Fluorite Phase Research Articles

Ceramics with eutectic microstructure in the ZrO2–PrOx system

AbstractPraseodymium oxides present redox properties analogous to those of Ce‐based systems and have been proposed for catalytic applications in combination with CeO2, ZrO2, or both. However, uncertainties remain concerning the nature and redox behavior of Pr‐rich mixtures, especially with ZrO2. Here we study the eutectic composites of the ZrO2–PrOx system, focusing on the sensitivity of their microstructure, phase symmetry, and composition to variations of the processing atmosphere from oxidizing to reducing. Mixed oxides have been produced by a laser‐assisted directional solidification technique in O2, air, N2, or 5%H2(Ar) environment, and the resulting materials have been analyzed by scanning electron microscopy/energy‐dispersive X‐ray spectroscopy, X‐ray diffraction, Raman spectroscopy, and magnetic susceptibility. In air, N2, or 5%H2(Ar) atmosphere, a lamellar, eutectic‐like microstructure forms, the major phase being the one with less Pr content. Both the Pr concentration in each phase as the PrOx molar percentage of the eutectic composites decrease as the atmosphere becomes more reducing. Both eutectic phases are fluorite‐like when processing in air, whereas in N2 or 5%H2(Ar), the phase with high Pr content is of the A‐R2O3 type, and the phase with low Pr content can be described as a fluorite phase containing C‐R2O3‐like short‐range‐ordered regions. The results obtained for samples processed in O2 suggest that for high enough pO2 no eutectic forms, in analogy with the ZrO2–CeO2 system. The evolution of the phase composition and symmetry is discussed in terms of the limited stability of the phases found in the ZrO2–Pr2O3 system, namely, A‐ or C‐R2O3‐like, beyond a certain Pr oxidation degree and oxygen content.

Crystal structure and thermophysical properties of (Gd0.25Sm0.25Yb0.25Y0.25)3TaO7 high-entropy oxide

An equimolar quaternary (Gd0.25Sm0.25Yb0.25Y0.25)3TaO7 high-entropy oxide was synthesised using the sol-gel technique and high-temperature calcination technology. The obtained nanopowder and compact bulk sample are all of the single fluorite phase. The relatively low thermal conductivity (1.170‒1.211W·m‒1·K‒1, 50‒1000 °C) of bulk sample is ascribed to the typical phonon scattering mechanisms and high-entropy distortion effect. The high average thermal expansion coefficient reaches 10.855 × 10–6 K−1 (1000–1400 °C) owing to the relatively weak ionic bond strength and lattice energy. The outstanding phase stability up to 1400 °C is in favour of lengthening the service life at high temperatures. These results reveal that (Gd0.25Sm0.25Yb0.25Y0.25)3TaO7 high-entropy oxide can be explored as a promising thermal barrier coating material.

Synthesis and Oxygen Mobility of Bismuth Cerates and Titanates with Pyrochlore Structure

Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi1.6Y0.4Ti2O7 were synthesized by the citrate route; Bi2Ce2O7 and Bi1.6Y0.4Ce2O7—by the Pechini method. The structural characteristics of materials after conventional sintering at 500–1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi1.6Y0.4Ti2O7, occurs after high-temperature calcination. Complex oxides Bi2Ce2O7 and Bi1.6Y0.4Ce2O7 have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO2-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes.

Dry Reforming of Methane over 5%Ni/Ce1-xTixO2 Catalysts Obtained via Synthesis in Supercritical Isopropanol.

A series of 5%Ni/Ce1-xTixO2 catalysts was prepared with nickel impregnation of mixed Ce-Ti oxides obtained via synthesis in supercritical isopropanol. All oxides have a cubic fluorite phase structure. Ti is incorporated into the fluorite structure. Small amounts of impurities of TiO2 or mixed Ce-Ti oxides appear with Ti introduction. Supported Ni is presented as the NiO or NiTiO3 perovskite phase. Ti introduction increases total samples reducibility and results in stronger interaction of supported Ni with the oxide support. The fraction of rapidly replaced oxygen and the average tracer diffusion coefficient also increase. The number of metallic nickel sites decreased with increasing Ti content. All catalysts except Ni-CeTi0.45 demonstrate close activity in tests of dry reforming of methane. The lower activity of Ni-CeTi0.45 can be connected to Ni decoration with species of the oxide support. The incorporation of Ti prevents detachment of Ni particles from the surface and their sintering during dry reforming of methane.

