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Origin of Appearance of PTCR Properties in Bi^|^ndash;Sr^|^ndash;Ti^|^ndash;O System
Abstract
(Bi0.85Sr0.15)4(Ti0.95Nb0.05)3O12 shows a large positive temperature coefficient of resistivity (PTCR) properties of almost 4 orders of magnitude at about 543 K. This temperature does not correspond to the Curic point of Bi4Ti3O12, but to the melting point of Bi metal. The microstructure of samples with or without showing PTCR properties observed by optical microscopy, showd the presence of Bi metal distributed in matrix. The various resistivity-temperature characteristics were obtained by controlling the distribution Bi metals in the matrix. Therefore, the new appearance of PTCR properties in Bi–Sr–Ti–O system would be related with the melting and solidification of Bi metal which is finely distributed in samples and isolated for each other. The equivalent electrical circuit for the samples were determind by complex impedance and modulus analysis. A model of this appearance of PTCR properties is proposed from these results as follows; Electrical current would flow in Bi metal at lower temperatures, but would not flow at higher temperatures. The resistivity of the sample at higher temperatures would be determined by matrix. Another RC component which acts only in the vicinity of PTCR temperature was also observed. This component may be due to the interface of Bi metals and the matrix.
A new type of PTC material based on Bi metal/ceramics composites has been developed. Bi/SrBi4(Ti0.95Nb0.05)4O15 composites with various Bi contents were fabricated by powder metallurgical process. It was found that the samples of x=0.73∼0.80 in xBi–(1−x)SrBi4(Ti0.95Nb0.05)4O15 composites fired at 1075°C for 2 h in Ar atmosphere can show the PTCR properties at the melting point of Bi metal of about 270°C. The composites consisted of the microstructures where Bi metal phases were dispersed in the SrBi4(Ti0.95Nb0.05)4O15 ceramics matrix. The same techniques were applied to other ceramics matrix. Both of Bi/TiO2 and Bi/Al2O3 composites were confirmed to show PTCR properties at the same temperature. Since the resistivity-temperature characteristics at higher temperatures were determined by the ceramic matrix, the resistivity of composites and the magnitudes of PTCR effect could be controlled by choosing the different resistivity of the ceramic matrix.
A method of characterising the electrical properties of polycrystalline electrolytes is described which enables grain boundary (intergranular) and bulk (intragranular) impedances to be separated and identified, by reference to an equivalent circuit which contains a series array of parallel RC elements. In the simple case of the “ideal solid electrolyte”, the equivalent circuit contains a single RC element. The impedance and modulus spectra, i.e. plots of Z″ and M″ versus log ω, are simple “Debye” peaks whose peak maxima coincide at an angular frequency ωmax=(τσ)−1, where τσ is the “conductivity relaxation time” and the complex modulus is the inverse complex perimittivity. For real solid electrolytes there is usually a distribution of relaxation times, in which case the maxima in the impedance and modulus spectra no longer coincide. An assignment of peaks in these more complex spectra is possible in principle, since the modulus spectrum effectively suppresses information concerning grain boundary (and electrode) effects. Experimental results are presented for cold-pressed lithium orthosilicate and germanate, and for sintered β-alumina. Some advantages of this new approach are demonstrated by comparison with conventional impedance and admittance plane methods of analysis.
Semiconducting BaTiO3 can be prepared by substituting small amounts of ions of higher valency for Ba or Ti ions. At higher concentrations the foreign ions are compensated by metal ion vacancies. Polycrystalline samples prepared in air show an enormous increase in resistivity above the ferro-electric Curie point. According to Heywang this is a result of surface barriers which are very sensitive to the value of dielectric constant. This theory is extended with a ferro-electric effect.RésuméLe TiO3 Ba semiconducteur peut être preparé en remplaçant les ions de Ba et de Ti par de petites quantités d'ions de plus forte valence. Aux plus fortes concentrations les ions introduits sont compensés par les vides créés par les ions de métal. Des échantillons polycristallins préparés dans l'air démontrent une augmentation énorme dans la résistivité au-dessus du point Curie ferro-électrique. D'après Heywang, cela est causé par des barrières de surface qui sont sensibles à la valeur de la constante diélectrique. On étend cette théorie à l'effet ferro-électrique.ZusammenfassungHalbleiter-BaTiO3 lässt sich herstellen, indem man kleine Mengen höherwertiger Ionen für Ba-oder Ti-Ionen substituiert. Bei höheren Konzentrationen werden die Fremdionen durch Metallionen-Leerstellen kompensiert. Polykristalline in Luft hergestellte Proben zeigen oberhalb des ferroelektrischen Curie-Punkts einen starken Anstieg des spezifischen Widerstands. Heywang erklärt dies aufgrund von Oberflächen-Schranken, die für den Wert der dielektrischen Konstanten sehr empfindlich sind. Diese Theorie wird durch einen ferroelektrischen Effekt erweitert.
A large number of compounds of the general form (Bi2O2)2+ (Mem−1RmO3m+1)2− are synthesized. Me and R represent ions of appropriate size and valency and m equals 2, 4 and 5. A dielectric study revealed ferroelectricityin the following compounds: MeBi2R2O2 (Me Ba,Pb, Sr and R Nb, Ta), MeBi4Ti4O15 (Me Ba, Ph, Sr), Me0.5Bi4.5Ti4O15 (Me K, Na) and Me2Bi4Ti5O18 (Me Pb, Sr). Curie temperatures exist over a wide range from about 100°C to perhaps as high as 900–950°C (for Bi3TiNbO9). The presence of Bi ions seems related to the high (Tc > 500°C) Curie temperatures of many members. In a given series, the Curie points decreased with the divalent ion in the order: lead-strontium-barium. At the Curie temperature, a symmetry change from tetragonal to orthorhombic takes place, with the largest orthorhombic distortion observed being 1-007 Bi3TiNbO9 at room temperature. Dielectric studies on single crystals indicate the ferroelectric axis to be the pseudo-tetragonal c-axis.