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High-Temperature Microfluidics for Space Propulsion
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Microsystems Technology.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, microfabrication methods and tools for analysis of heated cold-gas microthrusters are presented, with the aim of improving their reliability and performance. Cold-gas thrusters operate by accelerating pressurized gas through a nozzle. These thruster systems are very straightforward in both design and operation, relying on little more than a pressurized tank, a valve, and a nozzle. This makes them suitable for miniaturization, enabling their use on very small spacecraft. However, an inherent drawback with cold-gas thrusters is their low propellant efficiency – in thrusters known as specific impulse, or Isp.  This is compounded by the fact that when reducing length, the volume, e.g., that of the propellant tank, reduces with the cube of the length, meaning that the maximum amount of storable fuel reduces quickly. Hence, maximizing fuel efficiency is even more important in miniaturized systems. Still, because of their other advantages, they remain suitable for many missions. Schlieren imaging – a method of visualizing differences in refractive index – was used thrughout this thesis to visualize exhaust jets from microthrusters, and to find leaks in the components. It was found that effects of the processing of conventionally fabricated silicon nozzles, resulted in a misalignment of up to 3°  from the intended thrust vector, increasing propellant consumption by up to 5%, and potentially causing unintended off-axis acceleration of the spacecraft. Schlieren imaging was also used to verify that the exhaust from thrusters fabricated with close to circular cross-sections was well behaved. These nozzles did not suffer from the previous misalignment issue, and the shape of the cross-section decreased viscous losses. For applications requiring higher temperatures, a microthruster nozzle with an integrated flow sensor was fabricated from tape cast yttria stabilized zirconia. The ceramic substrate enabled heater temperatures of the nozzle exceeding 1000 °C, resulting in an increase in Isp  of 7.5%. Integration of a flow sensor allowed the elimination of couplings and reduced the number of interfaces, thereby reducing the overall risk of failure. Close integration of the sensor allowed moving the point of measurement closer to the nozzle, enabling improved reliability of the measurements of the propellant consumption. The temperature of the heater, in combination with the ion conductive properties of the substrate proved to be a limiting factor in this design. Two routes were explored to overcome these problems. One was to use the temperature dependence of the ion conductivity as a sensing principle, thereby demonstrating a completely new flow sensor principle, which results in better calibration, tighter integration, and 9 orders of magnitude stronger signal. The other was using hafnium oxide, or hafnia, as a structural material for high-temperature micro-electromechanical systems. This involved developing a recipe for casting hafnia ceramic powder, and determining the Young's modulus and thermal shock resistance of the cast samples, as well as studying the minimum feature size and maximum aspect ratio of cast microstructures.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 50 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1233
National Category
Other Engineering and Technologies
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
URN: urn:nbn:se:uu:diva-246057ISBN: 978-91-554-9186-4 (print)OAI: oai:DiVA.org:uu-246057DiVA: diva2:792825
Public defence
2015-04-24, 2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2015-04-01 Created: 2015-03-02 Last updated: 2015-04-17
List of papers
1. Schlieren Imaging of Microthruster Exhausts for Qualitative and Quantitative Analysis
Open this publication in new window or tab >>Schlieren Imaging of Microthruster Exhausts for Qualitative and Quantitative Analysis
2012 (English)In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 23, no 8, 085403- p.Article in journal (Refereed) Published
Abstract [en]

