Abstract: Pure and nickel doped (x = 0.05, 0.10, 0.15 mol%) polycrystalline potassium hexatitanate (K2Ti6O13) ceramics were synthesized using conventional solid state reaction route. Lattice constants have been evaluated from XRD data recorded at Room Temperature (RT), revealed its single phase formation in a monoclinic symmetry. XRD result corroborated the successful doping of Ni in the K2Ti6O13 matrix. Dielectric and electrical properties (relative permittivity ε’, loss tangent tanδ and ac conductivity σac) have also been studied as a function of frequency at room temperature for all specimens and it is found that these parameters generally decrease with increase in frequency and dopant concentration.
INTRODUCTION
Titanium oxide has attracted attention of researchers considerably due to its potential applications in various fields such as in solar cells (Barbe et al., 1997), electronics (Croce et al., 1998), photocatalysis (Hodos et al., 2004) and sensors (Mor et al., 2004). Photocatalytic degradation using semiconductors as catalysts have been shown as the most promising method for the destruction of pollutants in water (Fujishima et al., 2000). Among the semiconductor catalysts, TiO2 is the most used photocatalyst showing a high performance such as degradation of Rhodamine B in the presence of TiO2 powder and nanotubes (Pang et al., 2010), removal of arsenic from contaminated water by iron based titanium dioxide (Halim et al., 2008), photocatalytic activity over methyl orange dye (Fathima et al., 2009), photocatalytic degradation of β-naphthol (Qourzal et al., 2006), photocatalysis of phenolic compounds (Khuanmar et al., 2007), photocatalytic degradation of phenol (Zulfakar et al., 2011), removal of colour from landfill by solar photocatalytic (Makhtar et al., 2010). It has been observed that besides, TiO2, materials containing titanium oxide have wide applications in various fields. Among such materials alkali titanate family, M2Ti6O13 (M = Na, Li, K), is the most useful. Conventionally these materials are prepared by solid state reaction method using titanium oxide (TiO2) as a raw material and have interesting technological applications such as ceramic capacitors (Xu et al., 2005), dielectric sensors and biosensors (Dominko et al., 2006), biophysics and nanotechnology (Becker et al., 2007; Wang et al., 2003) ion exchange (Bavykin and Walsh, 2007) and photocatalysis (Stengl et al., 2006; Teshima et al., 2008). K2TinO2n+1, with 3<n<6, have a wide band gap of about 3.45 eV while that of TiO2 has 3.22 eV and thus K2Ti6O13 shows semiconducting behaviour, it is also known for its catalytic activity (Sayama and Arakawa, 1994). Other applications of K2Ti6O13 are reinforcements for metals, such as copper, to improve their wear resistance (Murakami and Matsui, 1996), plastics to improve their mechanical and dielectric properties (Yu et al., 2000) and automotive brake lining pad as a substitute for cancerogenic asbestos (Choy and Han, 1998). Luo et al. (2011) reported the mechanical and thermal insulating properties of resin-derived carbon foams reinforced by K2Ti6O13 whiskers. Physicochemical properties of ambient-dried SiO2 aerogels with K2Ti6O13 whisker have been investigated (Zhang et al., 2009). Physical properties of such compounds may be modified by doping aliovalent 3D transition-metal ions. Recently, effect of Na-substitution on the dielectric behaviour of layered K2-xNaxTi4O9 (0.05≤x≤0.15) ceramics (Vikram et al., 2009), structural and dielectric properties of Cu-doped K2Ti6O13 (Vikram, 2010) have also been reported by our research group. Besides bulk material, preparation and photocatalytic activity of nanomaterials of these pure alkali titanates M2TinO2n+1 (M = Li, Na, K etc.) and silver hexatitanates have also been reported (Song et al., 2007; Rodriguez-Gonzalez et al., 2009). In the present study synthesis, room temperature dielectric and electrical properties of pure and Ni doped K2Ti6O13 have been investigated.
