Chapter 7 102

CHAPTER 7

ELECTRICAL PROPERTIES OF ZnO DOPED

MAGESIUM ALUMIUM SILICATE

GLASS-CERAMICS

Chapter 7 103

CHAPTER 7

ELECTRICAL PROPERTIES OF ZnO DOPED

MAGNESIUM ALUMINUM SILICATE GLASS-CERAMICS

The effect of ZnO doping on structural, dielectric and electrical properties of magnesium

aluminum silicate (MAS) glass-ceramic has been investigated in this chapter.

7.1 MATERIAL PREPARATION

In the process of investigation on the doping effect of ZnO in MAS glass-ceramic, the

compositions studied were represented by the general formula (18.5 – x) MgO-xZnO-

16.5Al2O3-47.5SiO2-9.5K2O-8B2O3 with x = 0.0, 0.1 wt.%, 0.3 wt.% and 0.5 wt.%. The

components SiO2 (99.9 %), Al2O3 (99.9 %), MgO (99.9 %); KO (99.9 %); B2O3 (99.9 %)

and ZnO (99.9 %) in proper weight percentages were thoroughly mixed in agate mortar for

2 h, including wet mixing in acetone media for 1 h. The mixture was then calcined at 950

oC for 6 h. 7 wt.% MgF2 was added to the calcined charge and was pulverized using agate

mortar and pestle for 2 h. Finely milled charge was seasoned in 5% H3PO4 acid solution in

acetone medium for a period up to 72 h. Cylindrical pellets of diameter 12 mm and

thickness 3 mm were prepared in a hydraulic press using 80 MPa pressure. Those were then

sintered using a two-step heating program. In the first step, the compact was heated up to

670-700 oC for 2- 4 h to ensure good nucleation and to initiate crystal growth. These pellets

were then sintered with optimized temperature and time (1000 oC, 3 h) in air atmosphere.

The bulk densities and apparent porosities of the sintered samples were measured by

Archimedes method (i.e. immersion in deionised water).

Chapter 7 104

7.2 MATERIAL CHARACTERIZATION

7.2.1 X-ray diffraction

Figure 7.1 shows the X-ray diffraction patterns of the samples sintered at 1000 oC. The

XRD analysis of the samples revealed that the predominant crystalline phases were found

to be Magnesium Silicate Fluoride and Cordierite, together with minor traces of Phlogopite

and Sapphirine. Addition of small amounts of ZnO does not impede the formation of

Cordierite and Magnesium Silicate Fluoride. Thus, the basic structure of the sample

remains unchanged when doped with ZnO.

Fig. 7.1 XRD patterns of ZnO doped MAS glass-ceramics

The measured bulk densities of the samples sintered at 1000 oC are shown in Fig. 7.2. Bulk

densities of the samples increased with ZnO content and is found to be maximum for x =

0.5 wt% ZnO doped MAS glass-ceramics as shown in Fig. 7.2.

Chapter 7 105

Fig. 7.2 Bulk density of the MAS glass-ceramics doped with xZnO (x = 0.0, 0.1, 0.3 and

0.5) sintered at 1000 oC.

7.2.2 Scanning electron microscopy

Figure 7.3 shows the microstructures of samples with different ZnO content sintered at

1000 oC. The micrographs show that the percentage of ZnO addition affects the

microstructure. From picture (a), it can be seen that the specimen was not dense and the

grain did not grow for 0.0 wt% ZnO doped MAS glass-ceramics. Picture (b), picture (c)

and picture (d) show that the grain size increased with the increase of ZnO addition due to

the liquid phase effect resulting from addition of ZnO. It is seen that the porosity decreased

with increasing ZnO content and the sample with 0.5 wt% ZnO has the lowest porosity

compared to others. This result corresponds to the relative density of samples as shown in

Fig. 7.2. When the ZnO content increased to 0.5 wt%, the grains of samples were in close

contact and there was little porosity, which is consistent with the result of bulk densities.

Fig (c) and (d) shows plate-like microstructure with some needle like grains.

Chapter 7 106

Fig. 7.3 SEM micrographs of the samples with different ZnO contents sintered at 1000 oC.

