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THERMOELECTRIC MATERIALS

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THERMOELECTRICMATERIALS

Thermoelectricmaterials

Abstract

Withvarious developments in the mechanical engineering and desire to saveenergy, thermoelectric materials have turned to be significant invarious fields. This paper seeks to understand what thermoelectricmaterials are and where they can be used in the divers engineeringindustry. This paper also seeks to explore the properties ofmaterials that can be exploited for the thermoelectric principles ormaterials. In addition, the figure of a material will be explored aswell as the performance metrics. To put the topic in perspective,this paper will mention a few of the traditional materials in thethermoelectric materials topic. By the use of state of the artmaterials, this paper seeks to explore the latest material innovationespecially as employed in the energy harvesting.

Thermoelectricmaterials

Thermoelectricmaterials (TE) are special type of materials that are capable ofconverting thermal energy into electrical energy finding theirapplications in devices that generate power. These materials can beassembled into mechanical structures that can be used to transformheat to electrical energy. In modern day usage these devices can beused to tap energy from heat increasing energy efficiency andreducing the effects of carbon dioxide emissions [1]. The biggestchallenge is that a good TE material should be a good conductor ofelectricity e.g. a metal and a poor conductor of heat for instance aninsulator. This kind of property is not common to many materials [7].

Allthermoelectric materials depend on thermoelectric effect. This effectexists where there is a difference in temperature which creates anelectric potential. At the same time, the electric potential createsa difference in temperature. This phenomenon can be described byseebeck effect, peltier effect and Thomson effect. It is understoodthat all materials have a nonzero thermoelectric effect but in mostmaterials it is too small to be utilized. OPeil [2].

Accordingto Rowe [3], seebeck effect is experienced when two dissimilarconductors say A and B make up an electric circuit a current willflow provided that the junctions of the two conductors are atdifferent temperatures. This can be expressed by equation 1 below.This equation explains why all thermoelectric materials have seebeckeffect as part of their properties. This is experienced due to thethermoelectric effect that relates to the materials withthermoelectric properties.

V=α∆T (1)

WhereV is the thermoelectric voltage, ∆T the temperature gradient and αis the seebeck coefficient which is the open circuit voltage. Thisopen voltage is produced on a conductor between two points. These twopoints is where there is a uniform temperature difference of 1 Kbetween them. For peltier effect, when an electric current flowsthrough a junction of two dissimilar conductors either heat isliberated or absorbed [3]. Finaly Thomson effect is the change ofheat content of a single conductor of unit cross section when aquantity of electricity flows along it through a temperature gradientof 1 K. According to Bos [2], thermoelectric materials are still notwidely used. This is because the coupling between the electrical andheat current is weak in most thermoelectric materials thereforereducing the efficiency of energy conversion [1]. It would require alot of heat to generate very little electricity making the wholesituation uneconomical. However this is not the end ofthermoelectrically materials since more research has been carried outto yield new p-and n- type semiconductors which will improve energyconversion.

Usesof thermoelectric materials

Accordingto Bos [6], thermoelectric materials find uses in niche coolingapplications for instance to maintain stable temperatures in laserand optical detectors mostly found in office water coolers. TEmaterials are also found in space exploration to convert heat from aradioactive material into electricity. In other words these materialsfind applications in heat harvesting and refrigeration. As argued byTritt [10], thermoelectric refrigeration is an environmental friendlysmall scale localized cooling in computers, infrared detectors,electronics and opto-electronics as well as other applications.Utilization of peltier coolers in relation to refrigeration ofbiological specimens is an emerging application of thermoelectricity.

Agood example which demonstrates one great use of these special typesof materials is in the motor vehicles. In the internal combustionengine of a car approximately a third of the fuel is converted intomechanical energy the rest being lost as heat. In the case of heatharvesting a thermoelectric generator taps the lost heat energy fromthe exhaust gases which are at a temperature of about 300 -5000Cand turns this heat energy into electricity. State of the art modulesgenerate about 1 KW which can be used to power the electricalequipments in the car. This allows for improvement in fuel usage, itallows for a smaller alternator which in turn reduces roll frictionincreasing fuel usage efficiency and reducing CO2emissions.

