A bistable SMA microvalve for 3/2-way control

Abstract

In this paper the layout, fabrication, and characterization of a first-of-its-kind bistable 3/2-way shape memory microvalve are presented. A symmetric arrangement of two counteracting shape memory alloy (SMA) microbridges is used for switching. The stable states in power-off condition are realized by magnetostatic contact forces between a movable hard-magnetic cylinder and two soft-magnetic layers. A detailed design study of the magnetic retaining system is performed to meet the requirements of a large pressure range and identical flow rates in the open valve state and a low leakage in the closed valve state in both outlet ports. The overall dimensions of first demonstrators are 11 mm × 6 mm × 4 mm. The 3/2-way bistable microvalve performance is demonstrated, showing identical flow rates in both outlet ports for a pressure difference up to 200 kPa for gas (N₂) and up to 100 kPa for liquid (water) with flow rates of 1250 sccm and 11 ml/min, respectively.

Introduction

Bistable microvalves are of considerable interest for flow control, e.g., in portable pneumatic systems posing high demands on the pressure range [1] and in high-throughput bioanalytics [2]. Due to bistability, the continuous supply of power required to maintain a certain valve state is avoided, which is particularly important when operating at small duty cycles. A promising application is the integration of the microvalves on a backplane to control high flow rates in various channels and directions at pressure differences up to 100 kPa [3].

So far, several bistable microvalves have been developed based, e.g., on electromagnetic [4], [5], electrostatic [6] and thermopneumatic principles [7] showing either a low pressure range or rather complex designs, which hamper further miniaturization. Due to the energy density of shape memory alloy (SMA) actuators in the order of 10⁷ J/m³ [8], high forces and strokes may be generated in a small temperature range, which exceeds conventional principles based on thermal expansion by at least one order of magnitude. Recently, we introduced a novel bistable switching mechanism combining SMA actuation and magnetostatic retention [9]. We introduced the first-of-its kind bistable SMA microvalve combining SMA actuation and magnetostatic retention [10], optimized the performance by a detailed design study of functional layers and developed an extended layout of a bistable SMA microvalve for 3/2-way control of fluid flow [11]. The bistable 3/2-way SMA microvalve is of considerable interest for mixing [12] and diverting [13] applications that operate at small duty cycles.

In the following, the operation principle of the 3/2-way bistable SMA microvalve as well as the layout including SMA microbridge, SMA switching unit, the magnetic retaining system and fluidic unit are presented. Then, the fabrication is described and finally the characterization of the microvalve is discussed.