The present invention relates to a method of and an apparatus for depositing material, preferably a film, on a substrate and to a method of and an apparatus for fabricating a powder, preferably an ultrafine powder.
Material films, in particular ceramic films, have wide ranging structural and functional applications. These different applications often require films of different thickness, but there is no single commercially cost-effective film or coating deposition technique for depositing both thin films, typically films having a thickness of less than 1 xcexcm, and thick films, typically films having a thickness greater than 10 xcexcm.
Vapour processing techniques, including chemical vapour deposition (CVD) and physical vapour deposition (PVD), have been used to fabricate thin films, but, because of the slow deposition rate and expensive equipment, are not suited to the deposition of thick films of large area. Moreover, the coating of substrates of complex shape is particularly difficult using a PVD technique.
Sol-gel processing techniques have also been used to deposit thin films, but, while thin films can be achieved in a single coating run, thicker films provided by a single coating are cracked and thus thick solid films have to be built up by performing a plurality of successive coating runs.
A novel deposition technique, referred to as electrostatic spray assisted vapour deposition (ESAVD) and disclosed in WO-A-97/21848, has also been used particularly to deposit thin films. In this ESAVD technique, an aerosol is electrostatically generated from a nozzle unit and a temperature gradient and electric field are provided between the substrate and the nozzle unit such that the aerosol droplets undergo combustion and/or chemical reaction in the vapour phase close to the surface of the substrate. This deposition technique is capable of producing solid films which exhibit excellent substrate adhesion, but does have limitations as a consequence of electrostatically generating the aerosol, for example, with regard to the nature of the utilisable precursor solutions, the deposition rate and the droplet size distribution of the aerosols.
Spray pyrolysis, where a film is deposited by delivering an aerosol generated by ultrasonic atomisation to a heated substrate, has been used to deposit both thin and thick films as disclosed, for example, in EP-A-0103505 and GB-A-1362803, but the deposition efficiency is usually very low because of the very high loss of the aerosol to the environment, which loss is unacceptable both for environmental reasons and cost reasons where the precursor materials can be expensive and the deposition rate is very low. Furthermore, the deposition of very thick films, typically films having a thickness of greater than 150 xcexcm, by spray pyrolysis is difficult. In published articles entitled xe2x80x9cCorona Spray Pyrolysisxe2x80x9d Thin Solid Films, 121 (1984), pages 267 to 274 and xe2x80x9cProperties of Thin In2O3 and SnO2 Films Prepared by Corona Spray Pyrolysis and a Discussion of the Spray Pyrolysis Processxe2x80x9d Thin Solid Films, 121 (1984), pages 275 to 282, the deposition of thin films of doped In2O3 and SnO2 by corona spray pyrolysis with a claimed deposition efficiency of up to 80% has been discussed, but this deposition technique essentially requires the use of an organic precursor solution, the delivery of the aerosol vertically downwardly so as to utilise the gravitational effect on the aerosol droplets, and a specific electrode configuration comprising two electrodes each disposed at an angle of from 40 to 45xc2x0 relative to the vertically downward flow path of the aerosol.
It is an aim of the present invention to provide an improved method of and apparatus for depositing material, preferably one of thin or thick films, on a substrate, referred to as electrostatic assisted aerosol jet deposition (EAAJD), which in particular is low cost and exhibits a high deposition efficiency, and an improved method of and apparatus for fabricating a powder, preferably an ultrafine powder.
Accordingly, the present invention provides a method of depositing material, preferably a film, on a substrate, comprising the steps of: providing a substrate; heating the substrate; generating an aerosol comprising droplets of a material solution; providing a nozzle unit for delivering the aerosol to the substrate, the nozzle unit including at least one outlet through which a directed flow of the aerosol is delivered and at least one electrode; charging the aerosol droplets with a positive or negative charge; providing a flow of the aerosol through the nozzle unit so as to deliver a directed flow of the aerosol from the at least one outlet; and generating an electric field between the substrate and the at least one electrode such that the directed aerosol flow is attracted towards the substrate.
Preferably, the substrate is heated to a temperature of less than about 1050xc2x0 C., more preferably less than about 800xc2x0 C.
Preferably, the substrate is heated during deposition.
More preferably, the thermal environment is such as to maintain a decreasing temperature gradient in a direction away from the substrate towards the nozzle unit.
In one embodiment the material solution is an aqueous solution.
In another embodiment the material solution is a non-aqueous solution. Preferred non-aqueous solvents include acetylacetone, methanol and 2-methoxyethanol.
