Graphite Disk Research Articles

Elaboration of modified electrodes through a direct anodic oxidation of chitin

By means of cyclic voltammetry study at platinum, gold and graphite electrodes, it is shown that the addition of chitin to a mixed solvent system acetonitrile/dimethylacetamide with the presence of LiCl, leads to the appearance of a quasi-reversible oxidation step and a subsequent generation of coated surfaces, indicating a direct electron ejection from this biopolymer. The electrochemical patterns of this redox system on platinum and Fourier transform infrared spectroscopic characterization of the electrodeposits, on Pt and Au, reveal a coupled chemical conversion of chitin, consecutively to the electrons transfer. By contrast, an adsorption phenomenon is expected on graphite disk. Besides, the electrogenerated films are oxidized with a biggest difficulty than chitin. The macroscale electrolysis of chitin at platinum grid leads to new insoluble oxidation products, which precipitate in the electrochemical cell and on the working electrode.

Crack growth in pyrographite under the conditions of radiation

The damage induced by radiation in the pyrographite is accompanied by significant plastic deformation. Microcracks arise in the edges of the radiated areas that develop radially in the direction of the unaffected matrix. The peculiarities in the formation of the tension state of the radiated area and adjacent unaffected areas are analyzed in order to explain the reasons behind the growth of cracks. The analysis is carried out on graphite disks with constant thicknesses that are exposed to radiation with high-energy electrons in an HVTEM-JEOL 1000. A differential equation for specific load conditions is obtained from the analysis of equilibrium conditions of a disc-shaped element.

Metallic CoS2 nanowire electrodes for high cycling performance supercapacitors

We report metallic cobalt pyrite (CoS2) nanowires (NWs) prepared directly on current collecting electrodes, e.g., carbon cloth or graphite disc, for high-performance supercapacitors. These CoS2 NWs have a variety of advantages for supercapacitor applications. Because the metallic CoS2 NWs are synthesized directly on the current collector, the good electrical connection enables efficient charge transfer between the active CoS2 materials and the current collector. In addition, the open spaces between the sea urchin structure NWs lead to a large accessible surface area and afford rapid mass transport. Moreover, the robust CoS2 NW structure results in high stability of the active materials during long-term operation. Electrochemical characterization reveals that the CoS2 NWs enable large specific capacitance (828.2 F g−1 at a scan rate of 0.01 V s−1) and excellent long term cycling stability (0–2.5% capacity loss after 4250 cycles at 5 A g−1) for pseudocapacitors. This example of metallic CoS2 NWs for supercapacitor applications expands the opportunities for transition metal sulfide-based nanostructures in emerging energy storage applications.

In-situ synthesis of Ni/N-doped CNFs-supported graphite disk as effective immobilized catalyst for methanol electrooxidation

In this study, a facile, controllable, and efficient process (electrospinning followed by calcination at 1100 °C under argon atmosphere) is developed for in-situ synthesis of Ni/N-CNFs catalyst as thin film immobilized on graphite disk. Moreover, the influence of nitrogen content on the catalytic activity of the supported Ni-doped CNFs toward methanol oxidation was investigated. The composition and morphology of the prepared catalysts were characterized by XRD, EDX, FE-SEM and TEM techniques. The electrocatalytic activity and stability were evaluated by cyclic voltammetry (CV) and chronoamperometry, respectively. Compared to the powder form, the results indicated that the proposed in-situ immobilization of the introduced nanofibers on graphite disk reveals distinct enhancement in the electrocatalytic activity due to merging of the underneath nanofibers with the graphite support which eliminates the interfacial resistance. Numerically, the detected maximum current density was 270.44 and 80.59 mA/cm2 for Ni/N-CNFs/graphite and unsupported nanofibers, respectively. Moreover, the obtained results have also showed that nitrogen doping effectively enhances the electrocatalytic activity and stability of Ni/CNFs toward methanol oxidation in the alkaline medium. However, due to the influence of methanol content on the water-alcohol mixture fluidity, the optimum methanol concentration was observed to be 0.5 and 5.0 M for the supported and unsupported nanofibers, respectively. Overall, this study opens new avenue to prepare one-pot current collector/electrode plate to be utilized in the fuel cell technology.

