Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2014.
Supporting Information
for Adv. Mater., DOI: 10.1002/adma. 201400951
Large-Area Freestanding Graphene Paper for Superior
Thermal Management
Guoqing Xin, Hongtao Sun, Tao Hu, Hafez Raeisi Fard, Xiang
Sun, Nikhil Koratkar, Theodorian Borca-Tasciuc , and Jie
Lian*
Supporting Information
Large Area Free-Standing Graphene Paper for Superior
Thermal Management
Guoqing Xin, Hongtao Sun, Tao Hu, Hafez Raeisi Fard, Xiang Sun, Nikhil Koratkar,
Theodorian Borca-Tasciuc and Jie Lian*
1. Experimental
2. Experimental setup of the electrospray process for GPs.
3. A continuous roll-to-roll process for mass production of GPs
4. Mechanical properties of free standing GPs.
5. Measurement of thermal and electrical conductivity.
6. Record of infrared video of heat diffusion and heat transfer.
1. Experimental
Materials. Graphite powder (99.99%), N-methylpyrrolidone (NMP, 99.5%) and sodium nitrate (99%)
were purchased from Sigma Aldrich. HCl (36.5%), ethanol (95%) and H2SO4 (98%) were purchased
from Fisher chemical. KMnO4 (99.4%) was purchased from Mallinckrodt specialty chemicals. H2O2
(30%) was purchased from Macron Fine Chemicals.
Graphene Synthesis. Graphene was prepared by thermal exfoliation and reduction of the graphite
oxide (GO) prepared from graphite power following the Hummers’s method. [1,2] Graphite powder (5 g)
was added to a mixture containing concentrated H2SO4 (115 mL) and NaNO 3 (2.5 g) in an ice bath (0
°C). KMnO4 (15 g) was then added carefully to the solution and maintained for 30 min at 35 °C
followed by the slow addition of DI water (230 mL). The temperature of the reaction was maintained at
98 °C for 15 mins. After that additional deionized water (355 mL) containing H2O 2 (3 wt%, 5 mL) was
added. The solid obtained from centrifugation (3200 rpm, 5 mins) was washed with excess deionized
water, 20 vol% HCl, and ethanol. The washing process was repeated for several times until the pH of
the solution reached neutral. The final yellow brown GO powders were dried under vacuum at 40 °C
for 12 h.
Thermal exfoliation of GO was achieved by placing the GO powder (200 mg) in a 20-mm-innerdiameter, 1-m-long quartz tube that was sealed at one end. The other end of the quartz tube was closed
using a rubber stopper. An argon inlet was then inserted through the rubber stopper. The sample was
flushed with argon for 10 mins, and the quartz tube was quickly inserted into a tube furnace
(Thermolyne 79300, Thermo Fisher Scientific Inc., USA) preheated to 1050 °C and held in the furnace
for 30 s. Rapid heating (>2000 °C/ min) splits the GO into bulk quantities of few-layered graphene
sheets. The Brunauer–Emmett–Teller (BET) surface areas was measured as high as 1200m2/g,
indicating a well-exfoliation of the graphene sheets obtained from the physical thermal expansion.
Graphene was further dispersed in NMP with concentrations from 0.1 mg/ml to 2 mg/ml and sonicated
by a tip (bar type) sonication instrument at 400 W for 1 h to get a uniform solution.
Deposition of Graphene Film by ESD. During ESD process, mono-dispersed fine droplets are
generated due to repulsion forces between charges in the droplets. The size of droplets can be
well controlled by adjusting the flow rate and electric field applied to the injection nozzles
and substrates, and the diameter of the droplets can be as small as several hundred nanometers
in scale. Thus the thickness of film can be varied from hundreds of nanometers to hundreds of
micrometers.
