The Magneto-Mechanical Behavior of Active Components in ... - MDPI

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The Magneto-Mechanical Behavior of Active Components in Iron-Elastomer Composite Sneha Samal 1 , Marcela Kolinova 2 and Ignazio Blanco 3, * 1 2 3

*

ID

Institute of Physics of Czech Academy of Science, Na Slovance 1999/2, 182 21 Prague 8, Czech Republic; [email protected] Institute for Nano Materials, Technical University of Liberec, Studentská 1402/2, 461 17 Liberec 1, Czech Republic; [email protected] Department of Civil Engineering and Architecture and UdR-Catania Consorzio INSTM, University of Catania, Viale Andrea Doria 6, 95125 Catania, Italy Correspondence: [email protected]

Received: 29 July 2018; Accepted: 3 September 2018; Published: 6 September 2018

Abstract: The magneto-rheological effects in iron-elastomer composites (IEC) were investigated by simulation, surface topography, and 3D representation. The simulated behavior of magneto-rheological elastomeric composites in the presence of an external magnetic field was determined and the influence of magnetic intensity on the isotropic distribution of iron filler particles in IECs was investigated. The magnetic intensity distribution was analyzed from the edge of the surface towards the center of the IEC. The samples were characterized for microstructural images after experimental tests using both micro-computed tomography (µCT) and scanning electron microscopy (SEM). The adhesion of filler particles within the matrix of the magneto-rheological elastomer (MRE) composite and their distributions were also investigated. µCT showed the overall 3D representation of IEC and the inner distribution of filler particles revealed the presence of some porosity which may be due to bubbles and voids in the matrix of the composite. Finally, a mechanism was established governing particle–particle interactions on the basis of dipole–dipole interactions. Keywords: polymer composites; magnetorheological elastomer; simulation; micro-computed tomography; microstructure; magnetorheological effect

1. Introduction Recently increasing focus has been directed on magneto-rheological elastomer (MRE) composites with magneto-active particles embedded inside the matrix of polymer. MREs are the emerging alternative for magneto rheological (MR) fluids which suffer from some drawbacks such as particles settling down and loss of the MR effect [1,2]. The MREs are analogous materials to MR fluids with the carrier fluid replaced by the polymer matrix and the embedded ferromagnetic particles as filler. The isotropic distribution of filler particles in the MREs plays a crucial role in increasing the elastic modulus under an external magnetic field. Due to their property variability, these materials are considered as smart materials. Until now the maximum modulus increase for the MRE has been reported to be up to 0.6 MPa (40% of initial modulus) with iron filler concentration of 30 vol % [3,4]. The MRE materials can be used in various applications such as adaptive and tuned vibration absorbers, stiffness tunable mounts, as well as semi-active and active mounts in vehicles [5]. Due to the damping property, these materials are also used in many industrial applications as dampers and isolators [6]. However, the conventional fabrication of the MRE under the influence of magnetic field has faced many limitations in the industrial application. The anisotropic distribution of the particles as a chain shape must be considered under the application of an external magnetic field rather than prior to use J. Compos. Sci. 2018, 2, 54; doi:10.3390/jcs2030054

