Fluorite Phase Research Articles

Atypical decomposition processes of Sm2Zr2O7 + 8YSZ dual-phase TBCs during hot corrosion

The accelerated decomposition of an Sm2Zr2O7+8YSZ system in the form of air plasma spraying (APS) thermal barrier coatings of composites, double ceramic layer (DCL), and functional graded system (FGS) types under hot corrosion conditions were analyzed. Hot corrosion tests were performed using liquid sodium sulfate salt deposits and then X-Ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersion spectroscopy (EDS) were performed to identify the reaction products. The obtained data indicated that negligible amounts of sulfates or oxysulfates were formed, but significant decomposition of pyrochlore Sm2Zr2O7 to the fluorite phase was detected. These phenomena were also observed in a much higher temperature range during pure oxidation tests, but the lowest decomposition temperature (920 °C) was observed in the sodium sulfate assisted process. The described observations were confirmed during similar tests and realized using model powder mixtures. Microstructural analysis of thermal barrier coatings (TBC) cross-sections revealed a complex decomposition process that was localized to areas near the Sm2Zr2O7 splats. The decomposition of the observed pyrochlore phase was mainly related to interactions between Sm2Zr2O7 and 8YSZ in accordance with the fluxing mechanism typical for the hot corrosion process of Ni-based superalloys. Additionally, the effect of eutectic Na-Sm-O was determined.

Structure evolution, thermal properties and sintering resistance of promising thermal barrier coating material La2(Zr0.75Ce0.25)2O7

Rare-earth doped zirconates are promising candidate materials for high-performance thermal barrier coatings (TBCs). The phase and microstructure stability is an important issue for the materials that must be clarified, which is related to the long-term stable work of TBCs at high temperatures. In this work, La2(Zr0.75Ce0.25)2O7 (LCZ) ceramic coatings prepared by atmospheric plasma spraying present a metastable fluorite phase, which can transform into stable pyrochlore under high-temperature annealing. The detailed structure evolution of the ceramic coatings is characterized systematically by SEM, XRD and Raman. The associated thermal properties of LCZ ceramics were also reported. Results show that LCZ ceramic has an ultralow thermal conductivity (0.65 W/m·K, 1200 °C), which is only 1/3 of that of yttria-stabilized zirconia (YSZ). The thermal expansion coefficients of LCZ ceramic increase from 9.68 × 10-6 K-1 to 10.7 × 10-6 K-1 (300 - 1500 °C), which are relatively larger than those of La2Zr2O7. Besides, Long-term sintering demonstrates that LCZ ceramic coating has preferable sintering resistance at 1500 °C, which is desirable for TBC applications.

Effects of CaF2 on the sintering and crystallisation of CaO–MgO–Al2O3–SiO2 glass-ceramics

The effects of CaF2 (1–5 wt.%) on the glass structure, crystallisation behaviour, sinterability, microstructure, and properties of a CaO–MgO–Al2O3–SiO2 glass-ceramic system were investigated. The results show that the glass transition temperature, crystallisation peak temperature and the activation energy for crystallisation decreased with the addition of CaF2. The crystallisation of diopside was initiated at the particle boundaries and followed with progressive advancing towards the centre. The enhanced crystallisation for CaF2-containing samples was observed and is likely due to the depolymerisation of the glass structure. However, the enrichment of Ca–2F complexes resulted in fluorite phase precipitation at the crystallisation front of the diopside phase when the CaF2 addition exceeded 3 wt.%. The hindered viscous flow sintering from a lower crystallisation temperature led to a decline in the sintering shrinkage, the extent of which was proportional to the amount of added CaF2. Plastic deformation was observed along with the newly-formed amorphous phase as the sintering temperature approached the glass melting point, which benefits densification. The performance analysis reveals that the fabricated glass-ceramics with the proper addition of CaF2 exhibited significant improvements in its density and bending strength. Whereas, the Vickers hardness, water absorption and chemical resistance all decreased.

Photocatalytic degradation of methylene blue dye by fluorite type Fe2Zr2-xWxO7 system under visible light irradiation.

