Ce1−x Sm x O2−x/2—A novel type of ceramic material for thermal barrier coatings
Journal of Advanced Ceramics
Ce1xSmxO2x/2-A novel type of ceramic material for thermal barrier coatings
Hong-song ZHANG 0 1
a b Yong-de ZHAO
Gang LI 0
0 Department of Mechanical Engineering, Henan Institute of Engineering , Zhengzhou 450007 , China
1 Institute of Chemistry Henan Academy Sciences , Zhengzhou 450052 , China
2 Department of Construction Engineering, Henan Institute of Engineering , Zhengzhou 450007 , China
In this study, Ce1xSmxO2x/2 ceramics were synthesized by sol-gel route and solid state sintering method. The phase structure was analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. The morphologies of the synthesized powders and the corresponding bulk samples were observed using scanning electron microscopy (SEM). Their thermal diffusivities and thermal expansion coefficients were measured by the laser-flash method and the pushing-rod method, respectively. Results show that pure Ce1xSmxO2x/2 powders with single fluorite structure are synthesized successfully, and their microstructures of the corresponding bulk samples are very dense. With the increase of Sm2O3 content, their thermal expansion coefficients decrease due to the higher electro-negativity of Sm3+ ions as compared with that of Ce4+ ions. Their thermal conductivities at 1000 ℃ lie in the range of 1.62-2.02 W/(m·K) due to the phonon scattering caused by the substituted atoms and oxygen vacancies. The Ce1xSmxO2x/2 ceramics can be used as ceramic candidates for novel thermal barrier coatings (TBCs).
thermal barrier coatings (TBCs); CeO2 oxides; doping; thermophyscial properties
properties, 7–8 wt% yttria-stabilized zirconia (YSZ)
ceramic has been widely employed as the top coat
material by the current commercial thermal barrier
coatings in high temperature turbine components.
However, the thermal insulation ability and working
lifetime of the YSZ thermal barrier coating can be
injured severely for long-term application above
1200 ℃ due to its inherent phase transformation and
enhanced sintering [5,6]. Therefore, it is very urgent to
develop alternatives to YSZ for advanced TBC
The excellent ceramic candidates for TBCs must
possess a few important performances, such as low
thermal conductivity, appropriate thermal expansion,
good phase stability at high temperature, low sintering
rate, high melting point, chemical inertness, and good
adherence to the metal substrate . However, ceramic
materials matching all the requirements are still very
rare in light of the current standard. Now, low thermal
conductivity and appropriate thermal expansion
coefficient have been regarded as the primary selection
criterions of the ceramic materials for TBC applications.
In recent years, ceramic oxides with pyrochlore
structure or defect fluorite structure have been widely
studied [8–11]. Except for the A2B2O7-type (A = rare
earth element, B = Zr, Ce, Hf, Sn) oxides [1–10], the
cerium oxides with fluorite structure have recently
attracted extensive attention due to a diversity of
applications, such as conversion catalysts for selective
hydrogenation of unsatured compounds, catalysts for
three-way automobile exhaust systems, abrasives for
chemical polishing slurries, gates for metal-oxide
semiconductor devices, and luminescent materials for
violet/blue fluorescence [12–14]. Now, the rare earth
doped CeO2 (RE2O3–CeO2) have also been considered
to be new materials for TBCs and solid oxide fuel cells
due to the excellent electrical, mechanical, and
thermophysical properties [15–17]. For example, Cao et
al.  studied the thermal conductivity and thermal
expansion coefficient of La2Ce2O7. Patwe et al. 
reported the lattice thermal expansion of Gd2CexZr2xO7.
Zhang et al. investigated the thermophysical properties
of (Sm1xGdx)2Ce2O7  and (Sm1xDyx)2Ce2O7 .
Zha et al.  found that the electrical conductivities of
Ce1xGdxO2x/2 (GDC) and Ce1xSmxO2x/2 (SDC) at
700 ℃ are almost equal to the value of YSZ at 1000 ℃.
