Effect of WO₃ Nanoparticles on the Radiative Attenuation Properties of SrTiO₃ Perovskite Ceramic

Abstract

In the present work, an experimental study is performed to study the radiation shielding characteristics of SrTiO₃ (STO) perovskite ceramic added with different amounts (x = 0, 2, 5, and 10%) of tungsten trioxide nanoparticles (WO₃ NPs). The four ceramic samples were prepared using the solid-state reaction method. The structural properties were examined using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) techniques. The analysis showed the successful formation of WO₃- doped STO samples. The crystallite size, estimated using the Scherrer equation, was found in the range of 50.86–41.17 nm. The effect of WO₃ NPs on the radiation shielding performance of these ceramics was studied. Different parameters, such as linear attenuation coefficient (LAC) and other related factors, were experimentally determined. The linear attenuation coefficient results demonstrated that the additional amount of WO₃ in the ceramics correlates with an improvement in their shielding abilities. The half-value layer (HVL) values for the ceramics with 2% WO₃ nanoparticles are equal to 0.071, 1.760, 2.407, and 2.564 cm at 0.060, 0.662, 1.173, and 1.333 MeV, respectively. As the energy increases, more radiation can pass through the material; therefore, a larger thickness is required to absorb half of the total photons, leading to a greater HVL. The tenth value results reaffirmed that increasing the WO₃ content in the STO ceramics improves their shielding efficiency. The radiation protection efficiency (RPE) of the four prepared STO ceramics was reported. From the RPE, we found that more photons can be attenuated at lower energies.

1. Introduction

The use of radiation has been growing during the past decades in various areas, including agricultural, industrial, and medical fields. Despite the enormous profits of radiation, it has the prospect of causing a significant risk hazard to human safety and the environment. Three basic concepts are followed for better radiation protection: minimizing the exposure time, increasing the distance that separates the entity of question and the source of radiation as much as possible, and using a protective material to isolate the entity of question from radiation. These three concepts are applied as far as practicable to reduce the entire absorbed dose due to radiation exposure [1].

The rendering of a material as a radiation shield is frequently evaluated by its ability to stop the penetration of incident radiation through various mechanisms of interaction. γ radiation, categorized as high penetrating potential, interacts with the material in three distinct processes: Compton scattering (CS), pair production (PP), and photoelectric absorption (PA) [2]. The probability of each interaction occurring is dependent on the incident γ radiation power and the composition of the protective material. PA is the main process for low energy γ radiation that interacts with materials with high atomic numbers. For high-energy γ radiation, PP became the dominant process. The γ interaction behavior with the shield is characterized by the linear attenuation coefficient (LAC), which depends on the properties of the material, the energy, and the incident radiation. Appropriate shielding material is able to attenuate γ radiation with minimum change to its thermal, mechanical, and physical properties as well as its chemical stability. In general, many factors must be considered when designing a radiation shield, including the kind and intensity of radiation, its energy level, cost, toxicity, and weight of material [3,4,5,6].

The frequently employed shielding materials are lead, concrete, and alloys. Studies searching for developed materials for radiation shielding applications have increased owing to their important role in progressed technologies that need ionizing radiations, such as nuclear medicine, radiology, and the characterization of materials. This necessitates advanced materials to be employed in the fabrication of protective structures to shelter humans and the environment from the hurtful impact of ionizing radiation. Aygün et al. studied heavy concretes holding chrome-ore for nuclear radiation shielding applications [7]. Their results showed better shielding properties for concretes with aggregates and additives compared to some heavy concretes and standard concrete [7]. M.I. Sayyed et al. prepared four alloys, including CrTe, CrTe_0.95Sb_0.05, CrTe_0.90Sb_0.10, and CrTe_0.80Sb_0.20, using a solid-state reaction. The results showed that replacing Te with Sb in the different alloys causes an improvement in their linear attenuation coefficient [8].

