Combinatorial nanopowder synthesis along the ZnO-Al2O 3 tie line using liquid-feed flame spray pyrolysis

Article abstract of DOI:10.1111/j.1551-2916.2011.04585.x

Liquid-feed flame spray pyrolysis (LF-FSP) of mixtures of alumatrane [Al(OCH₂CH₂)₃N]/zinc acetate dihydrate [Zn(O₂CCH₃)₂·2(H₂O)] or zinc propionate [Zn(O₂CCH₂CH₃)₂]/aluminum acetylacetonate [Al(Acac)₃] dissolved in EtOH in known molar ratios can be used to combinatorially generate nanopowders along the ZnO-Al ₂O₃ tie-line. LF-FSP was used to produce (ZnO) _x(Al₂O₃)_1-x powders with x=0-1.0. Powders were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, thermal gravimetric analysis, differential thermal analysis, and BET. The resulting powders had average particle sizes (APSs) <100 nm with the majority being <50 nm. Analytical data suggest that at concentrations of interest for transparent conducting oxides, <10 mol% Al₂O₃ the particle morphologies are combinations of plates and rods that grow with c/a ratios close to 1. The spinel phase dominates at (ZnO)_x(Al ₂O₃)_1-x (x=0.5 and 0.3). In the latter case, the currently accepted phase diagram for the ZnO-Al₂O₃ couple indicates that phase separation should occur to form zinc spinel (ZnAl₂O₄) and α-alumina. It appears that the rapid quenching during LF-FSP helps to preserve the spinel phase at ambient temperature giving rise to kinetic nanopowder products along the ZnO ₂-Al₂O₃ tie-line. Finally, the solubility of ZnO in Al₂O₃ and vice versa in the materials produced by LF-FSP suggest apparent flame temperatures reached before quenching are 1700°-1800°C. Efforts to re-pass the spinel phase powders, (ZnO) _x(Al₂O₃)_1-x, x=0.5 and 0.3 through the LF-FSP system were made with the hope of generating core shell materials. However, instead the x=0.5 material generated materials closer to the x=0.3 composition and pure ZnO nanoparticles that coat the former materials. These results suggest that at LF-FSP flame temperatures ZnO remains in the vapor phase for sufficient times that Al³⁺ oxy-ions generated promote nucleation of finer particles leaving essentially phase pure ZnO still in the vapor phase to condense giving the two distinct particle morphologies observed.

Full text of DOI:10.1111/j.1551-2916.2011.04585.x

J. Am. Ceram. Soc., 94 [10] 3308–3318 (2011)
DOI: 10.1111/j.1551-2916.2011.04585.x
r 2011 The American Ceramic Society
ournal
J
Combinatorial Nanopowder Synthesis Along the ZnO–Al O Tie Line
2
3
Using Liquid-Feed Flame Spray Pyrolysis
Min Kim, Samson Lai, and Richard M. Lainew
Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136
Liquid-feed flame spray pyrolysis (LF-FSP) of mixtures of
alumatrane [Al(OCH CH ) N]/zinc acetate dihydrate [Zn
(
2 3
e.g. In O :Sn 5–6 at% Sn (ITO). Unfortunately, ITO is expen-
sive because of the scarcity of In. Thus, extensive efforts have
been made to find substitutes that provide similar properties at
lower costs.
2
2 3
.
O CCH ) 2(H O)] or zinc propionate [Zn(O CCH CH ) ]/
2 3 2 2 2 2 3 2
aluminum acetylacetonate [Al(Acac) ] dissolved in EtOH in
3
known molar ratios can be used to combinatorially generate
nanopowders along the ZnO–Al O tie-line. LF-FSP was used
Aluminum-doped zinc oxide or ZnO:Al was first proposed as
an ITO substitute by Wasa et al. in 1971, and has since been
the subject of numerous studies, typically in thin films prepared
1
1
2
powders with x 5 0–1.0. Powders
3
to produce (ZnO) (Al O )
x
2
3 1ꢀx
1
2–16
were characterized by X-ray diffraction, scanning electron mi-
croscopy, transmission electron microscopy, Fourier transform
infrared, thermal gravimetric analysis, differential thermal anal-
ysis, and BET. The resulting powders had average particle sizes
using a variety of deposition methods.
One key issue is the
thermal stability of ZnO which loses oxygen at temperatures
1
7
ꢃ1501C. In 1984, Minami and colleagues described the prep-
aration of ZnO:Al films stable to ꢁ4001C with resistivities of
ꢀ
4
8,17,18
(APSs) o100 nm with the majority being o50 nm. Analytical
data suggest that at concentrations of interest for transparent
ꢁ
2 ꢂ 10 O ꢄ cm.
ZnO:Al films have recently been used in
commercial flat panel displays and thin film solar cells offering
ꢀ
4
conducting oxides, o10 mol% Al O the particle morphologies
2
are combinations of plates and rods that grow with c/a ratios
3
resistivities of 1–3 ꢂ 10 O ꢄ cm at dopant concentrations of
8
,11–18
1
.6–3.2 at% Al in ZnO.
Considerable opportunity remains
close to 1. The spinel phase dominates at (ZnO) (Al O )
x
x 5 0.5 and 0.3). In the latter case, the currently accepted phase
2
3 1ꢀx
to further modify these materials to improve conductivity, ther-
mal stability, and transparency through further manipulation of
both chemical and phase composition as well as processing
(
diagram for the ZnO–Al O couple indicates that phase sepa-
2
3
ration should occur to form zinc spinel (ZnAl O ) and a-al-
8
2
4
methods motivate the efforts reported here.
We have previously demonstrated that liquid-feed flame
umina. It appears that the rapid quenching during LF-FSP helps
to preserve the spinel phase at ambient temperature giving rise
to kinetic nanopowder products along the ZnO –Al O tie-line.
spray pyrolysis (LF-FSP) offers a simple way to produce com-
Basically, in the
19–34
2
2
3
plex mixed-metal oxide nanopowders.
Finally, the solubility of ZnO in Al O and vice versa in the
2
3
LF-FSP process, alcohol solutions of metalloorganics and
occasionally organometallics at 2–10 wt% of ceramic content
are aerosolized with oxygen and thereafter ignited. Combustion
occurs at temperatures of 15001–20001C depending on the pre-
cursor, its concentration and processing conditions. Combus-
tion takes place within a 1.5 m quartz tube and the combustion
products are quenched at rates of 10001C/ms. The resulting ce-
ramic soot (metal oxide nanopowders) is collected downstream
in electrostatic precipitators. Production rates run 30–100 g/h.
The unaggregated and often single crystal nanopowder compo-
sitions are the same as those in the original precursor solution.
More details are available in the experimental and in previously
materials produced by LF-FSP suggest apparent flame temper-
atures reached before quenching are 17001–18001C. Efforts to
re-pass the spinel phase powders, (ZnO) (Al O ) , x 5 0.5
x
2
3 1ꢀx
and 0.3 through the LF-FSP system were made with the hope of
generating core shell materials. However, instead the x 5 0.5
material generated materials closer to the x 5 0.3 composition
and pure ZnO nanoparticles that coat the former materials.
These results suggest that at LF-FSP flame temperatures ZnO
31
remains in the vapor phase for sufficient times that Al oxy-
ions generated promote nucleation of finer particles leaving es-
sentially phase pure ZnO still in the vapor phase to condense
giving the two distinct particle morphologies observed.
1
9–34
published papers.
Most recently we demonstrated that LF-FSP processing of
MgO–, CuO–, CoO–, ZrO–, CeO–, NiO–Al O mixed-metal
I. Introduction
2
3
systems provides access to heretofore unknown and/or unusual
spinel phases because of the rapid quench kinetics that occur in
RANSPARENT conducting oxides or TCOs are used for a wide
variety of applications including various display applica-
tions as well as photovoltaic, electrochromic, RF shielding, and
T
27–33
the process.
Given that some of these materials and phases
may offer unique properties inaccessible using standard ceram-
ics processing methods, we sought to extend LF-FSP processing
sensor applications. TCOs typically have band gaps 43.1 eV
(
1
absorptions o400 nm) and can have conductivities of ꢁ1 ꢂ
to (ZnO)
comes. The current studies also serve as the basis for LF-FSP
processing of (ZnO) (Al
x 2 3
(Al O )1ꢀx nanopowders hopefully with similar out-
4
0 S/cm similar to good metal conductors.
1–10
Although func-
tionally useful TCOs have been made from a wide variety of
alloys or doped forms of In O , SnO , Ga O , ZnO, PbO ,
x
2
O
3
)
1ꢀx nanopowders incorporating ter-
2
3
2
2
3
2
tiary and/or quaternary dopants.
