In a particle-free ink, a solvated metal salt is reduced in-situ to produce a metal. In complete contrast to more conventional conductive inks, this means that the uncured particle-free ink is often transparent. The chemical reaction is induced by heat, light or plasma, and produces a smooth conductive metal layer.
Figure 1: A transparent metal salt solution is transformed into a metal by a chemical reaction, enabling smooth and thin metal layers to be printed. Image sources: Left - Tollens test, WikiCommons. Right - flexible printed electronics, iStock
Conductivity close to bulk metals
High conductivity is a key advantage of particle-free inks. Because the metal is formed in-situ, and the proportion of binder materials can be very low, conductivity can be as high as 80% of the bulk metal. This, of course, means that less ink can be used, with the additional benefit of there being less solvent to evaporate away during curing.
Permeability for e-textiles
Combining electronic functionality with textiles promises sensing of biometric parameters in a comfortable, conformal manner. While there are many different strategies to make fabrics conductive, such as applying inks printed onto a stretchable substrate, particle-free inks are well suited to directly printing on textiles. Unlike flake-based stretchable inks, their low viscosity and lack of particles enables particle-free inks to permeate textile fibers. Once the ink is metalized by curing, the fibers themselves become conductive, thus retaining the breathability and stretchability of the original fabric.
Highly smooth surfaces
A classic school chemistry experiment is using Tollens' reagent to produce a silver mirror on the inside of a test tube using a reducing agent (see left-hand picture) - this process is similar to that employed with particle-free silver inks. The reflectiveness of the silver coating is due to its smooth surface, which is a key benefit of particle-free inks over their flake-based and even nanoparticle-based alternatives.
Aside from making a reflective surface, smooth conductive traces are highly desirable for radio frequency (RF) applications. This is because as signal frequency increases, surface roughness rather than bulk conductivity increasingly dominates impedance. Furthermore, particle-free conductive inks can be deposited in very thin layers due to their low viscosity. This is advantageous since at higher frequencies, conduction only occurs close to the surface within the 'skin depth' of the material, so thinner layers reduce costs as less silver (or other metal) is needed.
Low viscosity enables emerging printing methods
Viscosity, and indeed rheology in general, are crucial in determining the compatibility of conductive inks with each printing method. Since particle-free inks can have very low viscosities and pose no risk of clogging nozzles, they are ideally suited to aerosol and electrohydrodynamic (EHD) printing. Both these techniques have nozzles with very small apertures and are capable of printing at lines as thin as 10 and 1 um, respectively. Depositing lines this narrow enables printed electronics to compete with subtractive methods such as photolithography and thus be used for semiconductor packaging.
Application opportunities
Given the outlined attributes, we suggest that the following applications are especially well suited to particle-free conductive inks:
Electromagnetic interference shielding.
High-resolution printing for advanced semiconductor packaging.
Printing directly onto fabrics.
Furthermore, particle-free inks can also be used for applications currently dominated by flake or nanoparticle-based conductive inks.
To learn more about existing and future applications for conductive inks, and indeed other solution-processable materials for printed/flexible electronics, please see the IDTechEx reports "Conductive Ink Markets 2020-2030: Forecasts, Technologies, Players" and "Materials for Printed/Flexible Electronics 2021-2031: Technologies, Applications, Market Forecasts". All IDTechEx's reports, including a wide selection on various aspects of printed/flexible electronics, contain detailed analysis of established and emerging technologies, their potential adoption barriers and suitability for different applications, and an assessment of technological and commercial readiness. Our reports also include multiple company profiles based on interviews with early-stage and established companies, along with 10-year market forecasts. A full list of IDTechEx's reports, covering many aspects of 3D printing and printed electronics, can be found at www.IDTechEx.com