Improved resistance of lanthanum zirconate coatings to calcium-magnesium-alumina-silicate corrosion through composition tailoring

Calcium–magnesium–alumina–silicate (CMAS) corrosion resistance is an important issue on the design of next-generation thermal battier coatings. As one of the promising thermal battier coatings, the lanthanum zirconate coating has attracted continuous attention. In this work, three lanthanum zirconate coatings with different La/Zr composition, i.e., La_1.8Zr_2.2O_7.1, La₂Zr₂O₇, and La_2.5Zr_1.5O_6.75, are fabricated by laser-enhanced chemical vapour deposition, and their resistance to CMAS corrosion at 1250?°C is investigated. Among them, La_2.5Zr_1.5O_6.75 shows the best CMAS corrosion resistance because increased La content is beneficial to the formation of a dense and continuous apatite Ca₂La₈(SiO₄)₆O₂ layer, which effectively slows down the subsequent molten CMAS penetration. This study clarifies the significant role of rare earth on CMAS corrosion resistance and is expected to guide the future design of rare-earth-based thermal battier coatings through composition tailoring.     

Effect of mechanical activation of coprecipitated precursor on synthesis of La2Zr2O7

A lanthanum zirconate (La₂Zr₂O₇, LZ) precursor has been prepared by coprecipitation of the relevant hydroxides from nitrate solution. The effect of the mechanical activation of the precursor on its thermal decomposition and LZ formation in the temperature range of 700–1100 °C has been investigated. After mechanical activation, the precursor releases volatile components (H₂O and CO₂) at lower temperatures. Mechanical activation accelerates the LZ crystallisation and results in the formation of phase-pure LZ without admixtures of unreacted zirconia and lanthana. The LZ powder prepared from the mechanically activated precursor is characterised by a larger BET surface area, in comparison to that synthesised from the as-prepared precursor under the same calcination conditions.     

Preparation and corrosion resistance of nonstoichiometric lanthanum zirconate coatings

Lanthanum zirconate is a promising thermal barrier coating material owing to its excellent thermophysical properties and La plays the key role in its corrosion resistance. Here, an amorphous precursor is used as raw feedstock material so as to synthesize lanthanum zirconate coatings with tailorable composition by atmospheric plasma spray (APS). Three lanthanum zirconate coatings of La_1.7Zr_2.3O_7.15, La_2.0Zr_2.0O_7.0 and La_2.3Zr_1.7O_6.85 are fabricated. Furthermore, the corrosion resistance of the as-sprayed coatings against CaO-MgO-Al₂O₃-SiO₂ at 1250℃ is investigated. The increased La content promotes the formation of a sealing layer of the crystalline Ca₂La₈(SiO₄)₆O₂ apatite, which slows down the penetration of molten CaO-MgO-Al₂O₃-SiO₂. Therefore, the infiltration rate of the La_2.3Zr_1.7O_6.85 coating decreased up to 42.6 % compared with the other two coatings. This work develops a feasible preparation strategy to control the La composition for the improved corrosion resistance, which is expected to guide the future coating design and synthesis for the materials with big composition changes during the APS process.     

A novel NTC ceramic based on La2Zr2O7 for high-temperature thermistor

A novel negative temperature coefficient material based on lanthanum zirconate ceramics was proposed for high-temperature applications. This material was synthesized through a solid-state reaction by sintering at 1923 K for 10 h in air. The X-ray diffraction and scanning electron microscopy results confirmed that La₂Zr₂O₇ ceramics exhibited a pyrochlore phase with a relative density of 98.2 %. The resistance–temperature characteristics of the material revealed that La₂Zr₂O₇ ceramics exhibited an NTC feature within the broad temperature range of 973–1773 K in addition to maintaining high thermal constant B, and resistivity to ensure good sensitivity at high temperatures. These properties, along with high ceiling temperature, unique oxygen insensitivity, and excellent ageing coefficient of <0.7 % at 1773 K, render La₂Zr₂O₇ ceramics a promising candidate as thermistor materials with high-temperature NTC.     

