KR100244231B1 - Shadow mask for cathode ray tube - Google Patents

Abstract

The present invention relates to a shadow mask that prevents color bleeding (doming) caused by heat deformation in a color cathode ray tube and improves image quality by having an initial permeability characteristic, the configuration of which is pure iron (aluminum-killed steel) material The shadow mask for cathode ray tube is a technology related to the shadow mask for cathode ray tube, characterized in that the composite thin film layer of Fe, Ni, W is formed on one or both surfaces of the surface.

Description

Shadow Mask for Cathode Ray Tube

The present invention relates to a shadow mask for a color cathode ray tube, and more particularly to a shadow mask that prevents color bleeding (Doming phenomenon) caused by thermal deformation of the shadow mask and has excellent initial permeability characteristics.

Color cathode ray tubes are required for the fineness of the screen and reproducibility of vivid colors, and as the development of the transmission method increases the number of scanning lines, cathode ray tubes are required to cope with this problem.

In the color cathode ray tube, as shown in FIG. 1, a panel 1 coated with a fluorescent film 3 on an inner side thereof, and a funnel 2 coated with conductive graphite on an inner side thereof are fused with frit glass. An electron gun 6 for generating an electron beam 5 is enclosed in the neck portion 4 of the funnel. A shadow mask 7, which is a color-selective electrode, is supported by the frame 8 inside the panel. The outer circumferential surface of 2) is equipped with a deflection yoke 9 for deflecting the electron beam from side to side.

An inner shield 10 is fixed to the frame 8 to prevent the electron beam 5 radiated from the electron gun from being deformed by a geomagnetic field or a leakage magnetic field.

In the color cathode ray tube configured as described above, when the image signal is input to the electron gun 6, hot electrons are emitted from the cathode of the electron gun, and the emitted electrons are accelerated and focused toward the panel 1 by the voltage applied to each electrode of the electron gun. Will proceed.

At this time, the electrons are adjusted by the magnetic field of the deflection yoke 9 mounted on the neck portion 4 of the funnel 2, and the adjusted electron beam 5 is prevented from being deformed by the inner shield 10. Scanned to the front of the panel 1, the deflected electron beam 10 passes through the holes of the shadow mask 7 coupled to the inner frame 8 of the panel and is subjected to color screening. 1) A video signal is reproduced by colliding with each of the fluorescent films 3 coated on the inner surface to emit light.

In the structure, the shadow mask 7 uses pure iron (Fe) (AK steel: Aluminum Killed steel) having a thermal expansion coefficient of 12 × 10 ⁻⁶ deg ⁻¹ , and the electron beam emitted from the electron gun 6 is a shadow mask ( In the case of 7), the effective electron beam passing through the hole formed in the shadow mask is about 20%, and the remaining electron beam collides with the shadow mask and is converted into thermal energy, so the surface temperature of the shadow mask is heated to about 80 ° C.

Therefore, the shadow mask 7 is thermally deformed as the temperature rises, and thus the position of the hole formed in the shadow mask is changed so that the electron beam passing through the shadow mask hole does not hit a predetermined phosphor but strikes a phosphor located nearby. It causes color bleeding called doming, which does not reproduce the original color but reproduces other colors.

Recently, a shadow mask made of Invar alloy, which is a Fe-Ni-based material having excellent thermal expansion characteristics (1.5 × 10 ⁻⁶ deg ⁻¹ ), has been used, but it is a problem that it is much more expensive than pure iron in terms of price. Inexpensive shadow masks with low expansion are required.

In particular, in the trend of developing large screens of cathode ray tubes, electron guns using high current have been developed to improve the brightness and lifetime of image quality. Therefore, the energy of electrons from these electron guns is larger than that of electrons from conventional electron guns. As the temperature rise is accelerated, the color bleeding problem of the shadow mask is more serious and the vacuum characteristics are also a problem due to the generation of gas due to the temperature rise.

The present invention has been made in order to solve the above-mentioned problems with the shadow mask made of pure iron material, by forming a composite thin film layer of Fe, Ni, W on the surface of the shadow mask made of pure iron material thermal deformation of the shadow mask Its purpose is to prevent color bleeding of cathode ray tubes by suppression.

1 is a cross-sectional view showing a cathode ray tube structure

2A to 3B are longitudinal cross-sectional views showing an embodiment of the present invention.

