Major Project In M-Tech.

Table of Contents

1.               Introduction to the Project.

2.               Block Diagram

3.               Circuit Diagram.

4.               Material picture

5.               Component List.

6.               Peltier effect.

7.               Thermoelectric cooling.

8.               Power Supply.

9.               Bibliography.

 

Acknowledgement

I take immense pleasure in thanking Er. Gautam Kocher our beloved H.O.D for having permitted us to carry out this project work.

I wish to express our deep sense of gratitude to our Internal guide,  Er. Harwinder Lal & Er. Amandeep Singh for  their able guidance and useful suggestions, which helped me in completing the project work, in time.

Finally, yet importantly, we would like to express our heartfelt thanks our beloved parents or their blessings, our friends & classmates for their help and wishes for the successful completion of this project.

Last but not least I would like to thank Er. Sukhjinder Singh (Instrumental Engineer) for all the work of designing and assembling the power sectors.

 Certificate

  TO WHOM IT MAY CONCERN

This is to certify that  Project  entitled “Peltier Effect Based Refrigeration System” is submitted in partial fulfillment for the award of degree M-TECH (Production Engineering) of Punjab Technical University has been successfully completed by Mr.Manpreet Singh having                              Roll No. 100628181034

 Certificate

This is to certify that Project entitled “Peltier Effect Based Refrigeration System” is submitted in partial fulfillment for the award of degree M-TECH (Production Engineering) of Punjab Technical University has been successfully completed by Mr. Manpreet Singh having                              Roll No. 100628181034.

He has done a good job under our guidance/supervision.

 

Er.Harwinder Lal                                                         Er. Amandeep Singh

Asst Prof. RIET Phagwara.                                                         Asst Prof. CTIEMT Jalandhar

1. Introduction:

While most traditional refrigeration systems use designs based on compressors and refrigerants, more and more applications are turning to thermoelectric cooling as an alternative to traditional refrigeration technology. While thermoelectric cooling isn't viable for every refrigeration, thermoelectric modules can significantly outperform traditional refrigerant-based cooling systems in certain applications.

In our project we completely ignore the need of the compressor, cooling is done on the bases of thermo electric effect. Thermoelectric solid-state air conditioners can be a cost-effective solution for cooling electronic/electrical equipment and devices housed in enclosures and cabinets. With relatively low energy requirements and the ability to provide both cooling and heating from the same device (when needed), these systems are becoming increasingly popular for enclosure cooling applications.

2. Block diagram :

Block Diagram

3.Circuit diagram:

Circuit Diagram

4.Thermo-electric Material picture:

Pictures of the project

5.Component List:

Component List
S.No.	Component		Value	Quantity
1	LED		2 pins	2
2	Diode		IN 4007	4
3	Capacitor		1000µf	1
4	Capacitor		470 µf	1
5	Transformer(step down)		220-12	1
6	Voltage Regulator		7805	1
7	Resistance		100Ω	1
8	Bi-metallic block			1

Various Components & materials used

6.Peltier effect :

The Peltier effect is the presence of heat at an electrified junction of two different metals and is named for French physicist Jean-Charles Peltier, who discovered it in 1834. When a current is made to flow through a junction made of materials A and B, heat is generated at the upper junction at T₂, and absorbed at the lower junction at T₁. The Peltier heat   absorbed by the lower junction per unit time is equal to

where Π_AB is the Peltier coefficient for the thermocouple composed of materials A and B and Π_A (Π_B) is the Peltier coefficient of material A (B). Π varies with the material's temperature and its specific composition: p-type silicon typically has a positive Peltier coefficient below ~550 K, but n-type silicon is typically negative.

The Peltier coefficients represent how much heat current is carried per unit charge through a given material. Since charge current must be continuous across a junction, the associated heat flow will develop a discontinuity if Π_A and Π_B are different. Depending on the magnitude of the current, heat must accumulate or deplete at the junction due to a non-zero divergence there caused by the carriers attempting to return to the equilibrium that existed before the current was applied by transferring energy from one connector to another. Individual couples can be connected in series to enhance the effect. Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators.

Seebeck effect:

The Seebeck effect is the conversion of temperature differences directly into electricity and is named for German physicist Thomas Johann Seebeck, who, in 1821 discovered that a compass needle would be deflected by a closed loop formed by two metals joined in two places, with a temperature difference between the junctions. This was because the metals responded differently to the temperature difference, creating a current loop and a magnetic field. Seebeck did not recognize there was an electric current involved, so he called the phenomenon the thermo magnetic effect. Danish physicist Hans Christian Worsted rectified the mistake and coined the term "thermoelectricity". The voltage created by this effect is on the order of several microvolts per Kelvin difference. One such combination, copper-constantan, has a Seebeck coefficient of 41 microvolt’s per Kelvin at room temperature.

The voltage V developed can be derived from:

Where SA and SB are the thermo powers (Seebeck coefficient) of metals A and B as a function of temperature and T1 and T2 are the temperatures of the two junctions. The Seebeck coefficients are non-linear as a function of temperature, and depend on the conductors' absolute temperature, material, and molecular structure. If the Seebeck coefficients are effectively constant for the measured temperature range, the above formula can be approximated as:

The Seebeck effect is used in the thermocouple to measure a temperature difference; absolute temperature may be found by setting one end to a known temperature.

A metal of unknown composition can be classified by its thermoelectric effect if a metallic probe of known composition, kept at a constant temperature, is held in contact with it. Industrial quality control instruments use this as thermoelectric alloy sorting to identify metal alloys. Thermocouples in series form a thermopile, sometimes constructed in order to increase the output voltage, since the voltage induced over each individual couple is small. Thermoelectric generators are used for creating power from heat differentials and exploit this effect.

Diagram of the circuit on which Seebeck discovered the Seebeck effect. A and B are two different metals.

Materials selection criteria

Figure of merit

The primary criterion for thermoelectric device viability is the figure of merit given by:

,

which depends on the Seebeck coefficient, S, thermal conductivity, λ, and electrical conductivity, σ. The product (ZT) of Z and the use temperature, T, serves as a dimensionless parameter to evaluate the performance of a thermoelectric material.

Phonon-Glass, electron-crystal behavior

Notably, in the above equation, thermal conductivity and electrical conductivity intertwine. G. A. Slack proposed that in order to optimize the figure of merit, phonons, which are responsible for thermal conductivity must experience the material as they would in a glass (experiencing a high degree of phonon scattering—lowering thermal conductivity) while electrons must experience it as a crystal (experiencing very little scattering—maintaining electrical conductivity). The figure of merit can be improved through the independent adjustment of these properties.

