M2YSi (M=Rh,Ir): Theoretically predicted damage‐tolerant MAX phase‐like layered silicides

Searching for layered MAX phase‐like materials with properties of both ceramics and metals is a topic in its infancy. Herein, through a combination of crystal structure, electronic structure, chemical bonding, and elastic property investigations, we report two MAX phase‐like layered materials Rh₂YSi and Ir₂YSi. Rh₂YSi and Ir₂YSi have bulk modulus B of 150 and 185 GPa, respectively, which are comparable to the typical MAX phases like Ti₂AlC, Ti₃AlC₂, and Ti₃SiC₂, but much lower shear modulus G (82 and 97 GPa for Rh₂YSi and Ir₂YSi, respectively) than MAX phases. The high stiffness is due to the presence of rigid Si2–M–Si3–M (M = Ir, Rh) units, while the low shear deformation resistance is due to the presence of metallic bonds and the weak bonds that link the rigid Si2–M–Si3–M (M = Ir, Rh) units. Based on the low shear deformation resistance and low Pugh's ratio, Rh₂YSi and Ir₂YSi are predicted as damage‐tolerant silicides and promising water vapor‐resistant interphase materials for SiC_f/SiC composites if yttria or yttrium silicates are formed to protect the SiC fibers in oxygen containing environments. The possible slip systems are {0001} <> and {} <0001> for both Rh₂YSi and Ir₂YSi.     

Synchrotron x-ray spectroscopy investigation of the Ca1−xLnxZrTi2−x(Al,Fe)xO7 zirconolite ceramics (Ln = La,Nd, Gd,Ho, Yb)

When incorporating actinides into zirconolite for high-level radioactive waste immobilization, Al³⁺ and Fe³⁺ ions generally act as charge compensators. In this study, we rationally designed a series of (Ln = La, Nd, Gd, Ho, Yb) to unravel the dopant solubility and evolutions of the crystalline phase and local environment of cations through synchrotron X-ray methods. It was found that single zirconolite phase is difficult to obtain and the fraction of perovskite have an increase with x from 0.1 to 0.9 in . Formation of both zirconolite-2M and zirconolite-3O phases was observed in and . Phase transformation from zirconolite-2M to 3O occurs at x = 0.7 for while x = 0.9 for . The solubility of and to form single zirconolite-2M can reach to 0.9 f.u. and 0.7 f.u., respectively. The evolution of lattice parameters of zirconolite in is greatly related to the ionic radii of cations and substitution mechanism among the cations. X-ray absorption near edge spectroscopy revealed that Fe³⁺ ions replace both five- and six-coordinated Ti sites and the ratio of TiO₅ to TiO₆ decreases when increasing dopant concentration in the . For the local environment of Zr⁴⁺, the major form is ZrO₇ with a trace of ZrO₈.     

Mechanical characterization of boron carbide single crystals

Out-of-plane anisotropy in the mechanical response of boron carbide was studied by performing nanoindentation experiments on four specific crystallographic orientations of single crystals, that is, , , , and . For each orientation of the single crystals, in-plane variations of indentation modulus and hardness were also studied by monitoring the relative rotation between the crystal surface and a Berkovich indenter tip. A significant out-of-plane anisotropy in indentation modulus was observed with ~80 GPa difference between the highest and lowest values. A smaller but measurable out-of-plane anisotropy in indentation hardness was also observed. In-plane anisotropy, on the other hand, was found to be significantly influenced by the scatter in the data and geometrical imperfections of the indenter tip. Investigations of indentation pop-in events suggested that deformation is entirely elastic prior to the first pop-in. Furthermore, quasi-plastic flow along the orientation of the single crystals was found to be more homogeneous than the other tested orientations. For select indents, cross-sectional transmission electron microscopy (TEM) of the indented regions showed formation of a quasi-plastic zone in the form of lattice rotation and various microstructural defects. The quasi-plastic zone grew in size with increasing the indentation depth. The TEM observations also suggested the crystal slip to be a potential mechanism of quasi-plasticity and a precursor for formation of amorphous bands that could eventually lead to cracking and fragmentation. The proposed failure mechanism provides valuable insights for calibrating constitutive computational models of failure in boron carbide.     

