Effect of Mn doping on structure, the dielectric and electric properties of BCZT ceramics

Abstract

The synthesis of (Ba_0.99Ca_0.01)(Zr_0.2Ti_0.8)O₃ (BCZT) and doped (Ba_0.99Ca_0.01)(Zr_0.2Ti_0.8-xMn_x)O₃ (BCZT-xMn) ceramics was successfully carried out by the solid-state method. The doping effect is followed by X-ray powder diffraction, scanning electron microscopy (SEM), and dielectric and conductivity measurements. Indeed, X-ray diffraction measurements show the crystalline structure of ceramics. SEM images indicate the doping effect on the studied perovskite microstructure. The results indicate that during doping the maximum value of the temperature (Tm) of the dielectric constant varies slightly, and there will be a considerable decrease in permittivity, dielectric losses and conductivity. Manganese ions are well integrated in the perovskite while maintaining the solid solution.

1 Introduction

Since the 1950s, the Pb-based ceramic perovskite compositions like Pb(Zr_1-xTi_x)O₃ (PZT) and PbMg_1/3Nb_2/3O₃ (PMN) have been frequently used worldwide for their dielectric properties [1, 2]. This material has a very high dielectric permittivity at the Curie temperature and a coupling coefficient too. For this reason, it is found useful in various fields of applications such as transducers and sensors [3,4,5]. However, these lead oxides have recently been found to be toxic and seriously dangerous to the environment and human health. Much research has therefore been conducted to find its lead-free alternative with similar ferroelectric and piezoelectric effects, generally titanium-based solid solutions such as Bi_0.5Na_0.5TiO₃(NBT), K_0.5Na_0.5NbO₃ (KNN), BaTiO3 (BT) and (Ba,Ca)(Ti,Zr)O₃ (BCTZ) [6]. Among piezoelectric materials, we can mention the most famous one, (1-x)Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT), a pseudo-binary system that had a very high piezoelectric coefficient of about 600pc/N at room temperature, an amazing value that even exceeds that of the PZT perovskites [7,8,9,10].

Among the important characteristics of this pseudo-binary ferroelectric BCZT is the existence of a triple point in the previously studied BZT-BCT diagram, located at x = 0.32 where this region is considered as a separation between the rhombohedral and the tetragonal symmetry and corresponds to a high permittivity and huge electromechanical coefficients [11].

In fact, at the end of the last century, Simon et al. focused their researches on the ternary form BaTiO₃-CaTiO₃-BaZrO₃, which led to a diagram of four zones with different structures and dielectric aspects [2, 12], as shown in Fig. 1:

Fig. 1

Schematic representation of the BT-BZ-CT ternary diagram

Zone 1: Dielectric behavior similar to BaTiO₃ in both its proximity and the Ba_1-xCa_xTiO₃ (BT-CT), with three anomalies corresponding to rhombohedral (R), tetragonal (T), orthorhombic (O) and cubic (C) transitions.

R-O transition temperature decreases remarkably as the substitution of Ba²⁺ by Ca²⁺ increases in perovskite A sites.

Zone 2: For solid compositions close to BaTi_1-xZr_xO₃ with 0.1 < x < 0.27, only one dielectric anomaly is detected related to the R–C transition; the Curie temperature peak is remarkable without dispersion by frequency effect.

Zone 3: In BaTi_1-xZr_xO₃ proximity with 0.275 < x < 0.42, there is a larger peak with higher temperature values and a frequency dispersion. At the macroscopic scale, the compositions keep their shape and polarization regardless of the temperature.

Zone 4: This region separates zone II from zone III where, depending on the composition, classic ferroelectric states and relaxing states coexist.

However, the formation of the pure perovskite phase BCZT requires a very high calcinations and sintering temperature of about 1420 °C for sintering. The studies carried out by the researchers aim at reducing the temperature and improving the electrical presets of the perovskite.

The doping with various metal ions such as Ca²⁺ [13], Cu²⁺ [14], Fe³⁺ [14], Bi³⁺ [11], Mg²⁺ [14] and Mn⁴⁺ [15] either in site A or/and B can improve the properties of BCZT ceramics. As mentioned, the addition of a dopant to (BCZT) ceramics can improve densification, dielectric and ferroelectric properties. MnO₂ is an interesting additive because of its multivalence which can be a donor or acceptor dopant [16]. Research studies show that the addition of Mn can reduce dielectric losses and increase the densification of dielectric metals [5].

Mn doping at B sites of the perovskite (Ba_0.99Ca_0.01)(Zr_0.2Ti_0.8-xMn_x)O₃ (BCZT-xMn) structure creates a variety of compositions with many properties similar to those of lead ceramics, such as cooling devices and dielectric responses enhanced by the rhombohedral (R) and tetragonal (T) phases separated by an intermediate orthorhombic (O) phase.

