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Design and construction of a Peltier temperature-control device
Advanced Energy Conversion.
Vol. 2, pp. 255-263.
Pergamon Press, 1962. Printed in Great Britain
DESIGN A N D CONSTRUCTION OF A PELTIER TEMPERATURE-CONTROL DEVICE IRVING I. SOCHARD*t Summary--A Peltier temperature-control device for use with micro-miniature electronics has been designed, built and tested. The principal features sought were (1) compatibility with conventional power supplies, particularly single-cell electrochemical systems, (2) techniques of assembly that are versatile with respect to physical configurations, and (3) techniques of assembly that are readily adaptable to production. The device contains a 1 ina temperature regulation chamber and operates from a supply voltage of 1"5-2"0V at input powers up to about 6 W. Eighteen thermoelectric couples were connected in series electrically and in parallel thermally. Aluminum was used as the structural material, serving as both the base and the temperature'controlled chamber. An anodized layer on the aluminum provided good electrical insulation and thermal contact between the structural materials and thermoelectric elements. The circuit patterns were placed on the anodized layer by printed-circuitry techniques. By using jigs that are adaptable to quantity production, all 72 solder joints connecting the elements in series and to the structural materials were made in two operations. Tests and evaluation of the device show the performance to be that predicted by simple theory: (1) a maximum temperature reduction of 48°C from a room-temperature ambient; (2) a maximum input power of less than 6 W to maintain the controlled chamber at 25°C while the ambient varies throughout the military range ( - 5 4 to ÷71°C). INTRODUCTION MOST military electronics are required to operate in ambient temperatures throughout the range o f - - 5 4 ° C to + 71 °C. I n m a n y circuits about half the components are needed to provide temperature compensation throughout this range, or to make up for loss due to such compensation. A properly designed Peltier device, together with appropriate sensing and control elements, can maintain the electronics at a temperature within a selected range, say 20°C-30°C, while the ambient varies throughout the military range. Use o f the Peltier device should, in addition, reduce the required development time by eliminating the temperature compensation problem. It will also increase the circuit reliability since fewer components are needed. A n additional benefit is a cost reduction, b o t h in development and in the produced item. The cost reduction can be substantial in dense electronic packaging where the electronics cost m a y be o f the order of $2000 per in3~ while a Peltier device to control the temperature o f this volume will cost less than $100. It is inherent in present o f thermoelectric materials that the voltage to be applied to a single couple for m a x i m u m cooling is about 0.1 V. A practical device, then, must consist o f a number o f couples connected electrically in series so that a conventional power supply m a y be used. In order to formulate and verify simple principles o f device design, and also to demonstrate the feasibility o f such temperature-control devices, an exploratory device has been designed, built and performance tested. Selection o f the major design parameters was as follows: (1) the device was to operate f r o m a standard power supply, a single-ceU electro* Diamond Ordnance Fuze Laboratories, Washington 25, D.C. Present address: Emerson Research Laboratories, Emertron Inc., 1140 East-West Highway, Silver Spring, Maryland. :~ Personal communication, N. Doctor, of Diamond Ordnance Fuze Laboratories. 255
256
IRVrNG I. SOCHARD
chemical battery of nominally 1.5 to 2.0 V, (2) the temperature-controlled chamber was to have a 1 in 3 volume, and (3) the maximum net heat-pumping capacity for cooling was to be about 4 W. In addition, considerable attention was given to making all construction techniques adaptable to mass-production. D E S C R I P T I O N AND C O N S T R U C T I O N An 18-couple temperature regulation device was designed and constructed. To keep the current requirements as low as possible, the smallest diameter thermoelectric material available at the time (nominally ~ in.) was used. All the internal connections to the 36 elements were made by printed-circuitry techniques. The circuitry was formed directly on an anodized layer on aluminum sheet. This technique allows good thermal contact to the aluminum base while providing electrical insulation from it. Figure 1 is a cross-sectional view of the device. The device weighs 3 oz. Figure 2 is a photo of the unassembled components. POLYURETHANE INSULATION
FOAM /
TEMPERATURE CONTROL CHAMBER (ANODIZED ALUMINUM)
.R,NTEO iiiiiiiiiiiiiiyiiiiiii2iiii Ei!iiiiiii!ii!!:!iiiii!i !ii iiiiiiiiii!iii!!i!i ! !ii!!iii! i CONNECTORS--. liiiiiiiiiiiiiii;1 THERMOELECTR,C ~ CIRCUIT
ELEMENT
BASE PLATE (ANODIZED ALUMINUM)
O
::::::::::::::::::::::::
.ii::ii:;:~;;......
iil
~ i i l ] I
2
CLEARANCEHO'E
/¢/
(FOR 4-401
3
IN.
