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Article

Study of Gas Swelling Processes under Irradiation with Protons and He2+ Ions in Li4SiO4–Li2TiO3 Ceramics

by
Inesh E. Kenzhina
1,2,3,4,*,
Artem L. Kozlovskiy
1,3,
Yevgen Chikhray
1,2,5,
Timur Kulsartov
2,5,
Zhanna Zaurbekova
2,5,
Meiram Begentayev
1 and
Saulet Askerbekov
2,5
1
Department of General Physics, Satbayev University, Almaty 050032, Kazakhstan
2
Institute of Applied Sciences and Information Technologies, Almaty 050032, Kazakhstan
3
Laboratory of Solid State Physics, The Institute of Nuclear Physics, Almaty 050032, Kazakhstan
4
Advanced Electronics Development Laboratory, Kazakh-British Technical University, 59 Tole bi St., Almaty 050000, Kazakhstan
5
Research Institute of Experimental and Theoretical Physics, Al-Farabi Kazakh National University, Almaty 050032, Kazakhstan
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(10), 1526; https://doi.org/10.3390/cryst13101526
Submission received: 28 September 2023 / Revised: 13 October 2023 / Accepted: 19 October 2023 / Published: 22 October 2023
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Abstract

:
One of the important areas of research in the energy sector is the study of the prospects for using new types of nuclear fuel, including tritium, which is one of the most promising types of fuel for thermonuclear energy. At the same time, for the production of tritium in the required quantities, the one that is the most optimal is the use of blanket materials based on lithium-containing ceramics. This is where tritium is released from lithium under the influence of neutron irradiation. The paper presents the results of an investigation of the influence of two-phase ceramics based on Li4SiO4–Li2TiO3 compounds on the resistance to external influences (mechanical loads) during the accumulation of hydrogen and helium (He2+) in the near-surface layer. The interest in such studies is primarily related to the search for solutions in the field of creating high-strength materials for tritium generation for its further use as nuclear fuel for thermonuclear fusion, as well as to the study of the mechanisms of the influence of different phases on the changes in the strength properties of ceramics, which provides an opportunity to expand fundamental knowledge in this area. The proposed method of obtaining two-phase ceramics by mechanical-chemical mixing and subsequent sintering into spherical particles enables the production of well-structured, high-strength ceramics of specified geometric dimensions (limited only by the dimensions of the mold) with a controlled phase ratio. During the experiments, it was found that increasing the content of Li4SiO4 phase in ceramics leads to an increase in strength characteristics (hardness, resistance to cracking) by 15–20% compared to single-phase ceramics. The most optimal composition of two-phase ceramics with high resistance to destructive embrittlement is the ratio of phases 0.75Li4SiO4–0.25Li2TiO3. One of the factors explaining the increase in resistance to destructive embrittlement under high-dose irradiation for two-phase ceramics is the increased dislocation density and the presence of interphase or intergranular boundaries, the high concentration of which leads to the creation of additional obstacles to the agglomeration of hydrogen and helium in the near-surface layer.

