Measurement of specific heat and energetic characterization of materials up to high temperatures

A new DSC system has been developed which not only allows quantitative results in the temperature of –160C to 700C, but also allows the quantitative determination of a variety of material properties up to 1500C. For example, the specific heat of materials can be measured to at least 1400C, while enthalpies, etc. can be measured to 1500C.

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Zusammenfassung Ein neuartiges DSC Messystem (Netzsch DSC 404) wurde entwickelt, das sich durch hohe Reproduzierbarkeit der Basislinie, grosse Empfindlichkeit und breiten Temperaturanwendungsbereich (–160C bis 700C resp. bis 1500C) auszeichnet. Die Messanordnung ermöglicht die Verwendung von unterschiedlichen GasatmosphÄren als auch Messungen im Vakuum. Es werden Beispiele der Bestimmung von SchmelzvorgÄngen, der Glasumwandlungstemperatur, der KristallinitÄt und der spezifischen WÄrme, sowohl für Polymere als auch für anorganische Materialien dargestellt und diskutiert.

     

A method for the determination of the specific heat and heat of decomposition of composite materials

A method has been developed to determine the specific heat of a material during thermal decomposition using a combination of DSC and TGA data obtained at the same heating rate. The heat of decomposition is calculated simultaneously using the same data. This technique was used to determine the specific heat and heat of decomposition of a widely used fiberglass-filled phenol—formaldehyde resin and a fiberglass-filled acrylonitrile—butadiene (AB) copolymer. Experimental data are presented for the specific heat of both the virgin and char components for temperatures between 60 and 730°C. The calculated specific heat of the mixture during decomposition for both materials is also presented.     

Response of a glass/phenolic composite to high temperatures

Determining the response of composite phenolic materials to fire remains a major unsolved problem that is important for high consequence safety analysis. Difficulties arise when thermophysical property measurements are obscured by decomposition reactions. This article presents several decomposition experiments and models for a phenolic resin impregnated into chopped 1.27-by-1.27 cm glass fabric. The thermal response of the material was measured using thermogravimetric analysis (TG), differential scanning calorimetry (DSC), and laser flash diffusivity (LFD). The TG data was used to develop a 5-step decomposition mechanism describing mass loss due to reaction; the DSC data was used to describe the energy changes associated with these reactions; and the LFD data was used to describe energy flow into the decomposing material. An effective thermal conductivity model was used to partition energy transport by gas conduction, solid conduction, and diffusive radiation. The dynamic gas volume fraction is treated as a field variable to extrapolate thermal transport properties at high temperatures where decomposition is prevalent. These various models have been implemented into a finite element response model with an example calculation that includes uncertainty.     

Incubation in cyclohexene decomposition at high temperatures

A detailed master equation simulation has been carried out for the thermal unimolecular decomposition of C₆H₁₀ in a shock tube. At the highest temperatures studied experimentally [J. H. Kiefer and J. N. Shah, J. Phys. Chem., 91, 3024 (1987)], the average thermal vibrational energy is greater than the reaction threshold and therefore 〈ΔE〉 (up and down steps) is positive for molecules at that energy, rather than negative; the converse is true at lower temperatures. The calculated incubation time, in which the decomposition rate constant rises to 1/e of its steady state value, is found to be only weakly dependent on temperature (at constant pressure) between 1500 K and 2000 K and to depend almost exclusively on 〈ΔE〉_d (down steps, only), and not on collision probability model. Simulations of the experimental data show the magnitude of 〈ΔE〉_d depends weakly on assumed collision probability model, but is nearly independent of temperature. The second moment 〈ΔE〉^½ is found to be independent of both temperature and transition probability model. The experimental data are not very sensitive to the possible energy-dependence of 〈ΔE〉_d for a wide range of assumptions. It is concluded that the observed experimental “delay times” probably can be identified with the incubation time; further experiments are desirable to test this possibility and obtain more direct measures of the incubation time.     

Ionic conduction in composite polymer electrolytes at subambient temperatures

The majority of investigations carried out on polymer(SINGLEBOND) salt systems have been on polyether electrolytes at moderate temperatures where such electrolytes exhibit macroscopic uniformity. Relatively little attention has been paid to the subambient temperature region where composite electrolytes based on polyethers exhibit much higher conductivities than their pure polyether electrolyte analogues. For all of the composite systems studied the conduction mechanism changes from one in which the ions are coupled to the polymer segmental relaxations to one in which the ions are decoupled and thermally activated ionic hopping produces higher conductivities than would be expected from ion-segmental coupling and higher than observed for the base polyether(SINGLEBOND) salt system. This change has been observed at temperatures between 10 and 80°C above the respective glass transition temperatures. The relationship between this interaction and these higher conductivities at subambient temperatures is explored and discussed. © 1996 John Wiley & Sons, Inc.     

