Effects of annealing on thermoelectric properties of Sb2Te3 thin films prepared by radio frequency magnetron sputtering

Antimony telluride (Sb₂Te₃) thin films were deposited on silicon substrates at room temperature (300 K) by radio frequency magnetron sputtering method. The effects of annealing in N₂ atmosphere on their thermoelectric properties were investigated. The microstructure and composition of these films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, respectively. The electrical transport properties of the thin films, in terms of electrical conductivity and Seebeck coefficient were determined at room temperature. The carrier concentration and mobility were calculated from the Hall coefficient measurement. Both of the Seebeck coefficient and Hall coefficient measurement showed that the prepared Sb₂Te₃ thin films were p-type semiconductor materials. By optimizing the annealing temperature, the power factor achieved a maximum value of 18.02 μW cm^?1 K^?2 when the annealing temperature was increased to 523 K for 6 h with a maximum electrical conductivity (1.17 × 10³ S/cm) and moderate Seebeck coefficient (123.9 μV/K).     

Preparation and microwave absorption properties of BiFeO<Subscript>3</Subscript> and BiFeO<Subscript>3</Subscript>/PANI composites

BiFeO₃(BFO) particle was successfully synthesized by normal citric acid sol–gel method and the size of BiFeO₃ particle is about 200 nm. BiFeO₃/polyaniline (PANI) composites with the different weight ratio were synthesized by in situ emulsion polymerization. The citric acid doped PANI is fibrous and form a loose structure outside the BFO particle. With the increasing of PANI, the conductivity value of composites are increasing to 9.34?×?10^?2 S/cm. Moreover, the permittivity also enhance with the increasing of conductivity, which contribute to the improvement of dielectric loss. Microwave absorbing properties were investigated with a vector network analyzer in 1–18 GHz. The minimum reflection loss (RL) value is about ?40.2 dB at 8.3 GHz when the thickness is 3.5 mm, and the maximum bandwidth less than ?10 dB is 3.5 GHz (from 13.5 to 18 GHz) at the thickness of 2 mm. 3 mm millimeter-wave-attenuation properties were also tested, and the maximum attenuation value of BFO/PANI composites reach 15.71 dB. The composites can dissipate microwave energy into heat effectively by dielectric relaxation because of the suitable conductivity. The interface scattering and multiple reflections also play a important role because of the increasing of a loose structure. The BFO/PANI composite can be taken as a promising lightweight and multiband microwave absorber.     

Enhancing the thermoelectric performance by defect structures induced in p-type polypyrrole-polyaniline nanocomposite for room-temperature thermoelectric applications

Organic thermoelectric materials mainly conducting polymers are green materials that can convert heat energy into electrical energy and vice versa at room temperature. In the present work, we investigated the thermoelectric properties of polymer nanocomposite of polypyrrole (PPy) and polyaniline (PANI) (PPy/PANI) by varying the pyrrole: aniline monomer ratios (60:40, 50:50, and 40:60). The PPy/PANI composite is prepared by in-situ chemical polymerization of PPy on PANI dispersion. It has been observed that the combination of two conducting polymers has enhanced the electrical and thermal properties in the PPy/PANI composite due to the strong π–π stacking and H-bonding interaction between the conjugated structure of PPy and conjugated structure of PANI. The maximum electrical conductivity of 14.7 S m^?1 was obtained for composite with high pyrrole content, whereas the maximum Seebeck coefficient of 29.5 μV K^?1 was obtained for composite with high aniline content at 366 K. Consequently, the PPy/PANI composite with pyrrole to aniline monomer ratio of 60:40 exhibits the optimal electrical conductivity, Seebeck coefficient, and high power factor. As a result, the maximum power factor of 2.24 nWm^?1 K^?2 was obtained for the PPy/PANI composite at 60:40 pyrrole to aniline monomer ratio, which is 29 times and 65.8 times higher than PPy (0.077 nWm^?1 K^?2) and PANI (0.034 nWm^?1 K^?2), respectively.

