Experimental study of the thermoelectric properties of YbH2

Journal Pre-proof Experimental study of the thermoelectric properties of YbH2 Yunxia Wang, Yuji Ohishi, Ken Kurosaki, Hiroaki Muta PII:

S0925-8388(19)34742-5

DOI:

https://doi.org/10.1016/j.jallcom.2019.153496

Reference:

JALCOM 153496

To appear in:

Journal of Alloys and Compounds

Received Date: 27 June 2019 Revised Date:

20 December 2019

Accepted Date: 20 December 2019

Please cite this article as: Y. Wang, Y. Ohishi, K. Kurosaki, H. Muta, Experimental study of the thermoelectric properties of YbH2, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2019.153496. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

CRediT author statement Yunxia Wang: Data curation, Investigation, Writing- Original draft preparation. Hiroaki Muta: Conceptualization, Supervision, Writing - Review & Editing Ken Kurosaki: Writing - Review & Editing Yuji Ohishi: Writing - Review & Editing

1

Experimental study of the thermoelectric properties of YbH2

2

Yunxia Wang1, Yuji Ohishi1, Ken Kurosaki1,2, and Hiroaki Muta1

3

1

4

0871, Japan

5

2

6

Nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-

Institute for Integrated Radiation and Nuclear Science, Kyoto University, 2, Asashiro-

7 8 9

ABSTRACT In this study, metal hydride YbHx was successfully fabricated by the Sievert’s method.

10

The crystal structure and composition were confirmed by X-ray diffraction pattern analysis

11

and the measured hydrogen pressure change during the hydrogenation process. The

12

Seebeck coefficient was negative and strongly depended on the H/Yb ratio. The thermal

13

conductivity was approximately 6 to 10 W m-1 K-1 at room temperature, which is lower

14

than the calculated thermal conductivity for YbH2 using a modified Slack’s model. This is

15

the first report on the experimental study of hydrides as TE materials.

16 17

Keywords

18

Ytterbium hydride, Thermoelectric properties, Thermal conductivity, Seebeck coefficient

19 20 21

1. Introduction The energy crisis is attracting attention because of population growth and industrial

22

development [1]. Meanwhile, a significant amount of energy is being wasted in the form of

23

heat from industrial processes, automotive exhaust and so on [2,3]. Thermoelectric (TE)

24

technology, an environmentally friendly and sustainable energy conversion technology that

25

can generate electricity from heat, provides a method to solve the energy waste problem [4].

26

The conversion efficiency of TE materials is dependent on the dimensionless figure of

27

merit,

28

the electrical resistivity and

=

/

, where

is the Seebeck coefficient, T is the absolute temperature,

is

is the thermal conductivity. To achieve a high enough ZT

1

29

value, a combined effort of enhancing the material’s Seebeck coefficient and electrical

30

conductivity while reducing its thermal conductivity is needed.

31

Thermoelectrics research has undergone significant development in the past two

32

decades with various strategies to enhance TE parameters and increase ZT values, including

33

engineering band structures to increase power factor [5], nanostructuring [6] and

34

controlling multiscale hierarchical architecture [7] for phonon scattering to reduce thermal

35

conductivity. While at the same time, searching for new potential materials with high

36

power factor/low thermal conductivity is also effective [8]. These cutting-edge strategies

37

have already resulted in a variety of TE materials with record-breaking performance. For

38

example, novel p-type TE materials like PbTe [9], GeTe [10] and SnSe single crystals [11]

39

can display a maximum ZT of 2.6 at 850 K, 2.4 at 600 K, and 2.6 at 923 K, respectively. In

40

addition, the recently reported n-type TE materials are also promising, with n-type SnSe

41

single crystals [12] showing a ZT of 2.8 at 773 K and n-type Mg3+δSbxBi2−x Zintl family

42

possessing an average ZT of 1.05 within the temperature range of 323-523 K [13].

43

However, several goals have not yet been achieved given the present status of TE

44

technology. Especially in practical applications, various challenges still exist, such as low

45

conversion efficiency, low reliability, high cost and negative environmental effects [14–19].

46

Therefore, additional effort needs to be made to explore new TE materials.

47

Metal hydrides with rare-earth metal elements have been widely investigated due to

48

their interesting structures and properties, such as the switchable optical properties of

49

yttrium and lanthanum hydrides [20]. The stability, thermodynamic properties, mechanical

50

properties and semiconducting behavior of these materials have been reported [21–24].

