=Paper=
{{Paper
|id=Vol-3126/paper9
|storemode=property
|title=Features of the implementation of methods for a comprehensive study of properties of thermoelectric materials
|pdfUrl=https://ceur-ws.org/Vol-3126/paper9.pdf
|volume=Vol-3126
|authors=Roman Dunets,Bogdan Dzundza,Liliia Turovska,Nazariy Senkiv
}}
==Features of the implementation of methods for a comprehensive study of properties of thermoelectric materials==
https://ceur-ws.org/Vol-3126/paper9.pdf
Features of the Implementation of Methods for a Comprehensive
Study of Properties of Thermoelectric Materials
Roman Dunets1, Bogdan Dzundza1,2, Liliia Turovska3, Nazariy Senkiv2
1
Lviv Polytechnic National University, Bandera str., 12, Lviv, 79013, Ukraine
2
Vasyl Stefanyk PreCarpathian National University, Shevchenko str., 57, Ivano-Frankivsk, 76018, Ukraine
3
Ivano-Frankivsk National Medical University, Halytska str. 2, Ivano-Frankivsk, 76018, Ukraine
Abstract
Methods for the implementation of software and hardware tools for comprehensive
nondestructive research of thermoelectric parameters of semiconductors have been analyzed
and adapted. An information-measuring system has been developed, in which, due to a
combination of different research methods, it is possible to perform the whole complex of
thermoelectric measurements in one technological cycle and on one sample of typical
configuration, in particular, thermo-EMF, electrical conductivity, Hall coefficient,
magnetoresistance, Nernst-Ettingshausen coefficient, thermal conductivity, and thermoelectric
figure of merit. The use of digital algorithms for filtering and processing the received data made
it possible to obtain important parameters that are difficult to measure directly, in particular,
the mobility and concentration of charge carriers, parameters of near-surface layers, to
reconstruct the profiles of the distribution of thermoelectric parameters over the thickness. An
important advantage of these methods is the absence of the need for accurate measurements of
heat fluxes, which greatly simplifies and reduces the time for conducting experimental studies.
Keywords 2
Thermoelectric properties, information-measuring systems, measurement methods, computer
tools, signal processing, defects identification
1. Introduction specialization of thermoelectric research.
Therefore, an urgent task is the adaptation of
methods and the development of tools both for the
Thermoelectric materials are becoming more
study of the main thermoelectric parameters of
widespread as simple and reliable energy
semiconductor materials and for express methods
converters, but their efficiency is still quite low.
for determining the operating characteristics of
Therefore, a large number of studies are carried
thermoelectric energy conversion modules.
out aimed at increasing the efficiency of
thermoelectric materials. Such studies require
measurements of electrical conductivity, Seebeck 2. Selection and adaptation of
coefficient, thermal conductivity, which, when measurement methods
using classical techniques, is a rather laborious
task, since samples of various configurations and
When studying thermoelectric and
accurate measurement of heat fluxes are required.
photoelectric properties, it becomes necessary to
A large number of universal tools for laboratory
measure a sufficiently large number of quantities
research have been developed, but their effective
of different nature (electrical conductivity,
use is not always possible due to the narrow
ISIT 2021: II International Scientific and Practical Conference
«Intellectual Systems and Information Technologies», September
13–19, 2021, Odesa, Ukraine
EMAIL: roman.b.dunets@lpnu.ua (A. 1);
bohdan.dzundza@pnu.edu.ua (A. 2); lturovska@ifnmu.edu.ua (A.
3); senabo33@gmail.com (A. 4)
ORCID: 0000-0002-3325-7908 (A. 1); 0000-0002-6657-5347 (A.
2); 0000-0002-3530-7518 (A. 3)
©️ 2021 Copyright for this paper by its authors. Use permitted under Creative
Commons License Attribution 4.0 International (CC BY 4.0).
