Study of the Phase Formation Processes and Their Influence on the Change in the Optical and Shielding Characteristics of 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ Ceramics

Abstract

The phase formation processes in 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ ceramics with variation in the thermal annealing temperature were evaluated in this study. According to the obtained data on the phase composition dependent on the annealing temperature, the phase transformation dynamics, which can be written in the form of ZnO/Bi₂O₃/WO₃/Al₂O₃ → ZnBi₃₈O₆₀/ZnO/Bi₂WO₆/WO₃ → Bi₂Al₄O₉/ZnBi₃₈O₆₀/Bi₂WO₆/ZnO/WO₃ → ZnWO₄/Bi₂WO₆/ZnAl₂O₄/ZnO → ZnWO₄/Bi₂WO₆/ZnAl₂O₄ → Bi₂WO₆/ZnWO₄/ZnAl₂O₄ → ZnAl₂O₄/Bi₂WO₆/Bi₂W₂O₉, were established. It has been found that the formation of phases of complex oxides of the ZnWO₄, Bi₂WO₆ and Bi₂W₂O₉ types in the composition of ceramics leads to an increase in the density of ceramics up to 8.05–8.10 g/cm³, which positively affects the shielding efficiency and strength characteristics. According to the data on the change in strength characteristics, it was found that a change in the density of ceramics from 6.3 to 8.05–8.10 g/cm³ leads to strengthening and an increase in the crack resistance of ceramics by 75–80%, which indicates a high strength of ceramics and their increased resistance to external influences. As shown by the evaluation of the shielding characteristics, an increase in the density of ceramics due to a phase composition change leads to an increase in the shielding efficiency and a decrease in gamma intensity by a factor of 3–3.5. At the same time, on the base of the data presented, it can be concluded that ceramics obtained in the range of 900–1100 °C have both high shielding characteristics and high strength and resistance to external influences.

1. Introduction

In the modern world, in view of the increasingly active use of various sources of ionizing and electromagnetic radiation in various sectors of human life, the issues of protection against negative impacts are quite acute. The basic principles of protection against exposure to various types of ionizing radiation, as well as requirements for installations operating ionizing radiation, are set out in the As Low As Reasonably Achievable (ALARA) concept, the basis of which were formulated in 1954 under the auspices of the International Commission on Radiological Protection [1,2,3]. The main goal of this concept is to minimize the impact of radiation and ionizing radiation on living organisms [4,5]. In this regard, in recent years, exploratory works and research haveve been carried out, as aimed at finding and creating the most effective shielding materials for protection against X-ray, electron and gamma radiation, which have the maximum penetrating power, and due to their nature are capable of causing a large amount of damage during the ionization process, both in living organisms and in electronics [6,7,8]. At the same time, in contrast to heavy ions or fission fragments of nuclear fuel, for which the maximum path length is no more than 50–100 μm, and the penetrating power itself has a pronounced dependence on the initial energy of the particles, for gamma and neutron radiation, the path length can reach tens of centimeters. In this regard, much attention is paid to various composite materials, the combination of elements of which makes it possible to approach the density of lead protective materials. At the same time, unlike lead, composite materials in the form of ceramics or glasses are not toxic, which makes their wide use in the medical sector acceptable, as well as in the conditions of contact between protective materials and living organisms [9,10,11,12,13]. Additionally, an important factor in the manufacture of new types of protective shielding materials is given to their transparency, which will allow them to be used as protective materials with the ability to access visual observation of the effects of radiation or the operation of ionizing radiation sources [14,15]. Great prospects in this research field are assigned to various types of telluride or borate glasses or ceramics, which, as a rule, consist of a large number of different oxide compounds [9,10,11,12,13,14,15,16,17,18,19,20], including refractory oxides, as well as rare earth elements [15,16,17,18,19,20]. Interest in these types of materials is due to high shielding characteristics in a wide energy range, as well as good transparency. High shielding characteristics are primarily due to a combination of various elements, as well as high density values, which can significantly reduce the intensity and penetrating power of ionizing radiation. At the same time, the emphasis of such studies has recently begun to shift to the search for cheaper components for protective shielding materials during their manufacture [21,22,23], since the use of tellurium oxide as the main component is quite expensive and energy consuming. In this regard, emphasis is placed on a number of components, such as zinc oxide (ZnO) [24], aluminum oxide (Al₂O₃) [25], tungsten oxide (WO₃) [26], which have not only good absorbing properties due to optical characteristics, but also sufficiently high strength and mechanical characteristics that are necessary to increase the resistance of shielding materials to external influences. It is also worth noting that, in most cases, in the manufacture of glasses or ceramics, bismuth oxide (Bi₂O₃) [23,27] is used, the use of which is due to its protective properties, which during sintering accelerate the process of phase formation, and in some cases, amorphization due to the acceleration of the melt.

