Design of Advanced Thermoelectric Materials
Shaabani, Laaya
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Publication date: 2018
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Shaabani, L. (2018). Design of Advanced Thermoelectric Materials. Rijksuniversiteit Groningen.
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Chapter
2
2
2.1
Synthesis details
Polycrystalline Ge1-xNaxSe (Pb1-xCexSe and Nax(Pb1-xTe)0.55(Pb1-xSe)0.35(Pb1-xS)0.1) samples were prepared using a melt alloying technique. The elements used as starting materials were Ge (99.999%, Alfa Aesar), Se (99.999%, Alfa Aesar), Pb (99.999%, Alfa Aesar), Te (99.999%, Alfa Aesar), S (99.999%, Alfa Aesar), Ce (99.9%, Aldrich) and Na (99%, Aldrich). The elements were weighed in an argon atmosphere glove box according to stoichiometric amounts of each composition with a total mass of 10 g and transferred to a carbon-coated quartz tube (as described below) and sealed under vacuum (10-4 Torr). The sealed tube was heated to 1223 K (1323 K) over 12
hours and held at that temperature for 10 hours. The samples were then quenched in cold water, followed by annealing at 673 K (823 K) for 72 hours. The resulting ingots were taken out of the tubes and then hand ground thoroughly for 1 h using an agate mortar and pestle to obtain a fine powder. The powder was loaded into a 12 mm diameter graphite die. The powders were then sintered using Spark Plasma Sintering (SPS) at 623 K for 30 minutes (793 K for 1 hour) under an axial pressure of 40 MPa in vacuum.
2.1.1 Carbon coating
Carbon coating process requires the following steps:
1. Cleaning the inside of the quartz ampule using a series of rinses with water to minimize impurities
2. Heat treatment of the tubes to remove any possible organic residue 3. Coating the tube with carbon by heating the tube rinsed with acetone,
doing this step for 3 times
2.1.2 Spark plasma sintering
The SPS process is the method used to compact dense pellets from powders. This technique which is a fast sintering technique,1,2 results in achieving highly dense
samples at lower processing temperature compared to conventional sintering techniques.3 SPS can produce materials with nano size grains which is beneficial to
significantly enhance thermoelectric properties.4-10 It simultaneously applies electric
current and mechanical pressure to consolidate powders with desired density. The SPS container (punches, mould and spacers) is made of graphite. The SPS process and geometrical configuration of the punches, mould and powder are illustrated in figure 2.1. Powders to be consolidated, are placed in a die and heated by applying the electric current.
2
2.2
Structural and chemical characterization
2.2.1 Powder X-ray diffraction (PXRD)
Powder X-ray diffraction (PXRD) is a rapid analytical method for phase identification and structure characterization of powder polycrystalline materials. The measurements in this thesis were carried out with GBC Scientific X-ray diffractometer and Bruker D8 Advance diffractometer operating with Cu (Kα) radiation at room temperature. The 2θ scans were taken from 10 to 1000 with a step size of 0.020 and an integration
time of 1 s per step. Phase analysis and structural refinements were performed on the obtained XRD data by the Rietveld method using the GSAS (General Structure Analysis System) software package.11
2.2.2 Scanning electron microscopy and energy-dispersive X-ray spectroscopy
Scanning electron microscopy (SEM) is a technique to analyze surface morphology and chemical composition. It provides images based on the interaction of a focused beam of high-energy electrons with the sample surface. The interaction of the focused accelerated electrons with the exposed area of the sample gives rise to various electron signals. These signals include secondary electrons (SE), backscattered electrons (BSE) and diffracted backscattered electrons (EBSD). SEM most commonly uses the secondary electrons and backscattered electrons for imaging the sample. SEs are produced by atoms near the surface of a sample when the high energy electron beam excites an electron from one of the constituent atoms of the sample. Images of the surface morphology and topography can then be obtained. Backscattered electrons are elastically scattered electrons that are backscattered from the surface of the sample and they are sensitive to the atomic mass of the nuclei that they scatter from. These electrons are used for imaging contrast between different phases and chemical compositions in the sample. The inelastic collision of the incident electrons with the sample results in the emission of characteristic x-rays from the different elements present in the sample, which are used for elemental analysis. An energy dispersive x-ray (EDS) analyzer integrated with the SEM can be used for qualitative
and quantitative analysis of the different elements of the sample. The SEM images presented in this thesis were obtained using a JEOL JSM-7001 scanning electron microscope (SEM) combined with an energy-dispersive X-ray (EDS) spectrometer.
