PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Thermal Diffusion”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4Linked to original sources
PubMedDate not supplied

Thermal diffusion through amalgam and cement base: comparison of in vitro and in vivo measurements.

Thermal diffusion was measured in vitro and in vivo through amalgam and amalgam underlaid with bases of zinc phosphate, zinc oxide-eugenol, and calcium hydroxide cements. Although the magnitudes differed, there generally was good agreement between in vitro and in vivo data with respect to the relative rates of thermal diffusivity through amalgam restorations underlaid with bases of each of the three materials. In all tests, both in vitro and in vivo, the zinc oxide-eugenol base proved to be the best thermal insulator. Calcium hydroxide was the next best thermal barrier and was followed by zinc phosphate cement. In vitro tests indicated dentin to be a better thermal insulator than zinc phosphate cement but inferior to the zinc oxide-eugenol and calcium hydroxide base materials used here. Although a method has been presented here for the in vivo assessment of the efficacy of thermal insulating bases and a number of in vivo experiments were conducted, much research remains to be done in this area. Additional investigation is needed to better define the parameters of thermal change beneath various types of restoratives and also to establish more exactly the role of base thickness in providing thermal protection beneath clinical metallic restorations.

Calcium Hydroxide↗
PubMed1984

The simultaneous measurement of thermal conductivity, thermal diffusivity, and perfusion in small volumes of tissue.

An improved technique is presented for the "in-vivo" determination of thermal conductivity, thermal diffusivity, and perfusion using a self-heated spherical thermistor probe. In the presence of flow, solution of the time-dependent, probe-tissue coupled thermal model allows the measurement of "effective" thermal conductivity and "effective" thermal diffusivity, which represent the thermal properties of the perfused tissue. Perfusion can be quantified from both "effective" thermal properties. In the presence of flow, it has been shown that the transient power responses does not follow t-1/2 as has been previously assumed. An isolated rat liver preparation has been developed validate the measurement technique. Radioactive microspheres are used to determine the true perfusion from the total collected hepatic vein flow. Experimental data demonstrates the ability to quantify perfusion in small volumes of tissue.

Animals↗
PubMed1993

Cerebral blood flow (CBF) monitoring in intensive care by thermal diffusion.

Continuous monitoring of cortical blood flow (CoBF) in the intensive care unit is possible with thermal diffusion techniques. The normal brain flow limits have been established when electrical activity ceases and when infarction is likely to occur. With continuous monitoring of CoBF one can see immediate changes in flow and approaching these levels may be anticipated. The thermal diffusion system we have employed is based on the thermal conductivity of cortical tissue. As blood flow increases through the tissue, the conduction of energy away from the flow probe allows the sensor to detect changes in flow. This form of monitoring has been carried out in patients with subarachnoid hemorrhage, resection of cerebral mass lesions, severe craniotrauma, and intractable epilepsy. In subarachnoid hemorrhage, vasospasm can be identified and the efficacy of treatment determined with continuous monitoring of CoBF. During resection of mass lesions, increases in blood flow can be readily detected to document the recovery of brain tissue. Continuous monitoring of CoBF in epilepsy patients is now possible with the implantation of subdural electrodes. The increase in blood flow can be documented and it is apparent that a period of elevation of blood flow is quite short. Therefore, this may be helpful in determining when other forms of CBF determination, such as Single Photon Emission Computed Tomographic (SPECT) scanning should be performed. In patients with cranial trauma, different patterns of CoBF changes are apparent. Some patients may develop increased CoBF prior to elevation of intracranial pressure (ICP); other patients demonstrate a drop in CoBF as a response to increased ICP.(ABSTRACT TRUNCATED AT 250 WORDS)

Body Temperature Regulation↗
PubMed1984

A problem in the measurements of pulmonary extravascular water volume by double indicator dilution method, using heat and dye--thermal diffusion into the left ventricular wall.

