DE2165169A1 - Substance for direct thermoelectric energy conversion with a high figure of merit - Google Patents

Description

DR. MICHAEL C. NICOLAOU, P.ENG.

SUBSTANCE FOR DIRECT THERMOELECTRIC ENERGY CONVERSION WITH A HIGH QUALITY CODE

DESCRIPTION '

The invention relates to a substance for direct thermoelectric Energy conversion.

The methods that are used for the direct conversion of energy make use of various physical principles or phenomena and are generally referred to as "immediate energy conversion ⁿ . One of these methods is that which makes the thermoelectric properties of the substances usable, namely the Seebeck or Peltier effect, which is why it is called »Immediate Thermoelectric Energy Conversion ^tf .

In its simplest form, a thermoelectric energy converter basically consists of a hot connection point (Solder joint), a cold junction, a positive branch and a negative branch. The hot solder joint is mostly a metal plate tied to each branch at one end and must be a good conductor of both heat and electricity. The electrical load is usually on the other end each branch is switched on and therefore forms the cold solder joint the device. The hot connection point is brought into contact with the heat source. Now causes the temperature difference a voltage difference between the hot soldering point and the cold soldering point and a flow of electrons from one of the legs after the other. The electrical energy generated in this way is used by

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the electrical load used. The quality of the energy conversion such a device essentially depends on:

(1) the temperature difference between the hot solder joint and the cold solder joint

(2) the physical properties of the substances used to manufacture be used by the positive or negative leg of the device,

away. The suitability of any material for thermoelectric energy conversion is called by a large, which can ^rt thermoelectric figure of merit Z ", examines the figure of merit is.:

e = the thermopower

Cf = the electrical conductivity

J ^ - the thermal conductivity

are. Accordingly, the grade of a thermoelectric device becomes dominated by the second main principle of thermodynamics and the figure of merit.

To date, several substances have been developed for this purpose been. In addition, many new substances are continuously being created. They belong to the class of substances at all are known as semiconductors, and are either simple metallic Elements or alloys thereof such as silicon and germanium, or intermetallic compounds. Of the latter group they like Compounds bismuth telluride, antimony telluride, germanium telluride, Lead telluride, silver antimony telluride, lead selenide, bismuth selenide, etc., as well as mixtures or solid solutions of these and other compounds, (American patents: U.S. Patent No. 3,434,203; No. 3,205,017; No. 3,279,954;

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No. 3,053,923; Mr. 3,287,100; No. 3,249,404; No. 3,414,405; No. 3,393,054; No. 3,059,040; No. 3,071,495; No. $ 3,200.87; No. 3,201,227; No. 2,990,439; No. 3,020,326; No. 2,957,937; No. 3,310,493; No. 3,228,805; No. 3,129,117; No. 3,000,441; No. 3,075,031; No. 3,232,719; No. 3,460,996; No. 3,373,061; No. 3,321,336; No. 3,208,835; No. 3,224,876; No. 3,201,235; No. 3,279,955; No. 3,070,644; No. 2,995,613; No. 3,256,699; No. 3,249,469; No. 3,073,883; No. 3,045,057; No. 3,021,378; No. 2,977,400; No. 3,261,721; No. 3,364,014; No. 3,061,657; No. 3,005,861; No. 3,018,312; No. 3,110,629; and No. 3,054,842; also the books: AF Joffe, "Semiconductor Thermocouples and Thermoelectric Cooling", Infosearch Limited, London, 1957; RS Heikes and R. ¥. Ure (Editors), "thermoelectricity: Science and Engineering ^n, Interscience Publishers, New Tork - London, 1961; PH EgIi (Editor), "Thermoelectricity", John Wiley, New Tork, 1960; and GW Sutton, "Direct Energy Conversion ^η , McGraw-Hill, New Tork, 1966).

However, none of these substances has been able to achieve a grade the conversion of thermal energy into electrical energy, which is higher than about 10%. As a result, can the application or use of thermoelectric energy conversion, as a decisive embodiment for medium or large Power plants, of course, are currently out of the question. this is caused by either the inability of these substances to give satisfactory performance at high temperatures (low melting point or the substance becomes intrinsic) or causes a low figure of merit or a combination of both reasons.

The invention is based on the object of this low energy

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U 2165163

Conversion grade of thermoelectric !! To increase devices and thereby expand their field of application.

This object is achieved according to the invention in that the Substance that is needed for direct thermoelectric energy conversion. an alloy or a solid solution of the following three intermetallic compounds:

Tin magnesium Mg Sn,

Germanium magnesium Mg Ge and

Silicon magnesium is composed of Mg Si and by the chemical formula:

“Mg Si Ge Sn
2 χ y 1-xy

where χ and y are the molecular fraction of each of Mg Si and Mg respectively represent Ge in the alloy.

