Nanocrystalline alpha alumina and method for making the same

Abstract

A process for producing metastable nanocrystalline alpha-alumina (-Al.sub.2O.sub.3) having particle sizes smaller than 12 nm. Starting crystallites of -Al.sub.2O.sub.3 having a particle size larger than 12 nm, typically on the order of about 50 nm, are ball-milled at low temperatures to produce a nanocrystalline -Al.sub.2O.sub.3 powder having a particle size of less than 12 nm, i.e., below the theoretical room temperature thermodynamic size limit at which -Al.sub.2O.sub.3 changes phase to -Al.sub.2O.sub.3, wherein the powder remains in the -Al.sub.2O.sub.3 phase at all times.

Claims

1. A process for producing a metastable -Al.sub.2O.sub.3 nanocrystalline powder having a particle size of less than 12 nm, comprising: placing starting crystallites of -Al.sub.2O.sub.3 having a particle size larger than 12 nm into a ball-milling jar containing a ball-milling media; and ball-milling the -Al.sub.2O.sub.3 starting crystallites at room temperature to produce a metastable ball-milled -Al.sub.2O.sub.3 nanocrystalline powder having a particle size of less than about 12 nm; wherein the room-temperature ball-milling prevents the -Al.sub.2O.sub.3 from undergoing a phase change to -Al.sub.2O.sub.3, such that the -Al.sub.2O.sub.3 starting crystallites and the ball-milled -Al.sub.2O.sub.3 nanocrystalline powder remain in the -Al.sub.2O.sub.3 phase at all times.

2. The process according to claim 1, wherein the ball-milling media is WC-Co.

3. The process according to claim 1, further comprising washing the ball-milled -Al.sub.2O.sub.3 nanopowders with nitric acid and hydrogen peroxide.

4. A process for producing a metastable -Al.sub.2O.sub.3 nanocrystalline powder having a particle size of less than 12 nm, comprising: placing starting crystallites of -Al.sub.2O.sub.3 having a particle size larger than 12 nm into a ball-milling jar containing a ball-milling media; and ball-milling the -Al.sub.2O.sub.3 starting crystallites at a cryogenic temperature at or below about 180 C. to produce a metastable ball-milled -Al.sub.2O.sub.3 nanocrystalline powder having a particle size of less than about 12 nm; wherein the cryogenic ball-milling prevents the -Al.sub.2O.sub.3 from undergoing a phase change to -Al.sub.2O.sub.3, such that the -Al.sub.2O.sub.3 starting crystallites and the ball-milled -Al.sub.2O.sub.3 nanocrystalline powder remain in the -Al.sub.2O.sub.3 phase at all times.

5. The process according to claim 4, wherein the ball-milling media is WC-Co.

6. The process according to claim 4, further comprising washing the ball-milled -Al.sub.2O.sub.3 nanopowders with nitric acid and hydrogen peroxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B are plots illustrating the thermodynamic instability (excess enthalpy) of the three alumina polymorphs as a function of surface energy (FIG. 1A) and particle size (FIG. 1B).

(2) FIGS. 2A AND 2B are plots of the X-ray diffraction patterns of lab-synthesized (FIG. 2A) and commercially prepared (FIG. 2B) -Al.sub.2O.sub.3 starting powder having a particle size of 50 nm, a 10 nm -Al.sub.2O.sub.3 powder produced from such starting powders after 270 minutes of ball milling, and the 10 nm ball-milled -Al.sub.2O.sub.3 powder after washing with nitric acid and hydrogen peroxide.

(3) FIG. 3 is a plot showing grain size and WC-Co contamination of the lab-synthesized and commercially prepared -Al.sub.2O.sub.3 powders as a function of milling time.

(4) FIG. 4 is a Transmission Electron Microscope (TEM) image of an -Al.sub.2O.sub.3 powder produced from 270 minutes of ball milling and washing in accordance with the present invention on top of a holey carbon grid, showing that the grain size of the powder is well below 50 nm.

