Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Elastic and thermodynamic properties of Mo2C polymorphs from first principles calculations

Transition metal carbides have unique physical and chemical properties and been widely used in engineering parts that need to work under high temperatures and pressures. o-Mo₂C, h-Mo₂C and t-Mo₂C are three critical molybdenum carbides polymorphs while remaining are largely unknown in their mechanical anisotropy, hardness and thermal properties. In this work, we investigated systematically the mechanical and thermodynamic properties of these three candidate carbides using first principles calculations based on density functional theory. Our results showed that the bonds in these compounds were mainly of metallic and covalent type. The Gibbs free energy analysis showed thermodynamically stable structures for all the three carbides. Their shear moduli were estimated to range from 149.1 to 153.4 GPa and hardnesses were expected to be less than 20 GPa. Young׳s moduli were analyzed to have more anisotropic features than bulk modulus for all the three compounds. In addition, heat capacities were calculated to predominate by phonon excitations at high temperature but electron excitations at low temperatures near 0 K.     

Theoretical consideration of the effect of porosity on thermal conductivity of porous materials

The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. The effects of porosity and pore size on the thermal conductivity are discussed. The thermal conductivity of insulating materials is determined by the heat capacity of phonons, the average phonon velocity and the phonon mean free path. We investigate the porosity dependence of these quantities, especially by taking into account phonon scatterings by pores, and present an expression for the thermal conductivity as a function of porosity. Our model consideration predicts that the thermal conductivity of nanoporous materials depends on the ratio of the pore size R _p to the phonon mean free path for zero-porosity, l ₀. The thermal conductivity for l ₀/R _p > 1 decreases steeply with increasing porosity because of effective phonon scatterings by pores. On the other hand, the thermal conductivity for l ₀/R _p < 0.1 decreases moderately with increasing porosity because phonon scatterings by pores are no longer effective. On the basis of the present theoretical consideration, we discuss the principal factor dominating the porosity dependence of thermal conductivity in nanoporous materials. We also discuss how one can design nanoporous materials with lower or higher thermal conductivity.     

Synthesis and characterization of GaN powder by the cyanonitridation of gallium oxide powder

Gallium nitride (GaN) powder was synthesized by the cyanonitridation of β-gallium oxide (β-Ga₂O₃) powder and characterized by powder X-ray diffraction, ⁷¹Ga magic-angle spinning nuclear magnetic resonance spectroscopy, Raman spectroscopy, and scanning electron microscopy. The cyano radical, which was formed by the thermal decomposition of organic compounds such as acetonitrile and melamine, was involved in the nitridation of β-Ga₂O₃. The formation of GaN via cyanonitridation commenced at slightly higher temperature than that via ammonolysis. The produced GaN powders showed five first-order phonon modes in Raman spectra. Particles’ morphologies were maintained during cyanonitridation, indicating that the β-Ga₂O₃ did not convert to GaN through gaseous species such as Ga₂O.     

Theoretical exploration of the abnormal trend in lattice thermal conductivity for monosilicates RE2SiO5 (RE = Dy,Ho, Er,Tm, Yb and Lu)

Rare earth monosilicates RE₂SiO₅ have been considered as promising environmental barrier coating materials for silicon-based ceramics due to their low thermal conductivity and good high-temperature stability. We herein performed a systematic study of the lattice dynamics for RE₂SiO₅ (RE?=?Dy, Ho, Er, Tm, Yb and Lu) using first-principles calculations. The loosely bound rare earth atoms provide large Grüneisen parameters and low phonon group velocities, both of which determine the low thermal conductivity. Theoretical exploration predicts an anomalous increase of lattice thermal conductivity with increment of RE atomic number and the mechanism is explained by the stronger atomic bonding and weaker phonon anharmonicity. Although incorporating heavier atoms has long been considered as an effective way to reduce lattice thermal conductivity, this work addresses the importance of bonding heterogeneity and anharmonicity rather than atomic mass variation. This theoretical study suggests an alternative approach towards the design of new thermal insulating materials.     

