U.S. patent application number 10/883277 was filed with the patent office on 2006-01-19 for microwave heating of energetic materials.
Invention is credited to Blaine W. Asay, William L. Perry, Steven F. Son.
Application Number | 20060011083 10/883277 |
Document ID | / |
Family ID | 35598077 |
Filed Date | 2006-01-19 |
United States Patent
Application |
20060011083 |
Kind Code |
A1 |
Perry; William L. ; et
al. |
January 19, 2006 |
Microwave heating of energetic materials
Abstract
Mixtures of high explosives with materials that readily absorb
microwaves ignite more readily when exposed to microwave energy
than the corresponding neat explosives. A charge of HMX (0.5 gram)
mixed with carbon nanotubes (1 percent by mass) ignited with 7.5
joules at an average rate of 750 W for 10 milliseconds. To raise a
charge of the same mass of neat HMX to an autoignition temperature
of 200 degrees Celsius would require much more energy (about 110
joules) for a longer duration (about 150 milliseconds).
Inventors: |
Perry; William L.; (Jemez
Springs, NM) ; Son; Steven F.; (Los Alamos, NM)
; Asay; Blaine W.; (Los Alamos, NM) |
Correspondence
Address: |
UNIVERSITY OF CALIFORNIA;LOS ALAMOS NATIONAL LABORATORY
P.O. BOX 1663, MS A187
LOS ALAMOS
NM
87545
US
|
Family ID: |
35598077 |
Appl. No.: |
10/883277 |
Filed: |
June 30, 2004 |
Current U.S.
Class: |
102/205 |
Current CPC
Class: |
C06C 7/00 20130101; C06B
23/001 20130101; B82Y 30/00 20130101 |
Class at
Publication: |
102/205 |
International
Class: |
C06C 9/00 20060101
C06C009/00 |
Goverment Interests
STATEMENT REGARDING FEDERAL RIGHTS
[0001] This invention was made with government support under
Contract No. W-7405-ENG-36 awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
Claims
1. A method for igniting high explosive comprising preparing a
mixture of high explosive having a first dielectric loss and a
material having a second dielectric loss that is higher than the
first dielectric loss of the high explosive, and thereafter
exposing the mixture to microwave energy.
2. The method of claim 1, wherein the explosive is selected from
the group consisting of TNT, PETN, RDX, HMX, TATB, and mixtures
thereof.
3. The method of claim 1, wherein the material that strongly
absorbs microwave energy is selected from the group consisting of
carbon nanotubes, metal powder, semiconductor powder, and mixtures
thereof.
4. The method of claim 3, wherein the metal powder comprises finely
divided powder.
5. The method of claim 3, wherein the semiconductor powder
comprises finely divided powder.
6. The method of claim 3, wherein the mixture comprises about 1
percent by mass or less of carbon nanotubes.
7. The method of claim 1, wherein the material that strongly
absorbs microwave energy comprises a semiconductor.
8. The method of claim 1, wherein the method further comprises
adding a binder to the mixture of high explosive having a first
dielectric loss and a material having a second dielectric loss that
is higher than the first dielectric loss of the high explosive
9. A mixture of high explosive having a first dielectric loss and
about 1 percent by mass or less of a material having a second
dielectric loss that is higher than the first dielectric loss of
the high explosive.
10. The mixture of claim 9, wherein the high explosive is selected
from the group consisting of TNT, PETN, RDX, HMX, TATB, and
mixtures thereof.
11. The explosive of claim 9, wherein the material that strongly
absorbs microwave energy is selected from the group consisting of
carbon nanotubes, metal powder, and semiconductor powder.
12. The explosive of claim 11, wherein the metal powder comprises
finely divided powder.
13. The explosive of claim 11, wherein the semiconductor powder
comprises finely divided powder.
14. The explosive of claim 9, further comprising a binder.
Description
FIELD OF THE INVENTION
[0002] The present invention relates generally to devices employing
energetic materials (i.e. explosives) and more particularly to
microwave heating of a charge of explosive.
BACKGROUND OF THE INVENTION
[0003] Microwave radiation is electromagnetic radiation with a
frequency in the range from about 1,000 MHz to about 30,000 MHz.
Microwave techniques have been employed for a wide variety of
applications that include radio astronomy, long distance
communication, navigation, microwave ovens, and the study of
physical and chemical properties of matter.
