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MECHANOCHEMICAL SYNTHESIS
Mechanochemically prepared reactive and energetic
materials: a review
Edward L. Dreizin1,* and Mirko Schoenitz1
1New Jersey Institute of Technology, Newark, NJ 07102, USA
Received: 15 December 2016
Accepted: 11 February 2017
Published online:
21 February 2017
� Springer Science+Business
Media New York 2017
ABSTRACT
Reactive and energetic materials are typically metastable and are expected to
transform into thermodynamically favorable reaction products with substantial
energy release. Preparation of such materials by mechanical milling is chal-
lenging: They are easily initiated by impact or friction. At the same time, milling
offers a simple, scalable, and controllable technology capable of mixing reactive
components on the nanoscale. In most cases, for reactive materials milling
should be interrupted or arrested to preserve the metastable phases. Arrested
reactive milling was exploited to prepare many inorganic reactive materials,
including nanocomposite thermite, metal–metalloid, and intermetallic systems.
Prepared materials are fully dense composites with unique properties, com-
bining high density with extremely high reactivity. Different milling devices
were used to prepare reactive materials and an approach was developed to
transfer the process conditions between different mills. Different milling pro-
tocols, such as milling at cryogenic temperatures or staged milling can be used
to prepare hybrid reactive materials with different components mixed on dif-
ferent scales; it was also used to tune the particle size distributions of metal-
based reactive material powders. Metal–halogen composites were prepared,
with metal matrix stabilizing a halogen (e.g., iodine) at temperatures substan-
tially exceeding its boiling point. Mechanochemically prepared reactive mate-
rials can be classified based on the energy of reaction between components and
the energy of oxidation of the bulk material composition. Work on
mechanochemical preparation of reactive and energetic materials is reviewed
with the focus on unique properties and ignition and combustion mechanisms
of the mechanochemically prepared reactive materials. An ignition mechanism
for nanothermites involving preignition reaction leading to a gas release pre-
ceding rapid temperature rise is discussed. A combustion mechanism is also
discussed, in which the nanostructure of the mechanochemically prepared
material is preserved despite the very high combustion temperatures.
Address correspondence to E-mail: [email protected]
DOI 10.1007/s10853-017-0912-1
J Mater Sci (2017) 52:11789–11809
Mechanochemical Synthesis
Introduction
Materials capable of an exothermic chemical reaction
releasing copious amounts of heat beyond that nec-
essary to self-sustain the reaction, are referred to as
reactive [1–4] and energetic [5, 6] materials. Typically,
reactive materials cannot detonate, while energetic
materials can. It is also common to refer to inorganic
compositions, such as thermites or intermetallics, as
reactive materials. Conversely, organic compounds,
such as trinitrotoluene or nitrocellulose, are com-
monly referred to as energetic materials. Here, the
focus will be on inorganic compositions and all non-
detonating materials with high heats of combustion
will be referred to as reactive materials or RMs. All
RMs are capable of a rapid exothermic reaction, and
thus such materials are thermodynamically meta-
stable. The heat is released when RMs transform into
their thermodynamically stable, chemically inert
products.
Mechanochemistry was recently developed as a
versatile methodology for synthesis of a broad range
of materials [7–13]. Metastable materials, in particu-
lar, are readily prepared using the relatively simple,
inexpensive, and scalable technique of ball milling.
For RMs of interest to this paper, the relaxation of
metastable states occurs volumetrically and is
accompanied with a rapid, strong heat release. The
metastability can be due to the presence of a dis-
torted/stressed crystal lattice or amorphous phases
and solid solutions. For such materials, the relaxation
of a metastable state does not generate much heat,
but serves as a trigger of a highly exothermic chem-
ical reaction, e.g., by generating fresh reactive sur-
faces exposed to external oxidizers. Examples of
such materials are Al�Mg [14–16] and Al�Ti [17]
metastable solid solutions. In those materials, the
mixing between components occurs on the atomic
scale. Conversely, composite metastable materials
can be prepared, in which components capable of an
exothermic reaction are mixed on a coarser scale
(10–100 nm), which is still fine enough for the
reaction to occur volumetrically upon initiation.
Respective examples are many aluminum-based
nanothermites, e.g., Al�MoO3 [18] or Al�CuO [19] and
boron-based nanocomposites, e.g., B�Ti [20].
Mechanochemically prepared RMs are of interest
to many diverse applications, including additives to
propellants, explosives, and pyrotechnics [21]; man-
ufacture of reactive structural components, such as
liners or munition cases [22–24], for advanced self-
propagating high-temperature synthesis [25–27], and
for generating hydrogen [28, 29]. Applications are
also possible for joining materials, e.g., for soldering,
brazing, and welding, replacing more expensive
reactive nanomaterials prepared by magnetron
sputtering [30–32].
For most applications, it is important to identify
methods, main challenges and limitations of the
mechanochemical synthesis. It is further important to
understand the conditions leading to ignition of RMs.
Finally, it needs to be known how rapidly is the heat
released upon ignition, what flame temperature is
achievable, and what the combustion products are.
These issues will be discussed in the present review.
Equipment for mechanochemical synthesis
High-energy ball mills have been used to prepare
energetic materials, including metastable alloys,
reactive composites, and fine powders. The three
main mill types are shaker, planetary, and attritor
mills, summarized briefly in Table 1. A common
feature for all types of mills is that the energy trans-
ferred from the milling tools to the powder being
milled is sufficient to cause plastic deformation of the
powder particles. This causes deformation and
breakage of the original particles, cold welding, and
formation of composites. These processes are similar
to all powder processing techniques involving
mechanical milling, discussed well in multiple
available reviews, e.g., [11, 33–35].
The type of mill determines the milling geometry
resulting in different aspects of sample handling and
containment. From a production point of view, only
attritor mills with stationary sample containers are
suitable for continuous production. All other mills
are operated as batch processes. Milling containers
are variably called milling vials or milling jars by the
manufacturers. The terms are used interchangeably
here.
Stationary milling containers readily enable real-
time measurements of milling characteristics, such as
temperature [41], torque [42], rate of rotation, and
consumed power [43, 44]. The temperature in the
moving milling vials of a Retsch PM400 planetary
mill used to prepare mechanically alloyed reactive
powders has been successfully monitored using
wireless transmitters mounted on the vial lids [45].