2A is a state diagram in which Ni-W and Ni layers are sequentially coated on the surface of a pure iron thin film

2B is a state diagram in which Fe, Ni-W, and Ni layers are sequentially coated on the surface of a pure iron thin film

2C is a state diagram in which Ni-W, Fe, and Ni layers are sequentially coated on the surface of a pure iron thin film

2D is a state diagram in which Fe and Ni-W layers are sequentially coated on the surface of a pure iron thin film

2E is a state diagram in which a Ni-W layer is sequentially coated on the surface of a pure iron thin film

3A is a state diagram in which a blackened film layer is coated on a surface after heat treatment according to FIG. 2D.

3B is a state diagram in which a blackened film layer is coated on the surface after the heat treatment according to FIG. 2E.

4 is a distribution diagram of Fe, N, W composite according to the heat treatment conditions

Figure 5a is an X-RAY diffraction analysis before heat treatment

5b is an X-RAY diffraction diagram after heat treatment

6 is an X-RAY diffraction diagram after the blackening film treatment

Explanation of symbols for main parts of the drawings

7: shadow mask

The present invention for achieving the above object is a composite thin film layer of Fe, Ni, W is formed on one or both surfaces of the shadow mask surface made of pure iron thin film material.

The composite thin film layer is composed of a mixture of γ (Fe, Ni) and W phases.

In forming the composite thin film layer of the present invention, the Ni layer and the Ni-W layer on one or both surfaces of a shrimp mask surface made of a pure iron sheet (AK steel: AK steel) as shown in FIGS. That is, any one selected from Ni-W // Ni layer), Fe // Ni-W // Ni layer, Ni-W // Fe // Ni layer, Ni-W layer, and Fe // Ni-W layer After the thin film layer is formed and subjected to heat treatment, a composite thin film layer made of Fe, Ni, and W is obtained according to the mutual diffusion of the respective alloys including Fe in pure iron.

In addition, the present invention is to reduce the thermal expansion by reducing the temperature rise of the shadow mask while having an excellent thermal conductivity, and to form a blackening film layer on the composite thin film layer to suppress the discharge failure, there is another feature.

The present invention is a spinel-type Ni _x Fe _3-x O ₄ (0 ≤ X ≤ 3), iron oxide, and W _y O _z (0 ≤ Y ≤) after coating the blackening material on the upper surface of the composite thin film layer A blackening film layer of 5, 0 ≦ Z ≦ 5) is formed.

The heat treatment condition for the blackening film layer is applied according to the existing method, but the blackening film layer of the result of the composition formula is the result shown in relation to the composite thin film layer.

In producing the shadow mask of the present invention described above, the raw material of the shadow mask (as shown in FIGS. 2A to 2C) is a thin plate obtained by cold rolling pure iron, and has a thickness of 0.05 to 0.3 mm.

At this time, if the thickness is 0.05mm or less, the yield strength of the shadow mask is lowered, so that mechanical deformation is easily caused by external force. If the thickness is more than 0.3mm, the yield strength is increased and deformation is difficult, resulting in poor moldability.

In order to make the surface of the pure iron sheet uniform and to remove trace impurities, surface cleaning is performed in order to remove the oils or contaminations adhered to the electropolished surface so that the metal film to be plated or deposited adheres to the surface of the pure iron sheet. .

Surface washing methods include solvent washing with an organic solvent such as trichloroethylene, alkali washing boiling in an alkaline liquid, or electrolytic cleaning in which an electrolyte is degreased in an alkaline liquid, and the like may be combined.

As a method for forming a thin film layer as shown in Figs. 2a to 2e on the surface of the pure iron plate subjected to the surface cleaning, the sputtering or electroplating or metal spraying method is applied, the electroplating method is more effective for mass production.

The total thickness of the thin film formed on the pure iron sheet is within 1 μm to 15 μm. If the thickness of the thin film is thicker than 15㎛, the hardness of the thin film may be increased and the moldability may be deteriorated. If the thickness of the thin film is less than 1㎛, the thermal expansion coefficient is similar to that of a pure iron thin plate, so that the thermal expansion characteristics are not improved, and thus no effect.

