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1. Characteristics of New Cement-Based Thermoelectric Composite for Low-Temperature Applications by A Novel Method

   Li, X. ( June 2021 , 2020 International High-Performance Buildings Conference)

   Thermoelectric (TE) cement composite is a new type of TE material. Unlike ordinary cement, TE cement can mutually convert thermal energy to electrical energy due to the addition of carbon fibers, metal oxide nanoparticles, etc. In hot summer or cold winter, the significant temperature difference between indoor and outdoor can be used by TE cement to generate electricity. On the other hand, given power input, the same material can provide cooling/ heating to adjust room temperature. Therefore, TE cement has certain energy-saving potential in the application of building enclosures and energy systems. Its ability to convert different forms of energymore »and use low-grade energy is conducive to the operation of net-zero buildings. In this study, a novel TE cement composite, MnO2 and graphite enhanced cement, was firstly fabricated. The surface morphology of the composites was analyzed by using the images taken by scanning electron microscopy. The performance indicators of TE materials include the power factor and dimensionless figure of merit ZT The values of five TE properties were measured and calculated by a Physical Property Measurement System at different temperatures. Compared with the cement reinforced by graphite alone, it is confirmed that MnO2 nanoparticles have a positive effect on the enhancement of the TE performance for cement composites. The 5wt.% graphite and 10wt.% MnO2 enhanced cement composite achieves the highest Z.T. of 6.2 × 10-6 at 350 K.« less
2. Thermoelectric Characteristics of Graphene and Aluminum Doped Zinc Oxide Nanopowder Enhanced Cement Composite for Low-Temperature Applications

   Liu, X. ( June 2021 , ASHRAE Annual Conference papers)

   Thermoelectric (TE) cement composite is a new type of TE material. Unlike ordinary cement, due to the inclusion of additives, TE cement can mutually transform thermal energy into electrical energy. In extreme weather, the large temperature difference between indoor and outdoor can be harvested by TE cement to generate electricity. In moderate weather, given power input, the same material can provide cooling/heating to adjust room temperature and reduce HAVC load. Therefore, TE cement has energy-saving potential in the application of building enclosures and energy systems. Its ability to convert different forms of energy and use low-grade energy is conducive tomore »the operation of net-zero buildings. In this study, the graphene nanoplatelets and aluminum-doped zinc oxide nanopowder enhanced cement composite, was fabricated. The performance indicator of TE materials includes the dimensionless figure of merit ZT, calculated by Seebeck coefficient, thermal conductivity, and electrical conductivity. These TE properties were measured and calculated by a Physical Property Measurement System at different temperatures. The highest ZT of 15wt.% graphene and 5wt.% AZO enhanced cement composite prepared by the dry method is about 5.93E-5 at 330K.« less
3. In Situ Characterization of Phase-Change Materials

   Tripathi, s. ; Janish, M. ; Noor, N. ; Jungjohann, K. ; Pete, D. ; Kotula, P. ; Silva, H. ; Carter, C. ( November 2018 , Materials Research Society symposia proceedings)

   To understand the mechanism underlying the fast, reversible, phase transformation, information about the atomic structure and defects structures in phase change materials class is key. PCMs are investigated for many applications. These devices are chalcogenide based and use self heating to quickly switch between amorphous and crystalline phases, generating orders of magnitude differences in the electrical resistivity. The main challenges with PCMs have been the large power required to heat above crystallization or melting (for melt-quench amorphization) temperatures and limited reliability due to factors such as resistance drifts of the metastable phases, void formation and elemental segregation upon cycling. Characterizationmore »of devices and their unique switching behavior result in distinct material properties affected by the atomic arrangement in the respective phase. TEM is used to study the atomic structure of the metastable crystalline phase. The aim is to correlate the microstructure with results from electrical characterization, building on R vs T measurements on various thicknesses GST thin films. To monitor phase changes in real-time as a function of temperature, thin films are deposited directly onto Protochips carriers. The Protochips heating holders provides controlled temperature changes while imaging in the TEM. These studies can provide insights into how changes occur in the various phase transformations even though the rate of temperature change is much slower than the PCM device operation. Other critical processes such as void formation, grain evolution and the cause of resistance drift can thereby be related to changes in structure and chemistry. Materials characterization is performed using Tecnai F30 and Titan ETEM microscopes, operating at 300kV. Both the microscopes can accept the same Protochips heating holders. The K2 direct electron detector camera equipped with the ETEM allows high-speed video recording (1600 f/s) of structural changes occurring in these materials upon heating and cooling. In this presentation, we will describe the effect of heating thin films of different thickness and composition, the changes in crystallinity and grain size, and how these changes correlate with changes in the electrical properties of the films. We will emphasize that it is always important to use low-dose and/or beam blanking techniques to distinguish changes induced by the beam from those due to the heating or introduction of an electric current.« less
4. Multivalent Hygrothermal Material System for Double-Skin Envelope Dehumidification Cooling and Daylighting