ESD was performed using a NaBond Electrospray unit. A roll-to-roll kit was integrated with the
electrospray unit for conveying the Al foil (see Figure S2). During ESD, a continuous and reciprocating
movement was applied on the spinneret; meanwhile, Al foil was moved slowly in the vertical direction
of spinneret’s movement in order to scan the whole substrate and obtain large area continuous graphene
films on Al foils. The separation of the metallic nozzles (diameter 0.15 mm) and the substrate was
varied between 2 and 3.5 cm during ESD process. To maintain a stable cone-jet mode (Taylor cone
mode), the electric field was controlled with electric potentials of 11.5 kV. The substrate was
maintained at 135 °C by a heating plate. The graphene solution was pushed by a syringe pump through
the spinneret at flow rate of 50~100 µL/min in a single nozzle. The concentration was controlled from
0.2 to 2 mg/ml. By changing the speed of rolling, samples with different thicknesses have been
achieved. Typically, to obtain an ordered and aligned structure inside of GPs, the concentration of
solution was reduced to to ~0.2 mg/ml at the flow rate of ~50 µl/min during ESD. When the droplets
were sprayed on the substrate, graphene sheets with planar structure stack up to form a layer-by-layer
structure.
Thermal Annealing GPs and Mechanical Press. Free-standing GPs from ESD were annealed in a
graphite crucible by using an electrical furnace. The samples were sandwiched again between polished
artificial graphite plates and heated up from room temperature to various temperatures (1600, 1800,
2000, 2200, 2500 and 2800 °C) at a rate of 1000 °C/h and kept at this temperature for 30 minutes in a
flow of argon. Mechanical press was carried out on a carver laboratory press under room temperature.
After applying 50 MPa ~300 MPa pressures with different time, GPs with various densities were
obtained.
Characterization of the free-standing GPs. The contact angles for the free-standing GPs and
substrates for ESD were measured by using a Ramé-Hart M500 digital goniometer equipped with a
dispensing needle (VICI Precision Sampling Co., CA, USA). A 1µL water droplet was generated using
the automatic dispenser of the goniometer, based on four time-controlled volume steps of 0.25 µL close
to the sample surface. The sessile droplet was formed by fixing the needle and approaching the
substrate parallel to the needle direction with a very gentle feed rate of a few micrometers per minute.
All the tests were performed in air at room temperature. The morphology and microstructure of
materials were determined by a field-emission SEM on a JEOL (JSM-6335). X-ray diffraction (XRD)
was performed using a PAN analytical X-ray diffraction system with the source wavelength of 1.542 Å
at room temperature. The surface area of graphene powder was measured using a Micromeritics
ASAP2000 instrument. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 Versa
Probe system. Raman study of GPs was performed with a LabRAM HR800 Raman microscope
-1
using a 532.18 nm green laser as the probing light source and 600 g mm grating. The scattered
light was collected in the back-scattering geometry using a CCD detector. A FLIR A325sc infrared
camera was used to record the time dependent temperature profile that develops on Al foil, Cu foil and
annealed GPs subjected to localized heating.
The effective thermal conductivities of the GPs were measured using a well-established self-heating
method.[3,4] The DC current and voltage across a GP bridge suspended between two heat sinks were
recorded by a Karl Suss Semi-automatic Probe Station during electrical self-heating of the GPs. Joule
heating in the sample generated a temperature profile detected by a high-performance infrared
temperature sensor (micro-epsilon, thermoMETER CT laser, the smallest spot size as 0.9 mm) with
laser marking. AZ heat conduction model was then used to back-out the thermal conductivity. Electrical
conductivity was measured from current-voltage curves under small currents that do not generate
significant self-heating.
2. Experimental setup of the electrospray process for GPs
Figure S1. Schematics of the electrospray process for GP.
3. A continuous roll-to-roll process for mass production of GPs.
Figure S2. Schematic description of the ESD system integrated with roll-to-roll for GPs.
The ESD was performed using a NaBond Electrospray unit. A roll-to-roll kit was integrated
with the electrospray unit for conveying the Al foil. During ESD, a continuous and
reciprocating movement was applied on the nozzle; meanwhile, Al foil was moved slowly in
the vertical direction of nozzle’s movement in order to scan the whole substrate and obtain
large area continues graphene films on Al foils.
