as a result of observing these samples with an optical microscope, fine (several tens of micrometers or less) point-like inclusions were dispersed, and these inclusions were segregated V, Fe, Cu, Ag, etc. It became clear. It is considered that the effect of preventing grain growth by these precipitates also contributes to the refinement of crystal grains due to the addition of elements.

In addition, the hot press conditions include a temperature of 550~
A temperature range of 850°C and a pressure of 1.5 to 4.0 ton/cm ² was used. If the temperature is below 550°C, sufficient densification of 98% or more of the theoretical density is not achieved, and the transmitted light is scattered by the remaining pores, resulting in a significant decrease in transmittance.

This is thought to be because the MgF ₂ powder does not undergo sufficient plastic deformation at this temperature. In addition, at temperatures exceeding 850°C, grain growth becomes significant and vacancies are trapped inside coarse crystals, so the effect of reducing vacancies due to diffusion through grain boundaries is reduced and the vacancies remain, resulting in sufficient light transmittance. It was found that it was not possible to obtain Furthermore, it was found that grain growth also caused a decrease in mechanical strength.

Regarding the pressure conditions of hot press, 0.5~
As a result of investigation in the range of 4.0 ton/cm ² , it was found that good translucency can be obtained at 1.5 ton/cm ² or more.

If the pressure is less than 1.5 ton/cm ² , sufficient densification of 98% or more of the theoretical density cannot be achieved, and if the pressure exceeds 4.0 ton/cm ² , it becomes uneconomical in terms of equipment capacity.

The chain line in Fig. 1 indicates the thickness of 3 mm obtained by the method of this invention.
An example of the infrared light transmission characteristics of MgF ₂ polycrystalline material is shown.
This shows that the transmittance in the 0.4-2 μm region is clearly improved compared to FIG. 1A without additive elements.

Example Chemical synthesis process of MgF ₂ raw material powder (solution reaction)
In each case, Cu was 500ppm, Ag was 500ppm, and V
After adjusting the content to contain 200ppm of Fe and 500ppm of Fe, dry and crush it to obtain a powder with a purity of 99.8% and a primary particle size of 0.2~
It was made into a powder of 0.4 μm. Both of these powders have an inner diameter of 15
It was filled into an alumina mold of mmφ and hot pressed in a vacuum of 10 ⁻³ torr at a temperature of 650° C. and a pressure of 2.5 ton/cm ² for 30 minutes. This sample was polished to a mirror finish on both sides using 0.3 μm alumina abrasive grains to a thickness of 3 mm, and spectroscopic measurements in the visible to infrared range were performed using a double beam spectrometer.

On the other hand, a sample hot-pressed in the same manner as above with the contents of V, Fe, Cu, and Ag reduced to a total of 10 ppm or less was similarly polished and subjected to spectroscopic measurements.

As a result, the latter showed light transmission characteristics equivalent to those in Figure 3A, and the former both showed light transmission characteristics of 0.4 to 0.4, which were equivalent to those in Figure 3B.
It showed improved light transmission properties in the 2μm band.

In particular, according to the present invention, MgF ₂ polycrystals with good translucency in the wavelength range of 0.4 to 2 μm can be easily obtained, and
This opens the door for applications that require optical transparency over a wide wavelength range of ~8 μm.

[Brief explanation of drawings]

Figure 1 shows the spectral characteristics of two typical types of MgF ₂ .
FIGS. 2A to 2C each show the relationship between the amounts of Cu, Ag, V, and Fe added and the transmittance of a 3 mm thick material at a wavelength of 1.5 μm.

Claims (1)

[Claims] 1 At least one of V, Fe, Cu, and Ag
_MgF2 polycrystal containing 10ppm or more and 500ppm or less. 2 At least one of V, Fe, Cu, and Ag
A method for producing MgF ₂ polycrystalline material, characterized in that the density is increased to 98% or more of the theoretical density by hot pressing raw material powder containing 10 ppm or more and 500 ppm or less.