Good temperature stability of K0.5Na0.5NbO3 based lead-free ceramics and their applications in buzzers

(K_0.5−xLi_x)Na_0.5(Nb_1−ySb_y)O₃ (KLNNSx–y, x = 0–4 mol% and y = 0–8 mol%) lead-free piezoelectric ceramics were prepared by the conventional mixed oxide method. The denser microstructure and better electrical properties of the ceramics were obtained as compared to the pure K_0.5Na_0.5NbO₃ ceramic. The temperature stability of the electrical properties of the ceramics was also investigated. The experimental results show that the KLNNS2.5–5 ceramic exhibits good electrical properties (k_p  49%, k₃₁  30% and , tan δ  0.019), and possesses good temperature stability in the temperature range of −40 to 85 °C. The related mechanisms for improved electrical properties and temperature stability were also discussed. Moreover, buzzers based on the KLNNS2.5–5 ceramic have been fabricated and their characterization is presented. These results show that the KLNNS2.5–5 ceramic is a promising lead-free material for practical application in buzzers.     

Piezoelectric behavior of (1−x)K0.50Na0.50NbO3−xBa0.80Ca0.20ZrO3 lead-free ceramics

(1−x)K_0.50Na_0.50NbO₃–xBa_0.80Ca_0.20ZrO₃ [(1−x)KNN–xBCZ] lead-free ceramics were prepared by the conventional solid-state method, and the effect of BCZ content on their phase structure and piezoelectric properties was studied. A coexistence of rhombohedral–orthorhombic phases was identified in the range 0.04<x<0.08. With increasing the BCZ content, their grain size becomes smaller, and their Curie temperature gradually decreases. An optimum piezoelectric behavior of d₃₃∼197 pC/N and k_p∼40.6% was demonstrated in the ceramic with x=0.06 because of the coexistence of two phases. As a result, the introduction of BCZ could further improve piezoelectric properties of KNN ceramics.     

Compositional dependence of phase structure and electrical properties in (K0.50Na0.50)0.97Bi0.01(Nb1−xZrx)O3 lead-free ceramics

(K_0.50Na_0.50)_0.97Bi_0.01(Nb_1-xZr_x)O₃ (KNBNZ) lead-free ceramics were prepared by the conventional solid-state sintering process. Their phase structure is dependent on the Zr content in the investigated range, and the ceramics endure a phase transition from pseudocubic to orthorhombic with increasing Zr content. Improved piezoelectric properties have been observed when the poling temperature is located at ~100 °C because of the coexistence of orthorhombic and tetragonal phases. Their dielectric and piezoelectric properties were enhanced by doping Zr, the ceramic with x=0.02 showing optimal electrical properties, i.e., d₃₃~161 pC/N, k_p~0.41, Q_m~81, T_c~370 °C, and T_o−t~130 °C. These results show that the KNBNZ ceramic is a promising lead-free piezoelectric material.     

Effect of HfO2 content on the microstructure and piezoelectric properties of (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃–xHfO₂ [BNBT–xHfO₂] lead-free ceramics were prepared using the conventional solid-state reaction method. Effects of HfO₂ content on their microstructures and electrical properties were systematically studied. A pure perovskite phase was observed in all the ceramics with x=0–0.07 wt%. Adding optimum HfO₂ content can induce dense microstructures and improve their piezoelectric properties, and a high depolarization temperature was also obtained. The ceramics with x=0.03 wt% possess optimum electrical properties (i.e., d₃₃~168 pC/N, k_p~32.1%, Q_m~130, ε_r~715, tan δ~0.026, and T_d~106 °C, showing that HfO₂-modified BNBT ceramics are promising materials for piezoelectric applications.     

Enhanced piezoelectric properties of (K0.40Na0.60)0.94Li0.06Nb0.94SbO3 lead-free ceramics by optimizing the sintering temperature

Effects of sintering temperature on the microstructure and electrical properties of (K_0.40Na_0.60)_0.94Li_0.06Nb_0.94SbO₃ (KNLNS) lead-free ceramics are investigated. The grain size gradually becomes larger with increasing sintering temperature from 1055 °C to 1105 °C, and the piezoelectric property could be enhanced by optimizing their sintering temperature. The ceramic sintered at 1075 °C has optimum electrical properties, i.e., d₃₃~272 pC/N, k_p~43.5%, ε_r~1152, tan δ~0.026, and T_C~346 °C. These results show that the sintering temperature can optimize electrical properties of KNLNS ceramics.     

