Nano–Nano Composite Powders of Lanthanum Zirconate and Yttria‐Stabilized Zirconia by Spray Pyrolysis

Powders composing of La₂Zr₂O₇ (LZ) and (Zr_0.8Y_0.2)O_1.9 (10YSZ) phases (volume ratio = 1:1) were synthesized by using a sol‐spray pyrolysis method. The effects of annealing temperature on the grain size and lattice parameter of the LZ–10YSZ powders were investigated. XRD results showed that the grain size of LZ and 10YSZ phases gradually grew from 10 to 95 nm and from 5 to 65 nm as the annealing temperature elevated from 900°C to 1200°C. The relative decreasing percentage of grain size comparing to that of the single‐phase LZ and 10YSZ powders were in the range 9%–36% and 37%–86%. The activation energy for grain growth of LZ and 10YSZ phases in the composite powders were 225 ± 12 and 382 ± 17 kJ/mol, which were 20% and 183% higher than that of the single‐phase counterparts. Obvious lattice contraction and lattice expansion for LZ and 10YSZ phases were observed at temperatures below 1100°C, respectively. SEM results revealed that LZ and 10YSZ phases were homogeneously distributed in the sintered bulk. The TEM results suggested that the grain growth was affected by the interaction on nanometer length scales of grain boundaries between LZ and 10YSZ phases in the composite.     

Effect of Al2O3 Addition on the Flexural Strength and Light‐Transmission Properties of Bone China

To improve the flexural strength and light‐transmission properties of bone china, the effects of adding different amounts of alumina (0–3%) to bone china bodies were studied and the phase composition and microstructure of different samples were studied by X‐ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, physical properties, such as the bulk density, the thermal expansion coefficient (TEC) and thermal shock resistance, were studied. It was found that adding alumina increased the overall sintering temperature while reducing the sintering temperature range of bone china. Furthermore, addition of 1% Al₂O₃ improved the tree‐point flexural strength from 120 MPa to 150 MPa, the light transmittance (at 2 mm thickness) from 6.7% to 7.5%, the thermal expansion coefficient from 8.4 × 10^?6°C^?1 to 8.1 × 10^?6°C^?1 and the thermal shock resistance from 140°C to 180°C. Higher corundum content results in similar high flexural strength but lower light transmittance.     

Sintering Mechanisms and Kinetics for Reaction Hot‐Pressed ZrB2

Sintering mechanisms and kinetics were investigated for ZrB₂ ceramics produced using reaction hot pressing. Specimens were sintered at temperatures ranging from 1800°C to 2100°C for times up to 120 min. ZrB₂ was the primary phase, although trace amounts of ZrO₂ and C were also detected. Below 2000°C, the densification mechanism was grain‐boundary diffusion with an activation energy of 241 ± 41 kJ/mol. At higher temperatures, the densification mechanism was lattice diffusion with an activation energy of 695 ± 62 kJ/mol. Grain growth exponents were determined to be ~4.5, which indicated that a grain pinning mechanism was active in both temperature regimes. The diffusion coefficients for grain growth were 1.5 × 10^?16 cm⁴/s at 1900°C and 2.1 × 10^?15 cm⁴/s at 2100°C. This study revealed that dense ZrB₂ ceramics can be produced by reactive hot pressing in shorter times and at lower temperatures than conventional hot pressing of commercial powders.     

New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.     

Synthesis and thermophysical properties of RETa3O9 (RE = Ce,Nd, Sm,Eu, Gd,Dy, Er) as promising thermal barrier coatings

Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa₃O₉ compounds have a defect‐perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa₃O₉ bulk ceramics were fabricated via solid‐state reaction. The crystal structure was characterized by X‐ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermophysical properties of RETa₃O₉ were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients, and high‐temperature phase stability. The thermal conductivities of RETa₃O₉ are very low (1.33‐2.37 W/m·K, 373‐1073 K), which are much lower than YSZ and La₂Zr₂O₇; and the thermal expansion coefficients range from 4.0 × 10^?6 K^?1 to 10.2×10^?6 K^?1 (1273 K), which is close to La₂Zr₂O₇ and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high‐temperature phase stability with these oxides, RETa₃O₉ ceramics were promising candidate materials for TBCs.     

