Ni composite particles and their electro-magneto responsive properties

Materials Science and Engineering B 221 (2017) 54–62

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Fabrication of BaTiO3/Ni composite particles and their electro-magneto responsive properties Yaping Lu a,b, Lingxiang Gao a,b,⇑, Lijuan Wang a,b, Zunyuan Xie a,b,⇑, Meixiang Gao c, Weiqiang Zhang a,b a

Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), Ministry of Education, Xi’an 710119, PR China School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710119, PR China c Yulin Vocational and Technical College, Yulin 719000, PR China b

a r t i c l e

i n f o

Article history: Received 8 January 2017 Received in revised form 29 March 2017 Accepted 3 April 2017

Keywords: BaTiO3/Ni Composite particles Electro-magneto response Hydrogel elastomer

a b s t r a c t BaTiO3 (BT)/Ni composite particles were made by one-step method through agglomerating the metal Ni (0) nanoparticles reduced by a specific reducing agent (N2H4
H2O) on the surface of BT sphere with diameter of 500 nm. The BT/Ni composite particles were characterized by the means of scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS). In BT/Ni particles, pure BT spherical particle was coated with Ni nanoparticles agglomerated on its surface. The average thickness of the Ni sheath was 30 nm and the content of Ni(0) and Ni (II) in the sheath were 70.2% and 29.8%, respectively. The responsive effects of BT/Ni particles filled in hydrogel elastomer were investigated by the viscoelastic properties. The results indicate that the BT/Ni particles exhibit electro and magneto coordinated responsive properties (E = 1 kV/mm, H = 0.1 T/ mm), which is superior to BT particles with individual electro response. Ó 2017 Published by Elsevier B.V.

1. Introduction Smart functional materials have attracted considerable attention in both academia and industry because of their precisely controllable responses to external stimulus, including mechanical stress, temperature, pH, light and electric or magnetic field (particularly for electric or magnetic fields) [1–7]. Among these, the electric and magnetic stimuli-responsive fluids possess smart responsive characteristics (the quick response and fine tuning) that are applied in various devices, such as clutches, absorbers, seismic vibration dampers, human muscle stimulators, actuators, optical finishing systems, medical therapies, micro-fluidic control, and viscosity reduction of crude oil [8–14]. Liu et al. [1]reviewed the mechanism, rheological analysis and dielectric characteristics of various electrorheological fluids, including organics, semiconducting polymers and their hybrids. Zhang et al. [3]systematically investigated many types of magneto rheological fluids as well as their fabrication strategies. Electromagnetic composite materials as an important intelligent material, which consist of dielectric materials, magnetic materials or double responses materials in electric and magnetic field, have become a new research hotspot in ferroelectric, ferro⇑ Corresponding authors at: School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710119, PR China. E-mail addresses: [email protected] (L. Gao), [email protected] (Z. Xie). http://dx.doi.org/10.1016/j.mseb.2017.04.001 0921-5107/Ó 2017 Published by Elsevier B.V.

magnetic materials field. These materials are widely used in electric field sensors, microwave devices and electromagnetic transformers [15–17]. In the past a few years, they have many complex patterns, such as physical-blended, laminated composite materials and composite films [18], and were studied for electromagnetic application. However, the practical applications of these materials are still hampered by irregular shape, broad size distribution and sedimentation of particles up to now. In order to overcome these defects, the stable coated composites of electromagnetic materials with uniform size and distribution should be studied. As an important dielectric material, BT is well-known for its high dielectric constant, low dielectric loss, ferroelectric, piezoelectric and positive temperature coefficient effect, which is a good electrorheological materials and have wide applications in sensors, actuators, many others electronic and electro-optical devices [19–25]. What’s more, the dielectric properties of BT can be tuned by controlling the size, shape, and phase easily [26–28]. Recently, BT-based multi-responsive particles have become a kind of important smart materials [29]. Wang et al. [30,31]reported the electrorheological behavior and magnetic properties of flower-like BT@Fe3O4 hierarchically structured particles and Fe3O4@TiO2 core-shell submicron-spheres. However, there are no report on the electro and magneto coordinated properties of BT particles coated by paramagnetic metal, for example, metal Ni which displays super-paramagnetic behavior [32].

