Nanostructured alumina from freeze-dried precursors

Article abstract of DOI:10.1111/j.1551-2916.2010.04021.x

Nanocrystalline alumina has been obtained on the 100 g scale by thermal decomposition of precursors resulting from the freeze-drying of aqueous solutions of different aluminum-containing products, namely aluminum acetate and aluminum l-lactate. Samples prepared at different temperatures (from 873 to 1573 K in steps of 100 K) were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and surface area measurements. In the acetate case, the transformation sequence involves the formation of θ-Al₂O₃ as an intermediate phase between γ-Al₂O₃ and α-Al ₂O₃, whereas this θ phase is not observed in the lactate case. TEM and SEM images show the nanoparticulate character of the aluminas obtained at relatively low temperatures, with typical particle size in the 5-10 nm range. Progressive grain growth occurs as temperature increases. Otherwise, the precursor characteristics have a clear influence on the microstructure of the resulting aluminas, as reflected also by the measured BET surface area values. Whereas long aluminum acetate fibers results in open arrays of low aggregated alumina particles, large aluminum lactate sheets lead to comparatively compact alumina microstructures. Nanostructured alumina obtained from the lactate precursor has been reconstituted in a granulated powder with sufficient consistence and flowability to allow it to be thermal sprayed and deposited on a stainless steel substrate. XRD data show that γ-Al ₂O₃ is the major phase in the coating, which includes also α-Al₂O₃ particles. SEM results offer evidences on the nanostructured character of the coating.

Full text of DOI:10.1111/j.1551-2916.2010.04021.x

J. Am. Ceram. Soc., 94 [1] 236–243 (2011)
DOI: 10.1111/j.1551-2916.2010.04021.x
r 2010 The American Ceramic Society
ournal
J
Nanostructured Alumina from Freeze-Dried Precursors
Regina Villanueva, Andre
´
s Go
´
mez, Pedro Burguete, Eduardo Martı
´
nez, Aurelio Beltra
´
n, and
aw
Fernando Sapin
˜
Institut de Ciencia dels Materials, Parc Cientific, Universitat de Valencia, PO Box 22085, 46071 Valencia, Spain
´
Monica Vicent and Enrique Sanchez
´ ´
´ ´ ´ ´ ´
Instituto de Tecnologıa Ceramica—Asociacion de Investigacion de las Industrias Ceramicas, Universitat Jaume I,
12006 Castellon, Spain
´
Nanocrystalline alumina has been obtained on the 100 g scale by
thermal decomposition of precursors resulting from the freeze-
drying of aqueous solutions of different aluminum-containing
products, namely aluminum acetate and aluminum L-lactate.
Samples prepared at different temperatures (from 873 to 1573 K
in steps of 100 K) were characterized by X-ray powder diffrac-
tion (XRD), scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM), and surface area
measurements. In the acetate case, the transformation sequence
involves the formation of h-Al2O3 as an intermediate phase be-
tween c-Al2O3 and a-Al2O3, whereas this h phase is not ob-
served in the lactate case. TEM and SEM images show the
nanoparticulate character of the aluminas obtained at relatively
low temperatures, with typical particle size in the 5–10 nm
range. Progressive grain growth occurs as temperature in-
creases. Otherwise, the precursor characteristics have a clear
influence on the microstructure of the resulting aluminas, as re-
flected also by the measured BET surface area values. Whereas
long aluminum acetate fibers results in open arrays of low ag-
gregated alumina particles, large aluminum lactate sheets lead
to comparatively compact alumina microstructures. Nanostruc-
tured alumina obtained from the lactate precursor has been re-
constituted in a granulated powder with sufficient consistence
and flowability to allow it to be thermal sprayed and deposited
on a stainless steel substrate. XRD data show that c-Al2O3 is
the major phase in the coating, which includes also a-Al2O3
particles. SEM results offer evidences on the nanostructured
character of the coating.