(a) 0 wt%, (b) 0.1 wt%, (c) 0.3 wt%, (d) 0.5 wt%

7.3 DIELECTRIC STUDIES

Figure 7.4 shows the variation of relative dielectric constant (εr) and dielectric loss (tanδ)

with ZnO content at 1 MHz at room temperature. The dielectric constant of the sample

strongly depends on its density [75]. The relative dielectric constant generally increases on

increasing ZnO concentration. From Fig. 7.4, it is evident that x = 0.5 possesses highest

dielectric constant, which may be due to its high density and low apparent porosity. From

Fig. 7.4, it is observed that tanδ decreases on increasing ZnO content, which may be

attributed to the porosity decrease owing to the increase in bulk density. From Fig. 7.4, it is

evident that 0.5 wt% ZnO doped MAS glass ceramic also possesses low dielectric loss.

Chapter 7 107

Fig. 7.4 Variation of relative dielectric constant (εr) and dielectric loss (tanδ) with ZnO

contents at room temperature.

The variation of relative dielectric constant (εr) and dielectric loss (tanδ) of all the samples

with temperature at 10 kHz is shown in Fig. 7.5(a) and (b) respectively. Both density and

the type of phase play a significant role in the dielectric constant of the samples. The

dielectric constant shows the same tendency of the bulk density. It is understood that higher

density will lead to higher dielectric constant owing to lower porosity. It is observed that,

initially, the value of εr increases with rise in temperature for all the samples. The peak

value of dielectric constant (εr) increases with increase in concentration of ZnO (x = 0.3 and

x = 0.5) in MAS glass-ceramic as compared to the pure one. The broadening of dielectric

peaks may be attributed to the disorder present in the systems. XRD analysis of the samples

revealed that no additional phases are formed in ZnO doped MAS glass-ceramics as

compared to the pure one. This suggests that increase in density would be responsible for

the variation in dielectric constant of samples. ZnO doped MAS glass-ceramics exhibit

interfacial or space charge polarization arising from the differences amongst the

conductivity of various phases. Due to the presence of phases of different conductivity,

motion of charge carriers occurs readily through one phase but is interrupted when it

arrives at the boundary of other phases. This causes a build-up of charge at the interface,

Chapter 7 108

which corresponds to a large polarization and high value of effective dielectric constant.

The increase of εr at higher temperatures may be due to weakening of binding force

between the ions leading to mobile ion contribution.

The dielectric loss (tanδ) is another parameter, which makes it possible to distinguish

between samples of different compositions. Density also plays an important role in

controlling the dielectric loss. Dielectric loss of the samples decreased subsequently with

increasing ZnO content. This might be due to the fact that the density of the samples

increased with increasing ZnO content. It is clear from Fig. 7.5(b) that the dielectric loss is

found to be minimum for x = 0.5, due to its high bulk density. The rapid increase of

dielectric loss at higher temperatures may be due to space charge polarization. It is found

that tanδ increases with increasing temperature because of fast movement of the ions in the

glass network and their increased response to an electric field with increasing temperature.

In addition, the high loss factor of ZnO doped MAS glass-ceramic at higher temperatures is

due to the large glass content and to the high mobility of alkali (K+) and F

- ions. It is clear

that the sample with 0.5 wt% ZnO has the best dielectric property, since; it possesses the

lowest dielectric loss.

Fig. 7.5 Variation in (a) εr and (b) tanδ of ZnO doped MAS glass-ceramics with

temperature at 10 kHz for different x.

Chapter 7 109

7.4 IMPEDANCE ANALYSIS

Figure 7.6 shows the complex impedance plots (Z′ vs Z′′) of ZnO doped MAS glass-

ceramics at different temperatures over a wide range of frequency (100 Hz – 5 MHz). The

Nyquist plots of all samples show a depressed semicircular arc whose center lies below the

real (Z′) axis. The nature of the plots confirms the presence of non-Debye and

polydispersive relaxation process in the samples. The shape of our plots suggests that the

impedance has contribution from the bulk (grain) as well as grain boundary at high

temperatures. From the graph it is clear that as the temperature increases intercept along the

real (Z′) axis shifts towards the origin indicating the increase in bulk conductivity of the

materials. The effect of ZnO concentration in the impedance plots of MAS glass-ceramic is

clearly seen in Fig. 7.7. Figure 7.7 shows the impedance plot as a function of composition

at 400 oC. It is observed that as the ZnO concentration increases the bulk resistance

increases. It is observed that the grain boundary contribution decreases for x = 0.5 wt%