51

Most importantly, the phase diagram of Yb-H shows that the YbH2 phase has a wide

52

compositional range from H/Yb = 1.7 to 2.2, which indicates that the carrier concentration

53

and vacancy at the hydrogen sites can be easily controlled by adjusting the hydrogen

54

concentration. Furthermore, it was found that hydrogenation could lead to volume change

55

and generate dislocations that would contribute to phonon scattering and result in a

56

decrease in thermal conductivity [25]. Therefore, the metal hydride YbH2 was fabricated

57

and studied as a TE material here.

2

58

In the present study, we successfully fabricated ytterbium hydride with different H/Yb

59

ratios using the Sievert’s method and studied the thermoelectric properties of these samples.

60

To the best of our knowledge, this is the first experimental study of the thermoelectric

61

properties of ytterbium hydride. The results of our study are expected to provide a unique

62

reference for future research of metal hydrides in the thermoelectric field.

63 64 65

2. Experimental procedures An ytterbium metal block with 99.9% purity was used as a precursor to fabricate the

67

bulk ytterbium hydride. The block was cut into slices (approximately 1 mm × 10 mm × 13

68

Sievert’s apparatus was used for the hydrogenation process [26]. The apparatus was

69

vacuumed at room temperature for 24 h after inserting the ytterbium metal, and the sample

70

was then annealed at 753 K under vacuum conditions (below 10−6 Pa) for 5 h to remove any

71

residual stresses and/or impurity gases. High-purity hydrogen gas (99.99999% pure) was

72

then introduced to the reaction chamber. The hydrogenation rate was kept very low to

73

prevent the sample from cracking due to expansion during hydrogen absorption. The H/Yb

74

ratio was determined from the changes in the hydrogen gas pressure during the

75

hydrogenation process.

66

76

mm), mechanically polished and degreased in acetone by an ultrasonic cleaner. A modified

To ensure the thermal stability of the samples, property measurements were

77

performed at relatively low temperatures (up to 420K). The samples were first

78

characterized by X-ray diffraction (XRD) analysis using an X-ray diffractometer (Ultima

79

IV, Rigaku Co.) with Cu Kα radiation (2 from 20 to 90 degrees). The lattice parameters of

80

the YbHx phase were calculated from the observed XRD peak positions based on Cohen’s

81

method [27]. Then, a ZEM-3 instrument (ULVAC-RIKO Inc.) was used to obtain the

82

electrical resistivity ( ) and Seebeck coefficient ( ) of the samples simultaneously in a He

83

atmosphere in the temperature range of 300-420 K by a four-point technique and static

84

direct-current method, respectively. The thermal diffusivity

85

flash method (Netzsch LFA-457) from room temperature to 420 K. The thermal

86

conductivity

was determined using

=

, where

was measured by the laser

is the heat capacity and

is the

3

87

density of the sample. The experimental data of

88

calculation of

89

dimensions. Multiple measurements were performed for each sample at each targeted

90

temperature and a deviation of 5%, 7% and 15% was observed for ρ, S, and κ, respectively,

91

resulting in an approximately maximum 30% deviation for ZT.

92 93

from reference were used for the

[28], and the density of the sample was calculated from its weight and

For comparison, the lattice thermal conductivity of YbH2 was estimated using a modified Slack’s model [28–32]: =

94

where

is the averaged crystal constant,

95

Grüneisen parameter, and

96

the following equations:

/

3

/

(1)

is the isothermal bulk modulus,

is the density of the unit cell. The value of

=9

=

!" #$ %

3

Θ'

( )

( )

01 /2

3

ℎ 3 !" 5 Θ= % (9 #$ 47 8

is the

is calculated from

(2) *+, (, - − 1) /

1 2 1 ? :; = 5 % + (9 3 : :>

*

(3)

:;

(4)

/

(5)

4 : = @% + A( / 3 :> = BA/

( ) = (1 +

(7) )

3

is the heat capacity;

(8) 3

and ( ) are the

97

where

98

and :; is the longitudinal, transverse, and average sound velocities, respectively; !" is

99

is linear thermal expansion coefficient;

(6)

molar volume at temperature 0 K and T, respectively; C' is the Debye temperature; : , :>

4

100 101 102

Avogadro’s number;

is the number of atoms in a molecule of YbH2; 8 is the molecular

weight; and A is the shear modulus. The values of

and A were calculated by the

Cambridge Sequential Total Energy Package (CASTEP) code.

