David Bahr
Purdue University, Materials Engineering, Faculty Member
Abstract The performance and the reliability of many devices are controlled by interfaces between thin films. In this study we investigated the use of patterned, nanoscale interfacial roughness as a way to increase the apparent... more
Abstract The performance and the reliability of many devices are controlled by interfaces between thin films. In this study we investigated the use of patterned, nanoscale interfacial roughness as a way to increase the apparent interfacial toughness of brittle, thin-film material systems. The experimental portion of the study measured the interfacial toughness of a number of interfaces with nanoscale roughness.
Two experiments that probe the nature of the rapid transition from elastic to plastic deformation are described. The load, and therefore stress, at which this yield point occurs is shown to be relatively independent of temperature in an... more
Two experiments that probe the nature of the rapid transition from elastic to plastic deformation are described. The load, and therefore stress, at which this yield point occurs is shown to be relatively independent of temperature in an iron alloy. When stresses lower than those required to generate a yield point during loading are applied for times between seconds and minutes, yielding occurs while the sample is under an applied stress. The time to generate a yield point increases as the applied stress is decreased.
ABSTRACT The performance of a piezoelectric generator in terms of the quality factor Q, the electromechanical coupling coefficient k2, and the efficiency were examined. The effect of design parameters such as membrane size, piezoelectric... more
ABSTRACT The performance of a piezoelectric generator in terms of the quality factor Q, the electromechanical coupling coefficient k2, and the efficiency were examined. The effect of design parameters such as membrane size, piezoelectric thickness, silicon thickness, and top electrode area are explored. The results show that both k2 and Q are sensitive to PZT thickness and electrode size. Keywords: pzt membrane, mems power, electromechanical coupling coefficient, quality factor, efficiency.
ABSTRACT This work details the effect of top membrane compliance on the performance of a MEMS based micro-heat engine and integrated thermal switch at operating speeds of 20, 40, and 100Hz and heat inputs of up to 60mJ per cycle. The... more
ABSTRACT This work details the effect of top membrane compliance on the performance of a MEMS based micro-heat engine and integrated thermal switch at operating speeds of 20, 40, and 100Hz and heat inputs of up to 60mJ per cycle. The engine consists of two flexible membranes encapsulating a volume of saturated working fluid. A thermal switch is used to intermittently reject heat from the engine to a constant temperature cooling sink.
Abstract The performance of AlN, ZnO, PZT, and PMN-33% PT as active layers in a piezoelectric device for energy conversion is examined in the context of electromechanical coupling coefficient. A figure of merit based on materials... more
Abstract The performance of AlN, ZnO, PZT, and PMN-33% PT as active layers in a piezoelectric device for energy conversion is examined in the context of electromechanical coupling coefficient. A figure of merit based on materials properties is used to identify PZT and PMN-33% PT as the most promising candidates. A full finite element model is then implemented to compare the performance of the PZT and PMN-33% PT.
Abstract We describe the micro-mechanical properties of vertically aligned carbon nanotubes (VACNTs) fabricated using a photolithographically patterned iron catalyst prepared using sol–gel techniques. The carbon nanotubes (CNTs) were... more
Abstract We describe the micro-mechanical properties of vertically aligned carbon nanotubes (VACNTs) fabricated using a photolithographically patterned iron catalyst prepared using sol–gel techniques. The carbon nanotubes (CNTs) were grown via chemical vapor deposition. The relative mechanical stiffness of the resultant structure was measured using nanoindentation based techniques and is shown to be related to the number of contact sites between tubes.
ABSTRACT A systematic study of interfacial fracture energies of thin gold films as a function of film thickness is presented in this paper. The films were sputter deposited onto sapphire substrates to thicknesses ranging from 10 nm to 200... more
ABSTRACT A systematic study of interfacial fracture energies of thin gold films as a function of film thickness is presented in this paper. The films were sputter deposited onto sapphire substrates to thicknesses ranging from 10 nm to 200 nm. Nanoindentation was used to measure mechanical properties and combined with stressed overlayers to trigger delamination and buckling. Fracture energies and interfacial fracture energies were then obtained from the buckles using mechanics-based models.
