Masters Thesis Presentation

A Bimetallic Valve Solution for a Hydrogen-Powered

Micro-Generator

Joseph M. MahoneyCandidate: BSME, MSME

July 18 2007

2

Contents

1. Project Background and Overview2. Steady-State Test3. Transient Test4. Computer Modeling and Simulation5. Conclusions and Future Work

Background and Overview

4

System Flowchart

5

Current System Design

6

Power Generating “Fin”

Catalyst

Metallic Layer

Electrical Conductor

Silicon Substrate

Thin-Film Piezoelectric

7

Cantilever Valve Design

H2 Flow

Air Flow

Thermal Actuator

Rigid Wall H2 Flow

Air Flow

“Hot” Position “Cold” Position

8

Actuation Options

Smart Memory Alloy (SMA)Metal transitions from a deformed to a set shape when heated above transition temperature

BimetallicDifference in coefficients of thermal expansion cause a strip to flex when heated or cooled from reference temperature

9

Bimetallic Cantilever Design

Pt CatalystGold Layer

Silicon Substrate

H2

Air

Pt CatalystGold Layer

Silicon Substrate

Pt CatalystGold Layer

Silicon Substrate

450 µmH2

Air

2 µm50 µmwide

~0.2 µm~0.02 µm

Steady-State Test

11

Bimetallic Equations

κκδ

2211 x−−=

( ) TBA Δ−=Δ ααε

( )42322342 464

61

BBBABABABABABAAA

BABABA

hEhhEEhhEEhhEEhEhhhhEE

R ++++Δ+

==εκ

hi: Thickness of Material iEi: Young’s Modulus of Material iαi: Coefficient of Thermal Expansion of Material iΔT: Temperature Deviation from ReferenceΔε: Misfit Strain

12

Deposition Thickness Determination

Available deposition materials were gold, titanium, titanium aluminate, and nickelAnalysis done at 50°C above reference temperature using Si cantilever 450μm long, 2μm thick, and 50μm wideMaximum tip deflection of 41μm was found using gold at a target deposition thickness of 165nm

13

SEM Measurements

14

Steady-State Test Apparatus

Heater Slot

ThermocoupleProbe

Chip Slot

15

Steady-State Test Images

Top: 25.1°CBottom 127°CMagnified 10x

16

Steady-State AnalysisLength of red lines parallel and perpendicular to cantilever are measured and the ratio between them is calculatedBecause the nominal length of the cantilever is known (450μm), the amount of deflection can be found by using the ratio found in the previous step

17

Steady-State Test ResultsChip 3

y = 3E-07x + 4E-06R2 = 0.9861

5.0E-6

10.0E-6

15.0E-6

20.0E-6

25.0E-6

30.0E-6

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Temperature (C)

Def

lect

ion

[m]

Measured Equation FEA (400) Linear (Measured)

Measured results show good linearityAt the reference temperature of 20°C, the cantilever still had a deflection due to residual stress from the fabrication process. This deflection was measured and accounted for in the analytic calculationAnalytic equation has average error of 7% over 100°C range

Transient Test

19

Transient Test Apparatus

Fuel-Air Inlet

Outlet

Chip Slot

Combustion Channel

20

Transient Test Calibration

Fuel flow through the test apparatus had to first be measured to correlate fuel velocity to combustion fluxVolume flow was measured using a bubble flowmeterVolume flow was converted to velocity using the nominal cross-sectional area of the combustion chamber

21

Transient Test CalibrationVelocity Calibration

y = 157.35E‐3x

R2 = 944.59E‐3

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250

Air Pressure [kPag]

Air Velocity

 [m/s]

22

Transient Test Images

Fuel Flow Off Fuel Flow On

23

Transient Test Movie

Movie recorded at 5 psi fuel flow (5.7 m/s) and 10x magnificationCaptured at 50 fps

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Heat Flux AnalysisUsing the same method as in the Steady-State test, the deflection of the cantilever at zero and full deflection was measured

The temperature of this cantilever was calculated using the relationship previously determined

Since the cantilever is at a constant deflection, and therefore, constant temperature at its full extension, the amount of heat into the cantilever must be the same as the amount of heat out of the cantilever

