Jung-Hoon Jang

Dispersion and Related Properties of Acid-Treated Carbon Nanotube/Epoxy Composites using Electro-Micromechanical, Surface Wetting and Single Carbon Fiber Sensor Tests

Studies of dispersion and related properties, in carbon nanotube/epoxy composites, were conducted using electro-micromechanical and wettability tests. Specimens were prepared from neat epoxy as well as composites with untreated and acid-treated carbon nanotube (CNT). The degree of dispersion and its standard deviation were evaluated by turbidity of the dispersing solution, as well as by volumetric electrical resistivity. Acetone was a better dispersing solvent than purified water and various acid treatments of the CNT also enhanced dispersion. Contact resistivity responded differently with dispersion degree. The apparent Youngs modulus was higher for composites with acid treated CNT. The interfacial shear strength between a single carbon fiber and CNT/epoxy was lower than that between a single carbon fiber and neat epoxy. This difference is attributed to increased viscosity and decreased bonding availability in the matrix due to the added CNT. The optimum CNT treatment, for maximizing interfacial adhesion while maintaining good electrical conductivity was the sulfuric acid treatment. The CNT composites can also sense micro-damage in terms of the stepwise increments of electrical resistivity combined with acoustic emission.

Stress and Cure Sensing of Single-Shape Memory Alloy (SMA) Fiber/Epoxy Composites using Electro-micromechanical Technique

SMA (shape memory alloy) is well known to change the microstructure from martensite to austenite with either temperature or stress. Stress and temperature response were investigated for SMA fiber/epoxy composites using an electro-micromechanical technique during curing. SMA fiber can be used in practical applications, including stress or cure-monitoring sensors, due to its inherent shape recovery properties, i.e., it exhibits a shape memory effect (SME) when subjected to applied stress or temperature. Superelasticity was observed for SMA fiber/epoxy composites under cyclic stress–strain curve. Under a certain stress, the original modulus was reduced steeply but then recovered under further stress. A sudden transitional change in electrical resistance was also observed around 87°C with an increase of temperature. Under cyclic loading the stress was suddenly leveled-off with a certain stress, which resulted in a different stress hysteresis repeatedly with two differing matrices and surface treatment. In SMA fiber/epoxy composites, residual stress of single-SMA fiber with and without embedding epoxy matrix exhibited the incomplete and complete recovery, respectively, during the curing process. Interfacial effect between SMA fiber and matrix can be important factor for practical applications including feasible sensing and actuator.

Dispersive evaluation and self-sensing of single-fiber/acid-treated CNT-epoxy nanocomposites using electromicromechanical techniques and acoustic emission

Self-sensing and dispersive evaluation were investigated with different dispersion solvents for single carbon fiber/acid treated carbon nanotube (CNT)-epoxy composites by electro-micromechanical technique and acoustic emission (AE) under cyclic loading/subsequent unloading. Gradient nanocomposite specimen was used to obtain contact resistivity using two- and four-probe method. Optimized dispersion procedure was set up to obtain improved mechanical and electrical properties. The case using good dispersion solvent exhibited higher apparent modulus and lower electrical contact resistivity for both the untreated and acid-treated CNT-epoxy composites. It is because of better stress transferring effect and enhanced interfacial adhesion. Micro-damage sensing was also detected simultaneously by AE combined with electrical resistance measurement. It exhibited the stepwise increase with progressing fiber fracture due to the maintaining numerous electrical contacts of CNT. Thin network of CNT by dipping method was formed on glass substrate to obtain conductive and transparent plate by UV transmittance.

Dispersive Evaluation and Self-Sensing of Single Carbon Fiber/CNT-Epoxy Composites using Electro-Micromechanical Techniques

Self-sensing and interfacial evaluation were investigated with different dispersion solvents for single carbon fiber/carbon nanotube (CNT)-epoxy composites by electro-micromechanical technique and acoustic emission (AE) under loading/subsequent unloading. Optimized dispersion procedure was set up to obtain improved mechanical and electrical properties. Apparent modulus and electrical contact resistivity for CNT-epoxy composites were correlated with different dispersion solvents for CNT. CNT-epoxy composites using good dispersion solvent showed higher apparent modulus because of better stress transferring effect due to relatively uniform dispersion of CNT in epoxy and enhanced interfacial adhesion between CNT and epoxy matrix. However, good solvent showed high apparent modulus but low thermodynamic work of adhesion, Wa for single carbon microfiber/CNT-epoxy composite. It is because hydrophobic high advanced contact angle was shown in good solvent, which can not be compatible with carbon microfiber well. Damage sensing was also detected simultaneously by AE combined with electrical resistance measurement. Electrical resistivity increased stepwise with progressing fiber fracture due to the maintaining numerous electrical contact by CNT.

Self-sensing of CNF and Ni nanowire/PVDF and cellulose composites using electro-micromechanical test

Self-sensing and actuation were investigated for carbon nanofiber (CNF) and Ni nanowire/polymer composites. Electro-micromechanical techniques can be used for evaluating self-sensing and interfacial properties indirectly under loading/subsequent unloading. Apparent modulus and contact resistivity for CNF/epoxy composites were evaluated as functions of different aspect ratio. CNF/epoxy composites with smaller aspect ratio shown to be higher apparent modulus due to high volume content in case of short aspect ratio. Surface energy via dynamic contact angle measurement was evaluated to obtain interfacial adhesion between nano-materials embedded matrix and carbon fiber sensor. Interfacial properties of CNF/epoxy with different aspect ratios were also obtained indirectly. CNF-PVDF, Ni nanowire-CNF-silicone and Ni nanowire-cellulose actuator were made successfully. Electrochemical actuator of CNF-PVDF was responded in electrolyte solution. Magnetic actuators of Ni nanowire-CNF-silicone and Ni nanowire-cellulose composites were monitored under electro-magnetic field with different frequency, wave function and voltage. Ni nanowire-CNF-silicone actuator with lightness and Ni nanowire-cellulose actuator with rapid frequency response having meaningful merits can be applied for various new smart structural materials.