Enhanced dielectric properties of sol-gel-BaTiO3/P(VDF-HFP) composite films without surface functionalization

Article abstract of DOI:10.1039/c4ra03715d

Barium titanate/poly(vinylidenfluoride-co-hexafluoropropylene) (BTO/P(VDF-HFP)) nanocomposite films were prepared by spin coating and their dielectric properties were investigated. Ferroelectric BaTiO₃was prepared by a low temperature sol-gel soft-chemistry method leading to spherical nanoparticles with a high degree of surface hydroxyl groups. Phosphonic acids and/or additional dispersant are typically used to cover the surface of hydrophilic oxide particles in order to improve the compatibility between the particles and the hydrophobic polymer. The effect of such surface modifications was studied using 2,3,4,5,6-pentafluorobenzyl phosphonic acid and additionally the dispersant BYK-W 9010 as surfactants. Against expectation, the use of pure sol-gel barium titanate led to the lowest particle agglomeration in the composite suspension and the best film quality resulting in a large enhancement of the relative permittivity by a factor of 10 compared to pure P(VDF-HFP). For this simple biphasic system without any further components a very high relative permittivity ε_rof 57 was obtained at 1 kHz for a 50 vol% BaTiO₃/P(VDF-HFP) composite. The ε_rvalues further increase at lower frequencies. These findings show that for the BTO nanoparticles, prepared by the sol-gel synthesis, no surface coating is required to obtain good dielectric properties, allowing for a considerable simplification of the preparation technique. This journal is

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H. Beige and S. G. Ebbinghaus, RSC Adv., 2014, DOI: 10.1039/C4RA03715D.
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BaTiO3–P(VDF-HFP) composite films without surface functionalization show a tenfold increase of the relative
permittivity compared to the pure polymer.
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Enhanced Dielectric Properties of Sol-gel-BaTiO3–
P(VDF-HFP) Composite Films Without Surface
Functionalization
Cite this: DOI: 10.1039/x0xx00000x
Claudia Ehrhardt,a Christian Fettkenhauer,a Jens Glenneberg,b Wolfram
Münchgesang,c Hartmut S. Leipner,b Gerald Wagner,b Martin Diestelhorst,c
Christoph Pientschke,c Horst Beigec and Stefan G. Ebbinghausa
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Barium titanate/poly(vinylidenfluoride-co-hexafluoropropylene) (BTO/P(VDF-HFP)) nano-
composite films were prepared by spin coating and their dielectric properties were
investigated. Ferroelectric BaTiO3 was prepared by a low temperature sol–gel soft-chemistry
method leading to spherical nanoparticles with a high degree of surface hydroxyl groups.
Phosphonic acids and/or additional dispersant are typically used to cover the surface of
hydrophilic oxide particles and to improve the compatibility between the particles and the
hydrophobic polymer. The effect of such surface modifications was studied using 2,3,4,5,6-
pentafluorobenzyl phosphonic acid and additionally the dispersant BYK-W 9010 as
surfactants. Against expectation, the use of pure sol-gel barium titanate led to the lowest
particle agglomeration in the composite suspension and the best film quality resulting in a
large enhancement of the relative permittivity by factor 10 compared to pure P(VDF-HFP). For
this simple biphasic system without any further components a very high relative permittivity r
of 57 was obtained at 1 kHz for a 50 vol.% BaTiO3/P(VDF-HFP) composite. The r values
further increase at lower frequencies. These findings show that for the BTO nanoparticles,
prepared by the sol-gel synthesis, no surface coating is required to obtain good dielectric
properties, allowing in a considerable simplification of the preparation technique.
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Keywords: polymer composite; barium titanate; nanoparticles; dielectric properties

Corresponding authors: Institute of Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-
Straße 2, D-06120 Halle (Saale), Germany.
Claudia Ehrhardt Tel.: +49 345 5525641; Fax: +49 345 5527028. E-mail: claudia.ehrhardt@chemie.uni-
halle.de;
Stefan G. Ebbinghaus Tel.: +49 345 5525870; Fax: +49 345 5527028. E-mail: stefan.ebbinghaus@chemie.uni-
halle.de
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Introduction
Composite dielectrics are promising materials for energy leads to better film homogeneities and a large enhancement of
storage systems because they combine the good processability the relative permittivity by a factor 2 compared to surface
and breakdown field strength of polymers with the high modified particles and even by a factor 10 compared to pure
permittivity of oxides. In a biphasic composite the dielectric P(VDF-HFP). A very large relative permittivity r of 57 was
properties are affected by both constituents. Therefore, the obtained at 1 kHz for a 50 vol.% BaTiO3/P(VDF-HFP)
challenge is to design a material with a high permittivity and composite with a low loss tangent in the order of 0.1. The
dielectric field strength without a large increase in dielectric possibility to avoid the coating of oxide particles strongly eases
1
loss, with the aim to enhance the resulting energy density.
the preparation technique, making the obtained films more
Nanocomposites are attractive because of their large interfacial attractive for technical applications.
