Coating of particulate substrates

Abstract

The present invention relates to a method for coating large area solid substrates with titanium by reacting the substrate surface with a mixture comprising titanium halide or subhalide powders in the presence of a reducing agent. The method is suited for coating large area substrates such as flakes, powder, beads and fibres with elemental Ti-base metals or alloys of Ti with coating additives based on any number of non inert elements from the periodic table.

Claims

1. A method for forming Ti-based coatings on a particulate substrate, including: a) forming an uncoated Ti-based powder by reacting at least one solid titanium subchloride with a solid Al reducing agent, wherein the Al reducing agent is an Al powder or an Al alloy powder; b) mixing the particulate substrate with the uncoated Ti-based powder; and c) heating the particulate substrate in contact with said uncoated Ti-based powder to a temperature less than 850 C. to produce a coating on said particulate substrate.

2. The method according to claim 1, wherein in a first step, the titanium subchloride reacts with the reducing agent to produce said uncoated powder, the uncoated powder is substantially free of oxygen and has a grain size less than 1 micron.

3. The method according to claim 1, comprising the steps of: mixing, stirring and heating a mixture of the at least one solid titanium subchloride, the Al reducing agent and the particulate substrate at temperatures between a first temperature T.sub.0 above 160 C. and a maximum temperature T.sub.max below 850 C. to form the coating on the substrate and by-products including aluminium chloride and titanium tetrachloride; to reduce titanium subchlorides to a composition with a chlorine content less than TiCl.sub.2; and the Ti-based coating comprises one or more of pure element, an alloy, an intermetallic compound, an inorganic compound, oxides, nitrides, carbides, borides or silicides; and condensing the by-products away from a reaction zone where the aluminium and precursor material are reacting; and collecting the resulting products, and separating the coated substrate from residual un-reacted materials and washing and drying coated substrate.

4. The method according to claim 1, wherein the uncoated Ti-based powder is a Ti-based nanopowder.

5. The method according to claim 4, further including a primary step of reducing TiCl.sub.4 to form the titanium subchlorides.

6. The method as claimed in claim 1, wherein the process is continuous and a stream of gas is passed in a direction away from solid reactants, and by-products are continuously removed away from the reactants.

7. The method as claimed in claim 1, wherein the method is carried out at a pressure between 0.01 mbar and 1.1 bar; and wherein the particulate substrate is in the form of a powder, flakes, beads, fibres, particles or a number of small objects.

8. The method according to claim 1, wherein the weight ratio of solid titanium subchlorides to substrate is between 0.01 to 1 and 5 to 1.

9. The method according to claim 1, wherein the Ti-based powder and an Al reducing agent starting materials are mixed with AlCl.sub.3 before mixing with the substrate, and wherein the weight of AlCl.sub.3 is between 10% and 500% of the weight of the Ti-based powder and the Al reducing agent.

10. The method according to claim 1, wherein the method includes an additional step of reacting the coated substrates with a reactive gas.

11. The method according to claim 1, wherein a mixture including TiCl.sub.xR.sub.a is heated at temperature up to 500 C. in step (a) to produce a mixture including metallic Ti-based species and then the resulting reactant mixture is mixed with the substrate in step (b), wherein TiCl is the titanium subchloride, and R.sub.a is the Al reducing agent.

12. The powder method according to claim 1, comprising: in a first step, a mixture comprising the titanium subchlorides, the Al powder, and optionally the particulate substrate in powder form, are heated at temperatures between T.sub.0 above 160 C. and T.sub.1 below 500 C. to form a mixture comprising metallic TiAl species in a fine powder or in a nanopowder form containing a component with a particle size below 1 micron; and adding the substrate powder if not added in the first step and mixing; and heating the resulting mixture comprising the metallic Ti-based species and the particulate substrate at temperatures between T.sub.2 above 160 C. and T.sub.max below 850 C. to induce reactions between the TiAl species and the substrate and cause a coating to form on the surface of the substrate.

13. The method according to claim 1, wherein the substrate is selected from the group consisting of itransition metal alloys, oxides, nitrides, carbides, or borides, iiglass, glass flakes, glass beads, quartz, borosilicate, soda-glass, silicon nitride, mica flakes, talc powder, iiigraphite powder, graphite flakes, carbon fibre and a combination thereof of i, ii, and iii; wherein the substrate is in the form of a powder, flakes, beads or fibres.

