Microstructure and electrical properties of Ti-modified (Na0.5K0.5)(TiχNb1-χ)O3 lead-free piezoelectric ceramics

Ti-Modified (Na0.5K0.5)(TixNb1-x)O3 (NKNT) piezoelectric ceramics were fabricated by double-layer buffed powder process at 1020℃ for 2 h. The microstructures, and piezoelectric and dielectric properties of the lead-free NKNT ceramics were investigated. X-ray diffraction re-suits indicated that Ti4+ had diffused into the (Na0.5K0.5)NbO3 lattices to form a solid solution with a perovskite structure. The introducing of Ti into the (Na0.5K0.5)NbO3 solid solution effectively reduced the sintering temperature and densified the microstructure with a decreased grain size. The highest relative density reached more than 90%. The highest piezoelectric dielectric coefficient d33 and planar mode electro mechanical coupling coefficient kp were 110 pC/N and 19.5%, which were obtained in the NKNT ceramic with 1 mol% Ti. The piezoelectric properties of the NKNT ceramics were enhanced by aging in air for a period of time owing to the compensation of oxygen vacancies.     

Structure and piezoelectric properties of (1–x)K0.5Na0.5NbO3–xLiBiO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1–x)(K_0.5Na_0.5)NbO₃–xLiBiO₃ [(1–x)KNN–xLB] (x=0, 0.0005, 0.002, 0.004, 0.006, 0.008, 0.010) were prepared by an traditional solid-state reaction. The microstructure and electrical properties of the ceramics were investigated. The results show that all (1–x)KNN–xLB ceramics possess pure perovskite structure when x=0.01, no trace of any secondary phase is detected, and the phase structure of the ceramics transits abnormally from orthorhombic to cubic. With the increase of the LB content, the size of grain gradually becomes small, the piezoelectric constant d₃₃ and the planar electromechanical coupling coefficient k_p first increases and then decreases. The d₃₃ and k_p of the ceramics reach their maximum values 115 pC/N at x=0.002 and 0.2701 at x=0.001, respectively. The dielectric constant e_r of the ceramics firstly increases evidently and then decreases with the increase of x, the maximum value 871.8 is obtained at x=0.006.     

Piezoelectric and dielectric properties of Lix（K0.46Na0.54） 1-xNb0.86Ta0.1 Sb0.04O3 lead-free ceramics

Lead-free piezoelectric ceramics Lix（K0.46Na0.54）1-xNb0.86Ta0.1Sb0.0403 （with x ranging from 0 to 0.1） were synthesized by conventional solid state sintering method. The effect of cationic substitution of Li for K and Na in the A sites of perovskite lattice on the structure, phase transition behavior and electrical properties were investigated. Morphotropic phase boundaries（MPB） between orthorhombic and tetragonal phase are found in the composition range of 0.06≤x≤0.08. Analogous to Pb（Zr, Ti）O3, the dielectric and piezoelectric properties are enhanced for the composition near the morphotropic phase boundary. The Li0.06（K0.46Na0.54）0.94- Nb0.86Ta0.1Sb0.04O3 ceramics show excellent electrical properties, that is, piezoelectric constant d33=215 pC/N, planar electromechanical coupling factor kp=41%, dielectric constant ε33^T /ε0=1 303, and dielectric loss tan δ=2.45%. The results indicate that Lix（K0.46Na0.54）1 -xNb0.86Ta0.1Sb0.0403 ceramic is a promising lead-free piezoelectric material.     

Preparation and properties of （K0.5Na0.5）NbO3-LiNbO3 ceramics

The lead-free piezoelectric ceramics （1-x）（K0.5Na0.5）NbO3-xLiNbO3（abbreviated as KNLN） were synthesized by a traditional solid state reaction. The effects of Li^＋ on the sintering characteristic, the phase structure and piezoelectric properties of KNLN ceramics were investigated. The sintering temperature of KNN-based ceramics is decreased by doping Li^＋ and the range of the sintering temperature is narrow. The KNLN ceramics exhibit an enhanced piezoelectric properties with the piezoelectric constant d33 value of 180-200 pC/N, The electromechanical coupling coefficients kp is 35%-40%. The results show that （1-x）（K0.5Na0.5）-NbO3-xLiNbO3 （x=0.05, 0.06） is a promising high-temperature lead free piezoelectric ceramic.     

