Controlled Synthesis of Mixed Oxide Nanoparticulas by Spray Pirolise
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Transcript of Controlled Synthesis of Mixed Oxide Nanoparticulas by Spray Pirolise
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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
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Rose, oh reiner Widerspruch, Lust,
Niemandes, Schlaf zu sein unter so vielen Lidern.
Rainer Maria Rilke
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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
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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
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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
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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
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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
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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
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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-
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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.
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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
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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
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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.
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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
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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.
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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.
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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
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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%),
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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
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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.
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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)
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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
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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
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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
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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.
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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-
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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
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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
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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
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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
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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
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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
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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
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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
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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
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46
to 10 mol%) has little influence on homogeneous YSZ particle size and morphology.
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