14. The method according to claim 13, wherein the substrate materials include silicon.

15. The method according to claim 14, wherein the substrate is a powder of glass flakes and the coating includes titanium silicides.

16. The method according to claim 14, wherein the substrate is made of borosilicate and heating of the particulate substrate is to a maximum temperature T.sub.max below 650 C., or the substrate comprises soda-glass and heating of the particulate substrate is to a maximum temperature T.sub.max below 600 C.

17. The method according to claim 13, wherein the substrate is based on carbon and comprises powder, beads, flakes or fibre and the coating includes titanium carbides.

18. The method according to claim 13, wherein the substrate comprises at least one transition metal in the form of powder, beads, flakes or fibre and the Ti-based coating includes titanium-transition metal compounds.

19. A method for forming Ti-based coatings on a particulate substrate, including: mixing, stirring and heating a mixture comprising at least one titanium subchloride, a solid Al reducing agent, a coating additive and a particulate substrate in the form of a powder, at temperatures between a first temperature T.sub.0 above 160 C. and a maximum temperature T.sub.max below 850 C. to form the Ti-based coating on the substrate and by-products including aluminium chloride and titanium tetrachloride; wherein the Al reducing agent is in a fine particulate form and an amount of Al is sufficient to reduce the at least one titanium subchloride to a composition with a chlorine content less than TiCl.sub.2; the coating additive includes sources for at least one of C, Si, B, O.sub.2 and N.sub.2, and the coating is based on titanium in the form of at least one of a titanium carbide, silicide, boride, oxide or nitride, and condensing the by-products away from a reaction zone where the Al reducing agent and the at least one titanium subchloride are reacting; and collecting reaction products, and separating the coated substrate from residual un-reacted materials, and washing and drying the coated substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features and advantages of the present invention will become apparent from the following description of embodiments thereof, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1: A block diagram for one embodiment illustrating general processing steps for forming Ti-based coating on a substrate surface.

(3) FIG. 2: A block diagram for one embodiment illustrating steps for forming of titanium based alloys on a substrate, starting from TiCl.sub.3 and Al.

(4) FIG. 3: An SEM micrograph showing Ti-based coating on glass flakes.

(5) FIG. 4: An SEM micrograph showing glass flakes coated with Ti.

DESCRIPTION OF PREFERRED EMBODIMENTS

(6) FIG. 1 is schematic diagram illustrating general processing steps for depositing a Ti coating on a powdery substrate. In a first step, the reducing agent R.sub.a (1), is mixed together with the titanium subchlorides (2), the substrate (3) and the additive precursors (4) in (5). The resulting mixture is then processed in (6) at temperature below 600 C. to produce an intermediate product comprising an uncoated powder and TiCl.sub.x, which is then progressed through the Coating Stage (7) to form a coating on the substrate surface. By-products (8) are discharged in (9) and the residual waste is processed through (10).

(7) In step (11), the products from (7) are sieved to remove any residual fines (12) which can be either recycled through (6) or withdrawn (13). Sieved coated products (14) can then washed and dried in necessary (15) leading to final product (16)

(8) FIG. 2 is a schematic diagram illustrating processing steps for one preferred embodiment for production of Ti coated glass flakes using Al as a reducing agent. Al and A/Cl.sub.3 are first mixed together at (1) to dilute the Al and spread its distribution within the reactant-substrate mixture. Precursors for coating additives (M.sub.z) (2) may also be added and mixed together with the AlAlCl.sub.3 depending on their compatibility with Al and AlCl.sub.3. The AlAlCl.sub.3-M.sub.z powder is then mixed with TiCl.sub.x (3) and the substrate powder (4) in step (5); for this embodiment, TiCl.sub.3 is the starting subchloride, glass flakes are used as substrate and the mixing can be before or during processing in step (5). The resulting TiCl.sub.3AlAlCl.sub.3-glass flakes mixture (5) is then processed at temperatures between 200 C. and 650 C. in a single cycle combining the uncoated powder production step and the Coating Stage (6). AlCl.sub.3 by-products are removed in a stream of inert gas away from the reaction zone and condensed elsewhere (7). A part of the AlCl.sub.3 might be recycled through (8) as shown in the diagram. The rest (9) are discharged and stored for disposal or other use.

