We report the temperature dependence of thermopower (S) in the ab plane of Bi2Sr2Co2O9 {BC-222} single crystalline whiskers under high pressure up to 1.2 GPa. Special modifications were made to our high-pressure transport measurement system to improve the measurement accuracy of S for whiskers with high resistance. Pressure improved the contact resistance dramatically. We observed a slight decrease of  S and four-wire electrical conductivity (sigma) under high pressure. As a result, the power factor (S2sigma) was decreased less than 20% by pressure. From the temperature dependence of sigma, the band gap of BC-222 whiskers increase very slightly under high pressure

Chen, F

Funahashi, Ryozi

Stokes, Kevin L

English

ScholarWorks @ The University of New Orleans

University of New Orleans 
ScholarWorks@UNO 
Physics Faculty Publications Department of Physics 
2002 
Thermoelectric properties of Bi2Sr2Co2O9 whiskers under 
hydrostatic pressure 
F Chen 
Kevin L. Stokes 
University of New Orleans 
Ryozi Funahashi 
Follow this and additional works at: https://scholarworks.uno.edu/phys_facpubs 
 Part of the Materials Science and Engineering Commons, and the Physics Commons 
Recommended Citation 
Appl. Phys. Lett. 81, 2379 (2002) 
This Article is brought to you for free and open access by the Department of Physics at ScholarWorks@UNO. It has 
been accepted for inclusion in Physics Faculty Publications by an authorized administrator of ScholarWorks@UNO. 
For more information, please contact scholarworks@uno.edu. 
Thermoelectric properties of Bi2Sr2Co2O9 whiskers under
hydrostatic pressure
F. Chena) and K. L. Stokes
Advanced Material Research Institute, University of New Orleans, New Orleans, Louisiana 70148
Ryoji Funahashi
National Institute of Advanced Industrial Science and Technology, Midorigaoka, Ikeda,
Osaka 563-8577, Japan
~Received 21 December 2001; accepted for publication 5 August 2002!
We report the temperature dependence of thermopower (S) in the ab plane of Bi2Sr2Co2O9
~BC-222! single crystalline whiskers under high pressure up to 1.2 GPa. Special modifications were
made to our high-pressure transport measurement system to improve the measurement accuracy of
S for whiskers with high resistance. Pressure improved the contact resistance dramatically. We
observed a slight decrease of S and four-wire electrical conductivity ~s! under high pressure. As a
result, the power factor (S2s) was decreased less than 20% by pressure. From the temperature
dependence of s, the band gap of BC-222 whiskers increase very slightly under high
pressure. © 2002 American Institute of Physics. @DOI: 10.1063/1.1510160#
In the past decade there has been a significant amount of
research directed toward finding materials with improved
thermoelectric performance.1–5 Both in systems that convert
heat into electrical energy ~Seebeck effect! and in solid-state
cooling systems ~Peltier effect!, the efficiency of a material is
determined by a dimensionless figure of merit
ZT5
S2s
k
T , ~1!
where T is the absolute temperature, S is the Seebeck coef-
ficient, s is the electrical conductivity, and k is the thermal
conductivity. It has been demonstrated that low dimensional
materials like superlattice structures, quantum wires, and
quantum dots can have a significantly higher thermoelectric
figure of merit due to an increase of the Seebeck coefficient
resulting from the altered electronic density of states ~relative
to bulk! and the lowering of the lattice contribution to the
thermal conductivity due to selective phonon scattering and
phonon confinement.5,6 These discoveries prompt us to study
the thermoelectric properties whiskers. Whiskers are
quasione-dimensional single crystals of ~typically! layer-
structured materials with the layer planes oriented parallel to
the preferred growth direction. The thermoelectric properties
of pentatelluride whiskers have been investigated for cooling
~low temperature! applications.7,8 Recently, it was discovered
that oxide materials can have reasonably high values of S
~100 mV/K! at high temperatures, making them potentially
useful for power generation applications. The thermoelectric
properties of polycrystalline samples Bi2Sr2Co2O9 ~BC-222!
