Recent development and applications of an optical method for measurements of thermophysical properties

The experimental determination of thermophysical properties has been greatly improved by the introduction of laser technology. The laser beam is used for sensing and also for heating (or exciting) the specimen. The advantage of using a laser beam is most strongly felt in the measurement of the thermal conductivity or the thermal diffusivity, which are some of the most difficult properties to measure. Interesting features of new techniques for investigating various aspects of thermal conductivity in fluids and solids are reviewed. An optical method, the so-called forced Rayleigh scattering method, or the laser-induced optical-grating method, has been developed and used extensively by the present author's group. The method is a high-speed remote-sensing method which can also quantitatively detect anisotropy, namely, direction dependence of heat conduction in the material. It was used for determination of the thermal diffusivity and its anisotropic behavior for high-temperature materials such as molten salts, liquid crystals, extended polymer samples, and flowing polymer melts under shear. Interesting applications of the method were demonstrated also for thermal diffusivity mapping and microscale measurement.Invited paper presented at the Twelfth symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Measurement of the Thermal Diffusivity of an Anisotropic Graphite Sheet Using a Laser-Heating AC Calorimetric Method

The thermal diffusivity of a graphite sheet having an extremely high anisotropy has been measured by a laser heating AC calorimetric method in the temperature range from 30 to 350 K. This graphite sheet has characteristics of high thermal diffusivity and high anisotropy, and it is only 100 m thick. Thus, it is difficult to apply the conventional AC technique. Therefore, we propose a simultaneous measurement method for the in-plane and out-of-plane thermal diffusivities, by analyzing the three-dimensional heat conduction process, which contains the effects of anisotropy and thermal wave reflections. This method was verified by checking with thermal diffusivity measurements of isotropic materials such as stainless steel and pure copper and was then applied to the anisotropic thermal diffusivity measurement of the graphite sheet.     

Measurement of transient behavior of the thermal diffusivity of flowing polymer melt

Molecular orientation of polymer molecules created by shear force due to the flow of the polymer melt in a duct rapidly relaxes after the change in the shear rate. This relaxation causes the changes in anisotropic behavior of mechanical. thermal, and optical properties which have strong effect in precision forming of casted polymer products. Among these properties, thermal diffusivity is one of the most difficult properties to measure by the conventional techniques. The present paper describes and discusses the subsecond measurement of relaxation characteristics in thermal dilfusivity anisotropy of flowing polymer melt in conjunction with measurements of some typical flow characteristics. The method used was the forced Rayleigh scattering method, an optical method which was developed and modified by the authors' group for applying to oriented polymer materials. The measuring lime of I ms can trace the relaxation occurred in the order of several seconds. Measured results of thermal diffusivity anisotropy were compared with transient characteristics of the now and quantitatively showed a good agreement with estimated behavior of oriented polymer molecules.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–39, 1995, Köln, Germany.     

The hydration of Portland and aluminous cements with added polymer dispersions

We have studied the hydration of ordinary Portland cement and Secar 71 aluminous cement with added polyvinyl alcohol, polyvinyl acetate, acrylic, poly (vinylidene chloride-acrylic), poly (styrene-acrylic) polymers and poly (styrene-butadiene) rubber latex. We have used the techniques of conduction and differential scanning calorimetry, infrared spectroscopy and X-ray diffraction in our studies.The nature of the interactions between the polymers and the two cements is significantly different. For Portland cement all polymers were found to retard the hydration rate to some extent, with the acrylics producing the maximum affect, whereas in the case of Secar 71, polymer additions had minimal effects on the hydration of or heat evolution from the cement. Our results suggest that many polymers interact with Portland cements in a chemical way although for aluminous cements the observations are less clear except in the case of polyvinyl alcohol-acetate.     

Measurements of the Thermal Conductivity and Thermal Diffusivity of Polymer Melts with the Short-Hot-Wire Method

In this paper, the thermal conductivity and thermal diffusivity of four kinds of polymer melts were measured by using the transient short-hot-wire method. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a wire with the same length-to-diameter ratio and boundary conditions as those in the actual experiments. The present method is particularly suitable for measurements of molten polymers where natural convection effects can be ignored due to their high viscosities. The results have shown that the present method can be used to measure the thermal conductivity and thermal diffusivity of molten polymers within uncertainties of 3 and 6%, respectively. Further, the thermal conductivity and thermal diffusivity of solidified samples were also measured and discussed.     