Design and development of ceramic-based composites with tailored properties for cutting tool inserts

A successful approach to the development of tailored cutting tool materials requires the development of innovative concepts at each step of manufacturing, from the material design, synthesis of composite powders, to their processing and sintering. In this paper, a computational design approach is applied in the development of reinforced ceramic-based cutting tool inserts with tailored structural and thermal properties. Several potential filler materials are considered at the material design stage for the improvement of structural and thermal properties of a selected matrix material. Properties, such as an improved thermal conductivity and reduced coefficient of thermal expansion are essential for an effective cutting tool insert to absorb thermal shock at varying temperatures. In addition, structural properties such as elastic modulus have to be maintained within a moderate range. A mean-field homogenization theory and effective medium approximation using an in-house code are applied for predicting potential optimum structural and thermal properties for the required application. This is done by considering the effect of inclusions as a function of volume fraction and particle size in the ceramic base matrix. Single inclusion composites such as alumina-silicon (Al₂O₃-SiC) and alumina-cubic boron nitride (Al₂O₃-cBN) as well as hybrid composite such as alumina-silicon-cubic boron nitride (Al₂O₃-SiC-cBN) are developed using the Spark Plasma Sintering (SPS) process in line with the designed range of filler size and volume fraction to validate the computational results. It is found that the computational material design approach is precise enough in predicting the target properties of a designed hybrid composite material for cutting tool inserts.     

Structural models and electronic properties of cage-like C3N4 molecules

We have proposed the atomic models and, using the Hückel approach, studied the electronic properties of various kinds of nonspherical C₃N₄ cage molecules (octahedral-, cubic- and dodecahedral-like structures) assembled from triazine rings and nitrogen bridges. Results of E_tot estimations reveal that octahedral-like cages (OLC) are the most stable ones. The carbon nitride cages have the HOMO–LUMO gaps in the interval 1.4–2.1 eV. This is the first reported comparison of the possible forms of nonspherical cage-like C₃N₄ molecules, which could be of interest to the current efforts in the synthesis of nanoscale-sized faceted forms of carbon nitride species.     

Effect of yttria on thermal transport and vibrational modes in yttria-stabilized hafnia

Low thermal conductivity plays an essential role in application relevant to thermal energy conversion and management. In this paper, we utilize molecular dynamics to investigate the thermal transport and lattice variation modes in yttria-stabilized hafnia, which only contains binary oxides of Y₂O₃ and HfO₂. It is found that the thermal conductivity κ of yttria-stabilized hafnia decreases significantly with the increase of doping ratio of Y₂O₃, and then reaches a limiting value (～2.1 W m^?1K^?1), because of the strong phonon scattering of oxygen vacancies. Importantly, a glass-like thermal conductivity κ is achieved in yttria-stabilized hafnia samples when the content of Y₂O₃ exceeds 15 mol%. By decomposing the phonon vibrational modes, we find that most of the heat is transported by diffusive modes. As a result, the κ exhibits a glass-like feature in yttria-stabilized hafnia samples with high content of Y₂O₃. Notably, the κ of yttria-stabilized hafnia is much lower than those of classical functional ceramics materials. The insight into the κ in yttria-stabilized hafnia system is beneficial for understanding and reducing the κ of materials through defect engineering. Despite its simple composition, yttria-stabilized hafnia with different doping ratios demonstrates unexpected high scattering rate of phonon vibration density states, which is confirmed by the diffused wavevector-frequency dispersion. Eigenvector periodicity and phonon participation ratio of phonon have been visualized to capture the distribution of phonon modes in yttria-stabilized hafnia with various dopant. This work investigates into the details of phonon vibrational modes in yttria-stabilized hafnia, which would be valuable for conducting experiments to acquire low thermal conductivity materials in laboratory.     

Numerical experiments on phonon properties of isotope and vacancy-type disordered graphene

