Precursor Dependent Microstructure Evolution and Nonstoichiometryin Nanostructured Cerium Oxide Coatings Using the Solution Precursor

Plasma Spray Technique

Virendra Singh,z Ajay Karakoti,z Amit Kumar,z Abhishek Saha,z Saptarshi Basu,y and Sudipta Sealw,z

zPlasma Nanomanufacturing Facility, Advanced Materials Processing and Analysis Center, Mechanical MaterialsAerospace Eng, Nanoscience and Technology Center Eng 381, University of Central Florida, Orlando, Florida 32816

yDepartment of Mechanical Engineering, Indian Institute of Science, Bangalore-560012

A solution precursor plasma spray (SPPS) technique has beenused for direct deposition of cerium oxide nanoparticles (CNPs)from various cerium salt solutions as precursors. Solution pre-cursors were injected into the hot zone of a plasma plume todeposit CNP coatings. A numerical study of the droplet injectionmodel has been employed for microstructure development duringSPPS. The decomposition of each precursor to cerium oxidewas analyzed by thermogravimetric-differential thermal analy-sis and validated by thermodynamic calculations. The presenceof the cerium oxide phase in the coatings was confirmed byX-ray diffraction studies. Transmission electron microscopystudies confirmed nanocrystalline (grain size o14 nm) charac-teristic of the coatings. X-ray photoelectron spectroscopystudies indicated the presence of a high concentration ofCe

31(up to 0.32) in the coating prepared by SPPS. The pro-

cessing and microstructure evolution of cerium oxide coatingswith high nonstoichiometry are reported.

I. Introduction

NANOCRYSTALLINE ceria has versatile properties for varioustechnological applications, such as, in (1) intermediate tem-

perature solid oxide fuel cells (IT-SOFC) as an electrolyte,1 (2)catalysis as a three way catalyst,2,3 (3) biomedical science as afree radical scavenger,4 (4) chemical mechanical planarization,5

(5) oxygen sensors,6 (6) high temperature oxidation resistance ofsteel,7 etc. These applications arise from the relative ease withwhich nanocerium oxide can manipulate oxygen vacancies in thelattice structure.8,9 It was indicated that these applications arefacilitated by high Ce31 concentration in cerium oxide nano-particles (CNPs) which results from the high-surface non-stoichiometry.10 Various synthesis technique have beenemployed in order to maintain high-surface nonstoichiometry(high Ce31) such as, sol–gel,11 hydrothermal,12,13 coprecipita-tion,14,15 combustion synthesis,16 microemulsion,8 spray pyroly-sis,17,18 and thermal spray.19

In this article, we report the processing and microstructureevolution of cerium oxide coatings with high nonstoichiometryusing the solution precursor plasma spray technique (SPPS).20

SPPS offers unique advantages on account of its ability toachieve considerably higher deposition rates than CVD andPVD methods and yet retain the nanostructures in the coatings.In the SPPS process, coatings are being fabricated in a singlestep by injecting a molecularly mixed precursor solution of

desired coating constituents into a plasma jet. The precursorchemistry can be changed to obtain desired chemical changes inthe thin films while obtaining a dense adherent coating. The ca-pabilities of this process have been demonstrated successfully toproduce thermal barrier coatings for turbines,21 electrolytes forSOFC,22 and materials for photo catalytic applications.23

Furthermore, numerous theoretical studies have been performedto understand the in flight thermophysical phenomena.24 It wasshown that the injected droplets of the solution undergo variousthermophysical treatments such as evaporation, precipitation,thermal decomposition, melting, and acceleration to the sub-strate followed by deposition on the substrate in the form ofsplats. The microstructure of SPPS coatings can be changed bycontrolling the size of the injected droplet. In the present article,a numerical study was also conducted for each precursor tosimulate the droplet injection process for microstructure evolu-tion. The characteristics of the coating, prepared by SPPS, aregreatly influenced by the precursor chemistry. Therefore, priorknowledge of chemical decomposition of precursor salts isessential for obtaining high-quality coatings. Thermogravimet-ric-differential thermal analysis (TG-DTA) studies of theprecursors have been carried out to determine the intermediatephase and decomposition behavior. Detailed phase analysis andmicrostructural characterization of the coatings were evaluatedby X-ray diffraction (XRD), scanning electron microscopy(SEM), and transmission electron microscopy (TEM), respec-tively. The role of precursor chemistry and decomposition onthe oxidation state of the final coating chemistry was studiedusing X-ray photoelectron spectroscopy (XPS).

II. Experimental Procedure

(1) Synthesis of Precursor Solutions

Three different precursor solutions were prepared by dissolvingcerium nitrate hexahydrate (Ce(NO3)3 � 6H2OB99%), ceriumsulfate tetrahydrate (Ce(SO4)2 � 4H2OB99%), and cerium ace-tate sesquihydrate (Ce(CH3CO2)3 � 1.5H2OB99.9%) in 1.5 L ofDI water at a molar concentration of (0.11M).

(2) SPPS

The direct current air plasma spraying process was used fordeposition of the oxides. Each precursor solution was injectedin the transverse direction through an air blast atomizer nozzlelocated in front of the plasma spray gun, (Model SG 100 PraxairTAFA, Concord, NH). The distance between the injection ofthe solution and the face of the gun was 13.0 mm, and theinjection height from the core of the plasma was 8.0 mm. Theseparameters were kept constant for each coating. Argon was usedas a primary gas, and hydrogen was used as a secondary gas toincrease the enthalpy of the plasma plume. Unlike the powderspray, in SPPS the liquid solution is injected in the form ofmicrometer size (1–100 mm) droplets into the plasma plume. The

N. Padture—contributing editor

This work was financially supported by the NSF DMII: 0500268 and Office of NavalResearch Young Investigator Award program ONR: N000140210591.

wAuthor to whom correspondence should be addressed. e-mail: sseal@mail.ucf.edu

Manuscript No. 27465. Received January 27, 2010; approved June 8, 2010.