Theoretical Prediction and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β‐Yb2Si2O7

The thermal and mechanical properties of β‐Yb₂Si₂O₇ were investigated using a combination of first‐principles calculations and experimental investigations. Theoretically, anisotropic chemical bonding and elastic properties, weak interatomic (010) and (001) planes in the crystal structure, damage tolerance, and low thermal conductivity are predicted. Experimentally, preferred orientation, superior mechanical properties, and damage tolerant behavior for hot‐pressed bulk β‐Yb₂Si₂O₇ are approved. Slipping along the weakly bonded {010}, {001}, or {100} planes, grain delamination, buckling, and kinking of nanolaminated grains are identified as main mechanisms for damage tolerance. The anisotropic linear thermal expansion coefficients (CTEs) are: α_a = (3.57 ± 0.18) × 10^?6 K^?1, α_b = (2.49 ± 0.14) × 10⁶ K^?1, and α_c = (1.48 ± 0.22) × 10^?6 K^?1 (673–1273 K). A low thermal conductivity of ~2.1 W (m·K)^?1 at 1273 K has been confirmed. The unique combination of these properties endow it a potential candidate for thermal barrier coating (TBC)/environmental barrier coating of silicon‐based ceramics.     

Semiconductor‐Insulator Transition in Undoped Rutile,TiO2, Ceramics

The electrical conductivity of undoped rutile ceramics is very dependent on sample processing conditions, especially the temperature and atmosphere during sintering and the subsequent cooling rate. Samples become increasingly semiconducting when quenched from temperatures above ~700°C without the need for a reducing atmosphere. Thus, samples quenched from 1400°C in air have conductivity ~1 × 10^?2 Scm^?1with activation energy ~0.01(1) eV over the temperature range 10–100 K, whereas similar samples that are slow cooled or annealed in air at 300°C–500°C are insulating with activation energy 1.67(2) eV and conductivity, e.g., 1 × 10^?7 Scm^?1 at 400°C. The very wide range of electrical properties is attributed to variations in oxygen content which are too small to be detected using thermogravimetry. Impedance analysis shows that, depending on cooling rate, partially oxidized samples may be prepared in which samples retain a semiconducting core, but have an oxidized outer layer.     

Fabrication and properties of macro-porous SiC using Al2O3–Y2O3–SiO2 as bonding additives

Macro-porous SiC was fabricated without using pore-forming agents by an in situ reaction bonding process. A bonding additive, Al₂O₃–Y₂O₃–SiO₂, with a low melting temperature was mixed with SiC particles and sintered at 1500 °C and 1600 °C for 1 h in Ar. Macro-porous SiC with a porosity of 32.7–45.9%, a pore size of 3.4–4.2 μm, and a relatively narrow and uniform pore size distribution was fabricated by varying the amount of bonding additive. The flexural strength of macro-porous SiC prepared at 1500 °C increased from 47.2 MPa to 71.2 MPa while the porosity decreased from 45.9% to 42.8%, respectively. When the sintering temperature of the macro-porous SiC was increased to 1600 °C, the flexural strengths were significantly reduced to 32.6–35.6 MPa, along with a reduction in porosity and pore size. The permeability of macro-porous SiC prepared at 1500 °C varied from 1.59 × 10^?12 m² to 1.26 × 10^?12 m², depending on the porosity. As the sintering temperature increased from 1500 °C to 1600 °C, the permeability decreased to less than 1.00 × 10^?12 m² because of the reduced porosity and average pore size. The electrical resistivity of macro-porous SiC prepared at 1500 °C and 1600 °C varied from 2.7 × 10⁸ Ω-cm to 1.4 × 10⁹ Ω-cm and from 1.3 × 10⁸ Ω-cm to 1.7 × 10⁹ Ω-cm, respectively, with increasing volume percent of bonding additives. The relatively high electrical resistivity was apparently due to neck bonding phase between SiC particles formed by phases consisting of Y₂Si₂O₇, YAG, and residual Al₂O₃.     

