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|Title: ||Heat treatments of NiTi shape memory alloy (SMA) thin films and its application as a micro-array for detection of infrared radiation|
|Other Titles: ||Nie tai xing zhuang ji yi he jin bo mo de re chu li ji qi ying yong yu wei xing lie zhen hong wai xian fa re chuan gan qi|
|Authors: ||Chan, Po Man (陳寶雯)|
|Department: ||Department of Physics and Materials Science|
|Degree: ||Master of Philosophy|
|Issue Date: ||2009|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Thin films.|
Infrared radiation -- Measurement.
Nickel-titanium alloys -- Heat treatment.
Shape memory alloys -- Heat treatment.
|Notes: ||CityU Call Number: TA418.9.T45 C427 2009|
xvi, 133 leaves : ill. 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2009.
Includes bibliographical references (leaves 124-128)
|Abstract: ||Uncooled imaging sensors that utilize the thermo-mechanical properties of thin film
NiTi shape memory alloys (SMAs) are fabricated for detection of infrared (IR)
radiation. The part of the spectrum relevant to this project is the far-infrared range.
This is different from the near-infrared range that can be detected by either a CCD
image sensor or a CMOS active-pixel sensor available in many domestic digital
cameras and video-cameras. Five micrometer thick NiTi SMA thin films are prepared
on glass substrates by direct current (DC) magnetron sputter deposition method. The
composition of the SMA films can be varied from 49.2 to 51.5 at. % Ti by changing
the target-substrate distance. The films deposited at longer target-substrate distance
contain higher Ti content. Since the as-deposited films are amorphous, they are heattreated
above the crystallization temperature. The thermal heat treatment is followed
by an aging process to induce two-way shape memory (TWSM) in the films. The
polycrystalline phases and microstructures of the films are analyzed by X-ray
diffraction (XRD) and scanning electron microscopy (SEM), respectively. The
transformation characteristics of the films are also studied by differential scanning
calorimetry (DSC). It is found that the films developed a multiple-stage
transformation (MST) behaviour depending on the annealing temperature and aging
time. For the sample, annealing at a higher temperature for a relatively short time has
led to one stage R-phase with reverse martensitic transformations above 0 ℃. This
one-step transformation is smoother and favours faster response as compared with the
multiple-stage transformation. The Ni- 49.8 at. % Ti SMA thin films are patterned to two-dimensional cantilever
micro-arrays by photolithographic technique. To ease the lift-off of the NiTi thin film
for subsequent heat treatments, a layer of photoresist is spin-coated on a glass
substrate before the SMA film deposition. Different baking time and temperatures are
applied to optimize the one-step baking process for the bottom photoresist. The higher
one-step baking temperature (120 ℃) promotes good interfacial adhesion, and the
film has good surface finish for the subsequent photolithographic and wet etching
processes. The width and length of each micro-cantilever are 50 and 100 μm,
respectively. The phase transformation temperatures are evaluated by DSC as well as
displacement measurements. The displacement and the sensitivity of the sample are
recorded. After the crystallization and constraint-aging heat treatments, the NiTi SMA
thin film cantilever micro-array exhibits a two-way shape memory behaviour in
association with the B2 « R transformation at near room temperatures. This two-way
shape memory effect has eliminated the need for an external biasing force. Upon the
absorption of IR radiation, the NiTi SMA thin film micro-cantilevers are heated up
leading to the reverse R-phase transformation. Illumination of the micro-array
increases the temperature locally and has caused curving up of the two-way shape
memory (TWSM) thin film.
In order to improve the resolution of the optical images, an optical collimator is
designed and used. In addition, another configuration (50 μm × 500 μm) of the NiTi
TWSM thin film cantilever micro-arrays are fabricated and examined. Based on the
dimensions of the micro-cantilever, a bending model which simulated the ideal
response of a straight micro-cantilever upon the absorption of an incident infrared
radiation is developed. It is noticed that a short aperture length will lead to a larger maximum change in the area between the original and the spontaneous reflecting
surface (Amax.) of the thin film micro-cantilever, but a longer aperture length would
be necessary for allowing sufficiently large deformation of the sample.
Accompanying with the long aperture length, the relatively smaller Amax. of the thin
film micro-cantilever can be raised by increasing the separation between the
collimator and the micro-array. Manipulating the collimator can lead to higher
resolution of the optical images by filtering the unwanted light rays off the screen.
The change in the light intensity is correlated with the deflection angle of the thin film
micro-cantilever, and the deflection angle of the micro-cantilevers is estimated as well.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b2374926|
|Appears in Collections:||AP - Master of Philosophy |
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