11790 J Mater Sci (2017) 52:11789–11809
Commercial designs using temperature and pressure
sensors and wireless transmitters are now available,
e.g., from Retsch or Fritsch. Wired temperature sen-
sors such as thermistors are possible to attach to
shaker mill vials, but connections need to be replaced
frequently due to fatigue failure after some hours of
use [46]. Monitoring temperature and pressure can be
used to track the milling progress without
interrupting the milling. The milling temperature can
change as a result of the reaction between energetic
components (such as in a thermite) [47, 48], or it can
change in characteristic ways as the milled powder
particles change shape or mechanical properties and
therefore serve as an indicator of the work being
performed on the milled powder [49]. An illustration
of temperature change in a milling vial during
Table 1 Milling equipment used for preparation of energetic powders
Shaker mills Designs- Spex SamplePrep (US) [36-38]- Aronov vibratory mill (Russia) [39]
Vial size: 50 mL: 1-10 gAdvantages: high collision energy, short milling �mes, can be pressure-proofedDisadvantages:
- batch process- small capacity- milling speed not variable- access for real �me measurements difficult- no provision for temperature control
Planetary mills Designs- Retsch PM series- Fritsch Pulverise�e series- AGO-mills (Russia) [40]
Vial sizes: 50 mL – 500 mL: up to 100 g/vialAdvantages:
- larger batch sizes than shaker mills- typically variable milling speed- can be pressure-proofed- milling compartment can be cooled- Some Frisch mills have variable transmission ra�os
Disadvantages: - real-�me measurements need wireless sensors- batch process
A�ritor mills Designs- Union Process, ver�cal axis- Zoz GmbH, horizontal axis
Vial size: 500 mL to 400 L for kg quan��esAdvantages:
- sta�onary vials enable real-�me monitoring- cooling jackets enable temperature control over wide
range, including cryogenic temperatures- batch and con�nuous processes feasible
Disadvantages:- milling container difficult to seal to higher pressures- cannot easily process small quan��es (single grams)- sample can be exposed to atmosphere during
loading/unloading
J Mater Sci (2017) 52:11789–11809 11791
preparation of a highly reactive Al�Fe2O3 composite
is shown in Fig. 1 [46]. The temperature first increa-
ses slowly from 20 to about 40 �C, while the initial
mixture of Al and Fe2O3 powders is milled. This
increase represents the dissipation of mechanical
energy from the milling tools (balls) balanced by the
heat removed from the milling vial. The temperature
is nearly stable, before exhibiting a sharp increase.
That increase, occurring at about 25 min represents
the mechanically triggered exothermic redox reaction
generating Al2O3 and Fe. For preparation of a reac-
tive material, the milling must be interrupted and
obtained metastable composite should be ‘‘arrested’’
just before such mechanically triggered reaction
occurs [50]. The milling conditions leading to such
self-sustaining reactions should be avoided in prac-
tical manufacturing of RMs. However, slow changes
in temperature, especially caused by difference in the
energy dissipation as a result of modification of
mechanical properties of the material being milled
are unavoidable and can be tracked for the process
control. Contemporary commercial devices are
designed to interface with the mill itself so that the
milling intensity (e.g., vial rotation rate) can be
automatically varied as the properties of the milled
powder changes, causing changes in temperature and
pressure, e.g., GrindControl vial design by Retsch.
Temperature control is possible for either moving
or stationary milling containers. In the stationary
case, a simple jacket around the container allows
continuous cooling by flowing cooling water, or liq-
uid nitrogen for milling at cryogenic temperatures,
e.g., [51–53]. Moving milling vials require airflow for
effective cooling, which limits the available temper-
ature range for milling. Finned heat sinks have been
mounted on planetary milling jars, and the whole
mill was outfitted with an air conditioning unit to
lower the vial temperature [45].
Moving milling vials in shaker and planetary mills
do not require an opening for an externally driven
impeller and can be hermetically sealed. Such vials
can be designed to withstand internal pressure. This
is of interest for the milling of energetic materials,
where accidental initiation of the material in the mill
could damage the milling equipment. It is also useful
if a pressurized gas is used to modify a metal powder
being milled [54]. Custom pressure-proof milling jars
have been designed for a Retsch PM400 planetary
mill, allowing exploratory work on thermites and
nitrate-based gas generators [55].
Milling vials and milling balls most typically used
for preparation of composite materials are made of
hardened or stainless steel. Iron contamination
depends on the milling time, the intensity and the
abrasiveness of the milled material. Milling contain-
ers made of other materials are available, such as
stabilized zirconia, alumina, or tungsten carbide.
Those are typically more expensive, and less readily
sealed. Steel jars coated with zirconia or tungsten
carbide are among more expensive while easily
sealed milling container options.
Milling parameters
The conventional milling parameters that determine
the properties of the resulting material are primarily
the type, size, and quantity of the milling media, the
operation speed of the mill, if it is variable, the
amount of milled powder, and the use of process
control agents. The milling media typically comprises
metal or ceramic balls. Their size, density, and
mechanical properties affect the rate at which the
material is refined as well as final properties of the
prepared material [49, 56, 57]. For a given type of mill
and milling vial, there are practical limits for the
range of milled powder and milling media that can
be used. Therefore, the amounts of milling media and
milled powder are often related using the ball-to-
powder mass ratio, or charge ratio, CR [58].
activated nanocompositeof Al and iron oxide
reduced iron+ Al oxide
initialmixture
Arrested Reactive Milling
Reactive MillingAl + Fe2O3
0 5 10 15 20 25 30 35 40
Milling Time [min]
20
30
40
50
60
70
80
90
Tem
pera
ture
of M
illin
g V
ial [
°C]
Figure 1 Changes in the temperature in the milling vial during
processing a blend of Al and Fe2O3 powders. The measurement is
for a SPEX 8000 shaker mill [46].
11792 J Mater Sci (2017) 52:11789–11809
Increasing the charge ratio, and increasing the mill
operation speed generally increases the milling
intensity, which is more quantifiably expressed as the
work performed on the milled powder by the milling
media [59], or the energy dissipation rate [46, 60]. A
free space must remain in the milling vials to enable
the milling media motion; this can be quantified via
the vial filling ratio [61].
The temperature in the milling vials is being mea-
sured, as discussed above, as a parameter affected by
and affecting both the milling progress and proper-
ties (strength, ductility) of the milled powder. In a
study on mechanically alloyed reactive Al–Mg pow-
ders, decreasing the ambient milling temperature by
50 �C resulted in an increase in the metastable solu-
bility of Mg in fcc Al from about 3–25% [45]. Cryo-
genic milling offers the possibility for preparing
reactive composites from materials that are either not
solid, e.g., elemental iodine [52] or cyclooctane [62],
or mechanically weak (e.g., polymers) at or slightly
above ambient temperatures [63]. Cryogenic milling
was also used to achieve a more uniform dispersion
of nanosized oxide inclusions in an aluminum-based
thermite [64].
