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Thermoelectric Materials Material Characterization, Phase Changes, Thermoelectricity – Figure of Merit (ZT), and Generation

Thermoelectricity – Thermoelectric Materials and Devices

Thermoelectricity refers to a class of energy from heat which has been Figure of Merit phenomena in which a released to the environment. For such difference creates an electric potential applications, thermoelectric materials Beneficial thermoelectric materials or an electric potential creates a with high working and should possess high Seebeck coeffi- temperature difference. In modern optimized efficiency need to be cients, high electrical conductivity and technical usage, the term refers developed. low thermal conductivity. A high collectively to the Seebeck effect, electrical conductivity is necessary to Peltier effect, and the Thomson effect. Very often, accurate knowledge of minimize Joule heating, while a low Various metals and certain thermal properties such as thermal conductivity helps to retain are generally employed in these thermal stability, thermal diffusivity or heat at the junctions and maintain a applications. One of the most thermal conductivity is of paramount high temperature gradient. These commonly used materials in such importance in the application of these three properties all factor into what is applications is telluride new materials. Measurements are known as the “figure of merit”, Z. Since

(Bi2Te3). necessary to resolve problems Z varies with temperature, a useful regarding a variety of issues, such as dimensionless figure of merit can be Over recent decades, efforts have been heat transfer within a given structure defined asZT . The dimensionless made to improve the efficiency of or the formation of thermally-induced figure of merit is calculated as follows: thermal processes and the output stresses between two different 2 energy of thermal engines. One materials which are in contact with ZT = (––––)S σ T approach is to generate electrical each other. λ where: S = or thermo power [µV/K] σ = electrical conductivity [1/(Ωm)] λ = total thermal conductivity [W/(m·K)].

2 ZT is a very convenient figure for (a) (b) comparing the potential efficiency of p-Type zT n-Type zT 2.0 2.0 devices constructed of different 1.8 PbTeSe 1.8 materials. Values of ZT = 1 are 1.6 1.6 PbTe considered good, but values in at least 1.4 PbTe 1.4 TAGS 1.2 1.2 Sb Te Yb MnSb Bi Te the 3-4 range would be considered 2 3 PbSe 14 11 2 3 La3Te4

zT 1.0 zT 1.0 PbTe CeFe4Sb12 CoSb SiGe essential in order to be competitive in (1960) 3 0.8 0.8 terms of efficiency with regards to 0.6 PbTe 0.6 (1960) mechanical energy generation and 0.4 SiGe 0.4 . To date, however, such 0.2 0.2 0.0 0.0 values have not been achieved; the 0 200 400 600 800 1000 0 200 400 600 800 1000 Temperature (°C) Temperature (°C) best reported ZT values have been in the 2-3 range. Approximate figure of merit (ZT) for various p-type and n-type thermoelectric materials. Source: G. Jeffrey Snyder, California Institute of Technology http://thermoelectrics.caltech.edu. Reproduced with permission.

Thermoelectric Generation Thermoelectric Cooling

The simplest If an electric current is applied to the consists of a thermocouple as shown, heat is () comprising a p-type and pumped from the cold junction to the an n-type connected hot junction. The cold junction will electrically in series and thermally in rapidly drop below ambient tempera- parallel. Heat is pumped into one side ture provided heat is removed from of the couple and discharged from the the hot side. The temperature gradient opposite side. An electrical current is will vary according to the magnitude produced which is proportional to the of generation current applied temperature gradient between the (Seebeck effect). hot and cold junctions.

Heat Cooled Source Surface

N – P + N – P + – + – +

Cool Dissi- Side pated Heat + –

Principle of thermoelectric generation Principle of thermoelectric cooling (Seebeck effect) (Peltier effect)

3 Thermophysical Properties – Laser Flash Method and Models

Thermoelectric Materials – The Focal Determination of Thermophysical Point for Energy Savings Properties