interactions, which have a significant effect on the degree of
polarisation and charge separation.2 The ferroelectric
polarisation determines the materials properties at high
Experimental procedure
Material preparation
frequencies and affects the polarisation mechanism, while for
lower frequencies or DC conditions the interfacial space-charge
effects are dominating.3–5 A uniform distribution of the filler
particles in the matrix enhances the interaction between the
components, resulting in an increased interfacial polarisation.6,7
This polarisation is due to the differences in the electrical
properties (permittivity and conductivity), leading to charge
accumulations at the boundaries to the less conducting regions.8
Different approaches have been used to enhance the composite
permittivity e.g. by thermal treatment of the oxide particles,
application of bimodal particle size distributions or increase of
the filler volume fractions.9–14 On the other hand, the large
contrast in permittivity or conductivity between filler and the
polymer host often generates highly inhomogeneous electric
fields, which reduce the effective breakdown strength of the
composites by electrical defect centres.15–17 The use of
nanoparticles decreases the occurrence of large local fields.18
Various investigations focused on the effect of oxide surface
functionalization, e.g. with phosphonic acids, to prevent
agglomeration and to improve the particles distribution and in
turn the composite homogeneity with the aim to enhance the
dielectric characteristics.11,19–23 It was shown that oxide
particles coated with organo-phosphonates resulted in an
improved dielectric breakdown resistance and energy storage
density.24 On the other hand, such surfactants can also cause an
increase of dielectric loss and leakage current due to residual
free molecules.25 Therefore, residual phosphonic acid
molecules have to be removed from the surface-functionalized
particles. Coating of the oxide particles thus requires a number
of additional preparation steps (synthesis of the phosphonic
acid, the coating itself and washing of the coated particles),
increasing the complexity and cost of the process. For this
reason, it would be desirable to omit the surfactant treatment.
In this article, we investigate the morphology and dielectric
properties of BaTiO3/poly(vinylidenfluoride-co-hexafluoro-
propylene) (BTO/P(VDF-HFP)) films prepared by spin-coating.
In contrast to earlier reports, the BTO nanoparticles were
synthesized by a sol-gel technique, leading to a high density of
surface hydroxyl groups.4,9,11,19,24,26 The effect of coating of
these oxide particles with pentafluorobenzyl phosphonic acid
(PFBPA) and the application of the dispersant BYK-W-9010
was investigated. We found that the untreated barium titanate
BaTiO3 (BTO) nanoparticles were synthesized by a sol–gel
method starting from barium chloride dihydrate (60 mmol,
BaCl2·2H2O, ≥99 %, Sigma-Aldrich) and freshly distilled
titanium (IV) chloride (60 mmol, TiCl4, 99.9 % Acros
Organics) in equimolar quantities as described in Ref. 27. TiCl4
was hydrolysed in 30 mL ethanol followed by the dropwise
addition of an aqueous solution of BaCl2·2H2O dissolved in
90 mL deionized and argon saturated water. Precipitation was
achieved by fast addition of an excess potassium hydroxide
solution (Sigma-Aldrich; 42 g in 100 mL water) and finally
100 mL water. The mixture was refluxed for 6 hours under
argon atmosphere. The crystalline particles were washed three
times with water, ethanol and acetone, respectively, and then
dried in air.
The phosphonic acid was synthesized by the Arbuzov reaction
of 2,3,4,5,6-pentafluorobenzyl bromide (>99 %, Sigma-
Aldrich) and triethyl phosphite (>95 %, Fluka).28 The obtained
phosphonic ester was distilled in vacuum (150 °C, 3 mbar) to
give the crude material as colourless oil. An excess of
bromotrimethylsilane (97 %, Sigma-Aldrich) was added for
conversion to the corresponding silyl ester. The phosphonic
acid was obtained by hydrolysis of the silyl ester with diluted
hydrochloric acid (0.01 M). The surface functionalization was
performed according to Kim et al..19 Typically 12 g oxide
powder was dispersed in 300 mL ethanol (96 %, analytical
grade purity) for 6 hours by ultrasonication to prevent particle
agglomeration. Afterwards, 6 mmol phosphonic acid dissolved
in 50 mL ethanol was added and the mixture was again
ultrasonicated for 1 hour at room temperature. The nano-
particles were separated by centrifugation and washed 3 times
with excess ethanol under ultrasonication followed by
centrifugation. Selected samples were additionally washed 3
times with water, afterwards with ethanol and finally with
acetone to remove unreacted and physisorbed phosphonic acid
as well as a water-dissoluble barium phosphonate byproduct
described below. After washing, the particles were dried for
4 hours under vacuum at 80 °C. Poly(vinylidenfluoride-co-
hexafluoropropylene) P(VDF-HFP) (averaged Mn ~ 130 000,
Sigma-Aldrich, contains 5 % HFP) was used as purchased.
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Preparation of composite films
focus on a sol–gel synthesized powder consisting of spherical
particles in the range of 20 – 50 nm as measured by TEM
(Fig. 1).
