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Synthesis by mechanical alloying and thermoelectric properties of Cu2Te
Journal of Alloys and Compounds 264 (1998) 293–298
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Synthesis by mechanical alloying and thermoelectric properties of Cu 2 Te K. Sridhar, K. Chattopadhyay* Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India Received 14 March 1997
Abstract Hexagonal Cu 2 Te has been synthesised by mechanical alloying from elemental powders. The milling time required for the synthesis is longer than that reported for other tellurides. The mechanical grinding of the bulk Cu 2 Te obtained by the melting route does not change the structure. Prolonged milling as well as grinding beyond 40 h lead to a decrease in grain size to nanometer level. The cold compaction of milled or ground powders exhibit much smaller Seebeck coefficient (thermopower). However, cold compaction of samples milled for longer time (.150 h) lead to the thermopower values close to that of the bulk indicating significant improvement of rheological properties at room temperature for powders milled for long times. 1998 Elsevier Science S.A. Keywords: Tellurides; Mechanical alloying; Thermoelectric properties; Cu 2 Te
1. Introduction Tellurides are attractive materials for thermoelectric applications due to their very high thermopower values and ability to yield both p and n type materials by doping [1–3]. The efficiency of these materials also depend on properties like electrical resistivity (s ) and thermal conductivity (K) [4,5]. An improvement of the efficiency of these materials for thermoelectric applications is possible by microstructural control involving reduction of grain size. Since major part of thermal conductivity is controlled by phonon conduction, introduction of large amount of grain boundaries lead to scattering and significant decrease in the conductivity [6,7]. Mechanical alloying which produces very fine grain size, therefore, provides significant opportunities for developing more efficient thermoelectric materials. Some work has been reported earlier in this direction [8–11]. In the present investigation we explore the formation of Cu 2 Te by mechanical alloying and study the influence of processing variables on the thermoelectric power. In particular we look into the possibility of cold compaction of the alloyed powders which can retain ultra fine grain size.
*Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00266-1
2. Experimental procedure The mechanical alloying was done using a high energy planetary mill (P7, Frisch make) operating at a rpm of 525. The elemental powders of Cu and Te with particle size less than 45 mm and purity higher than 99.9% were used as starting materials for mechanical alloying. The appropriate amounts of Cu and Te powders corresponding to stoichiometric Cu 2 Te composition were weighed and charged in WC vial. The grinding was done using WC balls having a ball to powder ratio of 8:1. Toluene was used as grinding medium in all cases. For the sake of comparison, Cu 2 Te compound was also prepared by induction melting the elements in appropriate proportion. The milled powders obtained at different intervals of time were characterized by X-ray powder diffraction with CuKa1 radiation using a Huber goniometer. The morphology of the powders were studied using a scanning electron microscope. The samples obtained after mechanical alloying of elemental powders Cu and Te were cold compacted using pressures of 550 MPa and 700 MPa. Typical size of the sample is 3–4 mm in width and 5 mm in diameter and exhibited no cracks at these pressures. These pellets were used to study thermopower. A transient temperature gradient was produced between the two ends of the sample. The thermal emf produced across the sample by raising the one end to approximately 5 K is measured and plotted against temperature difference across the two junctions.
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K. Sridhar, K. Chattopadhyay / Journal of Alloys and Compounds 264 (1998) 293 – 298
The Seebeck coefficient was obtained from the slope of a linear least squares fit of 200 individual voltage and temperature measurements. The absolute Seebeck coefficient was obtained by subtracting the contribution of Cu leads.
