美國N有關(guān)半導(dǎo)體熱電制冷器TEC的ppt課件

1、Presented ByEric W. Grob NASA Goddard Space Flight Center Eric.W.G,Thermo-Electric Coolers NASA GSFC,Thermal they provide temperature control for low noise amplifiers (LNAs), star trackers, and IR (infrared) sensors,TFAWS August 15-19, 2011,7,Seebeck Effect,The voltage difference, V, prod

2、uced across the terminals of an open circuit made from a pair of dissimilar metals, A and B, whose two junctions are held at different temperatures, is directly proportional to the difference between the hot and cold junction temperatures, THOT TCOLD V = (THOT - TCOLD) where = Seebeck coefficient Th

3、e temperature difference, produces an electric potential (voltage) that can drive an electric current in a closed circuit. Using the Seebeck effect, thermoelectric power generators convert heat to electricity. Very inefficient Used when waste heat is readily available, or in remote areas where depen

4、dability overrides efficiency,TFAWS August 15-19, 2011,8,Peltier Effect,When an electric current flows through two dissimilar conductors, depending on the direction of the current flow, the junction of the two conductors will either absorb or release heat. The heat absorbed or released at the juncti

5、on is proportional to the electrical current. The proportionality constant is known as the Peltier coefficient. Q = *I where = Peltier coefficient and I= junction current Thomson (Lord Kelvin) showed the relationship between the Seebeck and Peltier coefficients as: =TT =temperature of the junction (

6、K) =Seebeck Coefficient (V/K) Semiconductors are materials of choice for producing the Peltier effect. They are more easily optimized for pumping heat Designer can control the type of charge carrier employed within the conductor With semiconductor advancements, thermoelectric modules can now be prod

7、uced to deliver efficient solid state heat pumping for both heating and cooling,TFAWS August 15-19, 2011,9,But What Are These Coefficients ,Remember that the Seebeck Effect - a voltage is produced when a temperature difference is applied across a junction of dissimilar materials. This applied temper

8、ature difference causes charged carriers in the material, whether they are electrons or holes, to diffuse from the hot side to the cold side, similar to a gas that expands when heated. The efficiency with which a thermoelectric material generates electrical power depends on several material properti

9、es, of which perhaps the most important is the thermo-power, or Seebeck coefficient (). inversely related its carrier density - a higher results in decreased carrier concentration and decreased electrical conductivity. Therefore, insulators tend to have very high Seebeck coefficients, while metals h

10、ave lower values. depends on the materials temperature, and crystal structure and has units of V/K, or V/K. Typically metals have small coefficients because most have half-filled bands. Electrons (negative charges) and holes (positive charges) both contribute to the induced thermoelectric voltage th

11、us canceling each others contribution to that voltage and making it small. Superconductors have a zero coefficient because it is impossible to have a finite voltage across a superconductor, but can be doped with an excess amount of electrons or holes and thus can have large positive or negative coef

12、ficients, depending on the charge of the excess carriers. A larger induced thermoelectric voltage for a given temperature gradient will lead to a higher efficiency. There is an active research effort to find materials that could make cheaper and more efficient thermoelectric power generators,TFAWS A

13、ugust 15-19, 2011,10,Seebeck Coefficient - Examples,Bismuth telluride (Bi2Te3):287 V/K A gray powder that is a compound of bismuth and tellurium. It is a semiconductor which, when alloyed with antimony or selenium is an efficient thermoelectric material for refrigeration or portable power generation

14、. Furthermore, the Seebeck coefficient of bulk Bi2Te3 becomes compensated around room temperature, forcing the materials used in power generation devices to be an alloy of bismuth, antimony, tellurium, and selenium. Uranium dioxide (UO2): 750 V/K also known as urania or uranous oxide, is an oxide of

15、 uranium, and is a black, radioactive, crystalline powder that occurs naturally . Used in nuclear fuel rods. A mixture of uranium and plutonium dioxides is used as MOX fuel. Prior to 1960 it was used as yellow and black color in ceramic glazes and glass. Perovskite - SrRuO3 (Strontium/Ruthenate):36