The control of the milling vial temperature is a
particular concern for energetic composite materials.
While it is useful to know under what conditions the
whole powder charge will ignite during milling, and
this can help direct early, exploratory milling efforts,
milling to reaction stresses the milling equipment
and has the potential to cause damage even on the
laboratory scale. It is not feasible at all during pro-
duction. Process control agents (PCA) and particu-
larly endothermic organic liquids, e.g., hexane, act as
temperature moderators, and are an effective means
to prevent whole-sample ignition during milling. As
energetic composites are being milled, and the
intraparticle refinement increases, reaction between
the components still occurs, but proceeds gradually
and no longer has the potential to ignite the whole
sample at once. Solid PCA, such as stearic acid, are
also useful by providing lubricant, controlling cold
welding, and thus affecting the properties of the
prepared materials. However, solid PCA do not
prevent the self-sustained mechanically triggered
reactions.
PCA can serve additional purposes. Small addi-
tions of elemental iodine to energetic Al–Mg and Al–
Ti alloys were effective in reducing sizes of the pro-
duct particles [65]. Chemically active PCA were used
to prepare customized reactive materials, e.g., based
on aluminum [54].
Computational descriptions of millingprogress
Despite substantial experimental results for the types
of mills described so far, and despite the observation
that the milled products prepared in different mill
types are largely identical, transitioning milling
conditions from one mill to another requires experi-
mentation and validation. For larger scale produc-
tion, this is difficult and wasteful. Early efforts at
systematic quantitative description of the milling
process that would allow transferring milling condi-
tions were based on evaluating the energy trans-
ferred during milling [66–68] or a milling dose, Dm
[46, 69]. This quantity is defined as the work per-
formed on the milled powder relative to the powder
mass, mp. The work is approximated as the product
of the average energy dissipation rate Ed, and the
milling time, s:
Dm ¼ Edh i � s�mp
This simplified estimate is useful, although the
energy dissipation rate does change with time as the
properties of the milled powder change, and it
should properly be described as a distribution over
all interactions between the milling tools. The energy
dissipation rate has been systematically studied for
three specific mills using discrete element modeling
[70]. Operating parameters such as mill rotation rate
and properties of the milling tools were used directly.
Properties of the milled powder were modeled via
friction and restitution coefficients that were mea-
sured independently. The computed milling dose
was successfully related to experimentally observed
changes in the milled powder, such as grain refine-
ment and mechanical properties. The effort showed
that the work performed on the milled powder is
indeed a useful indicator for the milling progress,
and demonstrated the feasibility to determine scaled-
up milling conditions without large-scale trial-and-
error experimentation. It was shown also that using
only the total integrated energy transferred from the
milling tools to the powder is inadequate to compare
the milling progress achieved in different types of
mills. The energy is transferred in different types of
events, i.e., head-on collisions, gliding collisions, or
J Mater Sci (2017) 52:11789–11809 11793
by ball rolling, causing different types of deformation
and different rates of energy transfer per event. Only
those events need to be accounted for, for which the
rate of energy transfer to the powder exceeds a cer-
tain threshold, necessary to achieve plastic deforma-
tion of the material milled. The energy transfer in
different milling devices is dominated by different
types of events, leading to differences in the powder
refinement. Thus, a correction is necessary when
milling time in different devices is selected relying on
the calculated or measured bulk energy transfer rate
[70]. One challenge for this computational effort came
from the mechanical stiffness of the models, and
resulting energy distributions that showed unrealistic
outliers, which had to be screened out. Initial results
with mechanically responsive models that explicitly
allow for flexibility suggest improvements for future
work [60].
Reactive materials prepared by milling
Thermodynamic assessmentof mechanochemically prepared RMs
Commonly, mechanochemically prepared materials
are classified based on mechanical properties of the
starting components, e.g., ductile–brittle, ductile–
ductile, and brittle–brittle [11, 33, 34]. For RMs, a
convenient classification involves the heat of reaction.
In most applications, an external oxidizer is available
and thus the final useful heat of reaction is the heat of
complete oxidation of all starting components. The
external oxidizer is transported to the surface of a
reacting RM via convection and diffusion from sur-
rounding media, which typically occurs at a lower
rate than the transport of reactive components within
an RM particle containing components mixed on the
nanoscale. In other words, the relatively slow reac-
tion with an external oxidizer (combustion) com-
monly follows a faster reaction between the RM
components; the latter reaction is thus responsible
for initiation or ignition of the RM. Therefore,
mechanochemically prepared RMs can be assessed
comparing their heats of complete oxidation with
those released due to the reaction between the
material components. A higher heat of reaction
between components suggests a more sensitive and
readily igniting material. A higher heat of oxidation
suggests a material that is more attractive as a fuel.
Often it is desired to have both heats of reaction
maximized, although in some applications, less sen-
sitive materials are desired, for which the heat of
reaction between components should not be high.
Different mechanochemically prepared RMs are
presented in Fig. 2, where their heats of reaction
between components are plotted along the horizontal
axis, and their heats of complete oxidation are along
the vertical axis. Symbols show actual compositions
that have been prepared. Many groups of symbols
are connected by lines to indicate that the compo-
nents can be mixed in different proportions, yielding
RMs with different thermodynamic characteristics.
The lines for all aluminum-based composites cross at
the vertical axis, with the intersection point repre-
senting the heat of oxidation of pure Al. Similarly, the
lines for two shown boron-based compositions cross
at the point representing the heat of oxidation of pure
boron. For each Al-based composition, the points at
the right end of the line represent stoichiometric
compositions, such as 2Al�MoO3 or 2Al�3 CuO. Points
along the line shifting to the left side of the plot
represent more and more fuel-rich compositions,
prepared previously.
The data in Fig. 2 show that only Al�B and Al�Li
composites (or alloys) have higher heats of oxidation
than pure Al, the most popular metal additive to
energetic compositions. However, the heats of reac-
tion between components in both Al�B and Al�Li
materials are negligible compared to their heats of
oxidation. The low heat of metal–metalloid or inter-
metallic reaction suggests that ignition of such
materials can only be assisted by developed reactive
interface or selective oxidation of one of the compo-
nents, e.g., Li. Indeed, an enhancement of ignition
was observed for mechanochemically prepared Al�Li
powders [71, 72]; however, Al�B composites prepared
in preliminary unpublished experiments were found
to be difficult to ignite.
