DEVELOPMENT OF MEMS THERMOPILES AND RELATED APPLICATIONS

Ph.DDOCTOR OF PHILOSOPH

Thermal and Electrical Transport in Ferromagnetic Metal Thin Films

The recent emergence of spin caloritronics has focused considerable attention on the interplay between spin, charge, and temperature gradients in magnetic materials. A reliable and energy efficient method for generating pure spin currents would signify an important step toward future spin-based nano-electronics that may offer lower power consumption and greater processing capabilities. To develop new technology using thermoelectric effects in magnetic thin films, it is essential to understand thermal and electrical transport through these films. One possible source of pure spin currents is the so-called spin Seebeck effect (SSE) in which a thermal gradient (∇T) applied to a ferromagnet is thought to produce a pure spin current detectable by measuring a transverse voltage (VT) generated by the inverse spin Hall effect. However, recent work on spin-dependent transport in thin film nanostructures supported by bulk substrates has underscored the difficulty in understanding thermal gradients in these systems due to uncertainty in the direction of the applied thermal gradient through a substrate with a thermal conductance several orders of magnitude larger than the sample conductance. These results suggest that early SSE experiments may have been strongly affected by other effects such as the anomalous Nernst effect. They may also have been affected by thermoelectric effects generated from planar thermal gradients such as the planar Nernst effect which develops a VT in a film with a planar thermal gradient and magnetization. In this dissertation, I introduce the concepts of thermal conductivity, the Wiedemann-Franz law, and thermoelectric effects including the Seebeck effect, the Peltier effect, and the planar Nernst effect (PNE). Next, I describe our experimental method for measuring thermal and electrical transport in non magnetic and ferromagnetic metallic thin films using suspended Si-N membrane structures. Our membrane method reduces the background thermal conductance contribution by 5 orders of magnitude when compared with similar experiments conducted on thin films supported by bulk substrates. This confinement to the plane of the platform and film ensures a thermal gradient in the x- or y-direction only. The experiment therefore enables exploration of thermoelectric effects in a completely 2-D configuration. Next, I present results of several experiments probing thermal conductivity and the Lorenz number in thin films. Both the thermal conductivity and electrical conductivity of metallic thin films is lower that bulk values from literature. The deviation of the Lorenz number from the theoretically predicted Sommerfeld value in all films indicates imbalances between the heat and charge currents in the films from scattering or additional thermal conductivity contributions from magnons and phonons. I also present results from experiments measuring the Seebeck effect or thermopower, and anisotropic magnetoresistance in ferromagnetic thin films. In these films, the thermopower scales with resistance as predicted by the Mott equation, and the magnetic field dependence of the thermopower results from the same spin-dependent scattering responsible for the AMR. I present the first results from experiments designed to probe the PNE and related effects such as the SSE in ferromagnetic thin films. The results share features previously attributed to the SSE such as linear delta T dependence and sign reversal on hot and cold sides of the sample, however, the voltage generated transverse to the applied thermal gradient is always even in applied field due to spin-dependent scattering. The data display a sinθcosθ angular dependence predicted by the PNE rather than the cosθ angular dependence expected from the SSE. In these experiments, we observe no evidence of a thermally generated spin current, and the upper limit on the SSE coefficient in our experiment is 15-30 times smaller than previously reported by experiments conducted using bulk substrates. Finally, I present first results from experiments designed to measure the Peltier effect in thin films and test the interdependence between the Peltier and Seebeck effects predicted by Onsager reciprocal relations. These are the first measurements of the Peltier effect and Onsager reciprocity in ferromagnetic thin films near room temperature, and are an important step to confirm the validity of the theoretically predicted Onsager reciprocity in these systems

Electrical and thermal transport coefficients of CoFe thin films deposited on a microcalorimeter

