LaMgAl11O19 Research Articles

Preparation, electronic structure and thermal properties of Zn-doped LaMgAl11O19 materials

LaMgAl11O19 (LMA) with magnetoplumbite structure is a promising thermal barrier material due to its excellent thermal properties. In this work, Zn-doped LMA materials were studied in order to improve the thermal performance, and LaZnxMg1-xAl11O19 (x = 0, 0.1, 0.3, 0.5, 0.7, 1.0) were prepared by sol-gel method. The as-synthesized powder materials possess pure LMA phase and show the morphology of hexagonal plates. X-ray photoelectron spectroscopy result shows that there are AlO4 tetrahedra and AlO6 octahedra units in the target materials, and Zn2+ ions replace Mg2+ ions to occupy the tetrahedral position of magnetoplumbite structure. The composition LaZn0.3Mg0.7Al11O19 material exhibits the lowest thermal conductivity of 0.38 W/(m·K) at 1000oC while maintaining a high coefficient of thermal expansion (10.86 × 10−6 K−1). Therefore, the thermal properties of LMA materials can be improved by adding an appropriate amount of Zn ions.

Preparation and properties evaluation of high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)MgAl11O19 for advanced thermal barrier coating

LaMgAl11O19 (LMA) with magnetoplumbite structure has emerged as a promising candidate for thermal barrier coating (TBC). However, the inherent high thermal conductivity and relatively low thermal expansion coefficient (TEC) of LMA impose limitations on its further application. In this work, a novel high-entropy hexaluminate (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)MgAl11O19 (HE-LMA) has been designed and prepared to overcome these obstacles. HE-LMA with homogeneous chemical composition distribution possesses excellent phase stability up to 1600°C. Moreover, the TEC of HE-LMA (9.22 × 10−6 K−1 at 1300°C) is larger than that of LMA. The high-entropy strategy effectively decreases the thermal conductivity of HE-LMA (from 3.27 W/(m⋅K) at room temperature to 2.19 W/(m∙K) at 1000°C) in comparison to that of LMA (3.63–2.62 W/(m∙K)). Furthermore, HE-LMA demonstrates higher microhardness and fracture toughness due to the lattice distortion. These results indicate that (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)MgAl11O19 has the potential to be applied as a TBC.

Enhanced structural stability of yttria‐stabilized zirconia–LaMgAl11O19 dual ceramic coating by an α‐Al2O3 layer

AbstractTo prolong the service life and structural stability of CoCrNiAlY–yttria stabilized zirconia–LaMgAl11O19 (LaMA) dual ceramic coating during high temperature exposure, an AlY plating layer was deposited on top LaMA. Results revealed that this coating without AlY plating layer showed a rapid mass increase from 6.62 mg/cm2 at 20 h to 10.63 mg/cm2 at 80 h, but also suffered from a fast TGO growth and severe elemental diffusion, resulting in an exacerbating interface fracture of the coating. However, AlY plating layer was first oxidized, and Y2O3 induced the formation of a dense α‐Al2O3 reaction layer on the top LaMA layer, which greatly prevented the internal penetration of O2. This coating with AlY plating layer showed a relatively stable structure, a very slow TGO growth, and a slight mass increase trend from 8.19 mg/cm2 at 20 h to 8.73 mg/cm2 at 80 h. Potential oxidation damage mechanisms of the coatings with or without AlY plating layer were comprehensively clarified.

Enhanced oxidation resistance and structural stability of CoNiCrAlY - Yttria stabilized zirconia - LaMgAl11O19 dual ceramic coating by AlY planting layer and subsequent vacuum annealing treatment

AlY plating layer and vacuum annealing were designed to enhance the oxidation resistance and structural stability of CoCrNiAlY–yttria stabilized zirconia (YSZ)–LaMgAl11O19 (LaMA) dual ceramic coating. Vacuum annealing induced a prominent recrystallization of LaMA resulting in numerous micro-cracks. During the oxidation test, these micro-cracks acted as the infiltration channels of O2, accelerating the formation of brittle (Ni, Co)Cr2O4 and high stress initiation, and eventually led to serious fracture. However, AlY plating layer experienced a competitive oxidation during the vacuum annealing and thus led to the formation of α-Al2O3. During the oxidation test, these α-Al2O3 offered a strong barrier ability to block the infiltration of O2, leading to an enhanced oxidation resistance and good structural stability.