Novel thermoelectric materials have The thermal stability (analyzed with The world is turning its eye to the study already resulted in a new consumer TGA or DSC) yields information on the of thermoelectric energy. Thermo- product: a simple, efficient way of maximum service temperature. electric energy conversion is an cooling car seats in hot climates. The environmentally friendly solid-state devices, similar to the more familiar car Differential scanning calorimetry (DSC) technology with many promising seat heaters, provide comfort directly can further be used for the analysis of applications. One of the more straight- to the individual rather than cooling phase transitions or the specific heat forward of these lies in waste-heat

the entire car, saving on air-condi- capacity (cp). recovery in cars and trucks; the energy tioning and energy costs. now being lost from hot engines could Finally, DIL or TMA methods are used save billions of dollars if captured and To optimize a thermoelectric device, its to characterize the thermal and converted into electricity. thermal properties must be known. volumetric expansion of materials and The thermal conductivity (analyzed their change, allowing for the Research into the electrical and with LFA) is directly related to the analysis and prediction of thermal thermophysical properties of new efficiency of a thermoelectric material. stresses in a real device. materials could reduce the world’s reliance and overdependence on fossil fuels and has shown promise with two classes of materials: low-dimensional systems for enhanced electrical properties and materials with increased scattering which to inherently low thermal conductivity.

Thermophysical Properties

Light/Laser Flash Analysis (LFA) Differential Scanning Calorimetry (DSC) Dilatometry (DIL)

furnace detector LVDT protective system tube furnace furnace sample sample refer- thermo- sample ence couple QP heating R sample pushrod power element ΔT holder source sample

Thermal Diffusivity (a) Specific Heat (cp) Density (ρ) Specific Heat (cp) Coefficient of Thermal Expansion

λ(T) = a(T)·cp(T)·ρ(T) λ Thermal Conductivity

4 Light/Laser Flash Method LFA Analysis Information LFA 467 HyperFlash

The LFA method is illustrated in the The Laser Flash (LFA) technique is a The LFA 467 HyperFlash is a Xenon figure on page 4, on the left. The front fast, non-contact, absolute method flash-based system with a compact surface of a plane-parallel sample is for determining a complete set of design. It covers the temperature heated by a short light or laser pulse. thermophysical properties, including range from -100°C to 500°C. A variety The resulting temperature rise on the thermal diffusivity, specific heat of cooling options allow for measure- rear surface is measured versus time capacity and thermal conductivity. ments to be carried out across the full using an IR detector. The thermal This data can then be used: temperature range of the instrument diffusivity (a) and in most cases also without having to change either the the specific heat (cp) can be deter- As input data for numerical furnace or the detector. mined from the measured signal. ··simulations For optimization of thermoelectric The integrated automatic sample If the density (ρ) is known, the thermal ··materials with low lattice changer allows for unattended conductivity (λ) can be determined as conductivity and a high figure of analyses on up to 16 samples. The follows: merit. gas-tight design minimizes the risk of oxidation for measurements on

λ(T) = a (T) · cp(T) · ρ(T) oxygen-sensitive samples. where: The very high sampling rate and the λ = thermal conductivity [W/(m·K)] ρ = bulk density [g/cm3] unmatched ZoomOptics make for a c = specific heat [J/(g·K)]. p broad application range, even for thin samples.

LFA 467 HyperFlash

5 LFA for Fast and Reliable Thermal Diffusivity Measurements on Small and Thin Samples – LFA Software

LFA 457 MicroFlash®

The LFA 457 MicroFlash® incorporates The instrument accommodates both the latest state-of-the-art technology smaller and larger sample sizes (of up for laser flash systems. This bench-top to 25.4 mm in diameter) and with the instrument allows for measurements integrated sample changer, measure- from -125°C to 1100°C using two ments can be run on several samples different interchangeable furnaces. at the same time. The temperature increase on the back surface of the sample can be measured The vacuum-tight design allows for at very low sub-ambient temperatures tests under defined atmospheres. The thanks to the innovative infrared vertical arrangement of the sample sensor technology. holder, furnace and detector facilitates sample placement and simultaneously guarantees an optimum signal-to- noise ratio for the detector signal.

LFA 457 MicroFlash® LFA 427

The vacuum-tight LFA 427 is our most powerful and versatile LFA system, ideal for any application involving the characterization of standard and high-performance materials. The LFA 427 guarantees high precision and reproducibility, short measurement times and defined atmospheres over the entire application range from -120°C to 2800°C. The laser, sample and detector are configured in such a way as to function without the use of any mirrors.

