Frequency response and activation energy of palm-fatty acids at microwave region

Microwave technology has the potential to significantly enhance chemical reaction and knowledge of dielectric properties of materials plays a major role in microwave design for any process. In this paper, the dielectric properties (permittivity and conductivity) of palm-fatty acids were investigated. Palm-fatty acids were chosen being the most versatile and sustainable source of raw materials, also considered as the “gate-way” to other oleochemicals derivatives. Thus, the dielectric properties of commercial palm-fatty acids from various sources in flakes and liquid states were determined within the frequency range of 0.2 GHz to 4.5 GHz at temperature range of 303 K and 443 K (stabilized within 1 °C). The effects of temperature, moisture and unsaturation structure of palm-fatty acids on dielectric properties were also investigated. Real part of dielectric permittivity (ε_r^′) of all the palm-fatty acids increased with increasing temperature from 303 K to 443 K with correlation coefficient, R² > 0.95 to linear function; depending on temperature and moisture content of the palm-fatty acids. The conductivity increased with increase in temperature and following the model-fitting approach of Arrhenius equation with correlation coefficient (R² > 0.85) for all the measured samples. The calculated activation energy (E_a) for all measured samples was in the range between 0.06207 eV and 0.16442 eV. Hence, the data calculated would be useful to determine the quality of the palm-fatty acids and to specify their requirement for transportation, storage and process. Moreover, the information will also be useful in the design of a microwave (MW) reactor system for oleochemical downstream process as it is dependent on the dielectric properties of the materials being processed.     

Fabrication of MWCNT/Cu nanofibers via electrospinning method and analysis of their electrical conductivity by four-probe method

The objectives of the present study are (i) fabrication of multiwalled carbon nanotubes (MWCNT)/Cu composites via electrospinning method, better dispersion, and heat treatment; and (ii) to examine the effect of the heat treatment on the mechanical, thermal, and electrical properties of the electrospun fibers. In this study, we have fabricated polyacrylonitrile (PAN) based MWCNT/Cu carbon nanofiber composites by electrospinning method with different weight percentage of CNT (0.1%, 0.3%, 0.5%). The MWCNT/Cu composites were aligned in an electrified thin jet during electrospinning. Field Emission Scanning Electron Microscope and High Resolution Transmission Electron Microscopy was used to analyze the morphology of the as-prepared carbon nanofibers. The elemental analyses of fibers were determined by Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy, X-ray Powder Diffraction and X-ray Photoelectron Spectroscopy. Thermogravimetric analysis and mechanical strength were performed to study the thermal and mechanical properties of the nanofibers composites. Electrical conductivity of the as-prepared fibers was studied by four-point probe method. For the composites containing 0%, 0.1%, 0.3%, 0.5 wt% of MWCNT, the electrical conductivity was measured to be 0.021, 0.459, 1.155, 1.479 S cm^?1 respectively.     

Temperature effect on the electrical properties of undoped and vanadium-doped ZnTe thin films

Vanadium-doped ZnTe films of composition 0 to 10 wt% V, were prepared onto glass substrate by e-beam evaporation of the element in vacuum at 8×10⁻⁴ Pa. The effects of various deposition conditions on the electrical properties of the films have been studied in detail. The deposition rate of the ZnTe films was at about 2.05 nm s⁻¹. X-ray diffraction (XRD) study shows that the as-deposited ZnTe films are amorphous in nature.The effects of temperature on the electrical properties of the ZnTe and ZnTe:V films were studied in details. The heating and cooling cycles of the samples are reversible in the investigated temperature range after successive heat treatments in air. Thickness dependence of electrical conductivity is well in conformity with the Fuchs–Sondheimer theory. Temperature dependence of electrical conductivity shows a semiconducting behavior with a spectrum of activation energy. The value of activation energy for undoped ZnTe films do agree well with earlier reported values. Dopant vanadium concentration increases the conductivity of the samples. The composition and thickness dependence of the activation energy as well as thermoelectric power studies were done in the 300–413 K temperature range. The results of d.c. conductivity and thermopower obey an activated conduction mechanism. The thermopower of undoped ZnTe films indicates a p-type conductivity. Thermopower results of ZnTe:V films also suggest that the simultaneous bipolar conduction of both carriers take place.     

