ALMEIDA, L. A. L.; http://lattes.cnpq.br/5634939832159456; ALMEIDA, Luiz Alberto Luz de.
Resumo:
Microbolometers, operating at room temperature and employing vanadium-dioxide (V02)
thin film as the heat sensing material, have emerged as a promising solution for far infrared
imaging, providing a wide range of military, industrial and commercial applications. The V 0 2
thin film is characterized by a solid-state phase transformation from semiconducting type at low
temperatures into metallic type at higher temperatures. Recent fabrication techniques permit
the deposition of vanadium dioxide films, having a temperature coefficient of resistance (TCR
= ;|gp) of 3%C_ 1 in the semiconducting region that increases to values as large as 60%-C- 1 in
the hystcretic transition region. Despite this pronounced increase of the TCR, V02 - t h i n film
microbolometers are commonly operated outside the hysterctic region, in the semiconducting
part of its resistance-temperature (R x T) curve.
Experimental results, indicating a considerable increase in the sensitivity, have been recently
reported for a V 0 2 - t h i n film microbolometer operating in the hystcretic region. The theoretical
prediction of bolometric performance in the transition region is quite complex, due to the
hysteresis in the RxT characteristics. Thus, the analysis, design, and optimization of a V 0 2
microbolometer, operating in the hystcretic region, is critically dependent on the availability of
a hysteresis model that can mathematically describe both the major and minor loops.
This thesis is focused on the development of a hysteresis model which can be employed as
an analysis tool in the design of V 0 2 microbolometer and other related applications of this
material. For this purpose, it is first proposed a new algebraic model to describe magnetic
hysteresis, which I call Limiting Loop Proximity (L2P) model. With only four parameters,
it has low computational cost and reduced mathematical complexity, thus permitting a fast
numerical implementation and simple parameter estimation procedure.
Considering the V 0 2 thin film as a composite medium, containing semiconducting and
metallic microcrystals, the wellknown effective-medium approximation theory is employed to
relate the volume fraction of the semiconducting microcrystals to the effective film resistance.
The L2P model is first proposed for describing the hystcretic dependence of volume fraction
on temperature. From this, a model for hysteresis in the resistance-temperature characteristic
is then derived, and a procedure for estimating the model parameters is outlined. The model
reproduces the more important hysteretic characteristics such as the major, minor, and nested
loops, in good agreement with the experimental characteristics.