Infrared Thermography. Thermal Diffusivity Measurements of Templated Nanomaterials

Contents

Certificate

Acknowledgements

Abstract

List of Figures

List of Tables

Nomenclature and abbreviations

Chapter 1
Infrared thermography and nanomaterials: a review
1.1 Introduction to infrared thermography
1.2 Infrared thermography in thermal characterization
1.3 Thermography at reduced dimensions
1.4 Anodic alumina (AAO) templated nanostructures
1.5 Thermoelectrics
1.6 Bismuth telluride (Bi2Te3) nanowires
1.7 Motivation and objectives of the thesis
1.8 Thesis overview
1.9 References

Chapter 2
Synthesis and physical characterization of bismuth telluride nanowires
2.1 Bi2Te3: one dimensional nanostructure
2.2 Electrodeposition of Bi2Te3 nanowires
2.2.1 Electrolyte preparation
2.2.2 Working electrode preparation
2.2.3 Electrodeposition process
2.3 Physical characterization
2.3.1 X ray diffraction study (XRD)
2.3.2 Energy dispersive X-ray study (EDX)
2.3.3 Scanning electron microscopy (SEM)
2.3.4 Transmission electron microscopy (TEM)
2.4 Aging of Bi2Te3 nanowires
2.5 Single crystal bulk Bi2Te3
2.6 Conclusions
2.7 References

Chapter 3
Modeling, analysis and simulation of thermal waves in template nanomaterials
3.1 Introduction
3.2 AAO template thermal response
3.3 Periodic point source heating
3.4 Thermal diffusivity measurement
3.5 IR experiment with empty AAO template
3.6 Simulation studies
3.6.1 Electro-thermal mode
3.6.2 thermo SIM: software for thermal simulation
3.6.3 Phase image and phase image slope
3.6.4 Influence of convection and radiation effects
3.7 Experimental verification of convection effect
3.8 Conclusions
3.9 References

Chapter 4
Thermal characterization using infrared thermography
4.1 Thermal characterization techniques
4.1.1 Laser flash method
4.1.2 The 3-Omega method
4.1.3 Photo thermoelectric method
4.2 Infrared thermography system
4.2.1 Classification of IR thermography techniques
4.2.2 Infrared detectors and cameras
4.3 Lock-in thermography for thermal characterization
4.3.1 Phase and Amplitude images
4.3.2 Principle of operation
4.3.3 Experimental details
4.3.4 Data analysis and interpretation 4.4 Anisotropic AAO thermal diffusivities
4.4.1 In-plane thermal diffusivity of AAO template
4.5 Thermal diffusivity of single crystal Bi2Te3 sample
4.6 Thermal diffusivity of AAO/Bi2Te3 nanocomposite
4.7 Error analysis
4.8 First order lower bound model (FOLBM)
4.9 Thermal diffusivity of Bi2Te3 nanowires .
4.10 Thermal diffusivity of AAO/Ni, Co nanocomposites
4.11 Conclusions
4.12 References

Chapter 5
Frequency modulated thermal wave imaging for thermal diffusivity measurements
5.1 Introduction
5.2 Fundamentals of FMTWI
5.3 FMTWI for thermal characterization
5.4 Thermal diffusivity of empty AAO template
5.4.1 Experimental details
5.4.2FMTWI response in time and frequency domain
5.4.3Extraction of multiple phase and amplitude image
5.5 Thermal diffusivity estimation
5.5.1Phase slope measurement
5.5.2Thermal diffusivity calculation
5.6 FMTWI Time bandwidth influence
5.6.1 FMTWI: 2 second period & 1 - 3 Hz bandwidth
5.6.2 FMTWI: 5 second period & 1 - 3 Hz bandwidth
5.6.3 FMTWI: 5 second period & 0.5 - 1.5 Hz bandwidth
5.7 Conclusions
5.8 References

Chapter 6
Conclusions and future directions
6.1 Conclusions
6.2 Future directions

Abstract

The present document the efforts towards thermal characterization of templated nanostructures using an infrared (IR) thermography based non-contact technique. One dimensional bismuth telluride Bi2Te3 nanostructures were grown using anodic alumina (AAO) template assisted electrodeposition technique. An infrared thermography based technique was then developed for the in-plane thermal diffusivity measurements of these templated nanocomposites. The thermal diffusivity of the nanowires alone was estimated through comparing the experimental results with the predictions made by a first order lower bound model (FOLBM). A mathematical background of the proposed technique with simulation studies is also reported.

One dimensional Bi2Te3 nanowires were synthesized using a conventional three terminal electrodeposition system with empty AAO template as the working electrode, platinum counter electrode and saturated calomel electrode (SCE) as the reference electrode. The electrodeposition solution was prepared using precalculated composition of bismuth nitrate pentahydrate and tellurium powder in HNO3 electrolyte. The electrodeposition potential was determined from the cyclicvoltammetry (CV) and the nanowire deposition was done through Chronoamperometry (CA) process.

An infrared thermography based thermal characterization technique is developed for in-plane thermal diffusivity measurements of AAO templated nanowires. The aim was not to increase the spatial resolution directly to nanometers, but to use an indirect approach for estimating thermal parameters of nanostructures, from measurements done at millimeter scale. The technique is based on the principle of conventional active IR lock-in thermography. A periodic photo-thermal excitation of suitable frequency is applied on to the sample surface and the thermal response is monitored in radially outward direction, using an infrared camera. The observed thermal response carries both amplitude and phase information, which may be related to the thermal properties of the sample. There is a dependence of the relative phase and amplitude of the thermal response, on the radial distance of the sample. The in-plane thermal diffusivity of the sample can be determined from the slope of the plot of angular excitation frequency with square of the phase image slope. The illustrated technique utilizes only the phase information data for its results and thus, avoids the limitations associated with the amplitude information, such as sensitivity to local variation of surface illumination and emissivity. The underlying theoretical background has been developed, which has further been verified using an in-house developed simulation tool. Effect of convective losses has been modeled, simulated and later verified experimentally.

