thermoelectric panels

INTERN REPORT TOPIC : SYNTHESIZING THERMOELECTRIC MATERIALS AND MAKING PANELS

NAME : DEEPANSHU BHATIA

ENTRY NO. : 2012PH10837

PROFESSOR : CHESTA RUTTANAPUN

GUIDE : HAI RUDRADAWONG

INSTITUTE : KING MONGKUT INSTITUTE OF TECHNOLOGY LADKRABANG , BANGKOK

PREFACE

This report documents the work done during the summer internship at

Thermoelectric Laboratory of King Mongkut Institute of Technology

Ladkrabang (KMITL), Bangkok, Thailand under the supervision of Prof. Chesta

Ruttanpun. The report first shall give an overview of the tasks completed during

the period of internship with technical details. Then the results obtained shall be

discussed and analyzed. Report shall also elaborate on the the future works

which can be persuaded as an advancement of the current work.

Deepanshu Bhatia

2012PH10837

ACKNOWLEDGEMENT

The internship opportunity I had with KMITL was a great chance for learning by practical experiences. I am also grateful for having a chance to meet so many wonderful people and professionals who led me though this internship period.

I am using this opportunity to express my deepest gratitude and special thanks to Prof. Wichit Sirichote to guide me at their esteemed organization during the training.

I express my deepest thanks to Prof. Chesta Ruttanpun and Hai Rudradawong [Phd Student] for taking part in useful decision making & giving necessary advices and guidance and arranging all facilities to make the project go smoothly. I choose this moment to acknowledge his contribution gratefully.

I will strive to use gained skills and knowledge in the best possible way, and I will continue to work on their improvement, in order to attain desired career objectives. Hope to continue cooperation with all of you in the future.Sincerely,

Deepanshu Bhatia

ABSTRACT

The report presents the four tasks completed during summer internship at KMITL which are listed below:

1) Synthesis of Thermoelectric Material from Solid state reaction process.

2) Making Pellets from thermoelectric powder using Hydraulic press.

3) Making Thermo-electric panel of different sizes.

4) Testing of thermoelectric Panels.

All these Tasks have been completed successfully and the results obtained were according to expectations with some minor errors.

Also during the evaluation of pellets it was found that the pellet’s finishing depends not only on the quality of fine powder but also on the pressure applied on it in hydraulic press and the time for which it is kept.

We had limited testing facilities. Panel could be heated only upto 200 C. Glue used in the panels could only withstand up to 100C. Also we did not have the required equippments to test for Seebeck coefficient. So, we have a limited scope to test the panels and sure we have exhausted the opportunities to maximum extent.

INTRODUCTION

Thermoelectric substances :

Thermoelectric substances can be used to produce electric current from heat and also cooling and heating from electric current.When a conductive material is subjected to a thermal gradient, charge carriers migrate along the gradient from hot to cold; this is the Seebeck effect. In the open-circuit condition, charge carriers will accumulate in the cold region, resulting in the formation of an electric potential difference.The Seebeck effect describes how a temperature difference creates charge flow, while the Peltier effect describes how an electrical current can create a heat flow.

Electrons transfer heat in two ways: 1) by diffusing heat through collisions with other electrons, or 2) by carrying internal kinetic energy during transport.

The former case is standard heat diffusion, while the latter is thePeltier effect. Therefore, the Seebeck effect and the Peltier effect are the opposite of one another.

The thermoelectric effect is the fact that a temperature gradient in a conducting material results in heat flow , this results in the diffusion of charge carriers. The flow of charge carriers between the hot and cold regions in turn creates a voltage difference.

Basic Physics Behind the experiment:

When an end of a thermoelectric material is heated up, electrons in the warmer side of material have more energy. Since they are free to move, the excited electrons diffuse toward the colder side of the thermocouple leaving holes behind.

The electric field develops by the movement of these electrons point towards the colder side of the thermoelectric material which makes the cold side negative relative to the warmer side. The electrons will keep moving toward the colder side until the potential established by the charge separation counteracts the flow of electrons which creates equilibrium.

S = [ V(Plate1) – V(Plate2) ] ---------------------------------------------------

[ T(Plate1) - T(Plate2) ]

Therefore, One can say that to obtain a high power output from thermoelectric material , the potential difference obtained should be higher for minimum temperature difference , i.e the Seebeck Coefficient of a material should be higher for good results.

Delafossite structure and its significance :

Delafossite is a copper iron oxide mineral with formula CuFeO2 . . It is member of the delafossite mineral group with a general formula ABO2, a group characterized by a sheet of linearly coordinated A cations stacked between edge-shared octahedral layers (BO6). Delafossite along with other minerals of the ABO2 group has been recognized for its electrical properties from insulation to metallic conduction.

By heating the given sample at the specified temperature for the given time we get delafossite structure of the compound. Our compound is not CuFeO2, but it contains substituted Cr in the delafossite CuFeO2 to see the effect of Cr content on the thermoelectric properties.

