Multilayer heat rejection coating

Abstract

There is provided a multilayer coating comprising a plurality of layers comprising a) one or more layers of an elemental transition metal; b) one or more layers of an elemental metalloid; and c) two or more layers of an oxide; characterized in that the transition metal and metalloid layers are between the oxide layers and the plurality of layers does not need to contain an additional transparent conductive film (TCF). The multilayer coatings show high transparency in the visible light range combined with heat shielding without the need of transparent conductive oxide which have been previously used to achieve these properties. The multilayers can be produced with conventional physical vapor deposition methods on glass and polymer substrates. The coatings may therefore be used for applications on windows, plastic sheets and window shields. The invention relates also to the process for making the multilayer coatings, articles comprising them and their use in building and other applications.

Claims

1. A multilayer heat rejection coating comprising a plurality of layers comprising: one or more layers of elemental silver; layers of elemental germanium; and two or more layers of silicon dioxide (SiO.sub.2); wherein the silver and germanium layers are between the SiO.sub.2 layers, wherein a first layer of germanium is positioned on top of the silver layer and a second layer of germanium is positioned at the bottom of the silver layer, wherein a first layer of SiO.sub.2 is positioned on top of the outer germanium layer and a second SiO.sub.2 layer is positioned at the bottom of the inner germanium layer of the coating, and wherein the plurality of layers does not contain a transparent conductive oxide.

2. The multilayer coating of claim 1, comprising a multilayer consisting of SiO.sub.2/Germanium/Silver/Germanium/SiO.sub.2.

3. The multilayer coating of claim 1, wherein the coating consists of five to six layers.

4. An article comprising: a substrate; and a multilayer coating of claim 1 in the form of a multilayer on a surface of said substrate.

5. The article of claim 4, wherein the multilayer coating has a thickness in the range of 50 nm to 300 nm.

6. The article of claim 4, wherein the substrate is selected from materials in the group consisting of polymer, glass, wood, silicon, metal, metal alloy, and any mixture thereof.

7. The article of claim 4, further comprising a functional layer deposited on the multilayer coating.

8. The article of claim 7, wherein the functional layer is a self-cleaning layer comprising titanium dioxide.

9. The multilayer coating according to claim 1 for use as a coating for windows, windshields, plastic sheets, or building materials.

10. A process for making a multilayer coating of claim 1, comprising depositing the layers using physical vapour deposition.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The accompanying drawings illustrate a disclosed embodiment and serve to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

(2) FIG. 1 is a schematic diagram of a SiO.sub.2/Ge/Ag/Ge/SiO.sub.2/glass substrate structure.

(3) FIG. 2 shows the optical transparencies of the glass substrate, single layer TCO oxide, multilayers with TCO layers and multilayers with SiO.sub.2 layers.

(4) FIG. 3 shows the optical transparencies of a polycarbonate substrate, a single layer TiO.sub.2 on polycarbonate, multilayers with silicon dioxide on polycarbonate and TiO.sub.2 on multilayers with silicon dioxide on polycarbonate.

EXAMPLES

(5) Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

(6) A multilayer coating as disclosed above has been exemplified in FIG. 1. FIG. 1 shows a schematic diagram of a substrate, having a multilayer coating of five layers deposited thereon. The substrate is glass. The multilayer has the following layer structure: SiO.sub.2/Ge/Ag/Ge/SiO.sub.2. Heat rejection is shown as sunlight, which is projected towards the article as disclosed above, comprising the substrate with the multilayer coating. The heat radiation of the sunlight is sufficiently reduced.

(7) In this disclosure, there is demonstrated that using an insulating, widely available and high thermal stability material such as silicon dioxide, a good performance in terms of high transparency in the visible region, high infrared rejection and UV filtering as shown in FIG. 2 can be obtained.

(8) These silicon dioxide layers function as a perfect passivation layer against Oxygen to protect the critical Silver heat rejection layer. FIG. 2 further shows the optical transparency of the glass substrate, single layer TCO oxide, multilayers using TCO and multilayers using silicon dioxide. As such, the produced multilayer coatings represent an improvement over previously used TCO containing multilayer coatings.

(9) The exemplified structure has only five layers to achieve the desirable improvement in optical property. These five layers are all fabricated based on physical vapour deposition techniques which comprise only three different materials. If necessary, an additional TiO.sub.2 layer (6.sup.th layer) can be added to enhance the functionality of the multilayer coating.

(10) This multilayer coating can control the heat reflecting property of the film while maintaining high visible transparency due to the additional Germanium layer.

(11) Due to adhesion problem between materials, some thin films have difficulty to retain good adhesion on particular substrate. Peeling effect may occur, hence affect the intended performance of the overall structure. However, the multilayer thin coating including a TiO.sub.2 layer according to this embodiment has been deposited on glass and plastic at room temperature. A simple tape test has been done and it has been found that the multilayer coating adheres well and does not crack easily.

(12) Silicon dioxide used as the oxide layer in the embodiment is a material derived from sand. The most common silicon dioxide encountered is glass. Titanium dioxide as used is an ingredient in many cosmetics. Silver has been a well-known wearable accessory material for many years. Recently, Germanium has also been a popular material in bio-magnetic bracelet. All the components of the embodiment are advancing the development of disposable construction materials, automobile and consumer care electronics that require selective transparency in VIS-NIR region. The inventive coating is able to absorb UV and reject SWIR rays. This means that it can reduce the photo-degradation damage on materials.

(13) Silicon used in the embodiment is the most abundant electropositive element in the Earth's crust. Sand is an example of silicon dioxide which is used as source of the silicon produced commercially. In the proposed invention, only tens of nm of this silicon dioxide is used. Moreover, by deposition of a few nm of Silver and Germanium, selective transparency in the NIR wavelength is obtained.