1. Introduction

During the operation of blanket materials used for tritium production, these ceramics are subjected not only to the accumulation of radiation damage characteristic of helium and hydrogen agglomeration (which are products of nuclear reactions of neutrons with lithium) but also to high-temperature corrosion [1,2,3]. Moreover, degradation processes can be accelerated as a result of the accumulation of radiation damage as well as structural distortions associated with the deformation stresses of the ceramic lattice [4,5,6]. An important role in accelerating degradation is played not only by accumulation effects but also by the type of radiation exposure and the phase composition of ceramics. Variation of the phase composition of ceramics by combining two phases or by the appearance of impurity inclusions in the form of simple oxide compounds in the composition of ceramics, as shown in a number of works [6,7,8], leads to a significant increase in resistance to external influences as well as increased resistance to degradation during operation. An essential role in this is also played by size effects, characterized by the fact that at small grain sizes, there is the formation of a large number of boundary effects, which in turn prevent the diffusion of nuclear reaction products in the structure, thereby preventing them from agglomerating in the cavities and pores of the structure [9,10,11].
So, for example, size effects are quite well expressed in alloys, in particular high-entropy alloys, in which a decrease in grain size leads to strengthening due to dislocation as well as inter-boundary effects [12,13,14]. The use of layered structures, in particular carbide or nitride coatings, which are based on mechanisms for creating barrier effects, also helps to increase resistance to external influences, including ionizing radiation and mechanical stress [15,16]. An important factor in studying the mechanisms of radiation damage, in addition to dimensional factors, is the composition of structural materials that are used in nuclear physics installations, including for protection from the negative effects of neutron radiation, as well as products of nuclear reactions associated with their accumulation in the material of blankets [17,18]. A number of works [17,18,19] have shown that when irradiated with neutrons in lithium-containing ceramics used as blanket materials (for tritium multiplication), not only an accumulation of structural distortions occurs but also a change in the phase composition associated with polymorphic or transformation phase transformations, which are inextricably linked with the accumulation of radiolysis products. A number of works [20,21,22] also noted the role of boundary effects, a change in the concentration of which leads to a change in the rate of accumulation of radiation damage in ceramics as well as multilayer structures. This strengthening and increase in stability is based on the assumption of a change in the number of grain boundaries, the high density of which leads to an increase in the probability of recombination of defects and a decrease in their ability to agglomerate [20,21].
In this case, the diffusive nature of the migration of products of nuclear reactions of lithium with neutrons (tritium, hydrogen, and helium) associated with their displacement to the surface of the blanket material leads to a strong deformation distortion of the near-surface layer in which fission product agglomeration in pores and cavities occurs [23,24,25]. In turn, the agglomeration of helium and hydrogen in pores can lead to additional structural distortions due to deformation stretching of pores, leading to swelling and destruction of the near-surface layer. During high-dose irradiation or long-term operation of ceramics under conditions of increased radiation exposure, which can lead to the accumulation of hydrogen or helium in the structure due to their low solubility and high mobility, bubbles or gas-filled cavities can form in the surface layer. In this case, factors such as irradiation temperature and the rate of agglomeration of implanted ions can lead to an increase in the size of these inclusions, which in turn will lead to additional structural distortions and deformations and, under certain conditions, to the rupture of bubbles or peeling of the deformed surface.
The accumulation of these structural distortions under the action of high temperatures can be accompanied by the acceleration of processes of destructive change of strength properties as well as a decrease in thermal conductivity due to deformation distortion of the structure [26,27]. In the case of high-temperature impact in the damaged layer, the formed helium and hydrogen can diffuse much faster, thereby increasing the volume of gas-filled cavities, the opening of which occurs by explosive processes accompanied by the destruction of the near-surface layer, as well as increasing the concentration of microcracks and chips. In this case, the presence of impurity phases in the composition of ceramics or size effects can have a significant impact on the strengthening of blanket materials under high-dose irradiation. For example, in [28], the authors showed that the transition to nanosized grains in lithium-containing Li4TiO4 ceramics leads to an increase in resistance to softening as well as the creation of additional obstacles to migration processes associated with the accumulation of helium and hydrogen.
The purpose of this work is to study the influence of component variation in Li4SiO4–Li2TiO3 ceramics on the resistance to gas swelling under high-temperature and high-dose irradiation with protons and helium, simulating the degradation processes associated with the formation and agglomeration of decay products of nuclear reactions of neutrons with lithium, which are typical for the fuel cycle in tritium production. The main hypothesis of this work is to determine the influence of the presence of two phases in different ratios on the resistance to degradation of the strength properties of ceramics, as well as to establish the relationship between the presence of interphase boundaries (i.e., the presence of two phases in ceramics) and structural strengthening. Li4SiO4 and Li2TiO3 were chosen as the main components for creating two-phase lithium-containing ceramics, the combination of whose properties made it possible to obtain high-strength ceramics with increased resistance to external influences as well as resistance to radiation-induced degradation caused by the accumulation of helium and hydrogen in the surface layer.

