This journal is
c
The Royal Society of Chemistry 2013 Chem. Soc. Rev., 2013, 42, 7571--7637 7571
Cite this: Chem. Soc. Rev., 2013,
42, 7571
Hallmarks of mechanochemistry: from nanoparticles
to technology†
Peter Bala
´
z
ˇ
,*
a
Marcela Achimovic
ˇ
ova
´
,
a
Matej Bala
´
z
ˇ
,
a
Peter Billik,
bc
Zara Cherkezova-Zheleva,
d
Jose
´
Manuel Criado,
e
Francesco Delogu,
f
Erika Dutkova
´
,
a
Eric Gaffet,
g
Francisco Jose
´
Gotor,
e
Rakesh Kumar,
h
Ivan Mitov,
d
Tadej Rojac,
i
Mamoru Senna,
j
Andrey Streletskii
kl
and Krystyna Wieczorek-Ciurowa
m
The aim of this review article on recent developments of mechanochemistry (nowadays established as a
part of chemistry) is to provide a comprehensive overview of advances achieved in the field of atomistic
processes, phase transformations, simple and multicomponent nanosystems and peculiarities of
mechanochemical reactions. Industrial aspects with successful penetration into fields like materials
engineering, heterogeneous catalysis and extractive metallurgy are also reviewed. The hallmarks of
mechanochemistry include influencing reactivity of solids by the presence of solid-state defects,
interphases and relaxation phenomena, enabling processes to take place under non-equilibrium
conditions, creating a well-crystallized core of nanoparticles with disordered near-surface shell regions
and performing simple dry time-convenient one-step syntheses. Underlying these hallmarks are
technological consequences like preparing new nanomaterials with the desired properties or producing
these materials in a reproducible way with high yield and under simple and easy operating conditions.
The last but not least hallmark is enabling work under environmentally friendly and essentially
waste-free conditions (822 references).
1. Introduction
1.1 Definitions
Despite the long history of mechanochemistry, the definition of
a mechanochemical reaction was only recently incorporated
into the chemical literature. The recent IUPAC Compendium
of Chemical Terminology defines mechanochemical reaction as
a‘‘chemical reaction that is induced by mechanical energy’’.
1
However, the term mechanochemistry was introduced by
Ostwald
2a,b
much earlier. He was engaged in the systematiza-
tion of chemical sciences from the energetic point of view. He
understood mechanochemistry in a wider sense when com-
pared with the present view, regarding it as a part of physical
chemistry at the same level as thermochemistry, electro-
chemistry or photochemistry. After ten years, the practically
forgotten book by Pierce on mechanochemistry was published.
2c
He defined mechanochemistry as ‘‘the new science of mechanical
dispersion involving the use of principles in physical chemistry’’.
He hesitated to name this science, remarking ‘‘we shall call it
mechanochemistry for lack of a better name, as it involves disper-
sion or defflocculation by mechanical means, thereby bringing
about so-called colloidal dispersions’’. In this book, the relation-
ship between mechanochemistry and nanoscience can be traced
a
Institute of Geotechnics, Slovak Academy of Sciences , Watsonova 45, 04353,
Kos
ˇ
ice, Slovakia. E-mail: balaz@saske.sk; Fax: +421557922604;
Tel: +421557922603
b
Faculty of Natural Sciences, Comenius University, Mlynska
´
Dolina,
84215 Bratislava, Slovakia
c
Institute of Measurement Science, Slovak Academy of Sciences, Du
´
bravska
´
cesta 9,
84104 Bratislava, Slovakia
d
Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev St.,
Bldg. 11, 1113 Sofia, Bulgaria
e
Instituto de Ciencia de Materiales de Sevilla, C.S.I.C., Ame
´
rico Vespucio 49,
41092 Sevilla, Spain
f
Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali,
Universita
`
degli Studi di Cagliari, via Marengo 2, I-09123 Cagliari, Italy
g
Institut Jean Lamour, UMR 7198 CNRS, Ecole des Mines de Nancy,
Universite
´
de Lorraine, Parc de Saurupt – CS 14234, F54042 – Nancy Cedex, France
h
CSIR-National Metallurgical Laboratory, Jamshedpur-831 007, India
i
Joz
ˇ
ef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
j
Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku,
Yokohama, 223-8522, Japan
k
Institute Chemical Physics, Russian Academy of Sciences, Kosygina 4,
119991 Moscow, Russia
l
Moscow Institute of Physics and Technology, 9 Institute str., Dolgoprudniy,
Moscow region, 141700, Russia
m
Faculty of Chemical Engineering and Technol ogy, Cracow University of
Technology, 24 Warszawska Str., 31-155 Cracow, Poland
† Dedicated to the memory of Prof. Pavel Yurievich Butya gin.