MATERIALS AND METHODS
Synthesis: K2Ti6O13 (PTO) ceramic was prepared by conventional solid-state route taking stoichiometric amounts of the grinded AR grade K2CO3 and TiO2 powders (purity 99.9%), under acetone and calcined at 1000°C for 24 h followed by furnace cooling. To prepare nickel doped (x = 0.05, 0.10,0.15 mol%) specimens, desired amount of NiO powder (purity 99.9%; AR grade) was added to the mixture of potassium carbonate and TiO2 and the mass so obtained was recycled through the above process. The obtained powder was pressed into pellets of 13 mm diameter and 1.00 mm thickness which were further sintered at 1000°C for 1 h.
Characterization: Structural properties of all the specimens were studied by X-Ray Diffraction (XRD) spectrum on a X-ray powder diffractometer using Cu-Kα radiations (λ = 0.15406 nm) in 2θ range from 10-70° with scan rate of 2° min-1 at room temperature. The Lattice parameters were calculated using relation:
| (1) |
The dielectric and electrical measurements were carried out after applying the silver paste on the flat faces of the pellets, in the frequency range 100-1000 KHz using LCR HI-Tester (HIOKI 3532-50). The value of dielectric constant (εr) is calculated using the formula:
| (2) |
where, ε0 is the permittivity of free space, d is thickness of pellet, A is the cross-sectional area of the flat surface of the pellet and Cp is the capacitance of the specimen in Farad (F).
The complex dielectric constant (ε”) of all the samples were calculated using the relation:
| (3) |
where, tanδ is the dielectric loss tangent, proportional to the loss of energy from the applied field into the sample (this energy is dissipated as heat) and therefore denoted as dielectric loss.
The ac conductivity of the samples was determined using the relation:
| (4) |
where, ω is the angular frequency and tanδ denotes the dielectric loss tangent.
RESULTS AND DISCUSSION
Structural properties: Figure 1 shows the XRD patterns, achieved at RT, for pure and nickel doped (x = 0.0, 0.05, 0.10, 0.15) K2Ti6O13 ceramics. Further, no other impurity peak was observed in the XRD pattern showing the single phase sample formation. The peak position of each sample exhibits the monoclinic crystal symmetry which was confirmed from ICDS card No. 74-0275. The variations in the lattice parameters and cell volume have also been studied for different doping concentrations and presented in Table 1. The unit cell parameters reported by other researcher (Vikram, 2010) fairly coincides with the calculated values. The ionic radius of Ni2+ is 72 pm whereas that of Ti4+ is 68 pm. The Ni ions substitute the Ti4+ ions in the crystal due to comparable ionic radius. However, the increase in the lattice parameter may be due to the larger ionic radius of Ni ions. Hence, we can observe that doping invariably and consistently expands the unit cell volume.
| Table 1: | List of pure and nickel doped K2Ti6O13 ceramic samples with unit cell volume and lattice parameters |
| Fig. 1(a-d): | XRD patterns for pure and nickel doped K2Ti6O13 ceramics |
If we consider the schematic structure of K2Ti6O13, it may be described having K+ ions in the tunnel space contributing in the ionic conduction while TiO6 (or NiO6) assembly is attached with each other forming zigzag ribbons joined by edge-sharing (Xie and Lu, 2003).
Dielectric properties: The dielectric constant is represented by ε = εr -iε″ where εr is real part of dielectric constant (relative permittivity) and describes the stored energy while ε″ is imaginary part of dielectric constant which describes the dissipated energy. Figure 2a shows the frequency response of dielectric constant, recorded at RT, in the frequency range of 100-1000 kHz for x = 0.0, 0.05, 0.10 and 0.15, respectively. It is clear from the figure that dielectric constant decreases with the increase in frequency for all samples and this type of behavior can be explained on the basis of Maxwell-Wagner model (Prodromakis and Papavassiliou, 2009). According to this model, a dielectric medium is assumed to be made of well conducting grains which are separated by poorly conducting (or resistive) grain boundaries. Under the application of external electric field, the charge carriers can easily migrate the grains but are accumulated at the grain boundaries. This process can produce large polarization and high dielectric constant. The small conductivity of grain boundary contributes to the high value of dielectric constant at low frequency. The dielectric constant decreases with frequency as various polarisation processes ceases at higher frequencies. It has also been observed that the value of dielectric constant decreases with the increase in Ni dopant. It may be due to the small dielectric polarizability of nickel ions (1.23 Å3) compared to titanium ion (2.93 Å3) (Shannon, 1993). Hence, as the dopant concentration increases more titanium ions will be substituted by nickel ions and thereby decreasing the dielectric polarization which in turn decreases the dielectric constant.