CEUR Workshop Proceedings (CEUR-WS.org)
Seebeck coefficient, thermal conductivity, analysis of the "mobility spectrum" [1]. This
efficiency, etc.) depending on various factors method consists in the fact that the components of
(temperature, film thickness, production the conductivity tensor are represented in the form
parameters, type of substrate), which makes such of integral equations depending on the
experiments quite laborious. concentration and mobility of charge carriers.
Since the preparation of a sample makes up Such methods require a decrease in the
most of the labor costs in thermoelectric research, measurement error of the Hall voltage and
it is urgent to develop an automated computer magnetoresistance, since in the presence of noise
system that will allow combining direct and in the experimental data it is difficult to correctly
indirect research methods on one sample. In determine the search area for the parameters.
particular, the implementation of the Hall For a more visual analysis, we have
methods will make it possible to study the constructed a model of the measuring channel, in
galvanomagnetic, temperature, thickness and time particular, Fig. 1 shows a model of the measuring
dependences of the properties, in particular, to channel in the study of the galvanomagnetic
determine the electrical conductivity, the properties of the material, taking into account the
concentration of charge carriers, the Seebeck influence of parasitic effects, such as EMF of
coefficient, and magnetoresistance. nonequipotentiality, EMF of magnetoresistive
Despite the rather slow processes in the effect, thermo-EMF ΣUi(B,I,T), distortions
implementation of Hall methods, a number of caused by electronic nodes and inaccuracies of
difficulties arise associated with low signal levels actuators G(B,I,T), as well as the influence of
and the large influence of parasitic effects. external noise and interference η(B,I,T).
Spectral methods of analysis are especially
sensitive to signal noise, in particular, methods of
Figure 1: Model of the measuring channel in the study of the galvanomagnetic properties of the
material.
As a result, the measured Hall voltage will A special cryostat design, where the sample is
consist of a useful signal and noise, according to clamped between massive copper plates [2], and
the expression the measurements are carried out at a minimum
𝑈𝐻(𝐵, 𝐼, 𝑇) = 𝑈𝐻 + (1) sufficient current, at which there is no noticeable
+𝑆𝑖(𝐵, 𝐼, 𝑇)𝐺(𝐵, 𝐼, 𝑇) + 𝜂(𝐵, 𝐼, 𝑇), heating of the sample, the power released must not
where UH(B,I,T) is the Hall voltage signal with exceed a few mW, has been developed to deal
both linear and nonlinear distortion. A similar with the occurrence of uncontrolled temperature
situation will be in the study of other gradients. Low currents lead to a decrease in the
thermoelectric and photoelectric properties useful signal, but they can effectively deal with
associated with voltage measurement. the uncontrolled heating of the sample.
In thermoelectric materials, parasitic effects The automatic compensation circuitry has
that make the main contribution to the error been designed to eliminate the error associated
include the EMF of nonequipotentiality, with the nonequipotentiality voltage. In the
thermoelectric and thermomagnetic effects, absence of a magnetic field and a given current
which, even with a small change in the flowing through the sample, the
temperature gradient, significantly distort the nonequipotentiality voltage is measured by the
result due to the large coefficient of thermo-EMF. analog-to-digital converter ADC and
compensated using the compensation circuit on magnetoresistance do not change with time, but
the operational amplifier, which is guided by the with a change in the magnitude of the magnetic
voltage from the DAC. induction, the speed of the experiment, and
In the developed hardware-software complex, therefore the maximum frequency of the
several stages of data filtering are performed, in measured signal, can be controlled. This system
particular, at the first stage, the signal is amplified, can significantly reduce the noise component in
brought to the ADC range and passed through the the signal (Fig. 2).
hardware low-pass filter. After digitizing the Both modeling of the effect of noise on the
signal, a median filter is applied to the data results and experimental studies on real test
obtained from the ADC. This filter gets rid of the samples have been carried out to study the
random spikes associated with the operation of the effectiveness of noise control and the effect of
ADC. The third step is to apply a digital low-pass digital filtering on the parameter determination
filter to the entire measured relationship. To error. As a result of using the developed system,
reduce the noise component in the measured data, it was possible to significantly reduce the error in
including the quantization noise level, a digital determining the concentration and mobility of
low-pass filter with a finite impulse response charge carriers, in particular in the presence of
based on the Blackman weight function is used several types of charge carriers, the error in
[3]. The use of a low-pass filter is due to the fact determining the concentration and mobility of
that the measured signals change at a rather low heavy holes decreased by 4 times, light holes and
frequency, tenths and hundredths of hertz. electrons – by 1.5-2 times.