(5) FIG. 5 is a High Resolution TEM image of the individual alumina crystallites produced by a ball milling process in accordance with one or more aspects of the present disclosure.

DETAILED DESCRIPTION

(6) The aspects and features of the present invention summarized above can be embodied in various forms. The following description shows, by way of illustration, combinations and configurations in which the aspects and features can be put into practice. It is understood that the described aspects, features, and/or embodiments are merely examples, and that one skilled in the art may utilize other aspects, features, and/or embodiments or make structural and functional modifications without departing from the scope of the present disclosure.

(7) The present invention provides a process for producing metastable nanocrystalline alpha-alumina (-Al.sub.2O.sub.3) having a nanoparticle size smaller than 12 nm.

(8) The process of the present invention utilizes temperature-controlled high-energy ball milling and a simple acid washing technique to provide a simple, scalable, industrial process for producing metastable nanocrystalline -Al.sub.2O.sub.3 having a nanoparticle size at or below the room-temperature thermodynamic -Al.sub.2O.sub.3 particle size limit of 12 nm, i.e., below the size at which room-temperature thermodynamics would dictate that the alumina changes from the -phase to the -phase.

(9) The general concept behind the present invention is that the -Al.sub.2O.sub.3 particles are ball-milled at a low processing temperature, typically room temperature, where the low processing temperature kinetically hinders any phase transformation of -Al.sub.2O.sub.3 to -Al.sub.2O.sub.3 during the process because the activation barrier is too high.

(10) Thus, in accordance with the present invention, a starting powder of -Al.sub.2O.sub.3 crystallites having a particle size larger than 12 nm, typically about 50 nm, is placed into a ball-milling jar and is ball-milled at room temperature using a ball-milling media until the starting crystallites are reduced in size to a particle size of less than 12 nm. In most cases, the ball-milling media will be tungsten carbide-cobalt (WC-Co), though other containers and ball-milling media comprising materials harder than -Al.sub.2O.sub.3, such as diamond jars and media, diamond-coated jars and media, or cubic boron nitride jars and/or media, can also be used.

(11) In most embodiments, the starting -Al.sub.2O.sub.3 crystallites are ball milled in a series of short time intervals in order to prevent the temperature of the -Al.sub.2O.sub.3 crystals in the WC-Co ball-milling jar from increasing to a point where a phase change might occur. In a typical case, the starting crystals are ball-milled for about 30 minutes at time, with about 90 minutes being needed to reduce crystallites having a particle size of 50 nm to crystallites having a size of less than 12 nm. However, one skilled in the art will readily appreciate that one or both of the time intervals needed to prevent temperature increases of the -Al.sub.2O.sub.3 crystals or the total time needed to obtain a powder having the desired particle size may vary, depending on the volume of the WC-Co jar, the amount of the starting -Al.sub.2O.sub.3 powder, or the amount of the WC-Co ball-milling media.

(12) In other embodiments, the temperature of the -Al.sub.2O.sub.3 can be kept down by using active cooling measures, such as performing the ball milling at cryogenic temperatures, i.e., at or below about 180 C.

(13) In all cases, in accordance with the present invention, the ball-milling will be performed under a set of predetermined conditions configured to ensure that the starting -Al.sub.2O.sub.3 crystallites remain in a metastable -Al.sub.2O.sub.3 phase at all times and do not change to the -phase at any time during the ball-milling process.

(14) The sub-12 nm -Al.sub.2O.sub.3 powders produced by the ball-milling process in accordance with the present invention can then be washed with nitric acid and hydrogen peroxide to completely remove the WC-Co contamination on the surfaces of the particles.

(15) High Resolution TEM showed that the individual crystallites resulting from this ball milling process have differing crystallographic orientations with nominal sphericity, and are a truly nanocrystalline powder.