Ni-YSZ(001) solid-solid interfacial energy and orientation relationships

Equilibrated Ni particles were produced via solid-state dewetting of continuous Ni films deposited on the (001) surface of yttrium stabilized zirconia (YSZ). The solid-solid interface energy of the equilibrated Ni(111)-YSZ(001) interface was determined using Winterbottom analysis. Two low-index orientation relationships (ORs) were found using the selected area electron diffraction patterns in transmission electron microscopy: (OR₁) and (OR₂). However, many particles were found to deviate from these low-index ORs while maintaining the out-of-plane orientation, . The interface energy was measured to be 2.5 ± 0.1 J/m² regardless of the in-plane orientation. The orientation distribution was determined using electron backscattered diffraction for Ni particles on both (111) and (001) YSZ substrates and was found to correlate well with the interface energies measured in a previous study for the Ni-YSZ(111) interface and in the present study for the Ni-YSZ(001) interface.     

Direct Evidence for the Modulation Caused by Ti Substitution of Ta in a (Ta2O5)0.92(TiO2)0.08 Ceramic by Analytical Electron Microscopy

The high‐temperature tetragonal structure of Ta₂O₅ was observed in a Ta₂O₅ ceramic sample doped with 8 at.% TiO₂. Direct evidences for the modulation along [] direction caused by Ti replacing Ta are obtained based on atomic‐resolution high‐angle annular dark‐field lattice imaging and electron energy loss spectroscopy analysis. The atomic mechanism of Ti substitution of Ta is explicitly described based on the structure model along [110] direction projection.     

Sintering-driven effects on the band gap of (Pb,La)(Ti,Ni)O3 photovoltaic ceramics

In this work, the influence of the sintering temperature on the physical properties of (Pb_0.8La_0.2)(Ti_0.9Ni_0.1)O₃ (PLT-Ni) ceramics is reported. The experimental data revealed that the energy band gap of PLT-Ni ceramics could be tailored from approximately 2.7 to 2.0 eV by changing the sintering temperature from 1100°C to 1250°C. It is demonstrated that the simple substitution of Ti⁴⁺ by Ni²⁺ cations is effective to decrease the intrinsic band gap while increasing the tetragonality factor and the spontaneous polarization. However, the additional red-shift observed in the absorption edge of the PLT-Ni with increasing the sintering temperature was associated with a continuous increase in the oxygen vacancies () amount. It is believed that the impact of the creation of these thermally induced is manifold. The presence of and Ni²⁺ ions generate the Ni²⁺- defect-pairs that promoted both a decrease in the intrinsic band gap and an additional increase of the tetragonality factor, consequently, increasing the spontaneous polarization. The creation of Ni²⁺- defects also changed the local symmetry of Ni²⁺ ions from octahedral to a square pyramid, thus lifting the degeneracy of the Ni²⁺ 3d orbitals. With the increase in the sintering temperature, lower-energy absorbing intraband states were also formed due to an excess of , being responsible for an add-on shoulder in the absorption edge, extending the light absorption curve to longer wavelengths and leading to an additional absorption in “all investigated” spectrum as well.     

Single‐Crystal Elastic Properties of Aluminum Oxynitride (AlON) from Brillouin Scattering

The Brillouin light scattering technique was used to determine experimentally the three independent elastic constants of cubic aluminum oxynitride at the ambient condition. They are , and . Its bulk modulus is 221.2 GPa. The magnitude of Zener anisotropic ratio is 2.1 similar to other spinels. The anisotropic nature of the material is shown by a large variation in the Young's modulus and Poisson's ratio with crystallographic directions. The material was found to be auxetic in certain orientations.     

Fabrication and characterization of tetragonal yttria-stabilized zirconia single-crystalline thin film

Tetragonal yttria-stabilized zirconia thin film was successfully fabricated by a pulsed laser deposition method. The thin film grew heteroepitaxially with the orientation relationship of ZrO₂‖Al₂O₃. Energy dispersive X-ray spectroscopy mapping revealed that Y³⁺ ions were distributed homogeneously without local segregations. X-ray and electron-diffraction analysis confirmed a single crystalline structural feature of the film. On the other hand, high-resolution scanning transmission electron microscopy observations show that this film contains small-angle tilt grain boundaries, which is composed of the periodic array of dislocations with the Burgers vector .     

