Materials Science Forum Vol. 847

Abstract: Two group chromophores with strong methoxy electron donor and CF₃ electron acceptor were synthesized and reported. To investigate the effects of conjugative bond and its length between electron donor and acceptor on optical properties, thiophene ring was incorporated into the conjugated units of group B compared to group A. The results showed that the incorporation of thiophene rings could significantly enhance molecular first hyperpolarizability and permit a high thermal stability. It has been found that molecular first hyperpolarizability increased continually by the increase of bond length of group A. However, further increase of conjugated bond length between electron donor and acceptor in group B reduced the molecular hyperpolarizability. This study indicates that both the nature of the conjugative bridge and its bond length between electron donor and acceptor have inner correlation to optimize the molecular hyperpolarizability for electro-optical applications.

Abstract: The silver pastes containing Ag₂O powder, Ag powder, α-terpineol, ethyl-cellulose and Pb-free glass were synthesized for crystalline silicon (c-Si) solar cells. It was found that α-terpineol assisted the decomposition of Ag₂O powder and effectively lowered the decomposition temperature of Ag₂O. Ag nanoparticles were produced during the decomposition of Ag₂O, which helped to reduce the sintering temperature of the silver pastes. The Ag₂O-aided silver pastes were fired on polycrystalline silicon solar cells at various temperatures, and large plate-shaped Ag crystallites appeared at the interfaces between the sintered pastes and the emitter, which ensured a good electrical contact. The contact resistivity of Ag₂O-aided silver paste with an optimal ratio of Ag₂O to Ag was lower than that of the paste with pure Ag powder. The lowest contact resistivity of Ag₂O-aided Pb-free silver pastes sintered at 800°C was 0.029 Ω⋅cm², which was close to that of commercial silver paste that contained Pb-based glass (0.026 Ω⋅cm²). The experimental data demonstrated that the addition of Ag₂O reduced the contact resistance and promoted the sintering of Pb-free silver pastes, and Ag₂O-aided Pb-free silver paste could be a promising candidate used for front-contact electrode of c-Si solar cells.

Abstract: Oxygen-rich polycrystalline NiO_x films were prepared by means of magnetron sputtering. Thickness-dependent bipolar resistive switching behaviors revealed that the 20 nm-thick NiO_x film presented a clockwise current-voltage loop, while the 60 nm-thick NiO_x film achieved an anti-clockwise current-voltage loop. Redox reactions between penetrated Ag ions and drifted oxygen ions in the whole 20 nm-thick NiO_x films resulted in the clockwise current-voltage loops. Filamentary conducting paths composed by oxygen vacancies were responsible for the anti-clockwise resistive switching loops of the 60 nm-thick NiO_x film.

Abstract: Silver-coated carbon fibers successfully fabricated by a modified electroless plating process were characterized by scanning electron microscope (SEM) and X-ray diffraction analysis (XRD). The electroless plating process was modified by replacing the conventional pretreatment and sensitization steps by only using surface hydroxylation step. And the activation and electroless plating steps were merged into one step. The silver layer was successfully coated onto the surface of carbon fibers under the given coating conditions. The effects of AgNO₃ solution concentration and reaction temperature on the conductivity of silver-coated carbon fibers were studied in detail. The results indicated that the optimum AgNO₃ solution concentration was 0.05 mol/L, and the optimum range of reaction temperature was from 45°C to 65°C.

Abstract: CoSb₃ thermoelectric thin films were prepared on polyimide flexible substrate by radio frequency (RF) magnetron sputtering technology using a cobalt antimony alloy target. Ti and In were added into CoSb₃ thin films by co-sputtering. The influence of Ti and In on the thermoelectric properties of CoSb₃ thin films was investigated. X-ray diffraction result shows that the major diffraction peaks of all the thin films match the standard peaks related to the CoSb₃ phase. The sample has best thermoelectric properties when the Ti sputtering time was 1min and In sputtering time was 30 seconds.

