WO2013014423A1

WO2013014423A1 - Deposition process

Info

Publication number: WO2013014423A1
Application number: PCT/GB2012/051720
Authority: WO
Inventors: Troy Darrell Manning; Simon James Hurst; Gary Robert Nichol; Mathew WAUGH; Guillermo Benito Gutierrez
Original assignee: Pilkington Group Limited; University College London
Priority date: 2011-07-22
Filing date: 2012-07-18
Publication date: 2013-01-31
Also published as: GB201112648D0

Abstract

Processes are disclosed for the deposition of coatings on substrates, by heating the substrate to 80°C to 500°C, passing a fluid mixture of a silicon-containing precursor and a titanium-containing material through a flame and contacting the substrate with the fluid mixture, the ratio of silicon to titanium containing components is 10:1 to 1:10. The preferred substrate is glass. The coatings have anti reflective and self-cleaning properties.

Description

DEPOSITION PROCESS

This invention relates to processes for the deposition of a coating on a substrate and to coated substrates comprising a coating on at least one surface.

Photocatalytically active coatings on substrates (especially glass) are useful in producing self-cleaning substrates. Photocatalytic activity arises by the photogeneration, in a semiconductor, of a hole-electron pair when the semiconductor is illuminated by light of a particular frequency. The hole-electron pair can be generated in sunlight and can react in humid air to form hydroxy and peroxy radicals on the surface of the

semiconductor. The radicals oxidise organic grime on the surface. Photocatalytically active coatings when illuminated thus tend to destroy organic grime on the surface. They also tend to maintain their hydrophilic properties because of the active cleaning of the surface as a consequence of illumination. Photocatalytically active coatings may comprise a semi-conductor with a suitable band gap, for example, titanium oxide.

Titanium oxide photocatalytic coatings on glass are disclosed in EP 0 901 991 A2, WO 97/07069, WO 97/10186, WO 98/06675, WO 98/41480, WO 00/75087, in Abstract 735 of 187th Electrochemical Society Meeting (Reno, NV, 95-1, p.l 102) and in New Scientist magazine (26 August 1995, p.19). Mixed (silica titania oxide) coatings intended to be self cleaning are disclosed by Tricoli et al in Langmuir 2009, 25(21), ppl2578- 12584.

Titanium oxide coatings tend to have relatively high visible light reflection. In some applications, (e.g. in substrates for photovoltaic cells) it would be useful to reduce reflection (and thereby increase visible light transmission) whilst retaining some self- cleaning properties. Anti reflection coatings are discussed in WO-A-2011/077153, WO- A-2011/077157 and US-A-2007/0113881. It has in the past proved difficult to provide both higher transmission and retain self cleaning properties. It is important that such coatings on glass have relatively high durability especially to abrasion and weathering.

It is an aim of the present invention to overcome problems associated with the prior art and to provide processes for deposition of coatings which are self cleaning. It is also of great importance that the process is flexible enough to be used to deposit coatings before, during and/or after other coatings have been deposited, can be used both off-line and on- line (i.e. during the glass production process) yet are still able to provide durable and effective anti reflection coatings.

The present invention accordingly provides, in a first aspect, a process for the deposition of a coating on a substrate, the process comprising

a) providing a substrate at a substrate temperature of 80°C to 500°C, b) passing a fluid mixture comprising a silicon-containing coating precursor and a titanium-containing material through a flame, and

c) contacting at least one surface of the substrate with the fluid mixture during or after its passage through the flame,

wherein the ratio of the silicon-containing coating precursor and the titanium- containing material is in the range 1 : 10 to 10: 1.

Surprisingly, even coatings deposited at such relatively low temperatures according to the invention have good durability (e.g. to abrasion). Effective, relatively low temperature deposition also means that the process is relatively flexible and can be used to deposit coatings before, during and/or after other coatings have been deposited, and can be used both off-line and on-line (i.e. during the glass production process).

