Graphite Heater Research Articles

Optimising graphite composites and plate heat exchangers for latent thermal energy storage using measurements and simulation

Low thermal conductivity is a significant issue for the deployment of phase change based thermal energy stores. A structured methodology is required to find the best overall design, in order to assess its techno-economic feasibility. In this study the optimised structural configuration of graphite composites for this purpose are first considered from three different perspectives using both experimental measurements and numerical simulations. Both steady state and dynamic performance are taken into account, with the latter being considered both with and without phase change. In all three cases it is demonstrated that thin, parallel graphite sheets offer the most optimal results compared to other options. Resulting in the highest composite thermal conductivity measured to date at a loading of 3.5 mass%. The next step in achieving the best overall performance is the optimisation of this composite in the context of a plate and frame heat exchanger. For this purpose a simplified analytical model of this configuration was developed and validated. This was used in conjunction with the log mean temperature difference approach to optimise the exchanger for two typical applications: low temperature, low duty and high temperature, high duty. The results indicate that, even under an idealised configuration, the system will only be suitable for high temperature applications with gas heat transfer fluids and short discharge times (~1 h). For low temperature applications it may be possible to use liquid heat transfer fluids but also only for short discharge times. From an economic perspective, the range of graphite sheets currently available on the market will have to be reduced in cost by two or more orders of magnitude in order to be cost-effective.

Directional solidification behavior of turbine blades in DZ125 alloy: design of blade numbers on assembly

The influence of blades assembly, i.e., the blade number on the solidification process, heat exchange and grain structure on a directionally solidified Ni-based superalloy DZ125 was investigated by combining the experimental and simulation results. The casting process was simulated thermodynamically by ProCAST software, where the interface heat transfer coefficient was precisely determined by a measurement with thermocouples. There is a good agreement between experimental and simulation results. It was interestingly found that with the number of blades increasing from 6 to 10, due to the decrease in radiation absorption efficiency, the maximum temperature and the heating rate decrease in the mold, during the preheat process. During the withdrawal procedure, increased assembly numbers reduce the radiation exchange from mold to the enclosure, resulting in the decrease in cooling rate and temperature gradient of the blades. At the end of withdrawal, the slower cooling rate of the outside balances the temperature distribution of internal and external surfaces on the rabbet of blade. The interface heat transfer coefficient (IHTC) values of DZ125 casting system were investigated by inverse method. Then, a series of assembly blade groups were designed and being cast and calculated. Values of IHTC are proved to be suitable for different assembly groups of casting in this case. Based on simulation and experimental results, it was interestingly found that: In the preheating process, with the number of blades increased, the maximum temperature on the mold becomes lower and the heating rate becomes slower, which result from the decrease in radiation efficiency between the mold and graphite heater. During the withdrawal, increasing assembly blade number reduced the radiation exchange from the mold to the enclosure, and it results in the cooling rate and temperature gradient on the blades getting decreased. At the end of withdrawal, the slower cooling rate leads to the temperature distribution maintaining uniformity for internal and external surfaces on the rabbet of the blade.

Experimental Study of Surface Etching in Synthetic Diamond Microcrystals due to Presence of Cu and Fe at High Pressure

In the present work, microcrystals of synthetic diamond extracted from a metal-diamond composite were investigated. A composite based on Cu and Fe was obtained by sintering at a pressure of 4 GPa and a temperature of1300 °C. The experiments were carried out using a split-sphere high-pressure apparatus BARS. The high-pressure cell was made of refractory oxides ZrO2, CaO, and MgO using a tubular graphite heater. In the composite, diamond grains were in close contact with neighboring diamonds, and the metal phase filled the interstices. The study of the diamond crystals demonstrated the appearance of newly formed micromorphological structures on the surfaces in the form of numerous cavities of irregular shape on the faces of octahedron, as well as pyramids on the faces of cube, the morphological elements of which follow the contours of the cube face of the diamond. Thus, the results of the work evidence for the processes of etching of the diamond crystals during the experiments, which is associated with the presence of metallic iron in the composite. This type of etching forms a roughly cavernous surface on the diamond crystals, which can be considered as an additional factor for improving the metal-diamond bond in copper-based composites.

