The crucial challenge for oxide thermal insulators, such as Al2O3 and SiO2 nano-particle aggregates, is to solve the trade-off between extremely low thermal conductivity and unsatisfied sintering stability. We herein report the ultra-low thermal conductivities (0.068–0.1 W m− 1 K− 1) of β-SiC nanoparticle (~ 35 nm) packed beds. The breakthrough is realized by multiple heat blocking mechanisms in the nanostructures. The samples also possess good thermal stability as high as 1500 °C. Our results provide a new strategy to explore ultra-low thermal conductivity materials with excellent thermal stability, regardless of their high intrinsic lattice thermal conductivities.Download high-res image (266KB)Download full-size image

X2-RE2SiO5 orthosilicates are promising candidate environmental/thermal barrier coating (ETBC) materials for silicon-based ceramics because of their excellent durability in high-temperature environments and potential low thermal conductivities. We herein present the mechanical and thermal properties of X2-RE2SiO5 orthosilicates based on theoretical explorations of their elastic stiffness and thermal conductivity, and experimental evaluations of the macroscopic performances of dense specimens from room to high temperatures. Mechanical and thermal properties may be grouped into two: those that are sensitive to the rare-earth (RE) species, including flexural strength, elastic modulus, and thermal shock resistance, and those that are less sensitive to the RE species, including thermal conductivity, thermal expansion coefficient, and brittle-to-ductile transition temperature (BDTT). The orthosilicates show excellent elastic stiffness at high temperatures, high BDTTs, very low experimental thermal conductivities, and compatible thermal expansion coefficients. The reported information provides important material selection and optimization guidelines for X2-RE2SiO5 as ETBC candidates.

Thermal conductivities of γ-Y2Si2O7, β-Y2Si2O7, β-Yb2Si2O7, and β-Lu2Si2O7 were investigated by combined first-principles calculations and experimental evaluation. Theoretical calculation was used to predict the elastic properties, anisotropic minimum thermal conductivities, and temperature dependent lattice thermal conductivities. Experimentally, thermal conductivities of these disilicates were measured from room temperature to 1273 K. In addition, their experimental intrinsic lattice thermal conductivities were determined from the corrected thermal diffusivity data after removing the extrinsic contributions from phonon scattering by defects and thermal radiation. The experimental lattice thermal conductivities match well with the theoretical predictions. Furthermore, Raman spectra of the disilicates was measured and used to estimate the optical phonon relaxation time. The present results clearly disclose the specific material parameters that determine the low thermal conductivity of RE2Si2O7 and may provide guidelines for the optimal thermal conductivity of rare earth disilicates.

Nano-laminated Ti3AC2 (A = Si, Al) are highlighted as nuclear materials for a generation IV (GIV) reactor because they show high tolerance to radiation damage and remain crystalline under irradiation of high fluence heavy ions. In this paper, the energetics of formation and migration of intrinsic point defects are predicted by density functional theory calculations. We find that the space near the A atomic plane acts as a point defect sink and can accommodate lattice disorder. The migration energy barriers of Si/Al vacancy and TiSi anti-site defects along the atomic plane A are in the range of 0.3 to 0.9 eV, indicating their high mobility and the fast recovery of Si/Al Frenkel defects and Ti–A antisite pairs after irradiation. This layered structure induced large disorder accommodation and fast defect recovery must play an important role in the micro-structural response of Ti3AC2 to irradiation.

Porous γ-Y2Si2O7 ceramic with good mechanical and thermal properties is prepared by the in situ reaction sintering foam-gelcasting method using Y2O3 and SiO2 as the starting powders and non-toxic gelatin as the gel former. The porous sample has interconnected large pores (40–230 μm) and small pores (0.1–2 μm) in the skeleton. The porosity can be controlled from 64.3% to 89.3% by adjusting the solid content and amount of the slurry. The corresponding strength ranges from 46.5 to 3.4 MPa. Porous γ-Y2Si2O7 with a porosity between 57.2% and 90.0% shows a low thermal conductivity in the range of 0.918–0.147 W/(m K) at room temperature. Porous γ-Y2Si2O7 has excellent mechanical property at high temperature (the strength at 1100 and 1200 °C is maintained above 71% and 50%, respectively, of the value at room temperature) and good thermal stability (the reheating shrinkage is 1.3–1.7%). This work indicates that lightweight porous γ-Y2Si2O7 ceramic could be applied as a promising high-temperature thermal insulator with low thermal conductivity and high temperature reliability.

