A Smart Polymer Composite Based on a NiTi Ribbon and a Magnetic Hybrid Material for Actuators with Multiphysic Transduction

A smart composite material constituted of a magnetic hybrid film and a NiTi shape memory alloy (SMA) ribbon was obtained and characterized. The magnetic hybrid film was joined to the NiTi ribbon in order to combine the properties of both materials. This new composite material combines magnetic properties of the hybrid film, (Fe2O3-CMC)/(polyvinyl butyral), and the shape memory properties of the NiTi ribbon, which has a chemical composition of Ti-50.13 at. % Ni. This smart composite material has a mass of 18.3% NiTi ribbon and 81.7% magnetic hybrid film. Results obtained by DSC show that the smart composite material presents a small delay of transformation during warming and cooling because the magnetic hybrid film acts like a thermal insulator. Thermomechanical results indicate that the hybrid material also acts as a mechanical reinforcement, since it is observed that the Stress-Assisted Two-Way Memory Effect (SATWME) of the smart composite is lower than the SATWME of the SMA ribbon. The density current values of phase transformations were clearly identified with a thermomechanical apparatus developed in our laboratory. Finally, displacements of the smart composite material in cantilever configuration are obtained by applying an external magnetic field. All these results demonstrate that the smart composite material can be OPEN ACCESS Actuators 2015, 4 302 activated by temperature, electrical current, stress, and/or magnetic field, offering good expectations for actuating applications with multiphysic transduction.


Introduction
Composite materials should acquire properties of those materials forming the new material. They can even have different properties than the original ones. These kinds of materials are sometimes multifunctional or smart materials, and they can modify their mechanical or physical properties when a stimulus (thermal, electric, magnetic, etc.) is applied, and return to their original properties when the stimulus withdraws. Composites based on shape memory alloys (SMAs) are regarded as promising candidates for the so-called "smart composite materials" [1]. In this sense, shape memory composites, with either embedded or bonded shape memory material systems, provide tremendous potential for creating new paradigms in material-structural interactions [2][3][4][5]. The demand of shape memory composites for engineering and technical applications has been increasing in numerous commercial and industrial fields [2,3,6,7]. The possible applications include microgrippers, force sensors, microrelays, microvalves, micropumps, etc. [8][9][10][11][12]. The objective of this article is to present the first results of a smart composite material with multiphysic transduction that can be activated by several stimuli such as electrical current, temperature, stress, and magnetic field. In this novel material, the NiTi shape memory effect is combined with the viscoelastic, electrical and magnetic properties of a magnetic hybrid polymer consisting of iron oxide (Fe2O3) nanoparticles coated with carboxymethyl cellulose (CMC), and embedded in a polymeric matrix of polyvinyl butyral (PVB). The SMA, with a chemical composition of Ti-50.13 at. % Ni, is a ribbon-shaped material fabricated by the melt-spinning technique. The magnetic behavior of the (Fe2O3-CMC)/PVB hybrid material was described by López-Walle et al. [13]. Dynamic mechanical analysis (DMA) and thermomechanical tests for a similar composition of the SMA ribbon were discussed by Lomas-Gonzá lez et al. [14]. Here, the manufacturing and synthesis processes of each one of the constitutive materials and the composite material are first described. Then, the morphological characterization by SEM and the composition analysis of the smart composite material are shown. Differential scanning calorimetry (DSC) results for the NiTi ribbon, the hybrid material and the composite material are analyzed and compared. Finally, the multiphysic transduction by thermomechanical tests and magnetic experimentations is presented. Results validate multiphysic transduction for future interesting actuating applications.

Manufacturing and Synthesis Processes
As mentioned, the smart composite material is based on a NiTi SMA ribbon enclosed by a (Fe2O3-CMC)/PVB magnetic hybrid film. This section describes the manufacturing process of each one of these materials.

