Biomedical Engineering Reference
In-Depth Information
ceramic composites using thermoplastic and reactoplastic polymeric
matrices are described in various sources [15-26]. Haupt et al. [15], prepared
products by incineration of a polymeric matrix. Incineration of the organic
part, as a rule, is accompanied by oxidation and thermal destruction, which
consumes oxygen irreversibly, thus making the ceramic grains amorphous.
Loss of SC properties of materials produced via incineration can be
explained in this way. This demands a full circle restoration. It must be
noted that the coke formed after incineration also has a negative impact on
SC properties. This technique can be used to produce polymer-ceramic
composites designed to protect high-temperature superconductors against
humidity [13, 14].
It is known that particulate polymers, as a rule, improve a number of the
features of composites (hardness, impact strength and heat resistance, for
example). This is mainly related to the formation of a special interfacial
layer between the filler and polymeric binder. High-temperature SC
polymer-ceramic composites can be obtained both by the conventional
hot pressing of a ceramic mixture with some ready-made, highmolecular
weight binder and by the polymerization filler method [21]. Hot pressing
[18-20, 22-28] of the Y
1
Ba
2
Cu
3
O
7
x
oxide ceramic and superhigh molecular
polyethylene mixture at 200
C destroys the SC properties, which are
restored only after treatment of samples in a dry oxygen stream [22].
With regard to conventional fillers, several specific properties of
perovskite high-temperature superconductors (such as their layered
structure, developed surface of the ceramic grains, catalytic properties,
and free oxygen dislocated on the surface of the ceramic grains) have a
significant impact not only on the formation of a phase boundary and,
consequently, on the physical-mechanical properties, but on the SC
properties of the polymer-ceramic composites as well. Regardless of the
nature of the binder, the critical transition temperature (T
c
) into the SC state
of polymer-ceramic composites increases by 1-3K. This increase in the
transition temperature is due to the interaction of the polymer chains with
the surface of the ceramic grains.
It could be expected that such an interaction should change the packing
and structure of the polymer chains as well as the conformation at the
interphase. Interphase phenomena at the ceramic-polymer boundary have
been investigated, for example, for superhigh molecular weight polyethylene
+Y
1
Ba
2
Cu
3
O
7
x
ceramic. The influence of crystalline binders on the
valence state of Cu
2+
(I) in the ceramic has also been investigated.
It is interesting to study the influence of the environment on the SC
properties of SC ceramics and polymer-ceramic composites at ambient
temperatures. It is known that the bulk oxygen content determines the
properties of high-temperature oxide ceramics of the Y-Ba-Cu-O system
and the pattern in which oxygen fills the crystal structure. The presence of
8