(14) In the exemplified embodiment, other than the weight of the substrate, the multilayers are thin films of less than 200 nm of materials. Therefore, they do not add on much weight onto the substrate, keeping it as light weight as possible.

(15) All the films in the invention have been deposited at room temperature which means there is no need to heat up any substrate. This further means less time is used during deposition process as it does not need to ramp up or ramp down or hold the temperature during growth. This also means that the proposed structure can be deposited even onto flexible polymer substrates.

(16) Physical vapour deposition as used is a widely applicable deposition technique that can cover large areas with a coating of good uniformity. It has been used in glass coating, semiconductor and disk drive industry for many years. This represents excellent conformity with existing commercial technology that can yield high uniformity and high-throughput. Therefore, with regard to the manufacturability, high setup cost is not a hurdle for this invention which means that it has an extremely low barrier for its utilization unlike many alternatives.

(17) In addition, silicon dioxide is a very low cost material and in the exemplified embodiment, only a thin layer of this low cost material is needed.

Example 1

Depositing a Multilayer Coating

(18) The material sources for the transition metal layer(s) and metalloid layer(s) were used in their purest form and the oxide layers were produced by an oxide source with a purity of at least 99.99%.

(19) A vacuum chamber made of stainless steel by Korea Vacuum Technology was evacuated using a Leybold turbomolecular pump and an Edwards rotary pump. The substrate holder was cooled by a closed circulation chiller to maintain the substrate and substrate holder at a temperature of about 25° C. during deposition. The base pressure was reduced to at least 1×10.sup.−6 Torr. A 10 cm×10 cm glass substrate was pre-washed with acetone and isopropanol 10 min each before blow-drying with compressed air. Then, it was loaded into the vacuum chamber and the pressure was reduced until reaching the base pressure of at least 1×10.sup.−6 Torr. The deposition of a silicon dioxide layer was achieved by using a ceramic SiO.sub.2 target at a precise oxygen pressure of 1×10.sup.−4 Torr at room temperature, whereby the deposited layer was transparent. This was followed by deposition of a thin Germanium layer at a rate of 0.1 nm/s, then by deposition of a Silver layer at a rate of ˜1 nm/s and then by deposition of another layer of thin Germanium layer at a rate of 0.1 nm/s. The thicknesses of the Germanium and Silver layers were pre-calibrated using AFM and quartz crystal thickness monitor. Then, the rate of growth was calibrated. During the growth of the metalloid and transition metal layer, Oxygen was not introduced to prevent oxidation. Finally, this was followed by deposition of another layer of silicon dioxide at a precise Oxygen pressure of 1×10.sup.−4 Torr at room temperature. Various thicknesses of similar materials were deposited at different times. For example, a 50 nm of SiO.sub.2 was deposited first, followed by a 0.5 nm Germanium, then 7 nm of Silver, then 0.5 nm of Germanium and finally 50 nm SiO.sub.2.

(20) Polycarbonate substrates (10 cm×10 cm) were also deposited in the same manner and using the same parameters, but without acetone washing steps. The multilayers deposited were reasonably free of voids, cracks, pinholes or other defects.

(21) The transmission property of the multilayer samples were then measured by using a Shimadzu UV-VIS-NIR spectrophotometer of model UV-3600 and are shown in FIG. 2 and FIG. 3.

(22) A simple tape peel-of test (using 3M polyimide 5413 tape) was done on the film shown that the film is adhesive on the glass substrate and polycarbonate substrate.

(23) The process to produce the multilayer coating accordingly in general comprises the following steps:

(24) a) Cleaning the substrates with acetone and isopropanol before blow-drying with compressed air.

(25) b) Deposition of a silicon dioxide layer at a precise oxygen pressure of 1×10.sup.−4 Torr at room temperature of about 20 to 25° C., whereby a transparent deposited layer is obtained.

(26) c) Follow-up deposition of a thin Germanium layer, then by a Silver layer at a pressure below 1×10.sup.−6 Torr and then another layer of thin Germanium layer while carefully calibrating the thickness of Germanium and Silver layers. The thicknesses of the Germanium and Silver layers are measured using AFM and quartz crystal thickness monitor. Various thicknesses of similar materials are deposited at different times. Then, the rate of growth is calibrated.

(27) d) Finally, follow-up deposition of another layer of silicon dioxide at a precise Oxygen pressure of 1×10.sup.−4 Torr at room temperature of about 20 to 25° C., whereby a transparent deposited layer is obtained.

(28) A controlled deposition rate of 0.1 nm/s to 5 nm/s during the steps is desirable to ensure good quality film growth.

(29) Other than deposition on glass substrate, the room temperature deposited proposed multilayers have also been deposited on plastic (polycarbonate, PET, PMMA) as exemplified for polycarbonate in FIG. 3. The performance of the proposed multilayers on plastic has been found to be similar to deposition on glass. To enhance the functionality of the proposed multilayers, a layer of Titanium dioxide (TiO.sub.2) is also deposited. One line represents a TiO.sub.2 layer on bare polycarbonate which does not have any infrared rejection effect by itself. However, by depositing TiO.sub.2 on the proposed multilayer (the second lowest line), the proposed multilayer have additional functionality of having self-cleaning property while inheriting the high infrared rejection, high visible transparency and UV filtering.

INDUSTRIAL APPLICABILITY

(30) The multilayer coatings described in this disclosure may be useful as a facile and low-cost alternative to conventional window shields and window coatings. The good UV absorbing properties, visible light transparency and high infra-red light rejection of the ensuing substrates makes them very useful for all building material applications, since they reduce costs for electricity and air-conditioning.

(31) It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.