Search Results (22)

This work investigates the development of multifunctional bioceramic composites based on hydroxyapatite reinforced with the lead-free piezoelectric phase sodium potassium niobate, Na_0.5K_0.5NbO₃. Hydroxyapatite powder was synthesized by aqueous precipitation and combined with sodium potassium niobate synthesized by solid-state reaction to produce bulk ceramics containing 0, 10, 20, and 30 wt.% sodium potassium niobate, consolidated by conventional sintering. The materials were characterized in terms of density, linear shrinkage, phase composition, microstructure, elemental composition, flexural strength, and antibacterial response. X-ray diffraction analysis confirmed the coexistence of hydroxyapatite and orthorhombic sodium potassium niobate, with the intensity of sodium potassium niobate-related reflections increasing with its content, together with minor secondary phases. Scanning electron microscopy revealed the progressive incorporation of sodium potassium niobate grains into the hydroxyapatite matrix and showed a strong dependence of porosity on sodium potassium niobate content. Flexural tests showed that intermediate sodium potassium niobate additions improved strength compared with monolithic hydroxyapatite, whereas highly porous compositions exhibited reduced mechanical performance. Agar-diffusion tests against representative Gram-positive and Gram-negative strains showed measurable inhibition zones, although the differences between pure hydroxyapatite and composites containing sodium potassium niobate were limited and species-dependent. Slight increases were observed for P. aeruginosa and S. aureus, whereas no appreciable changes were detected for the other strains. Therefore, these findings should be regarded as preliminary evidence of antibacterial response rather than as confirmation of an intrinsic antibacterial effect of sodium potassium niobate. Overall, the structural and mechanical results indicate that hydroxyapatite/sodium potassium niobate composites are promising lead-free candidates for the development of multifunctional bioceramics, while their antibacterial response requires further validation. Full article

Lead-based materials are widely used in piezoceramics due to their high electromechanical properties. However, due to environmental protection and sustainable development, the use of the toxic element lead (Pb) in electronic devices is strictly restricted, therefore requiring the rapid development of piezoelectric-based devices with lead-free ceramics. In this context, a lead-free doped barium titanate was studied with a dual objective. First, a new sol–gel method to synthesize Hf⁴⁺-doped BaHf_xTi_1−xO (BHT) with x = 0.05, 0.075, and 0.10 is presented. Such BHT sols were prepared at high concentrations of up to 1 M. Dilution in ethylene glycol allowed parameters (viscosity, colloid sizes, etc.) to be controlled, which ensured a time-stable sol for several months at room temperature. Second, densified bulk ceramics with attrited powders were obtained from these sols and showed very good electromechanical properties, with a thickness coupling factor of k_t = 47% (BaHf_0.05Ti_0.95O₃ sintered at 1500 °C/6 h). These results are a first step that will allow the processing of lead-free piezoelectric thick films using a sol–gel composite method for vibrational energy harvesting applications. Full article

For the last several decades, energy harvesters based on piezoelectricity from mechanical vibration have emerged as very promising devices that are being explored extensively for their functionality in energy technologies. In this paper, a series of flexible lead-free BaZr_0.2Ti_0.8O₃ (BZT)/PVDF and lead-based PbZr_0.52Ti_0.48O₃ (PZT)/PVDF piezocomposites with variable filler content up to 50 vol% were prepared by a hot pressing method. The structure and morphology of the BZT and PZT powders, as well as the distribution of the piezo-active filler in the obtained flexible films were characterized by XRD and SEM analysis. In addition, the remnant polarization (P_r) and leakage current were also investigated to evaluate the breakdown strength in both types of flexible films. The calculations of storage energies and output voltage obtained for the investigated materials revealed an increasing trend with an increasing amount of BZT and PZT active phases. The maximum storage energy of 0.42 J/cm³ (and energy efficiency of 40.7 %) was obtained for the PZT–PVDF (40–60) films, while the maximum output voltage of about 10 V (~10 μA) was obtained for the PZT–PVDF (50–50) flexible film. In addition, a comparison between the properties of the lead-based and lead-free flexible films, as well as the potential use of these films as energy storage and energy harvesting systems were analyzed. Full article