Pulmonary extravascular water volume was obtained as the lung thermal volume (LTV) in mongrel dogs (n = 31) by double indicator dilution method, using negative heat and indocyanine green. We used a technic of one injection site (right atrium) and two sampling sites, i.e., pulmonary artery trunk and aortic root. In 13 dogs, cardiac outputs determined simultaneously from thermodilution curves in pulmonary artery trunk, aortic root and from dye dilution curve in aortic root, were in a good agreement with a coefficient of variation of +/- 12%. Lung thermal volume measured twice within a period of 5 minutes in 17 dogs, gave closely similar values with a coefficient of variation of +/- 9%. Lung thermal volume was 5.7 +/- 1.1 ml/kg (mean +/- SD) and corresponding "weighed lung water" (WLW), which was measured by destructive, direct method, was 5.0 +/- 1.0 ml/kg, the ratio of LTV to WLW (LTV/WLW) being 1.2 +/- 0.2 in control state. LTV/WLW ratio was 1.2 +/- 0.2 in 11 dogs loaded with dextran, and 1.1 +/- 0.1 in 6 dogs loaded with alloxan. Thus the lung thermal volume slightly exceeded WLW. In order to evaluate the extent to which the thermal diffusion into the left ventricular wall would cause LTV to slightly overestimate the pulmonary extravascular water volume, dye dilution curves and thermodilution curves were recorded simultaneously in the left atrium (LA) and aortic root (Ao) in a total of 25 runs in 5 dogs. The ratio of LTV obtained in LA to that in Ao was 0.9 +/- 0.1 (mean +/- SD). It was concluded that the pulmonary extravascular water volume was overestimated, by about 10% due to the thermal diffusion into the left ventricular wall.

Alloxan↗
PubMed1981

Thermal diffusivity in finite cylindrical specimens of dental cements.

1. A method has been developed and described for obtaining accurate thermal diffusivity values for dental cements and other restorative materials. This method employs specimens of a short cylindrical geometry which are readily fabricated from simply-constructed molds. 2. Results obtained for polycarboxylate and glass-ionomer cements show that the diffusivity values are mainly in the range of 2-3 x 10-3 cm2 s-1. Thus, the thermal characteristics of these materials are suitable if the cements are used as insulating bases below amalgam restorations. 3. thermal diffusivity values are a potential tool for investigating structure/property relationships in restorative materials, being sensitive to moisture content, powder/liquid ratio, and other compositional variables.

Dental Cements↗
PubMed1989

Effect of alumina concentration on the thermal diffusivity of dental porcelain.

The importance of the thermal diffusivity of restorative dental materials is well established. The laser flash method first described by Parker et al. (1961) is widely used to determine the diffusivity of industrial materials but has not hitherto been reported in the study of dental materials. The effect of the crystalline alumina content of dental porcelains has been investigated using this technique which permits rapid and accurate measurement. Values of 1.45 mm2/s and 0.66 mm2/s were determined for a commercial aluminous core porcelain and a dentine porcelain respectively. Mixtures of core and dentine powders exhibited intermediate levels. Increasing the alumina content resulted in increased diffusivity. The highest value was determined for a new high strength aluminous core porcelain which yielded a diffusivity of 1.74 mm2/s. This material was found to contain an increased concentration of alumina particles.

Aluminum↗
PubMed2002

Thermal diffusivity coefficient of glycerin determined on an acoustically levitated drop.

We present a technique that can be used to determine the thermal diffusivity coefficient of undercooled liquids that exist at temperatures below their freezing points. The technique involves levitation of a small amount of liquid in the shape of a flattened drop using an acoustic levitator and heating it with a CO2 laser. The heated drop is then allowed to cool naturally by heat loss from the surface. Due to acoustic streaming, heat loss is highly non-uniform and appears to mainly occur at the drop circumference (equatorial region). This fact allows us to relate the heat loss rate with a heat transfer model to determine the thermal diffusion coefficient. We demonstrate the feasibility of the technique using glycerin drops as a model liquid.

Journal Article↗
PubMed2002

Thermal diffusion and chemical kinetics in laminar biomaterial due to heating by a free-electron laser.

