Perovskite/Pyrochlore Composite Mineral-like Ceramic Fabrication for ⁹⁰Sr/⁹⁰Y Immobilization Using SPS-RS Technique

Abstract

A novel solid-phase synthetic approach was developed to produce a mineral-like composite ceramic based on strontium titanate (SrTiO₃) and yttrium titanate (Y₂Ti₂O₇) matrices for immobilizing radionuclides such as ⁹⁰Sr and its daughter product ⁹⁰Y, as well as lanthanides and actinides, via reactive spark plasma sintering technology (SPS-RS). Using XRD, SEM, and EDS analyses, the sintering kinetics of the initial mixed oxide reactants of composition Y_xSr_1–1.5xTiO₃ (x = 0.2, 0.4, 0.6 and 1) and structure-phase changes in the ceramics under SPS-RS conditions were investigated as a function of Y³⁺ content. In addition, a detailed study of phase transformation kinetics over time as a function of the heating temperature of the initial components (SrCO₃, TiO₂, and Y₂O₃) was conducted via in situ synchrotron XRD heating experiments. The composite ceramic achieved relatively high physicomechanical properties, including relative density between 4.92–4.64 g/cm³, Vickers microhardness of 500–800 HV, and compressive strength ranging from 95.5–272.4 MPa. An evaluation of hydrolytic stability and leaching rates of Sr²⁺ and Y³⁺ from the matrices was performed, demonstrating rates did not exceed 10⁻⁵–10⁻⁶ g·cm⁻²·day⁻¹ in compliance with GOST R 50926-96 and ANSI/ANS 16.1 standards. The leaching mechanism of these components was studied, including the calculation of solution penetration depth in the ceramic bulk and ion diffusion coefficients in the solution. These findings show great promise for radioactive waste conditioning technologies and the manufacturing of radioisotope products.

1. Introduction

The radioactive isotope ⁹⁰Sr finds wide application across various science, technology, and medicine fields. Production of ⁹⁰Sr occurs via the fission of ²³⁵U and ²³⁹Pu in nuclear reactors. ⁹⁰Sr emits beta radiation with an energy of 0.546 MeV and has a half-life of 28.8 years [1]. Upon decay, ⁹⁰Sr forms ⁹⁰Y, a strong beta emitter with an energy of 2.28 MeV and a half-life of 64 h [2]. The unique radiative characteristics of these radionuclides determine their extensive utilization in nuclear energy, medicine, industry, and scientific research. Specifically, an important use of ⁹⁰Sr and ⁹⁰Y is in radioisotope thermoelectric generator (RITEG) constructions as autonomous sources of electric power employed in space applications as well as ground-based navigation systems and meteorological stations [3,4]. In addition, these radionuclides are widely leveraged to develop sources of ionizing radiation (IRS) in radiation technology. In medicine, compounds of ⁹⁰Sr and ⁹⁰Y are applied in the diagnosis and radiotherapy of cancerous conditions [5,6].

The high radiological hazard presented by ⁹⁰Sr and ⁹⁰Y necessitates reliable isolation of these radionuclides from direct contact with users of relevant devices and the surrounding environment. Accordingly, significant focus is devoted to developing effective solid matrix materials for their secure immobilization. The most promising materials for these purposes are ceramic materials and their composites, which confer several advantages relative to alternative glass matrixes [7,8,9]. In particular, ceramic matrices have higher chemical stability compared to glasses. Ceramics also have higher thermal and radiation resistance, which increases the long-term reliability of radioactive waste immobilization. Another advantage of ceramic matrices is their higher mechanical strength, which is important for the safe transportation and disposal of waste. In addition, the use of ceramics provides a greater variety of compositions for the immobilization of different types of radionuclides. Finally, the use of powder metallurgy allows the production of denser and more homogeneous matrices with ceramics, which is critical for the long-term stability of immobilization [10,11,12,13,14,15]. Promising ceramic matrices for ⁹⁰Sr are mineral-like compounds based on titanates, zirconates, phosphates, and silicates. The potential of complex oxides with perovskite, pyrochlore, zirconolite, and monazite structures has been widely studied [16,17]. For example, the synthesis of SrTiO₃ ceramic possessing a perovskite crystal structure is of particular interest. Within these compounds, strontium ions (and others) are incorporated into the structure in place of calcium ions, forming stable solid solutions [18,19,20,21]. The regular crystalline lattice securely retains radionuclides within its nodes, preventing migration into the surrounding environment. In this regard, industrial experience exists utilizing SrTiO₃ in RITEG production [22].