In this study, non-stoichiometry lead-free piezoelectric ceramic Li_0.058(K_0.48Na_0.535)_0.966(Nb_0.9Ta_0.1)O₃ (LKNNT) thick films were deposited on Pt/Ti/Si substrates using spin-coating method technology to form a LKNNT/Pt/Ti/Si structure of the micro-pressure thick films. Additionally, the influence on the crystalline properties, surface microstructure images, and mechanical properties, and the piezoelectric properties of the non-stoichiometry lead-free piezoelectric ceramic Li_0.058(K_0.48Na_0.535)_0.966(Nb_0.9Ta_0.1)O₃ (LKNNT) thick films were observed, analyzed, and calculated using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), focused ion beam (FIB) microscopy, nano-indention technology, and other instruments. This study was divided into two parts: The first part was the investigation into the fabrication parameters and properties of the bottom layer (Pt) and buffer layer (Ti). The Pt/Ti/Si structures were achieved by the DC sputtering method, and then the rapid thermal annealing (RTA) post-treatment process was used to re-arrange the grains and reduce defects in the lead-free Li_0.058(K_0.48Na_0.535)_0.966(Nb_0.9Ta_0.1)O₃ (LKNNT) thick films. In the second part, lead-free Li_0.058(K_0.48Na_0.535)_0.966(Nb_0.9Ta_0.1)O₃ (LKNNT) powder was prepared by the solid-state reaction method, and then acetic acid (C₂H₄O₂) solvent was added to form a slurry for spin-coating technology processing. The fabrication parameters, thick film micro-structure, crystalline properties, nano-indention technology, and the piezoelectric coefficient characteristics of the developed lead-free Li_0.058(K_0.48Na_0.535)_0.966(Nb_0.9Ta_0.1)O₃ (LKNNT)/Pt/Ti/Si structure of the micro-pressure thick film devices a were investigated. According to the experimental results, the optimal fabrication processing parameters of the lead-free Li_0.058(K_0.48Na_0.535)_0.966(Nb_0.9Ta_0.1)O₃ (LKNNT) were an RTA temperature of 500 °C, a Ti buffer-layer thickness of 273.9 nm, a Pt bottom electrode-layer thickness of 376.6 nm, a theoretical density of LKNNT of 4.789 g/cm³, a lattice constant of 3.968 × 10⁻⁸ cm, and a d₃₃ value of 150 pm/V. Finally, regarding the mechanical properties of the micro-pressure devices for when a microforce of 3 mN was applied, the thick film revealed a hardness of 60 MPa, a Young’s modulus of 13 GPa, and an elasticity interval of 1.25 μm, which are suitable for future applications of micro-pressure devices. Full article

Piezoelectric materials are a class of compounds that is gaining increasing interest in various applications such as energy harvesting. During the last decade, lead-free ZnSnO₃ perovskite ceramic has gained attention among the scientific community thanks to its unique symmetry-dependent and spontaneous polarization properties such as piezoelectricity and ferroelectricity. Nevertheless, only a few studies successfully prepared pure ZnSnO₃, while most seem to mislead the product for its hydroxide precursor (ZnSn(OH)₆) or a mixture of Zn₂SnO₄ and SnO₂. In our work, we investigated the conversion of ZnSn(OH)₆ at different temperatures (500, 600, 700, 750 and 800 °C) by X-ray powder diffraction analysis, and in-situ using synchrotron radiation up to 950 °C under ambient atmosphere and in a vacuum, to reproduce conventional reaction conditions. SEM and TEM have been used to understand the evolution of the particle shape and surface structure before and after the thermal treatments. Our results show the instability of the ZnSn(OH)₆ phase, which converts into an amorphous structure at low temperature. Above 750 °C, the material segregates into Zn₂SnO₄ and SnO₂, supporting the hypothesis that the thermal treatment of the hydroxide phase under typical conditions results in the formation of an oxide mixture rather than the phase pure ZnSnO₃. Full article

The request for extremely low-temperature and short-time sintering techniques has guided the development of alternative ceramic processing. Atmosphere-assisted FLASH sintering (AAFS) combines the direct use of electric power to packed powders with the engineering of operating atmosphere to allow low-temperature conduction. The AAFS of nanometric Potassium Sodium Niobate, K_0.5Na_0.5NbO₃, a lead-free piezoelectric, is of great interest to electronics technology to produce efficient, low-thermal-budget sensors, actuators and piezo harvesters, among others. Not previously studied, the role of different atmospheres for the decrease in FLASH temperature (T_F) of KNN is presented in this work. Additionally, the effect of the humidity presence on the operating atmosphere and the role of the compact morphology undergoing FLASH are investigated. While the low partial pressure of oxygen (reducing atmospheres) allows the decrease of T_F, limited densification is observed. It is shown that AAFS is responsible for a dramatic decrease in the operating temperature (T < 320 °C), while water is essential to allow appreciable densification. In addition, the particles/pores morphology on the green compact impacts the uniformity of AAFS densification. Full article

The article presents the results of the synthesis of lead-free piezoceramic materials (Ba_0.9Ca_0.1)(Ti_0.9Zr_0.1)O₃ (BCZT system) in spherical powder form and their subsequent application in the binder jetting additive manufacturing process. Green models were manufactured using this powder material with binder jetting, different sintering modes were investigated, and the functional piezoelectric properties were measured. Lattice structures with triply periodic minimum surface topologies, such as Gyroid and Schwarz, were designed and manufactured. It is shown that the functional properties of lattice structures depend on the parameters of the cells and the chosen topology. Full article