Structural environment influence on Faraday effect in Tb3+ and Pr3+ co-doped fluorophosphate glass and glass-ceramics containing TbOF nanocrystals

The influence of structural environment of lanthanide ions on the co-doping Faraday effect is explored within fluorophosphate glasses and glass ceramics. In a first part, the crystallization process of a glass with a nominal composition 35 NaPO3 – 15 BaF2 – 50 TbF3 is explored in function of duration and temperature of the ceramization process. X-ray diffraction analysis and STEM-HAADF allowed the identification of crystalline TbOF cubic fluorite phase in these glass-ceramics, with a maximum crystalline fraction of 42 ± 2 wt% for a crystallite size of 46 nm for the sample heat-treated 10 h at 394 °C. In a second part, TbOF glass-ceramics following the composition law 35 NaPO3 – 15 BaF2 – 0.5 ((100-x) TbF3 – x PrF3) with x = 0, 1, 2, 3, 4 and 5 are prepared at the optimal heat treatment of 10 h at 394 °C corresponding to maximal crystalline fraction and optical transmission. The Verdet constants of these co-doped glass-ceramics are characterized and compared to the ones of the parent glasses. The maximum co-doping efficiency reaches 1.075 for the glass-ceramic, compared to a value of 1.037 for the corresponding co-doped glass samples. The structural organization around the lanthanide cation site increases the co-doping influence on the Faraday Effect by optimization of the superexchange interaction.

Mechanical Properties and Toughening Mechanisms of Promising Zr-Y-Ta-O Composite Ceramics

ZrO2-YO1.5-TaO2.5 (ZYTO) composite ceramics are considered to be a candidate for next-generation thermal barrier coatings (TBCs) due to their excellent thermal stability and low thermal conductivity in high temperatures; however, the mechanical properties and fracture toughness of the ZYTO system may be shortcomings compared with 7-8YSZ: the traditional TBC. In this study, ZYTO composite ceramics were successfully prepared by chemical coprecipitation reaction, and the microstructure of resulting composites was studied as a function of the doping of M-YTaO4. Mechanical properties, including the density, porosity, hardness and Young’s modulus, were all determinate; the toughening mechanism was verified by the crack growth behavior of the Vickers indentation test. The results suggest that M-YTaO4 refined the fluorite phase grain and strengthened the grain interface in the composite ceramic. The thermal mismatch between the second phase and matrix produced residual stress in the bulk and affected the crack propagation behavior. With the increase in M-YTaO4 doping, the grain coarsening and ferroelastic domains were observed in the experiments. The ferroelastic domains with orthogonal polarization directions near the crack tip evidenced the ferroelastic toughening mechanism. The competition among these crack behaviors, such as crack deflection, bridging and bifurcation, dominated the actual fracture toughness of the composite. The best toughening formula was determined in the two-phase region, and the highest indentation fracture toughness was about 42 J/m2, which was very close to 7-8YSZ’s 45 ± 5 J/m2.

Tuning of structural, optical/ spectral and thermal properties of cubic CeO2 nanoparticles via incorporation of zirconia

In this communication, the influence of Zr4+ ions on structural, microstructural, optical and thermal behavior of CeO2 was investigated. Structural parameters of sol-gel derived samples were assessed by XRD, FTIR and Raman spectroscopies. Refined profile parameters were determined through Rietveld refinement. XRD and Raman studies indicated cubic fluorite phase of CeO2 and occurrence of oxygen vacancies as defects attributed to distortion in local symmetry of O2− ions around Ce4+ ions, hence found applications in oxygen storage devices. UV–visible depicted the blue shifting of absorption edge on increasing Zr4+ ions concentration, into Ce4+ions lattice, hence found utility in photonic devices. Violet, blue and green PL emissions were observed and committed due to various defect states and oxygen vacancies. CIE and CCT coordinates demonstrated their potential applications in display devices. The synthesized materials with enhanced structural and optical characteristics have their wide employability as an active component in sophisticated optoelectronic device fabrication.

