Patent Application
20100258652
The invention relates to a device for forming aerosol, and particularly to a device according to the preamble of claim 1 for forming aerosol, the device comprising at least one gas-dispersing atomizer for atomizing a liquid into aerosol by means of gas at an atomizing head of the atomizer. The invention also relates to an apparatus for coating glass and particularly to an apparatus according to the preamble of claim 9 for providing a coating onto the surface of glass, the apparatus comprising at least one gas-dispersing atomizer for atomizing at least one liquid used for coating the glass into aerosol by means of gas at an atomizing head of the atomizer. The invention further relates to a method for coating a glass product, and particularly to a method according to the preamble of claim 23 for providing a coating onto the surface of a glass product from at least one liquid raw material.
It is known that coatings having the desired characteristics for the use of glass can be manufactured by using gaseous reactants that disperse into a hot glass surface. Such characteristics include for instance a refractive index fitting implemented with a coating having a refractive index between the glass and the coating, preferably the square root of the product of the refractive indices of the glass and the coating, electrochromism, i.e. a change in the colour of the glass when an electric current is conducted into the glass, a change in the absorption of solar radiation of the glass (so-called ‘solar control’ glass), and, particularly, coating glass with an electrically conductive coating, such as tin oxide doped with fluorine, antimony or indium or zinc oxide, doped with aluminium, the electrically conductive coating causing the glass to reflect infrared radiation, or such glass, coated with an electrically conductive, transparent coating, may be used in solar cell applications.
Flat glass may be coated with a plurality of known methods, such as Chemical Vapor Deposition (CVD), sputtering, plasma deposition or the spray pyrolysis method.
In the spray pyrolysis method, the raw material is typically a liquid containing the substances required for generating the coating that are sprayed onto the surface of the hot glass to be coated.
Coatings possessing the desired characteristics for use in glass may be manufactured, not only by using gaseous reactants, also by using liquid reactants. The use of liquid reactants enables a process that is generally simpler and more inexpensive than gaseous reactants, but the process rate is substantially slower than when gaseous reactants are used.
However, in practice, it has unfortunately proven difficult to provide sufficiently uniform coatings having the desired thickness. Although the manufacture of such coatings succeeds at a reasonable production rate provided the temperature of the glass is sufficiently high, typically more than 650° C., it has proven difficult to manufacture such coatings profitably and at a sufficient production rate when the temperature of the surface of the glass is less than 650° C. Herein, a sufficient production rate refers to a coating of the required thickness being provided onto glass whose rate is preferably more than 5 m/min., more preferably more than 10 m/min. and most preferably more than 15 m/min., whereby the coating can be provided onto the surface of a moving glass ribbon in a glass production process, i.e. in a so-called float process. A temperature of less than 650° C. is preferably as regards the manufacture of the coating, since in that case the coating may be manufactured in the float process after the tin bath, whereby the environmental conditions for the coating are substantially less demanding than in the area of the tin bath. A temperature of less than 650° C. is preferable also when a coating is manufactured in an offline process, since in the glass hardening process, the maximum temperature of the glass is typically 650° C.
Finnish published patent application 94620, Pilkington plc and Flachglas Aktiengesellschaft, 15 Apr. 1990, discloses a method of coating glass in which at least two gaseous reactants react together to form a coating on a moving ribbon of hot glass. In order for the method to be able to industrially produce coatings of a strength of more than 200 nm in a short coating zone, the process comprises establishing a first flow of a first reactant gas onto the hot glass surface, establishing a second flow of a second reactant gas as a turbulent flow, and directing the combined gas flow onto the surface of the hot glass as a turbulent flow. The process may be applied to producing a metal oxide coating on hot glass. The examples presented in the publication describe the production of a coating when the speed of the glass ribbon is 8 m/min. and the temperature of the glass 580° C. The thickness of the coating was 250 to 275 nm. In the second example, the thickness of the coating was slightly more than 300 nm. In practice, such a thin layer thickness does not result in a sufficiently low emissivity of the glass (less than 0.2) in order for the glass to be useful as low-emissivity glass (low-e). The publication does not disclose the emissivity or sheet resistance values of the coating. In the examples described in the publication, the first reactant gas was stannic tetrachloride in preheated dry air at 354° C. as a carrier gas. The stannic chloride was supplied at a rate of 84 kg per hour, and the dry air was supplied at a rate of 105 cubic metres per hour. The second reactant gas was fluoric acid also mixed into preheated air. The fluoric add was supplied at a rate of 34 kg per hour, and the air was supplied at a rate of 620 cubic metres per hour. The reactant gases mixed rapidly to provide a combined flow through the coating chamber. 84 kg stannic chloride requires about 7.3 cubic meters of oxygen for complete oxidation, so that, the oxygen content in the air being about 20%, it may be stated that stannic chloride reacts in the feed chamber at an atmosphere having no extra oxygen as regards the oxidation reaction. Such a reaction atmosphere is not advantageous for the formation of a stannic oxide layer, since it is preferable as regards conductivity that the structure of stannic oxide includes errors, preferably oxygen deficit.
