Thermal properties of pressure sintered alumina–graphene composites
Thermal properties of pressure sintered alumina-graphene composites
Pawe Rutkowski 0 1
Piotr Klimczyk 0 1
Lucyna Jaworska 0 1
Ludosaw Stobierski 0 1
Aleksandra Dubiel 0 1
0 Institute of Advanced Manufacturing Technology , ul. Wroclawska 37A, 30-011 Krako w , Poland
1 Faculty of Material Science and Ceramics, AGH University of Science and Technology , al. Mickiewicza 30, 30-059 Krako w , Poland
The work concerns the alumina-graphene materials sintered by two different pressure methods. The different particle sizes of graphene were used. The preparation route of the matrix-graphene mixture was discussed in the paper. The so-prepared compositions with different amount of graphene were hot-pressed and spark plasma sintered. The influence on uniaxial pressure during the sintering process on the microstructure was presented by the SEM microstructural observations and ultrasonic measurements. The material with unidirectional oriented graphene particles was prepared, and the anisotropy was even higher than 30 % for 10 mass% of graphene additive. The influence of graphene orientation as an effect of pressing process on the thermal properties was analysed. The anisotropy of thermal conductivity was 90 % for 10 mass% of graphene. The thermal diffusivity and thermal conductivity of composites manufactured by hotpressing and spark plasma sintering method were compared. The experiment-based calculation of the specific heat versus temperature was presented in the paper. The thermal expansion coefficient was determined by dilatometric method. The thermal stability was analysed by thermogravimetric method, and it showed that composites with up to 2 mass% of graphene can work at temperatures higher than 700 C.
Alumina; Graphene; Microstructure; Thermal stability; Thermal conductivity
In case of ceramic materials, the graphene particles are
used to improve the mechanical properties of such
wellknown material as alumina, silicon nitride and silicon
carbide . As a two-dimensional phase, graphene
shows very good electrical and thermal properties [21, 22].
That is, why this phase is very often deposited on the
ceramic or other kinds of surfaces. The electrical properties
of this phase, in case of ceramic material, can help at
shaping polycrystalline sintered materials by use of
electroerosion methods. For ceramics, as a materials working, at
high-temperature conditions, the addition of graphene
flake gives a hope of thermal properties improvement. The
typical methods of ceramicgraphene manufacturing
are hot-pressing (HP) and spark plasma sintering (SPS)
[9, 12, 18, 24]. The uniaxial pressing, which helps to
densify the materials during the sintering, becomes a
problem for ceramic2D phase materials. The one direction of
applied pressure causes the orientation of two-dimensional
phase in the ceramic matrix [23, 24]. This situation can lead
to the anisotropy of heat transport of ceramic matrix
composites. In the case of hexagonal boron nitride dispersed
phase, the thermal properties (conductivity) decrease in all of
directions in comparison with reference pure material . It
is caused mostly by agglomeration of hexagonal boron
nitride particles, where the agglomerates are porous. Different
situation is in case of graphene, where even well-packed
groups of oriented flakes lead to an improvement of thermal
conductivity in direction perpendicular to pressing axis so in
the direction of oriented graphene particles planes. It has
place in silicon nitridegraphene composites , where
introduce oriented graphene phase leads additionally to large
difference in thermal conductivity in different directions of
In this work, the experiments are focused on thermal
behaviour of alumina matrix composites with disperses
different size graphene particle. The thermal properties in
this work are discussed not only in the field of graphene
orientation in ceramic body but also in the field of
manufacturing process conditions. For that purpose, the
two-phase polycrystalline materials were obtained by HP
and SPS methods. On the sintered under uniaxial pressure
materials, the thermal diffusivity and thermal conductivity
were measured and compered with manufacturing method
and anisotropy of the composites.
The one very important and negative property of
graphene is lack of the resistance to reaction with oxygen.
That is why the produced composite materials were
thermogravimetry tested to show/determine maximal working
temperature in air conditions in function of graphene
The aim of the work was to investigate the influence of
technological conditions and graphene content on thermal
properties of aluminagraphene composites. The additional
parameter taken under discussion was the correlation
between material anisotropy and thermal conductivity.
Al2O3graphene composites were prepared using
commercial powders listed in Table 1. The graphene addition
Table 1 Materials characterization for composite preparation
Taimei CHEMICALS CO., LTD
Graphene Laboratories, Inc., USA
Cheap Tubes, USA
to the aluminium oxide matrix was established as follows:
from 0 to 10 mass%.
