Reduction dependent wetting properties of graphene oxide

This study reports contact angle measurements of standard, diol and aromatic solvents on graphene oxide thin films thermally reduced in ultra-high vacuum up to 900 °C. The films were chemically and morphologically characterized using respectively X-ray photoemission spectroscopy and atomic force microscopy. The characterization shows that the wetting occurs in the chemically heterogeneous regime, namely the surface roughness (3 nm) does not influence the wetting properties of the samples. Zisman, Owens–Wendt and Neumann methods have been applied in order to calculate the surface free energy of the thin films showing that the Owens–Wendt method best fit the data trends. The surface free energy varies from 51 mN/m (pristine graphene oxide) to 39 mN/m (900 °C reduced graphene oxide). A correlation between the surface chemical composition, the surface free energy and its polar and dispersive components is reported, giving a rationale to the wetting properties of graphene oxide and reduced graphene oxide.     

Pitch-based ribbon-shaped carbon-fiber-reinforced one-dimensional carbon/carbon composites with ultrahigh thermal conductivity

Ribbon-shaped carbon fibers have been prepared from mesophase pitch by melt-spinning, oxidative stabilization and further heat treatment. The internal graphitic layers of ribbon-shaped carbon fibers graphitized at 2800 °C show a highly preferred orientation along the longitudinal direction. Parallel stretched and unidirectional arranged ribbon-shaped carbon fibers treated at about 450 °C were sprayed with a mesophase pitch powder grout, and then hot-pressed at 500 °C and subsequently carbonized and graphitized at various temperatures to produce one-dimensional carbon/carbon (C/C) composite blocks. The shape and microstructural orientation of ribbon fibers have been maintained in the process of hot-pressing and subsequent heat treatments and the main planes of the ribbon fibers are orderly accumulated along the hot-pressing direction. Microstructural analyses indicate that the C/C composite blocks have a typical structural anisotropy derived from the unidirectional arrangement of the highly oriented wide ribbon-shaped fibers in the composite block. The thermal conductivities of the C/C composites along the longitudinal direction of ribbon fibers increase with heat-treatment temperatures. The longitudinal thermal conductivity and thermal diffusivity at room temperature of the C/C composite blocks graphitized at 3100 °C are 896 W/m K and 642 mm²/s, respectively.     

The effect of wetting on pore texture and methane storage ability of NaOH activated anthracite

The present work deals with the role of water commonly present in carbonaceous materials, or added to them, on the NaOH activation process. The preliminary wetting of an anthracite subsequently activated with NaOH in definite conditions (1 h at 730 °C, mass ratio NaOH/carbon = 3) and its consequence on the pore texture and resulting methane adsorption capacities are discussed. Water was added to a powder of anthracite according to wetting ratios water/carbon ranging from 0% to 30%. Significant effects on BET surface area, pore texture, packing density and corresponding methane storage capacity were evidenced. Among the investigated wetting ratios, an optimum of 20–25 wt.% of water added to the anthracite was found to lead to the highest adsorption properties for methane. An additional densification finally allowed reaching deliverable methane capacities higher than 150 V/V.     

Microporous alumina substrate with porosity >70% by gelcasting

Microporous alumina membrane substrate in tubular and planar configurations have been prepared by gelcasting of alumina powder slurry using high amount of urea–formaldehyde as gelling agent followed by humidity controlled drying, binder removal and sintering of the gelled bodies. Porosity of the substrate samples sintered at 1350 °C was more than 70% as measured by mercury porosimeter. More than 51% porosity could be retained even after sintering of the samples at 1450 °C. Average pores size of the membrane substrate samples sintered at temperature in the range from 1250 to 1550 °C varied between 0.42 and 0.56 with a maximum at 1350 °C. More uniform pores were observed in sample sintered at 1450 °C. Urea-formaldehyde polymer present in the gelcast body acts as template for micropores.     

The effect of the substrate on pitch wetting behaviour

The wetting behaviour of a petroleum pitch (modified with a surfactant) and a binder coal-tar pitch was studied using different substrates. The results show that both the pitch and the substrate have a significant influence on wetting behaviour, and consequently, on their mutual interactions during the mixing stage. Low values of surface tension and viscosity in the pitches lead to lower wetting temperatures. It was found that with petroleum coke and magnesia as substrates, the wetting occurs at lower temperatures than with graphite and carbon black for all pitches. Moreover, experiments carried out with amorphous and crystalline alumina revealed that the crystalline order affects the wetting behaviour considerably. Thus, whereas the pitch wets the corundum (crystallized alumina), it does not wet the amorphous alumina (basic, acid and neutral alumina).     

