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The goal of this work is to determine the widest bandgaps possible for Phononic Crystals (PnCs) operating in the MHz–GHz frequency range by using the Planes Approximation Method (PAM). MHz–GHz PnCs have micro/nanoscale features which must be fabricated in a cleanroom with cleanroom compatible materials. 1D and 2D simulations are performed, using PAM, for bimaterial (two-material) phononic crystals for 41 cleanroom compatible materials to determine the widest bandgaps. 1D results yield a monotonic, characteristic curve demonstrating a logarithmic relationship between the gap/midgap ratio and normalized impedance with an R2 value of 0.965. 2D simulations with circular inclusions on a square lattice demonstrate an increasing linear trend for the gap/midgap ratio and normalized impedance. The interplay between the ΓX\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$\Gamma {\text{X}}$$\end{document} and ΓM\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$\Gamma {\text{M}}$$\end{document} directions cause deviations from monotonicity. [ABSTRACT FROM AUTHOR]

The conceptual design of a microfabricated load cell for mechanical testing of materials is presented that utilizes radiation pressure for the application of load. Using a laser as the source of radiation pressure cyclic and quasi-static materials testing is possible in force, spatial and temporal ranges not previously accessible. The overall design consists of 3 subsystems working in unison for the testing of micro-/nanostructured materials in tension, compression, and bending. One subsystem applies radiation pressure (load); a second subsystem measures displacement optically; and the third subsystem is the optomechanical load cell (OMLC) itself with which the other two subsystems interact. The OMLC translates in one dimension and contains a thermal dissipation structure. A detailed design of the OMLC is presented that takes into consideration optical, mechanical, and thermal considerations when a 1 W laser at 532 nm is utilized. With analytical and computational models, force, displacement, and temporal resolutions are demonstrated. Specifically, it is shown that the design is capable of forces from fN to mN, with 10 pm of displacement resolution and cyclic loads of up to 1 MHz. [ABSTRACT FROM AUTHOR]

A laboratory course on the theory, fabrication, and characterization of microelectromechanical systems (MEMS) devices for a multidisciplinary audience of graduate students at the University of New Mexico, Albuquerque, has been developed. Hands-on experience in the cleanroom has attracted graduate students from across the university's engineering and science campuses, including students from the Nano-Science and Micro Systems program, itself an interdisciplinary program. This course has been offered yearly for the last four years (since 2007) at the University of New Mexico. In one project from the class, a MEMS actuator is fabricated. This paper details the theory, fabrication steps, and characterization of a silicon-on-insulator (SOI) comb drive actuator that relies on fixed-fixed beams for its restoring force. Discussions include force generation by comb drives and a linear and nonlinear description of the restoring force applied by the fixed-fixed beams. Course assessment data is given from 79 students from three semesters, based on which the effectiveness of the laboratory project is evaluated. [ABSTRACT FROM PUBLISHER]

Microcantilever beams are frequently utilized as sensor platforms in microelectromechanical system devices. These highly compliant surface-micromachined structures generally fail by adhering to the underlying substrate during processing or subsequent operation. Such failures, which are commonly known as "stiction" failures, can be prevented or repaired in a number of ways, including low adhesion coatings, rinsing with low surface energy agents, and active approaches such as laser irradiation. Gupta et al. [J. Microelectroinech. Syst. vol. 13, pp. 696-700, 2004] recently demonstrated that stress waves could be used to repair stiction-failed structures. This paper extends the work of Gupta et al. by developing a fracture mechanics theory of the repair process and compares that theory with corresponding experiments. We show that: 1) incremental crack growth is associated with each laser pulse, the extent of which is directly related to the laser fluence; 2) repeated pulsing fully repairs all of the microcantilevers; and 3) a fracture mechanics model accurately predicts the observed experimental results. [ABSTRACT FROM AUTHOR]

In this paper, we present experimental measurements of the strain energy release rate for stiction-failed polysilicon microcantilevers using a newly developed single cantilever beam peel test. Our experiments show that dry-contacting microcantilevers adhere exclusively as tip-stuck, ‘arc-shaped’ stiction failures, while adhesion under ‘wet’ conditions generate exclusively ‘s-shaped’ stiction failures. Microcantilevers were ‘peeled’ from the substrate under displacement control using a piezoelectric actuator attached to one end of an array of microcantilever beams. The crack length was monitored using interferometric imaging, and related to the applied displacement using established equations from linear elastic fracture mechanics. The pull-off forces associated with ‘arc-shaped’ stiction failures were an average value of 89.7 nN, for 1000 μm long beams, and an average value of 123 nN for 1500 μm long beams. Adhesion energies for s-shaped failures were measured as 13.7 mJ/m² for IPA released beams and 15.4 m J/m² for deionized water released beams. These values are in good agreement with previous measurements. The proposed experimental method enables application of a simple fracture mechanics model using a standard specimen geometry. These experiments, using both wet and dry adhesion failure conditions, show that the quality of the adhesive failure depends upon the magnitude of the forces pulling the microcantilever into contact with the underlying substrate. [ABSTRACT FROM AUTHOR]

