Micro and Nano Thermal Transport: Characterization, Measurement, and Mechanism

Micro and Nano Thermal Transport Research: Characterization, Measurement and Mechanism is a complete and reliable reference on thermal measurement methods and mechanisms of micro and nanoscale materials. The book has a strong focus on applications and simulation, providing clear guidance on how to measure thermal properties in a systematic way. Sections cover the fundamentals of thermal properties before introducing tools to help readers identify and analyze thermal characteristics of these materials. The thermal transport properties are then further explored by means of simulation which reflect the internal mechanisms used to generate such thermal properties.

Readers will gain a clear understanding of thermophysical measurement methods and the representative thermal transport characteristics of micro/nanoscale materials with different structures and are guided through a decision-making process to choose the most effective method to master thermal analysis. The book is particularly suitable for those engaged in the design and development of thermal property measurement instruments, as well as researchers of thermal transport at the micro and nanoscale.

• Includes a variety of measurement methods and thermal transport characteristics of micro and nanoscale materials under different structures
• Guides the reader through the decision-making process to ensure the best thermal analysis method is selected for their setting
• Contains experiments and simulations throughout that help apply understanding to practice

• Language: English
• Publisher: Academic Press
• Release date: Feb 9, 2022
• ISBN: 9780128236239

Book preview

Chapter 1: Introduction

Lin Qiu; Ning Zhu; Fengcheng Li    School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing, China

Abstract

This chapter mainly introduces the development process of micro- and nanoscale materials, their definitions and classifications, their differences with traditional materials, and their advantages in the field of thermal management. In addition, the special properties of micro- and nanoscale materials in the heat transfer process are reviewed in detail, including ballistic transport characteristics and interface effects. Finally, it focuses on the analysis of the limitations of traditional thermal measurement technique in heat transfer and the advantages of some advanced techniques.

Keywords

Micro- and nanoscale; Ballistic transport; Interface effect; Measurement techniques; Heat transfer; Size effect

Outline

1.1Micro- and nanoscale materials

1.1.1Definition and classification

1.1.2Advantages and disadvantages in thermal managements

1.2Thermal transport scale characteristics

1.2.1Conventional scale characteristics

1.2.2Unique features at the micro/nanoscale

1.3Demand for thermal properties research

1.3.1Limitations of traditional thermal measurement techniques

1.3.2The main purpose of thermal measurement at micro/nanoscale

References

The main content of this chapter will introduce the development process of micro- and nanoscale materials, the definition and classification of micro- and nanoscale materials, the differences among traditional materials, and their advantages in the field of thermal management. In addition, owing to the difference in scale, the thermal transport process is also different from traditional materials. Therefore we will review the characteristics of micro- and nanoscale materials in the heat transfer process in detail, which includes the two most important physical effects on the micro/nanoscale. One is the size effect of thermal conductivity caused by ballistic transport and the other is the interface thermal resistance effect. Finally, we will introduce the basic requirements for conducting thermal properties research, including the limitations of traditional thermal measurement techniques in the study of thermal transport of micro- and nanoscale materials and the importance of some advanced techniques in the thermal transport of micro- and nanoscale materials.

1.1: Micro- and nanoscale materials

Although the word nano was first coined in the 1980s, the origin of nanotechnology actually began in 1959. This new concept of nanotechnology was first proposed by Nobel Prize winner Richard Feynman, and then in 1981, the existence of atomic clusters was actually observed through a scanning tunnel microscope, a breakthrough discovery that also marked the birth of nanotechnology. Soon after, Iijima discovered a novel nanomaterial named carbon nanotubes (CNTs) in 1991 [1]. Since then, micro- and nanoscale materials with unique specifications have gradually attracted widespread attention from scientists and engineers around the world compared with traditional engineering materials, and more and more nanostructured materials have been discovered, prepared, and researched. At the same time, the physical properties of micro- and nanoscale materials (such as optical, electrical, thermal, mechanical, and magnetic properties) have also become research hot spots in the nanotechnology field. With the continuous new discoveries, it is believed that the micro- and nanoscale materials have undoubtedly become one of the most promising materials in the 21st century.

