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Erschienen in: Journal of Materials Science 2/2014

Open Access 01.01.2014

Acetylacetone stimulus effect on electrorheological properties of TiO2 aggregated nanoparticles

verfasst von: Alaedine Kossi, Jacques Persello, Bernard Cabane

Erschienen in: Journal of Materials Science | Ausgabe 2/2014

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Abstract

We present experimental results on electrorheological behavior under an ac electric field, and dielectric properties of colloidal suspensions composed of aggregated titanium dioxide nanoparticles dispersed in insulating PDMS silicon oil, as a function of adsorbed acetylacetone (Acac) dipolar molecules on TiO2 surface. The results show that the elastic modulus G′ of TiO2–Acac/PDMS ER fluid decreases when the electric field frequency is increased gradually from 10 to 104 Hz. In the range of tested electric field frequencies, we observe that the dielectric properties are given essentially by an interfacial polarization which is maximal at low frequency. At low frequency (υ = 10 Hz), G′ increases with the increasing Acac/TiO2 surface coverage θ from 0 to 0.45, due to the high polarization of the particles caused by the increase of the charge carriers that move to the interface between TiO2 particles and the insulating medium. For θ = 0.35, G′ reaches 1.80 MPa which is 3.6 times greater than that of pure TiO2/PDMS ER fluid. However, G′ decreases by increasing θ beyond 0.45; in such cases, the mismatch on dielectric constant and conductivity between dispersed phase (TiO2) and insulating medium (PDMS) begins to fall as θ increases because of the increase of Acac dipoles remaining free in PDMS. The performance of TiO2–Acac/PDMS ER fluid at low frequency was validated by a reversibility test where elastic modulus G′ increases dramatically and returns to its initial state as soon as several cycles of a switching electric field were applied between 0 and 1 kV/mm successively.

Introduction

Electrorheological (ER) fluids are made by suspending particles in a liquid dielectric constant and conductivity of which are mismatched to create dipolar particle interactions in the presence of an ac or dc electric field [1]. The ER phenomenon is related to changes in rheology of dispersions. Indeed, in response to an applied electric field, ER fluids increase their resistance to flow, and in some cases, they manifest in the conversion of fluid into solid behavior [2]. The main cause inducing this phenomenon is the polarization of particles which can be aligned in the direction of an applied electric field. For this reason, the ER effect, besides the nature of dispersed particles and insulating liquid, is sensitive to the electric field and its frequency and all additives concentration that can affect the dielectric properties of the fluid. Several authors have shown that the ER response decreases with the increasing frequency of the electric field beyond a frequency of 100 Hz [3]. Other workers have studied the effects of additives such as water, urea, or other polar molecules [49] on performance of an ER fluid. These studies have shown that these additives improve the ER response of ER fluids in terms of shear stress or elastic modulus.
In this study, our aim is to investigate the influences of adsorbed acetylacetone (Acac), as polar molecules, and electric field frequency, on the dielectric properties and their impacts on the ER response of titanium dioxide ER fluid, in terms of elastic modulus G′ and loss modulus G′′. The choice of Acac is motivated by its dielectric and polarizing properties (Acac has a high dielectric constant ε r ≈ 25.7 and a dipole moment μ ≈ 2.78 D (1D = 3.33564 10−30 cm)), and because of its good affinity with TiO2 surface, we expect that the adsorbed Acac can enhance the ER response of the TiO2 ER fluids.
The system consists of aggregated nanoparticles of titanium dioxide dispersed in silicone oil. In order to control the polarization forces, the dielectric constants of the titanium dioxide particles are modified by adsorbing different Acac concentrations on titanium dioxide surface. We use a scanning electron microscopy, X-ray diffraction, and small-angle neutrons scattering to characterize titanium dioxide morphology, and UV spectroscopy to determine the adsorption isotherm of Acac on titanium dioxide surface. The dielectric properties of titanium dioxide dispersed in silicone oil are measured using impedance spectroscopy. We analyze the ER response under ac electric field depending in adsorbed Acac amount onto TiO2 surface and in electric field frequency.