Composite suspensions were prepared by ball-milling of
P(VDF-HFP) dissolved in N,N-dimethylformamide (DMF,
analytical grade purity) and the BTO nanoparticles for typically
4 h with 300 rpm in a planetary ball mill (Retsch PM 400). For
selected batches, BYK-W 9010 (a phosphoric acid polyester
copolymer with acid groups, BYK-Chemie) was added as
additional dispersants prior to the ball-milling process.
Composite films were fabricated by spin-coating on aluminum-
coated substrates with 1000 rpm for 1 min. The aluminum
electrode was prepared by thermal evaporation of 150 nm
aluminum on a 24 mm x 24 mm glass substrate. The films were
dried at room temperature in air and afterwards annealed at
50 °C for 20 minutes. Parallel-plate capacitors were obtained by
deposing an array of 2 mm x 3 mm top aluminum electrodes
(350 nm thickness) on the composite films through a mask
using thermal evaporation (Vakuumtechnik Dresden model
B-30 HVT) with 1 nm/s deposition rate.
Measurements
Fourier transform infrared spectra with 2 cm-1 resolution were
collected on a Bruker TENSOR spectrometer with attenuated
total reflectance on a diamond crystal. Three-point BET gas
physisorption measurements were done on a Quantachrome
Corporation Nova 1000 using nitrogen as adsorbate in a range
of p/p0 0.1 – 0.3 to calculate specific surface areas and averaged
particle sizes. The powder density was determined by a Porotec
Pycnomatic Helium-pycnometer at room temperature. Particle
agglomeration was investigated with a Brookhaven Instruments
Corporation Particle Size Analyzer 90 Plus. The data was
averaged over three measurements. Thermal analysis including
thermogravimetry (TG) and diffential thermal analysis (DTA)
were performed in a Netzsch STA 449C system (heating rate
10 K/min, Al2O3 crucibles, flowing air with 20 mL/min). For
transmission electron microscopy a Philips CM200 STEM was
used. The film morphologies were investigated using an
environmental scanning electron microscope (Philips ESEM
XL 30 FEG). The elemental composition was analysed by
coupled energy dispersive X-ray spectroscopy (EDX). Cross-
sectional SEM images were prepared by fracturing liquid-
nitrogen cooled films. The film thickness was additionally
measured by means of a contact profilometer (Dektak 150
stylus surface profiler). Frequency-dependent capacitance and
loss tangent values were obtained with an Agilent 4284A LCR
meter in the range 120 Hz to 1 MHz at room temperature,
where the voltage was kept constant at 1 V. All presented
dielectric properties are averaged over four parallel-plate
capacitors.
Figure 1. TEM bright-field image of BaTiO3 particles synthesized by the sol–gel
procedure.
The sol–gel soft-chemistry synthesis leads to
a high
concentration of surface hydroxyl groups which are good
linkers for the surfactants. The particle size determined by TEM
confirms the value for the crystallite size of 32 ± 4 nm
calculated by the Scherrer equation using the (111) peak of the
XRD measurements (see Figure S1, Supporting information).
As described previously, the sol–gel synthesis of BaTiO3 leads
to a highly crystalline product already at low reaction
temperatures (i.e. without any calcination step) with a ratio of
approximately 35% tetragonal and 65% pseudo-cubic phase as
determined by Rietveld refinement.27 The formation of the
pseudo-cubic phase is well known for very small BaTiO3
particles.30 Using a helium pycnometer, a density of 5.10 g cm-3
was obtained. This corresponds to roughly 85 % of the
crystallographic density indicating the dense nanoparticular
structure and high crystallinity of the obtained powder.
Furthermore, the density of our BaTiO3 samples is very similar
to the one of commercially available nanopowders (e.g. Sigma-
Aldrich Barium titanate(IV) nanopowder, < 100 nm particle
size (BET): 5.56 g cm-3).12,14
A
fraction of the BTO powder was treated with
2,3,4,5,6-pentafluorobenzylphosphonic acid (PFBPA), which is
known to form stable bonds with the oxide surface, thus
forming a surface layer with a good compatibility to the
hydrophobic fluorinated polymer host. After surface treatment,
stable BTO dispersions without sedimentation over several
weeks in unpolar organic solvents were achieved. On the other
hand, SEM investigations revealed the presence of an
additional fraction of micrometer sized needles after the
PFBPA treatment as shown in Fig. 2.
Results and Discussion
Characterization of as-prepared and surfactant-coated BTO
particles
The properties of barium titanate are strongly influenced by
particle size and applied synthesis route.29 In this article, we
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remains in oxidic form in the sample.27 The weight loss of the
washed sample C-BTO@PFBPA is much smaller and more
diffuse (onset temperature 250 °C). For both samples the
observed weight loss can be correlated with the BET specific
surface area to calculate the degree of phosphonic acid surface
coverage ΘPA, assuming a specific area of 0.24 nm2 per
molecule phosphonic acid.31,32 In the corresponding
calculations the weight change during the exothermic reaction
was interpreted as the complete combustion of the organic
chain of the phosphonic acid. In this case, the average number
of bonded molecules per nm2 (PA) is given by
mPAchainNA
PA 
(1)
MPAchain
A
spec
where Δm is the weight change taken from thermogravimetry,
NA is Avogadro’s number, M is the molar mass of the
phosphonic acid organic chain and Aspec the specific surface
area of the modified powder. The obtained values are listed in
Table 1.
