Abdollah Hajaliloua*, Saiful Amri Mazlanb, Salihah Tan Shilanb, Ebrahim Abouzari-Lotfc

Enhanced magnetorheology of soft magnetic carbonyl iron suspension with binary mixture of Ni-Zn ferrite and Fe3O4 nanoparticles additive

Abdollah Hajaliloua*, Saiful Amri Mazlanb, Salihah Tan Shilanb, Ebrahim Abouzari-Lotfc

a Faculty of Mechanical Engineering, Department of Materials Engineering, University of Tabriz, 51666, Tabriz, Iran

bVSE Research Laboratory, Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia, 54100, Kuala Lumpur, Malaysia

cAdvanced Materials Research Group, Center of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 54100, Kuala Lumpur, Malaysia

cDepartment of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia

*Corresponding author, E-mail address:

(Abdollah Hajalilou)

Tel: +989366041987

Abstract

Fe3O4 and Ni0.5Zn0.5Fe2O4 nanoparticles were synthesized via precipitation and mechanical alloying, respectively and assessed as a potential magnetorheogical (MR) additive. X-ray diffraction and transmission electron microscopy were employed to evaluate the phase formation and structural and morphological changes. Vibrating sample magnetometer (VSM) was used to measure magnetic characteristics of the samples. The MR characteristics of carbonyl iron (CI)-based and 1wt.% (Ni0.5Zn0.5Fe2O4+Fe3O4)-CI based suspensions were measured from a steady and rotational rheometry by applying magnetic field strengths ranging from 0 to 558.39 kA/m with 79.77 kA/m increments. The results indicated that the MR effect of the micron-sized CI-based MR fluid significantly improved in the presence of nanoparticle additives; e.g., having higher yield characteristics. Chain-like structure formed in the presence of nanoscale additives improved the MR performance and sedimentation stability of the CI particles.

Keywords: Nanoparticles; Magnetic properties; Mechanical properties; Rheological properties

1. Introduction

Magnetorheological (MR) fluids, as reported by Rabinow for the first time in 1948 [1], are considered as smart materials with controllable properties. MR fluids are consisting of micron-sized magnetiazable particles and additives uniformly dispersed in the carrier fluid [1,2]. Without applied magnetic field, these liquid materials normally exhibit free-flow with a given apparent viscosity in the similar manner to Newtonian fluids [2,3,4]. Upon applying a magnetic field, the dispersed magnetic particles in the suspension tend to orientate, align and make a chain-like structure that results in a solid-like behavior representation in the MR fluids. This phenomenon leads to a remarkable variation in the rheological characteristics i.e. the apparent viscosity and the shear yield stress. On the other hand, the MR fluids would return to free-flowing liquids condition upon the elimination of externally applied magnetic field [2,5,6,7]. These transformations are considerably fast (milliseconds), reversible and controllable. Hence, these materials are applicable in an extensive range of engineering fields such as devices for torque transfer, which include dampers, clutches, brakes, and mounts for use in semi-active or adaptive vibration control and snubbing [1,2,8].

Soft magnetic carbonyl iron (CI) powders are mostly utilized as magnetizable particles in the MR suspensions because of their interesting magnetic properties, i.e. high saturation magnetization, high permeability and low coerctivity. Furthermore, these soft magnetic materials could be easily magnetized and demagnetized which results in a high yield stress under an externally applied magnetic field [4,5,6]. However, the higher density of CI compared to the carrier fluid results in a sedimentation problem and limits their use in engineering applications [2,4,5,6,9,10]. Various approaches have been considered to address the sedimentation with decreasing the density of magnetizable particles through coating with polymer/inorganic materials, modifying CI, introducing additives and bidispersing to reduce the direct contact of CI particles [2,4,5,6,7,9,10,11]. For example, Mrlik and Pavlinek modified the magnetic carbonyl iron (CI) particles with a thinpolymer shell utilizing the atom-transfer radical polymerization (ATRP) technique, which enables control of the molecular weight and polydispersity of the final grafted polymer chains on the surface of CI particles [11]. The molecular weight and conversion can be controlled by the molar ratio of monomer to initiator, reaction temperature and time [12]. This therefore allows tuning of the magnetorheological (MR) performance as well as stability properties (chemical, sedimentation) [11,12]. Indeed, the polymer coating procedure includes two steps, which are immobilization of initiator, e.g., 2-(4-chlorosulfonylphenyl)-ethyltrichlorosilane (CTCS) on the surface of magnetic particles and graft polymerization of butyl acrylate from the surface [4,12]. Although the modification of CI particles’ surface is complicated process and costly, it was found that the use of nanoparticels of soft and/or hard magnetic materials such as CI, Fe3O4 and Fe2O3 results in improved properties due to their superior magnetic properties and much lower density (~ 4-5 g/cm3) [2,9,12]. For example, better sedimentation stability was observed by introducing Fe3O4 nanoparticles into the CI-based suspension compared to the pristine systems [9,13]. On the other hand, Iglesias et al. by studying the stability and redispersibility of bidisperse magnetorheological fluids found that a volume fraction of nanoparticles not higher than 3% is enough to provide a long-lasting stabilization to MR fluids containing above 30% iron microparticles [14]. In other words, the inclusion of identical magnetite nanoparticles into a MR suspension, however, provides an excellent magnetorheological response compared to that obtained in more standard fluids, where the liquid carrier contains stabilizing and thickening compounds. An excess of nanoparticles (above 7% by volume) leads to a decrease in the MR response of the fluids. This is likely the consequence of the formation of an excessively thick cloud of magnetite nanoparticles around the iron spheres, resulting in a weaker magnetic interaction, probably lower than required for an optimum MR response. In another study, Sedlacik et al. by investigation the effect of the nanoparticles additive and coating of the magnetic particles found a significant improvement in the MR effect [15]. For example, the partial substitution of carbonyl iron (CI) spherical microparticles with Fe rod-like particles, which were synthesized via a surfactant-controlled solvothermal method, improved sedimentation stability in comparison with the application of CI particles alone. Besides, coating the CI and Fe particles with a polysiloxane layer through the hydrolysis–condensation polymerization of tetraethylorthosilicate represented better oxidation and chemical stability balance with an acceptable decrease in the MR effect. Machovsk´y et al. [16] by inclusion an even small amount of Fe3O4/ZHS hybrid composite sheets into MR suspension observed a remarkable improvement in the overall performance of MR suspensions. This is because the redispersibility properties of the suspensions increase with an increasing amount of magnetic additive, due to the sheet-like additive disturbs the packing perfection of bare CI particles thus impeding their agglomeration.

