Oxidation behavior of sintered tubular silicon carbide in pure steam Ι: Experiments

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Abstract

Experimental investigations are conducted on sintered tubular SiC in oxidizing environments containing pure steam at 1 atm with temperature range of 1140–1500 °C and velocity between 0.8 and 10 m/s. Linear weight loss was observed with time. The linear weight loss rates exhibit sensitive dependence on flow rate at a given temperature, demonstrating effects of flow boundary-layer diffusion rate on silica volatilization kinetics. Silica scale exhibits morphology change with respect to exposure time in an oxidizing environment, progressively demonstrating bubble formation and surface smoothing, extensive formation of cracks and pores, and crack reduction. Yet, strength measurements of pressure-less sintered SiC show no significant change after oxidation in the tested conditions. Hence, the primary life-time limiting factor for structural application of pressure-less sintered SiC in the tested environments is anticipated to be the loss of the material.

Introduction

Silicon carbide is a widely used refractory ceramic material that can retain its mechanical properties at high temperature (~1800 °C). Often, its high temperature application is limited by the recession of the material upon oxidation. The oxidation of SiC with water vapor can be categorized into the following two reactions – “passive oxidation” or “active oxidation”.SiC (s)+3H2O (g)=SiO2 (s)+CO (g)+3H2 (g)SiC (s)+2H2O (g)=SiO (g)+CO (g)+2H2 (g)

The passive oxidation shown in Eq. (1) accompanies the formation of a protective silica (SiO2) scale on the silicon carbide. The active oxidation presented in Eq. (2) occurs when the environment contains low oxidant pressures such that SiO2 cannot form [1]. The amount of the oxidants for steam is expressed as the partial pressure of water vapor and the active oxidation was commonly observed with water vapor pressure far below atmospheric (~10−4 atm) [1], [2], [3]. Most engineering applications of SiC undergo the passive oxidation with the appreciable presence of H2O or O2 in the environment. Today, SiC is being considered as light water reactor (LWR) fuel cladding candidate [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]. Hence, oxidation behavior of SiC cladding in pure steam environment in accident conditions has drawn attentions as key information to evaluate safety of SiC clad fuels in LWRs. Typically, the formed silica scale (SiO2) as a result of oxidation simultaneously volatilize by reacting with H2O to form silicon hydroxide or silicon oxyhydroxide species. SiO2 volatilization with H2O occurs through one of the following reactions, (3), (4), (5) [4].SiO2+H2O(g)=SiO(OH)2(g)SiO2+2H2O(g)=Si(OH)4(g)2SiO2+3H2O(g)=Si2O(OH)6(g)

These simultaneous reactions, one forming SiO2 and the other removing SiO2, are described by paralinear kinetics developed by Tedmon, Jr [5] and the SiC thickness recession rate is given indxdt=Kp2xKl

Where Kp is the parabolic rate constant of oxide formation, and Kl is the linear volatilization reaction rate constant. Once steady state oxide layer thickness is reached (xs=Kp2Kl), the recession of the underlying SiC occurs at a constant rate. Hence, weight loss of the specimen follows a linear behavior, which gives practically the same rate as the SiO2 volatilization. It is worth noting that the oxide scale grows in a parabolic manner until it reaches the steady-state limiting thickness, and as it gets closer to the steady-state thickness, the specimen weight loss behaves more linearly. It is believed that the thickness of the fluid boundary layer on the material surface strongly affects the volatilization rate of SiO2, making the SiC recession dependent on flow characteristics. This is because the reaction rate is affected by the outward diffusion of volatile products through the fluid boundary layer [6].

No dedicated investigation on oxidation behavior of tubular SiC in pure steam environments relevant to LWR accident condtions and their influences in mechanical strenghth have been conducted. The objective of this study is to (1) experimentally explore phenomenological aspects of silicon carbide (SiC) cladding linear weight loss behavior in a pure steam environment, and (2) assess its ramifications on structural strength of the material.

Section snippets

Description of the experiments

The material used in this experimental study was monolithic tubular pressureless-sintered SiC and CVD SiC. Two different manufacturer׳s monolithic SiC tubular samples were tested. The density of some of the SiC samples was 2.95 g/cc and were obtained from Ceramic Tubular Products (CTP). These tube samples were obtained by CTP from Saint-Gobain,Inc. The other samples were directly obtained from Saint-Gobain, Inc.; their density was 3.05 g/cc, and their trade name is SE type Hexoloy. Dimensions of

SiC material loss rates

Exposure of the tubular SiC at steam-oxidizing environment resulted in a linear weight loss of the materials for Saint-Gobain Hexoloy SiC as shown in Fig. 5. The Linearity of weight loss was found by plotting the data with respect to time.

Increased flow rates resulted in faster weight loss rates. This implies that SiC volatilization is dependent on the flow characteristics, for instance mass flux, velocity or Reynolds number. Indeed this trend of SiC weight loss has been commonly observed in

Discussion

Increasing the temperature causes higher weight loss rates. This is because the higher temperature increases the chemical reaction kinetics for both the oxide layer formation and volatilization, and it also affects the fluid properties including the diffusion coefficients [10]. The equilibrium thickness of the oxide layer depends on the temperature sensitivity of the volatilization process versus the oxide layer formation process. Two independently controlled factors – temperature and flow rate

Conclusions

It has been demonstrated that pressure-less sintered SiC undergoes weight loss in steam at 1 atm with a flow velocity range is 0.8–10 m/s. The weight loss rates exhibit sensitive dependence on flow rate at a given temperature, demonstrating volatilization-driven paralinear oxidation. Over the test periods, SiO2 morphology was observed to progressively undergo (1) bubbles formations+surface smoothing, (2) Extensive formation of cracks, pits, and pores, and (3) crack-reduction. Strength

Acknowledgments

Professor Mujid S. Kazimi suffered a heart attack and passed away while this article was being prepared. The authors acknowledge Professor Kazimi for his insights and guidance that have pervaded every corner of this work. Financial support from the Idaho National Laboratory Academic Center of Excellence at MIT and the AREVA Fellowship in Nuclear Energy Technology at MIT are appreciated. The authors are thankful for the samples provided for the tests by H. Feinroth of Ceramic Tubular Products.

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