For more than five decades, alkali niobate-based materials (K_xNa_1−xNbO₃) have been one of the most promising lead-free piezoelectric materials researched to be used in electronics, photocatalysis, energy storage/conversion and medical applications, due to their important health and environmentally friendly nature. In this paper, our strategy was to synthetize the nearest reproductible composition to K_xNa_1−xNbO₃ (KNN) with x = 0.5, placed at the limit of the morphotropic phase boundary (MPB) with the presence of both polymorphic phases, orthorhombic and tetragonal. The wet synthesis route was chosen to make the mix crystal powders, starting with the suspension preparation of Nb₂O₅ powder and KOH and NaOH alkaline solutions. Hydrothermal microwave-assisted maturation (HTMW), following the parameter variation T = 200–250 °C, p = 47–60 bar and dwelling time of 30–90 min, was performed. All powders therefore synthesized were entirely K_xN_1−xNbO₃ solid solutions with x = 0.06–0.69, and the compositional, elemental, structural and morphological characterization highlighted polycrystalline particle assemblage with cubic and prismatic morphology, with sizes between 0.28 nm and 2.95 μm and polymorphic O-T phase coexistence, and a d₃₃ piezoelectric constant under 1 pC/N of the compacted unsintered and unpoled discs were found. Full article

Lead-free piezoelectric powders (K_0.44Na_0.52Li_0.04)(Nb_0.82Ta_0.10Sb_0.04)O₃ were obtained by conventional and microwave-assisted reactive heating. Firstly, the synthesis of the material was carried out following the mixed oxide route and employing both traditional methods and microwave technology. Thermogravimetry, X-ray diffraction, field emission scanning electron microscopy and electrical properties analyses were evaluated. X-ray diffraction of the powders calcined by the microwave process shows the formation of perovskite structure with orthorhombic geometry, but it is possible to observe the presence of other phases. The presence of the secondary phases found can have a great influence on the heating rate during the synthesis on which the kinetics of the reaction of formation of the piezoelectric compound depend. The calcined powder was sintered at different temperatures by conventional and non-conventional processes. The microstructure of the ceramics sintered by microwave at 1050 °C for 10 min shows perovskite cubes with regular geometry, of size close to 2–5 µm. However, the observed porosity (~8%), the presence of liquid phase and secondary phases in the microstructure of the microwave sintered materials lead to a decrease of the piezoelectric constant. The highest d₃₃ value of 146 pC/N was obtained for samples obtained by conventional at 1100 °C 2 h compared to samples sintered by microwave at 1050 °C 10 min (~15 pC/N). Full article

We show how sintering in different atmospheres affects the structural, microstructural, and functional properties of ~30 μm thick films of K_0.5Na_0.5NbO₃ (KNN) modified with 0.38 mol% K_5.4Cu_1.3Ta₁₀O₂₉ and 1 mol% CuO. The films were screen printed on platinized alumina substrates and sintered at 1100 °C in oxygen or in air with or without the packing powder (PP). The films have a preferential crystallographic orientation of the monoclinic perovskite phase in the [100] and [−101] directions. Sintering in the presence of PP contributes to obtaining phase-pure films, which is not the case for the films sintered without any PP notwithstanding the sintering atmosphere. The latter group is characterized by a slightly finer grain size, from 0.1 μm to ~2 μm, and lower porosity, ~6% compared with ~13%. Using piezoresponse force microscopy (PFM) and electron backscatter diffraction (EBSD) analysis of oxygen-sintered films, we found that the perovskite grains are composed of multiple domains which are preferentially oriented. Thick films sintered in oxygen exhibit a piezoelectric d₃₃ coefficient of 64 pm/V and an effective thickness coupling coefficient k_t of 43%, as well as very low mechanical losses of less than 0.5%, making them promising candidates for lead-free piezoelectric energy harvesting applications. Full article

This article presents the results of manufacturing samples from barium titanate (BaTiO₃) lead-free piezoceramics by using the binder jetting additive manufacturing process. An investigation of the manufacturing process steps for two initial powders with different particle size distributions was carried. The influence of the sintering and the particle size distribution of the starting materials on grain size and functional properties was evaluated. Samples from fine unimodal powder compared to coarse multimodal one have 3–4% higher relative density values, as well as a piezoelectric coefficient of 1.55 times higher values (d₃₃ = 183 pC/N and 118 pC/N correspondingly). The influence of binder saturation on sintering modes was demonstrated. Binder jetting with 100% saturation for both powders enables printing samples without delamination and cracking. Sintering at 1400 °C with a dwell time of 6 h forms the highest density samples. The microstructure of sintered samples was characterized with scanning electron microscopy. The possibility of manufacturing parts from functional ceramics using additive manufacturing was demonstrated. Full article

