Liquid Phase Diffusion Bonding for Thermoelectric Material Pb-Sn-Te
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metal with superalloys is used for inserted material, 3)the joined interface is more tough than the simply
Fig.1
soldered one.
for low-n and high-n Pb-Sn-Te
Temperature
(K)
Variations of figure-of-merit with temperature
111
Mat. Res. Soc. Symp. Proc. Vol. 586 © 2000 Materials Research Society
In the present paper, liquid phase diffusion bonding has been applied to joining of Pb-Sn-Te, and power generating performance has been investigated on the joined Pb-Sn-Te consisting of low-n and high-n segments. EXPERIMENT
"Lever
The mixtures of Pb(6N), Sn(5N)
and
Te(6N)
We ight
were
encapsulated in evacuated quartz tubes under 1 X 103Pa. Mixing ratios of Pb:Sn:Te were
Heet pipe Specimen --
Chamber RF col1
0.75:0.25:1 and 0.5:0.5:1 for the
low-n and respectively.
high-n segments, The mixtures were
Graphite punch _'ChermocoupIe
melted at 1273K by the rocking RF ter furnace with a rocking cycle of I gener~ter Hz, and subsequently solidified unidirectionally at a cooling rate of 30K/h. The slope of temperature given to the quartz capsules during unidirectional solidification was Fig.2 Schematic view of liquid phase diffusion 0.5K/mm. The n of Pbo.75Sno.25Te bonding appratus and Pbo.sSno.sTe were 2.3 X 1025 26 3 and 1.2>x 10 /m , respectively. The low-n segment(Pbo.75Sno.25Te) and the high-n segment(Pbo.sSno.5Te) were joined by liquid phase diffusion bonding technique. A schematic view of the diffusion bonding apparatus is shown in Fig.2. A Sn-sheet 50iim thick was used for inserted material. The stacked specimen of Pbo.75Sno.25Te/Sn-sheet/Pbo.sSno.sTe was set between the graphite punches, loaded by the upper leverage, and heated by RF(40kHz, 10kW). Heating rate was 0.5K/s, and the joining was under 2.OMPa at 700K for 15min in Ar. p was measured by the DC method with high speed and high resolution, and K was by the Harman method[7]. ax was obtained from the temperature dependence of thermoelectromotive force Eo. Effective maximum power Pmax was also evaluated under the temperature difference AT. Internal resistance rmt and E6 were measured under AT to calculate Pmax from the following equation: Pmx = 1/4 X Eo2 / rin
(1)
The measurements were performed in the temperature range between 300K and 700K. RESULTS Thermoelectric Properties of the Segments Table 1 shows thermoelectric properties of the low-n and high-n segments at room temperature. The low-n segment has higher p and at, and lower K than the high-n one, as a result of even lower n. This tendency was maintained in the temperature range up to 700K. Power generated by a temperature difference of 1K can be expressed by a power factor(cC2/p). Ca2/p for both the segments was evaluated from the measured p and ca in the 112
Table 1
Thermoelectric properties of low-n and high-n segments at room temperature
Segment
Composition
Hole Concentration n(I/m')
Electrical Resistivity p(lutflm)
Low-n High-n
Pbo.75Sno.25Te Pbo.sSno.sTe
2.3 × 1025 1.2 X 10"
7.01 3.71
temperature range from 300K to 700K. The result is shown in Fig.3. ct2/p of the low-n segmen
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