G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
775–782
MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM
A REPRESENTATIVE 2D IMAGE AND/OR A 3D
RECONSTRUCTION BASED ON A STACK OF IMAGES
VREDNOTENJE MIKROSTRUKTUR Ni-SDC KERMETA Z 2D
IN/ALI 3D METODO
Gregor Kapun1,3, Marjan Marin{ek2, Franci Merzel1, Sa{o [turm3,4,
Miran Gaber{~ek1, Tina Skalar2
1National Institute of Chemistry, Laboratory for Materials Chemistry, Hajdrihova 19, Ljubljana, Slovenia
2University of Ljubljana, Faculty for Chemistry and Chemical Technology, Ve~na pot 113, Ljubljana, Slovenia
3Jo`ef Stefan International Postgraduate School, Jamova cesta 39, 1000 Ljubljana, Slovenia
4Jo`ef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
Tina.Skalar@fkkt.uni-lj.si
Prejem rokopisa – received: 2016-08-16; sprejem za objavo – accepted for publication: 2017-01-24
doi:10.17222/mit.2016.256
This work provides an in-depth discussion on the subject of deriving microstructural parameters from a realistic 2D image or a
3D volume reconstruction based on a stack of images. As a convenient model material, a nickel/samaria-doped-ceria (Ni-SDC)
cermet synthetized by the citrate-nitrate combustion reaction is tested. Field-emission scanning electron microscopy (FE-SEM)
for 2D microstructure evaluation or focused-ion-beam/scanning-electron-microscopy (FIB-SEM) for 3D imaging is used for the
cermet characterization. Microstructural parameters, such as the volume ratio of phases, grain morphology, contiguity of phases
and triple-point boundary density, are quantitatively evaluated. These microstructural parameters reveal that a 2D microstruc-
tural evaluation provides a relatively accurate and quick analytical approach. However, it is shown that a detailed microstruc-
tural quantification of the parameters that are closely related to transport phenomena and electrochemical reactions in a porous
cermet is only possible through the three-dimensional (3D) quantitative material characterization. Based on the microstructural
evaluation, optimization of the Ni-SDC material used is briefly discussed.
Keywords: solid-oxide fuel cell, Ni-SDC anode, microstructure, FIB tomography
Ni-SDC kermet (nikelj/cerijev oksid dopiran s samarijevim oksidom) je eden izmed najpogosteje uporabljenih anodnih
materialov v gorivnih celicah s trdnim elektrolitom (angl. SOFC). Med pripravo materiala sodi tudi kvantitativno vrednotenje
mikrostrukturnih parametrov kot so velikost in porazdelitev velikosti zrn, dele`i faz, kontinuitete, gostota trojnih to~k (angl.
TPB), gostota materiala, itd. V tem prispevku `elimo primerjati dva razli~na na~ina analiziranja teh parametrov. Prva t.i. 2D
metoda temelji na obdelavi slik, posnetih z vrsti~nim elektronskim mikroskopom s poljsko emisijo (SEM). Medtem, ko 3D
metoda upo{teva 3D rekonstrukcijo vzorca iz slik s pomo~jo sistema s fokusiranim ionskim sklopom (FIB-SEM). Omenjeni
mikrostrukturni parametri razkrivajo, da je 2D pristop razmeroma hiter in natan~en. Vendar se je izkazalo, da je za natan~nej{o
kvantitativno analizo mikrostrukture, ki je tesno povezana s prenosom snovi in elektrokemijskimi reakcijami v anodnem
materialu, potreben 3D pristop. Metodi sta bili ocenjeni glede na njuno te`avnost izvedbe in ~asovno zahtevnost.
Klju~ne besede: SOFC, Ni-SDC anoda, mikrostruktura, FIB-tomografija
1 INTRODUCTION
High energy-conversion efficiency, fuel flexibility
and environmental friendliness can be seen as great
advantages of solid-oxide fuel cells (SOFCs), which may
become one of the leading energy-conversion techno-
logies converting chemical energy into electricity using a
wide spectrum of fuels.1 An SOFC system consists of
two porous electrodes, which are separated by a solid
ceramic electrolyte. Most commonly, the anode material
is based on two-phase cermets. Due to reduced operating
temperatures (600–800 °C) in modern SOFC devices,
yttrium-stabilized zirconia (YSZ) based cermets, i.e.,
Ni-YSZ, have been typically replaced with samarium-
doped ceria (SDC) cermets, i.e., Ni-SDC 2; namely, at
comparable temperatures samarium-doped ceria exhibits
ionic conductivities approximately one order of magni-
tude greater than YSZ.3
Several synthesis approaches for cermet preparation
are described in the literature. Certain approaches such
as mechanical mixing4–6 of metal oxides are associated
with high costs due to fine particle milling and may lead
to a non-uniform particle size distribution or insufficient
continuity of ceramic, metal and pore phases in the final
cermet.7 Some of these issues may be avoided by using
alternative methods such as co-precipitation,4,8 sol-gel,9
Pechini synthesis,10–12 spray pyrolysis13 or citrate/nitrate
combustion synthesis.4 All of them rely on producing
small particles, which may eventually result in a homo-
geneous phase distribution within the cermet after
thermal treatment.
The most important requirements for efficient elec-
tro-oxidation at the anode are: i) high catalytic activity
for fuel electro-oxidation, ii) chemical-, morphological-
and dimensional-stability, iii) electronic- and ionic-con-
Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782 775
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:543.427.3:621.3.035 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)775(2017)
ductivity, iv) chemical-, thermal- and mechanical compa-
tibility to other cell components and v) open porosity.14,15
Many of those requirements are determined by the anode
microstructure. Specifically, the performance of com-
posite cermet anodes critically depends on features like
particle size distribution, contiguity and triple phase
boundary length.4,16 In attempts to find correlations
between the anode microstructure and selected measured
properties, authors usually rely on 2D cross-sectional
images obtained from optical or electron microscopy.17,18
From these images, 3D microstructural parameters are
estimated by using porous models based on geometrical
theories, such as general effective media (GEM),19 the
contiguity concept,20 the random register network mo-
del,21 the random packing spheres model22 and the
stochastic reconstruction.23 However, these models are
all based on various assumptions, which can only
indirectly describe the real 3D microstructure. Recently,
3D reconstructions of SOFC electrodes have been per-
formed by X-ray computed tomography as well as by
focused-ion-beam-coupled scanning electron microscopy
(FIB-SEM).24–26 The recently highly improved spatial
resolution of the latter method is based on the serial
sectioning by FIB and SEM imaging of a sample
followed by reconstruction of the SEM micrographs into
a single data set, which gives 3D information of the
probed volume. Direct observation of a real electrode 3D
structure is a key towards the establishment of a reliable
relationship between a porous anode microstructure and
the cell’s power-generation performance.
