Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001,3-8. 3

SPHERICAL CALCIA STABILIZED ZIRCONIAPOWDERSOBTAINED BY SPRAYPYROLYSIS

H. E. Esparza-Ponce, Á. Reyes-Rojas, M. Miki Yoshida

Centro de Investigación en Materiales Avanzados S.C, Miguel de Cervantes 120,Complejo Industrial Chihuahua, Chihuahua, Chih. CP. 31109, México

hUda.esparza@cimav.edu.mx

Abstract

Physical and chemical properties of ceramic powders are highly dependent on the morphology and chemical composition ofthe precursor powders. Therefore, properties of ceramic powders are expecting to be improved by controlling the most importantparameters of morphology such as composition of precursors, temperature and airflow. Promising techniques to produceceramic powders with controlled morphology are sol-gel, coprecipitation, and pyrolysis. Among these techniques, spray pyrolysisis one of the most suitable to control the morphology and the chemical compositionof the particles, because the spray pyrolysisteclinique integrates the evaporation, precipitation, decomposition and sintering stage of powder synthesis into a singlecontinuos process. Spherical powders were prepared from acetylacetonate ofzirconium and acetate calcium hydrated solutionsby pneumatic spray pyrolysis. The morphology and size of the powders change with variation of airflow, temperature, andsolution concentration.

Keywords: Spray pyrolysis technique, ultrajine powders, precursor ceramics, zirconia and calcia.

Resumen

Las propiedades fisicas y químicas de los polvos cerámicos dependen grandemente de la morfología y de la composiciónquímica de las soluciones precursoras. Por lo tanto, las propiedades de los polvos cerámicos están esperando ser mejoradascontrolando los parámetros que más influyen en la morfología como son la composición de las soluciones precursoras, temperaturay flujo de aire. Algunas de las técnicas que ofrecen producir polvos cerámicos con morfología controlada son: sol gel,coprecipitación y pirólisis.Dentro de estas técnicas la aspersión pirolítica 'es una de. las.más adecuadas para el control de lamorfología y la composición química de las partículas, ya que la aspersión pirolítica integra la evaporación, precipitación,descomposición y estado de sinterización para la síntesis de polvos en un solo proceso continuo. En este trabajo, los polvosobtenidos son esféricos y han sido preparados a partir de acetilacetonato de circonio y acetato de calcio en solución; mediantela técnica de aspersión pirolítica utilizando un aspersor neumático. La morfología y el tamaño de partícula cambian con lasvariaciones del flujo de aire, temperatura y la concentración molar de la solución.

Palabras clave: Zirconia, calcia, aspersión pirolltica, polvos nanométricos.

1. Introduction

In the past twenty years, a considerable interest has arisenin zirconia-based (Zr02-based) solid electrolyte materials fornumerous high-temperature technology applications. Highoxygen-ion conductivity combined with high thermochemi-cal stability are responsible for the use of stabilized zirconiamaterial s, for example in oxygen sensor or fuel cells [1,2].More applications as: knives and scissors, precision ballvalve balls.and seats,high-density ball and pebblernill-grind-ing media. Rollers and guides for metal tube forming, threadand wire guides, hot metal extrusion dies, deep well down-hole valves and seats, powder compacting dies, marine pumpseals and shaft guides, oxygen seasors, 'high temperatureinduction furnaee susceptors, fuel cell membranes, electricfumace heaters over 2000°C in oxidizing atrnospheres[3].

Zirconia exists in three 'crystal 'phases at different tem-peratures. At very high temperatures (>2370;C) the materialhas a cubic structure. At intermediate temperatures (1170 to2370°C) it has a tetragonal structure. At low temperatures(below 1170°C) the material transforms to the monoclinicstructure. The transformation from tetragonal to monoclinicis rapid and is accompanied by a 3 to 5 percent volume in-.crease that causes extensive cracking in the material. Thisbehavior destroys the mechanical properties of fabricatedcomponents during cooling and makes pure zirconia uselessfor any structural or mechanical application. Several oxidesdissolve in the zirconia crystal structure can eliminate thesecrystal structure changes. Commonly used effective addi-tives are MgO, CaO, and YP3' With sufficient amounts

2.2. Powder characterization

4 H E. Esparza y col. /Revista Latinoamericana de Metalurgia y Materiales

added, the high temperature cubic structure can be main-tained at room temperature. Cubic stabilized zirconia is a use-ful refractory and technical ceramic material because it doesnot go through destructive phase transitions during heatingand cooling.