Abstract. Schlieren imaging is a method used to visualize differences in refractiveindex within a medium. It is a powerful and straightforward tool for sensitiveand high-resolution visualization of, e.g., gas flows. Here, heated cold gasmicrothrusters were studied with this technique. The thrusters are manufacturedusing microelectromechanical systems technology, and measure 22×22×0.85 mm. Thenozzles are approximately 20 µm wide at the throat, and 350 µm wide at the exit.Through these studies, verification of the functionality of the thrusters, and directvisualization and of the thruster exhausts was possible. At atmospheric pressure,slipping of the exhaust was observed, due to severe overexpansion of the nozzle. Invacuum (3 kPa), the exhaust was imaged while feed pressure was varied from 100 to450 kPa. The nozzle was overexpanded, and the flow was seen to be supersonic. Theshock cell period was linearly dependent on feed pressure, ranging from 320 to 610 µm.With activated heaters, the shock cell separation increased. The effect of the heaterswas more prominent at low feed pressure, and an increase in specific impulse of 20%was calculated. It was also shown that schlieren imaging can be used to detect leaks,making it a valuable, safe, and noninvasive aid in quality control of the thrusters.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2012
Keyword
Schlieren imaging, microthrusters, MEMS, shock cells
National Category
Other Engineering and Technologies not elsewhere specified
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
urn:nbn:se:uu:diva-186252 (URN)10.1088/0957-0233/23/8/085403 (DOI)000306366600039 ()
Funder
VINNOVA
Available from: 2012-11-29 Created: 2012-11-28 Last updated: 2017-12-07Bibliographically approved
2. Investigation of exhausts from fabricated silicon micronozzles with rectangular and close to rotationally symmetric cross sections
Open this publication in new window or tab >>Investigation of exhausts from fabricated silicon micronozzles with rectangular and close to rotationally symmetric cross sections
2013 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 23, no 10, 105001- p.Article in journal (Refereed) Published
Abstract [en]

Close to rotationally symmetric in-plane silicon micronozzles with throat and exit diameters of 45 and 260 µm, respectively, have been fabricated using semi-isotropic SF6 etching through an array mask utilizing microloading and reactive ion etching lag. Comparison with nozzles fabricated using deep reactive ion etching (DRIE) and having a rectangular cross-section but a similar hydraulic diameter in the throat, showed that the Reynolds numbers were almost equal even though the DRIE-etched nozzle had an almost five times larger cross-sectional area, hence implying less viscous losses and higher efficiency with the nearly symmetrical nozzles. The nozzle shapes have been studied using x-ray computed tomography. Comparison of the nozzles' exhaust jets using schlieren imaging, showed that the rectangular nozzles' jets deviate from the nozzle axis direction. It is believed that it is caused by the inclined side walls resulting from the DRIE etching. The results from intentionally misaligning the wafers, each containing half a nozzle, 50 µm parallel with or perpendicular to the nozzle axis, showed that the exhaust deviated and widened, respectively. The findings show that the nozzle symmetry affects both the shape and the pointing direction of the exhaust and that schlieren imaging is a powerful tool for determining nozzle thrust vector deviations.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
urn:nbn:se:uu:diva-186774 (URN)10.1088/0960-1317/23/10/105001 (DOI)000324672700002 ()
Available from: 2012-11-29 Created: 2012-11-29 Last updated: 2017-12-07Bibliographically approved
3. High-temperature zirconia microthruster with integrated flow sensor
Open this publication in new window or tab >>High-temperature zirconia microthruster with integrated flow sensor
Show others...
2013 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 23, no 5, 055004- p.Article in journal (Refereed) Published
Abstract [en]

This paper describes the design, fabrication and characterization of a ceramic, heated cold-gas microthruster device made with silicon tools and high temperature co-fired ceramic processing. The device contains two opposing thrusters, each with an integrated calorimetric propellant flow sensor and a heater in the stagnation chamber of the nozzle. The exhaust from a thruster was photographed using schlieren imaging to study its behavior and search for leaks. The heater elements were tested under a cyclic thermal load and to the maximum power before failure. The nozzle heater was shown to improve the efficiency of the thruster by 6.9%, from a specific impulse of 66 to 71 s, as calculated from a decrease of the flow rate through the nozzle of 13%, from 44.9 to 39.2 sccm. The sensitivity of the integrated flow sensor was measured to 0.15 m Omega sccm(-1) in the region of 0-15 sccm and to 0.04 m Omega sccm(-1) above 20 sccm, with a zero-flow sensitivity of 0.27 m Omega sccm(-1). The choice of yttria-stabilized zirconia as a material for the devices makes them robust and capable of surviving temperatures locally exceeding 1000 degrees C.

