U.S. patent application number 11/165734 was filed with the patent office on 2006-03-02 for preparation of porous pyrophoric iron using sol-gel methods.
This patent application is currently assigned to The Regents of the University of CA. Invention is credited to Alexander E. Gash, Joe H. JR. Satcher, Randall L. Simpson.
Application Number | 20060042417 11/165734 |
Document ID | / |
Family ID | 35941154 |
Filed Date | 2006-03-02 |
United States Patent
Application |
20060042417 |
Kind Code |
A1 |
Gash; Alexander E. ; et
al. |
March 2, 2006 |
Preparation of porous pyrophoric iron using sol-gel methods
Abstract
New sol-gel methods can be employed to generate high surface
area porous iron (III) oxide-based solids. Chemical reduction of
such porous solids at low temperatures allows the preparation of
high surface area porous iron with little sintering, with the only
byproduct being water. The material is readily pyrophoric and has
utility in new decoy flares. The material, prepared by this
synthetic route, eliminates the use of hot caustic leaching
solutions. It does not require the incorporation of any hazardous
materials or processes that are not already used in current
production methods.
Inventors: |
Gash; Alexander E.;
(Brentwood, CA) ; Satcher; Joe H. JR.; (Patterson,
CA) ; Simpson; Randall L.; (Livermore, CA) |
Correspondence
Address: |
John P. Wooldridge;ATTORNEY
252 Kaipii Pl
Kihei
HI
96753
US
|
Assignee: |
The Regents of the University of
CA
|
Family ID: |
35941154 |
Appl. No.: |
11/165734 |
Filed: |
June 24, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60583155 |
Jun 24, 2004 |
|
|
|
Current U.S.
Class: |
75/415 |
Current CPC
Class: |
B22F 2999/00 20130101;
B22F 9/22 20130101; B22F 2998/10 20130101; B22F 2999/00 20130101;
B22F 2998/10 20130101; B22F 2201/10 20130101; B22F 9/22 20130101;
B22F 9/24 20130101; B22F 2201/013 20130101; B22F 3/1143 20130101;
B22F 9/22 20130101; B22F 3/1143 20130101; B22F 2999/00 20130101;
C22B 5/12 20130101; B22F 2201/04 20130101 |
Class at
Publication: |
075/415 |
International
Class: |
C22B 5/12 20060101
C22B005/12 |
Goverment Interests
[0002] The United States Government has rights in this invention
pursuant to Contract No. W-7405-ENG48 between the United States
Department of Energy and the University of California for the
operation of Lawrence Livermore National Laboratory.
Claims
1. A method for producing porous nanostructured pyrophoric metal,
comprising: forming a solution containing at least one
hydrated-metal ion inorganic salt and at least one solvent;
adjusting the pH of said solution with a proton scavenger to induce
gel formation of said solution to form a nanostructured
metal-oxide-based gel; drying said nanostructured metal-oxide-based
gel to form a nanostructured metal-oxide-based porous material;
thermally treating said nanostructured metal-oxide-based porous
material to produce a thermally treated nanostructured
metal-oxide-based porous material; and heating said thermally
treated nanostructured metal-oxide-based porous material in the
presence of a chemical reductant diluted with an inert carrier gas
to produce a porous nanostructured pyrophoric metal.
2. The method of claim 1, wherein said nanostructured
metal-oxide-based porous material is selected from the group
consisting of a xerogel and an aerogel.
3. The method of claim 1, wherein the step of drying comprises
drying said nanostructured metal-oxide-based gel by atmospheric
evaporation, wherein said nanostructured metal-oxide-based porous
material comprises nanostructured metal-oxide-based xerogel.
4. The method of claim 1, wherein the step of drying comprises
drying said nanostructured metal-oxide-based gel by super critical
solvent extraction, wherein said nanostructured metal-oxide-based
porous material comprises nanostructured metal-oxide-based
aerogel.
5. The method of claim 1, wherein the step of thermally treating
said nanostructured metal-oxide-based porous material includes
removing at least one impurity from said nanostructured
metal-oxide-based porous material
6. The method of claim 1, wherein the step of thermally treating
said nanostructured metal-oxide-based porous material includes
removing at least one surface bound chemical species from said
nanostructured metal-oxide-based porous material.
7. The method of claim 1, wherein the step of thermally treating
said nanostructured metal-oxide-based porous material is carried
out while said nanostructured metal-oxide-based porous material is
under a dynamic vacuum.
8. The method of claim 1, wherein said chemical reductant is
selected from a group consisting of hydrogen gas (H.sub.2) and
carbon monoxide (CO).
9. The method of claim 1, wherein said porous nanostructured
pyrophoric metal comprises porous nanostructured pyrophoric
iron.
10. The method of claim 9, wherein said at least one hydrated-metal
ion inorganic salt comprises Fe (III) salt.
11. The method of claim 10, wherein said Fe (III) salt is selected
from the group consisting of Ferric nitrate nonahydrate,
Fe(NO.sub.3).sub.3.9H.sub.2O, ferric chloride hexahydrate,
FeCl.sub.3.6H.sub.2O, and FeCl.sub.3
12. The method of claim 9, wherein said at least one solvent is
selected from the group consisting of ethanol (200 proof),
1-propanol, t-butanol, acetonitrile, water (distilled), ethyl
acetate, 2-ethoxy ethanol, N,N-dimethylformamide, methanol,
tetrahydrofuran (THF), acetone, ethylene glycol, propylene glycol,
formamide, 1,4-dioxane, benzyl alcohol, nitrobenzene, hexanes, and
dimethyl sulfoxide (DMSO).
13. The method of claim 9, wherein said at least one solvent is
selected from the group consisting of ethanol and water.
14. The method of claim 13, wherein said of ethanol is about 200
proof and said water is distilled.
15. The method of claim 9, wherein said nanostructured
metal-oxide-based gel comprises Fe.sub.2O.sub.3 gel.
16. The method of claim 9, wherein the step of forming a solution
is carried out under ambient conditions.
17. The method of claim 10, wherein said solution comprises about
0.65 g of Fe(NO.sub.3).sub.3.9H.sub.2O (1.6 mmol) dissolved in 3.25
mL of 200 proof ethanol.
18. The method of claim 17, wherein the step of adjusting the pH
comprises providing an epoxide/Fe ratio of about 11.
19. Porous nanostructured pyrophoric metal produced by a method
comprising: forming a solution containing at least one
hydrated-metal ion inorganic salt and at least one solvent;
adjusting the pH of said solution with a proton scavenger to induce
gel formation of said solution to form a nanostructured
metal-oxide-based gel; drying said nanostructured metal-oxide-based
gel to form a nanostructured metal-oxide-based porous material;
thermally treating said nanostructured metal-oxide-based porous
material to produce a thermally treated nanostructured
metal-oxide-based porous material; and heating said thermally
treated nanostructured metal-oxide-based porous material in the
presence of a chemical reductant diluted with an inert carrier gas
to produce a porous nanostructured pyrophoric metal.
