Impact of Gd₂O₃ Incorporation in Structural, Optical, Thermal, Mechanical, and Radiation Blocking Nature in HMO Boro-Tellurite Glasses ^†

Abstract

The glass system of B₂O₃-SiO₂-TeO₂-Bi₂O₃-ZnO-BaO doped with Gd₂O₃ (x = 0, 1, 2, 3, and 4 mol%) (BiTeGd-x) was prepared by using the melt-quench technique. The density of glasses increased from 5.323–5.579 g cm⁻³ for 0–4 mol% with an increase in Gd₂O₃ concentration. The simulation results obtained using Photon Shielding and Dosimetry (PSD) software (Phy-X version) produced the maximum mass attenuation coefficient (MAC) and minimum half-value layer (HVL) in the entire photon energy spectrum 0.015–15 MeV, suggesting the highest potential of BiTeGd-4 glass to act as a shield against low and high-energy radiation photons.

1. Introduction

The usefulness of glass materials for protection against radiation has been extensively explored by researchers. Simple fabrication techniques, non-toxicity, transparency, chemical durability, and chemical flexibility are the main contributing factors of glasses to enhance radiation shielding proficiency and achieve high density [1]. Certain glasses designed have transcended conventional shielding materials such as concretes and bricks with appropriate attenuation coefficients and half-value layers (HVL). Tellurite glasses show promising results for thermal and chemical stability with a low melting temperature and density [2].

Rare-earth oxide such as Gd₂O₃ has been reported to improve the physical, optical, and mechanical properties of tellurite glasses by increasing their density, refractive index, and hardness values [3]. Kaewjaeng et al. [4] suggested that Gd³⁺ doping reduced HVT significantly, allowing the glass to perform better than commercial X-ray windows, concrete, and bricks [5]. There are limited investigations on the effect of Gd³⁺ ions and the overall improvement of the stability of glasses and radiation shielding properties. The present study was carried out to explore the potential of this glass system in radiation field application by analyzing the physical, optical, and radiation-blocking development of B₂O₃-SiO₂-Gd₂O₃-TeO₂-Bi₂O₃-ZnO-BaO glass system (BiTeGd-x).

2. Materials and Methods

Melt quenching was conducted to produce the BiTeGd-x system where Gd₂O₃ mol% varied as 0, 1, 2, 3, and 4. A furnace temperature of 1100–1120 °C was used for its synthesis [6]. The density of prepared glasses was determined using Archimedes theory and distilled water. Carl Zeiss FESEM recorder was used to study its surface morphology through EDAX measurement. Theoretical values of terms to evaluate the gamma-ray shielding property such as mass attenuation coefficient (MAC), and HVL of the synthesized glasses were obtained by using Photon Shielding and Dosimetry (PSD)(Phy-X version) software in an energy region of 0.015–15 MeV [7].

3. Results and Discussion

3.1. Physical Properties

The synthesized Gd³⁺ tellurite glasses are shown in Figure 1, where Gd₂O₃ incorporation improved the transparency and changed the color of the glass from reddish-orange to light yellow. Archimedes’ principle was used to determine the density of the sample (

Table 1. Different physical parameters calculated for Gd³⁺-doped glasses.

Sample Code	BiTeHost	BiTeGd-1	BiTeGd-2	BiTeGd-3	BiTeGd-4
Average molecular weight, M (g/mol)	159.76	161.78	163.81	165.84	167.87
Density, ρ (g/cc) (±0.01)	5.2844	5.4229	5.4429	5.4935	5.5793
Molar volume, Vm (cm3)	30.231	29.8335	30.0967	30.1888	30.0882
Number density of Gd3+ ions in host glass, NGd (×1023 ions/mol)	0	0.202	0.400	0.598	0.801
Inter-ionic separation between Gd3+, ri (nm)	0	36.727	29.236	25.566	23.202

). Physical parameters such as molecular weight and molar volume were calculated for the fabricated glasses with other parameters using the following relations.

