TEM/STEM Observation of ZrC-Coating Layer for Advanced High-Temperature Gas-Cooled Reactor Fuel

The Japan Atomic Energy Agency (JAEA) has started to study and develop ZrC-coated fuel particles for advanced high-temperature gas-cooled reactors. The ZrC-coating layer was fabricated using the bromide process at JAEA. In the early stage of the project, however, the deposition temperature was varied. This paper mainly focuses on the microstructures of the ZrC-coating layer developed in the early stage of the project. Some circumferential stripes were observed in the ZrC-coating layer on optical micrographs. It was found that the stripes were caused by the nonuniform distribution of the free carbon phase. It was also revealed by means of transmission electron microscope /scanning transmission electron microscope observations that crystal grains of the ZrC were small and columner in shape, and were not equiaxed especially near the surface. It appears that the oscillated deposition temperature results in the nonuniform distribution of the free carbon region. The structure of the free carbon region formed in the ZrC-coating layer appeared to be such that the c -plane was roughly parallel to its lengthened direction. The ZrC-coating layer appeared to be bound to the PyC layer. Fibrous carbon existing at the PyC/ZrC boundary was also observed.     

Deposition Mechanism for Chemical Vapor Deposition of Zirconium Carbide Coatings

Zirconium carbide (ZrC) coatings were fabricated by chemical vapor deposition (CVD) using ZrCl₄, CH₄/C₃H₆, and H₂ as precursors. Both thermodynamic calculation results and the film compositions at different temperatures indicated that zirconium and carbon deposited separately during the CVD process. The ZrC deposition rates were measured for CH₄ or C₃H₆ as carbon sources at different temperatures based on coating thickness. The activation energies for ZrC deposition demonstrated that the CVD ZrC process is controlled by the carbon deposition. This is also proven by the morphologies of ZrC coatings.     

Performance of ZrC-Coated Particle Fuel in Irradiation and Postirradiation Heating Tests

The ZrC-coated UO₂ particle is a promising fuel for high-temperature gas-cooled reactors. Particle fuels with multiple layers of pyrolytic carbon and ZrC have been irratiation-tested to a maximum fast-neutron fluence exceeding 2 × 10²⁵/m² ( E < 29 fJ). In-reactor fission-gas release measured at a burnup of 1.5 at.% was minimal. The failure fraction by postirradiation examination was null for all samples. The ZrC-coated particles at 4 at.% burnup were postirradiation heated to 2400°C/min without failure until after 6000 s at the maximum temperature. It was found that the ZrC layer could sustain a large strain at such high temperatures. The behavior is in strong contrast with that of SiC of standard Triso coating, which is brittle to very high temperatures.     

Morphologies and growth mechanisms of zirconium carbide films by chemical vapor deposition

Zirconium carbide films were grown on graphite slices by chemical vapor deposition using methane, zirconium tetrachloride, and hydrogen as precursors. The growth rate of zirconium carbide films as a function of temperature was investigated. The morphologies of these films at different temperatures were also observed by scanning electron microscopy. The results indicated that the deposition of zirconium carbide was dominated by gas nucleation at temperatures below 1523 K, and by surface process at temperatures higher than 1523 K. By comparison of the deposition activation energies for zirconium carbide and deposited carbon, it was determined that the carbon deposition was the controlled process during the growing of zirconium carbide films. The effect of temperatures on the morphologies of zirconium carbide films was also discussed, based on the carbon deposition process.     

Deposition rates during the early stages of pyrolytic carbon deposition in a hot-wall reactor and the development of texture

Pyrolytic carbon layers were deposited from methane/oxygen/argon mixtures on planar substrates (silicon wafers) at a total pressure of 100 kPa, a maximum gas residence time of 2 s and a temperature of 1100 °C. The depositions were performed in a hot-wall reactor with the substrate oriented parallel to the gas flow. Particular attention was paid to factors that influence the reproducibility of the deposited layers. Scanning and transmission electron microscopy were applied to study the thickness profiles and the texture of the carbon layers. The surface topography was investigated by atomic force microscopy. For pyrolytic carbon deposited without oxygen, an alteration from medium- to high-textured carbon is observed with increasing residence time. Islands are observed on the surface of the layer whose size increases with the texture. For pyrolytic carbon deposited with 3% oxygen, lower deposition rates were obtained and a strong modification of the texture is found compared to gas mixtures without oxygen.     

