Date of Award

2015-01-01

Degree Name

Master of Science

Department

Mechanical Engineering

Advisor(s)

Arturo Bronson

Second Advisor

Vinod Kumar

Abstract

The transient surface temperature of particles within a packed bed was simulated to enable ultimately the prediction of plasma-surface reactions at ultrahigh temperatures. The motivation for the present study was along two fronts - the reactive infusion of liquid alloys at ultrahigh temperatures to process metal matrix or ceramic matrix composites and the plasma processing of materials to spike the surface temperatures to enable the formation of protective oxidizing scales on ultrahigh ceramic composites. The transient temperature simulations determined that a graphite crucible containing B4C spherical particles heated from 298 to 2128 K and 2504 K occurred in about 900 s. After placing a Hf disk on top of the packed bed, a thermal steady-state condition was reached in 900 s when the disk attained 2231â??. With a Zr disc replacing Hf, a thermal steady-state condition was reached in 900 s attaining 1855â??. This resulted because Hf and Zr have similar heat transfer coefficients and because above 1000â??, radiation is the main method of heat transfer. Even though the system with Hf was heated to a higher temperature, it took about the same time for both systems to reach steady-state. A parabolic behavior was seen at starting times since the temperature difference was greater. As time progressed, the temperature gradient became linear until reaching steady-state. The temperature spike for both Zr and Hf caused a sharp temperature gradient within approximately 0.3 cm, which could be significant in controlling the precipitation of phases (i.e., boride or carbide) during processing.

The oxidation of Ti particles to rutile within a packed bed were also studied at a constant temperature of 1700â??, the concentration of Ti and TiO2 species were plotted. processing temperature may cause a significant temperature gradient along the cross-section of the bed depending on its content of reactive metals, the effects of temperature however, should be simulated in future work.

The oxygen potential affects the plasma-surface reactions appear, as expected, though the adsorption and desorption of oxygen becomes more difficult to assess with decreasing oxygen atmospheres. The error in predictability with decreasing oxygen partial pressures less than 10-6 atm may become problematic, because plasma-surface reactions may occur at oxygen potentials less than 10-20 atm when processing elements from the Ti or rare earth metals family. The power used to generate the plasma atmosphere influences the adsorption and desorption of oxygen occurring on surfaces. The adsorption and desorption of the oxygen depends on a rate-determining step involving surface sites within a plasma atmosphere as similarly obtained for oxygen interacting with surfaces without a plasma atmosphere.

Language

en

Provenance

Received from ProQuest

File Size

50 pages

File Format

application/pdf

Rights Holder

Alejandro Garcia

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