Oxidation protection of C/C composites with a multilayer coating of SiC and Si + SiC + SiC nanowires

An oxidation protective Si–SiC coating with randomly oriented SiC nanowires was prepared on the SiC-coated carbon/carbon (C/C) composites by a two-step technique. First, a porous network of SiC nanowires was produced using chemical vapor deposition. This material was subjected to pack cementation to infiltrate the porous layer with a mixture of Si and SiC. The nanowires in the coating could efficiently suppress the cracking of the coating by various toughening mechanisms including nanowire pullout, nanowire bridging, microcrack deflection and good interaction between nanowire/matrix interface. The results of thermogravimetric analysis and thermal shock showed that the coating had excellent oxidation protection for C/C composites between room temperature and 1500 °C. These results were confirmed by two additional oxidation experiments conducted at temperature of 900 and 1400 °C, which demonstrated that the coating could efficiently protect C/C composites from oxidation at 900 °C for more than 313 h or at 1400 °C for more than 112 h.     

Growth mechanism of β-SiC nanowires in SiC reticulated porous ceramics

Polycarbosilane (PCS) was used as a precursor to prepare SiC reticulated porous ceramics (RPCs) with in situ growth of β-SiC nanowires at 1000–1300 °C. The nanowires in diameters of ∼50 nm exist on the surface of the strut and in the fracture surface of strut in SiC RPCs. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) indicate that the nanowire consists of a twinned β-SiC, which grows along the 〈1 1 1〉 direction. Field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS) reveal that β-SiC nanowire grows by the vapor–liquid–solid (VLS) process at low temperature. The morphologies of the nanowire formed at different temperatures testify the process. As the heat-treated temperature increased, the growth mechanism of the nanowire changes from VLS to vapor–solid (VS).     

Synchrotron X-ray diffraction study on a single nanowire of PX-phase lead titanate

Synchrotron X-ray diffraction (XRD) measurements of Pb–Ti–O fine acicular crystals (nanowires) of the so-called PX-phase have been performed. The θ–2θ patterns of the nanowires dispersed on substrates with two different geometries indicated that the PX-phase Pb–Ti–O nanowire has [1 1 0] along its short axis and [0 0 1] along its longitudinal axis. Using a focused micro-X-ray beam, we first demonstrated the XRD analysis of a single nanowire of PX-phase Pb–Ti–O. Bragg reflections from the same single nanowire with scattering vectors at two different angles suggested its tetragonal unit cell having the rather large a-axis lattice constant.     

Deposition and characterization of AZO thin films on flexible glass substrates using DC magnetron sputtering technique

Al-doped zinc oxide (AZO) thin films were deposited onto flexible ultra-thin glass substrates by using a direct current (DC) magnetron sputtering process. The effects of sputtering power, working pressure and substrate temperature on the morphology and optoelectronic performances of AZO films were investigated. The optimal sputtering power, working pressure and substrate temperature for AZO film were determined to be 100 W, 0.9 Pa and 150 ℃, respectively. Further increasing or decreasing the sputtering power, working pressure and substrate temperature degrades the quality of AZO films. XRD patterns show all as-sputtered AZO thin films are preferred to grow along <0002> direction. Moreover, the largest grain size, which depicts the best microstructure of AZO films, matches with the smallest stress value. It can be seen from SEM images that the surface is smooth and dense. The smallest value of the resistivity is 1.784×10⁻³ Ω cm and the average transmittance of all AZO films in the visible range is about 80%. The X-ray photoelectron spectroscopy spectra show that the amount of Al element in the AZO film is very small.     

Transparent gas senor and photodetector based on Al doped ZnO nanowires synthesized on glass substrate

In this work high density, well-aligned Al doped ZnO (AZO) nanowires are hydrothermally synthesized on glass substrate at 99 °C. The Al content is ~1.57 at%. The PL spectrum shows that Al impurities caused an increase in the number of oxygen vacancies. The spectral response results show that the maximum responsivity and quantum efficiencies η of AZO NWs are 3.61 A/W and 84.9%, at an incident light wavelength of 360 nm. These AZO NWs have less humidity sensitivity, thus decreasing the effect of humidity effect on gas sensing. Low gas concentrations of 10 ppm ethanol and 10 ppm acetone can be detected with good responses of 24.5% and 21.2%, using the AZO NW sensor at 200 °C and with 0.1 V applied bias.     

