Synthesis of millimeter-long vertically aligned carbon nanotube arrays on aluminum oxide buffer prepared by layer-by-layer assembly of boehmite nanoplates

To synthesize vertically aligned carbon nanotube (VA-CNT) arrays longer than a millimeter using chemical vapor deposition (CVD), aluminum oxide buffer has to be deposited on supporting substrates to prevent diffusion and aggregation of catalyst nanoparticles. Currently, reliable deposition has to be made using expensive and time-consuming e-beam evaporation or thermal sputtering. Here, we report a simple, low-cost, and scalable method for buffer preparation using layer-by-layer assembly of boehmite nanoplates followed by thermal annealing. On top of buffer prepared using this method, we have grown VA-CNT arrays consisting of CNTs with a length of 1.3(±0.1) mm, an inner diameter of 5.6(±1.3) nm, and a wall number of 4(±1) by using CVD with iron as catalyst and ethylene as carbon source.     

Growth of vertical carbon nanotubes according to the Al2O3 buffer layer preparation

Carbon nanotubes (CNTs) have been extensively studied for several decades due to its promising properties. In this study, effects of the Al₂O₃ buffer layer on the growth of the wellorganized CNTs forest were studied. The buffer layers were prepared by the two methods, direct sputtering using Al with oxygen feeding and annealing the sputtered pure Al films to obtain Al₂O₃ films. FE-SEM and AFM results showed that the surface of the buffer layer by the first method was more clean and smooth. The CNTs forest using the first buffer layer showed better results both in the growth length and CNTs quality.     

Characterization of evaporated and sputtered thin Au layers on poly(ethylene terephtalate)

Gold layers were prepared on poly(ethylene terephtalate) substrate by diode sputtering and vacuum evaporation. The mean layer thickness was determined by atomic absorption spectroscopy. Sheet electrical resistance and reflection of electromagnetic waves were used for the characterization of layers. Surface morphology of the layers was determined using atomic force and scanning electron microscopy. While the sputtering was found to proceed with two different rates, the vacuum evaporation proceeds at a constant rate. Rapid decrease of the sheet resistance was observed during sputtering, depending on the layer thickness, in contrast to vacuum evaporation. This can be due to different mechanisms of the Au deposition. According to the measured reflection of electromagnetic waves, the layers prepared by both techniques, i.e., sputtering and vacuum evaporation, are discontinuous for thicknesses below 4 nm, continuous but heterogeneous for thickness from 4 to 10 nm, and continuous and homogeneous for thickness above10 nm. The morphology of the layers prepared by vacuum evaporation does not depend on the layer thickness. Rounded clusters are observed on the surface of the evaporated layers. The layers prepared by sputtering exhibit significantly different morphology with much smaller, pointed clusters. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99:1698–1704, 2006     

Heat conduction across metal and nonmetal interface containing imbedded graphene layers

Thermal conductance at the interface between metal and non-metal materials in the presence or absence of an inserted graphene layer is measured using a time domain transient thermoreflectance technique. The insertion of a single layer graphene between thermal evaporation Al film and Si substrate enhances the interfacial thermal conductance, because the graphene works as a mask to prevent the metal atoms diffusing into the substrate and causes the reduction of the intermixing layer thickness. Conversely, for the Al/Si interface with the Al film prepared by magnetron sputtering, the insertion of a single layer graphene increases the number of interface and leads to the decrease of the interfacial thermal conductance.     