Micro- and nanoscale materials with various excellent properties are also considered to involve many subjects, including physics, biology, chemistry, electronics, nanomanufacturing, nanomechanics, engineering technology and medical science, etc. [2–4]. At present, the research direction of micro- and nanoscale materials is mainly their preparation and characterization. The purpose is to prepare more micro- and nanoscale materials with perfect structures and excellent properties in batches so that they are widely used in various fields. The application fields of micro- and nanomaterials mainly include microelectronics and computer technology, medical and health technology, aerospace, aviation and space exploration, energy and environment, biotechnology and agriculture, etc. Today, researchers around the world are working hard on great technological innovations.

1.1.1: Definition and classification

To put it simply, micro- and nanoscale materials refer to those materials with at least one dimension in the three-dimensional (3-D) space in nanometer (0.1–100 nm) size or composed of them as the basic unit, which is roughly equivalent to a scale where 10–100 atoms are closely packed together. There is another saying here: nanomaterials are defined as materials composed of unbound particles or particles in an aggregate or agglomerate state with one or more external dimensions with a size ranging from 1 to 100 nm [5]. They are a new generation of materials composed of nanoparticles with dimensions between atoms, molecules, and macroscopic systems and render the form of particles, tubes, rods, or fibers. To understand the definition of the scale of micro/nanoscale materials more intuitively, we have listed the diagram of some representative objects from micro- to macroscale, such as water and glucose molecules, DNA double-helix structure, viruses, microelectronic mechanical devices, human hair, ants, and table tennis ball, as shown in Fig. 1.1. Owing to the small size of the constituent units, the interface ratio is very high. As a result, nanomaterials have a variety of characteristics, which has led to the system consisting of nanoparticles exhibiting many special properties that are different from the conventional macroscopic materials. The continuous and extensive research on nanomaterials has brought our understanding of nature to a new level. In nature, it is an intermediate link between atoms, molecules, and macro systems and a new field that people have never explored before. In fact, during the evolution of materials composed of nanoparticles into a macroscopic system, changes in the degree of structural order make the properties of the system also vary greatly. Therefore indepth research on nanomaterials will lead to new understanding of the transition from micro- to macroscale.

Fig. 1.1

Fig. 1.1 Schematic diagram of the scale of some representative objects: from micro- to macroscale.

According to the spatial size of nanomaterials, they can be divided into the following four categories: zero-dimensional (0-D), one-dimensional (1-D), two-dimensional (2-D), and 3-D nanomaterials. In particular, 0-D, 1-D, and 2-D nanomaterials are collectively referred to as low-dimensional nanomaterials [6]. Fig. 1.2 clearly shows the classification of several representative micro- and nanoscale materials according to spatial dimensions.

•0-D nanomaterials are also commonly called quantum dots because their dimensions are in the nanoscale range in all three dimensions and equal to or smaller than the mean free path of the electrons. The electron is constrained in three directions. Fullerenes, nanoparticles, and clusters are typical 0-D nanomaterials. Most of these nanoparticles will be spherical with the diameter in the range of 1–50 nm. These nanomaterials can also be found in cube and polygon shapes.

•The 1-D nanomaterials indicate those which meet the nanoscale in only one dimension in space. Representatives of such materials include nanotubes, nanorods, nanowires, nanofibers, etc. They are usually very long, typically a few microns in length but only a few nanometers in diameter.

•The 2-D nanomaterials have two dimensions that fit the nanoscale range, such as nanofilms, coatings and thin-film multilayers, nanosheets, or nanowalls. The area of the nanofilms can be large (several square micrometers), but the thickness is always in nanoscale range.

•The 3-D nanomaterials refer to a block containing the abovementioned nanomaterial in a 3-D space, such as a nanoceramic material and mesoporous material.

Fig. 1.2

Fig. 1.2 Schematic diagram of several representative micro-nanoscale materials classified according to spatial dimensions. Reprinted with permission from Elsevier and Physics Reports. L. Qiu, N. Zhu, Y. Feng, E.E. Michaelides, G. Żyła, D. Jing, Xinxin Zhang, P.M. Norris, C.N. Markides, O. Mahian, A review of recent advances in thermophysical properties at the nanoscale: from solid state to colloids, Phys. Rep. 843 (13) (2020) 1–81. https://doi.org/10.1016/j.physrep.2019.12.001. Open Access.

