Diss. ETH No. 16401

Controlled Synthesis of Mixed OxideNanoparticles by Flame Spray Pyrolysis

A dissertation submitted to the

SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICHfor the degree of

DOCTOR OF TECHNICAL SCIENCES

presented by

RAINER JOSSENDipl. Chem. Ing. ETH Zurich

born on August 19th, 1974citizen of Naters VS, Switzerland

Accepted on the recommendation of

Prof. Dr. Sotiris E. Pratsinis, examinerProf. Dr. Greg Beaucage, co-examiner

Zurich, 2006

Rose, oh reiner Widerspruch, Lust,

Niemandes, Schlaf zu sein unter so vielen Lidern.

Rainer Maria Rilke

Acknowledgement

This work was carried out at the Particle Technology Laboratory at the ETH Zurich. I

would like to acknowledge the financial support by the TH-Gesuch No. 34/02-3, Willi-

Studer-Fonds, and the Millennium Chemical Research Center, USA. Furthermore, I would

like to express my thanks to the following people, who contributed significantly to the

success of this work:

Prof. Sotiris E. Pratsinis, for giving me the opportunity to carry out this Ph.D

thesis in his team, for his continuous encouragement and his valuable advice as well as

for creating a dynamic and stimulating atmosphere; Prof. Gregory W. Beaucage for

co-advising this work, for the introduction in small angle X-ray scattering and in light

scattering during his stay in Zurich and also the time I could spend with him.

Furthermore, I appreciate the work of help assistants: Sarah Lanfranchi, Tobias

Weber, Frederick Marxer, Tim Patey and Juan Carlos Andresen as well as the fruitful

discussion and collaboration with Dr. Hendrik K. Kammler, Dr. Lutz Madler, Dr. Roger

Muller, Martin C. Heine and Alexandra Teleki, Dr. Frank Krumeich for the TEM-images

and the whole PTL-team. I would like to acknowledge the work and support from the IPE

machine shop especially to Rene Pluss. Finally, to Ileana Eugster for helpful discussion

beside technology and science.

I

CONTENTS

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI

1 Criteria for flame spray synthesis of hollow, shell-like or inhomogeneous

oxides 1

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Powder synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.2 Powder characterization . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3.1 SiO2 Particle Size and Morphology . . . . . . . . . . . . . . . . . . 7

1.3.2 Bi2O3 Particle Size and Morphology . . . . . . . . . . . . . . . . . . 11

1.3.3 Morphology mapping of FSP-made oxides . . . . . . . . . . . . . . 17

1.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2 Morphology and composition of spray-flame-made yttria-stabilized zir-

conia nanoparticles 25

III

IV

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.2.1 Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.2.2 Precursor solution selection and preparation . . . . . . . . . . . . . 28

2.2.3 Powder characterization . . . . . . . . . . . . . . . . . . . . . . . . 30

2.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.3.1 Effect of precursor on particle morphology . . . . . . . . . . . . . . 31

2.3.2 YSZ crystal and primary particle sizes . . . . . . . . . . . . . . . . 38

2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3 Thermal stability of flame-made zirconia-based mixed oxides 49

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.2.1 Precursor preparation and thermal stability characterization . . . . 51

3.2.2 Apparatus and processing . . . . . . . . . . . . . . . . . . . . . . . 51

3.2.3 Powder characterization . . . . . . . . . . . . . . . . . . . . . . . . 52

3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.3.1 Pure ZrO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.3.2 Yttria-doped zirconia . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.3.3 Ceria-doped zirconia . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.3.4 Silica-doped zirconia . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4 Thermal Stability and Catalytic Activity of Flame-made Silica-Vanadia-

Tungsten oxide-Titania 67

VAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.2.1 Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.2.2 Material Characterization . . . . . . . . . . . . . . . . . . . . . . . 70

4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4.3.1 Powder characterization . . . . . . . . . . . . . . . . . . . . . . . . 71

4.3.2 Catalytic performance . . . . . . . . . . . . . . . . . . . . . . . . . 82

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

5 Non-intrusive droplet and particle dynamics during spray flame synthe-

sis of nano ZrO2 89

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

5.2.1 Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

5.2.2 Spray flame characterization . . . . . . . . . . . . . . . . . . . . . . 94

5.3 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

5.4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

5.4.1 Spray flame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

5.4.2 Droplet evaporation by SAXS . . . . . . . . . . . . . . . . . . . . . 103

5.4.3 Primary particle growth by SAXS . . . . . . . . . . . . . . . . . . . 104

5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

6 Research Recommendations 117

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

VI

A Reproducibility of nanoparticle production and color investigation on

titania particle made by pilot scale FSP 123

A.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

A.1.1 Reproducibility of pilot scale FSP . . . . . . . . . . . . . . . . . . . 124

A.1.2 Investigation on the yellowish color of titania made by pilot scale

FSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

A.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

A.2.1 Reproducibility of pilot scale FSP . . . . . . . . . . . . . . . . . . . 125

A.2.2 Investigation on the yellowish color of titania made by pilot scale

FSP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

B Flame spray scale-up investigation 127

B.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

C Systematical study of silica contaminated YSZ 131

C.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

C.1.1 Precursor preparation . . . . . . . . . . . . . . . . . . . . . . . . . 131

C.1.2 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

C.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

D Thermophoretice sampling in spray flames 135

D.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

D.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

Appendix: References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

Curriculum Vitae 140

Refereed Publications 141

Presentations 141

Zusammenfassung

Funktionnelle Oxide konnen mit Flammen-Spruh-Pyrolyse (FSP) Technik hergestellt wer-

den, die als Katalysatoren, stabile Quantenpunkte oder als Elektrolyt in Brennstoffzellen

angewended werden konnen. Jede dieser Anwendung benotig aber ein bestimmte durch-

schnittliche Partikelgrosse, Partikelverteilung, Agglomerate mit bestimmter fraktalen Di-

mension, Morphology und zuletzt bei Mehrkomponentensystem die richtige chemische

Zusammensetzung.

In FSP werden ein oder mehrere metallische Ausgangsprodukte in einem Losungs-

mittel gelost, mittels einer Duse zu Tropfen verspruht und anschliessend gezundet. Im

ersten Kapitel wird der Einfluss der Ausgangsprodukte und der Losungsmittelkomposition

erforscht. Hierzu wurde Silizumoxid und Bismuthoxid hergestellt und analysiert. Zur Her-

stellung von Siliziumoxid wurden Siliziumalkoxide mit verschieden Siedepunkte in Xylol

gelost oder Tetraethylorthosilikat wurde in verschiedenen Alkane gelost auch mit ver-

schieden Siedpunkte. Fur ein nicht verdampfbares Systhem wurde Bismuth Nitrat Pen-

tahydrat als Ausgangschemikalie verwendet und in verschiedenen Alkohlen mit verschiede-

nen Siedepunkten gelost. Mittels Transmission-Elektronenmikroskopieanalyise (TEM)

und Rontgenbeugung (XRD) wurde die Morphologie der Partikel bestimmt. Es hat sich

gezeigt, dass inhomogene und/oder hohle Partikel nur bei tiefer Verbrennungswarmedichte

und wenn der Siedepunkt des Losungsmittel tiefer als der Schmelz- oder Zerfallpunkt der

Ausgansprodukte ist, entstehen. Dies stimmt auch mit Daten aus der Literatur uberein.

VII

VIII

Im zweiten Kapitel wurden die Untersuchungen auf Yttriumoxid stabilisiertes Zirkon-

iumoxid (YSZ), einem Zweistoffsystem, erweitert. Es zeigte sich, dass homogenes YSZ

mit organometallischen Ausgangsprodukten oder mit Yttrium Nitrat Hexahydrat und

Zirkoniumkarbonat, behandelt mit 2- Ethylhexansaure, hergestellt werden kann. Wenn

organometallische Chemikalien mit Ausganstoffen, die Hydratwasser beinhalten gemis-

cht werden, werden Partikel mit inhomogener Morphologie und chemischer Komposition

produziert. Alkoxide sind sehr wasserempfindlich und reagieren rasch zu Hydroxiden,

welche in der Metalllosung ausfallen konne. Mit XRD konnten zwei durchschnitts Kristall-

durchmesser gemessen werden, deren Verhaltnis die Hohe der Inhomogenitat angibt.

Im dritten und vierten Kapitel wird das Gefundene von Kapitel eins und zwei

angewendet. Zirkonoxid basierende Partikel wurden auf ihre Thermostabilitat getestet

bezuglich Oberflachenverlust und Kristallinitat. Siliziumoxide und Vanadiumoxid dotie-

tes Wolframoxide/Titanoxide wird ebenfalls auf seine Thermostabilitat gepruft. Spezielle

Wert wurde auf den Effekt von Siliziumoxid auf die Thermo-, Kristallstabilitat und die

katalytisches Verhalten im DeNOx Pozess gelegt.

Zur Zeit werden nur die Endprodukte analytisch untersucht. Die Kombination

von ortlichen Messmethoden, die die Partikelbildung nicht beeinflussen, konnen Auf-

schluss geben, wie die Evolution der Partikelmorphologie im FSP vor sich geht. Fourie

transformierte Infrarot Spektroskopie (FTIR) wird eingesetzt um Gastemperaturen zu

messen, Phasen Doppler Anometrie (PDA) fur Tropfen- und Gasgeschwindikeit, Ther-

mophoretische Probenahme (TS) um das Partikelwachsum zu zeigen, und Kleinwinkel-

Rontgenstreuung (SAXS) und Lichtstreuung (LS) fur detailliertes Partikelwachstum und

Agglomerationsentwicklung zu studieren.

Partikle- und Tropfendynamik wird in Kaptiel funf mit ortlich aufgelosten und nicht

storenden (in-situ) Techniken studiert. In einer 15 g/h produzierenden Zirkoniumoxid

Spruhflamme wurde mit PDA Gasgeschwindigkeiten und mit FTIR Gastemperaturen

gemessen. Mit SAXS kann man gleichzeitig Primarpartikelradius, fractal Dimenssione

der Agglomerate, geometrische Standartabweichung, Partikelvolumenfraktion, Partikel-

IX

anzahlkonzentration, Agglomeratenradius und die Anzahl Primarpartikel pro Agglomer-

ate bestimmen. Fur SAXS konnte die dritte Synchroton Generation (ERSF, Grenobel)

benutzt werden, da diese genugend Energie liefert um Messungen mit einer Volumenfrak-

tion von weniger als 106 durchzufuhren.

