An abnormal incorporation behavior of Th in Gd2Zr2O7: A first-principles study

A theoretical study of Th accommodation in Gd₂Zr₂O₇ has been performed by density functional theory. Our calculations show that although thorium has only one charge state of Th⁴⁺, it can be incorporated into both Gd³⁺ and Zr⁴⁺ sites in Gd₂Zr₂O₇, depending on the chemical environments. Th occupation at Gd³⁺ site results in charge redistribution and the excess electrons introduced by Th are transferred to the neighboring Zr ions. As compared with the pure state, Th-containing Gd₂Zr₂O₇ pyrochlores are probably more inclined to undergo order-disorder transformation and are less susceptible to radiation-induced amorphization.     

Low energy ion‐solid interactions and chemistry effects in a series of pyrochlores

The effect of chemistry on low energy recoil events was investigated at 10 K for each type of atom in pyrochlores, using molecular dynamics simulation. Contour plots of the threshold displacement energy (E_d) in Gd₂Zr₂O₇ have been produced along more than 80 directions for each individual species. The E_d surface for each type of atom in Gd₂Zr₂O₇ is highly anisotropic; E_d of Zr exhibits the largest degree of anisotropy, while that of O_8b exhibits the smallest. The recommended values of E_d in Gd₂Zr₂O₇ based on the observed minima are 56, 94 and 25 eV, respectively, for Gd, Zr, and O. The influence of cation radius on E_d in pyrochlores A₂B₂O₇ (with A‐site ranging from Lu³⁺ to La³⁺ and B‐site ranging from Ti⁴⁺ to Ce⁴⁺) was also investigated along three directions [100], [110], and [111]. The E_d in pyrochlores strongly depended on the atom type, atom mass, knock‐on direction, and lattice position. The defects produced after low energy displacement events included cation antisite defects, cation Frenkel pairs, anion Frenkel pairs, various vacancies, and interstitials. Ce doping in pyrochlores may affect the radiation response, because it resulted in drastic changes in cation and anion displacement energies and formation of an unusual type of anti‐site defect. This work demonstrates links between E_d and amorphization resistance.     

Fabrication and characterization of novel excellent thermal-protection Gd2Zr2O7/ZrO2 composite ceramic fibers with different proportions of Gd2Zr2O7

Three kinds of Gd₂Zr₂O₇/ZrO₂ (GZC) composite fibers with different proportions of Gd₂Zr₂O₇ were prepared by electrospinning method through changing the amount of Gd³⁺ in precursor solutions. The thermal decomposition, crystallization process, high temperature stability and heat-conducting properties of GZC fibers were fully characterized. The results showed that there were three crystalline phases, tetragonal phase ZrO₂, cubic phase ZrO₂ and defect fluorite phase Gd₂Zr₂O₇ in all the GZC fibers. The content of Gd₂Zr₂O₇ increased gradually with the increase of Gd³⁺ in precursor solutions which led to the gradual slowing down of grain growth rate, the decrease of thermal conductivity and the increase of high temperature stability of the obtained composite fibers. The thermal conductivities of all the GZC fiber sheets were lower than that of 7YSZ fiber sheet. The sheets of all the GZC fibers could keep the high temperature stability up to 1300 °C.     

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.     

Thermal behaviour of multilayer and functionally-graded YSZ/Gd2Zr2O7 coatings

Zirconates with pyrochlore structure, such as Gd₂Zr₂O₇, are new promising thermal barrier coatings because of their very low thermal conductivity and good chemical resistance against molten salts. However, their coefficient of thermal expansion is low, therefore their thermal fatigue resistance is compromised. As a solution, the combination of yttria-stabilised zirconia (YSZ) and Gd₂Zr₂O₇ can reduce the thermal contraction mismatch between the thermal barrier coating parts.In the present study, two possible designs have been performed to combine YSZ/Gd₂Zr₂O₇. On the one hand, a multilayer coating was obtained where YSZ layer was deposited between a Gd₂Zr₂O₇ layer and a bond coat. On the other hand, a functionally-graded coating was designed where different layers with variable ratios of YSZ/Gd₂Zr₂O₇ were deposited such that the composition gradually changed along the coating thickness.Multilayer and functionally-graded coatings underwent isothermal and thermally-cycled treatments in order to evaluate the oxidation, sintering effects and thermal fatigue resistance of the coatings. The YSZ/Gd₂Zr₂O₇ multilayer coating displayed better thermal behaviour than the Gd₂Zr₂O₇ monolayer coating but quite less thermal fatigue resistance compared to the conventional YSZ coating. However, the functionally-graded coating displays a good thermal fatigue resistance. Hence, it can be concluded that this kind of design is ideal to optimise the behaviour of thermal barrier coatings.     