2.3
Magnetic properties measurement
The magnetic properties of the samples in this thesis were probed using a Quantum Design MPMS (magnetic properties measurement system) XL7 magnetometer. A SQUID (superconducting quantum interface device) is used to measure the magnetic dipole moment of a sample as a function of the temperature and the field. The operation temperature of the MPMS varies from 2 K to 350 K with a maximum magnetic field of ±7 T. Magnetic moments as low as 10-7 emu can be measured in the
MPMS. The temperature dependent magnetization measurements were conducted using zero field cooled (ZFC) and field cooled (FC) modes. In the ZFC mode, the studied sample was cooled to 5 K without an external magnetic field. Then, the magnetic field was applied and the magnetic moment as a function of temperature was measured while heating up the sample from 5 to 300 K (for high temperature measurements, the sample was heated from 300 K to 780 K). FC measurements were carried out in the same way, but the sample was first cooled down to 5 K in the presence of an applied magnetic field and measured on heating using the same applied field. Measurements of magnetization versus applied field were also carried out by applying field from 0.5 T to -0.5 T at constant temperature.
2
where e is the fundamental charge of the electron and RH is the Hall coefficient. The Hall mobility (μ) was calculated from:
μ = σ/ne = σRH
where σ is the electrical conductivity.
2.5
High temperature thermoelectric properties measurement
2.5.1 Seebeck coefficient and electrical resistivity measurement
The Seebeck coefficient is a fundamental electronic transport property of a material which describes the magnitude of a thermoelectric voltage built up when a temperature difference is applied across that material. This induced voltage can be measured, and with a known temperature gradient, the Seebeck coefficient is calculated. All high temperature Seebeck coefficient and electrical conductivity measurements were carried out simultaneously under helium atmosphere using a Linseis LSR-3 setup (figure 2.2a). The temperature dependence of the Seebeck coefficient and electrical conductivity can be measured on a cylindrical or bar shaped sample (at least 6 mm in length). Figure 2.2b shows the configuration of the thermocouples and sample.
2
Figure 2.3 illustrates the schematic of the measurement setup in the Linseis LSR-3. A sample is placed in a vertical position between the electrodes in the heating furnace. There are two source of heating in this setup. The primary furnace covers the entire measurement assembly and provides a specific temperature at which the measurement is taken. Once the sample has been heated and held at the desired temperature, a secondary heater placed in the lower electrode block generates a temperature gradient across the sample. The Seebeck coefficient is determined by measuring the upper and lower temperatures, T1 and T2, with the thermocouples in contact with the sample, and the electromotive force, dE, generated between the same wires in response to the temperature gradient. The multiple gradient method is implemented here. Based on the slope derived from the linear fitting of Seebeck voltage versus temperature gradient at a constant mean temperature, the absolute Seebeck coefficient of the sample is calculated as:
The electrical resistance at each specific temperature is measured using the dc 4-terminal method by applying a constant current between the ends of the sample and measuring the voltage drop, dV, between the same thermocouples connected to the sample by subtracting the thermo-electromotive force between leads. The electrical resistivity is then calculated from ρ = RA/l, where A is the area of the part of the sample in contact with the electrodes and l is the distance between the probe thermocouples.
Figure 2.3: Schematic illustration of measurement setup in Linseis LSR-3.12
2.5.2 Thermal conductivity measurement
Thermal conductivity is a property of a material that describes its ability to conduct heat. Here temperature dependent thermal conductivity was determined by the laser flash technique using a Linseis LFA 1000 (figure 2.4) and a Netzsch LFA at Tongji University and the University of New South Wales, respectively. The laser flash method a widely used technique for determining thermal conductivity by measuring thermal diffusivity at high temperatures. In this technique a short laser pulse or light pulse from a xenon flash lamp illuminates the bottom side of a disc-shaped sample and the temperature response on the top side of the sample is measured as a function of time (figure 2.5). The thermal diffusivity is calculated from the sample thickness and the time required to reach half of the maximum temperature increase. The thermal conductivity, κ, is then calculated by
2
Figure 2.4: The Linseis LFA 1000 system for high temperature thermal conductivity
measurement.13
Specific heat capacity can be estimated from
1. Experimental measurements 2. Dulong-Petit law
For this thesis study on Pb chalcogenides, heat capacity was estimated from the relation15,16
Cp (kB per atom) = (3.07+4.7 ×10-4(T-300))
based on experimental results that are consistent with theoretical calculated values within 2% error6 and is believed to be accurate for lead chalcogenides17-19, where T is
the temperature in Kelvin and kB is Boltzmann’s constant. Similarly, for the study of GeSe materials, this quantity was obtained from the relation20
Cp(GeSe, (298.15-940) K)=(46.777+15.099 ×10-3T-0.0316 ×10-6T2-1.231× 105 T-2)J.K-1.
2
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