Patent Application
20150219804
1. Field of the Invention
The present invention relates to a heat radiation control element used for spacecraft device, such as an artificial satellite, and a production method thereof.
2. Description of the Related Art
A heat radiation control element called an optical solar reflector (OSR) is used for a device for space exploration, such as an artificial satellite (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 05-77799). The OSR is configured to reflect sun light rays applied to the device for space exploration, and to emit self-generated heat of the device for space exploration to the space to prevent heat generation of the device for space exploration.
The ideal value of the radiative coefficient is better, as it is closer to 1. However, the normal emittance (∈n) at 300° K is about 0.845 with a conventional OSR structure. Quartz has relatively high emittance in the infrared light region, but the emittance thereof is low in the peak wavelength region of the black-body radiation at 300° K. This is due to the properties unique to the quartz substrate, and hence it has been difficult to increase the radiative coefficient of OSR.
The present invention is proposed based upon the aforementioned situation in the art, and aims to provide a heat radiation control element having excellent thermal radiation properties, and a production method thereof.
In order to solve the aforementioned problems, the heat radiation control element of the present invention contains an inorganic layer a surface of which has openings arranged two-dimensionally, and a reflecting film, in which the inorganic layer is either a quartz layer, or a glass layer.
Moreover, the production method of a heat radiation control element according to the present invention contains: forming a resist pattern, in which predetermined shapes are two-dimensionally arranged, on a surface of an inorganic substrate, which is either a quartz substrate or a glass substrate; etching the surface of the inorganic substrate using the resist pattern as a mask to form two-dimensionally arranged openings at the surface of the inorganic substrate; and forming a reflecting film on a surface of the inorganic substrate that is opposite to the surface thereof where the openings are formed through vacuum deposition or sputtering.
Furthermore, the production method of a heat radiation control element according to the present invention contains: laminating a dielectric film, a reflecting film, and a protective film in this order on one surface of an inorganic substrate that is either a quartz substrate or a glass substrate; forming a transparent conductive film on another surface of the inorganic substrate; forming a resist pattern, in which predetermined shapes are two-dimensionally arranged, on a surface of the transparent conductive film; and etching the surface of the inorganic substrate using the resist pattern as a mask to form two-dimensionally arranged openings at the surface of the inorganic substrate.
According to the present invention, the emittance of a certain wavelength region due to properties unique to a quartz substrate or glass substrate is improved by openings that are two-dimensionally arranged at a surface of the quartz or glass, and excellent thermal radiation properties can be obtained.
The embodiment of the present invention is specifically explained in the following order, hereinafter.
1-1. General description of the present technology
1-2. Radiation properties unique to inorganic (quartz and glass) substrate
1-3. Resonance of cavities at metal surface
1-4. Resonance of cavities at inorganic (quartz and glass) surface
1-5. Widths of cavities
1-6. Depths of cavities
1-7. Pitch of cavities
1-8. Shapes of cavities
2-1. First embodiment
2-2. Second embodiment
2-3. Another embodiment
The inorganic layer 21 is either a quartz layer or a glass layer.
The quartz layer is fused quartz or synthetic quartz formed of SiO2, and has high transmittance to light of a wide wavelength range.
The glass layer is, for example, non-alkali glass, or borosilicate glass, and has high transmittance to light of a wide wavelength range.
The openings 21a are holes, which are called cavities, and are two-dimensionally arranged at a surface of the inorganic layer 21. The two-dimensional arrangement may be a regular arrangement or irregular arrangement, but it is preferably a lattice arrangement, or a houndstooth arrangement, as a high planar filling factor can be attained in such arrangements.
The opening shapes of the openings 21a are ideally highly symmetric shapes, and are preferably circles or regular polygons. When the opening shapes are ovals, rectangles, or parallelograms, there is selectivity to polarized light, and radiation of the polarized light is varied depending on the wavelength. Therefore, the wavelength selectivity of the emittance is low.
The opening diameters a of the openings are preferably 3 μm to 11 μm. When the opening diameters are within this range, it is possible to attain a radiative coefficient (emittance) of 0.9 or greater. The opening diameters a of the openings are determined as a width W of one side in case of squares, but they are determined as diameters D in case of circles. Specifically, the opening diameters of the openings are diameters in the case where the opening shapes are circles, and are diameters of inscribed circles in the case where the opening shapes are regular polygons.
Moreover, the depths d of the openings are preferably 0.7 μm or greater, more preferably 1.0 μm or greater. The emittance can be improved by increasing the depths d of the openings.
