Al掺杂ZnO薄膜的结构和热电性质研究

用直流反应磁控溅射方法在透明平面玻璃上制备出Al掺杂ZnO薄膜,并对其结构和温差电动势率进行研究.实验结果表明,所制备样品具有C轴择优取向的多晶结构,温差电动势随着温差(△T)的增大而呈线性增加.并且样品的电阻越大,温差电动势率越小.在室温附近的环境温度对温差电动势率几乎无影响.退火处理后,温差电动势率增大.实验还发现,在外加磁场时,温差电动势率稍微减小.     

Influence of damp heat on the optical and electrical properties of Al-doped zinc oxide

Accelerated ageing in damp heat (DH) can reduce the stability of chalcopyrite solar modules. The decrease in lateral conductivity of the transparent Al-doped zinc oxide (ZAO) front contact contributes significantly to this effect. We present a study on the optical and electrical properties of ZAO without encapsulation on transparent quartz glass substrates with smooth and rough morphology before and after DH. Measured transmission/reflection curves are evaluated numerically to obtain the Drude mobility and carrier concentration. Comparison of optical and electrical data is helpful in evaluating the carrier transport in inhomogeneous ZAO layers, in particular the contributions of grains, grain boundaries and, what we have termed, ‘extended grain boundaries’. The latter are perturbations in the ZAO growth caused by the substrate microstructure.After 1000 h of exposure to DH, the lateral electrical conductivity of ZAO on smooth quartz decreases only by a factor of two, whereas it changes up to two orders of magnitude on rough quartz. Carrier concentration and mobility derived from the optical analysis show a systematic but only minor degradation within the grains. We thus conclude, that the substrate morphology is the dominant factor on the extent of ZAO degradation. Due to the significant difference in the decrease of conductivity on rough and smooth substrates, we postulate that the extended grain boundaries are the prevailing source of this degradation.     

Investigations of diffusion behaviour in Al-doped zinc oxide and zinc stannate coatings

Multi-layer dielectric/silver/dielectric coating systems have excellent proprieties as heat insulators and for solar energy reflection and electrical conductivity. The largest market is dominated by low-emissivity (low-E) coatings, which are applied to large area architectural glazing to reduce heat losses from buildings. They combine high visible transparency with high reflectance in the far-infrared region, where the thin (~ 10 nm) silver layer reflects long wavelength IR back into the building and the dielectric layers both protect the silver and act as anti reflectance layers.In this study, a range of dielectric coatings has been deposited onto soda-lime glass substrates by reactive sputtering from metallic targets. The magnetrons were driven in DC mode and also in mid-frequency pulsed DC and AC modes. Process variables investigated include operating pressure, oxygen flow rate and magnetron configuration. Selected coatings were annealed at 650 °C and analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscope (AFM).The oxide samples were then over-coated with silver and annealed for a second time. These coatings were analysed by secondary ion mass spectroscopy (SIMS) to determine the diffusion rates of silver and sodium (from the substrate) through the oxide coatings.The results to date, presented here, show the diffusion of silver and sodium atoms through zinc oxide and zinc stannate thin films deposited under a vast range of conditions. Preliminary attempts have been made to estimate diffusion coefficients for these coating systems and to relate these values to processing conditions and the structural variations observed.     

Chemically doped macroscopic graphene fibers with significantly enhanced thermoelectric properties

Flexible wearable electronics, when combined with outstanding thermoelectric properties, are promising candidates for future energy harvesting systems. Graphene and its macroscopic assemblies (e.g., graphene-based fibers and films) have thus been the subject of numerous studies because of their extraordinary electrical and mechanical properties. However, these assemblies have not been considered suitable for thermoelectric applications owing to their high intrinsic thermal conductivity. In this study, bromine doping is demonstrated to be an effective method for significantly enhancing the thermoelectric properties of graphene fibers. Doping enhances phonon scattering due to the increased defects and thus decreases the thermal conductivity, while the electrical conductivity and Seebeck coefficient are increased by the Fermi level downshift. As a result, the maximum figure of merit is 2.76 × 10^–3, which is approximately four orders of magnitude larger than that of the undoped fibers throughout the temperature range. Moreover, the room temperature power factor is shown to increase up to 624 μW·m^–1·K^–2, which is higher than that of any other material solely composed of carbon nanotubes and graphene. The enhanced thermoelectric properties indicate the promising potential for graphene fibers in wearable energy harvesting systems.