Experimental section

Synthesis of nanoporous TiO2 powder

Nanosized TiO2 powder was synthesized via a sol–gel method [10], using titanium tetraisopropoxide (Sigma-Aldrich, 98 %, solution), 2-Propanol (Sigma-Aldrich, ACS reagent, ≥99.5 %), aluminum isopropoxide (Aldrich, 99.99 %), ammonium hydroxide (Sigma-Aldrich, 28 % NH3 in H2O), Acac (Sigma-Aldrich, ReagentPlus®, ≥99 %), nitric acid (Sigma-Aldrich, ACS reagent, 70 %), acetic acid (Sigma-Aldrich, ACS reagent, ≥99.7 %), and ultrapure water (≥18 MΩ cm) as starting materials.
In a typical procedure, titanium (IV) tetraisopropoxide (0.388 mol) was rapidly added to ultrapure water (640 ml) and then stirred for 30 min. A white precipitate formed immediately upon the addition of titanium (IV) isopropoxide. The resultant colloid was recovered by centrifugation (10,000×g for 30 min). The centrifugation cake was added into a jacketed three-necked flask reactor equipped with a mechanical stirring containing 750 ml of an aqueous solution, of molar composition; 0.36 M nitric acid, 1 M acetic acid, and 0.05 M aluminum isopropoxide. The pH of the colloidal solution after addition of the cake was measured to be between 1 and 2.
Peptization occurred after heating the product at 80 °C for 1 h under medium stirring, whereupon the slurry became a stable sol. Then, the sol was cooled down to room temperature, and 17 M ammonium hydroxide solution was added drop wise into the TiO2 sol under mild stirring to form a white gel. The pH of the resultant colloidal suspension was measured to be close to 7.
The gel was then collected by centrifugation (10,000×g for 30 min) and dried at 110 °C over night. The powder was crushed and ground into fine powder using a mortar and pestle and was further calcined in air at 500 °C for 4 h.

Characterization of the nanoporous TiO2 particles

Electron microscopy was performed on the CCMA EM Core Facility (University of Nice Sophia Antipolis) using a scanning electron microscopy apparatus (JEOL 6700F) to characterize the TiO2 obtained after heat treatment at 500 °C. The crystal structure of the obtained TiO2 powder was analyzed using an X-ray diffractometer (CPS120/Cu) (XRD KαCu, λ = 0.15406 nm).
Morphologic characteristics of aggregated particles can be deduced from the small angle neutron scattering [11] spectra realized on a TiO2 heat treated at 500 °C and dispersed in silicone oil. The SANS experiments of a concentrated TiO2/PDMS suspension are realized at the ILL institute using the D11 instrument. In a typical signal of concentrated dispersions where isotropic interactions between identical particles are considered, the average distance between the nearest neighbors is given by r = 2π/q 0 where q 0 corresponds to the maximum in scattered intensity I max(q 0). In the case of a two-phase system, at large q vectors, the interface between particles and solvent is observed at small scale. The Porod limit can be used for a sharp interface (in the normal direction but smooth in any parallel direction) where the scattered intensity decreases as q −4 does at large q. The Porod limit in this case reaches a constant at large q values and is related to the specific area of particles (independently of the shape of the two phase) as [12]:
$$ \mathop {\lim }\limits_{q \to \infty } q^{4} I_{q} = cste = 2\pi (\Updelta \rho )^{2} S/V, $$
(1)
where (Δρ)2, given in m−4, is a constant related to the difference in contrast between the TiO2 and PDMS [13], and S/V is the surface per volume unit ratio. The specific area denoted S S (given in m2/g) is connected to S/V through the relation:
$$ S_{s} = \frac{1}{{d_{{{\text{TiO}}_{ 2} }}^{ 0} }}.{\raise0.7ex\hbox{$S$} \!\mathord{\left/ {\vphantom {S V}}\right.\kern-0pt} \!\lower0.7ex\hbox{$V$}}, $$
(2)
where \( d_{{{\text{TiO}}_{ 2} }}^{ 0} \) is the TiO2 density. \( \mathop {\lim }\limits_{q \to \infty } q^{4} I_{q} \) (q.a > 10) is determined experimentally by plotting q 4 Iq versus q.
For porous systems (aggregates formed from nanoparticles assembling), in some cases, the two length scales: nanopores (or nanoparticles) of few nanometer on one hand, and micronic grains on the other are largely separated. A typical signal may be obtained where three different regimes are observed on all samples: (I) a Porod’s regime at low q values (q < q 1) due to the envelope of the grains, (II) a transition regime in between, and (III) a second Porod’s regime at high q values (q > q 2) due to the inner pores of the grains [14]. Solvent PDMS is present in the inner pores and intergrain porosity. Introducing V e as the total volume of the envelopes of the grains, σ being the total inner surface of the solid grains, and Se as the total surface of the envelopes of the grains, one gets
  • from high-q Porod’s law
$$ \frac{\sigma }{{V_{e} }}(m^{ - 1} ) = \frac{{q_{ 2}^{4} I(q_{2} )}}{{2\pi (\Updelta \rho )^{2} }}, $$
(3)
  • from low-q Porod’s law
$$ \frac{{S_{e} }}{{V_{e} }}(m^{ - 1} ) = \frac{{q_{1}^{4} I(q_{1} )}}{{2\pi (\Updelta \rho )^{2} }}, $$
(4)
Finally, combining the two Porod’s law we obtain
$$ \frac{{q_{2}^{4} I(q_{2} )}}{{q_{ 1}^{4} I(q_{ 1} )}} = \frac{\sigma }{{S_{e} }}, $$
(5)