Composite nanofibers electrospun from cerium, titanium, and zinc precursors

Non-woven fibers were produced by sol-gel and electrospinning methods, from a solution containing cerium nitrate, zinc acetate, titanium isopropoxide, polyvinylpyrrolidone, acetic acid, ethanol, and water. The fibers were calcined at various temperatures ranging from 300 to 900 °C and were characterized using Scanning Electron Microscopy (SEM), X-Ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area analysis, Energy Dispersive X-ray (EDX), Raman spectroscopy, Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Structural characterization revealed the fluorite nanocrystalline phase of ceria (CeO2) at all temperatures, the wurtzite zinc oxide (ZnO) phase in the 300–500 °C range, and a variety of zinc titanate phases (such as ZnTiO3, Zn2Ti3O8 and Zn2TiO4) at higher temperatures. Titania (TiO2) phases were not observed following calcination up to 900 °C. The average ceria nanocrystallite size increases with calcination temperature, as revealed by XRD and confirmed by the Phonon Confinement Model (PCM) of Raman spectra. The opposite trend is observed for the BET specific surface area of the nanofibers, where this value decreases with calcination temperatures above 400 °C. These nanofibers containing ceria and zinc titanates are potential candidates for photocatalytic applications.

Structural analysis and properties of novel Terbium Samarium Zirconate functional ceramic

Synthesis and characterization of Terbium Samarium Zirconate (TbSmZr2O7) is reporting for the first time. XRD analysis along with Raman spectroscopy revealed that citrate-nitrate combustion synthesized TbSmZr2O7 powder and sintered sample crystallize in defect-fluorite structure with Fm3‾m space group. This composition lies at the pyrochlore-defect fluorite phase boundary. TEM analysis showed that the powder particles are in size range between 3 nm and 9 nm. FESEM image showed a well-sintered pellet surface with a grain size distribution between 0.1 μm and 2 μm. TbSmZr2O7 has an optical band gap of 1.66 eV and has an indirect allowed mechanism of optical transition. Due to intrinsic defect structure, TbSmZr2O7 exhibits photoluminescence with excitation bands at 270 nm and 313 nm and emission bands at 364 nm and 400 nm. Impedance spectroscopic analysis established that from 400 °C to 900 °C TbSmZr2O7 is a pure oxide ion conductor with a conductivity value of 1.9 × 10−3 S/cm at 900 °C.

Structure and redox properties of CeMnO3 thin films

Conformal thin films of a cerium-manganese mixed oxide with a stoichiometry of CeMnOx were deposited on a high-surface-area γ-Al2O3 support by Atomic Layer Deposition (ALD). Upon redox cycling, the CeMnOx film underwent a reversible phase transition between a reduced perovskite CeMnO3 phase and an oxidized fluorite CeMnO3.5 phase, as shown by X-ray diffraction (XRD) and High-Resolution Scanning/Transmission Electron Microscopy (HR-S/TEM). X-ray Photoelectron Spectroscopy (XPS) and Electron Energy Loss Spectroscopy (EELS) revealed that Ce and Mn were in +3-oxidation states in the reduced perovskite phase and +4 and + 3 respectively in the oxidized fluorite phase. The composition of the film as a function of P(O2) at 1073 K was measured by coulometric titration and showed that a transition between Ce3+ and Ce4+ occurred at a P(O2) of 10−9 atm, a value that is much higher than that required for oxidizing either reduced bulk ceria or thin-film ceria on γ-Al2O3. These properties make CeMnOx films an interesting candidate for various catalytic applications.

Rare-Earth-Zirconate Porous High-Entropy Ceramics with Unique Pore Structures for Thermal Insulating Applications.

Porous high-entropy ceramics are a new alternative material for thermal insulation. Their better stability and low thermal conductivity are due to lattice distortion and unique pore structures. In this work, rare-earth-zirconate ((La0.25Eu0.25Gd0.25Yb0.25)2(Zr0.75Ce0.25)2O7) porous high-entropy ceramics were fabricated by a tert-butyl alcohol (TBA)-based gel-casting method. The regulation of pore structures was realized through changing different initial solid loadings. The XRD, HRTEM, and SAED results showed that the porous high-entropy ceramics had a single fluorite phase without impurity phases, exhibiting high porosity (67.1-81.5%), relatively high compressive strength (1.02-6.45 MPa) and low thermal conductivity (0.0642-0.1213 W/(m·K)) at room temperature. Porous high-entropy ceramics with 81.5% porosity demonstrated excellent thermal properties, showing a thermal conductivity of 0.0642 W/(m·K) at room temperature and 0.1467 W/(m·K) at 1200 °C. The unique pore structure with a micron size contributed to their excellent thermal insulating performance. The present work provides the prospect that rare-earth-zirconate porous high-entropy ceramics with tailored pore structures are expected to be thermal insulation materials.

