Transient Electro-thermal Technique Research Articles

Thermal Transport in Extremely Confined Metallic Nanostructures: TET Characterization.

In recent years, the continuous development of electronic chips and the increasing integration of devices have led to extensive research on the thermal properties of ultrathin metallic materials. In particular, accurate characterization of their thermal transport properties has become a research hotspot. In this paper, we review the characterization methods of metallic nanomaterials, focusing on the principles of the transient electrothermal (TET) technique and the differential TET technique. By using the differential TET technique, the thermal conductivity, electrical conductivity, and Lorenz number of extremely confined metallic nanostructures can be characterized with high measurement accuracy. At present, we are limited by the availability of existing coating machines that determine the thickness of the metal films, but this is not due to the measurement technology itself. If a material with a smaller diameter and lower thermal conductivity is used as the substrate, much thinner nanostructures can be characterized.

Temperature Dependence of Thermal Conductivity of Giant-Scale Supported Monolayer Graphene.

Past work has focused on the thermal properties of microscale/nanoscale suspended/supported graphene. However, for the thermal design of graphene-based devices, the thermal properties of giant-scale (~mm) graphene, which reflects the effect of grains, must also be investigated and are critical. In this work, the thermal conductivity variation with temperature of giant-scale chemical vapor decomposition (CVD) graphene supported by poly(methyl methacrylate) (PMMA) is characterized using the differential transient electrothermal technique (diff-TET). Compared to the commonly used optothermal Raman technique, diff-TET employs joule heating as the heating source, a situation under which the temperature difference between optical phonons and acoustic phonons is eased. The thermal conductivity of single-layer graphene (SLG) supported by PMMA was measured as 743 ± 167 W/(m·K) and 287 ± 63 W/(m·K) at 296 K and 125 K, respectively. As temperature decreased from 296 K to 275 K, the thermal conductivity of graphene was decreased by 36.5%, which can be partly explained by compressive strain buildup in graphene due to the thermal expansion mismatch.

Molecular alignment induced high thermal conductivity in amorphous/ low crystalline polyimide fibers

It is of considerable scientific and technological importance to enhance the thermal conductivity (k) of polymers for the application in thermal management. In this work, we study the thermal transport in amorphous/low-crystalized polyimide (PI) composite fibers (crystallinity lower than 2%-10%) by using the transient electro-thermal (TET) technique and the steady-state electro-thermal (SET) technique from 320 K to 7 K. It is found that the amorphous/low-crystalized PI fibers can achieve a high k (2.13 W/m K) due to oriented chain alignment and inter-chain hydrogen bonds from the condensation polymerization, wet-spinning and low-ratio thermal drawing process. By using the thermal reffusivity (Θ) model, the phonon propagation in PI fibers is investigated. The structural domain size (SDZ) of the amorphous/low-crystalized PI fibers is determined based on the residual Θ at 0 K limit, which is calculated to be 3.85 nm and 9 nm for the undrawn and post-drawn PI fiber, respectively. The SDZ value is close to the radius of gyration (Rg) instead of the atomic scale characteristic length of PI. This result indicates that for amorphous/low crystalized PI fibers with hydrogen bonds, the Rg could have played a more important role in determining k.

Characterization of phonon thermal transport of Ti3C2T x MXene thin film

Two-dimensional MXene materials with high electrotonic conductivity, good chemical stability, and unique laminar structure show great potential in the field of electrochemistry. In contrast to the widely concerned electrical properties, studies on the thermal properties of MXene materials are very limited. This paper presents a comprehensive analysis of the thermal properties of Ti3C2T x MXene thin film. Thermal diffusivity and thermal conductivity of Ti3C2T x films are characterized by the transient electro-thermal technique. The experimental results show a 16% enhancement in thermal conductivity when the temperature is increased from 307 K to 352 K. The phonon transport contributes substantially to thermal conductivity compared with electron transport. Molecular dynamic simulation is employed to further investigate the role of phonon thermal transport of Ti3C2 layer. It is found that the combined effect of specific heat capacity, stacking structure and internal stress states is responsible for the thermal transport performance of Ti3C2T x MXene thin film.

Preparation and Bolometric Responses of MoS2 Nanoflowers and Multi-Walled Carbon Nanotube Composite Network.

