EP2451867A1

EP2451867A1 - Nanocomposite comprising boron nitride nanotubes

Info

Publication number: EP2451867A1
Application number: EP10716306A
Authority: EP
Inventors: Thomas Hammer; Frank Heinrichsdorff; Ursus KRÜGER; Volkmar LÜTHEN; Daniel Reznik
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2009-07-08
Filing date: 2010-03-31
Publication date: 2012-05-16
Also published as: WO2011003634A1

Abstract

The invention relates to a nanocomposite (15) comprising semiconducting nanoparticles (11a, 11b) that consist of boron nitride nanotubes (BNNT) and are distributed in an electrically insulating insulator, such as cellulose fibres (16a, 16b). Furthermore, the invention relates to a use of said nanocomposite as insulating material for a transformer. According to the invention, the insulator is a cellulose material or a polymer. In particular, the BNNT can be doped with dopants or provided with a coating made of metals or doped semiconductors. In this way, the specific resistance of the nanocomposite can be suitably altered, so that it is, for example, in the range of oil, and a combination of oil and the nanocomposite as electrical insulation has an improved dielectric strength in the event a DC voltage is applied. At the same time, in the event an AC voltage is applied, the insulation properties advantageously continue to be good.

Description

The invention relates to a nanocomposite with semiconducting nanoparticles, such as boron nitride nanotubes (BNNT), which are distributed in an electrically insulating insulating material.

Such a nanocomposite is described, for example, by N.P. Bansal et al. , "Boron Nitride Nanotubes-Reinforced Glass Composites", NASA / TM-2005-213874, pages 1 to 7, August 2005. Accordingly, it is possible to incorporate boron nitride nanotubes into glass as an electrically insulating insulator mechanical fiber stiffening of the glass.

Furthermore, it is known that nanocomposites can also be used as a field grading material when it comes to reducing peaks in the formation of electric fields, for example on the insulation of electrical conductors. According to WO 2004/038735 Al, for example, a material consisting of a polymer can be used for this purpose. In this, a filler is distributed whose particles are nanoparticles, so have a mean diameter of 100 nm hochtens. According to US 2007/0199729 A1, semiconducting materials whose band section lies in a range of 0 eV and 5 eV can be used for such nanoparticles, inter alia. By means of the used nanoparticles, which can consist of ZnO for example, the electrical resistance of the nanocomposite can be adjusted. If, during the admixture of the nanoparticles, a certain proportion of the volume is exceeded, which is between 10 and 20% by volume, depending on the size of the nanoparticles, the specific resistance of the nanocomposite is noticeably reduced, and in this way adjust the electrical conductivity of the nanocomposite and adapt it to the required conditions. In particular, I let a specific resistance in the order of 10 ¹ Ωcm set. This comparatively high electrical resistance leads to a load on an electrical component, which is coated with the nanocomposite, that when a DC voltage applied a certain leakage current must be accepted. However, a voltage drop across the nanocomposite is achieved, which results in a more uniform distribution of the potential and thus also grades the resulting electric field in a suitable manner. As a result, the resulting field peaks can be reduced, which advantageously increases the dielectric strength.

When the electrical conductor is subjected to an alternating voltage, a field-grading effect is also produced which, however, follows a different mechanism. The field-weakening effect of the nanocomposite depends on the permittivity of the nanocomposite, the permittivity ε being a measure of the permeability of a material for electric fields. The permittivity is also referred to as the dielectric constant, the term "permittivity" being used in the following: relative permittivity is the ratio of the permittivity ε of a substance to the electrical field constant εo, which is denoted by the permittivity number ε _r = ε / εo The higher the relative permittivity, the greater the felschwachende effect of the substance used in relation to the vacuum.Here only the Permittivitatszahlen of the substances used are treated. Also widely known are nanoparticle carbon nanotubes (hereinafter referred to as CNTs) and boron nitride nanotubes (hereinafter referred to as BNNTs). Although these structures can be present in a length of several micrometers, they have diameters of less than 100 nm and are therefore to be understood as nanoparticles. Such as Chang Chang et al. , "Physical Properties Letters 97," (2006), the intrinsic shafts of nanotubes are highly dependent on their diameters, for example, the thermal conductivity of CNT and BNNT increases with decreasing It is known from F. Du et al., "Effect of nanotube alignment on percolation conductivity in carbon nanotube / polymer composites", Physical Review B 72, (2005), that CNT in polymer composites has significantly lower percussion thresholds have electrical conductivity in the composite, such as spherical nanoparticles. The percolation threshold can also be increased by measures of alignment of the CNT in the matrix of the polymer and can be less than 1% by weight with a content of CNT in the matrix. In addition, C. Tang et al. , "Fluorination and

