New pathway for the preparation of pyrochlore Nd2Zr2O7 nanoparticles

Pyrochlore Nd₂Zr₂O₇ nanoparticles were prepared by complex-precipitation in aqueous media, followed by calcination in MgO matrix and subsequent dissolution processing. A suite of characterization techniques, including X-ray diffraction, Raman, TEM, SEM, dynamic light scattering, and nitrogen sorption, was employed to investigate the structure and particle size of the synthesized nano materials. Results show that calcination at 1200 °C for 20 h forms Nd₂Zr₂O₇ with pyrochlore structure. The matrix phase (MgO) had no effect on the formation of pyrochlore phase. The MgO phase was readily removed by dissolution at 0.5 M HNO₃ aqueous solution; and the remaining pyrochlore Nd₂Zr₂O₇ nanoparticles had a diameter of approximately 200 nm estimated by TEM and approximately 550 nm determined by light scattering due to slight aggregation. The bulk density of the pelletized powder reached approximately 99% of theoretical value, after uniaxial pressing at 2.0–2.5 MPa and sintering at 1400 °C for 48 h.     

The structure and electrical properties of lithium doped pyrochlore Gd2Zr2O7

The lithium-doped phases Gd_1.7Li_0.3Zr₂O_6.7 and Gd₂Zr_1.7Li_0.3O_6.55 with a pyrochlore structure were prepared by the modified Pechini method using citric acid and glycerol. Monitoring of the lithium content by using a nuclear microanalysis showed that a significant loss of lithium occurred after heat treatment above 1200 °C. Dense ceramics with a stoichiometric lithium content can be prepared by a low temperature microwave sintering (1100 °C). The introduction of lithium in the Gd-sublattice was accompanied by a decrease in the unit cell parameter (a = 10.5208 (1) Å vs 10.5346 (2) Å for Gd₂Zr₂O₇) and during doping at the Zr-sites with lithium, the cell parameter increased (10.5720 (1) Å). The doping in both cases led to an increase in the free cell volume. The impedance spectroscopy results showed that the bulk conductivity can be enhanced by the Li⁺-doping at the Gd³⁺-site by almost an order of magnitude. The sample Gd₂Zr_1.7Li_0.3O_6.55 had a conductivity lower than that of Gd_1.7Li_0.3Zr₂O_6.7 due to the possible trapping of oxygen vacancies by a high-charged acceptor defect LiZr???. The conductivity?pO₂ measurements showed that the Li-containing phase was a pure oxide-ion conductor at T < 800 °C.     

A novel Co-ions complexation method to synthesize pyrochlore La2Zr2O7

Pure pyrochlore Lanthanum zirconate (LZ) was synthesized by co-ions complexation method (CCM) at 1300 °C, which is 300 °C lower than that by solid-state method (SSM). At 1450 °C, the LZ prepared by CCM possessed lower thermal conductivity (1.15 W/m K) than that obtained by SSM (1.99 W/m K). This significant decrease may be caused by the different grain size, which is 300 nm and 2.5 μm synthesized by CCM and SSM, respectively. LZ precursor was belt-shaped and the belt shorten and the grain grown with the temperature increasing. Fourier transform infrared spectroscopy suggested the solidification in CCM forms from the complexation between La³⁺, Zr⁴⁺ and CH₃COO⁻, which is the key for solidification. Compared to SSM, CCM is a lower temperature and simpler technology to synthesize nano-size LZ and other rare-earth oxides.     

Reaction mechanisms of Gd2Zr2O7 in silicate melts derived from CAS

Infiltration mechanisms at high temperatures of sand & ashes (CAS) in aircraft thermal barrier coatings (TBC) have been widely studied using the pyrochlore phase Gd₂Zr₂O₇ (GZ) as TBC. Novelty of this study is GZ reactivity with each or a mix of oxides derived from CMAS have been scrutinized in order to better assess the diffusion, stability and role of each reactivity products in mitigation mechanisms across the CAS-GZ system. In particular, Gd^+III, Si^+IV, O^-II diffusion mechanisms at the GZ/SiO₂ interface have been discussed. These mechanisms shed light on why Gd-oxyapatites are the predominant phases at interfaces between silica-rich medium such as CAS and GZ when reactions are not complete, in contrast to thermodynamic data. Moreover, Gd₂Si₂O₇ phases are closely linked to the growth of Gd-oxyapatites and are the seat of the diffusion barrier to Si^+IV species, which further explains why GZ coatings shortly stops CMAS infiltration.     

