Phase morphology in electrospun zirconia microfibers

Article abstract of DOI:10.1111/j.1551-2916.2008.02264.x

Electrospinning of sol-gels has been used to produce zirconium-doped polymer microfibers from zirconyl chloride and poly(vinylpyrollidone) precursors. Calcination of these structures between temperatures of 370° and 930°C resulted in the formation of zirconia nanograined microfibers whose diameters ranged from 1200 to 800 nm at the higher temperatures and whose average grain size ranged from 9 to 33 nm. X-ray diffraction analysis revealed varying amounts of monoclinic and tetragonal zirconia present in the fibers and established how this varied with calcination temperature and time. The tetragonal phase was shown to be unstable and disappeared on heating the material beyond around 750°C. The amount of zirconia yielded from the precursor material was measured and was found to be consistently greater than the theoretical yield. Average grain size within the microfibers increased with increasing calcination temperature and is effectively doubled when a 10 kPa pressure was applied. The effect of pressure also results in the creation of new crystal structures within the nanofibers and, as with traditional zirconia processing, the addition of impurity ions was found to stabilize the tetragonal phase.

Full text of DOI:10.1111/j.1551-2916.2008.02264.x

J. Am. Ceram. Soc., 91 [4] 1115–1120 (2008)
DOI: 10.1111/j.1551-2916.2008.02264.x
r 2008 The American Ceramic Society
ournal
J
Phase Morphology in Electrospun Zirconia Microfibers
Erin Davies and Adrian Lowew
Department of Engineering, Australian National University, Canberra, ACT 0200, Australia
Meta Sterns
Department of Chemistry, Australian National University, Canberra, ACT 0200, Australia
Kazutoshi Fujihara and Seeram Ramakrishna
Division of Bioengineering, National University of Singapore, Singapore 117574, Singapore
Electrospinning of sol–gels has been used to produce zirconium-
doped polymer microfibers from zirconyl chloride and poly
(vinylpyrollidone) precursors. Calcination of these structures
between temperatures of 3701 and 9301C resulted in the forma-
tion of zirconia nanograined microfibers whose diameters ranged
from 1200 to 800 nm at the higher temperatures and whose av-
erage grain size ranged from 9 to 33 nm. X-ray diffraction
analysis revealed varying amounts of monoclinic and tetragonal
zirconia present in the fibers and established how this varied with
calcination temperature and time. The tetragonal phase was
shown to be unstable and disappeared on heating the material
beyond around 7501C. The amount of zirconia yielded from the
precursor material was measured and was found to be consis-
tently greater than the theoretical yield. Average grain size
within the microfibers increased with increasing calcination tem-
perature and is effectively doubled when a 10 kPa pressure was
applied. The effect of pressure also results in the creation of new
crystal structures within the nanofibers and, as with traditional
zirconia processing, the addition of impurity ions was found to
stabilize the tetragonal phase.
cessed using traditional methods, undergoes a morphological
change from tetragonal to monoclinic as it cools to below
11751C and the volume change associated with this causes
the material to disintegrate. To counter this, a small amount of
oxide impurity is added during the processing stage to stabilize
the tetragonal structure at room temperature through a complex
cation substitution mechanism.4 This leads to a material of quite
complex structure. Techniques such as the precipitation meth-
od1,5 have been used to characterize crystal structure and grain
size as a function of annealing temperature and dopant of zir-
conia nanoparticles, but recent studies6 have shown that high-
temperature zirconia phases can be produced from sol–gels at
relatively low annealing temperatures so long as the grain size is
small enough. Interestingly, it has been shown7 that if the
material is annealed at an insufficiently high temperature, then
incomplete conversion of the precursor sol–gel into pure zircon-
ia can result in point defects within the material that assist in the
stabilization of the tetragonal structure and that subsequent
removal of these defects causes tetragonal to monoclinic trans-
formation. By stabilizing the high-temperature phase structure
at room temperature, zirconia materials have been investigated
for use in solid oxide fuel cells8 and as sensors.9 As has been
shown with other systems, the electrical and mechanical prop-
erties are very dependent on crystal structure and dopant effects,
thus suggesting that stabilized zirconia is a material of interest.
