Measurement of thermal diffusivity and specific heat capacity of polymers by laser flash method

We measured thermal diffusivity and heat capacity of polymers by laser flash method, and the effects of measurement condition and sample size on the accuracy of the measurement are discussed. Thermal diffusivities of PTFE films with thickness 200–500 μm were the same as those data that have been reported. But, the data for film thickness less than 200 μm have to be corrected by an equation to cancel thermal resistance between sample film and graphite layers for receiving light and detecting temperature. Thermal diffusivity was almost unaffected by the size of area vertical to the direction of laser pulse, because heat flow for the direction could be negligible. Specific heat capacity of polymer film was exactly measured at room temperature, provided that low absorbed energy (< 0.3 J) and enough sample mass (> 25 mg) were satisfied as measuring conditions. Thermal diffusivity curve of PS or PC versus temperature had a terrace around T_g, whereas that of PE decreased monotonously with increasing in temperature until T_m. Further, we estimated relative specific heat capacity (RC_p) by calculating ratios of heat capacities at various temperatures to the one at 299 K. RC_p for PS obtained by laser flash method was larger than that obtained by DSC method, whereas the RC_ps for PE obtained by the both methods agreed with one another until T_m (305 K). RC_p for PS decreased linearly, with increase in temperature after it increased linearly until T_g (389 K), showing similarity to temperature dependency of thermal conductivity. RC_p for PE also decreased until T_m, similar to thermal conductivity. ©1995 John Wiley & Sons, Inc.     

Transport properties, specific heat and thermal conductivity of GaN nanocrystalline ceramic

The structural and transport properties (resistivity, thermopower and Hall effect), specific heat and thermal conductivity have been measured for GaN nanocrystalline ceramic prepared by hot pressing. It was found that the temperature dependence of resistivity in temperature range 10-300 K shows the very low activation energy, which is ascribed to the shallow donor doping originating in amorphous phase of sample. The major charge carriers are electrons, what is indicated by negative sign of Hall constant and Seebeck coefficient. The thermopower attains large values (−58 μV/K at 300 K) and was characterized by linear temperature dependence which suggests the diffusion as a major contribution to Seebeck effect. The high electron concentration of 1.3×10¹⁹ cm⁻³ and high electronic specific heat coefficient determined to be 2.4 mJ/molK² allow to conclude that GaN ceramic demonstrates the semimetallic-like behavior accompanied by very small mobility of electrons (∼0.1 cm²/V s) which is responsible for its high resistivity. A low heat conductivity of GaN ceramics is associated with partial amorphous phase of GaN grains due to high pressure sintering.     

Thermal diffusivity of some linear polymers

Summary Thermal diffusivity of several linear polymers and cross-linked polyester was measured as Function of temperature between 10‡ and 100 ‡C. Thermal diffusivity is lowered by reducing the interaction between chain molecules in amorphous polymers. In partially crystalline polymers, the crystallinity has a considerable effect on unsteady-state heat conduction. The variation of diffusivity with crystallinity was studied for polyethylene and polytrifluorochloroethylene.

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Zusammenfassung Die Temperaturleitf?higkeit verschiedener linearer Polymeren und vernetzter Polyester wurde als Punktion der Temperatur zwischen 10 und 100 ‡C gemessen. Die Temperaturleitf?higkeit wird durch Reduzierung der Wechselwirkung zwischen den Kettenmolekülen in amorphen Polymeren erniedrigt. In partiell kristallinen Polymeren spielt die Kristallinit?t in betr?chtlicher Weise bei der nichtstation?ren W?rmeleitung eine Rolle. Die Variation der Temperaturleitf?higkeit mit der Kristallinit?t wurde für Poly?thylen und Polytrifluorchlor?thylen untersucht.

