Antonio J. F. Bombard

PHOSPHATE COATING ON THE SURFACE OF CARBONYL IRON POWDER AND ITS EFFECT IN MAGNETORHEOLOGICAL SUSPENSIONS

Two types of carbonyl iron powders, (CIPs, BASF AG), the HS and HS-I (I = insulated, due a coating with phosphate), and two kinds of silica, one hydrophobic (Cab-O-Sil® TS610) and other hydrophilic (Cab-O-Sil® M5), were used to evaluate the influence of the surface treatment of the magnetic particle and the kind of fumed silica on the formulation of some magnetorheological suspensions (MRS). Oscillatory measurements at no field showed an evident difference between the silicas, but not a specific interaction with the phosphate coating on HSI. On the other hand, steady flow experiments also without magnetic field showed that the kind of silica and its specific interactions with the coating on iron powder drove the rheological behavior of the MRS on all region of the shear rate. Under magnetic field, the flow curves differences will be due to the iron particles and its magnetic properties, mainly on the region of higher shear rate.

Evaluation of Magnetorheological Suspensions Based on Carbonyl Iron Powders

The particle size distribution and magnetic susceptibility of some commercial carbonyl iron powders (code names CC, CS, HQ, OX and SM) were measured. The particle size of the powders increases as follows: HQ<SM<CC OX<CS. The magnetic susceptibility increases in the order: HQ OX SM CC CS. Magnetorheological suspensions (MRS) with 66% w/w of iron were prepared and their rheological properties were evaluated at no field, 100, 200 and 300 Oe. The yield stress under 300 Oe measured with strain-stress curves increases in the order: HQ OX<SM<CC<CS, showing direct correlation with the susceptibility. The plastic viscosity without field increases in the order: CS<CC<OX<SM<HQ, an inverse correlation with particle size. These results show that the particle size and/or size distribution can be another important property of the powders, together with magnetic susceptibility on the formulation of improved MRS.

Thin-Film Rheology and Tribology of Magnetorheological Fluids in Isoviscous-EHL Contacts

This article is concerned with an investigation of the tribological performance of magnetorheological (MR) fluids in pure sliding soft-elastohydrodynamic lubrication (EHL) steel/polytetrafluoroethylene (PTFE) point contacts. The lubricating properties of MR fluids were measured in thin film, lubricated conditions using a ball-on-three-plates tribometer and compared to base fluids in the form of Stribeck curves. A range of techniques was employed to interpret the possible mechanisms of friction and wear of dispersed iron microparticles. The friction surfaces were investigated using optical microscopy, environmental scanning electron microscopy (ESEM), X-ray photoelectron spectroscopy, and energy-dispersive spectroscopy (EDS). In all cases investigated, the friction coefficient is found to strongly depend on the viscosity of the lubricant oil. In the case of low-viscosity liquids, iron microparticles are entrapped in the contact and plow the PTFE surfaces resulting in a sensibly constant friction coefficient. For intermediate viscosities, friction decreases at low speeds because of the so-called “ball-bearing” effect, and later, friction increases as particles become embedded in the PTFE matrix. Finally, for high-viscosity fluids, iron particles either accumulate around the rubbing zone as a barrier that reduces the supply of oil available to the contact for boundary lubrication or the particles indent PTFE surfaces.

Magnetorheology of dimorphic magnetorheological fluids based on nanofibers

We report a systematic experimental investigation on the use of nanofibers to enhance the magnetorheological (MR) effect in conventional (microsphere-based) MR fluids formulated in polyalphaolefin oil/1-octanol. Two kinds of nanofibers are employed that have very similar morphology but very different magnetic properties. On the one hand we use non-magnetic goethite nanofibers. On the other hand we employ magnetic chromium dioxide nanofibers. For appropriate concentrations the on-state relative yield stress increases up to 80% when incorporating the nanofibers in the formulation. A similar yield stress enhancement is found for both nanofibers investigated (magnetic and non-magnetic) suggesting that the main factor behind this MR enhancement is the particle shape anisotropy. The relative yield stresses obtained by partial substitution of carbonyl iron particles with nanofibers are significantly larger than those measured in previous works on MR fluids formulated by partial substitution with non-magnetic micronsized spherical particles. We also demonstrate that these dimorphic MR fluids also exhibit remarkably larger long-term sedimentation stability while keeping the same penetration and redispersibility characteristics.

MAGNETORHEOLOGICAL FLUIDS WITH CARBONYL AND WATER ATOMIZED IRON POWDERS

Our aim in this work was to propose the use of a ternary blend of two carbonyl iron powder CIP, mixed with water atomized iron powder (WAIP), to reduce the off-state viscosity, without prejudice of MRF performance in terms of yield stress and torque output. The idea of mix water atomized iron powder with carbonyl iron powder is not new. The US Pat. # 5,900,184 by Weiss et al. (1999) describes that a binary blend, half-to-half, can reduces the viscosity of MRF in the absence of magnetic field, and increase the torque output under field.

