Patent Application
20130199339
A nanocomposite includes a matrix material and nanoparticles which have been added to the matrix material to improve a particular property of the material. For example, nanoparticles can be added to materials to keep them lightweight and make them ductile, while simultaneously increasing the strength of the materials. Nanocomposites having high strength-to-weight ratios are of interest to industries, such as the aerospace and automotive industries, provided they can be produced at lower cost with properties comparable to more conventional, heavier materials.
Metal matrix nanocomposites (MMNCs) are a type of nanocomposite in which nanoparticles, such as ceramic nanoparticles, are added to a metal matrix. MMNCs are desirable because they can be made from relatively inexpensive, abundant metals with strengths comparable to those of more expensive alloys. Although MMNCs have the potential for use in many industrial applications, their use has been limited by restrictions in batch size and process development that have hindered the ability to produce MMNCs in industrial-scale quantities.
MMNCs have been produced at the laboratory scale (i.e., in quantities of a few hundred grams or less) using a simple set-up where an ultrasonic probe is inserted into a small crucible containing a molten metal to which nanoparticles have been added. The ultrasonic probe uses cavitation to break-up nanoparticle agglomerates into nanoparticle agglomerates and individual nanoparticles, which are then dispersed within the molten metal. Unfortunately, the quantity of MMNC that can be processed in such a system scales with the probe diameter and it is impractical to scale-up the ultrasonic probe to a size that would allow for industrial-scale production. For this reason, methods for producing MMNCs in industrial-scale quantities based on ultrasonic cavitation have not been developed.
Apparatus for the production of metal matrix nanocomposites are provided. In one embodiment, an apparatus comprises a production chamber defining a cavity; a nanoparticle feeding system; a nanoparticle mixing system; a cavitation system and a pumping conduit. Components of the nanoparticle feeding system can comprise a nanoparticle source in communication with the production chamber cavity through a feeding system output port, and a nanoparticle flow rate controller configured to control the flow rate of nanoparticles from the nanoparticle source to the feeding system output port. Components of the nanoparticle mixing system can comprise a first impeller disposed within the production chamber cavity and configured to apply an axial shear force to nanoparticle agglomerates entering a molten metal held in the production chamber cavity through the feeding system output port, and to force the nanoparticle agglomerates downward into the molten metal; and a second impeller disposed within the production chamber and configured to apply a radial shear force to nanoparticle agglomerates forced downward into a molten metal held in the production chamber by the first impeller. Components of the cavitation system can comprise a cavitation cell disposed within the production chamber cavity and defining a cavitation cavity having an input aperture and an output aperture, wherein the cavitation cell is positioned within the production chamber cavity such that a sub-volume of molten metal held within the cavitation cavity could flow out through the output aperture and back into a larger volume of molten metal held in the production chamber cavity, and a cavitation source configured to create a cavitation zone within a molten metal held in the cavitation cavity.
The pumping conduit can be configured to conduct a flow of molten metal held in the production chamber cavity from the second impeller into the cavitation cavity through the cavitation cavity input aperture.
An example of a nanoparticle flow rate controller is an auger assembly comprising an auger housing that defines an opening in communication with the nanoparticle source and an auger blade received within the auger housing and configured to transport nanoparticles from the nanoparticle source to the feeding system output port when the auger blade is rotated. An example of a cavitation source is an ultrasonic probe.
In some embodiments of the apparatus, the cavitation cavity input aperture is centered directly below the cavitation source in the cavitation cavity and the cavitation cavity output aperture is disposed opposite the cavitation cavity input aperture. In such embodiments, the cavitation source can extend into the cavitation cavity through the cavitation cavity output aperture.
In some embodiments, the pumping conduit comprises a conduit housing that defines a pumping channel comprising an input aperture, sized and positioned to accept a flow of molten metal directed into it by the second impeller, and an output aperture in fluid communication with the input aperture of the cavitation cavity; and further defines an impeller cavity at least partially surrounding the periphery of the second impeller and in fluid communication with the pumping channel input aperture.
Also provided are methods for the production of metal matrix nanocomposites. In one embodiment, the method includes the steps of introducing nanoparticle agglomerates into a volume of molten metal contained within a cavity defined by a production chamber; mechanically mixing the nanoparticle agglomerates in the volume of molten metal, wherein the mixing reduces the size of the nanoparticle agglomerates; creating a cavitation zone within a sub-volume of the molten metal contained in a cavitation cell that is immersed in the larger volume of molten metal contained within the production chamber cavity; and dispersing the nanoparticles in the size-reduced nanoparticle agglomerates as individual nanoparticles in the molten metal by pumping the size-reduced nanoparticle agglomerates into the cavitation zone, wherein the dispersed individual nanoparticles pass out of the cavitation cell and back into the larger volume of molten metal.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.
Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements.
Apparatus and methods for industrial-scale production of MMNCs are provided. The apparatus and methods enable scaled-up MMNC production in an industrial-scale production chamber without the need for a concomitant scale-up of the cavitation device or cavitation zone used to disperse the nanoparticles within the metal matrix. The methods can be used for the batch production of an MMNC in a volume of molten metal housed within the cavity of a production chamber. Within the volume of molten metal, a flow is created which continuously carries agglomerates of nanoparticles, which have been introduced into the molten metal, through a cavitation zone formed in a cavitation cell housed within the production chamber.
While in the volume of molten metal, nanoparticles are simultaneously being exposed to different stages of processing. Thus, one basic embodiment of the method includes the steps of introducing nanoparticle agglomerates into a volume of molten metal contained within a cavity defined by an industrial-scale production chamber; mechanically mixing the nanoparticle agglomerates in the volume of molten metal, wherein the mixing reduces the size of the nanoparticle agglomerates; creating a cavitation zone within the volume of molten metal; and dispersing the nanoparticles in the size-reduced nanoparticle agglomerates as individual nanoparticles in the molten metal by forcing the size-reduced nanoparticle agglomerates to pass through the cavitation zone.
The above-referenced mechanical mixing and nanoparticle dispersion steps take place simultaneously in a single production chamber by a combination of integrated processing systems that allow the nanoparticle agglomerates and individual, dispersed nanoparticles to circulate, and then re-circulate, through the mechanical mixing and cavitation stages in a continuous fashion during the production of the metal matrix nanocomposite. This is achieved by forming the cavitation zone in a cavitation cell that is at least partially immersed in the volume of molten metal. This design creates a sub-volume of the molten metal housed in the cavitation cell, the sub-volume being in fluid communication with the larger volume of molten metal around the cavitation cell. When the apparatus is in operation, nanoparticle agglomerates and dispersed, individual nanoparticles in the molten metal are able to re-circulate through the sub-volume of the cavitation zone in the cavitation cell and then back out into the larger, surrounding, volume molten metal until a desired level of nanoparticle dispersion is achieved. The sub-volume of molten-metal in the cavitation cell is typically much smaller than the larger volume of molten metal in which it is formed. For example, in some embodiments of the present methods, the volume ratio of the sub-volume of molten metal in cavitation cell to the total volume of molten metal in the production chamber cavity is no greater than about 1:2. This includes embodiments in which the ratio is no greater than about 1:3, embodiments in which the ratio is no greater than about 1:4 and embodiments in which the ratio is no greater than about 1:5.
Metal matrix nanocomposites produced by the present methods are composite materials composed of a bulk metal matrix and nanoscale particles (nanoparticles) that are dispersed within the matrix. Examples of metals that can be used in the bulk metal matrix include, but are not limited to, aluminum, magnesium, nickel, copper and their alloys. Materials from which the nanoparticles can be made include, but are not limited to, ceramics, oxides, nitrides, carbides and other carbon-based particles. Specific examples of the types of nanoparticles that may be dispersed in the metal matrices include aluminum oxide nanoparticles, aluminum nitride nanoparticles, carbon nanotubes, silicon carbide nanoparticles, silicon nitride nanoparticles, titanium carbide nanoparticles and tungsten carbide nanoparticles.
For the purposes of this disclosure, the term “nanoparticle” is used to refer to a particle having at least one dimension that is no greater than about 100 nm. This includes particles having at least one dimension that is no greater than about 50 nm and further includes particles having at least one dimension that is no greater than about 10 nm. Some nanoparticles may have only a single dimension that is no greater than about 100 nm. These include thin flakes. Other nanoparticles may have two dimensions (e.g., height and width) that are no greater than about 100 nm. These include nanotubes and nanowires. Still other nanoparticles may have no dimension that exceeds 100 nm. In some embodiments, it is desirable that the longest dimension of the nanoparticle is no greater than about 100 μm. This includes embodiments in which the longest dimension of the nanoparticle is no greater than about 10 μm and further includes embodiments in which the longest dimension of the nanoparticle is no greater than about 1 μm. As evidenced by the description above, the term “nanoparticle” is not intended to refer to particles of a particular shape. Thus, the nanoparticles can take on a variety of forms including, but not limited to, spherical or substantially spherical, elongated, cylindrical, or planar. In some cases the shapes will be irregular.
