NONLINEAR PROPERTIES OF BARIUM TITANATE CERAMICS

The materials based on barium titanate have nonlinear dependence of polarization on the intensity of the electrical field. The properties of those materials are used when different automation elements such as primary converters, phase regulators for low and high frequencies, sensors, modulators etc. are derived. Ceramics based on barium titanate (BaTiO3) alloyed with H3BO3 and Bi2O3 have been derived and researched. The influence of the temperature of sintering on the values of the dielectric permittivity on the structural properties has been determined. The influence of the quantity of the added material on the temperature dependencies and the non-linear properties of the ceramics have been analyzed. Both legation additives decrease the sintering temperature. However, the values of the dielectric permittivity decrease while the non-linear properties of the doped barium titanate (BaTiO3) improve. The coefficient of non-linearity reaches its maximum for the H3BO3 composites, synthesized at temperature of 1100 C.


INTRODUCTION
Advances in modern automation systems require a search for new functional materials, including nonlinear dielectrics whose properties are influenced by an external electric field.Those materials have found application in dynamic RAM, tuneable microwave devices, such as tuneable oscillators, phase shifters, reactors with a large tuneable capacitance range and low dielectric loss, transducers and sensors [1][2][3], thermistors [4], and multilayer ceramic capacitors [5].
Barium titanate (BaTiO3) is a perovskite-type ceramics material, exhibiting very good properties like low dielectric losses, high values of the dielectric permittivity and high electromechanical coupling coefficients [1].
Lead-free piezoelectric barium titanate and its solid composites are in high demand due to the requirement to find a replacement for lead zirconate titanate (PZT) ceramics.These ceramic materials have high volatility and toxicity, which contaminate the environment [3].
The electrical properties of ferroelectric materials can be modified by doping in or-der to meet the requirements for some applications.The dopants (additions) like NiO, ZrO and Nb2O5 influence the grain size and phase transition temperature of the Barium titanate (BaTiO3) ceramics [6][7][8].The dielectric properties of these materials depend very much on grain size.Ceramic materials of Ba0.9Li0.1Ni0.05 by conventional state reaction were prepared.The high value for dielectric permittivity and low dielectric loss were obtained [9].
The addition of cadmium to barium zirconium titanate reduces dielectric losses and increase value of piezoelectric charge constant d33 [10].The legation additives used are Bi2O3 and H3BO3, which have relatively low melting temperature.They allow liquid phase synthesis of the ceramic materials and can stimulate the solidstate reactions.The density, the dielectric permittivity and the dielectric losses strongly depend of the sintered temperature.

EXPERIMENTAL DETAILS
The following composites have been derived in order to analyze the influence of bismuth trioxide Bi2O3 and boric acid H3BO3 on the non-linear properties of barium titanate (BaTiO3), Table 1 The samples of ceramic materials were prepared by conventional mixed oxide solid state reaction method.The raw materials of extra pure grade BaCO3, TiO2, H3BO3 and Bi2O3 were weighed in appropriate proportion to obtain the stoichiometric ratio.The barium carbonate BaCO3 and titanium dioxide TiO2 are mixed with distilled water in zirconia vials, using zirconia ball, by Reach PM-100 planetary mill.Homogenization was done for duration of 4 hours.Homogenized mixtures dried at a temperature of 150 o C after removing the zirconia ball.Prepared powder was pressed into pellets of 50 mm diameter and 20-25 mm thickness.As a binder have been using 3% water solution polyvinyl alcohol.The pellets of the made materials are sintered at a temperature of 900 o C for 3 hours.After that, the pellets were crushed using planetary ball mill, after adding the necessary amounts оf H3BO3 or Bi2O3.Milling powder was pressed into pellets of 10 mm diameter and 1.5-2 mm thickness and was calcined at a different temperature for 3 hours.

