Recently, graphene has been extensively researched in fundamental science and engineering fields and has been developed for various electronic applications in emerging technologies owing to its outstanding material properties, including superior electronic, thermal, optical and mechanical properties. Thus, graphene has enabled substantial progress in the development of the current electronic systems. Here, we introduce the most important electronic and thermal properties of graphene, including its high conductivity, quantum Hall effect, Dirac fermions, high Seebeck coefficient and thermoelectric effects. We also present up-to-date graphene-based applications: optical devices, electronic and thermal sensors, and energy management systems. These applications pave the way for advanced biomedical engineering, reliable human therapy, and environmental protection. In this review, we show that the development of graphene suggests substantial improvements in current electronic technologies and applications in healthcare systems.

Electronic and Thermal Properties of Graphene and Recent Advances in Graphene Based Electronics Applications

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power. Moreover, the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades. Among these compounds, layered two-dimensional (2D) materials, such as graphene, black phosphorus, transition metal dichalcogenides, IVA–VIA compounds, and MXenes, have generated a large research attention as a group of potentially high-performance thermoelectric materials. Due to their unique electronic, mechanical, thermal, and optoelectronic properties, thermoelectric devices based on such materials can be applied in a variety of applications. Herein, a comprehensive review on the development of 2D materials for thermoelectric applications, as well as theoretical simulations and experimental preparation, is presented. In addition, nanodevice and new applications of 2D thermoelectric materials are also introduced. At last, current challenges are discussed and several prospects in this field are proposed.

/pdf/recent-progress-of-two-dimensional-thermoelectric-materials-3ipszjttc0.pdf

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectrics (TE), the direct solid-state conversion of waste heat to electricity, is a promising field with potential wide-scale application for power generation. Intrinsic conflicts in the requirements for high electrical conductivity but (a) low thermal conductivity and (b) a large Seebeck coefficient have made enhancing TE performance difficult. Several recent striking advances in the field are reviewed. In regard to the former conflict, notable bottom-up nanostructuring methods for phonon-selective scattering are discovered, namely using nanosheets, dislocations, and most strikingly a process to fabricate nano-micropores leading to a 100% enhancement in the figure of merit (ZT ≈ 1.6) for rare-earth-free skutterudites. Porous materials are hitherto considered as having poor TE performance, so this is a new paradigm. In regard to the latter conflict, nanocomposite materials with hybrid effects and use of magnetism are emerging as novel bottom-up methods to enhance TE. Material informatics efforts to identify high-ZT materials are also reviewed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/smll.201702013

Novel Principles and Nanostructuring Methods for Enhanced Thermoelectrics.

Skutterudite materials are widely considered for thermoelectric waste heat recovery. While the skutterudite structure effectively scatters the high frequency phonons, grain-boundary engineering is needed to further reduce the thermal conductivity beyond simply decreasing grain size. Here, we show that reduced graphene oxide (rGO) increases the grain boundary thermal resistivity by a factor of 3 to 5 compared to grain boundaries without graphene. Wrapping even micron sized grains with graphene leads to such a significant reduction in the thermal conductivity that a high thermoelectric figure of merit zT = 1.5 was realized in n-type YbyCo4Sb12, while a zT of 1.06 was achieved in p-type CeyFe3CoSb12. A 16 leg thermoelectric module was made by using n- and p-type skutterudite–graphene nanocomposites that exhibited conversion efficiency 24% higher than a module made without graphene. Engineering grain boundary complexions with 2-D materials introduces a new strategy for advanced thermoelectric materials.

Skutterudite with graphene-modified grain-boundary complexion enhances zT enabling high-efficiency thermoelectric device

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Defect engineering in thermoelectric materials: what have we learned?