It is important to consider that the matrix properties are also influenced by the synthesis method. Conventional methods rely upon high-temperature sintering of powder mixtures, which may first be cold-pressed into compacted green bodies over several hours or days. This approach can result in significant energy expenditures as well as the possible generation of secondary wastes in the gas phase. It may also reduce the quality of the final product and lead to equipment degradation over time. Alternative processing routes could help to address such issues and are worth further exploration. Continued development of material fabrication techniques aims to promote sustainability, efficiency, and consistency in product quality.

As an alternative to conventional methods, scientists have explored spark plasma sintering technology (SPS) [23,24,25]. SPS is based on heating the compact through pulsed electric current under uniaxial pressing conditions, with high heating rates and short processing times measured in minutes. In particular, the potential of SPS has been experimentally demonstrated for synthesizing pyrochlore structure ceramic [26], scheelite [27,28,29], feldspar-like [30], sodium zirconium phosphate (NZP) [31], and glass-ceramic composites for immobilizing ⁹⁰Sr [32,33,34,35,36]. Additionally, our work has initially demonstrated high promise for applying SPS to produce a mineral-like SrTiO₃ perovskite matrix for ⁹⁰Sr immobilization, as well as to fabricate a prototype of IRS based on this approach [37]. SPS shows potential as a lower-energy and more efficient alternative processing route.

Among the more attractive methods is the process of synthesizing SrTiO₃ ceramic via reactive spark plasma sintering (SPS-RS) [38]. In this case, ceramic formation with the required composition occurs via solid-phase reaction in situ interaction of the initial powders during spark plasma heating. Our experiments [39] demonstrated that brief heating at relatively low temperatures serves to minimize the loss of volatile components from the original mixture. The resultant ceramic possesses a fine-grained, dense microstructure with high mechanical properties and hydrolytic durability. A prototype radiation source fabricated using this ceramic demonstrates superior operating characteristics compared to regulatory standards and analogous devices [40]. SPS-RS enables efficient, low-temperature synthesis with optimal material performance.

Considering the dual nature of ⁹⁰Sr existing together with its daughter product ⁹⁰Y, it is important to account for the ceramic matrix’s crystalline phase composition relative to both radionuclides. In this vein, the aim of the present study was to investigate the solid-phase synthesis of a mineral-like strontium-containing ceramic based on the SrTiO₃ perovskite composition containing varying Sr²⁺/Y³⁺ ratios via SPS-RS. Investigations were supplemented by examining phase transformation dynamics in the solid state with high temporal resolution using in situ synchrotron XRD heating experiments. This enables the determination of temperature ranges for phase formation, assessment of solid-state reaction rates, identification of possible intermediate compounds within the ceramic, and elucidation of influences on performance characteristics. Establishing optimal synthesis parameters can yield materials with tailored properties necessary for reliable immobilization of ⁹⁰Sr, ⁹⁰Y, and other radionuclides. Such research had not been previously conducted.

2. Materials and Methods

2.1. Reagents

The reaction mixtures (RM) were fabricated using powders of strontium carbonate (SrCO₃), titanium dioxide (TiO₂), and yttrium oxide (Y₂O₃) (99.9%, Sigma-Aldrich, St. Louis, MI, USA).

2.2. Synthesis of Initial Powders Reaction Mixtures

The RM were prepared by mechanochemical mixing of the starting powders in a planetary ball mill in a single 10 min cycle at 870 rpm. Zirconium dioxide milling jars and milling bodies were used for homogenization.

The quantitative mass ratios of initial reagents are presented in

Table 1. Chemical composition of starting reaction mixtures.

Initial Reagents	Ratio
	YxSr1–1.5xTiO3, x = 0.2	YxSr1–1.5xTiO3, x = 0.4	YxSr1–1.5xTiO3, x = 0.6	YxSr1–1.5xTiO3, x = 1
Y2O3	1.28	2.70	4.28	4.84
TiO2	4.56	4.79	5.05	5.15
SrCO3	5.90	3.54	0.93	-

.