Nano-sized Fe2Zr2-xWxO7 system was prepared using the Pacini method where x=0, 0.05, 0.1 and 0.15. All the samples were characterized using chemical analysis, X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), transmission electron microscopy (TEM), UV-vis diffuse reflectance measurements (DRS) and surface area measurements. The undoped Fe2Zr2O7 was crystallised in the cubic fluorite phase as a major phase in addition to rhombohedral phase of Fe2O3 and monoclinic phase of ZrO2 as the minor phases. Meanwhile, single cubic fluorite phase was defined for Fe2Zr0.85W0.15O7 sample. The last has the lowest band gap (1.69eV) and the highest surface area (106m2/g). From TEM, the average particle size of the prepared samples was in the range of (3-7nm). The photocatalytic efficiency of the prepared Fe2Zr2-xWxO7 system was manifested by the degradation of methylene blue and real textile wastewater of blue colour. Ascending degradation efficiency was exhibited with increasing tungsten concentration which is in accordance with their band gap as well as their surface area values. The degradation rate using Fe2Zr0.85W0.15O7 sample obeys the pseudo-first order kinetic at the optimum degradation conditions (1.5g/L catalyst and pH11). Fe2Zr0.85W0.15O7 showed promising photocatalytic activity for real textile wastewater where the 69% COD removal was obtained under the same conditions used for methylene blue degradation.

Micromechanical Characterization of Ce0.8Gd0.2O2‐δ–FeCo2O4 Dual Phase Oxygen Transport Membranes

Aiming toward an optimization of dual phase oxygen transport membrane materials for oxygen separation applications, ceramic composites consisting of a Ce1−xGdxO2−δ (0 < x < 0.2) fluorite phase, Gd0.9Ce0.1Fe0.8Co0.2O3 perovskite phase, FexCo3−xO4 (0 < x < 1) spinel phase, and CoO rock salt phase are developed and micromechanical properties (elastic modulus and hardness) of xCe0.8Gd0.2O2−δ: (1−x)FeCo2O4 (50 wt% ≤ x ≤ 90 wt%) composites are characterized via indentation testing at room temperature. The results obtained at low indentation loads indicate that the magnitude of the elastic moduli of the different phases is in the order Gd0.9Ce0.1Fe0.8Co0.2O3 > Ce1−xGdxO2−δ ≈ FexCo3−xO4 > CoO, and furthermore, hardness values are also in the same order. The hardness values of the obtained composites at higher impression loads reveal a stronger dependency on porosity than on composition due to similar hardness values of the main phases. Any compositional effect appears to diminish above a porosity of ≈1%.

Optical thermometry using up and down conversion photoluminescence mechanism in Y2Zr2O7: Er3+ phosphors with excellent sensing sensitivity

In this article, photoluminescence up and down conversion properties of Er3+ activated Y2Zr2O7 (YZO) phosphors have been investigated. All the compositions have been synthesized via hydrothermal method. Various characterization techniques have been used to characterize the samples. Powder X-ray diffraction reveals the crystallization of the fluorite phase of all samples. FESEM technique and EDX spectroscopy were performed for surface morphology and elemental analysis of the samples. The optical band gap is appeared to increase after doping. Photoluminescence down conversion (DC) excitation (PLE) spectra exhibit a strong near UV absorption at 378 nm. All the doped samples under 378 nm excitation display the featured emission of Er3+ ions. The variation of DC PL intensity and the lifetimes with temperature were also investigated within the temperature range 77K–475K. Excitation power dependent up conversion (UC) emission and temperature dependent UC emission spectra within 303K–483K of the optimized sample were examined with 980 nm laser excitation. The fluorescence intensity ratio (FIR) technique and the variation of the average lifetime with temperature have been used to study the temperature sensing behaviour of the reported phosphors. Comparatively high sensitivities suggest the reported phosphors could have potential applications in temperature sensing and also could be a UV and NIR to green converter phosphors material.

Decolourization of crystal violet using nano-sized novel fluorite structure Ga2Zr2-x W x O7 photocatalyst under visible light irradiation.

