Dielectric and thermal properties of AlN ceramics

The effects of slow-cooling and annealing conditions on dielectric loss, thermal conductivity and microstructure of AlN ceramics were investigated. Y₂O₃ from 0.5 to 1.25 mol% at 0.25% increments was added as a sintering additive to AlN powder and pressureless sintering was carried out at 1900 °C for 2 h in a nitrogen flowing atmosphere. To improve the properties, AlN samples were slow-cooled at a rate of 1 °C min⁻¹ from 1900 to 1750 °C, subsequently cooled to 970 °C at a rate of 10 °C min⁻¹ and then annealed at the same temperature for 4 h. AlN and YAG (5Al₂O₃/3Y₂O₃) were the only identified phases from XRD. AlN doped with 0.5 and 0.75 mol% Y₂O₃ had a low loss of <2.0 × 10⁻³ and a high thermal conductivity of >160 W m⁻¹ °C⁻¹.     

Cure and thermal properties of brominated epoxy systems

The addition of a brominated epoxy resin as a chain extender to di-, tri-, and tetrafunctional epoxy systems cured with diaminodiphenylsulfone has been investigated. The effect of increased BER content on the glass transition temperature of the cross-linked systems is discussed. The T_g of these systems is in the order T_g (tri) > T_g (tetra) > T_g (di). The contributions of the chemical structure and the cross-link density to the glass transition temperature are calculated. The kinetics of the reaction was followed dynamically by DSC. The overall order of the reaction and activation energies decrease with increased BER content. The thermal stability of these systems is also discussed.     

Dielectric and thermal expansion properties of LHPG grown potassium lithium niobate single crystals

Compositions of potassium lithium niobate K₆Li_4+xNb₁₀O₃₀ (x for excess mole % of Li₂O) with tetragonal tungsten bronze structure were prepared as crystal fibers by the Laser Heated Pedestal Growth technique. The lattice constants of KLN were measured by using X-ray diffraction and typical values for a = 12.5703 ± 0.0018 and c = 3.9485 ± 0.0012 were measured. This paper reports the dielectric permittivity measurements on single crystal along the a- and c- axis in the temperature range of 90-450K and frequency range of 10²-10⁵ Hz. The dielectric permittivity at room temperature along a- and c- axis was 430 and 330 respectively. Mechanism for the low temperature dielectric relaxation will be discussed. Thermal expansion behavier in the a- and the c-direction will also be reported.     

Dielectric,transport and thermal properties of clay based polymer‐ nanocomposites

The polymer nanocomposite films (PNC) with varying amounts of organically modified sodium montmorillonite (DMMT) clay in poly(methyl methacrylate) (PMMA) based polymer matrix were prepared by solution cast technique. Dielectric measurements were carried out on these films as a function of frequency at 30°C and 100°C. The addition of clay significantly improved the ionic conductivity. Transport parameters, such as the diffusion coefficient (D), number density (n) and mobility (μ) of charge carriers were determined using a new approach, which is based on impedance spectroscopy. The temperature‐dependent dc conductivity, relaxation and mobility plots obey the Arrhenius rule. The results suggest that the higher ionic conductivity of these PNC films at elevated temperature is not only due to increased mobility of ions, but it is accompanied by a significant increase in carrier concentration. Analysis of DSC thermogram reveals a very high percentage of amorphous content for all samples. A good correlation among dielectric permittivity, carrier concentration, mobility and ionic conductivity has also been observed. POLYM. ENG. SCI., 58:220–227, 2018. © 2017 Society of Plastics Engineers     

Dielectric and thermal properties of dicyclopentadiene containing bismaleimide and cyanate ester. Part IV