Thermoelectricmodule can be built for power generation or a cooling system. Athermoelectric module is made up of p- and n-type semiconductors legssandwiched between electrically insulating plates. For electricity toflow through and external circuit, a temperature gradient is requiredbetween the top and the bottom plates [1]. The properties of the p-and n- type legs semiconductors will greatly affect the efficiency ofthermoelectric module. The temperature gradient also plays a role inthe efficiency of energy conversion by these modules larger gradientyields the best results [6].

Thermoelectricpower generation

Thermoelectricpower generation is based on seebeck effect whereby if heat isapplied to circuit at the junction of two dissimilar conductors, acurrent will be generated. A simple thermoelectric generator consistof a thermocouple or a thermopile consisting of n-type elementswhich are materials with excess electrons and p-type elements whichare materials with deficit of electrons connected electrically inseries and thermally in parallel. Heat is pumped on one side of thethermocouple and rejected on the other side. This produces anelectrical current which is proportional to the temperaturedifference between the cold and hot junction. This voltage differenceis similar to that of a battery and can be used for electricalpurposes for instance powering the radio of a car. Figure 2 belowillustrates the power generation module [10].

Figure.2.Thermoelectric power generation. (seebeck effect) [10].

Thermoelectriccooling-refrigeration

Thermoelectricheating and cooling devices are based on peltier effect. This effectexists where a current is passed through a circuit made up of twoconductors that are not similar. When an electric input is applied toa thermocouple, electrons will move from the p-type material ton-type material absorbing thermal energy at the cold junction.Electrons will dump their extra energy at the hot junction as theyflow from n-type back to p-type material through the electricalconnector [3]. Removal of heat from the hot side drops thetemperature of the cold side rapidly and the magnitude of this dropin temperature depends on the electric current applied. Figure 3below illustrates thermoelectric cooling [10].

Figure.3.Thermoectriccooling (peltier effect), [10]

Thermoelectricwaste heat recovery

Thermoelectricmaterials are used in the process of waste heat recovery from theexhaust of the engine of an automotive. Engines Of automotives rejectquite a considerable amount of energy to the environment through theexhaust pipe [5]. This energy is lost as the exhaust gas that isemitted when an automotive engine burns to power off the machine. Byrecovering the exhausts heat that is lost away with the exhaust gas,the mechanism can save a lot of energy consumed by the engine [7].This recovery is done by the development of an efficient heatexchanger that will be responsible for the provision of optimalrecovery of the energy that would be lost in the exhaust gases.

Thisis done by the application of a prototype thermoelectric generatorthat is planted on the self-ignition engine such as the dieselengine. Through the use of the prototype, a tool of benchmarking theuse of energy is developed to measure the efficiency of the heatexchanger. In the process of normal combustion, ordinary cars enginesexchange an average of 30% to 40% of the heat that is generated bythe process [4]. This is the heat that is put into good mechanicaluse by the automobile. The rest of the heat is wasted away from theexhaust pipe to the environment through two main avenues the enginecooling systems and the exhaust gases.

Thismeans that the use of the wasted heat, even on partial basis willallow for considerable increase in the performance of the combustionengine. Furthermore, changing this heat energy contained in theexhaust gases would lead to measurable advantages. An observationmade that a normal car, equipped with combustion engines will haveseveral components that are electronically controlled [5]. Thereforethe tendency for the use of thermoelectric materials will be avoidingthe mechanical components and using electronic ones instead.

Propertiesof a thermoelectric material to be exploited

Accordingto Trit [10], a thermoelectric material constitutes the followingproperties,

  • Electrical resistivity or electrical conductivity.

  • Seebeck coefficient

  • Thermal conductivity.

Electricalresistivity or electrical conductivity

Thermoelectricmaterials have the feature of electrical resistance just as any otherelectrical conductor. Electrical resistance is the ability of amaterial to oppose the passage of currents of electricity through itas a conductor. The inverse of this property of the thermoelectricmaterials is electrical conductance, in which electric currentpasses. This feature of electrical resistance has the parallels ofconceptual properties with friction, as known mechanically. inaddition, a thermoelectric materials has to be a conductor ofelectricity. This is an important feature of a material that conductselectricity as it is present in the properties of the thermoelectricmaterials [5]. Despite this some thermoelectric materials have goodfeatures and properties yet they are semi-conductors of electriccurrent.

