10th IAA Symposium on Small Satellites for EO

20th – 24th April 2015 Berlin

Development and Testing of H2O2 MEMS based Microthrusters

D. Modenini1, R. Cocomazzi2, B. Margesin3, P. Tortora1 1 University of Bologna - Department of Industrial Engineering - Forlì Campus, Italy. Email: dario.modenini@unibo.it

2 Sitael Spa, Forlì, Italy 3 Fondazione B. Kessler - Center for Materials and Microsystems, Trento, Italy.

Overview • Motivations for H2O2 MEMS µThrusters

• Preliminary design

• Prototypes Manufacturing

• Catalytic decomposition kinetics experiments

• Numerical Modelling

• Status of the investigation

• Critical points and outcome for next steps

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Motivations for H2O2 µThrusters

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• Next generation micro/nanosat (

Consortium & Background

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• Investigation is jointly undertaken by 3 members:

1. University of Bologna (Italy)

2. Sitael Spa (Italy)

3. FBK - Bruno Kessler Foundation (Italy)

• Background of the consortium: The H2O2 MEMS thruster represents the the cold-gas MPS onboard ALMASat-1

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Modelling, design & testing

Manufacturing

Propellant: N2 stored at high pressure in a tank Thrusters: Silicon MEMS, 40μm throat size, 0.75 mN delivered thrust

Working principle

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• Working principle is based on the heterogeneous decomposition of aqueous solutions of H2O2 at high concentration, into water and oxygen:

𝐻𝐻2𝑂𝑂2 𝑙𝑙 → 𝐻𝐻2𝑂𝑂 𝒍𝒍 +12𝑂𝑂2 𝑔𝑔 − 98.2

𝑘𝑘𝑘𝑘𝑚𝑚𝑚𝑚𝑚𝑚

• Reaction is exothermic heat of reaction exploited to vaporize liquid product • In principle the temperature of products can be computed for an adiabatic decomposition,

as a function of the %[H2O2], however miniaturization increases the impact of heat (and viscous) losses lowering the max T attainable.

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THRUST

H2O2 Propellant Storage

Feeding System: Pump/Injector/Valve

Combustion Chamber with Catalyst

De Laval Nozzle Control Electronics

MEMS-based

command

power

liquid propellant

gas propellant

from Satellite Bus

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Prototypes Design & Manufacturing

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• Manufactured by DRIE (Deep Reactive Ion Etching) over a 525 µm thick, single side polished 4” silicon wafers.

• The prepared wafers are covered with a 500 µm thick polished Pyrex wafer to guarantee optical access.

• Decomposition chamber modelled as a dense array of pillars over which a catalytic layer (~100 µm of Ag or Pd) is deposited. • Deposition has been made through PVD in vacuum chamber equipped with planetary sample

holder. • Prior to PVD, an intermediate substrate of silicon oxide is grown for passivation. • Trade-off between pillars spacing:

• Having the smallest spacing between the pillars as to achieve the largest catalyst surface exposed to the propellant flow, but

• Catalyst deposition process employed is inherently not uniform among horizontal and vertical surfaces: minimum spacing must be limited to ensure vertical surfaces coverage.

Side view pillar path

catalyst layer silicon pillar

Prototypes Design & Manufacturing

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Figure 1: Decomposition chamber with inlet sieve and outlet nozzle of one of the design variants of microthrusters with Ag catalyst layer (left). Close up of the inlet sieve area (right).

Preliminary Thruster sizing: geometric parameter and estimate performance with AR = 10 and SP = 8

Throat Diameter: Dt 40 μm Constant Height: H 320 μm H2O2 initial concentration: %

87.5 %

Adiabatic Temperature: T 949 K Mass flow rate: �̇�𝑚 ~1 mg/s Thrust: F ~2 mN Specific Impulse: Isp 150 s

Prototype testing

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• The samples have been fed with in-lab distilled ~85% H2O2, injected through a syringe pump

• High speed camera imaging reveals that only an incomplete reaction occurs in the chamber, since a bubbly gas-liquid mixture is found at the nozzle rather than the expected warm gas stream

Detail of terminal part of the decomposition chamber and nozzle convergent under operation.

• SEM pictures of samples sections showed that catalyst deposition was not uniform all over the device, thus indicating the need for an improved deposition process.

Courtesy of ENEA Faenza

Prototype testing

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Catalytic Decomposition Kinetics

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• Motivations for experimental determination of reaction kinetics: a. Getting more insight on the decomposition process with “hands-on” experience b. Large literature on the subject, but data are very spread! c. Create our own data-base for benchmarking with the performance of the MEMS thrusters d. Drive the selection of catalyst materials for next generation prototypes

• Experiments based on pressure vessel method: • p measurements moles of O2 developed reaction advancement

• Works well at low T (O2 is the only gaseous phase)

• Assuming 1° order kinetics and Arrhenius-like temperature dependence:

𝑟𝑟 = −𝑑𝑑 H2O2𝑑𝑑𝑑𝑑

= 𝑘𝑘 𝑇𝑇 H2O2 = 𝐴𝐴𝑟𝑟𝑒𝑒−𝐸𝐸𝑎𝑎𝑅𝑅𝑅𝑅 H2O2

ln 𝑘𝑘 𝑇𝑇 = ln 𝐴𝐴𝑟𝑟 −𝐸𝐸𝑎𝑎𝑅𝑅𝑅𝑅

Catalytic Decomposition Kinetics

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∆𝑛𝑛𝑔𝑔𝑔𝑔𝑔𝑔 =∆𝑃𝑃 ∙ 𝑉𝑉𝑅𝑅𝑇𝑇

• Assuming water vapor pressure negligible:

