Ceramics for Sustainable Energy Technologies with a Focus on Polymer-Derived Ceramics - Novel Combustion Concepts for Sustainable Energy Development - page 509

Fig. 10 High emissivity
coating on the basis of a
silazane on a flexible CIGSe
solar cell

developed (G ü nthner et al. 2014 ). Due to their superior radiation hardness and high

speci c power to weight ratio, CIGSe cells have received much attention for power

generation in space applications, e.g., as electrical power supply for satellites. A
signi cant drawback of CIGSe solar cells is their low emissivity ( ʵ < 0.2) because

of the high infrared re ectance of the cell. This leads to an increase in the cell

temperature of more than 100

fl

ciency.
Additional optical coatings with a high emissivity like SiO 2 or Al 2 O 3 reduce the
increase of temperature remarkably. Therefore, a solution of a polysilazane was
used to apply a coating with a thickness of 1

° C during operation and a decrease in the ef

2 µ m on the CIGSe solar cell

-

(Fig. 10 ). After curing in air at 200 flexible

SiOC layer with a high emissivity of 0.8, thus reducing the operation temperature
by more than 40

° C, the silazane coating converts into a fl

° C and subsequently leading to a signi cant increase of the

ef ciency under space conditions (G ü nthner et al. 2014 ).

2.2.2 Ceramic Coatings Based on Polysilazanes and Polysiloxanes
(T > 600

°

C)

The biggest drawback of the PDC technology is the shrinkage of the precursor
during pyrolysis due to the mass loss of 10

30 wt% but especially due to the

-

increase in density from 1.1 g cm − 3 in the polymer state up to 2.4 g cm − 3 after

annealing at up to 1,000 C. These effects result in shrinkage of the coating

thickness up to 80 %, leading to a critical coating thickness of 1

°

2 µ m. Despite the

-

limited coating thickness of only 1 m, a silazane-based layer protects stainless

steel from oxidation up to an annealing temperature of 1,000

µ

° C (Fig. 11 )(G ü nthner

et al. 2009b ).
While the described thin coatings are very effective for protection of metals from
oxidation, the low thickness limits use in an abrasive environment typical for heat
exchangers or incinerators. Furthermore, a thicker coating also facilitates a better
protection of the metal surface from aggressive environments, especially at higher
temperatures. A simple strategy to increase the coating thickness is to use ceramic