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Article history: The role of different amount (from 1 to 20 mol%) of tantalum Ta(V) into the zirconia lattice has been
Received 14 October 2016 investigated in different powders systems prepared via co-precipitation and after calcination. The X-ray
Received in revised form 5 April 2017 diffraction and Raman analyses, have conrmed the presence of an orthorhombic phase in all the samples.
Accepted 7 April 2017
The stability of this phase has been investigated by in situ X-ray diffraction analysis from 100 C to
Available online xxx
1000 C and after the subsequent cooling. A phase transformation mechanism, with the formation of a
poor (monoclinic) and a rich phase (orthorhombic) in Ta has been consider in order to explain the XRD
Keywords:
patterns obtained at different temperatures. The characterization of the orthorhombic phase on bulk
Zirconia
Tantalum
ceramic specimens prepared via Spark Plasma Sintering (SPS) has been also investigated. SEM analysis
Orthorhombic has conrmed the presence of a two-phase system.
Raman 2017 Elsevier Ltd. All rights reserved.
Spark plasma sintering
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.017
0955-2219/ 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: G. Sponchia, et al., Orthorhombic phase stabilization and transformation
phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.017
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literature such as a better hot corrosion resistance for TBCs [3134] 2. Material and methods
concerning zirconia tantalum-doped materials.
In parallel to the research of new ZrO2 based systems with 2.1. Materials
enhanced macroscopic properties, some papers have evidenced the
presence of three orthorhombic phases into zirconia systems, sta- Zirconyl chloride octahydrate (ZrOCl2 8H2 O; Aldrich 98%), tan-
bilized also by some dopant such as MgO, at high pressure and talum(V) chloride anhydrous (TaCl5 ; Aldrich 99.8%), ammonium
temperature. For some of these systems, with specic composi- hydroxide (NH4 OH; Fluka 28 wt%), ethanol (EtOH; Aldrich 99.8%),
tion, detailed structural studies have been reported [3538 and and distilled water. All reagents were used as received without
reference therein]. In particular, pure zirconia system shows Pbca further purication.
(3.515 GPa) and Pnam (over 15 GPa) orthorhombic phases sta-
ble at high-pressure [36]. Moreover, different doped and co-doped 2.2. Synthesis of the powders
zirconia systems with Ca or Mg [39], or some alloy systems evi-
dence the Pbc21 (an allowed transformation of the Pca21 space Ta-doped ZrO2 powders were prepared via the base catalyzed
group) orthorhombic phase as reported by Kisi et al. [36]. Even coprecipitation method, already used for other similar works [21].
though zirconia Ta-doped systems have been previously studied, Briey, ethanolic solutions 0.1 M ZrOCl2 8H2 O and 12.5 mM TaCl5
very scarce and not always coherent data are available on phase were mixed together in different amounts while stirring at ambi-
analysis [30,33,4043]. The homogeneity range of orthorhombic ent temperature. Co-precipitation to the corresponding hydroxides
phase was found to be up to 33 mol% of Ta2 O5 in ZrO2 [44]. In was achieved by drop-wise addition of excess aqueous ammonia.
particular, on samples with low Ta content, some paper reports After 20 min of stirring at room temperature, the co-precipitated
that the addition of Ta brings about the gradual stabilization of the hydroxides were centrifuged (9000 rpm for 30 min) and washed
tetragonal ZrO2 , meanwhile, some others show the presence of an once with distilled water and three times with EtOH to remove
orthorhombic phase. Probably, since the differences between the excess ammonia and unreacted precursors, then dried at 130 C for
two phases are particularly evident at angles higher than 35 in 2, 12 h. The as-obtained powders were calcined in air at 1000 C for
the X-ray diffraction results have been variously interpreted due 6 h to promote the crystallization and transformation to the mixed
to the too limited range of the patterns investigated [45]. Recently, oxides.
