Special Heat Transfer Mechanisms: Active and Passive Thermal Diodes - Magnetocaloric Energy Conversion: From Theory to Applications - page 244

Fig. 6.27 a The magnetorheologic or electrorheologic fluid in the absence of electric and
magnetic elds, b The magnetorheologic fluid in the presence of an applied magnetic eld, c The
electrorheologic fluid in the presence of applied electric eld

volume fraction of the solid particles, the particle shape and size, the ratio of the
density of the two phases and the viscosity of the carrier fl uid. In most cases, the

shear stress also depends on the yield stress, which de nes the conditions under

which the fl uid will start to fl ow. However, with the magnetic eld being present,

the yield stress will drastically increase with the magnetic eld strength, trans-

forming the liquid magnetorheologic uid into a solid structure [ 129 ]. This trans-

formation can occur in a time period of milliseconds [ 130 , 131 ].
The magnetic particles can also be coated to prevent or reduce the effect of
sedimentation. Fang et al. [ 132 ] made analyses on two different MR suspensions,
among which the

fl

rst MR comprised carbonyl iron particles, and in the second, the
carbonyl iron particles were coated with multiwall carbon nanotubes (MWCNT) in
order to decrease the sedimentation effect. The addition of nanoparticles to the MR
can substantially in

fl uence the magnetorheology and other properties of MR fl uids.

136 ] exhibit similar behaviour to MRs. These
suspensions consist of a non-aqueous carrier

Electrorheologic (ER) fl uids [ 133

-

fl uid with suspended micron-sized

dielectric particles. As with the MR fl uids, the ER fl uids react strongly with the

applied electric eld, which leads to a substantial increase in the apparent viscosity,

yield stress and shear stress up to several orders of magnitude and the transition
from a liquid to a solid-like substance [ 134 , 135 ]. Also in this case, a chain-like
structure of particles is formed in the direction of the electric eld (Fig. 6.27 ). The

application uids is similar as that for magnetorheologics

[ 133 ]. There are also international conferences on Electrorheological Fluids and
Magnetorheological Suspensions, which deal with this important domain [ 137 ].
Since the aim of this subsection is to present the available technologies for
thermal diodes, magnetorheology and electrorheology can be potentially applied for
the following principles of thermal diodes:

eld of electrorheologic fl

￿ MR or ER thermal diode that applies a magnetically induced fl uid fl ow,

MR or ER thermal diode that acts as a thermal contact switch and
￿ MR or ER thermal diode that applies the anisotropy of thermal conductivity.

￿