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Chapter 4 Results and Discussions

4.2 Si-doped Fe 2 VGa

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4.2 Si-doped Fe

2

VGa

4.2.1 Crystal Structure Analysis

All polycrystalline samples presented here are prepared by arc-melting method. The constituent elements including Fe (99.9%, Alfa Aesar), V (99.7%, Alfa Aesar), Ga (99.9999%, Alfa Aesar), and Si (99.999%, CERAC) are placing on a water cooled copper plate in a high purity argon atmosphere. Each sample is re-melted 10 times in order to create the homogeneity, but the losing weight of each sample upon melting process is around 3%. After arc-melting process, each sample is post-annealed in the vacuum-sealed (10-6 torr) quartz tube at 1000 0C for 3 days followed by the furnace cooling.

Fig. 4.18 X-ray diffraction patterns of Fe2VGa1-xSix samples

30 40 50 60 70 80

(422)

(400)

(220)

2

Int ensi ty(a. u.)

(111)

x = 0 x = 0.05 x = 0.10 x = 0.15 x = 0.20

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In figure 4.18, the X-ray diffraction patterns of Fe2VGa1-xSix alloys are presented, where x are 0, 0.05, 0.1, 0.15, and 0.2 respectively. All Si-substituted samples exhibit the full-Heusler type L21 crystal structure, which dominant constructive diffracting peak is located in (2, 2, 0) phase, and there is no second phase existed in all compounds. For further understanding the successful substitution of the silicon for the gallium site, the enlargement of the (2, 2, 0) phase is showed in the figure 4.19. There is a trend of the peaks: when the silicon substituting level is increasing, the constructive diffracting peak is shifting toward larger angle. This can be explained by Bragg’s law, nλ=2dsinθ, which implies that the constructive diffracting angle is increasing as the lattice spacing is lowering. According to the fact that the atomic radius of the gallium atom (136 pm) is larger than the silicon atom (111 pm), the results is expected.

Fig. 4.19 Enlargement of the XRD pattern at (2, 2, 0) direction

44.0 44.2 44.4 44.6 44.8 45.0

(220)

2

Int ensity (a. u.)

x = 0 x = 0.05 x = 0.10 x = 0.15 x = 0.20

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Furthermore, the lattice parameter of Fe2VGa1-xSix compounds are calculated by the Rietveld refinement method under the software Highscore Plus and shown in figure 4.20. The lattice parameter of un-doped one, Fe2VGa, is around 5.7788 Å , and the lattice parameter is linearly decreasing while the doping content is increasing.

Therefore, from the above evidences, the series of Si-substituted is successful.

0.05 0.10 0.15 0.20

5.755 5.760 5.765 5.770 5.775 5.780 5.785

Lattice Parameter (A)

x

Fe2VGa

1-xSi

x

Fig. 4.20 Lattice parameter of Fe2VGa1-xSix samples

In this case, we substitute the gallium atom on the silicon atom which has one more valence electron compared with the gallium atom in the outer shell. The electrical resistivity of Fe2VGa1-xSix samples are shown in figure 4.21, and all samples are located in the range between 1.40 μΩ m and 2.4 μΩ m from 300 K to 700 K. The resistivity of the un-doped sample is around 1.4 μΩ m at room temperature, and it process the semiconductor-like behavior in the temperature range 300 ~ 400 K.

However, the electrical resistivity starts to act metallic behavior when the temperature goes high. This is because electrons have more kinetic energy when the temperature is high and this increases probability of the collision between electrons and atoms. The behavior of Fe2VGa0.95Si0.05 is much similar with the un-doped sample, but the resistivity at the room temperature is 15% higher than the un-doped one. For x = 0.1, 0.15, and 0.2 samples, they have the same electrical behavior: they all have a broad maximum in the temperature range 700 ~ 800K. We also found that the electrical resistivity is increasing while silicon atoms are substituting gallium sites, which is against our purpose for a good thermoelectric material.

Fig. 4.21 Electrical resistivity of Fe2VGa1-xSix samples

300 400 500 600 700 800

1.4

proportional to the slope of density of state at Fermi level, which is given by

S= -

p

2k2B substitution, but the results showed the maximum value of Seebeck coefficient.

Therefore, we try to replace gallium atoms by silicon atoms to perform the electron-substitution on Fe2VGa system.

Fig. 4.22 Seebeck coefficient of Fe2VGa1-xSix samples

300 400 500 600 700 800

-60

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Seebeck coefficients of Fe2VGa1-xSix samples are showed in figure 4.22, and a p-n transition is observed in this series. As we expected, the silicon atom has one more out-shell electron than the gallium atom, so the major carrier will become electron-type. Seebeck coefficient of the un-doped sample, Fe2VGa, at the room temperature is 22 μV K-1, and it exhibits a maximum value 24 μV K-1 at the 380 K, and then turns to a negative slope at the high temperature. The largest value of Seebeck coefficient is about -58 μV K-1 and is found in Fe2VGa0.85Si0.15 sample at room temperature.