20. The porous nanostructured pyrophoric metal of claim 19, wherein
said nanostructured metal-oxide-based porous material is selected
from the group consisting of a xerogel and an aerogel.
21. The porous nanostructured pyrophoric metal of claim 19, wherein
said porous nanostructured pyrophoric metal comprises porous
nanostructured pyrophoric iron.
22. The porous nanostructured pyrophoric metal of claim 21, wherein
said at least one hydrated-metal ion inorganic salt comprises Fe
(III) salt.
23. The porous nanostructured pyrophoric metal of claim 22, wherein
said Fe (III) salt is selected from the group consisting of Ferric
nitrate nonahydrate, Fe(NO.sub.3).sub.3.9H.sub.2O, ferric chloride
hexahydrate, FeCl.sub.3.6H.sub.2O, and FeCl.sub.3
24. The porous nanostructured pyrophoric metal of claim 21, wherein
said at least one solvent is selected from the group consisting of
ethanol (200 proof), 1-propanol, t-butanol, acetonitrile, water
(distilled), ethyl acetate, 2-ethoxy ethanol,
N,N-dimethylformamide, methanol, tetrahydrofuran (THF), acetone,
ethylene glycol, propylene glycol, formamide, 1,4-dioxane, benzyl
alcohol, nitrobenzene, hexanes, and dimethyl sulfoxide (DMSO).
25. The porous nanostructured pyrophoric metal of claim 21, wherein
said at least one solvent is selected from the group consisting of
ethanol and water.
26. The porous nanostructured pyrophoric metal of claim 21, wherein
said nanostructured metal-oxide-based gel comprises Fe.sub.2O.sub.3
gel.
27. The porous nanostructured pyrophoric metal of claim 22, wherein
said solution comprises about 0.65 g of
Fe(NO.sub.3).sub.3.9H.sub.2O (1.6 mmol) dissolved in 3.25 mL of 200
proof ethanol.
Description
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 60/583,155, filed Jun. 24, 2004, titled:
"Preparation of Porous Pyrophoric Iron Using Sol-Gel Methods,"
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to sol-gel chemistry, and more
specifically, it relates to sol-gel methods for producing porous
pyrophoric iron.
[0005] 2. Description of Related Art
[0006] Pyrotechnics can be grouped into six families; decoy flares,
illuminating flares, colored flares, smokes, igniters/starters and
miscellaneous pyrotechnic items. Decoy flares include infrared (IR)
and solid pyrophoric flares. Aircraft pyrophoric decoy flares are
solid pyrotechnic devices ejected as a precautionary measure or in
response to a missile warning system. The most significant
requirement of the device is that it develops a high-intensity,
characteristic signature, rapidly. In order to meet this
requirement, the energy radiated by the flare is typically provided
by a pyrotechnic reaction. Pyrotechnic compositions have been shown
to provide high energy densities and reasonable storage life at
moderate cost. The most common composition of a conventional
pyrotechnic flare consists of pyrophoric iron. This composition
provides the high energy density required for the decoy and also
produces solid combustion products for good radiation efficiency.
The net reaction is shown below: 2Fe(s)+
3/2O.sub.2.fwdarw.Fe.sub.2O.sub.3 (s)+heat
[0007] Decoy materials of this composition undergo the above
reaction to reach temperatures of 820.degree. C. in less than one
second and above 750.degree. C. for twelve seconds after their
exposure to air. The thermal response can be increased or decreased
with the addition of metals that undergo very exothermic reactions
when heated in air (e.g., B, Al, Zr, Ti) or inert metal oxides
(e.g., SiO.sub.2, Al.sub.2O.sub.3), respectively.
[0008] The current pyrophoric decoy flare is composed of pyrophoric
iron coated onto steel foil. The pyrophoric iron coating is
prepared by mixing Fe and Al powders in a slurry containing a
suitable solvent and binder. A very thin steel foil is then coated
with the slurry by either dip coating or spraying. The resulting
material is then rapidly heated to 500.degree. C. to drive off the
solvent and binder to yield a coating of the metallic powders. The
coated substrate is then heated to relatively high temperatures
(.about.800-1000.degree. C.) in both H.sub.2 and Ar atmospheres to
from an iron/aluminum alloy. The resulting alloy can be leached
with a hot (.about.100-200.degree. F.) caustic aqueous solution of
10-20% sodium hydroxide (by mass) to leach the aluminum from the
alloy and render the remaining iron porous and highly pyrophoric.
Some patent processes claim that use of stannite (dissolved as
SnCl.sub.2 or Sn(s)) in the aqueous leaching solution increases the
activity (i.e., makes the iron more pyrophoric) and the lifetime of
the active decoy. There are several variations of the described
manufacturing technique that allow the preparation of the
pyrophoric iron as a powder or a coating on a metal foil.
Pyrophoric foils are particularly attractive for their ability to
be dispersed from the aircraft in a cloud-like pattern. The high
surface area to mass ratio of the foils requires that they flutter
after being ejected from the aircraft and take on the appearance of
a moving hot cloud when several decoys are ejected in rapid
succession. This signal is attractive to the IR-seeking missile.
Current pyrophoric decoy composition and performance can be
modified through manipulation of the manufacturing process.
[0009] Having a small amount of a substance in intimate contact
with the pyrophoric iron that undergoes an exothermic reaction when
heated can increase the pyrophoric action of the decoy flare
material. Metals, such as boron or titanium, can be added to the
pyrophoric foils to achieve this desired result. Alternatively, the
pyrophoric iron can be coated with aqueous solutions of
commercially available alumina or silica sol that coat the porous
base metal. The inert oxide coating blocks O.sub.2 from getting to
the iron too rapidly and hence slows down the burn rate and makes
the pyrophoric response of the material less intense. The
pyrophoric iron generated by the above processes can be stored in
solvents such as acetone, ethanol, and methanol, under certain
conditions, with little loss in their pyrophoric performance.
Although this process is well documented and provides functional
and effective pyrotechnic flares it can and should be improved. The
current process relies heavily upon the use of hot caustic leaching
solutions to prepare the high surface area porous pyrophoric Fe
metal. These solutions are corrosive and represent both a safety
and environmental hazard.
[0010] Magnus reported that pyrophoric iron could be generated from
reduction of iron compounds in a stream of H.sub.2 at relatively
low temperatures (360-420.degree. C.) as early as 1825. Since then
many researchers have repeated this result using the iron (III)
oxides as the iron-containing reagent.
[0011] Sol-gel chemistry utilizes the hydrolysis and condensation
of molecular chemical precursors, in solution, to produce
nanometer-sized primary particles, called "sols". Through further
condensation the "sols" are linked to form a three-dimensional
solid network, referred to as a "gel", with the solvent liquid
present in its pores. Evaporation of the liquid phase results in a
dense porous solid referred to as a "xerogel". Supercritical
extraction of the pore liquid eliminates the surface tension of the
retreating liquid phase and results in solids called, "aerogels".