Publication U.S. Pat. No. 2,564,708, Corning Glass Works, 21 Aug. 1951, discloses a heat screen that reflects low temperature heat radiation and transmits high temperature heat radiation. The heat screen is based on a film deposited on the surface of a glass plate by heating the glass plate to a temperature of more than 500° C. by atomizing a solution containing the desired metal salts and by spraying the atomized liquid on the surface of the glass plate to produce a coating of the desired thickness. The desired film material is tin oxide, tin oxide doped with antimony or a tin oxide doped with indium oxide. The publication mentions that when a coating is produced on borosilicate glass at a temperature of 700° C., the production of a film of a thickness of 100 to 700 nm takes 10 to 20 seconds. The publication does not mention the drop size of the atomized raw material solution, but based on the thickness of the film, the production time and the temperature, it can be concluded that the average diameter of a drop was several dozens of micrometres, which is a typical drop size when producing drops with a conventional gas or pressure dispersing atomizer. For providing a coating layer of a sufficient thickness when the glass ribbon to be coated moves at a rate of 5 m/min., the length of the coating chamber should be about 1 metre, and at a rate of 15 m/min., up to about 3 metres, and the temperature distinctly more than 650° C. This makes the production process disclosed in the publication uneconomic in connection with a float process and impossible in connection with post-treatment of glass.
Published U.S. Pat. No. 2,566,346, Pittsburgh Plate Glass Co., 4 Sep. 1951, discloses a method of producing an electroconductive coating on glass by using an aqueous solution as the raw material. Said published patent further discloses a soda glass based glass product having an electroconductive tin oxide coating doped with fluorine on the surface thereof, the thickness of the coating being 25 to 600 nm and the specific resistivity being 200 to 500 μΩ-cm. The examples of the publication disclose a method of providing a coating based on conventional spraying of a liquid raw material on the surface of an object. According to the examples, a layer of a thickness of about 75 nanometres was produced by a 5-second spraying at a spraying rate of 120 ml/min. The specific resistivity of the coating produced was about 400 μΩ-cm. To achieve lower specific resistivities, thicker coatings are required. As was mentioned above, the thickness of the coating should typically be several hundreds of nanometres, which makes the method disclosed in the publication unpractical in connection with a glass production process.
Published U.S. Pat. No. 4,721,632, Ford Motor Company, 26 Jan. 1988, discloses a method of lowering the emissivity of a doped tin oxide film. However, the emissivities of the films disclosed in the publication are in the order of 0.25 to 0.29, which are too high for good low-emissivity glass.
