Composites of porous pyrophoric iron and ceramic and methods for preparation thereof

Abstract

Disclosed herein are pyrophoric composite materials comprising nanoporous pyrophoric alpha iron nanoparticles dispersed in a ceramic matrix for use as aerial decoys. The composite material is prepared using tape casting methods to produce a thin film. The iron precursor in the film is then activated by reduction under a hydrogen atmosphere. The composite nanoporous pyrophoric alpha iron nanoparticles and ceramic material is an improvement over current pyrophoric decoys as it eliminates the use of harmful chemicals and the need for a substrate to support the composite.

Claims

1. A composite material for use as an aerial countermeasure decoy comprising, nanoporous pyrophoric alpha iron nanoparticles dispersed in a ceramic matrix, wherein said composite material is monolithic.

2. The composite material of claim 1, wherein the ratio of the nanoporous pyrophoric alpha iron nanoparticles to the ceramic matrix is 41:59 to 93:7.

3. The composite material of claim 1, wherein the ceramic matrix is porous and having a porosity of between 10% to 40%.

4. The composite material of claim 1 wherein the ceramic is selected from the group consisting of aluminum silicate, sodium silicate, lithium silicate, magnesium silicate, bentonite, montmorillonite, Boehmite, and feldspar.

5. The composite material of claim 1 further comprising a fuel selected from group consisting of aluminum, silicon, tin and magnesium.

6. A composite material for use as a countermeasure decoy consisting essentially of, nanoporous pyrophoric alpha iron nanoparticles dispersed in a ceramic matrix, wherein said composite material is monolithic and a fuel wherein said fuel is selected from the group consisting of aluminum, silicon, tin and magnesium.

7. A canister comprising a plurality of thin flat composite material of claim 1 in an inert atmosphere.

8. A process for preparing the composite material of claim 1, comprising: preparing a slurry comprising alpha iron oxide nanoparticles and ceramic material dispersed in a liquid, wherein the ratio of alpha iron oxide nanoparticles to ceramic material is 50:50 to 90:10 by weight; tape casting the slurry into a thin flat material; drying the thin flat material to remove the liquid dispersant; sintering the thin flat material; and reducing the thin tape material under a hydrogen atmosphere to produce nanoporous pyrophoric alpha iron nanoparticles dispersed in a porous ceramic matrix.

9. The process of claim 8, wherein the ceramic material is aluminum silicate.

10. The process of claim 8, wherein the ceramic material is selected from the group consisting of sodium silicate, lithium silicate, magnesium silicate, bentonite, montmorillonite, Boehmite, and feldspar.

11. The process of claim 8, further comprising a binder selected from the group consisting of methylcellulose, hydroxypropyl methylcellulose, and ethylcellulose.

12. The process of claim 8, further comprising a fuel selected from the group consisting of aluminum, silicon, tin and magnesium.

13. The process of claim 8, wherein the liquid dispersant is water or alcohol.

14. The process of claim 8, wherein the thin flat material is about 0.014 inches to about 0.028 inches thick.

15. The process of claim 8, wherein the alpha iron oxide nanoparticle and ceramic material is dispersed using a resonant acoustic mixer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood, and further objects, features and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings.

(2) FIG. 1. Pyrophoric response of three separate iron/aluminum silicate composites exposed to flowing air.

(3) FIG. 2. Exemplary tape casting machine assembly.

(4) FIG. 3. Illustration of a pre-reduction alpha iron oxide and ceramic composite.

(5) FIG. 4. Illustration of a post-reduction nanoporous alpha iron nanoparticles and ceramic composite.

DETAILED DESCRIPTION

(6) The present invention discloses a pyrophoric composite material useful as countermeasure decoys and methods for preparing the same. The methods provided herein eliminates the need for using caustic chemicals for preparing nanoporous pyrophoric alpha iron nanoparticles. The disclosed method has the added benefit of a self-supporting matrix, thereby eliminating the need for a substrate. The process utilizes resonant acoustic mixing to disperse alpha iron oxide nanoparticles and ceramic into a mixture. The mixture is then tape casted into a desired thickness. The alpha iron oxide dispersed in the ceramic is then reduced to convert alpha iron oxide into porous pyrophoric alpha iron nanoparticles. The pyrophoric response of the iron can be adjusted (i.e. tuned) based on the iron/ceramic material weight ratio and/or the addition of other fuels such as Mg, Al etc.

(7) As used herein, the term pyrophoric means the ability to self-ignite spontaneously upon exposure to air.

(8) The general processing steps for preparing the nanoporous pyrophoric alpha iron nanoparticles and ceramic composite material is as follows: a. Preparing a mixture by dispersing alpha iron oxide nanoparticles, ceramic material, a liquid solvent and optional additives to form a slurry. b. Tape casting and drying the slurry mix into a desired thickness. c. Sintering the composite material to strengthen the tape casted mixture. d. Reducing the alpha iron oxide to form nanoporous pyrophoric alpha iron metal dispersed in a ceramic matrix.

(9) The composite material prepared by the process described herein comprises nanoporous pyrophoric alpha iron nanoparticles wherein the alpha iron spontaneously self-ignites upon exposure to air. The nanoporous pyrophoric alpha iron nanoparticles are dispersed in an interconnected matrix of porous ceramic material. It should be understood that the ceramic material should be sufficiently porous to permit oxygen in the atmosphere to permeate and react with the pyrophoric alpha iron disperse through the entire structure such that spontaneous self-ignition of the iron is achieve at under 2 seconds, with output temperatures reaching as high as about 600° C. to about 800° C. as illustrated in FIG. 1. The density of the composite material should be adjusted such that the material flutters like confetti when expelled from an aircraft. Recommended porosity of the ceramic material in the composite is about 10% to 40%, and preferably about 20% to 30%.