ZnO doped MAS glass-ceramic. The impedance data were analyzed in order to obtain the

bulk resistance (Rb), and bulk capacitance (Cb) of the samples. The value of Rb can be

obtained from the intercept on the Z′ axis, the variation of which with ZnO concentration at

400 oC is shown in Fig. 7.8. It is evident from Fig. 7.8 that the bulk resistance (Rb)

increases with an increase in ZnO concentration and is found to be maximum for 0.5 wt%

ZnO doped MAS glass-ceramic. These values were used to calculate the bulk capacitance

(Cb) using the relation ωmaxRbCb = 1, where ωmax (=2πfmax) is the angular frequency at the

maxima of the semicircle. The variation of Cb with ZnO concentration at 400 oC is shown

in Fig. 7.8 [inset]. It is clear that the bulk capacitance (Cb) increases with increase in ZnO

concentration and is found to be maximum for 0.5 wt% ZnO doped MAS glass-ceramic.

Chapter 7 110

Fig. 7.6 Nyquist plots of ZnO doped MAS glass-ceramics at four different temperatures

Fig. 7.7 Comparison of Nyquist plots of ZnO doped MAS glass-ceramics at 400 oC

Chapter 7 111

Fig. 7.8 Variation of Rb and Cb (inset) with ZnO concentration at 400 oC

Figure 7.9 shows the variation of imaginary part of impedance [Z′′] with frequency at four

different temperatures for ZnO doped MAS glass-ceramics. The curves show that the value

of Z′′ reaches maximum value of Z′′max at all temperatures. The average peak position

regularly changes towards the higher frequency side on increasing temperature for all the

samples. The Z′′ spectra are broadened on the low frequency side of the maximum peak

showing a departure from the ideal Debye-like behavior. The asymmetric peaks imply the

existence of electrical processes in the samples with spread of relaxation time. Furthermore,

as the temperature increases the magnitude of Z′′ decreases, the effect being more

pronounced at the peak position. The shift of the peak towards higher frequency on

increasing the temperature is due to the reduction in the bulk resistivity of all the samples.

The effect of increase of ZnO concentration on the electrical behavior of the samples can

clearly be seen in terms of variation in the magnitude of Z′′, peak broadening and

asymmetry.

Chapter 7 112

Fig. 7.9 Variation of imaginary part of impedance (Zʹʹ) of ZnO doped MAS glass-ceramic

as a function of frequency for different ZnO concentrations at different temperatures.

7.5 MODULUS STUDIES

Figure 7.10 show the variation of imaginary part of electric modulus (M′′) of ZnO modified

MAS glass-ceramics with frequency at different temperatures. The frequency region below

the M′′ peak indicates the range in which charge carriers are mobile over long distances. In

the frequency range (≥ peak frequency), the charge carriers are spatially confined to

potential wells and free to drift within the wells. It is observed that the maxima M′′max shifts

Chapter 7 113

towards higher frequencies with a rise in temperature. The observed M′′ peaks of the plots

are related to conductivity relaxation of the materials. Figure 7.11 shows that the height and

broadening of the modulus peak appear to decrease with an increase in ZnO concentration.

The decrease in the M′′ peak height on increasing x suggests an enhancement in the

capacitance value of the sample (Fig. 7.8) on substitution of Zn in the compound [66]. This

observation appears to be in good agreement with the complex impedance spectrum, and is

expected to cause an increase in the dielectric properties of the materials.

Fig. 7.10 Variation of imaginary part of modulus (M′′) with frequency of (a) 0.0 wt% (b)

0.1 wt% (c) 0.3 wt% and (d) 0.5 wt% ZnO doped MAS glass-ceramic at selected

temperatures

Chapter 7 114

Fig. 7.11 Variation of imaginary part of modulus (M′′) with frequency for different ZnO

concentrations at 400 oC and 450

oC [inset]

The impedance and modulus spectroscopic plots (Z′′ and M′′ versus frequency f) are

complementary to each other. As suggested by Sinclair and West [76], a combined plot of

imaginary modulus (M′′) and impedance (Z′′) as a function of frequency is useful to detect

the effect of the smallest capacitance and large resistance. It is advantageous to plot Z′′ and

