Inkjet printable constantan ink for the fabrication of flexible and conductive film

The formulation and inkjet printing of constantan ink is described. Due to the small nozzle sizes which are used for inkjet printing, nano-sized particles are typically required for ink formulation. However, it is well-known that most metallic nanoparticles would form an oxide layer on the surface during the nanoparticle fabrication process. In this work, a simple chemical process has been developed to remove the oxidation and cure the printed patterns. Ammonia chloride (NH₄Cl) is used to etch the surface oxidation and nanoparticles can be sintered together with thiourea dioxide (TD) additive which reduces the sintering process temperature. The resistivity of synthesized constantan sensor is around 4 μΩ m and temperature coefficient is as same as the bulk constantan. X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) show that the oxidation has been removed/ reduced successfully. Scanning electronic microscopy (SEM) result shows that printed particles are sintered very well; relative high resistivity is caused by the porous structure. The low temperature sintering is attributed to the reduced copper and nickel from etched oxidation surfaces. The reduced copper and nickel small particles have high surface energy and tend to sinter with each other; this process can cause solid diffusion among constantan particles. This process has been proved by Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA). Chemical reduction sintering method can also be used to sinter alloy particles at low temperature and make printed alloy sensors on flexible substrates.