Graphite Channel Research Articles

Analytical Modeling of Water Droplet Behavior at the Gas Channel Corner for Proton Exchange Membrane Fuel Cells

Abstract Water management is of significant importance to achieving high performance of proton exchange membrane fuel cells. In recent years, droplets emerged from the rib surface and accumulated at the channel corner have been found to be a crucial part of water flooding. In this study, an analytical model is first proposed to quantitatively estimate the variation in the morphology and dynamic behavior of growing droplets with consideration of the channel sidewall interaction. In order to predict the water geometry, the flow channel with compressed gas diffusion layer (GDL) is described mathematically, and water behavior at steady-state and dynamic state are both evaluated through the geometric and force analysis. The model results indicate that the droplet profile transforms from concave to convex when its size grows, in which process contact angles and channel shape play an important role. Compared with the graphite channel, the droplet in the metallic channel is more inclined to be adsorbed on the sidewall and GDL, resulting in a higher adhesion force and a lower gas shear force. The critical gas velocities for the detachment of droplets are quantitatively predicted to avoid water flooding. The model is helpful to understand the droplet behavior in the presence of channel sidewall interaction.

Phonon-drag in a graphite channel buried in diamond

While the phonon-drag effect can induce large Seebeck coefficients, it is associated with large mean free path phonons present in the vicinity of the maximum in temperature of the lattice thermal conductivity. In this paper, we initiate a new route by searching for the mutual drag between the electron and phonon-drag gases at the interface between two different media. In that respect, the temperature studies of the conductance and Seebeck coefficient of a model system consisting of an electrically conductive graphitic channel buried beneath the surface of a diamond crystal are shown. The observed behaviour is very similar to that of graphite, with a typical negative peak associated with the phonon-drag effect. Interestingly, this phonon-drag peak of the buried graphitic channel appears at a significantly higher temperature than that in pure graphite.

Real-time COVID-19 detection via graphite oxide-based field-effect transistor biosensors decorated with Pt/Pd nanoparticles

Coronavirus 2019 (COVID-19) spreads an extremely infectious disease where there is no specific treatment. COVID-19 virus had a rapid and unexpected spread rate which resulted in critical difficulties for public health and unprecedented daily life disruption. Thus, accurate, rapid, and early diagnosis of COVID-19 virus is critical to maintain public health safety. A graphite oxide-based field-effect transistor (GO-FET) was fabricated and functionalized with COVID-19 antibody for the purpose of real-time detection of COVID-19 spike protein antigen. Thermal evaporation process was used to deposit the gold electrodes on the surface of the sensor substrate. Graphite oxide channel was placed between the gold electrodes. Bimetallic nanoparticles of platinum and palladium were generated via an ultra-high vacuum (UHV) compatible system by sputtering and inert-gas condensation technique. The biosensor graphite oxide channel was immobilized with specific antibodies against the COVID-19 spike protein to achieve selectivity and specificity. This technique uses the attractive semiconductor characteristics of the graphite oxide-based materials resulting in highly specific and sensitive detection of COVID-19 spike protein. The GO-FET biosensor was decorated with bimetallic nanoparticles of platinum and palladium to investigate the improvement in the sensor sensitivity. The in-house developed biosensor limit of detection (LOD) is 1 fg/mL of COVID-19 spike antigen in phosphate-buffered saline (PBS). Moreover, magnetic labelled SARS-CoV-2 spike antibody were studied to investigate any enhancement in the sensor performance. The results indicate the successful fabrication of a promising field effect transistor biosensor for COVID-19 diagnosis.