3. Results
3.1. Characterization of the mechanically alloyed powders X-ray diffraction patterns of the as-mixed and mechanically alloyed powders are shown in Fig. 1a. The intensity of the reflections from the elemental powders decreases rapidly with milling time. At 6 h of milling the predominant peaks correspond to Cu 2 Te. However, weak reflections of elemental Cu and Te can still be seen. Although there exist a large number of phases near the Cu 2 Te composition (11 variations with composition Cu 22x Te), the observed lines can be fitted best with the phase having composition Cu 0.64 Te 0.36 . This phase has a hexagonal structure (P6 / mmm) with a50.8328 nm and c50.7219 nm and is stable at room temperature. It is difficult to decide when the elemental powders are completely replaced by the telluride. The strong Te (101) and Cu (111) peaks coincide with relatively strong telluride peaks of (102) and (113) respectively. A comparison of the ratios of the intensities indicate that Cu. probably coexist with telluride at 12 h while after 24 h, it is difficult to identify copper. Further milling with increasing time lead to considerable broadening of the peaks such that nearby peaks of telluride get merged into fewer broad peaks. At 128 h one can observe only three broad peaks in the diffractogram. An analysis of the grain size from the X-ray line broadening (where deconvolution of the merged peaks are possible) using the Scherrer formula indicates a rapid decrease of grain size in the first 20 h (Fig. 1b). Prolonged milling brings the grain size to less than 10 nm, with this value stabilizing beyond 40 h. Composition analysis of the milled powders did not reveal any contamination from the ball or the bowel materials. After 12 h of milling, all the powder particles show uniform composition. Fig. 2a shows the morphology of the powders particles milled for 1 h, 30 h and 50 h. Fig. 2b show a high magnification micrograph of a sample milled for 67 h. A careful observation reveals a layered structure. In order to compare the results of mechanical alloying we have also synthesized the same phase by melting and subjected it to mechanical grinding. Fig. 3 shows the X-ray diffraction patterns obtained from the as cast state and from the samples ground for different times. The pattern from the as cast material indicates the existence of the same Cu 2 Te phase in addition to small peaks of Cu and Te. These elemental peaks vanish quickly during mechanical grinding. No further structural change can be
observed. However, the line broadening with increasing milling time is similar to the mechanically alloyed samples.
3.2. Thermoelectric power measurements The thermoelectric power of all the samples was measured at room temperature. The as cast sample gives a value of 37.4 mV per K. This is higher than the reported value of 17 mV per K [12]. The discrepancy is most likely due to the existence of large number of structural variants for Cu 2 Te. The present result corresponds to hcp Cu 2 Te and we shall take it as the bulk value for comparison with milled samples. The thermoelectric power of the mechanical alloyed samples was measured by cold compacting the powder under two compaction pressures (550 MPa and 700 MPa). Fig. 4 shows a typical plot based on thermopower measurements carried out on a sample milled for 10.5 h. In all cases a linear behaviour was observed between the change in temperature difference in the sample and the emf generated. The thermopower was obtained from the slope of the linearly fitted line to the experimental data points. Fig. 5 shows the thermopower as a function of the milling time for both the compaction pressures. Under the lower compaction pressure of 550 MPa, the thermopower increases gradually and reaches a value of 32 mV per K only after 150 h of milling. In contrast, the samples compacted with the higher pressure show an increase in thermopower in the milling time range of 10.5–22 h, roughly corresponding to the formation of the Cu 2 Te in the X-ray diffraction pattern. We have also measured the thermopower of the Cu 2 Te samples obtained by the mechanical grinding and compacted at the same low pressure of 550 MPa. Fig. 6 shows the result as a function of grinding time. In all cases the value of the thermopower was low at small milling times and approached the bulk value at longer milling times under identical compacting conditions.
4. Discussion The effective use of thermoelectric materials demands a high figure of merit Z given by a 2 r /K [13,14]. Here a is the Seebeck coefficient or thermoelectric power, r is the resistivity and K is the thermal conductivity. The Seebeck coefficient in general is a materials property and reported to have not been greatly affected by microstructural changes. The reduction of thermal conductivity represents a good option for improving thermoelectric materials. It is well known that the mechanochemical effect associated with the process of mechanical alloying can lead to synthesis of compounds with high heat of formation. The tellurides, in general, have high heat of formation. An added advantage of synthesising compounds by the mechanical alloying route over conventional routes is
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Fig. 1. (a) X-ray diffraction patterns of the as-mixed and mechanically alloyed Cu and Te elemental powders for different time of milling. (b) A plot showing grain size vs milling time for mechanically alloyed powders.
