Pressure controlled droplet spraying (PCDS) method for forming particles of compound materials from melts

Abstract

A method and apparatus of forming compositionally homogeneous particles is provided. The method includes forming a homogenous melt from a plurality of constituent materials under a first pressure sufficient to prevent substantial vaporization of the constituent materials. Droplets are generated from the homogenous melt. The droplets are cooled under a second pressure sufficient to prevent substantial vaporization of the constituent materials at least until the homogeneous particles formed therefrom have stabilized.

Claims

1. A method of forming compositionally homogeneous layers, comprising: forming a homogeneous melt from a plurality of constituent materials that have different vaporization rates, the homogeneous melt being formed under a first pressure sufficient to prevent compositional changes in the homogeneous melt due to selective vaporization of the constituent materials, wherein the constituent materials are compound semiconductor materials; generating droplets from the homogeneous melt; and applying the droplets to a substrate under a second pressure sufficient to prevent substantial compositional changes in the droplets due to selective vaporization of the constituent materials so that a homogeneous layer is formed upon cooling.

2. The method of claim 1 further comprising applying the droplets to the substrate under the second pressure so that a plurality of homogeneous layers is formed.

3. The method of claim 2 wherein the plurality of homogeneous layers forms a preform structure.

4. The method of claim 1 wherein the homogeneous melt is formed above its liquidus temperature.

5. The method of claim 1 wherein the first pressure arises from an inert gas.

6. The method of claim 1 wherein the second pressure arises from an inert gas.

7. The method of claim 1 wherein the second pressure arises from a reactive gas.

8. The method of claim 1 wherein generating the droplets further comprises mixing the homogeneous melt with an inert gas stream while maintaining the homogeneous melt at a specified temperature at least until the droplets are formed.

9. The method of claim 1 wherein generating the droplets is performed using a spray nozzle.

10. The method of claim 1 wherein the homogeneous melt is formed from a solid body that is melted in a droplet generation vehicle that generates the droplets.

11. The method of claim 1 further comprising transferring the homogeneous melt from a melting vessel to a droplet generation vehicle using a gas pressure differential.

12. A method of forming compositionally homogeneous layers, comprising: mixing in a melting vessel a plurality of constituent materials that have different vaporization rates, wherein the constituent materials are compound semiconductor materials; forming a homogeneous melt from the plurality of constituent materials, the homogeneous melt being formed under a first pressure sufficient to prevent compositional changes in the homogeneous melt due to selective vaporization of the constituent materials; generating droplets from the homogeneous melt; and applying the droplets to a substrate under a second pressure sufficient to prevent substantial compositional changes in the droplets due to selective vaporization of the constituent materials so that a homogeneous layer is formed upon cooling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a simplified schematic diagram illustrating the components of an apparatus for performing a Pressure Controlled Droplet Spraying (PCDS) method.

(2) FIG. 2 is a flowchart illustrating one example of a PCDS method.

(3) FIG. 3 depicts an example of an apparatus for implementing a PCDS method.

DETAILED DESCRIPTION

(4) In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments or other examples described herein. However, it will be understood that these embodiments and examples may be practiced without the specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail, so as not to obscure the following description. Further, the embodiments disclosed are for exemplary purposes only and other embodiments may be employed in lieu of, or in combination with, the embodiments disclosed.

(5) As detailed below, a so-called Pressure Controlled Droplet Spraying (PCDS) technique is provided for the fabrication of particles, including but not limited to nanoparticles. The particles are formed from compound materials and have a controlled composition. Among other things, the method can overcome the limitations in compositional control that is encountered in commonly practiced methods for particle formation such as flame pyrolysis, plasma spraying and gas aggregation. Furthermore, the PDS technique can enable the formation of new compositions of particles, including nanoparticles, and new applications which derive therefrom. Although the technique is also applicable to larger particles, the focus of this discussion will be on nanoparticles.

(6) FIG. 1 is a simplified schematic diagram illustrating a melting vessel 10, droplet generation vehicle 20, and a particle generation vehicle 30 in which the techniques of the present invention may be performed. While these components are illustrated as physically distinct elements for purposes of illustration, those of ordinary skill in the art will recognize that the functions performed by each component may be combined or divided into any number physical elements.

