Characterization of composites containing NiAl2O4 spinel phase from Al2O3/NiO and Al2O3/Ni systems
Characterization of composites containing NiAl2O4 spinel phase from Al2O3/NiO and Al2O3/Ni systems
Justyna Zygmuntowicz 0 1 2
Paulina Wiecin´ ska 0 1 2
Aleksandra Miazga 0 1 2
Katarzyna Konopka 0 1 2
0 Faculty of Chemistry, Warsaw University of Technology , 3 Noakowskiego St., 00-664 Warsaw , Poland
1 Faculty of Materials Science and Engineering, Warsaw University of Technology , 141 Woloska St., 02-507 Warsaw , Poland
2 & Justyna Zygmuntowicz
This paper focused on the characterization of the composites containing nickel aluminate spinel from Al2O3/NiO and Al2O3/Ni systems. The composites were prepared by die pressing of powders and subsequent sintering of green bodies in air atmosphere. Composites were characterized by XRD, SEM, EDS and DTA/TG/MS analyses. The physical properties of the composites were measured by Archimedes method. Quantitative description of the composites microstructure was made on the basis of SEM images using computer image analysis. The XRD studies and SEM observations of composites confirmed the presence of two phases Al2O3 and NiAl2O4 in the whole volume of samples from both systems. Spinel phase was evenly distributed throughout the volume of the material. Morphology of NiAl2O4 obtained from both systems was characterized by the presence of voids. The DTA/TG/MS measurements showed the characteristics of organic binder decomposition and type of gases released to the atmosphere during thermal treatment. Moreover, the DTA/TG analysis showed the temperature of spinel-phase formation for both systems. It was found that the spinel-phase NiAl2O4 formation retards the process of densification. Therefore, it can be concluded that densification of samples with spinel phase depends mainly on the volume of spinel phase in composite material and does not depend on the substrates used to prepare spinel phase. The values of the selected properties of Al2O3-Ni- and Al2O3-NiO-based materials confirmed that the physical properties depend on the type of substrates used in the fabrication of composites. The type of powder influences the open porosity of samples. For composites produced using NiO powder, open porosity is lower than for samples formed with nickel powder.
Al2O3–NiO Al2O3–Ni Spinel-phase
In the past few years, the use of composite materials has
significantly increased in various applications due to the
unique characteristics of these materials in comparison
with metals, ceramics or polymers. The composites
containing spinel phase are characterized by stability in high
temperature, chemical resistance, abrasion resistance and
high hardness [
]. A particular type of such materials is
composites from Al2O3–Ni and Al2O3–NiO systems with
spinel-phase NiAl2O4. The composites containing spinel
phase are widely used due to their unique properties.
Moreover, in the last few years they are the subject of
many studies. For example, the composites NiAl2O4/Al2O3
are applied as structural ceramic materials [
with the nickel aluminate phase (NiAl2O4) are also used as
catalysts or precatalysts for steam reforming or as electrode
materials in high-temperature fuel cells due to their unusual
]. Composites with nickel aluminate phase
can be prepared by several methods, such as a powder
], sol–gel method [
], wet chemical methods
such as hydrothermal and solvothermal methods ,
powder coating process [
], conventional impregnation
solid-state reaction [
]. The formation of the spinel
depends on many factors, e.g., time, temperature, starting
substrates used in the process, and it is not still fully
described. On the basis of literature data, it was found that
the most important factor to generate spinel phase is
sintering atmosphere [
]. During sintering in an oxygen
environment, Ni or NiO particles can react with Al2O3 to
form spinel. The reaction of spinel-phase NiAl2O4
formation is according to the following equation:
O2 ? NiAl2O4, which proceeds in two Al2O3 ? Ni ?
I step: 2 Ni ? O2 ? 2 NiO
II step: NiO ? Al2O3 ? NiAl2O4.
The use of NiO powder allows to bypass the stage of
oxidation of Ni to NiO.
As reported by the literature and own research, the
spinel phase influences the mechanical properties of
composite and therefore it is important to characterize spinel
distribution and its morphology [
13, 15, 17–19
]. For this
reason, the presented research was focused on the
characterization of the composites containing NiAl2O4 spinel
phase from Al2O3/NiO and Al2O3/Ni system.
The aim of the present study was to characterize nickel
aluminate spinel in the composites obtained from Al2O3–
NiO and Al2O3–Ni systems. Composites were
characterized by XRD, SEM, EDS and DTA/TG/MS and
In the experiment, the following powders were used:
a-Al2O3 TM-DAR from Taimei Chemicals (Japan) of
average particle size 133 ± 30 nm and density
3.96 g cm-3, NiO powder from Alfa Aesar (USA) of
average particle size 5 lm and density 6.67 g cm-3, as
well as Ni powder from Sigma-Aldrich (Poland) of average
particle size 8.5 lm and density 8.9 g cm-3. The purity of
the powders was as follows: a-Al2O3—99.99 %, nickel
oxide—99.96 % and nickel—99.99 %. Figure 1 presents
SEM images of the starting powders. It was found that they
were firmly agglomerated.