we have reported a detailed X-Ray diffraction analysis on a ZrO2
sample doped with 16 mol% of TaO2.5 [46] and we have identied 2.3. Powder consolidation
the high pressure Pca21 orthorhombic structure reported by Kisi
et al. [36] as the main crystalline phase. The consolidation process of the zirconia Ta-doped powders
Despite the extensive Raman knowledge of the well-known (ZT5, ZT8, ZT12) via Spark Plasma Sintering (SPS) has been per-
zirconia polymorphs (i.e. monoclinic, tetragonal and cubic) the formed with a Dr. Sinter Lab SPS 515S equipment at two different
orthorhombic phase, in all his forms (Pnam, Pbca, Pca21 ) has not experimental conditions sets: a) T = 1250 C, P = 75 MPa, t = 5 min
been investigate in details. The only system published, in which (SPS1250 series), b) T = 1150 C, P = 100 MPa, t = 5 min (SPS1150
both calculated vibrations and experimental data match, concerns series). Lower sintering temperature needs higher sintering pres-
the orthorhombic Pnam system, which identies the cotunnite- sure in order to reach a good nal density of the specimen.
type phase [48,49]. Generally, other attempts, performed to
discriminate the orthorhombic Pbca, cannot be considered fully 2.4. Characterization
reliable. The main difculty arises from the restricted number of
high pressure-induced polymorphs, transformed from either mon- The microstructure of the powders was studied by means of X-
oclinic [50,51] or tetragonal [52] zirconia, studied so far. Different ray powder diffraction (XRPD). A Philips diffractometer with a PW
pressure-annealing treatments involves different grade of lattice 1319 goniometer (Almelo, The Netherlands) with BraggBrentano
distortions, leading to different crystallographic systems and con- geometry, connected to a highly stabilized generator (40 kV), was
sequently different Raman mode shifts. used for the preliminary XRPD measurements. A Bruker D8I-90
Aim of the paper is to investigate the role of Ta as a dopant with BraggBrentano geometry, connected to a highly stabilized
agent, in a specic range of composition into the ZrO2 structure as generator (40 kV), was used for the Wide-Angle X-Ray Scatter-
stabilizer of the orthorhombic phase in the powder and in the con- ing (WAXS) measurements of the sintered specimens. The in
solidated specimens. In detail, the study was carried out as follows: situ temperature XRPD measurements were collected at the ELET-
a) Seven ZrO2 -TaO2.5 doped powdered systems, with the amount of TRA Laboratories of Sincrotrone Trieste S.C.p.A (Line MCX) using
Ta ranging from 1 to 20 mol% of TaO2.5 (ZT1, ZT3, ZT4, ZT5, ZT8, ZT12, a quartz glass capillary as sample holder [53]. The patterns were
ZT20), were prepared by a catalyzed co-precipitation route; b) the acquired in the range 1001000 C every 100 C with a temperature
thermal evolution and the stabilization of the crystalline phases ramp of about 0.17 C/sec. Before any measurements, the temper-
were analyzed in situ and ex situ by XRD patterns using also ature of the samples was kept constant for 10 min.
a synchrotron line; c) a detailed Raman spectroscopy analysis was Grain size distribution, compositional Energy Dispersive X-ray
performed; d) the three more signicant powdered systems were Spectroscopy (EDS) analyses, Backscattered Electrons (BSE) and
consolidated via Spark Plasma Sintering (SPS) at two different tem- Secondary Electrons (SE) images on the sintered specimens have
peratures and pressures; e) a detailed XRD and SEM analysis of the been preformed with a Zeiss Sigma VP FE-SEM c/o the Centro di
consolidate systems were conducted to dene the role of Ta in the Microscopia Giovanni Stevanato in Venezia-Mestre (VE, Italy). In
nal microstructure. order to evidence the grain boundaries the surfaces have been pol-
The present study have also showed that the lower limit rela- ished (lapping with diamond paste till 1 m) and then have been
tive to the stabilization of the orthorhombic phase, in the system annealed at one hundred degree lower respect to the SPS temper-
ZrO2 -TaO2.5 , is 16 mol% instead of 25 mol% of TaO2.5 as reported ature.
