-
REVIEW
Preparation of multicomponent oxides by
mechanochemicalmethods
A. F. Fuentes L. Takacs
Received: 26 July 2012 / Accepted: 18 September 2012 / Published
online: 2 October 2012
Springer Science+Business Media New York 2012
Abstract A large variety of synthesis strategies and
processing techniques are currently being used to obtain
new multicomponent oxides and/or modify existing ones.
Among them, mechanochemical processing has become
very popular because it is simple to implement, solvent
free, and capable of providing enough volume of the target
material in an economically viable manner. The prepara-
tion of complex oxides can benefit from mechanochemical
methods for two important reasons: First, it is not a dif-
fusion-controlled process and thus, high-rate solid state
reactions can be promoted between oxides with different
physical and chemical properties without using high tem-
peratures; secondly, because reactants are processed under
non-equilibrium conditions, uncommon metastable phases
are frequently obtained featuring flexible crystal
structures,
small particle size, high concentration of defects, and
off-stoichiometry. Furthermore, conversion to the true
equilibrium phases induced by additional processing (e.g.,
firing) offers the possibility of isolating fairly stable
intermediate states with unusual and desirable properties
that are inaccessible for more conventional processing
techniques. As oxide particles are hard and brittle, the
number of oxide systems prepared by means of mechano-
chemical methods grew rapidly only in recent years when
more powerful milling devices and abrasion-resistant mill-
ing tools became available. This article summarizes recent
work carried out in the field; only dry milling of oxides
(and
occasionally carbonates) in the absence of additives is
considered. Some of the main challenges of mechano-
chemical processing are also highlighted and discussed.
Introduction
Multicomponent oxides constitute an important class of
inorganic solids exhibiting an extraordinary range of
crystal structures and functional properties, which are
behind many advances of modern technology. The obser-
vation that some of their physical and chemical properties
depend not just on intrinsic characteristics such as crystal
structure and chemical composition, but are also sensitive
to the presence of structural defects has fueled the search
for new synthesis strategies and processing techniques
aiming to understand and control defect structure. Because
of the slow diffusion kinetics in the solid state, most syn-
thetic approaches involve a previous dissolution step;
however, solid state reactions are gaining attention because
of ecological as well as energy reasons. Also, much interest
is focused nowadays on identifying and developing sol-
vent-free and low-waste procedures, which avoid the use
and generation of hazardous substances, while providing
the necessary volume of the target material in an eco-
nomically viable manner.
Several research groups worldwide have turned their
attention to mechanochemical processing, encouraged by the
simplicity of the process and the moderate cost of the
A. F. Fuentes (&)Cinvestav Unidad Saltillo, Carretera
Saltillo-Monterrey
Km. 13.5, 25900 Ramos Arizpe, Coahuila, Mexico
e-mail: [email protected]
Present Address:A. F. Fuentes
Department of Earth and Environmental Sciences,
University of Michigan, Ann Arbor, MI 48109, USA
e-mail: [email protected]
L. Takacs
Department of Physics, University of Maryland Baltimore
County, Baltimore, MD 21250, USA
e-mail: [email protected]
123
J Mater Sci (2013) 48:598611
DOI 10.1007/s10853-012-6909-x
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equipment needed to perform it. Even more interesting is the
possibility to obtain nanocrystalline materials and non-
equilibrium structures with different properties compared to
those prepared by conventional processing. Accordingly,
mechanical milling has become an attractive and powerful
method to produce new complex oxides or to modify
existing ones by inducing phase transformations, extended
solid solutions, disordering, or amorphization. The use of
mechanical energy to stimulate physical and chemical pro-
cesses in the solid state is certainly not a new
development.
Traditionally, the preparation of oxide systems used
mechanical milling mostly for mixing and grinding powders
and to induce phase transformations; however, the recent
availability of more powerful milling devices and appro-
priate milling tools and the advances gained in understand-
ing the basic phenomena behind mechanochemical reactions
have triggered renewed interest in the subject with still
many
challenging opportunities remaining for research.
Interest in the mechanochemistry of complex oxides
increased substantially during the last ten to fifteen years,
as
evidenced by the large variety of inorganic systems analyzed
and the increasing number of publications on the subject.
Unfortunately, the experimental details are often incom-
plete; thus, direct comparison of the results from different
laboratories is usually not possible. Moreover, the reaction
products are rarely characterized completely. These cir-
cumstances result in limited progress toward understanding
the fundamental features of mechanochemical effects on
oxides. Nevertheless, many general trends can be deduced
from the large body of available empirical data.
The first review ever published exclusively on the mech-
anochemistry of complex oxides was written by Zyryanov [1].
He provides a critical evaluation of the existing literature
using the crystal structure of the reaction product as the
organizing principle. Russian researchers have been active
in
this area for decades and Zyryanovs paper provides an
extensive summary of their work. Therefore, the current
paper
will reference only the latest and most important papers
from
the Russian literature. Another recent review [2]
concentrates
on the mechanochemical synthesis of ferroelectric ceramics
starting from different precursor materials. It includes
both
completely mechanochemical synthesis and thermal synthesis
using mechanically activated components. Research in this
area was triggered by the successful preparation of an
important ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) from PbO,
MgO, and Nb2O5 [3]. If PMN is prepared by conventional
solid state synthesis, various pyrochlore-type binary lead-
niobium intermediates form that have poor dielectric prop-
erties. Using mechanochemical synthesis, single-phase PMN
could be obtained.
The aim of the present work is to present a summary of
the mechanochemistry of complex oxides emphasizing
chemical composition and potential applications, mainly in
the area of electroceramics. After some general remarks,
iron oxides, bismuth oxides, titanates, etc. are considered
in
separate sections, followed by a few other examples of
practical interest. Only dry milling of oxides (and occa-
sionally carbonates) in the absence of additives is consid-
ered, except for a short section on soft mechanochemistry.
General remarks
It is well known that high-energy ball milling is an
efficient
method to mechanically activate materials and to affect
chemical reactions [4] and alloying [5]. Mechanochemical
processing can be carried out in several types of com-
mercially available milling devices, each operating based
on very different mechanical principles and offering vari-
ous capacities and milling efficiencies. Some research
groups have developed the ability to design and manufac-
ture their own milling equipment. While the existence of
various options has many benefits, it substantially com-
plicates the task of analyzing the literature as comparing
data published by different authors using different mills is
often difficult. The highest impact energies are achieved in
shaker mills and planetary mills, while drum mills, vibra-
tory mills, and attritors deliver lower energy and a
different
combination of impact, shear, and friction forces (Fig. 1).