Although several methods have already been used to date to
prepare nanostructured coatings (sputtering, sol–gel, etc.), the
process that is most likely to find industrial implementation in
the short term is thermal spraying and, in particular, plasma
spraying, owing to its great versatility.8 In plasma thermal
spraying (atmospheric plasma spraying (APS) being the most
widespread method), the nanostructured starting powder is
heated in a high-temperature gas stream to a semifused state,
while concurrently being accelerated toward the substrate to be
coated.9–14 When this material impacts the substrate, small lens-
like deposits form (known as ‘‘splats’’), approximately 20 mm in
diameter and 5 mm thick, from the semifused state. These splats
solidify quickly at cooling rates of approximately 100 K/s. Coat-
ings with thickness up to 200–300 mm can be produced by in-
creasing the number of spray gun passes. Because this is a rapid
process, in which the residence times during which the materials
are exposed to high temperatures are extremely short, it might
favor holding the nanostructure of the starting powder in the
coating. In fact, progress of feedstock particles melting (a pro-
cess that has been thoroughly studied) depends on both plasma
spraying conditions and the very nature of the feedstock.15,16
Thus, the maintenance of the starting powder nanostructure
should be practically guaranteed in nonmelted zones. We will
return on this point later on.
Under this approach, the seemingly obvious requirement for
obtaining nanostructured coatings is to have the adequate nano-
structured starting powder at our disposal. Nanostructured ma-
terials are ultrafine-grained solids with a high proportion of
atoms at grain boundaries. The key factor for industrial exploi-
tation of nanostructured materials lies in the development of
cost-effective, large-scale manufacturing processes.17,18 At pres-
ent, more than two dozen different processes for their prepara-
tion have been described. However, most of these methods are
expensive, require sophisticated equipment, and yield relatively
low production rates. Therefore, they have high production
costs and are conceived for obtaining high-value-added materi-
als, with typical applications in the fields of electronics and pho-
tonics. In other cases, difficulties may appear owing to operation
details. Indeed, a problem frequently associated with sol–gel-re-
lated methods lies in the stoichiometric control and the cation
distribution in multicomponent systems.
I. Introduction
HERE is an increasing awareness of the potential of nano-
structured coatings, both for replacing present coatings, due
T
to their superior properties in comparison with their conven-
tional (microstructured) counterparts, and for generating new
applications.1,2 Studies have centered on ceramic–metal com-
posites (cermets of the WC–Co type) and ceramic oxides.3–7 The
reason for this choice is based on the particular characteristics of
ceramic materials, such as high hardness and high thermal and
corrosion resistance, which provide many advantages compared
with polymers and metals when it comes to use as coatings, es-
pecially in applications in which wear, temperature, and corro-
sion resistance are simultaneously required.
In this context, powder processing methods for the prepara-
tion of submicrometer and nanostructured powders in a con-
trolled form, with a wide range of compositions, sizes, and
morphologies, need to be improved. The freeze-drying method
was first successfully applied by E. W. Flosdorf in 1935 in the
instant coffee industry. Nowadays, this method is applied in the
production of numerous advanced materials.19–32 The method
consists of (a) fast freezing of a sprayed solution, (b) vacuum
drying by sublimation of the solvent, and (c) decomposition of
the resulting precursor to give rise to oxides, nitrides, or carbides
by heat treatment in a controlled atmosphere. In comparison
with other chemical methods, one of the advantages of this
J. Smialek—contributing editor
Manuscript No. 27691. Received March 15, 2010; approved June 28, 2010.
This study was supported by the Spanish Ministry of Science and Technology and EU
FEDER Program (MAT2006-12945-C0-01, MAT2006-12945-C0-03, MAT2009-14144-
C03-01, and MAT2009-14144-C03-03).
wAuthor to whom correspondence should be addressed. e-mail: fernando.sapina@uv.es
236

January 2011
Nanostructured Alumina
237
technique is its capacity to prepare polycrystalline powders with
controlled characteristics. The commercial applications (elec-
tronics, pharmacy, catalysis, pigments, etc.) are determined by
the specific properties of the resulting materials. As far as other
competing synthesis processes are concerned, the disadvantages
of the sol–gel method (difficult control of the stoichiometry and
cation distribution in multicomponent systems) have been noted
above. In the spray-drying method, on the other hand, the
equipment has a high volume and a low production rate.
Alumina and mixtures of alumina and titania are used as
coatings that withstand severe high temperature, friction, or
corrosion environments.33–35 The present paper reports on how
the use of freeze-dried precursors allows the preparation of
nanostructured alumina powders at the 100 g scale. The nano-
structured alumina was reconstituted in a granulated powder
with sufficient consistency and flowability to allow it to be
sprayed and deposited on stainless steel substrates.
leaving the sample inside the furnace (slow cooling, ca. 2 K/
min). All products were stored in a desiccator over CaCl2.