(9) The remaining inert gas with any residues (10) is processed through a dedicated scrubber. At the end of the reaction, there is a separation step (11) where coated flakes (12) are separated from the TiClAl based fines (13) and then washed and dried in dedicated equipment (15) and the resulting end product (16) is discharged and ready for use.

(10) The TiClAl based fines (13) can be recycled (17) or discharged (18).

(11) Materials produced using the present invention have unique characteristics that may not be obtained using prior art methods. The invention includes materials made using the present coating invention and the use of such materials, without being limited by the examples provided in the specifications by way of illustration. Specific properties include the ability to produce coating for large area substrate of composition and structure usually unachievable with conventional physical vapour deposition or chemical vapour deposition.

(12) As an example for the specific quality and use of materials produced using the current technology is in production of metallic Ti-based pigment for use in the paint industry. Currently, there are no technologies capable of producing titanium metal based flakes at an affordable price. Such pigments would be very attractive for use in the automotive paint industry and in the architecture and paint industry in general. It is also possible to change the hue, the reflectance and the refraction of the coating by changing the composition. For example, adding chromium to titanium results in increased reflectance while adding other materials such vanadium provides a dull metallic colour to the film. It is also possible to change the colour and the interference characteristics of the pigment by addition of a varying amount of oxygen to the Ti film covering the substrate.

(13) The following are examples of preparation of titanium compounds in accordance with an embodiment of the present invention.

EXAMPLE 1

Ti on Glass Flakes

(14) The starting materials were 1 g of TiCl.sub.3 powder 170 mg of Ecka Al powder (4 microns) and 4 g of AlCl.sub.3 powder. The starting materials were mixed together and the resulting mixture was thoroughly mixed with 10 g of glass flakes. The resulting mixture was heated in a rotating quartz tube under argon at a temperature of 575 C. for 10 minutes. The powder was then sieved to remove un-deposited products and the remaining coated flakes washed in water and dried. The flakes have a darkish metallic titanium appearance. Examination under an SEM shows that the surface is thoroughly coated with metallic Ti but with the presence of metallic titanium particulates. SEM micrographs for coated flakes are in FIG. 3 and FIG. 4.

EXAMPLE 2

Ti on Mica Flakes

(15) The starting materials were 1 g of TiCl.sub.3, and 4 g of AlCl.sub.3. The starting materials were mixed together and the resulting mixture was thoroughly mixed with 10 g of mica flakes. The resulting mixture was heated in a rotating quartz tube under argon at a temperature of 575 C. for 10 minutes. The powder was then sieved to remove un-deposited products and the remaining coated flakes washed in water and dried. The flakes have shiny metallic appearance.

EXAMPLE 3

Ti on Carbon Fibres

(16) The starting materials were 1 g of TiCl.sub.3 powder, 170 mg of Ecka Al powder (4 microns) and 4 g of AlCl.sub.3 powder. The starting materials were mixed together and the resulting mixture was thoroughly mixed with 1 g of carbon fibres (cut to 1 cm length). The resulting mixture was heated in a rotating quartz tube under argon at a temperature of 750 C. for 10 minutes. The products were then sieved to remove un-deposited/unreacted materials and the remaining coated fibres washed in water and dried. SEM analysis shows the fibre are coated with a Ti-based coating. The fibres have very high resistance to oxidation and after burning a sample in air for 48 hours at 800 C., the residue are empty long tubular shells of titanium oxides.

(17) The present method may be used for production of coating or compounds of various compositions based on Ti including coatings of pure metal, alloys, oxides, nitrides, with additives including other coating additives as described above. Modifications, variations, products and use of said products as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

(18) In the claims which follow and in the preceding description of embodiments, except where the context requires otherwise due to express language or necessary implication, the word comprise and variations such as comprises or comprising are used in an inclusive sense, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

(19) It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention, in particular it will be apparent that certain features of embodiments of the invention can be employed to form further embodiments.