Claudia Walls

High-thermal-conductivity, mesophase-pitch-derived carbon foams: effect of precursor on structure and properties

Abstract Pitch-based carbon foams are not new, but the development of high thermal conductivity foams for thermal management applications has yet to be explored. The research reported here focused on a novel foaming technique and the evaluation of the foaming characteristics of two mesophase pitches (Mitsubishi ARA24 and Conoco Dry Mesophase). After graphitization to 2800°C, densities of the graphite foams ranged from 0.2 to 0.6 g/cm3, with average pore diameters ranging from 275 to 350 μm for the ARA24-derived foams, and from 60 to 90 μm for the Conoco-derived foams. Scanning electron microscopy and polarized light optical microscopy were performed to characterize the cell walls, revealing highly aligned graphitic-like structures along the axis of the ligaments. Analysis of X-ray diffraction results determined that the foams exhibited average interlayer (d002) spacings as low as 0.3355 nm, stack heights (Lc) up to 80 nm and crystallite sizes (La) up to 20 nm. Finally, thermal diffusivity measurements were performed revealing that the bulk thermal conductivity varied with density from 40 to 150 W/m K. The specific thermal conductivities of the graphitized foams were more than six times greater than solid copper.

Nitrogen impurity gettering in oxide dispersion ductilized chromium

Work by Scruggs in the 1960s demonstrated that tensile ductility could be achieved at room temperature in powder metallurgically-produced Cr alloyed with MgO. During consolidation, much of the MgO converted to the MgCr{sub 2}O{sub 4} spinel phase, which was hypothesized to getter nitrogen from the Cr, rendering it ductile. We have duplicated this effect, achieving room temperature tensile elongations of 4% for hot-pressed Cr-6MgO-(0-1)Ti (wt.%) and 10% for hot-pressed and extruded Cr-6MgO-0.75Ti. Direct incorporation of nitrogen into the MgCr{sub 2}O{sub 4} phase was not detected; however, impurities, particularly nitrogen and sulfur, were observed to segregate to and/or precipitate at interfaces between the MgO/MgCr{sub 2}O{sub 4} phases and the Cr matrix. Exploratory studies of other non-spinel forming oxide dispersions (La{sub 2}O{sub 3}, TiO{sub 2} and Y{sub 2}O{sub 3}) showed a similar pattern of impurity segregation/precipitation, suggesting that there is nothing unique about spinel dispersions in Cr with regards to impurities. However, none of these other dispersions resulted in similar levels of tensile elongation.

Silicon nitride containing rare earth silicate intergranular phases

Si[sub 3]N[sub 4] ceramics prepared with refractory grain boundary phases to improve high temperature properties are difficult to density by conventional sintering methods. Gas-pressure sintering may be used to promote densification and development of acicular grains for improved fracture toughness. The current study examined rare earth silicate sintering aids with the composition M[sub 2]Si[sub 2]O[sub 7], where M is a trivalent cation (Y, La, Nd). M[sub 2]O[sub 3] and SiO[sub 2] additions were varied to develop a number of compositions in the Si[sub 3]N[sub 4]-Si[sub 2]N[sub 2]O-M[sub 2]Si[sub 2]O[sub 7] ternary phase field. Pressureless sintering and gas-pressure sintering were used to density the samples. Densification, microstructure development, oxidation resistance, and mechanical properties were evaluated and compared with respect to compositional variations and processing conditions.

Effect of Powder Characteristics on Gas-Pressure Sintering of Si 3 N 4 With Rare Earth Additives

Several Si3N4 powders, synthesized by various methods and having different surface areas, oxygen contents and impurity levels, were examined. During early stage densification, all powders showed similar shrinkage with the diimide derived powder exhibiting delayed α/β transformation compared to the other powders. The diimide and gas-phase derived powders achieved the highest final densities. Improved densification was observed by increasing the oxygen content and this also resulted in high toughness for some materials with rare earth apatite additives. However, the increased oxygen resulted in reduced high temperature strength. Fracture toughnesses (KIc) up to 10 MPa✓m were obtained for some compositions.