Semiconductor Nanowires for Thermoelectric Applications

Ph.DDOCTOR OF PHILOSOPH

Improving processor efficiency through thermal modeling and runtime management of hybrid cooling strategies

One of the main challenges in building future high performance systems is the ability to maintain safe on-chip temperatures in presence of high power densities. Handling such high power densities necessitates novel cooling solutions that are significantly more efficient than their existing counterparts. A number of advanced cooling methods have been proposed to address the temperature problem in processors. However, tradeoffs exist between performance, cost, and efficiency of those cooling methods, and these tradeoffs depend on the target system properties. Hence, a single cooling solution satisfying optimum conditions for any arbitrary system does not exist. This thesis claims that in order to reach exascale computing, a dramatic improvement in energy efficiency is needed, and achieving this improvement requires a temperature-centric co-design of the cooling and computing subsystems. Such co-design requires detailed system-level thermal modeling, design-time optimization, and runtime management techniques that are aware of the underlying processor architecture and application requirements. To this end, this thesis first proposes compact thermal modeling methods to characterize the complex thermal behavior of cutting-edge cooling solutions, mainly Phase Change Material (PCM)-based cooling, liquid cooling, and thermoelectric cooling (TEC), as well as hybrid designs involving a combination of these. The proposed models are modular and they enable fast and accurate exploration of a large design space. Comparisons against multi-physics simulations and measurements on testbeds validate the accuracy of our models (resulting in less than 1C error on average) and demonstrate significant reductions in simulation time (up to four orders of magnitude shorter simulation times). This thesis then introduces temperature-aware optimization techniques to maximize energy efficiency of a given system as a whole (including computing and cooling energy). The proposed optimization techniques approach the temperature problem from various angles, tackling major sources of inefficiency. One important angle is to understand the application power and performance characteristics and to design management techniques to match them. For workloads that require short bursts of intense parallel computation, we propose using PCM-based cooling in cooperation with a novel Adaptive Sprinting technique. By tracking the PCM state and incorporating this information during runtime decisions, Adaptive Sprinting utilizes the PCM heat storage capability more efficiently, achieving 29\% performance improvement compared to existing sprinting policies. In addition to the application characteristics, high heterogeneity in on-chip heat distribution is an important factor affecting efficiency. Hot spots occur on different locations of the chip with varying intensities; thus, designing a uniform cooling solution to handle worst-case hot spots significantly reduces the cooling efficiency. The hybrid cooling techniques proposed as part of this thesis address this issue by combining the strengths of different cooling methods and localizing the cooling effort over hot spots. Specifically, the thesis introduces LoCool, a cooling system optimizer that minimizes cooling power under temperature constraints for hybrid-cooled systems using TECs and liquid cooling. Finally, the scope of this work is not limited to existing advanced cooling solutions, but it also extends to emerging technologies and their potential benefits and tradeoffs. One such technology is integrated flow cell array, where fuel cells are pumped through microchannels, providing both cooling and on-chip power generation. This thesis explores a broad range of design parameters including maximum chip temperature, leakage power, and generated power for flow cell arrays in order to maximize the benefits of integrating this technology with computing systems. Through thermal modeling and runtime management techniques, and by exploring the design space of emerging cooling solutions, this thesis provides significant improvements in processor energy efficiency.2018-07-09T00:00:00

High-density thermoelectric power generation and nanoscale thermal metrology

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (p. 299-305).Thermoelectric power generation has been around for over 50 years but has seen very little large scale implementation due to the inherently low efficiencies and powers available from known materials. Recent material advances appear to have improved the technology's prospects. In this work we show that significantly increased generated power densities are possible even for established material technologies provided that parasitic losses are controlled and effective strategies are found for handling the large resulting heat fluxes. We optimize the performance of a thermoelectric generator in this regime, and discuss fundamental performance limits in this context. We present a design of a thermoelectric generator using conventional material and a microchannel heat sink that we predict can generate many times the power of a conventional thermoelectric, at a comparable efficiency. A high temperature vacuum test station is used to characterize the power generation, efficiency, and material properties of thermoelectric materials and generators. The results of a series of studies on various bulk and thin-film materials are presented, as well as packaged generator performance. The method of CCD thermoreflectance imaging is pursued in this thesis as a quantitative means for making noncontact temperature measurements on solid-state samples at the micro- and nano-scale. We develop and test a theory of the instrument and the measurement process to rigorously characterize the accuracy and precision of the resulting thermal images. We experimentally demonstrate sub-micron spatial resolution and sub-20 mK temperature resolution with this tool. High-resolution thermal images of thermoelectric elements, polysilicon-gate field effect transistors, and other integrated electronic devices are presented.by Peter M. Mayer.Ph.D