M′′ versus frequency simultaneously. This helps us in distinguishing whether the relaxation

process is due to short range or long-range movement of charge carriers. If the process is

long range, then the peak in M′′ versus frequency and Z′′ versus frequency will occur at the

same frequency and if the process is localized these peaks will occur at different

frequencies. Fig.7.12 shows the impedance and modulus plots (Z′′ and M′′ versus

frequency f) of ZnO doped MAS glass-ceramics for different ZnO concentrations at a

particular temperature (450 oC). This figure exhibits appreciable mismatch between the Z′′

and M′′ peaks for different ZnO concentrations. Even though Zn has been substituted at the

Mg sites, the mismatch between the peaks of Z′′ and M′′ frequency plots are observed for

all compositions. The existence of an appreciable separation between these peaks suggests

Chapter 7 115

the presence of localized movement of charge carriers (via hopping type mechanism) and

departure from an ideal Debye-like behavior [77] for all ZnO modified MAS glass-

ceramics. The broad and asymmetric peaks of all the samples irrespective of ZnO

concentrations suggest the existence of a distribution of relaxation times.

Fig. 7.12 Variation in Z′′ and M′′ with frequency at a particular temperature for different x.

Figure 7.13 shows the variation of most probable relaxation time (determined from the

position of the loss peak in the Zʹʹ or M′′ versus ln(f) plots) with inverse of absolute

temperature (i.e., τ versus 103/T) for different ZnO concentrations at high temperature

region. The graph follows the Arrhenius relation, τ = τoexp (-Ea/KBT) (where τo pre-

exponential factor, Ea activation energy, KB Boltzmann constant and T absolute

temperature). The activation energy (Ea) values obtained from the impedance (Z′′) spectrum

represents the localized conduction (i.e., dielectric relaxation) and that obtained from the

Chapter 7 116

modulus (M′′) spectrum represents nonlocalized conduction (i.e., long range conduction).

The relaxation time is thermally activated process and the activation energy values of the

samples obtained from the Z′′ spectra (Fig. 7.13(a)) are found to be in the range of 0.62 eV

to 1.05 eV. The Ea values of the samples calculated from the impedance spectrum

decreases with the increase of ZnO content and is found to be minimum for x = 0.5 wt%

ZnO doped MAS glass-ceramic. When Zn2+

is substituted at the Mg2+

sites, there may be a

change in the concentration of oxygen vacancies due to the variable oxidation states of Mg

and Zn in MAS glass-ceramic. With the increase in Zn concentration, there may be an

increase in oxygen vacancies leading to an increase in the number of conducting electrons.

So the activation energy decreases with an increase in Zn concentration. This change in

behavior indicates the participation of Zn ions in the relaxation and conductivity process.

With the help of modulus plot, variation of most probable conduction relaxation time (τ)

with temperature is shown in Fig. 7.13(b). The values of Ea obtained from the M′′ spectra

are found to be in the range of 0.37 eV to 0.78 eV which is similar to those for ionic

conductivity and oxygen vacancy migration. It is clear that the activation energy of x = 0.5

wt% ZnO doped MAS glass-ceramic (calculated from impedance and modulus spectrum) is

equal and this suggests that the nature of species taking part in both localized and

nonlocalized conductions is same.

Chapter 7 117

Fig. 7.13 Variation of relaxation time (τ) obtained from (a) Z′′ spectra and (b) M′′ spectra of

samples with temperatures for x=0, 0.1, 0.3 and 0.5

7.6 CONDUCTIVITY STUDIES

The frequency dependence of ac conductivity, σ (ω), at various temperatures for all samples

is shown in Figure 7.14. The pattern of conductivity spectrum shows low-frequency plateau

and high-frequency dispersion (for x = 0.1 wt.%, 0.3 wt.% and 0.5 wt.%) with a change in

slope at x = 0.0 wt.%. At low frequency, the conductivity shows a flat response which

corresponds to the dc part of the conductivity. It is clear from the graphs that the flat region

increases with the increase in temperature. The attainment of low-frequency plateau in the

material was found to be more pronounced at higher concentration of ZnO. The decrease in

bulk resistance of all the samples with increase in temperature is also evidenced in the plot

since the conductivity increases with rise in temperature. The higher values of conductivity

at higher temperatures are possibly due to the movement of mobile ions. Although an

enhancement in the conductivity with the ZnO doping is evident in Fig. 7.14, the origin of

the enhancement is not clear at present. ZnO doped samples have higher density which may

contribute to the conductivity increase. It may be due to the easy migration of K+ and F

-

ions through the ZnO doped MAS glass-ceramics. The explanation of the enhancement,

