Fabrication and thermoelectric properties of SrTiO3–TiO2 composite ceramics

The paper presents the results of preparing biphase SrTiO₃–TiO₂ ceramics as a promising system for n-type thermoelectrics using the features of a two-dimensional electron gas. Ceramics was obtained by reactive spark plasma sintering of SrCO₃ and TiO₂. The dynamics of phase transformations are shown; it is clarified that phase transformations are not the driving force of sintering. The mutual stabilization of the SrTiO₃ and TiO₂ phases is shown. Unique data on the assessment of the temperature gradient in the system have been obtained. A comparison of the thermoelectric characteristics of biphasic ceramics and its constituent phases allows concluding that the role of the two-dimensional electron gas is reduced to modulating the properties of bulk phases. Clear signs of size quantization were detected by the X-ray luminescence method, which is expressed in the blueshift of the luminescence spectrum by 22.3 ± 0.8 meV.     

Coaxial structured Bi2S3–SnS2-MWCNT hybrid nanocomposite with its improved thermoelectric properties

Bismuth(III) sulfide (Bi₂S₃), tin(IV) sulfide (SnS₂), and a multi-walled carbon nanotube (MWCNT) are prepared as a hybrid composites (Bi₂S₃–SnS₂-MWCNT) using a simple hydrothermal method. The as-prepared hybrid composite exhibit n-type thermoelectric (TE) characteristics and an enhanced power factor (PF) value. Bi₂S₃–SnS₂ composites are anchored to the surface of MWCNT and form a coaxial structure. The nanostructure formation through bonding between these different materials is confirmed via scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS). The TE properties of the Bi₂S₃–SnS₂-MWCNT composite is improved by changing the Bi₂S₃, SnS₂ and MWCNT content. The maximum PF (∼150.8 μ·W/m·K²) was obtained for Bi₂S₃–SnS₂-MWCNT composite with Bi:Sn ratio of 7:3 and 2 wt% of CNT. The highest PF values were ∼34 and ∼58 times higher than the PF of Bi₂S₃ and SnS₂ at room temperature. The synthesized Bi₂S₃–SnS₂-MWCNT hybrid nanocomposite can provide important components for fabrication CNT-based TE composites with high conversion efficiency, further advancing TE device.     

Electrical properties and phase of BaTiO3–SrTiO3 solid solution

It has been known that ABO₃ type perovskite ferroelectrics, such as BaTiO₃ (BTO) and SrTiO₃ (STO), form a complete solid solution. In this study, Ba_1?xSr_xTiO₃ (BST, x=0.0–1.0) solid solution were sintered by a solid-state reaction method using BTO and STO raw powders with appropriate chemical composition. The crystal structure was investigated by a Rietveld refinement method; Fullprof, using X-ray diffraction data. Within the reasonable goodness of fit, tetragonal symmetry was found in BST with x≤0.2, while BST with x≥0.4 were found to be cubic symmetry. However, Ba_0.7Sr_0.3TiO₃ was difficult to decide whether it is cubic or tetragonal because of large uncertainties after final fitting. The composition ratios calculated from the fitted occupancies match well with those measured by EDS within experimental uncertainties. Remnant polarizations of BST with x<0.3 decrease with increasing Sr concentration. Furthermore, measured phase transition temperatures and maximum dielectric constant decrease as increasing Sr concentration. Measured electrical properties of BST were match well with the structural refinement investigations.     

One-step fabrication of Ce–N-codoped TiO2 nano-particle and its enhanced visible light photocatalytic performance and mechanism

Ce, N codoped TiO₂ nano-particles were fabricated through sol–gel strategy in the absence of water. Results revealed that Ce was not implanted into the lattice of TiO₂ and existed as the forms of small cluster CeO₂ which uniformly diffused onto the surface or interstitial site of TiO₂, while N dopants were successfully incorporated into the structure of TiO₂ by substituting the lattice oxygen atoms and existed as the forms of N–Ti–O bonds, thereby resulting in the formation of impurity level above the valence band of TiO₂ and enhancement of visible photocatalytic (PC) performance. The enhanced visible PC mechanism was discussed.     

