Adalbert Feltz

Spinel forming ceramics of the system FexNiyMn3–x–yO4 for high temperature NTC thermistor applications

Abstract The thermal stability and aging of spinels of the system Fe 2 O 3 –NiO–Mn-oxide are studied by measuring the electrical properties of ceramic samples obtained by sintering up to 1350°C. Based on the formula Fe III x Ni II y Mn z 3−x−y O 3+ x 2+δ , chemical analyses of ceramic samples with defined values of x and y make it possible to determine the average oxidation number z of manganese with z= 2δ 3−x−y +2 thus leading to the share of oxygen 2−x−2δ 4 O 2 which is liberated during sintering. X-ray diffraction measurements of samples with higher NiO and lower Fe 2 O 3 content indicate NiO separation. Hence, the analytical results are suitable to elucidate the phase constitution of the ceramics consisting of a mixture of NiO and of a spinel with modified composition. The formation of the spinel ceramics FeNi 0.5 Mn 1.5 O 4 ( x =1, y =0.5) and FeNi 0.7 Mn 1.3 O 4 ( x =1, y =0.7) in a single-phase state, i.e. without oxygen loss on sintering at 1350 or 1300°C in air, is due to the lower content of NiO at a sufficiently high fraction of Fe 2 O 3 in the composition. The compounds show ferrimagnetic behavior. FeNi 0.5 Mn 1.5 O 4 has a Curie temperature of 245 5°C. At increasing temperature, commonly above 150°C or 200°C up to 400°C the specific electrical resistivity ρ 25°C and the value of the B 25/100°C constant of both of the two compounds depend on time and on the thermal pre-treatment of the samples. Aging is due to the frozen-in state of the equilibrium of distribution of cations between the tetrahedral and octahedral sites of the spinel structure. This tends to shift towards equilibrium when the temperature is increased step by step during aging; it can be followed by measuring the electrical properties. The changes are observed to arise approaching the Curie temperature. Soaking of the samples at 650°C after aging for 72 and 144 h up to 500°C shows that the variations of the ρ 25°C and B 25/100°C values can be repeated in a following aging cycle, provided that the same cooling rate is applied. On the other hand, above 400 °C the ralaxation effects fail. Changes in this range of higher temperature involve fast rates of cation re-distribution thus leading to short waiting times until a constant value of the electrical resistivity is achieved. Hence, above 400°C, FeNi 0.5 Mn 1.5 O 4 ceramics appear completely stable against aging within common measuring times. Therefore, they should be suitable for high temperature NTC thermistor applications. The comparatively high value B 25/100°C =7470 K makes it possible to measure the temperature via the electrical resistivity with satisfactory sensitivity, e.g. α= 1 ρ d ρ dT =0.8% is achieved in the range around 750°C.

Perovskite forming ceramics of the system SrxLa1−x TiIVx+yCoIIyCoIII1−x−2yO3 for NTC thermistor applications

Abstract The paper reports the preparation, structure and electrical properties of oxide ceramic semiconductors based on the series SrxLa1−x TiIVx+yCoIIyCoIII1−x−2yO3 with perovskite type stucture: 0

Characterization of dielectric powders by a new defined form factor

Abstract Capacitance measurements of oxide powders in the frequency range of 1 kHz to 10 MHz have been performed in order to deduce a form factor which is related to the morphology of the particles and their arrangement in the powder. Using a capacitor measuring setup the frequency independent real part of the complex dielectric constant was measured in the range of 1 kHz to 2 MHz in dependence on the volume fraction. The interpretation follows the models derived for inhomogeneous dielectrics, which allow to calculate a morphology dependent parameter provided that the permittivity of the solid is known. A new defined form dependent weighting factor introduced in Bruggeman’s model of geometrical averaging between lamellar and spherical constituents is shown to supply the better approximation to measuring data than the aspect ratio of ellipsoids determining the depolarization factor. The results are verified for aggregated and partially stepwise ground powders of Mg2TiO4, MgAl2O4, SrTiO3, CaTiO3, Al2O3, and ZnO and related to other particle shape parameters derived from SEM studies indicating very clearly the suitability to describe the morphology of powders by this new form factor. The frequently applied Lichtenecker rule does not allow to consider particle morphology. For comparison the specific surface area BET data and the d50 values from grain size distribution measurements of the powders are also reported. The applicability of the method for checking the reproducibility of different lots of capacitor raw materials is shown for two COG powders.

Semiconductor properties of thin and thick film Ga2O3 ceramic layers

Abstract The semiconductor behavior of thin and thick film β Ga2O3 layers is studied by measuring the resistivity as a function of oxygen partial pressure and temperature in the range up to 900°C. As for ZnO and SnO2 a relatively high initial oxygen vacancy defect concentration has to be assumed for β Ga2O3. However, the conductivity is by many orders of magnitude lower and the activation energy by about 1 order of magnitude higher. With increasing temperature a change at about 810±50°C from a lower value of the activation energy EA(2)=1·6 ± 0·1 eV to a higher one EA(1)=2·4 ± 0·1 eV is observed at thin film ceramic layers thus leading to the assumption that oxygen cleavage in contact with the atmosphere is achieved in the upper range. Contrary to the band model which is convincingly founded for ZnO and SnO2 in the literature, polaron hopping seems to be the more suitable model for analysis of the conductivity data of β Ga2O3. The lower value EA(2) is interpreted as the polaron hopping energy at approximately constant charge carrier concentration. On the other hand, in the high temperature range above Tch the charge carrier density is varying. However, at the applied measuring conditions, this variation remains below the initial oxygen vacancy defect concentration. Corresponding to formula I GaIII2-2xGaII2x′O3-xV {O,x} and formula II GaIII2-xGaIx″O3-xV {O,x} two different structures for the oxygen vacancy defects in β Ga2O3 are discussed. The measurements seem to confirm formula I. However, provided that there is an equilibrium between states corresponding to formula II and I, the assumption of double occupied GaI states is also consistent with the experimental results.

Permittivity and the form factor of BaTiO3 and related powders

Abstract Capacitance measurements of ferro- and paraelectric oxide powders in the frequency range of 1 kHz–10 MHz have been performed in order to deduce a form factor which is related to the morphology of the particles and their arrangement in the powder. Using a capacitor measuring setup the frequency independent real part of the complex dielectric constant was measured in the range of 1 kHz–2 MHz as a function of the volume fraction. The interpretation follows a model derived for inhomogeneous dielectrics, which makes it possible to calculate a morphology dependent parameter provided that the permittivity of the solid is known. The results are verified for various BaTiO 3 powders and for aggregated and stepwise ground Ba 1 − x Sr x TiO 3 powders and are related to other particle shape parameters derived from SEM studies indicating the suitability of this method to describe the morphology of ferroelectric powders. For comparision the specific surface area BET data and the d 50 values from grain size distribution measurements of the powders are also reported. The applicability of the method for checking the reproducibility of different lots of ferroelectric capacitor raw materials is shown for Z5U and X7R powders.