The 500 MHz to 5.50 GHz complex permittivity spectra of single-wall carbon nanotube-loaded polymer composites

High Permittivity Polymer Composites on the Basis of Long Single-Walled Carbon Nanotubes: The Role of the Nanotube Length

Controlling the permittivity of dielectric composites is critical for numerous applications dealing with matter/electromagnetic radiation interaction. In this study, we have prepared polymer composites, based on a silicone elastomer matrix and Tuball carbon nanotubes (CNT) via a simple preparation procedure. The as-prepared composites demonstrated record-high dielectric permittivity both in the low-frequency range (102−107 Hz) and in the X-band (8.2−12.4 GHz), significantly exceeding the literature data for such types of composite materials at similar CNT content. Thus, with the 2 wt% filler loading, the permittivity values reach 360 at 106 Hz and >26 in the entire X-band. In similar literature, even the use of conductive polymer hosts and various highly conductive additives had not resulted in such high permittivity values. We attribute this phenomenon to specific structural features of the used Tuball nanotubes, namely their length and ability to form in the polymer matrix percolating network in the form of neuron-shaped clusters. The low cost and large production volumes of Tuball nanotubes, as well as the ease of the composite preparation procedure open the doors for production of cost-efficient, low weight and flexible composites with superior high permittivity.

Behaviors of Electromagnetic Wave Propagation in Double-Walled Carbon Nanotubes

In this study, behaviors of electromagnetic wave propagation in a double-walled carbon nanotube (DWCNT) are investigated theoretically. For this purpose, the effects of carbon nanotube’s inner and outer tubes’ material property parameters (μ, ε) on electromagnetic wave propagation are discussed. The effects of interaction between the carbon nanotube’s inner and outer tubes on the electromagnetic wave propagation are defined. Nonlocal effects of the DWCNT on electromagnetic wave propagation are examined. Besides, the electromagnetic wave propagation frequencies are specifically investigated, taking the DWCNT’s nonlocal effects and material property parameters (ε, µ) into account. When the wavenumber, k, is greater than 1.8 × 10¹⁰, the frequencies of the fundamental mode and the second mode converge to 3.554 × 10⁸ Hz. Additionally, the electromagnetic wave propagation frequencies decrease with the increase of the DWCNT’s nonlocal parameter (ν) and decrease with material parameter (D).

Redox-Responsive Nanobiomaterials-Based Therapeutics for Neurodegenerative Diseases

Redox regulation has recently been proposed as a critical intracellular mechanism affecting cell survival, proliferation, and differentiation. Redox homeostasis has also been implicated in a variety of degenerative neurological disorders such as Parkinson's and Alzheimer's disease. In fact, it is hypothesized that markers of oxidative stress precede pathologic lesions in Alzheimer's disease and other neurodegenerative diseases. Several therapeutic approaches have been suggested so far to improve the endogenous defense against oxidative stress and its harmful effects. Among such approaches, the use of artificial antioxidant systems has gained increased popularity as an effective strategy. Nanoscale drug delivery systems loaded with enzymes, bioinspired catalytic nanoparticles and other nanomaterials have emerged as promising candidates. The development of degradable hydrogels scaffolds with antioxidant effects could also enable scientists to positively influence cell fate. This current review summarizes nanobiomaterial-based approaches for redox regulation and their potential applications as central nervous system neurodegenerative disease treatments.

Microwave Absorbing properties of metal functionalized-CNT-polymer composite for stealth applications

In this study, we report on the electrical properties of multi-wall carbon nanotubes (MWCNT) composites functionalized with metal or metal alloy oxides and embedded in a polyurethane matrix to develop a lightweight material for microwave absorption and shielding. The CNT nanoparticles are functionalized with metallic oxides such as Cobalt oxide, Iron oxide, and Cobalt Iron oxide, at three different concentrations. Metallic oxides are used at 5%, 10%, and 20% concentration of the total CNT percentage weight. The resulting functionalized CNT is mixed with polyurethane polymer at 5% wt of the total composite weight. Three sets of cylindrical samples are developed, and each set contains three different metal oxide concentrations. The dielectric properties of the nine developed samples are obtained by measuring their permittivity spectra using an open-ended coaxial probe technique in the spectral range 5-50 GHz. The absorption efficiency of the composites is then obtained by calculating the reflection loss at normal incidence. The results show that the spectral range of absorption can be tuned by changing the CNT concentration, and the material thickness. Functionalized CNT with different alloyed metal oxides enhanced the absorption efficiency of the polyurethane/CNT composites. Such functionalized composites can be used to replace the common heavyweight materials used for microwave applications.