were found to be superior to most other oxides9 and single-
crystal BC-222 whiskers were found to have a power factor
(S2s) 5–7 times higher than that of the polycrystalline BC-
222 samples. In this letter, we extend the previous studies
and investigate the thermoelectric properties BC-222 whis-
kers under hydrostatic pressure. The use of high pressure in
material science can be a powerful tool to simulate hypo-
thetical chemical substitution, to uncover underlying system-
atics and to provide inputs and insights for band structure
theories.10,11 In fact, the thermoelectric properties of several
materials are found to improve upon application of uniaxial
pressure12,13 and hydrostatic pressure.14 For the BC-222
whiskers, the contact resistance is rather large and may be a
serious problem for the thermoelectric applications. We
found out that the pressure improves the contact resistance
dramatically and has very small side effect for the power
factor.
The Seebeck coefficient under high pressure was mea-
sured using a very low frequency ac two-heater method.15
Two surface mount resistors were driven by sinusoid currents
that differ in phase by p/2. The amplitude of the currents can
be adjusted to minimize the ac component of the base tem-
perature fluctuation (T˜ ). The ratio (dT/T˜ ) of ac temperature
gradient (dT) versus the base temperature fluctuation was
monitored to ensure the correct and precise S from ac
measurements.15 Two pairs of T-type thermocouples ~each
pair consists of TN and TP wires! were attached to the
sample directly using EPO-TEC H20E silver epoxy and
three independent signals were measured simultaneously to
solve for the base temperature, temperature gradient, and the
Seebeck coefficient. The copper leads of the T-type thermo-
couples were calibrated relative to Pb using data from
Roberts.16 Our Teflon pressure cell is about 3 mm in diam-
eter and 7 mm long, thus the sample size can be much larger
than that could be measured in an anvil cell and ensures
precise result together with other careful considerations.
Even with the surrounding of fairly good thermally conduct-
ing pressure medium, the measured Seebeck coefficient is in
good agreement with that which is measured outside of the
pressure cell. The small pressure effect on thermocouples
was also considered and corrected.
Bi2Sr2Co2O9 ~BC-222! single crystalline whiskers ~Fig.
1! were prepared using the glass-annealing method.17,18 The
samples for the measurement of ab-plane resistivity werea!Electronic mail: fchen@uno.edu
APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 13 23 SEPTEMBER 2002
23790003-6951/2002/81(13)/2379/3/$19.00 © 2002 American Institute of Physics
Downloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
about 1 mm in thickness, 10–20 mm in width, and 1–2 mm
in length. Since the whiskers have a very small cross section,
the intrinsic resistance is inevitably high. More seriously, the
contact resistance contributed an even larger portion of the
two-wire resistivity, probably due to the anisotropic nature of
the whiskers. At ambient pressure, the two-wire resistance
across the sample we measured was 10–40 kV at room tem-
perature and became higher at lower temperature. This con-
tributed a considerable amount of noise to the signals. We
adopted some improvements to our method described in Ref.
15. First, we picked different independent signals to mea-
sure. In general, the two thermocouple voltages are on the
order of millivolt and the Seebeck voltage measured between
the two TN or TP legs is on the order of microvolts. Only
three of these four voltages need to be measured and gener-
ally only one of the ~larger! thermocouple voltages is mea-
sured to avoid subtracting two large numbers to yield a small
number. However, this is no longer an advantage in the case
of a high-resistance sample in which the ~smaller! Seebeck
voltages are both very noisy. In this experiment, we picked
the two ~larger! thermocouple signals with low noise and one
~smaller! signal between the two TP legs with high noise.
Second, we have used a better Fourier-transform routine to
handle low signal-noise ratio and the results were satisfac-
tory.
We have measured the resistivity ~r! of BC-222 ~sample
No. 1! in ab-plane from 4 to 900 K. The inset of Fig. 2
shows the logarithm of r vs 1000/T . The curve shows typical
narrow gap semiconductor behavior with the gap Eg
53.53 meV from T530– 150 K. At higher temperatures, the
slope of dr/dT is positive, as plotted in the inset of Fig. 2.
This is not unusual for such a small gap material at low
temperature and indicates our BC-222 sample can be roughly
categorized as semimetal or very narrow gap semiconductor
in the temperature range 4–900 K. Materials with good ther-
moelectric properties are usually expected to be within this
regime.