Published U.S. Pat. No. 4,728,353, Glayerbel, 1 Mar. 1988, discloses an apparatus for pyrolytically forming a metal compound coating on a hot glass substrate. It is essential to the operation of the apparatus that the gaseous environment in the immediate vicinity of the glass substrate is controlled by feeding preheated gas thereto to form a protective atmosphere in the vicinity of the glass substrate. The protective atmosphere enables the prevention of surrounding air from penetrating into the coating area. The publication discloses that the preheated gas is preheated air, so the coating formation reactions occur in an oxygen-rich atmosphere. The publication discloses the feed of liquid coating material by spraying, but does not disclose the diameter of the mist drop. Since atomizers had not been developed to produce small droplets in 1988, it was evident to a person skilled in the art at that time that the diameter of the mist drop was several dozens of micrometers. Publication Arthur H. Lefebvre, Atomization and Sprays, Taylor&Francis, USA, 1989, discloses different atomizers, The word ‘mist’ generally used in patent publications refers to drops having a diameter of about 100 micrometers (said publication, page 80), and with pressure-dispersing and air-dispersing atomizers, the drop size distributions disclosed with in said publication (particularly pages 201 to 273) never show drops less than 10 micrometres, the average diameters typically being 30 to 80 micrometers. The evaporation of such a drop is possible during the 10-second time mentioned in the publication, provided that the air temperature is several hundreds of degrees, such as is indeed described in the publication. However, the heating of air makes the solution expensive, particularly when large quantities of air described in the publications are used.
Coated flat glass is used in different building applications, such as energy saving, heat radiation reflecting glasses (low-emissivity, i.e. low-e glass) or self-cleaning glasses. In the former case, the glass is coated in most cases with tin oxide doped with fluorine (FTO), in the latter case with titanium dioxide preferably having the crystal form of an anatase.
Different prior art atomizers are disclosed in publication Huimin Liu, Science and Engineering of Droplets—Fundamentals and Applications, William Andrew Publishing, LLC, New York, 2000, particularly pages 23 to 25. When reference is made later in the present text to the diameter of drops produced with prior art atomizers, reference is made to this publication.
Published U.S. Pat. No. 7,008,481 B2, 7 Mar. 2006, Innovative Thin Films, Ltd., discloses a method and an apparatus for preparing a homogeneous pyrolytic coating. The apparatus disclosed in the publication removes large drops from a mist of liquid droplets, whereby the uniformity of the coatings improves. The publication does not mention the size of the liquid drops used, but the publication makes reference to prior art atomizers, whereby the liquid drop size can be assumed to be more than 10 micrometres. The publication also refers to electrostatic and ultrasound atomizers, but the production rate of these atomizers is low and they are not as such suitable for a coating process for flat glass requiring a large droplet production.
Published U.S. Pat. No. 5,882,368, 16 Mar. 1999, Vidrio Pifano De Mexico, S.A. DE C.V., discloses a method and an apparatus for coating a hot glass substrate with a mist of fine droplets. The mist droplets are produced with a plurality of ultrasound atomizers. The description of the publication describes the production of droplets with an ultrasound atomizer having a frequency of 1 MHz, yielding droplets having a diameter of less than 10 micrometres, typically 5 micrometres, The temperature of the glass ribbon to be coated is 580 to 610° C.
A prior art problem is that a hot glass ribbon should be coated in a temperature of about 580° C. at the lowest, whereby, in practice, the coating unit should be situated, e.g. on the production line for flat glass (a float line), inside the tin bath or immediately after the tin bath, whereby the apparatus construction required is expensive. A further prior art problem is that the coating consumes relatively much time. A prior art problem is that it does not present a method of advantageously producing a coating on the surface of glass from liquid raw materials, the oxidation degree of the coating being lower than the oxidation degree of a completely oxidized coating. The coating should preferably be produced at the preparation or processing rate of the glass product, such as flat glass, at a temperature of at most 650° C. In prior art, droplets used for coating have been produced with electrostatic atomizers and ultrasound atomizers for producing sufficiently small droplets for achieving a high-quality coating. However, the problem in electrostatic atomizers and ultrasound atomizers is that they are incapable of high material production, which is also noted in publication Huimin Liu, Science and Engineering of Droplets—Fundamentals and Applications, William Andrew Publishing, LLC, New York, 2000. In addition, electrostatic atomizing and ultrasound atomizing require a separate carrier gas that does not directly mix with the droplets in the atomizing event. In coating based on ultrasound atomizing and also on electrostatic atomizing, droplets are conducted to a coating chamber and during the transport event, part of the material may be dried or react into solid particles, as is mentioned in publication U.S. Pat. No. 5,882,368. At least part of these reaction products and solid particles end up in the coating causing mistiness therein. In addition, these gas phase reactions impair the efficiency of the material use in the process. Prior art pneumatic atomizers, in turn, may achieve a sufficient material yield to produce a coating on a moving glass ribbon, but the size of the drops generated by these known pneumatic atomizers is too large for achieving a high-quality coating.
