Bui Duc Long

A review on the fabrication of polymer-based thermoelectric materials and fabrication methods.

Thermoelectricity, by converting heat energy directly into useable electricity, offers a promising technology to convert heat from solar energy and to recover waste heat from industrial sectors and automobile exhausts. In recent years, most of the efforts have been done on improving the thermoelectric efficiency using different approaches, that is, nanostructuring, doping, molecular rattling, and nanocomposite formation. The applications of thermoelectric polymers at low temperatures, especially conducting polymers, have shown various advantages such as easy and low cost of fabrication, light weight, and flexibility. In this review, we will focus on exploring new types of polymers and the effects of different structures, concentrations, and molecular weight on thermoelectric properties. Various strategies to improve the performance of thermoelectric materials will be discussed. In addition, a discussion on the fabrication of thermoelectric devices, especially suited to polymers, will also be given. Finally, we provide the challenge and the future of thermoelectric polymers, especially thermoelectric hybrid model.

Carbonate Hydroxyapatite and Silicon-Substituted Carbonate Hydroxyapatite: Synthesis, Mechanical Properties, and Solubility Evaluations

The present study investigates the chemical composition, solubility, and physical and mechanical properties of carbonate hydroxyapatite (CO3Ap) and silicon-substituted carbonate hydroxyapatite (Si-CO3Ap) which have been prepared by a simple precipitation method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF) spectroscopy, and inductively coupled plasma (ICP) techniques were used to characterize the formation of CO3Ap and Si-CO3Ap. The results revealed that the silicate (SiO4 4−) and carbonate (CO3 2−) ions competed to occupy the phosphate (PO4 3−) site and also entered simultaneously into the hydroxyapatite structure. The Si-substituted CO3Ap reduced the powder crystallinity and promoted ion release which resulted in a better solubility compared to that of Si-free CO3Ap. The mean particle size of Si-CO3Ap was much finer than that of CO3Ap. At 750°C heat-treatment temperature, the diametral tensile strengths (DTS) of Si-CO3Ap and CO3Ap were about 10.8 ± 0.3 and 11.8 ± 0.4 MPa, respectively.

Development of a bone substitute material based on alpha-tricalcium phosphate scaffold coated with carbonate apatite/poly-epsilon-caprolactone.

Interconnected porous tricalcium phosphate ceramics are considered to be potential bone substitutes. However, insufficient mechanical properties when using tricalcium phosphate powders remain a challenge. To mitigate these issues, we have developed a new approach to produce an interconnected alpha-tricalcium phosphate (α-TCP) scaffold and to perform surface modification on the scaffold with a composite layer, which consists of hybrid carbonate apatite / poly-epsilon-caprolactone (CO3Ap/PCL) with enhanced mechanical properties and biological performance. Different CO3Ap combinations were tested to evaluate the optimal mechanical strength and in vitro cell response of the scaffold. The α-TCP scaffold coated with CO3Ap/PCL maintained a fully interconnected structure with a porosity of 80% to 86% and achieved an improved compressive strength mimicking that of cancellous bone. The addition of CO3Ap coupled with the fully interconnected microstructure of the α-TCP scaffolds coated with CO3Ap/PCL increased cell attachment, accelerated proliferation and resulted in greater alkaline phosphatase (ALP) activity. Hence, our bone substitute exhibited promising potential for applications in cancellous bone-type replacement.

Work-Softening, High Pressure Phase Formation and Powder Consolidation by HPT

Among the various severe plastic deformation (SPD) processes, high pressure torsion (HPT) has several unique characteristics. These are applicability of very large strain and deformation under high pressure. Due to these abilities of HPT, several unique phenomena have been observed. In the present paper, three topics were reviewed; 1) work-softening in pure Cu, 2) high pressure phase formation in pure Ti and 3) synthesis of Cu-NbC composite. Work softening in pure Cu was observed when low strain rate and high pressure were applied. In Ti high pressure ω phase is obtained after unloading only when the deformation at high pressure was applied. The volume fraction of ω phase increased with the increase in the amount of strain. In pure Fe, high pressure ε phase was not retained at ambient pressure. The bulk Cu-NbC composite was synthesized starting from elemental powders. This demonstrates that HPT is an efficient tool for mechanical alloying and cold consolidation.

A DFT study on an alkali atom doped decahedral silver nanocluster for potential application in opto-electronics and catalysis