Figure 2 shows the normalized resistivity of BC-222
~sample No. 2! in ab-plane under different pressure. The
sample No. 2 has an Eg53.89 meV at ambient pressure
~curve X), which is slightly higher than sample No. 1. As a
result, the slope dr/dT was always negative, and the resis-
tivity at room temperature is also higher than sample No. 1.
The pressure for curve Y and Z were 0.40 and 1.20 GPa,
respectively. The pressure increased both resistivity and Eg
slightly for sample No. 2. Upon reducing the pressure back
to ambient pressure, the resistivity was not reversible but Eg
remained unchanged. However, we also observed in another
sample that the four-wire resistivity at room temperature was
decreased when applying pressure, then increased when fur-
ther pressure was applied. It is likely that the pressure re-
duced the intrinsic resistivity at room temperature, but also
induced defects in the whiskers. It was noticeable that the
two-wire resistance was much higher than the four-wire re-
sistance at ambient pressure, possibly due to the contact was
made through c direction of the whiskers and the fact that the
resistivity of BC-222 along the c axis (rc) is about 102 – 103
times of that in the ab plane (rab). However, the pressure
reduced the contact resistance dramatically. Upon applying
the pressure, the contact resistance was reduced by 1–2 de-
cades.
Figure 3 shows the thermopower of BC-222 whisker
~sample No. 3! versus temperature under high pressure.
Curve ~a! was measured at ambient pressure. Then pressure
FIG. 1. Scanning electron microscopic image of a BC-222 whisker. The
well-grown plane corresponds to the crystallographic ab plane. Inset: lay-
ered crystal structure of BC-222.
FIG. 2. Normalized ab-plane resistivity of BC-222 vs temperature under
hydrostatic pressures. The pressure in giga pascal for these curves are
changed in this sequence: 0.00 (X), 0.40 (Y ), and 1.20 (Z). Inset: Resis-
tivity vs temperature showing change of slope from negative to positive
when temperature increases.
FIG. 3. Thermopower of BC-222 whisker vs temperature under hydrostatic
pressures. The pressures in giga pascal for these curves are: 0.00 ~a!, 0.30
~b!, 0.60 ~c!, 0.90 ~d!, and 1.20 ~e!, respectively.
2380 Appl. Phys. Lett., Vol. 81, No. 13, 23 September 2002 Chen, Stokes, and Funahashi
Downloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
was increased in turn to 0.30 ~b!, 0.60 ~c!, 0.90 ~d!, and then
to 1.20 GPa ~e!. The thermopower data at ambient pressure
~a! had lowest quality due to its highest resistance across the
whisker. In addition, the whisker is ribbon-like and the inter-
nal stress might play an important role at ambient pressure.
As a matter of fact, the thermopower measured above 250 K
at ambient pressure was not fully repeatable for different
runs. Nevertheless, the data below 250 K agrees within dif-
ferent runs very well and the bending over above 250 K
shown in curve ~a! was always observed although the degree
of the bending varied. Upon applying a pressure of 0.30 GPa
~b!, the two-wire resistance across the sample decreased and
the bending over above 250 K shown in curve ~a! also dis-
appeared. The S – T curve was also repeatable between dif-
ferent runs. The curves ~a! and ~b! are almost identical below
250 K. Upon further increase the pressure to 0.6 GPa ~c!, S
at 290 K decreased from 100.0 to 97.3 mV/K and the whole
S – T curve shifted downwards noticeably. Further increase
of pressure does not have an apparent effect on the S – T
curve. We were able to measure the S – T curve after the
pressure was reduced to ambient pressure and though consis-
tent with the previous runs, the data resolution was insuffi-
cient to determine if S was fully reversible upon release of
pressure due to higher resistance between the measuring
leads.
For nondegenerate systems, it was shown both
theoretically19,20 and experimentally14 that the simple for-
mula
S5
kB
e
F S r1 52 D2 T*T G , ~2!
is meaningful, where kB is the Boltzmann constant, e is the
electron charge, and r is the energy dependence parameter
for the electron scattering. The characteristic temperature T*
describes the degree of temperature dependence. At ambient
pressure, T* extracted from our thermopower data is 43.6 K,
which corresponds to a kBT* of 3.76 meV. This agrees well
with the energy gap of Eg53.53 meV and Eg53.89 meV
from the resistivity data of sample Nos. 1 and 2. We can
conclude from both S – T and r – T that under high pressure,
the electronic band structure of BC-222 whiskers changes
very little.
