Weigang Ma

Simultaneous measurements of the specific heat and thermal conductivity of suspended thin samples by transient electrothermal method.

The electrothermal technique is developed to simultaneously measure the specific heat and thermal conductivity of individual thin samples suspended across two heat sinks, resorting to pulsed direct currents with or without a dc offset. The temperature evolution due to Joule self-heating is recorded and compared with the numerical solutions of transient heat conduction equations using the finite volume method. The thermal conductivity is determined by the steady temperature level and the specific heat by the transient temperature rise or relaxation. This technique is applied to a 10 microm thick platinum wire and the thermal conductivity and specific heat are in good agreement with the literature values. In addition, the influences of thermal radiation and thermal boundary resistance between the sample and heat sinks on the experimental results are discussed.

Simultaneous measurement of thermal conductivity and thermal contact resistance of individual carbon fibers using Raman spectroscopy

In this paper, a new method employing Raman spectroscopy to determine thermal conductivity (TC) and thermal contact resistance (TCR) of an individual fiber was developed. Laser absorption is accounted for, but there is no need to be determined in this method. The local temperatures along the fiber longitudinal direction were determined by Raman shift. Two independent equations related to TC and TCR were established through measuring the temperature variation induced by changing electrical heating power at the center of the sample and the local temperature rise induced by a focused laser heating from Raman spectroscopy at two different positions on the sample, respectively. By solving the two equations, TC and TCR can then be obtained. This method has been validated by measuring two suspended carbon fibers.

A T-type method for characterization of the thermoelectric performance of an individual free-standing single crystal Bi2S3 nanowire

A comprehensive method to evaluate the thermoelectric performance of one-dimensional nanostructures, called the T-type method, has been first developed. The thermoelectric properties, including the Seebeck coefficient, thermal conductivity and electrical conductivity, of an individual free-standing single crystal Bi2S3 nanowire have been first characterized by applying the T-type method. The determined figure of merit is far less than the reported values of nanostructured bulk Bi2S3 samples, and the mechanism is that the Seebeck coefficient is nearly zero in the temperature range of 300-420 K and changes its sign at 320 K.

Solvothermal synthesis of superhydrophobic hollow carbon nanoparticles from a fluorinated alcohol

A new and simple method of synthesizing fluorinated carbon at the gram scale is presented by reacting a fluorinated alcohol with sodium at elevated temperatures in a sealed Teflon reactor. The resulting carbon nanoparticles are around 100 nm in diameter, and display a hollow shell morphology, with a significant amount of fluorine doped into the carbon. The nanoparticles disperse easily in ethanol, and are thermally stable up to 400 °C and 450 °C under air and nitrogen, respectively. The nanoparticle dispersion was printed onto various substrates (paper, cloth, silicon), inducing superhydrophobicity.

Laser flash-Raman spectroscopy method for the measurement of the thermal properties of micro/nano wires

This paper introduces a new method for measuring the thermal diffusivity and thermal conductivity of individual micro/nano wires using Raman spectroscopy. This method uses a focused short pulsed laser and a continuous-wave laser in a Raman spectroscopy system as the local heater, Raman signal excitation source, and temperature sensor. Unsteady and steady thermal conduction models are used to get two independent equations for the thermal diffusivity (α) and laser absorptivity (η). This new method is verified by comparing experimental results for graphite carbon fiber with measurement using the 3ω method. The method was then used to measure the temperature dependent thermal diffusivity and thermal conductivity of individual carbon nanotubes.

Systematic characterization of transport and thermoelectric properties of a macroscopic graphene fiber

Graphene, a two-dimensional material with extraordinary electrical, thermal, and elastic performance, is a potential candidate for future technologies. However, the superior properties of graphene have not yet been realized for graphenederived macroscopic structures such as graphene fibers. In this study, we systematically investigated the temperature (T)-dependent transport and thermoelectric properties of graphene fiber, including the thermal conductivity (λ), electrical conductivity (σ), and Seebeck coefficient (S). λ increases from 45.8 to 149.7 W·m–1·K–1 and then decreases as T increases from 80 to 290 K, indicating the boundary-scattering and three-phonon Umklapp scattering processes. σ increases with T from 7.1 × 104 to 1.18 × 105 S·m–1, which can be best explained by the hopping mechanism. S ranges from–3.9 to 0.8 μV·K–1 and undergoes a sign transition at approximately 100 K.

