Electrical Percolation Network Research Articles

Laser sintering and patterning of gallium-doped zinc oxide/indium-tin oxide nanoparticle films with tailorable electrical and optical properties

Gallium-doped zinc oxide (GZO) and tin-doped indium oxide (ITO) nanoparticles (NPs) were combined to create bi-component suspensions for the drop-casting and CO2-laser sintering of transparent conducting oxide (TCO) thin films with significantly reduced ITO content. An aqueous dispersion of ITO NPs enabled the suspension of GZO NPs without surfactants. Transmission electron microscopy indicated the formation of high aspect-ratio segments of ITO NPs from the suspension through oriented attachment, that persisted in the deposited and sintered thin films to establish an efficient electrical percolating network within the less conductive GZO NP matrix. Rapid CO2-laser sintering under argon gas of approximately 800 nm thick NP films yielded resistivities of 7.34 × 10−3 Ω·cm and 116 Ω·cm for pure ITO and pure GZO respectively. However, a bi-component film with only 19.6 at.% indium (relative to zinc) achieved a resistivity of 3.21 × 10−1 Ω·cm. By changing the ITO content, the near-infrared transmittance could be adjusted between 13% and 82% and the optical bandgap energy between 3.93 and 3.33 eV, enabling fine-tuning of the properties. Finally, a fast and material/energy efficient processing route was demonstrated for the fabrication of a GZO-ITO circuit pattern using CO2-laser patterning of a mask and CO2-laser sintering of the NP films.

Influence of Different Nano-Structured Fillers on the Performance of Epoxy Nanocomposites

Nowadays, multi-functional materials are strongly needed to meet the requirements of next-generation electronic devices. In this work, two different nanostructured fillers, reduced graphene oxide (RGO) and nanoalumina, were chosen to study their effect on the thermal, electrical and mechanical properties of the prepared epoxy composites at different loadings (0.5 to 2 wt%). RGO was firstly prepared and characterized by XRD, Raman spectroscopy and TEM confirming its production. The results revealed that RGO showed excellent adhesion with the polymer. Whilst, alumina aggregated and debonded from the matrix, as confirmed by SEM images. Hence, at only 2 wt%, RGO/epoxy composites exhibited the highest thermal conductivity (0.391 W/m-K), which was 1.96 times higher than the neat epoxy. Whereas, the alumina/epoxy composites showed lower increment at the same loading (0.206 W/m-K). However, at 2 wt% RGO, electrical percolation networks had been formed across the matrix (DC conductivity = 2×10-7 S/cm). While, epoxy filled with alumina remained insulative at any loading (~ 10-12 S/cm at 100 Hz). Besides, the tensile strength of the composites was improved by 75% and 37% when filled with 0.5 wt% RGO and alumina, respectively. These results are very useful for preparing multi-functional polymeric materials, which are critically required for packaging industries.

Heating of Ti3C2Tx MXene/polymer composites in response to Radio Frequency fields

Here we report for the first time that Ti3C2Tx/polymer composite films rapidly heat when exposed to low-power radio frequency fields. Ti3C2Tx MXenes possess a high dielectric loss tangent, which is correlated with this rapid heating under electromagnetic fields. Thermal imaging confirms that these structures are capable of extraordinary heating rates (as high as 303 K/s) that are frequency- and concentration-dependent. At high loading (and high conductivity), Ti3C2Tx MXene composites do not heat under RF fields due to reflection of electromagnetic waves, whereas composites with low conductivity do not heat due to the lack of an electrical percolating network. Composites with an intermediate loading and a conductivity between 10–1000 S m−1 rapidly generate heat under RF fields. This finding unlocks a new property of Ti3C2Tx MXenes and a new material for potential RF-based applications.