Nanoindentation is widely used to explore the mechanical properties of small volumes of materials. For crystalline materials, there is a growing experimental and theoretical interest in pop-in events, which are sudden displacement-burst... more
Nanoindentation is widely used to explore the mechanical properties of small volumes of materials. For crystalline materials, there is a growing experimental and theoretical interest in pop-in events, which are sudden displacement-burst excursions during load-controlled nanoindentation of relatively dislocation-free metals. The first pop-in event is often identified as the initiation of dislocation nucleation, and thus the transition from purely elastic to elastic/plastic deformation.
ABSTRACT Adhesion is an important factor in controlling properties and performance of thin film devices. It is a critical factor in hybrid microcircuits with multilayer films and dissimilar metal interconnects where diffusion of copper... more
ABSTRACT Adhesion is an important factor in controlling properties and performance of thin film devices. It is a critical factor in hybrid microcircuits with multilayer films and dissimilar metal interconnects where diffusion of copper from leads during processing and environmental effects during service can modify the adhesion strength of the gold conductive films.
RESUMEN Un programa de verano en sitio sobre la Experiencia en Investigación para estudiantes universitarios (REU) ha estado operando en la Universidad Estatal de Washington durante los últimos cinco años. Durante este tiempo hemos... more
RESUMEN Un programa de verano en sitio sobre la Experiencia en Investigación para estudiantes universitarios (REU) ha estado operando en la Universidad Estatal de Washington durante los últimos cinco años. Durante este tiempo hemos experimentado con diversos modos de métodos organizados de investigación, incluyendo el que los alumnos trabajen individualmente y en grupos en sus proyectos.
Abstract Mechanical power production by a dynamic micro heat engine with integrated thermal switch is demonstrated. A microengine operated from a constant heat source of 60◦ C is shown to produce 350 µW of mechanical power. Employing an... more
Abstract Mechanical power production by a dynamic micro heat engine with integrated thermal switch is demonstrated. A microengine operated from a constant heat source of 60◦ C is shown to produce 350 µW of mechanical power. Employing an active thermal switch to control heat rejection from the microengine enables power to be increased to 2500 µW. Power consumption by the thermal switch is shown to be minimized by operating the cantilever switch at its resonant frequency.
Thin film adhesion energies can be calculated using buckles with the telephone cord geometry. The buckles can be measured through the point of inflection of the buckle or through the center of the buckle and modeled as a straight buckle... more
Thin film adhesion energies can be calculated using buckles with the telephone cord geometry. The buckles can be measured through the point of inflection of the buckle or through the center of the buckle and modeled as a straight buckle of uniform width and height. In the tungsten-silica system, a unique delamination morphology involved a transition from the straight to telephone cord buckle. This structure was used to compare various measurement techniques.
Nanoindentation is a powerful set of tools for the investigation of the mechanical properties of materials in small scales. Indentation tests are a very commonly applied means of testing the mechanical properties. The two mechanical... more
Nanoindentation is a powerful set of tools for the investigation of the mechanical properties of materials in small scales. Indentation tests are a very commonly applied means of testing the mechanical properties. The two mechanical properties measured most frequently using load and depth sensing indentation techniques are the elastic modulus, E, and the hardness, H. 1
RESULTS For mapping evoked responses across the cortical columns of the somatosensory and auditory cortex, we have initially relied on flat, highdensity electrode arrays made from Delrin blocks and placed directly on the dural surface. We... more
RESULTS For mapping evoked responses across the cortical columns of the somatosensory and auditory cortex, we have initially relied on flat, highdensity electrode arrays made from Delrin blocks and placed directly on the dural surface. We have used both silver and stainless steel wire in these measurements. Silver has the advantage of a low resistance pathway for the electrical recordings, whereas stainless steel exhibits better longevity in chronic preparations.
