Electrets were already mentioned (by Dan M?) in this thread.
A model according to which they are nothing more than
"frozen alignments of microscopic dipoles", is not
consistent with experimental data of Walker and
Jefimenko (J. Appl. Phys., vol. 44, August 1973, p. 3454).
The authors created many wax electretes (cylinders whose
thickness was 1.3 cm) , sliced them and measured NET
electric charges dQ in consecutive layers. The "frozen
dipoles" model would lead to dQ=0 in each of the 12
layers. The article shows distributions of dQ which are
avery different; they are perfectly consistent with the
1837 model of "penetrating charges" of M. Faraday.
Space charge densities in both outer layers were often
as high as 0.2 mC/cm^3. That is a lot. The electric
pressure in the dielectric must be very large for such
densities, even if the reducion of E by the factor 5 or
10, in comparison with the vacuum, is allowed.
Does anybody know about more recent work on space
charges in electrets? Or about older studies in other
types of electrets? Or any data on space charges in
dielectric materials? Please share what you know,
and your references.