The combustive heat flux can be estimated by setting it equal to the convective heat lossThe convection coefficient was calculated based on book equations for a flow over a perpendicular plate

This analysis was done for multiple fuel velocities to estimate the relationship between heat flux into the cantilever and fuel velocity

( )( )

Pt

ambbeamcombust

ambbeamPtcombustconvcombust

outin

LTTLhQ

TTAhAQQQ

QQQ

TTQdtdT

−=′′

−=′′→=∴

==

===

max

max

max

when 0

and 0 when 0

&&

&&&

&

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Heat Flux ResultsHeat Flux

y = 79894x - 168914R2 = 0.8951

000.0E+0

100.0E+3

200.0E+3

300.0E+3

400.0E+3

500.0E+3

600.0E+3

700.0E+3

800.0E+3

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Air Velocity [m/s]

Q''

[W/m

²]

Computer Simulation and Modeling

27

AssumptionsThe cantilever is treated as 2D: it is much longer (L=450μm) than it is wide (w=50μm); since thermal expansion is the focus, the flexure in the width is negligible compared to the flexure in the length

Heat loss is not calculated through the width as the height is small (h<w::2μm<50μm)

Cantilever is treated as a bulk object as its Biot number is extremely small (Bi≈1x10-6)Constant material properties: small temperature rangeCr layer is negligible: only 5-10nm compared to gold deposit of 150nm

Cr layer acts as intermediary layer for Au to bond to Si

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Boundary- and Initial Conditions

The convection is a function of h(u∞, Tamb, L, w), Tn and Tamb

When the cantilever is closed, there are no convective loses on the bottom surface

The heat flux is a function of tip position, time and air velocity

H2 flow takes time to diffuse away from and into the Pt when the cantilever is closed and opened, respectively

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Simplified Cantilever Model

qconvqcombustion

qconv

T∞ = 25°C

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Code Outline1. Initialize Variables for Material Properties2. Calculate Values of the Cantilever, such as Initial

Thermal Energy and Inertia3. Perform Time Loop

1. Calculate Deflection at tn Based on Temperature at tn-1

2. Calculate Stress at tn based on Deflection at tn

3. Calculate Temperature Based on Deflection at tn Temperature is calculated based on Total Thermal Energy of cantilever as changed from convective loss and combustive gain

4. Increase Time by Δt4. Plot Deflection and Temperature over Time5. Find Frequency and Maximum Stress

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Simulation Results

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

0.2

0.4

0.6

0.8

1

1.2

1.4x 10-5 Tip Deflection vs. Time

Time [sec]

Def

lect

ion

[m]

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

100

200

300

400

500

600Temperature vs. Time

Time [sec]

Tem

pera

ture

[C]

•Frequency of 4.8 Hz•Maximum Temperature of 530°C•Maximum Deflection of 12μm

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Design Optimization

Easily changed fabrication variables are reference temperature, gold deposition thickness and area of platinum depositionSimulation can be put inside an Nelder-Mead optimization algorithm

1. Designs a cantilever that will operate at a specified frequency, as required by the combustion chamber

2. Designs power generating “fin” to maximize strain in piezoelectric layer

Conclusions and Future Work

34

Conclusions

Using commercially available parts and nanofabrication methods, a proof-of-concept cantilever was tested Using the data collected from the experiments, a preliminary computer simulation was created

The simulation can be used within optimization algorithms to create a design to operate at a specified frequency or maximize strain

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Future Work

Using a high-speed camera and a higher contrast background, rerun the transient test to better determine the heat flux as a function of fuel velocityFollowing experiments run on the combustor side, update the simulation to reflect more accurate fuel diffusion lag times and convection coefficientsIntegrate cantilever into full system and test power output and efficiency

Questions

Backup

38

Previous Micro-Valves

Lo et al. (2001)Actively controlled micro-valve

Passes electric current through an area of Pt, keeping it at a constant temperatureThe more current required to heat the Pt, the greater the mass flow rate over itThe current also passes through the Si arms; heating and expanding them to open the valve

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Previous Micro-Valves

Yan (2002)Imitates a bimetallic cantilever using only one materialActively controlled micro-valve