The structure of thermotropic copolyesters

Highly oriented polymeric products have been produced over the past fifteen years by two very different processing routes; from conventional polymers processed to highly oriented extended chain structures, and from rod-like polymers which exhibit liquid crystalline behaviour. Gel spun polyethylene is an example of such a conventional polymer. There are three main types of liquid crystalline polymers (LCP) which have high orientation and modulus: lyotropic aramids, such as poly(-phenylene terephthalamide) (PPTA); lyotropic, aromatic heterocyclic polymers, or ordered polymers; and the family of thermotropic aromatic copolyesters. Extensive characterization of the thermotropic copolyesters has resulted in the delineation of a fibrillar, hierarchial structural model which accounts for the structures observed in a broad range of oriented fibres, extrudates and moulded articles. Three distinct fibrillar species are observed: microfibrils that are about 50 nm, fibrils about 500 nm, and macrofibrils about 5m, in size. Superimposed on the structural hierarchy is a defect hierarchy, defined by the regular meander of the molecular chain and a localization of defects within a microfibril at about a 50 nm periodicity. Orientational variations, layering and skin core structures, in thick specimens, are the result of local flow fields on the basic structural units during solidification. The fibrillar textures appear to be present prior to any preparation for microscopy. A wide range of specimen preparation methods, i.e. fractography, sonication, microtomy and etching, and microscopic techniques, i.e. optical, scanning and transmission electron microscopy, were applied to the characterization of the aromatic copolyesters and PPTA. Interestingly, the same basic hierarchy is observed for both the lyotropic and the thermotropic LCPs and the microfibrillar structures of all the highly oriented polymers, including polyethylene, appear quite similar.     

Mean Square End-to-End Distance of a Semiflexible Polymer on the Bethe Lattice

A combinatorial method to calculate the mean square end-to-end distance R ² of a polymer on a Bethe lattice is used. The case of an anisotropic lattice and semiflexible polymers is considered. The distance on the Cayley tree is defined by embedding the tree on an N-dimensional Euclidean space considering that every bend of the polymer defines a direction orthogonal to all the previous ones. The semiflexible polymer is effectively equivalent to a flexible one if one considers an effective (noninteger) coordination number. Although an analytical calculation is performed, a closed expression for R ² is possible only for the isotropic case. Numerical results are shown for the anisotropic case. Plots of R ² against N for different values of the anisotropy parameter y are shown. The power dependence for N does not depend on the anisotropy as expected, but the linear coefficient increases on increasing the anisotropy. The anisotropy tends to stretch the polymer.     

Miscibility and morphology of poly p-phenylene sulphide–liquid crystal polymer blends

Blends of poly p-phenylene sulphide (PPS) and a liquid crystalline polymer (LCP) were made by two methods: (i) mixing and capillary extrusion (samples A), and (ii) injection moulding (samples B). To study miscibility in the melt and solid states and the resulting morphology, techniques like polarized light optical microscopy, capillary rheometry, dynamic mechanical thermal analysis and scanning electron microscopy with X-ray microanalysis were used. It was observed that the miscibility of the amorphous fractions of both polymers increased with increasing intensity (rates and stresses) of deformational flow (shear and elongational). Samples A had a morphology composed of fibrils of both polymers, but a matrix made of only one polymer i.e. PPS. Samples B had a mainly fibrillar morphology, with no observable matrix, made of both polymers. Formation of pure LCP fibrils was not observed neither in the extruded blends nor in the injection moulded samples. The addition of LCP to PPS improved its mechanical properties. At a molecular level, these blends can be considered to be molecular composites.     

