Tingting Miao

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.

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.

Chemically doped macroscopic graphene fibers with significantly enhanced thermoelectric properties

Flexible wearable electronics, when combined with outstanding thermoelectric properties, are promising candidates for future energy harvesting systems. Graphene and its macroscopic assemblies (e.g., graphene-based fibers and films) have thus been the subject of numerous studies because of their extraordinary electrical and mechanical properties. However, these assemblies have not been considered suitable for thermoelectric applications owing to their high intrinsic thermal conductivity. In this study, bromine doping is demonstrated to be an effective method for significantly enhancing the thermoelectric properties of graphene fibers. Doping enhances phonon scattering due to the increased defects and thus decreases the thermal conductivity, while the electrical conductivity and Seebeck coefficient are increased by the Fermi level downshift. As a result, the maximum figure of merit is 2.76 × 10–3, which is approximately four orders of magnitude larger than that of the undoped fibers throughout the temperature range. Moreover, the room temperature power factor is shown to increase up to 624 μW·m–1·K–2, which is higher than that of any other material solely composed of carbon nanotubes and graphene. The enhanced thermoelectric properties indicate the promising potential for graphene fibers in wearable energy harvesting systems.

Thermal transport and thermal stress in a molybdenum film-glass substrate system

Three-dimensional integration with through-silicon vias is emerging as an approach for improving the performance of integrated circuits. Thermal transport and thermal stress in such designs currently limit their performance and reliability. In this study, the thermal dissipation and thermal stress in a 95.3-nm-thick molybdenum (Mo) film–glass substrate system were investigated using a picosecond laser pump–probe method with four different configurations. This allowed the thermal transport and the generation and propagation of coherent acoustic phonon waves in a Mo film–glass substrate system to be comprehensively studied for the first time. The universality of the superposition model previously proposed for a platinum film on a glass substrate was verified using the present Mo film–glass substrate system from the close agreement between experimental data and theoretical predictions. The thermal transport in the Mo film and the coherent acoustic phonon wave propagation in the Mo film and glass substrate, i...

Essential role of enhanced surface electron–phonon interactions on the electrical transport of suspended polycrystalline gold nanofilms

The electrical resistivity of suspended polycrystalline gold nanofilms with different lengths has been measured over the temperature range of 2 K to 340 K, which dramatically increases compared with bulk gold and slightly increases with length. Classical size effect theories considering surface and grain boundary scatterings cannot explain the increased film resistivity, especially the temperature dependence of resistivity, over the whole temperature range. Considering the fact that the reduction of the coordination number of atoms at the surface and the interface leads to a decrease of the phonon spectrum frequency and consequently affects the surface phonon spectrum, the electron–phonon interaction as a relatively independent surface effect is taken into account. The theoretical predictions and the experimental measured film resistivity match very well over the whole temperature range and the extracted surface Debye temperature decreases significantly compared to the bulk value, which illustrates the essential role of enhanced surface electron–phonon interactions on the electrical transport of the present gold nanofilms.

THERMAL AND ELECTRICAL TRANSPORT CHARACTERISTICS OF POLYCRYSTALLINE GOLD NANOFILMS

The in-plane thermal and electrical conductivities of several suspended polycrystalline gold nanofilms with thickness of 20.0–54.0 nm have been measured simultaneously at 100–310 K. Both the thermal and electrical conductivities drop greatly compared to the corresponding bulk value, and the electrical conductivity reduction is larger. Fits to the temperature-dependent electrical conductivity confirm that the scattering of electrons by softened phonons is significant and cannot be reconciled with the classical size-effect model considering only surface and grain boundary. Taking into account the enhanced electron-phonon scattering, the electrical conductivity is well predicted over the whole temperature range and the obtained Debye temperature agrees well with the calculated value from the elastic continuum model. Furthermore, a new model on the thermal transport of metallic nanofilm is proposed based on the Energy Conservation Law, in which the electron-phonon scattering induced electron energy decrease is supposed to be counteracted by the phonon energy increase. The present model greatly improves the prediction of thermal conductivity in thin films compared to the corresponding result directly from electrical thermal analogy applied to bulk metals.Copyright