Synthesis and oxidation kinetics of Cr2AlC, V2AlC & Ti3AlC2 MAX phases

Abstract

MAX phases are the promising class of materials owing to their superior combination of both metal and ceramic properties. This includes high thermal and electrical conductivity, good machinability, great damage tolerability, and outstanding thermal shock resistance. MAX phases are also highly resistant to fatigue, creep, and oxidation. The reports on MAX phases increased exponentially after the discovery of their derivative, i.e., MXenes. Attempts are being made to develop novel MAX phases and their derivatives. This work sheds light on the fundamental aspects, including structure and defects together with the synthesis protocols and oxidation resistance of the MAX phases. The prime objective is to attain a stronger understanding of the synthesis protocol and oxidation performance of the MAX phases. The efforts have been carried out to bring insight knowledge related to mechanisms associated during non-isothermal oxidation in the MAX phases. The oxidation kinetic analysis is performed to gain further knowledge related to oxidation in the MAX phases. The complete work conducted in the present thesis is systematically presented in the seven chapters. Chapter 1: In this chapter, the current state of the MAX phases (M is an early transition metals, A is a group 13 – 16 elements, X is either C or N) has been addressed. The objective is to introduce basic fundamental aspects related to the MAX phases. A comprehensive discussion referred to the crystal structure of the MAX phases is presented. The theoretical understanding associated to electronic structure and atomic bonding in MAX phases is elaborated. The classification of the MAX phases on the basis of chemical versatility is reviewed. Efforts are also made to bring insight knowledge related to superior properties of the MAX phases. Moreover, the emergent need and significance of Cr2AlC, Ti3AlC2 and V2AlC MAX phases has been discussed. Chapter 2: Presents the literature survey on the work done for MAX phases. The progress in the synthesis and characterization of MAX phases has contributed to a stronger understanding of the properties. This chapter focuses to address recent growth in the MAX phase. A better understanding related to the vital role of processing routes to obtain highly pure MAX phases is addressed. Novel synthesis approaches to prepare bulk as well as thin films are highlighted. In addition, some of the MAX phases have shown good resistant to oxidation. The isothermal and non-isothermal oxidation behavior of the MAX phases is elaborated. New strategies developed to design MAX phases with better oxidation resistance and use these phases in nuclear power plants and aerospace industries are discussed. According to literature survey, drawbacks associated to obtain Cr2AlC, Ti3AlC2 and V2AlC MAX phases and lack ofnon-isothermal oxidation studies are discussed. Finally, the objectives of the current thesis are framed on the basis of lacuna observed in the literature. Chapter 3: In this chapter, the materials and methods employed for the synthesis of Cr2AlC, Ti3AlC2 and V2AlC MAX phases are presented in details. The synthesis parameters such as temperature, composition and holding time are optimized. The synthesized MAX phases are characterized through numerous techniques such as X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), High resolution (HRTEM), Selected area energy diffraction (SAED), X-ray photoelectron spectroscopy (XPS), Thermogravimetry analysis (TGA) and Differential thermal analysis (DTA). A concise summary of all the characterization techniques is presented in this chapter. The procedure adopted to examine the non-isothermal synthesis and oxidation kinetics of Cr2AlC, Ti3AlC2 and V2AlC is elaborated. The theoretical background associated with the thermal kinetics analysis is presented. Chapter 4: The formation of nanolaminated Cr2AlC MAX phase by using solid state synthesis route has been investigated. The mixture of chromium (Cr), aluminum (Al) and graphite (C) in different compositions are sintered at various temperatures (800 – 1300 °C). The intermediate phases formed during synthesis of Cr2AlC phase are determined and reaction pathway is established. The synthesis kinetics involved during formation of Cr2AlC phase is also determined. The prepared Cr2AlC phase is characterized through XRD, FESEM, HRTEM and SAED techniques. The non-isothermal oxidation kinetics of the Cr2AlC phase is examined through a TGA/DTA technique, at variable heating rates. The TGA/DTA results show that the oxidation of the Cr2AlC phase occurred in two stages. The multi-stage kinetic analysis is performed to establish the nature of the oxidation process. The kinetic triplets (activation energy, pre-exponential factor and reaction mechanism) are estimated for the oxidation process in the Cr2AlC MAX phase. Chapter 5: The synthesis of Ti3AlC2 MAX phase is presented in the chapter. The role of experimental conditions to obtain highly pure Ti3AlC2 phase is investigated. The mixture of titanium (Ti), titanium carbide (TiC) and aluminum in different molar ratios are pelletized and heat treated in an argon atmosphere. The pathway responsible for the formation of Ti3AlC2 is identified. The synthesis kinetic parameters are also evaluated during formation of Ti3AlC2 MAX phase. The micrographs of fractured Ti3AlC2 demonstrated a layered structure corresponding to the MAX phases. HR-TEM, SAED and XPS analysis also confirmed the formation of Ti3AlC2 MAX phase. The oxidation stability of Ti3AlC2 is testified under non- isothermal condition through a DTA technique. The oxidation kinetics responsible for theoxidation of Ti3AlC2 MAX phases is evaluated. The kinetic triplets involved during oxidation process are determined. The oxidation reaction pathway is proposed and the oxidation reaction mechanism is evaluated. Chapter 6: The impact of synthesis parameters during the formation of V2AlC MAX phase is studied. The formation pathway of V2AlC is proposed with the help of XRD analysis. A highly pure V2AlC phase is observed when the vanadium, aluminum and carbon are mixed in 2:1.3:1 and sintered at 1500 °C in an argon atmosphere. The synthesized sample is characterized through XRD, FESEM, HRTEM, SAED, XPS and DTA techniques. The stability of V2AlC phase is examined in air atmosphere and oxidation kinetic analysis is performed. The reactions involved during oxidation of V2AlC phase are determined through thermodynamic calculations and XRD results. The DTA analysis demonstrated two exothermic peaks during oxidation of V2AlC MAX phase. The kinetic parameters are evaluated for both oxidation stages. Finally, the reaction mechanism responsible for the oxidation in V2AlC is proposed. Chapter 7: The outcome of the present thesis is concluded in this chapter. The results demonstrated that the synthesis parameters play vital role during synthesis of the MAX phases. The oxidation of the MAX phases under non-isothermal conditions is outlined. The reaction pathways responsible for the synthesis and oxidation of the MAX phases is compiled. The oxidation kinetic analysis of Cr2AlC, Ti3AlC2 and V2AlC MAX phases is summarized. The future prospective of the prepared MAX phases are suggested in the chapter.