Effects of Composition Variations on Mechanochemically Synthesized Lithium Metazirconate-Based Ceramics and Their Resistance to External Influences

Abstract

The study examines the influence of variations in the compositions of components for the production of lithium-containing ceramics based on lithium metazirconate obtained by the method of mechanochemical grinding and subsequent thermal sintering. For component variation, two compositions were used, consisting of zirconium dioxide (ZrO₂) and two distinct types of lithium-containing materials: lithium perchlorate (LiClO₄·3H₂O) and lithium carbonate (Li₂CO₃). Adjusting the concentration of these components allowed for the production of two-phase ceramics with varying levels of impurity phases. Using X-ray phase analysis methods, it was determined that the use of LiClO₄·3H₂O results in the formation of a monoclinic phase, Li₂ZrO₃, with impurity inclusions in the orthorhombic phase, LiO₂. On the other hand, when Li₂CO₃ is used, the resulting ceramics comprise a mixture of two phases, Li₂ZrO₃ and Li₆Zr₂O₇. During the studies, it was established that the formation of impurity inclusions in the composition of ceramics leads to an increase in the stability of strength properties with varying mechanical test conditions, as well as stabilization of thermophysical parameters and a decrease in thermal expansion during long-term high-temperature tests. It has been established that in the case of two-phase ceramics Li₂ZrO₃/Li₆Zr₂O₇ in which the dominance of the Li₆Zr₂O₇ phase is observed during high-temperature mechanical tests, a more pronounced decrease in resistance to cracking is observed, due to thermal expansion of the crystal lattice.

1. Introduction

The future progress of a country’s energy sector will be closely tied to the adoption of alternative energy sources, with nuclear, hydrogen, and thermonuclear energy emerging as the most promising for substantial energy generation [1,2,3]. The expansion of their role in the energy sector is contingent not only on worldwide efforts to diminish reliance on fossil fuels and reduce harmful emissions but also on the imperative to bolster energy production capacity in response to growing energy demands from the populace [4,5]. One of the ways to solve the problems in the energy sector is to create new types of nuclear reactors, including high-temperature reactors, in which ceramic materials are to be used as core and nuclear fuel, as well as thermonuclear reactors based on technological solutions related to the production of energy from tritium. At the same time, to produce tritium, which, along with hydrogen, is one of the key types of nuclear fuel for thermonuclear installations, it is proposed to use lithium ceramics. Interest in lithium ceramics for the production of tritium is primarily due to the possibility of nuclear reactions involving the interaction of neutrons with lithium, which results in the formation of tritium, which can be used to support thermonuclear reactions and fuel production. At the same time, interest in the development of ceramic materials containing lithium in this direction is primarily due to the possibility of creating sustainable materials for tritium multiplication (in order to maintain thermonuclear reactions), as well as those that are highly resistant to mechanical, thermal and radiation influences, which are integral factors accompanying the operation of ceramic materials.

A crucial aspect of ceramic production, or, in simpler terms, selecting a ceramic manufacturing method, involves examining the impact of manufacturing process variables and the specific procedures used in crafting ceramics [6,7,8]. The selection of these conditions plays a pivotal role in shaping the uniformity of the resultant ceramic compositions, along with a combination of physicochemical, structural, and thermophysical properties [9,10]. The methods currently available for obtaining lithium-containing ceramics can be categorized into several groups that share common technological processes. The most common method is the so-called melt method, combined with the sol-gel method, the use of which makes it possible to obtain spherical lithium-containing ceramics from compounds of lithium carbonate with zirconium, titanium or silicon oxides, depending on the type of ceramics that is planned to be obtained [11,12,13,14,15].

The aim of this work is to study the effect of component variations in the manufacture of lithium-containing ceramics based on lithium metazirconate on their phase composition, structural and strength characteristics, as well as stress resistance to temperature tests. The relevance of this work consists in the possibility of creating lithium-containing ceramics by mechanochemical synthesis with a controlled phase composition, which plays a very important role in determining the strength and thermophysical characteristics. Furthermore, the focus on ceramics based on lithium metazirconate primarily arises from its exceptional strength properties and its resistance to prolonged mechanical or radiation stress, along with its resistance to thermal expansion and degradation [16,17,18,19,20]. It is essential to emphasize that the objective of this study was not to produce single-phase ceramics but rather to precisely gauge the impact of impurity phases formed during mechanochemical mixing and subsequent thermal sintering on factors such as strength, thermophysical characteristics, thermal stability, and resistance to external influences [21,22,23]. This work is based on the hypothesis that the formation of impurity inclusions makes it possible to create additional barriers in ceramics for the propagation of microcracks under external influences, thereby increasing the strength of ceramics [24,25]. Moreover, since these inclusions also consist of the original components, this makes it possible to increase the concentration of lithium in the ceramic composition.

