Thermal Conductivity

PMIC specializes in both in-plane and through-thickness thermal conductivity evaluation of materials, including laminates, carbon fibers, ceramics, composites, and alloys.  Complex structures, such as bonded and fastened joints, including IC chips bonded to heat sink material, can be measured, as well as the thermal resistance of interfaces, multi-layer materials, and thin-film materials.  See the bottom of this page for a partial list of materials PMIC has tested.  Testing can be performed in air, partial atmosphere, vacuum, or inert gas.

WHICH STANDARD IS BEST?  Some industries require specific standards be used to confirm product properties.  For new application materials not ready for certification, product development, and general characterization, PMIC can recommend the best test method based on multiple criteria.

Full descriptions of testing capabilities and test methods are available below per system type.


TK-3 and TK-7 Capabilities

This describes the capabilities and limitations of PMIC's test systems which are used to perform testing per ASTM C177  and ASTM C1044.
Guarded-Hot-Plate SystemsCapabilities
Thermal Conductivity Range< 0.01 W/(m·K) to 2 W/(m·K)
(0.69 Btu·in/(h·ft2·°F) to 13.88 Btu·in/(h·ft2·°F)
Mean Sample Temperature Range98 K to 723 K
(-175 °C to 450 °C)
Specimen SizeIdentical pair of samples: both 152 mm × 152 mm, ≤25.4 mm thick
(6.0" × 6.0", ≤1.0" thick)
Reproducibility±2 %
Accuracy±3 % to ±5 %
Note 1ASTM C177 has a practical limit for thermal conductivity. Unless ASTM C177 is required, PMIC has observed best results from other test methods for materials with conductivity >2 W/(m·K).
Note 2Sample sizes and temperature range are limited by PMIC's current test system designs.

The Guarded-Hot-Plate Apparatus allows for direct measurement of heat flux through materials.  Specimens are placed in the space between the heater plate and cooling plates (Figures 1 and 2).  Thermal properties are calculated from measurements of the metering area, energy input, temperatures, and thickness of the specimen.  ASTM C177 uses two identical specimens, but a single-sided mode of measurement is available for the same system using Practice C1044.

Illustration of ideal heat flow in the C177 apparatus.
Figure 1: Diagram of idealized heat flow in a Guarded-Hot-Plate Apparatus.
General arrangement of Guarded-Hot-Plate apparatus,insulation wrapped around two cold surface assemblies sandwiching two specimens sandwiching the primary guards between which is the guarded hot plate.
Figure 2: Diagram for the general arrangement of mechanical components for the Guarded-Hot-Plate Apparatus (Fig. 1, ASTM C177-19).
ASTM C1114

ASTM C177 has practical measurement limits associated with higher temperatures.  ASTM C1114 uses a thin-heater setup with higher-temperature materials.  One of the greatest challenges associated with ASTM C1114 is accounting for edge heat loss in specimens.  PMIC’s head of research division, Dr. Ingo Stark, is currently in the final stages of testing a new custom ASTM C1114 test system so that PMIC may offer thermal conductivity testing at temperatures reaching 1523 K (1250 °C) and higher.


C518 and E1530 system limitations

This describes the capabilities and limitations of PMIC's test systems which are used to perform testing per ASTM C518 and E1530
ASTM C518, ASTM E1530, ISO 8301 Heat Flow Meter SystemsCapabilities
Thermal Conductivity Range0.01 W/(m·K) to 2 W/(m·K)
Mean Sample Temperature Range98 K to 548 K
(-175 °C to 275 °C)
Standard Specimen Sizes25.4 mm squares or 50.8 mm squares, <25.4 mm thick
(1" squares or 2" squares ≤ 1" thick)
Reproducibility±2 %
Accuracy±5 % to ±10 %
Note 1C518 has a smaller temperature range of 233 K to 548 K (-40 °C to 275 °C) due to flux sensor limitations.
Note 2Specimen size is limited to the extent to which adaptors can fit the specimen to existing flux sensor sizes.
Note 3Accuracy is dependent on the thermal resistances of available reference materials how well they match the thermal resistance of the specimen.

ASTM C518 measurements rely on the calibration of the heat flow meter apparatus and the quality of the heat flux transducers used, but the procedure itself is deceptively simple: a specimen is placed between two heat flux transducers with uniform thermal contact between a hot plate and a cold plate, as shown in Figure 3.  A steady-state heat flux is established through the test specimen.  The plate temperatures and plate separation are measured.

A sample is sandwiched between two heat flux transducers.
Figure 3: C518 Felt sample sandwiched between heat flux transducers in the heat flow meter system.