The concentration of nanoparticles in the MMNCs will depend, at least in part, on the desired properties (e.g., strength, wear-resistance, temperature stability, ductility and thermal and electrical conductivity) of the MMNC. By way of illustration only, the present apparatus and methods can be used to fabricate MMNCs having a nanoparticle concentration in the range from about 0.1 to 10 volume percent (vol. %). This includes embodiments in which the MMNCs have a nanoparticle concentration in the range from about 0.1 to 5 vol. % and further includes embodiments in which the MMNCs have a nanoparticle concentration in the range from about 1 to about 3 vol. %.
The present apparatus and methods can be designed to produce MMNCs on an industrial scale. For example, in some embodiments, the apparatus and methods can produce batches of MMNCs with batch sizes of at least 10 kg. This includes embodiments in which the MMNC are produced in batches of 100 kg, 500 kg, 1000 kg or greater. As described in greater detail, below, the present methods can be carried out in a volume of molten metal contained within the cavity of a single production chamber. Thus, if industrial-scale production is desired, the volume of molten metal can be large enough to produce the batch-sizes mentioned above. For example, in some embodiments the production chamber will be large enough to hold volumes of 3 liters or greater, 5 liters or greater, or even 10 liters or greater.
The industrial scale production of the MMNCs using the present apparatus can be carried out on time scales that are commercially practical. By way of illustration only, some embodiments of the present apparatus and methods can produce a quantity of at least 1 kg of MMNC, having the nanoparticle loadings recited herein, in a period of one hour or less. This includes embodiments in which at least 2 kg of the MMNC is produced in a period of one hour or less and further includes embodiments in which at least 5 kg of the MMNC is produced in a period of one hour or less.
An apparatus suitable for carrying out the present methods has three main, integrated systems—a nanoparticle feeding system, a mechanical mixing system and a cavitation system.
The nanoparticle feeding system is configured to introduce nanoparticles into a volume of molten metal contained within the cavity of a production chamber at a controlled, well-defined rate. The components comprising the nanoparticle feeding system include a nanoparticle source and a nanoparticle flow rate controller. The nanoparticle source is generally a container suitable for containing a quantity of nanoparticles before they are introduced into the molten metal. The flow rate of nanoparticles from the nanoparticle source into the molten metal, through a feeding system output port, is controlled by the nanoparticle flow rate controller. In a typical embodiment, the nanoparticle source opens into the nanoparticle flow rate controller, which is in communication with the feeding system output port. By “in communication with” it is meant that nanoparticle agglomerates from the nanoparticle flow rate controller are able to pass out of the flow rate controller and into the molten metal through the feeding system output port through one enclosed or partially enclosed pathway. The feeding system output port generally will be submerged in a volume of molten metal in the processing chamber when the apparatus is in operation. An auger is an example of a nanoparticle flow rate controller that can be used in the apparatus. However, other nanoparticle flow rate controllers, including known powder flow controllers can be employed.
The nanoparticles are introduced into the molten metal at a feed rate that allows the nanoparticles to agglomerate into relatively large agglomerates or ‘clusters’ as they are fed into the melt. It is desirable to introduce clusters having a size (diameter) of less than 1 mm, as larger clusters will float to the surface of the melt where they can react with the vapor above the melt. Thus, in some embodiments, the apparatus and methods are designed to introduce clusters with an average size in the range from about 300 to about 700 μm. Nanoparticle feed rates that are suitable for achieving a satisfactory introduction of nanoparticles into the melt include those in the range from about 1 to about 20 grams per minute (g/min) However, other feed rates can be used, including feed rates of 8 g/min or greater.
The mechanical mixing system is configured to force the nanoparticle clusters downward into the molten metal and to shear the nanoparticle clusters into nanoparticle agglomerates having a reduced size. The reduction in nanoparticle agglomerate size is advantageous because it prepares the nanoparticle agglomerates for introduction into the cavitation system and renders their dispersion more efficient. In some embodiments, the size-reduced nanoparticle agglomerates introduced into the cavitation system have an average particle size of 100 μm or less. For example, the average size of the nanoparticle agglomerates after mechanical mixing can be in the range from 10 μm to 100 μm.
The shear forces to which the nanoparticle clusters are exposed during the mechanical mixing step can be created by an impeller submerged in the volume of molten metal and disposed below the feeding system output port. In some embodiments the nanoparticle clusters are exposed to both an axial shear and a radial sheer during the mechanical mixing process. This can be accomplished by employing two or more impellers acting in concert to reduce the average nanoparticle agglomerate size and to create a flow channel in the molten metal that directs the nanoparticles exiting the feeding system downward and toward the cavitation system. The impeller or impellers can be designed to create turbulent flow in the molten metal, which aids agglomerate shear. As used herein, the term ‘impeller’ broadly refers to a rotating device, such as a rotor or blade, that is capable of forcing the molten metal in a desired direction.