     

Intercomparison of thermal conductivity and thermal diffusivity methods for plastics

An intercomparison of measurements of the thermal conductivity and thermal diffusivity of two poly(methyl methacrylates) is reported. A wide variety of methods were used: temperature wave analysis, laser flash, transient plane source (Hot Disk^®), transient line-source probe, and heat flux meter methods. Very good agreement of thermal conductivity results and, separately, of thermal diffusivity results was obtained. Similarly, good agreement between thermal conductivity and thermal diffusivity results, when converted using specific heat capacity and density values, was also obtained. Typically, the values were within a range of approximately ±10%. Considering the significant differences between the methods and the requirements on specimen dimensions, the level of agreement between results was considered to be good.     

The thermal diffusivity and thermal conductivity coefficient of methanol vapour

The thermal diffusivity of methanol vapour is measured by the thermal lens technique giving values of 0.76, 1.02, 1.70 and 1.78 N s⁻¹ at 321, 341, 377 and 405 K respectively. These thermal diffusivities are shown to be pressure-independent, the pressure dependence of the thermal conductivity merely reflecting the pressure-dependent heat capacity caused by the presence of dimers and tetramers.     

Temperature dependence of the heat diffusivity of proteins

In a combined experimental-theoretical study, we investigated the transport of vibrational energy from the surrounding solvent into the interior of a heme protein, the sperm whale myoglobin double mutant L29W-S108L, and its dependence on temperature from 20 to 70 K. The hindered libration of a CO molecule that is not covalently bound to any part of the protein but is trapped in one of its binding pockets (the Xe4 pocket) was used as the local thermometer. Energy was deposited into the solvent by IR excitation. Experimentally, the energy transfer rate increased from (30 ps)(-1) at 20 K to (8 ps)(-1) at 70 K. This temperature trend is opposite to what is expected, assuming that the mechanism of heat transport is similar to that in glasses. In order to elucidate the mechanism and its temperature dependence, nonequilibrium molecular dynamics (MD) simulations were performed, which, however, predicted an essentially temperature-independent rate of vibrational energy flow. We tentatively conclude that the MD potentials overestimate the coupling between the protein and the CO molecule, which appears to be the rate-limiting step in the real system at low temperatures. Assuming that this coupling is anharmonic in nature, the observed temperature trend can readily be explained.     

Determinants of thermal conductivity and diffusivity in nanostructural semiconductors

The origin of size effects in the thermal conductivity and diffusivity of nanostructural semiconductors was investigated through the establishment of a unified nanothermodynamic model. The contributions of size-dependent heat capacity and cohesive energy as well as the interface scattering effects were considered during the modeling. The results indicate the following: (1) both the thermal conductivity and diffusivity decrease with decreasing nanocrystal sizes (x) of Si and Si/SiGe nanowires, Si thin films and Si/Ge(SiGe) superlattices, and GaAs/AlAs superlattices when x > 20 nm; (2) the heat transport in semiconductor nanocrystals is determined largely by the increase of the surface (interface)/volume ratio; (3) the interface scattering effect predominates in the reduction of thermal conductivity and diffusivity while the intrinsic size effects on average phonon velocity and phonon mean free path are also critical; (4) the quantum size effect plays a crucial role in the enhancement of the thermal conductivity with a decreasing x (<20 nm). These findings provide new insights into the fundamental understanding of high-performance nanostructural semiconductors toward application in optoelectronic and thermoelectric devices.     

Dilatometric measurements on some polymers: The pressure dependence of thermal properties

Summary For linear polyethylene, polyethyleneoxide and isotactic polypropylene we report measurements of the specific volume as a function of temperature and pressure, up to 1 kbar (for PE up to 10 kbar) obtained by slow cooling from the melt (6 K/h). Expansivity and compressibility of the melt and the temperature of crystallization are evaluated. By means of the volume of fusion and the Clausius-Clapeyron equation — the applicability of which is discussed briefly — one gets the enthalpy and the entropy of fusion. For polyethylene and polyethyleneoxide these data are shown to decrease with increasing pressure.