2. Materials and Research Methods

The synthesis of lithium-containing ceramics was carried out by using two types of compositions of the initial mixtures: (1) ZrO₂ and LiClO₄·3H₂O and (2) ZrO₂ and Li₂CO₃ with varying the molar ratio of the components from 0.25 to 0.75 M. In the future, these compositions will be designated as 1 and 2. The selection of these two compositions, featuring different components, stems from the potential to create lithium-containing ceramics with diverse phase compositions, strength, and thermophysical properties. Additionally, the choice of using LiClO₄·3H₂O and Li₂CO₃ as the lithium component was made to evaluate the feasibility of the mechanochemical synthesis method in producing stable ceramics with high lithium concentrations, which constitutes a crucial criterion when selecting ceramics for tritium production.

The primary approach employed to manufacture lithium-containing ceramics was the mechanochemical solid-phase synthesis method. This method entails mechanically blending the initial components within a PULVERISETTE 6 planetary mill (Fritsch, Berlin, Germany) for 5 h at a grinding speed of 300 rpm, with the aim of achieving a uniform particle size distribution. Subsequently, the resulting powders were compacted into 10 mm diameter and 1 mm thickness tablets, considering the requirements of the measuring instruments used to determine properties such as hardness, fracture resistance, and thermal conductivity. The tablets were pressed using a mold specially made for this purpose under a pressure of 250 MPa. After pressing, the resulting tablets were subjected to thermal annealing at a temperature of 900 °C in a muffle furnace SNOL 39/1100 (AB UMEGA-GROUP, Ukmergė, Lithuania). Annealing was carried out for 8 h in an air atmosphere, followed by cooling for 24 h with the furnace. The selection of these annealing conditions, encompassing temperature, duration, and cooling process, was informed by prior experimental investigations [26,27], alongside a priori knowledge concerning phase and structural transformations in lithium-containing ceramics derived through mechanochemical or solid-phase synthesis.

The analysis of structural parameters and phase composition in the examined ceramics, contingent on the type of initial component composition and its variations, was conducted through an assessment of the X-ray diffraction patterns. These patterns were captured in the Bragg-Brentano configuration (2θ = 25–100°) with an increment of 0.03°. A Cu—kα X-ray tube with a wavelength of 1.54 Å was used as an X-ray source. The diffraction data was collected using a D8 Advance ECO X-ray diffractometer (Bruker, Berlin, Germany). The interpretation of the acquired diffraction patterns was facilitated by the Diffrac EVA v.4.2 software (Bruker, Berlin, Germany), enabling the determination of structural parameters such as crystal lattice dimensions, volume, and the structural order degree, along with the calculation of phase contributions within the samples. The weight contributions of each identified phase within the samples were ascertained by calculating the areas of all diffraction reflections characteristic of a given phase and then determining their ratio relative to other established phases. The degree of structural ordering (degree of crystallinity) was determined as the ratio of the contributions of the intensities of all diffraction reflections to the area of background radiation recorded during the angular dependence of the change in the intensity of X-ray diffraction patterns.

The mechanical properties, including alterations in hardness and resistance to cracking during a single compression, were evaluated for the investigated ceramics with regard to variations in their phase composition using established methods. Hardness assessments were conducted by applying a load of 100 N on a Vickers diamond pyramid indenter for 15 s, followed by the evaluation of the resultant indenter imprint on the sample. These experiments were carried out using a Duroline M1 microhardness tester (Metkon, Bursa, Turkey). The assessments of the hardening factors were based on the data concerning changes in hardness, attributable to adjustments in component ratios, as well as the introduction of impurity phases within the structure.

A single compression, based on the data of which the dynamics of changes in cracking with increasing pressure on the sample was established, was carried out using a mechanical testing machine LFM-L 10 kN (Walter + Bai AG, Leuningen, Switzerland). At the same time, to establish the effect of crack resistance when varying the compression rate, a series of experiments were carried out on samples in which the samples were subjected to compression at different compression rates. The influence of thermal expansion on crack resistance was determined by conducting experiments on single compression of samples heated in a chamber at a temperature of 500 °C.