Ionic liquid functionalized polymer composite nanotubes toward dye decomposition

Poly(ionic liquid) functionalized composite nanotubes are achieved by ATRP grafting ionic liquid monomers for example Vi Et Im+Bràfrom the poly(DVB-co-VBC) nanotubes surface. PW12O403àanion is introduced through anion exchange. The PW12O403àbased composite nanotubes can synchronously absorb and decompose water soluble dyes for example methyl orange(MO). The cooperative interplay is promising in highly efficient decomposition of dyes.     

The rate of methyl radical decomposition at high temperatures and pressures

Rate constants are calculated for CH₃ (+ Ar) ? CH₂ + H (+ Ar) at the limiting low-pressure, the limiting high-pressure, as well as the intermediate fall-off ranges. The results show that published experimental rate constants for methyl dissociation correspond to the fall-off region close to the low-pressure limit. At the low-pressure limit the activation energy is less than the bond dissociation energy, in agreement with experimental results. Forward and backward rate coefficients at the high-pressure limit are compared with other theoretical calculations. More theoretical and experimental work is necessary to understand the reverse reaction and its competing reactions, as well as the decomposition channel leading to CH + H₂. © 1994 John Wiley & Sons, Inc.     

Transesterification in polymer blends including polycarbonate at high temperatures

Miscibility of polycarbonate (PC)/poly (arylate) (PAr) was studied by differential scanning calorimetry (DSC), phase contrast microscopy with digital image analysis (DIA) system, FT-IR spectroscopy, and cloud points for the PC/PAr/methylene chloride ternary systems. PC and PAr were immiscible over the whole composition range from the thermodynamical viewpoint. However, PC/PAr is homogenized by transesterification between PC and PAr at high temperatures. There is observed a competition between phase separation due to thermodynamical factor and phase dissolution due to transesterification. The pattern of the phase separation was spinodal decomposition type and the structure was successfully analysed by DIA.     

Ultracentrifugal equilibrium measurement of high polymer solutions at elevated temperatures

To investigate the solution properties of polyethylene, which has the simplest structure of the vinyl polymers, experiments were made with a magnetically suspended equilibrium ultracentrifuge. Preliminary studies were carried out with a polystyrene–chloroform system at 25°C. and a polystyrene–methylcyclohexane system at 68°C. (which is close to the theta temperature) in order to check the difficulties involved in the flotation equilibrium in the former case and the high temperature measurement in the latter. However, no trouble was encountered in either system, and the results were discussed and compared with earlier results for polystyrene solutions. It was found that chloroform is a good solvent for polystyrene, and the measured weight-average molecular weight is somewhat smaller than the value obtained in a theta solvent. After overcoming some technical difficulties involved in studies at higher temperatures, we carried out experiments on polyethylene in α-chloronaphthalene at 130°C. The results are considered reasonable by comparison with results obtained by other methods. The sample employed, Marlex 50 of melt index 0.7, has a wide molecular weight distribution: i.e., M_z/M_w = 5.2 and M_z+1/M_z = 2.4.     

The specific heat of linear polyethylene after annealing at low temperatures

After annealing at low temperature, linear polyethylene displays an unusual feature in its specific heat curve. On heating, a maximum is observed just above the annealing temperature. The magnitude of this excess specific heat is dependent on the initial level of crystallinity and the temperature and time of annealing. The maximum does not reappear on subsequent cooling followed by rapid heating and represents the formation and disappearance of an unstable structure. These results can be interpreted as a crystallization-melting phenomenon and are consistent with the wide-angle x-ray diffraction patterns. The optimum effects are observed in the β-region (?50°C to 0°C) and could possibly lead to complications in interpreting other phenomena in this temperature range.     

Solid state NMR of a polymer composite and of a polymer blend

Although nuclear magnetic resonance may not seem the technique of choice to study interfaces between components in a polymer composite or polymer blend because of its inherent low sensitivity, for certain systems solid state NMR techniques can emphasize the signals from these interfaces. In case of a composite material with an inorganic filler (particles, fibers) cross-polarization or dipolar dephasing techniques from protons in the organic matrix or in the interphase to nuclei in the filler can be used to selectively observe the nuclei in the surface of the filler. Any changes at the filler surface caused by the presence of the matrix or coupling agent can then be detected. An example of glass reinforced nylon modelcomposites is discussed. The same techniques can also be used to study interphases in polymer blends when one of the components contains a NMR nucleus that is not present in the other component. As an example the blend poly(vinylidene fluoride)-poly(methyl methacrylate) is studied and it is shown that such techniques can provide very detailed information about the miscibility at a molecular scale.     

