|Author:||Wong, Man Chung|
|Title:||Magnetic-field induced luminescence via strain-mediated coupling|
|Advisors:||Hao, J. H. (AP)|
Materials -- Magnetic properties.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Physics|
|Pages:||v, 145 pages : color illustrations|
|Abstract:||Compared to photoluminescence (PL) and electroluminescence (EL), little work is reported on magnetoluminescence, because magnetoluminescence effects typically occur under extreme conditions of high magnetic field and low temperature. Owing to the excellent magneto-mechanical coupling property, magneto elastomer has been extensively used in magnetic sensors and actuators. It can be foreseen that the combination of phosphor materials and magneto elastomer is not only an alternative solution to direct magnetic-luminescence but also paving a way to better understanding of the relationship between magnetic field and luminescence. In this study, novel magnetic-induced luminescence (MIL) phenomenon has first been observed from the flexible magnetic composite laminates via strain-mediated coupling. Red-green-blue (RGB) and white light emissions were observed by the naked eyes from the magnetic composite laminates. The fabrication and morphology, magnetic, and elastic characterizations of the magneto composite laminates will be given. The basic luminescent mechanism, measurement of optical characters and emission modulation will be briefly introduced. Firstly, the magnetic composite laminates were fabricated by consisting of the metal-ion-doped ZnS microparticles phosphor mixed to polydimethylsiloxane (PDMS) and another phase consists of Fe-Co-Ni alloy particles mixed to PDMS to form magneto elastomer. In this work, ZnS is chosen as phosphor host because of its non-central symmetric wurtzite structure, which can result in piezoelectric potential when applying strain. On the other hand, the designed magnetic composite can response to external magnetic field and then exhibit large deformation, which can serve as a magnetic actuator to stimulate the doped ZnS in the hybrid system. Magnetization of the magneto elastomer was measured from vibrating sample magnetometer (VSM) at room temperature. The magnetic behavior of the composite can be approximately considered as linear behavior with no significant hysteresis because of negligible coercive field (Hc 9 Oe). The Young's modulus for the magnetic elastomer is calculated to be 70 MPa under a zero magnetic field. Measurement shows that both strain and stress of the magnetic elastomer increase monotonically and reproducible after many cycles of operation when the static magnetic field increases from 1.5 to 6 kOe, making it durable and reliable for MIL application.|
Secondly, MIL phenomenon has been observed from the phosphor composites via strain-mediated coupling. The pattern of the green colored logo and white color light emission can be seen by the naked eyes from the magnetic composite laminates under an AC magnetic field of Hrms = 3.5 kOe and frequency of 30 Hz. Interestingly, the achieved light emissions of the material systems can be modulated in reversible and dynamical manners under the control of low magnetic field at room temperature. The hybrid device performance parameters, including CIE, CCT, luminance, and power efficiency have been tested. The CIE coordinates are found to be (0.3462, 0.3735) and hence the CCT is determined to be 5027 K, indicating the obtained MIL phosphor composite emits a somewhat cool white color. Finally, optimization design has been done with the structure of magnetic composite laminates and measurement conditions. Therefore, RGB colors were observed with ZnS: Cu, Al and (Ca1-XSrX) S: Eu co-doped phosphor composite laminates under 3.5 kOe AC magnetic fields. Tunable white color was also realized with ZnS: Cu, Al and YAG: Ce co-doped phosphor composite laminates. In this work, the display and light source drive by a magnetic field, providing a new insight and possibility for display and solid state lighting applications in any specific situation where alternating magnetic fields are applicable. The hybrid magnetic composite laminates possess flexible, contactless and free of electric power features, which are attractive for future magnetic sensing and energy harvesting applications.
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