Considering that the Hall voltage and
a b
Figure 2: Dependence of the Hall voltage on the magnetic field: noisy (a) and after digital filtering (b).
The design of the cryostat provides for the measure thermal conductivity, heat capacity,
presence of a gradient heater and a differential thermoelectric figure of merit, and other
thermocouple to determine the thermo-EMF quantities. The combination of direct and indirect
coefficient and the Nernst-Ettingshausen methods makes it possible to determine electrical
coefficient by direct methods. Along with direct conductivity, carrier concentration, Seebeck
methods, indirect methods based on the modified coefficient, Nernst-Ettingshausen coefficient,
Harman method [4] and impedance spectroscopy magnetoresistance, thermal conductivity, as well
[5,6] are implemented in the measuring complex as thermoelectric figure of merit, and to carry out
for the complete characterization of express diagnostics of finished thermoelectric
thermoelectric material and automated express energy conversion modules on one sample in one
diagnostics of thermoelectric elements. These technological cycle.
methods are indirect measurement methods and
are favorably distinguished by a short experiment 3. Hardware and software tools for
time and do not require complex and laborious
measurements of heat fluxes through the sample. research implementation
This combination of methods made it possible to
solve two main problems of classical methods, The set of methods for complex nondestructive
namely, the need for accurate measurement of research of thermoelectric parameters of
heat fluxes through the sample and the need to semiconductors determines the characteristics of
prepare samples of various configurations to the system that it implements, namely, the amount
of input information, the speed of information operating systems. When constructing a system
receipt and the processing time of the input for complex nondestructive research of
information. thermoelectric parameters of semiconductors, it is
The process of measuring the parameters of the necessary to determine the solution of which
sample determines the maximum amount of input problems will be implemented in hardware, and
information and the maximum rate of its entry into which ones – by specialized and universal tools.
the system. The largest amount of input The criterion for this can be the comparison of the
information will be in the implementation of given time for solving the problem and the time
galvanomagnetic methods, and the maximum data for solving it by hardware or software and
input speed will be in the implementation of the hardware tools, which is given in [7].
impedance spectroscopy and will be no more than Taking this into account, the system that
20х106 counts/s. ensures the implementation of the set of these
The minimum processing time of the input methods is implemented as a three-level
information is determined by the process of specialized computer system (Fig. 3), optimized
controlling the operating conditions of the test to obtain the maximum number of parameters that
sample, that is, the formation of control signals to fully characterize the sample without destroying it
the actuators, and the shortest time between two and without the need to change the configuration
control signals will be when the Harman pulse of the sample for various research methods. This
method is implemented and will be 0.1 μs. concept, combined with modular structuring
That is, the system that ensures the techniques, makes it possible to design an
implementation of the set of these methods will information-measuring and control system that
not only be information-measuring, but also can be easily upgraded or expanded. At the lower
control at the same time. On the other hand, the level, the actuators are controlled to create the
implementation of the system should optimally necessary conditions for the experiment; sensor
combine the known approaches to the polling, analog processing and filtering of signals
construction of computer systems – purely are carried out. At the middle level, digital signal
hardware implementation and software and processing, control signal generation, and self-
hardware implementation. The hardware diagnostics are performed. Depending on the
implementation provides maximum performance, required performance, this level can be
but it requires redesigning each time in case of implemented both on a single microcontroller and
changes in the system's algorithm. The hardware on an FPGA or their combination. The upper level
and software implementation as a whole has a is implemented in software on a standard personal
lower performance, but its operation can only be computer, which made it possible to develop a
reprogrammed when the operation algorithm is convenient graphical interface for control and
changed. In turn, software and hardware tools are visualization of results. In addition, the transfer of
divided into universal and specialized – all calculations and simulations to the PC makes
microprocessor tools that work without operating it easy to expand the capabilities of software data
systems, and microcomputer tools that run under processing without interfering with the hardware.