Fluorite-type Zr-based oxides with the composition Ga2Zr2−xWxO7 (x = 0, 0.05, 0.1, 0.15 and 0.2) were prepared using the citrate technique. Appropriate characterizations of all prepared materials were carried out. X-ray diffraction clarified that the undoped and W-doped Ga2Zr2O7 samples were crystallized in the cubic fluorite phase structure. The average particle size of the samples was in the range of 3–8 nm. The lowest band gap (1.7 eV) and the highest surface area (124.3 m2 g−1) were recorded for Ga2Zr0.85W0.15O7. The photocatalytic impacts of the prepared systems were studied by removal of crystal violet (CV) dye employing visible light illumination and taking into consideration the initial dye concentrations, duration of visible irradiation treatment, catalysts dose and the dopant concentration. The obtained results showed higher dye removal with the boost of the catalyst dosage. W doping shifted the absorption to the visible light range by decreasing the band gap from 4.95 eV for parent Ga2Zr2O7 to 1.7 eV for 15 mol% tungsten-doped Ga2Zr2O7 enhancing the photocatalytic decolourization of CV from 4.2% to 83.6% for undoped and 15 mol% W-doped Ga2Zr2O7, respectively, at optimum operating conditions (pH 9, 1 g l−1 catalyst dose and 300 min) while heavily doped W sample containing 20 mol% W showed lower removal than 15 mol% W-doped Ga2Zr2O7. Complete CV degradation using 15 mol% W-doped Ga2Zr2O7 was attained with the assistance of 25 mmol l−1 hydrogen peroxide. The reaction is aligned to pseudo-first-order kinetics. Different scavengers were introduced to decide the significance of the reactive species in CV degradation. and h+ had the major role in the degradation of CV by Ga2Zr2−xWxO7 system compared with HO•.

Thermal analysis of the crystallization kinetics of lead zirconate titanate powders prepared via sol–gel route

The crystallization evolution of lead zirconate titanate (PZT) powders processed from two organic gel systems with nominal composition was investigated. The crystallization of the perovskite PZT form initiated as the calcining temperature reached 400 °C. The formation of PbO phase constituted seeding grains for PZT crystallization at such low temperature. The formation of lead-deficient fluorite or pyrochlore phases was completely avoided under judicious processing conditions. TGA/DTA analysis was carried out to evaluate the activation energy associated with the perovskite phase formation. The kinetic parameters of the PZT crystallization toward perovskite formation were followed by employing Kissinger and JMAK isoconversional kinetic models. The results revealed that the crystallization mechanism is related to the powder precursor system. The value of the activation energy for the perovskite formation was about 77 kJ mol−1 irrespective of the precursor system when the Kissinger model was applied.

Mixed-conducting ceramic-carbonate membranes exhibiting high CO2/O2 permeation flux and stability at high temperatures

This investigation demonstrates the feasibility to fabricate high quality ceramic-carbonate membranes based on mixed-conducting ceramics. Specifically, it is reported the simultaneous CO2/O2 permeation and stability properties of membranes constituted by a combination of ceramic and carbonate phases, wherein the microstructure of the ceramic part is composed, in turn, of a mixture of fluorite and perovskite phases. These ceramics showed ionic and electronic conduction, and at the operation temperature, the carbonate phase of the membranes is in liquid state, which allows the transport of rm{CO}_3^{2-} and O2− species via different mechanisms. To fabricate the membranes, the ceramic powders were uniaxially pressed in a disk shape. Then, an incipient sintering treatment was carried out in such a way that a highly porous ceramic was obtained. Afterwards, the piece is densified by the infiltration of molten carbonate. Characterization of the membranes was accomplished by SEM, XRD, and gas permeation techniques among others. Thermal and chemical stability under an atmosphere rich in CO2 was evaluated. CO2/O2 permeation and long-term stability measurements were conducted between 850 and 940 °C.The best permeation-separation performance of membranes of about 1 mm thickness, showed a maximum permeance flux of about 4.46×10−7 mol·m−2·s−2·Pa−1 for CO2 and 2.18×10−7 mol·m−2·s−1·Pa−1 for O2 at 940 °C. Membranes exhibited separation factor values of 150–991 and 49–511 for CO2/N2 and O2/N2 respectively in the studied temperature range. Despite long-term stability test showed certain microstructural changes in the membranes, no significant detriment on the permeation properties was observed along 100 h of continuous operation.