Structural evolution and physical properties of Bi1−xGdxFeO3 ceramics

The crystal structure and physical properties of Bi_1?xGd_xFeO₃ ceramics with x ≤ 0.5 have been studied by X-ray diffraction, differential scanning calorimetry, dielectric and magnetic measurements. Bi_1?xGd_xFeO₃ compounds crystallize at room temperature in R3c structure for x < 0.08 and in Pbnm structure for x ≥ 0.3. The R3c phase transforms to the Pbnm phase as temperature increases. Temperature-dependent dielectric response and low-temperature X-ray diffraction indicate a phase transition of the Pbnm phase at low-temperature. Detailed structural analysis indicates that the interaction between Bi and O plays an important role in the evolution of structure and physical properties. A phase diagram is constructed to illustrate the physical property–composition–temperature relations.     

Dielectric properties and relaxor ferroelectric behavior of (1–y)Ba(Zr0.1Ti0.9)O3–yBa(Zn1/3Nb2/3)O3 ceramics

The microstructure, dielectric and ferroelectric properties of (1–y)Ba(Zr_0.1Ti_0.9)O₃–yBa(Zn_1/3Nb_2/3)O₃ (y=0–0.05) ceramics prepared by traditional solid state method were investigated by X-ray diffractometer, scanning electron microscope, electric parameter testing system and ferroelectric tester. It is found that the barium zirconate titanate based ceramics are single-phase perovskites as y increases up to 0.05 and their average grain size decreases with the increase of y. The permittivity maximum ε_r,max is suppressed from 8948 to 1611 at 1 kHz with increasing y, and the ferroelectric–paraelectric phase transition temperature T_m decreases from 93 to –89 °C at 1 kHz as y increases. The composition-induced diffuse phase transition is enhanced with increasing y. The relaxor-like ferroelectric behavior with a strong frequency dispersion of T_m and permittivity at T<T_m accompanied by a strong diffuse phase transition is found for the system with high y value. The remnant polarization decreases with increasing y, while the coercive field decreases remarkably and then increases with the increase of y.     

Synthesis,characterization and electrical properties of the system LaMnxFe1−xO3

The system manganese substituted lanthanum ferrite viz. LaMn_xFe_1?xO₃ (1.0 ≤ x ≥ 0) was prepared by sol–gel autocombusion method. The structural characterization of the samples was carried out by X-ray diffraction technique and it is found that, the phase transfer from cubic to orthorhombic perovskite structure. The lattice parameter and crystallite size decrease with increasing Mn content. The phase formation of perovskite was revealed by thermal analysis technique. The surface morphology and elemental analysis of all the samples were carried out by scanning electron microscopy and energy dispersive X-ray spectroscopic technique, respectively. Electrical properties of the compounds show that, they exhibit semiconducting behavior. The substitution of manganese ions plays an important role in changing their structural, electrical and magnetic properties of lanthanum ferrite.     

Thermoelectric properties of solution-combustion-processed Na(Co1−xNix)2O4

We synthesize nano-sized Na(Co_1?xNi_x)₂O₄ (0 ≦ x ≦ 0.12) powders by the solution combustion method using aspartic acid as combustion fuel. The influence of Ni²⁺ on the thermoelectric properties for the Na(Co_1?xNi_x)₂O₄ is investigated. When the Ni²⁺ content is increased, the electrical conductivity (σ) is lowered mainly because of an increase in the porosity. In addition, the Ni²⁺ substitution leads to a significant increase in the Seebeck coefficient (α) up to x=0.06, and then a decrease with further increasing its content. The power factor (σα²) is thus improved by the Ni²⁺ substitution. We obtain the highest power factor (5.55×10^?4 Wm^?1K^?2 at 800 °C) for Na(Co_0.94Ni_0.06)₂O₄. This value is over five times higher than that of NaCo₂O₄ (1.08×10^?4 Wm^?1K^?2) at 800 °C.     

