The High Temperature Seebeck Probe (HTSP-2000) is designed for simoultaneous determination of both Seebeck coefficient (thermopower) and electrical conductivity of semiconducting materials, for both single crystals and polycrystals at elevated temperatures (up to 1100°C) and under controlled gas phase environments (i.e. under controlled oxygen activity) (Figure 1).
The Seebeck Coefficient may be used for the determination of defect-related properties of materials and it may also be used for monitoring of electrical properties as a function of time during reactions, such as adsorption, changes of non-stoichiometry, and equilibration.
Defect chemistry, non-stoichiometry, chemical diffusion, gas/solid reactions Charge and mass transport, energy conversion, equilibration kinetics, gas sensing Semiconducting properties, thermoelectric effects
Contents
The HTSP-2000 (Figure 1 and Figure 3) incorporates probe chamber (including sample holder, microheaters and thermocouples), and probe head (including electrical outlets and printed circuit board).
The sample is placed between two Pt plates attached to micro-heaters (serving as thermoelectrodes for the determination of the Seebeck coefficient, and current electrodes for the determination of electrical conductivity). The plates are pushed against the sample by a spring. A temperature gradient across the sample is imposed by microheaters. In order to determine electrical conductivity, the specimen is equipped with two (voltage) electrodes (wrapped around) and soldered with Pt connecting wires.
For functionality of the HTSP-2000, several ancillary items are required, including personal computer, multimeter, electrometer, scanners, current source, temperature controllers, tube furnace, oxygen sensor, gas flow system and software for data acquisition and processing.
Figure 1. Set up of HTSP-2000 and Ancillary Equipment
Figure 2. HTSP External View
Figure 3. HTSP Internal View
Seebeck coefficient is a basic quantity in the characterization of electronic structure of materials as well as their thermoelectric properties. This quantity may be related to the concentration of electronic charge carriers. Combined Seebeck coefficient and electrical conductivity data can be used for the determination of their mobility terms.
These data may be used for derivation of defect chemistry models and evaluation of the transport of charge and matter at elevated temperatures corresponding to the conditions of processing or performance of materials.
The principle of the determination of Seebeck Coefficient is illustrated in Figure 4.
Figure 4. Generation of Seebeck Voltage
Imposition of a temperature gradient, 
, across a specimen (using micro-heaters) results in generation of a potential difference, 
, which is termed Seebeck Voltage or Thermovoltage. Knowledge of both and are required for the determination of the Seebeck coefficient (thermopower), S [1-3]:
Eq(1)
Figure 5. Determination of Seebeck Coefficient (Example of CaTiO3)
Figure 5 shows a typical experimental plot of vs within both orientations of the T gradient. The following equations describe the relationship between S and semiconducting properties of materials:
Eq(2)
Eq(3)
Eq(4)
where k is Boltzmann constant 
; e is elementary charge 
; Sn, Sp denote Seebeck coefficient components related to electrons and electron holes, respectively; Nn, Np denote density of states in conduction band and valence band, respectively; An, Ap are kinetic constants related to scattering of electrons and electron holes, respectively; n, p denote concentrations of electrons and electron holes, respectively.
Assuming Maxwell-Boltzmann statistics of electrons the following expressions may be written between the concentration of electronic charge carriers and Fermi energy level, EF:
Eq(5)
Eq(6)
where Ec is the energy of the bottom of conduction band and Ev is the energy of the top of valence band. Therefore, S may be directly related to EF:
Figure 6. S vs Log P(O2) for BaTiO3
Figure 6 shows the plot of thermopower, S, as a function of oxygen partial pressure, p(O2), for undoped BaTiO3 within the n-p transition regime at 1090-1310
Eq(7)
Eq(8)
K [4]. As seen, thermopower exhibits rapid changes within the n-p transition regime. The slope within both n- and p- type regimes may be used for verification of defect chemistry models [5,6].
Electrical conductivity, 
, involves the components related to the species involved in the conduction, such as electrons, electron holes and ions. Therefore:
Eq(9)
where
Eq(10)
where µn and µp denote the mobilities of electrons and electron holes, respectively.