Thermal conductivity coefficient measurements were conducted in the temperature range of 25 to 700 °C using the longitudinal heat flow method. The KIT-800 instrument from Teplofon, Moscow, Russia, was employed for these measurements.

The coefficient of thermal expansion for the ceramic samples, influenced by prolonged thermal exposure during 500 h of heating at 700 °C, was determined by assessing changes in the crystal lattice volume before and after thermal exposure. Using Formula (1), alterations in material properties due to high-temperature degradation were calculated based on the acquired data.

$${\beta }_V\left(T\right)=\frac{1}{V_{initial}}\frac{∆V}{∆T},$$

(1)

where ∆V is the change in the volume of the crystal lattice before and after high-temperature heating, ∆T is the difference in measurement temperatures, and V_initial is the volume of the crystal lattice of the initial sample. The crystal lattice volume was determined by calculating the crystal lattice parameters for each test sample by analyzing the obtained X-ray diffraction patterns of the ceramic samples under study. Moreover, in the case of thermal tests, before they were carried out, the parameters and volume of the crystal lattice of each test sample were measured, after which, after thermal exposure, X-ray diffraction patterns were obtained, on the basis of which the parameters of the crystal lattice of the samples after thermal exposure were determined. Through comparative analysis, the values of changes in the volume of the crystal lattice were calculated, indicating changes in the properties of ceramics as a result of external influences.

3. Results and Discussion

In previous studies [28,29], it was demonstrated that modifications in the synthesis conditions, such as adjusting the proportions of the initial solution components, result in alterations in the ceramics’ phase composition. This, in turn, directly influences the material’s hardening effects and thermophysical properties. It is important to note that these studies explored the use of LiClO₄·3H₂O as a lithium-containing component. Changes in its concentration or variations in the thermal sintering conditions lead to the formation of impurity inclusions in the form of oxide phases. As indicated in [28], these inclusions enhance resistance to radiation-induced swelling during the accumulation of helium in the surface layer.

Figure 1 presents the X-ray diffraction results for the ceramic samples obtained from composition 1, with varying proportions of the components. These diffraction patterns reveal the impact of altering the initial component concentrations on the phase composition, specifically the changes in the relative amounts of the identified components within the samples. The X-ray diffraction data indicate that the primary phase in all three samples is the Li₂ZrO₃ (PDF-00-033-0843) monoclinic phase, and characteristic reflections of the LiO₂ (PDF-01-080-3472) orthorhombic phase are also detected. An analysis of the weight percentages of these phases, based on the determination of the weight ratio of the areas of diffraction reflections for each established phase, is detailed in Table 1. As per the provided data, an increase in the LiClO₄·3H₂O mixture composition results in a rise in the contribution of the LiO₂ impurity phase, increasing from 1.4 wt% to 5.7 and 7.3 wt%. Concurrently, the assessment of structural parameters, also depicted in Figure 2, signifies an improvement in the crystal lattice of the primary Li₂ZrO₃ phase, as evidenced by a reduction in the parameters and volume of the crystal lattice. Comparable alterations in structural parameters with variations in the component ratios are also observed for the LiO₂ impurity orthorhombic phase.

Analyzing the data obtained, we can conclude that when lithium perchlorate (LiClO₄·3H₂O) is used as a lithium-containing component, the main phase in the ceramics is the monoclinic phase Li₂ZrO₃. At the same time, at high concentrations of LiClO₄·3H₂O in the composition, the formation of the orthorhombic phase of LiO₂ in the form of inclusions is observed, the maximum content of which is no more than 7.3 wt%. Moreover, according to the data of works [28,29], the content of impurity inclusions, even in the case of varying thermal sintering conditions, does not exceed 10 wt%.

Therefore, upon examining the observed alterations in structural parameters, it can be deduced that as the concentration of LiClO₄·3H₂O increases, the ceramics become denser owing to the structural organization of the recognized phases. Furthermore, upon assessing the phase composition, an augmentation in the LiO₂ impurity phase within the ceramic composition was ascertained. This, in turn, can potentially impact modifications in the thermophysical and strength properties of the ceramics.

Figure 1. Results of X-ray phase analysis of the studied samples of lithium-containing ceramics depending on the variation in the concentration of the initial components.