Fabrication of patterned Ba0.71Sr0.29TiO3 thick film on Si substrate by tape casting method

Patterned barium strontium titanate (BST) thick films are fabricated in the grooved silicon substrate using tape casting method and sintered from 800 to 1250 °C for 2 h. The slurry used for the tape casting is from sol-precipitation method, and the crystallization of the as precipitated BST powders is improved by hydrothermal treatment at 200 °C for 5 h. The patterned BST thick films have a size of 800 × 300 μm and homogeneous thickness of 30 μm. After sintering below 1000 °C, the obtained BST thick films have a dielectric abnormality at about 30 °C and the dielectric loss is about 0.02.     

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.     

Synthesis of diamond at sub 300 °C substrate temperature

Sulfur-assisted hot-filament chemical vapor deposition (HFCVD) was employed to grow nanocrystalline diamond at low substrate temperature, ∼ 300 °C, on molybdenum (Mo), and ∼ 250 °C, on polyimide film. The polyimide films remained flexible and strong after the deposition, clearly indicating that they did not experience temperatures near or above the glass transition temperature at ∼ 360 °C. The relative intensity of the diamond peak in the Raman spectra increases when the substrate temperature is decreased from ∼ 500 to ∼ 300 °C, a result that is inverted with respect to HFCVD without sulfur. This behavior was employed to obtain microcrystalline diamond on Mo at ∼ 270 °C. Profound changes induced to the gas phase chemistry and surface reactions when a trace amount of H₂S is added to the HFCVD process seem to enable these results.     

Evaluation of the bond strength of different bonding agents to porcelain and metal alloy

This study was carried out with the purpose of testing the bond strength of different bonding agents bonded to different substrates.Substrates consisted of cylindrical specimens of three different materials: porcelain, metal, and a porcelain–metal combination. Specimens were all 10 mm in diameter and 4 mm thick. Surfaces to be bonded were air-abraded with Al₂O₃ and cleaned ultrasonically in distilled water for 10 min. After the preparation of the surface was complete, three different bonding agents were applied to the central region of the substrates. Composite resin of a 3.5 mm diameter and 2 mm thick was applied. All specimens were thermocycled between 5 and 55 °C for 200 cycles with a 30-s dwell time. After thermocycling, specimens were stored at 37 °C in distilled water for an additional 7 days before being subjected to a shear load. Shear testing was conducted Hounsfield test machine.The univariate analysis of variance and the Duncan multiple comparison test were used for statistical assessment. It was found that both type of bonding agents and of substrate led to statistically significant differences in bond strength (p<0.01).It was found that the highest bond strength was produced by Clearfil and on pure alloy substrate (33.36 MPa) and the lowest bond strength in Single Bond and porcelain–alloy substrate (4.25 MPa).     

Rheological behavior and thermal properties of pitch/poly(vinyl chloride) blends

The effect of adding poly(vinyl chloride) (PVC) and coke filler on the rheological behavior and thermal properties of a coal tar pitch was investigated with a view to developing an appropriate viscoelastic binder for the injection molding of graphite components. Dynamic mechanical analysis revealed that the pitch formed compatible blends with PVC featuring a single glass transition temperature (Tg) intermediate to the two parent Tg’s. Adding PVC to the pitch increased melt viscosity substantially and resulted in strong shear thinning behavior at high PVC addition levels. Adding coke powder as filler increased the melt viscosity even further and enhanced shear thinning trends. Pyrolysis conducted in a nitrogen atmosphere revealed interactions between the PVC and pitch degradation pathways: the blends underwent significant thermal decomposition at lower temperatures but showed enhanced carbon yields at high temperatures. Pyrolytic carbon yield at 1000 °C was further improved by a heat treatment (temperature scanned to 400 °C) in air or oxygen. However, carbon yield decreased with addition of PVC. In addition, the degree of ordering attained following a 1 h heat treatment at 2400 °C also decreased with increasing PVC content.     