Figure 7. Log vs Log P(O2) for BaTiO3
Figure 7 shows the plots of log as a function of p(O2) for undoped BaTiO3 [4]. The slopes of S vs log p(O2) may be used for verification of defect chemistry models [5,6]. Eqs. (9) and (10) may also include the component related to ionic defects.
Both S and (determined simultaneously within the n-to-p-type transition regime) allows the determination of the band gap, Eg the mobility terms and kinetic constants, using a Jonker analysis which is described by the following equations [2,3]:
Eq(11)
where
Eq(12)
Eq(13)
where
Eq(14)
the minimum of corresponds to n-p transition. The quantities b, d and Eg in Eqs (11)-(13) can be determined from the following relations:
Eq(15)
Eq(16)
Figure 8. Jonker Plot
Figure 8 shows schematic plot of S vs log illustrating the parameters that are critical for solving Eqs. (14) and (15).
Figure 9. Log S vs Log for BaTiO3
Figure 9 shows the Jonker plot for BaTiO3 (using the data in Figure 6 and Figure7). Details of the Jonker analysis for are reported in refs. [2-4].
The determination of both and S allows the determination of the n-p transition regime of amphoteric oxide semiconductors, such as TiO2, BaTiO3 and CaTiO3.
Figure 10. S and within N-P transition
Figure 10 illustrates the effect of p(O2) on both and S. As seen, the minimum of coincides with S=0. As also seen, the n-p transition regime is characterized by a complex dependence involving a steep part passing through S=0 which on both sides assume curvatures. The p(O2) range in the vicinity of S=0 
, in which S exhibits non-linear changes vs p(O2), is much larger than that demarcated by the changes of 
.
Doping with ions of different valency results in a modification of defect structure leading, in consequence, to change in the concentration of both electronic and ionic defects and related
semiconconducting properties.
Figure 11. Effects of Donors and acceptors on Log vs Log P(O2)
Figure 12. Effects of Donors and acceptors on S vs Log P(O2)
Figure 11 and Figure12 illustrate the effect of donorand acceptor-type dopants on the effect of p(O2) on both and S.
The HTSP-2000 may be used for the determination of thermoelectric properties of materials as well as defect-related properties. Specifically, the probe may be used for simultaneous determination of both thermopower and electrical conductivity of materials at elevated temperatures and under controlled oxygen activity. Combination of both S and may then be used for the determination of defect chemistry and related semiconducting quantities, such as concentration and mobility of both electronic and ionic defects [8].
Mr. Ross K. Druitt
Managing Director of Sialon Ceramics Pty. Ltd. and Wallarah Minerals Pty. Ltd.
130 Tall Timbers Rd, Doyalson North, NSW 2262, Australia
Tel. +61 2 4358 4994; Fax. +61 2 4358 1348Sialon Ceramics ABN: 57 002 988 543; Wallarah Minerals ABN: 77 002 503 399
1. B S Hong, S J Ford. T O Mason, in: Electrical Properties of Oxide Materials, J Nowotny, C C Sorrell,Eds., Trans Tech Publ, Zurich, 1998, p 163
2. T Bak, J Nowotny, C CSorrell, in: ref. [I], p. 1
3. J Nowotny, in: The CRC Handbook of Solid State Electrochemistry, P J Gellings, H J M Bouwmeester, Eds., CRC Press, Boca Raton, 1997, p 121
4. J Nowotny, M Rekas, Ceramics Intern, 20 (1994) 225
5. J Nowotny, M Rekas, in: Electronic Ceramic Materials, Trans Tech Publ, Zurich, 1991, p. 1
6. J Nowotny, M Rekas, Solid State lonics, 49 (1991) 135
7. J Nowotny,. M Rekas, H-I Yoo, in: Ceramic Interfaces: Properties and Applications, R St C Smart, J Nowotny, Eds., Institute of Materials, London, 1998,p 283
8. J Nowotny, M Radecka, M Rekas, S Sugihara, E R Vance, W Weppner, Ceram. International, 24 (1998) 553-59
UNSW • University of New South Wales • Charles C. Sorrell • Centre for Materials Research in Energy Conversion • School of Materials Science & Engineering • Januzs Nowotny • Tadeusz Bak
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