Journal

J. Am. Ceram. Soc., 93 [11] 3700–3708 (2010)

DOI: 10.1111/j.1551-2916.2010.03985.x

r 2010 The American Ceramic Society

3700

solution drop experiences various thermodynamic, chemical,and mechanical changes. The grit blasted 316-stainless steelsubstrate was preheated to 2501C using a plasma spray gunfor better adhesion of the coating. The coating substrate waskept at a distance of 55 mm from the gun. The plasma sprayparameters, atomizer pressure, and flow rates used in this studyare listed in Table I.

(3) Microstructure and Phase Identification

The microstructure of the coating was characterized using SEM(Carl Zeiss ultra 55, Germany). To evaluate the nanocrystallinecharacteristics of the coating, the TEM samples were preparedby stripping off portions of the coating from the substrate andgrinding it using a mortar and pestle. The grounded powderswere ultrasonically dispersed in ethanol, and TEM samples wereobtained by taking a few drops of the suspension on the holeycarbon coated copper grids and dried in a vacuum chamber forhigh resolution TEM analysis (Philips Tecnai F30, FEI Com-pany, Hillsboro, OR). Phase identification was performed usingXRD (Rigaku D-Max B diffractometer, Japan). XRD patternswere recorded for each coated surface using CuKa radiation(lB1.54 A) in the range of 201–901 with a scanning speed of0.51/min.

(4) Precursor Characterization

The 100 mL of each precursor solution was dried at 1051C for10 h. Simultaneous DTA and TG were carried out on driedprecursor solutions using TG-DTA analyzer (SDT Q600, TAInstruments Inc., Newcastle, DE) in flowing air (Dry air; Airgas,FL) (100 mL/min) at a heating rate of 101C/min. The simulta-neous measurement of weight change and heat flow was re-corded up to 8001C for nitrate and acetate precursors and up to10001C for sulfate precursors. The onset decomposition tem-perature was determined by Tangent (ascending peak slop lineintersection with the base line) method using inbuilt instrumentsoftware. The weight loss calculation in the corresponding tem-perature range was carried out to determine decomposition re-action. The area of the exothermic/endothermic peak is thechange in enthalpy calculated by performing peak integration.Linear base line was selected for numerical integration.

(5) X-Ray Photoelectron Spectroscopy of the Coating

XPS experiments were conducted using a Physical Electronics(PHI 5400 ESCA) spectrometer with an MgKa X-ray sourceoperated at 300 W and 15 kV. The XPS analysis was done atbase pressure typically below 5� 10�8 torr. The exposure timeof the sample was minimized (1-cycle and 10 sweeps) to avoidthe radiation-induced change in oxidation state of cerium.

(6) Numerical Modeling

To validate the effect of different precursor on the microstruc-ture development during plasma spraying, a numerical study of

droplet injection model has been employed. This model uses a2D plasma field as a continuous phase and calculates heat andmass transfer between a single stream of droplets and the plasmafield. The commercially available software ‘‘Fluent 6.3’’ wasused in this study. The interaction between the droplet and theplasma field has been simulated by using a ‘‘Discrete PhaseModel (DPM).’’ DPM is a method of solving interaction (en-ergy, momentum, and mass) between droplets/particles with asurrounding gas phase. DPM incorporates the effect of turbu-lence on the droplet trajectory using either a stochastic trackingmodel or particle cloud model.25,26 The properties for plasmaand different precursors were adopted from available litera-ture.27,28

III. Results and Discussion

(1) Phase Evaluation of SPPS Cerium Oxide Coating

The XRD pattern of each coating prepared by SPPS is presentedin Fig. 1. The coating prepared by cerium nitrate hexahydrateand cerium acetate sesquihydrate precursor exhibit the FCCfluorite structure, confirming the complete decomposition ofthe precursor solution to cerium oxide. The XRD pattern of thecoating prepared by cerium sulfate tetra hydrate also revealsthe presence of cerium oxide along with the presence ofintermediate decomposed sulfate phases. The TG-DTA analy-sis of the precursors indicates that complete decompositionof the sulfate precursor occurs at 8301C as compared with theacetate (B3301C) and nitrate (B3301C) precursors. As men-tioned earlier, during the plasma spray process of solution,droplets undergo various thermochemical stages before deposi-tion on the substrate along with unpyrolyzed solution droplets.The unpyrolyzed solution undergoes in situ decomposition onthe substrate due to the heat from the plasma plume. It isimportant to mention that in the SPPS process, the spraydistance used was 55 mm and the substrate temperature was4501–5501C. During the spray process this temperature issufficient to decompose the entrapped unpyrolyzed nitrate andacetate precursors in the coating but it is insufficient to pyrolyzethe entrapped sulfate precursor. The nitrate and acetate precur-sors decompose completely in to cerium oxide at 3301C as nofurther weight loss was observed, was confirmed by TG analysis(Section III(4)). As the decomposition temperature of thecerium sulfate precursor is significantly higher than the

Table I. Solution Precursor Plasma Spray Parameters forCerium Oxide Coating

Parameter Value

Solution injection rate (mL/min) 20Primary gas (Ar) flow rate (s.l.m.) 48Secondary gas (H2) flow rate (s.l.m.) 8Arc voltage (V) 50Arc current (A) 800Stand of distance (mm) 55Nozzle internal diameter (mm) 7.5Injection type TransverseInjection position (mm)From the torch axis 13From the nozzle exit 8Atomization pressure (psi) 15

Fig. 1. X-ray diffraction (XRD) patterns of the solution precursorplasma spray cerium oxide coating prepared by different precursorsviz (a) cerium acetate (b) cerium nitrate hexahydrate (c) cerium sulfatetetrahydrate. XRD patterns indicate typical fluorite crystal structure ofcerium oxide. �The presence of oxysulfate phases in coating prepared bycerium sulfate tetrahydrate precursor.

