1 Enhanced Thermoelectric Performance of Nanostructured Nickel -doped Ag 2Te

2025-04-30 0 0 984.29KB 21 页 10玖币

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Enhanced Thermoelectric Performance of

Nanostructured Nickel-doped Ag2Te

Vikash Sharma1,†,*, Divya Verma2,3, Ranu Bhatt4, Pankaj K. Patro5, Gunadhor

Singh Okram1,*

1UGC-DAE Consortium for Scientific Research, Indore, Madhya Pradesh-452001, India

2Government College Alote, District Ratlam, Madhya Pradesh-457114, India

3Department of chemistry, Vikram University, Ujjain, Madhya Pradesh-456010, India

4Technical Physics Division, Bhabha Atomic Research Centre, Mumbai-⁠⁠400085, India

5Powder Metallurgy Division, Bhabha Atomic Research Centre, Vashi Complex, Mumbai-

400703, India

*vikash.sharma@tifr.res.in, okram@csr.res.in

ABSTRACT We report on the thermoelectric properties of nickel-doped Ag2-xNixTe (x = 0,

0.015, 0.025 & 0.055, 0.115, 0.155) nanostructures in the temperature (T) range of 5 K to 575

K. The electrical resistivity (ρ) of Ag2Te nanostructure shows metallic behaviour in 5 K to 300

K initially that evolves into two metal to insulator transitions (MITs) at low and mid-

temperature regimes with increasing x due to Mott-variable range hopping (VRH) and

Arrhenius transports, respectively. Their Seebeck coefficient varies nearly in a linear fashion in

this temperature range, showing metallic or doped-degenerate semiconducting behaviour.

Notably, this behaviour of Seebeck coefficient () is in contrast to Mott-VRH conduction

(

) as observed in ρ. The steady increase in ρ and S with the sharp decrease in thermal

conductivity between 410 K to 425 K associated with the structural phase transition

accomplishes a maximum thermoelectric figure of merit (ZT) of 0.86±0.1 near 480 K in x =

0.155. This is ~ 83 % more compared to that of bulk Ag2Te, and shows a significant

improvement over the best value reported for Ag2Te nanostructures thus far. This study,

therefore, shows that simultaneous nanocomposite formation, doping and nanostructuring could

be an effective strategy for tuning the electron and phonon transports to improve the

thermoelectric properties of a material.

KEYWORDS: silver telluride, nanostructures, electrical conductivity, Seebeck coefficient,

thermal conductivity, figure of merit

INTRODUCTION

Thermoelectric (TE) materials can be used to generate electricity directly from temperature

gradient in a reliable way that has the potential for recovery of waste heat as electricity as well

as to overcome the current global energy crisis and future energy problems1,2,3,4. The

conversion efficiency of a TE material or device can be examined using dimensionless figure

of merit, 

 where  is electrical conductivity, and κ is the total thermal conductivity

contributions from its electronic ( and lattice  parts and T is the absolute temperature5.

The possible enhancement of ZT through the large electrical conductivity and Seebeck

coefficient with the minimum thermal conductivity is inherently in conﬂict in the usual

semiconductors or metals. This is possible fortunately in nanostructures (NSs) by

independently tuning the power factor (PF) S2σ and  as evident from, say, the size and

morphology-controlled complex materials6, nanocomposites7,8, hybrid materials with

inorganic/organic interfaces9,10, introduction of nanoparticles (NPs) in host matrix11, alloying

and doping12.

Silver telluride (Ag2Te) is an interesting and attractive nonmagnetic topological insulator at

ambient conditions13,14 with many intriguing properties such as the structural phase transition

from the low-temperature monoclinic phase β-Ag2Te to high temperature face-centred cubic

(fcc) phase α-Ag2Te near 417 K15,16, pressure-induced charge density wave (CDW) phase17,

structural17,18 and electronic topological19 phase transitions. While α-Ag2Te phase is a

superionic conductor, its β-phase is a narrow-band gap (~ 0.05 eV) semiconductor with high

carrier mobility and low  due to the Ag-atoms-induced disordered structure in the Ag2Te

lattice20 making it a suitable TE material16,15,21. Although its low effective mass (~ 10-2 of free

electron mass)16 favours the small S, high mobility makes both σ and  . As a result, ZT

of ⁓ 0.27 at 370 K22 and ⁓ 0.29 at 550 K23 for bulk Ag2Te along with several attempts to

improve upon its TE properties via nanostructuring, alloying and or doping20,23,24,25,26,27,28,29

have been reported so far. Cadavid et al.29 showed a peak ZT of ~ 0.66 at 450 K in surface-

functionalized Ag2Te NPs. Zhou et al.23 reported ZT of ⁓ 0.62 and ⁓ 0.47 at 550 K in sulphur-

doped 15 nm NP sample and sulphur-doped bulk Ag2Te, respectively. Yang et al. observed a

peak ZT of ⁓ 0.55 at 400 K in Ag2Te nanowires20. A highest ZT of ⁓ 0.68 at 573 K in Ag2Te/Ag

nanocomposite30 and ZT of ⁓ 0.067 and ⁓ 0.32 at 300 K in uncapped and 1,2-ethanedithiol-

capped Ag2Te nanocrystals, respectively have also been reported25. The maximum PF of ~1370

µWm-1K-2 at 425 K in Ag1.99TeSe0.01/Ag nanocomposites24, ~ 315 µWm-1K-2 at 410 K in Ag2Te

nanowire films27, and ~ 3.94 µWm-1K-2 at 300 K in Ag2Te/Ag nanofibers28 have been reported.

Although these reports focus only on TE properties for the temperature over 300 K, whereas

low temperature TE properties of this compound are not explored so far.