     

All-organic PANI–DBSA/PVDF dielectric composites with unique electrical properties

Novel all-organic polymer high-dielectric permittivity composites of polyaniline (PANI)/poly (vinylidene fluoride) (PVDF) were prepared by solution method and their dielectric and electric properties were studied over the wide ranges of temperatures and frequencies. To improve the interface bonding between two polymers, dodecylbenzenesulfonic acid (DBSA), a bulky molecule containing a polar head and a long non-polar chain was used both as a surfactant and as dopant in polyaniline (PANI) synthesis. Synthesized conducting PANI–DBSA particles were dispersed in poly(vinylidene fluoride) (PVDF) matrix to form an all-organic composite with different PANI–DBSA concentrations. Near the percolation threshold, the dielectric permittivity of the composites at 100 Hz frequency and room temperature was as high as 170, while the dielectric loss tangent value was as low as 0.9. Like typical percolation system, composites experienced high dielectric permittivity at low filler concentrations. However, their dielectric loss tangent was low enough to match with non-percolative ceramic filler-based polymer composites. Maximum electrical conductivity at 24 wt% of PANI–DBSA was mere 10^?6 S/cm, a remarkably low value for percolative-type composites. Increase in the dielectric permittivity of the composites with increase in temperature from 25 to 115 °C for different PANI–DBSA concentrations was always in the same range of 50–60 %. However, the degree of increase in the electrical conductivity with the temperature was more prominent at low filler concentrations compared with high filler concentrations. Distinct electrical and their unique thermal dependence were attributed to an improved interface between the filler and the polymer matrix.     

Influence of annealing on the structural and electrical transport properties of Bi0.5Sb1.5Te3.0 thin films deposited by co-sputtering

Bi_0.5Sb_1.5Te_3.0 thin films were deposited on silicon substrates at room temperature by co-sputtering and the effects of annealing temperatures on structure and thermoelectric properties were investigated. The composition, crystallinity, and microstructure of these thin films were characterized by energy dispersive X-ray spectroscopy, X-ray diffraction, and scanning electron microscopy. The crystalline quality of the thin films was enhanced with a rising annealing temperature. When annealed at 573 K, the layered structure of the Bi_0.5Sb_1.5Te_3.0 thin films with a preferred orientation along the (00l) plane was formed. However, excessive high annealing temperature caused the thin films to become porous due to the separation of substantial Sb-rich precipitates. The electrical transport properties of the thin films, in terms of electrical conductivity and Seebeck coefficient were determined at room temperature. The carrier concentration and mobility were calculated from the Hall coefficient measurement. By optimizing the annealing temperature and time to 573 K for 6 h, the thermoelectric power factor was enhanced to 22.54 μW/(cm K²) at its maximum with a moderate electrical conductivity of 6.21 × 10² S/cm and a maximum Seebeck coefficient of 190.6 μV/K.     

Pressure infiltration of boron nitride preforms with molten aluminum for low density heat sink materials

Cubic boron nitride (cBN) has outstanding mechanical and thermal properties. The previous research focused on mechanical properties, to data, the thermal property of cBN has rarely been reported. In this work, a wide range of aluminum/cubic boron nitride (Al/cBN) composites were fabricated by pressure infiltration at 5.0 GPa and 960–1600 °C. The microstructure, phase composition, thermal conductivity and coefficient of thermal expansion of the Al/cBN composites were investigated. The results showed that a maximum thermal conductivity of 266 W/mK and the coefficient of thermal expansion of 4–6 × 10^?6 K^?1 which matches well to semiconductors, indicating that the Al/cBN composites are promised heat sink materials of high efficiency for the wide band gap semiconductors.     

