Naveen K. Gupta

Demonstration of motionless Knudsen pump based micro-gas chromatography featuring micro-fabricated columns and on-column detectors

This paper reports the investigation of a micro-gas chromatography (μGC) system that utilizes an array of miniaturized motionless Knudsen pumps (KPs) as well as microfabricated separation columns and optical detectors. A prototype system was built to achieve a flow rate of 1 mL min(-1) and 0.26 mL min(-1) for helium and dry air, respectively, when they were used as carrier gas. This system was then employed to evaluate GC performance compromises and demonstrate the ability to separate and detect gas mixtures containing analytes of different volatilities and polarities. Furthermore, the use of pressure programming of the KP array was demonstrated to significantly shorten the analysis time while maintaining a high detection resolution. Using this method, we obtained a high resolution detection of 5 alkanes of different volatilities within 5 min. Finally, we successfully detected gas mixtures of various polarities using a tandem-column μGC configuration by installing two on-column optical detectors to obtain complementary chromatograms.

Thermal transpiration in zeolites: A mechanism for motionless gas pumps

We explore the use of a naturally occurring zeolite, clinoptilolite, for a chip-scale, thermal transpiration-based gas pump. The nanopores in clinoptilolite enable the required free-molecular flow, even at atmospheric pressure. The pump utilizes a foil heater located between zeolite disks in a plastic package. A 2.3mm thick zeolite disk generates a typical gas flow rate of 6.6×10−3 cc/min-cm2 with an input power of <300mW∕cm2. The performance is constrained by imperfections in clinoptilolite, which provide estimated leakage apertures of 10.2–13.5μm∕cm2 of flow cross section. The transient response of the pump is studied to quantify nonidealities.

A Si-Micromachined 162-Stage Two-Part Knudsen Pump for On-Chip Vacuum

This paper investigates a two-part architecture for a Knudsen vacuum pump with no moving parts. This type of pump exploits the thermal transpiration that results from the free-molecular flow in nonisothermal channels. For a high compression ratio, 162 stages are serially cascaded. The two-part architecture uses 54 stages designed for the pressure range from 760 to ≈ 50 Torr, and 108 stages designed for lower pressures. This approach provides greater compression ratio and speed than using a uniform design for each stage. Finite element simulations and analytical design analysis are presented. A five-mask single-wafer fabrication process is used for monolithic integration of the Knudsen pump that has a footprint of 12 × 15 mm2. The pressure levels of each stage are measured by integrated Pirani gauges. Experimental evaluation shows that, using an input power of ≈ 0.39 W, the evacuated chamber is reduced from 760 to ≈ 0.9 Torr, resulting in a compression ratio of ≈ 844. The vacuum levels are sustained during 37 days of continuous operation.

A Si-micromachined 48-stage Knudsen pump for on-chip vacuum

This paper describes a thermal transpiration-driven multistage Knudsen pump for vacuum pumping applications. This type of pump relies upon the motion of gas molecules from the cold end to the hot end of a channel in which the flow is restricted to the free molecular or transitional regimes. To achieve a high compression ratio, 48 stages are cascaded in series in a single chip. A five-mask, single silicon wafer process is used for monolithic integration of the designed Knudsen pump. The pump has several monolithically integrated Pirani gauges to experimentally measure the vacuum pumping characteristics of the pump. It has a footprint of 10.35 × 11.45 mm 2 . For an input power of 1350 mW, the fabricated pump self-evacuates the encapsulated cavities from 760 to ≈50 Torr, resulting in a compression ratio of 15. It also pumps down from 250 to ≈5 Torr, resulting in a compression ratio of 50. Each integrated Pirani gauge requires ≈3.9 mW of power consumption, and its response is sufficiently sensitive in the operating pressure range of 760‐1 Torr. (Some figures may appear in colour only in the online journal)

Porous ceramics for multistage Knudsen micropumps—modeling approach and experimental evaluation