In particular, in the Ni-W alloy film commonly formed in the above-described thin film structure, W is a high specific gravity of W in which electrons emitted from the electron gun collide with the shadow mask and are converted into thermal energy to cause a temperature increase of the mask, resulting in thermal expansion. Due to its excellent electron radiation and high heat capacity, it prevents the temperature rise of shadow mask and prevents thermal expansion.

Therefore, in the composition of Ni-W, W is 20 to 50% by weight, and Ni is 50 to 80% by weight.

If the content of W is less than 20%, the effect due to the above W is small and the coefficient of thermal expansion becomes large, making it difficult to achieve the purpose as a low thermal expansion shadow mask, and if more than 50%, the hardness increases and the workability worsens. Is generated.

The reason for forming the Fe thin film in the thin film structure is that when the heat treatment in the structure as shown in Figs. 2b to 2d, the diffusion of Fe, Ni, W ions between the Fe layer, the Ni-W layer, and the pure iron sheet (Fe) becomes active, and thus γ (Fe, This is because the formation of a composite thin film layer made of a mixture of Ni) and W becomes easier.

At this time, the thickness of the Fe thin film is 5 µm or less, preferably 1 µm or less, and more preferably 0.25 µm.

If the thickness of the Fe thin film is 5㎛ or more, it takes a long time for the diffusion of Fe, Ni, and W ions between the Fe and Ni-W layers during heat treatment, and the deformation of the tissue may occur due to the growth of crystal grains.

In addition, the role of the Ni layer is advantageous in improving geomagnetic properties because it forms a γ (Fe, Ni) phase by reaction with Fe as a result of heat treatment, thereby having low thermal expansion characteristics, low coercive force, high magnetic flux density, and high permeability. .

The thickness of the Ni layer is thinly formed to 1 μm or less.

Since Ni has a thermal expansion coefficient of 13 × 10 ⁻⁶ deg ^{−1, which} is similar to that of a pure iron thin plate, when the thickness is 1 μm or more, the thermal expansion characteristics may be deteriorated.

When Ni electroplating is applied to 1㎛ or less and uniform coating film is applied, Ni-W alloy film is easily formed on it, and when only Ni-W film is formed, surface strength of metal is improved and ductility is reduced. This difficult problem can be improved.

When the Ni layer is formed by electroplating, the strike plating method of instantaneously thin plating using a large current is effective, and the thickness is preferably 0.25 탆.

In the method of forming a Ni layer by the sputtering deposition method, the surface of the pure iron sheet is washed and dried, and then sputtered on the surface of the pure iron sheet using Ni or Ni-W alloy as a target in a vacuum of 10 ^-4 to 10 ^-8 torr. It can manufacture by forming a vapor deposition film.

At this time, if the vacuum condition is less than 10 ^-4 torr, the deposition is not good, the surface is also worse, and if it is more than 10 ^-8 torr, it takes a long time to reach the degree of vacuum and the deposition takes a lot of time.

The deposited film of the Ni layer is carried out at 1 µm or less as in the case of electroplating, preferably at 0.25 µm.

Alternatively, it can be produced by the metal spraying method.

As described above, a thin film layer as shown in FIGS. 2A to 2E is formed on the surface of the pure iron sheet, and then heat-treated.

The heat treatment may be performed before etching after forming the thin film layer, or may be performed after etching, or heat treatment before etching and heat treatment again after etching.

Heat treatment conditions are performed in 400 degreeC or more and 1,200 degreeC in oxygen-free atmosphere, such as hydrogen or argon atmosphere.

Preferably it is carried out at 700 ° C or more and 1,000 ° C and the heat treatment time is 1 minute or more and less than 2 hours.

Preferably it is within 5 minutes-1 hour.

If the temperature is below 400 ℃, the diffusion of atoms from the interface between the Ni-W layer and the pure iron is difficult to occur, and it takes a long time due to heat treatment, and there is almost no heat treatment effect. In addition, the grain growth of the Ni-W layer and the pure iron sheet is excessively grown and the structure becomes soft, making it unsuitable for use as a shadow mask.

In addition, the heat treatment time is not less than 1 minute and not more than 2 hours, and within 1 minute, the diffusion between atoms is insufficient, and thus it is difficult to form γ (Fe, Ni) and W alloys. There is a fear that the etching characteristics may deteriorate.

As the temperature increases, the heat treatment is performed in a short time.