A novel Co-ions complexation method to synthesize pyrochlore La2Zr2O7

Pure pyrochlore Lanthanum zirconate (LZ) was synthesized by co-ions complexation method (CCM) at 1300 °C, which is 300 °C lower than that by solid-state method (SSM). At 1450 °C, the LZ prepared by CCM possessed lower thermal conductivity (1.15 W/m K) than that obtained by SSM (1.99 W/m K). This significant decrease may be caused by the different grain size, which is 300 nm and 2.5 μm synthesized by CCM and SSM, respectively. LZ precursor was belt-shaped and the belt shorten and the grain grown with the temperature increasing. Fourier transform infrared spectroscopy suggested the solidification in CCM forms from the complexation between La³⁺, Zr⁴⁺ and CH₃COO⁻, which is the key for solidification. Compared to SSM, CCM is a lower temperature and simpler technology to synthesize nano-size LZ and other rare-earth oxides.     

Influence of LaMgAl11O19 On Solid Particle Impact Erosion Wear Behavior of 3YSZ Ceramic at Elevated Temperatures

To study the improvement in solid particle impact erosion wear resistances of 3 mol% yttria‐stabilized zirconia (3YSZ) ceramic at elevated temperatures up to 1400°C, 2 wt% LaMgA1₁₁O₁₉ was added into 3YSZ to prepare LaMgA1₁₁O₁₉‐3YSZ ceramic for erosion resistance tests with angular corundum abrasive particles. The testing results show that the volume erosion rates of 3YSZ and LaMgA1₁₁O₁₉‐3YSZ ceramic were similar in the temperature range from room temperature to 600°C, then exhibited a sharp increase from 600°C to 1200°C, and dropped again at 1400°C. It was mainly caused by the change in material removal mechanisms from plastic deformation below 600°C to the interaction of transverse cracks in the temperature range from 600°C to 1400°C. The solid particle impact erosion wear properties of 3YSZ ceramic in the temperature range from 600°C to 1400°C were successfully improved by the addition 2 wt% LaMgA1₁₁O₁₉ platelets. Comparing with the volume erosion rate of pure 3YSZ ceramic (0.687 mm³/g) at 1200°C, the value of LaMgA1₁₁O₁₉‐3YSZ ceramic (0.551 mm³/g) has been decreased by 20%.     

Effect of B2O3 on the properties of ferronickel melt and mineral wool

To utilize both ferronickel slag and its sensible heat, the melting and viscosity-temperature behaviors of the B₂O₃ modified slags were studied first. Results show that the initial melting temperature reduces from 1612 K (1339 °C) to 1534 K (1261 °C), and the complete melting temperatures decreases from 1744 K (1471 °C) to 1696 K (1423 °C) with the increase of B₂O₃ from 3 to 9 wt%. However, the temperature span of the viscosity ranging in 0.5–2.5 Pa s increases from 10 K (°C) to 39 K (°C) with increasing B₂O₃. Then, the characterizations of the slag fibers prepared by the modified slags suggest that the addition of B₂O₃ decreases the mean diameter of the slag fibers from 5.6 ± 0.45 μm to 3.4 ± 0.27 μm, whereas increases their tensile strength from 1724 ± 80 MPa to 2114 ± 136 MPa. Moreover, both the glass transition temperature (Tg) and crystallization temperature (Tc) are delayed by the addition of B₂O₃ since boron works as network former in the slag melt structure.     

