Zirconate Ceramics 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.

Effect of Yb3+ on the thermophysical and mechanical properties of gadolinium zirconate ceramics: First-principles calculations and experimental study

Thermal barrier coating materials require high phase stability, low thermal conductivity, and a thermal expansion coefficient that matches the bonding layer. Cation doping is a highly efficient method for enhancing the thermophysical and mechanical characteristics of materials used in thermal barrier coatings. The thermophysical and mechanical properties, as well as the phase stability at high temperatures of ceramic materials doped with Yb3+ in gadolinium zirconate, are examined by utilizing first principles calculations and solid-state reaction methods. As the Yb3+ content increases, the grain average particle size of gadolinium zirconate ceramics first decreases and then increases. Given that a smaller grain size results in more grain boundaries, thereby reducing the thermal conductivity of the material, it can be inferred that Yb0.20 ((Gd0.8Yb0.2)2Zr2O7) with the smallest grain size exhibits lower thermal conductivity. As the Yb3+ content increases, both the calculated and experiment results suggest the Young’s modulus of gadolinium zirconate initially declines to its minimum value when the Yb3+ content is 0.5 and subsequently experiences a little increase. Furthermore, as the concentration of Yb3+ increases, the material initially experiences a drop in both hardness and fracture toughness, followed by a subsequent increase. Both the calculation and experimental results indicate that the thermal conductivity of gadolinium zirconate initially decreases and then increases. Among the different compositions, Gd1.5Yb0.5Zr2O7 demonstrates the minimum thermal conductivity, measuring 1.029 W/(m⸱K) according to the Clark model and 1.152 W/(m⸱K) according to the Cahill model. At a temperature of 1273 K, the experimental results demonstrated that (Gd0.8Yb0.2)2Zr2O7 has the lowest thermal conductivity of 1.415 W/(m·K). Furthermore, both the calculated and experimental thermal expansion coefficients of the material decrease first until the Yb3+ concentration is 0.5 and then increase slightly when the Yb3+ content increases. Moreover, gadolinium zirconate ceramics have demonstrated a consistent single-phase fluorite structure, excellent stability at high temperatures, and remarkable structural integrity during repeated heating and cooling cycles. The calculation results exhibit strong congruence with the experimental data. The objective of this work is to improve the thermophysical and mechanical properties of gadolinium zirconate ceramics for their application as materials for thermal barrier coatings.

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.

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.

Low-toxic gelcasting of zircon ceramics

Zircon (ZrSiO4) ceramics are highly valued in various industries due to their exceptional physical and chemical properties. In the present work, zircon ceramics with high-density and desirable mechanical properties were successfully prepared using an aqueous gelcasting method, involving the use of a low-toxic and water-soluble monomer, N,N-dimethylacrylamide (DMAA), as the binder. A high solid loading and low viscosity zircon slurry was achieved by optimizing the pH and the dosage of the ammonium polyacrylate (PAA-NH3) dispersant. Zircon green bodies fabricated through the gelcasting method exhibited uniform microstructures and relatively high mechanical strength exceeding 11.0 MPa. The impact of solid loading on the density, decomposition, flexural strength, micro-hardness, and microstructure of the final ceramics were carefully studied. Notably, the ceramics, prepared by solidifying the slurry with a 45 vol% solid loading and sintering the green body at 1550 °C for 2 h, exhibited a relative density of 96.3 %, high phase purity, a uniform microstructure, and the highest flexural strength of 407.9 MPa. The results demonstrate that mechanically robust dense zircon ceramics can be effectively prepared using the low-toxic gelcasting method.

STUDY OF THE EFFECT OF THE FORMATION OF TWO-PHASE CERAMICS BASED ON NEODYMIUM ZIRCONATE DUE TO DOPING WITH MGO AND Y2O5 ON THE STABILITY OF STRENGTH AND THERMOPHYSICAL PARAMETERS UNDER IRRADIATION