Summary

The control of nanoparticle characteristics during flame synthesis is crucial since prop-

erties of the final product made from these particles depend on their size distribution,

morphology, extent of aggregation as well as chemical and phase composition. Flame

spray pyrolysis (FSP) for synthesis of functional oxide systems can be used for novel cat-

alysts, stable quantum dots and fuel cells to mention some of their potential applications.

The influence of metal precursor and solvent composition on the morphology of SiO2,

Bi2O3 and other oxide particles made by flame spray pyrolysis (FSP) was investigated in

the first chapter. Silica precursors with different boiling points were dissolved in xylene

and different solvents were used to dissolve tetraethyl-orthosilicate (TEOS). For Bi2O3,

non-volatile bismuth nitrate pentahydrate was dissolved in different alcohols with different

boiling points. From these data and from the literature of FSP synthesis it is inferred

that hollow/inhomogeneous particles were formed at low combustion enthalpy densities

and when the solvent boiling point was comparable or smaller than the precursor melting

or decomposition point.

In the second chapter the investigation of homogeneity and chemical composition

was extended to a multiple system like yttria stabilized zirconia (YSZ). While in chap-

ter one the production rate was relatively low (20 g/h), here, the production rate wasincreased up to 350 g/h. The effect of liquid precursor composition on product particle

morphology, composition and crystallinity was investigated. Flame-made YSZ nanopar-

XI

XII

ticles of homogeneous composition and morphology were formed when using either only

organometallic zirconium and yttrium precursors or 2-ethylhexanoic acid as solvent and

inexpensive zirconium carbonate and yttrium nitrate hexahydrate as precursors. In con-

trast, and consistent with the literature, hollow or inhomogeneous YSZ particles were

made when organometallic zirconium and yttrium nitrate precursors of high water con-

tent were employed, especially at high production rate. The ratio of XRD-determined

small to large sizes for inhomogeneous crystalline particles is an effective quantitative

measure of their degree of inhomogeneity.

In the third and fourth chapter the finding of chapter one and two will be applied

by making mixed oxide for catalytic applications. First, thermostability of zirconia based

materials were tested by sintering them in an oven at different temperatures and second

the effect of doped WO3/TiO2 powders with vanadia and silica on their specific surface

area, crystallinity, thermostability and catalytic behavior.

At the moment, most of the analysis of product particles was performed after the

final product has been formed. The combination and implementation of existing on-line

spray probing techniques has the potential to yield valuable local information on particle

size and morphology evolution without disturbing the process. This will be used to better

understand the FSP process itself that would lead to optimal process design and control

and finally tailor-made-products by FSP. Hence FTIR will be used for temperature mea-

surement, Phase Doppler Anemometry (PDA) will allow to estimate the droplet lifetime

in the spray by measuring droplet velocity and droplet size evolution, thermophoretic

sampling (TS) to measure primary particle growth, while small angle X-ray scattering

(SAXS) and light scattering (LS) will enable the on-line monitoring of the nanoparticle

morphology and aggregate size evolution. Except from TS, these techniques are all in-situ

on-line methods.

Therefore, droplet and particle dynamics were studied in chapter five in situand non-intrusively in a particle-laden spray flame producing 15 g/h zirconia. Droplet

velocities were measured by 2D Phase Doppler Anemometry (PDA) and droplets smaller

XIII

than 4 m were used to estimate gas velocity. Gas temperature was measured by Fourier

Transform Infrared (FTIR) emission/transmission spectroscopy. In situ small anglex-ray scattering (SAXS) was used to measure simultaneously the evolution of primary-

particle diameter, mass-fractal dimension, geometric standard deviation, particle volume

fraction, particle number concentration, agglomerate size and number of primary particles

per agglomerate of zirconia in the spray flame. For SAXS, a third-generation synchrotron

source is used where nano to micro scale measurement were possible.

CHAPTER

ONE

Criteria for flame spray synthesis of hollow, shell-like

or inhomogeneous oxides

Abstract

The influence of metal precursor and solvent composition on the morphology of SiO2,

Bi2O3 and other oxide particles made by flame spray pyrolysis (FSP) was investigated.

Silica precursors with boiling points Tbp = 299-548 K dissolved in xylene were used as well

as different solvents (Tbp = 308-557 K) with tetraethyl-orthosilicate (TEOS) as the silica

precursor. For Bi2O3, non-volatile bismuth nitrate pentahydrate was dissolved in solvents

with Tbp = 338-468 K. Product powders were characterized by nitrogen adsorption, X-ray

diffraction and scanning and transmission electron microscopy. From these data as well

as from the literature of FSP synthesis of Bi2O3, CeO2, MgO, ZnO, Fe2O3, Y2O3, Al2O3

and Mg-Al spinel it is inferred that hollow/inhomogeneous particles are formed at low

combustion enthalpy densities and when the solvent boiling point is comparable or smaller

than the precursor melting or decomposition point.

1

21.1 Introduction

Silica and titania nanoparticles are produced in mega ton quantities annually by flame

technology, a manufacturing route that is investigated currently for synthesis of other

commodity and novel functional metal and ceramic nanoparticles [1, 2]. While volatile

metal-chlorides are fed as gases in a flame resulting in their metal oxides, liquids are

sprayed during FSP [3]. Liquid precursors considerably broaden the range of accessible

products from simple and mixed oxides for lasing materials [4] to alumina-supported

platinum catalysts [5] for enantioselective hydrogenation of ethyl pyruvate in synthesis of

chiral pharmaceuticals to name just a couple of examples.

Recent research activities have improved the understanding of particle formation and

growth during FSP for control of product particle size, crystallinity and even scale-up [4-8].

For example, Madler et al. [7] found that the specific surface area of silica made by FSP of

hexamethyldisilane (HMDSO) at constant liquid volumetric flow rate was decreased using

methanol, ethanol or iso-octane as solvents in this order. As the combustion enthalpy per

unit volume of these solvents was increased, higher flame temperatures enhanced sintering

resulting in larger particles. As result, this process has been scaled up for synthesis up

to 1 kg/h of silica [9] and ZrO2 [10] nanoparticles. Limaya and Helble [11] made zirconia

particles by FSP of zirconium(IV) n-butoxide in butanol and they postulated liquid-

and vapor-phase pathways of the final particle formation depending on the applied flame

temperature.

In some cases, however, FSP has produced inhomogeneous powders. Suyama and

Kato [12] made mostly hollow Mg-Al spinel by spray pyrolysis using magnesium and

aluminum nitrate co-dissolved in ethanol. Madler and Pratsinis [13] obtained hollow

bismuth oxide particles by FSP of bismuth nitrate dissolved in nitric acid/ethanol. By

replacing ethanol with acetic acid, they produced homogeneous, non-hollow, solid Bi2O3

particles. Similarly, solid but inhomogeneous ceria particles were made by FSP using

cerium acetate dissolved in acetic acid [8]. Adding an iso-octane/2-butanol mixture to

the precursor solution resulted in rather homogeneous powders of 4-8 nm primary particle

3diameter. Tani et al. [14] produced thinly-shelled hollow (eggshell-like) or solid -Al2O3

particles by FSP of aqueous emulsion of aluminum nitrate when using air or oxygen,

respectively, as dispersion/oxidant gas. They also produced hollow TiO2, ZrO2, Y2O3

or solid ZnO, Fe2O3, CeO2 and MgO particles by FSP of aqueous emulsion in air of

the corresponding nitrate or chloride precursors [14]. Some of the solid product powders

(CeO2, ZnO and MgO) were quite inhomogeneous with a broad, nearly bimodal size

distribution.

Even though hollow or porous particles can be useful for insulation, catalyst supports

or encapsulation matrices [15], they are generally an undesired by-product. However,

systematic studies of the effect of process variables on FSP-made particle morphology

are still missing. It is therefore of interest to identify FSP conditions and criteria that

prevent synthesis of inhomogeneous powders. In this study the effect of liquid precursor

on product morphology is investigated during FSP synthesis of silica and bismuth oxide.

Design and operation criteria for synthesis of homogeneous particles are proposed and

compared with the present data and pertinent ones in the literature.

1.2 Experimental

1.2.1 Powder synthesis

Figure 1.1 shows a schematic of the experimental set-up of the small flame spray (FSP)

[7, 8]. Silica and bismuth oxide powders were produced by this FSP using air or oxygen (>

99.95% purity, Pan Gas, Luzern, Switzerland) as oxidant / dispersion gas. A constant 1.5

bar gas pressure drop at the burner nozzle tip was maintained by adjusting the nozzle-gap

width. The gas flow rates were controlled by mass flow controllers (Bronkhorst, Ruurlo,

The Netherlands). The liquid precursor solution was fed through the nozzle by a syringe

pump (IER-232, Inotech, Oberwinterthur, Switzerland). The resulting spray was ignited

by a ring of 18 supporting premixed flamelets sustained by 0.5 l/min CH4 and 2 l/min

O2 [8]. An additional oxygen sheath gas flow (5 l/min) was supplied through a sintered

4metal plate ring 8 mm wide with an inner diameter of 9 mm surrounding the supporting

flamelets [8] .The particles were collected on a glassfiber filter (diameter of 150 mm;

Wathman GF/A) using a vacuum pump (Vaccubrand, Type RZ 16, Germany).

The effect of precursor composition on flame-made silica characteristics was inves-

tigated by spraying 1.5 ml/min of various silicon precursors (Table 1.1a, 0.88 M Si in

xylene (> 98%, Sigma-Aldrich, Fluka Chemie GmbH, Buchs, Switzerland)) to the flame.

The combustion enthalpy density was defined as the ratio of the fed liquid precursor com-

bustion enthalpy (kJ/min) over the total gas flow (ggas/min) within the spray. The effect

of solvent composition (Table 1.1b) on silica characteristics was studied using tetraethyl-

orthosilicate (TEOS) as Si-precursor. Since the combustion enthalpy per mass of alkane

solvents is similar but their densities vary from 0.62 (pentane) to 0.77 g/cm3 (hexade-

cane), the silica precursor concentration was adjusted between 0.64 M and 0.79 M. All

solvent experiments were performed at combustion enthalpy density of 6.4 0.1 kJ/ggasand silica production rate of 4.65 g/h, silica with 3 l/min of air as dispersion gas using

liquid feed rates from 1.65 (hexadecane) to 2 ml/min (pentane).

