Solid electrolytes based on {1 - (x + y)}ZrO2-(x)MgO-(y)CaO ternary system: Preparation, characterization, ionic conductivity, and dielectric properties.

Different composition of composite material of zirconium dioxide co-doped with magnesium oxide [MgO(x)] and calcium oxide [CaO(y)] according to the general molecular formula {1 - (x + y)}ZrO2-(x)MgO-(y)CaO were prepared by co-precipitation method and characterized by different techniques, such as XRD, FTIR, TG-DTA, and SEM. Co-doping was conducted to enhance the ionic conductivity, as mixed system show higher conductivity than the single doped one. Arrhenius plots of the conductance revealed that the co-doped composition "6Mg3Ca" has a higher conductivity with a minimum activation energy of 0.003 eV in temperature range of 50-190 °C. With increasing temperature, dielectric constant value increased; however, with increasing frequency it shows opposite trend. Co-doped composition C2 exhibit higher conductivity compared to C3, owing to the concentration of Mg content (0-6%); the conductivity decreases thereafter. Zirconium oxide was firstly used for medical purpose in orthopaedics, but currently different type of zirconia-ceramic materials has been successfully introduced into the clinic to fix the dental prostheses.

Introduction

The problem associated with liquid electrolytes in practical applications, such as leakage, low energy, limited operating temperature range, and low power density are removed by solid form of electrolytes [1]. Solid electrolytes have become a widely studied field of solid state chemistry in recent years, due to their excellent suitability as electrically conductive material at high temperature. The most used solid electrolyte or fast ion conductor at present are those where oxygen ions are the charge carriers; namely oxide ion conductors. Oxide ion conductors aroused worldwide attention for its wide application domains as chemical sensor, solar cells, and oxygen separation membrane and in SOFCs [2]. The classical ion conducting oxide material are those based on ZrO2, CeO2 and ThO2. Recently, doped ZrO2 was the most studied solid ionic conductor, because of its attractive anionic conductivity, as well as good thermal stability. At room temperature, zirconium dioxide has a monoclinic structure, which undergoes transformation as the temperature increases. From 1170 °C to 2370 °C, zirconia has tetragonal modification whereas at a temperature higher than 2370 °C, it adopts cubic structure [3], [4]. Pure zirconia is basically a poor oxide ion conductor at lower temperature. Therefore, researchers are concentrating to develop a new material where high temperature ZrO2 cubic/tetragonal (high ionic conductivity) phases stabilized at lower temperature by doping [5]. It was observed that the stability of the high temperature modifications of zirconia with oversized divalent or trivalent cation dopants (such as Y3+, Ca2+, Mg2+, Ce3+) was much higher than that of undersized trivalent cation (such as Al3+, Fe3+ and Cr3+) dopants. Thence, cations used as dopant for stabilization of zirconia must have a large ionic size and lower charge state than Zr [6].

The effect of the dopant oxide on the ionic conductivity of ZrO2 based ternary system has been investigated extensively. It was reported that mixed oxides produced material with superior properties than single component [7], [8], [9], [10]. Therefore co-doping was carried out using suitable fluorite stabilizer oxide (MgO, CaO, Y2O3, and CeO2) to improve stability as well as promoting the formation of defects. In the present investigation, calcium and magnesium oxides are chosen as a dopant; not only because they are relevant to the oversized cations and are of lower charge state but also they are cheap precursors [6]. For doping of zirconium dioxide, different methods, such as co-precipitation [11] alkoxides [12], citrate routes, and powder mixing [13] are used. The present study reports the synthesis of CaO/MgO doped Zirconia and its characterization using various analytical techniques.

Experimental

Synthesis of zirconium dioxide was carried out using zirconium oxychloride (CDH, New Delhi, India) by co-precipitation method. Weighed amount of zirconium oxychloride (ZrOCl2·8H2O) was reconstituted in distilled water and stirred well. After obtaining homogeneous solution, precipitation was conducted by adding 100 mL of NaOH. The obtained precipitate was washed several times with distilled water until it become neutral and then placed in oven for drying at 200 °C for 3 h. The obtained raw material was grinded in an agate mortar in the medium of acetone with intermittent grinding into fine powder and heat at 800 °C for 24 h. For synthesis of Mg and Ca doped zirconia, requisite amount of precursors zirconium oxychloride, magnesium nitrate (Merck, Mumbai, India), and calcium nitrate (Otto Kemi, Mumbai, India) were dissolved in water and the above described procedure was carried out [14].

