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Results pertaining to the fabrication of seal-less, metal-supported, electrolytic reverse hydrolysis anode compartments via radio-frequency induction-coupled suspension plasma spraying (SPS) are presented herein.
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1
*Corresponding author Tel.: +1 819 821 8000ext63238; fax: +1 819 821 7095 E-mail:[email protected]
Radio-frequency induction-coupled suspension plasma spraying of dense
ceramic layers on metal substrates
Daniel L. Calabretta
Centre de recherche en énergie, plasma et électrochimie,
Département de génie chimique et de biotechnologique, Université de Sherbrooke, Sherbrooke, Qc., Canada, J1K 2R1
Available online 29 January 2013
Abstract
Results pertaining to the fabrication of seal-less, metal-supported, electrolytic reverse hydrolysis anode
compartments via radio-frequency induction-coupled suspension plasma spraying (SPS) are presented
herein. The glycine nitrate process (GNP) facilitated the synthesis of contemporary metal oxide powders.
Variable SPS parameters include: GNP-powder tailoring, substrate material, solvent, post-heat-treatment,
substrate cooling arrangement, and substrate holder type/arrangement. The latter most parameter required
a new design in order to eradicate objectionable, plasma-substrate arcing. Depositions were characterized
by energy dispersive x-ray spectroscopy, scanning electron microscopy, and x-ray diffraction. Optimized
anode compartments having La0.7Sr0.3Ga0.7Fe0.2Mg0.1O3-δ as its primary electrolyte, and up to five
additional functional layers, were achieved. The anode compartments may be considered for other
applications, such as intermediate temperature solid oxide fuel cells, and are in the early stages of
electrochemical performance testing.
Keywords: Radio-frequency induction-coupled suspension plasma spraying; Intermediate temperature
solid oxide fuel cell; LSGFM; Thermal spray processes; Coatings
1. Introduction
For over a decade, researchers at CREPE have been investigating the use of thermal spray
processes for the relatively rapid fabrication of metal-supported, intermediate temperature (773±100 ºK)
solid oxide fuel cells (IT-SOFCs). If the anode or cathode layer is deposited onto the porous metal
support prior to the deposition of the solid oxygen anion electrolyte (SOAE) layer, then the integrated
component can be used as anode compartments for the hypothetical electrolytic reverse hydrolysis (ERH)
or electrolytic reverse combustion processes, respectively. These hydrogen storage processes have been
described elsewhere [1-3] and are of principal interest to the author. But the investigations pertaining to
SPS are of interest to the IT-SOFC community, and, more generally, to those interested in thin, dense,
ceramic depositions on metal substrates.
Reviews [4, 5] show that double-doped LaGaO3 perovskites, doped ceria fluorites, and scandia
stabilized zirconia (ScSZ) are the most relevant SOAEs for the processes under investigation. In 1994,
Ishihara et al. [6] reported on the near pure oxygen anion (O2-
) conducting, double-doped, LaGaO3
perovskite family. A review [7] of the first decade of international investigation of this family is
available. Later, the same group reported [8-10] on the extraordinary electronic properties of
La0.7Sr0.3Ga0.7Fe0.2Mg0.1O3-δ (LSGFM). Although hardly investigated since its discovery, LSGFM has O2-
conductivities that are approximately two orders of magnitude higher than the conventional yttria
stabilized zirconia (YSZ) at the intermediate temperature range. Using LSGFM as their primary SOAE
[8], 0.5 mm thick, electrolyte-supported IT-SOFCs were fabricated. An additional La0.9Sr0.1Ga0.8Mg0.2O3-δ
(LSGM), SOAE interlayer was deposited on the LSGFM substrate by pulsed laser deposition (PLD) and
90wt%Ni-Fe electro-catalytic anode and Sm0.5Sr0.5CoO3-δ cathode layers were applied by screen-printing.
2
*Corresponding author Tel.: +1 819 821 8000ext63238; fax: +1 819 821 7095 E-mail:[email protected]
Despite the thickness of the LSGFM layer, their cells yielded power densities of 0.2 and 0.1 W cm-2
at
873 and 773 °K, respectively. LSGFM has partial electronic conductivity and the additional LSGM layer
(<1µm) blocks electron flow to the anode-SOAE interface which thereby affords nearly pure O2-
conduction through the multi-layer assembly; furthermore, a samarium or gadolinium doped ceria (SDC
or GDC) interlayer (<1µm) was required between the anode and LSGM layers in order to prevent the
formation of insulating, nickel lanthanate phases. Thus far, no group has attempted to fabricate metal-
supported IT-SOFCs having LSGFM as its primary SOAE via TSDPes.
A brief history of radio-frequency induction-coupled suspension plasma spraying (SPS)
deposition technology is warranted. In the patent’s invention summary [11], the physical steps that the
sheared suspension droplets experience upon exiting the atomization probe (into the plasma core) were
described consecutively as: (a) vaporizing the carrier substance; (b) agglomerating the small particles into
at least partially melted drops; (c) accelerating these drops; and (d) projecting the accelerated drops onto
the substrate to form the material deposit. This is true if inert gas or O2 plasmas with aqueous solvent
based suspensions are used.
Consideration of the solvent-plasma interaction(s) and its effect(s) on deposition quality has been
lacking. If hydrocarbons are injected into O2 rich plasmas, reactions tending towards the hydrocarbon’s
complete oxidation will occur. These processes are generally exothermic and should therefore cause
localized heating around the solid particles; hence, if compared to aqueous suspensions, an increase in the
solid’s heating and fusion rates can be expected. No mention of this significant factor was made by
Bonneau [12] who attempted to deposit conventionally sized (5≤dp≤26) electrolyte (e.g. CeO2, ZrO2,
La2O3, etc.) powders as dense SOAE layers by injecting suspensions comprised of glycol-water mixtures
(solvent) into an O2 rich plasma. Due to the difficulties in maintaining the powders in suspension,
negative results were obtained. In Jia’s bibliographic review [13], other than-“low torch power limits the
ability to use high solid content suspensions and often requires suspensions to be alcohol based to lower
the energy required to vaporize the suspending liquid”-no mention of the potential effect(s) of the
suspension’s solvent character on the overall deposition process’s energy balance were made. Schiller et
al. [14] did remark on the possibility of increasing the plasma’s enthalpy by combustion reactions, but this
point has not been sufficiently elaborated on in the literature. A recent review [15] on the preparation of
IT-SOFCs via thermal plasma processing did not discuss the possibility of harnessing exothermic,
solvent-plasma interactions either.
For SPS, suspension comprised of organic solvents being injected into an O2 rich plasma would
more appropriately be characterized as a hybrid between the high velocity oxy-fuel and SPS technologies,
and could be defined as radio-frequency induction-coupled oxygen-fuel vacuum suspension plasma
spraying. Due to the lengthiness of such an acronym, however, it is more practical to use the trademark
acronym, SPS. Rudimentary considerations are possible using the solvents’ available thermodynamic
properties [16]. Assuming the complete oxidation of two solvents, where H2O and CO2 are the reactions’
products, at standard temperature and pressure, the exothermic heat addition per unit time divided by the
power dissipation in the radio-frequency induction-coupled plasma torch, which is typically 45±5 % of
the plate power, is here defined as the maximum power of complete oxidation to power dissipation ratio
( PDCO PP ). In Fig. 1 the ratios have been plotted as a function of the solvent flow-rate for the solvents
methanol (CH3OH), and pentadecane (C15H32). The latter has been used to approximate mineral oil.
Clearly the type of organic solvent and its flow-rate are significant variables in the deposition processes’
overall energy balance. In the studies of Bonneau [12], Schiller [14], and Jia [13], the O2 sheath gas flow-
rates were always well in excess of the stoichiometric amounts required for the complete oxidation of
their respective solvents. More detailed considerations of the effect of the suspension’s wt% solids is
warranted but will not be treated here; rather, the results of a two-year experimental study of SPS of
dense, ceramic layers are presented.
3
*Corresponding author Tel.: +1 819 821 8000ext63238; fax: +1 819 821 7095 E-mail:[email protected]
flow rate (ml min-1
)
0 2 4 6 8 10 12 14 16 18
PC
O/P
PD
0.0
0.1
0.2
0.3
0.4
0.5
C15H32
CH3OH
Fig. 1 Maximum power of complete oxidation to power of heat dissipation ratios’ dependencies on
MeOH (672.14 kJ mol-1
; 0.792 g ml-1
) and C15H32 (9414.85 kJ mol-1
; 0.769 g ml-1
) flow rates; 45% overall
plasma circuit efficiency operating at 50 kW plate power
1.1 Preparation of integrated, metal-supported SOFCs by TSDPes
The IT-SOFCs that have achieved the highest electrochemical performances were prepared via
the vaporization deposition techniques RF-magnetron sputtering [17, 18] and pulsed laser deposition [19,
20] having deposition rates of 10-4
μm s-1
and 10-3
μm s-1
, respectively. The most exceptional IT-SOFCs
were developed by Ishihara et al. and employed the SOAE La0.9Sr0.1Ga0.8Mg0.2O3-δ. A thin interlayer of
Sm0.2Ce0.8O2-δ was deposited between the anode and LSGM layer in order to prevent the formation of
nickel lanthanates.
