Army

Heat stable electrical junctions

Novel bonds between components show excellent mechanical strength and low electrical resistance

Materials Electronics

Electronics within solar panels must operate at high temperatures and present a potential application for these novel electronic junctions. (U.S. Air Force photo/Eddie Green)

There are several commercial approaches for joining electrical components for stable operation at high-temperature including brazing and thermo-compression bonding. In the brazing approach, a mixture of metals is applied between the two parts to be joined. While extensively used, this approach is not without drawbacks. The metals are aggressive chemically, and the composition of the active components of an electrical element can be degraded by chemical interaction with the braze. Also, the most common braze (aluminum-silicon) is only stable to about 660° C., which limits the useful temperature range available to many uses. Finally, with brazing, voids can develop because of chemical inter-diffusion between the components and the braze, and embrittled layers with poor mechanical properties can result.

Thermo-compression bonding uses thin gold layers applied to the two components to be attached. The layers are placed in contact and heated to a high temperature to create one unified gold layer. Gold does not oxidize at standard temperature, pressure, and in common lab environments, so thermo-compression bonding can result in a low resistance electrical contact. However, gold is known to quickly diffuse along all the surfaces and deeply into electrical components, such as thermoelectric materials. This deep diffusion of gold is known to poison thermoelectric materials and degrade thermoelectric device performance.

Acknowledging a need for forming low electrical resistance junctions that are strong and stable to elevated temperatures such as 800° to 900° C., Army researchers have developed a strong, heat-stable junction formed of indium (In), tin (Sn), and nickel (Ni). The junction is made by heating a layer of indium and tin aligned against a layer of nickel to a temperature of 400° C or more thereby forming a stable intermetallic bond between the semiconductor components. There are several key characteristics of the junction:

  • Thermal stability – the In3Ni2 compound alone is stable up to 869° C
  • Mechanical strength – lab experience with a test sample and a thermoelectric prototype indicates the junction is unusually strong and robust
  • Thermal-match – the thermal expansion coefficient of a nickel shoe and the thermoelectric materials is very closely matched across the spanned temperature range, resulting in dramatically reduced thermal stress and resistance to breakage
  • Low electrical resistance – the electrical current is not significantly impeded at the junction between the shoe and the thermoelectric materials

One use for the invention is in energy recovery from the heat coming off an internal combustion engine’s exhaust. Such energy return can offset the parasitic load of the engine’s alternator. The invention provides the stability for the component n-type or p-type semiconductors needed for reliability over the lifetime of the car.

The strong stable junctions can be used in additional electrical devices including solar cells, where solar light is concentrated as much as 400 times, with resulting dramatic increases in temperature. The temperature of the solar cell may reach several hundred degrees C., above ambient temperature. For this and like applications, this electrical contact technology would offer a path to an increased lifetime without degradation in performance.

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