One Step Synthesis of Sr2Fe1.3Co0.2Mo0.5O6−δ-Gd0.1Ce0.9O2−δ for Symmetrical Solid Oxide Fuel Cells

Homogenous distribution of individual phases in the composite electrode is critical to the overall electrode performance. In this work, we report a one-step synthesis method to fabricate Sr2Fe1.3Co0.2Mo0.5O6−δ-Gd0.1Ce0.9O2−δ (SFCM-GDC) composite electrode material for symmetrical solid oxide fuel cells (SSOFCs). X-ray diffraction analysis reveals that the characteristic peaks of the perovskite and fluorite phases are obtained in the SFCM-GDC powder via the one-step synthesis method without any observable impurities. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) indicate a better phase distribution of the composite electrode of SFCM-GDC via one-step synthesis than that via a mechanically mixing process. As a result, the area specific resistance (ASR) of SFCM-GDC electrode is effectively decreased to 0.036 Ω cm2 and 0.047 Ω cm2 at 850 oC in air and hydrogen, respectively, compared to that of 0.041 Ω cm2 and 0.074 Ω cm2 of the mechanically mixed SFCM and GDC (SFCM+GDC). The maximum power density of a cell with configuration of SFCM-GDC∥LSGM∥SFCM-GDC can reach 0.986 W cm−2, higher than that with mechanically mixed electrode of 0.894 W cm−2 at 800 °C. This work shows that one step synthesis method is an effective way to fabricate composite electrode with enhanced electrode performance.

Novel materials for solid oxide fuel cells cathodes and oxygen separation membranes: Fundamentals of oxygen transport and performance

In the field of modern hydrogen energy, obtaining pure hydrogen and syngas and then being able to use them for green energy production are significant problems. Developing solid oxide fuel cells (SOFC) and catalytic membranes for oxygen separation as well as materials for these devices is one of the most likely ways to solve these problems. In this work, the authors’ recent studies in this field are reviewed; the fundamentals of developing materials for SOFC cathodes and oxygen separation membranes’ permselective layers based on research of their oxygen mobility and surface reactivity are presented. Ruddlesden – Popper phases Ln2–xCaxNiO4+δ (LnCNO) and perovskite-fluorite nanocomposites PrNi0.5Co0.5O3–δ–Ce0.9Y0.1O2–δ (PNC–YDC) were studied by isotope exchange of oxygen with C18O2 and 18O2 in flow and closed reactors. For LnCNO a high oxygen mobility was shown (D* ~ 10–7 cm2/s at 700 °C), being provided by the cooperative mechanism of oxygen migration involving both regular and highly-mobile interstitial oxygen. For PNC–YDC dominated a wide fast diffusion channel via fluorite phase and interphases due to features of the redistribution of cations resulting in superior oxygen mobility (D* ~ 10–8 cm2/s at 700 °C). After optimization of composition and nanodomain structure of these materials, as cathodes of SOFC they provided a high power density, while for asymmetric supported oxygen separation membranes – a high oxygen permeability.

Structure of HfO2 modified with Y, Gd, and Zr at ambient conditions and high pressures

High-resolution and high-pressure X-ray diffraction measurements were performed to determine the effects of 5% Y, 3% Gd, and 50% Zr substitution on the crystal structure and polymorphism of HfO2. High-resolution experiments at ambient pressure show a mixture of monoclinic and cubic fluorite phases in 5% Y:HfO2 and 3% Gd:HfO2, while 50% Zr:HfO2 formed a monoclinic, single-phase solid solution. Crystallographic refinement using the Rietveld method indicates that Y and Gd substitute for Hf in both the monoclinic and cubic phases. High-pressure X-ray diffraction was performed in situ up to 31 GPa and shows that the pressure-induced phase transitions were reduced with the addition of either Y, Gd, or Zr relative to the transition pressures in undoped HfO2. The pressure-induced changes in lattice parameter, relative volume, and spontaneous strain are reported.

Improved SMTB-Q model applied to oxygen migration and pressure phase transitions in UO2