𝑘𝑘 𝑇𝑇 =2∆𝑷𝑷 ∙ 𝑉𝑉

∆𝑡𝑡 ∙ 𝑅𝑅 ∙ 𝑻𝑻 ∙ 𝐻𝐻2𝑂𝑂2 ∙ 𝑉𝑉𝐻𝐻2𝑂𝑂2[1/𝑠𝑠]

manometer

Pressure vessel

Moles of Gas

Test Conditions

Vessel volume 0.24 L Tested Temperatures 23,36,50°C Catalyst Pt supported on

alumina powder, 5% wt

Catalyst mass 150 mg Catalyst surface/mass 110 m2/g H2O2 % 5% H2O2 Volume 3 cc

Catalytic Decomposition Kinetics

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• Pressure variations show a fairly linear behavior 1st order kinetics

• As expected, increase of T leads to increase of ∆𝑃𝑃 slope faster decomposition kinetics

• Best fit of the average k(T) over the time frame leads to the following activation energy and Arrhenius factor:

Ea = 57.5 kJ

Ar = 2e15 1/s

Numerical Modelling

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• Motivations: 1. Driving the design of the next thruster prototypes; 2. Getting more insight into how different parameters impact the complicated nature of the

problem (chemical, thermal, fluid dynamics…) • A code for the 1-D, steady, non-adiabatic, chemically reacting multiphase flow is being

developed • Assumptions and focus:

• Plug flow assumption (no diffusive effects) • Impact of heat losses, increasingly important at small scales • Interphases mass transfer through evaporation

• Solve for longitudinal variation of: 1. H2O2 flux (species conservation) 2. Evaporative molar flux (interphase mass transfer) 3. Temperature (energy balance) 4. Pressure (pressure drop along the chamber)

Numerical Modelling

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• Resulting set of 4 coupled ODE:

• Enthalpy conservation extended over 5 species (H2O2(l) ,H2O2(v), H2O(l), H2O(v), O2) • Evaporative heat flux according to a simplified Hertz-Knudsen-Langmuir formula • Pressure drop by analogy with flow across porous media (Ergun equation)

𝑑𝑑�̇�𝑛𝐻𝐻2𝑂𝑂2𝑑𝑑𝑑𝑑

= 𝐴𝐴cross ∙ 𝑟𝑟 = 𝐴𝐴cross ∙ 𝑘𝑘(𝑇𝑇)[H2O2] (MB)

𝑑𝑑�̇�𝑛𝑣𝑣𝑑𝑑𝑑𝑑

=𝜂𝜂𝑒𝑒𝑣𝑣

�2𝜋𝜋ℳ𝑣𝑣𝑅𝑅𝑇𝑇(𝑝𝑝𝑠𝑠𝑠𝑠𝑡𝑡 − 𝑝𝑝𝑣𝑣)𝑠𝑠𝑔𝑔𝑙𝑙𝐴𝐴cross (EvB)

𝑑𝑑𝑇𝑇𝑑𝑑𝑑𝑑

= −∑ 𝑑𝑑�̇�𝑛𝑖𝑖

𝛼𝛼

𝑑𝑑𝑑𝑑 ℎ𝑖𝑖𝛼𝛼

𝑖𝑖 + 𝑈𝑈𝑐𝑐𝑠𝑠𝑟𝑟(𝑇𝑇 − 𝑇𝑇w )𝐴𝐴cross∑ �̇�𝑛𝑖𝑖𝛼𝛼𝑐𝑐𝑝𝑝 ,𝑖𝑖𝛼𝛼𝑖𝑖

(EB)

𝑑𝑑𝑝𝑝𝑑𝑑𝑑𝑑

= −�1501 − 𝑒𝑒𝑝𝑝𝑅𝑅𝑒𝑒𝑝𝑝

+ 1.75�1 − 𝑒𝑒𝑝𝑝𝑒𝑒𝑝𝑝3

𝜌𝜌𝑢𝑢2

𝐷𝐷𝑒𝑒𝑒𝑒 (PD)

Numerical Modelling

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• Example of simulation output (87.5% H2O2, �̇�𝑚 = 1 mg/s, pin =1.2 bar, Tin = 298 K):

• Output strongly affected by tuning parameters (e.g. heat transfer, reaction kinetics…) • Still a useful tool for getting design trends

Current status and next steps

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• Updated status of the efforts towards the development of a MEMS HTP monopropellant µThuster have been presented.

• The investigation encompasses several research areas: 1. MEMS design and manufacturing 2. Catalytic decomposition kinetic experimental tests 3. Numerical modelling

• Tests over first batch of prototypes revealed only a partial degree of decomposition: deeper insight into the catalytic reaction is needed.

• Next steps: 1. Manufacturing: improved catalyst deposition, integration of a heating/sensing resistor 2. Catalytic decomposition: further kinetic studies (different concentrations, T, materials) 3. Numeric: from 1D to 2/3D full FEM model for the reacting multiphase fluid dynamics

around pillars • Short term objective: next generation of prototypes capable of delivering steady thrust. • Medium to long term objectives: from component level to equipment level

• embedding several functionalities into integrated MEMS (e.g. µValve, µPump).

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Current status and next steps

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THANK YOU!

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Foliennummer 1OverviewMotivations for H2O2 µThrustersConsortium & BackgroundWorking principleSchematics of H2O2 monoprop. systemPrototypes Design & ManufacturingPrototypes Design & ManufacturingPrototype testingPrototype testingCatalytic Decomposition KineticsCatalytic Decomposition KineticsCatalytic Decomposition KineticsNumerical ModellingNumerical ModellingNumerical ModellingCurrent status and next stepsCurrent status and next steps