Moreover, the aspect ratio (d/a) of the openings is preferably 0.2 or greater, more preferably 0.5 to 3.0. When the aspect ratio is too small, the obtainable result is the same as the case where there are no openings formed at the surface. When the aspect ratio is too large, it is difficult to form deep openings. Moreover, the thermal radiation properties are saturated at a certain depth or greater, and hence it is not necessary to form excessively deep openings.
The reflecting film 22 is provided on the opposite side of the inorganic layer 21 to the side thereof where the openings are formed. When the reflecting film 22 is provided at the side of the inorganic layer 21 where the openings are formed, an effect obtainable by resonances of the openings is reduced, and excellent thermal radiation properties cannot be obtained.
A material of the reflecting film 22 is not particularly limited, as long as it is a material having high reflectance to sun light. Examples thereof include a single metal (e.g., Ag, Cu, Al, Mo, Cr, Ti, Ni, W, and Fe), an alloy containing any of the above-listed metals, and a semiconductor material (e.g., Si, Ge, and Te).
In the dielectric reflecting film 23, a plurality of dielectric films having mutually different refractive indexes are laminated. The sun light absorptivity of the heat radiation control element can be reduced by setting the reflectance of the dielectric reflecting film high in the certain wavelength region at which the reflectance of the reflecting film 22 is low.
The protective film 24 is formed of a metal (e.g., Cr, and Ni), or an oxide (e.g., SiO2, and Al2O3), and is configured to protect the reflecting film 22. As a result, the reflecting film 22 is protected from being damaged, and letting sun light pass through.
The transparent conducting film 25 or 26 may not be formed on the side surfaces and bottom surfaces of the openings as in the third structural example, or may be formed on the side surfaces and bottom surfaces of the openings as in the fourth structural example. As for the transparent conducting films 25, 26, a zinc oxide-based transparent conducting film, an indium oxide-based transparent conducting film, or a tin oxide-based transparent conducting film can be used. Since the transparent conducting films 25, 26 let visible rays pass through, and reflect near infrared rays and mid-wavelength infrared rays, excellent heat radiation properties can be attained.
The heat radiation control element having the aforementioned structure can improve the emittance in a certain wavelength region due to the properties unique to a quartz substrate or glass substrate, and excellent thermal radiation properties can be attained, as two-dimensionally arranged opening are provided at a surface of the quartz or glass. In the case where the heat radiation control element is applied to an artificial satellite, for example, a number of electronic equipment loaded inside the artificial satellite can be increased because of a heat balance between heat generated from the electronic equipment inside the artificial satellite and heat radiating from the heat radiation control element. As a result, more functions can be provided to one artificial satellite. Moreover, a number of the heat radiation control elements loaded can be reduced by the improvement of the emittance, hence down-sizing and weight-reduction of the artificial satellite can be achieved.
The spectrum of the molecular vibration is complicated, not like a wide and smooth spectrum of free electrons of a metal. It has been considered that a resonance of cavities does not occur in a transparent material. Only limited in the narrow wavelength range of the molecular vibration, however, a resonance similar to that of a metal seems to occur.
It has been already known that emittance is controlled by providing cavities to a surface of a material having strong absorption, such as a metal.
Inside the cavity, there is a mode of electromagnetic waves locally existing. In case of a metal, the local mode is excited by the thermal energy (thermal vibration) of free electrons inside. Moreover, the local mode causes a resonance with the transmitted light at a certain wavelength, and the energy is passed from the local mode to the transmitted light. As a result, it can explain that the radiation is increased when cavities are formed at a surface of a metal.
Since a resonance occurs only at a certain wavelength, there is the selectivity of the wavelength of radiation. It can be said that a resonance occurs when a size of each cavity is approximately larger than a half the wavelength. Actually, the wavelength at which a resonance occurs and the strength thereof depend on a refractive index of the material or a shape of each cavity, and thus it is necessary to precisely carry out numerical computation.
In order to contribute to radiation, a presence of an imaginary part (extinction coefficient k) of the refractive index of the material is particularly important. In case of a metal, the thermal vibration of free electrons is coupled with light having a wavelength that is longer than a wavelength corresponding to the plasma frequency ωp. This means that the energy transfers between the thermal energy and the light inside the metal. Optically, this corresponds that the imaginary part of the refractive index is not 0.
To explain specifically, radiation increases through the following steps 1 to 5.