The cavitation system is designed to disperse size-reduced nanoparticle agglomerates into individual nanoparticles in the molten metal. During cavitation, the nanoparticles are dispersed by a cavitation effect resulting from the bursting of bubbles created inside the agglomerates within the molten metal, which enhances nanoparticle wettability. The cavitation process is carried out in a cavitation zone formed in a sub-volume of the larger volume of molten metal held in the production chamber. The volume of the cavitation zone corresponds to the volume of molten metal in which the nanoparticle agglomerates are subjected to the cavitation action of the cavitation source. In the present methods, the cavitation zone is sized and positioned within the flow of molten metal such that the nanoparticle agglomerates carried by the flow of molten metal are forced to pass through the cavitation zone before returning to the larger volume of molten metal.
The components comprising the cavitation system include a cavitation cell that defines a cavitation cavity and a cavitation source configured to create a cavitation zone within the sub-volume of molten metal held within the cavitation cavity. The cavitation cell can be immersed in the volume of molten metal held within the production chamber and is open to the production chamber cavity via openings that allow fluid flow between the sub-volume of molten metal within the cavitation cavity and the larger volume of molten metal around the cavitation cell. One such opening is the cavitation cavity input port which is positioned to receive a flow of molten metal containing the size-reduced nanoparticle agglomerates from the mixing system. In one embodiment, the cavitation cavity input port is centered directly below the cavitation zone when the apparatus is in operation. In addition, the cavitation cell will have at least one cavitation cavity output port through which the molten metal having individual nanoparticles dispersed therein can exit the cavitation cavity after passing through the cavitation zone.
Cavitation sources suitable for use in the present methods and apparatus include, but are not limited to, ultrasonic probes, electromagnetic probe and cyclic high pressure cavitation sources.
The cavitation cell is desirably sized such that the sub-volume of molten metal held within the cavitation cavity conforms to the volume of the cavitation zone generated by the cavitation source. In addition, the cavitation cavity input and output ports are positioned such that the flow of molten metal containing the size-reduced nanoparticle agglomerates will pass through the cavitation zone before it can exit the cavitation cell. The sub-volume of molten metal held within the cavitation cavity can be said to ‘conform to’ the volume of the cavitation zone when the cavitation zone extends across the cavitation cavity between the input and output ports, thereby preventing any significant portion of the flow of molten metal entering the cavitation cavity from passing around (rather than through) the cavitation zone and out of the cavitation cavity. An illustration of a cavitation cell containing a sub-volume of molten metal that conforms to the volume of the cavitation zone created by an ultrasonic probe is shown in
b) illustrates some example dimensions of the cavitation zone in the cavitation cavity. As shown in this figure, the cavitation zone 116 extends laterally and vertically across the cavitation cell such that nanoparticle agglomerates entering the cavitation cavity through the input port must traverse the cavitation zone before they can exit the cavitation cavity through the output port. In this figure, ‘d’ represents the diameter of the probe. Representative height and width dimensions (d and 2d) for the cavitation cell and for the probe immersion depth dimension (d/2) are shown in
A flow of molten metal containing size-reduced nanoparticle agglomerates can be delivered to the cavitation cavity by a pumping conduit which conducts the molten metal to the cavitation cell and forces (pumps) it into the cavitation cavity. As such, the pumping conduit will define a pumping channel that is sized and positioned to conduct a flow of molten metal containing dispersed, sized-reduced nanoparticle agglomerates from the mechanical mixing system toward the cavitation system. The pumping channel comprises an input aperture into which the flow of molten metal is directed by the mechanical mixing system and an output aperture from which the flow of molten metal exits into the cavitation cell. The flow of molten metal can be directed into the pumping channel by, for example, positioning the input aperture near an impeller of the mechanical mixing system, such that the rotation of the impeller directs the molten metal to flow into the input aperture. For example, when a mixing system comprising two or more impellers is employed, the pumping conduit can be configured to force molten metal to flow from the final impeller into the pumping channel.
The shapes and dimensions of the pumping channel, input aperture and output aperture are desirably designed to enhance the pumping action provided by the pumping conduit. For example, the pumping channel can have a cross sectional area which progressively decreases along at least a portion of its length from the input aperture toward the output aperture. In some embodiments, the pumping channel is continuously tapered from its input aperture to its output aperture. The output aperture is typically smaller than the input aperture and is sized to provide a desired, fixed molten metal flow rate into the cavitation cell. For example, the pumping conduit can be designed to provide molten metal flow rates into the cavitation cell in the range from about 0.5 m/s to about 2 m/s. By way of illustration only, in some embodiments the input aperture is a circular aperture having a diameter in the range from about d/4 to about ¾d, where d is the diameter of the probe in the cavitation cavity.
