Some improvements have been made in the formulation of holographically formed polymer-dispersed liquid crystals (H-PDLCs) and in the fabrication of devices made from these materials, with resulting improvements in performance. H-PDLCs are essentially volume Bragg gratings. Devices made from HPDLCs function as electrically switchable reflective filters. Heretofore, it has been necessary to apply undesirably high drive voltages in order to switch HPDLC devices.
Many scientific papers on H-PDLCs and on the potential utility of H-PDLC devices for display and telecommunication applications have been published. However, until now, little has been published about improving quality control in synthesis of H-PDLCs and fabrication of H-PDLC devices to minimize (1) spatial nonuniformities within individual devices, (2) nonuniformities among nominally identical devices, and (3) variations in performance among nominally identical devices. The improvements reported here are results of a research effort directed partly toward solving these quality-control problems and partly toward reducing switching voltages.
The quality-control improvements include incorporation of a number of process controls to create a relatively robust process, such that the H-PDLC devices fabricated in this process are more nearly uniform than were those fabricated in a prior laboratory-type process. The improved process includes ultrasonic mixing, ultrasonic cleaning, the use of a micro dispensing technique, and the use of a bubble press. The ultrasonic mixing (in contradistinction to other types of mixing) creates more nearly uniform H-PDLCs. The ultrasonic cleaning removes chips of indium tin oxide (which is electrically conductive), whereas, heretofore, chips of indium oxide remaining at the edges of H-PDLC devices have caused electrical short circuits. The micro-dispensing technique enables the emplacement of precisely the amount of H-PDLC required for a given cell volume so that the H-PDLC can be pressed between glass substrates to a precise inter-substrate distance defined Some Improvements in H-PDLCs Nonuniformities and required drive potentials have been reduced. Goddard Space Flight Center, Greenbelt, Maryland NTB 01ElecComp 1105 10/19/05 9:10 AM Page 42 the application of the correct pressure needed to push the substrates against the H-PDLC and the controlled amounts of spacers, thereby also helping to minimize nonuniformity of gaps among cells.
The drive-voltage problem has been partially solved by development of a formulation that includes an additive that reduces the magnitude of the required drive voltage. Devices that can be switched from a reflectivity of ˜0.5 to a reflectivity near zero by applying relatively low drive potentials (<100 V) have been demonstrated. It has been postulated that the reduction in the magnitude of the required drive voltage is attributable to a reduction in the surface anchoring energy.
In cases of fabrication of multilayer devices comprising stacked H-PDLC panels, the improved process also includes the use of thin glass substrates with appropriate optical coatings. Devices comprising, variously, five or ten stacked panels have been fabricated thus far.
This work was done by Gregory P. Crawford and Liuliu Li of Brown University for Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/ Computers category. GSC-14920-1