Received 15th November 2012
DOI: 10.1039/c3cs35468g
www.rsc.org/csr
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for the first time (colloidal dimensions = 1–1000 nanometers) and
the first colloidal mill (Fig. 1) was advertised there. Sub-
sequently, the boundaries of mechanochemistry contracted. For
instance, Hu
¨t
tig
2d
assumes that mechanochemistry includes only
the release of lattice bonds without any formation of new sub-
stances (i.e. he supports the physical approach), while Peters
2e
also
puts transformations due to mechanical stress of material that are
accompanied by chemical reaction into this category. It was
Butyagin
2f
who contributed a certain unification. He considered
the behaviour of solids exposed to the effect of mechanical energy
from the viewpoint o f three main aspects: structural disordering,
structure relaxation and structural mobility. In real conditions,
these three factors simultaneously affect the reactivity of solids.
At present, the definition of Heinicke
2g
is widely accepted:
‘‘mechanochemistry is a branch of chemistry which is concerned
with chemical and physico-chemical transformations of substances in
all states of aggregation produced by the effect o f mechanical energy’’.
There is another frequently used term in mechanochemistry –
mechanical activation (MA). The term was introduced by
Sme
´
kal,
4a
who regarded it as a process involving an increase
in reaction ability of a substance which remains chemically
unchanged. In this case, the MA precedes the reaction and has
no effect during the course of this reaction. Provided the
activation brings about a change in composition or structure,
it is a mechanochemical reaction. The definitions of mecha-
nical activation published later were always dependent on the
observed effect. Butyagin
4b
defined MA as an increase in
reaction ability due to stable changes in solid structure.
Structural relaxation plays an important role in mechanical
activation. The concept of slowly changing states after inter-
rupting the action of mechanical forces has been described by
Lyachov.
4c
He published a generalised relaxation curve for
activated solids where individual parts of the curve correspond
to processes with different characteristic times of relaxation
(Fig. 2).
By this theory there is no possibility of influencing the
reactivity of activated solids in states whose relaxation times
are shorter than the characteristic time of the reaction itself. On
the contrary, some long-living states (e.g. surface area) may be
regarded as constant during the course of reaction and their
influence has to be a subject of mechanical activation studies.
As for the kinds of relaxation processes, various processes were
described: heating, formation of a new surface, aggregation,
recombination, adsorption, imperfections, chemical reaction
Fig. 1 Plauson-Oderberg colloid mill for wet milling.
3
From left to right: (top row) Peter Bala
´
zˇ, Marcela Achimovic
ˇ
ova
´
, Matej Bala
´
zˇ, Peter Billik, Zara Cherkezova-Zheleva,
Jose
´
Manuel Criado, Francesco Delogu, Erika Dutkova
´
; (bottom row) Eric Gaffet, Francisco Jose
´
Gotor, Rakesh Kumar,
Ivan Mitov, Tadej Rojac, Mamoru Senna, Andrey Streletskii, Krystyna Wieczorek-Ciurowa
Professor Peter Bala
´
z
ˇ
graduated in chemistry at the Faculty of Natural Sciences, P.J. S
ˇ
afa
´
rik University, Kos
ˇ
ice, in 1971. He has been active
in the field of mechanochemistry since 1977 and his basic and applied research encompasses the field of solid-state chemistry,
nanoscience and minerals engineering. He is the author of 4 books and 240 papers in reviewed journals. His papers have been cited more
than 1100 times in Science Citation Index. He is a founding member of the International Mechanochemical Associaton. Since 1977 he has
worked at the Institute of Geotechnics of Slovak Academy of Sciences in Kos
ˇ
ice, Slovakia.
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between adjoining particles, etc.
4d,e
The rate of these relaxation
processes may be vastly different and the processes can change
from one way of relaxation to the other. Thus, MA can be
regarded as a multi-step process with changes in the energetic
parameters and the amount of accumulated energy of solids in
each step. The four processes, namely the accumulation of
defects, amorphization, the formation of metastable poly-
morphous forms and chemical reaction, are united by the
term mechanical activation. Especially, various types of defects
(Fig. 3) play important roles in mechanochemistry.
4f
Juha
´
sz proposed that processes under the influence of
mechanical activation can be subdivided into primary and
secondary ones.
