•To decrease the resonant frequency of the engine compared to a previous engine using Silicon Nitride, which had a resonant frequency of 110 Hz 3.

•To fabricate PDMS membranes

•To investigate the material properties of the PDMS membranes.

•To Investigate the effect of a PDMS expander membrane on the engine

OBJECTIVES

Fabrication and Characterization of PDMS Membranes for MEMS-based Micro Heat Engine

M. A. Brubaker, B. S. Preetham, R. F. Richards, M. J. Anderson, H. Bardaweel, C. D. Richards

Mechanical and Materials Engineering, Washington State University REU

CONCLUSION•The PDMS membranes were successfully fabricated and characterized . A Pressure-Deflection curve was found and the membranes showed to be very flexible. A modulus of elasticity of 6.89 N/m was measured for PDMS membranes.

•The membranes were incorporated into the micro heat engine and an estimated resonant frequency of 58.3 Hz was measured. This is a decrease by a factor of 2 from the previous measurments3.

(1) Whalen S, Thompson M, Bahr D, Richards C, Richards R 2003 Design, fabrication, and testing of P3 micro heat engine Sensors Actuators A 104 290-8

(2) Sang S, Witte H 2010 Fabrication of a surface stress-base PDMS micro-membrane biosensor Microsystem Technologies 16 1001-8

(3) Bardaweel H, Anderson A, Richards R, Richards C 2008 Optimization of the dynamic and thermal performance of a resonant micro heat engine J. Micromech. Microeng. 18 104014

REFERENCES

The PDMS expander membrane was created using MEMS-based fabrication techniques.

Membranes were then diced and assembled into the engine.

The membranes are 10mm by 10mm and approximately 1 micrometer thick.

FABRICATIONINTRODUCTION

Micro Heat Engine

• Converts heat into mechanical energy then electrical power

• Applicable for any situation where low grade waste heat exists.

•Small Military devices

•Portable Electronic Device

Semiconductor

tape

Working fluid

(3M PF-5060 L)

Wicks

Thermal

switch

Polymer

Added mass

Expander

membrane

Evaporator

membrane

Started with a silicon nitride wafer; Photolithography and development of photo resist (AZ5214)

Reactive Ion Etching of Silicon Nitride; remove photo resist

Sputter Gold; photolithography and development of photo resist

Gold Etch; remove photo resist

Chemical etching of Silicon using KOH

Reactive Ion Etch top layer of Silicon NitrideRecipe based off of 2

•As heat enters and leaves the engine the working fluid evaporates and condenses.

•Mechanical energy is created by the oscillation of the expander membrane.

•This creates mechanical energy that can be converted into electrical energy through the use of piezoelectric material which can be deposited on top of the expander membrane. 1

•The engine’s performance is heavily influenced by its resonant frequency, the smaller the resonant frequency the greater the performance.3

DESIGN

Expansion

Heat addition

Heat rejection

Compression

y = 0.0689x

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150 200 250 300

Deflection (micrometers)

Pre

ssu

re (

pascals

)

RESULTS

•The pressure-deflection curve reveals the flexibility of PDMS.

•From the linear portion of the graph, the modulus of elasticity (k) was estimated to be 6.89 N/m.

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500 600 700

Deflection (micrometers)

Pre

ss

ure

(p

as

ca

ls)

•The data shows the deflection of the PDMS membrane after heat was added to the Engine. The resonant frequency was estimated to be 58.3 Hz.

0

1

2

3

4

5

6

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

Time (s)

Defl

ecti

on

(m

icro

mete

rs)

-1

0

1

2

3

4

5

6

7

8

9

Vo

ltag

e (

V)

Deflection of Membrane

Voltage In

Voltage Out

Linear Portion

This work was supported by the National Science Foundation’s REU program under grant number EEC-0754370

EXPERIMENTAL SET UP

•The thermal switch was not used to control heat addition to the engine. However, a resistance heater enabled precise control of the magnitude and duration of the heat addition process.

•10 mJ of heat per cycle of energy was added into the engine at 10 Hz with a duty cycle of 1%.

Laser head

Vibrometer

PC

Oscilloscope

Engine

Power supply

Function

generator

Pulse circuit