The laser power, pulse width, gas and vacuum functions can all be set variably over a wide range, making it possible to effectuate optimum measurement conditions for a great LFA 427 variety of sample properties.

6 LFA Software

The Laser Flash systems run under Proteus® software on a Windows® operating system. The combination of easy-to-understand menus and automated routines makes this software very user-friendly while still allowing for sophisticated analysis. The LFA software includes:

Calculation models for thermal ··diffusivity: ··Adiabatic ··Cowan ··Improved Cape-Lehman (via consideration of multi- dimensional heat loss and non-linear regression) 2-/3-layer models (analysis by ·· LFA software including heat loss and pulse length correction. means of non-linear regression and consideration of heat loss) ··In-plane ··Radiation correction (for via various mathematical functions transparent and semi-transparent (polynomials, splines, etc.) samples) ··Heat-loss corrections Model wizard for selecting the ··Accurate pulse length correction, ··optimum evaluation model patented pulse mapping (patent no.: US7038209B2; Definition of an arbitrary number of US20040079886; DE1024241) ··temperature levels and number of ··Baseline correction shots per level

Determination of contact resistance Determination of specific heat by ··in multi-layer systems ··means of a comparative method and standard samples Ability to average shots at the same ··temperature level ··Integrated database ··Approximation of shots as a curve

7 Thermal Analysis – DSC and STA; Coupling to Evolved Gas Analysis

Heat Flow DSC Method Differential Scanning Calorimetry Thermogravimetric Analysis and Simultaneous Thermal Analysis

Based on a homogeneous temperature Differential scanning calorimetry (DSC) Simultaneous Thermal Analysis (STA) field in the DSC furnace, equal heat is one of the most frequently employed refers to the simultaneous application flows along the disc-shaped sensor are Thermal Analysis methods. It can be of Thermogravimetric Analysis (TGA) directed to the sample and reference used to analyze nearly any energetic and Differential Scanning Calorimetry crucibles. If the heat capacities on the effect occurring in a solid or liquid (DSC) to one and the same sample in a sample and reference sides differ, or if during thermal treatment. single instrument. The test conditions the sample shows a changed heat are of course identical for the two absorption or resulting difference in For standard applications between signals, since they share the same heat flow causes temperature gradients -180°C and 700°C, the easy-to-use atmosphere, heating rate, thermal at the sensor. Sensitive sensors record DSC 204 F1 Phoenix® can be used. contact to the sample crucible, etc. these temporary deviations, which are The sample throughput is also then depicted as either exothermal Our other models, the vacuum-tight improved, as more information is peaks, endothermal peaks or steps in DSC 404 F1/F3 Pegasus® systems, are gathered from each test run. the differential heat flow curves. designed for the precise determination of the of high- The STA 449 F1/F3 Jupiter® systems performance materials under defined are designed for measurements in the atmospheres. They can be configured temperature range from -150°C to for different furnace and sensor types 2400°C. Various interchangeable TGA, furnace and are easily interchanged by the user DTA and DSC sensors and furnaces

sample refer- to cover a broad temperature range make these vacuum-tight models to ence from -150°C to 2000°C. one of our most popular products. QP R

ΔT

Differential Scanning Calorimetry (DSC)

DSC 204 F1 Phoenix®

8 STA Analysis Information Combines Analysis Information from DSC and TGA

DSC Analysis Information TGA Analysis Information Standards

Specific heat capacity (cp) Mass changes Our DSC and STA systems are designed ··Melting/crystallization behavior ··Temperature stability in accordance with the majority of ··Solid-solid transitions ··Oxidation/reduction behavior instrument and application standards ··Polymorphism ··Decomposition for DSC and TGA systems, including: ··Degree of crystallinity ··Corrosion studies ISO 11357, ISO 11358, ASTM E967, ··Glass transitions ··Compositional analysis ASTM E968, ASTM E793, ASTM D3895, ··Cross-linking reactions ··TGA data as base for thermokinetic DIN 51004, DIN 51006, DIN 51007. ··Oxidative stability ··analysis (NETZSCH Thermokinetics ··Purity Determination software program) ··DSC data as base for thermokinetic ··analysis (NETZSCH Thermokinetics software program) Coupling to Evolved Gas Coupling Analysis Information Analysis