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Effect of crystallite size of zinc oxide on the mechanical,thermal and flow properties of polypropylene/zinc oxide nanocomposites

ZnO nanoparticles were prepared using zinc chloride and sodium hydroxide in chitosan medium. Prepared ZnO (NZO) and commercial ZnO (CZO) was characterized by scanning electron microscopic and X-ray diffraction studies. PP/ZnO nanocomposites were prepared using 0–5 wt% of zinc oxide by melt mixing. It was then compression moulded into films. Transparency of the composite films were improved by reducing the crystallite size of ZnO. Melt flow index studies revealed that NZO increased the flow characteristics of PP while CZO decreased. X-ray diffraction studies indicated α-form of isotactic polypropylene. An increase in mechanical properties, dynamic mechanical properties and thermal stability of the composites were observed by the addition of ZnO. Uniform dispersion of the ZnO was observed in the scanning electron micrographs of the tensile fractured surface of composites.     

Multifunctional polymer nanocomposites with enhanced mechanical and anti-microbial properties

The paper reports the results of a project aiming to obtain multifunctional binary and ternary polymer nanocomposites with enhanced mechanical and anti-microbial properties. To this end a DGEBA-based epoxy resin is loaded using montmorillonite clays and later used as matrix for glass fibre reinforced laminates. Both binary and ternary nanomodified specimens are manufactured and subjected to mechanical testing. An accurate analysis of the effect of nanomodification on the biological activity is carried out as well.     

Structural and optoelectronic properties of Al-doped zinc oxide films deposited on flexible substrates by radio frequency magnetron sputtering

Al-doped zinc oxide (AZO) thin films were deposited onto flexible polyethylene terephthalate substrates, using the radio frequency (RF) magnetron sputtering process, with an AZO ceramic target (The Al₂O₃ content was about 2 wt.%). The effects of the argon sputtering pressure (in the range from 0.66 to 2.0 Pa), thickness of the Al buffer layer (thickness of 2, 5, and 10 nm) and annealing in a vacuum (6.6 × 10^− 4 Pa), for 30 min at 120 °C, on the morphology and optoelectronic performances of AZO films were investigated. The resistivity was 9.22 × 10^− 3 Ω cm, carrier concentration was 4.64 × 10²¹ cm^− 3, Hall mobility was 2.68 cm²/V s and visible range transmittance was about 80%, at an argon sputtering pressure of 2.0 Pa and an RF power of 100 W. Using an Al buffer decreases the resistivity and optical transmittance of the AZO films. The crystalline and microstructure characteristics of the AZO films are improved by annealing.     

Ultrathin Al-doped transparent conducting zinc oxide films fabricated by pulsed laser deposition

Al-doped transparent conducting zinc oxide (AZO) films, approximately 20-110 nm-thick, were deposited on glass substrates at substrate temperatures between 200 and 300 °C by pulsed laser deposition (PLD) using an ArF excimer laser (λ = 193 nm). When fabricated at a substrate temperature of 260 °C, a 40-nm-thick AZO film showed a low resistivity of 2.61 × 10^− 4 Ω·cm, carrier concentration of 8.64 × 10²⁰ cm^− 3, and Hall mobility of 27.7 cm²/V·s. Furthermore, for an ultrathin 20-nm-thick film, a resistivity of 3.91 × 10^− 4 Ω·cm, carrier concentration of 7.14 × 10²⁰ cm^− 3, and Hall mobility of 22.4 cm²/V·s were obtained. X-ray diffraction (XRD) spectra, obtained by the θ-2θ method, of the AZO films grown at a substrate temperature of 260 °C showed that the diffraction peak of the ZnO (0002) plane increased as the film thickness increased from 20 to 110 nm. The full-width-at-half-maximum (FWHM) values were 0.5500°, 0.3845°, and 0.2979° for film thicknesses of 20, 40, and 110 nm, respectively. For these films, the values of the average transmittance in visible light wavelengths (400-700 nm) were 95.1%, 94.2%, and 96.6%, respectively. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) observations showed that even the 20-nm-thick films did not show island structures. In addition, exfoliated areas or vacant and void spaces were not observed for any of the films.     