F/G. 1. Cross-sectional view of the thermoelectric temperature control device. The chamber, 1.4 x 1.4 × ½ in., was constructed of 0.031 in. anodized aluminum. The chamber junction circuitry was printed on the anodized layer at the bottom of the chamber. The 36 cylindrical elements, arranged in a 6 x 6 square pattern 0.210 in. apart, center to center, were soldered between the base plate and the chamber. The elements are 0.400 in. long with a diameter averaging 0.130 in. The base plate is 2 x 3 in. ; clearance holes for No. 4 machine screws were drilled at each corner to allow the device to be fastened to a heat sink. The plate was made of -tk in. anodized aluminum and the plate junction circuitry was printed on top of it. Any metallic structure of sufficient size can be used as a heat sink. Examples of usable heat sinks are electronic chasses, air frame, vehicular structures, etc. For laboratory use, an 11 x 14 in. sheet of ¼ in. aluminum was used. The base plate and chamber were made of 52-S aluminum, which anodizes well. They were machined, using normal shop methods, then carefully finished to be free from scratches, but were not polished. After all machining operations were complete, the aluminum parts were heavily anodized. A thin coating of electroless nickel was then deposited on the anodized layer. This was used as a base for depositing a 2 mil layer of electrolytic copper (Fig. 3(a)). The aluminum parts were then covered with photo resist. The patterns representing the junction circuits were projected on them, and the unexposed portion of the photo-resist removed chemically. Next the parts were etched to remove the unprotected copper and
Ill
BA~=
I
~ = ~ , =
O-EL ELEMEN" /-JIG A
FIG. 2. Components ready for the first assembly step. The printed circuitry has already been formed and the parts solder dipped.
FIG. 4. Devices after first assembly step. Jig A has been removed.
[facing p. e.'i6]
Fro. 5. Device ready for final assembly step, utilizing jig B. After the solder melts, the device is pushed down the guide rails to complete assembly.
Design and Construction
of a Peltier Temperature-Control
Device
257
nickel. The remainder of the photo-resist was then removed* (Fig.(3b) ). The parts were then solder-dipped in a tin-bismuth-antimony solder. The solder adhered to the copper plating but did not wet the aluminum oxide (Fig. 3(c) and Fig. 2). The base plate and chamber were then ready for assembly.
ELECTROLESS NICKEL ELECTROLYTIC COPPER
ALUMINUM OXIDE
ALUrtrlNUM
(A) DEPOSITION FIG.
3. Major
SOLDER
(B)
PHOTO- ETCH
CC) SOLDER
DIP
steps in the preparation of the printed circuitry. (These drawings are not to scale.)
The alloyed bismuth telluride thermoelectric material was received from the manufacturer in the form of rods 9 in. long and approximately 4 in. dia.? These were cut to the correct length on a cut-off wheel. No supplemental grinding was needed since small variations in length were compensated by variations of the solder thickness in the finished device. The individual elements were then rolled in tape, leaving only the ends exposed. They were next dipped into a high-acidity chloride flux and then into the above solder. The solder adhered to the exposed ends only. The tape was then removed and the elements were ready to assemble. For quantity production, the rods could be epoxy-coated before cutting. The cutting process would expose the ends to be soldered. The first part of the assembly was carried out using the jig labelled A in Fig. 2. Jig A was attached to the base plate by four small screws. The elements were then dropped through the holes in the jig, alternating n- and p-type elements. The assembly was then placed on a hotplate just long enough for the solder to melt. The elements then seated themselves by gravity. Upon cooling, the jig was removed (Fig. 4). The second assembly step attached the chamber to the elements utilizing jig B (Fig. 5). The chamber and the assembly were put on the jig, the guideposts going through the screwclearance holes in the assembly. The entire unit was again put on the hotplate till the solder melted. The assembly was then pushed against the chamber and the entire unit removed from the hotplate. When the unit had cooled, the jig was removed and the assembly was complete except for insulation. The insulation is cut urethane foam 0.3 in. thick, except between .the elements where silica-gel was used, In a production model, the insulation would all be foamed in place.
* This process, used in forming the printed circuit, was developed by Miss E. L. Hebb of the Diamond Ordnance Fuze Laboratories. t The material was obtained from the Melcor Company of Trenton, New Jersey. 9
25
IRVING I. SOCHARD
TESTS
AND
EVALUATION
The
device was tested to determine performance characteristics under normal laboratory conditions of temperature and air movement. During most of the tests, the unit was mounted on the 11 in. x 14 in. aluminum heat sink. A “semi-infinite” heat sink* improved performance only a few percent. The most important parameters measured were current through the unit, voltage drop at the terminals of the unit, the temperature of the inside of the chamber, and the ambient temperature. The current and voltage measurements are correct to a few per cent. The temperature of the chamber was measured with a calibrated thermocouple and is accurate to within f 1 “C. For some tests, the difference between the chamber temperature and room temperature was calculated. This value, AT,, is the net temperature difference produced, which is always less than AT, the temperature difference from one end of the thermoelectric elements to the 0ther.t The net temperature difference is less than AT because of the following major reasons: (1) The temperature change in the heat sink due to the heat rejected to it by the device. (2) The thermal resistance of the pressed contact between the heat sink and the base plate. (3) The thermal resistance across the anodized layers at the hot and cold junctions. In another series of tests, the ability of the device to remove heat generated in the chamber was measured. This heat, Qe, is less than Q, the total amount of heat removed by the device, by the amount of heat leakage through the insulation. Figure 6 shows the relationship between the net temperature reduction, AT,, and the electric power consumed. The current at each point and the voltage at some points are indicated. There was no heat load, Qe, in the chamber. Nearly all the effective cooling obtainable can be achieved by a single-cell battery as indicated. Lesser degrees of cooling are obtainable by reducing the input power. It should be noted that the current-voltage relationship is ,not linear. The rate of cooling at different current levels is shown in Fig. 7. The time constant decreases with increasing current. The maximum rate of initial cooling occurs for currents higher than those which produced maximum steady-state cooling. The result of reversing the current at 1 A is shown also. The chamber was empty during these runs. The time constant would be increased by a factor of 2-3 if the chamber had been filled with microminiature electronics. The present model was not specifically designed for rapid cooling. The response time could have been decreased several times by using an alternate design. However, this result would be achieved at the cost of increased power consumption and size. Figure 8 indicates the relationship between the heat removed and the effective temperature difference obtained for various currents. The lines of constant current are, in addition, approximately constant power levels. These values are shown as well. The lines of equal COP, for a few values are also shown. COP, is defined as Qe over the power required by the device. Figure 8 was calculated by assuming a, T, K, and p were independent of temperature and is only approximate. However, all the temperature differences indicated * A 16 in.
x 20 in. sheet of + in. aluminum with a flow of air directed along the under side. t This is the AZ’ most often referred to in the literature on thermoelectricity.
Design and Construction of a Peltier TemperatureControl
Heat (Room
load
in
Device
chamber,
259
Qe=O
temp. =24-C
Coolingrange for
0
I
2
3
4
Power
5
required.
6
7
8
9
W
FIG. 6. Relationship between the net temperature reduction achieved from a room temperature ambient and the electric power required.
are accurate to a few degiees below room temperature. Similar curves can be calculated for other temperature ranges. In addition, they can be extended to the other quadrants where they apply to heating applications. This type of curve is useful in obtaining graphic solutions to many applications problems. Figure 9 indicates the power required to maintain a constant temperature of 25°C in the chamber through a wide raqge of ambients. The range of military ambients (- 54°C to 71 “C) is shown. The device would be capable of maintaining a constant chamber temperature throughout this range if coupled with the appropriate temperature sensing element, feedback, and power supply. A maximum power of approximately 6 W would be needed. If the requirements for a particular application were only that the temperature excursion be reduced, useful results can be achieved with less power.
,L
CURRENT REVERSED
AMt-
TIME,
MIN
FIG. 7. Thermal response time of the Peltier device.
260
IRMNG
I.
f&HARD
FOR ROOM TEMPERATURE
Q,=
FIG.
NET
RATE
OF HEAT
REMOVAL
FROM
AMBIENTS
CHAMBER
(W)
8. Linearized
cooling characteristics for the thermoelectric temperature control device. The lines of equal practical coefficients of performance are shown also.
6.0
5.0 z 4.0 8 .: s t
3.0
; k Q.
2.0
0
-80
-60
40
-20 Ambient
0
20
temp.,
40
60
“C
FIG. 9. The electric power required to keep the regulated chamber at a constant 25°C for a wide range of temperatures.
