Nanomechanical Properties and Thermal Conductivity Estimation of Plasma-Sprayed, Solid-Oxide Fuel...

Nanomechanical Properties and Thermal ConductivityEstimation of Plasma-Sprayed, Solid-Oxide Fuel CellComponents: Ceria-Doped, Yttria-Stabilized Zirconia Electrolyte

NEELIMA MAHATO,1 SAMIR SHARMA,1,2 ANUP KUMAR KESHRI,3

AMANDA SIMPSON,4 ARVIND AGARWAL,5 and KANTESH BALANI1,6

1.—High Temperature Fuel Cell Laboratory, Department of Materials Science and Engineering,Indian Institute of Technology, Kanpur, Kanpur 208016, India. 2.—Present address: AdvancedEngineering, Ashok Leyland, Chennai 600103, India. 3.—Bharat Heavy Electrical Limited,Hyderabad, India. 4.—Hysitron Inc., 9625 West 76th Street, Minneapolis, MN 55344, USA.5.—Mechanical and Materials Engineering, Florida International University, Miami, FL 33174,USA. 6.—e-mail: [email protected]

Solid-oxide fuel cell components were fabricated using an atmospheric plasmaspraying method. Lanthanum strontium manganite (LSM), 8 mol% yttria-stabilized zirconia (8YSZ), ceria (CeO2), and YSZ-NiO powders were used asfeedstock materials for layered deposition of cathode, electrolyte, and anode,respectively, to make a complete cell. In this work, two types of electrolytematerials were investigated, viz., 8YSZ and the one containing 10 wt.% CeO2.Because a high densification is expected in the solid oxide electrolyte (asopposed to observed porosity of �27%), current work focuses only on thenanomechanical evaluation of the same. Scanning electron microscopy (SEM)images show the retention of nanocrystallinity in the plasma-sprayed depos-its. Elemental analyses via energy-dispersive spectroscopy revealed chemi-cally distinct identities of the cell components ruling out diffusion or reactionat the boundaries. Porosity values vary between 29.0% and 35.4% in anodeand 42.9–48.4% in cathode, indicating appreciable achievement for high per-formance of electrode materials. The addition of 10 wt.% ceria to 8YSZ hasshown enhancement in the elastic modulus and hardness of the electrolytematerial by 18.4 GPa and 1.6 GPa, respectively. Theoretical estimation ofthermal conductivity of the plasma-sprayed materials has been found to be inthe order of 2.27–4.45 W/mK.

INTRODUCTION

Solid-oxide fuel cells (SOFCs) are electrochemicaldevices consisting of solid ceramic cell components,viz., porous electrodes and a dense, gas-tight elec-trolyte in between. These devices convert chemicalenergy of the fuel gases into electricity with negli-gible pollution.1 Recent developments in SOFCtechnology focus on improved power output, lower-ing operational temperature (650–850�C), enhanc-ing the life of a fuel cell stack in terms of reliabilityand durability, and most importantly, reducing thecost of fabrication. Among the most commonly usedSOFC materials, Ni/8-YSZ (8 mol% yttria-stabilizedzirconia), 8YSZ (with and without dopants), and

lanthanum strontium manganite (LSM) are used asanode, electrolyte, and cathode, respectively.

At the anode, the oxidation of fuel gases andproduction of electrons takes place by the virtue ofits chemistry and microstructure. Ni (30–50 vol.%)dispersed in YSZ matrix assists in the conductionmechanism (electrical conductivity2 �4 9 103), car-ries out electrocatalytic activity during the oxida-tion of fuel gases, and it offers low charge transferresistance.3 In the microstructure, an amount of40% porosity is desired to facilitate a fair supply offuel gases, to remove reaction products, and tomaintain triple-phase boundaries (TPBs) among theelectrolyte, electrode, and gas phase.1 Besides theseproperties, phase stability in fuel environment as

JOM

DOI: 10.1007/s11837-013-0601-8� 2013 TMS

well as high operational temperatures and a goodmatch of thermal expansion coefficients between theother cell components is also essential to abolishcracks/fractures and failure. The electrons generatedat the anode travel through an external load and reachthe cathode to combine with oxygen in the air beingpumped in during the operation and to produce oxideions. A functionally sound cathode usually consists ofthree functional layers: electrochemically active lay-ers with fine grain size and good interfacial bonding,diffusion layers with large open porosity (�40%), andcurrent-collecting layers with high electronic conduc-tivity (�89–103 S cm�1).4 In both cases, anodes aswell as cathodes, functionally graded or multilayeredstructures with variations in composition and micro-structure, are judiciously tailored to enhance electro-chemical and mechanical performance. The oxide ionsproduced at cathode traverse through electrolyte toreach anode. The electrolyte, thus, must be highlydense (>98% of the relative theoretical density), gastight (gas leak rate �1 9 10�6 mbar l s�1 cm2 orless),5 and as thin as possible (<10 lm) to reduce thediffusion length of oxide ions and electrolyte resis-tance (resistance is inversely proportional to conduc-tor length). At the same time, it must also be a badconductor for electrons so that maximum number ofelectrons can move across the external load (mini-mized leakage current).6,7

