Advances in Science and Technology Vol. 67

Abstract: In contrast with metals, the resistivity of ceramics decreases with increasing temperatures. This phenomenon was first discovered in 1833 by Faraday and remained a mere scientific curiosity until 1930, when Samuel Ruben proposed the fabrication of a pyrometer device, which explored the negative temperature coefficient (NTC) of resistance exhibit by Cu2O. Eight decades later, NTC ceramic thermistors constitute an important business segment for most electroceramic manufacturers. Here, we present a review of the most significant scientific and technological advances, which lead to the enormous commercial success of NTC thermistors. This review concludes with an outlook into future possible applications of NTC ceramics, providing that some current technological shortcomings (such as ageing) are resolved.

Abstract: Nb-doped and Nb-Mn-codoped (1-xmol%)BaTiO3-xmol%(Bi0.5Na0.5)TiO3 (BBNTx) lead-free positive temperature coefficient of resistivity (PTCR) ceramics were prepared by the conventional solid state reaction method. The XRD patterns indicated that all BBNTx samples formed a single perovskite structure with tetragonal phase. 0.25 mol% Nb doped BBNT1 ceramic, sintered at 1330°C for 1h in air, had low room-temperature resistivity (ρ25) of 80 Ω•cm and a high resistivity jump (maximum resistivity [ρmax]/minimum resistivity [ρmin]) of 4.2 orders of magnitude with Tc about 152°C. The Nb-doped BBNTx (10≤x≤60) ceramics also showed distinct PTC effect with Tc between 185 and 232°C by sintering in N2, which was shut off when samples were cooled to a low temperature. In addition, The Nb-Mn-codoped BBNT1 ceramics exhibited higher resistivity jump than the single Nb-doped ones, with increasing the room-temperature resistivity.

Abstract: Magnetic susceptibility measurements, magnetization and neutron diffraction results at low temperature for cobalt and manganese oxide spinel ceramics are presented. The Curie temperature varies similarly with the sample composition in ceramics and powders. The experimental molar Curie constant variation is explained by the presence of Co2+, CoIII, Mn3+ and Mn4+, and possibly Co3+ in the octahedral sites for the cobalt rich phases. The magnetic moments of the cations in tetrahedral and octahedral sites are not collinear and the global magnetization is oriented in a third direction.

Abstract: The coupling between the strain gradient and electric polarization is known as flexoelectricity in dielectrics materials. In case of magnetic media it takes the form of electric polarization induced by spin modulation and vice versa. This spin flexoelectricity causes new physical phenomena of micromagnetism such as electric field driven magnetic domain wall motion and electrical control of magnetic vortices in magnets as well as clamping of the magnetic domain walls at the ferroelectric ones in multiferroics.

Abstract: The paper reports on bulk and layered multiferroic composites based on cobalt ferrite and lead iron tantalate-lead titanate solid solution. Syntheses of CoFe2O4 and Pb(Fe1/2Ta1/2)O3 were performed by conventional solid state reactions. 0.5CoFe2O4–0.425Pb(Fe1/2Ta1/2)O3-0.075PbTiO3 bulk composites were prepared by sintering at 900°C. Multilayer composites were fabricated by tape casting, stacking and lamination of alternate ferrite and relaxor layers, followed by cosintering at 950°C. X-ray diffraction analysis and scanning electron microscopic observations confirm that the obtained ceramic samples are composed of ferrite spinel and relaxor perovskite phases. Impedance spectroscopic studies carried out in the temperature range 218-623 K at frequencies 10 Hz–2 MHz show high and broad maxima of dielectric permittivity. On the basis of investigations of magnetization versus magnetic field (up to 85 kOe) and temperature (4-400 K), the behavior typical of hard magnetic materials was found for CF-PFT-PT composites. The measurements carried out at room temperature as a function of the external dc magnetic field and frequency of the sinusoidal ac modulation field, reveal a distinct magnetoelectric effect of the investigated bulk and layered composites.

