Multiferroics with negative value of dielectric constant are very promising materials because of their modern applicability. These materials can be used as materials for the construction of electromagnetic radiation shields. The subject of the research is multiferroic BiFeO 3-PbFe 1/2 Nb 1/2 O 3 (BF-PFN) ceramics. For all multiferroic materials the following studies are conducted: SEM, EDS and the temperature dependence of dielectric constant ε′(T). Above a certain temperature (different for different chemical compositions) the value of dielectric constant reaches negative values. Such the behavior of the dielectric constant may indicate that the polarization inside the material has a reverse direction to the external electric field. That is, the electric field inside the material counteracts the applied external electric field. The obtaining materials also show negative dielectric losses. The Axelrod model is used to explain the mechanism that causes negative dielectric loss.

The fractional Zener model with two spring-pots is proposed to description of relaxation time spectrum of ferroelectric ceramic material. This model is based on fractional calculus. The influence of values of a and b parameters on the shape of the relaxation time spectrum was investigated.PACS numbers: 77.84. Dy, 72.15.Lh, 77.90.+k, 62.40.+i IntroductionIn recent years, there has been a considerable amount of works carried out on modelling the viscoelastic response of materials [1, 2] over range of temperature (T ) and time (t) of frequency (f ). These investigations deal primary with the modelling of various viscoelastic functions. These functions can be described by the generalised Maxwell or the generalised Kelvin-Voigt models [1,2]. Using these models to fit the data obtained in relaxation experiments requires the determination of number of fitting parameters.An alternative approach has been used to model the viscoelastic behaviour of material that is based on the concept of the fractional differential and integral method. The method uses the idea that the spring and dashpot elements in the phenomenological models are replaced by spring-pot element.Linear viscoelastic models are often presented as spectral models. Fractional models are amenable to analysis using the Fourier and Laplace transforms and they are described by the fractional derivative of a order with respect to time with evidently 0 < a ≤ 1.Although direct measurement of the relaxation time spectrum is impossible, the main advantage of this modelling is to ensure consistency with linear viscoelastic theory.In this paper we considered two fractional viscoelastic Zener models. One of these consisted of one spring-pot and two springs and second one consisted of two spring-pots and two springs. We apply the fractional Zener model with two spring-

The multiferroic (ferroelectric–ferromagnetic) composites (PFN–ferrite) based on ferroelectromagnetic PbFe1/2Nb1/2O3 powder and ferrite powder (zinc–nickel ferrite, NiZnFeO4) were obtained in the presented study. The ceramic PFN–ferrite composites consisted of 90% powder PFN material and 10% powder NiZnFeO4 ferrite. The ceramic powders were synthesized by the classical technological method using powder calcination, while densification of the composite powders (sintering) was carried by two different methods: (1) free sintering method (FS) and (2) spark plasma sintering (SPS). The composite PFN–ferrite samples were thermally tested, including DC electrical conductivity and dielectric properties. Besides, XRD, SEM, EDS (energy-dispersive spectrometry) and ferroelectric properties (hysteresis loop) of the composite samples were tested at room temperature. At the work, a comparison was made for the results measured for PFN–ferrite composite samples obtained by two methods. The X-ray examination of multiferroic ceramic composites confirmed the occurrence of the strong diffraction peaks derived from ferroelectric (PFN) matrix of composite as well as weak peaks induced by the ferrite component. At the same time, the studies showed the absence of other undesired phases. The results presented in this work revealed that the ceramic composite obtained by two different technological sintering methods (free sintering method and spark plasma sintering technique) can be the promising materials for functional applications, for example, in sensors for magnetic and electric fields.

Aurivillius-type ceramics Bi 6 Fe 2−x Mn x Ti 3 O 18 x = 0, 0.3, 0.9, 1.5 were obtained by a solid-state reaction method using high-purity TiO 2 , Bi 2 O 3 , Fe 2 O 3 and Mn 2 O 3 powders. The milled powder was calcined at 1113 K for 4 h. After calcination, the powder was milled again than pressed into pellets and sintered at 1213 K for 4 h. It was detected that the addition of manganese ions to the multiferroic five-layer Aurivillius-type structure affects the size of the grains. It was found that, the certain amount of manganese ions causes that the polarization of the material doped by them, have the direction opposite to the direction of the applied electric field. The doped material behaves like dia-electric material. The presented research complements the research concerning the Aurivillius ceramics doped with manganese. An attempt was made to explain the reasons for the negative values of dielectric constant and dielectric loss, that occur in manganese-doped five-layer Aurivillius type ceramics and which have not been described in the literature so far.

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The mechanical loss (Q-1) and Young's modulus (E) were measured as a function of temperature in the range 293K