Design and regularity research of MOCVD heating plate based on experiments and simulations

Metal organic chemical vapor deposition (MOCVD) is the key technique used in developing thin film materials, such as GaAs, InP, and ZnO. The heating system design plays an important role in the overall design of the complete MOCVD system. In this study, the structure of the MB-600D MOCVD resistance heater developed by us was considered as the research object. The distribution of current and temperature in the heating plate can be determined through thermoelectric simulation. Supported by the theory of thermodynamics and thermoelectricity, it can be used for the improved design and regularity research of the heating plate. On this basis, it was found that higher surface emissivity of the heater correlated to greater thermal radiation, stronger heating effect of the graphite disc, and higher overall temperature. A direct correlation was observed between the rotational speed of the graphite disc and the upper surface temperature distribution uniformity; the optimal vertical distance between the graphite disc and heater was also determined. These results provide a theoretical basis to improve the MOCVD heating plates and a solution for high-quality epitaxial growth.

Incipient stages of transformation of round natural diamonds under dissolution in Fe-S melt at high pressure

Research subject. The article presents the results of a microscopic and photogoniometric study of natural rounded diamonds of tetraghexahedral habit from the kimberlite pipe “Internationalnaya” (Yakutia). The diamonds was partially dissolved in a sulphur-containing iron melt (sulphur content of 15–30 wt %) at 4.5 GPa and 1450ºС.Methods. The experiments were carried out on a multi-puncheon apparatus of a “split-sphere” type in high-pressure solid-phase cells made of refractory oxides ZrO2, CaO, MgO using a cylindrical shape graphite heater. The crystals were studied using an MBS-10 optical microscope with a photo camera, and a Jeol JSM-6510LV scanning electron microscope. A goniometric study of diamond crystals was carried out by a photo method in a cylindrical chamber. It was found that when a sulphur content was 15 wt %, diamond crystals of tetrahexahedral habit were transformed into a curved shaped octahedroids with morphological features similar to natural diamonds found in kimberlites. When the sulphur content was 23–30 wt %, the rate of dissolution of diamonds in the Fe-S melt sharply reduced, while the diamond surface at the micro level became covered with numerous etching hillocks, whose sidewalls have surfaces similar to flat-faced {111} form. Dissolution of the rounded diamonds in the Fe-S melt at high pressure occurred by a “normal” mechanism, that is perpendicular to the surface of the dissolving crystal through trigonal dissolution layers, while a tangential-layered mechanism played a minor role.Conclusion. The natural diamond crystals could underwent dissolution in the mantle before they were captured by kimberlite magma. Two fundamentally different types of homomorphic and typomorphic features of the dissolution forms observed on natural diamonds can be determined, namely: on one side, those associated with storage in mantle before the crystals were captured by the kimberlite magma, and on the other side, with the kimberlite process itself. The presence of octahedral diamonds with parallel (trigonal) striation in kimberlite deposits may indicate on a high degree of diamond preservation due to relatively insignificant effect of the kimberlite magma. This, undoubtedly, should help to decipher the diamond genesis and, possibly, improve the mineralogical criteria used in diamond exploration.

Development of High Temperature Powder X-ray diffraction by Graphite Heater

Development of High Temperature Powder X-ray diffraction by Graphite Heater

Electron and Mass Transfer during Solid Carbon Reduction of Metals in Solid Complex Oxides

