DE102013112522A1 - Thermoelectrically active airgel - Google Patents

Abstract

The invention relates to a method for producing a thermoelectrically effective airgel, wherein the airgel is a doped with thermoelectrically active materials during the sol formation airgel, wherein a network of the doped thermoelectric material and the gelling agent is formed by gelation, subcritical or supercritical drying or a thermoelectrically active material is processed via a sol-gel process and a subcritical or supercritical drying process to form a thermoelectric airgel.

Description

The invention relates to a thermoelectrically active airgel.

Aerogels are highly porous solids that contain up to 99.98% of the volume of pores. There are various types of aerogels, with silicate-based ones being the most common. Other materials, such as plastic or plastic-based are used in special cases. In principle, all metal oxides, polymers and some other substances can serve as a starting point for the airgel synthesis by means of a known sol-gel process. Aerogels are usually nanostructured open-pore solids. The actual term "airgel" denotes, in contrast to articles usually produced in the sol-gel process, spongy bodies in which the pore content is greater than 90% by volume.

As aerogels based on silicates, for example, aerogels based on water glass and quartz glass are known. It is known that quartz glass aerogels can be rendered permanently water repellent when wet with a trimethylchlorosilane wash. If the most commonly used tetraethylorthosilicate or waterglass is replaced by methyltrimethylorthosilicate in the starting material for quartz glass aerogels and the sol-gel routine changes somewhat, it is possible to prepare rubber-like flexible aerogels from otherwise brittle aerogels.

In addition, it is known to produce organic aerogels, for example from resorcinol-formaldehyde, cellulose, alginate and starch. From resorcinol-formaldehyde and cellulose aerogels can be produced by pyrolysis at 1000 ° C with exclusion of air carbon aerogels.

So it is z. B. from the DE 10 2006 049 179 A1 known. Producing aerogels from cellulose fibers or to produce cellulose fibers as aerogels, wherein the density is in a range of 100 to 200 kg / m ³ . In these carbon aerogels, which were generated from the pyrolysis of cellulose aerogels, it was even surprisingly found that they contradict the Wiedemann-Franz's law, so the fibers have a high electrical conductivity despite very low thermal conductivity.

Out " 2012 2nd International Conference on Materials, Mechatronics and Automation, Lecture Notes in Information Technology, Vol. 15 ", pages 335 to 340 It is known to form thermoelectric composite materials of PEDOT: PSS or PEDOT: PSS / MWCNT materials, in particular thin films. Such materials, which are mixtures of poly-3,4-ethylenedioxythiophene and polystyrenesulfonates and may contain further fillers to improve the electrical conductivity, such as MWCNTs (multi-wall carbon nanotubes), have good thermoelectric properties and can also be produced industrially or in the process large quantities available.

All aerogels have properties that are determined only by their structure and are essentially independent of the material that forms the skeleton of the airgel.

All aerogels are significantly lighter than the solid starting materials, defined only by the porosity of up to 99.9% with a very low thermal conductivity, which is typically in the range of 5 mW / Km to 50 mW / Km. The inner surface of aerogels is usually around 500 m ² / g. Hydrophilic silica aerogels should no longer come in contact with a fluid medium after they have been prepared, otherwise they will pick up the fluid and subsequent drying will result in extreme shrinkage and cracking of the airgel. Plastic aerogels or oxidic aerogels are less problematic in this respect.

Oxidative aerogels can be used up to approx. 60% of the absolute melting temperature of the respective oxide. If they are thermally stressed, sintering reactions occur which cause the airgel to shrink extremely and thus make a more or less massive body. Previously known polymeric aerogels have operating temperatures of up to 300 ° C under an oxidizing atmosphere, with carbon aerogels being stable up to 2000 ° C under an inert atmosphere.

The efficiency of thermoelectric systems is determined by the ZT value, which is greater the greater the specific electrical conductivity σ and the smaller the specific thermal conductivity λ.

The ZT value is further determined by the Seebeck coefficient α, which is given on the basis of the material pairing used, at a mean temperature T defined by the application.

The low conversion efficiency currently known and available thermoelectric materials is extremely disadvantageous for the large-scale implementation of thermoelectric systems in industrial applications. According to the current state of the art, thermoelectric efficiencies of commercial thermoelectrics are within the range of 5 to 5. 8%, which represents very little for industrial implementation.

The object of the invention is to make use of temperature differences with high thermal insulation performance and low weight.

The object is achieved with thermoelectrically active aerogels having the features of claim 1.

Advantageous developments are characterized in the subclaims.

According to the invention, thermoelectric materials are integrated into the sol-gel process during airgel production or the thermoelectric material itself is processed into an airgel. On the one hand, this can be done by mixing solids which are thermoelectrically active into a gelling agent, with powder mixtures of the starting materials in the nano-scale range preferably being used as solids. In addition, it is possible to mix thermoelectric materials in the form of liquid solutions with a gelling agent.

The integration of the thermoelectrics is ideally carried out in the liquid sol before the gelation phase, in principle, all types of aerogels (silica, polymer, cellulose, metal oxides, etc.) are thermoelectrically activated in this way. With regard in particular to the highest possible efficiency of the overall system and a consideration of the manufacturing costs and the process complexity, silica aerogels can be used. Particularly suitable are cellulose aerogels because they can be formed by the addition of powdery gelling agent (e.g., ethyl cellulose, methyl cellulose) into the liquid thermoelectric phase, and other fillers such as CNTs can be added in advance.

As a liquid thermoelectric phase, mixtures of PE-DOT and PSS, optionally doped with carbon nanotubes, can be provided. However, the carbon nanotubes can also be accommodated in silica aerogels.