2020 NASA Technology Taxonomy

This document is an update (new photos used) of the PDF version of the 2020 NASA Technology Taxonomy that will be available to download on the OCT Public Website. The updated 2020 NASA Technology Taxonomy, or "technology dictionary", uses a technology discipline based approach that realigns like-technologies independent of their application within the NASA mission portfolio. This tool is meant to serve as a common technology discipline-based communication tool across the agency and with its partners in other government agencies, academia, industry, and across the world

2차원 금속 칼코겐 화합물의 합성과 열 및 전하 운송 특성에 대한 연구

학위논문(박사)--서울대학교 대학원 :공과대학 화학생물공학부,2020. 2. 정인.Both the rapid growth of the global population and environmental problems ask to develop sustainably and environmentally friendly alternative energy resources. Thermoelectric technology is a good candidate because it directly converts heat into electric energy without releasing undesirable gaseous chemical residues. In order for this technology to be practically and broadly applied, the conversion efficiency of thermoelectrics should be improved, which is a main interest in the field. In this dissertation, I will discuss the new thermoelectric materials and their thermoelectric properties. Among many thermoelectric materials, I will focus on the Pb and Cl introduced n-type SnSe and Cu doped n-type Bi2Te3 based compounds. In addition, I discuss the performances of the thermoelectric device with a new metallization layer for minimal energy loss. First, I introduce both Cl and PbSe to induce n-type conduction in intrinsic p-type SnSe. PbSe alloying enhances power factor and suppresses lattice thermal conductivity at the same time, giving a highest thermoelectric figure of merit ZT of 1.2 at 823 K for n-type polycrystalline SnSe materials. The best composition is Sn0.90Pb0.15Se0.95Cl0.05. Samples prepared by solid state reaction show a high maximum ZT (ZTmax) ~1.1 and ~0.8 parallel and perpendicular to the press direction of spark plasma sintering, respectively. Remarkably, post ball-mill and annealing processes considerably reduce structural anisotropy, thereby leading to a ZTmax ~1.2 along both the directions. Hence, the direction giving a ZTmax is controllable for this system using the specialized preparation methods for specimens. Spherical aberration-corrected scanning transmission electron microscopic analyses reveal the presence of heavily dense edge dislocations and strain fields, not observed in the p-type counterparts, which contribute to decreasing lattice thermal conductivity. Second, I report an ultrahigh carrier mobility ∼467 cm2 V−1 s−1 and power factor ~45 μW cm−1 K−2 in a new n-type Bi2Te3 system with the nominal composition CuxBi2Te3.17 (x = 0.02, 0.04, and 0.06). It is obtained by reacting Bi2Te3 with surplus Cu and Te and subsequently pressing powder products by spark plasma sintering (SPS). SPS discharges excess Te but stabilizes the high extent of Cu in the structure, giving unique SPS CuxBi2Te3.17 samples. The analyzed composition is close to CuxBi2Te3. Their charge transport properties are highly unusual. Hall carrier concentration and mobility simultaneously increase with the higher mole fraction of Cu contrary to the typical carrier scattering mechanism. As a consequence, the electrical conductivity is considerably enhanced with Cu incorporation. The Seebeck coefficient is nearly unchanged by increasing the Cu content in contrast to the general understanding of inverse relationship between electrical conductivity and Seebeck coefficient. These effects synergistically lead to a record high power factor among all polycrystalline n-type Bi2Te3-based materials. Third, the most reported thermoelectric modules suffer from considerable power loss due to high electrical and thermal resistivity arising at the interface between thermoelectric legs and metallic contacts. I devised the metallization layer of Fe-Ni alloy