Thermal conductivity & diffusivity FAQs

Click on the A-number for an answer...

A1. What is the connection between thermal conductivity and thermal diffusivity?

A2. What is the physical difference between thermal conductivity and thermal diffusivity?

A3. Where can I find cheap but accurate thermophysical property measurement equipment?

A4. How can I test loose-fill insulating materials?

A5. Is it possible to measure the thermal resistance/conductivity of products with air cavities?

A6. Why can I see different thermal conductivity values for foamed plastics?

A7. If I want a low thermal conductivity, which density should I choose?

A8. Is it possible to calculate thermal conductivity with knowledge of the density?

A9. How shall I perform a measurement on a moist material?

A10. Are there materials that can insulate better than air?

A11. Is there convection in insulation materials?

A12. Is there an optimum density for insulation materials?

A13. Is it possible to measure thermal conductivity on components of unusual size and shape?

A14. What are the reasons to measure thermal diffusivity instead of thermal conductivity?

A15. What are the most frequently used methods to measure thermal diffusivity or thermal conductivity?

A16. Has the thermal conductivity / diffusivity of all materials the same temperature dependence?

A17. Is it possible to extrapolate the thermal conductivity of a material beyond the temperature range for which the measurements are made?

A18. What are the most accurate measurement methods?

A19. In the laser flash method, why should the specimen be opaque to the laser beam?

A20. Is it possible to measure thermal diffusivity of a multilayered material by laser flash method?

A21. What are the problems in measuring thermal conductivity using pulse methods?

A22. Why do thermal conductivity/diffusivity measurement devices need special specimen geometry?

A23. What is the thermal equivalent of Ohm's law?

A24. What is the best method for thermal conductivity measurement of solid electrical conductors at very high temperature?

A25. What is the principle of thermal diffusivity measurement?

A1. What is the connection between thermal conductivity and thermal diffusivity?

Thermal conductivity (l) and thermal diffusivity (a) are related by l = a . r . c_p, where r is density and c_p is specific heat capacity. At high temperatures, or when the material is available only in small sizes, a, r and c_p are often measured to determine l.

A2. What is the physical difference between thermal conductivity and thermal diffusivity?

Thermal conductivity (l) is a property that determines HOW MUCH heat will flow in a material, while thermal diffusivity (a) determines HOW RAPIDLY heat will flow within it. A homely example is to imagine holding a poker that is suddenly put into a fire. How hot you feel at the handle end is determined by the poker's thermal conductivity and how quickly you feel the heat is determined by its thermal diffusivity.

A3. Where can I find cheap but accurate thermophysical property measurement equipment?

Ron Tye has written a brief note entitled "Cost and accuracy" in T²PAC News. See the page of the NPL newsletter under section 'A Word In Your Ear' (near the bottom of the left-hand-menu).

A4. How can I test loose-fill insulating materials?

The insulating material is blown (or cured depending on material type) into a frame of rigid material and a bottom with a thin material with negligible resistance. The blowing is carried out according to instructions (e.g. manufacturer, standard) so as to obtain the intended fibre distribution, weight and density.

A5. Is it possible to measure the thermal resistance/conductivity of products with air cavities?

Yes it is possible but heat flow direction and size of the cavities must be considered. The heat flow in a cavity is by radiation, convection and conduction. If convection occurs, the convection pattern/type will mainly depend on heat flow direction, temperature and size of the cavity.

A6. Why can I see different thermal conductivity values for foamed plastics?

If the product is manufactured with a blowing agent that has a lower thermal conductivity than air and it gradually leaves the product, the thermal conductivity will increase with time. The long-term change in thermal resistance will depend on the gas type, gas permeability and thickness of the product and type of covering (if any). For slabs an aged value is stated.

A7. If I want a low thermal conductivity, which density should I choose?

Thermal conductivity for insulating materials usually follows the expression lambda = a + b*density + c/density. At low densities the radiation dominates, yielding a high value of thermal conductivity. At high densities, the influence of thermal conduction in the solid phase will gradually increase. The curve (thermal conductivity versus density) will then have a U-shaped form with a minimum at a density of 40-60 kg/m³.

A8. Is it possible to calculate thermal conductivity with knowledge of the density?

An approximate estimation is possible. The equation lambda = a + b*density + c/density can be used for low density material. The third term represents the radiant heat transfer, which is considerable for low densities. For higher densities the equation lambda = a + b*density + c*density² is often used. The constants a, b, c are usually calculated from several measurements.

A9. How shall I perform a measurement on a moist material?

Measurement of moist materials is complicated, time-consuming and must be carried out by experienced personnel. Measurement causes normally a redistribution of moisture in the material which leads to two types of problem: A) Redistribution of moisture. The test is carried out on a specimen with a moisture distribution that is no longer uniform. B) Redistribution of the moisture simultaneously induces phase changes. These effects must be well known or negligible during the test. Small temperature gradients do not guarantee that phase-change effects will be negligible. In CEN/TR 14613 and ISO 10051 the measurement procedures are described in detail.

A10. Are there materials that can insulate better than air?

The thermal conductivity of air at ambient conditions is about 0.026 W/(m.K). Microporous insulation materials show a thermal conductivity below 0.025 W/(m.K) even at ambient conditions. The thermal conductivity of the air within the micropores is partially suppressed. Evacuated insulation systems could show even lower values of the thermal conductivity, e.g. less than 0.005 W/(m.K).