Nanomaterials can be classified into nanometals, nanocrystals, nanoceramics, nanoglasses, nanopolymers, and nanocomposites according to their chemical composition. According to the material properties, they can be divided into nanosemiconductors, nanomagnetic materials, nanononlinear optical materials, nanosuperconducting materials, nanothermoelectric materials, etc. According to their applications, they can be classified into nanoelectronic materials, nanophotoelectronic materials, nanobiomedical materials, nanosensitive materials, and nanoenergy storage materials.

1.1.2: Advantages and disadvantages in thermal managements

Compared with traditional materials, nanomaterials show a wide range of application prospects because of their excellent properties; especially they play a vital role in next-generation thermal managements for high-power electronics. Nanomaterials exhibit ultrahigh ratio of surface to volume, ultrahigh electrical conductivity, ultrahigh thermal conductivity, high mechanical strength, and good chemical stability. For nanoporous materials, the most representative are aerogels because they have ultrahigh porosity (95% or higher), ultralow density (0.02–0.32 g/cm³), and ultralow thermal conductivity (0.02–0.036 W/m K). Therefore they are considered as one of the most promising and efficient thermal insulation materials [7]. Although nanomaterials show many attractive properties, the key factors hindering their further development are manufacturing costs and technology. The cost of synthesizing nanostructured materials is much higher than traditional materials. For some unique nanomaterials, the synthesis and processing technologies are still immature and cannot be mass-produced. Moreover, the nanomaterials with a large number of defects hinder their application in various fields, such as missing atoms and doping. The following takes some of the most common and popular nanomaterials as examples to illustrate their advantages in the field of thermal management through practical applications.

In the field of microelectronics, the rapid development of information technology has significantly increased chip power consumption. At the same time, with the continuous miniaturization and integration of electronic devices, thermal management has become a crucial technical link. The reduction in external dimensions will amplify the intensity of the thermal loads and place extremely high requirements on the thermal management architecture to achieve reliable operation. Whether the chip can operate normally lies in the speed of heat removal. For example, the current mainstream way of dissipating heat inside mobile phones is to use graphite sheets with high thermal conductivity. The graphite heat sink distributes the heat of the mobile phone's heating center to a large area to dissipate heat evenly. With the advancement of nanoelectronics and the emergence of new application areas such as 3-D chip-stacking architecture and flexible electronics, developing novel materials to meet these pressing thermal management challenges has become a major priority. Up to date, some studies have proposed many cubic crystals, 2-D layered materials, nanostructured networks, and composites as reliable candidates for thermal management materials [8].

It is known that certain cubic crystals have high isotropic thermal conductivity, which is required for thermal management applications. In most modern electronic devices, Si used as a substrate is considered as a relatively good thermal conductor with a thermal conductivity of approximately 140 W/m K at room temperature [9]. However, diamonds maintain a record of high thermal conductivity in existing solid materials, with reported values of 2270–3450 W/m K [10]. This excellent heat transfer performance makes diamond films and substrates to be one of the best candidates to alleviate the local heat of high power density electronic devices, and the scarcity and cost of diamond have become the biggest challenge [11].

In addition, 2-D nanomaterials have been proven to be promising candidates for thermal management applications. To reduce thermal constraints and approach the performance limits of complex avionics products, Barako et al. [12] proposed a synthetic diamond material for rational engineering design of complex nanostructures consisting of heat sources, microfluidic chips, the vertically arranged copper nanowires, and nanoporous media. Through the rational integration of nanomaterials in the entire thermal resistant chains, the ultrahigh thermal load intensity of high-power electronic products in aerospace is achieved. Wu et al. [13] prepared a boron nitride (BN) nanosheet/polymer composite film with excellent flexibility and toughness through vacuum-assisted filtration. They reported the film exhibited strongly fire-resistant properties and anisotropic properties, with extremely low thermal conductivity of 1.0 W/m K in the out-of-plane direction and ultrahigh thermal conductivity of approximately 200 W/m K in the in-plane direction, making this material an excellent candidate for thermal management in electronics.

