Paraffin Matrix Research Articles

Synthesis and microwave absorption properties of carbon coil–carbon fiber hybrid materials

Uniform growth of carbon coils on the surface of carbon fibers was achieved by acetylene decomposition at 680°C with Ni nanoparticles as the catalyst. The homogeneous precipitation-reduction method was used to coat the carbon fibers with Ni nanoparticles. The scanning electron microscope images reveal that these carbon coils are mainly composed of spring-like microcoils, and include a few twist-shaped nanocoils. The carbon yield calculated from carbon yield=(mass of product/mass of Ni)×100% is about 34,000% in a run of 1h. By incorporating 10wt% of the carbon coil–carbon fiber hybrid in a paraffin matrix, a material with good microwave absorption is produced, despite the low mass percentage of filler.

Study on thermal properties of phase change material by an optical DSC system

An optical differential scanning calorimeter (DSC) system was used in studying the thermal properties of phase change material (PCM). In this study, several typical PCMs were studied on its thermal properties by the optical DSC system. These PCMs are sodium sulfate (Na2SO4·10H2O), paraffin, n-octadecane, and the paraffin/n-octadecane composites at different mass ratios. The thermal properties and micro-structure change of these PCMs were mainly investigated. The results showed that for Na2SO4·10H2O, paraffin and n-octadecane, the measured phase change temperatures and enthalpies appeared no large relative deviation (<5%) from the theoretic values. A high measurement accuracy of the optical DSC system has been obtained and concluded from the experimental results. For the paraffin/n-octadecane composites, the measured phase change temperatures appeared to be lower than that of paraffin, and could be adjusted from 24.2 °C to 49.6 °C by changing the mass fractions of n-octadecane. The lowest eutectic temperature was obtained at the mass fraction 40% of n-octadecane. The measured phase change enthalpies, however, appeared no large variation between 186.4 J/g (paraffin) and 230.5 J/g (n-octadecane). And according to the micro-photos before and after phase change, n-octadecane existed as crystal particles which disappeared with increasing temperature and finally combined with the paraffin matrix.

Preparation and Characterization of Diatomite Loaded Composite Phase Change Materials

Organic phase change materials like paraffin as phase change material, modified diatomite as carrier, composite phase change material with proper phase change temperature and larger phase change enthalpy is prepared by melt blending. The structure and performance of composite phase material are characterized using SEM, FI-IR and synthesized thermal analyzer DSC. The results show that the phase change temperature of composite phase change material is 30, and phase change enthalpy is 89.54J/g. With every part preserved, phase change particles are distributed in the diatomite/melted paraffin matrix evenly. Stable composite phase change materials are prepared with diatomite as carrier and paraffin as PCM, which are bonded with Vander Waals forces in the form of physical adsorption.

Three-dimensional SiO2@Fe3O4 core/shell nanorod array/graphene architecture: synthesis and electromagnetic absorption properties

We developed a new strategy, i.e., a seed-assisted method, to fabricate a three-dimensional (3D) SiO2@Fe3O4 core/shell nanorod array/graphene architecture. The fabrication processes involved deposition of β-FeOOH seeds on the graphene surfaces in the ferric nitrate aqueous solution, subsequent growth of β-FeOOH nanorod arrays on the graphene surfaces in the ferric chloride aqueous solution under hydrothermal conditions, deposition of SiO2 coating on the surfaces of β-FeOOH nanorods, and final formation of the 3D architecture by a thermal treatment process. Scanning electron microscopy and transmission electron microscopy measurements showed that the SiO2@Fe3O4 core/shell nanorods with a length and diameter of about 60 and 25 nm, respectively, were almost grown perpendicularly on both side surfaces of graphene sheets. The measured electromagnetic parameters showed that the 3D architecture exhibited excellent electromagnetic wave absorption properties, i.e., more than 99% of electromagnetic wave energy could be attenuated by the 3D architecture with an addition amount of only 20 wt% in the paraffin matrix. In addition, the growth mechanism of the 3D architecture was proposed, and thus, the strategy presented here could be used as a typical method to synthesize other 3D magnetic graphene nanostructures for extending their application areas.