2. Materials and Methods

Samples for research to test the hypothesis were prepared using the method of mechanical-chemical mixing with the subsequent pressing of samples into spheres with a diameter of about 1.0–1.5 mm. The mechanical-chemical stirring was carried out in a planetary mill, PULVERISETTE 6 (Fritsch, Berlin, Germany). The mixing was carried out at a speed of 250 rpm for 1 h in order to obtain a homogeneous mixture of ceramics, which was subsequently used to obtain tablets for further studies, including modeling of gas swelling processes under high-dose irradiation. Powders of lithium orthosilicate (Li4SiO4) and lithium titanate (Li2TiO3) were used as starting components for ceramics preparation. The chemical purity of the powders used was 99.95%, and the powders were purchased from Sigma Aldrich (Sigma, St. Louis, MO, USA). The ratio of component x to obtain xLi4SiO4–(1−x)Li2TiO3 ceramics was varied from 0 to 1 M in steps of 0.25 M. After mixing, the obtained mixtures were pressed into tablets using a special press mold, which allowed obtaining spheres of specified geometric dimensions. After pressing, the samples were annealed in a muffle furnace for 24 h at 400 °C in order to remove deformation and structural distortions associated with mechanical-chemical mixing and pressing processes. Using the standard Archimedes technique, the density of the ceramics was determined as a function of compositional variation as well as porosity, knowledge of which is necessary in the evaluation of swelling and degradation processes. Table 1 shows the results of the density and porosity of xLi4SiO4–(1−x)Li2TiO3 ceramics depending on the variation of the component ratio.
The morphology of the obtained ceramics, depending on the variation of the component ratio, was analyzed using scanning electron microscopy, which was used to obtain a series of images of samples at different enlargements. These images were obtained using a Hitachi TM3030 scanning electron microscope (Hitachi, Tokyo, Japan). Macroscale images of the studied spheres were also obtained to evaluate the influence of external influences, including radiation damage and its induced consequences for the accumulation of structural damage.
The method of X-ray diffraction was used to determine the phase composition of the studied ceramics as well as to establish the dependence of structural parameters depending on the changes in the phase composition. This method was realized on the X-ray diffractometer D8 Advance ECO (Bruker, Berlin, Germany). X-ray diffractograms were taken using the Bragg-Brentano method in the geometry 2θ = 15–100°, with a step of 0.03°. The analysis of the phase composition was determined using an estimation of the weight contributions of each phase, established by approximating the obtained diffractograms, followed by the determination of the weight contribution of all established reflexes for each phase.
The measurement of the strength characteristics of the ceramics under study, as well as the calculation of the hardening factor for variations in the ratio of components and the influence of the dimensional factor on the increase in strength characteristics, were carried out using two methods. The first method consisted of determining the resistance of ceramics to mechanical effects when an external force was applied to the sample (squeezing the sample) at a constant rate of loading until the stage of its cracking. This method provides an indication of the resistance to external loads as well as the ability of ceramics to withstand a certain amount of mechanical pressure. For such measurements, a special mechanical machine, LFM-1 (Walter + Bai AG, Löhningen, Switzerland), was used, equipped with holders for compressing the samples and monitoring the cracking processes with a high-resolution extensometer. The second method of measuring the strength characteristics consisted of indenting the ceramic samples at a constant load on the indenter in order to obtain an indenter imprint in the form of a diamond-shaped pyramid, the analysis of the diagonals of which allows us to determine the surface hardness of the ceramic samples. A LECO LM700 microhardness tester (LECO, Tokyo, Japan) was used for these measurements. A Vickers diamond pyramid was used as an indenter; the indenter load was 100 N and was selected for the purpose of measuring the imprints of the damaged layer in ceramics. On the basis of the obtained data, the values of hardening factors (change in hardness at a variation of the phase ratio) and increase in resistance to cracking (change in the values of the maximum pressure that the ceramic withstands when it is compressed at a constant rate) were determined. Also, a comparative analysis of changes in strength characteristics depending on the type of external influences (helium irradiation and protons simulating the effect of hydrogen accumulation) allowed us to establish the kinetics of degradation of the strength properties of the damaged near-surface layer with a thickness of about 1 micron.
The modeling of radiation damage processes in the near-surface and the kinetics of their accumulation in the damaged layer with a thickness of about 1 μm was carried out by irradiating the samples with protons (500 keV) and He2+ ions (40 keV). The irradiation fluence was from 1015 to 5 × 1017 ions/cm2. The choice of this range is due to the possibility of accumulation of radiation-induced damage in the near-surface layer of ceramics with sufficiently high defect concentrations (of the order of 1–20 sleep, according to the recalculation of the irradiation fluence in the value of atomic displacements). The maximum depth of the damaged layer in ceramics under irradiation with protons with a given energy is of the order of 1.0–1.5 μm, and in the case of irradiation with He2+ ions, it was of the order of 300–500 nm. The choice of He2+ ions is due to the possibility of modeling the effects of helium implantation in the near-surface layer with their subsequent agglomeration and deformation associated with the occurrence of microcracks and an increase in the number of gas-filled inclusions formed. The choice of protons with an energy of 500 keV is due to the possibility of modeling the effects of hydrogenation of a near-surface layer under high-dose irradiation, which is accompanied by deformation of structural properties and deterioration of strength characteristics.

