The performance of CNT as catalyst support on CO oxidation at low temperature

A carbon nanotube (CNT) was used as catalyst support impregnated with transition metal cobalt for CO oxidation at low temperature. Catalyst properties were analyzed by X-ray powder diffractometer (XRD), X-ray photoelectron spectrometer (XPS), and transmission electron microscope (TEM). Analytical results for TEM and XRD demonstrated that cobalt particles were highly dispersed on the carbon nanotube (20–30 nm) with nanosized cobalt particles (10–15 nm). These investigations indicated that Co/CNT generates about 99% of the high activity for CO conversion at 250 °C and thermally stability that is superior to Co/activated carbon (AC). The optimum reaction conditions for CO conversion were O₂ concentration 3%, operation temperature 250 °C, CO concentration 5000 ppm, and space velocity 156,000 h⁻¹. At 250 °C, CO may act as a reductant for NO reduction over Co/CNT in the presence of oxygen, whereas CO/NO = 2.5 showed that maximum NO reduction was 30%. Under H₂ rich conditions, the optimum reaction temperature for CO conversion was under 300 °C, and performance of CO₂ selectivity was better at 200 °C than 250 °C as the oxygen concentration increased.     

Pulsed laser deposition of hard and superhard carbon thin films from C60 targets

In the present study, carbon films were deposited by a pulsed laser deposition method. A C₆₀ fullerene target has been irradiated by a frequency doubled Nd:YAG laser with a pulse duration of 7 ns. The carbon films grown on Si(111) substrates at different substrate deposition temperatures (30, 300 and 500 °C) were characterized by Raman, X-ray Photoelectron and X-ray Auger Electron Spectroscopies, Energy Dispersive X-Ray Diffraction, Scanning Electron and Atomic Force Microscopies, and Vickers microhardness technique. The composition, structure, morphology and mechanical properties of films were found to be strongly dependent on the substrate temperature. At 30 °C and 300 °C deposition temperature, superhard and hard diamond-like films have been obtained, respectively. In the case of 500 °C deposition, a hard film, composed of crystalline C₆₀ and diamond-like carbon, has been prepared.     

Densification during hot-pressing of carbon nanotube–metal–magnesium aluminate spinel nanocomposites

The densification by hot-pressing of ceramic–matrix composites containing a dispersion of carbon nanotubes (CNT), mostly single-walled, is studied for the first time. Fifteen different CNT–Co/Mo–MgAl₂O₄ composite powders containing between 1.2 and 16.7 vol.% CNT were prepared by catalytic chemical vapour deposition. The in situ growth of CNT within the oxide powder made it possible to obtain a highly homogeneous distribution of CNT. Low contents of CNT (up to 5 vol.%) are beneficial for the first shrinkage step (up to 1100 °C), dominated by the rearrangement process, while higher contents are detrimental. At higher temperatures (1100–1300 °C), CNT clearly inhibit the shrinkage, and this detrimental effect regularly increases with the CNT content. Several explanations are proposed, in relation with the particular mechanical properties of CNT and their highly connected web-like distribution within the material.     

Low temperature processed SnO2 films using aqueous precursor solutions

We present a comparison study of the microstructure developments during aqueous solution deposition of SnO₂, particularly, through chemical bath deposition (CBD) and liquid phase deposition (LPD) at very low temperatures (40–75 °C). The effects of solution chemistry on the microstructural details and electrical properties of SnO₂ thin films are presented and discussed. Smooth, nanoparticulate SnO₂ films were obtained from supersaturated precursor solutions with lower precursor concentrations while more aggregated SnO₂ films were generated from higher precursor concentrations. Loosely-packed and porous structures were obtained from low supersaturation solutions with very low pHs. The deposition rates were also evaluated under various deposition conditions. XRD result shows that annealing process helps improve the degree of crystallinity of the as-deposited films that are composed of 3–10 nm nanocrystalline particles. One advantage of LPD of SnO₂ films is in-situ fluorine doping during deposition. The resulting electrical resistivity of F-doped SnO₂ films was about 18.7 Ω cm after the films were annealed at 450 °C.     