A11. Is there convection in insulation materials?

No. In insulation materials the convection of air is suppressed by the porous structure.

A12. Is there an optimum density for insulation materials?

At lower densities thermal radiation will become a dominant heat transfer mechanism and the apparent thermal conductivity increases with decreasing density. At higher densities the thermal conductivity increases due to the increasing solid conductivity (heat transfer via the solid structure). Thus, there exists a minimum of thermal conductivity also depending on temperature.

A13. Is it possible to measure thermal conductivity on components of unusual size and shape?

In-situ measurements are often not possible. Thermal conductivity measurement devices need special geometries which usually differ from that of ordinary objects. In the case of small components thermal diffusivity measurements are recommended, with samples of 8 mm diameter or so.

A14. What are the reasons to measure thermal diffusivity instead of thermal conductivity?

In order to calculate thermal stress during rapid temperature change you need the thermal diffusivity of the material. Other reasons are: 1) Only small samples in the range of 8 mm diameter and 2 mm thickness are available, 2) the temperature is higher than 1000 °C which is the upper limit for thermal conductivity measurements.

A15. What are the most frequently used methods to measure thermal diffusivity or thermal conductivity?

There is no universal method to measure thermal diffusivity or thermal conductivity. Basically, thermal conductivity is determined by steady state methods and thermal diffusivity by transient methods. The choice of a measurement method depends on many criteria such as sample geometry, temperature range, type of material, magnitude of thermal conductivity / diffusivity, uncertainty, etc..

A16. Has the thermal conductivity / diffusivity of all materials the same temperature dependence?

The variation of these thermal properties with temperature depends on the type of material. In the same temperature range the values of thermal conductivity / diffusivity can decrease for some materials and increase for others.

A17. Is it possible to extrapolate the thermal conductivity of a material beyond the temperature range for which the measurements are made?

The variation of thermal conductivity / diffusivity is not monotonous in a wide temperature range. Thermal diffusivity and thermal conductivity sometimes vary sharply with temperature, especially at low temperatures and in the vicinity of phase transition (Curie point of iron for example). So thermal properties values should not be extrapolated outside the temperature range for which the measurements are made.

A18. What are the most accurate measurement methods?

Measurement methods can be divided into two categories: absolute methods and comparative methods. An absolute method allows to perform a measurement that is directly traceable to the primary SI units (such as temperature, time, length, voltage, weight, etc.) without the use of an external reference of the same quantity. For comparative methods, the thermal conductivity / diffusivity measurement is carried out by comparison with reference materials that were beforehand characterised by an absolute method. So, absolute methods give smaller uncertainties of measurement than with comparative methods.

A19. In the laser flash method, why should the specimen be opaque to the laser beam?

Laser flash method is based on the comparison of experimental curves with a mathematical model that assumes that the energy of the beam is absorbed at the surface of the specimen. If the specimen is not opaque at the wavelength of the beam, then the energy pulse is absorbed inside it. Then the boundary and initial conditions of the mathematical model are different from experimental conditions. This leads to a wrong estimation of thermal diffusivity. This problem can be avoided by coating the specimen or by using a laser having a suitable wavelength (CO₂ laser for example).

A20. Is it possible to measure thermal diffusivity of a multilayered material by laser flash method?

Usual methods that estimate thermal diffusivity from experimental curves assume that the specimen is homogeneous. Thermal diffusivity of multilayered materials can be estimated by these methods if the number of layers is high enough to consider the specimen as homogeneous. This assumption is valid if the same thermal diffusivity values are obtained for specimens having different thicknesses. For multilayered materials which cannot be considered as homogeneous (specimen consisting of 2 or 3 layers for example), a specific mathematical model describing the thermal behaviour of the considered specimen should be used.

A21. What are the problems in measuring thermal conductivity using pulse methods?

The problems are not due to the experimental apparatus which is relatively simple, but the complexity of evaluation of thermal conductivity from thermal profiles inside the specimen. This requires a large matrix computation.

A22. Why do thermal conductivity/diffusivity measurement devices need special specimen geometry?

The uncertainty of thermal diffusivity / conductivity measurements depends on how well the theoretical model corresponds with the measurement set-up, in particular the validity of the assumptions concerning sample geometry (shape, size, thickness/diameter ratio, parallelism of specimen faces, etc.).

A23. What is the thermal equivalent of Ohm's law?

There is a close analogy between the flow of electricity (electrons) and heat. Heat flux Q corresponds to the electric current I. Temperature difference corresponds to voltage (potential difference). Thermal resistance corresponds to the electrical resistance. Thermal resistance is connected with thermal conductivity through geometrical parameters (area and length). Applying this model one can solve thermal problems in the same way as when analysing electrical circuits.

A24. What is the best method for thermal conductivity measurement of solid electrical conductors at very high temperature?

The most precise method from 1000 K up to 4000 K is pulse technique that overcomes problems connected with steady state techniques (furnace control, chemical reactions, evaporation, specimen containment, loss of mechanical strength, etc.).

A25. What is the principle of thermal diffusivity measurement?

The methods of measuring thermal diffusivity rely on analysing the temperature response of the specimen subjected to transient thermal conditions. The transients can be caused by a pulse, a periodically varying heat flow or a monotonic heating regime.

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