Date palm fiber composites with aligned, continuous fibers and epoxy matrix were prepared and mechanically characterized using micromechanical models. A hydrothermal process with microwave heating was utilized to extract the date palm frond (midrib) fibers, making the process eco-friendly as opposed to the commonly reported chemical extraction method. Single fibers were then characterized and found to have an average elastic modulus and ultimate tensile strength of 17.1 ± 1.5 GPa and 333 ± 27.4 MPa, respectively. Interfacial shear strengths ranging between 6 and 12 MPa were determined using a single-fiber pull-out test. Fibers were aligned to make uniaxial tensile samples with volume fraction ranging from 11% to 56%, elastic modulus ranging from 1.72 GPa to 9.52 GPa and ultimate tensile strengths ranging from 16.9 MPa to 43.8 MPa. All the results were found to obey "Rule of Mixtures" theory. The composite's mechanical properties are similar to or exceeding those of the commercially available wood products such as medium density fiberboard, oriented strand board, thick particle board, hard fiber board, and plywood, making the hydrothermal approach with microwave heating as a promising approach for preparation of date palm composites and further opens up similar process pathway for other natural fibers. [ABSTRACT FROM AUTHOR]

Composite material systems composed of a matrix of nanomaterials can achieve combinations of mechanical and thermophysical properties outside the range of traditional systems. The microstructure of the system dictates the rate, in which heat moves through the material. In this work, air/carbon nanofiber networks are studied to elucidate the system parameters influencing thermal transport. Thermal properties are measured with varying initial carbon fiber fill fraction, environment pressure, loading pressure, and heat treatment temperature (HTT) through a bidirectional modification of the 3ω technique. The nanostructure of the individual fibers is characterized with small angle X-ray scattering and Raman spectroscopy providing insight to individual fiber thermal conductivity. Measured thermal conductivity of the carbon nanofiber networks varied from 0.010 W/(m K) to 0.070W/(m K). An understanding of the intrinsic properties of the individual fibers and the interactions of the two-phase composite is used to reconcile low measured thermal conductivities with predictive modeling. Accounting for fiber-to-fiber interactions and the nuanced changes in the composite as pressure is applied is necessary to successfully model thermal transport in system. [ABSTRACT FROM AUTHOR]

Abstract: A unique type of nonwoven carbon material has been developed which is flexible, resilient, and produced at modest temperature and near ambient pressure using catalytic deposition. This material is comprised entirely of nanoscale carbon fibers, which are extensively interlaced to create a coherent, bulk material. The structure and basic mechanical and electrical properties of this material were investigated through cyclic compression and in situ resistance measurement. The material was highly elastic and capable of being repeatedly compressed without disintegration. The mechanical response varied with density, and the density was controlled by the amount of catalyst used. The material exhibited a high electrical resistivity, which varied nonlinearly with compression. [Copyright &y& Elsevier]

The design, fabrication and characterization of a recently developed test platform for the characterization of nanoscale properties of thin films are presented. Platforms are comprised of a microfabricated cascaded thermal actuator system and test specimen. The cascaded thermal actuator system is capable of providing tens of microns of displacement and tens of milli-Newton forces simultaneously while applying a relatively low temperature gradient across the test specimen. The dimensions of the platform make its use possible in both the scanning/transmission electron microscope environments and on a probe station under an optical microscope. Digital image correlation was used to obtain similar accuracy (~10 nm) for displacement measurements in both a SEM and under an optical microscope. Proof of concept experiments were performed on freestanding 250 nm thick Pt thin films. [ABSTRACT FROM AUTHOR]

Abstract: The rate of catalytic carbon nanofiber formation from a mixture of ethylene and hydrogen at 550°C was found to be dramatically faster over physical mixtures of palladium and cobalt micron scale particles than over either metal independently. The rate correlated with the metal fraction nearly identically for either Pd or Co rich mixtures. The highest rate increase over either pure metal was observed for a 1:1 mass ratio (∼150 times faster), although significant increases were found even at metal ratios of 11:1 (∼45 times faster). There was no direct evidence of extensive alloy formation from the mixed powders which suggests that a synergistic mechanism driven by proximity only may be responsible for the observed rate increases. It is thought a species (e.g. hydrogen atoms) formed at one metal (e.g. palladium) diffuses to the other where it accelerates carbon deposition by affecting the other catalyst material directly, or by generating favorable radical species. Kinetic synergism was also observed for Pd–Co alloys, although it was clearly less dramatic than that found for mixtures. Still, the fundamental similarity in behavior suggests that on the alloy surface two site types exist: one primarily Pd and one primarily Co. [ABSTRACT FROM AUTHOR]