Despite the amount of literature related to the preparation of metal-supported SOFCs by TSDPes,
there are few reports detailing the electrochemical performances of complete cells. Recent reviews [21,
22] on integrated, metal-supported SOFC preparation via TSDPes can be compared with the earlier
review by Henne et al. [23].
Takenoiri et al. [24] prepared nickel felt-supported SOFCs via atmospheric plasma spraying
(LaCoO3 cathode and YSZ SOAE) and high velocity oxy-fuel (anode) technologies. Ceramic-coated
metal separators were used in their thirty, seal-less stack that achieved power densities of 0.3 W cm-2
.
Stack degradation was significant while operating at 1243 °K for 2400 hrs. Tsukuda et al. [25]
constructed calcia-stabilized zirconia supported, tubular IT-SOFC having a Ni-YSZ cermet anode, YSZ
SOAE, and LaCoO3 cathode. Their cell performances of ~0.5 W cm-2
were similar to analogous cells
fabricated by co-sintering procedures.
4
*Corresponding author Tel.: +1 819 821 8000ext63238; fax: +1 819 821 7095 E-mail:[email protected]
Shiller et al. [26] and Lang et al. [27] reported on SOFCs prepared by vacuum plasma spraying
(using Laval-type nozzles) the functional layers on nickel-alloy, porous, metal filters and nickel-felt
substrates. Power densities were measured to be 0.3-0.4 W cm-2
between 1023-1073 °K. Compared to
atmospheric plasma spraying, vacuum plasma spraying can decrease the SOAE’s permeability by about
an order of magnitude. The high melting temperature YSZ and scandia stabilized zirconia (ScSZ) SOAEs
were sprayed directly into the arc while electrode materials were injected downstream. Electrochemical
characterization was later reported [28] for larger SOFCs using the same fabrication procedure. Schiller
et al. [29] assembled anode-supported SOFCs and achieved similar electrochemical performances to those
reported on by Lang et al. [27]. By obtaining a maximum power density of 0.5 W cm-2
at 1073 °K, Stöver
et al. [30] set a new benchmark for mostly TSDP-prepared SOFCs that were comprised of NiO-YSZ
anodes and YSZ SOAEs.
In regards to TSDP for the fabrication of SOFCs having double-doped, LaGaO3 electrolytes, Ma
et al. [31] reported on near theoretical open circuit voltages for their Ni-YSZ/LSM/LSM button cells.
SOAE layer thicknesses of 65±15μm were deposited by atmospheric plasma spraying, which rendered
power densities of 0.08-0.15 W cm-2
between 773-1073 °K; however, in order to obtain an acceptable
level of perovskite phase, a post-deposition, heat-treatment step was required. Hwang et al. [32] later
reported on Ni-YSZ/LSGM/LSCF SOFCs prepared by atmospheric plasma spraying technology on nickel
substrates. Compared to the cells prepared by Ma et al., they obtained superior power densities.
To the author’s knowledge, at the start of this project, a fully-functional, metal-supported IT-
SOFC had not yet been assembled and tested at CREPE. Efforts towards the development of small IT-
SOFCs have always been in regards to the individual cathode [33, 34] or electrolyte [11, 12, 35, 36]
layers.
2.0 Experimental
2.1 The glycine nitrate process
Several metal oxide powders were synthesized by the glycine nitrate process (GNP). These
materials’ coefficients of thermal expansion (CTEs), along with those of the principal metal substrate
materials, are listed in Table 1. Reagent grade starting materials having the following purities were
obtained from Alfa Aesar: La(NO3)3*6H2O (99.9 wt%), Fe(NO3)3*9H2O (98.1 wt%), Sr(NO3)2 (99.9
wt%), Mg(NO3)2*6H2O (99.9 wt%), Sm(NO3)3*6H2O (99.9 wt%), Ce(NO3)3*6H2O (99.5 wt%),
Ni(NO3)2*6H2O (99.5 wt%), Ga (99.999 wt%) and C2H5NO2 (98.5 wt%). Gd0.1Ce0.9O2-δ SOAE (Sigma
Aldrich; 734667) having particle diameters (dp)≤0.5µm was also used without further tailoring.
For the synthesis of powders by the glycine nitrate process (GNP), the vessel used at CREPE for
the synthesis of nano-powders [37] by radio-frequency induction-coupled solution plasma spraying was
slightly modified. Fig. 2 shows the block-schematic for the set-up. Briefly, metal nitrate salts of the
corresponding metal oxides are dissolved in water, or the metals, i.e. gallium, is reacted and dissolved in
[HNO3]. Glycine (C2H5NO2) is then dissolved into the solution which is subsequently heated and agitated
on a hot-stir-plate. After a few minutes, the majority of the solution’s water is evaporated and the
homogeneous, viscous fluid reacts exothermically rendering powders with sub-micron grain sizes.
During the synthesis of GNP powders, best results were obtained when the double-walled vessel
was placed in a horizontal position and maintained at 343±10 °K with a thermostatic water bath. This
avoided excess condensation on the reactor walls thus making it more agreeable to collect the powders.
The system’s pressure was maintained at 54±14 103 Pa and the reaction vessel’s temperature was
monitored with a hand-held recorder and a 3.175 mm o.d., SS-sheathed, K-type, grounded thermocouple
(Omega™) that was fitted into place with a Swagelok™ component. As well, a 1.7±0.1 C2H5NO2 to
metal mole ratio was used and 75±25 ml volumes of GNP solution were reacted each time. The reaction
vessel’s temperature would stagnate after several minutes of heating; shortly thereafter, simultaneous
increases in the system’s pressure and reaction vessel’s temperature indicated the initiation of the powder
formation process.
5
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Typically, in order to eliminate phase impurities, following their synthesis, ~100 g samples of
GNP powders were placed in MgO crucibles (Ozark Technical Ceramics, MO, USA) and heated to ≤1780
°K at 1.5±0.5 °K min-1
in a RHF 17/3M (Carbolite Ltd., London, UK) resistance furnace. Then, the
powders were pulverized in an 8000M Mixer/Mill (Spex SamplePrep LLC, NJ, USA), and subsequently,
with a Sonic Sifter (ATM Corp.; WI, USA), sieved to dp≤53 μm.
Table 1 mean CTEs for the ERC and ERH anode compartment materials
*mass basis
Fig. 2 Schematic of the arrangement used for the synthesis of GNP powders
2.2 SPS for the preparation of ERH anode compartments
The SPS apparatus’s cross-view schematic is shown in Fig. 3. Details on the apparatus’s
suspension atomizer, torch, and substrate holder, along with other relevant points on the preparation of
substrates have been given elsewhere [38]. Constant SPS parameters are listed in Table 2.
Material (mole basis) CTE (10-6
m m-1
°K-1
) Reference
Sm0.2Ce0.8O2-δ 12.7 45
La0.9Sr0.1Ga0.8Mg0.2O3-δ 11.6 7
La0.8Sr0.2Ga0.8Mg0.2O3-δ 12.1 46
La0.7Sr0.3Ga0.7Fe0.2Mg0.1O3-δ 13.6 10
Gd0.2Ce0.8O2-δ 11.6 5
Inconel™ 600 14.5 -
Hastelloy™ X 15.6 -
*Ni0.9Fe0.1 13.2 ca.
reaction
vessel
hot-plate
stirrer
double-walled
powder
collection-vessel
filtration-
vessel
thermostat
to vacuum
pump
6
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Table 2 Constant SPS parameter values
Parameter Value
icentral plasma gas(Ar) flow rate (slpm) 22±1
isheathe plasma gas (O2) flow-rate (slpm) 61±1
atomization gas (Ar) flow-rate (slpm) 11±0.5
deposition chamber pressure (kPa) 13.3±6.7
plasma plate power (kW) 50±1
suspension injection flow rate (ml min-1
) 5.5±1.5
spraying distance (cm) 7±3
atomization gas pressure (kPa) 345±10
Fig. 3 Cross-sectional schematic of the apparatus used for SPS-(1) electrically-grounded water-cooled
mask; (2) copper plaque; (3) SS304 water-cooled substrate holder; (4) double-walled, aluminum, water-
cooled jacket; (5) flexible SS316 hose; (6) dielectric fitting; (7) Teflon nipple; (8) metallic adapter
Initial investigations of the character of SPS depositions were done on commercial, porous or
solid, metal substrates, which were neither pre- nor post-heated by the plasma jet. While ideally the
water-cooled
automated arm
high-pressure
water inlet
vacuum
high-pressure
water outlet
high-pressure
water outlet
Faraday cage
peristaltic pump suspension
stir-plate
torch
deposition
chamber
1
2
3
5 4
6
7
8
7
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deposition rates would refer to the coatings’ thicknesses per unit area and unit time, and would be
reported in units of µm cm-2
s-1
, where the coatings’ thicknesses-determined from their cross-sectional
micrographs-, and the area and time factors- determined from the cooling mask’s inner diameter (Fig. 3)
and the substrate-plasma jet exposure-time during each loop, which are known to be 3.834 ± 0.001 cm
and 2.5 ± 0.1 s, respectively-, the accuracy of the time variable renders meaningless deposition rates;
hence, the deposition rate are given in µm loop-1
2.3 Design and preparation of ERH anode compartments
Both ‘pipe’ (Fig. 4(a)) and ‘NPT’ (Fig. 4(b)) anode compartment designs were investigated; after
several negative results, the former was discarded for the latter. It was imperative for the substrates’
interior and exterior walls to be intensively cooled amid SPS; this necessitated the NPT design.