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Fig. 2. (a) Scanning electron micrographs showing morphology of the powder particles milled for 1 h, 30 h, and 50 h. (b) Scanning electron micrograph showing layered structure for 67 h of mechanical alloying.
the production of extremely fine grained (nanometric) microstructures. The earlier efforts to synthesize tellurides by mechanical alloying indicate a very short milling time. For example, it takes only two minutes for the formation of the PbTe compound from the elemental mixture [9]. For Bi 2 Te 3 , twenty minutes of milling yields a completely single phase compound [10]. In comparison, the formation of copper telluride requires longer milling time. Although the phase is the major phase after 6 h of milling, it requires at least 24 h of milling to obtain a single phase without a trace of elemental powder. This clearly reflects the lower value of the free energy change associated with the compound formation (241.9 kJ mol -1 in comparison to 269.1 kJ mol -1 and 278.3 kJ mol -1 for PbTe and Bi 2 Te 3 respectively) [15]. Since one of the phases (Cu) is ductile, one expect that the mechanical alloying will promote a layered structure prior to the compound formation [16]. This is clearly established from the SEM micrograph (Fig. 2b). The layered morphology of the individual powder particles can be clearly seen. One of the most interesting result of the present work is the variation of the Seebeck coefficient or thermoelectric
power with cold compaction. The thermoelectric power for the mechanically alloyed samples compacted at a pressure up to 550 MPa approaches the bulk value only for samples milled for 150 h and beyond. The value does not approach the bulk value at 24 h when most of the powders have transformed into telluride. Even for the case of mechanical grinding of Cu 2 Te, the same behaviour could be observed. This clearly reflects that cold compacted specimens obtained at lower milling time do not get compacted properly and affect the electrical properties. The increase in the value of the thermoelectric power at higher pressure between 6 and 24 h also support the above conclusion. This is the reason why the ground tellurides are normally used after hot compaction sacrificing to a great extent the fine grains due to coarsening. One of the most important observation of the present work is the attainment of thermopower values close to the bulk value for samples milled beyond 150 h. These samples exhibit a very broad crystalline peak indicating the attainment of nanometer grain size. Clearly these powders have much superior rheological properties of die filling even at lower compaction pressure. Fig. 7 shows the microstructure of the
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297
Fig. 5. Comparison of thermopower obtained from cold compacted samples of mechanically milled powders for different times compacted with two different pressures.
Fig. 3. X-ray diffraction patterns of as-cast and mechanically ground samples of Cu 2 Te for different times.
can lead to a significant decrease in the thermal conductivity and subsequent improvement of the figure of merit of the materials.
5. Conclusions compacted (at 550 MPa) specimen of mechanically alloyed powders milled for 174 h. The microstructure clearly reveals the flow lines in the grains. This feature is absent in samples compacted from powders milled for lesser time. There exist a clear possibility of superplastic flow which requires further studies. However, the technological significance of this observation lies in the possibility of obtaining nanometer grain size compacts without grain growth. This
Fig. 4. A typical plot of change in emf as a function of change in temperature difference for 10.5 h mechanically alloyed sample showing linear behaviour. The slope gives the thermopower.
The present work permits following conclusions to be drawn.
Fig. 6. Thermopower variation with milling time for cold compacted mechanically ground samples of Cu 2 Te (compaction pressure 550 MPa).
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Acknowledgements The authors like to thank Dr. S.K. Tiwari for help in setting up the thermopower measuring unit and discussions. The financial help from the Department of Science and Technology, Government of India, is gratefully acknowledged.
References
Fig. 7. Scanning electron micrograph of the cold compacted sample (550 MPa pressure) from powders milled for 174 h. Note the flow lines marked by the arrow.
1. The hexagonal Cu 2 Te can be synthesized from the elemental mixture of copper and tellurium by mechanical alloying at room temperature. 2. The mechanical grinding of this phase did not lead to any structural change. However the broadened peaks suggest refinement of the grain size. 3. The cold compacts of the milled powders exhibit a very low thermopower at short milling times both for the mechanically alloyed and mechanically ground telluride powders. However, the value increases with milling time and approaches the bulk value beyond 150 h of milling. Although higher compaction pressure improves the thermopower, it is still much lower then the bulk value for short and intermediate milling times (up to 22 h of milling time). 4. The above observation indicates better rheological properties of powders prepared with longer milling time. This can be attributed to the nanometer grain size which is reflected by the significant broadening of the X-ray peaks.