Sol–gel-processed ceramics in fibrous form are proving to be
very interesting materials. Here, although the fiber diameters are
not necessarily on the nanoscale (i.e., o100 nm), the crystals
that constitute these fibers often are. There is overwhelming
evidence to suggest that materials on this scale have unique
physical, structural, electrical, and mechanical characteristics.
For instance, the promising thermoelectric properties of doped
cobaltite ceramics stem from complex layered structures within
the ceramic grains caused by atomic misfits, and the smaller the
grains, the greater the effect.10 Another example is the ferro-
electric polarization properties of nanograined perovskite bari-
um titanate.11 Processing and characterization of polymeric
microfibers, for instance, are now well-established phenomena,
with polymeric microfibers being used as biostructural scaf-
folds,12 protective filters,13 and polymeric batteries.14
I. Introduction
ANOSTRUCTURED ceramic oxides are receiving significant
research interest, due to the complex structural aspects that
N
can be achieved on a relatively small scale. Also, nanocrystalline
materials are often regarded as consisting of metastable phases
due to the high interfacial and grain boundary energies present.1
As a result, structure and associated properties will depend on
processing history. In addition, grain size is an important factor
insofar as grain size control can be used as a method to control
many properties. One common method for the fabrication of
nanostructured ceramics is the sol–gel approach. Here, a soluble
ceramic precursor material is dissolved in an appropriate solvent
that also contains an organic component, and subsequently an-
nealed to create a ceramic oxide.
One system of interest is zirconia (ZrO2). Traditionally, sol–
gel-processed zirconia has been used in film form as a high
refractive index coating or thermal barrier.2,3 However, the com-
plex structural nature of this material has suggested that it may
have other areas of application. Zirconia exists in three different
polymorphs: monoclinic (ambient to 11751C), tetragonal
(11751–23701C), and cubic (23701–26801C). Zirconia, when pro-
A simple method of producing nano and microfibrous mate-
rials is electrospinning.15 The electrospinning process has been
used for many years, but has reached a new level of interest re-
cently as a method for creating one-dimensional structures of
high aspect ratio. Electrospinning has seen extensive use in the
production of polymeric and bio-materials for numerous appli-
cations, and in recent years, has been used to produce small-scale
ceramic fibers16 through the use of sol–gels. The sol–gel method
allows for the production of ceramic precursor fibers, which can
then be annealed at relatively low temperatures to create ceramic
oxide-based fibers of reasonable structural integrity.
K. J. Bowman—contributing editor
Manuscript No. 23237. Received May 20, 2007; approved November 27, 2007.
wAuthor to whom correspondence should be addressed. e-mail: Adrian.Lowe@anu.
edu.au
1115

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Journal of the American Ceramic Society—Davies et al.
Vol. 91, No. 4
By definition, the aspect ratio of electrospun fibers, and in
Calcination of the fibers was performed using a standard
muffle furnace, with crucibled specimens placed in a cold oven,
and then heated at approximately 81C/min to the target temper-
ature. Once the heating cycle had reached completion, the furnace
was switched off and the specimens were left to cool overnight.
particular the fiber diameter, could provide an effective and
simple way of introducing a sizing control into ceramic nano-
structures. From a sol–gel precursor, electrospun fibers of
known diameters can be easily obtained either in random or
in aligned form, and subsequently annealed to produce oxide
fibers. Several ceramic systems produced through the electro-
spinning technique have been studied in recent years11,16–18 with
varying degrees of success. Recent work5 has shown that yttria-
stabilized zirconia can be successfully electrospun from a sol–gel
precursor and that annealing at 15001C for 1 h results in a
nanofibrous structure of pure zirconia. Shao et al.19 have pro-
duced zirconia fibers of around 200 nm in diameter using
poly(vinyl alcohol) as an organic precursor, whereas Dharma-
raj et al.20 used poly(vinyl acetate) to produce similar-sized fibers
at varying calcination temperatures.
The work presented here is based on the production of sol–
gel-derived zirconia fibers using poly(vinylpyrollidone) (PVP) as
a precursor. Compared with other polymer precursors, PVP
produces a prolific amount of electrospun material, suggesting it
has potential for large-scale ceramic fiber production, and it is
readily soluble in safe solvents, such as water. The study also
establishes the effect of calcination temperature on average grain
size, fiber diameter, and polymorphic composition.