[0004] Recently, the ignition of several important high explosives
by microwave irradiation has been reported (see Kazuo Hasue, Masami
Tanabe, Nobutune Watanabe, Shoji Nakahara, and Fumiaki Okada,
"Initiation of Some Energetic Materials by Microwave Heating,"
Propellants, Explosives, Pyrotechnics, vol. 15, pp. 181-186 (1990),
incorporated by reference herein). Samples of high explosive (PETN,
RDX, HMX, and TNT, for example) were confined in tubes and ignited
by microwave radiation having a frequency of 2450 MHz.+-.50 MHz.
The TABLE below the next paragraph summarizes some of the
properties of these explosives.
[0005] The uniformity of heating by microwave radiation is related
to the dielectric properties of the material being heated. The
absorbed power, which results in microwave dielectric heating, is
given by equation (1) P=(
5/9).times.f.times.E.sup.2.times..epsilon..times.tan
.delta..times.10.sup.-10[W/m.sup.3] (1)
[0006] where f is the frequency in Hertz (Hz), .epsilon.' is the
real part of the relative dielectric constant, tan .delta. is the
dielectric loss, and E is the electric field intensity in volts/cm.
Values for .epsilon.', tan .delta., and .epsilon.'' are given in
the TABLE for the high explosives PETN, RDX, HMX, and TNT. If the
frequency f and the electric field E are constant, then the
absorbed power P depends on .epsilon.'.times.tan .delta., which is
equal to .epsilon.'', which is the imaginary part of the relative
dielectric constant; .epsilon.' and tan .delta.'', however, may
change with changes in temperature and frequency. TABLE-US-00001
TABLE Melting Ignition material .epsilon.' tan.delta. .epsilon.''
point (.degree. C.) temperature PETN 2.1 3.0 .times. 10.sup.-3 6.3
.times. 10.sup.-3 141.3 200 RDX 2.5 6.7 .times. 10.sup.-4 1.7
.times. 10.sup.-3 204.1 200 HMX 2.4 2.9 .times. 10.sup.-5 7.0
.times. 10.sup.-5 276-277 241 TNT 2.0 1.2 .times. 10.sup.-4 2.4
.times. 10.sup.-4 80.75 330
[0007] The power reflectivity is given by equation (2)
|.GAMMA.|.sup.2=(P.sub.o'/P.sub.o).times.100[%] (2) where
|.GAMMA.|.sup.2 is the power reflectivity, P.sub.o (kW) is the
incident power and P.sub.o' (kW) is the reflected power. Hasue et
al. calculated an initiation energy, E.sub.i, using equation (3)
E.sub.i=(P.sub.o-P.sub.o').times.t[kJ] (3) where t is the
initiation delay time in seconds.
[0008] As the TABLE shows, TNT, PETN, RDX, and HMX exhibit low
dielectric loss, i.e. they have a small value for tan .delta.. For
these materials, the absorption of microwave radiation was low,
indicated by the small value of .epsilon.''.
[0009] When these energetic materials are exposed to microwave
energy, their temperatures increase as they absorb the microwaves
until the exothermic reaction takes over and they ignite. It is
believed that as microwaves are directed at a location of the
explosive charge, the temperature rises exponentially at that
location, ignition occurs and the reaction spreads out until it
consumes the entire charge. The reaction can range in intensity
from a non-violent burning to very violent thermal explosion; this
depends largely on the level of confinement of the explosive; an
initiated explosive contained in a strong, gas tight vessel will
explode violently while in most circumstances, an unconfined
explosive will simply burn.
[0010] According to Kazuo Hasue et al. "Initiation of Some
Energetic Materials by Microwave Heating," Propellants, Explosives,
Pyrotechnics, vol. 15, pp. 181-186 (1990), for all of the
explosives tested, a delay in ignition using microwaves was
observed. The delays observed are as follows: PETN 71 seconds, RDX
88 seconds, HMX 176 seconds, and TNT 100 seconds. These ignition
times are too long for a practical use using microwave
ignition.
[0011] If the delay time for microwave ignition for high explosives
were significantly reduced, ignition of high explosives using
microwave energy may be employed as an initiation mechanism for
practical devices. Therefore, there remains a need for reducing the
delay time for ignition of high explosives for microwave energy to
be useful as an initiation mechanism.
[0012] Therefore, an object of the invention is a method for
microwave ignition of high explosives with a reduced delay
time.
[0013] Another object of the invention is a high explosive mixture
sensitized to ignition by microwave energy.