(7) A batch of the desired compound material or constituent sub-compounds, elements or other suitable precursors is mixed and melted in the melting vessel 10 to form a homogeneous melt at a temperature above its liquidus temperature. The melt is maintained under an atmosphere of an inert gas or other vapor, which is at a sufficient pressure to suppress significant loss by vaporization of the sub-compounds or other by-products of the compound, after initial melt-vapor equilibrium is achieved. Furthermore, it is desirable that the melting temperature be sufficient for the melt to achieve a sufficiently low viscosity so that it can be transported through a siphoning tube or other appropriate delivery vehicle. It is also desirable that the material from which the melting vessel 10 is formed have a low reactivity with the melt during processing in order to minimize the possibility of contamination. The melting vessel 10 and/or the materials therein may be heated by any number of conventional techniques including resistive heating or induction heating. Furthermore, the melting vessel 10 may itself act as the pressurized container or may be contained within an outer vessel which is pressurized.

(8) The droplet generation vehicle 20 generates droplets, as small as nano-size, from the melt in the melting vessel 10. In some embodiments the droplet generation vehicle 20 may comprise a spray nozzle in which droplets are generated by mixing a stream of the melt with a high-pressure stream of inert gas or other appropriate carrier. Such nozzles can be similar in form to those used in other spraying applications, provided that they are formed from materials that are able to withstand elevated temperatures. The temperature of the droplet delivery vehicle 20 during processing should be high enough to permit the delivery of the melt until the point of droplet formation without detrimental cooling. The materials used for constructing the delivery vehicle should withstand the processing conditions including the temperatures employed and preferably have a low-reactivity with the melt during processing in order to reduce the possibility of contamination.

(9) The droplets formed by the droplet generation vehicle 20 are cooled and solidified in the particle generation vehicle without significant loss of constituents. This approach is quite different from particle generation from a solution or a dispersed medium in which the loss of a solvent or dispersing fluid from the droplets (i.e., a drying process) is an essential part of the particle formation process. The atmospheric pressure surrounding the droplets in the droplet generation vehicle 20 should be sufficient to suppress loss of material by volatilization of the sub-compounds or other constituents, at least up to the point where the droplet composition is stabilized by cooling. Typically, consistent with a flow condition, this pressure will be lower than the inlet pressure from a droplet delivery apparatus or other input device that transfers the droplets form the droplet generation vehicle 20 to the particle generation vehicle 30. The temperature, or temperature gradient, within the particle generation vehicle 30 should be sufficient to insure that the material remains molten through the critical phase of droplet formation and entry into the particle generation vehicle 30.

(10) FIG. 2 is a flowchart illustrating one example of the PCDS method described above. The method includes the compositional design and selection of the constituent materials (step 310) and the mixing and melting of the constituent materials in a suitable vessel (step 320). The resulting melt is delivered to a droplet generation vehicle (step 330). Particles are then generated under suitable environmental conditions which provide compositional control (step 340). The particle are then collected and, optionally, classified according to size (step 350).

(11) FIG. 3 depicts an example of an apparatus for implementing a PCDS method, which is schematically illustrated in FIG. 1. A homogeneous melt 110 is contained in a vessel 120 which is heated by an external heater 130. A delivery apparatus, which in this case is comprised of a siphoning tube 140 and includes an aspirating gas stream (150) fed through another supply tube 160, is used to deliver the melt through a nozzle 170. The melt and gas stream are combined and forced through the nozzle 170, which may be heated by an external heater 180. A droplet stream is produced and directed into a pressurized particle generation chamber 190. A particle stream 200 that is produced when the droplets cool may be entrained in a gas stream 220 supplied through a surrounding manifold 210, which is heated by an external heater 230.

(12) As previously mentioned, the nozzle 170 may be similar in form to those used in other spraying applications, provided that they are formed from materials that are able to withstand elevated temperatures. In addition, the size and configuration of the nozzle may be adjusted to produce appropriately sized droplets, facilitating the flow of the melt through the vehicle and the mixing of the melt in an appropriate ratio with the dispersing gas. Furthermore the rates of dispersing gas flow may be adjusted in order to facilitate the formation and transport of droplets of a given size from the nozzle. Such modifications are within the purview of those of ordinary skill in the art.

(13) Optionally, the above identified components of the apparatus shown in FIG. 3 may be integrated into one monolithic unit or used as discrete interconnected but separately operated parts.