Design and Construction of a Peltier Temperature-Control Device
261
A 2 W supply could be used to reduce the temperature excursion by 213 over the military ambient range. The temperature reduction that can be obtained with a particular device can be expressed as :
when
and where the subscript D indicates the terms applied to the assembled device. T is the average chamber temperature of the range being considered .The thermal resistance of the contacts is assumed to be negligible, thus AT = ATe. The terms QD,KD, and RD can be found by simple measurements. The Seebeck coefficient for the device aD, is determined from a measurement of the Seebeck voltage developed when the chamber and the base plate are kept at different fixed temperatures. UD was found to be O-0079 V/C. KD, the thermal conductance of the device is evaluated by putting a resistor of known dissipation in the chamber and observing the temperature rise at equilibrium. KD is found to equal 0.077 W/C. The device resistance, RD, was found from ohmmeter measurements made at low currents on the assembled unit to be 0.62 D. Substituting these values in the above equation, and using a T of 273°K (O’C), gives AT=281-4*012-
13Qe
Figure 10 shows the agreement between the simple theory presented and the actual data. Good agreement between theory and measured values was obtained over a wide range of currents and heat loads in the temperature interval from +50” to -25°C. A slight modification of the coefficients is necessary at other temperatures. The agreement obtained here indicates that the thermal resistance of the anodized layer and the pressed contact is small since AT and AT, are approximately equal. Some points taken from Fig. 8 are shown for comparison. A simple device figure of merit for cooling can be defined in two ways: 201 = ~
aD2
and
zD2
=
2(Ap
KDRD
where Tc is the temperature of the chamber. Substituting the appropriate values gives 201 = l-35 x 10-s “C-l and 20s = l-50 x 10-3 “C-l
If the value of K for the semiconductor is assumed to 1.2 x 1O-2W/C-cm and the value for the foam is 3 -6 x 10-4 W/C-cm, it can be shown from the geometry that approximately half of KD is due to conduction through the elements and half to conduction through the insulation. The figure of merit for the material would then be about twice that for the device. This is in reasonable agreement with the values of the figure of merit for the material obtained from the manufacturer. This agreement and other measurements indicate that electrical contact resistance is not a major factor in determining the performance of this device.
IRVINGI. !SOCHARD
262
CURVES:
AT =28I-4,01*-13c1, o=DATA
MEASURED
A-DATA
TAKEN
DIRECTLY FROM
CHARACTERISTICS
-I
LINEARIZED CURVE
i
1 2W
-20_I 0
DISSIPATION
3
2
I
4
(AMPS)
FIG. 10. A comparison
of the measured device performance and the simplified theory developed. Some points obtained from Fig. 8 are shown as well. The points from Fig. 8 that correspond to the zero watt line are omitted as they essentially fall on the measured values shown.
DISCUSSION
The fabrication and assembly techniques presented here can be applied to a variety of additional devices. Two of these will be briefly discussed : First, a thermoelectric temperature regulator that can be effectively used in conjunction with two-dimensional microminiaturized circuitry; and second, a three-stage thermoelectric cooler that can be used to cool small infrared detectors. Both these devices could be fabricated without any unusual’difficulties, and approximate performance characteristics for them calculated from available data. The problem of thermostatting close-packed two-dimensional circuitry can be simplified if the circuit is formed directly on anodized aluminum wafer boards using techniques similar to those discussed here. The junction pattern for the thermoelectric regulator would then be formed on the other side of the wafer. A variation of this idea would be to mount several circuit wafers at right angles to each regulator wafer. They would then each make contact to it along an edge. Such a device could be designed with power and performance characteristics similar to that of the chamber discussed. The sensitivity of most infrared detectors is greatly increased by cooling. The temperatures required are in some cases within the capability of a multi-stage thermoelectric cooler. Using present materials a small three-stage cooler could be designed to provide temperatures as low as -80°C starting from an ambient of 25°C with a power requirement as low as several watts. The techniques discussed here are particularly suitable for such a device. By using different sides of a single aluminum sheet for the hot junction of one stage and the cold junction of the stage below, the transfer loss between stages can be kept to a minimum. The assembly of such a device would be effected with a series of simple jigs. The &in. diameter cylindrical elements employed were the smallest currently available.
Design and Construction
of a Peltier Temperature-Control
Device
263
Smaller cross-sectional material should soon be available. This is important because of the ditkulties involved in machining these alloys to the desired size. Smaller diameter material will allow the construction of devices using higher voltages and lower currents in a particular application. A device with performance similar to those described that operates on 6-9 V at 3 - 3 A should be feasible shortly. Such a unit could easily be incorporated into normal transistor circuitry. The problem of reliability, however, may place an upper limit on the usable voltage. It was previously stated that approximately ten pairs of elements are needed per volt of power supply. This rule is based on fundamental considerations and should remain valid in spite of any probable material development. For a 9 V power supply, therefore, about 90 pairs or 180 elements would be needed. These would require 360 metal-semiconductor solder joints in series. The necessary quality control to insure reliable operation of such a device may not be feasible.
