DE202008006946U1 - Article having a coating of a clathrate compound - Google Patents

Abstract

Object, the one prepared by a quenching process coating from a clathrate compound.

Description

The The present invention relates to articles comprising a coating from clathrate compounds.

STATE OF THE ART

Clathrate or cage inclusion compounds have been known for about 200 years and their targeted production for about 60 years. The first representatives discovered were gas hydrates, ie inclusions of gas molecules (eg Cl ₂ ) in the ice grid. Nowadays, intermetallic clathrates, which have been known since the 1960s, are the subject of extensive research, since they are used inter alia in thermoelectric modules. See for example the US 5,800,794 A1 . US 6,188,011 B1 and US 6,525,260 B2 , Intermetallic clathrates are classified into 9 different structural types, of which type 1 is the most studied, and therefore numerous representatives are known. 1 shows schematically the structure of such a clathrate compound, wherein the larger balls in the right half of the drawing represent the embedded in the cage atoms.

to Definition of the term "clathrate" should be noted below are meant above all those compounds in which the Caged atoms form a space-filling network form, although specifically in English under the definition of "clathrate" sometimes all kinds of inclusion compounds be understood.

As components of intermetallic clathrates a variety of combinations of elements can function. Thus, for example, reveals EP 1,074,512 A1 , which is largely the US 6,461,581 B1 Cage inclusion compounds in which one or more elements of the 4th main group, in particular C or Si, together with so-called "exchange atoms" from almost all the other elements of the periodic table - with the exception of hydrogen, technetium, the noble gases and the transuranium - can be selected form the matrix, ie the "cage". The choice of storable atoms in the above documents is possible within a wide range: atoms of the 1st to 3rd main group and those of the transition elements - with the exception of the 7th subgroup (Mn, Tc, Re) and the transuranium - in the matrix will be included. In general, atoms of the 1st and 2nd main groups are currently preferred as inclusion components and, for example, Ga, Ge and Si as cage components.

The Production of such clathrates usually takes place by melting the elements, after cooling usually several phases are obtained below the melting range of the mixture, either not yet the desired clathrate compounds or too low levels of the desired solid clathrate phase in combination with unwanted, disturbing foreign phases include. To obtain phase-pure clathrate compounds is therefore a subsequent heat treatment at temperatures of several hundred degrees Celsius, mostly over a period of time required by a few days or even weeks, until essentially one pure, solid phase of the desired clathrate is present.

One variant of these methods involves hot compression molding or discharge plasma sintering of powders of the starting materials at around 700 ° C., which can speed up the manufacturing process (see, for example, US Pat US 6,525,260 B2 ).

Nevertheless suffer all known methods in addition to the disadvantage of mostly very high expenditure of time also under that of the high energy requirement to achieve and maintain the required temperatures of several hundred degrees Celsius during the heat treatment.

About that In addition, the clathrate compounds thus obtained are usually in the form of blocks or compact masses laborious for later use be mechanically processed. Thin layers of clathrate compounds were also already made, but are to elaborate pulsed laser ablation or helicon magnetron sputtering required, and the thicknesses of the resulting layers are only in the nanometer range.

Desirable would therefore be a process by which a) clathrate compounds in a short time with comparatively low energy consumption in layer thicknesses can be produced in the micrometer range, b) the clathrates directly on a workpiece or object to be coated therewith can be generated, creating such items be made quickly and inexpensively, and c) clathrates can be produced in higher purity and with better properties are.

DISCLOSURE OF THE INVENTION

These objects are achieved by the inventors by providing articles having a quaternary coating of a clathrate compound. The articles are directly coated by a process which comprises preparing a homogeneous melt containing the desired elements in the desired ratio and subsequently solidifying the melt to the clathrate compound by ra cooling, ie quenching or "quenching" of the melt on a surface of an article.

In diametrically opposed to the prior art, according to which the solidified Melt heated for a long time ("tempered") must be so that phases of the desired cage inclusion compounds and the resulting blocks of clathrate phases by means of further, more or less expensive process steps have to be applied to substrates, the present inventors found that desired clathrate compounds also form when a melt of the components quenched or is quenched, what enormous time, energy and thus cost savings in the production of the articles of the invention means. These are therefore already in the course of the manufacturing process The clathrates coated with it and can be immediate their further use are supplied.