Sol-gel materials are distinctive in that they typically posses
high surface areas, high porosities and small primary particle
size. The properties unique to sol-gel materials lead to their
enhanced reactivity. Therefore, sol-gel chemical routes are very
attractive because they offer low temperature routes to synthesize
homogeneous materials with variable compositions, morphologies, and
densities. A schematic representation of the sol-gel process and
materials is shown in FIG. 1.
[0012] Scientists at the Naval Research Laboratory have prepared
and characterized thermally emitting aerogels. Iron metal was
deposited into the framework of silica, resorcinol-formaldehyde,
and carbon aerogel materials using a metal organic chemical vapor
deposition (MOCVD) system. One aerogel, the iron doped-carbon
material, was a strong thermal emitter and burned at
600-700.degree. C. The results shown in this study are encouraging
that sol-gel techniques can be used to prepare thermal emitters.
However, the iron precursor used in the MOCVD process, iron
pentacarbonyl, is highly pyrophoric and toxic.
[0013] Production of pyrophoric iron in a simple and safe manner
would be advantageous from a safety and environmental point of
view.
SUMMARY OF THE INVENTION
[0014] It is an object of the present invention to provide
"sol-gel" chemical techniques that can be used in environmentally
acceptable solvents to prepare high surface area porous iron (III)
oxides.
[0015] It is another object of the invention to provide a "sol-gel"
methodology for producing nanostructured energetic materials while
minimizing or eliminating the health and environmental hazards
associated with their current fabrication.
[0016] These and other objects will be apparent to those skilled in
the art based on this disclosure.
[0017] The present invention provides combined sol-gel and elevated
temperature processing methods for producing porous nanostructured
pyrophoric metals. One or more hydrated-metal ion inorganic salts
and one or more solvents are combined in a solution. The pH of the
solution is adjusted with a proton scavenger to induce gel
formation. This results in the formation of a nanostructured
metal-oxide-based gel, which is then dried by one of two methods.
To produce xerogel, the gel is dried by atmospheric evaporation. To
produce a nanostructured metal-oxide-based aerogel, the gel is
dried by super critical solvent extraction. The dried
nanostructured metal-oxide-based material (aerogel or xerogel) is
then treated thermally, under a dynamic vacuum, to remove any
impurities or surface bound chemical species. A reduction step
includes heating the thermally treated material in the presence of
a chemical reductant (e.g., hydrogen gas H.sub.2, or carbon
monoxide (CO)) diluted with an inert carrier gas, to an elevated
temperature.
[0018] Highly porous sol-gel derived iron (III) oxide materials can
be reduced to sub-micron-sized metallic iron by heating the
materials to intermediate temperatures in a hydrogen atmosphere.
Through a large number of experiments, complete reduction of the
sol-gel based materials was realized with a variety of
hydrogen-based atmospheres (25-100% H.sub.2 in Ar, N.sub.2,
CO.sub.2, or CO) at intermediate temperatures (350.degree. C. to
700.degree. C. Sol-gel-derived metallic iron powders that were
produced were ignitable by thermal methods. The present invention
teaches techniques for producing sol-gel-derived metallic iron
powders that are pyrophoric. For comparison several types of
commercial micron sized iron oxides (Fe.sub.2O.sub.3, and
NANOCAT.TM.) were also reduced under identical conditions. All
resulting materials were characterized by thermal gravimetric
analysis (TGA), differential thermal analysis (DTA), powder X-ray
diffraction (PXRD), as well as scanning and transmission electron
microscopies (SEM and TEM). In addition, the reduction of the iron
oxide materials was monitored by TGA. In general the sol-gel
materials were more rapidly reduced to metallic iron and the
resulting iron powders had smaller particle sizes and were more
easily oxidized than the metallic powders derived from the micron
sized materials. Impurities in the smaller fine metallic powders
can prevent pyrophoricity if a passivation layer is on the
iron.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The accompanying drawings, which are incorporated into and
form part of this disclosure, illustrate embodiments of the
invention and together with the description, serve to explain the
principles of the invention.
[0020] FIG. 1 illustrates a sol-gel process and materials.
[0021] FIG. 2 illustrates pseudomorphic transition of iron oxide
aerogel material.
[0022] FIG. 3 shows a SEM image of iron (III) oxide aerogel
starting material.
[0023] FIG. 4 shows a TGA trace following reduction of iron (III)
oxide aerogel with H.sub.2.
[0024] FIG. 5 shows the FT-IR spectra of "as-made" and heat-treated
iron oxide aerogel.
[0025] FIG. 6 shows PXRD patterns of products from reduction of
iron oxide aerogels.
[0026] FIG. 7 shows a TEM image of metallic Fe from reduction of
sol-gel materials.
[0027] FIGS. 8A-C show SEM images of metallic Fe powder from
reduction of sol-gel.
[0028] FIG. 9 shows a TEM image of heat-treated iron (III) oxide
aerogel.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The invention demonstrates that "sol-gel" chemical
techniques can be used in water-, or another environmentally
acceptable solvent, based processing to prepare high surface area
porous iron (III) oxides. These materials can then be reduced using
molecular hydrogen, at elevated temperatures, to produce high
surface area porous pyrophoric iron metal. This material will be
used to provide a decoy with comparable performance characteristics
to that currently used without the environmental and health
concerns of using hot caustic leaching solutions that are needed in
the conventional production process of pyrophoric decoys.
Alternatively, "sol-gel" techniques can also be used to immobilize
the pyrophoric iron generated by reduction of the sol-gel-derived
iron (III) oxides, or from some alternative source, in an inert
matrix, which can be cast to parts with a variety of shapes and
sizes. This second approach allows the resulting pyrophoric
pyrotechnic to be easily and desirably released, as well as to have
the versatility to control the composition of the matrix and tailor
the material to provide a specific output response. The sol-gel
approach enables high control over chemical compositions and
reaction rates of energetic materials and that the process is
safe.
[0030] A "sol-gel" methodology is used for producing nanostructured
energetic materials (i.e., pyrotechnics) while minimizing or
eliminating the health and environmental hazards associated with
their current fabrication. This sol-gel approach for preparing
pyrotechnic formulations involves a fundamental change in the
conventional manufacturing and fabrication processes of energetic
materials. One particular application of this methodology can be
used to eliminate the use of caustic leaching solutions associated
with pyrophoric decoy flare manufacture, while maintaining or
improving performance of the final products. Low temperature
reduction of high surface area porous sol-gel-derived iron (III)
oxide with molecular hydrogen results in the formation of porous
pyrophoric iron metal, suitable for use in pyrophoric decoy flares.
Processing and preparation with environmentally acceptable media
under neutral conditions replaces the process currently used in
pyrophoric flare manufacturing.
[0031] Preparation of Fe.sub.2O.sub.3 gels from Fe(III) salts.