Interaction of spin, charge and heat currents in micro/nano structures opens opportunities for improving the performance of thermoelectric devices and future generation memory devices. For example, in magnetic insulators, coupled interaction of magnons with heat current generates the so called spin-Seebeck effect while magnon degree of freedom is utilized to transmit and process information in the sub-field of magnonics. In parallel, in magnetic metals, the charge carrying electrons interact with magnons and phonons to generate effects such as spin transfer torque and spin-dependent Seebeck effect. In the heart of the research on future generation thermoelectric and memory devices, lies the interplay of electrons, phonons and magnons along with external parameters of magnetization and thermal current. In this thesis, the roles played by the electrons, phonons and magnons in generating various electrical, thermal and magneto-thermal transport properties has been studied in ferromagnetic alloy thin films. Emphasis has been given to the development and utilization of SiN membrane based suspended microcalorimeter. In CoFe thin films, resistivity, thermopower and thermal conductivity were measured in the temperature range of 25 K-296 K. Contribution of electron scattering from phonons and magnons to these transport coefficients has been separated. The change of sign of the magnon drag contribution to thermopower from Co-rich to Fe-rich side of the material has been discussed. Additionally simultaneous measurement of resistivity and thermal conductivity has facilitated studying the validity of Wiedemann-Franz law in these films. In a second alloy of Permalloy, observation of transverse magneto-thermoelectric effects in the presence of in-plane temperature gradient and magnetization has been discussed. Various competing effects have been separated depending on their angular dependencies. Together with the results from bulk substrates (see PhD thesis by M. Schmid), the existence of transverse spin Seebeck effect has been discarded

Transition metal oxides - Thermoelectric properties

Transition metal oxides (TMOs) are a fascinating class of materials due to their wide ranging electronic, chemical and mechanical properties. Additionally, they are gaining increasing attention for their thermoelectric (TE) properties due to their high temperature stability, tunable electronic and phonon transport properties and well established synthesis techniques. In this article, we review TE TMOs at cryogenic, ambient and high temperatures. An overview of strategies used for morphological, compositing and stoichiometric tuning of their key TE parameters is presented. This article also provides an outlook on the current and future prospects of implementing TMOs for a wide range of TE applications

Self-Powered Infrared Detection in Low-Dimensional Carbon Assemblies

Room-temperature mid-infrared photodetection meet upcoming demands including real-time health condition monitoring, low-cost industrial inspection and distributive sensing for Internet-of-Things. Photo-thermoelectric (PTE) effect is a bandgap limitless photodetection mechanism which utilizes photons induced thermoelectric effect at material interfaces. The 1/f noise and shot noise in dark current can be significantly reduced in a zero-biased PTE detector. Carbon nanotubes (CNTs) and graphene are emerging low-dimensional materials with excellent PTE properties. Besides the strong and broadband light-matter interaction, their increased electrical to thermal conductivity ratio and electron density-of-states dependence on energy also lead to enhanced thermoelectric conversion efficiency. In this thesis, we present two self-powered PTE detection architectures. In the first one, vertical photo-thermoelectric effect of an anti-reflecting carbon nanotube forest (CNTF) is employed in a broadband mid-infrared detector. 99.4% average reflection suppression in the CNTF at 2.5~25 µm spectral range enables responsivity of 6 V W-1 and detectivity of 2.2×107 cm Hz1/2 W-1 under very weak illumination power, rendering sensitive weak infrared photodetection in real life. Top-electrode material, thickness and patterns are systematically studied related to the PTE response, and further improvement is possible by increasing the CNTF height and reducing the photosensitive area. In the second architecture, CNTs/Poly vinyl alcohol (PVA) composite based planar photodetector with asymmetric metallic electrodes is investigated. PTE voltage response is optimized via mixing 25 wt.% CNTs into PVA matrix attributed to the enhanced phonon scattering at CNTs/PVA interfaces. Moreover, crystallization of PVA around CNTs networks contributes to a rather stable photoresponse (variation < 4 %) under significant bending down to a 3.5 mm radius. This flexible, wearable photodetector also proves preliminary passive imaging of human body radiation. Finally, a unique and facile fabrication technique is demonstrated for the integration of a flexible, semi-transparent photodetector based on graphene nanoplatelets/PEDOT: PSS composite. This photodetector exhibits enhanced PTE response, high flexibility, and good optical transparency at a low loading of graphene