2
SbO ; the commercial material of choice is doped indium oxide,
It is important to note that there are now two books on the
gas phase synthesis of metal oxide nanopowders and a number
of reviews including those by several other groups that have
explored the use of LF-FSP for the production of nanooxide
R. Riedel—contributing editor
3
4–41
powders.
Although we were unable to identify other groups
using LF-FSP to produce Al-doped ZnO, numerous other
groups have developed diverse approaches to these materials
as mentioned below.
Manuscript No. 28519. Received August 27, 2010; approved March 27, 2011.
This work was supported by the Air Force Office of Scientific Research, through con-
tracts No. F49620-03-1-0389 and subcontract from UES Inc. F074-009-0041/AF07-T009,
Diamond Innovations Inc., the National Science Foundation (IGERT grant DGE-9972776
and DMR 9975542), and Maxit Corp., Oslo, Norway.
Three phases are found along the ZnO–Al O tie-line; ZnO
2
3
w
Author to whom correspondence should be addressed. e-mail: talsdad@umich.edu
2 4 2 3
zincite, ZnAl O spinel, and a-Al O alumina. Based on these
3
308

October 2011
Combinatorial Nanopowder Synthesis Using LF-FSP
3309
phases, a set of compositions was targeted for LF-FSP processing
focusing on two main regions of Al -doped ZnO
(ZnO) (Al O ) , x5 0.9–1.0] and compositions at the other
nanopowders that are largely unaggregated; although they are
lightly agglomerated. ‘‘Shooting’’ rates can be 200 g/h when us-
ing wire-in-tube electrostatic precipitators operating at 10 kV.
Typical powders are 15–100 nm APS with specific surface areas
2 3
O
[
x
2
3 1ꢀx
end of the tie line including the spinel phase region
(ZnO) (Al
2
[
x
2
O
3
)
1ꢀx, x50.1–0.5]. All LF-FSP powders were pro-
(SSAs) of 30–100 m /g. When combinations of elements are
duced at rates of 30–50 g/h using EtOH as the solvent and fuel.
The as-prepared powders were characterized by X-ray diffraction
used, the resulting nanopowders will have compositions identi-
cal to those of the precursor solutions. Because compositions of
chemical solutions can be changed intentionally, potentially
even during mixing just before aerosolization, it becomes pos-
sible to combinatorially produce mixed-metal oxide materials.
Hence it becomes possible to rapidly optimize materials for
given properties or for ease of processing.
(XRD), transmission electron microscopy (TEM), BET, Fourier
transform infrared (FTIR), thermal gravimetric analysis–differ-
ential thermal analysis (TGA-DTA), and chemical analysis.
II. Experimental Procedure
(
1) Materials
Zinc nitrate [Zn(NO
drate [(CH COO) Znꢄ 2H O, 99.9%] were purchased from Ald-
(4) Characterization
)
3 2
ꢄ x(H
2
O), 99%] and zinc acetate dihy-
(A) TGA/DTA: Phase transformations and mass loss
events occurring during heating of as-prepared samples were
investigated via simultaneous differential thermal (SDT) ana-
lyzer (TA Instruments Inc., New Castle, DE). The transforma-
tion temperatures determined by TGA were also done using a
model 2960 simultaneous thermogravimetric analyzer (TA In-
struments, Newcastle, DE).
As-prepared powders of about 20 mg were hand pressed in a
3 mm dual action die and placed inside Pt sample cups and
heated at ramp rates of 101C/min from room temperature to
3
2
2
rich (Milwaukee, WI). Triethanolamine N(CH
2
CH
OH, 99 %], and methanol were
purchased from Alfa Aesar. Aluminum tris (sec-butoxide),
Al(OsBu) , 97%] was purchased from Chattem Chemical
2 3
OH) ,
1
3 2
anhydrous ethanol [CH CH
[
3
Co. (Chattam, TN) Aluminum acetylacetonate [Al(C
9.9%] was purchased from Mackenzie Inc. (Bush, LA). All
compounds and solvents were used as received.
5 7 2 3
H O ) ,
9
1
4001C. The reference material was a pellet of a-alumina. A
(
2) Precursor Preparation and Pyrolysis
synthetic air flow of 50 mL/min was maintained during all SDT
experiments. Precursor samples were placed in alumina sample
cups with an empty alumina cup as the reference and heated at
ramp rates of 101C/min up to 10001C.
(
A) Zinc Propionate: Zn(O CCH CH ) : Zinc nitrate
2
2
3 2
[
Zn(NO
excess propionic acid (400 mL, 5.44 mol) in a 1 L flask equipped
with a still head and an addition funnel. N was sparged directly
)
3 2
2
ꢄ x(H O), 99%, 100 g, 0.33 moles] was reacted with
2
(
B) XRD: As-prepared and heat-treated (air/nitrogen)
through the solution (2 psi pressure) as the solution was heated
at 1201C/2 h to distill off B150 mL of liquid (water and prop-
ionic acid). The resulting solution was slowly heated to distill off
excess solvent and reactant. Thereafter the solution was reduced
to 100 mL of a viscous green gel by rotary evaporation, and then
the gel was dried to a solid under a dynamic vacuum at 701C.
The TGA of this product is discussed in Section III.
samples were characterized using a Rigaku Rotating Anode Go-
niometer (Rigaku Americas, The Woodlands, TX). Powder sam-
ples for the Rigaku were prepared by placing ꢁ100 mg in XRD
sample holders (amorphous silica slides) for data collection.
˚
CuKa (l5 1.54 A) radiation with a Ni filter was used with a
working voltage and current of 40 kV and 100 mA, respectively.
Scans were continuous from 101 to 801 2y with a step scan of 21
2y/min and increments of 0.051 2y. Peak positions and relative
intensities were characterized by comparison with PDF files for
(B) Alumatrane Al[N(CH CH O) ]: This was synthe-
sized from Al(OsBu) and N(CH CH OH) as described else-
2 2 3
3
2
2
3
1
9,22
where,
was 7.5 wt % by TGA.
C) (ZnO) (Al O )
then diluted with EtOH such that the ceramic yield
2 4
zincite (ZnO), normal spinel (ZnAl O ), nonstoichiometry spinel
[
(Zn0.3Al0.7)Al1.7O ], and d-Al O , (36-1451), (05-0669), (77-
4
2 3
(
:
A series of precursors corre-
1ꢀx (x 5 0–1.0) powder com-
x
2
3 1ꢀx
x
0732), and (42-1215), respectively.
C) SSA: SSA was measured on a Micromeritics ASAP
000 sorption analyzer (Micromeritics Instrument Corporation,
sponding to specific (ZnO)
2 3
(Al O )
(
positions were prepared. For ZnO-rich composition series,
alumatrane was used as the source of Al and methanolic
zinc acetate solutions for ZnO. For the remaining ZnO–Al O
2
2
O
3
Norcross, GA). Samples (200 mg) were degassed at 4001C until
the outgas rate was 5 mmHg/min. Analyses were run at 77 K with
2
3
tie-line compositions, we used aluminum acetylacetonate
Al(Acac) ] as the source of Al and Zn(O CCH CH for
ZnO, because zinc acetate is only slightly soluble in EtOH.
Measured volumes of the two solutions, based on the molar
ratio of Zn and Al, were mixed in the appropriate amounts to
make 500 mL solutions for each precursor compositions listed in
Table I Section III, and then 2.5 L of EtOH was added to give a
final volume of 3000 mL with stirring at room temperature.
2
N . SSAs were determined by the BET multipoint method using
[
3
O
2 3
2
2
3 2
)
at least five data points with relative pressures of 0.001–0.20. The
average particle size (APS) was derived using the formula hDi ¼
6
where hDi 5APS, and r is the density of the material.
rꢂSSA
Table I. Selected (ZnO) (Al O )
x
Compositions Produced by Liquid-Feed Flame Spray
Pyrolysis (LF-FSP)
Nanopowder
2
3 1ꢀx
Sample
Mol% of Al O
2 3
Mol% of ZnO
(3) LF-FSP
LF-FSP, as developed at the University of Michigan, has been
From zinc acetate and alumatrane
19–33,42
described in detail in published papers.
typically EtOH) solutions containing 1–10 wt % loading of ce-
ramic as precursors, e.g. single- or mixed-metal alkoxides, car-
boxylates or b-diketonates are aerosolized with O into a 1.5 m
Briefly, alcohol
9
9
9
9
9
1
5 mol% ZnO
7.5 mol% ZnO
9 mol% ZnO
9.5 mol% ZnO
9.8 mol% ZnO
00 mol% ZnO
5
2.5
1
0.5
0.3
0
95
97.5
99
99.5
99.8
100
(
2
quartz chamber where it is ignited with methane pilot torches.