2.3. Spark Plasma Sintering Reactive Synthesis of Ceramic Materials

Ceramic samples were synthesized via SPS-RS using a Dr.Sinter·LAB^TM SPS-515S system (Japan, Kyoto) according to the general procedure: 4 g of RM powder was placed in a graphite die (working diameter 15.5 mm), pre-consolidated (pressure 20.7 MPa), then transferred to a vacuum chamber (10⁻⁵ atm). Heating was achieved using unipolar low-voltage pulsed current in On/Off mode, with a 12 pulse/2 pause periodicity, resulting in a pulse packet duration of 39.6 ms and pause of 6.6 ms. The temperature during SPS was monitored using an optical pyrometer (lower limit 650 °C) focused on a hole in the outer wall of the die 5.5 mm deep. The heating rate was controlled in stages: 300 °C/min from 0 to 650 °C, then 50 °C/min above 650 °C (working range of pyrometer). Samples were held at maximum temperature for 5 min, and then cooled to room temperature over 30 min. Consolidation pressure was 21.5 MPa. Graphite foil (200 μm thick) was used to prevent adhesion and facilitate sample removal. Insulating cloth wrapped the die to minimize heat loss. The resultant cylindrical matrix samples measured 15.3 mm diameter and 4–6 mm height, depending on sintering parameters.

2.4. Characterization of the Research Methods

Particulate size distribution (PSD) of the RM powder was determined using a G3-ID unit of Malvern Instruments Ltd. (Malvern, UK). Scanning electron microscopy (SEM) was carried out using a Carl Zeiss CrossBeam 1540 XB (Jena, Germany) equipped with a Bruker energy dispersive X-ray spectroscopy (EDS) attachment (Mannheim, Germany). X-ray powder diffraction (XRD) analysis was conducted using Bruker D8 Advance (Mannheim, Germany) and Burevestnik Colibri (Moscow, Russia) diffractometers with CuKα1-Kα2 radiation (40 kV, 10 mA; mean wavelength λ = 1.5418 Å). The signal was detected using a Muthen2 detector with a Kβ Ni-filter, over a 2θ range of 20–100°, step size 0.0185° and dwell time per step of 244 s. Vickers microhardness (HV) was measured at 0.2 N load using a Shimadzu HMV-G-FA-D microhardness tester (Kyoto, Japan). Compressive strength (σ_cs) was studied using a Shimadzu Autograph AG-X plus 100 kN universal testing machine (Kyoto, Japan). Sample bulk relative density (RD) was determined using hydrostatic weighing on an OHAUS Corporation Adventurer™ balance (Parsippany, NJ, USA).

Hydrolytic durability of the matrices was assessed based on the leaching rate of strontium ions during prolonged contact (30 days) with distilled water (pH 6.8) at room temperature (25 °C) under static conditions according to GOST R 52126-2003, analogous to ANSI/ANS 16.1, which was updated in accordance with an earlier version recommended by the IAEA (ISO 6961:1982). The strontium ion concentration was determined using inductively coupled plasma mass spectrometry (ICP-MS) on a Thermo Scientific iCAP 7600 Duo spectrometer (Waltham, MA, USA).

The leaching rates are calculated according to the following equation:

$${\mathrm{R}}_{\mathrm{n}}^{\mathrm{S}\mathrm{r}}=\frac{{\mathrm{m}}_{\mathrm{n}}^{\mathrm{S}\mathrm{r}}}{{\mathrm{M}}_{\mathrm{o}}^{\mathrm{S}\mathrm{r}}\times {\mathrm{t}}_{\mathrm{n}}\times \mathrm{S}}$$

(1)

where ${\mathrm{R}}_{\mathrm{n}}^{\mathrm{Sr}}$—Sr leaching rate (g/cm²⋅day); ${\mathrm{m}}_{\mathrm{n}}^{\mathrm{Sr}}$—Sr mass, leached for nth time interval, g; ${\mathrm{M}}_0^{\mathrm{Sr}}$—Sr mass concentration in the matrix, g/g; S—sample’s surface area, cm²; tn—duration of the nth time interval, days.