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Based on the above research achievements, an inspired design spontaneously arises from our mind. In this study, we combined electro-response material with magneto-response material to fabricate the composite particles possessing electro-magneto double response performances. In this paper, Ni-coated BaTiO3 (BT/Ni) particles with excellent submicro-nano structure are fabricated through that pure BT spherical particle was coated with Ni nanoparticles agglomerated on BT surface by a one-step liquid phase reduction method. BT/Ni composite materials simultaneously covers the characteristics of BT and Ni, so compared with a single material, its performance will be obvious expansion, especially the dielectric and magnetic properties are improved together. The liquid phase reduction method is a common and effective method because the method consumes less resource with high production efficiency and the prepared particles has a small size in narrow distribution [33–36]. In our work, the Ni nanoparticles are prepared by the liquid phase reduction method of making Ni particles via the reduction of nickel hydrazine complex precursors. The method shows the advantages of using half dosage of hydrazine for complete reduction of nickel ions in solution and the obtained Ni particles with better dispersion. The optimum preparation conditions were obtained by studying the influences of the appropriate amount of reductant, reaction temperature, pH, concentration of Ni2+ ions on the Ni particle size, morphology, and the dispersion [37]. At the exploration stage, we adopted two-step method of reduction independent Ni particles and agglomerated them on the surface of BT via electrostatic interaction by adjusting pH value. But the result was not well that Ni particles self-reunite seriously due to their extremely small size. Based on the experience of exploring, we adopt the method of one-step cladding to restore nickel acetate from Ni(Ⅱ) to Ni(0) on the surface of BT by controlling the reduction rate in the liquid system. The SEM measurement reveals the coated composite particles obviously.

2. Experimental 2.1. Materials Tetrabutyl titanate (TBT, Ti(OC4H9)4) and acetonitrile (CH3CN) were received from Tianjin Kemiou Chemical Reagent. Ethanol (C2H5OH) and ammonia (NH3
H2O) were purchased from Sinopharm Group Chemical Reagent. The above four reagents were used as the precursor of Ti, solvent (CH3CN and C2H5OH mixture), hydrolysis agent to prepare TiO2 by sol–gel method. Barium hydroxide (Ba(OH)2
8H2O) was obtained from Sinopharm Group Chemical Reagent and it was the Ba raw materials to prepare BT particles with the precursor TiO2 via hydrothermal synthesis method. Nickel acetate (C4H6O4Ni
4H2O), ethylene glycol ((HOCH2)2), hydrazine hydrate (N2H4
H2O) and sodium hydroxide (NaOH) were supplied by Guangdong Guanghua Reagent, Sinopharm Group Chemical Reagent, Tianjin Fuchen and Tianjin Kemiou Chemical Reagent, respectively. They were used separately as the source of Ni, solvent, reducing agent and pH regulator to prepare Ni(0) nanoparticles by liquid phase reduction method. Gelatin, glutaraldehyde(C5H8O2), glycerol(C3H8O3) were the products of Aladdin, Tianjin Fuchen and Sinopharm Group Chemical Reagent, respectively. They were used to prepare the particles/gelatin composite hydrogel elastomers which act as polymer matrix, stabilizer and cross-linking agent. All chemicals used in this experiment were of analytical grade and used without further purification. The water used throughout this work was deionized produced by water purification system of Qianyan Technology.