Glass-based materials are also common shielding materials and are possible alternatives to concretes for radiation-shielding purposes [9,10,11,12,13,14]. Glass materials display unique properties that make them useful for some industrial and technical applications. For example, T. H. Khazaalah et al. used soda–lime–silica (SLS) glass waste to fabricate free-Pb glass shielding [8]. γ-ray shielding properties of borate and phosphate glasses containing BaO, Bi₂O₃, and PbO in different ratios were previously studied. The results showed good attenuation properties of these glass systems [10,11]. In addition, the radiation shielding efficiency of borosilicate glass, which consists of silica and boron oxide, has been studied [12]. A recent report showed that borosilicate glass (30 mol%) mixed with 20 mol% of Bi₂O₃ and 50 mol% of BaO displays the highest density value and better radiation shielding performances [14].

Perovskite-based ceramic materials have attracted great interest due to their unique characteristics [15,16,17]. SrTiO₃ (STO) is a model of the perovskite-like ceramic material with many important properties and possible uses in diverse applications such as optoelectronic [18], photovoltaic [19,20], photocatalytic [20,21], and energy storage applications [22]. STO displays a high dielectric constant, average permittivity, and low tangent loss [14]. STO has also proven its efficacity in the domain of radiation shielding [23,24,25]. Moreover, STO is a mixed conducting perovskite [23,24] which can be influenced by doping and/or addition, temperature, and oxygen partial pressure (pO₂). For instance, E. Hannachi et al. studied the radiation-protecting properties of STO doped with tin oxide. The results showed that tin oxide addition enhanced the radiation shielding capacity of STO ceramic [23].

Metal oxide semiconductors play an interesting role in different areas of science, such as engineering, medicine, physics, chemistry, etc. Metal oxide semiconductors have gained much interest owing to their particular optical, magnetic, and electrical properties. Among the most useful metal oxide semiconductors, WO₃ has been employed for different applications such as electrocatalyst [26], photocatalyst [27], electrochromic [28], anticancer [29], and gas sensing [30]. WO₃ is an n-type semiconductor with a large band gap (around 2.7–3.5 eV) and is non-toxic. One of the most important characteristics of WO₃ is its morphological assortment. It can be designed in different dimensional morphologies, such as nanoparticles [31], nanowires [32], nanofibers [33], nanorods [31], nanosheets [34], and nanoplates [35]. It is shown that some special morphologies of WO₃ nanostructures are more appropriate for particular applications. With the emergence of nanotechnology as a progressive branch of science in recent years, the radiation protection system has also used various types of nanoparticles [36,37,38]. The fundamental supposition of the investigations was that the application of nanoparticles leads to a more even distribution within the matrix compared to particles with micrometric size, and, hence, a better attenuation will be attained. Dong Yu found that the γ attenuation of nano-sized WO₃ was greater than the micro-sized particles [39]. Tekin et al. showed that the materials doped with nano-sized WO₃ displayed better shielding characteristics than materials doped with micro-sized WO₃ particles [37]. Kazemi et al. demonstrated, using the MCNPX model, that composites with WO₃ nanoparticles had mass attenuation coefficients greater than those of composites with WO₃ microparticles [40].

To the best of our knowledge, there is no study regarding the effect of WO₃ nanoparticles on the radiation-shielding characteristics of ceramic materials. Thus, we aim in this work to study, for the first time, the radiation attenuation of STO ceramic mixed with WO₃ nanoparticles. Different radiation shielding parameters were experimentally measured and discussed.

2. Experimental Procedure

2.1. Synthesis of Materials

All chemicals, strontium carbonate (SrCO₃), titanium dioxide (TiO₂), and tungsten trioxide nanoparticles (WO₃ NPs), were reagent grade and used without further purification. First, stoichiometric strontium titanate SrTiO₃ (STO for brevity) was synthesized using the solid-state reaction by mixing finely in ball milling media SrCO₃ and TiO₂ powders. The obtained mixture was sintered at 1300 °C for 5 h to form SrTiO₃ ceramic (STO for brevity), as shown in Equation (1).

