Patent Application
20110297203
This disclosure pertains to the making of thermoelectric elements (sometimes called thermoelectric “legs”) and their assembly into a thermoelectric device. Materials having thermoelectric properties are often brittle and susceptible to cracking. This disclosure pertains to the use of sintering techniques to produce fine-grained thermoelectric legs to their net design shape and to the assembly of the legs into a thermoelectric device without changing the net shape of the legs or otherwise increasing the susceptibility of the legs to fracture or cracking.
Thermoelectric devices are formed of two different (but complementary) thermoelectric materials and can produce an electrical current when separated junctions are subjected to a suitable temperature differential or can produce separate hot and cold junctions when powered with an electrical current. The power generation thermoelectric devices exploit the Seebeck effect, a phenomenon in which a temperature gradient is applied across a body and as a result an open circuit voltage, co-linear to the temperature gradient, is established. The sign of the voltage with respect to the applied temperature gradient is dependent on the nature of the majority charge carriers. Where a temperature difference exists between ends of a thermoelectric element, heated electrons (or holes) flow towards the cooler end. Where a pair of dissimilar thermoelectric semiconductor elements, that is a pair consisting of an n-type and a p-type element, are suitably connected together to form an electrical circuit, a direct current (DC) flows in that circuit.
Several families of crystalline thermoelectric material compounds have been discovered and developed. Among these compounds, skutterudite (CoSb3) is an example. Cubic CoSb3 possesses two voids in a crystallographic unit cell. The voids may be filled to some extent, for example, with rare-earth, alkaline-earth, or alkali metal elements. Such partial filling approaches may be used to adjust or tune thermoelectric properties of the crystalline material. The skutterudites display semiconductor properties and distinct compositions can be formed with p-type and n-type conductivity. Many other thermoelectric compositions are known and available.
To date, however, all practical such thermoelectric materials are brittle, narrow band gap semiconductors. In automotive applications, thermoelectric devices comprising thermoelectric legs of these ceramic materials are subjected to very harsh operating environments. The devices are often situated where they are subjected to substantial mechanical forces, including vibrational forces. And the thermoelectric legs, which are often only a few millimeters in length, are subjected to substantial temperature differences over that short length which induces stresses in the legs. Such stresses affect the viability of the brittle materials and, therefore, acceptance of their use in automotive vehicle applications.
Thermoelectric compositions are generally prepared by mixing powders of elemental or pre-combined constituents in a convenient mass or volume and fusing or sintering them into an billet of a suitable composition. Once suitable crystalline compositions have been formulated it is necessary to form p-type and n-type thermoelectric legs of a desired size and shape for incorporation into a thermoelectric device, comprising many legs arranged for electrical interconnections for producing an electrical potential or a heating or cooling effect. In order to more effectively produce thermoelectric materials and to shape them into thermoelectric legs for a device, machining of ingots of the brittle materials has been employed at some stage of manufacture. It is a purpose of this disclosure and invention to provide a method of preparing thermoelectric elements (legs) in which the legs of thermoelectric material are formed to a net shape without machining and thereafter assembled into a thermoelectric device without machining the net shape formed legs.
The inventors of the methods disclosed in this specification have carefully considered reasons for the relatively low strength and durability of thermoelectric legs as they have been made and assembled into thermoelectric devices. They have observed that thermoelectric legs made by prior practices have strength-limiting flaws in the ceramic material that lead to breakage when subjected to mechanical forces, such as vibration, or to thermal shock. Strength limiting flaws can be intrinsic in nature taking the form of large grains, agglomerates or cracks which are on the interior of the brittle, crystalline specimen and initiate failure internally. Other flaws include surface flaws in which failure of the leg initiates from flaws associated with surface imperfections. These surface flaws can be intrinsic flaws inherent in parallelepiped leg shapes or extrinsic flaws resulting from machining or other processes employed in fabricating legs of suitable shape. The inventors' goal is to use a method of making thermoelectric legs or other elements in which the size of the strength limiting flaws introduced into the legs are limited to the dimensions of the largest grains in the sample (up to ten to fifteen microns) of the thermoelectric material so as to obtain increased strength in the elements of a thermoelectric device. This increased strength is to enable application of the devices, for example, on automotive vehicles.
