Gadolinium Zirconate Research Articles

Sustainable synthesis of pyrochlore gadolinium zirconate through the molten salt method: A green approach

Gadolinium zirconate (GZO) with pyrochlore phase exhibits exceptional promise for thermal barrier coatings due to its notable low thermal conductivity. Current synthesis methods for GZO, however, are frequently complex, energy-intensive, and environmentally hazardous. A more sustainable and eco-friendly synthesis route for GZO is thus imperative. This research introduces a green synthesis process for pyrochlore GZO utilizing the molten salt method.Experimental synthesis at varying temperatures revealed changes in the phase composition of GZO via the molten salt method. Fluorite GZO formed at 1200 °C, while pyrochlore GZO appeared at 1400 °C. XRD refinement, TEM, and Raman spectroscopy confirmed these results. Density Functional Theory (DFT) calculations established the minimum theoretical synthesis temperature for pyrochlore GZO at 1370 °C. Limiting factors for reactions at different temperatures during the molten salt synthesis process were identified. DFT calculations and experiments verified that the fluorite structure GZO transitions to pyrochlore GZO at temperatures exceeding 1370 °C. Property tests on pyrochlore GZO samples synthesized through two pathways showed minimal differences. This study thus lays a theoretical foundation for the eco-friendly synthesis of pyrochlore GZO material using the molten salt method and offers insights for future process development.

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.

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.

Assessment of the microstructural features and electrochemical corrosion behaviour of nanostructured Gd2Zr2O7 EB-PVD TBCs under three salt conditions

Improving corrosion resistance is a challenging task, as it determines the life time of thermal barrier coatings (TBCs). In this regard, this study was aimed to characterize the electrochemical corrosion behaviour of nanostructured Gadolinium Zirconate in three different corrosive salt conditions such as Na2SO4 + 55 wt% V2O5 (SM1), Na2SO4 + 10 wt% NaCl (SM2), and Na2SO4 + 7.5 wt% NaCl +7.5 wt% V2O5 (SM3). The uniformity of the coated surface was verified by mapping the vibrational bands associated with Gd and Zr using FT-IR and Raman spectroscopy. The coated surface possessed a uniform columnar structure with leafy appearance. Potentiodynamic polarisation studies and electrochemical impedance spectroscopic data demonstrated that the nanostructured GZ exhibited superior corrosion resistance properties under SM1 salt condition as compared to SM2 and SM3 conditions. SEM and EDS analysis were used to evaluate the nanostructured GZ coatings that were subjected to electrochemical corrosion. In the case of SM2 and SM3 conditions, the surfaces were degraded with microcracks. The formation of corrosion products and the exposure of succeeding layers as a result of corrosion were also characterized using FT-IR, Raman spectroscopy, XRD technique and 3D optical microscope.

High temperature infiltration behavior and reaction characteristics of Colima volcanic ashes on gadolinium zirconate coatings deposited by atmospheric plasma spraying

Volcanic ashes are considered a serious threat to the aircraft industry. At high temperatures, they inflict severe thermochemical damage to the typical 7 wt.% yttria-stabilized zirconia thermal barrier coatings that protect the aircraft turbine. There is a need to evaluate alternative materials with excellent resistance to infiltration of molten siliceous particles, such as gadolinium zirconate. In this work, free-standing thermal barrier coatings of gadolinium zirconate were manufactured by atmospheric plasma spraying, varying deposit parameters to obtain different relative densities to evaluate the infiltration of molten Colima volcanic ashes for 1 h and 10 h at 1250 °C. The infiltration depth and the reaction products resulting from each interaction, were studied by different characterization techniques. In general, the coatings show high resistance to the infiltration of the volcanic ashes, reaching infiltration depths between 50 µm and 80 µm after 10 h of infiltration time. In this sense, gadolinium zirconate coatings are excellent candidates against the infiltration of Colima volcanic ashes compared to the typical yttria-stabilized zirconia-based thermal barrier coatings.

Comparative study on the synthesis and characterization of Gd2Zr2O7 nanoparticles by mechanical milling and Co-precipitation techniques

Gadolinium Zirconate nanoparticles were synthesized by Co-precipitation and Ball milling techniques. In this study, the effect of the synthesis technique and calcination temperature on the structural, morphological and thermal properties were investigated. The thermal properties examined showed that the GZ nanoparticles obtained from CP technique suffered two-fold weight loss as compared to BM technique. The SEM results revealed the formation of agglomerated particles. However, TEM results confirms the formation of nanoparticles of 10.5 nm and 8 nm by BM and CP techniques respectively. GZ particles obtained through CP technique showed better uniformity of particle size over BM technique. The functional groups of the nanoparticle were studied using FT-IR spectroscopy analysis. The XRD results confirm the formation of defect-fluorite structure and pyrochlore structure for the samples calcined at 1100 °C and 1400 °C (BM) respectively. The formation of the corresponding structures was further confirmed by Raman Spectroscopy. The thermal conductivity of the nanoparticle produced by ball milling and co-precipitation technique is almost same. As compared to NiCrAlY and uncoated substrate materials, the micro hardness of the GZ coatings has the maximum hardness of 594.6 HV, indicating the improved hardness behavior of TBC system.

