EP3020050A1

EP3020050A1 - Electrically insulating composite material, method for producing such a material and use thereof as en electrical insulant

Info

Publication number: EP3020050A1
Application number: EP14736806.2A
Authority: EP
Inventors: Sombel DIAHAM; Thierry Lebey; Marie-Laure LOCATELLI; François SAYSOUK
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Toulouse III Paul Sabatier
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Toulouse III Paul Sabatier
Priority date: 2013-07-08
Filing date: 2014-07-08
Publication date: 2016-05-18
Also published as: JP2016531972A; WO2015004115A1; FR3008223B1; CN105612587A; US20160152794A1; FR3008223A1

Abstract

The invention relates to an electrically insulating material comprising a thermostable and electrically insulating polymer matrix wherein electrically insulating inorganic nanoparticles of all sizes smaller than or equal to 200nm are dispersed. Said material is especially applicable as an electrical insulant, especially in the form of a film, in electrical, electronic or electrotechnical systems wherein it may be subjected to temperatures higher than 200°C and strong electric fields.

Description

ELECTRICALLY INSULATING COMPOSITE MATERIAL, METHOD FOR MANUFACTURING SUCH MATERIAL AND USE THEREOF

ELECTRICAL INSULATION

The present invention is in the field of electrical insulation, in particular components of electronic, electrical or electrotechnical systems capable of being subjected to high temperatures and to strong electric fields, in particular conversion or storage systems. of electrical energy. More particularly, the invention relates to an electrically insulating material, based on a polymer matrix and inorganic nanoparticles, and a method of manufacturing such a material. The invention furthermore relates to the use of such a composite material, in particular in the form of a film, as an electrical insulator, and an electrical, electronic or electrotechnical system in which this material is used as a that isolating electric.

Components of electrical, electronic or electrotechnical systems are frequently subjected to high temperatures, which it is desirable for them to be able to function, and moreover reliably. This need has become even more acute as the temperatures to which the components of such systems are subjected are becoming higher. For example, the trend toward miniaturization of high heat dissipation and / or embedded electronic systems, particularly in the aeronautics, rail traction and space sectors, is leading to an increase in the power density of their active components. . The components of these systems are therefore subjected to increasingly higher operating temperatures, at the same time as more stringent voltage and electric field conditions.

Conventionally, it is implemented to achieve intrinsic or extrinsic insulation of electronic system components, for example semiconductor surface insulation or inter-component insulation in the field of microelectronics, and including power microelectronics, electrically insulating polymeric materials, such as polyimides, chosen for their high thermal stability and their mechanical properties compatible with thin film application as electrical insulators within such systems. However, it has been observed by the present inventors that the electrical insulation properties of such polymers degrade under the effect of an increase in temperature, especially for temperatures above 200 ° C.

More particularly, the electrically insulating thermostable polymer materials conventionally used in electrical / electronic / electrotechnical systems, such as polyimides, exhibit a strong degradation of their dielectric properties and electrical insulation in the temperature range above 200. ° C, notably resulting in an increase in dielectric losses and DC resistivity, and a drop in the dielectric breakdown field. These materials thus become semi-insulating. By way of example, as regards the polyimides, it is observed that the volume electrical resistivity DC (p) becomes less than 10 ¹² Ω.cm, and the dielectric loss factor (tanδ) becomes greater than 10%, beyond 200 ° C. In addition, the breaking field collapses with increasing temperature, with decreases of up to more than 50% from its value at 25 ° C. These materials thus become technically inefficient, or even failing, above 200 ° C.

It is therefore desirable to have an electrical insulating material which has the advantageous properties of the polymer materials proposed by the prior art for the electrical insulation of the components of electronic systems, in terms of thermal stability and mechanical properties, all by presenting electrical insulation performance at high temperatures, typically above 200 ° C, including under strong electric field, so as to ensure the reliable operation of the systems in which it is implemented as a electrical insulation. The present invention aims to to propose a material having such properties.