The composites were prepared from the powder
mixtures containing 90 vol% of Al2O3 and 10 vol% of NiO or
Ni powder. Two series of samples were prepared: series I
with the addition of NiO and series II with the addition of
Ni. The use of NiO powder in series I allowed to bypass the
stage of oxidation of Ni to NiO before the formation of
NiAl2O4 spinel phase.
The powder mixtures were homogenized in ethanol in a
planetary ball mill with a rotating speed of 300 rpm for
60 min. The next step was drying at 40 C for 48 h.
Subsequently, the poly(vinyl alcohol) (PVA) was added to
the powders as a binder, and granulation was made. Then,
the samples were prepared by uniaxial pressing at a
pressure of 100 MPa. The dimensions of cylindrical samples
were as follows: diameter d = 20 mm and height
h = 3 mm. The samples were sintered in air atmosphere at
temperature of 1400 C. Dwell time was 2 h. The heating
rate and cooling rate were 5 C min-1.
X-ray diffraction (XRD) was used to identify the phases
present in the samples. The structural studies were carried
out by using a Rigaku MiniFlex II for 2h values ranging
from 10 to 80 and using CuKa1.54 radiation and
k = 1.54178 A˚ . The analyses were performed at the cross
sections of samples.
DTA/TG measurements were taken by using Netzsch
STA 449C coupled with Quadrupole Mass Spectrometer
Netzsch QMS 403C. The heating rate was 5 C min-1, and
the final temperature was 1400 C. The measurements
were taken in the constant flow of two gases: argon
10 mL min-1 (protective gas) and synthetic air (80:20
N2:O2) 60 mL min-1. Mass spectrometer was set to detect
m/z values in mass range 10–300.
The density of obtained composites was measured by
the Archimedes method according to the PN-76/E-06307.
The average values were calculated from measurements of
30 samples in each series. Additionally, the densities of the
sintered bodies were determined on helium pycnometer
AccuPyc II 1340 (Micromeritics), in a sequence of 100
purges and 100 measurement cycles. The shrinkage after
sintering process was also determined.
The microstructure of the longitudinal section of
sintered specimens was investigated by scanning electron
microscope Hitachi S-3500N and SU-70. The longitudinal
sections were prepared by cutting the samples along the
axial direction with diamond saw and then polishing with
diamond paste 1 lm.
Quantitative description of the microstructure of the
composites was made on the basis of SEM images of
randomly selected areas on the longitudinal sections using
computer image analyzer [
]. The quantitative and
stereological analysis was not performed for distinguished
grains, but concerned the characteristics of the spinel areas
(spinel particles). The distinction of spinel areas was
executed on the basis of the differences in BSE contrast of
NiAl2O4 and Al2O3 phases. The object of the study was to
use the stereological analysis to determine the volume
fraction of spinel phase in the composites. Moreover, based
on the stereological analysis shape parameters of voids
present in the spinel phase were defined: coefficient
describing the elongation (a = dmax/d2), surface
development (R = p/p d2) and convexity (W = p/pc) , where
dmax—maximum diameter of void projection [lm],
d2—diameter of circle of the same surface as the surface of
the analyzed grain [lm], p—perimeter of void [lm] and
pc—Cauchy perimeter [lm].
Results and discussion
Preliminary macroscopic observations of the composites
from both series after the sintering process have shown that
the samples surface was blue, which was the evidence of
the presence of the spinel phase.
The X-ray diffraction patterns of samples from the series I
and series II are shown in Fig. 2. For both the series, only
peaks corresponding to a-alumina and cubic aluminate
nickel spinel were shown in the XRD patterns. There were
no changes in the alumina structure after sintering process
(in comparison with initial powder). The absence of Ni and
NiO peaks indicates that nickel and nickel oxide have fully
reacted with alumina into nickel aluminate spinel phase.
The compositions of starting powders are shown in
beginning of the oxidation of Ni to NiO. The mass loss
amounting to 3.57 % is observed till 356 C which again
indicates that the first stage of thermal treatment is
connected with moisture evaporation and polymer
decomposition. Then, the increase in mass is observed till
1080 C which can be ascribed to the formation of
NiAl2O4 spinel phase. The mass increase in the
temperature range 356–1080 C equaled 4.91 %.
