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Rare-earth Ions Doping and Fabrication of CaBi4Ti4O15 Materials

Author: PengZongZhi
Tutor: HeXinHua
School: South China University of Technology
Course: Microelectronics and Solid State Electronics
Keywords: Bismuth layered structure ferroelectrics (BLSFs) rare-earth doping dielectric properties piezoelectric properties hydrothermal
CLC: TM282
Type: Master's thesis
Year: 2013
Downloads: 19
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Abstract


Ferroelectric ceramics,CaBi4Ti4O15(CBT),are well known as one of thehigh-performance materials,and have been widely used in micro-electronics andelectronic sensing technologies, etc. They are expected to be one of the potentialcandidate materials which can substitute for the PZT-based ferroelectric ceramics.Bismuth layered structure ferroelectrics CaBi4Ti4O15have high Curie temperature(Tc=790℃), low relative dielectric constant (εr=168) and dielectric loss(tanδ=13×10-4), which are suitable for their application in high-temperature andhigh-frequency devices. However, because of their low density (ρv=7.28g/cm3), highsintering temperature (1180℃), small piezoelectric constant (d33=7pC/N), smallelectromechanical coupling factor (kt<10%) and large temperature coefficient ofresonance frequency (frTc≥4×10-5/℃), their improved properties are demanded tofulfill various requirements.In this paper, the effect of rare-earth ions dopants on the properties and structureof CaBi4Ti4O15is investigated, with Ca1-xMxBi4Ti4O15+x/2(M stands for rare-earthelement) as chemical formulation. In the mean time, the fabrication process ofCaBi4Ti4O15powder by wet-chemical methods is also studied. The main contents areas follows:(1) The substitution of rare-earth ions Sc3+, Y3+and La3+for A site of CaBi4Ti4O15isinvestigated. The effect of Sc3+, Y3+and La3+on the crystalline phase, dielectricproperties and piezoelectric properties of CBT ceramics is investigated, whilerare-earth elements Sc, Y and La, with the same numbers of outer electrons butdifferent numbers of electronic layers, substitute for A site. The researchindicates that doping with a small amount of rare-earth ions Sc3+, Y3+and La3+does not change the main crystalline phase of ceramics, and can diminish theshape anisotropy of crystalline grain, increase the density of ceramics, decreasethe dielectric loss and conductivity at high temperature, therefore, improve thepiezoelectric activity. The variation of Curie temperatures of CBT ceramics(TSc>TY>TLa) with the radii of doping ions (rSc3+<rY3+<rLa3+) is in accordance with the tolerance-factor formula. The Curie temperature of ceramics doped withSc3+,Y3+increases while that of ceramics doped with La3+decreases. When largeamount of Y3+and La3+is added, a phase-transition peak will appear below theCurie temperature in the εr-T curve.(2) The substitution of rare-earth ions Dy3+and Yb3+for A site of CaBi4Ti4O15isinvestigated. The effect of Dy3+and Yb3+on the crystalline phase, dielectricproperties and piezoelectric properties of CBT ceramics is investigated, whilerare-earth elements La, Dy and Yb with decreasing ionic radii substitute for Asite. The research indicates that lattice distortion occurs by small amount ofrare-earth ions La3+, Dy3+and Yb3+doping, and therefore, results in the increasein Tcand improved properties at high temperature. With decreasing radii of thelanthanide elements, the smaller ionic radius, the larger lattice distortion,consequently, the higher Tc. The value of Tcis851℃at a doping amount of Ybof0.05mol. The solid solubility of rare-earth ions in the crystal lattice of CBTdecreases markedly with the decreasing radii of lanthanide elements. La3+showshighest solid solubility in crystal lattice among these three dopants, and thusbetter modification effect than that of Dy3+and Yb3+doping.(3) The fabrication of CaBi4Ti4O15powder by a sol-gel and hydrothermal method isinvestigated. Combining sol-gel method and hydrothermal process, wedeveloped a new two-hydrothermal process by optimizing raw materials,hydrothermal conditions, solvent and mineralizer, and plate-like CaBi4Ti4O15nano-powders with good crystallization are obtained. The effect of hydrothermalconditions on main crystalline phase of nano-powders is studied. TheCaBi4Ti4O15ceramics derived from the synthesized nanometer powders exhibitdense microstructure with fine grain.

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