Gd2Zr2O7 ceramics synthesized by solid-state reactive sintering: Effects of starting powders with different-scale particle sizes

In this work, the effects of starting oxide powders with different-scale particle sizes on the synthesis of gadolinium zirconate pyrochlore (Gd₂Zr₂O₇, GZO) and its physical properties were studied. Micron Gd₂O₃ (μG), micron ZrO₂ (μZ), nano Gd₂O₃ (nG), and nano ZrO₂ (nZ) powders were used. GZO ceramics were prepared by employing solid-state reactive sintering at 1300 °C, 1400 °C, 1500 °C and 1600 °C with mixed powders of different sizes (μGμZ, μGnZ, nGμZ and nGnZ). X-ray diffraction and Raman analyses of the ceramics revealed that nG has a more significant impact on the crystallization process than nZ. All ceramics synthesized with different sized oxide powders crystallized into pyrochlore phases except for those synthesized with μGnZ mixed powders, which resulted in a fluorite phase. The results indicated that decreasing the particle size of only ZrO₂ to synthesize pyrochlore-phase Gd₂Zr₂O₇ with high crystallinity may not be effective. Samples obtained at 1500 °C were further analyzed. Scanning electron microscopy results revealed that all four ceramics have a non-homogeneous grain size and that the average grain size ranges from 5.40 to 8.30 μm. In addition, the density and Vickers hardness measurements showed that the use of nanopowders significantly improves the mechanical properties.     

Modified Pechini method by PVP addition for Nd:Gd2O3 nanophosphors fabrication

Gadolinium oxide nanopowders doped with neodymium were prepared via modified Pechini method based on double stabilization of nanoparticles. Polyvinylpyrrolidone was used as an additional stabilizer at low-temperature stage of synthesis. This polymer also acts as a fuel during thermal treatment of precursors. Photoluminescence and FTIR spectroscopy, DTA/TG, XRD and SEM analysis were used for the analysis of thermal evolution of initial materials, chemical intermediates and final products at different stages of thermal treatment. Photoluminescence characterization included measurements of excitation and emission spectra of synthesized powders at different stages of thermal treatment.     

Performance of YSZ and Gd2Zr2O7/YSZ double layer thermal barrier coatings in burner rig tests

Double layer thermal barrier coatings (TBCs) consisting of a Gd₂Zr₂O₇ (GZO) top and an ytrria stabilized zirconia (YSZ) interlayer have been tested in a burner rig facility and the results compared to the ones of conventional YSZ single layers. In order to gain insight in the high temperature capability of the alternative TBC material, high surface temperatures of up to 1550 °C have been chosen while keeping the bond coat temperature similar. It turned out that the performance of all systems is largely depending on the microstructure of the coatings especially reduced porosity levels of GZO being detrimental. In addition, it was more difficult in GZO than in YSZ coatings to obtain highly porous and still properly bonded microstructures. Another finding was the reduced lifetime with increasing surface temperatures, the amount of reduction is depending on the investigated system. The reasons for this behavior are analyzed and discussed in detail.     