Bismuth trinitrate pentahydrate (Bi(NO3)35H2O, > 98%; Sigma-Aldrich, FlukaChemie GmbH, Buchs, Switzerland) was dissolved in HNO3 / alcohol mixtures (15 vol% of

nitric acid (65 %, Sigma-Aldrich, Fluka Chemie GmbH, Buchs, Switzerland) and 85 vol%

of the corresponding alcohol, Table 1.1c). Since the density and combustion enthalpy per

unit mass of these alcohols vary, the feed rate and bismuth concentration were adjusted

(Table 1.1c) to keep production rate at 11.5 g/h Bi2O3 and total combustion enthalpy

density of 2.2 0.03 kJ/ggas. The total combustion enthalpy density was increased to 4.0 0.09 and 4.7 0.2 kJ/ggas by increasing the precursor feed rates by a factor of 2 and2.65, respectively. For Bi2O3 3 l/min oxygen was used as dispersion/oxidant gas and 1.58

l/min CH4 and 1.38 l/min O2 for the supporting flamelets.

5Figure 1.1: Sketch of a typical flame spray pyrolysis unit. The precursor mixture is

rapidly dispersed by oxygen or air and fed into the premixed flame methane / oxygen

stream.

6Table 1.1: Process conditions and properties of solvents and precursors em-

ployed in the FSP synthesis of SiO2 and Bi2O3.

Solvent boiling feed rate conc. sourcea

point K ml/min mol/l purity (%)

(a) Si-Precursor properties in xylene (Tbp = 410-416 K)

TMS, Si(CH3)4 299-300 1.5 0.88 >99.5

HMDSO, C6H18OSi2 374 1.5 0.88 >99

TEMOS, Si(CH3O)4 393-341 1.5 0.88 >99

TEOS, Si(CH3CH2O)4 436-440 1.5 0.88 >98

TEPOS, Si(CH3CH2CH2O)4 548 1.5 0.88 >97

(b) Solvents for tertaethyl-orthosilcate (TEOS, Tbp = 436-440 K)

Pentane, C5H12 308-309 2.0 0.64 >99

Hexane, C6H14 342 1.89 0.68 >99.5

iso-Octane, C8H18 372 1.81 0.71 >99.5

Octane fraction 397-401 1.8 0.72

Decane fraction 441-451 1.73 0.75

Dodecane, C12H26 486-489 1.70 0.77 90-95

Tetradecane, C14H30 523-526 1.67 0.78 >97

Hexadecane, C16H34 556-559 1.65 0.79 >98

(c) Solvent properties employed in the FSP synthesis of Bi2O3

Methanol (MeOH) 338 1.31 0.31 >99.8

Ethanol (EtOH) 351 1.00 0.41 >99.9

Methoxy-2-propanol 391 1.03 0.39 >98

Ethoxy-ethanol 408-410 0.80 0.51 >99

Porpylene glycol propylether 413-423 0.89 0.46 >98.8

Diethylene glycol-monoethylether 468 0.90 0.45 >98

a Simga-Adrich, Chemie GmbH, Buchs, Switzerland. HMDSO, hexamethyl-

disilane; TEOS, tetraethyl-orthosilicate;TMS, tetramethylsilane; TEPOS,

tetrapropyl-orthosilica; TEMOS, tetramethyl-orthoslicate

71.2.2 Powder characterization

The powder specific surface area (SSA) was measured by nitrogen adsorption at 77 K

(BET-method, Micrometrics Inc., Gemini 2375, The Netherlands) after degassing samples

for at least 1 h at 150C under nitrogen. The average BET-equivalent primary particle

diameter is dBET = 6/(SSA p), where p is the density of SiO2 (2.2 g/cm3) or -Bi2O3(8.9 g/cm3). The crystallinity of bismuth oxide was measured by X-ray diffraction (40

kV, 40 mA) using a step size of 0.05 and a scan speed of 0.25/min (Burker Advanced

D8, Karlsruhe, Germany). The XRD spectra were analyzed using the Topas 2.0 software

(Bruker AXS, 2000). Measured XRD patterns were regressed with the crystalline data

of -Bi2O3 (PDF #78-1793 [16]). The morphology of product powders was analyzed by

scanning electron microscopy (SEM) operated at 30 kV (Hitachi Model S900). Both,

secondary-electron (SE) and back-scattered electron (BS) SEM images were taken to

distinguish from hollow and dense particles [13]. Further analysis was performed by

transmission electron microscopy (TEM, Zeiss 912 Omega; Jena, Germany operated at

100 kV using a slow CCD camera and the Proscan software). TEM pictures were prepared

by dipping the TEM grid into the product powder.

1.3 Results and Discussion

1.3.1 SiO2 Particle Size and Morphology

The effect of silica precursor/solvent composition on product primary particle size, dBET ,

was investigated at constant metal concentration, gas flow rate and combustion enthalpy

density. Using different silica precursors and solvents, the relative volatility of the sprayed

liquid was varied systematically. The relative volatility, TR, is the ratio of the boiling point

of the precursor over that of the solvent.

Figure 1.2 shows the dBET of silica as a function of TR for various precursors (Table

1.1a) dissolved in xylene (triangles) and for TEOS (circles) dissolved in various alkane

8Figure 1.2: Primary particle BET diameter of silica using TEOS/alkanes (circles)

and xylene/Si-precursors (triangles) as a function of the relative boiling point (Tbp(Si-

precursor)/Tbp(solvent)) at constant enthalpy density of 6.2 kJ/ggas. The volatility of

either precursor or droplet has no effect on particle size and morphology as homogeneous

particles were formed at all conditions.

9solvents (Table 1.1b). Error bars represent twice the standard deviation of reproduced

experiments. The total combustion enthalpy density was kept constant at 6.2 kJ/ggas.

Increasing Si-precursor Tbp from 299 to 548 K (increases TR from 0.75 to 1.37) had no

effect on the dBET (13.8 1 nm). Likewise, increasing the solvent boiling point (Tbp) from308 to 559 K (decreasing TR from 1.42 to 0.79) had also no significant influence on dBET

indicating that the employed solvent/precursor compositions were of minor importance

in flame spray synthesis of silica at these conditions. Figure 1.3 shows TEM images of

homogeneous silica agglomerates made using TEOS/pentane (a, TR=1.42), TMS/xylene

(b, TR=0.75), TEOS/hexadecane (c, TR=0.79) and TEPOS/xylene (d, TR=1.37). No

distinct size differences or hollow particles were observed in any silica products. This

morphology is similar to fumed silica made by feeding gaseous Si-precursor (e.g. SiCl4 or

HMDSO) to flames resulting in fumed silica [17].

Apparently, silica particle formation takes place in the gas phase as the precursor

fully evaporates and forms product particles by chemical reaction, coagulation and sinter-

ing [7]. If particle formation would have taken place at or within the droplet, mass transfer

to the gas phase would be the rate-limiting step and spherical or broken shells may be

formed [15]. This was not observed and therefore dBET was not affected by the precursor

and solvent composition (Figure 1.2). Even though the difference in vapor pressure of

TEPOS (Tbp = 548 K) and xylene (Tbp = 410-416 K) is about 5 orders of magnitude at

room temperature, no significant difference in dBET was observed as the total combustion

enthalpy density was rather constant [7]. The droplet evaporation history for volatile

silica precursors has no significant effect on particle formation (size and morphology) so

major process parameters are combustion enthalpy and metal concentration in the flame.

This is in agreement with Briesen et al. [18] who found that the combustion enthalpy or

adiabatic flame temperature determines the product silica primary particle size made in

a vapor-fed premixed flame reactor.

10

Figure 1.3: Transmission electron microscope images of FSP-made silica at constant

combustion enthalpy density and production rate from a) TEOS/pentane, b) TMS/xylene,

c) TEOS/hexadecane and d) TEPOS/xylene. At all conditions homogenous particles were

formed.

11

1.3.2 Bi2O3 Particle Size and Morphology

Since typically non-volatile metal precursors are employed in FSP, the effect of solvent

composition was investigated for synthesis of Bi2O3 from non-volatile bismuth nitrate

dissolved in different alcohols (Table 1.1c). Experiments were carried out at three com-

bustion enthalpy densities but constant metal concentration and gas flow rates.

Figure 1.4 shows the dBET of Bi2O3 as a function of the employed alcohol (solvent)

boiling point, Tbp, at combustion enthalpy density of 2.2 (diamonds), 4.0 (circles) and

4.7 kJ/ggas (triangles). The primary particle size is not affected by the alcohol boiling

point (Tbp) as with silica and remains constant at 14 1 nm, 17 1 nm and 23 1 nmfor combustion enthalpy densities of 2.2, 4.0 and 4.7 kJ/ggas, respectively. The increase in

particle size from 14 to 23 nm by increasing the combustion enthalpy density is consistent

with the current understanding of the effect of adiabatic flame temperature and product

particle size [18]. With increasing combustion enthalpy density from 2.2 to 4.7 kJ/ggas,

the flame height increased from 55 to 75 mm similar to vapor-fed flame synthesis of

silica [18]. The increased residence time at high temperature in the flame leads to longer

residence time for particle sintering resulting in larger primary particles and lower specific

surface area [1, 7] For the lower combustion enthalpy densities, 2.2 and 4.0 kJ/ggas and

for the lowest boiling point solvent (MeOH), a slightly larger dBET was measured at each

combustion enthalpy. The latter Bi2O3 powders, however, are inhomogeneous and contain

larger particles that decrease the SSA and increase the dBET as it will be shown shortly

by microscopy and X-ray diffraction.

Figure 1.5 shows TEM images of bismuth oxide particles made with a) EtOH as sol-

vent at a feed rate of 3.5 ml/min resulting in combustion enthalpy density of 4.7 kJ/ggas;

b) methoxy-2-propanol as solvent at a feed rate of either 1.80 ml/min resulting in com-

bustion enthalpy density of 4.0 kJ/ggas or c) 0.9 ml/min resulting in combustion enthalpy

density of 2.0 kJ/ggas and d) ethoxy-ethanol as solvent at a feed rate of 1.6 ml/min re-

sulting in combustion enthalpy density of 4.0 kJ/ggas. In all cases (Figure 1.5a-d), only

homogeneous particles were formed. Hollow or large particles were not found at high

12

Figure 1.4: Primary particle BET diameter of Bi2O3 made by FSP from bismuth nitrate

pentahydrate/HNO3/alcohol solutions as a function of the boiling point of the employed

alcohol solvent for three combustion enthalpy densities. At constant combustion enthalpy

density, the solvent composition has little influence on product primary particle size.