Inthis view, a good thermoelectric material has a high electricalconductivity or low electrical resistivity so that little heat isgenerated from the electrical current flow or in other terms tominimize rise in temperature from resistance to electric currentflowing through. This material has a large value of seebeckcoefficient which helps in maximizing heat conversion to electricityor electricity to cooling performance [4]. These materials also havea low thermal conductivity meaning that little heat is transferredfrom the hot junction to the cold junction.

Seebeckcoefficient or Thermopwer

Accordingto Trit (2006), the thermoelectric power or the seebeck coeffient αcan be thought of as the heat per carrier over temperature or moresimply the entropy per carrier. The best thermoelectric materialshave its seebeck coefficient very high. The Seebeck coefficient of amaterial is the measure of the level or degree at which an inducedthermoelectric voltage. The measurement of the seebeck coefficient interms of the SI units is the volts per Kelvin. The most importantapplication of this property in the thermoelectric materials is tooptimize the behavior of thermoelectric coolers and thermoelectricgenerators.

Theseebeck effect is characteristic with the thermoelectric materialsdue to the phenomena that they create. Thermoelectric materials ateknown of creating a thermaelectrical effect. This effect is whatrelates to the phenomenon that is created by the difference intemperature leading to a realization of electric potential. In otherwords or circumstances, the electric potential is what creates atemperature difference. For the thermoelectric materials to besignificantly useful, they have to possess nonzero values of theSeebeck coefficient. The usefulness of the thermoelectric materialsis the ability to cause the thermoelectric effect, which is definedby the magnitude of the Seebeck coefficient.

Thermalconductivity

Thermoelectricmaterials are thermal conductors or posses certain degrees of thermalconductivity. Thermal conductivity is the feature that enables amaterial to conduct heat. This means that good thermoelectricmaterials should be able to conduct heat, whose abilities differentwithy materials and the temperature of the heat being conducted [10].This is because the uses of the thermoelectric materials require themto conduct both the electricity and heat. For instance, the thermalelectric generators generate electricity from a gradient oftemperature. This means that these materials should be effective inthese two conductions for both the heat and electric conductivity[8]. Therefore, the more optimized a material is for theconductivities, the better the experience of the use of thethermoelectric materials.

Theefficiency of the thermoelectric materials is based on their abilityto conduct thermal and electric energies. Despite of this fact, thereexist useful materials that are semi-conductors. In fact,semiconductors are the ideal features of thermoelectric devices dueto their electronic features when exposed to high temperatures andtheir band structure. In terms of values, a thermoelectric materialsmaterial must show higher values of thermal conductivities of adevice. An ideal thermoelectric materials material should have highervalues of semi-conductor levels at higher temperatures [1]. This isbecause semiconductors have the most optimum combination of theseebeck coefficient. In addition, the semi conductors portray thebest electrical resistivity as well as thermal conductivity.Moreover, semi conducting materials have additional advantage ofbeing able to use electrons to conduct electricity. In the case theelectrons are absent the semi-conducting material uses holes toconduct electric current.

Figureof merit for thermoelectric materials

Theefficiency of a thermoelectric device is characterized by thedimensionless thermoelectric material’s figure of merit. This canbe defined by the following equation according to Bos (2012).

ZT=S2αTκ (2)

Where,S is the seebeck coefficient, α is the electrical conductivity, κthe thermal conductivity and T the absolute temperature. The seebeckcoefficient is also known as the thermal power and it is ameasurement of the amount of voltage generated per unit oftemperature. The electrical conductivity determines a material willconduct electricity better or worse and also determines the thermalconductivity. The efficiency of the thermoelectric cooler or thermalgenerator is given by the size of the larger the figure of metricthe larger the better [8]. This requires maintaining high electricalconductivity and thermo power or seebeck coefficient while limitingthe thermal conductivity [10]. For most commercial applications thefigure of merit ZT for a thermoelectric material should be higherthan 1.

