Liquid He based resistivity/thermoelectric power (5-330K)
Dr. Gunadhor S Okram
Electrical resistivity (and its inverse is conductivity) is a property of a material that quantifies how strongly the material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. In insulators and semiconductors, the atoms influence each other so that there exists a forbidden band of energy levels (without electrons) between the valence band and the conduction band. In order for a current to flow, a relatively large amount of energy is required for an electron to jump across this forbidden gap into the conduction band. Thus, even large voltages can yield relatively small currents. This energy gap is small and comparable to thermal energy in semiconductors while it is very large in insulators. In metals, the conduction band and valence band are overlapped and electrons are free to move for conduction. Ideally, this scenario could be for a single crystal, which is not normally available for practical purposes.
Temperature range: 5 - 300 K
Measurable range:~1 V/K to ~100 mV/K
Material type: metal or semiconductor (bulk, single crystal or thin film)
Sample size: preferably proper shape; like cylindrical 2mmLx0.2mm diameter or parallelopiped 2mmLx1mmWx0.1mmH (Ideal size: 5mm diameterx0.5mm thickness)
Photograph of the thermopower and resistivity set ups.
Practically, samples are of different types, which are not necessarily a single crystal. Howsoever, it should be generally a well-characterized compact solid material of good quality with very high density such as a sintered pellet piece of parallelopiped shape (10mmL x1mmW x0.5mmH) and rectangular shape thin film (say, 10mmL x1mmW x0.0002 mmH) for metallic samples, and a sintered circular pellet (10mm diameterx1mm thick) for semiconducting/ highly resistive samples. Metallic samples are normally measured by four-probe method to avoid lead and contact resistances as they are quite often comparable to or even larger than the actual resistance to be measured, and in forward and reverse current modes to take a mean voltage, which is to avoid any thermoelectric voltages due to possible temperature gradient along the sample length (Figure 2, a, b). Semiconducting or high resistive samples are normally measured by two-probe method in guarded mode
Schematic of the 4-probe measurement set up.
Resistivity of Nb; insets show transition region (bottom) and its derivative
Conductivity of conducting polymer (polyaniline) at various ion doping
Photograph of bottom portion of 2-probe measurement sample holder.
Thermoelectric power (or thermopower) is the conversion of heat directly into electricity or vice-versa (refrigeration). Curiously, when a metal rod is maintained at two different temperatures at its ends, the electrons from hot end tend to flow towards the lower temperature, creating a voltage difference. The voltage difference per unit Kelvin is the Seebeck coefficient (S) or thermopower. In metals, as the electrons are free to move, the Seebeck effect is small (in V/K range [1,2,4,5], Figure 3, right panel). However, in semiconductors, they are relatively very large (in mV/K ). Semiconductors that exhibit good thermoelectric properties are known as thermoelectrics. The sign of S is also used for characterization of the majority charge carriers in a material as its mathematical expression is directly related to the first power of the charge. The main interest in the study of thermoelectricity lies in the fundamental properties of a material and its potential applications such as thermocouple and thermoelectrics. Notably, thermoelectric devices can be considered as renewable source of energy devices as temperature gradient is ubiquitous, and electricity can be generated anywhere, at difficult or key situations/ locations as well!
Schematic of the measurement set up.
Thermopower of Pt wire
(Black circle: our result; Red star: earlier publication);
inset: scatter in the data, 3.76%
Temperature range: 5 - 300 K
Measurable range: 1 to G
Basic accuracy (25oC 5oC): 0.06%+ 1 count
1. Size Dependent Thermopower in Nanocrystalline Nickel, A. Soni and G. S. Okram, Appl. Phys. Lett. 95, 013101 (2009): Got selection for the July 20, 2009 issue of Virtual Journal of Nanoscale Science & Technology.
Resistivity and thermopower measurement setups in the Temperature range of 5-325 K, A. Soni and G. S. Okram, Rev. Sci. Instrum. 79, 125103 (2008).
Electrical properties of polyaniline doped with metal ions, J. B. M. Krishna, A. Saha, G. S. Okram, A. Soni, S. Purakayastha and B. Ghosh, J. Phys. D: Appl. Phys. 42, 095404-10 (2009).
Size-dependent resistivity and thermoelectricity of Cu nanoparticles, G. S. Okram and N. Kaurav, J. Appl. Phys. 110, 023713 (1-9) (2011).
Intrinsic thermopower of Group VB metals, G. S. Okram, AIP Advances 2, 012178(1-9) (2012).
Optoelectronic and Thermoelectric properties in Ga Doped b-PbS2 nanostructured thin films, R. Geethu, R. Jacob, T. Shripathi, G. S. Okram, V. Ganesan, S. Tripathi, A. Fatima, Sreenivasan P.V, Urmila K. S, B. Pradeep, R. R. Philip, Appl. Surf. Sci. 258, 6257-6260 (2012).