The purpose of this study is to establish the effect of thermal annealing temperature on the processes of phase formation in composite ceramics based on oxide compounds ZnO, Al₂O₃, WO₃, Bi₂O₃ selected in an equal stoichiometric ratio of 0.25 mol. The interest in the selected combination of oxide components for the preparation of composite ceramics using the mechanochemical synthesis method is due to the possibility of controlling the density of ceramics due to the ongoing phase transformations with an increase in the annealing temperature, as well as the great prospects of these components as alternatives to telluride and borate protective glasses and ceramics.

2. Experimental Part

For the synthesis of ceramics, ZnO, Al₂O₃, WO₃, Bi₂O₃ oxide compounds were chosen in the form of powders with a particle size of several microns, purchased from Sigma Aldrich (Sigma, St. Louis, MO, USA). The chemical purity of the compounds used was 99.95%. The samples were mixed using a PULVERISETTE 6 planetary mill (Fritsch, Berlin, Germany). Samples were stirred with a glass of tungsten carbide, with a volume of 80 mL, and filled with powders of the initial oxide components in the ratio to grinding balls with a diameter of 8 mm 1:2. The grinding speed was 250 rpm, the grinding time was 30 min.

Ceramics of the following composition 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ obtained by grinding followed by thermal sintering in a muffle furnace in the annealing temperature range of 500–1100 °C were chosen as the samples under study. The samples were annealed in a Nabertherm muffle furnace (Nabertherm GmbH, Lilienthal, Germany). The samples were annealed in alundum crucibles; after annealing for five hours, the samples were cooled together with the furnace for 24 h until reaching room temperature. The choice of the composition of ceramics is due to the possibility of obtaining compounds with a high absorbing capacity. The variation in thermal annealing conditions makes it possible to change the phase composition, and, as a result, tune the ceramics density and physicochemical, strength and optical properties. The selected stoichiometric ratio of the ZnO, Al₂O₃, WO₃, Bi₂O₃ oxide components will allow one to vary the phase composition by thermal annealing due to the processes of phase transformations during sintering of simple oxides into complex phases.

Figure 1 shows the results of change in the particle size of powders after grinding and thermal annealing, obtained using the ANALYSETTE 22 NeXT Nano particle analyzer (FRITSCH, Berlin, Germany). The particle size, as well as the standard deviation and the homogeneity degree, were determined using the method of laser optical diffraction.

Table 1. Grain size data.

Temperature, °C	Daverage, µm	SBET, m2/g
Pristine	1.51 ± 022	0.61 ± 0.03
500	1.41 ± 0.13	0.63 ± 0.04
600	1.25 ± 0.14	0.62 ± 0.02
700	1.22 ± 0.12	0.63 ± 0.03
800	1.16 ± 0.11	0.65 ± 0.03
900	1.07 ± 0.11	0.68 ± 0.02
1000	1.01 ± 0.09	0.74 ± 0.04
1100	0.92 ± 0.17	0.81 ± 0.03

presents data on the average grain size (D_average), as well as the specific surface area (S_BET) values obtained using the laser optical diffraction method.

As can be seen from the data presented, an increase in the annealing temperature leads to a decrease in the grain size of the objects under study, as well as an increase in the specific surface area. At the same time, in the case of the original samples, the presence of a small amount of large grains was observed, the presence of which may be due to the agglomeration of smaller grains. In the case of annealed samples with varying annealing temperature, the presence of large inclusions in the powders was not found.

To study the phase composition as a function of the thermal annealing conditions, the X-ray phase analysis was carried out using an D8 Advance ECO powder X-ray diffractometer (Bruker, Berlin, Germany). The contribution of each phase was obtained by calculating the area ratio of the reflection characteristics of the established phases, as well as the subsequent determination of its weight contribution.

After grinding and thermal annealing, the samples were pressed into pellets 10 mm in diameter. Pressing was carried out using a steel mold under a pressure of 300–400 MPa for 30 min.

X-ray diffraction patterns were measured on pressed samples before and after annealing. The measurement on pressed samples is due to the need to determine the phase composition of ceramics, which were subsequently used to assess the strength properties and shielding characteristics.

The density of the obtained samples was measured using the standard method of Archimedes, the obtained density values were in good agreement with the results of the estimated density in the analysis of phase diagrams.

The optical properties of the synthesized ceramics were studied using the transmission and reflection spectra measured on a Specord 250 plus UV-Vis spectrophotometer (Analytic Jena, USA). The spectra were recorded in the wavelength range from 300 to 1000 nm, with a step of 1 nm. An integrated sphere was used for shooting. Based on the obtained transmission and reflection spectra, optical characteristics were calculated, including the band gap, absorption coefficient, reflecting changes in the optical density of ceramics.