3. Results and Discussion

3.1. Characterization of Initial Samples Obtained Using Mechanical-Chemical Mixing and Subsequent Thermal Annealing

Figure 1 shows the results of the obtained data on the morphological features of synthesized ceramics, reflecting the influence of variation in the number of components in the composition of ceramics on the structure and geometrical sizes of grains, as well as their packing density. The analysis of extreme cases (one-component Li2TiO3 and Li4SiO4 ceramics) reflects large differences in the shape of grains and their sizes. So for Li4SiO4 ceramics, the grains are rather small and homogeneous in size, the average size of which is about 50–70 nm, while Li2TiO3 ceramics are characterized by large cubic or rhombic grains, the size of which is about 700–900 nm, and in some cases more than 1 μm. Consequently, we can conclude that the nature of grain formation for these types of ceramics during thermal sintering is different: in the case of Li2TiO3 ceramics during thermal annealing, grain enlargement occurs with the formation of a fairly dense packing of grains with well-defined grain boundaries, creating a pore space. In the case of Li4SiO4 ceramics, the fine-grained packing structure is a characteristic of the so-called nanostructural type of ceramics, with the formation of a large number of boundary effects and agglomerates of small grains. At variation of concentration of components in the composition of Li4SiO4–Li2TiO3 ceramics in the case when Li4SiO4 is 0.25 and 0.5 M, a slight decrease of grains characteristic for Li2TiO3 with small filling of interstitial space with small grains characteristic for Li4SiO4 is observed (this effect is most manifested for ceramics with an equal ratio of components). Similar effects related to the change of morphological features at variation of the component ratio in two-phase ceramics based on Li2TiO3–Li2ZrO3 compounds were also established [29]. In the case when the Li4SiO4 phase prevails in the composition of ceramics, the composition is dominated by small-sized grains, completely hiding the Li2TiO3 grains. In summary, having analyzed the data on the morphological features of the obtained grains, we can conclude that the change in the ratio of Li2TiO3/Li4SiO4 components leads to the formation of ceramics with different geometric grain sizes, which can play an important role in determining the strength characteristics, as it was shown in [30,31] that the transition to the fine-grained fraction of ceramics has a significant effect on the resistance to radiation damage during the accumulation of nuclear reaction products and also increases the stability of ceramics to external influences.
Figure 2 shows the results of X-ray phase analysis of the obtained samples of lithium-containing Li4SiO4–Li2TiO3 ceramics depending on the variation of initial components during preparation and further mixing to obtain two-component ceramics with different phase content. The general view of the obtained diffractograms depending on the variation of the ratio of ceramic components indicates the formation of highly ordered structures characterized by high indices of the degree of structural ordering (more than 90%), which is evidenced by the shape of diffraction reflections as well as the ratio of intensities to the background radiation. However, the general analysis of changes in the diffractograms depending on the variation of the ratio of components in comparison with the data obtained for single-component ceramics indicates the absence of any occurrence of phase transformations associated with the formation of complex oxides or solid solution-type structures. The obtained diffractograms of samples with different variations of components are typical for two-phase ceramics containing both observed phases (monoclinic Li2TiO3 (PDF-01-077-8280) and monoclinic Li4SiO4 (PDF-01-070-2340)) in different weight proportions. In the case of one-component ceramics Li4SiO4 or Li2TiO3, the presence of any impurity phases in the form of pure lithium, silicon, or titanium oxides was not found, which indicates that the selected conditions of synthesis (thermal sintering at a given temperature) do not lead to the decomposition of the initial compounds but only to their structural ordering, as evidenced by the shape of diffraction reflections as well as the established values of the degree of structural ordering (more than 90%). It should also be noted that the selected conditions of mechanical chemical mixing and subsequent thermal annealing do not lead to the initialization of phase transformation processes for two-component ceramics associated with the formation of impurity inclusions, and the main observed changes for two-component ceramics are associated with changes in the ratio of contributions of each identified phase as well as its structural ordering. In the case of the Li2TiO3 phase, the observed broadening of the reflections with an increasing contribution of Li4SiO4 in the composition of the samples correlates with the data on morphological features, according to which a decrease in grain size was observed with changing the ratio of Li2TiO3/Li4SiO4 components.
Figure 3 shows the phase composition distribution diagrams (i.e., the ratio of the installed phases) of the xLi4SiO4–(1−x)Li2TiO3 ceramics samples as a function of the variation of the component ratio during synthesis. The weight contribution of each phase was determined by evaluating the weight ratio of the diffraction reflections observed for each established phase, followed by calculating their ratio. The overall analysis of the calculations performed showed a good agreement between the obtained data on the weight contributions of each phase and the selected component ratios in the fabrication of two-phase ceramics.
Table 2 shows the results of the estimation of structural parameters for the established phases in ceramics depending on the variation of their ratio used to obtain ceramics with different phase compositions. The general analysis of the obtained parameters of the crystal lattice depending on the ratio of components in the ceramics indicates an insignificant variation of the parameters, the change of which is due to the effects of structural ordering in the process of thermal annealing.
Figure 4 shows the results of changes in the strength characteristics of the investigated xLi4SiO4–(1−x)Li2TiO3 ceramics at variation of the component ratio. These changes reflect the hardening effect associated with changes in grain sizes and interphase boundaries arising from changes in the ratio of phases in ceramic composition. As can be seen from the presented data comparing hardness values and resistance to single compression (i.e., the maximum amount of pressure the ceramics can withstand when compressed at a constant rate of 5 mm/min), Li4SiO4 ceramics have higher strength characteristics than Li2TiO3 ceramics in the case of single-component ceramics. This difference can be explained by both dimensional factors (for Li2TiO3, the grain sizes are much larger than for Li4SiO4 ceramics, which leads to a lower dislocation density) and structural features of the ceramics (Li4SiO4 ceramics, according to a number of studies have higher strength values [28,31,32,33]). When varying the ratio of Li2TiO3/Li4SiO4 components, it was found that an increase in the Li4SiO4 content leads to an increase in hardness and maximum load, the maximum of which is reached in the case of 0.75Li4SiO4–0.25Li2TiO3 ceramics, for which the increase in hardness in comparison with single-component ceramics amounted to more than 19% and the maximum load more than 15%. This behavior of change in strength characteristics for two-component ceramics can be explained not only by changes in the phase composition (increase in the contribution of the Li4SiO4 phase, which has higher hardness values) but also by dimensional factors, which appear quite strongly when the Li4SiO4 phase dominates. At small grain sizes, according to [28], the so-called dislocation hardening effect can be observed, which directly depends on the size and shape of grains, the reduction of which leads to the creation of a large number of grain boundaries, which subsequently prevent external influences under mechanical loads, thereby increasing stability and increasing resistance to external influences and cracking.
Based on these changes in strength characteristics as well as changes in morphological features, the hardening factors (change in hardness and increase in resistance to cracking) and dislocation density, the change of which also contributes to the increase in resistance to external influences, were calculated. These values are presented in Table 3.
As can be seen from the presented data, the increase in Li4SiO4 for two-component ceramics leads to an increase in resistance to cracking under mechanical external influence. This can also be explained by the factor of increasing dislocation density, which, according to the calculated data, also increases. However, in the case of Li4SiO4 ceramics, a large value of dislocation density (more than 2–3 orders of magnitude higher) leads to the so-called dislocation supersaturation effect, which leads to a decrease in strength characteristics. The analysis of the obtained hardening factors indicates that the increase in strength characteristics depends not only on dislocation density and related dimensional effects but also on the phase composition of ceramics. The increase in the contribution of Li4SiO4, as can be seen from the obtained values, leads to a fairly high increase in strength.