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.     

Characterization of microstructure and thermal properties of Gd2Zr2O7-type thermal barrier coating

We present herein a characterization of the microstructure and thermal properties of thermal barrier coatings (TBCs), which we obtained via plasma spraying of powder Gd₂Zr₂O₇. By using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD), we evaluated the phase composition of a ceramic layer and estimated the ceramic-layer stress state by the sin²ψ method. The tests revealed that the TBC layer consisted of a single-phase structure of Gd₂Zr₂O₇, namely, an Fd3m lattice. The thermal diffusivity of the outer ceramic layer was determined based on a bilayer model and corrected with a factor to account for the presence of pores. The results reveal that the use of the standard parameters in a standard spraying process gives good-quality Gd₂Zr₂O₇ TBCs with a thermal conductivity considerably lower than 8YSZ-type TBCs.     

Investigation of Native Point Defects and Nonstoichiometry Mechanisms of Two Yttrium Silicates by First‐Principles Calculations

First‐principles method is used to study the native point defects in Y₂SiO₅ and Y₂Si₂O₇ silicates. The calculated defect formulation energies show similar native point defect behaviors in Y₂SiO₅ and Y₂Si₂O₇: the oxygen Frenkel defect is predominant; and it is followed by the cation antisite and Schottky defects. The possible chemical potential range of each constituent is further considered in the calculation of defect formation energy. Oxygen interstitial (O_i) and oxygen vacancy (V_O) are the predominant native point defects under O‐rich and O‐poor condition, respectively. In addition, the mechanisms of accommodating composition deviations from stoichiometric Y₂SiO₅ and Y₂Si₂O₇ are investigated. For Y₂SiO₅, Y₂Si₂O₇ impurity may appear, together with the defects of Si_Y antisite, O_i interstitial, and/or V_Y vacancy when SiO₂ is excess; while Y_Si antisite appears together with Y_i interstitial and/or V_O vacancy in Y₂SiO₅ when Y₂O₃ is excess. For Y₂Si₂O₇, the main process is the formation of Si_Y antisite accompanied by O_i interstitial and/or V_Y vacancy when SiO₂ is excess; but Y₂SiO₅ impurity forms, together with Y_Si antisite, V_O vacancy, and/or Y_i interstitial in Y₂Si₂O₇ when Y₂O₃ is excess. We expect that the results are useful to control of processing conditions and further to optimization of performance of the two silicates.     

Influence of doping Mg2+ or Ti4+ captions on the microstructures,thermal radiation and thermal cycling behavior of plasma-sprayed Gd2Zr2O7 coatings

Gadolinium zirconate (Gd₂Zr₂O₇) coatings doped by the transition metal Ti and the alkaline earth metal Mg were expected to have improved thermal radiation performance, which could be combined with their excellent thermal barrier properties to comprehensively improve the thermal insulating performance. The results show that the parent Gd₂Zr₂O₇ powder as well as the Gd-site and Zr-site substituted powders crystallize as pyrochlore Gd₂Zr₂O₇ in Fd-3m space group, while all the as-sprayed coatings have the combination of fluorite and a little part of pyrochlore phase. Gd₂Zr₂O₇ ceramic has high mid-infrared emittance and the addition of Ti⁴⁺ into Gd₂Zr₂O₇ can enhance the infrared absorption/emittance in a specific wavenumber range, dominantly in the near-infrared (0.75–2.5 μm) band due to the enhancement of electron transition induced by the impurity energy levels linked to the widening of the conduction band. The normal spectral infrared emissivity of Gd₂Zr₂O₇-based coating was higher than 0.88 at 1073 K. The monolayered doped Gd₂Zr₂O₇ coatings present very low thermal cycling lifetime, similar with the parent coating, mainly related with their low fracture toughness, despite (Gd_1-xMg_x)₂Zr₂O₇ series display lower thermal conductivity than the parent one.     