Due to their broadband optical absorption ability and fast response times, carbon nanotube (CNT)-based materials are considered promising alternatives to the toxic compounds used in commercial infrared sensors. However, the direct use of pure CNT networks as infrared sensors for simple resistance read-outs results in low sensitivity values. In this work, MoS2 nanoflowers are composited with CNT networks via a facile hydrothermal process to increase the bolometric performance. The thermal diffusivity (α) against temperature (T) is measured using the transient electro-thermal (TET) technique in the range of 320 K to 296 K. The α-T curve demonstrates that the composite containing MoS2 nanoflowers provides significant phonon scattering and affects the intertube interfaces, decreasing the α value by 51%. As the temperature increases from 296 K to 320 K, the relative temperature coefficient of resistance (TCR) increases from 0.04%/K to 0.25%/K. Combined with the enhanced light absorption and strong anisotropic structure, this CNT–MoS2 composite network exhibits a more than 5-fold greater surface temperature increase under the same laser irradiation. It shows up to 18-fold enhancements in resistive responsivity ((Ron − Roff)/Roff) compared with the pure CNT network for a 1550 nm laser at room temperature (RT).

Effects of Current Annealing on Thermal Conductivity of Carbon Nanotubes.

This work documents the annealing effect on the thermal conductivity of nanotube film (CNTB) and carbon nanotube fiber (CNTF). The thermal properties of carbon nanotube samples are measured by using the transient electro-thermal (TET) technique, and the experimental phenomena are analyzed based on numerical simulation. During the current annealing treatment, CNTB1 always maintains the negative temperature coefficient of resistance (TCR), and its thermal diffusivity increases gradually. When the annealing current is 200 mA, it increases by 33.62%. However, with the increase of annealing current, the TCR of CNTB2 changes from positive to negative. The disparity between CNTB2 and CNTB1 suggests that they have different physical properties and even structures along their lengths. The high-level thermal diffusivity of CNTB2 and CNTF are 2.28–2.46 times and 1.65–3.85 times higher than the lower one. The results show that the decrease of the thermal diffusivity for CNTB2 and CNTF is mainly caused by enhanced Umklapp scattering, the high thermal resistance and torsional sliding during high temperature heating.

Structure Domain Size of Carbon Fibers

The internal micro-structure is an important factor affecting the thermal conductivity of materials. In this work, the relationship between micro-structure, thermal conductivity, and temperature variation for carbon fibers (CF) are systematically studied by using the transient electrothermal technique. When the temperature is below the critical temperature (about 110 K), the internal structure of carbon fiber deteriorates, resulting in a very different behavior of thermal conductivity variation with temperature. By introducing the thermal reffusivity theory, the structure domain size of CF can be represented by the phonon mean free path at 0 K. The thermal reffusivity change of two temperature ranges is studied, and the mean free path of phonons in CF before and after deterioration can be calculated as 317 nm and 234 nm, respectively. The results are in good agreement with the crystallite grain size of 288 nm determined by Raman study. The thermal reffusivity provides another feasible method to study the relationship between structure domain size and thermophysical properties of materials.

Characterization of Thermal and Electrical Transport in 6.4\u2009nm Au Films on Polyimide Film and Fiber Substrates

The surface and grain boundary scattering impact on the electrical and thermal conduction in the thin metallic films coated on organic substrates has not been studied thoroughly. In this work, we study heat and electron transport in the 6.4 nm thin Au films supported by polyimide (PI) substrate using the transient electro-thermal technique. Thermal and electrical conductivities of 6.4 nm thin Au film are much smaller than bulk value. The thermal and electrical conductivities of 6.4 nm Au film deposited on the PI fiber are reduced by 59.3% and 76.8% in the comparison with the value of bulk Au. For PI film, the reduction of thermal and electrical conductivities is 47.9% and 46.3%. Lorenz numbers of 6.4 nm Au film supported by PI fiber and PI film are 4.51 × 10−8 WΩK−2 and 2.12 × 10−8 WΩK−2, respectively. The thermal conductivities of PI fiber and PI film are 0.87 Wm−1K−1 and 0.44 Wm−1K−1. The results reveal that PI is a suitable substrate material in the flexible electronic devices field.

In situ investigation of annealing effect on thermophysical properties of single carbon nanocoil