2. Experimental Procedure

2.1. Preparation of Calcia stabilised Zirconia powder.

The experimental system used in this work is shown inFigure l. It mainly consists of a pneumatic atomizer (a), inwhich the precursor solution is converted in afine mist, thenit was carried out by the airflow and injected by a nozzle (b)into the quartz tube (e), which takes the place of a reactionchamber. The airflow was regulated with amass flow controller(d). The solution temperature was controlled precisely (± 1°C) by partially immersing the solution container in adeionized water bath (e). The quartz tube was located insideof a cylindrical furnace (f), which has a very precisetemperature control (± 1°C). Finally, at the outermost side ofthe quartz tube, the electrostatic precipitator (g) is placed.This is composed of a recollection tube (h) and anode wire(i) connected to a high voltage source (j).

(f)

(í)

Fig. l. Schematic diagram of the pyrolytic system

Calcia stabilized zirconia (CaSZ) Zr 0.85 CaO.l5 O 1.85powders were preparedfrom a solution ofZr[CH3COCH=C-O-CH3]4 and Ca(CH3COO)2 H20, in a mixture of 60%deionized water and 40% absolute ethanol. A Zr:Ca molarproportion of 0.86:0.14 was chosen and total precursorconcentrations ofO.01, 0.05 and 0.1 mol dm-3 were tested.The furnace temperature was varied between 800 to 1000 °C;differentcarriergas flows was tried among 33 to 167 cms.s-I.

The crystalline structure of the zirconia-based powderswas analysed by X ray diffraction (XRD) in a D5000 X-raydiffractometer, using Cu Ka radiation, at 40kV and 30 mA.The 2q angle were swept in a range from 5 to 80°, with a 2qstep size ofO.l ° and step time of 5 s.Particle composition was analysed by electron microscopy,using a JSM 5800LV scanning and a CM200 transmissionelectron rnicroscopes, both equipped with a DX prime energydispersive X rayspectrometer (EDS). In addition, crystallinestructures were studied by selected area electron diffractionpattems (SAED), and compared with XRD results.

Additionally, particle size, morphology and microstructurewere evaluated as functions ofthe experimental conditions.For the determination ofthe average particle sizes (APS) ofeach sample, around 200 particles were measured directlyfrom several bright field transmission electron micrographs,using a digital imaging anaIyser IMAGE-PRO PLUS 4.1.

3. Results

3.1 Parameters in powder preparation by spray pyrolysis

It is well established, for conventional pneumatic spraying,that the droplet sizes decrease when the airflow increases[2]. Ifthe sprayed droplets are focussed directly towards thereaction chamber, the droplets size corresponds to thatmentioned above. However, if the atomizer spray s down tothe solution container and the mist of solution is carried outin an upward direction by the airflow; the droplet sizeselection occurs due to the combined effect of gravitationaland Stokes' force [1]. When these forces reach equilibriumthe droplet travels with a constant terminal velocity, which isfunction of the particle size. Therefore, only the dropletssmaller than certain critical sizes can be carried out to thereaction chamber. This critical size (de) is airflow dependent,and for low and medium airflow (O< G>A < 8 dm' min') in factreverse the reliance between droplet size and airflow, i.e.average droplet size increase with the increase of airflow justto certain limit flow. Only if the gas carrier flow increasesfurther (G> A EE> 8 drrr' min') the average droplet size candecrease. The real residence time ('t) ofthe droplets is in theregio n ofhigh the temperatures ofthe cylindrical furnace, italso was estimated as a function of the gas carrier flow rate(G> A)' taking into account the geometry of the reactor. Gasexpansion correction was estimated by this factor, since thework ofMilosevic et al. [2]. Subsequently, at a fixed fumacetemperature the residence time decreased when airflowincreased. In addition, the rate of droplet heating (RH) hasbeen estimated considering furnace temperature distribution,

.... ::

Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001.

. ow rate and geometry ofthe reaction chamber.It can bencluded that, under the experimental conditions utilized in. work, very high increases of the heating rate (several

thousand of °C s-l) happens when the airflow increases.

3.2 Composition and crystalline structure ofZr02-CaO FinePowders.

Figure 2 shows characteristic EDS spectrum of CaSZpowders. The average Zr:Ca molar proportion in the powdersis almost the same as in the precursor solution; i.e. Zr:Ca and

approximately equal to 0~86:0~14.

2500_

2000

1500

1000O

500

OO

Zr

eaJ1.}

200 400 600

Energy (keV)

800 1000

Fig. 2. Characteristic EDS spectra ofCaSZ powder.