(3) Fiber Characterization
X-ray diffraction (XRD) experiments were performed using
a Siemens D5000 Diffractometer (Siemens Ltd., Bayswater,
˚
Australia) at a wavelength of 1.5406 A. Sharp XRD spectra
were obtained by grinding the specimen with ethanol to form a
slurry, which was poured onto a microscope slide and analyzed
once the ethanol had evaporated off. A quantitative determina-
tion program (SIROQUANT—Sietronics Pty. Ltd., Belconnen,
Australia), based on the Rietveld method,21 was used to deter-
mine the relative composition and the lattice parameters of the
phases present in the annealed sample. The SIROQUANT pro-
gram calculates a theoretical XRD profile and fits it to the mea-
sured pattern by full-matrix least-squares refinement of the
following Rietveld parameters: phase scales, line asymmetry,
phase preferred orientation, phase linewidths (U, V, W), instru-
ment zero, the lineshape parameter for each phase, and the
phase unit cell dimensions.
III. Experimental Results
(1) Processing Observations
II. Experimental Procedure
Originally, fibers were collected through the use of a horizontal
square collector plate. However, there were problems associated
with this method, typically caused by the ionic nature of the
solution and the relatively high stiffness of the fibers when com-
pared with polymeric electrospun fibers. The presence of the zir-
conyl chloride gave the solution an ionic characteristic, which
caused the fibrous material to preferentially align along the elec-
trical field. As this field acted perpendicular to the collector plate,
fibers tended to stand vertically between the plate and the needle,
a phenomenon enhanced by the high fiber stiffness. Eventually,
enough fiber mass would form so as to collapse the fibrous
structure onto the collector plate, but this would occur after sig-
nificant bunching and random coalescence of the fibers close to
the needle, leading to a very distorted, random collection of
fibers. Use of a horizontal rotary collection mechanism (e.g.,
drum winder) actually gained benefit from some of these issues,
resulting in homogeneous, reasonably well-aligned fibers.
(1) Materials
The sol–gel precursor materials consisted of zirconyl chloride
octahydrate (high purity grade, Sigma-Aldrich Pty. Ltd.,
Castle Hill, Australia) and PVP (Mw approximately 1 300 000,
Sigma-Aldrich Pty. Ltd.) dissolved in deionized water and eth-
anol (absolute grade, Ajax Chemicals, Seven Hills, Australia).
The composition used gave a clear, viscous liquid and contained
3.0 g ZrOCl2 ꢀ 8H2O, 0.3 g PVP, 1.3 g deionized water, and 0.7 g
ethanol, giving a solution of approximately 57 wt% ZrOCl2.
The solution was stirred at ambient temperature for 24 h to en-
sure complete miscibility.
(2) Fiber Processing
The solutions were subsequently electrospun using standard
electrospinning equipment at a feed rate of 0.3 mL/h through
a needle of diameter 0.21 mm. The accelerating voltage used was
20 kV and the distance between the needle tip and the collector
mechanism was 100 mm. To ensure fibers of reasonable consis-
tency and quantity, a drum winder mechanism of rotational
speed 80 rpm was used as the collector mechanism and a sche-
matic showing the essential features of the electrospinning pro-
cess is shown in Fig. 1. The viscosity of the solution coupled with
the use of PVP as the organic precursor ensured a large quantity
of fibers were obtained within a few minutes. The samples were
subsequently oven dried for 48 h before calcination.
(2) Percentage Yield of Fiber
The thermal decomposition mechanism of zirconyl chloride
is unclear. However, it is believed22 that the structure of this
compound is quite complex, consisting of constitution water
rather than hydration water. This would suggest that the
structure of zirconyl chloride is best represented as being
[Zr4(OH)8(H2O)16]81 and 8Clꢁ rather than ZrOCl2 ꢀ 8H2O.