The O2-dependent growth of ZnO nanowires and their photoluminescence properties

ZnO nanowires were massively synthesized on a Ni(NO₃)₂-coated silicon substrate under oxygen-containing argon atmosphere by a simple chemical vapor deposition method. The average diameter of the ZnO nanowires was about 50 nm and the average length was about 20 μm. The morphologies of the ZnO nanowires strongly depended on oxygen content in the growth atmosphere. At low oxygen concentration (about 5–10 ppm), ZnO nanocones and nanoneedles were obtained, while at high oxygen concentration (about ∼250 ppm), ZnO nanoparticles deposited on the substrate. The room temperature photoluminescence (PL) spectrum of the ZnO nanowires revealed that a strong UV band at 384 nm dominated the whole spectrum. These results indicate that the ZnO nanowires grown under oxygen-containing atmosphere possess better crystalline quality and UV luminescence properties than those grown in reducing hydrogen atmosphere. Based on the analysis of oxygen effect on the ZnO nanostructures, a vapor–liquid–solid mechanism assisted by the redox growth mode was proposed to understand the growth of the ZnO nanowires.     

Transparent and high infrared reflection film having sandwich structure of SiO2/Al:ZnO/SiO2

New transparent and high infrared reflection films having the sandwich structure of SiO₂/Al:ZnO(AZO)/SiO₂ were deposited on the soda-lime silicate glass at room temperature by radio frequency (R.F.) magnetron sputtering. The optical and electrical properties of SiO₂ (110 nm)/AZO (860 nm)/SiO₂ (110 nm) sandwich films were compared with those of single layer AZO (860 nm) films and double layer SiO₂ (110 nm)/AZO (860 nm) films. The results show that these sandwich films exhibit high transmittance of over 85% in the visible light range (380–760 nm), and low reflection rate of below 4.5% in the wavelength range of 350–525 nm, which is not shown in the conventional single layer AZO (860 nm) films and double layer SiO₂ (110 nm)/AZO (860 nm) films. Further these sandwich films display a low sheet resistance of 20 Ω/sq by sheet resistance formula and high infrared reflection rate of above 80% in the wavelength range of 15–25 μm. In addition, the infrared reflection property of these sandwich films is determined mainly by the AZO film. The outer SiO₂ film can diminish the interference coloring and increase transparency; the inner SiO₂ film improves the adhesion of the coating to the glass substrate and prevents Ca²⁺ and Na⁺ in the glass substrate from entering the AZO film.     

Fabrication of diamond nanowires for quantum information processing applications

We present a design and a top-down fabrication method for realizing diamond nanowires in both bulk single crystal and polycrystalline diamond. Numerical modeling was used to study coupling between a Nitrogen Vacancy (NV) color center and optical modes of a nanowire, and to find an optimal range of nanowire diameters that allows for large collection efficiency of emitted photons. Inductively coupled plasma (ICP) reactive ion etching (RIE) with oxygen is used to fabricate the nanowires. Drop-casted nanoparticles (including Au, SiO₂ and Al₂O₃) as well as electron beam lithography defined spin-on glass and evaporated Au have been used as an etch mask. We found Al₂O₃ nanoparticles to be the most etch resistant. At the same time FOx e-beam resist (spin-on glass) proved to be a suitable etch mask for fabrication of ordered arrays of diamond nanowires. We were able to obtain nanowires with near-vertical sidewalls in both polycrystalline and single crystal diamond. The heights and diameters of the polycrystalline nanowires presented in this paper are ≈ 1 μm and 120–340 nm, respectively, having a 200 nm/min etch rate. In the case of single crystal diamond (types Ib and IIa) nanowires the height and diameter for different diamonds and masks shown in this paper were 1–2.4 μm and 120–490 nm with etch rates between 190 and 240 nm/min.     