METHODS AND PROCEDURES FOLLOWED

Synthesis of thermoelectric Substances, fabrication and testing of Thermo-electric panelsPart 1 :Synthesis of thermoelectric SubstancesWe synthesised thermo-electric substances from chemicals. We mix the required chemical compounds in the required proportion. Then we followed the following procedures to get our thermoelectric material prepared in powdered form.

1. Ball milling :After mixing the compounds we put the mixture in a container with milling balls. The milling balls are of different sizes and help to mix the compound and make a fine powder mixture. Put all the contents into a plastic container along with the milling balls and ethanol and put the container for milling for 24 hours. Ethanol does not react chemically with the compounds, it just provides a medium for the milling balls to mix the substance properly. After 24 hours the milling is over we get a solution of the mixture of fine powder with ethanol.

2. Heating in heater:After ball-milling we need to remove the ethanol. So, we put our compound in a heater at about 90-100 C for 24 hours. The ethanol evaporates leaving behind the properly mixed compounds. Till now it is mixed uniformly but not in fine powdered form. We have to manually crush it by using mortar and pestle to very fine powder before the next step. After, it gets converted to very fine powder we move to the next step. We have to crush each sample for about 1-2 hours to get very fine powder using two type of mortar and pestle.

3. Chemical reaction in oven :Now comes the main transformation. This mixture is now heated at a specific temperature under certain conditions to get the required delafossite structure. We heat the sample at 1050 C for 20 hours in an electrical oven. The temperature vs time graph programmed in the oven is as follows :

0 5 10 15 20 25 300

200

400

600

800

1000

1200

Y-Values

Heating Electric Oven:

The oven rose from room temperature to required set temperature 1050 C in 3 hours then it stays at that temperature for 20 hours and then we take out the sample at this temperature to room temperature quickly to allow quenching.

After cooling down the sample at room temperature we get a hardened solid mass of the required thermoelectric substance. Now using the bigger mortar and pestle crush it into smaller pieces and then more fine pieces, finer powder and the very fine. It takes a lot of time about 6-7 hours to crush it into final usable fine powder.

Mortar and pestle :

This final fine powder is our thermoelectric substance. Part 2:

Making the pellet

Now, using the fine powder we can create thermoelectric pellets of different shapes from cylindrical or cubical. We are considering only cylindrical ones. We use iron moulds to make these pellets. Moulds look like this :

Using the moulds and applying a pressure of 9-12 *(10^6) Pascal, we get our pellets. We use a hydraulic press for the purpose :

We make pellets of different sizes to compare their properties. Right now the pellet is just compressed powder and it’s not very strong. We again repeat the heating in oven step to make them strong. The pellets were sintered in furnance at 1050 C under air atmosphere for 24h. After heat treatment the sample was rapidly quenched at room temperature. Now these pellets are ready to use in our thermoelectric panel.

Pellets :

Part 3 :

Making the panel To make the panel we have to keep the pellets in a grid pattern with a*b pellets.The base and head of panel are either made of ceramic thermally conducting substance or metallic plates (zinc).

First we make a series connection between the pellets using copper plates. These copper plates are joined with the pellets with conducting glue (Glue 1). After this connection the pellets are connected in series such that when the whole system is placed between temperature gradients the thermoelectric currents of all

the pellets add up. The connection made with this glue have to be kept in open at room temperature for 2-3 days. After that it was kept for 1 day in heater at 100 C.

Connecting the pellets in series using copper plates and copper wires :

After making the pellets in series this whole grid like system has to be joined to the ceramic or metallic plates. The glue (Glue 2) used is thermally conducting and electrically insulating. In case of metallic plate panel we have to be very careful because any error in insulating glue can lead to short circuiting the whole panel with zinc plates on top and bottom, hence hampering the performance. To ensure that the glue is properly applied it has to be dried one day in room temperature and subsequent day in oven at 100 C. After these procedures ensure that all connections are properly made and no connection is short circuited by any of glue thermally or electrically.

Panel (with ceramic base ) :

Panel (with metallic base ) :

Part 4 :

Testing the panel

To test the panel we apply a finite temperature gradient and measure the series current of the panel. We first measure the resistance of the panel at room temperature. Then apply an external resistance equivalent to this in our circuit. Add an ammeter in series, voltmeter in parallel and then gradually increase the temperature gradient and observe the change in current.The colder junction is maintained by keeping ice on it and hotter one by using heater.

This way we get the readings and plot the graph for various panel of different grid configurations, different thermoelectric substance, different heights of pellets and get our conclusions.

