Ceramic Composites: Qandilite MgTiO3 with Borosilicate Glass for Enhanced Electrical Properties

Abstract

Qandilite MgTiO3 ceramic was prepared form MgO-TiO2 powder/borosilicate glass composite. Composite materials were prepared from nominal MgTiO3 alone and with 10, 30 and 50% borosilicate glass. Sintering process gave qandilite alone in nominal MgTiO3 alone or with 10% glass whereas it gave crystalline magnesium titanate (Mg0.75Ti2.25O5) in both (i.e. 30% -and 50% - containing glass) and with forsterite (Mg2SiO4) in 30% or rutile with enstatite (MgSiO3) in that containing 50%-glass.

Their microstructure shows clear tetragonal, octahedral, in addition to rod-like crystals embedded in glassy matrix.

The dielectric constant of the composites increased in the samples containing > 30 %glass (⁓125 at room temperature and 1 kHz). The values of activation energy were in the 0.145 – 0.438 eV range. The results of ac conductivity indicated the dominance of electronic mechanism over the ionic transfer one in the composite samples which mean it can applied as semiconductor materials in electronic devices.

Introduction

Magnesium Titanate (MgTiO3) ceramics is a famous dielectric material used in microwave frequencies [1]. The MgTiO3 structure belong to ilmenite type structure of hexagonal space packing of oxygen atom [2] .

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There are different routes to get MgTiO3 material such as thermal decomposition of peroxide precursors, hydrothermalmechano-chemical complex ation routes, Sol-gel method MOSD and auto- igniting combustion technique [3- 6]. MgTiO3 ceramic have many applications in filters, antennas for communication, radar, direct broadcasting satellite and global positioning system operating at microwave frequencies [6-7].

B2O3 – doped MgTiO3was prepared by solgel route and its calcination from 650 to 900 °C gave particle size from 20-30 nm to 90-120 nm respectively, whereas calcination at 1100 C gave 40-60 nm.

Also authors supposed the incorporation of B in MgTiO3 structure [8]. Through solid reaction method, binaryMgTiO3-CaTiO3 ceramic was prepared and incorporation of CaTiO3 from 5 % to 10 % mole change volume density from 3.612 to 3.500 g/cm3 and dielectric constant from 17.8 to 16.6 [9]. Partial substitution of Mg by Zn in (Mg 0.95‐Zn 0.05) TiO3 ceramic lowered the sintering temperature and improved dielectric constant. Other said that the dielectric response of MZT ceramics is suitable for dielectric resonator and type-1 RF capacitor applications. [10-11]. Based on MgTiO3-LiF materials, humidity sensor was fabricated as thin ceramic film used spinning coating technology (Ceramic thick film humidity sensor based on MgTiO3 + LiF [12].

The present research deals with the study of preparation of MgSiO3 by ceramic route. Characterization of the ceramic samples was by x ray diffraction analysis (xrd), scanning electron microscopy(SEM) and energy dispersive x ray microanalysis (EDX).The electric properties include dielectric constant and conductivity were measured .

Results and Discussion

Characterization of the samples

Incorporation of borosilicate glass lowers the sintering temperatures from1300 °C in MT10 and MT9 samples to 1200 °C in the MT7 and MT5 composite samples (Fig.1). Qandilite MgTiO3 (ICCD, 82-1130, orthorhombic) was developed alone in MT10 and with little olivine (Mg2SiO4, ICCD, 87-0061, orthorhombic) in MT9 composite samples after sintering at 1300 °C. Increase of borosilicate glass to 30 % (MT7) and 50 %( MT5) led to decrease the sintering temperature to 1200 °C. Magnesium titanate (Mg0.75 Ti2.25O5, ICDD, 82-1127, orthorhombic) was developed as the major phase in the later samples but with olivine in MT7 and with enstatite (MgSiO3, ICDD 84-0653, orthorhombic) and rutile (TiO2, ICDD 82-0514, tetragonal) in MT5 sample (Fig. 1).