16、V/ K Constantan: 35 V/K Thallium tin telluride (Tl2SnTe5): 270 V/K,TFAWS August 15-19, 2011,11,Making a Semiconductor,One process for forming crystalline wafers is forming a cylindrical ingot of high purity monocrystalline silicon is formed by pulling a seed crystal from a melt. A wafer is a thin sl

17、ice of semiconductor material from these ingots. Wafers are formed of highly pure (99.9999% purity), nearly defect-free single crystalline material. Dopants (impurity atoms) such as boron or phosphorus can be added to the molten intrinsic silicon, thus changing it into n-type or p-type extrinsic sil

18、icon (more on this later). The wafer serves as the substrate for microelectronic devices built in and over the wafer and undergoes many microfabrication process steps such as doping or ion implantation, etching, deposition of various materials, and photolithographic patterning. Finally the individua

19、l microcircuits are separated (dicing) and packaged,TFAWS August 15-19, 2011,12,Doping to Make P- or N-Type Semiconductors,Semiconductor doping was formally first developed by John Robert Woodyard working at Sperry Gyroscope Company during World War II. N-Type:Doping pure silicon with Group V elemen

20、ts (nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), extra valence electrons are added that become unbonded from individual atoms and allow the compound to be an electrically conductive (N-Type). P-Type:Doping with Group III elements (boron (B), aluminum (Al), gallium (Ga), i

21、ndium (In), thallium (Tl), which are missing the fourth valence electron, creates broken bonds (holes) in the silicon lattice that are free to move. The result is an electrically conductive p-type semiconductor,TFAWS August 15-19, 2011,13,Making a Semiconductor,Semiconductor device fabrication is th

22、e process used to create the integrated circuits that are present in everyday electrical and electronic, including thermoelectric, devices. Multiple-step sequence of photographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducti

23、ng material. Silicon is almost always used, but various compound semiconductors are used for specialized applications. The entire manufacturing process, from start to packaged chips ready for shipment, takes six to eight weeks and is performed in highly specialized facilities. When feature widths we

24、re far greater than about 10 microns, purity was not the issue that it is today in device manufacturing. As devices became more integrated, cleanrooms became even cleaner. The workers in a semiconductor fabrication facility are required to wear cleanroom suits to protect the devices from human conta

25、mination. In an effort to increase profits, semiconductor device manufacturing has spread from Texas and California in the 1960s to the rest of the world, such as Europe, Middle East, and Asia,TFAWS August 15-19, 2011,14,Manufacturing a TEC,Semiconductor manufacturing process is beyond the scope of

26、this course (and the instructors capability!), but an quick overview is shown below,TFAWS August 15-19, 2011,15,Cool Picture,TFAWS August 15-19, 2011,16,Heat Flow in TE Module,The simplest form of a thermoelectric module:a single semiconductor “pellet is soldered to electrically conductive material

27、(plated copper) on each end. “N-Type” semiconductor material:electrons are repelled by the negative pole of the power supply and attracted by the positive pole. Electrons carry the heat “P-type” semiconductor material:“holes” are repelled by the positive pole of the power supply and attracted by the

28、 negative pole. “Holes” carry the heat. It is the charge carriers inherent in the material structure that dictate the direction of the heat flow. This thermoelectric effect and its application in thermoelectric devices involves very complex physics at the subatomic level,TFAWS August 15-19, 2011,17,

29、For you sub-atomic physicists out there, this is way outside the scope of this course,How to Use Semiconductors in TECs,Multiple pellets are needed in order to pump an appreciable amount of heat through a thermoelectric module,TFAWS August 15-19, 2011,18,One idea is to connect N-type or P-type mater

30、ial in parallel , both electrically and thermally. Unfortunately, this is not practical. Typical semiconductor pellet is only rated for “tens” of milli-volts. A single pellet can draw 5 amps or more with 60mV applied. A 254-pellet device will draw more than 1000 amps with 60mV applied. The only real

31、istic solution is to wire the semiconductors in series electrically and in parallel thermally. Interconnections between pellets introduce thermal shorting that significantly compromises the performance of the device,So how do they make this work ,Aha,Use both “N-type” and “P-type” materials. Arrange