3.2. Study of the Influence of Hardening Factors at Variation of Composition of xLi4SiO4–(1−x)Li2TiO3 Ceramics on Radiation-Induced Swelling at Helium and Hydrogen Accumulation in the Near-Surface Layer

When lithium-containing ceramics are used as materials for tritium generation, during their production in the process of lithium-neutron interaction, as it is known, helium and hydrogen are released, the accumulation of which in the structure can have a negative impact on the mechanical properties and resistance of ceramics to external influences. Such influence, expressed in partial embrittlement and degradation of the near-surface layer, can lead to the destruction of the near-surface layer of ceramics, and the formed delaminated particles, when moving into the core or plasma, can lead to its contamination. In this regard, the study of ceramics’ resistance to the accumulation of radiation-induced damage and its influence on the change in strength properties is one of the key tasks in assessing the prospects of lithium-containing ceramics as blanket materials. The most effective way of modeling the accumulation of radiation damage in the near-surface layer is high-dose irradiation with protons (simulating the effect of hydrogen) and He2+ ions. The use of these two types of irradiation can help to evaluate the resistance of ceramics to the accumulation of radiation damage as well as establish the dependence of changes in strength parameters with increasing irradiation dose (with the accumulation of atomic displacements). In this case, the main changes will be observed in the small near-surface layer; deformation due to the accumulation of radiation damage will affect the changes in hardness and crack resistance.
Figure 5 shows the results of changes in the magnitude of the maximum pressure under single compression, reflecting the changes in the resistance of ceramics to cracking under mechanical loading.
Figure 6 shows the results of changes in the hardness value of ceramics measured using the indentation method as a function of irradiation fluence. These changes reflect the kinetics of the hardness degradation of ceramics as a result of the accumulation of radiation damage in the near-surface layer. In fact, in certain cases, irradiation may cause the so-called radiation-induced hardening effect, which is associated with a change in dislocation density as well as the accumulation of structural distortions in the near-surface layer. These changes can lead to the creation of additional barriers to the propagation of microcracks and chips under external influences, which in turn leads to the effect of increasing hardness and crack resistance. In the case of the experiments carried out, the main changes in strength parameters are observed at fluences above 1016 ion/cm2, and they are associated with a decrease in strength characteristics. This change is due to the formation of structurally distorted areas in the damaged layer, which, with high-dose irradiation, can lead to partial amorphization and a sharp deterioration in strength parameters.
The general analysis of changes in the values of resistance to single compression and hardness indicates an accumulative effect of structural distortions, which have a negative impact on the change in strength characteristics. In this case, in the case of proton irradiation, the most significant changes in the strength characteristics expressed as a decrease in the values are observed at fluence above 5 × 1016 protons/cm2, while at irradiation with He2+ ions, the changes occur at fluence 1016 ions/cm2. The explanation for such a change can be the difference in energy losses as well as the size of the colliding particles, which, in interaction with ceramics, lead to the formation of point and vacancy defects. Also, due to their nature and weak solubility, hydrogen and helium are able to agglomerate in the structure due to implantation at high fluences of irradiation, which in turn leads to the formation of agglomerates in the pore space in ceramic structures. In this case, lower porosity values of biphasic ceramics as well as higher dislocation density contribute to the creation of additional obstacles for the agglomeration of implanted hydrogen and helium, thereby reducing the number of these gas-filled inclusions. At fluences above 1017 cm−2 in the case of irradiation with protons and helium, a sharp deterioration of strength characteristics is observed, which indicates destructive deterioration of the near-surface layer associated with agglomeration and subsequent deformation distortion due to the formation and increase in size of gas-filled cavities. Such phenomena are fairly well known, and a swelling threshold above 1017 cm−2 has been previously established for various types of ceramic materials [34,35,36].
Figure 7 shows a general analysis of changes in the strength characteristics depending on the type of external influences (under irradiation with protons and He2+ ions), reflecting the deterioration of ceramic properties during the accumulation of radiation damage in the near-surface layer. These dependencies were obtained by a comparative analysis of the values of the maximum pressure that can be withstood by ceramics under loading, as well as changes in hardness, with the data obtained for the original samples not subjected to irradiation.
The general trend of changes in strength characteristics, as mentioned above, indicates the accumulative nature of structural damage, but the transition from single-phase ceramics to two-phase ceramics leads to a change in the trends of deterioration of strength characteristics, both in the case of irradiation with protons and irradiation with He2+ ions. In general, it can be noted that the transition to two-phase ceramics with increasing concentrations of Li4SiO4 in the ceramic composition leads to an increase in resistance by 1.5–2.5 times compared to single-phase ceramics, with the most pronounced changes occurring at fluences above 1016 cm−2. In this case, the presence of interphase boundaries as well as the high packing density of smaller grains leads to an increase in resistance to degradation during the accumulation of implanted hydrogen and helium, which is expressed in small changes in the values of de-strengthening and resistance to cracking. In addition, when irradiated with He2+ ions, the changes in strength characteristics are practically 1.5–2 times higher than the similar changes in values when irradiated with protons, which indicates a more destructive ability of He2+, the accumulation of which leads to swelling and embrittlement, which is accompanied by the explosive opening of gas-filled bubbles [37,38].
Figure 8 shows, as an example, the effects of irradiation with protons (simulating hydrogen accumulation) and helium at a maximum irradiation fluence of 5 × 1017 ions/cm2 for two types of ceramics: Li2TiO3 and 0.75Li4SiO4–0.25Li2TiO3. This showed maximum (indicating the destructive effect of ionizing radiation on the near-surface layer) and minimum (indicating high stability) changes in the properties of ceramics during the studies.
As can be seen from the presented data, the irradiation of Li2TiO3 ceramics with protons leads to destructive surface changes with the formation of microcracks associated with oversaturation with structural distortions of the near-surface layer, as well as partial peeling of the damaged layer, which can lead to contamination of the core. An analysis of the peelings indicates the accumulation of hydrogen in the near-surface layer in certain places, with subsequent destruction of the layer with a thickness of about 0.3–0.5 μm, which corresponds to the depth of hydrogen penetration into the ceramic structure. For Li2TiO3 ceramics, He2+ irradiation with the maximum irradiation fluence (5 × 1017 ions/cm2) leads to more pronounced destruction of the near-surface layer, accompanied not only by delamination of the damaged layer but also by partial destruction of the surface as a result of the explosive evolution of gas-filled regions.
In the case of two-phase 0.75Li4SiO4–0.25Li2TiO3 ceramics, which showed maximum resistance to the accumulation of radiation-induced defects and little change in strength parameters, under proton irradiation, the main structural changes in the near-surface layer are associated with the formation of convex inclusions. This indicates the formation of gas-filled bubbles in the near-surface layer. This behavior indicates a higher resistance of two-phase 0.75Li4SiO4–0.25Li2TiO3 ceramics to swelling and degradation, which is due to the presence of interphase boundaries as well as high dislocation density, the presence of which prevents destructive embrittlement of the near-surface layer, which is observed for single-phase ceramics. In the case of irradiation by He2+ ions with the maximum irradiation fluence (5 × 1017 ions/cm2), partial delamination of the damaged layer is observed as a result of the explosive mechanism of opening of gas-filled regions in the near-surface layer. This has a good agreement with the classical theory of gas swelling proposed by Evans in a number of works [39,40,41]. It should be noted that two-phase ceramics are more resistant to degradation than single-phase ceramics, both in the case of proton irradiation, for which the maximum irradiation fluence leads to the nucleation of gas-filled regions, and for He2+ irradiation, under which irradiation-less damaged regions are observed for two-phase ceramics than for single-phase ceramics.
Hence, analyzing the established changes in strength characteristics as well as morphological features, we can conclude that the presence of two phases in lithium-containing ceramics not only increases the strength characteristics due to small grains and denser packing to reduce the number of pores in the structure but also has a significant impact on increasing the resistance to radiation-induced embrittlement and destruction of the near-surface damaged layer.