Gas Responsive Nanoswitch: Copper Oxide Composite for Highly Selective H2S Detection

AbstractA nanocomposite material based on copper(II) oxide (CuO) and its utilization as a highly selective and stable gas‐responsive electrical switch for hydrogen sulphide (H2S) detection is presented. The material can be applied as a sensitive layer for H2S monitoring, e.g., in biogas gas plants. CuO nanoparticles are embedded in a rigid, nanoporous silica (SiO2) matrix to form an electrical percolating network of low conducting CuO and, upon exposure to H2S, highly conducting copper(II) sulphide (CuS) particles. By steric hindrance due to the silica pore walls, the structure of the network is maintained even though the reversible reaction of CuO to CuS is accompanied by significant volume expansion. The conducting state of the percolating network can be controlled by a variety of parameters, such as temperature, electrode layout, and network topology of the porous silica matrix. The latter means that this new type of sensing material has a structure‐encoded detection limit for H2S, which offers new application opportunities. The fabrication process of the mesoporous CuO@SiO2composite as well as the sensor design and characteristics are described in detail. In addition, theoretical modeling of the percolation effect by Monte‐Carlo simulations yields deeper insight into the underlying percolation mechanism and the observed response characteristics.

Shape memory conductive electrospun PVA/MWCNT nanocomposite aerogels

Shape memory polymers have promising applications in different fields. In this work, polyvinyl alcohol/multiwall carbon nanotube nanocomposite aerogels were fabricated through supercritical CO2 drying of electrospun hydrogel counterparts. Mercury porosimetry measurements and scanning electron microscopy results indicated that the nanofibrous aerogel had a high porosity (more than 80%), low density (0.025–0.036 g cm−3), high surface area (more than 1700 m2 g−1) and a pore size less than 100 nm. By incorporating multiwall carbon nanotubes, the density and Young’s modulus of the polyvinyl alcohol/multiwall carbon nanotube nanocomposite aerogel were increased up to 0.03 g cm−3 and 38 MPa, respectively. A percolation threshold of 1.5 wt% of the multiwall carbon nanotube was achieved for the conductive polyvinyl alcohol/multiwall carbon nanotube nanocomposite aerogel. The nanofibrous aerogel showed a perfect shape memory effect under thermal stimulus, and the corresponding results revealed that the shape recovery ratio and recovery speed of the nanocomposite aerogel were significantly higher than pure polyvinyl alcohol aerogel owing to the stabilizing of the crystalline domains and improvements in mechanical properties due to the presence of the multiwall carbon nanotubes. PVA/MWCNT nanocomposite aerogels were fabricated via the ScCO2 drying method. The aerogels were fully characterized, and their shape memory behaviors were examined. As the MWCNT content was increased, the electrical conductivity increased abruptly by up to 4 orders of magnitude, implying that an electrical percolation network had formed in the nanocomposite aerogels. The shape memory results illustrated the shape recovery ratio, and the recovery speed of the nanocomposite aerogels increased, due to the addition of the MWCNTs.

Electrical dual-percolation in MWCNTs/SBS/PVDF based thermoplastic elastomer (TPE) composites and the effect of mechanical stretching

Dielectric thermoplastic elastomers (TPEs) offer a number of advantages over traditional dielectric elastomers or rubbers in terms of tailorable mechanical and electrical properties, higher mechanical strain, and ease of processing and shaping. Such a combination of properties has attracted increasing attention in flexible energy harvesting and storage applications. The combination of styrene–butadiene-styrene (SBS) and poly(vinylidene fluoride) (PVDF) has the potential to provide a combination of high elongation to break and increased relative permittivity, however the immiscibility between SBS and PVDF results in polymer blends with poor stretchability and processing properties. In this work, a dual percolated structure was created in a thermoplastic elastomer of SBS/PVDF (50/50 wt%), by coupling EVA as a compatibiliser for SBS/PVDF with multi-walled carbon nanotubes (MWCNTs) as a conductive filler that created an electrical percolation network. The elongation at break of SBS/PVDF was significantly enhanced by adding 20 wt% of EVA due to the reduced phase dimensions and enhanced interfacial adhesion. The addition of MWCNTs enabled the formation of a percolated network at 1 wt% in the SBS phase, followed by a second percolation at 3 wt% in both PVDF and SBS phases. The relative permittivity of the composite increased to 22.5 at 1 wt% MWCNT with a tan δ of 0.5, and further increased to 34.9 for a 2 wt% of MWCNT concentration while the tan δ remained constant. In-situ electrical testing for the SBS/PVDF thermoplastic elastomer under strain showed that, at 1 wt% MWCNT, the non-percolated PVDF islands acted as variable capacitors whose capacitance increased with degree of stretching. For the dual percolated structure formed at 3 wt% MWCNT, the capacitance and conductivity of the composites were unaffected up to 30% strain. The high relative permittivity and strains of over 100% means that the SBS/PVDF based thermoplastic elastomer is readily suitable for vibration control sensors, variable impedance devices, energy harvesters and artificial muscles and actuators.

