Role of sub-stochiometric carbon content on the structural, thermal and electrochemical properties of TaCx

Abstract

Interstitial transition metal carbides (ITMCs) display unique mechanical, thermal (high melting point) and electromagnetic properties. ITMCs of group IV and V metals show strong non-stoichiometric character i.e. the composition of the carbide can vary over a wide range without any change in the crystal structure. The change in composition of strongly nonstoichiometric compounds is a very important parameter in determining the properties of these materials. These materials are extremely hard, have the highest known melting points and are also radiation resistant. Out of the entire group V ITMCs Tantalum Carbide (TaC) finds its industrial applications because of its high temperature stability. Synthesis of nano sized powders of TaC with large specific surface area and their complete characterization is of paramount importance for ultra- high temperature (UHT) as well as catalytic applications. Apart from the precursors, the synthesis method and processing parameters like temperature, time, pressure, and catalyst etc. play very important role in determining the properties of the final powders and hence their performance. The stoichiometry of TaC plays an important role in determining its properties in UHT as well as electro catalytic applications. The present work deals with the synthesis and characterization of nano sized cubic TaC by single step chemical reaction synthesis route. Further, the prepared materials have been tested as electrocatalyst for HER in acidic media. The entire work of the thesis is divided into eight chapters. Chapter 1 deals with the interstitial transition metal carbides (ITMCs). Properties and industrial applications of ITMCs are discussed taking into consideration their crystal structure and bonding. Non stoichiometric nature of the group IV and V ITMCs and its influence on property determination is described. Among the ITMCs, the significance of TaC, its industrial applications, especially as ultra-high temperature material (UHTM), are discussed. The tantalum-carbon binary system with all possible structures (γ-TaC, α-Ta2C, β-Ta2C, δ- Ta4C3) is presented. The variation in nature of bonding within the TaC phase at substoichiometric carbon concentrations is discussed. Bulk synthesis methods for TaC are briefly described. The need for development of well characterized nano TaC for mechanical, UHT and electrocatalytic applications is discussed. Chapter 2 describes the literature available on the work being done in the field of synthesis and characterization of nano-sized TaC. Prevalent methods for the synthesis of single phase cubic-TaC and their salient features are presented. The understanding about the details of different parameters which affect the final powder characteristics such as precursor for tantalum, carbon source, their characteristics, their mixing ratios, temperature regime for synthesis, atmosphere during heating, presence/absence of catalysts, etc. have been described. The work done on electrochemical activity and properties of TaC for HER are also presented. Chapter 3 includes the protocols followed in the present investigation for the synthesis of TaC nanopowders from different Ta precursors. Basics of different characterization techniques, sample preparation for the same and data analysis methods used for the characterization of the synthesized nano-powders are also discussed in this chapter. The techniques discussed are: X-ray diffraction (XRD), Thermal analysis (DSC/TGA/DTG), Field-Emission Scanning Electron Microscope (FE-SEM), Transmission electron microscopy (TEM), BET analysis and Electrochemical measurements. Chapter 4 deals with the synthesis and characterization of nano TaC from tantalum-ethoxide precursor. The XRD results of the synthesized samples have been analysed and discussed. The carbon content of the lattice of TaC nanopowders, size and strain has been used to determine the evolution of the nanopowders with time as well as temperature. Based on XRD results the formation mechanism of the nano TaC has been proposed. From the thermal analysis results, the stability of the synthesized powders over range of temperature(s) as well as the external carbon content of the powders has been evaluated. FE-SEM and TEM results have been presented and discussed to analyse the microstructure of the synthesized powders and to confirm the crystal structure of product phase by measuring the lattice spacing using HR-TEM. Surface area and pore size distribution of the synthesized samples has been analysed by BET technique. Chapter 5 gives the details for the synthesis and characterization of nano TaC from Ta2O5 precursor using acetone as reducing and carburization agent. The effect of Mg and acetone in the initial mixture as well as the effects of time and temperature on the final product are studied. The powders obtained have been characterized using XRD, DSC-TG, FE-SEM, HRTEM and BET. Based on XRD results and thermodynamic parameters, the formation mechanism of the TaC has been proposed. The strain and size evolution of the powders with time has been used to explain the grain growth mechanism in the system. The role of carbon in structural evolution has been analysed and discussed. Chapter 6 comprises of the results obtained for the synthesis and characterization of the nano cubic TaC from TaCl5 precursor using acetone as reducing and carburization agent. The XRD and thermal analysis results of the synthesized samples have been analysed and discussed. The carbon content of the TaC nanopowders, size and strain has been used to determine the evolution of the nanopowders with time as well as temperature. Based on XRD results and thermodynamic parameters, the formation mechanism has been proposed. From the thermal analysis results, the stability of the synthesized powders over range of temperature(s) as well as the external carbon content of the powders has been evaluated. FE-SEM and TEM results have been presented and discussed to analyse the microstructure of the synthesized powders and to confirm the crystal structure of product phase. Surface area and pore size distribution of the synthesized samples has been analysed by BET technique.