Graphite Heater Research Articles

Comment on “Increase in specific heat and possible hindered rotation of interstitialC2molecules in neutron-irradiated graphite”

Iwata and Watanabe's model for the observed low-temperature specific heat of neutron-irradiated graphite [T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 (2010)] assumes that self-interstitial atoms exist as clusters of nearly free ${\text{C}}_{2}$ molecules. We suggest that their hypothesis is not supported by other experiments and theory, including our own calculations. Not only is it inconsistent with the long-known kinetics of interstitial prismatic dislocation loop formation, density-functional theory shows that the di-interstitial is covalently bonded to the host crystal. In such calculations no prior assumptions are made about the nature of the bonding, covalent or otherwise.

A new large area lanthanum hexaboride plasma source

A new 18x18 cm(2) active area lanthanum hexaboride (LaB(6)) plasma source for use in a dc discharge has been developed at UCLA. The cathode consists of four tiled LaB(6) pieces indirectly heated to electron emission (1750 degrees C) by a graphite heater. A molybdenum mesh anode 33 cm in front of the LaB(6) accelerates the electrons, ionizing a fill gas to create a 20x20 cm(2) nearly square plasma. The source is run in pulsed operation with the anode biased up to +400 V dc with respect to the cathode for up to 100 ms at a 1 Hz repetition rate. Both the cathode and anode "float" electrically with respect to the chamber walls. The source is placed in a toroidal chamber 2 m wide and 3 m tall with a major radius of 5 m. Toroidal and vertical magnetic fields confine the current-free plasma which follows the field in a helix. The plasma starts on the bottom of the machine and spirals around it up to four times (120 m) and can be configured to terminate either on the top wall or on the neutral gas itself. The source typically operates with a discharge current up to 250 A in helium making plasmas with T(e)<30 eV, T(i)<16 eV, and n(e)<3x10(13) cm(-3) in a background field of 100 G<B(o)<320 G, giving a magnetized plasma with 0.1<beta<1.

Investigation of Reduction and Smelting Mechanism in the Hi-QIP Process

A new coal-based reduction and smelting process for production of high quality iron pebbles in a rotary hearth furnace (Hi-QIP Process) was developed. The reduction, carburization, smelting, and separating mechanism of the Hi-QIP process were investigated. The experiments were carried out in a graphite heater furnace under rapidly heating up to 1773 K. A mixture of coal and ore produced molten metal and slag, which were held on the coal and did not come into contact with the refractory located under the coal layer. It is confirmed that the reduction of wettability between the iron and slag promotes the separation of them, when the content of FeO slag decreases. High productivity of the process is expected when using iron ore with small particle diameter and low gangue content. Favourable operating results were obtained in a pilot test using a rotary hearth furnace with a diameter of 7 m and a width of 1.5 m. This test demonstrated the possibility of continuous production of iron pebbles with high productivity (15t-iron/d).

Design evaluation of the semi-prototype for the ITER blanket first wall qualification

For the second qualification of the First Wall (FW) procurement of the International Thermonuclear Experimental Reactor (ITER), a semi-prototype of the FW has been designed with increased local surface heat flux up to 5 MW/m 2. With the given conditions, the new semi-prototype design was simulated with the commercial CFD code, the ANSYS-11. The results show that the semi-prototype temperature exceeds the melting temperature of Be, and the current design is required to be modified. In order to enhance cooling, a hypervapotron was added in the design and an analysis with the same code was performed. The results show that the temperature with the hypervapotron reduced by around 100 °C but it was still higher than the melting temperature of Be. The hypervapotron mock-up was fabricated and tested with a variance of inlet coolant flow rates and heat fluxes of up to 1.75 MW/m 2 using the second Korea Heat Load Test (KoHLT-2) facility, in which heat was loaded by a graphite heater through radiation heating. Wall and coolant temperatures were measured and compared with the simulation results. So far, there is a large difference between the experiments and the simulation, and a next experiment is being prepared.

