Sang-Dong Jang

Paper transistor made with covalently bonded multiwalled carbon nanotube and cellulose

We report a flexible paper transistor made with regenerated cellulose and covalently bonded multiwalled carbon nanotube (RC-MWCNT). MWCNT bonded to cellulose chains can act as electron channel paths in dielectric cellulose layers. It is found that the covalent bonding between cellulose and MWCNT can be modulated by reaction time and temperature. The RC-MWCNT paper transistor shows that the leakage current and the on/off ratio are strongly associated with the concentration of MWCNTs. The estimated electron mobility of RC-MWCNT paper is comparable to other organic transistor materials. The RC-MWCNT paper transistor has a potential for flexible electronic paper.

Electrically aligned cellulose film for electro-active paper and its piezoelectricity

Electrically aligned regenerated cellulose films were fabricated and the effect of applied electric field was investigated for the piezoelectricity of electro-active paper (EAPap). The EAPap was fabricated by coating gold electrodes on both sides of regenerated cellulose film. The cellulose film was prepared by dissolving cotton pulp in LiCl/N,N-dimethylacetamide solution followed by a cellulose chain regeneration process. During the regeneration process an external electric field was applied in the direction of mechanical stretching. Alignment of cellulose fiber chains was investigated as a function of applied electric field. The material characteristics of the cellulose films were analyzed by using an x-ray diffractometer, a field emission scanning electron microscope and a high voltage electron microscope. The application of external electric fields was found to induce formation of nanofibers in the cellulose, resulting in an increase in the crystallinity index (CI) values. It was also found that samples with higher CI values showed higher in-plane piezoelectric constant, d31, values. The piezoelectricity of the current EAPap films was measured to be equivalent or better than that of ordinary PVDF films. Therefore, an external electric field applied to a cellulose film along with a mechanical stretching during the regeneration process can enhance the piezoelectricity.

Frequency selective surface based passive wireless sensor for structural health monitoring

Wireless sensor networks or ubiquitous sensor networks are a promising technology giving useful information to people. In particular, the chipless passive wireless sensor is one of the most important developments in wireless sensor technology because it is compact and does not need a battery or chip for the sensor operation. So it has many possibilities for use in various types of sensor system with economical efficiency and robustness in harsh environmental conditions. This sensor uses an electromagnetic resonance frequency or phase angle shift associated with a geometrical change of the sensor tag or an impedance change of the sensor. In this paper, a chipless passive wireless structural health monitoring (SHM) sensor is made using a frequency selective surface (FSS). The cross type FSS is introduced, and its SHM principle is explained. The electromagnetic characteristics of the FSS are simulated in terms of transmission and reflection coefficients using simulation software, and an experimental verification is conducted. The electromagnetic characteristic change of the FSS in the presence of mechanical strain or a structural crack is investigated by means of simulation and experiment. Since large-area structures can be covered by deploying FSS, it is possible to detect the location of any cracks.

Passive wireless structural health monitoring sensor made with a flexible planar dipole antenna

Cheap and efficient wireless sensors have been widely studied by electronics and communication technology development. In this paper, a flexible planar dipole antenna based passive wireless strain sensor has been investigated. The planar dipole antenna is designed for X band and made on a flexible polymer substrate using a conventional photolithography process. The fabricated dipole antenna is attached to a nonmetallic cantilever beam and monitors its bending strain. Mechanical strain and load impedance of the dipole antenna can change its resonance frequency, return loss and reflected signal. The return loss and reflected signals of the dipole antenna sensor are characterized by using a network analyzer. The strain sensitivity of the sensor is proportional to the return loss variation with the bending strain of the cantilever beam. The magnitude of reflected signals increases as the bending strain increases.

Electrical Breakdown Studies on Electro-active Paper

The concept of using cellulose as a smart material for preparing electro-active paper (EAPap) has been a land mark discovery, since it exhibits an impressive magnitude of actuation at relatively modest voltages. Considering the small thickness of cellulose samples (35 mum) and smaller depth of electrodes (100 nm), even at relatively low operating voltages (7 V), high electrical stresses can exist at pointed asperities on electrode surface, non-uniform locations in cellulose structure and at triple points joining electrode, cellulose and air. This paper reports the results of our AC and DC breakdown tests on cellulose samples carried out according to ASTM standards. The variation of breakdown strengths with relative humidity levels has been investigated and the results are explained with the aid of Maxwell-Wagner effect.