The calculation of the effective diffusion coefficient (De) was performed using mathematical transformations of the second Fick law according to the method described in the paper [41]:

$$\frac{\sum \mathrm{m}}{{\mathrm{M}}_0}=2{\left(\frac{{\mathrm{D}}_{\mathrm{e}}}{\mathrm{\pi }}\right)}^{\frac{1}{2}}\times \left(\frac{\mathrm{S}}{\mathrm{V}}\right){\mathrm{t}}^{\frac{1}{2}}+\mathsf{\alpha }$$

(2)

where m—strontium weight, mg; t—leaching time, s; M₀—initial cesium content in the sample, mg; De—effective diffusion coefficient, cm²/s; S—the surface area of the sample, cm²; V—a volume of sample, cm³; α—parameter that takes into account the initial leaching of strontium, not related to diffusion (strontium leaches out at the initial contact of the leaching solution with the sample surface).

In the calculation, this equation was reduced to a linear form by introducing the coefficient K, which represents the tangent of the slope of the straight-line dependence of strontium leaching from the sample on the square root of the contact time of the material with the leaching agent:

$$\mathrm{K}=2{\left(\frac{{\mathrm{D}}_{\mathrm{e}}}{\mathrm{\pi }}\right)}^{0.5}\times \left(\frac{\mathrm{S}}{\mathrm{V}}\right)$$

(3)

The effective diffusion coefficient was calculated:

$${\mathrm{D}}_{\mathrm{e}}=\frac{{\mathrm{K}}^2\times \mathrm{\pi }}{4}\times {\left(\frac{\mathrm{V}}{\mathrm{S}}\right)}^2$$

(4)

The leaching index (L) was calculated as the decimal logarithm of the inverse diffusion value:

$$\mathrm{L}=\mathrm{l}\mathrm{g}\frac{1}{{\mathrm{D}}_{\mathrm{e}}}$$

(5)

Estimation of the dominant leaching mechanism based on the dependence of the decimal logarithm of the accumulated fraction of leached radionuclide (Bt, mg/m²) on the decimal logarithm of the leaching time t, s:

$$\mathrm{l}\mathrm{g}({\mathrm{B}}_{\mathrm{t}})=\frac{1}{2}\mathrm{l}\mathrm{g}\mathrm{t}+\mathrm{l}\mathrm{g}\left[{\mathrm{U}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\mathrm{d}\sqrt{\frac{{\mathrm{D}}_{\mathrm{e}}}{\mathrm{\pi }}}\right]$$

(6)

where is Umax—the maximum amount of leached radionuclide, mg/kg, d—matrix density, kg/m³.

The leaching depth of the matrix characterizes the destruction of the matrix.

Matrix when it is in aqueous medium and is calculated according to Equation (7):

$${\mathrm{L}}_{\mathrm{t}}^{\mathrm{i}}={\sum }_{\mathrm{i}}^{\mathrm{n}}\left({\mathrm{W}}_{\mathrm{n}}^{\mathrm{i}}\frac{{\mathrm{t}}_{\mathrm{n}}}{\mathrm{d}}\right)$$

(7)

where is ${\mathrm{L}}_{\mathrm{t}}^{\mathrm{i}}$—the leaching depth of the matrix reached during the time interval tn, cm; d—density of the sample, g/cm³.

2.5. In Situ Synchrotron Study

In situ synchrotron radiation (SR) XRD heating experiments were performed using beamline 5-B at the VEPP-3 storage ring [42] at the Siberian Centre for Synchrotron and Terahertz Radiation (SB RAS, Novosibirsk, Russia) based on the VEPP-4-VEPP-2000 electron-positron collider complex at the Budker Institute of Nuclear Physics of Siberian Branch of the Russian Academy of Sciences [43]. Diffraction patterns were recorded using a one-dimensional detector OD-3 [44]. The wavelength of monochromatic radiation was 1.51 Å over a 2θ range of 18–50°. Samples were placed in a special furnace heated at a constant rate of 15 °C/min to 400 °C and 5 °C/min to 1000 °C. The OD-3 exposure time was set to 60 s.

3. Results and Discussion

The ceramic formation was based on the initiation of a chemical reaction according to the in situ interaction principle of the starting RM powders during spark plasma sintering, according to the following parallel reactions:

SrCO₃ + TiO₂ → SrTiO₃ + CO₂↑

(8)

Y₂O₃ + 2TiO₂ → Y₂Ti₂O₇

(9)

According to the granulometric analysis data of the RM (Figure 1), all initial powders were found to have a polydisperse composition. SrCO₃ has particles in the size range of 0.2–5 μm and an insignificant amount (about 10%) in the range of 20–50 μm (Figure 1a). TiO₂ is represented by four particle size distribution areas in the ranges of 0.1–1 μm in the amount of 15 vol.%, 1–5 μm 25 vol.%, 5–20 μm 30 vol.%, and 20–50 μm with 30 vol.% (Figure 1b). Y₂O₃ has four particle size distribution areas in the ranges of 0.1–0.5 μm in the amount of 5 vol.%, 0.5–5 μm 25 vol.%, 5–20 μm 50 vol.%, and 20–100 μm with 20 vol.% (Figure 1c).

