Effect of PbO₂ and Bi₂O₃ on the Physical, Optical, and Gamma-Ray Shielding Properties of Boro-Tellurite Glasses

1 Introduction

The development of nuclear technology in energy production, medicine, the food irradiation industry and academic scientific research has made people more exposed to ionising or nuclear radiation [1,2]. Gamma radiation (γ) is regularly useful in some ways, but it is also very hazardous to living things as well as the environment. In living things, these radiations can pass through easily and cause serious health effects by damaging cells and molecules [3]. Due to this, developing effective radiation shielding targets or materials has become an important scientific and technological challenge in modern material science [4].

Concrete is widely used due to its very dense nature and can block γ rays efficiently [5]. But these common resources have a lot of problems. Concrete needs to be very thick to work as a shield, and it often is not clear and flexible in terms of structure [6]. These restrictions have led researchers to investigate other target materials that provide effective radiation shielding and are also stronger and safer for the environment.

Several researchers, including our group, show that HMO glasses have become auspicious materials for radiation shielding applications in recent decades [7,8,9,10]. Related to other shielding materials as mentioned above, glass has a series of benefits. These include ease of fabrication, structural homogeneity, optical transparency, adjustable composition and the ability to hold some amounts of different HMOs in a single composite [11]. It is possible to make effective glass materials that can be useful in nuclear radiation technologies.

Tellurite (Te) based glasses have gotten a lot of consideration due to their unique physical and structural features compared to other types of glass materials [12]. Also, it is well known for its high density, higher refractive index, great optical features, and absorption of nuclear radiation than other glass-forming systems such as silicate or borate glasses [13]. Pure Te glasses have some good features, but they often don’t last long in terms of chemical and mechanical stability. To address these constraints and further improve their nuclear radiation shielding efficacy, the incorporation of additional HMOs has been broadly investigated and reported [14]. Among them, lead oxide (PbO) and bismuth oxide (Bi2O3) are effective modifiers that have been studied for radiation shielding [15,16]. These two compounds are very interesting because they have high atomic numbers and atomic masses. The study on borate glasses containing cadmium and copper explored how potassium fluoride influences their physical and structural features [17]. The investigations into chromium-activated tellurite-borate glasses have demonstrated their high efficiency and stability for use in LED technology [18]. The nanocomposite films using copper bismuth oxide have been investigated for their ability to block radiation and their internal structural features [19]. The impact of Bi2O3 on the behaviour of phosphate-based glasses has been evaluated to improve their shielding performance [20].

Borate (B2O3) based glasses are also well studied because they can easily be made into glass, are very stable at high temperatures and can hold various modifying oxides. B2O3 forms a solid glass network out of BO4 and BO3 structural units [21]. Further, these glasses can have low melting points and good chemical stability, which makes them good hosts for glass systems that can do more than one thing [22].

A good way to make advanced radiation shielding materials that work better is to mix tellurite and borate glass networks with HMOs. This study deals with B2O3-TeO2-based glass composites with varying PbO and Bi2O3. Incorporating these parts in a systematic way figures out how composition, density, optical properties and radiation attenuation efficiency are related. This study’s results are projected to support the advancement of classy glass materials with improved shielding properties. Furthermore, this study presents potential uses in nuclear power plants, radioactive waste storage, etc.

2 Materials and Methods

2.1 Glass Synthesis

TeO2-PbO2-Bi2O3-B2O3 glasses were prepared via. melt-quenching technique. The TeO2, PbO2, Bi2O3, and B2O3 oxides were weighed, and a batch of 15 g was prepared. The oxides were then ground together in an agate mortar for 30 min. The mixture was transferred to an alumina crucible. The batch was melted at 1000°C for 20 min. The molten mixture was quickly poured onto a pre-heated brass plate. To prevent cracking due to thermal stress, the glass discs were annealed for 2 h, then allowed to cool slowly. The photo of the samples is shown in Fig. 1.

Figure 1: Picture of the samples.

