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At least 163 records · Page 9Linked to original sources
PubMed1996

Thermal diffusion flowmetry.

Continuous monitoring of cerebral blood flow (CBF) has been an area under research to observe immediate changes in blood flow under a wide variety of circumstances. Recently, however, continuous monitoring of blood flow in the trauma cases has provided prognostic and therapeutic help in the management of these patients. Changes in CBF prior to the electrical discharge in the epileptic focus imply that blood flow changes may help in defining the focus. In the neurologically compromised patient, for whatever reason, CBF is an important parameter to follow and may give clues to therapeutic efficacy.

Cerebrovascular Circulation↗
PubMed1999

On the assumption of negligible heat diffusion during the thermal measurement of a nonuniform specific absorption rate.

The problem of measuring a spatially varying specific absorption rate (SAR) by thermal techniques is treated both analytically and numerically. It is shown that, unless the measurement is attempted at an inflection point of the SAR distribution, it will be confounded by thermal diffusion within the medium. Rules of thumb are provided to enable the experimenter to gauge the thermal conduction contributions (i.e. error) to the thermally determined SAR near a spatial extremum. The simplest of these is that the width t(m) [s] of the time window, over which temperature variation associated with SAR is measured, should satisfy the inequality t(m) </approximately alpha(2)/pi(2)D, where alpha [m] is a characteristic length scale of the spatial variation of the SAR and D [m(2)/s] is the thermal diffusivity of the medium. The above considerations are valid, independent of the source of the deposited energy.

Absorption↗
PubMed2001

The effect of coupled transport phenomena in the Opalinus Clay and implications for radionuclide transport.

In this study, the potential effects of coupled transport phenomena on radionuclide transport in the vicinity of a repository for vitrified high-level radioactive waste (HLW) and spent nuclear fuel (SF) hosted by the Opalinus Clay in Switzerland, at times equal to or greater than the expected lifetime of the waste canisters (about 1000 years), are addressed. The solute fluxes associated with advection, chemical diffusion, thermal and chemical osmosis, hyperfiltration and thermal diffusion have been incorporated into a simple one-dimensional transport equation. The analytical solution of this equation, with appropriate parameters. shows that thermal osmosis is the only coupled transport mechanism that could, on its own, have a strong effect on repository performance. Based on the results from the analytical model, two-dimensional finite-difference models incorporating advection and thermal osmosis, and taking conservation of fluid mass into account, have been formulated. The results show that, under the conditions in the vicinity of the repository at the time scales of interest, and due to the constraints imposed by conservation of fluid mass, the advective component of flow will oppose and cancel the thermal-osmotic component. The overall conclusion is that coupled phenomena will only have a very minor impact on radionuclide transport in the Opalinus Clay, in terms of fluid and solute fluxes, at least under the conditions prevailing at times equal to or greater than the expected lifetime of the waste canisters (about 1000 years).

Aluminum Silicates↗
PubMed1992

Determination of thermal properties of impression materials.

Residual strains are important for dimensional accuracy of impression materials. When these strains are being calculated, knowledge of thermophysical constants is needed. This paper describes determination of thermal diffusivity, thermal conductivity, specific heat, heat of polymerization rate, and total heat of polymerization. The first two constants mentioned were obtained from an experiment in which transient heat conductivity was studied. The evaluation was based on an analytical solution of this problem. The remaining constants were derived by means of standard differential scanning calorimetry, DSC. Results from accurate, cost-efficient methods are given for three silicones and one polyether.

Calorimetry↗
PubMedDate not supplied

Phototoxicity to the retina: mechanisms of damage.

