Mechanism of significantly enhanced piezoelectric performance and stability in textured potassium-sodium niobate piezoelectric ceramics

In this study, the piezoelectric coefficient d₃₃ and planar electromechanical coupling coefficient k_p were enhanced 145% and 71%, respectively for the <001>-textured (K_0.5Na_0.5)_0.95Li_0.05Nb_0.93Sb_0.07O₃ piezoelectric ceramics compared with their randomly oriented counterpart. Significantly enhanced piezoelectric response in textured ceramics is originated from a combined effect of the intrinsic high piezoelectric activity of <001>-oriented grains in the tetragonal-orthorhombic phases, and easy polarization rotation of fine domains. Furthermore, a comparative analysis suggests that <001>-textured ceramics exhibit good thermal stability, benefiting from the weakened depolarization behavior via crystal orientation. The superior fatigue resistance in textured ceramics can be attributed to the reduced clamping effect as low defect density. These results show that high-performance textured ceramics reported in this work will be promising candidates in the field of lead-free piezoelectric materials.     

Selective growth and texturing of VO2(B) thin films for high-temperature microbolometers

Vanadium oxides exhibit a broad spectrum of physical properties due to their ability to form various compounds and polymorphs. To utilise a particular property, it is essential to selectively synthesise a desired phase. Herein, we demonstrate a method to selectively and reproducibly grow (00l)-textured VO₂(B) thin films using an amorphous SrTiO₃ buffer layer by sputtering at <350 °C, which enables their direct integration with read-out-integrated-circuits (ROICs), glass, and polymer substrates. The VO₂(B) films exhibit high temperature-coefficient-of-resistances (TCRs) (>−3.5%/K at 25 °C and >−1.5%/K at 95 °C) and low electrical resistivities (∼5 × 10⁻¹ Ω cm at 25 °C and <1 × 10⁻¹ Ω cm at 95 °C), which are favourable for realising highly-sensitive, low-noise, and high-temperature microbolometers. A robust thermal stability of these VO₂(B) thin films at ambient pressure will provide new opportunities to incorporate thermal sensing functions to various electronics.     

Crystalline texture analyses of sputtered BiFeO3 thick films on Si with a large polarization

In our work last year (H. Zhu et al., Rhombohedral BiFeO₃ thick films integrated on Si with a giant electric polarization and prominent piezoelectricity, Acta Materialia 200 (2020) 305–314), it was demonstrated that the rhombohedral-like, (110)-textured BiFeO₃ thick films (～2 μm) sputter-deposited at 450 °C and 500 °C exhibited ultrahigh polarizations of P_r ～ 115 μC/cm² and 135 μC/cm², respectively. However, it is not sufficient to explain these ultra-high polarizations by a preferential growth mechanism and the effect of a moderate compressive strain. To further clarify the polarization enhancement of the films, the texture characteristics of these BFO thick films were quantitatively analyzed by fitting the rocking curves and pole figures to the March-Dollase model. The results showed that, in addition to the (110)-textured growth of a BFO thick film under a moderate compressive strain, the minority non-(110)-textured grains also contributed to the enhancement of the total polarization. Our study demonstrates that, the ultra-high polarizations of our BFO thick films can be well explained by adding the contribution from non-textured grains to the preferential growth of the film under a compressive strain.     

3D printing orientation controlled PMN-PT piezoelectric ceramics

Piezoelectric textured ceramics have drawn increasing research and industry interests by balancing the production cost and material performances. A new approach to realize the texture in piezoelectric ceramics is developed based on 3D printing stereolithography (SL) technique and successfully applied in the preparation of < 001 > -textured 0.71(Sm_0.01Pb_0.985)(Mg_1/3Nb_2/3)O₃-0.29(Sm_0.01Pb_0.985)TiO₃ (1 %Sm-PMN-29PT) ceramics in this work. As a critical process in texture ceramic fabrication, the alignment of BaTiO₃ templates along the horizontal direction is achieved by the shear force produced from the relative motion between the resin container and the blade during SL. The textured ceramics with obvious grain orientation features are successfully obtained. The enhanced piezoelectric properties of d₃₃ ≈ 652 pC N^?1 and d₃₃* ≈ 800 pm V^?1 are achieved in the 3D printed textured ceramic, which are about 60 % and 40 %, respectively, higher than their non-textured counterparts. Moreover, the textured sample shows a significant improvement on thermal stability of d₃₃*_T, which varies by less than ± 6 % from RT to 110 °C. Furthermore, the introduction of 3D printing into the synthesis of textured piezoelectric ceramics shows great advantages over the traditional tape-casting method. This work is expected to provide a promising way for the future design of textured piezoelectric functional materials.     