Effect of pressure and temperature on viscosity of a borosilicate glass

During industrial glass production processes, the actual distribution of stress components in the glass during scribing remains, to date, poorly quantified, and thus continues to be challenging to model numerically. In this work, we experimentally quantified the effect of pressure and temperature on the viscosity of SCHOTT N‐BK7^® glass, by performing in situ deformation experiments at temperatures between 550 and 595°C and confining pressures between 100 and 300 MPa. Experiments were performed at constant displacement rates to produce almost constant strain rates between 9.70 × 10^?6 and 4.98 × 10^?5 s^?1. The resulting net axial stresses range from 81 to 802 MPa, and the finite strains range from 1.4% to 8.9%. The mechanical results show that the SCHOTT N‐BK7^® glass is viscoelastic near the glass transition temperature at 300 MPa of confining pressure. To elucidate the data, we incorporated both 1‐element and 2‐element generalized Maxwell viscoelastic models in an inversion approach, for which we provide MATLAB scrips. Results show that the 2‐element Maxwell model fits the experimental data well. The stress decreases with increasing temperature at 300 MPa and the temperature dependence yields a similar activation energy (601 ± 10 kJ mol^?1 or ?H/R = 7.2 × 10⁴ K) to a previously reported value at 1‐atm (615 kJ mol^?1 or ?H/R = 7.4 × 10⁴ K). The SCHOTT N‐BK7^® glass shows a limited linear increase in viscosity with increasing pressure of ~0.1 log₁₀ (Pa·s)/100 MPa, which is in agreement with the most recent 2‐internal‐parameter relaxation model (based on experiments).     

Liquid‐phase sintering of highly Na+ ion conducting Na3Zr2Si2PO12 ceramics using Na3BO3 additive

Na₃Zr₂Si₂PO₁₂ (NASICON) is a promising material as a solid electrolyte for all‐solid‐state sodium batteries. Nevertheless, one challenge for the application of NASICON in batteries is their high sintering temperature above 1200°C, which can lead to volatilization of light elements and undesirable side reactions with electrode materials at such high temperatures. In this study, liquid‐phase sintering of NASICON with a Na₃BO₃ (NBO) additive was performed for the first time to lower the NASICON sintering temperature. A dense NASICON‐based ceramic was successfully obtained by sintering at 900°C with 4.8 wt% NBO. This liquid‐phase sintered NASICON ceramic exhibited high total conductivity of ~1 × 10^?3 S cm^?1 at room temperature and low conduction activation energy of 28 kJ mol^?1. Since the room‐temperature conductivity is identical to that of conventional high‐temperature‐sintered NASICON, NBO was demonstrated as a good liquid‐phase sintering additive for NASICON solid electrolyte. In the NASICON with 4.8 wt% NBO ceramic, most of the NASICON grains directly bonded with each other and some submicron sodium borates segregated in particulate form without full penetration to NASICON grain boundaries. This characteristic composite microstructure contributed to the high conductivity of the liquid‐phase sintered NASICON.     

Thermal shock behavior of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The single-ceramic-layer (SCL) 8YSZ (conventional and nanostructured 8YSZ) and double-ceramic-layer (DCL) La₂Zr₂O₇ (LZ)/8YSZ thermal barrier coatings (TBCs) were fabricated by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal shock behavior of the three as-sprayed TBCs at 1000 °C and 1200 °C was investigated. The results indicate that the thermal cycling lifetime of LZ/8YSZ TBCs is longer than that of SCL 8YSZ TBCs due to the fact that the DCL LZ/8YSZ TBCs further enhance the thermal insulation effect, improve the sintering resistance ability and relieve the thermal mismatch between the ceramic layer and the metallic layer at high temperature. The nanostructured 8YSZ has higher thermal shock resistance ability than that of the conventional 8YSZ TBC which is attributed to the lower tensile stress in plane and higher fracture toughness of the nanostructured 8YSZ layer. The pre-existed cracks in the surface propagate toward the interface vertically under the thermal activation. The nucleation and growth of the horizontal crack along the interface eventually lead to the failure of the coating. The crack propagation modes have been established, and the failure patterns of the three as-sprayed coatings during thermal shock have been discussed in detail.     