Each of χ and y must be greater than zero and less than one, and their sum (χ + y) must again be less than one or less, respectively than be one hundred percent. The properties of the alloy are determined by the values of χ and y or by the atomic proportions or atomic ' Percentages of silicon, germanium and tin in the solid solution ab-P hang. If these proportions or ratios are the same, then the substance will have approximately the following properties:

Melting point = 1000 ° C

forbidden energy band gap = 0.6 eV The above facts make it clear that the material as a thermoelectric pair at a hot solder joint temperature of

ο ο

up to Ö00 C and possibly up to 900 C can be active. This, of course, it will produce a high Carnot quality grade according to the second main principle of thermodynamics.

The above-mentioned expression ⁿ melting point ^M is intended as the solidus

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temperature or, more precisely, the mean temperature at the transition of the Substance from the solid to the liquid state are understood to be indicative (In the English literature, the intermetallic Compounds Mg Si, Mg Ge and Mg Sn as magnesium silicide.

2 2 2

magnesium germanide or magnesium stannide).

To get the best possible performance of this substance as a thermoelectric To achieve energy converters, one must take into account the following factors:

(1) The relative proportions (proportions) of silicon, germanium and tin in the solid solution must be determined.

(2) The substance must be endowed with the appropriate impurities and at the appropriate level.

As far as factor No. (1) is concerned, there is the best solution in that the same atomic percentages should be used for silicon, germanium and tin. Nevertheless, as a result of difficulties when doping the substance so that it becomes either p-type (positive electrical conduction) or η-type (negative electrical conduction) is made, the molecular fractions (proportions) of Mg Si, Mg Ge and Mg Sn

2 2 2

be made unequal. This may be necessary to the best possible Figure of merit of this material to accomplish.

The factor no. (2) stands for the correct doping of the substance, with it it is transformed into a p-conducting or η-conducting material, in relationship. This can be done when there is an excess or deficiency of magnesium, silicon, germanium or tin in the solid solution is supplied. An excess of magnesium will cause a negative doping (n-type substance), while an excess will cause any one of the other three elements, on the other hand, becomes a positive doping

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(p-type substance). There can be any other doping impurity (Elements or compounds) are needed, if they prove to be more adequate dopants. The important consideration is the doping level that is set in this way

20th

should be that a carrier concentration of about 10 carriers cm is caused. This, of course, applies to both the donors and the acceptors. The doping of the substance, to transform it into a negatively conductive material (excess of electrons or donors) 'should not prove to be a problem due to the high ratio of electron mobility on punch mobility. The doping of the substance to transform it into a positively conductive material (excess of holes or acceptors ) to transform becomes without difficulty or more sufficient Way, by reducing its forbidden energy band gap. This can be done by increasing the molecular content of tin-magnesium and, to a lesser extent, that of germanium-magnesium in the alloy. In other In words, each of the branches (legs) of the device becomes one have different composition. This may be necessary in order to obtain approximately the same figure of merit for both branches of the device. Regardless, the degree to which this increase in the Mg Sn molecular fraction can be cited becomes the limit its solubility in the other two compounds (Mg Ge and

Mg Si) and the necessity of some excessive reduction the temperature at which the material begins to melt, to avoid. This degree is also determined by it held back that the transformation of the substance in the intrinsic state at relatively low Temperatures must be prevented.

Based on the previous explanation, the following information

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play appropriate compositions of the substance in thermoelectric Energy conversion devices can be used:

(1) For the negative branch of the device (n-type material):

(a) Mg Si Ge Sn

2 0.33 0.33 0.33

(b) Hg Si Ge Sn

⁶ 2 0.4 0.3 0.3

(c) Mg Si Ge Sn

2 0.5 0.3 0.2

(2) For the positive branch of the device (p-type material):

(a) Mg Si Ge Sn

^ Z 0.33 0.33 0.33.

(b) Mg Si Ge Sn

2 0.2 0.4 0.4

(c) Mg Si Ge Sn

^ 2 0.3 0.3 0.4

The preferred composition ranges for the alloy or solid solution, expressed as molecular percentages, are: from 20 to 60 percent silicon magnesium, from 20 to 40 percent germanium magnesium and from 10 to 40 percent tin magnesium

To create a breakthrough in the design or applications of direct thermoelectric energy conversion, a substance must be found that produces a figure of merit at least ten times that of the best used today thermoelectric materials is. This will eventually result in a heat-to-electricity conversion efficiency of about $ 40 (Of course, also from the temperature difference between the hot connection point and the cold connection point depending). I believe that this invention leads to such a development leads.

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Application of this invention to thermoelectric devices provides the following basic advantages:

(1) A high, ideal quality grade of the Carnot heat engine working between the same temperature limits.