Tensile mechanical properties of basalt fiber reinforced polymer composite under varying strain rates and temperatures

High-strength woven fabrics and polymers are ideal materials for use in structural and aerospace systems. It is very important to characterize their mechanical properties under extreme conditions such as varying temperatures, impact and ballistic loadings. In this present work, the effects of strain rate and temperature on the tensile properties of basalt fiber reinforced polymer (BFRP) were investigated. These composites were fabricated using vacuum assisted resin infusion (VARI). Dynamic tensile tests of BFRP coupons were conducted at strain rates ranging from 19 to 133 s⁻¹ using a servo-hydraulic high-rate testing system. Additionally, effect of temperature ranging from −25 to 100 °C was studied at the strain rate of 19 s⁻¹. The failure behaviors of BFRP were recorded by a Phantom v7.3 high speed camera and analyzed using digital image correlation (DIC). The results showed that tensile strength, toughness and maximum strain increased 45.5%, 17.3% and 12.9%, respectively, as strain rate increased from 19 to 133 s⁻¹. Moreover, tensile strength was independent of varying temperature up to 50 °C but decreased at 100 °C, which may be caused by the softening of epoxy matrix and weakening of interfaces between fibers and matrix when the glass transition temperature was exceeded.     

Segregated reduced graphene oxide polymer composite as a high performance electromagnetic interference shield

Herein, we report the synthesis of a graphene/polymer composite via a facile and straightforward approach for electromagnetic interference (EMI) shielding applications. Polystyrene (PS) beads were added in graphene oxide (GO)/water solution followed by the addition of hydroiodic acid (HI) for in situ reduction of GO. The composite solution (rGO/PS) was filtered, hot compressed and tested for EMI shielding and dielectric measurements. A 2-mm thick segregated rGO/PS sample with 10 wt% filler loading delivered a high EMI shielding effectiveness (SE) of 29.7 dB and an AC electrical conductivity of 21.8 S m^?1, which is well above the commercial requirement for EMI shielding applications. For comparison with the segregated rGO/PS composite, a control polymer composite sample utilizing a thermally reduced graphene oxide was synthesized by following a conventional coagulation approach. The as-synthesized conventional rGO/PS yield an EMI SE of 14.2 dB and electrical conductivity of 12.5 S m^?1. The high EMI shielding of segregated rGO/PS is attributed to the better filler-to-filler contact among graphene layers surrounded by PS beads and also to the better reduction and preservation of graphene structure during reduction process that makes the low temperature chemically reduced segregated rGO/PS approach a viable route compared to high temperature thermally reduced conventional rGO/PS approach.     

The effect of ozone and high temperature on polymer degradation in polymer core composite conductors

The next generation High Temperature Low Sag Polymer Core Composite Conductors can experience harsh in-service environments including high temperature and highly concentrated ozone. In some extreme cases, it is possible that the conductors will experience temperatures of up to 180 °C and ozone concentrations as high as 1% (10,000 ppm). Therefore, the goal of this work was to understand the degradation mechanisms in a high temperature epoxy, which could be used in the conductors at temperatures as high as 140 °C in the presence of 1% ozone. Then, the combined aging data for the epoxy were compared to the aging results from room temperature aging in 1% ozone and aging in air at 140 and 180 °C. In addition, important but limited aging testing was also performed on a set of PCCC rods to verify some of the observations from the neat resin experiments. It was determined that the mass loss, volumetric shrinkage, and flexural strength reductions of the epoxy aged at 140 °C were driven almost entirely by temperature and that the effect of 1% ozone at that temperature can be thought of as insignificant for aging times up to 90 days. The composite rods displayed postcuring at 140 °C and were also unaffected by the presence of ozone at aging time lengths of 90 days. Up to this time aging the polymer and composite specimens in atmospheric 180 °C resulted in the most drastic changes in both physical and mechanical properties, except viscoelasticity where the polymer specimens aged at 140 °C with 1% ozone showed the greatest increase in the storage modulus. The least amount of degradation to the materials was found to occur after aging at room temperature in 1% ozone.     

The solution of Pennes' bio-heat equation with a convection term and nonlinear specific heat capacity using Adomian decomposition

Journal of Thermal Analysis and Calorimetry - Pennes' bio-heat equation is the most widely used equation to analyze the heat transfer phenomenon associated with hyperthermia and cryoablation...     

Kinetics of polymer decomposition

Kinetic equations have been developed for polymers that decompose by the zipper mechanism. The usual assumption of steady state kinetics has not been made. Plots of the fraction decomposed α, as a function of time demonstrate a variety of patterns depending upon the relative values of the fraction k₁, of chains becoming activated per second and the fraction k₂ of an activated chain that decomposes per second.     

Thermal decomposition temperatures of metal sulfates

The thermal decomposition of sixteen metal sulfates was studied by thermogravimetry at heating rates of 2 and 5°C min^?1 in flowing air and high-purity nitrogen. Their decomposition behaviors, especially the initial decomposition temperatures, were examined with relation to the thermodynamic functions for decomposition. Of the factors possibly influencing the decomposition temperature, the equilibrium SO₃ pressures over the sulfates were evaluated: the equilibrium pressures at the initial temperatures for sulfates of metals, of which the oxidation state was unchanged during decomposition, were nearly equal to 1×10^?4 atm at 2°C min^?1 in flowing nitrogen.