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.     

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30 mol% LaPO₄ doped Gd₂Zr₂O₇ (Gd₂Zr₂O₇-LaPO₄) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd₂Zr₂O₇ and LaPO₄ phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250 °C for 1–8 h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd₂Zr₂O₇-LaPO₄ TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO₂ phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.     

CMAS (CaO–MgO–Al2O3–SiO2) resistance of Y2O3-stabilized ZrO2 thermal barrier coatings with Pt layers

Calcium–magnesium–alumina–silicate (CMAS) corrosion significantly affects the durability of thermal barrier coatings (TBCs). In this study, Y₂O₃ partially stabilized ZrO₂ (YSZ) TBCs are produced by electron beam-physical vapor deposition, followed by deposition of a Pt layer on the coating surfaces to improve the CMAS resistance. After exposure to 1250 °C for 2 h, the YSZ TBCs were severely attacked by molten CMAS, whereas the Pt-covered coatings exhibited improved CMAS resistance. However, the Pt layers seemed to be easily destroyed by the molten CMAS. With increased heat duration, the Pt layers became thinner. After CMAS attack at 1250 °C for 8 h, only a small amount of Pt remained on the coating surfaces, leading to accelerated degradation of the coatings. To fully exploit the protectiveness of the Pt layers against CMAS attack, it is necessary to improve the thermal compatibility between the Pt layers and molten CMAS.     

Plasma-sprayed La2Ce2O7 thermal barrier coatings against calcium–magnesium–alumina–silicate penetration

As one of promising thermal barrier coating (TBC) candidates, La₂Ce₂O₇ (LC) has attracted increasing attention because of its low thermal conductivity and potential capability to be operated above 1250 °C. In this paper, the microstructure evolution and mechanical properties of the plasma-sprayed LC TBC with calcium–magnesium–alumina–silicate (CMAS) glassy deposits at 1250 °C were investigated. Due to chemical reaction between the CMAS deposits and LC coating, a dense sealing layer, mainly composed of Ca₂(La_xCe_1−x)₈(SiO₄)₆O_6−4x and CeO₂, was formed on the coating after heat-treatment at 1250 °C and effectively prevented CMAS from further penetration. The interaction layer had the micro-hardness of ∼10–12 GPa, relatively harder than the LC coating.     

Preparation,structural and microstructural properties of Ba0.64Ca0.32Al2Si2O8 ceramics phase

Monoclinic celsian has been prepared from Ba-LTA zeolite precursor. The Ca²⁺-exchanged hexacelsian (HC_Ca) synthesized from Ba-LTA zeolite precursors was used for preparation of monoclinic celsian (MC_Ca). The partially ion exchange diphyllosilicate has a composition of Ba_0.64Ca_0.32Al₂Si₂O₈ (HC_Ca). It was found that prepared HC_Ca phase is stable between room temperature and 1300 °C. During prolonged heating this phase is polymorphic transformed to Ba, Ca-celsian feldspar. Synthesis of Ba, Ca-celsian and thermal behavior during transformation processes was observed by XRD method. The crystal structure and microstructural parameters were refined using Rietveld method. The crystal morphology of thermal treated samples was observed by SEM/EDAX analysis.     

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.     

Structure and dielectric properties of perovskite ceramics in the system Ba(Ni1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3

Ceramics in the system Ba(Ni_1/3Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ (BNN–BZN) were prepared by the mixed oxide route. Powders were mixed and milled, calcined at 1100–1200 °C then pressed and sintered at temperatures in the range 1400–1500 °C for 4 h. Selected samples were annealed or slowly cooled after sintering. Most products were in excess of 96% theoretical density. X-ray diffraction confirmed that all specimens were ordered to some degree and could be indexed to hexagonal geometry. Microstructural analysis confirmed the presence of phases related to Ba₅Nb₄O₁₅ and Ba₈Zn₁Nb₆O₂₄ at the surfaces of the samples. The end members BNN and BZN exhibited good dielectric properties with quality factor (Qf) values in excess of 25,000 and 50,000 GHz, respectively, after rapid cooling at 240 °C h⁻¹. In contrast, mid-range compositions had poor Qf values, less than 10,000 GHz. However, after sintering at 1450 °C for 4 h and annealing at 1300 °C for 72 h, specimens of 0.35(Ba(Ni_1/3Nb_2/3)O₃)–0.65(Ba(Zn_1/3Nb_2/3)O₃) exhibit good dielectric properties: τ_f of +0.6 ppm °C⁻¹, relative permittivity of 35 and quality factor in excess of 25,000 GHz. The improvement in properties after annealing is primarily due to an increase in homogeneity.     