For the number density of Gd³⁺ ions,

$$N_{Gd}=\frac{x{N}_{A\rho }}{M}$$

(1)

The interionic separation between Gd³⁺ ions was obtained with

$$r_i={\left(\frac{1}{N_{Gd}}\right)}^{\frac{1}{3}}$$

(2)

All calculated parameters are summarized in Table 1. Gd³⁺ increased the density of the glass from 5.323 to 5.5793 gcm⁻³ from 0 to 4 mol% of Gd₂O₃. The high molecular weight of Gd₂O₃ compared to TeO₂ caused a density increase in the glass. The tendency of Gd³⁺ ions to form a closed-packed network by filling the interstitial spaces has also increased the density. Moreover, the Gd³⁺ ions have an ionic radius of 1.19 Å which is greater than that of Te⁴⁺ and Bi³⁺ ions (0.99 and 1.03 Å), which also increased the density. Molar volume (V_m) initially decreased but increased depending on the concentration of Gd₂O₃. BiTeGd-1 glass with the minimum molar volume confirmed polymerization in this glass network [8]. The number density of Gd³⁺ ions increased with the increase in Gd₂O₃ concentration. The interionic radius (r_i) decreased with successive addition of Gd₂O₃ molecules, indicating the shrinkage of ionic clouds due to Gd³⁺ ions.

The XRD images of the Gd³⁺ glasses are shown in Figure 2a. Sharp crystalline peaks were absent in the XRD images, assuring the amorphous nature of the current glasses. In addition, the broad hump observed in Bragg’s angle of 20–30° also reflected the non-crystallinity of the glasses. The surface morphology of Gd³⁺-doped tellurium borosilicate glass BiTeGd-2 was examined by using SEM images (Figure 2b).

The occurrence of smooth and homogenous texture in the SEM images without any cluster of unresolved particles proved the amorphous character of the synthesized glasses. The compositional analysis of the BiTeEu-2 glass was performed using EDAX measurement. The EDAX spectrum shows properly distributed elements such as boron (B), oxygen (O), silicon (Si), europium (Eu), tellurium (Te), bismuth (Bi), barium (Ba), and zinc (Zn) (Figure 2c). Aluminium (Al) was detected in the glass composition because of the alumina crucible used in the glass melting process. A bar chart representing the weights of all constituent elements (Figure 2d) exhibited the highest weight of Bismuth (Bi) as Bi has the heaviest atomic weight.

3.2. Optical Properties

The function of Gd³⁺ ions in enhancing the optical properties of the glass was studied by calculating parameters such as refractive index (n), dielectric constant (ε), molar refractivity (R_m), reflectance loss (R in %), and molar electron polarizability (α_m) [9] using the following set of equations.

$$\mathsf{\epsilon }={\mathrm{n}}^2$$

(3)

$${\mathrm{R}}_{\mathrm{m}}=\left(\frac{{\mathrm{n}}^2-1}{{\mathrm{n}}^2+2}\right){\mathrm{V}}_{\mathrm{m}}$$

(4)

$$\mathrm{R}=\left(\frac{{\mathrm{n}}^2-1}{{\mathrm{n}}^2+2}\right)\%$$

(5)

$${\mathrm{\alpha }}_{\mathrm{m}}=\frac{3}{4\mathrm{\pi }{\mathrm{N}}_{\mathrm{A}}}{\mathrm{R}}_{\mathrm{m}}$$

(6)

The values are presented in

Table 2. Optical parameters of Gd³⁺-doped glasses.