Low thermal conductivity of SrTiO3−LaTiO3 and SrTiO3−SrNbO3 thermoelectric oxide solid solutions

Electron-doped SrTiO₃ has been attracting attention as oxide thermoelectric materials, which can convert wasted heat into electricity. The power factor of the electron-doped SrTiO₃, including SrTiO₃-LaTiO₃ and SrTiO₃-SrNbO₃ solid solutions, has been clarified. However, their thermal conductivity (κ) has not been clearly identified thus far. Only a high κ (>12 W m⁻¹ K⁻¹) has been assumed from the electron contribution based on Wiedemann–Franz law. Here, we show that the κ of the electron-doped SrTiO₃ is lower than the assumed κ, and its highest ZT exceeded 0.1 at room temperature. The κ slightly decreased with the carrier concentration (n) when n is below 4 × 10²¹ cm⁻³. In the case of SrTiO₃-SrNbO₃ solid solutions, an upturn in κ was observed when n exceeds 4 × 10²¹ cm⁻³ due to the contribution of conduction electron to the κ. On the other hand, κ decreased in the case of SrTiO₃-LaTiO₃ solid solutions probably due to the lattice distortion, which scatters both electrons and phonons. The highest ZT was 0.11 around n = 1 × 10²¹ cm⁻³. These findings would be useful for the future design of electron-doped SrTiO₃-based thermoelectric materials.     

Microstructure and mechanical properties of WC–C nanocomposite films

WC–C nanocomposite film was prepared by using a hybrid deposition system of r.f.-PACVD and DC magnetron sputtering. W concentration in the film was varied from 5.2 to 42 at.% by changing the CH₄ fraction of the mixture sputtering gas of Ar and CH₄. Hardness, residual compressive stress and electrical resistivity were characterized as a function of W concentration. Raman spectroscopy, XRD and high resolution TEM were employed to analyze the structural change in the film for various W concentrations. In the present W concentration range, the film was composed of nano-sized WC particles of diameter less than 5 nm and hydrogenated amorphous carbon matrix. Content of the WC particles increased with increasing W concentration. However, the mechanical properties of the film increased only when the W concentration was higher than 13 at.%. Structural analysis and electrical conductance measurements evidently showed that the increase in hardness and residual stress occurred as the WC particles were in contact with each other in the amorphous carbon matrix.     

Synthesis of TiO2–Ag nanocomposite with sol–gel method and investigation of its antibacterial activity against E. coli

TiO₂–Ag nanocomposite was prepared by the sol–gel method and an azeotropic distillation with benzene was used for dehydration of the gel. Because of gel dehydration by distillation method a nanopowder with a surface area of 230 m²/g was produced which decreased to 80 m²/g after calcination. TEM micrographs and XRD patterns showed that spherical nanosized Ag particles (≈ 10 nm) were deposited among TiO₂ particles. The antibacterial activity of calcined powder at 300 and 500 °C was studied in the presence and in the absence of UV irradiation against Escherichia coli as a model for Gram-negative bacteria. The antibacterial tests confirmed the powder calcined at 300 °C possessed more antibacterial activity than the pure TiO₂, amorphous powder and the powder calcined at 500 °C under UV irradiation. In the absence of UV, the reduction in viable cells was observed only with calcinated powder at 300 °C.     

Porous Ca3Co4O9 with enhanced thermoelectric properties derived from Sol–Gel synthesis

Highly porous Ca₃Co₄O₉ thermoelectric oxide ceramics for high-temperature application were fabricated by sol–gel synthesis and subsequent conventional sintering. Growth mechanism of misfit-layered Ca₃Co₄O₉ phase, from sol–gel synthesis educts and upcoming intermediates, was characterized by in-situ X-ray diffraction, scanning electron microscopy and transmission electron microscopy investigations. The Ca₃Co₄O₉ ceramic exhibits a relative density of 67.7%. Thermoelectric properties were measured from 373 K to 1073 K. At 1073 K a power factor of 2.46 μW cm⁻¹ K⁻², a very low heat conductivity of 0.63 W m⁻¹ K⁻¹ and entropy conductivity of 0.61 mW m⁻¹ K⁻² were achieved. The maintained figure of merit ZT of 0.4 from sol–gel synthesized Ca₃Co₄O₉ is the highest obtained from conventional, non-doped Ca₃Co₄O₉. The high porosity and consequently reduced thermal conductivity leads to a high ZT value.     