A Superior Method for Constructing Electrical Percolation Network of Nanocomposite Fibers: In Situ Thermally Reduced Silver Nanoparticles.

Nanocomposite fibers, composed of conductive nanoparticles and polymer matrix, are crucial for wearable electronics. However, the nanoparticle mixing approach results in aggregation and dispersion problems. A revolutionary synthesis method by premixing silver precursor ions (silver ammonium acetate) with polyvinyl alcohol is reported here. The solvation of ions-prevented aggregation, and uniformly distributed silver nanoparticles (in situ AgNPs, 77 nm) are formed after thermal reduction (155 °C) without using additional reducing or dispersion agents. The conductive fiber is synthesized by the wet spinning technology. After careful optimization, flower-shaped silver nanoparticles (AgNFs, 350-450 nm) are also employed as cofillers. The addition of in situ AgNPs (9.5 vol%) to AgNFs (30 vol%) increases electrical conductivity by 1434% (2090 to 32 064 S cm-1 ) through the efficient construction of percolation networks. The in situ AgNPs provide significantly higher conductivity compared with other secondary nanoparticle fillers. The gaseous byproducts dramatically increase flexibility with a moderate compromise in tensile strength (55 MPa). The particle-free ion-level uniform mixing of silver precursors, followed by in situ reduction, would be a fundamental paradigm shift in nanocomposite synthesis.

Ultralow electrical percolation in melt-compounded polymer composites based on chemically expanded graphite

It has been difficult to construct electrical percolation networks in melt-compounded polymer composites due to shear-induced network destruction, especially for low graphene content composites. To overcome this issue, here we employ chemically expanded graphite (CEG) to construct conductive networks in which the polymer molecules would penetrate into the inner part of CEG to form a polymer entrapped in CEG structure. This novel polymer entrapped in CEG structure is very useful for the effective charge transfer and further construction of conductive networks in the process of melt compounding. The final polymer composites exhibit an ultralow electrical conductive percolation threshold of 0.29 vol% after melt compounding which is almost among the lowest level compared with those of other melt-compounded polymer composites. Besides, the thermal conductivity of the composites is also significantly enhanced. This strategy here has provided a new way to maintain the conductive networks and decrease the electrical percolation threshold in the melt-compounded polymer composites through constructing polymer entrapped in conductive fillers structure.

Bridging electrode gaps with conducting polymers around the electrical percolation threshold

The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is grown electrochemically using cyclic voltammetry between the gaps of interdigitated Au electrodes with separations of 10, 20, and 50μm. Three electrical conductivity regimes are identified through resistance measurements and SEM imaging. The first is the insulating region where there are no complete conducting pathways between the electrodes. The second is the percolation region where a few localised conducting polymer bridges have formed. The third is the thin film region where a continuous conducting polymer film connects the electrodes. The demonstration of the ability to reliably generate conducting polymer electrical percolation networks is important for their future use in high sensitivity devices.