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of the material. According to many researchers [6,7], the anisotropic distribution in MRE samples showed a much larger MR effect than that of isotropic ones. The isotropic distribution of iron particles showed a low MR effect at the initial stage, therefore MRE isotropic distribution could represent a new area for this research. The dynamic properties of isotropic MREs depend on the matrix, the particles content, and the external magnetic field. The microstructural characterization of the filler, within the surface of the matrix, and the influence of filler shape have shown improvement in the mechanical properties of the resultant composite [8]. A simulation analysis was carried out before performing the experiment and the influence of magnetic field towards the MRE sample at various positions from bottom to top level was studied. The MRE samples after testing, in the presence of a magnetic field, were analyzed for microstructure and filler orientation. Micro-computed tomography (µCT) was implemented to examine an overall 3D representation of filler particles distribution in the MRE composite. The filler orientation within the MRE composite with self-assembly particles which may lead to dipole interactions was observed by using scanning electron microscopy (SEM). A correlation was established between the observations of simulation properties, µCT, and SEM images of the MRE samples, and their magnetic interaction arises from self-assembly filler particles [9,10]. In our previous study [11,12] we focused on the fabrication of isotropic distribution of the MRE samples without any influence of external magnetic field. The mechanical properties and microstructure of the MRE samples were briefly discussed with isotropic distribution [13]. In the present work we investigated the iron filler distribution and the adhesion throughout the matrix of the magneto rheological elastomeric composite. With the aim to describe the behavior of the magnet active component the configuration of the sample was studied without and with mesh condition, and the filler-matrix interaction, adhesion, surface feature analysis, orientation of distribution of the particles were analyzed. The composites were analyzed by scanning electron microscopy (SEM) for the distribution and orientation of the filler within the material. The influence of magnets creates porosity very widely throughout the composite, however fabrication of the MRE composite with magnets results in anisotropic distribution of filler within the matrix. 2. Experimental Iron particles (Havel Composites CZ s.r.o., Olomoucky Kraj, Czech Republic) of 50–100 µm (purity > 95%) were incorporated into silicon elastomer and two different composition of matrix, ZA 22 (polyaddition product, Figure S2) and N 1522 (polycondensation product, Figure S3) from Luˇcební závody, Kolín, Czech Republic were taken into consideration. The composites were prepared using 30 vol % filler particles with and without magnetic field influence on the fabrication methods. In optical microscope images of Figure 1 it is possible to observe that the MRE samples, fabricated by N 1522 matrix in the presence of a magnetic field, show an anisotropic structure. The silicon oil was used to improve the adhesive properties of the particles on the matrix surface. All the components such as filler with silicon oil, matrix, binder, and catalyst were stirred slowly until homogenization for 30 min at room temperature. Then, for the final fabrication of the sample, the mixture was put under vacuum, to remove air bubbles, and then cured for 24 h in the standard mold without any influence of magnetic field. The samples were designed cylindrically with length of 20 mm and radius of 5 mm. The induced magnetic field was applied perpendicularly to the sample thickness (Figure S1). The magnetic intensity and the sample orientation in the magnetic field were set up. The MRE sample was put in contact, by its upper and lower surface, with copper slabs in the circuit of the magnetic field as described elsewhere [14]. Simulation was carried out by the means of MSC MARC, non-linear dynamics Software (Newport Beach, CA, USA) and characterization of the MRE composites after testing was carried out using SEM and µCT analyses. The simulation parameter was carried out at boundary conditions of the magnetic potential and current of 1 A. The surface morphology of the samples was examined by SEM (Hitachi-model TM-3000, Hitachi High-Technologies Corporation, Tokyo, Japan), with a Secondary Electron Detector (Hitachi High-Technologies Corporation, Tokyo, Japan) and field emission source using 10 kV acceleration voltages. Sample fragments were mounted

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onto aluminum stubs andmounted out-gassed in a aluminum desiccator over h before being coated with a 4 nm layer Sample fragments were onto stubs48and out-gassed in a desiccator over 48 of h platinum prior to imaging in the SEM. before being coated with a 4 nm layer of platinum prior to imaging in the SEM.

Figure 1.1.Optical Optical images of the linear arrangement of within filler the particles within the Figure images (a,b) of(a,b) the linear arrangement of filler particles magneto-rheological magneto-rheological elastomers (MREs) composite (N 1522 as matrix) in the presence of magnetic elastomers (MREs) composite (N 1522 as matrix) in the presence of magnetic field during fabrication. field during fabrication. (c) The linear chain and branches of filler particles. (c) The linear chain and branches of filler particles.

The µCT (Bruker, R.M.I. s.r.o., Lázně Bohdaneč, Czech Republic) analysis was carried out on an The µCT (Bruker, R.M.I. s.r.o., Láznˇe Bohdaneˇc, Czech Republic) analysis was carried out on an open tube source with Tungsten, power of 10 kW, voltage source of 100 kV, and current of 100 µA. open tube source with Tungsten, power of 10 kW, voltage source of 100 kV, and current of 100 µA. The number of projections was 2849 with total test duration of 4 h. X-ray spot target was 16 µm with The number of projections was 2849 with total test duration of 4 h. X-ray spot target was 16 µm with rotation of 360° and camera resolution of 1632 × 1092. µCT analysis was carried out in a model: sky rotation of 360◦ and camera resolution of 1632 × 1092. µCT analysis was carried out in a model: sky scan with reconstructed images in 3D ways from slices of the two dimensional structure. scan with reconstructed images in 3D ways from slices of the two dimensional structure. 3. Results 3. Results 3.1. Simulation of the the Magnet Magnet and and MRE MRE Samples Samples 3.1. Simulation Behavior Behavior of In Figure Figure 2,2,the theexternal externalregion regionofof the cylindrical magnet is presented by coil current location In the cylindrical magnet is presented by coil current location and and its respective magnetic potential. Here it represents the outer magnetic potential as element or its respective magnetic potential. Here it represents the outer magnetic potential as element or entity entity and coil current that develops in the magnet. and coil current that develops in the magnet.


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Figure 2. Magnet with coil current and magnetic potential of magnet. Figure 2. Magnet with coil current and magnetic potential magnet. Figurethe 2. Magnet with coil and magnetic potential of magnet. Figure 3a portrays magnet and thecurrent MRE sample at the top position onofconsidering other factors such as coil winding, coil coren and coil skeleton, whilst in Figure 3b the schematic diagram Figure 3a portrays the magnet and the MRE sample at the top position on considering other in theFigure mesh position is reported. 3a portrays the magnet and the MRE sample at the top position on considering other factors such as coil winding, coil coren and coil skeleton, whilst in Figure 3b the schematic diagram factors such as coil winding, coil coren and coil skeleton, whilst in Figure 3b the schematic diagram in in the mesh position is reported. the mesh position is reported.