In accordance with preferred embodiments of the methods of this invention, thermoelectric material compositions are obtained as powders in a desired grain size by any suitable practice. For example, the average grain size of n-type and p-type skutterudite compositions may be of the order of about ten microns, and it is preferred that the quantity of grains significantly larger than the average grain size be minimized. A suitable weight or volume of a powder of a composition may then be carefully loaded into a forming cavity, defining the shape of the leg, for axial compaction and sintering into the net shape of a desired thermoelectric leg. One or many such legs are then formed in separate cavities by net shape sintering into durably compacted legs. The sintered legs are used in the assembly, or otherwise making, of a designed thermoelectric device without any machining or processing of the legs which would introduce additional flaws into their surfaces or internal structure. In a preferred practice, spark plasma sintering (SPS) is used to fuse the powder into the dense leg shapes. SPS passes very large currents through the compacted powder and the resulting Joule heating quickly consolidates the leg(s). It could be used to simultaneously produce many legs in side-by-side forming cavities. But hot pressing and subsequent sintering practices may also be used to achieve consolidated net shapes.
By the term “net shape” it is intended that the sintered elements be fabricated with such dimensional control that no additional shaping is required, so that no extrinsic flaw is introduced in the elements and no remediation of any damage from such shaping is necessary. Thus, each of the thermoelectric elements of a device, n-type and p-type, are prepared by net shape sintering and thereafter assembled into a thermoelectric device without machining or exposure to other processes which may inflict damage or deform the surfaces of any of the legs of the device.
In addition to the net shape, sinter forming of fine grain powder thermoelectric legs, consideration may also be given to their cross-sectional shape. Often, the legs of a thermoelectric device are square in cross-section and cut to a desired length for placement of many closely spaced, like-shaped p-type and n-type legs between electrodes to be bonded in electrical contact with the ends of the legs. In addition to avoiding machining of the thermoelectric material legs, it may be preferred to sinter them into round or rounded cross-sections without sharp edges.
Thermoelectric devices generate electricity by electrically connecting two thermoelectric elements of differing thermopower signs and exposing them to a temperature gradient. The capabilities of the device will depend both on the magnitude of the Seebeck coefficient of the thermoelectric elements, a material effect, and the magnitude of the temperature gradient. It is therefore desirable to have the absolute values of Seebeck coefficients be as large as possible.
Semiconductors are attractive candidate materials for thermoelectric elements because they may be doped with elements providing excess electrons or holes which results in large positive or negative values of the Seebeck coefficient of these materials depending on the charge of the excess carriers.
While doped semiconductors have suitable thermoelectric properties, they are generally brittle and prone to stochastic fracture under stress. Thermoelectric devices have utility only when exposed to a temperature gradient and temperature gradients are known to induce stresses in bodies. Thus, thermoelectric devices will, in the course of performing their intended function, experience stress. Further, the magnitude of the stress will increase with increasing temperature gradient. Thus, the stresses imposed on the thermoelectric elements will increase in proportion to the electrical output of the device. Conversely, any stress limitation imposed on the device will also limit its allowable electrical output. Accordingly, it is desired to make and assemble the thermoelectric elements so that they do not break or fracture in the handling or operation of a thermoelectric device.
In this representation it is intended that plate 20, the hot plate, is maintained at a higher temperature than plate 18, the cold plate. Obviously such a temperature gradient will produce a heat flow in the direction indicated by arrow 22. Electrical terminals 24 and 26 provide connection with an external load or with another thermoelectric device. In the configuration shown connector 26 will be at a more positive electrical potential that connector 24.