First-rate research and characterization Compositional analysis can be achieved by coupling thermo- ··Evaporation/sublimation analytical instrumentation for TGA, ··Decomposition/thermal stability DSC, STA, TMA or DIL to a mass ··Pyrolysis/combustion spectrometer (MS), gas chromato- ··Solid-gas reactions graph-mass spectrometer (GC-MS) or ··Desorption/absorption Fourier Transform Infrared spectrom- ·· eter (FT-IR). These techniques can also include simultaneous coupling of the MS and FT-IR or GC-MS and FT-IR to the thermal analyzer. The unique adapter allows for coupling even when used in combination with an automatic sample changer.

For molecules which are highly condensable (e.g., metal vapors), the STA 409 CD with SKIMMER® system is available, which allows for maximum temperatures of either 1450°C or 2000°C.

STA 449 F1 Jupiter® coupled to STA 409 CD QMS 403 C Aëolos® with SKIMMER®

9 Thermal Analysis – DIL and TMA

Dilatometry and Thermomechanical Dilatometer Method Analysis

Dilatometry (DIL) is used to measure In dilatometry, dimensional changes A pushrod is positioned directly the expansion or shrinkage of solids, are determined versus temperature or against the sample and transmits the powders, pastes or liquids under time while the sample is subjected to a length change to a linear variable negligible load. It is closely related to controlled temperature program. The displacement transducer (LVDT). As thermomechanical analysis (TMA), degree of expansion divided by the the sample length changes during the which determines dimensional change in temperature is called the temperature program, the LVDT core is changes under a defined mechanical material’s coefficient of expansion (α) moved, and an output signal propor- force. and generally varies with temperature. tional to the displacement is recorded. The temperature program is controlled We offer dilatometer systems for using a thermocouple located either 1 Δl measurements in the temperature α = ––– (–––) next to the heating element of the L ΔT range between approx. -260°C and 0 furnace or next to the sample. 2800°C. where: α = coefficient of expansion For the investigation of thermoelectric LVDT furnace L0 = initial sample length materials, either our DIL 402 C or our Δl = change in length system ΔT = change in temperature. dual/differential DIL 402 CD (-180°C to 2000°C) may be used. The specific needs of this application field are served by our interchangeable sample pushrod furnaces, protective tubes, and holder sample pushrods made of a variety of materials. Dilatometry (DIL)

The TMA 402 F1 and F3 Hyperion® are based on a modular concept, with interchangeable furnaces covering the temperature range from -150°C to 1550°C. The broad range of force can be varied between -3 N and 3 N.

DIL 402 C

10 TMA Method DIL and TMA Analysis Information Standards

Irrespective of the selected type of Linear thermal expansion All NETZSCH dilatometer and thermo- deformation (expansion, compression, ··Coefficient of thermal expansion (CTE) mechanical instruments are designed penetration, tension or bending), any ··Volumetric expansion in accordance with standards such as length change in the sample is ··Shrinkage steps DIN EN 821, DIN 51045, ASTM E831, communicated to a highly sensitive ··Glass transition temperature ASTM E228, ASTM D696, ASTM D3386, inductive displacement transducer ··Phase transitions ISO 11359 -Part 1 to 3. (LVDT) via a pushrod and transformed ··Sintering temperature/sintering step into a digital signal. The pushrod and ··Density change corresponding sample holders of fused ··Softening points silica or aluminum can be quickly ··Influence of additives/raw materials and easily changed out to optimize the ··Decomposition temperature system to a given application. ··Anisotropic behavior ··Caloric effects by using c-DTA® ··DIL/TMA data as base for ··thermokinetic analysis (NETZSCH Thermokinetics software program)

furnace

sample

push rod

force sensor sample carrier

F height setting actuator (static or displacement modulating) transducer

Thermomechanical Analysis (TMA)

TMA 402 F1 Hyperion®

11 Applications – Increasing the Figure of Merit, ZT

Targeting Low-Lattice Conductivities for Increased Figure of Merit, ZT

Nanostructured bulk thermoelectrics The lattice thermal conductivity is then have an enhanced thermoelectric calculated from: figure of merit ZT( ). The thermal

conductivity is reduced due to phonon λlattice = λtotal - λelec. scattering at nanoscale interfaces. The contribution to thermal conduc- The thermal diffusivity (a), density (ρ), tivity by charge carriers is described by

and specific heat (cp) are measured and the Wiedemann-Franz Law: used to calculate the total thermal

conductivity using the formula: λelec = L0 · σ · T

λ = a · ρ · cp. where L0 is the Lorenz number and σ the electrical conductivity.