Higher production rate corresponding to higher combustion enthalpy density and longer

flames prolong the particle residence time at high temperature resulting larger particles.

13

combustion enthalpy density ( 4.7 kJ/ggas) or with solvents having boiling points largerthan 391 K (Table 1.1c).

In contrast, Figure 1.6 shows images of inhomogeneous Bi2O3 powder made using

EtOH as solvent at a feed rate of 1 ml/min resulting in a combustion enthalpy density

of 2.2 kJ/ggas. Figure 1.6a shows a TEM picture of a shell-like particle together with

fine particles ( 10 nm). Figure 1.6b shows a secondary electron (SE)-SEM image andFigure 1.6c the corresponding backscattered electron (BS)-SEM image of larger, fine and

shell-like particles. Dense, large particles can be observed (white structures in Figure

1.6c) with particle diameters varying from 250 to 500 nm whereas the hollow ones appear

gray and the fine particles are barely visible. Similar results were obtained with MeOH

as solvent.

X-ray diffraction was used to further investigate particle homogeneity. Figure 1.7

shows XRD spectra of the Bi2O3 powders in Figure 1.5a (broken line) and Figure 1.6

(solid line). This figure shows a clear distinction between a smooth (broken line) and

a hump-containing (solid line) XRD corresponding to homogeneous and inhomogeneous,

respectively, powders made at 4.7 and 2.2 kJ/ggas combustion enthalpy density. For the

inhomogeneous powder (Figure 1.6), a bimodal crystal size distribution can be derived

from XRD [16] resulting in average crystal sizes of 108 nm (15 % by mass) and 3 nm (85%

by mass) for each size mode. This gives a quantitative measure of crystalline powder

homogeneity and is used throughout this study.

The above results can be placed in the FSP parameter space of combustion enthalpy

density and solvent boiling point, Tbp: Figure 1.8 shows that hollow, shell-like or inho-

mogeneous powders were produced at low combustion enthalpy densities (< 4.7 kJ/ggas)

and with solvents having low boiling points (open symbols). In all other cases, solid

homogeneous, nanostructured Bi2O3 powders were observed (filled symbols). Similarly,

Madler and Pratsinis [13] used acetic acid (AcOH, Tbp = 391 K) as solvent and produced

homogeneous powders. The particle morphology was independent of the combustion en-

thalpy density since they chose a high boiling point solvent (Figure 1.8, filled triangles).

14

Figure 1.5: Transmission electron microscope images of solid, nanostructured Bi2O3

particles made by FSP from a bismuth nitrate pentahydrate/ HNO3 solution in a) EtOH

at a combustion enthalpy density of 4.7 kJ/ggas, b) methoxy-2-propanol at 4.0 or c) at

2.0 kJ/ggas and d) ethoxy-ethanol at 4.0 kJ/ggas.

15

Figure 1.6: Images of hollow, large and fine solid Bi2O3 made by FSP from a bismuth ni-

trate pentahydrate/HNO3/EtOH solution at a combustion enthalpy density of 2.0 kJ/ggas:

a) TEM of a hollow Bi2O3 particle surrounded by very fine ones, b) secondary-electron

(SE) image and corresponding c) backscattered-electron (BS) images. The latter shows

solid particles (white contrast) of about 100-250 nm and a hollow broken shell of 2 m in

diameter (gray).

16

Figure 1.7: XRD spectra of Bi2O3 powder made by FSP from bismuth nitrate dissolved

in EtOH for a combustion enthalpy density of 2.0 (solid line) and of 4.7 (broken line)

kJ/ggas. There is a distinct difference (hump) between inhomogeneous (solid line) and

homogeneous (broken line) Bi2O3.

17

When using, however, more than 20% EtOH with the balance of AcOH as solvent, inho-

mogeneous particles were formed [13]. By calculating the boiling point of AcOH/EtOH

mixture with the Antoine equation, the latter data are in excellent agreement with the

present study (Figure 1.8, open triangles). The melting point of Bi(NO3)35H2O is 349 K[19] and in this study, solvents with Tbp = 338-468 K were used. Hollow particles were

formed if a concentration gradient is formed during solvent evaporation [20] and if the

molten oxide precursor decomposes within this layer. If the salt shells are impermeable to

the solvent, the pressure within the particle increases substantially and particles fragment

or explode [21] resulting in broken shells. From Figure 1.8 it may be inferred that bismuth

nitrate precipitates on the droplet surface forming a shell that can trap remaining sol-

vents (MeOH and EtOH) which evaporate within that shell. The resulting high pressure

inside that hollow sphere disrupts it forming shell-like fragments. For high boiling point

solvents (methoxy-2-propanol to diethylene glycol-monoethylether, Table 1.1c) no shells

were formed. The bismuth nitrate decomposed in the hot solvent and solid nuclei are

formed throughout the droplet. Upon solvent evaporation, these nuclei form extremely

fine oxide particles.

1.3.3 Morphology mapping of FSP-made oxides

The limit between the region of hollow and homogeneous particles can be found close to

the melting point of the bismuth nitrate (Tmp = 349 K), thus corroborating the outlined

mechanism. The decomposition/melting point of bismuth nitrate (Td/mp = 349 K) can be

used to relate the present results to other FSP-studies of particle morphology. Figure 1.9

maps the powder morphology in the FSP parameter space of combustion enthalpy density

as function of the ratio Tbp/Td/mp. Filled symbols represent homogeneous powders while

open represent hollow or inhomogeneous powders.

Circles represent the Bi2O3 data from this study (Figure 1.8). Hollow particles

and inhomogeneous Bi2O3 powders were only found at Tbp/Td/mp < 1 and at combustion

enthalpy densities < 4.7 kJ/ggas. The triangles refer to Madler and Pratsinis [13] who

18

Figure 1.8: Morphology mapping of solid (filled symbols) and hollow/shell-like (open

symbols) particles as a function of solvent boiling point and combustion enthalpy density

during FSP synthesis of Bi2O3 from bismuth nitrate pentahydrate.

19

prepared Bi2O3 by FSP as discussed before (Figure 1.8). Inhomogeneous silica powders

(filled squares) were not made here since the employed Tbp/Td/mp was > 1.4 and the

combustion enthalpy densities were about 6.2 kJ/ggas. The inverse triangles show data of

ceria [8] made by FSP from cerium acetate (Td/mp = 573 K) dissolved in acetic acid (Tbp

= 391 K) resulting in Tbp/Tmp = 0.68. By adding a mixture of iso-octane/2-butanol, the

combustion enthalpy density was increased from 1.2 to 1.8 kJ/ggas at a liquid precursor

feed rate of 1 ml/min. A mixture of large and small particles (open inverse triangles)

was produced at combustion enthalpy densities of 1.2, 1.8 and 3.2 kJ/ggas (2 ml/min

feed rate). When increasing, however, the liquid feed rate to 4 ml/min resulting in a

combustion enthalpy density of 5.9 kJ/ggas, solid and homogeneous ceria particles were

formed (filled inverse triangles).

Tani et al. [14] made nanoparticles by FSP of aqueous emulsions of precursor metal

nitrates mixed with kerosene and a surfactant that was sprayed in a flame reactor. Here,

the Tbp/Td/mp was calculated using the boiling point of water (Tbp = 373 K) and the

melting point of the nitrates. The diamonds represent Al2O3 powders with Tbp/Td/mp

= 0.92. Using oxygen as oxidant/dispersion gas, homogeneous alumina powder (filled

diamonds) was formed as the combustion enthalpy density was 5.4 kJ/ggas. Using air as

oxidant/dispersion gas, inhomogeneous alumina was formed (open diamonds) as the com-

bustion enthalpy density was decreased to 4.2 kJ/ggas. Homogeneous iron oxide powder

(filled butterfly) was produced even though air was used as dispersion gas (Tbp/Td/mp =

1.16, 4.5 kJ/ggas). Inhomogeneous powders were observed when making ceria (Tbp/Td/mp

= 0.88, 4.6 kJ/ggas, circles containing cross), zinc oxide (Tbp/Tmp = 0.92, 4.6 kJ/ggas,

square containing cross), yttria (Tbp/Tmp = 1.00, 4.5 kJ/ggas, triangle containing dot)

and magnesium oxide (Tbp/Tmp = 1.01, 4.6 kJ/ggas, triangle containing cross, Figure 1.9).

Suyama and Kato [12] (square containing dot) prepared Mg-Al spinel by spray pyroly-

sis of Mg(NO3)36H2O and Al(NO3)39H2O dissolved in EtOH. The melting point of thealuminum nitrate is 347 K, while the magnesium nitrate precursor decomposes at 371

K. They made hollow particles at Tbp/Tmp = 1.01 (Figur 1.9) and combustion enthalpy

20

Figure 1.9: Morphology mapping of various solid (filled symbols) and hollow/shell-like

(open symbols) ceramic oxide particles in the FSP parameter space of the ratio of the

solvent boiling point over the precursor decomposition or melting point (Tbp/Td/mp) and

combustion enthalpy density. Homogeneous particles were made for Tbp/Td/mp >1.05 and

for combustion enthalpy densities > 4.7 kJ/ggas.

21

density of 1.2 kJ/ggas. Figure 1.9 shows that homogeneous powders were only found at

high combustion enthalpy densities (> 4.7 kJ/ggas) and at Tbp/Td/mp > 1.05 for all these

oxides.

1.4 Conclusions

Formation of hollow/inhomogeneous ceramic oxide particles by flame spray pyrolysis

(FSP) was examined since FSP is one of the promising techniques for synthesis of a

wide spectrum of nanostructured oxides. By analyzing Bi2O3 and silica powders made

with various precursors and solvents, low process temperatures and low boiling point sol-

vents favor formation of hollow or inhomogeneous powders. These criteria were found to

match data with flame-spray-made Bi2O3, SiO2, CeO2, MgO, ZnO, Fe2O3, Y2O3, Al2O3

or Mg-Al spinel. In particular, inhomogeneous particles were formed at low combustion

enthalpy densities (< 4.7 kJ/ggas) and when the solvent boiling point is smaller than the

melting or decomposition point of the metal precursor (Tbp/Td/mp < 1.05).

References

[1] S. E. Pratsinis. Flame aerosol synthesis of ceramic powders. Prog. Energy Combust.

Sci., 24(3):197219, 1998.