Thermophysical properties and cyclic lifetime of plasma sprayed SrAl12O19 for thermal barrier coating applications

About 6-8 wt% yttria-stabilized zirconia (YSZ) is the industry standard material for thermal barrier coatings (TBC). However, it cannot meet the long-term requirements for advanced engines due to the phase transformation and sintering issues above 1200°C. In this study, we have developed a magnetoplumbite-type SrAl₁₂O₁₉ coating fabricated by atmospheric plasma spray, which shows potential capability to be operated above 1200°C. SrAl₁₂O₁₉ coating exhibits large concentrations of cracks and pores (~26% porosity) after 1000 hours heat treatment at 1300°C, while the total porosity of YSZ coatings progressively decreases from the initial value of ~18% to ~5%. Due to the contribution of porous microstructure, an ultralow thermal conductivity (~1.36 W m⁻¹ K⁻¹) can be maintained for SrAl₁₂O₁₉ coating even after 1000 hours aging at 1300°C, which is far lower than that of the YSZ coating (~1.98 W m⁻¹ K⁻¹). In thermal cyclic fatigue test, the SrAl₁₂O₁₉/YSZ double-ceramic-layer coating undertakes a thermal cycling lifetime of ~512 cycles, which is not only much longer than its single-layer counterpart (~163 cycles), but also superior to that of YSZ coating (~392 cycles). These preliminary results suggest that SrAl₁₂O₁₉ might be a promising alternative TBC material to YSZ for applications above 1200°C.     

Near‐Zero Thermal Expansion in In(HfMg)0.5Mo3O12

In(HfMg)_0.5Mo₃O₁₂, which can be considered as a 1:1 mole ratio solid solution of the low‐positive thermal expansion material HfMgMo₃O₁₂ and the low‐negative thermal expansion (NTE) material In₂Mo₃O₁₂ was prepared. From DSC and XRPD results, we show that In(HfMg)_0.5Mo₃O₁₂ exists in a monoclinic (P2₁/a) structure at low temperature and undergoes a phase transition at ~425 K to an orthorhombic phase (Pnma), with an associated enthalpy change of 0.89 kJ mol^?1. Thermal expansion is large and positive in the low‐temperature monoclinic phase (average α_? = 16 × 10^?6 K^?1 and 20 × 10^?6 K^?1, from dilatometry and XRPD, respectively). Remarkably, this material has a near‐zero thermal expansion (ZTE) coefficient over the temperature range ~500 to ~900 K in the high‐temperature orthorhombic phase, both intrinsically and for the bulk sample. The average linear intrinsic (XRPD) value is α_? = ?0.4 × 10^?6 K^?1, and the average bulk (dilatometric) value is α_? = 0.4 × 10^?6 K^?1 with an uncertainty of ± 0.2 × 10^?6 K^?1. The slight difference between intrinsic and bulk thermal expansion is attributed to microstructural effects. XRPD results show that the thermal expansion is more isotropic than for the parent compounds HfMgMo₃O₁₂ and In₂Mo₃O₁₂.     

A Novel Magnetodielectric Solid Solution Ceramic 0.4LiFe5O8–0.6Li2MgTi3O8 with Excellent Microwave Dielectric Properties

In this study, a novel spinel solid solution ceramic of 0.4LiFe₅O₈–0.6Li₂MgTi₃O₈ (0.4LFO–0.6LMT) has been developed and investigated. It is found that the 40 mol% LiFe₅O₈ and 60 mol% Li₂MgTi₃O₈ are fully soluble in each other and a disordered spinel phase is formed. The ceramic sample sintered at 1050°C/2 h exhibits both good magnetic and dielectric properties in the frequency range 1–10 MHz, with a permeability between 29.9~14.1 and magnetic loss tangent between 0.12~0.67, permittivity between 16.92~16.94 and dielectric loss tangent between 5.9 × 10^?3–2.3 × 10^?2. The sample also has good microwave dielectric properties with a relative permittivity of 16.1, a high quality factor (Q × f) ~28 500 GHz (at 7.8 GHz). Furthermore, 3 wt% H₃BO₃–CuO (BCu) addition can effectively lower the sintering temperature to 925°C and does not degrade the magnetodielectric properties. The chemical compatibility with silver electrode indicates that this kind of ceramics is a good candidate for the low‐temperature cofired ceramic (LTCC) application.     