Fine-grained polycrystalline MoTe2 with enhanced thermoelectric properties through iodine doping

Consisting of heavy elements and favorable electronic structure, MoTe₂ has great potential as a good thermoelectric material for heat-to-electricity conversion. While some experimental work has been performed on the p-type version, n-type MoTe₂ is theoretically predicted to have a great conversion efficiency and is crucial for eventual device functionality, yet has not been explored. Here, the preparation and thermoelectric properties of n-type iodine-doped nano-polycrystalline MoTe₂ are currently reported. Nano-polycrystalline MoTe_2???xI_x is obtained by ball milling and spark plasma sintering techniques. The composition, morphology and crystal structure of the prepared materials were analyzed by XRD and FESEM, which indicated a homogeneous single phase. The measured transport properties over the temperature range of 298–823 K indicate that iodine doping greatly enhances the carrier concentration and corresponding power factor, and drastically reducing the thermal conductivity. The ECR (Electrical conductivity ratios) carrier scattering analysis demonstrates that dislocation scattering is the main mechanism throughout the experimental temperature range. With the temperature and doping increasing, the thermal conductivity was reduced rapidly, and the minimum value was 1.19 Wm^??1 K^??1 at 673 K. The maximum value of the figure merit ZT?~?0.16 over 673–750 K, which is much higher than other reported values. These excellent properties imply that MoTe₂ will be an efficient candidate for thermoelectric applications.

     

Fabrication of thermoplastic polyester elastomer/layered zinc hydroxide nitrate nanocomposites with enhanced thermal,mechanical and combustion properties

The objective of this study is to explore the potential of layered zinc hydroxide nitrate modified with sodium benzoate as nanoparticle in thermoplastic polyester elastomer (TPEE). The organically modified zinc hydroxide nitrate was compounded with TPEE using solution blending method. The nanocomposite structure was characterized by means of X-ray diffraction and transmission electron microscopy. The results showed that the nanoparticle was homogenously dispersed in TPEE matrix, and partially exfoliated structure was formed. The thermal behavior, mechanical and thermal combustion properties of the novel nanocomposite were studied respectively through differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA) and microscale combustion calorimeter (MCC). For the nanocomposite containing 7 wt% nanoparticle, the crystallization temperature evaluated by DSC was increased by 10 °C. The storage modulus at −95 °C measured by DMA was improved by around 26%. The heat release capacity (an indicator of a material fire hazard) from MCC testing was reduced by about 56% (compared to the results of neat TPEE).     

Damp heat stability of Al-doped zinc oxide films on smooth and rough substrates

Properly encapsulated chalcopyrite-based solar modules from a number of manufacturers have passed the accelerated ageing tests in damp heat.Non-encapsulated modules after damp heat exposure show an increased series resistance which is contributed significantly from the increase in lateral sheet resistance of the transparent Al-doped zinc oxide (ZAO) front contact. We have shown previously, that a rough substrate morphology (on a μm-scale) is a major influence in the ZAO degradation mechanism, due to local perturbations of the ZAO growth (extended grain boundaries).To further model and examine the extended grain boundaries we present a study of the electrical properties of RF-sputtered ZAO films before and after damp heat, grown on rough quartz glass as well as on polished, texture-etched and patterned silicon. The pattern consisted of equidistant trenches and have a periodical two-dimensional geometry.The strongest decrease of ZAO conductivity occurs on the patterned silicon substrate perpendicular to the trenches. The conductivity parallel to the trenches, however, had the same trend as the conductivity of ZAO on smooth silicon. Measured activation energies of the conductivity of 40 meV or less after damp heat are not sufficient to explain the drastic decrease of conductivity in terms of a grain-boundary-barrier theory (as described, e.g., by Seto). We will summarise our current understanding of the extended grain boundaries and present a refined barrier model to explain our experimental observations.     