A systematic study of the structural, electronic and optical properties of the decahedral bimetallic Ag12X cluster is presented in the framework of density functional theory (DFT), where one atom of an alkali metal (X = Li, Na, K, Rb, Cs, Fr) is added, replacing a Ag atom in the decahedral Ag13 cluster in core (c-doped), vertex (v-doped) and surface (s-doped) positions. Geometrical optimization of the clusters indicated that Li and Na doped clusters exhibited the highest stability. The binding energy (BE), vertical ionization potential (VIP), vertical electron affinity (VEA) and HOMO–LUMO gaps were calculated to compare the electronic stability and chemical inertness of the doped clusters. In addition, the VIP and VEA values of the doped clusters revealed that the doped clusters exhibited more electronic and chemical reactivity than the undoped Ag13. Through optical spectra analysis, it is revealed that Ag12Na and Ag12Li clusters exhibited higher oscillation strength, whilst the s-doped Ag12Li exhibited 3 times higher oscillation strength with respect to undoped Ag13. In addition, a partial density of states (PDOS) calculation indicated that the red or blue shifting of the d-electrons are potentially responsible for this red and blue shifting of the optical peaks of the doped Ag12X clusters. Finally, these Ag12X clusters have promising electronic and optical properties; in particular, the Ag12Li dimer is a highly stable cluster with excellent optical absorption spectra. Thus, a neutral Ag12Li cluster might find good application in opto-electronics and its anion might be highly reactive and thus, can be a very good potential candidate for catalysis.

Comparison of Two Powder Processing Techniques on the Properties of Cu-NbC Composites

An in situ Cu-NbC composite was successfully synthesized from Cu, Nb, and C powders using ball milling and high pressure torsion (HPT) techniques. The novelty of the new approach, HPT, is the combination of high compaction pressure and large shear strain to simultaneously refine, synthesize, and consolidate composite powders at room temperature. The HPTed Cu-NbC composite was formed within a short duration of 20 min without Fe contamination from the HPT’s die. High porosity of 3–9%, Fe and niobium oxidations, from grinding media and ethanol during ball milling led to low electrical conductivity of the milled Cu-NbC composite. The electrical conductivity of the HPTed Cu-NbC composite showed a value 50% higher than that of milled Cu-NbC composite of the same composition.

The effect of nitric acid (HNO3) treatment on the electrical conductivity and stability of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) thin films

Abstract In this work, the posttreatment of an organic polymer is performed using an inorganic acid, nitric acid (HNO3). We picked poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the base material and improved its electrical conductivity by acid treatment with different concentrations of HNO3. The acid treatment was able to achieve the optimum electrical conductivity of 197 S/cm, which is 115.5 times higher than the base material when treated with an aqueous solution containing 65% of HNO3. Moreover, the films showed higher transparency in the visible range while conducting Fourier transform infrared analysis. In addition, the treated films showed improved stability against outdoor operating conditions in terms of sheet resistance compared with untreated PEDOT:PSS films. We tried to develop a hypothesis to describe the reason behind the electrical conductivity enhancement by studying the thicknesses of all the samples at different acid concentration levels. The results from atomic force microscopy, the Hall effect, and the trend of film thickness suggest that the conformational change, the removal of excess PSS from the polymer, and the increase in carrier concentration are the reasons behind the improvement in electrical conductivity.

Evaluation of Thermoelectric Properties of Cu3.21Bi4.79S9 Bismuth Chalcogenide

Cu3.21Bi4.79S9 was synthesized from Cu, Bi and S element powders using mechanical alloying method. The formation of Cu3.21Bi4.79S9 was identified using XRD and the changes of morphologies of the mixtures of Cu, Bi, and S powders during milling were observed using table top SEM. The milled powders were sintered using Hot-isostatic pressing at 230°C with a pressure of 50 MPa. Electrical resistivity and Seebeck coefficient of sintered samples were measured using ZEM-3 (Electrical resistivity and Seebeck Coefficient measuring System). Cu3.21Bi4.79S9 and some secondary phases were found in the 5h milled powder but single phase Cu3.21Bi4.79S9 was only obtained after milling for 15 h. A minimum electrical resistivity of sintered Cu3.21Bi4.79S9 sample was found to be 0.66 Ω.m at 170°C. We observed that a n- to p-type conversion at temperature of around 75 °C. However, a maximum n-type Seebeck coefficient of Cu3.21Bi4.79S9 was of -214 μV/K at 45 °C. The Seebeck coefficient decreases with increasing temperature and it reaches zero value at around 75 °C and then p-type Seebeck coefficient increases with increasing the temperature. The maximum p-type Seebeck coefficient was observed of 202 μV/K at 170°C.

Preparation of TEG Material Based on Conducting Polymer PEDOT: PSS through Treatment by Nitric Acid

Sustainable energy sources are gaining popularity due to the ever increasing energy demand along with escalating costs and the environmental issues associated with the conventional sources of energy. Among the technologies available today, TEGs (Thermoelectric Generators) are intensively investigated due to their capability of harnessing electricity from waste heat, such as power plants, industries, automobiles and even human bodies. The authors work focuses on the development of high efficiency organic thermoelectric materials to harness energy from ambient sources such as solar and domestic heat. In this present work, poly (3, 4-ethylenedioxythiophene)/(poly)styrenesulfonate (PEDOT: PSS) was post treated using nitric acid in order to increase its electrical conductivity, hence the efficiency. A maximum electrical conductivity of 197.51 S/cm was achieved which was approximately 115 times than that of the untreated material. Improvement of the electrical conductivity after post-treatment is due to the depletion of the non-conducting PSS after the acid treatment. Hence, it is concluded that it is an easy method to achieve comparatively good thermoelectric properties which can find its uses in flexible thermoelectric applications.