Synthesis,characterization and controlled release properties of zinc–aluminium-beta-naphthoxyacetate nanocomposite

An organic–inorganic nanohybrid nanocomposite was synthesized by co-precipitation method using beta-naphthoxyacetate (BNOA) as guest anion and zinc–aluminium layered double hydroxide (Zn–Al-LDH) as the inorganic host. A well-ordered nanohybrid nanocomposite was formed when the concentration of BNOA was 0.08 M and the molar ratio of Zn to Al, R = 2. Basal spacing of layered double hydroxide containing nitrate ions expanded from 8.9 to 19.5 Å in resulting of Zn–Al-BNOA nanocomposite was obtained indicates that beta-naphthoxyacetate was successfully intercalated into interlayer spaces of layered double hydroxide. It was also found out the BET surface area increased from 1.13 to 42.79 m² g^?1 for Zn–Al-LDH and Zn–Al-BNOA nanocomposite, respectively. The BJH average pore diameter of the synthesized nanocomposite is 199 Å which shows mesoporous-type of material. CHNS analysis shows the Zn–Al-BNOA nanocomposite material contains 36.2 % (w/w) of BNOA calculated based on the percentage of carbon in the sample. Release of BNOA from the lamella of Zn–Al-BNOA was controlled by the zeroth and first order kinetics at the beginning of the deintercalation process up to 200 min and controlled by pseudo-second order kinetics for the whole process. This study suggests that layered double hydroxide can be used as a carrier for organic acid herbicide controlled release formulation of BNOA.     

Coexistence of three ferroelectric phases and enhanced piezoelectric properties in BaTiO3–CaHfO3 lead-free ceramics

Perovskite ferroelectrics possess the fascinating piezoelectric properties near a morphotropic phase boundary, attributing to a low energy barrier that the results in structural instability and easy polarization rotation. In this work, a new lead-free system of (1-x)BaTiO₃-xCaHfO₃ was designed, and characterized by a coexistence of ferroelectric rhombohedral-orthorhombic-tetragonal (R-O-T) phases. With the increase amount of CaHfO₃ (x), a stable coexistence region of three ferroelectric phases (R-O-T) exists at 0.06 ≤ x ≤ 0.08. Both large piezoelectric coefficient (d₃₃～400 pC/N), inverse piezoelectric coefficient (d₃₃^*～547 pm/V) and planar electromechanical coupling factor (k_p～58.2%) can be achieved for the composition with x = 0.08 near the coexistence of three ferroelectric phases. Our results show that the materials with the composition located at a region where the three ferroelectric R-O-T phases coexist would have the lowest energy barrier and thus greatly promote the polarization rotation, resulting in a strong piezoelectric response.     

Preparation and properties of KH550-Al2O3/PI–EP nanocomposite material

First, polyimide (PI)–epoxy resin (EP) polymer matrix was prepared from 3,3′-diethyl-4,4′-diamino diphenyl methane (DEDADPM), benzophenone tetracarboxylic acid dianhydride (BTDA) and epoxy resin (E-51), through thermal imide process. Then, the nanometer alumina (Al₂O₃) modified by the coupling agent, (3-aminopropyl)triethoxysilane (KH550), was doped into the PI–EP polymer matrix, using an in situ sol–gel method to prepare a series of KH550-Al₂O₃/PI–EP nanocomposite materials based on different KH550-Al₂O₃ contents. Fourier transform infrared spectroscopy (FTIR) indicated that in the presence of chemical reaction between poly(amic acid) and epoxy resin, an imide ring was formed, the thermal imidization reaction of the materials was completed and the KH550-Al₂O₃ had doped into the PI–EP polymer matrix. The heat-resistance, dielectric specification and mechanical properties of KH550-Al₂O₃/PI–EP nanocomposite materials were evaluated. The results showed that the decomposition temperatures were ranged between 438 and 450 °C, dielectric constant and dielectric loss were in the range of 3.32–3.71 and 1.5 × 10^?3–2.5 × 10^?2, respectively, and they all increased with the increase of KH550-Al₂O₃ content (0–10 wt%), but the shear strength first increased and then decreased, attained its maximum value of 10.64 MPa at 8 wt%, which was about 119 % higher than that of undoped material. The adhesive forces of nanocomposite materials were all at higher level (one or two levels). Thus, the overall performance of KH550-Al₂O₃/PI–EP nanocomposites was the best when the doping amount of KH550-Al₂O₃ was 8 wt%. The properties such as high heat-resistance, dielectric properties and ready attachment of impregnating varnish to steel plate with very high strength fully met the necessary requirement.     