CeO2–x Nanoparticles Heated to 900 °C Afford Nanomaterials without Thermally Induced Mismatches: Implications for Catalyst Carriers

Nowadays, CeO2–x (0 < x < 0.5) nanoparticles are used for a wide range of applications, most of them using high temperatures. Since the phase diagram of nanoparticles differs from that of the corresponding bulk material and plays a pivotal role in many applications, the phase transition of CeO2–x nanoparticles exposed to a CO atmosphere at 900 °C was probed by in situ XRD measurements. The results show the presence of the fluorite and Cu2Se-like phases at this temperature. During the cooling process to room temperature in the He atmosphere, the Cu2Se-like phase turns metastable and transforms back to the fluorite phase. The phase transition occurs in 1D, with the crystallization mechanism controlled by volume diffusion. The Cu2Se-like phase presents a negative thermal expansion coefficient (α) for the temperature range of 580–300 °C due to the O diffusion from the Cu2Se-like to the fluorite crystal structure. Consequently, tuning the amounts of the fluorite and the Cu2Se-like phase allows reaching a tunable thermal expansion coefficient of the CeO2–x nanoparticles. In particular, the condition for a null thermal expansion coefficient was successfully achieved, which allows the use of smart CeO2–x nanoparticles without inducing thermal mismatches for applications like catalyst carriers.

Formation of nano-rod structures in manganese-rich ceria–manganese mixed oxides and their soot oxidation activity

EDTA-citrate synthesized manganese-rich ceria–manganese oxide catalysts (Ce1−xMnxO2−δ; where x = 1 to 0.6, termed CM100, CM90, CM80, CM70, CM60, CM50 and CM40 respectively) were obtained after calcination at 600 °C/5 h, characterized and tested for soot oxidation activity. Mn3O4 and Mn2O3 phases were observed in the XRD spectra; with the increase in the ceria concentration, cubic fluorite phase formation was observed from CM80 to CM40. BET and BJH analyses revealed that the pore diameter and volume had a meaningful impact on the soot oxidation activity, as seen for CM80 which had a BET surface area of 31 m2/g, pore diameter of 21.119 nm, and a pore volume of 0.1637 cc/g. The formation of the Mn3O4 peak in all the synthesized catalysts and ceria’s cubic fluorite F2g peak (452 cm −1) were observed from CM80 to CM40 from Raman spectral analysis. FESEM analysis of all the CMX samples revealed an intriguing aspect, the appearance of nano-rods. These nano-rods were observed in the HRTEM imaging as well. The high-resolution imaging and SAED analysis proved that the nano-rod structures were primarily due to the α-MnO 2 phase. The mixture of agglomerates and nano-rods profoundly affected the soot oxidation activity of the synthesized samples. CM70 had a minimum T50 of 389 °C among the CMX catalysts. The activation energy was reckoned with the Ozawa and KAS methods. CM50, with 131 kJ/mol for Ozawa and 127 kJ/mol for the KAS method, had the lowest values among the chosen catalysts. Synergistic interactions between ceria and the manganese oxides, considerable specific surface area, pore diameter, pore volume, substantially higher exposed (220) and (200) planes and the formation of the nano-rods helped to lower the activation energy for soot oxidation activity and consequently a reduction in the soot oxidation temperature of CM50 to CM70. The concentration of the nano-rods rendered a considerable role in reducing the T50 temperatures of the CMX catalysts.

Enhanced H2 permeation and CO2 tolerance of self-assembled ceramic-metal-ceramic BZCYYb-Ni-CeO2 hybrid membrane for hydrogen separation

Perovskite-type mixed protonic-electronic conducting membranes have attracted attention because of their ability to separate and purify hydrogen from a mixture of gases generated by industrial-scale steam reforming based on an ion diffusion mechanism. Exploring cost-effective membrane materials that can achieve both high H2 permeability and strong CO2-tolerant chemical stability has been a major challenge for industrial applications. Herein, we constructed a triple phase (ceramic–metal–ceramic) membrane composed of a perovskite ceramic phase BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb), Ni metal phase and a fluorite ceramic phase CeO2. Under H2 atmosphere, Ni metal in-situ exsolved from the oxide grains, and decorated the grain surface and boundary, thus the electronic conductivity and hydrogen separation performance can be promoted. The BZCYYbNi-CeO2 hybrid membrane achieved an exceptional hydrogen separation performance of 0.53 mL min−1 cm−2 at 800 °C under a 10 vol% H2 atmosphere, surpassing all other perovskite membranes reported to date. Furthermore, the CeO2 phase incorporated into the BZCYYb-Ni effectively improved the CO2-tolerant chemical stability. The BZCYYbNi-CeO2 membrane exhibited outstanding long-term stability for at least 80 h at 700 °C under 10 vol% CO2–10 vol% H2. The success of hybrid membrane construction creates a new direction for simultaneously improving their hydrogen separation performance and CO2 resistance stability.