They all cause activation and reactions in powder mixtures
by continuously exposing fresh surfaces and creating lat-
tice defects, surface radicals, and broken bonds.
There are many promising results on the application of
mechanochemical processing to complex oxides. Never-
theless, the method has some inherent difficulties when
compared to its application to alloys, intermetallic com-
pounds, and many other systems. Most importantly, oxide
particles are hard and brittle and consequently do not react
with each other unless a threshold stress is exceeded. In
practical terms, reactions between oxides require highly
energetic impacts of the milling balls; thus, only the most
powerful milling devices are suitable for the preparation of
mixed oxides. Extending the milling time in a low-energy
mill is usually not sufficient as weak impacts do not result
Fig. 1 Main forces acting on powder particles during milling
J Mater Sci (2013) 48:598611 599
123
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in any chemical change. It might also happen during
milling that a stable state of mechanochemical equilibrium
is reached at a given volume fraction of the phases
involved (e.g., reactants and reaction products) with
further
milling being ineffective in driving the reaction forward.
Contamination from the milling tools is a particularly
serious problem when hard and abrasive oxides are milled
at high intensity for a prolonged time. However, several
steps can be made to reduce the level of contamination.
The milling container and the balls can be made from a
material that is less detrimental to the properties of the
product. Thus, ceramic tools made of alumina are preferred
to steel and WCCo. If a high density is crucial, balls made
from zirconia or even hafnia can be used. Also, selecting
the lowest acceptable milling speed, the shortest possible
milling time, and optimizing the amount of powder charge
and the number and size of the milling balls can keep
contamination at an acceptable level.
If processing begins with a clean mill, much of the
contamination occurs during the early stages of the process
when the tool surfaces are still bare and the collisions
between them result in significant abrasion. After a few
minutes, a powder coating develops and the rate of con-
tamination decreases substantially. This phenomenon can
be utilized to coat the working surfaces before processing.
A preliminary batch is milled until a stable coating
develops. Then, the loose powder in the mill is discarded
and processing of the final batch is started with the tool
surfaces already protected by a layer of compatible powder
[1]. Nevertheless, contamination cannot be avoided com-
pletely, even if the most careful protocol is followed.
Finally, as mechanochemical reactions are inherently
complex, it is difficult to predict whether or not a desired
reaction product would be formed. Sometimes, even the
direction of a mechanochemical process seems to contradict
thermodynamical calculations, or the reaction proceeds
according to a route that is substantially different from
the
path of conventional solid state reactions. Chemical changes
caused by mechanical action result from various thermally as
well as mechanically induced processes and occur simulta-
neously at many reaction sites [6]. Designing a mathematical
model capable of describing the events taking place in such
reaction sites would entail correlating the milling
parameters
with the energy being transferred to the material subjected
to
milling and with the frequency of collisions. Given the
complex motion of the grinding media, the number of vari-
ables involved in determining the energetics of a
high-energy
ball mill is very large, as shown in Fig. 2 [7]. Also, the
con-
nection between the macroscopic operation of the mill and
the
local loading of the particles is very complicated.
Therefore,
developing a complete and predictive mathematical model of
milling-induced chemical reactions is quite unlikely. How-
ever, some progress has been made in metal systems to
ascertain whether or not a mechanochemical reaction is pos-
sible using a given set of milling parameters [5]; available
data
concerning multicomponent oxides are still scarce. A recent
theory of ultrafast mechanochemical synthesis in MOM0O3oxide
systems alleges that large molecular mass of the reac-
tants, small difference of their Mohs hardness, and large
enthalpy of the chemical reaction involved are optimal for
high yield mechanochemical reactions [8, 9].
In spite of all the difficulties, it would be a mistake to
overemphasize the problems and ignore the enormous
potential of mechanochemical processing as a powerful
method for obtaining multicomponent oxides. The prepa-
ration of complex oxides can benefit from mechanochem-
ical methods for two important reasons:
(1) Conventionally, complex oxides are prepared by solid
state reactions between oxides, carbonates, and nitrates
at high temperature (typically over 1000 C). Firingtimes as long
as days or even weeks can be necessary to
insure single-phase products, resulting in a very costly
procedure.
Take for example the case of apatite-type lanthanum
silicates La10-x(SiO4)6O02?y which are promising oxygen
ion-conducting materials for application as electrolytes in
intermediate-temperature solid oxide fuel cells [10]. As for
many silica-based compounds, conventional solid state pro-
cessing from a mixture of simple oxides requires several
days
of heat treatment at 1400 C to yield the oxygen ion-conducting
solid solution [11]. The same material can be
prepared by ball milling in less than 9 h using the same
chemicals, a planetary mill at moderate speed, and zirconia
milling tools [12].
(2) High-temperature synthesis produces large grains and
equilibrium phases, but many applications benefit
Fig. 2 Milling parameters determining the energetics in a
planetaryball mill with a cylindrical vial (adapted from Ref.
[7])
600 J Mater Sci (2013) 48:598611
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from the formation of uncommon metastable phases
with flexible crystal structures and the small particle
size, high concentration of defects, and off-stoichi-
ometric composition obtained by mechanochemical
methods.
Mechanochemical processing creates a unique and often
quite stable defect structure that is difficult to obtain
using
conventional processing. Furthermore, the transformation
to the true equilibrium phase induced by additional
processing (e.g., firing) often proceeds through a series of
transitions involving fairly stable intermediate states with
unusual physical and chemical properties. For example,
pyrochlore-type titanates and zirconates form a very
interesting family of compounds with potential applica-
tions as electrolytes for solid oxide fuel cells, thermal
barrier coatings, and matrix materials for the immobiliza-
tion of high-level radioactive waste. Pyrochlores prepared
by ball milling from mixtures of simple oxides present a
high degree of structural disorder that can be reduced by
thermal treatment in a controlled manner [1315]. That
way, the properties of the product can be controlled by
changing structural/microstructural characteristics, while
keeping the chemical composition constant.
