Methods and apparatus for producing nanometer scale particles utilizing an electrosterically stabilized slurry in a media mill

Abstract

Disclosed herein are methods and apparatus for producing nanometer scale particles utilizing an electrosterically stabilized slurry in a media mill. A method for producing nanometer scale particles includes adding to a media mill a feed substrate suspension. The feed substrate suspension includes a liquid carrier medium and feed substrate particles. The method further includes adding to the feed substrate suspension in the media mill an electrosteric dispersant. The electrosteric dispersant includes a polyelectrolyte. Still further, the method includes operating the media mill for a period of time to comminute the feed substrate particles, thereby forming nanometer scale particles having a (D.sub.90) particle size of less than about one micron, and recirculating for further grinding the nanometer scale particles from the media mill.

Claims

1. A method for producing nanometer scale particles comprising: providing a media mill; adding to the media mill a feed substrate suspension, wherein the feed substrate suspension comprises a liquid carrier medium and feed substrate particles, wherein the feed substrate particles comprise organic solids, glass, graphene, metals, ores, silica, diatomaceous earth, clays, organic pigments, pharmaceutical materials, or carbon black; adding to the feed substrate suspension in the media mill an electrosteric dispersant to separate the feed substrate particles in the feed substrate suspension in order to improve spacing of the feed substrate particles, wherein the electrosteric dispersant comprises a polyelectrolyte; operating the media mill for a period of time to comminute the feed substrate particles, thereby forming nanometer scale particles having a (D.sub.90) particle size of less than one micron; and recirculating for further grinding the nanometer scale particles from the media mill.

2. The method of claim 1, wherein the liquid carrier medium comprises water or an organic solvent.

3. The method of claim 1, wherein the feed substrate particles comprise glass, silica, or diatomaceous earth.

4. The method of claim 1, wherein the feed substrate particles are present in the feed substrate suspension in an amount of 20% to 70% by weight of the feed substrate suspension.

5. The method of claim 4, wherein the feed substrate particles are present in the feed substrate suspension in an amount of 20% to 40% by weight of the feed substrate suspension.

6. The method of claim 1, wherein the polyelectrolyte comprises a polymer or copolymer having electrically-charged functional groups or inorganic affinic groups.

7. The method of claim 1, wherein the period of time is from 10 minutes to 6,000 minutes.

8. The method of claim 1, wherein the nanometer scale particles have a (D.sub.90) particle size of less than 500 nm.

9. The method of claim 1, wherein the media mill comprises a milling media, and wherein recirculating for further grinding the nanometer scale particles from the media mill further comprises separating the nanometer scale particles from the milling media.

10. The method of claim 1, further comprising drying the nanometers scale particles after recirculating for further grinding the nanometer scale particles from the media mill.

11. The method of claim 1, further comprising separating the electrosteric dispersant from the nanometer scale particles after recirculating for further grinding the nanometer scale particles from the media mill.

12. The method of claim 1, further comprising adding to the feed substrate suspension in the media mill a defoaming agent.

13. The method of claim 1, wherein the electrosteric dispersant is added in an amount of 2% to 20% by weight of the feed substrate particles.

14. The method of claim 1, further comprising adding additional electrosteric dispersant during the period of time that the media mill is operating.

15. A media mill apparatus configured for producing nanometer scale particles comprising: a milling chamber; an agitator extending into the milling chamber; a milling media disposed within the milling chamber; a feed substrate suspension comprising a liquid carrier medium and feed substrate particles, and disposed within the milling chamber and interspersed with the milling media wherein the feed substrate particles comprise organic solids, glass, graphene, metals, ores, silica, diatomaceous earth, clays, organic pigments, pharmaceutical materials, or carbon black; and an electrosteric dispersant to separate the feed substrate particles in the feed substrate suspension in order to improve spacing of the feed substrate particles comprising a polyelectrolyte mixed within the feed substrate suspension, wherein the agitator is configured to apply mechanical work to the milling media for a period of time, thereby causing the milling media to comminute the feed substrate particles to form nanometer scale particles having a (D.sub.90) particle size of less than one micron.

16. The media mill apparatus of claim 15, wherein the milling chamber further comprises a screen, wherein the screen is sized to permit passage of the nanometer scale particles but not the milling media.

17. The media mill apparatus of claim 15, wherein the milling media comprises one or more of sand, steel, silicon carbide, ceramics, zirconium silicate, zirconium and yttrium oxide, glass, alumina, titanium, crosslinked polystyrene, and methyl methacrylate.