Observations and Results:From the above process we prepared Two different kind of thermoelectric substance 1) Cu Fe 0.75 Cr 0.25 O 2 (Sample 1)2) Cu Fe 0.9 Al 0.1 O 2 (sample 2) And from these two substances we prepared total 6 Modules3 From sample 1 and 3 from Sample Two as Follows

*here parameter used to compare the size of pellet is weight (in grams) of pellet because the height is directly proportional to wt., as other conditions like pressure, temp. were identical during its manufacturing. From Sample 1:Module No. Pellet’s weight

(in gm of sub. filled )

Type of Plates used

Sam1 Mod a 4gm CeramicSam1 Mod b 4gm MetallicSam1 Mod c 5gm Ceramic

From Sample 2:Module No. Pellet’s weight (in

gm of sub. filled )Type of Plates used

Sam2 Mod a 2gm MetallicSam2 Mod b 3gm MetallicSam2 Mod c 6gm ceramic

For testing the module’s performance, we prepared a following setup by applying a temp gradient on the opposite sides of modules we measured the output current as a function of temperature Gradient.Case 1: Sam1 Mod aBottom Surface Temp ( in C)

Top Surface Temp (in C)

Temp gradient Current Output ( in uA)

21.5 13 8.5 2130 17 13 3242.4 21.3 21.1 5056 24.8 31.2 5869 30.6 38.4 65.3

83 28.5 54.5 76.993 31 62 86.7

Case 2: Sam1 Mod bBottomSurface Temp ( in C)


Temp gradient

Current Output ( in uA)

70 30 40 5880 35 45 72115 28 87 163130 45 90 191150 50 100 203

Case 3: Sam1 Mod cBottom Surface Temp ( in C)


Temp gradient


46 13 33 11055 18 37 12056 18.4 37.6 12371.6 22 49.6 17284.4 32 52.4 192.2103.3 34 69.4 246125 50 75 279135 55 80 300

Case4: Sam2 Mod aBottom Surface Temp ( in C )


Temp gradient


45 20 25 1160 27 33 1277 32 45 15

Case 5: Sam 2 Mod cBottom Surface Temp ( in C)


Temp gradient


35.5 13.5 22 95.747.5 20.5 27 114.860 30.4 29.6 16070 34 36 18068 30.2 37.8 212

73 32 41 241105 42 63 280108 40 68 323124 50 74 345131 40 91 382

# Note: Out of all modules two Modules (Sam 2 Mod 1, Sam 2 Mod 2) were defective as we can see from the readings that the output current is very low and not increasing with increase in gradient and also the Internal Resistance is not as per the expectation there might be some short circuiting in the module.

Conclusion:From the above results one can conclude that the Current output of modules increases with increase in Temp. Gradient, i.e Temp gradient is providing an electromotive force implicitly.Other conclusion which can be drawn from the results is that the two modules named as Sam2 Mod a, Sam 2 Mod b were defective, may be the conducting glue (electrically) had made a contact between the top and bottom plates or maybe there was an electrical contact between the metallic plates and the pellets due to which the total internal resistance was too low and the whole module was short circuited. The thermally conducting glue used to connect pellets with the plates was not a good choice to be used in our modules because as we reached at temp. around 130 C the thermally conducting glue( Fiber Adhesive) started to break. Due to which we had to limit ourselves to Max limit of 130-150 C, and got the Maximum Current readings till this Temp range only.Also max. Current for a module was more in case of module where large pellet (larger in length) of same material were used.We got the Max current for module named as Sam 2 Mod c of 382 uA ( i.e for Sample 2) , that too at Temp gradient of 91 C , but if we had some better Thermally conducting glue that can with stand in Temp range of 400-500 C than , we would have got a far better readings.

Scope of Improvement in Future:

We tried a very new concept of using a Metallic plates instead of Ceramic Plates in the Modules and as result we got enhanced values of Max . current (almost Twice) through the Metallic Plates Modules than from the Ceramic plates module.We also tried to decrease the heat conduction between top and Bottom plates (as it degrades the Temp gradient produced by heating one side) by replacing the copper plates used to connect the pellets in series by copper wires, but unfortunately due to some manufacturing errors the circuit got short circuited and we were not able to take the real readings of modules and we didn’t have enough time in the end to make that type of module again but I think is proper care is taken while joining the pellets with copper wires instead of copper plates , the heat conduction can between two plates can be reduced upto an extent and we can get high temp gradient by heating upto a small range of Temp.

Sources Of Error:

During the preparation of pellets the powder was kept in a ceramic dish for days wrapped in Al Foil, due to which after some time moisture got into it and when we made pellets from that moisturized powder they we not as good in finishing as the one made by dry powder.

Also some times the pressure applied to Dies kept in Hydraulic Press to make Pellets was not same in each case some time it dropped to half of its actual value which might have brought non uniformity in pellets.

While preparing modules and joining pellets with copper plates and electrically conducting glue there were instances when electrical glue spread over the pellet to get is short circuited.

While applying thermally conducting Glue sometimes there were direct contacts made between top and bottom plates by the overflow of glue.

And there are manual error also while measuring the readings like contacts are not made proper while connecting external circuits.

# If these all errors are handled properly and the new methods we tried like using metallic plates and copper wires and using a high temp. resistant glues, we can reach to a whole new level of Thermoelectricity in Future.

REFERENCES :

Wikipedia : Thermoelectric substances

Research paper on Thermoelectric properties of Sn+2 –Substituted CuFeO2 Delafossite-Oxide (Advanced Materials Research Vol. 802(2013) pp 17-21 , doi:10.4028/www.scientific.net/AMR.802.17)

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