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Title: Study of lead-free ferroelectric ceramics with tunable photoluminescence properties
Authors: Chan, Yi Lok
Degree: M.Phil.
Issue Date: 2018
Abstract: Rare-earth (RE) dopants can not only improve the functional properties of ceramics but also produce photoluminescence (PL) emissions. Depending on the RE dopants, up-conversion (UC) and down-conversion (DC) PL emissions can be obtained. With the use of external stimuli, such as electric field (E-field) and chemical reaction, tuning PL can be achieved. Tunable PL property plays an important role in optical applications, such as biological labeling, volumetric 3D displays and long-distance quantum communication if rapid and reversible manipulation is realized. In this work, external E-field has been chosen as stimulus to tune PL. Applying external E-field, phase change might result, thus structural symmetry could be enhanced and the PL emission will be weakened. Therefore, it is anticipated that turning PL at room temperature might be possible when the transition temperature Tm (i.e., the temperature at which the dielectric constant becomes maximum) of the sample is close to room temperature. This work thus aims to develop new lead-free RE-doped barium zirconate titanate (BZT)-based ceramics with tunable PL properties under E-field at room temperature. Ferroelectric ceramics BZT has been chosen as the host mainly because of their adjustable Tm by the Zr/Ti ratio. Three RE ions, Dy, Er³⁺ and Eu³⁺, have been doped into the BZT ceramics as activators for producing PL emissions. The PL response of the RE-doped BZT ceramics have been studied under E-field. BaZrxTi1-xO₃ (BZT-x) ceramics with x = 0.15, 0.2, 0.25 and 0.3 have been fabricated by solid-state reaction for selecting suitable host for the doping of RE ions. In consistent with other research works, the Tm of the ceramics determined based on their temperature-dependent dielectric properties decreases with increasing Zr content. For the BZT-x ceramics with x = 0.15 and 0.2, their Tm are close to room temperature, and thus they are selected as the host ceramics in this work. Probably due to the decrease in Tm, both the ferroelectric and piezoelectric properties of the ceramics are weakened with increasing Zr content.
Three groups of RE-doped samples have been fabricated using solid-state reaction method; they are: (i) BZT-x ceramics (x = 0.15 and 0.2) doped with 0.2 mol% Dy³⁺ (BZT-x-Dy); (ii) BZT-x ceramics (x = 0.1, 0.15 and 0.2) doped with 0.2 mol% Er³⁺ (BZT-x-Er); and (iii) BZT-x ceramics (x = 0.15, 0.175 and 0.2) doped with 0.2 mol% Eu3+ (BZT-x-Eu). All the samples are also abbreviated as BZT-x-RE. The XRD results indicate that the three RE ions have successfully diffused into the host and no impurity phases are observed within the detection limit. The in-situ XRD results of BZT-0.15-Er ceramic reveals rhombohedral-to-cubic phase change and strain might be induced by external E-field. The Tm of the BZT-0.15-RE and BZT-0.2-RE ceramics are found in the range of 62-67°C and 30-37°C, respectively. The doping of Er³⁺ or Eu³⁺ ions significantly improves the piezoelectric properties of the BZT-x ceramics while the effect of the Dy³⁺-doping is relatively weak. The UC PL properties of the BZT-x-Er ceramics have been studied. Based on the power-dependent measurements, multi-photon absorption process is confirmed in the UC emission. Typical DC PL emissions are obtained for all the BZT-x-RE ceramics. For studying the effects of structural phase change, the UC PL emissions of the BZT-0.15-Er ceramic and the DC PL emissions of the BZT-x-Eu (x = 0.15 and 0.175) ceramics have been measured as functions of temperature. In general, the PL intensity decreases with increasing temperature as multiphonon relaxation rate is enhanced. Our results also show that the thermally induced rhombohedral-to-cubic phase change decreases the PL intensity. Moreover, the result of the BZT-0.15-Er ceramic reveals the occurrence of thermal agitation (TAG) between two thermally coupled emission levels ⁴S3/2 and ²H11/2. The PL responses under E-field for the BZT-x-RE ceramics have been studied. The observed PL intensities of all the BZT-x-RE ceramics are found diminished under a sufficiently high E-field. Phase change induced by E-field is believed to be the major cause for the weakening of PL intensity. Under a sufficiently high E-field, rhombohedral-to-cubic phase change happens for all the BZT-x-RE ceramics (except BZT-0.1-Er), and thus leading to an enhancement in structural symmetry and then a reduction in PL intensity. For the BZT-0.1-Er ceramic, rhombohedral-to-orthorhombic as well as orthorhombic-to-tetragonal phase changes occur under an E-field. As their effects on structural symmetry are opposed, the overall change in PL intensity becomes insignificant. Under an E-field of 2 kV/mm, the BZT-0.2-RE ceramics, which contain more cubic phase at room temperature, show less change (< 10%) in PL intensity as compared to the BZT-0.15-RE ceramics (≥ 20%). Among all the ceramics, the BZT-0.15-Er ceramic exhibits the largest reduction in PL intensity (75-80%) under an E-field of 2 kV/mm. Different from the other two dopants, Er³⁺ is reported to be more sensitive to the crystal symmetry of the host. However, the recovery of PL intensity for all the ceramics is small, generally less than 10% upon the removal of E-field. This suggests that the reversibility of the E-field-induced PL response is not high.
Subjects: Hong Kong Polytechnic University -- Dissertations
Ferroelectric crystals
Electronic ceramics
Pages: xv, 121 pages : color illustrations
Appears in Collections:Thesis

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