Preparation of (0 2 0)-oriented BaTi2O5 thick films and their dielectric responses

Barium dititanate (BaTi₂O₅) thick films were prepared on a Pt-coated Si substrate by laser chemical vapor deposition, and ac electric responses of (0 2 0)-oriented BaTi₂O₅ films were investigated using several equivalent electric circuit models. BaTi₂O₅ films in a single phase were obtained at a Ti/Ba molar ratio (m_Ti/Ba) of 1.72–1.74 and deposition temperature (T_dep) of 908–1065 K as well as m_Ti/Ba = 1.95 and T_dep = 914–953 K. (0 2 0)-oriented BaTi₂O₅ films were obtained at m_Ti/Ba = 1.72–1.74 and T_dep = 989–1051 K. BaTi₂O₅ films had columnar grains, and the deposition rate reached 93 μm h^?1. The maximum relative permittivity of the (0 2 0)-oriented BaTi₂O₅ film prepared at T_dep = 989 K was 653 at 759 K. The model of an equivalent circuit involving a parallel combination of a resistor, a capacitor, and a constant phase element well fitted the frequency dependence of the interrelated ac electrical responses of the impedance, electric modulus, and admittance of (0 2 0)-oriented BaTi₂O₅ films.     

Hot corrosion behavior of BaLa2Ti3O10 exposed to calcium-magnesium-alumina-silicate at elevated temperatures

Calcium-magnesium-alumina-silicate (CMAS) attack has been regarded as one of the significant failure mechanisms for thermal barrier coatings (TBCs). In this study, CMAS corrosion behavior of BaLa₂Ti₃O₁₀, a novel TBC material, is investigated at 1300?°C and 1350?°C for 0.5?h, 4?h, 12?h and 24?h. Results reveal that BaLa₂Ti₃O₁₀ has high resistance to molten CMAS infiltration, attributable to the formation of a dense reaction layer. X-ray diffraction, scanning electron microscope, energy dispersive spectroscope, transmission electron microscope confirm that the layer consists of apatite, celsian and perovskite phases. With increased corrosion duration, the layer retains good phase stability and the thickness increases. The formation of corrosion products and the reaction layer are discussed according to a dissolution-reprecipitation mechanism and the optical basicity theory.     

Celsian formation from barium-exchanged geopolymer precursor: Thermal evolution

In this paper, thermal evolution, including element & phase composition and microstructure, of Ba²⁺ exchanged K-based geopolymer precursor (BaGP) were systematically investigated during high-temperature treatment. The results proved that celsian precursor with lower residual alkaline cation content were obtained through amorphous geopolymer than traditional ion-exchanged celsian through crystallized zeolite. With the increase in temperature, weight loss of BaGP was due to evaporation of OH groups and decomposition of BaCO₃. Similar to K-based geopolymer, BaGP showed amorphous structure, and nanometer-sized celsian nucleuses first crystallized from the amorphous BaGP matrix after it was treated at 900 °C. In the treatment temperature range from 1000 to 1400 °C, hexagonal celsian became the main phase. After being treated at 1400 °C, hexagonal celsian grains were clearly noticeable with extra SiO₂ locating between celsian grains. It was therefore concluded that geopolymer precursor technique provides an alternative route for the preparation of celsian ceramics.     

