ताप-वैद्युतिकी - बल्क , नैनो तथा तनु फिल्में

This year research work was continued on the CSIR Network Project - Technologies and Products for Solar energy utilization through Networks (TAP-SUN) on “Development of novel thermoelectric materials and devices for harnessing solar energy and waste heat” (Network Project – NWP 54). The research was focused on the development of novel thermoelectric materials with enhanced figure-of-merit and was mainly focused on the design, synthesis, characterization and thermoelectric property evaluation of several novel nanostructured thermoelectric materials, such as, Mg3Sb2, Mg2Si-based compounds and other novel thermoelectric materials. The focus of research was to develop compatible n & p-type thermoelectric elements for integration as a thermoelectric device. As a part of XIIth Five Year Plan Network Project (PSC0109), which was initiated this year, research work was carried out on the development of rare-earth free permanent magnet materials. The work was concentrated on MnAl alloys and heat treatment parameters were optimized to obtain good permanent magnet properties.

Apart from two Network projects, an Indo-Hungarian International project (GAP123632) on “structural nanocomposites” was also sanctioned by DST. The research this year resulted in ten publications, mostly in high impact SCI journals and two international patents.

Development of novel thermoelectric material Bi doped Mg3Sb2 with a figure-of-merit ~ 0.6 at 750 K as an inexpensive thermoelectric material for power generation

A proof of principal has been established experimentally for a Zintl compound of Mg3Sb2 and its derivative of isoelectronically Bi doped Bi; Mg3Sb2-xBix (0 ≤ x ≤ 0.4) alloys in Mg3Sb2. Single phase p-type Mg3Sb2 compounds, with Mg and Sb powders as starting materials, have been prepared directly by spark plasma sintering (SPS) in a one step process. The structural refinements of this hexagonal Zintl compound by X-ray diffraction analysis (XRD) and high resolution transmission electron microscopy (HRTEM) investigation reveal that they are single phase devoid of any oxides or Sb precipitates. Transport measurements indicate low thermoelectric figure of merit (ZT = 0.26 at 750 K) for Mg3Sb2. However, an optimum doping of 0.2 at% with iso-electronic Bi ions at the Sb site enhances the ZT to 0.6 at 750 K, which is comparable with the present day industrial materials such as Bi based tellurides and selenides which are in this class of materials with appropriate doping. The substantial increase in ZT in Mg3Sb2-xBix (0 ≤ x ≤ 0.4) owes to a partial decoupling of the electronic and phonon subsystem, as expected for a Zintl phase compound. While the reduction in thermal conductivity in Mg3Sb2-xBix (0 ≤ x ≤ 0.4) accounts to mass fluctuations and grain boundary scattering, the enhancement in the electronic power-factor is attributed to the presence of heavy and light bands in its valence band structure. The latter has been confirmed by means of both X-ray photo electron spectroscopy studies and first-principles density functional based calculations. These measurements established that a high figure of merit can be achieved ingredients combined with its one step synthesis leads to a cost effective production and less toxicity makes the material an environmentally benign system for thermoelectric power generation.

X-ray diffraction patters of Mg3Sb2-xBix ; 0 ≤ x ≤ 0.4 alloys (figure 1) exhibit a single phase of Mg3Sb2 and single solid solution phase in Bi doped Mg3Sb2 . Figure 2 (a) which shows the bright field electron micrograph recorded from the specimen of Mg3Sb2 exhibits highly densified grains and sharp grain boundaries. SAED in the inset of figure 2 (a) clearly reveals of β-Mg3Sb2-type hexagonal structure. The atomic scale images from the same specimen exhibits the presence of different orientations of the crystallographic planes and their interface boundaries (figure 2(b)). Figure 2 (c) exhibits the bright field electron micrograph recorded from the specimen of Mg3Sb1.8Bi0.2 showing densely packed grains and the atomic scale image in the inset of this figure exhibits high crystallinity of the material. The EDS-TEM patterns recorded from the specimen of Mg3Sb1.8Bi0.2 (figure 2(d)) shows all the elements; Mg, Sb and Bi throughout in the microstructure.