We have studied phonon properties of graphene theoretically with different concentrations of ¹³C isotope and vacancy-type defects. The forced vibrational method, which is based on the mechanical resonance to extract the pure vibrational eigenmodes by numerical simulation, has been employed to compute the phonon density of states (PDOSs) and mode pattern of isotope-disordered graphene as well as a combined isotope and vacancy-type defective graphene structure. We observe a linear reduction of the E_2g mode frequencies with an increase in ¹³C concentration due to the reduced mass variation of the isotope mixture. We find a downshift of the E_2g mode of 65 cm^− 1, which is a very good agreement with the experimental results, and the phonon frequencies described by the simple harmonic oscillator model. The vacancy-type defects break down the phonon degeneracy at the Г point of the LO and TO modes, distort and shift down the phonon density of states significantly. The PDOS peaks for the combined isotope and vacancy-type defects show the remarkable increase in the low-frequency region induced by their defect formations. Due to phonon scattering by ¹³C isotope or vacancies, some graphene phonon wave functions become localized in the real space. Our numerical experiments reveal that the lattice vibrations in the defective graphene show the remarkably different properties such as spatial localization of lattice vibrations due to their random structures from those in the perfect graphene. The calculated typical mode patterns for in-plane K point optical phonon modes indicate that the features of strongly localized state depend on the defect density, and the phonon is localized strongly within a region of several nanometers in the random percolation network structures. In particular, for in-plane K point optical phonon modes, a typical localization length is on the order of ≈ 7 nm for isotope impurities, ≈ 5 nm for vacancy-type defects and ≈ 6 nm for mixed-type defects at high defect concentrations of 30%. Our findings can be useful for the interpretation of experiments on infrared, Raman, and neutron-diffraction spectra of defective graphene, as well as in the study of a wide variety of other physical properties such as thermal conductivity, specific heat capacity, and electron–phonon interaction.     

Crystallization by post-treatment of reactive r.f.-magnetron-sputtered carbon nitride films

Carbon nitride (CN_x) films have been deposited on single crystal ZrO₂(100) substrates by reactive r.f. magnetron sputtering. The effect of thermal annealing at 900°C on the structural properties of the films has been studied by atomic force microscopy (AFM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). AFM results show a fine spread of well-defined grains with an average size of 500 nm exhibited by the post-annealed films. TEM images confirmed that thermal annealing has transformed the predominantly amorphous as-deposited films into crystalline phases. The surface morphology observed is composed of carbon nitride nanofibres and p-C₃N₄ crystals in a less nitrogenated amorphous matrix. The size of the crystals ranges from about 0.05 to 1.5 μm. The smallest of the nanofibres appear to be approximately 20 nm in external diameter. Selected area electron diffraction indicates the fibre walls to be crystalline in nature. N 1s peaks in XPS spectra of the annealed films indicate the presence of two different bonding states, one attributed to nitrogen inserted into the graphitic ring structure, and the other attributed to nitrogen surrounded by three carbons in the NC network.     

High temperature ceramic radomes (HTCR) – A review

An electromagnetically transparent, structurally robust and environmentally resistant enclosure of radar antenna for ground based systems to modern avionics in military aircraft and missiles is called as radome. Radome materials are classified based on: (i) type of function - surface-based or flight-mode and (ii) speed of operation - subsonic, supersonic to hypersonic. The desired properties of these materials are low dielectric constant and low loss factor in addition to its capacity to withstand the high temperature of operation. Composite laminates of glass or aramid fibre reinforced polymeric resins are radome material candidates for applications in subsonic range. However, ceramics are the only viable option for military aerospace applications such as a fighter jet travelling at Mach 3 or an advanced hypersonic missile speeding up to Mach 5. This review outlines the hand-full of ceramic materials already in application as radome materials like high-purity-alumina, pyroceram, slip-cast-fused-silica, their processing technology, electromagnetic and mechanical properties, advantages and disadvantages with respect to advanced military vehicles. Use of silicon nitride based radome materials, that has exceptional mechanical strength and thermal stability up to 1400 °C is illustrated with respect to reaction bonded silicon nitride, hot pressed silicon nitride, silicon oxynitride, sialon and their composites. Design of new generation radome materials was conceptualized and discussed as applicable to silicon nitride and related ceramics, wherein incorporation of varied degree of porosity improves electromagnetic properties, simultaneously, maintaining the required mechanical strength. Multilayer and graded porosity and its influence on electromagnetic properties were briefly discussed. Si₃N₄ ceramics having controlled porosity leading to optimum electromagnetic and mechanical properties produced through systematic processing is proposed as the futuristic high temperature radome material for supersonic applications.     

Elastic and thermal parameters of lanthanide-orthophosphate (LnPO4) ceramics from atomistic simulations

We performed extensive and accurate atomistic simulations of elastic and heat transport properties of series of rare-earth orthophosphate ceramics LnPO₄ (Ln = La, …, Lu and Y) in monazite and xenotime structures. The results show clear trends in the elastic moduli along the lanthanide-series, which complement the existing experimental data on these materials. We found that the thermal conductivities of xenotimes are about two times larger than those of monazite, which is in agreement with the experimental measurements and explained by sizes of the primitive cells. Large sets of data allowed assessment of the validity of Slack's model as well as accuracy of molecular dynamics simulations of heat flow for prediction of thermal conductivity. Last, but not least, the separation of the intrinsic and extrinsic contribution to the measured thermal diffusivities allowed for a detailed analysis of the phonon mean free paths in the considered materials.     