The electrical resistivity of liquid carbon under rapid heating of dense isotropic graphite in different media

Experiments are considered which involve pulsed heating of graphite (in several microseconds) and registering of electrical resistance of liquid carbon. Samples of MF-307 dense (2 g/cm3) isotropic graphite with a cross section of ∼0.3 × 0.3 mm and 10–15 mm long are heated in water or in thick-walled (outside diameter D ∼ 10 mm, inside diameter d ∼ 0.5 mm) sapphire capillary tubes. It is confirmed that the heating in water at atmospheric pressure does not enable one to obtain and investigate liquid carbon: at best, only the beginning of the liquid state region is attained. The heating in sapphire tubes causes the emergence of pulsed pressure (up to ten kilobars) after expanding graphite comes against the tube wall. This growing pressure (within several microseconds) enables one to investigate the liquid state of carbon in a confined volume. The isochoric heating provided the possibility of measuring the electrical resistivity of liquid carbon at high specific energies (up to ∼32 kJ/g) and high pressures; such measurements are quite expensive in the case of stationary investigations.

High-pressure falling sphere viscosimetry of basaltic and dacitic rocks in conjunction with synchrotron radiation

In situ falling-sphere technique viscosity measurements in a DIA-type multi-anvil apparatus – MAX80 at beamline F2.1 at DESY / HASYLAB, Hamburg, Germany – have been performed. The viscosity was measured following Stokes law by evaluation of X-radiography sequences taken by a CCD-camera. Powdered basalt, dacite, and diabase samples were measured at pressures of 0.5 and 1 GPa and temperatures of 1890 K. After pressurization the temperature produced by an internal graphite heater was increased up to sample melting observed by X-ray diffraction and X-radiography. The results cover a data range from 3.5 Pa s (basalt at 1 GPa) and 195.5 Pa s (diabase at 0,5 GPa) and are in good agreement with published data.

Nanosized Silicon Carbide Obtained from Rice Husks

Two ways to obtain nanosized silicon carbide (SiC) powders from the products of thermal decomposition of rice hulls and posterior thermal and chemical treatment of SiO2-C precursors are shown in the present paper. The reagents and products were analyzed using BET, DTA, IR, XRD and SEM/TEM. Precursors obtained from rice husks containing pure SiO2 and a controlled SiO2-C ratio were used for the synthesis of SiC. The synthesis of SiC proceeded for 30-45 min in a graphite heater furnace under protective Ar atmosphere at relatively low temperatures (1450oC-1550oC). Nanosized dimensions of reagents obtained from rice husks and their high activity allow obtaining SiC in relatively milder thermal regimes. TEM and XRD analysis revealed synthesis of nanostructured mainly β-SiC with a mean crystallite size of 40-100 nm. Due to their purity and nano-scale properties, the products obtained are appropriate for production of bulk SiC or design of SiC–based ultra high-temperature materials using the methods of powder metallurgy.

Effect of expansion temperature of expandable graphite on microstructure evolution of expanded graphite during high-energy ball-milling

Two expanded graphites (EG), marked as EG-1 and EG-2, were prepared by rapid heating of expandable graphite to 600 and 1000 °C, respectively, and ball milled in a high-energy mill (planetary-type) under air atmosphere. The microstructure evolution of the ball-milled samples was characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). XRD analysis shows that the evolution degree of the average crystallite thickness along the c-axis ( L c ) of EG-2 is lower than that of EG-1 during the milling process. From the HRTEM images of the samples after 100 h ball-milling, slightly curved graphene planes can be frequently observed both in the two EGs, however, EG-1 and EG-2 exhibit sharply curved graphene planes and smoothly curved graphene planes with high bending angles, respectively.