Light damage to the retina occurs through three general mechanisms involving thermal, mechanical, or photochemical effects. The particular mechanism activated depends on the wavelength and exposure duration of the injuring light. The transitions between the various light damage mechanism may overlap to some extent. Energy confinement is a key concept in understanding or predicting the type of damage mechanism produced by a given light exposure. As light energy (either from a laser or an incoherent source) is deposited in the retina, its penetration through, and its absorption in, various tissue compartments is determined by its wavelength. Strongly absorbing tissue components will tend to "concentrate" the light energy. The effect of absorbed light energy largely depends on the rate of energy deposition, which is correlated with the exposure duration. If the rate of energy deposition is too low to produce an appreciable temperature increase in the tissue, then any resulting tissue damage necessarily occurs because of chemical (oxidative) reactions induced by absorption of energetic photons (photochemical damage). If the rate of energy deposition is faster than the rate of thermal diffusion (thermal confinement), then the temperature of the exposed tissue rises. If a critical temperature is reached (typically about 10 degrees C above basal), then thermal damage occurs. If the light energy is deposited faster than mechanical relaxation can occur (stress confinement), then a thermoelastic pressure wave is produced, and tissue is disrupted by shear forces or by cavitation-nonlinear effects. Very recent evidence suggests that ultrashort laser pulses can produce tissue damage through nonlinear and photochemical mechanisms; the latter because of two-photon excitation of cellular chromophores. In addition to tissue damage caused directly by light absorption, light toxicity can be produced by the presence of photosensitizing agents. Drugs excited to reactive states by ultraviolet (UV) or visible light produce damage by type I (free radical) and type II (oxygen dependent) mechanisms. Some commonly used drugs, such as certain antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), and psychotherapeutic agents, as well as some popular herbal medicines, can produce ocular phototoxicity. Specific cellular effects and damage end points characteristic of light damage mechanisms are described.

Animals↗
PubMed1982

Correlation of thermal shock resistance with thermal compatibility dat for porcelain-metal systems.

On the basis of this study the following conclusions may be made: (1) A strong linear relationship exists between the thermal shock resistance and compatibility index values of the systems studied. (2) Compatibility index analysis and thermal shock testing are promising approaches for evaluation of porcelain-metal compatibility. (3) Further refinements of the model are necessary to account for the effects of stress relaxation, thermal diffusivity, thermal history, elastic modulus, porcelain tensile strength, and geometry on the predicted stress state of porcelain-metal systems.

Dental Alloys↗
PubMed1990

[Thermal properties of human teeth and dental cements].

The thermal properties (thermal diffusivity and thermal conductivity) of human teeth and some dental cements were measured. The temperature change at the pulp cavity was also measured before and after the restoration with the full metal crown. The thermal insulating efficiency of dental cement was discussed. The thermal diffusivity of human dentin, enamel and dental cements was 2.04 x 10(-3), 3.22 x 10(-3), and 0.99-1.87 x 10(-3) cm2/sec, respectively. The thermal conductivity was 1.36 x 10(-3), 1.84 x 10(-3), and 0.50-1.51 x 10(-3) cal/cm.sec.degrees C, respectively. The dental cement beneath the crown insulated the thermal shock to dental pulp and the thermal insulating efficiency was increased with the decrease in the thermal diffusivity and thermal conductivity of dental cement.

Body Temperature↗
PubMed2004

Correlation between the Soret coefficient and the static structure factor in a polymer blend.

Mutual mass diffusion and thermal diffusion has been investigated in poly(dimethylsiloxane)/poly(ethylmethylsiloxane) (PDMS/PEMS) polymer blends of equal weight fractions. Molar masses ranged from below 1 to over 20 kg/mol. Both the mutual mass (D) and the thermal diffusion (D(T)) coefficient contain a thermally activated factor with an activation temperature of 1415 K. The molar mass dependence of D(T) is due to an end-group effect of the local friction coefficient. The thermal diffusion coefficient in the limit of long chains and infinite temperature is D(T)(0,infinity) = -1.69 x 10(-7) cm2(sK)(-1). The Soret coefficient S(T) of blends far enough away from a critical point is proportional to the static structure factor S(q = 0).

Journal Article↗
PubMed2006

Thermal-cycling-induced spectral diffusion and thermal barriers in anisole-doped cyclohexane, an unusual multiphase host-guest system.

The host-guest system of anisole incorporated into a cyclohexane matrix was investigated in a series of hole-burning experiments. This system is unusual in that cyclohexane can freeze into coexisting solid phases. The hole-burning experiments support the existence of two crystalline phases and one disordered phase. A second surprising characteristic of this system is that the quasi-line absorption features of the spectra appear inverted at low temperature because of unexpected dominance of fluorescence and phosphorescence.

Journal Article↗