DISPENSER PRINTED THERMOELECTRIC ENERGY GENERATORS
Alic Chen1, Deepa Madan2, Mike Koplow1, Paul K. Wright1,2 and James W. Evans3
1
Department of Mechanical Engineering, University of California, Berkeley, USA
2
Center for Information Technlogy Research in the Interest of Society (CITRIS),
University of California, Berkeley, USA
3
Department of Materials Science & Engineering, University of California, Berkeley, USA
Abstract: Thermoelectric energy generators are attractive as potential energy harvesters for converting waste
thermal energy into electrical power. Optimized thermoelectric device designs require 100-200 µm element
thicknesses currently unachievable with common manufacturing technologies. This work presents both a unique
direct-write dispenser printing technique and novel polymer-composite thermoelectric materials for fabrication of
optimized thermoelectric devices. Initial composite materials properties show the proposed method as a promising
low-cost, scalable method for manufacturing of thermoelectric energy generators.
Keywords: Thermoelectric, Energy harvesting, Direct-write, Dispenser-printing, Polymer-composites
INTRODUCTION
techniques. Traditional bulk thermoelectric elements
prepared by dicing and extrusion are limited by the
resolution of the dicing step to
>300µm
thermoelectric leg lengths [3]. Alternatively,
conventional microfabrication processes involving
lithography and thin-film vapor deposition are limited
to <60µm element leg lengths [3, 9-10]. There exists a
gap in the available technologies for producing
optimized thermoelectric devices with 100-200µm leg
lengths. Thus, a promising direct-write printing
technique has been developed to additively create
microscale generators. Direct writing is an alternative
method for fabricating thick film structures. It is a
simple and flexible additive deposition method for
patterning materials at ambient and room temperature
conditions [11]. Printing techniques involve the use of
specially tailored thermoelectric inks consisting of
slurry suspensions of active materials in polymer
binders and solvents. These inks enable fabrication of
low-cost thermoelectric devices [6, 12-13]. The
process energy input and waste generated are both
reduced substantially compared to thin-film
microfabrication methods [11, 12-15]. Earlier results
have shown this method to be both cost effective and
scalable for mass production [6, 13].
This work presents the use of a previously
developed dispenser printer to print custom-tailored
thermoelectric materials. The printer has been
successfully used for battery, capacitor and MEMS
sensor research [15-16]. The printer is capable of
printing within 1µm resolutions. Feature sizes down
to 50µm can be printed with film thicknesses ranging
from 10 to 200µm per pass, depending on process
parameters such as shot pressure, tip size, ink
rheology, and shot spacing. Printing can be
accomplished on a variety of substrates ranging from
The successful realization of reliable wireless
sensor systems in residential, industrial and medical
monitoring applications requires long-lasting power
solutions for autonomous nodes [1]. While wireless
sensor networks have made a new generation of ondemand wireless monitoring applications possible, the
practical deployment of such systems are ultimately
constrained by the device power supply. The power
supplies are typically fixed-energy primary batteries
that require replacement after 5-10 years depending
on use [2]. Battery replacement is thus undesirable
and inefficient in large-scale deployments of wireless
sensor nodes. This presents a special challenge for
power sources, as they must be robust enough to last
without fatigue and also provide enough power to
maintain a sufficient duty cycle.
Thermoelectric devices are attractive as potential
power sources because they directly convert thermal
gradients into electrical power [3]. They are silent,
require no moving parts, have proven reliability
through extended use, and are scalable. The bulk of
thermoelectric research is in novel material synthesis
[4, 5], while less attention is paid to device-level
manufacturing.
Previous work has shown that micro-scale
thermoelectric generators are capable of powering
low-powered wireless sensor radios through
optimized generator designs [6]. Design studies
suggest the optimal element leg lengths for microscale
generators to be between 100-200µm to maximize
power output [6-8].
While the necessary power output from microscale thermoelectric generators is theoretically
feasible, the required device dimensions present a
challenge for standard mass production manufacturing
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PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
printed circuit boards (PCB) to glass slides. While the
printer is mainly used for prototyping purposes, this
method is scalable for mass manufacturing using
multiple dispenser heads in an assembly line.