Prolonged thermal durability of plasma-sprayed lanthanum hexaluminate thermal barrier coating by optimizing crystallinity

LaMgAl11O19 (LMA) with magnetoplumbite structure has been identified as a potential candidate for the next generation of thermal barrier coatings (TBCs). However, the as-sprayed LMA coating presents a large amount of amorphous phase originating from the rapid quenching from the molten droplets, which may compromise the reliability of coating during high-temperature service. A logical approach to improve LMA TBC life, therefore, is to enhance its crystallinity. In the present study, three LMA powders with different particle size distributions (fine, medium and coarse feedstocks) were utilized to deposit TBCs to investigate whether the thermal durability of LMA can be improved by increasing particle size. It was based on a hypothesis that large particle size can decrease the melting degree of in-flight particles and thus result in high crystallinity of as-sprayed LMA coating, making less recrystallization stress to enhance the thermal shock resistance of LMA coating. Results show that prolonged thermal cycling durability can indeed be achieved by increasing particle size. However, excessive particle size could lead to higher porosity derived from unmelted particles, which function as the weak regions for the coating system, thereby damaging the structural integrity and adhesive strength between the topcoat and bond coat. This could cause microcrack linking when the accumulated thermal stress exceeds its fracture toughness under consecutive heating-cooling cycles despite its higher crystallinity. Concerning the coating fabricated by medium powder, it achieves the optimal balance of crystallinity and structure integrity, resulting in the longest thermal cycling lifetime. The results provide guidance for the development and design of high thermal durability LMA TBCs. When optimizing the recrystallization stress within the coating, attention should be paid to improving its bonding strength simultaneously.

White light emission and energy transfer of LaMgAl11 O19 :Eu3+ , Tb3+ phosphors.

White-light-tunable LaMgAl11 O19 :x%Tb3+ , y%Eu3+ series phosphors were prepared using the gel-combustion method. The structure and luminescence properties were studied, and the energy transfer of Eu3+ and Tb3+ in the LaMgAl11 O19 system was also discussed. The results showed that the LaMgAl11 O19 matrix exhibited strong emission in the blue-light region under the excitation of ultraviolet light, which resulted in conditions suitable for the preparation of white-light-tunable phosphors. The emission spectra of LaMgAl11 O19 :2%Tb3+ , y%Eu3+ (y = 2%-9%) series phosphors were obtained through optimization experiments. It could be seen from the CIE diagram that by adjusting the doping quantities of Eu3+ and Tb3+ in the LaMgAl11 O19 host, multicolor luminescence and white light emission in a single host could be achieved. By calculating the energy transfer efficiency and critical distance between Eu3+ and Tb3+ series phosphors, the mechanism of energy transfer between Tb3+ and Eu3+ was found to be the interaction between electric quadruples.

CaO–MgO–Al2O3–SiO2 (CMAS) corrosion behaviour of LaMgAl11O19/GdPO4 thermal barrier coating materials

CaO–MgO–Al2O3–SiO2 (CMAS) corrosion poses serious hidden dangers for the application of thermal barrier coatings (TBCs). In this study, LaMgAl11O19 (LMA) and GdPO4 were mixed at molar ratios of 2:1, 1:1 and 1:2 to prepare LMA/GdPO4 materials, and the CMAS corrosion behaviours of these materials were investigated at 1300°C–1500 °C for 20 h and 40 h. It was demonstrated that temperature was the main factor influencing the corrosion behaviours and products. The materials were damaged at 1300 °C by the crystallization of CMAS melts to form CaAl2Si2O8. In contrast, the materials were corroded by CMAS melts via the reaction between CMAS and GdPO4 at 1500 °C. These results indicate that the addition of GdPO4 to LMA can improve the resistance of the LMA material to CMAS corrosion.

Microstructures, thermophysical properties and thermal cyclic behaviors of Nd2O3 and Sc2O3 co-doped LaMgAl11O19 thermal barrier coating deposited by plasma spraying

In this study, La1-xNdxMgAl11-xScxO19 (x = 0.1, 0.2, 0.3; abbreviated as LNMAS-1, 2, 3) coatings which are supposed to possess better properties than LaMgAl11O19 (LMA) were plasma-sprayed and their high-temperature performance were comparatively investigated. Results show that addition of Nd3+ and Sc3+ as dopants to LMA endows corresponding coatings with reduced thermal conductivity and enhanced thermal expansion coefficient, while maintaining advantageous phase stability, although still being subjected to amorphization in plasma flame and following crystallization upon high-temperature service. Furthermore, the doping could cause adherence increasing between topcoat/bondcoat, benefiting from improved melting condition, especially in LNMAS-2 and LNMAS-3 coatings, which is related to the specific powder morphology and lowered melting point. During exposure to 1350°C, mechanical performance and structure integrity of doped free-standing LNMAS coatings can be well preserved even after 400 h aging. In thermal cyclic fatigue test, LNMAS-2 and LNMAS-3 coatings undertake thermal cycling lifetime of ∼181 and 191 cycles at 1100°C, respectively, 40% durable than that of LMA coating. These preliminary results suggest that LNMAS-2, 3 might be promising candidates for advanced thermal barrier coating applications.