Abstract: Carbon nanofiber growth on palladium particles from ethylene–oxygen mixtures was investigated with respect to thermal history. Electron microscopy, combined with focused ion beam cross-sectioning show particles sinter quickly, but can be stabilized by the addition of a short carbon deposition step at a temperature below the general reaction temperature. This step generates a thin layer of carbon on the catalyst which reduces sintering once the temperature is raised to the optimal reaction temperature. For example, high temperature (e.g. 500°C) catalyst pre-treatment leads to catalyst particle sintering, and subsequent fiber growth produces large diameter fibers. In contrast, small diameter fibers form on catalyst particles pretreated at low temperature (ca. 350°C), even if the fibers are grown at a temperature at which deposition rates are faster (e.g. 550°C). These results led to the development of unique multiple temperature fiber growth protocols that produce smaller diameter fibers while improving the deposition rate. [Copyright &y& Elsevier]

This paper identifies dynamic excitation parameters that promote decohesion of stiction-failed microcantilevers. The dynamic response of ‘s-shaped’ adhered beams subjected to harmonic loading is described using modal analysis; this model is then used to predict the onset of debonding in the context of a critical interface energy. These theoretical results are used to rationalize preliminary experiments, which illustrate that dynamic excitation may be used to affect partial or complete repair of stiction-failed microcantilevers. The theoretical results provide fundamental insight regarding regimes where resonant effects trigger debonding and can serve as a potential mechanism for stiction repair. The models illustrate that driving a structure at resonance is usually beneficial with regards to debonding. However, this is not universally true; there is no benefit to driving a device at frequencies with unfavorable mode-shapes. Thus, these results provide a reasonable physical and mathematical explanation for the preliminary experimental results, while providing a roadmap for identifying parameters in future tests. [ABSTRACT FROM AUTHOR]

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump–probe spectroscopies due to their limited thermal penetration depths. Here, we experimentally and numerically investigate the TPD of the recently developed optical pump–probe technique steady-state thermoreflectance (SSTR) and explore its capability for measuring the thermal properties of buried substrates. The conventional definition of the TPD (i.e., the depth at which temperature drops to 1/e value of the maximum surface temperature) does not truly represent the upper limit of how far beneath the surface SSTR can probe. For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump–probe techniques, exemplified by measuring three control samples. We also discuss the required criteria for SSTR to isolate the thermal properties of a buried film. Our study establishes SSTR as a suitable technique for thermal characterizations of sub-surface buried substrates in typical device geometries. [ABSTRACT FROM AUTHOR]

Abstract The triboelectrification of conducting materials can be explained by electron transfer between different Fermi levels. However, triboelectrification in dielectrics is poorly understood. The surface dipole formations are shown to be caused by the contact-induced surface lattice deformations. An Atomistic Field Theory (AFT) based formulation is utilized to calculate the distribution of the polarization, electric and potential fields. The induced fields are considered as the driving force for charge transfer. The simulation results show that a MgO/BaTiO 3 tribopair can generate up to 104 V / c m 2 , which is comprable with the data in the published literature. Highlights • Contact electrification is correlated with the surface dipole formation. • Atomistic Field Theory provides a coarse-grain tool for electro-mechanical coupling. • Electric characteristics are calculated from a first-principles perspective. • The atomistic calculation on potential is comparable with published experiments. [ABSTRACT FROM AUTHOR]

The objective of this paper is to formulate a simple analytical model that describes Bragg and Mie scattering leading to the creation of bandgaps in phononic crystals (PnCs). The model is a homogenized, mean-field 1D model, dubbed the Planes Approximation Method (PAM). Bragg and Mie scattering are decoupled into independent 1D models and then recombined graphically to understand better and visualize bandgap creation. Due to the model's simplicity, it allows for rapid solutions compared to other computational methods. This paper argues that acoustic impedance mismatches yield bandgaps based on the formulation. PAM is demonstrated using a square lattice of microscale circular inclusions of tungsten with a lattice constant of 45 μm in a SiO 2 matrix and then compared to results from finite difference time domain and plane wave expansion simulations. • Analytical model is developed describing interplay between Bragg and Mie Scattering in phononic crystals (PnCs). • Homogenized, Mean-Field 1D model is applied to 2D PnCs and compared to computationally expensive PWE and FDTD models. • Model shows that acoustic impedance mismatch is responsible for phononic bandgaps. • Combining Bragg and Mie Scattering mechanisms creates deep, wide phononic bandgaps. [ABSTRACT FROM AUTHOR]

