  To construct reliable material platforms and to uncover new rules to realize
spin-Seebeck effect (SSE) and thermal spin-filtering effect (SFE) are core
topics in spin caloritronics. Here we design several single-layer
boron-nitrogen nanotubes (BNNTs) with n boron (nitrogen) atoms substituted by
carbons in every unit cell. We find that for n = 1, the magnetic BNNTs generate
a good SSE with nearly symmetric spin-up and spin-down currents; while as the
carbon dopant concentration increases (c.f. n $\geq$ 2), a high rotational
symmetry of the carbons contributes to generate the SSE with more symmetric
thermal spin-up and spin-down currents, otherwise towards the thermal SFE.
Moreover, some metallic BNNTs can generate the SSE or the SFE with finite
threshold temperatures, due to the compensation effect around the Fermi level.
More importantly, we find that the compression strain engineering is an
effective route to improve these effects and to realize the transition between
them. These theoretical results about the SSE in nanotubes enrich the spin
caloritronics, and put forwards new material candidates to realize the SSE and
other inspiring thermospin phenomena