The reaction mixtures (RM) (Figure 2a–d) with varying ratios of TiO₂, Y₂O₃, and SrCO₃ following high-energy grinding comprised particles sized 0.1–1 μm. According to SEM images, the RM consisted of agglomerations of nanoparticles less than 100 nm (Figure 3a–d) as determined using EDS. Elemental mapping of RM particle surfaces from EDS (Figure 3a–d) indicated a uniform composition without foreign inclusions, corroborated using XRD analysis (Figure 4). With the changes in component contents outlined in Table 1, decreasing SrCO₃ concentration reduced XRD peak intensities with an inverse dependence observed for Y₂O₃.

X-ray diffraction analysis of the Y_xSr_1.5-xTiO₃ reaction mixture powders revealed the presence of orthorhombic SrCO₃ crystalline phase with space group Pmcn (DB# 01-078-4340), tetragonal TiO₂ phases with space group P42/mnm for Rutile (DB# 01-079-5859) and I41/amd, choice-2 for Anatase (DB# 01-076-3177), and cubic Y₂O₃ phase with space group Ia-3 (DB# 01-081-9987) (Figure 4).

Standard diffraction patterns and 3D structural models of the RM components were generated using the Vesta software (Figure 5) to facilitate phase identification and visualization of crystal structures. The unit cell parameters of the starting powders matched calculated values for Rutile tetragonal phase with a = b = 4.5933, c = 2.9592 Å, Anatase tetragonal phase with a = b = 3.78520, c = 9.51390 Å, Yttrium Oxide cubic phase with a = b = c = 10.60410 Å, and Strontianite orthorhombic phase with a = 6.02000 b = 5.09300 c = 8.37600 Å.

As previously shown by dilatometry, XRD, TGA, and synchrotron XRD [39], the optimal sintering temperature for reaction sintering of RM based on SrCO₃ and TiO₂ to produce a single phase SrTiO₃ perovskite ceramic via SPS was determined to be 1300 °C. The ceramic formation process occurred in three shrinkage stages of the RM: (1) particle rearrangement and packing under pressing pressure at low temperature; (2) in situ interaction of RM components, including partial decomposition of SrCO₃, and consolidation of the nascent SrTiO₃ phase under thermal and pressing conditions; (3) deep interaction of RM components to form a single phase SrTiO₃ composition and its consolidation via activation of solid state grain boundary diffusion and plastic flow above 1270 °C [39]. A similar dilatometric dependence was observed in the current study for Y_xSr_1.5–xTiO₃ RM compositions (Figure 6). Specifically, sintering RM with 0.2 mole Y₂O₃ content (Y_xSr_1.5–xTiO₃, where x = 0.2) showed shrinkage at 1000 °C at a rate of 0.6 mm/min or more due to partial mechanical densification of RM particles and decomposition of strontium carbonate. The shrinkage rate then increased to 1.4 mm/min at 1100 °C associated with initiation of in situ interaction of RM components and partial formation of SrTiO₃ perovskite and Y₂Ti₂O₇ pyrochlore phases. This was corroborated using XRD data (Figure 7, curve 1). Increasing Y₂O₃ content in the RM to 0.4 moles (Y_xSr_1.5–xTiO₃, where x = 0.4) also led to a sharp increase in shrinkage rate to 3.4 mm/min at 1100 °C. However, further increasing Y₂O₃ to 0.6 moles (Y_xSr_1.5–xTiO₃, where x = 0.6) decreased the powder shrinkage rate to 1.6 mm/min. A similar trend of slowing shrinkage rate was observed upon full replacement of Sr²⁺ with Y³⁺ (sample Y_xSr_1.5–xTiO₃, where x = 1) not exceeding 1 mm/min. This is attributed to insufficient SrCO₃ for the in situ reaction with TiO₂ to form the SrTiO₃ phase with a = b = c = 3.9046 Å and cell volume 59.5292 Å. Per XRD, the ceramic composition consisted of a single Y₂Ti₂O₇ phase with a = b = c = 10.0761 Å and cell volume 1023.0085 Å (Figure 7, curve 4).