A new method for production of glass-Ln2Ti2O7 pyrochlore (Ln = Gd,Tb, Er,Yb)

Glass-Ln₂Ti₂O₇ pyrochlore (Ln = Gd, Tb, Er, Yb) was fabricated by sintering the pelletized mixture of glass precursor and Ln-Ti composite at 1200 °C. The phase pure pyrochlore was in-situ crystallized in the amorphous glass matrix. The Ln-Ti composite was prepared by a simple soft chemistry route in an aqueous solution to ensure the homogeneity of the product. Thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy techniques were employed to investigate the glass-Ln₂Ti₂O₇ pyrochlore structure formation. The cell parameters for the pyrochlore structures by Le Bail fitting are in good agreement with the published data. The presence of the melting glass matrix facilitates the pyrochlore formation, with particle sizes in the range of 1–3 μm. This new aqueous synthetic method provides a simple pathway to produce glass-Ln₂Ti₂O₇ pyrochlore without using organic solvent and/or milling procedures, making it an attractive potential method for scale-up production.     

Titanium substitution in Gd2Zr2O7 for thermal barrier coating applications

The study underlines the impact of Ti⁴⁺ substitution in Gd₂Zr₂O₇ for applications in thermal barrier coatings (TBC). Depending on the Ti⁴⁺ content, two different crystal structures of Gd₂Zr₂O₇ namely pyrochlore and fluorite were determined. Ti⁴⁺ substitutions in the increasing order induced a gradual contraction of Gd₂Zr₂O₇ unit cell; however, with the accomplishment of concentration dependent crystal structures of either single phase pyrochlore or mixtures of pyrochlore and fluorite. Absorption measurements enunciated the enhanced infra-red reflectance behaviour of Gd₂Zr₂O₇ due to Ti⁴⁺ substitutions. A gradual increment in the concentration of Ti⁴⁺ substitutions in Gd₂Zr₂O₇ envisaged a simultaneous porous to dense morphological features, which reflected in the resultant mechanical data. Hot corrosion studies ensure the critical role of Ti⁴⁺ to retain the crystal structure of Gd₂Zr₂O₇.     

Characterizations of vacuum sintered Gd2Zr2O7 transparent ceramics using combustion synthesized nanopowder

Highly transparent Gd₂Zr₂O₇ ceramic was fabricated by vacuum sintering using combustion synthesized nanopowder with mean particle size of about 80 nm. The morphology and structure were analyzed by X-ray diffractometer, scanning electron microscopy, Raman spectroscopy and transmission electronic microscopy. The Gd₂Zr₂O₇ nanopowder and transparent ceramic are both in low ordered pyrochlore structure. The effects of sintering temperature on the density and transmittance of Gd₂Zr₂O₇ ceramic were investigated, and the optimum sintering temperature (1825 °C) was obtained. Gd₂Zr₂O₇ transparent ceramic sintered at 1825 °C for 6 h shows the highest transmittance of 77.3 % and the average grain size of about 80 μm.     

Rapid synthesis of Gd2Zr2O7 ceramics by flash sintering and its aqueous durability

The high radiation resistance and long time stability of Gd₂Zr₂O₇ ceramics make it a promising candidate for high level waste (HLW) immobilization materials. In this study, single phase nanocrystalline Gd₂Zr₂O₇ was successfully synthesized and consolidated at temperatures around 1050 °C for only 1 min by flash sintering for the first time. The phase evolution and microstructural development during flash sintering were systematically studied and compared with the conventionally sintered samples. The flash sintered Gd₂Zr₂O₇ exhibit defect fluorite structure, and a following heat treatment at 1400 °C could transform the Gd₂Zr₂O₇ ceramics from defect fluorite phase into pyrochlore phase. The MCC-1 leaching test shows that the flash sintered Gd₂Zr₂O₇ samples exhibit good aqueous durability.     

