Narrow Results

Ferroelectric ceramics, such as barium titanate, have garnered significant interest due to their unique electrical and mechanical properties. In particular, their ability to undergo significant deformation under an applied electric field aids in their utilization in many applications, including actuators and sensors. The deformation behavior of ferroelectric ceramics is complex and is influenced by various factors, such as crystal structure, defect density, and processing conditions. This study focuses on the mechanics of ferroelectric ceramics and seeks to offer a thorough comprehension of the barium titanate’s deformation behavior. The study begins by discussing the crystal structure of barium titanate and how it influences the ferroelectric behavior of the material. It then delves into the various mechanisms of deformation, including domain wall motion, phase transformation, and twinning. The study also discusses the effects of temperature, electric field strength, and microstructure on the deformation behavior of barium titanate. Furthermore, the study explores the relationship between the deformation behavior and the mechanical characteristics of barium titanate, including Poisson’s ratio and Young’s modulus. Finally, the study concludes with a discussion of the potential applications of ferroelectric ceramics and the need for further research in this area. Overall, this study provides a comprehensive understanding of the deformation behavior of barium titanate showcasing distinct influences of grain size, texture, and anisotropy. Notably, varying grain sizes significantly impact deformation behavior. For instance, smaller grain sizes ($<10\mu$m) exhibit superior deformation characteristics, correlating with higher permittivity values (2731–5801) compared to larger grain counterparts (18.4$\mu$m). Additionally, transition temperatures ($T_{\mathrm{\text{O}}–\mathrm{\text{T}}}$) for different grain sizes (18.0–$30.1^{\circ }$C for smaller grains, 21.5–$30.6^{\circ }$C for larger grains) underscore the impact of phase transitions on grain size. These results underscore the paramount importance of grain size, texture, and anisotropy in governing the mechanical traits of barium titanate, emphasizing their consideration during fabrication and processing for optimal performance in diverse applications.

Dry sliding wear tests of hot-pressure sintered and wrought cobalt were carried out to compare their wear characteristics. Cobalt powders with average size of 1.5µm were electro-pressure sintered to make sintered-cobalt disk wear specimens. A vacuum-induction melted cobalt ingot was hot-rolled at 800°C to a plate, from which wrought-cobalt disk specimens were machined. The specimens were heat treated at various temperatures to vary grain size and phase fraction. Wear tests of the cobalt specimens were carried out using a pin-on-disk wear tester against a glass (83% SiO₂) bead at 100N with the constant sliding speed and distance of 0.36m/s and 600m, respectively. Worn surfaces, their cross sections, and wear debris were examined by an SEM. The wear of the cobalt was found to be strongly influenced by the strain-induced phase transformation of ε-Co (hcp) to α-Co (fcc). The sintered cobalt had smaller uniform grain size and showed higher wear rate than the wrought cobalt. The higher wear rate of the sintered cobalt was explained by the more active deformation-induced phase transformation than in the wrought cobalt with larger irregular grains.

The purpose of the study was to comparatively investigate two NiTi orthodontic wires. It is valuable to determine the phase transformation temperature and corrosion characteristics of the orthodontic wires to further study the shape memory effect and corrosion resistance properties. Optical microscope and EDX analysis were used for microstructure characteristics and composition analysis. Differential scanning calorimetry (DSC) was carried out to identify the phase transformation behavior of the two wires. Electrochemical tests in artificial saliva at 37 ±1^°C including polarization and electrochemical impedance spectroscopy (EIS) were used to assess the corrosion resistance and corrosion mechanism of the wires. It was found that the transformation temperature range of A-wire (imported) is narrower while the A_s and A_f are close to the body temperature, which is more suitable in the orthodontic operation at early stage. The corrosion current density of A-wire is lower than that of B-wire (domestically made) while the corrosion potential is higher. EIS test results indicated that the corrosion mechanism was the same. However, the oxide layer formed on the surface of A-wire is more protective.

