Synthesis of Novel Magnetorheological Fluids
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viscosity of the MR fluids. The rationale for selection and the role of different components of MR fluids are briefly discussed in the following sections. Choice of Magnetic Dispersed Phase For the synthesis of high-strength MR fluids, iron has been a popular choice since it is the element with the highest saturation magnetization ^OMS = 2.1 T). Also, high-purity (>99.9%) iron powder prepared using decomposition of iron pentacarbonyl through a chemical vapor deposition (CVD) process is relatively inexpensive. The iron powders prepared using this CVD process often contain higher concentrations of such impurities as carbon, and these impurities also typically lead to an increase in mechanical and magnetic hardness of the particulate material. A reduction step is
normally used to lower the carbon content of iron powders. Such reduced carbonyi iron powders are magnetically softer and hence probably better suited for the synthesis of MR fluids (see Figure 1). The size of the particles useful for practical MR fluids is approximately in the l-10-/am range. Currently, very few sources (e.g., ISP and BASF in the United States) of iron particles that are suitable for MR fluids exist. This can be a concern for those involved in commercialization of MR fluids and devices. Particles larger than 10 /itn make it difficult to make MR fluids that are stable against settling. In the literature, virtually no experimental reports or theoretical models concern the influences of particle size, for a given volume fraction of the magnetic phase, on the yield stress and off-state viscosity of MR fluids. Apparently, for the same volume fraction of the magnetic phase, the yield stress for MR fluids based on smaller iron particles (~2 ^m) is somewhat lower than that for MR fluids based on larger particles (5-7 /im). On the other hand, when the particle size of the dispersed phase becomes smaller, the destabilizing effect of Brownian motion can become significant, leading to a decrease in the yield stress as the temperature of the MR fluid increases. The particle size distribution of the particulates used in the synthesis of MR fluids also appears to have an important effect on the "on-" and "off-" state rheological behavior of MR fluids. A potential advantage of using a multimodal particle size distribution is that the maxi-
Figure 1. Scanning-electron-microscopy micrographs of carbonyi iron particulates used in the synthesis of magnetorheological fluids.
MRS BULLETIN/AUGUST 1998
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Synthesis of Novel Magnetorheoiogical Fluids
mum possible volume fraction of the magnetic phase can be increased without causing an unacceptable increase in the offstate viscosity. This can lead to higher yield stress and temperature-stable MR fluids. The influence of average particle size and the beneficial effects of using a bimodal particle size distribution on the off-state (i.e., zero field) viscosity has been discussed recently by Foister.2 In addition to the process based on decomposition of iron pentacarbonyl, many other processes are available for
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