Ferric nitrate nonahydrate, Fe(NO.sub.3).sub.3.9H.sub.2O, ferric
chloride hexahydrate, FeCl.sub.3.6H.sub.2O, and FeCl.sub.3 salts
were obtained from Aldrich Chemical Co. and used as received. The
synthesis of Fe.sub.2O.sub.3 nanostructured gels was performed in
the following solvents, all of which were reagent grade or better:
ethanol (200 proof; Aaper), 1-propanol (J. T. Baker), t-butanol (J.
T. Baker), acetonitrile (EM Science), water (distilled), ethyl
acetate (Mallinckrodt), 2-ethoxy ethanol (Chemical Samples Co.),
N,N-dimethylformamide (Fluka), and the methanol, tetrahydrofuran
(THF), acetone, ethylene glycol, propylene glycol, formamide,
1,4-dioxane, benzyl alcohol, nitrobenzene, hexanes, and dimethyl
sulfoxide (DMSO) were all from Aldrich Chemical Co. Some of these
solvents, in particular ethanol and water, are highly desirable for
this process as they are environmentally benign compared to
solvents currently utilized such as acetone and benzene. The
propylene oxide was also obtained from Aldrich Chemical Co. All
syntheses were performed under ambient conditions.
[0032] In a typical experiment, 0.65 g of
Fe(NO.sub.3).sub.3.9H.sub.2O (1.6 mmol) was dissolved in 3.5 mL of
200 proof ethanol to give a clear red-orange solution that remained
unchanged upon storage, under room conditions, for several months.
If instead, a 1.0 g portion of propylene oxide (17 mmol; propylene
oxide/Fe=11) was added to the solution there was rapid (<1 min.)
color change as the solution became an intense dark red-brown
color. With time, the solution transformed into a rigid red-brown
gel. Gel formation usually occurred within several minutes. Unless
otherwise stated, all synthesis experiments used 3.5 mL of solvent,
[Fe(III)]=0.35 M, and an epoxide/Fe ratio of 11.
[0033] As an example of one embodiment of the present invention,
the processing of Fe.sub.2O.sub.3 nanostructured gels is described.
Aerogel samples were processed in a Polaron.TM. supercritical point
drier. The solvent liquid in the wet gel pores was exchanged for
CO.sub.2(I) for 3-4 days, after which the temperature of the vessel
was ramped up to .about.45.degree. C., while maintaining a pressure
of .about.100 bars. The vessel was then depressurized at a rate of
about 7 bars per hour. For aerogel processing we preferred to use
polyethylene vials to hold the gels during the extraction process.
This was done because much less monolith cracking was observed than
when Fe.sub.2O.sub.3 gels were processed in glass vials.
[0034] Xerogel samples were processed by drying in a fume hood at
room temperature for 14-30 days. Under these conditions high vapor
pressure solvents, like ethanol, were evaporated and the wet gels
were converted to xerogels.
[0035] A 1 gram portion of dried iron oxide aerogel was heat
treated under a vacuum atmosphere at 150.degree. C. Measurements
were preformed on an iron oxide nanostructured gel. That sample was
heated to 375.degree. C. while having an atmosphere consisting of
50% argon and 50% hydrogen flowing through the reactor. The oxide
can be reduced in pure H.sub.2 or mixtures of Ar/H.sub.2 and
N.sub.2/H.sub.2. For the same heating rates, the rate of reduction
was found to vary with the concentration of H.sub.2 in the gas
mixture, thus the reduction does not appear to be diffusion
limited. The sample had a weight loss of 48% at temperatures above
350.degree. C. According to calculations, based on the elemental
analysis of the nanostructured iron (III) oxide, this weight loss
corresponds to complete conversion of the metal oxide material to
iron metal. This is to be expected if the iron (III) oxide species
is converted quantitatively to Fe metal.
[0036] The powder X-Ray diffraction patterns were obtained using a
Siemens DIFF500 diffractometer. The sol-gel nanostructured
materials are amorphous to X-rays. For the sample that showed a
48-wt % loss, the X-ray diffraction pattern show the material to
consist of metallic iron. That is to say that the starting porous
iron (III) oxide was chemically transformed to iron metal. The
surface area of this material was 27 m.sup.2/g. This value
indicates that the material is still porous and has a high surface
area, characteristics that lend themselves to the pyrophoric nature
of the material. It also suggests that the nanoporous nature of the
metal oxide is maintained upon reduction.
[0037] Sol-gel techniques can be used to produce a substrate for
immobilization of the pyrophoric material in the flare and allow
suitable dispersion when deployed. The extremely versatile nature
of sol-gel chemistry allows for the reformulation of materials that
is not possible or practical with conventional systems, to allow
decoy flares with special features to be readily and safely
prepared. This invention advances the use of nanotechnology in
defense applications. Preliminary work has demonstrated that this
approach enables high control over chemical compositions, particle
size and distribution, and reaction rates and that the process is
safe. Although sol-gel technology has the potential to impact a
number of defense and energy needs, the focus will be on pyrophoric
pyrotechnic needs in decoys. The disclosed process provides a
pyrophoric decoy whose processing and composition is acceptable by
OSHA, EPA, the Clean Air Act, Clean Water Act, and Resource
Recovery Act standards.
[0038] One aspect of the invention includes the reduction of
sol-gel-derived high surface area porous iron (III) oxide aerogels
and xerogels to pyrophoric iron using molecular hydrogen at
elevated temperatures. Since first reported in 1825, many
researchers have found that reduction of powdered ferric oxide with
hydrogen gas at temperatures between 360-600.degree. C. yields
pyrophoric iron. Reduction at temperatures higher than 650.degree.
C. resulted in non-pyrophoric iron. The general hypothesis used to
rationalize these observations is that the temperature of reduction
is low enough so as to reduce the iron species to metallic iron
with minimal sintering of the final product. At the lower
temperatures the movement of atoms to orient them in a dense and
more compact crystalline state is so slow that it does not occur to
an appreciable extent. The result is the production of a very
fine-grained porous Fe(s) powder that ignites upon contact with
air.
[0039] The iron (III) oxides produced using the present method have
surface areas and porosities significantly higher than those
reported previously. The dry porous iron (III) oxides can be
prepared using benign and environmentally acceptable solvents like
water and ethanol, Fe (III) inorganic salts (chloride and nitrate),
and propylene oxide. This material can be reduced to porous iron
metal using hydrogen at temperatures between 360-600.degree. C.
Typical sol-gel particle and pore morphology is shown in FIG. 2
[0040] In FIG. 1 the porous iron (III) oxide is reduced to Fe(s)
while retaining a significant amount of the porous skeletal
framework of the precursor oxide material. This is feasible,
provided that the reduction temperature is kept low enough to
prevent sintering. Certainly there will be some sintering of the
porous solid; however, it is highly likely that material like that
shown on the right side of FIG. 2 would be pyrophoric as it would
have many of the characteristics (e.g., small particle size, high
surface area) of finely divided iron that is pyrophoric. FIG. 3
shows iron (III) oxide sol-gel materials with a microstructure
similar to that described and shown in FIG. 2.