To achieve the desired microstructure withrequired porosity, plasma spray coating is an eco-nomically and efficient technology.8 A completeSOFC is fabricated splat by splat, and coatingparameters can be tailored to achieve functionallygraded layers corresponding to different cell compo-nents, viz., electrodes and electrolyte. Electrolytesfabricated using plasma spray coating usuallyexhibit electrical conductivity of approximately one-fifth to one-third of the corresponding bulk materials.Aruna et al. reported electrical conductivity of plas-ma-sprayed 8YSZ electrolyte to be �0.94 S m�1 at800�C, whereas bulk samples exhibited conductivityof �1.7 S m�1, i.e., only a half value compared withbulk5 owing to porosity and poor bonding between thecoating layers. But at the same time, plasma-sprayedcoatings enable anodes to possess a higher amount oftriple-phase boundaries compared with those fabri-cated by any other technology, and hence, they ex-hibit better performance. It is also reported that thetotal ionic conductivity of plasma-sprayed YSZ elec-trolyte is�2.3 times higher than that of sintered YSZelectrolyte measured at 600�C in air.9

The effective area of contact between splats isapproximately 20%. These factors generate theoverall microstructure of the plasma-sprayed coat-ings, which in turn affects significantly variousproperties of the coatings, viz., electrical, mechani-cal, thermal, etc., and hence, it is distinctly differentcompared to the sprayed bulk material. In terms ofVickers microhardness and electrical conductivity,the plasma-sprayed Ni/YSZ coatings of spray-driedpowders show higher hardness and conductivity

values, viz., �280 kg mm�2 and 651 S cm�1 (elec-trical conductivity measured at 800�C) comparedwith blended powders that exhibit hardness of150 kg mm�2 and conductivity of 254 S cm�1 forsame conditions.10 Even though the conductivityvalue of the latter is less, but still higher than thestandard value of 160 S cm�1 required for SOFCelectrical conductivity.10 The hardness valuedecreases after oxidation (pore former graphiteparticles burn off) from 280 to 100 kg mm�2 andfurther decreases to �55 kg mm�2 after reduction ofNiO to Ni by fuel gases (H2) due to an increase inporosity.10 The hardness as well as wear propertiesare reported to have further improved by usingnanostructured YSZ powders. Upon a decrease ofaverage grain size from 62 nm (of conventional 8%YSZ) to 48 nm (nanostructured 7% YSZ prepared byco precipitation and agglomeration), the hardnessvalues of 7% YSZ increased from 722 (for conven-tional) to 960 kg mm�2 (for nanostructured), andthe wear rate declined from 12.8 9 10�5 (for con-ventional) to 3.3 9 10�5 mm3 N�1 m�1 (nanostruc-tured) under sliding against 100C6 steel.11 Damageaccumulation in SOFC components becomesimportant owing to the generation of thermalstresses during operation, which can be insinuatedby relativistic tribological performance of theadjoining surfaces.

In the current work, two types of SOFCs wereprocessed via layered deposition of electrodes andan electrolyte in between using the plasma spraydeposition method. Spray-dried powders of YSZ andNiO were blended together to be used as feedstockmaterial for the anode coating. Nanoparticles,because of their low mass, pose difficulty while beingcarried by a moving gas stream and cause undesir-able clogging of the nozzle owing to interparticlefriction. Hence, powders are spray dried prior coatingto facilitate consistent powder flow during plasmaspraying.9 Spray-dried powder of LSM was used tofabricate cathode coating. Two types of electrolyteswere investigated, viz., YSZ and YSZ-10 wt.% CeO2

fabricated between electrode layers of different cellsand the effect of CeO2 doping on phase and micro-structural evolution was elicited. It must be notedthat air plasma spraying may not be the optimumprocessing technique for the deposition of electrolyte,as a completely dense electrolyte is required for ac-tual application. For commercial applications, theelectrolyte can be sprayed using vacuum plasmaspraying, where greater than 95% relative theoreti-cal density can be achieved.

Young’s modulus (E) for sprayed coatings signifi-cantly depends on its porosity and stress state,which, in turn, determines the microstructural andthermal properties of the starting powders, spray-ing modes, and processing parameters. For compactmaterials, Young’s modulus is practically indepen-dent of the structure and is determined only by thestrength of atomic bonds. In addition, E values alsodepend upon methods using different indentation

Mahato, Sharma, Keshri, Simpson, Agarwal, and Balani

loads and coating thickness.12 Typically, Young’smodulus values vary from 5 to 100 GPa for 7–8 wt.%YSZ measured by a microindenter (much lower thanmonolithic zirconium dioxide (200 GPa),13 whereasE = 140–150 GPa as measured by a nanoindenter for

YSZ plasma-sprayed coatings.14,15 Further, tribo-logical characterization of all the individual layers ofSOFC was analyzed to evince the interfacial damagethat accrues from the stresses generated duringthermal cycling of SOFC. In plasma-sprayed coating,

Fig. 1. Scanning electron micrographs of initial powder feedstock, viz., spray dried YSZ at low and high magnification (a, b), LSM (c, d),YSZ-CeO2 (e, f), and YSZ-NiO (g, h).