Reaction Mechanism and Microstructure Development of Zr–Si Alloyed Melt‐Infiltrated ZrC‐Modified C/C Composite

ZrC ceramic modified‐C/C composite is prepared by a quick and low‐cost reactive melt infiltration process with a Zr‐Si8.8 alloy. Reaction kinetics and mechanism of pyrolytic carbon with the infiltrated Zr‐Si8.8 alloy are investigated. A continuous ZrC layer is found to be formed around pyrolytic carbon due to the in situ reaction and its thickness parabolically increases with an increase in reaction time period. Zr concentration in the alloyed melts decreases due to the reaction between Zr and pyrolytic carbon and Zr₂Si phase precipitates from the residual alloyed melts. A model for the growth of ZrC layer is established to describe the reaction kinetics of pyrolytic carbon with Zr‐Si8.8 alloy. The calculated thickness of the reaction‐formed ZrC layer is in good agreement with the experimental data. Based on the Arrhenius equation, the activation energy of the reaction between carbon and Zr‐Si8.8 alloy is 313.2 KJ/mol, smaller than that of the reaction between carbon and pure zirconium. The microstructure of the reactive melt‐infiltrated ZrC‐modified C/C composite is characterized by optical microscope, SEM, EDS, XRD, and TEM. The mechanism of the reaction between pyrolytic carbon and Zr‐Si8.8 alloy is discussed on the basis of the characterization results and phase diagram. A schematic is proposed to understand the reaction mechanism between pyrolytic carbon and Zr‐Si8.8 alloy and microstructure development of the ZrC‐modified C/C composite.     

Influence of the deposition parameters on the texture of pyrolytic carbon layers deposited on planar substrates

Pyrolytic carbon layers were deposited from methane on planar substrates (pyrolytic boron nitride) at various residence times, methane pressures and deposition temperatures. The depositions were performed in a cavity oriented perpendicular to the gas flow. The small surface area/reactor volume ratio of the reactor geometry allows depositions in the growth and nucleation mechanism. Transmission electron microscopy was applied to study the texture and microstructure of the carbon layers. A texture transition from medium- to high-textured pyrolytic carbon occurs as a result of increasing residence times, methane pressures and temperatures. Improved textures are generally correlated with increasing deposition rates, which are not necessarily constant during long-term depositions. Lower textures are observed in the vicinity of the substrate interface that are attributed to the influence of the substrate morphology and microstructure.     

Fluidized bed chemical vapor deposition of pyrolytic carbon - II. Effect of deposition conditions on anisotropy

Pyrolytic carbon coatings were deposited on top of alumina particles at deposition temperatures from 1250 °C to 1450 °C and with acetylene and acetylene/propylene mixtures at concentrations between 25% and 70%, v/v. The anisotropy of pyrolytic carbon coatings was quantified using electron diffraction and Raman peak intensities, and related to the deposition conditions and microstructure. In correlation with the TEM images, it was found that the value of orientation angle (OA) measured from selected area electron diffraction patterns, increased with increasing deposition temperature and precursor concentration corresponding to a reduction in texture. The effect of temperature was more pronounced for the acetylene/propylene mixture and at low temperatures highly anisotropic material was obtained on a local scale. However, the use of larger TEM selecting apertures showed that the majority of coatings were isotropic overall. A correlation was found between OA and Raman D band intensity which allows quick classification of pyrolytic carbon texture.     

Synthesis of fine zirconium carbide powder by carbothermal reaction of carbon‐coated zirconia particles

In this study, a novel method for the synthesis of fine ZrC powder was presented. It consists of chemical vapor deposition (CVD) of C from CH₄ on ZrO₂ particles followed by carbothermal reaction. Firstly, optimal CVD conditions (1300 K and 30 minutes) yielding the stoichiometric amount of C deposit (23 wt%) were determined. Carbothermal reaction behavior of the carbon‐coated oxide particles was then investigated in Ar flow at 1700‐1800 K for 0‐120 minutes. Mass measurements, XRD and SEM techniques were used to characterize the products at various stages of the process. Lattice constants and mass losses of the samples increased to the levels of ZrC with increasing temperature and time. Almost pure ZrC powder (oxygen content: 0.59 wt%) with a mean particle size of ~170 nm was synthesized at 1800 K within 120 minutes. The present study demonstrates that ZrC powder can be synthesized at lower temperatures and shorter reaction times using C‐coated ZrO₂ powders compared with the conventional method which uses a mixture of ZrO₂ and solid C particles.     