A novel bismaleimide (BMI), bis(4-maleimidophenoxy-3,5-dimethylphenyl)dicyclopentadiene (DCPDBMI), containing a large dicyclopentadiene (DCPD) and aryl ether linkage, was synthesized from bis(4-aminophenoxy-3,5-dimethylphenyl)dicyclopentadiene and maleic anhydride by the usual two-step procedure that included ring-opening addition to give bismaleamic acid, followed by cyclodehydration to bismaleimide. The monomers were characterized by Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (NMR), elemental analyses (EA), and mass spectra (MS). A series of bismaleimide-triazine (BT) resins were prepared from synthesized bismaleimide (DCPDBMI) and then cured with 2,6-dimethyl phenol-dicyclopentadiene dicyanate ester (DCPDCY) at various molar ratios. Thermal properties of cured BT resins (DCPDBMI/DCPDCY) were studied using dielectric analyzer (DEA), dynamic mechanical analyzer (DMA) and thermalgravimetric analyzer (TGA). These data were compared with that of commercial bismaleimide (DDMBMI) cured with bisphenol A dicyanate ester (BADCY). The cured DCPDBMI/DCPDCY exhibits lower dielectric constant, dissipation factor and moisture absorption than those of DDMBMI/BADCY. The effects of blend composition on the glass transition temperatures and thermal stability are discussed.     

Morphology and thermal properties of PPS/ABS blend systems

In this study the PPS/ABS blend system was investigated in order to collectively identify the relationship among blend morphology, chemical compatibilization, and thermal property. The ABS resin was chemically modified by the incorporation of maleic anhydride through reactive extrusion for enhanced compatibilization, and ABS/PPS and the modified ABS/PPS blends were prepared by a twin‐screw extruder. The effect of chemical modification of ABS on the morphological, mechanical, and thermal properties of the resulting blend was examined. A strong chemical interaction between PPS and MABS was observed by optical microscopy, scanning electron microscopy, and FTIR. The PPS/MABS blend showed a single glass‐transition temperature in dynamic mechanical analysis, demonstrating pseudo‐homogeneous phase morphology induced by chemical compatibilization. The PPS/MABS blend also exhibited an enhanced thermal stability and heat distortion temperature compared with the PPS/ABS blend. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 661–665, 2003     

Dielectric and thermal properties of Pb (Sc0.5Nb0.5)O3 ceramic

The phase transition behaviour of PSN depends on the order of ions in the B sublattice. Partially ordered PSN has two diffuse phase transitions.     

Elevated temperature thermal properties of advanced materials used in LSF systems

Lightweight cold-formed steel (CFS) construction solutions are increasingly adopted in low and mid-rise buildings. Many different materials are used to construct CFS wall systems, without a full understanding of their thermal properties. For many of these materials, only ambient temperature thermal properties are available from their manufacturers. This creates difficulty in classifying the materials for use at elevated temperatures. In this study, a series of elevated temperature thermal property tests to measure specific heat, thermal conductivity, and mass loss was conducted for a range building materials from wallboards, insulation, and phase-change materials (PCMs), used in Australia and several other countries. Simultaneous Thermal Analyser and Laser Flash Apparatus were used to determine the elevated temperature thermal properties of the selected materials, gypsum plasterboard, PCM incorporated gypsum plasterboard, magnesium sulphate board, fibre cement board, cellulose insulation, vacuum insulation panel, microencapsulated paraffin PCM, and bio-based PCM. Their elevated temperature thermal properties are presented in this article, which also includes analyses of their chemical composition and associated chemical reactions at elevated temperatures. These results can be used in the selection of suitable energy-efficient and fire-resistive materials, and in heat transfer modeling to identify wall configurations with increased fire resistance and energy efficiency.     

Dielectric properties of graphite nanocomposites

Polymer nanocomposites are an emerging class of multifunctional materials that have not been optimized for their functional potential. In this study, the dielectric properties of graphite polymer nanocomposites were evaluated. The objective was to develop for composite applications, an alternative cost‐effective nanoscale carbon material with properties comparable to those of carbon nanotubes. J. VINYL. ADDIT. TECHNOL., 12:127–130, 2006. © 2006 Society of Plastics Engineers     

Dielectric properties of sulphonated polystyrene

Polystyrene was sulphonated with chlorosulphonic acid and the dielectric properties of the films were measured as a function of degree of sulphonation and water content at 27°C at frequencies of 10³ to 10⁶ Hz. The sulphonated samples have a high dielectric constant as compared to the unsulphonated polystyrene. The hydration of polystyrene sulphonic acid is a stepwise process.     