Carbon nanotube scaffolds with controlled porosity as electromagnetic absorbing materials in the gigahertz range

Control of the microscopic structure of CNT nanocomposites allows modulation of the electromagnetic shielding in the gigahertz range. The porosity of CNT scaffolds has been controlled by two freezing protocols and a subsequent lyophilization step: fast freezing in liquid nitrogen and slow freezing at -20 °C. Mercury porosimetry shows that slowly frozen specimens present a more open pore size (100-150 μm) with a narrow distribution whereas specimens frozen rapidly show a smaller pore size and a heterogeneous distribution. 3D-scaffolds containing 3, 4, 6 and 7% CNT were infiltrated with epoxy and specimens with 2, 5 and 8 mm thicknesses were characterized in the GHz range. Samples with the highest pore size and porosity presented the lowest reflected power (about 30%) and the highest absorbed power (about 70%), which allows considering them as electromagnetic radiation absorbing materials.

Microwave pumped high-efficient thermoacoustic tumor therapy with single wall carbon nanotubes

The ultra-short pulse microwave could excite to the strong thermoacoustic (TA) shock wave and deeply penetrate in the biological tissues. Based on this, we developed a novel deep-seated tumor therapy modality with mitochondria-targeting single wall carbon nanotubes (SWNTs) as microwave absorbing agents, which act efficiently to convert ultra-short microwave energy into TA shock wave and selectively destroy the targeted mitochondria, thereby inducing apoptosis in cancer cells. After the treatment of SWNTs (40 μg/mL) and ultra-short microwave (40 Hz, 1 min), 77.5% of cancer cells were killed and the vast majority were caused by apoptosis that initiates from mitochondrial damage. The orthotopic liver cancer mice were established as deep-seated tumor model to investigate the anti-tumor effect of mitochondria-targeting TA therapy. The results suggested that TA therapy could effectively inhibit the tumor growth without any observable side effects, while it was difficult to achieve with photothermal or photoacoustic therapy. These discoveries implied the potential application of TA therapy in deep-seated tumor models and should be further tested for development into a promising therapeutic modality for cancer treatment.

Impact of foaming on the broadband dielectric properties of multi-walled carbon nanotube/polystyrene composites

This study investigated the impact of foaming on the broadband dielectric properties of multi-walled carbon nanotube/polystyrene (MWCNT/PS) nanocomposites. Different carbon nanotube concentrations were prepared by blending of a 20 wt.% MWCNT/PS masterbatch and pure PS using a twin-screw extruder. A chemical blowing agent was used to foam the nanocomposites in a micro injection molding machine. Compression molding was applied to fabricate unfoamed nanocomposites for comparison purposes. Comparing the dielectric properties of unfoamed and foamed nanocomposites showed that foaming increased the percolation threshold, reduced DC and AC conductivities, widened the insulator–conductor transition window, and reduced the dissipation factor of the MWCNT/PS composites. These were attributed to deteriorated conductive network and inferior dispersion and distribution of MWCNTs coming from the presence of foam cells in the nanocomposites. The obtained results propose foaming as a promising technique to improve the dielectric properties of MWCNT/polymer composites.