Thermal shock behavior of LaMgAl11O19/Yb2Si2O7/Si thermal/environmental barrier coatings with LaMgAl11O19-LiAlSiO4 transition layer

Thermal/environmental barrier coatings (T/EBCs) could be used to protect SiC-based ceramic matrix composites (CMCs) from chemical attack and lower the substrate temperature for improving their lifetime. To mitigate the mismatch of thermal expansion coefficient (CTE) between LaMgAl11O19 (LMA) TBC and Yb2Si2O7 (YbDS) EBC, a transition layer formed by positive thermal-expansion LMA and negative thermal-expansion LiAlSiO4 (LAS) was introduced. Thermo-mechanical performance of LMA-LAS composite coating was studied. LMA-LAS composite coating exhibited an intermediate CTE and an excellent fracture toughness, revealing that LMA-LAS coating was an acceptable transition layer. Thermal shock behavior of LMA/YbDS/Si without and with LMA-LAS transition layer were compared. The thermal cycling lifetime of T/EBC was improved by about 20% with LMA-LAS intermediate layer. The final failure of T/EBC coating system was mainly associated with thermal mismatch and thermally grown oxide (TGO) formation.

Infrared radiation and thermal cyclic performance of a high-entropy rare-earth hexaaluminate coating prepared by atmospheric plasma spraying

In this study, a high-entropy RMgAl11O19 (HE-RMA, R = La, Pr, Nd, Sm, Gd) and LaMgAl11O19 (LMA) coatings were fabricated by atmospheric plasma spraying. The phase composition, microstructure, thermal stability, infrared emissivity performance and shock resistance were comparatively characterized. The results showed that doping multiple rare-earth cations could be conductive to enhance the infrared emissivity. The as-sprayed HE-RMA coating exhibited the highest infrared emissivity, which reached up to 0.971 at 1000 °C. The reason for the improvement of the infrared emissivity was attributed to introduced impurity energy level resulting from doping cations, which could reduce the forbidden bandwidth and increase probability of electronic transition. Meanwhile, HE-RMA coating exhibited better shock resistance at 1100 °C due to superior fracture toughness (1.84 ± 0.41 MPa·m1/2) during thermal cycling test at 1100 °C. In addition, HE-RMA coating still exhibited high infrared emissivity (0.932 at 1000 °C) at 1100 °C annealing for 100 h with only a slight reduction.

Understanding the Enhanced Protective Mechanism of CoCrNiAlY–YSZ–LaMgAl11O19 Double-Ceramic Coating with Aluminum Plating

To understand the enhanced protection mechanism of CoCrNiAlY–YSZ–LaMgAl11O19 double-layer ceramic coating with aluminum plating, a finite element simulation method was used to simulate the distribution of thermal stress in the coating in all directions. The results show that in the air exposure of the un-aluminized coating, high temperature causes a large radial thermal stress on the surface of the LaMgAl11O19 (LMA) layer, and it increases with the increase in temperature, which is the main reason for the initiation of axial cracks. After arc aluminum plating, the aluminum plating layer effectively inhibited the volume shrinkage of the coating through good adhesion to the coating and internal diffusion; the thermal stress of the coating was considerably reduced; and the CoCrNiAlY–YSZ–LMA coating had an effective enhancement and protection effect. However, there was still a certain amount of shear thermal stress inside the LMA layer, the top of the crack, and the bottom of the crack. This thermal stress caused the initiation of radial microcracks in the LMA layer, which also becomes a risk point for the failure of the aluminum coating.

Effect of synthesis technique on developing lanthanum magnesium hexaaluminate powders as an alternative for thermal barrier coating

AbstractCitric acid sol-gel technique and solid-state reaction method were used for synthesizing high purity lanthanum magnesium hexaaluminate powders. The samples were calcined at 1150°C, 1250°C, and 1350°C to understand the effect of synthesis temperature on the LaMgAl11O19(LMA) samples. The collective effect of the synthesis technique and the synthesis temperature on developing lanthanum magnesium hexaaluminate powders were analyzed using XRD, FTIR, SEM, TGA-DSC, and Particle analyzer. LMA powders obtained through sol-gel techniques showed better uniformity and pure LMA phase, while the ball milling method displayed traces of intermediate phases. The synthesized powders in the crystallite size of 150 nm and 280 nm were obtained through the sol-gel and ball milling technique, respectively. Citric acid sol-gel technique aided in the early onset of phase formation and the required closely packed platelet formation with a particle size of around 150 nm with good uniformity.