With this structure in mind, a likely decomposition would be
according to the following equation:
8þ
½Zr4ðOHÞ8ðH2OÞ16ꢂ þ8Clꢁ ! 4ZrO2þHCl þ 16H2O (1)
This might also account for some of the complex hydroxyl-
based intermediate stages that are believed to occur during this
decomposition process.7 It is unclear what, if any, role the PVP
plays in this decomposition reaction, but assuming that all sol-
vents have been subsequently removed during the electrospin-
ning and drying process, zirconyl chloride octahydrate should
constitute 91% of the weight of the electrospun fibers. From
Eq. (1), 1 g of ZrOCl2 ꢀ 8H2O should yield 0.383 g of ZrO2.
Sample weights before and after annealing at temperatures
between 2701 and 9301C for 1 h revealed that the yield was
consistently underestimated by around 10%–17%. This could
have two reasons: incomplete removal of the PVP or incomplete
conversion of the ZrOCl2 ꢀ 8H2O.
At low annealing temperatures, the fibrous specimens showed
a dark tinge that was not apparent at higher temperatures. This
suggests that residual PVP is still present.
Fig. 1. Schematic showing the essential features of the electrospinning
experimental setup. The Taylor cone is the phenomenon through which
sol–gels are stretched electrostatically into small diameter fibers.

April 2008
Phase Morphology in Zirconia Microfibers
1117
Fig. 2. Scanning electron microscope images of sol–gel-processed zirconia nanofibers annealed at (a) 3701C, (b) 4501C, (c) 5401C, and (d) 9301C.
However, high-quality XRD spectra were obtained at these
temperatures, suggesting that the presence of the polymer was
not interfering with the creation of zirconia crystals. One anec-
dotal piece of evidence to propagate this view is that samples
annealed at temperatures higher than 6301C displayed signifi-
cantly more brittleness than material processed at lower temper-
atures. At this stage, any effect from the decomposition of PVP
on the calcination mechanism is unclear, although carbon or
carbon-based compounds were not spectroscopically detected,
other than at temperatures where calcination is yet to commence.
The second reason is probably due to the presence of impurity
hydroxide OHꢁ ions.7 These ions are a known impurity in zir-
conia. It is very likely that there are OHꢁ ions present in the
samples, as the mode of synthesis used is susceptible to incor-
poration of such impurities. Annealing zirconia from zirconyl
chloride octahydrate creates an environment where freed water
molecules can react with the chlorine ions to form hydrochloric
acid and OHꢁ ions. Therefore, if the hydroxide ions were pres-
ent in the final product, they would contribute to the higher than
expected yields.
Zirconia nanocrystals do not form from this precursor at tem-
peratures below 4001C,23 so the amorphous trace at 3701 is as
expected. At temperatures of 4501 and 5401C, the peaks are
representative of a tetragonal phase (characterized by the asym-
metry and slight splitting of the 110 and 112 peaks at 351 and
501). At temperatures of 5401 and 6301C, the tetragonal phase
starts to transform into a monoclinic pattern, and at tempera-
tures of 7301C and higher, the transformation is virtually com-
plete. This is clearly revealed in Fig. 4 through the increasing size
ꢀ
of the characteristic 111 and 111 peaks and the gradual disap-
pearance of the tetragonal 011 peak as a function of increasing
temperature.
Figure 5 reveals the percentage present of each phase as
a function of annealing temperature as determined by the
SIROQUANT software package. Clearly, the tetragonal phase
dominates at lower annealing temperatures, and the monoclinic
phase predominates at the higher temperatures. At an annealing
temperature of 5401C, the fibers are very much a composite
structure of the two crystal types. To investigate further the
crystal structure of the fibers in the intermediate temperature
range, the effect of annealing time on the percentage of mono-
clinic material present was determined at two sample tempera-
tures. Between annealing times of 30 min and 4 h, a slight
(3) Fiber Morphology
Figure 2 shows scanning electron microscope images of fibers
annealed at 3701, 4501, 5401, and 9301C. These images reveal
that fiber diameter quickly reaches a threshold minimum value
of around 850 nm, as shown in Fig. 3, and suggests that the
polymer has essentially been eliminated at temperatures of
5401C and higher. In addition, Fig. 2(d) reveals a distinctly
granular structure, absent from other images, which is most
likely due to larger grain size, rather than a polymer-based
phenomenon.