Permittivity of dielectric composite materials comprising graphene nanoribbons. The effect of nanostructure

New lightweight, flexible dielectric composite materials were fabricated by the incorporation of several new carbon nanostructures into a dielectric host matrix. Both the permittivity and loss tangent values of the resulting composites were widely altered by varying the type and content of the conductive filler. The dielectric constant was tuned from moderate to very high values, while the corresponding loss tangent changed from ultralow to extremely high. The data exemplify that nanoscale changes in the structure of the conductive filler result in dramatic changes in the dielectric properties of composites. A microcapacitor model most explains the behavior of the dielectric composites.

Microstructure and dielectric properties of biocarbon nanofiber composites

A kind of web-like carbon with interconnected nanoribbons was fabricated using bacterial cellulose pyrolyzed at various temperatures, and the microwave dielectric properties were investigated. Bacterial cellulose was converted into carbonized bacterial cellulose (CBC) with a novel three-dimensional web built of entangled and interconnected cellulose ribbons when the carbonization temperature was below 1,200°C; the web-like structure was destroyed at a temperature of 1,400°C. Composites of CBC impregnated with paraffin wax exhibited high complex permittivity over a frequency range of 2 to 18 GHz, depending on the carbonization temperature. Both real and imaginary parts were the highest for CBC pyrolyzed at 1,200°C. The complex permittivity also strongly depended on CBC loadings. For 7.5 wt.% loading, the real and imaginary permittivities were about 12 and 4.3, respectively, and the minimum reflection loss was -39 dB at 10.9 GHz. For 30 wt.% loading, the real and imaginary permittivities were about 45 and 80, respectively, and the shielding efficiency was more than 24 dB in the measured frequency range and could be up to 39 dB at 18 GHz. The electromagnetic properties were assumed to correlate with both the dielectric relaxation and the novel web-like structure.

Broad-band conductivity and dielectric spectroscopy of composites of multiwalled carbon nanotubes and poly(ethylene terephthalate) around their low percolation threshold

Composites of multiwalled carbon nanotubes with poly(ethylene terephthalate) (PET-MWCNT) with up to 3 vol% MWCNTs were prepared and characterized by broad-band AC conductivity and dielectric spectroscopy up to the infrared range using several techniques. A very low electrical percolation threshold of 0.07 vol% MWCNTs was revealed from the low-frequency conductivity plateau as well as from DC conductivity, whose values show the same critical power dependence on MWCNT concentration with the exponent t = 4.3. Above the plateau, the AC conductivity increases with frequency up to the THz range, where it becomes overlapped with the absorption of vibrational modes. The temperature dependence down to ~5 K has shown semiconductor behaviour with a concentration-independent but weakly temperature-dependent small activation energy of ~3 meV. The behaviour is compatible with the previously suggested fluctuation-induced tunnelling conductivity model through a thin (~1 nm) polymer contact layer among the adjacent MWCNTs within percolated clusters. At higher frequencies, deviations from the simple universal conductivity behaviour are observed, indicating some distribution of energy barriers for an electron hopping mechanism.

COMPARATIVE STUDY OF MICROWAVE AND THERMAL CURING OF HIGH TEMPERATURE EPOXY/CARBON NANOTUBES POLYMER NANOCOMPOSITES AND THEIR PROPERTIES

The high temperature epoxy resin system was cured using microwave and regular oven heating in the presence and absence of multiwalled carbon nanotubes (CNTs) and their properties are studied. For this study, a high temperature EPON-862 resin is used as matrix and CNTs as reinforcement. The EPON-862-part-A resin is used was reinforced with 0.1 wt.%, 0.2 wt.% and 0.3 wt.% of CNTs using a high-intensity ultrasonic irradiation. The curing agent epicure W (part-B) was mixed with EPON-862 resin using a noncontact mixing method (Thinky, Japan). The reaction mixture was cured using a 2.45 GHz (Sharp BP210) microwave oven for only 10 min instead of 8 h for conventional oven heating (curing schedule is 4 h at 120°C, post curing at 170°C for 4 h). The compression tests were performed for all the sample including thermally cured and microwave-cured neat EPON-862, EPON-862/0.1 wt.% CNTs, EPON-862/0.2 wt.% CNTs and EPON-862/0.3 wt.% CNTs. Microwave-cured samples of EPON-862/0.2 wt.% CNTs showed 28.57% and 5.57% increase in compression modulus and compression strength, respectively, as compared to the neat EPON-826 regular oven heating.