Structural (in)stability and spontaneous cracking of Li-La-zirconate cubic garnet upon exposure to ambient atmosphere

Among the Li-ion conducting inorganic materials, lithium lanthanum zirconate (LLZO) is believed to possess good chemical stability against Li metal and hence considered to be a promising solid electrolyte for Li-ion batteries. However, systematic sets of studies conducted here at regular intervals during storage of Al-doped LLZO (cubic garnet) sintered pellets in ambient atmosphere have raised serious concerns over their structural/mechanical stability/integrity upon exposure to air. Spontaneous cracking/disintegration/pulverization of LLZO pellets takes place after about three weeks of exposure, primarily due to formation of La₂Zr₂O₇ in the LLZO bulk; as found to be thermodynamically feasible at room temperature upon reaction with CO₂/moisture. Steep increase in La₂Zr₂O₇ content coincides with the spontaneous cracking/disintegration. Estimation suggests that internal stresses associated with the formation of La₂Zr₂O₇ from LLZO can be high enough to cause spontaneous fracture. This mandates the development/fabrication/usage of solid-state cells using LLZO under stringent controls against exposure to atmospheric species.     

Molten salt synthesis of La2Zr2O7 ultrafine powders

Ultrafine powders of pyrochlore-type La₂Zr₂O₇ were synthesized via a simple molten salt mediated process using zirconium oxide and lanthanum oxide as raw materials, and sodium chloride, potassium chloride and sodium fluoride to form a reaction medium. The effects of reaction temperature, salt/reactant ratio and salt type on the La₂Zr₂O₇ formation were investigated. Among the three attempted salt assemblies (KCl–LiCl, Na₂CO₃–K₂CO₃, and NaCl–KCl–NaF), NaCl–KCl–NaF showed the best accelerating effect on the La₂Zr₂O₇ formation. At a given temperature, the La₂Zr₂O₇ content in the final products increased with the increase in the salt amount. Phase pure submicron sized La₂Zr₂O₇ ultrafine powders were obtained after 3 h firing at 1100 °C with the salt/reactant weight ratio of 5:1 or at 1200 °C with salt/reactant weight ratio of 3:1. The synthesis temperature (1100 °C) was much lower than that required by the conventional solid-state mixing method or a wet chemical method. The “dissolution–precipitation” mechanism had dominated the synthesis process.     

Chemical interactions between 3 mol% yttria-zirconia and Sr-doped lanthanum manganite

The chemical interactions between porous (La_0.8Sr_0.2)MnO₃ (LSM) film and 3 mol% yttria tetragonal zirconia (TZ3Y) substrate have been investigated over the temperature range of 1300–1500 °C in air. Two distinct reaction layers of fluorite-type cubic zirconia solid solution c-(Zr,Mn,La,Y)O₂ and lanthanum zirconate pyrochlore (La,Sr)₂(Zr,Y)₂O₇ were observed at the interface of LSM/TZ3Y. It has been found that the diffusion/dissolution of Mn ions in TZ3Y leads to the formation of the fluorite-type cubic zirconia solid solution, while the interaction of lanthanum with TZ3Y results in the formation of the lanthanum zirconate pyrochlore phase. Phase studies in the (ZrY)O₂–La₂O₃–Mn₃O₄ system show that the fluorite-type cubic zirconia solid solution phase c-(Zr,Mn,La,Y)O₂, rather than the tetragonal 3 mol% Y₂O₃–ZrO₂ phase, is in equilibrium with LSM perovskite at high temperatures. A ternary phase diagram of the system at the (ZrY)O₂-rich end at 1400 °C in air was proposed based on the experimental results. It is suggested that the fundamental reason for the beneficial effect of A-site non-stoichiometry or Mn excess of LSM in the inhibiting of the lanthanum zirconate formation is due to the fact that Mn₃O₄ does not equilibrate with lanthanum zirconate at high temperatures.     