It must be notice that, other than crystalline magnesium titanate major phases, incorporation of borosilicate glass enhances the crystallization of Mg olivine (Mg2SiO4) , enstatite (MgSiO3) as well as rutile (TiO2) in case of the highest borosilicate ratio. Also, combination of glass meaning increase of the silica ratio and facilitates the crystallization of enstatite other than the olivine of low silica ratio. Rutile TiO2of tetragonal structure crystallized in medium of good mobility, in the highest borosilicate ratio, which considered in higher order after cubic structure other than the other formed orthorhombic phases developed in the present composite.

The SEMmicrographs of the composite sample had developed after sintering at 1200 and 1300 °C are shown in figures 2 and 3. In both MT10 and MT9 composite samples, orthorhombic crystals (1 to 5 um in width) of qandilite were developed in glassy matrix. In the MT7 and TM5 composite samples, orthorhombic or rod-like crystals of magnesium titanate were appearing in glassy groundmass respectively (Fig. 3).

The EDXmicroanalysis in the MT10 and MT7 samples were referred to the main crystalline phases MgTiO3 and Mg0.75 Ti2.25O5 in MT10 and MT7, respectively (Fig. 4).

Electrical Properties

Figure 5 shows the variation of dielectric constant with temperature at different frequencies (1 kHz, 100 kHz, 800 kHz, 1 MHz) for all the studied glass ceramic samples. It can be seen that there is an increase in the dielectric constant with increasing temperature for samples MT10 and MT9 (Fig. 5:a and b).This behavior is normal which can be attributed to the permitting of more vibration of the molecules/atoms as a result of weakening in the binding force between them with increasing the temperature. Accordingly, an increase in the polarization occurs, hence increasing the dielectric constant (ε′) [14], in addition to the increase of the number of charge carriers (mainly electrons) with temperature; giving rise to the observed increase in the dielectric constant (ε′) [15]. While for samples MT7 and MT5 the dielectric constant decreases with increase in temperature which is an unexpected behavior. However, this type of behavior is in quite agreement with that reported in other glasses [16-17].

It can be noticed (Fig.5) that the ε′ decreases with increasing frequency. This decrease can be related to the dielectric polarization mechanism of the material. Dielectric polarization occurs by electronic, ionic, interfacial or dipolar polarization. Electronic and ionic polarizations are active in the high frequency range, while the other two mechanisms prevail in the low frequency range. When the frequency is increased, the dipoles will no longer be able to rotate sufficiently rapidly, so that their oscillations begin to lag behind those of the field [18-19], consequently ε′ decreases.

Figure 6 shows the variation of dielectric constant, εʹ, with the glass fraction at 25 and 300 (°C) at different frequencies. It can be noticed that εʹ decreases with the increase of the glass fraction till 30wt% (sample MT7). This decrease may be attributed to presence of higher amount of glassy phase in the glass ceramics [20 -21] . Also this decrease revealed to the presence of the crystallized secondary phase of forsterite (Mg2SiO4), which confirmed by XRD (Fig.1), in both samples MT9 and MT7 with 10 and 30 (wt%) of glass, respectively. This phase possesses lower εʹ value [22] than that of MgTiO3 phase [23-24] present in sample MT10, consequently the ɛʹ values decreases. For example at room temperature and 1 kHz, the εʹ decreases from about 28 for sample MT10 to 14 and 12 for samples MT9 and MT7, respectively.

It was reported that the dielectric properties of ceramic materials depend on the average grain size as well as the processing conditions (sintering temperature) where the dielectric constant increases with sintering temperature [26]. The obtained results (Fig.5) show that the dielectric constant values of sample MT9 are higher than those of sample MT7 due to its higher sintering temperature (1300C). It is, also, observed (Fig.6) that sample MT5 with the highest glass fraction (50 wt%), has the highest values of dielectric constant. This can be related to the presence of TiO2 (rutile phase) (Fig.1) which is characterized by high εʹ [27- 28]. For example, the εʹ for sample MT5 reaches ⁓125 at room temperature at 1 kHz.