Adhesion properties of functionally gradient diamond composite films on medical and tool alloys

We have investigated the adhesion properties of microcrystalline diamond thin films on Ti-Al-V alloy, Co-Cr-Mo alloy and steel. Microcrystalline diamond possesses high hardness, a low coefficient of friction, extreme chemical inertness and biocompatibility; these properties can enhance the performance of metal alloys used in medical implants and in machine tools. We have adopted three methods for improving the adhesion of microcrystalline diamond to commonly used metal alloys: (1) by alloying the substrate surface to minimize graphitization; (2) by employing appropriate buffer layers between the diamond film and the substrate; and (3) by creating functionally gradient diamond-(titanium carbide, tungsten carbide, titanium nitride and aluminum nitride) composites. We have demonstrated that functionally gradient discontinuous buffer layers of titanium carbide, titanium nitride, aluminum nitride and tungsten carbide are able to control stress and graphitization in microcrystalline diamond thin films. This work on buffer layers and functionally gradient coatings should allow the development of more adherent crystalline diamond films for medical and tribological applications.     

非晶硅太阳能电池背反ZnO:Al薄膜制备

以ZnO:Al(2%Al2O3,质量分数)为靶材,用射频磁控溅射在玻璃衬底上制备ZnO:Al薄膜,分析了各沉积参数对薄膜光电性能的影响。结果表明:溅射功率对ZnO:Al的透过率影响最大,其次是反应腔室压力,而衬底温度对透过率几乎没有影响。ZnO:Al的电阻率主要取决于衬底温度和溅射功率。综合考虑透过率和电阻率,确定了背反ZnO:Al的最佳沉积参数(衬底温度为200℃,溅射功率为200W,反应腔室压力为0.6Pa),得到了透过率大于85%,电阻率最小为7.6×10-4Ωcm的ZnO:Al薄膜。制备了ZnO:Al/Ag/ss(stainless steel)背反电极,并将其用于非晶硅太阳能电池。与无背反的不锈钢衬底上的电池相比,非晶硅太阳能电池短路电流密度增加了16%。     

Catalytic growth of ZnO nanostructures by r.f. magnetron sputtering

The catalytic effect of gold seed particles deposited on a substrate prior to zinc oxide (ZnO) thin film growth by magnetron sputtering was investigated. For this purpose, selected ultra thin gold layers, with thicknesses close to the percolation threshold, are deposited by thermal evaporation in ultra high vacuum (UHV) conditions and subsequently annealed to form gold nanodroplets. The ZnO structures are subsequently deposited by r.f. magnetron sputtering in a UHV chamber, and possible morphological differences between the ZnO grown on top of the substrate and on the gold are investigated. The results indicate a moderate catalytic effect for a deposited gold underlayer of 4 nm, quite close to the gold thin film percolation thickness.     

Direct growth of vertically aligned carbon nanotubes on a planar carbon substrate by thermal chemical vapour deposition

Uniform, vertically aligned multiwalled carbon nanotube arrays (VACNTs) were grown on glassy carbon-like thin films by thermal chemical vapour deposition (CVD). Thin (5 nm) aluminum and iron catalyst layers were pre-deposited by evaporation on the carbon substrates and VACNTs were grown at 750 °C by water-assisted CVD using ethylene as the carbon source. The aluminum layer was shown to be essential for aligned nanotube growth. VACNT arrays adhered strongly to the carbon film with low contact resistance between the VACNTs and the substrate. The VACNT arrays grown directly on the planar conducting carbon substrate have attractive properties for use as electrodes. Excellent voltammetric characteristics are demonstrated after insulating the arrays with a dielectric material.     

Characteristics of Transparent Conducting Nano-Scaled Thin Films Based on ZnO

Different from the common method used for improving the conductivity of ZnO through doping and/or heat treatments, we have used an alternative, i.e., through the introduction of an Al mid-layer without any heat treatment, to enhance the electrical conductivity. We have investigated a nano-scaled sandwich structure consisting of two outer layers of ZnO and a mid-layer of an ultra-thin aluminum thin film for the improvement of the electrical conductivity of ZnO. The nano-scaled ZnO/Al/ZnO thin films were obtained using an RF sputter deposition method. A number of different RF sputter deposition pressures were used to deposit the ZnO layers with thicknesses ranging from 5 to 180 nm. Al layers with various thicknesses ranging from less than 30 to 210 Å were also obtained using the same RF sputter deposition system. The optical transmittance and electrical resistivity of ZnO films, Al films, and ZnO/Al/ZnO thin films were investigated and compared. We have shown that the use of an ultra-thin Al mid-layer enhances the electrical conductivity of ZnO without scarifying its optical transmittance. Furthermore, the electrical transport in ZnO/Al/ZnO films is believed to be dominated by the electrons in the Al but not by the carrier concentration in the ZnO.     