Here, we demonstrate the TE properties of Ag2-xNixTe (x = 0, 0.015, 0.025, 0.055,

0.115, 0.155) NSs in the temperature range of 5 K to 575 K. The metallic behaviour of undoped

nanostructure over T range of 5 K to 300 K, like bulk Ag2Te17, turns to two clear metal to

insulator transitions (MITs) for x = 0.115 and 0.155. Their structural phase transition from β-

Ag2Te to α-Ag2Te near 420 K is clearly evident in all ρ, S and. Even though the peak PF of

~ 1254 µWm-1K-2 at 580 K for x = 0 is larger than that of ~ 1175 µWm-1K-2 at 420 K for x =

0.155, the latter shows the maximum ZT of ~ 0.86 compared to ~ 0.43 at 580 K of the former;

the ZT of ~ 0.86 is associated with an ultralow value of κl ~ 0.15 Wm-1K-1 at 482 K. This ZT

value is significantly larger compared to achieved value in undoped bulk and nanostructured

Ag2Te20,23,25,29,30. This is attributed to the point defects, structural distortion, and anharmonicity

prevailed as x increases, leading to the substantial optimization of the electron and phonon

transports.

EXPERIMENTAL SECTION

Silver nitrate AgNO3 (≥ 99.0%), nickel chloride NiCl2 (98%), potassium tellurite

monohydrate K2O3Te.H2O (≥ 90%), and diethylene glycol (DEG, 99%) were used as received

without further purifications. Typically, 2:1.1 of AgNO3 and K2O3Te.H2O was mixed in 30 ml

of DEG in a three neck round bottom flask. This mixture was heated with rate of ⁓ 10 oC/min

up to ~235 oC. The reaction temperature of ~235 oC was maintained for 3 hrs, and product was

cooled down to room temperature. Ethanol was added in the reaction product and centrifuged

it at 12,000 rpm for 10 minutes. The resultant supernatant was discarded after retaining the

sample. This washing step was repeated for three times and product was then vacuum-dried.

The obtained powder sample was used for further characterizations. This sample gives x = 0.

To prepare the Ni-doped nanostructured Ag2Te, desired amount of NiCl2 was replaced for

AgNO3 and followed similar preparation procedure for the nanostructured Ag2-xNixTe (x =

0.015, 0.025, 0.055, 0.115, 0.155) samples; values of x shall denote samples in the text. Further,

doping of Ni with x = 0.205 led to the formation of the Ag2Te/Ni1.4Te nanocomposite.

Bruker D8 Advance X-ray diffractometer, TECHNAI-20-G2 (200 KV), FEI Nova

nanosem450, X-ray photoelectron spectroscope (SPECS, Germany) with Al Kα radiation and

SEM equipped with EDX JEOL JSM 5600 were used for x-ray diffraction (XRD), transmission

electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray

photoelectron spectroscopy (XPS) and energy dispersive analysis of x-ray (EDX), respectively.

A commercial Labram-HR800 micro-Raman spectrometer and vibrating sample magnetometer

(VSM) of Quantum Design were used for Raman and magnetization measurements,

respectively. Four probe and differential direct-current methods were used to measure the

resistance and Seebeck coefficient over the T range of 5–320 K in a home-made setup31. We

have also measured TE properties in the T range 300 K to 580 K. Linseis make LSR-3 measured

Seebeck coefficient and electrical resistivity (four-probe method) simultaneously on the cold-

pressed sintered (at 500 oC) rectangular pellets with dimension 5 mm (l) × 3 mm (b) × 2 mm

(t). Dynamic laser flash technique using Linseis make LFA-1000 measured the thermal

diffusivity (D) of the disc-shaped samples with a diameter of ~ 10 mm and thickness of ~ 2

mm to determine their thermal conductivities using the formula  = DCpd, where Cp is specific

heat and d is mass density of the sample; the cold-compressed pellets were sealed in a quartz

tube under a vacuum of ⁓ 10-6 torr, and annealed it at 500 oC for 24 hrs in a conventional

furnace. Typical uncertainty in the measurements of S, ρ and  were less than 3, 4 and 10%,

respectively. The mass densities of these samples were greater than 82 % of the bulk value (8.2

g/cm3) as listed in table S1.

RESULTS AND DISCUSSION

The XRD pattern of x = 0 confirms its monoclinic crystal structure with space group P21/c of

β–Ag2Te (figure 1a). The samples Ag2-xNixTe with x = 0.015, 0.025, 0.055, 0.115 and 0.155

retain the same β–Ag2Te phase with a small fraction (˂ 3 %) of elemental Ag metal (figure

S1b-f) that manifests the successful substitution of Ni up to about 15.5 % in the host Ag2Te

lattice (figure 1a). Therefore, these Ni-doped Ag2-xNixTe samples are effectively

nanocomposites of Ag (<3%) and Ag2Te (>97%). Their Rietveld refinements are shown in

figure S1. XRD of x = 0.205 (figure S2) shows additional peaks corresponding to Ni1.4Te

(P4/nmm, S.G. # 129) along with main phase of Ag2Te. This suggests that further doping of

Ni cannot be possible beyond x = 0.155, and forms the Ag2Te/Ni1.4Te nanocomposite. The

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1EnhancedThermoelectricPerformanceofNanostructuredNickel-dopedAg2TeVikashSharma1,†,*,DivyaVerma2,3,RanuBhatt4,PankajK.Patro5,GunadhorSinghOkram1,*1UGC-DAEConsortiumforScientificResearch,Indore,MadhyaPradesh-452001,India2GovernmentCollegeAlote,DistrictRatlam,MadhyaPradesh-457114,India3Departmentofche...

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1 Enhanced Thermoelectric Performance of Nanostructured Nickel -doped Ag 2Te

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