Preferably, the substrate temperature is 100 to 475°C, 100 to 450°C, 105 to 475°C, preferably 105 to 450°C. Deposition on to substrates at a temperature of greater than 100°C is advantageous because it results in good coatings without requiring a subsequent heating step (which is time consuming and relatively inefficient in production).

The fluid mixture will usually further comprise a comburant, and/or a carrier fluid, preferably a carrier gas. Preferably the fluid mixture further comprises an oxidant. Air is the most preferred oxidant although other oxidants including gases such as oxygen may be used depending on the chemistry of the coating precursors and other parameters of the process.

Preferably, the process is such that the refractive index of the coating is 1.1 to 2.5, preferably 1.25 to 1.7, more preferably 1.4 to 1.7. Such refractive indices may be obtained by varying the ratio of the components of the fluid mixture, especially the silicon-containing coating precursor and the titanium- containing material and the temperature of the substrate.

Preferably the process is continued until the thickness of the coating is 10 to 500 nm, preferably 80 to 200 nm more preferably 95 to 200 nm, most preferably 105 to 200 nm. The thickness of the coating is preferably that which will result in destructive interference between the light reflected from the surface of the coating and the surface of the glass. For optimum destructive interference the length of the optical path in the coating should be equal to one half of the wavelength of the light. This thickness can be calculated from the equation:

t = A 4n

where t is the thickness of the coating, λ is the wavelength of the incident light and n is the refractive index of the coating.

The preferred substrate is glass, including float glass or rolled glass. A preferred glass substrate (especially for use in photovoltaic cells) has an iron content of 0.015% by weight or lower.

The process of the invention may be conducted on line or off-line. If conducted on line during the float glass production the substrate will generally be a ribbon of float glass.

Whether conducted on line or off line, the coating, if deposited on float glass, may be deposited on the gas side surface of float glass, but is preferably deposited on the tin side surface of float glass. This is advantageous because subsequent coating of the coated substrate may then be performed directly on the gas side surface of the glass.

Whichever surface of the glass is coated, the process may further comprise a step of depositing at least one further coating either on the same surface as the coating or on another (or the other) surface. The, or each, further coating may be deposited before, during (if on the other surface), or after deposition of the coating. One possible further coating is a coating of a transparent conductive oxide. Transparent conductive oxide coatings include coatings of doped tin oxide, doped zinc oxide or indium oxides (e.g. indium tin oxide, ITO).

It has surprisingly been discovered by the inventors of the present invention that the coatings deposited by the process are of much better quality if the deposition is conducted when the flame is substantially stable. Thus, it is preferred if the process is conducted with a substantially stable flame.

Flame deposition processes usually comprise the steps of forming a fluid mixture comprising a precursor of an oxide of a metal or a metalloid, an oxidant and, optionally, a comburant. This fluid mixture may then be ignited at a point which is adjacent to the surface of the substrate. The precursor for the oxide may be any compound of a metal or metalloid which may be dispersed in the fluid mixture and which will decompose to form an oxide when the mixture is ignited. Processes in which at least some of the coating precursors are in the vapour phase are commonly termed combustion chemical vapour deposition processes (hereinafter for convenience CCVD processes). In a preferred embodiment the processes of this invention are CCVD processes.

The burner used in the flame deposition process preferably extends across the full width of the substrate although a series of smaller burners may be used. The burner is preferably positioned above the substrate in close proximity to the surface of the glass substrate.

The distance between the burner and the substrate will typically be in the range of from 2 to 20 mm and preferably in the range 3.0 to 15.0 mm. Such close proximity results in a coating having improved properties possibly because it minimises the amount of recombination between the species produced by burning the precursor before they are deposited upon the surface of the substrate. It may be necessary to adjust the distance between the burner and the surface of the glass substrate in order to optimise the properties of the desired coating. A plurality of burners positioned along the length of the substrate may be used in order to deposit a coating having the desired thickness.

The thermal output of the burners useful in the processes of this invention may be from 0.5 to 10 kW/10cm², preferably from 1 to 5 kW/10cm². The concentration of precursor in the fluid mixture which is delivered to the burner is typically from 0.05 to 25 vol%, preferably from 0.05 to 5 vol% gas phase concentration.

