Study of Structural, Strength, and Thermophysical Properties of Li2+4xZr4-xO3 Ceramics

The work is devoted to the study of technology that can be used to obtain lithium-containing ceramics of the Li2+4xZr4−xO3 type using the method of solid-phase synthesis combined with thermal annealing at a temperature of 1500 ◦C. A distinctive feature of this work is the preparation of pure Li2ZrO3 ceramics with a high structural ordering degree (more than 88%) and density (95–97% of the theoretical density). During the study, it was found that a change in the content of initial components for synthesis does not lead to the formation of new phase inclusions; however, an increase in the LiClO4·3H2O and ZrO2 components leads to changes in the size of crystallites and dislocation density, which lead to the strengthening of ceramics to external mechanical influences. The results of the measurements of thermophysical characteristics made it possible to establish that the compaction of ceramics and a decrease in porosity lead to an increase in the thermal conductivity coefficient of 3–7%.


Introduction
One of the important tasks in nuclear energy, set by the International Atomic Energy Commission, is the search for alternative ceramics to the main type, Li 4 SiO 4 ceramics, which are used as tritium breeder materials in new-generation reactors, particularly in heliumcooled pebble-bed reactors [1][2][3]. The search for alternative materials is associated with expanding the capabilities of materials such as blankets, as well as studying the possibilities of increasing the production of tritium in the reactor itself due to transmutation reactions of the 6 Li + n → He + T type and its subsequent accumulation [4,5]. At the same time, it is worth considering the fact that the short lifetime of tritium, due to rapid radioactive decay, makes its shelf life extremely limited, which leads to the need for constant reproduction of tritium in a thermonuclear reactor.
One of the ways to solve this problem is the use of lithium-containing ceramics as blanket materials, which are placed in the reactor walls for the continuous reproduction of tritium and the maintenance of thermonuclear reactions [6][7][8]. In this case, the ceramic material is subjected to continuous irradiation, accompanied by thermal heating and the accumulation of radiation damage associated with transmutation reactions, the main products of which are tritium and helium [9,10]. In turn, the accumulation of helium in the structure can lead to gaseous swelling of the near-surface layer of ceramics, the accumulation of stresses, and the deformations of the structure, which can subsequently lead to a decrease in strength and thermal characteristics [11][12][13][14]. In this regard, additional requirements are imposed on lithium-containing ceramics, related to their resistance, that is,

Experimental Section
LiClO 4 ·3H 2 O and ZrO 2 powders, purchased from Sigma Aldrich (Saint Louis, MO, USA), were chosen as initial reagents for synthesis; the powder sizes were no more than 1 µm, and their chemical purity was 99.95%.
The synthesis of ceramics was carried out using the method of mechanochemical synthesis, combined with thermal annealing of the resulting ground mixtures at an annealing temperature of 1500 • C. The synthesis was carried out by varying the content of the components xLiClO 4 ·3H 2 O and (1−x)ZrO 2 , where x = 0.1-0.5 in order to determine the effect of changes in the concentration of the components on the structural parameters and phase composition of the studied ceramics. The grinding of samples after weighing was carried out in a PULVERISETTE 6 planetary mill (Fritsch, Germany) under the following conditions: grinding speed 400 rpm and grinding time 1 h. Thermal annealing was carried out in a muffle furnace at a temperature of 1500 • C for one hour; the heating rate was 10 • C/min, and the cooling time was 24 h. Figure 1 shows a schematic representation of the main processes used to obtain lithium-containing ceramics, including component weighing, mechanochemical grinding, and subsequent thermal sintering at specified parameters.