Preparation of Gd2Zr2O7 nanopowders by polyacrylamide gel method and their sintering behaviors

Well-crystallized and good-dispersed Gd₂Zr₂O₇ nanopowders with defect-fluorite structure are successfully prepared by the polyacrylamide gel method. The mole ratio of acrylamide/Gd and calcination temperature have the significant effects on the phase composition, particle size, and agglomeration degree of the Gd₂Zr₂O₇ nanopowders. The sintering behaviors of the as-prepared Gd₂Zr₂O₇ nanopowders are investigated at temperatures of 1200?1500 °C. The Gd₂Zr₂O₇ nanopowders can be sintered into dense ceramics with a high relative density of 98 % at 1500 °C for 2 h. Moreover, the defect-fluorite phase of Gd₂Zr₂O₇ transforms to pyrochlore phase of Gd₂Zr₂O₇ with the increase of sintering temperature. In addition, the two-step sintering is performed to prepare the Gd₂Zr₂O₇ nanoceramics using the as-prepared Gd₂Zr₂O₇ nanopowders as starting materials. This work presents a simple and industrially feasible approach for preparing the Gd₂Zr₂O₇ nanopowders with excellent sinterability.     

Evolution of hot corrosion resistance of YSZ,Gd2Zr2O7, and Gd2Zr2O7 + YSZ composite thermal barrier coatings in Na2SO4 + V2O5 at 1050 °C

This paper compares the hot corrosion performance of yttria stabilized zirconia (YSZ), Gd₂Zr₂O₇, and YSZ + Gd₂Zr₂O₇ composite coatings in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1050 °C. These YSZ and rare earth zirconate coatings were prepared by atmospheric plasma spray (APS). Chemical interaction is found to be the major corrosive mechanism for the deterioration of these coatings. Characterizations using X-ray diffraction (XRD) and scanning electron microscope (SEM) indicate that in the case of YSZ, the reaction between NaVO₃ and Y₂O₃ produces YVO₄ and leads to the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. For the Gd₂Zr₂O₇ + YSZ composite coating, by the formation of GdVO₄, the amount of YVO₄ formed on the YSZ + Gd₂Zr₂O₇ composite coating is significantly reduced. Molten salt also reacts with Gd₂Zr₂O₇ to form GdVO₄. Under a temperature of 1050 °C, Gd₂Zr₂O₇ based coatings are more stable, both thermally and chemically, than YSZ, and exhibit a better hot corrosion resistance.     

Phase structure and thermal conductivities of Er2O3 stabilized ZrO2 toughened Gd2Zr2O7 ceramics for thermal barrier coatings

3.5 mol% Er₂O₃ stabilized ZrO₂ (ErSZ) and Gd₂Zr₂O₇ powders were produced by a chemical co-precipitation and calcination method, and ErSZ was used to toughen Gd₂Zr₂O₇. The phase structure, toughness and thermal conductivities of ErSZ toughened Gd₂Zr₂O₇ ceramics were investigated. When the ErSZ content was below 15 mol%, the compound consisted of pyrochlore phase, the ordering degree of which decreased with the increase of the ErSZ content. High ErSZ doping led to the formation of metastable tetragonal (t′) phase in the compound. The addition of ErSZ in Gd₂Zr₂O₇ benefited its toughness, mainly attributable to the presence of t′ phase in the compound. With the increase of the ErSZ content in the compound, the thermal conductivity first decreased and then showed an upward tendency, and 10 mol% ErSZ toughened Gd₂Zr₂O₇ exhibited the lowest thermal conductivity.     

