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At least 19 recordsLinked to original sources
PubMed2006

Photothermal gas analyzer for simultaneous measurements of thermal diffusivity and thermal effusivity.

In this paper, we describe a new, simple, and fast photothermal method for simultaneous measurements of two important gas thermal properties: thermal diffusivity and thermal effusivity. The method consists essentially in combining a photoacoustic cell and a thermal wave pyroelectric cell enclosed in a single compact gas analyzer. The photoacoustic cell is kept filled with synthetic air and sealed. The pyroelectric cell is also filled with synthetic air, and after some warm up time, the synthetic air is exchanged to the gas of interest. It is shown that the analysis of the transient and saturation signals of both photoacoustic and pyroelectric cells is capable of measuring the thermal properties with an accuracy of 3%. This particular capability of performing simultaneously the measurements of thermal diffusivity and thermal effusivity allows us to carry on the complete characterization of the thermal properties of gases.

Journal Article↗
PubMed2002

Diffusion and thermal diffusion of semidilute to concentrated solutions of polystyrene in toluene in the vicinity of the glass transition.

The approaching glass transition in polystyrene/toluene solutions leads to a sharp decay of both the collective diffusion coefficient D and the thermal diffusion coefficient D(T) at concentrations above 0.2 g/cm(3). The Soret coefficient S(T) = D(T)/D follows power-law scaling from semidilute to concentrated and is not influenced by the slowing down of the dynamics associated with the glass transition. Both D and D(T) are governed by the same friction coefficient. The scaling behavior of S(T) with concentration on approach of the glass transition is compared to the divergence of S(T) near a consolute critical point.

Journal Article↗
PubMed2002

Mass and thermal diffusivity algorithms: reduced algorithms for mass and thermal diffusivity determinations.

Mass and thermal diffusivity measurements conducted on Earth are prone to contamination by uncontrollable convective contributions to the overall transport. Previous studies of mass and thermal diffusivities conducted on spacecraft have demonstration the gain in precision, and lower absolute values, resulting from the reduced convective transport possible in a low-gravity environment. We have developed and extensively tested real-time techniques for diffusivity measurements, where several measurements may be obtained on a single sample. This is particularly advantageous for low gravity research were there is limited experiment time. The mass diffusivity methodology uses a cylindrical sample geometry. A radiotracer, initially located at one end of the host is used as the diffusant. The sample is positioned in a concentric isothermal radiation shield with collimation bores located at defined positions along its axis. The intensity of the radiation emitted through the collimators is measured versus time with solid-state detectors and associated energy discrimination electronics. For the mathematical algorithm that we use, only a single pair of collimation bores and detectors are necessary for single temperature measurements. However, by employing a second, offset, pair of collimation holes and radiation detectors, diffusivities can be determined at several temperatures per sample. For thermal diffusivity measurements a disk geometry is used. A heat pulse is applied in the center of the sample and the temperature response of the sample is measured at several locations. Thus, several values of the diffusivity are measured versus time. The exact analytic solution to a heat pulse in the disk geometry leads to a unique heated area and measurement locations. Knowledge of the starting time and duration of he heating pulse is not used in the data evaluation. Thus, this methodology represents an experimentally simpler and more robust scheme.

Journal Article↗
PubMed2006

Study of the thermal diffusion behavior of alkane/benzene mixtures by thermal diffusion forced rayleigh scattering experiments and lattice model calculations.

