Microstructure evolution and hot corrosion mechanisms of Ba2REAlO5 (RE = Yb,Er, Dy) exposed to V2O5 + Na2SO4 molten salt

Ba₂REAlO₅ (RE?=?Dy, Er, Yb) powders were synthesized by a solid state reaction method, followed by cold pressing and sintering to produce pellets for hot corrosion tests. When exposed to V₂O₅?+?Na₂SO₄ molten salt at 900?°C and 1000?°C for 4?h and 20?h, REVO₄, Ba₃(VO₄) ₂ and BaAl₂O₄ formed as corrosion products due to chemical interactions between the ceramics and the molten salt, which were temperature and time independent. After the hot corrosion tests at 1000?°C, continuous, dense reaction layers with a thickness of ～80?μm formed on the sample surfaces, which had an effective function on suppressing further penetration of the molten salt. The hot corrosion mechanisms of Ba₂REAlO₅ are proposed based on Lewis acid-base rule, phase diagrams and thermodynamics. From a thermodynamics perspective, the molten salt directly reacting with Ba₂REAlO₅ is difficult compared with Yb₂O₃, Al₂O₃ and BaO.     

Hot corrosion behavior of NdYbZr2O7 exposed to V2O5 and Na2SO4 + V2O5 molten salts

In order to evaluate the application prospects of NdYbZr₂O₇ as a novel TBC material, NdYbZr₂O₇ ceramic was synthesized via a solid-state reaction sintering method, and its hot corrosion behavior exposed to V₂O₅ and Na₂SO₄ + V₂O₅ molten salts at 900 °C, 1000 °C, and 1100 °C was comparatively investigated. For the V₂O₅ salt, the primary corrosion products were granular (Nd,Yb)VO₄ as well as cube-like m-ZrO₂. The corrosion layer consisted of two distinct layers, one of which was Zr-rich layer and another was V-rich layer. In the case of Na₂SO₄ + V₂O₅, NaVO₃, as an intermediate product, played an important role in dissolving the NdYbZr₂O₇ ceramic. Herein, the (Nd,Yb)VO₄ exhibited a rod/plate-like morphology, which could be attributed to the synergistic effect of low driving force and low nucleation rate. Since the molten salt infiltration rate was superior to the pore filling rate throughout the hot corrosion, the thickness of corrosion layer increased with the rise of temperature. The hot corrosion mechanisms of NdYbZr₂O₇ ceramic in various molten salts were discussed based on the phase diagram, Lewis acid-base rule and chemical thermodynamics. On this basis, the NdYbZr₂O₇ coating was prepared by atmospheric plasma spray (APS) and it exhibits a higher corrosion resistance compared to YSZ coating.     

Calcium-magnesium-alumina-silicate (CMAS) resistance characteristics of LnPO4 (Ln = Nd,Sm, Gd) thermal barrier oxides

Calcium-magnesium-alumina-silicate (CMAS) attack has been considered as a significant failure mechanism for thermal barrier coatings (TBCs). As a promising series of TBC candidates, rare-earth phosphates have attracted increasing attention. This work evaluated the resistance characteristics of LnPO₄ (Ln = Nd, Sm, Gd) compounds to CMAS attack at 1250 °C. Due to the chemical reaction between molten CMAS and LnPO₄, a dense, crack-free reaction layer, mainly composed of Ca₃Ln₇(PO₄)(SiO₄)₅O₂ apatite, CaAl₂Si₂O₈ and MgAl₂O₄, was formed on the surface of compounds, which had positive effect on suppressing CMAS infiltration. The depth of CMAS penetration in LnPO₄ (Ln = Nd, Sm, Gd) decreased in the sequence of NdPO₄, SmPO₄ and GdPO₄. GdPO₄ had the best resistance characteristics to CMAS attack among the three compounds. The related mechanism was discussed based on the formation ability of apatite phase caused by the reaction between molten CMAS and LnPO₄.     