Figure 3 shows the XRD spectrum ofthe calcia stabilizedzirconia (CaSZ) powder.It is observed that diffraction peakscorresponding to the cubic and tetragonal phases, indicatingthat a partially stabilized structure has been formed. SAEDresults also confirm this fmding. Table 1 shows interplanardistance calculated from SAED pattems ofCaSZ powders.Itcan be concluded, the presence of Cubic and Tetragonalphases. A preliminary estimation using the Rietveld methodreports a proportion of 5 weight percent for the tetragonalphase. An exhaustive study is in course.

18001600 111140012001000

200800600400200

o10 30

220 331

311 420

400

50 70 90 110

Fig. 3. XRD spectrum ofCaSZ powder obtainedat 900°C.

5

Table 1. Interplanar distance calculated from SAED pattems."T" Tetragonal and "C" Cubic phases.

d(A)Phase h k "l JPDS (71) (70) (73) (75)

T 1 O 1 2.995 2.99 2.99 2.99 2.99T O O 2 2.635 - 2.61 - -

e 2 O O 2.565 2.57 - 2.57 2.57T 1 1 2 1.841 1.84 1.87 1.85 1.86T 2 1 1 1.555 - 1.57 1.56 1.57e 3 1 1 1.548 1.54 -

/

3.3 Particle size ofCaSZ powders

In general, the growth and characteristics ofthe powdersprepared by spray pyrolysis are determined by the combinedeffect ofthe droplet size generated in the sprayer, the size ofthose effectively transported (de) to the reaction chamber,the residence time (t) and the rate of droplet heating (RH) inthe chamber. These parameters are, as well, dependent onthe geometry ofthe system, principally spraying and reactionchambers, solution composition and concentration, airflowand to the reactor temperature.

First of all, it is worthwhile to consider that the averageparticle size (APS) measured in all the samples ofthis workare nano-metric (40 to 350 nm), they were considerably smallerthan those calculated from the average droplet size using awell known approach [2]. This result can be explained bytwo possible mechanisms considering that each droplet didnot convert to a unique particle. The first one, due to theoccurrenceof'.an .important droplet broke down [2] orexplosive boiling [6]. These events probably take place whenthe droplets travel into the nozzle, before they have beeninjected into the reaction chamber. At this stage the residencetime (~10-3 s) is shorter than the drying time (~O.1 s [5]) andthe heating rate is very high (> 105 °C s-I), in these conditionsa vigorous boil of the droplets can happen, given as aconsequence, their broke down.

The second additional mechanism can occur afterdehydration ofthe dropletlparticle, at the beginning oftheirtravel in the quartz tube. Due to the high temperature andvery large surface area of the precursor salt particles, thetotal evaporation rate can be extremely high, given as aconsequence, the loose of the precursor material or theformation of many ultra fine particles. This mechanism wasalso suggested for ZnO powders by Liu et al. [7].1t is expectedthat precursor evaporation was more intense at low airflow,i.e. when the residence time in the reaction chamber washigh: Moreover, at high solution concentration the dropletstended to evaporate the solvent more quickly, thus, theyformed precursor salt particles more easily and consequentlythe precursor evaporation rate increa ledoThe experimental

6 H E. Esparza y col. /Revista Latinoamericana de Metalurgia y Materiales

results are consistent with these two considerations. TheAPS was a bimodal distribution of sizes, small sizes (»40 nm)at low 6 dm3 min-l and large size distribution (350 nm)at higherflows (see figure 4); additionally, theAPS increasedwith the increase of solution concentration (see figure 5).

600~--------------------------,

400

o 5

airflow drn' rnin'

10

Fig. 4. APS as a function of the airflow.

160 -r-----------------,120

80 ...•_ .•.......•••••••

"",....~_ ...

40o +- ~----------~--~

o 0,05 0,1

concentration mol dm-3

Fig. 5. APS as a function ofthe solution concentration.

3.4. Morphology and inner structure ofthe CaSZ particles.

Secondary electron SEM micrograph representative ofCaSZ powder is shown in figure 6.

Fig. 6. Secondary electron SEM micrograph of powders CaSZ.

