The Influence of Carbon Nanotubes on the Thermoelectric Properties of Bismuth Telluride

Thermoelectric materials are devices that have the ability to convert waste heat to electricity. The widespread use of thermoelectric materials is currently limited by the low value of their figure-of-merit (ZT). Bismuth Telluride (Bi2Te3) is a promising thermoelectric material in the near room temperature applications that provides a ZT value ~ 1. In order to overcome the limitation of utilizing thermoelectric materials in waste heat recovery, a ZT value > 2 is required. In this current study, multi-walled carbon nanotubes (MWCNT) were incorporated into the Bi2Te3 bulk matrix system to enhance its mechanical and thermoelectric properties through powder processing techniques. The nanocrystalline Bi2Te3/MWCNT composites were prepared using high energy ball milling and spark plasma sintering (SPS) techniques. The structural characterization and the average grain size values of both pristine Bi2Te3 and Bi2Te3/MWCNT were found to be approximately (~ 13 nm), and the average strain was found to be 0.2 using both X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) techniques. Vickers Microhardness test shows significant improvement of the nanocomposite hardness up to ~2 GPa as a function of increasing the MWCNT content. As for the dimensionless figure of merit (ZT) of the composite, it is expected to increase above the value of the pure binary Bi2Te3 in the temperature range of 298–498 K the addition of MWCNT increased the ZT value from 0.48 to maximum ZT value to 0.61 at 50oC, while at 150oC the ZT value was measured to be 0.35 and 0.43 for Bi2Te3 and MWCNT/Bi2Te3, respectively. It is considered that the enhancement of the thermoelectric performance of the composite mostly derived from the thermal conductivity, which is reduced by an active phonon-scattering at the MWCNT/Bi2Te3 interface

Evaluation of photothermal conversion performance of shape-stabilized phase change materials using a heat flux evolution curve

Phase change materials are promising alternatives for solar energy harvesting by photothermal conversion and thermal energy storage. In this work, a shape-stabilized phase change material (PCM) was prepared by hot-melt blending of paraffin wax (PW), high-density polyethylene (HDPE), and expanded graphite (EG) to investigate the photothermal conversion and storage performance using a heat flux evolution curve. This study introduced the heat flux evolution curve for the first time to accurately measure phase change duration, which is otherwise underestimated by the conventional temperature curves. The impact of various component compositions on thermal conductivity, energy storage density, PCM leakage, and photothermal conversion efficiency was evaluated experimentally. The results showed that the addition of 20 wt% EG enhanced the thermal conductivity of the composite by 305%. The total energy storage density of the composite varied in the range of 116.7-138.5 J/g during the photothermal conversion study. The composite with 15 wt% EG and 50 wt% PW exhibited a photothermal conversion efficiency of 79.8% when calculated from the temperature evolution curve and 61.8% from the heat flux evolution curve. This difference in efficiency indicates that the temperature evolution curve accounts only for the phase change of PCMs at the point of temperature measurement, while the heat flux evolution curve estimates the phase change of whole PCMs in the composite. Therefore, this work not only provides a shape-stabilized phase change material for the effective utilization of solar energy but also provides some guidelines to accurately calculate the photothermal conversion efficiency.This work was made possible by the Award NPRP13S-0127-200177 from the Qatar National Research Fund (a member of The Qatar Foundation). SEM measurements of the samples were accomplished in the Central Laboratories Unit, at Qatar University. The statements made herein are solely the responsibility of the authors.Scopu

Synthesis and characterization of vanadium (IV)-flavonoid complexes and its antioxidant ability toward superoxide and radical scavenging

In this project Vanadium complex -Vanadium (IV) - flavone was synthesized using vanadium(IV) acetylacetonate (VO(acac)2) complex and 3-hydroxy-6-methyl flavone ligand. The complex stability was checked using FTIR and UV-vis spectroscopies. Peackes around 990 cm-1 conforms the formation of (V=O) in the complex, as well as (V-O) around 790 cm-1. In UV-Vis spectrum peak around 400-450 nm was noticed, which conforms the formation of the vanadium complex that correspond to the ligand to metal charge transfer (LMCT) transition. The radical scavenging abilities of vanadium complex were investigated using DPPH. The anti-oxidant activity using (BHA) as a standard reference, the complex synthesized displayed strong DPPH antioxidant radical scavenging activity compared to VO(acac)2 and BHA, with IC50 value of (105, 95 and 96) mM respectively. The absorbance in which the reducing power occurred were found to be (0.397, 0.825 and 0.228) for the complex, VO(acac)2 and BHA

Evaluation of photothermal conversion performance of shape-stabilized phase change materials using a heat flux evolution curve

Phase change materials are promising alternatives for solar energy harvesting by photothermal conversion and thermal energy storage. In this work, a shape-stabilized phase change material (PCM) was prepared by hot-melt blending of paraffin wax (PW), high-density polyethylene (HDPE), and expanded graphite (EG) to investigate the photothermal conversion and storage performance using a heat flux evolution curve. This study introduced the heat flux evolution curve for the first time to accurately measure phase change duration, which is otherwise underestimated by the conventional temperature curves. The impact of various component compositions on thermal conductivity, energy storage density, PCM leakage, and photothermal conversion efficiency was evaluated experimentally. The results showed that the addition of 20 wt% EG enhanced the thermal conductivity of the composite by 305%. The total energy storage density of the composite varied in the range of 116.7–138.5 J/g during the photothermal conversion study. The composite with 15 wt% EG and 50 wt% PW exhibited a photothermal conversion efficiency of 79.8% when calculated from the temperature evolution curve and 61.8% from the heat flux evolution curve. This difference in efficiency indicates that the temperature evolution curve accounts only for the phase change of PCMs at the point of temperature measurement, while the heat flux evolution curve estimates the phase change of whole PCMs in the composite. Therefore, this work not only provides a shape-stabilized phase change material for the effective utilization of solar energy but also provides some guidelines to accurately calculate the photothermal conversion efficiency

An Enhancement of Compositional Stability of Phase Change Materials by Lamination with Aluminum Sheet

The wax leakage from shape-stabilized phase change materials (SSPCMs) is a limitation because it reduces their functionality. In this work, an enhancement of the compositional stability of SSPCMs formed by high-density polyethylene (HDPE) and paraffin wax blends through a lamination by aluminum (Al) foil was studied. The materials’ thermal conductivity was enhanced by adding expanded graphite (EG). The lamination of SSPCMs is the simplest method of reducing leakage, but it suffers from poor adhesion between polymer-based blends and protecting layers. The improved adhesion between SSPCMs and Al foil was achieved by adding 2 wt.% of maleated polyethylene (PE) acting as an adhesion promoter into SSPCMs or by plasma treatment of both SSPCMs and Al surfaces. Microscopic, spectroscopic, and optical techniques were used to analyze the surface and adhesion properties of SSPCMs. The peel resistance of SSPCMs after plasma treatment or modification by maleated PE increased from 2.2 N/m to 7.2 N/m or 55.1 N/m, respectively. The wax leakage from the treated or modified SSPCMs was suppressed significantly. The plasma-treated or maleated PE-modified SSPCMs showed leakage of 0.5 wt.% or 0.2 wt.%, respectively, after three days of leakage test. It indicates a good potential of this treatment/modification for industrially applied SSPCMs