It is thus an object of the invention to provide a method and an apparatus for implementing the method so as to solve the above problems. The object of the invention is achieved with a device according to the characterizing part of claim 1, which is characterized in that the atomizer further comprises one or more flow restraints for changing the hydrodynamic properties of the aerosol flow discharging from the atomizing head in a manner reducing the drop size of the drop jet. The object of the invention is also achieved with an apparatus according to the characterizing part of claim 9, which is characterized in that the apparatus further comprises one or more flow restraints for changing the hydrodynamic properties of the aerosol flow discharging from the atomizing head in a manner reducing the drop size of the drop jet before it is conducted onto the surface of the glass. The object of the invention is further achieved with a method according to the characterizing part of claim 23, which is characterized by the method comprising the steps of:
atomizing at least one liquid raw material by means of at least one gas-dispersing atomizer into aerosol that is discharged from an atomizing head of the atomizer;
reducing the drop size of the aerosol discharged from the atomizing head of the atomizer by changing the hydrodynamic properties of the aerosol flow by means of flow restraints; and
conveying the aerosol onto the surface of the glass product, wherein the aerosol reacts providing a coating onto the surface of the glass product.
Preferred embodiments of the invention are described in the dependent claims.
The present invention is based on the idea of pneumatically or gas-dispersedly producing or atomizing a drop jet or an aerosol from at least one liquid raw material by making the average drop size of the drop jet or aerosol 3 micrometres or less, preferably 1 micrometre or less. Producing small droplets in accordance with the present invention is based on the surprising observation that by subjecting a drop jet or an aerosol produced with a pneumatic atomizer to flow restraints, an aerosol can be produced, provided that the flow rate of the drop jet or aerosol is sufficient, wherein the average diameter of the liquid drops is less than 3 micrometres and preferably less than 1 micrometre. This may be implemented for instance by feeding an aerosol produced with a gas-dispersing atomizer into a tube containing a plurality of flow restraints disposed inside the tube, whereby mist having a very small drop size can be produced, provided that the drop-gas mixture, i.e. the aerosol, travels at a sufficiently high rate in the tube. The flow restraints are used to change the hydrodynamic properties of the aerosol produced in a manner reducing the average drop size of the aerosol.
The principle of the invention may be utilized in coating glass products at a temperature of less than 650° C., for example. In this case, a hot glass product may be coated, which may be a glass ribbon flowing in a float process, for example. In the float process, melt glass flows first on the surface of melt tin, after which it rises onto a roll conveyer and flows further to a cooling furnace. As regards the coating of the glass ribbon, the most advantageous place is between the tin bath and the cooling furnace, wherein the temperature of the glass is typically 630 to 530° C. The hot glass product may also be a glass product moving in a glass hardening process, for example. In the glass hardening process, the glass product is first heated typically to a temperature of about 650° C., whereupon the surface of the product is rapidly cooled with air jets. The glass product may also be heated to a temperature of 500 to 650° C. in a separate offline device for the coating according to the invention.
The atomizing liquid raw material may be a metal salt dissolved in water or alcohol, for example. Alcohol or another exothermic liquid is preferable, since is does not bind process heat as does water. The salt may preferably be a nitrate, since the solubility of nitrates into water and alcohols is generally good. The alcohol is preferably methanol. As regards functional coatings, preferable metals include tin, fluorine, antimony, indium, zinc and aluminium, which are used in the preparation of conductive coatings (coating of doped tin oxide or doped zinc oxide, antimony may be used to provide the coating also with solar absorption, i.e. a so-called ‘solar control’ property), vanadium, which is used in the preparation of an electrochromic coating (coating of oxygen deficit vanadium oxide, VO2), and silicon, used in the preparation of a refractive index adjustment coating (coating of oxygen deficit silicon oxide, SiOx). The metal-containing liquid raw material may also as such be a solution, for instance tin tetrachloride, silicon tetrachloride (SiCl4), tin tetrachloride (SnCI4), monobutyl tin chloride (MBTC), trifluoroacetic acid (TFA), hydrogen fluoride (HF), or the like. Raw materials having a high steam pressure at room temperature are preferable to the process. A metal-containing raw material may also be a colloidal solution, colloidal silica, for example. In this case, the diameter of the colloidal metal oxide particles is typically less than 100 nm.