The aim of this work is to compare a Ni-SDC micro-
structure obtained by the conventional 2D method using
SEM with the state-of-the-art FIB-SEM-based 3D
approach. The advantages and challenges of both me-
thods are discussed in significant detail, with a special
emphasis on the accuracy and applicability of the
obtained results as well as on the time required for pre-
paration and conducting the analysis. Both approaches
track well the characteristic microstructure parameters
that of a porous cermet structure, such as volume frac-
tion of phases, the average grain sizes and grain size
distribution in various directions, grain shape factor,
porosity (open and closed), contiguity of phases (C) and
triple-point boundary density (TPB).
2 EXPERIMENTAL PART
2.1 Material preparation
Anode materials were synthesized via the citrate-
nitrate combustion synthesis by mixing aqueous solu-
tions of metal nitrates and citric acid. Cation precursors –
hydrated metal nitrates Ce(NO3)3 · 6H2O (99 %, Sigma-
Aldrich), Sm(NO3)3 . 6H2O (99.9 %, Alfa Aesar) and
Ni(NO3)2 · 6H2O (99 %, Acros Organics) and citric acid
(100%, Carlo Erba) were separately dissolved in distilled
water. The mixture was prepared by taking care that the
molar ratio between cerium and samarium cations was
80:20, and considering that the Ni volume content in the
final composite was 50 %. In order to ensure subsequent
self-sustaining combustion the molar ratio between citric
acid and nitrates was set to 0.2. The prepared mixture
was then held over a water bath at 60 °C under vacuum
(8 mbar) for at least 6 h until it transformed into a light
green brittle gel. The dried gel was grinded and compres-
sed into pellets (f = 12 mm, h = 30 mm, p = 100 MPa),
which were then immediately ignited at the top with a
hot metal tip to start a self-sustaining combustion reac-
tion that spread as a reaction zone throughout the pellet.
The synthesized NiO-SDC powder was then crushed in
an agate mortar, wet-milled (isopropanol) in a planetary
mill (Fritsch pulverisette 7 premium line) for 15 min
with 10 mm grinding balls and 500 min–1 (1st milling)
and additional 15 min with 1 mm grinding balls and
500 min–1 (2nd milling). After drying the milled powder
was calcined at 800 °C for 1 h and subsequently
uni-axially pressed into pellets (f = 6 mm, h = 2.6 mm,
p = 100 MPa). The formed pellets were sintered at
1400 °C for 1 h. To determine the final composite micro-
structure, all the sintered tablets were polished, thermally
etched and reduced in an Ar-H2 (5 % of volume frac-
tions) atmosphere at 900 °C for 2 h.
2.2 Material characterization
The rapid increase in temperature during combustion
inside the reaction zone was measured as a temperature
profile using an optical pyrometer (Impac, IPE 140,
based on sample brightness). The measuring range of the
pyrometer is from 50 °C to 1200 °C, and it has a very
quick response time (1.5 ms). The accuracy of the op-
tically measured temperature was ±2.5 °C below 400 °C
and ±0.4 % of a measured value (in °C) above 400 °C.
The particle size observations of the as-synthesized
NiO-SDC dispersion were performed on a JEOL
JEM-2100 transmission electron microscope (TEM)
operated at 200 kV and equipped with EDX. The TEM
samples were prepared by dispersing the prepared
powder in ethanol using ultrasonication followed by
deposition of the obtained suspension on holey carbon-
coated copper grids. Particle size distributions of
unmilled and milled powders after the synthesis were
obtained using a Microtrac Bluewave particle sizer and
measured as aqueous suspensions. The shrinkage curve
of the NiO-SDC during sintering up to 1450 °C with a
constant heating rate of 10 K min–1 was obtained with a
heating microscope (Leitz Wetzlar). FE-SEM (Zeiss,
Ultra Plus) was used for a conventional 2D cross-section
imaging of sintered and reduced pellets. FIB-SEM
(Zeiss, Crossbeam 540) was used for the image data
stack acquisition of sintered anode material to obtain 3D
reconstruction of the probed volume.
2.3 2D microstructure analysis
The quantitative 2D microstructure analysis involved
the determination of the fraction of phases (jNi, jSDC),
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
776 Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
intercept grain lengths in x and y directions (Feret x and
Feret y), diameter of the area-analogue circle (d which
equals D-CIRCLE), sphericity (F-CIRCLE) and micro-
structural porosity () were performed on 4–5 digital
images using Axiovision 4.8 image-analysis software.
The definitions and equations of these parameters are
listed in Figure 1. Geometrical porosity (geometrical) was
calculated from the Archimedes densities of the pellets
using measured dimensions (diameter, height and mass)
relative to the theoretical densities. In order to determine
the contiguity values (C), which represent a frequency of
contacts between grains of one or various phases, in a
three-phase mixture the Gurland principle27,28 modified
by J. H. Lee et al.29 was used. SEM micrographs of the
given sample were lined (Figure 1) vertically and
horizontally. Along with these lines contacts between
different phases were counted. The final number of
contacts (N) was normalized to 1 mm length of sample.
Finally, contiguities were calculated using Equations (1),
(2) and (3):
C
N
N N NNi Ni
Ni Ni
Ni Ni Ni pore Ni SDC'
−
−
− − −
=
+ +
2
2
(1)
C
N
N N NSDC SDC
SDC SDC
SDC SDC SDC pore Ni SDC'
−
−
− − −
=
+ +
2
2
(2)
C
N
N N N NNi SDC
Ni SDC
Ni Ni Ni pore Ni SDC SDC SDC
−
−
− − − −
=
+ + +
2
2 2 + −N SDC pore
(3)
2.4 3D microstructure analysis
In addition to the conventional 2D microstructure
analysis, a FIB-SEM serial sectioning method was used
for a quantitative 3D examination of the anode micro-
structures. The method allows direct observation of
critical microstructure parameters like phase distribu-
tions, grain connectivity and triple phase boundary
density. The prepared Ni-SDC cermet was infiltrated
with epoxy resin (EpoFix, Struers) under vacuum in
order to easily distinguish the pores and avoid mistakes
due to the depth of field during SEM imaging. Sub-
sequently, a lamella with a thickness of 150 μm was cut
out, polished down to 50 μm and glued directly to an
aluminium stub to prevent sample drift during FIB ope-
ration. The configuration of the FIB-SEM setup is
presented in Figure 2. Initially, a layer of platinum was
"in situ" deposited on top of the region of interest to
protect the surface and prevent the curtaining effect of
the cross-section during milling. The volume of interest
was separated using an optimised U-pattern pre-milling
procedure to prevent material re-deposition and shadow-
ing of the signals used for imaging and microanalysis.
The sample was then serially sectioned using an auto-
mated slicing procedure with drift correction algorithms
to obtain a series of 2D images with narrow and repro-
ducible spacing between the individual image planes.30
Experimental milling and imaging parameters were
optimized in order to obtain a high-quality 3D recon-
struction (10 nm × 10 nm × 20 nm voxel resolution) with
phase-contrast information. Individual phases were
identified from EDXS elemental maps and further seg-
mented according to their grey level. Raw images were
pre-processed with "in-house" programing using ImageJ
followed by phase segmentation based on thresholding,
edge detection and a region growth algorithm.31,32 After
phase separation, the 3D microstructure with final di-
mensions of 10 μm × 10 μm × 10 μm was reconstructed
using the Amira 5.4.5. software package.33,34 The 3D
structure was additionally separated into particles based
upon a three-dimensional watershed algorithm applied to
a distance measurement.35,36 Volume fractions, feature
size and size distributions for each individual phase were
calculated directly from the 3D reconstruction. The
specific surface area was calculated inside the Amira
5.4.5. software using a triangular approximation based
on the marching cubes algorithm. A summary of the
results is listed in Table 1.