It was reported that the dielectric properties of ceramic materials are depend on the average grain size as well as processing conditions (sintering temperature) where the dielectric constant increases with sintering temperature [29].

Fig.7 represents the ac conductivity ( σac ( as a function of the reciprocal temperature 1000/T for the investigated samples at 1 kHz. Due to the thermal activation process that related to hopping of charge carriers which are bound in the localized states [30-31], the electrical conductivities of the samples increase with the increase in temperature, implying a typical semiconducting behavior [32]. It is also concluded from Fig.7 that, among the different samples studied, the conductivity increases in the following order: MT7 < MT9 < MT10 < MT5.

The conductivity values of sample MT10 without glass fraction decrease by increasing the glass weight fraction till sample MT7 which contain 30 wt% of glass then it increases in sample MT5 which contain 50 wt% of glass. For example at RT and 1 kHz, the electrical conductivity of sample MT10 decreases from 3.98 x 10-7 S m-1 to 1.11 x 10-7 and 5.38 x 10-8 (S m-1) for samples MT9 and MT7, respectively. This decrease of conductivity may be attributed to the presence of the crystallized phase of forsterite (Mg2SiO4) (Fig.1) which is characterized by extremely low electrical conductivity [33-35] in both samples MT9 and MT7.

However, the relative increase of conductivity in sample MT5 can be attributed to the role played in conduction mechanism by Li+ ions (ionic radius 90 pm) which accompanied with the increase of glass fraction in addition to the possibility of the formation of Ti4+ and Ti3+ ions. Consequently, the electron hopping between these ions may participate in the increase of conductivity.

From the slopes of the linear fit in the studied temperature range 25 - 300 (°C), the activation energies associated with ac conduction can be estimated. The obtained values of the ac activation energies for different samples are listed in Table 2.The activation energy (Ea(ac)) is found to have values in the range 0.15–0.44 (eV). These results may indicate the dominance of electronic mechanism over the ionic transfer one. For sample MT10, the value of activation energy suggests that the conduction process occurs through electronic mechanism by the hopping process of localized electrons between the Ti3+ and the Ti4+ ions.

The increased activation energy in the whole temperature region for samples MT9 and MT7 (Table 2) to values of 0.37 and 0.44 (eV), respectively, can be attributed to the participation of the ionic conduction mechanism via Li+ ions which needs relatively higher activation energy than the electronic ones. The increase in the Li+ ions is due to increased glass weight fraction. For sample MT5, which contain 50 wt% of glass, the activation energy decreased to 0.15 eV. This finding is consistent with the increase in conductivity value of this sample to 9.32 x 10-7 S m-1. This decrease of activation energy can be related to the possibility of the formation of Ti4+ and Ti3+ions which may cause the dominance of electronic mechanism over the ionic transfer mechanism.

Table 2 Activation energies (Ea(ac)), ac conductivities (σac ) and dielectric constants (ε′) at room temperature (RT) of the samples.

Conclusion

Qandilite (MgTiO3) was prepared as major phase with enstatite and olivine in composite samples within sintering temperature between 1200 to 1300 °C. Qandilite was developed as rod-like crystals in low micron size. The dielectric constant of the studied composite samples decreases with increasing the glass fraction till 30 wt%, then increased to show the highest values of dielectric constant (⁓125 at room temperature and 1 kHz) for glass fraction of 50 wt%.

The electrical conductivities of the samples increase with the increase in temperature, implying a typical semiconducting behavior. The conductivity of the composite samples seems to proceed dominantly by electronic conduction over the ionic transfer ones by the hopping process of localized electrons between the Ti3+ and the Ti4+ ions. Therefore they may represent good candidates to be applied as semiconductor materials in electronic devices.