Matjaz Valant

Dielectric characterisation of ceramics from the TiO2-TeO2 system

Abstract Compositions from TiO 2 –TeO 2 tie line were characterised using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and dilatometry. Results show that the only binary compound existing on the tie line is TiTe 3 O 8 . Single-phase TiTe 3 O 8 was synthesised at 700°C in air and sintered at 720°C to ∼95% of theoretical density. Such ceramics exhibit a relative permittivity of 50, a Q × f -value of 30,600 GHz and a temperature coefficient of resonant frequency ( τ f ) of +133 ppm/°C, measured at ∼5 GHz. The concentration of structural defects in the TiTe 3 O 8 grains is negligible which makes the dielectric properties of TiTe 3 O 8 ceramics insensitive to variations in the heat-treatment conditions. The TiTe 3 O 8 compound is chemically compatible with TeO 2 , which displays a negative temperature coefficient of resonant frequency (a relative permittivity of 19.3, a Q × f -value of 30,000 GHz and a temperature coefficient of resonant frequency of −119 ppm/°C for TeO 2 ceramics with ∼ 20% porosity) and can be used for compensation of the temperature coefficient of resonant frequency of the TiTe 3 O 8 compound. Ceramics from the TiTe 3 O 8 –TeO 2 subsystem can be sintered to >97% of theoretical density at temperatures as low as 670°C, which together with the fact that the ceramics exhibit a highly tunable τ f , relative permittivity around 30 and a Q × f -value of ∼22,000 GHz suggests a potential for use in LTCC technology.

Microscopic interpretation of sign reversal in the electrocaloric effect in a ferroelectric PbMg1/3Nb2/3O3-30PbTiO3 single crystal

With increasing temperature, PbMg1/3Nb2/3O3-30PbTiO3 (PMN-30PT) crystals change from pseudo-rhombohedral to tetragonal to cubic phases. In addition to the usual positive electrocaloric effect (ECE), a negative ECE, whose origin is uncertain, is observed. Here, these two types of the ECE contributions in PbMg1/3Nb2/3O3-30PbTiO3 crystals are modelled theoretically using a one dimensional statistical mechanical lattice model, which is solved by an exact matrix method. The quasi one-dimensional model reproduces the trends in the experimental behaviour and attributes the electrocaloric sign reversal to free energy changes induced by the electric field.

The effect of partial isovalent substitution in the A-sublattice on MW properties of materials based on Ba6–xLn8+2x/3Ti18O54 solid solutions

Abstract Investigations of materials based on the three systems: (Ba 1– y Pb y ) 6– x La 8+2 x /3 Ti 18 O 54 , (Ba 1– y Pb y ) 6– x Nd 8+2 x /3 Ti 18 O 54 , and (Ba 1– y Ca y ) 6– x Ln 8+2 x /3 Ti 18 O 54 revealed that a partial isovalent substitution in cation sublattices allows the control of the electrophysical parameters of microwave dielectrics. The investigations were carried out over a wide range of x values. Data on the relationship between the crystal-cell parameters and the microwave dielectric properties ( e , Q , and t f ) have been obtained and related to the size of the isovalent ion residing at crystallographic sites in the A-sublattice. Based on the data obtained, a non-random distribution of Ca 2+ and Pb 2+ ions on different A-sites as a function of their concentration in different systems was concluded. Various temperature-stable microwave dielectrics, based on the above three systems, have been synthesized and shown to exhibit excellent MW properties.

Formation and decomposition of the Bi2TeO6 compound

Abstract XRD and SEM studies of the formation of Bi 2 TeO 6 confirmed that the Bi 2 TeO 6 compound forms in an oxygen atmosphere and also during prolonged calcination in air. At elevated temperatures the compound decomposes with the degree of the decomposition being dependent on the oxygen partial pressure. The decomposition products were identified as Bi 6 Te 2 O 15 and the high-temperature phase 2Bi 2 O 3 ·3TeO 2+x . The sublimation of TeO 2 was proved not to be the main reason for the appearance of the Bi 6 Te 2 O 15 phase during the decomposition of Bi 2 TeO 6 .

Oxygen transport during formation and decomposition of AgNbO3 and AgTaO3

A thermogravimetric method was used to analyze intermediate processes involved in the formation and decomposition of AgNbO 3 and AgTaO 3 perovskites. Critical parameters that control the kinetics of the formation are associated with oxygen transport. The Nb 2 O 5 crystal structure has a capacity to trap molecular oxygen, which evolves during the decomposition of Ag 2 O that is present in a starting mixture. The formation of the perovskite phase involves a simultaneous reaction of three species: O 2 , Ag, and Nb 2 O 5 /Ta 2 O 5 . As the trapped molecular oxygen is in the immediate vicinity of the reaction site, the kinetics of the reaction is significantly accelerated. An absence of the molecular oxygen in the solid-state phase cannot be compensated for with an increase in a partial pressure of oxygen in the gas phase, that is, application of oxygen atmosphere.