Figure 2. SEM image of phosphonic acid surface treated powder on a Ba-
phosphonate needle.
EDX analysis of these needles revealed a high content of
barium and also fluorine, which apparently stems from the
phosphonic acid. It can therefore be concluded that the needles
consist of barium phosphonate. In fact, it was possible to
synthesize a Ba-phosphonate from Ba(OH)2·8H2O and PFBPA.
This byproduct was found to be water soluble. Therefore, some
samples were additionally washed with water after the surface
treatment reaction (denoted as C-BTO@PFBPA in the
following). Figure 3 shows the thermogravimetric analysis
(TG) of PFBPA-treated powders before and after the additional
washing step.
Table 1. BET surface, TG weight loss, temperature interval of the
combustion reaction, number of phosphonic acid molecules per nm2 and
degree of surface coverage of the modified BTO particles.
Aspec
(BET)
(m2g-1)
Δm
(%)
ΔTwt
(°C)
PA
ΘPA
Sample
(nm-2) (%)
BTO
25.86
20.45
23.06
BTO@PFBPA
C-BTO@PFBPA
4.51 350–500
2.81 250–500
7.33
4.05
176
97
The coverage higher than 100 % for the BTO@PFBPA sample
is striking. If the weight change is assigned to the loss of the
entire phosphonic acid molecule as done e.g. in Ref. 11, a
surface coverage below 100 % is obtained. However, mass
spectrometry of the evolved gas gave no hints for phosphorous-
containing volatile reaction products. For this reason, we
assume that phosphor remains in the sample in oxidic form
after decomposition and the weight change was assigned to the
decomposition of the organic chain. One possible reason for a
surface coverage larger than 100 % is the formation of
phosphonic acid multilayers.19,31,32 As an alternative
explanation, we propose that the large value is due to the
formation of the Ba-phosphonate shown in Fig. 2. This Ba-
phosphonate may also form in commercial and/or thermally
treated barium titanate powders by a leaching of barium ions,
which is clearly more severe for the highly reactive sol–gel
BTO. This interpretation is supported by the fact that after a
thorough water cleaning step, the C-BTO@PFBPA sample
showed a coverage of 97 %, which corresponds to a
monomolecular surface layer. Since the barium phosphonate is
water soluble, it is reasonable to assume that the rinsing
removes it completely.
Figure 3. TG/DTA analysis of surface-coated powder before (BTO@PFBPA) and
after (C-BTO@PFBPA) the water cleaning step.
The thermogravimetry of BTO@PFBPA showed a significant
weight loss between 350–500 °C accompanied by an
exothermic signal in the differential thermal analysis (DTA).
Mass spectrometry (not shown) revealed that this reaction step
is correlated with the release of water, fluorine and carbon
dioxide and can therefore be assigned to the oxidative
degradation of the phosphonic acid chain, while phosphor
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FT-IR spectroscopy provides additional information on the content of adsorbed water and surface hydroxyl groups (around
surface modification products. PFBPA is known to form stable 1350 cm-1). It can be assumed that the large amount of hydroxyl
bonds to the BTO particle surface.11,19,22 The sol–gel soft- groups causes an increase of dielectric loss and leakage current
chemistry method applied in this work leads to nanoparticles due to residual-free ionic groups.25 The high content of
with a high degree of surface hydroxyl groups, which are good adsorbed water also prevents the polymer wetting of these
linkers for the phosphonic acid.33 Figure 4 shows the FT-IR particles, despite of the phosphonic acid coverage. As a
spectra of the surface-modified particles before and after the consequence, the dielectric measurements of composites out of
cleaning step in comparison with the pure BTO particles and this powder showed a strong increase of the leakage current.
Ba-phosphonate obtained from the reaction of Ba(OH)2·8H2O For composite systems, the wetting interactions between oxide
with the phosphonic acid.
particles and the polymer host decide about the film
homogeneity and the dielectric performance. The successful
binding of the phosphonic acid is evidenced by hydrophobic
properties of the coated BTO@PFBPA particles noticeable by
the formation sedimentation-stable dispersions in different
organic solvents. On the other hand, pure sol–gel BTO also
formed stable particle dispersions in pure DMF and in the
polymer solution without sedimentation over several weeks.
This can be explained taking into account that the aprotic yet
polar F atoms in the polymer chain enable strong interactions
with the surface hydroxyl groups of the BTO nanoparticles.
For a better insight into the dispersibility, the agglomeration
behavior of pure and surface treated particles was investigated
by Dynamic Light Scattering (DLS). We studied both the
solvent-dispersed particles and the resulting composite
suspensions,
i.e.
oxide
particles
plus
polymer
dispersed/dissolved in DMF. The results are depicted in
Figure 5.
Figure 4. FT-IR spectra of pure BTO (grey), surface-modified BTO@PFBPA (black),
isolated Ba-phosphonate byproduct (blue) and the purified C-BTO@PFBPA (red).