Among different metal oxide oxidizers for alu-
minum, MoO3 appears to be the most attractive
thermodynamically based on Fig. 2. Respective
compositions were prepared mechanochemically and
characterized [18, 73]. Similarly, nanocomposite
thermites with other oxidizers, such as CuO, Fe2O3,
WO3 and others were prepared mechanochemi-
cally and tested in various laboratory experiments
[19, 21, 74]. Note that data shown in Fig. 2 do not
account for the reaction rate or other additional
processes accompanying reaction, such as gas release,
11794 J Mater Sci (2017) 52:11789–11809
which could affect the practical ignition or combus-
tion of the RM substantially. For example, one of the
most reactive metal oxide oxidizers, Bi2O3, causing
release of Bi gas upon ignition [75, 76], appears to be
the least attractive based on its heat of reaction with
aluminum. Specific properties of oxidizer should
always be considered, in particular when working
with such gas-generating oxidizers as NaNO3 [77].
Interestingly, Al�polytetrafluoroethylene (PTFE)
composites appear to be most attractive, among Al-
based RMs, based on their thermodynamic charac-
teristics, justifying the interest to preparing such
composites mechanochemically [53, 78–80].
Among aluminum-based intermetallic composi-
tions, Al�Ni appears to be the most attractive ther-
modynamically. Respective RMs were prepared
mechanochemically and characterized in Refs.
[81–83].
It is apparent from Fig. 2 that boron-based com-
posites, such as boron-titanium and boron zirconium,
are more attractive thermodynamically than many
Al-based thermites. Such RMs were prepared
mechanochemically and characterized in Refs.
[20, 47, 84].
Milling dose for mechanochemicallyprepared RMs
For selected materials prepared mechanochemically
in our research, the milling dose required for
preparation is plotted versus the heat of reaction
between the material components in Fig. 3. The mil-
ling dose represents practical milling conditions used
in different studies, and computed rate of energy
transfer from milling tools obtained for different ball
mills [60, 70]. The total computed energies for dif-
ferent mills are normalized for the conditions found
in the SPEX 8000 series shaker mill. The data shown
in Fig. 3 are scattered; however, it is clear that gen-
erally lower milling dose needs to be used when
working with materials with higher energies of
reaction between components. Part of the reason for
the substantial scatter in the data shown in Fig. 3 is
that different degrees of refinement were achieved
-ΔHR, kJ/g0 2 4 6 8 10
-ΔH
ox, k
J/g
0
10
20
30
40
50
60
Al.Bi2O3
Al.Ni
Al.WO3
Al.MoO3
Al.CuOAl.Fe2O3
Mg.SAl.S
Mg
Zr
B
Li
Al
Ti
B.TiB.Zr
Al.PTFE
Mg.NaNO3
Al.NaNO3
Al.Mg.NaNO3
Figure 2 Maximum heat of
oxidation versus heat of
reaction between components
of mechanochemically
prepared RMs.
-ΔHr, kJ/g0 2 4 6 8 10
Mill
ing
dose
, kJ/
kg
1
10
100
1000
Al.Li
Al.Ti
Al.Mg
Al.Ni
Al.10% PTFE
Al.Bi2O3 B.ZrAl.S
B.Ti
Al.CuO
Al.Fe2O3
Al.MoO3
Mg.SAl0.5Mg0.5
.NaNO3
Mg.NaNO3
Al.NaNO3
Figure 3 Milling dose versus heat of reaction between compo-
nents of selected mechanochemically prepared RMs.
J Mater Sci (2017) 52:11789–11809 11795
for different compositions; in some cases, the milling
conditions were not optimized as long as a suffi-
ciently reactive material was obtained.
Metallic alloys with negligible heatof intermetallic reaction
These materials include such mechanically alloyed
powders as Al�Mg [14, 16, 65, 85–88], Al�Ti [89–91],
and Al�Li [72]. Ignition of such materials can be
triggered by a phase change occurring upon heating
in the metastable structure produced by milling or by
selective rapid oxidation of an alloying additive, such
as Mg or Li. For example, mechanically milled Al�Mg
alloys typically consist of a metastable solid solution,
which transforms into a mixture of Al and Al3Mg2
upon heating [86, 87]. Examples of X-ray diffraction
patterns characterizing structures of the mechanically
milled aluminum-based alloys with different
additives are shown in Fig. 4. Depending on the
system and milling time (dose), powders contain
metastable intermetallic compounds or solid solu-
tions. For the Al�Li system, an intermetallic d-LiAl
forms relatively readily. However, the structure is
destroyed in longer milling, yielding an X-ray
amorphous material. For Al�Mg alloys, either a solid
solution or intermetallic c-Al12Mg17 is produced
when the starting composition is adjusted. Formation
of a metastable L12 phase of Al3Ti is observed in the
milled Al�Ti. In each case, the material structure
changes upon heating. The change in the structure
leads to formation of defects and fresh surface, prone
to rapid oxidation when the external oxidizer is
available. Kinetics of the phase transformation can be
identified from thermo-analytical measurements
performed with mechanically alloyed powders in
both inert and oxidizing environments. The same
kinetics can then be adapted to describe ignition of
these materials heated in practical situations at much
higher rates. Accelerated ignition may result in a
faster flame propagation or flame speed in the aero-
solized mechanically alloyed powders [90, 92]; the
effect of mechanical alloying on the burn rate of
individual particles is less well understood, however
[16].
Activated metals
Most of materials in this group of mechanochemically
prepared RMs is not represented in Fig. 2 because
they mostly contain one material with a small
amount of additive. Examples of such materials are
metal powders processed by mechanical milling to
obtain flake-like or nanosized particles [54, 93–95].
An example of flattened aluminum particles (flakes)
obtained as a result of milling a commercially avail-
able aluminum powder with 10 wt% PTFE is shown
in Fig. 5. Such flake-like particles that also contain
multiple defects and micro-cracks ignite much more
readily than the powder particles obtained by solid-
ification of a melt (i.e., atomized aluminum) due to
their developed surface.
Examples of activated metals include multiple
metal–polymer systems, such as less fuel rich, and
therefore highly reactive Al�PTFE (or Teflon�)
[53, 78, 79], Mg�PTFE [96], or the chemically much
2Θ (Cu-Kα), degrees30 35 40 45 50
Inte
nsity
, a.u
.
Al-20%Ti, 15 h
Al-30%Mg, 12 h
Al-50%Mg, 12 h
Al-30%Li, 6 h
Al-30%Li, 102 h
ss of Mg in fcc-Al
L12 phase of Al 3Ti
δ-LiAl
γ-Al12Mg17
Aluminum
Figure 4 X-ray patterns characterizing mechanically alloyed Al-
based RMs with negligible energy of intermetallic reaction
between components.