Exploration for the potential precursors for zirconium carbide atomic layer deposition via comprehensive computational mechanistic study of the gas phase decomposition of neopentyl zirconium derivatives

We previously reported the probable gas phase decomposition mechanism of tetraneopentyl zirconium (ZrNp₄) under typical MOCVD (metalorganic chemical vapor deposition) conditions (400 to 800 °C) using computational thermochemistry. By the same approach, we performed a mechanistic study of the gas phase decomposition of trineopentyl zirconium monochloride (ZrNp₃Cl) to evaluate its possibility as a CVD precursor for ZrC film growth. It was demonstrated that strong Zr-Cl bonding would require much higher growth temperatures to drive the gas phase decomposition of ZrNp₃Cl, compared to ZrNp₄. The higher temperature growth would pose the problem of accelerated gas phase parasitic reactions, which potentially hamper ZrC deposition on the surface. However, strong Zr-Cl bonding offers the possibility of ZrC ALD (atomic layer deposition) using dineopentyl zirconium dichloride (ZrNp₂Cl₂), and a postulated scheme is presented based on the results from the gas phase decomposition study of ZrNp₄ and/or ZrNp₃Cl.     

Palladium Migration Through a Zirconium Carbide Coating in TRISO‐Coated Fuel Particles

Reaction of ZrC with Pd at temperatures up to 1500°C was examined using ZrC/Pd composite, Pd/ZrC‐coated TRISO particles, and Pd/ZrC bulk diffusion couples experiments. Intermetallic phase (Pd₃Zr) and amorphous carbon at the ZrC–Pd interfaces were identified by X‐ray diffraction (XRD), Raman and scanning electron microscope (SEM). Moreover, thicknesses of Pd₃Zr layers were measured by energy‐dispersive X‐ray spectrometry (EDS). The validity of the reaction was proved by thermodynamic calculation. The reaction kinetics parameters, i.e., the activation energy (208.2–266.5 kJ/mol) and the reaction order (3.38–3.78) for Pd attacking through a ZrC coating in TRISO particles were determined based on both the DSC curves and the growth of the Pd₃Zr layer.     

The role of ZrCl4 partial pressure on the growth characteristics of chemical vapour deposited ZrC layers

ZrC layers were deposited in a chemical vapour deposition (CVD) reactor on graphite substrates using a ZrCl₄-Ar-CH₄-H₂ precursor mixture. The deposition was conducted at different ZrCl₄ partial pressures at a constant substrate temperature of 1400 °C for 2 h at atmospheric pressure. The deposited ZrC layers were characterised using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The effect of ZrCl₄ partial pressure on the growth rate, microstructure and surface morphology of the deposited layers was studied. The ZrCl₄ partial pressure was manipulated by changing the flow rate of the argon carrier gas through the sublimation chamber. The boundary layer thickness decreased as ZrCl₄ partial pressures increased due increased argon flows. The increased ZrCl₄ partial pressure increased the growth rate of ZrC layers linearly. It was found that the transport process of the source materials was laminar and forced convection flow. The flow process of source materials through the boundary layer to the reacting surface was also illustrated using a model. The average crystallite size increased with ZrCl₄ partial pressures, whereas the lattice parameter, lattice strain and dislocation density decreased as ZrCl₄ partial pressure increased. The surface morphology of the as-deposited ZrC layers varied with the ZrCl₄ partial pressure. The size of crystals grew larger and the cavities surrounding them decreased in number and size as the ZrCl₄ partial pressure increased.     