Brittle materials fracture when overloaded or over stressed in tension. The fracture stress of commercial brittle materials, however, is not an intrinsic property of the material itself, but depends on the size of the test sample, the multiaxial stress gradient, and on the number and severity of pre-existing flaws in the material. Strength limiting flaws may include, without limitation, large grains, agglomerates, pores, voids, surface scratches or cracks.
σf=KIc/(Y·c1/2),
where KIc is the fracture toughness of the material,
and Y is a unitless geometric factor associated with flaw shape.
Thus σf∝1/√{square root over (c)} and the fracture stress will depend upon the length of the largest suitably-oriented flaw.
Even in this simple configuration, it is clear that two nominally equivalent elements may, depending on their flaw density and distribution, have substantially different failure stresses or fracture strengths. Because of the series electrical connection employed in the device, any fracture has potential to at least severely degrade its performance. Thus, it is desired to fabricate thermoelectric elements which have consistent distributions of small flaws so that all elements will have substantially equivalent fracture strengths. It is of course intended that the element fracture stress be sufficiently greater than even extreme in-service stresses to assure device durability.
It is particularly advantageous to eliminate or reduce surface or external strength-limiting flaws. As
It is a goal of this invention to eliminate the necessity for any mechanical sawing or machining operation by fabricating thermoelectric elements to net shape using powder processing techniques. The term ‘net shape’ is intended to convey that the thermoelectric elements, as fabricated, fully satisfy all dimensional requirements and that no further shaping or dimensional adjustment, no matter how minimal is required or performed.
The process of fabricating the Bi2Te3 thermoelectric elements requires that the material possess large oriented grains. In general, the largest crack or other flaw dimensions are comparable to the grain size. Hence, the use of large-grained material may result in large internal flaws which may promote lower failure stresses and be detrimental to mechanical reliability.
It is a goal of this invention to minimize the dimensions of internal strength-limiting flaws by promoting a fine grain size or minimizing the size of the largest grains in an entire grain size distribution.
Finally, because conventional elements are sawed from ingots they will typically exhibit a rectangular cross-section resulting from sawing in two orthogonal directions. The corners or angular junctions formed by the two intersecting sawn surfaces are, unfortunately, very effective stress raisers, that is the local geometry is such that it induces a local stress which is greater than the applied stress. Since these angular junctions also tend to be a frequent origin of damaging cracks, fractures are readily initiated at these angular junctions.
It is a goal of this invention to minimize stress-raising shapes and, in particular, to avoid any angular junctions between surfaces.
Thermoelectric elements fabricated by consolidation and sintering of powders are fabricated as net shape entities and no further processing or manufacturing operations such as sawing is used. Further, the geometry of the powder processed elements is dictated by the geometry of the die and thus may be fabricated in forms with smoothly-varying curvature to ensure that no stress-raising angular features are present. A natural configuration for the sintered elements is a cylinder, but packing considerations, driven by a requirement to maximize the electrical output per unit area may lead to the adoption of other forms, for example an elliptic cylinder or a form 60 like that shown in
The thermopower or Seebeck coefficient of a thermoelectric material is frequently presented as a single value. However the Seebeck ‘coefficient’ is in fact a second rank tensor. Thus, in individual crystals the values of the ‘coefficient’ may vary appreciably with crystal orientation, especially for crystals of low symmetry, many of which, like Bi2Te3 find application as thermoelectric elements. Since powder processed thermoelectric elements will comprise a multiplicity of randomly oriented crystals (grains), the net Seebeck ‘coefficient’ will be some average of the Seebeck coefficients of all grains. In highly anisotropic crystals, which may exhibit radically different thermopowers in different crystal orientations, this may unacceptably reduce the overall thermopower of the compact. It is therefore preferred that materials exhibiting higher symmetry, and therefore greater isotropy be employed. Suitable candidate materials are filled skutterudites which have high thermopower and exhibit crystal structures with cubic symmetry rendering their thermoelectric response substantially isotropic.