One-step synthesis of conductive graphene/polyaniline nanocomposites using sodium dodecylbenzenesulfonate: preparation and properties

The preparation of conducting graphene/polyaniline–sodium dodecylbenzenesulfonate (PANI–SDBS) nanocomposites using synthesised graphene as the starting material is successfully conducted in the present study. The effect of the anionic surfactant SDBS on the properties of the graphene/PANI–SDBS nanocomposites is studied. The structure and morphology of the synthesised nanocomposites are characterised by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV–vis) spectrophotometry, X-ray diffraction and atomic force microscopy (AFM). The electrical conductivity properties of the resulting nanocomposites are determined using a resistance meter measurement system. The FESEM and TEM images reveal that the addition of SDBS surfactant to the PANI transforms the nanofibers of the PANI to a nanosphere morphology of PANI–SDBS. FTIR and UV–vis studies reveal that the conductive graphene/PANI–SDBS nanocomposites are successfully synthesised. AFM characterisation shows that the addition of graphene reduces the root mean square roughness of the surface of the PANI. The electrical conductivity and thermal stability of the PANI are improved after the introduction of SDBS. The nanocomposites containing a 5 wt% graphene loading exhibit the highest electrical conductivity of 2.94?×?10^?2 S/cm, which is much higher than that of PANI (9.09?×?10^?6 S/cm).     

The effect of Sr addition on the electrical properties of BaZn2Sb2 Zintl compounds

Sr doped BaZn₂Sb₂ polycrystalline materials with nominal compositions of Ba_1?xSr_xZn₂Sb₂ (x = 0.0, 0.25, 0.5) were prepared by synthesizing single crystals using a Sn-flux method followed by vacuum melting. The materials were characterized by powder X-ray diffraction and scanning electron microscopy equipped with electron energy dispersive spectroscopy, respectively. The electrical conductivity and Seebeck coefficient of the materials from room temperature to 773 K were measured. The electrical conductivity of all the materials decreased with increasing temperature and turned to increase with increasing temperature when temperature is above ~600 K, while the Seebeck coefficient increased with increasing temperature and turned to decrease with increasing temperature when temperature is above ~580 K. The electrical conductivity and Seebeck coefficient both increased after doping Sr into BaZn₂Sb₂. The maximum power factor for the sample with nominal composition of Ba_0.5Sr_0.5Zn₂Sb₂ reached 10.67 μW cm^?1 K^?2, which was about five times as high as that of the pure BaZn₂Sb₂.     

Electrical and mechanical properties of PMMA/reduced graphene oxide nanocomposites prepared via in situ polymerization

We report the effect of filler incorporation techniques on the electrical and mechanical properties of reduced graphene oxide (RGO)-filled poly(methyl methacrylate) (PMMA) nanocomposites. Composites were prepared by three different techniques, viz. in situ polymerisation of MMA monomer in presence of RGO, bulk polymerization of MMA in presence of PMMA beads/RGO and by in situ polymerization of MMA in presence of RGO followed by sheet casting. In particular, the effect of incorporation of varying amounts (i.e. ranging from 0.1 to 2 % w/w) of RGO on the electrical, thermal, morphological and mechanical properties of PMMA was investigated. The electrical conductivity was found to be critically dependent on the amount of RGO as well as on the method of its incorporation. The electrical conductivity of 2 wt% RGO-loaded PMMA composite was increased by factor of 10⁷, when composites were prepared by in situ polymerization of MMA in the presence of RGO and PMMA beads, whereas, 10⁸ times increase in conductivity was observed at the same RGO content when composites were prepared by casting method. FTIR and Raman spectra suggested the presence of chemical interactions between RGO and PMMA matrix, whereas XRD patterns, SEM and HRTEM studies show that among three methods, the sheet-casting method gives better exfoliation and dispersion of RGO sheets within PMMA matrix. The superior thermal, mechanical and electrical properties of composites prepared by sheet-casting method provided a facile and logical route towards ultimate target of utilizing maximum fraction of intrinsic properties of graphene sheets.     

Seebeck coefficient and electrical conductivity of mesoscopic nanocrystalline SrTiO3

In the present study, we investigate the effect of the grain boundaries on both the electrical transport and the thermoelectric properties. For this purpose, the Seebeck coefficient and the electrical conductivity of a model material, such as nominally pure SrTiO₃ (single crystal, microcrystalline, and nanocrystalline), is measured under oxidizing conditions. The impedance spectroscopy measurements reveal a strong change of the conduction properties of the nanocrystalline sample compared with the unperturbed bulk properties, namely a reduction of the p-type conductivity by two orders of magnitude at high oxygen partial pressure. Similarly, the Seebeck coefficient values of the nanocrystalline sample exhibit remarkable deviations from the single crystal ones: Under oxidizing conditions, values up to 2160 μV K^?1 (at 575 °C) are detected. More importantly, in the nanocrystalline sample, the dependence of the Seebeck coefficient on the concentration of the charge carriers is found to be four times larger than in the single crystal.     

