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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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Variation of electrical and thermal conductivities around the phase transition point.(a) Electrical conductivity of graphite suspensions as a function of volume fraction in the liquid state. The inset shows a logarithmic plot of σc versus (ϕ−ϕc), which indicates a percolation threshold of ϕc=0.05%. (b) Electrical conductivity of graphite suspensions as a function of temperature. The inset shows the relationship between the contrast ratio of electrical conductivity and graphite volume fraction. (c) Thermal conductivity at different graphite volume fractions as a function of temperature. The inset plot indicates the relationship between the contrast ratio of thermal conductivity and graphite volume fraction. (d) Cyclic EC (electrical conductivity) versus temperature measurement results of a 0.8% graphite/hexadecane suspension. Blue and red squares indicate the conductivity variance through the processes of freezing and melting, respectively. (e) TC (thermal conductivity) contrast after different cycles. Blue lines indicate the thermal conductivity of 0.8% graphite/hexadecane suspensions at 3 °C during different thermal cycles, whereas red lines show the thermal conductivity at 25 °C during different thermal cycles.
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f2: Variation of electrical and thermal conductivities around the phase transition point.(a) Electrical conductivity of graphite suspensions as a function of volume fraction in the liquid state. The inset shows a logarithmic plot of σc versus (ϕ−ϕc), which indicates a percolation threshold of ϕc=0.05%. (b) Electrical conductivity of graphite suspensions as a function of temperature. The inset shows the relationship between the contrast ratio of electrical conductivity and graphite volume fraction. (c) Thermal conductivity at different graphite volume fractions as a function of temperature. The inset plot indicates the relationship between the contrast ratio of thermal conductivity and graphite volume fraction. (d) Cyclic EC (electrical conductivity) versus temperature measurement results of a 0.8% graphite/hexadecane suspension. Blue and red squares indicate the conductivity variance through the processes of freezing and melting, respectively. (e) TC (thermal conductivity) contrast after different cycles. Blue lines indicate the thermal conductivity of 0.8% graphite/hexadecane suspensions at 3 °C during different thermal cycles, whereas red lines show the thermal conductivity at 25 °C during different thermal cycles.

Mentions: The electrical conductivity of the suspensions is measured using a four-point configuration. The electrical conductivities of the graphite suspensions in the liquid state prepared using different volume fractions of the graphite flakes are shown in Figure 2a. The inset of Figure 2a shows a logarithmic plot of electrical conductivity versus (ϕ−ϕc), demonstrating a percolating threshold of ϕc=0.05% (ref. 14). The electrical conductivity of the graphite suspensions varies significantly around 18 °C when hexadecane starts to freeze, as shown in Figure 2b for a graphite volume fraction loading between 0.2–1%, above the percolation threshold. In the liquid state, the electrical conductivity of the graphite suspensions varies little with temperature; from 18.5 to 17.5 °C, the electrical conductivity increases by 2 orders of magnitude. After the hexadecane is completely frozen, the electrical conductivity stabilizes. In both the solid and the liquid states, the electrical conductivity increases with an increase of the graphite volume fraction. However, the contrast ratio of electrical conductivity, defined as the ratio of electrical conductivity between the solid and the liquid states near the phase transition, peaks at 250 around a volume fraction of 0.8% (Fig. 2b, inset). The peak exists because at a lower graphite volume fraction, the electrical conductivity in the solid state is not sufficiently high. However, at a higher graphite volume fraction, the electrical conductivity in the liquid state is large, also reducing the contrast ratio. We anticipate that the peak value depends on the geometry of the graphite flakes, surface states and liquid properties, and believe that further improvements in the contrast ratio are possible through optimization.


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

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

Variation of electrical and thermal conductivities around the phase transition point.(a) Electrical conductivity of graphite suspensions as a function of volume fraction in the liquid state. The inset shows a logarithmic plot of σc versus (ϕ−ϕc), which indicates a percolation threshold of ϕc=0.05%. (b) Electrical conductivity of graphite suspensions as a function of temperature. The inset shows the relationship between the contrast ratio of electrical conductivity and graphite volume fraction. (c) Thermal conductivity at different graphite volume fractions as a function of temperature. The inset plot indicates the relationship between the contrast ratio of thermal conductivity and graphite volume fraction. (d) Cyclic EC (electrical conductivity) versus temperature measurement results of a 0.8% graphite/hexadecane suspension. Blue and red squares indicate the conductivity variance through the processes of freezing and melting, respectively. (e) TC (thermal conductivity) contrast after different cycles. Blue lines indicate the thermal conductivity of 0.8% graphite/hexadecane suspensions at 3 °C during different thermal cycles, whereas red lines show the thermal conductivity at 25 °C during different thermal cycles.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: Variation of electrical and thermal conductivities around the phase transition point.(a) Electrical conductivity of graphite suspensions as a function of volume fraction in the liquid state. The inset shows a logarithmic plot of σc versus (ϕ−ϕc), which indicates a percolation threshold of ϕc=0.05%. (b) Electrical conductivity of graphite suspensions as a function of temperature. The inset shows the relationship between the contrast ratio of electrical conductivity and graphite volume fraction. (c) Thermal conductivity at different graphite volume fractions as a function of temperature. The inset plot indicates the relationship between the contrast ratio of thermal conductivity and graphite volume fraction. (d) Cyclic EC (electrical conductivity) versus temperature measurement results of a 0.8% graphite/hexadecane suspension. Blue and red squares indicate the conductivity variance through the processes of freezing and melting, respectively. (e) TC (thermal conductivity) contrast after different cycles. Blue lines indicate the thermal conductivity of 0.8% graphite/hexadecane suspensions at 3 °C during different thermal cycles, whereas red lines show the thermal conductivity at 25 °C during different thermal cycles.
Mentions: The electrical conductivity of the suspensions is measured using a four-point configuration. The electrical conductivities of the graphite suspensions in the liquid state prepared using different volume fractions of the graphite flakes are shown in Figure 2a. The inset of Figure 2a shows a logarithmic plot of electrical conductivity versus (ϕ−ϕc), demonstrating a percolating threshold of ϕc=0.05% (ref. 14). The electrical conductivity of the graphite suspensions varies significantly around 18 °C when hexadecane starts to freeze, as shown in Figure 2b for a graphite volume fraction loading between 0.2–1%, above the percolation threshold. In the liquid state, the electrical conductivity of the graphite suspensions varies little with temperature; from 18.5 to 17.5 °C, the electrical conductivity increases by 2 orders of magnitude. After the hexadecane is completely frozen, the electrical conductivity stabilizes. In both the solid and the liquid states, the electrical conductivity increases with an increase of the graphite volume fraction. However, the contrast ratio of electrical conductivity, defined as the ratio of electrical conductivity between the solid and the liquid states near the phase transition, peaks at 250 around a volume fraction of 0.8% (Fig. 2b, inset). The peak exists because at a lower graphite volume fraction, the electrical conductivity in the solid state is not sufficiently high. However, at a higher graphite volume fraction, the electrical conductivity in the liquid state is large, also reducing the contrast ratio. We anticipate that the peak value depends on the geometry of the graphite flakes, surface states and liquid properties, and believe that further improvements in the contrast ratio are possible through optimization.

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