For a purpose other than that of the present invention, it has been proposed by the prior art to modify electrically insulating polymer materials, more particularly based on polyimide, by incorporating therein a mineral filler, more specifically nitrile particles. boron, in order to improve the thermal properties.

As an example of such prior art, mention may be made of the publication by Sato et al. (2010), which describes a composite material based on a polyimide matrix in which are dispersed boron nitride particles whose size is less than 0.7 μιη and a specific surface area of 13 m ² . g ^"1 or the publication of Li et al (201 1), which also describes a material based on a matrix of a polyimide in which particles of boron nitride are dispersed The average size of these particles is described as equal to 70 nm It appears from the figures of this document, especially from Figure 3, that a significant amount of these particles have dimensions of the order of several hundred nanometers. the object of this earlier work was to improve the thermal conduction properties of polyimide-based materials None of these documents are concerned with the electrical insulation properties of the materials proposed It has now been discovered by the present inventors that, quite surprisingly and remarkably, the electrical insulating properties of electrically insulative thermostable polymer materials, in particular polyimides, were strongly It was improved at temperatures above 200 ° C, including in high electric field conditions, when electrically insulating inorganic nanoparticles, having specific size characteristics, were incorporated into a matrix of such polymers.

Thus, it is proposed according to the present invention an electrically insulating material comprising a matrix of a thermostable and electrically insulating polymer in which nanoparticles are dispersed. electrically insulating inorganic. These electrically insulating inorganic nanoparticles are chosen from electrically insulating metal nitrides, diamond, the oxides of at least one metal of columns 1 to 1 1 of the periodic table of electrically insulating elements, and their mixtures, and the set of these inorganic electrically insulating nanoparticles have dimensions less than or equal to 200 nm. By this is meant that each of the nanoparticles is such that none of its spatial dimensions is greater than 200 nm. The material according to the invention differs in this respect from the materials proposed by the prior art, in particular by the publication of Li et al. (201 1), wherein a significant amount of the particles has at least one dimension greater than 200 nm.

By electrically insulating polymer is meant in the present description a polymer having electrical insulation properties at room temperature, that is to say at about 25 ° C.

By thermostable polymer is meant in the sense of the present invention a polymer whose mass is substantially preserved when it is subjected to a temperature rise, at least up to a temperature of 350 ° C, that is to say that the loss of mass is less than 10% in thermogravimetric analysis measured at 10 ° C / min. Depending on the applications for which the material is intended, especially for high temperature applications, for example above 250 ° C., a polymer is advantageously chosen according to the invention for the constitution of the material for which, at least until at a temperature of 400 ° C., the weight loss is less than 5% in thermogravimetric analysis measured at 10 ° C./min.

The electrically insulating nanoparticles are further defined here conventionally in themselves, such as nanoparticles of electrical conductivity less than or equal to 10 ^"11 Ω ^{" 1} .cm ^"1. Such a definition excludes nanoparticles of titanium oxide in particular. (TiO ₂ ), which are semi-insulating and have an electrical conductivity greater than this value, as described in the publication by Feng et al (2013). The material according to the present invention advantageously retains electrical insulation properties at high temperatures, including higher than 200 ° C, both in direct current (DC) and AC (AC), and in a strong electric field. In the temperature range of 200 to 400 ° C, it thus has electrical and dielectric properties greatly improved over the materials proposed by the prior art.

In particular, for the material according to the invention, the DC dielectric breakdown field is not degraded above 200 ° C., unlike materials of similar constitution but in which at least a portion of the particles have less than a dimension greater than 200 nm as proposed by the prior art. Compared with such materials, and materials made of the uncharged polymer, the values of the burst field are increased very significantly. In addition, all the other dielectric properties of the material according to the invention are also improved over the materials of the prior art. At high temperatures, higher than 200 ° C., the material according to the invention has, in particular, with respect to the uncharged polymers: values of the factor of dielectric losses at a frequency of 1 kHz reduced by a factor of 5 to 1000; volumetric electrical resistivity values in DC greatly increased by a factor of 1000 to 100,000; reduced leakage current densities, including in strong electric fields, more particularly decreased by a factor of 10 to 100000 under electric fields higher than 10 kV / mm. The material according to the invention, retaining its electrical insulation properties in a range of temperatures and electric fields in which the homogeneous polymers not loaded with nanoparticles, or the polymers loaded with nanoparticles of larger size, lose these same properties and become semi-insulating, thus makes it possible to overcome the disadvantages of the materials of the prior art in terms of electrical insulation at high temperatures. It thus satisfies the severe requirements areas of conversion and storage of electrical energy in the temperature range of 200 to 400 ° C, especially under strong electric field.