Vol. 2, pp. 255-263.
Pergamon Press, 1962. Printed in Great Britain
DESIGN A N D CONSTRUCTION OF A PELTIER TEMPERATURE-CONTROL DEVICE IRVING I. SOCHARD*t Summary--A Peltier temperature-control device for use with micro-miniature electronics has been designed, built and tested. The principal features sought were (1) compatibility with conventional power supplies, particularly single-cell electrochemical systems, (2) techniques of assembly that are versatile with respect to physical configurations, and (3) techniques of assembly that are readily adaptable to production. The device contains a 1 ina temperature regulation chamber and operates from a supply voltage of 1"5-2"0V at input powers up to about 6 W. Eighteen thermoelectric couples were connected in series electrically and in parallel thermally. Aluminum was used as the structural material, serving as both the base and the temperature'controlled chamber. An anodized layer on the aluminum provided good electrical insulation and thermal contact between the structural materials and thermoelectric elements. The circuit patterns were placed on the anodized layer by printed-circuitry techniques. By using jigs that are adaptable to quantity production, all 72 solder joints connecting the elements in series and to the structural materials were made in two operations. Tests and evaluation of the device show the performance to be that predicted by simple theory: (1) a maximum temperature reduction of 48°C from a room-temperature ambient; (2) a maximum input power of less than 6 W to maintain the controlled chamber at 25°C while the ambient varies throughout the military range ( - 5 4 to ÷71°C). INTRODUCTION MOST military electronics are required to operate in ambient temperatures throughout the range o f - - 5 4 ° C to + 71 °C. I n m a n y circuits about half the components are needed to provide temperature compensation throughout this range, or to make up for loss due to such compensation. A properly designed Peltier device, together with appropriate sensing and control elements, can maintain the electronics at a temperature within a selected range, say 20°C-30°C, while the ambient varies throughout the military range. Use o f the Peltier device should, in addition, reduce the required development time by eliminating the temperature compensation problem. It will also increase the circuit reliability since fewer components are needed. A n additional benefit is a cost reduction, b o t h in development and in the produced item. The cost reduction can be substantial in dense electronic packaging where the electronics cost m a y be o f the order of $2000 per in3~ while a Peltier device to control the temperature o f this volume will cost less than $100. It is inherent in present o f thermoelectric materials that the voltage to be applied to a single couple for m a x i m u m cooling is about 0.1 V. A practical device, then, must consist o f a number o f couples connected electrically in series so that a conventional power supply m a y be used. In order to formulate and verify simple principles o f device design, and also to demonstrate the feasibility o f such temperature-control devices, an exploratory device has been designed, built and performance tested. Selection o f the major design parameters was as follows: (1) the device was to operate f r o m a standard power supply, a single-ceU electro* Diamond Ordnance Fuze Laboratories, Washington 25, D.C. Present address: Emerson Research Laboratories, Emertron Inc., 1140 East-West Highway, Silver Spring, Maryland. :~ Personal communication, N. Doctor, of Diamond Ordnance Fuze Laboratories. 255
256
IRVrNG I. SOCHARD
chemical battery of nominally 1.5 to 2.0 V, (2) the temperature-controlled chamber was to have a 1 in 3 volume, and (3) the maximum net heat-pumping capacity for cooling was to be about 4 W. In addition, considerable attention was given to making all construction techniques adaptable to mass-production. D E S C R I P T I O N AND C O N S T R U C T I O N An 18-couple temperature regulation device was designed and constructed. To keep the current requirements as low as possible, the smallest diameter thermoelectric material available at the time (nominally ~ in.) was used. All the internal connections to the 36 elements were made by printed-circuitry techniques. The circuitry was formed directly on an anodized layer on aluminum sheet. This technique allows good thermal contact to the aluminum base while providing electrical insulation from it. Figure 1 is a cross-sectional view of the device. The device weighs 3 oz. Figure 2 is a photo of the unassembled components. POLYURETHANE INSULATION
FOAM /
TEMPERATURE CONTROL CHAMBER (ANODIZED ALUMINUM)
.R,NTEO iiiiiiiiiiiiiiyiiiiiii2iiii Ei!iiiiiii!ii!!:!iiiii!i !ii iiiiiiiiii!iii!!i!i ! !ii!!iii! i CONNECTORS--. liiiiiiiiiiiiiii;1 THERMOELECTR,C ~ CIRCUIT
ELEMENT
BASE PLATE (ANODIZED ALUMINUM)
O
::::::::::::::::::::::::
.ii::ii:;:~;;......
iil
~ i i l ] I
2
CLEARANCEHO'E
/¢/
(FOR 4-401
3
IN.