seamlessly securing skutterudite materials and metallic electrodes, allowing for a minimal loss of energy transferred from the former. It is applied to an 8 couple thermoelectric module that consists of n-type (Mm,Sm)yCo4Sb12 (ZTmax = 0.9) and p-type DDyFe3CoSb12 (ZTmax = 0.7) skutterudite materials. It performs as a diffusion barrier suppressing chemical reactions to produce a secondary phase at the interface. Consequent high thermal stability of the module results in the lowest reported electrical contact resistivity of 2.2–2.5 μC cm2 and one of the highest thermoelectric power density of 2.1 W cm–2 for a temperature difference of 570 K. Employing a scanning transmission electron microscope equipped with an energy dispersive X-ray spectroscope detector, we confirmed that it is negligible for atomic diffusion across the interface and resulting formation of a detrimental secondary phase to energy transfer and thermal stability of the thermoelectric module.최근 세계 인구의 빠른 성장과 심각한 환경 오염 문제로 인해 지속 가능하고 환경 친화적인 대체 에너지 기술 개발이 요구 되고 있다. 열전 기술은 원하지 않는 화학 잔류물을 방출하지 않고 열을 전기 에너지로 직접 변환할 수 있는 좋은 대체 에너지 기술 중 하나이다. 해당 기술이 보다 실용적이고 광범위하게 적용되기 위해서는 열전 소재의 성능 향상이 필수적이다. 본 학위 논문은 고효율 열전 소재 개발을 위한 새로운 합성법 개발 및 개발된 소재의 열전 특성에 대하여 기술하였다. 본 논문은 보고된 다양한 열전 소재 중에서 n형 SnSe 와 n형 Bi2Te3 소재에 대한 연구 및 새로운 금속 층이 도입된 열전 소자의 효율에 대해서 논의하고자 한다. 먼저, 고유한 p형 전도 특성을 보이는 SnSe 반도체에 n형 전도 특성을 유도하기 위해 Cl과 PbSe를 동시에 도입하였다. PbSe 합금은 역률(power factor)을 향상시킬 뿐만 아니라 동시에 격자 열전도도를 억제하여 823 K 에서 1.2의 최고 ZT 값을 얻을 수 있었다. 놀랍게도, 추가적인 볼밀 및 어닐링 공정을 통해서 구조적 이방성을 상당히 감소시킬 수 있었고, 그 결과 양방향에서 비슷한 ZTmax ~1.2 를 보이는 것을 확인하였다. 즉, 해당 시스템에서 높은 열전 효율을 보이는 방향을 특별한 합성 방법을 통해 조절할 수 있었다. 또한, 구면수차보정 주사투과전자현미경 분석을 통해서 격자 열전도도를 낮추는데 기여하는 매우 조밀한 edge dislocation과 strain field 를 관측할 수 있었다. 두번째로, 새로운 n 형 Bi2Te3 시스템에서 전하 이동도가 467 cm2 V-1 s-1 및 역률이 ~ 45 μW cm-1 K-2 값을 가지는 소재를 개발하였다. 해당 소재는 Bi2Te3와 과량의 Cu 및 Te을 함께 반응시킨 후 스파크 플라즈마 소결 (SPS) 장비를 통해 분말 생성물을 가압함으로써 얻을 수 있다. SPS 과정에서 과량의 Te은 방출되지만 높은 함량의 Cu를 구조 내에 안정화 시키는 중요한 역할을 한다. 그 결과 분석된 조성은 "CuxBi2Te3"에 가까우며, 독특한 전하 운송 특성을 보인다. 전형적인 전하 산란 메커니즘과는 다르게 전하 농도 및 이동도가 Cu의 함량에 따라 동시에 증가하는 것을 확인하였다. 그 결과, 전기 전도도가 Cu 도입을 통해 상당히 향상되었다. 하지만 전기 전도도와 제벡 계수 사이의 일반적인 이해와는 반대로 Cu 함량이 증가함에 따라 제벡 계수는 거의 변하지 않았다. 이러한 효과들이 협력적으로 작용하여 매우 높은 곡률을 얻을 수 있었으며, 이 값은 다결정 n형 비스무스 텔루라이드 중 가장 높은 값이다. 마지막으로, 보고 된 대부분의 열전 소자의 경우 열전 소재와 금속 전극 사이의 인터페이스에서 발생하는 높은 전기 및 열 저항으로 인해 상당한 전력 손실이 발생한다. 본 연구에서는 Skutterudite 재료와 금속 전극을 완벽하게 고정하는 Fe-Ni 합금의 금속 층을 고안하여 전달되는 에너지 손실을 최소화 시켰다. 개발한 금속 층은 계면에서의 화학 반응을 억제하는 확산 방지막 역할을 하는 것을 확인하였다. 결과적으로 개발한 소자는 보고된 값 중 가장 낮은 전기 접촉 저항을 가지며 (2.2–2.5 μC cm2), 온도 차이가 570K 에서 2.1W cm-2 의 높은 열전 전력 밀도를 보였다. 우리는 열전 모듈의 에너지 전달과 열 안정성에 유해한 2차상의 형성이 무시할 수준인 것을 주사투과전자현미경 분석을 통해 확인하였다.Chapter 1. Introduction: High performance bulk thermoelectric materials and Dissertation Overview 1 1.1 Introduction of Thermoelectrics 1 1.2 General background 7 1.3 Advanced approaches for thermoelectric materials 12 1.3.1 Doping and alloying 12 1.3.2 Nanostructuring 16 1.3.3 All-scale hierarchical architecturing e 20 1.3.4 Discoveries of promising materials with intrinsically low thermal conductivity 24 1.3.5 Increasing the power factor through band engineering 30 1.4 Dissertation Overview 37 Chapter 2. High Thermoelectric Performance in n-Type Polycrystalline SnSe via Dual Incorporation of Cl and PbSe and Dense Nanostructures 50 2.1 Introduction 50 2.2 Experimental section 56 2.3 Resutls and discussion 61 2.4 Conclusion 109 2.5 References 111 Chapter 3. Ultrahigh Power Factor and Electron Mobility in n‑Type Bi2Te3−x% Cu Stabilized under Excess Te Condition 121 3.1 Introduction 121 3.2 Experimental section 128 3.3 Resutls and discussion 133 3.4 Conclusion 172 3.5 References 173 Chapter 4. High Power Density Skutterudite-based Thermoelectric Modules with Ultralow Contact Resistivity Using Fe-Ni Metallization Layers 184 4.1 Introduction 184 4.2 Experimental section 189 4.3 Resutls and discussion 193 4.4 Conclusion 214 4.5 References 216 Biblography 227 국문 초록 (Abstract in Korean) 229Docto