The above heat treatment is carried out in an inert gas atmosphere such as hydrogen atmosphere or argon atmosphere or in a vacuum state so that the metal surface is not oxidized under the influence of oxygen.

When the heat treatment is performed as described above, Fe, Ni, W elements diffuse into each layer and the pure iron sheet at the interface between the pure iron sheet and the Ni layer, Ni-W, and Fe layer, and the atomic radius is similar to that of iron (atomic radius 1.24Å) and Ni (atomic radius 1.25Å) combines to form γ (Fe, Ni) crystal phase, and W (atomic radius 1.37Å), which has a larger atomic radius than iron and nickel, is precipitated separately at the interface of γ (Fe, Ni) crystal, resulting in low thermal expansion. A thin film layer made of a mixture of γ (Fe, Ni) alloy and W having characteristics is formed (FIG. 4).

Therefore, it has the effect of reducing the thermal expansion as a whole and improving the moldability due to the excellent mechanical ductility and reducing the metal back effect restored after molding, and also has a good influence on the geomagnetic shielding because it exhibits high permeability soft magnetic properties. .

The heat treatment as described above is followed by a blackening treatment by a known method. (FIGS. 3A and 3B).

That is, the shadow mask made by heat treatment and etching as described above is subjected to surface blackening in a gas mixed with city gas, nitrogen and oxygen at 500 to 700 ° C. near the recrystallization temperature of pure iron to form an oxide film.

In this way, the blackening treatment is performed by spinel-type Ni _x Fe _3-x O ₄ (0 ≤ X ≤ 3), iron oxide, and W _y O _z (0 ≤ Y ≤ 5, 0 ≤ Z ≤ 5). The blackening film which is a mixed layer is formed.

In this blackening film, a trace amount of impurity elements such as Mn, Co, P, Cu, Zr, Ti, S, C, Al, Cr, and Si contained in the pure iron sheet is blackened by the diffusion of the annealing and blackening treatment elements. It may be contained in.

The thickness of the blackening film is treated within 0.5 μm to 3 μm, but when the thickness is smaller than 0.5 μm, heat radiation effect and vacuum characteristics may be deteriorated, and gas may be generated.

The blackening film thus formed has a thermal radiation rate of 0.7 or more, and due to the excellent thermal conductivity of tungsten oxide and the high thermal conductivity of the black surface film, heat accumulation is reduced, thereby suppressing the temperature rise of the shadow mask and reducing thermal expansion. In addition, when the coating film is plated, the gas adsorbed in the plating film can be removed, thereby preventing the discharge failure due to the influence of the gas generated on the metal surface in vacuum.

The following is described according to the embodiment.

Example (1-3)

The surface of the pure iron sheet was electropolished and degreased to obtain each thin film layer by a coating method as shown in Table 1, to obtain a shadow mask which was subjected to heat treatment, etching and molding, followed by surface oxidation.

After measuring the initial permeability and thermal expansion coefficients showing the mechanical and magnetic properties of the shadow mask, and assembling on the frame and attaching the inner shield as shown in Fig. 1, a 29 "color cathode ray tube was fabricated along the centerline of the panel. Doming characteristics were evaluated at a point 11 cm to the left and the results are shown in Table 1 below.

As shown in Table 1, all of the present invention (Examples 1, 2 and 3) showed superior initial permeability, thermal expansion coefficient, and domming characteristics in comparison with Comparative Examples (1 to 2).

5A and 5B are X-RAY diffraction measurement results of the crystal phase of the thin film layer after the heat treatment result. FIG. 5A is before the heat treatment and FIG. 5B is the measurement result after the heat treatment.

Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Film Formation Method Spattering Electroplating Electroplating Spattering Not carried Membrane structure before heat treatment Fe // Ni-W // Fe Ni-W // Fe Fe // Ni-W // Fe Fe // Ni-W // Fe Fe Heat Treatment Electroplating Thickness (㎛) Fe: 0.5Ni-W: 5 Fe: 0.5Ni-W: 5 Fe: 0.25Ni-W: 5 Fe: 0.25Ni-W: 5 - Heat treatment atmosphere vacuum Hydrogen Hydrogen Hydrogen - Heat treatment temperature 750 ℃ 750 ℃ 550 ℃ 350 ℃ 750 ℃ Heat treatment time 5 minutes 30 minutes 50 minutes 30 minutes 30 minutes Crystal phase of thin film after heat treatment γ (Fe, Ni), W γ (Fe, Ni), W γ (Fe, Ni), W Fe-Ni-W, Ni-W - Yield strength after heat treatment (㎏) 16 17 16 18.5 14.0 Initial Permeability After Heat Treatment 4200 3800 4100 3000 1980 Thermal expansion coefficient after heat treatment (1 × 10 ⁶ deg ^-1 ) 6.5 6.9 6.8 7.8 12.3 Doming (μm) 40 43 38 49 98