The possibility of iron and chromium reduction in the composition of complex oxides by gaseous carbon monoxide and solid carbon was studied in a laboratory resistance furnace with a graphite heater. Holding only a CO atmosphere does not reduce chromium and iron in both chromites and silicates. In a mixture with solid carbon, chromium and iron were completely reduced in chromites, and iron precipitated in an iron–magnesia silicate matrix in the part having undergone hydration in nature and dehydration upon heating. Continuous carbide shells, which exactly reproduce a surface relief, appear on the surfaces of solid oxide and carbon in a mixture of solid reagents. Similar shells consisting of a carbon-free FeCr alloy form on the surfaces of chromite crystals disseminated in a silicate phase. These results do not correspond to the concepts of both direct and indirect reduction, and they are analyzed in terms of electronic reduction theory developed by us. According to this theory, a low-temperature plasma appears in a mixture of solid reagents due to thermionic electron emission from the carbon surface. As a result, charged particles, namely, electrons and carbon cations, appear near the reagent surfaces. Their interaction with oxide anions causes the formation of CO and anion vacancies, which are charged by two electrons and are scattered in the anion sublattice of oxide. The saturated vacancy solution in the anion sublattice of oxide decomposes to form anion void at the site where the chemical potential of free electrons reaches the Fermi level of certain cations. These cations in anion void regroup to form a metallic phase. The high reducibility of solid carbon is explained by the exponential temperature dependence of the number of electrons emitted by it. This dependence takes place when the plasma flows around the surfaces of both reagents. The formation of carbide shells on the reagent surfaces is caused by the secondary interaction of carbon with a reduced metal on the oxide surface or of oxide lattice fragments with carbon on the corrosion resistance surface. The formation of carbide shells “poisons” the reagent surface and retards and stops reduction. This process renews after breaking the shells upon melting and flowing carbides. The detected laws manifest themselves during both direct and indirect reduction, make the concepts of both mechanisms closer, and eliminate the theoretical discrepancies observed between them in practice.

Analysis on three typical abnormal microstructures of overhead power line suspension clamps

This article lists three typical abnormal microstructures of overhead power line suspension clamps, including inclusion, flake graphite and cementite network. A series of tests including macro check, hardness measurement, composition tests, Energy-dispersive x-ray analysis and metallographic observation have been carried out. The experimental results showed that the causes of abnormal microstructures mainly include: high content of impurities, flake graphite blank and unfulfilled annealing heat treatment. It is suggested to strengthen the supervision on suspension clamps before using, and focus on the quality inspection of metallographic structure and composition.

The behavior of radioactive metals (Am, Eu, Sr) during the processing of radioactive graphite in salt melts

The behavior of Am, Eu, Sr radionuclides was investigated by thermodynamic modeling method at heating of radioactive graphite in NaCl – KCl – Na2CO3 – K2CO3 molten salt with additives of nickel oxide NiO. The integrated thermodynamic analysis was carried out by means of TERRA software in temperature range 373-3273 K to determine possible composition of condensed and gaseous phases. It was established that americium is in gaseous state in temperature range 2773-3273 K. Europium is in the forms of gaseous EuO and Eu in temperature range 2373-3273 K. Strontium is in the forms of gaseous SrCl2, SrCl, Sr, SrO in temperature range 2373-3273 K.

The effect of graphite intercalated compound particle size and exfoliation temperature on porosity and macromolecular diffusion in expanded graphite

The pore structure of expanded graphite (EG) including pore volume, pore size distribution and surface area is investigated by mercury porosimetry, nitrogen adsorption and SEM. Also, the diffusion of silicone oil molecules as macromolecules with different molecular weights into EG pores is studied. Various EG samples were prepared by the sudden heating of graphite intercalated compound (GIC) with varying particle size of 35, 50, 80 and 200 meshes in an electrical furnace at temperatures of 700, 800 and 900 °C. The EGs were characterized by FTIR to evaluate the presence of functional groups. It was found that the exfoliation process has not significantly introduced oxygen functional groups such as epoxy and carboxyl groups to the EG structure. Therefore the chemical structure of the EG is very close to pristine graphite. The mercury porosimetry results showed a broad range of total pore area from 5 to 31 m2/g for the EGs. The particle size of GIC and exfoliation temperature showed strong effects on the pore size of EG. The mercury intrusion porosimetry and nitrogen adsorption isotherms revealed that μm-pores are dominant as compared with nm-pores in all EG samples. The diffusion of silicone oils as macromolecular guests with three different viscosities was experimentally studied to analyze the diffusion facts of EG as the host. It was observed that as the exfoliation temperature decreases, the sorption capacity decreases; and EG samples prepared from the GICs with smaller particle size have lower sorption capacity. Sorption experiments also showed that the whole pore volume of EG is not filled with silicone oil leading to this fact that EG includes relatively closed pores. A new model was suggested for the sorption capacity of EG as a function of the pore area of EG and the square root of molecular weight of silicone oil.