In this work the thermal diffusion behavior of binary mixtures of linear alkanes (heptane, nonane, undecane, tridecane, pentadecane, heptadecane) in benzene has been investigated by thermal diffusion forced Rayleigh scattering (TDFRS) for a range of concentrations and temperatures. The Soret coefficient ST of the alkane was found to be negative for these n-alkane/benzene mixtures indicating that the alkanes are enriched in the warmer regions of the liquid mixtures. For the compositions investigated in this work, the magnitude of the Soret coefficient decreases with increasing chain length and increasing alkane content of the mixtures. The temperature dependence of the Soret coefficient depends on mixture composition and alkane chain length; the slope of ST versus temperature changes from positive to negative with increasing chain length at intermediate compositions. To study the influence of molecular architecture on the Soret effect, mixtures of branched alkanes (2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane, and 2,2,4-trimethylpentane) in benzene were also investigated. Our results for the Soret coefficients show that the tendency for the alkanes to move to the warmer regions of the fluid decreases with increasing degree of branching. The branching effect is so strong that for 2,2,4-trimethylpentane/benzene mixtures the Soret coefficient changes sign at high alkane content and that equimolar 2,2,3-trimethylbutane/benzene mixtures have positive Soret coefficients in the investigated temperature range. In order to investigate the effect of molecular interactions on thermal diffusion, we adapted a recently developed two-chamber lattice model to n-alkane/benzene mixtures. The model includes the effects of chain-length, compressibility, and orientation dependence of benzene-benzene interactions and yields good qualitative predictions for the Soret effect in n-alkane/benzene mixtures. For the branched isomers, we find some correlations between the moments of inertia of the molecules and the Soret coefficients. PACS numbers: 66.10.Cb, 61.25.Hq.

Journal Article↗
PubMed2004

Mixing rules for multicomponent mixture mass diffusion coefficients and thermal diffusion factors.

Mixing rules are derived for mass diffusion coefficient and thermal diffusion factor matrices by developing compatibility conditions between the fluid mixture equations obtained from nonequilibrium thermodynamics and Grad's 13-moment kinetic theory. The mixing rules are shown to be in terms of the species mole fractions and binary processes. In particular, the thermal diffusion factors for binary mixtures obtained by the Chapman-Enskog expansion procedure are suitably generalized for many-component mixtures. Some practical aspects of the results are discussed including the utilization of these mixing rules for high pressure situations.

Journal Article↗
PubMed2004

Determination of thermal diffusion coefficients using thermal field-flow fractionation and Mark-Houwink constants.

In this paper, a new approach for determination of thermal diffusion coefficient D(T) values using thermal field-flow fractionation retention data and Mark-Houwink constants is reported. The method utilizes the availability of Mark-Houwink constants from the literature together with thermal field-flow fractionation retention data to calculate D(T) values for both narrowly and broadly dispersed polymeric samples. The proposed method was tested with thermal field-flow fractionation data from a number of published papers. In general, D(T) results obtained from the new approach agree well with those reported from the literature. Since Mark-Houwink constants have been extensively tabulated, the new method can be used to generate a broad database of D(T) values for use in the characterization of polymers and in studies of the thermal diffusion phenomenon.

Journal Article↗
PubMed1973

Cortical blood flow in controlled hypotension as measured by thermal diffusion.

A thermal diffusion flow probe which gave a continuous, dynamic, quantitative record of cortical blood flow (CBF) was used to assess CBF in experimental animals with controlled hypotension. Acute hypotension was produced by trimethaphan camsylate, halothane, and sodium nitroprusside. Halothane produced less reduction in CBF per drop in blood pressure than the other two agents.

Anesthesia, Intravenous↗
PubMed1980

Thermal diffusivity, specific heat, and thermal conductivity of A-150 plastic.

Some thermal properties of A-150 tissue-equivalent plastic have been determined. The results are: thermal diffusivity, 2.72 x 10(-3) cm2s-1 +/- 0.4%; specific heat, 1.72 J g-1 K-1 +/- 1.3%; and thermal conductivity, 5.3 x 10(-3) WK-1 cm-1 +/- 1.4%. The significance of the measurements for the design of a calorimeter core calibration heater is briefly described.

Calcium Fluoride↗
PubMed1992

Measurement of the thermal diffusivity of human epidermis by studying thermal wave propagation.

The thermal diffusivity of dry human epidermis was determined in vitro by studying thermal wave propagation in thin epidermal layers at frequencies between 10 and 200 Hz. Transmission measurements were performed on samples applied to a plane copper support at the underside of which thermal waves were generated by means of a square voltage controlled power transistor. Additionally, measurements were performed on epidermal layers with metal and air backing, in which thermal waves were generated by the absorption of intensity modulated light in a thin, superficially applied graphite layer (short and open circuit measurements). Thermal waves were detected by means of the laser beam deflection technique which allows the contactless measurement of the oscillatory surface temperature of a sample with respect to amplitude and phase. A critical discussion of methods shows that the thermal diffusivity is most reliably determined by transmission experiments. From experimental data obtained by this method a mean value alpha = (2.8 +/- 0.9) x 10(-4) cm2 s-1 was evaluated for the thermal diffusivity of dry epidermis.