Enhancement of thermoelectric properties by modulation-doping in silicon germanium alloy nanocomposites

Modulation-doping was theoretically proposed and experimentally proved to be effective in increasing the power factor of nanocomposites (Si(80)Ge(20))(70)(Si(100)B(5))(30) by increasing the carrier mobility but not the figure-of-merit (ZT) due to the increased thermal conductivity. Here we report an alternative materials design, using alloy Si(70)Ge(30) instead of Si as the nanoparticles and Si(95)Ge(5) as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 ± 0.1 at 900 °C.     

Improved exfoliation of surface-functionalized graphene oxide by epoxy monomer and enhanced mechanical properties of epoxy nanocomposites

Journal of Materials Science - The surface functionalization of graphene oxide (GO) provides an efficient way to improve the dispersion of GO in matrix and the interfacial properties of...     

Electrodeposition of Al-doped ZnO nanoflowers with enhanced photocatalytic performance

In this study, Al-doped ZnO nanoflowers were fabricated on conductive substrates via a simple electrodeposition process. The Al-doped ZnO nanoflowers are three-dimensional micro/nano hierarchical structures composed of numerous nanosheets. The chemical composition and crystal structure of the as-synthesized nanoflowers were characterized by EDS, XPS, XRD and HRTEM. It was found that the Al doping led to the decrease of the band gap of ZnO from 3.21 eV to 3.07 eV. Considering the large surface areas, the Al-doped ZnO nanoflowers were used as the photocatalyst for degradation of methyl orange, and exhibited a significantly enhanced performance comparing with the undoped ZnO nanostructures. The good photocatalytic performance should be related to the large surface areas of the nanoflowers and the more free carriers in the Al-doped ZnO, which are introduced by the dopants.     

Reduced graphene oxide enhanced magnetic nanocomposites for removal of carbamazepine

Ferrite, as a kind of common magnetic adsorbent, always tend to reuniting and lead to the poor performance for removing contaminant in aqueous. Meanwhile, reduced graphene oxide (rGO), a high-efficiency adsorbent used for water treatment, is hydrophobic and easy to stack because of the Van der Waals force, resulting in the low adsorption capacity. Herein, we prepare the magnetic CoFe₂O₄/rGO nanocomposites to solve the reuniting of CoFe₂O₄ and stacking of rGO simultaneously. The rGO nanosheets can improve the dispersion of CoFe₂O₄ nanoparticles. As sorbent, the adsorption behaviors of carbamazepine on the nanocomposites can be fitted well by the Langmuir and pseudo-second-order kinetic models. In addition, the CoFe₂O₄/rGO nanocomposites show enhanced adsorption performance than that of the pure CoFe₂O₄ nanoparticles, and the loading of rGO can affect their adsorption performance. The adsorption of carbamazepine on CoFe₂O₄/rGO is exothermic and mainly controlled by π–π interaction and hydrogen bond interaction. Furthermore, the CoFe₂O₄/rGO can be used to remove other organic pollutants simultaneously. Finally, the nanocomposites can be collected by external magnet, and the regenerated nanocomposites still showed a high adsorption capacity retention (90%) after five cycles.     

The characteristics of transparent conducting Al-doped zinc oxide thin films deposited on polymer substrates

Al-doped zinc oxide (AZO) transparent, conductive thin films were deposited on inexpensive polyethylene terephthalate substrates, using radio frequency (rf) magnetron sputtering, with an AZO ceramic target (the Al₂O₃ content is approximately 2 wt%). This paper presents an effective method for the optimization of the parameters for the deposition process for AZO thin films with multiple performance characteristics, using the Taguchi method, combined with grey relational analysis. Using the Taguchi quality design concept, an L₉ orthogonal array was chosen for the experiments. The effects of various process parameters (rf power, substrate-to-target distance, substrate temperature and deposition time) on the electrical, structural, morphological and optical properties of AZO films were investigated. In the confirmation runs, using grey relational analysis, the electrical resistivity of the AZO films was found to have decreased from 5.0?×?10^?3 to 1.6?×?10^?3?Ω-cm and the optical transmittance was found to have increased from 74.39 to 79.40%. The results demonstrate that the Taguchi method combined with grey relational analysis is an economical way to obtain the multiple performance characteristics of AZO films with the fewest experimental data. Additionally, by applying an Al buffer layer, of thickness 10?nm, the results show that the electrical resistivity was 3.1?×?10^?4?Ω-cm and the average optical transmittance, in the visible part of the spectrum, was approximately 79.12%.     