 Semiconductors

Semiconductors are ideal thermoelectric devices because of their band structure and electronic properties at high temperatures. Device efficiency is proportional to ZT, so ideal materials have a large Z value at high temperatures. Since temperature is easily adjustable, electrical conductivity is crucial. Specifically, maximizing electrical conductivity at high temperatures and minimizing thermal conductivity optimizes ZT.

 Thermal conductivity

κ = κ _electron + κ _phonon
According to the Wiedemann–Franz law, the higher the electrical conductivity, the higher κ _electron becomes. Therefore, it is necessary to minimize κ _phonon. In semiconductors, κ _electron < κ _phonon, so it is easier to decouple κ and σ in a semiconductor through engineering κ _phonon.

 

Electrical conductivity

Metals are typically good electrical conductors, but the higher the temperature, the lower the conductivity, given by the equation for electrical conductivity:
σ_metal = ne²τ/m

• n is carrier density
• e is electron charge
• τ is electron lifetime
• m is mass

As temperature increases, τ decreases, thereby decreasing σ_metal. By contrast, electrical conductivity in semiconductors correlates positively with temperature.
σ _{semiconductor} = neμ

• n is carrier density
• e is electron charge
• μ is carrier mobility

Carrier mobility decreases with increasing temperature, but carrier density increases faster with increasing temperature, resulting in increasing σ _{semiconductor}.

 State density

The band structure of semiconductors offers better thermoelectric effects than the band structure of metals.
The Fermi energy is below the conduction band causing the state density to be asymmetric around the Fermi energy. Therefore, the average electron energy is higher than the Fermi energy, making the system conducive for charge motion into a lower energy state. By contrast, the Fermi energy lies in the conduction band in metals. This makes the state density symmetric about the Fermi energy so that the average conduction electron energy is close to the Fermi energy, reducing the forces pushing for charge transport. Therefore, semiconductors are ideal thermoelectric materials

Materials of interest

Strategies to improve thermo electrics include both advanced bulk materials and the use of low-dimensional systems. Such approaches to reduce lattice thermal conductivity fall under three general material types: (1) Alloys: create point defects, vacancies, or rattling structures (heavy-ion species with large vibrational amplitudes contained within partially filled structural sites) to scatter phonons within the unit cell crystal. (2) Complex crystals: separate the phonon-glass from the electron crystal using approaches similar to those for superconductors. The region responsible for electron transport would be an electron-crystal of a high-mobility semiconductor, while the phonon-glass would be ideal to house disordered structures and dopants without disrupting the electron-crystal (analogous to the charge reservoir in high-Tc superconductors.) (3) Multiphase nano composites: scatter phonons at the interfaces of nano structured materials, be they mixed composites or thin film super lattices.
Materials under consideration for thermoelectric device applications include:

 Bismuth chalcogenides

Materials such as Bi₂Te₃ and Bi₂Se₃ comprise some of the best performing room temperature thermoelectrics with a temperature-independent thermoelectric effect, ZT, between 0.8 and 1.0. Nano structuring these materials to produce a layered super lattice structure of alternating Bi₂Te₃ and Bi₂Se₃ layers produces a device within which there is good electrical conductivity but perpendicular to which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type). Note that this high value has not entirely been independently confirmed.
Bismuth telluride and its solid solutions are good thermoelectric materials at room temperature and therefore suitable for refrigeration applications around 300 K. The Czochralski method has been used to grow single crystalline bismuth telluride compounds. These compounds are usually obtained with directional solidification from melt or powder metallurgy processes. Materials produced with these methods have lower efficiency than single crystalline ones due to the random orientation of crystal grains, but their mechanical properties are superior and the sensitivity to structural defects and impurities is lower due to high optimal carrier concentration.
The required carrier concentration is obtained by choosing a non stoichiometric composition, which is achieved by introducing excess bismuth or tellurium atoms to primary melt or by dopant impurities. Some possible dopants are halogens and group IV and V atoms. Due to the small bandgap (0.16 eV) Bi₂Te₃ is partially degenerate and the corresponding Fermi-level should be close to the conduction band minimum at room temperature. The size of the band-gap means that Bi₂Te₃ has high intrinsic carrier concentration. Therefore, minority carrier conduction cannot be neglected for small stoichiometric deviations. Use of telluride compounds is limited by the toxicity and rarity of tellurium.

Lead telluride

Jeffrey Snyder and his colleagues have shown in 2008 that with thallium doped lead telluride alloy (PbTe) it is possible to achieve zT of 1.5 at 773 K (Heremans et al., Science, 321(5888): 554-557). In an article published in January 2011, they showed that replacing thallium with Sodium zT~1.4 at 750 K is possible (Y. Pei et al., Energy Environ. Sci., 2011). In May 2011 they reported in Nature in collaboration with Chinese research group that PbTe_1-xSe_x alloy doped with sodium gives zT~1.8±0.1 at 850 K (Y. Pei et al., Nature, 473 (5 May, 2011)). Snyder’s group has determined that both thallium and sodium alter the electronic structure of the crystal increasing electric conductivity. The Snyder group also claims that selenium increases further electric conductivity and also reduces thermal conductivity. These works show that other bulk alloys have also potential for improvement, which could open many new applications for thermo electrics.

Inorganic clathrates

Inorganic clathrates have a general formula A_xB_yC_46-y (type I) and A_xB_yC_136-y (type II), in these formulas B and C are group III and IV atoms, respectively, which form the framework where “guest” atoms A (alkali or alkaline earth metal) are encapsulated in two different polyhedra facing each other. The differences between types I and II comes from number and size of voids present in their unit cells. Transport properties depend lot on the properties of the framework, but tuning is possible through the “guest” atoms.
The most direct approach to the synthesis and optimization of thermoelectric properties of semiconducting type I clathrates is substitution doping, where some framework atoms are replaced with dopant atoms. In addition, powder metallurgical and crystal growth techniques have been used in the synthesis of clathrates. The structural and chemical properties of catharses enable the optimization of their transport properties with stoichiometry. Type II materials should be investigated in future because their structure allows a partial filling of the polyhedron enabling a better tuning of the electrical properties and therefore a better control of the doping level. Partially filled variant can also be synthesized as semiconducting or even insulating.
Blake et al have predicted zT~0.5 at room temperature and zT~1.7 at 800 K for optimized compositions. Kuznetsov et al measured electrical resistance and See beck coefficient for three different type I clathrates above room temperature and by estimating high temperature thermal conductivity from the published low temperature data they obtained zT~0.7 at 700 K for Ba₈Ga₁₆Ge₃₀ and zT~0.87 at 870 K for Ba₈Ga₁₆Si₃₀.