One-step solution combustion synthesis of K0.5Na0.5NbO3 powders at a large-scale

In this work, fine powders of K_0.5Na_0.5NbO₃ (KNN) are produced by a single step of solution combustion synthesis (SCS) using glycine as the reductant fuel and potassium & sodium nitrates as the oxidizers, which are mixed with niobium oxide. Single phase of KNN products can be successfully prepared in a one-step SCS by adjusting the glycine-nitrate ratio. It is concluded that the stoichiometric or slightly fuel-excess condition is good for the single step production of KNN. The one-step SCS of KNN also shows good scaling-up productivity, which can be ignited by both electric heating and microwave heating. This work may provide a feasible large-scale production of various functional oxides by a one-step SCS process.     

Hydrothermal synthesis and characterization of Na0.5Bi0.5TiO3 microcubes

Na_0.5Bi_0.5TiO₃ microcubes with smooth faces and clear, well-defined edges have been successfully prepared for the first time by a simple hydrothermal method without any surfactants. The as-prepared Na_0.5Bi_0.5TiO₃ microcubes showed an obvious emission band compared with nanoparticles, which is attributed to the different NBT morphology and size.     

Improved ferroelectric photovoltaic effect in Mn-doped lead-free K0.5Na0.5NbO3 films

The pure and Mn-doped K_0.5Na_0.5NbO₃ (KNN) films were deposited using solution-gelation method. The crystal structure, ferroelectric properties, spectral response and J-V performance of photovoltaic effect were systematically investigated. Both the ferroelectric and leakage properties are obviously enhanced for Mn-doped KNN films. A fascinating phenomenon is observed that the ferroelectric photovoltaic effect is enhanced in Mn-doped KNN films, which is originated from the improved ferroelectric polarization and narrower band gap. The transition element Nb partially substituted by Mn results in the lattice distortion and further destroys the symmetry space structure, which enhances ferroelectric polarization. And the narrower band gap effectively decreases the internal potential barrier to separate the carriers. This work gives a clear relationship between the lattice distortion, ferroelectric and photovoltaic response. It is certain that lead-free transparent K_0.5Na_0.5NbO₃ films can be potentially applied in viable ferroelectric based solar cells.     

The optimum conditions for preparing the lead-free piezoelectric ceramic of Bi0.5Na0.5TiO3 using the Taguchi method

Using the Taguchi method, this study analyzes the optimum conditions for preparing the disk of bismuth sodium titanate (Bi_0.5Na_0.5TiO₃), BNT, which is prepared using the mixed oxide method. The controllable factors used in this study consisted of the following: (1) the duration of milling; (2) the temperature of calcining; (3) the rotation speed of mill; and (4) the temperature of sintering. According to the optimum conditions, the confirmation experiment was carried out, and the relative density of the BNT disk, which was prepared without adding the additive, was up to 89.05%. The percentage contribution of each controllable factor was also determined. Most interestingly, the temperature of sintering is the most influential factor to press the BNT powder together tightly, and its value of percentage contribution is up to 94.94%.     

Oxygen-enriched sintering for improved piezoelectric performance of (K0.5Na0.5)(Ta0.3Nb0.7)O3 lead-free ceramics: The impact of defects

In this work, (K_0.5Na_0.5)(Ta_0.3Nb_0.7)O₃ (KNNT) powders synthesized by the microwave-hydrothermal method were sintered under different oxygen partial pressure (P_O2 = 0, 0.498, 0.995?atm) atmospheres. Effects of P_O2 on the phase composition, microstructure, piezoelectric properties, and defects in the KNNT ceramics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis DRS). Results indicated that P_O2 had almost no effect on the phase structure and grain morphology of KNNT ceramics. Abnormal grain growth was observed with increasing P_O2; simultaneously, the density of the ceramics decreased. However, the d₃₃ value reached its maximum of 205 pC/N when sintered at the highest P_O2, and k_p showed a similar trend. TEM observations indicated that the microscopic domains had the most regular domain orientation at a P_O2 of 0.998?atm. These phenomena were due to the different oxygen vacancy concentrations caused by varying P_O2. UV–Vis DRS revealed that the defect concentration decreased with increasing P_O2; moreover, the oxygen vacancy concentrations were determined by XPS to be 23.5%, 21.1%, and 19.1% corresponding to P_O2 values of 0, 0.498, and 0.995?atm, respectively, indicating that oxygen-enriched sintering can reduce defects and enhance piezoelectric properties.     