The onset of plastic deformation was investigated using nanoindentation in single crystals of the explosive cyclotrimethylene trinitramine (RDX). Cleavage and habit planes were tested revealing a range of yielding behaviors. Smooth habit... more
The onset of plastic deformation was investigated using nanoindentation in single crystals of the explosive cyclotrimethylene trinitramine (RDX). Cleavage and habit planes were tested revealing a range of yielding behaviors. Smooth habit planes of unprocessed single crystals exhibited distinct yield points near the theoretical shear strength; planes produced by cleavage yielded at lower applied stresses.
ABSTRACT The Materials Science and Engineering program at Washington State University has developed and implemented four modes of involving undergraduates in research: a mandatory year long senior thesis for MSE students, paid part-time... more
ABSTRACT The Materials Science and Engineering program at Washington State University has developed and implemented four modes of involving undergraduates in research: a mandatory year long senior thesis for MSE students, paid part-time research during the school year funded through external research grants, research for course credit, and full time summer research as part of a Research Experience for Undergraduates program funded through the National Science Foundation.
Abstract Progress toward the realization of an external combustion dynamic micro heat engine is documented. First, previous work on three engine components: the piezoelectric membrane generator, the thermal switch and the wicking... more
Abstract Progress toward the realization of an external combustion dynamic micro heat engine is documented. First, previous work on three engine components: the piezoelectric membrane generator, the thermal switch and the wicking evaporator are summarized. Second, the internal irreversibility of the engine is quantified with measurements of viscous dissipation. Finally, the integration of a thermal switch with an engine is described.
Abstract The reliability of thin film systems is important to the continued development of microelectronic and micro-electro-mechanical systems (MEMS). The reliability of these systems is often tied to the ability of the films to remain... more
Abstract The reliability of thin film systems is important to the continued development of microelectronic and micro-electro-mechanical systems (MEMS). The reliability of these systems is often tied to the ability of the films to remain adhered to its substrate. By measuring the amount of energy to separate the film from the substrate, researchers can predicts film lifetimes.
ABSTRACT In thin film systems subject to mechanical strains, such as those experienced in Micro Electro Mechanical Systems (MEMS), failure often occurs via fracture mechanisms. Both through thickness cracking and interfacial delamination... more
ABSTRACT In thin film systems subject to mechanical strains, such as those experienced in Micro Electro Mechanical Systems (MEMS), failure often occurs via fracture mechanisms. Both through thickness cracking and interfacial delamination leading to failure of the device or layer are possible failure modes.
The mode I interfacial adhesion energy for as-deposited Au/SiO2 was measured using a stressed overlayer test, and ranged from 0.39±0.09 J/m2 for spontaneous blisters to 0.37±0.17 J/m2 for indentation-induced blisters. After these films... more
The mode I interfacial adhesion energy for as-deposited Au/SiO2 was measured using a stressed overlayer test, and ranged from 0.39±0.09 J/m2 for spontaneous blisters to 0.37±0.17 J/m2 for indentation-induced blisters. After these films were heated to 100° C and 300° C for 1h, the interfacial fracture energies increased, to 0.9 J/m2 and 9.9 J/m2, respectively. This was consistent with Au/SiO2 films aged over an 8-year period, which had a mode I interfacial fracture energy between 1.2 J/m2 and 1.9 J/m2.
As continuous indentation methods continue to develop for testing sub m volumes of materials, there is an increasing body of work developing nanoindentation techniques to examine the time dependent plastic deformation of metals [1].... more
As continuous indentation methods continue to develop for testing sub m volumes of materials, there is an increasing body of work developing nanoindentation techniques to examine the time dependent plastic deformation of metals [1]. During a continuous indentation test, the load and penetration depth of a tip driven into a sample are monitored, and material properties are extracted from this information.
Carbon nanotubes grown on a substrate form a turf-a complex structure of intertwined, mostly nominally vertical tubes, cross-linked by adhesive contact and few bracing tubes. The turfs are compliant and good thermal and electrical... more
Carbon nanotubes grown on a substrate form a turf-a complex structure of intertwined, mostly nominally vertical tubes, cross-linked by adhesive contact and few bracing tubes. The turfs are compliant and good thermal and electrical conductors. In this paper, we consider the micromechanical model of the turf (Mesarovic et al, 2007, Scripta Mat 56, 157-60), and develop a finite element implementation to simulate nanoindentation tests on CNT turfs.