Influence of the intermixed interfacial layers on the thermal cycling behaviour of atmospheric plasma sprayed lanthanum zirconate based coatings

In application as a thermal barrier coating (TBC), yttria stabilised zirconia (YSZ) approaches some limits of performance. To further enhance the efficiency of gas turbines, higher temperature capability and a longer lifetime of the coating are needed for the next generation of TBCs. Pyrochlore oxides of general composition, A₂B₂O₇, where A is a 3⁺ cation (La to Lu) and B is a 4⁺ cation (Zr, Hf, Ti, etc.) have high melting point, fair coefficient of thermal expansion, and low thermal conductivity which make them suitable for applications as high temperature thermal barrier coatings. Among those oxide materials lanthanum zirconate (LZ/La₂Zr₂O₇) offers very attractive properties. This work describes the fabrication, microstructure and high temperature (1280 °C) thermal cycling behaviour of lanthanum zirconate coatings with five different coating architectures, deposited using atmospheric plasma spray process. The coating architecture which had five layers with two intermixed interlayers had much longer life time than other considered architectures. The coatings were characterised using X-ray diffraction, energy dispersive spectrometry, optical and scanning electron microscopy, before and after thermal cycling tests, to study the coating failure mechanisms.     

Phase Formation,Microstructure, and Densification of Yttrium‐Doped Barium Zirconate Prepared by the Sonochemical Method

Spherical monodispersed yttrium‐doped barium zirconate (BaY_0.1Zr_0.9O₃; BYZ) particles were successfully prepared in single step without the requirement of calcination process by the sonochemical method in highly basic aqueous solution. The stoichiometric solution of BaCl₂.2H₂O, ZrOCl₂.8H₂O, and YCl₄.6H₂O was precipitated in a 20 M NaOH under high‐intensity ultrasonic irradiation (20 kHz, 150 W/cm²) for 15, 30, and 60 min. As‐prepared powders were identified by XRD, FT‐IR, and Raman spectroscopy as cubic perovskite BYZ. The microstructure examined by SEM and TEM showed spherical‐shaped BYZ particles formed by aggregation of primary nanocrystals, and this unique morphology was induced by the effects of the ultrasonication and the strong alkaline environment. BYZ powders prepared under ultrasonication for 60 min had narrow‐sized distribution with the average particle size of 267 ± 26 nm and the specific surface area of 40.2 m²/g. BYZ ceramics sintered in the air at 1550°C for 20 h showed good densification (95%) and consisted of large grain size (7.67 ± 2.79 μm).     

Improvement of Electrical Conductivity of Trivalent Rare‐earth Cation‐doped Neodymium Zirconate by Co‐doping Gadolinium and Ytterbium

Pyrochlore oxides of A₂Zr₂O₇, where A represents trivalent rare‐earth elements, have a high electrical conductivity, which makes them suitable for applications as high‐temperature solid electrolytes. The influence of Gd and Yb cations co‐doping at the Nd site on structure and electrical conductivity of a pyrochlore oxide Nd₂Zr₂O₇ is investigated using X‐ray diffraction and impedance spectra measurements. Different zirconate ceramics of Nd₂Zr₂O₇, Nd_1.8Gd_0.2Zr₂O₇, Nd_1.8Gd_0.1Yb_0.1Zr₂O₇, Nd_1.4Gd_0.6Zr₂O₇ and Nd_1.4Gd_0.3Yb_0.3Zr₂O₇ are prepared by pressureless‐sintering method at 1,973 K for 10 h in air. Nd₂Zr₂O₇ doped with Gd and Yb cations at the Nd site exhibit a single phase of pyrochlore‐type structure. The measured values of the total conductivity obey the Arrhenius relation. Nd₂Zr₂O₇ and its doped zirconate ceramics are oxide‐ion conductors in the oxygen partial pressure range of 1.0 × 10^–4 to 1.0 atm at all test temperature levels. The total conductivity increases with reducing average ionic radii of A‐site rare‐earth cations. The dual Yb+Gd intermix doping causes a distinctly enhanced total conductivity as compared to unmodified Nd₂Zr₂O₇ and singly Gd‐doped zirconate ceramics. The highest total conductivity value obtained in this work is 1.02 × 10^–2 S cm × ¹ at 1,173 K for Nd_1.4Gd_0.3Yb_0.3Zr₂O₇ ceramic.     

