Pressure sensitive Paints (PSP) are widely applied in aerodynamic surface tests, as they allow fast data acquisition and pressure mapping across the entire surface during flight simulation [1]. The PSP used in this work is mainly composed of Fluoro-Isopropyl-Butyl (FIB) (Figure 1.A), a polymer that allows oxygen permeability in the paint and platinum tetra(pentafluorophenyl)porphyrin (PtTFPP), responsible for pressure sensitivity (Figure 1.B) [2]. Considering the PSP unique properties, it is possible to obtain an electrical response that enables the use of this paint in airplanes pressure sensors, during flight. For this, an electrical charge of graphene derivative is added to the PSP.
Graphene has properties that make it interesting for several applications. Its final application is related to its production method. Electrochemical exfoliation of graphite was used to produce the carbonaceous electrical charge, which has advantages such as ecofriendly reagents, simple product purification and ease of subsequent steps [3, 4].
Seeking to reach the PSP application potential as an oxygen sensitive material that generates an electrical response on pressure, it is necessary that the added electrical charge is compatible with the paint, producing a homogeneous material, high electrical conductivity and mechanical properties suitable for aerospace applications. Thus, the interaction of graphene derivatives with paint and how this influence on its sensitivity to oxygen was studied. This sensitive material can be used in an aeronautical sensor redundant to the Pitot Tube. Therefore, the main objective of the proposed research is to increase the safety of air navigation using a graphene-derived composite with a pressure-sensitive metalloporphyrin paint (PSP).
Materials and methods
Initially, the production of graphene derivatives was carried out by electrochemical exfoliation of graphite, in which plates of thin graphite sheets pressed were used as working electrode and counter electrode. As an electrolyte solution, a mixture of salts (NH4)2SO4 and (NH4)2HPO4 was used at a concentration of 0.1 mol.L-1 and a proportion of 70/30 v/v%. Exfoliation took place at a fixed potential of 10 V, applied by an electrical power supply. At the end of this process, two drying methodologies were tested: kiln at 70°C (Graphene A) and lyophilization (Graphene B).
The composite production consists of mixing the graphene derivative and PSP, at a concentration of 1 mg.mL-1, with the aid of an ultrasound tip for 30 minutes. In addition to the carbonaceous material, a commercial graphene (Graphene C) was also used. After mixing, the material is dripped onto an aluminum surface.
The characterizations performed were Raman Spectroscopy and X-Ray Diffraction Spectroscopy to analyze the morphology of the graphene used and Stereo Microscopy, XRD and electrical impedance for the composites.
Results and discussion
Analyzing the Raman spectra, Graphene A has a more disorder in the carbon chain, higher degree of stacking between planes and a greater association of defects (D4, 2D1 bands, respectively). The lyophilized material showed less interactions between the edges (D1+D4 band) configuring smaller and less agglomerated particles. Graphene C also showed a certain degree of defects and functionalization by oxygenated groups. The diffractogram of Graphene B and C indicates greater disorder in the bonds of the carbon rings with sp2 hybridization than that presented in Graphene A, with A being the material with the highest number of layers and the smallest lamellar distance.
Stereo microscopy (Figure 2) images of the composites showed that the mixture is not homogeneous, especially after drying and presents roughness, being these characteristics that can be a problem for the final application.
The composite diffractograms show the same peaks only for the paint, with a difference in intensity. The amorphous peak at 16.4°, referring to the polymer, becomes more intense and crystalline with Graphene A and C indicating possible interaction and intercalation of graphene layers with the polymer.
The Electrical Impedance of the composites was performed in variable vacuum without flow. Composites with Graphene A and B are resistive, without variation in the presence of different amounts of oxygen. With Graphene C, the response was capacitive-resistive, and was sensitive to pressure changes, but still needs calibration (Figure 3).
[1] PUKLIN, E. et al. Ideality of pressure-sensitive paint. I. Platinum tetra(pentafluorophenyl)porphine in fluoroacrylic polymer. Journal of Applied Polymer Science, v. 77, n. 13, p.2795-2804, 2000.
[2] GAMAL, E.; et. al. Oxygen pressure measurement using singlet oxygen emission. Review of Scientific Instruments 76, 054101, 2005.
[3] KARIMOV, K. S.; et. al. Development of pressure-sensitive thermo-electric cell using graphene and n-Bi2Te3. Emergent Materials 2, pages 387–390, 2019.
[4] ANJU, M.; et. al. Graphene-dye hybrid optical sensors. Vol 17, 194 – 217, 2019.