EBM Technique Study on Microstructural Characterization of Al-Zr-N Coating

Through the implementation of the electron beam physical vapour deposition (EB-PVD) technique, an aluminium zirconium nitrate (Al-Zr-N) coating has been created in the current work on high speed steel (E19) substrates. Through the use of X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), nano indentation, and scratch tests, the structural morphological and mechanical properties of the substrate are investigated. The cause of tension relaxation and vacancy concentration is explained by the fluorite phase's lattice constant decreasing with rising deposition temperature. As the temperature of the substrate rises, the coating's increased surface roughness is explained by an increase in crystallite size [1]. TBCs are frequently applied to metal substrates using the Atmospheric Plasma Spraying (APS) and Electron Beam Physical Vapour Deposition (EB-PVD) methods. For the next generation of turbine engines, the high costs and relatively high thermal conductivities of EB-PVD coatings, as well as the short thermal lifetime of APS coatings, are significant drawbacks. Suspension Plasma Spraying (SPS) was evaluated in this study to enhance the thermal properties of TBC. The SPS process enables the creation of columnar micro-structures that are easily adjustable in terms of thermal conductivity and columnar structure compaction.

The Electrical Properties of Zr Doped Ce0.6Y0.4O2- δ Solid Electrolytes for Solid Oxide Fuel Cell Applications

Ionic conducting solid electrolytes based on doped ceria has been studied widely. In this work, we are studying the effect of Zr4+ substitution on the solid electrolyte system Ce0.6-xZrxY0.4O2-δ where x = 0, 0.2, 0.4, and 0.6. The samples were synthesized by mechanical milling and the evolution of their structure with milling time was analyzed by X-ray diffraction (XRD). The milled powders were uniaxially pressed and sintered at different temperatures (1200 and 1500 °C) and the morphology was analyzed by scanning electron microscopy (SEM) and the electrical properties were measured using impedance spectroscopy. XRD analysis shows the formation of fluorite type cubic phases after 20 h of milling and with increasing the sintering temperature from 900 to 1200 °C shows the transformation of cubic fluorite phase to a tetragonal phase. The analysis of electrical properties shows a decrease in conductivity with the substitution of Zr4+ in Ce0.6-xZrxY0.4O2-δ oxide.

Ce-doped SBA-15 supported Pd catalyst for efficient hydrogenation of 2-ethyl-anthraquinone

Hydrogenation of 2-ethyl-anthraquinone (EAQ) is a key reaction in the anthraquinone process for industrial production of H2O2, a green chemical with a variety of applications. Herein, Ce-doped SBA-15 was synthesized via a one-step hydrothermal method with ethylenediamine as pH regulator and explored as the support of Pd catalyst for EAQ hydrogenation. Part of Ce species was incorporated into SBA-15 framework and the rest existed as extra-framework Ce and fluorite phase CeO2 particles. The framework Ce species had a suppressing effect on the aggregation of Pd particles. Their amount increased with lowering Si/Ce molar ratio (SCA) from 50 to 20 and reached a plateau at 30. The content of Ce3+ and the number of oxygen vacancies showed the same trend as the amount of framework Ce with SCA, but monotonously increased with raising reduction temperature from 200 to 600 °C. In addition, hydrogen spillover from metal to support and an increase in the binding energy of Pd were observed. Compared with Pd/SBA-15, Ce-doped catalysts presented significantly improved catalytic performance. Among them, the catalyst with SCA of 30 and reduced at 400 °C exhibited the highest activity (3.8 times of Pd/SBA-15) and selectivity to 2-ethyl-anthrahydroquinone (94.2 % from 91.8 % over Pd/SBA-15).

