Dr. David CarrollProfessor of Physics Center for Nanotechnology and Molecular Materials at Wake Forest University501 Deacon Blvd. Winston-Salem NC 27105

Powe

rFelt…

Therm

o-Piez

o-

Electr

ics

PowerFelttm is a “TPEG”: a metamaterial – fabric that combines power scavenging from thermoelectrics with power scavenging from a piezoelectric. By combining these orthogonal functionalities, we are able to generate more power than the two approaches individually. The synergistic effect is referred to as the “TPEG” effect.

500 W Kinetic power

1kW Heat power

TEG + PEG or TPEG

1 W heat+5 W kinetic

6 Watts 8 Watts!

Example:

Power comparisons with competitive technologies. With no vibrations, the PowerFelt TPEG has a ZT > 0.1 and power factor ~ 200 W/m2K. However, its composition allows for a greater T across the active elements within the fabric. This translates into better performance overall.

Comparisons of PowerFelt™ with competitive technologies

Depends on stroke.

Power at T ~ 10K

Power at T ~ 50K

Max Power/g Radius of bend

Compression strength

Piezo-output @ 1Hz

PowerFeltTM* 0.1 W/m2 2.5 W/m2 700 mWg-1 <1 mm excellent 0.1 - 10 W/m2

gTEG** 0.03 W/m2 0.25 W/m2 200 mWg-1 ~1 cm good none

Fuji ~2 mW/m2 20 mW/m2 400 mWg-1 ~1 cm poor none

BiTe Ceramic NA NA 232 mWg-1 NA Excellent none

* Numbers have been vetted by NIST** Similar to KAIST construct

Piezo dominates at high frequency, but rolls off due to impedance mismatch. For a lower stroke this line moves lower, but doesn’t cross the TEG intercept.

At low frequency the thermoelectric effect dominates. As mechanical energy is added to the system, the power increase is faster than expected. The TPEG effect.

TPEGs are made from a set of doped nanocomposites. Samples shown here use carbon nanotubes but more powerful structures are under development that use an exotic 2D nanophase.

These composites are doped to form p-type and n-type conductors. They are then assembled into the meta-structure.

Modules can have many layers. Between each layer is a piezo-component.

A. Schematic diagram of the multilayered device structure. B. The alternating p-type/n-type conduction layers prior to adding the insulating layers.

The multilayered “device” is composed of alternating p-type and n-type thin film conduction layers, with insulation layers between each conduction layer (a). A p/n junction is formed on alternating sides of the device to allow conduction through the layers while adding the thermal voltage contribution from each layer in series. Each p/n junction is formed first (b) by pressing the junction at 400 K, just above the melting point of the binding polymer.

The TPEG Effect

The observation is simple: For a simple system as shown; with a piezoactive component in contact with two thermoelectric legs, one can bend or twist the system and generate piezoelectric power at the same time as generating thermoelectric power using the same components for different purposes. However:

Ptotal > Ppiezo + Pthermo !

WHY?

A simple diagram of what is happening in the system

n-type CNT matrix

p-type CNT matrix

Epiezo- - - - - - - - - - - - - - - - - -

- - - - - - - - - - - - - - -- - - - - - - - - - - - - - -- - - - - - - - -- - - - - -

+ ++ ++ ++ ++ ++ + + ++ + + ++ ++ ++ ++

+ ++ + ++ ++ ++

+ ++ +

Consider a thermal gradient across the piece as shown

cold hot

A simple hypothesis might be that the bound charge establishes an external bias across the thermoelectrically active electrodes thereby shifting the Fermi level and altering the Seebeck coefficient…

We might then expect that the total voltage observed from the former configuration to be:

Vtotal = Vthermo + Vpiezo where Vthermo = V’thermo + Vmodified

and V’ = the “native” thermoelectric voltage of the electrode materials.

We can write this as:

Vtotal = ’T + modifiedT + w({S} – [sE]{T})/[dt] Vpiezo

Where: ’ = native Seebeck coeff., modified = increase in Seebeck, w = thickness of piezo, and ({S} – [sE]{T})/[dt] is the stress-strain relations from the coupled equations for piezos. We will call this C and it is a function of temperature as well as strain; C(T,s).

Vtotal = ’T + modifiedT + C(T,s) Vpiezo

The “field doping” can be easily imagined to lead to large increases in the Seebeck Coefficient. As shown here the density of states has spikes associated with the dimensionality (Van Hove Singularities).

The Seebeck Coefficient S ~ d (DOS)/dE| Ef and so as the fermi level is pushed to the singularity by the applied field, the Seebeck Coeff. rises sharply.

Our experiment looks like this…

We use a pendulum to establish a driving force on the piezo that goes as:

Amp = A0e-t/sin(t) – a damped harmonic oscillator.

Assuming that the piezo has been poled only in one direction – from one electrode to the other, our expression for voltage looks like…

Vtotal = ’T + (o modifiede-t/sin(t) ) T + Coe-t/sin(t) Vpiezo

Now we want to find the order of magnitude of the o modified. To do this we set the voltage divider so that we can shunt off the thermoelectric constant part:

Vtotal- ’T = [o modified T + CoVpiezo] e-t/sin(t)

But recall that C0(T) is also temperature dependent. So the null experiment is to swing the pendulum with contacts that have no thermoelectric component – like Al contacts. Then we compare this to the system with thermoelectric components – CNTs.

This null experiments – not shown – gives only a 5% variation in the Vpiezo with T over the temperature range of the experiments. So this is the limits of our ability to measure o. However, according to literature, increasing temperature should lead to a reduction in the d31 coefficient. So we should see a decreasing voltage from this effect.

A second consideration is if the PVDF film swells under temperature change. This would lead to a dimensional change in the capacitor. But again, over the temperature swings of this experiments, such dimensional changes in PVDF are far less than a few percent according to literature.

Thus, we would expect for small T, positive changes in the absolute value of time dependent voltage with temperature should be attributable primarily to o.

Vtotal- ’T = [o modified T + CoVpiezo] e-t/sin(t)

Weak temperature dependences for T < 20K and less than the glass transition temperature of PVDF

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The combined effect

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The combined effect

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Summary:

We have shown that the TPEG represents a fundamentally different power scavenger with the ability to capture both: KINETIC and THERMAL waste power.

1)Our TEG performance starts at ZT > 0.1 (with a higher power coming soon)2)Our PEG depends on the KE available but can reach very high powers in energetic environments.3)The TPEG is resistant to water and oxygen showing little degradation over time.4)We are on the cusp of being able to produce very large pieces of this material. Estimated costs are around $25/m2 initially.

Disclaimer:

This presentation contains forward looking statements and estimates. The data was taken at the Center for Nanotechnology and Molecular Materials at Wake Forest University by the Carroll Research Group in cooperation with Streamline Inc. Commercial images used are for illustration only and do not imply endorsement by these companies or persons.

Funding:

Streamline Automation, NASA, AFOSR, NSF.