Experimental Section
LiClO4·3H2O and ZrO2 powders, purchased from Sigma Aldrich (Saint Louis, MO, USA), were chosen as initial reagents for synthesis; the powder sizes were no more than 1 μm, and their chemical purity was 99.95%.
The synthesis of ceramics was carried out using the method of mechanochemical synthesis, combined with thermal annealing of the resulting ground mixtures at an annealing temperature of 1500 °C. The synthesis was carried out by varying the content of the components хLiClO4·3H2O and (1−х)ZrO2, where x = 0.1-0.5 in order to determine the effect of changes in the concentration of the components on the structural parameters and phase composition of the studied ceramics. The grinding of samples after weighing was carried out in a PULVERISETTE 6 planetary mill (Fritsch, Germany) under the following conditions: grinding speed 400 rpm and grinding time 1 h. Thermal annealing was carried out in a muffle furnace at a temperature of 1500 °C for one hour; the heating rate was 10 °C/min, and the cooling time was 24 h. Figure 1 shows a schematic representation of the main processes used to obtain lithium-containing ceramics, including component weighing, mechanochemical grinding, and subsequent thermal sintering at specified parameters. The study of the structural parameters, as well as the phase composition of the synthesized ceramics, was carried out using the method of X-ray diffraction phase analysis. The diffraction patterns were taken in Bragg-Brentano geometry on a D8 Advance ECO powder diffractometer (Bruker, Germany). The recording conditions were as follows: 2θ = 25-75°, step 0.03°, and spectrum acquisition time at the point 1 s. An analysis of the structural parameters was carried out using the DiffracEVA v.4.2 program code, the phases were determined using the full-profile Rietveld method, and the phases were refined using the PDF-2 (2016) database.
The determination of ceramic density was carried out by analyzing changes in crystal lattice volume with further application of the obtained data in Formula (1): where Vo is the crystal lattice volume depending on the composition of the ceramics, Z is the number of atoms in a crystal cell, and A is the atomic weight of atoms. The crystallite sizes were determined using the Scherrer method, which is based on the determination of the FHWM value depending on the angular position of the diffraction maxima. The study of the structural parameters, as well as the phase composition of the synthesized ceramics, was carried out using the method of X-ray diffraction phase analysis. The diffraction patterns were taken in Bragg-Brentano geometry on a D8 Advance ECO powder diffractometer (Bruker, Germany). The recording conditions were as follows: 2θ = 25-75 • , step 0.03 • , and spectrum acquisition time at the point 1 s. An analysis of the structural parameters was carried out using the DiffracEVA v.4.2 program code, the phases were determined using the full-profile Rietveld method, and the phases were refined using the PDF-2 (2016) database.
The determination of ceramic density was carried out by analyzing changes in crystal lattice volume with further application of the obtained data in Formula (1): where V o is the crystal lattice volume depending on the composition of the ceramics, Z is the number of atoms in a crystal cell, and A is the atomic weight of atoms. The crystallite sizes were determined using the Scherrer method, which is based on the determination of the FHWM value depending on the angular position of the diffraction maxima.
The dislocation density was estimated using Formula (2): where L is the crystallite size, determined using the Scherrer method, taking into account the contribution of reflection shape distortions. The crystallinity degree was determined by calculating the ratio of the diffraction reflection areas to the background radiation area, which is characteristic of disordered regions in the ceramic structure.
The study of strength properties was carried out using two methods. The determination of hardness was carried out using the indentation method, which was implemented using a LECO LM 700 microhardness tester (LECO Corporation, Saint Joseph, MI, USA). A Vickers pyramid was used as an indenter, with an indenter load of 10 N.
Resistance to single compression was determined by testing the compression of samples in a press with a compression rate of 0.1 mm/min. The resistance limit was defined as the maximum pressure that the sample could withstand until cracking and partial destruction.
The study of the thermo-physical properties of the ceramics was carried out using the method of determining the temperature difference with a longitudinal heat flux through the sample during heating. The thermal conductive characteristics were evaluated using Formula (3), which makes it possible to determine the thermal conductivity coefficient.