Thermoelectric properties of p-type semiconductors copper chromium disulfide CuCrS2+x

A series of bulk samples CuCrS_2+x (x = 0, 0.01, 0.02, 0.06, 0.10) were prepared by combining mechanical alloying and spark plasma sintering. The effect of excessive sulfur content on the phase structure, microstructure, and thermoelectric and optical properties was investigated. The excessive sulfur initially entered into the lattice sites and then into the lattice interstices. A direct band gap semiconductor for CuCrS₂ material with an optical band gap of about 2.48 eV was proved. An improved electrical conductivity 2980 S m^?1 at 673 K reached along with an inversely varied Seebeck coefficient as increasing x value, which showed a maximum power factor of 104 μ W m^?1 K^?2 at 673 K for CuCrS_2.01 sample. In addition to the low thermal conductivity between 0.48 and 1.02 W m^?1 K^?1 in the whole temperature range, a peak ZT of 0.15 was achieved at 673 K for CuCrS_2.01 bulk sample, which was 36 % higher than that (0.11) of the CuCrS_2.00.     

Polyaniline–sodium montmorillonite clay nanocomposites: effect of clay concentration on thermal, structural, and electrical properties

A simple and facile method was used to synthesize polyaniline (PANI) nanocomposites with sodium montmorillonite clay (Na⁺-MMT) using in situ intercalative oxidative polymerization. Aniline was admixed with Na⁺-MMT at various concentrations, keeping the aniline monomer in the reaction mixture constant. The intercalation of PANI into the clay layers was confirmed by X-ray diffraction studies in conjugation with electron microscope techniques and FTIR spectra, particularly by the narrowing of the Si–O stretching vibration band confirmed the interaction between PANI and the clay. The employed route offers the possibility to improve the thermal properties with simultaneously controlled electrical conductivity. Thermal studies show an improved thermal stability of the nanocomposites relative to the pure PANI. Depending on the loading of the clay, the room temperature conductivity values of these nanocomposites varied between 2.0 × 10⁻⁴ and 7.4 × 10⁻⁴ S cm⁻¹, with the maximum at 44 wt% PANI concentration. The decrease of electrical conductivity at high PANI concentration was ascribed to the decrease of the structural ordering of PANI in the nanocomposite.     

Boosting the Thermoelectric Performance of the Doped DPP-EDOT Conjugated Polymer by Incorporating an Ionic Additive

The thermoelectric (TE) performance of organic materials is limited by the coupling of Seebeck coefficient and electrical conductivity. Herein a new strategy is reported to boost the Seebeck coefficient of conjugated polymer without significantly reducing the electrical conductivity by incorporation of an ionic additive DPPNMe₃Br . The doped polymer PDPP - EDOT thin film exhibits high electrical conductivity up to 1377 ± 109 S cm⁻¹ but low Seebeck coefficient below 30 µV K⁻¹ and a maximum power factor of 59 ± 10 µW m⁻¹ K⁻². Interestingly, incorporation of small amount (at a molar ratio of 1:30) of DPPNMe₃Br into PDPP - EDOT results in the significant enhancement of Seebeck coefficient along with the slight decrease of electrical conductivity after doping. Consequently, the power factor (PF) is boosted to 571 ± 38 µW m⁻¹ K⁻² and ZT reaches 0.28 ± 0.02 at 130 °C, which is among the highest for the reported organic TE materials. Based on the theoretical calculation, it is assumed that the enhancement of TE performance for the doped PDPP - EDOT by DPPNMe₃Br is mainly attributed to the increase of energetic disorder for PDPP - EDOT .     

