TERM PAPER ONMAGNETIC NANOCOMPOSITES

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Department of Mechanical EngineeringNational Institute of Technology, Rourkela-769008

Contents

1. Introduction32. Brief History43. Magnetic Properties64. Types of MNC105. Synthesis146. Applications187. Conclusion228. References23

CHAPTER IINTRODUCTIONMaterials with features on the scale of nanometer often have properties dramatically different from their bulk scale counterparts. Nanocrystalline materials are single phase or multiphase polycrystals, the crystal size of which is of the order of few nanometers so that about 40 to 80 % of the atoms are in the grain boundaries [1]. Nanostructure science and technology is a broad and interdisciplinary area of research and development activity that has been growing worldwide in the past decades. Important among these nanoscale materials are nanocomposites, in which the constituents are mixed at nanometer length scale. They often have properties that are different compared to conventional micro scale composites and can be synthesized using simple and inexpensive techniques. The study of nanocomposite materials requires a multidisciplinary approach with impressive technological promise, involving novel synthesis techniques and an understanding of physics and surface science.

During the last decade, the development of magnetic nanocomposite materials has been the source of discovery of spectacular new phenomena, with potential applications in the fields of information technology, telecommunication or medicine. Magnetic nanocomposite materials are generally composed of ferromagnetic particles (grain size in nanometer scale) distributed either in a non-magnetic or magnetic matrix. The shape, size and distribution of the magnetic particles play an important role in determining the properties of such materials [2]. The matrix phase separates the magnetic particles and changes the magnetic exchange interaction. This affects the transport and magnetic properties. Therefore, understanding and controlling the structure of materials is essential to obtain desired physical properties.

CHAPTER IIBRIEF HISTORYNanocomposite magnetic materials have their origins in the amorphous alloys that were brought to market in the 1970's. Amorphous materials are characterized by a lack of long range atomic order, similar to that of the liquid state. Production techniques include rapid quenching from the melt and physical vapour deposition is another. The lack of crystallinity causes amorphous materials to have a very low magnetic anisotropy. METGLAS 2605 Fe78Si13B9 is a common amorphous magnetic alloy, in which B acts as a glass forming element. The importance of anisotropy suggests searching for other materials with isotropic magnetic properties. In magnetic materials the ferromagnetic exchange length expresses the characteristic distance over which a magnetic atom influences it's environment, and has values on the order of 100 nm. If the magnet has a structure with grain diameters smaller than the exchange length, it becomes possible to "average" the anisotropy of the grains to a low bulk value. Such a material then realizes the high saturation magnetisation (Ms) of the crystalline state and low coercivity (Hc) due to randomized anisotropy.

In 1988 Y.Yoshizawa [3] developed the FINEMET alloy based on Fe73.5 Si13.5B9Nb3Cu1. This was an extension of the common Fe-Si-B alloy with Cu as a nucleation agent and Nb as a grain refiner. The material is produced in the amorphous state and then crystallized by annealing. Nb that segregates to the grain boundaries acts a diffusion barrier preventing grain growth. The structure is a nanocomposite of 10- 100 nm diameter bcc- FeSi grains embedded in an amorphous intergranular matrix.

In 1990 K.Suzuki [4] reported the development of the Fe88Zr7B4Cu1 alloy which was named NANOPERM. Zr and B act as glass forming agents in this alloy and the microstructure consists of -Fe grains embedded in an amorphous matrix. By eliminating Si, higher saturation inductions are achieved than in FINEMET, but the Hc are also higher. The amorphous intergranular phase in both FINEMET and NANOPERM have Curie temperatures lower than that of the nanocrystalline grains.

In 1998 M.A. Willard [5] reported the development of HITPERM, an alloy based on the composition Fe44Co44Zr7B4Cu1. The key distinction is the substitution of Co for Fe. HITPERM forms '- FeCo grains in a Co enriched amorphous matrix. The amorphous matrix has a Curie temperature higher than the primary crystallization temperature of the alloy. This allows the '-FeCo grains to remain exchange coupled at high operating temperatures. Due to the presence of Co, HITPERM alloy has an Ms higher than FINEMET or NANOPERM as well as a higher Hc.

CHAPTER IIIMAGNETIC PROPERTIES OF NANOMATERIALS

The effect of reducing the physical size of materials is of great importance from bothfundamental considerations and modern practice. A brief discussion of magnetic behavior oflow dimensional systems is focused based on literature. Magnetic nanoparticles exhibit specific properties such as coercivity and superparamagnetism, generally attributed to reduced dimensions.

3.1 Coercivity

The coercivity of fine particles has a striking dependence on their size. Fig. 1 showsvery schematically, how the size range is divided, in relation to the variation of coercivitywith particle radius r.