where q is the heat flux density, W/m 2 ; t c1 and t c2 are the temperatures on both sides of the sample, K; and δ is the thickness of the sample, m. The temperature difference was measured using a KIT-800 instrument (Moscow, Russia). Figure 2 shows the results of the X-ray diffraction phase analysis of the studied ceramics regarding the concentration of the components. A full-profile analysis of the obtained diffraction patterns made it possible to establish that the main reflections are characteristic of the Li 2 ZrO 3 monoclinic phase (PDF-01-070-8744) with C2/2(15) spatial syngony. According to the results in [28], thermal annealing at temperatures above 900 • C leads to the displacement of LiO and ZrO 2 impurity phases from the ceramic structure, and a further increase in the annealing temperature leads to the ordering of the structural parameters (lattice parameter and crystallinity degree) and a change in the grain size associated with their coarsening and increase in density. An analysis of the obtained diffraction patterns indicated that a change in the component concentrations in the composition of the ceramics, as well as subsequent thermal annealing, does not lead to the formation of new phases, as evidenced by the absence of new reflections in the diffraction patterns. Unlike commercial samples of Li 2 ZrO 3 ceramics, in the structure of which, according to the data in [18], the presence of impurity phases of the initial ZrO 2 and Li 2 CO 3 powders was observed, impurity inclusions were not found in the studied samples obtained at a temperature of 1500 • C. At the same time, the data of a comparative analysis of the areas of diffraction reflections and background radiation indicate a high crystallinity degree of the samples (more than 85%), which varies depending on the content of the components. Moreover, the content of ZrO 2 impurity inclusions in the structure of Li 2 ZrO 3 ceramics was reported in [19], the presence of which is due to manufacturing processes. Moreover, in most known works [20], thermal sintering occurs at temperatures of 700-1100 • C. The main changes in the diffraction patterns are mainly associated with changes in the intensity of reflections and their width; this is characterized by a change in the size of crystallites, as well as a shift of reflections to the region of large angles, which is typical for a change in crystal lattice parameters. For comparison, the changes in the main reflections and their positions are presented, according to which, an increase in the concentration of the LiClO 4 component in the initial mixture leads to a decrease in the intensity of reflections and their shift. At the same time, the greatest changes are observed for ceramics with a content of x = 0.4-0.5. Such a shift may be due to the substitution of zirconium ions (13.9 Å), which have a radius larger than that of lithium ions (7.1 Å). An increase in the lithium concentration in the structure, in turn, does not lead to the formation of impurity phases, and a decrease in the intensity of reflections and a change in their width (FWHM) indicate changes in the size of crystallites and their fragmentation, which, in turn, lead to an increase in the dislocation density due to an increase in grain boundaries. A change in the positions of reflections, including their shift to the large angle region, as is known, leads to a decrease in crystal lattice parameters, which can be due to several factors: the processes of substitution of atoms of one type for atoms of another type; the deformation processes of crystal lattice compression, leading to its compaction; and relaxation processes associated with changes in point defect concentration in the composition of ceramics as a result of external influences. It is also worth noting that the change in the crystal lattice parameters may be due to the effect associated with a change in the component concentration of O-vacancies. An increase in the lithium concentration in the structure can also lead to the formation of additional vacancy defects, which are formed as a result of the partial breaking of Zr-O chemical bonds. In this case, part of the free oxygen can be displaced from the structure during thermal annealing.  Table 1 presents the results of evaluating the structural parameters, as well as the crystallinity degree of the samples under study. It can be seen from the data presented that an increase in the X component content leads to a decrease in the crystal lattice parameters. However, a change in the crystal lattice parameters leads to a slight decrease in the structural ordering degree, which may be due to the partial replacement of zirconium ions by lithium ions at the lattice sites, as well as the fragmentation of crystallites, the size of which also decreases. In turn, a decrease in the size of crystallites leads to an increase in the dislocation density, which plays an important role in increasing the strength properties.  When estimating the shape of the diffraction reflection lines, it was determined that deformation effects associated with the structure distortion make a very small contribution to the change in structural parameters (no more than 5-10% according to the Williamson-Hall method), and the change in the shape of reflections is mostly due to size effects, leading to a change in the dislocation density.