Without to commit themselves to a certain theory, take the Inventors suggest that the formation mechanism of clathrates in the manufacturing process It is based on the fact that with such a rapid cooling the solid, crystalline phase of the matrix, ie the "cage", formed more rapidly than the phase separation of the uncrystallized inclusion component (s) The atoms of the latter are in the solid matrix be captured and trapped. That way, moreover Clathrates with very high phase purity, d. H. with by usual Methods, such. B. X-ray powder diffraction and scanning electron microscopy, barely detectable levels of foreign phases produced. Possibly Foreign phases detectable in the transmission electron microscope are typically a submicron structure, i. H. they lie between the grains or crystallites of the desired clathrate phase as intergranular Phase with thicknesses of 0.05 to 0.1 microns before, whereas after the The prior art usually impurities in the form of isometric grains 10 to 100 μm in diameter attack. This will ensure compliance with the desired Atomic ratios and properties, such. B. the thermoelectric power, of the produced by the process according to the invention Materials improved.

If desired such minor impurities by foreign phases in a conventional manner by heat treatment, d. H. "Tempering" the objects clathrate coated be removed, but due to the low share and Submicrostructure of impurities Duration of heat treatment significantly shortened compared to the prior art is and annealed at significantly lower temperatures can.

Under the "desired relationship" is not necessarily to understand an exact stoichiometry, as particularly good Results sometimes with clathrates of elements in non-stoichiometric, d. H. non-integer shares.

The In principle, objects can be coatings have almost all known clathrate compounds, d. H. either from clathrates based on organic compounds as well as inorganic Compounds, such. B. intermetallic clathrates. In the case of organic Compounds are previously with "elements" for the production of the melt respective "molecules" to form the lattice or inclusion components meant. In this context, it should further be noted that under "intermetallic" depending on the context and language in addition to the classic Metal elements also semiconductors and occasionally carbon be subsumed, this term being used herein in its broadest interpretation to understand. Preferably, the objects are with coated such intermetallic clathrates, since in this case the time and cost factor are particularly significant.

The Selection of the matrix and inclusion components is not specific limited. However, one or more are preferred elements selected from Si, Ge, Sn, Pb, Al, Ga, In and transition elements as matrix or cage components, as well as one or more of alkali metals, alkaline earth metals, transition elements, including lanthanides and actinides, more preferably from alkaline earth metals and lanthanides and sometimes also actinides, selected elements as inclusion components, as these Elements currently preferred for the preparation of clathrates used for thermoelectric and semiconductor applications become.

at suitable selection of the components is an additional by the invention Advantage achievable, namely the comparatively simple Solution for a highly topical and state of the art the technique unsatisfactorily solved problem: increasing the thermo-power of clathrates by introducing f-elements, d. H. Lanthanides or (less preferred) actinides, into the cages of clathrate structures.

While the clathrate phase is formed, according to the state of the art, the atoms of the f element to be stored in the cage are displaced from the clathrate structure, which is called segregation, because in addition to the clathrate phases, other thermodynamically more stable phases can form with the f element. As a result of the rapid quenching, the novel production process makes it possible to produce metastable crystalline phases with f-elements as inclusion atoms. The latter have because they caught in the cage and there are "frozen" within the clathrate phase, so to speak, not the possibility of segregation.

There the thermal conductivity of many clathrates clearly is lower than that of pure metals, in the new manufacturing process preferably thin layers or droplets of the melt with a thickness or a diameter of less than 300 μm, preferably below 100 microns, quenched, so as to their cooling rate to increase. The forming during rapid cooling, solid Clathrate connection also falls directly as Thin film or granules on the inventive Subject, resulting in some later processing steps can be omitted, according to the state of the art, where mostly Blocks of clathrate are obtained, mandatory are. The clathrate compounds obtained by the new preparation method Of course, if desired by known methods, for. As hot pressing or spark plasma sintering, also to Blocks are compacted.

As discussed above for the mechanism of clathrate formation, the gist of the invention resides in the very rapid cooling or quenching of the melt on the desired article. Although the required cooling rate depends on the components of the respective clathrate compound, it is not particularly limited as long as a desired clathrate is obtained. However, the inventors have found that particularly good results with respect to the phase purity with cooling rates of at least 10 ³ K / s, preferably at least 10 ⁵ K / s, can be achieved.

The Type of rapid cooling of the melt is not special restricted and this can be done by introducing the same into a coolant in which the object is present, take place. In this case, the melt, for example, through a nozzle, z. B. using an inert gas as a carrier, in a cooling gas or a cooling liquid, the / which surrounds the object injected and at the same time to fine droplets are atomized on the Deposit the surface of the object. Alternatively, it can the melt into an optionally heated, the object containing Mold filled or directly in it from powders of the starting components are generated, after which the mold is introduced into the coolant, z. B. is immersed. As a coolant can For example, nitrogen, noble gases or others, under the process conditions too no reactions with the clathrate components capable Gases or liquids, eg. B. liquefied inert gases, be used.