In reported high-temperature annealing of carbon nanocoils (CNCs), the samples studied before and after annealing are different ones. This significantly hinders annealing effect understanding due to unknown and remarkable sample-to-sample structure difference. Here using the transient electro-thermal technique (TET) and current-induced annealing, we report the first time in situ investigation of annealing effect on the thermophysical properties for the same individual CNC. Our dynamic annealing track uncovers an electrical resistance relation with annealing time as R∼−Rsln(t). The reaction rate (Rs) shows a normal distribution against the annealing power/temperature, proposing that the activation energy for structure reconstruction in CNCs follows a normal distribution. After annealing at 5–35€μA, the average thermal diffusivity (α) and electrical conductivity (σ) of CNCs show respective 50–160% and 100–170% increase. Normative linear relation between α and σ is discovered, which proposes axial-direction parallel structure in CNCs. The nonuniform temperature distribution along the sample during annealing creates different annealing levels and provides a great advantage to study the relation between structure and thermophysical properties. Our micro-scale Raman characterization reveals the nonuniform distribution of grain size along the length direction of CNCs after annealing and finds a rapid grain size increase from 4.0 to 7.8 nm near the sample's middle point. The middle point of the sample has the highest temperature rise (Tc), largest thermal conductivity (κ) increase, and the most dramatic structure improvement. Its κ shows a rapid improvement (8.7-fold maximum change) from 1200 K to 1800 K. A linear relation between κ−1 and Tc is observed and is attributed to the change of grain size during annealing. Using the concept of thermal reffusivity (Θ=1/α), a 1-fold increase of average grain size and a 197 K decrease of Debye temperature of CNCs after annealing are uncovered.

One-Dimensional Thermal Characterization at the Micro/Nanoscale: Review of the TET Technique

Thermal transport properties are important criteria for evaluating the properties of various fascinating micro/nanoscale materials. Over the past decades, characterization and study of the potential mechanisms of the in-plane and out-of-plane thermal conductivity of micro/nanostructures has attracted extensive attention in both experiment and theory. The transient electro-thermal (TET) technique is one of the fastest and accurate method for characterizing thermal conductivity, thermal diffusivity, thermal reffusivity and specific heat of one-dimensional materials simultaneously. In this review, the recent research progress in developing the TET technique for measurements of thermal transport in one dimensional micro/nanoscale structures is summarized. Also, the differential TET and extension of TET are reviewed in slightly more detail, including TPET, PLTR and PLTR2 techniques.

Temperature dependent thermal transport in graphene paper above room temperature

Graphene paper (GP) is more feasible to be integrated as a heat spreader than monolayer graphene due to its controllable size and moderate reduction in thermal transport properties. In this work, we fabricated and characterized GP to understand its thermal transport properties based on thermal bridge method combined with transient electro-thermal technique. The thermal conductivity of GP exhibits a highly temperature dependent property from 637 W/mK at room temperature to 757 W/mK at 80 °C, which is contrary to the trend of monolayer graphene. The increasing thermal property can be partially ascribed to the morphological and inner-structural change of the composite at elevated temperatures. Surface roughness is found to be reduced by 30% from atomic force microscope imaging. This feature is beneficial for many industrial applications such as the cooling film in smartphones. The implementation of GP is further verified in a smartphone to simulate the heat-dissipation performance. The results show that GP with temperature dependent properties exhibits much better cooling performance than conventional cooling films such as copper film.

19-Fold thermal conductivity increase of carbon nanotube bundles toward high-end thermal design applications

In high temperature annealing of carbon nanotube (CNTs) bundles for structure and thermal conductivity (κ) improvement, the statistical errors from sample-wide structure variation and sample transfer/preparation significantly overshadows the understanding of structure-κ correlation and change. In this work, the sequential process of current-induced thermal annealing (CITA) on improving the structure, electrical and thermal conductivities of chemical vapor deposition grown CNT bundles is studied for the first time. Our in-situ κ measurement using the transient electro-thermal technique uncovers the conjugated dynamic electrical, thermal, and structural properties. The electrical resistance and thermal diffusivity evolution of CNT bundles during CITA is studied. The thermal diffusivity and κ before and after CITA are measured from room temperature down to 10 K to uncover the reduction of defect density and enhancement of inter-tube connection strength after CITA. Our micro-Raman spectroscopy study from the most annealed region to the non-annealed region reveals significantly improved order in sp2 bonding carbon structure and reduced defects along the sample length. The resulting κ has 5–19 times increase at the most annealed region of CNT bundles. The intrinsic κ of CNT walls against the annealing temperature is also determined, which reaches a level as high as 754 W/m·K after CITA.

Characterization of Thermal Conductivity of Carbon Fibers at Temperatures as Low as 10 K

Due to their excellent thermal structural properties, carbon fibers have been extensively applied in the fields of mechanical and aerospace engineering. Many studies have been conducted at room temperature or higher, yet the literature gives very few data on the measurement of thermophysical properties at very low temperatures. The thermal diffusivity of two kinds of carbon fibers was measured by the transient electrothermal technique at very low temperatures (down to 10 K) in the work, the ρCp followed the Debye T3 law dependence quite accurately in agreement with theoretical investigations at very low temperatures(< 10 K). The thermal diffusivity of carbon fiber almost increases linearly with decreasing temperature, but when the temperature drops down below 90 K, the thermal diffusivity rapidly decreases. The thermal expansion mismatch between the different materials present in carbon fibers may influence the change of thermal diffusivity. The relationship between the impurity concentration and the thermal transport of the carbon fibers is discussed.