Magnesium group IV compounds

Mg₂B^IV (B^IV=Si, Ge, Sn) compounds and their solid solutions are good thermoelectric materials and their figure of merit values are at the level with established materials. Due to the lack of the systematic studies about their thermoelectric properties suitability of these materials, and in particular their quasi-ternary solutions, for thermoelectric energy conversion remains in question. The appropriated production methods are based on direct comelting but mechanical alloying has also been used. During synthesis, magnesium losses due to evaporation and segregation of components (especially for Mg₂Sn) need special attention. Directed crystallization methods can produce single crystalline material. Solid solutions and doped compounds have to be annealed in order to get homogeneous samples. At 800 K Mg₂Si_1-xSn_x may have a figure of merit about 0.9.

Silicides

Higher silicides seem promising materials for thermoelectric energy conversion, because their figure of merit is at the level with materials currently in use and they are mechanically and chemically strong and therefore can often be used in harsh environments without any protection. More detailed studies are needed to assess their potential in thermoelectric and possibly to find a way to increase their figure of merit. Some of possible fabrication methods are Czochralski and floating zone for single crystals and hot pressing and sintering for polycrystalline.

 Skutterudite thermoelectrics

Recently, skutterudite materials have sparked the interest of researchers in search of new thermoelectric These structures are of the form (Co,Ni,Fe)(P,Sb,As)₃ and are cubic with space group Im3. Unfilled, these materials contain voids into which low-coordination ions (usually rare earth elements) can be inserted in order to alter thermal conductivity by producing sources for lattice phonon scattering and decrease thermal conductivity due to the lattice without reducing electrical conductivity. Such qualities make these materials exhibit PGEC behavior.
The composition of skutterudites corresponds to the chemical formula LM₄X₁₂, where L is a rare earth metal, M a transition metal and X a metalloid, a group V element or pnictogen whose properties lie between those of a metal and nonmetal such as phosphorus, antimony, or arsenic. These materials could be potential in multistage thermoelectric devices as it has been shown that they have zT>1.0, but their properties are not well known and optimization of their structures is under way.

 Oxide thermoelectrics

Due to the natural super lattice formed by the layered structure in homologous compounds (such as those of the form (SrTiO₃)_n(SrO)_m—the Ruddleson-Popper phase), oxides have potential for high-temperature thermoelectric devices. These materials exhibit low thermal conductivity perpendicular to these layers while maintaining electrical conductivity within the layers. The figure of merit in oxides is still relatively low (~0.34 at 1,000K), but the enhanced thermal stability, as compared to conventional high-ZT bismuth compounds, makes the oxides superior in high-temperature applications.
Interest towards oxides as thermoelectric materials was reawakened in 1997 when Na_xCoO₂ was found to be a strong candidate for thermoelectric material. Some advantages of oxides are their thermal stability, non toxicity and high oxidation resistance. Research on oxides as thermoelectric materials is ongoing, but it seems that in order to simultaneously control both the electric and phonon systems nanostructures have to be used. Some layered oxide materials, which are built from several layers, might have zT~2.7 at 900 K. If these layers have the same stoichiometry, they will be stacked so that the same atoms will not be positioned on top of each other.

 Half Heusler alloys

Half Heusler alloys have potential for high temperature power generation applications especially as n-type material. These alloys have three components that originate from different element groups or might even be a combination of elements in the group. Two of the groups are composed of transition metals and the third group consists of metals and metalloids. Currently only n-type material is usable in thermoelectric but some sources claim that they have achieved zT~1.5 at 700 K, but according to other source only zT~0.5 at 700 Khas been achieved. They state that primary reason for this difference is the disagreement between thermal conductivities measured by different groups. These alloys are relatively cheap and also have a high power factor.

Electrically conducting organic materials

Some electrically conducting organic materials may have a higher figure of merit than existing inorganic materials. Seebeck coefficient can be even millivolts per Kelvin but electrical conductivity is usually very low resulting small figure of merit. Quasi one-dimensional organic crystals are formed from linear chains or stacks of molecules that are packed into a 3D crystal. It has theoretically been shown that under certain conditions some Q1D organic crystals may have zT~20 (Figure 13) at room temperature for both p- and n-type materials. In the Thermo electrics  Handbook chapter 36.4 this has been accredited to an unspecified interference between two main electron-phonon interactions leading to the formation of narrow strip of states in the conduction band with a significantly reduced scattering rate as the mechanism compensate each other causing high zT

Others

Silicon-germanium alloys are currently the best thermoelectric materials around 1000 ℃ and are therefore used in radioisotope thermoelectric generators (RTG) and some other high temperature applications, such as waste heat recovery. Usability of silicon-germanium alloys is limited by their high price and in addition, zT is also only in the mid-range (~0.7).
With functionally graded materials, it is possible to improve the conversion efficiency of existing thermoelectric materials. These materials have a non-uniform carrier concentration distribution and in some cases also solid solution composition. In power generation applications the temperature difference can be several hundred degrees and therefore devices made from homogeneous materials have some part that operates at the temperature where zT is substantially lower than its maximum value. This problem can be solved by using materials whose transport properties vary along their length thus enabling substantial improvements to the operating efficiency over large temperature differences. This is possible with functionally graded materials as they have a variable carrier concentration along the length of the material, which is optimized for operations over specific temperature range.