Fig. 1: Overview of the size dependence of coercivity exhibited by magnetic particles: HC = 0 below superparamagnetic (SP) particle size limit r0; single-domain behavior (SD) between r0 and the single domain limit rc; and multidomain behavior (MD) for r > rc.

Beginning at larger size the following regions can be distinguished:(i) Multi-domain (MD): It is observed for r > rc and in this region, the coercivitydecreases as the particle size increases and the coercivity Hc is found to vary withsize as ~ 1/ rn.(ii) Single-domain (SD): For r0 < r < rc, the particles become single domain and in thissize range, the coercivity reaches a maximum.(iii) Superparamagnetic (SP): Below a critical size r0, the coercivity is zero because ofthermal effect, which is strong enough to spontaneously demagnetize the assemblyof magnetic particles.

3.2 Superparamagnetism

The effective magnetic moment of a ferromagnetic particle is determined by its size. Aferromagnetic sample with a volume greater than a critical value Vc divides into multiplemagnetic domains, each magnetized along the local easy axis but in one of two oppositedirections. The multiple domain structure is, however, no longer favorable below the criticalvolume, and the particle becomes a single domain with ferromagnetic alignment of all itsmoments along the easy axis in the same direction. Thermal fluctuations of the moment existon a microscopic scale, but to reverse the direction of the single domain's magnetizationrequires an energy E to overcome the crystal-field anisotropy. If single domain particlesbecome small enough, KV would become so small that thermal fluctuations could overcomethe anisotropy forces and spontaneously reverse the magnetization of a particle from one easydirection to the other, even in the absence of an applied field. Each particle has a magneticmoment = MsV and, if a field is applied, the field will tend to align the moments of theparticles and the thermal energy will tend to disalign them. This is called superparamagnetism.The probability of such a reversal by thermal activation is proportional to exp (-E/kT). Thisdiffers from conventional paramagnetism because the effective moment of the particle is thesum of its ionic particles, which can be several thousand spins in a ferromagnetic particlesmall enough to show superparamagnetism .Very fine ferromagnetic particles have very short relaxation times even at roomtemperature and behave superparamagnetically; that is, their behavior is paramagnetic buttheir magnetization values are typical of ferromagnetic substances. The individual particleshave normal ferromagnetic moments but very short relaxation times so that they can rapidlyfollow directional changes of an applied field and, on removal of the field, do not hold anyremanent moment. Superparamagnetism is characterized by two experimental features:1. There is no hysteresis; (i.e., both the retentivity and the coercivity are zero) in the fielddependence of magnetization.2. Magnetization curves measured at different temperatures superimpose whenmagnetization (M) is plotted as a function of Field (H) / temperature (T).Superparamagnetism can be destroyed by cooling. This follows because the characteristicfluctuation time for a particle's moment varies exponentially with temperature, so themagnetization appears to switch sharply to a stable state as the temperature is reduced. Thetemperature at which this occurs is called the blocking temperature (TB), and it dependslinearly on the sample's volume and on the magnitude of the crystal-field anisotropy.In the case of superparamagnetic materials, the magnetization shows temperature andpath dependence which is shown schematically in Fig. 2.

Fig. 2: Schematic diagram of ZFC and FC magnetization curves as a function oftemperature taken in an applied field H. Arrow indicates blocking temperature, TB.

The two curves zero field cooled (ZFC) and field cooled (FC) show different behaviorat low temperatures. As the temperature increases the magnetic moment in the FC curvedecreases. However, as the temperature begins to rise from 5 K, the moment in the ZFC curve begins to increase. At a certain temperature, the ZFC curve reaches a peak and thistemperature is called the blocking temperature (TB). The divergence of ZFC and FC curve and the blocking temperature depend on the particle size and its distribution. The blockingtemperature of a substance should decrease with increasing applied field and eventuallydisappear when the field reaches a critical value. The higher field is expected to lower thebarriers between the two easy axis orientations.For a particle of constant size below the blocking temperature TB, the magnetizationwill be stable and shows hysteresis. It refers to particles which have relaxation time fordemagnetization longer than 100 sec. For uniaxial particles using the same criterion forstability gives,

TB =KV/25k

Where K = Anisotropy constantV = Volume of the particlek = Boltzmanns constant (1.38 10-23 J K-1)If one considers Ni as a classical example with an anisotropy constant K = 4.5 103 Jm-3 then for a size 20 nm, the particle will show a blocking temperature (TB) at ~ 55 K usingequation above. Below TB, the magnetization will have relatively stable and shows ferromagnetic behavior. While above TB, the thermal energy will be sufficient to suppress the ferromagnetic behavior and thus the particles become superparamagnetic.