Thermal Conductivity and Raman Spectra of Carbon Fibers

Due to its unique physical properties, carbon fiber (CF) has been widely studied for extensive application in aerospace and machinery. In this study, the thermal diffusivity of three kinds of CF sample is characterized by the transient electrothermal technique at room temperature. By subtracting the effect of radiative losses, the effective thermal diffusivity of CFs can be calculated as $$6.46\times 10^{-6}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}$$ , $$6.58\times 10^{-6}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}$$ and $$2.01\times 10^{-4}\,\hbox {m}^{2}\cdot \hbox {s}^{-1}$$ , respectively. For the first time, the emissivity coefficient of carbon fiber is calibrated as 0.78. Combined with Raman spectra and phonon scattering, we found that the better crystalline structure and low defect in CF have an obvious impact on its thermal diffusivity.

Thermal and Electrical Properties of 3.2 nm Thin Gold Films Coated on Alginate Fiber

The thermal transport in metallic thin films can be reduced by the electron scattering and there are very little available knowledge that can be used to explain the mechanism. In this work, we characterized the thermal and electron transport of 3.2 nm thin gold films coated on alginate fiber by the transient electrothermal (TET) technique. The results reveal that the thermal and electrical conductivities are reduced significantly from the respective values of bulk material by 76.2% and 93.9%. At the same time, the Lorenz number is calculated as 8.66 × 10−8 W Ω K−2 and it is almost three times increased from the value of bulk material. The intrinsic thermal diffusivity of alginate fiber is 3.25 × 10−7 m2 s−1 and the thermal conductivity is 0.51 W m−1 K−1.

Measurement on the Thermal Properties of Graphene Powder

We report on an in-plane thermal diffusivity study of suspended graphene powder (GP) measured by the transient electro-thermal (TET) technique. The GP with a density of 0.24 $$\hbox {g}\,\cdot \,\hbox {cm}^{-3}$$ is made up of five–six-layer graphene. And the average size of graphene flakes used in our study is 0.98 $$\upmu $$ m. The intrinsic thermal conductivity perpendicular to in-plane of GP is determined at 18.8 $$\hbox {W}\,\cdot \,(\hbox {m}\,\cdot \,\hbox {K})^{-1}$$ using the thermal conductivity instrument, and the range of the in-plane thermal diffusivity of GP is identified from $$0.86\times 10^{-5 }\,\hbox {m}^{2 }\,\cdot \,\hbox {s}^{-1}$$ to $$1.52\times 10^{-5 }\,\hbox {m}^{2}\,\cdot \,\hbox {s}^{-1}$$ measured by the TET technique. Accordingly, the corresponding intrinsic thermal conductivity is 13.5 $$\hbox {W}\,\cdot \,(\hbox {m}\,\cdot \,\hbox {K})^{-1}$$ –23.8 $$\hbox {W}\,\cdot \,(\hbox {m}\,\cdot \,\hbox {K})^{-1}$$ . It is obvious that the two methods used in the experimental research on the intrinsic thermal conductivity of GP in different directions are not only the same order of magnitude but also have a maximum difference of only 5 $$\hbox {W}\,\cdot \,(\hbox {m}\,\cdot \,\hbox {K})^{-1}$$ . The results of our experiments are about one order of magnitude lower than those reported for four–five-layer graphene. There are various porosities in the whole sample after the compaction steps in the preparation of the samples, which gives rise to a large thermal contact resistance. And widely uneven surface defects observed under an optical microscope for the studied GP lead to substantial phonon scattering. Those factors combine together to give the observed significant reduction in the thermal conductivity.

The structure of the “amorphous” matrix of keratins

Various keratin fibers, particularly human hairs, were investigated by transmission electron microscopy, TEM, solid-state 1H NMR and Transient Electro-Thermal Technique, TET. The results converge to suggest that the matrix of keratin fiber cortex, far from being amorphous, has a well-defined nano-scale grainy structure, the size of these grains being around 2-4nm. The size of the grains appears to strongly depend on the chemical treatment of the fiber, on the temperature and on the relative humidity of the environment, as well as on the physiological factors at the level of fiber production in follicle. By suggesting an organization at the nano-scale of the protein chains in these grains, likely to be Keratin Associated Proteins, the results challenge the view of matrix as a homogeneous glassy material. Moreover, they indicate the potential of further investigating the purpose of this structure that appears to reflect not only chemical treatments of keratins but also biological processes at the level of the follicle.