18. The media mill apparatus of claim 15, wherein the milling media are provided in the shape of one or more of balls, beads, and cylinders.

19. The media mill apparatus of claim 15, wherein the polyelectrolyte comprises a polymer or copolymer having electrically-charged functional groups or inorganic affinic groups.

20. A method for producing nanometer scale particles in a media mill comprising a milling media, the method comprising: providing a media mill; adding to the media mill a feed substrate suspension, wherein the feed substrate suspension comprises a liquid carrier medium and feed substrate particles; adding to the feed substrate suspension in the media mill an electrosteric dispersant to separate the feed substrate particles in the feed substrate suspension in order to improve spacing of the feed substrate particles, wherein the electrosteric dispersant comprises a polyelectrolyte, wherein the polyelectrolyte comprises at least one of: alkylammonium salts of polymers and copolymers, polymers and copolymers having acidic groups, functionalized comb copolymers and block copolymers, modified acrylate block copolymers, modified polyurethanes, modified and/or salified polyamines, phosphoric polyesters, polyethoxylates, polymers and copolymers having fatty acid radicals, modified polyesters polyphosphates, or a mixture thereof; operating the media mill for a period of time to comminute the feed substrate particles, thereby forming nanometer scale particles having a (D.sub.90) particle size of less than one micron; and recirculating for further grinding the nanometer scale particles from the media mill.

21. The method of claim 20, wherein the polyelectrolyte comprises a styrene-based co-polymer.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The present disclosure will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:

(2) FIG. 1 is a conceptual illustration showing product suspension particle separation utilizing electrostatic methods, as practiced in the prior art;

(3) FIG. 2 is a conceptual illustration showing product suspension particle separation utilizing steric methods, as practiced in the prior art;

(4) FIGS. 3A and 3B are schematic drawings of a wet media mill useful in milling particles in a continuous process in accordance with some embodiments of the present disclosure;

(5) FIG. 4 is a conceptual illustration showing product suspension particle separation utilizing electrosteric methods in accordance with some embodiments of the present disclosure;

(6) FIG. 5 is a flowchart illustrating a method for wet media milling in accordance with some embodiments of the present disclosure; and

(7) FIGS. 6A-6E are graphs illustrating average particle size diameters for particles produced in accordance with some examples of the present disclosure.

DETAILED DESCRIPTION

(8) The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Thus, any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Furthermore, as used herein, numerical ordinals such as “first,” “second,” “third,” etc., such as first, second, and third components, simply denote different singles of a plurality unless specifically defined by language in the appended claims. All of the embodiments and implementations described herein are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description.

(9) Disclosed herein are embodiments of methods and apparatus for producing nanometer scale particles utilizing an electrosterically stabilized slurry in a media mill. The disclosed embodiments makes use of electrosteric (electrostatic and steric) stabilization of ultra-fine (sub-micron) particles in a wet milling process using electrosteric dispersants. Electrosteric dispersants are polymers that are capable of stabilizing product particle suspensions electrostatically as well as sterically. With electrosteric dispersants, there is reduced use of the dispersant, the amount of dispersant used need to be controlled to an exacting standard, and agglomeration of the particles is efficiently avoided. This enables an increased milling efficiency and a reduced energy consumption for the wet milling process because the viscosity of the suspension remains low, and further there is a reduced probability of mill screen blockage because of the reduced probability of agglomeration.

(10) The nanometer scale particles in accordance with the present disclosure may represent a variety of substances useful in a variety of industries. For example, particles that may be milled as described herein may include inorganic and organic solids, minerals, ores, silica, diatomaceous earth, clays, organic and inorganic pigments, pharmaceutical materials, carbon black, paint additives, pigments, photographic materials, cosmetics, chemicals, metal powders useful as catalysts and supports, stationary phase particles useful in analytical and preparative chromatographic separations of chemical compounds, powdered toners, therapeutic and diagnostic imaging agents, medicinally active agents, medicaments, plant and herbal extracts, drugs, pro-drugs, drug formulations, and the like.