Importance of microstructure and interface in designing metal oxide nanocomposites for supercapacitor electrodes

In this work, we demonstrate that designing metal oxide nanocomposite with an optimized nanocarbon to metal oxide ratio is required to realize the best electrochemical performance. Using carbon nano-onion (CNO) and TiO2 composite as a model system, it is evidenced that the formation of interfacial structure and the existence of electrical percolation networks are essential to enhance electrical performance of the composite. When the aforementioned conditions are satisfied, CNO/TiO2 hybrid nanocomposites (CTHNs) with only 11wt% TiO2 loading exhibits excellent electrochemical performance. The maximum specific capacitance obtained for this composite electrode is 386Fg−1 at a scan rate of 5mVs−1, which is the highest reported value till date in a TiO2 composite electrode measured in a two electrode cell configuration. At this composition, excellent electrochemical stability is obtained with only 5% capacitance degradation over 2000cycles. As the TiO2 loading increases the electrochemical performance decreases.

Effect of Sorted Single-Walled Carbon Nanotubes on Rate Capability of Lithium-Ion Battery Cathodes

Lithium iron phosphate (LFP) is a promising cathode material for Lithium-ion batteries. LFP has many unique properties including great thermal stability, high energy density, and excellent cycle life. However, one disadvantage to LFP is its poor electrical conductivity which leads to low rate capability (C-rate). To overcome this disadvantage, we have incorporated single-walled carbon nanotubes (SWCNTs) into the electrodes of LFP batteries. SWCNTs have emerged as one of the leading additives for LFP batteries due to their ability to enhance electrode conductivity and mechanical stability. The incorporation of SWCNTs create a better electrical percolation network thereby enhancing the C-rate. To this end, LFP coin-cell batteries were fabricated with the addition of sorted metallic and semiconducting SWCNTs. C-rate and cycle stability measurements were made for comparison studies. Test showed that LFP with SWCNTs incorporated into the cathode material led to a modest improvement to the C-rate while good cycle stability was maintained.

Joule effect self-heating of epoxy composites reinforced with graphitic nanofillers

Self-heating of conductive nanofilled resins due to the Joule effect is interesting for numerous applications, including computing, self-reparation, self-post-curing treatment of resins, fabrication of adhesive joints, de-icing coatings and so on. In this work, we study the effect of the nature and amount of graphitic nanofiller on the self-heating of epoxy composites. The addition of graphitic nanofillers induced an increase in the thermal conductivity of the epoxy resins, directly proportional to the nanofiller content. Percolation was not observed because of the heat transport through phonons. In contrast, the electrical conductivity curves present a clear percolation threshold, due to the necessity of an electrical percolation network. The electrical threshold is much lower for composites reinforced with carbon nanotubes (CNTs, 0.1 wt.%) than for the resin filled with graphene nanoplatelets (GNPs, 5 %). This fact is due to their very different specific areas. The composites filled with CNTs reach higher temperatures than the ones reinforced with GNPs, applying low electrical voltage because of their higher electrical conductivity. In contrast, the self-heating is more homogeneous for the GNP/epoxy resins due to their higher thermal conductivity. It was also confirmed that the self-heating is repetitive in several cycles, reaching the same temperature when the same voltage is applied.

Improved densification and mechanical properties of spark plasma sintered carbon nanotube reinforced alumina ceramics

Dense magnesium oxide (MgO) doped multiwalled carbon nanotube (MWCNT) reinforced alumina (Al2O3) nanocomposites were fabricated using spark plasma sintering (SPS). Sintered nanocomposites possessed refined microstructure due to the presence of uniformly dispersed CNTs and ability of MgO to increase densification rate before onset of abnormal grain growth. Williamson-Hall analyses of XRD patterns indicated that matrix crystallite size (LC) and lattice micro-strain (εC) of the nanocomposites decreased by ∼40% and >30%, respectively, than those of pure Al2O3 (LC ≈ 75 nm, εC = 1.54 × 10−3). Present investigation also depicted the suitability of CNT addition in Al2O3 towards achieving higher density of nanocomposites using low temperature (1300 °C) SPS. Addition of CNT (especially, at ≥0.6 vol.%) in highly electrically insulating matrix established an electrical percolating network that helped in local heating of matrix particles during SPS and led to higher densification. The highest changes in indentation fracture toughness at 1 kgf and wear rate at 20N normal load were obtained at only 0.6 vol.% MWCNT loading which were >22% higher and 35% lower, respectively, compared to pure Al2O3.