All BTO samples exhibit the characteristic feature of remaining
surface hydroxyl groups (3512 cm-1) which apparently cannot
totally be saturated by the phosphonic acid. The broad P–OH
stretching bands (around 2750 cm-1 and 2300 cm-1) are only
found for pure PFBPA, but not for BTO@PFBPA, verifying
that these groups form bonds to the oxide surface in at least a
bidentate coupling mode.20,21,34 The comparison with the FT-IR
spectra of PFBPA is shown in the supplementary information
(Fig. S2). After a typical surface functionalization, aromatic
vibration modes (1526 cm-1 and 1502 cm-1) and the C–F
stretching mode (1126 cm-1) were detected indicating the
presence of PFBPA. In the finger-print region between
1300 cm-1 and 900 cm-1, major changes can be observed.
Especially the P═O stretching modes (1258 cm-1 and
1072 cm-1) and the P–O stretching modes (1020 cm-1 and
968 cm-1) of the pure PFBPA (marked by dashed blue and
black lines) collapses to two main signals of M–O–P vibration
(M=metal; marked by dashed red lines at 1094 cm-1 and
1047 cm-1) in case of a successful surface bonding.11,19,22 After
the surface modification, several additional signals remain
around 1300 – 1200 cm-1 and 1080 cm-1 (marked by a blue
box), which can be attributed to the barium phosphonate. This
byproduct can be eliminated by a washing step after the surface
treatment. The resulting particles (C-BTO@PFBPA; bottom
layer) show only signals, which can be attributed to the surface-
bonded phosphonic acid, but also reveal the presence of a high
Figure 5. Particle agglomeration determined by DSL of pure sol-gel synthesized
BTO and phosphonic acid surface modified BTO@PFBPA dispersed in water and
DMF as well as in the resulting polymer composite suspension. Bars represent
the variation of particle aggregate sizes.
Typically, BTO nanoparticles tend to form large agglomerates
in organic solvents or in composites because of their high
surface energy and lipophobic properties resulting in severely
inhomogeneous electric fields at the interfaces that can conduct
charges and in turn reduce the dielectric breakdown
strength.11,24,35 Therefore, numerous investigations focused on a
suitable surface functionalization to suppress particle
agglomeration and improve the polymer compatibility.
Dynamic Light Scattering on such particles generally reveal a
main fraction of almost isolated particles with a narrow size
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distribution and additionally a small fraction of much larger contrast, it was possible to prepare uniform composite thin
agglomerates. A similar particle size distribution was also films of BTO@PFBPA (i.e. containing the Ba-phosphonate
observed in this work as shown in Figure 5. In water the main byproduct). The resulting composite materials show isolated
fraction of BTO@PFBPA shows a slightly smaller particles and uniformly distributed particles (Fig 6).
size than pure BTO, what is surprising considering the
hydrophobic properties of the surfactant. We assume that this
effect is due to the long-term ultrasonication during the surface
functionalization. On the other hand, an increasing particle
agglomeration was found with progressing time before the DLS
measurement. Therefore, the seemingly different sizes may
simple be a time effect.
For both BTO and BTO@PFBPA, the size distribution of the
agglomerates in DMF is slightly increased compared to water.
On the other hand, the aggregation of BTO@PFBPA in DMF is
obviously decreased compared to the pure BTO due to the
hydrophobic surfactant layer.
The size distribution changes significantly when the polymer is
added. It is to be expected that the fluorinated aromatic ring of
the phosphonic acid creates a good chemical compatibility to
the highly fluorinated chains of the chosen polymer.19,21
Nevertheless, we found that the aggregate sizes of our sol-gel
BTO@PFBPA particles in the P(VDF-HFP)/DMF solution
become larger than in pure DMF. As shown in the Supporting
information these aggregates are also slightly larger than the
ones of phosphonic acid modified commercial BaTiO3
(Fig. S3). Surprisingly, the pure (highly hydroxylated) sol-gel
BTO particles form very small aggregates (< 400 nm). This
finding can be explained by the formation of hydrogen bonds
between the surface OH-groups of the BTO nano particles and
the F atoms of P(VDF-HFP), creating a good compatibility
between BTO and polymer matrix. Such strong interactions
have already been observed by Zhou et al.26 These authors
activated the surface of BaTiO3 particles by H2O2 to introduce
additional hydroxyl groups. This led to polymer composites
with lower loss tangent and higher dielectric strength compared
to the untreated material. It should be noted that the advantage
of our approach is the formation of spherical nanoparticles with
a high degree of surface hydroxyl groups in a simple one-step
synthesis, i.e. without any surfactant functionalization. As
expected, this good wetting leads to fewer voids in the
composite, and in turn to a more homogeneous film
morphology with better dielectric properties as described
below.26
Figure 6. TEM bright-field image of a thin section of a BTO@PFBPA/P(VDF-HFP)
composite film.
Surprisingly, the large Ba-phosphonate needles found in the
powder were not observed in the final composite films most
likely because the Ba-phosphonate is dissolved by DMF.
Therefore, these needles do not interfere with the composite
structure. More interestingly, it was found that homogeneous
composite thin films can also be prepared out of the pure BTO
powders, i.e. without phosphonic acid coating. Figure 7
compares SEM images of both composite systems (phosphonic
acid coated and pure BTO).
