Physicochemical characteristic of BaTi_0.95Zr_0.05O₃

Introduction

Barium titanium oxide, BaTiO₃ (BT), was discovered for the first time during World War II in 1941¹. This compound which crystallizes in the perovskitestructure was widely used because it’senhanced dielectric permittivities². BT presents a transition temperature of about 120 °C³, and for T Tc, it shows a tetragonal structure, with a ferroelectric behavior. It is also known that the dielectric properties are granulo-dependent⁴.

Substitution of another element in a pure matrix produces remarkable effects, either on structural characteristics such as crystalline phase, grain size and number of active phonon modes, or on physicochemical characteristics such as dielectric properties, dielectric constant, transition temperature and dielectric loss^5-6-7-8.

There is various method of preparation of the ceramic or the powder fine perovskite⁹.We chose to work with the sol-gel method since it is known that this method produces powder with small grain sizes¹⁰.

In this work, we focus on the role of zirconium substituted in the BT material. Physical and chemical properties are studied. We worked on a percentage of 5% of Zr; BaTi_0.95Zr_0.05O₃ (BZT0.05) material.

With the help of X-ray diffraction (XRD), we have studied the effect the Zr on the temperature crystallization and on the quality of the prepared powder.The dielectric measurements of sintered pellets are carried out between 30 to 200 °C, with the HP4284A impedance measuring bridge having a frequency range of 100 Hz to 2 MHz. The parallel faces of the samples are covered with electrodes conductive silver lacquer to form a parallel plate capacitor.The dielectric tests also showed the influence of Zr which is noted on several parameters that we will discuss later.

Procedure

The following precursors were used to prepare the BTZ0.05powders: barium acetate (Ba(CH3CO2)2; 3H₂O, purity 99:9%, Johnson Matthey GmbH Alfa, Karlsrube, Germany), zirconium acetate (Zr(CH3CO2)4; xH₂O) (purity 99.9%, x = 1.5, Johnson Matthey GmbH Alfa, Karlsrube, Germany), and titanium alcoxide (97 % (Assay) Johnson Matthey GmbH Alfa, Karlsrube, Germany). Distilled water was used as solvent. Preparation of the samples was needed two steps. The first step consisted of a preparation of a colloidal solution of TiO₂. Titanium alcoxide was added to 0.5 M lactic acid aqueous solutions. A white precipitate was then obtained under stirring, at 60 °C, during 20 hours, which transforms into a clear solution. The latter was adjusted, by adding water, to 1M the solution. A rapid stirring was necessary to obtain a homogeneous solution (colloidal solution). In a second step were added to the latter colloidal solution, in stoi­chiometric amounts, barium and zirconium acetates as we have shown in other work¹¹. Phase identification of the samples was performed using X-ray diffraction (Cu K ray, λ = 1.5418 Å). Dielectric measurements were carried out in the frequency range of 100 Hz to 2 MHz using an inductance capacitance resistance (LCR) bridge (HP, Model 4284 A).

Results and discussion

The first difficulty was to find the proper calcination temperature value for the complete crystallization, and without any secondary phase, for powder to prepare, which is verified by XRD analyzes.

Figure 1: Evolution of the XRD spectra of the BZT0.05 powder, calcined for 4 hours, depending on the temperature Click here to View Figure

 

Figure. 1 shows the XRD spectra of the sample BTZ0.05 calcined at different temperatures [500 °C to 1000 °C for 4 hours].These spectra clearly show the effect of increasing temperature on the quality of the prepared powder. In fact, the XRD pattern of the calcined powder at 500 °C is substantially amorphous, apart diffuse diffraction peaks and intense which was identified as peaks of reflection characteristics of the TiO₂ precursor, no crystalline phase is apparent. However, at 600 °C the peak characteristic of the reflection phase TiO₂ decreases almost completely and also registers the birth of the characteristic reflection peaks of BTZ0.05 phase. Toowe noted that the powder is completely crystallized into the perovskite phase, at 600 °C, with the presence of a secondary phase (ZrO₂) at 2θ » 24°, this crystallization temperature is significantly lower than other processes like classical pathway and the solid coprecipitation^12-13-14and also for sol-gel preparation method¹⁵.On the other hand, the intensity of the secondary phase decreases with increasing calcination temperature. Thereby, powder calcined at 1000 °C for 4 h, led to a solid solution BTZ0.05pure and without secondary phase.Also we observed, from Fig.2, the crystallization peak of the perovskite phase becoming narrower when the calcination temperature increases and the reflection peaks of ZrO₂ phase disappears completely.This shows a better crystallization of the phase.

