Room Temperature Electrical Analysis of Pr³⁺-Doped Silicate Glasses for Energy Storage Applications ^†

Abstract

Composite glasses possessing an amorphous nature and high dielectric constants exhibit properties suitable for optoelectronic and electrochemical applications. Multi-component silica–calcium phosphate glasses doped with 0.5 and 1 mol% of trivalent praseodymium (Pr³⁺) were synthesized using the sol-gel method. The Pr³⁺-doped and undoped glasses were compared at room temperature (300 K) to analyze their electrical variations. Dielectric studies predicted an increase in the dielectric constant and conductivity in the doped samples when compared to the undoped glass. A high dielectric constant of 89.2 was observed in the optimally doped glass at 1 kHz. The value of the capacitance increases to the order of nanofarads as the concentration of Pr³⁺ increases, indicating enhanced storage in the material. The AC conductivity of the highly doped sample evidenced a high value of 2.9 × 10⁻⁵ S/cm at 10 MHz. The Cole–Cole plot of the glasses demonstrated a single flattened semicircle due to the lack of grains. The equivalent circuitry constitutes a constant-phase element (CPE) in series with the parallel circuit of a resistor and CPE. This behavior is indicative of the suitability of the glasses as cathodes. The increase in capacitance with doping in the low-frequency region suggests the use of the glasses as dielectric energy-storage materials in condensers.

1. Introduction

The demand for sustainable energy grows alongside the development of substitute materials with higher efficiency for energy storage devices [1]. Crystalline electrodes that require higher annealing temperatures, resulting in unfavorable contact impurities could be replaced with glass materials, augmenting electrode–electrolyte interfacial contact and ionic conductivity levels [2]. The multi-functionality of glasses has also been extended in electrochemical industries as electrochemical sensors and batteries [3]. Silica–phosphate glasses are favored in electrochemical sectors as electrodes in solid-state batteries [4] due to enhanced polymerization by the non-bridging oxygen units of the silicate and phosphate tetrahedra. Faster ionic conduction in glass electrodes and solid electrolytes by overcoming the impediment due to grain boundaries could be achieved by the use of glass modifiers [4]. Electrical studies on glass matrices with rare earth dopants for electrochemical applications have been recently reported [5]. However, Pr³⁺-doped silica calcium phosphate glasses for low-loss energy sector applications have yet to be explored in detail.

The present article reports the analysis of the electrical properties of the Pr³⁺-doped silica–calcium phosphate multi-component glasses. The findings confirm the suitability of the glasses with high dielectric constants and increasing condenser values for inter-layer dielectric substrates. The impedance plots would suggest the potentiality of the glasses as cathode materials.

2. Materials and Methods

The glasses were synthesized as per the scheme reported [6]. The raw materials, namely tetra ethoxy orthosilicate, triethyl phosphate, calcium nitrate tetrahydrate, and praseodymium nitrate hexahydrate, were stirred vigorously for 2 h and left for gelation and aging for the formation of glass monoliths. The glasses were then annealed at 750 °C before being ground into powders for characterization. The glasses with 0, 0.5, and 1 mol% of Pr³⁺ were coded as 0P, 0.5P, and 1P, respectively.

The powered glasses were pelletized and subjected to broadband dielectric spectroscopy for electrical analysis using NOVOCONTROL (Novocontrol Technologies GmbH & Co. Montabaur, Germany, Concept 80) in the frequency range of 100 Hz to 10 MHz at room temperature (300 K).

3. Results and Discussion

3.1. Dielectric Studies

The complex dielectric parameter of a material is expressed by the equation,

ϵ = ϵ′ + j ϵ″

(1)

The dielectric constant, ϵ′ represents the energy storage capacity due to polarization during the application of electric field, while the loss factor ϵ″ represents energy dissipation. The dissipation loss could also be due to damping resistance, which prevents the orientation of dipoles with the applied electric field [7]. Figure 1 shows the variation of the dielectric constant and dissipation factor as a function of log f for all concentrations of Pr³⁺.

The onset of a larger dielectric constant at low frequencies is due to electrode polarization in the sample [8]. The effect increases with doping up to 1 kHz, after which the decrease is constant and independent of dopant concentration. This is because with increasing frequencies, the total polarization in the sample lags behind the rapidly changing alternating electric fields [9]. As the concentration of the dopant increases, the bonds become stronger due to the non-bridging oxygens held by the Pr³⁺ ions. This could be ascribed to the increase in electrode polarization at lower frequencies and the effect becoming more pronounced with higher levels of doping [5].

