Dielectric properties of Na1 − x K x NbO3 and Na1 − x Li x NbO3 ceramics

The dielectric properties of solid solutions based on sodium potassium and sodium lithium niobates have been studied in wide ranges of temperatures (25–750°C), frequencies (25 to 10⁶ Hz), and electric fields (up to 30 kV/cm). We have identified solid-solution regions differing in the temperature and frequency dependences of the dielectric permittivity. It is reasonable to take into account the present results in device applications where wide variations in dc bias field and frequency are needed.     

Ba,Sr掺杂对K0.47Na0.47Li0.06NbO3基无铅压电陶瓷结构、介电及压电性能的影响

采用传统的固态反应法制备了(K0.47Na0.47 Li0.06 )1-x(Ba0.5Sr0.5)xNbO3无铅压电陶瓷,研究了Ba,Sr掺杂对K0.47Na0.47Li0.06NbO3陶瓷的晶体结构、电畴结构、介电及压电性能的影响.随着Ba,Sr掺杂量的增加,陶瓷样品逐渐由正交相向四方相转变,同时居里温度(Tc)降低,剩余极化率(Pr)、矫顽场(Ec)、介电常数(εr)增加;压电常数(d33)、机电耦合系数(kp)先增加后减小.x=0.5％时陶瓷的压电常数d33达到221 pC/N,机电耦合系数kp为43.1％.     

LiTaO3掺杂对K0.5Na0.5NbO3基无铅压电陶瓷性能的影响

采用传统陶瓷工艺制备了LiTaO3掺杂的K0.5Na0.5NbO3基无铅压电陶瓷(记为KNN xLT,x=0～8%(摩尔分数)),并研究了陶瓷的晶相、显微结构和压电、铁电等性能.研究结果表明,KNN xLT陶瓷的正交相-四方相准同型相界(MPB)位于4%＜x＜6%处.随着LiTaO3含量的增加,陶瓷的正交→四方结构相变温度(TO-T)向低温方向移动,而四方→立方结构相变温度(Tc)向高温方向移动.陶瓷的压电常数d33和机电耦合系数kp随LiTaO3含量的增加均先增大后减小,而剩余极化强度Pr则随之逐渐减小,矫顽场Ec逐渐增大.当x=6%时,陶瓷具有较好的压电和铁电性能:d33=190pC/N,kp=40.0%,Pr=22.0μC/cm2,Ec=1.78kV/mm,Tc=440℃.该体系陶瓷具有较高的压电常数和比较大的平面机电耦合系数,是一种应用前景良好的压电铁电材料.     

Influence of zirconium addition on final properties of K0.5Na0.5NbO3-based ceramics

Microstructure, electrical and dielectric properties of K_1/2Na_1/2NbO₃ (KNN) modified with ZrO₂ were investigated. Powders were obtained by a conventional solid-state method. Samples doped with 0–2 mol% of ZrO₂ were sintered at 1,125 °C for 2 h. Through XRD spectra, the perovskite structure was observed, in addition to small peaks corresponding to secondary phases. It was also determined that zirconium drastically changed the microstructure and grain size of KNN ceramics. The addition of upto 1.0 mol% of Zr⁴⁺ produced a softening effect in the ferroelectric properties of the material, and increased its density. Conversely, samples prepared with contents higher than 1.0 mol% reduced piezoelectric and dielectric properties.     

Photoluminescence and electrical properties of Er-doped (K0.5Na0.5)NbO3-based transparent ceramics

Er-doped 0.98(K_0.5Na_0.5)NbO₃-0.02Ba(Bi_0.5Nb_0.5)O₃ transparent fluorescent ceramics were prepared using traditional solid-phase method. The (K_0.5Na_0.5)NbO₃ (KNN) ceramics were modified by introducing the second group elements Ba(Bi_0.5Nb_0.5)O₃ and rare earth ions Er³⁺. The effects of Er³⁺ on the structure, optical and electrical properties of transparent ceramics (K_0.5Na_0.5)NbO₃-Ba(Bi_0.5Nb_0.5)O₃ were investigated. The ceramics form a single perovskite structure and have a pseudo-cubic phase structure. Nanoscale grain size was obtained for ceramics, and the smallest average grain size is 80 nm. The ceramics have high transmittance. The ceramic 0.1 mol% Er-doped 0.98(K_0.5Na_0.5)NbO₃-0.02 Ba(Bi_0.5Nb_0.5)O₃ achieved the highest transmittance for this system with 62% and 52% at near-infrared light (1000 nm) and visible light (700 nm), respectively. The ceramics have up-conversion luminescence properties and also maintain good electrical properties. Under 980 nm excitation, the samples showed two green emission bands (518–536 nm, 536–557 nm) and one red emission band (646–677 nm). In addition, the ceramics have relaxation ferroelectricity, and a high dielectric constant. These functional ceramics with multiple properties will have greater research significance and application value.

