Jinren Song

Relationship between oxidation resistance and structure of B4C-SIC/C composites with self-healing properties

Abstract The oxidation behavior of B 4 C–SiC/C composites of various compositions at temperatures up to 1500°C was analyzed by the thermal gravimetric/differential thermal analysis (TG/DTA) technique and the surface morphology of the composites after isothermal oxidation at 800, 1000 and 1200°C was examined by scanning electron microscopy. The results indicated that the composites exhibited variable oxidation resistance at high temperatures depending on composition and oxidation temperature. The variance of self-healing properties was attributed to the difference of the compositions and the properties of the decarbonized layers including wetting ability, viscosity, volatility and oxygen permeability. The rate-limiting steps of oxidation and the concentration distribution of oxygen across the decarbonized layers of the composites after oxidation are discussed.

Selection of candidate doped graphite materials as plasma facing components for HT-7U device

Selection of candidate materials for plasma facing material (PFM) in HT-7U device and plasma–wall interactions are critically important to reach high plasma performance. Based on concentrated research on multi-element doped graphite containing B, Si and Ti, two kinds of doped graphites have been chosen as candidates for PFM in HT-7U. Doped graphite GBST1308 with the dopant concentration of 1% B, 2.5% Si, 7.5% Ti was developed as low-Z PFM for reducing the chemical sputtering and suppressing the radiation enhanced sublimation, and successfully used as the new limiter material in last two campaigns of HT-7 tokamak experiments. Doped graphite with the composition of 2.5% Si, 7.5% Ti has improved mechanical properties and thermal conductivity of 314 W/m K at room temperature. TDS and high heat flux experiments results demonstrated that such doped graphites are promising candidate plasma facing components for HT-7U. 2003 Elsevier Science B.V. All rights reserved.

The primary results for the mixed carbon material used for high flux steady-state tokamak operation in China

Abstract Several types of carbon mixed materials have been developed in China to be used for high flux steady-state tokamak operation. Performance evaluation of these materials is necessary to determine their applicability as PFCs for high flux steady state. This paper describes the primary results of carbon mixed materials and the effects of dopants on properties are primarily discussed. Test results reveal that bulk boronized graphite has excellent physical and mechanical properties while their thermal conductivity is no more than 73 W/m K due to the formation of a uniform boron–carbon solid solution. In case of multi-element doped graphite, titanium dopant or a decreased boron content is favorable to enhance thermal conductivity. A kind of doped graphite has been developed with thermal conductivity as high as 278 W/m K by optimizing the compositions. Correlations among compositions, microstructure and properties of such doped graphite are discussed.

The preparation of fine-grain doped graphite and its properties

Fine-grain doped graphite was prepared by the ball-milling dispersion method for the first time. Such composite has not only high thermal conductivity and excellent bending strength (116 MPa), but also better oxidation resistance at elevated temperature and outgassing properties than those of composite doped with normal size carbides. Correlations between microstructure and properties of such composites are also discussed in detail.

The preparation of Ti(C,N,O) nanoparticles using binary carbonaceous titania aerogel

Abstract The Ti(C, N, O) nanoparticles have been prepared by using binary carbonaceous titania aerogel as precursor of carbothermal reduction reaction. The binary aerogel precursor, composed of binary carbon–titania nanoparticles with particle sizes in the range 1–8 nm, is obtained by a sol–gel supercritical fluid drying method. This method provides a homogeneous distribution of titania within carbon and high contact area between the reactants, leading to the formation of loose agglomerated Ti(C, N, O) nanoparticles at a lower carbothermal reduction temperature. The BET surface areas of binary carbonaceous–titania aerogel and its carbothermal reduction products at various temperatures are investigated. The Ti(C, N, O) formed over a range of 1000–1300°C in flowing argon are also characterized by TEM, EDX spectrum, XRD and chemical analyses. The results show that the carbothermal reduction temperature is important for the crystallite growth of Ti(C, N, O) powders.