A 1.91 cm o.d., Inconel™ bar was machined, in-house, to the GKN™ flow-restrictor
specifications (1.27±0.01 cm i.d.), sand-blasted, cleaned, and shipped to the manufacturer where SIKA-
HX-5 powders were pressed and sintered into the part. There appeared to be excellent porous-solid joint
integrity, and further machining of the integrated part was done as required throughout the SPS
optimization. For the final preparation of the integrated parts, its non-porous region were sand-blasted
prior to washing it in alternating baths of 1 N HCl and 20 vol% H2O2 -as was deemed necessary-and
placing it in an EtOH, ultra sound bath for several hours.
Fig. 4 Cross sectional schematics of the metal-supported, ERH anode compartments substrates – (a) pipe
design; (b) NPT design
For the preparation of ERH anode compartments, protocols were taken from the studies of Shiller
et al. [26], Lang et al. [27] and Ishihara et al. [19, 39, 40]; the deposition of a dense layer of NiO-Fe2O3 on
nickel-alloy, GKN™ porous metal filters, followed by its H2 reduction, prior to or after the depositions of
the electrolyte layers, is essentially a hybrid of the research groups’ designs. The selection of SPS
feedstock materials coincided with those used by Ishihara et al; however, LSGFM typically took the place
of LSGM as the principal electrolyte.
2.4 Analyses
XRD patterns of were acquired using an X’Pert Pro™, multi-purpose, x-ray diffractometer
(Philips, Eindhoven, Netherlands); it utilized Cu Kα radiation (45 V; 45 mA).
A model S-4700 (Hitachi, Tokyo, Japan) field-emission scanning electron microscope (SEM),
equipped with an energy dispersive x-ray spectroscopy (EDS) apparatus, facilitated the micrographic and
elemental analyses of the GNP synthesized powders.
influent water
effluent water
exterior water-cooling
interior water-cooling
copper plaque
integrated NPT ERH anode
compartment (b)
0.48 cm OD SS316 tube
0.16 cm OD SS316 tube
TIG weld
1.27 cm OD
GKN™-HX-5
1.91 cm OD machined SS316 bar
1.91 cm OD machined Inconel bar
(a)
8
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Prior to acquiring an SPS coatings’ cross-sectional, SEM micrographs– with either the S-4700, a
JEOL J840A (JEOL Ltd., Tokyo, Japan) or a variable pressure N-3700 (Hitachi, Tokyo, Japan)
apparatus-a homogeneous mixture of epoxy resin and hardener (Precision Surfaces Int., TX USA) was
poured into the sample containing molds and left to cure at ambient conditions for two days. The
hardened, sample-containing epoxy was cut with an ISOMET 2000 (Buehler Co., IL, USA), diamond
saw; the corresponding face was manually polished with an Ecomet 3-variable speed grinder (Buehler
Co.) with 180-600 grit silica-papers, and the final polishing employed 6 µm diamond suspensions
(Precision Surfaces Int.). Lastly, a thin layer of 80wt%Au-Pd was applied with a Hummer IV Sputtering
System (Anatech; CA, USA), which was operated at 10 mA and 90±10 Pa for ~1 min.
On one occasion the adherence and porosity of SPS depositions on the ERH anode compartment
was investigated by measuring its open circuit voltage (OCV) using the set-up depicted in Fig. 5.
Following the depositions, 9912-G silver conductor paste (ESL Electroscience; PA, USA) was applied as
a cathode (SOFC) layer. Then a layer of Aremco 634-AL Pyro-paint™ was coated on the ERH anode
compartments’ exterior walls with a paint brush. A nickel wire, which was inserted in an Al2O3 tube and
coiled at its end, was used to make the electrical contact with the silver cathode. Electrical contact was
made with the anode via a Swagelok™, SS316, male, NPT fitting. The OCV measurements were taken
with a hand held multi-meter. Electrical contacts were made at the exterior of the furnace at the 1.56 mm
o.d., SS316, influent H2 tube and the nickel wire. 99.9 vol% H2 and 99.9 vol% O2 were passed through
their respective compartments at 200 ± 50 ml min-1
. Teflon tubing and Swagelok™ components were
used to electrically insulate the influent H2 line from its reservoir, and a grounded, k-type, Omega™
thermocouple was inserted in an alumina tube such that its end was in physical contact with the ERH
anode compartment’s electrically insulating layer. The arrangement was heated at 2 K min-1
to 923±10
°K in a Lindberg 55035 mini mite, horizontal, tubular furnace (Lindberg/MPH; MI, USA).
Fig. 5 Experimental arrangement used for OCV measurements
3. Results and discussion
Results and discussions regarding the synthesis and characterization of the SPS feedstock
powders will be given first. This will be followed by the results and commentary on the initial
investigations pertaining to SPS and succeeded by the presentation of its relative, follow-up research,
particularly as it relate itself to the eradication of plasma-substrate arcing. Consecutively, inquests
towards the fabrication of ERH anode compartments are given.
O2
Al2O3 tube
influent H2 effluent H2
quartz tube
Al2O3 tube Ni wire
634-AL pyro-paint™
tube furnace resistance coils
1.56 mm o.d.
SS316 tube
4.76 mm o.d.
SS316 tube
NPT-type ERH anode
Swagelok™-NPT fitting
sheathed k-type
thermocouple
9
*Corresponding author Tel.: +1 819 821 8000ext63238; fax: +1 819 821 7095 E-mail:[email protected]
3.1 Characterization of GNP powders
During the GNP synthesis of the supposed nickel/iron oxide mixture (NFO), there was a
remarkable increase in the reaction vessel’s temperature (673 ± 10 °K), and, as shown by its XRD pattern
in Fig. 6, the as-synthesized powder was identified as Awaruite (Ni3Fe). However, subsequent air
sintering at 1773 ± 10 °K for 8 hrs rendered the desired composition, which was ferro-magnetic and
tended to agglomerate which therefore made it arduous to sieve to the preferred particle sizes (dp < 32
μm).
Position (°2 theta)
20 30 40 50 60 70 80
Rela
tive I
nte
nsit
y
0
2000
4000
6000
8000
10000
Fe3O4
NiO
FeNi3
Fig. 6 XRD patterns for GNP synthesized NFO powder before (bottom) and after air sintering at 1773 ±
10 °K for 8 hrs
The La0.8Sr0.2Ga0.8Mg0.2O3-δ, La0.7Sr0.3Ga0.7Fe0.2Mg0.1O3-δ, and Sm0.2Ce0.8O2-δ GNP synthesized
powders that were obtained from the GNP collection vessel were ethereal. During the reaction, the
reaction vessel’s temperature did not exceed 473±10 °K, over-pressurization of the collection vessel did
not occur while applying the specified vacuum conditions, and about half of the materials could be
collected directly from the reaction vessel (Fig. 2).
Following the GNP synthesis of La0.8Sr0.2Ga0.8Mg0.2O3-δ, the resultant phases coincided to with
what was already known [44]. As shown in Fig. 7(a), the insulating, SrLaGa3O7 compound, which was
initially significant, diminished with the air-sintering temperature, and became negligible after soaking it
at 1748±10 °K for 6 hrs. Fig. 7(b) is the EDS spectrum of La0.8Sr0.2Ga0.8Mg0.2O3-δ following its air-
sintering at 1748±10 °K. EDS does not allow for the detection of magnesium; however, the subsequent
two-electrode conductivity tests affirmed its composition [41].
10
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Position (°2 theta)
20 30 40 50 60 70
Rela
tiv
e In
ten
sit
y
0
2000
4000
6000
8000
10000
12000
14000
16000
? ? ? ? ? ? ?
La0.8Sr0.2Ga0.8Mg0.2O3-
SrLaGa3O7
SrLaGaO4
uknown?
1523 ºK
1748 ºK
1623 ºK
(a)
keV
2 4 6 8 10
co
un
ts
0
200
400
600
800
1000
O
LaLa
La
La
La
La
La
La
Ga
GaSr
(b)
Fig. 7 Analyses for GNP synthesized La0.8Sr0.2Ga0.8Mg0.2O3-δ powder-(a) XRD patterns before (bottom)
and after air sintering for 6 hrs at 1523, 1623, and 1748±10 °K; (b) EDS spectrum for
La0.8Sr0.2Ga0.8Mg0.2O3-δ after air sintering at 1748±10 °K
For La0.7Sr0.3Ga0.7Fe0.2Mg0.1O3-δ (LSGFM), if the same sintering sequence as the one used for
LSGM is employed, it was not possible to completely eliminate the SrLaGa3O7 phase. This is
exemplified by LSGFM’s XRD patterns that are shown Fig. 8(a).