A lumped-parameter dynamic model for an enclosed incompressible squeeze film with a central gas bubble has been derived. A new approach was applied to derive closed-form expressions for the lumped-parameter mass and damping coefficients... more
A lumped-parameter dynamic model for an enclosed incompressible squeeze film with a central gas bubble has been derived. A new approach was applied to derive closed-form expressions for the lumped-parameter mass and damping coefficients caused by liquid motion. It was assumed that plate motions were small and the fluid behaved as a continuum. The values of the lumped-parameter mass and damping were found to depend on the aspect ratio and nondimensional squeeze-film thickness.
The mechanical properties of arrays of curved, intertwined, but nominally vertical carbon nanotubes (CNTs), referred to as turfs, have been measured using nanoindentation. The elastic properties appear to be non-linear; as noted in prior... more
The mechanical properties of arrays of curved, intertwined, but nominally vertical carbon nanotubes (CNTs), referred to as turfs, have been measured using nanoindentation. The elastic properties appear to be non-linear; as noted in prior studies the observed tangent modulus decreases with increasing strain. Decreasing adhesion between the turf and probe lowers the perceived stiffness of the material.
Objective Work has been done with plating thin films into NPG, but performance for catalysis was the focus. In this project, the hardness of plain NPG was measured and as well as NPG with the addition of other metals. Forming a core shell... more
Objective Work has been done with plating thin films into NPG, but performance for catalysis was the focus. In this project, the hardness of plain NPG was measured and as well as NPG with the addition of other metals. Forming a core shell structure from the NPG allows for varying thicknesses, differing metals, and potential layering. Nickel and gold were both chosen, as nickel is often worked with gold. The choice of gold kept p the chemistry of the NPG the same while only changing a physical characteristics.
Abstract In this two-part paper, the optimization of the electromechanical coupling coefficient for thin-film piezoelectric devices is investigated both analytically and experimentally. The electromechanical coupling coefficient is... more
Abstract In this two-part paper, the optimization of the electromechanical coupling coefficient for thin-film piezoelectric devices is investigated both analytically and experimentally. The electromechanical coupling coefficient is crucial to the performance of piezoelectric energy conversion devices. A membrane-type geometry is chosen for the study. In part I a one-dimensional model is developed for a membrane composed of two layers, a passive elastic material and a piezoelectric material.
Abstract Complex structures consisting of intertwined, nominally vertical carbon nanotubes (CNTs), referred to as turfs, have unique properties that arise from their complex nanogeometry and interactions between individual CNT segments.... more
Abstract Complex structures consisting of intertwined, nominally vertical carbon nanotubes (CNTs), referred to as turfs, have unique properties that arise from their complex nanogeometry and interactions between individual CNT segments. For applications such as contact switches for electrical or thermal transfer it is necessary to understand the properties that arise from the collective behavior of an assemblage of CNTs rather than the properties of a single tube.
The effects of hydrogen on the deformation of a BCC Ti alloy surrounding nanoindentations are examined using scanning probe microscopy. The surface deformation patterns surrounding indentations with 2600 ppm hydrogen are significantly... more
The effects of hydrogen on the deformation of a BCC Ti alloy surrounding nanoindentations are examined using scanning probe microscopy. The surface deformation patterns surrounding indentations with 2600 ppm hydrogen are significantly less than with 140 ppm hydrogen. The reduced pileup is attributed to hydrogen reducing the ability for cross slip to occur.
Surface roughness, grain size and chemistry of Pt/Ti bottom electrodes have been investigated for their contribution to interfacial adhesion with PbZr0. 52Ti0. 48O3 (PZT) thin films used in a microelectromechanical systems... more
Surface roughness, grain size and chemistry of Pt/Ti bottom electrodes have been investigated for their contribution to interfacial adhesion with PbZr0. 52Ti0. 48O3 (PZT) thin films used in a microelectromechanical systems power-generating application. Grain size of Pt bottom electrodes demonstrated rapid grain growth in the range of 550–700° C.