Nanomaterials

In addition to the nanostructure Bi₂/Bi₂Se₃ super lattice thin films that have shown a great deal of promise, other nonmaterial’s show potential in improving thermoelectric materials. One example involving PbTe/PbSeTe quantum dot super lattices provides an enhanced ZT (approximately 1.5 at room temperature) that was higher than the bulk ZT value for either PbTe or PbSeTe (approximately 0.5). Individual silicon nano wires can act as efficient thermoelectric materials, with ZT values approaching 1.0 for their structures, even though bulk silicon is a poor thermoelectric material (approximately 0.01 at room temperature) because of its high thermal conductivity
Not all nanocrystalline materials are stable, because the crystal size can grow at high temperatures ruining materials desired characteristics. In nanocrystalline material, there are many interfaces between crystals, which scatter phonons so the thermal conductivity is reduced. Phonons are confined to the grain, if their mean free path is larger than the material grain size. Measured lattice thermal conductivity in nanowires is known to depend on roughness, the method of synthesis and properties of the source material.
Nan crystalline transition metal siilcides are a promising material group for thermoelectric applications, because they fulfill several criteria that are demanded from the commercial applications point of view. In some nano crystalline transition metal silicides the power factor is higher than in the corresponding polycrystalline material but the lack of reliable data on thermal conductivity prevents the evaluation of their thermoelectric efficiency.
One advantage of nanostructure skutterudites over normal skutterudites is their reduced thermal conductivity but further performance improvements can be achieved by using composites and by controlling the grain size, the compaction conditions of polycrystalline samples and the carrier concentration. Thermal conductivity reduction is caused by grain boundary scattering. ZT values of ~ 0.65 and >0.4 have been achieved with CoSb₃ based samples, the former value is for 2.0 at.% Ni and 0.75 at.% Te doped material at 680 K and latter for Au-composite at T>700 K.
Due to the unique nature of grapheme, engineering of thermoelectric device with extremely high See beck coefficient based on this material is possible. One theoretical study suggests that the See beck coefficient might achieve a value of 30 mV/K at room temperature and zT for their proposed device would be approximately 20.
Super lattices and quantum wells can be good thermoelectric materials, but their production is too difficult and expensive for general use because of their fabrication is based on various thin film growth methods. Super lattice structures allow the independent manipulation of transport parameters by adjusting the structural parameters enabling the search for better understanding of thermoelectric phenomena in nano scale. Many strategies exist to decrease the super lattice thermal conductivity that are based on engineering of phonon transport. The thermal conductivity along the film plane and wire axis can be reduced by creating diffuse interface scattering and by reducing the interface separation distance, both which are caused by interface roughness. The interface roughness can be natural due to the mixing of atoms at the interfaces or artificial. Many different structure types, such as quantum dot interfaces and thin films on step-covered substrates, can act as source for artificial roughness.
However while engineering interface structures for reduced phonon thermal conductivity effects to electron transport has to be taken into account because the reduced electrical conductivity could negate the advantage received from phonon transport engineering. Because electrons and phonons have different wavelengths, it may be possible to engineer the structure in such a way that phonons are scattered more diffusely at the interface than electrons. This would reduce the decrease of the electrical conductivity.
Second approach is to increase phonon reflectivity and therefore decrease the thermal conductivity perpendicular to interfaces. This can be achieved by increasing the mismatch between the materials. Some of these properties are density, group velocity, specific heat, and the phonon spectrum between adjacent layers. Interface roughness causes diffuse phonon scattering, which either increases or decreases the phonon reflectivity at the interfaces. Mismatch between bulk dispersion relations confines phonons and the confinement becomes more favorable as the difference in dispersion increases. The amount of confinement is currently unknown as only some models and experimental data exist. As with a previous method, the effects on the electrical conductivity have to be considered.
In order to further reduce the thermal conductivity, the localization of long wavelength phonons can be attempted with a periodic super lattices or composite super lattices with different periodicities. In addition, defects, especially dislocations, can be used to reduce thermal conductivity in low dimensional systems.
Thermoelectric performance improvements in super lattices originate from various sources, usually at least the lattice thermal conductivity in the cross plane direction is very low but depending on the type of super lattice, the thermoelectric coefficient may also increase because the band structure changes. Low lattice thermal conductivity in super lattices is usually due to strong interface scattering of phonons. Electronic band structure in super lattices comprises the so called mini bands, which appear due to quantum confinement effects. In super lattices, electronic band structure depends on the super lattice period so that with very short period (~1 nm) the band structure approaches the alloy limit and with long period (≥ ~60 nm) mini bands become so close to each other that they can be approximated with a continuum.
Especially in multi quantum well structures the parasitic heat conduction could cause significant performance reduction. Fortunately, the impact of this phenomenon can be reduced by choosing the distance between the quantum wells correctly.
The See beck coefficient can change its sign in super lattice nano wires due to the existence of mini gaps as Fermi energy varies. This indicates that super lattices can be tailored to exhibit n or p-type behavior by using the same do pants as those that are used for corresponding bulk materials by carefully controlling Fermi energy or the dopant concentration. With nano wire arrays, it is possible to exploit semimetal-semiconductor transition due to the quantum confinement and use materials that normally would not be good thermoelectric materials in bulk form. Such elements are for example bismuth. The See beck effect could also be used to determine the carrier concentration and Fermi energy in nano wires.
In quantum dot thermoelectric, unconventional or non band transport behavior (e.g. tunneling or hopping) is necessary to utilize their special electronic band structure in the transport direction. It is possible to achieve zT~3 at elevated temperatures with quantum dot super lattices, but they are almost always unsuitable for mass production. Bi₂Te₃/Sb₂Te₃ super lattice as a micro cooler has been reported to have zT~2.4 at 300 K.
Nano composites are promising material class for bulk thermoelectric devices, but several challenges have to be overcome to make them suitable for practical applications. It is not well understood why the improved thermoelectric properties appear only in certain materials with specific fabrication processes.
K. Biswas et al. report in Nature Chemistry 3, 160–166 (2011) that SrTe nano crystals embedded in a bulk PbTe matrix so that rock salt lattices of both materials are completely aligned (endotaxy) with optimal molar concentration for SrTe only 2 % can cause strong phonon scattering but would not affect charge transport. They report maximum zT~1.7 at 815 K for p-type material.

 Production methods

Production methods for these materials can be divided into powder and crystal growth based techniques. Powder based techniques offer excellent ability to control and maintain desired carrier distribution. In crystal growth techniques dopants are often mixed with melt, but diffusion from gaseous phase can also be used. In the zone melting techniques disks of different materials are stacked on top of others and then materials are mixed with each other when a travelling heater causes melting. In powder techniques, either different powders are mixed with a varying ratio before melting or they are in different layers as a stack before pressing and melting.

Thermo power:

The thermo power or Seebeck coefficient, represented by S, of a material measures the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material, and the entropy per charge carrier in the material. S has units of V/K, though μV/K is more common. Values in the hundreds of μV/K, regardless of sign, are typical of good thermoelectric materials. The term "thermo power" is a misnomer since it does not measure power, but measures the voltage induced in response to a temperature difference. An applied temperature difference causes charged carriers in the material to diffuse from the hot side to the cold side. Mobile charged carriers migrating to the cold side leave behind their oppositely charged nuclei at the hot side thus giving rise to a thermoelectric voltage. Since a separation of charges creates an electric potential, the buildup of charged carriers onto the cold side eventually ceases at some maximum value since the electric field is at equilibrium. An increase in the temperature difference resumes a buildup of charge carriers on the cold side, leading to an increase in the thermoelectric voltage, and vice versa. The material's temperature and crystal structure influence S; typically metals have small thermo powers because of half-filled bands caused by equal negative and positive charges cancelling each other contributing to the induced thermoelectric voltage. In contrast, semiconductors can be doped with excess electrons or holes, causing the magnitude of S to be large. The sign of the thermo power determines which charged carriers dominate the electric transport.

If the temperature difference ΔT between the two ends of a material is small, then the thermo power of a material is defined approximately as::

and a thermoelectric voltage of ΔV is seen at the terminals.