Drop‐and‐catch (DnC) calorimetry using aerodynamic levitation and laser heating

Design, calibration, and operation of a system for drop‐and‐catch (DnC) calorimetry on oxides from temperature above 1500°C are described. This system allows the measurement of heat contents and heats of fusion by drop calorimetry of small (100 mg or less) samples held by containerless levitation at high temperature and dropped in a calorimeter at room temperature. The spheroids, 2‐3 mm in diameter, prepared by laser melting of powders, are aerodynamically levitated in a splittable nozzle levitator and laser heated to the desired temperature monitored by radiation thermometry. The sample is dropped by splitting the nozzle and caught by splittable water‐cooled calorimetric plates at 25°C, which provide complete enclosure of the sample to avoid heat loss by radiation. The drop time is ~0.1 seconds, calorimeter equilibration time after the drop is ~15 minute. DnC experiments are automated with software‐controlled laser power and programmable delay between splitting the nozzle and catching the sample. The fusion enthalpy of Al₂O₃ measured by DnC calorimeter, 120 ± 10 kJ/mol, agrees well with previously reported values. The system can be used for measurements of fusion enthalpies of refractory oxides amenable to laser heating as well as for splat quenching of oxide melts.     

Cold sintering process of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte

The recently developed technique of cold sintering process (CSP) enables densification of ceramics at low temperatures, i.e., <300°C. CSP employs a transient aqueous solvent to enable liquid phase‐assisted densification through mediating the dissolution‐precipitation process under a uniaxial applied pressure. Using CSP in this study, 80% dense Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) electrolytes were obtained at 120°C in 20 minutes. After a 5 minute belt furnace treatment at 650°C, 50°C above the crystallization onset, Li‐ion conductivity was 5.4 × 10^?5 S/cm at 25°C. Another route to high ionic conductivities ~10^?4 S/cm at 25°C is through a composite LAGP ‐ (PVDF‐HFP) co‐sintered system that was soaked in a liquid electrolyte. After soaking 95, 90, 80, 70, and 60 vol% LAGP in 1 M LiPF₆ EC‐DMC (50:50 vol%) at 25°C, Li‐ion conductivities were 1.0 × 10^?4 S/cm at 25°C with 5 to 10 wt% liquid electrolyte. This paper focuses on the microstructural development and impedance contributions within solid electrolytes processed by (i) Crystallization of bulk glasses, (ii) CSP of ceramics, and (iii) CSP of ceramic‐polymer composites. CSP may offer a new route to enable multilayer battery technology by avoiding the detrimental effects of high temperature heat treatments.     

First principles investigation on mechanical and thermal properties of α‐ and β‐YAlB4 ultra‐high temperature ceramics

Ultra‐high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties that makes them promising candidates for applications in extreme environments. Various UHTCs are needed due to diverse harsh conditions that UHTCs are faced with in different applications. Due to structural similarity to ZrB₂, possible high melting point and possible protective oxide scale formed in oxygen rich and water vapor environments, REAlB₄ (RE: rare‐earth) is suggested a good candidate for UHTCs. In the present work, temperature‐dependent mechanical and thermal properties of both α‐YAlB₄ (YCrB₄ type, space group Pbam) and β‐YAlB₄ (ThMoB₄ type, space group Cmmm) were investigated by first principles calculations in combination with quasi‐harmonic approach. Due to the structural similarity between α‐YAlB₄ and β‐YAlB₄, their properties are very similar to each other, which are approximately transverse isotropic with properties in (001) plane being almost the same and differing from properties out of (001) plane. The results reveal that resistance to normal strain in (001) plane (~460 GPa) is higher than that along [001] direction (~320 GPa) and thermal expansion in (001) plane (~10 × 10^?6 K^?1) is lower than that along [001] direction (~17 × 10^?6 K^?1), which is because the stiff boron networks are parallel to (001) plane. The average thermal expansion coefficient is around 12 × 10^?6 K^?1, which is fairly high among UHTCs and compatible with metallic frameworks. The combination of high thermal expansion coefficient and protective oxidation scale forming ability suggest that REAlB₄ is promising for practical applications not only as high‐temperature structural ceramic but also as oxidation resistant coating for alloys.