Thermal conductivity of bulk and dense oxynitride Lu4Si2O7N2 was measured at various temperatures. Results show that Lu4Si2O7N2 possesses low thermal conductivity and has potential applications as thermal insulation material. Detailed analysis shows that the experimental thermal diffusivity is significantly influenced by defects at low temperature and infrared radiation at high temperature. Therefore, we combined Debye model and Slack's equation to predict the temperature dependent intrinsic lattice thermal conductivity of Lu4Si2O7N2. All parameters indispensable in the calculation (equilibrium crystal structure, second order elastic constants, elastic modulus, Debye temperature, and Grüneisen constant) were predicted with the help of first-principles calculation methods. Finally, factors that influence the thermal conductivity of Lu4Si2O7N2 were discussed and possible explanations for the difference between experimental and theoretical thermal conductivity were illustrated.

β-Lu2Si2O7 is a promising candidate in the third generation of environmental barrier coating (EBC) materials for silicon-based ceramics due to its excellent high temperature environmental durability. However, the high temperature thermal and mechanical properties of β-Lu2Si2O7 are seldom reported, which hinders the design and evaluation of its EBC applications. In this paper, pure and dense β-Lu2Si2O7 sample was successfully fabricated and its mechanical properties, including Young's modulus, bulk modulus, shear modulus, Poisson's ratio, flexural strength, fracture toughness and hardness were investigated. The ball indentation test reveals that the main deformation mechanisms of β-Lu2Si2O7 at room temperature are deformation twinning and dislocation glide, which indicate that β-Lu2Si2O7 is a damage tolerant ceramic. In addition, the thermal expansion coefficient and thermal shock resistance were measured at high temperatures. β-Lu2Si2O7 possesses excellent high temperature elastic stiffness up to 1470 °C, and the critical temperature difference for thermal shock resistance is 270 K.

In this paper, Lu2SiO5 is reported as a promising rare-earth silicate with very low thermal conductivity. First-principle method is used to calculate the crystal structure, second order elastic constants and anisotropic elastic stiffness. Using these parameters, the whole profile of temperature dependent lattice thermal conductivity of Lu2SiO5 is predicted based on well-established models. The low lattice thermal conductivity of Lu2SiO5 originates from its complex crystal structure, significant bonding heterogeneity, low Debye temperature, and low sound velocity. Experimental intrinsic lattice thermal conductivity is also determined by successfully eliminating the extrinsic mechanisms for phonon scattering by point defects and grain boundaries, as well as the contribution of thermal radiation at high temperatures, from the measured lattice thermal diffusivity. The experimental result agrees well with theoretical prediction. The present method can illustrate how specific material parameters govern lattice thermal conductivity and provide quantitative guideline in searching novel candidates with low thermal conductivity.

Porous Y2SiO5 ceramic is a promising high-temperature thermal insulator in harsh environment. However, all the published relevant works faced serious problems, such as severe linear shrinkage, low porosity and low strength. In this study, porous Y2SiO5 ceramic with low sintering shrinkage and high porosity was successfully prepared by foam-gelcasting method using gelatin as the gelling agent. The effects of sintering methods, including in situ reaction sintering and direct sintering, and sintering temperatures on the phase composition, microstructure, shrinkage, porosity, and compressive strength of porous Y2SiO5 were investigated. Compared with samples fabricated by direct sintering, porous Y2SiO5 ceramic prepared via in situ reaction sintering method has the merits of the low linear shrinkage of 1.0%–4.7%, low bulk density of 0.79–0.88 g/cm3, high porosity of 82.1%–80.1%, and high strength of 3.54–8.03 MPa, when the sintering temperatures increase from 1350 to 1550 °C. Porous Y2SiO5 has unique multiple pore structures, especially containing the interconnected small pores in skeleton. The thermal conductivity of porous Y2SiO5 is very low, which is 0.228 W/(m⋅K) for the sample with a porosity of 79.6%. This work reports an optimal processing method of highly porous Y2SiO5 with the potential application as high-temperature thermal insulation material.

Lu2SiO5 is a promising candidate of environmental barrier coatings (EBC) for silicon based ceramics due to its excellent high temperature stability. However, little information is available for the mechanical and thermal properties of Lu2SiO5, which frustrated evaluation of its performances for EBC applications. In this paper, dense Lu2SiO5 ceramic is successfully fabricated from Lu2O3 and SiO2 powders by in situ hot pressing/reaction sintering at 1500 °C. Mechanical properties, including Young's modulus, bulk modulus, shear modulus, Poisson's ratio, fracture toughness, Vickers hardness, and bending strength are reported for the first time. Lu2SiO5 possesses excellent high temperature mechanical properties up to at least 1300 °C. Thermal stress for the case of Lu2SiO5 or Y2SiO5 coating on silicon bond coat and thermal stress resistance parameter are also estimated based on the experimental mechanical and thermal properties. The present results suggest that Lu2SiO5 has better reliability than Y2SiO5 in harsh thermal environment.