SrCO₃ + TiO₂ ⇒ SrTiO₃ + CO₂

(1)

In the second stage, different amounts of WO₃ NPs (x = 0, 2, 5, and 10 wt.%) were added to the STO sample. The mixtures were then compacted into pellets using PVA, which plays the role of the binder. The resulting pellets were placed in crucible alumina, then sintered at 1500° to obtain compact and dense ceramics of pristine STO ceramic (x = 0 wt.%) and WO₃-doped STO ceramics (x = 2, 5, and 10 wt.%). Then, the obtained ceramic samples were given for various characterizations.

2.2. Characterization of Ceramics

The crystallinity and phase analysis of the prepared ceramics were analyzed using a Rigaku Benchtop Miniflex powder XRD instrument with Cu Kα radiation. Using the Bruker alpha-II FT-IR spectrometer, the FT-IR spectra were recorded in the region of 400–4000 cm⁻¹. The γ-ray shielding parameters of STO-WO₃ NPs ceramics, such as linear attenuation coefficient (LAC), were experimentally determined using a high pure germanium (HPGe) detector and different point sources (Co-60, Cs-137, and Am-241). Readings resulting from the detector were processed using the Genie 2000 program. This program displays the intensity (count rate) for each photon that fell on the detector and was discovered during the measured time. From this point, the intensity of each specific energy was measured in the presence (I) and the absence (I₀) of the absorption sample. Then, by knowing the thickness of the absorption sample (t), the following law is used to calculate the LAC, which represents the possibility of the photon interaction through a certain distance within the absorption sample [41].

$$\mathrm{I}={\mathrm{I}}_0{ \mathrm{e}}^{-\mathrm{LAC} . \mathrm{t}} \hspace{1em}\hspace{1em}\hspace{1em}\mathrm{then},\hspace{1em}\hspace{1em}\hspace{1em}\mathrm{LAC}=\frac{1}{\mathrm{t}} \ln \frac{{\mathrm{I}}_0}{\mathrm{I}}$$

(2)

The other attenuation factors-based LAC determination, such as half and tenth value layers (HVL, TVL) and radiation absorption factor (RAP). The definition and laws of these factors were reported in previously published papers [42,43].

3. Results and Discussion

3.1. Structural and Functional Analysis

The synthesized ceramics were analyzed using XRD and FT-IR to investigate the structural and compositional properties. Figure 1 depicts the XRD patterns of SrTiO₃ (STO) doped with different amounts of WO₃ NPs. Pure STO exhibits (100), (110), (111), (200), (210), (211), (220), and (310) planes corresponding to cubic perovskite structure (space group: Pm-3 m) of SrTiO₃ which are in agreement with JCPDS card no. 35–0734. The ceramic doped with x = 2 wt.% of WO₃ NPs does not contain any impurity. However, the secondary phase of SrWO₄ (marked as ‘*’ in the figure; card no. 85–0587) appeared for x = 5 and 10% of WO₃ NPs, indicating that WO₃ NPs may react with the host material (STO) during the sintering process. The peak intensity of the impurity phase increases remarkably as the content of WO₃ NPs increases. The structural parameters, such as crystal cell parameter ‘a’ as well as the unit cell volume ‘V’ for all as-prepared ceramics calculated from the XRD data, are listed in

Table 1. Structural parameters deduced from XRD analysis.