The dependence of lattice thermal conductivity on phonon modes in pyrochlore-related Ln2Sn2O7 (Ln = La,Gd)

Rare-earth pyrochlore materials are promising thermal barrier coatings materials and fundamental understanding of their thermal transport is crucial for further improving its performance. In this work, using density functional theory (DFT) method, we calculated the intrinsic lattice thermal conductivities of Ln₂Sn₂O₇ (Ln = La, Gd) and conducted a comprehensive analysis on the mode thermal conductivity, relaxation time, Grüneisen parameters, group velocity, and specific heat, respectively. It is shown that in pyrochlore-type materials the number of the optical phonons is much larger than that of the acoustic phonon, and the thermal conductivity of acoustic phonons are suppressed, both of which increase the contribution ratio of optical phonons. Especially, through cumulative analysis, we found that the contribution of optical phonons is significant: the ratio of optical contribution is more than 50% and 64% in La₂Sn₂O₇ and Gd₂Sn₂O₇. This work provides a comprehensive picture illustrating the significant role of the optical phonons in the lattice thermal conduction in rare-earth pyrochlore materials, and points out an avenue to obtain low thermal conductivity in complex structural thermal insulation materials.     

Development of magnetically switchable high permittivity microwave dielectrics using La(Al1-xFex)O3

The introduction of transition metal doping, particularly Fe³⁺, into high-performance microwave dielectrics can make “smart” materials that switch between a high-Q, low loss state and a low-Q, high loss state using a small external magnetic field. In this study, the dielectric and magnetic properties of the high permittivity host material LaAlO₃ (ε_r = 22.5), when doped with Fe³⁺, are reported. Spin losses dominate the loss tangent at cryogenic temperatures and survive up to room temperature. Peaks in the loss tangent versus temperature relation are observed near 40, 75, and 215 K. Additional measurements of samples exposed to annealing in varying environments, combined with Debye analysis and the results of native defect energy predictions from density functional calculations[Phys Rev B. 2009;80:104115], allows us to associate the 40, 75, and 215 K peaks to the following reactions, , , and , respectively.     

Atomic structures of Ti-doped α-Al2O3 Σ13 grain boundary with a small amount of Si impurity

Doping is a fundamental technique to control sintering of α-Al₂O₃, and many types of elements have been used to control grain growth and material properties. Such dopants tend to be concentrated at the grain boundaries and modify grain boundary properties. It is therefore important to investigate the variation of grain boundary atomic structures induced by additives. However, in the sintering process it is difficult to prevent small amounts of unintentional impurity (such as Si), and the potential influence of additives and impurities on the grain boundary structure should be taken into account. Here we show that two types of grain boundary atomic structures are formed in a Ti-doped Σ13 []/() α-Al₂O₃, which has been determined by atomic-resolution scanning transmission electron microscopy. Combining with atomic-scale spectroscopic analyses, we elucidate that the local concentration of Si impurities significantly alters the grain boundary atomic structures and the valence state of Ti additives. The present results suggest that the subtle change in concentrations of both additives and impurity should have a strong impact on sintering processes and resultant properties in α-Al₂O₃.     

Combined electromechanical dynamic fracture behavior of lead zirconate titanate (PZT)

Coupon specimens of poled and depoled lead zirconate titanate (PZT) are examined under combined stress wave and electric loading conditions. Mode-I crack initiation and fracture behavior is examined using ultrahigh-speed imaging and two-dimensional digital image correlation. The dynamic critical stress intensity factor () is extracted using measured displacement fields ahead of the impulsively loaded crack tip, and compared between poled and depoled plates that were either under no electric field, positive 0.46 kV/mm electric field, or negative 0.46 kV/mm electric field. Poled specimens had a poling direction and applied electric field direction normal to the crack front. The addition of an electric field resulted in a crack-enhancing effect, where the dynamic fracture toughness of poled specimens under 0.46 kV/mm was almost half that of samples with no electric field. Depoled samples experienced almost no change in dynamic fracture toughness with the addition of an electric field.     

Effect of pressurization on the fracture toughness of borosilicate glasses

In this study, hot-compression is applied to two multicomponent borosilicate glasses, Borofloat33 (Boro33) and N-BK7, using molecular dynamics simulations. The effects of pressure on elastic properties, surface energy, and fracture toughness ( are investigated. It is found that the impact on is mainly dominated by the change of Young's modulus under pressure, which is proportional to the relative change in density. Between the two glasses under investigation, can be improved more effectively by the hot-compression process for Boro33, due to its higher concentration of 3-coordinated boron (B³), which facilitates densification via B³ to B⁴ conversion under compression.     