Morphology and dielectric properties of composite films
Thin films were prepared by spin-coating of BTO/polymer
suspensions. Cross-section SEM was used to characterize the
film morphologies. The particle distribution and overall
homogeneity of the composite material are the key parameters
for enhanced dielectric properties. We observed that
C-BTO@PFBPA/P(VDF-HFP) composites show highly
defective structures (voids, cracks, agglomerations) when
water-rinsed powder was used. In turn, these composites
exhibited a strong increase of the leakage current, which made
it impossible to evaluate their dielectric properties. For this
reason, these films are not further discussed in the following. In
Figure 7. Cross-section SEM images of spin-coated phosphonic acid modified
BTO@PFBPA/P(VDF-HFP) (left) and pure BTO/P(VDF-HFP) composites films
(right) with different BaTiO3 volume fractions.
A maximum oxide filling of 50 vol.% was chosen as upper
limit, which is near the percolation threshold evaluated by
numerical calculations.11,24,36 For the high-content composites,
the oxide particles are very densely packed, preventing a clear
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insight into the film morphology by SEM. Therefore, As earlier reported, the use of BYK leads to a homogeneous
composites of 50 vol.% and low-content composites of particle distribution in the phosphonic acid modified
10 vol.% are presented for both BTO powders in Figure 7. For BTO@PFBPA/P(VDF-HFP) system, resulting in an increase of
the BTO@PFBPA/P(VDF-HFP) composites areas with well- the permittivity.27 As shown in the left part of Figure 8, BYK
separated particles in the composite films were found but also effectively suppressed the formation of small defects. With this
regions with enriched oxide contents and polymer dispersant the particles are more uniformly distributed and
accumulations were identified, indicating an insufficient surrounded by polymer matrix, indicating that the phosphonic
homogenization of the dispersions in spite of the surface acid alone does not provide a sufficiently good connection
coating by the phosphonic acid. Especially, the high content between the oxide surface and the hydrophobic polymer.
composite tended to form undesired blowholes. Against Surprisingly, the SEM imaged in the right part of Figure 8 show
expectation, we observed that composites out of pure sol–gel that such morphological improvements are achieved for the
BTO form layers with a clearly improved homogeneity. For pure BTO/P(VDF-HFP) composites. In contrast even without
pure BTO/P(VDF-HFP) composites the particles are well BYK the BTO/P(VDF-HFP) films show a uniform and defect-
dispersed (as clearly visible in the image of the low oxide free morphology. Both samples, i.e. with and without BYK,
content composite) thus preventing percolation. In the high exhibited a similar particle distribution.
oxide content composite the particles are more densely packed The SEM investigations are corroborated by the dielectric
similar to the situation in BTO@PFBPA/P(VDF-HFP), but measurements discussed below, where a better homogeneity
without the unfavorable formation of flaws such as cracks or was found to lead to an increased permittivity (Fig. 9). In the
pores. This finding can be explained assuming that the small dielectric measurements, the voltage was kept constant at 1 V
particle size and the highly hydroxylated BTO particle surface and the obtained values were averaged over four parallel-plate
form a strong interaction with the F atoms of P(VDF-HFP), capacitors. The results of the composite films are compared to a
creating a good compatibility between BTO and polymer pure P(VDF-HFP) film, which was prepared in same way as the
matrix.
composite films.
Previous investigations revealed that an improved oxide-
polymer wetting can also be achieved by additional dispersants
such as BYK-W 9010 (BYK).37,38 BYK-W 9010 is known as a
commercially available dispersion additive consisting of a
phosphoric acid polyester copolymer with acid groups, It is
frequently used to enhance the dispersability of oxide particles
and was therefore tested in this study. Similar to other
surfactants, it is supposed to link the particle surface with the
polymer matrix. Therefore, we additionally investigated the
effect of BYK with respect to the composite homogeneity and
dielectric properties. Figure 8 compares SEM images of both
composite systems (phosphonic acid coated and pure BTO)
with and without addition of BYK.
Figure 9. Relative permittivities and loss tangents of spin-coated phosphonic acid
modified BTO@PFBPA/P(VDF-HFP) (a) and pure BTO/P(VDF-HFP) (b) composite
films with different BaTiO3 volume fractions compared to the pure polymer.
The observed relative permittivity of approximately r ~ 5 for
P(VDF-HFP) is less than half of the specified value (r ~ 11.4)
for the ideal crystalline polymer. This decrease can be
explained by a reduced crystallinity due to the spin coating
process without mechanically streching.39,40 For the composite
films of both the pure BTO and the phosphonic acid-coated
oxide the relative permittivities r strongly increase with rising
oxide content. For the BTO/P(VDF-HFP) composite system out
of untreated powder this increase is significantly larger.
Comparing the permittivities of the composites with 50 vol.%
BTO to the pure P(VDF-HFP) polymer, an enhancement up to
Figure 8. Cross-section SEM images of composite films from phosphonic acid
modified BTO@PFBPA/P(VDF-HFP) (left) and pure BTO/P(VDF-HFP) (right) with
and without addition of the BYK dispersant. The BTO content was 30 vol.% in all
cases.
a
factor of four for the phosphonic acid modified
BTO@PFBPA/P(VDF-HFP) system was found.