The pumping conduit can be integrated with an impeller of the mechanical mixing system via a pumping conduit housing that defines an impeller cavity (e.g., an arcuate cavity) that surrounds the periphery of the impeller and opens into the pumping channel.
The materials selected for each component of the apparatus should be tailored to meet the particular demands placed on that component. For example, any components that are directly exposed to the molten melt should be selected such that they have a low dissolution rate in the melt and are resistant to the high melt temperatures. Such components include, for example, the inner surfaces of the production chamber which define the production cavity, impellers and impeller shafts, portions of the feeding system that contact the melt (e.g., a helical auger blade), and the pumping conduit housing and shaft. Materials that are suitable for these components include titanium, titanium alloys and titanium-based ceramics (e.g., TiC). The components can be constructed from these materials or coated with them. For example, components such as impeller shafts and blades can be constructed from a low carbon steel (e.g., H13 or H21) coated with TiC. In addition, it is advantageous if the components of the feeding system are resistant to erosion by the nanoparticles with which they come into contact. One example of a titanium alloy that is resistant to nanoparticle erosion and has a low dissolution rate in aluminum and magnesium alloys is Ti-6Al-4V. The materials that are in contact with the cavitation zone during the operation of the apparatus (e.g., the cavitation cell and portions of the cavitaion source) should also be composed of materials that are resistant to cavitation-induced corrosion. Such materials include niobium, titanium and their alloys. One example of a suitable niobium alloy is C-103 (9.6 wt. % Hf, 0.85 wt. % Ti, balance Nb).
In order to illustrate some features of the present apparatus in more detail, exemplary embodiments are described below, in conjunction with
With reference to
In the embodiment of
As shown in
Connecting joint 508 can be configured such that certain components of the feeding system (e.g., the motor, motor controller, and nanoparticle source) are not positioned directly above the molten metal contained in the production chamber when the apparatus is in operation. This is advantageous because it reduces the exposure of these components to the heat emanating from the molten metal. For example, in the embodiment depicted in
Canister interior surface 602 may be a smooth surface that is generally cylindrical in shape, however other interior surface geometries are possible. The internal cavity formed by canister interior surface 602 may be narrower at the bottom than at the top. For example, the circumference of the opening 603 formed at the bottom of canister interior surface 602 may be smaller than the circumference of the opening 605 formed at the top of canister interior surface 602 to facilitate moving materials from canister 502 into connecting joint 508.
Impeller motor 702 is coupled to a first end of shaft 706 and radial shear impeller 710 is mounted on the second end of shaft 706. Axial shear impeller 708 is mounted on shaft 706 between shaft 706 first end and shaft 706 second end. The forward faces of the blades 709 of axial shear impeller 708 are angled downward at an angle θblade, relative to the longitudinal axis 707 of shaft 706 (i.e., they are forward-pitched), to induce turbulent flow within the molten metal matrix and to induce a flow of molten metal toward radial shear impeller 710. Axial shear impeller 708 can create turbulent flow within the molten metal held in canister 502, resulting in shearing stresses which act upon the nanoparticle agglomerates, breaking them up and reducing their size. A flow of the resulting mixture of molten metal and randomly-distributed, size-reduced nanoparticle agglomerates is directed toward radial shear impeller 710, traveling substantially in the direction of the longitudinal axis 707 of shaft 706. This flow can be accelerated by radial shear impeller 710, which also forces the flow toward the entrance of a cavitation cell. The blades of radial shear impeller 710 in this embodiment of the apparatus are not pitched. The flow of molten metal and size-reduced nanoparticle agglomerates directed by radial shear impeller 710 travels substantially in a direction of about 90° with respect to longitudinal axis 707. It is advantageous to position the axial shear impeller and the radial shear impeller sufficiently close together along the impeller shaft that the two impellers create an integrated and continuous flow pattern, rather that two spatially separated, independent flow zones.
As used herein, the term “mount” includes join, unite, connect, associate, insert, hang, hold, affix, attach, fasten, bind, paste, secure, bolt, screw, rivet, solder, weld, glue, form over, layer, and other like terms. The phrases “mounted on” and “mounted to” include any interior or exterior portion of the element referenced.
The invention was made with government support under 70NANB 10H003 awarded by National Institute of Standards and Technology. The government has certain rights in the invention.