Ceramics	STO	STO + (2%) WO3 NPs	STO + (5%) WO3 NPs	STO + (10%) WO3 NPs
a (Å)=	3.9020	3.9047	3.9032	3.9042
V (Å3)=	59.4103	59.5337	59.4651	59.5108
Structure	Cubic	Cubic	Cubic	Cubic
dXRD (nm)=	45.48	41.17	45.49	50.86

. It is evident that the structural parameters ‘a’ and V’ are higher for doped samples compared to the STO pristine sample. This effect can be attributed to the difference in ionic radii between the W element (0.66 Å) derived from WO₃ NPs dopant and Ti element (0.605 Å) derived from the host SrTiO₃. The presence of the SrWO₄ secondary phase can also be another reason for the lattice expansion in WO₃- doped STO ceramics.

The crystallites size (d_XRD) is estimated for all ceramics by using the Scherrer equation [44]:

$${\mathrm{d}}_{\mathrm{XRD}}=\frac{\mathrm{k} \mathsf{\lambda }}{\mathsf{\beta } \cos \mathsf{\theta }}$$

(3)

where $\mathrm{k}$ is a constant close to unity (k = 0.9), β is full width at half maximum, and θ is Bragg’s angle. The calculated d_XRD values for various ceramics are registered in Table 1 and are found to be in the range of 50.86–41.17 nm. Compared to the pristine sample, the crystallite size first reduces for the sample doped with a low amount of WO₃, suggesting that WO3 NPs act as inhibitors for the grain growth of STO ceramic [45]. With further increasing the amount of WO₃, the d_XRD return to increase. This unexpected result may be due to the transformation of tungsten oxide into metal tungstate. Indeed, for high amounts of WO₃, a new second phase of SrWO₄ was detected, which is mainly attributed to the possible reaction between the host STO phase and WO₃ NPs. It can be then assumed that the newly formed phase may result in initiating the growth process of aggregate particles by coalescence [46]. Further analysis is needed to validate this assumption, which will be the aim of our future work.

The functional groups of all ceramics were characterized using FTIR analysis, as depicted in Figure 2. In the measured range, several distinct absorption bands were detected for the pristine STO ceramic. The signal at about 560 cm⁻¹ is related to the characteristic stretching vibration of the Sr-Ti-O bond [47], which confirms the successful formation of the SrTiO₃ phase. This peak is maintained in the FTIR spectra of doped ceramics, confirming that the STO host perovskite structure is preserved. Another characteristic band with small intensity is observed around 1100 cm⁻¹, which can be attributed to the C=O stretching modes [48]. A large band around 3000 cm⁻¹ is also observed for STO ceramic, which may be assigned to the O–H stretching modes [49]. Additional bands around 825 cm⁻¹ appeared for ceramics doped with x = 5 and 10% of WO₃ NPs. This band is most probably a result of the signals of the W-O stretching vibration [50].

3.2. Radiation Attenuation Analysis

Figure 3 demonstrates the MAC (mass attenuation coefficient) of the prepared samples at four tested energies. At the first energy, the MAC values increase as more WO₃ nanoparticles are added to the ceramic. For example, the MAC of the WO₃-free ceramic is equal to 1.821 cm⁻¹ at this energy, while the MAC of the ceramic with 10% WO₃ nanoparticles is equal to 2.188 cm⁻¹. From MAC determination and the density of ceramic samples, the LACs (linear attenuation coefficients) of the prepared ceramic samples can be calculated. Figure 4 demonstrates the LACs of the prepared ceramic samples at four tested energies. At the first energy, the LAC values increase as more WO₃ nanoparticles are added to the ceramic. For example, the LAC of the WO₃-free ceramic is equal to 9.308 cm⁻¹ at this energy, while the LAC of the ceramic with 10% WO₃ nanoparticles is equal to 11.563 cm⁻¹. The same trend can be observed at higher energies, however, with a smaller difference. Nevertheless, these results show how the amount of WO₃ in the ceramics correlates with an improvement in their shielding abilities. In addition, this figure also shows that LACs have an inverse relationship with energy. More specifically, the STO ceramic with 2% WO₃ has an LAC equal to 9.810, 0.394, 0.288, and 0.270 cm⁻¹ at 0.060, 0.662, 1.173, and 1.333 MeV, respectively. Thus, ceramics are most effective against low-energy photons.