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2.2. Preparation of BT/Ni composite particles 2.2.1. Synthesis of BT core particles (BT-core) Firstly, mono-dispersed TiO2 particles were prepared by the sol–gel method. Typically, 0.5 mL of NH3
H2O was added into the mixture of 25 mL C2H5OH and 35 mL CH3CN, and was kept stirring for 30 min in ice-water bath, named solution A. 1.7 mL of TBT was added into the mixture of 25 mL C2H5OH and 35 mL CH3CN, and was kept stirring for 30 min at ambient temperature, named solution B. Then, the obtained solution B was dropped into the solution A with stirring for about 2 h, and aged in a closed container with static condition for 8 h at room temperature. Finally, the precursor TiO2 was centrifuged and washed with ethanol and de-ionized water several times, and dried at 80 °C in the oven [38–42]. Secondly, 0.95 g precursor TiO2 was dispersed into 70 mL water, and was kept stirring for 1 h in room temperature to get a uniform suspension. 3.79 g Ba(OH)2
8H2O was poured into the suspension and the reaction was stirred for another 1 h. The resulted suspension subsequently transferred to a Teflon-lined stainless steel autoclave. The autoclave was sealed and then kept heating at 100 °C for 24 h. After the completion, the reaction suspension was cooled naturally to room temperature and treated by filtration, washing and drying to get the BT spheres with diameter of  500 nm [42–44]. 2.2.2. Preparation of BT/Ni coated particles BT/Ni microspheres were prepared by a simple one-step method and the procedure as follows: Firstly, 0.25 g of C4H6O4Ni
4H2O was dissolved in 20 mL of ethylene glycol with vigorous stirring to form a uniform solution at 55 °C. BT spheres were dispersed into the mixture solution with the help of an ultrasonic bath and further mechanical stirring homogenously. Then, N2H4
H2O (0.97 mL, 85 wt%) was slowly dropped into the system with vigorously stirring until the purple color appears. Subsequently, NaOH solution (2 mL, 1 M) was added into the above reaction system to adjust the pH value of 10.0–10.7. After stirring for 45 min, the target product could be obtained when the color turned black. The precipitates were isolated by magnet, washed with ethanol and water several times, then dried at 50 °C in vacuum oven over 10 h. The obtained BT/Ni microspheres were sealed in vials [45,46] . 2.3. Material characterization The morphology of the products was examined by scanning electron microstructures (SEM, Quanta FEJ250) and transmission electron microscopy (TEM, JEM-2100). The zeta-potential was determined using by electrophoretic light scattering using the Delsa Nano C Analyzer (Beckman Coulter, USA). The surface component analysis of the particles was characterized by the X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra spectrometer, monochromatized Al Ka radiation source (ht = 1486 eV). All peak values have been calibrated by using the C 1s peak at 284.8 eV as reference). The relative dielectric property was indicated by the dielectric constants of the suspensions dispersed the dried particles with a volume fraction of 6% in silicone oil (g = 50 mPa s at 25 °C)[47]. The dielectric spectra of the suspensions were measured by an impedance analyzer (HP 4284A) in the frequency range from 20 Hz to 105 Hz using a measuring fixture (HP 16452A) for liquid. The magnetic property of the particles were measured by Agilent E4991A impedance analyzer and vibrating sample magnetometer (VSM, BHV-50HTI, Riken Denshi) at room temperature. The electro and magneto responses properties were investigated by Q800DMA dynamic viscoelastic spectrometer under static-press multi-frequency mode in the range of frequency 1– 10 Hz at room temperature. Each measurement was carried out

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and repeated at least three times. The responsive elastomers were prepared as follows: 1.0 wt% particles was dispersed uniformly in gelatin/glycerol/water mixture system at 65 °C. Subsequently, a small amount of glutaraldehyde was quickly added into the mixture as the cross-linking agent and the mixture was transferred

into two plexiglas boxes (40  20  8 mm3) quickly. Under the same condition, the mixture in one mold was cured with external dc electric field (1.0 kV/mm) and magnetic field (H = 0.1 T/mm) (Fig. 1) for 50 min (remained at 65 °C for 30 min, and then cooled down from 65 °C to room temperature for 20 min), while the other

Fig. 1. Schematic of custom plexiglas mold with co-ordinated electric field and magnetic field.

Fig. 2. Typical SEM and TEM images of as-synthesized (a, c) BT and (b, d) BT/Ni particles.

Fig. 3. Schematic of possible formation process of BT/Ni composite particles.

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Fig. 4. Zeta potentials of BT particles in EG as a function of pH. Fig. 5. XRD patterns of BT particle (a) and BT/Ni composite particle (b).

Fig. 6. XPS spectrum of BT/Ni composite particles (a)wide spectrum and (b) survey scan of the Ni 2p3/2 region.

