Piezoelectrics ejection process

The generation of droplets of controlled size is technologically easy today and uses either thermal or piezoelectric ejection process for conventional inks. The development of inkjet printing for the controlled deposition of functional materials is very interesting because it allows a cheap and infinitely modulable top-down synthetic method for designing the siuface of substrates or for building midtilayer 2D and 3D materials [73,74]. Its development for sol-gel inks is fairly recent, as it was reported at the end of the 1990s [75]. This first work, aimed at... 

Ink-jet describes small drops of ink driven to the substrate, usually by an electrostatic field. The drops are formed by a rapid pressure pulse in a small, nearly enclosed chamber. When the pressure increases, a small amount of liquid is ejected from the chamber through a nozzle (the only exit). When the pressure in the chamber decreases again, that liquid is separated from the bulk of the liquid in the chamber by surface tension in a process called pinch-off. The pressure pulse is commonly generated by a piezoelectric material or by heating the liquid via current through resistive material embedded in the wall of the chamber. The ejection process usually leaves the drop with insufficient velocity to drive it to the substrate, so another force, such as an electrostatic field, is used to complete the delivery. The result is a rapid stream of successive drops, controlled electronically. 

The ink-jet process relies on using a piezoelectric printhead that can create deformation on a closed cavity through the application of an electric field. This causes the fluid in the cavity to be ejected through the nozzle whose volume is determined by the applied voltage, nozzle diameter, and ink viscosity. The final width of the drop of the substrate is a result of the volume of fluid expelled and the thickness of the droplet on the surface. In addition, the drop placement is critical to the ultimate resolution of the display. Typical volumes expelled from a printhead are 10 to 40 pi, resulting in a subpixel width between 65 and 100 pm. Drop accuracies of +15 pm have been reported such that resolutions better than 130 ppi are achievable however, because the solvent to polymer ratio is so high, the drops must be contained during the evaporation process to obtain the desired resolution and film thickness. This containment can be a patterned photoresist layer that has been chemically modified so that the EL polymer ink does not stick to it. 

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