F/G. 1. Cross-sectional view of the thermoelectric temperature control device. The chamber, 1.4 x 1.4 × ½ in., was constructed of 0.031 in. anodized aluminum. The chamber junction circuitry was printed on the anodized layer at the bottom of the chamber. The 36 cylindrical elements, arranged in a 6 x 6 square pattern 0.210 in. apart, center to center, were soldered between the base plate and the chamber. The elements are 0.400 in. long with a diameter averaging 0.130 in. The base plate is 2 x 3 in. ; clearance holes for No. 4 machine screws were drilled at each corner to allow the device to be fastened to a heat sink. The plate was made of -tk in. anodized aluminum and the plate junction circuitry was printed on top of it. Any metallic structure of sufficient size can be used as a heat sink. Examples of usable heat sinks are electronic chasses, air frame, vehicular structures, etc. For laboratory use, an 11 x 14 in. sheet of ¼ in. aluminum was used. The base plate and chamber were made of 52-S aluminum, which anodizes well. They were machined, using normal shop methods, then carefully finished to be free from scratches, but were not polished. After all machining operations were complete, the aluminum parts were heavily anodized. A thin coating of electroless nickel was then deposited on the anodized layer. This was used as a base for depositing a 2 mil layer of electrolytic copper (Fig. 3(a)). The aluminum parts were then covered with photo resist. The patterns representing the junction circuits were projected on them, and the unexposed portion of the photo-resist removed chemically. Next the parts were etched to remove the unprotected copper and
Ill
BA~=
I
~ = ~ , =
O-EL ELEMEN" /-JIG A
FIG. 2. Components ready for the first assembly step. The printed circuitry has already been formed and the parts solder dipped.
FIG. 4. Devices after first assembly step. Jig A has been removed.
[facing p. e.'i6]
Fro. 5. Device ready for final assembly step, utilizing jig B. After the solder melts, the device is pushed down the guide rails to complete assembly.
Design and Construction
of a Peltier Temperature-Control
Device
257
nickel. The remainder of the photo-resist was then removed* (Fig.(3b) ). The parts were then solder-dipped in a tin-bismuth-antimony solder. The solder adhered to the copper plating but did not wet the aluminum oxide (Fig. 3(c) and Fig. 2). The base plate and chamber were then ready for assembly.
ELECTROLESS NICKEL ELECTROLYTIC COPPER
ALUMINUM OXIDE
ALUrtrlNUM
(A) DEPOSITION FIG.
3. Major
SOLDER
(B)
PHOTO- ETCH
CC) SOLDER
DIP
steps in the preparation of the printed circuitry. (These drawings are not to scale.)
The alloyed bismuth telluride thermoelectric material was received from the manufacturer in the form of rods 9 in. long and approximately 4 in. dia.? These were cut to the correct length on a cut-off wheel. No supplemental grinding was needed since small variations in length were compensated by variations of the solder thickness in the finished device. The individual elements were then rolled in tape, leaving only the ends exposed. They were next dipped into a high-acidity chloride flux and then into the above solder. The solder adhered to the exposed ends only. The tape was then removed and the elements were ready to assemble. For quantity production, the rods could be epoxy-coated before cutting. The cutting process would expose the ends to be soldered. The first part of the assembly was carried out using the jig labelled A in Fig. 2. Jig A was attached to the base plate by four small screws. The elements were then dropped through the holes in the jig, alternating n- and p-type elements. The assembly was then placed on a hotplate just long enough for the solder to melt. The elements then seated themselves by gravity. Upon cooling, the jig was removed (Fig. 4). The second assembly step attached the chamber to the elements utilizing jig B (Fig. 5). The chamber and the assembly were put on the jig, the guideposts going through the screwclearance holes in the assembly. The entire unit was again put on the hotplate till the solder melted. The assembly was then pushed against the chamber and the entire unit removed from the hotplate. When the unit had cooled, the jig was removed and the assembly was complete except for insulation. The insulation is cut urethane foam 0.3 in. thick, except between .the elements where silica-gel was used, In a production model, the insulation would all be foamed in place.