Broadband Photodetection in Graphene

Graphene, a single-atom-thick plane of carbon, has unique optoelectronic properties that result in a variety of potential photonic applications, such as optical modulators, plasmonic devices and THz emitters. In this thesis, the light-matter interaction in monolayer graphene and the subsequent photoexcited charge carrier transport are studied, and it is found that graphene has unique advantages for hot-electron photothermoelectric detection. Particularly promising is detection of terahertz (THz) radiation, in which graphene devices may offer significant advantages over existing technology in terms of speed and sensitivity. By using a tilted angle shadow evaporation technique, bi-metal contacted graphene photodetectors are realized experimentally. Efficient photodetection via the hot-electron photothermoelectric effect is demonstrated at room temperature across a broad frequency range (THz to near infrared). For THz detection, the best device shows sensitivity exceeding 10 V/W (700 V/W) and noise equivalent power less than 1100 pW/Hz1/2 (20 pW/Hz1/2), referenced to the incident (absorbed) power, implying a performance competitive with the best room-temperature THz detectors for an optimally absorbing device, while time-resolved measurements indicate that the graphene detector is eight to nine orders of magnitude faster than those. To increase the absorption and quantum efficiency, large area epitaxial graphene micro-ribbon array photodetectors are designed for resonant plasmon excitation in the THz range. By tailoring the orientation of the graphene ribbons with respect to an array of sub-wavelength bimetallic electrodes, a condition is achieved in which the plasmonic mode can be efficiently excited by an incident wave polarized perpendicular to the electrode array. The sensitivity of the detector is enhanced when the plasmon resonance frequency, which is tunable by adjusting the gate voltage, matches with the frequency of the incident radiation

Broadband Photodetection in Graphene

Graphene, a single-atom-thick plane of carbon, has unique optoelectronic properties that result in a variety of potential photonic applications, such as optical modulators, plasmonic devices and THz emitters. In this thesis, the light-matter interaction in monolayer graphene and the subsequent photoexcited charge carrier transport are studied, and it is found that graphene has unique advantages for hot-electron photothermoelectric detection. Particularly promising is detection of terahertz (THz) radiation, in which graphene devices may offer significant advantages over existing technology in terms of speed and sensitivity. By using a tilted angle shadow evaporation technique, bi-metal contacted graphene photodetectors are realized experimentally. Efficient photodetection via the hot-electron photothermoelectric effect is demonstrated at room temperature across a broad frequency range (THz to near infrared). For THz detection, the best device shows sensitivity exceeding 10 V/W (700 V/W) and noise equivalent power less than 1100 pW/Hz1/2 (20 pW/Hz1/2), referenced to the incident (absorbed) power, implying a performance competitive with the best room-temperature THz detectors for an optimally absorbing device, while time-resolved measurements indicate that the graphene detector is eight to nine orders of magnitude faster than those. To increase the absorption and quantum efficiency, large area epitaxial graphene micro-ribbon array photodetectors are designed for resonant plasmon excitation in the THz range. By tailoring the orientation of the graphene ribbons with respect to an array of sub-wavelength bimetallic electrodes, a condition is achieved in which the plasmonic mode can be efficiently excited by an incident wave polarized perpendicular to the electrode array. The sensitivity of the detector is enhanced when the plasmon resonance frequency, which is tunable by adjusting the gate voltage, matches with the frequency of the incident radiation

