engleski

Anisotropic thermal conductivity of the Y-Al-Ni-Co decagonal approximant

The anisotropic thermal conductivity  of Y-Al- Ni-Co decagonal approximant with composition Al76Co22Ni2.[1] has been measured along three orthogonal directions a*, b and c of the Y-Al- Ni-Co unit cell, where (a, c) monoclinic atomic planes are stacked along the perpendicular b direction. The phononic contribution  ph= - el was estimated by subtracting the electronic contribution  el from the total conductivity using the Wiedemann-Franz (WF) law,  el = Lo σ (T) T and the measured electrical conductivity data σ (T) on the same specimens [2]. Though the use of the WF law is a rough approximation, in this way determined  ph gives an indication of the anisotropy of the phononic spectrum. At room temperature (RT), we get the anisotropy:  a*=12.5 W/mK,  ela*=9.1 W/mK with their ratio ( ela*/ a*)RT=0.73,  b=46.3 W/mK,  elb=29.2 W/mK with ( elb/ b)RT=0.63 and  c=17.4 W/mK,  elc=12.2 W/mK with ( elc/ c)RT=0.70. Electrons are thus majority heat carriers for all three directions. The anisotropic thermal conductivities appear in the order  b> c> a*and similarly  elb> elc> ela*, which is identical to the order in which the anisotropic electrical conductivities appear σ b>σ c>σ a*. The anisotropic  ph's appear in the same order,  phb> phc> pha*, so that the phononic conductivity is the highest along the b direction perpendicular to the (a, c) atomic layers, whereas the two in-plane conductivities are lower and show smaller anisotropy. For all directions,  ph's show a typical phonon umklapp maximum at about 40 K. While it is expected that the anisotropic electronic thermal conductivities  el should appear in the same order as the anisotropic electrical conductivities (both are related to the electronic density of states), it is not a priori obvious that the anisotropic phononic thermal conductivities should also appear in this order, as  ph is related to the vibrational density of states. Striking anisotropy in thermal conductivity of the Y-Al- Ni-Co is due to the anisotropy of the structural symmetry.

thermal conductivity; decagonal approximants; Y-Al-Ni-Co

This work was supported in part by the 6th Framework EU Network of Excellence 'Complex Metallic Alloys' (Contract No. NMP3-CT-2005- 500140).