4. Conclusions

The proposed method of producing lithium-containing ceramics based on the mechanical-chemical mixing of the initial components Li4SiO4 and Li2TiO3 in a given stoichiometric ratio allows for the production of two-component ceramics with increased strength and resistance to external influences, including gas swelling at the accumulation of helium and hydrogen in the near-surface layer. In the conducted studies related to the application of the method of two-phase ceramic fabrication to increase their resistance to degradation under proton and helium irradiation, it was found that the optimal composition with the highest resistance to degradation is ceramics of the 0.75Li4SiO4–0.25Li2TiO3 type. The analysis of changes in strength parameters during the experiments showed that the accumulative effect of radiation damage during irradiation with protons and He2+ ions is most evident at fluences higher than 1017 cm−2, at which a sharp deterioration of resistance to external mechanical effects is observed, and gas-filled bubbles are formed in the near-surface layer, the opening of which occurs at the maximum irradiation fluence (5 × 1017 cm−2).
The practical significance of the presented results lies in the proposed methodology for creating two-component lithium-containing ceramics, the use of which is one of the solutions in the field of tritium production for thermonuclear energy. Moreover, in contrast to the known methods for producing two-phase ceramics, the proposed method of mechanochemical grinding of Li4SiO4 and Li2TiO3 compounds with different stoichiometric compositions makes it possible to obtain two-phase ceramics with high strength and resistance to degradation during hydrogenation and helium swelling. In general, the results obtained, in addition to having practical significance, also have theoretical significance, which consists of obtaining new data on changes in the properties of ceramics under radiation exposure. At the same time, in the future, these samples will be studied for tritium release during their neutron irradiation, as well as for the study of sorption and desorption processes.