Among soft magnetic materials, Ni-Zn ferrites due to their high magnetic permeability, high electrical resistivity, high Curie temperature and low power loss at high frequencies, have been widely used in many applications such as rod antennas, transformer cores, radio frequency circuits, read/write heads for high speed digital tapes and high quality filters [2,7,17,18]. On the other hand, Fe3O4 nanoparticles that contain both Fe2+ and Fe3+ possess satisfactory soft magnetization, large specific surface area and high surface energy [2,9,19,20]. In order to synthesize Fe3O4 and Ni-Zn ferrite nanoparticles, a variety of either dry routes such as mechanical alloying or wet chemical methods of hydrothermal, micro-emulsion, flame spray pyrolysis, hot-soap, electrospray pyrolysis, sol-gel and plasma-enhanced chemical vapor deposition have been employed [2,9,11,13,16,17,18, 19, 20]. Thus, in this study, in order to promote the MR fluids applications in the industry level and improve their performance, e.g. for mechanical transmission system, Ni-Zn ferrite and Fe3O4 nanoparticles were introduced into micron-sized CI-based MR fluid. In this aspect, nanoparticles of the Ni-Zn ferrite was fabricated through a mechanical alloying route and Fe3O4 nanoparticles was synthesized through a precipitation route. The rheological behaviors of the samples under various applied magnetic fields were evaluated by means of a rotational rheometer with a parallel-plate measuring cell. Steady-state experiments were conducted to measure the flow and yield behaviors and their improvements in the presence of 1wt.% (Ni-Zn ferrite + Fe3O4) nanoparticles additive. In order to understand the role of additives and the suspension stability, the profile of the sedimentation ratio as a function of time was considered and discussed in details.

2. Experimental

In order to synthesize Fe3O4 nanoparticles, the precursors of FeCl2.4H2O, FeCl3.6H2O, and NaOH were purchased from R&M Brand and used without further purification. The Fe3+ and Fe2+ molar ratio was chosen to be 2:1 in iron chloride solution. The white egg was used as a stabilizer. Indeed, 0.05 g white egg was stirred and homogenized in 25 mL distilled water. A 0.3850 g FeCl3.6H2O and 0.1417 g FeCl4.4H2O were dissolved in 25 mL of distilled water. Then the white egg was added and subsequently, 2 molar NaOH was added until the solution color changed to brownish and pH became constant at 11, as schematically shown in Fig. 1. The black solid product was retrieved with magnetic separation and washed several times with distilled water. Finally, the obtained products were dried in oven at 60 ˚C for 24 h.

For synthesizing Ni-Zn ferrite nanoparticles, powders of NiO (99.7 %), Zn (99.5 %) and Fe2O3 (99.5 %) were purchased form Alfa Aesar and used without further purification. Then, the powders were mixed in terms of the molar ratios using the following reaction:

0.5Zn + 0.5NiO + Fe2O3 = Ni0.5Zn0.5Fe2O4 (1)

A ball to powder ratio was chosen to be 10:1. The powder mixture was ball-milled in a home-made planetary ball mill for 60 h at ambient temperature and under argon atmosphere. The ball milling media was hardened chromium steel vial (150 mL) with five hardened steel balls (18 mm). The rotational speed of the vial was fixed at 600 rpm and the disc at 400 rpm for running the experiments.