(D) Preparation of an Alumina Coating from Nanostruc-
tured Powder
(a) Feed Powders: In most thermal spray methods, coat-
ings are obtained from feedstock in powder form. As a result, in
order to obtain nanostructured coatings by these techniques, it is
convenient to use nanoparticles as raw material. However,
nanoparticles cannot be sprayed directly owing to their low
mass: they must be agglomerated to form micrometric particles.
This process, which is critical for the success of the method, is
known as reconstitution process. In fact, agglomerate size and
density are critical for coating quality. The powder must be
dense enough and have an appropriate particle-size distribution.
Fine and porous powders cannot be fed into the center of the
plasma flame. Indeed, they will float on the flame surface or will
be evaporated by superheating before spattering on the sub-
strate, which results in poor deposition efficiency and poor coat-
ing bond strength.
II. Experimental Procedure
Nanostructured alumina agglomerates with good cohesion
and sufficient mechanical strength were prepared by forcing the
granular association of precursor particles. With this aim, an
aqueous solution (10 wt%) of polyvinyl alcohol (PVA, Mowiol
4/88 from Clariant, Mutlenz, Switzerland) was used. A sample
of the nanostructured alumina powder was deposited on a tray
and sprayed with the PVA solution. It was then transferred to a
granulating drum, which was rotated at 200 rpm for 10 min. The
amount of PVA added was ca. 1.0 wt% (referred to powder
sample). The resulting granulated powder was screened between
40 and 100 mm. Finally, in order to enhance powder flowability,
fumed silica (Aerosil 200 from Evonik, Essen, Germany) was
added to the granulated powder, and this mixture was again
rotated in the drum for 10 min. The amount of fumed silica
added was ca. 0.6 wt% (referred to powder sample).
(b) Coating Processing by APS: APS is one of the most
important and versatile techniques currently used by various
sectors of industry for the deposition of wear-resistant ceramic
coatings. During the process, the feedstock is injected in powder
form inside the flame at very high temperature (6000–12 000 K).
The powder is heated and accelerated until impact on the sub-
strate surface, where it cools rapidly, and forms the coating.
In this study, coatings were obtained by APS (Sulzer-Metco
F4 MB plasma gun, Wintherthur, Switzerland), mounted on an
industrial robot (IRB1400 from ABB, Zurich, Switzerland). Be-
fore spraying, the substrate was blasted with corundum grit and
cleaned with ethanol to remove any dust or grease residue from
the surface. The main spraying parameters were as follows: Ar
flow rate, 35 slpm (standard liter per minute); H2 flow rate, 12
slpm; arc intensity, 600 A; spraying velocity 1000 mm/s; and
spraying distance 120 mm.
(1) Synthesis
(A) Materials: The materials used as reagents in this
study were aluminum hydroxide, Al(OH)3 (Kemira Iberica,
´
Barcelona, Spain, 100%), aluminum L-lactate, Al(CH3CHOH-
COO)3 (Aldrich, St. Louis, MO, 97.0%), cerium (III) acetate
hydrate, Ce(CH3COO)3 ꢀ 1.5H2O (Aldrich, 99.9% pure), glacial
acetic acid (Panreac, Castellar del Valles, Spain 99.5%), and
acetic anhydride (Panreac 98%).
(B) Preparation of Precursors: Aluminum hydroxide
was transformed in a soluble acetate complex by refluxing for
24 h in a suspension containing Al(OH)3 (0.7 mol) in a 1:1 (v:v)
mixture of glacial acetic acid and acetic anhydride (50 mL).
Then, the suspension was filtered and the resulting solid was
washed with acetone. The precursor solution was prepared by
refluxing for 3 h in a suspension of the acetate complex (0.024
mol) in water (200 mL), which resulted in a clean solution whose
concentration in Al31 was 0.12M.
Aluminum L-lactate precursor solution (0.60M in Al31) was
directly prepared by dissolving the salt (0.15 mol) in water (250
mL) under continuous stirring at room temperature. We also
prepared a second precursor solution in which a small amount
of cerium was present (Ce31:Al31 5 1:100 mol; [Al31] 5 0.60M).
Addition of cerium has the purpose of avoiding the carbon de-
position observed on the products resulting from thermal de-
composition of the cerium-free precursor at low temperatures.