Advanced Energy Projects FY 1996 research summaries

The mission of the Advanced Energy Projects Division (AEP) is to explore the scientific feasibility of novel energy-related concepts. These concepts are typically at an early stage of scientific development and, therefore, are premature for consideration by applied research or technology development programs. The portfolio of projects is dynamic, but reflects the broad role of the Department in supporting research and development for improving the Nation`s energy posture. Topical areas presently receiving support include: alternative energy sources; innovative concepts for energy conversion and storage; alternate pathways to energy efficiency; exploring uses of new scientific discoveries; biologically-based energy concepts; renewable and biodegradable materials; novel materials for energy technology; and innovative approaches to waste treatment and reduction. Summaries of the 70 projects currently being supported are presented. Appendices contain budget information and investigator and institutional indices

Understanding the Global Energy Crisis

Central issues in global energy are discussed through interdisciplinary dialogue between experts from both North America and Europe with overview from historical, political, and socio-cultural perspectives, outlining the technology and policy issues facing the development of major conventional and renewable energy sources. We are facing a global energy crisis caused by world population growth, an escalating increase in demand, and continued dependence on fossil-based fuels for generation. It is widely accepted that increases in greenhouse gas concentration levels, if not reversed, will result in major changes to world climate with consequential effects on our society and economy. This is just the kind of intractable problem that Purdue University’s Global Policy Research Institute seeks to address in the Purdue Studies in Public Policy series by promoting the engagement between policy makers and experts in fields such as engineering and technology

Emerging semiconductor nanostructure materials for single-photon avalanche diodes

Detecting of light at the single photon level has a far-reaching impact that enables a broad range of applications. In sensing, advances in single-photon detection enable low light applications such as night-time operation, rapid satellite communication, and long-range three-dimensional imaging. In biomedical engineering, advancing single-photon detection technologies positively impacts patient care through important applications like singlet oxygen detection for dose monitoring in cancer treatment. In industry, impacts are made on state-of-the-art technologies like quantum communication which relies on the efficient detection of light at the fundamental limit. While the high impact of single-photon detection technologies is clear, the potential for improvement and challenges faced by prominent single-photon detection technologies remains. Superconducting single-photon detectors push the bounds of performance, but their high cost and lack of portability limits their prospect for far reaching applicability. Single-photon avalanche diodes (SPADs) are a promising alternative which can be made portable, absent of the need for cryogenic cooling, but they generally lack the performance of superconducting detectors. The materials in SPAD designs dictate operation, and conventional materials implemented being defined according to intrinsic material properties, limits SPAD performance. However, new classes of advanced materials are being realized which exhibit modified electromagnetic properties from the engineered arrangement of subwavelength structural units and low-dimensional properties. Such materials include metamaterials and low-dimensional materials, and they have been shown to enhance optoelectrical properties that are critical to avalanche photodiodes, like rapid photo response, enhanced absorption, and reduced dark current. In this work, the application of such advanced materials in SPADs is explored. Tapered nanowires and nanowire arrays are optimized for enhanced absorption and shown experimentally at room temperature to demonstrate high speed near-unity absorptance response at the single-photon level. In the metamaterial and nanowire devices, the gain and timing jitter are shown to be significantly improved over conventional bulk-based designs. Furthermore, the modelling of metamaterials in a SPAD device design and its operation with external single-photon detection circuitry is studied. The analysis is further shown to extend down to single nanowire devices which offers an elegant approach for integrated photonic circuits