Synthesis of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 powders through the molten salt method

The Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ powder synthesis through molten salt method was investigated in the temperature range of 650–700 °C for 2–4 h. The XRD results indicated that the optimal synthesizing temperature for molten salt method was 700 °C, significantly lower than that for conventional processing route of solid state reaction method, where a calcining temperature of 850 °C was needed. The SEM results revealed better crystallization of the powders obtained through molten salt method, compared with those through the conventional processing route of solid state reaction method.     

Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.     

Solid state metathesis synthesis of BaTiO3, PbTiO3, K0.5Bi0.5TiO3 and Na0.5Bi0.5TiO3

A solid state metathesis approach has been applied to synthesize perovskite oxides such as BaTiO₃, PbTiO₃, K_0.5Bi_0.5TiO₃ and Na_0.5Bi_0.5TiO₃, these were characterized by powder XRD, IR and energy dispersive spectra (EDS). Potassium titanium oxalate and metal chlorides are used as the starting materials. X-ray analysis shows the formation of a single phase with tetragonal structure for BaTiO₃, PbTiO₃, K_0.5Bi_0.5TiO₃ and a monoclinic structure for Na_0.5Bi_0.5TiO₃. The Infrared spectra of these compounds show the characteristic band due to Ti–O octahedron for all the compounds. The EDS spectra show the relative ratio of the metal ions. The morphology of synthesized compounds was obtained from SEM measurements.     

(Ba0.5Sr0.5)TiO3 modification on etched aluminum foil for electrolytic capacitor

A new method was proposed to form (Ba_0.5Sr_0.5)TiO₃–Al₂O₃ composite oxide film on etched aluminum foils. The specimens were covered with (Ba_0.5Sr_0.5)TiO₃ (BST) layer by dip-coating in citrate solution and subsequent heat-treatment under 400–650 °C, finally by anodizing in a hot boracic acid and borate solution. The BST powders heated under different temperatures were characterized by X-ray diffraction (XRD) and the specific capacitance of the coated specimens heat-treated under different temperatures and times was measured. It is found that the specific capacitance increases initially with enhancing the temperature and reaches to maximum at 550 °C, but slightly decreases with the heat-treatment time. The capacitance was increased by about 35% after BST coating.     

Fuel mixture approach for solution combustion synthesis of Ca3Al2O6 powders

Single-phase 3CaO·Al₂O₃ powders were prepared via solution combustion synthesis using a fuel mixture of urea and β-alanine. The concept of using this fuel mixture comes from the individual reactivity of calcium nitrate and aluminum nitrate with respect to urea and β-alanine. It was proved that urea is the optimum fuel for Al(NO₃)₃ whereas β-alanine is the most suitable fuel for Ca(NO₃)₂. X-ray diffraction and thermal analysis investigations revealed that heating at 300 °C the precursor mixture containing the desired metal nitrates, urea and β-alanine triggers a vigorous combustion reaction, which yields single-phase nanocrystalline 3CaO·Al₂O₃ powder (33.3 nm). In this case additional annealing was no longer required. The use of a single fuel failed to ensure the formation of 3CaO·Al₂O₃ directly from the combustion reaction. After annealing at 900 °C for 1 h, the powders obtained by using a single fuel (urea or β-alanine) developed a phase composition comprising of 3CaO·Al₂O₃, 12CaO·7Al₂O₃ and CaO.     

Carbon dioxide hydrogenation to methanol over the pre-reduced LaCr0.5Cu0.5O3 catalyst

The synthesis of methanol from CO₂ hydrogenation was carried out over the pre-reduced Cu-based LaCr_0.5Cu_0.5O₃ catalyst. It showed a much higher catalytic performance (X_CO2 = 10.4% and S_MeOH = 90.8%) at 250 °C than over 13% Cu/LaCrO₃ prepared by wet-impregnation method (X_CO2 = 4.8% and S_MeOH = 46.6%). XRD, H₂/CO₂-TPD and XPS measurements illustrated that hydrogen was adsorbed on the Cu^α+ sites and that CO₂ was activated on the medium basic sites for the reduced LaCr_0.5Cu_0.5O₃. This phenomenon was responsible for its catalytic activity.     