Enhancement of oxygen diffusion process on a rotating disk electrode for the electro-Fenton degradation of tetracycline

An electro-Fenton process was developed for wastewater treatment in which hydrogen peroxide was generated in situ with a rotating graphite disk electrode as cathode. The maximum H2O2 generation rate for the RDE reached 0.90mg/L/h/cm2 under the rotation speed of 400rpm at pH 3, and −0.8V vs SCE. The performance of this electro-Fenton reactor was assessed by tetracycline degradation in an aqueous solution. Experimental results showed the rotation of disk cathode resulted in the efficient production of H2O2 without oxygen aeration, and excellent ability for degrading organic pollutants compared to the electro-Fenton system with fixed cathode. Tetracycline of 50mg/L was degraded completely within 2h with the addition of ferrous ion (1.0mM). The chronoamperometry analysis was employed to investigate the oxygen diffusion on the rotating cathode. The results demonstrated that the diffusion coefficients of dissolved oxygen is 19.45×10−5cm2/s, which is greater than that reported in the literature. Further calculation indicated that oxygen is able to diffuse through the film on the rotating cathode within the contact time in each circle. This study proves that enhancement of oxygen diffusion on RDE is benefit for H2O2 generation, thus provides a promising method for organic pollutants degradation by the combination of RDE with electro-Fenton reactor and offers a new insight on the oxygen transform process in this new system.

Joining of Y<sub>2</sub>O<sub>3</sub>-Doped Aluminum Nitride with High Density Graphite by Spark Plasma Sintering

High density graphite disks and aluminum nitride ceramics powders have been utilized to obtain joints by Spark Plasma Sintering technique. The joining was carried out in vacuum, at temperatures of 1700°C, 1800°C and 1900°C, under the pressure of 50 MPa with a constant dwelling time of 5 minutes The AlN ceramics to be joined were also synthesized by ceramic technology standard route by using AlN powders and 2.5 % wt.Y2O3 powders as sintering additive, which were added in order to increase densification rate and by thus, thermal conductivity. The joining of AlN/C was performed both without and with the aid of a ceramic powder composite AlN+Y2O3+C film, as interlayer. Besides the crystalline phases (AlN and C), the Al5Y3O12 compounds with a cubic crystallographic structure were identified by X-ray diffraction. The optical microscopy images revealed that all samples, both without and with film as interlayer, had strong joined areas, without any defects and discontinuities at interfaces. The Vickers microhardness and Young Modulus values measured by nanoindentation have shown that using of the film as intermediary layer was leading to the highest values of mechanical properties (HV = 8 – 23 GPa and E= 227-512 GPa) at the AlN/C joints interfaces.

Three-Dimensional Porous NiO Nanosheets Vertically Grown on Graphite Disks for Enhanced Performance Non-enzymatic Glucose Sensor

Vertically aligned 3D porous NiO nanosheets (s-NiO) were directly grown on graphite disks (GD) by chemical bath deposition via a surfactant template under mild reaction conditions. Such porous s-NiO as active electrode materials for non-enzymatic glucose sensors manifested a much better electrocatalytic performance than most of the existing electrodes of non-enzymatic glucose sensors, including a fast response time (<1s), a high sensitivity (36.13μAmM−1mm−2), a wide linear range (up to 10.0mM) and a low detection limit (0.9μM at signal/noise ratio (S/N=3) at a potential of 0.50V (vs. Ag/AgCl). Significantly, in comparison with the porous NiO nanofilms (f-NiO) fabricated by the traditional drop-coating method, the as-prepared porous s-NiO showed better stability towards the oxidation of glucose. The high electrocatalytic performance and stability were possibly attributed to the unique 3D porous structure of s-NiO. The 3D porous s-NiO vertically grown on GD (s-NiO/GD) could be a desirable electrocatalyst for enhanced performance non-enzymatic glucose sensor.

