Nanotec 2005

Applications of Magnetic Nanocomposites

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Application of nanostructure materials is vast and fascinating. The range of application includes cosmetic and sun block, nanofilters, coatings for cutting tools, tennis balls, materials for vehicles, catalysis, sensors, vitamins and additives in food and drug delivery etc. All such applications are for the benefit of mankind.

The basic iron oxide and its alloys were the initial soft ferrites which were applied to high frequency applications - heavy duty and light duty multipliers used in cores of transformers, generators and other communication equipments/microwave system components. Microwave applications have revolutionized both the domestic (cooking purpose) and the communication sector. Iron oxide and their alloys embedded in polymer materials are the basics to films or the matrix medium, which lead to development of magnetic memories (audio-video tapes, floppy discs, etc.). The thrust to develop natural and butyl rubber based ferrite composites (Rubber ferrite composites - RFC) was done in/with a view to arrive at flexible elastomer magnets [21, 22, 37]. RFCs have been proved as potential materials in microwave absorbers, designing of versatile magnets, noise filters, etc.

Fig. 11.23 (a) (A) Hysteresis loop of Mn1_xZnxFe2O4 (MZF) (with reference to the sample in Fig. 11.5); (B) hysteresis loop of RFC (Rubber ferrite composites) having MZF (with reference to the sample in Fig. 11.5) [22]; (b) Representative hysteresis loop for A: Ceramic Ni1-xZnxFe2O4 (NZF) and B: RFC (NZF in NR (Natural rubber) [37])

Fig. 11.23 (a) (A) Hysteresis loop of Mn1_xZnxFe2O4 (MZF) (with reference to the sample in Fig. 11.5); (B) hysteresis loop of RFC (Rubber ferrite composites) having MZF (with reference to the sample in Fig. 11.5) [22]; (b) Representative hysteresis loop for A: Ceramic Ni1-xZnxFe2O4 (NZF) and B: RFC (NZF in NR (Natural rubber) [37])

Fig. 11.23. (continued)

The application for polymer-iron oxide based magnetic nanocomposites is highly vital in thrust areas viz. environment, bioscience, conventional/alternative sources of energy and high technology sectors (space science, etc.).

Concentrating on environment pollution, pollutants in wastewater effluents from industrial or domestic sources contain plenty of organic chemicals, which must be removed or destroyed before recycling or discharging back to the environment. Ground and surface water may also contain such pollutants and needs proper treatment for sufficient purity for drinking purpose. Pollutant degradation by virtue of photo catalysis is a promising process for treating wastewater. Magnetic photo catalyst is a process, which allows easy recovery of photo catalyst from the treated water by magnetic force. Magnetic iron oxide core coated with titanium dioxide (TiO2) forms the basis for magnetic photo catalyst.

Iron oxide mineral analogs for environmental research have found extensive application in environmental laboratory settings as substitutes for various subsurface soils. Behavior of these oxides is regulated by surface chemistry and morphology.

There are reports where iron oxide mineral analogs are characterized to represent natural iron oxide minerals in water distribution systems and in subsurface environments. Iron oxide mineral analogs were procured as crushed corrosion products from a water pipe and glass beads coated with iron oxide by a forced acid hydrolysis technique [43]. Corrosion products were found to be a mixture of five highly crystalline forms of iron oxide including goethite (a-FeOOH), ferrihydrite (Fe5O7(OH)4H2O), iron hydroxide (Fe (OH)3), ferroxyhite (8-FeOOH), and lepidocrocite (g-FeOOH).

Though we have been mainly discussing on the soft magnetic materials, hard magnetic nanoparticles have shown immense potential for increasing magnetic storage devices.

Magnetic nanomaterials have also been used in filters to remove selected impurities from various types of fluids. The separation of organic contaminants such as polyaromatic hydrocarbons from water and the removal of sulfur compounds from hydrocarbon fuels are also being investigated with magnetic fluids.