Ab initio computations of electronic,mechanical, lattice dynamical and thermal properties of ZrP2O7

A systematical ab initio analysis of ZrP₂O₇ is presented in this work. Density functional theory (DFT) computations were performed for the electronic, mechanical, lattice dynamical and thermal properties of ZrP₂O₇. The lattice constants determined from the theoretical calculation are consistent with the experimental results. Based on the analyses on the electronic density of states, charge density and electron localization function of ZrP₂O₇, heterogeneous bonding nature is revealed and confirmed by the phonon density of states. We also reported the second-order elastic constants and polycrystalline mechanical properties of ZrP₂O₇ for the first time. According to the calculated polycrystalline moduli, the minimum thermal conductivity of ZrP₂O₇ is estimated to be 1.15 W m⁻¹ K⁻¹. Our theoretical results illustrate that ZrP₂O₇ is a promising candidate as thermal barrier coating and high temperature binding material.     

Theoretical predictions on intrinsic lattice thermal conductivity of ZrB2

As a most important thermal management material, high thermal conductivity of ZrB₂ is expected. However, the reported values of thermal conductivity κ of ZrB₂ are quite scattering, and no consensus has been reached. The contribution from lattice separated by Wiedemann-Franz law is low and the relationship between electron and phonon contributions is still blurry. To explore the intrinsic κ of ZrB₂, in this work, two approaches, i.e. analytical Debye-Callaway model and iterative solution to the Boltzmann transport equation (BTE), are used to simulate the temperature-dependent theoretical lattice κ of ZrB₂. Our work demonstrates that the lattice thermal conductivity of ZrB₂ has been underestimated. The intrinsic lattice thermal conductivity of ZrB₂ is estimated to be 91 and 88 W m^?1 K^?1 at 300 K, by two different models, respectively. The effects of low lying optical phonon modes and grain boundary on the thermal conductivity of ZrB₂ are discussed. The thermal conductivity of ZrB₂ is controllable by designing effective grain size and microstructure. By casting light on the micro mechanism on lattice heat conduction of ZrB₂, our work will be constructive to the application of ZrB₂ as thermal management material.     

Thermal performance regulation of high-entropy rare-earth disilicate for thermal environmental barrier coating materials

We prepared the novel high-entropy (xRE_1/x)₂Si₂O₇ (RE = Y, Yb, Er, Sc, Gd and Eu, x = 2–6) ceramics by a two-step method for the application of thermal environmental barrier coatings (TEBCs), and the effect of configuration entropy and lattice distortion on microstructures and thermal properties at high temperature were investigated. The results showed that the configuration entropy resulted from mass disorder can only contribute to the stability of thermal properties and microstructure. Lattice distortion should be responsible for reduction in thermal properties, which may be due to the enhancement of atomic nonharmonic vibration, resulting in intensified phonon scattering and hindering atomic amplitude oscillation. As-prepared high-entropy (Y_1/6Yb_1/6Er_1/6Sc_1/6Gd_1/6Eu_1/6)₂Si₂O₇ ceramic exhibited the relatively low thermal diffusivity, thermal conductivity and coefficient of thermal expansion, which were 0.89–0.50 mm² s^–1, 1.99–2.50 W m^–1 K^–1 and (3.01–3.78) × 10^–6 K^–1 in the temperature range of 293–1373 K, respectively. This work provides a solid guarantee for the application of TEBC materials.     

One-pot fabrication of Ag–SrTiO3 nanocomposite and its enhanced thermoelectric properties

Ag–SrTiO₃ ceramic nanoparticles were fabricated by doping SrTiO₃ with various contents (0.5, 1, 3, and 5%, in mass ratio) of Ag. Composite samples were prepared through a one-pot solvothermal method and sintering process. The temperature-dependent thermoelectric properties of these sample were measured from 300 K to 500 K. The maximum power factor (843.3 μ·W/m·K²) at 500 K, which is ∼3.96 times higher than that of the pristine SrTiO₃ ceramics, was obtained for the Ag–SrTiO₃ composite sample with 1% of Ag. In addition, the thermal conductivity of the composites decreased due to the phonon scattering effect. The maximum thermoelectric figure of merit (ZT), i.e., ∼0.09, which was achieved with 1% of Ag at 500 K, yielded an enhanced power factor and a reduced thermal conductivity. This ZT value was ∼4.27 times larger than that of pristine SrTiO₃ at the same temperature.     