Methanol permeability and proton conductivity of Nafion membranes modified electrochemically with polyaniline

Nafion membrane was modified with polyaniline by electrochemical methods, including constant current, constant potential and potential cycling. The methanol permeability and the proton conductivity of the Nafion membranes with and without modification were determined by the electrochemical oxidation of the permeated methanol and electrochemical impedance spectroscopy, respectively. It is found from FTIR results that the polyaniline formed by electrochemical method is in emeraldine state. With the modification of polyaniline, the methanol permeability of the Nafion membrane is reduced by one decade but there is only a slight decrease in proton conductivity. The power density output of direct methanol fuel cell using polyaniline-modified Nafion membrane is 40% larger than that using pure Nafion membrane. The modification with polyaniline is better than that with polypyrrole. The methanol permeability of the polyaniline-modified Nafion is about half of that of the polypyrrole-modified Nafion but the proton conductivity of the former is higher than the later.     

Wide range tuning of electrical conductivity of RF sputtered CdO thin films through oxygen partial pressure variation

The effect of oxygen partial pressure variation on the electrical conductivity and the optical transparency of CdO thin films, deposited through RF magnetron sputtering were studied in detail. Thin films of CdO have been deposited through radio frequency magnetron sputtering of a prefabricated CdO target at a fixed pressure 0.1 mbar and at a substrate temperature 523 K. It was found that the electrical conductivity of the CdO films could be varied over three decades for a variation of oxygen partial pressure of 0–100%, without introducing any extrinsic dopants. X-ray diffraction (XRD) studies showed that the films were polycrystalline in nature with a preferential orientation along (1 1 1) plane. Compositional information was obtained by X-ray photoelectron spectroscopic studies. This wide range of variation of electrical properties was explained through the oxygen vacancies formation.     

Crystal structure and electrical conductivity of imidazolium succinate

Small single crystals of the imidazolium succinate were grown and their structure was re-examined using the X-ray diffraction method and a probable protonic conduction mechanism has been proposed. The electric conductivity of the powdered tablets was measured using the impedance spectroscopy method. The range of obtained values of conductivity as well as the activation energy (0.65 eV) of the imidazolium succinate is close to these of imidazolium malonate and imidazolium glutarate investigated previously [K. Pogorzelec-Glaser, J. Garbarczyk, Cz. Pawlaczyk, E. Markiewicz, Materials Science Poland 24 (2006) 245–253]. Metastable and virtual positions for proton transfer were indicated by means of calculations of the atomic displacement factors and the probability density function.     

Effect of synthesis process on the microstructure and electrical conductivity of nickel/yttria-stabilized zirconia powders prepared by urea hydrolysis

In this study, NiO/YSZ composite powders were synthesized using hydrolysis on two solutions, one contains YSZ particles and Ni²⁺ ion, and the other contains NiO particles, Zr⁴⁺, and Y³⁺ ions, with the aid of urea. The microstructure of the powders and sintered bulks was further characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicated that various synthesis processes yielded NiO/YSZ powders with different morphologies. The NiO precursors would deposit onto the surface of YSZ particles, and NiO-deposited YSZ composite powders were obtained. Alternatively, it was not observed that YSZ precursors deposited onto the surface of NiO particles, thus, a uniform powder mixture of fine NiO and fine YSZ particles was produced. After sintering and subsequent reduction, these powders would lead to the variations of Ni distribution in the YSZ matrix and conductivity of cermets. Owing to the core–shell structure of the powders and the higher size ratio of YSZ and NiO particles, the conductivity of cermet with NiO-deposited YSZ powders containing 23 wt% NiO is comparable to those with a NiO/YSZ powder mixture containing 50 wt% NiO.     

Effect of different graphite materials on the electrical conductivity and flexural strength of bipolar plates fabricated using selective laser sintering

Selective Laser Sintering provides a way to fabricate graphite composite bipolar plates for use in fuel cells. This significantly reduces time and cost at the research and development stage of bipolar plates, as compared with the conventional fabrication methods such as compression molding and injection molding. Different graphite materials, including natural graphite, synthetic graphite, carbon black, and carbon fiber, were investigated using the selective laser sintering process to fabricate bipolar plates. The effect of each material on the electrical conductivity and flexural strength of the bipolar plates was studied experimentally. With a proper combination of these materials, bipolar plates with electrical conductivity ranging from 120 to 380 S/cm and flexural strength ranging from 30 to 50 MPa have been obtained, which satisfy the requirements set by the Department of Energy and also are comparable with those developed by compression molding and injection molding. A modified percolation model was proposed to predict the electrical conductivity of the fabricated bipolar plates with different compositions. The analytical results calculated from the proposed model agree well with the experimental results. Finally, a single PEM (Proton Exchange Membrane) fuel cell unit was assembled using the fabricated bipolar plates, and its in-situ performance was studied.     