Compared with pure doped ceria oxide (DCO)
electrolyte, the DCO–chloride or DCO–carbonate
composite electrolyte not only has much higher ionic
conductivity, but also shows higher ionic transference
number at intermediate temperature range [23,24], and
these electrolytes also have good chemical stability
Although thermophysical properties of a few
rare earth stabilized CeO2 have been reported by
some researchers, the present reports about rare
earth stabilized CeO2 applications for TBCs are still
not systemic. Therefore, investigation of the
thermophysical properties of rare earth stabilized CeO2
is still of notable significance. Previous works have
discussed the electrical conductivity of Ce1xSmxO2x/2
system, but did not deal with thermophysical properties
of Ce1xSmxO2x/2 oxides. In the present study,
Ce1xSmxO2x/2 oxides were synthesized by sol–gel
method and pressureless sintering technology, and the
In the current investigation, Sm2O3 powders
(Rare-Chem Hi-Tech Co. Ltd., Guangdong, China;
purity ≥ 99.9%) and Ce(NO3)·6H2O (Zibo Huantuo
Chemical Co. Ltd.; analytical pure) were chosen as the
raw materials. Before weighting the raw powders, the
samarium oxide powders were firstly calcined at 800 ℃
for 2 h to remove the adsorptive water and carbon
dioxide in air, and then weighted samarium oxide
powders were dissolved in diluting nitric acid.
Ce(NO3)·6H2O was dissolved in distilled water and all
solutions were mixed with constant stirring.
Subsequently, the pH value of the mixed solution was
adjusted to 6 by adding ammonia hydroxide drop wise.
At the same time, ethylene glycol was put into the
resultant solution, and the mole ratio of ethylene glycol
to cerium was 1.8:1. The mixed solution was then
continuously evaporated on a water bath till a viscous
liquid was obtained, and the viscous liquid was heated
at 130 ℃ using air oven till a porous solid mass was
obtained. The obtained porous solid mass was ground
in an agate mortar and activated at 800 ℃ for 2 h in a
muffle oven. At the end, the achieved powders were
isostatically cold pressed into pellets at 100 MPa, and
the pellets were pressureless sintered at 1600 ℃ for
10 h in air to fabricate dense bulk samples.
An X-ray diffractometer (XRD, D8advance Bruker)
with Ni filtered Cu K radiation (0.1542 nm) was used
to analyze the phase structure of the synthesized
powders and the corresponding bulk samples. The
infrared spectra and Raman patterns of the synthesized
powders were recorded by a Fourier transform infrared
(FTIR) spectrometer (Nicolet 380) and a laser Raman
spectrometer (Renishaw inVia-Reflex), respectively. A
scanning electron microscope (SEM, Quanta-250, FEI)
was selected to observe the microstructure of the
synthesized powders and the corresponding bulk
A high temperature dilatometer (Model Netzsch DIL
402C/7, Germany) was utilized to measure the thermal
expansion coefficients (TECs) of bulk samples in the
temperature range of 20–1200 ℃. The thermal
diffusivity measurement () in 200–1000 ℃ was
carried out using laser-flash method (Model LFA1000,
Linseis, Germany) in an argon atmosphere. The specific
heat capacity (cp) from 20 to 1200 ℃ was calculated
using Neumann–Kopp rule in light of the reference
specific heat values of Sm2O3 and CeO2 . The
actual bulk density () of the sintered samples was
measured by the Archimedes drainage method at
room temperature. The thermal conductivity (k) of
Ce1xSmxO2x/2 oxides was achieved by Eq. (1), and the
actual thermal conductivity (k0) was computed using Eq.
(2)  in order to eliminate the influence of porosity ()
on thermal conductivity of bulk samples.
k cp (1)
3 Results and discussion
3. 1 Characterization about powders
The X-ray diffraction patterns of the synthesized
powders are displayed in Fig. 1 together with the data of
CeO2. Clearly, the X-ray diffraction patterns of
Ce1xSmxO2x/2 powders are consistent with that of
CeO2, which means that pure Ce1xSmxO2x/2 powders
with single fluorite structure are synthesized
successfully in the current study. The diffraction peaks
near 28.51°, 33.01°, 47.17°, and 55.88° can be indexed
to the (111), (200), (220), and (311) planes of fluorite
structure, respectively. With the increase of Sm2O3
content, the X-ray diffraction peaks corresponding to
the (111), (200), and (220) planes shift gradually to the
lower angles, which also implies that the Sm3+ ions have
entered the crystal lattice of CeO2, and this result can
also be confirmed by the increasing crystal lattice
parameters displayed in Fig. 2. From Fig. 1, the peak
width of Ce0.9Sm0.1O1.95 is greater than those of
Ce0.7Sm0.3O1.85 and Ce0.5Sm0.5O1.75, which signifies that
Ce0.9Sm0.1O1.95 has a small particle size as compared to
those of Ce0.7Sm0.3O1.85 and Ce0.5Sm0.5O1.75. In addition,
several weak peaks in the XRD pattern of
Sm0.5Ce0.5O1.75 near 30° and 32° can also be found,
which can be attributed to the tiny amount un-dissolved
Sm2O3 in the procedure of sol–gel synthesis.