Preparation and thermoelectric properties of Bi2SexTe3−x bulk materials

Bi₂Te₃ and Bi₂Se₃ nanoplates were synthesized by a microwave-assisted wet chemical method, and Bi₂Se_xTe_3−x (x=1, 2, 3) bulk nanocomposites were then prepared by hot pressing the Bi₂Te₃ and Bi₂Se₃ nanoplates at 80 MPa and 723 K in vacuum. The phase composition and microstructures of the bulk samples were characterized by powder X-ray diffraction and field-emission scanning electron microscopy, respectively. The electrical conductivity of the Bi₂Se_xTe_3−x bulk nanocomposites increases with increasing Se content, and the Seebeck coefficient value is negative, showing n-type conduction. The absolute Seebeck coefficient value decreases with increasing Se content. A highest power factor, 24.5 µWcm⁻¹ K⁻², is achieved from the sample of x=1 at 369 K among the studied samples.     

Preparation of proanthocyanidin–chitosan complex and its antioxidant and antibacterial properties

Chitosan (CS) is a natural polysaccharide with beneficial effects on human health. To further strengthen its biological properties, a CS-based complex was prepared by combining it with proanthocyanidin (PA) and named it as PA–CS in this study. The successful preparation of PA–CS was confirmed by Fourier transform infrared spectroscopy. The size of PA–CS particles was distributed between 10 and 100 µm with a mean value of 30.6 µm and low pH and high salt concentration could increase their swelling ratio. The loading capacity of PA in PA–CS particles was 82.2?±?1.7 mg/g, and an increased release of PA from the complex occurred at low pH and high salt concentration. After combination, PA–CS complex exhibited better antioxidant activity than CS, which is a result of the synergistic effects of PA and CS. The reducing power and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity of PA–CS complex are mainly attributed to PA, whereas the superoxide radical scavenging activity of PA–CS complex largely depends on CS. Moreover, PA–CS complex showed similar diameters of inhibitory zone and identical minimal inhibitory concentrations compared with CS, suggesting that the antibacterial potential of PA–CS complex is similar to that of CS. In addition, after 20 days storage, PA–CS complex did not exhibit significant changes in DPPH radical scavenging activity and antibacterial activity, indicating the good stability of this complex.     

Design,fabrication and properties of 1–3 piezoelectric ceramic composites with varied piezoelectric phase distribution

Here the 1–3 connectivity cement/polymer based piezoelectric composites with varied piezoelectric phase distribution were designed. The dielectric, piezoelectric and electromechanical properties of the composites were studied. The results indicate that the composite with varied distribution of piezoelectric ceramic has large relative permittivity, piezoelectric strain constant and electromechanical coupling coefficient at the thickness vibration mode. The composites with varied distribution of matrix phase have larger piezoelectric voltage constant, smaller mechanical quality factor and acoustic impedance value than those with varied distribution of piezoelectric ceramic phase. The electromechanical coupling property of the composites at the planar vibration mode shows obvious dependence on matrix phase distribution. The novel piezoelectric composites show potential applications in fabricating ultrasonic transducers with specific surface vibration amplitude.     

Green synthesis of bi-component Mn3O4–MnO2 nanorods and enhanced catalytic properties

Bi-component Mn₃O₄–MnO₂ nanorods with lengths of up to 30 μm are synthesized using a simple hydrothermal method in the absence of templates. Samples were characterized by FE-SEM, HR-TEM, XRD, XPS, and N₂ adsorption–desorption. The results indicate that the Mn₃O₄–MnO₂ and Mn₅O₈ samples with various morphologies can be easily obtained via modulating Cl⁻ ions and glucose contents. Compared with that of the Mn₅O₈ samples, the bi-component Mn₃O₄–MnO₂ nanorods exhibited higher catalytic properties for oxidative decomposition of MB owing to its adjustable valence state and oxygen content.     