Microstructure and electrical properties of sol–gel derived Ni-doped CaCu3Ti4O12 ceramics

Dielectric properties and varistor performance of sol–gel prepared Ni-doped calcium copper titanate ceramics (CaCu₃Ni_xTi₄O_12+x, x=0, 0.1, 0.2, 0.3) were investigated. SEM and XRD were used in the microstructural studies of the specimens and the electrical properties were investigated for varistors. XRD patterns show that the CCTO ceramics were single phase with no Cu-rich phase. SEM results indicated that the samples had smaller grain sizes than those synthesized by traditional solid-state reaction methods. The experimental results show that the highest dielectric constant and lower dielectric loss occur when x=0.2. When x=0.3, the lowest leakage current is obtained and the maximum value reaches 0.295; meanwhile, the lowest threshold voltage and nonlinear coefficient are found, the minimum values of them are 1326 V/mm and 3.1, respectively.     

Dielectric properties of Mg- and Mn-doped (Ba1−xSrx)(Ti1−yZry)O3

This study showed that the substitution of SrO and ZrO₂ in BaTiO₃ results in a change in the unit cell volume, Curie temperature shift and temperature dependence of dielectric constants. The unit cell volume decreased with an increasing SrO content (≤15 mol%), but increased with an increasing ZrO₂ (≤10 mol%). Substitution of Zr⁴⁺ ions for Ti sites in Ba(Ti_1?yZr_y)O₃ shifted the Curie point (T_c) to lower temperatures by 4.1 °C per mol% of ZrO₂ at y≤0.10. SrO substitution in the samples with 10 mol% ZrO₂ (Ba_1?xSr_x)(Ti_0.9Zr_0.1)O₃ brought about relatively small shifts in T_c (2.2 °C/mol%), and their dielectric constants decreased with an increasing SrO content near T_c. Mg- and Mn-doped (Ba_0.9Sr_0.1)(Ti_0.9Zr_0.1)O₃ broadened the dielectric constant peaks at T_c with uniform microstructures, which satisfies the temperature characteristics of Y5V specifications. The leakage currents of the Mg- and Mn-doped samples were significantly suppressed.     

Effects of cooling rate on the microstructure and electrical properties of Dy2O3-doped ZnO-based varistor ceramics

The microstructure, electrical properties, and density of Dy2O3-doped ZnO-based varistor ceramics, prepared using high-energy ball milling （HEBM） and sintered at 800℃, were investigated by increasing the cooling rate in the order of H （slow cooling in furnace） → L （cooling in furnace） → K （cooling in air）. With the increase in cooling rate, the grain size and density decreased, the breakdown voltage （VImA/mm） increased, and the nonlinear coefficient （α） and leakage current （IL） exhibited extremum. The sample with the cooling type L showed the best properties with the breakdown voltage of 2650 V/ram, o：of 20.3, IL of 5.2 laA, and density of 5.42 g/cm^3. The barrier height （ФB）, donor concentration （Nd）, density of the interface states （Nd）, and barrier width （ω） all exhibited extremum during the alteration in cooling rate. The different relative amount of Bi-rich phase and its distribution as well as the characteristic parameters of grain boundary, resulting from the alteration of cooling rate, led to the changes in the properties of varistor ceramics.     