Phase transformation, microstructure and mechanical property of Ni_50-x/2Ti_50-x/2Al_x (x=1, 2, 3 and 4 at.%) alloys have been investigated by electrical resistance (ER) measurement, scanning electron microscope (SEM), X ray diffraction (XRD) and tensile tests. The results show that a two-step phase transformation occurs from parent phase to R phase and subsequently to martensite upon cooling, and the transformation reverses upon heating. Both the martensitic transformation and R phase transformation temperatures decrease with increasing of Al content. The microstructure analysis indicates that the NiTi-Al alloys are composed of NiTi as the predominant phase and Ti₂Ni as the second phase mainly distributing at grain boundaries. The solid solution hardening effect of Al at room temperature has been demonstrated by the remarkable increasing of yielding strength and decreasing of tensile strain with increasing of Al content. Ni₄₉Ti₄₉Al₂ alloy deformed at the liquid temperature exhibits a maximum recoverable strain of 6.3%.

In the present work, a series of samples were prepared by pressureless sintering from the starting materials of Si₃N₄ (α and/or β phases), SiO₂ and Li₂CO₃. The phase transformation was studied with emphasis on the influence of sintering temperature and Li₂CO₃ content. Related phase transformation was observed and analyzed by SEM and XRD and probable mechanisms were given, which will be helpful for the further explanation to the oxidation mechanism of Si₃N₄ based ceramics at elevated temperature.

Two $\beta$-Sialons, with $z$-values of 1 and 4, respectively, were successfully synthesized by silicothermal reduction and nitridation method under 0.4 MPa nitrogen pressure. The effect of firing temperatures on the phase transformations and morphologies of $\beta$-Sialons were analyzed by XRD and SEM. For $\beta$-Sialons ($z=1$), the product was finally composed of targeted $\beta$-Sialon ($z=1$) and secondary phase $\alpha$-Si₃N₄; for $z=4$, $\beta$-Sialon ($z=4$) was the main phase, and 15R-Sialon and $\alpha$-Al₂O₃ co-existed as secondary phases. A higher firing temperature is more beneficial for the phase transformations and crystal growth of $\beta$-Sialons, however, the most suitable firing temperature was 1400${}^{\circ }$C.

The melting temperatures of alkali halides (LiCl, LiF, NaBr, NaCl, NaF, NaI, KBr, KCl, KF, KI, RbBr, RbCl, RbI and CsI) have been evaluated over a wide range of pressures. The solid–liquid transition of alkali halides is of considerable significance due to their huge industrial applications. Our formalism requires a priori knowledge of the bulk modulus and the Grüneisen parameter at ambient conditions to compute $T_m$ at high pressures. The computed values are in very good agreement with the available experimental results. The formalism can satisfactorily be used to compute $T_m$ at high pressures where the experimental data are scanty. Most of the melting curves ($T_m$ versus $P$) exhibit nonlinear variation with increasing pressure having curvatures downward and exhibit a maximum in some cases like NaCl, RbBr, RbCl and RbI. The values of $T_m^{max}$ and $P_{max}$ corresponding to the maxima of the curves are given.

In this paper, we give a microscopic view concerning influence of the growth conditions on the physical properties of nanocrystals (NCs) thin films made of CdS, prepared using chemical bath deposition CBD technique. We show a crystalline phase transformation of CdS NCs from hexagonal wurtzite (W) structure to cubic zincblende (ZB) when the growth conditions change, particularly the solution pH values. This effect was confirmed using X-ray diffraction (XRD), transmission electron microscopy (TEM), optical absorption and photoluminescence (PL) measurements. The optical absorption spectra allow calculation of the bandgap value, $E_g$, where significant increase $\sim$200 meV in the CdS bandgap when transforming from Hexagonal to Cubic phase was found.

The temperature-dependent structure and dielectric properties of Bi${}_{0.95}$Ba${}_{0.05}$Fe${}_{0.95}$ Ti${}_{0.05}$O₃ ceramic are studied systematically. At the temperature range of 25–800${}^{\circ }$C, the cell parameters are increasing, this indicates its distorted rhombohedral $R$3$c$ structure. A structural phase transformation from rhombohedral $R$3$c$ to cubic Pm3$m$ is observed at 850${}^{\circ }$C. The thermal expansion coefficients in the temperature range 25–700${}^{\circ }$C are calculated. Meanwhile, the temperature-dependent dielectric constant (${𝜀}^{\prime }$) and dielectric loss ($tan\delta$) at different frequencies are measured. This work attempts to show the thermodynamic stability and the effect of charge defects on the electric properties for doped BiFeO₃ ceramics.