Electrical Conductivity of BN Nanotubes ", Journal of American Chemical Society 127, (2005), pages 6552-6553 (including Supporting Information) discloses that BNNTs semiconducting properties can be influenced by doping with different dopants of their electrical conductivity similar to bulk semiconductors The object of the invention is to improve a nanocomposite of the type specified at the outset such that it is comparatively well suited for use as a field-grading material. This object is achieved according to the invention by the nanocomposite specified at the outset in that the insulating material which forms the matrix of the nanocomposite consists of a cellulose material or a polymer. The use of the semipermeable nanoparticles initially has the advantage that significantly lower degrees of fullness of at most 5% by volume, preferably even at most 2% by volume, in the insulating material are sufficient to cause percolation of the nanoparticles and thus to increase the electrical conductivity of the nanocomposite , This is possible, although the band gap of BNNT according to C. Tang is about 5.5 eV and according to US 2007/0199729 Al and WO 2004/038735 Al it is required that the nanoparticles of semiconductors used in field-grading nanocomposites have a Band gap between 0 eV and 5 eV should have.

The comparatively low degrees of fullness with BNNT necessary for increasing the conductivity have the great advantage that the nanocomposite largely retains its mechanical properties, since the BNNT only slightly disturbs the matrix material because of its low content. In comparison to the use of spherical nanoparticles such as ZnO or SiC, as proposed according to WO 2004/038735 A1 and US 2007/0199729 A1, it is therefore possible to produce a substantially more stable nanocomposite. This is of particular advantage, for example, when the nanocomposite is subjected to mechanical stress in the relevant application. This can be, for example, the vibrations of a machine or a transformer. Also, when used in electrical machines, the nanocomposite of a

Be exposed to centrifugal force. In these applications, the inventive nanocomposite advantageously leads to the components produced from the nanocomposite, such as. B. insulation gen, can reliably fulfill their task over a longer period of operation.

By using BNNT, the following advantages can also be obtained by using the obtained nanocomposite as a field-grading material.

BNNT are insulated by the band gap of 5.5 eV even at high temperatures, so that a temperature-induced breakdown can be avoided.

Furthermore, BNNTs have a high thermal conductivity of more than 300 W / mK. Like C.W. Chang et al. In addition, it can be expected that with thin BNNTs with a diameter of less than 20 nanometers, the thermal

Conductivity can be over 1000 W / mK. As a result, the nanocomposite, in addition to its property as field-grading material, can simultaneously ensure reliable heat dissipation of electrical power components such as transformers.

BNNTs have a permittivity of 8 _BNNT , which is about 4. Thus, the permittivity of common insulator materials such as polymers or cellulosic material is very similar. The introduction of BNNT into these insulator materials for producing the nanocomposite according to the invention thus does not change or only slightly changes the permittivity of the nanocomposite in comparison to the solid insulator material, whereby a fluctuation of the field strength in the interior of the nanocomposite can be kept small. This occurs, as already mentioned, in an overload of the module to be isolated with an alternating voltage and can lead to unwanted partial discharges, which ultimately destroy the insulation. Due to their dimensions in the nanometer range, BNNT have a high aspect ratio, which is comparable to that of CNT. The by F. Du et al. Percolation thresholds of 1% by weight and less for CNT are therefore also valid for BNNT. Therefore, the effect as a field-grading material with the already mentioned low concentrations of BNNT in the matrix of the insulating material can be achieved, whereby the concentration values indicated in connection with this invention are given in% by volume. Due to the structure of the nanocomposite made of an insulating material such as cellulose material or a polymer and BNNT in the present case, however, the data in% by weight and% by volume do not differ greatly from one another. Suitable materials for the electrically insulating insulating material as a polymer, for example, thermoplastics into consideration, such as. As polyethylene, polystyrene or PVC. As polymers also elastomers, silicones and resins (natural resins and synthetic resins) can be selected. If a zeolite material is selected as the insulating material, it is particularly advantageous if it is used as paper. This paper can be impregnated with the BNNT. In the broadest sense, impregnation means a connection between the fibers of the cellulosic material and the BNNT. For example, the BNNT may be attached to the fibers of the cellulosic material, which may occur during papermaking. However, impregnation can also take place in such a way that the BNNTs are added during papermaking and after the drying process of the paper are enclosed in the spaces formed by the fibers of the cellulose material.