To determine the strength characteristics, standard indentation methods and a single compression method to determine the maximum compression pressure that the ceramic can withstand were applied using a Vickers pyramid as an indenter. The indenter load was 100 N. To determine the hardness, a LECO LM700 microhardness tester (LECO, Tokyo, Japan) was used.

Determination of the resistance of ceramics to cracking was carried out using the method of single compression of ceramics at a constant compression rate. The formation of microcracks during compression was recorded using an extensometer. The compression rate was 0.1 min/min. Determination of fracture toughness was carried out using an LFM-L test machine (Walter + bai AG, Lohningen, Switzerland).

The shielding characteristics of the synthesized ceramics were determined using Co⁵⁷ (130 keV), Cs¹³⁷ (660 keV), Na²² (1270 keV) gamma radiation sources, which were used as generators of ionizing radiation with a given gamma photon energy. The classical experimental scheme was used to assess the shielding characteristics. This includes placing a gamma radiation source in a special lead container with an outlet, in front of which, at a distance of 10 cm, a protective ceramic shield was placed, behind which a detection system based on a NaI detector was placed. The dynamic range for the NaI detector was 0–2–2.5 Mimp/s. After the passage of gamma radiation through the protective shield, the transmitted radiation intensity was recorded for a given period of time, for comparison with the magnitude of the intensity of this radiation for the same period of time without shielding.

3. Results and Discussion

Figure 2 shows the results of X-ray diffraction of the investigated 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ ceramics depending on the thermal sintering temperature in the muffle furnace. For a comparative analysis of the change in the phase composition of ceramics depending on the annealing conditions, an X-ray diffraction pattern of the sample in the initial state after grinding in a planetary mill is given. The general view of the presented X-ray diffraction patterns reflects the change in the phase composition of ceramics, in view of the appearance of new diffraction reflections with a change in the annealing temperature, as well as rather high values of the structural ordering degree of the samples, expressed in a change in the ratio of background radiation and the area of the main diffraction reflections. In addition, according to the analysis of the obtained data, an increase in the annealing temperature leads not only to a change in the phase composition of ceramics during the phase formation processes, but also to a change in the concentration of deformation inclusions (point defects, vacancies, and dislocations), and its change is clearly seen in the form of a diffraction line shape change.

According to the assessment of the phase composition of the synthesized ceramics after mechanochemical grinding, it was found that there were no diffraction reflections characteristic of phases of complex oxides or solid solutions of substitution or interstitial, which indicates that during mechanochemical grinding, phase transformations do not occur. Determination of the weight contributions of the established contributions showed that the initial sample is a mixture of oxide components ZnO, Bi₂O₃, WO₃, Al₂O₃ in the ratio 50.1 wt: 28.7 wt: 11.4 wt: 9.8 wt, respectively.

In the case of thermal annealing of ceramic samples at a temperature of 500 °C, according to the data of X-ray phase analysis, it was established that there were no phase transformations associated with the phase formation. Thus, the observed partial ordering of diffraction reflections, which is expressed in a change of their shape, a decrease in the reflection asymmetry, as well as variations in the contributions (weight content of phases) for annealed samples, indicates the structural ordering on heating. In this case, according to the data on changes in the crystal lattice parameters presented in

Table 2. Structural parameters of crystal phases.

Phase	Cell Parameter, Å
	Temperature, °C
	Pristine	500 °C	600 °C	700 °C	800 °C	900 °C	1000 °C	1100 °C
ZnO–Hexagonal	a = 3.24489, c = 5.19884	a = 3.23789, c = 5.19883	a = 3.24189, c = 5.19776	a = 3.24761, c = 5.21101	a = 3.23484, c = 5.19364	a = 3.22406, c = 5.17836	–	–
Bi2O3–Monoclinic	a = 5.83269, b = 8.17140, c = 7.51377, β = 112.700°	a = 5.81553, b = 8.13135, c = 7.49756, β = 112.369°	–	–	–	–	–	–
WO3–Monoclinic	a = 7.27554, b = 7.55230, c = 7.67142, β = 90.857°	a = 7.25699, b = 7.51232, c = 7.64585, β = 90.589°	a = 7.27833, b = 7.54031, c = 7.67134, β = 90.820°	a = 7.25692, b = 7.52405, c = 7.66381, β = 90.660°	–	–	–	–
Al2O3–Cubic	a = 7.91675	a = 7.89346	–	–	–	–	–	–
ZnBi38O60–Cubic	–	–	a = 10.18689	a = 10.22085	–	–	–	–
Bi2Al4O9–Orthorhombic	–	–	a = 7.77947, b = 8.15873, c = 5.73945	a = 7.77794, b = 8.17313, c = 5.74733	–	–	–	–
Bi2WO6–Orthorhombic	–	–	a = 5.47499, b = 16.45805, c = 5.44780	a = 5.46103, b = 16.43546, c = 5.44460	a = 5.42355, b = 16.38712, c = 5.42645	a = 5.39696, b = 16.28751, c = 5.41901	a = 5.38744, b = 16.27154, c = 5.41156	a = 5.39906, b = 16.28749, c = 5.42536
ZnWO4–Monoclinic	–	–	–	–	a = 4.68416, b = 5.69505, c = 4.90631, β = 90.395°	a = 4.67773, b = 5.67830, c = 4.90150, β = 90.271°	a = 4.66947, b = 5.67719, c = 4.89478, β = 90.112°	–
ZnAl2O4–Cubic	–	–	–	–	a = 8.07055	a = 8.05314	a = 8.03777	a = 8.05941
Bi2W2O9–Orthorhombic	–	–	–	–	–	–	–	a = 5.42325, b = 5.44788, c = 23.75490