EXPERIMENTAL
Thermoelectric Materials Synthesis
Printable thermoelectric materials are synthesized
as composite systems consisting of active
thermoelectric particles (n-type Bi2Te3 and p-type
Bi0.5Sb1.5Te3 particles) within a polymer binder. To
achieve the 100-200µm feature sizes, 50-100µm
printer tips are required to dispense the materials. The
tip size thus limits the maximum particle size of the
active materials. Empirical testing has shown that the
optimum average particle size for this proposed
application is approximately 10µm [6].
To produce 10µm average Bi2Te3 and
Bi0.5Sb1.5Te3 particles, bulk bismuth, antimony and
tellurium particles (1-12mm) purchased from Sigma
Aldrich, Inc. were first mechanically alloyed using a
high-energy planetary ball-mill (Torrey Hills ND
0.4L). Mechanical alloying is a proven, low-cost
technique for synthesis of thermoelectric materials
from elemental components [5]. The starting powders
were placed in 100ml stainless steel jars with 20mm
stainless steel balls at a ball-to-powder mass ratio of
10:1. The jars were then placed in the planetary ball
mill and ran at 315rpm for 12 hours. All materials
preparation and extraction were performed under a
dry argon environment to prevent oxidation of
materials. The as-milled powders were measured
using a Coulter LS-100 Laser Sizer to determine the
particle size distributions. Initial as-milled particle
sizes ranged between 2-200µm. To further reduce the
particle sizes, the as-milled powders were ball milled
again in 100ml stainless steel jars with 3mm stainless
steel balls at a ball-to-powder mass ratio of 10:1. The
jars were placed in the ball mill and ran at 210 rpm for
60 min. Fig. 1 shows the particle size distribution of
the powders measured using the Coulter LS-100 and
the ball-milling parameters used. The resulting
average particle size after ball milling was 10µm
while the particles ranged between 2µm and 60µm.
The measured powder showed a bi-modal distribution
with peaks at 6µm and 30µm. Finally, a 500-mesh
sieve was used to remove the larger particles.
Epoxy resins were used as the polymer binder in
the formulation of the thermoelectric inks. Epoxy
resins are a class of thermoset polymers that are
proven and commonly used in commercially available
electrically conductive adhesives [6]. The epoxy
system was formulated using a bisphenol f epoxy
Fig. 1: Particle size distribution of mechanically
alloyed and ball-milled Bi2Te3 powders
resin (EPON 862, Hexion Specialty Chemicals, Inc.)
and an anhydride-based hardener (MHHPA, Dixie
Chemicals, Inc.). The epoxy-to-hardener equivalent
weight ratio was 1:0.85. 2E4MZCN (Sigma-Aldrich,
Inc.) was employed as the catalyst in the system. 1020 wt% of butyl glycidyl ether (Heloxy 61, Hexion
Specialty Chemicals, Inc.) was also used in the resin
blend as a reactive diluent to adjust the viscosity of
the ink to the desired properties.
The mechanically alloyed Bi2Te3 and Bi0.5Sb1.5Te3
powders were then added into the epoxy resin system
at 86 wt% of the slurry. The slurry was mixed using a
paint shaker and an ultrasonic bath to disperse the
particles. The thermoelectric inks were then deposited
on glass substrates to form 100-200µm thick films
using the dispenser printer. The films were finally
cured at 200°C in a vacuum oven for a minimum of 3
hours. Fig. 2 shows cross-section scanning electron
microscope (SEM) images of a dispenser printed
Bi2Te3/epoxy composite film. The images suggest that
the epoxy polymer binder forms a solid, dense matrix
when mixed and cured with the active Bi2Te3
particles. The particle distributions within the films
appear to be uniform and no voids are present.
Fig 2. Scanning electron microscope (SEM) images
of the ball-milled Bi2Te3/epoxy composite films
278
Device Fabrication
Fig. 3 shows the proposed fabrication steps for a
dispenser-printed
thermoelectric
device.