TraditionalTE materials

Accordingto Trit [10], early TE materials include

  • Bismuth telluride alloy (Bi2Te3 )

  • Lead telluride alloy (PbTe).

  • Silicon-germanium alloy (SiGe)

Thesematerials had the best figure of merit in different temperatureranges. Bi2Te3and its alloy have found their applications in TE refrigerationapplications and some niche low-power generation applications. Thesematerials have a useful temperature range of 180 K to 450 K. On otherhand PbTe and SiGe materials have found their application mainly inhigh power generation applications including spacecraft powergeneration and they have a useful temperature range of between 500 Kand 900 K and 800 K to 1300 K respectively.

Latestthermoelectric material innovation for energy harvesting

Theefficiency of thermoelectricmaterialis dependent on the dimensionless figure of merit (ZT) of TEmaterials as defined in equation 2 above. High ZT means high energyconversion by reducing the lattice thermal conductivity. To achievethese high ZT values seebeck coefficient and thermal conductivityneed to be as large as possible while the thermal conductivity needsto be as small as possible to be maintain the difference in thetemperature between two side the sold and hot one. This creates abig problem since all these three parameters cannot be optimizedindependently [6]. According to Chen et al, there are two recentdifferent approaches have been developed to establish the nextgeneration of thermoelectric materials. These include,

  • Finding and using new families of bulk thermoelectric materials with complex crystal structure.

  • Synthesizing and using low-dimensional thermoelectric material systems.

Outof these approaches ZT improvement have been achieved in PGECmaterials and nanostructure materials such as superlattices, quantumdots, nanowires and nanocomposite. These new methodologies are beingdeveloped to tackle the ZT=1 limit in thermoelectric performance andthe background of this research is the decoupling of electronic andthermal transport. This approach tries to come up with materials withlarge seebeck voltage, a large electronic conductivity and lowthermal conductivity. This property is not exhibited by traditionalthermoelectric materials such Bi2Te3[6].

PhononGlass Electron Crystal (PGEC) thermoelectric materials

Agood thermoelectric material would behave like as phonon glasselectron crystal which means that it would have the thermo propertiesof glass-like materials and electrical properties of crystalline,materials. In typical PGEC materials, there exists a range of highmobility electrons. These electrons are able to freely charge andheat [6]. Generally, these materials contain a large amount of largeinterstitial sites filled with other element atoms which act asrattler atoms. These atoms vibrate at relatively low frequencies andconsume thermal energy.

Therefore,the PGEC materials act like a crystal for electrons while stillefficiently scatter phonons making these materials retain highelectrical conductivities and also obtain low thermal conductivities.Examples of PGEC thermoelectric materials include, skutterudites,clathrates and β-Zn4Sb3[6]. A good example of sketterudites compound is CoSb3whichhas a crystal structure belonging to body centered cubic with 8 CoSb3formulaeunits which can be regarded as a simple cubic transition metal Co sublattice partially filled almost square ring Sb4[6]. These rings fill six out of the eight cubes formed by Co atomswhile the other voids are empty and can be filed by other atoms asrattlers.

Theoriginal CoSb3hasa very high power factor but its lattice thermo conductivity is toohigh to be an effective thermoelectric material. To improve the ZTfor these materials, a good approach would be the void filling indifferent elements with diverse structures which include lanthanide,actinide, alkaline-earth metals and group IV elements. These atomscan act as effective phonon scattering centers to substantiallyreduce the lattice thermal conductivity. These materials have animproved figure of merit of up to ZT=1.5 [1].

Nanostructuredthermoelectric materials

Accordingto Bos [6], it was predicted that large seebeck coefficient could beattained in nanostructured materials. This group of materialsconsists of composite materials where nanometer regions of say amaterial A are embedded in material B.The increase in seebeckcoefficient is created by the changes that occur after the reductionof dimensionality of a material. Although it found it difficult totranslate in bulk materials this concept was proven in carefullygrown quantum dot thin-films. However, the large number of interfacesin nanostructure materials are very good at reducing the latticethermal conductivity which is achieved due to lattice’s vibrationbehaving like waves in a solid [10]. The propagation of these waveswith wavelengths shorter with the size of the nanodomains is notaffected but waves of similar length are scattered very effectivelyand it is this effect that leads to large reduction in thermalconductivity.

