Lingfeng He

Validation of multiscale microstructure evolution models can be improved when standard microstructure characterization tools are coupled with methods sensitive to individual point defects. We demonstrate how electronic and vibrational properties of defects revealed by optical absorption and Raman spectroscopies can be used to compliment transmission electron microscopy (TEM) and x-ray diffraction (XRD) in the characterization of microstructure evolution in ceria under non-equilibrium conditions. Experimental manifestation of non-equilibrium conditions was realized by exposing cerium dioxide (CeO2) to energetic protons at elevated temperature. Two sintered polycrystalline CeO2 samples were bombarded with protons accelerated to a few MeVs. These irradiation conditions produced a microstructure with resolvable extended defects and a significant concentration of point defects. A rate theory (RT) model was parametrized using the results of TEM, XRD, and thermal conductivity measurements to infer point defect concentrations. An abundance of cerium sublattice defects suggested by the RT model is supported by Raman spectroscopy measurements, which show peak shift and broadening of the intrinsic T2g peak and emergence of new defect peaks. Additionally, spectroscopic ellipsometry measurements performed in lieu of optical absorption reveals the presence of Ce3+ ions associated with oxygen vacancies. This work lays the foundation for a coupled approach that considers a multimodal characterization of microstructures to guide and validate complex defect evolution models.

The early stage of microstructural evolution of ThO₂, under krypton irradiation at 600, 800, and 1000°C, was investigated using in situ transmission electron microscopy (TEM). Dislocation loops grew faster, whereas their number density decreased with increasing irradiation temperature. Loop density was found to decrease with ion dose. Interstitial dislocation loops, including Frank loops with Burgers vector of a/3〈111〉 and perfect loops with Burgers vector of a/2〈110〉, were determined by traditional TEM and atomic resolution–scanning TEM techniques. Atomistic and mesoscale level modeling are performed to interpret experimental observations. The migration energy barriers of defects in ThO₂ were calculated using density‐functional theory. The energetics of different dislocation loop types were studied using molecular dynamics simulations. Loop density and diameter were analyzed using a kinetic rate theory model that considers stoichiometric loop evolution. This analysis reveals that loop growth is governed by the mobility of cation interstitials, whereas loop nucleation is determined by the mobility of anion defects. Lastly, a rate theory model was used to extract the diffusion coefficients of thorium interstitials, oxygen interstitials, and vacancies.

Soft X-ray spectromicroscopy at the O K-edge, U N _4,5-edges and Ce M _4,5-edges has been performed on focused ion beam sections of spent nuclear fuel for the first time, yielding chemical information on the sub-micrometer scale. To analyze these data, a modification to non-negative matrix factorization (NMF) was developed, in which the data are no longer required to be non-negative, but the non-negativity of the spectral components and fit coefficients is largely preserved. The modified NMF method was utilized at the O K-edge to distinguish between two components, one present in the bulk of the sample similar to UO₂ and one present at the interface of the sample which is a hyperstoichiometric UO_2+x species. The species maps are consistent with a model of a thin layer of UO_2+x over the entire sample, which is likely explained by oxidation after focused ion beam (FIB) sectioning. In addition to the uranium oxide bulk of the sample, Ce measurements were also performed to investigate the oxidation state of that fission product, which is the subject of considerable interest. Analysis of the Ce spectra shows that Ce is in a predominantly trivalent state, with a possible contribution from tetravalent Ce. Atom probe analysis was performed to provide confirmation of the presence and localization of Ce in the spent fuel.

To improve the flexural strength of ceramic tiles, a surface strengthening method is presented in this work. By cofiring coating and substrate, residual compressive stress is introduced in the coating. The phase composition and morphological characteristics of coating materials and the influences of coefficient of thermal expansion (CTE) and coating thickness on the flexural strength have been investigated. The prestressed ceramic tile is prepared by pressureless sintering a green body coated with a low CTE coating at 1200°C. The CTE of green body and optimal coating is 7.85 × 10⁻⁶°C⁻¹ and 5.69 × 10⁻⁶°C⁻¹, respectively. The flexural strength of prestressed porcelain tiles (89 ± 3 MP) has been increased by 102% when the coating thickness is 67 µm compared to their uncoated counterpart (44 ± 3 MPa). This simple surface strengthening method of ceramic tiles is cost‐efficient and economical for large‐scale industrial applications.