This can be written in relation to the electric field E and the temperature gradient  by the approximate equation:

The absolute thermo power of the material of interest is rarely practically measured because electrodes attached to a voltmeter must be placed onto the material in order to measure the thermoelectric voltage, inducing a thermoelectric voltage across one leg of the measurement electrodes. The measured thermo power then includes the thermo power of the material of interest and the material of the measurement electrodes and is written as:

Superconductors have S = 0 since the charged carriers produce no entropy. This allows a direct measurement of the absolute thermo power of the material of interest, since it is the thermo power of the entire thermocouple. In addition, a measurement of the Thomson coefficient μ, of a material yields the thermo power through the relation

S is an important material parameter that determines the efficiency of a thermoelectric material; a larger induced thermoelectric voltage and a higher S mean a higher efficiency.

Charge-carrier diffusion:

The Seebeck effect is caused by two things: charge-carrier diffusion and phonon drag. Charge carriers in the materials will diffuse when one end of a conductor is at a different temperature than the other. Hot carriers diffuse from the hot end to the cold end, since there is a lower density of hot carriers at the cold end of the conductor, and vice versa. If the conductor were left to reach thermodynamic equilibrium, this process would result in heat being distributed evenly throughout the conductor . The movement of heat (in the form of hot charge carriers) from one end to the other is a heat current and an electric current as charge carriers are moving.

In a system where both ends are kept at a constant temperature difference, there is a constant diffusion of carriers. If the rate of diffusion of hot and cold carriers in opposite directions is equal, there is no net change in charge. The diffusing charges are scattered by impurities, imperfections, and lattice vibrations or phonons. If the scattering is energy dependent, the hot and cold carriers will diffuse at different rates, creating a higher density of carriers at one end of the material and an electrostatic voltage.

This electric field opposes the uneven scattering of carriers, and equilibrium is reached where the net number of carriers diffusing in one direction is canceled by the net number of carriers moving in the opposite direction. This means the thermo power of a material depends greatly on impurities, imperfections, and structural changes that vary with temperature and electric field; the thermo power of a material is a collection of many different effects.

Early thermocouples were metallic, but many more recently developed thermoelectric devices are made from alternating p-type and n-type semiconductor elements connected by metallic connectors. Semiconductor junctions are common in power generation devices, while metallic junctions are more common in temperature measurement. Charge flows through the n-type element, crosses a metallic interconnect, and passes into the p-type element. If a power source is provided, the thermoelectric device may act as a cooler by the Pettier effect described below. Electrons in the n-type element move opposite the direction of current and holes in the p-type

element will move in the direction of current, both removing heat from one side of the device. When a heat source is provided, the thermoelectric device functions as a power generator. The heat source drives electrons in the n-type element toward the cooler region, creating a current through the circuit. Holes in the p-type element then flow in the direction of the current. Therefore, thermal energy is converted into electrical energy.

Thomson effect:

The Thomson effect was predicted and subsequently observed by Lord Kelvin in 1851. It describes the heating or cooling of a current-carrying conductor with a temperature gradient.

Any current-carrying conductor (except for a superconductor) with a temperature difference between two points either absorbs or emits heat, depending on the material. If a current density J is passed through a homogeneous conductor, the heat production q per unit volume is:

where ρ is the resistivity of the material, dT/dx is the temperature gradient along the wire and μ is the Thomson coefficient. The first term is the Joule heating, which does not change in sign; the second term is the Thomson heating, which follows J changing sign.

where ρ is the resistivity of the material, dT/dx is the temperature gradient along the wire and μ is the Thomson coefficient. The first term is the Joule heating, which does not change in sign; the second term is the Thomson heating, which follows J changing sign.

In metals such as zinc and copper, whose temperature is directly proportional to their potential, when current moves from the hotter end to the colder end, there is a generation of heat and the positive Thomson effect occurs.

Conversely, in metals such as cobalt, nickel, and iron, whose temperature is inversely proportional to their potential, when current moves from the hotter end to the colder end, there is absorption of heat and the negative Thomson effect occurs.

If the Thomson coefficient of a material is measured over a wide temperature range, it can be integrated using the Thomson relations to determine the absolute values for the Pettier and Seebeck coefficients. This needs to be done only for one material; since the other values can be determined by measuring pair wise Seebeck coefficients in thermocouples containing the

reference material and then adding back the absolute thermo power of the reference material.

Lead is commonly stated to have a Thomson coefficient of zero; in fact, it is non-zero, albeit being very small. In contrast, the thermoelectric coefficients of all known superconductors are zero.

Thomson relations:

The Thomson coefficient is unique among the three main thermoelectric coefficients because it is the only one directly measurable for individual materials. The Peltier and Seebeck coefficients can only be determined for pairs of materials; hence, no direct methods exist for determining absolute Seebeck or Peltier coefficients for an individual material. In 1854, Lord Kelvin found relationships between the three coefficients, implying that only one could be considered unique.

The first Thomson relation is

Where T is the absolute temperature, μ is the Thomson coefficient and S is the Seebeck coefficient. The second Thomson relation is

Where Π is the Peltier coefficient. It predicted the Thomson effect before it was formalized.

Device efficiency:

The efficiency of a thermoelectric device for electricity generation is given by η, defined as

The maximum efficiency η_max is defined as

Where T_H is the temperature at the hot junction and T_C is the temperature at the surface being cooled.   is the modified dimensionless figure of merit, which takes into consideration the thermoelectric capacity of both thermoelectric materials being used in the device and is defined as

where ρ is the electrical resistivity,   is the average temperature between the hot and cold surfaces and the subscripts n and p denote properties related to the n- and p-type semiconducting thermoelectric materials, respectively. Since thermoelectric devices are heat engines, their efficiency is limited by the Carnot efficiency, hence the T_H and T_C terms in η_max. Regardless, the coefficient of performance of current commercial thermoelectric refrigerators ranges from 0.3 to 0.6, one-sixth the value of traditional vapor-compression refrigerators.

7.Thermoelectric cooling:

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the atomic scale, an applied temperature gradient causes charged carriers in the material to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence inducing a thermal current.

This effect can be used to generate electricity, measure temperature or change the temperature of objects. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices are efficient temperature controllers.

Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other side against the temperature gradient (from cold to hot), with consumption of electrical energy. Such an instrument is also called a Peltier device, Peltier heat pump, solid state refrigerator, or thermoelectric cooler . The Peltier device is a heat pump: when direct current runs through it, heat is moved from one side to the other. Therefore it can be used either for heating or for cooling (refrigeration), although in practice the main application is cooling. It can also be used as a temperature controller that either heats or cools.