Cerium was handled as cerium (III) acetate, and was dissolved
in the aluminum L-lactate solution. On the other hand, for con-
sidering possible effects on the microstructure of the final al-
umina associated with the concentration of the precursor
solution, we also prepared analogous solutions whose concen-
tration in Al31 was 0.12M. In order to improve efficiency for
further treatments, the precursor solutions must be as concen-
trated as solubility allow.
(3) Characterization
Droplets of these solutions were flash frozen by spraying on
liquid nitrogen and, then, freeze-dried at a pressure of 10ꢁ4 atm
in a Telstar Cryodos freeze-drier (Terrasa, Spain). In this way,
dried solid precursors were obtained as amorphous (X-ray
diffraction (XRD)) loose powders. The thermal evolution of
the precursors was monitored by thermogravimetric experi-
ments (TGA–DTA) under oxygen atmosphere (heating rate
5 K/min, flow rate 50 cm3/min), using a Setaram Setsys 16/18
system (Caluire, France).
(C) Synthesis of Nanostructured Alumina: Nanostruc-
tured alumina samples were synthesized by thermal decomposi-
tion of the amorphous precursor solids. A sample of the selected
precursor was placed into an alumina boat and introduced in the
furnace. Several runs under different experimental conditions
were performed in order to determine its influence on the struc-
ture and microstructure of the samples. The precursor powder
was heated at 5 K/min to a final temperature Tf (Tf 5 573–
1573 K, in steps of 100 K) that was held for a period of time thold
of 6 h under flowing oxygen. The solid was then cooled by
(A) Elemental Analysis: Metal ratios in the cerium-con-
taining samples were determined by energy-dispersive X-ray
analysis (EDAX) on a scanning electron microscope (Hitachi
S-4100, (Tokyo, Japan) BSE Autrata detector, EMIP 3.0 Soft-
ware, and a RONTEC microanalysis system, Berlin, Germany).
Operating voltage was 20 kV, ;and the energy range of the anal-
ysis was 0–20 keV. EDAX showed that the freeze-dried precur-
sors were homogeneous at micrometer scale.
(B) XRD: XRD patterns were obtained from a diffracto-
meter using CuKa radiation (Seifert XRD 3003 TT, Ahrensburg,
Germany). Samples were dusted through a sieve on the holder
surface. Routine patterns were collected with a scanning step of
0.081 at 2y over the angular range 2y 101–701 with a collection
time of 5 s per step. All graphical representations relating to
XRD patterns were performed using adequate software (DRX-
Win Program).36 This technique was also used to characterize the
as-sprayed coating.
(C) Microstructural Characterization: The morphology
of the freeze-dried precursors, the resulting oxides, and the

238
Journal of the American Ceramic Society—Villanueva et al.
Vol. 94, No. 1
as-sprayed coating were observed using a scanning electron
microscope–field emission (Hitachi 4100FE) operating at an
accelerating voltage of 30 kV, and using also a transmission elec-
tron microscope (JEOL JEM 1010, Tokyo, Japan) operating at an
accelerating voltage of 100 kV. All the scanning electron micro-
scopic (SEM) preparations were covered with a thin film of gold
for better image definition.
(D) Surface Areas: The BET surface areas of the prod-
ucts were determined by nitrogen adsorption at 77 K, assuming
a cross-sectional area of 0.162 nm2 for the nitrogen molecule
(Micromeritics ASAP 2000, Norcross, GA). Before adsorption
measurements, the samples were outgassed in vacuum at 423 K
for 18 h.
major phase at 1373 K, we can now also observe weak peaks
attributable to a-Al2O3. The a-Al2O3 phase becomes the major
one at 1473 K, but the presence of y-Al2O3 results in some weak
peaks. Finally, at 1573 K, a-Al2O3 is the only detectable phase.
Thus, the transformation sequence in the thermal decomposi-
tion of the acetate precursor can be written as follows:
Otherwise, dealing with the aluminum lactate samples, the
final products from thermal treatments ranging from 573 to 973
K were amorphous. Crystallization of the g-Al2O3 phase can be
observed at 1073 K. Formation of a-Al2O3 is not detected (as
minor phase) until 1273 K. Finally, a-Al2O3 appears as the only
crystalline phase at 1373 K and higher temperatures. In the case
of Al–Ce lactate samples, the transformation sequence is similar
to that described for the lactate precursor exempt from Ce.
