Novel synthesis of core–shell urchin-like ZnO coated carbonyl iron microparticles and their magnetorheological activity

Abstract

The overall stability (thermo-oxidation, sedimentation) of MR suspensions is a crucial problem decreasing their potential applicability in real life. In this study the unique functional coating of carbonyl iron (CI) particles with ZnO structures was presented in order to develop a new MR suspension based on the core–shell ZnO/CI urchin-like dispersed particles. The two-step synthesis provides the suitable core–shell particles with improved sedimentation and also thermo-oxidation stability. Moreover, due to the enhanced sedimentation stability core–shell based suspensions exhibit higher values of the yield stress than those of bare CI based suspensions at 20 wt% particle concentration. The suspension with 60 wt% particle concentration reaches values of the yield stress around 2.2 kPa at 272 mT. The excellent MR efficiency of the core–shell ZnO/CI based suspension at elevated temperatures was observed. Finally, the dimorphic particle based suspension was prepared when the ratio between the carbonyl iron and core–shell urchin-like particles was 1 : 1. The highest yield stress was obtained in the case of a dimorphic particle-based suspension due to the good magnetic properties of the bare carbonyl iron and mechanical gripping between core–shell ZnO/CI urchin-like particles.

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1. Introduction

Magnetorheological fluids (MRF) represent a class of smart materials consisting of magnetic particles and liquid medium, whose rheological characteristics show fast and reversible transition from a liquid-like to a solid-like state when they are exposed to external magnetic fields.^1–5 In the absence of a magnetic field, the particles are randomly dispersed in the carrier liquid and MR fluid exhibits Newtonian-like behaviour. After applying a magnetic field, the dispersed particles are magnetized and the interaction between the induced dipoles cause the particles to form internal field-induced structures, parallel to the applied field.^6–12 Development of such structures results in the increase of the suspension viscosity and the transition from Newtonian to pseudoplastic behaviour appears with a certain level of yield stress.^13–16

There is a wide range of technological applications that benefit from materials able to provide rapid-response interfaces between electronic controls and mechanical systems, including rotary brakes, dampers, shock absorbers, torque transducers, etc.^17–20 Besides established commercial use of MR fluids in automotive industry, developments might be expected in field of civil and aerospace engineering especially.^21,22 Moreover, presently the new medical applications of MR fluids in the cancer treatment by hyperthermia method started to be developed.²³

In spite of the essential advantages of such materials, there are still two main limitations inhibiting of their impulsive expansion to the common real-life applications. The first one is the poor stability against sedimentation, due to the considerable difference between solid particles and liquid medium densities. There are several options how to improve such behaviour i.e. using thixotropic agents, surfactants²⁴ or utilizing bidispersed or dimorphic particle mixtures.^25,26 Also employing of the core–shell particles based on inorganic-polymer^27–34 or inorganic-MWCNT^35–37 as well as inorganic–inorganic coatings enhance the mentioned sedimentation stability. The second disadvantage is insufficient thermal stability of the iron particles, whose oxidation after they are exposed to the elevated temperature considerably decreases the magnetorheological properties.³⁸ Thus, the modification of the particles especially, using various polymers^39–41 is usually applied.

In our preceding study⁴² the coating of the carbonyl iron particles with low molecular weight substance cholesteryl chloroformate was performed. Such coating provided particles with slightly enhanced thermooxidation stability and moderately increased sedimentation stability of their silicone oil suspension.

Therefore, this study is concentrated on the preparation of the core–shell inorganic–inorganic ZnO urchin-like coated carbonyl iron microparticles via two-step synthesis. The successful synthesis was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction spectroscopy (XRD) techniques. Rheological properties in the presence as well as in the absence of the external magnetic field of both bare carbonyl iron and the urchin-like ZnO/CI core–shell particles-based suspensions were investigated. Finally, the crucial parameters such as thermooxidation and long-term sedimentation stability were also evaluated.

2. Materials and methods

2.1. Materials

Carbonyl iron microparticles (SL grade, α-iron content >99.5%, BASF, Germany), zinc acetate dihydrate Zn(CH₃COO)₂·2H₂O (ZAD), zinc nitrate hexahydrate Zn(NO₃)₂·6H₂O and hexamethylenetetramine (CH₂)₆N₄ (HMTA) were all purchased from PENTA (Czech Republic) and used as received without further purification. Demineralized water with conductivity about 10⁻⁷ S cm⁻¹ and ethanol were used throughout experiments.