Initial combustion temperatures run 15001–20001C, depend-
ing on the processing conditions, generating nanopowder
From zinc propionate and aluminum acetylacetonate
‘
‘soot.’’ Gas phase temperatures in the flame were measured us-
1
3
5
8
9
9
0 mol% ZnO
0 mol% ZnO
0 mol% ZnO
0 mol% ZnO
5 mol% ZnO
9 mol% ZnO
90
70
50
20
5
10
30
50
80
95
99
ing a two-color Omega S3753 optical pyrometer (Omega
Engineering Inc., Stamford, CT) (calibrated using 304 steel Tm
and an external thermocouple) used in peak mode (recording
at highest temperature in the flame).
Temperatures decline to 3001–5001C over 1.5 m, equivalent to
a 10001C quench in r100 ms leading to kinetic products and
1

3
310
Journal of the American Ceramic Society—Kim et al.
Vol. 94, No. 10
(
D) TEM: An analytical high-resolution TEM (Model
composition pattern similar to other metal carboxylate precur-
2
9–33
3
011, JEOL, Osaka, Japan) was used to measure the particle
sors studied previously.
Figure 1 shows a TGA trace for Zn(O CCH CH ) . Initial
sizes and morphologies of as-prepared powders. Powder samples
were prepared by dipping a holey carbon grid in a vial of emul-
sion with as-prepared powder. The specimen was held in a Gatan
double tilt goniometer (Gatan Inc., Pleasanton, CA). An oper-
ating voltage of 300 kV was used.
2
2
3 2
mass losses (2%) are due to propionic acid of recrystallization.
Thereafter, mass loss events are attributed to the decomposition
of the propionate ligands as reaction suggested in reactions (1)
2
2,24,29–33
and (2).
Zn(O CCH CH )
2 2 3 2
(
E) Scanning Electron Microscopy (SEM): A high-
resolution SEM (FEI NOVA dual beam-focused ion beam work-
station and scanning electron microscope) was used to image
powder morphologies. Powder samples were dispersed in distilled
! ZnðO CCH CH ÞðOHÞ þ CH CH¼C¼O
2
2
3
3
(
1)
Calc: ðFoundÞ Mass Loss ¼ 26:0% ð25:0%Þ
2
H O using an ultrasonic horn (Vibra-cell, Sonics and Materials
ZnðCH CH2COOÞðOHÞ ! ZnO þ H2O þ CH3CH¼C¼O
3
Inc., Newton, CT). A drop of the dispersed powder/water was
placed on an aluminum SEM stub and allowed to dry for 4 h on a
hot plate. Powders were sputter coated with 1–5 nm of Au–Pd to
reduce charging effects. The operating voltage was 15.0 kV.
Calc: ðFoundÞ Mass Loss ¼ 34:4% ð35:0%Þ
(2)
Final ceramic yields [37 wt% (ZnO)] are within experimental
error of the calculated value (37.6 %) from the decomposition of
III. Results and Discussion
the precursor [Zn(O
expected based on previous studies.
2 2 3 2
CCH CH ) ] to the oxide (ZnO) and are as
2
2,24,29–33
The objectives of the current study were to develop structure–
property–processing parameters for LF-FSP production of
(
2) LF-FSP Processing Conditions
(ZnO)
tie line including spinel (ZnAl
x
(Al
2
O
3
)
1ꢀx oxide nanopowders along the ZnO–Al
2 3
O
2
O
4
) composition regions as listed
As we have described in previous publications, our LF-FSP
processing conditions typically involve aerosolizing ethanol so-
lutions of soluble precursors in the desired stoichiometric ratios
in Table I. Details of the LF-FSP process are given in Section II.
The first step in processing nanopowders by LF-FSP is in the
choice of a precursor. The following section briefly explores the
issues with precursor choice.
with oxygen. The resulting aerosol is ignited using methane/O
2
pilot torches inside a 1.5 m by 30-cm-diameter quartz tube.
Combustion occurs at temperatures of 15001–20001C followed
by rapid quenching to temperatures near 4001C at the end of the
quartz tube. The resulting powders are captured in electrostatic
precipitators thereafter and collected by hand. Rates of produc-
tion are typically 10–20 g/h but can be effected at up to 200 g/h
as desired. Gas phase temperatures in the flame were measured
using an optical pyrometer. However, these temperatures are
reaction system dependent and thus a range of temperatures
is given.
(
1) Precursor Choices
A wide variety of precursors have been used in the fabrication
of (ZnO) (Al O ) nanopowders, coatings, and nanowires.
x
2
3 1ꢀx
These include metalloorganics such as carboxylates, acetylac-
organometallics such as diethyl-
4
3–51
etonates and alkoxides,
5
2–54
zinc and trimethylaluminum
and simple nitrates and
Alternate processing approaches use simple sput-
tering of targets preformed from powders or direct oxidation of
5
1,55–57
sulfates.
LF-FSP processing allows the use of mixed-metal precursors,
mutually soluble in alcohol, of any composition to be aerosolized
and combusted. Therefore, LF-FSP offers access to high SSA
nanopowders consisting of single-phase, solid solutions and
56,58
zinc metal or its oxide in flowing gas.
Metal carboxylates, acetylacetonates, and alkoxides are often
chosen as precursors for processing metal oxide materials as an
alterative to metal halides, nitrates, and sulfates in an effort to
avoid halide and sulfate contamination of the resulting oxides.
Nitrates typically offer excellent solubility in water and polar
mixed-phase nanopowders in the (ZnO) (Al O )
2
oxide system.
x
3 1ꢀx
As noted above, several other groups also conduct gas phase
processing including spray pyrolysis and flame spray pyrolysis.
In these systems, the typical flame temperatures are closer to
x
solvents but often generate copious amounts of NO gases dur-
ing processing that present significant handling drawbacks.
Metal alkyls are very reactive and flammable and as such not
easily handled in a typical ceramics processing laboratory.
Of the metalloorganic precursors, metal acetylacetonates are
very useful for vapor processing of thin films and coatings but
are often not as polar as carboxylates or alkoxides and therefore
1
0001C; although in several recent reviews temperatures as high
40–42
as 30001C are discussed.
akin to those of plasma processing.
These latter temperatures are more
Zn(O CCH CH ) *xCH CH CO H
2
2
3 2
3
2
2
not as soluble [e.g. Al(Acac) ] in polar solvents such as alcohols
3
100
90
2
%
often used in sol–gel processing and or spray and flame spray
pyrolysis systems. Indeed, the simplest metal carboxylates such
as Zn(OAc) are not easily dissolved in alcohols. Thus from a
Zn(O2CCH2CH3)2
(MW : 211)
25%
2
solubility aspect, the simplest, lowest cost, and easily prepared
carboxylates are the propionates. We have reported that trietha-
nolamine complexes of transition metals (trane complexes) are
relatively insensitive to hydrolysis compared with most alkox-
ides and as such offer a simple alternative to most aluminum
8
7
0
0
Zn(O CCH CH )(OH)
2
2
3
(MW : 155)
2
2,26
alkoxide precursors.
Thus, we originally developed precursor systems based on
60
50
36%
2
2
Zn(OAc)
ing the course of these studies we replaced Zn(OAc)
Zn(O CCH(CH finding it more soluble and more easily han-
2
2 2 3
and Al(OCH CH ) N (alumatrane). However, dur-
2
with
2
3 2
)
ZnO
(MW : 81)
4
0
0
dled and alumatrane with Al(Acac) although its solubility is
3
limited. Thus, most of our LF-FSP precursor solutions were
made at low total concentrations of precursors.
We previously reported the characterization of alumatrane
3
200
400
600
800
1000
Temperature (°C)
[
N(CH CH O) Al] and its use as a precursor in LF-FSP for the
2 2 3
2
2,24,29–33,59
synthesis of d-alumina nanopowders. Here we report
on the zirconium precursor, Zn(O CCH synthesized as
discussed in the Section II. This precursor has a thermal de-
Fig. 1. TGA of Zn(O CCH
2
air. Note the 2 wt% mass loss results from propionic acid or waters of
crystallization.
2
CH ramped at 101C/min in synthetic
3 2
)
2
CH )
2 3 2

October 2011
Combinatorial Nanopowder Synthesis Using LF-FSP
3311
In general, LF-FSP processing as practiced at UM differs
considerably from other groups because we work with turbulent
flames rather than laminar flow systems. Furthermore, in addi-
tion to having somewhat higher temperatures, our precursor
throughput is much higher than other researchers who often
produce milligrams/hour. These differences appear to lead to
quite different products in some instances.