Keyword
Keywords: Zirconia, YSZ, HTCC, schlieren imaging, flow sensor, microthruster
National Category
Other Engineering and Technologies not elsewhere specified
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
urn:nbn:se:uu:diva-186257 (URN)10.1088/0960-1317/23/5/055004 (DOI)000317739100004 ()
Funder
Vinnova
Available from: 2012-11-28 Created: 2012-11-28 Last updated: 2017-12-07Bibliographically approved
4. Investigation of a zirconia co-fired ceramic calorimetric microsensor for high-temperature flow measurements
Open this publication in new window or tab >>Investigation of a zirconia co-fired ceramic calorimetric microsensor for high-temperature flow measurements
2015 (English)In: Journal of Micromechanics and Microengineering, ISSN 0960-1317, E-ISSN 1361-6439, Vol. 25, no 6, 065014Article in journal (Refereed) Published
Abstract [en]

This paper describes the design, fabrication and characterization of a flow sensor for high-temperature, or otherwise aggressive, environments, like, e.g. the propulsion system of a small spacecraft. The sensor was fabricated using 8 mol% yttria stabilized zirconia (YSZ8) high-temperature co-fired ceramic (HTCC) tape and screen printed platinum paste. A calorimetric flow sensor design was used, with five 80 mu m wide conductors, separated by 160 mu m, in a 0.4 mm wide, 0.1 mm deep and 12.5 mm long flow channel. The central conductor was used as a heater for the sensor, and the two adjacent conductors were used to resistively measure the heat transferred from the heater by forced convection. The two outermost conductors were used to study the influence of an auxiliary heat source on the sensor. The resistances of the sensor conductors were measured using four-point connections, as the gas flow rate was slowly increased from 0 to 40 sccm, with different power supplied through the central heater, as well as with an upstream or downstream heater powered. In this study, the thermal and electrical integrability of microcomponents on the YSZ8 substrate was of particular interest and, hence, the influence of thermal and ionic conduction in the substrate was studied in detail. The effect of the ion conductivity of YSZ8 was studied by measuring the resistance of a platinum conductor and the resistance between two adjacent conductors on YSZ8, in a furnace at temperatures from 20 to 930 degrees C and by measuring the resistance with increasing current through a conductor. With this design, the influence of ion conductivity through the substrate became apparent above 700 degrees C. The sensitivity of the sensor was up to 1 m Omega sccm(-1) in a range of 0-10 sccm. The results show that the signal from the sensor is influenced by the integrated auxiliary heating conductors and that these auxiliary heaters provide a way to balance disturbing heat sources, e.g. thrusters or other electronics, in conjunction with the flow sensor.

National Category
Other Engineering and Technologies
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
urn:nbn:se:uu:diva-246059 (URN)10.1088/0960-1317/25/6/065014 (DOI)000354803000014 ()
Available from: 2015-03-02 Created: 2015-03-02 Last updated: 2017-12-04Bibliographically approved
5. A high-temperature calorimetric flow sensor employing ion conduction in zirconia
Open this publication in new window or tab >>A high-temperature calorimetric flow sensor employing ion conduction in zirconia
2015 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 106, no 19, 194103Article in journal, Letter (Refereed) Published
Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2015
Keyword
Calorimetric flow sensor, Ion conductivity, Zirconia, YSZ8
National Category
Ceramics Embedded Systems Condensed Matter Physics
Research subject
Engineering Science with specialization in Microsystems Technology
Identifiers
urn:nbn:se:uu:diva-246105 (URN)10.1063/1.4921051 (DOI)000355008100059 ()
Funder
Swedish National Space Board
Available from: 2015-03-02 Created: 2015-03-02 Last updated: 2017-12-04Bibliographically approved
6. Hafnium oxide in high-temperature microelectromechanical systems
Open this publication in new window or tab >>Hafnium oxide in high-temperature microelectromechanical systems
(English)Manuscript (preprint) (Other academic)
National Category
Other Engineering and Technologies
Identifiers
urn:nbn:se:uu:diva-246061 (URN)
Available from: 2015-03-02 Created: 2015-03-02 Last updated: 2015-04-17

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