November 2010 Microstructure Evolution and Nonstoichiometry in SPPS Cerium Oxide Coating 3701

substrate temperature achieved during the coating, the completedecomposition of unpyrolized cerium sulfate salt that reachesthe substrate does not take place that results in retainingthe intermediate sulfur compounds indicated by the TG-DTA(Section 4(III)C and XRD pattern (Fig. 1(c))).

(2) Microstructural Characterization of as Sprayed SPPSCoatings

Unlike the plasma spray coating of the powders, the micro-structure of the SPPS coatings differs in splat size and degree ofmelting. Smaller splats (1–2 mm) and various melting productsare the characteristic features of the SPPS coatings. In the SPPSprocess, coatings may consist of various products resulting fromthe thermophysical interaction of solution droplets and plasmaflame besides splats such as unmelted spherical particles, brokenshells, unpyrolyzed solution, etc.25 The surface morphologiesof each coating sprayed by various precursors are shown inFigs. 2(a)–(c). Each micrograph reveals porosity, agglomerationof sintered particles, and a high degree of roughness in thecoating. This type of microstructure can be attributed to the

combined deposition of ‘‘solution and plasma flame interaction’’products and in situ decomposition/sintering of unpyrolyzedsolution at substrate. The SEM micrographs do not show anyremarkable changes in microstructure and surface morphologyof the coating with change in solution chemistry. These aspectsof microstructure development have been further studied usingnumerical modeling of droplet injection in Section III(5). Fur-thermore, HRTEM images depict the nanometric characteristicsof the coating. Grain size o14 nm were retained in all the coat-ing (see Fig. 3). The SAED confirm the polycrystalline nature ofthe coatings (Figs. 3(a)–(c), inset). Spherical nanograin geometryhas been observed in the coating prepared by usingCe(SO4)2 � 4H2O as precursor, whereas irregular nanograinswere observed in the coatings prepared by the other twoprecursors. Similar nanocharacteristics of the coating havebeen observed by various researchers for titanium oxide coat-ing (grain size o20 nm)23 and yttria-stabilized zirconia SPPScoating (grain size o100 nm).29 In the SPPS process, the atom-ized solution droplets undergo an aerodynamic break up in totiny droplets during the interaction with the plasma jet.30,31

These droplets further breakup until volume precipitation take

Fig. 2. Surface morphology of solution precursor plasma spray cerium oxide coating prepared by (a) cerium acetate (b) cerium nitrate hexahydrate (c)cerium sulfate precursors. Insets reveal the magnified images of smaller sections of the corresponding coating. (d) Magnified image indicates splatformation (1–2 mm) along with fine unmelted particles.

Fig. 3. HRTEM images of solution precursor plasma spray coating prepared by by (a) cerium acetate (b) cerium nitrate hexahydrate (c) cerium sulfateprecursors indicating that coating consists of nanoparticles of cerium oxide. Insets show the low magnification TEM images, fast Fourier transform oflattice and SAED patterns.

3702 Journal of the American Ceramic Society—Singh et al. Vol. 93, No. 11

place. Saha et al.27 defined a critical drop size, below which alldroplets undergo volumetric precipitation followed by meltingand deposition. Droplets larger than the critical size can frag-ment into daughter droplets. The volume precipitation occursfor the small droplets, whereas shell formation may be one ofthe possibilities depending on the heating rate of the dropletsand nature of the precipitation during heating and evaporatingphases that gives different morphologies including solid parti-cles, hollow shell, and fragmented shell.31 However, not all pre-cipitated particles deposit in the form of a pan cake (splat).Before impingement on the substrate, molten particles must haveenough momentum to deposit in the form of a splat. Low mo-mentum particles deposit as spherical particles, rather than splatand entrapped within the splats (Fig. 2(d)). The present SPPScoating development can be defined in the following stages.

Shear deformation of droplets at atomizer exit-vaporizationof water from droplets-decomposition of cerium salts solidparticles-disintegration of droplet in to finer droplets due tothermal decomposition (endothermic/exothermic)-sintering/shrinkage of fine solids-melting-splat formation/depositionupon impact.

(3) Grain Size Measurement of Coatings

The average crystallite size for each coating was calculated usingthe Scherrer equation

D ¼ 0:9lb cos y

(1)

where l is the wavelength of the X-rays, 2y is the diffractionangle. b is the corrected full-width at half-maximum (FWHM)obtained using the (111) line of the high-purity LaB2 as thestandard. In the present work, the following expression wasused to calculate b due to the Lorentzian (Cauchy) profile of theX-ray peaks

b ¼ bo � bi (2)

where bo is the FWHM observed and bi is the FWHM due tothe instrument.

Table II shows the average crystallite size, measured for the(111) XRD peak, are in agreement with the grain size measuredfrom TEMmicrographs. HRTEM images analyzed using imageanalysis software (IQmaterials 2.0 Software) confirms the grainsize distribution (Fig. 4). The size distribution was based onrandomly selected (80) data points in each image. Nanocrystal-line features of the SPPS coating can be ascribed to (1) very highcooling rate (1061C/min) during deposition of molten particleson the substrate and (2) time spent by the plasma gun for coat-ing development was very short (in milliseconds for one pass).The consequence of these factors restricts the grain growth eventhough the heat imparted by the plasma jet to the coating ishigh. Coatings prepared by cerium acetate precursors showed ahigher fraction of smaller (4–8 nm) grains as compared withother coatings (histogram, Fig. 4). The exothermic decomposi-tion of acetate precursors between 2001 and 3501C to ceriumoxide could generate a localized increase in the enthalpy asshown in Fig. 5(a). The localized increase in enthalpy may causea high rate of decomposition of injected droplets in to ceriumoxide and evolution of gaseous products leading to smaller par-

ticle deposit.18 The process can be assumed similar to the com-bustion synthesis of fine metal oxide particles.

(4) Thermal Decomposition of Precursors (TG-DTA Study)

To attain a complete oxide coating from a solution, it is requiredto choose the correct metal salt such that, during interactionwith the plasma flame it should decompose into the desiredmetal oxide. The thermal decomposition of the precursor salt inplasma decides the final composition of the coating in the SPPSdeposition process. Therefore TG-DTA studies in air on eachprecursor were conducted and are shown in Figs. 5(a)–(c). Thedecomposition of each precursor is discussed in the followingsections.