Dielectric properties of cellulose acylates

Thirteen kinds of cellulose acylates from the acetate to stearate were prepared by trifluoro acetic anhydride–fatty acid esterification, and their dielectric constant and loss were measured over wide temperature and frequency ranges. Two types of relaxation process (α_d and β_d) were recognized for all the acylates and an additional relaxation process (γ_d) was detected for the acylates with side chain length longer than that of the butyrate. These α_d to γ_d processes were attributed to the micro-Brownian motion of the main chain (α_d), the motion of oxycarbonyl group of the side chain (β_d) and the motion initiated by minimum three methylene groups in addition to the oxycarbonyl group (γ_d), respectively. In the transition map for the β_d process, inflection points were observed, and the temperature at the inflection point could be regarded as a freezing point of the micro-Brownian motion of the acyl side chain. In the temperature region above the inflection point, plots in transition map for all the acylates were found to be on the same straight line.     

Dielectric,thermal, and swelling properties of calcium ion‐crosslinked sodium alginate film

In this study, Ca²⁺‐crosslinked sodium alginate (SA) gel films (SA‐Ca/SA2‐Ca) have been prepared, and their structural and thermal characterizations were investigated using fourier transform infrared spectroscopy, X‐ray diffraction, differential scanning calorimetry, and thermogravimetric analysis, respectively. Dielectric characterization was performed at room temperature in the frequency range of 12 Hz–100 kHz. The equilibrium swelling value (ESV) of gel films was determined both in distilled water and in 0.1 M sodium chloride (NaCl) solution at room temperature. Sodium alginate films (SA/SA2) were also prepared for use as reference. The effect of the crosslinking of sodium alginate on the dielectric and thermal properties of gel films was investigated by comparing the properties of gel films with those of SA/SA2 films. Although ESV of Ca²⁺‐crosslinked SA film in distilled water is about 350 /g_polymer, it decreased into one third in 0.1 M NaCl solution. The crosslinking of SA did not significantly affect the thermal properties, but it decreased the β‐relaxation associated with the polar side groups. Frequency spectra of electric modulus, impedance and Cole‐Cole plots confirmed the higher conductance values of SA‐Ca films at low frequencies than those of SA film due to the presence of by‐product of crosslinking. POLYM. ENG. SCI., 54:1372–1382, 2014. © 2013 Society of Plastics Engineers     

Dielectric and mechanical properties of poly(vinyl chloride)–dioctylphthalate systems

The mechanical properties, tensile strength, and elongation were investigated for poly(vinyl chloride) (PVC) samples mixed with dioctylphthalate (DOP) at concentrations from 0 to 100 parts per hundred parts PVC at 23°C. It was found that the tensile strength decreased with the increase of concentration, and the elongation was increased until a concentration of 30 DOP content, and then decreased. This leads to the suggestion that intermolecular plasticization is dominant until 30 DOP content, while interstructural plasticization is prevailing for higher concentrations. The permittivity ε′ and the dielectric loss factor ε″ of the same samples have been measured in the frequency range 10²–10⁵ Hz at temperatures from 3 to 96°C. Results show that as the DOP content increases in PVC, the dielectric absorption becomes broader, and the glass transition temperature T_g is lowered. The magnitude of the loss peak decreases with an increase of DOP content to a minimum at concentrations from 40 to 60 DOP content. At higher concentrations the loss peak is increased and T_g is unaltered. Another absorption was observed at 100 Hz and at high temperatures, which was attributed to Maxwell–Wagner effect or direct current conductivity or both of them. It was found that the sample containing 40 parts DOP in 100 parts PVC possesses the best mechanical and electrical properties.     

Studies on rheological,thermal, and morphological properties of methylcellulose gel in aqueous medium

The rheological, morphological, and thermal behavior of aqueous methylcellulose (MC) gels was investigated by optical microscope, dynamic mechanical analyzer (DMA), and differential scanning calorimeter. The double crossover between the storage modulus and loss modulus during the frequency scan in DMA suggests the transition from gel to sol state. These gel networks are comprised of bead‐rod structures, which is visible under the optical microscope. Furthermore, the degelation mechanism of gels comprises of the thermodynamic viable bead burst phenomenon. The reheated samples of MC under dynamic compression show partial irreversibility of the MC aqueous system, indicating the possible presence of weakly coupled random coils and helix. The phenomenon of turbidity of MC aqueous gels is discussed with the help of a large bead model, in which the small‐sized beads present in aqueous solution collide with each other to form large beads. POLYM. ENG. SCI., 59:2024–2031, 2019. © 2019 Society of Plastics Engineers     