The density was measured using the Archimedes method as [23,24,25]:

The chemical composition and density are listed in Table 1. The physical parameters are calculated using the standard formulae described in previous works and are provided in Table S1 of the supplementary information [26,27,28].

2.2 Mechanical Analysis

The elastic moduli were calculated using the Makishima-Mackenzie model [29,30,31]. The calculations involve the total atomic packing density (Vt) and the dissociation energy (Gt) as two input parameters:

In these equations, xi is the mole fraction, and Vi is the packing density factor of the i-th oxide component.

The elastic moduli are determined as [26,27,28]:

Young’s modulus (E):

Bulk modulus (B):

Shear modulus (G):

Longitudinal modulus (L):

2.3 Optical Analysis

The absorption spectra were recorded using a PerkinElmer Lambda 19 UV-Vis spectrophotometer from 190–1000 nm. The Eg has been estimated using Tauc’s plot method. This method relates the absorption coefficient (α) and photon energy (hν) as [32,33]:

The quantity α h ν 1 / 2 is plotted against hν. The linear portion of this curve is then extrapolated to the x-axis where absorption is zero. The intercept provides the value of Eg. The Eg was utilized to calculate additional optical parameters and is shown in Table S2 of the supplementary information.

Optical electronegativity (χ*):

Linear dielectric susceptibility (χ(1)):

Third-order nonlinear optical susceptibility (χ3):

Nonlinear refractive index (n2optical):

2.4 Gamma Ray Shielding Evaluation

The linear attenuation coefficient (LAC) is obtained as: It=I0e−LAC.t(13) I0 and It are the incident and transmitted intensity of photons through a material of thickness t.

The MAC can be obtained as:

The HVL and MFP are obtained as:

The Phy-X software enables the accurate calculation of the MAC directly from a material’s chemical composition across various energy levels [34]. It automates the derivation of secondary shielding metrics, including the LAC, HVL and MFP. By replacing time-consuming manual calculations, Phy-X/PSD provides an efficient and reliable platform for the theoretical screening, comparison, and design of novel shielding products prior to physical fabrication. The obtained data is presented in Tables S3–S6 of the supplementary information.

3 Results and Discussion

3.1 Physical and Structural Parameters

The ρ increases from 4.759 (PbBi10) to 5.561 g cm−3 (PbBi16) as shown in Fig. 2. The lighter B2O3 (69.62 g mol−1) is being progressively replaced by the much heavier HMOs, PbO2 (239.2 g mol−1) and Bi2O3 (465.9 g mol−1) [35,36]. This also results in an overall rise in the mass of the glass matrix sharply with doping. The average molar mass (Mm) of the glass composition rises from 153.211 to 187.165 g mol−1. The Vm increases from 32.194 to 33.657 cm3 mol−1 (Fig. 2). This implies that the rate of volume expansion exceeds the rate of mass accumulation [35,36]. The addition of Pb4+ and Bi3+ ions expands the glass network, increasing the free volume (Fig. 2). The concentration of dopant metal ions (N) increases from 1.871 to 2.863 × 1021 ions cm−3. As the mole percentage of PbO2 and Bi2O3 increases, the N naturally rises. The separation between boron atoms (dB-B) decreases from 3.767 to 3.559 × 10−8 cm. As the content of B2O3 is reduced from 50 to 38 mol%, the boron structural units rearrange into smaller domains as shown in Fig. 3. This results in a slight reduction in the average inter-atomic distance between boron centers. The ri decreases from 8.116 to 7.042 × 10−8 cm. The polaron radius (rp) decreases from 3.270 to 2.838 × 10−8 cm. The ions come closer with increasing metal-ion number density, as shown in Fig. 3. This enhances the localisation of charge carriers, as indicated by the shrinking polaron radius [37]. The field strength (F) increases from 1.870 to 2.484 × 1015 cm2. The reduction in ri leads to a stronger local electric field. It influences the polarization of the surrounding oxygen ions. The OPD decreases from 80.761 to 75.468 as shown in Fig. 4. A lower OPD signifies a loosely packed structure. The addition of HMOs breaks the continuous B-O-B linkages. It results in the formation of NBOs and the disruption of the rigid network. The OMV increases from 12.382 to 13.251 cm3 mol−1 as shown in Fig. 4. It confirms the increase in volume available per mole of oxygen. The elastic moduli decrease with the addition of PbO2 and Bi2O3, as shown in Fig. 5. The mechanical stiffness reduces as the formation of NBOs disrupts the network. It is due to the replacement of stronger B-O bonds by weaker Pb-O and Bi-O bonds. The decrease in OPD also creates voids. It makes the material more susceptible to deformation under stress.

