Method of preparing aluminum nanorods

Description

BACKGROUND OF INVENTION

1. Field of Invention

Particles, and particularly nanoparticles find a wide range of use as fillers, active media, explosives, propellants, and reflective materials. The present invention related to the field of aluminum nanoparticle manufacture and in particular to aluminum nanorod compositions and hydrocarbon coated aluminum nanorod compositions.

2. Background of the Art

Aluminum powders are used in a broad range of applications including explosives, propellants and powder metallurgy parts for aircraft and automobiles. Since the reactivity of aluminum increases as the particle size decreases, small particles are desirable for aluminum used in propellants and explosives. Commercially available aluminum powders are generally several microns or larger in size. However, there are three general methods for obtaining aluminum particles with dimensions in the sub-micron range: mechanical attrition (high-energy ball milling), vapor condensation, and aluminum precipitation via chemical synthesis.

Nanosized aluminum powders have been made by mechanical attrition of 100 μm powders under different atmospheres and by gas condensation (See Eckert et al., NanoStructured Materials, vol. 2, pp 407-413 1993, and references cited therein). Aluminum particles prepared with ball milling with average grain size of 20 to 30 nm exhibited a reduced melting temperature in comparison to the bulk melting point regardless of whether argon, hydrogen or oxygen atmospheres were used in the milling process. Milling in oxygen atmosphere gave the most dramatic drop in melting point. Re-melting the samples several times did not recover the bulk melting temperature indicating that some amount of oxide phase was covering the grain boundaries.

Vapor condensation of aluminum nanoparticles is exemplified by U.S. Pat. Nos. 6,676,727 and 6,689,190, Pozarnsky, wherein aluminum metal is vaporized, condensed in an inert gas stream, and the metal nanoparticles collected in an inert liquid medium, such as a hydrocarbon. The hydrocarbon medium stabilizes the nanoparticles toward oxidation. The method has limitations in that a high energy demand is required, production rates are limited because of the need to carry the vapor in the inert gas stream, and the cost of the method is prohibitive for large volumes.

Precipitation via chemical synthesis of aluminum nanoparticles has been accomplished from different starting points. Buhro, et al., (J. Am. Chem. Soc., 1998, 120, 10847-10855) describe the generation of alane, AlH₃, by the reaction of lithium aluminum hydride with aluminum chloride followed by the in situ decomposition of alane to aluminum metal nanoparticles and hydrogen. The reaction suffers from the difficulty in removing the lithium chloride by-product and the resulting low purity of the particles (about 87% Al). An alternative decomposition method starting with AlH₃(NMe₂Et), an alane adduct of dimethylethylamine, gave high purity aluminum nanoparticles. However, preparing the starting alane-dimethylethylamine adduct was very hazardous on one mole scales. A similar process of preparing elemental aluminum particles from alane-trimethylamine has been described by Johnson et al., (1996 MRS Proceedings, Preparation of Nanometer Sized Aluminum Powders) and in an early report by Stecher (O. Stecher and E. Wiberg, Chem. Ber. 75, 2003-2012, 1942). Johnson, et al, passivated the aluminum nanoparticles by controlled oxidation with air and found that active aluminum content dropped rapidly as the particle size approached 100 nm. For instance, 100 nm aluminum particles exhibited only 70% active aluminum.

U.S. Pat. Nos. 3,954,443 and 3,860,415 Gautreaux et al., describe a metallurgical method of providing aluminum metal involving a pyrolysis of alkyl aluminum compounds to for aluminum powder. Steps in the process call for formation of trialkyl aluminum and dialkylaluminum hydride followed by pyrolyzing the mixture in an oil medium to form an aluminum powder, filtering the powder, washing with hexanes to remove residual oil, and casting the Al powder into suitable forms. U.S. Pat. No. 3,784,372 and related application U.S. Pat. No. 3,862,835, Scull, describe the pyrolysis of tialkylaluminum compounds in a neutral liquid solvent. Exemplary solvents for the process were aliphatic, alicyclic and aromatic hydrocarbons, and paraffin oil was preferred. There is no indication that aluminum nanoparticles or nanorods would be available in these processes.