Effects of zirconium carbide content on thermal stability and ablation properties of carbon/phenolic composites

Ceramic particles were utilized to improve thermal stability and ablation properties of carbon/phenolic (C/Ph) composites. In this study, zirconium carbide (ZrC) modified C/Ph composites were fabricated by vacuum impregnation method, and effects of ZrC content on thermal stability and ablation properties were investigated by thermogravimetry analysis and plasma wind tunnel test. Moreover, morphological characterization was carried out using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. Experimental results showed that increasing ZrC content could lead to an evident increase in char yield, but an observable reduction in linear ablation rates and back-face temperatures because of the formation of ZrO₂ layer on the ablation surface. The work provided an effective way to improve thermal stability and ablation properties of C/Ph composites.     

A study of the properties of pyrolytic carbons deposited from propane in a tumbling and stationary bed between 900 and 1230°C

In order to obtain low-temperature-isotropic (LTI) pyrolytic carbons, a new “Tumbling Bed” reactor has been developed. The characteristics of the pyrolytic carbons deposited in this tumbling bed have been studied by varying the gas composition, the total gas flow rate, the weight of bed particles and the rotational speed (RPM) of the reaction tube in the temperature range of 900–1230°C. It was found that all pyrolytic carbons deposited were isotropic in the temperature range of 1050–1230°C. The density of the Isotropie carbon increased slightly with temperature, but it was independent of the other variables with values of 1.9–2.0 g/cm³. The apparent crystallite size, L_e, of the isotropic carbon was about 30 Å regardless of coating conditions. The deposition rate increased with temperature, propane concentration, and total flow rate, showed a minimum with increasing RPM of reaction tube, and decreased with the weight of bed particles. The deposition mechanism of the isotropic pyrolytic carbon was suggested from the results. Additionally, a few experiments were carried out in a stationary bed in order to study the role of the rotating action in the tumbling bed reactor. Columnar, sooty and filamentous carbons were obtained in the stationary bed. From scanning electron micrographs of fracture surfaces of the filamentous carbons, it appeared that they are constructed by spiral growth of carbon flakes.     

Structure and energetics of ZrC(100)||c-ZrO2(001) interface: A combination of experiments,finite temperature molecular dynamics,periodic DFT and atomistic thermodynamic modeling

The oxidation process of ZrC is very important as it affects its initial excellent mechanical and physical properties. ZrC is an ultra-high temperature ceramic, but forms low refractory oxides at lower temperatures of 500–600 °C. To develop core/shell materials by coating the ZrC surface with another material that forms protective layers on ZrC and prevents it from oxidation (such as SiC), there is the need to study and characterize the oxidized layer surrounding ZrC particles. XPS, ToF-SIMS, TEM-ED and EDX analyses were used to investigate the covering oxidized layer, and polycrystalline ZrO₂(mainly cubic phase) was identified. Some traces of the tetragonal phase are observed to be present as shells around the ZrC particles with a thickness of about 4 nm on the average. Periodic DFT was subsequently used to characterize the interface formed between ZrC(100) and c-ZrO₂(001) phases. A strong interface was noticed mainly with charge transfer from Zr (c-ZrO₂ side) at the interface to O and C (ZrC side) atoms at the interface. The interfacial properties are local to only the first and second layers of ZrO₂, and not on the third and fourth layers of ZrO₂, as Bader charge analysis revealed substantial charge transfer at the interface region with no charge redistribution in the second ZrO₂ layer and subsequent bulk layers. The main physical quantity, ideal work of adhesion (W_ad), used to characterize the interface, remains quite constant for all ZrO₂ layers, and converges at three layers of ZrO₂. The interfacial bonds formed are observed to be stronger than the free surfaces in the corresponding ZrC and c-ZrO₂ used to generate the interface.     

Reaction kinetics and ablation properties of C/C-ZrC composites fabricated by reactive melt infiltration

Carbon/carbon-zirconium carbide (C/C-ZrC) composites were prepared by reactive melt infiltration. Carbon fiber felt was firstly densified by carbon using chemical vapor infiltration to obtain a porous carbon/carbon (C/C) skeleton. The zirconium melt was then infiltrated into the porous C/C at temperatures higher than the melting point of zirconium to obtain C/C-ZrC composites. The infiltration depth as a function of annealing temperature and dwelling time was studied. A model based on these results was built up to describe the kinetic process. The ablation properties of the C/C-ZrC were tested under an oxyacetylene torch and a laser beam. The results indicate that the linear and mass ablation rates of the C/C-ZrC composites are greatly reduced compared with C/SiC-ZrB₂, C/SiC, and C/C composites. The formation of a dense layer of ZrC and ZrO₂ mixture at high temperatures is the reason for high ablation resistance.     