By using the first-principles calculation, we studied the mechanisms of point defects in Y4Al2O9 (YAM), a promising ternary oxide with excellent optical and thermal properties. It is found that the predominant native defect species is closely dependent on the chemical potentials of each constituent. In the case of O-rich condition, the oxygen interstitial has the very low defect formation energy, followed by the anti-site defects and Al vacancy; in the case of Al-rich condition, the oxygen vacancy yields the lowest defect formation energy, followed by the anti-site defects and Al interstitial. The present result shows that in all the possible chemical potential ranges, anti-site defects have relatively low defect formation energy and might exist in high concentration in YAM. Furthermore, AlY anti-site has relatively lower defect formation energy than the YAl anti-site throughout. The behaviors of defect complexes under non-stoichiometric condition, such as the Al2O3 or Y2O3 excess, are also investigated. The results provide helpful guide to optimize the experimental synthesizing of YAM.

The native point defects and mechanism of accommodating deviations from stoichiometry of Si2N2O crystal have been investigated using atomistic simulation techniques. This work firstly provides a reliable classical interatomic potential model derived from density functional theory calculations. The force-field parameters well reproduce the crystal structure, elastic stiffness, and dielectric constants of Si2N2O. It is expected that the force-field parameters are useful in future investigations on Si2N2O by molecular dynamic simulation. The calculated formation energies for native defects suggest that intrinsic disorder in stoichiometric Si2N2O is dominated by antisites and a degree of oxygen Frenkel defect may also exist in this system. In nonstoichiometric Si2N2O, the calculated reaction energies indicate that excess SiO2 or Si3N4 is most likely accommodated by the formation of antisite in the lattice. And we also find that SiO2 excess is energetically more favorable than Si3N4 surplus in Si2N2O.

The mechanical and thermodynamic stabilities of M4AlC3 (M = V, Nb and Ta) and Ti4AlN3 polymorphs were investigated by means of the first-principles pseudopotential total energy method. All compounds satisfied the Born criteria for mechanical stability, but had different thermodynamic stabilities. Only Ta4AlC3 showed a possible polymorphic phase transformation driven by thermodynamic competition. The present theoretical results support the polymorphism of Ta4AlC3 in experimental synthesis and explain the underlying thermodynamic mechanism. Polymorphism was excluded from V4AlC3, Nb4AlC3 and Ti4AlN3.

We performed ab initio calculations for monovacancy formation and migration in Ti2AlC. Carbon and aluminum vacancies have almost equally low formation energies, respectively, at (Ti- and Al-rich) and (Ti- and C-rich) growth conditions, wherein both defects exhibit a high equilibrium concentration and structural tolerance to large off-stoichiometry in Ti2AlC. In contrast, VTi has the highest formation energy at all possible conditions. The intrinsic migration energies of various vacancies are determined to be in the sequence Em(VAl) < Em(VTi) < Em(VC).

Ti2AlC was predicted to bear Al-vacancy down to a sub-stoichiometry of Ti2Al0.5C. The phase instability beyond a critical Al content was attributed to occupation of the Ti–Al anti-bonding orbital, which reduces the coupling strength between Ti2C slab and Al atomic plane. The migration energy barrier of Al self-diffusion along the (0 0 0 1) plane was low, 0.83 eV, resulting in rapid out-diffusion of Al during oxidation and decomposition of Ti2AlC at high temperatures.

Al3BC3, an isostructural phase to Mg3BN3, experienced no pressure-induced phase transformation that occurred in the latter material (J. Solid State Chem. 154 (2000) 254–256). The discrepancy is not clear yet. Using the first-principles density functional calculations, we predict that Al3BC3 undergoes a hexagonal-to-tetragonal structural transformation at 24 GPa. The predicted phase equilibrium pressure is much higher than the previously reported pressure range, i.e., 2.5–5.3 GPa, conducted on phase stability of Al3BC3. A homogeneous orthorhombic shear strain transformation path is proposed for the phase transformation. The transformation enthalpy barrier is estimated to yield a low value, i.e., 0.129 eV/atom, which ensures that the transformation can readily take place at the predicted pressure.Relative enthalpy of tetragonal phase with respect to hexagonal phase at various pressures. The pressure-induced phase transformation occurs at about 2 and 24 GPa for Mg3BN3 and Al3BC3, respectively.

Nano-laminated Ti3AC2 (A = Si, Al) are highlighted as nuclear materials for a generation IV (GIV) reactor because they show high tolerance to radiation damage and remain crystalline under irradiation of high fluence heavy ions. In this paper, the energetics of formation and migration of intrinsic point defects are predicted by density functional theory calculations. We find that the space near the A atomic plane acts as a point defect sink and can accommodate lattice disorder. The migration energy barriers of Si/Al vacancy and TiSi anti-site defects along the atomic plane A are in the range of 0.3 to 0.9 eV, indicating their high mobility and the fast recovery of Si/Al Frenkel defects and Ti–A antisite pairs after irradiation. This layered structure induced large disorder accommodation and fast defect recovery must play an important role in the micro-structural response of Ti3AC2 to irradiation.