(11) In accordance with the methods of the present disclosure, nanoscale particles have been demonstrated having (D.sub.90) mean particle sizes below one micron, for example below 800 nanometers (nm), or below 500 nm. As set forth in the examples below, using input particles having a D.sub.90 mean particle size of about 5 microns, product particles have been prepared having D.sub.10 mean particle sizes of about 100 nm to about 200 nm, D.sub.50 mean particle sizes of about 150 to about 250 nm, and D.sub.90 mean particle sizes of about 250 nm to about 350 nm. It is expected that particles within the aforementioned size range, or anywhere between the aforementioned size range and an input size of (D.sub.90) about 100 microns or less (such as about 50 microns or less, or about 30 microns or less, or about 10 microns or less), will find application in almost any industrial or commercial application currently practiced. Greater detail regarding the wet media milling process, along with the electrosteric dispersants used in the milling process, is provided below. In particular, two embodiments of a mill are disclosed below in connection with FIG. 3A (vertical wet media mill) and FIG. 3B (horizontal media mill).

(12) Wet Media Milling

(13) In a wet milling process, repeated collisions of milling media with a solid particle material being milled, i.e., the milled substrate, result in repeated fracture of the substrate and concomitant substrate particle size reduction. When a wet media milling process is used to reduce the size of particles of the substrate, the process is usually carried out in a mill including a milling chamber containing milling media, the solid material or substrate that is to be milled, and a liquid carrier in which the media and substrate are suspended. The contents of the milling chamber are stirred or agitated with an agitator that transfers mechanical work and energy to the milling media. The accelerated milling media collide with the substrate in energetic collisions that may crush, chip, fracture, or otherwise reduce the size of the solid substrate material and lead to an overall reduction in substrate particle size, and an overall reduction in substrate average or mean particle size distribution. Examples of suitable wet milling systems include ball mills, planetary ball mills, circulating stirred media mills, basket stirred media mills, ultrasonic media mills, and the like.

(14) Milling media are generally selected from a variety of dense and hard materials, such as sand, steel, silicon carbide, ceramics, zirconium silicate, zirconium and yttrium oxide (e.g., yttria stabilized zirconia), glass, alumina, titanium, and certain polymers such as crosslinked polystyrene and methyl methacrylate. Media geometries may vary depending on the application, although spherical ball-shapes or cylindrical beads are commonly used. In some embodiments, milling media may be of various sizes and size distributions that include large milling media particles and smaller milling media particles. Suitable liquid carriers for the milling media and substrate include water, aqueous salt solutions, buffered aqueous solutions, organic solvents such as ethanol, methanol, butanol, hexane, hydrocarbons, kerosene, PEG-containing water, glycol, toluene, petroleum-based solvents, mixtures of aromatic solvents such as xylenes and toluene, heptane, and the like. Typically, the solvent will be selected based upon the substrate (product) particles.

(15) Wet media mills useful for reducing the particle size of a solid substrate may operate in a batchwise mode or in a continuous or semi-continuous mode. Wet media mills operating in a continuous mode may incorporate a separator or screen for retaining milling media together with relatively large particles of the solid substrate being milled in the milling zone or milling chamber of the mill while allowing smaller particles of the substrate being milled, i.e., product substrate particles, to pass out of the milling chamber in either a recirculation or discrete pass mode. Recirculation may be in the form of a slurry, suspension, dispersion, or colloid of the substrate suspended in a fluid carrier phase that moves from the milling chamber into a holding vessel and thence back to the milling chamber, for example with the aid of a pump. A separator or screen may be located at the outlet port of the milling chamber, including for example rotating gap separators, screens, sieves, centrifugally-assisted screens, and similar devices to physically restrict passage of milling media from the mill. Retention of milling media occurs because the dimensions of the milling media are larger than the dimensions of the openings through which the reduced size substrate particles may pass.

(16) FIG. 3A depicts an exemplary vertical wet media mill 15 configured for use in accordance with some embodiments of the present disclosure, wherein the reference numerals correspond with the following illustrated features: 10: motor 11: shaft 12: entry port 13: charging level 14: agitator 15: media mill 16: milling chamber 17: secondary screen 19: exit screen 20: exit port 31: inlet port 32: holding tank 33: piping system 34: pump 35: piping system

(17) The exemplary wet media mill 15 is now described in accordance with its usual operation. In an embodiment, a milling media (not shown) and a fluid carrier that contains an electrosteric dispersant may be added to milling chamber 16 of media mill 15 through entry port 12. (The electrosteric dispersant is described in greater detail, below.) During this charging of the media mill 15, agitator 14 may optionally be in operation, and exit port 20 may be open to allow fluid carrier to exit from the media mill 15, or it may be closed to contain the fluid carrier. Optionally, a secondary larger screen 17 including openings through which the milling media may pass may be provided in the media mill 15.