Enhanced piezoelectric properties and temperature-insensitive strain behavior of <001>-textured KNN-based ceramics

In this study, <001>-textured 0.99(K_0.5Na_0.5)_0.95Li_0.05Nb_0.93Sb_0.07O₃−0.01CaZrO₃ [abbreviated as 0.99KNLNS-0.01CZ] lead-free ceramics were prepared by templated grain growth (TGG) using plate-like NaNbO₃ templates and sintered by a two-step sintering process with different soaking time. All textured samples with high Lotgering factor (f >85%) presented orthorhombic and tetragonal coexisting phase, and the proportion of orthorhombic phase was varied with prolonged soaking time. A large piezoelectric constant d₃₃ (~ 310 pC/N) was obtained in the textured samples with a 12 h soaking time, which was almost twice larger compared to the randomly oriented one. Furthermore, the field-induced piezoelectric strain coefficient d₃₃^*(~ 440 pm/V) of the textured ceramics with 6 h soaking time was larger than the value of randomly oriented one (~ 298 pm/V) at room-temperature. Enhanced piezoelectric response and good temperature stability prove that <001>-textured 0.99KNLNS-0.01CZ ceramics are promising candidates in the field of lead-free piezoelectric materials.     

High‐Speed Preparation of <111>‐ and <110>‐Oriented β‐SiC Films by Laser Chemical Vapor Deposition

Highly oriented <111> and <110> β‐SiC films were prepared on Si(100) single crystal substrates by laser chemical vapor deposition using a diode laser (wavelength = 808 nm) and HMDS (Si(CH₃)₃–Si(CH₃)₃) as a precursor. The effects of laser power (P_L), total pressure (P_tot), and deposition temperature (T_dep) on the orientation, microstructure, and deposition rate (R_dep) were investigated. The orientation of the β‐SiC films changed from <111> to random to <110> with increasing P_L and P_tot. The <111>‐, randomly, and <110>‐oriented β‐SiC films exhibited dense, cauliflower‐like, and cone‐like microstructures, respectively. Stacking faults were observed in the <111>‐ and <110>‐oriented films, and aligned parallel to the (111) plane in the <111>‐oriented film, whereas they were perpendicular to the (110) plane in the <110>‐oriented film. The highest R_dep of the <111>‐oriented β‐SiC film was 200 μm/h at P_tot = 200 Pa and T_dep = 1420 K, whereas that of the <110>‐oriented film was 3600 μm/h at P_tot = 600 Pa and T_dep = 1605 K.     

Development of texture in Bi0.5Na0.5TiO3 prepared by reactive-templated grain growth process

The effect of sodium concentration on texture development in Bi_0.5Na_0.5TiO₃ [BNT] bulk ceramics was examined. The 〈1 0 0〉-textured specimens were prepared by the reactive-template grain growth process using platelike Bi₄Ti₃O₁₂ particles. Texture did not extensively develop in stoichiometric and Na-deficient BNT, but excess Na promoted extensive texture development together with densification. The role of excess Na was discussed based on the formation of a liquid phase.     

Texture effect on the electrochemical properties of LiCoO2 thin films prepared by PLD

LiCoO₂ thin films were prepared by pulsed laser deposition (PLD) on Pt/Ti/SiO₂/Si (Pt) and Au/MgO/Si (Au) substrates, respectively. Crystal structures and surface morphologies of thin films were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The LiCoO₂ thin films deposited on the Pt substrates exhibited a preferred (0 0 3) texture with smooth surfaces while the LiCoO₂ thin films deposited on the Au substrates exhibited a preferred (1 0 4) texture with rough surfaces. The electrochemical properties of the LiCoO₂ films with different textures were compared with charge-discharge, dQ/dV, and Li diffusion measurements (PITT). Compared with the (1 0 4)-textured LiCoO₂ thin films, the (0 0 3)-textured thin films exhibited relatively lower electrochemical activity. However, the advantage of the (1 0 4)-textured film only remained for a small number of cycles due to the relatively faster capacity fade. Li diffusion measurements showed that the Li diffusivity in the (0 0 3)-textured film is one order of magnitude lower than that in the (1 0 4)-textured film. As discussed in this paper, we believe that Li diffusion through grain boundaries is comparable to or even faster than Li diffusion through the grains.     