(14) Optionally, a separate carrier stream of gas may be delivered through, or around, the nozzle 170 to entrain the droplets and/or the resulting particles. This stream should be of appropriate temperature and pressure so that the generation and processing of the droplet stream is not detrimentally impacted.

(15) Optionally, the composition of the atmosphere in the particle generation vehicle 30 may be inert or reactive. The latter option can enable reactions at the surface of the droplets, which may introduce or enhance desired features in the resulting particles. For example, a suitable reactive gas or gases can facilitate the formation of an optically or electrically enhanced surface layer, enhanced chemical reactivity, anti-agglomeration behavior or enhanced environmental stability.

(16) The PCDS method as described above differs significantly from the droplet spraying techniques employed in the aforementioned references to Kasagi, Kerkhof, Chou et al., Wendt et al. and Roeker. For instance, none of these references attempt to actively control, through control of atmospheric pressure, the composition of the droplets and the resulting particles, which is one important element of the PCDS method. In addition, these references do not attempt to control the composition of the particles when they are formed from melts that are prone to compositional changes due to the selective vaporization of the constituent materials. An example of such materials is a II-VI compound semiconductor. In Kasagi, only single composition metals are employed, which of course will not exhibit compositional changes.

(17) The solution droplet spraying methods described in both Kerkhof and Chou et al. differ in several additional ways from the PCDS method described above. In these references, the feedstocks are solutions of materials dissolved in a solvent. The droplets that are formed undergo a drying process, whereby the solvent is evaporated and the dissolved materials form salt residues or similar aggregates. Moreover, these salts need not be compositionally homogeneous or fully reacted compounds. By contrast, the feedstocks employed in the PCDS method are homogeneous melts which generally have been reacted above their liquidus temperature. These feedstock are therefore well-suited to form a homogenous, chemically reacted compound material upon cooling. Depending on the cooling regime, the resulting particles may also be amorphous, polycrystalline or crystalline in nature.

(18) In Roeker, the droplet spraying of fuels results in a dispersion of combustable droplets which are pyrolysed in the vapor phase and do not lead to the formation of nanoparticles. Any pressurization arising during fuel injection is an artifact of the desire to form combustable gas mixtures, rather than a desire to suppress a loss of materials from the droplets.

(19) The droplet spraying methods cited above also do not address the application of such methods to the high temperatures appropriate for many compound materials, nor for droplet formation down to nanoparticle size. Furthermore, when solution spraying is employed to form particles, the initial droplets, which also contain a solvent that is later removed by drying, can be substantially larger than the final particles. There is no analogous size reduction mechanism for melt droplets when the PCDS method is employed.

(20) In some embodiments of the invention, instead of forming particles such as nanoparticles, the PCDS technique may be used to form compositionally homogeneous layers on a substrate. In this case, after the droplets are generated they are applied to a substrate while under a sufficient pressure to prevent substantial vaporization of the constituent materials so that a homogenous layer is formed on the substrate upon cooling. In some implementations this technique may be used to build up a structure (e.g., a fiber preform) in a layer-by-layer manner.

EXAMPLES

(21) For the purpose of illustration, the PCDS method as described hererin may be applied to compound semiconductor materials, which are useful for the fabrication of electro-optic devices. This should not be considered as a limitation in the scope of the invention but is simply used to illustrate a group of materials which are prone to the selective loss of constituents during processing and which would benefit from the compositional control provided by the PDS method. One sub-group of materials within this domain are CuInSe (CIS) compound semiconductors and related alloys, which are of interest for use as absorbers in Photovoltaic (PV) solar cells. Non-vacuum methods of CIS film formation have been reported by many authors including Eberspacher et al. (Eberspacher, C., Frederic, C. Pauls, K. and Serra, J., Thin Solid Films 387, 18, 2001) and are an area of great commercial interest. Because of compositional control issues in nanoparticle processing, the nanoparticles used in these approaches are typically comprised only of constituent metals and require additional processing to form the compound semiconductor.

(22) Fully reacted compound semiconductor particles and materials of the type formed by the PCDS method described herein may be used for a variety of purposes. As previously mentioned, such particles and materials may be advantageously employed in the method for forming compound semiconductor thin films described in co-pending U.S. patent application Ser. No. 12/185,369.