AEROSOL CHEMICAL PROCESSES IN THE ENVIRONMENT - CHAPTER 11 docx

Characterization of Zirconia Powder Produced by Atomization Claude Landron CONTENTS Introduction ...249 X-ray Absorption...250 Sample Elaboration ...251 EXAFS Experiments...251 Results

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Part III

Aerosol Synthetic Chemistry

L829/frame/ch11 Page 247 Tuesday, February 1, 2000 3:55 PM

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Characterization of Zirconia Powder Produced

by Atomization

Claude Landron

CONTENTS

Introduction 249

X-ray Absorption 250

Sample Elaboration 251

EXAFS Experiments 251

Results and Discussion 252

Conclusion 254

References 255

INTRODUCTION

New manufacturing techniques for ceramic processing have been carefully explored as means of producing powders with a variety of well-defined characteristics The quality of the final product can be significantly improved by the choice of an efficient production process The control of the shrinkage depends drastically on the size and on the microporosity of the particles It is well-known that the formation of hollow spheres has a negative effect on densifying and sintering Another important problem related to the particle shape is illustrated by the fact that strongly agglomerated powders give difficulties in sintering conventional powders Atomization is an alternative procedure that presents many advantages to produce ceramic.1 It has been used to elaborate high-Tc super-conductor powders.2 As a matter of fact, the synthesis of powders by the spray pyrolysis of the precursor solution can be considered a containerless experiment Thus, chemical segregation of cations is reduced to a single microsized drop, thereby ensuring good stoichiometry control Among the techniques for preparation of aerosols (such as condensation methods, dispersion methods, electrical atomization, vibrating orifice technique), we have selected ultrasonic atomization, which

is suitable for controlling the distribution of the powder particle size

Although aqueous solutions occupy an important place in material chemistry, relatively few studies have dealt with structural investigations of the liquid state, for which detailed, accurate information related to atomic distance and coordination number would be useful It has been shown3

that a liquid zirconium precursor used at high concentration reproduces an environment similar to that of crystallized compounds In recent years, new methods for the characterization at the micrometer scale of small liquid particles which play an increasingly important role in many fields

of material science, has been developed with high spatial and temporal resolution EXAFS L829/frame/ch11 Page 249 Tuesday, February 1, 2000 3:55 PM

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250 Aerosol Chemical Processes in the Environment

(Extended X-ray Absorption Fine Structure) is presently the best in situ technique that can be used

to detect differences on the Angstrom scale in precursor transformation during ceramic synthesis This spectrometric technique gives accurate information concerning the environment of the cations and the presence of anions in the coordination sphere, which has an effect on the structure of the final product This chapter describes the modification of the environment of Zr4+ cations in samples

developed in association with zirconia (ZrO2) powder processing

X-RAY ABSORPTION

Interest in local structure determination has recently stimulated the growth of EXAFS studies.4 The principal advantage of this spectrometry is that it is particularly suitable for the study of disordered, glassy, or vitreous materials.4 EXAFS is based on the measurement of the attenuation of X-ray beam propagation in a condensed medium Attenuation results from three principal processes: pair produc-tion, scattering, and photoelectric absorption In the energy range of the X-rays used in EXAFS (1

to 20 keV), photoelectric absorption dominates the attenuation process The atomic distribution of a selected atom is obtained by Fourier transforming the EXAFS signal as a function of incident photon wavenumber This oscillating signal is related to the variation of the absorption cross-section for the photoexcitation of an electron from a deep core state to a continuum state The oscillations of the absorption spectrum above an absorption edge of one particular type of atom is related to the final state of the excited atom They result from the interferences between the wave function of an outgoing photoelectron that is emitted during the photoemission process and the part of this wavefunction that

is reflected by the neighboring atoms Because of the shortness of its mean free path, the photoelectron can be considered a local probe for structure determination The oscillations essentially depend on the number, the distance, the disorder, and the type of nearest neighbor of the excited atom From theoretical considerations, the absorption function χ(k) = (µ – µ0)/µ0 for a given shell i is given by

(11.1) where

(11.2) and where

fi(k) is the back-scattered amplitude of the shell i

φi(k) is the phase shift related to the atoms of the shell i

σi results from the variation of the R i containing information related to static and dynamic disorder

EXAFS depends only on the local structure by the fact that this length is short, thus resulting in information that is particularly useful in the case of amorphous materials

The radial distribution functions around an excited atom are obtained by extracting5 the modulus

of the complex Fourier transform of k nχ(k):

(11.3)

χ( )k = ∑( [N kR i i2]f k B k i( ) ( )))