Effect of physical vapor deposited Al2O3 film on TGO growth in YSZ/CoNiCrAlY coatings

Thermal barrier coatings (TBCs) comprising of yttria stabilized zirconia (YSZ) ceramic top coat and CoNiCrAlY metallic bond coat have been widely used in gas turbines. However, the developed oxides layer in the interface of the top and bond coats during thermal exposure of the TBCs always results in the destruction of the system. In order to restrain the growth of oxides layer and improve the thermal shock resistance of TBCs, a thin Al₂O₃ film was pre-deposited on CoNiCrAlY bond coat by physical vapor deposition (PVD) technology. After thermal exposure, morphologies and phase compositions of the thermal growth oxides (TGO) layer in the conventional and pre-deposited Al₂O₃ film TBCs were examined by scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). The residual stresses in the coatings were analyzed using micro-Raman spectroscopy (LabRam-1B). It was found that TGO layer formed in the conventional TBCs was mainly composed of Al₂O₃, (Cr,Al)₂O₃ + (Co,Ni)(Cr,Al)₂O₄ + NiO (CSN), and (Cr,Al)₂O₃ + (Co,Ni)(Cr,Al)₂O₄ (CS), while in the treated TBCs, the formed TGO layer appeared more uniform and compact. The CSN and CS clusters, which are normally considered as a weakness for TBCs, were greatly limited. The residual stresses in the TBCs after thermal shock were also reduced by the deposition of Al₂O₃ film.     

Colossal permittivity in (In1/2Nb1/2)xTi1−xO2 ceramics prepared by a glycine nitrate process

(In + Nb) co-doped TiO₂ nanoparticles with very low dopant concentrations were prepared using a glycine nitrate process. A pure rutile—TiO₂ phase with a dense microstructure and homogeneous dispersion of dopants was achieved. By doping TiO₂ with 1.5% (In + Nb) ions, a very high dielectric permittivity of ε′ = 42,376 and low loss tangents of tanδ = 0.06 (at room temperature) were achieved. The large conduction activation energy at the grain boundary decreased with decreasing dopant concentration. The colossal permittivity was primarily attributed to the internal barrier layer capacitor (IBLC) effect. The dominant effect of interfacial polarization at the non–Ohmic sample–electrode contact was observed when the dopant concentration was ≤1.0 mol%. Interestingly, the sample–electrode contact and resistive–outer surface layer effects, i.e., surface barrier layer capacitor (SBLC) effect, has also an effect on the colossal dielectric response in (In + Nb) co-doped TiO₂ ceramics.     

Influence of stoichiometry on the dielectric and ferroelectric properties of the tunable (Ba,Sr)TiO3 ceramics investigated by First Order Reversal Curves method

The changes induced by the different stoichiometries in Ba_0.9Sr_0.1TiO₃ solid solutions, with (Ba,Sr)/Ti = 1 and (Ba,Sr)/Ti > 1, on the dielectric, ferroelectric and ac tunability characteristics are investigated. A small difference in the (Ba,Sr)/Ti ratio causes a shift of the Curie and Curie–Weiss temperatures of 16 and 19 °C, respectively, but does not change the diffuse character of the phase transitions. The FORC method is used for describing the local switching properties and the ac tunability characteristics. Irrespective of the stoichiometry, no clear separation between the reversible and irreversible contributions to the polarization are visible on the FORC diagrams. The maximum of the FORC distribution is located in almost the same position, at low fields, meaning that small fields are necessary to switch the majority of the dipolar units of these systems. The diagram obtained for the solid solutions with (Ba,Sr)/Ti = 1 shrinks towards smaller coercivities in comparison with the Ba-rich samples, due to the smaller Curie temperature, making it closer to the ferro–para phase transition. The tunability determined in the FORC experiment depends not only on the actual field, but also on the reversal field. A dependence of the FORC susceptibility on the two maxima corresponding to the reversal field was found for the stoichiometric samples, while one single maximum at low reversal fields is characteristic of the Ba-rich samples. These results are interpreted in relationship to domain wall mobility, which is higher for the ferroelectric sample, close to its ferro–para phase transition.     