Abstract: The Aurivillius phases in the Bi-Fe-Ti-O system showing multiferroic properties arouse an increasing interest due to their wide potential applications in electronics. These compounds were usually prepared by high temperature solid-state reaction method using respective oxide powders mixed in required stoichiometry. An excess of bismuth oxide was often added due to its evaporation during heat treatment. The mixture of the oxide powders were calcined in air between 700 and 900°C for several or even for several dozen hours. In the present paper the Bi6Fe2Ti3O18 Aurivillius phase was prepared by direct solid state reaction between respective oxides and by co-precipitation – calcination method. Mixture of the oxides and co-precipitated gels were calcined at different temperatures and X-ray diffraction analysis was used for identification of phase composition of the products.

Abstract: The multiferroic Aurivillius phases in the Bi-Fe-Ti-O system are built from alternate (Bi2O2)2+ and (Bin-1XnO3n+1)2 layers, where X = Fe3+, Ti4+ and “n” refers to the number of perovskite-like layers between Bi2O2 layers. Detailed magnetic studies should be done to understand electromagnetic interactions and multiferroic coupling effects. In the present paper, a powder composed of the Aurivillius phase with n = 5, Bi6Fe2Ti3O18, was successfully prepared by the hydrothermal method. The powder was sintered, obtaining dense polycrystalline materials. It was stated that both powder and sintered bodies were paramagnets with a possible antiferromagnetic ordering or a spin-glass state at the liquid helium temperatures.

Abstract: Relationship between magnetic ordering and ferroelectric polarization switching in YMnO3 epitaxial thin film was investigated. It was found that Néel point of the YMnO3 film is below 80 K, which is consistent with that of YMnO3 single crystal by neutron diffraction. From temperature dependence of the polarization-electric field hysteresis loops and the dielectric permittivity-voltage characteristics, ferroelectric polarization switching behavior was investigated from 300 to 10 K in detail. The ferroelectric polarization switching behavior accords with Ishibashi-Orihara’s theory. Moreover, it was found that the dielectric permittivity under bias electric field have a anomaly below 80 K and the nucleation density for the ferroelectric polarization switching decreases below 130 K, which is higher than 80K.

Abstract: The layered ruthenocuprate materials RuSr2LnCu2O8 and RuSr2(Ln1+xCe1-x)Cu2O10, with Ln=lanthanide or Y for both structures, consist of pairs of CuO2 planes alternating with perovskite-like sheets of vertex sharing RuO6 octahedra. Samples of Ru-1212 and Ru-1222 materials were known to show both superconducting and magnetic transitions. Here, we discuss the problem of the coexistence, considering a model which includes two types of carriers responsible separately for the ordered phases. By considering the interplay between a hybridization mechanism and the direct exchange coupling between these carriers, we look at the stability of the coexisting phase assuming an inhomogeneous superconducting phase.

Abstract: The magnetoresistance of n-type conducting, paramagnetic Co-doped ZnO films prepared by pulsed laser deposition on sapphire substrates has been studied experimentally and theoretically. Positive magnetoresistance (MR) of 124% has been observed in the film with the lowest electron concentration of 8.3·10¹⁷ cm⁻³, while only a negative MR of −1.9% was observed in the film with an electron concentration of 9.9·10¹⁹ cm⁻³ at 5 K. The positive MR is attributed to the quantum correction on the conductivity due to the s-d exchange interaction induced spin splitting of the conduction band. The negative MR is attributed to the magnetic field suppressed weak localization [1]. Voltage control of the electron concentration in Schottky diodes revealed a drastic change of the magnetoresistance and demonstrated the electrically controllable magnetotransport behavior in Co-doped ZnO [2]. The magnetically controllable spin polarization in Co-doped ZnO has been demonstrated at 5 K in magnetic tunnel junctions with Co-doped ZnO as a bottom electrode and Co as a top electrode [3]. There spin-polarized electrons were injected from Co-doped ZnO to a crystallized Al₂O₃ layer and tunnelled through an amorphous Al₂O₃ barrier. Our studies demonstrate the spin polarization and manipulation in Co-doped ZnO.