4e,h,i
The primary process (e.g. increase of internal
and surface energy, increase of surface area, decrease of the
coherence energy of solids) generally increase the reactivity of
the substance. The secondary processes (e.g. aggregation,
adsorption, recrystallization) take place spontaneously in acti-
vated systems and may appear even during milling or after
milling has been completed.
However, the terms mechanochemistry and mechanical
activation (MA) are not the only ones. Even contributors to this
critical review use different terminology. For this reason a list of
abbrevations has been inserted at the end of this paper.
1.2 History of mechanochemistry
Important players. There are several approaches on how
to deal with this topic. According to the paper by Takacs,
5a
the earliest documented mechanochemical reaction may have
been milling of cinnabar (HgS). Theophrastus of Eresus
(371–286 BC), student and successor of Aristotle at the Lyceum
in Athens, wrote in his book De Lapidibus (On Stones) that
‘‘native cinnabar was rubbed with vinegar in a copper mortar with
a copper pestle yielding the liquid metal’’. This is a very clear
description of a mechanochemical process. The first described
mechanochemical reduction probably followed the reaction
HgS + Cu - Hg + CuS (1)
where vinegar was added to prevent the side effects which
usually accompany dry milling in air. It remains a mystery
why the mechanochemical preparation of mercury from its
sulphide according to reaction (1) was forgotten during the
Middle Ages. However, as published in ref. 5a, examples of
other mechanochemical reactions between 300 BC and the end
of the 18th century can be also traced in medieval literature.
Agricola documented several examples of chemical reactions
under the influence of mechanical action which can be con-
nected with mining and metallurgical operations.
5b,c
It is
interesting to note that in the 17th century Bacon referred to
four treatments that, in essence, are still among the most
important procedures to prepare active solids. One of them is
milling.
5d
It was Wenzel who stressed the fact that the degree of
conversion of heterogeneous reactions depends mainly on the
surface area of the reacting solids and is not proportional to
their amount.
5e
The quick look into the history of mechanochemistry in
ancient and medieval times above is not exhaustive. Many other
important scientists contributed to the development of
mechanochemistry, e.g. Baramboin, Bowden, Carey Lea, Clark,
Dachille, Faraday, Flavickii, Fink, Fox, Grohn, Gutmann,
Heinicke, Hofman, Hu
¨
ttig, Juha
´
sz, Khodakov, Oprea, Ostwald,
Parker, Paudert, Peters, Rowan, Roy, Schrader, Simoniescu,
Sme
´
kal, Tabor, Tamman, Thiessen, Yoffe and Wanetig. This
list of mechanochemists is not complete but all of them
contributed to the establishment of mechanochemistry.
6,7
The small (and incomplete) excursion into history of mechano-
chemistry can be broadened by reading several review papers
given in Table 1.
In the modern history of mechanochemistry the Inter-
national Conference on Mechanochemistry and Mechanical
Alloying (INCOME) series was initiated by the International
Mechanochemistry Association IMA (an associate member of
IUPAC). IMA was established in 1988 in Tatranska
´
Lomnica,
Slovakia, see Fig. 4. The first INCOME conference was held in
Kos
ˇ
ice (1993) and it was followed by conferences in Novosibirsk
(1997), Prague (2000), Braunschweig (2003), Novosibirsk (2006),
Jamshedpur (2008) and Herceg Novi (2011).
Theories and models. Several theories and models in the
history of mechanochemistry have been elaborated.
6
In hot-spot
theory, the hypothesis for the reason of mechanical initiation of
chemical reactions was developed by Bowden, Tabor and
Yoffe.
9a–c
They found out that with friction processes, tempera-
tures of over 1000 K on surfaces of about 1 mm
2
can occur
and last for 10
4
–10
3
s and that they represent an important
Fig. 2 A generalised relaxation curve of a me chanically activated state.
4c
Fig. 3 Defects created by MA of solids. Reprinted with permission from ref. 4 g .
Copyright 2005, Wiley.
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cause of mechanically initiated reactions. These temperatures
can be also reached near the tip of a propagating crack.
9d
Later
this theory was expanded for other processes, e.g. oxidation
of metals. Several interpretations have been described.
7
For
example, it was proved that the processes occurring at the tip of
a crack during the cleavage of a crystal may proceed by different
mechanisms depending on the speed of crack motion.
9e,f
This
theory has been strongly criticized recently.
9g
In the sixties, the
first model in mechanochemistry – the magma-plasma model –
was proposed.