The phases were quantified using the Rietveld method on TOPAS software (version 3) (

Table 2. Phase content in the obtained samples calculated using the Rietveld method.

Sample Name	Phase Content, wt. %		Rwp. %	GOF, %
	SrTiO3	Y2Ti2O7
YxSr1–1.5xTiO3, x = 0.2	84.44	15.56	4.56	2.98
YxSr1–1.5xTiO3, x = 0.4	66.24	33.76	4.56	2.26
YxSr1–1.5xTiO3, x = 0.6	94.08	5.92	4.68	2.98
YxSr1–1.5xTiO3, x = 1	0	100	5.13	3.08

). The calculated phase ratio using the Rietveld method shows that the initial compositions are selected correctly (Table 1), considering the error factor R_wp. The “goodnesses of fit” were 2.98, 2.26, 2.98, 3.08 for x = 0.2, 0.4, 0.6, and 1, respectively. As can be seen, the profile calculated using the Rietveld method has a small discrepancy with the experimental data.

A detailed investigation of phase formation in the sintered RM system and the resulting composite ceramic was carried out using in situ synchrotron X-ray diffraction during heating up to 1000 °C (Figure 8). The diffraction patterns of Y_xSr_1–1.5xTiO₃ RM compositions (x = 0.2, 0.4, 0.6) showed diffraction peaks corresponding to SrCO₃ phase (Figure 8a–c), consistent with XRD of the initial RMs (Figure 4). A decrease in SrCO₃ peaks occurred over 850–900 °C, accompanied by the emergence of unidentified peaks near 29 and 34 ° that reached maxima around 900–950 °C. This correlates well with the DTA maximum at 941 °C seen previously [39]. These peaks likely relate to the rhombic → hexagonal phase transition of SrCO₃. SrO peaks may also be present despite SrCO₃ decomposition occurring at higher temperatures. Thus, conducting the in situ heating XRD experiment in air may have accelerated this process. Additionally, peaks for the final SrTiO₃ perovskite phase emerge around 850–900 °C, in agreement with the literature [44]. It is also indicated that an intermediate Sr₂TiO₄ phase forms, with decreasing intensity upon decreasing SrCO₃ concentration in RM. The Y₂Ti₂O₇ pyrochlore phase forms at 950 °C, observed for all RM compositions. TiO₂ phases in the anatase and rutile forms, as well as some residual Y₂O₃, were also present for all RMs, attributed to the incomplete reaction of oxides by 1000 °C. As shown previously using XRD, heating the RMs to 1300 °C is optimal as it achieves a complete chemical reaction of the initial RM components.

SEM analysis (Figure 9) revealed that the microstructure of the fabricated ceramics consisted of densely sintered grains with no porosity or large defects. The morphology of longitudinal cross-sectional surfaces of composite ceramic samples (Figure 9a,a*–d,d*) was identical for all samples and exhibited a monolithic structure. For the single-phase sample fabricated without SrCO₃, a larger grain size was observed, likely related to the specific sintering behavior of the pyrochlore phase alone (Figure 9d,d*). EDS analysis confirmed the component ratios in the original RM and resulting ceramics. The distribution of Sr²⁺, Y³⁺, and Ti²⁺ was uniform across the cross-sectional surface areas analyzed (Figure 9).

The physical and mechanical properties of the ceramic samples were determined. It was found that relative density, compression strength, and Vickers microhardness varied depending on ceramic composition (Figure 10). With decreasing strontium concentration in Y_xSr_1–1.5xTiO₃ for x = 0.4 and above, the relative density of the ceramic slightly decreased (Figure 10a). Compression strength also decreased from 272.5 to 95.6 MPa, with the pyrochlore sample showing the minimum value compared to all test samples (Figure 10b). The reduction in properties is attributed to compositional changes in the ceramic samples, as the content of the Y₂Ti₂O₇ pyrochlore phase increased. Pyrochlore has a lower Mohs hardness value (5–5.5) compared to SrTiO₃ (6–6.5).