Modelling the electro-mechanical properties of PPy/epoxy conductive composites

Modern trends in materials dedicated for aircraft applications are focused on development of highly multifunctional materials that allow not only to carry mechanical loading, but also to possess several other types of functionality, e.g. be thermal resistant, electrically conductive, etc. The material of such a type, with a possibility of conducting electrical current without loss of mechanical properties is the main topic of presented research results. The work presented here deals with the determination of the overall stochastic mechanical and electrical properties of a blend of polypyrrole (PPy) and epoxy. The material properties are obtained applying appropriate periodic boundary conditions (PBCs) to the representative volume elements (RVEs) of the material nano–structure. A strategy to model numerically the electrical percolation network in the framework of Finite Element Method (FEM) is proposed and discussed. Electrical and mechanical properties of the material are calculated as a function of the filler volume fraction. Finally, the numerical results are compared with experiments found in literature and an overall good agreement is found.

Rapid synthesis of ultra-long silver nanowires for tailor-made transparent conductive electrodes: proof of concept in organic solar cells

Rapid synthesis of ultralong silver nanowires (AgNWs) has been obtained using a one-pot polyol-mediated synthetic procedure. The AgNWs have been prepared from the base materials in less than one hour with nanowire lengths reaching 195 μm, which represents the quickest synthesis and one of the highest reported aspect ratios to date. These results have been achieved through a joint analysis of all reaction parameters, which represents a clear progress beyond the state of the art. Dispersions of the AgNWs have been used to prepare thin, flexible, transparent and conducting films using spray coating. Due to the higher aspect ratio, an improved electrical percolation network is observed. This allows a low sheet resistance (RS = 20.2 Ω/sq), whilst maintaining high optical film transparency (T = 94.7%), driving to the highest reported figure-of-merit (FoM = 338). Owing to the light-scattering influence of the AgNWs, the density of the AgNW network can also be varied to enable controllability of the optical haze through the sample. Based on the identification of the optimal haze value, organic photovoltaics (OPVs) have been fabricated using the AgNWs as the transparent electrode and have been benchmarked against indium tin oxide (ITO) electrodes. Overall, the performance of OPVs made using AgNWs sees a small decrease in power conversion efficiency (PCE), primarily due to a fall in open-circuit voltage (50 mV). This work indicates that AgNWs can provide a low cost, rapid and roll-to-roll compatible alternative to ITO in OPVs, with only a small compromise in PCE needed.

High performance benzoxazine/CNT nanohybrid network – An easy and scalable way to combine attractive properties

This work presents a scalable and solventless synthesis of a diamine based benzoxazine and the subsequent preparation of nanocomposites with pristine multiwalled carbon nanotubes (CNTs). The synthesized precursor showed an improved thermal stability upon curing temperature pushing aside the main drawback of benzoxazine resins. It exhibits excellent intrinsic properties and strong natural interactions with CNTs. The increase of thermomechanical stability was found to be dependent on the filler content with an optimum at 0.5wt.% of CNTs. An electrical percolating network was established with low CNT content, around 0.3wt.%. The volumic variation due to thermal degradation at high temperature was severely restricted in the presence of CNTs and was correlated to the formation of a percolating network acting as an internal reinforcing framework. The limiting oxygen indexes of nanocomposites were found to be remarkably high, above 38%, showing the potential to fabricate novel promising high performance materials. The resulting attractive properties were attributed to the strong interactions between CNTs and polybenzoxazine.

Monte Carlo simulation studies on the effect of entropic attraction on the electric conductivity in polymer nano-composites.