Barium Titanate (BaTiO₃) is one of the most promising lead-free ferroelectric materials for the development of piezoelectric nanocomposites for nanogenerators and sensors. The miniaturization of electronic devices is pushing researchers to produce nanometric-sized particles to be embedded into flexible polymeric matrices. Here, we present the sol-gel preparation of crystalline BaTiO₃ nanoparticles (NPs) obtained by reacting barium acetate (Ba(CH₃COO)₂) and titanium (IV) isopropoxide (Ti(OⁱPr)₄). The reaction was performed both at ambient conditions and by a hydrothermal process carried on at 200 °C for times ranging from 2 to 8 h. Doped BaTiO₃ nanoparticles were also produced by addition of Na, Ca, and Bi cations. The powders were annealed at 900 °C in order to improve NPs crystallinity and promote the cubic-to-tetragonal (c⟶t) phase transformation. The microstructural features of nanoparticles were investigated in dependence of both the hydrothermal reaction time and the presence of dopants. It is found that short hydrothermal treatment (2 h) can produce BaTiO₃ spherical and more homogeneous nanoparticles with respect to longer hydrothermal treatments (4 h, 6 h, 8 h). These particles (2 h) are characterized by decreased dimension (approx. 120 nm), narrower size distribution and higher tetragonality (1.007) in comparison with particles prepared at ambient pressure (1.003). In addition, the short hydrothermal treatment (2 h) produces particles with tetragonality comparable to the one obtained after the longest process (8 h). Finally, dopants were found to affect to different extents both the c⟶t phase transformation and the crystallite sizes. Full article

Due to the ever-increasing limitations of the use of lead-based materials, the manufacturing of lead-free piezoceramics with competitive piezoelectric properties and established nontoxicity is considered a priority for the scientific and industrial community. In this work, a lead-free system based on sodium potassium niobate (KNN), opportunely modified with MgNb₂O₆ (MN), was prepared through a combination of a mechanochemical activation method and air sintering, and its toxicity was evaluated. The effect of the mechanical processing on the microstructure refinement of the processed powders was established by X-ray diffraction and the average crystallite size content of the Nb₂O₅ species was evaluated. The experimental evidence was rationalized using a phenomenological model which permitted us to obtain the amount of powder processed at each collision and to optimize the activation step of the pre-calcined reagents. This influenced the final density and piezoresponse of the as-sintered pellets, which showed optimal properties compared with other KNN systems. Their toxicological potential was evaluated through exposure experiments to the pulverized KNN-based pellets, employing two widely used human and environmental cellular models. The in vitro assays proved, under the selected conditions, the absence of cytotoxicity of KNN-bases systems here studied. Full article

The considerable decrease in temperature and time makes FLASH sintering a more sustainable alternative for materials processing. FLASH also becomes relevant if volatile elements are part of the material to be processed, as in alkali-based piezoelectrics like the promising lead-free K_0.5Na_0.5NbO₃ (KNN). Due to the volatile nature of K and Na, KNN is difficult to process by conventional sintering. Although some studies have been undertaken, much remains to be understood to properly engineer the FLASH sintering process of KNN. In this work, the effect of FLASH temperature, T_F, is studied as a function of the particle size and impurity content of KNN powders. Differences are demonstrated: while the particle size and impurity degree markedly influence T_F, they do not significantly affect the densification and grain growth processes. The conductivity of KNN FLASH-sintered ceramics and KNN single crystals (SCs) is compared to elucidate the role of particles’ surface conduction. When particles’ surfaces are not present, as in the case of SCs, the FLASH process requires higher temperatures and conductivity values. These results have implications in understanding FLASH sintering towards a more sustainable processing of lead-free piezoelectrics. Full article

There is a strong need in the industry to develop lead-free piezoelectrics for sensors and actuators. Although these materials have become an important component of many electronic devices, it is very important for the industry to decarbonise ceramic technology, especially through the introduction of modern sintering technologies. Among the many piezoelectric compounds available, Calcium Barium Titanate (BCT) have been widely investigated because of its similar performance to lead-containing Lead Titanate Zirconate (PZT). In this paper, a modified Pechini method for obtaining ceramic Ba_0.9Ca_0.1TiO₃ nano-powders is described. Deviation from the established procedure resulted in the precipitation of the solution or obtaining of a low-quality (poorly crystallized) product with numerous impurities. The samples of BCT materials were examined to find their ideal microstructures and structures; these factors were confirmed by their outstanding X-ray diffraction spectra and high piezoelectric constant values that are comparable to commercial lead-containing materials. Full article