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On the other hand, for the pure BTO/P(VDF-HFP) system an case, BYK can effectively link both materials and increases the
even larger enhancement by a factor larger than 10 was wettability, leading to uniform composite film thicknesses for
achieved at 1 MHz with a further increase for lower various oxide contents.
frequencies. The loss tangents of phosphonic acid modified It is noteworthy that the permittivities of pure
BTO@PFBPA/P(VDF-HFP)
composites
are
almost BTO/P(VDF-HFP) samples were generally larger than those of
independent of the oxide content, which is probably the result the surface modified BTO@PFBPA/(PVDF-HFP) samples with
of the oxide surface treatment. Pure BTO/P(VDF-HFP) or without BYK. Regarding the film thicknesses, the
composites show a slightly increasing dielectric loss tangent BTO/P(VDF-HFP) samples without additional dispersant
with rising oxide filling at low frequencies. However, the exhibited very similar values around 4.2(4) μm and surface
values are not significantly higher than for the surface-coated roughnesses of approximately 0.2 μm for all oxide contents,
oxide particles. Therefore, the use of pure BTO particles that while samples with BYK-W 9010 as additive show decreasing
stem from the sol–gel approach strongly enhances the dielectric film thicknesses with increasing oxide contents, typically
permittivity while the loss tangent remains low.
ranging from approximately 4.6(6) μm for the 10 vol.%
Moreover, a reproducible effect of BYK on the permittivity of composites to 2.9(3) μm for 50 vol.%. The decreasing film
the pure BTO/P(VDF-HFP) composites was not observed thicknesses confirm an improved particle wetting by addition of
(Fig. 10). Instead, it was found that the relative permittivity of the BYK dispersant. On the other hand, the decreasing film
BTO/P(VDF-HFP) composite films increases with BYK only thicknesses worsen the relative permittivity. Especially the
for low oxide contents while it decreases for higher fillings. In residual free acid groups of the dispersant cause an increase of
contrast, the use of BYK in the phosphonic acid modified dielectric loss and leakage current, which results in a negative
BTO@PFBPA/P(VDF-HFP) system leads to an increased effect for the relative permittivity of composites of pure and
homogeneity and permittivity.
also functionalized BTO.25 These findings underline the
positive influence of pure sol-gel BTO without surfactant-
coating or additional dispersants on the composite
homogeneity. The loss tangents of both BTO systems
(phosphonic acid coated and pure BTO) with addition of BYK
are almost identical compared to corresponding samples
without BYK.
In this context it has to be noted that the dispersibility of BTO
particles strongly depends on the preparation technique. Typical
commercially available BTO particles do not possess a high
degree of surface hydroxyl groups. Therefore, the surface
modification of such BTO particles with phosphonic acids leads
to a better dispersibility in the polymer solution. The
corresponding composote systems with PVDF (or its
copolymers) show relative permittivities in the range of 35 –
40, while composites without functionalization achieve slightly
smaller relative permittivity up to 33.9,11,19 Previously, higher
permittivities close to our values could only be realized in thick
composite films or pellets from solution casting or hot-
pressing.23,26,41–45 For our BTO nanoparticles from the
precipitation method coating with phosphonic acids was found
to be not mandatory for a good dispersibility because the high
amount of hydroxyl groups already generates a good wettability
resulting in uniform particle distributions and enhanced
dielectric properties.
Figure 10. Relative permittivities of composite films from phosphonic acid
modified BTO@PFBPA/P(VDF-HFP) (a) and pure BTO/P(VDF-HFP) (b) containing
different BaTiO3 volume fractions with and without addition of the dispersant
BYK-W 9010.
These findings are supported by measurements of the film
thicknesses and surface homogeneities. BTO@PFBPA/P(VDF-
HFP) samples without additional dispersant show an increasing
film thickness with higher oxide contents, typically ranging
from 5.4(2) μm for the 10 vol.% composites to 10.7(4) μm for
50 vol.%. This effect is accompanied by an increasing surface
roughness from approximately 0.1 μm to 0.4 μm. In contrast,
the corresponding samples with BYK-W 9010 as additive
exhibit uniform film thicknesses around 4.5(5) μm and surface
roughnesses of 0.10(5) μm. This effect is not surprising because
BYK reduces the viscosity of highly filled formulations by
increasing the polymer wetting of particles. Additionally, we
assume that the formation of the Ba-phosphonate during the
functionalization of sol-gel BTO disturbs the surfactant-
polymer interactions so that the phosphonic acid alone is not
sufficient for linking the oxide surface and the polymer. In this
In addition to the increased permittivity and low loss tangent of
pure BTO/P(VDF-HFP) composites, we also found an
enhancement of the dielectric strength by almost factor 2
compared to BTO@PFBPA/P(VDF-HFP) composites. While
BTO@PFBPA/P(VDF-HFP)
films
exhibited
dielectric
strengths of approximately 90 V/μm (within a range of
 10 V/μm), the BTO/P(VDF-HFP) composites afforded a
significant improvement to approximately 170 V/μm (within a
range of  20 V/μm). For neither composite films of the pure
BTO nor for the phosphonic acid-coated oxide any systematic
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influence of the oxide content or additional dispersant on the tangents in the order of 0.1 and an enhanced dielectric strength
dielectric strength could be identified. of approximately 170 V/μm were found for these films.