(18) The milling chamber 16 may then be charged with the solid substrate to be milled and optionally additional fluid carrier (optionally including additional electrosteric dispersant). Additionally, the milling chamber 16 may further be charged with a defoaming agent that prevents bubble formation during the milling process, as known in the art. In embodiments, once all of the fluid carrier and the substrate has been added, the slurry may have a solids content from about 5 wt.-% to about 40 wt.-%, such as from about 10 wt.-% to about 40 wt.-%, or about 15 wt.-% to about 40 wt.-%, or about 20 wt.-% to about 40 wt.-%. The exit port 20 of the milling chamber 16 may then be closed and the media mill 15 may be charged to a level 13. Fluid carrier may be transferred using a piping system 35 with the aid of a pump 34 to a holding tank 32 via inlet port 31. The fluid carrier may be pumped from the holding tank 32 via the piping system 33 back to the inlet port 12 of the media mill 15.

(19) The contents of the media mill 15 are agitated or stirred, preferably at a high speed or with high acceleration and deceleration, by agitator 14 that is driven by motor 10 and coupled with shaft 11. The time period of agitation to produce a product in accordance with the present disclosure may range, for example, from about 10 minutes to about 6,000 minutes or more, such as about 10 minutes to about 3,000 minutes, or about 10 minutes to about 1,000 minutes. Fluid carrier is continuously recirculated from the milling chamber 16 to the holding tank 32. This recirculation may be continued until a minimum or a desired substrate particle size is obtained, for example within the mean particle size ranges described above. During this process, additional electrosteric dispersant may be added, as required.

(20) At the end of the process, the residual product particles of milled solid substrate remaining in the media may be transferred to the holding tank 32 as a dispersion in the fluid carrier, optionally under pressure or by means of pump 24 from the milling chamber 16. Essentially all milling media remain in the milling chamber 16, and the product substrate particles are isolated substantially free of milling media as a dispersion in the fluid carrier. The product substrate particles produced in accordance with the present disclosure may have a (D.sub.90) particle size of less than about one micron, such as less than about 800 nm, or less than about 500 nm. The fluid carrier may be removed by drying or baking, as is known in the art. The electrosteric dispersant may remain with the milled product after drying in some embodiments, whereas in other embodiments the electrosteric dispersant may be removed, for example by baking in a kiln. Removal of the electrosteric dispersant will depend on final product requirements and intended application.

(21) FIG. 3B presents an alternative embodiment of a stirred media mill, namely a horizontal media mill. Many of the physical components of the embodiment of FIG. 3B are similar to that of FIG. 3A, as both embodiments accomplish the same function. In FIG. 3B, however, attention is drawn to the particular functions that occur in each area of the mill, with reference to illustrated functions (A) through (E). As illustrated, at function (A), energy that is input to the mill through the shaft is dissipated inside the suspension. At function (B), friction occurs in the suspension where the agitator is near the chamber wall. At function (C), displacement occurs within the suspension during the approach of two or more pieces of grinding media towards one another. At function (D), the grinding media contact one another without causing stress to the suspended particles. Further, at function (E), the grinding media may be deformed temporarily after the contact.

(22) Electrosteric Dispersants

(23) Greater detail is now provided regarding the electrosteric dispersants utilized in the wet media milling processes of the present disclosure. The electrosteric dispersants provide electrosteric stabilization to the product particles. Electrosteric stabilization is a combination of electrostatic and steric stabilization. With reference to FIG. 4, electrosteric stabilization involves adsorbing charged polymers (polyelectrolytes) on the surface of the colloidal product particles. The surface of a particle typically is composed of negative as well as positive sites. For instance, such charged sites may include functional groups including but not limited to OH.sup.−, H.sup.+, O.sub.2.sup.−, and O.sup.−, among others. The relative concentration of each charge depends on a number of factors including the nature of particle, the oxidation state of the particle, and the pH of the system.