Proton (H⁺) irradiation effects in polycrystalline UO₂ have been studied. The irradiation was carried out using three ion energies and two different ion fluxes at 600 °C. Scanning electron microscopy (SEM) investigations showed that significant surface flaking took place. Focused ion beam (FIB) milling in SEM was successfully applied for extracting lamellas from uneven blistered surfaces for transmission electron microscopy (TEM) investigations allowing detailed investigations for the degradation mechanisms. High-resolution TEM for the flaked UO₂ surfaces revealed that the implanted H⁺ formed sharp two-dimensional cavities at the peak ion-stopping region instead of diffusing to the matrix. The resulting lateral stress likely caused UO₂ surface deterioration in good agreement with previous blistering and flaking studies on crystalline materials.

Graphical abstract

Thoria (ThO2) has lately gained attention due to its potential for use as a nuclear fuel. From a physics standpoint, ThO2 is an actinide-bearing material with no 5f electrons and is thus ideally suited as a baseline material for future studies of the physical properties of actinide systems with correlated electrons. Current investigations of ThO2 as a nuclear fuel focus on the influence of radiation-induced lattice defects on its thermal properties, especially the conductivity. This work presents a first investigation of the impact of point defect disorder on phonon thermal conductivity of ThO2 by solving the Boltzmann transport equation within the single-mode relaxation time approximation. The relaxation times of intrinsic, three-phonon scattering are calculated by a rigorous sampling of k-points within the irreducible Brillouin zone of the face-centered cubic crystal structure. The effect of point defects on the thermal conductivity of ThO2 is predicted using the classic model by Klemens for phonon relaxation times that result from the change in mass and induced lattice strain associated with point defects. Within this model, the change in force constants and atomic radii are computed using input from an atomistic model of ThO2. The defects considered are uranium substitution at a thorium site, oxygen vacancies and interstitials, and thorium vacancies and interstitials. The results show that the conductivity of ThO2 is highly sensitive to intrinsic point defects and less sensitive to U substitution on the cation sublattice.

Thermal transport is a key performance metric for thorium dioxide in many applications where defect-generating radiation fields are present. An understanding of the effect of nanoscale lattice defects on thermal transport in this material is currently unavailable due to the lack of a single crystal material from which unit processes may be investigated. In this work, a series of high-quality thorium dioxide single crystals are exposed to 2 MeV proton irradiation at room temperature and 600 °C to create microscale regions with varying densities and types of point and extended defects. Defected regions are investigated using spatial domain thermoreflectance to quantify the change in thermal conductivity as a function of ion fluence as well as transmission electron microscopy and Raman spectroscopy to interrogate the structure of the generated defects. Together, this combination of methods provides important initial insight into defect formation, recombination, and clustering in thorium dioxide and the effect of those defects on thermal transport. These methods also provide a promising pathway for the quantification of the smallest-scale defects that cannot be captured using traditional microscopy techniques and play an outsized role in degrading thermal performance.

Experimental work aimed at understanding the role of dislocation loops in limiting phonon mediated thermal transport in ceramics is presented. Faulted dislocation loops, having diameters of a few nanometers, were introduced by irradiating a polycrystalline cerium dioxide sample with 1.6 MeV protons at 700°C. XRD analysis indicated that irradiated samples retained their crystalline structure and exhibit very little lattice expansion suggesting a low concentration of point defects. Further microstructure characterization using transmission electron microscopy revealed that interstitial type faulted dislocation loops were primarily created as expected for these irradiation conditions. Thermal conductivity of the damaged layer was measured using a modulated thermoreflectance approach. Analysis of the experimental data using the classical Klemens‐Callaway approach reveals that the conductivity reduction is primarily due to dislocation loops, while point defects and voids play only a minor role. These results provide experimental confirmation that faulted loops offer a unique arrangement for displaced atoms that leads to an unusually large reduction of thermal conductivity.

