TaC Samples Research Articles

Experimental investigation and mathematical modelling of water vapour corrosion of Ti3SiC2 and Ti2AlC ceramics and their mechanical behaviour

Commercially available Ti3SiC2 and Ti2AlC ceramics were used in this study to investigate their wet corrosion and mechanical behaviour as they were under investigation for years for their applications in the field of nuclear as cladding materials and aerospace. The test coupons of dimension 3 × 4 × 40 mm3 and 3 × 4 × 20 mm3 were machined out from commercially available samples for the 3-pt bend test and wet corrosion test, respectively. The water vapour corrosion studies of these samples were carried out at 800 ℃, 1000 ℃, 1200 ℃ for 10, 20 and 100 h in gas flow condition containing 50 % steam + 50 % air. Phase analysis of the as-received Ti3SiC2 and Ti2AlC ceramics revealed the presence of other impurity phases such as TiC and TiSi2. The XRD patterns of the oxidised samples show the formation of rutile as the major phase in both materials. The oxidation layer formed on Ti3SiC2 sample was measured to be 280 μm after exposing the sample in steam for 100 h at 1200 °C. The water vapour corrosion studies reveal that Ti2AlC has high oxidation resistance compared with the Ti3SiC2 due to the formation of protective layers of TiO2 and Al2O3 which resulted in reduced weight gain and oxidation layer thickness. Three-point bend tests were conducted at room temperature for the samples after the water vapour corrosion test at 1000 °C/100 h. The TAC samples showed no degradation in the bending strength (244 MPa) whereas the TSC samples showed reduced strength of 320 MPa. The tensile strength of the samples was measured at room temperature and hydrothermal condition (250 °C and 250 bars pressure) and it was observed that Ti3SiC2 had high tensile strength (190 MPa) in hydrothermal conditions. The tensile strength results were validated using Finite element analysis (FEA) using ANSYS and the FEA results showed a negligible variance of 7 % compared with experimental method. Mathematical modelling based on one dimensional solution of diffusion equation combined with Deal-Grove model was employed to study and compare the oxidation thickness for the linear and parabolic models for the ceramics. The model was effective in validating the oxidation thickness of Ti3SiC2 showing that the experimental thickness was closer to that of mathematical model.

Development of oxide-free oxide materials under conditions of plasma-spark sintering of a mixture of oxide-free components and the addition of various metal powders

Effect of powder additives of W and Mo during spark plasma sintering of com-positions at pressing loading of 60 MPa in the range 1200‒1600 °С on the phase composition, microstructure, grain sizes of crystalline phases, relative density, linear shrinkage, physical-mechanical properties and linear correlation of modulus of Elasticity and fracture toughness of mullite‒ZrB 2 ‒β-SiC‒TaC samples was shown. Synthesised powders of ZrB 2 , β-SiC and TaC are characterisized by intensive crystal-lization of ZrB 2 , β-SiC and TaC cub , respectively. Sintered samples with additives of W and Mo powders show intensive mullitization, active crystallization of β-SiC, (Zr, Ta)B, C, β-WC and β-WB in comparison with lower crystallization of (Ta, Zr)C, B, β-MoC, β-MoB and MoB 2 in the range 1200‒1600 °С. Microstructure of sample with molybdenum additive at 1500 °С more uni-formly and densely sintered, crystalline, containing less quantity of pores and weakly sintered areas, more reinforced at the boundaries areas of oxide-nonoxide and nonoxide crystalline phases. Such sample is characterisized by smaller grains sizes of crystalline phases in the range 1400‒1600 °С and show uniform growing of relative density and linear shrinkage, physical-mechanical properties with larger their values, higher resistance to the cracking with propagation of micro-cracks by meandering path as well as larger linear correlation of modulus of Elasticity and fracture toughness in the range 1200‒1600 °С.

