Karmann Mills

Integration of data: the Nanomaterial Registry project and data curation

Due to the use of nanomaterials in multiple fields of applied science and technology, there is a need for accelerated understanding of any potential implications of using these unique and promising materials. There is a multitude of research data that, if integrated, can be leveraged to drive toward a better understanding. Integration can be achieved by applying nanoinformatics concepts. The Nanomaterial Registry is using applied minimal information about nanomaterials to support a robust data curation process in order to promote integration across a diverse data set. This paper describes the evolution of the curation methodology used in the Nanomaterial Registry project as well as the current procedure that is used. Some of the lessons learned about curation of nanomaterial data are also discussed.

Understanding chromaticity shifts in LED devices through analytical models

Abstract This paper demonstrates that chromaticity shifts in light-emitting diode (LED) devices arise from multiple mechanisms that produce chemical changes in the materials used to construct the LED devices. Each chromaticity change is shown to proceed over a finite period of time, and there is a limit on the impact of each shift. For example, chromaticity shifts in LED devices usually start with a fast-acting component that quickly reaches a maximum value, followed by one or more slower acting component(s). This behavior can be modeled analytically with a bounded exponential component to describe the fast-acting component, followed by one or more generalized logistic models. These analytical models contain several key parameters, including the limiting value of each chromaticity shift (A for the upper asymptote and L for the lower asymptote) and the rate of the change k. This approach to chromaticity modeling is demonstrated with analytical models of the chromaticity shifts caused by the irreversible degradation of phosphors. These analytical models provide insights into the kinetic processes responsible for green and red chromaticity shifts caused by phosphor degradation. A green shift is produced by the surface oxidation of the nitride phosphor that changes the emission profile to lower wavelengths. As the surface oxidation reaction proceeds, surface reactants are consumed thereby slowing the reaction rate, and the bulk oxidation processes become more prevalent. A red chromaticity shift can arise from quenching of the green phosphor which shift the emission in the red direction. This paper concludes by discussing the implications of these models for projecting chromaticity for different operational conditions.

Impact of California Fires on Local and Regional Air Quality: The Role of a Low‐Cost Sensor Network and Satellite Observations

Abstract PM2.5, or fine particulate matter, is a category of air pollutant consisting of particles with effective aerodynamic diameter equal to or less than 2.5 μm. These particles have been linked to human health impacts as well as regional haze, visibility, and climate change issues. Due to cost and space restrictions, the U.S. Environmental Protection Agency monitoring network remains spatially sparse. To increase the spatial resolution of monitoring, previous studies have used satellite data to estimate ground‐level PM concentrations, despite these estimates being associated with moderate to large uncertainties when relating a column measure of aerosol (aerosol optical depth) with surface measurements. To this end, we discuss a low‐cost air quality monitor (LCAQM) network deployed in California. In this study, we present an application of LCAQM and satellite data for quantifying the impact of wildfires in California during October 2017. The impacts of fires on PM2.5 concentration at varying temporal (hourly, daily, and weekly) and spatial (local to regional) scales have been evaluated. Comparison between low‐cost air quality sensors and reference‐grade air quality instruments shows expected performance with moderate to high uncertainties. The LCAQM measurements, in the absence of federal equivalent method data, were also found to be very useful in developing statistical models to convert aerosol optical depth into PM2.5 with performance of satellite‐derived PM2.5, similar to that obtained using the federal equivalent method data. This paper also highlights challenges associated with both LCAQM and satellite‐based PM2.5 measurements, which require further investigation and research.

Lifetime predictions for dimmable two-channel tunable white luminaires

Two-channel tunable white lighting (TWL) systems represent the next wave of solid-state lighting (SSL) systems and promise flexibility in light environment while maintaining the high reliability and luminous efficacy expected with SSL devices. TWL systems utilize LED assemblies consisting of two different LED spectra (i.e., often a warm white assembly and a cool white assembly) that are integrated into modules. While these systems provide the ability to adjust the lighting spectrum to match the physiology needs of the task at hand, they also are a potentially more complex lighting system from a performance and reliability perspective. We report an initial study on the reliability performance of such lighting systems including an examination of the lumen maintenance and chromaticity stability of warm white and cool white LED assemblies and the multi-channel driver that provides power to the assemblies. Accelerated stress tests including operational bake tests conducted at 75°C and 95°C were used to age the LED modules, while more aggressive temperature and humidity tests were used for the drivers in this study. Small differences in the performance between the two LED assemblies were found and can be attributed to the different phosphor chemistries. The lumen maintenances of both LED assemblies were excellent. The warm white LED assemblies were found to shift slightly in the green color direction over time while the cool white LED assemblies shifted slightly in the yellow color direction. The net result of these chromaticity shifts is a small, barely perceptible reduction in the tuning range after 6,000 hours of exposure to an accelerating elevated temperature of 75°C.

Modeling the impact of thermal effects on luminous flux maintenance for SSL luminaires

Meeting the longevity requirements of solid-state lighting (SSL) devices places extreme demands on the materials and designs that are used in SSL luminaires. Therefore, understanding the aging characteristics of lens, reflectors, and other materials is essential to projecting the long-term performance of LED-based lighting systems. Overlooking these factors at either the design or product specification stage can result in premature failure of the device due to poor luminous flux maintenance and/or excessive chromaticity shifts. This paper describes a methodology for performing accelerated stress testing (AST) on materials intended for use in SSL luminaires. This test methodology, which consists of elevated temperature and humidity conditions, produces accelerated aging data that can be correlated to expected performance under normal luminaire operating conditions. The correlations can then be leveraged to produce models of the changes in the optical properties of key materials including transmittance versus wavelength of lenses and reflectance versus wavelength for housings and other reflectors. This information has been collected into a lumen maintenance decision support tool (LM-DST) and together with user supplied inputs (e.g., expected operation conditions) can provide guidance on lifetime expectations of SSL luminaires. This approach has been applied to a variety of materials commonly found in SSL luminaires including acrylics, polycarbonates, and silicones used for lenses and paints, coatings, films, and composites used for reflectors.