Electric transport properties of rare earth doped YbxCa1-xMnO3 ceramics (part III: Point defect chemistry)

A defect chemical model based on charge neutrality and laws of mass action is proposed to clarify the details of the chemistry of point defects for donor-doped YbxCa1-xMnO3. DC-conductivity measurements were carried out in a wide range of partial pressure of oxygen p(O2) ≈ 10−1 down to 10-19 MPa at 750⁰C for the first time without disintegrating the ceramic sample through reduction. A comparison of the experimental observations and the theoretical defect chemical models clearly shows the possibilities for controlling charge carriers in dependence of partial pressure of oxygen p(O2) and dopant concentration. The origin of a plateau state, of a drastic decrease in conductivity in the intermediate and reduction p(O2) regimes are figured out, respectively. In addition, the kind and concentration of the electronic and ionic majority charge carriers are determined and formulated according to the proposed defect chemical model. Furthermore, phase transitions were studied in a wide range of p(O2) at elevated temperature

Electric transport properties of rare earth doped YbxCa1-xMnO3 ceramics (part I: Optimization of ceramic processing)

A comprehensive study was performed in order to find the effect of different calcination and sintering conditions on the physical properties of calcium manganite ceramics in dependence of temperature T and partial pressure of oxygen p(O2). The eventual formation of oxygen vacancies during sintering was investigated and the results were confirmed by monitoring the release of oxygen using a ZrO2 oxygen sensor. The phase transition behavior was studied using thermogravimetric analysis (TGA) in a wide range of p(O2) ≈ 10−1 MPa down to 10-19 MPa at high temperatures accompanied by dilatometry- and XRD-measurements. Furthermore, the present study reveals for the first time a way for reducing and preventing crack formation that may occur during sintering. The present systematic research provides essential fundamental information before performing electrical measurements necessary in order to understand important factors about charge carriers and electrical transport mechanis

Heavily donor-doped, optically translucent and ferroelectric barium titanate ceramics through defect chemical engineering

The possibility of preparing translucent ferroelectric heavily donor-doped BaTiO3 ceramics rich in TiO2 has been demonstrated by conventional ceramics processing based on the synthesis via the solid state route and on subsequent sintering and annealing at high pressures of oxygen partial pressure. The resulting ceramics possess very high densities and a very fine-grained microstructure with a remarkably narrow grain size distribution. Under oxidizing conditions large amounts of the donor – in this case La – can be incorporated into the perovskite lattice during sintering, as the variation of the lattice parameters determined by combined XRD (X-ray diffraction) and Rietfeld-refinements suggests. The electrons released from these donors are captured most probably by vacancies of titanium, which act as acceptors. The optical properties are largely determined by the defect chemistry and in particular by the concentration of free conducting electrons. Especially the reduction of the free electron concentration is believed to be mainly responsible for the high degree of optical translucency. The normalized values of transmittance almost reach the best values reported so far for nano-crystalline barium titanate powders that have been consolidated via pressure assisted sintering. Measurements of the refractive indices showed that they can be modified by the state of polarization of the ferroelectric BaTiO3-based ceramics. This newly developed material possibly opens a door to novel electro-optical applications

Defect chemistry of donor-doped BaTiO 3 with BaO-excess for reduction resistant PTCR thermistor applications : redox-behaviour

The electrical conductivity of donor-doped BaTiO3 thermistor ceramics with excessive BaO revealing a reduction-persistent PTCR effect has been carefully examined depending on materials’ composition and oxygen partial pressure at moderate temperatures between 973 and 1273 K. This thermal regime represents the range which is relevant for the realization of insulating grain boundaries in these electrically inhomogeneous ceramic materials through reoxidation. Based on the experimental results strong evidence for a general correlation between the PTCR characteristics, DC-conductivity and the herewith associated defect chemistry significant to thermistor applications is presented for the system (Ba, La)mTiO3, where m designates the BaO-excess (m ≥ 1). Nominal compositions with a relatively low (Ba + La)/Ti ratio m only show a rather poor PTCR effect and an overall donor-type response in conductivity can be observed at all levels of oxygen partial pressure considered in the present study. With increasing (Ba + La)/Ti ratio m the nonlinear resistivity jump with increasing temperature strongly improves and the acceptor-type behaviour seems to dominate the total conductivity at partial pressures of oxygen above approximately 10−6 MPa. This result for compositions with high BaO-excess can be understood by the local formation of point defect associates in the grain boundary regions that consist of both acceptor-type titanium vacancies and donor-type oxygen vacancies. Their origin is attributed to the preferential local incorporation of excessive BaO into the BaTiO3 lattice at the intergranular interfaces