Hot-pressed composites preparation
In the case of HP method, two kinds of mixtures were
prepared: alumina with 010 mass% of 8 nm graphene and
alumina with 02 mass% of 4 nm graphene. The powder
mixtures were homogenized for 10 h in propanol using a
rotaryvibratory mill and alumina grinding media. Dried
and granulated powders were hot-pressed (Thermal
Technology LLC) at 1400 C for 1 h under 25 MPa in argon
flow. The heating rate was 10 C min-1. Sintered bodies
with a diameter of 50 mm were obtained.
Spark plasma sintered composites preparation
In the case of SPS method, the alumina powder (A16SG,
Almatis) with addition of 0.3 mass% of MgO nanopowder
(Inframat) has been used as a starting material. Multilayer
graphene nanoparticlesGNP, characterized in Table 1,
were used as fillers for alumina ceramic matrix composites.
The mixtures, containing alumina with addition of
GPLs, grades Gn(12), Gn(8) and Gn(4), respectively, were
prepared using Fritsch Pulverisette 6 planetary mill
equipped with ZrO2 grinding vessel and balls. Powders
were milled in isopropanol with rotation speed of 200 rpm
for 8 h. The mixtures were dried and sieved through
0.5 mm mesh.
The composites were sintered using SPS (FCT system,
Germany) in the temperature 1550 C for 10 min and
Average particle size 0.1 lm
Colour black, purity 99.2 %
Average flake thickness 12 nm (3050 monolayers)
Average particle (lateral) size: *4.5 lm (1.510)
Specific surface area 80 m2 g-1
Colour black, purity 99.9 %
Average flake thickness 8 nm (2030 monolayers)
Average particle (lateral) size *0.5 lm (0.153.0)
Specific surface area 100 m2 g-1
Colour black, purity 99 %
Average flake thickness \4 nm
Average particle (lateral) size 12 lm
Specific surface area [700 m2 g-1
applying 35 MPa of uniaxial pressure during the whole
cycle. Sintered specimens were disk-shaped with
dimensions of 20 mm in diameter and *5 mm in thickness.
d diameter of the measuring aperture of the
sample/reference/mm. The thermal conductivity was calculated from
the following equation:
Apparent density of the sintered samples was calculated
basis on the Archimedes method. The phase composition of
the sinters was checked by XRD diffraction (production of
Philips with X-Pert HighScore software) and the Rietveld
refinement determining the phase content quantitatively.
Raman spectroscopy (Horriba Yvon Jobin LabRAM HR)
was used for graphene phase identification. The
morphology of samples was observed using the SEM
techniqueFEI Nova Nano SEM.
The anisotropy of elastic wave velocity was measured
by ultrasonic method using UZP-1 (INCO-VERITAS)
The thermal stability was measured in air flow by means
of thermogravimetric TG measurements using STA 449 F3
Jupiter . The measurement was taken up to temperature of
Heat measurements were taken on a Netzsch LFA 427
apparatus. To determine the specific heat by comparative
method, Pyroceram 9606 reference material, with the
known coefficient of thermal expansion and specific heat,
was used. Thermal diffusivity was determined using the
laser pulse method (LFA) for the reference and test
material at temperatures ranging from 25 to 900 C in argon
flow. The sintered bodies were measured in pressing
direction (the direction of applied pressure during
manufacturing process) using the Cape-Lehmann ? pulse
correction computational model and in perpendicular
direction to the applied pressure using Radiation ? pulse
correction model. At each temperature, three
measurements were taken for statistical purposes. Examination of
tested materials density changes as a function of
temperature in the range up to 900 C was performed by
determining the coefficient of thermal expansion using a
Netzsch DIL 402C dilatometer. Based on these
measurements, specific heat was determined using the following
where cp specific heat of the sample/reference/J g-1 K-1, T
temperature of the sample/reference/K, Q energy absorbed
by the sample/reference/J, V amplitude of signal gain for
the sample/reference, q apparent density of the sample/
reference/g cm-3, D thickness of the test material/mm and
kT aT cpT qT
where a(T) thermal diffusivity/mm2 s-1, cp(T) specific
heat/J g-1 K-1, q(T) density of the material/g cm-3.