Effects of deposition temperature and time on HfC nanowires synthesized by CVD on SiC-coated C/C composites

HfC nanowires were synthesized on SiC-coated carbon/carbon composites via a catalyst-assisted chemical vapor deposition (CVD) process from the HfCl₄–CH₄–H₂ system. The effects of deposition temperature (1273, 1323, 1373 and 1423 K) and time (20, 40, 60 and 90 min) on the formation and microstructure of HfC nanowires were investigated. The results showed that the diameter of HfC nanowires increased with the deposition temperature increasing; both the density and thickness of HfC nanowire films increased with the deposition time prolonging. The growth of HfC nanowires followed the bottom-type vapor–liquid–solid (VLS) mechanism.     

Transformation of CaCO3 to single crystalline micro/nanowires

Single crystalline CaCO₃ micro-nanowires were grown from the solution using poly-vinyl alcohol (PVA) to direct crystal growth and SiO₂ nanoparticles acting as seeds for the growth of amorphous CaCO₃. The aragonite and calcite micro/nanowire, diameter ranging from 70 to 700 nm, were transformed mostly from rhombohedral calcite and vaterite. The micro/nanowires or spicules grown on glass substrate had lengths ranging from 10 to 50 μm and the growth direction showed no orientational relationship to substrate. Without the use of either SiO₂ or PVA, there was no formation of either the amorphous CaCO₃ or the micro/nanowires or spicules.     

Large-scale AlN nanowires synthesized by direct sublimation method

Hexagonal aluminum nitride (AlN) nanowires were fabricated by direct sublimation method without a catalyst layer. The obtained nanowires have diameters of about 30–100 nm and length up to tens of micrometers. TEM observation indicates that these nanowires are single-crystalline and grow along [0 0 0 1] direction. It is thought that vapor–solid (VS) mechanism should be responsible for the growth of AlN nanowires. In addition, room temperature Raman scattering and photoluminescence spectra from AlN nanowires were studied. Photoluminence spectrum of the AlN nanowires shows a wide emission band centered of 517 and 590 nm, which is related to N vacancies and the transition from the level of V_N⁺ to ground state of the deep level of [V_Al^3? + 3O_N⁺] defects, respectively.     

Nanowire of hexagonal gallium oxynitride: Direct observation of its stacking disorder and its long nanowire growth

The crystal structure of gallium oxynitride nanowire was investigated by using scanning transmission electron microscopy. Gallium oxynitride nanowire was directly observed to have a biphasic wurtzite and zinc-blende structure. There was a stacking disorder of several atomic layers between the two phases. The new biphasic nanowire formed due to the presence of Ni in starting material because its nitride has a zinc-blende structure whereas gallium oxynitride has the wurtzite structure. Crystal growth of gallium oxynitride nanowires was studied using seed crystals. Seed crystals and amorphous gallium oxide precursors were annealed under different ammonia flow rates to grow gallium oxynitride nanowires. The nanowires grew to length of 150 μm but they did not grow laterally when the ammonia flow rate was 50 mL/min.     

Synthesis of high-quality AZO polycrystalline films via target bias radio frequency magnetron sputtering

The deposition rate, transmittance and resistivity of aluminium-doped zinc oxide (AZO) films deposited via radio frequency (r.f.) sputtering change with target thickness. An effective method to control and maintain AZO film properties was developed. The strategy only involved the regulation of target bias voltage of r.f. magnetron sputtering system. The target bias voltage considerably influenced AZO film resistivity. The resistivity of the as-deposited AZO film was 9.82×10⁻⁴ Ω cm with power density of 2.19 W/cm² at target self-bias of −72 V. However, it decreased to 5.98×10⁻⁴ Ω cm when the target bias voltage was increased to −112 V by applying d.c. voltage. Both growth rate and optical band gap of AZO film increased with the absolute value of target bias voltage – growth rate increased from 10.54 nm/min to 25.14 nm/min, and band gap increased from 3.57eV to 3.71 eV when target bias voltage increased from −72 V to −112 V at r.f. power density of 2.19 W/cm². The morphology of AZO films was slightly affected by the target bias voltage. Regulating target bias voltage is an effective method to obtain high-quality AZO thin films deposited via r.f. magnetron sputtering. It is also a good choice to maintain the quality of AZO film in uptime manufacturing deposition.     

Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction

The ethanol sensing properties of CuO nanowires prepared by oxidation reaction of copper plate have been examined. The characterization of CuO nanowires by FE-SEM, EDS, and TEM revealed diameters of 100–400 nm and a monoclinic structure with a growth direction along 〈1 1 0〉 direction. The ethanol sensing characteristics of CuO nanowires were studied at ethanol concentrations of 100–1000 ppm and working temperatures of 200–280 °C. An increase of resistance was observed under an ethanol vapor atmosphere due to the p-type semi-conducting property of CuO. It was found that the sensitivity, the response and the recovery time depended on the working temperatures and also ethanol concentration. The sensor exhibited the optimum sensitivity of 1.5 to ethanol vapor concentration of 1000 ppm at the working temperature of 240 °C with a response and recovery time of 110 and 120 s, respectively.     

Mechanical,structural and oxidation resistance enhancement of carbon foam by in situ grown SiC nanowires

A method for processing carbon foams containing both silicon carbide (SiC) nanowires and bulk SiC and silicon nitride (Si3N4) phases has been developed by reaction of powder mixtures containing precursors for carbon, sacrificial template, silicon (Si), short carbon fibers (SCF) and activated carbon (AC). In situ growth of Si nanowires during pyrolysis of the foam at 1000 °C under N2 changed the foam׳s microstructure by covering the porous skeleton inside and out. In situ-grown SiC nanowires were found smoothly curved with diameters ranging around two main modes at 30 and 500 nm while their lengths were up to several tens of micrometers. SCF were found effectively mixed and well-bonded to pore walls. Following density, porosity and pore size distribution analyses, the heat-treated (HT) foam was densified using a chemical vapor infiltration (CVI) process. Thereafter, density increased from 0.62 to 1.30 g/cm³ while flexural strength increased from 29.3 to 49.1 MPa. The latter increase was attributed to the densification process as well as to low surface defects, presence of SCF and coating, by SiC nanowires, of the entire SiC matrix porous structure. The foam׳s oxidation resistance improved significantly from 58 to 84 wt% residual mass of the heat treated and densified sample. The growth mechanism of Si nanowires was supported by the vapor–liquid–solid mechanism developed under pyrolysis conditions of novolac and reducing environment of coal cover.     

Room temperature preparation of high performance AZO films by MF sputtering

Aluminum-doped zinc oxide (AZO) thin films have been deposited by MF magnetron sputtering from a ceramic oxide target without heating the substrates. This study has investigated effects of sputtering power on the structural, electrical and optical properties of the AZO films. The films delivered a hexagonal wurtzite structure with (002) preferential orientation and uniform surface morphology with 27–33 nm grain size. The results indicate that residual stress and grain size of the AZO films are dependent on sputtering power. The minimum resistivity of 7.56×10^?4 Ω cm combined with high transmittance of 83% were obtained at deposited power of 1600 W. The films delivered the advantages of a high deposition rate at low substrate temperature and should be suitable for the fabrication of low-cost transparent conductive oxide layer.     

Combustion synthesis of high porosity SiC foam with nanosized grains

High porosity silicon carbide (SiC) foam with nanosized grains was synthesized by a newly developed process involving two steps: (i) preparation of Si/C foam by gel-casting technique and (ii) fabrication of SiC foam by combustion Si/C foam in nitrogen atmosphere. The as-synthesized SiC foam with a high porosity in the range 70–90% exhibited an attractive strength up to 1.6 MPa. SEM analysis showed that the foam struts consisted of tightly bonded SiC particles with a grain size of 80–300 nm.     

Growth of hexagonal prism ZnO nanorods on Zn substrates by hydrothermal method and their photoluminescence

Hexagonal prism ZnO nanorods were successfully grown on Zn substrates by the 120 °C and 24 h hydrothermal reaction of the solutions with pH of 9–12. Results from XRD, SEM, TEM, SAED and HRTEM showed that the as-synthesized products were wurtzite ZnO with the shape of hexagonal prism nanorods grown along the [0 0 1] direction with smooth prismatic side planes. The PL spectra showed strong emission band at 543 nm in the green-yellow region due to the recombination of electrons trapped in singly ionized oxygen vacancies and photoexcited holes. This facile, reproducible and effective low-cost approach is promising for the future large-scale synthesis of wurtzite ZnO nanostructures for different applications in nanotechnology.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.