Avoiding grain defects in single crystal components by application of a heat conductor technique

Abstract During single crystal solidification in turbine blades, grain defects are often formed in the platform region with abrupt variation in cross-section. In order to reduce grain defects arising from component geometries an HC (Heat Conductor) technique was developed. In the production process for ceramic shell moulds, a graphite heat conductor of excellent heat conductivity is inserted close to the inner corner of the platform. This effectively extracts local heat from this critical region during directional solidification. Numerical simulations show that by using HC the thermal condition in the platform region can be significantly improved. The single crystal growth in the blade body can spread into the platform more quickly before the melt at the platform extremity becomes deeply undercooled. Structural investigations reveal a remarkable reduction in grain defect formation, providing confirmation of the effectiveness of the HC technique in improving the casting quality of single crystal components.

핵융합로부품 시험을 위한 고열부하 시험시설 KoHLT-1 구축

본 한국원자력연구원에서는 국제열핵융합실험로(ITER)의 일차벽을 개발하기 위해 그라파이트 히터를 이용한 고열부하 시험시설 KoHLT-1(Korea Heat Load Test facility-1)을 구축하였으며, 현재 정상적으로 가동되고 있다. KoHLT-1의 주목적은 Be-CuCrZr-SS의 이종 금속이 HIP 방법에 의해 접합된 ITER 일차벽 mockup의 접합 건전성을 확인하는데 있다. KoHLT-1은 판형 그라파이트 히터, 냉각 jacket이 부착된 상자형 시험용기, 직류 전원, 냉각계통, He 기체 공급계통과 각종 진단계통으로 구성되어 있으며, 이 모든 시설은 Be 처리가 가능한 특수 정화계통이 설치된 실험실에 설치되었다. 그라파이트 히터는 두개의 시험 대상물 사이에 설치되며, 시험대상물과의 거리는 <TEX>$2{\sim}3\;mm$</TEX>이다. 시험 대상물의 크기와 요구되는 열유속에 따라 여러 가지의 그라파이트 히터를 설계, 제작하였으며, 전기 저항은 고온 운전 중에 <TEX>$0.2{\sim}0.5{\Omega}$</TEX>이 되도록 하였다. 히터는 100V/400 A의 직류전원에 연결되어 있으며, PC와 multi function module로 구성된 전류 조정계통에 의해 미리 프로그램되어 있는 패턴으로 전류를 자동 조절하게 된다. 두 시험대상물에 인가되는 열유속은 calorimetry법에 의해 냉각수의 입, 출구 온도와 유량을 측정하여 얻게 된다. 여러 가지 형태의 ITER 일차벽 Be mockups에 대해 고열부하 시험을 수행하였으며, 시험을 통하여 KoHLT-1 고열부하 시험 시설의 성능이 확인되었고, 24시간 이상의 연속 운전에 있어서도 그 신뢰성이 입증되었다. A high heat flux test facility using a graphite heating panel was constructed and is presently in operation at Korea Atomic Energy Research Institute, which is called KoHLT-1. Its major purpose is to carry out a thermal cycle test to verify the integrity of a HIP (hot isostatic pressing) bonded Be mockups which were fabricated for developing HIP joining technology to bond different metals, i.e., Be-to-CuCrZr and CuCrZr-to-SS316L, for the ITER (International Thermonuclear Experimental Reactor) first wall. The KoHLT-1 consists of a graphite heating panel, a box-type test chamber with water-cooling jackets, an electrical DC power supply, a water-cooling system, an evacuation system, an He gas system, and some diagnostics, which are equipped in an authorized laboratory with a special ventilation system for the Be treatment. The graphite heater is placed between two mockups, and the gap distance between the heater and the mockup is adjusted to <TEX>$2{\sim}3\;mm$</TEX>. We designed and fabricated several graphite heating panels to have various heating areas depending on the tested mockups, and to have the electrical resistances of <TEX>$0.2{\sim}0.5$</TEX> ohms during high temperature operation. The heater is connected to an electrical DC power supply of 100 V/400 A. The heat flux is easily controlled by the pre-programmed control system which consists of a personal computer and a multi function module. The heat fluxes on the two mockups are deduced from the flow rate and the coolant inlet/out temperatures by a calorimetric method. We have carried out the thermal cycle tests of various Be mockups, and the reliability of the KoHLT-1 for long time operation at a high heat flux was verified, and its broad applicability is promising.