Abstract: The CH₃NH₃PbI₃ light-absorbing layer shows preferably photovoltaic performance. However, comparing with CH₃NH₃PbI₃ film, the CH₃NH₃SnI₃ film show smaller band gap and wider light absorption range, meanwhile, it is non-toxic and environmental friendly solar cell material. In this paper, different proportions of tin (Sn) was doped into CH₃NH₃PbI₃ film, the atomic ratio of lead-tin strictly controlled to form CH₃NH₃SnI₃ from CH₃NH₃PbI₃. The five different proportions of doping CH₃NH₃Pb_1-xSn_xI₃ perovskite powders were studied. The morphology of powders was observed by scanning electron microscopy (SEM). X-ray diffraction (XRD) and energy dispersive spectrum (EDS) were used to analyze the crystallization and the proportion composition of powders, respectively. The experimental results show that the powder crystal orientation was very obvious at the ratio of lead-tin around 1:1.99 and the atomic ratio close to the ideal stoichiometric ratio of doped atoms.

Abstract: In this study, a PANI covalently grafted GO (GO-PANI) was obtained through amide reaction and a PANI covered GO (GO/PANI) was prepared via in-situ polymerization. The better thermoelectric properties were achieved in GO-PANI compared with GO/PANI under the same situation, i.e., the equal mass ratio of GO and PANI as well as the oxidative degree of polymerization reaction. Furthermore, polyaniline grafted from the reduced graphene [R(GO-PANI)] was prepared by reducing the above GO-PANI using hydrazine hydrate and the improved power factor as high as 3.6 × 10^-8 W m^-1 K^-2 was attained at 341 K.

Abstract: In this study, Mn doped Cu_12-xMn_xSb₄S₁₃(x=0, 0.5, 1.0, and 2.0) tetrahedrite samples were prepared by melting and annealing followed by hot press sintering. Powder X-ray diffraction and scanning electron microscopy and electron energy dispersive spectroscopy analysis were performed for the samples, and the thermoelectric transport properties of samples were characterized. The experimental results showed that the synthetic tetrahedrites were consisted of principal Cu₁₂Sb₄S₁₃ phase and a small amount of secondary Cu₃SbS₄ and CuSbS₂. The electrical conductivity of the tetrahedrites decreased with increasing the Mn doping amount. Contrary to the electrical conductivity, the Seebeck coefficient of the tetrahedrites increased with increasing Mn doping amount. The thermal conductivity decreased with increasing Mn doping amount due to the suppression of the carrier contribution, as well as due to the substitution effect of Mn on the Cu site. For the Mn doped Cu_12-xMn_xSb₄S₁₃ compounds with x=0.5, 1.0, and 2.0, the ZT values decreased with the increase of Mn doping amount, a maximum ZT=0.89 was obtained for the Mn doped compound with x=0.5.

Abstract: Due to the high-performance in the medium temperature application, β-Zn₄Sb₃ thermoelectric material has been received much attention. It is found that low dimensional thin film can improve the thermoelectric properties of materials by quantum local area effect and interface effect in recent years. In this paper, the β-Zn₄Sb₃ thin film was prepared on polyimide flexible substrate by DC magnetron co-sputtering method. The results showed that the thin film exhibited predominately ZnSb phases when the thin film was prepared by DC magnetron sputtering using Zn₄Sb₃ alloy target. It is suggested that the element Zn has high saturated vapor pressure and the thin film is lack of Zn due to the evaporation during the heat treatment process. We further adopted co-deposition Zn and Zn₄Sb₃ by DC magnetron co-sputtering to supplement the content of Zn. The sputtering power of Zn₄Sb₃ is fixed and Zn is set to 21W, 27W and 34W, respectively. The results indicated that the thin films transformed from ZnSb phase into β-Zn₄Sb₃ phase after Zn added. EDS analysis demonstrated that the atomic ratio of Zn:Sb was approach 4:3, and a slightly surplus of Zn. The thermoelectric properties of thin films with β-Zn₄Sb₃ phase were improved obviously.

Abstract: The band structures and transport properties of Half-Heusler compounds NbFeSb and NbRuSb were studied using ab initio calculations and the Boltzmann transport equation with constant scattering time approximation (CSTA). Both compounds were identified as good p-type thermoelectric materials because of the combination of heavy and light bands in the valence band maximum (VBM). The Seebeck coefficients for NbRuSb were lower than that for NbFeSb; while the electrical conductivities for NbRuSb were little higher than that for NbFeSb. Consequently, the power factors in the p-type regimes for both compounds were similar at a given temperature. NbFeSb and NbRuSb could be efficient materials for thermoelectric generators based on the results in the present investigation.