Preparation of ER fluids

In a typical procedure, several solutions of various amounts of Acac [15] in PDMS oil silicone (Dow Corning Corporation XIAMETER® PMX-200 silicone fluid, viscosity 10 cSt at 25 °C) were prepared (in the range of 0–3.215 wt%). The final ER fluids were obtained by dispersing 5 g of TiO2 powder in 5 g of Acac silicone oil solutions by means of a Sonifier Vibra-cell (75041) ultrasonic apparatus operating at 30 % amplitude for 4 min, to have a volume fraction of 0.2 of TiO2 in PDMS. Then, the ER fluid was out-gassed at room temperature and at 10−3 Pa for 1 h. Afterward, dry N2 was introduced into the bowl up to a pressure of 1 atm to avoid any contamination by the humidity of the air. The ER fluids were stored in desiccators before use.

Adsorption isotherms of Acac onto TiO2 dispersed in PDMS

Various amounts of Acac were added to TiO2—PDMS dispersions, and the part of Acac that had reacted with TiO2 in each sample was determined through the depletion method: after 48 h of equilibration, the TiO2 particles were separated through centrifugation, and the concentration of Acac in the supernatants was measured through UV spectroscopy after filtration of the samples through 0.22-μm filters.
We define surface coverage, θ, as the fraction of the adsorption sites of TiO2 occupied by Acac molecules; at equilibrium, we have
$$ \theta = \frac{{\frac{{m_{\text{Acac}}^{\text{ads}} }}{{M_{\text{Acac}} }}.N_{\text{a}} .A^\circ_{\text{Acac}} }}{{m_{{{\text{TiO}}_{ 2} }} S_{{{\text{TiO}}_{ 2} }} }}, $$
(6)
where \( A ^ {{\circ }}_{\text{Acac}} \) is the area occupied by one molecule of Acac, \( M_{\text{Acac}} \) is the Acac molar mass, N a is the Avogadro number, \( m_{{{\text{TiO}}_{ 2} }} \) is the TiO2 weight, and \( S_{{{\text{TiO}}_{ 2} }} \) is the TiO2 specific area.
The experimental surface coverage can be compared with θ being calculated from the Langmuir equation [16, 17], it is given as follows:
$$ \theta_{\text{Langmuir}} = \frac{{b\varphi_{\text{Acac}}^{\text{equil}} }}{{1 + b\varphi_{\text{Acac}}^{\text{equil}} }} $$
(7)
where \( \varphi_{\text{Acac}}^{\text{equil}} \) is the volume fraction of unadsorbed Acac in equilibrium with the dispersion. b is adsorption/desorption equilibrium constant of Acac on TiO2, it is given by the following:
$$ b = \left( {\frac{{h^{2} }}{{2\pi .m_{\text{Acac}}^{ \circ } .kT}}} \right)^{3/2} \frac{3}{{4\pi .a_{\text{acac}}^{ 3} }}{\text{e}}^{ - \varepsilon /kT} , $$
(8)
where \( m_{\text{Acac}}^{\text{o}} = \rho_{\text{Acac}} \frac{4\pi }{3}a_{\text{Acac}}^{3} \approx 2156.10^{ - 25} {\text{ Kg}} \) is the mass of one Acac molecule, h is the Planck constant, k is the Boltzmann constant, T ≈ 298 K is the temperature, and ε is the interaction energy between TiO2 and Acac.