Effect of modified ITO substrate on electrochromic properties of polyaniline films

In this work, we report the morphological and electrochromic properties of electrochemically synthesized polyaniline (PANI) thin films on bare and modified indium–tin oxide (ITO) glass substrates. In the last case, the surface of ITO glass was covered by a self-assembled monolayer of N-phenyl-γ-aminopropyl-trimethoxysilane (PAPTS). Atomic force microscopy images and perfilometry show that smoother and thinner PANI films were grown on PAPTS-modified ITO substrates. PANI-based electrochromic devices (ECDs) were assembled by using a viscous polymeric electrolyte (PE) of LiClO₄ and polymethyl methacrylate (PMMA) co-dissolved in a mixture of propylene and ethylene carbonate. The architectural design of the devices was glass/ITO/PANI/PE/ITO/glass. A dual ECD was also prepared by collocating a poly(3-methylthiophene) (P3MT) thin film as a complementary electrochromic element. The effect of the PAPTS-modified ITO substrate is reflected in a higher optical transmittance at bleach state and a little less color change at 550 nm of PANI-based ECDs.     

Role of porosity on the equilibration kinetics in electrical conductivity relaxation experiments

The role of porosity on the equilibration kinetics in electrical conductivity relaxation (ECR) experiments is highlighted. Both porous and dense conductivity bars are used to determine the chemical oxygen surface exchange coefficient (k_chem) of neodymium nickelate Nd₂NiO_4+δ (NNO) from 600 to 800 °C. Using porous bars allows for quicker ECR experiments during which the conductivity transient is rate limited only by oxygen surface exchange which enables the use of a simple transient model. Additionally, porous bars have similar microstructures to porous electrodes which means they are critical for determining the oxygen exchange kinetics of realistic electrodes. ECR results on dense bars with porous coatings are also presented. The conductivity transients of dense and porous bars both show similar trends with oxygen partial pressure. Additionally, both porous and dense bars show a difference between oxidation and reduction transients with oxidation transients being faster than reduction transients.     

Effect of film thickness on electrical property of microcrystalline silicon

We report dependences of electrical properties on SiH₄/H₂ dilution rate and film thickness for microcrystalline silicon films formed by a hydrogen radical-induced chemical vapor deposition (HRCVD) method. The electrical conductivity of the films at SiH₄ 18 sccm /H₂ 120 sccm was markedly increased to 10⁻³ S/cm as film thickness increased above 100 nm. Crystalline grains with (2 2 0) orientation were formed. Theoretical analysis revealed that grain boundaries among (2 2 0) grains had a low defect density of 1×10¹² cm⁻² so that the high conductivity was achieved.     

Effect of polytetrafluoroethylene-treatment and microporous layer-coating on the electrical conductivity of gas diffusion layers used in proton exchange membrane fuel cells

The purpose of this study is to investigate the effect of ploytetrafluoroethylene (PTFE)-treatment and microporous layer (MPL)-coating on the electrical conductivity of gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells (PEMFCs). The results show that, for PTFE-treated GDLs, the electrical conductivity in orthogonal in-plane directions is almost invariant with the PTFE loading. On the other hand, the in-plane conductivity of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be higher than that of the counterpart SGL 10BC (25% PTFE) and this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Further, the conductivity of each GDL sample was measured in two perpendicular in-plane directions in order to investigate the in-plane anisotropy. The results show that the electrical conductivity of the GDL sample in one direction is different to that in the other direction by a factor of about two. The contact resistance, the main factor affecting the through-plane conductivity, of PTFE-treated GDLs shows a different trend to the corresponding in-plane conductivity, namely it increases as the PTFE loading increases. On the other hand, the contact resistance of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be lower than that of the counterpart SGL 10BC (25% PTFE) and again this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Also, it was noted that the MPL coating appears to have a positive effect in reducing the contact resistance between the GDL and the bipolar plate. This is most likely due to the compressibility of the MPL layers that allows them to fill in the ‘gaps’ that exist in the surface of the bipolar plates and therefore establishes a good contact between the latter plates and the GDLs. Finally, good curve fitting of the contact resistance as a function of the clamping pressure has been achieved.     