Influence of the per pulse laser fluence on the optical properties of carbon nanoparticles synthesized by laser ablation of solids in liquids

In this work we present experimental results on the optical characterization of carbon-nanoparticles (CNPs) synthesized by the laser ablation of solids in liquids technique (LASL). A pulsed Nd-YAG laser, a graphite disk and acetone were used in the laser ablation experiments. The per pulse laser fluence was varied, while all the other irradiation parameters (irradiation time, repetition rate, etc.) were kept constant. Both the graphite target and the obtained CNPs were characterized by Raman micro-spectroscopy. The colloidal solutions were characterized by UV–vis and photoluminescence (PL) spectroscopies. Additionally, the CNPs were also characterized by TEM and HRTEM. Our results show that spherical nanoparticles in the range of 4–20 nm in diameter were obtained. UV–vis and PL results for the obtained CNPs colloidal solutions showed that the optical absorption and PL intensity are dependent on the per pulse laser fluence. We also found that the PL spectral emission of the CNPs can be tuned from blue to yellow by varying the excitation wavelength.

Deuterium/hydrogen permeation through different molecular sieve membranes: ZIF, LDH, zeolite

Different molecular sieve membranes have been evaluated in the separation of deuterium (D2)/hydrogen (H2) mixtures. Both the single gases and the binary D2/H2 mixture have been permeated at room temperature through supported Metal Organic Framework (MOF) membranes (ZIF-8 and ZIF-90) as well as through supported zeolite (SOD, LTA, FAU) and layered double hydroxide (LDH) membranes. Also the D2/H2 permeation through the non-coated bare α-Al2O3 supports, and pressed graphite discs has been studied. Since the molecular mass of D2 is twice the mass of H2, and D2 is slightly smaller than H2, D2 should show higher single component permeability than H2 because of its favored adsorption and diffusion compared with H2. For the MOF membranes (ZIF-8 and ZIF-90) as well as the LDH membrane, the single gas D2 permeabilities are indeed by the factor of 3 larger than those of H2 at room temperature. However, due to strong molecular interaction, no separation is found for the equimolar D2/H2 mixture. For comparison, we also studied the permeation of He/D2. Since these mixture components are of the same molecular mass and almost the same size, no separation can be expected and no separation has been found.

Mechanism of Electrochemical Hydrogenation of Epitaxial Graphene

Mechanism of electrochemical hydrogen adsorption on epitaxial graphene (EG) was observed to be dependent on defects in EG as observed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Raman spectroscopy. To determine the material dependence on electrochemical hydrogenation, a set of different EG samples (Si-face EG [∼2 ML], C-face EG [∼10 ML], M-plane EG [∼25 ML], defective Si-face EG [>50 ML]) and a graphite disk [>500 ML] were characterized using a home built electrochemical cell developed in previous work, dilute perchloric acid (HClO4) solution, silver/silver chloride (Ag/AgCl) reference electrode in saturated KCl (0.198 V vs. NHE) and potentiostat. The Nyquist plots obtained from EIS of epitaxial graphene with low defect density showed only one semicircle covering the entire frequency range attributed to adsorption of hydrogen at the relatively chemically inert basal plane surface and further supported by lack of hydrogen peaks in the CV. Samples with high defect density showed an additional semicircle at the intermediate to high frequency ranges linked to adsorption and charge transfer of hydrogen to graphene. The increased presence of point defects in epitaxial graphene augments the surface area of the material resulting in increased diffusion of hydrogen ions though the graphene lattice allowing for hydrogen to adsorb to additional sites within the lattice.

Maglev rotating disk laser

We propose and demonstrate a laser-diode-pumped, maglev rotating Nd:YAG disk laser. The disk of the laser crystal is attached to a maglev pyrolytic graphite disk and is rotated by compressed gas. In this rotating disk laser, the detrimental thermal effects are alleviated and the laser can be operated in the single transverse electromagnetic (TEM)00 mode with high brightness. In our proof-of-concept experiment, we achieve a 17.7 mW laser output at 447 mW of absorbed pump power and a ～4 Hz rotation frequency.