Hot corrosion behaviour of Gd2O3 doped LaMgAl11O19 thermal barrier coating exposed to molten V2O5 at 900 °C

This study investigates the hot corrosion behaviours of La1-xGdxMgAl11O19 (x = 0, 0.2, 0.4, 0.6, 0.8) bulk ceramics and ceramic coatings. The hot corrosion resistance of the ceramic coating was inferior to that of the bulk ceramic, owing to the formation of an amorphous phase during the plasma spraying process. The smaller size of Gd3+ in comparison with La3+ facilitated the ready combination of Gd3+ with VO43−. LaVO4 was obtained as the corrosion product in the LaMgAl11O19 (LMA) sample. Upon doping the LMA with Gd2O3, the LaVO4 was almost entirely replaced by GdVO4. Therefore, the sacrificial action of GdMgAl11O19 protected the LMA from hot corrosion, while also providing a new method for modifying the corrosion resistance of molten slag.

Solution Combustion Synthesis and Energy Transfer in LaMgAl11O19:Tb3+/Sm3+ Tunable Phosphor

Tb 3+ /Sm 3+ single as well as co-doped LaMgAl11O19 (LMA) phosphors have been synthesized by a low-temperature solution combustion method. The X-ray diffraction pattern, photoluminescence properties and energy transfer (ET) processes between rare earth (RE) ions, were investigated in detail. The mechanism of energy transfer (ET) from Tb 3+ to Sm 3+ was seen as the dipole–dipole interaction. Efficient ET from Tb 3+ to Sm 3+ ions was observed, leading to color-tunable emissions of LMA:0.02Tb 3+ , x Sm 3+ (x = 0.005 to 0.02 mol) phosphors. The efficiency of the ET gradually increased with increase in the Sm 3+ ion concentration, reaching a maximum of 78.28% at Tb 3+ ion concentration 0.02mol. The critical distance (Rc) among sensitizer and activator by the concentration quenching method was estimated to be 27.96 Å. The LMA: Tb 3+ , Sm 3+ phosphors exhibited the emission peaks in the blue ( 5 D3→ 7 FJ = 5, 4, 3, and 2), green ( 5 D4→ 7 FJ = 6, 5, and 4), and orange-red ( 4 G5/2→ 6 HJ = 5/2, 7/2, and 9/2) regions under the excitation wavelength of 375 nm. As a consequence of fine-tuning of the emission composition of the Tb 3+ and Sm 3+ ions, multicolor emitting luminescence properties can be achieved in a single host lattice LMA.

Thermal cycling failure of the multilayer thermal barrier coatings based on LaMgAl11O19/YSZ

Thermal cycling failure of three multilayer TBCs based on LaMgAl11O19 (LaMA)/YSZ was comparatively investigated by using the burner-rig testing method in this work. Results indicate that through optimizing the weight ratio and thickness of the intermediate LaMA/YSZ composite layers, a five-layer TBC with much improved thermal cycling life of 11,749 cycles at 1372 °C surface and 1042 °C bond coat testing temperature has been realized. While, thermal cycling lifetimes of the tri- and six-layer TBCs were 7439 and 7804 cycles at surface/bond coat testing temperatures of 1378 °C/1065 °C and 1367 °C/1056 °C, respectively. Factors related to the 60 wt.% LaMA + 40 wt.% YSZ (60LaMA + 40YSZ) intermediate composite layer with the highest thermal expansion coefficient than other composite layers generating higher internal stress level to the tri- and six-layer TBCs, different bond coat temperature and TGO growth, as well as long-term stability of the LaMA coating during thermal cycling tests, were characterized and compared to understand the different thermal cycling lifetime and failure modes among such three multilayer TBCs.