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mold without any field. The curing progress went on at room temperature for another 7 h without the external fields applied for each mixture. Finally, the particle/gelatin composite hydrogel elastomers were prepared, which were named FE (free-elastomer, cured without any field), ERE (electro-responsive elastomer, cured under 1.0 kV/mm electric field), MRE(magneto-responsive elastomer, cured under 0.1 T/mm magnetic field) and EMRE (electro-magneto-responsive elastomer, cured under coordinated 1.0 kV/mm electric field and 0.1 T/mm magnetic field), respectively. 3. Results and discussion 3.1. Characterization of particles Fig. 2 shows the SEM and TEM images of bare BT particles and BT/Ni composite particles. The bare BT particles, as shown in Fig. 2a, have a narrow size distribution of 500 nm and disperse uniformly. Their surfaces are smooth, as shown in the magnified TEM of Fig. 2c. When the Ni nanoparticles gathered on the surface of BT with liquid phase reduction reaction, the morphology of products has a significant change. The surfaces of the composite particles are no longer smooth and the average diameter of particles increases to 528 nm, as shown in Fig. 2b. Compared with the BT, the thickness of the Ni in BT/Ni composite particles is 30 nm, shown in Fig. 2d. The SEM and TEM photos indicate that the Ni sheath are evenly distributed on the surface of BT and no free Ni particles are observed, revealing that BT and Ni are combined successfully. A schematic illustration of the possible forming mechanism of BT/Ni core-shell particles is shown in Fig. 3. Follow it, there are two important reaction stages. At the first stage, hydrazine reacts with nickel acetate (C4H6O4Ni
4H2O) to form Ni-hydrazine complexes in suspensions (Eq.(1)). 2þ

Ni

þ xN2 H4 $ ½NiðN2 H4 Þx 

2þ

ð1Þ

When BT spheres were dispersed into the suspension and the pH value of was adjusted to pH > 10, [Ni(N2H4)x]2+ can combine well on the surface of BT particles. The reason can be gotten from

Fig. 4 that shows the zeta potential values of the BT particles in EG as a function of pH value. It is found that the zeta potentials of BT particle was f  13 mV when pH > 10, meaning the surface of BT particles has enough negative charge. Based on the result, [Ni (N2H4)x]2+ carrying positive charge combines BT particle well with the help of electrostatic interaction. At the second stage, [Ni(N2H4)x]2+ was reduced to Ni in the alkali system, as following equation:

2½NiðN2 H4 Þx 

2þ

þ 4OH ! 2Ni # þN2 þ 4H2 O þ 2ðx  1ÞN2 H4

ð2Þ

According to the literature [48,49], the zeta potential of Ni particles was f = +40–0 mV between 4.3 < pH < 10.7 in aqueous system, meaning that Ni particles has positive charge. So, for firm gathering Ni particles on BT surface while not destroying [Ni(N2H4)x]2+ combination, NaOH solution was dropped to the suspension for adjusting the pH value to 10–10.7. Finally, the compact and continuous Ni was constructed on the surface of BT particle. Fig. 5 shows XRD patterns of the BT particle and BT/Ni composite particle. The peaks in Fig. 5a are sharp and match well with JCPDS No. 89-1428, suggesting the high purity of BT particles. The XRD patterns of the as-precipitated BT/Ni composite particles showed in Fig. 5(b), which is different from Fig. 5a. Extra three characteristic peaks for nickel (2h = 44.5°, 51.8°, and 76.4°), corresponding to miller indices (1 1 1), (2 0 0), and (2 2 0) were observed. The appearance of those peaks demonstrated that the resultant particles have face-centered cubic Ni (JCPDS, No.04-0850). The results indicate that BT/Ni composite particles possess cubic BT as core and face-centered cubic Ni as sheath [23,50]. XPS spectrum analysis usually focuses on the surface elemental composition as well as its microenvironment, which regards as a powerful measurement to confirm the chemical states and valence of the elements. So the chemical states and valence of the elements in the volume of BT/Ni composite particles could be studied through XPS shown in Fig. 6. As seen in the XPS spectrum of the BT/Ni composite particles (Fig. 6a), these peaks corresponding to C, Ti, O, Ba and Ni are clearly identified, indicating that the surface of BT/Ni composite particles mainly contains these elements. The relative elements concentrations of C, O, Ni, Ba and Ti on the surface of particles were determined from the spectra of BT/Ni com-

Fig. 7. Dielectric spectra of suspensions containing BT/Ni composite particles and BT particles (6%, T = 25 °C).