(A) Thermal Decomposition of Cerium Acetate Sesquihy-drate Precursor: The TG-DTA for the acetate precursor isshown in Fig. 5(a). The decomposition of acetate precursors inair can be expressed in two stages. The first endotherm at 1521Cin the TG-DTA plot indicates dehydration of Ce(CH3

CO2)3 � 1.5H2O according to Eq. (3). A second sharp peak be-tween 2001 and 3001C corresponds to an exothermic reactionand validates the decomposition of anhydrous cerium acetate tocerium oxide, water vapor, and CO2. The final weight loss(B44.96%) from the TG plot agrees well with the weight loss(B45.76%) calculated from reaction (4)

CeðCH3 CO2Þ3 � 1:5H2O! CeðCH3 CO2Þ3 þ 1:5H2O (3)

2CeðCH3CO2Þ3 þ 12:5O2 ! 2CeO2 þ 9H2O

þ 12CO2 ðDH � þ546KJmol�1Þ(4)

The complete decomposition of the cerium acetate precursorin air occurs at a temperature of B3301C. The characteristicsharp exothermic decomposition of cerium acetate can be cor-related to the combustion process used for the synthesis ofCNPs.32 In such processes the energy liberated from the heatof reaction is high enough to directly render the cerium oxidephase from the precursor solution. The plasma spraying of suchprecursors has different chemical and physical nature. Besidesthe evaporation of water and the decomposition of the acetateprecursor, the plasma heat triggers an exothermic explosion ofthe droplets in to nanometer CNPs. The process is very quickand does not allow the growth of nucleated CNPs and anychanges in the stoichiometry of the CNPs.

Table II. Concentration of Ce31

in Coatings Calculated fromDeconvoluted XPS Peaks and Average Crystallite Size from

X-Ray Diffraction

Coating precursor

Concentration

of Ce31

Average crystallite

size (nm)

Ce(NO3)3 � 6H2O 0.26 13.3Ce(CH3 CO2) � 1.5H2O 0.32 9.7Ce(SO4)2 � 4H2O 0.23 14.3

Fig. 4. Histogram of grain sizes of nanocrystalline cerium oxide coatingfor each salt, measured in high-resolution TEM micrographs usingimage analysis software. A total of 80 data points were counted fromdifferent HRTEM images. For irregular particles longer dimension wasselected.

November 2010 Microstructure Evolution and Nonstoichiometry in SPPS Cerium Oxide Coating 3703

(B) Thermal Decomposition of Cerium Nitrate Hexahy-drate Precursor: Figure 5(b) represents the experimentalTG-DTA curve of Ce(NO3)3 � 6H2O decomposition in air.Two major mass loss peaks were observed. The first mass losspeak up to 2401C, is due to the removal of water of crystalli-zation given in Eq. (5). The second mass loss is observedbetween 2401 and 3301C, and is due to the decomposition ofanhydrous Ce(NO3)3 to CeO2 and oxides of nitrogen. Theobserved 46.4% mass loss was in agreement with the theoreti-cal mass loss of 47.2% as per Eq. (6). The difference in the massloss is due to the initial absorption of moisture by cerium nitrate

CeðNO3Þ3 � 6H2O! Ce ðNO3Þ3 þ 6H2O (5)

CeðNO3Þ3 þO2 ! CeO2 þOxides of nitrogen

ðDH � �121 kJmol�1Þ(6)

(C) Thermal Decomposition of Cerium Sulfate Tetrahy-drate: Amultistage decomposition was observed for the sulfateprecursor. This can be divided into four stages. The decomposi-tion products have different stability temperatures as depicted inthe TG-DTA plot shown in Fig. 5(c). The first two endothermscorrespond to the dehydration of the precursor to Ce(SO4)2 in thetemperature range between 601 and 3501C. The third endothermreveals that the decomposition of the anhydrous sulfate salt toCe2(SO4)3 occurred between 4451 and 5221C. The final decom-position took place at 8501C as indicated by the steep slope on thedecomposition curves. The enthalpy change of reduction of Ce41

to Ce31 sulfate is lower than the enthalpy change due to the lossof water and the final decomposition from Ce2(SO4)3 to CeO2.Ce2(SO4)3 compound was found to be stable between 5221 and6551C. The decomposition of the sulfate precursor can be shownby the following sequence and intermediate products

CeðSO4Þ2 � 4H2O! CeðSO4Þ2 � 2H2Oþ xH2O (7)

CeðSO4Þ2 � 2H2O! CeðSO4Þ2 þ xH2O (8)

2CeðSO4Þ2 ! Ce2ðSO4Þ3 þ SO3 þ 1=2O2

ðDH � �40 kJmol�1Þ(9)

Ce2 ðSO4Þ3 ! 2CeO2 þ 2SO3 þ SO2

ðDH � �296 kJmol��1Þ(10)

Several oxysulfate compounds formations have been reportedat intermediate temperature ranges while different temperaturesfor dehydration, reduction, and decomposition of sulfate precur-sor have also been reported (Eqs. (7)–(9)).32–35 The disagreementin the reported results can be ascribed to the different decompo-sition environments and heating rates. The calculated weight loss(39.43%) according to the Eq. (10) is in good agreement with theobserved weight loss (39.29%) in the corresponding TG plot. Thiscorroborates the formation of the intermediate Ce (III) com-pound according to Eq. (9). TG-DTA analysis supports the ob-servation of several intermediate sulfate peaks in the XRDpattern along with cerium oxide using cerium sulfate as the pre-cursor for SPPS. As explained earlier, depending upon the dropletsize, unpyrolyzed precursor droplets or partially decomposed pre-cursor can reach the substrate. After deposition, the unpyrolyzedprecursor decomposes when the coating temperature becomeshigher than the precursors decomposition temperature. Thedecomposition temperature (to CeO2) for cerium nitrate andcerium acetate precursors is well below the substrate tempera-ture. This facilitates the formation of single phase CeO2 coating,despite the deposition of unpyrolyzed solution during spray,whereas the decomposition temperature for cerium sulfate pre-cursor is higher than the coating temperature, which resultsin incomplete decomposition. For confirmation, the coatingprepared by sulfate precursor was scratched and investigatedunder TG-DTA (Fig. 5(c)). DTA plot of the coating shows asingle endothermic peak between 7001 and 8501C which corrob-orates the presence of oxysulfate in the coating along with CeO2.

