Precision Thermocouple Based Temperature Measurement System

Introduction

The aim of this project is to design, construct, and validate a thermoelectric measurement system for determining the Seebeck coefficient of bulk materials through the Seebeck effect. This system measures the voltage that a material produces when a temperature gradient is applied and turns it into useful thermoelectric parameters.

Unlike conventional temperature sensors, such as thermistors or integrated circuit sensors, thermoelectric measurements directly reveal material properties by correlating temperature differences with electrical voltage. However, the thermoelectric voltage generated typically resides in the microvolt range, making accurate measurement a challenge.

In this work, a high-precision measurement methodology is employed, using a nanovoltmeter to directly capture the thermoelectric voltage without the need for external amplification. A controlled temperature gradient is set up across the sample, and the voltage that comes out is measured to find the Seebeck coefficient. The system is designed to ensure accuracy, stability, and minimal noise interference in microvolt-level measurements.

Theoretical Background

Seebeck Effect

The Seebeck effect states that when two dissimilar conductors are joined to form a loop and their junctions are maintained at different temperatures, a voltage is generated.

The thermoelectric voltage is given by: $V=S\cdot \mathrm{\Delta }T$ where:

• $V$ = thermoelectric voltage
• $S$ = Seebeck coefficient (µV/°C)
• $\mathrm{\Delta }T$ = temperature difference between junctions

We measure $\mathrm{\Delta }T$ using two K-type thermocouples by making contacts on the sample. The $V$ is measured using the nanovoltmeter. We use the trigger function to directly download 4 values for each $\mathrm{\Delta }T$ in a csv format. The reason we take 4 values is to consider the uncertainty in the measurement.

To get the Seebeck Coefficient we plot Voltage vs $\mathrm{\Delta }T$ and calculate the slope of the linear fit curve plotted using the equation above.

Two-Probe Measurement Technique

In this work, a two-probe measurement technique is employed to measure the Seebeck voltage generated across the sample. In this method, the same pair of contacts is used for voltage measurement. The thermoelectric voltage is directly measured across the sample using a high-precision nanovoltmeter. Since the Seebeck effect inherently produces a voltage under open-circuit conditions, no external current is required, making the two-probe method well-suited for this application. Although contact resistance can influence measurements in general electrical characterisation, its effect on Seebeck voltage measurements is minimal because no current flows through the sample. Therefore, voltage drops associated with contact and lead resistances are negligible. As a result, the two-probe configuration provides a simple and effective approach for determining the Seebeck coefficient in this setup.

Why Two-Probe Measurement Technique over Four-Probe Measurement Technique?

A two-probe measurement technique is preferred in this study because the Seebeck voltage is measured under open-circuit conditions, where no external current flows through the sample. Consequently, errors arising from contact and lead resistances are insignificant. In contrast, the four-probe method is primarily used for electrical resistivity measurements, where current is passed through the sample and voltage drops due to contact resistance must be eliminated. Since resistivity measurement is not the objective of the present work, the additional complexity of a four-probe configuration is unnecessary. Thus, the two-probe method offers a simpler, reliable, and sufficiently accurate approach for Seebeck coefficient measurement in this experimental setup.

Material Selection

Zinc oxide (ZnO) is recognised as an n-type semiconductor and exhibits a pronounced Seebeck effect. When a temperature gradient is established across the material, charge carriers migrate from the hotter side to the cooler side, resulting in the generation of a thermoelectric voltage. The magnitude and polarity of this voltage depend on the properties of the material, with ZnO typically exhibiting a negative Seebeck coefficient due to the predominant conduction of electrons. The Seebeck coefficient for zinc oxide (ZnO) usually falls within the range of approximately –100 to –500 µV/K, which can vary based on factors such as temperature, doping, and the methods used in material preparation. The negative value indicates that ZnO functions as an n-type semiconductor, with electrons serving as the dominant charge carriers.

The ZnO was prepared by grinding 3 grams of Sigma-Aldrich 99% pure ZnO and making a pellet. The pellet was first annealed for 5 hours at 300°C. Since the pellet was not hard enough, it was re-annealed at 500°C for 3 hours. On cooling, it was cut into a rectangular shape of thickness 3 mm.

Experimental Setup

Schematic diagram of the experiment - two probe measurement

Schematic diagram of the experiment

Experimental setup

Experimental setup of the sample

The experimental setup comprises two copper blocks functioning as thermal reservoirs, separated by a gap of approximately 3–4 mm. Each copper block has dimensions of about 12–15 mm in width and 8–10 mm in height, providing mechanical stability while minimizing thermal mass. A ZnO slab with a thickness of 3mm and length about 6mm is positioned across the gap, overlapping slightly (~1 mm) on both blocks to ensure optimal thermal contact.

On the hot side, a layered structure is implemented, consisting of a copper block, a layer of Kapton tape for electrical insulation, and a power resistor serving as the heating element. The cold side features a similar configuration without the heater, allowing it to remain near ambient temperature. This arrangement establishes a controlled temperature gradient across the ZnO sample.

Four electrical contacts are applied to the top surface of the ZnO slab using conductive silver paste, arranged in succession from the hot side to the cold side as Tₕ, V⁺, V⁻, and T_c. The total probe span is maintained at approximately 4 mm to ensure that all contact points fall within the pellet surface. Thermocouples are connected at Tₕ and T_c to measure the temperature difference across the sample.

The Seebeck voltage is measured between the V⁺ and V⁻ contacts, while the temperature gradient is obtained from the thermocouple readings. This configuration facilitates accurate determination of the Seebeck coefficient while minimizing errors associated with contact resistance and thermal instability.

References

Rawat, P. K., & Paul, B. (2016). Simple design for Seebeck measurement of bulk sample by 2-probe method concurrently with electrical resistivity by 4-probe method in the temperature range 300–1000 K. Measurement, 94, 297–302. https://doi.org/10.1016/j.measurement.2016.05.104

Goldsmid, H. J. (2010). Introduction to thermoelectricity. Springer. https://doi.org/10.1007/978-3-642-00716-3

Rowe, D. M. (Ed.). (2006). Thermoelectrics handbook: Macro to nano. CRC Press.

Snyder, G. J., & Toberer, E. S. (2008). Complex thermoelectric materials. Nature Materials, 7(2), 105–114. https://doi.org/10.1038/nmat2090

Özgür, Ü., Alivov, Y. I., Liu, C., Teke, A., Reshchikov, M. A., Doğan, S., … Morkoç, H. (2005). A comprehensive review of ZnO materials and devices. Journal of Applied Physics, 98(4), 041301. https://doi.org/10.1063/1.1992666

Look, D. C. (2001). Recent advances in ZnO materials and devices. Materials Science and Engineering: B, 80(1–3), 383–387. https://doi.org/10.1016/S0921-5107(00)00604-8

Keysight Technologies. (2020). B2901A Precision Source/Measure Unit datasheet. https://www.keysight.com/