The second-moment tight-binding variable-charge (SMTB-Q) interatomic potentials have been implemented in the molecular dynamics (MD) code LAMMPS in order to study the static and dynamical properties of uranium dioxide UO2. With respect to a previous work on UO2 the SMTB-Q model has been slightly modified in introducing a splitting energy of the U 5f orbitals. This improvement results in a better description of the electronic structure of UO2 namely the gap estimation which is now close to the experimental value (~2 eV). The structural and mechanical properties along with the cohesive energy of bulk UO2 are in good agreement with the experimental data. The ionic charges on uranium and oxygen are respectively equal to 2.86 and −1.43, very close to the Bader charges derived from ab initio calculations. The migration energies and the diffusion coefficient calculated respectively for oxygen vacancy (VO) and oxygen interstitials (IO) in under and over stoichiometry compare well with ab initio calculations and experimental data. The oxygen diffusivity is consistent at high temperature when additional Frenkel thermally formed swamps the effect of single IO and VO defects with recent prediction from EAM semi-empirical potentials.Additionally, a study on phase transitions between high pressure polymorphs of UO2 has been performed and has shown the good transferability of the SMBT-Q potential over different coordination. It is found that the UO2 phases stability order under tensile and compressive stresses, compared with stable fluorite phase at 0 GPa, are respected.

Rapid synthesis of Gd2Zr2O7 ceramics by flash sintering and its aqueous durability

The high radiation resistance and long time stability of Gd2Zr2O7 ceramics make it a promising candidate for high level waste (HLW) immobilization materials. In this study, single phase nanocrystalline Gd2Zr2O7 was successfully synthesized and consolidated at temperatures around 1050 °C for only 1 min by flash sintering for the first time. The phase evolution and microstructural development during flash sintering were systematically studied and compared with the conventionally sintered samples. The flash sintered Gd2Zr2O7 exhibit defect fluorite structure, and a following heat treatment at 1400 °C could transform the Gd2Zr2O7 ceramics from defect fluorite phase into pyrochlore phase. The MCC-1 leaching test shows that the flash sintered Gd2Zr2O7 samples exhibit good aqueous durability.

Thermophysical properties of urania-zirconia (U,Zr)O2 mixed oxides by molecular dynamics

Molecular dynamics simulations were used to investigate the thermophysical properties of (U,Zr)O2 between 300 K and 3500 K. For compositions with <25% UO2 the tetragonal phase is stable and beyond 25% the cubic fluorite phase becomes stable for all temperatures. Thermal expansion, heat capacity and thermal conductivity have been predicted. The addition of ZrO2 to UO2 causes a reduction in thermal conductivity however this effect decreases with increased temperature and becomes insignificant beyond 1000 K. Thermal expansion of (U,Zr)O2 mixtures with >25% UO2, which are in the cubic fluorite phase, is similar to that of UO2. A superionic transition is observed in cubic (U,Zr)O2 at temperatures between 1500 K and 3000 K, occurring at progressively lower temperatures with increasing ZrO2 content. The heat capacity of these mixed oxides increases from 80 J/mol.K up to 130 J/mol.K at temperatures relevant to accident conditions, possibly retarding temperature increase in fuels with a significant pellet-clad bonding layer.

Complex Rare-Earth Tantalates with Pyrochlore-Like Structure: Synthesis, Structure, and Thermal Properties

The existence of a wide range of pyrochlore-like (PL) Ln2FeTaO7 compounds (Ln = Pr–Yb, including Y) with a hexagonal structure (space group R$$\overline{3}$$) was shown. Two new low-temperature methods for the synthesis of Ln2FeTaO7 were developed to reduce the production temperature and time, namely, coprecipitation followed by annealing and synthesis in molten salts NaCl/KCl. The synthesis of PL phases was found to involve the formation of a nanocrystalline metastable fluorite phase, the fluorite irreversibly transforming to a PL phase as the crystallization time or temperature increases. No other transitions in Ln2FeTaO7 were recorded by DSC up to the melting point of 1320–1450°C (depending on the composition of the Ln2FeTaO7 compound). The isobaric heat capacity Cp(T) was measured and the thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) were calculated in the temperature range 5–1300 K for Y2FeTaO7 by the way of example. In the low-temperature region of 2–25 K, an abnormal behavior of Cp(T), apparently related to the Y2FeTaO7 magnetic transition, was detected.