2 Experiment details

The lead-free BCZT-xMn ceramics were prepared by a solid state method where % x = (0, 0.25 and 1). The starting materials with highly pure (99.9%) powders BaCO₃, CaCO₃, ZrO₂, TiO₂ and MnO_2, were dried in an oven heated to 150 °C for one hour. Then, they were then weighed to obtain a mixture with stoichiometric proportions.

The mixture was milled for 2 h, then pressed (100 MPa) into a cylindrical shape and followed by calcinations at 1150 °C for 15 h. The resulting pellet was grounded again into powder for 2 h to increase reactivity and reduce particle thickness to the micrometer. Subsequently, the powders obtained were reformed into pellets with a diameter of 8 mm and reheated at 1420 °C for 4 h.

The phase purity of the prepared ceramic was confirmed by X-ray powder diffraction (Philips X'Pert, Pro X-ray Diffractometer) using CuKα radiation ($\lambda$ = 1.5418 A°). The evolution of grain size is observed by the SEM technique using a scanning electron microscope. A precision impedance analyzer shows the evolution of the dielectric permittivity in relation with the temperature.

3 Results and discussions

3.1 X-ray diffraction

Figure 2 depicts the X-ray diffraction of BCZT-xMn ceramics. Firstly, the diagram does not indicate the existence of other phases than the structured perovskite, which implies that the Mn⁴⁺ particles are well introduced into the unit cell of the perovskite in order to preserve the structure of the solid solution. The X-ray diagrams of the BCZT and BCZT-xMn ceramics reveal peak of diffraction at 45°, which is consistent with crystals (002) and (200), indicative of the tetragonal structure of BCZT ceramics.

Fig. 2

X-ray diffraction pattern of the Mn (0%, 0.25% and 1%) doped BCZT

In addition, the diffraction peaks are slightly shifted towards the widest angle for a higher Mn concentration indicating the decrease of the ceramic system constant (BCZT-Mn), which is linked in particular to the presence of Mn⁴⁺ occupying the B sites. In fact, the ionic radius of Mn⁴⁺(0.54 A°) is smaller than that of Ti⁴⁺ (0.605 A°) and Zr⁴⁺ (0.72 A°).

On the other hand, the diffraction peaks of BCZT-xMn ceramics exhibit a full width at half maximum that increases with Mn concentration. This can be linked to the size of the ceramic grains (which decreases if the Mn content becomes higher).

The reticular planes of a crystal are defined by their Miller indices (hkl), or there is an interference of planes for the triplets (h,k,l) in orthorhombic structure. The interreticular distance d_hkl was related to the Miller indices by the following relationship [17, 18]:

$$ \frac{1}{{d_{hkl}^{2} }} = \frac{{h^{2} }}{{a^{2} }} + \frac{{k^{2} }}{{b^{2} }} + \frac{{l^{2} }}{{c^{2} }} $$

(1)

with $d_{{{\text{hkl}}}}$ is calculated from the Bragg equation [17,18,19,20]:

$$ 2d_{hkl} \sin \theta = n\lambda $$

(2)

where n is the order of diffraction taken equal to one in general (n = 1), and $\lambda$ is the wavelength of X-ray diffraction equal to 1.5418A°.

The calculation of the lattice parameters a, b and c is shown in Table 1. These results indicate that the addition of Mn induces a slight variation in the lattice parameters. Indeed, the ionic radius of Mn⁴⁺ (0.54 A°) and Mn³⁺ (0.645 A°) is close to the ionic radius of Ti⁴⁺ (0.605A°). However, the radius of Mn²⁺ (0.83A°) is larger than that of Ti⁴⁺, and the occupation of the Ti site by Mn ions induces a weak deformation of the crystal lattice.

Table1 Lattice parameters of BCZT-xMn (% x = 0, 0.25 and 1)

The calculation of the nanometric dimensions of the BCZT-xMn ceramic particles is based on the study of ray diffraction; the size of the crystallites is given by the following Scherer’s law [18, 21]:

$$ D = \frac{K \lambda }{{\beta \cos {\uptheta }}} $$

(3)

where D is the size of the crystallites, $\lambda$ is the wavelength of the X-rays (1.5418 A°), K is the Scherer constant taken equal to 0.9, $\theta$ is the diffraction peak angle, and $\beta$ is the width at mid-height of the diffraction peaks (FWHM) determined by the Match refinement.