The X-ray diffraction data of the resultant material were collected in the range of 20 ≤ 2θ ≤ 80° using Bruker AXD D8 X-ray diffractometer with Cu Kα radiation (λ = 1.5406 °A) at room temperature for confirming the desired phase of samples. Scanning Electron Microscope (JEOL JSM-6510 LV) was used to evaluate the surface morphology features at an accelerating rate of 20 kV. The thermal decomposition of synthesized material was analysed through thermo-gravimetric and differential thermal analysis (TG/DTA) using “PerkinElmer Thermal Analyser” with heating rate of 20 °C min−1 from the temperature range of 40–800 °C in nitrogen flowing atmosphere. FTIR analysis was conducted by ‘‘Perkin Elmer Spectrum Version 10.4.00” in the wavelength range of 4000–400 cm−1 at room temperature. The finally obtained fine powder was pelletized by applying pressure of 5 tons cm−2. The prepared circular pellet has the radius 0.65 cm and thickness 0.1 cm. Before performing the electrical and dielectric measurements, opposite surfaces of the pelletized sample were coated by carbon paste to ensure good electrical contact with electrode capacitor. The temperature dependent electrical conductivity and dielectric measurements of the sample have been performed using a Wayne Kerr “43100” LCR meter from 30 °C to 1000 °C temperature range. The heating rate of the sample was controlled by Eurotherm C-1000 [15]. Different compositions of material used in this study are presented in Table 1.

Table 1

The nominal composition of the investigated samples.

Sample	Composition (mol%)
	ZrO2	MgO	CaO	Sample denotation
ZrO2	100	0	0	C0
8Mg	92	8	0	C1
6Mg3Ca	91	6	3	C2
4Mg6Ca	90	4	6	C3
12Ca	88	0	12	C4

Results and discussion

The purity and phase crystallinity of the prepared composite samples were confirmed by XRD analysis. The representative XRD patterns of synthesized material by co-precipitation method and annealed at 800 °C for 24 h was shown in Fig. 1. It can be clearly seen from the Fig. that two phase nature of the composite has been obtained and doping of MgO and CaO has no effect on the peak position, rather it only affects the peak height of pure zirconia. Phase composition analysis reveals that pure ZrO2 (C0) show co-existence of monoclinic and tetragonal phase; the monoclinic phase concentration was more than that of tetragonal phase. The observed diffraction pattern of pure ZrO2 having tetragonal crystal structure with lattice constant a = 0.35644 Å, c = 0.5176 Å and monoclinic phase with lattice cell parameter a = 0.5144 Å, b = 0.51964 Å and c = 0.51964 Å [16]. Additionally some new peaks detected in case of composite diffractograms (C1, C2 and C3) have a lattice constant a = b = c = 0.4195 Å, which allocates the presence of cubic structure of MgO [17]. After co-doping of zirconia with CaO and MgO (C2, C3), monoclinic phase of zirconia become the minor one and the high temperature cubic phase whose intensity increases as the doping level of CaO increases is the dominating one with same position of peak. However, the peaks of sample C4 become broad with increasing concentration of CaO and fully cubic stabilized zirconia ceramics was obtained after addition of 12 mol% CaO. That was due to the decrease in grain size. Along with cubic phases, at 2θ = 31.29° and 45.15°, extra peaks of CaZrO3 are also observed [6].

Fig. 1

X-ray diffraction patterns for the C0, C1, C2, C3, C4 composite solid electrolyte.

FTIR spectra for pure and composite samples were presented in Fig. 2. The observed strong absorption peak at approximately 452 cm−1 region is due to Zr—O vibration, which confirmed the formation of ZrO2 structure; prominent peak at 1383 cm−1 corresponds to the O—H bonding. The peak at 1621 cm−1 may be due to adsorbed moisture and broad band around 3346–3433 cm−1 are due to stretching vibrations of the O—H bond of water molecules [18], [19]. Further, composition C1, C2, C3, and C4 have some new IR bands at different wave numbers corresponding to MgO and CaO content. The absorption peaks at 1635 cm−1 and 1137 cm−1, 1012 cm−1 of spectra C1, C2, C3 correspond to bending vibration of OH bonds and Mg—OH stretching vibration, respectively. The peaks around 833–617 cm−1 were assigned to different Mg—O—Mg vibration modes of MgO [20], [21]. The peak at 595 cm−1 is associated with the vibration of Ca—O bonds. The transmission peak in spectra of C2, C3, and C4 located at 876 cm−1 is related to symmetric stretching vibration of Ca—O—Ca bonds. The sharp and intense peak at 1410 cm−1 was assigned to the asymmetrical stretching vibration of OH—Ca [22].