Spark plasma sintering of tungsten/graphite joints with a SiC ceramic interlayer

Tungsten powders were successfully sintered and simultaneously joined to a graphite disk using an interlayer of SiC ceramic powder, which contained Al2O3 and Y2O3 as sintering aids for the SiC. The tungsten/graphite joints were prepared at 30MPa and temperatures ranging from 1700°C to 2000°C for 5min using spark plasma sintering (SPS). The tensile strength of the joints prepared at 1700°C reached 21MPa. The suggested joining mechanism of the joints prepared below 1800°C is as follows: both the sintered W metal layer and the SiC ceramic interlayer are chemically bonded together accompanying reaction phases of WC, W2C and W5Si3. A physical bond is achieved between the SiC interlayer and graphite when the SiC powder fills the open pores of the graphite and is sintered there. Although a similar joining mechanism is suggested when the joining temperature exceeds 1900°C, the SiC interlayer disappears, and the broad reaction layer consisting of WC, W2C and trace W5Si3 is formed.

Multi-Mini-Eutectic Fixed-Point Cell for Type C Thermocouple Self-Calibration

Thermocouples are generally calibrated using a series of standard fixed-point cells. However, thermocouples in use, particularly base metal and refractory thermocouples, exhibit significant calibration drift due to factors such as inhomogeneity growth. It is not possible to reliably determine the magnitude of this drift by removing the thermocouple for recalibration and instead, must be quantified by some form of in situ calibration. Here, a multi-mini-cell for the use with a metal sheathed W–Re thermocouple (Type C) was developed. The cell contains two layers of different eutectic materials in the same crucible, one in each compartment, separated by a thin graphite disk. The cobalt–carbon and iron–carbon eutectic materials were selected to prove the concept. In addition, thermal modeling was performed to predict the performance of this design of this multi-fixed point. A prototype multi-mini-fixed-point cell was constructed and tested, and results are reported. The overall performance, despite only very small amounts of each fixed-point material being used, is that the repeatability of the W–Re thermocouple (melt-to-melt) was found to be 1.8 $$\upmu \hbox {V}$$ or $$0.11\,^{\circ }\hbox {C}$$ for the Fe–C $$(1153\,^{\circ }\hbox {C})$$ and 1.5 $$\upmu \hbox {V}$$ or $$0.10\,^{\circ }\hbox {C}$$ for the Co–C $$(1324\,^{\circ }\hbox {C})$$ . The lack of drift in the thermocouple enabled the repeatability and stability of the principle of multi-mini-fixed points for self-calibration to be demonstrated.

Multi-objective design of a power inductor: a benchmark problem of inverse induction heating

Purpose – The purpose of this paper is to investigate a bi-objective optimization problem characterized by coupled field analysis. The optimal design of a pancake inductor for the controlled heating of a graphite disk is considered as the benchmark problem. The case study is related to the design of industrial applications of the induction heating of graphite disk. Design/methodology/approach – The expected goal of the optimization process is twofold: to improve temperature uniformity in the disk and also electrical efficiency of the inductor. The solution of the relevant bi-objective optimization problem is based on multiphysics field analysis. Specifically, the direct problem is solved as a magnetic and thermal coupled problem by means of finite elements; a mesh-inspired definition of thermal uniformity is proposed. In turn, the Pareto front trading off electrical efficiency and thermal uniformity is identified exploiting evolutionary computing. Findings – By varying the problem targets, different Pareto fronts are identified trading off thermal uniformity and electrical efficiency of the induction-heating device. Practical implications – These results suggest how to improve the design of this kind of device for the epitaxial growth of silicon wafer; the advantage of using a magnetic concentrator placed close to the inductor axis is pointed out. Originality/value – The coupling of a multiphysics direct problem with a multiobjective inverse problem is presented as a benchmark problem and accordingly solved. The benchmark provides a simple analysis problem that allows testing various optimization algorithms in a comparative way.