The Coolant can - especially if a big one Volume of which is available for cooling stands and it during the cooling process for the better Heat distribution is optionally mixed - room temperature However, preferably, the coolant is the Increasing the cooling rate to one Temperature of <5 ° C (before) cooled.

Preferably the rapid cooling of the melt takes place simply Distributing them on a surface of the object, which thus serves as a solid coolant, on the the melt solidifies to the desired clathrate compound. The surface of the article is preferably made a material with high thermal conductivity, such as z. Copper or silver, but is not limited thereto. Analogous to cooling in liquid or gaseous form Coolants, the object surface before on and / or during the distribution (s) of the melt on it cooled, more preferably again to a temperature of <5 ° C. The Cooling the surface can be done in any way carried out, for example by inflating or spraying an auxiliary coolant, z. B. a - possibly liquefied - inert gas, or by one with the surface heat-conducting connected cooling device such as a cooling jacket or similar.

Preferably The melt passes through a nozzle on the surface sprayed and finely distributed on it, which in turn reduces the cooling rate elevated. For the same purpose of fine distribution can the surface, d. H. the object, and the nozzle during the application of the melt relative to each other be moved, bringing both movements only one of both are also included by both at the same time.

The type of movement is not specifically limited. Preferably, however, the surface is rotated at a defined distance from the nozzle, whereby the invention can be carried out using melt-spinning methods and devices. This technique is commonly used to make amorphous and nanocrystalline intermetallic or ceramic materials as well as polymer fibers, so it is well known and easily adaptable to the requirements of the new manufacturing process with minor adaptations. Melt spinning combines some of the preferred features of the new manufacturing process discussed hereinbefore and hereinafter; Among other things, cooling rates of 10 ⁵ to 10 ⁷ K / s can be achieved in this way.

The rotation of the article is preferably carried out with rotational speeds of at least 1300 U / min, which on the one hand ensures a particularly good distribution of the melt on the rotating surface and on the other hand, the usual rotational speeds in melt spinning represents wel the corresponding existing devices are designed.

alternative or in addition to the rotational movement of the surface the nozzle during the injection of the melt, z. B. motor driven, across the surface be led, which in a short time also a larger Surface for cooling available stands.

alternative For this purpose, the melt before cooling in electromagnetic Suspended and lifted by suspending the state of suspension in a coolant, preferably gas, surface present of the subject to be dropped, causing them to turn on the surface is distributed and thereby cooled. The melt can in this case, for example, already after a known Schweießmelzverfahren, z. B. by induction heating, getting produced. To further improve the distribution and Thus, the cooling will melt when lifting the limbo preferably between two surfaces, d. H. a surface of the object and z. B. a pressure plate, pressed flat, both can be optionally cooled again.

As As can be seen from the above, the goal becomes particularly rapid cooling, d. H. Deterrence of Clathrate precursor melt predominantly achieved by that this is distributed as finely as possible on the object. This can be achieved in different ways as mentioned, these can also be combined with each other. In the distribution The melt on a surface can do this by one Be surrounded by cooling gas and / or in its entirety or, preferably, only partially immersed in a cooling liquid and optionally in addition to one of the melt spraying nozzle to be moved.

By the direct spraying of the melt a surface of the The article to be coated with the clathrate compound becomes Production efficiency in the production of such items enormous improved. The objects or "substrates" for such direct coatings with clathrate compounds are not specially restricted. So, among others Semiconductor wafer or thermoelectric or electrothermal Components such as thermoelectric generators for the conversion of Waste heat into electrical energy, such. B. sheets for Exhaust pipes or chimney linings, as well as Peltier elements for active cooling to much more economical Be made way, as was previously possible, especially the objects to which the clathrate precursor melt is preferably sprayed on, have almost any shape can.

The Surface of the so to be coated with a clathrate compound Item may improve the adhesion of the clathrate layer be pretreated, whereby both surface structuring, z. As roughening and the like, as well as the application of an adhesive layer or primer are included. The latter is preferably also from a good heat-conducting material and can, for example from a thin layer, e.g. B. 10 to 200 microns, a melted metal, such as. Ga or Sn, this is superficial on contact with the hot melt melts, causing the adhesive layer and clathrate layer to solidify be connected to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

1 shows schematically the structure of a producible by the process according to the invention clathrate.

2 shows the X-ray powder diffraction of obtained in Process Example 1 Ba ₈ Pd ₄ Ge ₄₂ .

EXAMPLES

below The invention is based on specific embodiments described, which serve only for illustration and not as Limitation are to be understood.

example 1

General procedure

It were some intermetallic below Clathrate compounds using a melt spinning device made in the following way. First, powder mixtures were the desired elements in the desired ratios with a total weight of at least 1 g each in a water-cooled Copper trough melted by high frequency heating, in carbon transferred coated quartz ampoules, for homogenization kept in the liquid state for several hours and stiffened.