However, the a-Al2O3 appears at higher temperatures. In fact,
both a-Al2O3 and transition g-Al2O3 phase peaks are observed
at 1373 K. It is interesting to note that the CeO2 peaks are not
detected at relatively low temperatures (i.e., in the presence of
g-Al2O3). In fact, the CeO2 peaks are not observed before the
formation of some a-Al2O3 (1373 K). These two facts suggest
III. Results
The thermal evolution of the precursors was monitored by TGA
and XRD. Figure 1 shows the TGA curves corresponding to the
thermal evolution under oxygen atmosphere (heating rate 5 K/min,
flow
Aluminum lactate precursor ! amorphous product ! g-Al2O3 ! a-Al2O3
Al2Ce lactate precursor ! amorphous product ! g-ðAl; CeÞ2O3 ! a-Al2O3 þ CeO2
rate 50 cm3/min) of the different freeze-dried powder
that (a) g-Al2O3 is able to form a solid solution that incorporates
Ce(IV) ions, (b) the presence of dissolved Ce(IV) ions stabilizes
g-Al2O3 with respect to a-Al2O3, and (c) the solubility limit of
Ce(IV) in a-Al2O3 is very low. Thus, the transformation se-
quences in the thermal decomposition of the lactate precursors
are as follows:
The poor crystallinity of the low-temperature polymorphs
(g and y), as well as the influence on peak width of the disorder
in the cation positions, prevents a clear picture of the temper-
ature evolution of the crystallite size of these phases from being
obtained. However, information was obtained on the a-Al2O3
crystallite size. This size was calculated from XRD patterns by a
standard Scherrer analysis of the half-width of the XRD peaks.
LaB6 from NIST (SRM660a) (Gaithersburg, MD) was used as
standard to calibrate the intrinsic width associated with the
equipment.37 Crystallite sizes range, in all cases, from 70 to 200
nm, depending on the temperature and precursor. Thus, even at
1573 K, the a-Al2O3 crystallites are very small.
precursors. In all cases, the onset of the thermal decomposition
of the organic component of the precursor complexes occurs at
ca. 475 K, and decomposition is complete at 675–700 K. Before
this process (i.e., from room temperature until ca. 475 K), a
relatively small (3–12 wt%) and continuous weight loss is ob-
served for all samples, which could be associated with evolution
of water molecules retained in the precursors. After water re-
moval, decomposition of the precursors occurs in at least two-
step superimposed processes.
Figure 2 shows the XRD patterns of the products resulting
from thermal treatment of the precursors at temperatures rang-
ing from 1073 to 1473 K. The thermal treatments were per-
formed under dynamic oxygen atmosphere for 6 h. In the case of
the Al acetate samples, the presence of aluminum oxyhydroxide
was observed at 573 and 673 K. By rising the treatment tem-
peratures, it appeared firstly (773 K) an amorphous product,
whereas transition aluminas were detected from 873 to 1273 K.
Concerning these transition aluminas, g-Al2O3 is the only crys-
talline product between 873 and 1073 K, while y-Al2O3 is ob-
served at 1173 and 1273 K. Although y-Al2O3 remains as the
Figures 3–5 show characteristic transmission electron micro-
scopic and SEM images corresponding to some representative
alumina samples. In the case of the Al acetate precursor, the
Aluminum acetate precursor ! AlOðOHÞ ! amorphous product ! g-Al2O3 ! y-Al2O3 ! a-Al2O3
freeze-dried powder is made up of fibers, several hundred nano-
meters long, with a diameter of about 5–10 nm. The external
appearance of these fibers remains practically unchanged during
the thermal decomposition process, as can be clearly observed in
the micrograph of the sample prepared at 1073 K (Fig. 3(a)).
However, these fibers are now constituted by aggregates of
nanometric particles with a typical size of 5–10 nm. At higher
temperatures, a continuous small increase in particle size is ob-
served, together with a progressive collapse of the fibrillar mi-
crostructure. Between 1373 and 1573 K (Fig. 3(b)), the collapse
of the fibrillar array produces an open structure.
