A novel multifunction material with both electrorheological performance and luminescence property

JOURNAL OF RARE EARTHS, Vol. 32, No. 11, Nov. 2014, P. 1022

A novel multifunction material with both electrorheological performance and luminescence property CHEN Mingxing (陈明星)1, SHANG Yanli (商艳丽)2, JIA Yunling (贾云玲)1, LI Junran (李俊然)1,* (1. College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China; 2. College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China) Received 1 April 2014; revised 10 July 2014

Abstract: A multifunctional material with both electrorheological (ER) performance and luminescence property was synthesized by a simple coprecipitation. The tetrabutyl titanate, as well as the Tb(NO3)3⋅6H2O and sulphosalicylic acid (C7H6O6S⋅2H2O, SSA) were chosen as starting materials. The composition, ER performance and luminescence property of the material were studied. The results showed that a novel material (TiTbSSA) with both ER performance and luminescence property was obtained. The relative shear stress τr (τr=τE/τ0, τE and τ0 were the shear stresses of the suspension with and without an applied electric field) of the suspension (30 wt.%) of the material in silicone oil reached 32.7 at a shear rate of 12.5 s–1 and an electric field strength of 4 kV/mm (DC electric field). The material containing the rare earth (RE=Tb) complex exhibited fine luminescence performance and higher ER activity. Therefore, it is a novel multifunction material which would have wide application prospect. Keywords: multifunctional material; electrorheological performance; luminescence property; rare earth complex

Electrorheological (ER) fluid is an exceptional suspension, the dispersed particles in the suspension would be polarized and attracted to each other to form a fiber-shaped structure, consequently make an ER fluid increasing its viscosity, in an applied electric field[1]. Furthermore, the viscosity change of an ER fluid is reversible with and without an applied electric field[1]. In view of this reversible and quick response to external electric field, ER fluid has attracted much interest in the application in various mechanical devices, such as clutches, valves, damping devices, and so on[1–3]. However, ER fluid has not been used wide because it does not have a high enough ER effect which is suited to the requirements of most of the applications. In order to get high active ER material, various types of ER materials, including ER materials doping the rare earth elements such as La, Ce, Pr, Y, Er, Gd based on inorganic compounds[4–15] or organic polymer[16–18], have been synthesized and studied. The majority of the inorganic compound materials have better ER performance. Yin and Zhao have obtained a cerium-doped TiO2 ER material that has very high ER activity[4]. However, up until now, an ER material based on titanium compound doping a kind of terbium complexes has not been studied. Fluorescent material, particularly fluorescent rare earth complexes are of great interest owing to their broad applications in biochemistry, material chemistry, medicine

and so on[19]. However, the materials with both ER performance and luminescence property have not been reported except our research[20,21]. In order to make a material have more broad application, which not only has better ER performance, but also has better luminescence property, we selected titanium compound as the principal component of our materials, and synthesized ER material with luminescence property. The composition, ER property and luminescence property of the material were studied in this paper.

1 Experimental 1.1 Preparation of TiTbSSA material The TiTbSSA material was synthesized through the following process. First, a complex solution was prepared by mixing fully the ethanol solutions of Tb(NO3)3⋅6H2O and sulphosalicylic acid (SSA), and then to add dropwise the complex solution into ethanol solution containing tetrabutyl titanate under vigorous agitation. The molar ratio of Tb(NO3)3⋅6H2O, SSA and tetrabutyl titanate was 1:3:3. The sediment was separated and washed with absolute ethanol, then dried at 80 ºC for 48 h. The yellow product was ground and finally dried in vacuum for 72 h at 50 ºC. The material containing Tb(C7H5O6S)3 and TiO(OH)2 was thus obtained.

Foundation item: Project supported by National Natural Science Foundation of China (10704041, 90922033, 21071008) and the National Basic Research Program of China (2013CB933401, 2010CB934601) * Corresponding author: LI Junran (E-mail: [email protected]; Tel.: +86-10-62753517) DOI: 10.1016/S1002-0721(14)60177-0

CHEN Mingxing et al., A novel multifunction material with both electrorheological performance and …

The dried particle material was mixed as quickly as possible with dimethyl silicone oil (density ρ=0.98 g/cm3 and viscosity η=98 mPa·s at 25 ºC) under stirring, producing the ER fluid (suspension, 30 wt.%) sample. The ER fluid was then put in the gap between the cylinders of the apparatus as soon as possible for the ER measurement. 1.2 Characterization of material

2 Results and discussion 2.1 Composition of material The result from the elemental analysis of TiTbSSA material shows that the contents (wt.%) of carbon, nitrogen and hydrogen are C: 20.05, N: 0.78, H: 3.29, respectively, which is consistent with the composition [ Tb(C7H5O6S)3]⋅[TiO(OH)2]⋅2H2O⋅0.2HNO3, of which 3

the calculation contents (wt.%) are C: 20.11, N: 0.68 and H:2.71, respectively. The result mentioned above shows that the reaction scheme that corresponds to the synthetic procedure of the material is as follows: Ti(C4H9O)4+4H2O=Ti(OH)4+4C4H9OH Ti(OH)4=TiO(OH)2+H2O 3C7H6SO6⋅2H2O+Tb(NO3)3⋅6H2O=Tb(C7H5O6S)3+ 3HNO3+12H2O 1