Hot Corrosion Behavior of Plasma-Sprayed Gd2Zr2O7/YSZ Functionally Graded Thermal Barrier Coatings

The development of advanced thermal barrier coating (TBC) materials with better hot corrosion resistance, phase stability, and residual stresses is an emerging research area in the aerospace industry. In the present study, four kinds of TBCs, namely, single-layer yttria-stabilized zirconia (YSZ), single-layer gadolinium zirconate (GZ), bilayer gadolinium zirconate/yttria-stabilized zirconia (YSZ/GZ), and a multilayer functionally graded coating (FGC) of YSZ and GZ, were deposited on NiCrAlY bond-coated nickel-based superalloy (Inconel 718) substrates using the atmospheric plasma spray technique. The hot corrosion behavior of the coatings was tested by applying a mixture of Na2SO4 and V2O5 onto the surface of TBC, followed by isothermal heat treatment at 1273 K for 50 h. The characterization of the corroded samples was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) to identify physical and chemical changes in the coatings. GIXRD was used to analyze the residual stresses of the coatings. Residual stress in the FGC coating was found to be −15.2 ± 10.6 MPa. The wear resistance of TBCs is studied using a linear reciprocating tribometer, and the results indicate that gadolinium zirconate-based TBCs showed better performance when deposited in bilayer and multilayered functionally graded TBC systems. The wear rate of as-coated FGC coatings was determined to be 2.90 × 10−4 mm3/Nm, which is lower than the conventional YSZ coating.

Separation and determination of percentage level niobium in gadolinium zirconate using bifluoride fusion and spectrophotometry

A simple and reliable method for the determination of percentage level niobium (3.5-8 %) in highly refractive gadolinium zirconate (Gd2Zr2O7) doped with niobium, a burnable poison material of nuclear reactor has been proposed. A fusion procedure using ammonium bifluoride (ABF) with Gd2Zr2O7 was developed. Fusion conditions such as time, temperature and ABF to sample ratio were optimised for realizing quantitative recovery of Nb. The fused masses obtained under different conditions were characterized using SEM-EDX and XRD to comprehend the conversion of the refractive oxide form to soluble fluoro compounds and their identification. The real samples fused under optimized conditions were dissolved in 2 % tartaric acid solution and the same was used for quantifying Nb employing spectrophotometry using 5-bromo pyrogallol red as chromogenic agent. Precision of the measurements was better than 3 % (RSD) for Nb(V) at 0.5 ppm and the accuracy of the method is 99.8 %.

Randomised nano-/micro- impact testing – A novel experimental test method to simulate erosive damage caused by solid particle impacts

A novel randomised nano-/micro-scale impact test method has been developed to experimentally simulate particulate erosion where statistically distributed impacts with defined energy occur sequentially within the test area. Tests have been performed on two brittle glasses (fused silica and BK7) to easily highlight the interaction between impacts, as well as on two ceramic thermal barrier coating systems (TBCs, yttria stabilised zirconia, 7YSZ, and gadolinium zirconate, GZO) that experience erosion in service. Differences in erosion resistance were reproduced in the randomised impact tests, with GZO less impact resistant than 7YSZ, and BK7 significantly worse than fused silica. The impact data show that erosion resistance is influenced by different factors for the glasses (crack morphology, longer-length interaction of radial-lateral cracks in BK7 vs cone-cracking in fused silica) and TBCs (fracture toughness).

A green approach for synthesizing high pure gadolinium zirconate nano-particles using microwave energy

Nano crystalline high purity gadolinium zirconate powder has been prepared by wet chemical synthesis method using microwave energy. Co-precipitation of zirconium oxychloride and gadolinium nitrate under the influence of microwave energy is studied. The powder has been characterized using thermal TG-DTA/DSC, XRD, EDS and FESEM. The average crystallite size (ACS) of the ZrO2 (600 °C) nano-particles is found to be 26 nm. The characterization studies have shown the existence of non agglomerated nano-crystalline particles with regular and uniform spherical morphology. Microwave energy is found to assist in achieving the desired properties of gadolinium zirconate. The microwave assisted synthesis is faster, better and green chemistry approach which not only provides the enhanced reaction rates and high yields but also, it provides short reaction time, higher selectivity and reduction of pollutant solvents. The key features of obtaining the pure non agglomerated nano-structured gadolinium zirconate powder are the notable attractions of the present study.