Remarkably, our investigations show that when BTO According to our results a tremendous simplification of the
nanoparticles prepared by a sol–gel route are applied as fabrication process of composite films is possible. Highly
inorganic component, the best dielectric properties are achieved homogenous composite films from sol–gel synthesized BTO
by keeping the composite system as simple as possible, i.e. can be obtained without any surface modifications or
without surfactant-coating of the oxide or application of dispersants. This simple processability using a solution
additional dispersants. The permittivity of roughly 60 at 1 kHz technique leads to films with superior dielectric properties.
is among the highest values ever reported for BTO-based Therefore, these composites are promising candidates for film
nanocomposite dielectrics.4,9–11,19,26 The use of sol-gel BTO capacitors, e.g. in embedded applications or as a multilayer for
with a high degree of surface hydroxyl groups is therefore a electrical energy storage.
clear advantage making the fabrication process much easier,
faster and cheaper.
Acknowledgements
The work was supported by the Federal Ministry for Education and
Conclusions
Research (BMBF) within the ForMaT/SuperKon grant. For density
measurements, we thank Jenny Bienias within the group of Prof. Dr.
Thomas Hahn. Additional thanks are due to the German Federal
Environmental Foundation (DBU) for financial support of a PhD
scholarship.
High-quality composite films were obtained by spin-coating of
BaTiO3 embedded in P(VDF-HFP) matrix. BaTiO3
a
synthesized by a sol–gel route appears as well-crystalline
spherical nanoparticles with a high degree of surface hydroxyl
groups, which are good linkers for surfactant molecules such as
PFBPA, but also for the fluorinated polymer used as matrix
material. Phosphonic acids are often used as detergents to form
stable oxide surface bonds and to create a good connection to
the fluorinated organic polymer host. However, we found that
the surface treatment of the sol–gel synthesized BTO with
PFBPA leads to the formation of a barium phosphonate. The
surface-treated BTO@PFBPA powder exhibit an apparent
surfactant coverage higher than 100 % due to this barium
phosphate byproduct, while the washed C-BTO@PFBPA
powder led to a value of 97 %, which corresponds to a
complete monomolecular surface coverage. Unfortunately,
composites made from the washed powder showed severe
morphological defects and a strong increase of the leakage
current, precluding their application as dielectric material. In
contrast, homogeneous composite film can be prepared using
either unwashed BTO@PFBPA or pure BTO particles.
Composite films of phosphonic acid modified BTO show some
morphological defects. The formation of defects can be
suppressed by the addition of BYK-W 9010 as dispersant. This
indicates that the phosphonic acid itself is not sufficient for
linking the oxide surface and the hydrophobic polymer.
Notes and references
a
Institute of Chemistry, Martin-Luther-University Halle-Wittenberg,
Kurt-Mothes-Straße 2, D-06120 Halle (Saale), Germany.
b
Interdisciplinary Centre of Materials Science, Martin-Luther-University
Halle-Wittenberg, Heinrich-Damerow-Straße 4, D-06120 Halle (Saale),
Germany.
c
Institute of Physics, Martin-Luther-University Halle-Wittenberg, Von-
Danckelmann-Platz 3, D-06120 Halle (Saale), Germany.
Electronic Supplementary Information (ESI) available: [XRD pattern of
BTO obtained by the sol-gel synthesis. FT-IR spectra of pure BTO and
PFBPA as well as the surface-modified BTO@PFBPA and isolated
Ba-phosphonate byproduct and the purified C-BTO@PFBPA. Detailed
investigations of the particle agglomeration determined by DSL of pure
sol-gel synthesized BTO and phosphonic acid surface modified
BTO@PFBPA and modified commercially available nanopowders
BTOcom@PFBPA (Sigma-Aldrich Barium titanate(IV) nanopowder,
< 100 nm particle size (BET)) dispersed in water, DMF and in the
resulting polymer composite suspension.] See DOI: 10.1039/b000000x/
In contrast to previous investigations, in which mainly
commercially available BTO nanoparticles were used, it was
found that composites of sol–gel BTO form very homogeneous
layers with well-separated particles even without an application
of phosphonic acid or the additional dispersant BYK-W 9010.
This result can be attributed to strong hydrogen bonds between
the surface hydroxyl groups and the fluorine atoms of the
polymer. These BTO/P(VDF-HFP) composite films show a
doubling of the relative permittivity r compared to films from
PFBPA-functionalized particles and more than one order of
magnitude higher values compared to the pure polymer. An
extraordinarily high relative permittivity r of 57 was obtained
for a 50 vol.% BaTiO3 composite at 1 kHz with a further
increase for lower frequencies. In addition, relatively low loss
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