CHAPTER IVTYPES OF MAGNETIC NANOCOMPOSITES

4.1 Ceramic-matrix nanocompositesIn this group of composites the main part of the volume is occupied by aceramic, i.e. a chemical compound from the group of oxides, nitrides, borides, silicides etc.. In most cases, ceramic-matrix nanocomposites encompass ametalas the second component. Ideally both components, the metallic one and the ceramic one, are finely dispersed in each other in order to elicit the particular nanoscopic properties. Nanocomposite from these combinations were demonstrated in improving their optical, electrical and magnetic propertiesas well as tribological, corrosion-resistance and other protective properties. The binaryphase diagramof the mixture should be considered in designing ceramic-metal nanocomposites and measures have to be taken to avoid a chemical reaction between both components. The last point mainly is of importance for the metallic component that may easily react with the ceramic and thereby lose its metallic character. This is not an easily obeyed constraint, because the preparation of the ceramic component generally requires high process temperatures. The most safe measure thus is to carefully choose immiscible metal and ceramic phases. A good example for such a combination is represented by the ceramic-metal composite ofTiO2andCu, the mixtures of which were found immiscible over large areas in the Gibbs triangle of Cu-O-Ti. The concept of ceramic-matrix nanocomposites was also applied tothin filmsthat are solid layers of a few nm to some tens of m thickness deposited upon an underlying substrate and that play an important role in the functionalization of technical surfaces.Gas flow sputteringby the hollow cathode technique turned out as a rather effective technique for the preparation of nanocomposite layers. The process operates as a vacuum baseddepositiontechnique and is associated with high deposition rates up to some m/s and the growth of nanoparticles in the gas phase. Nanocomposite layers in the ceramics range of composition were prepared fromTiO2andCuby the hollow cathode techniquethat showed a highmechanical hardness, smallcoefficients of frictionand a highresistance to corrosion.

4.2 Metal-matrix nanocompositesMetal matrix nanocomposites can also be defined as reinforced metal matrix composites. This type of composites can be classified as continuous and non-continuous reinforced materials. One of the more important nanocomposites isCarbon nanotube metal matrix composites, which is an emerging new material that is being developed to take advantage of the high tensile strength and electrical conductivity of carbon nanotube materials. Critical to the realization of CNT-MMC possessing optimal properties in these areas are the development of synthetic techniques that are (a) economically producible, (b) provide for a homogeneous dispersion of nanotubes in the metallic matrix, and (c) lead to strong interfacial adhesion between the metallic matrix and the carbon nanotubes [6]. In addition to carbon nanotube metal matrix composites, boron nitride reinforced metal matrix composites and carbon nitride metal matrix composites are the new research areas on metal matrix nanocomposites. A recent study, comparing the mechanical properties (Young's modulus, compressive yield strength, flexural modulus and flexural yield strength) of single- and multi-walled reinforced polymeric (polypropylene fumaratePPF) nanocomposites to tungsten disulfide nanotubes reinforced PPF nanocomposites suggest that tungsten disulfide nanotubes reinforced PPF nanocomposites possess significantly higher mechanical properties and tungsten disulfide nanotubes are better reinforcing agents than carbon nanotubes. Increases in the mechanical properties can be attributed to a uniform dispersion of inorganic nanotubes in the polymer matrix (compared to carbon nanotubes that exist as micron sized agggregates) and increased crosslinking density of the polymer in the presence of tungsten disulfide nanotubes (increase in crosslinking density leads to an increase in the mechanical properties). These results suggest that inorganic nanomaterials, in general, may be better reinforcing agents compared to carbon nanotubes.Another kind of nanocomposite is the energetic nanocomposite, generally as a hybrid solgel with a silica base, which, when combined with metal oxides and nano-scale aluminum powder, can formsuperthermitematerials.

4.3 Polymer-matrix nanocompositesIn the simplest case, appropriately addingnanoparticulatesto a polymer matrix can enhance its performance, often dramatically, by simply capitalizing on the nature and properties of the nanoscale filler(these materials are better described by the termnanofilled polymer composites [7]). This strategy is particularly effective in yielding high performance composites, when good dispersion of the filler is achieved and the properties of the nanoscale filler are substantially different or better than those of the matrix. An example of this would be reinforcing a polymer matrix by much stiffer nanoparticles of ceramics, clays, or carbon nanotubes. It should be noted that the improvement in mechanical properties may not be limited to stiffness or strength. Time-dependent properties could be improved by addition of the nanofillers. Alternatively, the enhanced crystallization behavior under flow conditionsand other physical properties of high performance nanocomposites may be mainly due to the high aspect ratio and/or the high surface area of the fillers, since nanoparticulates have extremely highsurface area to volume ratioswhen good dispersion is achieved. Nanoparticle dispersibility in the polymer matrix is a key issue, which limits the appliciable particle volume fraction and there for also the multi-functionality of the composite material. Recent research on thin films (thickness