3.2. Electrical Conductivity Studies

The variation of real and imaginary components of electrical conductivity versus log f is shown in Figure 2, and the inset shows the Jonscher’s power law fit for conductivity in all the synthesized glasses. The increase in conductivity above the threshold frequency (hopping frequency, ω_p), and the related dielectric factors were estimated for each glass at 300 K and are given in

Table 1. Dielectric parameters.

Parameters/ Sample Code	Hopping Frequency ωp (kHz)	Frequency Exponent (s)	ϵ′ (at 1 kHz)	σAC (×10−5 S/cm) at 107 Hz	Bulk Resistance (MΩ)	σDC (×10−6 S/cm)
0P	0.70	0.7828	18.506	0.905	2.659	0.0283
0.5P	1.03	0.7409	30.202	1.18	1.311	0.0575
1P	2.25	0.6351	89.160	2.91	0.239	0.3161

, which are in close agreement with the values reported [10]. Below the threshold frequency, the conductivity is independent of frequency, representing minimal DC conductivity, which is demonstrated by the plateau in the graph. With an increase in Pr³⁺ concentration, ω_p shifts towards higher frequency ranges. The low conducting property at smaller frequencies is indicative of scattering; however, an increase in the conductivity value was noticed upon doping. This is due to the enhanced hopping mechanism, leading to structural variations with the addition of Pr³⁺ ions [5]. The exponent term (s), obtained from the slope of the inset, which determines the magnitude of conductivity, is a measure of the ionic interaction with the host matrix [11]. The pronounced AC conductivity in frequencies higher than the order of 10⁵ Hz, showing dispersion, could be due to the predominant influence of the electronic order of charge carriers with relatively lighter mass. This is indicative of dielectric relaxation. In the higher frequency domain, the ionic motion becomes suppressed due to the larger masses of ions and the corresponding inability to change in tune with rapidly changing electric fields.

The variation in capacitance of the glasses as a function of frequency is shown in Figure 3. The capacitance of the doped and undoped glasses is negligible at higher frequencies [12]. However, in the low-frequency region dominated by electrode polarization, the capacitance increases from the order of picofarads for the sample coded 0P to the order of nanofarads corresponding to the samples 0.5P and 1P. The exponential increase in capacitance for the highly doped concentration of 1 mol% in the glasses shows that the material could be used for energy storage, sensor applications, and filter circuits in the frequency range up to 10³ Hz.

3.3. Impedance Studies

Nyquist plots for all the samples are presented in Figure 4. The single depressed semicircle deduced from the Nyquist plot indicates the lack of lattice symmetry, defects, and inhomogeneities in the amorphous glasses [13,14]. The semicircle observed could be attributed to the effect of localized grains in the glass matrix. The center of semicircles positioned below the real impedance axis could be fitted to an equivalent circuit with a constant-phase element (CPE) in series with a parallel combination of resistor and CPE, and are shown in the inset of Figure 4. The depressed semicircle also validates the non-Debye relaxation mechanism of dielectric polarization in the samples [13]. It could be observed that the radius of the semicircles decreases with increasing concentrations of Pr³⁺, indicating a decrease in bulk resistance and subsequent increase in conductivity in the samples [13]. The DC conductivity values observed are in the order of μS/cm [12] and are found to increase with doping.

4. Conclusions

Ceramic-attributed calcium phosphate is blended with vitreous silica to selectively forge an amorphous composite glass system while being doped with different concentrations of Pr³⁺ by the sol-gel route. The increasing dielectric constant with doping substantiates the insulating behavior of the synthesized materials. At higher frequencies, the enhancement of AC conductivity could be ascribed to the hopping mechanism, with a value of 2.91 × 10⁻⁵ S/cm at 10 MHz. Nyquist plots with a single asymmetric semicircle predicts a non-Debye model of relaxation and represents an equivalent circuitry comprising of a resistor and constant-phase elements. Electrode polarization is dominant at low frequencies, contributing to the augmented capacitive values of the order of nanofarads, and is indicative of a proportional increment in energy storage. The composite material finds promising applications in the micro-component radio frequency capacitors. The capacitors could also be proved to be ideal for their use as filters, tuning devices, Q circuits, and glass antennas, and in sensor applications.