The work presents the study results of the determination of the resistance of neodymium zirconate doped with MgO and Y2O5 to radiation damage stemming from irradiation with heavy ions akin to nuclear fuel fission fragments. The attraction towards this type of ceramics stems from its potential to raise the operational temperatures within the core of next-generation nuclear reactors. This is owed to its superior thermal conductivity when compared to zirconium dioxide, coupled with the heightened strength parameters that signify the ceramics' resistance against external factors. The main results of this study are to determine the influence of the formation of substitution or interstitial phases when adding magnesium and yttrium oxides to the composition, on increasing the stability of the strength and thermophysical parameters of neodymium zirconate to the radiation defects accumulation in the damaged surface layer. During the studies, it was found that the formation of impurity phases in the form of MgO inclusions (when it is added to the composition) and a substitution phase of the Y2Zr2O7 type (with the addition of Y2O5) results in an elevation in the hardness and crack resistance stability of neodymium zirconate ceramics, which indicates the positive effect of doping linked to the formation of additional interphase boundaries that prevent strain embrittlement of the damaged layer under high-dose irradiation. During determination of the thermophysical parameters of the studied neodymium zirconate ceramics, it was observed that the formation of interphase boundaries during doping not only enhances thermal conductivity but also mitigates the decline in the thermal conductivity coefficient during irradiation for two-phase ceramics in comparison with undoped neodymium zirconate ceramics.

Structural, optical, photoluminescence and radiation-shielding investigation of magnesium doped barium zirconate (BaZrO3) ceramics

Magnesium-doped barium zirconate (BaZrO3) powders with various Mg atomic ratios (0.1, 0.2, 0.3, 0.4, and 0.5) have been synthesized by using the solid-state reaction method. The structure and microstructure were studied via scanning electron microscopy, X-ray powder diffraction (XRD), energy dispersive X-ray (EDX), Raman, and Fourier transform infrared (FTIR) spectroscopy. The optical properties were investigated using UV–Vis and photoluminescence spectroscopy. XRD results showed that the synthesized powders have a cubic crystal system with a grain size in the range of 23–27 nm. The band gap is estimated to be about 3.5–3.59 eV. The photoluminescence shows UV and visible emissions. The neutron and gamma shielding features were calculated and assessed based on Phy-X software of energies ranging from 0.015 to 15 MeV. The gamma shielding parameters were generally reduced by adding Mg instead of Ba. The neutron shielding parameter was enhanced gradually. Based on these results, the ability to use the synthesized samples in radiation and neutron shielding applications can be deduced.

Impact of Alumina-Based Binder on Formation of Dense Strontium Zirconate Ceramics

Strontium zirconate (SrZrO3) is a technical ceramic with potential for refractory applications due to its chemical stability at high temperatures, high melting temperature, and favourable thermal expansion coefficient. Practical use of SrZrO3 is limited by poor mechanical strength relative to ceramics such as alumina (Al2O3). Sintering of SrZrO3 with a hydratable Al2O3 binder is investigated as a method for improving mechanical performance. Density, phase composition, thermomechanical properties, and chemical stability in contact with alloys up to 1350°C are considered. Results are compared with SrZrO3 samples formed using a traditional polyvinyl alcohol (PVA) binder. SrZrO3 reacts with alumina during sintering to form SrAl2O4 and ZrO2. Compared to the PVA binder, the alumina binder drastically reduced porosity in the ceramic, while increasing the indirect tensile strength and fracture toughness. No significant detriment to thermal expansion or chemical stability was found, offering potential for high-temperature applications particularly as a dense crucible material.

Pressureless two-step sintering of ultrafine-grained high-entropy zirconate ceramics with excellent mechanical properties

The challenge of preparing dense ultrafine-grained high-entropy ceramics through conventional pressureless sintering is hereby addressed by a simple two-step sintering method. Ultrafine-grained (Ce0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 high-entropy zirconate with 99.2 % theoretical density and 135 nm grain size has been fabricated by pressureless two-step sintering for the first time. Compared to the conventional sintering, two-step sintering provides finer grain sizes and better microstructural uniformity, which yield excellent comprehensive mechanical properties with high hardness of 16.0 GPa, large elastic modulus of 262 GPa and high fracture toughness of 2.8 MPa·m1/2. It is believe that pressureless two-step sintering can be utilized to prepare other high-entropy ceramics with fine grain size and high quality.

Impact of sintering time on structure and electrical properties of Pb-free BNT-BKT-SZ ceramics