Improving room-temperature electrochemical performance of solid-state lithium battery by using electrospun La2Zr2O7 fibers-filled composite solid electrolyte

Low ionic conductivity at room temperature and poor interfacial compatibility are the main obstacles to restrain the practical application of polymer solid electrolytes. In this work, lanthanum zirconate (LZO) fibers were prepared by electrospinning method and used for the first time as fillers in sandwich polypropylene carbonate (PPC)-based solid electrolyte. Meanwhile, a graphite coating was applied on one surface of the composite solid electrolyte (CSE) membrane. The results show that the LZO fibers significantly increases the room-temperature electrochemical performance of the CSE, and the graphite coating enhances the interfacial compatibility between electrolyte and lithium anode. Furthermore, an ultra-thin PPC-LZO CSE with a total thickness of 22 μm was prepared and used in NCM622/CSE/Li solid-state cell, which shows an initial discharge capacity of 165.6 mAh/g at the current density of 0.5C and a remaining capacity of 113.0 mAh/g after 250 cycles at room temperature. Rise to 1C, the cell shows an initial discharge capacity of 154.2 mAh/g with a remaining capacity of 95.6 mAh/g after 250 cycles. This ultra-thin CSE is expected to be widely applied in high energy-density solid-state battery with excellent room-temperature electrochemical performances.     

High capacity immobilization of U3O8 in Gd2Zr2O7 ceramics via appropriate occupation designs

An optimal occupation of U⁴⁺ and U⁶⁺ in Gd₂Zr₂O₇ is necessary for a really high immobilization capacity of U₃O₈ in Gd₂Zr₂O₇ based waste forms. Based on four kinds of occupation methods, a series of U₃O₈-doped Gd₂Zr₂O₇ compositions have been synthesized. The effects of U₃O₈ content on the phase and microstructure evolution of Gd₂Zr₂O₇ pyrochlore waste forms were investigated. Detailed XRD analysis show that the four sets of samples exhibit a single defect fluorite structure within the range of 0<x≤0.4, 0<x≤0.66, 0<x≤0.6 and 0<x≤1, respectively. The highest solubility of U₃O₈ is about 82.29 wt% when the occupation design (U⁴⁺ and U⁶⁺ substitute for Gd and Zr, respectively) was employed. It was found that the cell parameters of compounds in Set A (Gd_2–3x(U⁴⁺_xU⁶⁺_2x)Zr₂O_7+7x/2) decrease with increasing _x, while those of the other compositions increase. Moreover, the uranium are almost homogeneously distributed in all samples.     

Role of structural ordering on the radiation response of Gd2Zr2O7 pyrochlore

Complex oxides with pyrochlore and fluorite phases offer several advantages for the Immobilization of actinides or high-level radioactive wastes. In this report, we present the different behavior of structural ordering/crystallinity of Gd₂Zr₂O₇ (GZO) ceramics upon sintering at two different temperatures (1400°C–1500 °C). XRD and Raman spectroscopy studies revealed the enhancement of structural ordering/crystallinity with the increase of sintering temperature. Further, the ion irradiation experiments using 100 MeV iodine at the fluence of 1.0 × 10¹⁴ ions/cm² were performed to investigate the radiation effects on both GZO ceramics. The irradiation studies insinuate that the GZO ceramic sintered at 1500 °C possesses relatively better radiation resistance than GZO ceramic sintered at 1400 °C. The variation in the radiation resistance response of GZO ceramics seems associated with the different degrees of structural ordering. These results suggest the role of structural ordering in the radiation resistance response of GZO ceramics. The relatively better radiation tolerance of GZO15 ceramic with some extant pyrochlore phase ordering may be suitable for applications in harsh environments.     

Solid-state reactive sintering of La2-xGdxZr2O7 transparent ceramics and their optical properties

Transparent polycrystalline La_2-xGd_xZr₂O₇ (x = 0.4–2.0) ceramics were fabricated by solid-state reactive sintering using commercially available La₂O₃, Gd₂O₃ and ZrO₂ nanopowders as the raw materials. The phase composition, microstructure evolution, densification and optical transmittance of the resulting ceramics were investigated. XRD and Raman results reveal that both the powders and ceramics are in single-phase pyrochlore structure. With the promotion of Gd content, the disorder degree of pyrochlore structure increases gradually while the cell parameters decrease. The x = 1.0, 1.2 and 1.6 samples exhibit high optical transmittance in the 450 nm-6.0 μm range, and the La_0.4Gd_1.6Zr₂O₇ sample shows the highest transmittance of 83.84%. The transparent La_2-xGd_xZr₂O₇ ceramics with high optical quality are available as scintillator matrice and high temperature window material.     

