Patent ID: 6149832
Filing Date: 2000-11-21
Classification: H01F

Abstract:
A magnetorheological fluid comprising a dispersion of low coercivity, generally spherical magnetic particles in a liquid vehicle,said particles consisting essentially of carbonyl iron particles with at least about 10.sup.19 surface hydroxyl groups per square meter of surface area,said fluid comprising at least 20% by volume of a first size group of said particles characterized by a mean diameter in the range of one to two micrometers, a standard deviation of particle diameters of less than half of said mean diameter value and a specific surface area greater than four square meters per cubic centimeter,said fluid optionally comprising a second size group of said particles characterized by a mean diameter, larger than the mean diameter of said first size group, but not larger than about 15 micrometers, the standard deviation of the diameters of said second size group being less than half of their mean diameter value,said liquid vehicle comprising at least 80% by volume of a composition incapable of forming hydrogen bonds with said hydroxyl group containing particles, said composition being selected from the group consisting of aliphatic hydrocarbons and glycol esters, said vehicle having a viscosity at 40.degree. C. no greater than 100 cP, andsaid liquid vehicle plus dispersed group of small particles having a yield stress at 25.degree. C. in the absence of a magnetic field of at least 200 Pa.