(3) Power Processing Studies
Below, we first discuss what is known about the ZnO–Al O
2
3
phase diagram, and then discuss the types of powders made and
their specific characteristics.
The ZnO–Al
2
O
3
phase diagram was first described by Bunt-
ing in 1932 but does not incorporate ZnO:Al solid solutions (up
to 5 mol% Al depending on the synthesis temperature) and
60
2 3
O
61
off-stoichiometric spinel phases. Hansson et al. recently revised
the phase diagram, adding two solid solution regions to Bunting’s
work. First, the maximum solubility of Al O in zincite was
2
3
found to be 4.7 mol% at 16951C and was found to decrease rap-
idly with decreasing temperatures to r0.5 mol% at 15501C and
below. The spinel composition is reported as the mineral gahnite
20
30
40
50
60
70
80
36
(
ZnAl
2
O
4
) when Al
2
O
3
is in equilibrium with zincite ZnO.
LF-FSP flame synthesis occurs at temperatures of 15001–
0001C, thus we are potentially able to explore the solubility
Fig. 3. XRD powder patterns of as-processed (ZnO) (Al
x
powders by LF-FSP wurtzite ZnO (PDF file: 36-1451), spinel ZnAl O
2 4
2
O
3
)
1ꢀx nano-
2
limit of Al in ZnO at much higher temperatures than possible
for standard ceramics processing methods and by virtue of the
rapid quenching that occurs in LF-FSP, produce powders with
relatively extreme compositions, e.g. at the edge of the currently
known solubility limits.
(
PDF file: 05-0669), nonstoichiometric spinel (Zn0.3Al0.7)Al1.7
O
4
(PDF
Ã
file: 77-0732), a-alumina (PDF file: 71-1124), d -Al O (PDF file:
46-1215).
2
3
However, nanopowders produced by LF-FSP show higher
(5) XRD Studies
solubility (up to 20 mol% Al
2
O
3
) at ZnO-rich compositions than
Here we begin with the ZnO-rich samples from zinc acetate and
alumatrane, which form Al-doped zincite nanopowders. The
XRDs for the majority of the compositions are given in Fig. 2.
100 mol% ZnO (pure ZnO) has the hexagonal wurtzite structure
in accord with PDF [36-1451]. The XRDs of the 95–100 mol
ZnO compositions show that hexagonal zincite is the major
phase. However, a very small (440) peak at 65.21 2y seen in the
XRD supports spinel phase formation for compositions 97.5
and 95 mol% ZnO.
36
the phase diagram. As with many of our previous studies, LF-
FSP materials generated by rapid quenching lead to novel kinetic
products not expected based on traditional processing methods.
Since traditional processing methods lead to thermodynamically
rather than kinetically defined phase compositions, these materi-
als may offer unique opportunities for a wide variety of applica-
tion including various display applications and/or photovoltaic,
electrochromic, RF shielding, and sensor applications.
Because of solubility issues with zinc acetate, we switched to
zinc propionate Zn(O CCH CH ) for the broader set of com-
(4) Compositions
2
2
3 2
positions including the spinel phase region. We also switched to
aluminum acetylacetonate [Al(Acac) ] from alumatrane. Figure
provides XRD patterns for all as-produced nanopowders
along the (ZnO) (Al O ) tie-line.
The set of compositions listed in Table I were produced using
LF-FSP. First, we made ZnO-rich compositions from zinc
acetate and alumatrane, then the overall composition series
with zinc propionate and aluminum acetylacetonate.
3
3
x
2
3 1ꢀx
In Fig. 3, the Zn-rich samples (99, 95 mol% ZnO) show only
wurtzite (PDF 36-1451) suggesting improved miscibility for this
precursor system. For 80 mol% ZnO, wurtzite is the main phase,
but traces of spinel ZnAl O are also observed as a weak (440)
2
4
9
5 mole% ZnO
peak appears at 65.21 2y supporting spinel phase formation.
For samples with 30 and 50 mol% ZnO, ZnAl spinel is
observed as the main phase. The formation of 100% spinel
2 4
O
3
6
97.5 mole% ZnO
phase at 50 mol% ZnO is expected. However, the 30 mol%
99 mole% ZnO
Table II. Particle Sizes (nm) Measured by FWHM of XRD
and the Aspect Ratios (c/a)
1
00 mol% 99.8 mol% 99 mol% 97.5 mol% 95 mol%
100 mole% ZnO
(hkl)
ZnO
ZnO
ZnO
ZnO
ZnO
(
(
(
(
(
(
(
(
100)
002)
101)
102)
110)
103)
112)
201)
134
150
114
100
101
90
68
59
54
47
55
41
47
51
44
41
38
34
36
28
31
34
39
32
32
30
32
27
27
33
33
34
29
31
32
27
29
29
Zn spinel [05-0669]
Wurtzite [36-1451]
88
88
20
30
40
50
60
70
80
Aspect ratio
c/a)
1.12
0.87
0.93
0.82
1.03
x 2 3
Fig. 2. XRD patterns of (ZnO) (Al O )1ꢀx nanopowders from zinc
acetate and alumatrane.
(

3
312
Journal of the American Ceramic Society—Kim et al.
Vol. 94, No. 10
Table III. Lattice Parameters Measured by X-Ray
Diffraction Using (100) and (101) Peaks
Table IV. Average Particle Sizes and Specific Surface Areas
of (ZnO) (Al O ) Nanopowders
x
2
3 1ꢀx
1
00 mol% 99.8 mol% 99 mol% 97.5 mol% 95 mol%
Particle size (nm)
ZnO
ZnO
ZnO
ZnO
ZnO
Sample ZnO
(mol %)
2
SSA (m /g)
a
c
3.2442
5.1900
94.6
3.2506
5.2084
95.3
3.2508
5.2068
95.3
3.2532
5.2128
95.6
3.2490
5.2022
95.1
XRD line broadening
BET-derived
9
9
8
5
3
9
5
0
0
0
16
14
13
13
12
12
36
32
30
26
22
22
30
33
39
50
59
63
Cell
volume
ZnO sample is also almost phase pure spinel in contrast to ex-
isting thermodynamic data where phase separation between
spinel and a-alumina should be found. In the phase diagram,
phase pure spinel with a 30 mol% ZnO composition forms only
between 17001 and 18001C. One could argue that in LF-FSP,
phase pure spinel forms and the spinel structure is preserved by
rapid quenching to ambient temperature, forming a kinetic
product.
10
rapid quenching (10001C/ms). A detailed discussion of the for-
mation of transition aluminas rather than a-Al O at these tem-
peratures is presented elsewhere.
2
3
5
9
Thus, we can suggest that the effective LF-FSP operating
temperature is 17001–18001C. The term ‘‘effective’’ is used be-
cause we are not able to estimate the effect(s) of the very small
particle sizes and therefore high surface energies on the solubility
of one oxide phase in the other.
(6) APSs and SSAs from BET
Table II provides APSs of samples from zinc acetate and al-
umatrane along the different plane directions as measured using
Debye-Scherer methods. 100 mol% ZnO (pure ZnO) has the
largest APSs of 90–150 nm depending on the crystallographic
plane.
The 10 mol% ZnO sample shows phase separation between
spinel and d-alumina; different from the phase diagram where
phase separation should lead to spinel and a-alumina. Accord-
Crystallite sizes decrease to B30 nm for 95 mol% ZnO.
XRDs of the (hkl) 100, 002, and 101 reflections of all LF-FSP
prepared samples showed no evidence of preferred growth ori-
entations in contrast to reports for sputtered materials with even
2
2
ing to Hinklin et al., the formation of d-alumina might be
favored during LF-FSP processing. One can argue that kinetic
transition alumina formed during LF-FSP is unable to trans-
form to the thermodynamically stable a-alumina, because of
1
3
less Al doping.
Fig. 4. SEM micrographs of as-prepared nanopowders made from zinc acetate and alumatrane. (a) 100, (b) 97.5, and (c) 95 mol% ZnO samples.

October 2011
Combinatorial Nanopowder Synthesis Using LF-FSP
3313
2 3 2 3
Fig. 5. SEM images of (a) 99 mol% ZnO with Al O ; (b) 50 mol% ZnO in Al O .