Low-loss, high-permittivity composites made from graphene nanoribbons

A new composite material was prepared by incorporation of graphene nanoribbons into a dielectric host matrix. The composite possesses remarkably low loss at reasonably high permittivity values. By varying the content of the conductive filler, one can tune the loss and permittivity to desirable values over a wide range. The obtained data exemplifies how nanoscopic changes in the structure of conductive filler can affect macroscopic properties of composite material.

Radiofrequency interaction with conductive colloids: permittivity and electrical conductivity of single-wall carbon nanotubes in saline

Conductive nanoparticles may enhance tissue heating during radiofrequency (RF) irradiation. Specific absorption rate (SAR) is known to rise with the electrical conductivity of tissue. However, no studies to date have measured the relationship between complex permittivity and nanoparticle concentration in tissue-like samples. The complex permittivities of colloids containing single-wall carbon nanotubes (SWCNTs) in normal (0.9%) saline were measured from 20 MHz to 1 GHz. Carbon concentrations ranged from 0 to 93 mM (0.06% volume), based on SWCNT weight per volume. Measurements were made with 0.02% Pluronic F108 surfactant added to the colloids to prevent SWCNT flocculation. The data were fit to the Cole-Cole relaxation model with an added constant phase angle element to correct for electrode polarization effects at low RF frequencies. Electrode polarization effects increased with carbon concentration. The real parts of the permittivities of the colloids increased with carbon concentration. The static conductivity rose linearly with carbon concentration, doubling from 0 to 93 mM. The SAR of the colloids is expected to increase with RF frequency, based on the properties of the imaginary part of the permittivity.

Ionizing Radiation Effects on Interfaces in Carbon Nanotube-Polymer Composites

The purpose of this research was to probe nanotube-polymer composites for evidences of radiation induced chemistry at the interface of the host polymer and the nanotube structures. Single wall carbon nanotube (SWNT) / poly (methyl methacrylate) (PMMA) composites were fabricated and exposed to gamma radiation with a Co60 source at a dose rate of 1.28 X 106 rad/hour in an air environment for a total dose of 5.9 Mrads. Neat nanotube paper and neat PMMA were also exposed. Spun coat films of SWNT/PMMA were exposed to gamma radiation with a Ce157at a dose rate of 4.46 x 103 rad/hr for a total dose of 3.86 Mrads. Both irradiated and non-irradiated samples were compared. Glass transition temperatures were characterized by differential scanning calorimetry. Dynamic mechanical analysis and dielectric analysis evidenced changes in relaxations induced by irradiation. Irradiated composites exhibited radiation induced chemistry distinct from degradation effects noted in the pure polymer. Scanning electron microscopy provided images of the SWNTs and SWNT/PMMA interface before and after irradiation. This investigation imparts insight into the nature of radiation induced events in nanotubes and nanocomposites.

Explosive percolation in a nanotube-based system

Using the percolation theory, we study the underlying mechanism in the formation of single-walled nanotube bundles with uniform diameter. By applying the cluster repulsion process to stick percolation, we find that the transition becomes explosive. To understand the transition nature, we first investigate the scaling behavior of transition interval Δ. By comparing the results with loopless and loop-allowed bond percolations, we find that the loops crucially affect the scaling behavior of Δ, and Δ is not universal. Moreover, the scaling behavior of Δ for the present nanostick systems is the same as that for loopless bond percolation. For more systematic studies on the transition nature, we also measure the changes in order parameter during the stick removal process and show that there exists a hysteresis. The results more clearly show that the transition of the stick system with cluster repulsion is discontinuous.