We observed both increment and decrement of the room
temperature resistivity, but the overall trend was dominated
by defects in the crystalline induced by applying pressure. As
a result, the power factor decreased slightly under high pres-
sure in most of the cases, but less than 20% in all cases.
However, the contact resistance was reduced by 1–2 decades
when applying pressure. This has great impact from the ap-
plication point of view. One key factor to reach material limit
of ZT in application is to have very small contact resistance.
Our experiment demonstrated that pressure can be a useful
way to improve the contact resistance with less than 20%
poisoning effect to the power factor.
In summary, we extended the studies of thermoelectric
properties of Bi2Sr2Co2O9 by applying hydrostatic pressure
to the whiskers. The two-wire electrical conductivity in-
creased 1–2 decades under pressure. Meanwhile electronic
band structure changes little under high pressure, results in
very little poisoning effect to the power factor. The experi-
mental data showed that pressure can be used to improve the
efficiency of the thermoelectric devices by reducing the con-
tact resistance. We demonstrated in this letter again the pos-
sibility of thermoelectric devices with better efficiency under
high pressure in another way, even with unavoidable
tradeoff.14
This work is supported by Research Grant No.
DAAD19-99-1-0001 from the Army Research Office.
1 D. A. Wright, Nature ~London! 181, 834 ~1958!.
2 G. A. Slack and V. G. Tsoukala, J. Appl. Phys. 76, 1665 ~1994!.
3 B. C. Sales, D. Mandrus, and R. K. Williams, Science 272, 1325 ~1996!.
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Bastea, C. Uher, and M. G. Kanatzidis, Science 287, 1024 ~2000!.
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and M. B. Korzenski, Phys. Rev. B 64, 121104 ~2001!.
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~2000!.
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Chu, J. H. Eggert, and H. K. Mao, Phys. Rev. B 50, 4260 ~1994!.
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2381Appl. Phys. Lett., Vol. 81, No. 13, 23 September 2002 Chen, Stokes, and Funahashi
Downloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions


Thermoelectric properties of Bi2Sr2Co2O9 whiskers under hydrostatic pressure

University of New Orleans

University of New Orleans ScholarWorks@UNO Physics Faculty Publications Department of Physics 2002 Thermoelectric properties of Bi2Sr2Co2O9 whiskers under hydrostatic pressure F Chen Kevin L. Stokes University of New Orleans Ryozi Funahashi Follow this and additional works at: https://scholarworks.uno.edu/phys_facpubs  Part of the Materials Science and Engineering Commons, and the Physics Commons Recommended Citation Appl. Phys. Lett. 81, 2379 (2002) This Article is brought to you for free and open access by the Department of Physics at ScholarWorks@UNO. It has been accepted for inclusion in Physics Faculty Publications by an authorized administrator of ScholarWorks@UNO. For more information, please contact scholarworks@uno.edu. Thermoelectric properties of Bi2Sr2Co2O9 whiskers underhydrostatic pressureF. Chena) and K. L. StokesAdvanced Material Research Institute, University of New Orleans, New Orleans, Louisiana 70148Ryoji FunahashiNational Institute of Advanced Industrial Science and Technology, Midorigaoka, Ikeda,Osaka 563-8577, Japan~Received 21 December 2001; accepted for publication 5 August 2002!We report the temperature dependence of thermopower (S) in the ab plane of Bi2Sr2Co2O9~BC-222! single crystalline whiskers under high pressure up to 1.2 GPa. Special modifications weremade to our high-pressure transport measurement system to improve the measurement accuracy ofS for whiskers with high resistance. Pressure improved the contact resistance dramatically. Weobserved a slight decrease of S and four-wire electrical conductivity ~s! under high pressure. As aresult, the power factor (S2s) was decreased less than 20% by pressure. From the temperaturedependence of s, the band gap of BC-222 whiskers increase very slightly under highpressure. © 2002 American Institute of Physics. @DOI: 10.1063/1.1510160#In the past decade there has been a significant amount ofresearch directed toward finding materials with improvedthermoelectric performance.1–5 Both in systems that convertheat into electrical energy ~Seebeck effect! and in solid-statecooling systems ~Peltier effect!, the efficiency of a material isdetermined by a dimensionless figure of meritZT5S2skT , ~1!where T is the absolute temperature, S is the Seebeck coef-ficient, s is the electrical conductivity, and k is the thermalconductivity. It has been demonstrated that low dimensionalmaterials like superlattice structures, quantum wires, andquantum dots can have a significantly higher thermoelectricfigure of merit due to an increase of the Seebeck coefficientresulting from the altered electronic density of states ~relativeto bulk! and the lowering of the lattice contribution to thethermal conductivity due to selective phonon scattering andphonon confinement.5,6 These discoveries prompt us to studythe thermoelectric properties whiskers. Whiskers arequasione-dimensional single crystals of ~typically! layer-structured materials with the layer planes oriented parallel tothe preferred growth direction. The thermoelectric propertiesof pentatelluride whiskers have been investigated for cooling~low temperature! applications.7,8 Recently, it was discoveredthat oxide materials can have reasonably high values of S~100 mV/K! at high temperatures, making them potentiallyuseful for power generation applications. The thermoelectricproperties of polycrystalline samples Bi2Sr2Co2O9 ~BC-222!were found to be superior to most other oxides9 and single-crystal BC-222 whiskers were found to have a power factor(S2s) 5–7 times higher than that of the polycrystalline BC-222 samples. In this letter, we extend the previous studiesand investigate the thermoelectric properties BC-222 whis-kers under hydrostatic pressure. The use of high pressure inmaterial science can be a powerful tool to simulate hypo-thetical chemical substitution, to uncover underlying system-atics and to provide inputs and insights for band structuretheories.10,11 In fact, the thermoelectric properties of severalmaterials are found to improve upon application of uniaxialpressure12,13 and hydrostatic pressure.14 For the BC-222whiskers, the contact resistance is rather large and may be aserious problem for the thermoelectric applications. Wefound out that the pressure improves the contact resistancedramatically and has very small side effect for the powerfactor.The Seebeck coefficient under high pressure was mea-sured using a very low frequency ac two-heater method.15Two surface mount resistors were driven by sinusoid currentsthat differ in phase by p/2. The amplitude of the currents canbe adjusted to minimize the ac component of the base tem-perature fluctuation (T˜ ). The ratio (dT/T˜ ) of ac temperaturegradient (dT) versus the base temperature fluctuation wasmonitored to ensure the correct and precise S from acmeasurements.15 Two pairs of T-type thermocouples ~eachpair consists of TN and TP wires! were attached to thesample directly using EPO-TEC H20E silver epoxy andthree independent signals were measured simultaneously tosolve for the base temperature, temperature gradient, and theSeebeck coefficient. The copper leads of the T-type thermo-couples were calibrated relative to Pb using data fromRoberts.16 Our Teflon pressure cell is about 3 mm in diam-eter and 7 mm long, thus the sample size can be much largerthan that could be measured in an anvil cell and ensuresprecise result together with other careful considerations.Even with the surrounding of fairly good thermally conduct-ing pressure medium, the measured Seebeck coefficient is ingood agreement with that which is measured outside of thepressure cell. The small pressure effect on thermocoupleswas also considered and corrected.Bi2Sr2Co2O9 ~BC-222! single crystalline whiskers ~Fig.1! were prepared using the glass-annealing method.17,18 Thesamples for the measurement of ab-plane resistivity werea!Electronic mail: fchen@uno.eduAPPLIED PHYSICS LETTERS VOLUME 81, NUMBER 13 23 SEPTEMBER 200223790003-6951/2002/81(13)/2379/3/$19.00 © 2002 American Institute of PhysicsDownloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsabout 1 mm in thickness, 10–20 mm in width, and 1–2 mmin length. Since the whiskers have a very small cross section,the intrinsic resistance is inevitably high. More seriously, thecontact resistance contributed an even larger portion of thetwo-wire resistivity, probably due to the anisotropic nature ofthe whiskers. At ambient pressure, the two-wire