(24) Polyelectrolytes have associated with them an overall electrical character (i.e., positive or negative). Polyelectrolytes adsorb strongly to the surface of particles by attaching themselves to oppositely charged sites on the surface of particles. Not all of the ionic sites on each polyelectrolyte, however, are used during the adsorption process. While some of the ionic sites are used to adsorb the polyelectrolyte to the surface of the particle, others of the ionic sites are in the part of the polymer that trails freely in the liquid medium. The combined like charges associated with the particle surface and polymer chains in solution give each particle an overall negative or positive charge for the particle-polymer composition. Each polymer-coated particle may repel the like charges associated with other polymer-coated particles because such particles experience an electronic repulsion. This electronic repulsion, in combination with the steric effect of the polymer, disperses the product suspension. Moreover, as both electrostatic and steric separation is achieved, particle separation is significantly stronger than either electrostatic or steric separation alone, resulting is less dispersant required, and less tight control requirements over the amount of dispersant used in the milling process.

(25) Polyelectrolytes suitable for use in accordance with the present disclosure as electrosteric dispersants include functional polymers that have a number-average molecular mass of at least about 500 g/mol, for example at least about 1,000 g/mol, such as at least about 2,000 g/mol. In some embodiments, the functional polymers may have a number-average molecular mass as high as about 5 million, or even 50 million g/mol. Typically, though, the number-average molecular mass will be less than about 500,000 g/mol, such as less than about 100,000 g/mol, or less than about 50,000 g/mol, or less than about 25,000 g/mol. In particular, the polyelectrolyte dispersant may be chosen from polymers and copolymers having electrically-charged functional groups or inorganic affinic groups, alkylammonium salts of polymers and copolymers, polymers and copolymers having acidic groups, functionalized comb copolymers and block copolymers, modified acrylate block copolymers, modified polyurethanes, modified and/or salified polyamines, phosphoric polyesters, polyethoxylates, polymers and copolymers having fatty acid radicals, modified polyacrylates such as trans-esterified polyacrylates, modified polyesters such as acid-functional polyesters, polyphosphates, and mixtures thereof. Suitable electrosteric dispersants are sold under the trade names: Disperbyk-199 and Disperbyk-2010 (BYK GmbH, Wesel, DE); and Flexisperse 225 and Flexisperse 300 (ICT, Cartersville, Ga., US), as non-limiting examples. In embodiments, the product suspension in the wet media mill may have an electrosteric dispersant content from about 2 wt.-% to about 20 wt.-%, such as from about 2 wt.-% to about 15 wt.-%, or about 5 wt.-% to about 15 wt.-%, based on the weight of the solid particles.

(26) Milling Method

(27) Referring to FIG. 5, illustrated is a flowchart for a method 500 for producing nanometer scale particles. The method 500 includes step 502 of pre-mixing, which is when the feed substrate suspension is pre-mixed with dispersant in a separate tank. The feed substrate suspension includes a liquid carrier medium and feed substrate particles. The liquid carrier medium may include water or an organic solvent. The feed substrate particles may include organic or inorganic solids, glass, graphene, metals, minerals, ores, silica, diatomaceous earth, clays, organic and inorganic pigments, pharmaceutical materials, or carbon black. The feed substrate particles may be present in the feed substrate suspension in an amount of about 5% to about 70% by weight of the feed substrate suspension, or about 5% to about 40% by weight. The electrosteric dispersant may be added in an amount of about 2% to about 20% by weight of the feed substrate particles. The electrosteric dispersant includes a polyelectrolyte. The polyelectrolyte may include a polymer or copolymer having electrically-charged functional groups or inorganic affinic groups.

(28) The method 500 further includes a step 504 of adding milling/grinding media to the mill, that is, the mill is filled with an appropriate amount of milling/grinding media. Milling media are generally selected from a variety of dense and hard materials, such as sand, steel, silicon carbide, ceramics, zirconium silicate, zirconium and yttrium oxide (e.g., yttria stabilized zirconia), glass, alumina, titanium, and certain polymers such as crosslinked polystyrene and methyl methacrylate. Media geometries may vary depending on the application, although spherical ball-shapes or cylindrical beads are commonly used. In some embodiments, milling media may be of various sizes and size distributions that include large milling media particles and smaller milling media particles.

(29) The method 500 further includes a step 506 of adding to a media mill the pre-mixed feed substrate suspension from step 502. The feed suspension may be added in a batch or continuous process. A defoaming agent may also optionally be added. Still further, the method 500 includes step 508 of operating the media mill for a period of time to comminute the feed substrate particles, thereby forming nanometer scale particles having a (D.sub.90) particle size of less than about one micron, or less than about 800 nm, or less than about 500 nm, or less than about 400 nm. The period of time may be from about 10 minutes to about 6,000 minutes, or from about 10 minutes to about 3,000 minutes, or from about 10 minutes to about 1,000 minutes. Additional electrosteric dispersant may be added during the period of time that the media mill is operating.