10
According to this model, a huge amount of
energy is set free at the contact spot of colliding particles. This
energy is responsible for the formation of a special plasmatic
state which is characterized by emission of fairly excited frag-
ments of solid substance, electrons and photons over a short
time (Fig. 5). The surface of colliding particles is rather dis-
ordered and local temperatures can reach more than 10 000 K.
Thiessen et al. distinguish the reactions which occur in the
plasma from the reactions taking place at the surface of
particles during the significantly excited state or immediately
after its expiration. These considerations led to an important
conclusion which is valid for mechanically activated reactions:
these reactions do not obey a single mechanism.
Later on, mainly German and Russian mechanochemists have
developed several other models and theories like the spherical
model,
2f
dislocation and phonon theory,
11a,b
the theory of short-l iving
active centers,
11c
the kinetic model
11d
(expanded recently
11e,f
), the
impulse model
4d,11d,g
and the analogy model.
11h
These models and
theories are described in more details in a monograph (ref. 7).
1.3 Mechanochemical tools and how to rule them
The multi-stage character of MA requires high-energy mills
with different working regimes (compression, shear, impact).
The principles of the most frequently applied mills are shown
in Fig. 6.
There are several variables which influence the milling
process, e.g. type of the mill, material of milling media, ball-
to-powder ratio, filling extent of the milling chamber, milling
atmosphere, milling speed, milling time, etc.
7
The purpose of an ideal device for a mechanochemical
synthesis (MCS) is to insert the maximum amount of energy
into the treated solid to enable the accumulation of the input
energy. This energy is responsible for the occurrence of defects,
which greatly affect chemical reactivity. This requires high-
energy inputs to be transferred from the working medium to
Table 1 Review papers on history of mechanochemistry
Title Author(s) and year Reference
Mechanochemische reaktionen Peters, 1962 2d
Review of the phase transformation and synthesis of inorganic solids
obtained by mechanical treatment (mechanochemical reactions)
Lin and Nadiv, 1970 8a
Mechanically initiated chemical reactions in solids Fox, 1975 8b
Mechanochemistry of inorganic solids Boldyrev, 1986 4d
Accelerating the kinetics of low-temperature inorganic synthesis Roy, 1994 8c
Colloid-chemical aspects of mechanical activation Juha
´
sz, 1998 4j
Mechanochemistry of solids: past, present and prospects Boldyrev and Tka
´
c
ˇ
ova
´
, 2000 4f
Mechanochemistry in extractive metallurgy: the modern science with old routes Bala
´
zˇ, 2001 8d
M. Carey Lea, the first mechanochemist Takacs, 2004 8e
Mechanochemistry: the mechanical activation of covalent bonds Beyer and Clausen-Schaumann, 2005 8f
Mechanochemistry and mechanical activation of solids Boldyrev, 2006 8g
The mechanochemical reduction of AgCl with metals: revisiting an
experiment of M. Faraday
Takacs, 2007 8h
Fig. 4 The foundation of International Mechanochemical Association (IMA) in
Tatranska
´
Lomnica, Slovakia, in 1988. From left to the right: A. P. Purga (Russia),
V. Jesena
´
k (Slovakia), I. Hocmanova
´
(Slovakia), L. G. Austin (USA), P. Bala
´
z
ˇ
(Slovakia), M. Senna (Japan), interpreter, E. G. Avvakumov (Russia), L. Opoczky
(Hungary), K. Tka
´
c
ˇ
ova
´
(Slovakia), N. Z. Lyachov (Russia), V. V. Boldyrev (Russia),
H.-P. Hennig (Germany), N. S
ˇ
tevulova
´
(Slovakia), H.-P. Heegn (Ge rmany), P. Yu.
Butyagin (Russia).
Fig. 5 Magma-plasma model: E – exo-electrons, N – undeformed solid,
D – highly deformed surface layer, P – plasma.
10
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the treated solid during the mechanochemical reaction. Due to
their high energy density, and simple set-up, handling and
cleanability, planetary ball mills (for example, manufactured by
Fritsch or Retsch, Germany) are especially suitable for various
MCS processes.
12–17
At the laboratory scale, the MCS can be
realized in a disc vibration mill as well.
18
Industrial planetary
mills with continuous action, characterized by productivity of
up to 3–5 tons per hour, are now commercially available.
The energy density in these mills is 100–1000 times higher
than the energy density used in earlier conventional milling
equipment.