Immobilization of simulated waste into pure Gd2Zr2O7 pyrochlore without space occupancy design

In this study, different molar ratios of Nd:Ce were directly mixed with prepared pure Gd₂Zr₂O₇ powders without space occupancy design. Samples were obtained by performing sparking plasma sintering (SPS) at 1750°C for 5 minutes. X-ray diffraction (XRD) results show that maximum solid solubility of simulated radionuclides can reach 50 mol%. In addition, all samples with the maximum solid solubility have high compactness, and all elements are evenly distributed on the surface of the samples. The samples show a better crystallization effect as the molar ratio of Nd:Ce increases. The maximum solid solubility increases from 42 mol% to 50 mol% when the amount of Nd₂O₃ reaches 66 mol%.     

Isothermal Oxidation Behavior of Gd2Zr2O7/YSZ Multilayered Thermal Barrier Coatings

Efficiency of a gas turbine can be increased by increasing the operating temperature. Yttria‐stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used in gas turbine applications. However, above 1200°C, YSZ undergoes significant sintering and CMAS (calcium magnesium alumino silicate) infiltration. New ceramic materials of rare earth zirconate composition such as gadolinium zirconate (GZ) are promising candidates for thermal barrier coating applications (TBC) above 1200°C. Suspension plasma spray of single‐layer YSZ, double‐layer GZ/YSZ, and a triple‐layer TBC comprising denser GZ on top of GZ/YSZ TBC was attempted. The overall coating thickness in all three TBCs was kept the same. Isothermal oxidation performance of the three TBCs along with bare substrate and bond‐coated substrate was investigated for time intervals of 10 h, 50 h, and 100 h at 1150°C in air environment. Weight gain/loss analysis was carried out by sensitive weighing balance. Microstructural analysis was carried out using scanning electron microscopy (SEM). As‐sprayed single‐layer YSZ and double‐layer GZ/YSZ showed columnar microstructure, whereas the denser layer in the triple‐layer TBC was not columnar. Phase analysis of the top surface of as‐sprayed TBCs was carried out using XRD. Porosity measurements were made by water intrusion method. In the weight gain analysis and SEM analysis, multilayered TBCs showed lower weight gain and lower TGO thickness compared to single‐layer YSZ.     

High density nano-grained Gd2Zr2O7 ceramic prepared by combined cold and microwave sintering

In this study, nano-grained Gd₂Zr₂O₇ (NGZO) ceramic with a high relative density was prepared by a novel cold sintering process (CSP) assisted by microwave sintering (CSP-MS). The CSP with water as the liquid phase at 280 °C yielded nano-grained ceramic with a relative density of 71.5%. NGZO with a high relative density of 97.1% and average grain size of 73 nm was obtained by subsequent microwave sintering of the cold-sintered sample at 1300 °C. Therefore, CSP-MS is feasible in preparing dense NGZO ceramics with small grain sizes at relatively low sintering temperatures.     

Rapid fabrication and phase transition of Nd and Ce co-doped Gd2Zr2O7 ceramics by SPS

A series of Nd and Ce co-doped Gd_2-xNd_xZr_2-yCe_yO₇ (0.0 ≤ x, y ≤ 2.0) ceramics were rapidly fabricated through spark plasma sintering (SPS) within 3?min. The effects of Nd and Ce contents on the phase composition, lattice parameter, active modes, microtopography and microstructure have been investigated in detail. XRD studies reveal that the compositions corresponding to 0.0 ≤ y ≤ 1.0 show a single phase and beyond 1.0 exhibit multiphase. The lattice parameters increase with elevated Nd and Ce content. The grains are densely packed on each other with cube-like shape, and the elements are almost homogeneously distributed in the compound. This synthetic method provides a simple pathway for the preparation of highly densified single phase ceramic at 1600–1700 ℃ for 3?min under pressure of 80?MPa.