Measuring Enthalpies of Formation Using Thick Multilayer Foils and Differential Scanning Calorimetry
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the exothermic heats from these SSRs, enthalpies of formation (Al-If) can be measured accurately at
low temperatures (< 800'C). Measuring AHf at low temperatures using solid state reactions greatly reduces the risk of environmental contamination compared to high temperature reaction calorimetry [5-61. This is particularly useful for transition metal compounds that contain reactive materials such as Ti, Zr and Hf. This paper reports measurements of AHf for one Cu-Zr compound and five Al-Zr alloys to demonstrate the ability to accurately measurement enthalpies of formation at low temperatures using multilayer foils and DSC. A series of thick multilayer foils were fabricated for each compound or alloy using a planetary magnetron sputtering system. The foils were heated in a DSC and the resulting exothermic heat flows were analyzed using a simple model to account for interfacial reactions that occur during sample fabrication. The resulting values agree with the available literature. EXPERIMENTAL PROCEDURES Multilayer samples were fabricated by magnetron sputter depositing Cu and Zr onto 3in and 6in (100) Si wafers and Al and Zr onto 27in glass substrates using a planetary system [7]. The purities of the Cu, Zr, and Al targets were 99.999%, 99.95%, 99.5%, respectively. The multilayer samples were 25 to 50.tm thick, and they were easily removed from the substrates as free standing foils. The Cu/Zr samples are limited to Cu-rich compositions (0.9 to 12.3 at% Zr) while the Al/Zr samples cover a broader range from 8 at% Zr to 66 at% Zr. Compositions of all samples were predicted using calibrated deposition parameters. The composition of most samples were also measured using a combination of wet chemistry and X-ray Fluorescence (XRF) [8]. In almost all cases the predicted compositions fell within the experimental uncertainty (±5%) of the measured compositions. Because the Cu/Zr multilayers were deposited on to small substrates, the Cu and Zr 21 Mat. Res. Soc. Symp. Proc. Vol. 382 01995 Materials Research Society
layer thicknesses were relatively uniform across the foils and only one layer thickness was tested for each sample. The Al/Zr multilayers were deposited on much larger substrates and large variations (>3x) in layer thicknesses developed across the complete foil. Thus, several different layer thicknesses were tested for each Al/Zr multilayer sample. The composition was assumed constant across a given foil. The Cu/Zr and Al/Zr multilayers are predominantly crystalline after deposition as shown in Figure 1. The individual layers are highly textured with Cu (111), Zr (002) and AI(l 11) planes lying parallel to the foil's layering. The Cu and the Zr grains scale in size with their layer thickness while the Al grains tend to be 2x wider than their layer thickness. Thin, reaction layers of amorphous Cu-Zr and AI-Zr could be seen in as-deposited samples using high-resolution, crosssectional TEM. This demonstrates that some fraction of the solid state reactions (SSRs) begins during the synthesis of the materials.
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