[0041] In the present invention synthesis, the pyrophoric iron is
produced using water or ethanol, Fe (III) salts, propylene oxide,
and hydrogen. This method is not without hazards. The flammable
nature of hydrogen requires that necessary safety steps be taken.
However, hydrogen is both used as a reagent and generated as a
byproduct of the caustic leach process in the current manufacturing
method.
[0042] The catalyzed hydrolysis and condensation of tin alkoxides
in alcoholic media followed by rapid high-temperature supercritical
extraction yields products that combust on exposure to air.
Preliminary investigations have indicated that the high temperature
extraction step results in the reduction of some of the oxide to
high surface area porous pyrophoric tin metal. Porous pyrophoric
metals can therefore be prepared utilizing aspects of the sol-gel
method. The pyrophoric tin oxide could be coated onto a variety of
different substrates for a myriad of energetic needs related to
decoy flares. The application of sol-gel methods to decoy
countermeasure devices extends beyond the preparation of pyrophoric
iron.
[0043] Sol-gel methodology can be utilized to provide an effective
medium for the dispersion of the pyrophoric iron in decoy flares.
Small particle sized native metals can be incorporated into a
sol-gel metal oxide network composite decoy materials can involve
the generation of the pyrophoric iron in the gel matrix in situ.
This has been performed previously on Fe.sub.2O.sub.3-doped
SiO.sub.2 and A.sub.2O.sub.3 gels. Reduction of the dried mixed
Fe.sub.2O.sub.3-doped metal oxide matrix with molecular hydrogen at
elevated temperatures gives nanoparticles of Fe(s) in the oxide
matrix. The particles generally have very small diameters (2-20
nm), high surface areas, and are pyrophoric. The spatial isolation
of Fe.sub.2O.sub.3 particles from one another precludes them from
diffusing together and sintering to larger particles. Numerous
metal oxide/iron (III) oxide gels have been readily prepared using
the epoxide addition method from solutions of mixed Al (III) or Si
(IV) and Fe(III) molecular precursors with high levels of iron. The
matrix oxide will act as a burn rate modifier in these types of
materials as well as a substrate for processing into decoy
parts.
[0044] The rheological properties of the sol allow gels of it to be
cast and processed into a variety of complex and precise sizes and
shapes. It is certain that the composite sols could be cast and
processed to give parts (e.g., thin discs or wafers) that have a
large surface area to mass ratio. Parts with this property respond
to ejection from a moving aircraft by fluttering in the air as do
the conventional pyrophoric decoy foils. In addition, the large
surface area to volume ratio of these composites ensures rapid
diffusion of air into the part and complete ignition of the
pyrophoric iron. The production of thin aerogel or xerogel
SiO.sub.2 discs containing pyrophoric iron is an appropriate way to
achieve a desirable dispersion of the decoy material once deployed.
The pore size of the matrix material is dependent upon its
processing conditions and can be readily varied with some degree of
precision. This might allow the preparation of decoy flares with
varied burn rates. For example, for faster burn rates one would
employ processing conditions that yielded larger pores and
conversely smaller pore sizes may result in slower burn times.
[0045] Iron (III) oxide aerogel and xerogel materials used in this
study were made from the salts FeCl.sub.3.6H.sub.2O and
Fe(NO.sub.3).sub.3.9H.sub.2O using sol-gel techniques described
elsewhere. As a control, samples of Fe.sub.2O.sub.3 (Aldrich) and
NANOCAT.TM. (a commercial source of 3 nm particle size amorphous
iron (III) oxide) were also reduced.
[0046] A thermogravimetric analyzer (TGA) was set up to accommodate
and monitor the reduction of iron (III) oxide aerogels. TGA
measurements were performed using a Cahn model 141 TGA balance.
Measurements were preformed on two iron oxide nanocomposites. The
oxides can be reduced in pure H.sub.2 or mixtures of Ar/H.sub.2 and
N.sub.2/H.sub.2. For the same heating rates, the rate of reduction
was found to vary with the concentration of H.sub.2 in the gas
mixture, thus the reduction does not appear to be diffusion
limited. The samples decrease to about 52-wt % for maximum
temperatures above 350.degree. C. This weight loss corresponds to 3
moles of oxygen to 1 mole of iron in the starting composite,
assuming only the presence of iron and oxygen. The reduced material
displays a small amount of weight gain on cooling, probably due to
the surface re-oxidation from reaction with water in the gases.
Background measurements confirm that this increase is not due to
buoyancy changes in the system.
[0047] The powder X-Ray diffraction patterns were obtained using a
Siemens DIFF500 diffractometer. The iron oxide aerogel materials
are amorphous. For samples reduced to 52-wt %, the patterns show
the presence of metallic iron. For samples not fully reduced, the
patterns show the presence of Fe.sub.3O.sub.4. Fourier
transform-infrared (FTIR) spectra were collected on pressed pellets
containing KBr (IR-grade) and a small amount of solid sample. The
spectra were collected with a Polaris.TM. FTIR spectrometer.
[0048] Surface area and pore volume and size analyses were
performed by BET (Brunauer-Emmet-Teller) methods using an ASAP 2000
Surface Area Analyzer (Micromeritics Instrument Corporation).
Samples of approximately 0.1-0.2 g were heated to 200.degree. C.
under vacuum (10.sup.-5 Torr) for at least 24 hours to remove all
adsorbed species. Nitrogen adsorption data was taken at five
relative pressures from 0.05 to 0.20 at 77K, to calculate the
surface area by BET theory.
[0049] Scanning electron microscopy (SEM) was carried out using a
Hitachi S-4500 cold field emission SEM. Typical accelerating
voltages used for aerogel samples ranged from 1.8-6 kV and depended
on sample conductivity. No sample preparation (i.e., coating with
conductive layer of Au) was performed on the samples. The SEM
micrographs showed the products of the TGA reduction to consist of
large (>100 micron) porous chunks. These pieces were found to
consist of clusters of smaller particles of about 200 nm. The
transmission electron microscopy (TEM) was performed on a Philips
CM300FEG operating at 300 kev using zero-loss energy filtering with
a Gatan energy Imaging Filter (GIF) to remove inelastic scattering.
The images where taken under BF (bright field) conditions and
slightly defocused to increase contrast. The images were also
recorded on a 2K.times.2K CCD camera attached to the GIF.
[0050] Consideration of the composition and phase of the initial
iron (III) oxide material is extremely important. The presence of
trace impurities can affect the properties of materials
dramatically. To complicate the situation, there are thirteen known
phases of iron oxides, each having distinct properties and chemical
characteristics. A sol-get derived iron (III) oxide material made
by the present method was determined to consist mainly of the
compound Ferrihydrite, Fe.sub.5HO.sub.8.4H.sub.2O (Formula
Weight=480 g/mol). This is a highly hydrated poorly crystalline
iron (III) oxide phase that was determined by XRD. Elemental
analyses on the sol-gel derived materials indicated significant
levels of carbon (2-6 wt %) and hydrogen (1-3 wt %) and chloride
(1-5%; note chloride only present in the samples made from
FeCl.sub.3.6H.sub.2O).