The most promising aspect of pharmaceuticals and medicine as it relates to nanotechnology is currently drug delivery. In the area of biomedicine, nano-scale dimensional features are those for cell (10-100 mm), virus (20-450 nm), protein (5-50 nm). This is quite indicative that magnetic nanoparticles be associated with biomedicines. Magnetic nanoparticles can be coated with biological molecules to make them interactive with or bind to a biological entity, thereby providing a specific tag or address for controllable mechanism. The nanoparticles are magnetic, which means they obey Coulomb's law and can be manipulated by an external magnetic field gradient. This action at a distance, combined with the intrinsic penetrability of magnetic fields into human tissue, opens up many applications involving the transport and immobilization of magnetic nanoparticles, or of magnetically tagged biological entities. Thus a package can be delivered (say an anticancer drug) and targeted to the affected region of the body i.e., a tumor. There are reports on some special physical properties of magnetic nanoparticles for their potential application in biomedicine [44, 45].

In the area for cancer treatment, a relatively non-specific feature of chemotherapy is one of its major disadvantages. The therapeutic drugs are administered intravenously leading to general systematic distribution, resulting in deleterious side effects as the drug attacks normal, healthy cells in addition to the target tumor cells. However, if such treatments could be localized or brought exactly to the affected site, then the continued use of very potent and effective agents could be made possible. This was probably the main impetus to recognize the purpose to use magnetic carriers to target specific sites (generally cancerous tumors) within the body. In magnetically targeted therapy, a cytotoxic drug is attached to a biocompatible magnetic nanoparticle carrier [46] . The PMAA coated maghemite nanoparticles are recognized as potential magnetically targeted drug carriers by adsorbing an anticancer drug (carboplatin) by virtue of the ion-dipole interaction between CO.- of PMAA and carboplatin [35].

Towards mid-twentieth century, impetus for research in space and astronomy grew rapidly. There arose a necessity for controlling liquids in space (zero gravity problems). This led to the development of ferrofluids, which are colloidal suspensions of magnetic nanoparticles (e.g., Fe3O4 of size about 10 nm) in a liquid. The response of ferrofluids to external magnetic fields offered smooth control of fluid movement required for mechanical or other physical action. Magnetic elements and their alloy oxides (Fe, Co, Zn, and Mn) were basic to ferrofluids. Mostly ferrofluids refer to magnetite (Fe3O4) nanoparticles in some liquid medium. It is essential that the nanoparticles remain separate from one another in the liquid medium forming a homogenous system. If there is any tendency for agglomeration of magnetic nanoparticles in the medium, it will cause anisotropic behavior of the ferrofluid, which in turn will spoil its purpose. Magnetic and van der Waals interactions must be overcome to prevent the particles from agglomeration into larger particles. Superparamagnetic property is highly dependent on particle size. Thermal motion of magnetic particles that are smaller than 10 nm in size is sufficient to prevent agglomeration due to magnetic interactions. Surfactants (oleic acid or tetramethylammonium hydroxide) are added to oil-based or aqueous-based ferrofluids to prevent the nanopar-ticles from approaching one another very closely. The surfactants are basically long-chain hydrocarbon with a polar head that attaches itself to the surface of the magnetite particle. The long chains comprising the tails, act as a repellent cushion and prevent the close approach of another particle. Spikes are observed when the ferrofluids are subjected to strong magnetic fields. Such spikes arise from a tendency of the particles to line up along the magnetic field lines to lower their energy. The presence of surface tension in fluids restricts the extent to which the particles can align themselves within the field. The main application areas of ferrofluids are:

1. Mechanical movements in regions not easily approachable viz. rotating shaft seals in X-ray generators and in vacuum chambers.

2. High speed computer disk drives.

3. Improving performance of loudspeakers (damping of unwanted resonance and dissipating heat in the coils).

4. Migratory sense of animals.

5. Biomedical applications (targeted medicines and magnetic resonance imaging MRI).

Magnetic materials (FeBO3 and FeF3) transparent in the visible region (absorption coefficient <40 cm-13 at room temperature have caught up tremendous interest (Ziolo 1992) and [47]. This area included colored magnets, bead like magnets and low-density magnetic microphores.

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