Figure 4 presents DTA/TG/DTG curves together with
mass spectra for Al2O3/NiO green body. The total mass
loss was 3.63 % which corresponds to the moisture and
polymer content in the analyzed sample. The mass loss was
observed till ca. 430 C. The endothermic peak on DTA
curve with the minimum at 92 C is visible (similarly to
Al2O3/Ni sample). This peak overlaps with the peak from
mass spectrometer of m/z value 18 which indicates
dehydration process. The peaks on MS 44 curve at 87 C and
the second one at 362 C indicate the decomposition of
According to DTA curves, the formation of spinel phase
was observed at 891 and 897 C, respectively, for Al2O3/Ni
and Al2O3/NiO systems. At higher temperatures, the
sintering process of grains proceeded. The slight difference in
reaction temperature between NiO- and Ni-based samples
(7 C) can be caused by the difference in particles size
between nickel oxides; the NiO resulting from Ni oxidation
is of smaller particles size than NiO used as starting
material. According to the literature, the smaller the
particles size, the slightly lower the reaction temperature [
Selected physical properties of the composites are
presented in Table 2. The theoretical density was calculated
according to the rule of mixtures. Calculations were made
for the composition listed in Table 1. The following
densities were used: 3.96 g cm-3 for Al2O3 and 4.51 g cm-3
for NiAl2O4 (based on JCPDS file No. 10-0339). It was
found that in the case of the series II, the relative density is
significantly lower than in case of series I. Moreover, the
open porosity is about 3 times higher than in series I. In
order to determine whether the type of substrate affects the
density and shrinkage of the composite with spinel, a
comparative series of samples from the Al2O3–NiO system
was made. A composition (mol%) of comparative samples
was the same as the samples of series II. Selected physical
properties have shown that density and shrinkage of
composites from series II and III are of the same order of
magnitude. These results confirmed the fact that the
spinelphase formation retards the process of densification
]. Therefore, it can be concluded that densification
of samples with spinel phase depends mainly on the
volume of spinel phase in composites and does not depend on
the substrates used to obtain the spinel phase. The type of
powder influences the open porosity of samples. For
composites produced using NiO powder, open porosity is
lower than for samples formed with nickel powder. This
was also confirmed by pycnometric measurements. This
difference may also result from the smaller particle size of
NiO than that of Ni.
Nevertheless, the aim of the conducted research was to
produce spinel phase from initial NiO or Ni powders. It was
observed that the linear shrinkage for series I (11.82 %
regarding samples height) is higher than for series II (6.28 %
regarding samples height). It is in good agreement with the
relative density values, because the higher the density, the
higher the sintering shrinkage. Figure 5 shows the SEM
micrographs of the fracture of the obtained composites. The
SEM observations of the microstructure confirmed that the
composite specimens were homogenous. There were no
visible cracks in each series. In both, it was observed that the
NiAl2O4 is characterized by two forms: the full oval shape
of spinel phase and the spinel-phase areas with characteristic
voids (shape of ‘‘doughnuts’’). The literature reports that the
shape of NiAl2O4 may result from the differences in the
values of the thermal expansion coefficient and diffusion
coefficient of components [
The volume fraction of the spinel phase in composites
was determined by stereological analysis. Series II had a
higher value of the volume fraction (61.6 %) of the
NiAl2O4 spinel phase than that of series I (30.71 %). The
size of spinel grains was calculated with regard to voids
present in the middle part of the spinel. For series I, the
diameter (d2) of voids (4.09 ± 0.46 lm) is slightly smaller
than for series II (5.65 ± 0.57 lm). The values of
parameters describing shape showed that the voids present in the
spinel areas are spherical. A higher value of the variation
coefficient was obtained for the series I (a = 1.48) than for
series II (a = 1.33). The results obtained through the
quantitative description of voids present in the spinel areas
showed that curvature of grain boundary and convexity had
the same value (for series I: R = 1.21 and W = 1.01,
whereas for series II R = 1.15 and W = 1.08). It showed
that the voids in the both series have similar shape. The
SEM images give also the confirmation that the sintered
bodies are not fully densified. Nevertheless, the
determination of sintering conditions of samples containing spinel
phase requires separate research.
Sintering of green bodies obtained from Ni or NiO and
Al2O3 powders in the air atmosphere leads to homogeneous
distribution of NiAl2O4 spinel phase in Al2O3 matrix.
There was no delamination at the interface of NiAl2O4/
According to DTA/TG/MS analysis, spinel phase is
formed at 891 and 897 C, respectively, for Al2O3/Ni and
Al2O3/NiO systems. The oxidation Ni to NiO was observed
as mass increase on TG curve. Mass spectrometer allowed
to observe that H2O and CO2 are released to the
atmosphere as the products of thermal decomposition of
polymeric binder used in die pressing of samples.
It was observed in each series that the spinel phase is
characterized by two forms: the full oval shape of spinel
phase and spinel with characteristic void inside. The results
obtained through the quantitative description of voids
present in the spinel areas have shown that voids have
similar spherical shape. Further, it was found that the
spinel-phase NiAl2O4 formation retards the process of
densification in both series.
Acknowledgements This work was financially supported by the
Faculty of Material Science and Engineering Warsaw University of
Technology (statute work). The authors thank Professor M. Szafran
and his group from the Faculty of Chemistry, Warsaw University of
Technology for the scientific support.
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