  Ultra-high temperature ceramics (UHTCs) are interesting materials for a large
variety of applications under extreme conditions. This paper reports on the
production and extensive characterization of highly dense, pure zirconium and
tantalum diborides, with particular interest to their potential utilization in
the thermal solar energy field. Monolithic bulk samples are produced by Spark
Plasma Sintering starting from elemental reactants or using metal diboride
powders previously synthesized by Self-propagating High-temperature Synthesis
(SHS). Microstructural and optical properties of products obtained by the two
processing methods have been comparatively evaluated. We found that pure
diborides show a good spectral selectivity, which is an appealing
characteristic for solar absorber applications. No, or very small, differences
in the optical properties have been evidenced when the two investigated
processes adopted for the fabrication of dense TaB2 and ZrB2, respectively, are
compared.