11
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Fig. 8 Analyses for GNP synthesized La0.7Sr0.3Ga0.7Fe0.2Mg0.1O3-δ powder − (a) XRD patterns after GNP
synthesis (bottom) and after air sintering for 8 hrs at1373, 1623 1673, 1743, and 1773±10 °K; (b) EDS
spectrum after air sintering at 1773±10 °K for 8 hrs; (c) SEM micrograph after GNP; (d) SEM
micrograph after air sintering at 1623±10 °K for 8 hrs; (e) SEM micrograph after air sintering at 1748±10
°K for 8 hrs, pulverizing, and sieving to dp≤32µm
Position (°2 theta)
20 30 40 50 60 70 80
Re
lati
ve
In
ten
sit
y
0
2000
4000
6000
8000
10000
12000
14000
? ? ? ? ?
La0.7Sr0.3Ga0.7Mg0.2Fe0.1O3-
SrLaGa3O7
La4Ga2O9
unknown?
1373 ºK
1623 ºK
1673 ºK
1743 ºK
1773 ºK
(a)
keV
2 4 6 8 10
co
un
ts
0
100
200
300
400
500
O
LaLa
La
La
La
La
LaLa
Ga
Ga
Sr
Fe
Fe
(b)
(c) (d) (e)
50µm 50µm 5µm
12
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Iron’s presence can be found in LSGFM’s EDS spectrum (Fig. 8(b)). The SEM micrographs for
the as-synthesized powders (Fig. 8(c)) shows that the dp is greater than several hundred microns and that
the particles are comprised of interlocking grains having sizes in the order of 0.20±0.05 µm. Although dp
had remained essentially the same after sintering at 1623±10 °K (8 hrs), the grain sizes augmented to
0.75±0.25 µm. It was discovered that, for SPS, it was essential to pulverize and sieve the powders such
that their dp≤32µm. The SEM micrograph shown in Fig. 8(e) exemplifies the conventional sized powders
that were used for the majority of the SPS depositions.
Following its GNP synthesis, Sm0.2Ce0.8O2-δ (SDC) was sintered at 1773±10 °K for 8 hrs. Its
XRD patterns, given in Fig. 9, shows that a purely fluorite phase was obtained.
Position (°2 theta)
20 30 40 50 60 70 80
Rela
tive
In
ten
sit
y
0
1000
2000
3000
4000
CeO2
Fig. 9 XRD patterns for GNP synthesized Sm0.2Ce0.8O2-δ powder before (bottom) and after air sintering at
1773±10 °K for 8 hrs (top)
3.2 Initial investigations of SPS
Initial attempts at SPS depositions of double-doped, LaGaO3 SOAE employed parameters that
were very similar to those used by Jia and Gitzhofer [42]. Early on, however, it was found that the back-
pressure in the atomization probe’s suspension inlet-caused by the pressure exerted by the injection gas
line (Fig. 3)-often prevented the EtOH-based suspension from flowing altogether. Furthermore, EtOH,
GNP powder suspensions were not uniform; thus, at the time of the depositions, EtOH flowed faster than
the solids while the latter obviously settled in the peristaltic tube; therefore, it was impossible to know,
even approximately, what the suspensions’ wt% solids were at the atomization probe’s exit. This
13
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occurred if solid GNP powders were added to the solvent either directly following their synthesis or
following their sintering and subsequent placement in an ultrasound bath. The problem was particularly
severe using the former arrangement. The effect of the addition of dispersant(s) on EtOH suspensions’
stabilities and fluidities was not investigated as white mineral oil was discovered to be a far superior
solvent. For the flow-rates used in this study (2.5-8.0 ml min-1
), mineral oil suspensions were uniform;
this stems from mineral oil’s kinematic viscosity (34.5 cSt), which is high compared to EtOH’s (1.5 cSt).
Furthermore, as shown in Fig. 1, mineral oil has a higher volumetric power density. All of the SPS
results given here are in regards to mineral oil suspensions.
A summary of the SPS (i-xi) parameters are given in Table 3. The corresponding samples’
analyses were done using the given protocols (2.3). For this preliminary study, 7 wt% solid suspensions
were used. As well, the substrate type and the GNP powders’ post-synthesis manipulations varied for this
portion of the optimization. Either a mortar and pestle (MP) or a miniature ball-mill (MBM) was used to
pulverize (~ 1 g) the sintered, GNP powders.
Table 3 Summary of the varied parameters’ values during the initial SPS investigations
Deposition ST FR (ml min-1) SOAE SD NL Analyses PT
Ii Ti 4 LSGFM 10 55 SEM, EDS MBM
iii Ti 4 LSGM 10 20 SEM, EDS MBM
iii Ti 4 LSGMF 10.5 20 SEM MBM
iiv Ti 4 LSGM 10 23 SEM, EDS MBM
iv Ti 4 LSGMF 14 15 SEM, EDS MBM
ivi Ti 8 LSGM 20 4 SEM, EDS MBM
ivii SIKA-SS-10 4 LSGMF 10 20 SEM MP
iviii SIKA-HX-10 4.5 LSGMF 10 10 SEM, EDS MP
iix SIKA-HX-1 4 LSGFM 11 7 SEM 2.1
ix SIKA-HX-1 4 LSGFM 11 60 SEM 2.1
ixi SIKA-HX-1 4 LSGFM 11 55 SEM 2.1
ST – substrate type; FR – flow rate; SD – spraying distance; NL – number of deposition loops; PT
– powder tailoring.
Figs 10(a)-(c) shows that the coating suffers from a high level of open porosity; despite the
number of passes, its EDS, elemental spectrum indicated the presence of titanium. The distribution of
elements, however, was similar to the feedstock’s (Fig. 8(b)).
14
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Fig. 10 Analyses for deposition i (Table 3) − (a)-(c) SEM micrographs (top section)
Deposition ii’s cross sectional, SEM micrographs are shown in Fig.s 11 (a)-(b). The LSGFM
layer is well adhered to the Ti substrate, but there is significant variation in its thickness. Furthermore,
the coating is not homogeneous. Regions with significant open porosity abruptly become dense. Copper,
which assuredly came from the fusion of the copper, cooling insert that Jia [13] used for his SPS
depositions, was present in the EDS, elemental spectrum; hence, Jia’s claim that the copper cooling mask
rendered superior coatings could not be substantiated.
10 µm 10 µm
(a) (b)
I(c)
1 µm
LSGFM LSGFM
LSGFM
15
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Fig. 11 Analyses for deposition ii (Table 3) − (a)-(b) SEM micrographs (cross section); (c) SEM
micrograph (top section)
The global, SEM micrograph of deposition iii’s top-section is given in Fig. 12(a).
Although the coating has a high surface density of caverns, their characters do not correspond
well with those stemming from the discovered, plasma-substrate arcing. In addition, the
coating’s microstructure is highly porous and it appears as though the powders were heated to a
quasi-vapour state amid SPS. The effect may be a result of the amount of time the post-sintered,
GNP powders were subjected to ball-milling-which, for this portion of the optimization, varied
between several minutes to several hours. A substantial portion of the powders were lost during
the ball-milling. Powders having an excessively small particle size distribution would be more
susceptible to vaporization; hence, careful control over the powder’s post-tailoring process is
critical. The SEM micrograph of the deposition’s cross section (Fig. 12(c)) shows that the rate of
deposition was in the order of 1.0 ± 0.1μm loop-1
. However, the coating did not adhere to the
titanium substrate and suffers from a significant level of open porosity.
10 μm
Ti
LSGFM
(a)
LSGFM
(b)
10 μm Ti
29.4 μm
23. μm
1 μm
LSGFM (c)
16
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Fig. 12 Analyses for deposition iii (Table 3) (a)-(b) SEM micrographs (top section); (c) SEM
micrograph (cross section)
As shown in Figs. 13(a)-(c), deposition iv’s top global and micro characteristics are
similar to those of deposition iii’s. The layer’s thickness, which is approximately half of the
previous two depositions, indicates that either its density is significantly higher or that a
significant portion of the in-flight particles were rebounded upon contacting the substrate.
Again, the coating has poor adherence to the titanium substrate and its non-homogeneity is
presumably due to the powder’s post-tailoring sequence. As well, the top-section’s EDS,
elemental spectrum was similar to the feedstock’s (Fig. 8(b)).
100 μm
LSGM LSGM
10 μm
1 μm
18 μm
LSGM
(a) (b)
(c) epoxy
17
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Fig. 13 Analyses for deposition iv (Table 3) − (a)-(b) SEM micrographs (top section); (d) SEM
micrograph (cross section)
In comparison to the previous depositions, the only remarkable parameter change for deposition v
was the greater stand-off spraying distance. Two different types of microstructures can be observed. On
the one hand, as shown in Fig. 14(a), there are regions in which splats having diameters ranging from a
tenth to several tens of microns are deposited upon seemingly larger particle deposits that had reached a
quasi-liquid state. A vertical crack is also present in the top left hand side of the micrograph. On the
other hand, as shown in Fig. 14(b), there are regions that correspond to the previous depositions where the
material attained a quasi-vaporized state prior to being deposited. There is substantial variation in the
deposition’s thickness and the cross-section micrographs (Figs. 14(c)-(d)) shows that, along with open
porosity, the deposition suffers from both vertical and horizontal cracking. In addition, either during or
after the deposition, portions of the coating having lengths and widths in the order of several microns
broke away from the rest of the deposition. The initial layer of the deposit, however, is well adhered to
the titanium substrate. Finally, other than the presence of titanium, the deposition’s top-section, EDS
spectrum coincided with the feedstock’s.