It is typical of the glass products described in the present application that the coating is substantially composed of an oxygen deficit metal oxide, which may be doped or undoped. It is preferable that the electrically conductive metal oxide coatings have crystal defects in the coating structure, generating conductivity in the coating. Typically, such crystal defects are oxygen deficit in the crystal structure. An electrochromic coating is mainly composed of vanadium oxide, VO2, which is accomplished only if a deficit of oxygen is usable for the oxidation of the vanadium. If more oxygen exists, vanadium is oxidized into the form V2O5, which has no electrochromic properties. A reflective index adjustment coating is composed of oxygen deficit silicon oxide SiOx, wherein x is between 1≦x≦2. The coating may also be a completely oxygen deficit compound, such as magnesium fluoride, MgF2, which produces an antireflection coating having a low reflective index. For achieving an oxygen deficit metal oxide coating, an oxygen deficit gas atmosphere is generated in the coating chamber by feeding inert or reducing gas into the coating chamber, such as at least nitrogen, carbon dioxide, carbon monoxide, hydrogen, methane or propane. It is most preferable to use this gas feed also for the atomizing of the liquid raw material.
In some coating applications, such as in electrically conductive coatings of solar cells, it is preferable that the coating also comprises small particles whose diameter is typically less than 200 nm. Such particles scatter light, and due to the small particle size, the majority of the scattering is directed forward, whereby the sunlight can be collected into the solar cell more efficiently. Such a fine particle may be produced in the coating along with the raw material, for instance by using a raw material solution also containing colloidal particles. The material of the particles is preferably the same as the material of the coating.
An advantage of the present invention is that it enables the production of small droplets having a diameter of less than 3 micrometres or less. Small droplets are preferable as regards the process, since their diffusion rate in the coating chamber is substantially higher than that of usual mist drops. Small droplets evaporate faster, which is preferable as regards the speed of the process. Since gravitation affects small droplets less than mist drops, no defects are generated on the glass surface to be coated, as does from mist drops settling on the surface by the action of gravitation. Since the mass of a liquid drop of the size 1 micrometre, for example, is only one thousandth of the mass of a liquid drop of the size 10 micrometres, the smaller liquid drop evaporates and burns in the pyrolysis process substantially more easily than the larger one, allowing the coating to be made at a lower temperature and/or at a higher rate. In addition, since these small droplets may be produced pneumatically, with a gas-dispersing atomizer, for example, a large material output can be achieved combined with the production of small droplets, which has not been possible in accordance with the prior art. In addition, the use of a separate carrier gas is avoided, allowing the apparatuses to be made simpler. Furthermore, the solution of the invention, wherein the drop size of the drop jet or aerosol is reduced by means of flow restraints, the particle size distribution of the drop jet or aerosol can be reduced, which in prior art gas-dispersing atomizers is wide.
In the following, the invention will be described in more detail in connection with preferred embodiments with reference to the accompanying drawings, in which
In accordance with the above, the device for forming aerosol comprises at least one gas-dispersing atomizer 6 for atomizing liquid 3 into aerosol by means of gas at the atomizing head 34 of the atomizer 6. The atomizer 6 comprises at least one liquid conduit 14 for feeding at least one liquid 3 to be atomized into the atomizing head 34 and at least one gas conduit 8 for feeding at least one gas into the atomizing head 34 for atomizing the liquid into aerosol. The atomizing gas atomizes the liquid 3 into aerosol at the atomizing head 34, particularly as the result of the rate difference between the atomizing gas and the liquid 3 discharging at the atomizing head 34. The atomizer 6 further comprises one or more flow restraints 36 for changing the hydrodynamic properties, such as rate and pressure, for example, of the flow of aerosol discharging from the atomizing head 34 in a manner reducing the drop size of the drop jet. The atomizer 6 may be provided with an atomizing chamber 35 provided with the flow restraints 36 and in flow connection with the atomizing head 34. In
The atomizer 6 is situated in a chamber 7 substantially separating the inner gas atmosphere of the chamber from the surrounding atmosphere. An inert or reducing gas is fed into the chamber 7 from a gas conduit, which is preferably the gas conduit 8 used for atomizing the liquid raw material. It is evident to a person skilled in the art that the gas can also be conducted to the chamber from elsewhere and that there may be more than one gases and feed conduits.