     

BiFeO3-doped (Na0.5K0.5)NbO3 lead-free piezoelectric ceramics

Abstract

Lead-free piezoelectric ceramics (1?x)(Na_0.5K_0.5)NbO₃-xBiFeO₃ (x=0～0.07) were synthesized by the solid-state reaction. Differential scanning calorimetry (DSC) measurements revealed that an increase in the amount of BiFeO₃ dopant resulted in a decrease in the orthorhombic-tetragonal and tetragonal-cubic phase transition temperature of the material. One percent BiFeO₃ additive suppressed grain growth, which not only benefits the sintering of ceramics but also enhances the piezoelectric and ferroelectric properties, where d₃₃=145pC/N, k_p=0.31, Q_m=80, P_r=11.3 μC cm^?2 and E_c=16.5 kV cm^?1. As x_BF>0.01, both piezoelectric and ferroelectric properties decreased rapidly with an increasing amount of dopant.     

BiFeO3-doped (Na0.5K0.5)NbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1−x)(Na_0.5K_0.5)NbO₃-xBiFeO₃ (x=0∼0.07) were synthesized by the solid-state reaction. Differential scanning calorimetry (DSC) measurements revealed that an increase in the amount of BiFeO₃ dopant resulted in a decrease in the orthorhombic-tetragonal and tetragonal-cubic phase transition temperature of the material. One percent BiFeO₃ additive suppressed grain growth, which not only benefits the sintering of ceramics but also enhances the piezoelectric and ferroelectric properties, where d₃₃=145pC/N, k_p=0.31, Q_m=80, P_r=11.3 μC cm⁻² and E_c=16.5 kV cm⁻¹. As x_BF>0.01, both piezoelectric and ferroelectric properties decreased rapidly with an increasing amount of dopant.     

(Na0.5K0.5)NbO3–LiTaO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics were prepared in the system (1−x)(Na_0.5K_0.5)NbO₃–xLiTaO₃ following the conventional mixed oxide route. The effect of cationic substitution of lithium for sodium and potassium in the A sites and tantalum for niobite in the B sites in (Na_0.5K_0.5)NbO₃ (NKN) perovskite lattice on symmetry and physical properties were investigated. The materials with perovskite structure are in orthorhombic phase when x<5 mol.% and transform to tetragonal phase when x>6 mol.%; when x≥8 mol.%, a K₃Li₂Nb₅O₁₅ phase with tetragonal tungsten bronze structure begins to appear and becomes dominant with increasing content of LiTaO₃. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases appears at x=5–6 mol.%. Analogous to Pb(Zr,Ti)O₃, the piezoelectric and electromechanical properties are enhanced for compositions near the morphotropic phase boundary. Piezoelectric constant d₃₃ values reach ∼200 pC/N. Electromechanical coefficients of planar mode reach ∼36%, respectively. Our results show that (Na_0.5K_0.5)NbO₃ with small amount of LiTaO₃ (x=7 mol.%) is a good lead-free piezoelectric ceramic.     

A Rosen-type piezoelectric transformer employing lead-free K0.5Na0.5NbO3 ceramics

Lead-free K_0.5Na_0.5NbO₃–K_5.4Cu_1.3Ta₁₀O₂₉–MnO₂ (KNN–KCT–Mn) ceramics have been prepared by a conventional ceramic sintering technique. The ceramics show excellent piezoelectric properties for application in power devices, and the optimum properties measured are as follows: piezoelectric constant d ₃₃ = 90 pC/N, planar electromechanical coupling factor k _p = 0.40, mechanical quality factor Q _m = 1900, remanent polarization P _r = 11.8 μC/cm², coercive field E _c = 0.85 kV/mm. A Rosen-type piezoelectric transformer with a dimension of 21 mm × 6 mm × 1.2 mm was fabricated using the KNN–KCT–Mn ceramics. Properties of the piezoelectric transformer operating in the first and second modes have been characterized. For the first mode, the transformer has a maximum output power of 0.7 W with a temperature rise of 14 °C. For the second mode, the maximum output power of the transformer is 1.8 W with a temperature rise of 33 °C. KNN–KCT–Mn ceramics have shown to be a potential lead-free candidate to be used in high-voltage–low-current devices.     