Even if,we see that the spectrum of the powder calcined at 1000 °C, for 4 h, is virtually a reproduction of the XRD spectrum corresponding to the calcined powder at 600 °C. However, from Fig.2, there is a displacement of the position of the peak (110) to the highest angles, reflecting the effect of the calcination temperature on the crystallization of the BTZ_0.05 powder.

Figure 2: The zoom peak (110) in the range 28°< 2θ < 35° Click here to View Figure

 

In order to make a comparison of the XRD spectra of the pure BT powder calcined at the same temperature values that the BTZ_0.05 sample, we proceed to develop a pure BT powder (Fig.3).Fig. 3 shows that, BT spectra are isotype of that BTZ0.05.Indicating that pure perovskite phase, without the presence of secondary phase, is fully attained at the calcination temperature 600 °C.However,it may be noted two major differences between powders BT and BZT: First to 600°C, even 900°C,spectrum of BTZ0.05_, also shows the presence of the ZrO₂ phase while for BT the secondary phase (TiO₂) has completely disappeared at 600 °C,indicating that the zirconium doping increases the calcining temperature of the pure perovskite phase.This is due to the atomic number of zirconium size which is larger or the nature of the bonds.These results are in good agreement with those found in the literature^16-17. And second, With regard to the spectrum of the BT powder calcined at 600 °C, we see that the intensity of the reflection peaks, characteristics of pure perovskite phase,is smaller than that of BTZ0.05.This can be interpreted by the effect of atomic radius of Zr(R_Zr R _Ti).

Figure 3: X-ray diffraction of pure BaTiO3 and doped, calcined at different temperature for (4h)Click here to View Figure

 

In other words the substitution of Ti by Zr can increase the intensity peaks at certain crystallization temperatures.In the same way, to highlight the influence of zirconium on the structural state of BaTiO₃ we calculated the lattice parameters a and c of BTZ0.05 by the use of CMPR program.The parameters a and c and their ratio c/a (quadracite) and their difference is illustrated in Table.1. Doping zirconium thus produced a reduction in quadracite and growthin volume. The Scherrer equation enabled us to determine the size of the powder grains PTZ0.05we found a value of 24.72 nm.

Table 1: Parameters of mesh and quadracite of BTZ0.05 (1000 °C (4h))

Rate Zr (%)	Parameters a and c		Quadracite (c/a)	c-a  (Å)	Volume (Å3)
x	c (Å)	a (Å)	c/a
0	4,0421	4,0014	1,0102	0,0407	64,7189
5	4,0423	4,0029	1,0098	0,0394	64,7718

 

The results of dielectric analyzes performed on pellets sintered at 1100 °C (8H) are shown schematically in Fig.4 .The shape of the dielectric constant as a function of the temperature of the pure sample (Fig.4.a) shows an increase in the dielectric constant values a function of the temperature to a maximum at a temperature of the order 150 °C with a maximum value of about 1900.This transition temperature is independent of the frequency;confirming thatit is the Curie ferroelectric-paraelectrictransition.The curves of Fig.4 .b and Fig.4.c of the dielectric permittivity  which corresponds to the BZT0.05 show a similar trend to that observed in the case of BaTiO₃ (pure sample), although the values of the dielectric permittivity measured at different frequencies are different. In addition, it is noted that the value of the transition temperature (Tc) decreases sharply. However, doping Zr produces a second anomaly at 60 °C (Fig.4 b). And the Fig.4.c shows the persistence of the second anomaly during cooling. Indeed, the work done by Baptisaet al.¹⁸we confirm that it is a structural transition and that such anomaliecan not be allocated to the vacant oxygen sites in the crystal structure. But recently Nanakornet al.^14-15-19 have mounted that it is a orthorhombic-tetragonal structural transition (To-q).

Figure 4: dielectric constant as a function of the temperature of the pure sample and of the BZT0.05 sample Click here to View Figure

 

The choice of 1100 °C for 8 hours in the sintering step is not random, but after a study of function the sintering time. In fact, we prepared two samples one sintered at 1100 °C for 4 hours and the other at 1100 °C for 8 hours. The table.2shows the result of this study.It is thus observed that sintering at 1100 °C for 8 hours improves the dielectric constant obtained about 113% relative to the sintered for 4 h.

Table 2: Sintering time of effect on the dielectricproperties of BZT0.05

	Sintering time (h)	ε’max at Tc	Tc
BZT0.05	4	2976	130°C
BZT0.05 (mounted)	8	6740	120°C

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