4. Mechanical properties of free standing GPs.
Figure S3. Mechanical properties of Pristine GPs and annealed GP at various temperatures. a)
Stress-strain curves and b) Young’s modulus.
5. Measurement of thermal and electrical conductivity
Figure S4. Schematic illustration of the measurements of GP’s thermal conductivity.
The thermal conductivity of the GPs was obtained from a self-heating method, as described
previously.[3,4] The specific testing device has been shown in Figure S4. Typically, the GPs
were cut into slices with the radio of width/length larger than 10, which can be considered as
a one-dimensional heat transfer problem as the length is much longer than the width and
thickness. The sample was suspended across two copper blocks which can maintain at certain
stable temperature. High-purity silver paste was used to adhere GP slices and copper blocks in
order to get high quality contacts. A DC current in the range of 10-70 mA was applied on the
blocks and the heating temperature of GP samples would correspondingly change. When the
temperature rise is not large (below 15 °C in this study), we can assume that the volumetric
heat generation due to Joule heating, q =UI/2Lwt, is uniform, where w, t and L are the width,
thickness and half length of the sample, and U, I are the voltage and current measured across
the contacts, respectively. When the origin of x was set at the middle point of the suspended
strip, the one-dimensional steady-state heat transfer equation can be written as:
k
d 2T
2 wh r (T  T 0 )
 q 
0
2
dx
A
where A, k and hr are the cross-section area of GP, thermal conductivity and the effective
radiation heat transfer coefficient. The tests were carried out in an airtight testing environment
in order to prevent the effect of air flow. When the temperature rise is small, the heat loss to
the surrounding air accounted from convention and radiation can be neglected. We can then
rewrite the equation as follows:
k
d 2T
 q  0
dx 2
With boundary conditions at two ends of the strips as T L =T0 and T-L=T0, temperature
distribution of the strip can be expressed as:
Tx  T 0 
q 2
(L  x2 )
2k
Temperature was distributed symmetrically with respect to the middle point (x=0) of the strip,
where the maximum temperature TM was achieved. After measuring the temperature TM at the
middle point and T0 at the end of the strip with a high performance infrared thermometer, the
thermal conductivity of the GP can be obtained by:
k
UIL
4 wt (TM  T0 )
Meanwhile, by measuring the current and voltage of samples, electrical conductivities could
also be computed.
In order to cross-check the thermal conductivities of the GPs measured by the self-heating
method, laser flash method by Netzsch LFA 447 NanoFlash instrument was employed to
measure thermal conductivity of the GPs. Less than 10% difference was identified between
these two methods (e.g., ∼600 Wm-1K-1 by laser flash vs. 635 Wm-1K-1 by self-heating
method), which is within the range of the experimental uncertainty.
6. Record of infrared video of heat diffusion and heat transfer.
In the experimental setup for the supplementary video S1, an electrical iron with tunable
temperature was used as hot spot. In order to concentrate the heat and protect samples, iron
head was insulated with a quartz tube. The end of quartz tube was higher than iron head by
~0.5 cm. The heat diffusion experiments were conducted on Al foil, copper foil and annealed
GPs with same thicknesses (~25 m) and punched into circular geometries with one-inch
diameter. Samples were coated by graphite paint in order to get a same emissivity. Al, Cu and
GPs circular sheets were put on the quartz tube without directly contact with iron head, as
seen in Figure S8. Temperature was controlled with the iron head in order not to oxidize Al
and copper samples. Heat was transferred from iron heat to the sample mainly by radiation.
Since the surfaces of samples were coated with graphite and the geometries were also same,
heat transfer can be considered as same for all of the samples. An infrared camera was held on
the top to record the temperature change. In the video, the first sample is Al, second one is Cu
and the last one is GPs.
Figure S5. Schematic illustration of the experimental setup for record of heat spreading on
Al, copper and GPs sheets.
Supplementary video S1: Comparison of heat diffusion between Al, Cu and GP in circular
shape. The first sample is Al sheet, the second one is Cu sheet and the last one is GP sheet.
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