Development of Oxide-Free Oxide Materials under Spark-Plasma Sintering Conditions of a Mixture of Oxide-Free Components and Various Metal Powder Additives

The effects of Wand Mo power additives during spark-plasma sintering of compositions at compression force 60 MPa at 1200 – 1600°C on the phase composition, microstructure, crystalline-phase grain size, relative density, linear shrinkage, physicomechanical properties, and the linear correlation of the elasticity modulus and fracture toughness of mullite–ZrB2–β-SiC–TaC samples were studied. The synthesized ZrB2, β-SiC, and TaC powders were characterized by extensive crystallization of ZrB2, β-SiC, and TaCcub, respectively. Sintered samples withWand Mo powder additives showed extensive mullite formation and more active crystallization of β-SiC, (Zr, Ta)B, C, β-WC, and β-WB than (Ta, Zr)C, B, β-MoC, β-MoB, and MoB2 at 1200 – 1600°C. The microstructure of the sample with the Mo additive at 1500°C was more uniformly and densely sintered and crystalline; contained fewer pores and weakly sintered areas; and was more reinforced at the boundaries of oxide-free oxide and oxide-free crystalline phases. This sample had characteristically smaller grain sizes of the crystalline phases at 1400 – 1600°C and showed uniform growth of the relative density, linear shrinkage, and physicomechanical properties; higher crack resistance (with propagation of microcracks along a meandering path), and a better linear correlation of the elasticity modulus and fracture toughness at 1200 – 1600°C.

Prenatal exposure to legacy contaminants and visual acuity in Canadian infants: a maternal-infant research on environmental chemicals study (MIREC-ID)

BackgroundPrenatal exposure to environmental contaminants can have deleterious effects on child development. While psychomotor, cognitive and behavioural outcomes have been investigated in relation to chronic exposure, the associations with visual functions remains unclear. The present study’s aim was to assess the associations of prenatal exposure to legacy persistent organic pollutants and heavy metals with visual acuity in Canadian infants. The potential protective effects of selenium against mercury toxicity were also examined.MethodsParticipants (mean corrected age = 6.6 months) were part of the Maternal-Infant Research on Environmental Chemicals (MIREC) study. Concentrations of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), lead and mercury were measured in maternal blood during pregnancy, as well as in the cord blood. The Teller acuity card test (TAC) (n = 429) and the visual evoked potentials in a sub-group (n = 63) were used to estimate behavioural and electrophysiological visual acuity, respectively. Multivariable linear regression models were used to investigate the relationship between exposure to each contaminant and visual acuity measures, while controlling for potential confounders. Breastmilk selenium, which was available for about half of the TAC and VEP samples, was also taken into account in the mercury models as exploratory analyses.ResultsWe observed no significant associations between exposure to any contaminants and TAC. Analyses revealed a negative trend (p values < 0.1) between cord blood lead and mercury and electrophysiological visual acuity, whereas PCB and PBDE showed no association. When adding breastmilk selenium concentration to the mercury models, this association became statistically significant for cord concentrations (β = − 3.41, 95% CI = − 5.96,-0.86), but also for blood levels at 1st and 3rd trimesters of pregnancy (β = − 3.29, 95% CI = − 5.69,-0.88). However, further regression models suggested that this change in estimates might not be due to adjustment for selenium, but instead to a change in the study sample.ConclusionsOur results suggest that subtle, but detectable alterations of infant electrophysiological visual acuity can be identified in a population prenatally exposed to low mercury concentrations. Compared to behavioural visual acuity testing, electrophysiological assessment may more sensitive in detecting visual neurotoxicity in relation with prenatal exposure to mercury.

Mechanical properties of tantalum carbide from high-pressure/high-temperature synthesis and first-principles calculations.

As a member of the refractory metal carbide family of materials, TaC is a promising candidate for ultra-high temperature ceramics (UHTC) with desirable mechanical strength. TaC sample quality and therefore mechanical properties are strongly dependent on synthesis method, and atomistic origins of mechanical failure are difficult to assign. Here, we have successfully synthesized high quality densified TaC samples at 5.5 GPa and 1400 °C using the high pressure and high temperature (HPHT) sintering method, with Vickers hardness determined to be 20.9 GPa. First-principles calculations based on the recently developed strain-stress method show that the ideal indentation strength of TaC is about 23.3 GPa in the (11[combining macron]0)[001] direction, in excellent agreement with experimental results. The detailed indentation shear deformation analysis and structural snapshots from the calculations indicate that the slip dislocations of TaC layers are the main structural deformation mode during the Vickers indentation process, and that the strong directional Ta-C bonds are responsible for the high mechanical strength of TaC. HPHT synthesis is shown to produce TaC samples with superior strength, and together with accurate first-principles calculations offers crucial insights for rational design and synthesis of novel and advanced UHTC materials.