In the final step of the 3D quantification, the seg-
mented data cube with a 3D data matrix/grid of dimen-
sions Np × Np × Np, with Np = 400 at a resolution of 20
nm/pixel was exported for the TPB length calculation
using an "in house" programing algorithm based on the
centroid method.32,37,38 Each grid element (pixel) gets
associated with one of the three different phases: the
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782 777
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Schematic view of FIB-SEM settings and the measuring
procedure
Figure 1: In the left-hand part the definitions of FERETX, FERETY,
DCIRCLE and FCIRCLE are presented. Lined SEM micrograph in the
right-hand part (horizontal lines A, B, C, D, … and vertical lines a, b,
c, d, …) with marked Ni-SDC phase contact (mark) and listed num-
ber of Ni-SDC contacts for lines "I" (left) and "g" (right)
ductivity, iv) chemical-, thermal- and mechanical compa-
tibility to other cell components and v) open porosity.14,15
Many of those requirements are determined by the anode
microstructure. Specifically, the performance of com-
posite cermet anodes critically depends on features like
particle size distribution, contiguity and triple phase
boundary length.4,16 In attempts to find correlations
between the anode microstructure and selected measured
properties, authors usually rely on 2D cross-sectional
images obtained from optical or electron microscopy.17,18
From these images, 3D microstructural parameters are
estimated by using porous models based on geometrical
theories, such as general effective media (GEM),19 the
contiguity concept,20 the random register network mo-
del,21 the random packing spheres model22 and the
stochastic reconstruction.23 However, these models are
all based on various assumptions, which can only
indirectly describe the real 3D microstructure. Recently,
3D reconstructions of SOFC electrodes have been per-
formed by X-ray computed tomography as well as by
focused-ion-beam-coupled scanning electron microscopy
(FIB-SEM).24–26 The recently highly improved spatial
resolution of the latter method is based on the serial
sectioning by FIB and SEM imaging of a sample
followed by reconstruction of the SEM micrographs into
a single data set, which gives 3D information of the
probed volume. Direct observation of a real electrode 3D
structure is a key towards the establishment of a reliable
relationship between a porous anode microstructure and
the cell’s power-generation performance.
The aim of this work is to compare a Ni-SDC micro-
structure obtained by the conventional 2D method using
SEM with the state-of-the-art FIB-SEM-based 3D
approach. The advantages and challenges of both me-
thods are discussed in significant detail, with a special
emphasis on the accuracy and applicability of the
obtained results as well as on the time required for pre-
paration and conducting the analysis. Both approaches
track well the characteristic microstructure parameters
that of a porous cermet structure, such as volume frac-
tion of phases, the average grain sizes and grain size
distribution in various directions, grain shape factor,
porosity (open and closed), contiguity of phases (C) and
triple-point boundary density (TPB).
2 EXPERIMENTAL PART
2.1 Material preparation
Anode materials were synthesized via the citrate-
nitrate combustion synthesis by mixing aqueous solu-
tions of metal nitrates and citric acid. Cation precursors –
hydrated metal nitrates Ce(NO3)3 · 6H2O (99 %, Sigma-
Aldrich), Sm(NO3)3 . 6H2O (99.9 %, Alfa Aesar) and
Ni(NO3)2 · 6H2O (99 %, Acros Organics) and citric acid
(100%, Carlo Erba) were separately dissolved in distilled
water. The mixture was prepared by taking care that the
molar ratio between cerium and samarium cations was
80:20, and considering that the Ni volume content in the
final composite was 50 %. In order to ensure subsequent
self-sustaining combustion the molar ratio between citric
acid and nitrates was set to 0.2. The prepared mixture
was then held over a water bath at 60 °C under vacuum
(8 mbar) for at least 6 h until it transformed into a light
green brittle gel. The dried gel was grinded and compres-
sed into pellets (f = 12 mm, h = 30 mm, p = 100 MPa),
which were then immediately ignited at the top with a
hot metal tip to start a self-sustaining combustion reac-
tion that spread as a reaction zone throughout the pellet.
The synthesized NiO-SDC powder was then crushed in
an agate mortar, wet-milled (isopropanol) in a planetary
mill (Fritsch pulverisette 7 premium line) for 15 min
with 10 mm grinding balls and 500 min–1 (1st milling)
and additional 15 min with 1 mm grinding balls and
500 min–1 (2nd milling). After drying the milled powder
was calcined at 800 °C for 1 h and subsequently
uni-axially pressed into pellets (f = 6 mm, h = 2.6 mm,
p = 100 MPa). The formed pellets were sintered at
1400 °C for 1 h. To determine the final composite micro-
structure, all the sintered tablets were polished, thermally
etched and reduced in an Ar-H2 (5 % of volume frac-
tions) atmosphere at 900 °C for 2 h.
2.2 Material characterization
The rapid increase in temperature during combustion
inside the reaction zone was measured as a temperature
profile using an optical pyrometer (Impac, IPE 140,
based on sample brightness). The measuring range of the
pyrometer is from 50 °C to 1200 °C, and it has a very
quick response time (1.5 ms). The accuracy of the op-
tically measured temperature was ±2.5 °C below 400 °C
and ±0.4 % of a measured value (in °C) above 400 °C.
The particle size observations of the as-synthesized
NiO-SDC dispersion were performed on a JEOL
JEM-2100 transmission electron microscope (TEM)
operated at 200 kV and equipped with EDX. The TEM
samples were prepared by dispersing the prepared
powder in ethanol using ultrasonication followed by
deposition of the obtained suspension on holey carbon-
coated copper grids. Particle size distributions of
unmilled and milled powders after the synthesis were
obtained using a Microtrac Bluewave particle sizer and
measured as aqueous suspensions. The shrinkage curve
of the NiO-SDC during sintering up to 1450 °C with a
constant heating rate of 10 K min–1 was obtained with a
heating microscope (Leitz Wetzlar). FE-SEM (Zeiss,
Ultra Plus) was used for a conventional 2D cross-section
imaging of sintered and reduced pellets. FIB-SEM
(Zeiss, Crossbeam 540) was used for the image data
stack acquisition of sintered anode material to obtain 3D
reconstruction of the probed volume.
2.3 2D microstructure analysis
The quantitative 2D microstructure analysis involved
the determination of the fraction of phases (jNi, jSDC),
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
776 Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
intercept grain lengths in x and y directions (Feret x and
Feret y), diameter of the area-analogue circle (d which
equals D-CIRCLE), sphericity (F-CIRCLE) and micro-
structural porosity () were performed on 4–5 digital
images using Axiovision 4.8 image-analysis software.