Lattice parameters determined using the (100) and (101)
peaks (Fig. 2) show c/a axis ratios of o2.0 as listed in Table
III. In Table III, it can be seen that the d-spacings of both the a-
volume decrease. However, the 95 mol% ZnO cell volume is
lower than that for 99.8 mol% ZnO, which indicates that the
concentration of Al ions is above the solubility limit in ZnO at
31
64
and c-axes increase when the amount of Al increases indicat-
this composition (maximum is 4.7 mol% per Hansson et al.).
The maximum cell volume is found for 97.5 mol% ZnO. For
99.8, 99.0, and 97.5 ZnO mol% compositions, our results sug-
31
ing that the cell volume increase is proportional to the Al ion
concentrations. Note that these values are the averages gleaned
from XRD data and may average in data for pure ZnO with
31
gest that Al dissolves in ZnO, likely due to the rapid quench-
ing during the LF-FSP process. Unfortunately, the exact
explanation for these observations is beyond the scope of this
paper.
31
data for nanoparticles that have somewhat higher Al concen-
trations based on the arguments made below concerning the
mechanisms for particle formation.
3
1
21
Since the Al radius (0.053 nm) is smaller than that of Zn
0.075 nm), the expansion of the lattice constants in both direc-
The APSs for samples from zinc propionate and aluminum
acetylacetonate were estimated from Debye-Scherer line broad-
ening and their SSAs (Table IV). The BET particle sizes are
about twice the Debye-Scherer APSs perhaps because some
necking occurs between single crystal particles. This would
lead to lower surface areas than expected and reduced APSs.
(
31
tions might be explained in terms of the incorporation of Al
ions in interstitial sites as has been suggested by other au-
6
2,63
21
31
31
not by substitution of Zn by Al . If Al ions
thors,
were to substitute for Zn , one would expect that the total cell
21
Fig. 6. TEM micrographs of as-prepared powders: (a) 100, (b) 99.8, and (c) 95 mol% ZnO.

3
314
Journal of the American Ceramic Society—Kim et al.
Vol. 94, No. 10
2 3 2 3
Fig. 7. TEM images of (a) 50 mol% ZnO in Al O ; (b) 30 mol% ZnO in Al O .
(
7) SEM and TEM
TEM micrographs of 100, 99.8, and 95 mol% ZnO samples are
shown in Fig. 6. While small particles (o15 nm in particle sizes)
are not observed in pure ZnO powders, the 99.8 mol% ZnO
powders show significant amounts of fine particles with APSs o
High-resolution microscopy was used to assess particle mo-
rphologies as a function of chemical and phase composition.
SEMs of the 100, 97.5, and 95 mol% ZnO samples made from
zinc acetate and alumatrane are shown in Fig. 4. Pure ZnO
3
0 nm compared with the pure ZnO powders.
The increase in number of small sized particles in 99.8 and 95
(ZA00) powders consist of hexagonal rods and platelets. The
mol% ZnO samples also matches with XRD line broadening
results and SEM images. It appears that there is a bimodal size
distribution at 25 and 250 nm in these powders, while the com-
positions close to the spinel region show more uniform and
smaller APSs.
addition of small amounts of alumina in the 97.5 and 95 mol%
ZnO powders results in significant reductions in APSs from the
pure ZnO powders. Similar reductions in particle size on in-
creasing Al content have been observed before. This APS
reduction in the SEM images matches with the APS data from
XRD line broadening in previous section. However, particles
31
45
31
The morphologies also change with Al doping. As the
amount of Al increases, the resulting particles tend toward
rods indicating enhanced growth along the c-axis perhaps indi-
4100 nm are still seen in both samples.
SEM was also used to demonstrate powder uniformity for the
31
cating somewhat selective addition of Al ions to one growth
face. However, detailed studies show that correlate particle size
samples from zinc propionate and aluminum acetylacetonate.
Figure 5 shows that SEM resolution is insufficient to reveal in-
dividual particles but does provide a view of the general particle
population.
31
and shape as a function of Al content were not pursued
further.
TEM images were also used to gather information on particle
morphologies and sizes of as-prepared powders at the spinel
composition. Discussions of actual size/size distributions are not
appropriate if based solely on TEM micrographs, unless com-
bined with the XRD results and SEM images. Figure 7 offers
For a ZnO-rich sample (Fig. 5(a)) shows dominant cylindri-
cal-shaped particles with irregular c/a ratios, and a bimodal dis-
tribution of APSs. For a spinel region sample (Fig. 5(b)), most
particles are spherical and APSs are less than 50 nm. These
SEMs indicate that the particle populations produced here do
not include any obvious micrometer size particles.
high-resolution TEM images of (ZnO) (Al O )
x
nanopowders
2
3 1ꢀx
x10
1
0 mole % ZnO
0 mole % ZnO
3
5
8
0 mole % ZnO
0 mole % ZnO
9
9
5 mole % ZnO
9 mole % ZnO
4000
3500
3000
2500
2000
1500
1000
500
Fig. 9. TGAs of as-processed (ZnO)
from zinc propionate and aluminum acetylacetonate (101C/min, in air).
x 2 3
(Al O )1ꢀx nanopowders made
x 2 3
Fig. 8. DRIFTs of as-processed (ZnO) (Al O )1ꢀx nanopowders.

October 2011
Combinatorial Nanopowder Synthesis Using LF-FSP
3315
powder surfaces. These spectra offer little evidence for either
nC–H or nO–H bands.
The as-processed nanopowders have no significant organic
ꢀ
1
species on the surface, as the absence at 2900–2700 cm nC–H
vibrations suggests.
6
5
ꢀ1
)
nO–H vibrations (3800–3200 cm
show both chemi- and physisorbed water on the surface of
nanopowders.
6
6
ꢀ1
nM–O bands are observed in the 1200–400 cm region. The
spinel structure (80, 50, and 30 mol% ZnO) is confirmed by
FTIR spectra, as there are three characteristic peaks at 510, 570,
ꢀ
1
and 690 cm corresponding to a normal spinel structure with
.
31 67,68
octahedrally coordinated Al
ꢀ1
The bands at 450–600 cm
show nZn–O bands typical of the wurtzite structure, which
dominate Zn-rich samples (95, 99, 80 mol% ZnO).
6
9,70
(
9) TGA–DTA Studies
TGAs were performed in air on all as-prepared powder samples
to determine the relative amounts of surface species and thermal
behavior. The thermal stability of the powders was investigated
using DTA. Figure 9 shows the TGA for the series of powders.
The DTAs are not shown, because they are not informative, no
phase transformations are observed.
20
30
40
50
60
70
80
All as-processed powders exhibit 0.5–5.0 wt% mass losses up
to ꢁ3001C, typical of LF-FSP produced nanopowders that can
be attributed to evolution of both physi- and chemisorbed water
Fig. 10. XRDs of as-prepared and reprocessed nanopowders of spinel
composition.
6
6
as seen in the FTIR (Fig. 8). Mass losses between 3001 and
001C are due to elimination of carbonate species as CO . The
amount of mass loss is proportional to the amount of alumina as
4
2
2
9–33
from LF-FSP. Particle sizes here are typically o30 nm in diam-
eter with the vast majority o20 nm. Figure 7(a) shows clear
seen previously.
˚
unidirectional lattice planes, [d 52.4 A for (311) planes, cubic
(10) Reprocessing Spinel Composition Nanopowders
by LF-FSP
spinel phase]. The d-spacing values from TEM images confirm
our XRD results concerning spinel phase at 50 mol% ZnO com-
position. Multifaceted single particles are likely a consequence of
particle formation during rapid quench from the gas phase.
We recently re-shot d-alumina using LF-FSP causing d-alumina
5
9
transform to a-alumina. Based on this result, we realized that
given that 30 mol% ZnO composition gives pure nonstoichio-
metric spinel phase (PDF file: 77-0732) as a kinetic product, we
might expect core-shell nanostructured nanoparticles to form
with spinel and a-alumina using LF-FSP reprocessing based on
the phase diagram thermodynamic data and the nucleation
differences between spinel and a-alumina due to different boil-
(8) Diffuse Reflectance Infrared Fourier
Transform Spectroscopy
Once the particle morphologies were characterized by XRD
and high-resolution microscopy, the particle surface chemistries
3
2–42,59
ing/melting points.
Therefore, we re-shot as-produced
were characterized using FTIR. DRIFT spectra for (ZnO)
Al O ) nanopowders are presented in Fig. 8. Higher wave-
number (41500 cm ) regions were first normalized and then
multiplied 10ꢂ in an effort to observe nC–H or nO–H bands on
x
spinel composition nanopowders (30 and 50 mol% ZnO), be-
cause we anticipated that the as-processed spinel nanoparticles
might phase separate to spinel and a-alumina during reprocess-
ing. The existing phase diagram suggests that phase separation
(
2
3 1ꢀx
ꢀ1
2 3
Fig. 11. SEM images of re-shot 50 mol% ZnO in Al O .