(16) The process in accordance with the present invention thus provides an industrial, scalable, and economical procedure that provides a new avenue for processing of nanocrystalline -Al.sub.2O.sub.3. The success provided by the process of the present invention in WC-Co ball milling of large starting crystallites of -Al.sub.2O.sub.3 down to metastable sub-12 nm crystallites without those crystallites undergoing an undesirable phase change was unexpected by the inventors and is contrary to what would normally be expected from the thermodynamic characteristics of the various phases of alumina. The results provided by the process of the present invention are vitally important since -Al.sub.2O.sub.3 does not ordinarily exist at these crystallite sizes, and the current bottom-up synthesis approaches can only produce -Al.sub.2O.sub.3, which is an undesired phase for many applications, at these small crystal sizes.

Example

(17) The process in accordance with the present invention was employed on lab-synthesized and commercially available -Al.sub.2O.sub.3 powders to demonstrate the universal nature of the approach and to re-reinforce its industrial scalability.

(18) Lab-synthesized -Al.sub.2O.sub.3 nanopowders were synthesized via a modified reverse strike co-precipitation route similar to other systems reported in the literature. See J. W. Drazin et al., Phase Stability in Nanocrystals: A Predictive Diagram for Yttria-Zirconia, Journal of the American Ceramic Society 98, 1377-1384 (2015); and J. W. Drazin et al., Phase Stability in Calcia-Doped Zirconia Nanocrystals, Journal of the American Ceramic Society 99, 1778-1785 (2016).

(19) Aluminum nitrate nonhydrate (>98%, Sigma-Aldrich, St. Louis, Mo.) was dissolved in de-ionized water to form a clear, dilute solution. The cationic solution was then added drop-wise to a stirred 5M excess ammonium hydroxide solution where aluminum hydroxide formed as a white precipitate. The final mixture was washed with 190 proof denatured ethanol and centrifuged (5000 rpm for 4 min) 3 times to remove and replace the excess ammonia solution. The white precipitate was then dried at 70 C. for 48 hours. The dried powder was heated to 1200 C. and held at temperature for 2 hours to calcine the hydroxide to -alumina with a crystallite size of 50 nm.

(20) It is noted that 50 nm was the smallest possible crystallite size that we were able to produce in the lab such that the powder was 100% -Al.sub.2O.sub.3. Unfortunately, all attempts at seeding the synthesis with 50+nm -Al.sub.2O.sub.3 powder (as reported by Li and Sun) were unable to lower the calcining temperature or the crystallite size. See J. G. Li et al., Synthesis and sintering behavior of a nanocrystalline -alumina powder, Acta Materialia 48, 3103-3112 (2000). Therefore, to test the efficacy of the ball-milling method of the present invention, the inventors decided to use a commercial -Al.sub.2O.sub.3 powder having the smallest grain size commercially available (99.9+% -Al.sub.2O.sub.3 powder obtained from Stanford Advanced Materials, Irvine, Calif.) and ball mill this commercial powder to shear or grind the agglomerates and grains and compare the results obtained to results obtained for the lab-synthesized powder.

(21) The starting crystallite sizes for the lab-synthesized and commercial -Al.sub.2O.sub.3 were 50 and 60 nm, respectively, as determined by diffraction peak broadening analysis.

(22) The -Al.sub.2O.sub.3 powders were ball milled in a SPEX 8000M using a cobalt cemented tungsten carbide vial in 3 g batches. The milling media consisted of four 6 mm and two 12 mm tungsten carbide (WC) balls (WC-Co, McMaster-Carr, Elmhurst, Ill.) giving a ball to powder mass ratio (BMR) was 10:1. The batches were milled in 30 minutes intervals and re-oriented at each interval to ensure that the powder did not preferentially collect inside the vial during milling. The crystallite size of the lab-synthesized and commercial powders after milling was about 9.6 nm and 8.7 nm, respectively.