Structural,optical, and electrical properties of conducting p-type transparent Cu–Cr–O thin films

Cu–Cr–O films were prepared by DC magnetron co-sputtering using Cu and Cr targets on quartz substrates. The films were then annealed at temperatures ranging from 400 °C to 900 °C for 2 h under a controlled Ar atmosphere. The as-deposited and 400 °C-annealed films were amorphous, semi-transparent, and insulated. After annealing at 500 °C, the Cu–Cr–O films contained a mixture of monoclinic CuO and spinel CuCr₂O₄ phases. Annealing at 600 °C led to the formation of delafossite CuCrO₂ phases. When the annealing was further increased to temperatures above 700 °C, the films exhibited a pure delafossite CuCrO₂ phase. The crystallinity and grain size also increased with the annealing temperature. The formation of the delafossite CuCrO₂ phase during post-annealing processing was in good agreement with thermodynamics. The optimum conductivity and transparency were achieved for the film annealed at approximately 700 °C with a figure of merit of 1.51×10⁻⁸ Ω⁻¹ (i.e., electrical resistivity of up to 5.13 Ω-cm and visible light transmittance of up to 58.3%). The lower formation temperature and superior properties of CuCrO₂ found in this study indicated the higher potential of this material for practical applications compared to CuAlO₂.     

The effect of annealing on electrical properties of fluorinated amorphous carbon films

Fluorinated amorphous carbon (a–C:F) films have been deposited by electron cyclotron resonance chemical vapor deposition (ECR–CVD) at room temperature using C₄F₈ and CH₄ as precursor gases. The chemical compositions and electrical properties of a–C:F films have been studied by X-ray photoelectron spectroscopy (XPS), capacitance–voltage (C–V) and current-voltage (I–V) measurements. The results show that C–CF_x and C–C species of a–C:F films increase and fluorine content decreases after annealing. The dielectric constant of the annealed a–C:F films increases as a result of enhancement of film density and reduction of electronic polarization. The densities of fixed charges and interface states decrease from 1.6 × 10¹⁰ cm^− 2 and (5–9) × 10¹¹ eV^− 1 cm^− 2 to 3.2 × 10⁹ cm^− 2 and (4–6) × 10¹¹ eV^− 1 cm^− 2 respectively when a–C:F films are annealed at 300 °C. The magnitude of C–V hysteresis decreases due to reduced dangling bonds at the a–C:F/Si interfaces after heat treatment. The conduction of a–C:F films shows ohmic behavior at lower electric fields and is explained by Poole–Frankel (PF) mechanism at higher electric fields. The PF current increases indicative of reduced trap energy when a–C:F films are subjected to higher annealing temperatures.     

Hydrothermal and electrochemical growth of complex oxide thin films for electronic devices

Thin film growth of complex oxides including BaTiO₃, SrTiO₃, BaZrO₃, SrZrO₃, KTaO₃, and KNbO₃ were studied by the hydrothermal and the hydrothermal–electrochemical methods. Hydrothermal–electrochemical growth of ATiO₃ (A = Ba, Sr) thin films was investigated at temperatures from 100 to 200 °C using a three-electrode cell. Current efficiency for the film growth was in the range from ca. 0.6 to 3.0%. Tracer experiments revealed that the ATiO₃ film grows at the film/substrate interface. Thin films of AZrO₃ (A = Ba, Sr) were also prepared on Zr metal substrates by the hydrothermal–electrochemical method. By applying a potential above ca. +2 V versus Ag/AgCl to the Zr substrates, AZrO₃ thin films were formed uniformly. Thin films of KTaO₃ and KNbO₃ were prepared on Ta metal substrates by the hydrothermal method. Perovskite-type KTaO₃ thin films were formed in 2.0 M KOH at 300 °C. Pyrochlore-type K₂Ta₂O₆ thin films were formed at lower temperatures and lower KOH concentrations.     