The half-value layer (HVL) of the investigated samples is graphed in Figure 5 against energy. For all the tested ceramics, their HVL values correlate with energy. For example, for the ceramic with 2% WO₃ nanoparticles, its HVL values are equal to 0.071 cm, 1.760 cm, 2.407 cm, and 2.564 cm at 0.060 MeV, 0.662 MeV, 1.173 MeV, and 1.333 MeV, respectively. As the energy increases, more radiation can pass through the material, which is why this relationship occurs. Therefore, a larger thickness is required to absorb half of the total photons, leading to a greater HVL. The figure also shows that HVL decreases with increasing WO₃ content. This trend is most evident at higher energies, such as 1.173 MeV. Moreover, at 1.173 MeV, the WO₃ values are equal to 2.483, 2.407, 2.334, and 2.259 cm for the samples with 0, 2, 5, and 10% WO₃ nanoparticles. Thus, increasing the amount of WO₃ nanoparticles in the ceramics improves their HVL values, lowering how thick a material needs to be to attenuate the same number of photons.

The tenth value layer (TVL) is like HVL, except it is defined as how thick a material needs to be to absorb nine-tenths of the incoming photons. Because of this, similar trends can be observed between the two figures; however, all the TVL values are higher than their HVL counterpart. The same two trends can be observed in Figure 6. Mainly, the TVL values increase with energy. For example, the TVL values for the STO with no WO₃ nanoparticles are equal to 0.247, 6.158, 8.250, and 8.809 cm at 0.060, 0.662, 1.173, and 1.333 MeV, respectively. The second trend involves the relationship between TVL and photon energy. At 0.662 MeV, the STO ceramics have TVL values of 6.158, 5.848, 5.724, and 4.604 cm for the samples with 0, 2, 5, and 10% WO₃ nanoparticles. Thus, increasing the WO₃ content in the STO ceramics improves their shielding efficiency.

The mean free path (MFP) of the STO ceramics is shown in Figure 7 at the four tested energies. The maximum MFP values occur for the STO ceramic, while the ceramic with 10% WO₃ nanoparticles has the minimum MFP values. For example, at 0.060 MeV, the STO ceramic has an MFP value of 0.107 cm, while the ceramic with 10% WO₃ NP has an MFP value of 0.086 cm. Thus, the amount of WO₃ in the ceramic mixture correlates with more collisions occurring within the sample, leading to greater attenuation. Furthermore, for any single ceramic, its MFP values increase with energy. For instance, STO-5WO₃^’s is equal to 0.099 cm, 2.486 cm, 3.367 cm, and 3.628 cm at 0.060, 0.662, 1.173, and 1.333 MeV. Higher energy radiation tends to pass through the samples more easily, decreasing the number of collisions and increasing the distance between collisions. Thus, ceramics are able to attenuate lower energy photons than higher energy ones. The results of LAC, HVL, and MFP, as well as the mass attenuation coefficient (MAC), are reported in

Table 2. The experimental values of the attenuator parameters for STO-ceramic samples.

Code	Energy (MeV)	STO	STO + 2% WO3 NPs	STO + 5% WO3 NPs	STO + 10% WO3 NPs
LAC, cm−1	0.060	9.308	9.810	10.097	11.563
	0.662	0.374	0.394	0.402	0.500
	1.173	0.279	0.288	0.297	0.307
	1.333	0.261	0.270	0.276	0.279
HVL, cm	0.060	0.074	0.071	0.069	0.060
	0.662	1.854	1.760	1.723	1.386
	1.173	2.483	2.407	2.334	2.259
	1.333	2.652	2.564	2.515	2.484
TVL, cm	0.060	0.247	0.235	0.228	0.199
	0.662	6.158	5.848	5.724	4.604
	1.173	8.250	7.997	7.753	7.505
	1.333	8.809	8.517	8.353	8.253
MAC, cm2.g−1	0.060	1.821	1.904	1.943	2.188
	0.662	0.073	0.076	0.077	0.095
	1.173	0.055	0.056	0.057	0.058
	1.333	0.051	0.052	0.053	0.053

.