Results and Discussion
Relaxation processes also do not make a large contribution to the change in structural parameters, since the crystallinity degree does not increase with changes in component concentrations, but it has a slight downward trend, which indicates the absence of defect annealing processes.
Based on this, it was assumed that the main changes in diffraction reflections associated with a maxima position shift to the large angle region are due to the effects of the substitution of zirconium ions by lithium ions in the lattice sites with an increase in the lithium-containing component concentration in the ceramic during its synthesis. Table 1 presents the results of evaluating the structural parameters, as well as the crystallinity degree of the samples under study. It can be seen from the data presented that an increase in the X component content leads to a decrease in the crystal lattice parameters. However, a change in the crystal lattice parameters leads to a slight decrease in the structural ordering degree, which may be due to the partial replacement of zirconium ions by lithium ions at the lattice sites, as well as the fragmentation of crystallites, the size of which also decreases. In turn, a decrease in the size of crystallites leads to an increase in the dislocation density, which plays an important role in increasing the strength properties.  The results of changes in the structural parameters ( Figure 3) for high annealing temperatures are in good agreement with the results in [34], according to which, at high annealing temperatures, an anisotropic change in the crystal lattice parameters was observed due to thermal effects, as well as the difference in cationic radii. In this case, changes in the parameter a depending on the content of the component have an isotropic nature of distortions, while the parameters b and c change anisotropically, which may be due to the difference in the filling of lattice sites, as well as inhomogeneity in the replacement of zirconium ions by lithium ions.
A decrease in the crystal lattice parameters, as is known, leads to a decrease in its volume and, consequently, to a decrease in porosity and an increase in the density of ceramics. Figure 4 shows the results of changes in the density and porosity of the ceramics depending on the X component concentration. A change in density, including its decrease at high annealing temperatures of 1500 • C, is due to the fact that with a change in the components used for synthesis, by varying them with a subsequent increase in LiClO 4 ·3H 2 O, the crystal lattice volume decreases, which leads to ceramic compaction. At the same time, the difference from the density values presented in [28], which are close to the theoretical values, is that, at high lithium concentrations, there is an overall increase in the lattice volume, which leads to lower ceramic density values.
As can be seen from the data presented, a change in the crystal lattice parameters leads to the compaction of ceramics, as well as the approximation of the experimentally obtained density values to the theoretical value (95-97%). In this case, porosity decreases from 4.1% to 2.6%, with an increase in the X component, which, in turn, results in the ceramics strengthening to mechanical stress. Figure 5 shows the measurement results of the ceramics' single compression resistance. The change in the resistance value depending on the concentration of X varies in the range from 29.3 to 38.4 N, while for concentrations X = 0.4-0.5, an increase in resistance of 25-30% is observed. Such an increase in single compression resistance is due to an increase in the density of the ceramics, as well as a change in the dislocation density, which leads to a strengthening effect and an increase in fracture resistance. A comparative analysis with the known literature data on measurements of resistance to single compression, taken from [20,35,36], indicated that the synthesized ceramics have an increased resistance to mechanical stress, as well as greater strength. At the same time, as shown in [30], in which the properties of two-component Li 4 SiO 4 -Li 2 ZrO 3 ceramics were studied, an increase in density led to an increase in strength, which confirms the obtained results. The authors found that an increase in the resistance of ceramics to mechanical stress was not only due to an increase in density but also due to an increase in the content of Li 2 ZrO 3 in the composition of the ceramics. A similar effect was also observed in [36], where it was shown that a change in component content in the composition of ceramics leads to an increase in resistance to mechanical stress. The results of changes in the structural parameters ( Figure 3) for high annealing temperatures are in good agreement with the results in [34], according to which, at high annealing temperatures, an anisotropic change in the crystal lattice parameters was observed due to thermal effects, as well as the difference in cationic radii. In this case, changes in the parameter a depending on the content of the component have an isotropic studied, an increase in density led to an increase in strength, which confirms the obtained results. The authors found that an increase in the resistance of ceramics to mechanical stress was not only due to an increase in density but also due to an increase in the content of Li2ZrO3 in the composition of the ceramics. A similar effect was also observed in [36], where it was shown that a change in component content in the composition of ceramics leads to an increase in resistance to mechanical stress.   The results of the ceramic hardness obtained by indentation are shown in Figure 6. studied, an increase in density led to an increase in strength, which confirms the obtained results. The authors found that an increase in the resistance of ceramics to mechanical stress was not only due to an increase in density but also due to an increase in the content of Li2ZrO3 in the composition of the ceramics. A similar effect was also observed in [36], where it was shown that a change in component content in the composition of ceramics leads to an increase in resistance to mechanical stress.   The results of the ceramic hardness obtained by indentation are shown in Figure 6. As can be seen from the presented data on changes in the hardness value of the surface layer of ceramics, an increase in dislocation density leads to an increase in resistance to mechanical stress. At the same time, in the case of component concentrations of X = 0.4-0.5, the ratio of dislocation density to hardening is 3:1. The hardening effect is due to the creation of additional obstacles to the propagation of microcracks due to the high density of dislocations, which leads to an increase in resistance to mechanical stress. Moreover, an important role in strengthening is played by a decrease in the porosity of the ceramics and an increase in density; this causes a decrease in the concentration of disordered structural inclusions, which, in turn, leads to the strengthening of ceramics.
The results of the ceramic hardness obtained by indentation are shown in Figure 6. As can be seen from the presented data on changes in the hardness value of the surface layer of ceramics, an increase in dislocation density leads to an increase in resistance to mechanical stress. At the same time, in the case of component concentrations of X = 0.4-0.5, the ratio of dislocation density to hardening is 3:1. The hardening effect is due to the creation of additional obstacles to the propagation of microcracks due to the high density of dislocations, which leads to an increase in resistance to mechanical stress. Moreover, an important role in strengthening is played by a decrease in the porosity of the ceramics and an increase in density; this causes a decrease in the concentration of disordered structural inclusions, which, in turn, leads to the strengthening of ceramics.
The important characteristics for the determination of the operational parameters of lithium-containing ceramics are their thermophysical parameters, which play an important role in the processes of heat transfer and heat removal. Figure 7 shows the results of the change in the thermal conductivity coefficient depending on the concentration of the X component.
As can be seen from the data presented in Figure 7a, the value of the thermal conductivity coefficient in a wide measured temperature range remains almost unchanged, with a slight increase in thermal conductivity (no more than 0.5%) at elevated measurement temperatures. At the same time, the dependences obtained indicate a high stability of thermal conductivity over the entire temperature range. A comparative diagram of the average value of thermal conductivity shows that a change in the X component concentration leads to an increase in thermal conductivity of 3.5-7%, which indicates an increase in thermal conductivity due to structural changes, as well as a decrease in porosity. For example, as a comparison, Figure 7b shows the results of the thermal conductivity for Li2ZrO3 used as a blanket material [37]. According to this work, the change in thermal conductivity is influenced by two factors that are associated with heat transfer and heat transfer processes, as well as the contact area. In our case, the contact area was the same for all the samples under study; thus, the main changes are associated with the processes of heat transfer and heat transfer, which directly depend on the structure of the ceramics, as well as porosity. A decrease in porosity as a result of a change in the X The important characteristics for the determination of the operational parameters of lithium-containing ceramics are their thermophysical parameters, which play an important role in the processes of heat transfer and heat removal. Figure 7 shows the results of the change in the thermal conductivity coefficient depending on the concentration of the X component. component concentration leads to an increase in heat transfer and, accordingly, in the thermal conductivity. It should be noted that such a significant difference from the results in [32] is due to the fact that the studied ceramics in this work were highly porous samples with a packing density of 1.921 (at a density of 3.1 g/cm 3 , 53.4% of the theoretical value).

