The basic premise of measuring the thermal conductance of straps is to apply a measurable amount of heater power (Qhtr) to one end of the strap while securing the other end to a heat sink. The temperatures of the heat source and sink blocks are then measured just under the interfaces of the strap’s end fittings.
Given that external heat leak paths to and from the strap under test are minimized and predictable, the thermal conductance of the strap can be calculated as:
Cstrap = Qstrap/ΔTstrap
TAI uses a thermal interface material (TIM) (HITHERM™ HT-) in order to minimize the thermal resistance at the interface between the strap’s end fittings and the source and sink blocks. This is done because we cannot always control or replicate the exact attachment method and interface used in the specific application and because we are mainly interested in the thermal conductance of the strap itself.
As shown in Figure 1 (top right), and Figure 2 (left), the temperature sensors are embedded in the heat source and heat sink blocks. This puts the temperature measurements directly within the heat flow path. The effects of the blocks, temperature sensor locations, and the bolted interfaces can be determined and removed from the reported strap thermal conductance once the data is reduced.
This measurement technique is important because temperature probes attached to the exterior of the end fittings do not result in accurate measurements. Externally mounted temperature sensors result in measurements that are outside of the heat flow path.
To minimize heat leak paths (Qleak) that can compromise test results, TAI uses these design and configuration practices:
A radiation shield is not normally used in typical test configurations. Without a shield, the heat leak due to radiation is a straightforward calculation. With a shield, radiation would be reduced, but the heat transfer becomes more difficult to predict. TAI has experienced that test results with a radiation shield are highly variable and unpredictable. Therefore, we do not use a radiation shield in our thermal test configuration.
In our standard test configuration, we limit total heat leak to less than 3% of the total heater power for GFTS® and CuTS®. The total heat leak is calculated and taken into account when thermal conductance of the strap is reduced from the test data.
For the strap thermal conductance calculation, the heat leak (Qleak) through the various paths (wires, radiation, and source block support) is determined and subtracted from the measured heater power. The result is the heat flow through the thermal strap from source block to the sink block (Qstrap). The measured temperatures from the temperature sensors embedded in the heat source and sink blocks are used to calculate a raw thermal conductance (Craw). Craw is the value of the direct temperature measurements:
Craw = (Qhtr – Qleak) / ΔT = [(Ihtr)(Vhtr) – Qrad – Qhtr leads – QTC leads] / (Th – Tc)
The physical properties of the test blocks (material conductivity, heat flow area, depth of the temperature sensors) are used to calculate the thermal resistance due to the fact that the temperature sensors are not in direct contact with the thermal strap itself. Removing these effects gives the source-surface-to-sink-surface thermal conduction: Csc-sk, which includes the thermal resistance of the bolted interface, using the thermal interface material.
The thermal resistances across the bolted interfaces can be determined empirically using test data and the contact areas of the thermal strap. When the interface resistances are removed from Csc-sk, the resulting value (shown as CS) is the thermal conductance of the thermal strap alone. See Figure 3 for a detailed view of the thermal conductance values reported by TAI.
Pictured: Figure 3 - thermal strap conductance projection definitions.
Csc-sk is typically reported as it is directly related to the measurements obtained from the test configuration. Cs is also reported because it is the value of most interest to the end user.
To learn more about our standard thermal conductivity test procedures, contact TAI today. Our experts have standard work instruction packages for all of our strap qualification and processing procedures (stiffness, conductance, bagging & packing, etc.).
Our copper and graphite strap design, test, and manufacturing heritage spans three decades. It began with our own Scott Willen and Richard Jetley, who developed the first Graphite Fiber Straps (GFTS®), in SBIR Ph I and II contracts with the USAF in (this research was later published in Cryocoolers 11). GFTS® products gained popularity in - , During this time, several dozen assemblies, designed and manufactured by TAI Quality Manager, Trevor Sperry, were used to cool the phased antenna arrays and data acquisition systems on the ORION spacecraft, and compressors on JAXA's Astro-H satellite. Since , GFTS® have played vital roles in notable programs such as Boeing's CST-100 Starliner, NASA's IXPE and GRACE-FO satellites, ESA's Solar Orbiter, DLR’s EnMAP, and several other spaceflight missions (in addition to ground-based applications in the medical and cryogenic engineering industries).
Our Copper Cabled Strap (CuTS®) heritage began in , with our first-generation solderless copper braided straps (offered until ). TAI was the first and only supplier to offer a fully customizable standard product line and catalogs, both created by our Director of Business Development, Tyler Link, in . Two years later, TAI developed OFHC UltraFlex cabling, optimized end fitting design, and improved swage & final machining methods. In , this new generation of straps was studied extensively by Fermi National Lab and other universities and laboratories (research was later published in volume 86 of the Journal Cryogenics, and co-authored by TAI's Tyler Link and Jamie Deal).
Want more information on Flexible graphite wire supplier? Feel free to contact us.