A promising molten silicate resistant material: Rare-earth oxy-apatite RE9.33(SiO4)6O2 (RE = Gd,Nd or La)

In this work we reported a new class of rare earth oxy-apatite, RE_9.33(SiO₄)₆O₂, with superior molten silicate resistant capability which shows promising application for thermal barrier coatings (TBCs). Three RE elements with different ion radius, i.e., Gd, Nd and La, were selected to prepare RE_9.33(SiO₄)₆O₂ bulk using co-precipitation and pressless-sintering. After 8-h CMAS attack at 1300 °C, the specimens exhibited a dense, continuous reprecipitated layer at RE-apatite/CMAS interface, which is predominantly controlled by a dissolution-reprecipitation mechanism distinguishing from the sacrificial material which is usually with reaction layer being formed. With an increase of ion radius, apatite is easier to crystallize in the melt.     

Hot corrosion behavior of Ba2REAlO5 (RE = Dy,Er, Yb) ceramics by vanadium pentoxide at 900–1000 °C

Hot corrosion behavior of Ba₂REAlO₅ (RE = Dy, Er, Yb) ceramics exposed to V₂O₅ molten salt at 900 °C and 1000 °C was investigated, providing a better understanding of their corrosion resistance as promising thermal barrier coatings. Obvious surface reactions occurred forming continuous, dense reaction layers on the top surfaces of the samples, the types of corrosion products being temperature and time independent. After heat treatment for 4 h and 20 h in V₂O₅ salt at the two temperatures, the corrosion products consisted of REVO₄, Ba₂REV₃O₁₁ and BaAl₂O₄ (RE = Dy, Er, Yb). Prolonged heat treatment and elevated temperature promoted the growth of Ba₂REV₃O₁₁ and REVO₄ grains. The reaction layer had a positive function on suppressing further penetration of the molten salt. The mechanism by which the corrosion reaction occurs is proposed based on Lewis acid-base rule, phase diagrams and thermodynamics.     

Phase and microstructure evolution of Sc2O3–CeO2 –ZrO2ceramics in Na2SO4 + V2O5 molten salts

In this study, the destabilization resistance of Sc₂O₃ and CeO₂ co-stabilized ZrO₂ (SCZ) ceramics was tested in Na₂SO₄ + V₂O₅ molten salts at 750°C–1100 °C. The phase structure and microstructure evolution of the samples during the hot corrosion testing were analyzed with X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS), and X-ray photoelectron spectroscopy (XPS). Results showed that the destabilization of SCZ ceramics at 750 °C was the result of the chemical reaction with V₂O₅ to produce m-ZrO₂ and CeVO₄, and little ScVO₄ was detected in the Sc₂O₃-rich SCZ ceramics. The primary corrosion products at 900 °C and 1100 °C were CeO₂ and m-ZrO₂ due to the mineralization effect. The Sc₂O₃-rich SCZ ceramics exhibited excellent degradation resistance and phase stability owing to the enhanced bond strength and the decreased size misfit between Zr⁴⁺ and Sc³⁺. The destabilization mechanism of SCZ ceramic under hot corrosion was also discussed.     

Crystal Structure and Microwave Dielectric Properties of LiRE9(SiO4)6O2 Ceramics (RE = La,Pr, Nd,Sm, Eu,Gd, and Er)

The crystal structure and microwave dielectric properties of apatite‐type LiRE₉(SiO₄)₆O₂ ceramics (RE = La, Pr, Nd, Sm, Eu, Gd, and Er) have been investigated. The densification of lithium apatites has been greatly improved with the addition of 1 wt% LiF. Selected area electron diffraction and X‐ray diffraction (XRD) Rietveld analysis confirm that these compounds belong to the P6₃/m (No. 176) space group with hexagonal crystal symmetry. The porosity‐corrected relative permittivity was found to decrease with decreasing ionic polarizability of RE³⁺ ions. Relationships between the structural parameters and microwave dielectric properties have been examined. The observed variation in the quality factor of LiRE₉(SiO₄)₆O₂ + 1 wt% LiF ceramics (RE = La, Pr, and Nd) was correlated with average cation covalency (%). The temperature coefficient of resonant frequency was found to depend on the bond valence sum of cations. LiEr₉(SiO₄)₆O₂ + 1 wt% LiF ceramics showed good microwave dielectric properties with ε_r = 12.8, Q_u × f = 13000 GHz and τ_f = +17 ppm/°C. All the compositions showed low coefficient of thermal expansion with thermal conductivity in the range 1.3–2.8 W (m K)^?1.     