* This process, used in forming the printed circuit, was developed by Miss E. L. Hebb of the Diamond Ordnance Fuze Laboratories. t The material was obtained from the Melcor Company of Trenton, New Jersey. 9
25
IRVING I. SOCHARD
TESTS
AND
EVALUATION
The
device was tested to determine performance characteristics under normal laboratory conditions of temperature and air movement. During most of the tests, the unit was mounted on the 11 in. x 14 in. aluminum heat sink. A “semi-infinite” heat sink* improved performance only a few percent. The most important parameters measured were current through the unit, voltage drop at the terminals of the unit, the temperature of the inside of the chamber, and the ambient temperature. The current and voltage measurements are correct to a few per cent. The temperature of the chamber was measured with a calibrated thermocouple and is accurate to within f 1 “C. For some tests, the difference between the chamber temperature and room temperature was calculated. This value, AT,, is the net temperature difference produced, which is always less than AT, the temperature difference from one end of the thermoelectric elements to the 0ther.t The net temperature difference is less than AT because of the following major reasons: (1) The temperature change in the heat sink due to the heat rejected to it by the device. (2) The thermal resistance of the pressed contact between the heat sink and the base plate. (3) The thermal resistance across the anodized layers at the hot and cold junctions. In another series of tests, the ability of the device to remove heat generated in the chamber was measured. This heat, Qe, is less than Q, the total amount of heat removed by the device, by the amount of heat leakage through the insulation. Figure 6 shows the relationship between the net temperature reduction, AT,, and the electric power consumed. The current at each point and the voltage at some points are indicated. There was no heat load, Qe, in the chamber. Nearly all the effective cooling obtainable can be achieved by a single-cell battery as indicated. Lesser degrees of cooling are obtainable by reducing the input power. It should be noted that the current-voltage relationship is ,not linear. The rate of cooling at different current levels is shown in Fig. 7. The time constant decreases with increasing current. The maximum rate of initial cooling occurs for currents higher than those which produced maximum steady-state cooling. The result of reversing the current at 1 A is shown also. The chamber was empty during these runs. The time constant would be increased by a factor of 2-3 if the chamber had been filled with microminiature electronics. The present model was not specifically designed for rapid cooling. The response time could have been decreased several times by using an alternate design. However, this result would be achieved at the cost of increased power consumption and size. Figure 8 indicates the relationship between the heat removed and the effective temperature difference obtained for various currents. The lines of constant current are, in addition, approximately constant power levels. These values are shown as well. The lines of equal COP, for a few values are also shown. COP, is defined as Qe over the power required by the device. Figure 8 was calculated by assuming a, T, K, and p were independent of temperature and is only approximate. However, all the temperature differences indicated * A 16 in.
x 20 in. sheet of + in. aluminum with a flow of air directed along the under side. t This is the AZ’ most often referred to in the literature on thermoelectricity.
Design and Construction of a Peltier TemperatureControl
Heat (Room
load
in
Device
chamber,
259
Qe=O
temp. =24-C
Coolingrange for
0
I
2
3
4
Power
5
required.
6
7
8
9
W
FIG. 6. Relationship between the net temperature reduction achieved from a room temperature ambient and the electric power required.
are accurate to a few degiees below room temperature. Similar curves can be calculated for other temperature ranges. In addition, they can be extended to the other quadrants where they apply to heating applications. This type of curve is useful in obtaining graphic solutions to many applications problems. Figure 9 indicates the power required to maintain a constant temperature of 25°C in the chamber through a wide raqge of ambients. The range of military ambients (- 54°C to 71 “C) is shown. The device would be capable of maintaining a constant chamber temperature throughout this range if coupled with the appropriate temperature sensing element, feedback, and power supply. A maximum power of approximately 6 W would be needed. If the requirements for a particular application were only that the temperature excursion be reduced, useful results can be achieved with less power.
,L
CURRENT REVERSED
AMt-
TIME,
MIN
FIG. 7. Thermal response time of the Peltier device.
260
IRMNG
I.
f&HARD
FOR ROOM TEMPERATURE
Q,=
FIG.
NET
RATE
OF HEAT
REMOVAL
FROM
AMBIENTS
CHAMBER
(W)
8. Linearized
cooling characteristics for the thermoelectric temperature control device. The lines of equal practical coefficients of performance are shown also.
6.0
5.0 z 4.0 8 .: s t
3.0
; k Q.
2.0
0
-80
-60
40
-20 Ambient
0
20
temp.,
40
60
“C
FIG. 9. The electric power required to keep the regulated chamber at a constant 25°C for a wide range of temperatures.