3
316
Journal of the American Ceramic Society—Kim et al.
Vol. 94, No. 10
2 3
Fig. 12. TEM images of re-shot 30 mol% ZnO in Al O .
between spinel and a-alumina should be observed at 30 mol%
ZnO composition.
relatively weak Zn peak in XEDS [Fig. 14(a)] matching the
nonstoichiometric spinel in the XRD data. Cu peaks in XEDS
come from Cu grid in TEM sample holder.
As-produced LF-FSP (ZnO) (Al
x
2
O
3
)
1ꢀx (x 5 0.3, 0.5) pow-
s
ders (3.0 g) were dispersed with 5 mg DARVAN C-N (R.T.
Vanderbilt Company Inc., Norwalk, CT) in 100 mL EtOH using
a 1.2-cm-diameter 500 W ultrasonic horn (Sonics and Materials
Thus, our speculation on what happens in the gas phase on
re-shooting and perhaps also during initial particle formation
seems to be correct. That is, at the flame temperatures used,
ZnO remains in the gas phase long enough to produce two
6
00 VCX, Newtown, CT) at 40% of full power for 12 h. The
31
dispersion was allowed to settle for 24 h. Dispersed nano-
powders were mixed with ethanol to a 3.0 wt% ceramic load-
ing, and re-shot in the LF-FSP as described in Section II.
We find that re-shooting produces powders quite different
from our anticipated results. The XRDs (Fig. 10) show the
phases of the as-produced and re-shot nanopowders at 30 and
different particle morphologies ones with higher Al content
and particles that are mostly pure ZnO. This then provides a
reasonable explanation of the evolution of particle morphology
seen in the high ZnO content nanopowders (e.g. 495 mol%). It
is important to note that other efforts see well-defined particle
compositions, even those using LF-FSP-like approaches.
The best explanation is that these studies do not reach the
same combustion temperatures used here.
4
4,46
5
0 mol% ZnO compositions. For 50 mol% ZnO samples, phase
separation of wurtzite (zincite) and the nonstoichiometric spinel
phase (PDF 77-0732) were observed in contrast to the existing
thermodynamic data showing phase pure stoichiometric spinel.
For 30 mol% ZnO samples, two main phases, wurzite and spinel
IV. Conclusions
(
PDF file: 77-0732), with traces of d-alumina phase form.
Based on the phase diagram, heating phase pure spinel
44
The utility of LF-FSP processing for combinatorial studies of
nanopowders along the (ZnO) (Al O )
above approximately 17001C causes the formation of a liquid
phase coincident with the formation of the nonstoichiometric
spinel phase (PDF 77-0732). Thus, during LF-FSP reprocessing,
this appears to be what happens and on quenching the spinel
component retains its nonstoichiometry, whereas the liquid por-
tion is quenched to form wurtzite ZnO. One would therefore
expect two different particle morphologies as discussed below
but it also may mean that the two particle morphologies ob-
served above arise for similar reasons, the ZnO remains liquid
tie line was demon-
strated focusing on composition regions including Al-doped
x
2
3 1ꢀx
31
31
longer than Al -doped materials. Since at low Al loadings
their incorporation leads to an as yet unexplained increase in
unit cell dimensions that may arise from formation of intersti-
tials, a better explanation of the above analysis of c/a ratios may
be that it is actually averages particles that are pure ZnO and
31
those with higher Al doping. At the moment, this remains an
open question.
High-resolution SEM and TEM were used to study the mor-
phology of re-shot nanopowders. In Figs. 11 and 12, we observe
two different morphologies in the re-processed nanopowders.
Comparing with images in previous sections, we expect the cy-
lindrical shape nanoparticles to be zincite and the spherical
nanopowders to be nonstoichiometric spinel.
In order to characterize the two different morphologies in the
re-shot nanopowders, STEM and XEDS were used. XEDS data
(
were obtained for re-shot 50 mol% ZnO in Al O . For cylin-
Fig. 14) of four different spots in the STEM image (Fig. 13)
2
3
drical shapes (spots 2, 3, and 4 in Fig. 13), we observed Zn peaks
and no Al peaks in the XEDS [Fig. 14(b)–(d)]. For the spherical
particles (spot 1 in Fig. 13), we observe a strong Al peak and a
2 3
Fig. 13. STEM images of reprocessed 50 mol% ZnO in Al O . Note
points marked 1–4 which are used for the XED studies in Fig. 14.

October 2011
Combinatorial Nanopowder Synthesis Using LF-FSP
3317
2 3
Fig. 14. XEDS of 50 mol% ZnO in Al O in spot 1 (a), 2 (b), 3 (c), and 4 (d) from Fig. 13.
ZnO for TCO applications and zinc spinel (ZnAl O ). The re-
2
therefore high surface energies on the solubility of one oxide
phase in the other. A more detailed discussion of the thermo-
dynamics of cooling during nanoparticle formation is given
4
sulting data are different from the thermodynamic phase dia-
gram because the resulting nanopowders are kinetic products
made by rapid quenching of LF-FSP.
4
4
elsewhere.
The as-produced powders were characterized in terms of
phase, size, composition, and morphology by chemical analy-
sis, FTIR, XRD, SEM, TEM, and TGA–DTA. The XRD data
31
References
support previous conclusions suggesting that Al ions incor-
porate into interstitial sites of zincite resulting in octahedrally
1
H. Hosono, H. Ohta, M. Orita, K. Ueda, and M. Hirano, ‘‘Frontier of Trans-
parent Conductive Oxide Thin Films,’’ Vacuum, 66 [3–4] 419–25 (2002).
I. Hamberg and C. G. Granqvist, ‘‘Evaporated Sn-Doped In O Films: Basic
2 3
31
coordinated Al . At 80 mol% ZnO, small amounts of spinel
phase segregate, while the XRD results suggest formation of a
single zincite phase at Zn-rich compositions of 99, 95 mol%
ZnO. However, the observation of two different particle
morphologies in the last two compositions and the results of
the re-shooting of phase pure spinel and off-stoichiometry spinel
leading to mixed phase nanopowders as a consequence of the
low melting temperature of the spinel phase suggest that even in
the original powder production gas phase equilibria are of suffi-
cient importance during quenching as to control particle mor-
phologies and the chemical compositions of those particles.
2
Optical Properties and Applications to Energy-Efficient Windows,’’ J. Appl. Phys.,
6
0 [11] R123–59 (1986).
P. P. Edwards, A. Porch, M. O. Jones, D. V. Morgan, and R. M. Perks, ‘‘Basic
3
Materials Physics of Transparent Conducting Oxides,’’ Dalton Trans., [19] 2995–
3
002 (2004).
H. Hono, ‘‘Built-in Nanostructures in Transparent Oxides for Novel Photonic
4
and Electronic Functions Materials,’’ Int. J. Appl. Ceram. Technol., 1 [2] 106–18
2004).
D. S. Ginley and C. Bright, ‘‘Transparent Conducting Oxides,’’ MRS Bull., 25
(
5
[
8] 15–8 (2000).
6
B. G. Lewis and D. C. Paine, ‘‘Applications and Processing of Transparent
Conducting Oxides,’’ MRS Bull., 25 [8] 22–7 (2000).
H. Kawazoe, H. Yanagi, K. Ueda, and H. Hosono, ‘‘Transparent p-Type
Conducting Oxides: Design and Fabrication of p–n Heterojunctions,’’ MRS Bull.,
5 [8] 28–36 (2000).
T. Minami, ‘‘New n-Type Transparent Conducting Oxides,’’ MRS Bull., 25 [8]
8–44 (2000).
R. G. Gordon, ‘‘Criteria for Choosing Transparent Conductors,’’ MRS Bull.,
25 [8] 52–7 (2000).
Based on the solubilities of Al
2
O
3
in ZnO (up to 20 mol%
) and vice versa, we can suggest an effective operating
7
Al
O
2 3
2
temperature for LF-FSP processing that ranges between 17001
and 18001C in ZnO–Al O system from the two compositional
regions studied. The term ‘‘effective’’ is used because we are not
8
2
3
3
9
able to estimate the effect(s) of the very small particle sizes and

3
318
Journal of the American Ceramic Society—Kim et al.
Vol. 94, No. 10
1
0
41
T. J. Coutts, D. L. Young, and X. Li, ‘‘Characterization of Transparent Con-
ducting Oxides,’’ MRS Bull., 25 [8] 58–65 (2000).