Electrorheological measurements

The dynamic properties of ER fluids were measured by means of a Rheometrics Dynamic Analyzer II (RDA II, Rheometrics, Inc., NJ) in time sweeps mode. These measures consist of applying an alternating electric field (Trek Model 609E-5 High Voltage amplifier and Hameg function generator HM 8030-5 on a sample placed between two parallel plane–plane geometry electrodes and measuring moduli as a function of time until a stationary state is reached. The diameter of plane electrode was 25 mm, and the gap between electrodes was 1 mm. The moduli are measured at low shear strains, where dynamic properties are independent of shear strain amplitude. The mobile electrode (upper electrode) applies an oscillatory shear strain with γm = 0.2 % of amplitude, at constant temperature of 25 °C and with an angular velocity of ω = 100 rad/s. An ammeter (TENMA 72-7735 Multimeter) is used to measure the electrical current passing through the sample during rheological measurements under applied electric field.

Complex relative permittivity measurement

We use an HP 4192A LF impedance analyzer to measure the dielectric properties of the prepared ER fluids. The frequency range of the applied alternating field was between 5 Hz and 1 MHz. The dielectric cell consists of parallel stainless steel electrodes separated by a distance l that we can change. The electric circuit can be represented by a parallel equivalent schema (Fig. 1) where we consider the contribution, in total measured impedance (G, C), an impedance (G k, C k) due to the sample polarization, a stray impedance (G e, C e) due to electrode polarization, and a residual capacitance (C r) due to the field lines that are not located in the sample.
We can determine G e and C e by measuring the total impedances at different electrode distances for a given frequency. To calculate C r, we measure capacitance C of an empty cell as a function of the inverse of distance between electrodes; C is given by
$$ C = C_{\text{R}} + \frac{{\varepsilon_{0} S^{ * } }}{l}, $$
(9)
where ε 0 is the vacuum permittivity, S * is the electrode effective surface, and l is the distance between parallel electrodes.
Henceforth, once these corrections are made, we can determine the samples impedance (G k, C k) [18, 19] that can be related to the complex relative permittivity by
$$ \varepsilon_{rk}^{\prime } = \frac{{C_{\text{k}} }}{{C_{0} }}\, \;{\text{and}}\; \,\varepsilon_{rk}^{\prime \prime } = \frac{{G_{\text{k}} }}{{\omega C_{0} }}, $$
(10)
where C 0 is the vacuum capacitance, and ω is the ac electric field angular frequency.
Experimental measurements of \( (\varepsilon_{rk}^{\prime }) \) \((\varepsilon_{rk}^{\prime\prime} )\)and (ε rk ) obtained from Eq. (10) are fitted by using the Maxwell–Wagner equations [20], yielding \( \varepsilon_{rk}^{\prime } \) and \( \varepsilon_{rk}^{\prime\prime } \) as
$$ \varepsilon_{rk}^{\prime } = \frac{{(\varepsilon_{rk}^{\prime } )_{s} - (\varepsilon_{rk}^{\prime } )_{\infty } }}{{1 + \omega^{2} \tau }} + (\varepsilon_{rk}^{\prime } )_{\infty } , $$
(11)
$$ \varepsilon_{rk}^{\prime \prime } = \frac{{(\varepsilon_{rk}^{\prime } )_{s} - (\varepsilon_{rk}^{\prime } )_{\infty } }}{{1 + \omega^{2} \tau^{2} }}\omega \tau + \frac{{\sigma_{k}^{\prime } }}{{\omega \varepsilon_{0} }}, $$
(12)
where \( (\varepsilon_{rk}^{\prime } )_{s} \) and \( (\varepsilon_{rk}^{\prime } )_\infty\) are, respectively, the static value (when ω tends to 0) and the optic value (when ω tends to ∞) of relative permittivity, σ k is the sample conductivity, and τ is the relaxation time of the dielectric function.