In the case of the aluminum lactate precursor, the evolution
of the microstructure displays some differences. Indeed, the
freeze-dried precursor consists of sheets with typical dimensions
of 4000 nm ꢂ 4000 nm ꢂ 500 nm. Again, the external appear-
ance of the precursor particles remains practically unchanged
during relatively low temperature treatments (1073 K), as can be
clearly observed in the micrograph shown in Fig. 4(a). A higher
magnification image reveals that the product sheets consist of
aggregates of nanoparticles with a typical size of 5–10 nm
(Fig. 4(b)). At higher temperatures, a certain increase in particle
Fig. 1. Characteristic TGA curves corresponding to Al acetate (dotted
line), aluminum lactate (solid line), and Al–Ce lactate (dashed line)
freeze-dried precursors.

January 2011
Nanostructured Alumina
239
from an aluminum L-lactate precursor solution 0.12M in Al31
.
It must be stressed that neither XRD patterns nor SEM micro-
graphs corresponding to analogous products in these series
(0.60 or 0.12M aluminum lactate) reveal any noticeable differ-
ence. So, the observed differences in the behavior of samples
coming from aluminum acetate and aluminum L-lactate precur-
sors should be related to the proper precursor nature, and not
associated with concentration effects.
In the Al–Ce lactate case, the evolution of the microstructure
is very similar to that of the aluminum lactate precursor: the
large sheets of the original precursor are observed until 1373 K
(Fig. 5(c)), and consist of nanoparticle aggregates that grow
slowly between 1073 and 1373 K (Figs. 5(a) and (b)). At 1573 K,
strong grain growth and sintering is observed (Fig. 5(d)). How-
ever, there is a significant difference with respect to the alumi-
num lactate case: the presence on the surface of sintered grains
of small particles with a typical size of about 80 nm. According
to XRD data, the presence of these small particles can be
attributed to the segregation of CeO2 during the g-Al2O3 to
a-Al2O3 transformation.
These differences in the final microstructures, either open or
more compact depending on the precursor, are clearly reflected
in the evolution of the BET surface area of the samples (Fig. 6).
At a given temperature, the BET surface area of alumina sam-
ples coming from the acetate precursor are significantly larger
than those corresponding to lactate-derived samples.
Figure 7 shows the XRD of the coating prepared by using as
feedstock nanostructured alumina obtained from the aluminum
lactate precursor at 1073 K. As observed, g-Al2O3 is the major
phase in the coating but the presence of a-Al2O3 is evident. Ac-
cording to previous results in the literature, this is a usual sit-
uation: independent of the type of alumina used to apply the
coating, the presence of mixtures of g-Al2O3 and a-Al2O3 is al-
ways observed, the relative contents of both phases being mainly
related to the sprayed aggregate size.38,39
Figure 8 shows micrographs of the coating corresponding to
both the outer layer (Figs. 8(a) and (b)) and an inner surface as
resulting from a transversal fracture (Figs. 8(c) and (d)). As can
be observed (Fig. 8(a)), the porous as-sprayed surface exhibits
an irregular topography. One can clearly appreciate the final
image of the sprayed droplets (splats) together with zones in
which comparatively small pseudofaceted particles prevail. Very
likely, the as-observed splats became deformed and partially
disaggregated by hitting the substrate. When these splats are
examined at higher magnification (Fig. 8(b)), their surface ap-
pears as consisting of a highly compact array of nanoparticles.
Otherwise, the surface generated after fracture shows the same
microstructural features (splats and particle zones) than the
outer layer (Fig. 8(c)). Figure 8(d) shows a representative im-
age, at high magnification, focused on a zone exempt from
splats. As observed, the surface of the pseudofaceted particles is
also composed of nanoparticles. By comparison with the splats
case, these nanoparticles are higher and adopt a less compact
disposition.
IV. Discussion
It is well known that alumina crystallizes in many poly-
morphs.40–47 All these polymorphs can be classified into two
groups, based on the arrangement of oxygen anions. There are
structures based on a cubic compact packing (fcc), such as the
g, Z, y, or d forms, or on a hexagonal compact packing (hcp),
such as the a, k, and w forms. The differences between the
polymorphs belonging to the same group are related to the ar-
rangement of the cations in the holes of the compact packing of
oxide anions. It is, thus, very difficult to differentiate among the
polymorphs in each group. The different polymorphs are
obtained by thermal decomposition of a variety of precursors.