Branched polyaniline/molybdenum oxide organic/inorganic heteronanostructures: synthesis and electromagnetic absorption properties

In this work, we developed an efficient and facile method to fabricate branched polyaniline (PANI)/α-MoO3 organic/inorganic heteronanostructures. Scanning electron and transmission electron microscope measurements showed that PANI nanorods with an average diameter and length of about 55 and 110 nm respectively were grown perpendicularly on the surfaces of α-MoO3 nanorods and the density of the PANI nanorods could be readily controlled by simply changing the addition amount of aniline in the reaction system. The minimal reflection loss for the electromagnetic wave absorbent material was up to −33.7 dB at 16.88 GHz for the branched heteronanostructures/paraffin composites with a thickness of 2.0 mm, and all of the minimal reflection losses were less than those of the pure PANI nanorods in the frequency range of 2–18 GHz. More importantly, the content of the branched nanostructures in the paraffin matrix was only 10 wt%. Thus, the method presented here is a very efficient strategy to fabricate lightweight materials for strong electromagnetic wave absorbents.

Efficiency of Microwave Heating of Weakly Loaded Polymeric Nanocomposites

Electrical and thermal conductivity of materials are typically correlated, while some applications, including thermoelectrics, require these parameters to be controlled independently. Such independent control of thermal and electromagnetic properties can be achieved by using nanocomposites. In this study, nanocomposites were produced by mixing a small amount of carbon nanofibers (CNF), carbon coated cobalt (Co), and nickel nanowires (NiNW) with paraffin, which has low thermal and almost zero electrical conductivity. The fraction of nanoinclusions in the paraffin matrix was very low (below 1%). We showed that the thermal properties of nanocomposites are essentially the same as those of pure paraffin, while electromagnetic properties are significantly different. To determine the dependence of the heating rate on filler concentration, paraffin-based samples were heated in a microwave oven. We found that the heating rate of nanocomposites made of carbon nanofibers is much greater than that of any other nanocomposites. These findings suggest that at 2.45 GHz frequency, the heating rate is mostly controlled by the electrical losses in the fillers. The theoretical model predicts that the heating rate increases linearly with the particle concentration, which is in agreement with the experimental data.

Multi-resonances behavior of Ni nanobelt/paraffin composites at microwave frequencies

Ni nanobelts are synthesized through a facile hydrothermal method to form magnetic composites for high frequency applications. Four resonance peaks of the relative complex permeability illustrate the multi-resonance behavior in Ni nanobelt/paraffin composites in a frequency range of 2–18GHz. The resonance absorption characteristics are analyzed by fitting the permeability spectrum with the well-known Landau–Lifshitz–Gilbert (LLG) equation and Maxwell–Garnett mixing rule. Unlike previous studies, LLG equation has been deduced on account of the random distribution of Ni nanobelts in paraffin matrix first, and then applied for data fitting and resonance identification. Correspondingly, the first band is attributed to natural resonance, while the other three bands are considered to originate from non-uniform exchange resonance.

The influence of Fe content on the magnetic and electromagnetic characteristics for Fex(CoNi)1−x ternary alloy nanoparticles

Fex(CoNi)1−x (x = 0.14, 0.20, 0.25) ternary alloy nanoparticles were fabricated by a self-catalyzed reduction method at a low temperature. The investigation of static magnetic properties revealed both saturation magnetization and coercive force enhanced with increasing Fe content. By dispersing alloy nanoparticles into paraffin matrix homogeneously, the electromagnetic properties of them were investigated experimentally and the electromagnetic absorption performances were calculated according to transmission line theory. Significant dielectric relaxation was found in the low Fe content sample, which is dominant in dielectric loss. The magnetic loss was attributed to natural resonance and the resonance peaks’ shift to high frequency region with increasing Fecontent. The enhanced electromagnetic absorption performances were obtained by adjusting Fe content to balance electromagnetic parameters, because the electromagnetic parameters can be varied by structural and magnetic properties.