1400
1200
1000
800
600
400
200
0
(4) XRD Analysis
0
200
400
600
800
1000
Figure 4 presents XRD patterns obtained from specimens an-
nealed at various temperatures for 1 h. Annealing temperature
was the only experimental variable in this analysis. The units of
intensity are arbitrary and hence are excluded from the ordinate.
Annealing temperature (degrees C)
Fig. 3. Influence of the annealing temperature on the average fiber di-
ameter of sol–gel precursor electrospun zirconia nanofibers.

1118
Journal of the American Ceramic Society—Davies et al.
Vol. 91, No. 4
Fig. 4. Comparison of X-ray diffraction analysis patterns of specimens annealed at various temperatures for 1 h. The material is essentially tetragonal
at low temperatures and monoclinic at high temperatures. There is a range of temperatures at which both crystal phases exist.
increase in the monoclinic phase was detected at 4501C, whereas
no discernable variation in monoclinic phase amounts was
detected at 6301C. These results are consistent with the obser-
vations reported by Whitney,24 who showed that at 5971C and
beyond, conversion is effectively complete after 30 min, whereas
conversion at lower temperatures continues slowly for several
hours. This analysis suggests that for the heating regime adopted
(i.e., sample placed in a cold oven and then slowly heated over
a period of several hours), the time spent at the thermal soak
(annealing) temperature does not significantly affect the crystal-
lographic composition of the zirconia fibers.
presents the XRD spectra obtained from both heating regimes,
and from reference spectra obtained at 4501 and 7301C. This
analysis shows that although tetragonal zirconia can be easily
formed at 4501C, it is metastable, and is converted to monoclinic
zirconia through subsequent heating processes.
(6) Crystal Size Effects
One possible explanation for the presence of the tetragonal
phase is the influence of crystal size. It has been suggested25
that the size of an unconstrained zirconia crystal affects the sta-
bility of the tetragonal phase, and that for crystals smaller than
30 nm, the excess energy provided by the inner surface energy of
the crystal is sufficient to stabilize the tetragonal phase. There-
fore, as crystal size increases, this would suggest that the tetrag-
onal structure eventually transforms into more stable
monoclinic phase as the inner surface energy changes.
The average crystal size of the zirconia nanofibers obtained at
different annealing temperatures can be approximated by the
Scherrer method. This method relies on smaller crystals pro-
ducing broader XRD peaks than larger ones and relates the size
(5) Effect of Varying the Heating Regime
Studies were performed to establish the effect of multi-stage
heating regimes on the final crystal structure of the nanofibers.
Two regimes were adopted at two temperatures: regime A
involved annealing at 4501C for 1 h and then subsequent heating
to 7301C for 1 h followed by a furnace cool; regime B involved
cooling to room temperature between the two heating stages, as
detailed in Fig. 6. The reason for this was to establish if anneal-
ing at lower temperatures is final, or if it could be superseded by
a higher temperature annealing treatment. This would show if
the tetragonal transformation is a permanent one. Figure 7
100
90
80
70
60
50
40
30
20
10
0
400
500
600
700
800
900
1000
Annealing temperature (deg. C)
Fig. 6. Schematic representation of the multi-stage heating regimes
used in this study. Regime A involves a staged heating process and re-
gime B involves an intermediate cooling stage.
Fig. 5. Percentage of monoclinic phase present versus annealing tem-
perature (annealing time51 h).

April 2008
Phase Morphology in Zirconia Microfibers
1119
Fig. 7. X-ray diffraction analysis patterns for multistage heating regimes A and B and reference spectra taken at 4501 and 7301C. Both regimes create a
mostly monoclinic material.
of the crystal (t) to the width of the peaks at 50% of their height
(B), i.e.,
Figure 8(b) was obtained from a sol–gel containing 5 wt% cal-
cium acetate, thus introducing a small amount of calcium
impurity to the calcined material. Clearly, this material is te-
tragonal, and subsequent heating of the specimen showed this
material to be stable. Figure 8(c) was obtained from the pure
sol–gel, pressure-fired at 8001C for 4 h under 10 kPa pressure in
a graphite capsule. The material is clearly mostly monoclinic,
but there are several differences between this material and the
standard material worth discussing.