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posite particles integrated peak areas, leading the conclusion that the relative elements concentrations of Ni is about 13%, C is about 36%, O is about 47%, and the remaining part about 4%, which may be Ba and Ti element. Products were washed by ethanol as the detergent to prevent oxidation in the last washing process, amounts of ethanol was stranded in the surface of the particles. That is why did C and O element exist on the surface of BT/Ni composite particles . Fig. 6b shows a 2p3/2 (850-870 eV region) high resolution XPS spectrum (a) of Ni element. Ni 2p3/2 presents a main peak (b) at 852.5 eV with an intense satellite structure (d) at 858.2 eV, this signature is characteristic of Ni(0) [51,52]. The prominent bands (c) observed at 855 eV can be attributed to Ni(II) 2p3/2 [53]. By comparing to the spectral data for NiO [54], these bands indicate the presence of a small amount of NiO which can be formed as a thin oxide film on the surface of Ni(0) nanoparticles during XPS testing when the sample are exposed to air for a few minutes [55,56]. The other reason probably that C4H6O4Ni were adsorbed onto the sur-

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face of BT core and not be reduced. The peak areas were calculated by XPS software that the contents of the core-level spectra for Ni(0) and Ni(II) are 70.2% and 29.8%, indicating that the surface of the particles present large amounts of Ni(0) element. The electric field frequency dependence of dielectric constant of the BT and BT/Ni composite particles has been studied with a broadband dielectric spectroscopy at room temperature. As shown in Fig. 7, in this work the relative dielectric constant of particles was indirectly presented, and a higher dielectric constant of the suspension indicated a higher relative dielectric constant of the corresponding dispersed particles. By comparison, the relative dielectric constant of BT/Ni composite particles is weaker than that of BT particles. The value of magnetic permeability gives an indication of how easily a given material can be magnetized. The relationship between magnetic conductivity (lr) of the particles and magnetic field frequency is shown in Fig. 8a. For bare BT, as is shown, the lr keeps about 1, during to the non-magnetic nature of BT. After

Fig. 8. (a) The magnetic permeability spectra for BT/Ni and BT particles and (b) hysteresis loop measured at room temperature for BT/Ni particles.

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coating Ni shell on the surface of BT, it is clearly found that the obtained BT/Ni composite particles exhibit obviously increased lr value (3.5–5.0), although the value is affected by the frequency fluctuation with slight declining between the adequate frequency range of 3 MHz–10 MHz. Through the analysis of the magnetic permeability data, the conclusion can be drawn as follows: BT/Ni composite particle is a kind of magnetic material, and Ni sheath contributes to the magnetic permeability of BT/Ni composite particles. The M-H curve of the as-synthesized BT/Ni composite particles measured at room temperature is shown in Fig. 8b. From the figure, it can be clearly seen that the saturation magnetization (Ms) value of the BT/Ni composite particles is 2.5 emu
g1 at room temperature, suggesting the BT/Ni composite particles possesses strong magnetism. 3.2. Electro and magneto response of particles in hydrogel elastomer The polarizing microscope images from the sliced hydrogel ealstomers filled with different mass fraction BT/Ni composite particles (0.8 wt%, 1.0 wt%, 1.2 wt%) are shown in Fig. 9. The BT/Ni composite particles (Fig. 9(a–a2)) are freely dispersed in the elastomers with no applied field (FE) during the curing progress and its structure is isotropic. In ERE or MRE, when an electric or magnetic field applied during the curing process, BT/Ni composite particles are arranged into aligned chains paralleling to the direction of extra field, indicating that BT/Ni composite particles in hydrogel elastomer has response to electric and magnetic field, but the chain structure is loose, low density and the potential of aggregation of chain structure is not obvious (shown in Fig. 9(b–b2) and (c–c2)). When an electric and magnetic field were applied coordinately during the curing progress, the structure of BT/Ni composite particles with a thick and dense chain in the direction of the coupling fields (Fig. 9(d–d2)), resulting that the particles has strong electro-magneto response. With the increasing mass fraction of BT/Ni composite particles, the chain alignments were more and more obvious and compact. But when beyond a certain value, the