A realistic interaction potential model (RIPM) has been formulated to theoretically predict the pressure-induced phase transition, elastic properties and thermophysical properties of AlAs and AlSb, including temperature effect (300 K). This model exhibits a better agreement with the available experimental rather than theoretical data for obtained calculations of phase transition pressures and volume collapses. We have achieved elastic moduli, anisotropy factor, Poisson’s ratio, Kleinman parameter, on the basis of the calculated elastic constants. Apparently, this is the first time when thermophysical properties of these compounds are explored at temperature effect by using a single model. Our results are justified by available measured and other reported data which support the validity of our model.

This paper’s aim is to describe the results of the phase transformation of BCuAl₉Fe₄ alloy after casting, quenching and aging. After casting, the microstructure of this alloy consists of $\alpha$ phase with grain size about 100 $\mu$m, mixture $(\alpha +\gamma )$ and the inter-metallic phase Fe₃Al. However, the proportion of the $\alpha$ phase in the casting alloy is coarse. The alloy was heated at $850^{\circ }$C for 2 h then quickly cooling in water. After quenching, the microstructure of alloy shows that the grain size reduced to about 40 $\mu$m. After quenching, the alloy was aged at $350^{\circ }$C for 2 h, the martensite phase of this alloy decomposed into order phase ($\alpha +{\gamma }_2)$ with fine grain size, dispersed in the matrix. The intermetallic phase was fine and evenly dispersed in the matrix. By TEM analysis, after heat treatment, the structure of martensite and the inter-metallic phase in this alloy which had small grain size were formed.

Lead-free piezoceramics ($1-x$)Bi${}_{0.5}$(Na${}_{0.80}$K${}_{0.20}{)}_{0.5}$TiO₃–xLiNbO₃ (BNKT–LN) (where $x=0$, 0.025, 0.050 and 0.075mol fraction) were examined for their ferroelectric stability and subsequent modifications. Traditional solid-state reaction techniques were used to create the BNKT–LN piezoceramics, which were then sintered at a temperature of 1200°C. We concentrated on the structural, dielectric, piezoelectric and ferroelectric properties of the BNKT–LN ceramics. The powder X-ray diffraction pattern revealed a single perovskite structure with no secondary phases, as was observed. The temperature-dependent dielectric curves demonstrated that the maximum dielectric constant ($\epsilon$) is present in the pure BNKT sample and diffuses to a low value when the LN concentration rises. SEM confirms the material’s surface shape, and decreasing grain sizes are seen as the LN content rises. The BNKT–LN ceramics’ P–E hysteresis loops demonstrate good ferroelectric characteristics, with a maximum at 20 kV/cm. With an increase in LN concentration, remnant polarization ($P_r$) and coercive field ($E_c$) show a trend of increasing, reducing and then increasing. The maximum value of the piezoelectric coefficient ($d_{33}$) for the combined samples was 147pC/N. As a result, the lead-free ceramic BNKT (BNKT–LN) doped with LN is an auspicious choice for piezoelectric sensor applications.

Lead-zirconate-titanate PbZr_1-xTi_xO₃ has superb ferroelectric properties in x~0.48 morphotropic phase boundary phase. In bulk form it has a P4mm tetragonal crystal structure of lattice parameters a=0.4036 nm and c=0.4146 nm at room temperature or above, and a Cm monoclinic structure of a=0.5722 nm, b=0.5710 nm, c=0.4137 nm and β=90.50° at lower temperatures. A new polymorph lies in an orthorhombic structure of a=0.4038 nm, b=0.4017 nm, and c=0.4148 nm in small particles of size such as 20 nm. It has an enhanced value of density of 8.033 g/cm³ relative to 8.006 g/cm³ in the tetragonal or 8.000 g/cm³ in the monoclinic bulk structure in a rather incompressible and hard material. The result is analyzed with X-ray diffraction of the sample of nanoparticles.

A liquid fuel high velocity oxy-fuel (HVOF) thermal spray process has been used to deposit TiO₂ photocatalytic coatings utilizing a commercially available anatase/rutile nano-powder as the feedstock. The coatings were characterized in terms of the phases present, its crystallite size and coating morphology by means of X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy, respectively. The results indicate that the sprayed TiO₂ coatings were composed of both TiO₂ phases, namely anatase and rutile with different phase content and crystallite size. A high anatase content of 80% by volume was achieved at 0.00015 fuel to oxygen ratio with nanostructure coating by grain size smaller than feedstock powder.