Example (4-5)

The surface of the pure iron sheet is electropolished and degreased to obtain each thin film layer by the coating method as shown in Table 2, followed by etching, followed by heat treatment to form a black mask at a temperature of 600 ° C. under a mixed gas (city gas, oxygen + nitrogen). The adhesion evaluation and the heat radiation rate of the blackening film were evaluated.

And the emission worm up time was evaluated to determine the vacuum characteristics.

The thickness of the film was measured by copper plating on both surfaces of the shadow mask specimen on which the blackening film was formed, and then mirror-polished the cross-section, and etching and removing copper on the cross-section, and measuring the thickness of the blackening film by electron microscope.

The emission worm up time means that the electrons are radiated from the cathode of the electron gun when the voltage is applied to the heater and each electrode of the color cathode ray tube electron gun in the cooling state, and the cathode current reaches 450 ㎂, resulting in burns on the inner surface of the panel of the color cathode ray tube. From this time, the electron radiation ability and the color level of the color cathode ray tube, which are vacuum characteristics of the color cathode ray tube, are judged, and the emission worm up time is shorter.

The adhesiveness of the blackening film was bent by 90 °, the cellulose tape was attached to the bent portion, rubbed under a load of 100 g, and the tape was pulled out to evaluate the adhesion.

And heat emissivity of blackening mask was evaluated to evaluate heat radiation rate.

Then, the blackened shadow mask was assembled to the frame as shown in FIG. 1, the inner shield was attached, the panel coated with the phosphor and the funnel, and the electron gun was sealed to prepare a 29 "color cathode ray tube.

X-RAY diffraction measurement was performed to observe the crystal phase of the blackened film, and the result was as shown in FIG. 6.

Yield strength, initial permeability, and coefficient of thermal expansion, which show the mechanical, magnetic, and thermal expansion properties of the shadow mask, are measured, and the domming characteristics are evaluated 25cm away from the center of the panel surface after the color cathode ray tube is manufactured. The results are shown in Table 2.

As can be seen from Examples (4 to 5) and Comparative Examples (3 to 5) of the present invention, γ (Fe, Ni) and W phases were formed in the thin film layer, and Ni _x Fe _3-x O _4, Fe ₂ O ₃ and Fe ₃ O ₄ phases were observed.

If blackening is sufficient, the thermal radiation characteristics, adhesion, emission worm up time characteristics are excellent, and the doming characteristic of the color cathode ray tube is also improved.

Example 4 Example 5 Comparative Example 3 Comparative Example 4 Comparative Example 5 Film Formation Method Spattering Electroplating Electroplating Spattering Not carried Membrane structure before heat treatment Fe // Ni-W // Fe Ni-W // Fe Fe // Ni-W // Fe Fe // Ni-W // Fe Fe Heat Treatment Electroplating Thickness (㎛) Fe: 0.5Ni-W: 5 Fe: 0.5Ni-W: 5 Fe: 0.25Ni-W: 5 Fe: 0.25Ni-W: 5 - Heat treatment atmosphere vacuum Hydrogen Hydrogen Hydrogen Heat treatment temperature 900 ℃ 800 ℃ 350 ℃ 700 ℃ 750 ℃ Heat treatment time 5 minutes 30 minutes 50 minutes 30 minutes 30 minutes Blackening temperature 600 ℃ 600 ℃ 450 ℃ 450 ℃ 600 ℃ Black film thickness 1.5 μm 1.7 μm 0.5 μm 0.6 μm 1.8㎛ Adhesion Evaluation Results Good Good Defective Defective Good Thermal emissivity evaluation 0.89 0.90 0.50 0.65 0.85 emission wormup time 7 sec 8 sec 15 seconds 12 seconds 10 sec Crystallization of Blackening Film After Blackening Ni _x Fe _3-x O ₄ Fe ₂ O ₃ Fe ₃ O ₄ Ni _x Fe _3-x O ₄ Fe ₂ O ₃ Fe ₃ O ₄ Ni _x Fe _3-x O ₄ Fe ₂ O ₃ Fe ₃ O ₄ Ni _x Fe _3-x O ₄ Fe ₂ O ₃ Fe ₃ O ₄ Fe ₂ O ₃ Fe ₃ O ₄ Crystal phase of thin film after annealing γ (Fe, Ni), W γ (Fe, Ni), W Fe-Ni-WNi-W γ (Fe, Ni) W - Yield strength after annealing (kg) 16 17 18 17.5 14.0 Initial investment after annealing 4200 3800 2400 3200 1980 Thermal expansion coefficient after heat treatment (1 × 10 ⁶ deg ^-1 ) 6.5 6.9 7.8 7.2 12.3 Doming (μm) 40 43 62 55 98