The burner is preferably associated with means for extracting the exhaust gases from the area adjacent to the surface of the substrate. In the preferred embodiments at least one means for extraction is positioned adjacent to each burner. The extraction means is typically a conduit associated with a fan which produces an updraft at the mouth of the conduit. Each extraction means is preferably provided with control means whereby the draft provided may be adjusted. In the preferred embodiments of the invention the extraction means are controlled so as to isolate the burner flames from each other, to control the direction of the flame so as to optimise the impingement of the flame over the surface of the substrate and to efficiently remove the by products which are generated by the combustion. The inventors have discovered that the quality of the coating which is deposited can be improved by extracting the exhaust gases in a manner which causes the tail of the flame to be positioned above the surface of the substrate i.e. when the burner is located above the substrate surface the tail of the flame is also located above the substrate surface and when the burner is located below the substrate surface the tail of the flame is also below the substrate surface. Extracting the gases in this way has been found to reduce powder formation and to improve the uniformity of the coating. These are significant advantages, especially in an on line coating process where a high deposition speed is advantageous.

The temperature of the flame may vary with the choice of the optional comburant. Any gas which can be burnt to generate a sufficiently high flame temperature to decompose the precursor is potentially useful. Generally the comburant will be one which generates a flame temperature of at least 1700°C. The preferred comburants include hydrocarbons such as propane, butane, acetylene, methane and natural gas or hydrogen.

Examples of silicon containing precursors include compounds having the general formula SiX₄ wherein the groups X, which may be the same or different, represent a halogen atom especially a chlorine atom or a bromine atom, a hydrogen atom, an alkoxy group having the formula -OR or an ester group having the formula -OOCR wherein R represents an alkyl group preferably comprising from 1 to 4 carbon atoms. Preferred precursors for silicon include silicon alkoxides, silicon halides and/or silanes. Particularly preferred precursors for use in the present invention include tetraethoxysilane (TEOS), hexamethyldisiloxane (HMDSO) and/or silane.

Preferred examples of titanium containing materials include pre-formed

nanoparticles, preferably nanoparticles comprising titanium oxide. Other titanium containing materials that may be used in the invention instead of or in addition to preformed nanoparticles are coating precursors including titanium alkoxides or titanium halides. Particularly preferred titanium precursors are titanium tetraalkoxides, in particular titanium tetrapropoxide (either n propoxide or i propoxide or a mixture) or titanium chlorides e.g. titanium tetrachloride.

Preferably, the titanium containing coating precursors are directed to the substrate surface at a surface flow rate of 2 x 10^"4 to 100 x 10^"4 m/min, preferably 2 x 10^"4 to 80 x 10^"' m/min, more preferably 2 x 10^"4 to 25 x 10^"4 m/min and most preferably 4 x 10^"4 to 16 x 10 ⁴ m/min (where the units are derived from chemical flow (in m³min^"1) ÷ burner face area (in m²) to take account of flow rates in burners of different face areas.) These flow rates are advantageous because they result in excellent coatings with good properties.

The process uses an oxidant which may comprise a source of oxygen. Typically the oxidant may be air. The ratio of precursor and/or comburant to oxidant (e.g. air) may be adjusted so that the flame is either oxygen rich or oxygen deficient. The use of an oxygen rich flame favours the production of a fully oxidised coating whereas the use of an oxygen deficient flame favours the production of a coating which is less than fully oxidised.

The ratio of silicon containing precursor to titanium containing material may be varied over a wide range depending upon the properties required in the coating. The ratio of silicon to titanium containing precursor may be l : 10 to 10: 1, preferably 1 :5 to 5: 1 and most preferably 1 :4 to 4 : 1.

Surprisingly, it has been discovered that even at low levels of titanium containing materials the coating still has some self cleaning properties.