(30) Additionally, the method 500 includes step 510 of recirculating for further grinding the nanometer scale particles from the media mill. Part of this step may further include removing the nanometer scale particles from the media mill may include separating the nanometer scale particles from the milling media. Optionally, the method 500 may include a step 512 of drying the nanometers scale particles after removing the nanometer scale particles from the media mill. Optionally, the method 500 may include a step 514 of, using a kiln, separating the electrosteric dispersant from the nanometer scale particles and removing any organic matter after removing the nanometer scale particles from the media mill. It should be appreciated that various steps in method 500 may be repeated one or more times throughout the operation of the method.

ILLUSTRATIVE EXAMPLES

(31) The present disclosure is now illustrated by the following non-limiting examples. It should be noted that various changes and modifications may be applied to the following examples and processes without departing from the scope of this invention, which is defined in the appended claims. Therefore, it should be noted that the following examples should be interpreted as illustrative only and not limiting in any sense.

(32) Five different example particle suspensions were prepared including a water (as the liquid medium), crystalline silica/quartz particles or diatomaceous earth particles (as the solid substrate), a defoaming agent, and various types and amounts of polyelectrolyte (as the electrosteric dispersant). The composition of each example slurry is presented below in TABLE 1.

(33) TABLE-US-00001 TABLE 1 .sup.4Example 1 .sup.4Example 2 Example 3 Example 4 Example 5 Feed Crystalline Crystalline Crystalline Crystalline Diatomaceous Silica/Quartz Silica/Quartz Silica/Quartz Silica/Quartz Earth Feed Size (D.sub.90).sup.1 5 microns 5 microns 5 microns 5 microns 50 microns Solids Concentration 30 wt.-% 35 wt.-% 37.5 wt.-% 35 wt.-% 20 wt.-% Dispersant Flexisperse.sup.2 225 Flexisperse 225 Disperbyk.sup.3 199 Disperbyk 199 Disperbyk 199 Dispersant Concentration  5%  5%  5%  5% 10% (by weight of solids) Grinding Media Volume 80% 80% 67% 67% 67% (% of Mill Volume) Grinding Media Size 0.1-0.2 mm 0.1-0.2 mm 0.1-0.2 mm 0.1-0.2 mm 0.1-0.2 mm Mill Tip Speed 14.7 m/s 14.7 m/s 17.6 m/s 17.6 m/s 8.8 m/s .sup.1Feed size measured using a laser particle analyzer (Microtrac S3500; available from Microtrac Retsch GmbH (Haan, Germany)) .sup.2Flexisperse 225 available from Innovative Chemical Technologies (Cartersville, GA, USA) .sup.3Disperbyk 199 available from BYK-Chemie GmbH (Wesel, Germany) .sup.4No defoaming agent used

(34) Each of the example particle suspensions was placed into a circulating stirred media mill (VMA Dispermat SL12, available from VMA-GETZMANN GmbH (Reichshof, Germany)) that also included yttria stabilized zirconia (YSZ) beads as the grinding media. Each example was subjected to wet media milling in the stirred media mill for a time period ranging from about 150 minutes to about 1,000 minutes. After the milling was completed, the product particles were measured for D.sub.10, D.sub.50, and D.sub.90 mean particle size using a nanoparticle analyzer (Anton-Paar Litesizer 500 (available from Anton Paar GmbH, Graz, Austria)). The mean particle sizes, as a function of milling time, for each of Examples 1-5, are presented in FIGS. 6A-6E, respectively. As shown in those Figures, methods in accordance with the present disclosure are readily able to achieve D.sub.10 mean particle sizes of about 100 nm to about 200 nm, D.sub.50 mean particle sizes of about 150 to about 250 nm, and D.sub.90 mean particle sizes of about 250 nm to about 350 nm.

(35) As such, the present disclosure has provided embodiments of methods and apparatus for producing nanometer scale particles utilizing an electrosterically stabilized slurry in a media mill. The methods and apparatus beneficially maintain particle separation as the particle size decreases below about 1 micron to avoid agglomeration and mill screen blockage. Moreover, the methods and apparatus are beneficially suitable for industrial scale manufacturing to the extent that tight control of any additives is not required to prevent product suspension flocculation or steep increases in viscosity.

(36) While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the inventive methods and apparatus. It is understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.