Thermal management of a semiconductor laser array based on a graphite heat sink.

Thermal analysis of a semiconductor laser array is carried out by establishing a packaging structure model of the array based on the use of copper as the base heat sink. A novel composite heat sink structure for a high-power semiconductor laser array with copper-implanted graphite microholes is proposed. The relationship between the location and quantity of microholes and the junction temperature of the semiconductor laser array is analyzed by numerical simulation. The packaging structure is found to effectively reduce the temperature generated by the semiconductor laser array in the process of operation, and the highest temperature is reduced by 4.52K. Since the laser diode bar is composed of multiple emitters, the temperature of the intermediate emitters is higher than that of the edge emitters. The structure of the graphite heat sink is optimized to improve the temperature uniformity of the semiconductor laser array emitter. Without changing the parameters of the semiconductor laser array device, the temperature difference is calculated, and a reduction from 7.68 to 2K is shown.

Resistive heater element made of highly durable TaC-coated graphite for high-temperature and highly corrosive processes: application to MOCVD GaN epitaxial growth

Ultra-thick TaC-coated graphite prepared via a novel process was verified for use as a highly durable heater material in high-temperature and highly corrosive process environments such as those for SiC, GaN, and AlN bulk/epitaxial growth. As fundamental data for heater design, the temperature dependence of the electrical resistivity of the ultra-thick TaC layer was assessed. Estimation based on resistivity data indicates that the TaC-coated graphite heater element requires a slightly higher electrical current than that for conventional heaters. A coil-shaped ultra-thick TaC-coated heater for a MOCVD GaN growth system was fabricated using the proposed process. It was found to successfully heat the system to the target temperature for GaN epilayer growth. The quality of the MOCVD-grown GaN epilayers obtained with the TaC-coated heater was confirmed to be almost identical to that of those obtained with a conventional heater in terms of crystallinity, thickness homogeneity, intrinsic defects, impurity incorporation, and contamination.

Re-Melting Nb–Si-Based Ultrahigh-Temperature Alloys in Ceramic Mold Shells

In furnaces with different heating elements, Nb–Si based ultrahigh-temperature alloy rods were re-melted in pure yttria mold shells and zirconia face-coat mold shells at 1850 °C for 30 min. The results evidenced that in the furnace with a tungsten heating element, the microstructure of the re-melted alloy became coarser, and the composition varied depending on the type of mold shell. Although the interface reaction layer between the re-melted alloy and the zirconia face-coat mold shell was much thicker, the deformability of the mold shell and the sand burning phenomenon of the alloy inside it were improved and ameliorated, respectively. However, after being re-melted in the furnace with a graphite heating element, the misrun phenomenon occurred in both specimens. Both re-melted alloys inside the mold shells were divided by a gap into an internal and an external part, with totally different microstructures and compositions. No reaction layer emerged at the interface between the re-melted alloy and the mold shells. Instead, infiltration zones arose in the mold shells adjacent to the interface.