Figure 2: Variation of ρ and V_m for the present samples.

Figure 3: Variation of d_B-B, r_i and r_p for the present samples.

Figure 4: Variation of OPD and OMV for the present samples.

Figure 5: Variation of elastic moduli for the present samples.

3.2 Optical Properties

The Tauc’s plot is shown in Fig. 6. The Eg decreases from 2.969 eV to 2.813 eV as shown in Fig. 7. It is a direct consequence of structural depolymerization. The formation of NBOs creates localized defect states at the top of the valence band. The electrons in NBOs are less tightly bound than those in bridging oxygens. The energy required for an electronic transition is reduced. The n increases from 2.405 to 2.449 as shown in Fig. 7. This is due to the high concentration of HMOs. Pb4+ and Bi3+ are highly polarizable cations with larger ionic radii. It increases the electron density and polarizability resulting in a stronger interaction with incident light. The dielectric constant (ε) increases from 5.786 to 5.999. The optical dielectric constant (εopt) increases from 4.786 to 4.999. Since the dielectric behavior follows the refractive index trend. The Rm and αm increase from 19.789 to 21.035 cm3 mol−1 and 7.849 to 8.343 × 10−24 cm3. The increase in Rm and αm confirms the electronic softening of the glass. The electron clouds surrounding the oxygen and HMOs become more deformable due to formation of NBOs. The electronic polarizability (αe) increases from 8.726 to 8.871 × 1023. It is due to the specific contribution of electronic displacements to the total polarizability, further supporting the increase in the refractive index. The reflection loss (RL) and transmission (T) increases from 0.170 to 0.177 and decrease from 0.709 to 0.699, respectively, as shown in Fig. 8. The reflection at the air-glass interface becomes more significant with a rise in refractive index. It leads to higher reflection losses and a reduction in optical transmission. The Metallization (M) decreases from 0.385 to 0.375. A less than 1 value of M indicates that the glasses are insulators. The decrease in M indicates a shift towards semi-conducting behavior [35,36]. The decrease in Eg increases the metallic character. The χ* decreases from 0.798 to 0.756. This parameter quantifies the ability of the anion matrix to hold valence electrons. It indicates that the electrons are less tightly held in the high-Pb/Bi glasses compared to the base glass. It is consistent with the formation of loosely bound NBOs. The χ(1) increases from 0.381 to 0.398. This linear optical parameter is directly derived from the refractive index and confirms the material’s increasing linear response to an optical field. The χ3 decreases from 1.751 to 1.548 × 10−15 esu. The n2optical decreases from 2.742 to 2.382 × 10−14 esu. It is due to the structural constraints imposed by the higher field strength (F) and reduced inter-nuclear distances, which may dampen the anharmonic motion of the electrons required for non-linear effects [37].

Figure 6: Tauc’s plot for the present samples.

Figure 7: Variation of E_g and n for the present samples.

Figure 8: Variation of R_L and T for the present samples.