Nanomechanical Properties and Thermal Conductivity Estimation of Plasma-Sprayed, Solid-OxideFuel Cell Components: Ceria-Doped, Yttria-Stabilized Zirconia Electrolyte

most of the pores are perpendicular to the heat fluxand, hence, are expected to provide better insulationproperties. Besides thermal insulation, these poresprevent the thermal and mechanical stress frombuilding up and restrict further propagation. Inaddition to the total porosity, the shape, dimension,orientation, and distribution of the pores also influ-ence the thermal conductivity of the coated layers.The thermal conductivity of 7–8 wt.% YSZ (porosity�3–8%) has been reported to be below 1 W/mK,which is significantly low compared to fully densemonolithic 7YSZ (>2.5 W/mK).16–18 Therefore, the-oretical models are utilized to estimate the thermalconductivity of the coated layers considering theinfluence of pore geometries, dimensions, and align-ment with respect to the operational heat flux.

EXPERIMENTAL DETAILS

Powder Feedstock, and Microstructural andPhase-Characterization of Materials

Nano and spray-dried powders (plasma spraygrade) of 8 mol% YSZ, NiO, LSM, and CeO2 wereprocured from Inframat Advanced Materials (Man-chester, CT). The morphology and particulars of thepowder feedstock and blend proportions used inplasma spray coating are shown in Figs. 1 and 2 andsummarized in Table I. The average particle sizewas determined using laser particle size analyzer(Analysette 22; Fritsch GMBH, Idar-Oberstein,Germany). The microstructural characterization ofinitial materials as well as fabricated cell compo-nents were carried out using scanning electronmicroscopy (SEM; ZEISS EVO 50 operated at 5–20 kV; Carl Zeiss, Oberkochen, Germany) with en-ergy-dispersive spectroscopy (EDS) facility (INCAPenta FETx3; Oxford Instruments, Oxfordshire,UK) and shown in Fig. 2. Phase characterizationwas carried out using x-ray diffraction (XRD; 2000D

diffractometer; Rich Siefert & Co., Ahrensburg,Germany) operated at 25 kV and 15 mA usingCu-Ka (k = 1.541) radiation. The data were recordedat step size of 2�/min. The crystallite sizes were cal-culated using Scherrer’s formula (B(2H) = kk/l cosH), where, B is full width at half maximum, H is theBragg angle, k is the x-ray wavelength, and L is thethickness of the crystallite.

Plasma Spraying of Solid-Oxide Fuel Cell

Two SOFCs were processed via atmosphericplasma spraying of the powder feedstock using SG100 plasma gun (Praxair Surface Technologies,Indianapolis, IN). The three different layers, cath-ode, electrolyte, and anode, were plasma sprayed onAISI 1020 (mild steel substrate) with the dimen-sions 100 9 20 9 3 mm3. A layer of LSM followedwith deposition of YSZ (SOFC 1: without CeO2, andSOFC 2: with 10 wt.% CeO2 electrolyte) as a middlelayer, and YSZ-NiO as the top layer. The cells werecathode supported type. The parameters used inplasma spray coating are listed in Table II.

Physical, Micromechanical, and Nanome-chanical Properties of Processed LSM/YSZ-CeO2/YSZ-NiO Plasma Sprayed Coatings

The coating porosity was determined by digitalimage analysis using ImageJ (National Institutes ofHealth, Bethesda, MD). A cross-sectional surfaceprofile of the two unpolished (leveled on emery clothwithout any polishing media) SOFC coatings wasgenerated by dynamically focusing of a laser beam(infra-red light k = 780 nm) and evaluation of theobjective position (profile) using three-dimensionallaser surface profilometry (Model: PGK 120; MAHR,Gottingen, Germany) with spatial and vertical res-olution of 0.1 lm and 5 nm, respectively. Approxi-mately 20 line profiles of length 400 lm were used(in each region of the coating) for evaluating theaverage surface roughness using MAHR Perthom-eter Concept image analyzing software. Bulk hard-ness was determined via Vicker’s macroindentationusing BAREISS-V-Test (Bareiss PrufgeratebauGmbH, Oberdischingen, Germany). The sampleswere first polished smoothly using micrometer-range diamond pastes, and indentation experimentswere carried out at a load of 0.98 N (100 g). Fiveindentations were made on each sample with adwell time of 10 s. The indent diagonals were mea-sured from the optical micrographs, and hardnessvalues were calculated using the equation:

HV ¼ 1:854P

d2

� �

where d ¼ d1þd2

2 and d is the average diagonal lengthin mm, Hv is the Vickers hardness, and P is theapplied load (in Newton). Nanomechanical proper-ties were investigated via nanoindentation usingHysitron TI900 Triboindenter equipped with a

Fig. 2. Phase characterization of the initial powders used for plasmaspraying.


Ta

ble

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cle

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spra

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rie

dp

ow

ders

(lm

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em

ark

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8m

ol%

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ate

d)

40–60

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ite

pow

der

ofm

olec

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rw

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ol,

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a15–40

m2/g

.D

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the

init

ial

nan

opow

der

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m3

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2(n

an

opow

der

)50–80

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low

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ula

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50

vol

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SZ

,aft

erre

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ctio

n(N

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Ni)

40–60

(YS

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mol%

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an

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wt.

