Significance and Use

This standard measures the steady state thermal impedance of electrical insulating materials used to enhance heat transfer in electrical and electronic applications. This standard is especially useful for measuring thermal transmission properties of specimens that are either too thin or have insufficient mechanical stability to allow placement of temperature sensors in the specimen as in Test Method E1225 .

This standard imposes an idealized heat flow pattern and specifies an average specimen test temperature. The thermal impedances thus measured cannot be directly applied to most practical applications where these required uniform, parallel heat conduction conditions do not exist.

This standard is useful for measuring the thermal impedance of the following material types.

Type I Viscous liquids that exhibit unlimited deformation when a stress is applied. These include liquid compounds such as greases, pastes, and phase change materials. These materials exhibit no evidence of elastic behavior or the tendency to return to initial shape after deflection stresses are removed.

Type II Viscoelastic solids where stresses of deformation are ultimately balanced by internal material stresses thus limiting further deformation. Examples include gels, soft, and hard rubbers. These materials exhibit linear elastic properties with significant deflection relative to material thickness.

Type III Elastic solids which exhibit negligible deflection. Examples include ceramics, metals, and some types of plastics.

The apparent thermal conductivity of a specimen can be calculated from the measured thermal impedance and measured specimen thickness if the interfacial thermal resistance is insignificantly small (nominally less than 1 %) compared to the thermal resistance of the specimen.

The apparent thermal conductivity of a sample material can be accurately determined by excluding the interfacial thermal resistance. This is accomplished by measuring the thermal impedance of different thicknesses of the material under test and plotting thermal impedance versus thickness. The inverse of the slope of the resulting straight line is the apparent thermal conductivity. The intercept at zero thickness is the sum of the contact resistances at the two surfaces.

The contact resistance can be reduced by applying thermal grease or oil to the test surfaces of rigid test specimens (Type III).

1. Scope

1.1 This standard covers a test method for measurement of thermal impedance and calculation of an apparent thermal conductivity for thermally conductive electrical insulation materials ranging from liquid compounds to hard solid materials.

1.2 The term thermal conductivity applies only to homogeneous materials. Thermally conductive electrical insulating materials are usually heterogeneous and to avoid confusion this test method uses apparent thermal conductivity for determining thermal transmission properties of both homogeneous and heterogeneous materials.

1.3 The values stated in SI units are to be regarded as standard.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

ASTM Standards

D374 Test Methods for Thickness of Solid Electrical Insulation

E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method

E1225 Test Method for Thermal Conductivity of Solids by Means of the Guarded-Comparative-Longitudinal Heat Flow Technique

Keywords

apparent thermal conductivity; guarded heater method; thermal conductivity; thermal impedance; thermally conductive electrical insulation; Guarded heater method; Roiseland method; Apparent thermal conductivity; Electrical insulating solids; Thermal conductance/conductivity; Thermal impedance; Thermal insulating materials (electrical)

ICS Code

ICS Number Code 29.035.01 (Insulating materials in general)

DOI: 10.1520/D5470-12

ASTM International is a member of CrossRef.

ASTM D5470