3.3 Gamma Ray Shielding Properties

The MAC of the TeO2–Bi2O3-PbO2-B2O3 glass systems was computed utilizing Phy-X software. The energy interval (in MeV) employed in this investigation was 0.015 ≤ E ≤ 15. Fig. 9 depicts the MAC pattern of the TeO2–Bi2O3-PbO2-B2O3 glass systems. The MAC for all samples diminished as energy rose. The discontinuities in MAC at 0.04 and 0.1 MeV are due to the K-shell energy of heavy metals like Te, Pb, and Bi. The principles in radiation physics indicate that photons interact with a glass through three principal mechanisms. The initial phenomenon is termed the photoelectric effect (PE), illustrated in Fig. 9 for energy less than 0.8 MeV. It exerts significant effects on low-energy photons. For PbBi10 at 0.015 MeV, the MAC (in cm2/g) was 59.94, but diminished to 3.93 at 0.06 MeV. Identical behavior was noted in the PbBi12, PbBi14 and PbBi16 samples within this energy range. The second phenomenon, termed Compton scattering (CS), exhibits a cross section that is largely independent of elemental composition. As illustrated in Fig. 9, all samples within the range of 0.8 to 4 MeV exhibited roughly comparable MAC values. As an illustration, for PbBi10 and PbBi12 at 2 MeV, the MAC was 0.0439 and 0.0441 cm2/g, respectively. Pair production (PP) is the most crucial component for high photon energy region. The probability of occurrence of this phenomenon is proportional to Z2. The MAC at the last few energies grew gradually up to 15 MeV, the maximum energy used in this investigation, as shown in Fig. 9. The findings of the attenuation graph showed that MAC rose when the concentrations of PbO2 and Bi2O3 varied between 10 and 16 mol%. The sample containing the highest concentrations of PbO2 and Bi2O3 (PbBi16) exhibited the greatest attenuation factor, while conversely, the sample with the lowest concentrations showed the least.

Figure 9: The MAC of the TeO₂-Bi₂O₃-PbO₂-B₂O₃ glasses.

The LAC value, as seen in Fig. 10, was variable with energy and typically diminished sharply as the energy rose from 15 keV to 6 MeV. The glass reduces the intensity of low energy photons, preventing most from penetrating it. With increasing energy, the photons traversed the sample easily, indicating that the LAC was comparatively low for high-energy photons. As an illustration, for PbBi10 at 0.15 and 8 MeV, the LAC values were 4.99 and 0.171 cm−1, respectively. The high-density material offers superior shielding compared to low-density glass. The PbBi16 sample possesses the greatest LAC values across all energy levels. PbBi10 exhibited the lowest density and the lowest LAC. Consequently, PbBi16 offers superior shielding owing to enhanced contacts compared to the PbBi10, PbBi12 and PbBi14 samples.

Figure 10: The LAC of the TeO₂-Bi₂O₃-PbO₂-B₂O₃ glasses.

Two further parameters commonly employed to describe photon attenuation are the HVL and MFP. A reduced HVL and MFP are advantageous as they require a smaller space to attenuate an equivalent quantity of photons. The HVL of the TeO2-Bi2O3-PbO2-B2O3 glass systems is plotted in Fig. 11. It illustrates that PbBi16 exhibited the lowest HVL and optimal attenuation, attributable to the elevated weight fractions of Bi2O3 and PbO2, as well as its comparatively high density. Conversely, the PbBi10, which had the lowest density and the least quantity of Bi2O3 and PbO2, showed the worst attenuation. The HVL decreased with a rise in energy, leading to better attenuation against gamma rays. Additionally, Fig. 11 shows that the HVL increased with increasing energy. This implies that low-energy photons can be adequately shielded by a thin layer, but high-energy photons require a thicker layer. The HVL of the PbBi10–PbBi16 glasses at 0.06 MeV was 0.0371, 0.0341, 0.0315 and 0.0293 cm, whereas at 10 MeV, the values increased to 3.94, 3.62, 3.35 and 3.12 cm, respectively.

Figure 11: The HVL of the TeO₂-Bi₂O₃-PbO₂-B₂O₃ glasses.

