Browsing by Author "Dr. Jacqueline Krim, Committee Chair"

Now showing 1 - 3 of 3

Fabrication of a Graphitic Layer for Nanotribological Studies of Temperature Rise in a Frictional Contact Area
(2005-12-01) Walker, Matthew James; Dr. Thomas Pearl, Committee Member; Dr. Harold Ade, Committee Member; Dr. Jacqueline Krim, Committee Chair
In this thesis I use a quartz crystal microbalance (QCM) to investigate the interfacial heat rise of an adsorbed Kr layer on a single layer of graphite called graphene. The graphene surface is made by reacting CO to a 1000 Å thick Ni(111) surface at a temperature of 400 °C. A 100 Å Ti layer is the base layer the Ni is deposited onto. The surface is characterized using Auger electron spectroscopy (AES) under ultra high vacuum conditions. The change in frequency vs. pressure/coverage graphs on a linear scale shows at what pressures a monolayer of Kr forms. The frequency vs. pressure/coverage graphs on a log scale show phase changes that can be compared to well known static phase changes. The comparison of the static phase change to the dynamic phase change yields an inferred temperature at the interface. This inferred temperature remained the same regardless of the sliding velocity. The latter observation, which is one principal point of this thesis, remains true irrespective of surface quality.
Impact of Environmental Conditions on the Contact Physics of Gold Contact RF Microelectromechanical Systems (MEMS) Switches.
(2008-12-05) Brown, Christopher John; Dr. Jacqueline Krim, Committee Chair; Dr. Thomas P. Pearl, Committee Member; Dr. Angus I. Kingon, Committee Member; Dr. Lubos Mitas, Committee Member
RF MEMS switch technology is poised to create a new generation of devices capable of vastly outperforming current mechanical and semiconductor switching technology. Despite the efforts of top industrial, academic, and government labs, commercialization of RF MEMS switches has lagged expectations. This dissertation focuses on issues associated with switch contact physics. Understanding the failure mechanisms for metal contact switches is a complex challenge. There is strong interplay between variables such as mechanical creep, deformation, contact heating, contact asperity size, real contact area, and current flow leading to the eventual failure of the switch. Stiction failures moreover are highly sensitive to ambient conditions and absorbed film layers at the switch contact. The experiments in this thesis seek to isolate individual failure mechanisms and tie them to the physics driving that behavior through correlation of experimental data and theoretical modeling. Four experiments in controlled environments were performed: 1) the impact of cryogenic temperatures on RF MEMS contacts, 2) a correlation between experimental data and theoretical modeling for gold asperity creep at room and cryogenic temperatures, 3) a power law relationship between contact resistance and time dependent creep, and 4) the pressure dependence of switch closure. Cryogenic temperatures were used to isolate contaminant film effects. Contaminant films were found to have less mobility at 77 K, and contact resistance measurements showed that the film could be reduced on the contact surface through mechanical cycling and high temperatures at the gold asperities. It was also noted at cryogenic temperatures that the choice of atmosphere was important. A nitrogen atmosphere at liquid nitrogen temperature produced variable contact resistance as the condensed liquid boiled off the switch contacts. Data was correlated with a single asperity creep model to show that change in contact resistance as a function of time is related to the creep of gold asperities at the contact interface. The change in contact resistance over time can be described by a power law relation derived from the single asperity creep model that takes into account the surface topography, material characteristics, and contributions from additional sources such as adsorbed film layers. Additionally, it was shown that the creep mechanism was temperature dependent and that creep was significantly decreased at cryogenic temperatures. A drop in pressure as a result from cryogenic temperatures was observed to create switch bounce at closure. This was explored in a set of room temperature experiments which established the onset of bounce at 60 Torr. The results were and correlated to the damping coefficient and the ratio of the damping force to the electrostatic force of actuation. This work contributes to the field of contact physics and MEMS switch technology by firmly establishing the role of creep in contact mechanics and quantifying its time and temperature dependent impact on contact resistance.
Superconductivity Dependent Friction
(2006-05-10) Highland, Matthew J.; Dr. Jacqueline Krim, Committee Chair; Dr. Hans Hallen, Committee Member; Dr. Lubos Mitas, Committee Member; Dr. Gerald Iafrate, Committee Member
In the work reported on here, Quartz Crystal Mircobalances are used to study the change in friction between a sliding layer and a substrate at a superconducting transition. Studies of this nature are one of the few able to isolate electronic and phononic contributions to friction. Superconductivity dependent friction was first observed in 1998 for N2 sliding on a Pb surface. This discovery generated much interest both experimentally and a theoretically. However success in reproducing and modeling the results was difficult due to the unique care required for this experiment. The most aggressive efforts to reproduce the affect where made by Renner, Taborek, and Rutledge. In their experiment Renner et al. tried to reproduce the experiment with improved the temperature stability. Renner et al. designed a system, in which the sample would be cooled over many hours. The improved thermal stability came at the expense of surface quality. The slow cooling of samples by Renner et al. most likely led to increased surface contamination which pinned the sliding layers. Pinning prevented Renner et al. from reproducing the experiment and reports of their unsuccessful attempts led many to question the existence of superconductivity dependent friction. Marginal success in reproducing the experiment was made in 2001 , while still using the same sample transfer technique of blowing Argon as utilized in the original experiment. In an effort to mitigate some of the expense associated with the large amount of liquid helium consumed during this experiment a temperature control loop was implemented and a number of other traditional low temperature physics techniques. Once again improved thermal stability came at the cost of surface quality, which led to pinning problems in that work as well. Many theoretical efforts were made to explain superconductivity dependent friction within the established understanding of electronic friction. Modeling the experiment was made difficult by ambiguity in two values that would prove crucial to many of the models. The electron mean free path l of the samples used was not known and could only be estimated from bulk values for Pb. Additionally, since the samples were exposed to atmosphere during transfer an unknown amount of oxide formed on the surface. This made the distance between the adsorbate and the superconducting substrate unknown, which introduced large variable to the numerical estimates made by a number of models. A few models were able to make verifiable experimental predication based on there work. Bruch predicted in 2000 that the magnitude of the change in frictional force experience by a sliding layer at a superconducting transition would depend on the polarity of the adsorbates in the sliding layer. It is Bruch's predication about the nature of superconductivity dependent friction that has motivated much of the work I present here. I have studied the change in friction felt at a superconducting transition by layers composed of absorbates with varying polarities (N2, He, and H2O) sliding on a Pb surface transferred to an experimental cell in-situ from the deposition chamber. The work in this dissertation shows that superconductivity dependent friction is present in all systems were the effects of pinning can be mitigated. I compare these results to the predictions made by Bruch and show that our results agree with his theory of superconductivity dependent friction. Additional comments are made on the unique superconducting behavior of the samples in this study, and on apparent coupling between sliding layers and magnetic fields. Finally comments are made as to the state of theoretical treatments to this problem and possible future experiments are suggested.