From the kinetic point of view, solid state reactions
induced by ball milling can progress in two different ways,
either (i) gradually extending to a very small volume with
each collision or (ii) suddenly through a mechanically
activated exothermic reaction. The latter are termed
mechanically induced self-sustaining reactions (MSR) and
are characteristic of highly energetic powder mixtures
including oxidemetal systems (e.g., Fe2O3Al), refrac-
tory-forming metalmetalloid mixtures (e.g., TiC), and
metalchalcogen mixtures (e.g., ZnS) [16]. Chemical
reactions in these systems are self-sustained, propagate
throughout the entire powder charge after some activation
period, and reach completion in a very short time; however,
additional milling is frequently needed to achieve a fully
reacted and homogeneous product. On the contrary, grad-
ual mechanochemical reactions are spatially limited to a
small volume around the point of collision between balls or
a ball and the inner wall of the container; thus, reaction
rates are time dependent, reaching a maximum at an
intermediate milling time and then decreasing as reaction
approaches completion or until a steady state of mecha-
nochemical equilibrium is reached. These are the reactions
commonly observed in oxide systems.
Iron oxides and related systems
The mechanochemical synthesis of ferrites has attracted the
interest of many research groups for some time now
because of the strong correlation between magnetic prop-
erties and structural disorder in AB2O4 spinels (e.g.,
[17]).
Mossbauer spectroscopy provides unique information
about the local coordination and the magnetic and oxida-
tion states of iron atoms that, combined with XRD and
other methods, allows for a comprehensive characterization
of ferrites [18]. The ideal spinel structure is formed by a
cubic close-packed array of oxygen atoms in which one-
eighth of the tetrahedral and half of the octahedral inter-
stitial sites (referred in the chemical formula as A and B,
respectively) are occupied by cations. The physical prop-
erties of spinels are determined by chemical composition
and cation distribution over the two available positions.
There are two types of oxides with spinel structure: In
normal spinels, the tetrahedral and octahedral sites are
commonly occupied by divalent (e.g., Mg, Ni, Co, etc.) and
trivalent cations (e.g., Al, Ga, Fe, etc.), respectively,
while
in inverse spinels, the divalent ions occupy octahedral
sites, whereas half of the trivalent cations are located in
the
tetrahedral sites. In general, inverse spinels have been
found to present the most valuable soft magnetic proper-
ties. Many intermediate cation distributions are possible,
with the degree of inversion often determined by the
preparation technique. For example, mechanical milling
can influence cation distribution and consequently the
magnetic properties.
Low conversion yields and partial reduction were a
common concern when using steel milling tools to combine
simple oxides to form ferrites. However, high yields could
be achieved when using tungsten carbide (WC) jars and
balls. For example, NiFe2O4 was obtained by milling the
appropriate mixture of NiO and a-Fe2O3 with almostcomplete
conversion [1921]. Thorough characterization
revealed that the product consisted of 613-nm grains with
a non-uniform coreshell structure, where an ordered inner
core is surrounded by a 12-nm thick disordered grain
boundary region. The core has the inverse spinel structure
of bulk NiFe2O4, while the cation distribution in the shell
is
almost random.
This coreshell configuration is typical of mechanically
milled nanopowders, but not nanopowders prepared by
other techniques [22]. It has been observed in MnFe2O4prepared
by a milling-induced displacement reaction [23],
in Ca2SnO4 prepared by reactive milling from the corre-
sponding elemental oxides [24], and in LiNbO3 and
ZrO2nanopowders prepared by milling commercially available
microcrystalline powders [2527]. But, X-ray absorption
spectroscopy studies (XAS) have shown that LiNbO3nanoparticles
prepared by the solgel method have well-
ordered microstructures with interfaces similar to the grain
boundaries in normal bulk solids [25].
The coreshell structure has a very strong influence on
the magnetic behavior of mechanochemically prepared
J Mater Sci (2013) 48:598611 601
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spinel ferrites [28]. For example, nanoparticulate
NiFe2O4presents lower saturation magnetization (*55 %), butenhanced
magnetic hardness compared to bulk nickel fer-
rite, both attributed to parasitic ferromagnetism due to
spin canting. Firing above 400 C relaxes the materialtoward the
equilibrium structure and consequently the
magnetic properties approach those typical of bulk
NiFe2O4.
MgFe2O4 spinel has been prepared by milling a mixture
of MgO and a-Fe2O3 powders [2931]. Complete conver-sion was
achieved when using WC milling tools. In this
case, the increase of the magnetization is mostly due to
cation disorder, while spin canting has a secondary effect
[32]. Interestingly, milling a mixture of CaO and a-Fe2O3under
similar conditions produced only 23 % conversion to
CaFe2O4 [33]. Spinel-type ZnFe2O4, MnFe2O4, and their
solid solutions have been prepared starting from a-Fe2O3,ZnO,
and/or different manganese oxides [34, 35]. Verdier
et al. [36] investigated the effect of milling conditions
using a planetary ball mill with separately adjustable
speeds for the supporting disk and grinding jars. They
observed different degrees of conversion depending on the
combination of impact and friction generated by the mill.
At low impact energy, partial reduction of Fe3? to Fe2?
took place, resulting in the formation of a wustite-like
(Fe,Zn)O phase. A similar reduction from Cu2? to Cu?
could be the reason why attempts to prepare CuFe2O4spinel were
unsuccessful [37]. Harris et al. [35] used a
high-energy shaker mill and steel tools to achieve almost
complete transformation to MnZn ferrites starting from
MnO, ZnO, and a-Fe2O3. As expected, the mechano-chemically
prepared ferrites showed non-equilibrium cat-
ion distribution with a significant fraction of Zn atoms
located at the octahedral site.
Mixed iron-containing oxides with the perovskite
structure were also prepared by mechanochemical meth-
ods. Weakly ferromagnetic LaFeO3 was obtained by
co-milling La2O3 and either Fe3O4 or a-Fe2O3 using hardenedsteel
vials and balls [38]. Several LnFeO3 phases (Ln = Pr,
Nd and Sm) were synthesized starting from the appropriate
rare-earth oxide and freshly fired FeOOH [39]. Mechanical
milling has also been used to prepare multiferroic BiFeO3powders
starting from an equimolar mixture of Bi2O3 and
Fe2O3 with complete conversion reached after milling for
12 h using WC tools [40, 41]. The as-obtained powders
consisted of spherical nanoparticles with the coreshell
structure; the amorphous shell had a thickness of about
1 nm. The amorphous fraction proved to be highly reactive
and the authors were able to observe its rapid crystalliza-
tion under irradiation with electrons in the transmission
electron microscope [41]. Milling a mixture of Y2O3 and
Fe2O3 powders generates yttrium iron garnet Y3Fe5O12with YFeO3
perovskite as an intermediate [42].