Peptide Assembly of Al/CuO Nanothermite for Enhanced Reactivity of Nanoaluminum Particles

Biological self-assembly procedures, which are generally carried out in an aqueous solution, have been found to be the most promising method for directing the fabrication of diverse nanothermites, including Al/CuO nanothermite. However, the aqueous environment in which Al nanoparticles self-assemble has an impact on their stability. We show that using a peptide to self-assemble Al or CuO nanoparticles considerably improves their durability in phosphate buffer aqueous solution, with Al and CuO nanoparticles remaining intact in aqueous solution for over 2 weeks with minimal changes in the structure. When peptide-assembled Al/CuO nanothermite was compared with a physically mixed sample in phosphate buffer for 30 min, the energy release of the former was higher by 26%. Furthermore, the energy release of peptide-assembled Al/CuO nanocomposite in phosphate buffer showed a 6% reduction by Day 7, while that of the peptide-assembled Al/CuO nanocomposite in ultrapure water was reduced by 75%. Taken together, our study provides an easy method for keeping the thermal activity of Al/CuO nanothermite assembled in aqueous solution.     

Improved adhesion of aluminum layers deposited on plasma and thermally treated poly(paraphenylene-vinylene) films substrates

Searching for better adhesion properties of metallic thin films to polymer substrates, we have studied the influence of the plasma and thermal treatments of poly(paraphenylene-vinylene) thin films on their adhesion to aluminum layers. The adhesion was found to be substantially increased when the polymer surface was treated with oxygen by RF sputtering, or when it was kept at high temperature prior to the metal deposition. An attempt has been made to explain the adhesion improvement in terms of surface analysis (XPS) and scanning electron microscopy (SEM) results of the treated surfaces. Both the metal-oxygen-carbon complex formation at the interface and the roughness induced by the oxygen treatment were found to be the reasons for the improved adhesion properties.     

Microscale Inhomogeneities in Aluminum Solution-Doping of Silica-Based Optical Fibers

The microstructure of aluminum (Al) solution-doped soot layers in modified chemical vapor deposition fabrication of silica-based optical fibers has been studied. It is shown that such Al doping is predominantly determined by deposition temperature. Radial and longitudinal Al doping distributions have been investigated in soot layers, in fully sintered glass layers, and in collapsed preforms. Formation mechanisms are discussed.     

Effects of Al interlayer coating and thermal treatment on electron emission characteristics of carbon nanotubes deposited by electrophoretic method

The effects of aluminum (Al) interlayer coating and thermal post-treatment on the electron emission characteristics of carbon nanotubes (CNTs) were investigated. These CNTs were deposited on conical-shaped tungsten (W) substrates using an electrophoretic method. The Al interlayers were coated on the W substrates via magnetron sputtering prior to the deposition of CNTs. Compared with the as-deposited CNTs, the thermally treated CNTs revealed significantly improved electron emission characteristics, such as the decrease of turn-on electric fields and the increase of emission currents. The observations of Raman spectra confirmed that the improved emission characteristics of the thermally treated CNTs were ascribed to their enhanced crystal qualities. The coating of Al interlayers played a role in enhancing the long-term emission stabilities of the CNTs. The thermally treated CNTs with Al interlayers sustained stable emission currents without any significant degradation even after continuous operation of 20 h. The X-ray photoelectron spectroscopy (XPS) study suggested that the cohesive forces between the CNTs and the underlying substrates were strengthened by the coating of Al interlayers.     

