THE EFFECT OF VARIOUS SUBSTITUTIONS ON THE MICROSTRUCTURE AND PROPERTIES OF SrTiO 3 -Sr 2 Nb 2 O 7 BASED ELECTROCERAMICS FOR MICROWAVE APPLICATIONS

Abstract

Sr 5 Nb 4 TiO 17 is known to exhibit low loss in comparison to other compounds in the SrTiO 3 –Sr 2 Nb 2 O 7 system; therefore, the aim of this work was to investigate the effect of various cation substitutions on the phase, microstructure and electrical properties of Sr 5 Nb 4 TiO 17 for possible microwave (MW) applications. All the compositions were prepared through a mixed oxide solid state sintering route. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for phase and microstructural analysis, and LCR meter and a vector network analyzer for the measurement of electrical properties. Compositions in the Sr 5-x Ca x Nb 4 TiO 17 (x = 0-5) series formed single phase ceramics at ~1400°C; however, at temperatures ≥1500°C, secondary phases with general formula (Sr,Ca) 6 Nb 4 Ti 2 O 20 formed along with the parent Sr 5-x Ca x Nb 4 TiO 17 phases with x = 0 – 5. Optimum MW dielectric properties ( r ~54  f ~6.5 ppm/°C, Q u ×f o ~1166 GHz) were achieved for the ~97% dense Sr 2 Ca 3 Nb 4 TiO 17 ceramics. Compositions in the Sr 5-x La x Nb 4-x Ti 1+x O 17 (x = 0-4) series also formed single phase ceramics at 1450°C to 1550°C. Optimum values for Q u ×f o (~ 9969 GHz),  r (~ 61) and  f (~ 117 ppm/°C) were achieved for the Sr 5-x La x Nb 4-x Ti 1+x O 17 composition with x = 4 (i.e. SrLa 4 Ti 5 O 17 ). In an attempt for further improvement in the electrical properties, compositions in the SrLa 4-x A x Ti 5 O 17 series (A = Nd or Sm, x = 0 ≤ x ≤ 4) were also investigated. An analysis of the properties vs. composition plots indicated that the optimum MW dielectric properties i.e.  r ~ 54, Q u ×f o ~ 6000 GHz and  f ~ 0 ppm/°C corresponded to the SrLa 4-x Nd x Ti 5 O 17 composition with x ~ 1.6. SrLa 4-x Sm x Ti 5 O 17 compositions formed single phase ceramics at x ≤ 3 and a mixture of Sm 2 Ti 2 O 7 and SrTiO 3 at x = 4. Here again, an analysis of the results indicated that the optimum properties i.e.  r = 53,  f ~ 0 ppm/ o C and Q u ×f o = 3200 GHz corresponded to the SrLa 4- x Sm x Ti 5 O 17 composition with x~1.4. Compounds in the Sr 1-x Ca x La 4 Ti 5 O 17 (x = 0 - 1) and SrLa 4 Ti 5-x Zr x O 17 (x = 0 – 0.1) composition series exhibited  r ~53.7, Q u ×f o ~11532 GHz and  f ~-1.4 ppm/°C, and  r ~ 57.3,  f ~ +71.5 ppm/°C and Q u ×f o ~9841 GHz for x = 0.6 (Ca 2+ ) and x = 0.1 (Zr 4+ ). Consistent with previous, the substitutions of smaller cations for larger ones led to the development of fine grained microstructures indicating improved density.