in literature [31,34,43,47]. The XRD analysis of the two systems Raman spectra were acquired with using a confocal (optical)
show that the same orthorhombic phase (Pca21 ) had to be used to microprobe from different areas of the samples. All Raman spectra
describe both the crystalline structures. were collected at room temperature using a single monochromator
(T-64000, Jobin-Ivon/Horiba Group, Kyoto, Japan) equipped with
a nitrogen-cooled 1024 256 pixels CCD camera (CCD3500 V,
Please cite this article in press as: G. Sponchia, et al., Orthorhombic phase stabilization and transformation
phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.017
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ZT20
ZT12
ZT8
ZT5
ZT4
ZT3
ZT1
15 20 25 30 35 40 45 50 55 60 65 70 75 80
2 ()
3. Results and discussion internal stress and lacking of oxygen vacancy. In particular: a)
the Ta(V) doping seems to generate internal local compressive
3.1. Ex situ X-ray powder diffraction stresses that prevent the monoclinic to tetragonal transformation
and stabilize the orthorhombic form of zirconia [54]; b) unlike
As shown in Fig. 1 the X-ray powders diffraction patterns can be the well-known case of Y(III) doping, in which two yttrium atoms
grouped in three different sets according to the crystalline phases introduce one oxygen vacancy in the zirconia structure leading the
evidenced by the samples. To the rst group belong the two samples tetragonal or cubic stabilization, Ta(V) does not introduce any com-
(ZT1 and ZT3) richer in the monoclinic phase; to the second group, pensating vacancy. The Zr(IV) maintains its sevenfold coordination
the samples (ZT4 and ZT5) that show the co-presence of monoclinic and only structural changes are introduced due to the size of the
and the orthorhombic phase, as prevalent phase; to the last group, doping cations.
the three samples (from ZT8 to ZT20) that show only the presence
of orthorhombic phase. 3.2. In situ X-ray powder diffraction
The Rietveld analysis, of all the samples, has been performed on
the full-range X-ray patterns (5 < 2 < 140 ). After a careful evalua- The three most signicant samples of each group, ZT3, ZT5
tion of the different zirconia polymorphs, the orthorhombic Pca21 and ZT8, were further investigated in situ by synchrotron XRPD
(space group no.29, ICSD 67004) phase has been selected by the measurements. Fig. 2 shows the thermal evolution of the XRPD pat-
tting procedures [46,54]. Fig. S1, into Supporting Information (SI), terns of the three samples, from 100 C to 1000 C and after the
shows an example of a full prole renement of the patterns per- subsequent cooling to room temperature (RT). The patterns were
formed by the Rietveld method (DBWS9600 computer program recorded every 100 C.
written by Sakthivel and Young and modied by Riello et al. [55]). All the as prepared powders are amorphous at RT. The crys-
The same gure shows also the single contributions of the two tallization process starts above 400 C for all the samples. At
phases (orthorhombic Pca21 and monoclinic P21 /c) on the total t 500 C, crystalline peaks are clearly detectable in the patterns. The
of the sample ZT3. Table 1 reports the weight fractions of the two orthorhombic phase starts to grow at the same temperature for
phases for all the powder samples. all the samples, evidencing the role of the Ta(V) cation as stabi-
The analysis of the patterns can be summarized as follow: when lizer of this phase. As expected, the RT patterns, after the thermal
the Ta content increases, the monoclinic phase decreases and the treatments at 1000 C in situ and ex situ, are the same.