Microwave response of magnetized hydrogen plasma in carbon nanotubes: multiple reflection effects

We derived simple sets of equations to describe the microwave response of the magnetized hydrogen plasma slab embedded inside carbon nanotubes, which were grown by iron-catalyzed high-pressure disproportionation. These equations, which are useful when interference effects due to multiple reflections between plasma film interfaces are small, were used to analyze the reflection, absorption, and transmission coefficients of the magnetized hydrogen plasma slab. A discussion on the effects of the continuously changing external magnetic field and hydrogen plasma parameters on the reflected power, absorbed power, and transmitted power in the system is presented.

Single-walled carbon nanotubes as a multimodal-thermoacoustic and photoacoustic-contrast agent

We have developed a novel carbon nanotube-based contrast agent for both thermoacoustic and photoacoustic tomography. In comparison to deionized water, single-walled carbon nanotubes exhibited more than twofold signal enhancement for thermoacoustic tomography at 3 GHz. In comparison to blood, they exhibited more than sixfold signal enhancement for photoacoustic tomography at 1064 nm wavelength. The large contrast enhancement of single-walled carbon nanotubes was further corroborated by tissue phantom imaging studies.

Improved Electromagnetic Interference Shielding Properties of MWCNT-PMMA Composites Using Layered Structures

Electromagnetic interference (EMI) shielding effectiveness (SE) of multi-walled carbon nanotubes-polymethyl methacrylate (MWCNT-PMMA) composites prepared by two different techniques was measured. EMI SE up to 40 dB in the frequency range 8.2-12.4 GHz (X-band) was achieved by stacking seven layers of 0.3-mm thick MWCNT-PMMA composite films compared with 30 dB achieved by stacking two layers of 1.1-mm thick MWCNT-PMMA bulk composite. The characteristic EMI SE graphs of the composites and the mechanism of shielding have been discussed. SE in this frequency range is found to be dominated by absorption. The mechanical properties (tensile, flexural strength and modulus) of the composites were found to be comparable or better than the pure polymer. The studies therefore show that the composite can be used as structurally strong EMI shielding material.

The effect of multi-wall carbon nanotubes on electromagnetic interference shielding of ceramic composites

Multi-wall carbon nanotubes (MWCNTs)-3 mol% yttria-stabilized zirconia (3Y-TZP) (MWCNTs-3Y-TZP) composite was prepared by spark plasma sintering. The complex permittivities of the composite have been measured in the Ku-band range (12.4-18 GHz) and it is found that both the real and imaginary permittivities of the composite increase with the increasing content of MWCNTs. The effect of the content of MWCNTs on the electromagnetic interference (EMI) shielding effectiveness (SE) of the composite has been evaluated, and it is found that the EMI SE of the composite increases with the increasing content of MWCNTs. An EMI SE value as high as 25-30 dB has been achieved in the Ku-band range for the composite with 9 wt% content of MWCNTs, indicating that the MWCNTs-3Y-TZP composite can be used as an effective EMI shielding material.

Electromagnetic interference (EMI) shielding of single-walled carbon nanotube epoxy composites

Single-walled carbon nanotube (SWNT)-polymer composites have been fabricated to evaluate the electromagnetic interference (EMI) shielding effectiveness (SE) of SWNTs. Our results indicate that SWNTs can be used as effective lightweight EMI shielding materials. Composites with greater than 20 dB shielding efficiency were obtained easily. EMI SE was tested in the frequency range of 10 MHz to 1.5 GHz, and the highest EMI shielding efficiency (SE) was obtained for 15 wt % SWNT, reaching 49 dB at 10 MHz and exhibiting 15-20 dB in the 500 MHz to 1.5 GHz range. The EMI SE was found to correlate with the dc conductivity, and this frequency range is found to be dominated by reflection. The effects of SWNT wall defects and aspect ratio on the EMI SE were also studied.

Novel carbon nanotube-polystyrene foam composites for electromagnetic interference shielding

A novel carbon nanotube-polystyrene foam composite has been fabricated successfully. The electromagnetic interference (EMI) shielding effectiveness measurements indicated that such foam composites can be used as very effective, lightweight shielding materials. The correlation between the shielding effectiveness and electrical conductivity and the EMI shielding mechanism of such foam composites are also discussed.