SEM-Guided Finite Element Simulation of Thermal Stresses in Multilayered Suspension Plasma-Sprayed TBCs

This study presents novel insights into thermal stress development and crack propagation mechanisms in single- and multilayered suspension plasma-sprayed (SPS) coatings of gadolinium zirconate (GZ) and yttria-stabilized zirconia (YSZ), thermally treated at 1150 °C. By combining image processing with finite element simulation, we pinpointed sites of high-stress concentration in the coatings, leading to specific cracking patterns. Our findings reveal a dynamic shift in the location of stress concentration from intercolumnar gaps to pores near the top coat/thermally grown oxide (TGO) interface with TGO thickening at elevated temperatures, promoting horizontal crack development across the ceramic layers. Significantly, the interface between the ceramic layer and TGO was found to be a critical area, experiencing the highest levels of both normal and shear stresses. These stresses influence failure modes: in double-layer SPS structures, relatively higher shear stresses can result in mode II failure, while in single-layer systems, the predominant normal stresses tend to cause mode I failure. Understanding stress behavior and failure mechanisms is essential for enhancing the durability of thermal barrier coatings (TBCs) in high-temperature applications. Therefore, by controlling the interfaces’ roughness along with improving interfacial toughness, the initiation and propagation of cracks can be delayed along these interfaces. Moreover, efforts to optimize the level of microstructural discontinuities, such as intercolumnar gaps and pores, within the creaming layer and close to the TGO interface should be undertaken to reduce crack formation in the TBC system.

Inductively coupled plasma optical emission spectroscopic determination of trace rare earth elements in highly refractory gadolinium zirconate (Gd2Zr2O7).

Gadolinium zirconate (Gd2Zr2O7) belongs to the category of burnable absorber (BA) material in nuclear reactors. The high neutron-absorption cross sections of Gd isotopes (155Gd and 157Gd) implement negative reactivity in the reactor core to control the excess reactivity of the fuel at the beginning of the fission cycle. The presence of other rare earth elements, which can come from the starting material and/or may be taken up during the synthesis steps, will affect the negative reactivity calculation. Thus the chemical quality assurance of Gd2Zr2O7 concerning the other trace rare earth impurities is indispensable. Here in this work, we decided to quantify thirteen trace rare earth elements in Gd2Zr2O7 by inductively coupled plasma optical emission spectroscopy (ICP-OES). One of the matrix elements viz., zirconium was separated via precipitation using d,l-mandelic acid. The trace rare earths were determined in the presence of gadolinium matrix in solution. It was found that all thirteen rare earth elements can be quantified in the range of 0.25 to 2.5 mg L-1 in the presence of 516 mg L-1 of Gd with a relative standard deviation of 1-3%. This corresponds to the determination of a minimum of 0.25 mg analyte per g of Gd2Zr2O7. The method detection limits of all thirteen rare earth elements range between 0.01 and 0.075 mg g-1. The proposed methodology was validated by analyzing synthetic standards and real samples with spike addition.

Amorphization of Gd2Zr2O7 induced by mechanical deformation in nanoindentation

Amorphization has become an important plastic deformation mechanism for covalently-bonded ceramics, which can disturb their crystalline order and affect their mechanical behaviors. Unlike the covalently-bonded ceramics, however, amorphization induced by mechanical deformation is rarely observed in ionically-bonded ceramics. In this work, we show direct experimental evidence that nano-scale amorphous zones in Gadolinium Zirconate ceramic are the ultimate stage of plasticity and relate to an accumulation of dislocations. In addition, the occurrence of amorphization also causes the unusual indentation size effect.

Internal Diameter Atmospheric-Plasma-Sprayed High-Performance YSZ-Based Thermal Barrier Coatings

Thermal barrier coatings (TBCs) play a crucial role in enhancing the operating temperature of key equipment. However, when TBCs are sprayed on the internal surface of these components, there are issues related to the imperfect internal diameter atmospheric plasma spray (ID-APS) process and the unsatisfactory performance of TBCs in internal holes. Yttria-stabilized zirconia (YSZ), YSZ + Gadolinium zirconate (GZ) and YSZ + tantalate (RETaO4) TBCs were prepared using the SM F210 ID-APS gun. The thermal shock and particle erosion resistance of the coatings were compared, and the relationship between the structure and properties was investigated. The results show that the hardness, fracture toughness and erosion resistance of the three coatings are the largest in the YSZ coating and the smallest in the GZ coating. YSZ coating also has the best thermal shock resistance. These research findings have important guiding significance for the improvement of the ID-APS process.

