Rare Earth Zirconates Research Articles

The effect of entropy on the structure and aqueous durability of nanocrystalline rare-earth zirconate ceramics

A series of compositionally complex A2Zr2O7 nanocrystalline ceramics were successfully prepared using sol-gel and co-precipitation methods. The resultant ceramics possess a cubic defect fluorite structure, with the sol-gel method yielding an average grain size of approximately 50–70 nm, while the co-precipitation method result in an average grain size of about 40–60 nm. Leaching tests revealed that the smaller grain sizes are correlated with higher leaching rates. Furthermore, for ceramics with similar grain sizes, those with higher entropy values exhibited higher leaching rates. The increase in grain boundaries was found to reduce the leaching performance of the rare earth zirconate ceramics, and this effect became more pronounced with increasing entropy. This work provides insights into the selection of entropy values and grain sizes for the high-level radioactive waste matrices, which can be considered as a potential substrate for the simultaneous immobilization of multiple radionuclides.

Preparation of spherical Gd2Zr2O7 powders by RF induction thermal plasma process

Spherical Gd2Zr2O7 (GZO) powders with a uniform particle size distribution are successfully prepared using a novel industrial approach, which combines spray-drying process and thermal plasma sintering technology together. In this, GZO powders with pyrochlore structure as the main phase are first synthesized via a solid-state reactive route using a mixture of the milled Gd2O3 powders and ZrO2 powders as starting materials. The influence of sintering temperature on the microstructure of prepared powders is researched. Then, GZO granules are prepared after wet-grinding and spray-drying process, which exhibit a spherical shape with the average particle size of 46.5 μm. RF induction thermal plasma is finally used to sinter the granulated particles, and the influence of feeding rate on the properties of spherical powders is investigated. The apparent density of plasma sintered GZO spherical powders is increased from 1.53 g/cm3 to 2.29 g/cm3 when the feeding rate of granulated powders is 80 g/min, and further increased up to 3.90 g/cm3 when the feeding rate is 15 g/min. Such powders are in potential demand for plasma sprayed coatings and additive manufacturing techniques, and the successful synthesis of spherical GZO powders may guide the way toward the preparation of many other spherical rare earth zirconates powders.

Effect of Yb2O3 doping on the thermal and mechanical properties GdAlO3-Gd2Zr2O7 composite with eutectic composition

Rare earth zirconates exhibit potential as thermal barrier coatings, but their limited toughness impedes their widespread use. To address this issue, rare earth aluminates are regarded as effective toughening agents for zirconates. However, the thermal conductivity of composites containing rare earth aluminates and zirconates tends to rise. To mitigate this issue, we have developed a series of Yb2O3-doped GAO-GZO composites. The introduction of Yb2O3 results in the formation of a fluorite phase, with Yb3+ ions occupying Gd sites in both GAO and GZO components. Although the fracture toughness remains relatively unchanged, the hardness of the composite improves. The thermal conductivity decreases with increasing Yb2O3 content due to enhanced phonon-defect scattering. Additionally, the coefficient of thermal expansion is reduced due to the stronger Yb-O bonding compared to Gd-O. This study presents a viable strategy for regulating the mechanical and thermal properties of rare earth zirconates reinforced with aluminates through defect engineering.

Thermophysical properties of Yb3+-doped nonstoichiometric gadolinium zirconate ceramics