Thermoelectric properties and figure of merit of perovskite-type Sr1−xLaxSnO3 ceramics

The thermoelectric properties of perovskite-type Sr_1−xLa_xSnO₃ ceramics with x=0.01–0.05 were evaluated from the Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ measured at high temperatures. The La-doped ceramics were n-type semiconductors exhibiting thermally activated electrical conduction behaviors in the temperature range of 473–1073 K. Eelectron carriers were introduced into the conduction band from doped La atoms up to x=0.03, which was the solubility limit of La at Sr site. The temperature dependence of the κ values for the ceramics was unaffected by both the La content and the microstructures. Estimations of the electronic thermal conductivities by the Wiedemann-Franz law revealed that the phonon thermal conductivities were dominant for all ceramics. The dimensionless figure of merit ZT increased with increasing temperature for all ceramics and reached 0.02–0.05 at 1073 K. In contrast to cubic Ba_1−xLa_xSnO₃ ceramics, bending of the Sn–O–Sn bonds due to octahedral tilting distortion in Sr_1−xLa_xSnO₃ lowered the electron mobility, decreasing the power factor S²σ and ZT values, although it effectively reduced the phonon mean free path, decreasing the κ values.     

Anisotropic properties of textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y₂O₃-5Al₂O₃ (mole ratio of 3:5) and 3Y₂O₃-5Al₂O₃-4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c-axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y₂O₃-5Al₂O₃-4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y₂O₃-5Al₂O₃ is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties.     

A prediction of the thermodynamic,thermophysical, and mechanical properties of CrTaO4 from first principles

It is reported that the self-forming CrTaO₄ oxide scale can protect refractory high-entropy alloys from oxidation, superior to Cr₂O₃. In this paper, the phase stability, mechanical, and thermal properties of three polymorphous phases of CrTaO₄ are systematically investigated from first-principles density functional theory calculations. The mechanical properties predicted using the strain–energy methods indicated that all three phases are mechanically stable. The temperature dependence of elastic constants and polycrystalline moduli of three phases demonstrated the thermal softening as temperature increase. The Helmholtz-free energies as a function of volume and temperature are derived from phonon dispersions within the quasi-harmonic approximation at six strained volumes. The calculated apparent bulk coefficients of thermal expansion of these three phases are evaluated, the highest value approximately 13.4× 10⁻⁶ K⁻¹ within a temperature range of 500–2000 K for the rutile I4₁md phase. The lattice thermal conductivity calculated by the Debye–Callaway model suggested that the rutile type I4₁md phase has the lowest value of approximately 2.1 W/m/K at 1800 K. The other two phases, C2/m and P2/c, exhibit higher values due to relatively lower Grüneisen parameters and larger phonon velocities. The melting point of CrTaO₄ is predicted to be between 1975 and 2449 K using ab initio molecular dynamics simulations. This work provides a comprehensive theoretical understanding of the thermodynamic, mechanical, and thermal properties for the new material CrTaO₄ and serves as an example of a viable computational design strategy for improved oxidation resistance of refractory alloys at high temperatures.     

Compositional tailoring effect on crystal structure,mechanical and thermal properties of γ-AlON transparent ceramics

In view of the practical application of γ-AlON as a promising transparent structural ceramic, in-depth insight into its mechanical and thermal properties is essential. The solid-state MAS NMR technique was combined with XRD Rietveld refinement to confirm the crystal structure of Al_(8+x)/3O_4-xN_x (x = 0.299–0.575). These structural parameters were further applied to predict hardness and elastic properties based on theoretical exploration, which are in good agreement with the experimental values. A slight enhancement of mechanical properties with increasing nitrogen concentration is attributed to the stronger chemical bond in octahedra. The experimental thermal conductivity of γ-AlON transparent ceramics was improved slightly with the rise of x in the temperature range from 298 K to 1074 K. The intrinsic lattice thermal conductivity was determined by eliminating the extrinsic phonon scattering as well as the thermal radiation. The reason for the discrepancy between experimental and intrinsic thermal conductivity was revealed. The present methods provided powerful and accessible guidelines in optimizing the mechanical and thermal properties of oxynitride materials.