B k( )=exp(−2k2σi2)exp(−2R i)sin{2kR i+φi( )k}

k

n

min

max

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Synthesis and Online Characterization of Zirconia Powder Produced by Atomization 251

SAMPLE ELABORATION

The starting solution was prepared by dissolving reagent-grade zirconium nitrate salts in distilled water to yield a 0.5 N solution The experimental set-up used for in situ X-ray absorption mea-surements of precursors during ceramic synthesis comprises an ultrasonic nebulizer coupled with

a detection chamber6 as shown in Figure 11.1 and Figure 11.2 Note two important advantages of the ultrasonic nebulizer: (1) the aerosol is practically monodispersed, and (2) both the aerosol production rate and the carrier gas flow rate can be independently varied

The micrometer-sized spheres are circulated in the X-ray irradiation cell for XAS experiments The spray chamber is designed to promote good mixing and transport of the aerosol toward the analysis cell where the droplets are irradiated

EXAFS EXPERIMENTS

Spectroscopic measurements were performed at the synchrotron radiation facilities of LURE (Orsay, France) The spectra were recorded above the Zr K-edge in the energy range 179000 to 19000 eV The X-ray beam provided by the 1.85-GeV storage ring of DCI was monochromatized by a double Bragg reflection (311) of two parallel Si crystals The energy resolution was estimated to be 2 eV The maximum intensity of the positron beam was about 300 mA The experimental device designed

FIGURE 11.1 Outline of the experimental set-up used for the X-ray absorption measurements of an aerosol

of micrometer-sized droplets of solutions The apparatus is composed of an ultrasonic atomizer producing the mist from a 5-cm high geyser generated from the solution The spray chamber is designed to promote a good mixing and transport of the aerosol toward the analysis cell where the droplets are irradiated in various situations A closed circuit permits one to keep a constant level in the solution and a stable density of particles

in front of the X-ray source.

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for in situ EXAFS analysis of aerosols is placed after the first ionization chamber of the X-ray beam line In situ absorption measurements of nebulized solutions were thus performed using the fluorescence detection mode, which is more efficient than the transmission mode The conditions for the absorption measurements of nebulized solutions were similar to those of trace analysis in very dilute samples The EXAFS measurement of samples produced by atomization and dried at different temperatures were performed, post mortem, in the transmission mode, on powders They are deposited on a series of tapes in order to obtain a sample thickness, x, which in optimal conditions

is determined by µx = 1, where µ is the absorption coefficient of the powder

RESULTS AND DISCUSSION

of the report on the International Workshop on Standards and Criteria in XAS A Victoreen-type baseline of the pre-edge was removed, and the oscillations were extracted from the background approximated by a cubic spline function and then Fourier transformed as seen in Figure 11.3 The structural parameters were determined through a fitting procedure using the theoretical amplitude

preferred this structural form of zirconia among its various polymorphs (cubic,10 tetragonal,11 or

standards because yttrium atoms used in substitution to stabilize these structures introduce oxygen vacancies that alter the coordination shell of the cations

FIGURE 11.2 Schematic illustration of the device used for in situ EXAFS measurements on nebulized solutions This cell was developed for an adaptation to the EXAFS IV station of the DCI storage ring at LURE (Orsay, France).

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Our structural results concerning an aerosol of a 0.5 N aqueous solution of zirconium nitrate presented are in quantitative accordance with the mean values of the bond lengths obtained by

coordination sphere around the zirconium atoms contains four OH groups, two water molecules, and two oxygen atoms belonging to one bidentate nitrate ligand We find a similar environment for zirconium in the atomized solution of zirconium nitrate Note that previous published data3 are also in agreement; these authors have given d(Zr-O) = 0.222 nm, which is the average of those values found by EXAFS

We have noted that the structure of the precursor depends on its state in the starting solution The anion complexing power has an effect on the formation of regularly shaped powders by avoiding hollow sphere formation The high degree of polymerization of the sulfate precursor solution of hydrolyzed Zr(IV) species is related to the viscosity of the liquid By heating the powder at different temperatures, we have obtained precursors at different polymerization states Another fact is able

to explain the morphology differences observed according to the precursor salt used: a comparison

ZrSO4 clearly shows that HCl continuously leaves the spheres, while the SO2 remains up to the crystallization of zirconia

Figure 11.4 shows the EXAFS spectra of the samples produced by atomization and dried at different temperatures For the amorphous samples elaborated between 200°C and 650°C, no significant changes in the Zr-O distances are observed before crystallization and d(Zr-O) is close

to 0.221 nm The coordination number in the amorphous samples seems to increase with the temperature of elaboration The treatment of the spectrum of the sample elaborated at 800°C shows

FIGURE 11.3 Zirconium K-edge EXAFS of an aerosol of a 0.5 N aqueous solution of zirconium nitrate.