Design and Construction of a Peltier Temperature-Control Device
261
A 2 W supply could be used to reduce the temperature excursion by 213 over the military ambient range. The temperature reduction that can be obtained with a particular device can be expressed as :
when
and where the subscript D indicates the terms applied to the assembled device. T is the average chamber temperature of the range being considered .The thermal resistance of the contacts is assumed to be negligible, thus AT = ATe. The terms QD,KD, and RD can be found by simple measurements. The Seebeck coefficient for the device aD, is determined from a measurement of the Seebeck voltage developed when the chamber and the base plate are kept at different fixed temperatures. UD was found to be O-0079 V/C. KD, the thermal conductance of the device is evaluated by putting a resistor of known dissipation in the chamber and observing the temperature rise at equilibrium. KD is found to equal 0.077 W/C. The device resistance, RD, was found from ohmmeter measurements made at low currents on the assembled unit to be 0.62 D. Substituting these values in the above equation, and using a T of 273°K (O’C), gives AT=281-4*012-
13Qe
Figure 10 shows the agreement between the simple theory presented and the actual data. Good agreement between theory and measured values was obtained over a wide range of currents and heat loads in the temperature interval from +50” to -25°C. A slight modification of the coefficients is necessary at other temperatures. The agreement obtained here indicates that the thermal resistance of the anodized layer and the pressed contact is small since AT and AT, are approximately equal. Some points taken from Fig. 8 are shown for comparison. A simple device figure of merit for cooling can be defined in two ways: 201 = ~
aD2
and
zD2
=
2(Ap
KDRD
where Tc is the temperature of the chamber. Substituting the appropriate values gives 201 = l-35 x 10-s “C-l and 20s = l-50 x 10-3 “C-l
If the value of K for the semiconductor is assumed to 1.2 x 1O-2W/C-cm and the value for the foam is 3 -6 x 10-4 W/C-cm, it can be shown from the geometry that approximately half of KD is due to conduction through the elements and half to conduction through the insulation. The figure of merit for the material would then be about twice that for the device. This is in reasonable agreement with the values of the figure of merit for the material obtained from the manufacturer. This agreement and other measurements indicate that electrical contact resistance is not a major factor in determining the performance of this device.
IRVINGI. !SOCHARD
262
CURVES:
AT =28I-4,01*-13c1, o=DATA
MEASURED
A-DATA
TAKEN
DIRECTLY FROM
CHARACTERISTICS
-I
LINEARIZED CURVE
i
1 2W
-20_I 0
DISSIPATION
3
2
I
4
(AMPS)
FIG. 10. A comparison
of the measured device performance and the simplified theory developed. Some points obtained from Fig. 8 are shown as well. The points from Fig. 8 that correspond to the zero watt line are omitted as they essentially fall on the measured values shown.
DISCUSSION
The fabrication and assembly techniques presented here can be applied to a variety of additional devices. Two of these will be briefly discussed : First, a thermoelectric temperature regulator that can be effectively used in conjunction with two-dimensional microminiaturized circuitry; and second, a three-stage thermoelectric cooler that can be used to cool small infrared detectors. Both these devices could be fabricated without any unusual’difficulties, and approximate performance characteristics for them calculated from available data. The problem of thermostatting close-packed two-dimensional circuitry can be simplified if the circuit is formed directly on anodized aluminum wafer boards using techniques similar to those discussed here. The junction pattern for the thermoelectric regulator would then be formed on the other side of the wafer. A variation of this idea would be to mount several circuit wafers at right angles to each regulator wafer. They would then each make contact to it along an edge. Such a device could be designed with power and performance characteristics similar to that of the chamber discussed. The sensitivity of most infrared detectors is greatly increased by cooling. The temperatures required are in some cases within the capability of a multi-stage thermoelectric cooler. Using present materials a small three-stage cooler could be designed to provide temperatures as low as -80°C starting from an ambient of 25°C with a power requirement as low as several watts. The techniques discussed here are particularly suitable for such a device. By using different sides of a single aluminum sheet for the hot junction of one stage and the cold junction of the stage below, the transfer loss between stages can be kept to a minimum. The assembly of such a device would be effected with a series of simple jigs. The &in. diameter cylindrical elements employed were the smallest currently available.
Design and Construction
of a Peltier Temperature-Control
Device
263
Smaller cross-sectional material should soon be available. This is important because of the ditkulties involved in machining these alloys to the desired size. Smaller diameter material will allow the construction of devices using higher voltages and lower currents in a particular application. A device with performance similar to those described that operates on 6-9 V at 3 - 3 A should be feasible shortly. Such a unit could easily be incorporated into normal transistor circuitry. The problem of reliability, however, may place an upper limit on the usable voltage. It was previously stated that approximately ten pairs of elements are needed per volt of power supply. This rule is based on fundamental considerations and should remain valid in spite of any probable material development. For a 9 V power supply, therefore, about 90 pairs or 180 elements would be needed. These would require 360 metal-semiconductor solder joints in series. The necessary quality control to insure reliable operation of such a device may not be feasible.