The requirements set on the choice of particle material are that the particles have to be magnetically multi-domain and they exhibit low levels of magnetic coercivity. In addition, maximizing the inter-particle forces and thus maximizing the MR effect can be achieved by choosing the particle material of the saturation magnetization JsTesla. The higher Js, the higher the inter-particle forces and the higher the MR effect is. The material most used today is high purity carbonyl iron (Fe) powder, made by chemical vapor deposition (CVD) of iron pentacarbonyl (Fe (CO) 5). The reasons for this are:
– The high chemical purity (>99.9%), which leads to less domain pinning.
– The mesoscale dimensions, which have many magnetic domains.
– The spherical shape, which minimizes the magnetically shape anisotropy.
– Its high magnetization saturation (Js = 2.4Tesla).
– The particles are magnetically soft and thus non-abrasive.
The best available particles are alloys of iron and cobalt that have saturation magnetization of about 2.4 Tesla. Unfortunately, such alloys are prohibitively expensive for most practical applications. The best practical particles are simply pure iron, as they have a saturation magnetization of 2.15 Tesla. Virtually all other metals, alloys and oxides have saturation magnetization significantly lower than that of iron, resulting in substantially weaker MR fluids.
Metal particles are in on-state (with magnetic field) guided by the magnetic field to a chain-like structure. This chain-like structure restricts the motion of the fluid and therefore changes the rheological behavior of the fluid. This structure resistance in counteracting with carrier liquid is the MR-effect. The metal particles are usually made of carbonyl iron, powder iron, iron/ cobalt alloys for large magnetic saturation. The amount of metal powder in MRF could be up to 50% by volume. The particle size is some ??meter and varies depending of manufacturing. The particle size could be chosen and combined differently for various purposes. In case of carbonyl iron the particle size is 1-10?meter.Basically larger particles and higher fraction of powder in MRF will provide higher torque in an on-state.
For proper utilization of this technology we need such type of particles which can magnetized easily and quickly therefore we use metal particles. Metal particles used in the MR- technology are very small. Size of the particle is approximate of the order of 1?m to 7?m. Commonly used metal particles are carbonyl iron, powder iron and iron cobalt alloys. Metal particles of these materials have the property to achieve high magnetic saturation due to which they are able to form a strong magnetizing chain. The concentration of magnetic particles in base fluid can go up to 50% (approx.)
The magnetic particles for MR fluids should have high saturation magnetization and low coercivity. Carbonyl iron powder 13,14,15,16,17, Nickel Zink ferrite 18, Iron oxide coated polymer composite particles 19, and Iron Cobalt alloy 20, 21 go well with these requirements. Saturation magnetization of Nickel Zink ferrite, Iron powder and Iron Cobalt alloy are 0.4T,2.1T and 2.43T respectively 22,20i.e. Iron Cobalt alloy has highest saturation magnetization but its density (8.1g/cm3) is greater than Iron therefore it aggravates gravitational settling. All the above mentioned magnetic materials are costly and therefore don’t suit the present synthesis of low cost MR fluid. Therefore for present synthesis Iron powder produced by electrolytic process has been chosen as this process yields the Iron of very high purity at very economical price (US$ 10 per Kg).
.All of the studies mentioned above, deal with micron-sized particles. When the size of the particles becomes smaller, Brownian motion may reduce the strength of the MR fluids, although it can aid in stabilizing against sedimentation. Several researchers have studied the effect on rheological behavior of adding nano-sized particles to MR fluids. Lemaire et al. (1995)24 studied the influence of particle size on the rheological behavior and found that if the ratio of magnetic interaction energy to thermal energy is much larger than unity, the yield stress increases with particle size. Kormann et al. (1996)25 made stable fluids with nano particles in polar liquids, but reported a low yield stress. Rosenfeld et al. (2002)26 and Poddar et al. (2004)27 prepared fluids with nano-sized iron powders, micron-sized powder and hybrid fluids, that is, a mixture of micron-sized and nano-sized particles. Both groups found that the micron-scale fluid exhibited the highest yield stress. However, Chaudhuri et al. (2005)28 and Wereley et al. (2006)29 found that replacing micro particles with nano particles, in small concentrations, tended to increase the field dependent yield stress. Furthermore, the nano particles reduced the sedimentation rate (Wereley et al., 2006)29. Similar results were seen by Park, Song and Choi (2009)30 Burguera et al. (2008)31 found that yield stress decreased with increasing concentration of nano particles, although stability was improved. Fang et al. (2009) 32 have studied the effect of carbon nanotubes on sedimentation stability and yield stress. Finally, Lopez-Lopez et al. (2009)33 analyzed the dependence on rheological behavior of cobalt powder for particle diameters in the range 50 nm to 1?m and found that particle size did not have much influence on the MR response for particles larger than 100 nm. Intrigued by the studies mentioned above, the focus of this project is on developing MR fluids suitable for a particular application in a prosthetic device. An MR fluid composition is sought that gives a suitable balance between the shear yield stress and off state viscosity. As previous studies on MR fluids, containing nano-sized particles, have focused on improved yield strength and sedimentation stability, this work provides a comprehensive experimental investigation of the field-induced shear yield stress versus the off-state viscosity for a number of different fluid mixtures.