2.2. Synthesis of core–shell urchin-like ZnO/CI particles

ZnO/CI composite particles were prepared in two steps. The first step involved “seeding” of CI particles with ZnO nanocrystals via modified procedure proposed originally by Spanhel.⁴³ Briefly, 0.005 M solution obtained by dissolving 0.165 g of ZAD in 150 ml of ethanol was added to the flask with 1 g of CI particles and sonicated for 2 minutes. After that, the flask was placed on the hot plate and heated at 80 °C. The dispersion was agitated mechanically by stirrer speed set at 900 rpm for 2 h before being left to cool down to the room temperature naturally. Than, the dispersion was washed several times with ethanol, filtered, and collected particles were dried at 60 °C.

After seeding procedure, CI_seed particles coated with ZnO nanocrystals were used as a substrate for the hydrothermal growth of ZnO nanorods. The growth was carried out in the mixture of aqueous solution of zinc nitrate hexahydrate (0.05 M) and HMTA (0.05 M) at 80 °C under mechanical stirring with agitator speed set up 650 rpm for 2 h. After cooling, the particles were washed thoroughly with distilled water, filtered and dried at 60 °C. Prepared CI/ZnO particles were characterized and used as a dispersed phase for magnetorheological suspensions.

2.3. Particles characterization

Crystalline phase of particles was characterized by the powder X-ray diffractometer X'Pert PRO X-ray (PANalytical, The Netherlands) with a Cu-K_α X-ray source (λ = 1.5418 Å) in the diffraction angle range 5–85° 2θ. The morphology was examined by scanning electron microscope Vega II/LMU (Tescan, Czech Republic) equipped with EDX-System OXFORD INCA Energy 200 (OXFORD INSTRUMENTS, UK).

The magnetic properties were studied using a vibrating sample magnetometer VSM 7400 (Lake Shore, United States). Thermogravimetric analysis (TGA) was carried out by the thermogravimeter Q500 (TA instruments, United States) in the temperature range from 25 to 800 °C at a heating rate 20 °C min⁻¹ in flowing air (30 sccm).

2.4. Suspension preparation and rheological measurements

ZnO/CI core–shell particles were suspended in silicone oil (Lukosiol M 200, viscosity η_c = 194 mPa s, density d_c = 0.970 g cm⁻³, relative permittivity ε′ = 2.89, loss factor tan δ = 0.0001, Chemical Works Kolín, Czech Republic) with 20, 40, and 60 wt% particle concentrations. The suspensions were mechanically stirred before each measurement. The rheological properties under an external magnetic fields in the range 0–300 mT were investigated using a rotational rheometer Physica MCR502 (Anton Paar GmbH, Austria) equipped with a Physica MRD 170/1T magneto-cell. The true magnetic flux density was measured using a Hall probe. Rheological measurements were performed at temperature 25, 45, 65, 85 and 105 °C.

2.5. Particle sedimentation

The silicone oil suspensions containing 40 wt% of both bare CI and ZnO/CI urchin-like particles were firstly mechanically stirred and then placed to the vials and sedimentation test was performed. The sedimentation ratio, as a ratio between the height of the suspension in the vial at the beginning of test and the height of the particle rich phase after certain time period, was evaluated as a measure of sedimentation stability within the 30 hours.

3. Results and discussion

3.1. XRD characterization

Powder XRD patterns of bare CI, CI seed, and CI/ZnO composites are shown in Fig. 1. For CI particles, peaks observed at 2θ = 44.6°, 65°, and 82.3° (marked by diamonds) correspond to (110), (200), and (211) reflections of iron with the cubic structure (ICDD PDF-2 entry 01-087-0722). The diffraction patterns of seeded CI sample seem to remain unchanged comparing to diffractogram of bare CI and no peaks corresponding to ZnO crystal phase were detected. On the other hand the presence of seeds on the CI particle surface was further confirmed by both SEM as well as by EDX. Diffraction patterns of CI/ZnO composite contain peaks related to iron cubic structure peaks at 2θ = 44.6°, 65°, and 82.3° and as expected also peaks located at 2θ = 31.7°, 34.3°, 36.1°, 47.4°, 56.5°, 62.8°, 67.8° and 68.9° (marked by stars) which correspond well to hexagonal wurtzite crystal structure of ZnO (ICDD PDF-2 entry 01-079-0207).