Heteroepitaxial growth of thick 3C-SiC (110) films by Laser CVD

Previously, we found 3C-SiC films favor to grow in <111> orientation on Si (110) ( https://doi.org/10.1111/jace.15260 ). However, epitaxial growth of thick <110>-3C-SiC is still a big challenge. In this study, thick 3C-SiC (110) epitaxial films were prepared on Si (110) substrate by laser chemical vapor deposition (LCVD) using hexamethyldisilane (HMDS) in H₂ atmosphere. The investigation of growth mechanism showed that the laser of LCVD played an important role during the depositions. Observation by high-resolution transmission electron microscopy (HRTEM) revealed that the interface of 3C-SiC (110)/Si (110) exhibited rough texture at atomic level. The atomic roughness on Si (110) surface could be a key factor for 3C-SiC (110) nucleation. The growth of thick 3C-SiC (110) epitaxial films could be very promising for new development in power electronics applications.     

Structure and electronic properties of δ-Bi2O3 tuned by vacancy and doping: A first-principles study

Pure Bi₂O₃ with high ionic conductivities is considered as a candidate material for an electrolyte in solid oxide fuel cells and oxygen separation membranes. However, its lower structural and thermal stability prevent it application in ion conductivity and photocatalysis at suitable temperatures. Metal oxides are usually used to stabilize its structure to lower temperatures and the underlying mechanism is still unclear. To shed light on the issue, vacancy ordered structures of pure and doped δ-Bi₂O₃ have been studied by first-principles calculations. It have been shown that the structure with combined <110> and <111> vacancy arrangements is energetically favorable compared to either <100>, <110> or <111> vacancy ordered structures. Electronic structure analyses have further verified that δ-Bi₂O₃ has a semiconductor character with an energy gap of 2.0 eV, consistent with the experiment results. The site occupation of doping ions is further analyzed by formation energy, geometry and electronic structures. It is evident that the substitution sites of doping ions depend on the type of the doping ions. The ions with large ion sizes tend to occupy the Bi(2) sites while the ions with small ion sizes tend to occupy the Bi(1) sites. At the same time, the probability of the Y ions occupying the oxygen vacancy sites and the optical properties of the Y-doped Bi₂O₃ are explored. Our investigations reveal that the electronic structure of oxides could be tuned by vacancy and interstitial defects for better conductivity, photocatalytic properties.     

Extrusion of AlSBA-15 molecular sieves: An industrial point of view

AlSBA-15 in the powder form with different n_Si/n_Al ratios (45, 136 and 215) were synthesized by hydrothermal technique. The powdered materials were made into cylindrical extrudates with the addition of bentonite as a binder. The AlSBA-15 materials were characterized by XRD, N₂ adsorption, AAS and thermogravimetric analysis. The orderly growth of AlSBA-15 is evidenced by its XRD. The surface area of the powder catalyst is around 950 m²/g and that of extrudate is close to 600 m²/g. Vapor phase alkylation of phenol with tert-butanol was carried out over the extrudates of AlSBA-15 as a model reaction. The activity of AlSBA-15 extrudates follows the order: AlSBA-15 Si/Al = 45 > AlSBA-15 Si/Al = 136 > AlSBA-15 Si/Al = 215. The reaction products were found to be 2-TBP, 4-TBP and 2,4-DTBP. The selectivity to para tertiary butylation is higher than other reactions.     

Growth-microstructure relationship in MgO-MgAl2O4 eutectic fabricated by micro-pulling down method with MgAl2O4 seed crystals

The MgO-MgAl₂O₄ eutectic was directionally solidified via micro-pulling down method in the form of rods with 2–3 mm diameter. MgAl₂O₄ single crystals (with <111> orientation) were used as crystallization seeds. At low pulling rates, especially 0.15 mm/min triangle-like cross-section was observed, which was linked to the eutectic MgAl₂O₄ following the crystallographic direction of the seed. MgO precipitates in the form of lamellae and rods with median equivalent diameter ranging from 0.19 to 0.85 μm, depending on the pulling rate. The preferred crystallization direction was <111> for both phases, however notable traces of other directions, e.g. <100>, <110> and <331> were found as well.     