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three-sided Berkovich diamond indenter (Hysitron,Minneapolis, MN). The load–displacement datawere analyzed utilizing the Oliver and Pharr19

method (depth-sensing indentation).

RESULTS AND DISCUSSION

Characterization of Phase and Microstructureof the Fabricated Cell

The SEM of the cross-sectioned samples showsfour distinct regions (Fig. 3) with different contrastscorresponding to anode, electrolyte, cathode, andsubstrate. The layers are flat and the boundariesare uniform after deposition, and there occurred nobending/deformation of either the substrate orSOFC component layers and layers show goodadhesion. This indicates that the plasma spraycoating conditions and parameters were well ad-justed.

Microstructures show the presence of a bimodaltype of grain structure in all the three cell compo-nent layers, i.e., fully melted (FM) surface andpartially melted (PM) surface. This is because ofpoor thermal conductivity of the material, whichrenders the surface particles fully melted and res-olidified, whereas the particle core partially meltsand sinters in a solid state. Image analyses of thelayered deposits show the sufficient amount ofporosity (or low densification) in SOFC 1 and SOFC2 (Tables III and IV), which is necessary to supplyair/oxygen at reaction sites (triple phase boundary).Pores are created by three different mechanisms.Macropores are developed due to the presence ofnonmelted or partially melted particles; microporesand submacropores are formed due to the entrap-ment of gas between the splat layers and by reduc-tion of NiO in anode by the fuel gas (volumeshrinkage, and can also form by graphite burn-off incase when graphite is present in the initial feedstock) during working of SOFC during operation.The reduction of NiO to Ni in the coating also has asignificant effect on the conductivity behavior of thecoating. The conductivity changes from that ofceramic insulator to metallic conductor. Pore form-ers or precursors, such as Na2CO3, could also beadded to generate/enhance porosity.20 Submicron-size pores are distributed throughout the coating.

An increase in the number of these tiny pores en-hances the amount of TPB. The porosity of SOFCanode must be �40% for developing better functionin terms of electrical conductivity. Porosity is animportant factor for improving anode fabrication,

Table II. Plasma-spraying parameters

Parameters Value

Plasma power (kW) 24–32 kWCurrent (amperes) 600–800Voltage (V) 40Stand-off distance (mm) 100Feed rate (g/min) VariablePrimary gas, argon (slm or standard liters per minute) 32Secondary gas, helium (slm) 60Carrier gas, argon (slm) 30

Fig. 3. Scanning electron micrographs of the cross section of thetwo plasma spray coated (a) SOFC 1 and (b) SOFC 2, with com-ponents.


Ta

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eff

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of

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SO

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1Y

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(an

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71.0

174.6

±8.8

YS

Zfi

74

±05

9.3

±1.0

205.6

±6.7

15.8

±0.9

31

±1.3

0.7

0±

0.0

20.1

03

±0.0

04

NiO

fi57

±25

YS

Z(e

lect

roly

te)

80.1

107.9

±8.8

YS

Zfi

50

±20

7.4

±0.9

139.0

±9.3

11.9

±1.4

38.2

±3.1

0.6

8±

0.0

30.1

02

±0.0

07

LS

M(c

ath

ode)

51.6

211.8

±13.5

LS

Mfi

82

±16

5.6

±0.7

188.3

±15.4

15.5

±2.4

31.2

±3.7

0.6

9±

0.0

20.1

28

±0.0

02

Mn

Ofi

88

±05

SO

FC

2Y

SZ

-NiO

(an

ode)

64.6

162.9

±10.8

YS

Zfi

91

±05

10.1

±1.0

197.4

±7.6

15.0

±0.8

32.1

±1.8

0.7

1±

0.0

20.0

95

±0.0

02

NiO

fi81

±30

YS

Z-1

0C

eO2

(ele

ctro

lyte

)73.4

84.4

±9.8

YS

Zfi

53

±13

7.8

±0.9

157.4

±2.7

13.5

±0.5

34.5

±0.8

0.6

6±

0.0

10.1

06

±0.0

08

CeO

2fi

67

±12

LS

M(c

ath

ode)

57.1

215.9

±14.6

LS

Mfi

67

±20

6.3

±0.7

176.7

±6.0

16.8

±0.7

29.9

±1.0

0.6

9±

0.0

10.1

21

±0.0

08

Mn

Ofi

71

±17

Ta

ble

IV.

Est

ima

tio

no

fth

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co

nd

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vit

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(%)

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f 2(l

am

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f 3(s

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k0

(W/m

K)

k(W

/mK

)

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1A

nod

e(N

iO-8

YS

Z)

29.0

0.3

20.1

30.5

55.0

52.2

7E

lect

roly

te(8

YS

Z)

19.9

0.5

70.1

70.2

73.5

02.8

4C

ath

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(LS

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48.4

0.5

80.1

30.2

99.6

04.4

5S

OF

C2

An

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(NiO

-8Y

SZ

)35.4

0.2

90.2

40.4

75.0

52.5

4E

lect

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te(8

YS

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10

wt.