Thermophysical Properties of Neodymium and Gadolinium Zirconate Hafnates

Pyrochlore-type neodymium and gadolinium zirconate hafnates have been prepared and identified. The heat capacities of the prepared samples have been measured by differential scanning calorimetry in the range 310–1800 K. Temperature-dependent cubic unit cell parameters have been determined and thermal expansion coefficients assessed in the range 298–1273 K using high-temperature X-ray diffraction. The thermal diffusivity of the samples was measured by the laser flash method, and the temperature-dependent thermal conductivity was calculated taking into account the porosity of the samples.

A two-material thermal barrier coating spatially tailored for high-efficiency GCI combustion

Continued research into thermal barrier coatings (TBCs) for internal combustion engines has generated insights into the design of TBCs that can increase fuel conversion efficiency. In this work, two low thermal inertia TBCs (a commercially available Gen 1 material, Gadolinium Zirconate, and a proprietary Gen 2 material) were experimentally tested in gasoline compression ignition (GCI) on a light-duty single cylinder research engine. Compared to a metal baseline, a 250-micron thick Gen 1 coated piston produced an efficiency benefit of 0.4 percentage points (pp) at 6 and 10 bar net indicated mean effective pressure (IMEPn) and a 0.2 pp penalty at 15 bar IMEPn. The Gen 1 coating also showed an uHC and smoke penalty. An 80-micron thick Gen 2 coated piston showed an efficiency benefit of 0.9 pp at 6 bar IMEPn and 0.4 pp at 10 bar IMEPn, but an efficiency penalty of 0.5 pp at 15 bar IMEPn. The Gen 2 coating, which has a lower thermal inertia than the Gen 1 coating, displayed durability problems while the Gen 1 coating did not. A third piston was coated with a 120-micron thick Gen 2 coating everywhere except the high failure outer bowl region, which was coated with a 120-micron Gen 1 coating. This hybrid coating displayed good durability, an uHC emission penalty, a smoke benefit, an efficiency benefit of 0.1 pp at 6 bar IMEPn, an efficiency benefit of 0.4 pp at 10 bar IMEPn, and no change in efficiency at 15 bar IMEPn. These results indicate that a durable Gen 1/Gen 2 coating can provide an efficiency benefit to GCI combustion, and that further improvements on piston-spray interaction optimization and surface sealing of the TBC, as well as increasing the coated combustion chamber surface area (e.g. head and valves) can add further improve efficiency.

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.

Microstructure Refinement of EB-PVD Gadolinium Zirconate Thermal Barrier Coatings to Improve Their CMAS Resistance

Rare-earth zirconates are proven to be very effective in restricting the CMAS attack against thermal barrier coatings (TBCs) by forming quick crystalline reaction products that seal the porosity against infiltration. The microstructural effects on the efficacy of Electron Beam-Physical Vapor Deposition gadolinium zirconate (EB-PVD GZO) against CMAS attack are explored in this study. Four distinct GZO microstructures were manufactured and the response of two selected GZO variants to different CMAS and volcanic ash melts was studied for annealing times between 10 min and 50 h at 1250 °C. A significant variation in the microstructural characteristics was achieved by altering substrate temperature and rotation speed. A refined microstructure with smaller intercolumnar gaps and long feather arms lowered the CMAS infiltration by 56%–72%. Garnet phase, which formed as a continuous layer on top of apatite and fluorite, is identified as a beneficial reaction product that improves the CMAS resistance.

Thermal expansion coefficient of nonstoichiometric gadolinium zirconate: First-principles calculations and experimental study

A high thermal expansion coefficient (TEC) is highly desirable for a thermal barrier coating material to prolong the life of the entire coating system. The TECs of Gd2-xZr2+xO7+0.5x (x = 0, ±0.125, ±0.25, ±0.375) have been studied by first-principles calculations and solid-state reactions. Due to the strong bond energy and interatomic interaction force between Zr and O, and Gd3+ having a higher thermal diffusion coefficient and lower solid solubility, the TEC of a material with higher Gd3+ content and lower Zr4+ content is improved. Calculated results show that the highest Gd3+ content (x = −0.375) increases the TEC of Gd2-xZr2+xO7+0.5x from 7.26 × 10−6/K (x = 0.375) to 8.95 × 10−6/K (1500K). The Debye temperature decreases with increasing Gd3+ content. The experimental TEC is highest (11.593 × 10−6/K) at a content of x = −0.375, far exceeding that at x = 0.375 (10.690 × 10−6/K). The experimental results agree well with the calculated results. This work lays a foundation for obtaining rare-earth zirconate materials with outstanding thermal physical properties, and it provides a novel method for their creation.