Electromagnetic and Microwave Absorption Properties of Carbonyl-Iron/Fe91Si9 Composites in Gigahertz Range

Carbonyl-iron/Fe91Si9 composites for thin microwave absorbers were firstly prepared by a simple blending technique. The patterns of carbonyl-iron and Fe91Si9 were characterized by scanning electron microscope (SEM). The complex permittivity, permeability and microwave absorption properties of the composites were studied in the frequency range of 2 - 7GHz by a HP8720B vector network analyzer. Complex permittivity and permeability decrease gradually with increasing weight percentage of Fe91Si9 in the composites, the variation of permittivity was very large but the variation of permeability was very small. The composites exhibited excellent microwave absorption properties with increasing Fe91Si9 content. The reflection loss (RL) values less than –20 dB were obtained in the 3.7 - 6.7 GHz frequency range for the paraffin matrix composites with 80 wt% carbonyl-iron/Fe91Si9 powders (weight ratio of carbonyl-iron to Fe91Si9 was 1:1), with thickness of 4.0 - 2.4 mm, respectively. The optimal RL of –45 dB was observed at 5.2 GHz with a matching thickness (dm) of 3.0 mm. The excellent microwave absorption properties were attributed to a better electromagnetic impedance match and a higher electric resistivity.

Synthesis and Characterization of Paraffin Wax Microcapsules with Acrylic-Based Polymer Shells

Microencapsulation of PRS paraffin wax was carried out by means of suspension-like homopolymerization of methyl methacrylate (MMA) and by the copolymerization of this monomer with methyl acrylate (MA) and methacrylic acid (MAA). The influence of the type of monomers and their proportion as shell materials, the mass ratio of polyvinylpyrrolidone to monomers (PVP/monomers), and the mass ratio of PRS paraffin wax to monomers (PRS/monomers) on the properties of phase change material (PCM) microcapsules has been studied. The analytical techniques used for the characterization of the particles were differential scanning calorimetry (DSC) and modulated DSC, dynamic light scattering, environmental scanning electron microscopy (ESEM) and gel permeation chromatography (GPC). The chemical property differences between the encapsulating “shell” material, acrylic polymer in this case, compared to the “core” material (PRS paraffin wax), such as polarity and interfacial tensions, largely determine the thermodynamically favored morphology of the microcapsules. While the equilibrium morphology was a core/shell structure, kinetic factors relating to the competition between the fast acrylic polymerization rate and the diffusion limited rate of phase separation were found to constrain the ability of the system to approach that equilibrium structure. For both homopolymerization of methyl methacrylate (PMMA) and copolymerizations of methyl methacrylate with methyl acrylate (P(MMA-co-MA)), “pomegranate” microspheres were observed with a thin acrylic shell surrounding an inner composite structure constituted by a paraffin matrix with a dense assortment of PMMA spherical domains. Ter-polymerizations incorporating methacrylic acid (P(MMA-co-MA-co-MAA) had a remarkable effect on the morphology and average particle size, resulting again in “pomegranate” microparticles but where the internal PMMA spheroids were more densely populated closer to the outer portion of the capsule; approaching a core/shell structure. As the amount of polyvinylpyrrolidone stabilizer in the aqueous medium was increased and the PRS/monomers proportion was decreased, the mean particle size of the microcapsules decreased. However, higher PVP/monomer and PRS/monomer ratios led to less efficient encapsulation of the PRS paraffin wax. Near perfect encapsulation efficiency was obtained for two pairings of these ratios via P(MMA-co-MA-co-MAA) polymerization, but only up to an equal mass ratio of PRS to acrylic polymer.

Synthesis, characterization and electromagnetic properties of Fe 1− xCo x alloy flower-like microparticles

Fe 1− x Co x alloy microparticles with size 3–5 μm and novel flower-like shapes were prepared by a simple low temperature reduction method. The electromagnetic properties for the paraffin matrix composites containing Fe 1− x Co x alloy microparticles were measured using a vector network analyzer in the 2–18 GHz frequency range. As a consequence of large surface- and shape-anisotropy energy for the flower-like shaped 3D microstructures, the strong natural resonance around 8–12 GHz and remarkable dielectric relaxation were observed in the complex permittivity and permeability spectrum, which are dominant in the enhanced electromagnetic wave absorption (EMA) performance. It was found that both the electromagnetic parameters of complex permittivity and permeability and the intensity and location of absorption band were remarkably dependent on the Co/Fe molar ratio. The enhanced EMA performance was obtained in these Fe 1− x Co x –paraffin ( x=0.4, 0.5, and 0.6) composites system. For the Fe 0.5Co 0.5 alloy, the reflection loss ( RL) exceeding −20 dB was obtained in the broad frequency range of 5.4–18 GHz with a thin sample thickness of between 1.0 and 2.9 mm. In particular, an optimal RL of −59 dB was obtained at 3.61 GHz with a thin thickness of 3.6 mm for the Fe 0.4Co 0.6 sample. The Fe 1− x Co x alloy microparticles may be attractive candidates for applications of microwave absorption materials with a wide frequency range and strong absorption in the high frequency region.