NiTi SMA Ribbons
The SMA ribbons were prepared at the Universidade Federal de Campina Grande, Brazil, by the single-roller melt-spinning technique. First, NiTi ingots were fabricated via a vacuum induction melting (VIM) process using a chemical composition of Ti-50.13 at. % Ni. Then, the alloy ingots were loaded in a quartz crucible with a nozzle diameter of 1.0 mm under a 200 mbar argon atmosphere. Finally, the molten alloy was ejected onto the surface of a polished copper wheel with a tangential velocity fixed at 30 m/s. The dimensions of the continuous obtained ribbons are 30 ± 5 μm in thickness and 1.7 ± 0.3 mm in width. Figure 1a shows a schema of the melt-spinning process and Figure 1b shows a sample of the NiTi ribbon. More information about the preparation of the SMA ribbons is described by Nuñez-Mendoza et al. [15].

Magnetic Hybrid Material
The magnetic hybrid material was prepared at Centro de Investigación, Innovación y Desarrollo en Ingenierí a y Tecnologí a of the Facultad de Ingenierí a Mecá nica y Elé ctrica (CIIDIT-FIME), at the Universidad Autónoma de Nuevo León (UANL). It consists of iron oxide (Fe2O3) nanoparticles (~4 nm) coated with carboxymethyl cellulose (CMC), embedded in polyvinyl butyral (PVB). López-Walle et al. reported all the details about the synthesis process [16,17]. Figure 2 depicted this process. In summary, a film of Fe2O3-CMC is synthetized, pulverized and dispersed in a polymer matrix of PVB, finally obtaining the (Fe2O3-CMC)/PVB magnetic film.

Smart Composite Material
In order to combine the magnetic and dielectric properties of the polymer film with the shape memory properties of the ribbons, the first one is joined to the second one by compressing them over 30 min at 348 K, as illustrated in Figure 3a. A sample of the obtained composite material, called M1, is shown in Figure 3b.
The next section deals with the characterization of this smart composite material.

Characterization
Several characterization techniques have been employed to identify the morphology, composition and transformation temperatures of the composite material.
The morphology of the composite material was observed in a NOVA NANO SEM 200 (FEI) ion scanning electron microscope (SEM).
Then, the mass fractions of its composing materials (percentage of NiTi ribbon mass and percentage of magnetic hybrid film mass) are calculated.
The transformation temperatures of the NiTi ribbons, the magnetic hybrid film and the composite material are determined by differential scanning calorimetry (DSC), using a Diamond DSC from Perkin Elmer. These tests are carried out at 5 K/min cooling and heating rates and the temperature scanning range was from 273 K to 393 K. The masses of the samples are approximately between 5 and 8 mg. Results and discussion of these characterizations are presented in next paragraphs.

Morphology and Composition Analysis of the Smart Composite Material
The smart composite material has been observed transversely by SEM in order to verify the adhesion between the magnetic hybrid material and the NiTi ribbon. In the section in Figure 4, red arrows show the zones where the magnetic hybrid film is not fully adhered to the SMA ribbon. Nevertheless, as described in the next sections, the adhesion between both materials is sufficient to consider that the obtained material is a composite material whose behavior is the result of the contribution of both materials: the magnetic hybrid material and the SMA ribbon.
Additionally, the mass fractions of both materials were empirically calculated. The length of the obtained material is about 3.5 cm and the masses of the NiTi ribbon and the magnetic hybrid film are 8.5 mg and 46.5 mg, respectively. Thus, the NiTi ribbon represents 18.3% of the mass of the composite material, and the magnetic hybrid film corresponds to 81.7%.

