| dc.description.abstract | This doctoral thesis is concerned with rapid solidification of AB5 materials suitable for electrochemical hydrogen storage. The primary objective of the work has been to characterise the microstructure and crystal structure of the produced AB5 materials as a function of the process parameters, e.g. the cooling rate during rapid solidification, the determination of which has been paid special attention to.
The thesis is divided in to 6 parts, of which Part I is a literature review, starting with a short presentation of energy storage alternatives. Then a general review of metal hydrides and their utilisation as energy carriers is presented. This part also includes more detailed descriptions of the crystal structure, the chemical composition and the hydrogen storage properties of AB5 materials. Furthermore, a description of the chill-block melt spinning process and the gas atomisation process is given.
In Part II of the thesis a digital photocalorimetric technique has been developed and applied for obtaining in situ temperature measurements during chill-block melt spinning of a Mm(NiCoMnAl)5 hydride forming alloy (Mm = Mischmetal of rare earths). Compared with conventional colour transmission temperature measurements, this technique offers a special advantage in terms of a high temperature resolutional and positional accuracy, which under the prevailing experimental conditions were found to be +/- 29 K and +/- 0.1 mm, respectively. Moreover, it is shown that the cooling rate in solid state is approximately 2.5 times higher than that observed during solidification, indicating that the solid ribbon stayed in intimate contact with the wheel surface down to very low metal temperatures before the bond was broken. During this contact period the cooling regime shifted from near ideal in the melt puddle to near Newtonian towards the end, when the heat transfer from the solid ribbon to the wheel became the rate controlling step.
In Part III of the thesis the changes of the crystal structure and the grain structure of La0.60Ce0.29Pr0.04Nd0.07Ni3.37Co0.79Mn0.25Al0.74 with increasing cooling rate during chill-block melt spinning are described. Totally, the material was rapidly solidified at 9 different cooling rates. The grain structure, crystallographic texture and the lattice parameters were studied by means of electron microscopy and powder X-ray diffraction. Additionally, the density of the rapidly solidified materials was measured by a gas pycnometer. All these properties were found to change with increasing cooling rate. The grain size decreased continuously with increasing cooling rate and was in the range of 1-5 μm. The strength of the crystallographic texture first increased and then decreased with increasing cooling rate. Transmission electron microscopy studies revealed that the grains contained a large amount of crystallographic twins and that the solidification morphology changed from cellular to plane front at a cooling rate during solidification of approximately 6·104 Ks-1. The unit cell volume and the density followed the same pattern with increasing cooling rate and decreased within each solidification morphology, but at the cooling rate from which the morphology changed, both these parameters suddenly increased. The identical variations in the unit cell volume and the density is explained by formation of excess lattice vacancies during rapid solidification.
In Part IV of the thesis rapid solidification of the materials La0.60Ce0.27Pr0.04Nd0.09Ni4.76Sn0.24 and LaNi4.76Sn0.24 at 7 different cooling rates are described. The materials were analysed by means of electron microscopy and powder X-ray diffraction. The grain structures of both alloys were found to be in the nanometer range, and the grain sizes were almost invariant with increasing cooling rate. Furthermore, the lattice parameters of these materials were almost unaffected by increasing cooling rate. However, elemental line scans showed that the tin containing materials were not chemically homogeneous after chill-block melt spinning. The tin and nickel level fluctuated in an opposite manner, and the origin of these fluctuations is possibly due to inhomogeneities in the master alloys produced prior to rapid solidification.
Part V of the thesis deals with the effect of heat treatment of the rapidly solidified materials presented in Part III and IV. The first material was heat treated at 400°C and the latter two at 1000°C and 900°C respectively. Electron microscopy investigations showed that the grain structure of the first material remained unchanged during the heat treatment while the latter two were subject to sincere grain growth. The inhomogeneities were removed during the heat treatment, and X-ray powder diffraction showed that the lattice constants were changing towards equilibrium values during the heat treatment. Furthermore, the density variations in the rapidly solidified material in Part III were removed by the heat treatment. This change and the change of the lattice parameters were probably due to annihilation of excess lattice vacancies during the heat treatment.
Finally, in Part VI of the thesis the measured variations in the lattice parameters with increasing cooling rate are compared with the electrochemical hydrogen storage properties of the materials, which has been studied in a parallel work. It is shown that the hydrogen storage capacity and the absorption pressure of the material in Part III are controlled by the unit cell volume and hence the cooling rate during solidification. | nb_NO |