100 μm
LSGFM
10 μm
LSGFM (a) (b)
1 μm
LSGFM (c) (d)
1 μm
epoxy
Ti
9.7 μm
18
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Fig. 14 Analyses for deposition v (Table 3) (a)-(b) SEM micrographs (top section); (c)-(d) SEM
micrograph (cross section)
During deposition vi, the suspension’s flow-rate, the SPS stand-off distance, and the number of
deposition loops differed. High-resolution, SEM micrographs of the coating’s cross-view are shown in
Fig.s 15(a)-(c); it can be seen that the well-adhered deposit suffers from a significant level of open
porosity. Additionally, the substrate was not well centered amid the deposition; hence, the coating’s
thickness varies greatly (7.9 ± 6.9 μm). Other than the presence of titanium, the deposition’s cross-
sectional, EDS, elemental spectrum corresponded to its feedstock (Fig. 7(b)).
--
LSGFM
1 μm
(a) LSGFM (b)
(c) (d) Ti
LSGFM
1 μm
1 μm
Ti
LSGFM
8.2 μm
1 μm
19
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Fig. 15 Analyses for deposition vi (Table 3) − (a-c) SEM micrographs (cross section);
Due to the substantial powder loss that was experienced while pulverizing the post-sintered, GNP
powders in the miniature ball-mill, attempts were made to reduce the powders’ particle sizes with a
mortar and pestle. As well, commercial GKN™ porous metal filters made from both SS316 (SIKA-SS-
10) and Hastelloy X (SIKA-HX-10 and SIKA-HX-1) were used throughout the remainder of the initial
optimization procedure.
Deposition vii’s global top-view, SEM micrograph, given in Fig. 16(a), shows that the powder
sizes are excessively large; hence, they are insufficiently melted in the plasma core, which gives rise to
discontinuous coatings. The high-resolution, top-sectional, SEM micrograph (Fig. 16(b)) suggests that
particle splat formation did not occur and that the particles were slightly liquefied in the plasma’s core.
The global, cross section, SEM micrograph (Fig. 16(c)) infers that the SS316 substrate’s pore size is
excessive and that the coating is poorly adhered to the substrate. As well, its thickness (Fig. 16(d)) is less
than what was achieved while spraying the ball-milled powders under the same operating conditions. This
indicates that solid particles were rebounded off of the substrate.
LSGM Ti
(a) (b)
LSGM
Ti
10 μm 1 μm
14.7 μm
LSGM
(c)
10 μm
Ti
20
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Fig. 16 Analyses for deposition vii (Table 3) − (a)-(b) SEM micrographs (top section); (c)-(d)
SEM micrographs (cross section)
Fig. 17 contains the analyses for deposition viii. While operating with the specified parameters, it
can be seen that a large percentage of the solid particles, especially those having diameters in excess of 30
µm, are not sufficiently fused in-flight, and therefore the coating is not dense. That is to say that only the
particles’ exterior surfaces attain the liquid state as the heat transfer rate was insufficient to liquefy their
cores’. However, as indicated by Fig. 17(d), the larger particles fuse together during the quenching
process. Clearly, the suspension’s large particle size distribution results in irregular depositions hence
further tailoring of the synthesized powders is required.
100 μm
(a) LSGM (c)
1 μm
LSGM
10 μm
SIKA-SS-10
LSGM
SIKA-SS-10
1 μm
7 μm
6.4 μm
21
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Fig. 17 Analyses for deposition viii (Table 3) − (a)-(b) SEM micrographs (top section); (c)-(d) SEM
micrographs (cross section)
Other than the severe plasma-substrate arcing, which results in “micro-lunar” surfaces, SPS of
post-sintered, GNP-synthesized, LSGFM powders, tailored to dp≤32μm, on SIKA-HX-1 substrates
renders depositions of uniform thicknesses (Fig. 18). The author is unable to specify the reason(s) for the
seemingly absence of the arcing in the prior depositions. In the plasma core, the outer volumes of the
feedstock powders with 32 μm < dp < 10 μm attain a majority liquid state in their outer volumes, while
smaller particles are completely liquefied. The quenched mixture results in depositions having minor
amounts of open porosity (e.g. Fig. 18(e)).
LSGFM LSGFM
300 μm 20 μm
50 μm 50 μm
SIKA-HX-10
LSGFM
LSGFM
SIKA-HX-10
(a) (b)
(c) (d)
22
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Fig. 18 Analyses for deposition ix (Table 3) − (a)-(f) SEM micrographs (top sections)
The SEM micrograph of deposition ix’s global top-view (Fig. 18(a)) indicates that the arc rate
density is 1.5 ±0.25 arcs loop-1
mm-2
. Microstructures for three of the deposition’s craters are displayed in
Figs. 18(b)-(d); the arcs penetrate the depth of the ceramic layer leaving craters with diameters of 45 ±15
μm. Two types of craters are noted. One type, which is exemplified by Figs. 18(b)-(c), is where only the
removal of material occurs during the arcing. The other type is where the arc results in both the removal
of material in combination with its vaporization, which, upon quenching, results in ceramic bubble
formation in the crater’s exterior ring (Fig. 18 (d)).
500 μm
LSGFM (a)
50 μm
(b) LSGFM
50 μm 100 μm
(c) (d)
1 μm
(e)
23
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Deposition x’s global, top section SEM micrograph (Fig. 19(a)) shows the augmentation in crater
density, compared to deposition ix’s, because of the greater number of deposition passes. The severe
environment also renders a high density of vertical cracks (Fig. 19(b)).
Fig. 19 Analyses for deposition x (Table 3) − (a)-(d) SEM micrographs (top section)
Cross-view, SEM micrographs for a 55 loop, LSGFM deposition are given in Fig. 20. The
operating conditions allow for a deposition rate of 0.8±0.1 μm loop-1
and the use of SIKA-HX-1
substrates largely eliminates the coating discontinuities and depressions that occur on SIKA-HX-10
substrates. Although the level of open porosity is very minor and the LSGFM coating adheres well to
Hastelloy X, the presence of vertical cracks-caused by either the arcs or the mismatch of the materials’
CTEs, or a combination of the two effects-are highly objectionable.
500 μm
LSGFM
200 μm
LSGFM
100 μm
LSGFM LSGFM
50 μm
(a) (b)
(c) (d)
24
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Fig. 20 Analyses for deposition xi (Table 3) − (a)-(c) SEM micrographs (cross section)
In summary, as it pertains to obtaining dense ceramic coatings by SPS, despite what was reported
[42], major parameter adjustments were required. Firstly, GNP powder-EtOH suspensions are not
homogeneous. Rather, viscous solvents that can be pumped at greater than 4 ml min-1
, such as mineral
oil, are required. Secondly, to obtain coatings of uniform thicknesses, post-air-sintered, GNP synthesized
powders must be pulverized and sieved to dp≤32 μm, and commercial substrates having low pore sizes,
such as SIKA-HX-1, are required. Thirdly, the plasma-substrate arcing, which produces craters at a rate
1.5±0.5 arcs loop-1
mm-2
, is highly objectionable as it is responsible for significant amounts of coating
loss and cracking–both horizontal and vertical.
The results from the secondary investigations of SPS of dense ceramic layers on commercial,
porous, metal substrates, particularly as it relates to the eradication of the plasma-substrate arcing, is
presented in the following sub-section.
3.3 SPS on GKN™ porous, metal filter substrates
Although several problems associated with SPS of dense ceramic layers on metal substrates were
exposed and resolved in the previous sub-section, several issues still need to be addressed before the
fabrication of metal-supported, ERH anode compartments can be considered. In this sub-section, findings
from the optimization of dense ceramic coatings on GKN™ porous metal filters by SPS are elaborated.
This particularly relates itself to material compatibility issues, the eradications of the discovered plasma-
substrate arcing, and multi-layer depositions.
LSGFM
100 μm
LSGFM
SIKA-HX-1
(a) (b)
SIKA-HX-1 100 μm
20 μm
(c)
LSGFM
SIKA-HX-1
25
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Table 4 Summary of the varied parameters’ values for the SPS of dense, ceramic layers on GKN™
porous metal filters
Deposition Powder Wt% solids iNo. loops Analyses
Ii NFO 10.6 50
SDC 10 3
LSGM 10 3
LSGFM 10.1 60 SEM, EDS
Iii NFO 10.6 50
SDC 10 3
LSGM 10 3
LSGFM 10.1 60 SEM
Iiii NFO 10.6 50
LSGM 10 3
LSGFM 10.1 80 SEM, EDS
Iiv NFO 9.7 60
SDC 10 2
LSGM 10 2
LSGFM 10.1 60 SEM
Iv NFO 6.9 25
LSGFM 8 25 SEM
Vi NFO 8 20
GDC 7.4 2
LSGM 7.4 2
LSGFM 8 70 SEM, EDS, XRD
The remainder SPS results presented in this text involves the use of sintered, GNP synthesized
powders that were tailored to dp<53 μm (NFO) and dp<32 μm (LSGM, SDC, LSGFM) using the protocol
given in 2.1. Depositions i-iv and v-vi were done using the electrically-non-insulated (old) and
electrically-insulated (Fig. 3) substrate holders, respectively. During every deposition, the cooling mask
was electrically grounded and neither the pre-heating of the metal substrates nor the post heating of the
coatings was done. As well, the spraying distance (nozzle tip to substrate) was 5.5±1.5 cm, and the
suspension flow-rate was 4.0±0.1 ml min-1
. A summary of the varied parameters’ values for the SPS of
dense, ceramic layers on GKN™ porous metal filters are listed in Table 4.