The definitions and equations of these parameters are
listed in Figure 1. Geometrical porosity (geometrical) was
calculated from the Archimedes densities of the pellets
using measured dimensions (diameter, height and mass)
relative to the theoretical densities. In order to determine
the contiguity values (C), which represent a frequency of
contacts between grains of one or various phases, in a
three-phase mixture the Gurland principle27,28 modified
by J. H. Lee et al.29 was used. SEM micrographs of the
given sample were lined (Figure 1) vertically and
horizontally. Along with these lines contacts between
different phases were counted. The final number of
contacts (N) was normalized to 1 mm length of sample.
Finally, contiguities were calculated using Equations (1),
(2) and (3):
C
N
N N NNi Ni
Ni Ni
Ni Ni Ni pore Ni SDC'
−
−
− − −
=
+ +
2
2
(1)
C
N
N N NSDC SDC
SDC SDC
SDC SDC SDC pore Ni SDC'
−
−
− − −
=
+ +
2
2
(2)
C
N
N N N NNi SDC
Ni SDC
Ni Ni Ni pore Ni SDC SDC SDC
−
−
− − − −
=
+ + +
2
2 2 + −N SDC pore
(3)
2.4 3D microstructure analysis
In addition to the conventional 2D microstructure
analysis, a FIB-SEM serial sectioning method was used
for a quantitative 3D examination of the anode micro-
structures. The method allows direct observation of
critical microstructure parameters like phase distribu-
tions, grain connectivity and triple phase boundary
density. The prepared Ni-SDC cermet was infiltrated
with epoxy resin (EpoFix, Struers) under vacuum in
order to easily distinguish the pores and avoid mistakes
due to the depth of field during SEM imaging. Sub-
sequently, a lamella with a thickness of 150 μm was cut
out, polished down to 50 μm and glued directly to an
aluminium stub to prevent sample drift during FIB ope-
ration. The configuration of the FIB-SEM setup is
presented in Figure 2. Initially, a layer of platinum was
"in situ" deposited on top of the region of interest to
protect the surface and prevent the curtaining effect of
the cross-section during milling. The volume of interest
was separated using an optimised U-pattern pre-milling
procedure to prevent material re-deposition and shadow-
ing of the signals used for imaging and microanalysis.
The sample was then serially sectioned using an auto-
mated slicing procedure with drift correction algorithms
to obtain a series of 2D images with narrow and repro-
ducible spacing between the individual image planes.30
Experimental milling and imaging parameters were
optimized in order to obtain a high-quality 3D recon-
struction (10 nm × 10 nm × 20 nm voxel resolution) with
phase-contrast information. Individual phases were
identified from EDXS elemental maps and further seg-
mented according to their grey level. Raw images were
pre-processed with "in-house" programing using ImageJ
followed by phase segmentation based on thresholding,
edge detection and a region growth algorithm.31,32 After
phase separation, the 3D microstructure with final di-
mensions of 10 μm × 10 μm × 10 μm was reconstructed
using the Amira 5.4.5. software package.33,34 The 3D
structure was additionally separated into particles based
upon a three-dimensional watershed algorithm applied to
a distance measurement.35,36 Volume fractions, feature
size and size distributions for each individual phase were
calculated directly from the 3D reconstruction. The
specific surface area was calculated inside the Amira
5.4.5. software using a triangular approximation based
on the marching cubes algorithm. A summary of the
results is listed in Table 1.
In the final step of the 3D quantification, the seg-
mented data cube with a 3D data matrix/grid of dimen-
sions Np × Np × Np, with Np = 400 at a resolution of 20
nm/pixel was exported for the TPB length calculation
using an "in house" programing algorithm based on the
centroid method.32,37,38 Each grid element (pixel) gets
associated with one of the three different phases: the
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782 777
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Schematic view of FIB-SEM settings and the measuring
procedure
Figure 1: In the left-hand part the definitions of FERETX, FERETY,
DCIRCLE and FCIRCLE are presented. Lined SEM micrograph in the
right-hand part (horizontal lines A, B, C, D, … and vertical lines a, b,
c, d, …) with marked Ni-SDC phase contact (mark) and listed num-
ber of Ni-SDC contacts for lines "I" (left) and "g" (right)
SDC phase, the Ni phase and the pore/void "phase",
respectively. We identify the three-phase boundary
(TPB) elements/pixels satisfying the following condi-
tion: an element belonging to the pore is a TPB element
if it has pixels of both other two phases among its first
neighbours on the grid and at least one of its first
neighbours also belongs to the TPB. In order to avoid
multiplication of the TPB points it is important to
assume that the boundary element should refer to one
phase only (pore in our case). If a given TPB element is
not connected to any other TPB element, it does not form
a TPB line. The TPB length can be obtained, to a
reasonable approximation, as a product of the number of
TPB points and the grid spacing, NTPB × d0, that
corresponds to the edge summation. The corresponding
TPB density is then simply  = NTPBNP3d0–2. Here we
define one cubic layer of neighbouring elements around
the tagged pixel represented either by a single element
(Figure 3), leading to  = 7.23 μm μm–3.
Table 1: Microstructural and morphological parameters of Ni-SDC
determined by 2D or 3D approach
bulk 2D 3D
Ni / % 36.1 37.7 34.7
SDC / % 36.1 42.8 37.4
feret / μm
x
Ni 0.55 0.49
SDC 0.42 0.40
y Ni 0.44 0.44
SDC 0.38 0.34
z Ni / 0.51
SDC / 0.40
d / μm
Ni 0.52 0.48
SDC 0.38 0.37
F CIRCLE /
Ni 0.90 0.85*
SDC 0.78 0.71*
 / %
geometrical 27.8 / /
microstructural 21.5 27.9
open / 27.2
closed / 0.7
C /
Ni-Ni 0.05 /
Ni-SDC 0.18 /
SDC-SDC 0.47 /
TPB density / μm μm–3 / 3.5
SSA / μm μm–3
Ni / 15.4
SDC / 18.7
pore / 15.7
SIA / μm μm–3
Ni-pore / 6.2
Ni-SDC / 9.2
SDC-pore / 9.5
* In xy projection
3 RESULTS AND DISCUSSION
The combustion synthesis of NiO-SDC preparation is
chosen due to its major advantages over the classic cal-
cination method. Namely, the citrate-nitrate combustion
is a one-step, relatively simple method with rapid mate-
rial synthesis inside the combustion zone. Typically, in
the head of the combustion zone, changes from ambient
temperature to combustion temperature happen in a
fraction of a second, while after the combustion the
product ash is also quickly cooled. In the case of
NiO-SDC preparation, the measured peak combustion
temperature is 1169.2 °C, the propagation wave velocity
is 0.6 mm s–1 and the temperature gradient in the head of
the reaction zone is calculated to be 1053 °C s–1. Such
rapid temperature variations limit significantly the time
span during which diffusion of species in the system
plays an important role. This leads to a typical nano-
scaled mixture of NiO and SDC phases (Figure 4),
which serves as the starting material for the final
Ni-SDC cermet preparation.