(a)

(b)

Figure 3. (a) Magnet with MRE sample(a) and (b) cross section view of magnet with core, skeleton, coil Figure 3. (a) Magnet with MRE sample and (b) cross section view of magnet with(b) core, skeleton, coil winding, and MRE sample position. winding, and MRE sample position. Figure 3. (a) Magnet with MRE sample and (b) cross section view of magnet with core, skeleton, coil winding, samplepresentation position. Figure 4a displaysand theMRE schematic of the MRE sample position on the lower part of

Figure 4a domain. displays the of the MRE sample position on is thedisplayed lower parton of the magnetic Theschematic position presentation of the sample with cross sectional view Figure 4a displays the schematic presentation of the MRE sample position on the lower part of the magnetic domain. The position of the sample with cross sectional view is displayed on considering considering the mesh size of the magnet and the surrounding atmosphere is sketched in Figure 4b. the magnetic domain. Thesurrounding position ofatmosphere the sample with cross sectional view is displayed on the mesh size the magnet the is sketched in Figure 4b.and Figure 5aofdisplays the and distribution of magnetic induction within the magnet the MRE considering the mesh size of the magnet and the surrounding atmosphere is sketched in Figure 4b. Figure 5a displays the distribution of magnetic induction within the magnet and the MRE sample sample in the presence of atmospheric effect. Simultaneously the distribution of magnetic induction Figure 5a displays the distribution of magnetic induction within the magnet andinthe MRE in the presence of atmospheric effect. Simultaneously distribution magnetic induction (Tesla) in the presence of magnetic field is calculated the considering theof force lines within the (Tesla) magnet, sample in the presence of atmospheric effect. Simultaneously the distribution of magnetic induction the presence of magnetic field which is presented in Figure 5b.is calculated considering the force lines within the magnet, which is in the presented(Tesla) in Figure 5b. presence of magnetic field is calculated considering the force lines within the magnet, which is presented in Figure 5b.

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Figure 4. Schematic presentation of the magnet with the MRE sample position on the top of the Figure 4. Schematic presentation of the magnet with the MRE sample position on the top of the Figure 4. Schematic presentation of the sample position on the top of the magnetic and respective atmosphere, (a) magnet without with mesh,the (b)MRE with mesh. magnetic and respective atmosphere, (a) without mesh, (b) with mesh. magnetic and respective atmosphere, (a) without mesh, (b) with mesh.

(a) (a)

(b) (b)

Figure 5. Distribution of magnetic induction on the above configuration of MRE in presence of Figure 5. induction on the configuration of MREofinMRE presence of magnetic Figure 5. Distribution Distributionofofmagnetic magnetic induction on above the above configuration in presence of magnetic field (a) with mesh and considering surrounding atmosphere (b) without mesh and using field (a) with mesh and considering surrounding atmosphere (b) without mesh and using lines force magnetic field (a) with mesh and considering surrounding atmosphere (b) without mesh andofusing lines of force as external parameter. as external lines of forceparameter. as external parameter.

It was observed that the magnetic induction reached a range of the interval 0.7–0.3 Tesla. The ItIt was that magnetic induction reached a range of the interval 0.7–0.3 Tesla.Tesla. The wasobserved observed thatthe magnetic induction a various range ofpositions the interval distribution of the values ofthe magnetic induction was reached studied at in the0.7–0.3 MRE sample. distribution of theofvalues of magnetic induction was studied at various positions in the MRE sample. The distribution the values of magnetic induction was studied at various positions in the MRE The magnetic induction distribution is shown on the upper part of the magnet including the MRE on The magnetic inductioninduction distribution is shownis on the upper part of the magnet the MRE the on sample. TheThe magnetic distribution on the upper of theincluding magnet including position. calculation showed that theshown average value ofpart magnetic induction reached position. The calculation showedshowed that the average value of of magnetic induction reached MRE on position. The calculation thatminimum the average value induction reached 0.3 Tesla on considering the maximum and ranges frommagnetic the bottom to the top level 0.3 Tesla on on considering considering the maximum and minimum ranges from the bottom to the top level 0.3 Tesla minimum ranges from the bottom to the top level (Figure 6a). The magnetic induction lines of the MRE sample force are also shown in Figure 6b, with (Figure 6a). magnetic induction lines of the MRE sample forceforce are also shown in Figure 6b, with (Figure 6a).The The magnetic induction lines of base the MRE arepresented also shown Figure 6b, the maximum and minimum values, from the to thesample top level, which an in average value the maximum and minimum values, from the base tobase the to topthe level, which presented an average value with the maximum and minimum values, from the top level, which presented an average of 0.3 Tesla. The magnetic induction of the magnet and the position of the MRE on the sample were of 0.3 Tesla. magnetic induction of the of magnet and the position of the of MRE theon sample were value of 0.3 The Tesla. The magnetic induction the magnet and the position the on MRE the sample distributed throughout the system. distributed throughout the system. were distributed throughout theinduction system. at various positions from the bottom to the top is plotted in The influence of magnetic The influence of magnetic induction atatvarious positions from the bottom to to thethe toptop is plotted in The influence of magnetic induction various from bottom is plotted the MRE on consideration of the boundary region positions with mesh sizethe and stiffness boundary, as the the MRE on consideration of the boundary region with mesh size and stiffness boundary, as the incomposite the MRE was on consideration the boundary region with mesh size and stiffness boundary, as the affected by anofelastomeric effect (Figure 7a,b). composite composite was was affected affected by by an an elastomeric elastomeric effect effect (Figure 7a,b).