R. Strobel and S. E. Pratsinis, ‘‘Flame Aerosol Synthesis of Smart Nano-
structured Materials,’’ J. Mater. Chem., 17, 4743–56 (2007).
R. M. Laine, ‘‘Liquid-Flame Spray Pyrolysis (LF-FSP) in the Synthesis
11
42
K. Wasa, S. Hayakawa, and T. Hada, ‘‘Electrical and Optical Properties of
Sputtered n–p ZnO–Si Heterojunctions,’’ Jpn. J. Appl. Phys., 10 [12] 1732 (1971).
of Single and Mixed-Metal Oxide Nanopowders’’; in Ceramics Science and
Technology, Vol. 3, Edited by R. Riedel, and I.-W. Chen. Wiley VCH Weinham,
in press.
12
Z. Jin, I. Hamberg, and C. G. Granqvist, ‘‘Optical Properties of Sputter-
Deposited ZnO:Al Thin Films,’’ J. Appl. Phys., 64 [10] 5117–32 (1988).
M. J. Alam and D. C. Cameron, ‘‘Preparation and Properties of Transparent
13
43
E. Arca, K. Fleischer, and I. V. Shvets, ‘‘Influence of the Precursors and
Chemical Composition of the Solution on the Properties of ZnO Thin Films
Conductive Aluminum-Doped Zinc Oxide Thin Films by Sol–Gel Process,’’
J. Vac. Sci. Technol. Sect. A, 19 [4] 1642–6 (2001).
Grown by Spray Pyrolysis,’’ J. Phys. Chem. C, 113, 21074–81 (2009).
J. U. Brehma, M. Winterer, and H. Hahn, ‘‘Synthesis and Local Structure of
14
44
T. Schuler and M. A. Aegerter, ‘‘Optical, Electrical and Structural Properties
of Sol Gel ZnO:Al Coatings,’’ Thin Solid Films, 351 [1–3] 125–31 (1999).
S. H. Jeong, J. W. Lee, S. B. Lee, and J. H. Boo, ‘‘Deposition of Aluminum-
Doped Nanocrystalline Zinc Oxides,’’ J. Appl. Phys., 100, 064311, 9pp (2006).
S. Suwanboon, P. Amornpitoksuk, A. Haidoux, and J. C. Tedenac, ‘‘Struc-
15
45
Doped Zinc Oxide Films by RF Magnetron Sputtering and Study of their Struc-
tural, Electrical and Optical Properties,’’ Thin Solid Films, 435 [1–2] 78–82 (2003).
tural and Optical Properties of Undoped and Aluminium Doped Zinc Oxide
Nanoparticles Via Precipitation Method at Low Temperature,’’ J. Alloys Compd.,
462, 335–9 (2008).
T. Ogi, D. Hidayat, F. Iskandar, A. Purwanto, and K. Okuyama, ‘‘Direct
Synthesis of Highly Crystalline Transparent Conducting Oxide Nanoparticles by
16
T. Suchiya, T. Emoto, and T. Sei, ‘‘Preparation and Properties of Transparent
Conductive Thin Films by the Sol–Gel Process,’’ J. Non-Cryst. Solids, 178, 327–32
1994).
46
(
17
T. Minami, H. Nanto, S. Shooji, and S. Takata, ‘‘The Stability of Zinc Oxide
Low Pressure Spray Pyrolysis,’’ Adv. Powder Technol., 20, 203–9 (2009).
B. Houng and C.-J. Huang, ‘‘Structure and Properties of Ag Embedded Alu-
47
Transparent Electrodes Fabricated by R. F. Magnetron Sputtering,’’ Thin Solid
Films, 111 [2] 167–74 (1984).
T. Minami, H. Nanto, and S. Takata, ‘‘Highly Conductive and Transparent
minum Doped ZnO Nanocomposite Thin Films Prepared Through a Sol–Gel
Process,’’ Surf. Coat. Technol., 201, 3188–92 (2006).
18
48
Aluminum Doped Zinc Oxide Thin Films Prepared by RF Magnetron Sputter-
ing,’’ Jpn. J. Appl. Phys., 23 [5] L280–2 (1984).
Z. Ben Ayadi, L. El Mir, K. Djessas, and S. Alaya, ‘‘The Properties of Alu-
minum-Doped Zinc Oxide Thin Films Prepared by rf-Magnetron Sputtering from
19
C. R. Bickmore, K. F. Waldner, D. R. Treadwell, and R. M. Laine, ‘‘Ultrafine
Spinel Powders by Flame Spray Pyrolysis of a Magnesium Aluminum Double
Nanopowder Targets,’’ Mater. Sci. Engg. C, 28, 613–7 (2008).
S. Cho, S.-H. Jung, J-W. Jang, E. Oh, and K.-H. Lee, ‘‘Simultaneous Syn-
49
Alkoxide,’’ J. Am. Ceram. Soc., 79 [5] 1419–23 (1996).
C. R. Bickmore, K. F. Waldner, R. Baranwal, T. Hinklin, D. R. Treadwell,
thesis of Al-Doped ZnO Nanoneedles and Zinc, Aluminum Hydroxides through
Use of a Seed Layer,’’ Cryst. Growth Design, 8, 4553–8 (2008).
X. L. Cheng, H. Zhao, L. H. Huo, S. Gao, and J. G. Zhao, ‘‘ZnO Nanopar-
ticulate Thin Film: Preparation, Characterization and Gas-Sensing Property,’’
20
50
and R. M. Laine, ‘‘Ultrafine Titania by Flame Spray Pyrolysis of a Titanatrane
Complex: Part I,’’ J. Eur. Ceram. Soc., 18 [4] 287–97 (1998).
21
A. C. Sutorik, S. S. Neo, T. Hinklin, R. Baranwal, D. R. Treadwell, R.
Narayanan, and R. M. Laine, ‘‘ ‘Synthesis of Ultrafine b’-Alumina Powders Via
Flame Spray Pyrolysis of Polymeric Precursors,’’ J. Am. Ceram. Soc., 81 [6] 1477–
Sensors Actuators B, 102, 248–52 (2004).
J. B. McPeak and K. M. Baxter, ‘‘Microreactor for High-Yield Chemical Bath
51
Deposition of Semiconductor Nanowires: ZnO Nanowire Case Study,’’ Ind. Eng.
Chem. Res., 48, 5954–61 (2009).
J.-S. Na, Q. Peng, G. Scarel, and G. N. Parsons, ‘‘Role of Gas Doping Se-
8
6 (1998).
22
52
T. Hinklin, B. Toury, C. Gervais, F. Babonneau, J. J. Gislason, R. W. Mor-
ton, and R. M. Laine, ‘‘Liquid-Feed Flame Spray Pyrolytic Synthesis of Nan-
quence in Surface Reactions and Dopant Incorporation During Atomic Layer
Deposition of Al-Doped ZnO,’’ Chem. Mater., 21, 5585–93 (2009).
oalumina Powders,’’ Chem. Mater., 16 [1] 21–30 (2004).
R. M. Laine, R. Baranwal, T. Hinklin, D. Treadwell, A. Sutorik, C. R. Bick-
23
53
S. Hartner, M. Ali, C. Schulz, H. Winterer, and M. Wiggers, ‘‘Electrical Prop-
erties of Aluminum-Doped Zinc Oxide (AZO) Nanoparticles Synthesized by
more, K. Waldner, and S. S. Neo, ‘‘Making Nanosized Oxide Powders from Pre-
cursors by Flame Spray Pyrolysis’’; pp. 17–24 in Novel Synthetic and Processing
Routes to Ceramics. Key Engineering Materials, Vols. 159–160, Edited by K. Ue-
matsu, and H. Otsuka. K. Trans Tech Publ. Ltd., Switzerland, 1998.
Chemical Vapor Synthesis,’’ Nanotechnology, 20, 445701 (2009).
A. B. F. Martinson, J. W. Elam, J. T. Hupp, and M. J. Pellin, ‘‘ZnO Nanotube
54
Based Dye-Sensitized Solar Cells,’’ Nano Lett., 7, 2183–7 (2007).
T. Strachowski, E. Grzanka, W. Lojkowski, A. Presz, M. Godlewski, S.
24
55
R. Baranwal, M. P. Villar, R. Garcia, and R. M. Laine, ‘‘Synthesis, Charac-
terization, and Sintering Behavior of Nano-Mullite Powder and Powder Com-
pacts,’’ J. Am. Ceram. Soc., 84 [5] 951–61 (2001).