A self-heating 2ω method for Seebeck coefficient measurement of thermoelectric materials

A novel and reliable self-heating 2ω method has been developed to measure the Seebeck coefficient of the microscale/nanoscale thermoelectric materials. Based on the analytical solution of the transient heat-conduction equation of the specimen heated by a harmonic current, two measurement modes have been developed: (1) the Seebeck coefficient can be directly extracted from the ratio of experimentally measured 2ω Seebeck voltage to theoretically predicted 2ω temperature drop oscillation; and (2) the Seebeck coefficient can be steadily extracted from the measured 2ω and 3ω voltages. This approach has been applied to a 25.4 μm thick K-type thermocouple and the measured Seebeck coefficient corresponds well with the nominal value.

Integrative characterization of the thermoelectric performance of an individual multiwalled carbon nanotube

Carbon nanotube-based organic composites and carbon nanotube networks are important flexible and lightweight thermoelectric materials. Characterization of the thermoelectric performance of individual carbon nanotubes is of vital importance for exploring the coupling mechanism between carbon nanotubes and organic composites, and proposing further improvement measures. The thermoelectric performance of an individual multiwalled carbon nanotube with a diameter of 66 nm has been comprehensively studied by applying our T-type method from 260 K to 420 K, using the same measurement configuration. The figure of merit increases from 4.84 × 10−8 to 1.32 × 10−6 on increasing the temperature, which is smaller than previous experimental results on carbon nanotube samples. The thermal conductivity increases from 706 W m−1 K−1 at 260 K to 769.3 W m−1 K−1 at 320 K, and then stays nearly constant until 420 K. The phonons dominate the thermal transport. The electrical conductivity exhibits thermally activated carrier gener...

Study on the cross plane thermal transport of polycrystalline molybdenum nanofilms by applying picosecond laser transient thermoreflectance method

Thin metal films are widely used as interconnecting wires and coatings in electronic devices and optical components. Reliable thermophysical properties of the films are required fromthe viewpoint of thermalmanagement. The cross plane thermal transport of four polycrystalline molybdenum nanofilms with different thickness deposited on glass substrates has been studied by applying the picosecond laser transient thermoreflectance technique. The measurement is performed by applying both front pump-front probe and rear pump-front probe configurations with high quality signal. The determined cross plane thermal diffusivity of the Mo films greatly decreases compared to the corresponding bulk value and tends to increase as films become thicker, exhibiting significant size effect. The main mechanism responsible for the thermal diffusivity decrease of the present polycrystalline Mo nanofilms is the grain boundary scattering on the free electrons. Comparing the cross plane thermal diffusivity and inplane electrical conductivity indicates the anisotropy of the transport properties of the Mo films.

ac heating–dc detecting method for Seebeck coefficient measurement of the thermoelectric micro/nano devices

A novel ac heating–dc detecting method is developed to measure the Seebeck coefficient of thermoelectric micro/nano devices. The suspended thermoelectric device in vacuum is heated by an ac current to generate a temperature difference composed of static and harmonic components and corresponding dc and harmonic thermoelectric voltage. The Seebeck coefficient can be extracted from the ratio of the dc thermoelectric voltage and the static temperature difference. Furthermore, it has been deduced that the dc thermoelectric voltage is proportional to the square of the heating current and the Seebeck coefficient can be directly extracted from the corresponding slope. This approach has been verified by numerical simulation on a 22.0 nm thick Au-Pt heterojunction and experiment applied on a 25.4 μm thick Chromega–Alomega thermocouple, and the measured Seebeck coefficient corresponds well with the nominal value.