The MFP of the TeO2-Bi2O3-PbO2-B2O3 glasses is represented graphically in Fig. 12. The MFP for all glasses exhibited a quick increase up to 0.08 MeV with rising energy, followed by a slowing in the rate of growth. The reduction in MFP was attributed to the increased PbO2 and Bi2O3 levels. A comparison of the MFP values of PbBi10 and PbBi16 revealed that PbBi10 needed a greater MFP than PbBi16. The MFP ratio between PbBi10 and PbBi16 was 1.41 at 0.1 MeV, 1.34 at 0.3 MeV, 1.23 at 0.6 MeV and 1.18 at 2 MeV. This signifies that the MFP ratio between PbBi10 and PbBi16 was comparatively elevated at lower energy, indicating that PbO2 and Bi2O3 influenced the MFP, resulting in enhanced shielding efficiency at lower energies.

Figure 12: The MFP of the TeO₂-Bi₂O₃-PbO₂-B₂O₃ glasses.

The comparative analysis was performed against several previously reported glass matrices to determine the practical standing of the synthesized glass system among existing radiation shields. The chemical compositions and heavy-metal contents of these reference materials differ. The evaluation of the current glasses against glasses modified with WO3, TeO2, GeO2, MoO3, or SrO allows for a direct assessment of the competitiveness and suitability for advanced industrial and medical applications. Fig. 13a compares the MAC of the TeO2-Bi2O3-PbO2-B2O3 glasses with those reported for WO3-MgO-B2O3-TeO2-GeO2 glasses [38]. PbBi16 and PbBi14 have higher MAC than all the WO3-MgO-B2O3-TeO2-GeO2 glasses, since it contains relatively high amounts of Bi2O3 and PbO2. PbBi12 has comparable MAC with 15WO3-35MgO-20B2O3-20TeO2-10GeO2 glass. Fig. 13b compares the MAC of the TeO2-Bi2O3-PbO2-B2O3 glasses with those reported for SrO-B2O3-TeO2 with different modifiers [39]. All the TeO2-Bi2O3-PbO2-B2O3 glasses have higher MAC than SrO-B2O3-TeO2 glasses with V2O5, MnO2, MoO3 and TiO2. Fig. 13c compares the MAC of the TeO2–Bi2O3-PbO2-B2O3 glasses with those reported for zinc boro tellurite glasses modified with PbO, Bi2O3 and MoO3 [40]. PbBi16 has comparable MAC with 10ZnO-35B2O3-35TeO2-20Bi2O3. The zinc boro tellurite glass modified with MoO3 has lower MAC than all the TeO2-Bi2O3-PbO2-B2O3 glasses, while the zinc boro tellurite glass modified with PbO has comparable MAC with PbBi12.

Figure 13: The MAC of the TeO₂-Bi₂O₃-PbO₂-B₂O₃ glasses in comparison with (a) B₂O₃-TeO₂-GeO₂-MgO-WO₃ glasses (b) SrO-B₂O₃-TeO₂ with different modifiers (c) zinc boro tellurite glasses modified with PbO, Bi₂O₃ and MoO₃.

4 Conclusion

The addition of PbO2 and Bi2O3 results in an increase in density (4.759–5.561 g cm−3) and molar volume (32.194–33.657 cm3 mol−1). The decrease in OPD (80.761–75.468 g-atom/L) confirmed expansion of the network rich in NBOs. The elastic moduli are in a decreasing trend. The Eg decreases from 2.969 eV to 2.813 eV. The n rises from 2.405 to 2.449. The metallization criterion decreased from 0.385 to 0.375. It indicates a shift toward semi-conducting behavior. Gamma-ray shielding evaluations using Phy-X software confirmed that the attenuation capacity significantly improved with the addition of HMOs. The high-density PbBi16 sample exhibited the highest MAC. The MAC reaches a maximum of 72.00 cm2/g at 0.015 MeV as compared to 59.94 cm2/g for the PbBi10 sample. Consequently, the PbBi16 glass provided the most space-efficient shielding, demonstrating the lowest HVL. It establishes that the PbBi16 glass is a highly effective material for advanced radiation protection applications.