Thermal Conductivity and 3.5 Separated Lattice Conductivity 1.6 of Ag Pb MTe 1.4 1-x 18 20

3.0 K)) . 1.2 K))

. This plot shows measurements carried 1.0

(W/(m out on Ag1-xPbMTe20 (M=Bi, Sb) in the

2.5 latt 0.8 K temperature range from 150°C to 0.6 tot 370°C. The lattice conductivity can be 0.4 2.0 300 400 500 600 700 calculated from the measured thermal Temperature (K) conductivity. Here, the temperature

1.5 dependence of the total thermal latt Thermal Conductivity / (W/(m conductivity (λtot, ■) and lattice thermal conductivity (λ , ) of AgPb BiTe is 1.0 latt 18 20 illustrated.

0.5 The inset indicates the temperature 300 400 500 600 700 Temperature (K) dependence of the lattice thermal

conductivity of Ag1-xPb18BiTe20 (x = 0, 0.3), compared with the lattice thermal Ag1-xPb18MTe20 (M = Bi, Sb); published by Kanatzidis et al., Northwestern University, IL, USA [1]. Measurements were carried out using the NETZSCH LFA 457 MicroFlash®. The typical sample dimension conductivity (λlatt) of AgPb18BiTe20 with a diameter of 12.7 mm and thickness of 2 mm was used. (presented in + symbol).

[1] Han, M.K.; Hoang, K.; Kong, H.; Pcionek, R.T.; Uher, C.; Paraskevopoulos, K.M.; Mahanti, S.D.; Kanatzidis, M.G., Chemistry of Materials (2008), 20(10), 3512-3520

12 PbTe-Ge and PbTe-Ge1-xSix

In the telluride materials PbTe-Ge Plot A shows that Ge has a significant reduction of the lattice conductivity can and PbTe-Ge1-xSix the thermal conduc- influence on the lattice conductivity of be observed (plot B). Similar behavior can tivity is easily tuned by alloying Ge with PbTe. With a decreasing Ge content, be seen by keeping a constant mixing Si and reducing the Ge content [2]. The the lattice conductivity decreases over ratio of Ge and Si while decreasing results shown below are obtained in the entire temperature range. In Ge0.8Si0.2 content (plot C). Plot D shows the temperature range between 25°C addition, by alloying Ge with Si of the that an amount of 5% Ge-/Ge-Si achieves and 320°C. PbTe-Ge (20%) composition, a further an optimum lattice thermal conductivity.

2.4 2.4 PbTe - Ge (20%) PbTe - Ge (20%) PbTe - Ge Si (20%) PbTe - Ge (10%) 0.95 0.05 2.2 2.2 PbTe - Ge Si (20%) PbTe - Ge (5%) 0.80 0.20 PbTe - Ge Si (20%) 2.0 0.70 0.30 2.0 K)) K)) . . 1.8 1.8

/ (W/(m 1.6 / (W/(m 1.6 lattice lattice λ λ 1.4 1.4 1.2 1.2 1.0 300 350 400 450 500 550 600 300 350 400 450 500 550 600 Temperature (K) Temperature (K)

A) Lattice Thermal Conductivity of PbTe-Ge (5%, 10%, 20%); LFA 457 B) Lattice thermal conductivity of PbTe-Ge1-xSix (20%); LFA 457 MicroFlash® MicroFlash® measurements. measurements.