Studies on mechanical and wear properties of alloyed hypereutectic gray cast irons in the as-cast pearlitic and austempered conditions

The mechanical and wear behavior of a series of as-cast gray iron alloys were compared with properties obtained after austempering at 360 °C. The austempered alloys showed equivalent or moderately enhanced mechanical strength than the as-cast pearlitic gray irons. The specific wear rates of all the austempered alloys decreased significantly by 7–15 times and friction coefficient reduced by 30–50% compared to pearlitic alloyed gray irons. The dry sliding wear studies of as-cast alloys against high carbon 1%Cr through-thickness hardened steel shows that the specific wear rate ranged from 5.6 to 19.1 (×10 −7) g/Nm with friction coefficient from 0.55 to 0.7. While, the austempered alloys showed wear rates from 0.5 to 2.6 (×10 −7) g/Nm with friction coefficient ranging from 0.23 to 0.4. The improved wear resistance was attributed to the layer wise surface phase transformation associated with strain induced martensite formation of the stabilized austenite in the austempered matrix, lubrication of the interface by the flake graphite and better heat conduction from the rubbing interface by higher volume fraction of the graphite. Cast iron alloyed with Ni shows enhanced mechanical properties and wear resistance. The tensile strength shows decreasing trend with increase in carbon equivalent and graphite volume. The specific wear rate and friction coefficient shows decreasing trend with increase in hardness and graphite flake volume.

Design and Performance of an LPCVD Reactor for the Growth of 3C-Silicon Carbide

This paper discusses the design of a low pressure chemical vapor deposition (LPCVD) reactor and growth of β silicon carbide (3C-SiC) thin films on Si. The reactor’s hot-zone configuration radiatively couples the graphite heater (no physical contact) to Si substrates with realized temperatures . We implemented a four-stage growth procedure using silane and propane precursors and hydrogen as the carrier gas. Temperatures, pressures, and flows varied from , from , and from during growth, respectively. Growth of 3C-SiC was confirmed using X-ray diffraction (XRD) with the observation of a (200) peak located at 41.4°. Activation energies of 21.5 and were calculated for the kinetically and mass-transport-limited regimes for polycrystalline 3C-SiC growth. The polycrystalline 3C-SiC films with fewest defects and smallest XRD full width at half-maximum were deposited at with a growth rate of using flow rates of , , and of (conc. 1.16%). The composition of the films measured by X-ray photoelectron spectroscopy shows that the films are slightly carbon rich with oxygen as the main source of contamination. Voids were observed at the film–substrate interface for samples grown at high precursor concentrations.

Growth and characterisation of BNC nanostructures

BNC nanostructures were produced by the resistive heating of graphite and boric acid powder in a high isostatic pressure apparatus at a temperature of ∼1650 °C, with a nitrogen pressure of 12 MPa. The resultant nanostructures were examined using transmission electron microscopy, electron energy-loss spectroscopy and X-ray photoelectron spectroscopy. The growth mechanisms of the nanostructures are discussed.