DSC Study of SMA Ribbons, Magnetic Hybrid Material and Smart Composite Material
In this section, the phases and transformation temperatures obtained by DSC are described. For this, the NiTi ribbon (sample M1), the magnetic hybrid film and the developed composite material were tested. Figure 5 shows a comparison of their three respective DSC curves. As observed in these curves, the interesting temperatures, described in the next paragraphs, occur in a similar range of values.
The DSC lets us know the thermal transitions of the magnetic hybrid material, such as its glass transition temperature (Tg). The Tg for the PVB is 342 K, approximately [18]. Furthermore, it has been found that the magnetic nanoparticles cause a significant decrease in Tg until about 327 K. This phenomenon has been reported by Kirchberg et al. [18] and is validated here.
The DSC curve of the SMA ribbon M1 is also presented. It shows that the reversible martensitic transformation is observed for the NiTi ribbon fabricated by melt-spinning. This reversible transformation produces the shape memory effect. During cooling, a small imperfection is observed on the peak (see the red circle on Figure 5). We assume that this imperfection is caused by the presence of the R-phase [15]. Thus, the transformation seems to occur in three steps, from austenite to R-phase and finally to martensite. R-phase is relatively common in this kind of alloy [19,20]. The transformation temperatures obtained from this DSC curve are displayed in Table 1.  Finally, the composite material was analyzed in order to compare its response to those of its constituent materials. As previously mentioned, the mass of the magnetic hybrid material is predominant. The behavior of the composite material is then affected by the insulating properties of the polymer. On the one hand, the intensity of the peaks is clearly reduced. On the other hand, the transformation temperatures are also affected. The temperature of the beginning of the austenitic transformation increases about 5 K (325.4 K for the composite material and 319.9 K for the NiTi ribbon) and the beginning of the martensitic transformation increases its temperature by 2 K (309.2 K for the composite material and 311.9 K for the NiTi ribbon). Table 2 shows the transformation temperatures for the composite material. In summary, the shape memory transformations and polymer transformation are still present in the DSC curve corresponding to the composite material. So, at this point, it is expected that the composite material could be activated by mechanical, thermal (by temperature or electrical current via Joule effect) and magnetic excitations.

Multiphysic Transduction
The composite material can be activated by electrical current, temperature, stress or magnetic field. In this section we observe, and sometimes compare, the thermomechanical and magnetic behaviors of the composite material when it is stimulated by electrical current, considering different amounts of stress and magnetic fields. This study is necessary for future applications of this composite material.

Thermomechanical Behavior by Electrical Stimulus of SMA Ribbons and Composite
In order to compare the thermomechanical behavior of the NiTi ribbons and the composite material, we use an automated system developed in our laboratory [17]. In this equipment, the sample is heated by Joule effect and cooled in air by natural convection. Every cycle is performed under different amounts of constant stress in order to obtain complete cycles of the Stress-Assisted Two-Way Memory Effect (SATWME). During heating, a change of phase from martensite (B19') to austenite (B2) is produced, promoting a dimension change (principally in length) in the SMA. This change is measured with a linear variable differential transformer (LVDT) sensor. The reverse transformation occurs throughout cooling. The initial dimensions of the SMA ribbon are 1.2 × 12 × 0.03 mm 3 . An example of a curve of strain (%ε) vs. current density (A/mm 2 ) of a complete cycle of a SATWME cycle obtained with this equipment is presented and explained in Figure 6. In order to assure electrical continuity in the circuit, both extremities of the composite were grabbed directly from the NiTi ribbon with the metallic clamps of the thermomechanical machine. This allows us, on the one hand, to heat and achieve the phase transformations of the SMA ribbon without damaging the hybrid material and, on the other hand, to compare the mechanical response of the ribbon with and without hybrid material, both at the same level of applied stress. Figure 7 shows the SATWME plots of the NiTi ribbon and the composite sample for two stresses, 0 MPa and 85 MPa.
The current density values for phase transformations were obtained from these curves using the tangent method [21][22][23][24]; results are displayed in Table 3. From these results, the effect of the stress on current transformations can be deduced: the greater the applied stress, the greater the current density and, consequently, the transformation temperatures. In fact, transformation temperatures are analogous to current densities.
Additionally, the start phase transformations are retarded in the composite material due to the thermal and electrical insulating properties of the polymers. So, comparing both samples, the NiTi ribbon needs smaller amounts of current density to start the austenitic transformation during heating and, during cooling, to begin the martensitic transformation.
Concerning strain, the effect on it caused by the stress when the current density increases is reduced in the composite material compared to the NiTi ribbon. That is because the polymers in the composite materials act as a mechanical baking.  The thermomechanical study reported in this section let us observe the behavior of the developed composite material when it is activated by electrical current and temperature, via Joule effect, also considering the effect of stress. These results show that martensitic transformation is carried out in the NiTi ribbon inside the composite material. So, the SMA ribbon provides the composite material, for instance, with the ability to do work, to lift loads and to conduct electricity. The response of an external magnetic field is presented in the next section.