1. Free electrons are vibrated by heat inside the metal.
2. Light is excited inside the metal by the vibration of the free electrons.
3. The local mode inside the cavity is excited by the light.
4. The local mode causes a resonance with the transmitted light at a certain wavelength.
5. Finally, the transmitted light radiates the thermal energy inside the metal to the outside.
A material always has dispersion, i.e., the wavelength dependency of a refractive index. A spectrum of radiation significantly changes depending on a resonance of cavities, and a refractive index value at the wavelength. In order to qualitatively determine at which wavelength a resonance occurs with a single cavity, it is assumed here that there is no dispersion.
As illustrated in
To increase radiation with cavities at a surface of a metal, a presence of free electrons inside the metal is important. Optically, it is important that the imaginary part of the refractive index is not 0 because of the presence of free electrons.
In a transparent material (the imaginary part of the refractive index is 0), such as quartz, and glass, vibration (most of it is lattice vibration) by heat is not coupled with light transmitting inside, and thus the light can travel inside the material without turning into thermal motion. Namely, the material is transparent. Accordingly, the local mode of the electric field in the cavity is not excited, and the presence of cavities does not largely affect radiation. Specifically, the aforementioned steps 1 and 2 do not exist.
However, with the quartz and glass, there is a wavelength range at which the imaginary part of the refractive index is not 0 within the infrared light region. This is due to molecular vibration, but not the vibration of free electrons as in a metal. The wavelength regions of the molecular vibration are limited, and the wavelength regions are wavelengths of around 9 μm and around 23 μm of the refractive index illustrated in
At the wavelengths of around 9 μm and around 23 μm, the molecular vibration inside the quartz is coupled with light. If cavities are present at a surface of the quartz, it is considered that the local mode of the electric field in the cavity is excited, and the wavelength thereof is a wavelength at which a resonance occurs, then energy is passed to the transmitted light. It is assumed that the same tendency can be observed with glass.
Specifically, the emittance, particularly that at the wavelength region, where the emittance reduces in case of a flat surface, can be improved by providing cavities at a surface of the quartz or glass. On the other hand, providing cavities does not make any change at other wavelength regions.
It can be understood that the emittance is increased by the resonance of the cavities in the wavelength range (especially around 9 μm) where the emittance is reduced by the molecular vibration. The normal emittance ∈n at 300° K for this emittance is about 0.962, and it can be seen that it is improved by 10% or more by providing the cavities.
Table 1 depicts the normal emittance ∈n at 200° K, 300° K, and 400° K.
As depicted in Table 1, without cavities, the normal emittance ∈n at 200° K, 300° K, and 400° K were respectively 0.846, 0.846, and 0.846. On the other hand, with cavities, the normal emittance ∈n at 200° K, 300° K, and 400° K were respectively 0.956, 0.962, and 0.967.
As described above, the emittance could be improved by providing the cavities. It is considered that this is because the energy of the molecular vibration is coupled with the light of the radiation mode (transmitted light) by the cavities, to improve the emittance in the certain wavelength region originated to the properties unique to the quartz substrate. The same tendency can be observed with the glass substrate. Examples of samples each prepared by forming cavities at a surface of a glass (TEMPAX, manufactured by SCHOTT AG) substrate, and measurement values thereof are presented below.
In this graph, Nos. 1 and 2 represent the emittance, which is actually measured on the actually prepared two samples, in which the cavities of the houndstooth arrangement illustrated in
Nos. 3 and 4 in this graph represent the emittance, which is actually measured on the actually prepared two samples each having a flat structure, where no cavity is formed at a surface of glass (TEMPAX, manufactured by SCHOTT AG). Specifically, the two samples of the same conditions were produced and evaluated in order to confirm reproducibility.
Nos. 3 and 4 exhibited the same radiative behaviors. Therefore, the radiation curves thereof are overlapped.
It can be seen that the emittance at the wavelength range (particularly around 9 μm), where the emittance of the glass is reduced by the molecular vibration, is increased by the resonance of the cavities. The normal emittance ∈n of the sample No. 1 at 300° K is about 0.968, and the normal emittance ∈n of the sample No. 2 at 300° K is about 0.956. It is therefore understood that the normal emittance is improved by 10% or greater by providing the cavities.
Table 2 depicts the normal emittance ∈n of Nos. 1 to 4 of
The cavities provided at a surface of a metal are configured to narrow a wide radiation spectrum due to free electrons, with utilizing a wavelength selectivity the cavities have. The cavities provided at a surface of quartz or glass are configured to couple the energy of the molecular vibration to the light of the radiation mode (transmitted light). So far, it has been considered that a resonance of cavities does not occur in a transparent material. It is a novel insight that, if the attention is focused on the molecular vibration, the cavities provided at a surface of quartz or glass causes a resonance as in the metal and behave the same way, only in the narrow wavelength range of the molecular vibration.