The monoclinic lattice parameters appear to have slightly
changed as a result of the pressure firing. Initial calculations
have shown that the unit cell shrinks by between 0.5% and
0.9%. In addition, basic analysis of the peak widths shows that
pressure firing effectively doubles the average grain size of the
material, whereas addition of 5% calcium impurity has negligi-
ble effect on the grain dimensions. Clearly, increased pressure
has the effect of promoting intergranular diffusion, leading to
larger grains. This would therefore suggest that if firing could
occur in a vacuum, then grain size may be reduced. This would
be of great importance for materials such as cobaltites, whose
properties are very dependent on the size of the complex layered
oxide structures formed during calcination.
A major difference between the pressure-fired zirconia and
the normally calcined zirconia is the presence of extra diffraction
peaks, most notably at 2y 5 451 and at 2y 5 551 where a doublet
is seen to occur. These peaks may correspond to orthorhombic
zirconia whose presence has been reported elsewhere26,27 in
materials processed with impurities through traditional or thin
film deposition techniques, and usually through high-pressure
means. It is rare to detect orthorhombic zirconia in pure,
dopant-free material, and the stability of this phase is not fully
known. These peaks are also characteristic of perovskite-type
structures, but as there are no impurity metallic cations in the
material, the existence of this structure is unlikely.
The process of pressure firing involves placing the material in
a porous graphite capsule, and initial XRD analysis of the final
product revealed a large quantity of residual carbon present in
the material. The pattern presented in Fig. 8(c) has been created
by removal of the characteristic hexagonal graphite lines, so
there is the possibility that these extra peaks are in fact caused by
the presence of zirconium carbide or zirconium carbonate in the
material. This is to be investigated further.
Kl
B cos yB
t ¼
(2)
where K is a constant depending on the crystallite shape, l is the
X-ray wavelength, and yB is the Bragg angle. By comparison of
the characteristic monoclinic peak at approximately yB 5 281,
approximate average grain sizes were obtained as detailed in
Table I. Clearly, average grain size increases with increasing
annealing temperature, strengthening the previous hypothesis.
However, the ‘‘transformation size’’ of 30 nm is not achieved in
this study. Although the Scherrer method is only an approxi-
mate method, there is still most likely a cause other than energy
factors for the tetragonal to monoclinic transformation. Perhaps
the most likely influence is the presence of hydroxide impurities,
as detailed previously.
(7) Stabilization of the Tetragonal Phase and the Influence of
Pressure
Although tetragonal zirconia is obtainable from zirconyl chlo-
ride sol–gel by calcination at temperatures of around 5001–
6001C, this phase is metastable and will transform to monoclinic
phase on subsequent heating at higher temperatures. In tradi-
tional zirconia processing, the tetragonal phase is stabilized by
the addition of small amounts of impurity oxide, such as calcium
and yttrium. Figure 8 presents three XRD patterns obtained
from material calcined at 8001C. Figure 8(a) is obtained from
the pure zirconyl chloride-based sol–gel used throughout this
study and reveals the predicted monoclinic-dominated structure.
Table I. Average Crystallite Thickness Versus Annealing
Temperature According to the Scherrer Equation (annealing
time 5 1 h)
Annealing temperature (1C)
Average crystallite thickness (nm)
450
540
630
730
830
930
9.00
10.80
16.20
23.13
26.99
32.39
The effect on mechanical and electrical properties of these
various structures is the subject of future work.

1120
Journal of the American Ceramic Society—Davies et al.
Vol. 91, No. 4
Fig. 8. X-ray diffraction analysis patterns for (a) standard zirconia (ZrO2) nanofibers calcined for 1 h at 8001C, (b) standard ZrO2 nanofibers containing
5 wt% calcium oxide impurity calcined for 1 h at 8001C, and (c) standard ZrO2 nanofibers calcined at 8001C for 4 h under a pressure of 10 kPa.
6S. Shukla and S. Seal, ‘‘Thermodynamic Tetragonal Phase Stability in Sol–Gel
Derived Nanodomains of Pure Zirconia,’’ J. Phys. Chem. B, 108, 3395–9 (2004).
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Acknowledgments
The authors wish to thank the Australian National University Electron Micro-
scope Unit and Dr. Jorge Hermann at the Research School of Earth Sciences at the
Australian National University for their assistance in the preparation of this paper.
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