2.0 2.5 PbTe - Ge Si (5%) 0.8 0.2 2.4 PbTe - Ge (X%) 1.8 PbTe - Ge Si (10%) 0.8 0.2 2.3 PbTe - Ge0.8Si0.2 (X%) PbTe - Ge0.8Si0.2 (20%) 1.6 2.2

K)) K)) 2.1 . . 1.4 2.0 1.9 / (W/(m 1.2 / (W/(m

lattice lattice 1.8 λ λ 1.0 1.7 1.6 0.8 1.5

250 300 350 400 450 500 550 600 650 0 5 10 15 20 Temperature (K) X%

C) Lattice Thermal Conductivity of PbTe-Ge0.8Si0.2; LFA 457 MicroFlash® D) Lattice Thermal Conductivity of PbTe-Ge and PbTe-Ge0.8Si0.2 under measurements. increasing Ge0.8Si0.2 content; LFA 457 MicroFlash® measurements.

[2] Sootsman, Joseph R.; He, Jiaqing; Dravid, Vinayak P., Li, Chang-Peng; Uher, Ctirad; Kanatzidis, Mercouri G. High Thermoelectric Figure of Merit and Improved Mechanical Properties in Melt Quenched PbTe – Ge and PbTe – Ge1-xSix Eutectic and Hyper-eutectic Composites, J. Appl. Phys. (2009), 105, 083718.

13 Applications – Skutterudite with Nanoparticles, Thermal Expansion and Specific Heat of

Skutterudite Thermoelectrics Lattice Thermal Conductivity and

Figure of Merit of La0.9CoFe3Sb12

Cubic skutterudite materials of the form The effect of introducing a nanopar-

(Co,Ni,Fe)(P,Sb,As)3 have a potential for ticle layer in La0.9CoFe3Sb12 in order to high ZT values due to their high reduce the thermal conductivity was mobility and high Seebeck coefficient. investigated up to 550°C in this

Unfilled CoSb3-based skutterudites are example. The thermal conductivity (λ) disadvantaged by their inherently high was calculated by using the heat

thermal conductivity, which lowers their capacity (cp), which was predetermined ZT value. However, these materials with the DSC 404 F1 Pegasus®. The contain voids into which low-coordi- lattice thermal conductivity was found nation ions (usually rare earth elements) by calculating the electrical thermal can be inserted. These alter the thermal conductivity using the Wiedemann- conductivity by producing sources for Franz relationship and subtracting it

lattice phonon scattering and decrease from λtotal. thermal conductivity due to the lattice without reducing electrical conductivity. At 452°C (725 K), the ZT exhibits its This makes these materials behave like a maximum, and the 5 wt.-% nano- PGEC (phonon-glass, electron ). It composite shows the highest ZT with is proposed that in order to optimize ZT, an improvement of nearly 15% over which are responsible for that of the control sample which thermal conductivity must experience contains no nanoparticles (orange the material as they would in a glass dots). These results show that nanopar- (high degree of phonon scattering – ticles introduced into skutterudite lowering the thermal conductivity) while systems which are already optimized must experience it as they can further reduce the thermal would in a crystal (very little scattering – conductivity and therefore improve ZT maintaining the electrical conductivity). across a broad temperature range.

6 0,5 K)) . La0.9CoFe3Sb12 5 2% nanoparticles 0,4 5% nanoparticles 10% nanoparticles 4 0,3 ZT

3 0,2

La0.9CoFe3Sb12 2 0,1 2% nanoparticles 5% nanoparticles Lattice Thermal Conductivity / (W/(m 10% nanoparticles 1 0 0 100 200 300 400 500 600 700 800 300 400 500 600 700 800 Temperature (K) Temperature (K)

Systematic decrease of λlattice with increasing the wt-% of nanoparticles Clearly improved ZT by introduced nanoparticle layers up to 800K. (LFA 457 MicroFlash® measurements).

Lattice thermal conductivity and ZT of CoSb3-based skutterudites; P. N. Alboni, X. Ju, J. He, N. Gothard & Terry M. Tritt, J. Appl. Phys. 103, 113707_2008

14 Bismuth Telluride – Thermal Expansion and Specific Heat

In this example, two bismuth telluride thermal expansion (dL/L0, plot A) thermoelectric materials n-B and specific heat (cp, plot B) in the

(Bi2Se0.2 Te2.8) and p-A (Bi0.5Se1.5Te3) temperature range between -150°C were analyzed with regard to their and 300°C.

