Thermal Conductivities and Conduction Mechanisms of Sb-Te Alloys
R. Lan, R. Endo, M. Kuwahara*, Y. Kobayashi and M. Susa
Dept. of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Ookayama,
Meguro, Tokyo 152-8552, Japan
E-mail: lan.r.aa@m.titech.ac.jp* Photonics Research Institute, National Institutes of Advanced Industrial Science and Technology,
1-1-1 Higashi, Tsukuba-shi, Ibaraki 305-8562, JapanABSTRACT
The thermal conductivities of Sb-x at%Te alloys (x = 14, 25, 44, 60, 70, and 80) have been measured by the hotstrip method from room temperature up to temperature just below the respective melting points. For theintermetallic compound Sb2Te3 (x = 60), the thermal conductivity decreases from room temperature up to about
500 K and then increases up to temperature just below the melting temperature. For other Sb-x at%Te alloys,
where x > 60, the temperature dependence of the thermal conductivities is negative. The thermal conductivities ofthe alloys are close to that of tellurium at room temperature. On the other hand, where x < 60, the temperaturedependence is positive. The thermal conductivities of the alloys increase remarkably with decreasing Te
concentration at room temperature.
Keywords Sb-x at%Te alloys, Thermal conductivity, Hot strip method

1. INTRODUCTION
Phase change random access memory (PCRAM) is a promising nonvolatile data storage technology for next
generation memory owing to higher performance characteristics such as fast operation and higher scalability1). Sb-Te
binary alloys have been suggested to be suitable candidates for PCM devices because of a dramatic change in electric
resistivities associated with transformation between amorphous and crystalline phases, which forms the basis for datastorage2, 3). The phase transformation is controlled by Joule-heating and cooling processes and, thus, accurate data forthermal conductivity of Sb-Te binary alloys are indispensable to optimal designing for PCM devices. Furthermore, thephysical properties of Sb-Te alloys have drawn much attention. Onderka and Fitzner4) have measured the electricalresistivity of Sb2Te3, the intermetallic compound in Sb-Te alloys, and found that, with increasing temperature, theresistivity increases in the solid state but decreases in the liquid state. On the basis of this temperature dependence of theresistivity, Onderka and Fitzner 4) have proposed that Sb2Te3 exhibits metallic conduction in the solid state andsemiconductor characteristics in the liquid state. For better understanding of the physical properties of Sb-Te alloys,
more accurate thermal conductivity data are desperately required. Against this background, the present work aims todetermine thermal conductivities of Sb-Te binary alloys as functions of temperature and composition.

2. EXPERIMENTAL
Samples used were Sb-x at% Te (x = 14, 25, 44, 60, 70, and 80). Thermal conductivity was measured by the hot strip
method as shown in Figure 15). Cylindrical samples of Sb-Te (20mm diameter and 40-50mm length) were preparedfrom Sb and Te powders with 99.9 mass% purity. A powder mixture (100 g) was melted in a quartz crucible (20mminner diameter) at 973K for 4 h in vacuum, followed by furnace cooling. Subsequently, the sample was taken from thetube and cut into two along the vertical axis, and the cross sections were mechanically polished using emery papers up
to #2000. Thermal conductivity measurements were conducted in argon atmosphere from 298 K up to temperatures justbelow the respective melting points. Prior to measurements of Sb-Te alloys, the thermal conductivity of fused silica wasmeasured to confirm the reliability of the hot strip method because the thermal conductivity of fused silica is close tothat of Sb-Te binary alloys. Two pieces of fused SiO2 block (40×20×10 mm) were used as the samples.