Low-temperature anisotropy of thermal conductivity of tin single crystals doped with zinc

The thermal conductivity of tin single crystals with zinc admixtures has been measured in the temperature range 3.5–25 K for concentrations up to 0.1 wt%. The anisotropy of thermal conductivity for two orientations, [001] and [010], has been determined. It was found that the influence of zinc admixture on the thermal conductivity anisotropy is of a complex, temperature-dependent character.Nomenclature T ₁ T ₂ Temperature differences in the specimen - Thermal conductivity coefficient - W Thermal resistivity - A, B, C Constants in Eq. (1) - T Temperature - ^th Residual electrical resistivity calculated from W-F law - ₀ Residual electrical resistivity from measurements - L ₀ Lorenz constant - _th Anisotropy coefficient of thermal conductivity - _el Anisotropy coefficient of electrical conductivity - c Admixture concentration     

Cyclodextrin polyurethanes polymerized with multi-walled carbon nanotubes: Synthesis and characterization

Insoluble cyclodextrin polymers co-polymerized with multi-walled carbon nanotubes were synthesized by polymerizing β-cyclodextrin with acid-functionalized multi-walled carbon nanotubes and diisocyanate linkers; hexamethylene- and toluene-2,4-diisocyanate. The polymers are useful in removing some organic pollutants from water, and we now report the full characterization of these polymers using infrared spectroscopy (IR), Raman spectroscopy, scanning and transmission electron microscopy (SEM and TEM) and thermal techniques such as thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC).The polymers could be synthesized as either powders or amorphous solids. Results of the IR analysis showed the presence of functional groups such as CO, CC, CH and CO, indicating that polymerization indeed took place. Characterization of the polymers by scanning electron microscopy and BET analysis showed that these polymers had a spongy appearance indicating a hierarchical pore structure. Incorporation of small amounts (<5%) of multi-walled nanotubes (MWNTs) improved the thermal stability of the polymers. This observation was further confirmed by differential scanning calorimetry (DSC) measurements.     

Anisotropically conductive polymer composites with a selective distribution of carbon black in an in situ microfibrillar reinforced blend

A carbon black (CB) based anisotropically conductive polymer composite (ACPC) was fabricated using an easy method comprising an extrusion-hot stretch-quenching process. The resultant ACPC exhibited a very low percolation and a strong anisotropy in conductivity due to its unique structure in which the dispersed polymer phase was deformed in situ into oriented conductive microfibrils, whose surface region holds the majority of CB particles. The oriented conductive microfibrils constructed different conductive networks in the parallel and perpendicular directions of ACPC, thus leading to a strong electrical anisotropy. The conductive mechanism underlying this material took effect in a complex manner including contact conduction and tunneling conduction.     

Electrical properties of poly(n-butylamino) (di-allylamino) phosphazene

A new organic–inorganic hybrid polymer, poly(n-butylamino) (di-allylamino) phosphazene, is synthesized and electrical properties of the polymer are reported. From the results of Fourier transform infrared (FTIR), 1H-nuclear magnetic resonance (NMR) and 31P-NMR measurements, it was confirmed that the synthesized polymer was poly(n-butylamino) (di-allylamino) phosphazene. From an Arrhenius plot of d.c. conduction it was found that experimental points fit to two straight lines, that the discontinuous point could be the glass transition (Tg=268.9 K) and that the activation energies are 0.74 and 1.5 eV at temperatures higher and lower than Tg, respectively. Dielectric constant and dielectric loss factors were measured over the temperature range from –20°C to 70°C with frequency 100 Hz100 kHz, and a dielectric loss contour was obtained. A relaxation process with the activation energy of 1.44 eV was found from the dielectric loss contour, which can be attributed to relaxation. A comparison was made between the present results and those of other phosphazene polymers. Because the polymer was non-flammable and shows high resistance and low dielectric constant, it could be used as an insulator in the field of electronics. © 1998 Kluwer Academic Publishers     

Conduction in thin polymer films

Thin polymer films have been made from a variety of starting monomers and their electrical conduction has been studied.The films are produced both by ultraviolet surface photolysis and by a cold-cathode glow-discharge technique. The samples are made in the form of a capacitor sandwich on a glass substrate with aluminium electrodes. The whole sample is made in a high-vacuum system, without breaking vacuum, using out-of-contact masking techniques.Generally, depending on the starting monomer, the polymer films are short-free down to a thickness of about 100 Å, and electric fields up to 10⁷ V/cm may be sustained.The current/voltage characteristics of the films have been investigated as a function of temperature and thickness. One theory to account for the conduction in these materials is presented and compared with other approaches to the problem.     