Figure 3: General concept of a specialized computer system for comprehensive thermoelectric and
photoelectric research.
Guided by the principles of modular parameters by analyzing the scatter of the
structuring, all the implemented methods for received data, which reduces the probability of
studying the properties of semiconductors are receiving incorrect data and outliers.
combined into groups and implemented as For automated processing and visualization of
separate subsystems. the results, a software analytical module has been
In particular, the subsystem for studying the developed for the application of physical and
galvanomagnetic properties of semiconductors mathematical models to determine the main
implements the classical methods of Hall thermoelectric parameters, including those that
measurements in constant magnetic fields, cannot be measured directly, such as the
measurements of thermo-EMF, electrical concentration and mobility of charge carriers, and
conductivity, Hall coefficient, magnetoresistance, the reconstruction of the profiles of these
and Nernst-Ettingshausen coefficient. parameters over the sample thickness [9,10].
The subsystem for express studies of Approximation and fitting were carried out by the
thermoelectric elements implements pulse least squares method using the algorithm for
methods and, for sufficiently thin film samples, minimizing functions of many variables by the
requires high frequencies and, accordingly, high Nelder-Mead deformed polyhedron method. The
speed of the system for generating and processing analytical module also provides automatic
signals. In addition, a highly stable source of both decoding of spectrograms and determination of
DC and AC of 10 μA to 500 mA, with a frequency thermoelectric parameters that are difficult to
of up to 2 MHz [7], and voltage measurement measure directly, for example, thermal
from 1 μV up to 1 V with a resolution of 12 bits, conductivity and thermoelectric figure of merit, as
up to 100 Mps with noise filtering and taking into well as automatic diagnostics and defects
account errors from parasitic physical processes identification of thermoelectric elements.
has been implemented. Also, based on the
obtained data and the adaptive algorithm, which 4. Conclusions
compares not only the absolute values, but also
their deviations from those typical for a given
The analysis has been carried out, methods for
series, defects identification of the studied
element with the determination of the probable studying the thermoelectric properties of
type of defect is provided [7]. semiconductors have been selected and adapted,
The generation and synchronous detection of and a computer system has been implemented,
signals with a frequency of up to 2 MHz and their which makes it possible to obtain all the necessary
mathematical processing based on fast Fourier parameters of the test sample in one technological
transforms to determine the amplitude and phase cycle on a sample of one configuration by
nondestructive methods, which several times
shift between them have been implemented for the
reduces the time for preparing and conducting an
subsystem for studying thermoelectric properties
based on impedance spectroscopy [8]. The main experiment.
task of fast digitalization of an analog signal is A special design of a cryostat and a sample
solved using a high-speed analog-to-digital holder, an algorithm and a circuit for
converter, for example, AD9643 (Analog compensation of nonequipotential voltage have
Devices, USA), which has two independent high- been developed, which makes it possible to
quality sample-hold devices and a built-in minimize parasitic effects during Hall
reference voltage source. Generation of a signal of measurements. It is shown that the use of digital
the required frequency, filtering and mathematical signal filtering algorithms to effectively reduce
processing of data have been implemented on the the noise component in the measured data has
FPGA, and a 32-bit microcontroller has been made it possible to reduce the error in determining
chosen for signal generation, control of switching the concentration and mobility of charge carriers
and operation of operational amplifiers, and by 2-4 times.
communication with a computer [7,8]. The use of indirect methods for studying
For analyzing the quality of contacts and self- thermoelectric properties made it possible to
diagnostics, algorithms and a subsystem for avoid the need to measure heat fluxes, to
automated analysis of the ohmicity of contacts implement algorithms for fast diagnostics of
have been developed, in particular, for analysing thermoelements and to reduce the error in
the I–V curve for linearity by software methods, measuring thermoelectric figure of merit by 1.5-2
times.
for detecting breakage and instability of contact
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