Results and discussion
The calculated, on the based on hydrostatic measurement,
densities values of alumina-based composites obtained by
HP and SPS methods are listed in Table 2. The relative
density shows that in case of HP, the materials with
addition up to 4 mass% of Gn(8) graphene have more than
98 % of theoretical density. The further increase in
graphene content leads to a decrease in material densification,
which reaches 95 % for 10 mass% of graphene. The results
obtained for SPSed composites confirm that low addition of
graphene phase, independently from type of graphene,
allow obtaining relative densities above 98 %.
The XRD measurements, made on aluminagraphene
materials, allowed to detect the graphite phase in case of its
higher content. The carried Raman analysis confirmed the
existence of graphene in all of the manufactured sintered
bodies. The Raman analyses, shown on the example HP
materials, are presented in Fig. 1. The Raman spectra of
graphene in manufactured composites are compatible with
Table 2 Densification of hot-pressed and spark plasma sintered
Fig. 1 Raman spectra of hot-pressed aluminagraphene composites
with various addition of graphene Gn(8)
Fig. 2 SEM observations of hot-pressed composites with 4 mass%
Gn(8) graphene content
the wavenumbers of pure graphene (powder) bands at:
3243, 2717, 1578 and 1349 cm-1.
The microstructural observations were made on the
polished surfaces and fractures. The examples of the results
are illustrated: for hot-pressed (HP) alumina/graphene
composites in Figs. 24 and for spark plasma sintered
(SPS) in Figs. 6 and 7.
The observed microstructures in Figs. 24 indicate on
graphene flake or group of flakes orientation
perpendicularly to the applied pressure during the HP process.
That situation takes place for whole range of the graphene
content. To prove the orientation of the graphene particles
in alumina matrix, the ultrasonic wave velocity was
measured on the samples in different direction.
In the case of hot-pressed alumina matrix composites,
the results of ultrasonic measurements, shown in Fig. 5,
indicate on a significant increase in anisotropy of
longitudinal wave velocity versus graphene content. The value
exceeds even 30 % for sample containing 10 mass% of
Fig. 3 SEM observations of hot-pressed composites with 8 mass%
Gn(8) graphene content
graphene. The results of anisotropy illustrated in Fig. 5
confirm the microstructural observation of oriented
graphene flakes/groups of flakes in hot-pressed composites.
The similar situation of anisotropy was recorded also in
previous work of the author in case of silicon nitride
graphene composites . In case of spark plasma sintered
materials, it was impossible to measure the anisotropy
because of too small dimensions of samples and too
The microstructural observation (Figs. 6, 7) made on
SPS obtained composites shows that SPS method did not
allow to obtain the graphene orientation in such scale
like in the hot-pressed materials. In the case of alumina
matrix composite containing 10 mass% of graphene,
there are visible large not oriented agglomerates of
graphene. In case of SPSed materials, the white inclusions
visible in Figs. 6 and 7 are zirconia phase coming from
the milling agent.
The hot-pressed alumina matrix composites were tested
in air condition at elevated temperature to determine the
maximal working temperature. The mass loss results
presented in Fig. 8 show that the materials containing up to
2 mass% can work up to 1000 C, because the graphene is
still protected from oxygen by alumina matrix. For higher
quantities of dispersed graphene phase, the obtained
composites can work up to temperature from 550 to 700 C
dependently on graphene content and the time of work. In
case of argon flow, the composites were stable up to
Table 3 presents data of thermal expansion coefficient
that was used to calculate thermal conductivity. The results
show that the additions of graphene do not change
significantly the value of CTE for the same processing
method. Comparing data recorded for sintered bodies
obtained by HP method and SPS one, the thermal expansion
for composites manufactured by SPS is lower. That can be
Fig. 4 SEM observations of fracture of hot-pressed composites with
6 mass% Gn(8) graphene content
Fig. 5 Anisotropy
Fig. 6 SEM observations of spark plasma sintered (SPS) alumina
composite with 2 mass% Gn(4) graphene content
explained by finer grains in the microstructure of SPSed
material. The data listed in Table 3 can be used for future
computer simulation of manufactured working parts.
