Bed Chemical Vapor Deposition Method Research Articles

(Invited) Production and Functionalization of Carbon Nanotubes for Electrochemical Energy Storage Devices

Self-supporting, sponge-like paper of carbon nanotubes (CNTs) can be a new platform for high energy/power density batteries. Presently, battery electrodes are built on metallic foils having only two faces and large mass with the aid of polymeric binders and conductive fillers. CNT paper can replace all these components. Furthermore, it captures nanomaterials at high loading, activates insulative materials, and moderates volumetric change of active materials.We have developed semi-continuous fluidized bed chemical vapor deposition method [1], which yields submillimeter-long few-wall CNTs with carbon purity >99 wt% and metal impurity <0.1 wt% [2]. By capturing capacitive particles (99 wt%) with CNTs (1 wt%), a lithium ion full cell with graphite-CNT anode and LiCoO2-CNT cathode is demonstrated (Figure 1b) [3]. This platform is also effective to create electrodes of the emerging active materials such as Si anode [4] and S cathode [5] with practically high gravimetric, areal, and volumetric loadings, and realized Li x Si-S and Li-Li2S x full cells [6,7]. In addition, by combining with the insulative sponge of boron nitride nanotubes (BNNTs), the graphite-CNT|BNNT| LiCoO2-CNT full cell with high thermal stability was developed [8].[1] D.Y. Kim, et al., Carbon 49, 1972 (2011).[2] Z. Chen, et al., Carbon 80, 339 (2014).[3] K. Hasegawa and S. Noda, J. Power Sources 321, 155 (2016).[4] T. Kowase, et al., J. Power Sources 363, 450 (2017).[5] K. Hori, et al., J. Phys. Chem. C 123, 3951 (2019).[6] K. Hori, et al., Carbon 161, 612-621 (2020).[7] Y. Yoshie, et al., Carbon 182, 32-41 (2021).[8] K. Kaneko, et al., Carbon 167, 596-600 (2020).Figure 1. (a) Semi-continuous production of submillimeter-long few-wall CNTs by fluidized bed [1]. (b) Lithium ion full cell made of graphite-CNT anode and LiCoO2-CNT cathode [3]. Figure 1

Microstructure of SiC–ZrC composite coatings on TRISO particles via fluidized bed chemical vapor deposition

A SiC and ZrC composite coating layer on a tristructural-isotropic (TRISO) coated particle was deposited by fluidized bed chemical vapor deposition method. Methyltrichlorosilane (MTS) and ZrCl4 powder were used as the source materials of the SiC coating and the ZrC coating, respectively. Two kinds of the SiC and ZrC composite coating layers were fabricated with other source gas fraction. The composite coating layer, with a similar ratio of SiC and ZrC, which was observed in the SEM microstructure, was fabricated, and each phase was confirmed by XRD, Raman, and TEM analysis. Only the SiC and ZrC phases were co-deposited; the ZrSi and free carbon phases were not. The composite coating layer with very small ZrC particles embedded in a SiC matrix was fabricated by decreasing the ZrCl4 source gas ratio. Nano-sized ZrC particles in the coating layer were confirmed by high resolution TEM and EDS analysis.

Large-scale synthesis of monodisperse SiC nanoparticles with adjustable size, stoichiometric ratio and properties by fluidized bed chemical vapor deposition

A facile fluidized bed chemical vapor deposition method was proposed for the synthesis of monodisperse SiC nanoparticles by using the single precursor of hexamethyldisilane (HMDS). SiC nanoparticles with average particle size from 10 to 200 nm were obtained by controlling the temperature and the gas ratio. An experimental chemical vapor deposition phase diagram of SiC in the HMDS-Ar-H2 system was obtained and three regions of SiC-Si, SiC and SiC-C can be distinguished. The BET surface area and the photoluminescence properties of the SiC nanoparticles can be adjusted by changing the nanoparticle size. For the SiC nanospheres with free carbon, a novel hierarchical structure with 5 ~ 8 nm SiC nanoparticles embedded into the graphite matrix was obtained. The advantages of fluidized bed technology for the preparation of SiC nanoparticles were proposed based on the features of homogenous reaction zone, narrow temperature distribution, ultra-short reactant residence time and mass production.