Preferably, the water contact angle after irradiation of the coatings with UV light for 24 hours is in the range 0 to 40°, preferably 5 to 40°. Most preferably the water contact angle are determined using 1 μΐ droplets of water.

Preferably the photocatalytic activity of coated substrates produced in accordance with the invention (e.g. according to Draft International Standard ISO/D1S 10678 using methylene blue as test molecule) are above 0.04 nmol/cm²h.

Substrates coated by means of the process find uses in many fields of use. A particularly important use is in forming a photovoltaic module using the substrate after deposition of the coating.

The inventors have discovered that deposition of metal oxide mixed

compositions is possible to achieve by premixing a gas stream with the two desired precursors.

The burner head is placed above the moving glass at a critical distance, consequently a durable hard coating with a refractive index between 1.40 and 2.5 is deposited with some photocatalytic and/or hydrophilic properties.

The present invention is illustrated by the accompanying drawings in which:

Figure 1(a) is a graph of light transmission (%) against wavelength for the Examples 1 to 6, (b) is a magnified portion of the transmission curves. Figure 2 is a graph showing static water contact angles determined after 24 hours of

UV irradiation for samples prepared using just HMDSO, varying ratios of HMDSO : TTIP and just TTIP. The Reference is the Pilkington product ACTIV™ self-cleaning glass.

Examples

The invention is also illustrated by the following Examples.

Examples 1 to 6

In Examples 1 to 6 a fluid mixture comprising propane (3.5 standard litres per minute SLM), air (75 SLM) and hexamethyldisiloxane (HMDSO) and/or titanium

tetraisopropoxide (TTIP) (total of both HMDSO and TTIP 12 cm³/hour) was fed to a burner for flame deposition of the coatings. Six passes of the substrate under the burner were made. The substrate was float glass. Substrate temperature was 180°C. The HMDSO and/or TTIP precursor was introduced by syringe drive/evaporator (200 °C) directly into gas flow. The burner to glass surface distance was 5 mm.

T_vis of base glass (without coating) for the examples was 89%. T_vi_s values were calculated from the spectra of the samples to ISO9050 and EN410/673.

The optical properties of the Examples were analysed and visible light transmission for the substrate (14) and Examples 1 to 6 are shown in Fig 1(a) and 1(b). Transmission improvement was noticeable in the visible transmission with the coatings substrates having up to 2% transmission improvement over the uncoated substrate especially at the blue end of the spectrum. The % improvement of weighted transmission values were calculated for standard photovoltaic cells of varying types. The results are as indicated in Table 1 as a function of the HMDSO :TTIP ratio.

The coating significantly improves the transmission of light into PV cells with consequent improvement in PV efficiency. Surprisingly, the durability of the coatings did not appear to be affected by temperature of deposition and was good even at the low temperature (180°C ) of deposition. Curve HMDSO: Tyis

in Fig TTIP (%)

Example 1 ratio % improvement in weighted transmission values (calculated) c-Si CdTe a-Si uc-Si CIGS

1 2 4: 1 91.1 0.8 1.0 1.4 0.7 0.7

2 4 3:2 90.7 0.5 0.6 0.9 0.4 0.4

3 6 1 : 1 90.9 0.7 0.8 1.1 0.6 0.6

4 8 2:3 90.2 0.2 0.3 0.5 0.2 0.2

5 10 1 :4 90.8 0.6 0.7 1.1 0.5 0.6

6 12 0: 1 89.9 0.0 0.1 0.0 0.0 0.0

5

Table 1

In Table 1, c-Si refers to crystalline silicon PV cell, CdTe to a cadmium telluride PV cell, a-Si to amorphous silicon PV cell, uc-Si to micro crystalline silicon PV cell and

CIGS to copper indium gallium selenide PV cell.

Examples 7 to 11

In Examples 7 to 11, preformed nanoparticles were added to the silica precursor to form coatings on glass.