The material was then inductively melted in vertical conical quartz tubes with a 0.3-0.5 mm x 2-5 mm orifice and a 10 to 20 mm diameter upper orifice slit serving as a nozzle, the melt due to the capillary forces remained in the quartz tube. The quartz tube was placed in a melt spinning apparatus at a distance of 0.3 mm above the tread of a copper wheel 30 cm in diameter and 8 cm in width, and then the wheel was rotated at a speed of 1,300 rpm. The above The opening of the tube was connected to an argon bottle and the melt was injected by means of argon overpressure through the nozzle onto the rotating copper wheel. The solidified on the wheel material was in the form of 20 to 50 microns thick slats with an area of up to 4 × 20 mm. The copper wheel had room temperature before the melt was sprayed, and due to the large mass difference between the wheel and the melt, the temperature of the wheel barely changed during the sharpening process. Despite the enormous cooling rates, all samples prepared had a well-defined crystal structure with no detectable levels of amorphous phases.

The following intermetallic clathrate compounds were prepared by the above melt spinning technique, the underlined elements forming the atoms included in the crystal lattice of the other elements:
Eu ₈ Ga ₁₆ Ge ₃₀ , Sr ₈ Ga ₁₆ Ge ₃₀ , Ba ₈ Ga ₁₆ Ge ₃₀ , Ba ₈ Pd ₄ Ge ₄₂ , Ba ₈ Ni ₄ Ge ₄₂ , Ba ₈ Au ₆ Si ₄₀ , Ba ₈ Au ₆ Ge ₄₀ , Ba ₈ Cu ₆ Si ₄₀ , Ba ₈ Ga ₁₆ Sn ₃₀ , Eu ₂ Ba ₆ Ga ₁₆ Ge ₃₀ and Eu ₄ Sr ₄ Ga ₁₆ Ge ₃₀ .

The phase purity of the obtained thin layers of the desired clathrate compounds was checked by X-ray powder diffractometry, whereby no foreign phases could be detected, especially since the measured data corresponded completely with simulated diffractograms in position and intensity. That is, the amount of impurities, if any, was in any case below the detection limit of the detection method (3%). 2 shows by way of example the X-ray powder diffractogram of Ba ₈ Pd ₄ Ge ₄₂ obtained in Example 1.

Consequently were 11 different copper wheels coated with clathrate compounds receive.

Example 2

example 1 was repeated but with a thin copper sheet with 5 cm width was stretched around the circumference of the wheel and at the same time with the spraying of the melt through a second nozzle Nitrogen gas at a temperature of about -180 to -170 ° C. was inflated on the sheet. To improve the liability of Clathrate layer on the sheet was this by melt spinning with a thin adhesive layer of Sn with a thickness of about 50 μm precoated.

Subsequently, the wheel was rotated at 1,300 rpm as in Example 1, and a melt was sprayed onto the plate and rapidly cooled, whereby a layer of
Sr ₈ Ga ₁₆ Ge _{30 was formed} on the copper sheet. A sample of the clathrate layer again showed no foreign phases in the X-ray powder diffractogram.

The thus obtained copper plate coated with the clathrate compound was removed from the wheel and rolled flat and is for manufacturing various types of thermoelectric modules ideally suited.

The The above examples clearly show that the new manufacturing process quenching precursor melts for the first time directly leads to objects, which with clathrate compounds coated in excellent form in the phase purity obtained from thin layers with thicknesses in the micrometer range become.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• US 5800794 A1 [0002]
• US 6188011 B1 [0002]
• US 6525260 B2 [0002, 0006]
• - EP 1074512 A1 [0004]
• - US 6461581 B1 [0004]

Claims (7)

Subject, one by a quenching method prepared coating of a clathrate compound. Article according to claim 1, characterized the coating has a layer thickness of 10 to 300 μm, preferably 10 to 100 microns. Article according to claim 1 or 2, characterized that between the surface of the object and the coating from the clathrate compound a primer layer to improve adhesion is provided. An article according to any one of claims 1 to 3, characterized in that the clathrate compound is an intermetallic Clathrate compound is. Article according to claim 4, characterized that the matrix or cage components one or more selected from Si, Ge, Sn, Pb, Al, Ga, In and transition elements Elements are. Article according to claim 4 or 5, characterized that the inclusion components one or more of alkaline earth metals and lanthanides are selected elements. An article according to any one of the preceding claims, characterized in that any unwanted Fremphasen in the clathrate coating in the form of intergranular phases with thicknesses of 0.05 to 0.1 microns.