LeadTelluride (PbTe)

Accordingto Bos [6], PbTe is one of the traditional thermoelectric materialsand the best studied bulk nanocomposite material is based on thiscompound.PbTe forms nanocomposite and this happens when it is mixedwith small amount of AgSbTe2andcooled from a melt. These materials have thermal conductivity closeto the glass limit and a figure of merit of about 2.This approach ledto huge interest in nanostructured materials which already had knowthermoelectric properties. For instance, the Bi2Te3isa figure of merit which has increased by 50% in the sample of thenanostructure [10].

BismuthTelluride Bi2Te3

Thisis one of the most commonly used thermoelectric material and itsallow with antimony telluride or bismuth selenide. This materialcrystallizes with layers that are formed by Te and Bi atoms in arhombohedral structure. These layers follow the sequenceTE-Bi-Te-Bi-Te. A strong covalent bonding strongly connects the atomswithin this sequence while there is a weak van der Waals interactionbetween the sequences [9]. This material can easily be cleavedperpendicular to the c-axis and its properties are highlyanisotropic.Bi2Te3willdisplay a smallenergygap of about 0.15 eV and a seebeck coefficient of approximately200mV/K[10].

Atroom temperatures or even below this material gives the bestthermoelectric performance therefore this makes its applicationmainly in thermoelectric cooling. To reduce the thermal conductivityof this material it is alloyed by Sb or Se atoms or even bothlikewise to achieve variations of its structural themeelectropositive alkali metals are introduced [8]. The electropositivemetals act as electron donors to the layers building blocks whichrearrange and forms additional bonds and from this complexcompositions and structures are achieved. Materials that have shownpromising thermoelectric properties include the chalcogenidesespecially through a low thermal conductivity. The materials canreach a maximum ZT of 0.8 at 225K when thermally doped thereforegetting use in low temperature application[10].

Sketterudites

Thesestate of the art materials are binary compounds with general formulaMX3whereM is the group 9 transition metals and X is a heavier pnictogenelement (P, As, Sb).These binary sketterudites crystallizes in thebody centered space group Im3.The structure corresponds tothree-dimensional array of corner connected MX6 octahedral which aremutually tilted. Due to tilting pnictogen atoms approach and thusform planer rectangular rings, tilting a octahedral also createsvoids at unit cell corners and center[10]. Thesematerials reveal diamagnetic and semiconducting behavior.

Themajor bonding interaction is between the transition metal andpnictogen atoms. The band gap is as a result of the stronginteractions between d and p states (dp hybridization) with thecompletely filled valence bond hosting 18 electrons[4]. Dueto the high thermal conductivity of binary skutterudites, they havenot been considered as promising thermoelectric materials. Untilternary derivatives were found, voids were partially occupied withguest atoms. These guest atoms are the rare earth metal atoms such asLa, Ce, and Yb.which allow for a tuning of the doping level towardsoptimum carrier concentrations.

Withlarger amounts of guest atoms an exchange of Co for more electronpoor Fe becomes possible and filled skutterudites can be p-and n-typeconductors. Guest atoms are loosely bonded in the voids and willscatter lattice phonons by their thermal motion[10]. Thisrattling effect of void-filling atom associated with a great decreasein thermal conductivity increasing energy conversion efficiency.Examples of filled skutterudites include, CeFeSb3 and CeFeAs3.Andthese materials have a ZT of around unity.

Conclusion

Thermoelectricmaterials are the materials that posses characteristics of showingthermoelectric effect. The thermoelectric materials are used in thethermoelectric power generation as well as thermoelectric coolinggeneration. In addition, these materials are important in therecovery of waste heat especially by automotives. In addition tothese uses, these materials have properties of electrical resistivityas well as electrical conductivity. Thermoelectric materials alsohave a Seebeck coefficient property as well as thermal conductivity.Despite having some traditional materials included in the category ofthermoelectric materials, a number of innovations have been done onthe field especially in energy harvesting.

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