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Figure 6. (a) Magnetic induction distribution on the top portion of the configuration exposing the Figure 6. (a) Magnetic induction distribution on the top portion of the configuration exposing the magnet and MRE sample (b) closure view. magnet and MRE sample (b) closure view.

Figure7.7. Magnetic a magnetic field showing the Figure Magnetic induction induction of of the theMRE MREsample sampleininthe thepresence presenceofof a magnetic field showing distribution of of magnetic lines force considering the the mesh mesh size sizeasas the distribution magnetic lines forcefrom fromthe thebottom bottomto to top top layer layer considering boundarylayer. layer. boundary

Simultaneously the magnetic induction was observed on the cross-section length of the magnetic Simultaneously the magnetic induction was observed on the cross-section length of the magnetic bar as a function of the arc length (Figure 8). The deduction shows the lower position (blue line) has bar as a function of the arc length (Figure 8). The deduction shows the lower position (blue line) has a a maximum strength of magnetic induction of 0.7 Tesla, however the top position shows (black lines) maximum strength of magnetic induction of 0.7 Tesla, however the top position shows (black lines) the the minimum value of magnetic induction. This may arise due to the contact area of the MRE sample minimum value of magnetic induction. This may arise due to the contact area of the MRE sample with with the magnetic domain. The value of the magnetic induction was derived from the magnetic coil the magnetic domain. The value of the magnetic induction was derived from the magnetic coil towards towards the MRE sample and is reported in Figure 9, where the magnetic induction is plotted as a the MRE sample and is reported in Figure 9, where the magnetic induction is plotted as a function function of the distance in combination with the magnet and the sample position. It was observed of the distance in combination with the magnet and the sample position. It was observed that the that the magnetic induction is maximum for the MRE sample at the contact position and then magnetic induction is maximum for the MRE sample at the contact position and then becomes lower becomes lower towards the top position. This representation shows the maximum strength to towards the top position. This representation shows the maximum strength to minimum position minimum position of magnetic induction as a function of displacement. The overall statistics of the of magnetic induction as a function of displacement. The overall statistics of the analysis is 2% of analysis is 2% of the results. the results.

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Figure 8. Magnetic induction as a function of arc length from bottom, middle, and top positions in the

Figure 8. Magnetic induction asasa afunction middle,and andtop toppositions positionsininthe the Figure Magnetic induction functionofofarc arclength length from from bottom, bottom, middle, MRE8.sample. MRE sample. MRE sample.

Figure 9. Magnetic induction in the middle of the coil of the MRE sample from the coil position towards the sample.

Figure 9. Magnetic induction in middle the middle theofcoil the sample MRE sample from coil position Figure 9. Magnetic induction in the of theofcoil theof MRE from the coilthe position towards 3.2. Microstructural Observation of MRE Composite the towards sample. the sample. The microstructure of the MRE composites was examined by SEM. The top surface and even the 3.2. Microstructural Observation MRE Composite to verify the iron particles distribution within the cross section ofObservation the samplesof of were investigated 3.2. Microstructural MRE Composite matrix of the silicon couch polymer. Figure 10a,b displays the microstructure of the elastomer matrix The microstructure of the MRE composites was examined by SEM. The top surface and even the The of the MRE composites wasuniform examined by SEM.distribution The top surface even the (ZAmicrostructure 22 and N1522). The MREs composite showed and isotropic of fillerand particles cross section of the samples were investigated to verify the iron particles distribution within the cross section of the samples were investigated to verify the iron particles distribution within the matrix within the matrix (Figure 10c,d). The isotropic distribution of iron particles showed adherence to the matrix of the silicon couch polymer. Figure 10a,b displays the microstructure of the elastomer matrix structure of the polymer matrix. Figure 11a–d displays the self-assembled structure of iron of the network silicon couch polymer. Figure 10a,b displays the microstructure of the elastomer matrix (ZA 22 (ZA 22 and N1522). The MREs composite showed uniform and isotropic distribution of filler particles particles (fibril) thatcomposite developedshowed due to the affine coupling naturedistribution of the magnetic field (two particle and N1522). The MREs uniform and isotropic of filler particles within within thein matrix (Figure 10c,d). The isotropic distribution of iron particles showedof adherence to the model) the10c,d). microscopic configuration of the MRE composite under the influence the matrix (Figure The isotropic distribution of iron particles showed adherencethe to magnetic the network network structure of thesmall polymer matrix. Figure displays self-assembled structureinofthe iron field. series of very chains of three or displays two11a–d particles fillerthe arrangement were observed structure ofAthe polymer matrix. Figure 11a–d theofself-assembled structure of iron particles