Microwave dielectric properties of tungstenbronze type like (Ba1−αSrα)6−3xR8+2xTi18O54 (R = Sm,Nd) solid solutions

Tungstenbronze type like Ba_6−3xR_8+2xTi₁₈O₅₄ (R = Sm or Nd) dielectric ceramics reveal high quality factor Q·f as well as high dielectric constant ɛ_r. We have investigated the effect of Sr substitution for Ba ions on the microwave dielectric properties of the compounds. (Ba_1−αSr_α)_6−3xR_8+2xTi₁₈O₅₄ (R = Sm or Nd) ceramics were prepared in the composition ranges of x = 0–0.2 and α = 0–0.312 and the microwave dielectric properties were investigated. (Ba_1−αSr_α)_6−3xSm_8+2xTi₁₈O₅₄, where x = 0.1 and α = 0.298, and (Ba_1−αSr_α)_6−3xNd_8+2xTi₁₈O₅₄, where x = 0.2, α = 0.296 revealed remarkably higher Q·f value among the solid solutions, indicating that Q·f increased with substituting Sr ions into Ba ions at the rhombic A1-site. This fact suggests that relaxation of local distortions at the A1-sites is closely related to improvement of Q·f.     

Effect of non-stoichiometry on the densification,phase purity,microstructure, crystal structure,and dielectric loss of Ba(Co1/3Nb2/3)O3 ceramics

The structural, vibrational, densification, and microwave properties of Ba(Co_1/3Nb_2/3)O₃ ceramics with small compositional variations along several tie lines in the ternary BaOCoONb₂O₅ diagram were studied. The results showed that very small deviation from stoichiometric Ba(Co_1/3Nb_2/3)O₃ composition has profound effect on Q × f, degree of ordering, densification, and phase assemblage. The 0.94 Ba(Co_1/3Nb_2/3)O₃–0.06 Ba₅Nb₄O₁₅ ceramic has the highest Q × f value (71 THz) – a value two times larger than that of stoichiometric Ba(Co_1/3Nb_2/3)O₃ (36 THz). Transformation from the (partial) disordered distribution of Co and Nb cations to 1:2 ordered arrangement in the octahedral sites was found to increase the Q factor of the high density and single phase ceramics. It was also observed that formation of very small amount of Ba₉CoNb₁₄O₄₅ second phase degraded Q × f value severely for the dense and highly ordered Nb-rich and Ba-deficient ceramics.     

The phase stability and thermophysical properties of InFeO3(ZnO)m (m = 2, 3, 4, 5)

The phase stability and thermophysical properties of InFeO₃(ZnO)_m (m = 2, 3, 4, 5) compounds were investigated, which are a general family of homologous layered compounds with general formula InFeO₃(ZnO)_m (m = 1–19). InFeO₃(ZnO)_m (m = 2, 3, 4, 5) ceramics were synthesized using cold pressing followed by solid-state sintering. They revealed an excellent thermal stability after annealing at 1450 °C for 48 h. No phase transformation occurred during heating to 1400 °C. InFeO₃(ZnO)₃ exhibited a thermal conductivity of 1.38 W m⁻¹ K⁻¹ at 1000 °C, which is about 30% lower than that of 8 wt.% yttria stabilized zirconia (8YSZ) thermal barrier coatings. The thermal expansion coefficients (TECs) of InFeO₃(ZnO)m bulk ceramics were in a range of (10.97 ± 0.33) × 10⁻⁶ K⁻¹ to (11.46 ± 0.35) × 10⁻⁶ K⁻¹ at 900 °C, which are comparable to those of 8YSZ ceramics.     

Microwave dielectric properties and crystal structure of the tungstenbronze-type like (Ba1 − αSrα)6(Nd1 − βYβ)8Ti18O54 solid solutions

A study of the dielectric properties, especially the Q × f value, of the tungstenbronze-type like (Ba_1 − αSr_α)_6 − 3xNd_8 + 2xTi₁₈O₅₄ solid solutions in x = 0 system was carried out. These compositions near x = 0 have very low Q × f values. To improve the Q × f value of these materials, we tried two substitutional systems, which are (Ba_1 − αSr_α)₆Nd₈Ti₁₈O₅₄ and Ba₄Sr₂(Nd_1 − βY_β)₈Ti₁₈O₅₄. In the former composition, the Q × f value was increased from 206 to 5880 GHz in the range of 0 ≤ α ≤ 0.5. And we found that Sr ions substituted for Ba ions in A1 sites have good effect on increasing the Q × f value, but Sr ions substituted for Ba ions in A2 sites have poor effect on increasing it. The latter composition also has a small effect on increasing the Q × f value.     