A) The dilatometer samples had an

dL/L0 / % initial sample length of 20 mm. The room-temperature bulk of the samples were 7.824 g/cm3 (n-B) and 6.829 g/cm3 (p-A), respectively. Plot A clearly shows that samples n-B and p-A exhibit very similar thermal expansion behavior. This proves that the considerable differences in composition have no impact on the thermal expansion behavior.

Temperature / °C

A) The thermal expansion measurements were carried out using the DIL 402 C equipped with a sample holder and pushrod made of fused silica along with a liquid nitrogen cooling device.

B) The evaluation of the specific heat Cp / (J/(g.K)) was carried out using the ratio method. Plot B illustrates the specific heat capacities of two samples, n-B and p-A. Here, the antimonide content in p-A (solid line) results in a significantly higher specific heat than is the case for n-B (dashed line). The content apparently results in a higher density and a lower specific heat. Furthermore, no phase transition can be observed in the two materials. This means that no structural changes can be estimated based on DSC tests.

Temperature / °C

B) Specific heat (cp) of two different bismuth tellurides (Ø 5 mm, 1 mm thick) measured at heating rates of 10 K/min in platinum crucibles in the DSC 204 F1 Phoenix®

15 Applications – Thermal Diffusivity and Thermal Conductivity of Bismuth Telluride; Characterization of PbTe

Bismuth Telluride — Thermal Diffusivity and Thermal Conductivity

In this example, the thermal diffusivity Plot A shows the thermal diffusivity of two bismuth telluride discs (Ø 12.60 (a), thermal conductivity and specific

mm, 2 mm thick), n-B (Bi2Se0.2 Te2.8) and heat (DSC results, see page 15) of n-B.

p-A (Bi0.5Se1.5Te3), were determined. The specific heat increases slightly The measurement results were used to with temperature. The thermal calculate the thermal conductivity (λ). diffusivity decreases versus temper- ature in the sub-ambient temperature range but increases sharply at high temperatures. This exemplifies a behavior which occurs in pure phonon-conducting materials at low temperatures. At high temperatures, K)) . the temperature dependence of the K)) /s) . 2 thermal diffusivity is dominated by free electrons which are generated at increasing temperatures. Their Heat /((J/g contribution to the heat transfer

Specific results in a sharp increase in thermal diffusivity. The thermal conductivity Thermal Diffusivity / (mm

Thermal Conductivity / (W/(m (λ) follows the temperature depen- dence of the thermal diffusivity.

In plot B, the thermal conductivities Temperature / °C (λ) of n-B and p-A are compared. At -150°C, the thermal conductivities of A) Thermophysical properties of n-B determined at a heating rate of 10/Kmin in the DSC the two materials are very similar. Up to 25°C, n-B exhibits less of a decrease in λ than p-A. Apparently the phonon contribution to λ decreases more sharply for p-A. The increase in λ at higher temperatures is almost parallel K)) . in the two materials. It can be assumed that the electron contribution to the thermal conductivity is nearly the same for both.

The sharp increase at higher tempera- tures is an indication for high electrical Thermal Conductivity / (W/(m conductivity at elevated temperatures. Therefore, a high figure of merit can be expected.

Temperature / °C

B) Thermal conductivity of the two bismuth telluride thermoelectric materials.

16 Characterization of PbTe

Due to its very low thermal conduc- material in thermoelectric applications. tivity, the variety of different doping Comprehensive characterization of the materials which can be used, and the material is thus of paramount resulting high figure of merit, doped importance. (PbTe) is an important

Plot A shows the thermophysical 2.0 4.0 1.0 properties of PbTe. The thermal 3.5 diffusivity (a) and the calculated 0.8

K)) thermal conductivity (λ) decrease with .

/s) 1.5 3.0 2 increasing temperature. The specific K)) . 2.5 heat capacity (c ) exhibits only a small a 0.6 p increase. 1.0 2.0 Heat / (J/(g 0.4 λ 1.5 Specific

Thermal Diffusivity /(mm 0.5 c 1.0 p Thermal Conductivity /(W/(m 0.2 0.5

0.0 0.0 0.0 0 50 100 150 200 250 300 Temperature /°C

A) Thermophysical properties of PbTe determined in the LFA system (a, λ) and DSC 204 F1 Phoenix® (cp) between room temperature and 250°C.

