Decomposition behaviour of blowing agent (TiH2, ZrH2)
Temperature dependent gas evolution studied using mass spectroscopy, phase transformation in blowing agent during heat-treatment studied in-situ by synchrotron-based X-ray diffraction
TiH2 or ZrH2 are used as blowing agent to prepare Al-based metallic foams. Powders are mixed and hot pressed yielding a dense compacted precursor. The foaming process is initiated by fast heating to above the melting temperature of the alloy. TiH2 and ZrH2 decompose releasing H2 gas which foams the liquid metal undergoing a phase transformation. Typical foaming temperature profiles are highly dynamic, which affect the kinetics of TiH2 or ZrH2 decomposition. The phase transformation sequence occurred during fast heating rates is studied in-situ using diffraction methods combined with the high photon flux available at the synchrotron facility Berlin (BESSY).
The decomposition of ZrH2 was studied at the surface by angle dispersive diffraction at the KMC2 beamline where a 6-circle diffractometer operates combined with a 2θ detector suitable for fast measurements in reflection mode using monochromatic radiation of wavelength 1.03 Å (equal to a photon‑energy of 12 keV). ZrH2 was heated from ambient temperature to 700 °C applying 140 K.min‑1 under Ar flow. The 2D detector scanned a 2q range of 16° and each diffractogram was acquired with a time resolution of 7 seconds. In Figure 1, every diffractogram is stacked with increasing temperature, so that the diffraction peaks appear as lines which evolve as temperature increases. Further analysis of these diffracted lines allow following the phase transformation. In this example, the initial stoichiometric tetragonal hydride ε‑ZrH2 transforms into the cubic hydride δ‑ZrHx<2 , and later into the metallic phases hexagonal-closed-packed α‑Zr and body-centered-cubic β‑Zr as H2 gas is released.
In order to obtain the phase transformation sequence in bulkier samples, the transmission-mode is more suitable. For transmission‑mode, high energies are required which are available at the EDDI beamline. The combination of energy dispersive diffraction, high energy and high photon flux allows a time resolution of 10 seconds per scan. In Figure 2, a typical foaming temperature profile T(t) was applied to study in‑situ the phase transformation sequence of TiH2 during decomposition under Ar flow. In this example on TiH2, it was relevant to detect the re‑precipitation of the hydride d during cooling.