[0051] The central hypothesis of this work was to demonstrate that
sol-gel iron (III) oxide materials would be reduced by molecular
hydrogen to metallic iron while maintaining the small particle size
and porosity that are characteristics of the reactant sol-gel
material. To demonstrate this a number synthetic experiments were
performed. The objective being to optimize the synthetic and
processing conditions that would result in Fe production, while at
the same time minimizing the concentration of hydrogen needed as
well as keeping the reduction temperature low. This objective
serves two purposes: 1) safety and 2) materials performance.
[0052] By using the minimum amount of H.sub.2 (mixed with an inert
gas like Ar or N.sub.2) to affect the reduction, any hazards
associated with the use of pure hydrogen would be diminished. The
use of the lowest possible reduction temperature would minimize all
of the processes involved in sintering (surface, vapor, and volume
diffusion). This leads to the production of elemental Fe that
retains the high surface area porous structure of the reactant iron
(III) oxide sol-gel material. Such a material would be
pyrophoric.
[0053] As can be seen in Table 1 numerous synthetic experiments
were run to determine optimal conditions for the reduction of iron
(III) oxide aerogel to porous Fe metal. Parameters that were varied
included the composition of the reduction gas, the heating rate,
and temperature.
[0054] For the sake of brevity, and in accordance with our
objective, the synthesis results will be summarized. Samples of
iron (III) oxide aerogel can be reduced to metallic Fe in the
following reducing atmospheres at given temperature ranges: 25-100%
H.sub.2 in Ar or N.sub.2 at temperatures between 350.degree. C. and
700.degree. C., 75% H.sub.2/25% CO.sub.2 at 700.degree. C., and 75%
H.sub.2/25% CO at 650.degree. C. Table 1 also contains the final
weight percent as well as the weight percent at maximum temperature
during the reduction process. These values were monitored by TGA
and will be discussed shortly.
[0055] From the entries in Table 1 it is apparent that hydrogen
levels below .about.20% are not sufficient to bring about complete
reduction of the sol-gel iron (III) oxide material. Even at
temperatures up to 600.degree. C. the reaction does not go to
completion at lower hydrogen concentration levels (2.5% H.sub.2).
That temperature is significantly higher than those reported in
patents from several decades ago. Certainly lower reduction
temperatures would be more desirable (e.g., 200-400.degree. C.)
[0056] It appears that there is a temperature threshold for
complete reduction. For example at 300.degree. C. and 100% H.sub.2
the reduction is incomplete. However, at 350.degree. C. the
reaction goes to completion. From the entries in Table 1 it is not
clear that heating rate affects the reaction to a discernable
degree. TABLE-US-00001 TABLE 1 This is a summary of experimental
conditions for experiments run to reduce iron (III) oxide aerogel
with H.sub.2. Wt % Heating Final at Rate Pro- Com- Temp Atmosphere
Wt % max T (C./min.) ducts ments 600 2.5% H.sub.2/N.sub.2 68.4 5 Fe
Reduc- tion not complete 600 2.5% H.sub.2/N.sub.2 55 5 600 2.5%
H.sub.2/N.sub.2 68.5 5 Fe3O4 Reduc- tion not complete 600 2.5%
H.sub.2/N.sub.2 71.4 5 Fe3O4 Reduc- tion not complete 600 2.5%
H.sub.2/N.sub.2 50.6 5 300 2.5% H.sub.2/N.sub.2 75.9 5 Fe + Reduc-
Fe3O tion not complete 400 2.5% H.sub.2/N.sub.2 72.3 5 Fe Reduc-
tion not complete 500 2.5% H.sub.2/N.sub.2 59.5 5 600 5.0%
H.sub.2/N.sub.2 51.9 5 550 5.0% H.sub.2/N.sub.2 52.1 5 350 18%
H.sub.2/Ar 54.1 5 Fe 375 25% H.sub.2/Ar 55.2 55.2 5 Fe 375 50%
H.sub.2/Ar 55.9 55.9 5 Fe 375 50% H.sub.2/Ar 56.5 55.9 5 400 50%
H.sub.2/Ar 56.7 56.4 5 400 75% H.sub.2/Ar 54.8 54.4 5 400 100%
H.sub.2 57.2 56.6 5 500 100% H.sub.2 53.9 55.4 5 600 100% H.sub.2
53.9 53.7 5 600 100% H.sub.2 53.7 53.6 5 Fe 500 100% H.sub.2 54
54.2 5 500 25% H.sub.2/Ar 52 50.9 5 400 25% H.sub.2/Ar 51.8 52 10
400 25% H.sub.2/Ar 55.6 52 20 400 100% H2 56.6 52.7 1 400 100% H2
56.9 54.8 0.5 Fe 350 100% H.sub.2 58.3 53.5 0.5 Fe 350 100% H.sub.2
55.6 55.2 1 Fe 350 50% H.sub.2/Ar 56 55.4 0.5 350 25% H.sub.2/Ar 56
55.4 0.5 400 100% H.sub.2 56.2 54.3 0.5 400 50% H.sub.2/N.sub.2 60
52.3 2 400 50% H.sub.2/N.sub.2 53.1 48.9 20 400 50% H.sub.2/N.sub.2
62.1 58.6 25 700 75% H.sub.2/CO.sub.2 56.9 55.6 10 400 100% H.sub.2
57.3 54.6 2.5 700 100% H.sub.2 52.6 51 2.5 Fe 300 100% H.sub.2 79.7
79.1 2.5 Reduc- tion not complete
[0057] Thermogravimetric analysis (TGA) proved to be a very
valuable technique for monitoring the progress of the reduction
reaction. This technique permits monitoring of the extent of
reaction with time and thus, determines when the reaction is
completed. When iron oxide is reduced to metallic iron and water
(see FIG. 1), at elevated temperatures, there is a net loss in the
solid mass of the system. By monitoring the mass of the sample
under reducing conditions, the onset and end of the chemical
reactions that produce the transformation can be determined. Once
the mass loss levels off, one can reasonably conclude that the
reaction has gone to completion under those conditions. In
addition, by examining any weight gain of the sample as it cools,
the fidelity of the experimental system to atmospheric impurities
(e.g., O.sub.2 or water) can be evaluated. If the system is
pristine there should be no weight change. If there is a source of
contamination (e.g., a leak) the sample will oxidize, and likely
passivate, and as such a weight gain would be observed.
[0058] FIG. 4 shows a typical TGA trace for the reduction of an
iron (III) oxide aerogel in a 100% hydrogen atmosphere at
700.degree. C. From this TGA trace it can be seen that the mass
loss levels out after about four hours under these conditions.