Figure 3 reveals the FTIR spectra of Ce1xSmxO2x/2
powders calcined at 800 ℃ for 2 h in the wave number
range of 500–4000 cm1. Obviously, several typical
infrared absorption bands can be found at about
Fig. 2 Relationship between lattice parameter and doping
content in Ce1xSmxO2x/2 system.
Fig. 3 FTIR spectra of Ce1xSmxO2x/2 powders.
570–590, 620–680, 1620–1640, and 3400–3500 cm1.
Another absorption band at about 1010 cm1 can be
observed in FTIR pattern of Ce0.5Sm0.5O1.75, which can
be attributed to the little residual Sm2O3 powders .
The absorption band near the 3400–3500 cm1 is the
evidence of water molecules contained in the powders
, and the band located at 1620–1640 cm1
represents another vibration of the water molecules .
The bands near 570–590 and 620–680 cm1 are the
typical absorption peaks of CeO2, and the variation of
intensity and wave number of these two infrared bands
can be attributed to the doping of Sm2O3 .
The typical Raman patterns of Ce1xSmxO2x/2
powders are plotted in Fig. 4 together with the data of
micron-size CeO2. In the case of micron-size CeO2, the
main peak at 461.49 cm1 can be attributed to the F2g
Raman band from the space group Fm3m of cubic
fluorite structure [31,32]. With the increasing content of
Sm2O3, the width of the main Raman band enhances
clearly, which means that a large number of oxygen
vacancies are created [31,33]. Furthermore, the main
peak of Ce0.5Sm0.5O1.75 obviously shifts to higher
position compared to those of Ce0.9Sm0.1O1.95 and
Ce0.7Sm0.3O1.85, which can be attributed to the small
distortions of the atomic positions caused by Sm2O3
doping . In the Raman spectra of Ce0.7Sm0.3O1.85
and Ce0.5Sm0.5O1.75, a small shoulder at 600 cm1 can be
assigned as a longitudinal optical mode arising due to
the relaxation of symmetry rules , and the additional
low intensity Raman bands around 250.65 and
375.22 cm1 are usually assigned to the presence of
extrinsic oxygen vacancies generated into the ceria
lattice improving diffusion rate of bulk oxygen after
samarium addition .
The micro-morphology of the Ce1xSmxO2x/2
powders is displayed in Fig. 5. Obviously, the
synthesized Ce1xSmxO2x/2 powders exhibit a certain
agglomeration. Ce0.7Sm0.3O1.85 and Ce0.5Sm0.5O1.75 have
a size of about 50 nm; however, the average particle size
of Ce0.9Sm0.1O1.95 is only about 15 nm. The average
particle size obtained from SEM is consistent with the
analytical results of XRD.
Fig. 4 Raman patterns of Ce1xSmxO2x/2 powders.
Fig. 5 Micro-morphology of Ce1xSmxO2x/2 powders:
(a) x = 0.1, (b) x = 0.3, (c) x = 0.5.
3. 2 Characterization of bulk samples
The XRD patterns of the sintered Ce1xSmxO2x/2
samples are plotted in Fig. 6. Obviously, the X-ray
diffraction patterns for bulk samples are very close to
those displayed in Fig. 1, which means that the
densified samples still remain the single fluorite
structure. From Fig. 6, the weak peaks near 30° and 32°
in the XRD pattern of Ce0.5Sm0.5O1.75 disappear, which
means that the residual Sm2O3 also enters the lattice of
CeO2 in the procedure of sintering. It can be observed
clearly from Fig. 7 that the grain size of these bulk
ceramics is inhomogenous, and the average grain size is
several micrometers. The obtained bulk samples have
dense microstructure; however, some apparent pores
can still be seen in Fig. 7. Their relative densities
determined by actual density and theoretical density in
sequence are 93.7%, 92.8%, and 95.6%; the grain
Fig. 6 XRD patterns of the Ce1xSmxO2x/2 bulk samples.
boundaries are very clean and no other phases can be
found in these interfaces.