Exemplary n-type skutterudites are those based on Co and Sb; exemplary p-type skutterudites are those based on Co, Fe and Sb. Both n-type and p-type skutterudites will exhibit enhanced thermoelectric properties when filled with Na, K, Ca, Sr, Ba, La, Ce, Pr, Nd, Eu, Yb and Tl or combinations thereof.
It will be appreciated by consideration of
The procedure entails charging a pre-measured quantity of p-type or n-type semiconductor thermoelectric powder as fine particles of suitable size, on the order of 10 micrometers and a size distribution in the range of 5 to 20 micrometers, fabricated by ball milling, into a pre-shaped chamber; compacting the powders under pressure in the range of 30 to 60 MPa; and consolidating or densifying the powder compact by sintering. Samples are sintered under dynamic vacuum or an inert atmosphere by heating, at a rapid rate between 50 and 100° C./minute with the application of a constant uniaxial pressure to a final temperature where consolidation is complete. Samples are generally allowed to age at this final temperature for 1 to 3 minutes before cooling to room temperature by convection with the pressure removed.
The compacting pressure may be applied in any suitable direction. If the primary concern is ease of die design and freedom from parting lines, visible features on the part along the line of contact of die segments which may result from die wear or from minor mismatch between die halves, compaction along the cylinder axis may be preferred. This configuration is shown in
Compacting in a direction perpendicular to the cylinder axis is shown at
Again dimensional consistency of the sintered powder compact may be promoted by utilizing a die with a plurality of substantially-identical die cavities to enable simultaneous fabrication of a plurality of elements. It will be appreciated that the use of a die with multiple cavities also enables production efficiencies.
Sintering is commonly used in the consolidation of powder compacts. Sintering seeks to consolidate powders by solid-state diffusion under heat, often with pressure-assistance, and sometimes promoted by addition of minor proportions of a liquid accelerant. In principle, the grain size of the resulting compacted and sintered solid would be substantially equal to the particle size of the powder charge. However, conventional powder processing approaches such as hot pressing, though not excluded, may result in grain growth in the powder charge during consolidation, possibly thereby prejudicing the fracture behavior of the resulting sintered and compacted solid. A preferred process is Spark Plasma Sintering Processing, a sintering process which promotes short process times and minimizes grain growth and coarsening.
Spark Plasma Sintering Processing applies a series of high frequency DC pulses through the compacted powder while it is under pressure. It is believed, but not relied on, that this generates a high current, low voltage spark and momentarily generates a plasma generally localized at points of particle-particle contact. This produces high localized temperatures between particles promoting solid state diffusion at surfaces and consolidation. The sintering process need only be continued for periods of up to 30 minutes, more typically for about 10 minutes, to achieve near complete densification of the compact. Because heating is localized, the bulk temperature of the compact is reduced and only minimal grain growth is generally observed. Hot pressing or other sintering approaches generally result in higher bulk temperatures in the compact during sintering and are thus more likely to promote grain growth. However any sintering process may be used provided that it promotes an average grain size in the sintered compact of less than several micrometers. The process is further illustrated by consideration of the following example.
P-type skutterudite powder was fabricated by first melting Ce, Co, Fe, and Sb metal in atomic proportions Ce:Co:Fe:Sb of 1.05:1:3:12.05 by induction melting them under an argon atmosphere to form an ingot of CeCoFe3Sb12. The CeCoFe3Sb12 was then comminuted by ball milling in acetone under a protective atmosphere of argon for 5 minutes to achieve particle sizes in the a range of 5 to 40 micrometers and annealed for 168 hours at 750° C. under a reduced atmosphere of 10−6 Torr.