resistanceacross the sample we measured was 10–40 kV at room tem-perature and became higher at lower temperature. This con-tributed a considerable amount of noise to the signals. Weadopted some improvements to our method described in Ref.15. First, we picked different independent signals to mea-sure. In general, the two thermocouple voltages are on theorder of millivolt and the Seebeck voltage measured betweenthe two TN or TP legs is on the order of microvolts. Onlythree of these four voltages need to be measured and gener-ally only one of the ~larger! thermocouple voltages is mea-sured to avoid subtracting two large numbers to yield a smallnumber. However, this is no longer an advantage in the caseof a high-resistance sample in which the ~smaller! Seebeckvoltages are both very noisy. In this experiment, we pickedthe two ~larger! thermocouple signals with low noise and one~smaller! signal between the two TP legs with high noise.Second, we have used a better Fourier-transform routine tohandle low signal-noise ratio and the results were satisfac-tory.We have measured the resistivity ~r! of BC-222 ~sampleNo. 1! in ab-plane from 4 to 900 K. The inset of Fig. 2shows the logarithm of r vs 1000/T . The curve shows typicalnarrow gap semiconductor behavior with the gap Eg53.53 meV from T530– 150 K. At higher temperatures, theslope of dr/dT is positive, as plotted in the inset of Fig. 2.This is not unusual for such a small gap material at lowtemperature and indicates our BC-222 sample can be roughlycategorized as semimetal or very narrow gap semiconductorin the temperature range 4–900 K. Materials with good ther-moelectric properties are usually expected to be within thisregime.Figure 2 shows the normalized resistivity of BC-222~sample No. 2! in ab-plane under different pressure. Thesample No. 2 has an Eg53.89 meV at ambient pressure~curve X), which is slightly higher than sample No. 1. As aresult, the slope dr/dT was always negative, and the resis-tivity at room temperature is also higher than sample No. 1.The pressure for curve Y and Z were 0.40 and 1.20 GPa,respectively. The pressure increased both resistivity and Egslightly for sample No. 2. Upon reducing the pressure backto ambient pressure, the resistivity was not reversible but Egremained unchanged. However, we also observed in anothersample that the four-wire resistivity at room temperature wasdecreased when applying pressure, then increased when fur-ther pressure was applied. It is likely that the pressure re-duced the intrinsic resistivity at room temperature, but alsoinduced defects in the whiskers. It was noticeable that thetwo-wire resistance was much higher than the four-wire re-sistance at ambient pressure, possibly due to the contact wasmade through c direction of the whiskers and the fact that theresistivity of BC-222 along the c axis (rc) is about 102 – 103times of that in the ab plane (rab). However, the pressurereduced the contact resistance dramatically. Upon applyingthe pressure, the contact resistance was reduced by 1–2 de-cades.Figure 3 shows the thermopower of BC-222 whisker~sample No. 3! versus temperature under high pressure.Curve ~a! was measured at ambient pressure. Then pressureFIG. 1. Scanning electron microscopic image of a BC-222 whisker. Thewell-grown plane corresponds to the crystallographic ab plane. Inset: lay-ered crystal structure of BC-222.FIG. 2. Normalized ab-plane resistivity of BC-222 vs temperature underhydrostatic pressures. The pressure in giga pascal for these curves arechanged in this sequence: 0.00 (X), 0.40 (Y ), and 1.20 (Z). Inset: Resis-tivity vs temperature showing change of slope from negative to positivewhen temperature increases.FIG. 3. Thermopower of BC-222 whisker vs temperature under hydrostaticpressures. The pressures in giga pascal for these curves are: 0.00 ~a!, 0.30~b!, 0.60 ~c!, 0.90 ~d!, and 1.20 ~e!, respectively.2380 Appl. Phys. Lett., Vol. 81, No. 13, 23 September 2002 Chen, Stokes, and FunahashiDownloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionswas increased in turn to 0.30 ~b!, 0.60 ~c!, 0.90 ~d!, and thento 1.20 GPa ~e!. The thermopower data at ambient pressure~a! had lowest quality due to its highest resistance across thewhisker. In addition, the whisker is ribbon-like and the inter-nal stress might play an important role at ambient pressure.As a matter of fact, the thermopower measured