[0014] Additional objects, advantages and novel features of the
invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art upon examination of the following or may be learned by practice
of the invention. The objects and advantages of the invention may
be realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
[0015] In accordance with the purposes of the present invention, as
embodied and broadly described herein, the present invention
includes a method for igniting high explosive. The method includes
preparing a mixture of high explosive having a first dielectric
loss and a material having a second dielectric loss that is higher
than the first dielectric loss of the high explosive, and
thereafter exposing the mixture to microwave energy.
[0016] The invention also includes a mixture of high explosive
having a first dielectric loss and about 1 percent by mass or less
of a material having a second dielectric loss that is higher than
the first dielectric loss of the high explosive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The accompanying drawings, which are incorporated in and
form a part of the specification, illustrate the embodiment(s) of
the present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
[0018] FIG. 1 shows a schematic representation of an apparatus that
was used to demonstrate microwave ignition according to the present
invention.
[0019] FIG. 2 shows a graph of the absorbed power, and intensity of
light output, as a function of time for a sample of HMX (0.5 gram)
mixed with 1 percent by mass of carbon nanotubes.
[0020] FIG. 3 shows a graph of the absorbed microwave power, and
absorbed energy, as a function of time for the sample of FIG.
2.
[0021] FIG. 4 shows a photomacrograph of fragments generated by
microwave initiation of a sample of HMX and carbon nanotubes;
and
[0022] FIG. 5 shows a photo macrograph of fragments generated by
microwave initiation of another sample of HMX and carbon
nanotubes.
DETAILED DESCRIPTION
[0023] The invention relates to the ignition of explosives using
microwave energy. For the purposes of this invention, the terms
"energetic material" and "explosive" are interchangeable. One
aspect of the invention involves preparing mixtures of high
explosive that can be ignited using much less energy than what
would is required for the neat high explosive. Another aspect of
the invention relates to reducing the delay for ignition. High
explosives contemplated with this invention include well-known high
explosives such as PETN, RDX, HMX, TNT and TATB, and mixtures of
these explosives. These high explosive materials do not absorb
microwave radiation readily and do not ignite promptly, as
previously described (see K. Hasue et al. in "Initiation of Some
Energetic Materials by Microwave Heating," Propellants, Explosives,
Pyrotechnics, vol. 15, pp. 181-186 (1990)). They are low loss
dielectric materials and require ignition times that, according to
K. House, vary from 71 seconds to 176 seconds. For this reason,
devices and methods employing microwave ignition of these types of
materials are not practical.
[0024] According to the invention, high explosive is mixed with
materials that absorb microwave radiation strongly. These mixtures
can be ignited using microwave energy, do not require as much
microwave energy as the neat explosive does for ignition, and have
a much shorter ignition time than the neat explosive does.
[0025] The materials that can be mixed with high explosive to
sensitize the explosive to microwave radiation include carbon
nanotubes, finely divided metallic particles, and semiconductor
particles. These are examples of materials that absorb microwave
energy strongly. It is expected that any solid metal could be used.
Only a very small percentage of the mixture, one percent by total
mass and even less, is needed to sensitize an explosive charge to
microwave radiation. Example of metals may be useful with the
invention include, but are not limited to, aluminum, iron, and
tungsten.
[0026] The preparation of mixtures of high explosive with carbon
nanotubes and the ignition of these mixtures using a flash of light
has been described in U.S. patent application Publication
US2004/0040637. According to the '637 Application, energetic
mixtures containing from 3 percent to 20 percent by weight of
carbon nanotubes with explosives (ammonium perchlorate, RDX, TNT,
and black powder) were prepared and ignited using a light flash.
This type of ignition is a surface ignition; the flash promotes
ignition on the surface and not throughout the bulk of the material
because the light does not penetrate the material. By contrast, the
present invention relates to volume ignition using microwaves. The
microwaves penetrate the entire volume of the mixture, and
microwave radiation is absorbed by carbon nanotubes throughout the
mixture. Volume ignition results in rapid consumption of the entire
charge, not just ignition and burning from the surface of the
mixture.
[0027] The invention was demonstrated by preparing a mixture of HMX
and a small amount of carbon nanotubes. Only about 0.1% percent by
mass of carbon nanotubes relative to the HMX produced a mixture
with an ignition less than one tenth the time required for neat
HMX.