Low thermal conductivities and high thermal expansion coefficients are desirable for rare-earth zirconate thermal barrier coating materials. Introduction of cation doping and adjustment of the nonstoichiometry are efficient methods for enhancing the thermophysical characteristics of materials. Herein, the thermophysical properties of Yb3+-doped nonstoichiometric gadolinium zirconate ceramic materials were investigated using first-principles calculations and solid-state reaction techniques. As the Yb3+ content increases, additional cations enter the interstitial spaces (for excess Gd3+ gadolinium zirconate) and form vacancy defects (for excess Zr4+ gadolinium zirconate), which serve as phonon scattering centres to decrease the thermal conductivity. When Yb3+ is doped at the same concentration, gadolinium zirconate with excess Zr4+ exhibits a lower thermal conductivity. Specifically, Gd1.875Yb0.25Zr2.125O7 shows the lowest minimum thermal conductivity of 1.133 W/(m⸱K) (according to the Clark model) and 1.241 W/(m⸱K) (according to the Cahill model). In addition, the experimental results also suggest that the optimal content of Yb3+ is 0.25 excess Zr4+ gadolinium zirconate, which has the lowest intrinsic thermal conductivity of 1.037 W/(m⸱K) at 1300 K. Moreover, the thermal expansion coefficient of the Yb3+-doped excess Gd3+ nonstoichiometric material is greater than that of the doped excess Zr4+ material, this is due to the greater electronegativity difference between Yb3+ and Zr4+ than that between Yb3+ and Gd3+ and the lower binding energy of Gd-O than that of Zr-O. The experimental results and the calculated results are in good agreement. The aim of this work is to enhance the thermophysical characteristics of gadolinium zirconate ceramics for use as thermal barrier coating materials.

Investigations on the mechanical and thermal properties of multicomponent dual-phase rare-earth zirconate ceramics

In this work, a series of multicomponent rare-earth zirconate (RE2Zr2O7, RE=La, Nd, Sm, Eu, Gd or Yb) ceramics were synthesized using solid-state reaction method. The phase stability and key factors affecting the thermal conductivity and mechanical performances were investigated. These materials possess pyrochlore-fluorite dual-phase structure and show excellent high-temperature phase stability. (La1/2Yb1/2)2Zr2O7 displays a lower amorphous-like thermal conductivity (1.39–1.67 Wm−1K−1, 25–1000 °C), while other specimens present a 1/T-like dependence at low temperatures. (La1/5Nd1/5Sm1/5Gd1/5Yb1/5)2Zr2O7 exhibit higher hardness (11.37 ± 0.35 GPa) and fracture toughness (2.10 ± 0.11 MPa·m1/2) in comparison with others. Size disorder shows stronger correlations with the thermal conductivity of multicomponent dual-phase RE2Zr2O7 ceramics, while the influence of configurational entropy is more significant for mechanical properties. A synergistic increase in configurational entropy and size disorder is an effective strategy to enhance the comprehensive performance of multicomponent dual-phase rare-earth zirconates.

Enhancing mechanical properties of lanthanide zirconates through the cold sintering assisted sintering process

This study evaluates the impact of incorporating varying contents (10–40 wt%) and molar concentrations (0.001–1 M) of citric acid solutions, as transient liquid phases in the Cold Sintering Assisted Sintering (CSAS) process of dysprosium zirconate (Dy2Zr2O7). CSAS processed samples achieved relative densities up to 98% of the theoretical maximum and significantly increased Vickers microhardness by over 2.5 times, compared to the traditional ‘press and fired’ sintering method. The Dy2Zr2O7 crystal structure remained consistent with the fluorite-type, with no secondary phases detected. Our findings underscore the benefits of using CSAS to enhance the mechanical strength of Dy2Zr2O7, while reducing the lengthy processing times at very high temperatures typically required for sintering refractory materials such as lanthanide zirconates.

Enhanced thermophysical and mechanical properties of gadolinium zirconate ceramics via non-stoichiometric design

Rare-earth zirconate ceramics are potential materials for thermal barrier coatings due to their low thermal conductivity and excellent structural stability. However, the low thermal expansion coefficient and fracture toughness limit their application and further improvements are needed. In this work, non-stoichiometric gadolinium zirconate ceramics with fluorite phase structure were successfully prepared using solid-state reaction method. HRTEM, iDPC-STEM, and in-situ XRD were employed to investigate nanostructure, lattice defects, and phase stability. The non-stoichiometric ratio introduces more defects and forms special nanostructures, enhancing the thermophysical properties of gadolinium zirconate ceramics. The non-stoichiometric gadolinium zirconate ceramics have lower phonon thermal conductivity with higher hardness and fracture toughness compared to stoichiometric ratio ceramics. In addition, the non-stoichiometric gadolinium zirconate ceramics exhibit excellent structural stability in a wide temperature range. These performances provide further applications for non-stoichiometric gadolinium zirconate ceramics as thermal barrier coatings.

