Lindong Zhai

Transparent and Flexible Cellulose Nanocrystal/Reduced Graphene Oxide Film for Proximity Sensing

The rapid development of touch screens as well as photoelectric sensors has stimulated the fabrication of reliable, convenient, and human-friendly devices. Other than sensors that detect physical touch or are based on pressure sensing, proximity sensors offer controlled sensibility without physical contact. In this work we present a transparent and eco-friendly sensor made through layer-by-layer spraying of modified graphene oxide filled cellulose nanocrystals on lithographic patterns of interdigitated electrodes on polymer substrates, which help to realize the precise location of approaching objects. Stable and reproducible signals generated by keeping the finger in close proximity to the sensor can be controlled by humidity, temperature, and the distance and number of sprayed layers. The chemical modification and reduction of the graphene oxide/cellulose crystal composite and its excellent nanostructure enable the development of proximity sensors with faster response and higher sensitivity, the integration of which resolves nearly all of the technological issues imposed on optoelectronic sensing devices.

Preparation and characterization of hydrogels from polyvinyl alcohol and cellulose and their electroactive behavior

ABSTRACT This paper reports polyvinyl alcohol/cellulose-based electroactive hydrogels for actuator applications. The polyvinyl alcohol/cellulose electroactive hydrogels were obtained by physical crosslinking of polyvinyl alcohol and cellulose. The formation of the polyvinyl alcohol/cellulose hydrogel structure, its thermal stability, crystallinity, and mechanical properties were studied by using the Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, and mechanical test. The studies reveal that the cellulose is uniformly reacted with hydroxyl groups of polyvinyl alcohol by intermolecular bond formation. The cellulose content along with actuation voltage and frequency of electroactive hydrogels influence their displacement behavior.

Electrode effects of a cellulose-based electro-active paper energy harvester

The possibility of cellulose-based electro-active paper (EAPap) as a vibrational energy transducer was investigated in this paper. Thin cellulose EAPap film specimens were prepared by the regenerating process. Three different metal electrodes of gold, silver and aluminum were deposited on a 50 ? 50?mm2 cellulose film using a thermal evaporator. An aluminum cantilever beam was used as a vibrational bender and EAPap was attached close to the root of the cantilever beam. The voltage output of the EAPap was measured under harmonic base excitation of the cantilever beam. The EAPap with aluminum electrode provided the largest open circuit voltage output compared to those with gold or silver electrodes. The output voltages of the EAPap increased linearly with increase of the area of the electrodes. The output voltages also increased with increasing input acceleration but became saturated at a certain magnitude. From the experimental results, we conclude that EAPap with metal electrodes can be used as a flexible energy harvesting transducer by external mechanical stress, and the output voltage is related to the electrode material due to its work function.

The effects of width reduction on the damping of a cantilever beam and its application in increasing the harvesting power of piezoelectric energy harvester

Previous work shows that when a cantilever piezoelectric energy harvester with a given width is split into several pieces and then electrically connected in parallel, the output power increases substantially compared with when it acts in a single piece with a similar total width. It was hypothesized that this increase is due to the reduction in the damping of the width-reduced beam. As a result, the beam with the smaller width vibrates with higher amplitudes and therefore has higher energy harvesting capability. In this paper, this hypothesis is examined by measuring the damping of the cantilever beam as its width is reduced. It is shown that as the width decreases, the damping is reduced, which contributes to the increase in the harvested power. It is then shown that the harvested energy from an array of cantilever piezobeams with a certain total width is higher than that from a single-piece harvester of similar width.

Flexible and transparent strain sensor made with silver nanowire–coated cellulose

Simple and versatile method of layer-by-layer deposition is used to coat silver nanowire on a cellulose film to fabricate a flexible and transparent strain sensor. Strain-sensing behaviors of such a simply fabricated cellulose film are analyzed in both stretching and bending modes. When 0.01 wt% silver nanowire is coated on the cellulose film, 70% transmittance is maintained with 2.4 kΩ/sq of sheet resistance, which is applicable for transparent electrode of the strain sensor. Conductivity of the transparent electrode is maintained after mechanical stretching, which demonstrates that the silver nanowire coating is securely adhered on the surface of cellulose film. The strain sensor shows high strain sensitivity and good gauge factor maintaining good transparency at low silver nanowire concentration, which might be associated with the tunneling resistance change in the silver nanowire. The morphology of the silver nanowire–coated cellulose strain sensor is investigated using an atomic force microscopy with an increase in silver nanowire concentration.