Entangled networks of carbon nanofibers are characterized both mechanically and electrically. Results for both tensile and compressive loadings of the entangled networks are presented for various densities. Mechanically, the nanofiber ensembles follow the micromechanical model originally proposed by van Wyk nearly 70 years ago. Interpretations are given on the mechanisms occurring during loading and unloading of the carbon nanofiber components. [ABSTRACT FROM AUTHOR]

A technique based on suspended islands is described to measure the in-plane thermal conductivity of thin films and nano-structured materials, and is also employed for measurements of several samples with a single measurement platform. Using systematic steps for measurements, the characterization of the thermal resistances of a sample and its contacts are studied. The calibration of the contacts in this method is independent of the geometry, size, materials, and uniformity of contacts. To verify the technique, two different Si samples with different thicknesses and two samples of the same SiNx wafer are characterized on a single device. One of the Si samples is also characterized by another technique, which verifies the current results. Characterization of the two SiNx samples taken from the same wafer showed less than 1% difference in the measured thermal conductivities, indicating the precision of the method. Additionally, one of the SiNx samples is characterized and then demounted, remounted, and characterized for a second time. The comparison showed the change in the thermal resistance of the contact in multiple measurements could be as small as 0.2 K/μW, if a similar sample is used. [ABSTRACT FROM AUTHOR]

Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m−1 K−1 to as low as 6.8 W m−1 K−1). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics of carbonaceous materials deposited in fuel rich ethylene–oxygen mixtures on three types of palladium: foil, sputtered film, and nanopowder, are reported. It was found that the form of palladium has a dramatic influence on the morphology of the deposited carbon. In particular, on sputtered film and powder, tight ‘weaves’ of sub-micron filaments formed quickly. In contrast, on foils under identical conditions, the dominant morphology is carbon thin films with basal planes oriented parallel to the substrate surface. Temperature, gas flow rate, reactant flow ratio (C2H4:O2), and residence time (position) were found to influence both growth rate and type for all three forms of Pd. X-ray diffraction, high resolution transmission electron microscopy, temperature-programmed oxidation, and Raman spectroscopy were used to assess the crystallinity of the as-deposited carbon, and it was determined that transmission electron microscopy and X-ray diffraction were the most reliable methods for determining crystallinity. The dependence of growth on reactor position, and the fact that no growth was observed in the absence of oxygen support the postulate that the carbon deposition proceeds by combustion generated radical species. [Copyright &y& Elsevier]

A planar neutron spectrometer, the Miniaturized Fast Neutron Detector, has been conceptualized. The device consists of a plastic converter layer followed by several stacked layers of thin-silicon diode detectors. Incident neutrons interact in the plastic converter layer to produce recoil ions which subsequently deposit energy in the diode detectors that follow. The device can be used to reconstruct incident neutron energy spectra with no prior knowledge of the spectrum via the SPUNIT unfolding algorithm. In this work, a Miniaturized Fast Neutron Detector with 20 stacked diode detectors is considered. Particle transport simulations were conducted to generate absorbed dose response functions for each of the 20 diode detectors and to compute the absorbed dose in each layer due to incident 252Cf, AmBe, and AmB neutron energy spectra as a function of integral neutron fluence. The results of this work demonstrate that the MFND is capable of unfolding three different, increasingly complex neutron energy spectra via the SPUNIT unfolding method with no a priori information. Additionally, the "effectiveness" of the MFND as a spectrometer is quantified as a function of integral neutron fluence. Spectrometer effectiveness metrics are useful for determining the minimum integral neutron fluence required to obtain acceptable resulting unfolded energy spectra, absorbed dose values, and risk-related quantities. [ABSTRACT FROM AUTHOR]

We investigate thickness-limited size effects on the thermal conductivity of amorphous silicon thin films ranging from 3 to 1636 nm grown via sputter deposition. While exhibiting a constant value up to ~100 nm, the thermal conductivity increases with film thickness thereafter. The thickness dependence we demonstrate is ascribed to boundary scattering of long wavelength vibrations and an interplay between the energy transfer associated with propagating modes (propagons) and nonpropagating modes (diffusons). A crossover from propagon to diffuson modes is deduced to occur at a frequency of ~1.8 THz via simple analytical arguments. These results provide empirical evidence of size effects on the thermal conductivity of amorphous silicon and systematic experimental insight into the nature of vibrational thermal transport in amorphous solids. [ABSTRACT FROM AUTHOR]