Growth and characterization of Ag–Al2O3 composites thin films for thermoelectric power generation applications

Thin films of Ag–Al₂O₃ composites were successfully grown on Si substrate by thermal evaporation method and their thermoelectric performance was modulated using post growth annealing technique. Pellet of Ag and Al mixture having 1:4 ratio was evaporated on Si substrate using the vacuum tube furnace. As grown sample was cut into pieces and post-growth annealing was performed at different temperatures using muffle furnace. XRD results suggested that as-deposited sample has amorphous nature, but crystallinity of the samples increase as an annealing temperature increase from 600 to 900^oC. This structural behavior of annealed samples was further verified by Raman spectroscopy measurements. We have reported an optimal annealing temperature (800 ⁰C) for the best thermoelectric performance of investigated composites. At this specific annealing temperature, charge carriers are highly mobile which resulted in the enhancement of thermoelectric power generation performance of Ag–Al₂O₃ composite. The value of power factor (1.38x10^?2 W/m-K^?2) reported in the current study is the highest value for Ag–Al₂O₃ composites so for reported in the literature according to the best of our knowledge.     

Novel method of fabrication of polyaniline–CdS nanocomposites: Structural,morphological and optoelectronic properties

Polyaniline–CdS nanocomposites have been synthesized by spin coating technique. The nanocrystalline CdS powder of particle size 40–50 nm was synthesized by sol–gel technique and the polyaniline was synthesized by chemical oxidative polymerization of aniline. The composite films were characterized by X-ray diffraction (XRD), field effect scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), UV–Vis spectroscopy and Four probe method. The results were compared with corresponding data on pure polyaniline films. The intensity of diffraction peaks for PANi–CdS composites is lower than that for CdS. The conductivity measurement shows that molecular chain constitution of polyaniline is the most important carrier in polyaniline–CdS nano composite. The optical studies showed that variation in band gap of polyaniline (3.40 eV) to 2.54 eV CdS which is attributed to the interaction of CdS nanoparticles with PANi molecular chains.     

MoSi2–Si3N4 composites: Influence of starting materials and fabrication route on electrical and mechanical properties

The fabrication method and the mechanical and electrical properties of different MoSi₂–Si₃N₄ composite materials were investigated. Commercially available individual compounds, one-stage combustion synthesized MoSi₂–Si₃N₄ and submicron MoSi₂ powders were used as starting materials, followed by hot pressing. It was found that the sintering atmosphere used, nitrogen or argon, had a significant effect on the phase composition, mechanical and electrical properties of the final materials. It was shown that in some cases partial nitridation of MoSi₂ occurred with the formation of MoSi₂–Mo₅Si₃–Si₃N₄ ternary composites. The electrical conductivity of the composites depends also on the microstructure of materials. It was shown that the composites fabricated using combustion synthesized MoSi₂ powders (500 nm) are characterized by higher flexural strength at room temperature compared to those from commercial powders. On the other hand, the composites fabricated from the commercial powders had higher strength and fracture toughness at elevated temperatures (up to 1200 °C). For all composites, the strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi₂ phase.     

Thermal–mechanical property and fracture behaviour of plasticised PLA–CaCO3 nanocomposite

Abstract

This study investigates the influence of two plasticisers, polyethylene glycol (PEG) and tributyl citrate (TbC), on the thermomechanical properties and fracture behaviour of nanosized calcium carbonate blended poly(lactic acid). Various compositions of nanocomposites were compounded and processed using co-rotating twin screw extrusion and compression moulding. DMA analysis shows that adding nano-CaCO₃ reduced the storage modulus (E′) of the nanocomposite while the glass transition temperature (T_g) of the samples was not affected. Furthermore, plasticised poly(lactic acid) (PLA) showed an improvement in elongation at break in all samples, and the impact resistance of the nanocomposites was also improved by 1·6 times with the addition of 20 phr PEG plasticiser and by 1·4 times with the addition of 20 phr TbC plasticiser. Morphological study reveals that the fracture behaviour of PLA-CaCO₃ nanocomposites changed from brittle to ductile after plasticisers were incorporated.