Conduction and relaxation mechanisms in gadolinium oxide nanoparticle doped polyvinyl alcohol films

Highlights

• Doping in nano form decreases ${\epsilon }^{\prime }$ in contrast to doping with ions which were reported to increase ${\epsilon }^{\prime }.$

• Activation energy decreases monotonically where as resistivity increases with increasing concentration.

• The films exhibit negative resistance co-efficient with temperature.

• Relaxation is seen to be non-Debye in nature.

• Correlated Barrier Hopping is the preferred mechanism for conduction.

Abstract

Concentration dependent dielectric and electrical properties of PVA-Gd₂O₃ nanocomposite films over a wide range of frequencies (100Hz-1 MHz) and temperatures (303 K–423 K) is reported here. TEM of the Gd₂O₃ nanoparticles showed that they could be indexed to the cubic phase. The frequency variation of the dielectric permittivity and dielectric loss could be explained by the Maxwell Wagner Sillar’s model. The dielectric permittivity decreased in the composite films contrary to observations in other similar systems. The complex impedance showed Arrhenius behavior and the activation energies decreased monotonically with increasing concentration. However the resistivity of the nanocomposite films shows an increase with concentration, the 4 wt% film showing the highest resistivity which indicates other contributions to conductivity. The Nyquist plot could be fitted to a simple parallel RC circuit showing the presence of long range conduction in these samples. The relaxation times varied from 10⁻⁴ to 10⁻⁶s and indicated a non-Debye type of relaxation. The master curve showed good overlap indicating that the same relaxation processes were active at all temperatures. The ac conductivity obeys Jonscher’s law and the exponent shows a monotonic decrease with temperature. This indicates that Correlated Barrier Hopping is the probable mechanism for conduction in these samples.

Introduction

The combination of inorganic nanoparticles and organic polymers forms an interesting class of materials called polymer nanocomposites. Polymers contribute properties such as ease of processing, low cost and flexibility while small quantities of the dopant nanoparticles add useful properties which help to tailor the material for specific applications [[1], [2], [3], [4]]. Nanocomposites using a wide variety of polymers have been studied [[1], [2], [3], [4], [5], [6], [7]] and the ones using the polymer poly vinyl alcohol (PVA) are biodegradable and environment friendly. PVA has high mechanical strength, good environmental stability and forms nanocomposites with ease. There is abundant literature reporting the effect of inorganic dopants like ZnBr₂, nano ZnO, nano CeO₂, CdS, Gd³⁺, Mn²⁺, EuCl₃, TbCl₃, YCl₃, Ag [[8], [9], [10], [11], [12], [13], [14], [15], [16]] etc. on the structural, optical, dielectric and electrical properties of PVA so as to enhance the suitability of the material for particular applications. The addition of semiconductor nanoparticles such as ZnO and HgSe to PVA have shown increased conductivity in these films paving the way for possible applications in the electronic industry [9,16]. The addition of nanoparticles of the rare earth oxide CeO₂ to PVA gave rise to enhanced photoluminescence even at low concentrations of the dopant [10]. Rare earth sesquioxides are one of the highly studied materials that have a wide range of applications in optoelectronics [[17], [18], [19]]. They are chemically and thermally very stable. Among them Gd₂O₃ is of particular interest due to its completely half filled 4f electronic shell which is expected to give rise to interesting optical, dielectric and electrical properties. The photoluminescence (PL) in lanthanides are in general sharp and intense as these arise from transitions within the f shell energy levels as well as from defect centers, and this is true in gadolinium also. Amongst the lanthanides, half filled shell has the maximum magnetic moment of I = 7/2 as all the electron spins are parallel. This makes it useful as a contrast agent in MRI [18]. The large magnetic moment also affects its electrical properties [20]. Recently Sudip Mukherjee et.al [21] reported high dielectric constant and low loss in a system of Gd₂O₃ nanocrystal dispersed in a silica matrix. Other reports with similar results are also available in literature [22]. These types of materials are promising candidates for memory device applications. Researchers such as Taha A. Hanafy [23] and M. Obula Reddy et.al [20] have reported the effect of lanthanide ions like La³⁺, Gd³⁺ and Er³⁺ on the dielectric relaxation and ac conductivity of PVA matrix. They observed significant increase in the dielectric permittivity and conductivity of PVA. The effect of Gd₂O₃ nanoparticles and Sm₂O₃ nanoparticles on PVA films was to give rise to good photoluminescence in the blue and red region with sufficient color purity for optoelectronic applications [24,25]. Because these materials have such a multitude of applications it is of interest to study the conductivity and relaxation mechanisms operative in them. The ac conductivity in such nanocomposites can be modeled in many ways, the main mechanisms being quantum mechanical tunneling (QMT), small and large poloron assisted tunneling and Correlated Barrier Hopping (CBH). The most suitable model can be inferred from the temperature dependence of the exponent obtained from fitting the dielectric data to Jonscher’s power law. An analysis of the dielectric data in the light of various formalisms helps delineate the relaxation processes in the material and gives an idea of the mechanisms occurring in these materials. Hence it was felt that a detailed dielectric analysis of this material will enhance our understanding of the processes taking place and lead to a better design of materials for new applications. With this view, a concentration dependent study of the dielectric and electrical properties of nanocomposite films of Gd₂O₃ with PVA was undertaken and to the best of our knowledge this is the first report on such a rare earth system. The results of the study point to an uncommon decrease in the value of dielectric permittivity and an increase in the resistivity of the films with concentration of Gd₂O₃.