Controlled synthesis of core-shell,yolk-shell and hollow spheres C@La2Zr2O7 via the combination of hard templating and co-precipitation method

Core-shell and yolk–shell architectures are attracting great attention owing to their unique structure and infusive applications in the nanotechnology field. In this research, core-shell structured C@La₂Zr₂O₇ nanospheres were synthesized through a facile technique using carbon spheres as templates. Lanthanum nitrate (La(NO₃)₃·6H₂O) and zirconium oxychloride (ZrOCl₂·8H₂O) co-precipitated homogeneously on the surface of modified carbon spheres by adding ammonia dropwise. The close-to perfect spherical La₂Zr₂O₇ hollow spheres were obtained by a two-step calcination of the C@La₂Zr₂O₇ core-shell components, which were calcined at a high temperature in argon atmosphere and followed by low temperature oxidization calcination. SEM, TEM, XRD, and FTIR were used to characterize the morphology, size, composition, and crystal structure of synthesized products. The results show that La₂Zr₂O₇ nanoparticles were attached tightly on the surface of carbon spheres through the functional groups, such as -OH, and O˭C-O-. After modified by NaOH, the carbon spheres surface possessed more abundant oxygen containing groups, resulting in much more La³⁺ and Zr⁴⁺ co-precipitated uniformly on their surface. Ultimately, an average diameter of the La₂Zr₂O₇ hollow spheres was about 220 nm and the shell thickness was about 40 nm. Through controlling the oxidation calcination time, the morphology of the powders exhibited core-shell structured, yolk-shell structured and hollow structured spheres. Further, the formation mechanism of the hollow La₂Zr₂O₇ spheres was elucidated.     

MgO or Al2O3 crucible: Which is better for the preparation of LLZ-based solid electrolytes?

Lithium lanthanum zirconate (Li₇La₃Zr₂O₁₂, LLZ) solid electrolytes were usually prepared by using Al₂O₃ crucibles and/or Al³⁺ dopants in order to stablize the formation of cubic garnet phase. In this work, we argue that Al-free MgO crucible could be better for the preparation of LLZ-based solid electrolytes through a detailed comparison of LLZ-Ga_0.2 samples prepared respectively by using MgO and Al₂O₃ crucibles. The solid-state reaction method was firstly used to prepare Li_7-3xGa_xLa₃Zr₂O₁₂ (x = 0–0.4) pellets by using Al-free MgO crucibles, which were sintered at 1100 °C for 12 h. The highly conductive cubic phase was obtained at Ga³⁺ doping content as low as x = 0.05 and the highest room-temperature conductivity was obtained at x = 0.2. Al₂O₃ crucible was then used to prepare the LLZ-Ga_0.2 pellet for comparing the influence of these two different crucibles. The results show that a higher lattice parameter (a = 12.9762 (8) Å), a higher relative density (95.5%) and a higher room-temperature ionic conductivity (1.352 (3) mS/cm) were achieved by using the MgO crucible.     

Particle Size and Crystal Phase Dependent Photoluminescence of La2Zr2O7:Eu3+ Nanoparticles

Herein, La₂Zr₂O₇:5% Eu³⁺ nanoparticles (NPs) with different sizes have been synthesized for the first time through a modified facile molten salt process using a single‐source complex precursor of La(OH)₃·ZrO(OH)₂:Eu(OH)₃·nH₂O. It was found that the concentration of the added ammonia to co‐precipitate the corresponding metallic ions to form the precursor can influence the final particle size of the fluorite La₂Zr₂O₇:5%Eu³⁺ NPs. Furthermore, the crystal phase of the La₂Zr₂O₇:5%Eu³⁺ NPs was transferred from fluorite to pyrochlore after thermal treatment at 1000°C. The relationship between photoluminescence (PL), quantum yield (QY), particles size and crystal phase has been further investigated through fluorescence decay, site symmetry, and Judd–Ofelt (J–O) analysis. Specifically, PLQY and lifetime increase with increasing particle size of the fluorite La₂Zr₂O₇:5%Eu³⁺ NPs. Additionally, crystal phase transfer from fluorite to pyrochlore resulted in large PLQY decrease and moderate lifetime increase in the La₂Zr₂O₇:5%Eu³⁺ NPs.     