Thermal characterization of natural and synthetic spider silks by both the 3ω and transient electrothermal methods

Thermal conductivity, thermal diffusivity, and volumetric heat capacity of three spider silks are measured in this paper as a benchmark for further studies. These silks include the major and minor ampullate silks of the Nephila clavipes spider, and a synthetic spider silk fiber made from recombinant dragline silk proteins purified from transgenic goats' milk. Two complementary measurement techniques are employed in the thermal characterization of these microscale single fibers for self-verification. One is the transient electrothermal technique (TET) and the other is the 3ω method. Experimental measurements indicate that thermal properties of the dragline silk are very close to those of the minor ampullate silk, whereas the ones for the synthetic silk are much lower due in part to its low crystallinity. The directly measured thermal conductivity, thermal diffusivity, and volumetric heat capacity of the major and minor ampullate silks are 1.2–1.26Wm−1K−1, 5.7–6×10−7m2s−1, and 2–2.17 MJm−3K−1, respectively. The thermal conductivity and thermal diffusivity of the as-spun synthetic silk are 0.24Wm−1K−1 and 1.6×10−7m2s−1 respectively. As part of this study, a detailed comparison of the TET and 3ω methods is provided showing the complementary nature of the techniques and illustrating the strengths and weaknesses of each.

Development of steady-state electrical-heating fluorescence-sensing (SEF) technique for thermal characterization of one dimensional (1D) structures by employing graphene quantum dots (GQDs) as temperature sensors

A fluorescence signal has been demonstrated as an effective implement for micro/nanoscale temperature measurement which can be realized by either direct fluorescence excitation from materials or by employing nanoparticles as sensors. In this work, a steady-state electrical-heating fluorescence-sensing (SEF) technique is developed for the thermal characterization of one-dimensional (1D) materials. In this method, the sample is suspended between two electrodes and applied with steady-state Joule heating. The temperature response of the sample is monitored by collecting a simultaneous fluorescence signal from the sample itself or nanoparticles uniformly attached on it. According to the 1D heat conduction model, a linear temperature dependence of heating powers is obtained, thus the thermal conductivity of the sample can be readily determined. In this work, a standard platinum wire is selected to measure its thermal conductivity to validate this technique. Graphene quantum dots (GQDs) are employed as the fluorescence agent for temperature sensing. Parallel measurement by using the transient electro-thermal (TET) technique demonstrates that a small dose of GQDs has negligible influence on the intrinsic thermal property of platinum wire. This SEF technique can be applied in two ways: for samples with a fluorescence excitation capability, this method can be implemented directly; for others with weak or no fluorescence excitation, a very small portion of nanoparticles with excellent fluorescence excitation can be used for temperature probing and thermophysical property measurement.

Thermal conductivity of giant mono- to few-layered CVD graphene supported on an organic substrate.

The thermal conductivity (k) of supported graphene is a critical property that reflects the graphene-substrate interaction, graphene structure quality, and is needed for thermal design of a graphene device. Yet the related k measurement has never been a trivial work and very few studies are reported to date, only at the μm level. In this work, for the first time, the k of giant chemical vapor decomposition (CVD) graphene supported on poly(methyl methacrylate) (PMMA) is characterized using our transient electro-thermal technique based on a differential concept. Our graphene size is ∼mm, far above the samples studied in the past. This giant graphene measurement eliminates the thermal contact resistance problems and edge phonon scattering encountered in μm-scale graphene k measurement. Such mm-scale measurement is critical for device/system-level thermal design since it reflects the effect of abundant grains in graphene. The k of 1.33-layered, 1.53-layered, 2.74-layered and 5.2-layered supported graphene is measured as 365 W m(-1) K(-1), 359 W m(-1) K(-1), 273 W m(-1) K(-1) and 33.5 W m(-1) K(-1), respectively. These values are significantly lower than the k of supported graphene on SiO2, and are about one order of magnitude lower than the k of suspended graphene. We speculate that the abundant C atoms in the PMMA promote more ready energy and momentum exchange with the supported graphene, and give rise to more phonon scattering than the SiO2 substrate. This leads to a lower k of CVD graphene on PMMA than that on SiO2. We attribute the existence of disorder in the sp(2) domain, graphene oxide (GO) and stratification in the 5.2-layered graphene to its more k reduction. The Raman linewidth (G peak) of the 5.2-layered graphene is also twice larger than that of the other three kinds of graphene, indicating the much more phonon scattering and shorter phonon lifetime in it. Also the electrical conductivity of the 5.2-layered graphene is about one-fifth of that for the other three. This further confirms the poor graphene quality of sample 4S, explaining its much lower k.