The ER experiments were performed on a rotary viscometer (Type Physica McR501s, Anton Paar of Germany). In this study, the sample shear stresses and viscosities have been determined under different electric field strengths (E, dc field) at a given temperature (20 ºC) and a shear rate (γ) range of 0–300 s–1. Infrared spectra (IR) of the materials were recorded with a Nicolet Magna-IR 750 spectrometer at 295 K. X-ray diffraction (XRD) analyses of the materials were fulfilled on a Bruker D8 ADVANCE X-ray diffractometer with Cu Kα radiation at a wavelength 0.15406 nm in a range of 10°–60°. The elemental analysis of the material was carried out using a German Elementar Vario EL instrument. The excitation and emission spectra of sample were determined on an Edinburgh FLS920 lifetime and steady state spectrometer. Excitation source is a Xe lamp with power of 450 W. The fluorescence lifetime of sample was obtained by using a microsecond flash lamp at a proper excitation wavelength and a frequency of 100 Hz. The micrograph of the material was obtained using a Desk scanning electron microscope (SEM) (Type PhenomproX, Phenom-World).

1

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TiO(OH)2+ Tb(C7H5O6S)3+0.2HNO3+2H2O= 3

1

[ Tb(C7H5O6S)3]⋅[TiO(OH)2]⋅2H2O⋅0.2HNO3 3

The result from elemental analysis indicates that there are water and nitric acid molecules in TiTbSSA material. Due to strong hygroscopicity of sulphosalicylic acid and rare earth nitrate, there are water molecules in the chemical formulae [C7H6O6S⋅2H2O and Tb(NO3)3⋅6H2O] of sulphosalicylic acid and terbium nitrate as the original materials because the water molecules strongly bind with C7H6SO6 or Tb(NO3)3 molecule by hydrogen bonds. Therefore, in the resultants of the coordination reactions of sulphosalicylic acid with rare earth nitrate, there are not only rare earth complex and HNO3 molecules, but also H2O molecules. Consequently, the HNO3 and H2O molecules, which have not been eliminated fully in our experimental conditions, appear in the TiTbSSA material due to the hydrogen bond action. Fig. 1(a) illustrates IR spectra of TiTbSSA, SSA and Tb(NO3)3 in a range of 650–4000 cm–1. Comparing with SSA, characteristic peak of –COOH group at 1668 cm–1 disappears in IR spectra of TiTbSSA, and the characteristic peaks of –COO− group at 1568 and 1473 cm–1 appear in IR spectra of TiTbSSSA[22], which should be the result that the carboxyl group of SSA is coordinated to the Tb3+ ion with forming the rare earth complex by the reaction between SSA and the rare earth nitrate. To compare IR spectra of TiTbSSA with that of Tb(NO3)3, the characteristic peak of NO–3 at 1383 cm–1 appears in IR spectra of TiTbSSA[22]. This result shows that there are HNO3 molecules in TiTbSSA material, which is consistent with the result of the elemental analysis. The pres-

Fig. 1 IR spectra of TiTbSSA, SSA and Tb(NO3)3

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ence of water molecules in TiTbSSA material is proved by the strong and broad vibration peak of –OH group at 3375 cm–1 [22]. In comparison with IR spectra of SSA, from Fig. 1(b), we can see that a vibration peak of O–Tb bond at 311 cm–1 appears in the IR spectra of TiTbSSA, which proves further the formation of the coordination bond between the –COO− group and the rare earth ion[23]. Fig. 2 illustrates the XRD patterns of TiTbSSA, SSA and Tb(NO3)3 samples. The XRD patterns show clearly that the characteristic peaks of SSA and Tb(NO3)3 are not present in the XRD pattern of TiTbSSA. This result can prove that there are no SSA and Tb(NO3)3 in TiTbSSA material, and SSA has reacted with Tb(NO3)3 to produce the rare earth complex, which is consistent with the result of the IR spectra. The results from the elemental analysis, XRD analysis and IR spectra can confirm that TiTbSSA is a novel material including rare earth complex. 2.2 Luminescence property of material In order to study the luminescence property of TiTbSSA, fixing the emission wavelength of the Tb3+ ion at 543 nm, the excitation spectra of TiTbSSA was obtained in the range 250–400 nm (see Fig. 3(a)). The maximum excitation wavelength of TiTbSSA is 310 nm. Fixing the maximum excitation of TiTbSSA at 310 nm, the emission spectra of TiTbSSA was obtained (see Fig. 3(b)), from which it can be seen that there are four emission fluorescence spectra bands at 487, 541, 581 and 617 nm, which are assigned to 5D4→7F6, 5D4→7F5, 5D4→7F4 and 5D4→7F3 electron transfers of the Tb3+ ion[19,24]. Fig. 3(b) illustrates the fluorescence emission spectra of TiTbSSA. From Fig. 3, we can find that TiTbSSA has higher fluorescence emission intensity. Moreover, fluorescence lifetime (647.13 µs) of TiTbSSA is higher. That is to say, TiTbSSA is a material with satisfactory luminescent performance. It is well known that the complexes of fluorescence ion Tb3+ with some carboxylic acid, especially SSA, have good luminescence property[19,25]. The coordination of H2O molecule with a fluorescence ion can weaken the fluorescence intensity of the fluorescence complex[19]. The influence of HNO3 molecule may be the same with that of H2O molecule. However, the