orthorhombic phase increases; in the samples with a Ta content The analysis of the patterns conrms that the amount of Ta
higher than 8 mol%, only the orthorhombic phase is stable till in the samples ZT3 and ZT5 is not enough to stabilize only the
1000 C; in all the samples no tetragonal phase is clearly detectable. orthorhombic phase. The increase of the monoclinic phase, at the
As reported in the introduction, this last nding disagrees with higher temperature and during the cooling, can be consistent with
some results [30,33,4042] published on similar systems, but the formation of a zirconia structure containing an amount of
where a detailed phase analysis is only partially reported. Ta distributed in a non-homogeneous way. In particular, the Ta
The reason why the Ta(V) stabilizes the orthorhombic, and can be simply heterogeneously dispersed in the sample or it can
not the tetragonal or cubic, phase could be explained in terms of form a two-phase system with a poor (monoclinic) and a rich
Please cite this article in press as: G. Sponchia, et al., Orthorhombic phase stabilization and transformation
phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.017
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Fig. 2. In situ XRPD thermal patterns of the samples from 100 C to 1000 C and after cooling. ZT3 (A), ZT5 (B) and ZT8 (C).
(orthorhombic) phase in Ta that, during the heating, start to develop analysis are reported in the SI. Fig. 3 shows well-dened linear
and to growth. The absence of the monoclinic peaks relative to the behaviors as reported by Kisi and Howard [56].
ZT5 sample, at the lower temperatures, can be due to the small The very similar slopes of the straight lines suggest the same
amount of the phase into the sample. At higher temperatures the behavior for each phase independently from the doping amount
monoclinic polymorph is well detectable and growth until 1000 C and that a higher doping amount corresponds to a bigger cell
and during the cooling. When the Ta increases, from ZT3 to ZT5, the volume, underling the presence of tantalum inside the cells. Consid-
amount of the monoclinic phase reduces. A more detailed picture of ering the properties for a good blades protection and for an active
the process can be found in the Sintered SPS specimens paragraph. In Thermal Barrier Coating (TBC), the top coat should have a) high
the sample with a higher content of Ta (ZT8), only the orthorhom- melting point, b) low density, c) low coefcient of thermal con-
bic phase is stable in all the range of the temperatures from 500 C ductivity, d) good corrosion, oxidation and wear resistance and e)
to 1000 C and from1000 C to RT. a volume CTE value very similar to the substrate one [57]. Usu-
The analysis of the cooling process is particularly interesting ally the zirconia YSZ, employed for this application, has a CTE of
and very useful for the study of the thermal stability of crystalline 31.5106 K1 . This value has to be compared with the values of
phases, but also for determining some other useful properties of 3644106 K1 relative to superalloy metal substrates of the tur-
ceramic materials. From the Rietveld analysis the unit cell param- bine blades, such as Hastelloy or Inconel . As shown in Table 2,
eters can be rened and the cell volume can be determined at the orthorhombic phase has a CTE higher than the YSZ one, con-
different temperature for both the phases. When a material is sequently the difference between the substrate and the coating
heated, the thermal agitation causes the bonds lengthening, the CTE values is smaller and a better adhesion should be achieved.
volume of crystalline materials increases and a correlation between On similar issues, some examples in literature have been reported
volume and temperature, known as the volume Coefcient of Ther- the improvement of the corrosion resistance in the Ta-doped zir-
mal Expansion (CTE) , can be determined. All the details of our conia materials [3134] and the lowering the thermal conductivity
coefcient [58].
A 146
B 141
ZT3 ZT3
ZT5 ZT5
145 Literature 140 ZT8
Cell Volume Pca21 ( )
Cell Volume P21/c ( )
3
3
144 139
143 138
142 137
141 136
140 135
200 400 600 800 1000 1200 1400 200 400 600 800 1000 1200 1400
Temperature (K) Temperature (K)
Fig. 3. Zirconia monoclinic (A) and orthorhombic (B) unit cell volume dependences with temperature.