Diffusion↗
PubMed2000

Thermal diffusion and soret feedback of gold-doped polyorganosiloxane nanospheres in toluene

We have investigated diffusion and thermal diffusion properties of light-absorbing colloidal polyorganosiloxane microgels containing tiny nanometer-sized gold clusters dispersed in toluene. Transient holographic gratings allow for very subtle perturbations in the linear regime where Soret feedback is negligible. Gold-doped colloids of different size and crosslink ratios show different Soret coefficients but identical thermal diffusion coefficients D(T). Undoped colloids tend to aggregate, but a consistent interpretation is obtained if an identical D(T) is assumed for the doped, the undoped, and the aggregated particles. Previously reported Soret feedback measurements on similar systems incidentally yielded comparable Soret coefficients. We show, however, that they suffer from strong convective perturbations.

Journal Article↗
PubMed2005

Measurements of molecular and thermal diffusion coefficients in ternary mixtures.

Thermal diffusion coefficients in three ternary mixtures are measured in a thermogravitational column. One of the mixtures consists of one normal alkane and two aromatics (dodecane-isobutylbenzene-tetrahydronaphthalene), and the other two consist of two normal alkanes and one aromatic (octane-decane-1-methylnaphthalene). This is the first report of measured thermal diffusion coefficients (for all species) of a ternary nonelectrolyte mixture in literature. The results in ternary mixtures of octane-decane-1-methylnaphthalene show a sign change of the thermal diffusion coefficient for decane as the composition changes, despite the fact that the two normal alkanes are similar. In addition to thermal diffusion coefficients, molecular diffusion coefficients are also measured for three binaries and one of the ternary mixtures. The open-end capillary-tube method was used in the measurement of molecular diffusion coefficients. The molecular and thermal diffusion coefficients allow the estimation of thermal diffusion factors in binary and ternary mixtures. However, in the ternaries one also has to calculate phenomenological coefficients from the molecular diffusion coefficients. A comparison of the binary and ternary thermal diffusion factors for the mixtures comprised of octane-decane-1-methylnaphthalene reveals a remarkable difference in the thermal diffusion behavior in binary and ternary mixtures.

Journal Article↗
PubMed1990

[Thermal diffusivity of dental cements].

Thermal insulative efficiency, is one of the desirable properties of the dental cements. In this study, the thermal diffusivity of three types of dental cements, were measured. Thermal diffusivity was determined by the following method. Two thermo-couples were used and connected to a chart record, the first was embedded in the cylindrical block of the cement specimen and the other in a mixing of ice and water (reference thermocouple). All them were set in a apparatus consisting of a double cooling bath. Calculation of thermal diffusivity were based on the curve provided of the record during cooling of the cement and a theoretical mathematic model. Values were ranged from 2,985 to 3,934 cm2.sec-1. ZOE cement exhibited the highest value, the glass-ionomers the lowest and the poly-carboxylates were average. The results showed that the thermal diffusivity of the cements is dependent from the type of the cement but the differences between them were not statistically significant. Additionally, the values obtained were about the same as the dentin, so the dental cements may consider as good thermal insulators.

Dental Cements↗
PubMed1994

Measurement of thermal diffusivity of biomaterials by focused ultrasonic beams (thermal pulse decay method by focused ultrasonic beams).

This study propose a new simple method of measuring the thermal diffusivity of living tissue by thermal pulse decay technique with focused ultrasonic beam, which does not require the accurate knowledge of Gaussian variance within the focal region. The measurement of temperatures at two different locations outside the focal region replaces the elaborate measurement of the size of the focal region and gives a thermal diffusivity with reasonable accuracy and automatically avoids the artifact due to beam-thermocouple interaction. The focused ultrasound was generated by the bowl-shaped ceramic piezoelectric transducer with the diameter of 30 mm. The focal lengths of transducer were 40 mm and 60 mm and the frequencies 1.7 and 3.6 MHz. The values of thermal diffusivity of biological tissues obtained in this method are fairly close to the previously published values and are also compared to the values obtained by heated thermistor method.

Animals↗