Other than viscosity and surface tension which dominate the behavior of conductive ink during printing, it is also important to understand the miscibility of liquid-liquid mixtures or the solubility of solids in liquids (Hughes et al., 2013).
Hilderbrand and Hansen solubility parameters are commonly used to investigate suitable solvents for the dispersion of graphene and other solutes. Successful solvents are governed by two criteria; (1) for nonpolar solutes, the Hilderbrand solubility parameters (�) of solvent and solute must be very close to each other and (2) for polar solutes, good solvents are those where solvent and solute have similar values of all three Hansen solubility parameters (�, � and � are the dispersion forces, polar forces and hydrogen-bonding forces) (Barton, 1983; Hansen, 2007). Hernandez et al. (2010) reported the dispersibility of graphene in 40 solvents and shown that good solvents for graphene were characterized by a Hilderbrand solubility parameter of � ~ 23 MPa1/2 and Hansen solubility parameters of � ~ 18 MPa1/2, � ~ 9.3 MPa1/2 and � ~ 7.7 MPa1/2.
Ultraviolet-visible (UV-Vis) spectrophotometer is one of the most popular analytical techniques because it is very versatile and able to examine the behavior of the sample. The spectrophotometers send the UV-Vis light through a sample and then the intensity of the beam is measured to identify the absorbance. Besides that, the concentration of the sample (in liquid or solid form) can be measured too (Johnson et al., 2015).
As reported by Johra et al. (2014), the absorption peak at 235 nm for GO sample in Figure 2.11 corresponds to –* transitions of the remaining sp2 C=C bonds. The spectrum is further shifted to longer wavelength around 265 nm after reduction of GO to graphene. The reduction of GO to graphene increased the –conjugation which reduces the required energy for the transition that corresponds to the observed shift of the absorption to the longer wavelength region. Meanwhile, the absorption peak at 265 nm for graphene corresponds to sp2 C=C bonds (Zhou et al., 2009). GO has higher intensity peak than graphene due to higher degree of oxidation. The higher degree in oxidation of GO is attributed to the effect of concentration of the oxidizing agent, i.e. potassium permanganate which increased the oxidation level (Peng et al., 2013; Emiru and Ayele, 2017).
Zeta potential analysis
According to Johnson et al. (2015), a charge may be developed at the interface creating a potential when the material is immersed in a solvent. The model can be presented as an electric double layer with three distinct regions known as Stern layer (a closely bound layer of oppositely charged ions), a diffuse layer and the bulk solvent.
The zeta, potential is defined as the potential at the shear plane within the diffuse layer, in which all components move as a single kinetic unit with the particle, as illustrated in Figure 2.12.
Graphene can be positively or negatively charged when it was dispersed in an organic solvent and the used solvents covered a wide range of electron-donor properties (Liu et al., 2012). Graphene in water is typically negatively charged (Liu et al., 2012; Konkena and Vasudevan, 2012). Negatively charged graphene is attributed to the Lewis acid-base interaction, which was originally defined by Lewis (Lewis, 1923). As reported by Gutmann (1978), the Lewis concept can be quantified through the donor and acceptor number. The charging was negatively charged in solvents with higher donor numbers and positively charged in lower donor numbers. Liu et al. (2012) reported zeta potentials of graphene dispersed in water and various organic solvents with respect to donor and acceptor numbers, as shown in Figure 2.13. The surface charge of graphene in water and various solvents were negative due to higher donor numbers than acceptor numbers.
Flexible electronics often called as flex circuits have created a vibrant market over the past few years. Several factors have contributed towards the increment of flexible electronics, including more ruggedness, bendable, lightweight, portable and less cost, with respect to production as compared to rigid substrate electronics. In order to make the structure flexible, all the components must bend up to some degree without losing their function.
Figure 2.14 shows several images of recent developments in the field of flexible electronics including (a) stretchable strain sensor for patients to monitor their health at home and (b) flexible circuit film displayed by research group in Penn State’s University Park Campus (Penn State, 2013; Diabetes Queensland, 2017). Two basic approaches have been widely used to fabricate flexible electronics (Cheng and Wagner, 2009); (1) transfer and bonding of completed circuits to a flexible substrate and (2) fabrication of the circuits directly on the flexible substrate.
Methods to fabricate flexible electronics
Two main approaches involve in fabrication of flexible electronics; (1) contact printing and (2) non-contact printing. In contact printing, the patterned structures with inked surfaces acquired physical contact with the substrate. Several examples of contact printing technologies are flexography, gravure, soft-lithography and roll-to-roll (R2R) printing. Meanwhile for non-contact printing, the solution is dispensed through openings or nozzles and structures are defined by moving the stage (substrate holder) in a pre-programmed pattern. It involves screen printing, aerosol-jet, 3D printing, spray coating, inkjet printing (Khan et al., 2015).
Among these two techniques, non-contact printing technologies receive more attentions due to its simplicity, affordability, speed, adaptability to the fabrication process, reduced material wastage, high resolution of patterns and easy control by adjusting few parameters. In this section, screen printing, spray coating and inkjet printing technologies will be explained in detail.
Inkjet printing, known as digital printing, is a non-contact printing technique where micro-sized ink droplets are ejected directly onto a substrate from a jet device driven by an electronic signal. Inkjet printing is considered as a popular method in the conventional printing industry due to fine pattern generation, non-contact injection, solution saving effects, high repeatability and scalability. Inkjet printing technologies can be classified as continuous or drop-on-demand, as illustrated in Figure 2.18 (Singh et al., 2010; Aleeva and Pignataro, 2014; Mariani et al., 2015).