(23) After ball milling, the powder was washed using a 5% (v/v) 15.4M HNO.sub.3 in 30% H.sub.2O.sub.2 solution to dissolve the residual tungsten carbide-cobalt (WC-Co) contamination from the ball-milled powder in a manner similar to that done by Archer et al. in their previous work. See M. Archer et al., Analysis of cobalt, tantalum, titanium, vanadium and chromium in tungsten carbide by inductively coupled plasma-optical emission spectrometry, Journal of Analytical Atomic Spectrometry 18, 1493-1496 (2003). The dissolution of the residual WC-Co contamination was performed at 95 C. while under magnetic stirring and was finished in under an hour. The mixture was then washed with 190-proof denatured ethanol 3 times.

(24) X-ray diffraction (XRD) analysis of the resulting powders was performed on a 18 kW rotating anode Rigaku X-Ray Diffractometer (Rigaku, Tokyo, Japan) operated at 50 kV and 200 mA using a copper target. The crystallite size was calculated using JADE 9.6 v software to perform a whole pattern fitting (WPF) refinement using PDF #10-0173 for -Al.sub.2O.sub.3 and PDF #51-0939 for WC-Co (when presenting the diffraction pattern). Specific surface area measurements were made on an ASAP 2020 sorption apparatus (Mircomeritics, Norcrosss, Ga.) using a five point liquid nitrogen adsorption measurement (BET theory). The reported values are an average of three consecutive measurements.

(25) The results of the XRD analysis of the two -Al.sub.2O.sub.3 samples after ball milling for 270 minutes are shown in FIGS. 2A and 2B.

(26) As can be seen from the plots in FIGS. 2A and 2B, both the milled the lab-synthesized and commercial powders retained their -structure even though their crystallite size (as determined by the width of the peaks in the XRD analysis using a Halder-Wagner analysis) was reduced below the thermodynamic phase-crossover size limit of 9.6 nm and 8.7 nm for the lab-synthesized (FIG. 2A) and commercial powders (FIG. 2B), respectively.

(27) The powders, which were white before the milling, developed a greyish color after milling as a result of the WC-Co milling media wearing against and so contaminating the surface of the nanopowder. This contamination is also observable in the diffraction pattern. For example, the major peak seen at 31.8 in FIG. 2B is associated with this WC contamination.

(28) In order to remove the WC contamination, a modified washing procedure was employed using nitric acid and hydrogen peroxide. The hydrogen peroxide oxidized the WC such that the product was dissolvable in nitric acid. See Archer, supra. With the low concentrations of cobalt cement used in this example, nitric acid was sufficient to prevent a cobalt passivation layer from forming; however, aqua regia, instead of nitric acid, can also be employed where appropriate to remove excess cobalt media from the milled nanopowder.

(29) After this acid washing, the powders returned to their bright white color. The XRD of the powders after acid washing are shown by the dotted lines in FIGS. 2A and 2B, where there was no phase or grain size change in the powders. The elimination of the WC contamination peak and the return of the powder to a bright white color signify that the washing was sufficient to remove the WC-Co contaminate.

(30) The plots in FIG. 3 show that there is a practical limit to the minimum obtainable crystallite size. The time-dependency of the crystallite size (solid lines) and the WC contamination levels (dotted lines) of the two powders show that the crystallite size of both powders followed very similar, almost overlapping, exponential decaying functions plateauing near 10 nm.

(31) The similarity in trends of the two powders suggests that the technique of the present invention may not be well suited to produce nanopowders much smaller than 9 nm. The plateau in the final sub-10 nm crystallize size can be explained by a Hall-Petchian-type phenomenon: as the size of the powder decreases, the hardness of the individual crystals and agglomerates increases to a point where it is significantly harder than the milling media, wearing the WC balls in the ball-milling chamber further. See D. Chrobak et al., Deconfinement leads to changes in the nanoscale plasticity of silicon, Nat Nano 6, 480-484 (2011); and Y. Tian et al., Ultrahard nanotwinned cubic boron nitride, Nature 493, 385-388 (2013).