In-plane thermal diffusivity of empty AAO templates, Bi2Te3/AAO nanocomposites and the bulk Bi2Te3 sample were measured using the proposed technique. Moreover, a change in the thermal property of Bi2Te3 nanowires as compared to its bulk counterpart was theoretically predicted. This fractional change in thermal diffusivity value of the nanowires was estimated through comparing the experimental results with the predictions made by FOLBM model. A fivefold reduction in thermal diffusivity of 200 nm Bi2Te3 nanowires as compared to bulk samples was inferred. Similar studies were also performed on Ni, Co / AAO nanocomposites, results of which are discussed.

The lock-in thermography based technique requires experiments to be done at different excitation frequencies independently. A relatively recent frequency modulated thermal wave imaging (FMTWI) technique has been introduced to overcome this limitation of the lock-in thermography based approach. This improves considerably the operational time of the complete experiment and makes it more efficient while retaining reliability. In FMTWI the applied excitation signal is a frequency modulated chirp signal. Multiple phase information corresponding to different excitation frequencies may be extracted from a single run of the experiment, from which thermal diffusivity of the sample can be determined using the phase image slopes. Experimental results from empty AAO templates and Bi2Te3/AAO nanocomposites are discussed. The influence of chirp time period and bandwidth on the final thermal diffusivity results are also included.

The distinct achievement of the present thesis is in-plane thermal diffusivity measurements of one dimensional templated nanostructures using infrared thermography, for the first time. The efficient and reliable technique developed and illustrated is well supported by theory, modelling and simulation. It has potential for very wide applicability.

List of figures

2.1 Schematic of the (a) empty AAO template (b) electrodeposition system used for the Bi2Te3 nanowire growth

2.2 (a) Cyclic voltammetry (CV) and (b) Chronoampereometry (CA) curves from electrodeposition process with respect to a quasi-reference electrode

2.3 X-ray diffraction patterns of electrodeposited AAO/Bi2Te3 nanowires matrix

2.4 EDX result of electrodeposited Bi2Te3 nanowires

2.5 SEM images of electrodeposited Bi2Te3 nanowires

2.6 TEM images of the electrodeposited Bi2Te3 nanowires

2.7 SEM images (a) CAD design for making contacts on single Bi2Te3 Nanowire (b) four point contact on a nanowire made using lift-off technique

2.8 XRD & EDX results for ten days old electrodeposited Bi2Te3 nanowires samples

2.9 TEM image showing thin oxide layer on top of the Bi2Te3 nanowire

2.10 EDX spectra from single Bi2Te3 nanowire using TEM instrument

3.1 Schematic of a circular AAO template

3.2 A typical IR image of an empty AAO template

3.3 Experimental temperature-time plots at different radial distances for an applied pulsed photothermal excitation to an empty AAO template; Inset is zoom-in of the marked region

3.4 Analogy between electrical and thermal parameters: Electro-thermal modeling

3.5 An element of the electro-thermal model mesh and the 3-D mesh network

3.6 A typical simulated phase image and derived plot of phase image slope

3.7 Screenshot of the simulator GUI showing phase plots

3.8 Electro-thermal representation of the template incorporating convection and radiation effects

3.9 Simulated comparison of the phase image slopes with (red) and without (green) convection and radiation effects

3.10 Experimental temperatures – time plot with forced convection effects

3.11 Experimental phase image slope with and without forced convection effects

4.1 Atmospheric infrared window [http://en.wikipedia.org/wiki/Infrared_window]

4.2 Schematic diagram of the experimental setup

4.3 Experimental setup for thermal characterization using IR thermography

4.4 (a) Phase image; Inset: phase profile across the line (b) Variation of phase slope for different excitation frequency, for 200 nm pore empty AAO sample

4.5 Representation of anisotropic thermal diffusivity of AAO samples in directions parallel and perpendicular to nanochannel axis

4.6 Excitation signal angular frequency ω plotted against (Ø/r)[2] for empty AAO templates with varying pore dimensions (a) 20 nm (b) 100nm (c) 200nm

4.7 Excitation signal angular frequency ω plotted against (Ø/r)[2] for single crystal Bi2Te3 sample

4.8 Excitation signal angular frequency ω plotted against (Ø/r)[2] for AAO/nanowire matrix from two different samples

4.9 Figure 4.9. Angular excitation frequency ω versus square of the phase slope (a) Ni/AAO nanocomposite (b) Co/AAO nanocomposite

5.1 Schematic of the experimental setup for FMTWI experiments

5.2 Temperature variation on AAO template on application of chirp photo-thermal signal

5.3 Fast Fourier Transform (FFT) response of applied chirp showing fundamental frequency of 0.25 Hz and its harmonic components

5.4 Phase images for a) 0.25 Hz, b) 0.5Hz, c) 0.75Hz, d) 1Hz, e) 1.25Hz, f) 1.5Hz, g) 1.75Hz, and h) 2Hz, excitation frequency

5.5 Amplitude images for a) 0.25 Hz, b) 0.5Hz, c) 0.75Hz, d) 1Hz, e) 1.25Hz, f) 1.5Hz, g) 1.75Hz, and h) 2Hz, excitation frequency