%C

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26.6

0.3

20.2

00.4

83.7

72.6

5C

ath

ode

(LS

M)

42.9

0.4

10.1

40.4

59.6

03.4

2


but at the same time, it affects the bonding strengthof the coating and leads to poor contacts between thephases, and therefore, poor electrical conductivity.SOFC components fabricated by plasma spraycoating exhibit lower relative theoretical density(73–84%) compared to those processed by sinteringmethods, e.g., spark plasma sintering (94–98%)technique employed by our group for similar mate-rials and composition.21 It must be mentioned herethat air plasma spraying is not ideal for the depo-sition of electrolyte, as an ideal SOFC would requirea very dense (near to complete theoretical density)electrolyte. In actual application, vacuum plasmaspraying may be utilized for the deposition of elec-trolyte. Current work focuses only on the nanome-chanical performance and estimates the theoreticalthermal conductivity of the air plasma processedcomposite layers in SOFC.

Spray-dried agglomerates of 8 mol% YSZ seemperfectly spherical, and the presence of YSZ can beidentified by two major characteristic peaks at30.11�, 50.18� and 59.65�, while the CeO2 charac-teristic peak matches well at 28.56�, 47.51�, and56.38�. The major characterization peaks for NiOand LSM are 37.28�, 43.32�, 62.9�, and 32.52�,46.83�, and 58.15�, respectively. No other peakswere found in the XRD pattern for the initial pow-ders. The XRD patterns confirm that zirconia was infully stabilized form. No peak shifts of the majorpeaks of YSZ and CeO2 phase have been detected.

The XRD results reveal the presence of nano-crystallites of different phases (in size range of 40–80 nm). Nanocrystalline YSZ electrolytes have beenreported to exhibit enhanced electrical conductivityof �2–3 orders of magnitude higher as compared tomicrocrystalline specimen.22

Magnified images of the pores show denselysprayed splats and many globular structures thatare remnants of unmolten or partially molten pow-der particles embedded at the floor of the pores(Fig. 4). Anode and cathode materials are the samefor both the SOFCs (Fig. 4a–d), whereas only elec-trolyte is different. In SOFC 1 (Fig. 4e, f), electrolyteis 8YSZ, whereas in SOFC 2 (Fig. 4g, h), electrolyteis 10 wt.% CeO2 reinforced 8YSZ. The addition ofceria in 8YSZ does not exhibit any apparent changein the microstructure and proportion of partiallyand fully melted regions. Only a few pores/microp-ores at the floor of the pore are visible, and thesurface elicits a compact appearance ruling out anypreexisting cracks. Interfacial bonding betweencathode and electrolyte layer shows a discontinuouslayer in SOFC 1, whereas electrolyte coating ofSOFC 2 is completely intact with cathode materialshowing good adhesion (Fig. 3). The absence of thediscontinuity in SOFC 2 suggests that the additionof CeO2 improves adhesion with the cathode. It isalso possible that the loss of material could haveoccurred at the interface during cross sectioningand polishing of the deposited layers. The densifi-cation of the electrolyte layers, as indicated earlier,

can be enhanced via utilization of other depositiontechniques (such as vacuum plasma spraying) toachieve enhanced solid-oxide fuel cell performance.

EDS and elemental mapping of the coated cellcomponent layers (for SOFC 1 and SOFC 2 inFigs. 5, 6, respectively) reveal chemically distinctidentities of the cell components with clean bound-aries ruling out diffusion or reactions at theboundaries. The interface between the layers is thinand continuous for both SOFC 1 (Fig. 5a) and SOFC2 (Fig. 6a). So after efficiently sustaining the ther-mal shocks during the processing, an essentialrequirement of SOFC is, therefore, met here. Addi-tionally, the net chemical analysis of each layers ofSOFC 1 (Fig. 5b1/2/3) and SOFC 2 (Fig. 6b1/2/3)indicates an absence of any diffusion between thelayers during deposition. Because all componentsare basically oxides, the distribution of oxygen(Figs. 5c1, 6c1) is uniform throughout with some-what greater concentration in the middle electrolytelayer, which looks darker due to its greater density.Cathode, electrolyte, and anode regions exhibituniform distribution of Mn, Ni, Zr, La, and Y(Fig. 5c2/3/4/5/6) in SOFC 1 and that of Mn, Ni, Zr,La, Ce, and Y (Fig. 6c2/3/4/5/6/7) in SOFC 2. Dis-tribution of yttrium and zirconium is observed inboth anode and electrolyte regions (Figs. 5c4/6 inSOFC 1 and 6c4/7 in SOFC 2). The appearance ofcerium in the electrolyte part of SOFC 2 (Fig. 6c6) isdiffused and not distinct, which might be due to lowconcentration (10 wt.%), but it is well detected inthe point EDS shown in Fig. 6b2. In the SEM imageof SOFC cross section (Fig. 5a), a discontinuity isalso observed (also see Fig. 4a/c). It is possible thatdiscontinuity might have occurred during cutting/polishing of the sample prior to imaging or evenearlier during processing due to the difference inthermal conductivity, which is explained in the lastsection. Such a crack is not present in the micro-structure of SOFC 2 (Fig. 6a). The latter showsbetter adhesion of the component layers, whichhelps in improving the life of the cell and enhancesits performance.