Temperature-dependent R-curve behavior of Pb(Zr1−xTix)O3

The crack growth resistance behavior of polycrystalline Pb(Zr_1?xTi_x)O₃ has been characterized in a novel experimental arrangement between 24 and 140 °C. Experimental measurements were carried out on compact tension specimens submerged in a temperature-controlled silicone oil bath. The results show a decrease in the observed shielding toughness, leading to an overall reduction in the maximum toughness. The temperature-dependent stress–strain behavior and elastic properties were characterized, providing an insight into the effect of the changing ferroelastic properties on the temperature-dependent fracture behavior.     

Nanoscaled grain boundaries and pores,microstructure and mechanical properties of translucent Yb:[LuxY(1−x)O3] ceramics

Translucent ceramics of Yb:[Lu_xY_(1?x)O₃] system doped by ZrO₂ was sintered from nanopowder synthesized by laser evaporation. The relative density of the ceramics was 99.97%, residual pores had sizes from 8 nm to 20 nm, Young modulus was 200 GPa at the applied load of 2000 mN, the microhardness was 12.8 GPa. The grains of ceramics had sizes 1–10 μm, but the thickness of grain boundaries was about 1 nm. The transcrystalline type of the crack propagation was detected in the specially broken ceramics. The results indicated high strength of grain bonds and good perfection of grain boundaries in the studied ceramics but an increased content of pores (higher than 10^?3 vol.%) and stoichiometry deviation (Lu:Y:O = 0.21:0.79:3) from the required one (Lu:Y:O = 0.25:0.75:3).     

Effect of poling condition on piezoelectric properties of （K0.5Na0.5）NbO3 ceramics

The lead-free piezoelectric ceramics （K0.5Na0.5）NbO3 （abbreviated as KNN） with the relative density of 97.6% were synthesized by press-less sintering owing to the careful control of processing conditions. The phase structure of KNN ceramics was analyzed. The results show that the pure perovskite phase with orthorhombic symmetry is in all ceramics specimens. The effect of poling conditions on the piezoelectric properties of KNN ceramics was investigated. The results show that the piezoelectric constant d33 and electromechanical coupling factor kp increase with poling field, poling temperature and poling time increasing, then decrease because of electric broken. Take into account of poling conditions and piezoelectric properties of pure KNN ceramics, the optimum poling conditions for pure KNN ceramics are poling field of 4 kV/mm, poling temperature of 140 ℃ and poling time of 20-25 min.     

Influence of CaF2 on the structure and dielectric properties of Ag（Nb0.8Ta0.2）O3 ceramics

Ag(Nb0.8Ta0.2)O3 ceramics were prepared by the traditional solid-state reaction method. The effect of CaF2 addition on the structure and di-electric properties of Ag(Nb0.8Ta0.2)O3 ceramics was investigated. The addition of CaF2 led the ceramics to a larger grain size and distortion of lattice. With the addition of 4.5 wt.% CaF2, the permittivity of the ceramics increased from 442 to 1028, the dielectric loss decreased sharply from 6.12 × 10-3 to 8.6 × 10-4, and the temperature coefficient of capacitance decreased from 1834 ppm/°C to-50 ppm/°C (at 1 MHz). These results indicated that the high permittivity was related with a large grain size, a low grain boundary density, and the weak Ta-O or Nb-O bond strength caused by the addition of CaF2.     

Phase transformation behavior and mechanical properties of Ti50Ni49−x Fe1Co x shape memory alloys

In this article, the influence of Co addition on phase transformation behavior and mechanical properties of TiNiFe shape memory alloy was investigated extensively. Differential scanning calorimetry (DSC) measurements shows that martensitic start transformation temperatures (Ms ) decrease drastically with increasing Co content, while the R phase transformation start temperatures (Rs ) vary slightly. Nevertheless, the substitution of Ni with Co does not exert substantial influence on the two-stage transformation behavior of the TiNiFe alloy. The results from stress-strain curves indicate that higher critical stress for stress-induced martensitic transformation (rSIM ) has been obtained because of Co addition. In such cases, the Ti50Ni48Fe1 Co1.0 alloy maintains a good shape memory effect, and a maximum recoverable strain of 7.5 % can be obtained.     