Graphene, another representative 2-D nanomaterial, also shows excellent performance in the field of thermal management. When the temperature rises beyond the normal operating range, it will negatively affect the performance of high-power lithium-ion batteries [14,15]. If they are overheated, the battery may experience thermal runaway and the battery may rupture or explode [16]. Researchers have proposed that the conventional method for thermal management of high power density ion batteries is based on the usage of thermal phase change materials (PCMs). They reduce battery temperature rise by means of latent heat storage and phase changes in smaller temperature ranges [17]. The use of PCMs in a battery cell can also buffer the battery from sharp fluctuations in ambient temperature. However, the thermal conductivity of common PCMs is very low, which is approximately 0.17–0.35 W/m K at room temperature. Therefore the graphene can be used as thermal interface materials (TIMs) or heat sinks [18], which is beneficial for transferring heat from computer chips to heat sinks and, thus, reducing the temperature of computer chips. It can also be used as a highly thermally conductive filler to enhance the overall thermal conductivity of PCMs. Yan et al. [19] reported that the thermal management of GaN transistors can be greatly improved via introduction of several graphene layers using few layers of graphene as alternative heat-escaping channels. Graphene quilts perform better in GaN devices on sapphire substrates, and a major change in thermal management was achieved.

Huang et al. proposed a CNT composite film for thermal management applications based on CNT arrays. As the aligned CNT tips protrude from both surfaces, forming an ideal thermal conduction path between the surfaces, they also become an ideal thermal management material, which is superior to dispersed CNT composites under low CNT loads [20]. As early as 2002, the Oak Ridge National Laboratory (ORNL) has produced a unique carbon foam with high thermal conductivity up to 40–180 W/m K. Owing to its low density, high thermal conductivity, relatively high surface area, and porous structure, it is also an ideal material for thermal management applications [21]. It also performed well in experiments, and the total heat transfer coefficient of carbon foam-based radiators can be up to two orders of magnitude higher than traditional radiators.

1.2: Thermal transport scale characteristics

1.2.1: Conventional scale characteristics

In general, from a macroperspective, heat conduction can basically be summarized as solving the heat diffusion equation based on energy conservation and Fourier law. As is known to all, Fourier law is not convincing enough. In fact, Fourier law tends to be an empirical formula, which represents that the heat flux is proportional to the temperature gradient. However, the law lacks a deeper physical basis because thermal conductivity can be defined only when Fourier law is assumed. Generally speaking, the macroheat conduction is only concerned with a specific temperature range, and in a small temperature interval, the thermal conductivity can be approximately considered as a constant physical parameter. In the micro- and nanoscale viewpoint, when the Fourier law is assumed to be true, thermal conductivity is no longer a constant physical parameter. At the macroscale, the nature of the heat conduction process is the heat diffusion process, as shown in Fig. 1.3A. Diffusion means that the particle has experienced multiple deflections during the transport process. During the transmission process, the particle will undergo multiple collisions, and each collision will cause the particle's momentum to change. It can also exchange energy with surrounding particles. The process of thermal transport is the process of diffusion transport of phonons [22]. According to the Bose-Einstein distribution, there are more phonons in regions with high temperature and fewer phonons in regions with low temperature. The phonons diffuse from a large number of regions to a small number of regions, and a heat flux is formed. The scattering of phonons, defects, boundaries, and electrons will continuously cause the change of momentum, thus forming the process of diffusion transport.

Fig. 1.3

Fig. 1.3 Schematic diagram of (A) Diffusive thermal transport process, (B) Ballistic thermal transport, (C) Effect of size effect on thermal conductivity. Reprinted with permission from American Chemical Society. W. Jang, Z. Chen, W. Bao, C.N. Lau, C. Dames, Thickness-dependent thermal conductivity of encased graphene and ultrathin graphite, Nano Lett. 10 (10) (2010) 3909–3913. https://doi.org/10.1021/nl101613u, Copyright @ 2010.