(2) A high figure of merit, due to

(a) a very low thermal conductivity

(b) a high mobility of the electrons

(c) a high electrical conductivity

(d) a high thermal force or a high Seebeck coefficient.

(3) The possibility of changing the composition of the substance (both χ and y, or at least one of them) along both, or at least one, of the branches of the device between the hot junction and the cold junction to check. This change in composition, in accordance with the temperature gradient prevailing in the device insure the best possible performance.

(4) Good mechanical strength due to the presence of Mg Si in the fabric.

The starting elements necessary for the manufacture of this substance (magnesium, silicon, germanium and tin) should be of the highest possible purity. The necessary intermetallic Compounds are preferably single crystals produced by means of a modified Bridgman method or some other embodiment to be grown.

The purity of the starting elements (basic elements), as a percentage In terms of weight, it should be no less than 99.99 and 99.999 for magnesium and tin, respectively, and should be substantially higher than that be the latter number for both silicon and germanium. The magnesium,

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as it is obtained with the aforementioned purity, must by distillation further cleaned under high vacuum to achieve a level of purity similar to that of the other three elements before it is used to make the intermetallic connections. The latter are concerned by mixing together the Elements according to the associated stoichiometric quantities and consequently made by melting them to bring about the necessary chemical effects (magnesium and silicon, Magnesium and germanium, and magnesium and tin)

The starting elements, after they have been mixed together,

should reach temperatures that are approximately 50 C higher than the melting points of the intermetallic compounds in question are heated and may be held at these temperatures for about an hour in order to allow sufficient time for the occurrence of the absolutely necessary effects. Accordingly, the connections become Tin magnesium and germanium magnesium processed by heating the essential elements up to about # 30 C or II65 C.

The germanium magnesium can still be produced by heating its

o ο

Ingredients up to about 1250 ° C or, preferably, up to about 1200 ° C

be made. The production of silicon magnesium is different differs from the above in that the basic elements up to βίο

should be heated to a temperature that must be higher than I4IO C, so that the complete melting of silicon is only guaranteed

and then the temperature can gradually increase up to about II50 C decrease, and consequently it will be sufficient at this level Time recorded to complete the chemical exposure. A strong shaking of the melt and a Excess of magnesium are precautionary measures that are used to achieve uniform (homogeneous) and stoichiometric compounds are necessary.

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The material is either according to the necessary proportions or amounts of mixing of the starting elements themselves (magnesium, Silicon, germanium and tin) and furthermore the melting together of them or, preferably, through the independent processing of each of the three intermetallic compounds (tin-magnesium, germanium-magnesium and silicon-magnesium), their according to the required Molecular proportions mixing and consequently the fusing thereof is produced. In either of these methods, the dopant can or the impurity in the desired amount of the basic components, which may be powdery or granular, added and mixed with it completely before the melting process is started. The doping impurity can can optionally also be added during melting.

If the first of the above two types of travel is chosen, then

should keep the ingredients up to a temperature higher than

ο
I4IO C, so that the complete melting of silicon is first completed, and then they are allowed to a temperature level that must be a few degrees higher than the melting point of germanium magnesium, which is the compound that has the highest melting temperature (1115 C) has to be cooled down gradually

the. Then the melt should be at this temperature, which is almost 1125 ° C may be held at a sufficient interval for the intermetallic compounds to mix completely permit. Afterwards the substance can be cooled very slowly to ambient temperature, and then the single-crystalline solid Solution to be obtained.

If the second type of procedure is needed, then the

Components, as mentioned above, heated up to around 1125 C. and kept at this temperature for a sufficient period of time,

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to a complete mixing of the intermetallic compounds assure. The crystal growth process as well as the preparation of the solid solution can be started immediately afterwards. A Thorough shaking of the melt is a necessary requirement in order to obtain a uniform solid solution. An excess of Magnesium in excess of the amount required by stoichiometry is allowed to procure be to any excessive loss of this element by evaporation, because of it, in relation to that of the others three elements, relatively high evaporation capacity, occur can replace. For best results, both the melting process and the crystal growth process should be performed in an inert gas environment or in a reducing gas environment or in a vacuum. The fabric should preferably be made in a vacuum. In addition, excessive attention should be exercised in order to avoid containers or To use crucibles that will not pollute or act on the basic components during melting and crystal growth.

Significant types of metallurgical processes can be used during and applied after the fabric has been fabricated to ensure that the resulting alloy or solid solution

Mg Si Ge Sn similar (homogeneous) and stoichiometric 2 χ y 1-x-y

will. These types of execution can be a thorough when melting Shaking the ingredients as well as the zone refining or Zone fusing include well known types.