Enhanced thermal stability of Bi2Te3-based alloys via interface engineering with atomic layer deposition

The ease of Te sublimation from Bi₂Te₃-based alloys significantly deteriorates thermoelectric and mechanical properties via the formation of voids. We propose a novel strategy based on atomic layer deposition (ALD) to improve the thermal stability of Bi₂Te₃-based alloys via the encapsulation of grains with a ZnO layer. Only a few cycles of ZnO ALD over the Bi₂Te_2.7Se_0.3 powders resulted in significant suppression of the generation of pores in Bi₂Te_2.7Se_0.3 extrudates and increased the density even after post-annealing at 500 °C. This is attributed to the suppression of Te sublimation from the extrudates. The ALD coating also enhanced grain refinement in Bi₂Te_2.7Se_0.3 extrudates. Consequently, their mechanical properties were significantly improved by the encapsulation approach. Furthermore, the ALD approach yields a substantial improvement in the figure-of-merit after the post-annealing. Therefore, we believe the proposed approach using ALD will be useful for enhancing the mechanical properties of Bi₂Te₃-based alloys without sacrificing thermoelectric performance.     

Thermal behavior of hydrostatically extruded polycarbonate and high-impact polystyrene

Effects of hydrostatic extrusion on the thermal properties of polycarbonate (PC) and of high-impact polystyrene (HIPS) were studied using differential scanning calorimeter (DSC) measurements. A glass transition temperature (T_g) and a peak temperature were determined from the DSC curves for both PC and HIPS extrudates. The T_g values of the PC extrudates, with a percentage reduction in area, R, from 40 to 50%, change appreciably from the value for the as–received PC. The results of the hydrostatic extrusion of the PC billets suggest that a two stage deformation process of molecular chains may be involved. Shear-banding is observed for HIPS extrudates with R = 30 to 60%; this fact indicates that a sub-glass transition (β-transition) occurs at temperatures below T_g. It is suggested that the molecular chains of the HIPS extrudate with R = 70% are oriented in the direction of hydrostatic extrusion. The deformation mechanism of molecular chains caused by the hydrostatic extrusion is discussed.     

Structure,Electrical, and Optical Properties of (Na1/2Bi1/2)TiO3–1.5at.%Bi(Zn1/2Ti1/2)O3 Lead‐free Single Crystal Grown by a TSSG Technique

Lead‐free single crystal (Na_1/2Bi_1/2)TiO₃–1.5 at.%Bi(Zn_1/2Ti_1/2)O₃ (NBT–1.5BZT) with dimension of Φ35 mm × 12 mm was successfully grown by a top‐seeded solution growth technique. The average and local structure were studied by a combination of X‐ray diffraction and Raman spectroscopy. The electric and optical properties of <001>‐oriented single crystals were investigated systematically. Compared with pure NBT, the piezoelectric constant and transmission coefficient were both enhanced, that is, from 62 pC/N and ~60% to 121 pC/N and ~70%, respectively. Furthermore, domain structure observation suggested that the <110>‐oriented tetragonal ferroelastic domains in NBT were suppressed at room temperature with addition of BZT, which was responsible for the improved piezoelectric and optical properties of NBT–1.5BZT single crystal.     

Processing and Properties of Textured Potassium Sodium Niobate [K,Na]NbO3 Ceramic Ribbons by Alginate Gelation Method

Random and <001> textured potassium sodium niobate – [K,Na]NbO₃ (KNN) ceramics with 1 mole% CuO sintering aid were fabricated in ribbon form through a combination of novel alginate gelation process and templated grain growth methods using platelike sodium niobate ‐ NaNbO₃ (NN) template particles. The platelike NN template particles were prepared by a two‐step molten salt synthesis method. Ribbons were drawn from alginate‐based slurries without or with 10 wt% NN template particles using 50 mm long slit nozzle with a rectangular orifice of 10 mm × 1 mm. Development of crystallographic texture as a result of varying sintering time and temperature was evaluated through the calculation of the degree of orientation as measured by the Lotgering factor (?₍₀₀₁₎) and an ?₍₀₀₁₎ of 0.81 was achieved. The electrical properties of textured ribbons were evaluated with polarization and strain versus electric field measurements.     