A newly designed graphite-polyaniline composite current collector to enhance the performance of flow electrode capacitive deionization

Flow-electrode capacitive deionization (FCDI) is an efficient and low-energy ions separation technology. However, the unsatisfactory conductive connection of the circuit in graphite (GP) channel and inevitable flow electrode (FE) sediment impedes the application of FCDI. Herein, a simple electrochemical polymerization method was adopted to build a polyaniline-graphite (PGP) composite current collector. The average salt removal rate (ASRR) of PGP-FCDI was increased by 1.46 times than GP-FCDI. The ohmic resistance of PGP-FCDI was found to be 3.5 Ω, which is much lower than that of GP-FCDI (12.9 Ω). The high conductive capacity of polyaniline provides excess conductive sites, which contributes to enhanced charge transfer efficiency. Furthermore, the composite material changes the polarity of the graphite surface and destroys the electrostatic interaction between graphite and activated carbon. As a result, electrode sediment is reduced by 15%. In addition, the performance of PGP-FCDI remains stable after continuous deionization cycles. The new features significantly contribute to the improved deionization performance and long-term stability of FCDI.

Effect of Serpentine Channel Pouring Process on the Microstructure of Semi-Solid 6061 Aluminum Alloy Slurry

The semi-solid slurry of 6061 aluminum alloy was prepared by the serpentine channel pouring process. The influence of graphite serpentine channel and copper serpentine channel on the slurry was comparative analyzed. The effect of pouring temperature on the slurry microstructure was also investigated. The results indicate that both copper and graphite serpentine channel can be used to prepare semi-solid slurry with spherical primary grains. Compared with a permanent casting, the microstructure of the semi-solid slurry was significantly improved and refined. With the increase of pouring temperature, the average equivalent grain diameter of the primary phase grains in the semi-solid slurry increases gradually, but the shape factor decreases gradually. When the pouring temperature increased from 675 °C to 690 °C, a high quality semi-solid slurry can be obtained. Comparing the two kinds of serpentine channel, it is found that the copper serpentine channel can make the primary grains finer, and the average equivalent grain size was 63 μm. However, the solidified shell near the inner graphite serpentine channel surface was thinner than that of the copper serpentine channel. In conclusion, the graphite serpentine channel is more suitable for preparing semi-solid 6061 aluminum alloy slurry.

Semi-Solid 6061 Aluminum Alloy Slurry Prepared by Serpentine Channel Pouring Process and its Rheo-Diecasting

Semi-solid 6061 aluminum alloy slurry was prepared by a graphite serpentine channel and its rheo-diecasting experiment was carried out on the slurry. The influence of pouring temperature on the microstructure evolution and mechanical properties of the rheo-diecasting were investigated. The microstructure and fracture mechanism of traditional die cast tensile specimens and rheo-diecast tensile specimens were compared and investigated. The results indicate that the microstructure of rheo-diecast tensile specimens is composed of spherical primary α-Al grains and fine secondary solidified α2-Al grains. When the pouring temperature increased from 660 °C to 720 °C, the average equivalent grain diameter of primary α-Al grains increased from 42 μm to 58 μm, and the shape factor decreased from 0.82 to 0.73. As the pouring temperature increases, the as-cast tensile strength and elongation of tensile specimens both increase first and then decrease. When the pouring temperature was 690 °C, the best mechanical properties were obtained, with as-cast tensile strength of 142.93 MPa and as-cast elongation of 4.86%. The fracture mechanism of traditional die casting is mainly ductile fracture, and the fracture mechanism of rheo-diecasting is a mixed fracture of intergranular fracture and ductile fracture.

Real-time nucleic acid detection via field-effect transistor sensors based on graphite oxide decorated with trimetallic nanocluster of gold, silver, and platinum

Field effect transistor (FET) based sensors are witnessing robust evolutions in the fabrication and characterization for the purpose of biomedical applications. In this article, a graphite oxide FET based sensor is designed, fabricated, and characterized for real-time detection of various concentrations of nucleic acid with a detection limit of 1.28 nM of DNA. This sensor consists of two gold electrodes connected through a channel of graphite oxide. Moreover, the sensor sensitivity is enhanced by decorating the graphite oxide channel with composite trimetallic nanoclusters that include gold, silver, and platinum. The developed sensor is investigated by both simulation and experiment. Both experimental and simulation agree where the current signal is higher for sensors decorated with trimetallic nanoclusters, which indicate higher sensitivity. Moreover, increasing the concentration of DNA results in an increment in the current signal thus the response is proportional to DNA concentration. The results indicate a promising sensor for real-time, reliable, and cost-effective DNA detection.

