Synthesis of materials for energy application focusing on MAX phases

Research Doctorate - Doctor of Philosophy (PhD)This thesis is primarily concerned with a series of experimental investigations into the synthesis of materials for energy conversion and related applications in hostile environments. The <i>M</i>_n+1<i>AX</i>_n (MAX) phases contain an early transition metal (<i>M</i>-element) a group 3 or 4 element (<i>A</i>-element) and either C or N (<i>X</i>-element) and are a group of ceramics with interesting properties that make them perfectly suited to many difficult and demanding applications. The high potential of the MAX phases has been largely frustrated by difficulties in large scale, economic synthesis. The formation of <i>M</i>_n+1<i>AX</i>_n phases was extensively studied throughout this thesis. Use of <i>M</i>-element oxides as reactants has been intensively investigated with great success. The processing involved in obtaining the metallic form of the <i>M</i>-elements contribute considerably to the high cost of the MAX phases, along with complex and small scale synthesis methodologies currently used. Methods have been developed throughout this work as a means of reducing the <i>M</i>-element oxides, considerably cheaper starting materials, and producing MAX phases via a single step pressureless reactive sintering process. Aluminium has been extensively explored as a reducing agent and aluminothermic reduction was proven capable of forming the majority of tested systems. Separation of the MAX phase alumina composite formed by the exchange reaction has been demonstrated in simple sedimentation experiments, allowing for purification of the MAX phase product. Alternatively carbothermal reduction has been shown in selected systems to produce self-separating products. This process has been shown to produce pure MAX phase products in a single step reaction, a highly desirable trait, and the first time a pure MAX phase has been produced by carbothermal reduction. Additionally investigations into the synthesis and stability of MAX phases in general lead to the discovery of two new compounds belonging to this family, Ti₃GaC₂ and Ti₃InC₂. Issues of energy conversion have been addressed in two ways, through the creation of a novel thermal energy storage material using immiscible materials known as Miscibility Gap Alloys and through the development of a thermionic converter for conversion of heat directly into electricity. Thermal energy storage is critical as it allows for the intermittency of a concentrator solar power plant to be overcome. Misibility Gap Alloys provide high energy density, constant temperature storage in a highly thermally conductive material. A thermionic converter, although in its most preliminary stages with very low power output and efficiency, was designed for high temperature energy conversion. This device can be used as a test bed for the design of a system which could be used as a topping cycle on a concentrated solar thermal power plant. To enhance the power output of the thermionic device, low work function hexaboride materials were investigated and synthesised at considerably lower temperatures than conventionally used. Overall several contributions have been made in novel and potentially economic methods for the production of MAX phases. The development of these synthesis methodologies may alloy for these materials to fulfil their long desired place in demanding environments such as efficient energy conversion. A new class of thermal storage materials was developed which can be used for overcoming the intermittency of concentrated solar thermal power production, which could be coupled with low work function materials in a thermionic energy conversion device in order to improve the efficiency of electricity generation