[2] W. J. Stark and S. E. Pratsinis. Aerosol flame reactors for manufacture of nanopar-

ticles. Powder Technol., 126(2):103108, 2002.

[3] M. Sokolowski, A. Sokolowska, A. Michalski, and B. Gokjeli. The in-flame-reaction

methode for Al2O3 aerosol formation. J. Aerosol Sci., 8:219230, 1977.

[4] R. M. Laine, C. R. Bickmore, D. R. Treadwell, and F. Waldner. Ultrafine metals

oxide powders by flame spray pyrolysis. Patent #5614596, Sep. 28 1999.

[5] R. Strobel, W. J. Stark, L. Madler, S. E. Pratsinis, and A. Baiker. Flame-made plat-

inum/alumina: Structural properties and catalyticlal behaviour in enantioselective

hydrogenation. J. Catal., 213:296304, 2003.

22

[6] C. R. Bickmore, K. F. Waldner, R. Baranwal, T. Hinklin, D. R. Treadwell, and R. M.

Laine. Ultrafine titania by flame spray pyrolysis of a titanatrane complex. J. Eur.

Ceram. Soc., 18(4):287297, 1998.

[7] L. Madler, H. K. Kammler, R. Mueller, and S. E. Pratsinis. Controlled synthesis

of nanostructured particles by flame spray pyrolysis. J. Aerosol Sci., 33(2):369389,

2002.

[8] L. Madler, W. J. Stark, and S. E. Pratsinis. Flame-made ceria nanoparticles. J.

Mater. Res., 17(6):13561362, 2002.

[9] R. Mueller, L. Madler, and S. E Pratsinis. Nanoparticle synthesis at high prodcution

rates by flame spray pyrolysis. Chem. Eng. Sci., 58(10):19691976, 2003.

[10] R. Mueller, R. Jossen, S. E. Pratsinis, M. Watson, and M. K. Akthar. Zirocnia

nanoparticles made in spray falmes at high production rates. J. Am. Ceram. Soc.,

50(12):30853094, 2004.

[11] A. U. Limaye and J. J. Helble. Morphological control of zirconia nanoparticles

through combustion aerosol synthesis. J. Am. Ceram. Soc., 85(5):11271132, 2002.

[12] Y. Suyama and A. Kato. Characterization and sintering of Mg-Al spinel prepared

by spray-pyrolysis technique. Cheram. Inter., 8(1):1721, 1982.

[13] L. Madler and S. E. Pratsinis. Bismuth nanopartilces by flame spray pyrolysis. J.

Am. Ceram. Soc., 85(7):17131718, 2002.

[14] T. Tani, N. Watanabe, K. Takatori, and S.E. Pratsinis. Morphology of oxide particles

made by the emuslsion combustion methode. J. Am. Ceram. Soc., 86(6):898904,

2003.

[15] G. L. Messing, S.-C. Zhang, and G. V. Jayanthi. Ceramics powder synthesis by spray

pyrolysis. J. Am. Ceram. Soc., 76(11):27072726, 1993.

[16] S. K. Blower and C. Greaves. The structure of -Bi2O3 from powder neutron-

diffraction data. Acta Cryst., 44:587589, 1988.

[17] H. K. Kammler and S. E. Pratsinis. Scaling-up the produciton of nanosize SiO2partilces in a double difusion flame aerosol reactor. J. Nanoparticle Res., 1(4):467

477, 1999.

[18] H. Briesen, A. Fuhrmann, and S. E. Pratsinis. The effect of precursor in flame

synthesis of SiO2. Chem. Eng. Sci., 53(24):41054112, 1998.

23

[19] D. R. Lide, editor. CRC Handbook of Chemistry and Physics. CRC Press, INC, Boca

Raton, Florida, 3rd electronic edition, 2000.

[20] I. W. Lenggoro, T. Hata, F. Iskandar, M. M. Lunden, and K. Okuyama. An experi-

mental and modeling investigation of particle production by spray pyrolysis using a

laminar flow aerosol reactor. J. Mater. Res., 15(3):733743, 2000.

[21] A. Gurav, T. Kodas, T. Pluym, and Y. Xiong. Aerosol processing of materials.

Aerosol Sci. Tech., 19:411452, 1993.

24

CHAPTER

TWO

Morphology and composition of spray-flame-made

yttria-stabilized zirconia nanoparticles

Abstract

Homogeneous yttria stabilized zirconia (YSZ) of 8-31 nm of average crystallite and particle

diameter containing 3-10 mol% yttria are made by flame spray pyrolysis (FSP) of various

yttrium and zirconium precursors at production rates up to 350 g/h. Product particles are

characterized by N2 adsorption (BET), transmission electron microscopy (TEM), energy-

dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The effect of liquid

precursor composition on product particle morphology, composition and crystallinity is

investigated. The yttria content does not affect the product primary particle and crys-

tal sizes of homogeneous YSZ. These are determined, in turn, by the process enthalpy

content and overall metal concentration. Flame-made YSZ nanoparticles of homogeneous

composition and morphology are formed when using either only organometallic zirconium

and yttrium precursors or 2-ethylhexanoic acid as solvent and inexpensive zirconium car-

bonate and yttrium nitrate hexahydrate as precursors. In contrast, and consistent with

the literature, hollow or inhomogeneous YSZ particles are made when organometallic zir-

conium and yttrium nitrate precursors of high water content are employed, especially at

high production rate. The ratio of XRD-determined small to large sizes for inhomogeneous

25

26

crystalline particles is an effective quantitative measure of their degree of inhomogeneity.

For such inhomogeneous particles nitrogen adsorption is not a reliable technique for the

average grain size as it relies on integral properties of the particle size distribution.

2.1 Introduction

Stabilized zirconia is important for its outstanding thermal stability, chemical resistance,

mechanical characteristics and ionic conductivity [1]. The high temperature cubic ZrO2

phase can be stabilized down to room temperature by the addition of yttria (Y2O3), mag-

nesia, calcium oxide or rare earth oxides [2]. Cubic ZrO2 is detected at room temperature

with as little as 1.5 mol% Y2O3. This is the limiting Y2O3 concentration for solid solution

in monoclinic zirconia while at Y2O3 concentrations higher than 7.8 mol% only cubic

ZrO2 is present. A yttria content of 17 to 40 mol% leads to the formation of cubic ZrO2

and Y4Zr3O12 while above that leads to pure Y4Zr3O12 [3]. Cubic ZrO2 has the highest

ion conductivity and yttrium-stabilized zirconia (YSZ) has been widely used in oxygen

sensors, solid oxide fuel cells (SOFC) [4] or as supporting material for platinum catalyst

for nitrogen oxide sensors [5]. Monodispersed nanoscale zirconia powder is favored for

preparation of advanced ceramics with uniform nanostructure and properties [6].

Yttria-stabilized zirconia (YSZ) is made by reaction sintering of the constituent ox-

ides but this time-consuming method may result in a product with undesirable particle

size distribution and purity [1]. Zhang et al. [7] made solid, spherical, uniformly dispersed

4 mol% YSZ by spray pyrolysis of zirconyl hydroxyl chloride and yttrium nitrate. The

product average BET-particle diameter was controlled from 16 nm to about 1m. Pebler

[1] ultrasonically atomized a nitrate solution of zirconium and yttrium into a multipass

quartz reactor designed to extend the residence time up to 15 s. He reported the forma-

tion of pure cubic (10 mol% YSZ) solid spherical particles with an average diameter of

0.5 m. Shukla et al. [8] synthesized YSZ particles by a rapid-combustion route where

a saturated solution containing zirconyl nitrate, yttrium nitrate and carbohydrazine is

27

introduced into a mue furnace at 620 K. The YSZ product consisted of cubic-fluorite

when made with more than 8 mol% of Y2O3. Karthikeyan et al. [9] made pure ZrO2 and

4.5 mol% YSZ powders by spraying solutions of 2.5 wt% zirconium butoxide/n-butanol

and 2 wt% zirconium/yttrium acetate/acid acetic/water, respectively, into a hydrogen

flame. Mostly tetragonal or monoclinic crystal structures were formed with sizes of 12

or 21 nm (zirconia) and 17 or 30 nm (YSZ), respectively. Yuan et al. [10] made pure

and yttria-stabilized zirconia by flame-assisted ultrasonic spray pyrolysis. They used

zirconium n-propoxide and yttrium nitrate hexahydrate dissolved in ethanol/HNO3 as

precursor at a Zr/Y ratio of 84:16 (8.5 mol% YSZ) with a total precursor concentration

(Zr+Y) of 0.1 and 0.2 M to make micron- and submicron-sized hollow or porous particles.

Xie [11] made well-dispersed YSZ nanoparticles with crystal sizes of around 12 nm by

sol-gel reactions of zirconyl chloride and yttrium chloride. Here, the effect of precursor

composition on product particle morphology, crystallinity and size is investigated dur-

ing continuous, dry synthesis of YSZ at relatively high production rate. A focus is on

identifying conditions that would allow controlled synthesis of homogeneous, single or

polycrystalline, nanostructured YSZ from inexpensive precursors.

2.2 Experimental

2.2.1 Apparatus

Figure 2.1 shows the experimental set-up of the large flame spray pyrolysis (FSP) con-

sists of an external-mixing gas-assisted stainless-steel nozzle (Schlick-Duse, Gustav Schlick

GmbH + Co, 970/4-S32) that is made of a capillary tube of 0.5 mm ID and an annular

gap that can be adjusted to keep a constant pressure drop (1 bar) across the nozzle [12].

Liquid precursor (13.5 to 81.1 ml/min) is fed through the capillary by a pulsation-free

precision piston pump (Isco Inc., 1000D). That liquid is dispersed (atomized) by 50 l/min

O2 (Pan Gas, Switzerland, 99.95%) unless otherwise noted. The resulting spray is ignited

by a supporting CH4/O2 diffusion flame (CH4 = 2 l/min (Pan Gas, Switzerland, 99.5%),

28

O2 = 4.5 l/min) surrounding the nozzle [12]. Additional sheath O2 (15 l/min) is supplied

through an outer sintered metal plate ring to assure complete precursor conversion. The

dispersion O2 flow rate is controlled by a mass flow controller (Bronkhorst) while that

of sheath O2 by a calibrated rotameter (Vogtlin Instruments AG). Product powders are

collected with a Jet filter (FRR 4/1.2, Friedli AG, Switzerland) containing four baghouse

filters which are cleaned periodically by sequential air pressure shocks. Additionally, a

check valve is used to avoid disturbance of the spray flame by these shocks during par-

ticle collection. Small samples of the product powder (1 g) are collected on-line usinga bypass connected to the inlet pipe of the baghouse filter upstream of the check valve

[12]. For this, a vacuum pump (Vacubrand RE 16) is used and particles are collected on

a glass fiber filter (Whatman GF/A), 150 mm in diameter, that is located in a stainless

steel holder.