This paper describes the evaluation of four types of porous ceramics for use as thermal transpiration materials in Knudsen pumps that operate at atmospheric pressure. Knudsen pumps are motionless gas pumps that utilize thermal transpiration along a channel or a set of channels; the channels must have a temperature gradient and must constrain the flow to remain within the free molecular or transitional flow regimes. Of the ceramics evaluated, a clay-based, 15 bar synthetic ceramic (15PC) presents the most favorable properties for Knudsen pumps. For an input power of 3.4 W, a 25 × 25 mm 2 nine-stage Knudsen pump that uses this material provides a maximum pressure head of 12 kPa and a maximum gas flow rate of ≈3.7 μ Lm in −1 . Reliability tests demonstrate more than 11 750 h of continuous operation without any deterioration in their gas pumping capabilities. A fitted model suggests that the temporal evolution of pressure at the sealed outlet of a Knudsen pump can be captured adequately using four parameters. These parameters correspond to various nonidealities that play dominant roles in the transient response of these pumps. (Some figures in this article are in colour only in the electronic version)

Dynamic Braille Display Utilizing Phase-Change Microactuators

In this paper a phase-change microactuator is presented for use in a dynamic Braille display. The state of the art for phase-change actuators is briefly discussed. Then, key design parameters are specified which lead to the formation of a concept. The concept is characterized in terms of the design parameters, and key performance metrics such as actuation time (135-285 ms), and average power consumption (30-40 mW) have been simulated. In comparison to recent literature, the response time is expected to be improved by over two orders of magnitude, whereas average power consumption is also reduced.

A monolithic 48-stage Si-micromachined Knudsen pump for high compression ratios

This paper describes a microchip of 10.35×11.45 mm² footprint that can self-evacuate on-chip cavities from 760 Torr to <;50 Torr or from 250 Torr to ≈5 Torr. The power consumed is ≈1,350 mW. These compression ratios of 15 and 50 offer >;10× improvement above those previously reported. The microfabrication process utilizes a single silicon substrate and requires five masks. A process is described for hermetic sealing of plasma enhanced chemical vapor deposited (PECVD) oxide/nitride layers using atomic layer deposited (ALD) Al₂O₃.

A planar cascading architecture for a ceramic Knudsen micropump

This paper describes a 9-stage Knudsen pump with planar architecture that uses nanoporous ceramic for thermal transpiration. While operating at 55 K above room temperature, the pump provides a maximum pressure head exceeding 12 kPa at a sealed outlet, or a gas flow rate of ≈3.8 µL/min. against a pressure head of 160 Pa. Experiments also demonstrate the capability of the pump to steer water droplets at speeds exceeding 1200 µm/s through a 250 µm fluorinated ethylene propylene capillary. The packaged volume for the 9-stage pump discussed here is 25×25×7.25 mm3. These characteristics indicate that the pump is potentially useful in microfluidic systems intended for both gas and liquid phase chemical sensing.

A Knudsen pump using nanoporous zeolite for atmospheric pressure operation

This paper describes the use of naturally occurring nanoporous zeolite (clinoptilolite) for a miniature Knudsen pump. Based on the principle of thermal transpiration, these pumps have no moving parts, and are attractive for applications ranging from gas analyzers to cooling systems. The Knudsen pump requires flow channels that are in the free molecular or transitional flow regimes. Consequently, at atmospheric pressure, the pore diameters should be les100 nm, and large numbers of pores are necessary to permit meaningful flow. The initial prototype, operating at ap50 K above room temperature, achieves a flow rate of ap0.12 seem with a small pressure load at the output, or a maximum pressure of ap2.5 kPa when the flow is blocked. Its packaged volume is 55times55times12 mm .

A high-flow Knudsen pump using a polymer membrane: Performance at and below atmospheric pressures

This paper describes a miniature gas (Knudsen) pump that utilizes thermomolecular flow through a nanoporous membrane. A temperature gradient along the length of a pore that supports free molecular gas flow at atmospheric pressure pumps gas molecules from the cold end to the hot end. In contrast with past work, the membrane material is mixed cellulose which provides superior uniformity in pore diameter and porosity. The final packaged volume of the Knudsen pump is 14x14×4.4 mm3. For an input power of 1.4 W, a single stage Knudsen pump based on these nanoporous polymer membrane has a temperature bias of 30 K across the thickness of the membrane, which provides 0.4 sccm flow against a 330 Pa pressure head. The load characteristics of the pump suggest that the pump can provide as much as 0.93 sccm gas flow in the absence of a load. Knudsen pump operation at sub-atmospheric pressures is also reported.