3. RESULTS AND DISCUSSION
Figure 2 shows the thermal conductivity for silica glass as a function of temperature, together with the reference data.
The present results show a good agreement with the reference data6), which validates the reliability of hot strip method.
Figure 3 shows the temperature change with time recorded during thermal conductivity measurements on Sb2Te3 at
298 K and 789 K. There are good linearity between temperature change and time in the time period 0.7-2 s. From theslope of the linear portion, the thermal conductivity of Sb2Te3 has been determined.
Figure 4 shows the thermal conductivity for solid Sb2Te3 as a function of temperature, in comparison with values
measured by other investigators7–11) and calculated from the Wiedemann–Franz (WF) law12) using the resistivity datareported by Onderka4) and the theoretical Lorentz number, 2.45×10-8 W/K2. Thermal conductivity data obtained
during the heating cycle are in very good agreement with those obtained during the cooling cycle. The data obtained in
the present work show an interesting temperature dependence: the thermal conductivity decreases with increasing
temperature up to approximately 500 K and then increases. The data obtained at room temperature in the present work
are close to those reported from Yokota and Katayama11). The present data are also in fairly good agreement with values
calculated by the WF law at temperatures lower than about 600 K, above which they deviate from each other.
Figure 5 shows the present thermal conductivity results of Sb2Te3 along with the thermal conductivity due to freeelectron (e), phonon (ph) and ambipolar diffusion (ab) estimated by Yokota and Katayama11). These results indicatethat the contribution from the free electrons is relatively small, i.e., 1 W m-1 K-1, and thus phonon diffusion dominatesthermal conduction at temperatures lower than 300 K. With increasing temperature, however, both contributions to thethermal conductivity become smaller, whereas the contribution from ambipolar diffusion increases. This contribution isunnoticeable at low temperature but increases remarkably with increasing temperature to become comparable to thecontribution from phonon diffusion. The increase of thermal conductivity of Sb2Te3 above 600 K would be due to theambipolar diffusion.
For other Sb-x at%Te alloys, where x > 60, the temperature dependence of the thermal conductivities is negative. Thethermal conductivities of the alloys are close to that of tellurium at room temperature. On the other hand, where x < 60,
the temperature dependence is positive. The thermal conductivities of the alloys increase remarkably with decreasing Te
concentration at room temperature.

REFERENCES
1) S. Lai: IEDM Tech. Dig., 2003, p. 10.1.1. 2) M. Wuttig: Nature Materials, 4 (2005) 265. 3) M. H. R. Lankhorst et al.: Nat. Mater., 4 (2005) 347.
4) B. Onderka and K. Fitzner: Phys. Chem. Liq. 36 (1998) 215. 5) R. Lan et al. : CAMP-ISIJ, 23 (2010), 79. 6)Japan Society of Thermophysical
Properties：Thermophysical Properties Handbook, Yokendo（2008）292. 7) Y. S. Touloukian et al.: Thermal Conductivity of Metallic Elements andAlloys (Plenum Press, New York, 1970) p.1241. 8) T. C. Harman,et al.: J. Phys. Chem. Solids 2 (1957) 181. 9) C. H. Champness et al.: Can. J. Phys.
43 (1965) 653. 10) Sh. Movlanovet al.: Dokl. Akad. Nauk Azerbaidzhan 17 (1961) 375 [in Russian]. 11) K. Yokota and S. Katayama: Jpn. J. Appl.
Phys. 12 (1973) 1205. 12) R. Franz and G. Wiedemann: Ann. Phys. (Leipzig) 165 (1853) 497 [in German].

(a) 298 K

T/K

6
4
20 -1
10100t/s

101

1t/s

10(b) 789K

T/K

Fig. 1 Schematic diagram for hot strip method

/Wm-1K-1

4Hot strip method
6)Reference data

10-1

3

Fig.3 Temperature rise of heater during2

thermal conductivity measurements on Sb2Te3
with time at (a) 298 K and (b) 789 K

10400

600
Temperature/K800

Fig. 2 Thermal conductivity of silica
glass as function of temperature

TeSbO87)
Johnson8)
HarmanHugh 7)
Champness10)9)
MovlanovYokota 11)
W-F law12)This study-HeatingThis study-Cooling

Si/Wm-1K-1

64200200400600800

Temperature/K
Fig.4 Temperature dependence of thermal conductivity for solid Sb2Te3 by
hot-strip method along with data measured and calculated from W-F law8Yokota 11)11)ph by Yokota11)
e by Yokota 11)
ab by Yokota
This study-HeatingThis study-Cooling-1

/Wm K-164200200400600800

Temperature/K
Fig.5 Present thermal conductivity results of Sb2Te3 along withthermal conductivity due to free electron (e), phonon (ph) andambipolar diffusion (ab) estimated by Yokota and Katayama