2.2.2 Precursor solution selection and preparation

The total metal concentration (Zr +Y) in the liquid precursor solution is kept constant at

0.5 M in all experiments. Typically a molar ratio of Zr/Y = 4.5 is used corresponding to

10 mol% YSZ in the product powder. Yttrium nitrate hydrate (Y(NO3)3xH2O = YNx,99.9%, Aldrich (x=6) and ChemPur (x=0.5)) is dissolved ultrasonically in EtOH (Al-

cosuisse, 99.9%, denatured with 2% methylethylketone) and then zirconium n-propoxide

(Zr(OC3H7)4 = ZP, 70 wt% in n-propanol, ChemPur) is added resulting in a clear solution.

Precursor solutions using YN6 and ZP at such ratios that correspond to 3, 5, 7, 8 and 9

mol% YSZ product powders are prepared as well. YSZ is made also by removing the H2O

from YN6 with acetic anhydride (C4H6O3 = AcAn, Riedel de Haen, 99-100%). The AcAn

is added slowly to YN6 at room temperature under N2 and the resulting NOx is bubbled

through a NaOH solution. The yttrium precursor solution is then mixed with ZP and

EtOH to form the YSZ precursor solution. In addition, yttrium butoxide (Y(C4H9O)3,

YB, Aldrich, 0.5 M in toluene) is mixed with ZP and toluene (C7H8, Fluka, >99.5%) to

make another YSZ precursor mixture to be investigated here. Inexpensive precursors are

29

Figure 2.1: YSZ (Schematic of the flame spray pyrolysis (FSP) reactor for synthesis of

yttrium stabilized zirconia at high production rates using a commercially available nozzle.

30

examined also such as zirconium carbonate hydroxide oxide (Zr(OH)2(CO3)2 ZrO2 = ZC,

ZrO2 content 44.4 wt%, LU United Intl Inc.) that is first dissolved in acetic acid (Fluka,

99.8% AcOH) at 50C and then 2-ethylhexanoic acid (2-EHA, Fluka, 99%) is added. This

solution is distilled at 160C for 6 hours. For yttrium, YN6 is dissolved first in EtOH

and then 2-EHA is added and distilled at 120C until a clear solution is formed. The

YSZ precursor is prepared by mixing these two precursor solutions using EtOH as an

additional solvent. Such precursor solutions are prepared containing yttria of 3, 5, 8 and

10 mol% in YSZ.

2.2.3 Powder characterization

The specific surface area (SSA) is determined from a five-point N2 adsorption isotherm at

77 K (Gemini III 2375 and Tristar 3000, Micromeretics Instruments Corp.) after degassing

the samples with N2 for 1 hour at 150C. Assuming monodisperse spherical particles,

the average BET-equivalent primary particle diameter, dBET , is calculated by dBET=6/(

SSA) where is the density of cubic 10 mol% YSZ (5.8 g/cm3 [13]). The densities of the3 to 9 mol% YSZ powders are calculated by a linear interpolation from pure tetragonal

zirconia (6.1 g/cm3 [13]) to 10 mol% YSZ. The crystallinity of YSZ is measured by X-ray

diffraction (Bruker, D8, 40 kV, 40 mA, Karlsruhe, Germany) over a 2 range of 20 to 70,

step size 0.03 and scan speed 0.6/min. The crystalline characteristics are obtained from

the XRD spectra using Topas 2.0 software (Bruker AXS, 2000) by the Rietveld method

[14] in which the effect of the equipment (e.g. X-ray source, slits) are incorporated. The

crystal size, dXRD, is calculated from the full width half maximum (FWHM) of the (111)

peak using Scherrers equation [14, ]: dXRD =0.9/ cos ), where is the wavelength of theX-ray (0.154186 nm) and and represent the measured FWHM and the diffraction angle,

respectively. Measured XRD patterns are regressed with the crystalline data of cubic

YSZ structure (PDF #77-2288 [15]) while for powders with bimodal size distribution

the Rietveld method is used to calculated both crystal sizes (dXRD,l, dXRD,s) [16]. The

morphology of the product powder is obtained by transmission electron microscopy (TEM)

31

with a Hitachi H600 or a JEOL 2000FX II electron microscopes operating at 100 or 200

kV, respectively, using magnification ranging from 50 to 800 k. The energy-dispersive X-

ray spectroscopy (EDS) analysis are made with an EDAX Genesis system with a resolution

of 130 eV to analyze the yttria content in the solid YSZ solution. During imaging special

attention is given to morphology and yttria distribution.

2.3 Results and Discussion

2.3.1 Effect of precursor on particle morphology

Figure 2.2 shows TEM images of 10 mol% YSZ powders made at production rates of a) 43

and b) 342 g/h using YN0.5/ZP/EtOH and at production rates of c) 54 and d) 324 g/h

using YN6/ZP/EtOH. Using YN0.5 fine solid, agglomerated YSZ nanoparticles with a

few large ones are made at 54 g/h (Fig. 1a). Increasing the production rate (that means

increased particle concentration and process enthalpy content) increases the size of all

particles. Using YN6 at a low production rate (54 g/h), large, hollow, shell-like and very

fine (only few nanometer in size) YSZ particles are made (Fig. 2.2c). At high production

rate (324 g/h) inhomogeneous powder is made also but without hollow particles (Fig.

2.2d, 250 nm particles and very fine ones). This is in agreement with Yuan et al. [10] who

reported also the formation of hollow or porous YSZ particles by hot-wall spray pyrolysis

of YN6/ZP/EtOH.

Here, it is shown (Fig. 2.2a, c) that the water content of YNx affects distinctly

the product powder morphology. Two mechanisms seem dominant for the formation of

inhomogeneous powder: Larger particles are formed directly form precursor droplets that

do not completely evaporate in the flame [17] while smaller ones are formed in the gas

phase. Possibly when YN6 is dissolved in EtOH and then mixed with ZP, one of the

four propoxides in ZP reacts with water forming an OH group [18]. The decomposition

point of the partially hydrolyzed ZP (Tdp = 500C to ZrO2 for Zr(OH)4 [13]) and the YN

(Tdp = 600C [6]) are much higher than the boiling point of EtOH (Tbp = 79C). Hollow

32

Figure 2.2: Yttria stabilized zirconia (10 mol% Y2O3) powders made by FSP of solutions

of yttrium nitrate (a, b) 0.5 hydrate (YN0.5) or (c, d) hexahydrate (YN6) and zirconium

n-propoxide (ZP) in EtOH made at a) 43, b) 342, c) 54, and d) 324 g/h.

particles are formed if a solute concentration gradient is created within a precursor droplet

during solvent evaporation so the molten precursor decomposes within this layer. The

partially hydrolyzed ZP precipitates first near the more supersaturated droplet surface

and forms a crust resulting in hollow or shell-like particles [17] consistent with Figure 2.2c.

These hollow or shell-like particles solidify at higher production rates as the combustion

33

enthalpy density within the spray is increased from 4.4 to 10.3 kJ/ggas (corresponding

to increasing the production rate from 54 to 324 g/h) and the flame height increases

from 9 to 40 cm. The increased particle residence time at high temperature results in

densification that leads to inhomogeneous particle size distribution (Fig. 2.2d) consistent

with Tani et al. [19] who monitored the evolution of hollow particles made by FSP. When

using YN0.5, the hydrate water concentration is rather low so that formation of partially

hydrolyzed zirconium is less favorable and therefore more homogeneous powder is made

(Fig. 2.2a, b). As the combustion enthalpy density increases from 3.7 to 10.3 kJ/ggas

(corresponding to increasing the production rate from 43 to 342 g/h) larger particles

are obtained. As the FSP gas-to-liquid mass ratio (GLMR) between dispersion gas and

liquid feed rate decreases from 7.1 to 0.9 for that increase in powder production rate,

larger droplets are formed [12, 20] leading to larger particles as expected for the increased

solids concentration and enthalpy content. The overall yttria content in the particles made

from YN6 is consistent (10 mol% measured by EDS) with the precursor inlet Zr/Y ratio.

These particles have, however, inhomogeneous composition as the average Y content in

the large and small particles is 11.2 and 2.9 mol%, respectively. EDS analysis showed

that the Y-content was constant within the large particle fraction of the inhomogeneous

powders. The large particles are formed directly from the droplets. This indicates also the

simultaneous co-precipitation of yttrium and zirconium precursors. The high H2O content

of YN6 favors formation of Zr(OH)4 and simultaneous co-precipitation with YN resulting

in a slightly enhanced Y-content in the precipitate near the surface of the precursor

droplets that would formed the large particles. The smaller ones are formed in the gas

phase from unreacted zirconium propoxide and yttrium nitrate which reacts also in the

gas phase. Only small amounts of the yttrium nitrate can go into the gas phase to form

a solid solution with the zirconia. This may explain the low yttria content in the small

particles. Using YN0.5 there is less H2O to react with ZP and form precipitated Zr(OH)4.

As a result, both precursors evaporate and form small and homogeneous particles (Fig.

2.2a, b). Here an average yttria content of 8.3 mol% is obtained that is consistent for all

34

particles in contrast to YN6-made particles. The slightly smaller yttria content in YSZ

made from YN0.5 compared to the theoretical 10 mol% results from incomplete dissolution

of yttrium nitrate in EtOH. The solution is filtered before filling into the piston pump

and therefore some of the metered yttrium nitrate is removed from solution. As a matter

of fact, YN0.5 crystals were observed at the bottom of the container and on the filter.

In contrast, YN6 dissolves better than YN0.5 in EtOH so all the yttrium added into the

precursor solution ends up in the particles as discussed above since no precipitates in that

precursor solution were observed.

Figure 2.3 shows TEM images of a) pure zirconia [21] as well as YSZ containing b)

3, c) 5 and d) 8 mol% yttria using YN6/ZP/EtOH at a production rate of 200 g/h. The

YSZ morphology changes from homogeneous (3 mol%) to inhomogeneous powder with

increasing yttria content. Increased water amount from YN6 in the precursor solution

leads to formation of Zr(OH)4 and hence to inhomogeneous product particles. This is in

agreement with Yuan et al. [10] who reported the formation of spherical and dense, pure

ZrO2 particles while for 10 mol% YSZ they found hollow and porous particles.

