Transient Hotwire Measurement of Diamondoid Thermal Conductivity

Esteban Pacheco • Matthew Powell-Palm

Transient Hotwire Raw Data

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Experimental Setup

Thermal Conductivity Results

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ta T

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Time [s]

AbstractMaterials of ultralow thermal conductivity have considerable potential as thermal insulators in micro and nanoscale electromechanical systems and as thermoelectric generators. We have developed a method of experimentally determining the thermal conductivities of powdered crystalline substances at high pressures and low temperatures using a transient hotwire technique. We have successfully determined the thermal conductivity of the diamondoid adamantane with this method and refined a novel experiemtnal procedure. We have attained good agreement with published values and will test other diamdoids in the coming future.

λ =q4π

dΔTd ln(t)

Motivation Transient Hotwire

Adamantane is the most structurally simple member the diamondoids. It undergoes a high-entropy phase change at 208.7 K which results in an increase in density from 1.08 to 1.18 g/cm3 and a transition from a plastic face centered cubic to a normal tetratgonal crystalline structure. It has a notably low room temperature thermal conductivity of 0.21 W/m-K. Having validatated our experimental method with adamantane, we plan next to develop polycrystalline diamondoid alloys, taking advantage of unique phonon mismatches to create substances of ultralow thermal conductivity.

Theory

Derived from the Heat Conduction Equation in a Radial Coordinate System assuming infinite wire length.

Experimental Method1. Compress powdered adamantane to 80,000 psi, forming single crystal.

2. Drill a tunnel through crystal.

3. Thread platinum wire probe through tunnel.

4. Recompress crystal with 8,000 psi, collapsing tunnel around wire.

5. Cool entire apparatus in cryostat, taking transient hotwire measurements.

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0.8Adamantane Thermal Conductivity

Temperature (K)

Thermal Conductivity (W/m−K)

Powell−Palm/Pacheco 2015Powell−Palm/Pacheco 2015Powell−Palm/Pacheco 2015Wigren 2011Cahill−Pohl Lower Limit

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