Miniaturized Knudsen Pumps for Micro Gas Chromatography and Other Applications

Abstract

Gas pumping is essential for many microsystems that require various types of gas flow and manipulation. As an integral part in microsystems, a gas micropump must satisfy the requirement of gas flow with modest power consumption and small footprint. Compared to other gas micropumps, thermally-driven miniaturized Knudsen pumps (KP) are desirable because of the motionless nature, which provides high reliability, silent operation, and flexibility of scaling. Despite the past successes in miniaturization of Knudsen pumps based on porous materials (KP.P) or lithographically micromachined channels (KP.L), a number of challenges still remain to be addressed. For the former type, the challenges include 1) thermal management for bidirectional pumping, and 2) architectures for simple and reliable assembly. For the latter type, the main challenge is concurrent generation of both high pressure head and sufficient flow rate, which limits the utility of this type of KPs in many scenarios. In this research, solutions to addressing the challenges of both types of KPs are investigated. The first effort is directed at enhancing the thermal management of KP.P in both pumping directions. In particular, a bidirectional KP module is designed to incorporate customized heat sinks that match the pumping need in each flow direction of a micro-gas chromatograph (μGC). This KP produces a maximum air flow rate of ≈0.82 sccm and a maximum blocking pressure of ≈880 Pa at 2 W. Proper thermal management effectively improves the bidirectional performance of the KP module by 2X-3X compared to prior work. Similar thermal management can be employed to improve the bidirectional performance involved in other microsystems. The second effort investigates the first 3D printed bidirectional KP. Metal 3D printing technology is leveraged to enable customization of structures, which contributes to a KP architecture that fulfills the contradicting requirements of thermal management in opposing pumping directions. With mixed-cellulose-ester (MCE) membranes serving as the pumping medium, it achieves a maximum flow rate of 0.39 sccm and blocking pressure of 818.2 Pa at 2 W. This KP presents the highest bidirectional performance among all reported prior work. The KP architecture is also simple, reliable, and facile for assembly. In this effort, anodic aluminum-oxide (AAO) membranes are also tested for the first time as a high-temperature (tested up to ≈200 ºC) pumping medium. The testing results demonstrate that AAO membranes can be used as an alternative to conventionally used mixed cellulose ester (MCE) membranes (max. temp. up to ≈75ºC) in high-temperature applications. The third effort is focused on developing a lithographically micromachined KP that, relative to comparable micropumps, provides both high pressure head (≈3.3 kPa) and modest flow rate (≈0.75 sccm). The performance is provided by both tailored pumping structures and a monolithic multi-stage architecture. The small footprint (7.5×5 mm2) and high pressure head property are desirable in many gas analysis systems such as gas chromatography. A flow rate sensor is also designed to be integrated with the KP to enable feedback control.