Hot corrosion behavior of TiO2 doped,Yb2O3 stabilized zirconia exposed to V2O5 + Na2SO4 molten salt at 700–1000 °C

3.5 mol% Yb₂O₃ stabilized zirconia (YbSZ) doped with 10 mol% TiO₂ (Ti-YbSZ) was produced, and its hot corrosion behavior exposed to Na₂SO₄ + V₂O₅ molten salt was investigated. The as-fabricated ceramic mainly consists of metastable tetragonal (t′) phase. When exposed to the molten salt at 700 °C, 800 °C, 900 °C and 1000 °C for 2 h and 10 h, YbVO₄ and m-ZrO₂ formed as corrosion products due to chemical reactions between the ceramics and the salt. Ti⁴⁺ in Ti-YbSZ solid solution keeps stable during the hot corrosion tests, which acts as a stabilizer for ZrO₂, preventing total decomposition of the t′ phase. After the hot corrosion tests, Ti-YbSZ has an apparently lower m phase content than Y₂O₃ doped Zirconia and YbSZ, indicative of better corrosion resistance. The hot corrosion mechanism of Ti-YbSZ is proposed based on Lewis acid-base rule, phase diagrams and thermodynamics.     

Comparison of hot corrosion resistance of Sm2Zr2O7 and (Sm0.5Sc0.5)2Zr2O7 ceramics in Na2SO4+V2O5 molten salt

Sm₂Zr₂O₇ and (Sm_0.5Sc_0.5)₂Zr₂O₇ ceramics were fabricated by a chemical co-precipitation and calcination method, and their hot corrosion behaviors in Na₂SO₄+V₂O₅ molten salt were investigated. Hot corrosion tests were carried at 700 °C, 800 °C and 900 °C for 4 h, and corroded surfaces were investigated using X-ray diffractometer and scanning electron microscopy. The corrosion products of Sm₂Zr₂O₇ ceramics were composed of SmVO₄ and monoclinic-ZrO₂, while those of (Sm_0.5Sc_0.5)₂Zr₂O₇ ceramic consisted of SmVO₄ and Zr₅Sc₂O₁₃. Considering the fact that Zr₅Sc₂O₁₃ is more desirable than monoclinic-ZrO₂ for thermal barrier coating applications, (Sm_0.5Sc_0.5)₂Zr₂O₇ showed better corrosion resistance to Na₂SO₄+V₂O₅ salt than Sm₂Zr₂O₇. The hot corrosion mechanisms of Sm₂Zr₂O₇ and (Sm_0.5Sc_0.5)₂Zr₂O₇ in Na₂SO₄+ V₂O₅ salt were discussed in detail.     

Hot corrosion behavior of Yb2Si2O7 ceramic under NaVO3 salt attack

Hot corrosion behavior of Yb₂Si₂O₇ bulk exposed to NaVO₃ molten salt was investigated at 1000–1500 °C for 2 h. Results showed that YbVO₄ was produced on the bulk surface at 1000–1200 °C because of the acidity of the corrosion medium. The corrosion product of Yb₂SiO₅ was yielded at 1300–1500 °C due to the transformation of the corrosion medium from acidity to alkalinity. The content of YbVO₄ in the products decreased as the temperature increased, while that of Yb₂SiO₅ increased with an increase in temperature. In this paper, various hot corrosion mechanisms of Yb₂Si₂O₇ bulk exposed to NaVO₃ molten salt are discussed based on the formation of YbVO₄ and Yb₂SiO₅.     