The solid solution obtained in this way should preferably be a single crystal obtained by means of a modified Bridgman method or by any other suitable method. If the Bridgman or temperature gradient freezing method is selected, the following measures must be taken into account when growing the single crystal:

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(1) The interface separating the solid phase and liquid phase should be regulated precisely and carefully. An arcuate interface that becomes hollow (concave) into the liquid phase extends into it, should be held fast. In other words, the interface should lie on an arcuate plane and extending into the liquid phase at the isothermal surface existing between the liquid phase and the solid phase.

(2) A relatively consistent or linear temperature gradient should be held along the longitudinal axis of the container carrying the crystal.

Satisfying the above two conditions will make it possible to produce single crystals with relatively few displacements and the possibility of crystal imperfections such. B. microscopic cracks and uneven crystal growth, essentially decrease. The alloy or solid solution can still be polycrystalline.

The fabric, if desired, can also be made by a powder metallurgy finish. If this Method is selected, then the intermetallic compounds are first established, furthermore they become a close-meshed one (thin) powder crushed and pulverized. The dense material is made by the complete mixing of the components and consequently their hot pressing or obtained by cold pressing and then sintering. The dopant should be one of the basic ingredients in the Mixing can be added.

The substance, as it is explained in the description, must have the appropriate impurities and the appropriate level

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be doped so that the best possible thermoelectric performance can be achieved. Doping with copper, silver, cadmium or zinc will cause a positive-type electrical conductivity or a p-conductive material. The aluminum, gallium, indium and gold can also be used as p-type additional impurities. With lithium, sodium or potassium as doping impurities, attempts can also be made to produce an excess of acceptors or holes, and consequently to produce a p-type substance. In addition, the compounds of lithium, sodium and potassium with each of boron, silicon, germanium and tin can also be used as p-type additive impurities. It can be also tried using magnesium boride as an additional p-type impurity i. The copper and silver are the preferred p-type dopants. In order to achieve an even more powerful doping effect, at least one of the elements copper, silver, cadmium and zinc can be mixed with at least one other element, which can be selected from the group of elements consisting of aluminum, gallium and indium, and then the resulting mixture or alloy can be used as the dopant. Doping with any of the majority of the elements forming the main groups V, VI and VII of the periodic table of the i elements will bring about a negative-type electrical conductivity or an η-conductive substance. These elements include phosphorus, arsenic, antimony, bismuth, sulfur, selenium, tellurium, chlorine, bromine and iodine. They can either be used in their pure elementary form or as compounds with other elements, preferably with magnesium. For example, magnesium chloride (Mg Cl), magnesium bromide (Mg Br) and magnesium iodide (Mg J) can be used as η-type dopants. To achieve better results, more than one doping element or doping compound can be used

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Can be applied. This can be applied to both η-type and Apply p-type dopants, and becomes even more important as the Substance consists of four different elements, the atomic radius and atomic weight of which are very different. Any other element can be Dopants (elements or compounds) can be selected if they cause more efficient doping.

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Claims (1)

PATENT CLAIMS 1. Substance for direct thermoelectric energy conversion, characterized in that the substance consists of an alloy or a solid solution of the following three intermetallic compounds: Tin magnesium Mg Sn, Germanium magnesium Mg Ge and Silicon magnesium Mg Si is composed and by the chemical formula: Mg Si Ge Sn 2 x y 1-x-y where χ and y are the molecular fraction of each of Mg Si and Mg respectively represent Ge in the alloy. 2. Fabric according to claim 1, characterized in that the molecular fraction of Mg Si in the alloy is larger than that of each of Mg Ge and Mg Sn. 3. Fabric according to claim 1, characterized in that the molecular fraction of each of Mg Sn and Mg Ge in the alloy are larger than that of Mg Si 2 2 2 4. Fabric according to claim 1, characterized in that the molecular fraction of Mg Sn in the alloy is larger than that of each of Mg Ge and Mg Si. 2 2 5 fabric according to claim 1, characterized in that the molecular fraction of Mg Si in the 209831/0588 Alloy is between 20 percent and "60 percent, that of Mg Ge is between 20 percent and 40 percent, and that of Mg Sn is between 10 percent and 40 percent is. 6. Fabric according to claim 1, characterized in that the molecular proportions of Mg Si, Mg Ge and Mg Sn in the alloy are the same. 7. Fabric according to claim 1, 2, or 5, characterized in that the substance used to make the negative Branch of the thermoelectric energy converter is needed. 3. Fabric according to claim 1, 3, 4, or 5, characterized in that the substance used to produce the positive Branch of the thermoelectric energy converter is needed. 9 · Fabric according to claim 1 or claim 51 characterized in that the molecular fractions are at least two at least one of the three intermetallic compounds extends longitudinally change one of the branches of the thermoelectric energy converter between the hot junction and the cold junction to the. 10. device for direct thermoelectric energy conversion, characterized in that the device branches from one the alloy claimed in claim 1 or claim 5 and one Dopants are made of compound substance. 209831/0588