Deposition of zinc–zinc phosphate composite coatings on aluminium by cathodic electrochemical treatment

The role of cathodic electrochemical treatment as an energy efficient and eco-friendly method to deposit zinc–zinc phosphate composite coatings on Al is addressed. The deposition of zinc–zinc phosphate composite coatings was carried out under galvanostatic conditions using an electrolyte solution containing 5g/l of ZnO and 12ml/l of o-phosphoric acid (85%), maintained at 27±1°C and two graphite disc anodes, placed on the two sides of the Al cathode. The applied current density (5 to 50mA/cm2), pH of the electrolyte (1.50, 2.30 and 3.00), treatment time (5 to 60min) and concentration of ZnO (1 to 5g/l) were used as the process variables. The methods of evaluation include, assessment of colour and uniformity by visual observations, adhesion by pull-off adhesion test, coating weight by a measure of gain in weight after deposition, morphological features by scanning electron microscopy, chemical composition by energy dispersive X-ray analysis and phase contents by X-ray diffraction studies. The corrosion resistance of the coatings, in 3.5% NaCl, was evaluated by potentiodynamic polarisation studies. The findings of this study reveal that the surface morphology, phase content and corrosion resistance of the composite coatings largely depend on the proportions of Zn and zinc phosphate in them. The cathodic electrochemical treatment method provides ample avenues to manipulate the volume fraction of Zn and zinc phosphate in the resultant composite coatings by a careful choice of the process parameters and hence, it is possible to deposit coatings on Al with varying degrees of corrosion protection.

Photoelectrochemical Solar Energy Conversion and Hydrogen Evolution Catalysis Using Earth-Abundant Iron Pyrite and Cobalt Pyrite Nanostructures

The scale of the energy challenges demands earth-abundant and inexpensive solar absorber materials and catalysts. Iron pyrite (FeS2) is an earth-abundant semiconductor that has generated renewed interest due to its promising properties for solar energy conversion (band gap of 0.95 eV, high absorption coefficient, and high carrier mobility). However, the solar conversion efficiency achieved for even single crystal pyrite has remained < 3%, limited by a low open circuit voltage (<200mV). To understand the fundamental bulk and surface defects intrinsic to pyrite semiconductor, we performed systematic photoelectrochemical and surface property studies on pyrite single crystals and electrical transport (temperature dependent Hall effect and field-effect) studies using single crystal pyrite nanorods and nanoplates as study platforms. The combined electrochemical impedance spectroscopy, surface property and transport studies suggest the heavily doped p-type conduction behaviors observed for intrinsic n-type pyrite originates from strong Fermi level pinning due to surface defects. Such understanding allows us to develop strategies to mitigate these issues in order to improve the performance and eventually fulfill pyrite’s promise as a solar material. Furthermore, I will also show that the pyrite-phase of transition metal disulfides (MS2, M= Fe, Co, Ni) on graphite disk substrates are highly efficient HER electrocatalysts. Several morphologies of CoS2, film, microwire, or nanowire, were controllably synthesized and these micro- and nanostructures with high surface areas substantially boost the catalytic performance.

Structural and mechanical characteristics of the Al–23Si–8Fe–5Mn alloy prepared by combination of centrifugal spraying and hot die forging

The structure, mechanical properties and thermal stability of the hypereutectic Al–23Si–8Fe–5Mn (wt%) alloy were studied. The alloy was prepared by the unique method of combining centrifugal spraying and hot die forging. The rapidly solidified semiproduct was prepared by pouring the melt onto a rapidly rotating graphite disc. This semiproduct was then compacted by die forging at the temperature of 550°C. It was shown that the preparation method led to a compact pore-free material with very good compressive mechanical properties. The results of thermal stability testing revealed that the mechanical properties do not change significantly at high temperature, even after 100h of annealing at 400°C. In addition, the Al–23Si–8Fe–5Mn alloy exhibited very good creep resistance. The structure contained a very fine intermetallic phase, which was identified as α-AlFeMnSi. This phase had a significant impact on the hardness and the yield strength of the material, but the primary impact of this phase was on the thermal stability of the material. The Al–23Si–8Fe–5Mn alloy was compared to a commercial casting Al–12Si–1Cu–1Mg–1Ni alloy, which is used in high temperature automotive applications. The room temperature mechanical properties of the two alloys were comparable, but the Al–23Si–8Fe–5Mn alloy exhibited considerably better thermal stability. Therefore, this alloy can be considered a promising alternative to conventional alloys. It was also demonstrated that the combination of centrifugal spraying and die forging is a suitable method for processing Al–Si based alloys with high iron contents.