(10) Composite Mix.

(11) Alpha iron oxide nanoparticles and ceramic material, at ratio of 50:50 to 90:10 by weight, may be dispersed with liquid into a free flowing slurry. The materials may be uniformly mixed using any known methods that produces a homogenous mixture. Resonant acoustic mixer (RAM) is preferred. Such mixers are available from Resodyn Acoustic®. Nanoparticles of alpha iron oxide having a size no greater than 100 nm and preferably between 20 nm-60 nm may be used. The liquid may be water or an organic solvent, however, any liquid that promotes flowability of the slurry during the tape casting process may be utilized. A preferred liquid is water. Ceramic material such as aluminum silicate may be mixed with the alpha iron oxide nanoparticle, however, sodium silicate, lithium silicate, magnesium silicate, bentonite, montmorillonite, Boehmite, and feldspar may be used as well. Polymeric binders may optionally be added to the mixture. Such binders include methylcellulose, hydroxypropyl methylcellulose, and ethylcellulose as well as other high viscosity binders soluble in the selected solvent may be used. Fuel components may optionally be added to modify the dynamic combustion of the self-supporting pyrophoric material. Such fuel additives include aluminum, silicon, tin and magnesium with tin being preferred.

(12) Tape casting. After the composite slurry is prepared, it may be tape casted using an apparatus having the general features illustrated in FIG. 2. Generally, the composite slurry is placed into a holding chamber 100 having a gap controlled by a doctor blade 200. The composite slurry passes under the doctor blade which spreads the slurry into a thin, flat, even, tape-like layer having a thickness of about 0.002″ to about 0.2″ and preferably about 0.045″. The tape-like composite 300 exiting the doctor blade rests atop a nonstick plate or platform which is moved by a conveyor system 500 to a heating element assembly 400 to remove the liquid dispersant. Recommended drying conditions of 70°−72° F., 15%-25% relative humidity for 16 hours may be used, however, these drying conditions may be adjusted to avoid cracks or surface stress on the composite material. A final thickness of 0.014″ to 0.028″ composite material tape/film is recommended to maintain structural integrity of the material. The thickness of the film may be adjusted to control the burn time of the pyrophoric material. After the composite tape is dried, it may be cut to the desired shape and length, or spooled for storage and further processing.

(13) Sintering.

(14) The composite tape is further processed by partial sintering to remove the binder, initiate inter-particle connections, and reduce the porosity which all leads to increased strength. Sintering of the composite is conducted at temperatures between 980° C. to 1200° C. with soak times between 10 minutes and 5 hours in air or under an inert atmosphere. The temperature and soak times may be varied, in accordance with generally known methods.

(15) Iron Reduction.

(16) The alpha iron oxide in the composite material is reduce to nanoporous pyrophoric alpha iron nanoparticles using hydrogen to produce water as a byproduct in this step. Reduction of alpha iron occurs under a flowing hydrogen environment, preferably at temperatures of 400° C. to 550° C. for a minimum of 3 hours. The reduced composite is then cooled under a flowing hydrogen, nitrogen or a mixed hydrogen/nitrogen atmosphere. FIG. 3 and FIG. 4 are illustrations of iron oxide in the ceramic matrix before reduction and after reduction respectively.

(17) Packaging.

(18) The iron and ceramic composite tape or film may be further processed into strips and packaged as a plurality of geometric shaped strips under a dry inert atmosphere (hydrogen and/or nitrogen) into canisters that can be ejected from an aircraft. Typical geometric shape strips include 1″×1″ squares packaged into a canister having inner dimensions of 1″×1″×8″.

EXAMPLE 1

(19) Example 1 is an illustration of the disclosed invention. Weighing out of the following components: i) alpha iron oxide (27 grams), ii) aluminum silicate (3 grams) iii) methylcellulose (1.5 grams) and iv) water (67.5 mL). In certain cases, additional fuels such as Al, Mg, Ti etc. may be added to the mixture. Disperse the methylcellulose powder in the water and allow for complete hydration of the methylcellulose over a 16-hour period. Add the alpha iron oxide and aluminum silicate to the mixing container containing the methylcellulose solution and disperse using an acoustic mixing technique. The composite slurry is then tape casted onto a Teflon or other suitable non-sticking plate or film using a doctor blade. The composite tape is allowed to dry for 12 hours under ambient conditions. The composite tape is then cut into pieces (size dependent on end application). The substrates are sintered in air at 10-15° C. above the silicate melting point with a soak time of 10 minutes to 3 hours. Hydrogen reduction of the alpha iron oxide is conducted, followed by testing or packaging. FIG. 1 is a chart of the pyrophoric response for the composite material prepared by Example 1.

EXAMPLE 2

(20) In another example, 80% alpha iron oxide nanoparticles and 20% sodium silicate by weight were dispersed in water. The mixture was tape casted as described in the procedures above and sintered at 1100° C. for 30 minutes and further reduced under flowing hydrogen.

(21) Although the subject invention has been described above in relation to embodiments thereof, it will be understood by those skilled in the art that variations and modifications can be effected in these preferred embodiments without departing from the scope and spirit of the invention.