5.6 Phase variation with radial distance for different excitation frequencies

5.7 Angular excitation frequencies plotted against square of phase slope

5.8 (a) FFT of the FMTWI IR response for 100 nm pore empty AAO template, inset: temperature – time plot for (2 s, 1-3 Hz bandwidth) chirp signal

(b) Angular excitation frequency plotted against square of phase slope

5.9 (a) FFT of the FMTWI IR response for 100 nm pore empty AAO template, inset: temperature – time plot for (5 s, 1-3 Hz bandwidth) chirp signal

(b) Angular excitation frequency plotted against square of phase slope

5.10 (a) FFT of the FMTWI IR response from 100 nm pore empty AAO template, inset: temperature – time plot for (5 s, 0.5 - 1.5 Hz bandwidth) chirp signal

(b) Angular excitation frequency plotted against square of phase slope

List of tables

4.1 Thermal diffusivity values for the different pore size empty AAO sample

4.2 Thermal diffusivity values of empty AAO and Bi2Te3 sample

4.3 Table 4.3 Thermal diffusivity of AAO / Ni, Co nanowire matrix in radial direction

5.1 Comparison of measured thermal diffusivity values of AAO / Bi2Te3 samples through lock-in and FMTWI techniques

Nomenclature and abbreviations

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Chapter 1

Infrared thermography and nanomaterials: a review

1.1 Introduction to infrared thermography

Thermography was initially developed to image temperature distributions, generally at a distance and through the air. Thermography literally means „writing with heat‟, similar to optical photography which means „writing with light‟. Infrared (IR) thermography may further be defined as the thermal study of an object surface through monitoring its emitted IR radiations. The invisible infrared radiations emitted from the surface of an object can be detected using suitable sensors and are represented as thermal images or thermograms.

The principle of operation of IR thermography is based on Planck‟s law and Stefan-Boltzmann law [1]. Every physical body having a finite temperature above absolute zero, continuously emits electromagnetic radiation. Planck‟s law describes the distribution of this emitted radiation as a function of the wavelength at a given temperature, which for a perfect black body may be expressed as, where E is the spectral radiance, T is the absolute temperature, KB is the Boltzmann constant, h is the Planck constant, and c is the speed of light. It may be further stated that for a given temperature the amplitude of the emitted radiation varies with the wavelength.

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The Stefan-Boltzmann law, also known as Stefan's law further quantifies the total power emitted per unit area at the sample surface, which is achieved by integration of Plank‟s law for all wavelengths. For a perfect black body, where P is the total power emitted per unit area of the sample surface, T is the absolute temperature, and σ is the Stefan–Boltzmann constant ( 5.67 x 10-[8] Wm-[2]K-[4]) or Stefan's constant. For practical non blackbody samples the governing equations are modified incorporating an emissivity (ɛ) factor.

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From a historical perspective, the world of infrared technology came into existence after Frederick William Herschel discovered the existence of infrared rays in the year 1800 [1]. He found that the temperature of a mercury thermometer was increasing strongly when it was incident with electromagnetic radiation beyond the red band of the sunlight spectrum, and no visible radiation could be seen. In 1840, Herschel‟s son John produced the first infrared image using an evaporograph by differential evaporation of a thin film of oil [1]. In 1880 the bolometer was invented by Samuel Pierpont Langley and perfected by Charles Greeley Abbot, which later became the heart of any IR camera. Max Planck‟s theory of radiation in 1900, clarified Herschel‟s experiment. World War I brought the first photoconductive detector. Post World Wars I and ,II progress in infrared and thermal technologies accelerated. Between 1960s and 1970s the first infrared camera became commercially available [1]. Early IR cameras used pyroelectric tube technology. However, after evolution of semiconductor technology from early 1980s bolometer focal plane array (FPA) IR cameras became available. Technological improvement since have significantly improved the price-performance ratio of IR systems. Some of the latest standard includes 640 x 512 and 1K x 1K pixel arrays.

1.2 Infrared thermography in thermal characterization

Infrared thermography techniques are widely used for non destructive testing (NDT) and evaluation [2]. The application areas are greatly varied: from water entrapment and moisture evaluation of buildings, degradation of EPROM (erasable programmable read only memory) chips and printed circuit boards, defect detection and characterization in turbine blades etc, medical and veterinary applications such as thermal coronary angiography, humane breast tumors, neuromuscular disorders, public services as forest fire detection and monitoring of traffic roads etc, to name a few [1].

The present section discusses a major application of IR thermography for thermal characterization of materials, in general, and nanomaterials in particular. Material thermo physical properties such as thermal conductivity, thermal diffusivity and heat capacity are of great importance in many industrial applications. They are also very important parameters for the thermoelectric materials in particular. A reliable measurement of these properties is still a great challenge before the scientific community. IR thermography not only provides a fast and contactless approach but is more user-friendly, simple design and a broad range of characterization property adds to its advantages.