A comparative investigation on the corrosion degradation of plasma sprayed YSZ and LnMgAl11O19 (Ln = Nd,Sm, Gd) coatings exposed to the molten V2O5 + Na2SO4 salt mixture at 1100 °C

Corrosive attack of the molten 50 wt. % V₂O₅ + 50 wt.% Na₂SO₄ salt mixture has been comparably studied for the APS YSZ and LnMgAl₁₁O₁₉ (LnMA, Ln = Nd, Sm, Gd) thermal barrier coatings upon a 10 h anneal at 1100 °C in air. The YSZ coating suffered from a deepest infiltration of the molten salt along its thickness direction through the open and connected pores as well as inter-lamellae microcracks. A large number of newly formed voids were widely distributed in the YSZ coating due to the corrosion degradation followed by the t, t’ to m-ZrO₂ phase transformation. While, a relative thin corrosion layer mainly consisting of α-Al₂O₃ and LnVO₄ were present for the corroded LnMA coatings. The much reduced number of open pores and connected microcracks together with the rapid chemical reaction between the molten salt and LnMA coatings, especially for the NdMA coating, preventing further infiltration of the molten salt, was beneficial to mitigate further attacks to the inner coating at the expense of sacrificing a thinner top layer. The presences of amorphous phases were thought to further accelerate the corrosion reaction and strengthen such a corrosion protection mode for all the APS LnMA coatings.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.     

Preparation of plasma sprayed nanostructured GdPO4 thermal barrier coating and its hot corrosion behavior in molten salts

Nanostructured GdPO₄ coatings, designed as the outer layer of double-ceramic-layer thermal barrier coatings (DCL-TBCs), were produced by air plasma spraying (APS). The coatings have close chemical composition to that of the agglomerated particles used for thermal spray. Nanozones with porous structure are embedded in the coating microstructure, having a percentage of ~30%. Hot corrosion tests of the coatings were carried out in V₂O₅ and Na₂SO₄+V₂O₅ salts at 900 °C for 4 h. Results indicate that dense reaction layers, consisting of GdVO₄ and Gd₄(P₂O₇)₃, form on the coating surfaces, which could suppress further penetration of the molten salts. In the V₂O₅ molten salt, the reaction layer is thicker and less molten salt trace could be found beneath the layer.     

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.     

Low-temperature synthesis of CdCu3Ti4O12 powders with high dielectric permittivities

The molten salt method was adopted in this work to synthesize CdCu₃Ti₄O₁₂ precursor powders and was found capable of notably reducing the sintering temperature. It was also found that the grain size gradually increases and the grain shape changes from spherical to cubic with increasing proportions of molten salt. The optimal sintering conditions were found to be at approximately 730?°C with the addition of 20% molten salt. After further sintering at 975?°C, the obtained perovskite ceramics exhibited a high dielectric constant at low frequencies.     

High velocity oxygen fuel sprayed Cr3C2-NiCr coatings against Na2SO4 hot corrosion at different temperatures

Cr₃C₂-NiCr/NiCrAlY coating was prepared by high velocity oxygen fuel (HVOF) spraying. The microstructure and the Na₂SO₄ hot corrosion behavior at different temperatures of the coating were investigated. The Na₂SO₄ hot corrosion mechanism of Cr₃C₂-NiCr coating was also discussed. The results showed that HVOF Cr₃C₂-NiCr coating was relatively dense and mainly composed of Cr₃C₂, NiCr and a small amount of Cr₇C₃ three phases. The dense Cr₂O₃ layer was formed on the surface of Cr₃C₂-NiCr coating after Na₂SO₄ corrosion at 750 °C to further prevent corrosion. The coating had produced the obvious longitudinal crack with the increase of hot corrosion temperature up to 900 °C. The corrosion mechanism of Cr₃C₂-NiCr coating against the Na₂SO₄ salt at high-temperature was as follows: firstly, the protective oxidizing film was formed at 750 °C, then the protective oxide film dissolved at the interface between the coating and Na₂SO₄ salt with the hot corrosion temperature increasing up to 900 °C, and subsequently the dissolved anions and cations could migrate to the interface between molten salt and air and the loose and unprotected oxides were regenerated, thereby exacerbating the failure of the coating.     