Figure 1. Results of X-ray phase analysis of the studied samples of lithium-containing ceramics depending on the variation in the concentration of the initial components.

Table 1. Data from phase analysis and structural parameters of the studied ceramics obtained from composition 1.

0.25–0.75	0.5–0.5	0.75–0.25
Li2ZrO3 Monoclinic, C2/c(15) (PDF-00-033-0843)
a = 5.3936 Å, b = 8.9584 Å, c = 5.3201 Å, β = 112.477°, V = 237.53 Å3	a = 5.3883 Å, b = 8.9548 Å, c = 5.3191 Å, β = 112.387°, V = 237.30 Å3	a = 5.3839 Å, b = 8.9477 Å, c = 5.3148 Å, β = 112.297°, V = 236.89 Å3
LiO2 Orthorhombic, Pnnm(58) (PDF-01-080-3472)
a = 3.9875 Å, b = 4.8722 Å, c = 2.9581 Å, V = 57.37 Å3	a = 3.9803 Å, b = 4.8742 Å, c = 2.9558 Å, V = 57.34 Å3	a = 3.9771 Å, b = 4.8683 Å, c = 2.9534 Å, V = 57.18 Å3

Table 1. Data from phase analysis and structural parameters of the studied ceramics obtained from composition 1.

0.25–0.75	0.5–0.5	0.75–0.25
Li2ZrO3 Monoclinic, C2/c(15) (PDF-00-033-0843)
a = 5.3936 Å, b = 8.9584 Å, c = 5.3201 Å, β = 112.477°, V = 237.53 Å3	a = 5.3883 Å, b = 8.9548 Å, c = 5.3191 Å, β = 112.387°, V = 237.30 Å3	a = 5.3839 Å, b = 8.9477 Å, c = 5.3148 Å, β = 112.297°, V = 236.89 Å3
LiO2 Orthorhombic, Pnnm(58) (PDF-01-080-3472)
a = 3.9875 Å, b = 4.8722 Å, c = 2.9581 Å, V = 57.37 Å3	a = 3.9803 Å, b = 4.8742 Å, c = 2.9558 Å, V = 57.34 Å3	a = 3.9771 Å, b = 4.8683 Å, c = 2.9534 Å, V = 57.18 Å3

Figure 2 shows the data on changes in X-ray phase analysis of the studied ceramic samples obtained from composition 2 (in which Li₂CO₃ was used as a lithium-containing component). According to the assessment of structural parameters, as well as the shape and position of diffraction reflections, a change in the concentration of the Li₂CO₃ component (due to its increase in content) leads to more pronounced changes in the phase composition during mechanochemical stirring and subsequent thermal annealing. At a low concentration of Li₂CO₃ in the ceramic composition, the presence of two monoclinic phases, Li₂ZrO₃ (PDF-00-033-0843) and Li₆Zr₂O₇ (PDF-01-081-2375), is observed. Simultaneously, the dominant phase, Li₂ZrO₃, comprises over 90% of the composition. It is noteworthy that the close-to-symmetrical configuration of the diffraction reflections suggests a minimal presence of structural distortions within the structure. As the Li₂CO₃ concentration within the ceramic composition changes, there is a more than twofold surge in the Li₆Zr₂O₇ phase contribution in the composition when the component ratios in composition 2 are equal. It exhibits dominance, exceeding 80%, when the component ratio is 0.75 to 0.25. This shift in the phase composition signifies that the inclusion of Li₂CO₃ in the initial mixtures, followed by mechanochemical milling and subsequent thermal annealing, triggers phase transformation processes. These processes are linked to the transformation of the Li₂ZrO₃ phase into Li₆Zr₂O₇, all while preserving the structural motif and crystal lattice type. Simultaneously, an examination of the crystal lattice parameters and volume, as detailed in Table 2, highlights that the prevalence of Li₆Zr₂O₇ results in more pronounced structural ordering. The changes in lattice parameters and volume are more significant at concentrations of 0.75 to 0.25 than at other concentrations.

Figure 2. Results of X-ray phase analysis of the studied samples of lithium-containing ceramics depending on the variation in the concentration of the initial components.

Figure 2. Results of X-ray phase analysis of the studied samples of lithium-containing ceramics depending on the variation in the concentration of the initial components.