A facile method to synthesize silicon oxide clad uranium oxide nanowires is presented. U₃Si₂, used as a precursor, was oxidized to produce uranium oxide nanocrystallites and amorphous silicon oxide under hydrothermal conditions at 300°C and a pressure of 7.8 × 10⁶ Pa. The growth of uranium oxide nanowires was assisted by silicon oxide via assembling the uranium oxide nanocrystallites in an amorphous silicon oxide matrix. The microstructure and composition of silicon oxide clad uranium oxide nanowires were characterized by XRD, SEM, TEM, and EDS. The uranium oxide in the nanowires was determined as UO_2.34 with a fluoride cubic structure.

A two‐step processing was developed to prepare Yb₂Si₂O₇‐SiC nanocomposites. Yb₂Si₂O₇‐Yb₂SiO₅‐SiC composites were first fabricated by a solid‐state reaction/hot‐pressing method. The composites were then annealed at 1250°C in air for 2 hours to activate the oxidation of SiC, which effectively transformed the Yb₂SiO₅into Yb₂Si₂O₇. The surface cracks purposely induced can be fully healed during the oxidation treatment. The treated composites have improved flexural strength compared to their pristine composites. The mechanism for crack healing and silicate transformation have been proposed and discussed in detail.

Corrosion testing of UNS N10003 in molten fluoride salt was performed in purified molten 27LiF-BeF2 (66–34 mol%) (FLiBe) salt at 700°C for 1,000 h, in pure nickel and graphite capsules. In the nickel capsule tests, the near-surface region of the alloy exhibited an approximately 200 nm porous structure, an approximately 3.5 μm chromium-depleted region, and MoSi2 precipitates. In the tests performed in graphite capsules, the alloy samples gained weight because of the formation of a variety of Cr3C2, Cr7C3, Mo2C, and Cr23C6 carbide phases on the surface and in the subsurface regions of the alloy. A Cr-depleted region was observed in the near-surface region where Mo thermally diffused toward either the surface or the grain boundary, which induced an approximately 1.4 μm Ni3Fe alloy layer in this region. The carbide-containing layer extended to approximately 7 μm underneath the Ni3Fe layer. The presence of graphite dramatically changes the mechanisms of corrosion attack in UNS N10003 in molten FLiBe salt. In terms of the depth of attack, graphite clearly accelerates the corrosion, but the results appear to indicate that the formation of the Cr23C6 phase might stabilize the Cr and mitigate its dissolution in molten FLiBe salt. Moreover, a thermal diffusion controlled corrosion model that was fundamentally derived from Fick's second law was applied to predict the corrosion attack depth of 17.2 μm/y for UNS N10003 in molten FLiBe in the pure nickel capsule at 700°C.

The thermal conductivity of nanocrystalline ceria films grown by unbalanced magnetron sputtering is determined as a function of temperature using laser‐based modulated thermoreflectance. The films exhibit significantly reduced conductivity compared with stoichiometric bulk CeO₂. A variety of microstructure imaging techniques including X‐ray diffraction, scanning and transmission electron microscopy, X‐ray photoelectron analysis, and electron energy loss spectroscopy indicate that the thermal conductivity is influenced by grain boundaries, dislocations, and oxygen vacancies. The temperature dependence of the thermal conductivity is analyzed using an analytical solution of the Boltzmann transport equation. The conclusion of this study is that oxygen vacancies pose a smaller impediment to thermal transport when they segregate along grain boundaries.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Ti₃SiC₂shows a unique combination of the properties of both metals and ceramics. However, its stiffness and strength lose rapidly above 1050°C, which is the main obstacle for the high‐temperature application of this material. To improve the high‐temperature mechanical properties of Ti₃SiC₂, Zr, Hf, or Nb were used as dopants in Ti₃(SiAl)C₂. At room temperature, the Zr‐, Hf‐, or Nb‐doped Ti₃(SiAl)C₂ceramics have comparable stiffness, hardness, strength, and fracture toughness with those of Ti₃(SiAl)C₂. At high temperatures, however, a significant improvement in stiffness and strength has been achieved for (Ti_1−xT_x)₃(SiAl)C₂(T=Zr, Hf, or Nb). (Ti_1−xT_x)₃(SiAl)C₂can retain high degrees of stiffness and strength up to 1200°C, which is 150°C higher than those for Ti₃(SiAl)C₂.