In one embodiment the aerosol droplets are at least partially charged prior to exiting the at least one outlet.
In another embodiment the aerosol droplets are charged prior to exiting the at least one outlet.
In a further embodiment the aerosol droplets are at least partially charged after exiting the at least one outlet.
Preferably, the aerosol droplets are charged by the at least one electrode.
Preferably, the at least one electrode is disposed at least partially in each aerosol flow.
Preferably, the at least one electrode extends upstream of the at least one outlet.
Preferably, the at least one electrode comprises an elongate element.
Preferably, the distal end of the at least one electrode is located at substantially the centre of the at least one outlet.
In one embodiment the distal end of the at least one electrode includes a single tip.
In another embodiment the distal end of the at least one electrode includes a plurality of tips.
Preferably, the nozzle unit includes a tubular section upstream of each outlet.
More preferably, the tubular section is an elongate section.
More preferably, the tubular section is a linear section.
More preferably, the tubular section is substantially cylindrical.
More preferably, the at least one electrode extends substantially entirely through the associated tubular section.
More preferably, the at least one electrode extends substantially along the central axis of the associated tubular section.
More preferably, at least the inner surface of the tubular section is composed of an insulating material.
In one embodiment the aerosol flow is provided by entraining the aerosol in a flow of a carrier gas fed to the nozzle unit.
In another embodiment the aerosol flow is provided by applying a reduced pressure to the at least one outlet so as to entrain the aerosol in a flow of a carrier gas drawn through the nozzle unit.
In one embodiment the carrier gas is a gas reactive to the material solution. In another embodiment the carrier gas is a gas non-reactive to the material solution.
Preferably, the flow of the carrier gas is provided, typically by controlling the flow rate, temperature and/or direction, such as to maintain the decreasing temperature gradient.
Preferably, the aerosol is delivered to the substrate such as to achieve a film growth rate of at least 0.2 xcexcm per minute.
More preferably, the aerosol is delivered to the substrate such as to achieve a film growth rate of at least 1 xcexcm per minute.
Still more preferably, the aerosol is delivered to the substrate such as to achieve a film growth rate of at least 2 xcexcm per minute.
Preferably, the flow rate through the at least one outlet is at least 5 ml per minute, more preferably at least 50 ml per minute.
Preferably, the nozzle unit is configured such that the aerosol flow from the at least one outlet is directed upwards, more preferably substantially vertically upwards.
Preferably, the nozzle unit includes a perforated member upstream of the at least one outlet. In a preferred embodiment the perforated member comprises a mesh.
Preferably, the applied voltage is less than about 35 kV, more preferably less than about 20 kV.
Preferably, the distance between the at least one outlet and the substrate is less than about 100 mm, more preferably less than about 50 mm.
In one embodiment the substrate is held stationary relative to the nozzle unit.
In another embodiment the method further comprises the step of moving the nozzle unit relative to the substrate.
Preferably, the substrate is rotated, tilted and/or translated relative to the nozzle unit
In one embodiment deposition is performed at atmospheric pressure.
In another embodiment deposition is performed below atmospheric pressure.
In a further embodiment deposition is performed above atmospheric pressure.
Preferably, the method further comprises the step of varying one or both of the composition and concentration of the material solution during deposition.
Preferably, the method further comprises the step of reversing the polarity between the substrate and the at least one electrode at intervals during deposition.
Preferably, the method further comprises the step of locally heating at least one area of the substrate.
Preferably, the method further comprises the step of one or both of electrically or magnetically steering the aerosol droplets in transit from the nozzle unit to the substrate.
Preferably, the film is one or both of a structural film or a functional film; typically, for use in engineering and medical applications.
Preferably, the film is one of a dense or porous film.
Preferably, the film is one of an amorphous or crystalline film.
Preferably, the film is one of a simple film, a doped film or a multi-component film; typically, non-oxide or oxide films.
Preferably, the film is a composite film.
Preferably, the film is a compositionally-graded film.
Preferably, the film is a multi-layered film.
In one embodiment the film is an inorganic film.
Preferably, the film is a ceramic film, more preferably an electroceramic film.
In another embodiment the film is an organic film.
Preferably, the film is a polymer film.
In a further embodiment the film is a hybrid film, such as an organic/inorganic film.