SEM micrograph of deposition i’s cross-sectional view, shown in Fig. 21(a), does not focus on
the craters that were still present to a minor degree. For the NFO, SDC, and LSGFM deposits, respective
deposition rates of 0.72±0.02, 1.0±0.02, and 1.45±0.02 μm loop-1
were obtained. Although the NFO layer
does not adhere to the SIKA-SS-5 substrate, there is excellent adherence at the three ceramic interfaces.
Other than the LSGM/LSGFM interface, it is possible to visually distinguish the distinct layers by the
abrupt colour variations. While the level of open porosity is low, the vertical crack that extends itself
across the SOAE layer is objectionable.
26
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Fig. 21 Analyses for deposition i (Table 4) − (a) SEM micrographs (cross section)
Deposition ii was done on a SIKA-HX-1 substrate. The SEM micrographs of its top section are
shown in Fig. 22. A crater formation rate of 0.06±0.01 craters loop-1
mm-2
is low compared to the ones
obtained while performing depositions with an ungrounded, cooling mask. Note that the calculation was
made using the number of LSGFM passes as it is believed that craters in the previous layers were
undetectable. The high resolution micrographs (Figs. 22(b)-(c)) indicate that the vast majority of the
feedstock is liquefied in the plasma’s core and that the coating is essentially free of vertical cracks.
Fig. 22 Analyses for deposition ii (Table 4) − (a)-(c) SEM micrographs (top-section)
1000μm
LSGFM (a) (b)
50μm
50μm
LSGFM
LSGFM
NFO
50μm epoxy
SIKA-SS-5 (a)
SDC
LSGM
(c) LSGFM
50μm
27
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Deposition iii was done on a SIKA-HX-1 substrate. From its cross-sectional, SEM micrograph,
shown in Fig. 23(b), deposition rates of LSGM/LSGFM and NFO are 1.34±0.14 and 0.92±0.10 μm loop-1
,
respectively, have been determined. The contrasting layers are firmly attached to one another and there
are no signs of discontinuities along their interfaces. Additionally, the EDS, elemental mapping of the
individual elements, which are shown in the bottom inserts, affirms that excellent phase separation is
achieved. Furthermore, the EDS spectrum (Fig. 23(a)) of the LSGFM layer coincided with its feedstock.
However, unwanted vertical cracks, which are only present in the SOAE layer(s), exist at intervals of one
for every 100±20 μm and extend themselves through the layer’s thickness, appear to have initiated
themselves at the top surface of the coating.
Fig. 23 Analyses for deposition iii (Table 4) − (a) EDS, elemental spectrum of the LSGFM layer (cross
section); (b) SEM micrograph (cross section); EDS, elemental mapping of the elements Fe (Ka1), Ga
(La1,2), Ni (Ka1), O (La1), and Sr (La1)
Deposition iv’s top section, SEM micrographs are shown in Figs 24(a)-(f). Again, while
operating with the grounded, cooling mask, and assuming 60 deposition loops (LSGFM), the crater
formation rate is 0.06±0.01 craters loop-1
mm-2
. The high resolution micrographs shows that not all of the
feedstock powders attain a purely fused state in the plasma core, particularly those having dp>20 μm.
100μm
LSGFM (b)
NFO
SIKA-HX-1
(a)
Ni (Ka1) La (La1) Ga (La1_2)
O (Ka1) Sr (La1)
Fe (Ka1)
28
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Fig. 24 Analyses for deposition iv (Table 4) − (a)-(f) SEM micrographs (top section)
For deposition v, to investigate the possibility of liquefying a greater portion of the feedstock’s
particles, the suspension’s solid wt% was lowered. Furthermore, this was the first trial that employed the
electrically insulated sample holder (Fig. 3). Numerous depressions are shown by the global SEM
micrographs of their top sections (Fig.s 25(a)-(c)). The cavernous character differs from the craters that
result from the plasma-substrate arcing and it can be concluded that they are a corollary of the larger pore
size of the SIKA-SS-10 substrates. Ostensibly, the plasma-substrate arcing can be eliminated by
operating with the new sample holder in combination with the grounded, cooling mask. As well, as
anticipated and exemplified by the high-resolution micrographs given in Figs 24(c)-(d), the percentage of
200μm
LSGFM LSGFM (a) (b)
1000μm
LSGFM (c)
50μm 20μm
LSGFM (d)
LSGFM (e)
10μm
(f)
20μm
LSGFM
29
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the feedstock that is liquefied in the plasma core is augmented when operating with suspensions
comprised of lower wt% solids.
Fig. 25 Analyses for deposition v (Table 4) − (a)-(d) SEM micrographs (top section)
Deposition vi’s top-sectional, XRD pattern coincides with the feedstock’s (Fig. 8(a)). Figs.
26(a)-(f) are its top- and cross- sectional, SEM micrographs of varying resolutions. Because a SIKA-HX-
1 substrate was used, the deposition is devoid of the caverns previously found (i.e. deposition v). Judging
by the quantity of splat formations (Figs. 26(c)-(d)), presumably, the vast majority of the feedstock
powders were liquefied in the plasma core. The deposition rates for the NFO, GDC, and LSGFM/LSGM
layers were 0.99±0.15, 0.65±0.30, 1.00±0.50 μm loop-1
, respectively. While there is some slight open
porosity at the GDC-LSGM interface, the deposition is dense. Only one vertical crack-which again
appears to have commenced at the top of the coating-can be found along the depositions cross-section;
but, to a lesser extent, horizontal cracks are also present. The EDS, elemental line-scans, which are
shown in the bottom panels of Fig. 27, demonstrate the excellent separation between the material
deposits. Using these optimized SPS parameters, similar results were achieved (i.e. reproducible);
multilayer, anodic, IT-SOFC half-cells of varying LSGFM thicknesses were prepared for further
investigations [37]. Notably, for these coatings, the coating thicknesses were an order of magnitude less
than those presented here and they did not suffer from vertical or horizontal cracks.
500μm
LSGFM
500μm
LSGFM
(b)
50μm
LSGFM
(a)
10μm
LSGFM (c) (d)
30
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Fig. 26 Analyses for deposition vi (Table 4) − (a)-(d) SEM micrographs (top sections); (e)-(f) SEM
micrographs (cross sections)
100μm
LSGFM (b)
50μm
LSGFM (c)
5μm
LSGFM (d)
100μm
LSGFM
(e)
50μm
LSGFM
(f)
500μm
LSGFM (a)
31
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Position (°2 theta)
20 30 40 50 60 70 80
Re
lati
ve
In
ten
sit
y
0
100
200
300
400
500
600
LSGFM
SrLaGa3O7
(b)
Fig. 27 Analyses for deposition vi (Table 4) − (a) SEM micrographs (cross section); (b) XRD pattern (top
section); EDS, elemental line-scans for the composition line shown in (a) are given in the bottom panels
The experimental findings from this sub-section can be summarized as follows:
grounding the cooling mask drastically diminishes the rate of crater formation;
grounding the cooling mask in combination with the electrically-insulated sample-holder
eliminates the plasma-substrate arcing;
depositions on SIKA-HX-10 substrates results in cavernous coatings;
depositions on SIKA-HX-1 substrates renders coatings having uniform depths;
feedstock liquefaction in the plasma core is largely dependent on the suspension’s solid content;
compared to the relevant TSDP literature pertaining to dense, ceramic coatings, the optimized,
multi-layer depositions (e.g vii) are of very high quality.
At this juncture, there are several recent claims that have been made by CREPE researchers
that need to be addressed. Jia and Gitzhofer [42] stated that the lowest viscosity suspensions
were ideal for SPS, which contradicts this investigation’s findings. In regards to their high
resolution, 79 µm2 micrographs of the SPS deposition of GDC, they claimed that negligible gas
permeability along with low coating porosity (1.4%) was attainable while using both induction
coupled plasma and GNP synthesized powders, which indicates that they did not observe the
(a)
NFO
SIKA-HX-1
GDC
Ce (La1) Gd (La1)
Ga (La1_2) Sr (La1) La (La1) O (Ka1)
Ni (Ka1) Fe (La1)
20μm
LSGFM
32
*Corresponding author Tel.: +1 819 821 8000ext63238; fax: +1 819 821 7095 E-mail:[email protected]
plasma-substrate arcing. However, this does not correspond to the author’s understanding of
matters. In their description of the test rig they used for the measurement of gas permeability, no
mention of how their coated substrates were fixed into the test rig’s enclosure was given. The
author deemed that open circuit voltage readings would be more accurately allow for the
determination of gas permeability and coating porosity hence their test rig was not used.