HfAl₄C₄, a new ternary aluminum carbide, was discovered and its crystal structure was determined by a combination of X‐ray diffraction, transmission electron microscopy, and first‐principles calculations. The crystal structure is trigonal belonging to thespace group. The refined lattice constants area=0.3308 nm,c=2.190 nm. First‐principles method was used to calculate the theoretical second‐order elastic constants, bulk modulus, shear modulus, and the Young's modulus of HfAl₄C₄. It shows that HfAl₄C₄has relatively high elastic stiffness.

TaC–TaSi₂ composites were fabricated at 1700°C by an in situ reaction/hot pressing method using Ta, Si, and graphite as initial materials. TaSi₂ content was 0–100 vol%. The microstructure and mechanical properties of the composites were investigated. It was found that the relative densities of composites were above 97.5% when the volume content of TaSi₂ was above 10%. The TaC/10 vol% TaSi₂ composite presented the highest flexural strength of 376 MPa. When the TaSi₂ content was 30–50 vol%, the composites showed the highest fracture toughness of about 4.3 MP·am^1/2. In addition, the composites could retain high Young's modulus up to at least 1525°C.

Hydrothermal oxidation of bulk Ti₃SiC₂in continuous water flow was studied at 500°–700°C under a hydrostatic pressure of 35 MPa. The oxidation was weak at 500°–600°C and accelerated at 700°C due to the formation of cracks in oxides. The kinetics obeyed a linear time‐law. Due to the high solubility of silica in hydrothermal water, the resulting oxide layers only consisted of titanium oxides and carbon. Besides general oxidation, two special modes are very likely present in current experiments: (1) preferential hydrothermal oxidation of lattice planes perpendicular to thec‐axis inducing cleavage of grains and (2) uneven hydrothermal oxidation related to the occurrence of TiC and SiC impurity inclusions. Nonetheless the resistance against hydrothermal oxidation is remarkably high up to 700°C.

The oxidation behavior of Hf–Al–C ceramics containing 37.5 wt% Hf₃Al₃C₅, 30.5 wt% Hf₂Al₄C₅, and 32.0 wt% Hf₃Al₄C₆ has been investigated at 900°–1300°C in air. The oxidation kinetics approximately follows a linear law with the activation energy of 194±12 kJ/mol. The oxidation resistance of Hf–Al–C ceramics at high temperature is superior to HfC. The oxide scale is porous and composed of well‐mixed t‐HfO₂ and Al₂O₃ as well as residual free carbon. The stabilization mechanisms of t‐HfO₂ and free carbon have been discussed. The simultaneous oxidation of Hf and Al in Hf–Al–C ceramics can be attributed to their close oxygen affinity as well as the strong coupling between Hf–C blocks and Al–C units in the crystal structures.

The reciprocating sliding friction and wear properties of two novel materials of Zr₂[Al(Si)]₄C₅ ceramic and Zr₂[Al(Si)]₄C₅–30 vol% SiC composite against Si₃N₄ ball were investigated. The sliding friction process of Zr₂[Al(Si)]₄C₅ against Si₃N₄ experiences two different stages under constant normal load and involves friction and wear mechanism transition. The static coefficient of friction increases with an increasing normal load. The friction force mainly comes from the interfacial shear between Si₃N₄ ball and Zr₂[Al(Si)]₄C₅, which changes with varied sliding distances and normal loads. In contrast, the friction process of the composite experiences one stage and the friction coefficient is not related to the test durations and normal loads. The friction force between Zr₂[Al(Si)]₄C₅–30 vol% SiC composite and Si₃N₄ is mainly from the plough between SiC particles and Si₃N₄ ball, which appears not to be influenced significantly by different normal load and sliding distance. In addition, microfracture induced mechanical wear is the rate‐control wear mechanism in both Zr₂[Al(Si)]₄C₅ and Zr₂[Al(Si)]₄C₅–30 vol% SiC composite. Adding SiC improves the wear resistance of the single‐phase material, because the second phase bears normal load and slows down material removal.