Graphite foam from pitch and expandable graphite

Graphite foams were prepared from a coal tar pitch that was partially converted into mesophase. Expandable graphite was used instead of an inert gas to “foam” the pitch. The resulting foam was subjected to a series of heat treatments with the objective of first crosslinking the pitch, and thereafter carbonizing and graphitizing the resulting foam. XRD confirmed that the graphitization at 2600 °C resulted in a highly graphitic material. The porosity of this foam derives from the loose packing of the vermicular exfoliated graphite particles together with their internal porosity. During the foaming process the pitch tends to coat the outside surface of the expanding graphite flakes. It also bonds them together. The graphite foam prepared with 5 wt.% expandable graphite had a bulk density of 0.249 g cm⁻³, a compressive strength of 0.46 MPa and a thermal conductivity of 21 W m⁻¹ K⁻¹. The specific thermal conductivity (thermal conductivity divided by the bulk density) of this low-density carbon foam was 0.084 W m² kg⁻¹ K⁻¹ which is considerably higher than that of copper metal (0.045 W m² kg⁻¹ K⁻¹) traditionally used in thermal management applications.     

Pulsed laser deposition BTS thin films: The role of substrate temperature

BaSn_0.15Ti_0.85O₃ (BTS) thin films were deposited on Pt/Ti/SiO₂/Si(1 0 0) substrate by pulsed laser deposition and the effects of substrate temperature on their structure, dielectric properties and leakage current density were investigated. The results indicate that the substrate temperature has a significant effect on the structural and dielectric properties of the BTS thin films which exhibit a polycrystalline perovskite structure if the substrate temperature ranges within 550–750 °C. The dielectric constant and loss tangent of the BTS thin films deposited at 650 °C are 341 and 0.009 at 1 MHz, respectively, the tunability is 72.1% at a dc bias field of 400 kV/cm, while the largest figure of merit (FOM) is 81.1. The effect of the substrate temperature on the leakage current of the BTS thin films is discussed.     

Microstructure formed by suspension plasma spraying: From YSZ splat to coating

In the current work, by means of suspension plasma spraying (SPS), yttria stabilized zirconia splats and coatings were deposited onto mirror-polished and rough substrates, respectively. The splats were characterized by scanning and transmission electron microscopes (TEM). The influence of substrate temperature on splat morphology was analyzed via comparing the splats deposited onto substrates held at different temperatures, namely room temperature (RT), 300 °C, 500 °C, 700 °C and 900 °C. As the substrates were heated from RT to 300 °C, the feature of splats changed rapidly. However, when the substrate temperature further increased, the variation of morphology was found less conspicuous. Integrated splats were peeled off from the substrate using a “plating and etching” approach for TEM examination. The observation on morphological and crystallographic features of splats revealed that the cooling rate during impacting and spreading of SPS splats was lower than that of splats deposited by conventional atmospheric plasma spraying. Based on the characteristics of splats, a modified mechanism consisting changing of droplets trajectory and shadowing effect was proposed to explain the formation of the microstructure of as-sprayed SPS coating. In addition, the coatings were also examined by X-ray diffraction and nano-indenter to determine the phase composition and microhardness, respectively.     

Microwave dielectric properties of the BaTi5O11 thin film grown on the poly-Si substrate using rf magnetron sputtering

BaTi₅O₁₁ thin films were grown on the poly-Si/SiO₂/Si substrate using rf magnetron sputtering. The BaO-TiO₂ thin film deposited on the poly-Si substrate had an amorphous phase even though the growth temperature was high at 550 °C. The amorphous film was crystallized into the BaTi₅O₁₁ phase when the film was post annealed above 800 °C. The post annealing temperature is one of the most important factors for the formation of the crystalline BaTi₅O₁₁ thin film. The homogeneous BaTi₅O₁₁ thin film was obtained when the film was grown at 550 °C and rapid thermal annealed (RTA) at 900 °C for 3 min. The dielectric constant (ɛ_r) of the BaTi₅O₁₁ film measured at 100 kHz was about 35 and the dissipation factors of all the films were smaller than 4.0%. The dielectric properties of the BaTi₅O₁₁ thin film were also measured at microwave frequencies. For the BaTi₅O₁₁ thin film grown at 550 °C and RTA at 900 °C for 3 min, the ɛ_r of 34–30 and dielectric loss of 0.025 ± 0.005 were obtained at 1–6 GHz.     