[1] J.W. Gardner, World Power Engineering 1 (1963) 22. [2] J.W. Gardner, Electrical Review 168 (1961) 569. [3] A.F. Ioffe, in: Semiconductor Thermoelements and Thermoelectric cooling, Infosearch Limited, London, 1957. [4] J.F. Goff, J.R. Lowney, Proceedings of the International Conference on Thermoelectric energy conversion, K.R. Rao (Ed.), IEEE, New York, 1976, 47. [5] W.F. Leonard, Proceedings of the International Conference on Thermo-electric energy conversion, K.R. Rao (Ed.), IEEE, New York, 1976, 50. [6] C.M. Bhandari, D.M. Rowe, Phys. C 11 (1978) 1787. [7] D.M. Rowe, V.S. Shukla, N. Savvides, Nature 290 (1981) 765. [8] B.A. Cook, B.T. Beaudry, J.L. Harringa, W.J. Barnett, Proceedings of 24th Intersociety Energy conversion Engineering conference, vol. 2, IEEE, New York, 1989, 693. [9] T.S. Oh, J.S. Choiand, D.B. Hyun, Scripta Metall. Mater. 32 (1995) 595. [10] K. Hasezaki, M. Nishimura, M. Umata, H. Tsukuda, Mater. Trans. JIM 35 (1994) 428. [11] J. Chitralekha, K. Raviprasad, E.S.R. Gopal, K. Chattopadhyay, J. Mater. Res. 10 (1995) 1897. [12] A.S. Okhotin, A.S. Pushkarasky, Proceedings of the International conference on Thermoelectric energy conversion. K.R. Rao (Ed.), IEEE, New York, 1976, 36. [13] P.F. Taylor, C. Wood, J. Appl. Phys. 32 (1961) 1. [14] R.D. Barnard, in: Thermo-electricity in metals and alloys, Taylor and Francis Ltd, London, 1972. [15] I. Barin, O. Knacke, O. Kubaschewski, Thermochemical properties of Inorganic substances(supplement), Springer Verlag, Berlin, 1977. [16] J.S. Benjamin, Scientific American 40 (1976) 234.
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Synthesis by mechanical alloying and thermoelectric properties of Cu 2 Te K. Sridhar, K. Chattopadhyay* Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India Received 14 March 1997
Abstract Hexagonal Cu 2 Te has been synthesised by mechanical alloying from elemental powders. The milling time required for the synthesis is longer than that reported for other tellurides. The mechanical grinding of the bulk Cu 2 Te obtained by the melting route does not change the structure. Prolonged milling as well as grinding beyond 40 h lead to a decrease in grain size to nanometer level. The cold compaction of milled or ground powders exhibit much smaller Seebeck coefficient (thermopower). However, cold compaction of samples milled for longer time (.150 h) lead to the thermopower values close to that of the bulk indicating significant improvement of rheological properties at room temperature for powders milled for long times. 1998 Elsevier Science S.A. Keywords: Tellurides; Mechanical alloying; Thermoelectric properties; Cu 2 Te
1. Introduction Tellurides are attractive materials for thermoelectric applications due to their very high thermopower values and ability to yield both p and n type materials by doping [1–3]. The efficiency of these materials also depend on properties like electrical resistivity (s ) and thermal conductivity (K) [4,5]. An improvement of the efficiency of these materials for thermoelectric applications is possible by microstructural control involving reduction of grain size. Since major part of thermal conductivity is controlled by phonon conduction, introduction of large amount of grain boundaries lead to scattering and significant decrease in the conductivity [6,7]. Mechanical alloying which produces very fine grain size, therefore, provides significant opportunities for developing more efficient thermoelectric materials. Some work has been reported earlier in this direction [8–11]. In the present investigation we explore the formation of Cu 2 Te by mechanical alloying and study the influence of processing variables on the thermoelectric power. In particular we look into the possibility of cold compaction of the alloyed powders which can retain ultra fine grain size.
*Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 97 )00266-1
2. Experimental procedure The mechanical alloying was done using a high energy planetary mill (P7, Frisch make) operating at a rpm of 525. The elemental powders of Cu and Te with particle size less than 45 mm and purity higher than 99.9% were used as starting materials for mechanical alloying. The appropriate amounts of Cu and Te powders corresponding to stoichiometric Cu 2 Te composition were weighed and charged in WC vial. The grinding was done using WC balls having a ball to powder ratio of 8:1. Toluene was used as grinding medium in all cases. For the sake of comparison, Cu 2 Te compound was also prepared by induction melting the elements in appropriate proportion. The milled powders obtained at different intervals of time were characterized by X-ray powder diffraction with CuKa1 radiation using a Huber goniometer. The morphology of the powders were studied using a scanning electron microscope. The samples obtained after mechanical alloying of elemental powders Cu and Te were cold compacted using pressures of 550 MPa and 700 MPa. Typical size of the sample is 3–4 mm in width and 5 mm in diameter and exhibited no cracks at these pressures. These pellets were used to study thermopower. A transient temperature gradient was produced between the two ends of the sample. The thermal emf produced across the sample by raising the one end to approximately 5 K is measured and plotted against temperature difference across the two junctions.
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K. Sridhar, K. Chattopadhyay / Journal of Alloys and Compounds 264 (1998) 293 – 298
The Seebeck coefficient was obtained from the slope of a linear least squares fit of 200 individual voltage and temperature measurements. The absolute Seebeck coefficient was obtained by subtracting the contribution of Cu leads.
3. Results
3.1. Characterization of the mechanically alloyed powders X-ray diffraction patterns of the as-mixed and mechanically alloyed powders are shown in Fig. 1a. The intensity of the reflections from the elemental powders decreases rapidly with milling time. At 6 h of milling the predominant peaks correspond to Cu 2 Te. However, weak reflections of elemental Cu and Te can still be seen. Although there exist a large number of phases near the Cu 2 Te composition (11 variations with composition Cu 22x Te), the observed lines can be fitted best with the phase having composition Cu 0.64 Te 0.36 . This phase has a hexagonal structure (P6 / mmm) with a50.8328 nm and c50.7219 nm and is stable at room temperature. It is difficult to decide when the elemental powders are completely replaced by the telluride. The strong Te (101) and Cu (111) peaks coincide with relatively strong telluride peaks of (102) and (113) respectively. A comparison of the ratios of the intensities indicate that Cu. probably coexist with telluride at 12 h while after 24 h, it is difficult to identify copper. Further milling with increasing time lead to considerable broadening of the peaks such that nearby peaks of telluride get merged into fewer broad peaks. At 128 h one can observe only three broad peaks in the diffractogram. An analysis of the grain size from the X-ray line broadening (where deconvolution of the merged peaks are possible) using the Scherrer formula indicates a rapid decrease of grain size in the first 20 h (Fig. 1b). Prolonged milling brings the grain size to less than 10 nm, with this value stabilizing beyond 40 h. Composition analysis of the milled powders did not reveal any contamination from the ball or the bowel materials. After 12 h of milling, all the powder particles show uniform composition. Fig. 2a shows the morphology of the powders particles milled for 1 h, 30 h and 50 h. Fig. 2b show a high magnification micrograph of a sample milled for 67 h. A careful observation reveals a layered structure. In order to compare the results of mechanical alloying we have also synthesized the same phase by melting and subjected it to mechanical grinding. Fig. 3 shows the X-ray diffraction patterns obtained from the as cast state and from the samples ground for different times. The pattern from the as cast material indicates the existence of the same Cu 2 Te phase in addition to small peaks of Cu and Te. These elemental peaks vanish quickly during mechanical grinding. No further structural change can be
observed. However, the line broadening with increasing milling time is similar to the mechanically alloyed samples.