Influence of sintering environment on the spark plasma sintering of Maxthal 312 (nominally-Ti3SiC2) and the role of powder particle size on densification

Herein, we report on the synthesis of Ti3SiC2 via the Maxthal 312 (nominally-Ti3SiC2) commercial powder route using spark plasma sintering (SPS). The SPSed bulk samples contained ancillary phases (TiCX and TiSi2) irrespective of the sintering environment as confirmed by XRD phase analysis. The optimum sintering temperature below which intrinsic decomposition starts appears to be ∼1350 °C in vacuum and ∼1450 °C in argon atmosphere, respectively. The role of powder particle size on densification during spark plasma sintering (SPS) was investigated. Analysis of the SPS processing parameters (piston movement and piston speed) indicated that the densification of Maxthal 312 powder (nominally-Ti3SiC2) can be improved by reduction in particle size, thus promoting the Joule effect, necking, mechanical movement of particles and plastic deformation. Further investigation on the role of graphite heating element as well as environment during the SPS synthesis showed that the graphite paper employed during SPS as well as possible silicon loss during vacuum sintering were responsible for the decomposition at temperature below the melting point of Si and/or shifting of the resultant bulk sample composition away from monolithic phase.

Overview of first Wendelstein 7-X high-performance operation

The optimized superconducting stellarator device Wendelstein 7-X (with major radius , minor radius , and plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of with central electron temperatures were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. In a first stage, plasma densities up to were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of a temperature of with was transiently accomplished, which corresponds to with a peak diamagnetic energy of and volume-averaged normalized plasma pressure . The routine access to high plasma densities was opened with boronization of the first wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above line integrated density and central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed.

Atomic absorption spectrometry for the quantification of cadmium in thermoformed and biodegradable flexible films made from cassava (Manihot esculenta crantz)

A simple method for determining the presence of cadmium (Cd) in thermoformed and biodegradable flexible films and flour and starch samples from cassava, using graphite furnace-atomic absorption spectrometry (GF-AAS), is proposed. The biopolymers used in contact with foods can contain Cd due to contamination from the origin of the material used to make them. The nitric: perchloric acid digestion with reflux was optimized, and the statistical quality parameters were standardized. The GF-AAS instrumental conditions and the graphite furnace heating program were adjusted through pyrolysis and atomization curves, and the optimal temperatures were 530 and 1750°C, respectively. In addition, under optimal conditions, the working range was from 0.1 µg/L to 1.0 µg/L, with limits of detection and quantification of 0.02 and 0.07 µg/L, respectively. The precision was evaluated using intermediate precision and repeatability of the method, which showed standard deviations of 1.6–4.8% and 2.1–4.8%, respectively. The percentage of recovery of Cd ranged from 96.2% to 97.3%. The results obtained support the suitability of the method for determining the presence of Cd in these samples. Cd concentrations in flexible films and starch were below the quantification limit. The other samples (thermoformed and flours) contained less than 1 mg/kg (maximum level set by the NTC 4096), indicating that these polymers meet this standard and therefore can be used for food containers and packaging for fresh fruits and vegetables.

Infrared dust arcs around stars – I. Effect of the radiation pressure

In this paper we consider the distribution of the interstellar dust in the vicinity of the star under an influence of the stellar gravitation and radiation pressure. This study is applicable to the stars with relatively weak stellar wind and strong radiation, when the stellar radiation swept out the interstellar dust much further from the star than the position of the bow shock created by an interaction of the stellar and interstellar plasma flows. In this case the number density of dust for a certain dust grains radius can be calculated analytically based on the classical `cold model'. The dust density distribution for the mixture of dust grains with different radii is calculated. We also calculated intensity maps of the thermal infrared emission at 24~$\mu$m from dust due to heating by the stellar radiation. It is shown that the obtained maps of the infrared emission extremely depends on the model parameters: material of dust grains, dust size distribution assumed in the interstellar medium, the approach used to calculate the dust temperature. The bright distinct arc at the intensity maps is seen for graphite dust grains and almost disappears for silicates. Absolute values of intensity in the case of graphite are several order of magnitudes larger than for silicates due to more intensive heating of graphite. Possible application of the presented theory is proposed as an algorithm for analysis of the observational images of the infrared arc around the star.