Sample Codes	R. I n	Dielectric Constant ε	Rm (cm3)	R (%)	Molar Electron Polarizability εm (Å3)	Band Gap Eg (eV)
BiTeHost	1.975	3.901	14.7544	0.9508	5.854	3.153
BiTeGd-1	1.989	3.955	14.8053	0.9778	5.875	2.957
BiTeGd-2	1.991	3.9663	14.9635	0.9832	5.938	3.121
BiTeGd-3	1.998	3.9939	15.0792	0.9969	5.984	3.196
BiTeGd-4	2.01	4.0407	15.1455	1.0204	6.01	2.791

. The refractive index continuously increased with doping Gd₂O₃. The refractive index was influenced by a larger atomic radius (1.79 Å) of Gd [10] which is greater than that of tellurium (1.6 Å) and boron (0.98 Å). The higher polarization ability of cations resulting from higher cation radius of Gd³⁺ ions induces high n values, providing a platform for current Gd³⁺-doped glasses in the non-linear optical application. An enhanced refractive index was also associated with a high dielectric constant, molar refractivity, reflectance loss, and molar electron values as shown in Table 2.

The absorption spectra recorded in the UV-visible region for the Gd-doped glasses are shown in Figure 3a. The synthesized glasses including the undoped glass showed maximum absorption in the UV region (300–400 nm). In addition, all the glasses showed a broad low intense absorption peak around 500 nm which corresponds to the absorption of Bi³⁺ ions. Higher transmittance observed in the visible of all samples was proof of improved transparency of the Gd-doped glass.

The relationship between absorbance and optical band gap E_g was provided by Tauc’s relation as follows.

$$\mathsf{\alpha }={\frac{\mathrm{B}(\mathrm{h}\mathsf{\nu }-{\mathrm{E}}_{\mathrm{g}})}{\mathrm{h}\mathsf{\nu }}}^{\mathsf{\gamma }}$$

(7)

where α is the absorption coefficient, also given by α = 2.303 A/t, A is the absorbance, t is the thickness of the sample material, B is the band tailing parameter, and the exponent γ depends on the kind of electronic transition mechanism. Because glasses are amorphous, we took γ = 2 as the transitions are indirect in nature. The resulting Tauc plot is represented in Figure 3b, where the extrapolation of the linear part of the curve is taken as E_g (Table 2). The E_g values showed that the band gap of Gd³⁺-doped glass was less than that of the undoped glass except for the BiTeGd-3 sample.

3.3. Radiation Shielding Parameters

MAC and HVL data simulated by Phy-X/PSD software in the 0.015–15 MeV photon energy spectrum are represented in Figure 4 [7,10]. The influence of energy on the MAC values of the glass was evident from the rapidly falling trend in the lower energy range, constancy in the intermediate range, and an increase at the higher end of the spectrum. This showed the occurrence of photoelectric absorption, Compton scattering, and electron–positron pair formation in the three energy regions. The two sharp peaks at 0.035 and 0.1 MeV were the K-edges associated with Te and Bi. Furthermore, the effect of varying the Gd₂O₃ content in MAC graphs on the continuous increase in MAC with the Gd³⁺ content was studied. Such an effect was observed because of the increasing density values of Gd₂O₃ from 5.323 to 5.579 gcm⁻³ from 0 to 4 mol% in concentration. Therefore, BiTeGd-4 glass exhibited the maximum attenuation compared to other Gd glasses.

In Figure 4b, low-energy photons generated low HVL values due to the high photon absorption mechanism. HVL increased continuously in the energy range of 0.1–5 MeV, showing the maximum value at 5 MeV and a decrease to the energy of 15 MeV. This implied that when the energy increased, the radiation penetrated deeper into the glass, and therefore it was essential to increase the thickness of the material to shield it from the high radiation photons. The maximum HVL was found at 5 MeV for BiTeGd-4 glass as 3.4619 cm, which was thick enough to protect the glass from high-energy radiation. Additionally, the HVL decreased with successive doping of Gd₂O₃ in the range of 0.015–15 MeV suggesting that BiTeGd-4 glass had the lowest HVL.

4. Conclusions

Improved transparency with enhanced density values was proved with tellurite glasses doped with Gd₂O₃. Non-crystalline nature and smooth glass surface morphology were verified by XRD and FESEM results. Optical parameters including the refractive index continuously increased with an increase in Gd₂O₃. MAC and HVL parameters computed by PSD software in the energy range of 0.015–15 MeV showed that BiTeGd-4 was the optimum glass for gamma radiation shielding.