Microstructure and enhanced electrical properties of (1−x)(Na0.5K0.44Li0.06)NbO3–x(Ba0.85Ca0.15)(Zr0.10Ti0.90)O3 lead-free ceramics

Environment-friendly lead-free piezoelectric ceramics (1?x)(Na_0.5K_0.44Li_0.06)NbO₃–x(Ba_0.85Ca_0.15)(Zr_0.10Ti_0.90)O₃ doped with 1.0 mol% MnO₂ were synthesized by conventional solid-state sintering method. The phase transition behavior and electrical properties of the ceramics is systemically investigated. It was found that all the ceramics formed pure perovskite phase with 0.0 ≤ x ≤ 0.1, and the phase structure of the ceramics gradually transformed from orthorhombic to tetragonal phase with increasing x. Coexistence of the orthorhombic and tetragonal phase is formed in the ceramics with 0.04 ≤ x ≤ 0.06 at room temperature, and enhanced dielectric, ferroelectric and piezoelectric properties are achieved in the two phase’s region. The ceramics in the mixed phase region exhibits the following optimum electrical properties: d ₃₃  = 130–147 pC/N, ε _r  = 642–851, P _r  = 5.51–12.44 μC/cm². The Curie temperature of the ceramics with mixed phase region was found to be 353–384 °C. The significantly enhanced dielectric properties, ferroelectric properties and piezoelectric properties with high cubic-tetragonal phase transition temperatures (T _c ) make the KNLN–xBCZT ceramics showing the promising lead-free piezoelectrics for the practical applications.     

Preparation and piezoelectric properties of CuO-doped (Na0.5K0.5)NbO3 ceramics by the citrate precursor method

A low-cost citrate so-gel route was investigated to synthesize nano-sized crystalline powders (<100 nm) of 1 mol% CuO modified (Na_0.5K_0.5)NbO₃ compositions. It was found that amorphous gels can be transformed into crystallite powders with single-phase perovskite structure when calcined at 500?C600 °C for 3 h. The transmission electron microscopy observation showed that the particles are column-like and well dispersed, depending on the calcination condition. The as-pressed samples exhibit improved densification behavior and finer grain morphology after sintering. Electrical properties of the samples sintered at 1,060 °C are as follows: dielectric constant ?? _r  = 605, piezoelectric constant d ₃₃ ~ 117 pC/N, planar electromechanical coupling factor k _p  ~ 0.38 and mechanical quality factor Q _m  ~ 725.     

(1-x)K0.48Na0.52NbO3-xBi0.45Nd0.05(Na0.92Li0.08)0.5ZrO3无铅压电陶瓷相结构及压电性能

为了在获得较高压电性能的同时又不大大降低陶瓷的居里温度(T_C), 设计和制备了Bi_0.45Nd_0.05(Na_0.92Li_0.08)_0.5ZrO₃改性的K_0.48Na_0.52NbO₃系无铅压电陶瓷((1-x)KNN-xBNNLZ), 研究了BNNLZ含量对KNN基无铅压电陶瓷相结构和电学性能的影响。研究结果表明, 所有陶瓷样品均具有较高的居里温度T_C(>300℃)。随着BNNLZ含量的增加, 陶瓷的正交-四方相变温度(T_O-T)不断向低温方向移动, 而三方-正交相变温度(T_R-O)不断向高温方向移动, 最终在陶瓷中形成了三方-四方(R-T)共存相, R-T共存相处于0.05<x<0.07范围。BNNLZ的加入引起陶瓷相结构的演化改变导致压电常数(d₃₃ )、介电常数(ε_r )、剩余极化强度 (P_r )和机电耦合系数(k_p )都先增大后减小, 当x=0.06时陶瓷具有最佳压电性能: d₃₃=313 pC/N, k_p=42%, P_r=25.48 μC/cm², ε_r=1353, tanδ=2.5%, T_C=327℃。     