3. 3 Thermal expansion coefficients
The dilatometric measurement data of Ce1xSmxO2x/2
ceramics with calibration are presented in Fig. 8.
Clearly, the typical linear thermal expansion property
can be noted in the measuring temperature range of
20–1200 ℃, which also means that there is no phase
transformation occurred in the measuring temperature
range. In order to minimize the mismatch between the
ceramic layer and the metal substrate, a high thermal
expansion coefficient for ceramics of TBCs is required.
The temperature dependence of the thermal expansion
coefficient of Ce1xSmxO2x/2 ceramics is exhibited in
Fig. 9, together with the data of 8YSZ which were
measured in the former research of the authors. As
shown in Fig. 9, the thermal expansion shows an
increasing temperature tendency owing to the increasing
atomic spacing at high temperatures. From Fig. 9, the
thermal expansion coefficient of Ce1xSmxO2x/2
decreases gradually with increasing Sm2O3 content, and
Ce0.5Sm0.5O1.75 has the lowest thermal expansion
coefficient, which is still higher than that of 8YSZ. It is
well known that the thermal expansion has close
relationship with the ionic bond strength, and the ionic
bond strength is affected by the electro-negativity of
cations composing the crystal expressed as the
following equation :
(xA xB )
where IA-B represents the ionic bond strength between
ions at A site and B site, and xA and xB are
electro-negativity of ions at A site and B site
respectively. For CeO2, the ions at sites A and B are
Fig. 7 Microstructure of the sintered Ce1xSmxO2x/2
samples: (a) x = 0.1, (b) x = 0.3, (c) x = 0.5.
Fig. 9 Thermal expansion coefficient of Ce1xSmxO2x/2 as
a function of temperature.
Ce4+ and O2, respectively; partial substitution of Sm3+
for Ce4+ can increase the electro-negativity of cations at
A sites owning to the higher electro-negativity of Sm3+
ions (1.17) compared with that of Ce4+ ions (1.12).
Therefore, it can be concluded that the thermal
expansion coefficient of Ce1xSmxO2x/2 ceramics
decreases with increasing Sm2O3 content. However, the
thermal expansion coefficients of Ce1xSmxO2x/2
ceramics are still higher than that of 8YSZ, which still
fulfills the basic requirement for thermal barrier
3. 4 Thermal conductivity
Based on the specific heat values of CeO2 and Sm2O3,
the computed specific heat capacities of Ce1xSmxO2x/2
ceramics according to the Neumann–Kopp rule are
plotted in Fig. 10. Obviously, the specific heat capacity
of Ce1xSmxO2x/2 ceramics increases with the
increasing temperature, and decreases with Sm2O3
content at identical temperatures.
The dependence of thermal diffusivity of
Ce1xSmxO2x/2 ceramics on temperature is shown in
Fig. 11, and the data displayed in Fig. 11 are average
values of every three measurements at identical
temperature. It can be observed clearly that the thermal
diffusivities decrease gradually with increasing
temperature in the present measuring temperature range,
which shows a typical phonon thermal conduction
In light of the values of thermal diffusivity, density,
and specific heat capacity of Ce1xSmxO2x/2 ceramics,
the final values of thermal conductivity are plotted in
Fig. 12. It can be noted that the thermal conductivity is
inversely proportional to the increasing temperature in
the current temperature range, and the thermal
conductivities of Ce1xSmxO2x/2 ceramics decrease
obviously with the increase of Sm2O3 content. However,
the thermal conductivity of Ce0.5Sm0.5O1.75 is slightly
higher than that of Ce0.7Sm0.3O1.85. According to the
phonon thermal conduction theory, the thermal
conductivity in electrical insulation solids is
proportional to the mean free path of phonon. The
Fig. 11 Thermal diffusivity of Ce1xSmxO2x/2 ceramics.