Amperometric urea biosensor based on covalently immobilized urease on an electrochemically polymerized film of polyaniline containing MWCNTs

In this work, polyaniline-multiwalled carbon nanotubes (PANI/MWCNTs) composite were fabricated by electropolymerization method, as a matrix for entrapment of enzyme. Urease has been immobilized by using physical adsorption and electrochemical entrapment technique. A conventional three-electrode system was used. Pencil graphite disk electrode (PGDE), Ag/AgCl (3M KCl) and platinum wire were used as working electrode, reference electrode and counter electrode respectively. The influence of several experimental parameters such as applied potential, type and concentrate of dopant and enzyme loading was explored to optimize the electroanalytical performance of the biosensor. The responses of the enzyme electrodes were measured via monitoring amperometric current at +0.3V (vs. Ag/AgCl). Kinetic parameters operational and storage stabilities were determined. The value of the apparent Michaelis–Menten constant, Imax and sensitivity (Imax/Km) experimentally determined to be 2.02mM, 2.5×10−5Acm−2 and 12×10−5AmM−1cm−2, respectively. The optimized urea biosensor shows a good sensitivity from 10−2M to 10−5M urea concentration range and a response time of about 50s. The detection limit was 0.04mM. The proposed biosensor retained 50% of its original response after 15 days.

Optical Properties of Amorphous Silicon–Carbon Alloys (a-Si x C1-x ) Deposited by RF Co-Sputtering

Amorphous silicon–carbon alloy (a-SixC1-x) thin films have been deposited by radio frequency (RF) sputter deposition. These films were obtained, from a composite target consisting of silicon fragments regularly distributed on the surface of a pure graphite disc, for different values of silicon surface fraction RSi/C, at an RF power of 250 W. X-ray diffraction diagrams show that all the as-deposited SixC1-x thin films are amorphous. The optical properties of the a-SixC1-x films were investigated by optical transmission measurements in the ultraviolet–visible–near infrared wavelengths range. The optical band gap Eg varies, with RSi/C, from 1.4 to 1.9 eV and presents a wide maximum at an RSi/C value of about 35 %. This maximum value is attributed to amorphous silicon carbide a-SiC as confirmed by theoretical correlation between the molar fraction x and RSi/C. The refractive index n follows well the Cauchy law and the extrapolated value, at infinite wavelengths, increases from 2.1 to 2.65 as the RSi/C fraction increases.

A novel mechanism for the oxidation reaction of VO2+ on a graphite electrode in acidic solutions

With the consideration of optimizing the performance of the all-vanadium redox flow battery (VRB), the oxidation reaction mechanism of VO2+ on a rotating graphite disk electrode has been investigated by potentiodynamic polarization in sulfuric acid solutions with various pH and vanadium concentrations. Furthermore, the reaction orders of VO2+ and H+ for the oxidation reaction of VO2+ have been calculated from the polarization results and compared with the theoretical results according to the possible reaction mechanisms available in the literature. However, a new oxidation reaction mechanism has been proposed to describe the oxidation of VO2+ at last, and the theoretic reaction orders of VO2+ and H+ based on the new mechanism are consistent with the experimental results when the electrochemical reaction is the rate-limited process. Moreover, a corresponding kinetic equation has been established for the oxidation reaction of VO2+ on a spectroscopically pure graphite electrode, and can be well used to predict the polarization behavior in V (IV) acidic solutions.