ASTM E1225 and D5470 Systems

This describes the capabilities and limitations of PMIC's test systems which are used to perform testing per ASTM E1225 and D5470 testing.
ASTM E1225, ASTM D5470 Guarded-Comparative-Longitudinal Heat Flow SystemsCapabilities
Thermal Conductivity Range0.2 W/(m·K) to 400 W/(m·K)
Mean Sample Temperature RangeCurrently tested from 45 K to 873 K
(-228 °C to 600 °C)
Standard Specimen SizesSolids, liquids, pastes
Reproducibility±2 %
Accuracy±5 % to ±10 %
Note 1Specimen size is desired to fit 25.4 mm square or Ø 25.4 mm adaptors for standard E1225 and D5470 testing.
Note 2Accuracy is dependent on specimen and material conductivity.

The testing stack is made by inserting the specimen between two reference materials (meter bars) with a uniform compressive load.  A temperature gradient is formed by maintaining a temperature difference between two actively heated and cooled plates, as shown in Figures 4 and 5.  A minimum of two temperature sensors are placed in each stack element in order to determine the steady state temperature gradient.  Apparent heat flow is calculated per unit area in the meter bars.  Specimen thermal conductivity at temperature is then calculated.

Heat flows from the upper stack heater through a stack of two meter bars sandwiching a specimen. Insulation and a guard shell help control heat losses. Temperature sensors account for heat at the top and bottom of the insulation next to the guard shell, the top and bottom of both meter bars, and the top and bottom of the specimen.

Example of an E1225 test stack prior to installation of insulation.

Example of three, four, and five thin specimens stacked to derive thermal conductivity of the material by excluding thermal resistance.
Figure 6: Three specimen stacks of increased layers and thicknesses are tested to calculate the thermal conductivity of materials too thin to accurately measure at thickness.  Specimens typically desired but not required to fit 25.4 mm square adapters.

For thin materials the apparent thermal conductivity of a specimen can be calculated from the measured thermal resistance and measured specimen thickness.  Interfacial thermal resistance must be 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 resistance of three different thicknesses d of the material and plotting thermal resistance versus thickness (Figure 6). 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.

This stacking method can also be used to calculate the thermal conductivity of coatings.  Alloy substrates are coated with the customers’ material and tested.  The results are then calculated against bare alloy blocks and bare alloy stacks as shown below in Figure 7.

The thermal conductivity of coatings can be calculated by comparing the results of alloy substrates coated against bare alloy blocks and bare alloy stacks.
Figure 7: Visual diagram of the calculation of thermal conductivity of coatings.  Flux sensors are used in place of meter bars when testing per ASTM E1530.

Thermal conductivity can be measured for TIM at temperature for thermal pads, greases, adhesives, gap fillers, tapes, phase change materials, and metal thermal interface materials.  PMIC customers frequently request testing to multiple bond line thicknesses (BLTs), specific aging history, and curing conditions for best data of their materials in use.


Xenon Flash DXF-900

Table showing showing the machine capabilities and limits of PMIC's xenon flash laser test system.
Xenon Flash DXF-900 by TA InstrumentsCapabilities
Thermal Diffusivity Range0.01 mm2/s to 1000 mm2/s
Thermal Conductivity Range0.1 W/(m·K) to 2000 W/(m·K)
Temperature Range296 K to 1173 K
(23 °C to 900 °C)
Sample SizeØ 12.7 mm or Ø 25.4 mm, <5 mm thick
(Ø 0.5" or Ø 1.0" disks, <0.2" thick)
Special fixtures for liquid, powder, laminate, film, etc.
Diffusivity Reproducibility±2 %
Conductivity Reproducibility±3.5 %
Diffusivity Accuracy±2.3 %
Conductivity Accuracy±4 %
Link to TA Instruments brochureThe values above come from PMIC's in-house validation of the Xenon Flash DXF-900 and may not match values available from TA Instruments.
Designed to meet ASTM E1461, ASTM C714, ASTM E2585, ISO 13826, ISO 22007-Part 4, ISO 18755, BS ENV 1159-2, DIN 30905, and DIN EN821.
The ideal ASTM E1461 specimen is a 12.5 mm diameter disk, 1 mm to 6 mm thick, with flat and parallel faces to within 0.025 mm.
Figure 8: Diagram of an ideal ASTM E1461 flash specimen for thermal diffusivity testing.

PMIC uses the Xenon Flash DXF-900 by TA Instruments to perform thermal diffusivity and thermal conductivity measurements.  Figure 8 shows the most common specimen geometry used in the instrument.

Small, thin specimens are subjected to a high-intensity energy pulse.  The top of the specimen absorbs the energy of the pulse and the instrument records the temperature of the specimen’s bottom.  Thermal diffusivity is calculated from specimen thickness and the time required for the bottom of the specimen to reach a relative increase in temperature.

Per ASTM E1461, the density and specific heat of a sample must be provided for the DXF-900 to calculate the thermal conductivity from its thermal diffusivity measurement.  PMIC can perform density and specific measurements.