Author Contributions

Conceptualization, I.E.K., A.L.K., Y.C., T.K., Z.Z., M.B. and S.A.; methodology, I.E.K., A.L.K., Y.C., T.K., Z.Z., M.B. and S.A.; formal analysis, I.E.K., A.L.K., Y.C., T.K., Z.Z., M.B. and S.A.; investigation, I.E.K., A.L.K., Y.C., T.K., Z.Z., M.B. and S.A.; resources, I.E.K., A.L.K., Y.C., T.K., Z.Z., M.B. and S.A.; writing—original draft preparation, review, and editing, I.E.K. and A.L.K.; visualization, I.E.K. and A.L.K.; supervision, I.E.K. and A.L.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan grant number AP19679905.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Results of changes in the surface morphology of ceramics as a function of the concentration of components in the composition of: (a) Li2TiO3; (b) 0.25Li4SiO4–0.75Li2TiO3; (c) 0.5Li4SiO4–0.5Li2TiO3; (d) 0.75Li4SiO4–0.25Li2TiO3; (e) Li4SiO4.
Figure 1. Results of changes in the surface morphology of ceramics as a function of the concentration of components in the composition of: (a) Li2TiO3; (b) 0.25Li4SiO4–0.75Li2TiO3; (c) 0.5Li4SiO4–0.5Li2TiO3; (d) 0.75Li4SiO4–0.25Li2TiO3; (e) Li4SiO4.
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Figure 2. Results of X-ray phase analysis of investigated Li4SiO4–Li2TiO3 ceramics as a function of variation in concentration of two components.
Figure 2. Results of X-ray phase analysis of investigated Li4SiO4–Li2TiO3 ceramics as a function of variation in concentration of two components.
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Figure 3. Phase analysis results of the investigated xLi4SiO4–(1−x)Li2TiO3 ceramics: (a) Li2TiO3; (b) 0.25Li4SiO4–0.75Li2TiO3; (c) 0.5Li4SiO4–0. 5Li2TiO3; (d) 0.75Li4SiO4–0.25Li2TiO3; (e) Li4SiO4.
Figure 3. Phase analysis results of the investigated xLi4SiO4–(1−x)Li2TiO3 ceramics: (a) Li2TiO3; (b) 0.25Li4SiO4–0.75Li2TiO3; (c) 0.5Li4SiO4–0. 5Li2TiO3; (d) 0.75Li4SiO4–0.25Li2TiO3; (e) Li4SiO4.
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Figure 4. Results of the variation of strength characteristics as a function of the variation of component concentration in the composition of xLi4SiO4–(1−x)Li2TiO3 ceramics.
Figure 4. Results of the variation of strength characteristics as a function of the variation of component concentration in the composition of xLi4SiO4–(1−x)Li2TiO3 ceramics.
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Figure 5. Results of measurements of the value of resistance to single compression to determine cracking as a function of irradiation: (a) under proton irradiation; (b) under He2+ irradiation.
Figure 5. Results of measurements of the value of resistance to single compression to determine cracking as a function of irradiation: (a) under proton irradiation; (b) under He2+ irradiation.
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Figure 6. Results of hardness measurements of ceramic samples depending on irradiation: (a) under proton irradiation; (b) under He2+ irradiation.
Figure 6. Results of hardness measurements of ceramic samples depending on irradiation: (a) under proton irradiation; (b) under He2+ irradiation.
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Figure 7. Results of comparative analysis of changes in strength characteristics depending on irradiation fluence: dependence of a decrease in resistance to cracking under single compression: (a) under proton irradiation; (b) under He2+ irradiation; dependence of change in the value of unstrengthening during accumulation of structural damage; (c) under proton irradiation; (d) under He2+ irradiation.
Figure 7. Results of comparative analysis of changes in strength characteristics depending on irradiation fluence: dependence of a decrease in resistance to cracking under single compression: (a) under proton irradiation; (b) under He2+ irradiation; dependence of change in the value of unstrengthening during accumulation of structural damage; (c) under proton irradiation; (d) under He2+ irradiation.