Graphene oxide-filled conducting polyaniline composites as methanol-sensing materials

Polyaniline/graphene oxide (PANI/GO) composites were prepared by polymerization of aniline monomer in the presence of GO under acidic conditions. The synthesized samples were characterized by Fourier transform infra red spectroscopy, ultraviolet–visible absorption, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis. The direct current electrical conductivity of the composite was calculated by a four-probe technique. It is found that the conductivity dramatically increased to 241 S m^?1 for PANI/GO (5 wt%) composite at 110 °C compared to pure PANI (7.5 S m^?1). The composite material was investigated as a methanol vapour sensor and compared with pure PANI. The methanol-sensing characteristics of the prepared composite was monitored by measuring the change in electrical resistivity on exposure to methanol vapour at different concentrations. The resistivity of PANI increases on exposure to methanol vapour because of strong hydrogen bonding between methanol with the polymer chain. A density functional theory study was carried out to verify the proposed concept of hydrogen bonding between the polymer chains and methanol. The presence of GO in PANI/GO composite increases the sensitivity towards methanol as compared with the pure PANI.     

Structural and magnetic properties of nanocomposites based on nanostructured polyaniline and titania nanotubes

Nanocomposites consisting of self-assembled polyaniline (PANI) nanostructures and titania nanotubes (TiO₂-NT) were synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in an aqueous dispersion of TiO₂-NT (outer diameter ~10 nm), without added acid. The influence of initial mole ratio of aniline to TiO₂ (80, 20, and 5) on the morphology, electrical conductivity, molecular structure, crystallinity, and magnetic properties of synthesized PANI/TiO₂ nanocomposites was studied. Transmission electron microscopy, Raman spectroscopy, and X-ray powder diffraction proved that the shape and structure of TiO₂-NT in the final nanocomposites were preserved. The shape of PANI nanostructures formed in the nanocomposites was influenced by the initial aniline/TiO₂-NT mole ratio. Nanotubes and nanorods are predominant PANI nanostructures in the nanocomposite prepared with the highest aniline/TiO₂ mol ratio of 80. The decrease of aniline/TiO₂ molar ratio induced more pronounced formation of nanorod network. The electrical conductivity of PANI/TiO₂ nanocomposites was in the range (1.3–2.4) × 10^?3 S cm^?1. The nanocomposites exhibit weak ferromagnetic behavior. Approximately order of magnitude lower values of coercive field and remanent magnetization were obtained for nanocomposite samples in comparison to pure PANI.     

Effect of calcination temperature on structure and thermoelectric properties of CuAlO<Subscript>2</Subscript> powders

Copper aluminum oxide (CuAlO₂) with delafossite phase was synthesized by the Pechini method using different calcination temperatures to evaluate its influence on the structure and thermoelectric material properties. X-ray diffraction and Raman spectroscopy confirm that delafossite phase was formed at 1100 °C with the presence of 2H-CuAlO₂ and Al₂O₃ impurities, while at lower calcination temperatures (900 and 1000 °C), a mixture of CuO + CuAl₂O₄ (spinel phase) was observed. Energy-dispersive X-ray elemental maps display an even distribution of copper, aluminum and oxygen in the sample calcined at 1100 °C. Direct optical band gap, E _g = 3.6 eV, was calculated from reflectance diffuse spectra by Kubelka–Munk and Tauc methods. An absorption band at 1.7 eV accounts for defect levels, masking the characteristic indirect transition. The thermoelectric properties, such as Seebeck coefficient, and thermal and electrical conductivities of the sample calcined at 1100 °C were measured at different temperatures. Hall voltage and positive values of the Seebeck coefficient (425.8–434.4 µV K^?1) confirm the material’s p-type character. The independence of the Seebeck coefficient on the operation temperature indicates a small polaron electrical conduction mechanism. Thermal conductivity decreases exponentially with the temperature from 43.45 to 23.9 W m^?1 K^?1, where the principal contribution is due to phonons. Figure of merit ZT of sample calcined at 1100 °C between 100 and 800 °C increases from 1.42 × 10^?8 to 4.94 × 10^?4 in the order of the literature reports. From the Arrhenius plot ln(σT) versus 1000/T, an activation energy E _a = 0.32 eV for the electrical conductivity was calculated.     