Electric current passes through the entire armThe smaller cross-section heats up more than the larger cross-section, therefore expanding more

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Previous Micro-Valves

Krulevitch et al. (1996)Passive Control System

Fluid heats the SMA film, causing it to open when it passes through its transition pointThe opening connects the inlet and the outlet

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References (A-L)Appel, Mantzaras, Schaeren, Bombach, and Inauen. “Catalytic Combustion of Hydrogen-Air Mixtures over Platinum: Validation of Hetero/homogeneous Chemical Reaction Schemes” Paul Scherrer Institute, Combustion Research. Ayhan, A. F. “Design of a Piezoelectrically Actuated Microvalve for Flow Control in Fuel Cells” 2002 University of Pittsburgh“Bimetallic Strip Applications” 5 June 2007 <http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/bimet.html> Callister, William D. Materials Science and Engineering, an Introduction. Catchmark, Jeff. Personal Interview. 13 December 2005, 02 March 2006 and 30 March 2006.Clyne, TW. “Residual stresses in surface coatings and their effects on interfacial debonding.” Key Engineering Materials (Switzerland). Vol. 116-117, pp. 307-330. 1996Duerig, Melton, Stockel, Wayman. Engineering Aspects of Shape Memory Alloys. 1990 Butterworth-Heinemann LtdGōkin, Keijō Kioku. Shape Memory Alloys. 1984 Sangyo ToshoHibbeler, R. Mechanics of Materials. 2004 Prentice Hall. Incropera & Dewitt. Fundamentals of Heat and Mass Transfer. 1996 John Wiley & Sons Inc.Krulevitch et al. “Thin Film Shape Memory Alloy Microactuators” Journal of Microelectromechanical Systems, Vol. 5, No. 4, pp. 270-282. December 1996Kung, C and Chen, R. “Fatigue Analysis of U-Shaped Flexural Electro-Thermal Micro-Actuators”Journal of the Chinese Institute of Engineers, Vol. 28, No. 1, pp. 123-130. 2005Lo, Tsai, Tsai, Fan, Wu, Huang. “A silicon mass flow control micro-system.” 4 May 2001.Lutter, Stefan. “AW: tipless cantilevers.” E-mail to Alex Imhof. 23 March 2006.

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References (M-Z)Machada & Savi. “Medical applications of shape memory alloys” Brazilian Journal of Medical and Biological Research Vol. 36, pp. 683-691. 2003Moorlegham et al. “Shape memory and superelastic alloys: The new medical materials with growing demand” Bio-Medical Materials and Engineering Vol. pp. 55-60. 1998Nitonol Devices and Components. “Technology.” 25 September 2005 <http://www.nitinol.com/3tech.htm>Norton, Robert L. Machine Design: An Integrated Approach. 2000 Pearson Education Ltd.“PSU Nanofab – Kurt L. Lesker E-Gun Thermal Evaporator”. Pennsylvania State University 17 May

2007 <http://www.nanofab.psu.edu/equipment/equipment.pages/kurtlesker.egun.thermal.evaporator.htm> “Shape Memory Alloys.” SMA/MEMS Research Group. 1 April 2006

<http://www.cs.ualberta.ca/~database/MEMS/sma_mems/sma.html>“Thermal expansion and the bi-material strip.” University of Cambridge. 1 April 2006 <http://www.doitpoms.ac.uk/tlplib/thermal-expansion/printall.php>Thomas, George.“Overview of Storage Development DOE Hydrogen Program”. Sandia National Laboratories, 9 May 2000.Warnatz, Allendorf, Kee, Coltrin. “A Model of Elementary Chemistry and Fluid Mechanics in the Combustion of Hydrogen on Platinum Surfaces” Combustion and Flame. Vol. 96, pp. 393-406. 1994Yan, D. “Mechanical Design and Modeling of MEMS Thermal Actuators for RF Applications” University of Waterloo. 2002Yu, Shebeko, Trunev, et al. “Flameless Combustion of Hydrogen on the Surface of a Hydrophobized Catalyst” Combustion, Explosion, and Shockwaves. Vol. 31 No. 5, pp. 537-542. 1995