Here, a screen-printed functional ink, comprising a combination of semiconducting acicular particles, electrically insulating nanoparticles and a base polymer ink, is described that exhibits pronounced pressure sensitive electrical properties for applications in sensing and touch sensitive surfaces.
The combination of these components in the as-printed ink yield a complex structure and a large and reproducible touch pressure sensitive resistance range. In contrast to the case for some composite systems, the resistance changes occur down to applied pressures of 13 Pa.
Current–voltage measurements at fixed pressures show monotonic non-linear behavior, which becomes more Ohmic at higher pressures and in all cases shows some hysteresis. The physical basis for conduction, particularly in the low pressure regime, can be described in terms of field assisted quantum mechanical tunneling.
Printable electronics is now an established area of technology with significant commercial potential in a wide range of application sectors that includes photovoltaics, super capacitors and RFID [1–3].
The development of technologies with electronic functionality that can be mass produced by printing processes has the potential to provide technical benefits, including lighter weight components, and can lead to significant cost benefits from the manufacturing process.
A wide range of materials and functionalities are currently under investigation for printable manufacture, from simple electrically conductive tracks [4], to flexible FETs [5]. An area of growing applications importance is touch sensitive components and surfaces for switching and position sensing.
In its simplest form, touch sensitivity requires some contact-sensitive switching capability. The addition of pressure sensitivity adds a third dimension to conventional two-dimensional touch surface interfaces, providing an additional input parameter based upon the applied force in addition to the surface location.
Functional force and location sensing touch pad technology based upon contact-resistance produced by printing has been demonstrated elsewhere [6, 7]. However, this approach involved additional patterning, through lithographic and printing methods, on top of a resistive layer or with the addition of features to create regular structures that were needed to create the pressure and location sensing capacity [6, 7].
An alternate approach is to develop an ink that is insulating when no pressure is applied, but has an intrinsic pressure sensitive electrical response, allowing direct printing of pressure sensitive components without the need for additional patterning and structures.
In bulk form, pressure sensitive electrically conducting switching composites have been a focus of research for over 50 years [8]. The first conductive composites were composed of metal or other conductive filler powders, such as nickel or carbon black, dispersed within insulating elastomers, such as rubber
Figure 1. (a) Schematic cross section of printed structure for the pressure sensitive test device (not to scale) and (b) photograph showing top view of a printed test device. The force probe is applied to the center of the printed disk.
The variation of conductivity behavior of such composites with filler loading has been described using percolation and effective medium theories, whereby the conductivity of the composite takes a value close to that of the bulk insulating matrix at low filler loading, and rises dramatically at the percolation threshold, a critical filler loading where direct electrical connections form between filler particles throughout the insulating matrix [11].
Such carbon black based conducting composites have been developed as inks for pressure sensing [12]. However, as the composite is loaded above the percolation threshold printed films have a finite start resistance and exhibit an inverse relationship between the starting resistance and pressure sensitivity of the resistive behavior.
Other research has shifted towards composites with filler particles of varying geometries and a wider range of insulating matrices [13–15]. For printing applications the challenge is to develop composite materials with appropriate pressure sensitive electrical behavior coupled with the structural requirements needed for preparation as inks that can be printed using conventional print technologies.
Such composite materials are not uncommon, having been a focus of research for use as conductive tracks and electrodes, chemical sensors and thin film transistors, and often comprise nanoscale conductive filler particles dispersed in a flowing polymer [16–20].
The choice of nanoscale filler particles has enabled the use of ink-jet printing for many of these composite materials; larger particles tend to clog the printer nozzles. This work presents a description of the functional behavior and physical structure of a new type of screen-printed (possible due to the choice of nanoscale filler particles) composite ink with electrical properties that are highly sensitive to applied force.
Investigation of the I–V behavior at constant pressure provides some insight into the physical mechanisms that may be responsible for the pressure sensitive electrical behavior.
没有评论:
发表评论