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Access Type
WSU Access
Date of Award
January 2023
Degree Type
Dissertation
Degree Name
Ph.D.
Department
Physics and Astronomy
First Advisor
Prof. Boris Nadgorny
Abstract
Percolation is a random probabilistic process that triggers a continuous phase transition from a finite cluster within a network below the critical value of the percolation threshold p_c to an infinite cluster above p_c. While classical percolation is well understood, percolation effects in randomly packed or jammed disordered systems are much less well-explored. Disordered systems exhibit different packing fractions or filling factors compared to ordered systems, determining the system's critical behavior, which in turn, influences its functionality. By selecting a particular kind of matrix and filler, one can tailor the desired properties of a composite for specific applications.
In this study, we investigated experimentally the electrical percolation in a binary composite system of disordered spherocylinders composed of conducting (CrO2) and insulating (Cr2O3) rod-like nanoparticles, with nominally identical geometries. By preparing composite samples with varying volume fractions of the two constituents and performing four-point electrical transport measurements, we determined the classical percolation threshold of CrO2, p_c=0.305±0.026 and the conductivity critical exponent t=2.52±0.03. Our experimental observations align well with theoretical simulations and modeling.
Furthermore, we investigated the tunneling percolation in CrO2-Cr2O3 binary composites below the classical percolation threshold. We observed the characteristic tunneling staircase associated with the presence of the tunneling percolation transitions in this system through interparticle contacts, with the two tunneling thresholds determined at p_T^1 = 0.259 and p_T^2 = 0.165, the latter corresponding to the conductivity onset in this system. We discussed possible mechanisms of higher order tunneling and proposed a simple model which is in good agreement with the experimental data. We also performed detailed magnetotransport measurements in CrO2-Cr2O3 nanoparticle composites at different temperatures. We found significant variations of magnetoresistance (MR) which was highly sensitive to the constituents composition near the tunneling percolation threshold p_T^1, with the maximum value of MR = 44.37% at T = 5 K near the tunneling threshold p=0.26. This research contributes to our understanding of percolation phenomena and its effects on electrical and magnetotransport properties in metal-insulator nanocomposites. These findings have significant implications for tailoring and optimizing composite materials for diverse technological applications.
Recommended Citation
Pokhrel, Shiva Prasad, "Electrical Percolation And Magnetoresistance In Randomly Packed Metal-Insulator Nanocomposites" (2023). Wayne State University Dissertations. 3933.
https://digitalcommons.wayne.edu/oa_dissertations/3933