The presence of major characteristic peaks in thecoated layers validates the presence of undisturbedYSZ (Fig. 7). But the CeO2 peaks are missing, whichindicates that CeO2 is forming a solid solution withYSZ and goes into the YSZ matrix. The majorcharacterization peaks of YSZ have been shiftedfrom its standard position, which may arise due tothe stress developed in the YSZ lattice. This peakshifts also indicate the formation of CeO2-YSZ solidsolution. The XRD results show the presence ofsome different phases, i.e., MnO2 in LSM cathode,which shows the decomposition of perovskite phases(Fig. 7). This is not desirable because it reduces thecatalytic property of the cathode in SOFC for thereduction of oxygen.

Peak broadening observed in the coated samplesrelative to the initial powder suggested that particlesize reduces during plasma spray deposition. This is


supposed to help improve the performance of thecell. The XRD pattern of anode coating shows themain composition NiO and YSZ, and a small

amount of metallic Ni is formed by the reduction ofNiO during spraying. The phase composition anal-ysis reveals that nanocrystallinity was achieved

Fig. 4. Scanning electron micrographs of different coating layer interfaces of SOFC 1, viz., cathode–substrate interlayer and pore interior at highmagnification (a, b); electrolyte–cathode interface and pore interior (c, d); anode–electrolyte interface and pore interior (e, f); electrolyte–cathodeinterface and pore interior of SOFC 2 (g, h). Addition of ceria to 8YSZ in the electrolyte of SOFC 2 does not seem to have imparted any apparentchange in the microstructure and proportion of PM and FM regions.


after plasma spraying and all the peaks associatedwith prime YSZ powder are apparent in the fabri-cated cell. This signifies that the main phase andchemical compositions are retaining their respectiveindividual identities and no undesirable secondphase is observed.

Mechanical Properties of Solid-Oxide FuelCells

The hardness of different coating layers wasmeasured by Vickers indentation method on crosssections of the cells. An applied load of 100 g wastaken toward the lower end to avoid the cracking at

the edges of the indent utilizing all the energy todeform the material. The results of bulk hardnessand nanoindentation are summarized in Table III.A minimal improvement is observed in themechanical properties of the SOFC2 as most of theerror values are overlapping. This slight improve-ment cannot directly be correlated with the densi-fication of the different layers. In case of electrolyte,it is observed that CeO2 addition decreases thedensification by �8.4%, whereas the hardnessincreases from 7.41 to 7.79 GPa. A similar trend isobserved with anode that shows an enhancement of�9%. However, in case of cathode of SOFC 2, withan increase in densification, hardness enhances by

Fig. 5. (a) SEM image, and EDS maps of (b1) anode, (b2) electrolyte, and (b3) cathode, and elemental mapping with (c1) O, (c2) Mn, (c3) Ni,(c4) Zr, (c5) La, and (c6) Y in plasma-sprayed SOFC 1 coating. Elemental mapping shows uniform distribution of elements in the respectivelayers (anode, electrolyte, and cathode) as well as sharp and clean boundaries ruling out any possibility of reaction between the componentmaterials at the interface.


Fig. 6. (a) SEM image and EDS maps of (b1) anode, (b2) electrolyte, and (b3) cathode, and elemental mapping with (c1) O, (c2) Mn, (c3) Ni, (c4)Zr, (c5) La, and (c6) Ce, and (c7) Y in plasma sprayed SOFC 2 coating. Elemental mapping shows uniform distribution of elements in therespective layers (anode, electrolyte, and cathode) as well as sharp and clean boundaries ruling out any possibility of reaction between thecomponent materials at the interface.


�12%. A higher hardness value implies good coatingquality in SOFC 2. Coatings prepared from spray-dried powders have been reported to exhibit greaterhardness values than those made of blended pow-ders.10 This is attributed to the larger size of thespray-dried powder compared with the blendedpowders. Therefore, the transfer of heat flux is alsogreater. On the other hand, in case of blendedpowders, owing to low dwell time, higher particlesize, irregular morphology, and density differenceamong the phases, the heat flux cannot penetratethe core, causing a large deviation in hardnessmeasurement in the resultant coating.10 In the caseof spray-dried powders, the deviation is less. Theaddition of 10 wt.% ceria to 8YSZ enhances themodulus and hardness of the electrolyte material by18.4 GPa and 1.6 GPa, respectively. SOFC 2 showsa smooth transition in the reduced modulus values.This reduces the probability of failure of the cell atthe operating temperature. SOFC 1 shows a drasticvariation in the elastic modulus values for the an-ode, electrolyte, and cathode part. The hardnessvalues obtained from nanoindentation experimentsare 1.6–2.7 times higher than bulk. One plausiblereason is the difference between the two techniques.Vickers’ hardness elicits the resistance to plasticdeformation in the bulk of the plasma coated layers,i.e., splats. The splat length and thickness rangefrom 12.6 lm to 36.9 lm and 4.2 lm to 7.6 lm,respectively. Thus, the area impacted under Vick-ers’ indenter covers multiple splats on the surface. Asingle splat layer as well as successive layers maycontain fully melted, partially melted, or sinteredregions, which have different hardness values.Moreover, there are currently many pores of dif-ferent geometries that tend to lower the hardness ofthe material. On the other hand, the Berkovichindenter impacting a nanoregion on the splat meetsthe nanocrystallites present in the splat (as

suggested by XRD results, Fig. 7), and hence, themeasured hardness values are higher.