As described above, in the present invention, the Fe layer, the Ni-W layer, and the Ni layer are laminated on the surface of the pure iron sheet and heat treated to form a composite thin film layer in which Fe, Ni, and W are mixed. doming).

In addition, by forming the blackening film layer at an appropriate temperature, a shadow mask having excellent effects of thermal radiation characteristics, adhesion, emission wormup time characteristics is obtained.

Claims (15)

A cathode ray tube having a shadow mask formed from a cold rolled thin plate made of pure iron, wherein the shadow mask for cathode ray tube is characterized in that a composite thin film layer made of Fe, Ni, and W is formed on one or both surfaces of the pure iron surface. The method of claim 1, The composite thin film layer is a shadow mask for cathode ray tube, characterized in that consisting of a mixture of γ (Fe, Ni) and W phase. The method according to claim 1 or 2, The composite thin film layer was selected from among Ni-W layer, Ni-W // Ni layer, Fe // Ni-W layer, Fe // Ni-W // Ni layer and Ni-W // Fe // Ni layer on the pure iron surface. Shadow mask for cathode ray tube, characterized in that the heat treatment after forming any one of the thin film layer. The method of claim 3, wherein The composite thin film layer is a shadow mask for cathode ray tube, characterized in that formed by electroplating or spatter deposition or metal spraying method. The method of claim 3, wherein A shadow mask for cathode ray tubes, wherein the Fe layer has a thickness of 5 μm or less. The method of claim 3, wherein A shadow mask for cathode ray tubes, wherein the thickness of the Ni layer is 1 µm or less. The method of claim 3, wherein A shadow mask for cathode ray tubes, wherein the Ni-W layer has a thickness of 1 to 15 µm. The method of claim 1, A shadow mask for cathode ray tubes, characterized in that the thickness of the thin steel plate is 0.05 to 0.3 mm. The method of claim 3, wherein A shadow mask for a cathode ray tube, wherein W is 20 to 50% by weight and Ni is 50 to 80% by weight in the Ni-W layer. The method of claim 4, wherein Shadow mask for cathode ray tube, characterized in that the degree of vacuum 10 ^-4 ~ 10 ^-8 torr when sputtering deposition or metal spraying method is applied. The method of claim 3, wherein Heat treatment shadow mask for cathode ray tube, characterized in that carried out for 2 minutes to 2 hours at a temperature of 400 ~ 1200 ℃ in an oxygen-free atmosphere such as nitrogen or hydrogen or argon or vacuum. The method according to claim 1 or 2, A shadow mask for cathode ray tubes, characterized in that a blackening film layer is formed on the upper surface of the composite thin film layer and heat treated at a temperature of 500 to 700 ° C. under a gas mixed with city gas, nitrogen, and oxygen. The method of claim 12, Cathode ray characterized in that the heat treated blackening film layer is a mixture of Ni _x Fe _3-x O ₄ (0 ≤ X ≤ 3), iron oxide, and W _y O _z (0 ≤ Y ≤ 5, 0 ≤ Z ≤ 5) Tolerant shadow mask. The method of claim 13, A shadow mask for cathode ray tubes, characterized in that the thickness of the blackening film layer is 0.5 to 3㎛. The method of claim 13, The blackening film layer is a shadow mask for cathode ray tubes, characterized in that the thermal emissivity is 0.7 or more.

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