Phase Separation Phenomenon and Mechanism of Ce0.6Zr0.4O2 Powders Prepared Using Chemical Coprecipitation Method

Ce_0.6Zr_0.4O₂ (C60Z) powders were synthesized using the chemical coprecipitation method to investigate the phase separation mechanism. The phase separation of C60Z powders occurred during the heating step (>1100°C), suggesting that it might be a thermally activated process. The activation energy for the phase separation of C60Z was 398 (±20) kJ/mol and the Avrami parameter was ~1.0, indicating that the phase separation may be interface controlled. We suggested that Zr⁺⁴ diffusion played an important role in C60Z phase separation due to the ionic radius of Zr⁺⁴ being smaller than that of Ce⁺⁴. The C60Z powders tended to aggregate, which induced the driving force for the phase separation resulting from the surface energy difference between the particle surface and the interface. The phase separation behavior of C60Z powders was very similar to the initial stage of sintering. After high‐temperature treatment, more Zr⁺⁴ ions diffused to the interface between C60Z particles to lower the surface energy of the system than did Ce⁺⁴ ions, which resulted in the composition gradually being changed and phase separation. Therefore, the phase separation of C60Z powders can be effectively decreased by the addition of Al⁺³, which suppresses aggregation.     

Microstructure and thermal & mechanical properties of La2Zr2O7@YSZ composite ceramic

La₂Zr₂O₇ used as a top coat material has low thermal conductivity and high stability at high temperature, but it also has a low fracture toughness, which limits its application. To improve the fracture toughness of La₂Zr₂O₇, a La₂Zr₂O₇@YSZ core–shell structured composite ceramic was designed and prepared. The morphology of the La₂Zr₂O₇@YSZ composite ceramic was investigated using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The resulting images show that the YSZ is coated on the surface of the La₂Zr₂O₇. The phases were analyzed by X-ray diffraction (XRD), and the XRD patterns show that pyrochlore and fluorite structures coexist in the La₂Zr₂O₇@YSZ composite material without any chemical reaction. Differential scanning calorimetry (DSC) was used to detect the heat change of the composite ceramic during heat treatment. The properties of the La₂Zr₂O₇@YSZ composite ceramic, such as the thermal conductivity, coefficient of thermal expansion (CTE), and mechanical properties were investigated using a laser flash method, high-temperature dilatometer, and nano-hardness tests, respectively. The thermal conductivity of the composite ceramic is in the range of 1.7745–2.3076 W m⁻¹ K⁻¹ in the temperature regime of 200–1000 °C. The maximum CTE of the composite ceramic is 10.3 × 10⁻⁶/°C. Owing to the thin YSZ coating on the La₂Zr₂O₇ surface, the hardness and Young's modulus of the composite ceramic are 8.17 GPa and 168.3 GPa, respectively. The nucleation and propagation of micro-cracks are investigated using a micro-hardness tester. Compared to La₂Zr₂O₇, the micro-cracks in the composite ceramic are shorter and more tortuous. The weak interface between the YSZ and La₂Zr₂O₇ results in the nucleation and tortuous propagation of micro-cracks, which depletes part of the energy and improves the fracture toughness of the composite ceramic. The results reveal that the La₂Zr₂O₇@YSZ composite ceramic has good mechanical and thermophysical properties.     

A novel porous La2Zr2O7 ceramic aerogels with ultralow thermal conductivity and photocatalytic property

Porous La₂Zr₂O₇ ceramic aerogels (CAs) were prepared by sol-gel template method and thermal treated process. The microstructure and crystallisation behavior of the samples were systematically characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. The results indicated that the as-prepared porous La₂Zr₂O₇ CAs had a single-phase pyrochlore structure with typical three-dimensional (3-D) porous structure. Meanwhile, the formation mechanisms of the as-prepared porous La₂Zr₂O₇ CAs were investigated. At the same time, the as-prepared porous La₂Zr₂O₇ CAs presented an ultralow room-temperature thermal conductivity of 0.07 W/(m K), high specific surface areas of 325.17 m²/g, and a relatively high compressive strength of 11.95 MPa. What's more, the as-prepared porous La₂Zr₂O₇ CAs possessed ideal photocatalytic activities due to its high crystallinity, large surface area as well as unique 3-D porous structure. Therefore, the present work is proposing some new insight to prepare rare-earth zirconates CAs with porous structures for thermal insulation and dye degradation applications.     