Here in this research work magnetisable particle as EC 10 TR and DPR 325 electrolytic iron powder (Industrial metal powder Pune) is used.
Additives form the third part of a MR fluid. Because the magnetic polarization mechanism, the working principle of MR fluids, is not affected by the surface chemistry of surfactants, it is relatively straightforward to use additives in MR fluids for all kind of purposes, such as:
– Prevention or minimization of sedimentation.
– Prevention or minimization of coagulating of the particles:
To maintain a coating on the particles in order to enhance redispersibility.
To enhance anti-oxidation.
In water-based carrier, liquids additives are used to control the pH-value.
The generally used Surfactants for the preparation of MR fluid are as follows.
1. Oleic acid
2. Tetramethylammonium hydroxide
3. Citric acid
4. Soy lecithin
5. White lithium based grease
Additives form the third component of the Magneto Rheological Fluid and are used in the fluids for many purposes, e.g. prevention and minimization of sedimentation, prevention and minimization of coagulating of the particles, maintain a coating on the particles in order to enhance re-dispersibililty and to enhance anti-oxidation. The prevention of sedimentation is one of the most important aspects. For the practical reason, the sedimentation rate is to be kept at minimum possible level. White lithium grease is a good additive with carrier liquid silicone oil. This white lithium grease is used in many automotive applications and easily available in automobile spare part shops. White lithium grease is thus used as an additive for the sample.
Highly viscous materials such as grease or other thixotropic additives are used to improve settling stability. Ferrous naphthanate or ferrous oleate can be used as dispersants and metal soaps such as lithium stearate or sodium stearate as thixotropic additives. Magnetic particles are coated with some materials like polystyrene (PS), gaur gum etc. to prevent CI particles from coming in contact with each other and to decrease the CI particle density to improve the sedimentation stability
The prevention of sedimentation is one of the most important aspects. If this is not prevented, MR fluids will alter their properties significantly over time. Some examples of additives are given which help to prevent this. Sedimentation is typically controlled by the use of thixotropic agents and surfactants such as xantham gum, silica gel, stearates and carboxylic acids. The thixotropic networks disrupt the flow at ultra low shear rates (the viscosity becomes nearly infinite), but thins as the shear rate is increased. The stearates form a network of swollen strands when used in conjunction with mineral oil and synthetic esters that serve to entrap particles and immobilize them. Fine carbon fibers have also been used for this purpose. The fibers increase the viscosity through physical entanglement but exhibit shear thinning due to shear-induced alignment. In this way they all contribute to keep the particles suspended in the carrier liquid and in this way the MR fluid will not alter its properties much over time. An important conclusion that was derived from the literature is that for each application and for each device, a specially formulated MR fluid should be developed. This because each MR fluid application or device has its own distinct working conditions, such as the environment in which it has to operate and the forces it is subjected to.
2.5)SELECTION OF ADDITIVES
As discussed earlier, many additives have been attempted by researchers for synthesis of MR fluid. Generally thixotropic agents are added to prevent particle sedimentation. In addition, anti wear, anti corrosion, friction modifier and antioxidant agent are also added. Following are some important additives used in the synthesis of MR fluids for different purposes as reported in the literature:
• To overcome sedimentation,
Fumed Silica 34, Lithium stearate, Aluminum distearate, Thiophosphorus, Thiocarbomate, Phosphorus, Guar gum 35, Organoclay36 Poly vinyl pyrolidone 37, Poly vinyl butyl 38
• To overcome agglomeration,
Fumed silica 34, Fibrous carbon 39, Stearic acid 40, Sodium dodecyl sulphate 41, Viscoplastic media 42
o To increase abrasion wear resistance, ZDDP (Zink dialkyl dithio phosphate ) 33-34
• To reduce oxidation, ZDDP 35
• To reduce friction resistance. Organomolybydnums (Moly) 34
Here in this research work Grease as an additive one
Conclusions: Many researchers are worked on the synthesis and characterization of MRFluid using different carrier oils such as synthetic oils, mineral oils, hydrocarbon oil etc and its cost is very high and there is very less work on the edible oils such as soybean oil, cotton seed oil, safflower oil etc its cost is very less as compared with the above without compromising in its properties. Most of the researchers are used carbonyl iron as a magnetisable particle in preparation of MRFluid ad its cost is also high. So here in this research work low cost electrolytic iron powder such as EC10TR,DPR325 is used. Many types of additives are used to control the sedimentation phenomenon by researchers and its cost is very high but here in this research work commercial grease is used in the preparation process.