Magnetic Response of the Composite Material
The displacement of the magnetic hybrid material and the composite material as a response of an applied external magnetic field is the focus of this section. Each sample is placed in a cantilever beam configuration as observed in Figure 8. An external magnetic field produced by applying an electrical current to an electromagnetic coil causes its bending (see Figure 9); when the magnetic field is turned off, the composite material returns to its original position. The movements of the samples are recorded with a Leica IC80 HD camera mounted on a Leica M80 stereomicroscope. The analysis of the images is done with the LAS Interactive Measurement software.  The samples have been placed at different initial distances from the electromagnet. Figure 10a depicts the displacement curves of the magnetic hybrid samples, with different lengths, as a function of the initial distance between the sample and the electromagnet. The applied magnetic field is 500 Oe. This value has been measured with a Tesla/Gauss Meter KOSHAVA 5. As expected, if the sample is closer to the electromagnet or if it is longer, the displacement increases. The relationship between the physical properties and the displacement response is a topic of actual research. Table 4 summarizes the maximal displacement of each sample, according to its length. Figure 10b shows the displacement curves of the composite material versus the initial distance between the sample and the electromagnet for three applied magnetic fields: 500, 2000, and 5000 Oe. The length of the tested sample is 35 mm. The response of the composite material is similar to that of the magnetic hybrid material. Additionally, by applying 5000 Oe, the maximal displacement is near 5 mm, a good displacement for actuating applications, despite the high energy required. This composite should also be used in sensing applications. For example, it could be implemented as a sensor during magnetic flux leakage inspection for the oil and gas industry in order to detect corrosion zones in steel by distinguishing changes in the magnetic field between steel and steel oxides.  In this section, the experimental response of the smart composite material has been evaluated. On the first hand, results confirm that this novel material can be activated by different stimuli: electrical current, temperature (via Joule effect), stress and magnetic field. On the other hand, the multiphysic transduction is also verified. Thus, the smart composite material combines the shape memory and the magnetic properties, in addition to the polymeric properties of the hybrid material.

Conclusions
A smart composite material with multiphysic transduction has been presented. It could be activated by the stimuli of electrical current, temperature, stress and magnetic field. The relation of mass is 18.3% NiTi ribbon and 81.7% magnetic hybrid film. DSC results show that transformation temperatures of the NiTi ribbon and the glass transition temperature of the magnetic hybrid material are nearby. This is very important for future possible applications because both effects can merge, obtaining only one macroscopic mechanical response on the smart composite material. The smart composite material presents a small delay of transformation during warming and cooling because the hybrid film acts like a thermal insulator. Thermomechanical results indicate that the hybrid material acts as a mechanical reinforcement, since it is observed that the SATWME of the smart composite is lower than the SATWME of the SMA ribbon. The density current values of the phase transformations during cooling and heating were clearly identified with the thermomechanical apparatus developed in our laboratory, even if during DSC tests this was not possible. Finally, the displacements obtained by applying external magnetic fields are interesting for actuating applications. Even the sample is separated from the electromagnet with a distance of some millimeters, the sensibility of the magnetic response is maintained. This is one of the most interesting advantages of magnetic actuators. In fact, this composite should be used in actuating applications where the magnetic field is the controlling signal or in sensing applications where the magnetic field must be measured.
Thus, obtained results demonstrate that the smart composite material can be activated by temperature, electrical current, stress and/or magnetic field, offering good expectations for actuating applications with multiphysic transduction. Further works will deal with the study of the mass relation between the SMA ribbon and the magnetic hybrid material, and the concentration of iron oxide nanoparticles in the hybrid material, in order to improve the mechanical and magnetic response of the composite material. Other magnetic tests will also be done to analyze the relationship between the physical properties and the magnetic behavior of the composite. Besides this, a dedicated application availing multiphysic transduction will be designed, simulated and fabricated.