Yatsunenko, H. Matysiak, R. R. Piticescu, and C. J. Monty, ‘‘Morphology and
Luminescence Properties of Zinc Oxide Nanopowders Doped with Aluminum
Ions Obtained by Hydrothermal and Vapor Condensation Methods,’’ J. Appl.
Phys., 102, 073513 (2007).
25
G. Williams, S. C. Rand, T. Hinklin, and R. M. Laine, ‘‘Laser Action in
Strongly Scattering Rare-Earth-Doped Dielectric Nanophosphors,’’ Phys. Rev. A,
5 [1] 013807–6 (2002).
56
6
J. D. Merchant and M. Cocivera, ‘‘Preparation and Doping of Zinc Oxide
Using Spray Pyrolysis,’’ Chem. Mater., 7, 1742–9 (1995).
T. Masaki, S.-J. Kim, H. Watanabe, K. Miyamoto, M. Ohno, and K.-H. Kim,
26
S. Kim, J. J. Gislason, R. W. Morton, X. Pan, H. Sun, and R. M. Laine,
‘Liquid-Feed Flame Spray Pyrolysis of Nanopowders in the Alumina-Titania
System,’’ Chem. Mater., 16 [12] 2336–43 (2004).
57
‘
‘‘Synthesis of Nano-Sized ZnO Powders Prepared by Precursor Process,’’
J. Ceram. Process. Res., 4, 135–9 (2003).
27
J. Marchal, T. Hinklin, R. Baranwal, T. Johns, and R. M. Laine, ‘‘Yttrium
Aluminum Garnet Nanopowders by Flame Spray Pyrolysis,’’ Chem. Mater., 16 [5]
22–31 (2004).
58
R. R. Piticescu, R. M. Piticescu, and C. J. Monty, ‘‘Synthesis of Al-Doped
ZnO Nanomaterials with Controlled Luminescence,’’ J. Eur. Ceram. Soc., 26,
2979–83 (2006).
8
28
R. M. Laine, J. Marchal, H. J. Sun, and X. Q. Pan, ‘‘A New Y
Produced By Liquid-Feed Flame Spray Pyrolysis (LF-FSP),’’ Adv. Mater., 17 [7]
30–3 (2005).
3 5
Al O12 Phase
59
2 3
R. M. Laine, J. C. Marchal, H. P. Sun, and X. Q. Pan, ‘‘Nano-a-Al O by
8
Liquid-Feed Flame Spray Pyrolysis (LF-FSP) of Nano-Transition Aluminas,’’
Nat. Mater., 5 [9] 710–2 (2006).
29
J. A. Azurdia, J. C. Marchal, P. Shea, H. Sun, X. Q. Pan, and R. M. Laine,
‘Liquid-Feed Flame Spray Pyrolysis (LF-FSP) as a Method of Producing Mixed-
Metal Oxide Nanopowders of Potential Interest as Catalytic Materials. Nano-
powders Along the NiO–Al Tie-Line Including (NiO)0.22(Al 0.78, a New
Inverse Spinel Composition,’’ Chem. Mater., 18 [9] 731–9 (2006).
60
‘
2 2 3
E. N. Bunting, ‘‘Phase Equilibria in the System SiO . ZnO–Al O ,’’ Bur.
Stand. J. Res., 8, 279–9 (1932).
R. Hansson, P. C. Hayes, and E. Jak, ‘‘Experimental Study of Phase
61
O
2 3
2 3
O )
Equilibria in the Al–Fe–Zn–O System in Air,’’ Metall. Mater. Trans. B, 35 [4]
633–42 (2004).
H. Kim, A. Pique, J. S. Horwitz, H. Murata, Z. H. Kafafi, C. M. Gilmore, and
30
J. A. Azurdia, J. C. Marchal, and R. M. Laine, ‘‘Combinatorial Processing of
Mixed-Metal Oxide Nanopowders Along the Co –Al Tie Line Using Liq-
uid-Feed Flame Spray Pyrolysis (LF-FSP),’’ J. Am. Ceram. Soc., 89 [9] 2749–56
2006).
62
O
3 4
O
2 3
D. B. Chrisey, ‘‘Effect of Aluminum Doping on Zinc Oxide Thin Films Grown by
Pulsed Laser Deposition for Organic Light-Emitting Devices,’’ Thin Solid Films,
377–378 [1] 798–802 (2000).
(
31
T. R. Hinklin and R. M. Laine, ‘‘Synthesis of Metastable Phases in the Mag-
nesium Spinel–Alumina System,’’ Chem. Mater., 20 [2] 553–8 (2008).
63
H. Cao, C. Sun, Z. Pei, A. Wang, L. Wen, R. Hong, and X. Jiang, ‘‘Properties
32
M. Kim and R. M. Laine, ‘‘Combinatorial Processing of Mixed-Metal Oxide
Nanopowders Along the ZrO –Al Tie Line Using Liquid-Feed Flame Spray
Pyrolysis (LF-FSP),’’ J. Ceram. Process. Res., 8 [2] 129–36 (2007).
of Transparent Conducting ZnO:Al Oxide Thin Films and their Application for
Molecular Organic Light-Emitting Diodes,’’ J. Mater. Sci.: Mater. Elect., 15 [3]
169–74 (2004).
2
2 3
O
33
64
M. Kim, T. R. Hinklin, and R. M. Laine, ‘‘Core-Shell Nanostructured Nano-
powders Along (CeO (Al
1ꢀx Tie-Line by Liquid-Feed Flame Spray Pyroly-
sis (LF-FSP),’’ Chem. Mater., 20 [16] 5154–62 (2008).
R. Hansson, P. C. Hayes, and E. Jak, ‘‘Experimental Study of Phase Equi-
libria in the Al–Fe–Zn–O System in Air,’’ Metall. Mater. Trans. B, 35 [4] 633–42
(2004).
)
x x
2 3
O )
34
65
T. T. Kodas and M. J. Hampden-Smith, Aerosol Processing of Materials.
Wiley-VCH, New York, 1999.
X. Liu and R. E. Truitt, ‘‘DRFT-IR Studies of the Surface of g-Alumina,’’
J. Am. Chem. Soc., 119 [41] 9856–60 (1997).
35
66
M. Winterer, Nanocrystalline Ceramics, Synthesis and Structure. Springer,
Lausanne, 2002.
J. B. Peri, ‘‘Infrared and Gravimetric Study of the Structure Hydration of g-
Alumina,’’ J. Phys. Chem., 69 [1] 211–9 (1965).
36
67
F. E. Luborsky, ‘‘Highly Coercive Materials, Development of Elongated
Particle Magnets,’’ J. Appl. Phys., 32, S171–9 (1961).
S. Shen, K. Hidahat, L. Yu, and S. Kawi, ‘‘Simple Hydrothermal Synthesis of
Nanostructured and Nanorod Zn–Al Complex Oxides as Novel Nanocatalysts,’’
Adv. Mater., 16 [6] 541–5 (2004).
37
B. L. Cushing, V. L. Kolesnichenko, and C. J. O’Connor, ‘‘Recent Advances
in the Liquid-Phase Syntheses of Inorganic Nanoparticles,’’ Chem. Rev., 104,
893–946 (2004).
68
2 4
A. Adak, A. Pathak, and P. Pramanik, ‘‘Characterization of ZnAl O Nano-
3
crystals Prepared by the Polyvinyl Alcohol Evaporation Route,’’ J. Mater. Sci.
Lett., 17 [7] 559–61 (1998).
S. V. Tsybulya, L. P. Solov’eva, L. M. Plyasova, and O. P. Krivoruchko,
38
M. T. Swihart, ‘‘Vapor-Phase Synthesis of Nanoparticles,’’ Curr. Opin. Coll.
Inter. Sci., 8, 127–33 (2003).
F. E. Kruis, H. Fissan, and A. Peled, ‘‘Synthesis of Nanoparticles in the Gas
69
39
‘‘Complete-Profile Analysis Applied to Nonstoichiometric Spinels,’’ J. Struct.
Chem., 32 [1] 386–8 (1991).
Phase for Electronic, Optical and Magnetic Applications—A Review,’’ J. Aerosol
Sci., 29, 511–35 (2004).
70
A. N. Tsvigunov, V. G. Khotin, A. S. Krasikov, and B. S. Svetlov, ‘‘Shock-
Wave Synthesis of Nonstoichiometric Aluminizing Spinel and Gahnite,’’ Glass
Ceram., 58 [9–10] 353–5 (2002).
40
L. Madler, ‘‘Liquid-Fed Aerosol Reactors for One-Step Synthesis of Nano-
Structured Particles,’’ Kona, 22, 107–20 (2004).
&