1.2.2: Unique features at the micro/nanoscale

Compared with diffusion thermal transport, the mechanism of ballistic transport is completely different, as shown in Fig. 1.3B. Assume that a system's characteristic length is small enough that the phonons do not undergo any scattering process from one side to the other; it is called ballistic transport, and thus the thermal conductivity will increase linearly with the increase in the size of the system [23–27]. When the size of the system is extremely small, the thermal conductivity will increase linearly with the increase of the system, and as the system continues to increase, it will gradually reach the diffusion thermal transport, which is the macroscopic thermal conduction process. Thermal transport can be described by Knudsen number (Kn), Kn = Λ/L, where Λ is the average free path of phonon transport, that is, the average phonon transmission distance between two collisions, and L is the characteristic length of the system [28]. In the study of the thermal conductivity of bulk materials, the product of the scattering time and the velocity of the phonon group is defined as the mean free path of the phonon. When Kn is > 1, it is ballistic transport; when Kn is < 1, it is diffusion transport; when Kn is ~ 1, it is somewhere in between. It is worth noting that the mean free path of the phonon varies greatly in different materials and generally ranges from nanometers to micrometers at room temperature [29]. It is generally believed that the average free path of crystalline materials with high thermal conductivity is larger, whereas the average free path of amorphous materials with low thermal conductivity is smaller, which results in different size effects for the thermal conductivity of different types of materials [30,31].

A long time ago, theoretical and experimental research found that the thermal conductivity of materials was strongly related to the size of materials at the micro/nanoscale, which is the called the size effect. Take the normal heat conduction of a thin film material as an example: when the thin film is macroscopic, the temperature shows a uniform linear distribution between the two ends of the material in the case of 1-D heat conduction. For nanoscale film, temperature jumps occur at the film interfaces, and the temperature distribution within the film becomes more and more gentle as the size decreases. Previous studies found that the larger the thickness of the film, the higher the thermal conductivity [23,24]. Until the thickness exceeds a certain size, the thermal conductivity is independent of the size. In addition, the representative nanomaterials, such as CNTs and graphene, also have the same size effect [25–27]. Some studies have found that the thermal conductivity of the CNTs will gradually increase with the increasing length; until the length reaches a certain value, the thermal conductivity gradually tends to a constant value and continues to increase without being affected by the size [26]. Other studies have also found that the thermal conductivity of graphene and ultrathin graphite wrapped in silica (1–20 layers thick) increases with the number of graphene layers (Fig. 1.3C), approaching the in-plane thermal conductivity of the bulk graphite of the thickest samples [25]; such a characteristic length is related to the specific material. For common silicon materials, the characteristic length should be in the order of 1–10 μm. After all, this unique feature is essentially due to the ballistic transport of energy-carrying particles at the micro/nanoscale.

Another very important thermal transport phenomenon is the interface thermal resistance at the micro/nanoscale, that is, it has a unique interface effect. When solving 1-D heat conduction at the macroscale, the temperature at the interface between two materials is generally considered to be continuous, as shown in Fig. 1.4A. In practical applications, the influence of contact thermal resistance is generally considered. The contact thermal resistance is caused by the rough interface between the two materials and the air gap [32,33]. If the two materials are in close contact at the atomic scale, the existence of contact thermal resistance is effectively avoided. In fact, even if the two materials are in close contact at the atomic scale, there will still be a temperature difference at the interface, as shown in Fig. 1.4B. This temperature difference is caused by the existence of interfacial thermal resistance [34,35], which is usually negligible at the macroscale. When the scale studied is down to tens of nanometers, the thermal resistance obviously needs to be taken into account. The total thermal resistance is largely derived from the interfacial thermal resistance, which is also called thermal boundary resistance (Kapitza resistance) [34]. Interfacial thermal resistance is more important at the nanoscale because it causes many contact interfaces and is more complex at the nanoscale. In addition, it is also mainly caused by the difference in the electrons/phonons and vibration characteristics of the two substances in contact. Boundary scattering occurs when carriers (phonons or electrons, determined by the properties of the material) try to cross the contact interface. The motion of the scattered carriers depends entirely on the state of energy available to the material at the interface, and the collision of heat carriers with the material boundary produces boundary scattering of phonons, thereby weakening the thermal conductivity [35,36]. For nanocrystalline materials, phonon scattering is dominant. When carriers in the crystal move, they will be scattered by thermal vibration atoms. Phonon scattering is divided into phonon-phonon scattering, impurity point defect scattering, and boundary scattering. These are the main factors that cause the interface thermal resistance. The intensity of the scattering is related to temperature and phonon frequency. In general, phonon-phonon scattering is ubiquitous and occupies a major part of the scattering mechanism, and boundary scattering is related to the characteristic size material [37].

Fig. 1.4

Fig. 1.4 Schematic diagram of temperature distribution of (A) two bulk materials at the macroscale, (B) two micro/nanoscale structured materials with interface effect.