The MATHIS TCi can be used to perform testing per ASTM D7984, ASTM D5334, ASTM D5930, and IEEE 442-1981.  Specimens may have a thermal conductivity from <0.01 W/(m·K) to 500 W/(m·K).  Testing may be performed over a temperature range of 223 K to 473 K (-50 °C to 200 °C).

PMIC uses the MATHIS TCi by C-Therm (link to brochure) to perform thermal conductivity measurements by the modified transient plane source and transient line source techniques.  This instrument can be used to test solids, liquids, powders, and pastes in various environmental enclosures when material or sample geometries restrict the use of PMIC’s preferred precision test methods and as the above ASTM standards recommend.


TDTR testing is a non-contact, laser-based technique that measures the thermal conductivity of materials (particularly thin films). This method can be applied to materials with smooth surfaces and thin films with a thickness range from nanometers to microns.

The idea behind this technique is that a pump laser pulse heats the surface of a material and a subsequent probe laser pulse measures a change in optical reflectivity that is created by the temperature excursion generated by the pump laser pulse. The reflectivity signal is measured as a function of time and the data are analyzed using a heat diffusion model to extract the thermal conductivity [1].

TDTR testing can be performed over the temperature range from 296 K to 623 K (23 °C to 350 °C).

The requirements for this test are as follows:

  • Specimens must have optically smooth surfaces (Mirror-like, ≤15 nm RMS roughness)
  • The volumetric heat capacity must be provided for each specimen

Dr. Cahill has partnered with PMIC to offer this testing to customers and performs testing as a subcontractor. David Cahill [at the University of Illinois at Urbana-Champaign] is the Willett Professor of Engineering and Professor of Materials Science and Engineering (MatSE) [2]. He received his Ph.D. in condensed matter physics from Cornell University, and worked as a postdoctoral research associate at the IBM Watson Research Center” [3].

Figure 1: Time Domain Thermoreflectance (TDTR) Test System [4].


[1] D. G. Cahill, “Thermal-conductivity measurement by time-domain thermoreflectance,” MRS Bulletin, vol. 43, no. 10, pp. 782–789, Oct. 2018, doi: 10.1557/mrs.2018.209.
[2] “Cahill Research Group.” (accessed Aug. 24, 2020).
[3] “David G. Cahill – Cahill Research Group.” (accessed Aug. 25, 2020).
[4] “cahill.thermal,” cahill.thermal. (accessed Aug. 24, 2020).


TK ASTM Standards

A searchable list of the ASTM standards by which PMIC performs thermal conductivity, thermal diffusivity, and thermal resistance testing.
ASTMC177Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded Hot-Plate ApparatusGuarded Heater
ASTMC1044Practice for Using a Guarded-Hot-Plate Apparatus or Thin-Heater Apparatus in the Single-Sided ModeGuarded Heater
ASTMC1114Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Thin-Heater ApparatusThin Heater
ASTMC518Standard Test Method for Steady State Thermal Transmission Properties by Means of the Heat Flow Meter ApparatusHeat Flow Meter
ASTME1225Standard Test Method for Thermal Conductivity of Solids Using the Guarded-Comparitive-Longitudinal Heat Flow TechniqueHeat Flow Meter
ASTMD5470Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation MaterialsHeat Flow Meter
ASTME1530Standard Test Method for Evaluating the Resistance to Thermal Transmission of Materials by the Guarded Heat Flow Meter TechniqueHeat Flow Meter
ASTME1461Standard Test Method for Thermal Diffusivity by the Flash SystemDXF-900
ASTMC714Standard Test Method for Thermal Diffusivity of Carbon and Graphite by Thermal Pulse MethodDXF-900
ASTME2585Standard Practice for Thermal Diffusivity by the Flash MethodDXF-900
ASTMD7984Standard Test Method for Measurement of Thermal Effusivity of Fabrics Using a Modified Transient Plane Source (MTPS) InstrumentMATHIS TCi
ASTMD5334Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe ProcedureMATHIS TCi
ASTMD5930Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source TechniqueMATHIS TCi
ASTMC1113Standard Test Method for Thermal Conductivity of Refractories by Hot Wire (Platinum Resistance Thermometer Technique)Hot Wire


TK Materials Tested

A partial list of materials for which PMIC has characterized the thermal conductivity, using various measurement techniques.
A Partial list of materials for which PMIC has characterized the thermal conductivity, using various measurement techniques.
Graphite, CarbonCarbon FibersSilicon CarbidePotting Compound
Porous CeramicsSilica AerogelsSiliconPolymers
ConcreteAsphaltTextilesTextile Composites
Epoxy ResinsFiberglassGlassDense Ceramics
Dense CeramicsLiquidsPlasticsFirebrick
Cellulose InsulationMineral FiberWoodInvar
AluminumCopperAlloys (mult.)Emulsified Oils
Food, solidsFood, fillingsFood, drinksTransparent (mult.)
Thermal Interface (TIM)Aerogels (mult.)