Flash joining of (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2Zr2O7 high-entropy ceramic to 3YSZ

(La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2Zr2O7 high-entropy ceramic (HEC) and 3 mol% yttria-stabilized zirconia were directly joined in seconds using an innovative pressure-assisted flash joining technique. An optimized shear strength of 26 ± 3 MPa was obtained at 1200 °C under uniaxial pressure of 9 MPa in 30 s. The contact resistance at the interface was found to induce intense local Joule heating, which accelerated interfacial mass transport and the joining process. However, this local heating effect also led to segregation and precipitation of the HEC near the interface, forming large La/Yb-rich rare-earth zirconate grains and intergranular pores. Increasing the pressure inhibited these issues by lowering the contact resistance, suppressing grain growth, and promoting grain sliding, rearrangement, and coalescence of grain boundaries. The proposed mechanism, based on the local heating effect, provides new insights into flash joining, highlighting the critical role of this phenomenon in triggering thermal runaway and mass transport.

Stable preparation of defective fluorite structure high entropy (Y0.2Sm0.2Eu0.2Er0.2Yb0.2)2Zr2O7 ceramic powders by molten salt synthesis

Rare earth zirconate ceramic has been considered as one of the potential thermal barrier coating materials to replace 6–8% YSZ material (6–8% yttrium oxide stabilized zirconia), because of its low thermal conductivity and good high temperature phase stability. However, there is a problem that the coefficient of thermal expansion does not match the matrix. In this paper, rare earth high entropy zirconate ceramic powder with defective fluorite structure was prepared by molten salt synthesis(MSS). The results show that the reaction temperature and the ratio of reactants to molten salt will affect the synthesis effects. With the action of the high entropy effect, the synthesized ceramic powder has low thermal conductivity, improved thermal expansion coefficient and high infrared reflectivity.

Design and experimental investigation of potential low-thermal-conductivity high-entropy rare-earth zirconates

Design and experimental investigation of potential low-thermal-conductivity high-entropy rare-earth zirconates

The synthesis of the rare earth A2Zr2O7 single crystals by simplified laser-heated floating hot zone and pedestal methods

The condensed matter community is in constant search for efficient and feasible methods for the synthesis of diverse compounds, especially in the single-crystalline form. We present a recipe for the simplified synthesis of high-quality single crystals of rare-earth zirconates A2Zr2O7, important from both the fundamental and technical application viewpoints,. The developed preparation route allows to effectively, repeatedly, and reliably synthesise the high-quality precursors and single crystals of desired compositions, avoiding contamination and difficulties connected with standard polycrystalline-precursor preparation. Simultaneously, the method is straightforward, saving time, energy, and personnel. In addition to previously reported single crystals, several new single crystals from the A2Zr2O7 series were prepared, including the previously unreported compound Tm2Zr2O7. The high quality of the synthesised samples was proven employing electron microscopy, X-ray diffraction, laboratory and synchrotron radiation diffraction, and neutron powder and Laue diffraction techniques. The crystal structures and structural parameters of the individual compounds were determined and discussed in view of previous reports on A2Zr2O7 and other A2B2O7 series (B stands for d- or p-elements). The introduced synthesis route is proposed to serve well not only for several other A2B2O7 series, but also for a variety of unrelated compounds.