D. Almond in his book [2] discusses determination of thermal properties (thermal diffusivity, thermal effusivity, thermal conductivity and specific heat), by photothermal techniques using IR detectors. The application of IR camera for determination of thermal diffusivity in metal plates was first presented by Welch et al. [3] and later for biomedical application by Milner et al. [4] and Telenkov et al. [5]. Peng-fei Qiu et al. [6] used a non-contact infrared thermography technique to determine the thermal diffusivities of non-metal materials, such as Lead zirconium titanate (PZT) ceramics, insulators like porcelain and Teflon plates with rough surfaces and nonhomogeneous microstructures. In his experiment, a pulsed laser was used to excite a thermal source in the sample, and an IR camera system was used to record the IR thermal images of the following heat diffusion and loss processes. J. M. Laskar et al. [7] reports thermal diffusivity measurement of solid samples by using a continuous heat source and infrared thermal imaging. Wolf et al. [8] proposed a very fundamental approach for thermophysical analysis of thin films by lock-in thermography. The technique is based on a periodically modulated thermal excitation, first introduced by Ångström [9]. The concept considers the geometric mean of the thermal diffusion length of the amplitude and phase images for measurement of the thermophysical properties of thin films under ambient conditions. Arantza Mendioroz et al. [10] reported photo thermal radiometry based technique for thermal diffusivity measurements of thin plates and filaments. The concept is based on the “Phase method” [11, 12] with a linear relation between the phase of the surface temperature „Ø‟ and the lateral distance to the heating spot, with a slope m = − (π f / D) [1]/[2], from which the thermal diffusivity (D) can be obtained. Photothermal methods combined with infrared detection have been used to measure thermal diffusivity of materials with anisotropic properties by Salazar [13], and samples which are non-opaque to thermal radiations by Tom et al. [14].

1.3 Thermography at reduced dimensions

Three major parameters underline any IR thermography system: space (spatial), temperature (thermal) and time (frame rates). Limits in resolution of any and all of these three parameters, characterize the IR system. The temperature resolution, expressed as the minimum temperature equivalent of the noise (NETD), is guided by the sensor‟s properties. The other key factor is the spatial resolution, which is controlled by the utilized band of IR wavelengths. In situations demanding micron order resolution, the conventional methods fails due to either the dimension limit of the thermal probe or the diffraction limit of excitation source. Constant efforts have been made to improve upon the spatial resolution and operational speed (frame rates) of temperature measurements [15].

The demand for nanoscale thermal characterization has stimulated the development of related measurement techniques. There can be various approaches in temperature measurements pertaining to nano regime. One primary approach is to fabricate a nanoscale thermal probe e.g. a nano-thermocouple. The development of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) lead to fabrication of temperature sensors with sub-micron size. By attaching it to the tip of atomic force microscope (AFM) [16], the thermal probe is capable of doing nanoscale temperature mapping, which is called the scanning thermal microscopy (SThM) [17]. The SThM technique developed more than 20 years is widely applied for different areas for temperature mapping [18-20] and thermal properties measurement [21-23]. The emergence of photolithography techniques has made possible the fabrication of temperature sensor tip with stable properties, which greatly improves the spatial resolution of SThM to less than 50 nm [20]. The SThM approach working on the thermal expansion effect features the highest spatial resolution with 10 nm, where an external heating source is needed [24]. The SThM technique has achieved high accuracy in thermal measurements with an uncertainty of about 10-[3] K and its temporal resolution is shorter than 1 ms, which is controlled by the lock-in bandwidth and the response time of the thermal probes [17]. One major limitation of STM based system is that it works only on metallic samples. For semiconductors materials, AFM is employed to replace STM, for better convenient operation in maintaining constant gap between the tip and sample [25].

M. S. Unlu et al. at Boston University have reported about high resolution backside imaging and thermography using a numerical aperture increasing lens (NAIL) [26-29]. The concept of NAIL was developed to study semiconductors at very high spatial resolution. The NAIL clubbed on the surface of a planar sample behaves as a solid immersion lens. The proposed design worked in 3-5μm wavelength imaging range and demonstrated a high lateral resolution of 1.4 μm along with a focal depth of 7μm. One very important feature of NAIL microscopy is the improved light collection efficiency (scales with the square of numerical aperture), which is important in the study of quantum dots and a variety of semiconductor failure analysis modalities including thermal imaging.

1.4 Anodic alumina (AAO) templated nanostructures

Anodic alumina templates or the AAO templates are membranes having nano sized pores of varying dimensions. The membrane is composed of a high purity alumina Al2O3 matrix that is manufactured electrochemically through anodization process. When pure aluminum is anodized in an acidic electrolyte, porous alumina membranes are formed with nearly uniform diameter and spacing of the ordered hexagonal holes [30, 31]. The pore spacing and size can be controlled by the conditions such as the chosen electrolyte, time of anodization, temperature, and applied voltage. The important parameters are its pore dimension and porosity. The commercially available templates have pore dimensions varying between 20-200 nm. The average pore density of these templates is of the order of 2 × 10[9] cm-[2]. It is observed that the pore dimensions within the same template have a statistical distribution as made evident from the scanning electron microscopy images. The other important parameter of these inorganic membranes is the porosity representing the fraction of empty space present. It is possible to have membranes with different pore dimensions but the same porosity. All these parameters are regulated through the controlling steps of the anodization process.

The AAO templates are extensively used for electrodeposition of one dimensional nano structures such as the nanowires and nano-tubes. It provides the necessary mechanical support to the nanostructures and protects them from the ambient static charges. Working prototypes are proposed consisting of the nanocomposites of nanowires embedded AAO matrix. A major concern relating to these materials is their extremely delicate and fragile nature. Since they have thickness of around 60 micron, extra care and caution is demanded while dealing with them. This makes characterization of these templates through any contact method, very challenging.

1.5 Thermoelectrics

Human energy demand has increased significantly in recent times, driven by population growth and increase in living standards. At this pace, energy fulfillment will be one of the major challenges before the whole world, in the years to come. Moreover environmental friendliness of the energy source is also of great concern and importance. The needs for alternate energy sources and for more efficient use of our current fossil fuels demand different types of energy conversion technologies. Thermoelectrics which is direct interconversion of heat and electrical energy may prove to be a better alternative. Their greatest advantage is that they do not use any moving parts or environmentally harmful fluids. They are a great means for energy harvesting especially in situations where heat energy is lost to the environment unnecessarily increasing the total entropy.