Focused ion beam optical patterning of ta-C films

Abstract Optical contrast formation by Ga+ ion implantation has been used for focused ion beam (FIB) writing of nano-scale optical patterns in tetrahedral amorphous carbon (ta-C). UV-vis optical spectroscopy results with Ga+ broad-beam ion implantation have shown a significant shift of the optical absorption edge to lower photon energies as obtained from optical transmission measurements of ta-C samples, implanted with Ga+ at ion energy E = 20 keV and ion fluences D = 3 × 1014 ÷ 3 × 1015 cm− 2. This shift is accompanied by a considerable increase of the absorption coefficient (photo-darkening effect) in the measured photon energy range (0.5 ÷ 3.0 eV). The obtained optical contrast (between implanted and unimplanted film material) could be used in the area of high-density optical data storage using computer controlled focused Ga+ ion beams. The underlying structural modifications, induced by the Ga+ ion bombardment, have been investigated by X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Focused ion beam (FIB) implanted patterns in ta-C samples, obtained with a fluence of 5 × 1015 cm− 2, are also presented.

Electrical conduction of ion tracks in tetrahedral amorphous carbon: temperature, field and doping dependence and comparison with matrix data

This paper gives an extended overview of the electrical properties of ion tracks in hydrogen-free tetrahedral amorphous carbon (ta-C) with a sp3 bond fraction of about 80%. The films were grown by mass selected ion beam deposition of 100 eV 12C+ ions. The ion tracks are generated by irradiation of ta-C films with uranium ions of 1 GeV kinetic energy. Along the ion path a conversion from diamond-like (sp3) carbon to graphite-like (sp2) carbon takes place. Topography and current measurements of individual ion tracks were performed by atomic force microscopy at ambient temperature. The temperature dependence of the electric conductivity was studied between 15 and 390 K by means of 0.28 mm2 large contact pads averaging over about 107 tracks. For each sample and at each temperature the conductivity as a function of the applied electrical field (non-ohmic behaviour) was measured separately and the data were extrapolated to field zero. In this way, the zero-field conductivity was determined independent from the field dependence. In spite of large differences in the absolute values, the temperature dependence of the zero-field conductivities is found to be very similar in shape for all samples. The conductivities follow a law up to temperatures slightly below room temperature. At higher temperatures a transport mechanism based on over-barrier hopping dominates with an activation energy of about 220 meV for tracks and 260 meV for the ta-C matrix. The field dependence measurements show that the deviation of the I–V characteristics from ohmic behaviour decreases with increasing zero-field conductivity. We also tested Cu-doped ta-C samples and found that they conduct significantly better than pure ta-C. However, the doping also increases the zero-field conductivity resulting in a weaker contrast between the track and matrix. The data are interpreted within the so-called ‘barrier model’ where the electrons are assumed to move fairly freely in well-conducting sp2 regions but encounter barriers in track sections consisting of more sp3-like bonds.

Electrochemical detection of hydrogen peroxide on platinum-containing tetrahedral amorphous carbon sensors and evaluation of their biofouling properties.

Hydrogen peroxide is the product of various enzymatic reactions, and is thus typically utilized as the analyte in biosensors. However, its detection with conventional materials, such as noble metals or glassy carbon, is often hindered by slow kinetics and biofouling of the electrode. In this study electrochemical properties and suitability to peroxide detection as well as ability to resist biofouling of Pt-doped ta-C samples were evaluated. Pure ta-C and pure Pt were used as references. According to the results presented here it is proposed that combining ta-C with Pt results in good electrocatalytic activity towards H2O2 oxidation with better tolerance towards aqueous environment mimicking physiological conditions compared to pure Pt. In biofouling experiments, however, both the hybrid material and Pt were almost completely blocked after immersion in protein-containing solutions and did not produce any peaks for ferrocenemethanol oxidation or reduction. On the contrary, it was still possible to obtain clear peaks for H2O2 oxidation with them after similar treatment. Moreover, quartz crystal microbalance experiment showed less protein adsorption on the hybrid sample compared to Pt which is also supported by the electrochemical biofouling experiments for H2O2 detection.