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.     

Structure and dielectric properties of barium titanate thin films for capacitor applications

BaTiO₃ is a typical ferroelectric material with high relative permittivity and has been used for various applications, such as multilayer ceramic capacitors (MLCCs). With the tendency of miniaturization of MLCCs, the thin films of BaTiO₃ have been required. In this work, BaTiO₃ thin films have been deposited on Pt-coated Si substrates by RF magnetron sputtering under different deposition conditions. The films deposited at the substrate temperature from 550 °C–750 °C show a pure tetragonal perovskite structure. The films deposited at 550 °C–625  °C exhibit (111) preferential orientation, and change to (110) preferential orientation when deposited above 650 °C. The film morphologies vary with working pressure and substrate temperature. The film deposited at 625 °C and 4.5 Pa has the relative permittivity of 630 and the loss tangent of 2% at 10 kHz.     

Morphology of water-based chemical solution deposition (CSD) lead titanate films on different substrates: Towards island formation

This study aims at the deposition of PbTiO₃ (PT) islands prepared by a water-based chemical solution deposition (CSD). Two aqueous citrato-based PbTiO₃ precursor solutions, either with or without peroxide, are deposited by spin coating. The effect of different substrates on the formation of separated grains or islands is examined. It is observed that spin coating of a 0.6 M precursor solution on a Pt(1 1 1)/IrO₂/Ir/SiO₂/Si substrate gives the best results towards island formation. For this substrate, crystallizations are carried out between 600 °C and 900 °C. A final crystallization at 800 °C results in the highest degree of separation for the islands, while keeping the platinized substrate intact. The deposition of diluted precursors shows that it is possible to form islands from a precursor solution with a concentration down to 0.3 M. Solutions with a lower concentration result in irregularly shaped structures.     

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.     

High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method

Diamond film on titanium substrate has become extremely attractive because of the combined properties of these two unique materials. Diamond film can effectively improve the properties of Ti for applications as aerospace and biomedical materials, as well as electrodes. This study focuses on the effects of process parameters, including gas composition, substrate temperature, gas flow rate and reactor pressure on diamond growth on Ti substrates using the hot-filament chemical vapor deposition (HFCVD) method. The nucleation density, nuclei size as well as the diamond purity and growth tendency indices were used to quantify these effects. The crystal morphology of the material was examined with scanning electron microscopy (SEM). Micro-Raman spectroscopy provided information on the quality of the diamond films. The growth tendency of TiC and diamond film was determined by X-ray diffraction analysis. The optimal conditions were found to be: CH₄:H₂ = 1%, gas flow rate = 300 sccm, substrate temperature T_sub = 750 °C, reaction pressure = 40 mbar. Under these conditions, high-quality diamond film was deposited on Ti with a growth rate of 0.4 μm/h and sp² carbon impurity content of 1.6%.     

Vertical-type Schottky-barrier photodiode using p-diamond epilayer grown on heavily boron-doped p+-diamond substrate

We investigate electrical and optical properties of a vertical-type Schottky-barrier photodiode (SPD) using a boron (B)-doped p-diamond epilayer grown on a heavily B-doped p⁺-diamond (100) substrate with B concentration ([B]) of 1 × 10²⁰ cm^− 3 by microwave plasma chemical-vapor deposition. Surface morphology and [B] in the epilayer are strongly affected by the growth temperature (T_g). Smooth surface and low [B] of (3 ± 2) × 10¹⁵ cm^− 3 with an abrupt [B] profile at epilayer interface are reproducibly obtained for T_g lower than 900 °C. The vertical-type SPD with a semitransparent WC Schottky contact is fabricated on an oxidized surface of the p-diamond epilayer. The SPDs with an ideality factor lower than 1.1 and a reverse leakage current less than 10^− 14 A are reproducibly obtained. The SPD is operable at zero or reverse bias mode with a fast response speed less than 1 s. The external quantum efficiency for 220-nm light illumination is measured to be almost constant value of 3.5 ± 0.5% with increasing the reverse bias voltage from 0 to 5 V before and after annealing at 400 °C for 10 min. It is found that the vertical-type SPD using p⁺-diamond substrate does not provide the photoconductivity gain property.