This article was submitted to Wearable Electronics, a section of the journal Frontiers in Electronics
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Understanding the sintering process of conductive inks is a fundamental step in the development of sensors. The intrinsic properties (such as thermal conductivity, resistivity, thermal coefficient, among others) of the printed devices do not correspond to those of the bulk materials. In the field of biosensors porosity plays a predominant role, since it defines the difference between the geometric area of the working electrode and its electrochemical surface area. The analysis reported so far in the literature on the sintering of inks are based on their DC characterization. In this work, the shape and distribution of the nanoparticles that make up the silver ink have been studied employing a transmission electron microscopy. Images of the printed traces have been obtained through a scanning electron microscope at different sintering times, allowing to observe how the material decreases its porosity over time. These structural changes were supported through electrical measurements of the change in the trace impedance as a function of drying time. The resistivity and thermal coefficient of the printed tracks were analyzed and compared with the values of bulk silver. Finally, this work proposes an analytical circuit model of the drying behavior of the ink based on AC characterization at different frequencies. The characterization considers an initial time when the spheric nanoparticles are still surrounded by the capping agent until the conductive trace is obtained. This model can estimate the characteristics that the printed devices would have, whether they are used as biosensors (porous material) or as interconnections (compact material) in printed electronics.
Printed electronics (PE) is a term that defines the printing of circuits not only on flexible substrates such as paper, textiles, Kapton, PEN and PET but also on a large number of potential media
In order to obtain an ink with rheological properties compatible with inkjet printing, silver nanoparticles stabilized powder (AgNPs), with an average size of 100
First, 400
Schematic of the drying process, first the sample is placed in a spin coater, the solvent is evaporated at room temperature, and the Ag NPs are isolated from each other due to the capping agent (capacitive behavior). When heat is added, the sintering process begins with the elimination of the capping agent and the interconnection of the Ag NPs.
An analysis of an ink drop was carried out using a TEM, Brand: JEOL, model 100 CX II, operated at an acceleration voltage of 100
The ink was characterized during the sintering process by impedance measurements performed at constant temperature inside the probe station, using an Agilent E4980A LCR meter and a triaxial thermal-chuck probe station. The samples were dried at 90°
First, current-voltage (I-V) measurements were carried out on the different segments of the TLM structure. The purpose of these measurements was to determine the contact resistance as well as the resistivity of the ink. Measurements ranged from −40
The proposed model can predict the time and temperature the ink must be dried in order to reach a certain impedance. In addition, both the material’s porosity and the time needed for the ink to behave fully resistive could be determined. It is important to note that this is a handy tool for predicting the behavior and usefulness of a custom ink. Once the resistance has been determined, through the percolative model, it is possible to estimate the porosity and the electrochemical area that a given trace of silver will have, which is of great interest for sensors and biosensors. Based on the data obtained in the present work and the information collected from the works
Comparison between drying at 120°
Then, the traces begin to join at the same rate as the resistance decreases, resulting in a continuous film. The expression for the inductance is:
After replacing Eq.
Superposition of the analytical model and the measurements made at three different frequencies [10 kHz (Blue), 100 kHz (Green) and 1 MHz (Red)], the capacitive behavior at the beginning of the measurement is observed and after 40 min the system becomes totally resistive. SEM images showing the evolution in the sintering of the ink are observed.
When developing a biosensor, it is essential to know the electrochemical area of the working electrode, which is possible to infer through the material’s porosity. The main application of this model is to determine the impedance that a printed trace will have through its physicochemical properties, the temperature and drying time. Furthermore, when the process of elimination of the capping agent and therefore capacitive behavior is completed, it is possible to infer the porosity of the material through the impedance value.
In this paper it was shown that it is feasible to use commercial nanoparticles and of-the-shelf chemicals to generate the aqueous suspension for developing a custom ink. As it was shown by the physical and electrical characterization, the developed ink has properties that are comparable to those of commercial inks. This has great value due to the low price of nanoparticles in relation to a commercial ink. It was also shown that a straightforward circuit can model the ink’s drying with great fidelity. It was shown that capacitive and inductive behaviors can be observed throughout the drying process, not only in agreement with the experimental measurements and the data reported in the literature, but also in correlation with the physics of drying. It is important to highlight that knowing the physical parameters of the ink, such as the size of the nanoparticle, its surface energy, the diffusivity, the nature of the dielectric that coats the nanoparticle, its volatility, and the volatility of the solvent, allowed us to simulate the drying behavior that the ink would present at different temperatures. A compact model for the sintering process is helpful to characterize the quality of the ink and optimize the sintering step. In this way, we were able to determine if the substrate and the process were compatible with these parameters. Finally, it was shown how this analytical circuit model, in combination with a percolative model can be a valuable tool for the determination of the porosity of the material.
The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.
GM contributed to the acquisition and analysis of the data as well as wrote the manuscript and led the project throughout. SB contributed to the acquisition and analysis of the data. HG contributed to the fabrication of the samples. SP contributed to the analysis of the data. JG, AO, MV, and PJ contributed to drafting the paper. FP conceived the original idea and supervised the project.
This work received funding from the following institutions: UTN.BA under projects CCUTIBA5219TC, CCUTIBA4764TC, CCUTNBA0005182, MATUNBA4936, and CONICET under projects PIP11220130100077CO and MINCyT under projects PICT2016/0579 and PICT 2017/2526. GM would like to express his gratitude to the Carolina Foundation for the financial support through the scholarship “Doctorado 2020”.
Special thanks to Ing. Maria Julia Yañez of the electron microscopy laboratory of the CONICET Bahía Blanca Scientific Center (CCTBB). To Ing. Juan José Ortiz and Lic. Fabián De Vita of the Argentine Nanotechnology Foundation. Lastly, to PLAPIQUI-CONICET-UNS and the UTN-FRBA Department of Chemical Engineering that have provided their facilities for this development.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
The Supplementary Material for this article can be found online at: