A High‐Temperature Neutron Diffraction Study of Nb2AlC and TiNbAlC

Herein, we report on the crystal structures of Nb₂AlC and TiNbAlC—actual composition (Ti_0.45,Nb_0.55)₂AlC—compounds determined from Rietveld analysis of neutron diffraction patterns in the 300–1173 K temperature range. The average linear thermal expansion coefficients of a Nb₂AlC sample in the a and c directions are, respectively, 7.9(5) × 10^?6 and 7.7(5) × 10^?6 K^?1 on one neutron diffractometer and 7.3(3) × 10^?6 and 7.0(2) × 10^?6 K^?1 on a second diffractometer. The respective values for the (Ti_0.45,Nb_0.55)₂AlC composition—only tested on one diffractometer—are 8.5(3) × 10^?6 and 7.5(5) × 10^?6 K^?1. These values are relatively low compared to other MAX phases. Like other MAX phases, however, the atomic displacement parameters (APDs) show that the Al atoms vibrate with higher amplitudes than the Ti and C atoms, and more along the basal planes than normal to them. When the predictions of the APDs obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Al atoms. In case of the Nb and C atoms, the agreement was more qualitative.     

Silicon Carbide Oxidation in High‐Pressure Steam

Silicon carbide is a candidate cladding for fission power reactors that can potentially provide better accident tolerance than zirconium alloys. SiC has also been discussed as a host matrix for nuclear fuel. Chemical vapor–deposited silicon carbide specimens were exposed in 0.34–2.07 MPa steam at low gas velocity (~50 cm/min) and temperatures from 1000°C to 1300°C for 2–48 h. As previously observed at lower steam pressure of 0.15 MPa, a two‐layer SiO₂ scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO₂ surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO₂ scale growth, whereby the amorphous layer is formed through a diffusion process and linearly consumed by an amorphous to crystalline phase transition process. Paralinear kinetics of SiC recession were observed after exposure in 0.34 MPa steam at 1200°C within 48 h. High‐pressure steam environments are seen to form very thick (10–100 μm) cristobalite SiO₂ layers on CVD SiC even after relatively short‐term exposures (several hours). The crystalline SiO₂ layer and SiC recession rate significantly depend on steam pressure. Another model is presented to describe the SiC recession rate in terms of steam pressure when a linear phase transition k_l governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.     

Microstructure and High‐temperature Oxidation Behavior of Ti3AlC2/W Composites

Using spark plasma sintering, Ti₃AlC₂/W composites were prepared at 1300°C. They contained “core‐shell” microstructures in which a Ti_xW_1?x “shell” surrounded a W “core”, in a Ti₃AlC₂ matrix. The composite hardness increased with W addition, and the hardening effect is likely achieved by the Ti_xW_1?x interfacial layer providing strong bonding between Ti₃AlC₂ and W, and by the presence of hard W. Microstructural development during high‐temperature oxidation of Ti₃AlC₂/W composites involves α‐Al₂O₃ and rutile (TiO₂) formation ≥1000°C and Al₂TiO₅ formation at ~1400°C while tungsten oxides appear to have volatilized above 800°C. Likely due to exaggerated, secondary grain growth of TiO₂‐doped alumina and the effect of W addition, fine (<1 μm) Al₂O₃ grains formed dense, anisomorphic laths on Ti₃AlC₂/5 wt%W surfaces ≥1200°C and coarsened to large (>5 μm), dense, TiO₂‐doped Al₂O₃ clusters on Ti₃AlC₂/10 wt%W surfaces ≥1400°C. W potentially affects the oxidation behavior of Ti₃AlC₂/W composites beneficially by causing formation of Ti_xW_1?x thus altering the defect structure of Ti₃AlC₂, resulting in Al having a higher activity and by changing the scale morphology by forming dense Al₂O₃ laths in a thinner oxide coating, and detrimentally through release of volatile tungsten oxides generating cavities in the oxide scale. For Ti₃AlC₂/5 wt%W oxidation, the former beneficial effects appear to dominate over the latter detrimental effect.     