Phase composition and structure of grain boundary of oversintered Y3Al5O12 ceramics

Phase composition and microstructures of grain boundary of oversintered yttrium aluminum garnet （Y3Al5O12, YAG） ceramics by vacuum sintering at 1 850 ℃ were investigated. For synthesizing YAG, grain boundary is a key factor for YAG ceramics. The morphology of grain boundary was observed by SEM, TEM and its composition was analyzed by EDS. It is identified that the grain boundary is composed of a-AI2O3 and yttrium aluminum perovskite （YAP, YAlO3） eutectics. At the edge of YAG crystal grain, YAG phase is decomposed into perovskite YAP and α-Al2O3 during high temperature sintering. Due to refractive indexes of YAP and α-Al2O3 phases in wide grain boundary are different from those of YAG, the transmittance of oversintered YAG ceramics is lower than that of YAG ceramics sintered at 1 750 ℃.     

Microstructure and properties of Cu−2Cr−1Nb alloy fabricated by spark plasma sintering

Cu?2Cr?1Nb alloy was fabricated by spark plasma sintering (SPS) using close coupled argon-atomized alloy powder as the raw material. The optimal SPS parameters obtained using the L₉(3⁴) orthogonal test were 950 °C, 50 MPa and 15 min, and the relative density of the as-sintered alloy was 99.8%. The rapid densification of SPS effectively inhibited the growth of the Cr₂Nb phase, and the atomized powder microstructure was maintained in the grains of the alloy matrix. Uniformly distributed multi-scale Cr₂Nb phases with grain sizes of 0.10?0.40 μm and 20?100 nm and fine grains of alloy matrix with an average size of 3.79 μm were obtained. After heat treatment at 500 °C for 2 h, the room temperature tensile strength, electrical conductivity, and thermal conductivity of the sintered Cu?2Cr?1Nb alloy were 332 MPa, 86.7% (IACS), and 323.1 W/(m·K), respectively, and the high temperature tensile strength (700 °C) was 76 MPa.     

Evolution of dielectric properties in BaZrxTi1−xO3 ceramics: Effect of polar nano-regions

BaZr_xTi_1?xO₃ (x = 0.2, 0.25, 0.3) ceramics were prepared by a citrate method. The ferroelectric phase transition behavior and bias electric-field-induced dielectric nonlinearity of the ceramics were investigated. With increasing zirconium content, the phase transition behavior of the ceramics changed from a diffusion phase transition to relaxor-like behavior. Applying bias electric field in diverse manners led to differing effects on the dielectric nonlinearity, depending on the zirconium content of the ceramics. These dielectric phenomena were related to the polarization response of polar nano-regions (PNRs) embedded in the ceramics, which are macroscopically in the paraelectric state at room temperature. The characteristic parameters of PNRs were determined by fitting the dielectric constants under bias electric field to a multipolarization mechanism model. It was found that the size of PNRs was decreased with increasing zirconium content. The evolution of the phase transition behavior and dielectric nonlinearity with zirconium content was interpreted in relation to the size change of PNRs.     

In situ synthesis and electrical properties of CuW–La2O3 composites

CuW–La₂O₃ composites were fabricated using an in-situ synthesis. The breakdown voltage in vacuum, moving trajectory of cathode spots, electrical conductivity, and hardness of CuW–La₂O₃ composites were carefully examined. The microstructures were characterized by a scanning electron microscope (SEM). The results show that CuW–La₂O₃ composites have the maximum hardness of 220HB and the electrical conductivity of 45% IACS when the content of La₂O₃ is 0.75 wt.%. In comparison with CuW alloy, the dielectric strength, arc life and the arc mobility of the CuW–La₂O₃ composites increased by 36.9%, 9.7% and 46.6%, respectively. As a result, the addition of La₂O₃ is useful to improve the properties of CuW alloys and the in situ synthesized CuW–La₂O₃ composites should have excellent arc erosion resistance.