Figure 5 Aluminum milled with 10 wt% PTFE, showing forma-
tion of Al flakes.
11796 J Mater Sci (2017) 52:11789–11809
less reactive Al�cyclooctane [62] or Al�polyethylene
[97]. Aluminum activated by milling with carbon was
also prepared [98]. Because the amount of activating
additive is commonly lower than necessary for the
complete reaction with the metal, the systems are fuel
rich and require an external oxidizer. In many cases,
mechanically activated metal powders are prepared
to assist the following self-propagating high-tem-
perature synthesis [99–104].
Highly reactive intermetallic, metal–metalloid, and thermite systems
For such systems, e.g., Al�Ni [81–83, 105–109], B�Ti
[20, 21, 110], B�Zr [48], or Al-based thermites, the heat
of reaction becomes comparable to that of the heat of
oxidation as the composition approaches that of sto-
ichiometric reactions (cf. Fig. 2). During milling,
components can ignite readily and thus special care
should be taken to avoid the reaction. The mixing
between reactive components achievable by milling
is typically on the scale of 100 nm. A characteristic
cross section of a composite thermite particle is
shown in Fig. 6. The morphology illustrated is typical
and includes a matrix of a more ductile component
(in this example, Al) with finely divided inclusions of
the harder material (MoO3). The structure of the
interface formed between reactive components is not
well understood or characterized. There must be a
layer in which the components are mixed on the
atomic scale (i.e., reacted); however, the thickness of
this layer formed during milling is not determined.
The transport of reactants through this layer defines
the kinetics of material ignition and thus is important
to understand and characterize. It is hypothesized
that this layer produced at relatively low ball milling
temperatures is thinner than any natural oxide layers
coating metals or, for example, intermixed materi-
als layers forming in nanofoils with the same com-
positions but produced by magnetron sputtering
[111, 112]. Further work is of interest exploring the
structure and properties of the mechanochemically
generated layers between components capable of
highly exothermic reactions.
Materials with customized properties
Materials with custom properties can be prepared,
e.g., containing components with biocidal character-
istics, such as iodine, or including customized parti-
cle size distributions. Halogen-containing RMs are of
interest for biological agent defeat munitions [113].
A number of such materials, including binary
Al�I2, Al�CHI3, and others as well as ternary, e.g.,
Al�B�I2 and Mg�B�I2 composites were prepared
mechanochemically. In such materials, highly volatile
iodine could be retained upon heating close to the
melting point of the matrix metals. This stabilization
of a volatile compound is illustrated in Fig. 7.
Aside from composition, mechanical milling can be
used to customize particle size distributions of RM
powders. For example, hybrid large particles were
prepared with a metal matrix and inclusions of pre-
liminarily prepared nanocomposite thermite [114].
Ignition of such particles is assisted by the thermite
reaction; however, the particles contain large amount
of unoxidized metal, which can react when the
1 µm
Al MoO3
Figure 6 Cross section of a milled Al-MoO3 thermite nanocom-
posite particle.
Temperature, °C200 400 600 800 1000
Mas
s ch
ange
, % (A
lI 3, I
2)
-100%
-80%
-60%
-40%
-20%
0%
Mas
s ch
ange
, % (A
l-I2)
-5%
-4%
-3%
-2%
-1%
0%
Al-I2(nom. 5 wt%)
I2
AlI3
Figure 7 Thermogravimetric trace of an Al-iodine composite
heated in a flow of argon (right axis). Elemental iodine and
commercial AlI3 are shown on the left axis for comparison. Axes
are scaled relative to nominal iodine content.
J Mater Sci (2017) 52:11789–11809 11797
external oxidizer becomes available. On the opposite
end of the spectrum, one can prepare nanosized
particles with unique surface properties and tunable
reactivity [54, 94]. Custom size distributions can also
be achieved, as was shown for Al�Mg alloys when a
specific PCA was used [115].
In the future, the mechanochemical approach can
be extended for preparation of customized organic
reactive and energetic materials as well as for
developing environmentally friendly material syn-
thesis techniques in general.
Ignition and combustion mechanisms
Although ignition and combustion characteristics
define the utility of RMs, these characteristics cannot
be treated as their properties; they depend on the
specific experimental configurations and ambient
conditions. For example, ignition of a material or
component is expected to occur differently when
initiated by impact of a projectile or by an external
heat source, such as a flame torch. Similarly, com-
bustion of the same RM will occur at different rates
and possibly in different regimes at various external
pressures and with different types and concentra-
tions of the ambient oxidizers. Packaging of RM, e.g.,
considering a single particle, particle pile, or a con-
solidated pressed sample will also affect both com-
bustion and ignition. Thus, ignition and combustion
characteristics of RMs can only meaningfully ana-
lyzed when the experimental configurations are
clearly identified. The configurations used in labora-
tory experiments, such as ignition of a powder-
like material using an electrically heated filament
[116–118], ignition of individual RM particles in
flames [85] or by a laser beam [119], ignition of
individual RM particles by a shock wave [120], and
others are particularly useful for understanding
processes causing ignition. Once such processes are
understood, an ignition and combustion model can
be developed describing reactions in RMs while
taking into account the heat and mass transfer pro-
cesses affected by the external conditions.
The reactions leading to ignition and occurring
during combustion are affected by both the RM
compositions and by methods used to prepare the
RMs. In particular, unusual and interesting charac-
teristics are expected for mechanochemically pre-
pared RMs, associated with relaxation of the
metastable states achieved during the material syn-
thesis. This part briefly discusses related processes
and mechanisms.
Ignition of mechanochemically preparedRMs
Phase changes and reactions leading to ignition
Thermo-analytical measurements, such as DSC and
TG, are used routinely to identify reactions leading to
ignition of RMs. The advantages of these techniques
are the capability of identifying specific reactions and
quantifying their kinetics, e.g., using measurements
at different heating rates and applying isoconver-
sional methods of data analysis. A detailed review
of relevant methods and techniques is available
[121]. For all mechanochemically prepared,
metastable RMs, it is common to detect one or more
exothermic phase changes or reactions occurring
upon heating in an inert environment. Examples of
such exothermic features detected by DSC are shown
in Fig. 8. Results are shown for various aluminum-
based materials heated in argon. Only the exothermic
processes occurring at temperatures below the alu-
minum melting point are shown. Such processes are
the most significant in defining the ignition delays.
Among the least exothermic reactions, are subsolidus
transformations generating intermetallics in Al�Mg,
Al�Li, Al�Ti and other similar systems. More
exothermic reactions are detected for Ni�Al and
metal–metalloid systems, such as B�Ti (not shown).