The effect of non-conductive nano-particles on the electrical percolating network formation and the electrical conductivity of conductive nano-particles in polymer matrices is investigated using Monte Carlo simulations and a percolation theory. Both conductive and non-conductive nano-particles are modeled as spheres but with different diameters. Non-conductive nano-particles are up to four times bigger than conductive nano-particles. Equilibrated configurations for mixtures of nano-particles are obtained via Monte Carlo simulations and are used to estimate the probability (P) of forming an electrical percolating network and the percolation threshold conductive nano-particle volume fraction (phi(c)). As the volume fraction (phi(nc)) of non-conductive nano-particles increases, phi(c) decreases significantly, thus increasing the electrical conductivity. When non-conductive nano-particles mix with conductive nano-particles, they make the effective interaction energy W(r) between conductive nano-particles attractive, which should facilitate the formation of the electrical percolating network. For a given phi(nc), phi(c) increases slightly with an increase in the non-conductive nano-particle diameter (sigma(nc)). We also carry out simulations with non-conductive nano-particles of different structures and find that phi(c) is relatively insensitive to the non-conductive nano-particle structure.

Multifunctional polyimide/graphene oxide composites via in situ polymerization

ABSTRACTIn this article, we detail an effective way to improve electrical, thermal, and gas barrier properties using a simple processing method for polymer composites. Graphene oxide (GO) prepared with graphite using a modified Hummers method was used as a nanofiller for r‐GO/PI composites by in situ polymerization. PI composites with different loadings of GO were prepared by the thermal imidization of polyamic acid (PAA)/GO. This method greatly improved the electrical properties of the r‐GO/PI composites compared with pure PI due to the electrical percolation networks of reduced graphene oxide within the films. The conductivity of r‐GO/PI composites (30:70 w/w) equaled 1.1 × 101 S m−1, roughly 1014 times that of pure PI and the oxygen transmission rate (OTR, 30:70 w/w) was reduced by about 93%. The Young's modulus of the r‐GO/PI composite film containing 30 wt % GO increased to 4.2 GPa, which was an approximate improvement of 282% compared with pure PI film. The corresponding strength and the elongation at break decreased to 70.0 MPa and 2.2%, respectively. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40177.

Electromagnetic interference shielding effectiveness of MWCNT filled poly(ether sulfone) and poly(ether imide) nanocomposites

Multiwalled carbon nanotube (MWCNT) filled poly(ether sulfone) (PES) and poly(ether imide) (PEI) composites were prepared with different MWCNT weight fractions (0.5–5wt%) by a solution mixing technique. Their electrical conductivities, electromagnetic interference (EMI), shielding effectiveness (SE), return loss (RL), and absorption loss (AL) were investigated. Morphologies of the fracture surfaces of nanocomposites studied by scanning electron and transmission electron microscopy showed relatively good MWCNT dispersion and distribution. The electrical conductivity of compression molded samples measured at room temperature indicated that the electrical percolation network was achieved already at 0.5% loading. The measurements of shielding effectiveness (SE) carried out in the frequency range of 8 to 12 GHz (X-band range) showed that SE increases with measurement frequency and with filler loading, whereby no significant differences could be observed between PES and PEI as matrices. The nanocomposites based on both matrices with 5 wt% loading of MWCNT exhibited shielding levels at 8 GHz between 42 and 45 dB in comparison with the pure polymers which showed value in the range of 1 to 2 dB. RL and AL showed significantly lower values for the composites as compared to unfilled polymers, but no systematic trends were observed on frequency. POLYM. ENG. SCI., 54:2560–2570, 2014. © 2013 Society of Plastics Engineers

Synthesis and characterization of polybenzoxazole/graphene oxide composites via in situ polymerization

In this study, poly(amic acid) was prepared via a polycondensation reaction of 3,3'-dihydroxybenzidine and pyromellitic dianhydride in an N-methyl-2-pyrrolidone solution; reduced graphene oxide/polybenzoxazole (r-GO/PBO) composite films, which significantly increased the electrical conductivity, were successfully fabricated. GO was prepared from graphite using Brodie's method. The GO was used as nanofillers for the preparation of r-GO/PBO composites through an in situ polymerization. The addition of 50 wt% GO led to a significant increase in the electrical conductivity of the composite films by more than sixteen orders of magnitude compared with that of pure PBO films as a result of the electrical percolation networks in the r-GO during the thermal treatment at various temperatures within the films.