A “box-and-whiskers” diagram of microhardness values was plotted to indirectly assess the strength micro-inhomogeneity of the material (Figure 10c). The data show that with changing ceramic composition, the hardness of ceramic materials changes only slightly in the range of 500–800 HV. Some scatter in microhardness values was observed for samples with a mixed SrTiO₃ and Y₂Ti₂O₇ composition, attributed to anisotropy in local volume regions of the material. For the sample consisting solely of the Y₂Ti₂O₇ phase without SrTiO₃, this scatter was more significant. This is attributed to both the ceramic composition due to the lower Mohs hardness of the Y₂Ti₂O₇ phase and its structure—larger grain growth during sintering was observed above for this phase alone.

An assessment of the hydrolytic stability of the fabricated composite ceramic samples was conducted, as this is a key parameter for their effectiveness in immobilizing radionuclides. It was determined that all samples exhibited low leaching rates of Sr²⁺ (Figure 11a). This value was 10⁻⁵–10⁻⁶ g·cm⁻²·day⁻¹, meeting the requirements of GOST R 50926-96 for solidified high-level waste.

The effective diffusion coefficient (De) of Sr²⁺ in the consolidated ceramic sample, the leachability index, and matrix leaching depth were calculated using mathematical transformations of Fick’s second law according to the methodology described in reference [45], with results presented in

Table 3. Sr²⁺ leaching parameters on Day 30.

Sample Name	Diffusion Coefficient (De), cm2/s	Error, cm2/s	Leachability Index (L)	Error	Leaching Depth, cm	Error, cm
YxSr1–1.5xTiO3, x = 0.2	8.81 × 10−12	±0.44 × 10−12	10.72	±0.53	3.00 × 10−5	±0.15 × 10−5
YxSr1–1.5xTiO3, x = 0.4	4.65 × 10−11	±0.23 × 10−11	9.86	±0.49	9.65 × 10−5	±0.48 × 10−5
YxSr1–1.5xTiO3, x = 0.6	5.44 × 10−9	±0.27 × 10−9	7.42	±0.37	3.76 × 10−4	±0.18 × 10−4

. Additionally, in accordance with references [45,46], an assessment of the dominant leaching mechanism was conducted. Following the de Groot and der Slot model [46], a dependence of the tangent of the inclination angle of the tangent line to the logarithmic dependence of the fraction of the leached component on the logarithm of time was plotted (Figure 11b), which relates to the matrix leaching mechanism. The results determined that the magnitude of the tangent of the inclination angle of the direct dependences remains unchanged over time. For the Y_xSr_1–1.5xTiO₃ sample where x = 0.2, the tangent of the inclination angle was equal to 0.756, indicating strontium leaching predominantly by a surface dissolution mechanism. For Y_xSr_1–1.5xTiO₃ samples where x = 0.4 and 0.6, the tangents of the inclination angles were equal to 0.515 and 0.542, respectively, signifying strontium leaching primarily via a diffusional mechanism.

The leaching depth indicators (Figure 11c, Table 3) show that the fabricated materials are characterized by a high degree of stability in an aqueous environment, attributed to their chemical composition and dense microstructure. The leachability index (L) of Y_xSr_1–1.5xTiO₃ samples where x = 0.2 and 0.4 are above 8, allowing the conclusion that Sr²⁺ is reliably fixed within the ceramic volume. This characterizes the samples as promising for application as immobilizing matrices [47].

It was determined that the leaching rate of Y³⁺ from the ceramic volume was also within low limits, not exceeding 10⁻⁵ g·cm⁻²·day⁻¹ (Figure 12a). A similar regularity was observed for the magnitudes of the tangent of the inclination angle of the tangent line to the dependence of the logarithm of the leached component fraction on the logarithm of time, which remained unchanged over time (Figure 12b). In addition, these values were identical for all samples and were 0.56. This indicates that the leaching rate of Y³⁺ is limited by its diffusion from the volume to the surface of the sample.

The Y³⁺ diffusion coefficients (

Table 4. Yttrium leaching parameters on day 30.