Fig. 2

FTIR spectra for the C0, C1, C2, C3, C4composite solid electrolytes.

The DTA curves for pure ZrO2 and its composite were illustrates in Fig. 3. The thermogram of pure ZrO2 indicates a broad endothermic peak at temperature 70 °C, which is due to evolution of absorbed water from the prepared powder. With increase in temperature, sharp exothermic peak was observed at 475 °C that was related to the lower temperature phase transition of pure ZrO2 to tetragonal/cubic phase (high temperature phases). However, for doped samples the intensity of exothermic peaks increases and peaks shifts to higher temperature, the shift increases with increase in conductivity [23], [24], [25].

Fig. 3

DTApeaks of the C0, C1, C2, C3, C4 composite solid electrolyte.

Electron microscopy is a versatile tool capable of providing structural information over a wide range of magnification. SEM micrograph of undoped and doped zirconia samples prepared via co-precipitation method was shown in Fig. 4. The SEM image (a) of pure zirconia clearly demonstrate that powder consist of irregular shape agglomerates covered by smaller particles [26]. After substitution of Mg to ZrO2, smooth and uniform surface was obtained. It can be seen clearly from the image that magnesium oxide has been mixed properly with zirconium dioxide phase and form a homogeneous mixture. The particles are closely packed together and form hard agglomerates on addition of Ca and therefore conductivity of the composite decreases, within the grains formation of isolated micro pores were also observed [27]. The EDX spectra of (b) and (c) indicate the presence ZrO2, MgO, and CaO, however existence of Cl was also noticed as impurity, which may be due to entrapped unreacted chlorides of zirconium during precipitation process [28].

Fig. 4

SEM images of (a) ZrO2, (b) 8Mg, (c) 12Ca.

The technique of AC impedance is well suited for the measurement of oxide ion conductivities of solid materials. Two point probe AC measurements were carried out in frequency range of 20 Hz to 1 MHz at an applied voltage of 1V. Impedance graph involve plotting of the imaginary part (Z″) against the real part (Z′). Fig. 5 shows the complex impedance plots for two compositions C2 and C3 at temperatures 300 °C, 400 °C, and 500 °C. Impedance spectra of the composites shows a single semicircle with vertical spike, indicating that the electrode are probably blocked and therefore electronic conduction is negligible or small compared to the magnitude of ionic conductivity. Single semicircle at high frequencies region was attributed to the bulk properties of the material, whereas the inclined spike is the characteristic of the impedance of oxide ion conductor electrode electrolyte reaction. It was observed from the plots that as the temperature increases the diameter of these semi circles become smaller and resistivity decreases, which ultimately increases ionic conductivity [29], [30]. It has to be noted that the complex impedance plot for composition C2 exhibit lower value of resistivity at constant temperature compared to composition C3. This is because resistivity decreases with increasing concentration of Mg and maximizes at lower concentration of Ca. Fig. 6 represents the Arrhenius plots of oxygen ion conductivities for pure and doped samples. Ionic conductivity of samples is expressed by an Arrhenius equation aswhere σT is the total conductivity, the pre-exponential factor is σ0, activation energy is denoted by Ea, and k is the Boltzmann constant [31]. At lower temperature, pure ZrO2 was not a good oxide ion conductor; for conductivity enhancement anionic vacancies are promoted by doping [32]. Above 190 °C, the drop in the conductivity was observed due to collapse of fluorite framework. This supports the argument of lattice collapse, as reported earlier [33]. Co-doped sample shows a significantly higher conductivity and outperformed the single doped and undoped ones. The conductivity obtained for C2 sample (6 Mg3Ca) is higher than C3. This is because grain boundary conductance increases as Mg content increases and maximizes at relatively lower concentration of CaO [34]. From the graph, it has been observed that the conductivity of Mg doped Zirconia (C1) is higher than Ca-doped ZrO2 (C4), owing to small ionic size of Mg compared to Ca. The high ionic radius of Ca results in blockage of oxide ions mobility, due to which conductivity decreases [35]. A second rise in the conductivity above 450 °C indicate phase transition in ZrO2 because on dopping with aliovalent cation high temperature phase transition are maintain at lower temperature [24]. Linear regression method was used to calculate activation energy at low and high temperatures as presented in Table 2. The decrease in activation energy was observed from 0% to 6% increases in the content of MgO; owing to doping production of oxygen vacancies, which make ionic conduction easier.