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Zusammenfassung Messungen des spezifischen Volumens als Funktion der Temperatur und des Drucks wurden an linearem Polyäthylen, an Polyäthylenoxid und an isotaktischem Polypropylen durch langsames Abkühlen aus der Schmelze (6 K/h) in einem Bereich bis 1 kbar (PE bis 10 kbar) durchgeführt. Daraus ausgewertet wurden die Ausdehnung und die Kompressibilität der Schmelze, sowie die Kristallisationstemperatur. Der Volumensprung sowie die Schmelzenthalpie und die Schmelzentropie wurden berechnet — die beiden letzteren mit Hilfe der Clausius-Clapeyronschen Gleichung, deren Anwendbarkeit diskutiert wird. Für Polyäthylen und Polyäthylenoxid nehmen diese Werte mit steigendem Druck ab.

     

Thermal diffusivity and thermal conductivity studies on the zirconate,cerate and uranate of barium

The thermal diffusivity and thermal conductivity of the zirconate, cerate and uranate of barium were investigated by employing the laser flash technique. The variation in the thermal resistivity as a function of temperature was examined and the heat transport behaviour of these materials is discussed.     

Thermal conductivity, thermal effusivity, and specific heat capacity near the lower critical point of the binary liquid mixture n-butoxyethanol-water

Experimental investigations on binary liquid mixtures near the critical mixing point are presently leading to a controversy about the anomaly in the thermal conductivity. A photopyroelectric technique is used to determine the thermal conductivity and the effusivity of the binary liquid mixture n-butoxyethanol-water at its critical concentration near the critical mixing point. It is proven that, contrary to previous reports, there is no critical enhancement in the thermal conductivity. The specific heat capacity is calculated from these results and compared with the results from measurements performed by adiabatic scanning calorimetry.     

Effect of pressure on the thermal conductivity of polymers

The thermal conductivity of polyolefins and halogen-substituted polymers was studied in a broad temperature interval spanning both solid and melt states, in the range of pressures from 0.1 up to 100 MPa with the aid of a high-pressure-calorimeter in the continuous heating regime. Treatment of data on the pressure dependence of the thermal conductivity of melts in terms of Barker's equation yielded the values of quasilattice Grueneisen parameter _B which exhibited the same dependence on molecular structure of a polymer as the parameter 3C/p from the Simha-Somcynsky equation of state (number of external degress of freedom per chain repeat unit). Analysis of the dependence of the thermal conductivity of polyethylene on the degree of crystallinity revealed the inadequacy of the current two-phase model which does not account for the microheterogeneity of the amorphous phase. It was concluded that interchain heat transfer makes the dominant contribution to the thermal conductivity of polymers both in amorphous and in crystalline states.

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Zusammenfassung Mit Hilfe eines Hochdruck-- Kalorimeters mit kontinuierlicher Aufheizung wurde im Druckintervall 0,1 bis 100 MPa und in einem breiten Temperaturbereich, in den sowohl feste als auch flüssige Zustände gehören, die Wärmeleitfähigkeit von Polyolefinen und halogenierten Polymeren untersucht. Drückt man die Druckabhängigkeit der Wärmeleitfähigkeit der Schmelzen mit Hilfe der Barkerschen Gleichung aus, erhält man die Werte für den Quasigitter Grueneisen-Parameter_b, der die gleiche Abhängigkeit von der Molekular-struktur eines Polymers zeigt, wie der Parameter 3C/p aus der Gleichung von Simha-Somcynsky (Zahl der externen Freiheitsgrade geteilt durch Kettenstruktureinheit). Eine Untersuchung der Abhängigkeit der Wärmeleitfähigkeit von Polyethylen von Kristallinitäts-grad zeigt die Mängel dieses Zwei-Phasen-Modelles, was die Mikroheterogenität der amorph-en Phase nicht erklärt. Man zog die Schlußfolgerung, daß ein Wärmetransport zwischen den Ketten sowohl im amorphen als auch im kristallinen Zustand den entscheidenden Beitrag zur Wärmeleitfähigkeit von Polymeren liefert.