Cellulose long fibers fabricated from cellulose nanofibers and its strong and tough characteristics

Cellulose nanofiber (CNF) with high crystallinity has great mechanical stiffness and strength. However, its length is too short to be used for fibers of environmentally friendly structural composites. This paper presents a fabrication process of cellulose long fiber from CNF suspension by spinning, stretching and drying. Isolation of CNF from the hardwood pulp is done by using (2, 2, 6, 6-tetramethylpiperidine-1-yl) oxidanyl (TEMPO) oxidation. The effect of spinning speed and stretching ratio on mechanical properties of the fabricated fibers are investigated. The modulus of the fabricated fibers increases with the spinning speed as well as the stretching ratio because of the orientation of CNFs. The fabricated long fiber exhibits the maximum tensile modulus of 23.9 GPa with the maximum tensile strength of 383.3 MPa. Moreover, the fabricated long fiber exhibits high strain at break, which indicates high toughness. The results indicate that strong and tough cellulose long fiber can be produced by using ionic crosslinking, controlling spinning speed, stretching and drying.

Electroactive and Optically Adaptive Bionanocomposite for Reconfigurable Microlens

This paper introduces an electroactive bionanocomposite based on poly(diethylene glycol adipate) (PDEGA) and cellulose nanocrystals (CNCs). The bionanocomposites were made using CNCs extracted from cotton and by optimizing its concentration in terms of the optical transmittance and viscosity. The characteristic properties of the materials were analyzed using contact angle measurements and Fourier transformation infrared spectra. Using the PDEGA/CNC bionanocomposite at a very low concentration of CNCs, a configurable lens having a robust, self-contained tunable optical structure was developed. The shape and curvature of the soft PDEGA/CNC device were controlled by applying voltage, and the focal length was measured. The simple structure, high optical transparency, biodegradability, thermal stability, high durability, and low power consumption make the new material particularly useful in fabricating a reconfigurable lens for future electronic and optical devices.

Flexible cellulose and ZnO hybrid nanocomposite and its UV sensing characteristics

Abstract This paper reports the synthesis and UV sensing characteristics of a cellulose and ZnO hybrid nanocomposite (CEZOHN) prepared by exploiting the synergetic effects of ZnO functionality and the renewability of cellulose. Vertically aligned ZnO nanorods were grown well on a flexible cellulose film by direct ZnO seeding and hydrothermal growing processes. The ZnO nanorods have the wurtzite structure and an aspect ratio of 9 ~ 11. Photoresponse of the prepared CEZOHN was evaluated by measuring photocurrent under UV illumination. CEZOHN shows bi-directional, linear and fast photoresponse as a function of UV intensity. Electrode materials, light sources, repeatability, durability and flexibility of the prepared CEZOHN were tested and the photocurrent generation mechanism is discussed. The silver nanowire coating used for electrodes on CEZOHN is compatible with a transparent UV sensor. The prepared CEZOHN is flexible, transparent and biocompatible, and hence can be used for flexible and wearable UV sensors.

Cellulose electro-active paper fabricated by facile solvent exchange pretreatment and its physical and electromechanical properties

AbstractThe cellulose electro-active paper (EAPap) has been discovered as a smart material that can be used for sensors, actuators and flexible electronics. EAPap is made from regenerated cellulose that requires dissolving cellulose pulp using solvents with a heating process. However the heating process in dissolving cellulose is very critical to the performance of EAPap. In this paper, we introduce a new fabrication process of cellulose EAPap with a pretreatment process of cellulose, which simplifies the fabrication process and does not require the heating process. The pretreatment process includes the first immersion of cellulose into deionized water and the second immersion it into N,N-dimethylacetamide (DMAc) solvent, followed by twice repeat of the immersions. The pretreated cellulose was dissolved using lithium chloride (LiCl)/DMAc solvent without heating, which gives a cellulose solution. The pretreated cellulose solution was compared with another cellulose solution made with the LiCl/DMAc solvent and a heating process in terms of viscosity and solubility. Cellulose EAPap samples were made by using two cellulose solutions and their physical and electromechanical properties were compared in terms of transparency, ion concentration, surface morphology, Young’s modulus, dielectric constant and piezoelectric charge constant. The pretreatment of cellulose results in similar or better physical properties of EAPap, which simplifies the cellulose EAPap fabrication.

Green all-cellulose nanocomposites made with cellulose nanofibers reinforced in dissolved cellulose matrix without heat treatment

Green all-cellulose nanocomposites were fabricated by adding reinforcing cellulose nanofiber (CNF) to a matrix of dissolved cellulose. CNFs were isolated from one dried native hardwood bleached Kraft pulp and office waste recycled deinked copy/printing paper (DIP) by using the TEMPO oxidation method. The cellulose was dissolved by using DIP and DMAc/LiCl solvent without heat treatment and solvent exchange to form a matrix of the all-cellulose nanocomposites. The DIP was not only selected for CNF isolation, but also for the cellulose matrix. The isolated CNFs and the all-cellulose nanocomposites were characterized by atomic force microscopy, thermogravimetry–differential thermal analysis, X-ray diffraction and mechanical tensile testing. The green all-cellulose nanocomposites made without heat treatment offered better thermal stability, crystallinity and mechanical properties than the heat treated ones. CNFs isolated from two resources show similar reinforcement capacity in all-cellulose nanocomposites. All-cellulose nanocomposite fabrication by dissolving cellulose without heat treatment and solvent exchange is a simple way that saves energy and chemicals.