phonon mean free path can be reduced when they
interact with lattice defects existed in actual crystal
lattice, and the influence of lattice defects including
vacancies, dislocations, grain boundaries, and
substituting atoms, on phonon mean free path can be
1 1 1 1 1 (4)
l( ,T ) li ( ,T ) lp ( , t) lv ( ,T ) lgb
where 1 , 1 , 1 , and 1
li ( ,T ) lp ( ,T ) lv ( ,T ) lgb
represent the phonon mean free paths due to interstitial
scattering, point defect scattering, vacancy scattering,
and grain boundary scattering, respectively .
Because only the nanometer grain boundary can result
in significant influence on phonon mean free path, so
the influence of grain boundary can be ignored
according to the microstructure plotted in Fig. 7 .
Thus, only point defects can result in obvious influence
on the phonon mean free path. In crystal lattice of
Ce1xSmxO2x/2 ceramics, there exist two types of point
defects, including oxygen vacancies and substituting
atoms, due to the substitution of Sm3+ cation for Ce4+
cation. On one hand, the oxygen vacancies can increase
the effective phonon scattering and decrease the phonon
mean free path. On the other hand, the differences of
atomic mass and ionic radius between Sm3+ and Ce4+
can also decrease the phonon mean free path in light of
Eq. (5) and Eq. (6) , which contributes to the lower
thermal conductivity of Ce1xSmxO2x/2 ceramics.
where a3 is the volume of each atom, v the transverse
wave speed, the phonon frequency, c the
concentration per atom, J the constant, the
Grüneisen parameter, M and R the average atomic
mass and ionic radius of the host atom respectively,
M and R the difference of mass and ionic radius
between the substituting and the substituted cations
respectively. Thus, doping of Sm2O3 oxide clearly
reduces the thermal conductivities of the Ce1xSmxO2x/2
ceramics. The slightly higher thermal conductivity of
Ce0.5Sm0.5O1.75 as compared to that of Ce0.7Sm0.3O1.85,
can be attributed to the formation of oxygen vacancy
pairs, which means the reduction of effective-oxygen
number in the Ce0.5Sm0.5O1.75 crystal lattice [40,41].
The thermal conductivities of Ce1xSmxO2x/2 ceramics
are in the range of 1.62–2.02 W/(m·K) at 1000 ℃,
which are clearly lower than that of dense 7.0 wt% YSZ
(3.0 at room temperature to 2.3 W/(m·K) at 700 ℃
reported by Wu et al. ). Therefore, the synthesized
Ce1xSmxO2x/2 ceramics are promising candidate
materials for future thermal barrier coatings.
(1) Pure fluorite-type Ce1xSmxO2x/2 powders and the
corresponding dense bulk ceramics were prepared
successfully by sol–gel route and pressureless sintering
method, respectively. The synthesized powders exhibit
a certain agglomeration, and the bulk samples have
dense microstructure whose relative densities are
greater than 90%.
(2) Because of the higher electro-negativity of Sm3+
ions as compared to that of Ce4+ ions, the thermal
expansion coefficients of Ce1xSmxO2x/2 ceramics
decrease gradually with the increasing Sm2O3 content.
Their thermal expansion coefficients are higher than
that of 8YSZ, which still fulfills the basic requirement
of thermal barrier coatings.
(3) The thermal conductivities of the Ce1xSmxO2x/2
ceramics lie in the range of 1.62–2.02 W/(m·K) at
1000 ℃, which are obviously lower than that of
7.0 wt% YSZ. The lower thermal conductivities can
mainly be attributed to the phonon scattering caused by
substituted atoms and oxygen vacancies in
Ce1xSmxO2x/2 crystal lattice.
(4) The excellent thermophysical properties indicate
that the Ce1xSmxO2x/2 ceramics are promising
candidates for the next generation thermal barrier
The authors would like to thank the financial support
from the National Natural Science Foundation of China
(No. U1304512), the Scientific and Technological
Projects of Henan Province (No. 132102210142), the
Program for Science & Technology Innovation Talents
in Universities of Henan Province (No. 13HASTIT018),
and the Postaldoctoral Research Sponsorship in Henan
Province (No. 2014069).
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