3.2. Thermoelectric power measurements The thermoelectric power of all the samples was measured at room temperature. The as cast sample gives a value of 37.4 mV per K. This is higher than the reported value of 17 mV per K [12]. The discrepancy is most likely due to the existence of large number of structural variants for Cu 2 Te. The present result corresponds to hcp Cu 2 Te and we shall take it as the bulk value for comparison with milled samples. The thermoelectric power of the mechanical alloyed samples was measured by cold compacting the powder under two compaction pressures (550 MPa and 700 MPa). Fig. 4 shows a typical plot based on thermopower measurements carried out on a sample milled for 10.5 h. In all cases a linear behaviour was observed between the change in temperature difference in the sample and the emf generated. The thermopower was obtained from the slope of the linearly fitted line to the experimental data points. Fig. 5 shows the thermopower as a function of the milling time for both the compaction pressures. Under the lower compaction pressure of 550 MPa, the thermopower increases gradually and reaches a value of 32 mV per K only after 150 h of milling. In contrast, the samples compacted with the higher pressure show an increase in thermopower in the milling time range of 10.5–22 h, roughly corresponding to the formation of the Cu 2 Te in the X-ray diffraction pattern. We have also measured the thermopower of the Cu 2 Te samples obtained by the mechanical grinding and compacted at the same low pressure of 550 MPa. Fig. 6 shows the result as a function of grinding time. In all cases the value of the thermopower was low at small milling times and approached the bulk value at longer milling times under identical compacting conditions.
4. Discussion The effective use of thermoelectric materials demands a high figure of merit Z given by a 2 r /K [13,14]. Here a is the Seebeck coefficient or thermoelectric power, r is the resistivity and K is the thermal conductivity. The Seebeck coefficient in general is a materials property and reported to have not been greatly affected by microstructural changes. The reduction of thermal conductivity represents a good option for improving thermoelectric materials. It is well known that the mechanochemical effect associated with the process of mechanical alloying can lead to synthesis of compounds with high heat of formation. The tellurides, in general, have high heat of formation. An added advantage of synthesising compounds by the mechanical alloying route over conventional routes is
K. Sridhar, K. Chattopadhyay / Journal of Alloys and Compounds 264 (1998) 293 – 298
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Fig. 1. (a) X-ray diffraction patterns of the as-mixed and mechanically alloyed Cu and Te elemental powders for different time of milling. (b) A plot showing grain size vs milling time for mechanically alloyed powders.
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Fig. 2. (a) Scanning electron micrographs showing morphology of the powder particles milled for 1 h, 30 h, and 50 h. (b) Scanning electron micrograph showing layered structure for 67 h of mechanical alloying.
the production of extremely fine grained (nanometric) microstructures. The earlier efforts to synthesize tellurides by mechanical alloying indicate a very short milling time. For example, it takes only two minutes for the formation of the PbTe compound from the elemental mixture [9]. For Bi 2 Te 3 , twenty minutes of milling yields a completely single phase compound [10]. In comparison, the formation of copper telluride requires longer milling time. Although the phase is the major phase after 6 h of milling, it requires at least 24 h of milling to obtain a single phase without a trace of elemental powder. This clearly reflects the lower value of the free energy change associated with the compound formation (241.9 kJ mol -1 in comparison to 269.1 kJ mol -1 and 278.3 kJ mol -1 for PbTe and Bi 2 Te 3 respectively) [15]. Since one of the phases (Cu) is ductile, one expect that the mechanical alloying will promote a layered structure prior to the compound formation [16]. This is clearly established from the SEM micrograph (Fig. 2b). The layered morphology of the individual powder particles can be clearly seen. One of the most interesting result of the present work is the variation of the Seebeck coefficient or thermoelectric
power with cold compaction. The thermoelectric power for the mechanically alloyed samples compacted at a pressure up to 550 MPa approaches the bulk value only for samples milled for 150 h and beyond. The value does not approach the bulk value at 24 h when most of the powders have transformed into telluride. Even for the case of mechanical grinding of Cu 2 Te, the same behaviour could be observed. This clearly reflects that cold compacted specimens obtained at lower milling time do not get compacted properly and affect the electrical properties. The increase in the value of the thermoelectric power at higher pressure between 6 and 24 h also support the above conclusion. This is the reason why the ground tellurides are normally used after hot compaction sacrificing to a great extent the fine grains due to coarsening. One of the most important observation of the present work is the attainment of thermopower values close to the bulk value for samples milled beyond 150 h. These samples exhibit a very broad crystalline peak indicating the attainment of nanometer grain size. Clearly these powders have much superior rheological properties of die filling even at lower compaction pressure. Fig. 7 shows the microstructure of the
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297
Fig. 5. Comparison of thermopower obtained from cold compacted samples of mechanically milled powders for different times compacted with two different pressures.