We have theoretically investigated the role of thermal diffusion and chemical kinetics as a possible dynamic explanation for the preferential ablative properties of infrared radiation from a free-electron laser (FEL). The model is based on a laminar system composed of alternating layers of protein and saline. We have compared exposure to 3 microm where water is the main absorber and 6.45 microm where both water and protein absorb. The picosecond pulses of the superpulse are treated as a train of impulses. We find that the heating rates are sufficient to superheat the outer saline layers on the nanosecond time scale, leading to explosive vaporization. We also find that competition between the layer-specific heating rates and thermal diffusion results in a wavelength-dependent separation in layer temperatures. We consider the onset of both chemical bond breaking and the helix-coil transition of protein prior to vaporization in terms of the thermal, chemical, and structural properties of the system as well as laser wavelength and pulse structure. There is no evidence for thermal bond breaking on these time scales. At 6.45 microm, but not 3 microm, there is evidence for a significant helix-coil transition. While the native protein is ductile, the denatured protein exhibits brittle fracture. This model provides a dynamic mechanism to account for the preferential ablative properties observed with FEL radiation tuned near 6.45 microm.

Journal Article↗
PubMed2001

Molecular origin of thermal diffusion in benzene + cyclohexane mixtures.

The isotope effect in thermal diffusion (Soret effect) of benzene+cyclohexane mixtures has been investigated by a holographic grating technique. The Soret coefficient can be split into additive contributions. One contribution, the isotope effect, stems from the differences of both mass and moment of inertia, and is independent of composition. An additional "chemical" contribution depends on concentration and even changes its sign at a benzene mole fraction x(benz) approximately 0.7. The mass effect is in agreement with molecular dynamics calculations: the heavier component migrates to the cold side.

Journal Article↗
PubMed2005

Thermal diffusion shock waves.

The Ludwig-Soret effect or thermal diffusion, which refers to the separation of liquid mixtures in a temperature gradient, is governed by a nonlinear, partial differential equation in space and time. It is shown here that the solution to the nonlinear differential equation for a binary mixture predicts the existence of shock waves completely analogous to fluid shocks and obeys an expression for the shock velocity that is an exact analogue of the Rankine-Hugoniot relations. Direct measurements of the time dependent, spatial absorption profile of a suspension of nanometer sized particles subjected to a sinusoidal temperature field generated by a pair of continuous laser beams, as well as self-diffraction experiments, show motion of the particles in agreement with the predictions of nonlinear theory.

Journal Article↗
PubMed1993

Measurement of thermal conductivity, thermal diffusivity, and perfusion.

This paper describes an experiment technique for the measurement of thermal conductivity, thermal diffusivity and perfusion using self-heated thermistors. Thermal probes are constructed by placing a miniature thermistor at the tip of a plastic catheter. The volume of tissue over which the measurement occurs depends on the surface area of contact between the thermistor and the tissue. Electrical power is delivered to a spherical thermistor positioned invasively within the tissue of interest. The electrical power and resulting temperature rise are measured by a microcomputer-based instrument. When the tissue is perfused by blood, the thermistor heat is removed both by conduction and by heat transfer due to blood flow near the probe. In vivo, the instrument measures effective thermal properties which are the combination of conductive and convective heat transfer. The accuracy of the conductivity and diffusivity measurements was evaluated by operation of the probe in media of known thermal properties. Perfusion measurements in canine liver, prostate, and spleen are presented.

Animals↗
PubMed2000

Effects of thermal diffusion on sound attenuation in evaporating and condensing gas-vapor mixtures in tubes

An investigation of sound propagation in an air-water vapor mixture contained in a cylindrical tube with wet walls was recently presented [Raspet et al., J. Acoust. Soc. Am. 105, 65-73 (1999)]. The formulation of the problem paralleled the "low reduced frequency method" of Tijdeman [J. Sound Vib. 39, 1-33 (1975)]. It was pointed out that a term of reduced frequency order had been neglected in the radial component of the diffusion equation [G. Swift, personal communication (1999)]. This term represents the additional mass diffusion driven by the temperature gradient, or Soret effect, and is proportional to the thermal diffusion ratio. The solution for the complex wave number of the acoustic mode with this additional term is presented here. Numerically calculated predictions for the air-water vapor mixture show little change in acoustic attenuation due to the coupling. Therefore, a description of the acoustic attenuation where the viscous, thermal, and diffusion processes are decoupled is adequate for the specific case previously discussed by Raspet et al.

