Effect of diamond-like carbon (DLC) on the properties of the NiTi alloys

NiTi alloy has found wide application in the biomedical field due to its unique shape memory effect, superelasticity and biocompatibility. However, the materials are vulnerable to surface corrosion and the most serious issue is out-diffusion of toxic Ni ions from the substrate into body tissues and fluids. In this paper, NiTi alloys were coated with diamond-like carbon (DLC) fabricated by plasma immersion ion implantation and deposition (PIIID) to improve their corrosion resistance and blood compatibility without sacrificing their shape memory effect and superelasticity. The structure of the films and the depth profiles between the films and substrate were studied using Raman spectroscopy and XPS, respectively. The phase transformation temperature, superelasticity, anticorrosion behavior and Ni ions release of the coated and uncoated sample were investigated by DSC, tensile tests, potentiodynamic polarization and AAS, respectively. The hemocompatibilty of the coated and uncoated samples was measured using clotting time and platelet adhesion. The results shows that the films is DLC accompanying with the formation of the mixing layer, and the DLC films can markedly improve the corrosion resistance and the hemocompatibility, obviously increase the ratio of albumin-to-fibrinogen and effectively block the Ni ions release of the NiTi alloys without sacrificing its superelasticity and changing its phase transformation temperature. The research results suggest DLC films prepared by PIIID could improve the in vivo performance of NiTi alloys implanted into the human body.     

Effect of temperature and moisture on electrical conductivity in polyaniline/polyurethane (PANI/PU) blends

Miscible blend of conductive polyaniline/polyurethane (PANI/PU) showed preferable electrical property at low percolation threshold compared to immiscible blend of PANI/polystyrene-isoprene-copolymer (PANI/SIS) and carbon black/PU composite (CB/PU). The time dependence of the electrical conductivity was investigated with these samples aged under different humidity and temperatures. The electrical conductivity of PANI/PU (11.5/88.5, v/v) decreased with aging time and the morphology changed with time in the coexistence of high moisture and high temperature. After the aging treatment, the film of the miscible blend was re-dissolved and re-cast. The morphology and electrical conductivity were found to recover to the same state as the original film. In addition, the recovery mechanism of the morphology and the conductivity was also proposed here.     

A feasible directions method for computing Bayes E-optimal block designs

The Bayes extension of the classical E-optimality design criterion is considered for the two-way analysis of variance model. A specialized variant of Topkis and Veinott's feasible directions algorithm is used to solve a convex programming problem arising under special prior assumptions. Numerical examples are computed.     

Self-limited growth of large-area monolayer graphene films by low pressure chemical vapor deposition for graphene-based field effect transistors

During the last decade, fabrication of high-quality graphene films by chemical vapor deposition (CVD) for nanoelectronics and optoelectronic applications has attracted increasing attention. However, processing of large-area monolayer and defect-free graphene films is still challenging. In this work, we have studied the effect of processing conditions on the self-limited growth of graphene monolayers on copper foils during low pressure CVD both experimentally and theoretically based on thermokinetics and kinetics of Langmuir adsorption. The effect of copper pre-treatment, growth time, and carbon potential of the atmosphere (indicated by the methane-to-hydrogen gas ratio, r) on the quality of graphene nanosheets (number of layers, surface roughness and the lateral size) were studied. Microscopic studies show that careful pre-treatment of the copper foil by electropolishing provides a suitable condition for the self-limited growth of graphene with minimum surface roughness and defects. Raman spectroscopy and atomic force microscopy determine that the number of graphene sheets decreases with increasing the carbon potential while smother surfaces are attained. Large-area monolayer graphene films are obtained at relatively high carbon potential (r=1) and controlled growth time (10 min) at 1000 °C. Measurement of the electrical response of the prepared monolayer graphene films on SiO₂ (300 nm)/Si substrates in a field effect transistor (FET) device shows a high mobility of 2780 cm² V⁻¹ s⁻¹. Interestingly, the device exhibits p-type semiconducting behavior with the Dirac point at a gate voltage of 25 V. The finding show a great promise for graphene-based FET devices for future nanoelectronics.     

Advanced deposition characteristics of kinetic sprayed bronze/diamond composite by tailoring feedstock properties

In this study, the effect of protective nickel film on diamond particles was studied with the goal of preventing fracture and obtaining a uniform diamond distribution in a bronze/diamond composite coating during kinetic spraying. Two types of bronze/diamond composite were deposited on aluminum substrate. For comparison with experimental results, the impact behavior between diamonds in the gas flow field was simulated by finite element analysis (using ABAQUS/Explicit 6.7-2). Size distribution and deposition efficiency of the diamond particles in the composite coatings were analyzed through scanning electron microscopy and image analysis methods. Diamond fracturing was avoided because the impact energy was mostly absorbed by the outer protective nickel film on the diamond particle during impact. The uniform distribution and deposition efficiency of diamond particles in the coating layer could also be achieved by tailoring the physical properties (density, size, etc.) of the feedstock.     

Galvanic corrosion of carbon steel coupled to antimony

Galvanic corrosion of carbon steel coupled to antimony was studied in aerated and N₂-purged electrolytes at ambient and 60 °C temperatures. Free corrosion potential of antimony and carbon steel shifts to more active values with increasing temperature and N₂ purging of the electrolyte. Under all experimental conditions, antimony remains less electronegative than carbon steels. Aeration and temperature affect potentiodynamic behaviour of both materials. As a consequence, the corrosion current for the antimony–carbon steel couple increases with increasing temperature and with aeration. There was a good agreement between the corrosion currents obtained through the Evans’ experiment and super-imposed potentiodynamic scans.     

Inhibited grain growth in hydroxyapatite–graphene nanocomposites during high temperature treatment and their enhanced mechanical properties

Nanostructured hydroxyapatite (HA)–graphene nanosheet (GN) composites have been fabricated by spark plasma sintering consolidation. Nanostructual evolution of the bioceramic-based composites during further high temperature heat treatment is characterized and enhanced mechanical strength is assessed. GN keeps intact after the treatment and its presence at HA grain boundaries effectively inhibits HA grain growth by impeding interconnection of individual HA grains. Microstructural characterization discloses strong coherent interfaces between GN and the (300) plane of HA crystals. This particular matching state in the composites agrees well with the competitive theoretical pull-out energy for single graphene sheet being departed from HA matrix. The toughening regimes that operate in HA–GN composites at high temperatures give clear insight into potential applications of GN for ceramic matrix composites.     

Tuning the photoluminescence of large Ti3C2Tx MXene flakes

Ti₃C₂T_x MXene has been reported to be a metallic two-dimensional (2D) material with high conductivity, whereas its photoluminescence (PL) mechanism is still under debate. Herein, we demonstrate that large Ti₃C₂T_x MXene flakes exhibit tunable PL under ambient conditions. The as-prepared Ti₃C₂T_x MXene flakes emit blue, yellow-green and red light under different excitation wavelengths. Their PL emission wavelengths redshift as the excitation wavelength changes from violet to red light. Surface modification of the MXenes can further tune the PL peak wavelength into the near infrared region. Using density function theory (DFT) calculations, this excitation wavelength-dependent PL can be correlated to TiO₂ defects that exist on the surface of Ti₃C₂T_x. Our study expounds on the optical properties of Ti₃C₂T_x MXene and is helpful for comprehensively understanding this novel material.