particles (fibril) that developed due to the affine coupling nature of the magnetic field (two particle (fibril) thatin developed due toconfiguration the affine coupling nature of the magnetic field (two particle model) in model) the microscopic of the MRE composite under the influence of the magnetic the field. microscopic of the MRE composite under the influence of the magnetic field. Ainseries A seriesconfiguration of very small chains of three or two particles of filler arrangement were observed the of very small chains of three or two particles of filler arrangement were observed in the microstructure

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of the MRE samples. This may lead to dipole– dipolelead interaction or contract interaction thatcontract gives rise microstructure microstructure of of the the MRE MRE samples. samples. This This may may lead to to dipole– dipole– dipole dipole interaction interaction or or contract to coupling of the spin particle direction as a function of the magnetic field. interaction that gives rise to coupling of the spin particle direction as a function of the magnetic interaction that gives rise to coupling of the spin particle direction as a function of the magnetic field. field.

Figure (a,b) Microstructuralimage image ofelastomer elastomer matrix and Figure 10.10. (a,b) Microstructural and (c,d) (c,d) scanning scanningelectron electronmicroscopic microscopic Figure 10. (a,b) Microstructural imageof of elastomer matrix matrix and (c,d) scanning electron microscopic image of isotropic distribution of filler particles within the matrix of the composite. image of isotropic distribution of the the composite. composite. image of isotropic distributionofoffiller fillerparticles particleswithin within the the matrix matrix of

Figure 11. (a–d) Self-assembled structure of iron particles (affine coupling, microscopic behavior) in Figure (a–d) Self-assembledstructure structureof ofiron ironparticles particles (affine (affine coupling, in in Figure 11.11. (a–d) Self-assembled coupling,microscopic microscopicbehavior) behavior) MRE composite. MRE composite. MRE composite.

3.3. Micro Computed Tomography MRE Micro Computed Tomographyofof ofMRE MRE 3.3.3.3. Micro Computed Tomography Figure 12 displays the µCT image Figure 12 displays the µCT image of of the the elastomer elastomer matrix, matrix, without without any any porosity. porosity. In In the the same same Figure 12 displays the µCT image of the elastomer matrix, without any porosity. In the same figure figure the the 3D 3D image image of of the the MRE MRE composite composite at at various various positions positions of of the the X-, X-, Y-, Y-, and and Z-axis Z-axis is is visualized. visualized. figure the 3D imageanalyzed of the MRE composite at various positions of the X-, Y-,the and Z-axisdistribution is visualized. Furthermore, Furthermore, we we analyzed samples samples using using micro-computed micro-computed tomography tomography for for the overall overall distribution Furthermore, we analyzedthe samples using micro-computed tomography for thecharacterization overall distribution of of of filler filler particles particles within within the matrix matrix of of the the composite. composite. Figure Figure 13 13 shows shows the the µCT µCT characterization of of the the filler particles within the matrix of the composite. Figure 13 shows the µCT characterization of the

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MRE composite The 3D 3D image imageshows showsthe theporosity porosityand and iron filler MRE composite(ZA (ZA22) 22)with withiron ironnanoparticles. nanoparticles. The iron filler volume distribution of the materials. The report of the observations is summarized in Table 1. volume distribution of the materials. The report of the observations is summarized in Table 1. MRE composite (ZA 22) with iron nanoparticles. The 3D image shows the porosity and iron filler volume distribution of the materials. The report of the observations is summarized in Table 1.

Figure12. 12.Micro-computed Micro-computed tomography tomography investigation of ZA 22 Figure investigation 22elastomer. elastomer. Figure 12. Micro-computed tomography investigation of of ZAZA 22 elastomer.

Figure 13. Three-dimensional image of the MRE composite (ZA 22 as matrix) using micro-computed

Figure 13. Three-dimensional image of the MRE composite (ZA 22 as matrix) using micro-computed tomography (µCT). Figure 13. Three-dimensional image of the MRE composite (ZA 22 as matrix) using micro-computed tomography (µCT). tomography (µCT).