Influence of water activity on belite (β-C2S) hydration

The hydration of the two most reactive phases of ordinary Portland cement (OPC), tricalcium silicate (C₃S), and tricalcium aluminate (C₃A) is successfully halted when the activity of water () falls below critical thresholds of 0.70 and 0.45, respectively. It has been established that the reduction in relative humidity (RH) and  suppresses the hydration of all anhydrous phases in OPC, including less explored phases like dicalcium silicate, that is, belite (β-C₂S). However, the degree of suppression, that is, the critical threshold, for β-C₂S, standalone has yet to be established. This study utilizes isothermal microcalorimetry and X-ray diffraction techniques to elucidate the influence of on the hydration of -C₂S suspensions via incremental replacements of water with isopropanol (IPA). Experimentally, this study shows that with increasing IPA replacements, hydration is increasingly suppressed until eventually brought to a halt at a critical threshold of approximately 27.7% IPA on a weight basis (wt.%_IPA). From thermodynamic estimations, the exact critical threshold and solubility product constant of -C₂S () are established as 0.913 and 10^−12.68, respectively. This study enables enhanced understanding of β-C₂S reactivity and provides thermodynamic parameters during the hydration of β-C₂S-containing cementitious systems such as OPC-based and calcium aluminate-based systems.     

Crystal chemistry and ion-irradiation resistance of Ln2ZrO5 compounds with Ln = Sm,Eu, Gd,and Tb

The previously unattained fabrication of single phase Ln₂ZrO₅ (Ln = Sm, Eu, Gd, and Tb) compounds via relatively low sintering temperature (1400°C) is achieved in this study using a coprecipitation method. The crystal structures have been investigated by neutron, synchrotron X-ray powder diffraction, and electron diffraction techniques. While the general long-range structure may be well described by the defect-fluorite type structure with Fmm symmetry, electron diffraction has highlighted a complex underlying modulated structure that varies between each compound. These compounds have been tested for ion-irradiation response using in situ 1 MeV krypton ions and transmission electron microscopy characterization. None of the compounds undergo a crystalline to amorphous transition, even holding at 50 K. Both the underlying fluorite and modulated superstructures are little affected by the irradiation. However, some atomic rearrangements are observed in the postirradiated electron diffraction patterns for the Sm₂ZrO₅ specimen.     

Crystal structure,phonon modes,and bond characteristics of AgPb2B2V3O12 (B = Mg,Zn) microwave ceramics

AgPb₂B₂V₃O₁₂ (B = Mg, Zn) ceramics with low sintering temperature were synthesized via the conventional solid-state reaction route. Rietveld refinements of the X-ray diffraction patterns confirm cubic symmetry with space group . The number of observed vibrational modes and those predicted by group theoretical calculations also confirm the space group. At the optimum sintering temperature of 750°C/4 hours, AgPb₂Mg₂V₃O₁₂ has a relative permittivity of 23.3 ± 0.2, unloaded quality factor () of 26 900 ± 500 GHz (), and temperature coefficient of resonant frequency of 19.3 ± 1 ppm/°C, while AgPb₂Zn₂V₃O₁₂ has the corresponding values of 26.4 ± 0.2, 28 400 ± 500 GHz () and –18.4 ± 1 ppm/°C at 590°C/4 hours. Microwave dielectric properties of a few reported garnets and Pb₂AgB₂V₃O₁₂ (B = Mg, Zn) ceramics were correlated with their intrinsic characteristics such as the Raman shifts as well as width of A_1g Raman bands. Higher quality factor was obtained for lower full width at half-maxima (FWHMs) values of A_1g modes. The increase in B-site bond valence contributes to high and low |τ_f| with the substitution of Zn²⁺ by Mg²⁺. Furthermore, the high ionic polarizability and unit cell volume with Zn²⁺substitution contribute to increased relative permittivity.     