Journal Article↗
PubMed2004

Thermal diffusion in a sinusoidal temperature field.

Separation of liquid mixtures in a thermal gradient, known as the Ludwig-Soret effect or thermal diffusion, is governed by a nonlinear, partial differential equation. It is shown here that the nonlinear differential equation for a binary mixture can be reduced to a Hamiltonian system of equations and that a solution can be obtained for the linear problem. The calculation gives a closed form expression for the space and time dependence of the concentration profile of the mixture, valid at short times.

Journal Article↗
PubMed1991

Modified thermal diffusion flow probe for the continuous monitoring of cortical blood flow.

A small thermal diffusion flow probe has been developed to monitor the dynamic changes in cerebral blood flow in small animals. Constantan wire was used as a heat source to make a miniature probe. The pair of thermocouples used to detect the heat gradient between two gold plates was elongated to avoid heat conduction between them, and this improvement allowed us to make quantitative measurements. After several basic experiments, local cerebral blood flow was measured simultaneously, using both the modified thermal probe and the hydrogen clearance method in four rabbits. A close relationship was obtained between the local cerebral blood flow values measured by hydrogen clearance (F, ml/100g/min) and the reciprocal of the thermocouple voltage (1/V;1/mV). The regression line was F = 29111(1/V - 1/226), (r = 0.92, P less than 0.001). We suggest that the modified thermal probe is a reliable and quantitative means of measuring flow. In addition, another probe modified for clinical use was evaluated. Continuous monitoring of local cerebral blood flow in postoperative patients was performed, and some illustrative cases are described.

Aged↗
PubMed1996

Thermal diffusion blood flow monitoring during aneurysm surgery.

Cortical blood flow (CoBF) monitoring with a thermal diffusion flow probe was performed during the clipping of aneurysms of the ICA and MCA regions, on a series of patients during the acute stage of subarachnoid haemorrhage. Emphasis was placed on the CoBF recovery after temporary clip release. Since the absolute value in this technique is unreliable, recovery of blood flow after temporary clipping is represented as %CoBF according to the following equation: [see text]

Adult↗
PubMed2006

Thermal diffusion of dilute polymer solutions: the role of solvent viscosity.

We have performed measurements of the thermal diffusion coefficient D(T) in the dilute limit on polystyrene in cyclo-octane, cyclohexane, benzene, toluene, tetrahydrofuran, ethyl acetate, and methyl ethyl ketone and of poly(dimethyl-siloxane) in toluene. These data have been combined with literature data to test various theoretical predictions. The viscosity is identified as the dominating and only relevant solvent parameter. On the polymer side, the size or mass of an effective correlated segment determines the strength of the Soret effect. Large and heavy effective segments, as found in stiffer chains, lead to higher D(T).

Journal Article↗
PubMed1992

Comparison of laser Doppler fluxmetry and the thermal diffusion method of measuring skin blood flow with hydrogen clearance.

Laser Doppler fluxmetry (LDF) and the thermal diffusion method (TDF) were compared with the hydrogen clearance method in ten rabbit ears. The LDF signal followed the rapid change in blood flow caused by aortic occlusion and the signal obtained by TDF showed a time delay and the response was less steep compared with the LDF signal. Simultaneous recording showed both methods correlated well with the hydrogen clearance method, but the relationship varied among the studies (the slope 0.0189-0.0967 for LDF, the slope 0.052-0.682 for TDF). Both methods appear to provide a good indicator of the response pattern of blood flow in the region tested. However, variability among the subjects indicates that the quantitative application of both techniques is difficult.

Animals↗