Coefficient of friction measurements exhibit noapparent change on adding 10 wt.% CeO2 in YSZelectrolyte and show better adherence between thecoated layers. Higher surface roughness is beneficialin the case of electrodes as it increases the reactionarea and performance of the SOFC cell. On the otherhand, surface roughness of the electrolyte should below in order to reduce the leakage and improve theionic conductivity. The average roughness determinedfor SOFC 1 is 99.8 ± 68.5 lm, 17.7 ± 9.3 lm, and20.9 ± 8.5 lm, respectively, for cathode, electrolyte,and anode. Similarly, average roughness values weredetermined for SOFC 2 as 103.2 ± 73.9 lm, 5.6 ±2.7 lm, and 31.4 ± 20.7 lm, respectively, for cathode,electrolyte, and anode. These observations suggestthat the average roughness remains similar for thecathode, anode, and electrolyte regions, but it can alsobe observed that ceria addition has resulted in asmoother surface (with a lower standard deviation).

Theoretical Estimation of Thermal Conduc-tivity

The SOFC cell fabricated by the atmosphericplasma spray method bears pores of more than onetype. During the process, the spray-dried powdersare exposed to plasma and get melted and sprayedwith a momentum against a flat substrate. Thespherical molten particles splat on the substrate andflatten to make a deposition. Numerous subsequentsplats make a, overall coating of a certain definedthickness. After adhering to the surface, the tem-perature of the matrix tends to decrease and shrink alittle. On such cooling splats when another big splathits with a momentum, it either gets glued with theformer or sits as a separate splat. However, strictbinding among subsequent splats causes mechanicalstress and may generate a discontinuity or delami-nation of layer as observed in Fig. 4c. Certain poresare observed to distribute randomly, whereas certainlocations show lamellar porosity or even cylindricalmorphology (cylindrical pores). Under operationalconditions at an elevated temperature, these createnonhomogeneous temperature distributions andlarge internal stresses inside the cell componentmatrix, which may generate bigger cracks andmaterial failure. Therefore, it is very important toconsider the matching between the coefficient ofthermal expansion and thermal conductivitybetween materials of the cell components. Duringthermal cycles, phase transition or corrosion phe-nomena of both metallic bound coat and the substratemust not occur. An increase in the temperature mayalso lead to a progressive destabilization of the coat-ing material. To minimize the internal stress, thethermal diffusivity of the material has to be kept aslow as possible.23 In plasma-sprayed coatings,it depends on the microstructural characteristics,viz., grain size, morphology, porosity, and phase

Fig. 7. Phase characterization of the two plasma-sprayed SOFCcoatings.


composition. The relation between thermal conduc-tivity k and thermal diffusivity is given byk Tð Þ ¼ a Tð Þ� Cp Tð Þ�qB, where, a(T) is thermal dif-fusivity, Cp(T) is specific heat and qB is the bulkdensity of the coating.24 Both thermal conductivityand diffusivity decrease in a similar fashion with theincrease in the content of lamellar and cylindricalpores oriented perpendicular the thermal flux,whereas in the case of randomly oriented cylindersand spheres, there lies little difference in the trend(Figs. 10 and 11 of Ref. 23). In plasma-sprayed coat-ings, besides grain size, morphology, porosity, phasecomposition, etc., thermal conductivity also dependson cell parameters. Stack design and operating con-ditions, such as cell geometry and operating voltage,influence temperature distribution in a very compli-cated way. To estimate thermal conductivity, themicrostructural properties are taken into consider-ation, viz., percentage, type shape, and orientation ofporosities. Inside the pores, there exists a radiativemode of thermal conduction, and hence, its contri-bution to the overall thermal conduction of the coatedmaterial is negligible. Since plasma-sprayed coatingpossesses more than one type of porosities, viz.,spheroid (spherical, oblate, and prolate), lamellar,and cylindrical, all these are to be taken into consid-eration. Since it was very difficult to classify theporosities into different categories form the micro-structural images, a stereological analysis of thebinary images of real material cross sections wasemployed to calculate porosity. Among the differentmodels available in the literature,16,23,25 the oneincorporating an iterative approach to extend thealready existing models for a material possessingsingle type of porosity (spherical or lamellar) to thematerials containing many types of porosities(spherical, lamellar, and cylindrical) is taken in thisarticle. This model is a hybrid between symmetrical(for spheroid pores) and asymmetrical (for lamellaeand cylindrical pores) approaches. An approximationthat most of the pores are aligned perpendicular tothe heat flux is taken in this investigation. Theexpression giving the final thermal conductivity k ofthe plasma-sprayed coating with three types ofporosities existing in the coating is16:

k ¼ k0

6fU f2

1� f1 þ f3ð Þð Þ

� �W

f1

1� f3ð Þ

� �H f3ð Þ

þWf1


� �U

f2

1� f 3ð Þ

� �H f3ð Þ

þ Uf2


� �H

f3

1� f1

� �W f1ð Þ

þWf1

1� f2 � f3ð Þð Þ

� �H

f3

1� f2ð Þ

� �U f2ð Þ

þHf3


� �U

f2

1� f1ð Þ

� �W f1ð Þ

þHf3


� �W

f1

1� f2ð Þ

� �U f2ð Þg

where W(f), U(f), and H(f) are the functionsdescribing the effects of the three different types ofporosities on the thermal conductivity of the matrix.W, H, and U can be defined in terms of morphol-ogy of the porosity; e.g., U(f) = (1 � f)X, and