The radiation protection efficiency (RPE) of the four prepared STO ceramics was calculated and graphed in Figure 8 at a thickness of 0.5 cm. RPE increases with WO₃ content at all tested energies. For instance, at 1.333 MeV, the RPE values are 12.25%, 12.64%, 12.87%, and 13.02% for the samples with 0, 2, 5, and 10% WO₃ nanoparticles. A greater RPE indicates a more effective shielding capability, so raising the amount of WO₃ in the ceramics leads to greater attenuation. Furthermore, RPE decreases as the energy of the incoming photons increases. The RPE of the sample with no WO₃ is equal to 99.05 at 0.060 MeV and drops to 12.25% at 1.333 MeV. Meanwhile, the RPE of the sample with 10% of WO₃ nanoparticles is equal to 99.69% and 13.02% at 0.060 MeV and 1.333 MeV, respectively. This conclusion reinforces some of the results from the previous figures that more photons can be attenuated at lower energies.

To compare the shielding abilities of the prepared ceramics with other radiation shields, Figure 9 graphs the LACs of the STO and STO + 10% of WO₃ NPs ceramics alongside four other shielding materials at 0.662 MeV. The STO and STO + 10% of WO₃ NPs ceramics had a greater LAC value than granite and marble [51,52]. SnTe and PbTe shields have a greater LAC than the STO ceramic but less than the STO + 10% of WO₃ ceramic [53]. Thus, ceramics have an overall very desirable shielding ability compared to other commonly used materials for radiation shielding purposes.

4. Conclusions

In this work, we proposed a strategy for developing new ceramic materials consisting of STO doped with WO₃ nanoparticles for the purpose of radiation protection. The different samples were prepared using the simple solid-state reaction method. The successful formation of the required compositions was confirmed by XRD and FTIR analyses. All as-prepared ceramics crystallized into the cubic structure. A new phase belonging to SrWO₄ was detected for 5 and 10% doped ceramics suggesting an interaction between the host STO and the dopant WO₃ NPs has occurred. The lattice constant ‘a’ and the unit cell volume ‘V’ were found to be higher for doped ceramic compared to the STO pristine ceramic. The crystallite size varied with the content of WO₃ NPs. In the second part, we focused on the role of adding WO₃ NPs in improving the radiation-protective characteristics of STO ceramics. The LAC was experimentally measured using a high pure germanium (HPGe) detector and three-point sources (⁶⁰C, ²⁴¹Am, and ¹³⁷Cs). The LAC of the WO₃-free ceramic at 0.06 MeV is equal to 9.308 cm⁻¹, while it is 11.563 cm⁻¹ for the ceramic with 10% WO₃ nanoparticles. This shows that the LAC values increase as more WO₃ nanoparticles are added to the ceramic. We investigated the relationship between the LAC and the energy, and we concluded that the ceramics are most effective against low-energy photons. In order to examine the thickness of the ceramics that required attenuating certain levels of the photons, we evaluated the HVL and TVL. From the findings of both factors, we found that a larger thickness is required to absorb the photons with higher energies. Moreover, we examined the impact of WO₃ nanoparticles on these two factors. In brief, increasing the amount of WO₃ nanoparticles in the ceramics improves their HVL and TVL values. In addition to these factors, the MFP results also proved that the ceramics are able to attenuate lower energy photons than higher energy ones. At 1.333 MeV, the RPE for the samples with 0, 2, 5, and 10% WO₃ nanoparticles are 12.25%, 12.64%, 12.87%, and 13.02%, respectively. Hence, from the RPE data, we found that increasing the amount of WO₃ in the ceramics leads to greater attenuation.