(5) Numerical Modeling of Precursor Injection forMicrostructure Evolution

We studied the heat and mass transfer to the precursor dropletsbased on their different chemistries in order to understand the in-flight thermophysical phenomena that ultimately determine thefinal microstructure.24 A numerical study was used to simulatethe droplet injection process. Ozturk and Cetegen24 have per-formed an experimental study using phase doppler anemometry

Fig. 5. Thermal gravimetric-differential thermal analysis curves ofdried (a) cerium acetate sesquihydrate (b) cerium nitrate hexahydrate(c) cerium sulfate tetrahydrate precursor and coating at heating rate of101C/min in flowing zero grade air.

3704 Journal of the American Ceramic Society—Singh et al. Vol. 93, No. 11

at the exit of an air-blast nozzle to determine the statistical dis-tribution of droplet sizes which indicates the size and velocitydistribution of the droplets injected from the nozzle. It clearlyshows that the diameter distribution of the droplets generated bythe nozzle follows a bell-shaped curve with a mean around 35 mmand mean velocity around 12 m/s. As the present study usesa similar kind of nozzle, we have assumed the mean size of theinjected droplets close to 30 mmwith an injection velocity of 12 m/s for numerical simulation. It is important to mention here thatcurrent model does not consider intermediate surface precipita-tion and shell breaking phenomena as reported in our previouswork.27 This being a conservative modeling considers continuousvaporization of solvent until the droplet reaches critical temper-ature or the solvent vaporizes completely. However, this model iscapable of predicting the global temperature rise and final tem-perature of the particle very closely to the detailed modeling

Figure 6 shows the temperature rise of the droplets of all threeprecursors with time. It shows two distinct zones. The first zone,termed as ‘‘vaporizing phase’’ has a slower rise in temperature.The temperature in this zone increases up to 3741C, which is thecritical point of water. At this temperature, water from the drop-let completely vaporizes which in conjunction with pyrolysis leadsto a solid particle formation. After this point the second phase,namely ‘‘solid particle heating’’ starts. This phase is marked byvery sharp rise in temperature reaching the melting point of ceria.It is important to notice that the temperature of the droplet fol-lows a similar trend for all of the precursors considered. Adetailed analysis of the thermodynamic and thermogravimetricproperties of these precursors shows that the primary differencebetween the precursors is the heat of pyrolysis. However, com-parison of heat transfer from the plasma field to the droplet andrequired heat of pyrolysis shows that the latter is almost few or-ders of magnitude smaller. An order of magnitude calculation forthe amount of heat transfer from the plasma environment to thedroplet is more than 500 times of required heat of pyrolysis ofsolutes. Thus, the effect of heat of pyrolysis does not have mucheffect on temperature rise which results in almost the same tem-perature profiles as shown in Fig. 6. This plot suggests that for allthree precursors, 30 mm droplets will reach the substrate, pyr-olyze, and melt during the flight.

A parametric study on cerium nitrate droplet diameter has alsobeen performed to understand what happens to droplets whichare either too small or too large. Ten and 50 mm droplets ofcerium nitrate precursor were selected for a parametric study.Figure 7 corresponds to the trajectories of these droplets. Itreveals that larger droplets with higher inertia have a larger po-tential to penetrate the outer shear layer of the plasma and reachthe core, where the temperature and velocity are the highest. Thetemperature rise for these three droplets are shown in Fig. 8. Itcan be noted from the figures that 30 mm droplets attain themelting temperature of CeO2 before reaching the substrate. How-

ever, for both 10 and 50 mm droplets, the temperature remains inthe lower range. Droplets of 10 mm do not have enough inertia topenetrate the shear layer of the plasma, thus these remain at thelower temperature zone and do not reach a very high tempera-ture. On the other hand, 50 mm droplets succeed in reaching thehigh-temperature plasma core. However, the droplets being largerin size contain a significant amount of solvent, which takes alonger time to vaporize. As a result, by the time the droplet turnsinto a solid particle, it reaches far down stream of the plasmawhere the temperature is comparatively lower. Thus, comparingthese results for different sized droplets it can be commented thatdroplets around 20–45 mm diameter have a higher probability ofreaching the substrate in a pyrolyzed and molten state.

A comparison of our numerical simulation with the SEMimages (Fig. 2) shows good agreement with very little change inmircostructure as functions of the precursor chemistry. Simi-larly, surface morphology of the coatings (Fig. 2) is in agreementwith the simulation results shown in Fig. 6 which shows that thetemperature rise is almost identical for all the precursors. SEMimages, however, indicate traces of unmelted deposit which canbe attributed to the deposition of unpyrolyzed smaller droplets(around 10 mm) or droplets with diameters 445 mm. The pres-ence of too large or too small droplets is unavoidable due to theconstraints in the spray nozzle design that invariably lead tounpyrolyzed deposits. The difference in the grain size distribu-tion among three precursors in Fig. 4 can only be attributed todifferent heats of pyrolyzation. The smaller grain size for acetateprecursors is due to the positive heat of pyrolysis that can lead to

Fig. 6. Temperature profiles for 30 mm droplets of different precursorswith time, indicate water evaporation up to critical point followed byheating of the solid particles up to the melting point of ceria.

Fig. 7. Trajectory of different size droplets of cerium nitrate precursor.This indicates that only larger size (420 mm) droplets penetrate plasmaplume. Smaller droplets remain in the shear layer of the plasma. For aclear representation, the plasma nozzle is shown away from the centerline.