Examples 7 and 8 were performed according to the methods described in Table 2, below, and Method B in which titania nanoparticles (P25 obtained from Degusa) were mixed with HMDSO and methanol (to aid suspension) in solution ratio of

HMDSO:MeOH of 1 :2.5. A saturated solution of titania was used. The solution was used to form the coating on the glass surface.

Examples 9 and 10 were performed according to the methods described in Table 2 below and Method C in which titania nanoparticles were activated by heating at 300°C for 1 hour to remove water and contaminants from the surface and to activate the OH groups.

The titania was then immersed in pure HMDSO and stirred at 70°C for 1 hour before use.

Example 11 was performed according to the methods of Table 2 and Method D in which hexyl acetate was used as the solvent.

The flow rate at the surface = chemical flow/burner face area. The burner face area is 0.000492m² (single row of nozzles) and 3 x 0.000492m² for a 3 burner row. In Table 2, the flow rates of 27.5cm³/hour to 106cm³/hour are equivalent to surface flow rates of 3.1 x 10^~4 m/min to 12 x 10^~4 m/min for a single nozzle row burner and 9.3 x 10^~4 m/min to 36 x 10^~4 m/min for a three nozzle row burner.

Examples 12 and 13

In these examples titanium tetra isopropoxide (TTIP) was added to the HMDSO solution at HMDSO: TTIP ratio of 7:4. The two reagents are miscible. The deposition conditions were as indicated in Table 2.

The flow rate of the HMDSO and TTIP solution was varied so that the rate of HMDSO entering the flame was kept at 17.5 cc/hour.

Table 2

The photocatalytic activity of Examples 7 to 13 was determined according to the

Draft International Standard ISO/DIS 10678 using a Philips black light source with λ = 364nm at ImW/cm² measured at sample surface using methylene blue as the probe molecule. The results are as indicated in table 3. Table 3

Example (sample) Photocatalytic Activity (nmol / cm h)

7 0.172

8 0.111

9 0.100

10 0.078

11 0.044

12 0.078

13 0.144

Claims

1. A process for the deposition of a coating on a substrate, the process comprising a) providing a substrate at a substrate temperature of 80°C to 500°C,

b) passing a fluid mixture comprising a silicon-containing coating precursor and a titanium-containing material through a flame, and

c) contacting at least one surface of the substrate with the fluid mixture during or after its passage through the flame.

2. A process as claimed in claim 1, wherein the substrate temperature is 100 to 475°C, preferably 100 to 450°C.

3. A process as claimed in either claim 1 or claim 2, wherein the substrate temperature is 105 to 475°C, preferably 105 to 450°C.

4. A process as claimed in any one of the preceding claims wherein the fluid mixture further comprises a comburant.

5. A process as claimed in any one of the preceding claims wherein the fluid mixture further comprises a carrier fluid, preferably a carrier gas.

6. A process as claimed in any one of the preceding claims wherein the fluid mixture further comprises an oxidant.

7. A process as claimed in any one of the preceding claims, wherein the refractive index of the coating is 1.25 to 1.7.

8. A process as claimed in any one of the preceding claims, wherein the thickness of the coating is 10 to 500 nm, preferably 80 to 200 nm more preferably 95 to 200 nm, most preferably 105 to 200 nm.

9. A process as claimed in any one of the preceding claims, wherein the substrate comprises glass, preferably float glass or rolled glass.

A process as claimed in any one of the preceding claims, wherein the titanium- containing material comprises pre-formed nanoparticles, preferably titanium oxide nanoparticles.

11. A process as claimed in any one of the preceding claims wherein the titanium- containing material comprises a titanium-containing coating precursor.

A process as claimed in any one of the preceding claims wherein the flame substantially stable.

A process as claimed in any one of the preceding claims further comprising a step of depositing at least one further coating, wherein preferably the further coating is deposited on the same surface as the coating.

A process as claimed in any one of the preceding claims wherein the ratio of the silicon-containing coating precursor and the titanium containing material is in the range 1 :4 to 4: 1.

15. A process as claimed in any one of the preceding claims, further comprising forming a photovoltaic module using the substrate after deposition of the coating.