(W1-x,Mx)C carbides with desired combinations of compatible density and properties – A first-principles study

Tungsten monocarbide (WC) is one of the highly valuable hard materials for industry, widely used as reinforcement in hardfacing overlays, thermal spray coatings, composites, and various alloys. However, its large density leads to the inhomogeneous distribution of WC particles in the metal-matrix hardfacing overlays. It is highly wished to have appropriate reinforcing phases with an optimal combination of high strength, compatible density, and physical properties. In this study, we tailored WC by partially substituting W with 3d and 4d transition metals through first-principles calculations. It is demonstrated that WC can be tailored by element-substitution with desired properties. Identified stable carbides possess lowered density and mechanical properties that are comparable to those of WC. Physical properties, for example, the Debye temperature, Grüneisen parameter, and thermal conductivity, of the tailored carbides are also studied for widened applications. Efforts are made to generate comprehensive information on metal-substituted with elucidated underlying mechanisms through analyzing the corresponding electronic characteristics.     

Diffused and successive phase transitions of (K,Na)NbO3-based ceramics with high strain and temperature insensitivity

Phase boundaries realize enhanced piezoelectricity in lead-free (K, Na)NbO₃-based ceramics but suffer from the weakness of undesirable temperature sensitivity. Here, an effective method is designed to develop temperature-insensitive piezoelectricity (small signal piezo-coefficient [d₃₃] and large signal piezo-coefficient []) in KNN-based piezoceramics by constructing the diffused and successive phase transitions, which results in a broadness of the optimal temperature range of the electrical properties. The room-temperature value in KNN-based ceramics modified with BaZrO₃ and (Bi_0.5Na_0.5)HfO₃ reaches as high as 540 (±10) pm V⁻¹, which is higher than PZT-5H and most reported KNN-based systems. Notably, superior temperature insensitivity of the and P_r values is also observed among the diffused and successive phase transitions region (20-100°C), with <5% fluctuation. In addition, the in situ temperature-dependent d₃₃ measurement shows a high-temperature reliability and less fluctuation (<15%) in a wide temperature range (20-120°C). These results open a new window for further development of highly temperature-insensitive lead-free piezoceramics.     

Microwave dielectric properties and thermally stimulated depolarization of Al-doped Ba4(Sm,Nd)9.33Ti18O54 ceramics

Ba₄(Sm_0.15Nd_0.85)_9.33Ti_18-zAl_3z/4O₅₄ (BSNT-zAl, 0.0 ≤ z ≤ 2.5) ceramics were prepared via a solid-state reaction, and the effects of Al doping on the microwave dielectric properties and defect behavior of the title compound were studied. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) photographs suggested that Al ions successfully entered the lattice to form tungsten-bronze-like solid solutions. With a small amount of Al substitution, the relative dielectric constant (ε_r), and the temperature coefficient of resonant frequency (τ_f) values decreased, whereas the quality factor (Q × f) substantially increased by approximately 50%. The defect-related extrinsic dielectric loss was clarified via the thermally stimulated depolarization current (TSDC) technique. With Al doping, the TSDC relaxation of across-grain-boundary oxygen vacancies () vanished, whereas that of defect dipoles () appeared at relatively low temperatures. Therefore, in the BSNT-zAl ceramics, oxygen vacancies were more inclined to interconnect with to form defect dipoles. This could reduce the activity of and account for the notable improvement in the Q × f values. In particular, the excellent characteristics of ε_r = 67.33, Q × f = 16 530 GHz, and τ_f = +0.87 ppm/°C were achieved in the specimens with z = 1.5 sintered at 1350°C for 4 hours.     

Role of oxygen-vacancy in piezoelectric properties and fatigue behavior of (Bi0.5Na0.5)0.93Ba0.07Ti1+xO3 ceramics

In this reported study, (Bi_0.5Na_0.5)_0.93Ba_0.07Ti_1+xO₃ (abbreviated as BNBT_1+x) ceramics, containing Ti-nonstoichiometry that ranged from a 2% deficiency to a 1% excess, were designed and systematically characterized. The results of the X-ray diffraction Rietveld refinement and X-ray photoelectron spectroscopy analysis of these materials revealed that the amount of Ti in the BNBT_1+x ceramics significantly affected the degree of coexistent rhombohedral/tetragonal phases and also affected the content of singly/doubly charged oxygen-vacancy (/) in the ceramics. After poling and 10⁵ fatigue cycles, the variation in Raman resonance line-width of the Ti–O bond in the BNBT_1+x ceramics was found to be strongly dependent on the amount of Ti in the ceramic. The → transformation and clustering of the defects under electrical loading were considered to be a critical factor in electric field-induced structural transition and fatigue properties of the material.