Magnetic Susceptibility and Saturation Magnetization of some Carbonyl Iron Powders used in Magnetorheological Fluids

Some commercial carbonyl iron powders, from BASF AG, were characterized through their magnetic susceptibility and saturation magnetization. The saturation magnetization increases in the order: HQ < OX < CS < CC < CL SM. Although all the powders have 96% minimum of iron content, this result suggests that an investigation at higher magnetic fields, under simultaneous shear is very important. The reversibility and the reproducibility of the MR effect were satisfactory, confirming that carbonyl iron powders are good ferromagnetic materials to prepare magnetorheological fluids.

Magneto-rheological fluids redispersibility – a factorial design study of phosphate shell on carbonyl iron powder with dispersing additives

We showed, in a previous paper, that Magneto-Rheological Fluids (MRFs) have different rheology when prepared with Carbonyl Iron Powders (CIP) phosphate (coated or uncoated). This was especially so when done without a magnetic field. This paper employs factorial design to examine the redispersibility and rheology of some MRF formulations; we use the same CIPs but with different dispersing additives. The factors are: CIP A (uncoated) or B (phosphate shell); additives with carboxylic acid or primary amine as the polar group; and n-octyl (C8H17) or n-dodecyl (C12H25) as the alkyl hydrocarbon chain (R-). CIP B was much more redispersible than CIP A, especially with amine additives; typical work values were > 5mJ @ 20 mm depth. In terms of viscosity, CIP A generated lower values, at shear rates above 100 s-1. It also realized higher yield stress values (Ho = 300 kA/m) than CIP B (50% and beyond).

CHAPTER 6:Thin-film Rheology and Tribology of Magnetorheological Fluids

This chapter is concerned with an investigation of the tribological performance of magnetic suspensions. We begin with a state-of-the-art review of related research available in the literature. Then we show that magnetic nanoparticles can both reduce friction and control starvation in lubricated contacts under the presence of non-uniform magnetic fields. Next, we move to the case of pure sliding soft-elastohydrodynamic (EHL) point contacts were friction is not as severe as the case in hard contacts. We show that at low speeds iron particles employed in the formulation of MR fluids are entrapped in the contacts and plow the soft surfaces resulting in a constant friction. This low speed (boundary) regime is dominated by the carbonyl iron grade employed in the formulation. However, at large sliding speeds, well in the full film lubrication regime, the friction coefficient is found to strongly depend on the viscosity of the lubricant oil.

Bi4Ti3O12 Particles Size Influence on Electrorheolocical Response

Electrorheological uids (ER) are commonly known as suspensions composed of semiconducting particles dispersed in insulating oil that respond to electric fields by gelling. The increase in suspension viscosity on application of the field is typically rapid and reversible and as a result, the ER response is amenable to applications where real time control of stress transfer properties is required. Ferroelectric particles are interesting in this application due to the presence of spontaneous polarization and high dielectric constant. Particularly, Bismuth Titanate (Bi4Ti3O12 - BIT) is well-known as layer-structured ferroelectrics, so the typical morphology of these crystals is lamellar. Therefore, these particles dispersed in oil, in the presence of an electric field must produce an interesting ER response. Thus, BIT powders were prepared by the conventional solid state reaction method and the particles size was adjusted using ball milling process. Different ER fluids containing average particles size about 2.5 to 0.5 μm were dispersed in silicon oil about 10% vol and were submitted to AC and DC electric field. The relation between the BIT particles size with the ER response was observed, presenting an increase of the shear stress with the reduction on particle size.

ELECTRORHEOLOGY OF DISPERSIONS OF BaxSr(1 x)TiO3 IN SILICONE OIL UNDER AC OR DC ELECTRIC FIELD

Electrorheology (ER) of ferroelectric materials such as nanometric BaTiO3 is still not fully understood. In this paper, nanoparticles of BaxSr(1-x)TiO3 (where x = 0.8, 0.9 or 1.0) were synthesized using the method of Pechini, calcinated at 950°C, and after, lixiviated under pH 1 or pH 5. A controlled stress rheometer (MCR-301) was used to make the ER characterization of dispersions made of Bax Ti1-xO3 in silicone oil (30% w/w), where (a) shear stress as a function of DC electric field (under constant shear rate) or (b) shear stress as a function of shear rate (under constant AC or DC electric field) were measured. We observed that electrophoresis occurred under electric field DC, creating a concentration gradient which induced phase separation in ER fluid. On the other hand, under AC fields above 1 kV/mm, the ER effect is stronger than for DC field, and almost without electrophoresis. Furthermore, there is an AC frequency, dependent on the disperse phase, where the ER effect has a maximum.