, the largest changes indicating structural ordering (decrease in parameters) were observed for the Bi₂O₃ and WO₃ phases. In the case of the ZnO phase, the ordering occurs most pronounced along the crystallographic axis a, while the decrease in the c parameter is insignificant. Almost no ordering was observed for the Al₂O₃ phase.

Annealing of the samples up to 600 °C led to new diffraction reflections, which indicates the initialization of phase transformations in ceramics, which are associated with thermal effects, as well as structural re-arrangement. According to the phase composition of samples annealed at 600 °C, the ceramics are a mixture of hexagonal ZnO, cubic ZnBi₃₈O₆₀, and two orthorhombic Bi₂WO₆ and Bi₂Al₄O₉ phases. The appearance of complex oxide phases is due to the fact that, with an increase in temperature, the oxide compound of bismuth (melting point 817 °C) begins to react with other compounds, thereby forming complex oxides and rearranging the crystal structures. As is known, bismuth oxide is used in ceramics and glasses in order to accelerate the phase transformations at lower temperatures, which is observed in this case. A similar phase composition for ceramics is also observed for the samples annealed at 700 °C, with only one difference associated with a decrease in the intensity reflections characteristic of the hexagonal phase of zinc oxide. The formation of phases with a change in the annealing temperature agrees fairly well with the phase diagrams of binary systems [28].

Figure 2. (a) X-ray diffraction patterns of 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ ceramics as a function of sintering temperature (to estimate the phase composition, the data of reference values of various phases from the PDF-2 (2016) database were used: Bi₂O₃ [29], ZnO [30], Al₂O₃ [31],WO₃ [32], ZnBi₃₈O₆₀ [33], Bi₂Al₄O₉ [34], ZnWO₄ [35], ZnAl₂O₄ [36], Bi₂W₂O₉ [37], Bi₂WO₆ [38]); (b) detailed change in the main diffraction reflections in the angular range of 2θ = 20–45°.

Figure 2. (a) X-ray diffraction patterns of 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ ceramics as a function of sintering temperature (to estimate the phase composition, the data of reference values of various phases from the PDF-2 (2016) database were used: Bi₂O₃ [29], ZnO [30], Al₂O₃ [31],WO₃ [32], ZnBi₃₈O₆₀ [33], Bi₂Al₄O₉ [34], ZnWO₄ [35], ZnAl₂O₄ [36], Bi₂W₂O₉ [37], Bi₂WO₆ [38]); (b) detailed change in the main diffraction reflections in the angular range of 2θ = 20–45°.

After annealing at 800–900 °C, an almost complete transformation of the zinc oxide phase into the monoclinic ZnWO₄ and cubic ZnAl₂O₄ is observed, and it can be explained by the partial replacement of zinc ions by tungsten ions in the structure in the case of the ZnWO₄ phase and zinc ions by aluminum ions in the case of the ZnAl₂O₄ phase. It should be noted that no reflection characteristics of the hexagonal phase of ZnO were found, which indicates that the entire zinc oxide reacted at the given annealing temperatures. Moreover, at a given annealing temperature, a phase transformation is observed, associated with the formation of a cubic spinel of the ZnAl₂O₄ type, and complete displacement of the ZnBi₃₈O₆₀ and Bi₂Al₄O₉ phases. At the same time, Bi₂WO₆ phase is appeared among the main phases, and the increase in its contribution is associated with the structural ordering.

The third stage of phase transformations is observed with an increase in the annealing temperature to 1000 °C, which leads to the complete disappearance of the hexagonal ZnO phase and the dominance of the ZnWO₄ and ZnAl₂O₄ phases, the weight contributions of which are more than 40% each. At 1100 °C, the complete dissolution of the monoclinic ZnWO₄ phase occurs, as well as the appearance of diffraction reflections characteristic of the orthorhombic Bi₂W₂O₉ phase. At the same time, the weight contribution of the formed Bi₂W₂O₉ phase in the composition of ceramics is dominant.