The effect of chitosan concentration on the electrical property of chitosan-blended cellulose electroactive paper

We studied the effect of chitosan blending on the electrical property of chitosan-blended cellulose electroactive paper (EAPap) under different humidity conditions. As the chitosan blending ratio increased, the real part of the dielectric constant of chitosan-blended cellulose EAPap increased while the dielectric loss factor decreased. From the curve fitting of the measured data using an electrode polarization model, it was found that increasing the chitosan ratio in the EAPap might promote a decrease in the relaxation time of the EAPap, resulting in an increase of the ion mobility and dc conductivity. Over 30% of the chitosan blending ratio, a gradual increment of the ion mobility of the EAPap was observed at 40% relative humidity, while a quadratic increment of the mobility was found at 60% relative humidity condition. This kind of ion-mobility-enhanced cellulose EAPap can be used not only for bending actuators but also for medical applications such as blood clotting patches.

Modeling of electromechanical behavior of chitosan-blended cellulose electroactive paper (EAPap)

Electromechanical bending actuation of chitosan-blended cellulose (CBC) electroactive paper (EAPap) was studied using a theoretical model, followed by an experimental comparison. The bending displacement of the model was calculated based on an ion traveling phenomenon and multilayered cantilever beam. By comparing the bending model and experimental data, we found that the bending model could predict the electromechanical actuation behavior as well as redistribution of ions inside of CBC EAPap under different humidity levels and electric fields. The bending actuation model of EAPap can be useful to investigate the electromechanical actuation behavior of EAPap devices such as artificial muscles, microrobots, and other various actuators.

Cellulose electro-active paper: actuator, sensor and beyond

Recently, cellulose has been discovered as a smart material that can be used as sensors and actuators. This newly discovered material is termed as electro-active paper (EAPap) that has merits in terms of lightweight, flexible, dryness, biodegradable, biocompatible, easy to chemically modify, cheap and abundance. The actuation principle of cellulose EAPap bending actuator is known to be a combination of piezoelectric effect and ion migration effect. This paper presents further investigation of cellulose EAPap for its possibilities in biomimetic actuator, sensor, MEMS, acoustic devices and others. Biomimetic actuator is made with cellulose EAPap by fabricating rectifying antenna (rectenna) array on it. Cellulose EAPap material is customized to satisfy the material requirement for actuators and other devices. The material improvement all about cellulose EAPap is introduced. To fabricate the rectenna array, micro patterning of metallic layer in conjunction with Schottky diode fabrication on the cellulose was made. The Schottky diode fabrication allows possibility of thin film transistor fabricated on a cellulose paper. Microwave power transmission is demonstrated by using rectenna arrays, which can be used for many applications. Some of the device fabrication along with brief demonstrations is illustrated.

Haptic device development based on electro static force of cellulose electro active paper

Haptic is one of well-considered device which is suitable for demanding virtual reality applications such as medical equipment, mobile devices, the online marketing and so on. Nowadays, many of concepts for haptic devices have been suggested to meet the demand of industries. Cellulose has received much attention as an emerging smart material, named as electro-active paper (EAPap). The EAPap is attractive for mobile haptic devices due to its unique characteristics in terms of low actuation power, suitability for thin devices and transparency. In this paper, we suggest a new concept of haptic actuator with the use of cellulose EAPap. Its performance is evaluated depending on various actuation conditions. As a result, cellulose electrostatic force actuator shows a large output displacement and fast response, which is suitable for mobile haptic devices.

Simulation studies of internal mechanisms in the static deflection of a cellulose electroactive paper actuator

Studies of voltage-induced deflections in electroactive paper EAPap have been carried out. On the experimental side, measurements of bias-dependent deflections and strain, water absorption as a function of time, and relative humidity were obtained for the cellulose EAPap actuator. In addition, model simulations have also been carried out to probe and quantify the role of the various internal mechanisms responsible for the deflection. Our simulation predictions yield good agreement with the measured deflection data for the EAPap. The modeling suggests that internal ion content and its migration, water absorption leading to a nonuniform permittivity, random variations in the transverse piezoelectric-coupling coefficient d31,i, and the modulus of elasticity all collectively contribute to the EAPap deflection electrophysics. It also appears that higher sensitivity, with a minimal bias dependence, could be achieved by deliberately adding ions during EAPap processing.