New route for preparation of SiC-B4C/C composite with excellent oxidation resistance up to 1400°C

Graphite materials are attractive materials for high temperature applications because of their high sublimation temperature above 3000 ◦C and high thermal shock resistance. They have been used as the structural materials at high temperature, such as rocket nozzles and hightemperature heat exchangers, etc. [1]. However, the application of carbon materials is limited by oxidation under oxidizing conditions at temperatures above 500 ◦C. This oxidation process results in the erosion of structure and eventually in the degradation of properties. Therefore, the improvement of oxidation resistance of graphite materials is one of the important problems to be solved, as reported by many workers [2–6]: It was well known that graphite with multilayered or functionally gradient coatings exhibited excellent oxidation resistance behavior [7], whereas there are still some critical problems such as cracking and debonding caused by thermal expansion mismatch between coating and substrate. The doping of ceramic particles in carbon substrate can solve the above cracking problem. Among the various combination of carbon and different kinds of ceramics, the B4C-SiC/C system has been found to possess an excellent inhibitory effect against air oxidation at temperature above 1000 ◦C due to formation of glassy borosilicate coating covered the active sites on the surface [6, 8]. However the temperature at which the B4C-SiC/C composite shows excellent oxidation resistance is limited, to our knowledge, there is no report on the B4C-SiC/C composite with excellent oxidation resistance above 1200 ◦C to date. Therefore it is essential

High-Temperature Joining of Carbon/Carbon Composites by an Organic Resin Adhesive

Carbon/carbon (C/C) composite materials are widely used in high-temperature fields because of their outstanding thermal–physical properties. But many difficulties have been experienced in the joining of C/C composites because of their brittleness and the harsh application environments. Use of high-temperature adhesives has been shown to be one of the most convenient and promising methods for the joining of materials employed at high temperatures. In this work, C/C composites were bonded by an organic resin adhesive using phenol-formaldehyde (PF) resin as matrix and boron carbide as modification additive. The shear bond strength of C/C composite joints, after being treated at 1200°C, were tested at room temperature, 1000°C, 1400°C and 1800°C. Results show that the above organic resin adhesive possesses satisfactory bond properties. The bond strength of C/C composite joints tested at room temperature was as high as 13.2 MPa. Because of the modification effect of boron carbide, the thermal stability and integrity of the residue derived from the carbonization of PF resin are improved considerably, which is responsible for the achievement of satisfactory bond strength. As to the C/C composite joints tested at high temperatures, the bond strength is obviously lower than that of C/C composite joints achieved at room temperature due to the melting of boron oxides which were the products of boron carbides modification reactions. The propagation of failure is much easier in boron oxides glass phase and is an important factor resulting in the decrease of bond strength at high temperatures. With the increase of test temperature, the amount of the boron oxides produced decreased because of the carbon thermal reduction reaction and its volatilization. As a result, the bond strength of C/C joints increased from 2.3 to 5.2 MPa with the increase of test temperature from 1000 to 1800°C.

The reaction behavior of carbon fibers and TaC at high temperatures

Chopped carbon fibers (CFs) and TaC particles were dispersed uniformly and then sintered at temperatures of 1773, 2123, 2298, 2473 and 2823 K, respectively. The effect of sintering temperature on the microstructure of CFs was investigated. The results showed that the reaction between CFs and TaC particles was controlled by solid diffusion. When the temperature was lower than 1773 K, mainly carbon diffused into TaC. However, Ta diffused into the CFs while carbon diffused into TaC particles and then precipitated as graphite when the sintering temperature was above 2123 K. During the process, the structure of TaC was not influenced. The preferential crystalline orientation of the graphite precipitated from TaC particles increased with the increase of sintering temperature. The R value determined by Raman spectra decreased from 0.12 to 0 as the temperature increased from 2123 to 2823 K, meaning the formation of perfect graphite crystallites.