The
fabrication steps are detailed as follows. (1) The
dispenser printer first additively deposits the bottom
interconnects on a substrate using a conductive silver
paste. (2) A polydimethylsiloxane (PDMS) mold with
the required geometry design is aligned and bonded to
the substrate using an oxygen-plasma bonding process
(Tegal Plasmod). (3) Next, holes in the mold are
selectively filled with thermoelectric materials using
the dispenser printer and cured to form the active
elements. (4) The top silver interconnects are then
printed to link the active elements. (5) Finally, the top
substrate is bonded to the mold using the same
method as step 2 to complete the device. The PDMS
(Sylgard 184, Dow Corning Corp.) molds with
optimal device geometries are fabricated using a
double-casting process from an aluminum mold with
the final design [7]. Fig. 3 also shows a PDMS mold
filled with thermoelectric inks. The holes have 1 mm
diameters and 1.5 mm heights. Holes as small as
500µm have been fabricated while smaller holes for
higher couple density are possible. Currently, only a
single-couple device has been fabricated, but further
work is being performed to overcome processing
difficulties involved in the printing step.
Fig. 3: Fabrication steps and image of dispenserprinted thermoelectric modules
orders of magnitude less than that of bulk. This is
likely due to the electrically insulating properties of
the epoxy, and the contact resistance due to the fine
particles. The thermal conductivities of the
composites are much lower than that of the bulk
materials, and are due to the thermally insulating
properties of the epoxy. Low thermal conductivity is a
very desirable factor in thermoelectric materials for
maintaining heat gradients. This presents trade-offs in
the composite materials properties, and as a result,
both the power factors and figure of merit (Z) values
of the composites are at least one to two orders of
magnitude less than that of bulk materials. The p-type
Bi0.5Sb1.5Te3/epoxy also shows better overall
thermoelectric
properties
than
the
n-type
Bi2Te3/epoxy composite. The current limiting factor
in the performance of the composite materials is the
RESULTS & DISCUSSION
X-ray powder diffraction (XRD) (Siemens
D5000) was performed on the as-milled powders to
confirm the chemical nature of the materials. Fig. 4
shows the XRD patterns of the as-milled n-type
Bi2Te3 and p-type Bi0.5Sb1.5Te3 powders. The patterns
appear to match with their corresponding materials,
suggesting that the materials are completely alloyed.
Electrical conductivity measurements of the
printed thermoelectric materials were carried out
using a standard four-point probe method to determine
the sheet resistance of the material. Seebeck
measurements were performed using a custom
Seebeck testing device to determine the voltage output
of the material for a given temperature gradient.
Thermal conductivity measurements were performed
using an Anter Corp. Model 2021 bulk thermal
conductivity tester.
Table 1 shows the measured material properties of
the
n-type
Bi2Te3/epoxy
and
p-type
Bi0.5Sb1.5Te3/epoxy composites at 300 K with respect
to the bulk material properties. The epoxy composite
materials appear to have relatively close Seebeck
coefficients compared to bulk materials. However, the
electrical conductivities of the composites are two
Fig. 4: X-ray powder diffraction (XRD) patterns of
as-milled thermoelectric powders
279
Seebeck
Coefficient
(µV/K)
-200
Electrical
Conductivity
(S/cm)
770
Thermal
Conductivity
(W/m-K)
1.2
Power
Factor
(W/m-K2)
3.08 x 10-3
Z (K-1)
N-type Bi2Te3
2.56 x 10-3
N-type
-159
8
0.48
0.20 x 10-4
0.42 x 10-4
Bi2Te3/Epoxy
P-type Bi0.5Sb1.5Te3
230
1100
1.4
5.82 x 10-3
4.16 x 10-3
P-type
272
14.4
0.52
1.07 x 10-4
2.05 x 10-4
Bi2 Sb1.5Te3/Epoxy
Table 1. Material properties of printed thermoelectric/epoxy composites at 300K compared to bulk materials.
low electrical conductivity. Further work is being
performed to improve the materials properties through
composite optimization and additives.
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CONCLUSION
The initial results of the work presented are
promising for dispenser printed thermoelectric
materials. While initial materials properties are not at
the level of bulk materials, the printability of the
materials allow for a low-cost and scalable fabrication
method for optimally designed thermoelectric
generators. Further work is currently being performed
on both the composite thermoelectric materials and
generator device fabrication.
ACKNOWLEDGEMENTS
The authors thank the California Energy
Commission for supporting this research under
contract 500-01-43. We would also like to thank
Brian Mahlstedt, Christine Ho and Kevin Huang for
their contributions.
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