above 250 Kat ambient pressure was not fully repeatable for differentruns. Nevertheless, the data below 250 K agrees within dif-ferent runs very well and the bending over above 250 Kshown in curve ~a! was always observed although the degreeof the bending varied. Upon applying a pressure of 0.30 GPa~b!, the two-wire resistance across the sample decreased andthe bending over above 250 K shown in curve ~a! also dis-appeared. The S – T curve was also repeatable between dif-ferent runs. The curves ~a! and ~b! are almost identical below250 K. Upon further increase the pressure to 0.6 GPa ~c!, Sat 290 K decreased from 100.0 to 97.3 mV/K and the wholeS – T curve shifted downwards noticeably. Further increaseof pressure does not have an apparent effect on the S – Tcurve. We were able to measure the S – T curve after thepressure was reduced to ambient pressure and though consis-tent with the previous runs, the data resolution was insuffi-cient to determine if S was fully reversible upon release ofpressure due to higher resistance between the measuringleads.For nondegenerate systems, it was shown boththeoretically19,20 and experimentally14 that the simple for-mulaS5kBeF S r1 52 D2 T*T G , ~2!is meaningful, where kB is the Boltzmann constant, e is theelectron charge, and r is the energy dependence parameterfor the electron scattering. The characteristic temperature T*describes the degree of temperature dependence. At ambientpressure, T* extracted from our thermopower data is 43.6 K,which corresponds to a kBT* of 3.76 meV. This agrees wellwith the energy gap of Eg53.53 meV and Eg53.89 meVfrom the resistivity data of sample Nos. 1 and 2. We canconclude from both S – T and r – T that under high pressure,the electronic band structure of BC-222 whiskers changesvery little.We observed both increment and decrement of the roomtemperature resistivity, but the overall trend was dominatedby defects in the crystalline induced by applying pressure. Asa result, the power factor decreased slightly under high pres-sure in most of the cases, but less than 20% in all cases.However, the contact resistance was reduced by 1–2 decadeswhen applying pressure. This has great impact from the ap-plication point of view. One key factor to reach material limitof ZT in application is to have very small contact resistance.Our experiment demonstrated that pressure can be a usefulway to improve the contact resistance with less than 20%poisoning effect to the power factor.In summary, we extended the studies of thermoelectricproperties of Bi2Sr2Co2O9 by applying hydrostatic pressureto the whiskers. The two-wire electrical conductivity in-creased 1–2 decades under pressure. Meanwhile electronicband structure changes little under high pressure, results invery little poisoning effect to the power factor. The experi-mental data showed that pressure can be used to improve theefficiency of the thermoelectric devices by reducing the con-tact resistance. We demonstrated in this letter again the pos-sibility of thermoelectric devices with better efficiency underhigh pressure in another way, even with unavoidabletradeoff.14This work is supported by Research Grant No.DAAD19-99-1-0001 from the Army Research Office.1 D. A. Wright, Nature ~London! 181, 834 ~1958!.2 G. A. Slack and V. G. Tsoukala, J. Appl. Phys. 76, 1665 ~1994!.3 B. C. Sales, D. Mandrus, and R. K. Williams, Science 272, 1325 ~1996!.4 D. -Y. Chung, T. Hogan, P. Brazis, M. Rocci-Lane, C. Kannewurf, M.Bastea, C. Uher, and M. G. Kanatzidis, Science 287, 1024 ~2000!.5 R. Venkatasubramanian, E. Silvola, T. Colpitts, and B. 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Instrum. 72,4201 ~2001!.16 R. B. Roberts, Philos. Mag. 36, 91 ~1977!.17 R. Funahashi and I. Matsubara, Appl. Phys. Lett. 79, 362 ~2001!.18 R. Funahashi, I. Matsubara, and M. Shikano, Chem. Mater. ~to be pub-lished!.19 D. K. C. MacDonald, Thermoelectricity: An Introduction to The Prin-ciples ~Wiley, New York, 1962!.20 H. J. Goldsmid, Electric Refrigeration ~H. Temple, London, 1986!.2381Appl. Phys. Lett., Vol. 81, No. 13, 23 September 2002 Chen, Stokes, and FunahashiDownloaded 31 Mar 2011 to 137.30.164.181. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

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Thermoelectric properties of Bi2Sr2Co2O9 whiskers under hydrostatic pressure

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