The beginning of thermoelectrics was marked by a simple observation made by Thomas Johann Seebeck way back in 1821. His observation is now popularly known as the Seebeck effect. A junction of two different metals (e.g. iron and copper) when heated gives rise to a (very small) voltage. The combination of these two wires is called a thermocouple. Thirteen years later in 1934 Jean Charles Athanase Peltier, a French watchmaker, observed the second of the thermoelectric effects. His observation was a complementary effect in the form of one junction getting cooled and the other one getting hot under the effect of applied electricity. Finally the interdependence of Seebeck and Peltier phenomenon was ascertained by William Thomson [32] (later became Lord Kelvin) in 1855. He observed that there is a heat exchange with the surroundings under the joint influence of temperature gradient and electric current, in a conductor. Applying the theory of thermodynamics he established a relationship between the Seebeck and Peltier coeffcients. These relations are known as the Kelvin relations [33].

The term "thermoelectric effect" encompasses the above three separately identified effects: the Seebeck effect, Peltier effect, and Thomson effect. Joule heating which is the heat generated whenever a voltage is applied across a resistive material, is not one of the thermoelectric effects. The basic difference lies in the fact that the three thermoelectric effects are thermodynamically reversible where as joule heating is not. The performance of any thermoelectric material is judged through its dimensionless figure of merit ZT, which is defined as, where T is the absolute temperature, S is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity. From the above expression it is evident that the thermal properties of any thermoelectric material are very important, which ultimately regulates its dimensionless figure of merit. Thus the thermoelectric materials are one of the ideal candidates for investigation of the thermal properties. In general the improvement of dimensionless figure of merit is inferred from the reduction in thermal parameters of these samples.

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1.6 Bismuth telluride (Bi 2 Te 3 ) nanowires

Bi2Te3 is presently amongst the most promising materials for thermoelectric applications, having the highest dimensionless figure of merit ZT close to 1, at room temperature [34]. Theoretical studies by Dresselhaus et al. [35] found that the thermoelectric figure of merit (ZT) of Bi2Te3 and related materials could be significantly enhanced in nanocrystals having one-dimensional morphologies such as nanowires and nanotubes. It is thus no surprise that present day commercial thermoelectric modules widely use bismuth telluride. Focus on this material for further studies and with the aim to finally design a thermoelectric device module operating at room temperatures, is thus desirable.

The challenge to improve the thermoelectric efficiency has lead to two different approaches either playing with the material aspect or to seek help of nanotechnology. Reduced dimension thermoelectric materials showed marked improvements in their figure of merit [35]. Theoretical predictions have shown that the Seebeck coefficient can be increased because of the enhancement of electronic state density near Fermi level and the reduction in the lattice contribution to the thermal conductivity [35]. It was appealing to study this effect of reduction in thermal properties of the templated Bi2Te3 nanowires as compared to their bulk samples, for which a novel technique based on non-contact approach is developed.

1.7 Motivation and objectives of the thesis

Infrared thermography based techniques for material characterization provide many advantages like non-contact approach, fast data evaluation, simple system design etc. to name a few. However it has not yet achieved nano scale resolution directly and has also not been used widely for nanostructures.

On the other hand AAO templated nanostructures are an important class of nano materials. The average thickness of these samples is around 60 μm, which makes them very delicate and fragile. Thermal characterizations of these materials through contact methods like 3ω method and photothermoelectric techniques [36] are very challenging. Fabricating metal patterns on AAO templates also poses a great challenge. Moreover, even the sample preparation for contact measurements may modify the basic properties of the templates.

The present thesis aims at studying the feasibility of assessing thermal properties of nanostructures using an infrared thermography based non-contact techniques with the following specific objectives.

- Synthesis and structural characterization of one dimensional nanostructure with focus on AAO template assisted electrodeposited Bi2Te3 nanowires.
- Design and development of an infrared thermography based noncontact technique for thermal characterization of templated nanostructures in general and bismuth telluride nanocomposites in particular.
- Development of mathematical model and simulation background in support of the potential technique for thermal diffusivity measurements of AAO templated samples.

1.8 Thesis overview

The thesis, including results and observations of the present study is arranged in six chapters.

Chapter 1: Infrared thermography and nanomaterials: a review

An introduction to the world of infrared thermography and its potential applications and advantages are provided. It also includes a detailed discussion on the different approaches of implementing IR thermography for materials characterization. Modern applications of thermography at reduced dimensions are discussed, along with the technology involved. Study of thermoelectric phenomenon and associated materials are included. The details of AAO templated nanostructures and bismuth telluride nanowires are also discussed. Based on the literature survey and potential working area, the objective of the current research work is proposed.

Chapter 2: Synthesis and physical characterization of bismuth telluride nanowires

This chapter discusses synthesis and characterization of bismuth telluride nanowires reporting various approaches used for nanostructure synthesis. Details of bismuth telluride nanowires synthesized through AAO template assisted electrodeposition technique are presented and the inherent advantages highlighted [C8]. The electrodeposited nanowires are initially subjected to structural characterization using tools like XRD, EDX, SEM, and TEM, results of which are included. Efforts were made to get electrical characterization of a single Bi2Te3 nanowire leading to its aging study [P1]. This motivated further experiments through which measurements could be made while the nanowires are still inside the AAO matrix.

