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Please use this identifier to cite or link to this item: http://hdl.handle.net/2031/4631

Title: Fabrications of shape memory alloy thin films and NiTi microvalves based on Micro-Electro-Mechanical System (MEMS) techniques
Other Titles: Xing zhuang ji yi he jin bo mo ji ji yu wei dian ji xi tong (MEMS) ji shu de NiTi wei xing fa men zhi zao
形状记忆合金薄膜及基于微电机系统 (MEMS) 技术的 NiTi 微型阀门制造
Authors: You, Liang (尤良)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2004
Publisher: City University of Hong Kong
Subjects: Electromechanical devices -- Materials
Microelectronics -- Materials
Nickel-titanium alloys
Shape memory alloys
Thin film devices -- Materials
Notes: CityU Call Number: TA487.Y68 2004
Includes bibliographical references (leaves 132-138)
Thesis (Ph.D.)--City University of Hong Kong, 2004
xvi, 138 leaves : ill. ; 28 cm.
Type: Thesis
Abstract: Pulsed laser deposition (PLD) and Magnetron sputtering were employed to fabricate shape memory alloy thin film as promising material for microdevices. A Pulsed laser deposition (PLD) system was designed, built and tested in our laboratory. Laser ablation was first employed to deposit shape memory alloy NiTi and NiTiHf thin films using composite targets. The crystal structure and phase transformation of the as-deposited and annealed thin films were investigated. The growth rate was found related to many factors. The most important impact factors are target-substrate distance d and laser energy p. In this research, an approach of composite target with adjustable fraction of desired Ni, Ti and Hf was used to obtain the desired film composition. In addition, the atomic ablation rates of Ti and Ni in NiTi alloy were found equal because of the congruent evaporation by a pulsed laser with high power density. Ni composition was lowered with the increase of laser energy and then increased when the laser energy was further increased. Ni composition decreased monotonically with the increase of substratetarget distance. This phenomenon can be attributed to the fact that different elements have different ablation plume shapes due to their atomic bonding strength and surface conditions. It was found that the as-deposited SMA thin films by PLD are in the amorphous state. The NiTiHf thin films crystallized from the amorphous state by annealing. The xray diffraction revealed a mixture of B2 structure and B19 structure phases in the annealed NiTiHf films at room temperature. It was also found that the ratio of peak intensity of martensitic phase to that of austenitic phase increased when the Hf content was increased. The transition temperatures of the laser ablated shape memory thin film were about 100°C lower than that of the bulk material with the same chemical composition. This phenomenon mainly resulted from the refined grain size of the thin films due to the dimensional constraints during the thin film crystallization process. The presence of particles on the pulsed laser deposited thin films can severely deteriorate the film quality and morphology. Although the use of lower laser power density has proven to be effective in reducing the amount of particles, this approach has lowered the film growth rate. In NiTi and NiTiHf laser ablation, the target surface became rough quickly. This rough surface resulted in the peeling-off from the target under further laser irradiation and generated large particles flying onto the opposite substrate. Polishing the target surface will lessen the tendency of over-heating and therefore decrease the formation of particles. In a parallel work with pulsed laser deposition, NiTiHf thin films were successfully deposited on different substrates by magnetron sputtering. To obtain the desired thin film compositions, pure metal plates such as Ti and Hf were placed on NiTi alloy target during the sputtering process. By this approach, NiTiHf films with Hf content in films ranging from 10.7 at% to 30.6 at% have been obtained. The transformation temperatures increased significantly with the increase of Hf content. The transition temperatures for thin films have been found much lower than the bulk alloy. Addition of Hf reduced the amount of available transformable martensite. It is believed that the formation of a second phase particles, which were dispersed inside grains and along grain boundaries, has impeded the martensitic transformation. Transformation stability in sputtered NiTiHf thin films was systematically studied in this research. With the increase of thermal cycles, the Mp and Rp decreased, and Ap increased. The R-phase became very indistinct and too obscured to be measured at the end. The dislocations developed in the cycling of temperature change can become barriers to impede the martensitic transformation and also its reversion. However, when the thermal cycling proceeds to a certain value, the transition temperatures will be independent with the thermal cycling. Hf element can promote the formation of martensite structure of B19'. At high Ni content (over 50 at%), the effect of Hf for promoting martensite phase has been suppressed and most of its grains are in austenite phase. The martensitic phase in NiTiHf thin film was indexed by (001)M type I twinning structure. The grain size in NiTiHf thin films was significantly smaller than that of bulk materials. Under the same annealing condition, the NiTiHf thin films had a typical grain size 10 times smaller than similar NiTi films. Microvalves using shape memory thin film actuators have been designed, fabricated and tested. The magnetron sputtering deposition method was selected because of the formation of micron size particles on PLD fabricated SMA films. The composition of thin film was selected as equiatomic NiTi since NiTiHf thin films were found too brittle especially when Hf content was higher than 15 at%. A model was established to describe the relationship among the force, movement, and bias spring in an ideally packaged valve. Analysis and calculation based on this model can provide useful information in the design and fabrication of NiTi thin film microvalves. NiTi films with different thickness have been successfully deposited on Si wafer. Micromachining technology was employed to make all components of microvalve. Individually fabricated components were assembled by a modified procedure to improve valve performance. Finite Element Analysis (FEM) was carried out to characterize the non-uniformly distributed thermal stress induced by deposition and crystallization of the NiTi films under high temperatures. The NiTi actuator approximates a spring: the force increases with applied deflection. The "relaxation" at the initial stage of the load-deflection curve may be related to experimental setup and pre-existing intrinsic plastic deformation. The valve fabricated with whole diaphragms of NiTi was capable of modulation by 99.0 % for water flows below a pressure of 12 kPa. An on/off ratio at this pressure was found up to 2000. Inserted soft polymer spacer between boss and orifice plate may improve the valve sealing performance. The power dependence of NiTi actuators was determined to show the possibility of reducing the power consumption of NiTi micro devices to an acceptable level. Transition performance has been investigated based on the assembled microvalve. The transformation characteristics for both martensitic, austenitic phases and Rphase varied with increasing cycle number. Training was effective for stabilizing the shape memory behavior. Fixed load and fixed deflection thermal cycling tests have been performed to find the effect of stress level and deformation on fatigue. Experimental results showed that the stability of shape memory characteristics strongly depended on the applied stress level.
Online Catalog Link: http://lib.cityu.edu.hk/record=b1871459
Appears in Collections:AP - Doctor of Philosophy

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