Ferroelectric conversion makes use of the peculiar properties of
_dielectric_[12] materials. Barium titanate, for example, has good
dielectric properties at low temperatures, but, when its temperature is
raised to more than 120°C, the properties get worse rapidly. We cannot
discuss dielectric behavior thoroughly in this booklet; suffice it to
say that in this process heat is absorbed in a realignment of molecules
within the barium titanate latticework.
If we now place a slab of barium titanate between the two plates of an
electrical condenser and charge the condenser, as shown in Figure 14, we
have a unique way of converting heat into electricity directly. When the
barium titanate is heated above its _Curie point_[13] of 120°C, the
condenser’s capacitance is radically reduced as the dielectric constant
falls. The condenser is forced to discharge and move electrons through
an external circuit consisting of the load and the original source of
charge. Useful electrical energy is delivered during this step. Figure
14 shows the process schematically and mathematically. When the
dielectric is cooled, waste heat is given up by the barium titanate, and
the cycle is complete.
[Illustration: Figure 14 FERROELECTRIC ENERGY CONVERSION
_The ferroelectric converter is really an electrical capacitor whose
capacitance is changed by temperature. When heat is added,
capacitance drops, voltage rises, and the capacitor is made to
discharge through the load. CYCLE: ① Switch #1 closed, #2 open.
Condenser charges from battery to charge Q₂ at voltage V₁ with
capacity C₁. ② All switches open. Heat added, capacity changes from
C₁ to C₂, charge remains constant, so voltage changes from V₁ to V₂.
③ Switch #2 closed, #1 open. Condenser discharges through load and
battery to charge Q₁ at voltage V₁ with capacity C₂. ④ All switches
open. Heat rejected, capacity changes from C₂ to C₁, charge remains
constant, so voltage changes from V₁ to V₀. CYCLE THEN REPEATS.
Energy supplied from battery each cycle is E₁. Energy delivered to
load and battery each cycle is E₂. Net energy converted is then
E₂ - E₁, the difference in the shaded areas._]
(a) CIRCUIT
HEAT IN
BARIUM TITANATE DIELECTRIC
WASTE HEAT OUT
SWITCH #2
LOAD
SWITCH #1
BATTERY
(b) CYCLE DIAGRAM
charge
volts
Q₂, Q₁, E₁, E₂, V₀ V₁ V₂
GENERAL INFORMATION:
C₂ < C₁
V = Q/C