Chapter 3: Modeling, analysis and simulation study of thermal waves in templated nanomaterials

Knowledge of thermal properties of any material is of great importance for deciding on its industrial applications. Thermal conductivity and the thermal diffusivity are two closely related thermal properties. In this chapter, a mathematical background for an IR thermography based non-contact technique for in-plane thermal diffusivity measurement of thin plate nanomaterial (as AAO template), is developed. The idea is not to physically improve the spatial resolution to nano scale, rather to estimate the properties of nanostructures from measurements done at mm level. Discussion on simulation studies of the proposed technique, based on an electro-thermal model is also included [J4], in support. The influence of convective and radiative heat loss components on the final thermal diffusivity results are also studied through both experiments and simulations.

Chapter 4: Thermal characterization using infrared lock-in thermography

An active infrared lock-in thermography based non-contact technique is proposed and used for thermal diffusivity measurements of thin plate nanomaterials in general and AAO templated Bi2Te3 nanowires in particular [J1, C3, C6, C7]. The basic working principle and experimental details are discussed in this chapter. The thermal properties of nanowires alone are estimated from the measurements done on empty AAO template, single crystal Bi2Te3 and AAO/Bi2Te3 nanocomposite and then comparing the results with a prediction made by first order lower bound model [J2]. A fivefold reduction in the thermal diffusivity value of 200 nm Bi2Te3 nanowires is estimated as compared to their bulk counterpart, in the present study. Similar studies were also performed on Ni, Co / AAO nanocomposites. A nearly seven- and six fold reduction, respectively, of thermal diffusivity in direction perpendicular to the NW axis is estimated for the synthesized Ni and Co NWs.

Chapter 5: Frequency modulated thermal wave imaging for thermal diffusivity measurements

Conventional lock-in thermography based thermal characterization technique (discussed in chapter-4) demands experiments be done at different excitation frequencies independently. This brings in a limitation in terms of the time required for the complete experiment. A potential solution to this limitation is proposed in the form of frequency modulated thermal wave imaging technique [J3, C1, C2]. The principle of operation of the technique along with the experimental details is explained in this chapter. Further, the effect of chirp bandwidth and period on the final thermal diffusivity results is also studied. Practical measurement results from empty AAO templates as well as AAO/Bi2Te3 nanocomposites are included.

Chapter 6: Conclusions and potential future directions are highlighted.

1.9 References

[1] Xavier P. V. Maldague, Theory and Practice of Infrared Technology for Nondestructive testing, John Wiley & Sons, USA, 2001.

[2] D. P. Almond and P. Patel, Photothermal science and techniques, Chapman & Hall, London, UK, 1996.

[3] C. S. Welch, D. M. Heath and W. P. Winfree, “Remote measurement of in-plane diffusivity components in plates, “ J. Appl. Phys. 61, 895, 1987.

[4] T.E. Milner, D. M. Goodman, B.S. Tanenbaum, B. Anvari, and J. S. Nelson," Noncontact determination of thermal diffusivity in biomaterials using infrared imaging radiometry," J. Biomed. Opt. 1, 92, 1996.

[5] S. A. Telenkov, J. Youn, D. M. Goodman, A. J. Welch and T. E. Milner,” Non-contact measurement of thermal diffusivity in tissue,” Phys. Med. Biol. 46, 551, 2001.

[6] Peng-fei Qiu, Shu-yi Zhang, Su-zhen Wu, Zhao-jiang Chen, Zhong-ning Zhang and Xiu-ji Shu,"Characterisation of thermal property of materials by infrared thermography," Nondestructive Testing and Evaluation 25 (1), 91, 2010.

[7] J. M. Laskar, S. Bagavathiappan, M. Sardar, T. Jayakumar, John Philip, Baldev Raj,"Measurement of thermal diffusivity of solids using infrared thermography," Materials Letters 62, 2740, 2008.

[8] A. Wolf, P. Pohl, and R. Brendel,” Thermophysical analysis of thin films by lock-in thermography,” J. Appl. Phys. 96, 6306, 2004.

[9] M. Ångström, Philos. Mag. 25, 130, 1863.

[10] Arantza Mendioroz, Raquel Fuente-Dacal, Estibaliz Apiñaniz, and Agustín Salazar,

“Thermal diffusivity measurements of thin plates and filaments using lock-in thermography," Rev. Sci. Instrum. 80, 074904, 2009.

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Publications from the thesis

Refereed Journal

J1. Lalat Indu Giri, Suneet Tuli, Manish Sharma, Philippe Bugnon, Helmuth Berger, and Arnaud Magrez,” Thermal diffusivity measurements of templated nanocomposite using infrared thermography,” Materials Letters 115, 106, 2014.

J2. Lalat Indu Giri, Sachin Pathak, and Suneet Tuli, “Thermal Diffusivity of Electrodeposited Ni And Co Nanowires Using Infrared Thermography,” J. Phys. Chem. Solids 75, 921, 2014.

J3. Lalat Indu Giri and Suneet Tuli, “Frequency modulated infrared imaging for thermal characterization of nanomaterials,” Infrared Physics & Technology 67, 526, 2014.

J4. Lalat Indu Giri, Krishnendu Chatterjee and Suneet Tuli, “Thermal diffusivity of templated nanomaterials under convective and radiative loss components,” Under preparation.

International Conferences

C1. Lalat Indu Giri and Suneet Tuli, ”Thermal diffusivity of templated nanocomposite using frequency modulated infrared imaging”, SPIE Smart Structures/NDE 2014, San Diego, California, United States, 9 - 13 March 2014, Proc. SPIE 9062, 906216, 2014; doi:10.1117/12.2059046.