Dielectric properties of oligomers: 4. Dielectric properties of vinyl acetate and methyl methacrylate oligomers

Dielectric properties of vinyl acetate and methyl methacrylate oligomers were studied in order to compare the dielectric properties of an oligomer with those of the corresponding high polymers. The two oligomers showed asymmetric dielectric relaxations at room temperatures. The complex dielectric constants vs. angular frequencies for these oligomers were well represented by the Havriliak-Negami equation:

$\text{?}{}^{\mathrm{ast;}}\text{??}{}_{\mathrm{\infty }}\text{=}\text{?}{}_0\text{??}{}_{\mathrm{\infty }}\text{[1+(jωτ}{}_0\text{)}{}^{\mathrm{1?\alpha }}\text{]}{}^{\mathrm{\beta }}$

The distribution parameters (1 - α) and β of vinyl acetate and methyl methacrylate oligomer are almost equal to those of poly(vinyl acetate) and poly(methyl methacrylate), respectively. It was concluded that the distribution of relaxation times was independent of the molecular weight of the polymers.     

Dielectric properties of segmented polytetramethyleneoxide-based polyurethanes

Three series of segmented polyurethanes, based on polytetramethylene oxide (PTMO) soft segments, having molecular weights of 650, 1000, and 2000 and MDI/butane diol hard segments, were synthesized and their dielectric properties examined. The effect of the soft segment length, soft segment concentration (ssc) as structural variables, and frequency and temperature as experimental variables, on relative permittivity and tan δ, were examined. The results were discussed in terms of the structural parameters such as the degree of phase separation and soft segment phase state. It was found that both soft segment length and ssc strongly affect dielectric behavior.     

Dielectric properties of polymer nanoparticle composites

Well-dispersed high dielectric permittivity titanium dioxide (TiO₂) nanoparticles were synthesized utilizing a block copolymer as a template. The nanoparticles were confined within microphase separated domains of sulfonated styrene-b-(ethylene-ran-butylene)-b-styrene (S-SEBS) block copolymers. A crosslinker (vinyltrimethoxysilane) was incorporated into the block copolymer matrices in order to decrease the dielectric loss from the free sulfonic acid groups. Dynamic mechanical analysis experiments confirmed that nanoparticles and crosslinker were confined within the crosslinked sulfonated styrene blocks and had no effect on the chain relaxation behavior of [ethylene-ran-butylene] blocks. Dielectric experiments showed that higher permittivity composites can thus be obtained with a significant decrease in loss tan δ (<0.01) when crosslinked with vinyltrimethoxysilane.     

Dielectric properties of Jordanian oil shales

Microwave heating has been suggested by various authors as a suitable technology for extraction of organic material from oil shales. However, one of the limiting factors in the development of this technology is a lack of accurate dielectric property data for design purposes. In this study the dielectric behaviour of El-lajun oil shale is quantified. The dielectric constant and loss factor of El-lajun oil shale were measured at 2470 and 912 MHz using the cavity perturbation technique. The effects of organic content, temperature, and moisture content on the microwave heating efficiency were quantified. Coaxial probe technique was also employed to study the effect of frequency on dielectric properties of oil shale. Generally, it was found that all samples were of low dielectric loss at room temperature with the imaginary part of permittivity falling significantly after the moisture was removed. This suggests that the major contribution in the dielectric loss is due to the presence of free and/or interlayer water. It was found that both the real and imaginary part of complex permittivity increased with a rise in temperature from 20 up to 80 °C, then dropped significantly at about 100 °C before staying approximately constant up to a temperature of about 480 °C. From this temperature both the real and imaginary parts of complex permittivity increased sharply with further increase in temperature. An attempt was also made to correlate the dielectric properties of the bulk shale sample with the organic content. However, no correlation between dielectric properties and organic matter content was found.     

Dielectric properties of polar rubber mixtures

A systematic study of the dielectric properties of Perbunan N mixed with silica in increasing quantities is carried out in the frequency range 500 Hz to 10¹⁰ Hz at room temperature. The data are analysed using the Cole-Cole method and the results are interpreted.