Results and discussion

Morphology and structure of TiO2 aggregated nanoparticles

SEM performed on TiO2 after heat treatment at 500 °C (Fig. 2 ) shows nanosized particles (between 10 and 20 nm) highly aggregated forming a porous system. The X-ray spectra (Fig. 3 ) show that the obtained TiO2 powder had an anatase crystalline structure. The density of TiO2 was determined using a standard flask pycnometer of 5 cm3, and we found a value of 3.84 g/cm3 that is close to the TiO2 anatase density (3.89 g/cm3).
The SANS curve giving the scattered intensity Iq versus wave vector q is shown in (Fig. 4a). A maximum of the scattered intensity is observed for an intermediate value of wave vector (q(I max) ≈ 3.56.10−1 nm−1), which characterizes the average interparticle distance r = 2π/q max; in our case (adhesive particles heat treated at 500 °C) we can confuse r with the average size of the nanoparticles. We obtain the average radius of the TiO2 nanoparticles about a ≈ 8.8 nm which is in the same range as that observed in the SEM picture. From the Porod plot (Fig. 4b) at very high value of q (where Iq is proportional to q −4 and q.a > 10), we can use the Porod limit (Eqs. (1) and (2)) to calculate the specific area of TiO2 particles. We obtain a specific area S S  ≈ 117 m2/g. Figure 4b shows also two Porod’s regimes: the first for q < q 1 = 0.09 nm−1 which characterizes the interface of the envelope of the grains S e, and the second for q > q 2 = 0.53 nm−1 which can be attributed to the surface of inner porous structure σ. Using Eq. (5), a high ratio (σ/S e) = 263 was obtained indicating that the porosity of the system is dominated by inner nanoporous structure.

Influences of electric field and its frequency on electrorheological properties

The elastic modulus G′ and loss modulus G′′ for TiO2–Acac/PDMS ER fluid measured in the applied electric field range 0–4 kV/mm versus electric field frequency are shown in (Fig. 5a, b).
We note that G′ and G′′ decrease with the increasing electric field frequency, and this occurs whatever be the applied electric field supportable by the sample. We also note that the increase in electric field frequency is accompanied by a decrease in the maximum of electric field E max supportable by the apparatus. This limitation of applied electric field is due to the appearance of an electric current density which is insignificant at low frequency, but becomes increasingly important as soon as the frequency increases. Figure 6 illustrates this situation where at low frequency (υ = 10 Hz) Emax can be up to 4 kV/mm, and the electric current density in the sample is very low (\( j_{{(E_{\hbox{max} } )}} \) = 0.01 μA/mm2), while at high frequency (υ = 10 kHz) E max is limited to 1.5 kV/mm (\( j_{{(E_{\hbox{max} } )}} \) ≈ 1.53 μA/mm2); beyond this limit, the measurement is interrupted because of the high conduction in the sample.