Based on the obtained X-ray diffraction patterns, the weight contribution of each phase, formed as a result of thermal annealing, was calculated. The changes in the contributions depending on the annealing temperature, which also reflects phase transformations, is presented as a diagram in Figure 3a. In Table 2, each cell parameter should be supplied by possible error range.

Analyzing the weight contributions of various phases in the composition of ceramics at different temperatures, we can formulate the following mechanisms of phase transformations: ZnO/Bi₂O₃/WO₃/Al₂O₃ → ZnBi₃₈O₆₀/ZnO/Bi₂WO₆/WO₃ → Bi₂Al₄O₉/ZnBi₃₈O₆₀/Bi₂WO₆/ZnO/WO₃ → ZnWO₄/Bi₂WO₆/ZnAl₂O₄/ZnO → ZnWO₄/Bi₂WO₆/ZnAl₂O₄ → Bi₂WO₆/ZnWO₄/ZnAl₂O₄ → ZnAl₂O₄/Bi₂WO₆/Bi₂W₂O₉.

Figure 3b presents the results of a comparison of the density values of the studied samples, measured using the Archimedes method and calculated based on the assessment of the contributions of various phases.

As can be seen, in the initial sample after grinding, the density is estimated to be less than 6.5 g/cm³. At the same time, thermal annealing at 500 °C leads to a slight increase in density, which is primarily due to structural ordering, which is reflected in cell parameters decrease, as well as variations in the contributions of the simple oxide phases. The main changes in density occur with the temperature increase to 600 °C and higher, which, as can be seen from the XRD data, are characterized by phase formation processes, including the formation of complex oxide phases, as well as changes in their ratio depending on the temperature. The stabilization of the density in the region of 8.05–8.10 g/cm³ is observed above 800 °C, and it is characterized by the formation of Bi₂WO₆/ZnWO₄/ZnAl₂O₄ and ZnAl₂O₄/Bi₂WO₆/Bi₂W₂O₉ phases. Such a change in the density indicates the effect of annealing temperature not only on the phase formation, but also on the compaction of ceramics, which can positively affect both the strength and shielding characteristics of ceramics.

An analysis of the changes in the cell parameters depending on annealing temperature indicates the structural ordering in the case of a decrease in grain size with the temperature increase from 500 to 700 °C, as well as partial substitution and subsequent phase transformations with the formation of complex oxides and their ordering. Moreover, above 900 °C, a partial increase in the parameters can be explained by the thermal expansion effects, as well as by cationic substitution with the formation of more complex crystal lattices. At the same time, comparing the changes in density and the degree of structural ordering presented in Figure 3b, one can conclude that, as the temperature rises to 1000–1100 °C, thermal expansion plays a dominant role in the structural parameter changing, since the degree of structural ordering remains practically unchanged, as well as the density of ceramics.

Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10 show the SEM images of the ceramic surfaces, together with the results of mapping, reflecting the element distributions depending on the sintering temperature. All images were taken in the same scale at the same magnification in order to be able to conduct a comparative analysis of the morphological features of the grains, as well as their placement. In the sample annealed at 500 °C, the structure of the ceramics is a finely dispersed powder with large agglomerates in the form of Bi₂O₃ particles of irregular shapes close to rhomboids or pyramidals. At the same time, these particles are embedded in the bulk of ceramics, which is a homogeneous mixture of oxide compounds of zinc, aluminum and tungsten.

For the samples annealed at 600–700 °C, the transformation of Bi₂O₃ particles is observed, which consists first in their size decrease at 600 °C, followed by incorporation into the main matrix in the form of finely dispersed spherical particles at 700 °C. It should also be noted that at 700 °C, the formation of large grains with a single shape, consisting of large agglomerates of particles, is observed.

For the samples annealed at 800 °C, the formation of a single matrix consisting of aluminum, oxygen, tungsten, and zinc is observed, which can be characteristic of the ZnAl₂O₄ and ZnWO₄ phases, which, according to phase analysis, are the dominant in the ceramic. At the same time, particles embedded in the matrix in the form of separate grains containing bismuth, tungsten and oxygen are clearly distinguished, which can be characteristic of grains of the Bi₂WO₆ phase, which formation occurs above 600 °C.

The dominance of the Bi₂WO₆ phase above 900 °C leads to the change in the structure of ceramics at these annealing temperatures, which is a matrix of the Bi₂WO₆ and ZnWO₄ phases with embedded inclusions in the form of grains of the ZnAl₂O₄ phase. These grains are clearly seen in the mapping patterns for samples annealed at 1000–1100 °C, for which a decrease in the contribution of this phase in the ceramic composition is observed.

Making a general conclusion about the observed changes in the morphological features of the synthesized ceramics, as well as their elemental composition, we can say that thermal annealing at temperatures above 700 °C leads to the formation of a solid ceramic, which is an interstitial solid solution. In this case, with an increase in the annealing temperature, the phase composition and morphological features of the ceramics are transformed.