The as-synthesized NiO-SDC product is partially
agglomerated. The size of the average agglomerate size
is about 1350 mm. The formed agglomerates of
NiO-SDC are weak and can be easily reduced in size
during milling (Figure 5). The first and second milling
cycles produce particles with average sizes of 5.23 mm
and 2.80 μm, respectively, which are still small agglo-
merates of nano-sized NiO and SDC one-phase regions.
The sintering behaviour of NiO-SDC during a dyna-
mic temperature rise is presented in Figure 6.
Densification of the tablet starts slightly above 800 °C,
proceeds over two separate steps, and is practically
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
778 Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Left: an example of the TPB element defined as one central
pixel surrounded by one layer of neighbouring pixels (pixels are
represented by balls, dark grey balls denote the SDC phase, black the
Ni phase and light grey the pores). Right: an image showing a distri-
bution of the TPB points in the sample ( = 7.23 μm μm–3)
Figure 4: High-resolution TEM image of as synthesized product
(some individual grains are encircled) and corresponding selected area
electron diffraction pattern for the image (insert)
completed at 1400 °C. The final shrinkage reached at
1450 °C is 24 %. Some features found in the sintering
curve may be clearly associated with the state of the
reacting matter during the synthesis. For example, the
relatively early start of the densification is a consequence
of the highly sinterable nano-sized mixture. However,
due to the presence of some agglomerates, the densifi-
cation first proceeds mainly as an inter-agglomerate
sintering with the maximal sintering rate at 950 °C.
Intra-agglomerate sintering starts at rather higher tempe-
ratures (above 1100 °C), reaching the maximum sinter-
ing rate at 1300 °C.
Among the crucial criteria for high-quality SOFC
anodes are the maximization of the TPB density, as well
as of the electronic and ionic conductivities. This means
that sintering must provide very good contacts between
the particles. Good particle-to-particle contact should
appear both inside one phase as well as between both
phases. Furthermore, the continuity of the Ni and SDC
phases throughout the cermet additionally contributes to
improved conductivities (ionic and electronic). At the
same time, the TPB density can be increased by prevent-
ing excessive grain coarsening during sintering. From
this point of view, the best compromise for the sintering
temperature for final cermet preparation was found to be
1400 °C. Any sintering temperature below 1400 °C may
result in the formation of continuous phases inside the
agglomerates only, whereas the intra-agglomerate con-
nection remains poor. Sintering temperatures above
1400 °C, however, result in exaggerated grain growth.
Microstructures of sintered NiO-SDC and sub-
sequently reduced Ni-SDC cermet are shown in Figure
7. Such a final Ni-SDC cermet was submitted to an
in-depth microstructure analysis. The dark-grey particles
seen in Figures 7a and 7b are NiO or Ni in the sintered
or reduced samples, respectively. The light grey is the
SDC phase, whereas the pores are black. It is evident
that NiO or Ni grains are homogeneously distributed
between the SDC phase. Furthermore, pores are also
uniformly distributed, especially in the reduced sample.
The parameters important for ann exact cermet 2D
analysis were extracted from the SEM micrographs by a
detailed quantitative microstructure analysis (Table 1).
Each parameter was determined on 5–10 different
regions on the surface of the analysed tablets. As a
whole, any quantitative calculation was based on several
hundreds of grains. After sintering at 1400 °C, the
remaining porosity in the NiO-SDC was around 97 %,
which means that the sample was densely sintered. After
the reduction, the microstructural porosity was increased
to 21.5 %. The value for the porosity, however, depends
to some extent on the determination method. The
microstructural porosity, obtained from conventional 2D
image analysis, tends to be lower than the geometrical
porosity. This may be explained by the fact that micro-
structural porosity is determined on a limited number of
images (which may be considered also as limited number
of slices throughout the sample), while the geometrical
porosity is a bulk property. To minimize this discrepancy
one has to increase the number of images for 2D micro-
structure analysis. Similarly, approximate numbers are
also determined for the phase fraction values (Ni and
SDC) and all the morphological values (Feret-values, –
F-CIRCLE and d). An interesting phenomenon may be
deduced from the contiguity values’ calculation. Namely,
the relatively low CNi-Ni value may indicate a poor contact
between the Ni grains. Such poor contact is highly
undesired, since it also results in a lower electrical
conductivity. However, the relatively high Ni content in
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782 779
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Particle size distribution of ungrounded powder (a), powder
after 1st milling cycle (b) and after 2nd milling cycle (c)
Figure 6: Sintering curve of NiO-SDC material calcined at 800 °C
(full line) and derivative sintering curve (dashed line)
SDC phase, the Ni phase and the pore/void "phase",
respectively. We identify the three-phase boundary
(TPB) elements/pixels satisfying the following condi-
tion: an element belonging to the pore is a TPB element
if it has pixels of both other two phases among its first
neighbours on the grid and at least one of its first
neighbours also belongs to the TPB. In order to avoid
multiplication of the TPB points it is important to
assume that the boundary element should refer to one
phase only (pore in our case). If a given TPB element is
not connected to any other TPB element, it does not form
a TPB line. The TPB length can be obtained, to a
reasonable approximation, as a product of the number of
TPB points and the grid spacing, NTPB × d0, that
corresponds to the edge summation. The corresponding
TPB density is then simply  = NTPBNP3d0–2. Here we
define one cubic layer of neighbouring elements around
the tagged pixel represented either by a single element
(Figure 3), leading to  = 7.23 μm μm–3.
Table 1: Microstructural and morphological parameters of Ni-SDC
determined by 2D or 3D approach
bulk 2D 3D
Ni / % 36.1 37.7 34.7
SDC / % 36.1 42.8 37.4
feret / μm
x
Ni 0.55 0.49
SDC 0.42 0.40
y Ni 0.44 0.44
SDC 0.38 0.34
z Ni / 0.51
SDC / 0.40
d / μm
Ni 0.52 0.48
SDC 0.38 0.37
F CIRCLE /
Ni 0.90 0.85*
SDC 0.78 0.71*
 / %
geometrical 27.8 / /
microstructural 21.5 27.9
open / 27.2
closed / 0.7
C /
Ni-Ni 0.05 /
Ni-SDC 0.18 /
SDC-SDC 0.47 /
TPB density / μm μm–3 / 3.5
SSA / μm μm–3
Ni / 15.4
SDC / 18.7
pore / 15.7
SIA / μm μm–3
Ni-pore / 6.2
Ni-SDC / 9.2
SDC-pore / 9.5
* In xy projection
3 RESULTS AND DISCUSSION
The combustion synthesis of NiO-SDC preparation is
chosen due to its major advantages over the classic cal-
cination method. Namely, the citrate-nitrate combustion
is a one-step, relatively simple method with rapid mate-
rial synthesis inside the combustion zone. Typically, in
the head of the combustion zone, changes from ambient
temperature to combustion temperature happen in a
fraction of a second, while after the combustion the
product ash is also quickly cooled. In the case of
NiO-SDC preparation, the measured peak combustion
temperature is 1169.2 °C, the propagation wave velocity
is 0.6 mm s–1 and the temperature gradient in the head of
the reaction zone is calculated to be 1053 °C s–1. Such
rapid temperature variations limit significantly the time
span during which diffusion of species in the system
plays an important role. This leads to a typical nano-
scaled mixture of NiO and SDC phases (Figure 4),
which serves as the starting material for the final
Ni-SDC cermet preparation.