The present invention also provides an apparatus for depositing material, preferably a film, on a substrate, comprising: a substrate holder for holding a substrate; a heater for heating the substrate; an aerosol generator for generating an aerosol comprising droplets of a material solution; a charge applicator for applying a positive or negative charge to the aerosol droplets; a nozzle unit in communication with the aerosol generator for delivering the aerosol to the substrate, the nozzle unit including at least one outlet through which a directed flow of the aerosol is in use delivered and at least one electrode; and a high voltage supply for generating an electric field between the substrate and the at least one electrode such that the directed aerosol flow is in use attracted towards the substrate.
Preferably, the apparatus is configured to maintain a decreasing temperature gradient in a direction away from the substrate towards the nozzle unit.
Preferably, the at least one electrode extends upstream of the at least one outlet.
Preferably, the at least one electrode comprises an elongate element.
Preferably, the distal end of the at least one electrode is located at substantially the centre of the at least one outlet.
In one embodiment the distal end of the at least one electrode includes a single tip.
In another embodiment the distal end of the at least one electrode includes a plurality of tips.
Preferably, the nozzle unit includes a tubular section upstream of each outlet.
More preferably, the tubular section is an elongate section.
More preferably, the tubular section is a linear section.
More preferably, the tubular section is substantially cylindrical.
More preferably, the at least one electrode extends substantially entirely through the associated tubular section.
More preferably, the at least one electrode extends substantially along the central axis of the associated tubular section.
More preferably, at least the inner surface of the tubular section is composed of an insulating material.
Preferably, the apparatus further comprises a gas supply unit in communication with the aerosol generator for supplying a flow of a carrier gas for entraining the aerosol and delivering the same through the nozzle unit.
Preferably, the at least one outlet is directed upwards, more preferably substantially vertically upwards.
Preferably, the distance between the at least one outlet and the substrate is less than about 100 mm, more preferably less than about 50 mm.
In one embodiment the nozzle unit and the substrate holder are held in fixed relation.
In another embodiment the nozzle unit and the substrate holder are configured so as to be movable relative to one another.
More preferably, the substrate holder is rotatable, tiltable and/or translatable relative to the nozzle unit.
Preferably, the nozzle unit includes a perforated member upstream of the at least one outlet. In one embodiment the perforated member comprises a mesh.
The present invention further provides a method of fabricating a powder, preferably an ultrafine powder, comprising the steps of: providing a heated zone; generating an aerosol comprising droplets of a material solution; providing a nozzle unit for delivering the aerosol to the heated zone, the nozzle unit including at least one outlet through which a directed flow of the aerosol is delivered and at least one electrode; charging the aerosol droplets with a positive or negative charge; providing a flow of the aerosol through the nozzle unit so as to deliver a directed flow of the aerosol from the at least one outlet; and generating an electric field between the heated zone and the at least one electrode such that the directed aerosol flow is attracted towards the heated zone where the aerosol droplets react homogeneously in the gas phase to form a powder.
The present invention still further provides an apparatus for fabricating a powder, preferably an ultrafine powder, comprising: a heater for providing a heated zone; an aerosol generator for generating an aerosol comprising droplets of a material solution; a charge applicator for applying a positive or negative charge to the aerosol droplets; a nozzle unit in communication with the aerosol generator for delivering the aerosol to the heated zone, the nozzle unit including at least one outlet through which a directed flow of the aerosol is in use delivered and at least one electrode; and a high voltage supply for generating an electric field between the heated zone and the at least one electrode such that the directed aerosol flow is in use attracted towards the heated zone where the aerosol droplets react homogeneously in the gas phase to form a powder.
The present invention is able, unlike the ESAVD technique disclosed in WO-A-97/21848, to utilise both aqueous and non-aqueous precursor solutions and particularly colloidal sol solutions, and allows much higher deposition rates, typically at least twice the rate possible using the ESAVD technique. Further, unlike the deposition technique disclosed in EP-A-0103505, crystalline, in particular dense, films can be produced in a single run without requiring a post-deposition heat treatment. Still further, unlike the corona spray pyrolysis deposition technique mentioned hereinabove, an organic precursor solution is not essentially required and in delivering the aerosol upwardly, preferably substantially vertically upwardly, to a downwardly-facing substrate, a more stable thermal environment can be maintained at the surface of the substrate so as to allow for a more precise control of the film deposition and hence provide an improved film.
In preferred embodiments of the present invention a deposition efficiency of at least 90% has been obtained, which enhanced deposition efficiency reduces the product cost and minimises the loss of the possibly harmful precursor materials to the environment.