Redox reactions in thermites and similar composi-
tions are the most exothermic.
The exothermic features observed in DSC repre-
sent reactions accompanying relaxation of the
metastable states obtained in the mechanochemically
prepared RMs. Specific phases formed in such reac-
tions can be usually identified by examining partially
reacted samples. Examples of such reactions for dif-
ferent ball-milled RMs, all occurring in solid phase,
are given in Table 2. Most importantly, the heat effect
and reaction kinetics can be determined processing
the DSC measurements.
Reaction mechanisms obtained from thermo-analytical
measurements
Identified reaction kinetics can serve as a foundation
for the model, describing ignition of RMs. Ignition
11798 J Mater Sci (2017) 52:11789–11809
can be predicted as a thermal runaway analyzing the
heat balance of an RM exposed to an ignition stim-
ulus. In the simplest case, the effect of the ignition
stimulus can be reduced to heating the RM at a
specified rate. Heat transfer with the ambient envi-
ronment along with the exothermic reactions in the
material, quantified from thermo-analytical mea-
surements should be accounted for, while predicting
the time to the thermal runaway, or ignition delay. A
specific ignition criterion is usually required, which
establishes a threshold temperature at which the
reaction mechanism considered during preignition
process ceases being prevailing. For example, when
the boiling temperature of a metal, such as alu-
minum, is reached, vapor phase oxidation, which is
neglected at lower temperatures, becomes a pre-
dominant reaction pathway.
Such models have been discussed in the literature,
employing the results of processing of the DSC
measurements to construct single or multistep reac-
tions leading to ignition. For example, models eval-
uating the rate of diffusion of reacting species
through a growing layer of aluminum oxide were
considered, accounting for the changes in properties
associated with polymorphic phase changes from
amorphous to c, and to a-alumina occurring at
increasingly higher temperatures. An example of
related calculation for Al–CuO thermite is shown in
Fig. 9a [135]. Experimental DSC traces are plotted
along with the ones predicted by the model. The
model describes successfully a multistep exothermic
reaction and the effect of heating rate on the position
and shape change of the exothermic peaks. In this
calculation, the temperature increases linearly, as in a
thermo-analytical experiment. In Ref. [136], this same
reaction model was coupled with a heat transfer
analysis considering heating the reactive powder on
top of an electrically heated filament. The powderTable 2 Examples of mechanochemically prepared composites
and respective observed reactions
Material Reaction References
Al�Ni Al ? Ni ? NiAl [125–128]
Al�Ti Al ? Ti ? Al3Ti [17, 129]
Al�Mg Al ? Mg ? A12Mg17, Al3Mg2 [16, 87, 130, 131]
B�Ti B ? Ti ? TiB2, TiB [110, 131]
Al�MoO3 Al ? MoO3 ? Al2O3 ? Mo [73, 79, 132, 133]
Al�PTFE Al ? (C2F4)n ? AlF3 ? C [78, 79]
Mg�S Mg ? S ? MgS [134]
Temperature, K300 600 900 1200
0.0E0
5.0E3
1.0E4
1.5E4Hea
t flo
w, W
/g
0.0
1.0
2.0
3.0
4.0
Wire Ignition, 16033 K/s
Model Experiment2 K/min5 K/min10 K/min
a
b
Figure 9 Heat flow as a function of temperature: DSC measure-
ments and respective calculations from Ref. [135] (top) and
calculations for a wire ignition experiment from Ref. [136].
Temperature, ºC
Hea
t Flo
w, m
W/m
g
Temperature, K
100 200 300 400 500 600
400 500 600 700 800 900
exo
Al.Ti
Al.Li
Al.Mg
Al.Ni
Al.MoO3
Al.CuO
Figure 8 Characteristic DSC traces for aluminum-based
mechanochemically prepared nanocomposite materials heated in
argon. Al�Mg [87], Al�Ti [122], and Al�Li [72] composites were
heated at 15 K/min, Al�Ni composite at 10 K/min [105], and
Al�MoO3 [123] and Al�CuO [124] thermites at 5 K/min. Vertical
scales are adjusted to fit all traces together; traces are shifted
vertically for clarity.
J Mater Sci (2017) 52:11789–11809 11799
was in thermal contact with a metal wire; the top
layer of the powder was cooled convectively by the
ambient air. The heating rate for the wire in the cal-
culation was selected to match that of a respective
experiment. The result of the calculation is shown in
Fig. 9b [136]. The exothermic peaks shift to higher
temperatures and merge. At about 950 K, a sharp
exothermic spike is predicted, occurring at about the
same temperature, at which ignition of this material
is observed experimentally. However, the predicted
heat flow spike fails to generate a substantial increase
in the powder temperature. Instead, the filament
serving as a heat source for a cold powder becomes
an effective heat sink when the powder starts self-
heating. Because of the contact with the filament, the
powder temperature remains very close to that of
the filament, despite a rapid exothermic reaction.
According to this analysis, powder can reach a higher
temperature and ignite only if the thermal coupling
between it and the filament is disrupted. This can
occur under conditions typical for high heating
rate experiments, but not in thermo-analytical
measurements.
Additional processes occurring upon rapid heating/
alternative initiation
Gas release from the mechanochemically prepared
thermite powders in contact with an electrically
heated filament was characterized in Refs. [136, 137].
The heating rates in such experiments vary in the
range of 103–105 K/s. The powder-coated wire was
placed in a small, evacuated chamber, and the pres-
sure increase caused by the gas released from the
powder was measured. For Al�CuO, Al�Fe2O3, and
Al�MoO3, it was observed that the gas release
occurred prior to detectable optical emission accom-
panying ignition. Typically, the pressure increase
was detected at temperatures just exceeding 900 K.
The released gas was assumed to be oxygen, pro-
duced by the decomposing oxidizers. No gas release
was detected in experiments in which oxidizers alone
(either micron or nanosized powders) were heated
up with the same rates and to the same maximum
temperatures.
Thus, the oxidizers, such as CuO, Fe2O3, and MoO3,
are relatively stable and do not rapidly decompose
upon heating unless they are mechanochemically
mixed with aluminum. To understand the mechanism
of gas release, consider Fig. 10 [136], comparing the
experimental ignition temperatures, calculated tem-
peratures of the heat flow spike (cf. Fig. 9), and tem-
peratures, at which specific percentages of oxygen are
predicted to be consumed in a Al�CuO thermite sub-
jected to different heating rates. The consumption of
oxygen is governed by the reaction proceeding with
the rate quantified from thermal analysis. As noted
above, the experimental ignition correlates well with
the heat flow spike predicted in calculations. More
importantly, ignition occurs when 3–5% of oxygen
contained in the original oxidizer, CuO have been
consumed. Shown are percentages based on the total
amount of oxygen in CuO; the oxygen consumption
could be even higher in the CuO layers adjacent to Al.
