Papers by Keyword: High Curie Temperature

Abstract: (K,Na)NbO₃(KNN)-based lead-free piezoelectric ceramic could be fabricated by ordinary solid sintering method.This paper reported preferable properties of solid solutions (1-x)(Na_0.535K_0.480)NbO₃-xLiNbO₃ (x=0.050,0.055,0.060,0.065 and 0.070 successively) all sintered at 1060°C.The nonconservation of charge suppressed negative effects caused by evaporations of K&Na and doped Li occupied A sites in ABO₃ perovskite structure lattices led to the intensification of lattice distortion.XRD result showd phase transition from coexistence of orthorhombic and tetragonal symmetry to tetragonal.Polymorphic phase boundary could be observed when 0.050≤ x≤ 0.060.Coexistence of orthorhombic and tetragonal phases brought improvements of piezoelectric coefficient d₃₃ and electromechanical coupling factor kp,whose top values were 162 pC/N and 28.1% in this paper. Positive correlation between the Curie temperature and Li content proved that a distorted crystal lattice needed more energy to accomplish its phase transition from tetragonal to cubic than a normal one.

Abstract: BaTiO₃-(Bi_0.5Na_0.5)TiO₃ (BTBNT) solid solution ceramics with the Curie temperature higher than 150°C were prepared, which were promising for X9R MLCCs application. (Bi0.5Na0.5)TiO3 (BNT) was first synthesized by the conventional solid state reaction and then it was mixed with BaTiO₃ (BT) with increasing BNT content from 0 to 12 mol%. BaTiO₃-(Bi_0.5Na_0.5TiO₃ solid solutions were obtained after calcining at 1100°C. The structural and dielectric properties of BTBNT and Nb-doped BTBNT ceramics were investigated.

Abstract: In this paper, Nb-doped BaTiO₃-(Na_0.5Bi_0.5)TiO₃ (BBNT-Nb) based PTCR ceramic material has been prepared using high purity Nb₂O₅, Bi₂O₃, Na₂CO_3, TiO₂ and BaTiO₃ powders as starting materials, and its structural and electrical properties are investigated. The results indicated that the Nb₂O₅ dopant is helpful not only to promote the formation of tetragonal BBNT structure without any second phase, but also to decrease the room-temperature resistivity. For the 0.10at% Nb-doped BBNT material with BNT content 35mol%, the properties of room-temperature resistance of 10⁴Ω·cm, temperature coefficient (α_k) of 6.3%/°C, PTCR anomaly lg(R_max/Rmin) of 3.6 and Curie temperature (Tc) of 193°C are obtained.

Abstract: (1-x)(0.6BIT-0.4BT)-xBiYbO3(BTBY) ceramics with high Curie temperature were prepared by the conventional processing. The effect of BiYbO3 content on the microstructure and electrical properties was investigated. The results show that the main phase of BTBY ceramics is BIT-BT, and a new phase BaBi4Ti4O15 (BBT) appeared. The grain morphology of BTPY ceramics are platelike and the grain size was significantly increased with increasing content of BiYbO3 .Tc of all the BTBY samples are above 440 . The BTBY ceramics show obvious dielectric relaxor characteristic. The dispersion factor γ, the dielectric constant εr, and the dielectric loss tanδ decreased with increasing the content of BiYbO3. When the content of BiYbO3 is 0.06, the optimal properties of BTBY ceramics are obtained, Tc is 457 , εr is 165, tanδ is 0.0223 and d33 is 10 pC·N-1.

Abstract: Ceramic materials based on lead titanate, lead niobate and bismuth layer-structured ferroelectrics (BLSF) were studied to develop piezoelectric ceramics for high temperature sensor applications. Compositional modification enabled lead titanate and lead niobate type ceramics to exhibit good piezoelectric properties at 500°C . The Curie temperature for one BLSF, CaBi4Ti4O15 was close to 800°C, though the piezoelectric constant was smaller than those of lead titanate and lead niobate ceramics. These ceramics seem to be good candidates for use as high temperature sensor materials. In addition, textured SrBi2Nb2O9 (SBN), another BLSF, ceramics with various orientation factors were fabricated through the templated grain growth (TGG) method. The resonant frequency of 76% textured SBN varied linearly with temperature and exhibited stable temperature characteristics. The temperature coefficient of the resonant frequency was –0.85 ppm/°C from –50 to 250°C, and was smaller than that of a quartz oscillator. Therefore, textured SBN ceramics are suitable for use as a resonator material when stable resonant frequency is needed in a high temperature range.

Abstract: PTCR ceramic samples of two systems, Bi4Ti3-xNbxO12+x/2(BTNO, 0≤x≤0.01) and Bi4Ti3O12 + (1-z)BaTi1-xNbxO3+x/2+2.0mol%TiO2 (BTO+(1-z)BTNO, 0≤x≤0.01, 0≤z≤1.0), were prepared by mixed oxide method. The properties of the sapmples were characterized by XRD, AFM and electrical properties testing. The results showed that single BaBi4Ti4O15 was obtained when x=0.05 and z=0.5. The roomtemperature resistivity of BTO+(1-z)BTNO ceramic sintered in air was 102-1045·cm. The minimum resistivity occurred at Nb content of about 0.3mol%.

Abstract: Growth of ferroelectric Bi2WO6 (BWO) mono-domain bulk crystals was attempted by the vertical-Bridgman (VB) method below the phase transition (ferro- to paraelectric) temperature of 940oC using Li2B4O7 as a flux. In this method, Pt crucibles with different shapes were used. The crucible with a wedged tip bottom produced BWO crystal with a thickness of over 4 mm along the crystallographic c-axis (perpendicular to the spontaneous polarization axis). Using BWO mono-domain crystals grown by slow cooling technique, on the other hand, their electric properties were characterized. The dielectric constants, ε ij, and electromechanical coupling factor, k33, of the crystals were 70-100 and 36% at room temperature, respectively.