Fig. 8. Temperature profiles for different size droplets of cerium nitrateprecursor, show size dependent behavior of injected droplets. Larger sizedroplets (B50 mm) pass through the hot zone of the plasma andattain low temperature (B5001C). Smaller droplets (B10 mm) remainin the low temperature shear layer and reach the substrate in unmeltedcondition.

November 2010 Microstructure Evolution and Nonstoichiometry in SPPS Cerium Oxide Coating 3705

secondary pyrolyzation for this precursor, compared with theother two precursors which have negative heats of pyrolysis.

(6) Thermodynamic Feasibility of Cerium Oxide Formationfrom Solution

The enthalpy of the plasma jet, evaporates the water contentof the injected droplets, decomposes anhydrous droplets andintermediate products and finally heats the products to theirmelting temperatures for deposition on the substrate. Enthalpyof the plasma varies depending on the proportion of the primaryand secondary gases. The specific mass enthalpy of the plasmaflame can be expressed by the following expression for the mix-ture of argon and hydrogen plasma gases36

Dh ¼Xki¼1

xiDHi=Xki¼1

xiDMi (11)

where xi is the molar fraction of the k chemical species present inthe mixture, DHi is the corresponding molar enthalpy, and Mi isthe atomic mass. The enthalpy of H/ArB1/6 mixture is 25 MJ/kgcalculated according to the Eq. (11) which is more than sufficientfor the conversion of different cerium salts precursor to ceriumoxide. Net enthalpy required for the formation of cerium oxidefrom different precursors is calculated from the TG-DTA plotand listed in Table III. Qualitatively it can be observed from theTG-DTA plot that the enthalpy change of dehydration is highestfor the cerium nitrate hexahydrate due to its hygroscopic natureand six molecules of water of hydration compared with the othercerium salts. In plasma spray most of the heat is consumed forthe evaporation of the water content of the droplets. After that,heat is utilized for decomposition of the anhydrous salts and

melting of the cerium oxide. The enthalpy required for meltingof cerium oxide particles can be calculated from the followingexpression

DH ¼Z T

298

Cp dt (12)

where Cp, J � (mol �K)�1571.15210.01335T�1145425T�2.37Assuming no phase transformation occurs in cerium oxide up

to its melting point (2773 K), its enthalpy of fusion will beDHB237.2 kJ/mol. It is confirmed from the enthalpy data pre-sented in Table III that plasma energy is sufficient for enthalpyof formation of cerium oxide from precursors and melting of thecerium oxide. However, partially molten (spherical particles)and unpyrolyzed solution observed in the coatings, are typicalcharacteristic of the SPPS process. These features are dependenton the various factors such as size and distribution of the pre-cursor droplets, and its correct injection to the core of theplasma assuming correct plasma energy.

Fig. 9. XPS spectrum of Ce (3d) for cerium oxide coating prepared by (a) cerium (III) acetate (b) cerium (III) nitrate hexahydrate (c) cerium (IV) sulfateprecursors. Peaks were deconvoluted to get the distinct spin-orbit doublet of 3d3/2 and 3d5/2. (d) S 2p spectra indicating the presence of SO4.

Table III. Net Enthalpy of CeO2 Formation and DehydrationCalculated from DTA Plot

Precursor

Net enthalpy of formation

(CeO2) (kJ/mol)

Enthalpy of

dehydration (kJ/mol)

Ce(NO3)3 � 6H2O B�268 B�99Ce(CH3 CO2) � 1.5H2O B1503 B�43Ce(SO4)2 � 4H2O B�377 B�60

3706 Journal of the American Ceramic Society—Singh et al. Vol. 93, No. 11

(7) Determination of Oxidation State of Cerium in SPPSCoated CNPs

As mentioned earlier, oxygen nonstoichiometry can be intro-duced by stabilizing cerium in the trivalent oxidation state inCNPs. We hypothesized that by changing the precursor chem-istry, we can alter the oxidation state of cerium in CNPsand thereby introduce oxygen nonstoichiometry in the coatings.Figures 9(a)–(c) shows the XPS spectra, the fitted curve, and thedeconvolution of peaks. Charging of samples was corrected bysetting the binding energy of adventitious carbon (C 1s) at 284.6eV. The complicated cerium XPS spectra were deconvoluted bypeak fit (4.1) software for clear identification of Ce31 and Ce41

peaks. The characteristic peaks of Ce31 are denoted by vo, v0 uo

and u0, whereas v, v00 v000, u, u00 and u000 correspond to the Ce41.The XPS spectra for each coating indicates the mixed oxidationstate of Ce31 3d3/2 and 3d5/2 binding energy, main peaks at(901.4 and 881.5 eV) and Ce41 3d3/2 and 3d5/2 at (916.7 and898.2 eV). The Ce31 concentration is calculated by taking theratio of the integrated peak areas corresponding to Ce13, to thetotal area under the XPS spectrum.38 The concentration of Ce31

in each coating is shown in Table II. The highest Ce31 concen-tration (32%) has been found in the coating prepared by ceriumacetate. No trace of nitrogen was observed in case of CeO2

coatings prepared by a nitrate precursor, whereas XPS analysesindicated the presence of sulfur in the CeO2 coatings preparedby sulfate precursors. The binding energy of S 2p3/2 and 2p1/2(168.23 and 169.38 eV) corresponds to the presence of SO4 typespecies (Fig. 9(d)).39 The presence of sulfur in the coatings is dueto the deposition of unpyrolyzed sulfate precursors with ceriumoxide particles.

In the present study, it is important to take cognizance of theretention of high concentrations of Ce31 in CeO2 coatings de-veloped by SPPS process. The nonstoichiometry in the fluoritestructure may exist due to very high cooling rates. Similarly un-melted and partially melted particles may also have high non-stoichiometry due to high temperature exposure during travel inthe plasma jet. It has been reported by various authors that thenonstoichiometry in CeO2 increases with increase in the temper-ature.40,41 Furthermore, it is also established that the Ce31 con-centration increases as the particle size is reduced.8,38 Thereforethe ability to retain nanosized particles by SPPS could be one ofthe reasons for higher concentration of Ce31 ceria coatings.