The work focuses on preparing and characterising BNT-based ceramics via a solidstate method. To investigate the phase, microstructure, and physical and electrical properties of BNT-based ceramics.Lead-free piezoelectric bismuth sodium titanate – bismuth potassium titanate – stronsium zirconate (BNT-BKT-SZ) ceramics were fabricated by the solid-state reaction method. The effect of sintering temperature with soaking times of 2, 4, and 6 h at 1150C on structural, microstructure, density, porosity, and electrical properties was examined. The phase formation of the ceramics was examined using X-ray diffraction (XRD). Scanning electron microscopy (SEM) (JEOL JSM5910LV) was employed to investigate ceramic microstructure. The bulk density and mechanical properties of the sample were measured using Archimedes’ method, respectively. The electrical properties of ceramics, such as dielectrics, ferroelectrics, and piezoelectrics, were investigated.XRD showed all samples had a single perovskite structure and no secondary phase. All sintered samples at different temperatures have a coexisting phase boundary between the rhombohedral phase and the tetragonal phase. The sintered ceramic at 1150C with a soaking time of 4 h shows a maximum density of 5.89 g/cm3. In addition, the temperature at which the sintering process is carried out substantially impacts the electrical characteristics. Dielectric and electric field-induced strain (Smax) properties that sintered at 1150C with a soaking time of 4 h exhibited the highest values of 4.489 and 0.39% (d33* of 650 pm/V), respectively.The impact of the coercive field on the electrical breakdown characteristics of ceramics should be investigated further in the course of research that has to be carried out.The characterisation confirmed the effects of sintering temperature on the physical, phase, microstructure, and electrical properties of BNT-based ceramics.Such research demonstrates a suitable sintering temperature for producing BNT-BKT-SZ. The mechanical and electrical properties of a material are dependent on its sintering parameters. The ceramic system is suitable for piezoelectric and/or energy storage applications.

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.

Highly conductive grain boundaries in cold-sintered barium zirconate-based proton conductors

A novel processing route for barium zirconate based protonic ceramics using cold sintering enables decreased sintering temperatures as well as high grain boundary conductivity.

Study of the relationship between entropy value and properties of lanthanum zirconate ceramics

In this paper, by regulating the A-site doped rare earth elements to achieve the transformation of entropy, the composition of (La1/3Nd1/3Eu1/3)2Zr2O7 (LNEZ) and (La1/4Nd1/4Eu1/4Y1/4)2Zr2O7 (LNEYZ) medium entropy ceramic system and (La1/5Nd1/5Eu1/5Y1/5Gd1/5)2Zr2O7 (LNEYGZ) high entropy ceramic system were designed and prepared. The results show that the thermal and mechanical properties of the ceramics are enhanced as the entropy value increases, and from this it is demonstrated that the crystal structure of the high-entropy LNEYGZ ceramics is the most disordered. Severe lattice distortions have the effect of hindering and enhancing the behavior of elemental expansion, phonon scattering, etc., and have the effect of enhancing the resistance of ceramics to sintering and heat insulation. Moreover, the small grain size enables the ceramic to have an enhanced ability to deflect cracks, and the fracture toughness of the ceramic is thus enhanced.

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.

Formation of single-phase multicomponent zirconate with colossal atomic radius difference via reactive flash sintering

In this study, multicomponent rare-earth zirconate ceramics (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Zr2O7 and (La0.2Eu0.2Gd0.2Yb0.2Y0.2)2Zr2O7 were synthesized via conventional sintering and reactive flash sintering, respectively. Single-phase (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Zr2O7 ceramics, with the defect fluorite structure, were successfully obtained via conventional sintering and reactive flash sintering, while secondary phase segregation and precipitation were observed only in conventionally-sintered (La0.2Eu0.2Gd0.2Yb0.2Y0.2)2Zr2O7 ceramics. This study proposes that the critical electric field of reactive flash sintering introduces defects to soften the lattice, which not only improves the mass transportation, but also relieves the lattice stress induced by the atomic radius difference, resulting in the single-phase defect fluorite structure of (La0.2Eu0.2Gd0.2Yb0.2Y0.2)2Zr2O7. Thus, reactive flash sintering is an efficient route for synthesizing and developing novel multicomponent oxides that cannot be synthesized via conventional sintering due to pronounced lattice stress.

Rapid and low-temperature preparation of zircon ceramic by hydrothermal-assisted sol-gel process and microwave sintering

During the traditional preparation of zircon, higher sintering temperatures and longer holding times are inevitable. A novel hydrothermal-assisted sol-gel process and microwave sintering (HM, microwave sintering precursors derived from hydrothermal-assisted sol-gel process) were provided to prepare zircon ceramics hereof. The influences of sintering temperature and holding time on the phase composition, microstructure, and compactness of ceramics were investigated. Data display that the properties of the obtained ceramics were highly dependent on the temperature and time, and the ZrSiO4 formation efficiency can be improved by increasing these two factors. The sample prepared by the HM process has the formation efficiency of ZrSiO4 up to 91.74 wt% (HM-1350-300), which far exceeds that of conventional microwave sintering (M-1350-300, 45.02 wt%). The results demonstrate that the HM process can be employed to obtain ZrSiO4 ceramics rapidly at low temperatures.