It is found that fuel to oxygen ratio strongly influenced on temperature and velocity of particles in stream jet consequently on phase transformation of anatase to rutile and their crystallite size and by optimizing the ratio which can promote structural transformation and grain coarsening in coating.

A Ni${}_{49.8}$Ti${}_{34.4}$Hf${}_{15.8}$ film was synthesized through magnetron sputtering followed by crystallization and thermal cycling. Microstructure evolution and phase transformation were investigated through transmission electron microscopy and differential scanning calorimetry. Crystallization can be partially completed when heating temperature was increased to 540${}^{\circ }$C, which is higher than the usually thought crystallization temperature of 503.6${}^{\circ }$C. After crystallization occurs, the film consists of large Ni–Ti–Hf grains and small granular particles of (Ti,Hf)₂Ni precipitates dispersed in an amorphous pattern. As the number of thermal cycles increases, crystallization is completed gradually and martensite transformation temperature decreases gradually.

Fe-doped TiO₂ crystals were successfully prepared using a sol–gel technique in reducing and oxidizing atmospheres. The effects of sintering atmosphere on phase transformation, oxygen vacancy concentration and photoabsorption behaviors were investigated. The results indicate that upon sintering in reducing atmosphere, Ti and Fe ion valences were decreased and highly Fe ions (12 mol%) were entirely dissolved into titania crystals, increasing oxygen vacancy concentration and leading to increased photoabsorption capability. In contrast, sintering in oxidizing atmosphere causes precipitation of the Fe₂O₃ phase, which is detrimental to the photoabsorption capability. The best photoabsorption performance is obtained by sintering 12 mol% Fe-doped TiO₂ in reducing atmosphere, resulting in an absorption edge of approximately 435 nm, which is much higher than that of undoped TiO₂ in the oxidizing atmospheres with the absorption edge 352 nm.

High-entropy alloys (HEAs) have shown considerable promise from both a scientific and an application perspective due to their outstanding comprehensive properties. In this study, an equiatomic FeCoCrNi HEA is used as input material for laser cladding on Ti-6Al-4V alloy. The HEA coating is characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) to investigate the bonding region, element distribution and microstructure evolution. The results show that the HEA coating is mainly composed of face-centered cubic (FCC) phase and body-centered cubic (BCC) phase, precipitating a small amount of (Fe, Cr)-rich phase and (Ni, Ti)-rich phase. Otherwise, the bonding region, which is between coating and substrate, is emphatically concerned in this paper. The bonding region is formed by the convection zone which is resulted from the density difference of HEA and TC4. In addition, the convection in molten pool plays a key role in the morphology of bonding region.

The pressure-driven phase transformation in polycrystalline BaTi${}_{1-x}$Fe_xO₃ ($x=1$%), synthesized by a solid-state reaction method, was investigated using synchrotron X-ray diffraction and Raman spectroscopy. All measurements were performed at room temperature within a large pressure range up to 36.5 GPa. The results clearly demonstrate a phase transformation from the ferroelectric tetragonal structure of P4mm symmetry to the paraelectric cubic structure of Pm-3m symmetry in the studied sample at the pressure of about 18 GPa. This phase transformation pressure is much higher than those previously reported for BaTiO₃-based systems, indicating the stabilization of the ferroelectric phase by doping 1% Fe into BaTiO₃. Feature modes of the tetragonal phase still persist while some new modes appear in the Raman spectra at pressures above 18 GPa. This is probably due to accumulated stress and/or local microscopic disorders in the sample, which are associated with the displacement of the Ti atoms along unit-cell diagonals.

In this paper, some methodology in nonlinear dynamics is used to study a boundary-value problem of a nonlinear model arisen in phase transitions in a slender cylinder composed of a compressible hyperelastic material. We transform the original system of boundary-value problem to an initial-value (dynamical) problem of finding periodic solutions of coupled nonlinear autonomous oscillators in a four-dimensional space. Hopf-like bifurcation analysis of the periodic solutions of the system is studied. Both analytical and numerical solutions are obtained by using a nonlinear transformation formulation. The accuracy of analytical solutions is investigated by comparing with the numerical solutions. The engineering stress-strain curve is plotted and compared with that from the normal form equation, which is a simplification of the original system.

We develop a computational model for the martensitic first-order structural phase transformation in a single crystal thin film, and we use this model to study the effect of spatial compositional fluctuation, spatial temporal noise, and the loss of stability of the metastable phase at temperatures sufficiently far from the transformation temperature.