Enhancement of oxygen vacancy sites of La2-xMxCe2O7-δ (M = Ca, Ba, Sr) catalyst for the low temperature oxidative coupling of Methane: A combined DFT and experimental study

This study investigated the effect of different alkaline earth metals doped La2-xMxCe2O7-δ (M = Ca, Sr, Ba and x = 0.30) catalysts prepared by one-step hydrothermal method for the low-temperature methane oxidative coupling (OCM) to olefins. The presence of a disordered cubic fluorite phase of synthesized catalysts was detected by powder XRD and Raman analysis. The surface analysis by XPS and Raman shows the presence of oxygen vacancy sites, which are responsible for the formation of electrophilic superoxide O2– species in presence of molecular oxygen. We observed that the ratio of active oxygen species (O2− and O22−) to lattice oxygen (O2–) was linearly correlated with the number of basic sites. The catalytic activity of different catalysts for the OCM reactions follows the order: La1.7Ca0.3Ce2O7-δ > La1.7Sr0.3Ce2O7-δ > La1.7Ba0.3Ce2O7-δ > La2Ce2O7. The reactivity order follows the same trend as the amount of basic sites and reactive oxygen species. According to this study, the best catalyst is La1.7Ca0.3Ce2O7-δ, which gives the highest C2 yield and C2 selectivity of 23.6 % and 78.8 %, respectively, at 650 °C. DFT studies revealed that substitution of La by Ca in the La2Ce2O7 catalyst significantly reduces the oxygen vacancy formation energy by about 100 kJ/mol compared to the undoped LaCe-oxide. We believe that the superior activity of La1.7Ca0.3Ce2O7-δ catalyst is attributed to the higher amount of basicity and electrophilic oxygen content, which are the governing factors in OCM for CH4 conversion and C2 product selectivity as well.

Oxidation Behavior of Single-Layer Gd2Zr2O7, and Gd2Zr2O7/YbSZ, and Double-Layer YSZ + Gd2Zr2O7/YbSZ Thermal Barrier Coatings

Oxidation is one of the failure mechanisms of gas turbines under operating temperature. Yttria-stabilized zirconia (YSZ) is well-known ceramic topcoat material of thermal barrier coatings (TBCs), but it has some drawbacks. From this point of view, other new materials such as rare earth zirconate are interesting. In the present research, single-layer Gd2Zr2O7 (GZO), and GZO/YbSZ (Gd2Zr2O7 + 50 wt% YbSZ), and double-layer YSZ + GZO/YbSZ (YSZ/Gd2Zr2O7 + 50 wt% YbSZ) topcoats as TBC were deposited by air plasma spraying. To estimate the phase stability of homogeneous mixtures of GZO (pyrochlore) and YbSZ (tetragonal), heat treatment was conducted at 1400 °C and showed complete conversion to a single-cubic (fluorite) phase after 100 h. The as-sprayed TBCs were subjected to oxidation for different times at 900 °C, and their features were investigated by different analysis techniques (SEM and XRD). The microstructural analysis indicated that the thickness of the thermally grown oxide layer was less for GZO coating due to reduced oxygen diffusion. Conversely, because of the lower fracture toughness and lower thermal expansion of GZO, it had a considerably less lifetime. It was concluded that coatings with the new composition GZO/YbSZ exhibited higher oxidation resistance and longer lifetime than GZO coatings.

Multi-method characterization of rare blue quartz-bearing metavolcanic rocks of the Rio dos Remédios Group, Paramirim Aulacogen, NE Brazil

The Rio dos Remédios Group comprises a supracrustal sequence that occupies the base of the Espinhaço Supergroup, São Francisco Craton, Brazil. Its basal formation, Novo Horizonte, crops out in the Paramirim region mainly as metavolcanic rocks that represent one of the fewer occurrences of blue quartz phenocrysts in South America. Their mineralogy consists of quartz and K-feldspar phenocrysts, whereas biotite, muscovite, fluorite, allanite, chlorite, sericite, zircon, and opaque phases occur immersed in a quartz-feldspar-rich groundmass. Such heterogeneous composition is also supported by x-ray diffraction and chemical data. Electron probe microanalysis in some samples revealed the presence of two distinct groups of biotite (magmatic and neoformed), in addition to the presence of iron-rich white mica and almost pure orthoclase feldspar. Our data suggest that the studied metavolcanic rocks have maintained their magmatic characteristics, which were progressively overprinted by hydrothermal fluids and ductile-to-brittle deformation. The magmatic mineralogy is akin to strongly peraluminous and alkaline magmas, common in anorogenic settings – a fertile site for the origin of blue quartz-bearing rocks worldwide.