The performances of higher alcohol synthesis over nickel modified K2CO3/MoS2 catalyst

Ni modified K₂CO₃/MoS₂ catalyst was prepared and the performance of higher alcohol synthesis catalyst was investigated under the conditions: T = 280–340 °C, H₂/CO (molar radio) = 2.0, GHSV = 3000 h^− 1, and P = 10.0 MPa. Compared with conventional K₂CO₃/MoS₂ catalyst, Ni/K₂CO₃/MoS₂ catalyst showed higher activity and higher selectivity to C₂⁺OH. The optimum temperature range was 320–340 °C and the maximum space-time yield (STY) of alcohol 0.30 g/ml h was obtained at 320 °C. The selectivity to hydrocarbons over Ni/K₂CO₃/MoS₂ was higher, however, it was close to that of K₂CO₃/MoS₂ catalyst as the temperature increased. The results indicated that nickel was an efficient promoter to improve the activity and selectivity of K₂CO₃/MoS₂ catalyst.     

Hydrothermal synthesis and piezoelectric property of Ta-doping K0.5Na0.5NbO3 lead-free piezoelectric ceramic

Ta-doping K_0.5Na_0.5Nb_1−xTa_xO₃ (x = 0.1, 0.2, 0.3, 0.4) powder was synthesized by hydrothermal approach and its ceramics were prepared after sintering and polarizing treatment in this work. The K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics near morphotropic phase boundary (MPB), which exhibited optimum piezoelectric properties of d₃₃ = 210 pC/N and good electromechanical coupling factors of K_p = 0.3. The domain structure has been observed from TEM images which indicates that the K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics have good piezoelectric and ferroelectric properties for it is near the MPB.     

Phase transition behavior and electrical properties of lead-free (1 − x)(0.98K0.5Na0.5NbO3–0.02LiTaO3)–x(0.96Bi0.5Na0.5TiO3–0.04BaTiO3) piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(0.98K_0.5Na_0.5NbO₃–0.02LiTaO₃)–x(0.96Bi_0.5Na_0.5TiO₃–0.04BaTiO₃) (KNN–LT–BNT–BT) with x = 0–0.10 have been synthesized by a conventional sintering technique. All samples possess pure perovskite structure, showing room temperature symmetries of orthorhombic at x < 0.02, and tetragonal at 0.05 ≤ x ≤ 0.10. A coexistence of orthorhombic and tetragonal phases in the composition range of 0.02 ≤ x < 0.05 in this system is caused by the temperature of the polymorphic phase transition (PPT) decreasing to around room temperature but not the behavior of the morphotropic phase boundary (MPB). The samples near the coexistence region exhibit improved properties, which are as follows: piezoelectric constant d₃₃ = 155 pC/N, remnant polarization P_r = 24.2 μC/cm², and coercive electric field E_c = 2 kV/mm. The results indicate that although this kind of ceramics displays good properties, further study is needed to promote the stabilities of the ceramics in order to utilize them in varying temperature environments.     

Microstructure and electrical properties of (Na0.5K0.5)1−2xMgxNbO3–Bi0.5Na0.5TiO3 lead-free piezoelectric ceramics

0.975[(Na_0.5K_0.5)_1−2xMg_xNbO₃]–0.025(Bi_0.5Na_0.5TiO₃) (KNMN–BNT, x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) lead-free piezoelectric ceramics were fabricated by the conventional solid-state sintering method. The dependence of Mg content on the microstructure and electrical properties of the ceramics is investigated. The X-ray diffraction (XRD) analysis revealed that an appropriate amount of Mg diffused into the KNN–BNT lattice to form a stable solid solution, the ceramics possessed a pure perovskite structure, and a morphotropic phase boundary (MPB) between the orthorhombic and tetragonal phases was observed with the composition of 0.02≤x≤0.05. The orthorhombic–tetragonal transition temperature (T_O–T) is less than 95 °C and the Curie temperature (T_c) is almost unchanged (~360 °C) with the increase of MgO content. The ceramics with x=0.02 showed enhanced piezoelectric and ferroelectric properties because of close proximity to the MPB, i.e., d₃₃~210 pC/N, k_p~0.41, 2E_c~22.4 kV/cm and 2P_r~39.2 μC/cm². Moreover, the dielectric properties exhibited optimal effects with x=0.02, that is ε_r~637 and tan δ~0.09. These results indicate that the introduction of MgO is an effective method to improve the density as well as the electrical properties and the temperature stability of the KNN–BNT ceramics. As a result, the KNMN–BNT ceramic is a promising candidate for lead-free piezoelectric materials.