In summary, there are two most important physical effects at the micro/nanoscale, one is the size effect of thermal conductivity caused by ballistic transport and the other is the interface thermal resistance effect. Therefore many researches on micro/nanoscale thermal transport are devoted to studying how to obtain the change of thermal conductivity with size and interfacial thermal resistance through theoretical and experimental means. It is generally considered that when the characteristic length of the thermal transport is less than 10 μm, it is urgent to consider the effects of the abovementioned two effects on the thermal conductivity. For different materials, such as most metal materials, liquids, amorphous materials, and organic materials, the characteristic length is different and may be smaller, which is related to the special properties of the material.

1.3: Demand for thermal properties research

1.3.1: Limitations of traditional thermal measurement techniques

With the rapid development of material synthesis and processing technology, it is extremely important to scientifically understand heat transfer performance in various micro/nanoscale structured materials/devices. Several commonly used thermal analysis techniques include thermocouples, differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermogravimetry (TG), and thermomechanical method (DMA). Thermophysical properties measurement techniques at the macroscale have been unable to meet the demands of nanoscale structured materials. The main reasons include the following: (1) materials with micro/nanoscale structures are often presented in the form of collectives, and conventional methods that are suitable for independent sample testing cannot be satisfied, such as thermal barrier coating materials deposited with stainless steel substrates; (2) the sensors for macroscale measurement have exceeded the size of some measured micro/nanoscale materials, for example, the spatial resolution of thermocouples is generally not less than 10 μm, which is already larger than the size of the micro/nanoscale materials to be measured; (3) conventional methods are usually only applicable to the form of existing materials (such as bulk, liquid, and powder) at the macroscale and cannot be used for some newly synthesized materials, such as micro/nanowires, nanotubes, and films; (4) micro/nanoscale structured materials are generally tested with a small amount of heat because of the large specific surface area; the proportion of heat loss is also large, so it is necessary to develop a high-resolution heat sensor, and conventional techniques are also no longer applicable; (5) finally, the energy may no longer be continuous at the micro-nanoscale, and temperature may not be applicable as a characteristic parameter of the average molecular kinetic energy, which poses a challenge to traditional heat transfer theory. With the development of micro/nanotechnology, thermophysical property measurement methods with high spatial resolution and thermal resolution have become one of the key technologies in the field of heat transfer research at domestic level and abroad. The development of thermal property characterization and gathering and transmission suitable for new micro/nanoscale structured materials have become the primary task of micro/nanoscale heat transfer studies [38].

1.3.2: The main purpose of thermal measurement at micro/nanoscale

Micro/nanoscale thermal transport is the scientific frontier in the field of engineering thermophysics, which is mainly manifested in the research of basic scientific issues in the heat transfer process. In addition, it has been widely used in fields facing major national needs such as thermal management of semiconductor microelectronic devices, efficient energy transmission and conversion, and new thermal energy-saving and environmental protection technologies. Accurate characterization and evaluation of the thermophysical properties of advanced materials/devices have always been a bottleneck issue affecting materials/devices performance improvement and structural optimization. Especially with the development of techniques such as synthesis, micromachining, and microanalysis, it is particularly important to characterize the heat transfer and heat storage characteristics of materials/devices with micro/nanoscale structures. One aspect is the new thermophysical characterization techniques, which has evolved into an important applied research direction in this field. Therefore the characterization of thermal properties of micro/nanoscale materials can make us go deeper into the excellent performance of micro/nanoscale materials and their differences from traditional materials and help us to understand the microscopic thermal transport mechanism of new materials. In general, micro/nanoscale structures are relatively complex and because of the very small scale, there are many structural factors that affect the heat transfer performance. Therefore the accurate characterization and evaluation of their thermal physical properties is of great significance to further explore their potential applications.

To accurately measure the temperature, heat flow, thermal resistance, and thermophysical properties of materials, including thermal conductivity and thermal diffusivity, various thermal property testing methods and calorimetry techniques have been developed. Currently reported methods applicable to the thermophysical properties of micro/nanoscale structured materials include T-type method [39], T-3ω method [40], 3ω method [41–52], H-type method [53], laser flash method [54,55], Raman spectroscopy method [56,57], laser flash Raman spectroscopy method [58], time-domain thermoreflectance (TDTR) method [59], photothermal resistance method [60], transient electrothermal (TET) technique [61], infrared thermography [62], hot strip method [63,64], and hot disk method [65,66].