Preparation of Silicon Carbide Coating Layer by Fluidized Bed Chemical Vapor Deposition Using a Halogen-Free Precursor

Tristructural-isotropic (TRISO) particle, with spherical ceramic fuel particle kernels followed by three layers of pyrolytic carbon and one layer of silicon carbide (SiC), has been successful now in high temperature gas cooled reactor (HTGR). The silicon carbide (SiC) layer used in TRISO coated fuel particles is normally produced at high temperatures (~1600°C) via fluidized bed chemical vapor deposition from methyltrichlorosilane (MTS) in a hydrogen environment. The precursor is strong corrosive and the process is not environmentally friendly. In this work, hexamethyldisilane (HMDS) was used instead of MTS and the deposition behavior was investigated via fluidized bed chemical vapor deposition method. Different experimental parameters were tested, such as deposition temperature (800~1450°C) and gas flow ratio of Ar: H2. The deposition rates were obtained and compared. It was found that the optimization parameters of highest deposition rate is 1000°C with the ratio of Ar: H2 of 1:1. The microstructures of the products were further investigated by SEM, XRD and Raman scattering. From the X-ray diffraction pattern it could be inferred that the β-SiC phase was obtained, and free carbon was also found in deposition products. Different types of SiC layer, including dense and porous layer can be prepared. The experimental results validated that HMDS was an alternative precursor for preparing the SiC layer in producing the TRISO particle and other SiC-coated materials in lower temperatures

Preparation and Performance of Fully Ceramic Fuel Element Using Silicon Carbide as Coating Layers

Tristructural-isotropic (TRISO) coated fuel particles with spherical ceramic fuel kernels followed by three layers of pyrolytic carbon and one layer of silicon carbide (SiC) is successfully used in high temperature gas cooled reactor (HTGR) system. In this paper, an improved design concept of fully ceramic fuel is proposed. The kernels were coated by SiC layers with different densities. Two kinds of SiC coatings, namely porous SiC and dense SiC were prepared by a fluidized bed chemical vapor deposition method. The densities of porous SiC and dense SiC were 1.40 g/cm3 and 3.20 g/cm3, respectively. By controlling experimental parameters, densities of the SiC layer can be adjusted. The structure and micromorphology of the coated particles were investigated by various technologies such as XRD, SEM, and EDX. The coated particles were dispersed in SiC matrix and further sintered to dense compacts. High temperature oxidation test confirmed the robustness of the fuel elements.

Microstructure evolution of a ZrC coating layer in TRISO particles during high-temperature annealing

The influence of high-temperature annealing on the microstructure of zirconium carbide (ZrC) was investigated in relation to its application as a coating layer of a nuclear fuel in a very high temperature gas cooled reactor. ZrC was deposited as a constituent coating layer of TRISO coated particles by a fluidized bed chemical vapor deposition method using a ZrCl4-CH4-Ar-H2 system. The grain growth of ZrC during high-temperature annealing was strongly influenced by the co-deposition of free carbon. Sub-stoichiometric ZrC coatings have experienced a significant grain growth during high-temperature annealing at 1800 °C and 1900 °C for 1 h. On the other hand, a dual phase of stoichiometric ZrC and free carbon experienced little grain growth. It was revealed that the free carbon of the as-deposited ZrC was primarily distributed within the ZrC grains but was redistributed to the grain boundaries after annealing. Consequently, carbon at the grain boundary retarded the grain growth of ZrC. Electron backscatter diffraction (EBSD) results showed that as-deposited ZrC had (001) a preferred orientation that kept its favored direction after significant grain growth during annealing. The hardness slightly decreased as the grain growth progressed.

Preparation of Highly Flexible SiC Nanowires by Fluidized Bed Chemical Vapor Deposition

High flexible SiC nanowires with diameters in the range of 10 to 50 nm and lengths of several hundred micrometers to several millimeters are synthesized using a novel catalyst-assisted fluidized bed chemical vapor deposition method. Methyltrichlorosilane (MTS) is used as source material and particles containing cobalt were used as catalyst. The fluidized bed is specially designed to form a perpendicular uneven temperature distribution. Suspended nanosized liquid catalyst droplets are produced at middle high temperature zone and SiC nanowires are formed in situ on the catalyst droplets then accumulated at the upper part of fluidized bed. The nanowires exhibit a single crystal feature with some stacking faults. Transmission electron microscopy (TEM) analysis verifies a growth direction along the <111> axis of the nanowires. A blue shift optical emission band at about 420 nm is detected using photoluminescence spectroscopy. From the high-resolution TEM investigation, a nanoscale smooth plane between the nanowire and the catalyst tip is observed and the two phases exhibit highly epitaxial growth interface. The growth process of the nanowires is discussed and a possible formation mechanism is proposed.