Dielectric response of carbon coated TiC nanocubes at 2–18 GHz frequencies

Microwave frequency (2–18 GHz) dielectric response has been investigated in the carbon-coated cubic TiC nanoparticles embedded in paraffin matrix with different weight fractions. DC conductivity measurements showed that the TiC nanocubes/paraffin composites have a percolation threshold of about 30 wt%. Absorption property was found to be improved with increasing mass ratio in the present system, up to a mass ratio of 50 wt%. Reflection loss exceeding −20 dB in the frequency range of 6–16 GHz, with layer thicknesses from 2 to 4 mm, was found in the 50 wt% loaded sample. TiC nanocube/paraffin composite with a proper mass ratio can be very good prospective microwave absorption agent.

Catalytic pyrogenation synthesis of C/Ni composite nanoparticles: controllable carbon structures and high permittivities

Catalytic pyrogenation of methane gas in the presence of Ni nanoparticles was employed to synthesize C/Ni composite nanoparticles at various reaction temperatures. The Ni nanoparticles prepared by the arc-discharge method served as a catalyst to decompose the hydrocarbon molecules and also provided isolated templates for the formation of carbon nanocapsules at 400 and 500 °C or multi-walled carbon nanotubes at 600 and 650 °C. The generation and growth mechanism of the carbon shells are discussed on the basis of structure evolution. By dispersing the nanoparticles homogeneously into a paraffin matrix, the electromagnetic parameters of the nanoparticles have been investigated in the frequency range 2–18 GHz. The samples exhibit high permittivities varying with the microstructures of the nanoparticles. The relationship between the dielectric properties and diverse carbon structures is indicated. The high permittivities of the nanoparticles are attributed to the better conductivity of the carbon shells and the charge polarizations at the defects or interfaces between metal cores and carbon shells.

Electromagnetic wave absorption by H2O‐filled SiO2 shell microcapsule‐dispersed paraffin matrix composites

AbstractElectromagnetic wave absorption of H2O‐filled SiO2 shell microcapsule‐dispersed paraffin matrix composites has been examined in a frequency range from 18 to 30 GHz. The composites showed electromagnetic wave absorption potential, and the electromagnetic wave absorption can be controlled by a volume fraction of liquid state H2O in the composite. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 770–772, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24175

Enhanced Thermal Conductivity of Phase Change Materials Modified by Exfoliated Graphite Nanoplatelets

AbstractComposite phase change materials (PCM) were prepared by mixing exfoliated graphite nanoplatelets (xGnP) into paraffin wax. The two types of graphite nanoplatelets that were investigated were xGnP-1 having thickness of 10 nm and a diameter of 1 um and xGnP-15 having the same thickness with a platelet diameter of 15 um. Direct casting and two roll milling were used to prepare samples. Scanning electron microscopy images show that the nanofillers disperse very well in paraffin matrix without noticeable agglomeration. Paraffin/xGnP-15 PCM consistently exhibited higher thermal conductivity than xGnP-1 PCM. The improvement in thermal conductivity was as high as 5 fold for xGnP-15 composites and 2 fold for xGnP-1 composites at 4 vol%. The aspect ratio, particle orientation, and interface density between the conductive particles and the polymer matrix were found to be the critical parameters in determining the conductivities of the resulting nanocomposites. The thermal physical properties of the nanocomposites were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). It was found that the latent heat of nanocomposites was not negatively affected in the presence of xGnP particles and the thermal stability improved.