Fig. 1 XRD patterns of particles, where peaks representing CI are marked as (diamonds) and peaks representing ZnO are marked as (stars).

3.2. SEM analysis

Scanning electron micrographs of bare CI powder are shown in Fig. 2(a and b). It can be seen that particles are of the spherical shape with bimodal particle size distribution with two boundary fractions of CI powder, being about 5 μm in diameter and the second approximately about 1 μm. Fig. 2(c and d) shows morphology of CI particles after seeding (seeded CI) in ZAD ethanol solution. Nanoparticles with the size of about 150 nm are clearly seen in the enlarged image (Fig. 2d) although no ZnO diffraction pattern was detected by XRD measurement. Therefore, energy dispersive X-ray (EDX) elemental microanalysis was performed to confirm the presence of zinc and oxygen. Zinc as well as carbon traces in the material were clearly revealed besides the strong signal of iron dominating the whole spectrum in Fig. 3. However, oxygen's Kα emission can be only found hidden in the low energy shoulder of the broad iron's L series emission peak. Precise quantification of zinc and oxygen mass ratio cannot be made due to the aforementioned poor spectral line resolution and because of the generally low sensitivity and semi-quantitativeness of the used EDX technique. The product of hydrothermal growth of ZnO on seeded CI particles is shown in Fig. 2(e and f). Finally ZnO microrods are growing radially onto CI particles surface forming urchin-like structure of ZnO decorated spherical CI particles. The ZnO microrods are about 100–200 nm in diameter and 1–1.5 μm in length. It should be noted that the diameter of microrods is approximately the same as dimensions of seeded ZnO nanocrystals. However, the microrods are crowded closely together much more than it can be expected according to the surface coverage density of nanoseeds observed in Fig. 2(c and d). It seems that the seeded particles are multifaceted centers of branched growth of single rods.

Fig. 2 SEM micrographs of bare CI (a and b), ZnO seeded CI (c and d), ZnO/CI urchin-like (e and f) particles at various magnifications.

Fig. 3 EDX spectrum of a surface spot on seeded CI particle, magnified details in inset graph: 1 – C Kα line, 2 – O K Kα line, 3 and 4 – Fe L lines, 5 – Zn L lines, the inset image shows the spot on the CI particle, where the point spectrum was collected from.

3.3. Magnetic properties

Magnetostatic properties were measured by vibrating sample magnetometry and the resulting hysteresis loops of bare CI, ZnO seeded CI and CI/ZnO particles are shown in Fig. 4. All samples exhibit a soft magnetic behavior with negligible remanence and coercivity. Whereas the values of remanence and coercivity remained almost unchanged, the saturation magnetization (M_s) decreased significantly from 182.6 emu g⁻¹ for bare CI to 129 emu g⁻¹ for CI/ZnO particles. The decrease in saturation magnetization can be reasonably attributed to diluting contribution of non-magnetic ZnO in sampling volume. However, the saturation magnetization value of ZN seeded CI sample (159.5 emu g⁻¹) is significantly lower than that of bare CI (182.6 emu g⁻¹) although the ZnO fraction is negligible. Another mechanism than dilution but related with the surface thin semiconducting film must be active in such phenomena. Magnetic domain wall pinning can be employed for its explanation similarly as it was already demonstrated being crucial for polyaniline thin film covered core–shell CI particles.

Fig. 4 VSM spectra of the bare CI (dashed line), ZnO seeded CI (solid line) and ZnO/CI urchin-like (dash dot line) particles.

3.4. Thermo-oxidation and sedimentation stability

Thermogravimetric analysis under dynamic air atmosphere is an indicative experiment of thermo-oxidative stability. Obtained results are shown in Fig. 5. The dense surface coverage of reactive and easily corrodible CI particles by semiconducting microrods enhanced the stability of CI towards oxidation even at elevated temperatures. First, the total mass increase due to oxidation was lower for CI/ZnO than for bare CI particles over the same temperature range. In next, the onset temperature as well as the maximum rate of oxidation is shifted of more than about 200 °C towards higher temperature for CI/ZnO which can be considered as a significant effect.

Fig. 5 TGA curves and their derivatives of bare CI (dash dot line) and ZnO/CI urchine-like (solid line) particles.