Figure 14 displays the overall and inner position of the MREs composite with matrix ZA 22 and N1522, where it is possible to observe that the ZA 22 matrix shows better adhesion and distribution


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Figure 14 displays the overall and inner position of the MREs composite with matrix ZA 22 and N1522, where it is possible to observe that the ZA 22 matrix shows better adhesion and distribution of of filler filler particles particles within within the the composite composite matrix. matrix. However, However, the the N N 1522 1522 matrix matrix develops develops poor poor adhesion adhesion while a lot of porosity was observed in the overall display of the MRE composite. while a lot of porosity was observed in the overall display of the MRE composite.

Figure 14. 14. µCT µCT characterization characterization of of iron iron filler filler MRE MRE composites composites at at different different orientation orientation for for MREs MREs with with Figure matrices ZA 22 (a,b) and N1522 (c,d). matrices ZA 22 (a,b) and N1522 (c,d).

4. Discussions 4. Discussions The self-assembled self-assembled structure The structure was was developed developed by by filling, filling, in in the the presence presence of of the the magnetic magnetic field, field, exploiting its contribution along the sample in the parallel direction. As the filler particle is already exploiting its contribution along the sample in the parallel direction. As the filler particle is already stabilized in in the the cured cured matrix, matrix, the the movement movement of of the the filler filler particle particle arises arises at at the the resting resting position position from from stabilized magnetic torques. As a result, the dipolar and contact interactions between magnetic moments play magnetic torques. As a result, the dipolar and contact interactions between magnetic moments play role in in the the improved improved behavior behavior of of the the mechanical mechanical properties. properties. Strongly Strongly and and loosely loosely coupled coupled filler filler aa role particles generate coupling of the magnetic moment that arises in the presence of the magnetic field. particles generate coupling of the magnetic moment that arises in the presence of the magnetic field. Table11reports reportsvarious various information of MRE the MRE sample (ZA 22)µCT from µCT observation. A total Table information of the sample (ZA 22) from observation. A total porosity porosity of 17.4% was in observed in the sample. sample position during analysis is represented of 17.4% was observed the sample. The sampleThe position during analysis is represented with a total 3 and surface area of 1166.2 mm2. Both the lower and upper vertical with a total volume of 15.7 mm 3 2 volume of 15.7 mm and surface area of 1166.2 mm . Both the lower and upper vertical positions of positions the sample centroid position are explained. the sampleofwith centroidwith position are explained. Table 1. observation of the magneto-rheological elastomer (MRE)(MRE) samplesample with microTable 1. Experimental Experimental observation of the magneto-rheological elastomer with computed tomography (µCT). micro-computed tomography (µCT).

Number of Layers Number Layers TotalofVOI Volume Total VOI Volume Object Surface Obj.S Object Surface Obj.S Surface Convexity Index Surface Convexity Index SCv.I SCv.I Structure Separation Structure Separation St.Sp St.Sp Surface of Closed PoresPores Po.S(cl) Surface of Closed Po.S(cl) Total Volume of Pore SpaceSpace Po.V(tot) Total Volume of Pore Po.V(tot) Lower Vertical Position Lower Vertical Position Object Volume Object Volume Intersection Surface i.S

Intersection Surface i.S Centroid (x) Crd.X Number of Objects Obj.N

1101 1101mm3 15.7 15.7 mm3 2 1166.2 mm 1166.2 mm2 −1−1 −314.3 mm −314.3 mm 0.01mm mm 0.01 2 2 57.3 57.3mm mm 3 2.7 mm 2.7 mm3 0.1 mm 0.1 mm 12.9 mm3 2 3 12.9 mm 29.2 mm

29.2 mm2 −0.3 mm 2913

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Table 1. Cont.

−0.3 mm 2913 0.6% 17.4% 2.3 mm 82.5% 90.0 mm−1 0.02963 mm (cl) 82215 2.6 mm3 2 µm 38.7 mm2 74.2 mm−1 1.2 mm 0.1 mm3 16.9%

Centroid (x) Crd.X Number of Objects Obj.N Closed Porosity Po(cl) Total Porosity Po(tot) Upper Vertical Position Object Volume Obj.V/TV Object Surface/Volume Ratio Obj.S/Obj.V Centroid (y) Crd.Y Number of Closed Pores Po.N Volume of Open Pore Space Po.V (op) Pixel Size Total VOI Surface (TS) Object Surface Density Obj.S/TV Centroid (z) Crd.Z Volume of Closed Pores Po.V(cl) Open Porosity Po(op)

The isotropic distribution of the iron particles was observed in the MRE composite during the fabrication stage without any influence of the magnetic field. The additive silicon oil improved the iron particles adhesion within the matrix, and during the measurement of mechanical properties under the influence of the external field, the particles provided a slight influence. As a result, self-assembled microstructure (fibrils) resulted in the microscopic range of the particle region within the larger or microscopical area of the MRE sample. This “two particles model” microstructure was generated by the strong coupling between the iron particles, due to their magnetic interactions, which is known as the affine coupling effect. The magnetic field arises due to the dipole of the initial particle [15]: B1 = ∇ × A I

(1)