The mechanical and thermophysical properties of quaternary‐layered carbides, Zr₂[Al(Si)]₄C₅ and Zr₃[Al(Si)]₄C₆ have been investigated and compared with those of Zr₂Al₃C₄ and Zr₃Al₃C₅. These four carbides are generally alike in mechanical and thermophysical properties due to their similar crystal structures that consisting of alternatively stacked ZrC layers and Al₃C₂/[Al(Si)]₄C₃ slabs. The layer thickness of zirconium carbide and aluminum carbide has effects on their properties. Thicker layer of zirconium carbide and/or thinner layer of aluminum carbides are in favor of stiffness, hardness, thermal, and electrical conductivities, but go against density, specific stiffness, Debye temperature, and coefficient of thermal expansion.

In this work, reciprocating ball‐on‐flat sliding friction and wear tests as well as two‐body abrasive wear tests were performed on Zr₂Al₃C₄, a new type of ceramic material. In the sliding wear tests, a Si₃N₄ball and an AISI 52100 steel ball were used as counter materials. When Zr₂Al₃C₄was slid against an AISI 52100 ball, the coefficient of friction (COF) was as low as 0.20–0.42, being independent of normal loads, and the wear rates of Zr₂Al₃C₄and AISI 52100 steel were in the range of 10⁻⁴–10⁻⁵mm³/m. A tribofilm between the tribopair was presumed to be responsible for the low COF and wear rate. According to Raman and energy‐dispersive spectroscopy analysis, the tribofilm consists of a mixture of oxides of Zr, Al, and Fe as well as amorphous carbon. When Zr₂Al₃C₄was slid against a Si₃N₄ball, a transition from mild wear to severe wear was observed as the normal load increased. The transition occurs under certain contact stresses after a damage‐accumulating period. In the two‐body abrasive wear test, surface chipping and fragmentation resulting from coalescence of surface and subsurface microcracks were the main material removal mechanisms of Zr₂Al₃C₄.

ZrO₂–Al₂O₃ nanocrystalline powders have been synthesized by oxidizing ternary Zr₂Al₃C₄ powders. The simultaneous oxidation of Al and Zr in Zr₂Al₃C₄ results in homogeneous mixture of ZrO₂ and Al₂O₃ at nanoscale. Bulk nano‐ and submicro‐composites were prepared by hot‐pressing as‐oxidized powders at 1100°–1500°C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO₂ in the composites increased from 7.5 nm for as‐oxidized powders to about 0.5 μm at 1500°C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al₂O₃ in the composites transformed from an amorphous phase in as oxidized powders to θ phase at 1100°C and α phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100°C to 13.5 GPa at 1400°C due to the increase of density.

Silicon pack cementation has been applied to improve the oxidation resistance of Zr₂Al₃C₄. The Si pack coating is mainly composed of an inner layer of ZrSi₂ and SiC and an outer layer of Al₂O₃ at 1200 °C. The growth kinetics of silicide coating at 1000–1200 °C obey a parabolic law with an activation energy of 110.3 ± 16.7 kJ/mol, which is controlled by inward diffusion of Si and outward diffusion of Al. Compared with Zr₂Al₃C₄, the oxidation resistance of siliconized Zr₂Al₃C₄ is greatly improved due to the formation of protective oxidation products, aluminosilicate glass, mullite, and ZrSiO₄.

A dense V₂AlC ceramic was synthesized by an in situ reaction/hot pressing method using V, Al, and C powders as starting materials. The reaction path was investigated. It was found that V₂AlC was produced by a reaction between Al₈V₅ and C. Single ‐phase V₂AlC could be prepared using the optimized molar ratio of V:Al:C=2:1.2:0.9. The grain sizes of as‐prepared V₂AlC samples were temperature dependent. On increasing the temperature from 1400° to 1700°C, the mean grain size of V₂AlC increased from 49 μm in length and 19 μm in width to 405 μm in length and 106 μm in width. The V₂AlC sample sintered at 1500°C exhibited the highest flexural strength of 289 MPa and a fracture toughness of 5.7 MPa·m^1/2, while the V₂AlC sample prepared at 1400°C showed the highest compressive strength of 742 MPa. The sample sintered at 1700°C possessed the highest damage tolerance. Additionally, V₂AlC could retain a high degree of Young's modulus up to 1200°C. Below 800°C, V₂AlC showed excellent thermal shock resistance.