Low-temperature properties of magnetically frustrated rare-earth zirconates A 2Zr2O7

Rare-earth A 2Zr2O7 zirconates have attracted considerable attention of the scientific community for their complex magnetic, electronic and material properties applicable in modern technologies. The light rare-earth members of the series, crystallising in the pyrochlore variant of cubic crystal structure, have been studied in detail. The heavier A 2Zr2O7 compounds have been investigated mainly from the material properties viewpoint, focussing on their thermal properties and stability at high temperature and pressure. Low-temperature studies were mostly missing until recently. We present the low-temperature magnetic and thermodynamic properties of A 2Zr2O7 with A = Y, La, Nd, Eu, Gd, Tb, Dy, Ho, Tm, Yb, and Lu, well covering the whole series, newly synthesised by high-temperature sintering and melting methods. X-ray diffraction reveals and confirms the ordered pyrochlore structure in the light members, the disordered cubic structure of the defect-fluorite type in A 2Zr2O7 with A = Y, Gd–Yb, and finally the lower symmetry rhombohedral structure in the end-member Lu2Zr2O7. The specific heat of the investigated compounds is dominated by a low-temperature anomaly associated with magnetic ordering: long-range in light rare-earth zirconates; and short-range in heavier members. The effective magnetic moment in the studied compounds, determined by fitting the magnetisation data to the Curie–Weiss formula, is in good agreement with the expected value of the A 3+ free ion. The magnetic properties have been revealed to be strongly influenced by the geometric frustration of the magnetic moments of both the pyrochlore structure, as well as the face centred cubic lattice created by the cations of the defect-fluorite structure, but connected also to intrinsic atomic disorder. The experimental results are discussed in the framework of previous studies on A 2Zr2O7 zirconates, as well as other A 2 B 2O7 compounds.

Improvement strategy on thermophysical properties of A2B2O7-type rare earth zirconates for thermal barrier coatings applications: A review

Improvement strategy on thermophysical properties of A2B2O7-type rare earth zirconates for thermal barrier coatings applications: A review

Heterophase synthesis of rare-earth zirconates

This study focuses on developing a heterophase process for synthesizing rare-earth zirconates, specifically R2Zr2O7 /R2O3·2ZrO2 (R = La, Sm, Gd, Dy). We investigated the sorption properties of low-hydrated zirconium hydroxide, a precursor for complex-oxide phases, towards rare-earth elements' ions (La, Sm, Gd, Dy). The results indicate that sorption by low-hydrated zirconium hydroxide is a multifaceted process, involving the incorporation of rare-earth cations into the pores of low-hydrated hydroxide and ion exchange. The paper details the synthesis of R2Zr2O7 /R2O3·2ZrO2 (R = La, Sm, Gd, Dy), considering both «light» and «heavy» elements. The process process involves the interaction between Zr(OH)3÷1O0.5÷1.5·(1.6÷2.6)H2O, low-hydrated zirconium hydroxide, and an aqueous solution of rare-earth acetate (С(La3+) = 0.155 mol/l, С(Sm3+) = 0.136 mol/l, С(Gd3+) = 0.141 mol/l, С(Dy3+) = 0.120 mol/l) followed by heat treatment. The resulting phases and their thermolysis products were analyzed using differential thermal analysis and X-ray phase analysis. Single-phase rare-earth zirconates R2Zr2O7 (R = La, Sm, Gd) and the Dy2O3·2ZrO2 solid solution were only obtained at 800 °С. The lattice parameters are calculated for each phase. Lanthanum, samarium, and gadolinium zirconates exibited a cubic pyrochlore structure (Fd3–m), while dysprosium displayed a fluorite structure (Fm3–m). The average particle size of all zirconates was 1.14 ± 0.02 μm.