Sedimentation stability is very important factor influencing the magnetorheological (MR) activity of the suspensions. Systems with poor long-term behaviour also exhibit reduced MR efficiency, due to fast particle settling. Such systems are not able to further create internal structures of suitable toughness after application of the external field and their application in real-life application is therefore limited. Thus the sedimentation ratio as a parameter representing the stability of the suspension in the time was investigated. As can be seen in the Fig. 6 the sedimentation ratio of suspension containing bare CI particles is nearly on 40% of the maxima after 5 hours while ZnO/CI urchin-like are on the 60%. Furthermore, within the another 25 hours of the test suspensions exhibit nearly same behaviour and the values of the sedimentation ration was fixed on the 30% for bare CI-based suspension and 50% for ZnO/CI urchin-like based suspensions, probably due to the partial decreased density of the core–shell particles and also because of the extraordinary urchin-like shape contributing to the enhanced sedimentation stability.

Fig. 6 Sedimentation stability of the bare CI particle suspension measuring jug A (□) and ZnO/CI urchin-like particle suspension measuring jug B (△).

3.5. Magnetorheological behaviour

With the help of the steady shear rheological experiments, the magnetorheological behaviour of the both suspensions based on bare CI particles and suspensions based on ZnO/CI core–shell urchin-like ones in the absence as well as in the presence of the various magnetic flux densities was investigated. As can be seen in the Fig. 7 both samples exhibit nearly Newtonian behaviour in the absence of the external field. However, after application of the external field the shear stresses increase with increasing magnetic flux density (Fig. 7a) for suspension based on bare CI particles. Values of the yield stress represented by the presence of the internal structures are not as high as would be expected for the material with such large magnetization saturation. This behaviour is probably connected with the very poor sedimentation stability of the suspension, as was discussed in the previous chapter and particles are not able to develop internal structures with appropriate toughness even at high magnetic flux densities. On the other hand, ZnO/CI core–shell urchin-like particles based suspensions, exhibit higher values of the yield stress than that of bare ones, even the magnetization saturation of ZnO/CI core–shell particles was considerably lower because of presence of coating layer. This behaviour is connected to enhanced sedimentation stability and suspension containing only 20 wt% particles reaches the value of the yield stress around 0.5 kPa at 272 mT.

Fig. 7 Dependence of the shear stress, τ, on the shear rate, g, for suspensions of 20 wt% of bare CI particles (a) and ZnO/CI urchin-like particles (b) at various magnetic flux densities, B (mT): (◁, ◀) 0, (●, ○) 45, (▲, △) 132, (■, □) 272.

Therefore the further investigation was concentrated only on the core–shell particles based suspensions. Fig. 8 represents the behaviour of the suspensions of different particle weight fractions (20, 40 and 60) under various magnetic flux densities. Yield stress of all suspensions increases with increasing magnetic flux density and also with particle weight fraction. To properly investigate the impact of the particle concentration on the development of the internal structures the data of the yield stress were fit with power–law relation τ_y ≈ B^a and values of the slope are summarized in the Table 1. As was observed and is usually stated in the literature^35,44,45 at low magnetic flux densities the behaviour of suspensions follow dipole mechanism where yield stress, τ_y varies rather with B² while at the high magnetic flux densities behaviour of suspensions exhibit local magnetization saturation of the particles and development of the internal structures follows saturation mechanism, where τ_y further varies only with B^1.5. In this case vectors of the magnetization are saturated and ability of the suspensions to develop considerable tougher internal structures with increased magnetic flux densities is not that high as is at lower flux densities. Finally, it can be concluded that suspension consisted of 60 wt% of ZnO/CI core–shell particles reaches the values of the yield stress nearly around 2.2 kPa at 272 mT. Moreover, critical magnetic flux density (B_c), is shifted towards higher values of B and suspensions with higher particle concentration need higher magnetic flux density to became magnetically saturated. Such values are very promising for the utilizing of these suspensions in the real-life applications.

Fig. 8 Dependence of the yield stress, τ_y, on the magnetic flux density, B, for suspensions of different particle weight fractions (wt%): (△) 20, (○) 40, (□) 60. Lines represent the power law model fit of the data for low (solid) and high (dashed) magnetic flux densities.