Analysis of Melting for Alkali Halides Based on Diffusional Force Theory

The theory for melting based on the concept of diffusional force is used for studying the melting of alkali halides. Values of the thermal expansivity and the Anderson-Grüneisen parameter are used to predict the interionic distances for 19 alkali halides at melting with the help of Kumars formula. A simple model for melting has been developed by estimating the diffusional force from knowledge of interionic potentials. The values of T_m thus obtained are found to show fairly good agreement with experimental values of melting temperatures and to be better than those obtained by Bosi.     

On the determination of the thermal diffusion factor from column measurements

A new method for experimental determination of the thermal diffusion factor _T for binary gas mixtures with a thermal diffusion column (TDC) is developed, based on A. M. Rozen's equation of TDC. The experimental results for _T are obtained in a reduced form in this approximation. An experimental reference point, determined in the same TDC with a standard gas mixture, is used for the transformation of the results for _T in absolute units. The proposed method is applicable for arbitrary gas mixtures, irrespective of the mass difference of the components.     

An improved correlation for the calculation of liquid thermal conductivity

Eighteen correlations appearing in the literature for the prediction of thermal conductivity, , of liquids are critically analyzed, and their reliability is checked using coherent input data and selected experimental values. The best results are obtained using the Reid, Sherwood, and Prausnitz correlation with a mean deviation of about 8% between predicted and experimental values. An improved correlation is proposed starting from the Viswanath equation, chosen because of its simplicity and convenience. The values of thermal conductivity obtained by this new correlation agree with the experimental values within 1%.     

Thermal Behavior of a Stagnant Gas Confined in a Horizontal Microchannel as Described by the Dual-Phase-Lag Heat Conduction Model

The transient thermal behavior of a stagnant gas confined in a horizontal microchannel is investigated analytically under the effect of the dual-phase-lag heat conduction model. The microchannel is formed from two infinite horizontal parallel plates where the upper plate is heated isothermally and the lower one is kept adiabatic. The model that combines both the continuum approach and the possibility of slip at the boundary is adopted in this study. The effects of the Knudsen number Kn, the thermal relaxation time _q, and the thermal retardation time _T on the microchannel thermal behavior are investigated using three heat conduction models. It is found that the deviations between the predictions of the parabolic and the hyperbolic models are insignificant. On the other hand, the deviations between the parabolic and dual-phase-lag models are significant under the same operating conditions.     

Measurements of Thermal Conductivity and Thermal Diffusivity of CVD Diamond

The thermal conductivity of natural, gem-quality diamond, which can be as high as 2500 Wm^–1 K^–1 at 25°C, is the highest of any known material. Synthetic diamond grown by chemical vapor deposition (CVD) of films up to 1 mm thick exhibits generally lower values of but under optimal growth conditions it can rival gem-quality diamond with values up to 2200 Wm^–1 K^–1. However, it is polycrystalline and exhibits a columnar microstructure. Measurements on free-standing CVD diamond, with a thickness in the range 25–400 m, reveal a strong gradient in thermal conductivity as a function of position z from the substrate surface as well as a pronounced anisotropy with respect to z. The temperature dependence of in the range 4 to 400 K has been analyzed to determine the types and numbers of phonon scattering centers as a function of z. The defect structure, and therefore the thermal conductivity, are both correlated with the microstructure. Because of the high conductivity of diamond, these samples are thermally thin. For example, laser flash data for a 25-m-thick diamond sample is expected to be virtually the same as laser flash data for a 1-m-thick fused silica sample. Several of the techniques described here for diamond are therefore applicable to much thinner samples of more ordinary material.     

Bubble Nucleation in Polymeric Liquids Under Shock Processes

This paper studies the phenomenon of spontaneous boiling-up of polymeric liquids on heating at rates up to 10⁷ K·s^–1. A model of the thermal equation of state for (polymer + monomer) systems and a procedure for determination of the spontaneous boiling-up temperature T* for polymeric liquids, taking decomposition into account, are proposed. The experimental data on T ^* for a number of polymer melts are compared with results calculated from the model.