Carbon nanotubes grown on a substrate form a turf–a complex structure of intertwined nanotubes cross-linked by adhesive contact. We analyze the physical mechanism of deformation on the basis of:(i) standard and continuous stiffness... more
Carbon nanotubes grown on a substrate form a turf–a complex structure of intertwined nanotubes cross-linked by adhesive contact. We analyze the physical mechanism of deformation on the basis of:(i) standard and continuous stiffness nanoindentation; and (ii) micromechanical scaling analysis. At moderate strains, deformation is fully reversible, comprising small viscoelastic relaxation caused by the thermally activated sliding of contacts.
Nanoindentation for measuring thin film mechanical properties is probably the most popular yet ill-understood method due to its inherent complexities. As opposed to burst pressure or microtensile tests of lithographed structures, where... more
Nanoindentation for measuring thin film mechanical properties is probably the most popular yet ill-understood method due to its inherent complexities. As opposed to burst pressure or microtensile tests of lithographed structures, where relatively uniform stress fields may be generated, the indentation-induced stress gradients can produce unique challenges. Because of the test's simplicity and ability to mechanically probe the smallest of scales, it is becoming increasingly applied.
The development and testing of a micro heat engine is presented. For the first time the production of electrical power by a dynamic micro heat engine is demonstrated. The prototype micro heat engine is an external combustion engine that... more
The development and testing of a micro heat engine is presented. For the first time the production of electrical power by a dynamic micro heat engine is demonstrated. The prototype micro heat engine is an external combustion engine that converts thermal power to mechanical power through the use of a novel thermodynamic cycle. Mechanical power is converted into electrical power through the use of a thin-film piezoelectric membrane generator.
ABSTRACT The effects of dissolved hydrogen on dislocation motion in stainless steel have been studied in an attempt to understand how hydrogen impacts deformation, which is important for understanding hydrogen embrittlement in these... more
ABSTRACT The effects of dissolved hydrogen on dislocation motion in stainless steel have been studied in an attempt to understand how hydrogen impacts deformation, which is important for understanding hydrogen embrittlement in these alloys. Indentation tests of stainless steels before and immediately after exposure to high hydrogen gas pressures have been conducted to examine the effects of dissolved hydrogen on indentation induced slip steps.
RESUMEN Se desarrolló una metodología basada en nanoindentaciones y análisis de la ultraestructura de células de madera de Eucalyptus nitens (Deane et Maiden) para explorar los factores que generan la aparición de fallas en la madera... more
RESUMEN Se desarrolló una metodología basada en nanoindentaciones y análisis de la ultraestructura de células de madera de Eucalyptus nitens (Deane et Maiden) para explorar los factores que generan la aparición de fallas en la madera debido a tensiones de crecimiento. Se estudió madera normal y madera con mesogrietas. Existieron diferencias significativas en la frecuencia de vasos y módulo de elasticidad de la pared S2 entre los dos tipos de madera estudiadas.
ABSTRACT The operation and testing of an extemal-combustion, micro heat engine, the P3 micro engine is presented. In particular, measurements are given for low frequency operation of the micro heat engine. Production of electrical power... more
ABSTRACT The operation and testing of an extemal-combustion, micro heat engine, the P3 micro engine is presented. In particular, measurements are given for low frequency operation of the micro heat engine. Production of electrical power by a dynamic micro heat engine is demonstrated. The prototype micro heat engine is an external combustion engine in which thermal power is converted to mechanical power through a novel thermodynamic cycle.
Abstract The elastic and adhesive properties of nominally vertically aligned carbon nanotube (CNT) turfs have been measured using nanoindentation. The perceived stiffness of a CNT turf is dependent on the unloading rate, which decreases... more
Abstract The elastic and adhesive properties of nominally vertically aligned carbon nanotube (CNT) turfs have been measured using nanoindentation. The perceived stiffness of a CNT turf is dependent on the unloading rate, which decreases at slower unloading rates. Depth-controlled nanoindentation was used to examine adhesion effects. Adhesive loads between the turf and the probe tip increased as the time the tip is in contact with the turf increased.