Fig. 2 XRD patterns of TiTbSSA, SSA and Tb(NO3)3

Fig. 3 Excitation (a) and emission (b) spectra of TiTbSSA

fluorescence emission intensity of TiTbSSA is better. The luminescence property of the composite TiTbSSA indicates that maybe H2O and HNO3 molecules do not coordinate with Tb3+ ion. The results mentioned above demonstrate that TiTbSSA as a composite containing the rare earth complex Tb(C7H5O6S)3 also displays favorable luminescent performance, even though the fluorescence complex has been included in the composite, namely the luminescence property of the complex would not be quenched by TiO(OH)2, H2O, and HNO3 in the material. Therefore, the composite material TiTbSSA with ER performance can be utilized as optical function material. 2.3 ER performance of material The dependences of the shear stress on shear rate under different electric field strengths for the suspension of TiTbSSA are illustrated in Fig. 4. The shear stress of the suspension increases linearly with increasing shear rate without an applied electric field, showing the behavior of a Newton fluid for the suspension. However, in an applied electric field, the shear stress increases with increasing the electric field strength at all shear rates, which is the characteristic of an ER fluid, the suspension shows the behavior of a non-Newton fluid. In addition, as shown in Fig. 4, under four different applied electric fields of E=1–4 kV/mm, the change in the shear stress is very small with increasing the shear rate in the range

CHEN Mingxing et al., A novel multifunction material with both electrorheological performance and …

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Fig. 4 Dependence of shear stress on shear rate under different electric field strengths for suspension of TiTbSSA

Fig. 6 Dependence of state yield stress on electric field strength for suspension of TiTbSSA

from 12.5 to 75 s–1, the shear stress increases slowly with increasing shear rate in the range from 75 to 300 s–1, indicating that the interfacial polarization between the dispersed phase and the medium in the suspension is a more important factor than the shear rate in influencing the shear stress for the studied ER fluid. That is to say, the electrostatic interaction force between the particles, which originated from the induced dipole moment caused by the interfacial polarization, dominates the shear force[26]. Namely, the fiber-shaped structure, which is established by the electrostatic interaction between the particles, is not fully broken even at higher shear rates and lower electric field strength. Moreover, from Fig. 5, the ER fluid has higher ER activity in a range of the shear rate from 12.5 to 50 s–1, the relative shear stress τr values are larger than 9, the τr value reaches 32.7 at γ=12.5 s–1 at E=4 kV/mm. Fig. 6 illustrates the dependence of the static yield stress (τy) on electric field strength for the suspension of material TiTbSSA. Like many other ER fluids, sample TiTbSSA also possesses the property that the yield stress increases with increasing the electric field strength, as the result of an increase in the polarization interactions between particles[27]. From Fig. 6, the α value of 1.53 in

equation τy=Eα has been obtained. The exponent α value of 1.53 is different from that obtained from the polarization model (i.e., 2.0)[27]. The value of α<2 can be due to non-spherical particles shape and a broad particle size distribution of TiTbSSA material, as can be seen in the SEM image from Fig. 7, and the high electric field strength (E=1–4 kV/mm) also can be a reason for making lower α value of 1.53[27]. The small polarity molecules H2O and HNO3 can enhance ER effect of an ER material[1]. According to the composition (see Section 2.1) of TiTbSSA, the contents (wt.%) of water and nitric acid in the composite are 8.65 and 3.02, respectively. Maybe the H2O and HNO3 have a role in promoting ER effect of the composite. However, the result obtained in our previous a research[21] shows that the contents of the H2O and HNO3 molecules are not a crucial factor on influencing ER performance for some composites. The influence of small polarity molecules such as H2O and HNO3 on ER performance of a material still needs to be studied in depth. The TiTbSSA composite is a material that has higher ER activity, and it has high breakdown strength (The current density is approximately 10 μA/cm2) in a large shear rate range under an electric field. These characteristics of TiTbSSA composite are beneficial in its application as an ER material.

Fig. 5 Dependence of relative shear stress on shear rate under an electric field of E=4 kV/mm over zero-field for suspension of TiTbSSA material

Fig. 7 SEM image of TiTbSSA material

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3 Conclusions

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A novel multifunction material was synthesized. The composition, ER performance and luminescence property were studied. The results showed that the material containing Tb(C7H5O6S)3, TiO(OH)2 and a small amount of water and HNO3 was a material with both ER performance and luminescence property which would have wide application prospect.

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