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Abstract
This paper presents a number of novel methods to measure the junction temperature and to estimate the health of gallium nitride light-emitting diodes (LEDs). The methods are based on measurements of the dynamic impedance and optical output. Our experimental analysis reveals temperature sensitive parameters of the electrical and optical responses. Moreover, a correlation between the non-radiative current characterizing the active region defects and the small-signal impedance is shown. The demonstrated methods can be applied to enhance existing temperature-monitoring techniques. The derived dependencies also build a foundation for advanced in-field health monitoring of the LEDs using the infrastructure of visible light communication systems. Such methods and techniques are valuable for predictive maintenance of solid state lighting systems.
Highlights
Polyelectrolyte membranes (PEMs) are widely used in fuel cells and flow batteries
Previous experiments of water vapor penetration into polyelectrolyte membrane (PEM) thin films have indicated the influence of the water concentration gradient and polymer chemistry on the interface evolution, which will eventually affect the efficiency of the fuel cell operation
The evolution of the interface between water and polyelectrolyte membranes (PEMs), which are used in fuel cells and flow batteries, of three different side-chain lengths has been studied using dissipative particle dynamics (DPD) simulations
Summary
Polyelectrolyte membranes (PEMs) are widely used in fuel cells and flow batteries. They are used to provide a pathway for proton transport and separate gases/electrolytes from mixing. The most commonly used PEM is Nafion, which has a polytetrafluoroethylene (PTFE) hydrophobic backbone and a highly hydrophilic (due to the presence of a sulfonic acid group) side chain; see Figure 1b. Hsu, and other co-workers [1,2] suggested water attached to ionic groups, forming spherical clusters of 4 to 6 nm that are distributed in the hydrophobic phase. The side-chain length has been varied to reproduce different types of PEMs. Some of common short side-chain PEMS are Dow (Figure 1a) and Aquivion, both of which have side chains shorter than those in Nafion. Long side-chain PEMs include Aciplex (Figure 1c), which has a side chain longer than in Polymers 2020, 12, 907; doi:10.3390/polym12040907 www.mdpi.com/journal/polymers
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