µ0 I 4πr2

(2)

the vector potential A I is defined as: AI =

I

r 0 cos θ 0 dI 0

where θ’ is the angle between r’ and dr(r’ − r). So A I can be written as: AI =

µ0 I 4πr2

I

r 0 ·rˆdI 0 =

µ0 I a × rˆ 4πr2

(3)

The magnetic moment of each filler particle is represented by m = A I , so the dipole potential can be converted [16] to: µ0 AI = m × rˆ (4) 4πr2 According to the magnetic field which plays the induction role due to dipole interaction: BI = ∇ × A I =

µ0 rˆ ∇× m× 2 4π r

(5)

here rˆ is the unit vector and r is the position vector for the individual filler particle is defined as follows: rˆ =

1 ˆ + yy ˆ + zz ˆ ) and r = ( xx r

q

x 2 + y2 + z2

(6)

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with respect to the 3D coordinate (x, y, z). The magnetic induction that arises within the MRE due to filler dipole interaction is: µ0 (7) B1 = [3(m·rˆ)rˆ − m] 4πr3 The interaction energy between two magnetic dipoles is given by: U12 = −m2 · B1 =

µ0 m1 ·m2 − 3(m1 ·rˆ) m2 · jˆ 3 4πr

(8)

where rˆ and jˆ are the unit vectors corresponding to the two particles in nearby positions. So the internal energy between two particles, due to dipole–dipole interaction, is nominated based on the position vector and the respective angle [17]: U12 =

µ0 m1 m2 (cos(θ1 − θ2 ) − 3 cos θ1 cos θ2 ) 4πr3

(9)

On taking the derivative with respect to the angular position of particle 1 and 2 defined as: ∂U12 = − sin(θ1 − θ2 ) + 3 sin θ1 cos θ2 = 0 ∂θ1

(10)

∂U12 = sin(θ1 − θ2 ) + 3 cos θ1 sin θ2 = 0 ∂θ2

(11)

and

So, the value of θ1 and θ2 falls in the range 0–π. The coupling along the x- and y- axis, results in a change of orientation spin from α to β. The magnetic dipole of the interaction between two iron particles defined as the magnetic dipole moment of a particle under external field H0 [18] is ma = 4πµm µ0 R3 βH0

(12)

where µ0 is the vacuum permeability, β = (µp − µm )/µp + 2µm ), µp and µm are the relative permeability of the particles and the matrix respectively. For an iron particle and silicone rubber, µp ≈ 1000, µm ≈ 1 and β ≈ 1. The self-assembled microstructure can also decrease the initial shear modulus. The initial modulus of the MRE without magnetic field [19] can be written as Ge = G0 1 + 2.5Φ + 14.1Φ2

(13)

where G0 is the modulus of the matrix and Φ is the volume percentage of the particles. The particle volume percentage in the MRE was assumed to be 30%. On this basis, the probability of formation of SC or HCP microstructures is effective in the composite. However, some exceptions were observed in the MRE composites during the fabrication process. Due to the higher density of iron particles, during the curing process at room temperature, their settlement was observed more towards the bottom with respect to the top layer (Figure 15).

where G0 is the modulus of the matrix and Φ is the volume percentage of the particles. The particle volume percentage in the MRE was assumed to be 30%. On this basis, the probability of formation of SC or HCP microstructures is effective in the composite. However, some exceptions were observed in the MRE composites during the fabrication process. Due to the higher density of iron particles, during the curing process at room temperature, their settlement was observed more towards the J. Compos. Sci. 2018, 2, 54 13 of 16 bottom with respect to the top layer (Figure 15).


13 of 15

15. MRE with ironeffect particles and some exceptions during fabrication. The Figure magneto-rheological wasdistribution attributedand tosome the exceptions dipole–dipole Figure 15. MRE with iron particles distribution during interaction fabrication. of particles or particles self-assembled phenomena in the MRE composite. The magneto-rheological properties arise The magneto-rheological interaction of influence particles or from the magnetic alignmenteffect of thewas ironattributed particles to in the dipole–dipole MRE composites under the of a particles self-assembled phenomena in the MRE composite. magneto-rheological magnetic field. This magnetic alignment increased the The stiffness of the material,properties observed arise as an from the magnetic alignment the ironduring particles the MRE compositeson under the influence of a increase in the shear storageof modulus thein shear test, depending the magnetic properties, magnetic field. This alignment stiffness of the material, observed as anofincrease morphology, and magnetic concentration of theincreased particles the in the composite. A schematic diagram the filler inparticles the shearbehavior storage modulus during test,magnetic depending on the magnetic properties, in response to the theshear applied field is portrayed in Figure morphology, 16, where the and concentration the particles in the composite. A schematic filler particles behavior influence of theofsize of regular and irregularly shaped diagram particles ofinthethe composite and their inintermediate response to the applied magnetic field is portrayed in the Figure 16, where the influence of the size of states for the magnetic induction toward formation of fibrils are shown. regular and irregularly shaped particles in the composite and their intermediate states for the magnetic induction toward the formation of fibrils are shown.

