Thermal diffusivity - A143

Thermal diffusivity
Foundational knowledge article
Document Type	Article
Document Identifier	143
Themes	Thermal management
Relevant Class	Material
Tags	Conductive heating Material parameters Thermal properties
Prerequisites	Heat transfer Specific heat capacity Thermal conductivity

Introduction

Thermal diffusivity, \(\alpha\), is a quantitative measure of how a material will respond to transient thermal conditions. It is defined as the ratio of thermal conductivity to the volumetric heat capacity of the material (density times the specific heat capacity).

Scope

This page defines thermal diffusivity, explains its significance in composites processing, and provides some typical values.

Significance

Materials with large thermal diffusivity have a high thermal conductivity relative to their heat capacity (i.e. their capacity to store thermal energy). As a result, they will rapidly distribute the thermal energy throughout their volume and change temperature quickly during heating or cooling. On the other hand, materials with a low thermal diffusivity have a low thermal conductivity relative to their hear capacity and will have a slow temperature response to a change in thermal conditions and will develop larger temperature gradients.

Thermal diffusivity is a key property in composites processing. For example, the thermal diffusivity of the tooling material dictates the rate at which heat is transferred through the tool between the hot side to the cold side. Tooling materials with a greater thermal diffusivity will react to localized heat change faster compared to a tooling material with low thermal diffusivity, reducing the internal thermal gradient that can form within the tool and subsequently within the part. To learn more about this interaction between tool heating and part curing, please see the effect of tooling in a thermal management system page in the Systems Knowledge volume.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Heat transfer
• Thermal conductivity
• Specific heat capacity

Definition

Thermal diffusivity is the measure of the rate of temperature movement through a material. It is used in the heat transfer equation describing the change of temperature for transient (non-steady state) heat flow.

Thermal diffusivity \(\alpha\) is defined as the ratio of thermal conductivity to the volumetric heat capacity of the material (density times the specific heat capacity) and calculated as\[\alpha=\frac{k}{\rho c_p}\]

Where,

\(k=\) Thermal conductivity [W/m·K]

\(\rho=\) Material density [kg/m³]

\(c_p=\) Specific heat capacity [J/kg·K]

Together, the bottom terms (\(\rho c_p\)) represent the volumetric heat capacity [J/m³·K].

Units

The general units of thermal diffusivity can be represented as\[\alpha=\frac{Length\, unit^2}{Time\, unit}\]

The following are common International System of Units (SI) and US Customary Units found in the literature for thermal diffusivity:

	SI Units	US Customary Units
Base units	m2/s	ft2/s
Other common forms	m2/h	ft2/h
	mm2/s

Typical Values

Examples of typical thermal diffusivity values seen in a various materials.

Material	Typical Value (SI units) m2/s	Typical Value (US Customary Units) ft2/s	[ref.]
Carbon Fibre (transverse)	1.9 x 10-7	2.0 x 10-6
Glass Fibre (S2)	8.0 x 10-7	8.6 x 10-6
Epoxy	1.5 x 10-7	1.6 x 10-6
PEEK	1.5 x 10-7	1.6 x 10-6
Cork	2-5 x10-7	2-5 x10-6
Steel (304 stainless)	4.0 x 10-6	4.3 x 10-5
Aluminum	9.7 x 10-5	1.0 x 10-3
Invar	2.6 x 10-6	2.8 x 10-5

Measurement

The following methods to measure thermal diffusivity are recommended by the Composites Materials Handbook - 17 (CMH-17) :

ASTM E1461: (Applicable to wide range of materials) Standard Test Method for Thermal Diffusivity by the Flash Method

ASTM C714: (Applicable to carbon and graphite only) Standard Test Method for Thermal Diffusivity of Carbon and Graphite by Thermal Pulse Method

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles	Basic Definitions of Material Properties - A211 Composite properties - A214 Conduction - A118 Glass transition temperature (Tg) - A210 Heat of reaction - A114 Material properties - A150 Polymer properties - A212 Reinforcement properties - A213 Specific heat capacity - A117 Thermal conductivity - A116 Thermal diffusivity - A143 Viscosity (resin) - A203
Foundational Knowledge Method Documents	How to measure gel time - M101 How to measure reinforcement content (and corresponding matrix content) - M109
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment	Rheometer - A357
Practice Documents
Case Studies
Perspectives Articles	Heat Transfer in Composites Processing - A321

References

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A quantitative measure of how a material will respond to transient thermal conditions. It is defined as the ratio of thermal conductivity to the volumetric heat capacity of the material (density times the specific heat capacity).

Materials with large thermal diffusivity have a high thermal conductivity relative to their capacity to store energy (heat capacity) and will rapidly distribute the thermal energy throughout its volume and rapidly change temperature to reach the new equilibrium temperature.

Materials with small thermal diffusivity have a low thermal conductivity relative to their capacity to store energy (heat capacity) and will have a sluggish temperature response to a change in thermal conditions (large temperature lag). This is because it takes more energy per unit volume to change the temperature, and the low thermal conductivity means it takes longer to distribute the thermal energy throughout its volume.