A1: χ l (k) function extracted from experimental EXAFS data; B1: Fourier transform TFl(R) of the k3 χ l (k) function

Fourier transform TF2(R) of the filtered oscillations k3 χ 2 (k); A3: Least-squares fit χ 3 (k) of the filtered spectrum; and B3: Fourier transforms TF3(R) of the function k3 χ 3 (k).

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that the structure of this sample is close to that of the monoclinic zirconia, in which Zr atoms are

surrounded by four oxygen atoms at a distance d = 0.211 nm and three atoms by distance d =

0.230 nm We can conclude that there is no significant modification of the zirconium environment

during this first stage of drying The variation of the coordination number can be related to the

growth of crystallites and by the modification of the chains [Zr(OH)2(SO4)2(H2O)4]n linked by water

molecules We have pointed out that the strongly concentrated solution exhibits an order around

the zirconium atoms that can be compared to the order around zirconium in a crystallized zirconium

sulfate The local structure in the solution is built from chains of dodecahedrons formed by

zirconium atoms that are eightfold oxygen coordinated The present analysis gives a Zr-O average

distance of 0.224 nm, which is not far from the sum of the ionic radii of O2– and VIIIZr4+ The final

zirconia is phase pure, as indicated by XRD

CONCLUSION

Zirconia powders have been synthesized with well-defined characteristics such as hardness, and

resistance useful for high temperature technical applications The elaboration process is based on

atomizing a solution containing the precursors A good conditioning of the aerosol is necessary to

improve our strategy of reducing the defects that can profoundly affect the sintering The application

of the ultrasonic nebulization technique to the synthesis of zirconia leads to materials composed

of homogeneous and narrow size distributed powders with the required particle morphology and

FIGURE 11.4 Comparison of the partial radial distribution function around a zirconium atom of a nebulized

650°C, and (E) 800°C.

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size distribution suitable for sintering Structural study by X-ray absorption spectroscopy has shown

that the different stages of the application of heat treatment appear to control the alteration of

particle morphology Absorption experiments have evidenced that crystallite growth in the

individ-ual particles is strongly related to the local structural development around the Zr4+ cations One

can characterize the zirconium environment during a heating process between its precursor liquid

state and its final monoclinic ZrO2 form Interesting results has been derived about the correlation

between the complexing power of an active anion and the final particle morphology, that is an

important research challenge in ceramic science

REFERENCES

1 Dubois, B., Ruffier, D., and Odier, P., Preparation of fine, spherical yttria-stabilized zirconia by the

3 McWhan, D.B and Lundgren, G., The crystal structures of some zirconium hydroxide salts, Acta

Cryst., 16, A36, 1963.

4 Koningsberger, D.C., General principle of analysis, X-Ray Absorption, J.D Winefordner, Ed., Chap 6,

John Wiley & Sons, New York, 1989, 243.

5 Teo, B.K., EXAFS: basic principles and data analysis, Inorg Chem Conc., Vol 9, Springer-Verlag,

Berlin, 1986, 26.

6 Landron, C., Odier, P., and Bazin, D., In situ XAS of aerosol systems: application to the structural

study of a zirconia precursor, Europhysics Lett., 21, 859, 1993.

7 Landron, C., Ruffier, D., Dubois, B., Odier, P., Bonnin, D., and Dexpert, H., Etude par EXAFS du

8 McKale, A.G., Veal, B.W., Paulikas, A.P., Chan, S.K., and Knapp, G.S., Generalized

Ramsauer-Townsend effect in extended XAS, Phys Rev B, 38, 10919, 1988.

9 Catlow, C.R.A., Chadwick, A.V., Greaves, G.N., and Moroney, L.M., EXAFS study of yttria stabilized

zirconia, J Am Ceram Soc., 69, 272, 1986.

10 Clearfield, A., Structural aspects of zirconium chemistry, Rev Pure and Appl Chem 14, 91, 1964.

11 Berthet, P., Berton, J., and Revecolevschi, A., Etude par EXAFS de l’environnement du zirconium

dans des materiaux prepares … partir d’alcoxydes metalliques, J Phys., 47, C8-729, 1987.

12 Livage, J., Doi, K., and Mazieres, C., Nature and thermal evolution of amorphous hydrated zirconium

oxide, J Am Ceram Soc., 51, 349, 1968.

X-ray powder diffraction data, J Solid State Chem., 94, 27, 1991.

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