   Ida, Aletheia ( April 2018 , Building Enclosure Science and Technology Conference)

   A unique paradox exists between the early and modern architectural and mechanical means of dealing with humidity in and around buildings. Traditional architectural conditions of biopolymeric thatch dwellings accommodated human thermal comfort through dehumidification by the materials employed at the building envelope. Original mechanical developments for dehumidification processes were developed specifically for removing moisture from materials in industrial applications. However, today, the modes by which humidity is treated for human comfort and material protection is inverted: buildings now utilize mechanical conditioning for dehumidification cooling functions and moisture protective materials in the building enclosure systems. Emerging multivalent hydrophilic materials are ablemore »to process humidity and moisture transport in new ways to allow for a systemic return to dehumidification cooling functions integrated in the building envelope system. The hydrophilic polymers are synthesized with low energy methods and poured into molds and then lyophilized to create macroporous networks that enhance both the sorption and thermal characteristics in the proposed application. The thermal and optical properties of novel hygrothermal materials are identified and used as inputs for simulation modeling for the proposed multivalent building enclosure system. The initial results provide improvements on annual energy loads and consumption for hot-humid climate conditions (Miami and Mumbai). The introduction of a sorption coefficient for the dehumidification function provides a unique contribution to design and performance analyses of double-skin envelopes. In addition, the dynamic modeling for temporal variations of material properties at the envelope also provides a new contribution to the field of building performance simulation. The challenges of these novel modes for moisture processing in the building envelope materials are addressed, with future work required for microbial identification and monitoring. The advantages from initial analytical and simulation modeling convey improvements for building energy conservation, natural daylighting, and water recuperation potential.« less
5. Materials selection rules for optimum power factor in two-dimensional thermoelectrics

   https://doi.org/10.1088/2515-7639/ab4600  

   Kommini, Adithya ; Aksamija, Zlatan ( November 2019 , Journal of Physics: Materials)

   Two-dimensional (2D) materials have emerged as the ideal candidates for many applications, including nanoelectronics, low-power devices, and sensors. Several 2D materials have been shown to possess large Seebeck coefficients, thus making them suitable for thermoelectric (TE) energy conversion. Whether even higher TE power factors can be discovered among the ≈2000 possible 2D materials (Mounetet al2018Nat. Nanotechnol.13246–52) is an open question. This study aims at formulating selection rules to guide the search for superior 2D TE materials without the need for expensive atomistic simulations. We show that a 2D material having a combination of low effective mass, higher separation inmore »the height of the step-like density of states, and valley splitting, which is the energy difference between the bottom of conduction band and the satellite valley, equal to 5k_BTwill lead to a higher TE power factor. Further, we find that inelastic scattering with optical phonons plays a significant role: if inelastic scattering is the dominant mechanism and the energy of the optical phonon equals 5k_BT, then the TE power factor is maximized. Starting from a model for carrier transport in MoS₂and progressively introducing the aforementioned features results in a two-orders-of-magnitude improvement in the power factor. Compared to the existing selection rules or material descriptors, features identified in this study provide the ability to comprehensively evaluate TE capability of a material and helps in identifying future TE materials suitable for applications in waste-heat scavenging, thermal sensors, and nanoelectronics cooling.

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