Moreover, despite claiming that powder synthesis was achieved by two different methods, the
high-resolution SPS micrograph that were presented in the separate articles [42, 43] were the
same, and no low-resolution micrographs were presented. Lastly, the SPS apparatus set-up for
the coatings that were presented in the Gitzhofer et al.’s follow-up article [37] did not correspond
to reality; rather, in order to eradicate plasma-substrate arcing, the set-up shown in Fig. 3 was
used.
Attempts at extending these promising results to ERH anode compartments will now be
elaborated.
3.4 SPS for the fabrication of ERH anode compartments
Table 5 lists the parameters/post-deposition analyses used in the investigations of, ERH anode
compartment (2.3) prepared by SPS.
For these investigations, the SPS stand-off distance was 4.5 ± 0.5 cm and, unless stated otherwise,
the suspension flow-rate was 4.0 ± 0.1 ml min-1
. Furthermore, in order to achieve a denser GDC inter-
layer, its suspension’s solid content was diminished from 8 to 3 wt%. GDC has a significantly higher
fusion temperature (~3000 °K) compared to those of NFO and the double-doped LaGaO3 (~1800 °K).
All other suspensions were comprised of 8 wt% solids. Finally, as will be discussed in more depth below,
the sequencing of the post-deposition, H2 reduction (H2R in Table 4) plays an important role in the
successful fabrication of the ERH anode compartments. Components were placed under a flowing (100 ±
25 ml min-1
) H2 (99.9 vol%) atmosphere at 923 ± 10 °K for 6 hrs. In order to determine if the reduction
of the anode layer(s) had indeed taken place in the allotted time-frame, electrical resistance measurements
were taken before and after their depositions. Similar tests proved that the incorporated Fe2O3, in
LSGFM is not reduced by H2 to its metallic state under the aforementioned conditions.
At the time of this writing, Ishihara et al.[19] had obtained the highest power densities in the IT-
SOFC literature. This was achieved by the in-situ reduction of the NiO-Fe2O3 substrate after the
deposition of the SOAE layers were deposited by pulsed laser deposition. Being that the ERH process to
be tested was to be done with a metal-supported, anode compartment fabricated by TSDP, which are
susceptible to vertical cracks, relatively thick (≥ 100 μm) SOAE layers were deposited so to lessen the
possibility of catholyte cross-over.
The SEM micrographs of deposition i’s top-section are shown in Fig. 28. Judging by the
coating’s surface, ostensibly, while in the plasma core, the feedstock had reached a quasi-vaporized state.
The post-deposition measurement of the suspension’s flow-rate rendered an unexpected value of 2.5 ± 0.1
ml min-1
due to a worn out tube.
33
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Table 5 Summary of the varied parameters’ values during the SPS on ERH anode compartments having
the NPT design
**3NFO7SDC-30wt%NFO-SDC; *7NFO3SDC-70wt%NFO-S
Deposition Powder no. loops H2R
Ii NFO 15
LSGFM 20
Iii NFO 35
LSGFM 99 R
Iiii NFO 35 R
GDC 4
LSGM 3
LSGFM 110
Iiv NFO 35 R
GDC 4
LSGM 3
LSGFM 99 R
Iv NFO 35
GDC 4
LSGM 3
LSGFM 99 R
Ivi NFO 35 R
*7NFO3SDC 3
**3NFO7SDC 3 R
GDC 4
LSGM 3
LSGFM 150 R
Ivii NFO 30
*7NFO3SDC 3
**3NFO7SDC 3 R
GDC 3
LSGM 3
LSGFM 99 R
Iviii NFO 30
*7NFO3SDC 3
**3NFO7SDC 3 R
GDC 3
LSGM 3
LSGFM 70
34
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Fig. 28 Analyses for deposition i (Table 5) − (a)-(d) SEM micrographs (top section)
To test the functionality of the ERH anode compartment that resulted from deposition ii, OCV
measurements were taken using the arrangement depicted in Fig. 5. As the system’s temperature was
augmented between 873±10 and 923±10 °K, the OCV increased from 0.385±0.003 and 0.645±0.003 V.
Thereafter, it diminished rapidly (~3 mV min-1
) until a zero voltage reading was obtained. The measured
value at 923±10 °K is in reasonable accordance with the one obtained by Ishihara et al. [19]. This was
the only ERH anode compartment that was characterized in this way. The subsequent SEM analysis of
the sample’s cross-section (Figs. 29(a)-(e)) shows that, during the examination period, other than the far
left portion of the coating (Fig. 29(a)), the LSGFM layer had completely delaminated itself from the
quasi-dense, 30±5 μm thick, 90wt%Ni-Fe, anode layer. The latter material remained flush and fully
adhered to the well adjoined SIKA-HX-1 and Inconel materials that constitute the integral, flow-restrictor
substrate. Five minor vertical cracks of variable spacing between each (50-400 μm) can be observed
along the 100±10 μm thick, LSGFM layer (Fig. 29(c)), which was therefore deposited at a rate of
1.01±0.10 μm loop-1
. From these analyses it can be concluded that the custom cooling arrangement can
achieve the goal of substrate temperature management but a protocol that allows for an adherent, multi-
layer deposition following the H2 reduction of NFO is still required.
LSGFM (b)
50μm
LSGFM (c)
20μm
LSGFM (d)
5μm
LSGFM (a)
200μm
35
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Fig. 29 Analyses for deposition ii (Table 5) following the OCV testing − (a)-(d) SEM micrographs (cross
sections)
Deposition iii’s H2 reduction sequence allows for the investigation of the cohesion of the SOAE
layer(s) on the 90wt%Ni-Fe anode layer. So to conform, as much as possible, to the desired final product,
GDC and LSGM inter-layers were included. The SEM micrographs of its cross sections are displayed in
Figs. 30(a)-(f).
The apparent LSGFM/LSGM and GDC deposition rates are 1.19±0.18 and 0.19±0.06 μm loop-1
,
respectively. As the deposition of the NFO layer was succeeded by its H2 reduction, a corresponding
value cannot be assigned; hence, only a “superficial” (i.e. 90wt%Ni-Fe) deposition rate of 0.74±0.12 μm
loop-1
can be given. The anode’s sub-micron porosity is particularly notable (Fig. 30(f)). Although there
is excellent coherence between the various layers, it can be seen that, in comparison with the previous
LSGFM
(a)
500μm
SIKA-HX-1
Inconel
Ag
90wt%Ni-Fe LSGFM
(b)
500μm
SIKA-HX-1
Inconel
Ag
90wt%Ni-Fe
LSGFM
(c)
500μm
SIKA-HX-1
Ag
90wt%Ni-Fe
LSGFM
(e)
200μm
SIKA-HX-1
Inconel
Ag
90wt%Ni-Fe
LSGFM (f)
100μm
Inconel
90wt%Ni-Fe SIKA-HX-1
LSGFM
(d)
500μm
SIKA-HX-1
Inconel
Ag
90wt%Ni-Fe
36
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depositions of simple substrates, the number of vertical and horizontal cracks is extensive; a significant
percentage of the former type, which occur at a frequency of 5-6 cracks mm-1
, penetrate the multi-layer
coating all the way up to the substrate; the latter type, which often but not always span themselves
between two neighbouring vertical cracks, appear to coincide with an individual layer resulting from a
single, deposition pass. Speculative reasons for the formation of these macro-fissures include: (a)
insufficient substrate cooling at the time of the deposition which results in the coating’s subsequent, rapid,
post-deposition cooling; (b) abrasive, post-deposition handling of the component during its removal from
the cooling apparatus; (c) CTE mismatch between the substrate and deposited layers; and (d) the inherent
vibrations that the robotic arm perpetuates during the deposition.
A Swagelok™, SS316, bottom ferrule was used to make the thermal contact between the
underside of the substrate and the interior cooling apparatus. The ferrule’s thermal conductivity (16.3 W
m-1
K-1
), in combination with its irregular shape, lends credence to (a). Moreover, the obnoxious macro-
fissures are more probably a consequence of reasons (a) and (b) together. Because of copper’s excellent
ductility and high thermal conductivity (386.0 W m-1
K-1
), it is a preferable thermal contact material;
hence, copper foil was used for depositions iv, v, vii, and viii. The remaining aforementioned reasons are
regarded with less confidence but are worthy of mention for future researchers.
Prior to depositing its three SOAE layers, deposition iv’s NFO layer was H2 reduced to its
metallic state. The completed deposition was then exposed to a H2 environment; thereafter, the SEM
micrographs for both the coating’s top (Figs 31(a)-(d)) and cross (Figs 31(e)-(h)) section were taken. Due
to the cross-sectional micrographs’ poor colour resolution it is not possible to ascertain the GDC layer’s
thickness. However, the LSGFM/LSGM and superficial anode deposition rates are 1.04±0.07 and
0.79±0.07 μm loop-1
, respectively. The anode layers sub-micron porosity is visible in Figs 31(e)-(f).