C2. Lalat Indu Giri and Suneet Tuli, ”Frequency modulated infrared imaging for thermal characterization of thin plate nanomaterials”, 3rd International Conference on Physics at Surfaces and Interfaces PSI 2014, Puri, Orissa India, 24-28 February 2014 (Accepted for POSTER).

C3. Lalat Indu Giri and Suneet Tuli, “ Thermal Diffusivity of Templated Nanomaterials”, Indo-US Workshop on Recent Advances in Micro/Nanoscale Heat Transfer and Applications in Clean Energy Technologies, Indian Institute of Technology Ropar, Rupnagar-140001, India, 21-22 December 2013 (POSTER).

C4. Priyanka Tyagi, Lalat Indu Giri, and Suneet Tuli,” Infrared Thermography for Nanoscale”, Asia Pacific Conference on Non-Destructive Testing APCNDT 2013, Mumbai, India, 18-22 November 2013 (Invited talk).

C5. Lalat Indu Giri, Manish Sharma, and Suneet Tuli,” Temperature Dependent Powerfactor of Thermoelectric Nanocomposite” 7th International Conference on Materials for Advanced Technologies ICMAT-2013, Suntec Singapore, 30 June – 05 July 2013 (POSTER).

C6. Lalat Indu Giri, Manish Sharma, and Suneet Tuli, ” Thermal Diffusivity Measurements of Thermoelectric Nanocomposite Using Infrared Thermography,” Materials Research Society (MRS) 2013 Spring Meeting, San Francisco, California,1-5 April 2013 (Abstract accepted for POSTER).

C7. Lalat Indu Giri, Pramod Kumar Sharma, Andleeb Zahra, Anwesha Bose, Manish Sharma, and Suneet Tuli,” Thermal Properties of Electrodeposited Bismuth Telluride Nanowires,” International Conference on Quantum Effects in Solids of Today (I-ConQuEST 2010), National Physical Laboratory, New Delhi, India, 20-23 December 2010 ( POSTER).

C8. Pramod Kumar Sharma, Andleeb Zahra, Lalat Indu Giri, and Manish Sharma, ”Electrodeposition of Bismuth Telluride Nanowires for Thermoelectric Applications,” XV International Workshop on the Physics of Semiconductor Devices IWPSD 2009, New Delh, India, 15-19 December 2009 (POSTER).

National Conferences

P1. Lalat Indu Giri, Manish Sharma, and Suneet Tuli,”Aging Study of Thermoelectric Bismuth Telluride Nanowires,” Sixth ISSS National Conference - 2013, Pune, India, 6-7 September 2013, Proc. paper no. 106 (POSTER).

P2. Lalat Indu Giri, Manish Sharma, and Suneet Tuli, ”Thermal diffusivity measurement of nanopore membranes,” National Seminar & Exhibition on Non-Destructive Evaluation (NDE 2011), Chennai, India, 8-10 December,2011, Proc. page no. 213-215 (ORAL PRESENTATION).

Chapter 2

Synthesis and physical characterization of bismuth telluride nanowires

2.1 Bi 2 Te 3 : One dimensional nanostructure

Nano-crystalline materials are attracting increasing attention from research community all over the world because of their novel properties and varied potential applications [1-3]. These nanostructures can be synthesized by a number of techniques, such as inert gas condensation, plasma processing, physical and chemical vapor deposition, electrodeposition, mechanical alloying, rapid solidification, Electrochemical Step Edge Decoration (ESED), Lithographical Patterned Nanowire Electrodeposition (LPNE), sol-gel, micro-emulsion, spark erosion, and severe plastic deformation [4]. Amongst them inert gas condensation, mechanical alloying, and the electrodeposition are the methods that are available for commercial use.

For bismuth telluride nanostructures numerous approaches for nanowires synthesis using both top-down and bottom-up approaches have been tried. Chemical vapor deposition (CVD), physical vapor deposition , thermal annealing, advanced nanolithography techniques such as electron beam (e-beam) or focused-ion-beam (FIB) writing, proximal probe patterning and X-ray or extreme-UV lithography are commonly used methods to synthesize nanowires [5,6]. However, in all these techniques inability for mass production and cost effectiveness is a great limitation. Recent years have seen wide spread application of electrodeposition technique for Bi2Te3 nanowire synthesis because of ease of the process, economic viability, scalability and the purity of the end products.

2.2 Electrodeposition of Bi 2 Te 3 nanowires

Electrodeposition is an established and versatile technique for nanomaterials preparation [7-9]. In this approach the starting material is in liquid state and it follows an AAO template assisted nanowire growth. These AAO templates shown in figure 2.1 (a) show remarkable characteristics for nanowire array synthesis through providing thermal and mechanical stability. The templates can also withstand high temperatures, are insoluble in organic solvents and their geometrical parameters like pore dimensions can be easily tuned by changing the synthesis conditions. It was preferred to grow bismuth telluride nanowires using the electrodeposition technique [10-14] as it provides good control over stoichiometry, can be used to deposit high aspect-ratio structures (having control over nanowire length), and results in electrically continuous nanowires. It also provides the unique possibility of tailoring the resulting alloy composition to form either a p-type or n-type Bi2Te3 sample by simply changing the deposition potential or the electrolyte composition [15], which can be utilized for realizing a practical device module using nanowires. Some of the other advantages include low cost and simplicity of the system. Importantly it permits easy adjusting of material composition by controlling the deposition potential or current.

