On the nonlinear behavior of Young's modulus of carbon‐bonded alumina at high temperatures

Abstract

The origin of the nonlinear behavior of the Young's modulus (E) of carbon‐bonded alumina at high temperatures was addressed, based on the microstructural changes observed during processing and their thermo‐mechanical properties. Impulse excitation technique, thermogravimetric analysis, porosity measurement, and scanning electron microscopy were conducted in order to highlight and explain the E behavior. The finite element model of a virtual microstructure was simulated and the results attained are in good agreement with the experimental data. The tests revealed that the Young's modulus of a cured sample heated from room temperature up to 500°C was governed by the release of volatiles. Above this temperature, the thermal expansion mismatch among alumina, graphite, and the carbon matrix is dominant resulting in an increase in the effective Young's modulus. During cooling, crack networks and gaps between alumina particles and the carbon matrix were developed. The former were induced by volatile release and by the graphite's highly anisotropic thermal expansion. The latter was derived by the thermal expansion mismatch between the alumina and the carbon matrix. The closure of the gaps and cracks governed the expansion behavior during the second heating cycle and a nonlinear effective Young's modulus increase as a function of temperature was observed.

Graphene-Reinforced Carbon-Bonded Coarse-Grained Refractories

“…A possible reason why, in the case of better packing, the GO addition resulted in lower strength (A–GO) might be that, in the denser structure, the gas release from the GO during coking led to internal stresses, causing microcracking in addition to generating pores. A similar effect was reported for the carbon bond by Luchini et al [ 15 ]. In a less dense structure (AC–GO), the developed stresses were smaller and did probably not cause cracking apart from leaving void space.…”

“…One reason behind the high thermal shock resistance of carbon-bonded alumina (compared to fully dense alumina) is indeed the presence of gaps, which gives flexibility to the material and acts as a damper for the in situ thermal stress generation [ 15 ]. It can be expected that additional thermal shock cycles would weaken the microstructure by increasing the total length of the crack network and the width of gaps between graphite and alumina grains.…”

“…In the case of a carbon-bonded refractory, the microstructure (in coked state) is characterized by ceramic grains with a specific grain size distribution (aggregates), graphite flakes, voids (pores and cracks) and kept together by a carbonaceous partly-graphitized bonding phase (matrix) that is formed during curing and coking from resin or pitch components [ 12 , 13 ]. Due to differential expansion and shrinkage during the thermal treatment, delamination gaps and microcracks develop around the grains and flakes within the carbonaceous matrix, which limit the mechanical strength but are also responsible for the excellent flexibility and thermal shock performance [ 14 , 15 ].…”

Optimisation of The Value in-use of The Refractory Submerged Entry Nozzle: Implication of preheating failures

“…The coking temperature of carbon bonded refractories have an impact on their strength and elastic Young's modulus [47,48,64]. The dynamic elastic Young's modulus is known to decrease in the initial stages of heating up to 500 °C due to the effects of volatile release from resins [47].…”

“…The limitation in other methods not being able to probe disordered carbons sufficiently (an example being X-ray diffraction) gives Raman this precedence [45]. In this study the carbonaceous material consists of a mixture of resin (disordered) and graphite (crystalline) embedded within a refractory aggregate mixture (comprising mainly of alumina aggregate) [46][47][48]. By utilising Raman with the Witec cluster image analysis technique, a better insight can be observed in terms of distinguishing the carbon phases, their chemical structures and surface area changes impacted by preheating.…”