Ultralight (<10 milligrams per cubic centimeter) cellular materials are desirable for thermal insulation; battery electrodes; catalyst supports; and acoustic, vibration, or shock energy damping. We present ultralight materials based on periodic hollow-tube microlattices. These materials are fabricated by starting with a template formed by self-propagating photopolymer waveguide prototyping, coating the template by electroless nickel plating, and subsequently etching away the template. The resulting metallic microlattices exhibit densities ρ ≥ 0.9 milligram per cubic centimeter, complete recovery after compression exceeding 50% strain, and energy absorption similar to elastomers. Young's modulus E scales with density as E ~ ρ(2), in contrast to the E ~ ρ(3) scaling observed for ultralight aerogels and carbon nanotube foams with stochastic architecture. We attribute these properties to structural hierarchy at the nanometer, micrometer, and millimeter scales.

Compressible media can mitigate impulsive loads due to their ability to absorb energy and lower the intensity of an impulse by extending its duration. A variety of cellular materials are currently used to protect persons or structures from impulsive loads in automotive, sporting, and defense applications. While foams are the most common energy absorption materials in use today, emerging capability to fabricate well-defined, ordered lattice structures offers opportunity to create new energy absorption materials with previously unachieved properties. [1] We use a novel approach to fabricate microlattices with a range of architectures and truss diameters in the 0.05->5 mm range. [2] Because the spatial configuration of voids and solid (referred to here as the cellular architecture) is not stochastic as in foams, microlattices offer much more control over their mechanical performance than other cellular materials. For example, the unit cell symmetry, truss angle, truss diameter, and node-to-node spacing can be controlled independently and the trusses can be solid or hollow. [3] Depending on the application, different performance characteristics are required of the energy absorbing material. The injury criterion, or damage threshold s th , determines the maximum allowable stress transmitted, s tr , through the energy absorber such that s tr < s th . For energy absorbers in direct contact with the human body the injury criteria is generally on the order of 1 MPa. [4,5] Safety standards often define limits on acceleration, e.g., 150 Gs at an impact velocity of 5.2 m s À1 for motorcycle helmets, [6] and the maximum acceleration is determined by the transmitted stress and the relevant mass by Newton's second law. Space limitations and light weight considerations typically call for a material with maximum energy absorption per unit volume and unit mass. These requirements call for a stress-strain performance with [*] Dr.

No abstract

A deployment-scale array of locally resonant membrane-type acoustic metamaterials (MAMs) is fabricated. The acoustic performance of the array is measured in a transmission loss chamber, and a complex interaction between the individual cell and the array length scales is shown to exist. Transmission behavior of both the membrane and the array are independently studied using analytical models, and a method for estimating transmission loss through the structure that combines vibroacoustic predictions from both length scales is presented and shown to agree with measurements. Degradation of transmission loss performance often associated with scaling individual MAM cells into arrays is explained using analytical tools and verified using laser vibrometry. A novel design for hierarchical locally resonant acoustic metamaterials is introduced, and experimental and analytical data confirm this approach offers an effective strategy for minimizing or eliminating the efficiency losses associated with scaling MAM structures.

Single crystalline 4H-SiC is a wide-gap semiconductor with optical properties that are poised to enable new applications in microelectromechanical systems (MEMS) and quantum devices. A number of key hurdles remain with respect to the micro and nano-fabrication of SiC to prepare precise photonic structures with nanometer-scale precision. These challenges include development of a fast, scalable etching process for SiC capable of producing a sub-nanometer roughness semiconductor surface while simultaneously reducing the total thickness variation across a wafer. Our investigation into UV photoelectrochemical processing of SiC reveals high dopant-type selectivity and the advantage of multiple etch stops to reduce layer thickness variation. We demonstrate dopant-type selectivities >20:1 using a single step and a >100x reduction in surface variation by combining two etch stops. Moreover, the etch rate is fast (>4 μm/hr) and the etched surface is smooth (~1 nm RMS). These results demonstrate a scalable path to the fabrication of precise nanoscale SiC structures and electronic devices that will enable the next generation of MEMS and photonic quantum devices.

The crime of vagrancy has deep historical roots in American law and legal culture. Originating in 16th-century England, vagrancy laws came to the New World with the colonists and soon proliferated throughout the British colonies and, later, the United States. Vagrancy laws took myriad forms, generally making it a crime to be poor, idle, dissolute, immoral, drunk, lewd, or suspicious. Vagrancy laws often included prohibitions on loitering—wandering around without any apparent lawful purpose—though some jurisdictions criminalized loitering separately. Taken together, vaguely worded vagrancy, loitering, and suspicious persons laws targeted objectionable “out of place” people rather than any particular conduct. They served as a ubiquitous tool for maintaining hierarchy and order in American society. Their application changed alongside perceived threats to the social fabric, at different times and places targeting the unemployed, labor activists, radical orators, cultural and sexual nonconformists, racial and religious minorities, civil rights protesters, and the poor. By the mid-20th century, vagrancy laws served as the basis for hundreds of thousands of arrests every year. But over the course of just two decades, the crime of vagrancy, virtually unquestioned for four hundred years, unraveled. Profound social upheaval in the 1960s produced a concerted effort against the vagrancy regime, and in 1972, the US Supreme Court invalidated the laws. Local authorities have spent the years since looking for alternatives to the many functions vagrancy laws once served.

WORK/FAMILY

No abstract