Fig. 3. X-ray diffraction patterns of as-cast and mechanically ground samples of Cu 2 Te for different times.
can lead to a significant decrease in the thermal conductivity and subsequent improvement of the figure of merit of the materials.
5. Conclusions compacted (at 550 MPa) specimen of mechanically alloyed powders milled for 174 h. The microstructure clearly reveals the flow lines in the grains. This feature is absent in samples compacted from powders milled for lesser time. There exist a clear possibility of superplastic flow which requires further studies. However, the technological significance of this observation lies in the possibility of obtaining nanometer grain size compacts without grain growth. This
Fig. 4. A typical plot of change in emf as a function of change in temperature difference for 10.5 h mechanically alloyed sample showing linear behaviour. The slope gives the thermopower.
The present work permits following conclusions to be drawn.
Fig. 6. Thermopower variation with milling time for cold compacted mechanically ground samples of Cu 2 Te (compaction pressure 550 MPa).
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K. Sridhar, K. Chattopadhyay / Journal of Alloys and Compounds 264 (1998) 293 – 298
Acknowledgements The authors like to thank Dr. S.K. Tiwari for help in setting up the thermopower measuring unit and discussions. The financial help from the Department of Science and Technology, Government of India, is gratefully acknowledged.
References
Fig. 7. Scanning electron micrograph of the cold compacted sample (550 MPa pressure) from powders milled for 174 h. Note the flow lines marked by the arrow.
1. The hexagonal Cu 2 Te can be synthesized from the elemental mixture of copper and tellurium by mechanical alloying at room temperature. 2. The mechanical grinding of this phase did not lead to any structural change. However the broadened peaks suggest refinement of the grain size. 3. The cold compacts of the milled powders exhibit a very low thermopower at short milling times both for the mechanically alloyed and mechanically ground telluride powders. However, the value increases with milling time and approaches the bulk value beyond 150 h of milling. Although higher compaction pressure improves the thermopower, it is still much lower then the bulk value for short and intermediate milling times (up to 22 h of milling time). 4. The above observation indicates better rheological properties of powders prepared with longer milling time. This can be attributed to the nanometer grain size which is reflected by the significant broadening of the X-ray peaks.
[1] J.W. Gardner, World Power Engineering 1 (1963) 22. [2] J.W. Gardner, Electrical Review 168 (1961) 569. [3] A.F. Ioffe, in: Semiconductor Thermoelements and Thermoelectric cooling, Infosearch Limited, London, 1957. [4] J.F. Goff, J.R. Lowney, Proceedings of the International Conference on Thermoelectric energy conversion, K.R. Rao (Ed.), IEEE, New York, 1976, 47. [5] W.F. Leonard, Proceedings of the International Conference on Thermo-electric energy conversion, K.R. Rao (Ed.), IEEE, New York, 1976, 50. [6] C.M. Bhandari, D.M. Rowe, Phys. C 11 (1978) 1787. [7] D.M. Rowe, V.S. Shukla, N. Savvides, Nature 290 (1981) 765. [8] B.A. Cook, B.T. Beaudry, J.L. Harringa, W.J. Barnett, Proceedings of 24th Intersociety Energy conversion Engineering conference, vol. 2, IEEE, New York, 1989, 693. [9] T.S. Oh, J.S. Choiand, D.B. Hyun, Scripta Metall. Mater. 32 (1995) 595. [10] K. Hasezaki, M. Nishimura, M. Umata, H. Tsukuda, Mater. Trans. JIM 35 (1994) 428. [11] J. Chitralekha, K. Raviprasad, E.S.R. Gopal, K. Chattopadhyay, J. Mater. Res. 10 (1995) 1897. [12] A.S. Okhotin, A.S. Pushkarasky, Proceedings of the International conference on Thermoelectric energy conversion. K.R. Rao (Ed.), IEEE, New York, 1976, 36. [13] P.F. Taylor, C. Wood, J. Appl. Phys. 32 (1961) 1. [14] R.D. Barnard, in: Thermo-electricity in metals and alloys, Taylor and Francis Ltd, London, 1972. [15] I. Barin, O. Knacke, O. Kubaschewski, Thermochemical properties of Inorganic substances(supplement), Springer Verlag, Berlin, 1977. [16] J.S. Benjamin, Scientific American 40 (1976) 234.