[0028] In order to construct a practical explosive device,
materials referred to herein as "sensitizers" were mixed with
energetic material at low concentrations in order to significantly
increase the local dielectric loss. When exposed to microwave
energy, the sensitizer creates "hot spots," and ignition occurs a
many locations throughout the entire charge of energetic material.
Adding the sensitizer to the energetic material also increases the
overall dielectric constant.
[0029] Sensitizers of the present invention include, but are not
limited to, carbon nanotubes, metallic and semiconductor particles.
Lossy liquid, i.e. a liquid having a higher dielectric loss than
that of the neat explosive material (lossy liquids such as water
and acetone, to name a few) or plastics, can also increase the
overall dielectric loss of the mixture.
[0030] It is believed that the sensitizer allows the quasi-uniform
absorption of microwaves such that the temperature of the energetic
material increases uniformly, and/or that the sensitizer provides
many locations in the mixture for localized absorption, with
induced ignition at these well-distributed discrete "hot spot"
locations. If the absorbing sensitizer causes large thermal
gradients in a small region, detonation may occur. In any case, the
presence of the sensitizer reduces the ignition delay time to such
an extent that microwave ignition in a device employing this type
of sensitized charge becomes practical.
[0031] Binders may be used with the invention. Useful binders
include organic and/or inorganic materials that have a higher
dielectric loss than the dielectric loss of the explosive. Some, or
all, of the sensitizer could be the binder.
[0032] This technique allows for a wide range of energy release
rates. Traditional detonating explosives release their energy very
rapidly, at a steady rate, creating high peak pressures and a short
pressure pulse duration. The characteristic blast from detonating
explosives is very effective for destroying some types of
structures, but improved coupling occurs for many structures by
lowering peak pressures and increasing the duration of the pressure
pulse. In practice, it is difficult to accomplish this with a
device that has good timing control. The invention described here
allows for the rapid release of explosive energy over a wide range
of impulse characteristics in a single device.
[0033] Another aspect of the invention relates to a system that
includes a microwave source capable of producing microwave
radiation and any energetic material that is sensitive to microwave
ignition. These materials can include classical CHNO explosives,
insensitive high explosives, high-nitrogen materials, thermites,
metastable intermolecular composites, and the like. The energetic
material may have intrinsic high dielectric loss, or may include
low dielectric loss explosive in combination with sensitizers such
as metallic particles, semiconducting particles, nanoscale and/or
microscale fibers, including carbon nanotubes. The sensitizer may
be distributed throughout the energetic material uniformly or in
such a way such that some desired performance characteristic is
achieved.
[0034] The energetic material may contain more than one type of
sensitizer, where each sensitizer may absorb microwave energy of
some particular frequency, or within a particular frequency range,
or have a particular microwave power response, such that different
frequencies or power levels induce different behaviors.
[0035] To facilitate extremely fast energy release rates, the
energetic material may be a composite consisting of various
energetic materials, some sensitized and some not, again to provide
flexibility in performance. An example of this type of material may
be a HMX/PETN composite, where the PETN contains the sensitizer
(the HMX could be uniformly loaded with 100 micron PETN crystals
that are uniformly loaded with 0.1% carbon nanotubes, for
example).
[0036] The common exploding-wire detonator uses PETN. A particulate
mixture of PETN crystals and sensitizer (carbon nanotubes or finely
divided metallic or semiconductor particles, for example) would be
employed as a microdetonators that could be distributed in a bulk
charge of a different explosive, say HMX. The distribution could be
uniform, or have characteristics that would lead to some specific,
desired performance. Using sensitized PETN in such a way would
produce detonation waves from many well-distributed locations
throughout the composite.
[0037] The following EXAMPLES demonstrate the operability of the
invention. The EXAMPLES demonstrate the performance of a mixture of
0.5 g HMX and 1 percent by mass carbon nanotubes when exposed to
microwave radiation. For comparison, 0.5 g HMX exposed to the same
or higher power microwaves over the same or longer duration did not
ignite. The EXAMPLES were performed using a microwave initiation
apparatus similar to that described by Kazuo Hasue, Masami Tanabe,
Nobutune Watanabe, Shoji Nakahara, and Fumiaki Okada, "Initiation
of Some Energetic Materials by Microwave Heating," Propellants,
Explosives, Pyrotechnics, vol. 15, pp. 181-186 (1990). The used to
demonstrate the invention included a microwave generator, a
microwave tuner, a power measurement apparatus and a shorted
waveguide that provided a region of standing-wave microwave energy
to facilitate the absorption of microwave energy into the energetic
material. The energetic material and associated fixtures perturb
the impedance of the waveguide, and a three-stub tuner was employed
to induce maximum power transfer to the energetic material.