Thermodynamic Modeling of Thermal Processes Involving Chlorine, Calcium, Beryllium, Nickel, and Cesium Radionuclides in the Course of Heating Radioactive Graphite in Steam

The behavior of Cl, Ca, Be, Ni, and Cs radionuclides in the course of heating of radioactive graphite in steam was studied by thermodynamic modeling. Complete thermodynamic analysis at temperatures in the interval from 373 to 3273 K was performed using the TERRA program with the aim of determining the possible composition of the gas phase. The modeling shows that chlorine occurs in the form of gaseous HCl in the temperature interval from 873 to 1973 K and in the form of gaseous HCl and Cl at temperatures from 1973 to 3273 K. Calcium in the temperature range from 1973 to 3273 K occurs in the form of gaseous Ca(OH)2, Ca, and CaOH and ionized CaOH+ and Ca+. Beryllium in the temperature interval from 1873 to 3273 K occurs in the form of gaseous Be(OH)2, BeOH, Be, and BeOH. Nickel at temperatures from 1673 to 2173 K occurs in the form of gaseous NiH, Ni(OH)2, NiOH, and Ni, and at temperatures from 2173 to 3273 K, in the form of gaseous NiH, NiOH, and Ni. Cesium at temperatures from 973 to 1473 K occurs in the form of gaseous CsCl and CsOH and of ionized Cs+, and at temperatures from 1473 to 3273 K, in the form of ionized Cs+. The main reactions within separate phases and between the condensed and gas phases were determined, and their equilibrium constants were calculated.

Design of Thermal Ground Planes for Cooling of Foldable Smartphones

Foldable smartphones are expected to be widely commercialized in the near future. Thermal ground plane (TGP), known as vapor chamber or two-dimensional flat heat pipe, is a promising solution for the thermal management of foldable smartphones. There are two approaches to designing a TGP for foldable smartphones. One approach uses two TGPs connected by a graphite bridge and the other approach uses a single, large, and foldable TGP. In this study, different thermal management solutions are simulated for a representative foldable smartphone with screen dimensions of 144 × 138.3 mm2 (twice the screen of iPhone 6 s with a 10 mm gap). In addition, the simulation includes two heat sources representing a main processor with dimensions of 14.45 × 14.41 mm2 and power of 3.3 W (A9 processor in iPhone 6S) and a broadband processor with dimensions of 8.26 × 9.02 mm2 and power of 2.5 W (Qualcomm broadband processor). For the simulation, a finite element method (FEM) model is calibrated and verified by steady-state experiments of two different TGPs. The calibrated model is then used to study three different cases: a graphite heat spreader, two TGPs with a graphite hinge, and a single, large, and foldable TGP. In the fully unfolded configuration, using a graphite heat spreader, the temperature difference across the spreader's surface is about 17 °C. For the design using two TGPs connected by a graphite bridge, the temperature difference is about 7.2 °C. Finally, for the design using a single large TGP with a joint region, the temperature difference is only 1–2 °C. These results suggest that a single foldable TGP or a configuration with two TGPs outperform the graphite sheet solution for the thermal management of foldable smartphones.

The role of carbon in the formation mechanism of F-type centers in α-Al2O3:C,Mg single crystals

In this study, an α-Al2O3:C,Mg single crystal was grown using the Czochralski method with graphite resistance heating, Al2O3, MgO, and graphite powder as raw materials. Both the graphite heating unit and the shield served as the carbon source during the growth process. The structure and the optical properties of the crystal were investigated. The as-grown crystal shows a prominent absorption band at 206 nm, 230 nm, and 256 nm. The excitation-emission (EE) spectrum reveals a weak luminescence center (435/510 nm) that is attributed to the $$ {\mathrm{F}}_2^{2+} $$ (2 Mg). We calculate the concentration of the F-type centers using Smakula’s equation. Using the first principles simulation method, we studied the relation between the C atoms and the absorption properties of the crystal, and we discuss the role of carbon in the formation mechanism of the F-type luminescence centers for a α-Al2O3:C,Mg crystal.