Oh, nice; I was thinking of what I believe is this, though: https://sumitomoelectric.com/products/cu-diamond https://sumitomoelectric.com/products/cu-diamond :

Oh, nice; I was thinking of what I believe is this, though: https://sumitomoelectric.com/products/cu-diamond https://sumitomoelectric.com/products/cu-diamond : size-selected diamonds to match the thickness profile of the power module/brick base plate, then encased in copper. Benefit is arguably more the expansion matching to GaN/SiC than the thermal conductivity improvements over plain copper.