Dielectric properties of TiO2–Acac/PDMS ER fluid

To understand the link between the ER behavior depending on the frequency and the dielectric properties, we have made measurements of the complex impedance versus frequency for TiO2–Acac/PDMS ER fluid. Figure 7 shows the experimental and the calculated results (using the Maxwell–Wagner equations) of the real part of relative permittivity (ε r ) (Figure 7a) and imaginary part ε r of relative permittivity (Figure 7b) versus frequency (ν m) for TiO2–Acac/PDMS ER fluid.
We observe that at low frequency, we have a macroscopic polarization due essentially to an interfacial polarization where its contribution to dielectric constant is maximal because the motions of the charge carriers that are accumulated at the interface between TiO2 and insulating medium. At high frequency, the electric field alternates so fast such that the interfacial polarization cannot increase, and there is no contribution to the dielectric constant. However, between these two limits (low and high frequency), at intermediate frequencies, the interfacial polarization begins to lag behind the electric field causing dissipative losses [21]. This energy dissipation is maximal when the loss tangent tgδ = ε r /ε 4 reaches a maximum value at an angular frequency ω m that can be related to the characteristic relaxation time τ as follows:
$$ \tau = \frac{1}{{2\pi \nu_{\text{m}} }} = \frac{1}{{\omega_{\text{m}} }}. $$
(13)
For TiO2–Acac/PDMS ER fluid, we found τ ≈ 7.10−5s corresponding to ν m about 2.3 kHz. The frequency below ν m determines the range in which charge carriers are mobile on long distances. At frequency above ν m, the charge carriers are confined to potential wells being mobile on short distance.
The influence of frequency on the polarization in terms of dielectric losses caused by the energy dissipation in the sample may explain the ER behavior where the elastic modulus G’ decreases as the electric field frequency increases between 10 Hz and 10 kHz.
The ER results and dielectric characterization clearly show that ER effect is optimal at low frequency. The following ER experiences will be investigated with a fixed applied electric field E = 1 kV/mm and with a frequency υ = 10 Hz to have an ER effect satisfactorily without the occurrence of a current strong enough, energy consumer and disruptive of ER measurement.

Influence of the added Acac on electrorheological properties

To quantify the role of Acac on the TiO2/PDMS ER properties, the Acac onto TiO2 surface adsorption isotherm is shown in (Fig. 8 ). The experimental results giving the surface coverage (θ) as a function of the volume fraction of free Acac in equilibrium with the dispersion are compared to those calculated from Langmuir equation. The best fit between experiment and Langmuir equation was obtained using an interaction energy ε = −8.85 10−20 J (−21.5 kT) and an adsorption/desorption equilibrium constant b = 6.88 103 where the adsorption of Acac onto TiO2 surface reaches a plateau for θ ≈ 0.90 corresponding to a maximum adsorption about 2Acac/nm2. However, a discrepancy was observed between the calculated and the experimental results for θ > 0.65 suggesting that Acac is adsorbed onto TiO2 surface in a multilayer manner. The results show also that the Acac adsorption onto TiO2 surface is virtually total for θ ≤ 0.45, while beyond this limit, the Acac remaining free in PDMS becomes significant and increases with surface coverage θ.
According to the Acac onto TiO2 surface adsorption isotherm, four ER fluids samples were investigated where different situations are represented: pure TiO2/PDMS ER fluid  = 0), total Acac adsorption onto TiO2 surface (θ = 0.35), and two samples with partial Acac adsorption onto TiO2 surface, respectively: θ = 0.59 and θ = 0.81. In all samples, Φ V (TiO2/PDMS) = 0.2. We carried out measurements of elastic modulus G′ and loss modulus G′′ on the four samples prepared above. Figure 9 represents the experimental results of G′ and G′′ obtained from the various samples prepared according to surface coverage rate, θ, of Acac adsorbed onto TiO2.
Figure 9 shows that the intrinsic ER properties of TiO2 ER fluid (without adding Acac θ = 0) are very correct and reach an elastic modulus G′ of about 500 kPa and G′′ of about 225 kPa with an applied ac electric field of 1 kV/mm and 10 Hz of frequency. It also shows that, for the same electric field and dynamic shear rheology conditions, the increase of adsorbed Acac onto TiO2 surface leads to an increase of G′ until it reaches a maximum Gmax ≈ 1.80 MPa which is 3.6 times greater than that for pure TiO2/PDMS ER fluid. This maximum corresponds to the surface coverage θ ≈ 0.45 (about 1 Acac/nm2). Beyond this limit, G′ decreases. As observed (Fig. 8), we can attribute this behavior to the TiO2/Acac/PDMS adsorption equilibrium that may affect the dielectric properties of ER fluid. Indeed, for θ ≤ 0.45 the amount of Acac, remaining free in the dispersing medium is negligible. Hence, the increase in surface coverage, from 0 to 0.45, results in an increase of interaction energy between particles due to the improvement of the interfacial polarization of the particles, by increasing the number of charge carriers that move to the interface between the TiO2 particles and the insulating medium. While for θ > 0.45, the mismatch on dielectric constant and conductivity between dispersed phase (TiO2) and insulating medium PDMS begins to fall as θ increases due to the increase of Acac dipoles remaining free in PDMS. Therefore, the part of the interaction energy that is dissipated through the insulating medium becomes significant leading to a decrease in G′. Figure 7 shows also that loss modulus G′′ remains virtually constant over the entire θ range. This means that by adsorbing Acac dipoles onto TiO2 surface, only the elasticity (dependence of G′ on the interaction energy) was influenced by adding Acac, and in this dynamic oscillation conditions (γ = 0.2 % and ω γ = 100 rad/s), there are no restructuring (no change on viscosity) and the deformation of the aligned structures developed by the applied electric field that still remains unchanged.
To validate the TiO2–Acac/PDMS ER performance, we carried out measurements of elastic modulus (Fig. 10) where the applied ac electric field (υ = 10 Hz) switches between 0 and 1 kV/mm successively. We find that the elastic modulus G′ increases dramatically with the electric field and returns to its initial state as soon as the electric field is removed, and this is reproducible over several cycles where E switches between 0 and 1 kV/mm successively. This result confirms that at low frequency (υ = 10 Hz), the studied systems are reversible.