The as-synthesized NiO-SDC product is partially
agglomerated. The size of the average agglomerate size
is about 1350 mm. The formed agglomerates of
NiO-SDC are weak and can be easily reduced in size
during milling (Figure 5). The first and second milling
cycles produce particles with average sizes of 5.23 mm
and 2.80 μm, respectively, which are still small agglo-
merates of nano-sized NiO and SDC one-phase regions.
The sintering behaviour of NiO-SDC during a dyna-
mic temperature rise is presented in Figure 6.
Densification of the tablet starts slightly above 800 °C,
proceeds over two separate steps, and is practically
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
778 Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Left: an example of the TPB element defined as one central
pixel surrounded by one layer of neighbouring pixels (pixels are
represented by balls, dark grey balls denote the SDC phase, black the
Ni phase and light grey the pores). Right: an image showing a distri-
bution of the TPB points in the sample ( = 7.23 μm μm–3)
Figure 4: High-resolution TEM image of as synthesized product
(some individual grains are encircled) and corresponding selected area
electron diffraction pattern for the image (insert)
completed at 1400 °C. The final shrinkage reached at
1450 °C is 24 %. Some features found in the sintering
curve may be clearly associated with the state of the
reacting matter during the synthesis. For example, the
relatively early start of the densification is a consequence
of the highly sinterable nano-sized mixture. However,
due to the presence of some agglomerates, the densifi-
cation first proceeds mainly as an inter-agglomerate
sintering with the maximal sintering rate at 950 °C.
Intra-agglomerate sintering starts at rather higher tempe-
ratures (above 1100 °C), reaching the maximum sinter-
ing rate at 1300 °C.
Among the crucial criteria for high-quality SOFC
anodes are the maximization of the TPB density, as well
as of the electronic and ionic conductivities. This means
that sintering must provide very good contacts between
the particles. Good particle-to-particle contact should
appear both inside one phase as well as between both
phases. Furthermore, the continuity of the Ni and SDC
phases throughout the cermet additionally contributes to
improved conductivities (ionic and electronic). At the
same time, the TPB density can be increased by prevent-
ing excessive grain coarsening during sintering. From
this point of view, the best compromise for the sintering
temperature for final cermet preparation was found to be
1400 °C. Any sintering temperature below 1400 °C may
result in the formation of continuous phases inside the
agglomerates only, whereas the intra-agglomerate con-
nection remains poor. Sintering temperatures above
1400 °C, however, result in exaggerated grain growth.
Microstructures of sintered NiO-SDC and sub-
sequently reduced Ni-SDC cermet are shown in Figure
7. Such a final Ni-SDC cermet was submitted to an
in-depth microstructure analysis. The dark-grey particles
seen in Figures 7a and 7b are NiO or Ni in the sintered
or reduced samples, respectively. The light grey is the
SDC phase, whereas the pores are black. It is evident
that NiO or Ni grains are homogeneously distributed
between the SDC phase. Furthermore, pores are also
uniformly distributed, especially in the reduced sample.
The parameters important for ann exact cermet 2D
analysis were extracted from the SEM micrographs by a
detailed quantitative microstructure analysis (Table 1).
Each parameter was determined on 5–10 different
regions on the surface of the analysed tablets. As a
whole, any quantitative calculation was based on several
hundreds of grains. After sintering at 1400 °C, the
remaining porosity in the NiO-SDC was around 97 %,
which means that the sample was densely sintered. After
the reduction, the microstructural porosity was increased
to 21.5 %. The value for the porosity, however, depends
to some extent on the determination method. The
microstructural porosity, obtained from conventional 2D
image analysis, tends to be lower than the geometrical
porosity. This may be explained by the fact that micro-
structural porosity is determined on a limited number of
images (which may be considered also as limited number
of slices throughout the sample), while the geometrical
porosity is a bulk property. To minimize this discrepancy
one has to increase the number of images for 2D micro-
structure analysis. Similarly, approximate numbers are
also determined for the phase fraction values (Ni and
SDC) and all the morphological values (Feret-values, –
F-CIRCLE and d). An interesting phenomenon may be
deduced from the contiguity values’ calculation. Namely,
the relatively low CNi-Ni value may indicate a poor contact
between the Ni grains. Such poor contact is highly
undesired, since it also results in a lower electrical
conductivity. However, the relatively high Ni content in
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782 779
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Particle size distribution of ungrounded powder (a), powder
after 1st milling cycle (b) and after 2nd milling cycle (c)
Figure 6: Sintering curve of NiO-SDC material calcined at 800 °C
(full line) and derivative sintering curve (dashed line)
the sample (Ni = 37.7 %) and its homogeneous dis-
tribution throughout the cermet should guarantee a good
Ni-to-Ni particle contact. The latter suggests that a
contiguity determination should consider a real 3D
microstructure, since contacts between particles may
continue also in the direction perpendicular to the
investigated 2D plane. Generally, it is clear that after the
reduction the average particle sizes remain well beneath
the micrometre range; however, more precise micro-
structural quantization requires a more sophisticated
approach.
More precise quantitative microstructural evaluation
is possible by using the real 3D microstructure recon-
struction based on FIB-SEM serial sectioning technique.
It has to be stressed that for 3D microstructure recon-
struction the back-scattered electron detector (BSC-ED)
is more appropriate than the commonly used in-lens
secondary electron detector (IL-SED), since BSC-ED
successfully copes with the shadowing effect, which
sometimes appears on the images when using IL-SED.
Considering the obtained phase-extraction images
(Figure 8), it is evident that the distribution of all three
phases, namely, Ni, SDC and pores (pores may be
considered as an additional phase in the cermet), is
homogeneous. No larger areas of one-phase domination
can be found in the cermet. Moreover, average particle
sizes and pore diameters in any direction are in the
nanometre range. Similar to the 2D analytical approach,
the 3D microstructural description also allows the deter-
mination of morphological parameters (Table 1);
however, the 3D analysis additionally enables a descrip-
tion of microstructure also in the z axis. It appears that
the d and Feret-values (x, y and z) are somewhat lower
when determined by the 3D approach. There are two
main reasons for such a slight discrepancy in the results.
Firstly, the computational algorithms employed for grey
image segmentations and thus the separation of phases
are different; and secondly, 2D statistical calculation is
based on several hundreds of analysed grains, while the
3D statistics involves several tens of thousands of re-
cognized particles. This fact makes the 3D microstruc-
tural analysis more reliable. Similar is true also for the
Ni and SDC determination.