French watchmaker, Jean Charles Athanase Peltier, discovered thermoelectric cooling effect,

also known as Peltier cooling effect, in 1834.  Peltier discovered that the passage of a current through a junction formed by two dissimilar conductors caused a temperature change.  However, Peltier failed to understand this physics phenomenon, and his explanation was that the weak current doesn’t obey Ohm’s law. Peltier effect was made clear in 1838 by Emil Lenz, a member of the St. Petersburg Academy.  Lenz demonstrated that water could be frozen when placed on a bismuth-antimony junction by passage of an electric current through the junction.  He also observed that if the current was reversed the ice could be melted.

In 1909 and 1911 another scientist Altenkirch derived the basic theory of thermoelectric.  His work pointed out those thermoelectric cooling materials needed to have high Seebeck coefficients, good electrical conductivity to minimize Joule heating, and low thermal conductivity to reduce heat transfer from junctions to junctions.  Shortly after the development of practical semiconductors in 1950’s, bismuth telluride began to be the primary material used in the thermoelectric cooling.

Thermoelectric cooler (TEC), or Peltier Cooler is a solid-state heat pump that uses the Peltier effect to move heat. The modern commercial TEC consists of a number of p- and n- type semiconductor couples connected electrically in series and thermally in parallel. These couples are sandwiched between two thermally conductive and electrically insulated substrates. The heat pumping direction can be altered by altering the polarity of the charging DC current. TEC schematic is illustrated in Figure 1. The typical materials used for constructing TEC are:

1. Substrate: aluminum oxide (Al2O3), aluminum nitride (AlN), or barium oxide (BaO)

2. Conductor: Copper

3. Thermoelectric semiconductor

i. n-type: bismuth-telluride-selenium (BiTeSe) compound

ii. p-type: bismuth-telluride-antimony (BiTeSb) compound

4. Assembled and joined by solder.

The TEC can be made in different shapes and sizes, but most common shape is a square or a rectangular substrate device. The practical size of a single stage TEC ranges from 3 mm x 3 mm up to 60 mm x 60 mm. The size limitation of 60 mm x 60 mm is due to the thermal stress. This stress comes from thermal expansion deformations between the cold and the hot junctions of the TEC. To obtain a larger temperature difference, a multistage TEC can be build. The multistage TEC is usually in cascade shape and 6 stages are the maximum practical limit.

TEC can be used in different application where cooling or temperature control of an object is

Required. In general, TEC is most often used when an object:

1. Needs to be cooled below the ambient temperature, or

2. Requires to be maintained at a consist temperature under a fluctuating ambient temperature.

TEC is perfect for cooling a small, low heat load object. Due to the low COP (Coefficient of Performance) compared with compressor cooling, TEC loses its advantage if the cooling load is higher than 200 watts. But, because TECs have no moving parts, they are lightweight and reliable, they create no electrical noise, and can be operated at any orientation or environment, in some instances TECs are used to cool kilowatts of heat. TEC is exceptionally suitable for a precision temperature control of an object such as a laser diode, CCD or other small objects. Paired with a DC power supply and an electronics proportional/integral (PI) controller packaged in a single chip device, TEC is able to control an object to +/- 0.1oC accuracy. Today, no other cooling method yet can provide such precise, simple and convenient temperature control.

THERMOELECTRIC COOLING MODULES:

A practical thermoelectric cooler consists of two or more elements of semiconductor material that are connected electrically in series and thermally in parallel. These thermoelectric element and their electrical interconnects typically are mounted between two ceramic substrates. The substrates serve to mechanically hold the overall structure together and to electrically insulate the individual elements from one another and from external mounting surfaces. After integrating the various component parts into a module, thermoelectric devices ranging in size from approximately 2.5-50 mm (0.1 to 2.0 inches) square and 2.5-50 mm (0.1 to 0.2 inches) in height may be constructed.

Figure 2-1

Both N-type and P-type Bismuth Telluride thermoelectric materials are used in a thermoelectric cooler. This arrangement causes heat to move through the cooler in one direction only while the electrical current moves back and forth alternately between the top and bottom substrates through each N and P element. N-Type material is doped so that it will have an excess of electrons (more electrons than needed to complete a perfect molecular lattice structure) and P-Type material is doped so that it will have a deficiency of electrons (less electrons that are necessary to complete a perfect lattice structure). The extra electrons in the N material and the “holes” resulting from the deficiency of electrons in the P material are the carriers which move the heat energy through the thermoelectric material.

Figure (2-1) shows a typical thermoelectric cooler with heat being moved as a result of an applied electrical current (I). Most thermoelectric cooling modules are fabricated with an equal number of N-Type and P-Type elements where one N and P element pair form a thermoelectric “couple.” The module illustrated in Figure (2-1) has two pairs of N and P elements and is termed a two couple module.

Heat flux (heat actively pumped through the thermoelectric module) is proportional to the magnitude of the applied DC electrical current. By varying the input current from zero to, it is possible to adjust and control the heat flow and temperature.

APPLICATIONS FOR THERMOELECTRIC COOLING:

Applications for thermoelectric devices cover a wide spectrum of product areas including military, medical, industrial, consumer, scientific/laboratory, and telecommunications. Uses range from simple food and beverage coolers for an afternoon picnic to extremely sophisticated temperature control systems in missiles and space vehicles. Occasionally, the use of a thermoelectric cooler was not anticipated during the planning stage of a new product, but later was found necessary to assure proper product operation. More typical, however, the use of a TE device is the only practical solution to a thermal management problem. Unlike a simple heat sink, a thermoelectric cooler permits lowering the temperature of an object below ambient as

well as stabilizing the temperature of objects which are subject to widely varying ambient conditions. A thermoelectric cooler is an active cooling device whereas a heat sink provides only passive cooling. Thermoelectric coolers generally may be considered for applications that require heat removal ranging from mill watts up to several hundred watts. Most single-stage TE coolers, including both high and low current devices, are capable of pumping a maximum of 3 to 6 watts per square centimeter (20 to 40 watts per square inch) of module surface area. Multiple modules mounted thermally in parallel may be used to increase total heat pump performance. Large thermoelectric systems in the kilowatt range have been built for specialized applications such as cooling submarines and railroad cars, but systems of this magnitude are unusual.

Typical applications for thermoelectric devices include:

a) Cooling of Computer Chips and Microprocessors

b) Cooling of CCDs

c) Cooling of Low-Noise Amplifiers

d) Temperature Stabilization of Electronic Components

e) Cooling Infrared Detectors

f) Semiconductor Wafer Probers

g) Infrared Calibration Sources & Black-Body References

h) Temperature Stabilization of Laser Diodes

THE THERMOELECTRIC COOLING ADVANTAGE:

Unlike conventional compressed refrigeration system, thermoelectric cooling modules are a form of solid state cooling that incorporates both semiconductor technologies and electronic assembly techniques. Therefore thermoelectric cooling modules are solid state, vibration-free, and noise free heat pump. They are modular devices, so are simple to install and operate. Since they consist primarily of thermoelectric material sandwiched between ceramic plates and have no moving parts, they are inherently reliable.