Bismuth(III) oxide and related systems
Bismuth(III) oxide Bi2O3 exists in at least five different
crystal forms, identified as a, b, c, d, and e-phases. Theform
stable at room temperature is the monoclinic a phase.It transforms
to the cubic fluorite-type (face-centered)
d phase at 729 C, which remains stable to the meltingpoint (825
C). Two metastable forms may occur oncooling between 500 and 650 C,
the tetragonal b phaseand a body-centered cubic c phase; either can
be stabilizedby certain impurities. The e phase has been
synthesized byhydrothermal methods; its crystal structure is
closely
related to the a and b forms. Interestingly, the
differentpolymorphs of Bi2O3 exhibit very different optical and
electrical properties. In particular, d-Bi2O3 is a veryeffective
oxygen ion-conducting material, and conse-
quently it is of interest for applications such as gas moni-
toring and solid oxide fuel cells. As d-Bi2O3 cannot beretained
at room temperature by quenching, strategies like
aliovalent chemical substitution and non-conventional
powder processing methods have been attempted for this
purpose.
Bismuth oxide is considered a good candidate for
mechanochemical synthesis because of its high molecular
mass [1], and thus numerous studies have been carried out
on Bi-containing materials, including attempts at stabiliz-
ing the high-temperature modifications of Bi2O3 by
mechanochemical methods. For example, c-Bi2O3 wasobtained by
milling the a form with different oxides suchas SiO2, PbO, ZnO, and
a-Fe2O3 in a 12:1 Bi-to-dopantcation atomic ratio, using either
hardened steel or zirconia
milling tools [43]. It was shown that contamination from
the milling tools plays an important role in determining the
obtained phases. For example, milling pure a-Bi2O3 withzirconia
tools resulted in a mixture of a and b-Bi2O3instead of the c form,
showing that Zr4? is unable to sta-bilize c or d-Bi2O3, contrasting
with the behavior of Fe
3?.
Ball milling a a-Bi2O3:HfO2 powder mixture with a 2:3molar ratio
produced some b-Bi2O3 already after a shortmilling time [44]. Yet,
although the b phase can be con-sidered a version of the d-phase
with ordered oxygenvacancies, 50 h of continuous milling were
required to
obtain the d form.Fluorite-type solid solutions based on d-Bi2O3
have also
been prepared by milling mixtures of different oxides with
a-Bi2O3 [45]. Thus, milling a-Bi2O3 with CaO, SrO, Y2O3,In2O3,
or La2O3 yielded metastable single-phase
Bi1.6M0.4O3-x powders (M = Ca, Ca0.5Sr0.5, Y, In, La)
with the fluorite structure for every dopant except indium,
the ion with the largest size difference compared to Bi3?.
Formation of a new metastable orthorhombic-distorted
fluorite structure d0-Bi2O3 has also been detected whenmilling
Bi2O3-containing systems [46].
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As the Bi2O3V2O5 system presents a large number of
stoichiometric compounds and solid solutions with very
interesting physical and chemical properties, it is also of
interest for mechanochemical investigations [47]. For
example, milling mixtures close to the 1:1 Bi2O3:V2O5molar ratio
in an AGO-2 mill with steel vials and balls led
to the highly disordered clinobisvanite (monoclinic) BiVO4as the
dominant phase, which is the form stable at room
temperature. In off-stoichiometric mixtures, several addi-
tional phases were formed, including cubic and tetragonal
fluorite-like phases. Monoclinic BiVO4 has also been
obtained using VO2 as vanadium source [47], although
prolonged milling yielded either fluorite-type phases or the
oxygen-deficient Bi2VO5 material, featuring a mixture of
V5? and V4?.
An interesting case is the Bi4V2O11 compound (2:1
Bi2O3:V2O5 molar ratio) since this oxide is the parent
compound of the BIMEVOX family of excellent oxygen
ion-conducting materials [4749]. The Bi4V2O11 phase
occurs in at least four different crystal forms, of which
the
c form is the most interesting due to its high ionic
conduc-tivity. After extended milling of an appropriate
Bi2O3V2O5mixture in a vibratory mill, only an amorphous material
was
obtained which crystallized upon annealing at 385 C.However,
Shantha et al. [50] were allegedly able to obtain
30-nm nanoparticles of tetragonal c-Bi4V2O11 by
millingstoichiometric mixtures of the elemental oxides using
agate
vials and balls. The inclusion of Si4? ions from the milling
tools might have aided the formation of Bi4V2O11.
Partial replacement of V5? by some other metal ions
(e.g., Cu2?, Ni2?) suppresses the c ? b ? a transition
oncooling, allowing the highly conductive c-phase to bestabilized
at room temperature. Therefore, ball milling has
also been used to prepare substituted Bi4V2O11. Zhang
et al. [51] prepared nanosized (*19 nm) Bi4V1.8Cu0.2O10.7powders
by milling appropriate mixtures of V2O5, Bi2O3,
and CuO using WC vials and balls. Below 600 C, thiscompound
presents the highest oxygen ion conductivity
ever measured (10-2 Scm-1 at 350 C), two orders ofmagnitude
higher than any other known solid oxide ion-
conducting material in the same temperature range.
Zyryanov and Uvarov [52] were also able to prepare a large
group of BIMEVOX phases, Bi4V1.8M0.2O11-x (M = V4?,
Zn2?, Sc3?, Sb3?, In3?) and Bi1.8Pb0.2VO5.4-x, by reactive
milling starting from the corresponding oxides. The
as-prepared materials presented a high degree of compo-
sitional disorder, which helped to stabilize the c
phase.Although a fraction of these structural defects relaxed
during sintering at 723 C, the c phase never transformedto the
orthorhombic or monoclinic forms. In other words,
the ordering of oxygen vacancies observed on cooling in
materials prepared by conventional synthesis did not occur.