Whole-core transient analysis methodology for prismatic high temperature gas reactors

A new transient thermalhydraulic method for simulating prismatic high temperature gas cooled reactors (HTGRs) during a loss-of-forced-circulation (LOFC) accident is presented. Whole-core, three-dimensional temperature profiles are computed at an individual fuel pin and lattice assembly block levels. A method that computes the fluid mass, velocity (momentum), and energy throughout a transient is implemented using a well-documented, semi-implicit pressure-correction scheme. Models for reactor cavity cooling system (RCCS) heat transfer and decay heat generation are also included. Pressurized (P-LOFC) and de-pressurized (D-LOFC) accidents are simulated. This method provides two significant advantages over available thermalhydraulic analysis techniques. The first is its ability to compute whole-core results and capture the important transient core-level phenomena such as bypass flow and heat redistribution into the reflector assemblies after reactor SCRAM. The second is its ability to compute each fuel assembly in detail, computing the heat rates and temperature profiles for every fuel pin, graphite, and coolant channel. These features produce realistic, 3-D transient results for prismatic HTGRs during a LOFC. A RELAP model is also developed and implemented for the HTGR. The limitations of existing methods for the simulation of the prismatic fuel are identified and the need for and novelty of the present method are highlighted.

Development and Characterization of a Micro Redox Fuel Cell

The design approach of catalysed flow field channel walls as an additional surface reaction area for the micro fuel cells was demonstrated in an air-breathing micro iron-oxygen redox fuel cell (μIOFC). The μ IOFC with a graphite channel improved the maximum power density by 280% when compared to the μIOFC with inactive channel walls for the redox couple reaction. In the micro redox anode fuel cell with a graphite channel, the performance is limited by the mass transfer of the redox species but not the anode reaction rate. The proton transfer to the cathode is also a limiting factor determining the μIOFC performance, and the anode layer on the ion exchange membrane should be eliminated to reduce the resistance of the proton transfer.

A 200-W permanent magnet Hall thruster discharge with graphite channel wall

A 200-W Hall thruster was designed using permanent magnets placing the axial maximum magnetic field outside of the channel, and the anode/gas distributor as an integrated U-shaped structure. A split structure is adopted for the discharge channel to conveniently change the wall material. With the initial conditions that the anode mass flow is maintained at 0.8 mg/s, 1.0 mg/s and 1.2 mg/s over a voltage range of 150–400 V (at 50 V intervals), the discharge properties were determined for the two thruster models with the channel walls composed of graphite and boron nitride (BN). The results demonstrate that under identical operating parameters, the properties of the thruster with the graphite channel walls are similar to the properties of the thruster with the BN channel walls. The maximum difference of the discharge current between the two wall materials is 6.2%; the maximum difference of the thrust and the specific impulse is 3.3%, and that of the anode efficiency is 1.7% (absolute value). These differences are smaller than the corresponding parameter differences observed from changing wall materials in other common Hall thrusters.

Performance comparison between standard and magnetically shielded 200 W Hall thrusters with BN-SiO2 and graphite channel walls

Magnetic shielding is a specific magnetic topology that can increase the lifespan of Hall thrusters by an order of magnitude. Magnetically shielded thrusters have similar performances as standard unshielded thrusters in the 5–15 kW power range. A comparison of the performances of two 200 W shielded (ISCT200-MS) and unshielded (ISCT200-US) Hall thrusters is presented here. The effects of replacing the usual BN-SiO2 walls with graphite are investigated on both thrusters. The ISCT200-US thruster has a peak anode efficiency of 39% and a specific impulse of 1400 s at 250 W with the ceramic discharge channel. However with graphite wall, the discharge current increases by 25% which reduces the efficiency down to 31% (1360 s specific impulse). The ISCT200-MS performances are significantly lower than the ISCT200-US at similar operating points. It only reaches 24% anode efficiency and 1020 s specific impulse at 250 W. The switch to graphite has little effects on its performances below 300 V. The lower performances are due to a low propellant utilization. We propose that this low propellant utilization is caused by lower ionization near the walls which as a large impact in small thrusters due to their higher surface to volume ratio.