The skutterudite powder was then placed, in a plurality of like shaped and dimensioned cavities contained within a three-part cylindrical graphite die. The die consisted of a smooth-surfaced backing plate; a solid cylindrical die body with recesses on each of its flat, end surfaces, and at about its mid-length a plurality of die cavities open on either end to the flat surfaces of the cylinder; and a combination punch comprising a plurality of individual punches, one for each of the die cavities, extending from a common die plate. The die plate was generally disc-like with one flat surface and with integral punches extending from the opposing surface. The die included alignment features including matched cylindrical cavities in the die body and die plate to releasably secure closely fitting pins, and recesses in the cylinder ends of the die body to tightly accommodate the perimeters of the complementarily-formed combination punch and the backing plate.
The die was assembled by first inserting the backing plate, generally resembling a flat-faced disc into a centered recess in one of the flat faces of a cylindrically-shaped die. The depth of the recess was less than the thickness of the backing plate so that one of the flat surfaces of the backing plate would protrude beyond the end of the cylinder while the second would terminate one end of the open die cavities. After placing the fine-grain skutterudite powder in the die cavities, now closed on one end by the backing plate, the combination punch was installed.
The combination punch was fitted into a centered recess on the second of the flat faces of the cylindrical die. Insertion was guided by the guide pins commonly engaged with the combination punch and die body to align the combination punch and die body and ensure that the individual punches were inserted into their respective die cavities. The flat surface of the combination punch extended beyond the cylindrical body of the die. Thus, compaction pressure, applied to the protruding faces of the backing plate and the combination punch, for example in a press, was transmitted to the skutterudites. The compaction pressure used in fabricating these skutterudite elements was 50 MPa.
The powder compact was then sintered by spark plasma sintering while under an axial load of 50 MPa using a pulsed DC current at a pulse frequency of 70 Hertz and a pulse duration of 12 milliseconds with a 2 millisecond pause. The powders were heated at a rate of 75° C./minute to a final temperature of 675° C. and held for an additional 2 minutes. The total heating time was 10 minutes while under an applied pressure of 50 MPa to form many net-shape processed, square-section shaped 2.5 millimeters×2.5 millimeters×11 millimeters bars at 98% densification. The average grain size was determined to be 7.6 micrometers with a maximum grain size of less than 40 micrometers.
These net-shape processed bars were tested in three-point bending over an 8 mm span. For comparison, equivalently-sized, samples of the same CeCoFe3Sb12 composition and grain size distribution were sawn from larger sized similarly Spark Plasma Sintering-processed blanks. The blank-sawn samples will therefore exhibit a distribution of strength-limiting extrinsic flaws resulting from the sawing process. These extrinsic flaws are analogous to those of conventionally-processed ingot sawn thermoelectric devices, but with a smaller average flaw size due to the smaller average grain size of the sintered blanks. No such extrinsic flaws are expected for the net-shape processed samples.
The fracture strength of the blank-sawn samples, at a 95% confidence level ranged from 32 to 45 MPa; the net-shape processed samples exhibited fracture strengths between 92 and 112 MPa. The powder processed samples also exhibited appreciably greater consistency in their mechanical strength properties than the ingot sawn samples. Thus the net-shape processed samples exhibited both superior strength and less sample-to-sample variation than the ingot sawn samples.
It will be appreciated that it was intended to directly compare these net-shape processed sintered bars with sawn bars representative of current fabrication procedures. Thus, the sintered bar cross-section was chosen to replicate the square cross-section of the sawn bars although it is recognized that the sharp corners necessarily consequent upon a square cross-section render it sub-optimal for the practice of the invention. Even greater demonstrated benefits of the powder-fabricated thermoelectric elements of the invention over sawn ingot thermoelectric elements are realized when shapes are fabricated without sharp corners.
Although the practices of this invention have been described with reference to specific examples, it will be appreciated that these are intended to be exemplary only and are not intended as limitations on the scope of the invention.
This invention was made with U.S. Government support under Agreement No. DEACO50000R22725 awarded by the Department of Energy. The U.S. Government may have certain rights in this invention.