Effect of Nafion on the preparation and capacitance performance of polyaniline

With manganese dioxide (MnO₂) as the oxidant, perfluorinated sulfonic acid ion exchange resin (Nafion) as the doping agent and emulsifier, Nafion doped polyaniline (PANI-Nafion) was prepared by emulsion polymerization method. Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were carried out to characterize the structure and morphology of PANI-Nafion. Symmetric redox supercapacitor was assembled with PANI-Nafion as active electrode material and 1.0 mol L⁻¹ H₂SO₄ aqueous solution as electrolyte. The electrochemical characteristics of these supercapacitors were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge tests. These results show that the diameter of PANI-Nafion nanofiber is about 30 ∼ 40 nm and the pores between PANI-Nafion composite materials are distributed uniformly. The specific capacitance of PANI-Nafion electrode is about 385.3 F g⁻¹, which is higher than that of undoped PANI (235.8 F g⁻¹). After 1000 charge/discharge cycles the specific capacitance of PANI-Nafion electrode is 272.4 F g⁻¹, its capacity retention is 70.7%, which is significantly better than that of PANI electrode materials.     

Semi-empirical modeling of electrical conductivity for composite bipolar plate with multiple reinforcements

Carbon composite bipolar plates were developed by compression molding of novolac type phenol formaldehyde resin with natural graphite, carbon black, and carbon fiber. The General Effective Media equation was adapted to model the electrical conductivity of the bipolar plate. The experimental values of the electrical conductivity of the composites with different reinforcements were well predicted by the model. For resin-graphite system (2-component), the most effective in-plane and through-plane electrical conductivities for 70% graphite content were found to be 201.26 and 40.91 S cm⁻¹, respectively. Similarly, for optimum resin-graphite-carbon black system (3-component), these values were found as 269.55 and 82.77 S cm⁻¹, respectively. The most effective in-plane and through-plane electrical conductivities were found to be 285.54 and 91.79 S cm⁻¹, respectively, for the composite with resin-graphite-carbon black-carbon fiber system (4-component). The predicted electrical conductivities for all the three systems were found to be in well agreement with the experimental values.     

Understanding CO2 electrochemical reduction kinetics of mixed-conducting cathodes by the electrical conductivity relaxation method

Electrochemical reduction reaction is an important approach to utilize CO₂ and convert it into valuable products. Exceptional reaction kinetics at a high temperature of solid oxide electrolysis cells (SOECs) attracts particular attention. In this work, we propose to investigate CO₂-RR kinetics using a new theoretical method based on the electrical conductivity relaxation (ECR) technique on a typical mixed-conducting Sr₂Fe_1.5Mo_0.5O_6-δ (SFM) electrode. Three kinetic parameters that are commonly adopted in the typical electrochemical test experiments consisting of overpotential, current density and area-specific resistance (ASR) are derived. The overpotential resulted from the difference in the oxygen partial pressure is caused by the change of CO₂ partial pressure, while current density from the surface reaction rate constant. Accordingly, area-specific resistance, as well as overpotential-current density relationship, can be derived. We believe that this work brings a new method to study the kinetic process of CO₂ electrolysis and to evaluate the electrocatalyst activity of developed new electrode materials as well as to benefit the designing of novel electrode electrocatalysts for highly efficient solid oxide electrolysis cells.     