Figure 2.4 shows TEM images of YSZ particles using YN6/AcAn/ZP/ EtOH solu-

tions. Although the hydrate is completely removed from the precursor solution by the

AcAn, inhomogeneous product powder is made. Here the AcAn reacts with the YN-

hydrate to form AcOH that reacts with yttrium nitrate to form yttrium acetate (YA).

The remaining AcOH reacts then with ZP to form zirconium acetate (ZA) [22]. The ZA

(decomposition point = 320C, [7]) and YA precipitate at the supersaturated surface of

the shrinking droplet (EtOH boiling point = 79C) and form a crust resulting in hollow or

shell-like particles [17] that solidify within the flame. This is supported by EDS showing

that the yttria content of the large particles is higher than that of the small ones that are

made by ZP that evaporated from the droplets. Therefore inhomogeneous particles with

a bimodal size distribution are formed as it is shown on TEM images (Fig. 2.4). This is

consistent with Karthikeyan et al. [9] who made YSZ with a broad size distribution using

yttrium and zirconium acetate as precursors.

35

Figure 2.3: a) Pure zirconia [21] and YSZ powders with b) 3, c) 5, and d) 8 mol%

Y2O3 made by FSP of a solution of yttrium nitrate hexahydrate (YN6) and zirconium

n-propoxide (ZP) in EtOH at a production rate of 200 g/h.

Figure 2.5 shows homogeneous YSZ powder containing 10 mol% of yttria made by

spraying a solution of YN6/EtOH/ZC/AcOH/2-EHA at a) 54 and b) 342 g/h powder

production rates. The 2-EHA solvent reacts with the Y- or Zr-precursor to form Y- and

Zr-octoate (2-ethylhexnoate). These octoates have lower melting point than the solvent

boiling point preventing solid precipitation in the droplet [23]. Using precursor with car-

36

Figure 2.4: a) YSZ powders containing 10 mol% Y2O3 made by FSP of a solution of

yttrium nitrate hexahydrate (YN6) in acetic anhydride and zirconium n-propoxide (ZP)

in EtOH at production rates of a) 54 and b) 342 g/h.

boxylate ligands (2-ethylhexanoate in this work) facilitates evaporation and subsequent

particle nucleation in the gas phase followed by coagulation, sintering and agglomeration

[24]. Here, the zirconium and yttria precursors have similar volatility and decomposition

pathways. This leads to a uniform metal distribution in the gas phase so yttria and zir-

conia can be formed together resulting in YSZ solid solution. The EDS analysis shows

homogeneous yttria distribution at any measured location within these particles. Stark et

al, [25] produced by FSP homogeneous mixed ceria/zirconia nanocrystals with narrow size

distribution using cerium (III) acetate hydrate and zirconium tetraacetylacetonate dis-

solved in lauric/acetic acid mixture. Schulz et al. [26] also produced homogeneous silica-

and alumina-doped ceria/zirconia nanoparticles of high crystallinity using zirconium car-

bonate, cerium (III) acetate hydrate, aluminum 2-ethylhexanoate and tetraethoxysilane

as precursor in 2-EHA and toluene.

Figure 2.6 shows 10 mol% YSZ powders made by spraying YB/ZP/ toluene at pro-

duction rates of a) 108 and b) 216 g/h. Here, also homogeneous YSZ powder is produced

37

Figure 2.5: YSZ containing 10 mol% Y2O3 made by FSP of a solution of yttrium nitrate

hexahydrate (YN6) and zirconium carbonate hydroxide oxide (ZC) in acetic acid and

2-ethylhexanoic acid at production rates of a) 54 and b) 342 g/h.

and the morphology is comparable to that made using YN6/EtOH/ZC/AcOH/2-EHA

(Fig. 2.5. The melting point of ZP (-70C) and even the boiling point of YB (109C)

are lower than the boiling point of the solvent toluene (110C) resulting in homogeneous

particles. At these conditions no precipitation can take place on the droplet surface dur-

ing its evaporation as there is no concentration gradient in the droplet. The precursor

remains in solution as the droplet evaporates. This is in agreement with Ishizawa et al.

[27] who prepared homogeneous YSZ powder by spray pyrolysis of zirconium n-butoxide

and yttrium isopropoxide in EtOH and never observed hollow or inhomogeneous product.

Although alkoxide precursors result in a homogeneous product, these are too expensive

to be used in YSZ manufacturing. If the ratio of the solvent boiling point to the pre-

cursor melting/decomposition point, TR, is smaller than one, inhomogeneous powders

are expected [23]. Here, in all cases TR < 1 when inhomogeneous powders are formed.

Jossen et al. [23] reported the formation of homogeneous powders when increasing the

combustion enthalpy density above 4.7 kJ/ggas. In our study the combustion enthalpy is

38

Figure 2.6: YSZ (10 mol% Y2O3) made by FSP of a solution of yttrium butoxide (YB)

and zirconium n-propoxide (ZP) in toluene at production rates of a) 108 and b) 216 g/h.

about 10 kJ/ggas for the highest production rate and still inhomogeneous powders were

formed. Madler et al. [28] reported for ceria a TR = 0.68 and homogeneous powders were

not formed until a combustion enthalpy density of 5.6 kJ/ggas is used. Here the TR for

EtOH/ZA is 0.59 and for EtOH/Zr(OH)4 it is 0.46. This indicates that for low TR, the

combustion enthalpy density has to be larger than 4.7 kJ/ggas homogeneous particles to

be formed.

2.3.2 YSZ crystal and primary particle sizes

Figure 2.7 shows XRD patterns of YSZ powder made from a) YN6/ZP/EtOH at 54

g/h (Fig. 2.2c), b) YN6/AcAn/ZP/EtOH at 54 g/h (Fig. 2.5a), c) YN0.5/ZP/EtOH

(Fig. 2.2a) at 43 g/h and d) YN6/EtOH/ZC/AcOH/2-EHA at 54 g/h (Fig. 2.6a). All

patterns show the formation of a solid YSZ solution regardless of its morphology. The

XRD of inhomogeneous YSZ (Fig. 6a,b) shows very sharp peaks (full width half maximum

(FWHM) < 0.2) corresponding to large particles. The peak shift in Figure 2.7b may come

39

Figure 2.7: XRD spectra of YSZ powders containing 10 mol% Y2O3 made

from a) YN6/ZP/EtOH, b) YN6/AcAn/ZP/EtOH, c) YN0.5/ZP/EtOH and d)

YN6/EtOH/ZC/AcOH/2-EHA. In all cases the stable cubic structure was identified.

from the inhomogeneous yttria distribution in the YSZ powder. All XRD patterns from

inhomogeneous powders as those made from YN6/ZP/EtOH and YN6/AcAn/ZO/EtOH

showed always peak shifts (not shown here). The shifts come from the irregular yttria

distribution into the zirconia. Stark et al. [29] showed that increasing the ceria fraction

in CexZrx-1O2 shifted the peak position towards higher diffraction angles.

In contrast, the XRD of homogeneous (Fig. 2.7d) and the mostly homogeneous

40

Figure 2.8: Effect of production rate on the a) dBET or dXRD and b) large YSZ particle

mass fraction made from YN6/ZP/EtOH, YN6/AcAn/ZP/EtOH or YN0.5/ZP/EtOH

solutions. Increasing the production rate increases the solids concentration and process

enthalpy density as liquid feed rate increases while the dispersion gas flow rate is constant.

(Fig. 2.7c) YSZ show broad peaks (FWHM > 0.6) that are typical for nanosized crystals.

Ishizawa et al. [27] showed that cubic and only minor amounts of tetragonal structures

are formed by spray pyrolysis when using 6 mol% Y2O3. Yuan et al. [10] reported also

the formation of cubic 10 mol% YSZ as well as Shukla et al. [8] for 8 and 10 mol% YSZ.

For the inhomogeneous (Fig. 2.7a,b) and for the mostly homogeneous powders (Fig. 2.7c)

two average cubic crystal sizes can be derived from XRD [28].

Figure 2.8a shows the dXRD of the small (filled symbols) and large (open symbols)

size fractions of YSZ made from YN6/ZP/EtOH (squares), YN6/AcAn/ZP/EtOH (tri-

angles), and YN0.5/ZP/EtOH (diamonds) as a function of production rate. Figure 2.8b

shows the mass fraction of the large crystal fraction as a function of production rate. Us-

ing YN6/AcAn/ZP/EtOH and YN6/ZP/EtOH, large particles constitute about 60 and

85 wt%, respectively, of the XRD mass fraction of powders made at low production rate

while the large particle mass fraction increases to 85 and 99 wt%, respectively, at high

41

production rate. Increased production rates are achieved by increased supply of liquid

precursor resulting in larger droplets amplifying, thus, the inhomogeneity of the prod-

uct. For YSZ made at 54 g/h using YN0.5/ZP/EtOH (Fig. 2.8a) also a bimodal crystal

size distribution was obtained that may appear inconsistent with TEM (Fig. 1a). The

corresponding XRD mass fraction of large particles is 17 wt% (Fig. 2.8a) which means

that only few larger particles exist by count that is typical for TEM. Increasing the pro-

duction rate (e.g. 342 g/h, Fig. 2.8b) increases the fraction of larger particles to about

85 wt% consistent with TEM (Fig. 2.2b). Anyway, the dXRD for the particles made at

high production rate is about 60 nm and that for the small ones is 40 nm. This dif-

ference in dXRD is much less than that from YSZ powder made using YN6/ZP/EtOH

(120 and 6 nm, squares). As a result YSZ made from YN0.5 will tend to appear vi-

sually more homogeneous than that made from YN6. Taking the XRD diameter ratio,

RXRD = dXRD,s/dXRD,l, of small to that of large particles shows that inhomogeneous

(made from YN6/EtOH and YN6/AcAn) powders have a very small such ratio (RXRD 10 mol%) on

particle size and morphology is not shown here. Adding more than 17 mol% yttria to

zirconia leads to the formation of Y4Zr3O12 which is not of interest for sensor or fuel cell

applications. For the mostly inhomogeneous YSZ particles made from YN6/ZP/EtOH

(Fig. 2.3b, c, d), the dBET (diamonds in Fig. 2.9a) increases from 20 nm for pure zirconia

[21] to 97 nm for 10 mol% YSZ. For small amounts of yttria (3 mol%) the dBET remains

constant at 20 nm while above that it increases to 24 (5 mol%), 40 (8 mol%) and finally

43

Figure 2.10: Average BET-equivalent and XRD (diamonds) diameter of homogeneous,

pure (open symbols) or yttria-stabilized zirconia (filled symbols) made from ZP/EtOH

[21] or YN6/EtOH/ZC/AcOH/2-EHA respectively, as a function of powder production

rate or process enthalpy and particle concentration) at O2 dispersion gas flow rate of 25

and 50 l/min.

to 97 nm (10 mol%) which is inconsistent with TEM and XRD. This indicates that the

dBET is not a reliable measure of particle size for inhomogeneous powders.