From the foregoing summary, it is clear that a suitable method for large scale production of pure aluminum nanoparticles is needed. Several issues must be addressed for a viable solution including availability of starting materials, safety of the process, throughput, passivation of the particles, and ultimately the purity of the nanoparticulate aluminum. Disclosed here is a simple method for preparing aluminum nanorods, from readily available starting materials, that provides hydrocarbon passivated nanorods of about 2 nm to 300 nm in diameter in high purity with low oxygen content.

SUMMARY OF INVENTION

The present invention provides a method of preparing a hydrocarbon coated aluminum nanorod composition comprising the steps: mixing a trialkylaluminum, R₃Al, or dialkylaluminum hydride, R₂AlH, with a high boiling hydrocarbon solvent under an inert atmosphere to provide an aluminum precursor mix, heating the aluminum precursor mix to a specific reaction temperature to induce pyrolysis of the trialkylaluminum or dialkylaluminum hydride to provide the aluminum nanorod composition, wherein the nanorods have an average diameter from about 20 to about 300 nm and an aspect ratio of about 10 to about 25, and wherein R is selected from the group: ethyl and propyl.

The invention further provides an aluminum nanorod composition comprising aluminum rods with an average diameter from about 2 to 300 nm, an aspect ratio of from about 10 to about 25, and an oxygen content of about 2% to about 5%. In a further embodiment, the aluminum nanorod composition further comprises a high boiling hydrocarbon wherein said composition has an aluminum content of about 5% to about 95 wt % and a hydrocarbon content of about 2 to about 95 wt %.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a transmission electron microscope picture of the aluminum nanrods.

FIG. 2 shows a TGA scan of the hydrocarbon coated aluminum nanorods in air.

FIG. 3 shows the DSC of the hydrocarbon coated aluminum nanorods in air.

DETAILED DESCRIPTION OF THE INVENTION

In the method of the invention a trialkylaluminum, R₃Al, or dialkylaluminum hydride, R₂AlH, is mixed with a high boiling hydrocarbon solvent under an inert atmosphere to provide an aluminum precursor mix. The trialkylaluminum and dialkylaluminum starting materials may be selected from the group: ethyl and propyl. A most preferred R group is ethyl. Specific materials useful as starting materials are tripropylaluminum, triethylaluminum, and diethylaluminum hydride with triethylaluminum being most preferred.

The high boiling hydrocarbon solvent may be selected from the group: C₁₀-C₃₀ straight chain and branched chain hydrocarbon, and C₇-C₃₀ alkyl substituted aromatic and alicyclic hydrocarbon. The hydrocarbon solvent may have a boiling point between 100° C. and 400° C. and preferred are solvents with boiling points between 150° C. and 300° C. The solvents preferably are dried to remove water before use. For instance, the solvents may be refluxed over sodium metal, or stored over molecular sieves. Hexadecane and docosane are preferred solvents for the method. However, many other hydrocarbon solvents and mixtures thereof may be used including: toluene, o-, p- and m-xylenes, 1,3,5-trimethylbenzene, decalin, naphthalene, t-butylbenzene, sec-butylbenzene, butylbenzene, decane, dodecane, and octadecane.

The inert atmosphere may comprise almost any atmosphere that effectively excludes water and oxygen including nitrogen, argon, helium, xenon, and mixtures thereof. Pressure vessels may be useful in the method, especially wherein the more volatile trialkyl aluminums are to be used. A pressure vessel also allows the use of lower boiling solvents. However, with triethyl aluminum, the method may be run at 1 atmosphere and no special apparatus is required. Mixing may be accomplished with any conventional method such as magnetic stirrer or motor driven mechanical stirrer.