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phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
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Table 2
Linear t parameters for P21 /c and Pca21 phases volume CTE calculation.
in Raman spectroscopy of solid solutions when disorder due to SPS1150 series SPS1250 series
a substituting dopant is introduced in the crystal structure [61].
BSE dark zones% WAXS P21 /c% BSE dark zones% WAXS P21 /c%
As different authors recently suggested [62,63], the orthorhom-
bic phase should be considered as mandatory intermediate step ZT5SPS 55 4 53 3 61 4 59 3
ZT8SPS 38 4 41 3 40 4 42 3
in the monoclinic to tetragonal phase transformation. Actually
ZT12SPS 11 2 10 3 13 4 11 3
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phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
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ZT20
ZT8
+
+ +
++ + + + +
* ZT5
+ +
+ *
++ + + +
* + ZT4
*
* ZT3
*
* ZT1
Fig. 4. Raman spectra of the powder samples. The star-marked (*) peaks are related to monoclinic phase, the cross-marked (+) peaks are related to orthorhombic one.
A B
ZT12 ZT12
ZT8 ZT8
ZT5 ZT5
15 20 25 30 35 40 45 50 55 60 65 70 15 20 25 30 35 40 45 50 55 60 65 70
2() 2()
Fig. 5. WAXS patterns of SPS specimens sintered at 1150 C (A) and 1250 C (B).
equilibrium temperatures of the phases for the powder and the the lighter grains are associated to the orthorhombic phase and the
sintered samples are strongly related to the preparation method. darker to the monoclinic one. The results of the statistical analysis
A SPS sample prepared at 1000 C with a uniaxial pressure of pre- of the volume fractions of the two phases, based on the grayscale
consolidation at 1.24 GPa (10tf ) showed the presence only of the quantication of the two zones and made with the ImageJ pro-
orthorhombic phase. However, its low bulk density (around 80%) gram, has been reported in Table 4. The quantitative estimation
prevented any further mechanical tests. This underlines that ther- matches very well also with the phase volume fractions calculated
modynamic equilibrium needs high temperature for a complete by Rietveld renements.
stabilization of the phase. In order to emphasize the grain boundaries the Secondary Elec-
Fig. 6 shows the FE-SEM images of the polished and annealed trons (SE) mode, which displays a morphological contrast, has also
surface of SPS specimens. In order to emphasize a compositional been used. Fig. 7 reports the comparison between the BSE and SE
contrast, the images have been recorded in Backscattered Electrons mode of the same surface. It is evident that the two compositional
(BSE) mode. zones correspond perfectly to the grain structure and no mixed
An evident weight contrast is visible on the polished surface of phase grain are detectable. From the picture, the grain size D and the
the sintered specimens. This result supports the presence of a two- shape factor F for the two different EDS zones have been estimated
phase system as anticipated by the XRD analysis of the powder (for the complete analysis see also SI) and reported in Table 5.
samples. The lighter zones are related to the heavier composition The larger size of the dark grains could be related to a faster
grains, so they are richer in heavier element, i.e. Ta. The punctual growth of the monoclinic phase or to a smaller number of nucle-
Energy Dispersive X-ray Spectroscopy (EDS) analyses of the two ation seed, meanwhile the shape factor indicates, for both the
zones (see SI Fig. S3), conrm a higher percentage of Ta on the phases, a similar growth path, without any preferential elongation.
lighter grains. As reported in the discussion of the powder sam- From EDS analysis an estimation of the elemental percent-
ples and considering tantalum as orthorhombic phase stabilizer, age composition has also been determined. The quantitative
Please cite this article in press as: G. Sponchia, et al., Orthorhombic phase stabilization and transformation
phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.017
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Fig. 6. FE-SEM images of SPS polished surfaces in BSE mode. Sample ZT5SPS1150 (A), ZT8SPS1150 (B), ZT12SPS1150 (C), ZT5SPS1250 (D), ZT8SPS1250 (E) and ZT12SPS1250
(F).