Table 1 Values of the slope for fitting of data with power–law relation at various magnetic flux densities

Particle concentration [wt%]	20	40	60
Low magnetic flux densities	1.90	1.84	1.79
High magnetic flux densities	1.24	1.36	1.52

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In applications, the suspensions are usually operating at elevated temperatures. Thus the rheological investigation was performed in the temperature range from 25 to 105 °C. For comparison of obtained results MR efficiency e = (η_E − η₀)/η₀ rather than absolute values of the yield stresses was selected (Fig. 9). The η_E represents the viscosity in the presence of the external magnetic field at low shear rate g = 1 s⁻¹, while η₀ is the viscosity in the absence of the external field at the same shear rate value. There are two reasons of MR efficiency e enhancement at elevated temperatures. In the absence of the external field, viscosity of suspensions η₀ decrease with increasing temperature. In the presence of the external field, magneto-attractive forces between particles are enhanced at elevated temperature and viscosity η_E rises. Therefore, the MR efficiency e increases with increasing temperature and with increasing magnetic flux density.

Fig. 9 Dependence of the MR efficiency, e, on the magnetic flux density, B, for suspensions of 60 wt% of core–shell ZnO/CI particles, at various temperatures T (°C): (□) 25, (△) 45, (○) 65, (◁) 85, (◇) 105.

In order to prepare suspensions with improved MR behaviour the bare carbonyl iron particles were mixed with core–shell ZnO/CI urchin-like particles in the ratio 1 : 1 and silicone oil and the suspension with 20 wt% of dimorphic particles was obtained. As can be seen in the Fig. 10, in the absence of the external magnetic field strength all suspensions exhibit nearly Newtonian behaviour, the lowest value of viscosity was obtained for bare CI particle-based suspension. Core–shell ZnO/CI particle-based suspensions exhibit higher values of the shear stress due to the ZnO coating of CI which results in enhanced mechanical gripping between particles. In the case of dimorphic particle-based suspension, the dimorphic suspension character, when bare carbonyl iron particles can fill free space between the core–shell ZnO/CI particles, contributing to the enhanced field-off values of the shear stress. Furthermore, after application of the external magnetic field, the suspension including dimorphic based particles exhibits the highest yield stress in comparison to suspensions consisting of both individual components. This behaviour is probably caused due to the synergism effect coming from the good magnetic properties of bare carbonyl iron and mechanical gripping of core–shell ZnO/CI urchin-like particles.

Fig. 10 Dependence of the shear stress, τ, on the shear rate, g, for suspensions of 20 wt% of (■, □) bare CI particles, (◁, ◀) ZnO/CI urchin-like particles and (●, ○) dimorphic particles at 0 mT (b) and (a) 272 mT.

4. Conclusion

A novel two-step synthesis of the core–shell ZnO/CI urchin-like particles was presented. The magnetic properties were shielded by small amounts of ZnO coating on the surface of the CI particles. Furthermore, the thermo-oxidation as well as sedimentation stability were enhanced in the case of core–shell based silicone oil suspensions. The rheological investigation also confirmed that core–shell particles based suspensions were able to develop considerably tougher internal structures at 20 wt% particles concentration than that of bare CI based ones. With increasing core–shell particle concentration in suspension, the yield stress increases and reached values around 2.2 kPa at 272 mT. Furthermore, the critical magnetic flux density (B_c) increases with increasing particle concentration therefore particles need higher B to be magnetically saturated. Finally the suspension based on dimorphic particles was prepared and due to the good magnetization saturation of bare carbonyl iron particles and mechanical gripping of the core–shell ZnO/CI urchin-like particles provides the system with the highest yield stress in comparison to the both individual components.

Acknowledgements

The authors wish to thank the Grant Agency of the Czech Republic for the financial support of Grant no. 202/09/1626. This article was written with the support of the Operational Programme ‘Education for Competitiveness’ co-funded by the European Social Fund (ESF) and the national budget of the Czech Republic, within the project ‘Advanced Theoretical and Experimental Studies of Polymer Systems’ (reg. number: CZ.1.07/2.3.00/20.0104). This article was written with the support of Operational Programme ‘Research and Development for Innovations’ co-funded by the European Regional Development Fund (ERDF) and the national budget of the Czech Republic, within the ‘Centre of Polymer Systems’ project (reg. number: CZ.1.05/2.1.00/03.0111).

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