The isothermal oxidation behavior of Zr₂Al₃C₄ in the temperature range of 500 to 1000 °C for 20 h in air has been investigated. The oxidation kinetics follow a parabolic law at 600 to 800 °C and a linear law at higher temperatures. The activation energy is determined to be 167.4 and 201.2 kJ/mol at parabolic and linear stages, respectively. The oxide scales have a monolayer structure, which is a mixture of ZrO₂ and Al₂O₃. As indicated by x-ray diffraction and Raman spectra, the scales formed at 500 to 700 °C are amorphous, and at higher temperatures are α-Al₂O₃ and t-ZrO₂ nanocrystallites. The nonselective oxidation of Zr₂Al₃C₄ can be attributed to the strong coupling between Al₃C₂ units and ZrC blocks in its structure, and the close oxygen affinity of Zr and Al.

The oxidation behavior of Zr₂[Al(Si)]₄C₅ and Zr₃[Al(Si)]₄C₆ in air has been investigated. The oxidation kinetics of bulk Zr₂[Al(Si)]₄C₅ and Zr₃[Al(Si)]₄C₆ at 900–1300 °C generally follow a parabolic law at a very short initial stage and then a linear law for a long period with the activation energy of 237.9 and 226.8 kJ/mol, respectively. The oxide scales have a duplex structure, consisting of mainly an outer porous layer of ZrO₂, Al₂O₃, and aluminosilicate/mullite, and a thin inner compact layer of these oxides plus remaining carbon. The oxidation resistance of Zr₂[Al(Si)]₄C₅ and Zr₃[Al(Si)]₄C₆ has been improved compared with Zr₂Al₃C₄, and is much better than ZrC due to larger fraction of protective oxidation products, Al₂O₃ and aluminosilicate/mullite.

In this work, a bulk Nb₄AlC₃ ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb₄AlC₃ were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10⁻⁶ K⁻¹ in the temperature range of 200°–1100°C. The thermal conductivity of Nb₄AlC₃ increased from 13.5 W·(m·K)⁻¹ at room temperature to 21.2 W·(m·K)⁻¹ at 1227°C, and the electrical conductivity decreased from 3.35 × 10⁶ to 1.13 × 10⁶Ω⁻¹·m⁻¹ in a temperature range of 5–300 K. Nb₄AlC₃ possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m^1/2. Most significantly, Nb₄AlC₃ could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C.

In this work, a bulk Nb₄AlC₃ ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb₄AlC₃ were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10⁻⁶ K⁻¹ in the temperature range of 200°–1100°C. The thermal conductivity of Nb₄AlC₃ increased from 13.5 W·(m·K)⁻¹ at room temperature to 21.2 W·(m·K)⁻¹ at 1227°C, and the electrical conductivity decreased from 3.35 × 10⁶ to 1.13 × 10⁶Ω⁻¹·m⁻¹ in a temperature range of 5–300 K. Nb₄AlC₃ possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m^1/2. Most significantly, Nb₄AlC₃ could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C.

This article presents the results of combined experimental and theoretical studies of elastic and thermal properties of Zr2Al3C4 carbide. The full set of second order elastic constants, bulk modulus, shear modulus, and Young’s modulus of Zr2Al3C4 were calculated and compared with those of Zr3Al3C5 and ZrC. The experimentally measured Young’s modulus and shear modulus are in good agreement with theoretical ones. The calculated Debye temperature from elastic constants of Zr2Al3C4 is 830 K, which is slightly higher than that of Zr3Al3C5, and exhibits pronounced enhancement in comparison with that of ZrC. The highest Debye temperature of Zr2Al3C4 is related with its highest specific stiffness, i.e., the stiffness-to-weight ratio. The heat capacity and thermal conductivity of Zr2Al3C4 were measured by means of the flash method. The thermal conductivity of Zr2Al3C4 decreases with increasing temperature, for instance the values at room temperature and 1600 K are 15.5 and 10.1 W/m K, respectively. The investigations provide information on elastic and thermal properties of Zr2Al3C4 with promising high temperature applications.