19
Mechanochemical vials or reactors and balls are
available in different materials such as agate (SiO
2
), silicon
nitride, sintered corundum, zirconia (tetragonal ZrO
2
stabilized
with Y
2
O
3
), chrome steel, Cr-Ni steel, tungsten carbide, or
polyamide.
14,15
The ball-to-powder ratio (BPR) (weight of the
balls to the powder) has been varied by different investigators
from a value as low as 1 : 1 to as high as B200 : 1. The ratio of
10 : 1 is most commonly used in small capacity mills such as
a SPEX mill.
14
Supplementary technical details of mechano-
chemical parameters such as the type of material of the
milling bodies and vials, ball filling ratio, milling atmosphere,
processing control, organic agents, milling speed, etc. can be
found in ref. 12, 14 and 15, and useful contributions to details
and techniques to determine and calculate the physics,
kinetics, and energetics between milling media during high-
energy milling (HEM) can be found in ref. 12 and 20–22. The
need to use a protective inert atmosphere in the case of
moisture- or oxygen-sensitive materials has been discussed in
many reviews.
14,23–25
In such a case, the vial is usually charged
and purged in a glove box, which is rather cumbersome. An
alternative would be the development of devices that allow
operating the mill while it is permanently connected to the gas
container. Ogino et al.
26
and Ching and Perng
27
measured the
pressure in a gas container that was connected to the milling
vial by means of a plastic tube. They used this method to
study the kinetics of the mechanochemical reaction between
titanium and nitrogen. The kinetics of mechanochemical
hydrogenation of metal alloys with potential applications
as hydrogen storage materials has been also studied by
several authors
28–32
with similar procedures. Shaker mills were
used in all of the mentioned works. A procedure that allows the
operation of a planetary mill with the vial connected to a gas
cylinder has been also described.
33
The vial and the gas
cylinder were connected by a flexible polyamide tube through
a rotary valve to avoid the problem of the spinning movement
of the connecting tube. This system has been successfully used
for the mechanochemical synthesis of refractory nitrides and
carbonitrides under nitrogen pressures up to 10 bar.
34,35
1.4 Accompanying phenomena
Several typical phenomena connected with MA and mechano-
chemical synthesis (MCS) processes can be mentioned here.
7,14
Since diffusion processes are involved in the formation of a
nanostructure, it is expected that the temperature of milling will
have a significant effect. Two kinds of temperature effects
during milling are usually taken into account: local tempera-
ture pulses due to ball collisions and the overall temperature in
the vial.
14
The temperature increase of the milling balls in two
laboratory mills was studied recently.
36
The ball temperature
remains below 100 1C in a SPEX mixer mill. Temperatures over
200 1C are typical for a planetary mill operating at similar
milling intensities. A temperature decrease is expected at very
high speeds, as the balls stay attached to the vial wall for
longer, reducing both heating and efficiency of milling.
37
A serious problem that usually occurs in mechanochemical
research is contamination. The small size of milled particles, the
availability of large surface area and the formation of new
surfaces during milling all contribute to the contamination
of the powder.
14
Using a ‘‘seasoned’’ milling vial (i.e. media
coated with the product powder) resulted in very low values for
Fe contamination.
38
During high-energy milling, the size of crystals decreases to
some critical value. Further energy supply to these crystals of
limiting size causes further deformation of crystals, energy
accumulation in the volume or at the surface of crystals, and
subsequently amorphization.
4f
The particle size reduction is in many cases complicated by
particle size enlargement, where smaller particles are put
together to form larger entities in which the original particles
can still be identified. This phenomenon, called equilibrium
state of milling, was experienced with solids and is closely
related to the effects of aggregation and agglomeration.
In the course of the milling process, a gradual deterioration
of effectiveness is observed. Thorough investigation of this
process on several solids has shown that three stages can be
clearly distinguished:
39a–c
the Rittinger stage (a), in which the
interaction of particles can be neglected; the aggregation stage (b),
in which the new surface area produced is not proportional to
the energy input because of particle interaction (aggregation);
and the agglomeration stage (c), in which the increase of
dispersion first drops to a negligible value and then stops
altogether. While in stage (b) the aggregates are mechanically
connected via van der Waals forces of magnitude 0.04–4 kJ mol
1
,
in stage (c) the particles are kept together by chemical bonds of
magnitude 40–400 kJ mol
1
. Mechanochemical reactions and
Fig. 6 Types of mills for high-energy milling: A – ball mill, B – planetary mill,
C – vibration mill, D – attritor (stirring ball mill), E – pin mill, F – rolling mill.
4k
Modified from ref. 4d.
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