Ferrogel

Magnetic field sensitive gels, or as we call them "ferrogels", are typical representatives of smart materials [Xulu, 2003; Zrínyi, 2000 & 1998]. In a ferrogel finely distributed colloidal particles having superparamagnetic behavior are incorporated into the swollen network. These particles couple the shape of the gel to the external magnetic field. Shape distortion occurs instantaneously and disappears abruptly when the external field is removed. A discontinuous elongation and contraction in response to infinitesimal change in external magnetic field has been observed. Many kinds of such gels have been developed and studied with regard to their applications to several biomedical and industrial fields such as controlled drug delivery systems and muscle-like soft linear actuators. Saslawski et al [Saslawski, 1988] reported the gelatin microsphere that was cross-linked by polyethylenimine for the pulsed delivery of insulin by oscillating magnetic field. The release rate of insulin from the alginate sphere with strontium ferrite microparticles (1 µm) dispersed can be much enhanced compared with that in the absence of magnetic field. Zrínyi et al.

[Zrínyi, 1998]reported that the magnetically-sensitive hydrogels can undergo quick, controllable changes in shape by introducing magnetic particles into the chemically cross-linked PVA that can be used as a new type of actuator to mimic muscular contraction, as shown in Fig. 2.17.

Fig. 2.17 (a) Snapshots from the motion of a magnetic-field-sensitive polymer gel, and the rule behind the gel indicates the amplitude of the oscillation, which is more than 4 cm; (b) Snapshots from the motion of a magnetic gel, and the rule behind the gel indicates the amplitude of the oscillation, which is about 3 cm [Zrínyi, 1998]

(a)

(b)

2.4.1 Synthesis of magnetic iron oxide nanoparticles

It has long been of scientific and technological challenge to synthesize the magnetic nanoparticles of customized size and shape. Physical methods such as gas phase deposition and electron beam lithography are elaborate procedures that suffer from the inability to control the size of particles [Gupta, 2005] in the nanometer size range. The wet chemical routes to magnetic nanoparticles are simpler, more tractable and more efficient with appreciable control over size, composition and sometimes even the shape of the nanoparticles. Iron oxides (either Fe3O4 or γ-Fe2O3) can be synthesized through the co-precipitation of Fe²⁺ and Fe³⁺ aqueous salt solutions by addition of a base [Gupta, 2005]. The control of size, shape and composition of nanoparticles depends on the type of salts used (e.g. chlorides, sulphates, nitrates, perchlorates, etc.), Fe²⁺ and Fe³⁺ ratio, pH and ionic strength of the media [33,34].

Conventionally, magnetite is prepared by adding a base to an aqueous mixture of Fe²⁺ and Fe³⁺ chloride at a 1:2 molar ratio. The precipitated magnetite is black in color.

The chemical reaction of Fe3O4 precipitation is given in Fig. 2.18. The overall reaction may be written as follows [Gupta, 2005]:

Fe²⁺+2Fe³⁺+8OH^-Î Fe3O4+4H2O (2.1) According to the thermodynamics of this reaction, a complete precipitation of Fe3O4 should be expected between pH 9 and 14, while maintaining a molar ratio of Fe³⁺:Fe²⁺ is 2:1 under a non-oxidizing oxygenfree environment. Otherwise, Fe3O4

might also be oxidized as reaction (2.2)

Fe3O4 +0.25O2+ 4.5H2OÎ3Fe(OH)3 (2.2) This would critically affect the physical and chemical properties of the nanosized magneticpartic les. In order to prevent them from possible oxidation in air as well as from agglomeration, Fe3O4 nanoparticles produced by reaction (2.1) are usually coated with organic or inorganic molecules during the precipitation process.

Fig. 2.18 Scheme showing the reaction mechanism of magnetite particle formation from an aqueous mixture of ferrous and ferricc hloride by addition of a base [Gupta, 2005]

2.4.2 Hysteresis

When a ferromagnetic material is magnetized by an increasing applied field and then the field is decreased, the magnetization does not follow the initial magnetization curve obtained during the increase. The irreversibility is called hysteresis. An example of a full or major (i.e., M is taken to near Ms) hysteresis curve (or loop) is giveb in Fig.

2.19. At extremely high applied fields, the magnetization approaches the saturation magnetization, Ms. Then if the field is decreaed to zero, the M versus H curve does not follow the initial curve but instead lags behind until, when H=0 again, a remanant magnetizatipn remains, the remanence Mr. If the field is now applied in the teverse direction (a negative field), M is forced to zero at a field magnitude called the hysteresis coercivity, Hc. Increasing this negative field still further forces the magnetization to saturation in the negative direction. Symmetric behavior of this hysteresis curve is obtained as H is varied widely between large positive and negative curve is obtained as H is varied widely between large positive and negative values.

One could say that hystersis is due to internal froction. Hence the area insude the loop is the magnetic energy that is dissipated while circling the loop.

Fig. 2.19 Full-loop hystersis curve. Ms is the saturation magnetization, Mr is the magnetization remanence (at H=0), and Hc is the coercivity [Klabunde, 2001]

Table 2.1 Critical diameter of single-domain (Ds) and super paramagnetic (Dsp) in the magnetic materials [Klabunde, 2001]

In addition, the “closure” effects of ferrogel in the DC mageic fields and the

“bursting” effect of ferrogel in the AC magnetic fields were dependent on the particle size of magnetic nanoparticles, which means the saturation magnetization (Ms) hysteresis coercivity (Hc) were very important factor in the ferrogel. As Fe3O4 for example, it exhibited the superparamagnetic behavior (Hc is near to zero) while the particle size is lower than the 5 nm. Moreover, Hc arrive the maximum while the particle size is 128 nm (the critical point of single domain and multi domain of iron particles). Hc displayed the saturation while the particle size is higher than 200nm, as showin in Table 2.1 and Fig. 2.20.

Fig. 2.20 Relatioship of the particle size and hysteresis coercivity (Hc) [Klabunde, 2001]

In addition, Ms would increase with the particle size increased. Therefore, the better “clouser” effect in the DC nagnetic fields would be observed in the higher Ms and lower Hc, but the rapider “bursting” effect (inductive heating effect) in the DC nagnetic fields would be found in the higher Hc, implying the faster heating conducting.