103 104 105

3. Results and discussion

Ytterbium hydride (YbH- , * = 1.85 and 1.88) with orthorhombic structure was

106

fabricated successfully. The X-ray diffraction patterns are shown in Fig. 1 and the peaks of

107

both samples matched well with the peaks of the ytterbium hydride reference [33]. Some

108

peaks from Yb2O3 were also present because the surface of bulk sample might be oxidized

109

[33]. As shown in Table I, the lattice constants of the present samples agreed well with the

110

experimental and theoretical results ever reported [23, 33]. The lattice parameters decreased

111

slightly with increasing H content in the hydride, which is similar to the phenomenon

112

reported in a study on yttrium hydrides [34].

Intensity (a.u.)

YbH1.85

YbH1.88

YbH2: 00-009-0256

Yb2O3: 01-077-0456 20

30

113

40

50 60 2θ (degrees)

70

80

90

114

Fig. 1. The XRD patterns of Yb1.85, YbH1.88, YbH2 [33] and Yb2O3 [35].

115

Table 1. The lattice parameters of the Yb1.85, YbH1.88 sample and the reference data. Species

a

Lattice parameters (Å) b c

5

YbH1.85 YbH1.88 Other experimental data of YbH2 [33] Theoretical data of YbH2 [23] 116

5.9044 5.8935 5.8710 5.844

3.5906 3.5714 3.5610 3.546

6.7775 6.7794 6.7630 6.707

The temperature dependence of the electrical resistivity of the ytterbium hydride

117

samples is shown in Fig. 2. We can see that the resistivity of the YbH1.85 and YbH1.88

118

samples showed metallic behavior, slightly increasing with temperature, and also depended

120

on the H/Yb ratio. The resistivity of YbH1.85 was in the range of 0.19-0.21 Ω m, which is

121

semiconductor [22] and carriers in the samples were generated by the deviation from the

122

composition. Hydrogen behave as anions in the ytterbium hydride and thus hydrogen

123

vacancies generate conductive electrons. Therefore, the YbH1.85 sample showed lower

124

resistivity than that of YbH1.88 due to its higher carrier density.

119

lower than the values of 0.25-0.27 Ω m obtained for YbH1.88. As reported, YbH2 is a

0.40 YbH1.85

Resistivity ( Ω m)

0.35

YbH1.88

0.30 0.25 0.20 0.15 0.10 300

125 126 127 128 129

320

340

360 380 Temperature (K)

400

420

Fig. 2. The resistivity of YbH1.85 and YbH1.88 as a function of temperature. Fig. 3 shows the Seebeck coefficients of YbH1.85 and YbH1.88 as functions of temperature. The Seebeck coefficient was negative over the entire temperature range which confirmed that the hydrogen vacancies generated carrier electrons. The | | showed a linear

6

130

increasing trend with increasing temperature, which can be explained by the following

131

equation: =

87 #$ ∗ 7 OP R S 3,ħ 3

/

(9)

is the carrier concentration, OP∗ is the density of states effective mass, , is the

132

where

133

charge of an electron,

134

Planck constant. The YbH1.85 sample had a Seebeck coefficient of approximately -14 to -17

135

is the temperature, #$ is the Boltzmann constant and ħ is the

136

TV/K in the temperature range of 330-420 K. YbH1.88 had a Seebeck coefficient of

137

approximately -54 to -62 TV/K, and the absolute value was much larger than that of

YbH1.85, probably due to its higher carrier concentration which can be confirmed from the

138

electrical resistivity results. -10

Seebeck coefficient (µV/K)

YbH1.85 YbH1.88

-20

-60 -70 -80 300

320

139

340

360 380 Temperature (K)

400

420

140

Fig. 3. Seebeck coefficient for YbH1.85 and YbH1.88 as a function of temperature.

141

The calculated power factors

142 143

/ were shown in Fig. 4, showed that YbH1.88 had a

higher power factor than that of YbH1.85 due to its higher absolute Seebeck coefficient

value. In this study, the highest power factor of YbH1.88 was approximately 14.2 × 10-9 7

144

W/cm K2. This value is quite low compared with those of normal TE materials and further

145

optimizations on carrier concentrations are needed.