Table 2. Data from phase analysis and structural parameters of the studied ceramics obtained from composition 2.

0.25–0.75	0.5–0.5	0.75–0.25
Li2ZrO3 Monoclinic, C2/c(15) (PDF-00-033-0843)
a = 5.3953 Å, b = 9.0068 Å, c = 5.3889 Å, β = 112.414°, V = 242.09 Å3	a = 5.4006 Å, b = 8.9803 Å, c = 5.3773 Å, β = 112.171°, V = 241.51 Å3	a = 5.3826 Å, b = 8.9645 Å, c = 5.3657 Å, β = 111.973°, V = 240.10 Å3
Li6Zr2O7 Monoclinic, C2/c(15) (PDF-01-081-2375)
a = 10.4216 Å, b = 5.9710 Å, c = 10.1699 Å, β = 99.994°, V = 623.25 Å3	a = 10.3909 Å, b = 5.9559 Å, c = 10.2358 Å, β = 99.975°, V = 623.88 Å3	a = 10.3644 Å, b = 5.9336 Å, c = 10.1937 Å, β = 100.034°, V = 617.31 Å3

Table 2. Data from phase analysis and structural parameters of the studied ceramics obtained from composition 2.

0.25–0.75	0.5–0.5	0.75–0.25
Li2ZrO3 Monoclinic, C2/c(15) (PDF-00-033-0843)
a = 5.3953 Å, b = 9.0068 Å, c = 5.3889 Å, β = 112.414°, V = 242.09 Å3	a = 5.4006 Å, b = 8.9803 Å, c = 5.3773 Å, β = 112.171°, V = 241.51 Å3	a = 5.3826 Å, b = 8.9645 Å, c = 5.3657 Å, β = 111.973°, V = 240.10 Å3
Li6Zr2O7 Monoclinic, C2/c(15) (PDF-01-081-2375)
a = 10.4216 Å, b = 5.9710 Å, c = 10.1699 Å, β = 99.994°, V = 623.25 Å3	a = 10.3909 Å, b = 5.9559 Å, c = 10.2358 Å, β = 99.975°, V = 623.88 Å3	a = 10.3644 Å, b = 5.9336 Å, c = 10.1937 Å, β = 100.034°, V = 617.31 Å3

The main difference in the use of lithium-containing components in the form of Li₂CO₃ and LiClO₄·3H₂O is that when lithium carbonate is used, the phase composition of the ceramics is presented in the form of two monoclinic phases Li₂ZrO₃ and Li₆Zr₂O₇, the contributions of which vary with changes in the concentration of lithium carbonate. Moreover, in the case of using lithium perchlorate, the dominant phase in the composition of the resulting ceramics is the monoclinic phase Li₂ZrO₃, and also in the composition of the ceramics, the presence of impurity inclusions in the form of the orthorhombic phase LiO₂ is observed, the content of which varies from 1.4 to 7.3 wt%.

Figure 3 depicts the results concerning the variations in the strength characteristics of the lithium-containing ceramics studied. These ceramics were obtained from different compositions where the components were altered to achieve two-phase ceramics. The general trend observed in the strength parameter data indicates a favorable impact from the introduction of impurity phases, such as LiO₂ (for composition 1) and Li₆Zr₂O₇ (for composition 2). Furthermore, in the case of composition 2, where Li₆Zr₂O₇ predominates in the composition of the ceramic, a more noticeable shift in strength parameters is observed. This can be attributed to the effects of structural ordering, which ranges from 89% to 93% in ceramic samples. Additionally, the formation of interphase boundaries hinders the propagation of microcracks and fractures within the ceramics. Moreover, in the case of using composition 1, the formation of the LiO₂ impurity phase at high concentrations of the lithium-containing component LiClO₄·3H₂O leads to maximum strengthening of no more than 7–8%, while when using composition 2, strengthening and resistance to cracking under external load exceeds 9–11%.