Energy storage and generation from thermopower waves

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references.The nonlinear coupling between an exothermic chemical reaction and a nanowire or nanotube with large axial heat conduction guides a self-propagating thermal wave along the nano-conduit. The thermal conduit accelerates the wave by rapidly transporting energy to un-reacted fuel. The reaction wave induces what we term a thermopower wave, resulting in an electrical current in the same direction. At up to 7 W/g, peak power density is larger than that of many present micro-scale power sources (e.g. fuel cells, batteries) and even about seven times greater than commercial Li-ion batteries. Thermopower waves also tend to produce unipolar voltage pulses, although conventional thermoelectric theory predicts bipolar voltage. These waves also generate thermopower in excess of previous measurements in carbon nanotubes (CNTs) and therefore could increase figures of merit in a variety of thermoelectric materials. In this thesis, I have developed the theoretical framework to describe the thermal and chemical profiles of propagating reaction waves, and their electrical properties. My analysis yielded a new analytical solution for one-dimensional reaction and thermal diffusion systems with nth order kinetics that obviates many approximate or numerical approaches from the past 80 years. A generalized logistic. function describes the temperature and concentration profiles within the solid fuel and provides a solution for the wave velocity for a wide range of conditions. This approach offers new insight into such problems spanning several fields in science and engineering, including propulsion and self-propagating high temperature synthesis (SHS) of materials, as well as the dynamics of thermopower waves. Temperature and voltage measurements of thermopower waves on CNTs show that they can generate power as much as four times greater than predictions based on reference measurements of the Seebeck coefficient for static temperature gradients. We hypothesize that the excess thermopower stems from a chemical potential gradient across the CNTs. The fuel (e.g. picramide) adsorbs and dopes the CNTs ahead of the wave and desorbs and reacts behind the wave front. Furthermore, the excess thermopower depends on the mass of fuel added (relative to CNT mass), and the chemical potential difference matches the magnitude of the excess thermopower. Thus, a major conclusion of this thesis is that coupling to a chemical reaction can boost the performance of thermoelectric materials through differential doping. Thermopower waves can have well defined velocity oscillations for certain kinetic and thermal parameter values. Cyclotrimethylene-trinitramine (fuel) on multiwalled CNTs (conduit) system generates voltage oscillations of 400 to 5000 Hz. These frequencies agree with velocity oscillations predicted by my thermochemical model of the reaction wave, extended to include thermal transport within the conduits. Thermopower waves could thus find applications as new types of alternating current (AC) batteries and self-powered signal generators, which could easily be miniaturized. Microelectromechanical systems and sensors would benefit from thermopower wave generators to enable functions such as communications and acceleration that currently require large power packs. Additionally, the "self-discharge" rate of thermopower wave generators is extremely low in contrast to electrochemical storage, since their energy is stored in chemical bonds. Thermopower waves thus enable new energy storage devices and could exceed limitations of conventional thermoelectric devices.by Joel T. Abrahamson.Ph.D