Preparation and characterization of materials in the system xCuO-(50-x) CdO-50B2O3

New glassy materials in the system xCuO-(50-x) CdO–50B₂O₃ were prepared by a melting-quench technique. Their UV–vis, FTIR, electrical, dielectric, SEM, XRD patterns and density properties were investigated. SEM and XRD studies confirmed their amorphous nature and the presence of crystalline phases in the sample with 50 mol% CuO. Replacing CdO with increasing concentrations of CuO decreased the density and increased the molar volume. Optical reflectance spectra revealed the presence of Cu²⁺ ions in octahedral coordination, as well as the presence of Cu¹⁺ and Cuº in the samples with greater than 30% CuO. FTIR measurements confirmed the conversion of BO₄ units to BO₃ units with increasing CuO contents. The conduction in the CuO-containing samples increased as the CdO was completely replaced by CuO. The CuO-containing samples exhibited a slight increase in the ɛʹ values with increasing temperature and a decrease with increasing frequency. The ɛʹ values gradually increase upon replacing CdO with up to 40 mol% CuO. An abrupt increase in ɛʹ was recorded for the sample with 50 mol% CuO, particularly at high temperature. The latter sample showed an εʹ value of 927at 100 Hz and 298 K. Prepared samples with high ɛʹ values are promising candidates for capacitor materials in electronic devices.     

X-ray diffraction,dielectric, pyroelectric,piezoelectric and Raman spectroscopy studies on (Ba0.95Ca0.05)0.8875Bi0.075TiO3 ceramic

The (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ ceramic composition was prepared using the conventional mixed-oxide technique. X-ray diffraction at room temperature and dielectric permittivity in the temperature range from 85 to 450 K and frequency range from 10² to 2 × 10⁵ Hz, respectively, were studied. The X-ray spectra were investigated by profile refinement technique with the use of specialized software at room temperature, the (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ composition crystallizes in quadratic perovskite structure. The dielectric measurements show classical ferroelectric behavior. The pyroelectric and piezoelectric results confirm the dielectric measurements. The pyroelectric coefficient is about 69.2 nC/cm² K at the transition temperature (T_C = 367 K). The piezoelectric constant is d₃₁ = 31.1 pC/N and the electromechanical coupling factor is k_P = 0.14679. Raman spectra of (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ ceramic were taken at various temperatures and measured over the wave number range from 50 to 1000 cm^?1. All the Raman bands were assigned as the transitional modes of Ba²⁺, Ca²⁺, Bi³⁺ and Ti⁴⁺ cations. The temperature evolution of Raman spectra across the transition shows an important evolution characterizing the disorder of the high temperature phase.     

Low temperature sintering of the Ba2Ti9O20 ceramics using B2O3/CuO and BaCu(B2O5) additives

The effects of B₂O₃/CuO and BaCu(B₂O₅) additives on the sintering temperature and microwave dielectric properties of Ba₂Ti₉O₂₀ ceramics were investigated. The B₂O₃ added Ba₂Ti₉O₂₀ ceramics were not able to be sintered below 1000 °C. However, when both CuO and B₂O₃ were added, they were sintered below 900 °C and had the good microwave dielectric properties. It was suggested that a liquid phase with the composition of BaCu(B₂O₅) was formed during the sintering and assisted the densification of the Ba₂Ti₉O₂₀ ceramics at low temperature. BaCu(B₂O₅) powders were produced and used to reduce the sintering temperature of the Ba₂Ti₉O₂₀ ceramics. Good microwave dielectric properties of Qxf = 16,000 GHz, ɛ_r = 36.0 and τ_f = 9.11 ppm/°C were obtained for the Ba₂Ti₉O₂₀ ceramics containing 10.0 mol% BaCu(B₂O₅) sintered at 875 °C for 2 h.