Fig.7. Bright field TEM micrographs of calcia stabilized zirconiaobtained at airflow: a) 4, b) 6, e) 8 and d) 10 dm' min'. The solutionconcentration was 0.05 mol dm', furnace temperature 900°C and

voltage of electrestatic precipitator 4.6 kV

Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001. 7

For this sample obtained at 900 oe, 0.05 mol dm-3, 4.6 kV,5 dm3 min-l, it can be observed that spherical particles arefue predominant morphology; only a small number ofthemshow a hollow or irregular shape.Airflow has also a noticeable effect on the morphology ofthe particles . It is shown in figure 7, that the number ofirregular hollow shaped particles increases as the airflowincreases. The formation ofhollow particles is related to thelocalized super-saturation ofthe solute with the consequentformation of a crust on the droplet surface [1]; whichdepends on the evaporation rate of the solvent, and on the.strength and permeability of the crust. When theconcentration of precursor solution is high enough, the crustis formed earlier on the droplet surface. The internal solvent,encapsulated by the forming crust, expands when heatedand increases the particle size of the powder; rose after thecomplete evaporation of solvent ofhollow particles that canbreak into irregular particles. It is expected that the influenceof the residence time is higher when it is shorter than thetime needed for complete evaporation ofthe solvent. In thiscase, with high enough airflow, the particle size tends to

grow, because the formation of a crust occurs earlier by thecooperative effect of low t and high RH. This proposal is inconcordance with the particle size (as discussedbefore) andporosity ofthe particles (see below), and with the results ofotherworks [2,7,8,1 ].

Figure 8 shows the variation of particles porosity as afunction of the airflow. It is observed as a decrease of theporosity, hence the permeability, ofthe particles as the airflowincreases. The logical consequences are the increase ofirregular hollow particles and particle sizes with the increaseof airflow, as described above.

In addition, TEM analysis demonstrates that the sphericalparticles contain inner small crystallites. Figure 9 shows thatinner crystallite size growth as furnace temperature increasesand airflow decreases. Figures 9a and 9b show bright fieldTEM micrographs of powders obtained at 900 and 1000 "Crespectively. (6 dm3 min-l, 4.6 kV, 0.1 mol dm-3). In this case,it can be noted as a slight increment of crystallite size, from 2- 6 nm to 3 - 8 nm approximately. In contrast, for samplesobtained at 900 "C and an airflow of2 dm3 min-l the crystallitesize increases to 24 - 41 nm (see figure 9c).

Fig. 8. Bright field TEM micrographs of calcia stabilized zirconia obtained at airflow: a) 4, b) 6, e) 8 and d) 10 dm' min'.The solution concentration was 0,05 mol dm', furnace temperature 900°C and voltage of electrostatic precipitator 4.6 kV

4. ConclusionsThe feasibility of produces Zirconia and Calcia stabilized

zirconia nanopowders without post-heat treatment by spraypyrolysis has been demonstrated. Compared with thetraditional method the powders are obtained in a step, savingtime and energy.

The size, morphology, and structural characteristics ofzirconia and CaSZ nanopowders depend principally on thesource solution concentration, reaction chamber temperature,and airflow. Average particle size range between 40 to 350nm.

Each particle is composed by many small crystallites ofsizes varying between 2 to 40 nro.In spite of porous or dense ceramic powder production, anappropriate precursor solution concentration is needed tofit the requirements.

8 H. E. Esparza y col. /Revista Latinoamericana de Metalurgia y Materiales

Fig.9. Bright field TEM micrographs of calcia stabilized zirconiaobtained at: a) 900 re, airflow 6 dm' min'; b) 1000 re, airflow 6dm' min'; and e) 900 De and airflow 2 dm' min'. The solutionconcentration was 0.1 mol dm.3.

Acknowledgment

The authors particularly acknowledge the experimentalassistance of W. Antúnez-Flores, K. Gómez-Fierro, D.Lardizabal, and F. Paraguay D.

References.

1. E.C. Subbarao, Zirconia-An Overview; pp 1-24 inAdvances in Ceramics, Vo1.3,Science and Technology ofZirconia 1. Edited by A.H. Heurer and L.w. Hobbs.American Ceramic Society, Columbus, OH. 1981.

2. K.Okada, A. Tanaka, S. Hayashi, N. Otsuka, Joumal ofMaterials Science Letters, 12 (1993) 854-857.

3.4. M. Miki- Yoshida and E. Andrade, Thin Solid Films, 224,

(1993) 87-96.5. O. Milosevic et al., Thin Solid Films, 296 (1997) 44 - 48.6.M. Shusser, T. Ytrehus, D. Weihs, Fluid Dynamics

Research 27 (2000) 353.7. T-Q. Liu, O. Sakurai, N. Mizutani, M. Kato, Joumal of

Materials Science (1986) 3698-3703.8. S. y. Cho, J. H. Lee, S. J. Park, J. ofMaterials Science 30

(1995) 3274-32789. B. Zhao, R. Zhang, L. Lu, H. Xie, Material Science and

Engineering B49 (1997) 36-41.