The moving, hot glass ribbon 2 enters the coating chamber from a tin bath 9 of the float line, the temperature of the glass ribbon 2, when it rises from the bath, being at most 650° C.
The liquid raw material used in the method of the invention may be a mixture, an emulsion or a colloidal solution. An emulsion refers to a mixture of at least two liquids that are inherently immiscible with one another. A colloidal solution refers to a solution composed of two different phases: a dispersed phase and a continuous phase. The dispersed phase contains small particles or droplets evenly distributed into the continuous phase. In other words, a colloidal solution is a solution containing colloidal particles.
In the following, the operation of the method and the device of the invention will be described with examples.
A fluorine-doped tin oxide coating was produced with the embodiment of the invention according to
The measurements, conducted with a Keithley 2400 General Purpose Source-Meter, provided with an Alessi CPS-05 measuring head, showed that the sheet resistance of the glass product was less than 20 μ/□, The emissivity measurements conducted with a Mk2 Emission meter (Sten Löfving Optical Sensors, Sweden) measured the emissivity of the coating P to be 0.09 to 0.14.
A fluorine-doped tin oxide coating was produced by the embodiment according to
The liquid raw material 3 was a solution containing, as weight fractions, 30 parts of methanol (MOON), 20 parts of monobutyl tin chloride (MBTC, CAS number 1118-46-3) and 9 parts of trifluoroacetate acid (TFA, CAS number 76-05-1). A bottle 11 containing the raw material liquid was placed into a pressure tank 10, which was pressurized with nitrogen gas (N2) flowing through a regulator 13 from line 12 to a pressure of 3 bar. Because of the pressure, the raw material 3 flowed through line 20 to a liquid flame spray nozzle 21. The flow amount was 30 ml/min, per liquid flame spray nozzle 21 width metre. From conduit 22, nitrogen gas (N2) also flowed to the liquid flame spray nozzle 21, the flow amount being 300 l/min. per liquid flame spray nozzle width metre, and oxygen gas (O2) from conduit 23, the flow amount being 100 l/min. per liquid flame spray nozzle width metre. By means of the liquid flame spray nozzle 21, oxide particles containing tin and fluorine were produced from the raw material 3, the average diameter of the particles being less than 100 nm and part of which ending up onto the surface of the glass product 2. Because of the pressure, the raw material 3 flowed through line 14 and a flow meter 15 to the atomizer 6. The flow amount was 150 ml/min. per atomizer width metre. From line 8, nitrogen gas (N2) was fed through a flow adjuster 16, the flow amount being 500 l/min. per atomizer width metre. The glass product 2 was heated in an oven 9 to a temperature of 550° C., after which the glass product moved first under the liquid flame spray nozzle 21 and then under a coating chamber 7 at a rate of 3 m/min. After the coating, the glass product 2, coated with nanoparticles and the coating P, was placed in a stress-relieve oven at a temperature of 500° C., and the product was allowed to cool slowly to room temperature.
Haze was measured from the coated product in accordance with standard ASTM D 1003, and the haze value was found to be 5 to 10%.
A vanadium oxide coating VO2 was produced by the embodiment of the invention according to
An oxygen deficit, carbon-doped SiOx coating was produced by the embodiment of the invention according to
It is obvious to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in a variety of ways. Consequently, the invention and its embodiments are not restricted to the above examples, but may vary within the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20071003 | Dec 2007 | FI | national |
| 20080217 | Mar 2008 | FI | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI2008/050772 | 12/19/2008 | WO | 00 | 5/27/2010 |