Thermoelectric measurements of PEDOT:PSS/expanded graphite composites

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/expanded graphite films were cast as thin films with different expanded graphite contents at room temperature. The thermoelectric properties of the composites were investigated as a function of the graphite concentration. The electrical conductivity and Seebeck coefficient were measured as a function of the graphite concentration. The electrical conductivity and power factor show similar trends with a sharp increase at around 55 wt% of expanded graphite content. The Seebeck coefficient does not show a significant dependence with the graphite content. SEM and TEM images indicate a nearly homogenous distribution of the filler in the matrix. The initial thermal stability is not modified with the filler.     

Preparation and characterization of poly(3-octylthiophene)/carbon fiber thermoelectric composite materials

Poly(3-alkylthiophene) (P3AT) with a high Seebeck coefficient has recently been reported. However, P3AT/inorganic conductive composites exhibit relatively poor thermoelectric performance because of their low electrical conductivity. In this work, carbon fiber sheets with a high electrical conductivity were chosen as the inorganic phase, and poly(3-octylthiophene)(P3OT)/carbon fiber composites were prepared by casting P3OT solution onto the carbon fiber sheets. The carbon fiber sheets incorporated into the composites can provide good electrical conductivity, and P3OT can provide a high Seebeck coefficient. The highest power factor of 7.05 μW m⁻¹ K⁻² was obtained for the composite with 50 wt% P3OT. This work suggests a promising method for preparing large-scale thermoelectric composites with excellent properties.     

On the importance of the structure in the electrical conductivity of fishbone carbon nanofibers

Carbon nanofibers (CNFs) have a remarkable electrical conductivity resulting highly attractive for different applications such as composites or electronics due to their high quality/price ratio. Although it is known that their graphitic character provides a high conductivity, very little is known about the influence of the nanofibers structure on that property. In this study, CNFs characterized by different physical properties are prepared at diverse synthesis temperatures within a range (550–750 °C) in which significant structural and dimensional changes are accomplished and homogeneous nanofiber growth takes place. The electrical conductivity is determined on the powdery as-grown materials modifying the compaction degree by applying pressure. Because of a combination of structural features, the apparent electrical conductivity increases with synthesis temperature of CNFs, ranging from 50 S m^?1 for the worst conducting CNFs at a low compaction degree (25 % of solid volume fraction) to 3 × 10³ S m^?1 for the best conducting CNFs at a high compaction degree (60 % of solid volume fraction). Further analysis is carried out applying the percolation theory to analyze the experimental data and the results suggest that both the orientation of the graphenes and the filament diameter distribution play a determining role in the intrinsic electrical conductivity with values in the interval 1.5 × 10³ to 1.3 × 10⁴ S m^?1. These intrinsic values of electrical conductivity are found between one and two orders of magnitude higher than that of the powder, highlighting the also important effect of porosity.     

Improved thermoelectric performance of PEDOT:PSS films prepared by polar-solvent vapor annealing method

To improve thermoelectric performance, polar-solvent vapor annealing (PSVA) method was introduced into the preparation of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. The solvent vapors included dimethyl sulfoxide, ethylene glycol, N,N-dimethylformamide, N-methyl-2-pyrrolidone, and deionized water (H₂O). The PSVA-treated PEDOT:PSS films exhibited significantly enhanced electrical conductivity and the maximum value was up to 496 S cm^?1. Especially, utilizing the PSVA method, H₂O could also remarkably enhance the electrical conductivity of pristine PEDOT:PSS film from 0.2 to 57 S cm^?1. There was no distinct change for the Seebeck coefficient of PSVA-treated films with the significantly enhanced electrical conductivity, thereby a maximum power factor of 9.47 μW m^?1 K^?2 at room temperature was obtained. The effects of PSVA method on thermoelectric performance of PEDOT:PSS films were also investigated systematically by analyzing the changes in morphology, carrier mobility and carrier concentration. The results confirmed that PSVA-treated PEDOT:PSS films could obtain smoother morphologies and realize the simultaneous increase of carrier mobility and carrier concentration, which results in the improvement of the thermoelectric performance.