Depending on the precursor (e.g., polymorphs of aluminum oxy-
hydroxide or hydroxide), different Al2O3 structures are formed,
which follow diverse sequences of phase transformations with
Fig. 2. X-ray diffraction patterns of materials prepared by thermal de-
composition in oxygen of (a) Al acetate freeze-dried precursor, (b) alu-
minum lactate freeze-dried precursor, and (c) Al–Ce lactate freeze-dried
precursor. The temperatures of the thermal treatment are 1073–1473 K,
in 100 K intervals, from bottom to top, in all cases.
size is observed (Fig. 4(c)). At 1573 K, strong grain growth and
sintering is observed (Fig. 4(d)).
Insofar as differences in precursor concentration salts (0.12M
for Al acetate and 0.60M for aluminum lactate) could have some
influence on the microstructural characteristics of the final prod-
ucts (because of effects due to the agglomeration degree after
freeze-drying), we carried out comparative experiments starting

240
Journal of the American Ceramic Society—Villanueva et al.
Vol. 94, No. 1
Fig. 3. Transmission electron microscopic (TEM) and scanning electron microscopic (SEM) images showing the microstructure of the materials ob-
tained from Al acetate freeze-dried precursors: (a) 1073 K (TEM); (b) 1573 K (SEM). Scale bars correspond to 100 and 1000 nm, respectively.
increasing temperature. In all cases, the a form is the the-
rmodinamically stable at high temperatures under normal con-
ditions. The detailed sequences and stability ranges have been
attributed to many factors, such as crystallinity degree, presence
of impurities, thermal history, among others.
Without prejudice to the above, there is abundant informa-
tion in the literature supporting the idea that the alumina par-
ticle size has a great influence on the crystalline form adopted:
nanoparticulate alumina crystallizes in the g form and micro-
particulate alumina in the a form.48 This is not an unique sit-
uation: nanoparticulate TiO2 and ZrO2 crystallize in anatase
and tetragonal forms, respectively, and microparticulate TiO2
and ZrO2 in rutile and monoclinic forms, respectively.49,50
Most researchers indicate that the microparticulate samples
of these oxides adopt the thermodynamically stable structural
variety, whereas those shown by nanoparticulate samples are
Fig. 4. Scanning electron microscopic images showing the microstructure of the materials obtained from aluminum lactate freeze-dried precursors:
(a and b) 1073 K, (c) 1273 K, and (d) 1573 K. Scale bars correspond to 2.5 mm, 250, 250, and 700 nm, respectively.

January 2011
Nanostructured Alumina
241
Fig. 5. Scanning electron microscopic images showing the microstructure of the materials obtained from Al–Ce lactate freeze-dried precursors: (a) 1073
K, (b and c) 1373 K, and (d) 1573 K. Scale bars correspond to 200, 300, 400, and 800 nm, respectively.
metastable.49 However, in 1993, molecular dynamic simulation
of different g-Al2O3 and a-Al2O3 surfaces indicated that the
surface energies of a-Al2O3 were significantly greater than those
of g-Al2O3.51 These data suggested that g-Al2O3 becomes the
energetically stable polymorph of nanoparticulate alumina, as it
was claimed by Navrotsky and colleagues in 1997.52 After few
months, the group of Navrotsky reported experimental results
showing that the surface energy of a-Al2O3 is actually signifi-
cantly greater than that of g-Al2O3 (2.64 vs 1.67 J/m2, respec-
tively). As they concluded, this implies, in practice, that surface
energy differences thermodynamically stabilize nanoparticulate
g-Al2O3 over nanoparticulate a-Al2O3.53 Since then, this group
has applied the same methodology, using high-temperature
solution calorimetry data coupled with other techniques, to
demonstrate that similar surface energy differences also explain
the TiO2, ZrO2, and anhydrous and hydrated ferric oxides
cases.54–56
Because the samples in this study are aggregated, we cannot
obtain information on the surface area at which stability cross-
over occurs. Note, furthermore, that the aggregation state of the
studied alumina samples depends on the precursor: it is low in
the acetate case, and very high in the lactate case. Indeed, al-
uminas obtained from acetate, lactate, and Ce lactate precursors
have BET surface areas of 156, 21, and 41 m2/g, respectively, at
1173 K. These differences in the aggregation state of the samples
are in turn due to differences in the microstructure of the orig-
inal precursor: the fibrillar microstructure of the acetate precur-
sors produces an open and poorly aggregated microstructure in
the resulting aluminas, whereas the lamellar microstructure of
lactate precursors produces a compactly aggregated microstruc-
Fig. 6. Evolution of the BET area with the preparation temperature of
the samples. Closed circles, acetate precursor samples; open circles, lac-
tate precursor samples; and closed square, Ce lactate precursor samples.