Fast anodization fabrication of AAO and barrier perforation process on ITO glass

Thin films of porous anodic aluminum oxide (AAO) on tin-doped indium oxide (ITO) substrates were fabricated through evaporation of a 1,000- to 2,000-nm-thick Al, followed by anodization with different durations, electrolytes, and pore widening. A faster method to obtain AAO on ITO substrates has been developed, which with 2.5 vol.% phosphoric acid at a voltage of 195 V at 269 K. It was found that the height of AAO films increased initially and then decreased with the increase of the anodizing time. Especially, the barrier layers can be removed by extending the anodizing duration, which is very useful for obtaining perforation AAO and will broaden the application of AAO on ITO substrates.     

Research on the Combustion Properties of Propellants with Low Content of Nano Metal Powders

A comparison of various experimental results for combustionrelated properties evaluation, including burning rates, deflagration heat, flame structures and thermal decomposition properties, of AP/RDX/Al/HTPB composite propellants containing nano metal powders is presented. The thermal behavior of n‐Al (nano grain size aluminum) and g‐Al (general grain size aluminum i.e., 10 μm) heated in air was also investigated by thermogravimetry. The burning rates results indicate that the usage of bimodal aluminum distribution with the ratio around 4 : 1 of n‐Al to g‐Al or the addition of 2% nano nickel powders (n‐Ni) will improve the burning behavior of the propellant, while the usage of grading aluminum powders with the ratio 1 : 1 of n‐Al to g‐Al will impair the combustion of the propellant. Results show that n‐Al and n‐Ni both have a lower heating capacity, lower ignition threshold and shorter combustion time than g‐Al. In addition n‐Al is inclined to burn in single particle form. And the thermal analysis results show that n‐Ni can catalyze the thermal decomposition of AP in the propellant. The results also confirm the high reactivity of n‐Al, which will lead to a lower reaction temperature and rather higher degree of reaction ratio as compared with g‐Al in air. All these factors will influence the combustion of propellants.     

Nucleation and growth of diamond films on aluminum nitride by hot filament chemical vapor deposition

Nucleation and growth of diamond films on aluminum nitride (ALN) coatings were investigated by scanning electron microscopy, Raman spectroscopy and scratch test. ALN films were grown in a magnetron sputtering deposition. The substrates were Si(111) and tungsten carbide (WC). Chemical vapor deposition (CVD) diamond films were deposited on ALN films by hot filament CVD. The nucleation density of diamond on ALN films was found to be approximately 10⁵ cm⁻², whereas over 10¹⁰ cm⁻² after negative bias pre-treatment for 35 min was −320 V, and 250 mA. The experimental studies have shown that the stresses were greatly minimized between diamond overlay and ALN films as compared with WC substrate. The results obtained have also confirmed that the ALN, as buffer layers, can notably enhance the adhesion force of diamond films on the WC.     

Photoactivity enhancement of zinc sulfide ceramics thin films through ultrathin buffering engineering

Zinc-sulfide (ZnS) thin films 200 nm-thick with various crystal features were fabricated using RF sputtering onto patterned sapphire substrates with and without ultrathin homo-ZnS and hetero-zinc oxide (ZnO) ultrathin buffer layers (approximately 45 nm in thickness). Microstructural analyses revealed that the crystalline ZnS thin films with a columnar grain feature were deposited on the various ultrathin buffer layers-coated substrates through RF sputtering. The surface morphology of the ZnS thin films became rough and the crystal defect density of the ZnS thin films increased when the ZnS thin films were grown on the buffer layers. Comparatively, the rugged and island-like ZnO buffer layer engendered the crystal growth of the ZnS thin film with a higher degree of structural disorder than that of the crystal growth on the ZnS buffer layer. An increased crystal defect number together with the highly rugged film surface of the ZnS thin film buffered with ultrathin ZnO layers efficiently enhanced the photoactivity of the 200 nm-thick ZnS thin film in this study.     