32、 N and P-type pellets in a “couple” and form a junction between them with a copper tab. Free end of P-type pellet connects to the positive voltage potential Free end of the N-type pellet connects to the negative side of the voltage. Electrons flow continuously from negative pole of the supply, throu

33、gh the N pellet, through the copper tab junction, through the P pellet, and back to the positive pole of the voltage supply. Configure a series circuit that keeps all of the heat moving in the same direction. The two different types of semiconductor material keep the charge carriers and heat flowing

34、 in the same direction through the pellets,TFAWS August 15-19, 2011,19,Simplest TEC,The simplest TEC consists of two semiconductors: One p-type and one n-type (one couple) semi-conductor, connected by a metallic conductor. When a positive DC voltage is applied as shown, electrons pass from the p-typ

35、e to the n-type element, and the cold-side temperature decreases as the electron current absorbs heat, until equilibrium is reached. Heat is pumped from the cold junction to the hot junction. The net cooling is diminished by the effects of Joulean losses generated by the current, and heat conduction

36、 through semiconductor material from the hot to the cold junction (back conduction,TFAWS August 15-19, 2011,20,Brilliant,Using this special properties of the thermoelectric “couples”, many pellets can be arranged in rectangular arrays to create practical thermoelectric modules. The device can pump a

37、ppreciable amounts of heat Series electrical connection is suitable for commercially available DC power supplies. Most common thermoelectric devices 254 alternating P and N-type pellets Use 12 to 16 VDC supply Draw 4 to 5 amps,TFAWS August 15-19, 2011,21,Thermal Gradients Within a Thermo-Electric Co

38、uple,Same as “couples” just covered, except packaging ceramic layers added. Temperature extremes are at the “junctions,TFAWS August 15-19, 2011,22,Constructing a TEC,A typical single stage cooler consists of two ceramic plates with an array of these p-type and n-type semiconductor materials (bismuth

39、 telluride alloys) between the plates. These multiple elements of semiconductor materials are connected electrically in series and thermally in parallel. When a positive DC voltage is applied as shown, electrons pass from the p-type to the n-type element, and the cold-side temperature decreases as t

40、he electron current absorbs heat, until equilibrium is reached. The heat absorption (cooling) is proportional to the current and the number of thermoelectric couples. This heat is transferred to the hot side of the cooler, where it is dissipated into the heat sink and surrounding environment,TFAWS A

41、ugust 15-19, 2011,23,From Schematic to Packaging,So, the most efficient scheme is to connect our P-N pellets electrically in series, but thermally in parallel. Metalized ceramic substrates provide the platform for the pellets and the small conductive tabs that connect them. The pellets, tabs and sub

42、strates thus form a layered configuration,TFAWS August 15-19, 2011,24,Real TEC Modules - Single,A typical thermoelectric module consists of an array of Bismuth Telluride semiconductor pellets that are “doped” with P and N pellets,. Module size varies from less than 0.25” x 0.25” to approximately 2.0

43、” x 2.0,TFAWS August 15-19, 2011,25,Real TEC Modules - Multiple,Some applications require: multiple single-stage: or stacked multistage modules,TFAWS August 15-19, 2011,26,Multistage TECs (also called cascade or stacked thermoelectric modules) are used where larger delta Ts are required,Peltier Vide

44、os,Illustrates the basic function, i.e.- create a temperature difference to generate voltage (power). http:/youtu.be/pgIOUXKyzFE And this one illustrates applying a voltage to get cooling. http:/youtu.be/u46VGSw9v1I,TFAWS August 15-19, 2011,27,THATS GREAT, BUT HOW DO I USE ALL THIS KNOWLEDGE,Design

45、Guide Online Application S/W Desktop Software,TFAWS August 15-19, 2011,28,Some Terminology,Temperature Difference ( T):hot side temperature of the module minus the cold side temperature of the module. Temperatures are referenced at the ceramic substrates of the module. Qc:total rate of heat being re