RESULTS AND DISCUSSION
An X-Ray Diffractometer with a monochromatic Cu-Kα radiation was used to ex-amine the crystal structure of the sintering ceramic materials over a 2θ angle from 20 o to 80 o .From the diffractograms can be seen that the researching composition have single phase perovskite structure.Similar results were derived for the samples of the other analyzed composites, synthesized at different temperatures.
The Scanning Electron Microscope (SEM) was used for researching the microstructures of the surfaces.The average grain size of the specimen, doped with 2 mol% H3BO3, was about 1-2μm and there was no dependences on the variation of the sintering temperature (Figure 2).The temperature dependencies of the dielectric permittivity have been analyzed.For those dependencies for a composite containing 2 mol % H3BO3, synthesized at different temperatures, phase transitions at Curie point have been observed.The phase transition is most evident at the highest sintering temperature.The higher quantity of the boric acid leads to decrease the values of the dielectric permittivity and to diffuse phase transition.For the materials sintered at low temperatures have been obtained diffuse phase transition.Table 2 shows the composites containing H3BO3, their dielectric permittivity and the Curie point for samples synthesized at different temperatures.
The temperature dependencies of the dielectric permittivity for compositions of doped barium titanate with 2 mol%H3BO3 and 0.05mol%Bi2O3 are showed in the Figure 4 and the Figure 5.The addition of 0.05mol%Bi2O3 to the barium titanate (BaTiO3) significantly reduces the dielectric permittivity of the materials and leads to a diffuse phase transition.For evaluation of the nonlinear properties of the ferroelectrics, a non-linearity coefficient K is defined, which shows how many times the dielectric permittivity increases when the electric field strength increases compared to its initial value [11]. ( Figure 6 and Figure 7 present the dependencies of the dielectric permittivity on the electric field strength for the materials which have been obtained good non-linear properties.For the non-linear ceramics, an important parameter is maximum values of the dielectric permittivity on the electric field strength.The values of the electric field for the analyzed materials, at which the dielectric permittivity reaches its maximum, are presented in Table 3. Table 3 shows a comparison of the non-linear properties of the ceramic materials, containing BaTiO3 at different sintering temperatures.The addition of H3BO3 to ceramic materials containing barium titanate leads to higher values of the non-linear coefficient compared to the addition of Bi2O3 to the same materials.The optimal sintering temperature for the first group of materials is 1100 o C, whereas for the materials doped with Bi2O3 that temperature is 1050 o C. Both of those additions significantly increase the non-linear properties of the barium titanate (BaTiO3), but decrease the values of the dielectric permittivity.That increase is higher when was added Bi2O3.
The sintering temperature decreases by 200 -250°C when H3BO3 и Bi2O3 are added to the barium titanate (BaTiO3).That temperature is 1350 o C for pure barium titanate (BaTiO3).Simultaneously, the non-linear properties improve while the dielectric permittivity decreases.

CONCLUSIONS
Ceramic materials based on Barium titanate with 1,2,3 and 4 mol% H3BO3 as well as BaTiO3 with 0.05 mol% Bi2O3, have been synthesized at temperatures 1050, 1100, 1150, 1200 and 1250 o C. The diffractograms show the synthesis of mono phase structure with reflexes typical for the barium titanate.According to SEM the structure of the materials is relatively homogeneous and the size of the beans is up to 10 μm.

Figure 1
Figure1shows the diffractogram of BaTiO3, containing 2% of H3BO3, the synthesized at temperature 1100 o C. From the diffractograms can be seen that the researching composition have single phase perovskite structure.Similar results were derived for the samples of the other analyzed composites, synthesized at different temperatures.

Fig. 7 .
Fig. 7. Dependency of the dielectric permittivity on the electric field strength for materials BaTiO3+3mol%H3BO3, sintered at temperature1100 o C.

Table 2 .
Dielectric permittivity and Curie temperature at a different sintering temperature.

Table 3 .
Comparison of the non-linear properties of the ceramic materials, containing BaTiO3 at different sintering temperatures.