Moreover,
It can be understood from the graph of
Moreover,
Moreover, Table 3 depicts the depths of the cavities, the aspect ratio, and the normal emittance.
It can be understood from
Moreover, it is understood that the normal emittance is significantly improved when the depths of the openings are 0.7 μm or greater, compared to the normal emittance in case of a flat surface. Moreover, it is understood that the normal emittance is significantly improved when the aspect ratio is 0.2 or greater, compared to the normal emittance in case of a flat surface.
The pitch of the cavities seems to have an optimal value relative to a width of each cavity. Here, the electric field inside the cavity is considered.
As illustrated in
In this case, it is considered that a resonance occurs when the width of each cavity is N/2 (N is an integer) times the wavelength. Even when the cavities are densely arranged (a thickness of a side wall is 0), the electric field E inside each cavity is independent, and all of them contribute to radiation.
However, an imaginary part of a refractive index of a general material is a finite value. As illustrated in
In this case, the electric field E is affected by the electric field E of the adjacent cavity. Therefore, the electric field E has less freedom in view of a state thereof (conditions for generating a larger amplitude are more limited), and the emittance is reduced. In the case where the cavity width is just ½ the wavelength, for example, the vibration is caused in a reverse phase, and thus radiation becomes 0 (because the reverse phase is added).
In the aforementioned case, cavities act independently as in the case where the imaginary part of the refractive index is infinite, and emittance is increased, if a wall is thickened by a certain degree, and the entry of the adjacent electric field is sufficiently reduced. However, the surface density of the cavities is reduced by the amount of the thickness of the wall. Therefore, it is considered that radiation is lower than the radiation obtained when the imaginary part of the refractive index is infinite.
From the aforementioned qualitative discussions, it is assumed that the widths of the cavities, the value of the imaginary part of the refractive index of the material, and the pitch (thickness of the side wall) of the cavities are related, and there is an optimal pitch determined by the width and the material. The specific value of the optimal pitch can be attained by numerical computation.
In the above, a calculation is performed on cubic cavities, each of which has an opening shape of a square. This is because there is less computational complexity in RCWA, as these cavities are highly symmetric and a longitudinal direction and a transverse direction are independent from each other. In practice, however, it is difficult to produce cavities of the aforementioned shapes by a simple method.
When cavities are produced by etching without mechanical processing, opening shapes of the cavities are preferably circles (cylindrical cavities). In the case where the opening shapes thereof are circles, there is high computational complexity, but qualitative discussions thereof are possible.
Considering the qualitative discussions (the electric field inside the cavity) in “1-7. Pitch of cavities,” it is assumed that the same emittance profiles are attained when the opening shapes of the cavities are squares as illustrated in
In case of circular cavities, a filling factor of the cavities can be increased more with the houndstooth arrangement as illustrated in
As illustrated in
Note that, other shapes, such as a rectangle, a parallelogram, an oval, can be considered, but such asymmetric shapes give selectivity to polarized light. Specifically, the polarized light of radiation is polarized depending on a wavelength to thereby lower the selectivity of wavelength. If it is specifically explained, to lower the selectivity of the wavelength means that the curve of the spectrum of the emittance depicted in
Moreover, the data obtained by measuring produced actual samples is introduced.
The actual samples to be introduced are each a sample where cavities are formed at a surface of quartz. Shapes of the cavities in the samples are as described in Table 4.
The emittance properties of the plate-shaped quartz substrate (No. 10), and the actual samples Nos. 11 to 16 were investigated. The results are depicted in
Moreover, it could be confirmed from the infrared emittance properties when a ratio of the cavity diameter to the pitch was 0.7, 0.8, or 0.9, that the infrared emittance was higher when the cavity diameter was larger (ratio: 0.9), with both pitches of 4.5 μm, and 9.0 μm.
As explained above, the heat radiation control element of the present embodiment can improve emittance in a certain wavelength range due to the properties unique to the quartz substrate or glass substrate, and have excellent thermal radiation properties.
The production method of a heat radiation control element according to the first embodiment: forming a resist pattern, in which predetermined shapes are two-dimensionally arranged, on a surface of an inorganic substrate, which is either a quartz substrate or a glass substrate; etching the surface of the inorganic substrate using the resist pattern as a mask to form two-dimensionally arranged openings at the surface of the inorganic substrate; and forming a reflecting film on a surface of the inorganic substrate that is opposite to the surface thereof where the openings are formed through vacuum deposition or sputtering.