IV. Summary

The SPPS process has been established to synthesize high qualitynanostructured cerium oxide coatings with desired chemistry.The role of precursor chemistry and decomposition behaviorhas been demonstrated for microstructure development anddifferent oxidation states of the coatings. Use of cerium acetateprecursor for coating has shown the highest concentration ofCe31 (0.32) and the smallest crystallite size (9.7 nm). Ceriumacetate and cerium nitrate precursors have shown single-phasenanocrystalline cerium oxide coating, whereas cerium sulfateprecursors introduce traces of sulfur in the coating. It has beenshown that substrate/coating temperature during plasma sprayplays an important role in obtaining single-phase cerium oxidecoating. The difference in the concentration of Ce31 oxidationstate in the coating is due to the differences in the decompositionbehavior in the plasma plume. Numerical simulation resultssuggest that the smaller droplets (o10 mm) do not penetrate theshear layer and travel within the low momentum and low-tem-perature zone of the plasma. Larger droplets (440 mm) pene-trate the plasma zone. However, they remain unmelted due to ashort residence time in the hot zone of the plasma. Droplets ofsizes 20–40 mm have been calculated for complete melting anddeposition in the form of splat. The microstructure of the SPPScoating consisted of a variety of structures which wereconfirmed by the numerical modeling due to the wide distribu-tion of injected droplets constrained by the nozzle design.

Acknowledgments

We would like to thank the Materials Characterization Facility at the Univer-sity of Central Florida.

References

1H. Inaba and H. Tagawa, ‘‘Ceria-Based Solid Electrolytes,’’ Solid State Ionics,83, 1–16 (1996).

2M. Fathi, E. Bjorgum, and O. A. Rokstad, ‘‘Conversion of Ethane to Ethyleneand Synthesis Gas with Cerium Oxide. Promoting effect of Pt, Rh and Ru,’’ Catal.Lett., 72, 25–31 (2001).

3J. Kaspar, P. Fornasiero, and M. Graziani, ‘‘Use of CeO2-Based Oxides in theThree-Way Catalysis,’’ Catal. Today, 50, 285–98 (1999).

4A. Karakoti, N. Monteiro-Riviere, R. Agarwal, J. Davis, R. Narayan, W. Self,J. McGinnis, and S. Seal, ‘‘Nanoceria as Antioxidant: Synthesis and BiomedicalApplications,’’ J. Miner. Met. Mater. Soc., 60, 33–7 (2008).

5P. B. Zantye, A. Kumar, and A. K. Sikder, ‘‘Chemical Mechanical Planarizat-ion for Microelectronics Applications,’’ Mater. Sci. Eng. R: Rep., 45, 89–220(2004).

6F. H. Garzon, R. Mukundan, and E. L. Brosha, ‘‘Solid-State Mixed PotentialGas Sensors: Theory, Experiments and Challenges,’’ Solid State Ionics, 136–137,633–8 (2000).

7S. Patil, S. C. Kuiry, and S. Seal, ‘‘Nanocrystalline Ceria Imparts Better High-Temperature Protection,’’ Proc. R. Soc. London A: Math. Phys. Eng. Sci., 460,3569–87 (2004).

8S. Deshpande, S. Patil, S. V. N. T. Kuchibhatla, and S. Seal, ‘‘Size DependencyVariation in Lattice Parameter and Valency States in Nanocrystalline CeriumOxide,’’ Appl. Phys. Lett., 87, 133113, 3pp (2005).

9S. Tsunekawa, R. Sivamohan, S. Ito, A. Kasuya, and T. Fukuda, ‘‘StructuralStudy on Monosize CeO2�x Nano-Particles,’’ Nanostruct. Mater., 11, 141–7(1999).

10S. Kim, R. Merkle, and J. Maier, ‘‘Oxygen Nonstoichiometry of NanosizedCeria Powder,’’ Surf. Sci., 549, 196–202 (2004).

11M. Alifanti, B. Baps, N. Blangenois, J. Naud, P. Grange, and B. Delmon,‘‘Characterization of CeO2–ZrO2 Mixed Oxides. Comparison of the Citrate andSol–Gel Preparation Methods,’’ Chem. Mater., 15, 395–403 (2002).

12Y. C. Zhou and M. N. Rahaman, ‘‘Hydrothermal Synthesis and Sintering ofUltrafine CeO2 Powders,’’ J. Mater. Res., 8, 1680–6 (1993).

13H. Masanori and K. Etsuro, ‘‘Hydrothermal Synthesis of NanocrystallineCerium(IV) Oxide Powders,’’ J. Am. Ceram. Soc., 82, 786–8 (1999).

14C. Pei-Lin and I. W. Chen, ‘‘Reactive Cerium(IV) Oxide Powders bythe Homogeneous Precipitation Method,’’ J. Am. Ceram. Soc., 76, 1577–83(1993).

15N. Uekawa, M. Ueta, Y. J. Wu, and K. Kakegawa, ‘‘Synthesis of CeO2

Spherical Fine Particles by Homogeneous Precipitation Method with PolyethyleneGlycol,’’ Chem. Lett., 31, 854–5 (2002).

16P. Duran, F. Capel, D. Gutierrez, J. Tartaj, and C. Moure, ‘‘Cerium (IV)Oxide Synthesis and Sinterable Powders Prepared by the Polymeric Organic Com-plex Solution Method,’’ J. Eur. Ceram. Soc., 22, 1711–21 (2002).

17D. Perednis and L. J. Gauckler, ‘‘Solid Oxide Fuel Cells with ElectrolytesPrepared Via Spray Pyrolysis,’’ Solid State Ionics, 166, 229–39 (2004).

18L. Madler, W. J. Stark, and S. E. Pratsinis, ‘‘Flame-Made Ceria Nanoparti-cles,’’ J. Mater. Res., 17, 1356–62 (2002).