Development of a High Heat Flux Test Facility for Plasma Facing Components

A high heat flux test facility using a graphite heating panel was constructed and is presently in operation at Korea Atomic Energy Research Institute, which is called KoHLT-1. One of the major purposes of this facility is to carry out a thermal cycle test of the ITER (International Thermonuclear Experimental Reactor) FWQM (first wall qualification mockup). The facility is equipped with a graphite heating element, a water cooled box-type vacuum chamber, and diagnostic systems. Two mockups are installed in the chamber; two facing mockups are simultaneously heated by a graphite heater installed between two mockups. The graphite heating element has an effective irradiation area of 244 mm × 80 mm and its electrical power is provided by a 40 kW DC power supply. The diagnostic system consists of two independent calorimetric power measuring systems, thermocouples, a vacuum gauge, and a CCD camera. Water cooling, Be treatment, and vacuum pumping systems are also equipped. We performed thermal cycle tests for two Cu mockups, and for Cu and Cu/SS mockups ensure the performance of the KoHLT-1. After that, we carried out thermal cycle tests up to 220 cycles for Cu mockup and FWQM at 0.65 MW/m2, from which the reliability of the KoHLT-1 was verified.

High rate capability of carbonaceous composites as anode electrodes for lithium-ion secondary battery

Anode materials with high rate capability for Li-ion secondary batteries were investigated by using the mixture of graphite, cokes, and petroleum pitch. Since obvious potential plateaus were obtained at graphite contents above 40 wt.%, which would cause difficulties in perceiving the capacity variations as a function of electrical potential, the graphite content were determined at 20–30 wt.%. The composites with a given content of graphite and remaining content of petroleum pitch/cokes mixtures at 1:4, 1:1, and 4:1 mass ratios were heated at a temperature range of 800–1200 °C. For a given composition of carbonaceous composite, the discharge rate capability improved but the reversible capacity decreased with increasing the heat treatment temperature. Although the reversible capacity increased with increasing content of the petroleum pitch for given graphite content and heat treatment temperature, the discharge rate capability decreased. The carbonaceous composites prepared by the mixture of 30 wt.% graphite and 70 wt.% petroleum pitch/cokes mixture at 1:4 mass ratio with the heat treatment at 800 °C showed relatively high electrochemical properties, of which reversible capacity, initial efficiency, discharge rate capability (retention of discharge capacity in 5 C/0.2 C) and charge capacity at 5 C were 312 mAh/g, 79%, 89% and 78 mAh/g, respectively.

A slide-type multianvil ultrahigh pressure apparatus and calibrations of its pressure and temperature

A slide-type ultrahigh pressure apparatus with 6/8 anvils is described. The three-supporting pillars of the main framework and the slide-type multi-anvils have conformable mechanical symmetry. Under the loading uniaxial pressure, the first-stage anvils can be self-corrected and run with excellent synchronicity and reproducibility. In the present experiments, the edge length of pyrophyllite octahedron is 125 mm and the truncation edge length of tungsten carbide anvil is 8 mm. The cell pressure was calibrated at room temperature by means of the known pressure-induced phase transitions of Bi and ZnTe at 255, 77, 96 and 120 GPa. The cell temperature under 10 GPa was measured up to 1560 ℃ by WRe3-WRe25 thermocouple. The temperature calibration was reconfirmed and extended to the higher range by observation of diamond formation on the interface between graphite heater and steel plug, and by comparison of the variational details of recorded data and the Fe—C phase diagram, and then the axial temperature gradient was estimated to be about 21 ℃/mm.

MATHEMATICAL MODELING FOR COMPUTATIONAL SIMULATION OF THE TEMPERATURE DISTRIBUTION DURING THE SYNTHESIS OF POLYCRYSTALLINE DIAMOND

For more than half century, synthetic diamonds have been successfully produced for a large range of technological applications. This synthesis is, today, a well known process associated with a system capable of simultaneously generating the high pressure and high temperature, HPHT, necessary to produce diamond from graphite in the presence of a catalyst metallic alloy. The graphite to diamond transformation, G→D, is usually performed at pressures and temperatures that may reach, respectively, 6 GPA and 2000 K. These severe conditions require a special high pressure device, HPD that takes the load from press equipment and converts it to the high pressure inside a reaction chamber where the G→D transformation takes place. The high temperature, which is also necessary for this transformation, is attained via the heat generated by an electrical current established from an applied voltage. In the past, studies have investigated the association of the voltage with the temperature established inside high pressure equipment. The desired level of temperature is obtained by controlling the applied voltage to the electrical resistance circuit, which includes metallic conductors as well as the graphite parts, heater sleeve and bulk material, inside the chamber. Since the bulk graphite partially transforms into diamond, the value of the applied voltage will change during transformation. As a consequence, heating gradients will be formed within the reaction chamber resulting in a specific local distribution of temperatures. This distribution depends on factors such as the geometrical shape and the materials of both, the HPD and the chamber, as well as the physical arrangement of graphite plus catalytic alloy. A relevant point is that this distribution of temperature inside the Abstract