An enhanced thermoelectric figure-of-merit (ZT) around 0.60 at 750 K is realized for the composition Mg3Sb1.8Bi0.2 (figure 3). The enhancement in ZT in this Zintl compound is due to an increase in power-factor with a simultaneous reduction in the thermal conductivity. The increase in the power-factor is attributed to the presence of light and heavy holes in the valence band as revealed by the theoretical band structure calculations. The computed valence band spectrum of Mg3Sb2 is in good agreement with the X-ray photoemission spectroscopy (XPS) data. Furthermore, angle dependence XPS was used to ensure that the surface composition of the alloy is same as in the bulk. On the other hand, the reduction in the thermal conductivity with Bi doping is attributed to mass fluctuation and grain boundary scattering. The measured thermal and electrical transport in these alloys can be correlated with the observed microstructures, as revealed by the transmission microscopy investigations. The electronic structure of Mg3Sb2 as revealed by X-ray photo-emission spectroscopy and DFT based calculations render consistent features of the valence band spectrum.

Figure 1: X-ray diffraction pattern of Mg3Sb2-xBix ; 0 ≤ x ≤ 0.4 alloys showing single phase of Mg3Sb2 and single solid solution phase in Bi doped Mg3Sb2 .

Figure 2 a) Bright field electron micrograph recorded from the specimen of Mg3Sb2 showing highly densified grains and sharp grain boundaries. SAED in the inset clearly reveals of β-Mg3Sb2-type hexagonal structure b) atomic scale images from the same specimen exhibits the presence of different orientations of the crystallographic planes and their interface boundaries c) Bright field electron micrograph recorded from specimen of Mg3Sb1.8Bi0.2 shows densely packed grains. The atomic scale image in the inset exhibits high crystallinity of the material d) EDS-TEM patterns recorded from the specimen of Mg3Sb1.8Bi0.2 shows all the elements; Mg, Sb and Bi throughout in the microstructure.

Figure 3 : Temperature dependence of the electronic properties of Mg3Sb2-xBx (0 ≤ x ≤ 0.4) -electrical conductivity σ(T), Seebeck coefficient S(T), power factor (σ2S (T)), total κ (T) & lattice thermal conductivity κL (T)

Synthesis of Rare Earth free Permanent Magnetic Material

The main objective of this project is to synthesize Rare-Earth Free Permanent Magnetic Material Mn-Al, Mn-Bi employing high energy ball milling, arc melting, conventional melting and spark plasma sintering techniques.

High energy ball milling and arc melting were used separately as well as in combination as a primary processing technique for synthesizing MnAl as a permanent magnetic material. Followed by primary processing high temperature (1150oC) solutionizing was done to obtain ε-phase (hcp structure) followed by water quenching. The ε phase transform to magnetic metastable τ phase by isothermal annealing at temperatures between 400 and 650°C in combination with varying annealing time. To achieve this transformation, annealing of water quenched sample was done for varying length of time ranging from 10 min. to 70 min. Annealing temperature was kept constant at 500oC for all the three conditions to study the dependence of annealing time on the final microstructure and to correlate it with the synthesis route. The samples were characterized by X-ray diffractometer (Fig.7) and vibrating sample magnetometer.

Ball milled sample required maximum time of annealing as compared to arc melted sample or sample prepared using combination of ball milling and arc melting. Ball milled sample shows higher percentage of γ2- and β-phases as indicated by their XRD pattern shown in Fig 8. However for the sample prepared employing ball milling and arc melting the hexagonal ε phase was completely transformed to metastable τ-phase although some amount of γ2- and β-phases was found to be still present.

Fig. 8 shows the magnetic hysteresis curves of τ-Mn-Al alloys for samples obtained from 3 different processing routes. The saturation magnetization, and coercivity values of all the samples obtained via different processing routes are given in Table I.

Work is presently underway to improve the magnetic properties of MnAl permanent magnetic material by employing different primary processing techniques optimizing their process parameter.

Fig. 7: XRD patterns of Mn54Al46 alloys after annealing of quenched sample. (a) Ball milled sample annealed for 50min. (b) Arc melted sample annealed for 40min. (c) Combination of High Energy Ball Milled and Arc Melted sample annealed for 30min.

Fig. 8: Hysteresis curves of Mn–Al followed by annealing at 500°C (a) Ball milled sample annealed for 50min. (b) Arc melted sample annealed for 40min. (c) Combination of High Energy Ball Milled and Arc Melted sample annealed for 30min.

Table I: Magnetic properties of Mn–Al followed by annealing at 500°C (a) Ball milled sample for 50 min. (b) Arc melted sample annealed for 40min. (c) Combination of High Energy Ball Milled and Arc Melted sample annealed for 30min.

Synthesis Route

Hc (kOe)

Ms (emu/g)

(a) High Energy Ball Mill

1.01

3

(b) Arc Melting

1.06

10.5

(c) High Energy Ball Mill + Arc Melting

1.7

28

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