  The 3$\omega$ method is a dynamic measurement technique developed for
determining the thermal conductivity of thin films or semi-infinite bulk
materials. A simplified model is often applied to deduce the thermal
conductivity from the slope of the real part of the ac temperature amplitude as
a function of the logarithm of frequency, which in-turn brings a limitation on
the kind of samples under observation. In this work, we have measured the
thermal conductivity of a forest of nanowires embedded in nanoporous alumina
membranes using the 3$\omega$ method. An analytical solution of 2D heat
conduction is then used to model the multilayer system, considering the
anisotropic thermal properties of the different layers, substrate thermal
conductivity and their thicknesses. Data treatment is performed by fitting the
experimental results with the 2D model on two different sets of nanowires
(silicon and BiSbTe) embedded in the matrix of nanoporous alumina template,
having thermal conductivities that differ by at least one order of magnitude.
These experimental results show that this method extends the applicability of
the 3$\omega$ technique to more complex systems having anisotropic thermal
properties.