Low-temperature hydro/solvothermal synthesis of Ta-modified K0.5Na0.5NbO3 powders and piezoelectric properties of corresponding ceramics

K_0.5Na_0.5Nb_1–xTa_xO₃ (KNNTx, x = 0–0.4) powders were synthesized by a novel hydro/solvothermal method at a low reaction temperature (180 °C) and the corresponding ceramics were obtained by normal sintering. Compared with conventional solid-state reaction technique, the optimal sintering temperatures of these ceramics were reduced at least 150 °C. Crystalline structures and surface morphologies were analyzed by X-ray diffraction and scanning electron microscopy. The excellent piezoelectric properties could be obtained by selecting poling temperature near the orthorhombic–tetragonal polymorphic phase transition temperature region. Ta-modified KNN ceramics exhibited better piezoelectric properties than those of pure KNN, and the piezoelectric coefficient d₃₃ showed the maximum value of 156 pC/N for KNNT0.3 ceramics. In addition, the sintering temperature for maximum d₃₃ value differed from that for maximum density. The present hydro/solvothermal method provides a new potential route for preparing KNN-based materials at relatively low temperature.     

Reactive templated grain growth and anisotropic electrical properties of (Na0.5K0.5)NbO3 ceramics without sintering aids

(Na_0.5K_0.5)NbO₃ (KNN) ceramics with {100} orientation without sintering aids were fabricated by a conventional process and reactive templated grain growth using tabular NaNbO₃ template particles. The KNN specimen sintered at 1,170?°C for 15?h were found to have a relative density of 95.6%. The experimental results show that the textured ceramics have a pseudo-cubic {100} orientation degree of 96.2% and a microstructure with brick-like grains aligning in the direction parallel to the casting plane. The dielectric constant is much higher than the value for non-textured ceramics. Transition temperatures (T _o–t, T _c) are reduced and dielectric peaks are greater breadth. Remanent polarization P _r decreased from 19.40 (the random KNN) to 13.77 (textured KNN) μC/cm². The textured ceramics show anisotropic electrical properties between different directions of textured KNN ceramics, and show a very high electromechanical coupling factor k _p?=?0.58 and a high piezoelectric constant d ₃₃?=?225?pC/N, compared to the random counterparts (k _p?=?0.31, d ₃₃?=?115?pC/N).     

Phase transition and piezoelectric properties of (1 − x)K0.5Na0.5NbO3–xLiSbO3 ceramics by hydrothermal powders

KNbO₃, NaNbO₃ and LiSbO₃ powders were synthesized by a hydrothermal route have been used to prepare (1 ? x)K_0.5Na_0.5NbO₃–xLiSbO₃ (KNN–LS; x = 0.00–0.08) ceramics. The effects of LiSbO₃ doping on the structures of KNN–LS ceramics have been systematically investigated by X-ray diffraction (XRD) and Rietveld refined XRD patterns. A gradual phase transition from orthogonal to tetragonal with the increase of LiSbO₃ content is demonstrated. Thereinto, the monoclinic phase is identified for the KNN–LS ceramic with the LiSbO₃ content of x = 0.08. Meanwhile, the XRD pattern reveals that the intensity ratio of (200)/(002) crystal face of the ceramic with x = 0.08 was bigger than one, which is different from the tetragonal phase. The tetragonal phase is revealed in the KNN–LS ceramic in the vicinity of x = 0.07, accompanying with relatively higher piezoelectric and ferroelectric properties. Tetragonal phase is beneficial to improve the piezoelectric properties of the KNN–LS ceramics.     

Phase transitions in Li0.13Na0.87NbO3 crystals

Li_0.13Na_0.87NbO₃ crystals were studied by dilatometric, dielectric, acoustic emission, and optical methods on heating in a temperature range from 20 to 700°C. Based on the results of these measurements, the following sequence of phases was established: 20–35°C, ferroelectric orthorhombic Pmm2; 350–40°C, ferroelectric orthorhombic Pmmm; 400–63°C, ferroelectric tetragonal, 14/mmm; above 63°C, cubic Pm3m. The latter two transformations can be classified as second-order ferroelectric phase transitions.