Flexible electronics for the strain sensor
In recent years, flexible strain sensors have attracted considerable attention with increasing demand in areas such as wearable electronics, soft robotics and healthmonitoring devices. According to the statistic of scientific journals related to flexible strain sensors which were taken from Scopus as in Figure 2.19, it shows that the number of scientific publications exhibited 416% improvement specifically from 2009 to 2018.
Strain sensor is a component of soft electronics that is integrated in flexible substrate and being used to monitor the shape, stress, strain, force and pressure. Filippidou et al. (2015) reported that strain sensors respond to the applied strain with different mechanisms which depend on the type of material, micro/nano-structure and fabrication process.
Solvents and chemicals
All of the solvents used to disperse graphene-based materials and chemicals involved in this research were purchased from local and oversea suppliers. Polyester varnish (PV) was used as a binder in the preparation of graphene-based material inks for spray coating and supplied by AEV Ltd., United Kingdom. Xylene thinner was used as a solvent for PV and also supplied by AEV Ltd., United Kingdom. Table 3.4 presents the general properties of PV. Meanwhile, solvents including ethylene glycol (EG), 2-Propanol (IPA), propylene glycol (PG), N,N-Dimethylformamide (DMF) and N-Methyl-2-pyrrolidone (NMP) were used to disperse graphene-based materials in the preparation of conductive ink for inkjet printing. EG, IPA and DMF solvents were supplied by Merck. PG and NMP solvents were supplied by Sigma Aldrich. Table 3.5 presents the details and chemical formula of the solvents and chemicals.
Table of contents :
CHAPTER 1 INTRODUCTION
1.2 Problem statements
1.4 Scope of study
1.5 Thesis overview
CHAPTER 2 LITERATURE REVIEW
2.1 Conductive ink materials
2.1.1 Graphene-based ink
2.1.2 Other conductive materials-based ink
2.1.2(a) Ink based on conductive nanomaterials
2.1.2(b) Ink based on conductive polymers
2.1.3 Graphene hybrid-based ink
2.2 Conductive ink properties
2.2.2 Surface tension
2.2.3 Solubility parameters
2.3 Conductive ink stability
2.3.1 Ultraviolet-visible spectrophotometer
2.3.2 Zeta potential analysis
2.4 Flexible electronics
2.4.1 Methods to fabricate flexible electronics
2.4.1(a) Screen printing
2.4.1(b) Spray coating
2.4.1(c) Inkjet printing
2.4.2 Flexible electronics for the strain sensor
CHAPTER 3 MATERIALS AND METHOD
3.1.1 Raw materials
3.1.1(a) Graphene-based materials
3.1.1(b) Silver nanoparticles
3.1.2 Solvents and chemicals
3.2 Experimental methods
3.2.1 Synthesis of graphene foam
3.2.2 Production of graphene-based ink for spray coating
3.2.2(a) Preparation of ink
3.2.2(b) Fabrication of conductive ink pattern
3.2.3 Production of graphene-based ink for inkjet printing
3.2.3(a) Different types of solvents
3.2.3(b) Mixed solvents
3.2.3(c) Fabrication of conductive ink pattern
3.3 Characterization techniques
3.3.1 Scanning electron microscopy
3.3.2 High resolution transmission electron microscopy
3.3.3 X-ray diffraction
3.3.4 Raman spectroscopy
3.3.5 X-ray photoelectron spectroscopy
3.3.6 Fourier-transform infrared spectroscopy
3.3.7 Physisorption of Nitrogen at 77 K
3.3.8 Thermogravimetric analysis
3.3.9 Visual observation
3.3.10 Zeta potential analysis
3.3.11 Ultraviolet-visible spectrophotometer
3.3.13 Measurement of contact angle
3.3.14 Electrical conductivity
3.3.15 Mechanical properties
CHAPTER 4 RESULTS AND DISCUSSION
4.1 Properties of graphene foam and commercial graphene-based materials
4.1.2 X-ray diffraction analysis
4.1.3 Fourier-transform infrared spectroscopy analysis
4.1.4 Raman spectroscopy analysis
4.1.5 X-ray photoelectron spectroscopy analysis
4.1.6 Thermal properties
4.2 Properties of graphene-based materials mixed with polyester varnish binder
4.2.1 Visual observation
4.2.2 Viscosity analysis
4.2.3 Surface wettability analysis
4.2.4 Electrical conductivity properties
4.3 Properties of graphene-based inks
4.3.1 Effect of GNPs dispersed in various types of common solvents
4.3.1(a) Visual observation
4.3.1(b) Zeta potential analysis
4.3.1(c) UV-Vis spectrophotometer analysis
4.3.1(d) Viscosity analysis
4.3.1(e) Surface wettability analysis
4.3.2 Effect of GNPs and GF dispersed in mixed solvents
4.3.2(a) Visual observation
4.3.2(b) Zeta potential analysis
4.3.2(c) UV-Vis spectrophotometer analysis
4.3.2(d) Viscosity analysis
4.3.2(e) Surface wettability analysis
4.4 Properties of graphene-based ink and graphene hybrid-based inks
4.4.1 Stability of GF ink, GF/AgNPs and GF/PEDOT:PSS hybrid inks
4.4.2 Physical properties of GF ink, GF/AgNPs and GF/PEDOT:PSS hybrid inks
4.4.3 Properties of printed GF ink and GF hybrid inks
4.4.4 Properties of printed GF/PEDOT:PSS hybrid ink for strain sensor
CHAPTER 5 CONCLUSIONS AND FUTURE RECOMMENDATIONS
5.2 Recommendations for future research