Insights into High‐Temperature Uniaxial Compression Deformation Behavior of Ti3AlC2

We report on strain‐rate‐dependent compression deformation behavior of Ti₃AlC₂ at 1000°C–1200°C. At 1000°C and high strain rate (10^?2 or 10^?3 s^?1), Ti₃AlC₂ deforms in a nonplastic manner. Upon increasing temperature and reducing strain rate, Ti₃AlC₂ exhibits a limited plasticity. For instance, the true plastic strain at 1200°C and 10^?4 s^?1 is only 3%, beyond which strain softening following a short hardening regime occurs. The softening results from the formation of localized microvoids and microcracks. Decreasing the strain rate further to 10^?5 s^?1 at 1200°C, strain hardening instead of softening is identified. Under such conditions, the plastic strain remarkably increases, reaching a value as high as 27%. Postdeformation microstructural analyses of the dislocation configurations explicitly evidence the dislocation reactions, formation of hexagonal dislocation networks and dislocation entanglements. These account for the strain hardening. The extraordinary plasticity at 1200°C and 10^?5 s^?1 benefits from the initiation of nonbasal slip systems. Finally, a complete high‐temperature deformation scenario for nanolaminated Ti₃AlC₂ is elaborated.     

Oxidation Resistance of AlPO4 Bonded Ceramic Coating Formed on Titanium Alloy for High‐Temperature Applications

AlPO₄ based coatings were prepared on Ti‐6Al‐2Zr‐1Mo‐1V titanium alloy using aluminum phosphate as a binder and Al₂O₃/Cr₂O₃ based mixing particles as the fillers. The microstructure, phase and chemical composition of the coatings were analyzed by SEM, XRD and EDS techniques. The high temperature infrared emissivity values of coated and uncoated titanium samples were tested. The results show that the coating had a higher infrared emissivity value (>0.8) than titanium substrate (0.15–0.3) in the wide wavelength range of 5–20 mm, which is attributed to the uniform dispersion of high emissivity Al₂O₃ and Cr₂O₃ particles in the AlPO₄ binder matrix. The coated titanium samples exhibited excellent oxidation resistance performance with significantly decreased oxidation rates at 600 and 800°C. The mass gain of the coated sample kept at a low and stable constant of 0.15 mg/cm², significantly lower than that of titanium substrate (0.54 mg/cm²) when oxidized at 600°C up to 100 h.     

Improved Crack Healing and High‐Temperature Oxidation Resistance of Ni/Al2O3 by Y or Si Doping

Recovery of mechanical strength was investigated for 5 vol% Ni/α‐Al₂O₃ nanocomposites that had improved resistance to high‐temperature oxidation by doping with Y or Si (Ni/Al₂O₃‐Y and Ni/Al₂O₃‐Si). Surface cracks disappeared completely because of the oxidation product, NiAl₂O₄. The fraction of crack disappearance was comparable between Ni/Al₂O₃‐Y and Ni/Al₂O₃‐Si. The apparent activation energy of crack healing is similar to the grain‐boundary diffusion of Ni ions in an Al₂O₃ matrix. The rate‐controlling process of crack healing is the grain‐boundary diffusion of cations in an internally oxidized zone (IOZ) of the Ni/Al₂O₃ system. The bending strengths of the as‐sintered and as‐cracked Ni/Al₂O₃‐Y samples were 561 and 232 MPa, respectively. Heat treatment at 1200°C for 6 h resulted in a recovery of the bending strength up to 662 MPa for Ni/Al₂O₃‐Y as well as 606 MPa for Ni/Al₂O₃‐Si. Y and Si dopants were segregated into the Al site at the Al₂O₃ grain boundaries, and then, enhanced covalent bonding occurred with neighboring oxygen. While the flux of Ni ions was retarded slightly by doping with Y and Si, a shorter IOZ provided enough Ni ions to form NiAl₂O₄ on the surface. Ni/Al₂O₃‐Y and Ni/Al₂O₃‐Si have the desirable properties of crack healing and resistance to high‐temperature oxidation.     

Fracture Resistance Behavior of High‐Thermal‐Conductivity Silicon Nitride Ceramics

Silicon nitride ceramics were prepared from a high‐purity silicon powder doped with 2 mol% Y₂O₃ and 5 mol% MgO as sintering additives via a route of sintering of reaction‐bonded silicon nitride (SRBSN). The materials sintered at 1900°C for 3, 6, 12, and 24 h had thermal conductivities of 109, 125, 146, and 154 W/m/K, and four‐point bending strengths of 786, 676, 608, and 505 MPa, respectively. The fracture toughness values, determined by the single‐edge‐precracked‐beam (SEPB) method, were 8.4, 8.6, 9.7, and 10.7 MPa m^1/2 for the materials sintered for 3, 6, 12, and 24 h, respectively, which were similar to the results measured by the chevron‐notched‐beam (CNB) test method. The materials sintered for longer times (12 and 24 h) showed stronger R‐curve behaviors over longer range of crack extension, in comparison with the materials sintered for shorter times (3 and 6 h).     

High‐Temperature Isothermal Oxidation of Ultra‐High Temperature Ceramics Using Thermal Gravimetric Analysis

Oxidation of ZrB₂ + SiC composites is investigated using isothermal measurements to study the effects of temperature, time, and gas flow on oxidation behavior and microstructural evolution. A test method called dynamic nonequilibrium thermal gravimetric analysis (DNE‐TGA), which eliminates oxidation during the heating ramp, has been developed to monitor mass change from the onset of an isothermal hold period (15 min) as a function temperature (1000°C–1600°C) and gas flow (50 and 200 mL/min). In comparing isothermal to nonisothermal TGA measurements, the scale thicknesses from isothermal tests are up to 4 times greater, indicating that oxidation kinetics are faster for isothermal testing, where the oxide scale thickness is 110 μm after 15 min at 1600°C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000°C–1600°C. The mass gain increased from ~5 to 45 g/m² and parabolic rate constants increased from 0.037 to 2.2 g²/m⁴·s over this temperature range. The effect of flow velocity on oxidation is not significant under the given laminar flow environment where the gas boundary layer is calculated to be 4 mm. These values are consistent with diffusion of oxygen through the glass‐ceramic surface layer as rate limiting.     

High‐Temperature Na2SO4 Deposit‐Assisted Corrosion of Silicon Carbide – I: Temperature and Time Dependence

Time and temperature dependence of Na₂SO₄‐induced hot corrosion were studied for sintered‐α (Hexoloy) as well as CVD‐SiC at temperatures between 900°C and 1100°C and at times from 0.75 to 96 h. The extent of corrosion was quantified using mass change measurements, removal of corrosion products using sequential water, HCl, and HF dissolution steps followed by ICP‐OES analysis, and by optical profilometry of corroded materials to characterize pitting on the sample surface. In addition, SEM, EDS, and XRD were used to better understand the morphology, distribution, and phase composition of corrosion products. It was found that hot corrosion of Hexoloy was more severe than that of high‐purity CVD‐SiC. Hot corrosion is initially rapid until a continuous silica layer is formed underneath the mixed silicate layer. Once a continuous silica layer was formed the temperature dependence of the corrosion rate was consistent with diffusion of oxygen through the silica layer.     

Silicon Diffusion in Silicide Coatings Deposition by the Pack Cementation Method on AISI D2 Tool Steel

Silicon - In the current research, pack cementation method for deposition of silicide on the AISI D2 steel has been investigated. The chemical, phase composition of coatings and microstructure,...     

Heat and Mass Transfer in Ceramic Lattices During High‐Temperature Oxidation

This work reports on the heat and mass transfer evolution of ceramic lattices during their oxidation at 1400°C and 1600°C in air. Si–SiC and Si–SiC–ZrB₂ systems were employed as skeleton material because they, previously produced as monolithic bars, showed promising oxidation behavior at high temperatures. Regular arrays of tetrakaidecahedra were first designed by CAD, then 3D printed and finally converted into ceramic by replica technique followed by reactive silicon infiltration. The surface area of each sample was calculated and specific weight variations were evaluated as a function of time. During oxidation, effective thermal conductivity and pressure drop of each sample were measured. Finally, results were correlated with the phenomena occurring during high‐temperature oxidation.     

Influence of ordered carbon‐vacancy networks on the electronic structures and elastic properties of Nb4AlC3−x

Carbon‐vacancy‐bearing Nb₄AlC_3?x has the best high‐temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb₄AlC_3?x have been identified to be ordered, establishing an ordered compound Nb₁₂Al₃C₈. Here, the spatial distribution of the ordered vacancies and their influences on bonding characteristics and elastic properties are unraveled by thoroughly comparing Nb₁₂Al₃C₈ and vacancy‐free Nb₄AlC₃. In Nb₁₂Al₃C₈, the carbon vacancies break only relatively weak Nb–C bonds and form ordered equilateral triangular carbon‐vacancy networks (OETCVNs) to maximize the bond strengthening effect. The networks slightly shift partial and total density of states toward the Fermi energy level, and bring about a feature of “de‐metallization”. Meanwhile, the presence of OETCVNs results in the softening of elastic modulus, decreasing of the anisotropy of Young's modulus, yet increasing that of shear modulus. These results shed lights on the carbon‐vacancy ordering behavior of MAX phases, and provide an opportunity to tailor their electronic structures and elastic properties through defect engineering.     

Mechanical and thermal properties and microstructural evolution of Ta-doped Nb4AlC3

(Nb_1-xTa_x)₄AlC₃ (x = 0–0.5) ceramics were prepared by the hot press sintering method. The XRD results show that the second phase (Nb_1-xTa_x)C is formed when the Ta content increases to 25 mol%. The SEM micrographs show that (Nb_1-xTa_x)C has a core/rim structure, whose formation mechanism was also investigated. Substituting some Ta for Nb can significantly improve the mechanical properties of Nb₄AlC₃. (Nb_0.75Ta_0.25)₄AlC₃ exhibits an excellent fracture toughness of 8.3 ± 0.3 MPa m^1/2 at room temperature (RT). The highest Young's modulus (349 ± 16 GPa) and Vickers hardness (4.5 ± 0.3 GPa) at RT are exhibited by the (Nb_0.5Ta_0.5)₄AlC₃ sample, which correlate to increases of 18% and 80%, respectively, compared with those of Nb₄AlC₃. The flexural strengths of (Nb_0.5Ta_0.5)₄AlC₃ are 439 ± 18 MPa at RT and 344 ± 22 MPa at 1100 °C, which correlate to increases of 27% and 45%, respectively, compared with those of Nb₄AlC₃. The solid solution of Ta and the formation of (Nb_1-xTa_x)C are beneficial to the strengthening of Nb₄AlC₃. The coefficient of thermal expansion (CTE) increases slightly from 7.08 × 10⁻⁶ K⁻¹ for Nb₄AlC₃ to 7.24 × 10⁻⁶ K⁻¹ for (Nb_0.75Ta_0.25)₄AlC₃ at 25–1400 °C. The thermal conductivity of (Nb_0.75Ta_0.25)₄AlC₃ (28.4–29.8 W/m·K) is higher than that of Nb₄AlC₃ (18.1–21.2 W/m·K) over the whole test range (25–1000 °C). Owing to their excellent mechanical and thermal properties, Ta-doped Nb₄AlC₃ ceramics have good potential as structural materials.     

Micro Four‐Layer SiC Coating on Matrix Graphite Spheres of HTR Fuel Elements by Two‐Step Pack Cementation

A micro four‐layer SiC coating, which includes inner transition layer, fine‐grained layer, dense bulk layer, and outer loose layer, was fabricated on the matrix graphite spheres of high‐temperature gas‐cooled reactor fuel elements to improve the oxidation‐resistant property by a two‐step pack cementation process. According to the experiment results, the micro four‐layer can be differentiated by SiC grain size and microstructure variation. The oxidation tests at 1773 K for 200 h reveal that the coating structure could effectively improve the oxidation resistance of matrix graphite spheres with a weight gain of 0.52 wt%, and the fine‐grained and dense bulk layers are evidenced as two main antioxidation layers. Although the thermal expansion coefficients of SiC and matrix graphite do not match each other so well, no obvious stress cracking was observed after thermal shocking tests from 1773 K to room temperature for 100 times.     

BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for High‐Temperature Capacitor Applications

Solid solutions of (1?x)BaTiO₃–xBi(Mg_2/3Nb_1/3)O₃ (0 ≤ x ≤ 0.6) were prepared via a standard mixed‐oxide solid‐state sintering route and investigated for potential use in high‐temperature capacitor applications. Samples with 0.4 ≤ x ≤ 0.6 showed a temperature independent plateau in permittivity (ε_r). Optimum properties were obtained for x = 0.5 which exhibited a broad and stable relative ε_r ~940 ± 15% from ~25°C to 550°C with a loss tangent <0.025 from 74°C to 455°C. The resistivity of samples increased with increasing Bi(Mg_2/3Nb_1/3)O₃ concentration. The activation energies of the bulk were observed to increase from 1.18 to 2.25 eV with an increase in x from 0 to 0.6. These ceramics exhibited excellent temperature stable dielectric properties and are promising candidates for high‐temperature multilayer ceramic capacitors for automotive applications.     

Preparation,Microstructure, and Mechanical Properties of Nb4AlC3–Nb5(Si,Al)3 Composites

Niobium aluminum carbide (Nb₄AlC₃), as a member of the MAX phases, can retain its stiffness and strength up to over 1400°C, however, the strength at room temperature is relatively low. In this work, Nb₅(Si, Al)₃ was used to strengthen Nb₄AlC₃. Nb₄AlC₃–Nb₅(Si, Al)₃ composites with different amount of Nb₅(Si, Al)₃ were synthesized from the elemental powders by in situ hot‐pressing/solid–liquid reaction synthesis process. The RT flexural strength was significantly improved from 370 MPa of monolithic Nb₄AlC₃ to 432 MPa of Nb₄AlC₃‐15 vol% Nb₅(Si, Al)₃ composite without the degradation of its high‐temperature mechanical properties. While a slightly decrease in fracture toughness occurs with the increment of Nb₅(Si, Al)₃ content. Meanwhile, Transmission electron microscopy (TEM) observations reveal that the interfaces between Nb₄AlC₃/Nb₄AlC₃, Nb₄AlC₃/Nb₅(Si, Al)₃, and Nb₅(Si, Al)₃/Nb₅(Si, Al)₃ are free of amorphous layers, which is also beneficial to its high‐temperature mechanical properties.     

Low‐Cost Silicon Nitride from β‐Silicon Nitride Powder and by Low‐Temperature Sintering

Aiming to manufacture low‐cost silicon nitride components, a low‐cost β powder was chosen as a raw powder and low‐temperature sintering at 1550–1600°C under atmospheric pressure nitrogen was carried out. The silicon nitride from β powder with 5 wt% Y₂O₃ and 5 wt% MgAl₂O₄ additives and sintered at 1600°C for 8 h was successfully densified, and it exhibited moderate strength and toughness of 553 MPa ± 22 and 3.5 MPa m^1/2, respectively. The results indicate that the low‐temperature sintering of the low‐cost β powder has a potential to reduce cost of components.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

High‐Temperature Dielectric Relaxation in Pb(Mg1/3Nb2/3)O3–PbTiO3 Single Crystals

We reported the dielectric properties of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ single crystal in the temperature range of 300–1073 K and the frequency range of 100 Hz–10 MHz. Our results showed the coexistence of both true‐ and pseudo‐relaxor behaviors in the crystal. The true relaxor behavior related to the paraelectric‐ferroelectric phase transition occurs at~423 K. The pseudo‐relaxor behavior appearing at~773 K was found to be related to oxygen vacancies. Further investigation reveals that the pseudo‐relaxor behavior has fine structure: it contains two oxygen‐vacancy‐related relaxation processes. The low‐temperature relaxation process is a dipolar relaxation created by the hopping motions of the oxygen vacancies, and the high‐temperature relaxation process is a Maxwell‐Wagner relaxation caused by the sample/electrode contacts.     

Carbon Interlayer Between CVD SiC and SiO2 in High‐Temperature Passive Oxidation

The oxidation behavior of high‐purity silicon carbide (SiC) prepared by chemical vapor deposition was investigated by thermogravimetry, transmission electron microscopy, and Raman spectroscopy in the temperature range 1534–1902 K in pure O₂. The carbon layer formed at the SiC/SiO₂ interface upon oxidation above 1784 K. Raman peaks corresponding to D‐ and G‐bands could be identified from the carbon layer. Bubbles were observed in the SiO₂ scale after the oxidation at 1873 K. This could be attributed to the accumulation of CO gas at the SiC/SiO₂ interface, resulting in the formation of the carbon layer and bubbles. These suggest that the oxidation rate of SiC is limited by the outward diffusion of CO in the SiO₂ scale in this temperature range.