This material finds particular, but not limited to, application as an electrical insulator in electrical, electronic and electrotechnical systems, such as: high temperature and high voltage energy storage capacitors with low dielectric losses; high power, high voltage and strong electric power electronics systems; systems in the field of electrical engineering, such as motors, electrical machines, operating under severe constraints of temperature, voltage, pressure, etc., including for the isolation of transformers, cables, etc. ; high power density systems such as integrated, optical, optoelectronic, photovoltaic, microwave, etc. ; and more generally any system requiring electrical insulation solutions under high temperature and strong electric field, particularly in the fields of transport, industry, oil exploitation, geothermal research, space, etc. It can also be used for the passivation or encapsulation isolation of metallized substrates, such as chips made of silicon carbide, diamond or gallium nitride, and intermetallic insulation layers, etc.

The use of the material according to the invention in such systems notably allows:

- to increase the life of conversion systems by improving the reliability of the insulation systems implemented there, as well as to reduce the costs related to maintenance;

- Decrease the mass and volume of electrical energy conversion systems, allowing their integration and / or increase their ability to work at higher temperatures. This is reflected in particular by a decrease in fossil fuel consumption, an increase in the number of passengers on board vehicles, such as airplanes or trains, etc. According to particular embodiments, the material according to the invention also meets the following characteristics, implemented separately or in each of their technically operating combinations.

In particularly advantageous embodiments of the invention, the set of electrically insulating inorganic nanoparticles dispersed in the matrix have dimensions less than or equal to 100 nm, that is to say that none of their spatial dimensions n ' is greater than 100 nm.

In particular embodiments of the invention, the electrically insulating inorganic nanoparticles that are dispersed in the polymer matrix have a generally spherical shape. These nanoparticles may furthermore have any crystallographic form, in particular cubic or hexagonal.

According to an advantageous characteristic of the invention, in terms of the effectiveness of electrical insulation at high temperature and in a strong electric field, the electrically insulating inorganic nanoparticles have a monomodal size distribution.

Their density is also preferably less than 2 g / cm ³ . Such a characteristic advantageously facilitates their dispersion in the polymer matrix, as well as the implementation of the material according to the invention, in particular for high rates of volume loading of the polymer matrix into nanoparticles.

In particular embodiments of the invention, the electrically insulating inorganic nanoparticles are present in the polymer matrix in a volume ratio of 0.1 to 95%, in particular from 1 to 95%, preferably from 20 to 60%, more preferably from 20 to 50%, and preferably from 35 to 45%.

For certain types of particles, a volume loading rate between 35 and 45% is particularly advantageous in view of the dielectric field of rupture, which has the highest values for this volume concentration range, while ensuring great ease of handling of the material.

Otherwise, the volume ratio of electrically insulating inorganic nanoparticles in the polymer matrix may be between 0.1 and 45%.

According to a particularly advantageous characteristic of the invention, the electrically insulating inorganic nanoparticles are dispersed in the polymer matrix so as to form no agglomerate of size greater than or equal to 2 μιτι, preferably greater than or equal to 1 μιτι. The thermostable and electrically insulating polymer used in the constitution of the matrix of the material according to the invention can be chosen from any polymer, including copolymers, which satisfies such characteristics. It may as well be a polymer of the thermosetting type, as a polymer of the elastomer type. When the polymer is of the thermosetting type, it preferably has a glass transition temperature greater than or equal to 200 ° C., in particular greater than or equal to 250 ° C., depending on the application intended for the material and the temperatures at which it is likely to be submitted. The thermostable and electrically insulating polymer according to the invention may in particular consist of a silicone material, for example in the form of gel, elastomer or polydimethylsiloxane (PDMS).