37
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Fig 30 Analyses for deposition iii (Table 5) − (a)-(f) SEM micrographs (cross sections)
LSGFM
(b)
300μm
90wt%Ni-Fe
SIKA-HX-1
GDC
(c)
LSGFM
300μm
90wt%Ni-Fe
SIKA-HX-1
GDC
LSGFM
300μm
90wt%Ni-Fe
SIKA-HX-1
GDC
LSGFM
300μm
90wt%Ni-Fe
SIKA-HX-1
GDC
LSGFM (f)
50μm
Inconel
90wt%Ni-Fe
SIKA-HX-1
GDC
(e)
(d)
LSGFM
(a)
500μm
Inconel
90wt%Ni-Fe
SIKA-HX-1
GDC
LSGM
LSGM
LSGM
LSGM
LSGM
LSGM
38
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o
Fig 31 Analyses for deposition iv (Table 5) − (a)-(d) SEM micrographs (top section); (e)-(h) SEM
micrographs (cross section)
LSGFM (a)
1000μm
Inconel
LSGFM (b)
200μm
LSGFM LSGFM
50μm
200μm
SIKA-HX-1
90wt%Ni-Fe
GDC
(d) (c)
LSGFM
(e)
500μm
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(g)
LSGM
LSGM
(f)
500μm
LSGFM
SIKA-HX-1
90wt%Ni-Fe GDC
Inconel
LSGM
90wt%Ni-Fe GDC
(h)
500μm
LSGFM
SIKA-HX-1
LSGM
39
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Deposition v’s cross-sectional micrographs are shown in Fig. 32. The SOAE layers completely
delaminate themselves from the anode, which remained adhered to the integral metal component. Despite
the addition of the LSGM and GDC SOAE interlayers, the overall result is similar to what occurred in the
case of deposition ii. Tentatively, one may conclude that it is essential to reduce the anode layers prior to
the deposition of the SOAE layers as the volumetric change that the anode layer endures during the
reduction step renders the undesirable result.
Fig. 32 Analyses for deposition v (Table 5) − (a)-(f) SEM micrographs (cross sections)
Deposition vi’s cross-sectional SEM micrographs are shown in Fig 33. As a Swagelok™, SS316,
bottom ferrule was used to make thermal contact between the substrate and the interior cooling apparatus
GDC
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(b)
Inconel
LSGM
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(a)
Inconel
LSGM
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(d)
LSGM
200μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
(e)
Inconel
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(c)
LSGM
LSGM LSGFM
200μm
SIKA-HX-1
90wt%Ni-Fe
GDC
(f)
Inconel
LSGM
40
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(Fig. 4), the deposition’s character resembles deposition iii’s. This is illustrated by the prevalence of
horizontal (6-7 cracks mm-1
) and vertical fissures that are of the particularity previously described.
However, there is good coherence between the anode, cermet, and electrolyte layers; therefore, the cermet
layers are effective in diminishing the interlayer tension that results from materials’ differing CTEs.
Finally, the loss of the electrolyte layers shown in Fig. 33(b) may have occurred while removing the
component from cooling apparatus following the final depositions.
Fig. 33 Analyses for deposition vi (Table 5) − (a)-(f) SEM micrographs (cross section)
With the exception of the use of copper foil as the cooling apparatus-substrate thermal connection
and the number of LSGFM deposition passes, the protocol used for deposition vi and vii were the same;
(a)
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
GDC
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(c)
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(d)
200μm
SIKA-HX-1 90wt%Ni-Fe
LSGFM
GDC
(e)
Inconel
100μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(f)
Inconel
(b)
500μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
LSGM
3NFO7SDC
7NFO3SDC
LSGM
3NFO7SDC
7NFO3SDC
Inconel
LSGM
3NFO7SDC
7NFO3SDC
LSGM
3NFO7SDC
7NFO3SDC
LSGM
3NFO7SDC
7NFO3SDC
LSGM
3NFO7SDC
7NFO3SDC
41
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the latter’s cross-sectional, SEM micrographs, along with its top-sectional XRD pattern, are shown in Fig.
34.
Although present, the incidence and character of the vertical and horizontal cracks was less than
what was found in the previous deposition. This supports the hypothesis that: amid deposition, maximum
substrate cooling increases the uniformity of the coating. The reason for the phenomenon may be because
the transformation of energy that is endured during the relaxation of residual stresses in a “macro-
quench”-i.e. deposited material resulting from several deposition passes-is in excess of what would
otherwise be experienced if the liquefied particles from an single pass are quenched and relieved of the
their residual stresses prior to the deposition of the succeeding layer.
The electrolyte layers are coherent to the anode layer. If compared with the previous depositions,
the addition of the cermet layers is responsible for the positive result. Due to substrate misalignment,
there is considerable variation in the electrolyte layer’s thickness ranges between 33±3 to 90 ± 5 μm,
going from left to right.
Other notable observations include: (I) the sub-micron porosity of the anode layer (Figs.
34(a),(c)-(d)); (II) the LSGFM, perovskite phase was present following the deposition (Fig. 34(h)); (III)
the poor coherence between the porous SIKA-HX-1 and solid Inconel housing at the integrated
substrate’s left side (Figs. 34(a)-(c)); and (IV) the detachment of the SOAE layers at the outer edge (Figs.
34(a),(d)). The latter two observations may be a direct result of the handling of the post-deposition
component. Further efforts are required to resolve minor issues surrounding the custom cooling
apparatus.
42
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Position (°2 theta)
20 30 40 50 60 70 80
Re
lati
ve
In
ten
sit
y
0
5
10
15
20
LSGFM
SrLaGa3O7
(h)
Fig. 34 Analyses for deposition vii (Table 5) − (a-g) SEM micrographs (cross section); (h) XRD pattern
(top section)
LSGM
3NFO7SDC
7NFO3SDC
100μm
SIKA-HX-1 90wt%Ni-Fe
LSGFM
GDC
(c)
Inconel
LSGM
100μm
90wt%Ni-Fe
LSGFM
GDC
(d)
Inconel
100μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(a)
Inconel
LSGM
300μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(b)
Inconel
LSGM
LSGM
3NFO7SDC
7NFO3SDC
LSGM
3NFO7SDC
7NFO3SDC
LSGM
3NFO7SDC
7NFO3SDC
50μm
SIKA-HX-1
90wt%Ni-Fe
LSGFM
GDC
(e)
LSGM
3NFO7SDC
7NFO3SDC
50μm
SIKA-HX-1 90wt%Ni-Fe
LSGFM
GDC
(f)
LSGM
3NFO7SDC
7NFO3SDC
SIKA-HX-1 90wt%Ni-Fe
GDC
(g)
50μm
LSGFM LSGM
3NFO7SDC
7NFO3SDC
(h)
43
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Points worth reiterating from the investigations pertaining to the fabrication of NPT type, ERH anode
compartment via SPS include:
NFO experiences a volumetric change during its H2 reduction to its corresponding sub-micron,
porous metallic form which causes the SOAE layer(s) to delaminate; hence, the deposition of the
electrolyte layers must succeed the reduction of the NFO layer;
degrees to which substrate cooling occurs reflects upon the homogeneity of coatings, particularly
as it pertains to the incidence of both horizontal and vertical cracking -where maximum cooling
amid deposition renders the best result;
diminishing the interlayer CTE mismatch via additional cermet layers mitigates the SOAE
layer(s) from delaminating from the anode;
unlike the optimal depositions in sub-section , crack-free coatings were not achieved during the
course of this optimization;
modifications to the SPS’s cooling apparatus (Fig. 3.4) that will allow for more intensive
substrate cooling during SPS and less abrasive addition and removal of the substrates are
required.
A TSDP coating study that unequivocally demonstrates the outright eradication of the fissuring
phenomena is unknown to the author. This, of course, does not bode well for the experiments that were
supposed to be at the heart of this multidisciplinary study. Only a superficial investigation of the
hypothetical process was possible in the allotted time-frame.
4. Conclusions
Significant gains were made in the field of SPS, multi-layered, ceramic-to-metal deposition
technology. Necessarily, many of the previously stated parameters were significantly modified. This
includes: (a) the suspension’s solvent; (b) the post-tailoring of the GNP-synthesized powders; (c) the
anode’s post-deposition, H2 reduction sequence; (d) the substrate type; (e) the substrate’s cooling amid
deposition; and (f) the substrate holder. Particularly, the undisclosed, deleterious, plasma-substrate arcing
was completely eradicated via the development of an electrically-insulated substrate-holder.
In regards to the TSDP literature pertaining to the deposition of dense, ceramic layers on metal
substrates, the optimized depositions given in 3.3 are of the highest quality known to the author.
Although the results were not fully transposed to the ERH anode compartments that were desired for a
fundamental investigation of a novel electrochemical process, they could instead be considered for
intermediate temperature water electrolysis, IT-SOFC, or other applications. For economic reasons,
Hastelloy X could be substituted by cheaper iron substrates, and air, as opposed to O2, could be
considered as the principal plasma gas.
More generally, because of the increase in the volumetric power density that results when
operating SPS with hydrocarbon solvents and O2 rich plasmas, this sub-family of TSDPes is very
versatile.
Acknowledgements
The financial support received from Kingston Process Metallurgy Inc., AUTO21, and NSERC made this
project possible. Along with F. Gitzhofer, CREPE technicians F. Barrette, S. Gutierrez, K. Béré, B.
Couture, I Kelsey-Léveque, and N. Simard were instrumental in the manifestation of this work.
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