Search

Densification behaviour and microstructural evolution of TiO 2 /CAS (CaO-Al 2 O 3 -SiO 2 )-incorporated alumina has been investigated systematically. The experimental results show that TiO 2 can greatly enhance the densification rates of alumina but trigger severely abnormal grain growth (AGG) at 0.60 wt%. On the other hand, small amounts of CAS can effectively inhibit abnormal grain growth, but provoke an abnormal densification owing to the formations of in situ pores left by liquid phase. With additional 0.50 wt% of CAS incorporated into 0.60 wt% TiO 2 -doped sample, the aspect-ratios of anisotropic growth grains increase by reduced growth rates of flat boundaries, accomplished by basal planes much smoother and straighter. This microstructure subsequently disappears when CAS incorporation is up to 4.0 wt%, substituted by that with low aspect-ratio and fine-equiaxed grains. Such occurrences are discussed in term of levels of liquid phase and 2-dimension (2-D) nucleation processes.

The effects of etching with HCl on the hydrogen sorption properties of Mg 2 Ni have been investigated. On being etched with HCl, Mg 2 Ni can be activated and hydrided easily at room temperature and its low temperature desorption kinetics are improved. However, its hydriding is incomplete at room temperature and its room temperature hydrogen absorption capacity degrades rapidly on cycling. X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy analyses show that the etching treatment removes the magnesium oxide overlayer and preferentially dissolves away magnesium; this results in a new surface layer composed mainly of porous, metallic nickel microspheres. We believe that the nickel microspheres act as the active catalyst for hydrogen absorption and desorption at lower temperatures. The incomplete hydriding and the cycling degradation of the hydrogen absorption capacity at room temperature are caused, we suggest, by the cracking and disintegration of the alloy powder on each cycle, with the newly generated surfaces deactivating very quickly.

In order to recover helium from the purge gas of synthetic ammonia production unit using Sichuan natural gas as the raw material, the present authors developed several multicomponent misch-metal hydrogen storage alloys with widely different plateau pressures. Experiments were carried out on the removal of hydrogen from simulated gas mixtures composed of H 2 -Ar in different proportions. The effect of the change in various technological parameters, involving pressure, temperature and the rate of flow, and the effect of various combinations of alloys with different plateau pressures was investigated. The results showed that, with favourable technological parameters, the concentration of remnant hydrogen could be reduced to less than 0.2%.