Figure 4 demonstrates the results of a comparative analysis of alterations in the strengthening efficiency of ceramics obtained from different compositions, depending on the concentration of the impurity phase (in the case of composition 1—LiO₂ phase, in the case of composition 2—Li₆Zr₂O₇ phase). As a comparison, the values of hardness and resistance of ceramics to cracking under single compression were taken from works [28,29]. The hardening values were calculated taking into account changes in hardness values and the maximum pressure that the ceramics could withstand during a single compression at a speed of 0.1 mm/min. The overall trend observed in the data suggests that the most significant alterations in the strength characteristics of ceramics due to variations in impurity inclusions occur at low concentrations. This is true for ceramics obtained from both composition 1 and composition 2. Furthermore, in the case of composition 2, when the Li₆Zr₂O₇ phase dominates at high concentrations, the strengthening effect is minimal, not exceeding 10%. This can be attributed to the dominance of the Li₆Zr₂O₇ phase in the ceramic composition, which results in a reduction of its strengthening properties. The strengthening effect in ceramics resulting from an increase in impurity inclusions is attributed to the presence of interphase boundaries. A higher concentration of these boundaries hinders the propagation of microcracks and cleavages under external loads and compression of the samples. However, when the Li₆Zr₂O₇ phase prevails in the ceramic composition, a slight decrease in the rate of strength characteristics’ growth is observed. This can be explained by the dominance of this phase, which has lower strength and a decrease in interphase boundaries. Thus, by analyzing the general alterations in strength characteristics depending on the production conditions (i.e., with variations in the ratio of the initial components), the following conclusion can be drawn. The formation of impurity inclusions in the form of LiO₂ or Li₆Zr₂O₇ results in a growth in strength characteristics (hardening of ceramics), which can be explained by the effects of interphase boundaries, the formation of which leads to a rise in resistance to cracking under external mechanical influences. In this case, the most pronounced changes are observed for two-phase ceramics, in which the Li₆Zr₂O₇ phase dominates.

A change in the speed of mechanical influence on samples leads to an acceleration of the propagation of microcracks in the samples, which in turn leads to accelerated cracking at lower values of external load. In this case, the accelerated mechanical impact on the samples leads to the rapid spread of deformations and an increase in stress in the samples, which is accompanied by an acceleration of the destructive propagation of cracks. The results of experiments on changing the loading rate on samples under single compression are presented in Figure 5. The compression rate varied within the range of 0.1 mm/min, 0.3 mm/min, 0.5 mm/min, 1 mm/min, and 2 mm/min.

An examination of the data displayed in Figure 5 revealed that at low compression rates, there are minimal alterations in the crack resistance value (the maximum pressure ceramics can endure during a single compression). This suggests that ceramics exhibit remarkable resistance to minor fluctuations in the speed of external influences. However, as the compression rate escalates, a reduction in crack resistance becomes evident (refer to Figure 6), signifying that at elevated rates of external influences, there is a rapid surge in the propagation of microcracks and structural deformations, resulting in cracking at lower external load values. It is worth mentioning that when the concentration of impurity inclusions increases, a reduction in the extent of changes in crack resistance at high speeds is evident. This signifies a favorable trend in the impact of interphase boundaries on resistance to structural deformations under external loads and underscores the enhanced stability of ceramics’ strength properties. Additionally, an analysis of these variations, as depicted in Figure 5 and Figure 6, highlights that the ceramics obtained from composition 1 exhibit the highest resistance to external influences. This is evident as the decline in resistance to external influences under heavy loads is less pronounced for them.

An evaluation of the thermal conductivity variation in ceramics concerning different compositions and component concentrations is depicted in Figure 7. These measurements encompassed a temperature range from 25 to 700 °C and employed the longitudinal thermal conductivity flow measurement method. It is noteworthy that the increase in temperature from 25 to 700 °C did not yield substantial alterations in the thermal conductivity coefficient. This underscores the remarkable stability of thermophysical parameters across the entire temperature range under consideration.

As evident from the data presented, the variations in the thermal conductivity coefficient are indicative of the beneficial impact of minor impurity inclusions, which contribute to an increase in thermal conductivity. When using Li₂CO₃ as the lithium-containing component at elevated concentrations, leading to the dominance of the Li₆Zr₂O₇ phase, a marked escalation in thermal conductivity is observed in comparison to composition 1. This decrease can be elucidated by the predominance of the Li₆Zr₂O₇ phase, characterized by superior thermal conductivity properties when contrasted with the Li₂ZrO₃ phase [30]. It is noteworthy that the thermal conductivity values for the acquired ceramic samples surpass the thermal conductivity values of analogous samples obtained through the Hot Wire Method [31]. These disparities can be ascribed to not only the presence of interphase boundaries but also an upsurge in the degree of structural ordering. A modification in the degree of structural ordering results in fewer structural deformations in the crystal lattice, subsequently leading to a reduction in the defective inclusions that impede phonon heat transfer.