Analysis of biomass residues potential for electrical energy generation in Albania

This paper presents the status of research of biomass potential for producing electrical energy in Albania. Biomass potential can be generated by different sources. Three types of biomass energy sources are included: dedicated bioenergy crops, agricultural and forestry residues and waste. The technical electrical energy considered in this study was calculated with two converting techniques: (1) combustion of the feedstock directly in an incinerator and then driving a steam generator for producing electrical energy and (2) production of biogas from an anaerobic digester and running a turbine for electrical energy generation. Analysis of the potential biomass resource quantity was computed according to statistical reports, literature review and personal investigations. From the biomass residue potential was calculated in terms of the theoretical energy content (total heating value) of every type of feedstock and the technical energy content for every Albanian prefecture according to different burning processes and different operation efficiencies. Results show that Albania was producing around of 4.8 million tons of dry biomass in year 2005. The theoretical energy content of biomass in Albania was 11.6 million MWh/a, and the technical electrical energy production was 3 million MWh/a. The electrical energy produced is equivalent to 45.8% of total Albania Country annual electrical consumption. In Albania Country, residues from agriculture, forest and urban waste represent a large biomass potential. By actual conversion techniques it is possible to generate one third of the theoretical heat energy into technical electrical energy. The use of heat from cogeneration plants depends on local heat provision conditions. It is another big energy potential but excluded in this study, so the rest of energy is considered as heat losses.     

On the equation of electrical conductivity relaxation method to measure kinetic parameters of solid oxide fuel cell materials with a three-dimensional rectangular geometry

The electrical conductivity relaxation method has been widely used to measure kinetic properties of species transport process in solid oxide fuel cell materials using rectangular dense samples. It is found that the equation of ECR used in some studies is flawed. An improved equation is presented in this communication.     

Temperature effect on the electrical properties of undoped NiO thin films

Undoped NiO thin films have been prepared onto glass substrate by e-beam evaporation of the element Ni in vacuum at 2 × 10⁻⁴ Pa. The as-deposited Ni films were then oxidized in air by heating about 2 h at a temperature of 470 K and then the oxidized Ni films are turned into NiO thin films. From the deposition time and film thickness after annealing in air, an effective deposition rate of NiO thin films was about 6.67 nms⁻¹. X-ray diffraction (XRD) study shows the NiO films are amorphous in nature. SEM studies of the surface morphology of NiO films exhibit a smooth and homogeneous growth on the entire surface. The elemental composition of NiO films is estimated by Energy Dispersive Analysis of X-rays (EDAX) method. The effects of temperature on the electrical properties of NiO thin films were studied in details. The heating and cooling cycles of the samples are reversible in the investigated temperature range after successive heat-treatment in air. Thickness dependence of conductivity is well in conformity with the Fuchs–Sondheimer theory. Temperature dependence of electrical conductivity shows a semiconducting behavior with activation energy. The thickness dependence of activation energy as well as thermopower studies was done within 293–473 K temperature range, respectively. Thermopower study indicates the NiO films a p-type semiconductor. Optical study in the wavelength range 0.3 < λ<1.2 μm range exhibits a high transmittance in the visible as well as in the near infra-red. Calculation from the optical data, the NiO sample exhibits a band gap at 3.11 eV, which does agree well with earlier reported values. These studies may be of importance for the application of this material in energy efficient surface coating devices.     

A generalized activation energy equation for torrefaction of hardwood biomasses based on isoconversional methods

In this work an extended kinetic analysis involving both experimental measurements and modelling procedures to describe the thermal degradation process of biomass is proposed. Three biomasses belonging to the hardwood family are investigated: ash-wood, beech-wood and hornbeam. The experiments are performed with a thermogravimetric balance working at four constant heating rate: 3, 5, 10 and 20 °C/min. This study is specifically dedicated to investigate the thermal behaviour of the selected biomasses when they undergo to torrefaction temperature conditions. The modelling analysis focuses on investigating the capability of consolidated models, usually applied to solids, in determining the activation energy (E_a) of the indicated biomasses within the torrefaction range. The adopted methods belong to the so-called isoconversional “model free” methods and, in this contest, both the differential ones as those of Friedman and Flynn and the integral ones as those of Kissinger-Akahira-Sunose, Doyle and Starink have been applied. The performances reached by adopting the integral methods widely satisfy the accepted accuracy level conventionally set at values lower than 10%. At the same time it is verified even that, when the methods are applied to biomasses belonging to the same family, the resulting E_a vs. α trends are very close for all the biomasses. This condition is exploited to propose a generalized predictive approach for the E_a calculation based on the knowledge of only the conversion fraction α. The presentation of the results includes also the investigation of the limits of the proposed methods in view of indicating their reliable application range when utilized for torrefaction design procedures.