Figure 11 shows the results of measurements of the optical transmission and absorption spectra, presented in the form of UV-Vis spectra measured in the wavelength range of 350–1000 nm. The optical spectra are characterized by a fundamental absorption edge in the region of 350–420 nm, as well as a sufficiently high transmittance in the visible and near-IR ranges. At the same time, the transmission variation has a clear dependence on the phase composition of ceramics, and the variation leads to an increase or decrease in the transmission. In turn, the shift of the fundamental absorption edge indicates a change in the band gap, as well as variations in the electronic and optical density of ceramics. An analysis of the absorption spectra indicates that a change in the phase composition leads to an increase in the absorption capacity in the region of 300–400 nm, which is characteristic of the boundary between visible light and ultraviolet.

Figure 12 shows the Tauc plots for the 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ ceramics synthesized at selected temperatures. The relation (1) was used to determine the band gap [39,40,41]:

$$\alpha =A{(h\nu -E_g)}^{1/2},$$

(1)

where A is a constant, hν is the photon energy.

Table 3. Optical characteristics of 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ ceramics.

Temperature, °C	Band Gap, eV	Linear Refractive index	Refraction Loss	Optical Transmission	Static Dielectric Constants	Molar Refraction	Metallization Criterion
Pristine	2.54	2.53	0.188	0.684	6.401	20.88	0.35
500 °C	2.31	2.61	0.199	0.668	6.812	20.33	0.40
600 °C	2.25	2.63	0.202	0.664	6.917	28.27	0.42
700 °C	2.05	2.71	0.212	0.650	7.344	31.28	0.48
800 °C	2.76	2.46	0.178	0.698	6.052	36.61	0.31
900 °C	2.71	2.48	0.181	0.694	6.150	39.86	0.32
1000 °C	2.65	2.49	0.182	0.692	6.200	41.86	0.33
1100 °C	2.64	2.50	0.184	0.690	6.250	66.76	0.33

presents the optical characteristics of the studied 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ ceramics made at different annealing temperatures. The calculation of the values of optical characteristics based on the data on changes in the optical spectra was carried out according to the formulas and methods presented in [42,43].

The general form of the dependences obtained indicates that the variation in the optical characteristics has a pronounced dependence on the phase composition, and these dependences are most pronounced for samples annealed at a temperature of 500–700 °C, which are characterized by phase transformations with the formation of complex oxides and the transformation of simple oxides in the composition of ceramics. These phase transformations are characterized by a decrease in the band gap due to a shift in the fundamental absorption edge, as well as an increase in the linear refractive index, an increase in which indicates an increase in optical absorption, as well as an increase in the static dielectric constant. The displacement of simple oxides (ZnO, WO₃, Bi₂O₃, Al₂O₃) at temperatures above 700 °C leads to a sharp increase in the band gap, which is due to a change in the electron density induced by the formation of complex oxides. At the same time, in the entire range of annealing temperatures, the change in the metallization criterion, which characterizes the propensity of a material to a metallic (Metallization criterion ≥ 1) or non-metallic (Metallization criterion ≤ 1) nature, as well as describing the insulating properties of materials, is no more than 0.31–0.48. Moreover, an increase in the annealing temperature above 800 °C leads to a decrease in the metallization criterion to 0.31–0.33, which indicates an improvement in the insulating properties, as well as the formation of vitreous ceramics that do not have a metallic nature. The metallization criterion is calculated for most vitreous ceramics, as well as glasses obtained from oxide compounds at high temperatures. This criterion makes it possible to qualitatively assess the nature of the obtained samples, as well as the change in their insulating properties (changes in electronic conductivity). A decrease in the metallization criterion indicates an increase in the insulating and heat-insulating properties of ceramics, which makes them promising materials not only as shielding materials, but also as insulating substrates.

In Figure 13a, the strength characteristics of the studied ceramics depending on the annealing temperature, reflecting the effect of annealing and changes in the phase composition on the mechanical properties and hardening of ceramics. The data are presented as indentation hardness values, as well as the maximum pressure that ceramics can withstand under a single compression. Based on the hardness change data and the maximum pressure value, the hardening and crack resistance values were calculated, and the results are shown in Figure 13b in comparison with the density change data.

The general view of the trends in strength characteristics reflects the influence of a change in the phase composition, as well as the strengthening of ceramics and an increase in resistance to external influences, including compression. In this case, as can be seen from the data presented, the changes are non-linear depending on the annealing temperature, which is expressed in a sharp increase in the strength characteristics with an increase in the annealing temperature from 500 to 700 °C, followed by a smooth increase in strength and resistance to cracking with increasing temperature. Moreover, the obtained dependences of the strengthening factors are similar in nature to the change in the density of ceramics depending on the annealing temperature. It should also be noted that, at annealing temperatures above 900 °C, there are practically no changes in the strength characteristics, which can be explained by the stabilization of the phase composition, as well as small changes in the density and degree of structural ordering. The effect of hardening of ceramics with a change in the phase composition can be explained by the morphological features of the samples obtained at temperatures above 700 °C, for which the formation of a dense structure with interstitials in the form of Bi grains is clearly visible (see the data in Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9), and at temperatures above 900 °C, the formation of ZnAl₂O₄ grains embedded in a matrix of Bi₂WO₆ and ZnWO₄ phases.

