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.
ASTM C177 and ASTM C1044 GUARDED-HOT-PLATE SYSTEMS
TK-3 and TK-7 Capabilities
| Guarded-Hot-Plate Systems | Capabilities |
|---|---|
| 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 Range | 98 K to 723 K |
| (-175 °C to 450 °C) | |
| Specimen Size | Identical 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 1 | ASTM 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 2 | Sample 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.
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.
ASTM C518, ASTM E1530, ISO 8301 HEAT FLOW METER SYSTEMS
C518 and E1530 system limitations
| ASTM C518, ASTM E1530, ISO 8301 Heat Flow Meter Systems | Capabilities |
|---|---|
| Thermal Conductivity Range | 0.01 W/(m·K) to 2 W/(m·K) |
| Mean Sample Temperature Range | 98 K to 548 K |
| (-175 °C to 275 °C) | |
| Standard Specimen Sizes | 25.4 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 1 | C518 has a smaller temperature range of 233 K to 548 K (-40 °C to 275 °C) due to flux sensor limitations. |
| Note 2 | Specimen size is limited to the extent to which adaptors can fit the specimen to existing flux sensor sizes. |
| Note 3 | Accuracy 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.
ASTM E1225 and ASTM D5470 GUARDED-COMPARATIVE-LONGITUDINAL HEAT FLOW SYSTEMS
ASTM E1225 and D5470 Systems
| ASTM E1225, ASTM D5470 Guarded-Comparative-Longitudinal Heat Flow Systems | Capabilities |
|---|---|
| Thermal Conductivity Range | 0.2 W/(m·K) to 400 W/(m·K) |
| Mean Sample Temperature Range | Currently tested from 45 K to 873 K |
| (-228 °C to 600 °C) | |
| Standard Specimen Sizes | Solids, liquids, pastes |
| Reproducibility | ±2 % |
| Accuracy | ±5 % to ±10 % |
| Note 1 | Specimen size is desired to fit 25.4 mm square or Ø 25.4 mm adaptors for standard E1225 and D5470 testing. |
| Note 2 | Accuracy 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.
ASTM D5470 and ASTM E1530 THIN MATERIALS and COATINGS STACKING TESTS
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.
THERMAL INTERFACE MATERIAL (TIM) TESTING
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
| Xenon Flash DXF-900 by TA Instruments | Capabilities |
|---|---|
| Thermal Diffusivity Range | 0.01 mm2/s to 1000 mm2/s |
| Thermal Conductivity Range | 0.1 W/(m·K) to 2000 W/(m·K) |
| Temperature Range | 296 K to 1173 K |
| (23 °C to 900 °C) | |
| Sample Size | Ø 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 brochure | The 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. |
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.
MATHIS TCi – MODIFIED TRANSIENT PLANE SOURCE and TRANSIENT LINE SOURCE
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.
TIME DOMAIN THERMOREFLECTANCE (TDTR)
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].
References:
[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.” https://cahill.matse.illinois.edu/ (accessed Aug. 24, 2020).
[3] “David G. Cahill – Cahill Research Group.” https://cahill.matse.illinois.edu/david-cahill/ (accessed Aug. 25, 2020).
[4] “cahill.thermal,” cahill.thermal. https://cahill-thermal.com/ (accessed Aug. 24, 2020).
TK ASTM Standards
| ASTM Standard | Name | System |
|---|---|---|
| C177 | Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded Hot-Plate Apparatus | Guarded Heater |
| C1044 | Practice for Using a Guarded-Hot-Plate Apparatus or Thin-Heater Apparatus in the Single-Sided Mode | Guarded Heater |
| C1114 | Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Thin-Heater Apparatus | Thin Heater |
| C518 | Standard Test Method for Steady State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus | Heat Flow Meter |
| E1225 | Standard Test Method for Thermal Conductivity of Solids Using the Guarded-Comparitive-Longitudinal Heat Flow Technique | Heat Flow Meter |
| D5470 | Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials | Heat Flow Meter |
| E1530 | Standard Test Method for Evaluating the Resistance to Thermal Transmission of Materials by the Guarded Heat Flow Meter Technique | Heat Flow Meter |
| E1461 | Standard Test Method for Thermal Diffusivity by the Flash System | DXF-900 |
| C714 | Standard Test Method for Thermal Diffusivity of Carbon and Graphite by Thermal Pulse Method | DXF-900 |
| E2585 | Standard Practice for Thermal Diffusivity by the Flash Method | DXF-900 |
| D7984 | Standard Test Method for Measurement of Thermal Effusivity of Fabrics Using a Modified Transient Plane Source (MTPS) Instrument | MATHIS TCi |
| D5334 | Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure | MATHIS TCi |
| D5930 | Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique | MATHIS TCi |
| C1113 | Standard Test Method for Thermal Conductivity of Refractories by Hot Wire (Platinum Resistance Thermometer Technique) | Hot Wire |
TK Materials Tested
| Graphite, Carbon | Carbon Fibers | Silicon Carbide | Potting Compound |
| Porous Ceramics | Silica Aerogels | Silicon | Polymers |
| Concrete | Asphalt | Textiles | Textile Composites |
| Epoxy Resins | Fiberglass | Glass | Dense Ceramics |
| Dense Ceramics | Liquids | Plastics | Firebrick |
| Cellulose Insulation | Mineral Fiber | Wood | Invar |
| Aluminum | Copper | Alloys (mult.) | Emulsified Oils |
| Food, solids | Food, fillings | Food, drinks | Transparent (mult.) |
| Thermal Interface (TIM) | Aerogels (mult.) |
