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Reversible temperature regulation of electrical and thermal conductivity using liquid-solid phase transitions.

Zheng R, Gao J, Wang J, Chen G - Nat Commun (2011)

Bottom Line: Internal stress generated during a phase transition modulates the electrical and thermal contact resistances, leading to large contrasts in the electrical and thermal conductivities at the phase transition temperature.With graphite/hexadecane suspensions, the electrical conductivity changes 2 orders of magnitude and the thermal conductivity varies up to 3.2 times near 18 °C.The generality of the approach is also demonstrated in other materials such as graphite/water and carbon nanotube/hexadecane suspensions.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.

ABSTRACT
Reversible temperature tuning of electrical and thermal conductivities of materials is of interest for many applications, including seasonal regulation of building temperature, thermal storage and sensors. Here we introduce a general strategy to achieve large contrasts in electrical and thermal conductivities using first-order phase transitions in percolated composite materials. Internal stress generated during a phase transition modulates the electrical and thermal contact resistances, leading to large contrasts in the electrical and thermal conductivities at the phase transition temperature. With graphite/hexadecane suspensions, the electrical conductivity changes 2 orders of magnitude and the thermal conductivity varies up to 3.2 times near 18 °C. The generality of the approach is also demonstrated in other materials such as graphite/water and carbon nanotube/hexadecane suspensions.

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Mechanisms in reversible temperature control of both electrical and thermal conductivities.(a) Schematic diagram of measurement of electrical resistance between two graphite flakes peeled from highly ordered pyrolytic graphite. The dimensions of the graphite flakes are ~1 μm×3 mm×3 mm. Graphite flakes are contacted by 30-μm diameter gold wires. (b) Measured variation of electrical resistance as a function of temperature, from 18.5 to 17.5 °C. The resistance of the circuit decreased about 460 times. Inset 1 is an optical image of graphite flakes submerged in solid hexadecane, and inset 2 shows the graphite flakes submerged in liquid hexadecane. (c) Stress distribution map in frozen hexadecane, showing that the stress is unevenly distributed from 74–400 p.s.i., with an average of ~160 p.s.i. (d) Conceptual illustration of the contact area variation between graphite flakes submerged in hexadecane through the process of hexadecane freezing and remelting. The anisotropic growth of hexadecane crystals generates pressure on the surface of the graphite flakes, which increases the contact area and reduces the flake separation. When the frozen hexadecane remelts, the pressure is released and the contact area decreases.
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f3: Mechanisms in reversible temperature control of both electrical and thermal conductivities.(a) Schematic diagram of measurement of electrical resistance between two graphite flakes peeled from highly ordered pyrolytic graphite. The dimensions of the graphite flakes are ~1 μm×3 mm×3 mm. Graphite flakes are contacted by 30-μm diameter gold wires. (b) Measured variation of electrical resistance as a function of temperature, from 18.5 to 17.5 °C. The resistance of the circuit decreased about 460 times. Inset 1 is an optical image of graphite flakes submerged in solid hexadecane, and inset 2 shows the graphite flakes submerged in liquid hexadecane. (c) Stress distribution map in frozen hexadecane, showing that the stress is unevenly distributed from 74–400 p.s.i., with an average of ~160 p.s.i. (d) Conceptual illustration of the contact area variation between graphite flakes submerged in hexadecane through the process of hexadecane freezing and remelting. The anisotropic growth of hexadecane crystals generates pressure on the surface of the graphite flakes, which increases the contact area and reduces the flake separation. When the frozen hexadecane remelts, the pressure is released and the contact area decreases.

Mentions: We set up an experiment (Fig. 3a) to measure the electrical contact resistance between two highly ordered pyrolytic graphite (SPI-1) flakes in a hexadecane environment. The resistance of the circuit decreases 400 times as the temperature decreases from 18.5 to 17.5 °C (Fig. 3b). The stress distribution (Fig. 3c) measured in the frozen hexadecane (see Method) shows that the stress is unevenly distributed in regions where the average pressure is ~160 p.s.i. We believe the non-uniform pressure distribution is due to the anisotropic growth of hexadecane crystals. The conceptual process is shown in Figure 3d. Similar to exfoliated graphite flakes, naturally peeled highly ordered pyrolytic graphite flakes are uneven and curved. In the liquid state, the contact area is small and hence the electrical resistance between two flakes is high. Hexadecane crystals exhibit strong anisotropic growth kinetics. During the freezing of hexadecane needle-like structures are formed, with the aspect ratio depending mainly on the freezing speed. The anisotropic growth of the hexadecane crystals generates stress and increases the contact area of the graphite flakes. Stress can also improve the electrical contacts by reducing the thickness of the insulating liquids in between graphite flakes. After freezing, the contact area and electrical resistance stabilize. When the hexadecane remelts, the pressure on the graphite flakes is released and the contact area is reduced because of the elastic recovery of the graphite flakes and interparticle repulsion. A similar trend exists for the thermal conductivity as well.


Reversible temperature regulation of electrical and thermal conductivity using liquid-solid phase transitions.