(32) Consequently, there is an equilibrium where the -Al.sub.2O.sub.3 powder will be as hard as or harder than the milling media such that continued milling will not significantly alter the grain size. This assumption is supported by the WC contamination curve in FIG. 3, which is essentially zero for the lab-synthesized powder produced in accordance with the present invention until minute 175, but then increases rapidly while the grain size barely decreases.

(33) Interestingly, the commercial -Al.sub.2O.sub.3 powder accumulated WC contamination at an earlier milling time compared to the lab-synthesized powder having similar grain sizes. One reason for this difference in contamination levels could be the agglomeration state of the powders: i.e., the amount of free surfaces in the powder.

(34) Brunauer-Emmett-Teller (BET) surface area analysis showed that the lab-synthesized and commercial powder, after milling and washing, had a surface area per gram of 62.1010.217 and 42.9830.120 m.sup.2.Math.g.sup.1 respectively. Consequently, even if the crystallite size plateau of 10 nm is an artifact of the milling time, the amount of WC contamination would grow too fast to produce significantly smaller grain sizes. Regardless, this is the first time that a simple procedure, with high yield, has demonstrated the feasibility of producing -Al.sub.2O.sub.3 with high specific surface areas and crystallite sizes at or below the -phase thermodynamic crossover limit.

(35) In another examination of the metastable -Al.sub.2O.sub.3 nanopowders produced in accordance with the method of the present invention, TEM samples were prepared by dispersing the nanopowders in methonal and then pipetting the nanopowder dispersion onto a holey carbon grid. All specimens were examined in a FEI Tecnai G2 TEM with LaB6 filament operated at 300 kV.

(36) FIG. 4 shows a TEM micrograph of an -Al.sub.2O.sub.3 nanopowder produced from lab-synthesized -Al.sub.2O.sub.3 starting powder, after ball-milling for about 270 minutes at approximately 30-minute intervals and removal of contaminants. The crystallite size of this powder is less than 10 nm, consistent with the reported XRD grain size. In addition, as can be easily seen from the image in FIG. 4, the powder exhibits a high level of agglomeration, which would be expected for sub-10 nm size particles because of their high electrostatic attraction to one another.

(37) FIG. 5 is a high-resolution TEM image of the powder depicted in FIG. 4, and reveals that the small grains shown in FIG. 4 have unique crystallographic orientations and are distinct individual particles. The observed crystallite size of the particles, on the order of 10 nm, matches well with the XRD grain size. The image further suggests that the crystals are not related by low angle misorientations, further showing that the produced powder has a sub-10 nm grain size.

(38) Thus, although it is not presently feasible to directly synthesize nanocrystalline -Al.sub.2O.sub.3 having a sub-12 nm grain size, the present invention provides a method for producing a metastable -Al.sub.2O.sub.3 nanocrystalline powder having a crystallite size of less than 12 nm using a novel WC-Co ball milling and nitric acid and hydrogen peroxide washing procedure. The procedure utilizes high-energy room-temperature ball milling that is conducted at a series of short intervals in order to avoid the - to -phase transition despite the final grain size being below the thermodynamic size limit for the -phase, while the washing effectively removed the milling contamination from the final powders. The procedure worked equally well for the lab-synthesized powders as for the commercial powders, producing similar crystallite sizes with sufficiently high yields that the overall procedure could be scaled up with minimal modifications. Therefore, this novel technique is the first to provide a pathway for the industrial production of metastable nanocrystalline -Al.sub.2O.sub.3 below the thermodynamic size limit.

(39) Although particular embodiments, aspects, and features have been described and illustrated, one skilled in the art would readily appreciate that the invention described herein is not limited to only those embodiments, aspects, and features but also contemplates any and all modifications and alternative embodiments that are within the spirit and scope of the underlying invention described and claimed herein. The present application contemplates any and all modifications within the spirit and scope of the underlying invention described and claimed herein, and all such modifications and alternative embodiments are deemed to be within the scope and spirit of the present disclosure.