The cellulosic material is the raw material for the paper. In the context of this invention, paper should be understood in the broadest sense to mean any product made of the cellulosic material. the. In particular, this means thinner paper sheets or thicker cardboard or cardboard. It is also possible to produce three-dimensional structures from papier mache, which are then to be understood as paper products.

However, the cellulosic material can also be used as a wood product. In the context of the invention, a wood product is understood to mean a further processing of the raw material wood from wood components glued together. In particular, this can be pressboard, which is designed in particular as a block chip. In addition, laminates can be produced by gluing thin layers of wood together (plywood). According to the invention, the BNNT can be introduced into the adhesive for the purpose of jointing the press chip or the wood layer.

According to one embodiment of the invention it is provided that to increase the effective conductivity of at least part of the distributed in the insulating BNNT doping of this BNNT with dopants or a coating with metals or doped semiconductors is provided on these semiconducting BNNT. This measure advantageously serves to influence the specific resistance of the nanocomposite according to the invention by selecting suitable dopants. It is desirable, for example, to set a resistivity in the order of 10 ¹² Ωcm, which should be achieved with a degree of fill of BNNT of less than 5% by volume, preferably less than 2% by volume. The doping of the BNNT or the coating can be carried out as described by C. Tang et al. described described. Doping can be achieved by modifying the BNNT by adding suitable dopants such that the dopant atoms form electronic states that turn the BNNT into a p-conductor (ie, electronic states are formed, the electrons from the valence band edge capture) or to an n-conductor (ie, reaching electronic states that emit electrons by thermal excitation across the conduction band edge). As a dopant for a p-doping, for example Be comes into question, as a dopant for n-doping Si comes into question. Such doping of the BNNT can be done in situ, during the growth of the BNNT z. B. from the gas or Flussigphase the dopant atoms are incorporated. It is also possible to carry out the doping in a further step after the growth of the BNNT, wherein the dopants are typically taken up by the BNNT under the influence of a heat treatment. By introducing the dopants into the BNNT, the resistivity can be lowered to values typical for doped semiconductors between 0.1 and 1000 Ωcm.

Another possibility is to provide the BNNT after its production with a thin layer of a metal or a highly doped semiconductor. As a result, there is a higher electrical conductivity in this layer of the nanoparticle than in the BNNT itself. This higher conductivity influences the electrical behavior of the nanocomposite when the prepared BNNTs are introduced into the insulating matrix of the insulating material. However, the specific resistance of the nanocomposite is again higher due to the low degree of fullness of BNNT, since the electrical insulating material has a much higher specific resistance than the introduced doped BNNT. By doping the BNNT with suitable dopants or their coating with metals or highly doped semiconductors, it also being possible to carry out doping and coating at the same time, so to speak, two degrees of freedom arise for influencing the conductivity of the nanocomposites according to the invention. One possibility is to change the full degree of BNNT in the nanocomposite. The specific resistance decreases with increasing concentration of BNNT in the matrix of the electrically insulating insulating material. The second possibility lies in the treatment according to the invention of the BNNT, wherein the doping and / or the coating reduces the specific resistance of the BNNT so that it leads to a greater reduction in the specific resistance of the nanocomposite at the same concentration in the nanocomposite. As a result, for example, the required maximum full degrees of BNNT in the nanocomposite can be maintained, for example, so that it satisfies the mechanical requirements of the application. In addition, the doping of the BNNT can be used specifically for the fact that the resistivity of the nanocomposite does not change abruptly with an increasing concentration of BNNT in the matrix of the insulating material but continuously changes over a certain concentration range. As a result, a more precise setting of the resistivity of the nanocomposite is advantageously possible since it is avoided that production-related, comparatively small fluctuations in the concentration of BNNT in the matrix of the insulating material lead to large deviations from the desired specific resistance of the nanocomposite.

