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: dreizin@njit.edu

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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