X ¼ 1�cos2 a1�F þ cos2 a

2F . Here, F is the shape factor of the

spheroid and a is the angle between the revolutionaxis of the spheroid and the nonperturbed heat flux.For sphere (a = c), F is 1/3, for oblate spheroids(c > a), it varies between 0 and 1/3. For prolatespheroids (a > c), it is between 1/3 and 1/2. In thisarticle, X values are taken for spheroid, lamellar,and cylindrical pores as 3/2, 1, and 2, respectively, fororientation perpendicular to the heat flux.16 Thethermal conductivity (k0) of crystalline and undopedmatrix without porosity is taken from the literatureand is calculated using rule of mixtures.26–30 Per-centages of the types of porosities, viz., cylindrical,lamellar, and spheroid, are denoted as f1, f2, and f3,respectively. The results of the calculation have beentabulated (Table IV). The estimated thermal con-ductivities are found to be very low, influenced sig-nificantly by pores and cracks, and they seem to befavorable. The overall structural and mechanicalproperties of the plasma-sprayed coatings are basi-cally dependent on the particle size distribution andmaterial properties of the starting powders. Besidesthese spraying modes, coating parameters also playkey role in achieving the overall desired properties ofthe fabricated coatings. It is, therefore, essential toperform a comparative and systematic study andanalysis of the coating obtained with differentstructures using finer starting powders with thesame chemical composition but with different physi-cal and technical properties. This would bring clearerinsight to achieve better adherence and anchoragebetween successive splats, different layers of thecoating and control optimum porosity, bettermechanical properties, and crack resistance at thesame time. If suspension plasma spraying is incor-porated to fabricate the electrolyte in combinationwith atmospheric plasma spraying employed to fab-ricate the two porous electrodes, then it would helpachieving highly densified coating. Young’s modulusis expected to increase significantly provided thatheat treatment is given to the coated layers. A con-siderably high porosity level (30–45%) of the coatingcreates a strong barrier against heat flux and protectsthe material from sintering and densification. Thetheoretical model for estimation of thermal conduc-tivity used in this article considers porosity only andnot the scattering effects by the sintered, semimol-ten, or partially molten agglomerated particlesembedded in the coated microstructure (Fig. 4).Thus, it is very important to analyze the micro-structures of the coated materials at various angles.There is scope for the development of thermome-chanical models of the fabricated coating layers andsimulation of their behavior at various temperatures.This, in turn, requires reliable values for the thermal


and mechanical properties of the materials at differ-ent elevated temperatures.

CONCLUSIONS

The plasma-sprayed coating layers of the two typesof SOFCs show distinct identities in terms of elemen-tal distribution, phase, and microstructure signifyingthe attainment of optimal processing parameters. Theprocessed SOFCs have been shown to retain thenanocrystallinity and well exhibit appreciable hard-ness values of the coated layers. Addition of 10 wt.%ceria to 8-YSZ in SOFC 2 marginally enhanced thehardness and exhibited a smooth transition in thereduced elastic modulus. Apparently, no change isobserved in the roughness or coefficient of frictionafter the addition of 10 wt.% CeO2 in YSZ electrolyte.SOFC 2 also exhibited good adherence between thecoated layers compared with that of SOFC 1. Besidesthis, the addition of ceria in SOFC 2 electrolyte is alsobound to enhance the oxide ion conductivity, i.e.,electrolyte efficiency. Although air plasma spraying isnot an optimal technique for deposition of dense elec-trolyte, this work has analyzed the nanomechanicalbehavior of deposited SOFC composite layers. Bulkhardness values are lower (5.6–10 GPa) comparedwith the hardness values obtained from nanoinden-tation results (139–205 GPa), indicating a contribu-tion of randomly scattered pores in the coating.Thermal conductivity estimations ranged between2.27 W/mK and 4.45 W/mK, which are lower whencompared with their bulk counterparts (3.5–9.6 W/mK). The thermal conductivity values for SOFC 2 arecomparatively lower than SOFC 1, suggesting higherresistance to thermal damage during operation. Aquantitative estimation of the thermal conductivity ofthe coated layersusing poregeometries (viz., spheroid,cylindrical and lamellar), along with inclusion of totalporosity content, renders a realistic contribution of thepore types on thermal conductivity.

ACKNOWLEDGEMENTS

K.B. acknowledges funding from Department ofScience and Technology, Ministry of Human ResourceDevelopment, Government of India, and CARE grant,Indian Institute of Technology Kanpur, India.

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Nanomechanical Properties and Thermal Conductivity Estimation of Plasma-Sprayed, Solid-Oxide Fuel...

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