Effect of Gd and Nb co-substitution on enhancing the thermoelectric power factor of nanostructured SrTiO3

Oxide thermoelectric materials have attracted researchers in recent decade due to their attractive features such as low toxicity, low cost and high chemical robustness. Perovskite based oxide thermoelectric are considered as the promising materials, especially for high temperature thermoelectric applications. In the present work, pure SrTiO₃, Sr_1-xGd_xTiO₃ (0 < x < 0.09) and Sr_1-xGd_xTi_1-yNb_yO₃ were prepared by varying Gd concentration (0 < x < 0.09) using hydrothermal method. The XRD analysis confirmed the high crystalline cubic structured nanocomposite with Gd and Nb substitution. The FESEM images revealed cubic morphology of the particles and the size of the cubes varied with the concentration of the dopant. The chemical compositions of the samples were confirmed by EDX analysis. The binding states and elemental composition of the samples were analyzed by XPS. Both the pure SrTiO₃, Sr_1-xGd_xTiO₃ samples show low electrical resistivity and the co-substituted sample exhibited relatively high resistivity. Seebeck coefficient of the samples increased with Gd concentration. The Gd and Nb co-substituted sample shows relatively higher Seebeck coefficient value compared to Gd substituted samples. The power factor of the nanocomposite were calculated from the obtained Seebeck coefficient and resistivity; Gd and Nb co-substituted sample shows relatively high power factor of 311.7 × 10^?6 Wm^?1K^?2 at 550 K compared to other samples.     

Solid-state extrusion of isotactic polypropylene through a tapered die: 2. Structure and some properties of extrudates

The structure and some properties of the solid-state extrudates of isotactic polypropylene (PP) were examined. The crystal modifications of the PP extrudates differed as the extrusion temperature changed. The formation of smectic crystals was observed in the samples extruded at temperatures below 70°C, while the monoclinic modification was predominant above 70°C. The crystal orientation factor, f₆, increased with increasing extrusion ratio (ER) and reached 0.988 when ER was 6.3, which was considered to be an upper limit of ER of PP extrusion. The mechanical properties and the thermal shrinkage of the extrudates were also examined. From these measurements the PP extrudates were considered to have structures similar to the drawn PP.     

Preparation of (110)-oriented SrTiO3 film on quartz glass by laser chemical vapor deposition

SrTiO₃ (STO) film was prepared on quartz glass by laser chemical vapor deposition at a deposition temperature (T_dep) ranged from 760 to 1104 K. Effect of the T_dep on the orientation, crystallinity, texture, and microstructure of the STO film was investigated. As the T_dep was increased, the preferred orientation of the STO film tended to be (110)-orientated with corresponding texture coefficient (TC) on the (110) reflection enhanced from 2.3 to 6; meanwhile, the full width at half maximum of the ω-scan on the (110) reflection decreased from 0.85° to 0.59°. The (110)-oriented grains were in wedge shape about 60 × 150 nm in size, which tended to be flat at an elevated T_dep of 1104 K.     

Thermoelectric properties of Mo-doped bulk In2O3 and prediction of its maximum ZT

In a search for new thermoelectric materials, indium oxide (In₂O₃) was selected as a candidate for high-temperature thermoelectric oxide materials due to its intrinsically low thermal conductivity (<2 W/mK) and ZT values around 0.05. However, low electrical conductivity is a factor limiting the thermoelectric performance of this oxide, and was addressed in this study by Mo doping. It was found that Mo is soluble in In₂O₃ but forms secondary phases at a fraction near x = 0.06 and higher. Mo was found to be unsuitable for heavy n-type doping necessary to improve the thermoelectric performance of the oxide to the desired level (ZT = 1). However, the experimental data enabled us to analyze the electrical conductivity behavior and the Seebeck coefficient of doped In₂O₃ with different carrier concentrations, predicting a theoretically achievable maximum power factor value of 1.77 × 10^?3 W/mK² at an optimum carrier concentration. This estimation predicts the highest ZT value of 0.75 at 1073 K, assuming the lattice thermal conductivity value remaining at an amorphous level.