Performance improvement in direct borohydride/peroxide fuel cells

Direct borohydride/peroxide fuel cells (DBPFCs) show progressively deteriorating performance during operation for various reasons such as decreasing reactant concentrations, gas evolution and uneven distribution of liquids. The present study aims to emphasize the importance of certain design parameters, such as bipolar plate materials, flow fields and manifold design, in determining the DBPFC performance. Bipolar materials and flow channel design have been investigated. A power density of 67 mW cm−2 has been obtained with composite graphite and parallel flow channel bipolar plates. It has increased to 87 mW cm−2 using sintered graphite and then to 93.3 mW cm−2 using sintered graphite with serpentine flow fields. The stacking of DBPFCs results in a loss of performance and unstable output. The performance has remained nearly unchanged as the cell number was increased by applying an independent cell liquid distribution network (ICLDN). Using an ICLDN, power densities of 98.3, 83.3 and 82 mW cm−2 have been obtained for single-cell, 3-cell and 6-cell stacks, respectively. Finally, a controlled oxidant feeding system (COFS) has been developed to provide stable output power, and it has demonstrated a stable output power of 6 W for 2.5 h.

Phase composition and structure of sodium aluminophosphate based glass materials depending on their synthesis conditions

The effect of the method of synthesis on the structure of aluminophosphate glasses intended for immobilization of high level waste from uranium–graphite channel reactor spent nuclear fuel reprocessing has been studied by X-ray powder diffraction and Fourier transform IR spectroscopy. Slow cooling in a furnace, both spontaneous cooling in a switched-off furnace and cooling simulating the conditions of glass cooling in their disposal canisters, or additional heat treatment (annealing) led to partial crystallization of glasses with segregation of different modifications of aluminum orthophosphate. For orthoand pyrophosphate based glasses with similar anionic motifs, their phase composition and degree of crystallization depend not so much on the chemical composition of a sample as on its heat treatment history.

A non-isothermal transient model for a metal-free quinone–bromide flow battery

An enhanced non-isothermal transient model for a metal-free quinone–bromide flow battery is developed. The graphite plate and channel are also included in the geometric model to represent an actual cell more accurately and increase the generality and usefulness. Rather than applying Darcy’s law typically used in a two-dimensional model to calculate the relation of velocity and pressure, we applied the Brinkman equation to handle the interface between the porous electrode and channel. The model includes a comprehensive description of energy transport and presents time-dependent characteristics of voltage and overpotential changes. The performance changes at different applied current density, temperature and flow rate values are also investigated. At a low applied current density, the flow rate has little effect on cell performance. The voltage and overpotential increase followed by a small increase with the increase in temperature. The current density distribution indicates the importance of cell structure optimization. The model is validated using data obtained from experiments in the literature. The model may be used to predict the cell performance and enable the development of a quinone–bromide flow battery.

The Influence of Channel Wettability on Two-Phase Flow and Polymer Electrolyte Membrane Fuel Cell Performance

Water management is one of the critical issues affecting polymer electrolyte membrane fuel cell (PEMFC) performance, durability and cost. Modifying the surface wettability of flow field channels in PEMFC can improve the water management and fuel cell performance. In present work, an ex-situ investigation of the effects of channels with different surface wettability on the two phase-flow characteristics has been conducted. Horizontal graphite channel (slightly hydrophobic) and other four channels coated with polytetrafluoroethylene (PTFE) (hydrophobic), silica/Polydimethylsiloxane (PDMS) (super-hydrophobic), silica/PDMS at the bottom wall but with the side walls being raw graphite (combined surface wettability channel), and silica coated channel (superhydrophilic), are tested and visualized at room temperature and atmospheric pressure. Pressure drop measurements and two-phase flow visualization using high speed camera have been performed. Super-hydrophobic channel has shown desirable positive effect on the two-phase flow and on the PEMFC's performance compared with the other channels, especially at high current densities.