High temperature performance of RE2Zr2O7 high-entropy ceramics designed by thermophysical performance-oriented principle

This study utilizes modulation of ionic disorder and electronegativity differences for targeted component design of high-entropy ceramic compositions. A series of novel multicomponent ceramics with the general formula (LaGdYSm)x2Ybx1Zr2O7 (x1+4x2 = 2, x1 = 0.6,0.4,0.2,0) were prepared by solid-state reaction. The evolution patterns of their microstructures were analyzed using XRD and TEM, and the anti-sintering ability and thermophysical properties were evaluated. The results confirm that the multicomponent ceramics gradually transform from the coexistence of pyrochlore/fluorite dual-phase structure to pyrochlore structure with the tuning of the size disorder degree. After 50h of exposure to air at 1500 °C, the ceramics retained excellent phase stability. Segregation of La and Yb elements with large differences in radius size occurs, forming a mixture of the La-rich pyrochlore and the Yb-rich fluorite phases. The specimen with the maximum size disorder exhibits the lowest grain growth rate and the greatest thermal insulation properties. Large lattice distortions and sluggish diffusion effects are responsible for the mutual inhibition of grain growth in the two-phase coexistence zone. The thermal insulation properties of the ceramics show well correlated with the degree of the size disorder, further validating the effectiveness of the component design. Meanwhile, the thermal expansion coefficients of prepared ceramic can reach 11.49 × ∼11.58 × 10−6 K−1 at 1000–1100 °C. The customization principles used in this work fill a gap in the compositional design methodology for multicomponent lanthanum zirconate-based high-entropy ceramics. It is essential for developing rare-earth zirconates with tunable thermophysical properties in the area of thermal barrier coatings.

One pot synthesis of high entropy rare earth zirconate ceramics with low thermal conductivity for high performance thermal-barrier coatings

By using the one pot synthesis method, zirconate rare earth ceramic powders with high entropy and low thermal conductivity were prepared. The phase and microstructure of the prepared powder and sintered ceramics were analyzed by x-ray diffraction, Raman spectroscopy and scanning electron microscopy. The results show that the prepared powder may be used directly for atmospheric plasma spraying. The ceramic prepared from this powder exhibited a density of 96.87 %, improved thermal conductivity and thermal expansion coefficient. Coating materials were prepared using an atmospheric plasma spraying system, with the LSZC coating demonstrating the highest bond strength of 48.81 MPa and thermal shock resistance of 18 cycles.

Magnetic frustration in rare-earth zirconates A2Zr2O7, the case of laser heated pedestal method synthesised A = Er, Tm, Yb, and Lu single crystals

Heavy rare-earth A2Zr2O7 zirconates have been studied for their high application potential, including thermal barrier coating, radiation waste storage, or solid oxide fuel cells. However, previous investigations have focused mostly on the high-temperature properties of polycrystalline samples. The low-temperature properties, important for a full understanding of these materials, remains understudied. We have newly synthesised good-quality single crystals of A2Zr2O7 with A = Er, Tm, Yb, and Lu using the laser heated pedestal method and have characterised them by means of X-ray diffraction, magnetisation, and specific heat measurements. The first three members crystallise in a cubic disordered defect-fluorite structure. Lu2Zr2O7 adopts lower-symmetry rhombohedral structure. Similar magnetic properties of all magnetic zirconates were observed at low temperature. In the unperturbed paramagnetic state, they follow the Curie-Weiss law with an effective magnetic moment close to the respective A3+ free ions. The negative paramagnetic Curie-Weiss temperature, in the case of A = Yb member highly negative, strongly suggests an antiferromagnetic ordering in the compounds. Below 30 K, the magnetic response becomes non-linear in the applied magnetic field, suggesting magnetic correlations between rare-earth moments or/and crystal field effects. Finally, at lowest measured temperature, Er2Zr2O7 and Yb2Zr2O7 members reveal a well-pronounced anomaly in specific heat at 0.9 and 1.5 K, respectively. The specific heat anomaly shifts to higher temperature in an applied magnetic field, suggesting ferromagnetic correlations – in contradiction to the negative Curie-Weiss temperature. We discuss these observations in the framework of freezing of magnetic moments on a disordered geometrically frustrated lattice. In contrast, the low-temperature properties of Tm2Zr2O7 are dictated mostly by its crystal electric field; a crystal-field-singlet ground state which does not provide degrees of freedom for a spin-freezing is proposed. No clear low-temperature anomaly in specific heat is observed down to 0.4 K for the A = Tm member. Lu2Zr2O7 does not bear any magnetic moment, therefore no magnetic signal was observed in any measured data. The results are discussed in the framework of heavy rare-earth A2B2O7 oxides and other magnetically frustrated systems.