One of the big advantages of 3D microstructural
analyses over the 2D approach is the fact that 3D
microstructure reconstruction also enables an in-depth
porosity description. Due to the large voxel matrix
(2.6×108 voxels) in 3D reconstruction, the determined
value for the microstructural porosity comes very close
to the determined value of the geometrical porosity. The
discrepancy of the geometrical and microstructural is much
bigger for the 2D approach. Moreover, a detailed exami-
nation of the separated pore phase enables the overall
porosity microstructural to be separated into open- and
closed-porosity. According to the results only 0.7% of
the porosity belongs to closed pores, making the pre-
pared microstructure interesting for SOFC applications.
The contiguity values may no longer be calculated
from the 3D microstructure reconstruction. Instead,
using an appropriate computational algorithm the much
more interesting TPB density value (TPBL) can be
obtained. In the analysed Ni-SDC cermet the TPBL is
determined as 7.23 μm μm–3, making this material fully
comparable to other Ni-based cermets described in the
literature. Further interesting information that may also
be obtained from 3D microstructural reconstruction is
the specific surface areas SSA of the phases (i.e., surface
areas of individual phases normalized to the sample
volume). For the investigated cermet the SSA values for
Ni- SDC and pores are determined 15.4 μm2 μm–3,
18.7 μm2 μm–3, 15.7 μm2 μm–3, respectively. The obtained
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
780 Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 8: 3D microstructure reconstruction of Ni-SDC cermet and
extracted phases Ni, SDC and pores
Figure 7: Microstructures of a) sintered (1400 °C) and b) reduced
(900 °C)
values are rather high compared to some data available in
the literature and are a consequence of the relatively
small grains of each individual phase. Furthermore, each
phase in the cermet is in contact with the two other
phases, meaning that by simple combination of the
obtained SSA values specific interface areas SIA
(normalized to the sample volume) between the three
neighbouring phases can be calculated. For instance, the
SSA of the Ni phase is divided into Ni-pore SIA
6.2 μm2 μm–3 and Ni-SDC SIA 9.2 μm2 μm–3. Thus, about
40 % of the total Ni surface is exposed to the porous
phase and can be used for surface catalytic reaction with
fuel. Namely, oxidation of the fuel involves numerous
reaction steps, which are not necessarily located at the
TPB (i.e., adsorption and dissociation of hydrogen or
hydrocarbon reforming inside an anode layer of a
SOFC).
4 CONCLUSIONS
This work focused on a 2D and 3D microstructural
analysis of a Ni-SDC anode cermet material using SEM
analysis and FIB tomography, respectively. The Ni-SDC
sample was prepared by citrate-nitrate combustion
synthesis, followed by milling, sintering and NiO to Ni
reduction. The 2D microstructure investigation employed
grey image-analysis and made it possible to obtain
quantitative values of the fraction of phases, microstruc-
tural porosity, grain sizes of all phases, sphericity and
contiguity. On the other hand, the 3D analysis addition-
ally provided information on specific surface areas and
specific interface areas of all the phases, as well as the
triple phase boundary density. When comparing the 2D
and the 3D microstructure analysis, the latter turned out
to be more challenging to perform, difficult to interpret
and extra time consuming in the initial step when
quantification algorithms are established. Due to the fact
that 3D microstructural analysis takes into the calcula-
tion a huge number of individual particles, the obtained
information is also more reliable than in the case of 2D
analysis. However, the biggest advantage of 3D
microstructure analysis is the possibility of a complete
microstructural reconstruction and determination of
interface connected properties that directly determine the
cermet activity in an operating SOFC.
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G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782 781
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
the sample (Ni = 37.7 %) and its homogeneous dis-
tribution throughout the cermet should guarantee a good
Ni-to-Ni particle contact. The latter suggests that a
contiguity determination should consider a real 3D
microstructure, since contacts between particles may
continue also in the direction perpendicular to the
investigated 2D plane. Generally, it is clear that after the
reduction the average particle sizes remain well beneath
the micrometre range; however, more precise micro-
structural quantization requires a more sophisticated
approach.
More precise quantitative microstructural evaluation
is possible by using the real 3D microstructure recon-
struction based on FIB-SEM serial sectioning technique.
It has to be stressed that for 3D microstructure recon-
struction the back-scattered electron detector (BSC-ED)
is more appropriate than the commonly used in-lens
secondary electron detector (IL-SED), since BSC-ED
successfully copes with the shadowing effect, which
sometimes appears on the images when using IL-SED.
Considering the obtained phase-extraction images
(Figure 8), it is evident that the distribution of all three
phases, namely, Ni, SDC and pores (pores may be
considered as an additional phase in the cermet), is
homogeneous. No larger areas of one-phase domination
can be found in the cermet. Moreover, average particle
sizes and pore diameters in any direction are in the
nanometre range. Similar to the 2D analytical approach,
the 3D microstructural description also allows the deter-
mination of morphological parameters (Table 1);
however, the 3D analysis additionally enables a descrip-
tion of microstructure also in the z axis. It appears that
the d and Feret-values (x, y and z) are somewhat lower
when determined by the 3D approach. There are two
main reasons for such a slight discrepancy in the results.
Firstly, the computational algorithms employed for grey
image segmentations and thus the separation of phases
are different; and secondly, 2D statistical calculation is
based on several hundreds of analysed grains, while the
3D statistics involves several tens of thousands of re-
cognized particles. This fact makes the 3D microstruc-
tural analysis more reliable. Similar is true also for the
Ni and SDC determination.
One of the big advantages of 3D microstructural
analyses over the 2D approach is the fact that 3D
microstructure reconstruction also enables an in-depth
porosity description. Due to the large voxel matrix
(2.6×108 voxels) in 3D reconstruction, the determined
value for the microstructural porosity comes very close
to the determined value of the geometrical porosity. The
discrepancy of the geometrical and microstructural is much
bigger for the 2D approach. Moreover, a detailed exami-
nation of the separated pore phase enables the overall
porosity microstructural to be separated into open- and
closed-porosity. According to the results only 0.7% of
the porosity belongs to closed pores, making the pre-
pared microstructure interesting for SOFC applications.
The contiguity values may no longer be calculated
from the 3D microstructure reconstruction. Instead,
using an appropriate computational algorithm the much
more interesting TPB density value (TPBL) can be
obtained. In the analysed Ni-SDC cermet the TPBL is
determined as 7.23 μm μm–3, making this material fully
comparable to other Ni-based cermets described in the
literature. Further interesting information that may also
be obtained from 3D microstructural reconstruction is
the specific surface areas SSA of the phases (i.e., surface
areas of individual phases normalized to the sample
volume). For the investigated cermet the SSA values for
Ni- SDC and pores are determined 15.4 μm2 μm–3,
18.7 μm2 μm–3, 15.7 μm2 μm–3, respectively. The obtained
G. KAPUN et al.: MICROSTRUCTURAL EVALUATION OF Ni-SDC CERMET FROM A REPRESENTATIVE 2D IMAGE ...
780 Materiali in tehnologije / Materials and technology 51 (2017) 5, 775–782
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 8: 3D microstructure reconstruction of Ni-SDC cermet and
extracted phases Ni, SDC and pores
Figure 7: Microstructures of a) sintered (1400 °C) and b) reduced
(900 °C)
values are rather high compared to some data available in
the literature and are a consequence of the relatively
small grains of each individual phase. Furthermore, each
phase in the cermet is in contact with the two other
phases, meaning that by simple combination of the
obtained SSA values specific interface areas SIA
(normalized to the sample volume) between the three
neighbouring phases can be calculated. For instance, the
SSA of the Ni phase is divided into Ni-pore SIA
6.2 μm2 μm–3 and Ni-SDC SIA 9.2 μm2 μm–3. Thus, about
40 % of the total Ni surface is exposed to the porous
phase and can be used for surface catalytic reaction with
fuel. Namely, oxidation of the fuel involves numerous
reaction steps, which are not necessarily located at the
TPB (i.e., adsorption and dissociation of hydrogen or
hydrocarbon reforming inside an anode layer of a
SOFC).