The electrochemical assembly of Bi2Te3 nanowire arrays was synthesized using a three electrode device EG&G PAR model 273 potentiostat / galvanostat with empty AAO template as the working electrode, platinum counter electrode and saturated calomel electrode (SCE) as the reference electrode, as shown in figure 2.1(b). The different steps involved in the nanowires synthesis are the electrolyte and working electrode preparation followed by the two electrodeposition process.

2.2.1 Electrolyte preparation

Bismuth (III) nitrate pentahydrate Bi (NO3)3.5H2O and tellurium powder (Aldrich) were used to prepare the deposition solution according to the reported procedure [10, 11]. Tellurium powder (calculated amount) was first dissolved in 1.0 M nitric acid (300 mL) at 80 oC with vigorous stirring in a fume hood. Initially some sediments of Te powder observed at the bottom of the beaker which dissolved completely after some time. Bi (NO3)3.5H2O was then added to have the final solution (starting material) for the electrodeposition process. This Bi[3]+/ HTeO2+ solution was then stored in air tight container under ambient laboratory conditions until further use. Before use, the electrolytes were systematically degassed by an argon flux for 20 min in order to remove dissolved oxygen.

2.2.2 Working electrode preparation

Anopore alumina membranes (20nm, 100nm and 200nm pore diameter) Anodisc 25 were purchased from Whatman and used as received without any modification. A gold film (~ 150nm thick) was sputtered onto one side of the AAO templates and plain glass slides. The Au sides of both the AAO and glass slide were pasted using commercial nail polish. A drop of DI water was used to ensure proper contact between the surfaces, which vaporized later on. The whole template assembly was then washed with DI water. The AAO portion of the assembly was then dipped into 1 M HNO3 solution for an hour to have proper pore openings and to ensure good deposition into the pores.

2.2.3 Electrodeposition process

Electrodeposition process primarily involves two different steps namely the cyclic voltammetry (CV) and the chronoampereometry (CA). CV was carried out on the electrolyte solution to identify the deposition potential. Figure 2.2 below shows the CV plot with reference to a quasi reference electrode. CA is then done with the same reference electrode immediately after the cyclic voltammetry to avoid any change of deposition potential.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2.1 Schematic of the (a) empty AAO template (b) electrodeposition system used for the Bi 2 Te 3 nanowire growth.

First the cyclic voltammetry (CV) of the deposition solution containing Bi[3]+/HTeO[2]+ was done in the window of -0.9 volts to +0.6 volts. From the CV plot electrodeposition potential was identified within the reduction cycle. A distinct oxidation and reduction cycles for the deposition solution was observed as shown in figure 2.2 (a). The deposition potential was chosen just close to the onset of the reduction cycle.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2.2 (a) Cyclic voltammetry (CV) and (b) Chronoampereometry (CA) curves from the electrodeposition process with respect to a quasi-reference electrode.

After CV process the actual synthesis of the nanowires was done through the CA method at the selected potential of -0.28 volts. After electrodeposition process the samples were kept in vacuum condition till further characterization.

2.3 Physical characterization

Bismuth telluride nanowires were synthesized using the electrodeposition process, with the aim of observing the change in thermal diffusivity of the nanowires as compared to its bulk counterpart. Before thermal characterization, the nanowires were examined through various physical characterization tools to study their chemical composition, crystallinity, morphology and dimensions etc.

2.3.1 X-ray diffraction (XRD) study

The crystal structures of the nanowires were studied using XRD.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2.3 X-ray diffraction pattern of electrodeposited AAO/Bi 2 Te 3 nanowires matrix.

After electrodeposition the over deposited Bi2Te3 were mechanically removed. X-ray diffraction measurement was performed on nanowires embedded in the AAO membranes with the BRUKER D8 Advance with a α-Cu X-ray source (λ = 1.54182 Å). Figure 2.3 shows the XRD pattern of one of the Bi2Te3 samples. The XRD pattern exhibits polycrystalline bismuth telluride in the as deposited state with (110) as the prominent plane parallel to the substrate. The strong (110) peak suggests that the c-axis is parallel to the film surface and perpendicular to the axis of the nanowire, respectively. All of the detected peaks are indexed as those from the rhombohedral Bi2Te3 crystal {space group R 3 m} with hexagonal crystal structures.

2.3.2 Energy dispersive X-ray study (EDX)

Energy dispersive X-ray spectroscopy was used for the elemental analysis of the electrodeposited Bi2Te3 nanowire samples. The nanowires were etched out of the AAO template using NaOH solution for the EDX sample preparation.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2.4 EDX result of electrodeposited Bi 2 Te 3 nanowires.

Above figure 2.4 shows the EDX result from one batch of electrodeposition, which shows the presence of both Bi and Te. A quantitative analysis of the spectrum indicates that the Bi: Te atomic ratio is close to 2:3. In addition to Bi and Te, fractions of other elements as C, O, Na and Au are also shown in the EDX spectra which might have got associated during the AAO etching and EDX/SEM sample preparation process steps.

2.3.3 Scanning electron microscopy (SEM)

The sample's surface topography and structure were studied using the SEM. The conventional sample preparation approach was implemented wherein the electrodeposited nanowires were first treated with 5 M NaOH solution for AAO removal. The samples were then washed with multiple changes of DI water and ethanol before finally mounting on the SEM stub using a carbon tape. A thin gold layer was sputtered on the samples before being imaged. The nanowire dimension especially the diameter could be well studied from the SEM images.

Abbildung in dieser Leseprobe nicht enthalten

Figure 2.5 SEM images of electrodeposited Bi 2 Te 3 nanowires.

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