Figure 2b shows the frequency response of dielectric loss, achieved at RT, in the frequency range of 100-1000 kHz for x = 0.0, 0.05, 0.10 and 0.15, respectively. We can mark the exponential demise of losses for all the ceramic specimens. However, losses decrease exponentially with the application of ac field owing to the fact that at higher frequencies, ceramic specimens offer low reactance to the sinusoidal signal and hence minimize the conduction losses (Bogoroditsky et al., 1979). Therefore, dielectric losses decrease at higher frequency. These types of variations in the dielectric losses are characteristic of the dipole orientation and electrical conduction (Lingwal et al., 2003). Doping concentration x = 0.10 increases dielectric losses to very high values as compared to x = 0. 05 and x = 0.0. However, further doping causes the decrease in losses to a significant limit. For heavy doping (x = 0.15) any further, dielectric losses are decreased to very low values even below the values for undoped sample (x = 0). Similar behavior of dielectric constant and loss tangent is also reported by Mahato et al. (2006) for Pb1-3x/2 Smx (Zr.53 Ti.47)O3 ceramic system.
Electrical investigation: Figure 2c shows the room temperature frequency response of ac conductivity in the range of 100-1000 kHz for x = 0.0, 0.05, 0.10 and 0.15, respectively. The ac conductivity increases with the increase in frequency for all compositions. It has been observed that ac conductivity gradually increases with the increase in frequency of applied ac field because the increase in frequency enhances the electron hopping frequency. It can also be seen from Fig. 2c that conductivity increases with the increase in dopant concentration up to 10% and then decreases for 15%. It may be attributed to the fact that the dopants of Ni2+ are acceptors for K2Ti6O13 and are usually compensated by the formation of oxygen vacancies. Thus, the increase in dopant concentration increases the oxygen vacancies which results in an increase of free electron density and conductivity. However, the substitution of Ti4+ with Ni2+ can take place up to a certain limit.
| Fig. 2(a-c): | (a)Variation of real dielectric constant with frequency for K2Ti6O13 and its nickel doped derivatives, (b) Variation of loss tangent with frequency for K2Ti6O13 and its nickel doped derivatives and (c) Variation of ac conductivity with frequency for K2Ti6O13 and its nickel doped derivatives |
When the introduction of Ni2+ exceeds this limit, the superfluous Ni2+ which cannot substitute Ti4+ further will segregate to grain boundary interfaces. Thus, the segregation of Ni2+ blocks the building and transportation of electrons and other defects and thereby decreases the conductivity.
CONCLUSION
K2Ti6O13 and its nickel doped (x = 0.05, 0.10, 0.15 mol%) ceramic specimens were synthesized using solid-state route. Lattice constants evaluated from room temperature XRD spectra revealed the phase formation in a monoclinic symmetry. Lattice constant have been found to increase with the increase in nickel concentration. The data revealed that the dielectric constant exhibit the normal dielectric behaviour and decrease with the increase in frequency and dopant concentration. Due to variation of doping concentration dielectric loss was primarily found to increase due to space charges but further it decreases due to inhibition of domain-wall motion. The ac conductivity shows the frequency and composition dependent behaviour. It increases with the increase in frequency and dopant concentration.
ACKNOWLEDGMENT
Authors are thankful to the Council of Science and Technology, Govt. of UP, India for the financial support in the form of centre of excellence in material science (nanomaterials), A.M.U., Aligarh, India.