     

Anomalous changes in the specific heat and thermal conductivity of LiKSO4 crystals around the high-temperature phase

The thermophysical properties of LiKSO₄ crystals were studied around the high-temperature phase, atT _{c 2}=943 K. A Heraeus (DSC) technique was used to measure the specific heat,c _p, while the thermal conductivity,K, was measured by the linear heat flow steady-state method. The measured parameters showed an anomaly in the temperature dependence of bothc _p andK. Anisotropy in the thermal conductivity coefficient was also observed in the different crystallographic axes.     

Low temperatures lattice thermal conductivity on non-crystalline polymers

The density fluctuation model is used to analyze the lattice thermal conductivity data of two samples of polycarbonate between 0.04 and 1K. The study is carried out by calculating the latice thermal conductivity of a noncrystalline polymer as the sum of two contributions asK=K _BM+K _Em, whereK _BE is attributed to phonons which interact with the crystal boundaries,K _EM is due to phonons which interact with the empty spaces. The relative importance of each contribution has also been examined by estimating their percentage contributions to the lattice thermal conductivity. An excellent fit to the experimental data was obtained over the whole temperature range.     

Thermal conductivity and thermal expansivity of thermotropic liquid crystalline polymers

The thermal conductivity and thermal expansivity of a thermotropic liquid crystalline copolyesteramide with draw ratio λ from 1.3 to 15 have been measured parallel and perpendicular to the draw direction from 120 to 430 K. The sharp rise in the axial thermal conductivity K_par; and the drastic drop in the axial expansivity α_∥ at low λ, and the saturation of these two quantities at λ > 4 arise from the corresponding increase in the degree of chain orientation revealed by wide-angle x-ray diffraction. In the transverse direction, the thermal conductivity and expansivity exhibit the opposite trends but the changes are relatively small. The draw ratio dependences of the thermal conductivity and expansivity agree reasonably with the predictions of the aggregate model. At high orientation, K_par; of the copolyesteramide is slightly higher than that of polypropylene but one order of magnitude lower than that of polyethylene. In common with other highly oriented polymers such as the lyotropic liquid crystalline polymer, Kevlar 49, and flexible chain polymer, polyethylene, α_par; of the copolyesteramide is negative, with a room temperature value differing from those of Kevlar 49 and polyethylene by less than 50%. Both the axial and transverse expansivity show transitions at about 390 and 270 K, which are associated with large-scale segmental motions of the chains and local motions of the naphthalene units, respectively. ©1995 John Wiley & Sons, Inc.     

Thermal diffusivity of polymers

Summary The thermal diffusivity of a homologous series of polymethylmethacrylates in the molecular weight range 2· 10²–10⁶ was investigated. The thermal diffusivity shows several transitions: The main glass transition and a transition from one liquid phase to another one. The decrease of the thermal diffusivity in the glass transition range is discussed. The two transitions are explained as caused by blocks of syndiotactic and isotactic segments contained in the atactic structure.

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Zusammenfassung Die Temperaturleitfähigkeit einer homologen Reihe von Polymethylmethacrylaten des Molekulargewichtsbereichs 2·10²–10⁶ wurde gemessen. Die Temperaturleitfähigkeit zeigt mehrere Übergänge: Die Glasumwandlung und einen Übergang von einer flüssigen Phase in eine andere. Die Abnahme der Temperaturleitfähigkeit im Einfrierbereich zum Glas wird diskutiert. Der Übergang im flüssigen Bereich wird mit Anteilen syndiotaktischer und isotaktischer Blöcke erklärt, die in der ataktischen Struktur vorkommen.

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HerrnD. Ziegler sind wir für die sorgfältige Durchführung der Messungen zu sehr großem Dank verpflichtet.

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Dem Verband der Chemischen Industrie danken wir für die Unterstützung dieser Arbeit.