The Bi2O3GeO2 system attracted attention because of
its various stoichiometric phases with attractive optical
and
electrical properties. Milling mixtures of GeO2 and Bi2O3with
different stoichiometries lead first to a layered Auri-
villius-type Bi2GeO5 material and then to different phases
including basically eulytite Bi4Ge3O12 and a cubic sillenite
phase of composition Bi12GeO20 [53]. All three phases
contained a high concentration of vacancies and a high
degree of compositional disorder. In the Bi2O3TiO2 sys-
tem, several attempts have been made to prepare the
Aurivillius-type Bi4Ti3O12 compound, perhaps the most
popular ferroelectric material ever, starting from mixtures
of Bi2O3 and different titanium sources [5457]. Various
Bi2O3:Nb2O5 compositions (1:1, 5:3, and 3:1 molar ratios)
were milled using WC jars and balls, but the only mixed
oxide identified by XRD was cubic Bi3NbO7 with an
excess of poorly crystalline Nb2O5 remaining in the mix-
tures [58]. Brankovic et al. [59] prepared multiferroic
BiMnO3 powders starting from Bi2O3 and Mn2O3 using
stainless steel vials and balls. Although obtaining single-
phase BiMnO3 by traditional solid state reaction of the
simple oxides requires pressures over 40 kbar, it was
readily obtained by high-energy ball milling. Apparently,
the as-prepared powder presents a tetragonal unit cell,
similar to that obtained when heating the triclinic form
above 490 C in traditional materials.
TiO2 and related systems
Titanium(IV) oxide has three naturally occurring crystal-
lographic modifications, namely anatase, rutile, and
brookite. A high-pressure polymorph with the a-PbOstructure is
known as srilankite. As titanium dioxide and
other titanium-containing compounds are very common
industrial ceramics, their behavior under high-energy ball
milling attracted significant attention.
The evolution of TiO2ZrO2 mixtures was studied by
several groups [6062]. Milling an equimolar mixture of
monoclinic ZrO2 and anatase TiO2 using zirconia jars and
balls produced ZrTiO4. The first step was a polymorphic
transformation of TiO2, followed by the formation of
mutual and partially amorphized solid solutions, with either
TiO2 or m-ZrO2 as solute, which evolved to ZrTiO4 on
further milling [60]. Complete transformation was
achieved only by firing at temperatures close to 1100 C.When
milling mixtures of TiO2 and ZrO2 using WC vial
and balls, (Zr1-xTix)O2 (0.44 B x B 0.60) solid solutions
with the orthorhombic srilankite structure were obtained
[61]. The as-prepared metastable solid solution was stable
up to 700 C and even to 1100 C in the 0.44 B x B 0.52composition
range.
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Zinc titanates are useful dielectric materials that can be
sintered at relatively low temperature. There are three
mixed compounds in the TiO2ZnO phase diagram,
namely Zn2TiO4 (a cubic spinel), ZnTiO3 (hexagonal
ilmenite type), and Zn2Ti3O8 (also cubic spinel). Manik
and Pradhan [63] investigated the evolution of an equi-
molar ZnO-anatase powder mixture while milling with
hardened chrome steel balls and obtained a mixture of
Zn2TiO4, ZnTiO3, and a small amount of rutile. Qian et al.
[64] analyzed the effect of the Ti source (anatase or
rutile)
on the evolution of different ZnOTiO2 mixtures during
high-energy ball milling with WC vials and balls. Inde-
pendent of the Ti source, mixtures with a 2:1 molar ratio
yielded a new cubic phase, identified as Zn4-xTi2?yO8 and
thought to be either a solid solution or a mixture of two
similar cubic phases, Zn2TiO4 and Zn2Ti3O8. The ZnTiO3content of
the final powder was larger when starting with
rutile, whereas the use of anatase favored the formation of
Zn4-xTi2?yO8. The structural similarities between rutile
and ZnTiO3 on one side and anatase and Zn2TiO4
Zn2Ti3O8 on the other may explain the preferential
formation of either ZnTiO3 or Zn4-xTi2?yO8.
The behavior of alkaline earth oxide-TiO2 mixtures is
also interesting. Milling BaOTiO2 mixtures in a high-
energy shaker mill resulted in nanocrystalline BaTiO3powders
[6567], both in nitrogen and in air. Although the
formation of some BaCO3 is expected in air, continuous
milling decomposes the carbonate, allowing the formation
of single-phase barium titanate. Milling mixtures of SrO
and TiO2 led to the formation of SrTiO3 in a broad com-
position range [68]. The crystalline product was obtained
embedded in an amorphous matrix. Poorly crystalline
Sr2TiO4 was observed when milling a (2 SrO):TiO2mixture.
Nanocrystalline CaTiO3 has also been prepared by
milling CaO and TiO2 [69]. If rutile was the Ti source,
CaTiO3 formed, while a mixture of Ca(OH)2, anatase, and
CaTiO3 was obtained when milling anatase with CaO. The
presence of CaO seems to inhibit the polymorphic trans-
formation from anatase to rutile. Brankovic et al. [70]
reduced the time needed to obtain pure CaTiO3 by opti-
mizing the experimental conditions. They were also able to
obtain CaTiO3 by milling CaCO3 and rutile TiO2. In this
reaction, the decomposition of the carbonate is the first
step; thus, it is necessary to let CO2 escape from the
milling
vial. Only an amorphous product formed when mixtures of
MgO and TiO2 were milled, but the crystalline MgTiO3and Mg2TiO4
phases could be obtained by annealing [71].
Perovskite-type CaTi1-xMnxO3-d was prepared by
milling appropriate mixtures of CaO, anatase TiO2, and
Mn2O3. The product formed with no apparent amorphous
or crystalline intermediate [72]. Another important low-
temperature-sinterable dielectric material is CaCu3Ti4O12.
It has the perovskite structure and can be prepared by
milling stoichiometric mixtures of CuO, TiO2, and either
CaO, Ca(OH)2 or CaCO3 using stainless steel jars and balls
[73, 74].
Mixed oxides of Ti and lanthanides
Mixed oxides form relatively easily when milling mixtures
of TiO2 and Ln2O3 (Ln = Y3? and lanthanides) [15, 75,
76]. There are only two stoichiometric compounds in the
corresponding phase diagrams, Ln2TiO5 and Ln2Ti2O7.
The crystal structure of Ln2TiO5 is related to the mineral
cuspidine [Ca4(Si2O7)(OH,F)2], while Ln2Ti2O7 titanates
belong to the pyrochlore family. The ideal pyrochlore
crystal structure can be visualized as a superstructure of
an
anion-deficient fluorite structure, where a double unit cell
has 1/8th of the anion positions empty in an ordered way
and the cations and anions are distributed in four crystal-
lographically non-equivalent sites. The main difference
between anion-deficient fluorites and pyrochlores is that
vacancies are randomly distributed in the anion sublattice
of the first, whereas ordered in a particular way in the
second. Many pyrochlores have partially disordered atomic
arrays and phase transition to a fluorite structure is
possible
by fully disordering the cations and anions in their
respective sublattices. As pyrochlore-type oxides present
good high-temperature oxygen ion conductivity, very low
thermal conductivity, and enhanced radiation resistance,
they are important materials for possible applications in
solid oxide fuel cells, thermal barrier coatings, or even as
immobilization matrices for high-level radioactive waste.