A wide range of concentrations can be used in the process to provide aluminum nanorods. However at concentrations greater than 33 wt % alkylaluminum, the reaction mixture forms a powder in the latter stages of the reaction. Preferred concentrations for maintaining a hydrocarbon slurry throughout the process are between about 5 wt % and 30 wt % alkylaluminum and the most preferred range is about 15 to 25 wt % alkylaluminum.

Heating the aluminum precursor mix to a specific reaction temperature provides the aluminum nanorod composition of the invention. The specific reaction temperature is that required to initiate and complete pyrolysis of the trialkyl aluminum or dialkyl aluminum hydride to liberate hydrocarbon and/or hydrogen. Heating can be done by using any conventional heat source. The specific temperature range and time for heating depends upon the hydrocarbon solvent and trialkyl aluminum or dialkylaluminum hydride used. Usually the temperature is increased until a reasonably controlled evolution of hydrogen and low molecular weight hydrocarbon is produced. Heating is stopped when the gas evolution subsides. The production of aluminum nanorod is evident in the formation of a black fine powder. A preferred specific reaction temperature is about 150° C. to about 300° C., and more preferred is from about 250° to about 280° C. when hexadecane is the solvent. The mixing and heating steps may also be done simultaneously, that is, the hydrocarbon solvent may be pre-heated to the desired reaction temperature and the alkyl aluminum mixed with the hydrocarbon at the reaction temperature.

The specific reaction temperature also may be modified by the use of a catalyst. Catalysts useful for decomposing alane-trialkylamine adducts, for instance, titanium(IV) isopropoxide, may be useful for decomposition of the trialkylaluminum or dialkylaluminum hydride.

After completion of the reaction the solvent may be removed to isolate the aluminum nanorod composition. Solvent removal may be accomplished by a variety of means including filtration, decanting, and centrifugation. Preferred methods include filtering the suspension under an inert atmosphere and decanting the solvent from a settled precipitate. Generally residual solvent is removed by washing with a low boiling solvent such as hexanes, pentane, toluene, benzene, and the like, followed by drying the composition under vacuum or in an inert atmosphere. However, it is clear from chemical, infrared, and thermal analysis that the solvent wash and drying does not remove all hydrocarbon from the aluminum nanorod composition. Preferably, enough solvent is removed in the process to provide an aluminum nanorod composition having an aluminum content of about 20% to about 95 wt % and a hydrocarbon content of about 2 to about 80 wt %.

In another embodiment of the invention the method provides an aluminum nanorod composition comprised substantially of nanorods having an average diameter from about 2 nm to about 300 nm and an aspect ratio of about 10 to about 25. Preferably the method provides nanorods having an average diameter of about 20 nm to 100 nm and more preferable are nanorods with an average diameter of about 50 nm to about 80 nm. The aluminum nanorod composition has unique properties. For instance, the oxygen content of the composition is about 2% to about 5%, much lower than oxygen content reported by Johnson et al, (above) wherein oxygen was used to passivate the particle. In the nanorod composition of the invention, no oxygen passivation is used or is necessary. The nanorod composition appears to be passivated, in situ, by residual high boiling hydrocarbon that may not be easily removed by vacuum drying. Thus, the composition is stable in air and exhibits hydrophobic character in water. Preferred aluminum nanorod compositions further comprise a high boiling hydrocarbon and have aluminum content of about 5% to about 95 wt % and a hydrocarbon content of about 2 to about 95 wt %. More preferred are compositions having about 50 to about 80 wt % aluminum and about 15 to about 50 wt % hydrocarbon.