4 CONCLUSIONS
This work focused on a 2D and 3D microstructural
analysis of a Ni-SDC anode cermet material using SEM
analysis and FIB tomography, respectively. The Ni-SDC
sample was prepared by citrate-nitrate combustion
synthesis, followed by milling, sintering and NiO to Ni
reduction. The 2D microstructure investigation employed
grey image-analysis and made it possible to obtain
quantitative values of the fraction of phases, microstruc-
tural porosity, grain sizes of all phases, sphericity and
contiguity. On the other hand, the 3D analysis addition-
ally provided information on specific surface areas and
specific interface areas of all the phases, as well as the
triple phase boundary density. When comparing the 2D
and the 3D microstructure analysis, the latter turned out
to be more challenging to perform, difficult to interpret
and extra time consuming in the initial step when
quantification algorithms are established. Due to the fact
that 3D microstructural analysis takes into the calcula-
tion a huge number of individual particles, the obtained
information is also more reliable than in the case of 2D
analysis. However, the biggest advantage of 3D
microstructure analysis is the possibility of a complete
microstructural reconstruction and determination of
interface connected properties that directly determine the
cermet activity in an operating SOFC.
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H. Y. JIN et al.: A FACILE METHOD TO PREPARE SUPER-HYDROPHOBIC SURFACES ON SILICONE RUBBERS
783–787
A FACILE METHOD TO PREPARE SUPER-HYDROPHOBIC
SURFACES ON SILICONE RUBBERS
PREPROSTA METODA ZA PRIPRAVO SUPERHIDROFOBNIH
POVR[IN PRI SILIKONSKIH GUMAH
Hai Yun Jin1, Yu Feng Li1, Shi Chao Nie1, Peng Zhang1, Nai Kui Gao1, Wen Li2
1Xi’an Jiaotong University, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an, 710049, China
2Hubei Polytechnic University, Hubei Key Laboratory of Mine Environmental Pollution Control & Remediation and School of Chemical and
Materials Engineering, Huangshi, 435003, PR China
lxfzzl@126.com
Prejem rokopisa – received: 2016-09-02; sprejem za objavo – accepted for publication: 2016-10-24
doi:10.17222/mit.2016.267
Fabricating large-scale super-hydrophobic surfaces for commercial applications is challenging due to certain limitations. In this
paper, a simple and inexpensive method is developed to fabricate super-hydrophobic surfaces on silicone rubbers. Rough micro-
structures were prepared on the mould’s inner surfaces and then sample super-hydrophobic surfaces on silicone rubbers with
different surface roughness were achieved using the standard moulding process. Furthermore, the effects of roughness on the
wettability were investigated. The results showed that by controlling the roughness, the fabricated surfaces exhibited a static
contact angle of 150.9° and a sliding angle of 8°. Finally, the property of hydrophobicity recovery for the silicone-rubber sam-
ples was also studied. The surfaces of the samples could recover well after a sand-blasting experiment. The proposed method is
low-cost, environmentally friendly and suggests promising industrial applications.
Keywords: thin films, coatings, chemical techniques, surface properties
Izdelava obse`nih superhidrofobnih povr{in za komercialne namene je zaradi dolo~enih omejitev zahtevna. V ~lanku je razvit
preprost in poceni na~in za izdelavo superhidrofobne povr{ine na silikonskih gumah. Grobe mikrostrukture so bile pripravljene
na notranjih povr{inah kalupa in nato je bila dose`ena vzor~na superhidrofobna povr{ina na silikonski gumi z razli~no hrapavo
povr{ino z uporabo standardnega postopka modeliranja. Nadalje so bili raziskani u~inki hrapavosti na omo~ljivost. Rezultati so
pokazali, da so s krmiljenjem hrapavosti izdelane povr{ine razstavljene pod stati~nim kontaktnim kotom 150.9° in drsnim kotom
8°. Nazadnje je bila preu~evana tudi lastnost okrevanja hidrofobnosti vzorcev silikonskih gum. Povr{ine vzorcev se lahko dobro
obnovijo tudi po poskusu s peskanjem. Predlagana metoda je poceni, okolju prijazna in ka`e obetavne mo`nosti apliciranja v
procese v industriji.
Klju~ne besede: tanke plasti, prevleke, kemijske tehnike, povr{inske lastnosti
1 INTRODUCTION
Superhydrophobic surfaces, with a water contact
angle (CA) greater than 150° and a sliding angle (SA)
less than 10°, have aroused increasing research interest
for their promising applications.1–6 By observing the
microstructure of a lotus leaf surface, it was found that
the combination of micro papilla and a thin wax film
leads to the self-cleaning properties.4 L. Jiang et al.1 dis-
covered that on top of the micro papilla also exist
branch-like nanostructures and pointed out that the
micro- and nanoscale hierarchical structures were the
fundamental mechanism for lotus leaf’s unique wetting
properties. There were two main measures to fabricate
superhydrophobic surfaces:
1) preparation of a rough surface followed by a low
free-energy material coating step,
2) creation of a rough surface from low-surface-energy
materials.1
Up to now, many approaches to fabricate rough sur-
faces had been developed, including the template
method7, phase separation8, self-assembly9, vapour-phase
deposition10, chemical etching11, laser etching12, hydro-
thermal method13, and electrospinning14. In the power
system, the surfaces of insulators always work under
high voltage and the sand-dust climate condition. How-
ever, some superhydrophobic coats (especially rubber-
based coatings) synthesized by techniques on large-area
and environmentally friendly cannot meet the operating
requirements of composite insulators.15–17 Therefore, it is
a good way to solve this problem by synthesizing a
superhydrophobic surface on the basis of not changing
the insulator material. Silicon rubber was a hydrophobic
material; therefore, a superhydrophobic surface could be
created by only forming a special nm-μm geometry
structure. The template method was a simple and con-
venient technique that could be used as a method to
prepare superhydrophobic surfaces, and the quality of
this method was easy to control; therefore, template
method was suited for industrial production.
In this study, to fabricate a superhydrophobic surface
on the composite insulators, a silicone-rubber super-
hydrophobic surface was developed with a facile tem-
plate method. The creation of a rough structure on the
inner surface of the mould and a special geotroy mor-
phology was prepared on the silicone-rubber surface
Materiali in tehnologije / Materials and technology 51 (2017) 5, 783–787 783
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:621.315.617:544.722.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)783(2017)