Nanocalorimetry Experiments and First-Principles Theoretical Studies of Solid-State Reactions in Nanolaminates

Abstract

The extraordinary sensitivity and extremely small thermal mass of chip-based nanocalorimetry sensors allow the study of reactions in thin films over a broad range of heating rates, from isothermal to 10^5 K/s. First-principles calculations provide insight in the phase transformation and diffusion behavior of a material at the atomistic scale. Combination of nanocalorimetry and first-principles, therefore, is highly efficient and reliable to determine the atomistic-to-macroscopic response of materials. This thesis explores, through use of this combined approach, reactions in reactive multilayers to synthesize ultra-high temperature ceramic coatings. We employ scanning AC and DC calorimetry techniques to investigate the synthesis of ZrB2 and carbon-doped ZrB2 using Zr/B and Zr/B4C multilayered reactive nanolaminates (MRNL). The solid-state reactions in these multilayers are shown to proceed in two distinct steps: an interdiffusion/amorphization step followed by a crystallization step. Measurements performed at heating rates ranging from 1,000 to 55,000 K/s allows determination of the kinetic parameters of the multilayer reactions, such as the activation energies of interdiffusion and crystallization. Low activation energies in the interdiffusion processes in the Zr/B MRNLs are found and amorphization is shown to facilitate fast transport of B atoms into Zr lattice. It has also been shown that C impurity atoms in the Zr/B4C MRNLs further reduce activation energies of interdiffusion and crystallization. First-principles theoretical modeling provides insight in the amorphization processes in the Zr/B MRNLs and confirms the relatively low activation energies associated with the processes. The simulations further elucidate the effects of concentration (ZrBx, 0<x≤2) and temperature on the diffusion kinetics and phase evolution of the Zr-B alloys. The effect of additives such as C and N on the synthesis of ZrB2 using MRNLs has also been investigated using the first-principles theoretical approach. It has been revealed that both impurities facilitate amorphization, enhancing intermixing of the constituent layers of the MRNLs.