Zheng R, Gao J, Wang J, Chen G - Nat Commun (2011)

Mechanisms in reversible temperature control of both electrical and thermal conductivities.(a) Schematic diagram of measurement of electrical resistance between two graphite flakes peeled from highly ordered pyrolytic graphite. The dimensions of the graphite flakes are ~1 μm×3 mm×3 mm. Graphite flakes are contacted by 30-μm diameter gold wires. (b) Measured variation of electrical resistance as a function of temperature, from 18.5 to 17.5 °C. The resistance of the circuit decreased about 460 times. Inset 1 is an optical image of graphite flakes submerged in solid hexadecane, and inset 2 shows the graphite flakes submerged in liquid hexadecane. (c) Stress distribution map in frozen hexadecane, showing that the stress is unevenly distributed from 74–400 p.s.i., with an average of ~160 p.s.i. (d) Conceptual illustration of the contact area variation between graphite flakes submerged in hexadecane through the process of hexadecane freezing and remelting. The anisotropic growth of hexadecane crystals generates pressure on the surface of the graphite flakes, which increases the contact area and reduces the flake separation. When the frozen hexadecane remelts, the pressure is released and the contact area decreases.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3104514&req=5

f3: Mechanisms in reversible temperature control of both electrical and thermal conductivities.(a) Schematic diagram of measurement of electrical resistance between two graphite flakes peeled from highly ordered pyrolytic graphite. The dimensions of the graphite flakes are ~1 μm×3 mm×3 mm. Graphite flakes are contacted by 30-μm diameter gold wires. (b) Measured variation of electrical resistance as a function of temperature, from 18.5 to 17.5 °C. The resistance of the circuit decreased about 460 times. Inset 1 is an optical image of graphite flakes submerged in solid hexadecane, and inset 2 shows the graphite flakes submerged in liquid hexadecane. (c) Stress distribution map in frozen hexadecane, showing that the stress is unevenly distributed from 74–400 p.s.i., with an average of ~160 p.s.i. (d) Conceptual illustration of the contact area variation between graphite flakes submerged in hexadecane through the process of hexadecane freezing and remelting. The anisotropic growth of hexadecane crystals generates pressure on the surface of the graphite flakes, which increases the contact area and reduces the flake separation. When the frozen hexadecane remelts, the pressure is released and the contact area decreases.
Mentions: We set up an experiment (Fig. 3a) to measure the electrical contact resistance between two highly ordered pyrolytic graphite (SPI-1) flakes in a hexadecane environment. The resistance of the circuit decreases 400 times as the temperature decreases from 18.5 to 17.5 °C (Fig. 3b). The stress distribution (Fig. 3c) measured in the frozen hexadecane (see Method) shows that the stress is unevenly distributed in regions where the average pressure is ~160 p.s.i. We believe the non-uniform pressure distribution is due to the anisotropic growth of hexadecane crystals. The conceptual process is shown in Figure 3d. Similar to exfoliated graphite flakes, naturally peeled highly ordered pyrolytic graphite flakes are uneven and curved. In the liquid state, the contact area is small and hence the electrical resistance between two flakes is high. Hexadecane crystals exhibit strong anisotropic growth kinetics. During the freezing of hexadecane needle-like structures are formed, with the aspect ratio depending mainly on the freezing speed. The anisotropic growth of the hexadecane crystals generates stress and increases the contact area of the graphite flakes. Stress can also improve the electrical contacts by reducing the thickness of the insulating liquids in between graphite flakes. After freezing, the contact area and electrical resistance stabilize. When the hexadecane remelts, the pressure on the graphite flakes is released and the contact area is reduced because of the elastic recovery of the graphite flakes and interparticle repulsion. A similar trend exists for the thermal conductivity as well.

Bottom Line: Internal stress generated during a phase transition modulates the electrical and thermal contact resistances, leading to large contrasts in the electrical and thermal conductivities at the phase transition temperature.With graphite/hexadecane suspensions, the electrical conductivity changes 2 orders of magnitude and the thermal conductivity varies up to 3.2 times near 18 °C.The generality of the approach is also demonstrated in other materials such as graphite/water and carbon nanotube/hexadecane suspensions.

View Article: PubMed Central - PubMed

Affiliation: Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.

ABSTRACT
Reversible temperature tuning of electrical and thermal conductivities of materials is of interest for many applications, including seasonal regulation of building temperature, thermal storage and sensors. Here we introduce a general strategy to achieve large contrasts in electrical and thermal conductivities using first-order phase transitions in percolated composite materials. Internal stress generated during a phase transition modulates the electrical and thermal contact resistances, leading to large contrasts in the electrical and thermal conductivities at the phase transition temperature. With graphite/hexadecane suspensions, the electrical conductivity changes 2 orders of magnitude and the thermal conductivity varies up to 3.2 times near 18 °C. The generality of the approach is also demonstrated in other materials such as graphite/water and carbon nanotube/hexadecane suspensions.

Show MeSH
Related in: MedlinePlus