Effects of adding different morphological carbon nanomaterials on supercapacitive performance of sol–gel manganese oxide films

In this study, the supercapacitive performances of manganese oxide films were investigated by adding different carbon nanomaterials, including carbon nanocapsules (CNC), multiwalled carbon nanotubes (MWCNTs) and multi-layered graphene. The manganese oxide films were prepared with manganese acetate precursor by sol–gel method, and the post-treatment effects were also examined. With a heat-treatment above 300 °C, the as-prepared amorphous films transformed to a compound of Mn₃O₄ and Mn₂O₃ phases, and the smooth surface became rough as well. Cyclic voltammogram (CV) tests showed that the manganese oxide film, which was mixed with 0.05 wt% MWCNTs and annealed at 350 °C for 1 h, exhibited the optimized specific capacitance, 339.1 F/g. During 1000CV cycles, the specific capacitances of original manganese oxide film decreased gradually from 198.7 to 149.1 (75%) F/g. After same number of cycle tests, the modified films containing 0.025 wt% CNC, 0.05 wt% MWCNTs and 0.1 wt% graphene retained 201.8 (64.2%), 267.4 (78.9%) and 193.1 (57.4%) F/g respectively. The results indicates that the supercapacitive performance of manganese oxide films were significantly modified by carbon nanomaterials; in addition, the MWCNTs additive could also reduce the decay rate.     

Effect of particle size,heating rate and CNT addition on densification,microstructure and mechanical properties of B4C ceramics

Monolithic B₄C ceramics and B₄C–CNT composites were prepared by spark plasma sintering (SPS). The influence of particle size, heating rate, and CNT addition on sintering behavior, microstructure and mechanical properties were studied. Two different B₄C powders were used to examine the effect of particle size. The effect of heating rate on monolithic B₄C was investigated by applying three different heating rates (75, 150 and 225 °C/min). Moreover, in order to evaluate the effect of CNT addition, B₄C–CNT (0.5–3 mass%) composites were also produced. Fully dense monolithic B₄C ceramics were obtained by using heating rate of 75 °C/min. Vickers hardness value increased with increasing CNT content, and B₄C–CNT composite with 3 mass% CNTs had the highest hardness value of 32.8 GPa. Addition of CNTs and increase in heating rate had a positive effect on the fracture toughness and the highest fracture toughness value, 5.9 MPa m^1/2, was achieved in composite with 3 mass% CNTs.     

Influence of substrate bias voltage on structure and properties of hard Si–B–C–N films prepared by reactive magnetron sputtering

We systematically investigated the effect of the rf induced negative substrate bias voltage, U_b, on characteristics of novel quaternary Si–B–C–N films. The films were deposited on Si(100) or glass substrates by reactive dc magnetron co-sputtering of silicon, boron and carbon from a single C–Si–B or B₄C–Si target in nitrogen–argon gas mixtures at substrate temperatures of 180–350 °C. Elemental compositions of the films, their surface bonding structure, and mechanical and electrical properties were primarily controlled by the U_b values, varied from a floating potential (being between − 30 and − 40 V) to U_b = − 700 V. The energy and flux of ions bombarding the target and the growing films were evaluated on the basis of the measured discharge characteristics. The films were found to be amorphous with thickness up to 5 μm and density around 2.4 g/cm³. They exhibited hardness up to 44 GPa, modified Young's modulus between 170 and 280 GPa, elastic recovery up to 82% and good adhesion to substrates at a low compressive stress (0.6–1.8 GPa). The results of stress measurements were compared with predictions of the model developed by Davis and a beneficial role of silicon in reducing the compressive stress in the films was proved. Electrical conductivity of the semiconductive Si–B–C–N films with a high (approximately 40 at.%) carbon content was controlled by the nitrogen–argon gas mixture composition and the U_b values.     

Preparation of antireflective SiO2 nanometric films

Antireflective nanometric SiO₂ films were formed on glass substrates by dip coating from a colloidal SiO₂ sol having an average particle size of 9 nm. Withdrawal speed of dip coating was varied between 100 and 200 mm/min with 25 mm increments, and baking temperature of the films was altered between 300 and 550 °C with 50 °C increments. Obtained SiO₂ films were in 80–200 nm thickness range. Film thickness was seen to increase with increasing withdrawal speed and to decrease with increasing baking temperature. A maximum light transmittance of 95% was obtained with 4.5% points increase, from the films which were withdrawn at 100 mm/min and baked at 450 or 500 °C. It was seen from SEM observations that the films exhibited full coverage on glass surface and contained no voids or cracks. Size of SiO₂ particles in the film was seen in the AFM analyses to increase with baking temperature. Sintering of SiO₂ particles appeared to accelerate at temperatures over 450 °C.     