In order to improve the mechanical properties of Ti₃AlC₂, near‐fully dense Ti₃AlC₂/TiB₂composites were synthesized using Ti, Al, graphite, and B₄C powders as the initial materials. Compared with monolithic Ti₃AlC₂, the composites exhibit a much higher strength (for the compressive strength, from initial 723 MPa to maximal 2205 MPa; for flexural strength, from initial 340 MPa to maximal 861 MPa), and the strengthening effect can be held at least up to 1100°C. Moreover, besides the enhancement of the elastic modulus and hardness of Ti₃AlC₂, the introduction of a TiB₂phase makes a positive contribution to its electrical conductivity.

Layered stacking characteristics of ternary Zr–Al–C carbides were investigated using scanning transmission electron microscopy (STEM). Three previously unknown compounds, i.e., Zr₄Al₃C₆, Zr₅Al₆C₉, and Zr₇Al₆C₁₁ were identified. The present study extends the structural information of ternary Zr–Al–C ceramics. The influence of the thickness of the NaCl-type Zr-C slab on the elastic properties of ternary Zr–Al–C ceramics is discussed based on first-principles calculations. In addition, direct atomic-resolution observations illustrate the process for forming the unique layered crystal structures of ternary Zr–Al–C ceramics. These results also provide insights into the formation mechanism of layered ternary Zr–Al–C carbides.

Bulk Ta₄AlC₃ ceramic was prepared by an in situ reaction synthesis/hot‐pressing method using Ta, Al, and C powders as the starting materials. The lattice parameter and a new set of X‐ray diffraction data were obtained. The physical and mechanical properties of Ta₄AlC₃ ceramic were investigated. Ta₄AlC₃ is a good electrical and thermal conductor. The flexural strength and fracture toughness are 372 MPa and 7.7 MPa·m^1/2, respectively. Typically, plate‐like layered grains contribute to the damage tolerance of Ta₄AlC₃. After indentation up to a 200 N load, no obvious degradation of the residual flexural strength of Ta₄AlC₃ was observed, demonstrating the damage tolerance of this ceramic. Even at above 1200°C in air, Ta₄AlC₃ still retains a high strength and shows excellent thermal shock resistance, which renders it a promising high‐temperature structural material.

Polycrystalline Zr₂Al₃C₄was fabricated by anin situreactive hot‐pressing process using zirconium (zirconium hydrides), aluminum, and graphite as starting materials. The investigation on reaction path revealed that the liquid Al played an important role in triggering the formation of ternary zirconium aluminum carbides. The mechanical properties of Zr₂Al₃C₄at room temperature were measured (Vickers hardness of 10.1 GPa, Young's modulus of 362 GPa, flexural strength of 405 MPa, and fracture toughness of 4.2 MPa·m^1/2). The electrical resistivity and thermal expansion coefficient were determined as 1.10 μΩ·m and 8.1 × 10⁻⁶K⁻¹, respectively.

Anin situreactive hot‐pressing process using zirconium (zirconium hydride), aluminum, and graphite as staring materials and Si and Y₂O₃as additives was used to synthesize bulk Zr₃Al₃C₅ceramics. This method demonstrates the advantages of easy synthesis, lower sintering temperature, high purity and density, and improved mechanical properties of synthesized Zr₃Al₃C₅. Its electrical and thermal properties were measured. Compared with ZrC, Zr₃Al₃C₅has a relatively low hardness (Vickers hardness of 12.5 GPa), comparable stiffness (Young's modulus of 374 GPa), but superior strength (flexural strength of 488 GPa) and toughness (fracture toughness of 4.68 MPa·m^1/2). In addition, the stiffness decreases slowly with increasing temperature and at 1600°C remains 78% of that at ambient temperature, indicating that Zr₃Al₃C₅is a potential high‐temperature structural ceramic.