Some significant features of thermoelectric devices include:

a) Small Size and Weight

b) Ability to Cool Below Ambient

c) Ability to Heat and Cool with the same Device: Thermoelectric coolers will either heat or cool depending upon the polarity of the applied DC power.

d) Precise Temperature Control: With an appropriate closed-loop temperature control circuit, TE coolers can control temperatures to better than ± 0.1°C.

e) High Reliability: Although reliability is somewhat application-dependent, the life of typical TE coolers is greater than 200,000 hours.

f) Electrically “Quiet” Operation

g) Convenient Power Supply

h) Spot Cooling: With a TE cooler it is possible to cool one specific component or area only, thereby often making it unnecessary to cool an entire package or enclosure.

I) Environmentally Friendly: Refrigeration systems can be fabricated without using chlorofluorocarbons or other chemicals that may be harmful to the environment.

HEAT SINK CONSIDERATIONS:

A heat sink is a term for a component or assembly that transfers heat generated within a solid material to a fluid medium, such as air or a liquid. Examples of heat sinks are the heat exchangers used in refrigeration and air conditioning systems and the radiator (also a heat exchanger) in a car. Heat sinks also help to cool electronic and optoelectronic devices, such as higher-power lasers and light emitting diodes (LEDs).

A heat sink is physically designed to increase the surface area in contact with the cooling fluid surrounding it, such as the air. Approach air velocity, choice of material, fin (or other protrusion) design and surface treatment are some of the design factors which influence the thermal resistance, i.e. thermal performance, of a heat sink. One engineering application of heat sinks is in the thermal management of electronics, often computer central processing unit (CPU) or graphics processors. For these, heat sink attachment methods and thermal interface materials also influence the eventual junction or die temperature of the processor(s). Thermal adhesive (also known as thermal grease) is added to the base of the heatsink to help its thermal performance. Theoretical, experimental and numerical methods can be used to determine a heat sink's thermal performance

   Heat Sink

Rather than being a heat absorber that exotically consumes all applied heat, a thermoelectric cooler is a heat pump which moves heat from one area to another. By reducing the temperature of the “cold” face of a TE device, heat will flow the (warmer) article being cooled into the TE module and then pass through the module to the “hot” face. To complete the thermal system, the hot face of the TE cooler must be attached to a suitable heat sink that is capable of carrying away both the heat pumped by the module plus Joule heat from the electrical power supplied to the module.

Since all optional characteristics of TE devices are related to heat sink temperature. heat sink selection and/or design should be considered carefully. A heat sink temperature rise of 5 to 15°C above ambient is typical for many thermoelectric applications.

Heat sink performance usually is specified in terms of thermal resistance (θs):

Where: θs = Thermal Resistance in Degrees C per Watt

Ts = Heat Sink Temperature in Degrees C

Ta = Ambient or Coolant Temperature in Degrees C

Q = Heat Input to Heat Sink in Watts

INSTALLATION OF THERMOELECTRIC MODULES:

Techniques used to install thermoelectric devices in a cooling system are extremely important and failure to observe certain basic principles may result in unsatisfactory performance or reliability.

Some of the factors to be considered in system design and device installation include the following

a) Thermoelectric devices have high mechanical strength in the compression mode but shear Strength is relatively low. As a result, a TE cooler should not be mechanical structure:

b) All interfaces between system components must be flat, parallel, and clean to minimize thermal resistance.

c) The “hot” and “cold” sides of standard thermoelectric modules may be identified by the position of the wire leads. Wires are attached to the hot side of the module, which is the module face that is in contact with the heat sink. For modules having insulated wire leads, when the red and black leads are connected to the respective positive and negative terminals of a DC power supply, heat will be pumped from the module’s cold side, through the module, and into the heat sink. Note that for TE modules having bare wire leads, the positive connection is on the right side and the negative connection is on the left when the leads are facing toward the viewer.

d) The object being cooled should be insulated as much as possible to minimize heat loss to the ambient air. To reduce conductive losses, fans should not be positioned so that air is blowing directly at the cooled object. Conductive losses also may be minimized by limiting direct contact between the cooled object and external structural members.

e) When cooling below the dew point, moisture or frost will tend to from on exposed cooled surfaces. To prevent moisture from entering a TE module and severely reducing its thermal performance, a suitable moisture seal should be installed. This seal should be formed between the heat sink and cooled object in the area surrounding the TE module(s). Flexible foam insulating tape or sheet material and/or silicone rubber RTV is relatively easy to install and makes an effective moisture seal.

CLAMPING:

The most common mounting method involves clamping the thermoelectric module(s) between a heat sink and flat surface of the article to be cooled. These approaches may be applied as follows:

a) Clean the module(s) and mounting surfaces to insure that all burrs, dirt, etc. have been removed.

b) Coat the “hot” side of the module(s) with a thin layer (typically 0.02mm / 01” or less thickness) of thermally conductive grease and place the module, side down, on the heat sink in the desired location. Gently push down on module and apply a back and forth turning motion to squeeze out excess thermal grease. Continue the combined downward pressure and turning motion until a slight resistance is detected. Suggested thermal greases include Wakefield Engineering Type 120, General Electric Type G641, Dow Corning Type 340, and American Oil and Supply Type 300. Note: Make sure that there are no chips, dirt, etc. at the grease interface as the presence of such material will degrade thermal performance.

c) Coat the “cold” side of the module(s) with thermal grease as specified in Step (b) above. Position and place the object to be cooled in contact with the cold side of the module(s). Squeeze-out the excess thermal grease as previously described.

d) It is important to apply uniform pressure across the mounting surfaces of the heat sink and cooled object together so that good parallelism is maintained. If significantly uneven pressure is applied, thermal performance may be reduced, or worse, the TE module(s) may be damaged.

8.POWER SUPPLY REQUIREMENTS:

7805 REGULATOR IC

This regulator IC is used to convert 12v or 9v into 5v dc supply  . This 5v supply is used for whole circuit . Basically it is used to overcome the fluctuations present in the input supply .

RESISTORS

The flow of charge (or current) through any material, encounters an opposing force similar in many respect to mechanical friction. This opposing force is called resistance of the material. It is measured in ohms.  In some electric circuits resistance is deliberately introduced in the form of the resistor.