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4.2.4 Thermal Conductivity

The thermal conductivity of Fe2VGa1-xSix samples is measured in the temperature range from 300 K to 820 K and is depicted in figure 4.23. The thermal conductivity of the un-doped sample is around 24.3 W m-1 K-1 at 300 K, and tends to decrease while the temperature increases. The behaviors of all samples are same as the un-doped sample. On the other hand, we found that the substitution of Si for Ga site results in the reduction of the thermal conductivity by 50% in x = 0.2 sample at room temperature.

Fig. 4.23 Thermal conductivity of Fe2VGa1-xSix samples

300 400 500 600 700 800

10 12 14 16 18 20 22

24 x=0

x=0.05 x=0.10 x=0.15 x=0.2

Temperature (K) Thermal Conductivity (W m-1 K-1 )

into two parts, the lattice thermal conductivity and the electronic thermal conductivity.

Figure 4.24 shows the comparisons between the lattice and electronic thermal conductivity of all Si-substituted Fe2VGa compounds.

Fig. 4.24 Comparisons between κ, κph, and κe of all Si-substituted samples

300 400 500 600 700 800

4 Thermal Conductivity, (W m-1 K-1 )

ph e

300 400 500 600 700 800

5 10 15

20 x=0.05

Temperature (K) Thermal Conductivity, (W m-1 K-1 )

ph e

300 400 500 600 700 800

2 Thermal Conductivity, (W m-1 K-1 )

ph e

300 400 500 600 700

5

Thermal Conductivity, (W m-1 K-1 )

Temperature (K)

x=0

300 400 500 600 700 800

2

Thermal Conductivity, (W m-1 K-1 )

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From the figures above, the lattice thermal conductivity of Fe2VGa is almost 4 times larger than the electronic part at room temperature. However, it is prone to decline with the temperature increasing and the electronic part starts to take control of the thermal conductivity when the temperature is above 631 K. We observed that when the substituting level increases, the temperature which the electronic part is dominated has a trend except x = 0.15 sample and we also plot this temperature versus composition of all samples in the figure 4.25. We also observed a strange electrical transportation on x = 0.15 sample, and we think the reason is same as the Ti-substituted series. The intersection of the DOS and Fermi level would result in the change of the electrical topological configuration, and further influence the electrical transportation.

Fig. 4.25 Temperature verse Si substituting level

0 0.05 0.1 0.15 0.2

450 500 550 600 650

Temperatur e (K)

x

Fe

2

VGa

1-x

Si

x

The temperature dependent power factor of Si-substituted Fe2VGa samples is depicted in figure 4.26. The maximum value of the power factor for the un-doped sample is only 0.4 mW m-1 K-2 at 340 K. However, when silicon atoms are going into this system, the power factor will decrease first, and then dramatically increase as the doping level increases. The reason why Fe2VGa0.95Si0.05 shows a smaller power factor is probably because the major carrier changes from hole-type to electron-type, and the power factor is proportional to the square of Seebeck coefficient. Therefore, the power factor would exhibit a small value during this transformation. The largest power factor is 2.1 mW m-1 K-2 and is found in Fe2VGa0.8Si0.2 at room temperature.

Compared with the Ti-substituted samples, although the power factor is smaller than Ti-substituted samples, the largest power factor is happened in Fe2VGa0.8Si0.2, and it seems to increase with the substituting level increased. Therefore, we will try to substitute more silicon atoms for further research.

Fig. 4.26 Power factor versus temperature of Fe2VGa1-xSix samples

300 400 500 600 700 800

0.0

Power Factor (mW m-1 K-2 )

Temperature (K)

4.2.6 Figure of Merit (zT)

We use the results of the power factor as well as the thermal conductivity to determine the figure of merit from room temperature to 820 K and plot it in the figure 4.23. The un-doped sample exhibits low value of zT because of the combination of the small power factor and a huge thermal conductivity. The largest value of zT happens in Fe2VGa0.8Si0.2 sample at room temperature and is about 0.052. The figure of merit is proportional to the ratio of the power factor over the thermal conductivity, and the power factor seems to increase as the doping level is increased, and, simultaneously, the thermal conductivity seems to decrease. Therefore, we will try to substitute more silicon atoms for further research.

Fig. 4.27 Figure of merit (zT) versus temperature of Fe2VGa1-xSix samples

300 400 500 600 700 800

0.00

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