[0038] A schematic representation of the apparatus is shown in FIG.
1. Apparatus 10 includes microwave generator 12, which supplied the
2.45 GHz, 6 kWatt microwaves. Microwaves pass from generator 12
through waveguide 14, also known as the microwave applicator, to
cylinder 16 and then to the sample of energetic materials 18.
Cylinder 16 was a 1-inch diameter quartz cylinder that was used to
contain the sample of energetic material 18 that was being exposed
to the microwaves. Forward and reflected powers are illustrated and
were measured using directional couplers. With further regard to
sample containment, a Pyrex tube (not shown) having an inner
diameter of about 3/16 inch was used to confine the sample.
Cylinder 16 surrounded the Pyrex tube, protected the waveguide and
other components of the microwave initiation apparatus, and also
acted as a "witness" for energy release rate by fragmenting during
the explosion event that followed ignition. The term "witness" is a
commonly used term that is used herein to describe a passive object
that is affected in some way by an explosive event. The witness was
used in order to provide some measure of the explosive event. In
the EXAMPLE below, the quartz `witnesses` the explosive event and
provides information about it by how it fragments as a result of
the explosion.
EXAMPLE 1
[0039] Ignition of a mixture of HMX with carbon nanotubes. A
mixture of HMX (0.5 g) and carbon nanotubes (1 percent by mass) was
prepared and ignited by microwave radiation. FIG. 2 shows a graph
of absorbed power as a function of time (solid line, power in units
of Watts, time in units of milliseconds) of the mixture. FIG. 2
also shows the photodiode signal (i.e. the light output) as a
function of time (dashed line) from the mixture, along with the
absorbed power. As FIG. 2 shows, the microwave power was turned on
at a time of minus 10 milliseconds, and ignition occurred about 10
milliseconds later.
[0040] FIG. 3 shows a graph of the absorbed microwave power as a
function of time (solid line) and a graph of the absorbed energy as
a function of time (the dashed line) of the mixture. The absorbed
energy is the integral of the absorbed power). The total absorbed
energy was 7.5 Joules (J) at an average rate of 750 Watts for a
duration of 10 milliseconds. For comparison, raising the same mass
of neat HMX to a conservative autoinitiation temperature of 200
degrees Celsius would require about 110 J for a duration of 150
milliseconds. The neat HMX would also require much higher electric
field strengths due to a weaker interaction of neat HMX with
microwave energy.
EXAMPLE 2
[0041] Evidence for variable energy release rate. The size of
container fragments generated during an explosive event provides a
measure of the energy release rate (see, for example, P. R. Lee,
"Hazard Assessment of Explosives and Propellants" in Explosive
Effects and Applications, J. A. Zukas and W. P. Walters,
Springer-Verlag (New York, 1998) p. 327, incorporated by
reference). For this example, two experiments were performed to
examine the fragment size of the pyrex sample container and the
quartz containment cylinder. In both experiments, the nominal
conditions were the same: incident power, sample size and sample
loading were nominally identical. Absorbed power data was not
recorded but inconsistencies between the two experiments resulted
in a difference in impedance matching such that the absorbed power
was different, which is indicated by the difference in sizes of the
fragments in FIG. 4 and FIG. 5. FIG. 5 (experiment 1) shows larger
fragments than those of FIG. 4 (experiment 2). A scale bar was not
included in these FIGURES, but the penny provides some indication
of the fragment sizes. These qualitative results indicate that the
energy release rate was significantly faster in EXAMPLE 1 compared
to EXAMPLE 2.
[0042] In summary, the present invention relates to igniting
energetic materials using microwave radiation. High explosives were
rendered more sensitive to microwave heating by the addition of
sensitizers. Effective sensitizers have a dielectric loss that
ranges from one to several orders of magnitude greater than that of
the explosive. The addition of these lossy dielectric materials
provides the user with the ability to better control the overall
behavior of the subsequent explosive event, i.e. the explosion. The
added sensitizers allow tuning of the energy release rate, up to
detonation of the explosive. The added sensitizers also allow for
prompt ignition of energetic materials.
[0043] 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, and obviously many modifications and variations are
possible in light of the above teaching.
[0044] The embodiment(s) were chosen and described in order to best
explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto.
* * * * *