Investigation of thermopower waves based energy sources

Miniaturisation of energy sources is critical for the development of the next generation electronic devices. However, reduction in dimensions of none of the commonly used energy generation technologies including batteries, fuel cells, heat engines and supercapacitors have resulted in efficient and reliable energy sources with high specific powers (power-to-mass ratio). Recently, the new concept of energy generation based on thermopower waves has shown promise for miniaturization. In such sources, exothermic chemical reactions of a reactive fuel are coupled to charge carriers of a thermoelectric (TE) material in its affinity, resulting in an intense thermal wave that self-propagates along the surface of the TE materials. This wave simultaneously entrains charge carriers, resulting in a large current. If the TE material also has a high Seebeck coefficient, a large output voltage and subsequently large specific power output are obtained. As the thermal wave results in a power output, it is called a thermopower wave. In the first stage of the PhD research, the author demonstrated thermopower wave systems based on thin films of Bi 2 Te 3 . Bi 2 Te 3 was implemented due to its high S (~ –200 μV/K) and σ (10 5 S/m). As Bi 2 Te 3 exhibits a low κ , the author devised a novel strategy by placing it on thermally conductive alumina (Al 2 O 3 ) substrate to compensate for this deficiency. The Bi 2 Te 3 based thermopower wave sources generated voltages and oscillations higher (at least 150 %) than the previously reported multi-walled carbon nanotube (MWNT) based thermopower wave sources, while maintaining a high specific power in the order of 1 kW/kg. In the second stage, the author implemented a novel combination of p-type Sb 2 Te 3 and n-type Bi 2 Te 3 as the core TE materials with complimentary semiconducting properties, to show the generation of voltage signals with alternating polarities. In the third stage, the author implemented zinc oxide (ZnO), which is a TE transition metal oxide (TMO), for the first time as the core material in thermopower wave sources. It was shown that both S (~ –500 μV/K at 300 °C) and σ (~ 4×10 3 S/m at 300 °C) of ZnO increased at elevated temperatures. By incorporating ZnO as the core TE material, the PhD candidate obtained voltages and oscillation amplitudes at least 200 % higher than any previously demonstrated thermopower wave systems (in the order of &amp;gt; 500mV), while maintaining a high specific power (~ 0.5 kW/kg). In the final stage, in order to exceed voltages larger than 1 V, the PhD candidate identified that manganese dioxide (MnO 2 ), which is another TE TMO, can exhibit exceptionally large S and moderate σ at elevated temperatures. As a result, the author implemented MnO 2 as the core TE material. It was shown that the S of MnO 2 increased dramatically with temperature, exhibiting a peak value of approximately –1900 μV/K at 350 °C. Consequently, voltages large enough (~1.8 V) to drive small electronic circuits were obtained, while maintaining high specific powers in the order of 1 kW/kg

Developing high-efficiency multiphase thermoelectric materials

This thesis explores strategies for improving the efficiency of thermoelectric materials, with a particular focus on multiphase bismuth chalcogenide compounds. The introduction and literature review provide the necessary background on thermoelectricity and its applications, define key parameters such as the dimensionless figure of merit zT, and outline established methods for optimising single and multiphase thermoelectric materials. The literature review chapter delves into important concepts and mechanisms such as energy filtering, modulation doping, and phonon scattering in multiphase systems. The experimental methods chapter then outlines the different material synthesis techniques used, such as melting, ball milling, and spark plasma sintering, as well as the analytical approaches used to study the materials, including structural, electronic, and thermal transport characterisation. The subsequent results chapters examine the effects of incorporating magnetic dopants and secondary phases into bismuth sulphide, telluride, and selenide host systems. A notable finding was that magnetic co-doping of Bi2S3 with chromium and chlorine significantly increased the thermopower and power factor, attributed to a magnetic drag effect that increases the effective carrier mass. The addition of a Bi14Te13S8 secondary phase to Bi2Te3 matrix compounds was also investigated; the presence of this phase led to an energy filtering effect that improved the thermopower but also introduced additional phonon scattering at phase interfaces that reduced the lattice thermal conductivity. Further studies of sulphur-containing Bi2Te2.7Se0.3 revealed that sulphur inclusion dramatically alters the density of states and native defect concentrations in both single and multiphase samples. Interestingly, multiphase Bi2Te2.7Se0.3 samples exhibited complex electronic behaviour, suggesting possible impurity band formation at higher secondary phase contents. Further investigations Bi0.5Sb1.5Te3 with added CrSb compounds showed that small amounts of the magnetic secondary phase increased the thermopower via an increased effective mass, but higher CrSb contents degraded the performance due to reduced carrier mobility. Finally, iodine doping of single phase Bi14Te13S8, an important component of the multiphase materials studied, was found to effectively optimise the power factor and reduce the lattice thermal conductivity, culminating in an improved figure of merit zT. In summary, this work provides compelling evidence that strategies such as energy filtering, modulation doping, and phonon scattering can be successfully exploited to improve the efficiency of multiphase bismuth chalcogenide thermoelectric materials. The results provide valuable insights to guide ongoing research and development efforts towards higher performance thermoelectric materials for real-world applications