First, a quartz substrate serving as an inorganic substrate 31 is washed to prepare the inorganic substrate 31 as illustrated in
Subsequently, predetermined openings are formed at the surface of the inorganic substrate 31 by performing etching using the resist pattern as a mask, as illustrated in
Finally, a reflecting film 32 is formed on a back surface of the inorganic substrate 31 to which the openings have been formed by vacuum deposition or sputtering, as illustrated in
According to the first embodiment, a heat radiation control element of the first structural example illustrated in
First, a glass substrate serving as an inorganic substrate 31 is washed to prepare the inorganic substrate 31 as illustrated in
Examples of a material of the metal layer 29 include Ni, Cr, Al, Ti, Ta, Cu, Au, and a composite material containing any of the above-listed materials.
Next, predetermined openings are formed at a surface of the inorganic substrate 31 through etching performed over the etching mask, as illustrated in
Formation of the openings is performed, for example, through dry etching. The gas used for the dry etching is obtained by using Ar as a main gas, and mixing Ar with a radical reaction gas, such as C4F8, and CF4.
Finally, a reflecting film 32 is formed on a back surface of the inorganic substrate 31, to which the openings have been formed, by vacuum deposition or sputtering, as illustrated in
According to this embodiment, a heat radiation control element of the first structural example illustrated in
Moreover,
The production method of a heat radiation control element according to the second embodiment contains: laminating a dielectric film, a reflecting film, and a protective film in this order on one surface of an inorganic substrate that is either a quartz substrate or a glass substrate; forming a transparent conductive film on another surface of the inorganic substrate; forming a resist pattern, in which predetermined shapes are two-dimensionally arranged, on a surface of the transparent conductive film; and etching the surface of the inorganic substrate using the resist pattern as a mask to form two-dimensionally arranged openings at the surface of the inorganic substrate.
First, a quartz substrate serving as an inorganic substrate 41 is washed to prepare the inorganic substrate 41 as illustrated in
Next, a transparent conducting film 45, which is, for example, formed of ITO, and has a thickness of 5 nm or greater, is formed on the other side of the inorganic substrate 41 by sputtering or vacuum deposition, as illustrated in
Next, a resist 40 is applied onto the transparent conducting film 45, as illustrated in
Next, predetermined opening are formed at the surface of the inorganic substrate 41 by performing etching using the resist pattern as a mask, as illustrated in
The production method of a heat radiation control element according to the second embodiment can produce a heat radiation control element of the third structural example illustrated in
In the formation of the openings in the first and second embodiments, a passivation mode and an etching mode are alternately performed. The passivation mode is a mode where a side surface protective film having a resistance to wet etching is formed on side walls of the openings of the N stage (N is natural number), and the etching mode is a mode where the bottom surfaces of the openings of the N stage, to which the side surface protective film has been formed, are etched through wet etching to form openings of the N+1 stage. As a result, the shape having a large aspect ratio can be obtained.
In the passivation mode, after forming a surface protective film having a resistance to wet etching on the opening of the N stage (N is a natural number) and the resist pattern, it is preferred that a side surface protective film be formed on side walls of the openings of the N stage, as well as exposing the bottom surfaces of the openings of the N stage through ion etching.
As for the wet etching, electrolytic etching or chemical etching may be used. In the case where the substrate is a metal or ally having high corrosion resistance, or the case where openings having opening diameters (widths) of 20 μm or less are formed, electrolytic etching is preferably used.
Moreover, the ion etching may be performed using Ar ions in a vacuum atmosphere by means of a vacuum device, or in the atmospheric pressure atmosphere by means of an atmospheric pressure plasma device. By ion etching, a side surface protective film can be formed on side walls of the openings of N step (N is a natural number), which have been formed by wet etching. Since the side surface protective film contains a product generated by a reaction of a material, such as the resist material, and a base material of the bottom surface of the opening (etching residues), the side surface protective film has a resistance to wet etching.
By alternately performing the passivation mode and the etching mode after the initial etching mode, the etching can be carried out in the depth direction with suppressing the side etching, and the predetermined opening shapes (high aspect shapes) can be obtained.
This application claims priority to Japanese application No. 2014-019571, filed on Feb. 4, 2014 and incorporated herein by reference, and Japanese application No. 2014-246939, filed on Dec. 5, 2014 and incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-019571 | Feb 2014 | JP | national |
| 2014-246939 | Dec 2014 | JP | national |