Thin film growth on nanostructured polymer webs for anti-reflection purposes

Nanostructures offer an alternative way of imparting anti-reflection properties to a surface, but most of the structuring techniques are quite complex and cost-intensive. At the Fraunhofer-IOF in Jena, a method for generating nanostructures on a PMMA surface by using a simple plasma-etching procedure has been developed. In the work described herein, we have attempted to transfer this technique to polymer webs in a roll-to-roll process with the intention of creating a highly effective and low-cost anti-reflection surface. Important for prospective applications of the webs is the protection of the nanostructured polymer surface by a thin glass-like layer. The challenge of this task is to enhance the mechanical stability without deteriorating the anti-reflection effect at the same time. First experiments involving the deposition of SiO2 layers in a thickness range from 15nm to 45nm on nanostructured TAC samples showed noticeable differences in the spectral reflectances obtained. While for “bump-like” nanostructures the average residual reflection could be minimized with increasing thickness of the protective layers, the reflectance curves of “sponge-like” samples were simply shifted to longer wavelengths. This could be rationalized in terms of different growth patterns of the SiO2 coatings, which were identified from SEM micrographs. While on a “bump-like” nanostructure the film enwraps the bumps and boosts the material/air fill factor in a suitable range for anti-reflection, a coating on a “sponge-like” structure closes the voids and grows predominantly on top of the structure. On such a “sponge”-type structure, the abrasion resistance can be significantly increased, almost reaching the value of the untreated surface.

Spark plasma sintered tantalum carbide: Effect of pressure and nano-boron carbide addition on microstructure and mechanical properties

TaC and TaC–1wt.% B4C powders were consolidated using spark plasma sintering (SPS) at 1850°C and varying pressure of 100, 255 and 363MPa. The effect of pressure on the densification and grain size is evaluated. The role of nano-sized B4C as sintering aid and grain growth inhibitor is studied by means of XRD, SEM and high resolution TEM. Fully dense TaC samples were produced at a pressure of 255MPa and higher at 1850°C. The increasing pressure also resulted in an increase in TaC grain size. Addition of B4C leads to an increase in the density of 100MPa sample from 89% to 97%. B4C nano-powder resists grain growth even at high pressure of 363MPa. The formation of TaB2/Carbon at TaC grain boundaries helps in pinning the grain boundary and inhibiting grain growth. The effect of B4C addition on hardness and elastic modulus measured by nanoindentation and the indentation fracture toughness has been studied. Relative fracture toughness increased by up to 93% on B4C addition.

A composite modulated structure mechanism for Ag + fast ion conduction in pearceite and polybasite mineral solid electrolytes

A careful electron diffraction investigation of pearceite- Tac and polybasite- Tac samples in the Ag + fast ion conducting state reveals that the average hexagonal [Ag I 9] 9+ sub-structure in these mineral solid electrolyte systems is mutually incommensurable with respect to the remaining [(Ag I,Cu I) 6(As III,Sb III) 2S 7] 2−[Cu IS 4] 7− framework sub-structure. It is shown how the mutual incommensurability of the two component sub-structures can be compatible with crystal chemical commonsense, with previously reported average structure refinements of these phases as well as providing an insight into their Ag + fast ion conductivity at such low temperatures. In addition, Ag +/Cu + ion ordering (and associated displacive shifts of the centreing S 2− ions) in the S 2−(Ag,Cu) 6 octahedral sites is shown to provide a mechanism for low temperature superstructure ordering.

Implantation of D and He in W-Coated Refractory Carbides

The effect of high temperature (700-1200°C) implantation of deuterium and helium in candidate fusion first wall materials was studied in the University of Wisconsin Inertial Electrostatic Confinement (IEC) device. Tungsten coated TaC and HfC ’’foam’’, single crystal tungsten, and high-emissivity tungsten coated ’’foam’’ were compared to previous tungsten powder metallurgy samples studied in the IEC device for the High Average Power Laser (HAPL) program. Scanning electron microscopy was performed to evaluate changes in surface morphology for various ion fluences at temperatures comparable to first wall temperatures. Single crystal tungsten was shown to exhibit less damage than polycrystalline samples at a fluence of 4×1016 He+/cm2. It was found that no significant deformations occur with deuterium implantation up to ~1018 D+/cm2 at 800°C on W-coated TaC and HfC foam samples. However, helium fluences in excess of 6×1017 He+/cm2 show extensive pore formation at 800°C and higher. These changes may have an impact on the lifetime of tungsten coatings on the first walls of inertial and magnetic confinement fusion reactors.

Macrophage responses to vascular stent coatings.