To determine the effect of the phase composition and density of ceramics, which are dependent on the annealing temperature, on the shielding characteristics, we carried out a series of experiments on shielding gamma radiation of different (fixed) energies. For calculations of shielding characteristics, such as the magnitude of the shielding efficiency, as well as the linear attenuation coefficient, the methods proposed in [44,45] were used. The choice of these values for comparative analysis is due to the possibility of assessing the decrease in the intensity of ionizing radiation, as well as determining the effectiveness of increasing the thickness by the amount of attenuation of ionizing radiation. The choice of three gamma-ray study sources capable of generating gamma-quanta with different (fixed) energies is primarily due to the possibility of modeling the main types of interaction of gamma radiation with matter when passing through it, including the photoelectric effect, the Compton effect and the formation of electron-positron pairs. At the same time, as is known from a number of works, protective glasses and ceramics are most efficient in shielding low-energy gamma quanta, while shielding of gamma quanta with an energy above 1 MeV, when the process of interaction is accompanied by the formation of electron-positron pairs, occurs with a significantly lower efficiency, and it is the thickness and density of the shielding materials that make the main contribution to the intensity decrease [21,22,23]. In this regard, in a large number of studies, the main emphasis is on increasing the density of protective materials by adding rare earth elements with a large molar mass and density, respectively [46,47,48,49].

Figure 14 shows the data for determination of the efficiency of reducing the intensity of the gamma radiation of fixed energy with a variation in the thickness of protective glasses: 1, 1.5 and 5 mm. The samples for shielding were obtained by pressing the synthesized annealed powders in special molds at a pressure of 300–400 MPa, the samples were pressed at room temperature, and the holding time under pressure was no more than 30 min. The thickness variation in protective ceramics was carried out in order to determine the effect of an increase in thickness on the shielding efficiency, taking into account the phase composition and density of the ceramics.

Figure 15 shows comparison diagrams of screening efficiency for various protective glass thicknesses. The data are presented in the form of comparative diagrams of changes in shielding efficiency values for each source of gamma rays separately depending on the thickness.

As can be seen for small thicknesses of 1–1.5 mm, the shielding efficiency is dependent on the density of ceramics, which varies with changes in the annealing conditions. In this case, for samples with a density of 6.0–6.5 g/cm³, the shielding efficiency for low-energy gamma rays does not exceed 40%, while for gamma rays with an energy of 1270 keV, the shielding efficiency is less than 10%. At the same time, in the case of thicknesses of 1 mm, an increase in the density of ceramics to 8.05–8.10 g/cm³ leads to an increase in efficiency up to 70–90% in the case of low-energy gamma rays, for which almost complete absorption of all gamma rays occurs, and 40–70% for high-energy gamma rays, for which a half attenuation of the intensity of the gamma radiation flux is observed. In the case of ceramic thicknesses of 1.5 mm, the shielding efficiency increases significantly, and for samples with a density of 8.05–8.10 g/cm³, the shielding efficiency is more than 95%, which indicates the almost complete absorption of gamma radiation in ceramics. At the same time, it should be noted that for samples obtained at 900–1100 °C, the shielding efficiency practically does not change with sintering temperature increase. This is due to the fact that at this annealing temperature range, the main changes in the properties of ceramics are associated more with the effect of hardening than with a change in the phase composition of ceramics. This indicates that these ceramics obtained at annealing temperatures of 900–1100 °C can be considered as effective shielding materials with increased strength and crack resistance. In the case of samples obtained at annealing temperatures of 500–700 °C, the shielding efficiency increases insignificantly with an increase in thickness from 1 to 1.5 mm. At the same time, the main effect of increasing the shielding efficiency, especially for low-energy gamma rays, can be due to the presence of zinc oxide in the ceramic structure, which, despite a lower molar mass than other oxides, has an increased absorption capacity due to a higher mass attenuation coefficient, which was shown in [24].

An increase in the thickness of ceramics to 5 mm leads to an increase in the shielding efficiency from 5–10% to 35–60% for samples annealed at temperatures of 500–700 °C, as well as an increase in the shielding efficiency for samples with a density of 8.05–8.10 g/cm³ to 80–97% for all types of gamma rays, which is in good agreement with the fact that an increase in the thickness of protective materials leads to an increase in the shielding efficiency [23].