Chapter 7 118

however, is still controversial and further study is needed to elucidate the conductivity

enhancement mechanism. At higher frequencies, the conductivity starts increasing and a

remarkable dispersion has been observed. It is clearly seen that the frequency at which the

dispersion becomes predominant shifts towards higher frequency regions as the

temperature increases. The conductivity dispersion, suggests that the electrical conduction

of the compounds is a thermally activated process which obeys the Jonscher’s law,

σac = σ0 + Bωn, where σ0 is the dc conductivity at a particular temperature, B is the

temperature dependent constant and n is the power law exponent which generally lies

between 0 and 1. The exponent n represents the degree of interaction between the mobile

ions.

The variation in exponent n as a function of temperature is represented in Fig. 7.15.

It is well-known that the mechanism of conductivity in any material can be understood

from the temperature dependent behavior of n. The decrease in the value of n with the

increase in temperature suggests that the charge transport between localized states takes

place due to hopping over the potential barriers. This suggests that the conductivity

behavior of ZnO doped MAS glass-ceramics can be explained using the correlated barrier

hopping (CBH) model.

Chapter 7 119

Fig. 7.14 Variation of ac conductivity of samples with frequency at different temperatures

Fig. 7.15 Variation of n as a function of temperature for all the samples

The temperature dependence of DC conductivity shown in Fig. 7.16 for all the samples

obeys Arrhenius relation. The activation energies for all the compositions are calculated

and listed in Table 7.1. It is observed that the activation energy is higher in ZnO doped

samples compared to the pure one. The conductivity of ZnO doped MAS glass-ceramic is

appreciably greater than that of pure MAS glass-ceramic. It is also observed that the

enhancement in conductivity is three to four orders within the composition range studied as

shown in Table 7.1 at 500 oC. It is worth noting that the activation energies for long range

Chapter 7 120

conduction and dc conduction are in close agreement. It suggests that similar energy

barriers are involved in both the long range and dc conduction processes. It is observed that

for x = 0.5 wt% ZnO doped MAS glass-ceramics; the activation energies for relaxation

process and dc conduction are in close agreement. It suggests that similar energy barriers

are involved in both the relaxation and conduction processes of 0.5 wt% ZnO doped MAS

glass-ceramics.

Fig. 7.16 Variation of σ dc with inverse of absolute temperature of all the samples at high

temperature

Table 7.1 DC conductivity and activation energy of ZnO doped MAS glass-ceramic

x

Ea (eV) σ (Ω-1

m-1

) at 500 oC

x = 0.0

x = 0.1

x = 0.3

x = 0.5

0.58

0.75

0.72

0.67

1.93 × 10-7

1.57 × 10-3

1.01 × 10-3

7.22 × 10-4

Chapter 7 121

7.7 CONCLUSIONS

The polycrystalline ZnO doped MAS glass-ceramics were prepared via sintering route.

Addition of ZnO can greatly improve the dielectric properties of MAS glass-ceramic. The

porosity is an important factor affecting the dielectric loss of the samples. The dielectric

constant of the samples was determined by the dual action of the density and the type of

crystalline phase. The dielectric constant increases and the dielectric loss decreases

significantly as the ZnO concentration increases. From the dielectric studies, one may

assume 0.5 wt% ZnO doped MAS glass-ceramic sample as a good dielectric material since

it possesses low dielectric loss. The electrical parameters such as the bulk resistance (Rb)

and bulk capacitance (Cb) were obtained using complex impedance spectroscopy. The bulk

resistance (Rb) decreases with a rise in temperature for all the compositions under study. It

is observed that as the ZnO concentration increases the bulk resistance increases. It is clear

that the bulk capacitance (Cb) increases with increase in ZnO concentration and is expected

to cause enhancement in the dielectric properties of the material with increase in ZnO

concentration. The Jonscher’s power law formalism was used to analyze the ac

conductivity. The conductivity increases with the increase in ZnO concentration. This may

be due to the easy migration of K+ and F

- ions in the ZnO doped MAS glass-ceramics. The

temperature dependence of DC conductivity obeys Arrhenius behavior for all the samples

under study. It is observed that similar energy barriers are involved in both the relaxation

and conduction processes of 0.5 wt% ZnO doped MAS glass-ceramics. The activation

energy associated with the dielectric relaxation determined from the electric modulus

spectra was found to be close to that of the activation energy for dc conductivity.