Therefore, it can be inferred that the reduction is complete at
this time. Inspection of the region of the weight loss/gain curve
in the cooling region (from 10-16 hours) indicates a weight gain of
.about.1.5%. It is difficult to assign any significant meaning to
this weight gain, as it is very slight
[0059] It was observed that the sol-gel derived iron (III) oxide
materials had mass losses of between 44-48% weight percent upon
completion of the reduction. Again completion was inferred from a
lack of weight gain after prolonged reaction time. If one considers
the iron (III) oxide aerogel material to be Ferrihydrite, the
reduction of this iron oxide phase to metallic iron would result in
a 42% mass loss. Fe.sub.5HO.sub.84H.sub.2O (F.W.=480
g/mol).fwdarw.5 Fe (F.W.=55.9 g/mol)+H.sub.2O
[0060] As previously stated, elemental analyses indicate a
background level of organic contaminant (C and H) of 4-9 wt %.
Talking this into account, as well as the reduction mass loss and
the dehydration, the expected weight loss of the iron (III) aerogel
should be range from 46 to 51 weight percent. It is therefore
reasonable to infer that the mass losses seen in these experiments
and tabulated in Table 1 are consistent with the reduction and
dehydration of Ferrihydrite. The presence of water and hydrocarbon
based impurities in the base iron (III) oxide aerogel material was
also confirmed using Fourier-Transform infrared (FT-IR)
spectroscopy.
[0061] FIG. 5 is an overlay of the FTIR spectra of the iron (III)
oxide aerogel and its vacuum dried (200.degree. C.) product. The
spectrum of the "as-is" aerogel (Trace A in FIG. 5) contains
several prominent absorptions. The intense and broad absorption in
the 3200-3600 cm.sup.-1 region likely corresponds to .nu.(O--H)
stretching vibrations of adsorbed water (sample was synthesized,
stored, and FTIR spectrum was taken under room conditions) and O--H
moieties present in the solid. In addition, the absorption at
.about.1630 cm.sup.-1 is likely due to the bending mode of water
.delta.(H.sub.2O). The presence of O--H groups in the IR of iron
(III) oxides synthesized by solution methods is very common.
[0062] The absorptions present at 2800-3000 cm.sup.-1 are due to
.nu.(C--H) vibrations. These, as well as the absorptions present
from 1400-800 cm.sup.-1 are probably due to ethanol (solvent used),
residual propylene oxide, or side products of the ring opening of
the propylene oxide. The propylene oxide is used in the synthesis
of the aerogel materials as a gelation agent. The absorptions
between 700 cm.sup.-1 and 500 cm.sup.-1 are those from the Fe--O
linkages that make up the framework of the aerogel. All of the
phases of iron oxides and oxyhydroxides have characteristic IR
vibrations in this region. The assignment of the spectrum shown in
Trace B of FIG. 5 to one particular phase of iron oxide is not
straightforward. Notwithstanding, with the FTIR evidence shown here
we tentatively conclude that the non-heat-treated aerogel material
is probably an iron oxyhydroxide phase.
[0063] The spectrum shown in Trace B of FIG. 5 is that of the
aerogel material that has been heated to 200.degree. C. under a
dynamic vacuum. This heat treatment results in a mass loss of
.about.30% of the material. There are three notable differences
between this spectrum and that of the iron (III) oxide aerogel.
First, the absorption in the 3200-3600 cm.sup.-1 region of the
spectrum is much less intense in the heat-treated sample. This is
possibly due to the removal of a large percentage of the O--H
moieties present in the original aerogel through condensation of
two neighboring OH groups to give a single oxygen bridge. Second,
there is no trace of the absorptions assigned to C--H bonds present
in the heat-treated sample. These organic constituents have also
been removed in the heating process. And finally, the two intense
absorptions at 510 and 615 cm.sup.-1 in the original aerogel have
shifted and split into three peaks at 565, 585, and 630 cm.sup.-1
respectively. The location of the IR bands present in the
heat-treated sample match very well to those reported for
maghemite, the .gamma.-phase of Fe.sub.2O.sub.3. It is worthwhile
to note that maghemite is magnetic and that the heat-treated
material in Trace B of FIG. 5 is also magnetic.
[0064] The primary analytical tools used to evaluate the relative
success of each experiment to reduce the aerogel materials to
metallic iron were TGA and powder X-ray diffraction. By monitoring
the mass loss or gain under reducing or oxidizing conditions and
knowing the composition of the starting material one could
determine if the reaction went to completion. This has already been
discussed. Analyzing the reaction products by PXRD and comparing
the results to known standards allowed additional confirmation.
[0065] Representative XRD patterns for reaction products are shown
in FIG. 6. The top XRD pattern indicates prominent lines for the
compound Fe.sub.3O.sub.4, magnetite, a well-known magnetic form of
iron oxide in which the iron atoms in the lattice have either a +2
or +3 oxidation state. This compound is often observed as an
intermediate in the reduction of iron (III) oxides to elemental
iron and is representative of incomplete reduction. The bottom XRD
pattern in FIG. 6 has diffraction peaks from metallic iron and is a
fine example of what is observed when reduction is complete.
[0066] Using TGA-monitored reduction several iron oxide based
powders were examined. Powders from commercial sources, different
phases of iron oxides, as well as sol-gel derived aerogels and
xerogels were evaluated. According to the results a sol-gel based
composition, iron (III) oxide aerogel (made with
Fe(NO.sub.3)9H.sub.2O precursor) reduced to metallic iron the most
rapidly under constant conditions (50% H.sub.2/50 Ar @ 450.degree.
C.). This is possibly related to the extremely high surface area of
the aerogel material.
[0067] Simultaneous differential thermal analysis (combination of
TGA and DTA) was shown to be an effective method to monitor the
oxidation of native iron powders produced via this approach. It
appears that iron produced from the reduction of aerogel iron (II)
oxide material oxidizes at .about.340.degree. C. This temperature
is at least 75.degree. C. less than is seen for the oxidation of
iron particles made from commercial Fe.sub.2O.sub.3 (Aldrich)
(T.sub.oxidation.about.415.degree. C.). This is potentially is very
interesting result. It is known that ultra-fine grained Al powders
prepared by vapor phase condensation oxidize at much lower
temperatures than micron sized powders. The UFG grained Al has
shown exceptional enhancement in energy release rates in mixtures
with oxidizers and is currently being examined for a myriad of
applications in energetic compositions.
[0068] To more fully characterize the final Fe metallic powders
both scanning and transmission electron microscopies (SEM and TEM)
were utilized. These methods will allow good characterization of
the particle size, morphology, and distribution of the metallic Fe
products from reduction of sol-gel iron (III) oxide materials. FIG.
7 contains a TEM image of the Fe metal powder product from the
reduction of an iron (III) oxide aerogel material. This TEM image
is a typical image obtained from this analysis and provides a fine
representation of the overall sample analyzed. The sample appears
to consist of nominally spherical particles with a diameter or
approximately 200-500 nm. These diameters are submicron but are
significantly larger than the primary particle size of the aerogel
starting material (.about.5-20 nm). This indicates that significant
sintering has taken place upon transformation. The particles are so
thick that suitable surface imaging with the TEM is difficult. For
a good look at the surface of these types of materials, SEM was
utilized.