The aerogels can in this case be made monolithic, but can also be formed in the form of relatively thin layers, in particular formed on a support.

Such relatively thin airgel layers can also be formed as multilayer layers with their respective (electrically conductive) carrier, wherein the heat difference over the entire layer packings of each individual layer is utilized, wherein the electrical carrier layers of the individual thermoelectrically active aerogels are insulated from one another such that a corresponding one Yield can be achieved.

The layer systems or the layer composite can be designed to be rigid or flexible, in particular for flexible airgel-based thermoelectric layers, according to the invention, there are application areas which extend into the field of thermal shielding systems, in particular in motor vehicles.

In the invention, it is advantageous that in addition to the apparent increase in the efficiency of thermoelectric systems with a relatively easy process thermoelectric elements can be generated, which allow particularly good in car screen elements of a high efficiency and in addition to excellent thermal insulation and shielding an energy contribution to the Total energy requirements of a motor vehicle can afford.

The invention will be explained with reference to a drawing. It shows:

1 : The problem in the production of aerogels with thermoelectric properties;

2 : The process steps in the production of thermoelectric aerogels.

3 : The types of possible aerogels (organic / inorganic).

In the preparation of thermoelectric elements, there is basically a conflict of objectives, which is that the lowest possible thermal conductivity and the highest possible electrical conductivity should be present.

The electrical power of such a system is influenced on the one hand by the thermoelectric voltage, d. H. the larger the temperature difference between the hot side and the cold side, the higher the thermal voltage. The electric power is negatively influenced by the electrical resistance. In the case of aerogels, it is also to be expected that a relatively high electrical resistance is present due to the low solids content.

The thermoelectric voltage is u. a. influenced by constructive conditions, in particular via the temperature connection on the hot and on the cold side, d. H. in how far the heat conduction takes place and in which form and in which way the heat transfers are designed.

Regarding the electric power are also constructive conditions crucial, especially in conventional thermoelectric elements, the length, the width and the thickness or the thigh diameter of the thermocouple. In the case of aerogels, this naturally also applies, wherein, depending on the embodiment, the properties of the airgel are to be set here instead of the properties of the thermal legs.

It should be emphasized that with aerogels or when using aerogels, the thermoelectric voltage can be influenced in a particularly positive manner, since there is a very high insulation performance.

For example, aerogels are manufactured by 2 ) the so-called liquid precursors are mixed to form a sol. This sol is a colloidal solution. In addition, cellulose aerogels can be initiated by the addition of powder, which is then broken down.

The dissolved colloids are then crosslinked to a gel. This is followed by either subcritical or supercritical drying, the result of which is the airgel, which then has solids contents of about 0.1 to 2% over the typical features of this class of material.

Such aerogels have different base materials ( 3 ), which may be either organic or inorganic. For the present application, for example, silica aerogels, metal oxide aerogels and organic aerogels and mixtures thereof come into question.

In addition, thermoelectrically active airgel can be produced in fiber form, so that very flexible shaped bodies, even in mixing systems with non-thermoelectrically active fibers, can be realized by means of fabrics or scrims. In particular, the mixture with electrically conductive flexible airgel carbon fibers (obtained by the pyrolysis of airgel cellulose fibers) is possible, but also all other metal threads are conceivable.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102006049179 A1 [0005]

Cited non-patent literature

• 2012 2nd International Conference on Materials, Mechatronics and Automation, Lecture Notes in Information Technology, Vol. 15 ", pages 335 to 340 [0006]

Claims (11)

 Thermoelectrically active airgel, wherein the airgel is an airgel doped with thermoelectrically active materials during the sol formation, wherein a network of the doped thermoelectric material and the gelling agent is formed by gelation, subcritical or supercritical drying or a thermoelectrically active material via a sol Gel process and a subcritical or supercritical drying process is formed into a thermoelectric airgel. Airgel according to claim 1, characterized in that the airgel is a mixture of silica and / or polymer and / or cellulose and / or metal oxide airgel formers. Airgel according to claim 1 or 2, characterized in that the airgel PEDOT: PSS (poly-3,4-ethylenedioxythiophene, polystyrene sulfonates) or other thermoelectrically active substances. Airgel according to one of the preceding claims, characterized in that the airgel contains electrically very good conductive fillers such as carbon nanotubes (CNTs) or nanosilver particles. Airgel according to one of the preceding claims, characterized in that the airgel contains electrically conductive thermoelectrically active fibers or aerogels.  A process for producing a thermoelectrically active airgel, wherein liquid precursors are mixed to give a colloidal sol, then gelation is effected via cross-linking of the colloids and, if appropriate, a gel modification is carried out, the airgel being produced by subcritical or supercritical drying, the sol thermoelectrically active oxides and / or thermoelectrically active plastics or plastic mixtures are admixed or as a precursor thermoelectrically active plastic such as PEDOT: PSS is used. A method according to claim 6, characterized in that the sol thermoelectrically active fibers, in particular thermoelectrically active aerogels are added. A method according to claim 6 or 7, characterized in that the gel is produced from PEDOT and PSS or contains PEDOT and PSS and also carbon nanotubes (CNT) are added. Method according to one of claims 6 to 8, characterized in that the airgel contains silica, polymers, cellulose or metal oxides as Aerogelbildner. Method according to one of claims 6 to 9, characterized in that the thermoelectrically active airgel is manufactured in fiber form and forms as fabric or scrim flexible moldings. A method according to claim 10, characterized in that fibers are produced from thermoelectrically active airgel and aerogels or other fibers as blended fabric or Mischgelege.

Images