One of the important parameters for assessing the resistance of ceramics to external influences is the determination of their resistance to external influences under conditions of elevated temperatures corresponding to conditions as close as possible to operating conditions. Under such circumstances, the impact on the samples may be more severe, given that the ceramic specimens are subject to thermal expansion. This, in turn, can have adverse effects on the ceramics’ strength properties and stability dynamics. The experiments were conducted using a testing apparatus equipped with a heating chamber, which facilitates the heating of samples up to 500 °C, enabling temperature stabilization, followed by a single compression test. Figure 8 illustrates a comparative assessment of variations in the resistance to single compression, both at room temperature and at 500 °C, offering insights into the data related to resistance to external forces.

The data presented indicate that when dealing with ceramics containing the LiO₂ impurity phase, raising the testing temperature from room temperature to 500 °C results in a minor reduction in the resistance to single compression (within a range of 4–6%). Furthermore, an increase in the concentration of the LiO₂ impurity phase diminishes the disparities in the data concerning variations in resistance to single compression, which can be attributed to heightened resistance to thermal expansion of ceramics when subjected to heat. For two-phase ceramics of the Li₂ZrO₃/Li₆Zr₂O₇ composition, an elevated concentration of the Li₆Zr₂O₇ phase with rising test temperatures during single compression results in a significant decrease in stability. This phenomenon can be attributed to variations in the thermal expansion of the crystal lattice, alongside a reduced resistance of ceramics to external forces stemming from thermal expansion.

Figure 9 shows the results of changes in the value of volumetric expansion of samples after thermal tests for resistance to long-term temperature heating of 500 h.

The data analysis reveals that an increase in the concentration of impurity inclusions in the form of the LiO₂ phase in the ceramic composition leads to enhanced stabilization of the crystal lattice’s thermal expansion. This is substantiated by the findings on the stability of strength characteristics, as shown in Figure 9b. Conversely, the prevalence of the Li₆Zr₂O₇ phase in the ceramic composition (derived from composition 2) disrupts both thermal stability (significantly elevating the coefficient of volumetric thermal expansion by over 1.5 times) and the hardness of the ceramics. These findings indicate a decline in strength characteristics’ stability during high-temperature aging.

In the case of two-phase ceramics, it was found that an increase in the contribution of the Li₆Zr₂O₇ phase leads to increased stability to high-temperature aging and degradation, due to increased stability to softening, as well as a decrease in the effect of thermal volumetric expansion of the crystal structure. In turn, the increase in resistance to cracking during long-term high-temperature degradation (high-temperature aging) for two-phase ceramics in the case of the dominance of the Li₆Zr₂O₇ phase can be explained by interphase boundaries, the presence of which is an obstacle to the propagation of microcracks under external influences (mechanical pressure), and the presence of interphase boundaries leads to an increase in resistance to volumetric thermal expansion of the crystal structure as a result of prolonged thermal effects.

4. Conclusions

During the X-ray phase analysis studies, it was determined that incorporating LiClO₄·3H₂O as a lithium-containing component in the ceramic manufacturing process results in the formation of a predominant monoclinic phase, Li₂ZrO₃, with impurity inclusions in the shape of the LiO₂ orthorhombic phase. The maximum proportion of LiO₂ impurity does not surpass 7–8%. Conversely, when employing Li₂CO₃ as the lithium-containing component in ceramics production using the proposed methodology, it was possible to create two-phase ceramics composed of monoclinic phases, Li₂ZrO₃ and Li₆Zr₂O₇. Analysis of mechanical tests revealed a positive effect of impurity inclusions on resistance to mechanical tests and an increase in hardness, both at room temperatures and during long-term high-temperature aging. The change in resistance to mechanical damage (under single compression) is due to the presence of impurity inclusions, an increase in the concentration of which leads to an increase in the contribution of interphase boundaries that prevent the propagation of microcracks in the samples.

The potential for practical application of the research results obtained consists of the proposed technology for creating lithium-containing ceramics by simple and inexpensive mechanochemical solid-phase synthesis combined with thermal annealing of the resulting powders. Moreover, the use of the proposed technology for creating ceramics by varying different concentrations of the initial components makes it possible to obtain ceramics with a given phase composition, the alteration of which affects the strengthening and increasing the resistance of ceramics to external influences.