19L. Pawlowski, The Science and Engineering of Thermal Spray Coatings, 2 edi-tion, Wiley-VCH, New York, 2008.

20L. Pawlowski, ‘‘Suspension and Solution Thermal Spray Coatings,’’ Surf.Coat. Technol., 203, 2807–29 (2009).

21N. P. Padture, K. W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E. H.Jordan, M. Gell, S. Jiang, T. D. Xiao, P. R. Strutt, E. Garcıa, P. Miranzo, andM. I. Osendi, ‘‘Towards Durable Thermal Barrier Coatings with Novel Micro-structures Deposited by Solution-Precursor Plasma Spray,’’ Acta Mater., 49,2251–7 (2001).

22R. Gadow, A. Killinger, and J. Rauch, ‘‘New Results in High VelocitySuspension Flame Spraying (HVSFS),’’ Surf. Coat. Technol., 202, 4329–36 (2008).

23D. Chen, E. H. Jordan, and M. Gell, ‘‘Porous TiO2 Coating Using the So-lution Precursor Plasma Spray Process,’’ Surf. Coat. Technol., 202, 6113–9 (2008).

24A. Ozturk and B. M. Cetegen, ‘‘Modeling of Plasma Assisted Formation ofPrecipitates in Zirconium Containing Liquid Precursor Droplets,’’ Mater. Sci.Eng. A, 384, 331–51 (2004).

25A. A. Mostafa and S. E. Elghobashi, ‘‘A Two-Equation Model for Jet Flowswith Vaporizing Droplets,’’ Fluid Dyn., 25, 73–8 (1990).

26D. I. Kolaitis and M. A. Founti, ‘‘A Comparative Study of Numerical Modelsfor Eulerian–Lagrangian Simulations of Turbulent Evaporating Sprays,’’ J. HeatFluid Flow, 27, 424–35 (2006).

27A. Saha, S. Seal, B. Cetegen, E. Jordan, A. Ozturk, and S. Basu, ‘‘Thermo-Physical Processes in Cerium Nitrate Precursor Droplets Injected into High Tem-perature Plasma,’’ Surf. Coat. Technol., 203, 2081–91 (2009).

28A. Ozturk and B.M. Cetegen, ‘‘Modeling of Axially and Transversely InjectedPrecursor Droplets into a Plasma Environment,’’ Int. J. Heat Mass Transfer, 48,4367–83 (2005).

29D. Chen,M. Gell, E. H. Jordan, E. Cao, and X.Ma, ‘‘Thermal Stability of AirPlasma Spray and Solution Precursor Plasma Spray Thermal Barrier Coatings,’’J. Am. Ceram. Soc., 90, 3160–6 (2007).

30L. Xie, X. Ma, E. H. Jordan, N. P. Padture, D. T. Xiao, and M. Gell, ‘‘Iden-tification of Coating Deposition Mechanisms in the Solution-Precursor Plasma-Spray Process Using Model Spray Experiments,’’ Mater. Sci. Eng. A, 362, 204–12(2003).

November 2010 Microstructure Evolution and Nonstoichiometry in SPPS Cerium Oxide Coating 3707

31S. Basu and B. M. Cetegen, ‘‘Modeling of Thermo-Physical Processes in Liq-uid Ceramic Precursor Droplets Injected into a Plasma Jet,’’ Int. J. Heat MassTransfer, 50, 3278–90 (2007).

32L. Gu and G. Meng, ‘‘Powder Synthesis and Characterization of Nanocrys-talline CeO2 Via the Combustion Processes,’’ Mater. Res. Bull., 42, 1323–31(2007).

33Y. Ying and Y. Rudong, ‘‘Study on the Thermal Decomposition of Tetrahy-drated Ceric Sulphate,’’ Thermochimic. Acta, 202, 301–6 (1992).

34B. M. Casari and V. Langer, ‘‘Two Ce(SO4)2 � 4H2O Polymorphs: CrystalStructure and Thermal Behavior,’’ J. Solid State Chem., 180, 1616–22 (2007).

35J. A. Poston, R. V. Siriwardane, E. P. Fisher, and A. L. Miltz, ‘‘Thermal De-composition of the Rare Earth Sulfates of Cerium(III), Cerium(IV), Lantha-num(III) and Samarium(III),’’ Appl. Surf. Sci., 214, 83–102 (2003).

36B. Pateyron, M. F. Elchinger, G. Delluc, and P. Fauchis, ‘‘Thermodynamicsand Transport Properties of Ar–H2 and Ar–He Plasma Gases Used for Sprayin at

Atmospheric Pressure I: Properties of the Mixtures,’’ Plasma Chem. Plasma Pro-cess, 12, 421–48 (1992).

37R. Venkata Krishnan and K. Nagarajan, ‘‘Heat Capacity Measurements onUranium-Cerium Mixed Oxides by Differential Scanning Calorimetry,’’ The-rmochimic. Acta, 440, 141–5 (2006).

38S. Tsunekawa, T. Fukuda, and A. Kasuya, ‘‘X-Ray Photoelectron Spectros-copy of Monodisperse CeO2-x Nanoparticles,’’ Surf. Sci., 457, L437–40 (2000).

39J. Rodriguez, T. Jirsak, A. Freitag, J. Hanson, J. Larese, and S. Chaturvedi,‘‘Interaction of SO2 with CeO2 and Cu/CeO2 Catalysts: Photoemission, XANESand TPD Studies,’’ Catal. Lett., 62, 113–9 (1999).

40I. Riess, M. Ricken, and J. Nolting, ‘‘On the Specific Heat of Nonstoichio-metric Ceria,’’ J. Solid State Chem., 57, 314–22 (1985).

41M. Mogensen, N. M. Sammes, and G. A. Tompsett, ‘‘Physical, Chemical andElectrochemical Properties of Pure and Doped Ceria,’’ Solid State Ionics, 129, 63–94 (2000). &

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