The measurement taken by the laser flash analysis
(LFA) method allowed directly to determine the thermal
diffusivity of tested materials. The earlier ultrasonic
experiments and microstructural observations showed the
graphene orientation in the manufactured materials. That is
Fig. 7 SEM observations of spark plasma sintered (SPS) alumina
composite with 10 mass% Gn(4) graphene content
0; 0.5; 1; 2 %
6 mass% Gn(8) 9.7
8 mass% Gn(8) 9.4
100 200 300 400 500 600 700 800 900 1000
Fig. 8 Thermal stability of hot-pressed aluminagraphene Gn(8)
Table 3 Thermal expansion coefficient versus graphene addition and
why the thermal diffusivity was also measured in two
different directions: in pressing axis (in direction of applied
pressure) and in perpendicular direction to the applied
Table 4 Thermal diffusivity of aluminagraphene Gn(x) composites at 25 C
Thermal diffusivity in
parallel direction/mm2 s-1
Thermal diffusivity in
perpendicular direction/mm2 s-1
Anisotropy/% (in comparison
with parallel direction)
Table 5 Specific heat of hot-pressed aluminagraphene Gn(8) composites
Specific heat/J g-1 K-1
Table 6 Specific heat of SPS sintered aluminagraphene composites
Specific heat/J g-1K-1
pressure. The results listed in Table 4 are for the
measurement at room temperature. The calculated anisotropy
of thermal diffusivity shows how different thermal
diffusivity is in perpendicular direction (in graphene flakes
orientation) in comparison with date measured in pressing
direction (minus indicates decreased values, plus
indicates increased values). The calculated anisotropy
shows that in the case of hot-pressed material, even small
Table 7 Thermal conductivity and anisotropy of aluminagraphene Gn(x) composites at 25 C
Thermal conductivity in
parallel direction/W m-1 K-1
Thermal conductivity in
perpendicular direction/W m-1 K-1
Anisotropy/% (in comparison
with parallel direction)
Fig. 9 Thermal conductivity versus temperature of hot-pressed
aluminagraphene Gn(8) composites, in pressing direction
additions of graphene improve thermal diffusivity in
perpendicular direction to the applied pressure during the
Looking at Table 4, the diffusivity in parallel direction
to pressing axis of hot-pressed composites decreases with
increasing addition of graphene, so in perpendicular
direction to the oriented graphene flakes. In the flake
direction, the graphene slightly improves thermal diffusivity in
comparison with pure alumina. The difference in thermal
diffusivity between different directions increases with
quantity of introduced graphene. The anisotropy listed in
Table 5 reaches even 89 % for addition of 10 mass% of
Gn(8). Compering different direction for the same
graphene content of hot-pressed samples, the addition of
graphene improves thermal diffusivity in direction of
100 200 300 400 500 600 700 800 900 1000
Fig. 10 Thermal conductivity versus temperature of hot-pressed
aluminagraphene Gn(8) composites, perpendicular pressing
graphene flakes orientation. In the case of spark plasma
sintered composites, the thermal diffusivity is lower than
for the same hot-pressed compositions. The difference in
density, different graphene particle size and low grain size
of matrix can be a plausible reason of lower value of this
parameter. Compering measurement directions in case of
SPS process, the improvement of thermal diffusivity
perpendicularly to pressing direction is visible only for higher
concentrations of graphene and probably, it is a result of
not oriented agglomerates of graphene Figs. 6 and 7.