Conclusion

ER fluids have been prepared by suspending porous aggregates of TiO2 nanoparticles in PDMS silicone oil. The porous nature of TiO2 aggregates is exploited to adsorb dipolar molecules of Acac onto TiO2 surface. The TiO2 volume fraction Φ in PDMS was about 0.2. Electrorheological properties of ER fluids are investigated under ac electric field. The results show that the elastic modulus G′ of TiO2–Acac/PDMS ER fluid decreases with the gradually increasing electric field frequency from 10 to 104Hz. In the range of the tested electric field frequencies, we observe that the dielectric properties are given essentially by an interfacial polarization which is maximal at low frequency. We studied the influence of the adsorbed Acac in terms of surface coverage θ on ER response of TiO2 ER fluids, the applied ac electric field, and its frequency are, respectively: E = 1 kV/mm and υ = 10 Hz. The result shows that G′ increases with the increasing surface coverage θ, from 0 to 0.45, because of the high polarization of the particles caused by the increase of the charge carriers that move to the interface between TiO2 particles and the insulating medium. For θ = 0.35, G′ reaches 1.80 MPa which is 3.6 times greater than that of pure TiO2/PDMS ER fluid. However, G′ decreases by increasing θ beyond 0.45; in such cases, the mismatch on dielectric constant and conductivity between dispersed phase (TiO2) and insulating medium (PDMS) begins to fall as θ increases because of the increase of Acac dipoles remaining free in PDMS. The performance of TiO2–Acac/PDMS ER fluid at low frequency was validated by a reversibility test where elastic modulus G′ increases dramatically and returns to its initial state as soon as several cycles of a switching electric field were applied between 0 and 1 kV/mm successively.

Acknowledgements

The authors wish to thank Ralph Schweins from the Institut Laue-Langevin (ILL) for technical assistance in SANS experiments. The authors also gratefully acknowledge the financial support from the ANR CNRS.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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Metadaten
Titel
Acetylacetone stimulus effect on electrorheological properties of TiO2 aggregated nanoparticles
verfasst von
Alaedine Kossi
Jacques Persello
Bernard Cabane
Publikationsdatum
01.01.2014
Verlag
Springer US
Erschienen in
Journal of Materials Science / Ausgabe 2/2014
Print ISSN: 0022-2461
Elektronische ISSN: 1573-4803
DOI
https://doi.org/10.1007/s10853-013-7764-0

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