Thermophysical and mechanical properties of dual-phase medium- and high-entropy rare-earth zirconate ceramics

The medium-entropy ceramic (La1/3Nd1/3Gd1/3)2(Zr3/4Ce1/4)2O7 and high-entropy ceramic (La1/5Nd1/5Sm1/5Eu1/5Gd1/5)2(Zr3/4Ce1/4)2O7 were successfully synthesized. The two ceramics had a pyrochlore-fluorite dual-phase structure, and they exhibited excellent phase stability at elevated temperature. Grain refinement or solution strengthening improved the Vickers hardness, fracture toughness and elastic modulus of the dual-phase ceramics. Compared with the single-element ceramic La2Zr2O7, multicomponent doping resulted in a significant increase in the thermal expansion coefficient. Simultaneously, the thermal conductivity of the dual-phase ceramics was lower than that of single-phase ones. In addition, on account of the large size disorder parameter of the medium-entropy ceramic, the thermal conductivity could outperform its high-entropy counterpart. These results indicate that pyrochlore-fluorite dual-phase rare-earth zirconates have enormous potential in applications concerning next generation thermal barrier coatings.

Impact of nonstoichiometry on the mechanical properties and thermal conductivity of gadolinium zirconate ceramics

Low elastic properties, high hardness, and low thermal conductivity are desirable for a rare-earth zirconate thermal barrier coating material. Realizing nonstoichiometry is a significant way to improve the elastic properties, hardness and thermal conductivity of rare-earth zirconate to further meet the requirements of thermal barrier coating materials, nonstoichiometric Gd2-xZr2+xO7+0.5x (x = 0, ±0.125, and ±0.25) was studied by first-principles calculations first and then verified by the solid-state reaction methods. Calculations revealed that excess Gd3+ and Zr4+ decrease the elastic modulus, sound velocity, and Debye temperature of Gd2Zr2O7, and increase its ductility. The elastic properties of the material exhibit significant anisotropy, which further increases with increasing Gd3+ and Zr4+ contents, resulting from lattice distortion caused by excess Gd3+ and Zr4+ entering the lattice. The hardness of Gd2Zr2O7 increases with a moderate excess of Zr4+, while it decreases with an excess of Zr4+ and Gd3+. In addition, with the disordered occupation of lattice atoms, the anharmonicity of lattice waves is increased, and phonon scattering is enhanced as the concentrations of Gd3+ and Zr4+ increase, resulting in the reduction of the phonon thermal conductivity. Moreover, Zr4+ is more conducive to the decrease in the thermal conductivity. The experimental consequences match well with the calculation consequences. This work is expected to provide support for improving mechanical properties and thermal conductivity of rare-earth zirconate materials.

Evaluation of the reactivity of dense lanthanum‑gadolinium zirconate ceramics with Colima volcanic ashes

The effects of ingestion of airborne particles from pyroclastic events of active volcanoes by aircraft turbines and their subsequent reaction with thermal barrier coatings have attracted the attention of the scientific community in recent years. The reaction products of infiltration experiments of lanthanum‑gadolinium zirconate (LGZO) ceramics with molten ashes from the active Colima volcano at 1250 °C for 10 h are presented and discussed as a function of the Gd3+ content. Five ceramic compositions, varying the Gd3+ content in solid solution were synthesized by the chemical coprecipitation and calcination method of pressed powders. These compositions include pure lanthanum and gadolinium zirconates, LZO, and GZO, respectively. Penetration depth and identification, and in some cases quantification of the reaction products between the molten ash and LGZO ceramics were performed by scanning electron microscopy, chemical composition with energy dispersive X-ray spectroscopy, grazing incident X-ray diffraction as well as micro-Raman spectroscopy. The LZO ceramic exhibited the greatest infiltration resistance with an infiltration depth of approximately 23 μm from the surface. The phase characteristics of the reaction layers were dependent on the gadolinium content. LZO, LGZO25, and LGZO50 (x = 0, 0.25, and 0.5) showed the presence of apatite as well as monoclinic and tetragonal zirconia, while LGZO75 and GZO (x = 0.75 and 1), additionally showed the presence of cubic zirconia and anorthite. As the Gd3+ content increases in the LGZO solid solutions, the wavenumber value corresponding to the stretching vibrational mode of the silicon tetrahedra in apatite shifts from 862 to 877 cm−1, which is associated with a decrease in SiO bond lengths. These findings indicate that the amount and kind of rare earth cations dissolved in the melt plays an important role in the precipitation of the reaction products.