Table 6
Tantalum molar estimation from EDS peaks for different zone (dark ang light) and
global content (bulk).
Ta% bulk Ta% dark Ta% light Ta% bulk Ta% dark Ta% light
ZT5SPS 51 21 15 1 51 21 15 1
ZT8SPS 81 21 16 1 81 21 16 1
ZT12SPS 12 1 21 16 1 12 1 21 15 1
It is worth noting that dark and light zones have a constant con-
tent of Ta independently from the main Ta content. In particular, the
dark zones, poor in Ta, can be associated to the monoclinic phase.
The light zone, instead, have a Ta content around 16 mol% that could
be identied as the minimal amount necessary to stabilizes the
orthorhombic phase in the sintered materials.
Keeping in mind also the results of the powder samples anal-
ysis, we can conclude that the heating treatments on ZrO2 -TaO2.5
Fig. 7. Comparison between FE-SEM image of polished surface of sample
structures induces the formation of a two-phase system of a poor
ZT8SPS1250 in SE (right) and BSE (left) modes. and a rich phase in Ta. The results of the sintered samples agree
with the previous evidences [46] and support also the ndings rel-
Table 5 ative to the powder samples [64]. For a system with 16 mol% of
SPS specimens grain size D statistical estimation and shape factor F. TaO2.5 , the heating treatments induce the formation of a single crys-
talline phase: the Pca21 orthorhombic phase. This seems contradict
SPS1150 series SPS1250 series
the literature on the existing ZrO2 -TaO2.5 phase diagram, where is
Dark grain Light grain Dark grain Light grain foreseen a single crystalline phase (orthorhombic) for sample with
D(nm) F D(nm) F D(nm) F D(nm) F an amount of TaO2.5 close to 25 mol% and corresponding to the
Zr6 Ta2 O17 phase reported by Kim [47]. In order to give a useful
ZT5SPS 550 80 0.69 320 60 0.66 600 80 0.69 350 40 0.68
ZT8SPS 560 70 0.69 340 50 0.67 580 60 0.70 330 60 0.67 contribution to the problem, we prepared a powder sample with
ZT12SPS 540 50 0.68 350 40 0.67 590 30 0.69 340 30 0.68 25 mol% of TaO2.5 (Zr6 Ta2 O17 ), labeled in Fig. S4 as ZT25, using the
same co-precipitation synthesis reported in the present paper and
we compared it with a sample with 16 mol% of TaO2.5 labeled as
evaluation has been made on the peaks areas relative to Zr and Ta ZT16 sample. The Fig. S4 shows that the patterns are quite simi-
energy lines. In order to work on a more supercial signal, minimiz- lar and the orthorhombic Pca21 crystalline phase is the same for
ing the pear-shaped interaction volume, the lowest beam energy the two compositions, meanwhile the lattice parameters and the
(5 keV) has been used. The quantitative results, on the average com- atomic coordinates (see Table S1) are slightly different due to the
position of the specimens reported in Table 6, match very well the different doping amount. This can considered a signicant result,
nominal content of the as prepared powders. All the EDS spectra since we have evidenced that the range of stability of the single
are reported in Fig. S3 in SI. orthorhombic phase relative to the ZrO2 -TaO2.5 system has a lower
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phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
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Acknowledgements
The authors thank N. Mazzucco for the FE-SEM images and PhD
E. Ambrosi for EDS spectra. Financial support from Junta de Andalu-
cia under grant no. P12-FQM-1079 is gratefully acknowledge.
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Please cite this article in press as: G. Sponchia, et al., Orthorhombic phase stabilization and transformation
phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.017
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Please cite this article in press as: G. Sponchia, et al., Orthorhombic phase stabilization and transformation
phase process in zirconia tantalum-doped powders and spark plasma sintering systems, J Eur Ceram Soc (2017),
http://dx.doi.org/10.1016/j.jeurceramsoc.2017.04.017