Thus, prior to ignition, the nanocomposite material
no longer contains thermodynamically stable CuO;
instead, it contains CuO1-x, where x is at least
0.03–0.05. It is proposed that the off-stoichiometric
oxide may be unstable on heating and decompose,
forming Cu2O and O2 gas. This reaction explains the
observed pressure rise.
Release of oxygen disrupts packed particles and
breaks their thermal contact with other surfaces,
including that of the wire, making it possible for the
predicted heat release to raise the temperature of RM
particles more significantly. Accounting for this pro-
cess enables one to predict ignition theoretically, in
line with the experimental results. Although not
observed in DSC experiments, the release of oxygen
Figure 10 Effect of heating rate on the experimental ignition
temperature (points with error bars) and calculated constant
oxygen consumption curves (solid lines). The constant oxygen
consumption curves and the trend showing occurrence of a sharp
heat flow spike caused by the polymorphic phase change in
alumina (dashed line) are calculated for 200 nm CuO inclusions.
Reproduced from Ref. [136] with permission from Begell House.
11800 J Mater Sci (2017) 52:11789–11809
by mechanochemically prepared materials is intrin-
sically associated with their structures, specifically a
large interface area between metal and oxidizer.
Because of the large interface area, a relatively low-
rate, low-temperature reaction consumes enough
oxygen to generate a metastable CuO1-x phase prior
to ignition. The low-temperature reaction leading to
formation of CuO1-x is observed in DSC experiments,
its rate is quantified, so that it can be used to predict
ignition directly, once the metastability of CuO1-x is
included in the model.
Other processes accompanying rapid heating may
be important for ignition of different RMs. Examples
of such processes are disruption of the continuity of
the oxide layers due to the thermal expansion mis-
match, release of gasified impurities, reactions
induced by strong thermal gradients in the heated
samples. A complete ignition model can often be
developed accounting for such processes along with
quantified reaction rates employing thermo-analyti-
cal measurements.
Combustion of mechanochemicallyprepared RMs
Combustion of metal-based RMs occurs typically at
high temperatures, exceeding the melting points of
the primary material components, such as Al, Mg,
and Ti. Thus, the nanostructure of the starting
material prepared mechanochemically may be lost. If
so, the initially finely mixed phases may separate,
leading to combustion of a material, which com-
pletely ‘‘forgets’’ its initial structure and thus the
method used for its preparation.
Indeed, for many mechanochemically prepared
RMs, the effect of structure is important during
ignition only. Once high combustion temperatures
are reached, the reaction proceeds similarly to what
would be expected for materials with the same
compositions obtained by other methods, e.g., cast
alloys.
A different observation was reported in experi-
ments [133, 138, 139] describing ignition of
fully dense nanocomposite thermites prepared
mechanochemically. When heated relatively slowly
with rates up to 106 K/s, aluminum-rich, micron-
sized Al�MoO3 particles were observed to burn with
rates characteristic of aluminum particle combustion.
Characteristic burn times were a few ms. A typical
optical emission trace that was used to obtain a burn
time for a particle ignited by a CO2-laser beam is
shown in Fig. 11a. Conversely, when these same
particles were ignited by electrostatic discharge
(ESD), being heated at about 109 K/s, they were
observed to burn much faster, with typical burn
times of about 0.1 ms. A measured emission trace
indicating a complete combustion event for the ESD-
ignited powder is shown in Fig. 11b. The difference
in the observed durations of the complete combus-
tion events for the same powder particles is striking.
When initiated very rapidly, the particles are con-
sumed nearly twenty times faster than it takes them
to burn after being heated in a laser beam.
This observation is consistent with the idea that
when heated relatively slowly, the reactive nanos-
tructure of the composite is lost before the reaction
can come to completion. However, when heated with
a sufficiently strong stimulus, the reaction propagates
before the structure has time to change.
Schematically, the difference between reactions
occurring for slowly and rapidly initiated nanocom-
posite particles is illustrated in Fig. 12. A starting
particle contains an aluminum matrix with embed-
ded nanoscale oxidizer inclusions. When heated
slowly, the temperature gradients in the heated par-
ticle are negligible. An aluminum molten drop is
Time, ms10-5 10-4 10-3 10-2 10-1 100 101
PMT
volta
ge
a. Laser ignition
b. ESD ignition
Figure 11 Optical emission traces produced by a single nanocom-
posite Al�MoO3 particle ignited by passing through a focused CO2
laser beam with the heating rate of 106 K/s [119] (a), and by
nanocomposite Al�MoO3 powder placed in a monolayer on a
conductive substrate and ignited by an electrostatic discharge with
the estimated heating rate of 109 K/s [138] (b).
J Mater Sci (2017) 52:11789–11809 11801
formed that is in contact with agglomerated or coa-
lesced oxidizer inclusions. Aluminum surface tension
pools the metal into a single droplet, pushing oxide
inclusions together, or even to the outer surface of the
droplet. The area of the reaction interface between the
aluminum and condensed phase oxidizer is mark-
edly reduced. The reaction occurs both at the surface
of the formed aluminum particle and across the
reduced aluminum/oxidizer interface. When the
particle is heated rapidly, and when the temperature
gradients are significant inside the particle, it is pos-
sible that a portion of the particle is molten and even
heated above the melting point, while the rest of the
particle remains solid. Thus, reaction between alu-
minum and oxidizer will initiate across the initial
metal–oxide interfaces existing in the nanocomposite
material. Once the reaction is initiated rapidly, tem-
perature will rise locally. Gasified reaction products
may be released and expelled along the grain
boundaries in the metal matrix. The heat transfer
between grains will be reduced; however, the local
heat generation within grains will continue. The
composite structure will thus be preserved and
additional fragmentation can occur. The resulting
reaction rate will thus be determined by the surface
interface between aluminum and oxidizer inclusions;
it can further increase depending on the effective size
of the produced fragments.
A characteristic time, s, separating fast and slow
heating regimes can be roughly evaluated as that
necessary for the temperature to equilibrate across
the particle, s ¼ d2
j , where d is characteristic particle
dimension, e.g., 10 lm, and j is the characteristic
thermal diffusivity, that can be conservatively taken
as that of the pure aluminum, e.g., 9.6 9 10-5 m2/s.