To calculate thermal conductivity, the specific heat was
calculated on the base of diffusivity data obtained from
standard samples, diffusivity of tested material and change
of density versus temperature calculated from thermal
Fig. 11 Thermal conductivity versus temperature of SPS sintered
alumina with different addition of graphene Gn(4) measured in
Fig. 12 Thermal conductivity versus temperature of SPS sintered
alumina with different addition of graphene Gn(4) measured in
perpendicular direction to pressing axis
expansion coefficient. The data are presented in Table 5 for
HP manufactured material and in Table 6 for SPSed
composites. The value at room temperature is 0.790.82
J g-1 K-1 for hot-pressed materials and little higher
0.840.90 J g-1 K-1 for spark plasma sintered ones. At
900 C, the specific heat is in the range between 1.2 and
1.6 J g-1 K-1.
Thermal conductivity was calculated on the base of
thermal diffusivity data, calculated specific heat
(Tables 5, 6) and the change of apparent density versus
temperature. The results at room temperature are
presented in Table 7. The apparent/relative density of the
sintered bodies plays very important role. For the same
sintering temperature (HP process) and increasing
graphene content, the density becomes lower, so the
calculated thermal conductivity in the direction
perpendicular to pressing axis stays almost on the same level as
it is for reference material. For direction in pressing axis,
so perpendicular to graphene flakes, the thermal
conductivity at room temperature decreases dramatically with
increasing quantity of graphene. The maximal anisotropy
is almost 90 %. In case of SPS material, the sample with
10 mass% Gn(4) shows higher values of thermal
conductivity in comparison with HPed material, what can be
explained by higher densification of the material. For
small additions of graphene, the situation in pressing
direction is similar to hot-pressed materials.
The behaviour of thermal conductivity of hot-pressed
materials versus graphene addition, measuring temperature
and tested direction is presented in Figs. 9 and 10. The
graphene addition, in case of measurement taken in pressing
axis direction, leads to a decrease of thermal conductivity in
temperature function reaching 10 W m-1 K-1 at 900 C
independently from graphene addition, and the value is
much lower than for pure alumina (Fig. 9). The results
recorded in perpendicular direction to the applied pressure
(during HP process) show the thermal conductivity at
900 C depends strongly of graphene content and for 10 %
of dispersed phase is even higher than for pure alumina
(measured in the same direction) (Fig. 10).
The thermal conductivity results of spark plasma
sintered material for 4 nm graphene are presented in Figs. 11
and 12. In the pressing direction, the situation is similar
like for the hot-pressed materials (Fig. 11). In this case,
also some inclusions of zirconia, coming from milling
agent, can have a negative influence on thermal properties.
This impurity can make an increase in intergranular
boundaries, which probably results in a decrease in thermal
conductivity. Also the value of zirconia thermal
conductivity is very low around 2 W m-1 K-1. For the values
measured in perpendicular direction to applied pressure
(Fig. 12), the increase of thermal conductivity is significant
for 10 mass% of graphene and the value at elevated
temperatures is much higher than for pure alumina.
In case of SPSed composite materials, also the influence
of type of graphene on thermal conductivity was showed.
The results for 2 mass% of 4, 8 and 12 nm are presented in
Fig. 13 and Table 7. The data illustrated in Fig. 13 show
that the type of graphene has almost no difference on
thermal conductivity at room and elevated temperatures.
The shape of the curve is the same. All of used graphene
types, in quantity that the materials are thermally stable in
air condition (for application purpose), give almost the
same decrease in thermal conductivity in comparison with
100 200 300 400 500 600 700 800 900 1000
Fig. 13 Thermal conductivity versus temperature of SPS sintered
alumina with 2 % addition of different GPLs
The hot-pressed aluminagraphene composites show
very high microstructural anisotropy, where graphene
flakes are perpendicularly directed to the pressing axis.
The anisotropy was confirmed by microstructural
observations and ultrasonic measurements.
In the case of HP and SPS techniques, the addition of
different amount of various types of graphene leads to a
decrease in thermal diffusivity/thermal conductivity
measured in pressing direction.
The increasing content of graphene in hot-pressed
alumina matrix composites results in an increase in
thermal conductivity in perpendicular direction to
pressing axis in comparison with measurement taken
in pressing direction for the same graphene content.
At room temperature, the conductivity in perpendicular
direction to pressing axis in case of HP obtained
materials does not vary a lot for different content of
graphene in comparison with pure polycrystalline
In case of SPS obtained composites, the conductivity in
perpendicular direction to pressing axis is lower than
for pure alumina material and also lower than values
measured in pressing direction for the same graphene
content (except of 10 mass%). It is probably caused by
lower orientation of graphene flake in comparison with
The thermogravimetric measurement taken on
hotpressed composites shows that materials with up to
2 mass% of graphene can be used as working parts in
air at high temperatures. Higher additions of graphene
decrease the working temperature 550 C. For argon
flow, all composites were stable at the temperature of
The thermal conductivity measurement of spark plasma
sintered composites shows that the kind of graphene
has no significant influence on thermal conductivity at
low its content.
Acknowledgements The study constitutes a part of the Project No.
GRAF-TECH/NCBR/03/05/2012 Ceramicgraphene composites for
cutting tools and devices parts with unique properties. Thank to Dr.
Wojciech Piekarczyk for help with ultrasonic measurements.
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