Influence of alloy components on electromagnetic characteristics of core/shell-type Fe–Ni nanoparticles

Fe–Ni alloy nanoparticles with various alloy components were fabricated by a direct current arc-discharge method. By dispersing the nanoparticles homogeneously into a paraffin matrix, the complex permittivity (εr=εr′+iεr″) and permeability (μr=μr′+iμr″) of the nanoparticles have been investigated in the frequency range of 2–18 GHz and the effects of alloy components on the electromagnetic parameters were discussed. It is found that the permittivities of the nanoparticles are lower than those of the microscale counterparts and almost independent of frequency. The magnetic loss is attributed to natural resonance and the resonance peak shifts to high frequency range with the increase in Fe content. Better microwave absorption performances can be obtained by adjusting the composition and tailoring the core/shell structures to balance the electromagnetic parameters. The calculated results indicate that the Fe–Ni nanoparticles with 49 wt % Ni exhibit excellent electromagnetic wave (EMW) absorption properties (reflection loss &amp;lt;−20 dB) over the range of 7.6–16.0 GHz in the thickness of 1.02–1.70 mm. The mechanism of effective EMW introduction and attenuation is discussed on the basis of the experimental results.

Preparation of PVA/paraffin thermal regulating fiber by in situ microencapsulation

In situ microencapsulation, a novel method, was applied to prepare thermal regulating fiber. The in situ microencapsulation of paraffin in PVA (fiber matrix) was performed by treating the as-spun fiber to promote the hydrolysis and polycondensation of ethyl orthosilicate (TEOS) at the interface between paraffin and PVA matrix. The mechanism of hydrolysis and polycondensation of TEOS was studied by a simulation experiment. The structure and properties of the thermal regulating PVA fibers were characterized by SEM, IR, DSC and tensile strength tester. Results showed that the paraffin phases tightly wrapped by TEOS hydrolysates were present in the PVA matrix when the as-spun fibers were treated in the acidic condition. The thermal regulating PVA fiber prepared by this method has a relatively high latent heat value (23.7 J g −1) and good latent heat stability. The mechanical property of the fiber is also good enough for application in textile.

Fine control of the growth and optical properties of CdSe quantum dots by varying the amount of stearic acid in a liquid paraffin matrix

Although stearic acid is a widely used amphiphilic compound in hot-matrix syntheses of quantum dots, the effect of its concentration has not been studied in details in order to achieve better control over the nanocrystal growth process. Here we systematically investigate various mixtures of stearic acid and non-coordinating solvent (paraffin) as the matrix for nucleation and growth of CdSe nanocrystals. The smallest nanoparticles with highest particle concentration in the reaction solution are formed at ca. 35wt.% of stearic acid. This peculiar trend is confirmed with two different ratios of selenium (Se):Cd precursors. Our findings allow precise tuning of nanoparticle sizes and size distribution, particle concentrations and optical properties in this synthesis, which is among the best candidates for industrial applications.

Machining Carbon Nanotubes into Uniform Slices

Shearing the carbon nanotubes (CNTs) to desired size or trimming the CNT tips conveniently is usually necessary for many applications. CNTs are normally believed possessing very high strength and toughness. In this paper we present a simple and novel method to actualize this process. In this method, aligned CNT arrays were embedded in paraffin matrix, and then the materials were carefully sliced up along the direction normal to the CNTs with a microtome. These slices consisted of vertically aligned CNTs with desired and uniform length. The experiments proved that there were enough interaction forces between the CNTs and the paraffin matrix to prevent the CNTs from being pulled out during the machining process. These sheared CNTs have shown better performance for thermal interface materials and field emission applications. This process may redound to unlocking the great potential of CNT applications.

FMR Study of Carbon Coated Cobalt Nanoparticles Dispersed in a Paraffin Matrix

Agglomerated cobalt magnetic nanoparticles coated with carbon, dispersed in a paraffin matrix, were prepared and investigated by FMR (ferromagnetic resonance) at room temperature. Four samples with different C/Co content, ranging from 0.175 to 1.011, dispersed at low concentration in paraffin were investigated. Very intense and broad FMR spectra with different intensities, line widths and positions of the resonance fields were recorded for the samples. A strong dependence of the FMR signal intensity and resonance on the concentration of magnetic nanoparticles was observed. Various magnetic interactions affecting the observed FMR spectra have been analyzed. It was found that with increasing concentrations of magnetic nanoparticles the magnetic dipole interaction between the agglomerates plays a more important role.