Thermal diffusivity

Thermal diffusivity (α) is a thermophysical property that describes the rate at which a temperature variation propagates within a material. 

Unlike thermal conductivity (λ), which describes heat transfer under steady-state conditions, diffusivity characterizes the behavior of a material under transient conditions, i.e., how quickly heat is distributed throughout the volume.

In practical terms, a material with high thermal diffusivity heats up and cools down rapidly; a material with low diffusivity responds more slowly to temperature variations, storing and releasing energy gradually. This property is related to thermal inertia and is relevant in non-steady-state applications (thermal cycles, rapid heating, thermal shocks, periodic temperature variations).

Formula and units of measurement

Thermal diffusivity is defined by the relationship between thermal conductivity (λ), density (ρ), and specific heat (cₚ):
α = λ / (ρ ⋅ cₚ)

where:

• λ = thermal conductivity [W/(m·K)]
• ρ = density [kg/m³]
• cₚ = specific heat at constant pressure [J/(kg·K)] 

The unit of measurement in the International System is m²/s, sometimes expressed in mm²/s for materials with very low values.

This relationship shows that thermal diffusivity depends both on conductivity (λ) and on the volumetric heat capacity of the material, defined as the product of density and specific heat (ρ·cₚ).

For the same conductivity, a material with high volumetric heat capacity exhibits lower diffusivity, since it absorbs a greater amount of energy before changing its temperature. Conversely, materials with low density and low specific heat may exhibit high diffusivity even in the presence of moderate conductivity.

Diffusivity, Conductivity and Specific Heat: an interconnected system 

Thermal diffusivity is part of a system of closely related thermophysical properties. Understanding them together is essential for a complete characterization of the thermal behavior of a material:

A practical example: cast iron vs aluminum 

A typical comparison is between cast iron and aluminum.

Aluminum exhibits significantly higher thermal conductivity (λ ≈ 205 W/(m·K)) and high diffusivity, resulting in rapid heating and heat distribution.

Cast iron, although having lower conductivity (λ ≈ 50 W/(m·K)), is characterized by higher density and volumetric heat capacity (ρ·cₚ). As a result, it heats up more slowly but retains heat longer and more uniformly.

The same principle applies to construction materials, industrial components, and systems subject to variable thermal conditions.

How thermal diffusivity is measured in the laboratory: Light Flash Analysis (LFA) 

The experimental determination of thermal diffusivity requires specific methods capable of analyzing the response of a material to a transient thermal input, rather than steady-state conditions on which conductivity methods are based. The most widely used and accurate method, particularly for high-temperature applications, is Light Flash Analysis (LFA), governed by ASTM E1461 and the international standard ISO 22007-4.

A short-duration energy pulse, generated by a laser or xenon lamp, is applied to one face of the sample. The temperature variation on the opposite face is measured over time using an infrared sensor. Analysis of the thermal response curve allows determination of thermal diffusivity through mathematical models that take into account sample geometry and test conditions, including possible heat losses.

The main advantages of the LFA technique are: 

• Non-destructive: the sample is not altered during the test and can be reused or subjected to further analyses.
• Fast: each individual measurement lasts only a few seconds, allowing high productivity and the characterization of multiple samples in a short time.
• Wide temperature range: the technique is applicable from cryogenic temperatures up to over 1250 °C in standard configurations, with advanced systems reaching 2800 °C. This makes it suitable for the characterization of refractories, high-temperature ceramics, and materials for aerospace applications.
• Versatile: suitable for metals, alloys, ceramics, polymers, composites, construction materials, and thin films. The sample typically requires a disc geometry with controlled thickness and diameter.

Thermal diffusivity determined by LFA, in combination with density (ρ) and specific heat (cₚ) - the latter preferably obtained by DSC or, alternatively, estimated by LFA through comparison with a reference material according to validated procedures - allows the indirect determination of thermal conductivity (λ), thus completing the thermophysical characterization of the material. 

Application fields and industrial relevance 

Thermal diffusivity is a key parameter in all contexts where materials operate under non-steady-state conditions. The main application areas are: 

• Automotive e aerospace: the design of components subjected to rapid thermal cycles - such as braking systems, engines, thermal shields, and structural components - requires knowledge of diffusivity to predict the temporal distribution of temperature and to assess thermal fatigue phenomena.

• Electronics and thermal management: in high power-density electronic components, diffusivity determines how quickly heat is dissipated from critical areas. It is a fundamental parameter in the selection of thermal pastes, substrates, and heat sink materials.

• Refractory materials and high-temperature ceramics: in industrial furnaces, reactor linings, and high-temperature components, the dynamic response of materials to thermal shock directly depends on diffusivity. LFA is the standard technique for this characterization.

• Building sector and summer comfort: in construction, diffusivity contributes to the evaluation of the thermal inertia of the building envelope. Materials with low diffusivity and high volumetric heat capacity - such as concrete, brick, or clay - attenuate and delay external temperature variations, improving summer comfort without active cooling systems.

• Research and development of new materials: in the characterization of advanced composites, phase change materials (PCM), aerogels, and functional materials, diffusivity is often the first parameter measured to understand thermal behavior under real operating conditions. 

Regulatory and technical framework 

• ASTM E1461: reference American standard for the determination of thermal diffusivity using the flash method. It defines measurement procedures, sample requirements, mathematical models for thermal response analysis, and validation criteria. It is the most widely cited international reference for this technique in industrial and research contexts.

• ASTM E2585: standard practice for thermal diffusivity measurement using the flash method for high-conductivity materials, such as metals and alloys.

• ISO 22007-4: international standard governing the determination of thermal conductivity and diffusivity of polymers and plastic materials using the laser flash method. It complements ASTM E1461 for polymer applications.

• ISO 18755:2022: international standard specifying a method for the determination of thermal diffusivity of monolithic ceramic materials by applying a thermal pulse (flash method) and analyzing the thermal response as a function of time.

• UNI EN ISO 10456: reference standard for the determination of declared and design thermal values of construction materials. It provides procedures to convert laboratory-measured values into values usable in energy calculations, taking into account temperature and humidity.

Certimac support

Certimac is equipped with advanced Light Flash Analysis (LFA) instrumentation for the determination of thermal diffusivity over a wide range of materials and across a temperature range covering applications from building insulation characterization to high-temperature industrial refractories.

Tests are performed in compliance with ASTM E1461 and ISO 22007 standards. Certimac operates as an ISO 17025 accredited laboratory by ACCREDIA, ensuring a quality system compliant with international requirements.

Based on results obtained through LFA, integrated with specific heat (DSC) and density measurements, Certimac provides a complete thermophysical characterization of the material, determining diffusivity (α), thermal conductivity (λ), and specific heat (cₚ). All parameters are obtained within the same measurement campaign, ensuring full consistency and metrological traceability. Results are issued in the form of a Test Report, usable for product qualification, R&D activities, quality control, and failure analysis.

Our team of experts supports companies not only during the testing phase, but also in the interpretation of results and in the practical application of data for product development and technological innovation.

For more information on our thermal diffusivity analysis services or to request a consultation, please visit our dedicated thermal analysis page, contact our experts by filling out the form at the bottom of the page, or write to materiali@certimac.it.