As in spinel-type oxides, many properties are influenced by
ion distribution and site occupancy, and thus mechanical
milling is a promising method for their synthesis.
Single-phase pyrochlore-type Ln2Ti2O7 (Ln = Gd, Dy,
Y) titanates were obtained by milling Ln2O3:TiO2 mixtures
(1:2 molar ratio) using zirconia containers and balls [15].
As
shown in Fig. 3a and b, the formation of the target
materials
(e.g., Gd2Ti2O7) started with the polymorphic transforma-
tion of Ln2O3 from its highly symmetric cubic form (C), that
is stable at room temperature, to the more dense and
monoclinic B-form. Normally, this transformation takes
place at high temperature and the transformation tempera-
ture increases as the size of the lanthanide ion decreases
[77]. Accordingly, temperatures close to 1850 C are neededto
induce the C to B transformation in Dy2O3 compared to
1200 C for Gd2O3. The thermal stability of C-Y2O3 is evenhigher.
The only existing solid state phase transition takes
place at 2240 C, yielding a fluorite-type Y2O3; monoclinicB-Y2O3
can only be obtained at high pressure. The same C
to B transformation can be brought about by milling with
steel tools relatively easily, and the transformation is
faster
604 J Mater Sci (2013) 48:598611
123
-
for larger cations as anticipated from the behavior upon
heating [78, 79]. Interestingly, fluorite-type Y2O3 was
obtained when milling C-Y2O3 with zirconia tools.
XRD results (Fig. 3b) suggest nucleation and crystalli-
zation of the rare-earth titanates from an amorphous matrix
after the amorphization of the initial mixture of oxides. A
thorough characterization of these pyrochlores prepared by
mechanical milling suggested that the thermally induced
transition to a more ordered state proceeds through two
types of transformations taking place between 750 and
950 C (Fig. 3c, d); the first one just below 800 C isassociated
with an ordering process affecting mostly the
oxygen sublattice, whereas the second one at slightly
higher temperatures (differential thermal analysis shows
generally only a broad exothermic peak) is related to cation
ordering, grain growth, and the release of strain energy
[13,
14, 80]. Therefore, temperatures close to 1000 C are
needed to transform these metastable forms into the ther-
modynamically stable pyrochlores. Previously, disorder in
these pyrochlore titanates could only be achieved by
replacing Ti4? by a larger cation, ion irradiation, or high
pressure.
Solid solutions of Ln2Ti2-xZrxO7 and Gd2Ti2-xSnxO7with
pyrochlore structure were prepared under similar
experimental conditions [13, 76, 8183]. In these systems,
disorder generated by milling adds to the chemical disorder
introduced by substituting Zr4? or Sn4? for Ti4?. A typical
example is the Gd2Ti2-xZrxO7 system, where anion-deficient
fluorite-like materials form upon milling, irrespective of
the
zirconium content. This is surprising as both limiting com-
pounds are pyrochlores and mixed systems formed
by high-temperature reactions also crystallize with pyroch-
lore structure. These rather stable materials could
be progressively ordered by annealing. For example,
Fig. 3 Mechanochemical synthesis of Gd2Ti2O7 a XRD pattern ofthe
starting mixture and b its evolution with milling time. c DTAcurve
of the reaction product obtained after milling for 19 h and
d evolution of its XRD pattern with post-milling thermal
treatments.
Numbers in parenthesis are the Miller indexes of each
reflection.Reflections labeled in d are those characterizing the
pyrochloresuperstructure and their intensity increases with the
degree of
structural ordering
J Mater Sci (2013) 48:598611 605
123
-
Gd2Zr2O7 prepared by ball milling a stoichiometric mixture
of monoclinic ZrO2 and C-Gd2O3 preserves the fluorite
structure even after annealing at 1200 C [13], whileGd2Zr2O7
prepared by solid state reaction at high tempera-
ture has a pyrochlore structure and disorders only above
1550 C. Intermediate pyrochlore oxides with very unusualcation
distributions appear during the annealing of
mechanochemically prepared powders. Their Gd atoms are
distributed between the 6- and 8-coordinated positions and
the Zr atoms are relegated to the larger A site. Similar
results
were obtained in the Dy2Ti2-xZrxO7 system [82]. In general,
mechanochemical synthesis combined with post-milling heat
treatments make controlling disorder and crystallite size
possible; these are the characteristics that determine con-
ductivity [84], resistance to ion-beam-induced amorphization
[85], and compressibility in pyrochlores [86].
Compounds with the Ln2TiO5 composition were also
obtained by milling the appropriate starting mixtures [75].
While only one crystal form exists for Ln2Ti2O7 pyroch-
lores, at least three polymorphs have been identified for
Ln2TiO5, depending on the annealing temperature and the
size of the trivalent ion involved. Specifically, as the
ionic
radius of the lanthanide component decreases with the
increasing atomic number, three structures with increasing
symmetry are observed: the a- or orthorhombic low-tem-perature
form, the hexagonal b- or high-temperature form,and an F-phase,
with a cubic fluorite-type structure [87].
Waring and Schneider [88] reported a reversible transi-
tion between a and b-Gd2TiO5 at 1712 C, which makespreserving
the high-temperature form by quenching difficult
due to the proximity of the melting point (1765 C). ForDy2TiO5,
a relatively narrow stability range from 1330 to
1650 C has been established for the hexagonal b phase.But, the
Ln2TiO5 phases obtained by milling had the high
temperature and hexagonal forms as a rule, illustrating the
potential of mechanochemical methods to prepare these
phases. In addition, powders obtained by ball milling show
high stability. For example, b-Gd2TiO5 was converted to
theorthorhombic low-temperature form only by firing above
1000 C. This high degree of stability is a general feature ofany
ball-milled Ln2TiO5 and Ln2Ti2O7 material.
Milling a mixture of SnO2 and TiO2 using WC vials and
balls led to Sn0.5Ti0.5O2 with rutile structure [89], while
tetragonal PbTiO3 was obtained [90] by milling PbOTiO2mixtures.
In the latter case, intermediate Pb3O4 and Ti10O18phases were also
observed at the early stages of the milling
process.