Resistors are of following types:

1.      Wire wound resistors.

2.      Carbon resistors.

3.      Metal film resistors.

Wire Wound Resistors:

Wire wound resistors are made from a long (usually Ni-Chromium) wound on a ceramic core. Longer the length of the wire, higher is the resistance. So depending on the value of resistor required in a circuit, the wire is cut and wound on a ceramic core. This entire assembly is coated with a ceramic metal. Such resistors are generally available in power of 2 watts to several hundred watts and resistance values from 1ohm to 100k ohms. Thus wire wound resistors are used for high currents.

Carbon Resistors:

Carbon resistors are divided into three types:

a.       Carbon composition resistors are made by mixing carbon grains with

binding material (glue) and moduled in the form of rods. Wire leads

are inserted at the two ends. After this an insulating material seals the

resistor. Resistors are available in power ratings of 1/10, 1/8, 1/4 ,

1/2 , 1.2 watts and values from 1 ohm to 20 ohms.

b.      Carbon film resistors are made by deposition carbon film on a ceramic

rod. They are cheaper than carbon composition resistors.

c.       Cement film resistors are made of thin carbon coating fired onto a

solid ceramic substrate. The main purpose is to have more precise

resistance values and greater stability with heat. They are made in a

small square with leads.

Metal Film Resistors:

They are also called thin film resistors. They are made of a thin metal   coating deposited on a cylindrical insulating support. The high resistance values are not precise in value; however, such resistors are free of inductance effect that is common in wire wound resistors at high frequency.

Variable Resistors:

Potentiometer is a resistor where values can be set depending on the requirement. Potentiometer is widely used in electronics systems. Examples are volume control, tons control, brightness and contrast control of radio or T.V. sets.

Fusible Resistors:

These resistors are wire wound type and are used in T.V. circuits for protection. They have resistance of less than 15 ohms. Their function is similar to a fuse made to blow off whenever current in the circuit exceeds the limit.

Resistance of a wire is directly proportional to its length and inversely proportional to its thickness.

RESISTOR COLOR CODE

Example:   1k or 1000 ohms

COLOUR CODES

COLOUR	NUMBER	MULTIPLIER	COLOUR	TOLERANCE
Black Brown Red Orange Yellow Green Blue Violet Grey White Gold Silver	0 1 2 3 4 5 6 7 8 9	100 101 102 103 104 105 106 107 108 109 10-1 10-2	Gold Silver No colour	5% 10% 20%

CAPACITORS

A capacitor can store charge, and its capacity to store charge is called capacitance. Capacitors consist of two conducting plates, separated by an insulating material (known as dielectric). The two plates are joined with two leads. The dielectric could be air, mica, paper, ceramic, polyester, polystyrene, etc.  This dielectric gives name to the capacitor. Like paper capacitor, mica capacitor etc.

Types of Capacitors:

Capacitors can be broadly classified in two categories, i.e., Electrolytic capacitors and Non-Electrolytic capacitors as shown if the figure above.

Electrolytic Capacitor:

Electrolytic capacitors have an electrolyte as a dielectric. When such an electrolyte is charged, chemical changes takes place in the electrolyte. If its one plate is charged positively, same plate must be charged positively in future. We call such capacitors as polarized. Normally we see electrolytic capacitor as polarized capacitors and the leads are marked with positive or negative on the can. Non-electrolyte capacitors have dielectric material such as paper, mica or ceramic. Therefore, depending upon the dielectric, these capacitors are classified.

Mica Capacitor:

It is sandwich of several thin metal plates separated by thin sheets of mica. Alternate plates are connected together and leads attached for outside connections. The total assembly is encased in a plastic capsule or Bakelite case. Such capacitors have small capacitance value (50 to 500pf) and high working voltage (500V and above). The mica capacitors have excellent characteristics under stress of temperature variation and high voltage application. These capacitors are now replaced by ceramic capacitors.

Ceramic Capacitor:

Such capacitors have disc or hollow tabular shaped dielectric made of ceramic material such as titanium dioxide and barium titanate. Thin coating of silver compounds is deposited on both sides of dielectric disc, which acts as capacitor plates. Leads are attached to each sides of the  dielectric disc and whole unit is encapsulated in a moisture proof coating. Disc type capacitors have very high value up to 0.001uf. Their working voltages range from 3V to 60000V. These capacitors have very low leakage current. Breakdown voltage is very high.

Paper Capacitor:

It consists of thin foils, which are separated by thin paper or waxed paper. The sandwich of foil and paper is then rolled into a cylindrical shape and enclosed in a paper tube or encased in a plastic capsules. The lead at each end of the capacitor is internally attached to the metal foil. Paper capacitors have capacitance ranging from 0.0001uf to 2.0uf and working voltage rating as high as 2000V.

Conclusion

Measurements of the electrical conductivity, Hall coefficient, thermoelectric power and Nernst coefficient on specimens cut from zone melted Bi₂ Te₃ and on a single crystal are described. The current flow was parallel to the cleavage planes and the galvanomagnetic effects of the single crystal were measured with the magnetic field perpendicular and parallel to the cleavage planes. For the other specimens the Hall and Nernst coefficients were measured with the magnetic field perpendicular to the cleavage planes. The temperature range was 100°K to 600°K and specimens with a wide variation of impurity content were used. Analysis of the thermoelectric power and conductivity in the extrinsic range indicated that the specimens were partially degenerate, and when allowance is made for this, the temperature dependence of mobility is T^-1.63 for electrons and T^-1.94 for holes. In the high temperature range the conductivity measurements yield a value of the energy gap at 0°K of 0.21 eV. The temperature dependence of the Hall coefficient in the extrinsic range is anomalous and the explanation is not known. The Nernst coefficient Q exhibits the general behaviour expected for a semiconductor, and it is shown that it obeys a relation which can be derived from Moreau's relation, Q being proportional to TRσd²E/dT² (thermoelectric power = dE/dT, Hall coefficient = R, conductivity = σ).

Measurements have been made of the thermal conductivity and thermoelectric power of a range of single crystal samples of p- and n-type B1₂Te₃ between 6 and 200 K. The observed lattice conductivity could be interpreted in terms of scattering by the tellurium isotopes. Measurements on the iodine doped samples showed the existence of appreciable impurity scattering. Interpretation of the thermoelectric power results suggests a multiband structure for Bi₂Te₃.

But practically the temperatures are noted down by employing the thermometers in this arrangements, And it has been observed that the temperature range with respect to voltage variation is dependent on atmospheric conditions in which the arrangement is kept but ideally at room temperature(i.e.-at 24 C) the cooling teamperature can approeach to 2 deg C when supplied a voltage of 12 V.

It can be improved by using the multiband structure of the Bismuth telluride.

9. References:

• www.Google.com
• www.Youtube.com
• www.magaland.com
• www.ferrotec.com/technology
• www.Wikipedia.org.
• Refrigeration and air conditioning by Domkundwar