Crystal structure of rare-earth silicon-oxynitride J-phases,Ln4Si2O7N2

Rare-earth silicon-oxynitride J-phases, Ln₄Si₂O₇N₂ (Ln=Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), were prepared by the N₂ gas-pressured sintering method at 1 MPa of N₂ and 1500–1700 °C. The Rietveld analysis was carried out for X-ray powder diffraction data measured at room temperature. The crystal structures of Ln₄Si₂O₇N₂ were refined with the structure model of La₄Si₂O₇N₂ for Ln=La, Pr, Nd, and Sm, and with that of Lu₄Si₂O₇N₂ for Ln=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The refined monoclinic unit-cell parameters (lengths a, b, c, angles β, and volume V) increased linearly in their two series of Ln with increasing ionic radii of rare-earth atoms. Discontinuities of the unit-cell parameters were found between the two Ln series.     

Thermal Conductivity of β-Si3N4: III, Effect of Rare-Earth (RE = La, Nd, Gd, Y, Yb, and Sc) Oxide Additives

β-Si₃N₄ ceramics sintered with a series of rare-earth (RE = La, Nd, Gd, Y, Yb and Sc) oxide additives were fabricated by hot pressing and subsequent annealing. Their microstructures, lattice oxygen contents, and thermal conductivities were evaluated. Mean grain size increased, while lattice oxygen content decreased, and hence, thermal conductivity increased with decreasing ionic radius of the rare-earth element. In all cases, a marked change was observed in the order of ionic radius from La to Nd to Gd, and a little change was observed below them. Rare-earth oxide additives significantly influenced the thermal conductivity of β-Si₃N₄, unlike in the case of AlN.     

Structure,thermal properties and hot corrosion behaviors of Gd2Hf2O7 as a potential thermal barrier coating material

In this work, Gd₂Hf₂O₇ ceramics were synthesized and investigated as a potential thermal barrier coating (TBC) material. The phase composition, microstructure and associated thermal properties of Gd₂Hf₂O₇ ceramics were characterized systematically. Results show that the thermal conductivity of Gd₂Hf₂O₇ ceramics is 1.40 Wm⁻¹K⁻¹ at 1200 °C, ~25% lower than that of 8 wt% yttria partially stabilized zirconia (8YSZ). Gd₂Hf₂O₇ ceramics also present large thermal expansion coefficients, which decrease from 12.0 × 10⁻⁶ K⁻¹ to 11.3 × 10⁻⁶ K⁻¹ (300–1200 °C). Besides, the hot corrosion behaviors of Gd₂Hf₂O₇ ceramics exposed to V₂O₅ and Na₂SO₄ + V₂O₅ salts at temperatures of 900–1200 °C were discussed in great detail. We pay much attention on the corrosion process, corrosion mechanism and corrosion damage of Gd₂Hf₂O₇ ceramics subjected to molten V₂O₅ and Na₂SO₄ + V₂O₅ salts at different temperatures.     

Spark plasma sintering of (5RE.2)Ta3O9 high-entropy ceramics and their thermal properties

Six rare-earth tantalate high-entropy ceramics of (5RE_.2)Ta₃O₉ (RE represents any five elements selected from La, Ce, Nd, Sm, Eu, Gd) were designed and prepared by spark plasma sintering process at 1400°C in this study. The (5RE_.2)Ta₃O₉ ceramics only consist of a single-phase solid solution with perovskite structure. Their relative densities are all above 90%, and the average grain size is in the range of 1.47–2.92 μm. The thermal conductivity of (5RE_.2)Ta₃O₉ ceramics is in 2.24–1.90 W m⁻¹ K⁻¹ (25°C–500°C), which is much lower than that of yttria-stabilized zirconia. In six samples, (La_.2Nd_.2Sm_.2Gd_.2Eu_.2)Ta₃O₉ possesses a thermal conductivity of 1.90 W m⁻¹ K⁻¹, a thermal expansion coefficient of 3.47 × 10⁻⁶ K⁻¹ (500°C), a Vickers hardness of about 7.33 GPa, and a fracture toughness of about 5.20 MPa m^1/2, which are suitable for its application as thermal barrier coatings.