Rare-earth silicates
Rare-earth apatite-type silicates have attracted attention
since the first reports by Nakayama [91, 92] about their
high oxygen ion conductivity. Their general formula is
Ln10-x(SiO4)6O02?y (Ln = lanthanide) and their crystal
structure (hexagonal symmetry) is built of isolated
SiO4tetrahedra, with the extra oxide ions (denoted by O0)occupying
the center of one-dimensional channels running
through the structure along the c-axis. These O0 ions
areresponsible for the ionic conduction. In addition, there are
two possible positions for the Ln3? cations, one of them
coordinated by 7, the other by 9 nearest neighbors. A
variety of chemical substitutions as well as non-stoichi-
ometry cation vacancies and/or oxygen excess are possible
in such a crystal structure, both having an important effect
on the electrical properties of these materials. The syn-
thesis of these silicates was successfully attempted by
milling stoichiometric mixtures of the corresponding ele-
mental oxides, Ln2O3 (Ln = La3?, Nd3?, Gd3?, and Dy3?)
and SiO2; ZrO2 containers and balls were used [12, 93, 94].
Both amorphous and low cristobalite SiO2 were evaluated
as silicon source in the mechanochemical preparation of
the La-containing silicate since that is the one showing the
highest conductivity.
XRD, infrared, and Raman spectra were used to follow the
reaction and to establish the time needed to achieve
complete
conversion in each case (Fig. 4). Milling La2O3 with
SiO2(amorphous or low cristobalite) in a 4:5 molar ratio yielded
a
product with the chemical formula La9.60(SiO4)6O2.4 after
short milling. Thus, the characteristic XRD reflections of
hexagonal La2O3 were completely replaced by the typical
peaks of an apatite-type silicate after milling for 6 h (Fig.
4a,
b). After 1 h of milling, the IR spectra still show the
char-
acteristic absorption bands of the SiO bond in silica (e.g.,
1105, 795, and 484 cm-1). The sample after 3 h of milling is
a mixture, and for longer milling times (Fig. 4c, d), the
spectrum is dominated by the peaks characteristic of the SiO
bond in the silicate (asymmetric stretching modes at 988,
915, and 881 cm-1) [12]. The conversion time was shorter
when using amorphous silica as the Si source, e.g., after
milling for 9 h, the 1105 cm-1 strong absorption band of
SiO bonds in silica is absent in 4c, but still present when
starting from low cristobalite SiO2 (4d). As opposed to the
pyrochlore case already discussed, the starting chemicals
and
the reaction product coexist during the mechanochemical
synthesis of apatite-type silicates and could be observed by
XRD after short milling times.
The same experimental conditions were used to prepare
silicates with Ln3? = Dy3?, Gd3?, and Nd3?. Interest-
ingly, previous attempts at preparing lanthanum silicates by
milling La2O3 with amorphous SiO2 using agate milling
tools resulted in complete amorphization of the reaction
mixture, and crystalline lanthanum silicates were only
obtained after firing at temperatures close to 1000 C [95].This
result suggests that the energy transferred to the
powder particles during milling with agate tools is not
606 J Mater Sci (2013) 48:598611
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sufficient to induce the crystallization of the target mate-
rials. Kharlamova et al. [96] analyzed the possibility of
using the same method to prepare Fe or Al-doped apatite-
type lanthanum silicates. Although the apatite-type silicate
did form on milling in all cases, Fe3? was not incorporated
into the silicate network when using a-Fe2O3 or FeO(OH)as iron
sources. However, milling a mixture of La2O3 with
either Fe(NO3)3/SiO2 or Fe(HCOO)3 precursors yielded
single-phase Fe-doped lanthanum silicate. Aluminum
incorporation seems to be much easier and Al-doped lan-
thanum silicates with an apatite structure were prepared by
milling appropriate mixtures of La2O3, SiO2, and Al(OH)3for a
very short time. Similar apatite-type germanates,
which are also very good oxygen ion-conducting materials,
have been readily obtained by reactive milling [97] starting
from La2O3 and GeO2.
ZrO2 and zirconia-based systems
The milling-induced stabilization of tetragonal and/or cubic
zirconia, t/c-ZrO2, has been documented for some time now
[27, 98101]. Research was initially encouraged by reports
that the room temperature monoclinic modification,
m-ZrO2, becomes unstable as the crystallite size decreases
below 30 nm. Conversely, the high temperature and more
symmetric tetragonal and cubic forms would be stabilized
when the crystallite dimension is below this critical value.
However, there was some controversy on whether this phase
stability inversion takes place due to the lower surface
energy of the latter forms because of kinetic reasons or
even
as an impurity effect. Interestingly, such phase transitions
were only observed when milling m-ZrO2 using steel tools,
but not when using zirconia. According to Karagedov et al.
Fig. 4 Mechanochemical synthesis of apatite La9.60(SiO4)6O2.4a
XRD pattern of the starting mixture and b its evolution withmilling
time. Evolution of IR spectra with milling time when using
La2O3 and amorphous (c) and low cristobalite SiO2 (d) as the
silicon
source. The Miller indexes of the reflections are given in a.
Thereflection marked with s is the most intense line of the SiO2
pattern.
Asterisks in c and d mark the main absorption bands of the
SiObands in silica
J Mater Sci (2013) 48:598611 607
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[102], the stabilization of t/c-ZrO2 by milling with steel
tools is the result of two parallel processes: decreasing of
the
particle size and the oxidation and incorporation of iron
impurities into the ZrO2 crystal network. Annealing the
milled zirconias suppresses the stabilizing role of Fe and
the
tetragonal or cubic form reverts to the monoclinic phase.
The influence of reactive milling on phase transforma-
tion in the ZrO2Y2O3MgO system has been examined by
Stubicar et al. [103] using WC milling tools. Solid solu-
tions with either cubic or tetragonal symmetry were
obtained when milling ZrO2 with Y2O3 and/or MgO, but
not in binary mixtures of yttria and magnesia.
Perovskite-type CaZrO3 was obtained by milling an
equimolar mixture of CaO and monoclinic ZrO2 [104]. Cubic
ZrO2 appeared after a short milling time, stabilized by the
presence of CaO, whereas the characteristic reflections of
CaZrO3 started to emerge at a later stage. The amount of
orthorhombic CaZrO3 grows steadily upon further milling to
become the only crystalline phase seen by XRD after 18 h.