movement of the particles would be limited with the polymer network and resulting of poor response. From the following pictures, Fig. 9d1 owns the optimum chain alignment orderly. The Fig. 10a shows the dependence of storage modulus(G) of different elastomers filled with 1.0 wt% of BT and BT/Ni composite particles under different curing conditions. Curve k (k0, k1, k2, k3) presents G of the gelatin hydrogel elastomers without any particle. It is shown that the curves k (k0, k1, k2, k3) almost coincide with each other, indicating that pure gelatin hydrogel elastomer has no obvious response to extra field. Curves a0–a3 presents G of FEs, EREs, MREs and EMREs filled with the BT particles, and curves b0–b3 presents G of corresponding elastomers filled with BT/Ni composite particles respectively. It is found that curves a0–a3 and b0–b3 are all above curves k, illustrating the filling role of particles in the elastomers. The G of ERE, MRE and EMRE filled with BT/Ni composite particles are increased in the order of electric, magnetic and electromagnetic fields, illustrating that BT/Ni composite particles shows superior response to electric and magnetic field. In that, the response to magnetic field is stronger than that to electric field, attributed to a amount of Ni(0) on the surface of BT/Ni composite particles. While for BT particles, in Fig. 10a, GFE  GMRE and GERE  GEMRE, illustrating that BT particles do not have obvious response to the magnetic field and the main contribution of electromagneto-response is electro-response. As shown in Fig. 10b, it is worth noting that the modulus increment sensitivity(DG/GFE) of the elastomers filled by the BT/Ni composite particles compared with that filled by the bare BT particles. Under electric field, DG/GFE of the elastomer filled by BT particles is larger than that filled by BT/Ni composite particles, however, under magnetic field and coupling field, DG/GFE of the elastomer filled by BT/Ni composite particles is larger than that filled by BT particles. Further, DG/GFE of the elastomer filled by BT/Ni composite particles reach the values of 55% under coupling fields while that the latter reach the value of 30%. The result reveals again that BT/Ni composite particles have significant electro and magneto response coordinately, superior to BT particles.

Fig. 9. Polarizing microscope images: the sliced ealstomers filled with BT/Ni composite particle of the mass fraction is 0.8 wt% (a-d), 1.0 wt% (a1-d1), 1.2 wt% (a2-d2), respectively. (a, a1, a2), (b, b1, b2), (c, c1, c2), (d, d1, d2) separately represent FE, ERE, MRE and EMRE.

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Fig. 10. (a)The Storage modulus (G) of FE/ERE/MRE/EMRE- gelatin (k0, k 1, k 2, k 3), FE/ERE/MRE/EMRE-BT (a0, a1, a2, a3) and FE/ERE/MRE/EMRE-BT/Ni (b0, b1, b2, b3), and (b) modulus increment sensitivity of FE/ERE/MRE/EMRE-BT (a0 1, a0 2, a0 3) and FE/ERE/MRE/EMRE-BT/Ni (b0 1, b0 2, b0 3) as a function of frequency.

4. Conclusions

Acknowledgements

This paper demonstrates a facile approach to fabricate multifunctional BT/Ni composite particles by a liquid phase reduction method. The as-prepared BT/Ni composite particles were characterized by SEM, TEM, XRD, XPS, etc. leading to conclude that the BT and Ni particles have excellent cubic structure, the average thickness of Ni was 30 nm, and the content of Ni elements were 13% (70.2% to Ni(0), 29.8% to Ni(II)). Importantly, the BT/Ni composite particles has fine dielectric and magnetic properties, and so it possesses significant responses both to electric and magnetic field simultaneously in hydrogel elastomer. Inorganic/inorganic particles, like as BT/Ni composite particles, have outstanding responses both to the electric and magnetic field. This result is used to boost the development of the particles/polymer composite hydrogel elastomer possessing electro-magneto-responsive properties.

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