A comparative analysis of the screening efficiency presented in Figure 14 showed that the greatest effect of the thickness increase is manifested for the samples annealed at 500–700 °C, where an increase in thickness from 1.5 to 5 mm leads to a threefold increase in efficiency. While for samples obtained at annealing temperatures of 900–1100 °C, an increase in thickness leads to an increase in efficiency by 15–20%. However, small changes in efficiency in this case are due to the high absorbing ability of ceramics due to a change in density, as well as the presence of a complex structure, which also makes it possible to screen secondary radiation arising from the formation of electron-positron pairs in the structure. Moreover, it should also be noted that ceramics obtained at given annealing temperatures due to their phase composition have higher strength indicators, which, together with shielding characteristics, makes them suitable candidates for shielding materials in order to increase the effectiveness of protection against the negative effects of gamma radiation.

Figure 16 shows the results of assessing the change in the value of the linear attenuation coefficient (LAC), which reflects the effectiveness of the absorbing and shielding ability of protective materials depending on the type of ionizing radiation.

As can be seen from the presented diagrams showing the results of changing the LAC value, an increase in the ceramic thickness from 1 to 1.5 mm does not lead to significant changes in the shielding attenuation value for low-energy ions, which can be explained by the fact that the shielding efficiency for low-energy ions is more than 85–90%. A similar situation is also observed with an increase in thickness from 1.5 to 5 mm for both low-energy and high-energy gamma rays. In this case, the main differences in the efficiency of changing the LAC value are when changing the density of ceramics, the variation in which occurs with an increase in the annealing temperature. In this case, a change in density to 8.05–8.10 g/cm³ leads to a three–five-fold increase in the attenuation efficiency with variation in the thickness of the ceramics. As a result, the obtained LAC value data can be used in comparison with other literature data. The obtained LAC values are in good agreement with the related values for telluride glasses with partial replacement of tellurium dioxide with zinc oxide [24], as obtained by sintering with a thickness of 1.5 mm. In this case, an increase in density for the synthesized ceramics leads to a 2–2.5-fold excess of the LAC value for telluride glasses, which indicates a higher attenuation and screening efficiency. Additionally, the obtained results of the LAC value for the synthesized ceramics have good convergence in the obtained values with the results of [23]. The results of a comparative analysis showed that the ceramics obtained at 900–1100 °C have a higher shielding efficiency than the Bi₂O₃–CaO–K₂O–Na₂O–P₂O₅-based glasses proposed in the work, which are also considered as one of the alternative materials to telluride and borate glasses for protection against ionizing radiation.

4. Conclusions

The paper presents the results of studying the processes of phase formation in 0.25ZnO–0.25Al₂O₃–0.25WO₃–0.25Bi₂O₃ ceramics, obtained by mechanochemical grinding and subsequent thermal annealing at different temperatures, the change in which, as it was established, initiates the processes of phase transformations with subsequent compaction of ceramics and the formation of interstitial solid solutions. Using X-ray phase analysis methods, it was found that the variation in annealing temperatures leads to the formation of ceramics containing Bi₂WO₆/ZnWO₄/ZnAl₂O₄ or ZnAl₂O₄/Bi₂WO₆/Bi₂W₂O₉ phases, and their appearance leads to an increase in density from 6.3 to 8.05–8.10 g/cm³. Moreover, an increase in the annealing temperature above 900 °C leads to a change in the shape of the grains and the formation of solid solutions of the introduction of ZnAl₂O₄ grains into the main matrix consisting of Bi₂WO₆ and Bi₂W₂O₉ phases. According to the evaluation of the strength properties, it was found that a change in the phase composition, and therefore, an increase in density, leads to the strengthening of ceramics, as well as an increase in their resistance to cracking under external pressures. An evaluation of the shielding characteristics showed that the variation in the thickness of the ceramics most effectively affects the shielding of high-energy gamma rays, which are characterized not only by an increase in the projective path length in the substance, but also by the possibility of the formation of electron-positron pairs that can cause secondary ionization effects. At the same time, an increase in the density of ceramics leads to a three- to five-fold increase in the weakening efficiency with a variation in the thickness of the ceramics.

According to the data obtained for evaluating the shielding of gamma radiation, the synthesized vitreous ceramics have a higher absorbing shielding capacity than analogous structures based on compounds of tellurium [16,17], bismuth [9], tungsten [26], boron [14], and lead [9] oxides. The higher shielding performance is due to density effects as well as absorption capacity change. At the same time, the ceramics presented in this study, in contrast to telluride or borosilicate glasses, have a higher crack resistance, as well as mechanical strength, which makes it possible to use them under conditions of high mechanical loads.

Further work in this direction will be aimed at studying the radiation resistance of ceramics during long-term irradiation, since it is known that ionization processes caused by irradiation can have a negative impact on the stability and safety of ceramics.