Ball milling has also been used to prepare nanosized powders
of pure and yttria-doped BaZrO3 starting from BaO2, ZrO2,
and commercially available zirconias partially stabilized
with
yttria [105]; the milling time needed to achieve complete
conversion increased as the yttrium content increased. No
mechanochemical reaction was observed when using BaCO3instead of
barium peroxide as the Ba source. Faster reaction
rates were obtained if the milling vial was opened regularly
to
allow the escape of excess oxygen.
Some other interesting examples
The synthesis of perovskite-type LaAlO3 was attempted,
starting with La2O3 and different sources of Al. No
mechanochemical reaction was observed when a-Al2O3was the Al
source, but a transient alumina obtained by
heating Al(OH)3 at temperatures between 400 and 800 Cas
reactants [106] gave the desired product. Other LnAlO3perovskites
(Ln = Nd, Sm, Dy) were also obtained by
milling the corresponding Ln2O3 oxides with partially
dehydrated Al(OH)3 as the Al source. Milling mixtures of
Al2O3 (a or c) and Y2O3 resulted in either an amorphousmaterial
or the YAlO3 perovskite, depending on the milling
conditions [107]. Magnesium aluminate spinel, MgAl2O4,
has been successfully prepared in a low-energy milling
device using a stainless steel container and balls, starting
from mixtures of MgO and c-Al2O3 or AlO(OH) [108].Conversion
took place with almost linear kinetics, and
complete conversion was achieved. The spinel was
obtained even when a-Al2O3 was used as Al source,although the
reaction rate was considerably lower. Milling
time was greatly reduced by milling a similar MgO plus
c-Al2O3 starting mixture using more energetic conditions
[71]. However, attempts to prepare SrAl2O4 and BaAl2O4by milling
amorphous Al2O3 and SrCO3 or BaCO3 using
stainless steel tools were unsuccessful [109]. Other spinel-
like phases prepared by mechanochemical methods include
ZnCr2O4 [110] and ASbO4 (A = Fe, V) [111].
Sodium niobate, NaNbO3, which is an interesting
material for its phase transitions and electrical behavior,
has been prepared by milling Na2CO3 and Nb2O5 using
zirconia vials and balls [112]. The vials had a small aper-
ture of about 5 mm in the cover to allow the escape of CO2during
milling. The formation of orthorhombic NaNbO3goes through an
intermediate XRD amorphous phase and
the reaction is incomplete even after prolonged milling.
This outcome does not depend strongly on the impact
energy of the balls [113].
The perovskite LaMnO3 (LSM) has been prepared by
milling La2O3 and different Mn sources, namely MnO,
Mn2O3, or MnO2, in a high-energy shaker mill [114, 115].
While no crystalline phase was obtained when starting with
MnO, LaMnO3 was obtained from Mn2O3 and MnO2. Milling
promoted the reduction of Mn(IV) to Mn(III), but not the
oxidation of Mn(II) to Mn(III). Consequently, the perovskite
formed when using MnO2, but not with MnO. The product
obtained with Mn2O3 and MnO2 was a mixture of the ortho-
rhombic and cubic polymorphs. Perovskite-type ErMnO3 and
ErMn0.9Ni0.1O3 have also been obtained by milling stoichi-
ometric mixtures of Mn2O3, Er2O3, and NiO [116]. The
as-prepared single-phase materials presented an orthorhombic
unit cell as opposed to the same material prepared by con-
ventional solid state reaction which is hexagonal.
Soft mechanochemistry
The observation that hydroxides and hydrated compounds
are much softer than the corresponding anhydrous oxides,
and consequently react more easily during mechanical
activation, led to the development of soft mechanochem-
istry [117]. Mechanochemical reactions between oxides
require highly energetic milling for an extended length of
time. The process is slow, energy inefficient, and results
in
substantial contamination of the product. Reacting of the
softer and more reactive hydroxides requires a much lower
milling intensity and shorter milling time, saving energy
and providing cleaner products.
Of course, the product of milling hydroxides is not the
desired final product, but a precursor, a mixed, often
amorphous oxide-hydroxide that has to be annealed to
obtain the final product. The benefits of the milling step
are
the formation of an intimately mixed precursor and a sub-
stantial decrease in the annealing temperature and time
compared to the conventional solid state reaction of the
oxides. During heat treatment, the unusual defect structures
608 J Mater Sci (2013) 48:598611
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are lost and it is difficult to achieve the formation of
metastable phases. Consequently, if the objective of milling
a mixture of oxide powders is to prepare an unusual meta-
stable phase with unique defect structureas is often the
casethe utility of soft mechanochemistry is limited. But,
if the goal is only to obtain a certain mixed compound as a
uniform single phase quickly and without much contami-
nation, the combination of low-energy milling of hydrox-
ides combined with annealing is an attractive option.
Conclusions
Undoubtedly, mechanical milling has become a powerful
powder processing method for the room temperature syn-
thesis of a number of important multicomponent oxides.
Recently, research activity in the field has grown steadily,
fueled by the development of new and more powerful mills
and the availability of new materials for milling media
(vials and balls) with increasing impact energy and resis-
tance to abrasion. Contamination from the milling tools
might be a particularly serious problem when hard and
abrasive oxides are milled at high intensity for a prolonged
time, although several steps can be made to reduce the
effects of contamination. Many multicomponent oxides
have been prepared featuring a broad variety of crystal
structures (spinels, pyrochlores, fluorites, apatites, per-
ovskites, etc.) and chemical composition. In some cases,
nucleation and crystallization of the reaction product takes
place from an amorphous matrix containing the reactants,
whereas in some others, both reactants and reaction prod-
ucts exist simultaneously during milling. The as-prepared
phases are frequently metastable, featuring a defect struc-
ture/microstructure that is difficult to obtain by means of
any other powder processing method. For example, some
authors have shown that mechanochemically prepared
oxides consist of grains presenting a non-uniform coreshell
structure, where an ordered inner core is surrounded by a
12-nm thick disordered grain boundary region. This core
shell configuration is typical of mechanically milled
nanopowders, but not of nanopowders prepared by other
techniques. In most cases, these metastable oxides show
rather good thermal stability; thus, heat treatments at high
tem-
peratures can be used to adjust the defect structure without
inducing transition to more stable crystallographic phases.
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Preparation of multicomponent oxides by mechanochemical
methodsAbstractIntroductionGeneral remarksIron oxides and related
systemsBismuth(III) oxide and related systemsTiO2 and related
systemsMixed oxides of Ti and lanthanidesRare-earth silicatesZrO2
and zirconia-based systemsSome other interesting examplesSoft
mechanochemistryConclusionsReferences