Otherwise, the polymer may consist of an epoxy-type polymer, cyanate ester type, or any other thermostable and electrically insulating polymer, especially polymers whose precursors can be dissolved in a solvent. By way of examples of such polymers, mention may be made of polymers of polyimide (PI), polyamideimide (PAI), polyetherimide (PEI), polyetheretherketone (PEEK), benzocyclobutene (BCB), polyethersulfone (PES), polyaryletherketone ( PAEK), polyimide siloxane, polyisoindolo-quinazolinedione, polyphenylquinoxaline, polyquinixalone, polyquinoline, polyquinoxaline, polybenzimidazole (PBI), polybenzoxazole (PBO), poly (arylene ethers), polysilane, poly (perfluorocyclobutane), and derivatives thereof.

In particular embodiments of the invention, the electrically insulating thermostable polymer is a polyimide, for example of biphenyltetracarboxylic acid dianhydride (BPDA) / p-phenylenediamine (PDA) type.

In particular embodiments of the material according to the invention, the electrically insulating inorganic nanoparticles comprise nanoparticles of metal nitride, or consist of nanoparticles of metal nitride.

Inorganic electrically insulating nanoparticles are for example chosen from nanoparticles of aluminum nitride (AlN), boron nitride (BN), silicon nitride (Si ₃ N ₄ ), or mixtures thereof. They comprise in particular nanoparticles of boron nitride. For example, they consist solely of nanoparticles of boron nitride.

Inorganic electrically insulating nanoparticles can be, or include, diamond nanoparticles (C).

They may also be, or contain, nanoparticles of a metal oxide of columns 1 to 1 1 of the periodic table of elements, for example a metal of column 1, a metal of column 2 , such as magnesium, beryllium, strontium or calcium, a metal of column 3, a metal of column 4, such as zirconium or hafnium, of a metal of column 5 , a metal from column 6, a metal from column 7, a metal from column 8, a metal from column 9, a metal from column 10, or a column 1 1 metal of the periodic table of elements, such as copper.

The electrically insulating inorganic nanoparticles are for example chosen from nanoparticles of zirconium oxide (Zr0 ₂ ), magnesium oxide (MgO), copper oxide, beryllium oxide, of strontium oxide and titanium, etc., or mixtures thereof. Such metal oxides may optionally include one or more additional metals, belonging or not belonging to columns 1 to 1 1 of the periodic table of elements. The material according to the invention can be in various forms, depending on the intended application. It can in particular be presented in the form of granules, to be shaped according to the desired configuration.

In particular embodiments of the invention, the material is shaped into a film. This film preferably has a thickness of between 100 nm and 1 cm, preferably between 100 nm and 1 mm, preferably between 1 and 100 μιη, and preferably still between 1 and 10 μιτι.

Otherwise, the material according to the invention can be shaped with a greater thickness, in particular for the encapsulation of electrical and / or electronic components.

According to another aspect, the present invention relates to a method of manufacturing a material according to the invention, corresponding to one or more of the above characteristics. This process comprises successive steps of:

dispersion of electrically insulating inorganic nanoparticles all having dimensions of less than or equal to 200 nm in a liquid composition comprising one or more precursors of a heat-stable and electrically insulating polymer, optionally in solution in a solvent, especially when the or the precursors are not in liquid form, - shaping of the dispersion thus obtained, in particular by deposition in the form of a film,

and heating under conditions suitable for carrying out the crosslinking of the polymer and the elimination of the solvent.

Where appropriate, electrically inorganic nanoparticles insulators may be pre-dispersed in a solvent, prior to their mixing with the precursor (s) of the thermostable and electrically insulating polymer.