Plot B depicts the thermal conductivity 4.0 of two PbTe samples between room 3.5 temperature and 250°C. Slight differences can only be observed K)) . 3.0 around room temperature.

2.5

2.0 These measurements demonstrate the high reproducibility for determination 1.5 of thermal diffusivity in the LFA system 1.0 and specific heat in the DSC 204F1 Thermal Conductivity /(W/(m Phoenix®. 0.5

0.0 0 50 100 150 200 250 300 Temperature /°C

B) Thermal conductivity of two PbTe samples

17 Applications – Characterization of PbTe

Thermal Stability of PbTe

In this example, the thermal stability of decomposing at around 600°C. The PbTe was analyzed using the STA 409 plotted mass numbers represent the CD coupled to a mass spectrometer via combination of the Pb and Te isotopes. the SKIMMER® system. This sophisti- The following gaseous products were cated coupling system is specially detected: designed to investigate the release of large molecules and highly 333 u: 208Pb + 125Te; 207Pb + 126Te condensable gases such as metal 334 u: 208Pb + 126Te; 206Pb + 128Te vapors. 335 u: 207Pb + 125Te; 206Pb + 126Te 332 u: 207Pb + 128Te Each plot shows the TGA curve of the 336 u: 208Pb + 128Te PbTe sample but with different mass 337 u: 207Pb + 130Te spectrometer results. PbTe starts 338 u: 208Pb and 130Te.

TG /mg Ion Current *10 -12 /A TG /mg Ion Current *10 -12 /A

0 0 1.0 1.2 -1 -1 332 1.0 335 0.8 -2 -2 PbTe 0.8 PbTe -6.43 mg -6.43 mg 0.6 -3 0.23455E-09A*s -3 0.6 0.20489E-09A*s 0.28191E-09A*s 333 337 0.4 -4 -4 0.4 0.19645E-09A*s -5 -5 0.2 0.2 -6 -6 0 0

200 400 600 800 1000 1200 200 400 600 800 1000 1200 Temperature /°C Temperature /°C

TG /mg Ion Current *10 -12 /A TG /mg Ion Current *10 -12 /A

2.5 3.5 0 0 3.0 -1 2.0 -1 334 PbTe 336 2.5 -2 PbTe -2 1.5 -6.43 mg 2.0 -6.43 mg 0.71867E-09A*s -3 -3 1.5 338 1.0 0.50572E-09A*s 338 -4 -4 0.52077E-09A*s 1.0 -5 0.50302E-09A*s 0.5 -5 0.5 -6 -6 0 0 200 400 600 800 1000 1200 200 400 600 800 1000 1200 Temperature /°C Temperature /°C

STA-MS SKIMMER® measurements on PbTe; TGA curve in correlation with the detected metal vapors

18 PbTe Thermoelectric Module on a Substrate

For a better overview, one can present the mass spectrometer data (obtained with the SKIMMER® coupling) along with the corresponding TGA, DTG and/or DSC curves in a 3D-plot. For a direct correlation, the mass numbers of interest were imported into the NETZSCH Proteus® software (instrument software) as continuous MID curves.

The PbTe sample was heated at a rate of 10 K/min across the temperature range from room temperature to 600°C. The MID curves selected here, which were directly imported from the coupled mass spectrometer, are in good correlation with the low mass loss of 0.08% (TGA signal). This low

STA-MS SKIMMER® measurements on a PbTe module deposited on a substrate; extract of the TGA mass loss can be attributed in part to curve and scan-bargraph in the mass number range from m/z 100 to 350. impurities but also to Pb and Te fragments of PbTe, such as Pb 208 u. Mass number 80 u can be attributed to the stable isotope 80Se (natural abundance 49.6%).

The other mass numbers detected – e.g., 58 u, 60 u, etc. – can be attributed to organic impurities and the substrate material.

STA-MS SKIMMER® measurements on a PbTe module deposited on a substrate; TGA curve in correlation with repeated MID curves for various mass numbers.

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NGB · Thermoelectric Materials · EN · 1000 · 08/13 · LH · Technical specifications are subject to change.