Diamond-like carbon (DLC) films have been proposed as potential coatings for blood-contacting devices. In this study, tetrahedral amorphous carbon (ta-C) films deposited by filtered cathodic vacuum arc system (FCVA) were compared with commercially deposited polyurethane coatings (PU) and uncoated stainless steel samples. X-ray reflectivity (XRR) measurements were performed to check density and thickness of the ta-C coatings, and contact angles measurements were used to assess surface wettability. J774 macrophages were used to assess the cell responses to the materials. Cell number, metabolic activity, and hydrogen peroxide production were measured by using biochemical assays, and the cell attachment and morphology were determined by using scanning electron microscopy and confocal laser scanning microscopy. Wettability measurements showed that of the materials, the stainless steel was the most hydrophilic, and the ta-C coatings were the most hydrophobic. Although the initial attachment and morphology did not appear to be dependent on the wettability, the cell numbers did increase with increasing wettability. Macrophages on the stainless steel samples were the most active in producing hydrogen peroxide. These data show that ta-C samples performed as well as commercial PU-coated samples in blocking cell reactions to the substrate and may prove to be effective coatings for blood-contacting materials.

Low-temperature investigation of paramagnetic centres in tetrahedral amorphous carbon deposited by S-bend FCVA

The electron-spin-resonance (ESR) features of tetrahedral amorphous carbon (ta-C) samples deposited by S-bend filtered cathodic vacuum arc (FCVA) are investigated in a wide temperature range (4.3–295 K) before and after post-deposition annealing at 500 °C. Special attention is given to the temperature dependence of the linewidths. While the sp 3 content does not change with annealing, the temperature trend of the linewidth becomes much smoother for post-deposition annealed samples. The results are discussed in terms of different possible mechanisms of motional line-narrowing and line-narrowing due to hopping motion.

Field emission from undoped and nitrogen-doped tetrahedral amorphous carbon film prepared by filtered cathodic vacuum arc technique

Abstract The field emission results from undoped and nitrogen doped tetrahedral amorphous carbon (ta-C and ta-C:N) prepared by the filtered cathodic vacuum arc (FCVA) technique, deposited on both n+ and p+-type Si are reported. The effect of different types of Si substrate and the film thickness on the onset electric field has been investigated. Three sets of ta-C samples with differing doping concentrations were used in the study: undoped p-type ta-C (p-ta-C), nitrogen weakly doped intrinsic ta-C (i-ta-C) and nitrogen heavily doped n+-type ta-C (n+-ta-C). The heterojunction-based field emission model gives a reasonable explanation for the behavior of the onset electric field measured. The heavily doped hetero-junction, n+-ta-C/p+-Si, demonstrated the lowest onset field of 10 V μ−1 with current densities of 0.1 mA mm−2 at 50 V μm−1 due to the Zener tunneling arising from the severe band bending. A film thickness of 30–40 nm is more favorable for field emission due to the ease with which the film can be fully depleted. At some locations of i-ta-C films, various types of craters were formed after an electrical discharge at a high field (∼58 V μm−1) followed by a subsequent reduction in the onset field to about 15 V μm−1.

Chemical reactions of carbon atoms and molecules from laser-induced vaporization of graphite, TaC and WC

The vapor composition above graphite, tantalum carbide and tungsten carbide together with the chemical reactions of C1, C2 and C3 have been studied by the techniques of cw laser vaporization, matrix isolation, and FTIR spectroscopy. The carbon species were scavengered by cocondensing reactive gases (N2, H2 and CO) with the vapors of graphitic samples in Ar or N2 matrices and then identified in the infrared. From these experiments, N2 is shown to be an ideal scavenger molecule for detecting C1, by the formation of CNN, while H2 is shown to scavenge only the C2 molecule, by forming acetylene. However, CO is shown to be potentially the most useful scavenger molecule since it reacts with both C1 and C2 to form C2O and C3O, respectively. The formation of CNN and C2H2 in N2 matrices is explained by excited state carbon reactions, whereas the formation of all products in reactant gas/Ar matrices is explained by ground state carbon reactions. By using neat N2 matrices, we have measured the CNN:C3 ratio for graphite, TaC and WC samples. From these results, the activity of carbon in WC was determined to be 0.4 while the activity of carbon in TaC was 0.2. The results of this study provide a foundation for understanding the elementary reactions of carbon atoms and molecules and for determining the vapor composition over graphites and carbides at very high temperatures.