Composition-dependent intrinsic defect structures in pyroclore RE2B2O7 (RE = La,Nd, Gd; B = Sn,Hf, Zr)

Point defects are closely correlated with various properties of pyrochlore oxides and therefore play a key role on their engineering applications. Here, the native point defect complexes in RE₂B₂O₇ (RE = La, Nd, Gd; B = Sn, Hf, Zr) under stoichiometric and nonstoichiometric compositions are studied by first-principles calculations. The O Frenkel defect complex is predicted to be the predominant defect structure in stoichiometric zirconates and hafnates, whereas the cation antisite defect complex is the predominant one in stannates. In the case of BO₂ excess, the formation of the B-RE antisite defect together with the RE vacancy and the oxygen interstitial is energetically favorable, whereas the RE-B antisite defect together with the oxygen vacancy and the RE interstitial is preferable under the RE₂O₃ excess environments. Additionally, the formation energies of the native defect complexes are greatly affected by the B-site and/or RE-site cations. The strategy on tailoring the intrinsic defect structures of these pyrochlore oxides is proposed. It is expected to guide the experiments on the defect-related property optimization through stoichiometric and nonstoichiometric compositions, so as to meet the specific engineering requirements and promote their commercial applications.     

La2Zr2O7 based high entropy ceramic with a low thermal conductivity and enhanced CMAS corrosion resistance

Herein, five new La₂Zr₂O₇ based high-entropy ceramic materials, such as (La_0.2Ce_0.2Gd_0.2Y_0.2Er_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Gd_0.2Y_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Y_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Gd_0.2Y_0.2Sm_0.2)₂Zr₂O₇), were synthesized using a sol-gel and high-temperature sintering (1000 °C) method. The spark plasma sintered (SPS) (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇ pellet shows a low thermal conductivity of 1.33 W m^-1 K^-1 at 773 K, and it also exhibits better CaO–MgO–Al₂O₃–SiO₂ corrosion resistance than that of Y₂O₃ stabilized ZrO₂. It shows that (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇ has a promising application potential as a thermal barrier coating.     

Hot corrosion behavior of V2O5-coated Gd2Zr2O7 ceramic in air at 700–850 °C

Gd₂Zr₂O₇ ceramic was prepared by solid state reaction at 1650 °C for 10 h in air, and exhibited a defect fluorite-type structure. Reaction between molten V₂O₅ and Gd₂Zr₂O₇ ceramic was investigated at temperatures ranging from 700 to 850 °C using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Molten V₂O₅ reacted with Gd₂Zr₂O₇ to form ZrV₂O₇ and GdVO₄ at 700 °C; however, in a temperature range of 750–850 °C, molten V₂O₅ reacted with Gd₂Zr₂O₇ to form GdVO₄ and m-ZrO₂. Two different reactions observed at 700 °C and 750–850 °C could be explained based on the thermal instability of ZrV₂O₇.     

Development of brannerite glass‐ceramics for the immobilization of actinide‐rich radioactive wastes

Brannerite‐based glass‐ceramics have been developed as potential waste forms for the immobilization of actinide‐rich radioactive wastes. For the first time, the formation of brannerite phases in glass has been demonstrated using uranium (U) and plutonium (Pu) with additions of gadolinium and hafnium as neutron absorbers. Both XRD and SEM‐EDS confirm that brannerite is the dominating phase with compositions close to Y_0.5U_0.5Ti₂O₆, Gd_0.2Pu_0.3U_0.5Ti₂O₆, and Gd_0.1Hf_0.1Pu_0.2U_0.6Ti₂O₆ internally crystallized in the glass. TEM SAED and Raman spectroscopy reveal the typical structure and vibration modes for brannerite. In addition, the presence of U⁵⁺ species as designed in the formulations has been confirmed by diffuse reflectance spectroscopy. More importantly, the U and Pu were partitioned exclusively in the ceramic phases with no detectable actinide in the glass.     

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.     

