Ben Huybrechts

The positive temperature coefficient of resistivity in barium titanate

Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.

Influence of high oxygen partial pressure annealing on the positive temperature coefficient of Mn-doped Ba0·8Sr0·2TiO3

Abstract A barium strontium titanate compound which exhibited positive temperature coefficient resistivity (PTCR) behavior was annealed in high oxygen partial pressures, in order to obtain a high acceptor density at the grain boundaries. The PTCR behavior changed significantly. Rises in maximum resistivity up to 200 times were obtained. The temperature of maximum resistivity decreased by 40–50°C. From the Heywang model the changes in PTCR behavior due to the high oxygen partial pressure annealing are explained by a rise in acceptor density possibly accompanied by an increase in acceptor energy.

Novel oxygen sensor using hot spot on ceramic rod

A novel oxygen sensor using hot spot on ceramic rod of high-Tc superconductor RBa2Cu3O7−δ (R: rare earth element) has been developed. The hot spot appears by the self-heating of the local part on the RBa2Cu3O7−δ ceramic rod when a voltage above threshold is applied at room temperature. This sensor operates without any separate heater by taking advantage of the high temperature of the hot spot wherein oxide ions can diffuse easily. The oxygen concentration is determined from the value of the current flowing through the rod by utilizing the change in the resistivity of the hot spot depending on oxygen partial pressure in atmosphere. Oxygen concentration of 0∼100% can be detected with high sensitivity and the response time is several seconds. The response performance of this oxygen sensor is almost the same as that of limiting-current-type zirconia sensor operating at 500°C.

Electric Field Sensitive Moving Hot Spot in GdBa2Cu3O7- δ Ceramics

An electric field sensitive moving hot spot appeared on a GdBa2Cu3O7-δ bar with following dimension, 0.65 mm×0.65 mm×40 mm, when a dc voltage of 7 V was applied at room temperature. The spot moved to the negative electrode, and the direction of movement could be reversed time after time by switching the positive and negative electrode. The average velocity of the hot spot was 1.9 mm/min. The spot size increased with increasing voltage. This phenomenon is believed to be related to the ionic conduction by oxygen.

Synthesis of 90 K superconducting Y0.9Ca0.1Ba2Cu4O8 in oxygen at ordinary pressure by Ag2O addition

Abstract A Y 0.9 Ca 0.1 Ba 2 Cu 4 O 8 superconductor was successfully synthesized in oxygen at ordinary pressure (0.1 MPa) by the addition of Ag 2 O. T c(onset) was 90 K and T c(zero) was 77 K. The volume fraction of superconductivity was increased by Ag 2 O addition to the calcined material before final sintering. The above observation showed that the addition of Ag 2 O has an ability to improve the superconducting properties of Y 0.9 Ca 0.1 Ba 2 Cu 4 O 8 ceramics.

Influence of annealing on the superconductivity in (Pb,Cu)Sr2(Y,Ca)Cu2O7−δ ceramics

Abstract The influence of annealing on the superconductivity in (Pb,Cu)Sr 2 (Y,Ca)Cu 2 O 7−δ compounds was investigated in terms of the O(3) and O(4) oxygen site occupancy. Samples with a nominal composition of (Pb 0.5 Cu 0.5 )Sr 2 (Y 0.5 Ca 0.5 )Cu 2 O 7−δ were annealed in oxygen at various temperatures. The superconducting transition temperature ( T c ) increased with an increase in the annealing temperature. Based on the finding by Maeda et al. that the occupation of the O(4) site occupancy is detrimental to the superconductivity and from our thermogravimetric analysis, it is demonstrated that the increase in T c was caused by an increase in the O(3) site occupancy and/or a decrease in the O(4) site occupancy.

Effect of intragrain current on low‐field magnetic‐flux distributions of zero‐field‐cooled polycrystalline YBa2Cu3O7−δ

Magnetic‐flux distributions B(r) below the lower critical field of the grains in a zero‐field‐cooled disk and a ring of polycrystalline YBa2Cu3O7−δ are measured at 77 K with a Hall probe. The measured distribution in the disk had a steep gradient on the sample surface, and the distribution in the ring, over the applied magnetic‐field range of 1.5–4 mT, was ‘‘W’’ shaped. These results can only be explained consistently by considering the intergrain current, the intragrain currents, and their interaction. It is shown that the magnetic‐flux density measured by a Hall probe with an active area much larger than the grain size is not the intergranular magnetic‐flux density Binter but μBinter, where μ is effective magnetic permeability (intergranular volume fraction), and dB(r)/dr gives μ times the intergrain (transport) critical current density Jct and not Jct as is assumed by many authors.

Grain Boundary Properties of Oxygen Hot Isostatically Pressed Barium Strontium Titanate with Positive Temperature Coefficient of Resistivity

The influence of annealing at high oxygen partial pressures (O2-HIP) on the Positive Temperature Coefficient of Resistivity (PTCR)-behavior of Mn-doped Ba0.8Sr0.2TiO3 was examined. Although no significant microstructural or density changes could be observed as a consequence of the O2-HIP annealing, the electrical resistivity characteristics changed markedly. The maximum resistivity was increased about 160 times and the temperature of maximum resistivity decreased from 209°C to 149°C. The grain resistivity is not changed, which shows that O2-HIPping only modifies the grain boundary structure. From the Heywang model and the gradient in the Arrhenius plot i.e., resistivity as a function of the reciprocal of the temperature, an increase of 44% in the acceptor state density was estimated.

Moving hot zone in aluminium foil

order to avoid breaking of the sample. The time interval between each voltage increase was at least 2 min to allow the sample to reach its new thermal equilibrium. Fig. 2 shows that the resistance increases gradually with increasing voltage. This is due to the joule heat generated in the sample. When an a.c. voltage of 4 V was reached a hot zone appeared and the current decreased significantly. However, when the voltage was not lowered, samples broke almost immediately after the hot zone appeared. Fig. 3 shows photographs after the appearance of the hot zone for a sample positioned vertically. As can be seen, the temperature distribution in the hot zone is not homogeneous. A bright spot in the upper part of the bot zone is clearly visible in Fig. 3b, taken after 90 s. Preliminary experiments showed that the hot zone in aluminium wires in a vacuum obtained by a rotary pump did not move. Therefore, the movement of the molten zone in aluminium foils is believed to be related to the convection of air around the molten zone, and not to convection in the melt itself. Due to the convection, the temperature at the top of the hot zone was higher than in the rest of the sample. As a result the resistance of the top region of the bot zone was higher and the voltage concentrated over that region. When the sample was positioned horizontally a hot zone appeared that did not move. This suggests that the above explanation is indeed plausible. Fig. 4 shows that the velocity of the hot zone is not constant. This is believed to be caused by the roughness of the sample edge and inhomogeneities introduced by handling the foil, both of which introduce local changes in the resistance. The

Influence of High Oxygen Partial Pressure on the Positive Temperature Coefficient Resistivity of BaTiO 3

The influence of high oxygen partial pressures on the PTC behavior of non acceptor doped BaTiO 3 is studied by using oxygen-hot-isostatic-press (O 2 -HIP). Annealing in high oxygen partial pressures increased the maximum resistivity with a factor 3, also the minimum resistivity and the gradient in the Arrhenius plot, i.e. resistivity versus the reciprocal of the temperature, increased. The results are analyzed using the well accepted Heywang model. The changes after O 2 -HIPping can be explained with an increase in acceptor density at the grain boundaries.