Surfing around,

This blogpost discusses the Multi-Hundred Watt RTG (MHW-RTG) which was developed for Voyager:

SiGe thermocouples were doped with Boron and Phosphorous. The main mode of failure in them was the germanium migrating out of solution over time, but the extent to which this occurred over the lifetime of the missions is unclear. To prevent sublimation and degradation, the thermocouples were coated with silicon nitride, which eliminated the need for the xenon cover gas used in earlier SiGe-based thermocouples.

 

The power conversion efficiency of the thermocouples was 6.5% at beginning of life, decreasing to 5.9 % at the end of design life (14 years, which the Voyager spacecraft have more than doubled). I am unable to find information about the current conversion efficiency of these systems.

The NASA Radioisotope Power System Program Office 2015 booklet Radioisotope Power Systems Reference Book for Mission Designers and Planners says (one of several links in previous reference)

Thermocouple performance may degrade over time due to precipitation of dopants in the material, sublimation of the thermocouple material, or changes in thermal conductivity of unicouple alloys. The output power degradation due to thermocouple degradation is ~0.8% per year, depending on the material and the operating conditions. Radioactive decay of the Pu-238 causes additional degradation at ~0.8% per year.

Surfing around,

This blogpost discusses the Multi-Hundred Watt RTG (MHW-RTG) which was developed for Voyager:

SiGe thermocouples were doped with Boron and Phosphorous. The main mode of failure in them was the germanium migrating out of solution over time, but the extent to which this occurred over the lifetime of the missions is unclear. To prevent sublimation and degradation, the thermocouples were coated with silicon nitride, which eliminated the need for the xenon cover gas used in earlier SiGe-based thermocouples.

The power conversion efficiency of the thermocouples was 6.5% at beginning of life, decreasing to 5.9 % at the end of design life (14 years, which the Voyager spacecraft have more than doubled). I am unable to find information about the current conversion efficiency of these systems.

The NASA Radioisotope Power System Program Office 2015 booklet Radioisotope Power Systems Reference Book for Mission Designers and Planners says (one of several links in previous reference)

Thermocouple performance may degrade over time due to precipitation of dopants in the material, sublimation of the thermocouple material, or changes in thermal conductivity of unicouple alloys. The output power degradation due to thermocouple degradation is ~0.8% per year, depending on the material and the operating conditions. Radioactive decay of the Pu-238 causes additional degradation at ~0.8% per year.

Srfing around,

NERVA says

SiGe thermocouples were doped with Boron and Phosphorous. The main mode of failure in them was the germanium migrating out of solution over time, but the extent to which this occurred over the lifetime of the missions is unclear. To prevent sublimation and degradation, the thermocouples were coated with silicon nitride, which eliminated the need for the xenon cover gas used in earlier SiGe-based thermocouples.

The power conversion efficiency of the thermocouples was 6.5% at beginning of life, decreasing to 5.9 % at the end of design life (14 years, which the Voyager spacecraft have more than doubled). I am unable to find information about the current conversion efficiency of these systems.

A NASA booklet (one of several links in previous reference) says

Thermocouple performance may degrade over time due to precipitation of dopants in the material, sublimation of the thermocouple material, or changes in thermal conductivity of unicouple alloys. The output power degradation due to thermocouple degradation is ~0.8% per year, depending on the material and the operating conditions. Radioactive decay of the Pu-238 causes additional degradation at ~0.8% per year.

Surfing around,

This blogpost discusses the Multi-Hundred Watt RTG (MHW-RTG) which was developed for Voyager:

SiGe thermocouples were doped with Boron and Phosphorous. The main mode of failure in them was the germanium migrating out of solution over time, but the extent to which this occurred over the lifetime of the missions is unclear. To prevent sublimation and degradation, the thermocouples were coated with silicon nitride, which eliminated the need for the xenon cover gas used in earlier SiGe-based thermocouples.

 

The power conversion efficiency of the thermocouples was 6.5% at beginning of life, decreasing to 5.9 % at the end of design life (14 years, which the Voyager spacecraft have more than doubled). I am unable to find information about the current conversion efficiency of these systems.

The NASA Radioisotope Power System Program Office 2015 booklet Radioisotope Power Systems Reference Book for Mission Designers and Planners says (one of several links in previous reference)

Thermocouple performance may degrade over time due to precipitation of dopants in the material, sublimation of the thermocouple material, or changes in thermal conductivity of unicouple alloys. The output power degradation due to thermocouple degradation is ~0.8% per year, depending on the material and the operating conditions. Radioactive decay of the Pu-238 causes additional degradation at ~0.8% per year.