In order to prepare 15 mL MR fluids, 4 µ-sized particles of carbonyl iron (CI), having density about 7.87 g/cm3, were purchased from BASF with OM series. Silicon oil (density: 0.971 g/ml3) and oleic acid (density of 0.9 g/ml3) were used as carrier fluid and surfactant, respectively. Nanoparticles of Fe3O4 and Ni0.5Zn0.5Fe2O4 were used as additives. Two types of samples containing 30 wt% magnetizable particles and 70 wt% carrier fluid were prepared; without additives (CI-based MR fluid) and with nanoparticles additives (1wt% (Ni0.5Zn0.5Fe2O4 +Fe3O4)-CI-based MR fluid). The details of producer are shown in Fig. 2.

Phase and structural evaluation were performed with X-ray diffraction (XRD) at a diffraction angular range of 2θ=20˚-80˚ and using CuKα. The particles size and morphology were evaluated through transmission electron microscopy (TEM) using a 75 kV Hitachi 7100 TEM (Tokyo Japan). A vibrating sample magnetometer (VSM; Home-made, Malek Ashatr, Isfahan, Iran) was employed to measure the magnetic behaviors of the samples at 25 ˚C.

A rotational rheometer (MCR 302 Anton Paar) with a magnetorheological equipment (MRD 70 Anton Paar) was employed to characterize the MR features. Parallel-plate configuration with a diameter of 20 mm at a gap of 1 mm was used and the magnetic field from 79.99 kA/m to 558.39 kA/m in the perpendicular direction to the flow was applied to assess the rheological behaviors of the suspensions at 25 ˚C.

3. Results and discussion

Fig. 3 shows the XRD patterns of the as-synthesized powders of Fe3O4 and Ni-Zn ferrites. All of the peaks can be indexed to a single-phase spinel structure with the space group of Fd-3m. The series of characteristic peaks are indexed for (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes of Fe3O4 and Ni-Zn ferrite. The samples can be totally indexed to the cubic spinel structures (JCPDS card No. 85-1436) and (JCPDS Card No. 019-0629), respectively. There is no extra peak in the XRD patterns, suggesting a single spinel phase formation for both cases.

The XRD pattern for a 60 h-milled sample indicated that the thermal energy provided through the mechanical alloying is sufficient to form a single phase Ni-Zn ferrite during the milling process. Furthermore, its formation during the process suggests that a large number of internal lattice imperfections such as grain boundaries and dislocations are created at such milling time, and consequently provide the short-circuit paths, which result in faster diffusion and matter transport. This is while a desired phase was not observed for durations less than 30 h milling; detecting Fe2O3 as second phase in the XRD patterns [21].

The TEM images exhibited almost spherical-shaped particles for both samples (Fig. 4). The range of particles size distribution was in 3-45 nm with the average size of 17 nm for the Ni-Zn ferrite sample. This is while the range was 3-25 nm with the average particle size of 13 nm for Fe3O4 sample.

A room temperature M-H hysteresis curves of the as-synthesized samples and CI powder particles are shown in Fig. 5. All the samples are showing an S-shaped hysteresis loop that suggests a ferromagnetic behavior of the samples. Furthermore, a little hysteresis behavior (low coercievity) indicates that Fe3O4, Ni0.5Z0.5Fe2O4 and CI have the soft magnetic materials behavior. The saturation magnetization (Ms) values found to be 196, 49.77 and 20.72 emu/g for CI, Ni0.5Z0.5Fe2O4 and Fe3O4, respectively. Their corresponding coercivity (Hc) values were 975, 18.80 and 1.92 Oe, accordingly. CI had the highest Ms value compared to Fe3O4, Ni0.5Z0.5Fe2O4. This could be explained in terms of the average particles size. In general, the magnetic properties such as Ms and permeability are attributed to two main magnetization mechanisms of spin rotation and domain wall motion [17,20]. Since the CI particles have greater size (micron-sized), the possibility for transition of single-domain into multi-domain regime increases. Furthermore, the larger particles possess greater domain walls. Therefore, an enhancement is achieved in the contribution of wall movement to magnetization and demagnetization, which needs less energy than that of domain rotation. Thus, it was expected that the Fe3O4 shows the lowest and CI shows the highest saturation magnetization because of possessing the lowest and highest particles size, respectively. Moreover, the Ms value of Fe3O4 nanoparticles was smaller than that of the values reported elsewhere [9]. For example, Chae et al. [9] reported a saturation magnetization value of 80 emu/g for Fe3O4 synthesized by solvothermal method. This indicates that the magnetic properties are dependent on not only the average particle size but also the preparation route. It simply means that synthesis method can have significant effect on the morphologic parameters of the sample, especially in nanostructured materials. This, in turn, influences the cation distribution over the tetrahedral and octahedral sites and consequently varies the magnetic properties of the sample.