Ultra-high-temperature ceramics for transpiration cooling
File(s)
Author(s)
Hedgecock, Rowan
Type
Thesis
Abstract
Porous ultra-high-temperature ceramics (UHTCs) have been studied for their use in transpiration cooling of the leading edges of hypersonic vehicles. The envisioned design consists of a layered hybrid material: a dense UHTC substructure with channels providing coolant fluid to a partially sintered, permeable surface layer to evenly distribute coolant across the leading-edge surface.
To assess sintering during the fabrication of zirconium diboride (ZrB2) for the surface layer, two size grades of commercially available ZrB2 powders were heated in air to 450 – 540 °C to vary oxygen content in the powder from 0.5 – 5.3 wt%O. Monitoring of density during hot-pressing showed that densification during initial stage sintering was plastic flow, with grain boundary diffusion from 1730 °C and volume diffusion from 1850 °C. The onset of rapid densification during hot-pressing of oxidised powders was delayed until after the onset of volume diffusion and prevented entirely with 5.3 wt%O in the fine power and 2.2 wt%O in the coarse powder up to 2000 °C/ 24 MPa.
Increasing powder size increased permeability from 0.79 m2 to 2.59 m2 in 33 vol% porous ZrB2 hot-pressed at 1700 °C/ 12 MPa due to an increase in pore size from 0.66 to 1.1 µm. Increased surface transport also increased pore size, allowing for permeability of 1.9 x 10-14 m2 at 31 vol% porosity. The resulting reduction in pore surface area prevented further densification of the partially sintered microstructures at intended application temperatures of 2000 °C.
The elastic modulus, flexural strength, and thermal conductivity of partially sintered ZrB2 increased linearly with density after the initial development of properties above 40 vol% porosity. Flexural strength and thermal conductivity were improved due to the expansion of inter-particle neck area when hot-pressing with oxidised powders...
To assess sintering during the fabrication of zirconium diboride (ZrB2) for the surface layer, two size grades of commercially available ZrB2 powders were heated in air to 450 – 540 °C to vary oxygen content in the powder from 0.5 – 5.3 wt%O. Monitoring of density during hot-pressing showed that densification during initial stage sintering was plastic flow, with grain boundary diffusion from 1730 °C and volume diffusion from 1850 °C. The onset of rapid densification during hot-pressing of oxidised powders was delayed until after the onset of volume diffusion and prevented entirely with 5.3 wt%O in the fine power and 2.2 wt%O in the coarse powder up to 2000 °C/ 24 MPa.
Increasing powder size increased permeability from 0.79 m2 to 2.59 m2 in 33 vol% porous ZrB2 hot-pressed at 1700 °C/ 12 MPa due to an increase in pore size from 0.66 to 1.1 µm. Increased surface transport also increased pore size, allowing for permeability of 1.9 x 10-14 m2 at 31 vol% porosity. The resulting reduction in pore surface area prevented further densification of the partially sintered microstructures at intended application temperatures of 2000 °C.
The elastic modulus, flexural strength, and thermal conductivity of partially sintered ZrB2 increased linearly with density after the initial development of properties above 40 vol% porosity. Flexural strength and thermal conductivity were improved due to the expansion of inter-particle neck area when hot-pressing with oxidised powders...
Version
Open Access
Date Issued
2023-02-16
Date Awarded
01/04/2023
Advisor
Vandeperre, Luc
Giuliani, Finn
Bouville, Florian
Sponsor
Engineering and Physical Sciences Research Council
Grant Number
EP/P000878/1
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)