The applications of strontium titanium oxide based thermoelectric materials are currently limited by their high operating temperatures of >700 °C. Herein, we show that the thermal operating window of lanthanum strontium titanium oxide (LSTO) can be reduced to room temperature by the addition of a small amount of graphene. This increase in operating performance will enable future applications such as generators in vehicles and other sectors. The LSTO composites incorporated one percent or less of graphene and were sintered under an argon/hydrogen atmosphere. The resultant materials were reduced and possessed a multiphase structure with nanosized grains. The thermal conductivity of the nanocomposites decreased upon the addition of graphene, whereas the electrical conductivity and power factor both increased significantly. These factors, together with a moderate Seebeck coefficient, meant that a high power factor of ∼2500 μWm–1 K–2 was reached at room temperature at a loading of 0.6 wt % graphene. The highes...

Thermoelectric power generation from lanthanum strontium titanium oxide at room temperature through the addition of graphene

Spark plasma sintered bismuth telluride-based thermoelectric materials incorporating dispersed boron carbide

Legislative CO2 emission penalties and the desire to increase overall efficiency using the least expensive technology for automotive vehicles has led manufacturers to research and develop thermoelectric generators (TEGs). TEGs will play an important role in achieving a large-scale, sustainable energy solution. However, the conflicting material characteristics needed for TEGs pose a formidable challenge. The suitability and opportunities for thermoelectric devices in automotive applications are discussed, with particular emphasis on systems for electrical energy generation from exhaust gases. The significant challenges for integrating thermoelectric devices into a device for wide scale automotive deployment are outlined, including the balancing of the many different requirements for the system such as thermal management, thermoelectric materials, design and packaging constraints, etc. Some of the failure modes of thermoelectric modules in such a system are also reviewed.

Chapter 9:Automotive Power Harvesting/Thermoelectric Applications

Improved efficiencies during waste heat recovery using currently available thermoelectric devices by increased hot-side temperatures push device materials to their operating limits. Magnesium silicide is one such material that is of interest for improved devices which can provide increased efficiencies by withstanding far higher temperatures.
Within this report, progress has been made in achieving ohmic contacts for high-performance tin-antimony doped magnesium silicide and higher manganese silicide. To achieve this, a lab scale prototyping and characterisation facility was designed and tested. Furthermore, a COMSOL model was developed for module prototyping verification and has been validated using empirical data. In addition, to achieve the fabrication of a prototype module a joining process is developed for pre-contacted co-sintered antimony doped magnesium silicide and undoped higher manganese silicide. Moreover, assembly and characterisation of a prototype 7-couple module is performed followed by its characterisation data being used to validate a COMSOL model.
Results showed contact sheet resistances of 8.58 ± 0.61 μΩ.cm2 and 32.7 ± 0.95 μΩ.cm2 being achieved for silver brazed antimony doped magnesium silicide and undoped higher manganese silicide, respectively. Furthermore, the prototype module created within this study has a power density of 130.29 mW/cm2 under a 300oC temperature differential. Moreover, barrier coating studies were additionally carried out for high figure of merit tin-antimony doped magnesium silicide and undoped higher manganese silicide investigating electrolytic plating, electroless plating, and sputter coating deposition techniques. Results demonstrated a sputter-coated nickel layer at a thickness of 1.2 μm with a 300 nm titanium adhesion layer results in a contact sheet resistance of 467.00 ± 8.35E μΩ.cm2 and 17.00 ± 0.13 μΩ.cm2 for silver brazed tin-antimony doped magnesium silicide and undoped higher manganese silicide, respectively. Furthermore, from the empirically verified COMSOL model a final COMSOL simulation is carried out with ideal values for the high-performance silicide material and a power density of 143mW/cm2 is calculated.
These results show that improvement of the barrier layer can lead to a significantly more efficient device by incorporating higher performance, higher operating temperature materials, however with increasing dopant levels effective joining processes becomes increasingly difficult.

Development of thermoelectric devices: design, fabrication and characterisation

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Thermoelectric power generation from lanthanum strontium titanium oxide at room temperature through the addition of graphene

Spark plasma sintered bismuth telluride-based thermoelectric materials incorporating dispersed boron carbide

Chapter 9:Automotive Power Harvesting/Thermoelectric Applications

Development of thermoelectric devices: design, fabrication and characterisation