Figure 16. Dipole–dipole interaction of two filler particles on the various angles and position vectors onFigure alignment of the z-axis for the coupling mechanism in theon MRE 16. Dipole–dipole interaction of two filler particles the sample. various angles and position vectors

on alignment of the z-axis for the coupling mechanism in the MRE sample.

The self-assembly model is used to explain the microstructure of iron particles in the MRE The self-assembly model14 is in used to explain microstructure of iron particles in the MRE composites, as shown in Figure agreement withthe literature [18,19]. The isotropic distribution of composites, shown in Figure 14 in agreement with within literature The isotropic filler particles as facilitates microscopic behavior changes the[18,19]. MRE composite, thusdistribution resulting inof fillercoupling particlesof facilitates microscopic behavior the changes within the MRE composite, thusdimension resulting in affine the particles, likely following two-particle model, within the broad coupling ofdeformation the particles,oflikely following the two-particle within the dimension ofaffine the macroscopic the samples [9,15]. The effect of model, the formation of abroad self-assembled of the macroscopic deformation of the samples [9,15].ofThe of the formation a self-assembled microstructure is observed in the isotropic distribution the effect MRE composite, whichofagrees well with microstructure observed in the isotropic distribution of the MRE composite, which agrees well with our previous datais[20]. our previous data [20]. The bonds between the particles and matrix influence the mechanical behavior of the composite material: if they are weak, then failure will occur at the surface of the composite. Weak interfacial bonding was observed as a pit at the interface between the iron filler and silicon couch matrix. A weak contact surface (Figure 15) was primarily observed in the sample with iron particles, showing isotropic particles distribution in the MRE composite. The magnetic dipole moment of each magnetic

J. Compos. Sci. 2018, 2, 54

14 of 16

The bonds between the particles and matrix influence the mechanical behavior of the composite material: if they are weak, then failure will occur at the surface of the composite. Weak interfacial bonding was observed as a pit at the interface between the iron filler and silicon couch matrix. A weak contact surface (Figure 15) was primarily observed in the sample with iron particles, showing isotropic particles distribution in the MRE composite. The magnetic dipole moment of each magnetic particle m is directed toward the horizontal direction with a polar angle. The balance between the elastic and magnetic forces is the factor influencing magnetic interactions (such as the magnetic properties, particle size, and spatial density of the filler) and which control the MR effect. Iron particles with a high saturation magnetization can be considered as one of the best filler materials. Usually, micrometer-sized particles are used, although recently MREs based on sub-millimeter iron particles were fabricated with a more pronounced MR effect. Smaller filler particles size and lower magnetic fields result in stronger magnetic networks. Both factors result in the extension of the linear viscoelastic regime to larger strain amplitudes and lead to higher shear storage and loss of moduli values [21,22]. The particle–particle dipolar interaction is explained in the schematic diagram (Figure 16) based on the various vector and angular positions that play the active role in the dipolar moment, contributing to the magneto-rheological effect in the MRE sample in the presence of a magnetic field [23], and also according to the continuum based model proposed in the literature [24–27]. 5. Conclusions The simulation behavior of the magnet and sample position were explained and the values of magnetic induction were calculated at various stages in the configuration of the sample without and with mesh condition. Magnetic force alignment was induced in the MRE composite in the presence of an external cylindrical magnetic field, resulting in magnetic interaction between two magnetically active particles with co-aligned dipoles induced by a uniform magnetic field. The isotropic distribution of iron particles in the MRE composites was observed from microstructural images, showing the formation of fibrils of particle chains (two or three particles series arrangement) within the elastomer. With the influence of magnetic field, a binary or ternary arrangement of the self-assembled microstructure was formed by the iron particles which exhibited the influence of the magnetic effect in the composite due to dipolar interaction. In a uniform magnetic field, particles become magnetized and assemble into chain-like microstructures due to dipole–dipole interactions, thus tending to align with the direction of the external field. The self-assembled magneto active particles induce a magnetic effect in the elastomeric composite. The normalized dipole–dipole interaction forces developed on various angles of the particles with relative angular positions to each other. This mechanism also explains the origin of the magneto-rheological effect that can contribute towards the better damping properties of the MRE composites, resulting in a great advantage for smart materials application, such as the damper in automotive vehicles. Supplementary Materials: The following are available online at http://www.mdpi.com/2504-477X/2/3/54/s1, Figure S1: Schematics/directions of the application of the magnetic field, Figure S2: Scheme of polyaddition chemical reaction for ZA 22, Figure S3: Scheme of polycondensation chemical reaction for N 1522. Author Contributions: S.S., M.K. and I.B. conceived and designed the experiments; S.S. and M.K. performed the experiments; S.S. and M.K. analyzed the data; S.S., M.K. and I.B. wrote the paper. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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