Sample Name	Diffusion Coefficient (De), cm2/s	Error, cm2/s	Leachability Index (L)	Error	Leaching Depth, cm	Error, cm
YxSr1–1.5xTiO3, x = 0.2	7.37 × 10−7	±0.36 × 10−7	6.13	±0.3	1.63 × 10−3	±0.08 × 10−3
YxSr1–1.5xTiO3, x = 0.4	1.30 × 10−8	±0.06 × 10−8	7.88	±0.39	5.54 × 10−4	±0.27 × 10−4
YxSr1–1.5xTiO3, x = 0.6	3.51 × 10−9	±0.17 × 10−9	8.46	±0.42	4.27 × 10−4	±0.21 × 10−4
YxSr1–1.5xTiO3, x = 1	8.86 × 10−10	±0.44 × 10−10	9.05	±0.45	3.34 × 10−4	±0.16 × 10−4

) in the ceramic samples varied significantly depending on the predominant cation in the ceramic composition. Presumably, increasing the Y³⁺ content stabilizes the structure of Y₂Ti₂O₇, increasing its strength of retention within the matrix volume.

The leachability index (L) of Y_xSr_1–1.5xTiO₃ samples (Table 4) where x = 0.6 and 1 are above 8, allowing the conclusion that Y³⁺ is reliably fixed within the ceramic volume.

Based on the obtained results of chemical resistance, as well as the composition of ceramics and its structure, it can be stated that an increase in the content of one of the presented phases leads to an increase in the rate of leaching from the ceramic volume of the component of the other phase. This is due to a number of factors, namely defects in the structure, which lead to an increase in the proportion of surface leaching of strontium due to an increase in the contact area with the solution. As indicated, the increase in the proportion of Y⁺³ leads to stabilization of the structure of pyrochlore Y₂Ti₂O₇ and, consequently, to higher parameters of chemical stability. The opposite is also true for perovskite SrTiO₃. The high fraction of Sr²⁺ ions leads to the stabilization of the perovskite phase, which is consistent with the leaching and SEM data (Figure 9, Figure 11 and Figure 12).

4. Conclusions

The work implemented a solid-state synthesis of mineral-like composite ceramics based on perovskite SrTiO₃ and pyrochlore Y₂Ti₂O₇, promising for immobilization of radionuclides ⁹⁰Sr and ⁹⁰Y, as well as lanthanides and actinides. A study was conducted on the kinetics of sintering a reaction mixture composition of Y_xSr_1–1.5xTiO₃ (x = 0.2, 0.4, 0.6, and 1), as well as structural-phase changes in the resulting ceramics under spark plasma sintering conditions using XRD, SEM, EDS and in situ heating synchrotron XRD experiment methods. It was established that the formation of SrTiO₃ begins already at 850 °C (although it cannot proceed intensely at this temperature), with the formation of an intermediate Sr₂TiO₄ phase in the interval 850–900 °C, which was not determined by XRD data. The formation of the pyrochlore phase Y₂Ti₂O₇ occurs at 950 °C and is observed for all the investigated RM compositions. A study of the densification dynamics of RM for the specified compositions was carried out at a constant temperature of 1300 °C and pressure of 21.5 MPa, where it was shown that the in situ reactive interaction and simultaneous sintering of the initial powder mixture proceeds through three stages of densification: (1) regrouping and packing of particles of the initial powder under the influence of pressing; (2) decomposition of SrCO₃ and formation of SrTiO₃ and Y₂Ti₂O₇ through possible intermediate products upon thermal effects above 850 °C; (3) activation of diffusion, plastic deformation and viscous flow in the solid body at high heating of the sample above 1270 °C. It was determined that the resulting ceramics possess high physicomechanical characteristics: specific density of ceramics 4.92–4.64 g/cm³; Vickers microhardness 500–800 HV; compression strength 95.5–272.4 MPa. Variation of these characteristics depends on the composition and structure of the ceramics; in particular, their decrease is due to the increase in the content of the Y₂Ti₂O₇ phase, which has a lower density on the Mohs scale compared to SrTiO₃. It has been proven that this ceramic has high hydrolytic stability, as the leaching rate of Sr²⁺ reaches 10⁻⁶ g∙cm⁻²∙day, and Y³⁺ 10⁻⁵ g∙cm⁻²∙day, which meets the requirements of GOST R 50926-96 and ANSI/ANS 16.1 for solidified high-level wastes. The mechanism of leaching of these ions from the ceramic volume has been established, indicating diffusive leaching with minimal depth of diffusion of ions from the volume to the ceramic surface.

The applied result of the study is of interest for technologies of radioactive waste conditioning and the production of radioisotopic products.