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Figure 8. Examples of ceramic surface degradation after radiation exposure in the case of proton irradiation with a fluence of 5 × 1017 ion/cm2: (a) Li2TiO3 ceramics; (b) 0.75Li4SiO4–0.25Li2TiO3 ceramics; and helium irradiation with a fluence of 5 × 1017 ion/cm2: (c) Li2TiO3 ceramics; (d) 0.75Li4SiO4–0.25Li2TiO3 ceramics.
Figure 8. Examples of ceramic surface degradation after radiation exposure in the case of proton irradiation with a fluence of 5 × 1017 ion/cm2: (a) Li2TiO3 ceramics; (b) 0.75Li4SiO4–0.25Li2TiO3 ceramics; and helium irradiation with a fluence of 5 × 1017 ion/cm2: (c) Li2TiO3 ceramics; (d) 0.75Li4SiO4–0.25Li2TiO3 ceramics.
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Table 1. Density and porosity data of ceramics.
Table 1. Density and porosity data of ceramics.
ParameterComponent Ratio of xLi4SiO4–(1−x)Li2TiO3 Ceramics
0 M0.25 M0.5 M0.75 M1 M
Density, g/cm33.33 ± 0.123.11 ± 0.142.85 ± 0.112.62 ± 0.092.32 ± 0.13
Porosity, %2.91 ± 0.061.92 ± 0.111.73 ± 0.130.72 ± 0.052.52 ± 0.14
Table 2. Data of the structural parameters of xLi4SiO4–(1−x)Li2TiO3 ceramics.
Table 2. Data of the structural parameters of xLi4SiO4–(1−x)Li2TiO3 ceramics.
ParameterComponent Ratio of xLi4SiO4–(1−x)Li2TiO3 Ceramics
0 M0.25 M0.5 M0.75 M1 M
Li2TiO3 *a = 5.0360 Å, a = 5.0251Å, a = 5.0507 Å, a = 5.0401 Å,
b = 8.8008 Å, b = 8.7750 Å, b = 8.7405 Å, b = 8.7835 Å,
c = 9.7137 Å, c = 9.7193 Å, c = 9.6585 Å, c = 9.7060 Å,
β = 99.792°, β = 99.615°.β = 95.147°.β = 99.832°
Li4SiO4 **-a = 11.5345 Å, a = 11.5006 Å, a = 11.4872 Å, a = 11.4985 Å,
b = 6.0948 Å, b = 6.0947 Å, b = 6.0876 Å, b = 6.1007 Å,
c = 16.5567 Å, c = 16.5667 Å, c = 16.5537 Å, c = 16.58952 Å,
β = 98.567°β = 98.567°β = 98.993°β = 99.285°
Parameter determination was performed by comparing the position of diffraction lines and their corresponding values of interplanar distances with data for reference values from the PDF-2(2016) database. The following cards were selected as reference values: * PDF-01-077-8280–Li2TiO3 Monoclinic, Space Group C12/c1(15); ** PDF-01-070-2340–Li4SiO4 Monoclinic P21/m(11).
Table 3. The changes in the strength characteristics.
Table 3. The changes in the strength characteristics.
ParameterComponent Ratio of xLi4SiO4–(1−x)Li2TiO3 Ceramics
0 M0.25 M0.5 M0.75 M1 M
Hardening factor, % *-4.615.219.82.9
Crack resistance increase, % *-3.99.416.12.1
Dislocation density, 109 cm−20.00160.00220.00480.0570.164
* These factors were calculated by comparing these values for two-component ceramics and Li4SiO4 ceramics with the values of hardness and maximum load under a single compression obtained for Li2TiO3 ceramics possessing the minimum values of strength parameters for the studied systems.
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Kenzhina, I.E.; Kozlovskiy, A.L.; Chikhray, Y.; Kulsartov, T.; Zaurbekova, Z.; Begentayev, M.; Askerbekov, S. Study of Gas Swelling Processes under Irradiation with Protons and He2+ Ions in Li4SiO4–Li2TiO3 Ceramics. Crystals 2023, 13, 1526. https://doi.org/10.3390/cryst13101526

AMA Style

Kenzhina IE, Kozlovskiy AL, Chikhray Y, Kulsartov T, Zaurbekova Z, Begentayev M, Askerbekov S. Study of Gas Swelling Processes under Irradiation with Protons and He2+ Ions in Li4SiO4–Li2TiO3 Ceramics. Crystals. 2023; 13(10):1526. https://doi.org/10.3390/cryst13101526

Chicago/Turabian Style

Kenzhina, Inesh E., Artem L. Kozlovskiy, Yevgen Chikhray, Timur Kulsartov, Zhanna Zaurbekova, Meiram Begentayev, and Saulet Askerbekov. 2023. "Study of Gas Swelling Processes under Irradiation with Protons and He2+ Ions in Li4SiO4–Li2TiO3 Ceramics" Crystals 13, no. 10: 1526. https://doi.org/10.3390/cryst13101526

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