46、moved from the cold side of the module. Waste Heat:this is the rate of heat that the heat sink attached to the hot side of the module must dissipate. It is the sum of the heat removed from the cold side of the module plus the Joulean losses from the power input to the module plus parasitics. Input V

47、oltage:the voltage applied to the module. Input Current:the current the module will draw at a particular input voltage. Coefficient of Performance (COP):ratio of heat removed to power input. QMAX:The amount of heat that a TEC can remove at a 0temperature difference when the hotside of the elements w

48、ithin the thermoelectric module are at 300K (27C). IMAX:The current that produces Tmax when the hotside of the elements within the thermoelectric module are held at 300K (27C). VMAX:The voltage that is produced at DTmax when Imax is applied and the hotside temperature of the elements within the ther

49、moelectric module are at 300 K. TMAX:The maximum obtainable temperature difference between the cold and hot side of the thermoelectric elements within module when Imax is applied and there is no heat load applied to the module. This parameter is measured with the hotside of an element at a temperatu

50、re of 300K (27C). . In reality, it is virtually impossible to remove all sources of heat in order to achieve the true Tmax. Therefore, the number only serves as a standardized indicator of the cooling capability of a thermoelectric module,TFAWS August 15-19, 2011,29,What Do All These Terms Mean,The

51、terms Imax, Vmax , Qmax and Tmax never cease to confuse people. All these terms are defined at a given hot side temperature Thot of the TEC. Imax, Vmax and Tmax occur at the same time with Q = 0. Imax, Vmax and Qmax occur at the same time with T = 0. Qmax and Tmax are shown on the performance curve

52、as the end points of the Imax line,TFAWS August 15-19, 2011,30,Illustration of Terms,IMAX corresponds to the current that gives TMAX at QC = 0. QMAX: is the amount of heat that a TEC can remove at a 0temperature difference when the hotside of the elements within the thermoelectric module are at 300K

53、 (27C). TMAX is the maximum obtainable temperature difference between the cold and hot side of the thermoelectric elements within module when Imax is applied and there is no heat load applied to the module. This parameter is measured with the hotside of an element at a temperature of 300K (27C,TFAWS

54、 August 15-19, 2011,31,Pump Analogy ,TFAWS August 15-19, 2011,32,Another View of Same Type of Data,Melcor,What Does Imax Mean ,Imax is the direct current level which will produce the maximum possible T, (i.e. Tmax), across the TEC with no heat load (Q=0). . Imax Operating below Imax there is insuffi

55、cient current to deliver the Tmax Operating above Imax the power dissipation within the thermoelectric begins to elevate the system temperature and diminish T. Imax and Vmax occur at the same operating point, i.e. Imax is the current level produced by applying Vmax to the thermoelectric device. Imax

56、 is not especially temperature-dependent. It tends to be fairly constant throughout the operating range of a thermoelectric device. Imax is NOT the maximum current that the TEC can withstand before failing. Thermoelectrics generally operate within 25% - 80% of the maximum current,TFAWS August 15-19,

57、 2011,33,What Does Vmax Mean ,Vmax is the DC voltage which will deliver the maximum possible T (i.e. Tmax), across the TEC with no heat load (Q = 0) At voltages below Vmax, there is insufficient current to deliver Tmax At voltages above Vmax, the power dissipation within the thermoelectric begins to

58、 elevate the system temperature, and diminish T. Imax and Vmax occur at the same operating point, i.e. Imax is the current level produced by applying Vmax to the thermoelectric device. Vmax is temperature-dependent. The higher the Thot , the higher the Vmax rating for a specific device. Vmax is NOT

59、the maximum voltage that the thermoelectic device can withstand before failing,TFAWS August 15-19, 2011,34,What Does Qmax Mean ,Qmax is the maximum load to the thermoelectric device that produces T = 0. Qmax, Imax, and Vmax occur at the same operating point to produce T= 0. It specifies an end point

60、 on the load line. Qmax does not project the maximum heat that can be handled by the thermoelectric device. If the load goes beyond, the TEC will still pump the heat, but the thermal load simply winds up at an above-ambient temperature. (i.e. you have to look at different performance curve.,TFAWS Au