Conclusions
In conclusion, the results of the research can be summarized as follows: 1. The proposed conditions to obtain Li2ZrO3 ceramics with different contents of the initial components хLiClO4·3H2O and (1−х)ZrO2 (х = 0.1-0.5) at an annealing temperature of 1500 °С make it possible to obtain highly ordered pure ceramics with a monoclinic phase (Li2ZrO3) and a density close to the theoretical value (95-97%). 2. The decrease in the crystal lattice parameters for the Li2ZrO3 monoclinic phase is due to the substitution of zirconium ions (atomic radius = 13.9 Å ) by lithium ions (atomic radius = 7.1 Å ), and it indicates an increase in lithium concentration in the ceramic structure. At the same time, changes in the crystal lattice parameters de- As can be seen from the data presented in Figure 7a, the value of the thermal conductivity coefficient in a wide measured temperature range remains almost unchanged, with a slight increase in thermal conductivity (no more than 0.5%) at elevated measurement temperatures. At the same time, the dependences obtained indicate a high stability of thermal conductivity over the entire temperature range. A comparative diagram of the average value of thermal conductivity shows that a change in the X component concentration leads to an increase in thermal conductivity of 3.5-7%, which indicates an increase in thermal conductivity due to structural changes, as well as a decrease in porosity. For example, as a comparison, Figure 7b shows the results of the thermal conductivity for Li 2 ZrO 3 used as a blanket material [37]. According to this work, the change in thermal conductivity is influenced by two factors that are associated with heat transfer and heat transfer processes, as well as the contact area. In our case, the contact area was the same for all the samples under study; thus, the main changes are associated with the processes of heat transfer and heat transfer, which directly depend on the structure of the ceramics, as well as porosity. A decrease in porosity as a result of a change in the X component concentration leads to an increase in heat transfer and, accordingly, in the thermal conductivity. It should be noted that such a significant difference from the results in [32] is due to the fact that the studied ceramics in this work were highly porous samples with a packing density of 1.921 (at a density of 3.1 g/cm 3 , 53.4% of the theoretical value).

Conclusions
In conclusion, the results of the research can be summarized as follows: 1.
The proposed conditions to obtain Li 2 ZrO 3 ceramics with different contents of the initial components xLiClO 4 ·3H 2 O and (1−x)ZrO 2 (x = 0.1-0.5) at an annealing temperature of 1500 • C make it possible to obtain highly ordered pure ceramics with a monoclinic phase (Li 2 ZrO 3 ) and a density close to the theoretical value (95-97%).

2.
The decrease in the crystal lattice parameters for the Li 2 ZrO 3 monoclinic phase is due to the substitution of zirconium ions (atomic radius = 13.9 Å) by lithium ions (atomic radius = 7.1 Å), and it indicates an increase in lithium concentration in the ceramic structure. At the same time, changes in the crystal lattice parameters depending on the X component concentration are anisotropic.

3.
A change in the X component content leads to an increase in the dislocation density, a change in which leads to the strengthening of ceramics and an increase in resistance to mechanical stress and cracking.

4.
An increase in the density of ceramics, as well as a decrease in density, leads to an increase in the thermal conductivity coefficient of 3.5-7%.