Characteristics of Optimized Al:ZnO sputtering targets prepared by nanostructured powder

Optimized Al:ZnO sputtering target was prepared by cold isostatic pressing (CIP) using nanostructured zinc oxide powder and aluminum oxide powder as raw material. Compared with the target prepared by conventional raw materials, the performance of the optimized Al:ZnO sputtering target is greatly improved. The microstructure of the optimized Al:ZnO sputtering target is refined and its average grain size is less than 5 μm with 99.7% theoretical density. Al:ZnO thin films of both optimized and conventional targets were prepared by RF magnetron sputter and their properties were characterized, respectively. The Al:ZnO thin films obtained by optimized target feature better uniformity and compactness, and the internal stress is −378.8 MPa, which is nearly 2/3 lower than that of the conventional target. The film obtained by optimized targets also features a 97% IR transmittance, 1.71 nm Rq surface roughness and non-offset (002) XRD peak. It can be speculated that the optimized Al:ZnO target has great potential to prepare micrometer scale Al:ZnO films and employed in thin-film ZnO device industry.     

Fabrication and characterization of aluminum nitride thick film coated on aluminum substrate for heat dissipation

For effective heat dissipation in high-power LED applications, aluminum nitride (AlN) thick films as thermally conductive dielectric layers were developed, which were deposited on an Al substrate by aerosol deposition (AD). The aerosol-deposited AlN thick films on Al substrates have advantages over conventional polymer-based dielectric substrates or ceramic substrate mounted heatsink systems including an epoxy adhesive, such as excellent heat dissipation capacity and low thermal resistance. AD is an effective method to fabricate high-quality AlN thick film without the Al₂O₃ phase because the film is formed at room temperature. Highly dense and well-adhered, pure AlN thick films with thicknesses up to 30 µm were deposited on an Al substrate. AlN-Al₂O₃ and AlN-polyvinylidene fluoride (PVDF) composite films were also deposited on an Al substrate in order to investigate the effect of Al₂O₃ and polymer on the microstructure and thermal properties. Among the films, pure AlN thick film exhibited the highest dielectric strength, the highest thermal conductivity, and the lowest thermal resistance. Therefore, it can be expected that the aerosol-deposited AlN thick film on Al substrate could be used as a powerful heatsink.     

Thermal degradation of urea‐formaldehyde cellulose composites filled with aluminum particles: Kinetic approach to mechanisms

This article reports a study on structural characterization and thermal degradation kinetics of insulating/conducting urea‐formaldehyde cellulose (UFC) composites filled with aluminum particles. Structural characterization of UFC/Al composites carried out by SEM, XRD, and FTIR analyses reveals that composites are fairly homogenous, and the interactions between UFC and aluminum in UFC/Al composites are more probably physical in nature. Measurements of inherent thermal stabilities, probing reaction complexity, and thermal degradation kinetics of UFC and UFC/Al composites have been undertaken by thermogravimetric (TG)/differential thermogravimetric (DTG) analyses under nonisothermal conditions. The integral procedure decompositions temperature (IPDT) elucidates significant thermal stability of UFC, and higher aluminum contents in composites are capable of enhancing the thermal stability of UFC resin. TG/DTG analyses suggest highly complicated thermal degradation profiles of UFC and UFC/Al composites, which consist of various parallel/consecutive reactions. Generalized linear integral isoconversional method has been employed to determine the activation energies of thermal degradation processes. Substantial variations in activation energies of UFC and UFC/Al composites with the advancement of reaction verify their multi‐step reaction pathways. Advanced reaction model determination methodology with the help of a novel kinetic function F(α,T) reveals that the multi‐step thermal degradation of UFC goes to completion by principally following intricate nucleation/growth mechanisms. It is also found that aluminum more likely participates in the thermal degradation of resin and tends to alter its reaction mechanism. Detailed interpretations of the obtained kinetic parameters are given, and their probable physical significances are discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44826.