The dielectric properties of dense ceramics of the “twinned” 8H‐hexagonal perovskite Ba8Nb4Ti3O24 are reported. Single‐phase powders were obtained from the mixed‐oxide route at 1325°C and ceramics (>92% of the theoretical X‐ray density) by sintering in air or flowing O2 at 1400°–1450°C. The ceramics are dc insulators with a band gap >3.4 eV that resonate at microwave frequencies with relative permittivity, ɛr∼44–48, quality factor, Q × fr∼21 000–23 500 GHz (at fr∼5.5 GHz) and temperature coefficient of resonant frequency, TCf,∼+115 ppm/K.

Rietveld refinement of room temperature (RT) neutron diffraction (ND) data reveals 12R-type hexagonal perovskites Ba3LaNb3O12 (BLN) and Sr3LaNb3O12 (SLN) to adopt space group R3¯ with tilted NbO6 octahedra. The presence of an octahedral tilt transition (Ttilt) at 465 K in BLN from R3¯ to R3¯m is proposed from a combination of high temperature ND data and fixed frequency permittivity measurements. Ttilt is estimated to be much higher at ∼720 K for SLN. The large difference in the RT temperature coefficient of the resonant frequency (τf), −100 ppm/K for BLN compared to −5 ppm/K for SLN, is attributed to the closer proximity of Ttilt to RT for BLN. τf in these 12R-type hexagonal perovskites can therefore be tuned by controlling the tolerance factor and therefore Ttilt in a manner similar to that used for many Ba- and Sr-based 3C-type ABO3 perovskites.

The dielectric properties of dense ceramics of the n=0 member of a newly identified homologous series Ba3+nLaNb3TinO12+3n, where n=0, 1, and 2, are reported. Single‐phase powders can be obtained from the mixed‐oxide route at 1350°C and dense ceramics (>97% of the theoretical X‐ray density) with uniform microstructures (3–5 μm) can be obtained by sintering in air at 1500°C. The ceramics are excellent dc insulators with a band gap >2.6 eV that resonate at microwave frequencies with a relative permittivity, ɛr∼44, a quality factor, Q×fr, of ∼9000 at fr∼5.5 GHz and a temperature coefficient of resonant frequency, TCf,∼−100 ppm/K.

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Platinum-rhodium Type R and S thermocouples fail in service because of two main reasons: excessive grain growth in the platinum limb at high temperatures (causing wire breakage) and temperature drift, i.e. deviation of electromotive force (emf) generated over time because of deposition of rhodium oxide on the platinum limb, out of accepted industry tolerances. Finding a solution to the problem is challenging because of the contradictory requirements of strength and narrow permissible emf range: emf is extremely sensitive to any dopants or alloying elements that can be used to improve strength at high temperatures. Johnson Matthey has developed a new thermocouple wire (platinum limb) called 'HTX ™ ' by doping platinum with a small quantity of zirconium which is oxidised during processing to electrically neutral zirconium oxide. Zirconium oxide improves the high temperature strength of platinum wire by restricting the grain growth but does not significantly affect the emf. The result is a wire which has the potential to last many times longer at high temperatures (>1200 °C) than standard platinum wire, whilst still achieving Class 1 tolerance (i.e. ±1 °C at 1000 °C). It also improves the thermoelectric drift characteristics of the thermocouple by counteracting any reduction in emf due to rhodium contamination by an increase in emf as any remaining zirconium is converted to zirconium oxide. We describe an impartial long-term assessment (up to 1500 h) of the thermoelectric stability of the HTX ™ wire by exposure to high temperature with periodic in situ calibration using a Co-C (1324 °C) high temperature fixed point. This yields stability measurements with very high precision.