COUNCIL FOR THE CENTRAL LABORATORY OF THE RESEARCH COUNCILS Dimples Day Meeting – 25 April 2006 Graham Arthur Central Microstructure Facility Rutherford

COUNCIL FOR THE CENTRAL LABORATORY OF THE RESEARCH COUNCILS

Dimples Day Meeting – 25 April 2006

Graham ArthurCentral Microstructure Facility

Rutherford Appleton LaboratoryChilton, Didcot

Oxon OX11 0QX


Outline

1. Dimples Fabrication

2. Pressure Sensor Design

3. Other itemsi. SPIE Smart Structures and Materials Symposium

• Two new mechanisms for dimple actuatorsii. Asymmetric dimple


Dimple Fabrication


Characterisation of elastomers

As supplied, MED-4905 and Sylgard 186 are highly viscous (up to 600,000cP). They must be diluted to allow spin-coating onto wafers at the required thicknesses (10-50um).Dilutions ranging from 25 – 60% solids are being tested.

Results (see following slides)

All spin-coating was at 1500rpm for one minute.

Elastomer 20m film 40m film

MED-4905 48% solids, w/w 58% solids, w/w

Sylgard 186 52% solids, w/w n/a



0

5

10

15

20

25

30

35

40

45

50

20 25 30 35 40 45 50 55 60 65

Dilution, %Solids

Th

ick

ne

ss

, um

y = 0.0702x - 0.4156

R2 = 0.9957

0.4

0.9

1.4

1.9

2.4

2.9

3.4

3.9

4.4

20 25 30 35 40 45 50 55 60 65

Dilution, %solids

Ln

Th

ickn

ess,

um

(left) MED-4905 Dilution vs. Coating Thickness

(right) MED-4905 Dilution vs. Ln Coating Thickness



(left) Sylgard 186 Dilution vs. Coating Thickness

(right) Sylgard 186 Dilution vs. Ln Coating Thickness

y = 0.0767x - 0.9576

R2 = 0.9975

1.9

2.1

2.3

2.5

2.7

2.9

3.1

3.3

3.5

35 40 45 50 55 60

Dilution, %solids

Ln T

hick

ness

, um

2

7

12

17

22

27

32

35 40 45 50 55 60

Dilution, %solids

Thic

knes

s, u

m



Thickness vs. Radial location

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35 40 45 50

Radial location, mm (0=centre)

Thic

knes

s, u

m

40% solids

45% solids

50% solids

55% solids

Thickness variation vs. Dilution

0.25

0.28

0.31

0.34

0.37

0.40

0.43

0.46

35 40 45 50 55 60

% solids(m

ax-m

in)/(

max

+min

)

Data for Sylgard 186 shows that increasing viscosity gives poorer uniformity of coating – multi-layer coatings should lead to improvements.


Inspection of Carbon Powder

The carbon powder used at Imperial for the dimple electrodes has been inspected using an scanning electron microscope (SEM) at RAL.

This reveals (see next slide) that there is a vast range of particle sizes (~50m to <250nm)

Is this a problem?

An alternative (finer) “carbon nano-powder” material is available from Sigma Aldrich and a small quantity have been obtained by Stella. Particle size is stated as <30nm. Cost is £75 for 25g


Inspection of Carbon Powder


New Dimple Design

Following input from Stella, a new dimple wafer has been designed. Key points are:

• Top edge of substrate has been rounded to avoid high strain at edge causing elastomer failure.

• Carbon powder electrodes will be used (and sealed to surface if necessary)

• 4 dimples - diameter is 10mm and separation is 10mm

Design layout is shown on next slide. Full process is shown in an accompanying Powerpoint file.


New Dimple Design


Pressure Sensor Design


New Pressure Sensor Design

Existing sensor:Silicon Nitride membrane (typically 0.5 m thick)Four strain gauges – two sensing and two referenceLow sensitivity

New sensor:Elastomer membrane (10-50 m thick)Improved sensitivity (>2 orders of magnitude possible)Possibly too much strain – but tuneable via film

thickness……will dynamic range be a problem?New strain gauge arrangement (via Lorenzo) – four sensing (two radial strain, two tangential strain)






Other Items


SPIE SS&M Symposium

SPIE’s Smart Structures and Materials symposium consisted of 15 conferences. Two had double sessions making 17 sessions in all, with 12-13 running in parallel – lots of running between sessions and still missed some!

