The present disclosure relates to systems, devices, and methods that augment a thermosiphon system with a thermally conductive matrix material to increase the surface area to volume ratio for heat conduction at a predetermined region(s) of the thermosiphon system while minimizing capillary forces that are isolated to those region(s). The thermosiphon system has tubing including a condenser region, an evaporator region, and an adiabatic region (e.g., a region between the condenser and evaporator regions). The tubing can contain a heat transport medium and can provide passive two-phase transport of the heat transport medium between the condenser and evaporator regions according to thermosiphon principles. The system also includes a thermally conductive matrix material contained in the condenser region and/or the evaporator region but not in the adiabatic region, such that the thermally conductive matrix material increases a surface area for heat transfer in the condenser region and/or the evaporator region.