Preparation and fine thermal insulation performance of Gd2Zr2O7/ZrO2 composite fibers

Gd₂Zr₂O₇/ZrO₂ (GZC) composite fibers were prepared by electro-spinning method. The XRD, XPS and Raman results showed that there were three crystalline phases, tetragonal phase ZrO₂, cubic phase ZrO₂ and defect fluorite phase Gd₂Zr₂O₇ in GZC composite fibers. GZC fibers remained an intact fiber texture up to 1400 °C according to SEM photographs. The thermal conductivity of GZC fibers was between 0.173 W/(m·K) at 400 °C and 0.309 W/(m·K) at 800 °C, which was lower than that of 7YSZ under the same experimental conditions. The fiber sheet with density about 3.5 g/cm³ has thermal shrinkage less than 3% at 1400 °C. Hence, GZC fibers could be used as refractories for heat protection.     

Volcanic Ash‐Induced Decomposition of EB‐PVD Gd2Zr2O7 Thermal Barrier Coatings to Gd‐Oxyapatite,Zircon, and Gd,Fe‐Zirconolite

The resistance of EB‐PVD Gd₂Zr₂O₇ thermal barrier coatings against high‐temperature infiltration and subsequent degradation by molten volcanic ash is investigated by microstructural analysis. At 1200°C, EB‐PVD Gd₂Zr₂O₇ coatings with silica‐rich, artificial volcanic ash (AVA) overlay show a highly dynamic and complex recession scenario. Gd₂O₃ is leached out from Gd₂Zr₂O₇ by AVA and rapidly crystallizes as an oxyapatite‐type solid‐solution (Ca,Gd)₂(Gd,Zr)₈(Si,Al)₆O₂₆. The second product of Gd₂Zr₂O₇ decomposition is Gd₂O₃ fully stabilized ZrO₂ (Gd‐FSZ). Both reaction products are forming an interpenetrating network filling open coating porosity. However, first‐generation Gd‐oxyapatite and Gd‐FSZ are exhibiting chemical evolution in the long term. The chemical composition of Gd‐oxyapatite does evolve from Ca,Zr enriched to Gd‐rich. AVA continuously leaches out Gd₂O₃ from Gd‐FSZ followed by destabilization to the monoclinic ZrO₂ polymorph. Finally, zircon (ZrSiO₄) is formed. In addition to the prevalent formation of Gd‐oxyapatite, a Gd‐, Zr‐, Fe‐, and Ti‐rich oxide is observed. From chemical analysis and electron diffraction it is concluded that this phase belongs to the zirconolite‐type family (zirconolite CaZrTi₂O₇), exhibiting an almost full substitution Ca²⁺ + Ti⁴⁺ <> Gd³⁺ + Fe³⁺. As all Gd₂Zr₂O₇ decomposition products with the exception of ZrSiO₄ exhibit considerable solid solubility ranges, it is difficult to conclusively assess the resistance of EB‐PVD Gd₂Zr₂O₇ coatings versus volcanic ash attack.     

Interaction between gadolinium zirconate and LiCl–Li2O melt

The interaction between Gd₂Zr₂O₇ and molten LiCl–Li₂O (2 wt%) was studied for 24–52 h at 650–710 °C in an argon atmosphere. Gd₂Zr₂O₇ is analyzed as a promising structural material for sensors used during pyrochemical reprocessing of spent nuclear fuel and for long-term storage or final disposal of high-level nuclear wastes.The chemical stability of Gd₂Zr₂O₇ relative to the components of the LiCl–Li₂O melt was thermodynamically evaluated. The surface morphology and structure of the samples before and after the experiment were analyzed using an X-ray diffractometer and scanning electron microscopy. The formation of a new Li⁺-doped phase based on Gd₂Zr₂O₇ and the Gd₂O₃ evolution onto the material surface was revealed by the X-ray diffraction analysis (XRD). Changes in the microstructure of the samples confirm the presence of large particles in the surface layer corresponding to the Gd₂O₃ phase, which is in good agreement with the XRD data. A profilometer was used to measure the roughness of the ceramics. Presumably, the thickness of the lithium-doped Gd₂Zr₂O₇ film, which is inhomogeneously distributed over the surface of the samples, was 3 μm. Therefore, it was found that dense Gd₂Zr₂O₇ (F) and Gd₂Zr₂O₇ (P) ceramics can be used in LiCl–Li₂O (2 wt %) as a structural material resistant to the high-temperature chemical attack.