[0069] SEM has proven to be a more useful method of surface
characterization of these materials. FIGS. 8A-C show several SEM
images of Fe metallic materials. From these images one can get an
estimation of the nature of the surfaces.
[0070] It appears that the reduced metallic iron powder is made up
of submicron-sized particles. From FIG. 8B it seems that the
metallic iron has retained some of the porosity of its precursor
material.
[0071] The objective of this work was to produce pyrophoric Fe for
decoy flares in a safe and non-toxic manner using sol-gel methods
and materials. Therefore, the phenomenological behavior of the
metallic powders on exposure to the atmosphere is of critical
importance. After reduction in the TGA, samples were cooled and
kept in an inert environment (Ar or N.sub.2). Once cool, the sample
was removed from the TGA apparatus and rapidly exposed to room
atmosphere.
[0072] The fine metallic powders, produced via the described
synthesis and processing conditions, could be burned with the
application of a thermal source (flame, and soldering iron were
used). Once ignited, the powders burned smoothly with a blue flame
and left behind a red residue, a telltale sign of hematite
Fe.sub.2O.sub.3. Initially there was some concern that the sample
size of the powders may not be sufficient to facilitate
self-heating to combustion. That is, the surface area to volume
ratio of a small amount (.about.100-200 mg) of metallic powder may
be high enough that localized heating did not occur to an
appreciable extent; any heat generated by oxidation of the
submicron Fe particles was rapidly dissipated to the surroundings.
To try and mitigate this potential scale effect, larger samples of
sol-gel iron (III) oxide aerogel were reduced in a tube furnace (up
to 2500 mg at a time).
[0073] As a set of control experiments, the same TGA and tube
furnace set up were used to reduce some commercial sources of iron
oxide. Hematite (Fe.sub.2O.sub.3-50 microns) from Aldrich, and
NANOCAT.TM. (a commercial source of 3 nm diameter iron oxide
particles from MACH L Inc., King of Prussia, Pa.) were reduced
under the same conditions as the iron oxide aerogel materials.
These experiments were effective in reducing the oxide to the base
Fe metal.
[0074] The reduction of iron oxide ores to iron, being a major step
in the commercial production of steel, is the subject of an
extremely large number of patents. Many of these patents refer to
processing conditions that leave the final Fe metal in a variety of
forms (e.g., consolidated brick, powders, pellets). These
conditions are particularly important to determine, as they dictate
the final form of the product metal. However, to our knowledge
there are no reports of the conditions needed to effect the
reduction of sol-gel-derived iron oxides to metallic iron. Sol-gel
materials are unique in that they typically posses high surface
areas, high porosities and small primary particle size. The
properties unique to sol-gel materials lead to their enhanced
reactivity. In our estimation, the iron powder products from the
reduction of sol-gel iron oxides may be highly reactive and will be
very useful in applications involving energetic materials.
[0075] One important result of this study was the identification of
optimum reduction conditions for the production of sub-micron Fe
powders from sol-gel derived-iron (III) oxide precursors. Previous
publications indicate that finely divided iron powders can be
pyrophoric. Taking the characterization done here into account,
there is little doubt that the particle sizes of the powders made
by this approach are as, or more finely divided than pyrophoric
powders. The starting materials in those reports were micron-sized
iron oxides. It is very likely that significant agglomeration and
consolidation occurred upon reduction. While in our case, the
reactant oxide particles are much smaller and more highly
porous.
[0076] Iron metal readily reacts with oxygen or water to passivate
its surface and generate heat. With high surface area powders, the
heat generated can be significant enough to ignite the entire iron
particle. These are the processes that lead to the pyrophoric
nature of finely divided iron. However with a suitable oxide
coating the iron particles can be very stable. The oxidation can
come from the interaction of the newly formed Fe surface with water
or O.sub.2 impurities in the reduction gases or with the water
produced as a byproduct of the reduction. Additionally, the sol-gel
material is the compound Ferrihydrite (Fe.sub.5HO.sub.8.4H.sub.2O),
which contains highly levels of water in it
[0077] Percival and co-workers report in a patent issued in 1959 on
the importance of low water content in the system used to reduce
finely divided Fe.sub.2O.sub.3 to pyrophoric iron.
[0078] The reduction of porous, high surface area iron (III) oxide
sol-gel materials gives submicron metallic iron powders. The
production of iron powders via this approach is beneficial from a
safety and environmental standpoint as it eliminates the need for
caustic leaching solutions used in the current production of
pyrophoric decoy flares.
[0079] The present inventors recommend that sol-gel derived
starting materials be heated to and held at elevated temperatures
for some time before starting reduction. Temperatures used will be
high enough to drive off organic impurities as well as any bound or
unbound water without causing the sintering of the porous iron
(III) oxide network. Heat treatment to temperatures below
300.degree. C. lead to contaminant removal and phase changes in the
iron (III) oxide sol-gel material without a significant reduction
in porosity or increase in primary particle size (See FIG. 9).
[0080] It will be useful to use two separate reduction steps. That
is initial reduction followed by cooling and then a second
reduction step. This type of methodology is used in current
production of pyrophoric foils and serves to reduce any last small
amounts of surface oxide on the metallic powders.
[0081] All sol-gel materials used should be derived from
chloride-free precursors. This is to minimize the chance for
residual chloride ions in the solid forming iron chloride species
that are believed to inhibit the pyrophoric nature of the solid. In
the future, residual chloride in the starting material may be used
to tune the degree of pyrophoricity; however, for this initial work
it should be avoided.
[0082] Impurities play a role in the behavior of these materials
and therefore all attempts to make this material must emphasize
rigorous elemental analyses on all reactants and products.
[0083] To increase the pyrophoricity of these materials,
incorporate small amounts of more reactive metals into the final
product powders as an igniter, using sol-gel techniques. Two
specific materials, tungsten and or tin oxide, are of particular
interest. Both tungsten and tin oxide precursors can be
incorporated into the iron oxide sol and gelation will create a
mixed oxide. Upon reduction, after drying, the tin or tungsten
oxide particles will be reduced to their native metal along with
the iron powder. When exposed to air the reactive tungsten or tin
metal will ignite which will help ignite the less reactive Fe
powders. This approach does not make the process any less
acceptable from an environmental and safety standpoint. It has been
already been demonstrated that pyrophoric tin oxide materials can
be made with sol-gel techniques.
[0084] The foregoing description of the invention has been
presented for purposes of illustration and description and is not
intended to be exhaustive or to limit the invention to the precise
form disclosed. Many modifications and variations are possible in
light of the above teaching. The embodiments disclosed were meant
only to explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best use
the invention in various embodiments and with various modifications
suited to the particular use contemplated. The scope of the
invention is to be defined by the following claims.
* * * * *