Abstract

Microvalves are a promising field of application for shape memory alloy (SMA) microactuators as they require a large force and stroke in a restricted space. The performance of SMA-actuated microvalves does not only depend on SMA material properties, but also requires a mechanically and thermally optimized design as well as a batch fabrication technology that is compatible with existing microsystems technologies. This chapter gives an overview of the different engineering aspects of SMA microvalves and describes the ongoing progress in related fields. Different valve types based on various design-material-technology combinations are highlighted. The examples also demonstrate the opportunities for emerging new applications.

Introduction

Shape memory alloys (SMAs) belong to the category of smart materials as they exhibit multi-functional properties, which can be used simultaneously to generate actuation, sensing and adaptive functions. The high energy density and favorable scaling behavior of their mechanical properties upon miniaturization make these materials particularly attractive for actuator applications in micro-dimensions.

In this realm, however, the close neighborhood of functional parts as well as fabrication constraints have a strong impact on physical properties and overall performance. In addition, the related research fields of materials development, design engineering and microtechnology are strongly interlinked and cannot be considered separately. Currently, each of these research fields faces a number of obstacles and constraints.

1. Despite the tremendous success in the development of SMA thin films and foils, considerable limitations due to temperature range, hysteresis and fatigue exist.

2. SMA microactuators usually consist of a monolithic structure with quasi-two- dimensional shape. In order to make an optimum use of the shape memory effect, homogeneous stress distributions should be generated upon loading, which requires some kind of shape optimization.