Corrosion mechanisms of high-entropy rare earth zirconate (Gd0.2Y0.2Er0.2Tm0.2Yb0.2)2Zr2O7 exposed to CMAS and multi-medium (NaVO3+CMAS)

The wettability and thermal corrosion behavior of CMAS and CN (CMAS+NaVO3) on high-entropy rare-earth zirconate (Gd0.2Y0.2Er0.2Tm0.2Yb0.2)2Zr2O7 (HEZ) at high temperature were studied and compared. The results reveal that HEZ is a promising TBCs material with good anti-CMAS performance. However, HEZ exhibits poor resistance to CN, and the reaction layer fails to effectively alleviate the melt penetration. Based on the calculation of OB (optical basicity) theory, the possibility that NaVO3 can improve the reaction activity of CMAS is ruled out. It was determined that NaVO3 depolymerizes the internal network structure of the CMAS, resulting in a lower melting point and viscosity, stronger wettability, and permeability of CN compared to CMAS, thus posing greater harmfulness. The influence of rare earth elements with different ionic radii on the formation of multicomponent RE-crystal was researched by crystal structure analysis and first-principles calculation, providing a theoretical basis for the reliability of the corrosion mechanism.

A novel dual-phase high-entropy (La0.2Nd0.2Gd0.2Er0.2Yb0.2)2Zr2O7 ceramic for thermal barrier coatings applications: Preparation, microstructure, and thermo–physical properties

High-entropy rare earth zirconates (RE2Zr2O7) have attracted significant research attention for thermal barrier coatings (TBCs) application due to their superior performance. However, currently used high-entropy RE2Zr2O7 shows only the modest improvements, thus limiting their overall performance enhancement. Therefore, novel high-entropy RE2Zr2O7 (La0.2Nd0.2Gd0.2Er0.2Yb0.2)2Zr2O7 (HEC) was synthesized in this study by chemical co-precipitation method, which was characterized by dual-phase and significant differences in ionic radius and atomic mass. Compared with single-component RE2Zr2O7 (RE = La, Nd, Gd, Er, and Yb) ceramics, HEC demonstrates significantly superior performance, including higher hardness (11.705 GPa), fracture toughness (1.693 MPa m1/2), and thermal expansion coefficient (11.1926 × 10−6/K, 1400 °C), and lower thermal conductivity (1.33–1.43 W/(m•K), 26–1000 °C). The comprehensive performance improvement can be attributed to the high-entropy effect, dual-phase nature, and significant differences in ionic radius and atomic mass. This study provides valuable insight into the development of high-entropy RE2Zr2O7 and suggests their significant potential in TBCs.

Effect of multi-component at the A site on the phase and sintering resistance of high entropy rare earth zirconates

A series of high entropy rare earth zirconate ceramics, (5A0.2)2Zr2O7, with single- or dual-phase structures were synthesized using the solid-state reaction method. The cations at the A site varied in terms of average ionic radius RA, atomic mass MA, radius (size) disorder δR, and mass disorder δM. Our results demonstrate that increasing the structural disorder degree promotes the transition from an ordered pyrochlore structure to a defective fluorite structure, and leads to poor sintering resistance. The structural disorder degree of the high entropy zirconates could be increased by decreasing the ionic radius ratio RA/RZr, increasing the size disorder δR and introducing Ce4+ at the A site. Meanwhile, large size disorder leads to strong partitioning of rare earth ions and deviation of the lattice constant from the predicted value. The presence of Ce4+ ions at A site could significantly increase the structural disorder degree and promote the sintering process, and the underlying mechanisms are proposed and discussed.