A particularly advantageous use of the nanocomposite is that this is used as insulation material for a transformer. In transformers, the live parts, such as the coils, must be electrically isolated from each other. For example, in the case of high-voltage transformers, oil fillings are used, into which walls of paper impregnated with the oil or also pressboard boards are additionally introduced. The resulting insulation must be during both operation of the transformer Applying to an AC voltage as well as for example in case of disturbances of operation with a DC voltage to ensure the electrical insulation. If the composite according to the invention is used as the insulating material, sufficient electrical insulation properties can be ensured both when the transformer is subjected to an alternating voltage and to a direct voltage. This is due to the fact that the BNNT in the insulating material scarcely or not change the permittivity number ε _C o _mP in comparison to the permittivity ε _{p of} the untreated paper because of its permittivity number 8 _BNNT of 4 with the insulating material, in particular paper. Therefore, the insulation of the transformer using the nanocomposite can be designed in a similar manner as is possible with the untreated papers. At the same time, however, the insulating properties of the combination of oil and paper can be improved in the case of the application of a DC voltage. In this case, the specific resistances are of importance, which amount to a factor of 1000 for oil (po) at 10 ¹² Ωcm and for untreated paper (p _p ). By introducing the BNNT with a specific conductivity of P _BNNT between 0.1 and 1000 Ωcm (possibly influenced by a suitable doping of the BNNT), the specific conductivity of the inventive nanocomposite can be, for example, in the form of an impregnated paper (p _C o _mP ) also at 10 ¹² Ωcm. As a result, the voltage stress is distributed evenly when exposed to a DC voltage to the oil and the paper, whereby voltage peaks and resulting breakthroughs are avoided.

Further details of the invention are described below with reference to the drawing. The same or corresponding drawing elements are each provided with the same reference numerals in the individual figures and are only insofar explained several times, how differences arise between the individual figures. Show it

Figures 1 and 2 schematically examples of functionalized

BNNT, as they can be used in an exemplary embodiment of the nanocomposite according to the invention, as a three-dimensional view,

FIG. 3 schematically shows an exemplary embodiment of the nanocomposite according to the invention, consisting of cellulose fibers and BNNT, in a three-dimensional view and FIG

Figure 4 schematically an exemplary embodiment of the inventive use of the nanocomposite as a transformer insulation in section.

In Figure 1, a BNNT 11 is shown schematically as tubes. This BNNT has also been treated with a dopant 12, wherein it is indicated in FIG. 1 that the dopant 12 is incorporated in the lattice 13 of the BNNT formed by the boron nitride.

An alternative embodiment of the BNNT 11 is shown in FIG. The BNNT 11 according to FIG. 2 is provided with a sheath 14, which itself consists of a semiconductor provided with a dopant 12.

The BNNT 11 according to FIGS. 1 and 2 can be processed into a nanocomposite 15 as shown in FIG. This consists of cellulose fibers 16a, 16b which were produced as paper. The paper is impregnated with the BNNT IIa and / or IIb. An impregnation with the BNNT IIa proceeds in such a way that the BNNT IIa is deposited on the cellulose fiber 16a. the. The percolation threshold is achieved by having enough BNNT on the surface of the cellulose fiber 16a to form a two-dimensional net on the surface of the cellulose fiber 16a.

Alternatively, BNNT IIb may also be included in the interstices 17 between different cellulosic fibers 16a, 16b. This results in the interstices 16, a three-dimensional network of BNNT IIb, where the concentration of BNNT IIb must be high enough to reach the percussion threshold, ie the formation of a closed network.