Disordered zinc in Zn4Sb3 with phonon-glass and electron-crystal thermoelectric properties

By converting waste heat into electricity, thermoelectric generators could be an important part of the solution to today's energy challenges. The compound Zn(4)Sb(3) is one of the most efficient thermoelectric materials known. Its high efficiency results from an extraordinarily low thermal conductivity in conjunction with the electronic structure of a heavily doped semiconductor. Previous structural studies have been unable to explain this unusual combination of properties. Here, we show through a comprehensive structural analysis using single-crystal X-ray and powder-synchrotron-radiation diffraction methods, that both the electronic and thermal properties of Zn(4)Sb(3) can be understood in terms of unique structural features that have been previously overlooked. The identification of Sb(3-) ions and Sb(2)(4-) dimers reveals that Zn(4)Sb(3) is a valence semiconductor with the ideal stoichiometry Zn(13)Sb(10). In addition, the structure contains significant disorder, with zinc atoms distributed over multiple positions. The discovery of glass-like interstitial sites uncovers a highly effective mechanism for reducing thermal conductivity. Thus Zn(4)Sb(3) is in many ways an ideal 'phonon glass, electron crystal' thermoelectric material.     

Synthesis and characterization of binary transition metal oxide/reduced graphene oxide nanocomposites and its enhanced electrochemical properties for supercapacitor applications

SnO₂/Co₃O₄ (BTMO) with reduced graphene oxide (rGO) nanocomposite were synthesized by co-precipitation method to determine its electrochemical properties for the betterment of Supercapacitor applications. The XRD pattern of BTMO/rGO nanocomposite shows tetragonal rutile and spinal cubic structure. The XRD peak of BTMO/rGO nanocomposite is comparatively broader than the BTMO nanocomposite and bare nanoparticles due to the presence of high surface area rGO. From the SEM image it is observed that the BTMO nanocomposite has comparatively larger particles than the bare nanoparticles and BTMO/rGO nanocomposites. Hence, the BTMO/rGO nanocomposite has alteration in surface to volume ratio and improved electron conductivity were observed with increased integral area and current such as 2.5117?×?10^?4 A/s and 3.1686?×?10^?4 A respectively in CV behavior, when it is compared to BTMO nanocomposite and bare nanoparticles. The BTMO/rGO nanocomposite also has an increased specific capacitance value of 317.2 F/g at 1 A/g. The increased specific capacitance value of BTMO/rGO nanocomposites are mainly due to the synergistic effect between SnO₂/Co₃O₄ and rGO. Hence, it may be responsible for the improved electron conductivity, due to the free diffusion pathway for the fast ion movement and also it has easily ion accessibility nature to the storage sites makes the materials with both the electric double layer capacitance and pseudocapacitance behavior. Hence, BTMO/rGO nanocomposite would be a promising candidate material for energy storage supercapacitor application.     

Thermal and aqueous stability improvement of graphene oxide enhanced diphenylalanine nanocomposites

Abstract

Nanocomposites of diphenylalanine (FF) and carbon based materials provide an opportunity to overcome drawbacks associated with using FF micro- and nanostructures in nanobiotechnology applications, in particular their poor structural stability in liquid solutions. In this study, FF/graphene oxide (GO) composites were found to self-assemble into layered micro- and nanostructures, which exhibited improved thermal and aqueous stability. Dependent on the FF/GO ratio, the solubility of these structures was reduced to 35.65% after 30 min as compared to 92.4% for pure FF samples. Such functional nanocomposites may extend the use of FF structures to e.g. biosensing, electrochemical, electromechanical or electronic applications.