Thus, we can follow the epsilon micro-deformation according to the law [18]:

$$ \varepsilon = \frac{\beta }{4\cos \theta } $$

(4)

According to the figure illustrating the X-ray diffraction, the FWHM increases with the addition of Mn, which induces a remarkable decrease in the size of the nano-crystallites with the increase in the amount of dopant from 144.366 to 57.729 nm for the high intensity peak, corresponding to the Miller index (110) (Table2).

Table 2 Grain size and micro-strain of BCZT-xMn ceramics

In fact, the diffusion of manganese ions into the ceramic leads to a reduction of the grain size which is mainly related to the appearance of oxygen vacancies. These results are grouped in Table 2.

3.2 Surface morphology

Figure 3 displays the micrographs of the BCZT and BCZT-xMn ceramics using the SEM analysis. The images taken of the different samples clearly depict a non-homogeneous particle size distribution. The addition of a small amount of Mn in doped ceramics leads to a larger grain size than in BCZT ceramics. Such addition promotes the growth of the ceramic grains, and therefore, a substitution of the B-site with Mn³⁺ and Mn⁴⁺ ions will occur, leading to the creation of oxygen vacancies and an improved mass and energy transfer.

Fig. 3

SEM micrographs of surface BCZT-xMn ceramics (a: x = 0%, b: 0.25% and c: 1%)

3.3 Dielectric study

To monitor the effect of Mn doping on the dielectric permittivity and dielectric losses of BCZT-xMn ceramics, measurements will be made for BCZT ceramics and both doped, and recorded in Fig. 2 for a frequency range from 1 to 1 MHz.

Figure 4 show that:

Fig. 4

Temperature dependence of dielectric constant and loss for BCZT-xMn ceramics

(i) Maximum dielectric permittivity decreases with frequency.

(ii) For the BCZT ceramics (Fig. 4a), the maximum temperature is close to room temperature and there is a slight dispersion at maximum.

(iii) Both curves correspond to Mn-doped ceramics (Fig. 4b and c); the peak widens as soon as the amount of Mn increases and the dielectric maximum decreases with a temperature shift associated with the maximum permittivity. It is also noted that there is a slight dispersion of the permittivity for T ≤ T_m and the dielectric losses decrease with temperature for all samples.

Indeed, Ti⁴⁺ ions are less chemically stable and are reduced to Ti³⁺ ions according to Eq. (5):

$$ Ti^{4 + } + \frac{1}{2}O_{o}^{x} \to Ti^{3 + } + \frac{1}{2}V_{o}^{ - } $$

(5)

When the Mn⁴⁺ ions diffuse into the BCZT ceramic, they are substituted with the Ti³⁺ ions according to the reaction (6):

$$ Mn^{4 + } + \, Ti^{3 + } \to \, Mn^{3 + } + \, Ti^{4 + } $$

(6)

This is notably linked to Mn additions, which limit the concentration of vacant oxygen positions and causes the decrease of the permittivity [22].

The dielectric permittivity in the paraelectric zone for a normal ferroelectric is given by the following Curie Weiss formula (7):

$$ {\upvarepsilon }_{{\text{r}}}^{^{\prime}} (T) = \frac{{\text{C}}}{{{\text{T}} - {\text{Tm}}}} $$

(7)

where C and T₀ are the Curie constant and Curie temperature, respectively. The deviation from the Curie law is due to the assembly of short-range nanopolar domains [23].

Uchino and Nomura proposed another form for the previous law of Curie–Weiss given by Eq. (8) [23, 24]:

$$ \frac{1}{{\varepsilon_{r}^{^{\prime}} }} - \frac{1}{{\varepsilon_{r\max }^{^{\prime}} }} = \frac{{\left( {T - Tm} \right)^{\gamma } }}{C} $$

(8)

where ${\upvarepsilon }_{{{\text{rmax}}}}^{^{\prime}}$ is the maximum permittivity of ceramics, and C is a multiplicative constant.

$\gamma$ is a constant corresponding to the degree of diffuseness [22, 25, 26], and it gives information on the phase transition in perovskite determined by the linear fit of the experimental data.

A plot of $\ln (\frac{1}{{{\upvarepsilon }_{{\text{r}}}^{^{\prime}} }}{ } - { }\frac{1}{{{\upvarepsilon }_{{{\text{rmax}}}}^{^{\prime}} }}) = f\left( {\ln T - T_{m} } \right)$ drawn for T$>$T_c for a frequency of 1 kHz allows us to obtain the value of $\gamma$ (Table 3).

Table 3 Dielectric properties of BCZT-xMn ceramics

The relaxation behavior is described by the quantity $\Delta$ T_relax defined by:

$$ \Delta T_{{{\text{relax}}}} = \, T_{m} \left( {100\;kHz} \right) \, {-} \, T_{m} \left( {1\;kHz} \right) $$

(9)

This relaxation parameter increases from 1 to 2 with the growth of the quantity of Mn added (Table 3).