Figure 2.10 shows the dBET for 10 mol% YSZ powder (filled symbols) made from

44

YN6/ETOH/ZC/AcOH/2-EHA and pure zirconia (open symbols [21]) made from ZP/

EtOH for 25 (triangles) and 50 (squares) l/min O2 dispersion gas. The dBET of homoge-

neous YSZ and ZrO2 are identical when made at the same production rate. For example,

at a production rate of 324 g/h the combustion enthalpy density is 10.6 kJ/ggas and the

flame height is 391 cm for both FSP processes. This indicates that combustion enthalpydensity and precursor concentration are the main process parameters to control particle

size for homogeneous powders as is with vapor-fed flame reactors [30]. This similarity in

particle size of pure zirconia and YSZ indicates that the yttria content has little influ-

ence on particle size of homogeneous powders as also can be seen in Figure 2.9a (circles,

triangles). Using 25 l/min O2, the dBET increases from 12 to 29 nm when increasing

the production rate from 54 to 216 g/h. At O2 dispersion gas flow rate of 50 l/min the

dBET is controlled from 8 to 31 nm when increasing the production rate from 54 to 324

g/h. At higher O2 flow rates mixing is intensified and combustion is accelerated short-

ening the flame height [31]. When increasing the O2 dispersion gas flow rate from 25

to 50 l/min, the flame height decreases, for example, from 40 to 30 cm at a production

rate of 217 g/h. The increase of the O2 dispersion gas flow rate decreases the droplet

concentration of the spray and the flame enthalpy content, thus, particle concentration

and particle residence time at high temperature decreases. This leads to faster quenching

and therefore smaller particles are formed. The corresponding dXRD (circles, diamonds)

closely follow the dBET at low production rate (up to 150 g/h) indicating single crystals of

YSZ while for production rates higher that 200 g/h dXRD is smaller than dBET indicating

polycrystallinity or increased degree of agglomeration. The dXRD of the inhomogeneous

product powder increases from 75 to 120 nm (YN6/ZP/EtOH) and from 75 to 110 nm

(YN6/AcAn/ZP/EtOH) as shown in Figure 2.8a. For the inhomogeneous YSZ powders,

large particles are formed directly from droplets as the solids precipitate at the droplet

surface. The average droplet diameter is 14 m at low production rate (54 g/h, 13.5 ml/min

liquid feed rate) while it increase to 22 m at the highest production rate (354 g/h, 81.1 liq-

uid feed rate) [12, Appendix A1.1]. The large particles are directly related to the droplet

45

size. The above droplet size ratio at high to low production rate is 1.57 which is in good

agreement with the dXRD ratio of the large particles at high to low production rates, 1.6

(Fig. 2.8a: YN6/ZP/EtOH squares). At higher production rate also the residence time

in the hot temperature zone is increased which has a minor influence on large particles

growth. For the homogeneous particles (Fig. 2.10), particle formation takes place in the

gas phase from evaporated precursors resulting in smaller particles. For the latter, the

sintering time is smaller than for large particles ( d4p, [32]) resulting in faster particlegrowth. By increasing the liquid flow rate (increasing the powder production rate) par-

ticle concentration and high temperature residence time are increasing leading the larger

particles. This may explain the increase of dXRD of homogeneous FSP-made YSZ powders

by a factor of 2-4 in Figure 2.10.

2.4 Conclusions

A systematic investigation of flame spray synthesis of nanostructured yttria-stabilized

zirconia (YSZ) was carried out at production rates up to 350 g/h. The precursor com-

position affects the morphology of YSZ powders made at identical combustion enthalpy

density, precursor concentration and constant process conditions. Inhomogeneous YSZ

powders exhibit a bimodal crystal size distribution. The diameter ratio between the two

modes can be used to estimate the degree of inhomogeneity. The average grain diameter

as determined by nitrogen adsorption of inhomogeneous YSZ powders does not represent

well the product particle characteristics.

The homogeneity of YSZ particles was improved by reducing the water content

in the precursor solutions. Flame-made YSZ particles have homogeneous composition

and morphology when using either organometallic precursors or 2-ethylhexanoic acid as

solvent and inexpensive zirconium carbonate as precursor. Increasing the production rate

increased drastically the dBET and dXRD of YSZ as both enthalpy content and metal

concentration increased resulting in larger particles and crystals. The yttria content (up

46

to 10 mol%) has little influence on homogeneous YSZ particle size and morphology.

References

[1] A. R. Pebler. Preparation of small particle stabilized zirconia by aerosol pyrolysis.

J. Mater. Res., 5(4):680682, 1990.

[2] R Nielsen, T. Wah, and A. Albany. Ullmanns encyclopedia of industrial chemistry,

volume 28 A. Wile-VCH Veralg GmbH, 1996.

[3] C. Pascual and P. Duran. Subsolidus phase-equilibria and ordering in the system

ZrO2-Y2O3. J. Am. Cem. Soc., 66(1):2327, 1983.

[4] G. S. Pang, E. Sominska, H. Colfen, Y. Mastai, S. Avivi, Y. Koltypin, and

A. Gedanken. Preparing a stable colloidal solution of hydrous YSZ by sonication.

Langmuir, 17(11):32233226, 2001.

[5] S. Benard, L. Retailleau, F. Gaillard, P. Vernoux, and A. Giroir-Fendler. Supported

platinum catalysts for nitrogen oxide sensors. Appl. Catal. B-Env., 55(1):1121, 2005.

[6] M. Gaudon, E. Djurado, and N. H. Menzler. Morphology and sintering behaviour

of yttria stabilised zirconia (8-YSZ) powders synthesised by spray pyrolysis. Ceram.

Inter., 30(8):22952303, 2004.

[7] S. C. Zhang, G. L. Messing, and M. Borden. Synthesis of solid, spherical zirconia

particles by spray pyrolysis. J. Am. Ceram. Soc., 73(1):6167, 1990.

[8] A.K. Shukla, N.A. Dhas, and K.C. Patil. Oxides ion coduction of calcia and yttria

stabilized zirconia prepared by rapid combustion rout. Mater. Sci. Eng., B40:153

157, 1996.

[9] J. Karthikeyan, C. C. Berndt, J. Tikkanen, J. Y. Wang, A. H. King, and H. Herman.

Nanomaterial powders and deposits prepared by flame spray processing of liquid

precursors. Nanostruct. Mater., 8(1):6174, 1997.

[10] F. L. Yuan, C. H. Chen, E. M. Kelder, and J. Schoonman. Preparation of zirconia

and yttria-stabilized zirconia (YSZ) fine powders by flame-assisted ultrasonic spray

pyrolysis (FAUSP). Solid State Ion., 109(1-2):119123, 1998.

[11] Y. Q. Xie. Preparation of ultrafine zirconia particles. J. Am. Ceram. Soc., 82(3):768

770, 1999.

47

[12] R. Mueller, L. Madler, and S. E. Pratsinis. Nanoparticle synthesis at high production

rates by flame spray pyrolysis. Chem. Eng. Sci., 58(10):19691976, 2003.

[13] D. R. Lide. CRC Handbook of Chemistry and Physics. CRC Press, Inc., Boca Rota,

Florida, 3rd electorn edtion edition, 2000.

[14] R.C Rau. Routine crystallite-size determination by X-ray diffraction broadening.

Adv.X-Ray Anal., 5:105, 1962.

[15] M. Morinaga, J. B. Cohen, and J. Faber. X-ray-diffraction study of Zr(Ca,Y)O2x.1. Average structure. Acta Crystallogr. Sect. A, 35:789795, 1979.

[16] R. W. Cheary and A. A. Coelho. Axial divergence in a conventional X-ray powder

diffractometer: I. Theoretical foundations. J. Appl. Crystal., 31:851861, 1998.

[17] A. Gurav, T. Kodas, T. Pluym, and Y. Xiong. Aerosol processing of materials. Aeros.

Sci. Technol., 19(4):411452, 1993.

[18] M. Z. C. Hu, J. T. Zielke, C. H. Byers, J. S. Lin, and M. T. Harris. Probing the

early-stage/rapid processes in hydrolysis and condensation of metal alkoxides. J.

Mater. Sci., 35(8):19571971, 2000.

[19] T. Tani, K. Takatori, and S. E. Pratsinis. Dynamics of hollow and solid alumina

particle formation in spray flames. J. Am. Ceram. Soc., 87(3):523525, 2004.

[20] R. Mueller. Characterization and synthesis of nanoparticles made in vapor and spray

flames. Ph.D. Thesis (ETH No. 15147), Department of Mechanical and Process

Engineering, 2003.

[21] R. Mueller, R. Jossen, S. E. Pratsinis, M. Watson, and M. K. Akhtar. Zirconia

nanoparticles made in spray flames at high production rates. J. Am. Ceram. Soc,

87(2):197202, 2003.

[22] J.R Lacher, J. Schwarz, and J.D Park. Nitration with uranium nitrate-nitrogen

tetraoxide-water complexe in the precense of acetic anhydride. J. Org. Chem.,

26:25362537, 1960.

[23] R. Jossen, S. E. Pratsinis, W. J. Stark, and L. Madler. Criteria for flame-spray syn-

thesis of hollow, shell-like or inhomogenous oxides. J. Am. Ceram. Soc., 88(6):1388

1393, 2005.

[24] S. E. Pratsinis. Flame aerosol synthesis of ceramic powders. Prog. Energy Combus.

Sci., 24(3):197219, 1998.

48

[25] W. J. Stark, L. Madler, M. Maciejewski, S. E. Pratsinis, and A. Baiker. Flame

synthesis of nanocrystalli