Molecular dynamics simulations have been used to investigate the nature of heat pulse propagation through single and multi wall nanotube. For the comparison, these simulations were carried out on armchair nanotube, (5,5) and (10,10),... more
Molecular dynamics simulations have been used to investigate the nature of heat pulse propagation through single and multi wall nanotube. For the comparison, these simulations were carried out on armchair nanotube, (5,5) and (10,10), zigzag nanotube, (10,0) and multi wall nanotube, which is built by (5,5) and (10,10). All of the simulations were run at 0K, and the length of the pulse was 1ps. Results have shown that the heat pulse excites a variety of phonon modes. The speed of leading wave packets of single nanotube is about 20Km/sec and this is corresponding ...
A strategy to increase the performance of diaphragm-type transducers is to provide an initial curvature to the diaphragm, which converts in-plane strains to motion normal to the diaphragm surface. In this paper, we analyze and measure the... more
A strategy to increase the performance of diaphragm-type transducers is to provide an initial curvature to the diaphragm, which converts in-plane strains to motion normal to the diaphragm surface. In this paper, we analyze and measure the actuation characteristics of composite piezoelectric diaphragms that are biased by a static pressure that induces diaphragm curvature. To analyze the actuation characteristics of the curved-diaphragm transducer, a mathematical model for thin diaphragms relating the applied static pressure ...
Research Interests:
The effects of varying chemistry and individual layer thickness on the mechanical properties of Cu-based nanocomposites, of interest for a number of thin film applications, were investigated. Two different sets of samples were used. In... more
The effects of varying chemistry and individual layer thickness on the mechanical properties of Cu-based nanocomposites, of interest for a number of thin film applications, were investigated. Two different sets of samples were used. In the first case the thickness of the individual layers was kept constant at 20 nm while three different stacking sequences were used; Cu/Ni, Cu/Nb and Cu/Ni/Nb. Bulge testing and nanoindentation were utilized to evaluate the mechanical properties and especially the strength of the nanocomposites ...
ABSTRACT Strength, friction, and wear are dominant factors in the performance and reliability of materials and devices fabricated using nickel based LIGA and silicon based MEMS technologies. However, the effects of frictional contacts and... more
ABSTRACT Strength, friction, and wear are dominant factors in the performance and reliability of materials and devices fabricated using nickel based LIGA and silicon based MEMS technologies. However, the effects of frictional contacts and wear on the mechanical performance of microdevices are not well-defined. To address these effects on performance of LIGA nickel, we have begun a program employing nanoscratch and nanoindentation. Nanoscratch techniques were used to generate wear patterns using loads of 100, 200, ...
ABSTRACT Liquid metal microscale switches, often using mercury, are sometimes used in place of solid-solid contact switches because of the ability to minimize damage from switching and the ability to make good contacts for electrical and... more
ABSTRACT Liquid metal microscale switches, often using mercury, are sometimes used in place of solid-solid contact switches because of the ability to minimize damage from switching and the ability to make good contacts for electrical and thermal conductivity. However, mercury has potential health and safety problems, and is difficult to use at high frequency (kHz) operation due to poor adhesion between the liquid-solid contacts. One alternative to the mechanical and chemical problems of a liquid mercury switch is using ...
Description/Abstract Development of flexible thin film systems for biomedical, homeland security and environmental sensing applications has increased dramatically in recent years [1, 2, 3, 4]. These systems typically combine traditional... more
Description/Abstract Development of flexible thin film systems for biomedical, homeland security and environmental sensing applications has increased dramatically in recent years [1, 2, 3, 4]. These systems typically combine traditional semiconductor technology with new flexible substrates, allowing for both the high electron mobility of semiconductors and the flexibility of polymers. The devices have the ability to be easily integrated into components and show promise for advanced design concepts, ranging from innovative ...