Thermal characterization of carbon nanotube fiber by time-domain differential Raman

Abstract

Most conventional Raman thermometry for thermal properties measurement is on steady-state basis, which utilizes either Joule heating effect or two lasers configurations coupled with increased complexity of system or measurement uncertainty. In this work, a new comprehensive approach including both transient and steady-state Raman method is proposed for thermal properties measurement of micro/nanowires. The transient method employs a modulated (pulsed) laser for transient heating and Raman excitation, and is termed time-domain differential Raman. The average elevated temperature during the transient heating period is probed simultaneously based on Raman thermometry. Thermal diffusivity can be readily determined by fitting normalized temperature rise against heating time with a transient heat conduction model. On the other hand, thermal conductivity can be obtained in the steady-state measurement by adjusting modulation settings. To verify this method, a carbon nanotube (CNT) fiber is measured with the thermal diffusivity of ${1.74}_{-0.20}^{+0.20}\times {10}^{-5}$ m²/s and the thermal conductivity of ${34.3}_{-0.4}^{+0.4}$ W/m K. The relatively low thermal transport values stem from numerous CNT-glue matrix and CNT–CNT thermal contact resistances. Compared with the conventional steady-state Raman method, the transient method requires no detailed laser absorption value and no temperature coefficient calibration. It can be easily applied to study transient thermal transport in materials.

Introduction

In recent years, Raman spectroscopy has been widely employed for temperature probing/mapping and thermal properties measurement [1], [2], [3], [4]. Raman scattering is an inelastic scattering generated in laser–material interaction, and is temperature dependent in terms of peak intensity, peak shift, and peak width (full width at half maximum, FWHM) [5]. Therefore, Raman signals arising from temperature variation can be used to measure thermal properties of materials, such as in the studies of carbon nanotubes (CNTs) [6], [7], graphene [8], [9], [10], silicon [11] and other nano-materials [12], [13]. For example, Zhang's group developed a non-contact T-type Raman spectroscopy method for the simultaneous measurement of micro/nano fibers' laser absorption and thermal conductivity [14]. Beechem et al. used micro-Raman spectroscopy to simultaneously map the complete temperature and biaxial stress distributions of a functioning polysilicon microheater [15]. Recently, they reported a comprehensive investigation of error and uncertainty in Raman thermal conductivity measurements based on numerical simulation [16]. In another work, Yue et al. investigated the thermal response of a Si substrate under tip-induced near-field laser heating using Raman spectroscopy, and for the first time achieve the resolution down to sub-10 nm [17].

Most optical thermometry techniques for thermal diffusivity determination are transient methods, such as laser flash method and time-domain thermoreflectance (TDTR, or pump-probe) method etc [18], [19], [20], [21], [22]. The thermal diffusivity, which is based on transient heat transfer model, is usually obtained in these optical thermal characterizations. Different from abovementioned optical thermometry, Raman thermometry works as a steady-state measurement tool since the continuous probe laser is employed and a relative long integration is required for a sound Raman signal. Steady-state heat transfer models are introduced for thermal transport description and precise measurement of heat flux is needed to calculate thermal conductivity of the material. In Yue et al.’s works [2], [23], a steady-state electro-Raman-thermal (SERT) technique was developed to characterize thermal transport in micro/nanoscale materials. In this method, the middle-point temperature of a microwire was recorded using Raman spectroscopy when the sample was heated by electric current. Thermal conductivity was derived from the relationship between the middle-point temperature and the Joule heating energy. However, the electrical/thermal contact resistance at sample-electrode conjunctions, and additional heating effect from the probing laser would contribute to unexpected measurement errors. Moreover, the electric circuit and electrodes are difficult to design for micro/nanoscale materials. Besides electrical heating, laser heating was employed to induce temperature difference for steady-state Raman spectroscopy measurements [3], [24], [25], [26]. This method eliminates the fabrication difficulties in electrical heating method and the highly focused laser spot shows great advantages in thermal probing at extremely small scales. But we need the precise calculation of laser absorption for calculating thermal conductivity. This requires the knowledge of the specific laser absorbance at certain wavelengths for the measured materials, which is usually unknown for novel materials [7].

Therefore, developing an optical method which is capable of measuring both thermal conductivity (steady-state thermal characterization) and thermal diffusivity (transient thermal characterization) is necessary and significant. Conventional Raman thermometry are based on continuous laser for heating and temperature probing, which gives us an idea that if using pulsed laser instead for Raman signal excitation, transient Raman thermometry could be possible [27], [28]. Furthermore, if the excitation laser is adjustable from continuity to pulse, both steady-state measurement and transient characterization based on Raman thermometry can be achieved. Recently, it was reported that a time-domain differential Raman (TD-Raman) thermometry was developed by modulating the excitation laser and probing transient Raman scattering during the pulsed heating circle [29]. In this work, the thermal diffusivity of a silicon cantilever was measured to validate the measurement capacity. However, most industrial materials do not have such a good crystalline structure as single crystalline silicon. The lower Raman excitation efficiency due to their less crystalline structure would increase the difficulty of the TD-Raman measurement. For example, the carbon nanotubes fiber (CNT fiber), which is a scale-up material from single CNTs, presents exceptional macroscopic properties. It has wide applications and is of great interest to explore its thermal properties [30], [31], [32], [33]. In this work, we use TD-Raman method to measure the thermal diffusivity of CNT fiber, and combine the steady-state Raman method to study its thermal properties comprehensively. Most importantly, we expand the application of the recently developed transient Raman method [29], and achieve both transient measurement and steady-state thermal characterization based on Raman thermometry at the same experimental configuration.