High lithium ionic conductivity in the garnet‐type oxide Li7−2xLa3Zr2−xMoxO12 (x=0‐0.3) ceramics by sol‐gel method

Lithium garnet‐type oxides Li_7?2xLa₃Zr_2?xMo_xO₁₂ (x=0, 0.1, 0.2, 0.3) ceramics were prepared by a sol‐gel method. The influence of molybdenum on the structure, microstructure and conductivity of Li₇La₃Zr₂O₁₂ were investigated by X‐ray diffraction, scanning electron microscopy, and impedance spectroscopy. The cubic phase Li₇La₃Zr₂O₁₂ has been stabilized by partial substitution of Mo for Zr at low temperature. The introduction of Mo (x≥0.1) can accelerate densification. Li_6.6La₃Zr_1.8Mo_0.2O₁₂ sintered at lower temperature 1100°C for 3 hours exhibits highest total ionic conductivity of 5.09 × 10^?4 S/cm. Results indicate that the Mo doping LLZO synthesized by sol‐gel method effectively lowers its sintering temperature and improves the ionic conductivity.     

Single-phase rare-earth high-entropy zirconates with superior thermal and mechanical properties

Emerging of high-entropy ceramics has brought new opportunities for designing and optimizing materials with desired properties. In the present work, high-entropy rare-earth zirconates (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ and (Yb_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ are designed and synthesized. Both high-entropy ceramics exhibit a single pyrochlore structure with excellent phase stability at 1600 °C. In addition, the Yb-containing system possesses a high coefficient of thermal expansion (10.52 × 10^?6 K^-1, RT～1500 °C) and low thermal conductivity (1.003 W·m^-1 K^-1, 1500 °C), as well as excellent sintering resistance. Particularly, the Yb-containing system has significantly improved fracture toughness (1.80 MPa·mm^1/2) when compared to that of lanthanum zirconate (1.38 MPa·mm^1/2), making it a promising material for thermal barrier coatings (TBCs) applications. The present work indicates that the high-entropy design can be applied for further optimization of the comprehensive properties of the TBCs materials.     

Glycine–nitrate synthesis of Sr doped La2Zr2O7 pyrochlore powder

Abstract

Abstract

Materials with A₂B₂O₇ (pyrochlore) structure have received significant attention for their applications as new protonic conductors and materials used in electronic devices. One of the unique synthesis routes for La₂Zr₂O₇ (pyrochlore) powders is the glycine–nitrate combustion method, which shows superior properties of the synthesised powder using glycine as a complexing agent. The Sr doped La₂Zr₂O₇ powders in pure pyrochlore structure were produced using this approach. Selected characteristics of the synthesised powders, such as crystal structure, lattice parameters, crystallite size, the vibrational properties, the morphology of the particles, along with the specific surface area and particle size, have been investigated. The dependence of some properties on annealing temperatures of the powders has been studied.     

Highly porous (La1/5Nd1/5Sm1/5Gd1/5Yb1/5)2Zr2O7 ceramics with ultra-low thermal conductivity

The high-entropy rare earth zirconate (La_1/5Nd_1/5Sm_1/5Gd_1/5Yb_1/5)₂Zr₂O₇ porous ceramics ((5RE_1/5)₂Zr₂O₇ PCs) were prepared using a foam-gel casting-freeze drying method combined with segmented calcination process. The results of SEM, TEM, and XRD analyses of the (5RE_1/5)₂Zr₂O₇ PCs indicated the formation of a defective fluorite crystal structure, with the rare earth elements homogeneously distributed. Meanwhile, the as-prepared (5RE_1/5)₂Zr₂O₇ PCs exhibited high porosity, low bulk density, low thermal conductivity, and relatively high compressive strength. Moreover, the high-temperature thermal conductivity of the samples was evaluated, and the results showed that the (5RE_1/5)₂Zr₂O₇ PCs maintain a thermal conductivity of 0.150 ± 0.002 W m^?1 K^?1 even at 1000 °C. The strategy used in this paper can be extended to the synthesis of other high-entropy porous ceramics with high porosity and low thermal conductivity, which is suitable for applications as thermal insulation materials.