Fig. 5

Impedance spectra for the C0, C1, C2, C3, and C4 composite solid electrolytes.

Fig. 6

Electrical conductivity as a function temperature for the C0, C1, C2, C3, C4 composite solid electrolyte.

Table 2

The activation energies for various molar ratios of composite solid electrolytes at low and high temperature phase.

Sample	Activation Energy (Ea) in eV
	50–190 °C	450–700 °C
C1	0.012	0.327
C2	0.003	0.263
C3	0.007	0.325
C4	0.011	0.287

Dielectric constant expressed the extent of distortion or polarization of electric charge distribution in the material as a function of frequency of applied electric field and is given aswhere capacitance in Farad is expressed by C, t is pellet’s thickness, and surface area of pellet is given by A. Fig. 7a shows a variation of dielectric constant with temperature at 1 MHz for doped samples. The highest dielectric constant was observed for the composition C2, which is slightly higher than composition C3. A significant increase in defect site and dipole take place with increase in concentration of dopant. Dielectric constant first increases to 100 °C temperature and then decreases, above 150 °C it slightly increases till 300 °C and than rapidly increases with increase in temperature, due to increase in oxide ion mobility through solid electrolyte, this process was thermally activated. The same pattern of plot was obtained for dielectric constant as observed for conductivity [36]. Increase in temperature results in increasing value of dielectric constant, which attribute to the onset of dipole in the composite system that create a suitable path for migration of ions. Additionally, it indicates the space charge polarization near interfaces of grain boundary [37], which results in large dielectric constant value of composite material at high temperatures [38].

Fig. 7a

Temperature dependent dielectric constant at 1 MHz for the C1, C2, C3, and C4 composite solid electrolyte.

The value of dielectric constant also varies when plot against different frequencies at constant temperature. Fig. 7b illustrates the plot of logarithmic ε vs. frequency for the composition 6Mg3Ca. It shows the highest value for dielectric constant and conductivity when calculated in respect to temperature. However, with respect to frequency, dielectric constant shows a decrease in values as the frequency increases, due to lower polarization. In temperature range 300–550 °C, there is a sharp increase in the value of ε, which might be due to space charge polarization in the materials [39].

Fig. 7b

Dielectric constant at different frequencies as a function temperature for the 6Mg3Ca composition.

The electrical modulus formalism of solids having ion conductivity are widely analysed in term of electric modulus (M), and is the reciprocal of dielectric constant and was used to investigate the space charge relaxation process. The electrical modulus spectrum represents the measure of the distribution of ion energies and it also describes the electrical relaxation and microscopic properties. The electrical modulus has been calculated using the following relation

Fig. 8 shows the electrical modulus at different frequencies as a function of temperature. As the temperature rises, the value of M decreases; however, the opposite trend was noticed in frequency. At low frequency (due to single relaxation process), the value of M rapidly decreases and at high temperature it becomes slow. Small contribution of electrode polarization brings M value closer to zero at low frequency and at high frequency, gradual increase in M value was observed due to saturation [40], [41].

Fig. 8

Electrical modulus formalism at different frequencies as a function temperature for the 6Mg3Ca composition.

Conclusions

Zirconia based solid electrolyte with general formula {1 − (x + y)}ZrO2-(x)MgO-(y)CaO} have been synthesized with the help of co-precipitation method. Impedance graph consist of single semicircle with a spike. Semicircle in high frequency region indicates the bulk resistance value and spike in lower frequency attributed to the oxide ion conductor electrode electrolyte reaction. The co-doped composition “6Mg3Ca” have higher conductivity compared to “4Mg6Ca”. In lower temperature region, C2 composition show minimum activation energy of 0.003 eV, which confirm that this composition has higher charge mobility within this range of temperature. With increment of frequency, dielectric constant value decreased and with increasing temperature it shows the opposite trend. On raising the temperature, the electric modulus of the sample decreases while frequency was increased.

Conflict of interest

The authors have declared no conflict of interest.

Compliance with Ethics Requirements

This article does not contain any studies with human or animal subjects.