Figure 5 illustrates the variations of permittivity as a function of temperature for f = 1 kHz; it allows us to go back to the values of maximum temperature T_m relative to the maximum of dielectric permittivity, critical temperature T₀ and temperature T_d from which the dielectric constant starts to deviate from Curie's law Figure 6. For this purpose, the degree of deviation from the Curie–Weiss law can be described by means of parameters $\Delta$ T_m defined as follows:

$$ \Delta T_{m} = \, T_{d} - \, T_{m} $$

(10)

Fig. 5

Temperature dependence of inverse dielectric constant for BCZT-xMn ceramics

Fig. 6

ln(1/ɛ—1/ɛ_max) as a function of Ln(T—T_m) for the BCTZ-xMn (x = 0%, 0.25% and 1%) ceramics at 10 kHz

The values of this parameter increase if Mn concentration increases which suggests that Mn doping induces a diffusion of the ceramic phase transition BCZT, where the diffuseness character of the phase transition is described by the following empiric parameter:

$$ \Delta T_{{{\text{diff}}}} = T_{{0.9\varepsilon_{r\max } }} \left( {1\;kHz} \right) - T_{{\varepsilon_{r\max } }} \left( {1\;kHz} \right) $$

(11)

The value of this parameter indicates the effect of doping ceramics BCZT-xMn. The main values extracted from the study on the ceramic dielectric of BCZT doped Mn are summarized in Table 3.

Figure 7 describes the variations of dielectric permittivity and dielectric losses as a function of frequency for T = T_m for a range from 1 to 1000 kHz. In this range, the dielectric constant is reduced as the frequency increases and decreases with Mn content due to the reduction in particle size with MnO₂ doping. The dielectric losses vary slightly with frequency, and a minimum value of around 0.01 can be recorded.

Fig. 7

Dielectric constant and dielectric loss as functions of frequency for BCZT-xMn ceramics at T = T_m

Figure 8 shows the conductivity measured at the critical temperature T_m as a function of the frequency of the BCZT ceramic. It is defined according to the following relationship:

$$ \sigma_{{{\text{ac}}}} = \omega \varepsilon_{0} \varepsilon_{1} {\text{tan}} \varepsilon_{1} $$

(12)

where $\omega$ is the angular frequency, both $\varepsilon$ ₀ and $\varepsilon$ ₁ are the dielectric constants of vacuum and the relative dielectric constant, and $tan \varepsilon_{1}$ is the dielectric loss. However, it can be seen that conductivity is highly frequency dependent and follows a linear variation. It increases with frequency, which implies a decrease in resistance according to the law:

$$ R = \frac{l}{{\sigma {\text{ S}}}} $$

(13)

Fig. 8

AC conductivity as function of frequency for Mn doped for BCZT-xMn (%x = 0, 0.25 and 1) at T_m

The conductivity also reduces with Mn doping as a result of the presence of defects or impurities in the ceramic.

4 Conclusion

BCZT and BCZT-xMn-doped ceramics are successfully prepared by the solid–solid method. The DRX spectra highlight the presence of the pure phase of the material, indicating that the Mn⁴⁺ ions are well integrated in the perovskite while maintaining the solid solution.

The addition of Mn reduces the permittivity of doped ceramics and reduces dielectric losses. In fact, the addition of manganese oxide reduces the concentration of vacant oxygen by occupying the perovskite site B while substituting with Ti⁴⁺ ions and also reduces the size of the grains.

In fact, DRX shows that the Mn doping decreases the granular size, in accordance with the values of the crystallite size which evolves from 144.36 nm for the parent compound up to 57.729 nm for an addition of 1% of Mn for the main peak (110). The low deformation of the perovskite is given by constant epsilon called micro-deformation. The calculation of the mesh parameters indicates an orthorhombic structure of the ceramics.

SEM characterization allows us to trace the microstructure of ceramic compounds. The study of the dielectric part as a function of temperature for a frequency range from 1 kHz to 1 MHz indicates that the maximum relative permittivity (at temperature T_m for 1 kHz) decreases by adding Mn of 3570.91 for x = 0%, 1570.32 for x = 0.25% to 850.29 for x = 1%. The curve ln(1/ε–1/ε_m) = f(ln(T–T_m)) allows us to know the nature of the phase transition by following the values of the diffusion coefficient which varies between 1 and 2. As well as the degree of deviation and the character of diffusion are, respectively, given by ∆T_diff and ∆T_d at the frequency 1 kHz. The decrease in permittivity and dielectric losses as a function of frequency at maximum temperature can also be marked for each composition.

A study of the conductivity shows that it varies linearly as a function of the frequency at temperature Tm for each proportion of Mn.