This estimate yields s � 1 ls; for composite particles,
this time would certainly be noticeably longer
because of lower thermal diffusivity. Characteristic
time of heating for ESD-initiated particles is of the
order of 1 ls or shorter; therefore, the heating is fast
and the nanostructure existing in the starting mate-
rial is likely to be preserved.
The idea that two different combustion scenarios/
mechanisms are possible is supported by different
shapes and morphologies of particles captured after
ESD ignition experiments and shown in Fig. 13. Both
product particles shown were produced by combus-
tion of the same composite 2Al�3CuO powder and
captured on a thin aluminum foil. The particle on the
left was ignited by ESD striking a monolayer of the
nanocomposite powder placed in a brass sample
holder. In this case, the ESD directly heats particles
resulting in the heating rates of 109 K/s. The particle
shows multiple inclusions with the typical Cu–Al2O3
morphology of thermite combustion products, which
remain embedded on the scale resembling that of the
original composite nanostructure. Some of such
inclusions are magnified and shown as insets in the
same image. Rounded shapes for both Cu and Al2O3
clearly suggest that the particle was reacting at a high
temperature, while its nanocomposite structure
remained mostly intact. Conversely, the particle
shown on the right originated from a thicker powder
layer and was likely heated slower, by combustion of
the few initially spark-ignited particles. The heating
rate for this case will not exceed 106 K/s. The phases
of copper and aluminum oxide are separated from
each other on a scale of several lm, much coarser
than the scale of mixing in the initial ball-milled
material.
The feasibility of a rapid heterogeneous reaction
while maintaining the nanocomposite structure can
be supported further considering the time scales
involved [140]: lattice vibrations, and therefore ther-
mal effects have equilibrium times in the range of
10-9–10-14 s; atomic transport in the absence of any
reactions equilibrates over short distances within
10-2–10-6 s. Therefore, energy can be delivered to the
nanocomposite much faster than its geometry canFigure 12 Schematic illustration of structures of reacting com-
posite particles initiated at different heating rates.
11802 J Mater Sci (2017) 52:11789–11809
change, and if the reaction between the components
occurs faster than the atomic transport time scale, a
qualitatively different bulk reaction rate is expected.
A related argument was put forward recently in
the context of aluminum nanoparticle combustion
and the typically observed d-power law combustion
behavior where the exponent becomes non-integer,
and less than unity as the particle size decreases
[141]. It was argued that multiple Al nanoparticles
rapidly fuse together into larger particles, which then
burn more slowly than expected for the original
particle size. The time scale s for nanoparticles to
coarsen can be phenomenologically related to Fren-
kel’s law [142], s ¼ gd=r, where g is the temperature-
dependent viscosity, d is the particle diameter (or
more generally a representative length scale), and r is
the surface tension. Surface tension can be substan-
tially different for interfaces of molten aluminum and
different solid oxides; it can be further affected by
small amounts of dopants, added to either aluminum
or oxide. Manipulating the surface tension offers
potentially a way of tuning the rate of fusion of
molten aluminum, and thus the rate of its ensuing
combustion defined by the available surface area of
the reaction interface.
The effects in the nanocomposites considered here
are complex, as different phases (metal fuel, oxide
oxidizer) have complex interfaces, and a priori
unknown materials properties such as viscosity and
interface energies. Some relevant literature can be
found in the context of nanoparticle sintering, and
annealing of metastable and amorphous solids (e.g.,
[140]). Nevertheless, the time scale of the complete
reaction for nanocomposites decreases with increas-
ing available reactive interface area, which is readily
controlled during preparation. Therefore, some of the
qualitative changes in the nanocomposite combustion
rate caused by different initiation may also be
observable by tuning the nanostructure while keep-
ing the initiation stimulus constant.
Although a detailed theoretical description of
the dramatic acceleration of combustion for the
nanocomposite thermite particles ignited by ESD has
not yet been developed, an ability to tune the burn
rate of RMs experimentally by varying their initiation
stimulus is of great practical interest. In insensitive
munitions development, such a capability can guide
design of a ‘‘smart’’ booster. When the booster is
activated, the charge would react rapidly, otherwise,
if initiated by other stimuli, the charge would react
slowly, minimizing any incurred damage. The same
capability can be used to design multiple initiators
for a single warhead, tuning its performance
depending on the specified target. Finally, under-
standing greatly accelerated reaction rates observed
in Refs. [138, 139] may help achieving long-sought
detonation in metal-based RMs.
Conclusions
A broad range of reactive materials prepared
mechanochemically includes intermetallics, ther-
mites, metal–metalloid, and metal–polymer compos-
ites. Such materials typically comprise fully dense
micron-sized particles with reactive components
Figure 13 Combustion products of Al�CuO nanocomposite pow-
der ignited by ESD and collected on an aluminum foil. Left particle
ignited from a powder monolayer, burn time *100 ls; right
particle ignited from a thicker powder layer, burn time *5 ms.
Note the respective scale bars.
J Mater Sci (2017) 52:11789–11809 11803
mixed on the nanoscale. The mechanochemical
method is versatile and readily scalable, and attracts
significant interest in the energetics community.
Multiple practical challenges, including safety of
operation, scaling-up process parameters from labo-
ratory to commercial devices are being addressed in
the current research. The key feature of the
mechanochemically prepared composite reactive
materials is their metastability. The relaxation of
metastable states generates fresh reactive surface;
often it is also accompanied with heat generation.
Therefore, external ignition stimuli are supported
and the materials ignite readily. Unique reaction
mechanisms can be enabled for the mechanochemi-
cally prepared reactive materials due to modification
of their composition occurring at relatively low tem-
peratures, prior to their ignition. Because of the fine
scale of mixing between components, even a rela-
tively low-rate reaction proceeding at low tempera-
tures in such materials causes their substantial
modification. The composition is thus altered com-
pared to the initial one, which could change
the reaction kinetics and even mechanism. An unu-
sual combustion mechanism specific for the
mechanochemically prepared nanocomposite mate-
rials is also discussed, for which the nanostructure is
partially preserved in the material when it burns at a
temperature exceeding melting points of all starting
components. In this case, the exothermic chemical
reaction propagates through the fully dense com-
posite particles faster than the melting front.
Acknowledgements
This work was funded in parts by Defense Threat
Reduction Agency (Grant No. HDTRA1-11-1-0060),
US Army Research Office (Grant No. W911NF-12-1-
0161), and Air Force Office of Scientific Research
(Grant No. FA9550-16-1-0286).
Compliance with ethical standards
Conflict of interest The authors declare that they
have no conflict of interest.
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