The T-type method (Fig. 1.5A) is suitable for the measurement of thermophysical properties of microscale filamentary materials, including thermal conductivity, thermal diffusivity, specific heat capacity, thermal effusivity, etc. Owing to the significant differences between the thermal properties of different samples, it is best to measure all properties of the same single sample. At this time, multiple and separate measurements of different physical properties on the same sample will damage the morphology of the microscale samples, thereby affecting its inherence of the thermophysical properties. To tackle this issue, Ma et al. [40] developed the integrated T-3ω method (Fig. 1.5B), which successfully achieved in situ measurement of multiple parameters of a single Bi2S3 nanowire. Moreover, the 3ω method (Fig. 1.5C), which was first proposed by Cahill [41,42], can be used to measure thermal conductivity of solid bulks and thin films. With the continuous updating and upgrading of techniques (Fig. 1.5D), it has also been applied to the thermal conductivity, thermal diffusivity, specific heat capacity, and thermal effusivity measurement of various micro/nanoscale materials [43–52], including nanofilms, nanoporous materials, superlattices, anisotropic crystals, nanotubes, nanowires and nanofluids, etc. The H-type method (Fig. 1.5E) is mainly used to measure the thermal conductivity of the nanofilms. By changing the heating power of the sensors on both sides of the nanofilm, the temperature difference between the sensor and the nanofilm can be obtained and then the thermal conductivity can be further calculated. The larger the temperature difference between the thin film and the metal sensor, the smaller the thermal conductivity of the nanofilm [53]. The laser flash method (Fig. 1.5F) first proposed by Parker et al. in 1961 has become one of the most classic thermophysical property measurement techniques [54] and has been widely used in the measurement of the thermal diffusivity of homogeneous thin films materials [55]. Because traditional thermal measurement methods, such as the laser flash method and the thermal bridge method, cannot measure the thermal conductivity of single-atomic layer graphene, Balandin et al. [56] successfully achieved the measurement of thermal conductivity for suspended single-layer graphene (SLG) based on Raman spectroscopy. Raman spectroscopy method (Fig. 1.5G) is a noncontact, nondestructive, and rapid thermal material performance measurement method, which is also very suitable for micro/nanoscale materials, such as graphene [57]. However, compared with Raman spectroscopy, laser flash Raman spectroscopy is more advanced and proposed by Li et al. [58] (Fig. 1.5H), and the advantage is that the sample can be nonconductive and suitable for the characterization of thermophysical properties of thermal insulation materials. Another more advanced thermophysical property characterization technique is TDTR method (Fig. 1.5I), which has high accuracy and wide applicability to various micro/nanoscale materials. With the continuous development of technique, it is now mainly used to measure the thermal conductance (G) at the solid interface [59]. The photothermal resistance technique (Fig. 1.5J) was first proposed by Hou et al in 2006 [60], which has the advantage that it can be used to characterize the thermal diffusivity of 1-D micro/nanoscale conductive and nonconductive materials. Unlike photothermal resistance technique, TET (Fig. 1.5K) is more suitable for measuring thermal diffusivity of fibers or narrow strips. It also tackled the long test time and weak signal shortcomings of other methods [61]. Then, the infrared thermography method (Fig. 1.5L) first proposed by Welch et al. [62] developed to be an effective technique for measuring thermal diffusivity. The measurement principle is based on the temperature change caused by photothermal excitation, and the temperature distribution is recorded by an infrared camera. In addition, the hot strip method (Fig. 1.5M) is usually used to measure the thermal conductivity or thermal diffusivity of some nanoporous materials/devices or nanopowders [63,64]. Similar to the hot strip method, the hot disk method (Fig. 1.5N) is suitable for the measurement of thermal conductivity or thermal diffusivity of nanoporous materials/devices and nanofluids [65,66].