Interplay between adhesion and interfacial properties of diamond films deposited on WC-10%Co substrates using a CrN interlayer

In this study, the effect of deposition temperature on the adhesion of diamond films deposited on WC-10%Co substrates with a Cr-N interlayer is investigated. Diamond films were deposited at different temperatures (550, 650 and 750 °C), using a hot filament chemical vapor deposition reactor. It was found that the optimal adhesion is obtained for the film deposited at 650 °C. The interplay between carbon interfacial diffusion and the adhesion of diamond films deposited at different deposition temperatures were investigated. The combined use of different characterization techniques (Indentation tests, SIMS, XPS, XRD and SEM) shows that the adhesion strength depends on the thickness of Cr-C layer formed at the interface during diamond deposition, which is strongly influenced by the deposition temperature. It is suggested that at the optimum deposition temperature, thickness of the Cr-C layer is too low to introduce a large thermal stress at the interface and sufficiently thick enough to withstand the propagation of indentation induced cracks.     

自蔓延高温技术制备ZrC粉体(英文)

采用自蔓延高温合成(self-propagating high-temperature synthesis,SHS)技术,以 Zr+C 为反应体系合成了 ZrC 粉末。研究了实验参数对 SHS过程中点火电流、燃烧温度的影响。采用了 3 种碳源,研究了其对最终产物形貌及化学组成的影响。通过添加不同含量的 NaCl 作为 SHS 稀释剂,控制产物粒径及形貌。结果表明:炭黑是高温自蔓延法制备 ZrC 粉体的最佳碳源。由该体系制备的 ZrC 粉末粒径在 0.5～1 μm之间,氧含量为 0.38%。随稀释剂 NaCl 含量增加,体系燃烧温度降低,产物粒径减小。当 NaCl 含量为 30% (质量分数)时,体系燃烧温度下降至 1 810 K,产物 ZrC 粉末的粒径减小至 50 nm。     

Deposition rate and temperature of carbon films during laser-induced CVD on a moving substrate

The feasibility of using pyrolytic laser-induced chemical vapor deposition (LCVD) to deposit carbon coatings on moving fused quartz substrates is investigated. This LCVD system uses a CO₂ laser to locally heat a substrate in open air to create a hot spot. Pyrolysis of hydrocarbon species occurs and subsequently deposits a layer of carbon film onto the substrate surface. The results of this study indicate that growth kinetics and the geometry of uniform carbon stripes deposited by pyrolytic LCVD strongly related to the laser power, the traverse velocity of the substrate, the type of hydrocarbon species used in deposition, and the diameter of the substrate. The deposition rate of carbon film increases exponentially with the laser power, while an increase in traverse velocity of the substrate will also increase the deposition rate until a maximum deposition rate is reached; further increases in the traverse velocity will decrease the deposition rate. We suspect that this optimal deposition rate is caused by substrate motion, which affects the substrate surface temperature, and consequently the effective surface area available for film deposition. The substrate temperature is observed to behave linearly with the deposition parameters considered in this study.     

Kinetics of pyrolytic carbon infiltration for the preparation of ceramic/carbon and carbon/carbon composites

Carbon/carbon and zeolite/carbon composites have been prepared by pyrolytic carbon infiltration of organic and inorganic substrates with different porous structures. The chemical vapour infiltration kinetics of these substrates has been studied in a thermogravimetric system at atmospheric pressure, using benzene as pyrolytic carbon precursor. The rate of pyrolytic carbon infiltration seems to depend on the porosity of the substrate available to the pyrolytic carbon precursor, irrespective of the nature of the substrate studied. Activation energy values of about 180 kJ/mol were found for the different substrates used in the temperature range of 700-800 °C, where the cracking reaction of benzene takes place, predominantly, in a heterogeneous form. At higher temperatures homogeneous reactions compete with heterogeneous ones and higher values of activation energies (280-380 kJ/mol) were obtained. The oxidation of the pyrolytic carbon deposited on the different substrates studied takes place in the same range of temperature, which suggests the presence of a similar pyrolytic carbon structure on substrates of different nature or a similar accessibility to the deposited layer.