The two mechanisms of impregnating the paper with BNNT IIa, IIb are shown together in FIG. These mechanisms can be used individually or jointly. The BNNT IIa and IIb may be identical in construction or have differences. An electrical insulation 18 according to Figure 4 consists of several layers of paper 19, between which Olschichten 20 lie. The papers 19 are also saturated with oil, which is not shown in detail in FIG. For this, the impregnation with BNNT 11 can be seen in FIG. 4 within the papers. The insulation shown according to FIG. 4 surrounds, for example, the windings used there in a transformer, which must be electrically insulated from the outside and from each other. The electrical insulation of a transformer must prevent electrical breakdown in case of application of an AC voltage. In this case, the isolation behavior of the insulation depends on the permittivity of the components of the insulation. For oil, the permittivity is ε _o approximately at 2, for the paper ε _p at 4. When a stress on the insulation with an alternating voltage, therefore, it results for the load of the individual insulation components that the voltage U ₀ applied to the oil is approximately twice as high as that on the paper applied voltage U _p . If the nanocomposite according to the invention is used in which the paper 19 is impregnated with BNNT in the manner shown in FIG. 4, then the BNNTs do not influence the stress distribution in the insulation according to the invention since the permittivity ε _BNN τ is also approximately 4, and therefore the permittivities - tat ε _{cop of} the impregnated paper is also at about 4. Thus, even in the case of the insulation according to the invention, the voltage U ₀ acting on the oil is approximately twice as great as the voltage U _COm p applied to the nanocomposite (paper).

If Storfalle occur on the transformer, so the dielectric strength of the insulation when DC voltages are important. The distribution of the applied voltage to the individual insulation components is then no longer dependent on the permittivity, but on the resistivity of the individual components. The specific resistance p _o of oil is 10 ¹² Ωcm. In contrast, P _p of paper is three orders of magnitude higher and is 10 ¹⁵ Ωcm. This causes that when a DC voltage applied the voltage at the Ol U ₀ amounts to the thousand times the voltage on the paper U _p . This imbalance involves the risk that when a DC voltage is applied to the insulation, breakdown in the oil occurs and the electrical insulation fails.

The inventively introduced into the paper 19 BNNT 11 are z. B. by a suitable doping with their resistivity (between 0.1 and 1000 Ωcm) adjusted so that the specific resistance of the paper p _p down- is set. This makes it possible to set a specific conductivity p _C o _mP for the composite _{according to the invention} , which is approximated to the specific resistance p _o and ideally corresponds to this approximately. At a specific resistance p _C o _mP of approximately 10 ¹² Ωcm, the voltage U ₀ applied to the oil is in the region of the voltage Ucomp applied to the composite, so that a balanced voltage profile is established in the insulation. This advantageously improves the dielectric strength of the insulation, since the loading of the oil is noticeably reduced.

Claims

Claims / Patent Claims

1. nanocomposite with semiconducting nanoparticles which at least partially consist of boron nitride nanotubes (IIa, IIb) and are distributed in an electrically insulating insulating material (16a, 16b),

characterized ,

that the insulating material consists of a cellulose material or a polymer.

2. nanocomposite according to claim 1,

characterized ,

in that to increase the effective conductivity of at least some of the boron nitride nanotubes (IIa, IIb) distributed in the insulating material (16a, 16b), a doping of these boron nitride nanotubes with dopants (12) or a coating (14) with metals or doped semiconductors is provided on these semiconducting Bornitπd nanotubes.

3. nanocope of claim 2,

characterized,

the doped boron nitride nanotubes (IIa, IIb) or the coating (14) of doped semiconductors on the nanoparticles have a specific electrical resistance p of at least 0.1 Ωcm to at most 1000 Ωcm.

4. Nanocomposite according to one of the preceding claims, characterized

that the Bornitπd nanotubes (IIa, IIb) are distributed in the insulating material (16a, 16b) with a concentration of at most 5% by volume, preferably at most 2% by volume.

5. nanocomposite according to any one of the preceding claims, characterized the insulating material is a thermoplastic, in particular polyethylene, polystyrene or PVC, an elastomer, a silicone or a resin.

6. Nanocomposite according to one of claims 1 to 4,

characterized,

in that the insulating material (16a, 16b) is a cellulose material-containing paper (19) or wood product, which is impregnated with the boron nitride nanotubes (11).

7. Use of a nanocomposite (15) with semiconducting, at least partially boron nitride nanotubes (IIa, IIb) existing nanoparticles, which are distributed in an electrically insulating insulating material (16a, 16b) of a cellulosic material or a polymer, as an insulating material for a transformer.