A magnetic microrheometer is used to characterize the development of viscosity at different depths in UV‐cured epoxy coatings. Lateral magnetic particle velocities are tracked at different depths to quantify viscosity gradients. In general, viscosity build‐up is faster near the coating surface, tending to produce a “skin”. The effects of process conditions on the viscosity gradient development, on the rate of viscosity increase, and on surface defects are studied. More severe gradients develop in thicker coatings and in those with higher photoinitiator concentration. Under some conditions, the skin layer wrinkles, indicating the development of local compressive stress. Curing at higher temperature, however, increases cure rates while reducing the viscosity gradients and wrinkling defects.
a b s t r a c tSag is a coating phenomenon characterized by gravity-driven flow after deposition; excessive amounts of sag can lead to coating defects. In this work, a new method for evaluating and quantifying sag is investigated. The motion of micron-sized Lycopodium spores on an inclined coating surface is tracked during drying, and the resulting surface velocity data is used to determine sag length. This in situ particle tracking method is minimally invasive and permits real time measurements. Measured sag lengths and real time surface velocities in aqueous polyvinyl alcohol solution coatings compare well with a theoretical model. The model is also used to develop a predictive sag regime map, which anticipates the extent of sag given coating properties and process-specific parameters. This map also identifies viable processing windows and aids in intelligent coating design given specific process constraints. The predictions of the sag regime map are compared against experimental sag results from polyvinyl alcohol solution coatings as well as four commercial latex paints, revealing good agreement for coatings with Newtonian or 'Newtonian-like' rheologies.
A magnetic microrheometer has been designed to characterize the local viscosity of liquid-applied coatings in situ during solidification. The apparatus includes NdFeB magnets mounted on computer-controlled micropositioners for the manipulation of ∼1 μm diameter superparamagnetic particles in the coating. Magnetic field gradients at 20-70 T/m are generated by changing magnet size and the gap distance between the magnets. A specimen stage located between two magnets is outfitted with a heater and channels to control process conditions (temperature and air flow), and a digital optical microscope lens above the stage is used to monitor the probe particle position. Validation studies with glycerol and polyimide precursor solution showed that microrheometry results match traditional bulk rheometry within an error of 5%. The viscosities of polyvinyl alcohol (PVA) solution and polyimide precursor solution coatings were measured at different shear rates (0.01-5 s(-1)) by adjusting the magnetic field gradient. The effect of proximity to the substrate on the particle motion was characterized and compared with theoretical predictions. The magnetic microrheometer was used to characterize the time-viscosity profile of PVA coatings during drying at several temperatures. The viscosity range measured by the apparatus was 0.1-20 Pa s during drying of coatings at temperatures between room temperature and 80 °C.
We investigated the composition dependence of the band structure of Ti-alloyed Al oxide (TiAlOx), tunneling magnetoresistance (TMR) behavior of the magnetic tunnel junctions (MTJs) with TiAlOx barrier, and the microstructural evolution of Ti–Al alloy films. X-ray absorption spectroscopy indicated that TiAlOx had localized d states in the band gap below the conduction band. As the Ti concentration increased, the resistance×area value and effective barrier height of the MTJs were reduced owing to the band-gap reduction of TiAlOx caused by the formation of extra bands, mainly composed of Ti3d orbitals, within the band gap. The TMR ratio increased up to 49% at 5.33at.% Ti. Ti alloying enhanced the barrier∕electrode interface uniformity and reduced microstructural defects. These structural improvements enhanced not only the TMR effect but also the thermal stability of the MTJs.
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