Molecular simulation of liquid water confined inside graphite channels: Thermodynamics and structural properties

We carried out molecular dynamics simulations to describe the properties of water inside a narrow graphite channel. Two stable phases were found: a low-density one made of water clusters adsorbed on the graphite sheets and a liquid one that fills the entire channel, forming several layers around a bulk-like region. We analyzed the interfacial structure, orientational order, water residence times in several regions, and hydrogen bonding of this last water phase, calculating also a quantity of electrochemical interest, the probability of electron tunneling through interfacial water. The results are in good qualitative agreement with the available experimental data.

Critical heat flux prediction in rod bundles under upward low mass flux densities

The analysis of experimental data and results of calculations for heat transfer crisis in heated channels under low upward coolant mass flux densities is presented. This analysis allows the determination of the basic features of the boiling crisis phenomenon. It is shown that the methods currently used for critical heat flux (CHF) prediction have insufficient accuracy in the given range of parameters. A new relationship for the CHF calculation is presented. It should be used for the water–water energy reactor (WWER) and uran–graphite channel reactor—Chernobyl-type (RBMK) rod bundles, and is verified by the test data. The comparison of results obtained by a new CHF correlation and the relationship used in RELAP5/MOD3.1 Code is presented. It is shown that the latter overpredicts the CHF values at atmospheric pressure and for x cr>0.4 and does not provide conservative estimations for the RBMK fuel bundles.

ＨＴＴＲ用燃料棒の流体振動実験

This study was performed in order to investigate flow-induced vibration characteristics of a fuel rod in HTTR (High Temperature engineering Test Reactor). The fuel rod used this experiment is a graphite fuel rod which simulated a specification of the HTTR's fuel rod. The experiment carried out up to Re=31, 000 using air at room temperature and pressure. Air flowed toward downstream in an annular passage consisted of the fuel rod and the graphite channel. Vibration amplitudes and frequencies of the fuel rod were measured by two noncontact displacement sensors fixed on a side wall of the graphite channel. The following conclusions were drived:(1) The fuel rod amplitudes increase with a increase of the flow rate and with a decrease of the fuel rod weight.(2) The fuel rod amplitudes are obtained by δ/De=2.22×10-10Re1.43 (9, 000≤Re≤31, 000), where δ is the vibration amplitude, De the hydraulic diameter and Re Reynolds number.(3) The fuel rod frequencies shift from lower natural frequency to higher as the flow rate increases.

Safety of power plants with boiling-water graphite channel reactors

Safety of power plants with boiling-water graphite channel reactors

Air oxidation studies in a long graphite channel

Experiments on the oxidation of short tubes of graphite in air (1–4) have been extended to long channels of graphite where the oxygen is depleted as it flows through the channel. The theory of reaction in porous solids which was developed for the earlier work has been extended to cover this more general case. It is shown that a useful approximation to the general theory can be obtained by considering the channel as a series of short sections and introducing the axial concentration effects via the problem boundary conditions. The theory was tested by experiments carried out in a vertical channel of graphite 14 ft long, 2 in. i.d. and 8 in. o.d. constructed from six blocks of British nuclear grade graphite. The inner graphite surface was exposed to oxygen nitrogen mixtures which flowed through the bore of the tube. Evidence of the radial diffusional effect was obtained from gas concentration measurements at the outer surface. The oxygen was depleted by chemical reaction as it flowed through the channel bore giving rise to an oxygen concentration profile which was experimentally measured. No corresponding axial concentration gradient at the exterior surface was observed. The derived theory allowed the diffusional effects to be quantitatively estimated. Good agreement between the present results and previously reported values for similar graphites was obtained.