Synthesis and Characterization Studies of Ca2+ and Y3+ Co-Doped Ceria-Na2CO3 Nanocomposite Electrolytes for Low Temperature SOFCs

Doped ceria-carbonate composites have been developed for low temperature (300-600 °C) solid oxide fuel cells (SOFCs) due to its enhanced performance compared to single phase doped ceria electrolytes and the possibility to avoid the Ce4+ reduction. In this study, Y3+ and Ca2+ doped and co-doped ceria Ce0.8Y0.2-xCaxO2-δ (x = 0 and 0.1) samples were synthesized through a simple and cost-effective molten salt method. Nanocomposites based on doped and co-doped ceria-Na2CO3 were then synthesized through mechanical milling. The prepared materials were characterized by X-ray diffraction, thermal analysis, scanning electron microscopy and electrochemical impedance spectroscopy. XRD result shows a single cubic fluorite phase of ceria with carbonate exist as amorphous phase. SEM micrographs also exhibited the homogeneous distribution of ceria and amorphous carbonate phase. Electrical conductivity analysis at different sintering temperatures shows high conductivity value is obtained for co-doped ceria-carbonate nanocomposite sample sintered at 700°C.

Doping effects on the structure and electrical properties of La2Ce2O7 proton conductors

Recent studies have reported that the conductivity of La2Ce2O7 proton conductors is improved by aliovalent doping, despite the high concentration of oxygen vacancies in the pristine material. In this study, we provide new insights into the structural and conducting properties of La2-x AxCe2O7±δ (A = Na+, Ca2+, Sr2+, Ba2+, Y3+ and W6+; x ≤ 0.2). The materials are apparently single phase compounds with a disordered fluorite-type structure by conventional X-ray diffraction; however, the local structure, studied by Raman spectroscopy, reveals a biphasic mixture of fluorite and C-type phases, which depends on dopant type and the sintering temperature. In addition, phase segregations are observed for Ca, Sr and Ba-doped samples by scanning electron microscopy. The different substitutions do not improve the bulk conduction at low temperature, and the conductivity differences between the samples are mainly ascribed to microstructural changes. High sintering temperatures produce a significant reduction of the bulk conductivity, i.e. 1.38 and 1.01 μS cm−1 at 300 °C for samples sintered at 1400 and 1500 °C, respectively. The use of Zn as sintering aid greatly improves the densification of La2Ce2O7 at lower temperature, without affecting negatively the conductivity.

Color tunable emission and temperature dependent photoluminescence properties of Eu3+ co-doped Gd2Zr2O7:Dy3+ phosphors

The present study focuses on the color-tunable properties through energy transfer mechanism of single-phase Gd2Zr2O7:Dy3+, Eu3+ phosphors prepared by hydrothermal method. For this intent, we have taken Eu3+ ion as an activator in 2 mol% Dy3+ doped Gd2Zr2O7 (GZO) phosphors. The formation of the defect fluorite phase of the prepared phosphors was confirmed from XRD diffraction pattern. UV–Vis diffuse reflectance spectra were utilized to estimate the optical bandgap of the materials. The characteristic photoluminescence (PL) properties related to the electronic transition of both Dy3+ and Eu3+ ions were observed under direct excitation of Dy3+ ions. In addition, the effect of Eu3+ concentration on the luminescence properties of GZO: 0.02Dy3+ was studied. The average lifetimes of both the excited states, 4F9/2 of Dy3+ and 5D0 of Eu3+ were estimated. PL and lifetime measurements illustrate that an energy transfer is possible between sensitizers and activators ions and mechanism involving the energy transfer process has been discussed. Thermal stability was also examined from temperature dependent PL spectra within the temperature range 80 K–450 K. As we increase the Eu3+ concentration within GZO: 0.02Dy3+ phosphors, the CIE co-ordinates shift toward the warm white region from the natural white region and finally shift toward the orange pink region. All the experimental results indicate that the proposed phosphors can be a possible choice for colour converting materials in solid state lighting applications.

Preparation and fine thermal insulation performance of Gd2Zr2O7/ZrO2 composite fibers

Gd2Zr2O7/ZrO2 (GZC) composite fibers were prepared by electro-spinning method. The XRD, XPS and Raman results showed that there were three crystalline phases, tetragonal phase ZrO2, cubic phase ZrO2 and defect fluorite phase Gd2Zr2O7 in GZC composite fibers. GZC fibers remained an intact fiber texture up to 1400 °C according to SEM photographs. The thermal conductivity of GZC fibers was between 0.173 W/(m·K) at 400 °C and 0.309 W/(m·K) at 800 °C, which was lower than that of 7YSZ under the same experimental conditions. The fiber sheet with density about 3.5 g/cm3 has thermal shrinkage less than 3% at 1400 °C. Hence, GZC fibers could be used as refractories for heat protection.