Power factor (10−9 W/cm K2)

25 YbH1.85 YbH1.88

20

15

10

5

0 300 146

320

340

360 380 Temperature (K)

400

420

147

Fig. 4. The power factor of YbH1.85 and YbH1.88 as a function of temperature.

148

We also obtained the total thermal conductivity, as shown in Fig. 5. The thermal

149

conductivity decreased with increasing temperature. For the YbH1.85 sample, the value

150

decreased from 10.0 W/m K to 6.9 W/m K when the temperature increased from 298 K to

151

413 K. For the YbH1.88 sample, in the same temperature range from 298 K to 413 K, the

152

value of

153

YbH1.88 might be caused by the increase in the density of dislocations in the sample when

154

an increased amount of H2 was absorbed during hydrogenation. Fig. 5 also shows the

155

theoretical results of lattice thermal conductivity calculated by the modified Slack’s model.

156

The calculated lattice thermal conductivity shows a similar temperature dependence, but the

157

value was higher than our experimental data. The contribution from electronic thermal

158

conductivity can be neglected in the experimental data, so this reduction may be attributed

changed from 6.2 W/m K to 3.9 W/m K. The lower thermal conductivity of

8

159

to the hydrogen vacancies and/or generated dislocations during the hydrogenation process,

160

as reported in the study in reference [25].

161

The thermal conductivity of the ytterbium hydride in the present work was higher

162

than those of the state-of-the-art TE materials, such as bismuth telluride and lead telluride.

163

However, the value was comparable with those of other nontoxic TE materials, such as

164

half-Heusler alloys, several silicides, and oxide materials. Additional reduction in the

165

thermal conductivity of these hydrides may be realized through optimizations on the

166

hydrogenation conditions. 18 Thermal conductivity (W/m K)

YbH1.88 15

YbH1.85 YbH2 (from calculation)

12 9 6 3 0 280

300

320

167

340

360 380 Temperature (K)

400

420

440

168

Fig. 5. Thermal conductivity of YbH1.85, YbH1.88 and the calculated lattice thermal

169

conductivity as a function of temperature.

170

Finally, the dimensionless figure of merit

as a function of the temperature was

171

also determined, as shown in Fig. 6. The resulting

value of YbH1.88 was higher than that

172

of YbH1.85. YbH1.88 had a maximum

173

values are thought to be caused by the samples’ high resistivity and slightly elevated

174

thermal conductivity. However, it was confirmed that both the electrical properties and

of 18 × 10-7 at a temperature of 416 K. The low

9

175

thermal conductivity can be easily controlled by adjusting the hydrogen amount, which can

176

improve the ZT value. 30 25 YbH1.85

20

YbH1.88

ZT (×10-7)

15 10 5 3 2 1 0 300

177 178 179 180

320

340

360 380 Temperature (K)

400

420

Fig. 6. ZT values of YbH1.85 and YbH1.88 as a function of temperature. 4. Conclusions YbHx (x=1.85 and 1.88) hydride materials were fabricated successfully by the

181

hydrogen gas absorption method using a handmade Sieverts apparatus. The crystal structure

182

was confirmed by XRD pattern analysis and their thermoelectric properties were

183

investigated for the first time. The hydrogen vacancies generated carrier electrons in the

184

samples, which decreased the electrical resistivity and the absolute Seebeck coefficient.

185

The thermal conductivity was also estimated using the Slack’s model. The experimental

186

thermal conductivities were lower than those obtained from the calculation. The ZT values

187

were not as high as those of some of the current TE materials. However, the hydride

188

showed a relatively high Seebeck coefficient and comparable thermal conductivity, and

189

both properties were controllable by adjusting the hydrogenation conditions. Hence, this

190

study indicates that metal hydrides have the possibility to be a new category of nontoxic TE

10

191

material and that it might be worthwhile to explore TE performance enhancement

192

techniques in future studies.

193 194

Acknowledgments The author Yunxia Wang would like to express her gratitude towards China

195 196

Scholarship Council (Grant No. 201604910683) and the Suganuma laboratory members,

197

especially Associate professor Jinting Jiu for private communications.

198 199

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Highlights • • • •

The hydride material YbHx (x=1.85 and 1.88) was fabricated successfully using a hand-made Sieverts apparatus. The thermoelectric properties of YbHx were investigated for the first time. The hydride YbHx showed relatively high Seebeck coefficient and comparable thermal conductivity. This study indicates that the ytterbium hydride has potential to be a new non-toxic TE material.