Crystallization mechanism of plasma-sprayed LaMgAl11O19 coating

Amorphous phase is often detected in the plasma-sprayed LaMgAl11O19 (LMA) coating, the crystallization of which can cause the volume reduction of the coating, resulting in the failure of the coating. However, it is still not clear for the crystallization mechanism of the amorphous phase. In this paper, the coating samples were deposited using atmospheric plasma spraying. Adequate evidence showed that the crystallization of amorphous LMA occurred in two stages at about 900 and 1170 °C, respectively, by studying the phase transition of Al2O3 and the synthesis process of LMA. A novel crystallization mechanism on the amorphous LMA coating was also proposed. Results showed that highly charged La3+ in LMA magnetoplumbite structure controls the crystallization rate. The higher the La content in the composition the harder the crystallization occurs.

Preparation and properties of LaMgAl11O19 thermal barrier coatings doped with Gd2O3

As a rare earth hexaaluminate, LaMgAl11O19 (LMA) has been one of the most promising materials used as thermal barrier coatings (TBCs). A large amount of amorphous phase, however, often exists in the plasma-sprayed LMA coating and significantly reduces the service lifetime of TBCs. In this study, La1-xGdxMgAl11O19 (x = 0, 0.2, 0.4, 0.6, and 0.8) ceramic powders are synthesised by solid-state reaction, and all of these powders are employed to prepare the corresponding coatings. The phase compositions and microstructures of samples are examined by X-ray diffraction and scanning electron microscopy, respectively. The linear thermal expansion behaviour and thermal cycling behaviour of the coatings are also analysed. The results show that the amorphous phase content is decreased and the thermal expansion behaviour is improved by doping the coatings with Gd2O3. The thermal cycling lifetime of the coating, however, basically remains unchanged.

Crystal growth and spectroscopic characterization of Sm:LaMgAl11O19 crystal

LaMgAl11O19 (LMA) crystal with nominal Sm3+ concentrations of 8 at.% was grown by the Czochralski method. Polarized absorption spectra, polarized emission spectra and fluorescence decay curve of Sm:LMA crystal were recorded at room temperature. The absorption cross-sections at 404 nm were calculated to be 2.33 × 10−20 cm2 and 0.86 × 10−20 cm2 for σ and π polarization, respectively. The spontaneous transition rates, branching ratios and the radiative lifetime of the 4G5/2 multiplet, were calculated. The Judd-Ofelt theory has been applied to evaluate the Ωt (t = 2, 4, 6) intensity parameters. The highest stimulated emission cross-sections for the 4G5/2 → 6H7/2 transition around 594 nm were calculated to be 1.67 × 10−21 cm2 for σ polarization and 1.15 × 10−21 cm2 for π polarization, with full width at half maximum (FWHM) of 7.68 nm and 7.27 nm, respectively. The fluorescence lifetime of the 4G5/2 manifold was 1.84 ms. All the results show that Sm:LMA crystal would be a promising orange-red solid state laser material.

Heat-treated lanthanum magnesium hexaaluminate coatings exposed to molten calcium-magnesium-alumino-silicate

Abstract LaMgAl11O19 (LMA) coatings usually have a large amount of amorphous phase due to the rapid cooling during spraying, which will not benefit the service lifetime in aeroengine. Fortunately, the thermal cycling lifetime of LMA TBCs can be improved by heat treatment. Moreover, the hot corrosion degradation with calcium-magnesium-alumino-silicate (CMAS) attracts much attention recently. Therefore, to clarify the influence of heat treatment on CMAS corrosion behaviour, plasma-sprayed LMA coatings were isothermally heat-treated and then exposed to CMAS. Results indicate that heat treatment promoted a crystallization of LMA coatings, but the molten CMAS had completely penetrated into LMA coatings along the increased and widened vertical microcracks. It is the isotropic pores that determine the penetration kinetics of CMAS to LMA. CaAl2Si2O8 and MgAl2O4 were the predominant reaction products in the superficial layer, while Ca3La6(SiO4)6 interpenetrated with the residual LMA in the inner layer, forming a corroded bilayer for LMA coatings.

LaMgAl11O19 synthesis using non-hydrolytic sol-gel methods

Polycrystalline LaMgAl11O19 (LMA) was prepared by four different non-hydrolytic sol-gel methods. From stable solutions, four powder precursors containing an amorphous and nanocrystalline phase with specific reactivity were obtained. The particle size, morphology, thermal behaviour, and phase composition of the powder precursors were studied using DLS, TEM, DSC/TG and XRD. Bulk ceramic samples containing LMA were prepared at 1200 °C for 16 h and examined in terms of phase purity and microstructure using XRD, SEM, and TEM. Raman spectroscopy of pure LMA was used to study the structure in detail. A mechanism of LMA formation and a relation between powder precursor properties and final phase composition is proposed. These findings may be useful for designing modern technologies for fabrication of LMA for optical or protective coating applications.