Fig. 1.5

Fig. 1.5 Schematic diagram of some representative thermophysical properties measuring techniques for micro/nanoscale structured materials. (A) Reprinted with permission from Elsevier and International Heat and Mass Transfer. S. Wu, Q.-Y. Li, T. Ikuta, K. Morishita, K. Takahashi, R. Wang, T. Li, Thermal conductivity measurement of an individual millimeter-long expanded graphite ribbon using a variable-length T-type method, Int. J. Heat Mass Transf. 171 (2021) 121115. https://doi.org/10.1016/j.ijheatmasstransfer.2021.121115.5a, Open Access. (b) Reprinted with permission from AIP and Review of Scientific Instruments. J. Wang, M. Gu, X. Zhang, G. Wu, Measurements of thermal effusivity of a fine wire and contact resistance of a junction using a T type probe, Rev. Sci. Instrum. 80 (7) (2009) 076107. https://doi.org/10.1063/1.3159863.5b, Open Access. (d) Reprinted with permission from Elsevier and Carbon. L. Qiu, K. Scheider, S. A. Radwan, L. S. Larkin, C. B. Saltonstall, Y. Feng, X. Zhang, P. M. Norris, Thermal transport barrier in carbon nanotube array nano-thermal interface materials, Carbon 120 (2017) 128–136. https://doi.org/10.1016/j.carbon.2017.05.037.5d (E) Open Access. (F) Reprinted with permission from Elsevier and Physical Letters A. H. Xie, A. Cai, X. Wang, Thermal diffusivity and conductivity of multiwalled carbon nanotube arrays, Phys. Lett. A 369 (1) (2007) 120–123. https://doi.org/10.1016/j.physleta.2007.02.079.5f, Open Access. (G) Reprinted with permission from Elsevier and Analyst. R. Smith, K. L. Wright, L. Ashton, Raman spectroscopy: an evolving technique for live cell studies, Analyst 141 (12) (2016) 3590–3600. https://doi.org/10.1039/C6AN00152A.5g, Open Access. (H) Reprinted with permission from AIP and Review of Scientific Instruments. J. Liu, H. Wang, Y. Hu, W. Ma, X. Zhang, Laser flash-Raman spectroscopy method for the measurement of the thermal properties of micro/nano wires, Rev. Sci. Instrum. 86 (1) (2014) 014901. https://doi.org/10.1063/1.4904868.5h, Open Access. (I) Reprinted with permission from AIP and Review of Scientific Instruments. P. Jiang, X. Qian, R. Yang, Time-domain thermoreflectance (TDTR) measurements of anisotropic thermal conductivity using a variable spot size approach, Rev. Sci. Instrum. 88 (7) (2017) 074901. https://doi.org/10.1063/1.4991715.5i, Open Access. (J) Reprinted with permission from AIP and Review of Applied Physical Letters. J. Hou, X. Wang, C. Liu, H. Cheng, Development of photothermal-resistance technique and its application to thermal diffusivity measurement of single-wall carbon nanotube bundles, Appl. Phys. Lett. 88 (2006) 181910. https://doi.org/10.1063/1.2199614, Open Access. (K) Reprinted with permission from Elsevier and Materials & Design. C. Xing, T. Munro, C. Jensen, H. Ban, C.G. Copeland, R.V. Lewis, Thermal characterization of natural and synthetic spider silks by both the 3ω and transient electrothermal methods, Mater. Des. 119 (2017) 22–29. https://doi.org/10.1016/j.matdes.2017.01.057, Open Access. (L) Reprinted with permission from Elsevier and Materials Letters. L. I. Giri, S. Tuli, M. Sharma, P. Bugnon, H. Berger, A. Magrez, Thermal diffusivity measurements of templated nanocomposite using infrared thermography, Mater. Lett. 115 (2014) 106–108. https://doi.org/10.1016/j.matlet.2013.10.042.5l, Open Access. (M) Reprinted with permission from Elsevier and International Heat and Mass Transfer. G. Wei, Y. Liu, X. Zhang, F. Yu, X. Du, Thermal conductivities study on silica aerogel and its composite insulation materials, Int. J. Heat Mass Transf. 54 (4) (2011) 2355–2366. https://doi.org/10.1016/j.ijheatmasstransfer.2011.02.026, Open Access. (N) Reprinted with permission from AIP and Review of Scientific Instruments. S.E. Gustafsson, Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials, Rev. Sci. Instrum. 62 (1991) 797–804. https://doi.org/10.1063/1.1142087, Open Access.

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