Dielectric properties and electrical behaviors of tunable Bi1.5MgNb1.5O7 thin films

The Bi_1.5MgNb_1.5O₇ (BMN) thin films were prepared on Au-coated Si substrates by rf magnetron sputtering. We systematically investigated the structure, dielectric properties and voltage tunable property of the films with different annealing temperatures. The relationships of leakage current and breakdown bias field with annealing temperature were firstly studied and a possible explanation was proposed. The deposited BMN thin films had a cubic pyrochlore phase when annealed at 550 °C or higher. With the increasing of annealing temperature, the dielectric constant and tunability also went up. BMN thin films annealed at 750 °C exhibited moderate dielectric constant of 106 and low dielectric loss of 0.003–0.007 between 10 kHz and 10 MHz. The maximum tunability of 50% was achieved at a bias field of 2 MV/cm. However, thin films annealed at 750 °C had lower breakdown bias field and higher leakage current density than films annealed below 750 °C. The excellent physical and electrical properties make BMN thin films promising for potential tunable capacitor applications.     

Low temperature atomic layer deposition of SiO2 thin films using di-isopropylaminosilane and ozone

Silicon dioxide (SiO₂) films are deposited by atomic layer deposition (ALD) at low temperatures from 100 to 200 °C using di-isopropylaminosilane (SiH₃N(C₃H₇)₂, DIPAS) as the Si precursor and ozone as the reactant. The SiO₂ films exhibit saturated growth behavior confirming the ALD process, showing a growth rate of 1.2 Å/cycle at 150 °C, which increases to 2.3 Å/cycle at 250 °C. The activation energy of 0.24 eV, extracted from temperature range of 100–200 °C, corresponds to the reported energy barrier for reaction between DIPAS and surface –OH. The temperature dependence of the growth rate can be explained in terms of the coverage and chemical reactivity of the thermally activated precursor on the surface. The ALD-SiO₂ films deposited at 200 °C show properties such as refractive index, density, and roughness comparable to those of conventionally deposited SiO₂, as well as low leakage current and high breakdown field. The fraction of Si–O bond increases at the expense of Si–OH at higher deposition temperature.     

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.     

A novel CO and C3H8 sensor made of CuSb2O6 nanoparticles

Trirutile-type CuSb₂O₆ nanoparticles were synthesized by a simple and economical route, starting from copper nitrate, antimony chloride, ethylenediamine, and ethyl alcohol as solvent. The latter was evaporated by microwave radiation at 140 W. The precursor material was calcined at 200, 300, 400, 500, and 600 °C, and analyzed by powder XRD. The oxide phase was obtained at the last calcination step (600 °C), whose powders were analyzed by field-emission scanning electron (FE-SEM) and transmission electron (TEM) microscopies. Microrods, hexagonal microplates, and nanoparticles with an average size of ~ 51.2 nm were observed. A forbidden bandwidth of 3.41 eV was detected for the direct transition with UV–vis. Tests were carried out on pellets made of the powders in carbon monoxide (CO) and propane (C₃H₈) atmospheres at different concentrations and operating temperatures, obtaining high response at 300 ppm of CO and 500 ppm of C₃H₈, both at 300 °C.     

Carbon nanotube and nanofiber syntheses by the decomposition of methane on group 8–10 metal-loaded MgO catalysts

We have found that several precious metal-loaded MgO catalysts are active in the formation of carbon nanotube (CNT) by the chemical vapor deposition (CVD) of methane. The catalysts were prepared with nine metals (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt) by impregnation onto a high surface area MgO. CNT synthesis was carried out in the temperature range from 600 °C to 1000 °C after reduction with H₂ at 800 °C.The amount of carbon deposited and crystallinity in the produced CNT on nine metals showed interesting tendencies: (i) The amount of carbon formed increased in the following transition series metals: first < second < third row transition elements, and (ii) the index of crystallinity I_G/I_D in Raman-bands of the CNTs decreased in the following order: 8 > 9 > 10 in the Periodic Table. Group 8 and 9 metals produced tube type fibers composed of the graphite layers arranged parallel to the fiber axis. On the other hand, carbon nanofibers (CNFs) grown on group 10 metals had herringbone type graphene sheets.     

Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films

Diamond-like carbon (DLC) coatings were successfully deposited on carbon nanotube (CNT) films with CNT densities of 1 × 10⁹/cm², 3 × 10⁹/cm², and 7 × 10⁹/cm² by a radio frequency plasma-enhanced chemical vapor deposition (CVD). The new composite films consisting of CNT/DLC were synthesized to improve the mechanical properties of DLC coatings especially for toughness. To compare those of the CNT/DLC composite films, the deposition of a DLC coating on a silicon oxide substrate was also carried out. A dynamic ultra micro hardness tester and a ball-on-disk type friction tester were used to investigate the mechanical properties of the CNT/DLC composite films. A scanning electron microscopic (SEM) image of the indentation region of the CNT/DLC composite film showed a triangle shape of the indenter, however, chippings of the DLC coating were observed in the indentation region. This result suggests the improvement of the toughness of the CNT/DLC composite films. The elastic modulus and dynamic hardness of the CNT/DLC composite films decreased linearly with the increase of their CNT density. Friction coefficients of all the CNT/DLC composite films were close to that of the DLC coating.     

Polymeric semiconducting carbon from fullerene by pulsed laser ablation

The polymeric semiconducting carbon films are grown on silicon and quartz substrates by excimer (XeCl) pulsed laser deposition (PLD) technique using fullerene C₆₀ precursor. The substrate temperature is varied up to 300 °C. The structure and optical properties of the films strongly depend on the substrate temperature. The grain size is increased and uniform polymeric film with improved morphology at higher temperature is observed. The Tauc gap is about 1.35 eV for the film deposited at 100°C and with temperature the gap is decreased upto 1.1 eV for the film deposited at 250 °C and increased to about 1.4 eV for the film deposited at 300 °C. The optical absorption properties are improved with substrate temperature. Raman spectra show the presence of both G peak and D peak and are peaked at about 1590 cm^− 1 and 1360 cm^− 1, respectively for the film deposited at 100 °C. The G peak position remains almost unchanged while D peak has changed only a little with temperature might be due to its better crystalline structure compared to the typical amorphous carbon films and might show interesting in device such as, optoelectronic applications.     

One-step synthesis of ternary MnO2–Fe2O3–CeO2–Ce2O3/CNT catalysts for use in low-temperature NO reduction with NH3

In this article, a facile one-step strategy for the synthesis of ternary MnO₂–Fe₂O₃–CeO₂–Ce₂O₃/carbon nanotubes (CNT) catalysts was discussed. The as-prepared catalysts exhibited 73.6–99.4% NO conversion at 120–180 °C at a weight hourly space velocity (WHSV) of 210 000 ml·g_cat^− 1·h^− 1, which benefited from the formation of amorphous MnO₂, Fe₂O₃, CeO₂, and Ce₂O₃, as well as high Ce^3 + and surface oxygen (O_ε) contents. The mechanism of formation of MnO₂–Fe₂O₃–CeO₂–Ce₂O₃/CNT catalysts was also proposed.     

Piezoresistivity of n-type conductive ultrananocrystalline diamond

Recent developments of a piezoresistive sensor prototype based on n-type conductive ultrananocrystalline diamond (UNCD) are presented. Samples were deposited using hot filament chemical vapor deposition (HFCVD) technique, with a gas mixture of H₂, CH₄ and NH₃, and were structured using multiple photolithographic and etching processes. Under controlled deposition parameters, UNCD thin films with n-type electrical conductivity at room temperature (5 × 10^− 3 – 5 × 10¹ S/cm) could be grown. Respective piezoresistive response of such films was analyzed and the gauge factor was evaluated in both transverse and longitudinal arrangements, also as a function of temperature from 25 °C up to 300 °C. Moreover, the gauge factor of piezoresistors with various sheet resistance values and test structure geometries was evaluated. The highest measured gauge factor was 9.54 ± 0.32 at room temperature for a longitudinally arranged piezoresistor with a sheet resistance of about 30 kΩ/square. This gauge factor is well comparable to that of p-type boron doped diamond; however, with a much better temperature independency at elevated temperatures compared to the boron-doped diamond and silicon. To our best knowledge, this is the first report on piezoresistive characteristics of n-type UNCD films.