Synthesis of single crystal (Mg 1−xFe x) 1−δO ( x=0.001–1.00) solid-solution and electrical conduction mechanism at high temperature and pressure

The (Mg,Fe) 1− δ O rock salt-type solid solution is believed to be one of the major constituents in the Earth's lower mantle. The rock salt-type (Mg 1− x Fe x ) 1− δ O ( x=0.001–0.43) single crystals (∼8 mm in diameter) were made by a melt-growth method using high temperature graphite heater (up to 3300 K) in the Ar atmosphere and by a FZ method in the CO 2–H 2 atmosphere ( pH 2/ pCO 2=0.1). Electrical conductivity measurements were carried out as functions of temperature and frequency by a the complex impedance method under pressure (5 GPa and ∼1800 K, and, 0.1 MPa and ∼1400 K). The ratio of Fe 3+/(Fe 2++Fe 3+) in the solid solution decreases with MgO contents under the same oxygen fugacity. Our experimental results show a change in charge transport mechanism in the (Mg 1− x Fe x ) 1− δ O solid solution at high temperature. The temperature of inflection point of the slope in Arrhenius plots depend greatly on both composition and extrinsic factors of crystals. Chemical composition and homogeneity of specimen rather than pressure greatly influence the electric conductivity. The activation energy for the (Mg 0.99Fe 0.01) 1− δ O solid solution is 2.4 eV corresponding with a migration enthalpy of O ions vacancies. It is proposed that possible dominant electrical conduction mechanism in ferropericlase under the lower mantle conditions, at least in the higher temperature region, is a super ionic conduction.

Exploratory study of the new B-doped diamond heater at high pressure and temperature and its application to in situ XRD experiments on hydrous Mg-silicate melt

We demonstrate the successful application of a new heating material, B-doped graphite (boron 10 wt%), to 23 GPa and 2000 °C. The heater resistance increases significantly at temperatures greater than 1200–1400 °C, similar to the diamond transition observed for a pure graphite heater. However, we find that the B-doped graphite furnace can be used at temperatures and pressures higher than that of diamond stability region, and that is stable for at least 3 h. Because this new furnace material is X-ray transparent and stable under high-pressure and high-temperature conditions, it is suitable for in situ X-ray synchrotron applications at high pressure and high temperature, including for structural refinement of hydrous silicate melt. High quality diffraction patterns of hydrous Mg-silicate melt were collected at 14.9 GPa using the B-doped diamond heater. Hence, B-doped graphite (diamond) is a promising heater material for high-pressure and high-temperature applications.

Edge‐Defined, Film‐Fed Growth (EFG) Technique for the Preparation of α‐Al2O3:C Crystal with Highly Sensitive Thermoluminescence Response

In this work, α‐Al2O3:C, a highly sensitive thermoluminescence dosimetry crystal, was grown by the EFG method in which a graphite heating unit and shield acted as the carbon source during the growth process. The optical, luminescent properties and dosimetric characteristics of the crystal were investigated. The as‐grown crystal shows a single glow peak at 536 K, which is associated with Cr3+ ions. After annealing in H2 at 1673 K for 80 h, the crystal shows a single glow peak at 460 K and a blue emission band at 415 nm. The thermoluminescent response of the annealed crystal shows linear–sublinear‐saturation characteristics in the dose range from 5 × 10−6 to 100 Gy.