  Dielectric materials, with high tunability at microwave frequencies, are key
components in the design of microwave communication systems. Dense
Ba0.6Sr0.4TiO3 (BST) ceramics, with different grain sizes, were prepared in
order to optimise the dielectric tunability via polar nano cluster effects.
Dielectric permittivity and loss measurements were carried at both high and low
frequencies and were supported by results from X-ray powder diffraction,
scanning and transmission electron microscopies, Raman spectroscopy and
piezoresponse force microscopy. The concentration of polar nano clusters, whose
sizes are found to be in the range 20 to 50 nm, and the dielectric tunability
increase with increasing grain size. A novel method for measurement of the
microwave tunability in bulk dielectrics is presented. The highest tunability
of 32% is achieved in ceramics with an average grain size of 10 um. The
tunability of BST ceramics with applied DC field is demonstrated in a prototype
small resonant antenna.