Hydrocarbon passivated aluminum nanorods have use in propellant and explosive formulations. Aluminum powder is a common ingredient in energetic materials. Aluminum is used to increase the energy and raise the flame temperature in rocket propellants. It is also incorporated in explosives to enhance air blast, increase bubble energies in underwater weapons and create incendiary effects. Nanoparticles of Aluminum, because of their large surface area, can increase the bum rate in some types of propellants. However, as Johnson, et al., (cited above) indicated, aluminum nanoparticles passivated by controlled oxidation with air exhibit lower active aluminum contents as the particle size approaches 100 nm. The more oxygen present the less aluminum is available as a fuel. Typical is an aluminum nanoparticle from Argonide Corporation referred to as ALEX that has an average size of about 140 nm and an active aluminum content of about 79%. Thus, an important attribute of aluminum nanoparticles is the active aluminum content. This may be measured by thermal gravimetric analysis (TGA), wherein the weight gain from oxidation of aluminum to aluminum oxide in air is measured; or by DSC by direct measure of the exotherm of the reaction. In general, large particle size aluminum powders (>1 μm) show slow uptake of oxygen but exhibit high active aluminum contents (>95%).

Another important and complementary attribute is the oxygen content of the aluminum particles. This gives a direct measure of the amount of surface oxidation present in the aluminum nanoparticle composition. The composition of the invention exhibits a low oxygen content, averaging about 2 to about 5%, and is an important attribute for propellants and explosives.

The following examples are meant to illustrate the invention and are not meant to limit the scope of the invention.

EXAMPLE 1

Hexadecane (500 mL, anhydrous) and Al(C₂H₅)₃ (100 mL) were mixed in 1000 ml schlenk flask in an argon filled glovebox at room temperature for 5 min. The mixture was heated to reflux temperature (ca. 250-270° C.) under argon. Black precipitate formed and gas (hydrogen and ethylene) was evolved. Heating and stirring were continued until no more gas evolved (about 3 h). After cooling, the black precipitate was separated by filtration under argon. The black powder was washed three times by anhydrous hexane, and dried under vacuum to give 27 g. The Infrared (IR) spectrum indicated the existence of —CH₃ and —CH₂ group. Combustions analysis with a LECO CS-200 instrument was used to determine carbon content. Inert gas fusion analysis was used to determine oxygen and hydrogen content using a LECO TCH-600 instrument. C, H: C: 33.0%. H: 6.5%. The C:H ratio of 5.1 was similar to that of hexadecane (5.4). The oxygen content was 5.0% The hydrocarbon contents varied from 20-40% by weight, depending on the treatment of the precipitate under vacuum. The surface area, as measured by BET nitrogen absorption with a Beckman Coulter SA-3100 instrument, was 22 m²/g. The black powder was stable in air and hydrophobic in water at room temperature. FIG. 1 shows an transmission electron microscope image (12000×) of the aluminum nanorod composition of the invention. The composition comprises substantially rods with small amounts of nanoparticles as well. However, in the sample preparation, sonication is used to separate rods from agglomerated rods and some rods may have broken.

EXAMPLE 2

In a similar manner to Example 1, docosane (50 mL) and Al (C₂H₅)₃ (10 mL) were processed to give 4.0 g black powder. The black powder was stable in air and hydrophobic in water at room temperature.

EXAMPLE 3

In a similar manner to Example 1, a black powder was prepared that was analyzed by DSC and TGA to estimate the % active aluminum in the composition. FIG. 2 shows the TGA curve of a 0.44 mg sample heated at 5° C./min in air. Between room temperature and 150° C. the sample weight drops to about 55% due to release of the residual nydrocarbon coating the nanorod composition. Weigh gain initiates at about 400° C. and increases to 104% which corresponds to 100% active aluminum based upon a initial 45 wt % hydrocarbon and 55 wt % aluminum. The DSC trace that measures the exotherm is shown in FIG. 3. The sample exhibited an exotherm of 8.0 kJ/g. The results of the TGA analysis were sensitive to the sample placement in the pan and the weight of the sample analyzed. Smaller samples in the range of 0.3 to 0.5 mg spread out over the pan gave the most consistent measurements. Larger samples, for instance 2-3 mg, gave lower active aluminum contents and concomitant smaller exotherms in the DSC. Samples that were concentrated in a pile, rather than spread thin across the pan, also gave lower % active aluminum and lower exotherms.

While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications as incorporate those features that come within the spirit and scope of the invention