Fig. 7. X-ray diffraction pattern of the plasma-sprayed coating.

242
Journal of the American Ceramic Society—Villanueva et al.
Vol. 94, No. 1
Fig. 8. Scanning electron microscopic images showing the microstructure of the plasma-sprayed coating: (a and b) surface, and (c and d) fracture views.
Scale bars correspond to 20 mm, 200 nm, 20 mm, and 200 nm, respectively.
ture. Owing to aggregation, it is clear that not all the surface is
accessible for BET measurement. Thus, interfacial energies must
contribute significantly to the total surface energy in the lactate-
derived samples, whereas their contribution must be relatively
low in the acetate-derived samples. Even more, the transforma-
tion sequence in the case of the acetate precursor includes the
presence of y-Al2O3, whereas this phase is not observed in the
transformation sequences of the lactate precursors. This
strongly suggests that the presence of y-Al2O3 can be attributed
to a stabilization of this phase due to an energy crossover fa-
vored at low aggregation degrees, but suppressed at high aggre-
gation degrees.
Finally, some remarks need to be made concerning the coat-
ing characteristics. As noted above, although our powder feed-
stock was conformed by agglomeration of nanoparticulate
g-Al2O3, the XRD data show that the coating is built up from
both g-Al2O3 and a-Al2O3. This is a systematic result concerning
nanostructured alumina thermally sprayed coatings: indepen-
dent of the structural variety used as feedstock, the resulting
coating consists of mixtures of g-Al2O3 and a-Al2O3.38,39 In the
thermal plasma treatment, the nanoparticle agglomerates are
subjected to very high temperatures. Depending on both the
agglomerate features and plasma parameters, partial or total
melting of the agglomerates occurs. Because of the high tem-
perature conditions, the possible nonfused (solid) regions in the
generated droplets should mainly consist of particles of the sta-
ble a-Al2O3. The short residence time does not make probable
significant further particle growth by sintering. On hitting the
substrate, quenching of the melt, at rates exceeding 100 K/s,
leads to the formation of a great number of crystallization nu-
clei, which have low chance for growing. Thus, solidification
(crystallization) of liquid droplets at considerable undercooling
should result in the formation of g-Al2O3 rather than a-Al2O3,
according to the stability crossover characterized by Navrotsky
et al.53 The observed two-scale structure of the thermally
sprayed coating (splats from molten regions of the agglomer-
ates cementing retained solid particles) is what could be expected
on a thermodynamic basis. In practice, this would make unnec-
essary kinetic ad hoc arguments to explain the presence of
g-Al2O3 when a-Al2O3 is used as feedstock.57 In any case, the
key role played by the feedstock powder microstructure features
in determining the coating microstructure and thermomechan-
ical properties make necessary further studies in order to adjust
spray parameters for improving coating performances.16
V. Conclusion
In summary, alumina can be successfully prepared as nanopar-
ticles on the 100 g scale by thermal decomposition at relatively
low temperatures of precursors obtained by freeze-drying solu-
tions of aluminum acetate or aluminum L-lactate. The formation
process shows that y-Al2O3 is produced as an intermediate phase
in the acetate case, whereas this phase is not observed in the
lactate case. The authors attribute this fact to the different ag-
gregation state of the alumina particles, which depends on the
microstructure of the corresponding precursor. A Ce-containing
lactate precursor was also prepared in order to avoid the depo-
sition of residual carbon in the course of the precursor decom-
position at low temperature. Whereas Ce seems to be dissolved
in g-Al2O3, cerium oxide is clearly observed once a-Al2O3 has
formed. Finally, nanostructured alumina obtained from lactate
precursor calcined at 1073 K has been reconstituted into a gran-
ulated powder with sufficient consistency and flowability to al-
low it to be sprayed and deposited on stainless steel substrates.
Thermal sprayed coating partly retains the nanostructured char-
acter of the alumina powder.
Acknowledgments
The SCSIE of the Universitat de Valencia is gratefully thanked for the use of
the X-ray diffraction, electron microscopy, and analytical facilities.
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