Topics covered included:

Electroactive polymers (EAPs) and devices, Smart wings, (Bio)mimetics, Nastic structures, Active control, Non-destructive evaluation (NDE), Energy harvesting, Ferroelectrics, Magneto-rheological fluids, Nanostructures, Sensors, BioMEMS, Shaped memory materials,…….


SPIE SS&M Symposium

With regard to EAPs:

• AMI is marketing EAP devices (eg “Universal Muscle Actuator, UMA – diameter ~5cm)• Most EAP actuators are centimetre-scale – we are millimetre-scale (and smaller)• Most actuators are “push-pull” (muscle-type) actuation• University of Pisa is making a bump-type structure to use as a fluidic pump and mechanical actuator• In comparison with most other researchers we appear to be at a comparable stage but working at a smaller scale


SPIE SS&M Symposium

Ideas from conference have inspired two new dimple actuator ideas:

1. Pull-pull dimple using a pre-strained actuation films.

- Modification of AMI’s UMA.

2. Multi-layer stack giving a reciprocating action

- EAP stack similar to Univ. of Pisa


Basic Principle

A voltage is applied to the compliant electrodes either side of the EAP material. The electrostatic pressure compresses the EAP and as it is incompressible (=0.5) it spreads in X & Y, increasing in area. EAP has low E (typically 0.2-4 MPa)

EAP Basic Principle

PCompliant electrodes

EAP

V


Universal Muscle Actuator

If an EAP film is pre-strained (stretched) across a frame, when the voltage is applied, movement is seen in the plane of the film as the strain is relaxed – the area of the electrodes increases.

The UMA consists of two such films, back-to-back and joined at their centres. There are now forces acting out-of-plane.


UMA Operating Principle

Insulating substrate (annulus)

Pre-strained EAP #1

Insulating spacer disc

Pre-strained EAP #2

Voltage "off" Apply voltage to LH EAP (green) - film expands and pre-strain is relaxed. Tension in red actuator pulls to right

Apply voltage to RH EAP (red) - film expands and pre-strain is relaxed. Tension in green actuator pulls to left

Switching voltage rapidly between actuation films provides an oscillating action. The DA50 has a diameter of >5cm


New Dimple Actuator #1

Modifying this idea, with only the lower side being active (upper surface has no electrodes) a dimple actuator can be formed:

Insulating substrate (annulus)

Pre-strainedEAP #1

Insulating spacer disc

Pre-strained EAP #2 (passive - no electrodes)

Voltage "off" - pre-strains in both films creates dimple and depth is where forces are balanced

Apply voltage to green EAP actuator - film expands and pre-strain is relaxed. Tension in red film pulls upwards and dimple recess disappears

Airflow



Dimple design #2 requires a long thin strip of EAP material.

• Top and bottom are coated with carbon or carbon grease to create the electrodes.

• The strip is folded back and forth many times to form a stack.

When a voltage is applied the stack is squeezed by the electrostatic forces. Some spreading occurs, but the height change is the desired movement:



Voltage “off” Voltage “on”

Electrode #1

Electrode #2

EAP



The actuating mechanism can be incorporated into a piston-style dimple with a downward-only reciprocating motion.

Airflow


The actuating mechanism can be incorporated into a piston-style dimple with a downward-only reciprocating motion.


Airflow


Other items

Following some questions, at an earlier meeting, about the possibility of making dimples with an asymmetric cross section, some simple FEAs were carried out:

• Square dimple – for simplicity at this stage

• Elastomer membrane – properties similar to those of MED-4905

• Grooves etched into upper surface of membrane (depth = 50% of film thickness)


Other items

Grooves etched into upper surface of elastomer membrane

Dimple outline


Other items

Applying pressure gives the dimple shape shown here

Documents

COUNCIL FOR THE CENTRAL LABORATORY OF THE RESEARCH COUNCILS Dimples Day Meeting – 25 April 2006 Graham Arthur Central Microstructure Facility Rutherford