In particular embodiments of the invention, the electrically insulating inorganic nanoparticles are introduced into the liquid composition in an amount such that the final volume loading rate of the nanoparticle matrix is between 0.1 and 95%, in particular between 1 and 95%, preferably from 20 to 60%, more preferably from 20 to 50%, and preferably between 35 and 45%. Prior to their introduction into the liquid composition, the nanoparticles may have undergone any appropriate pretreatment, for example a surface pretreatment to facilitate their dispersion in the liquid composition. In the particular case in which the material contains boron nitride nanoparticles, it is particularly advantageous that these nanoparticles have been subjected to a preliminary drying step, in particular by heat treatment, because of their intrinsic hygroscopic nature.

In particular embodiments of the invention, the step of dispersing the nanoparticles in the liquid composition comprises the mechanical mixing of the nanoparticles in this liquid composition, then the sonication of the mixture thus obtained, so as to ensure, by a phenomenon of cavitation occurring under the action of ultrasound, breaking nanoparticle agglomerates and thus a good dispersion of the latter in the composition. According to a particularly advantageous characteristic of the invention, the step of dispersing the nanoparticles in the liquid composition may be followed by a step of eliminating agglomerates of size greater than or equal to 2 μιη, preferably greater than or equal to 1 μιτι. . This step of removing agglomerates of micrometric size is preferably carried out by decantation by centrifugation. It is the responsibility of the person skilled in the art to to determine the conditions of the centrifugation operation, in particular as regards its speed and duration, so as to effect the elimination of agglomerates of micrometric size. The supernatant containing the so-called nanometric phase, free of agglomerates of micron size, is then used for the subsequent shaping step.

This shaping is in particular carried out by depositing the dispersion obtained in the form of a film, in particular of thickness between 100 nm and 1 cm, preferably between 100 nm and 1 mm, preferably between 1 and 100 μm, and preferentially still between 1 and 10 μιτι. Another aspect of the invention is an electrically insulating film formed based on a material according to the invention. This film can be obtained by a process as described above. It preferably has a thickness of between 100 nm and 1 cm, preferably between 100 nm and 1 mm, preferably between 1 and 100 μιτι, and preferably between 1 and 10 μιτι. According to another aspect, the present invention relates to the use of a material according to the invention, corresponding to one or more of the above characteristics, as an electrical insulator, in particular in an electrical, electronic or electrical engineering, for example in a system for converting or storing electrical energy. This use may especially be performed at a temperature above 200 ° C, the material according to the invention having electrical and dielectric properties quite advantageous at such high temperatures. It can also be performed under stringent electrical conditions, especially under strong electric field, for example at least 10 kV / mm.

The material according to the invention may in particular be applied to a support to be electrically insulated, in the form of a film of thickness between 100 nm and 1 cm, preferably between 100 nm and 1 mm, preferably between 1 and 100. μιτι, and preferably between 1 and 10 μιτι. The present invention also relates to an electrical system, electronic or electrotechnical system, which comprises, as electrical insulation of at least one of its components, whether it is an active component or a passive component, a film of a material according to the invention , responding to one or more of the above characteristics. Such a system may notably consist of a system for converting or storing electrical energy, such as a capacitor, a power module, etc., which may have to operate in a high temperature environment, and under a strong electric field, a semiconductor system, an integrated system, etc. Examples of such systems have been listed above in detail, as well as the advantages of implementing the material according to the invention as an electrical insulator within such systems.

The features and advantages of the invention will emerge more clearly in the light of the following examples, provided for illustrative purposes only and in no way limitative of the invention, with the support of FIGS. 1 to 21, in which:

FIG. 1 represents transmission electron microscopy images obtained for two batches of boron nitride nanoparticles not in accordance with the invention (BN-1) and (BN-2) and for two batches of nanoparticles of conformal boron nitride. to the invention (BN-3) and (BN-4); FIG. 2 shows a graph showing the size distribution of the nanoparticles, measured by laser particle size at a wavelength of 633 nm, on a dispersion of 0.1 g of particles in 10 ml of ethanol, for two batches of boron nitride nanoparticles not in accordance with the invention (BN-1) and (BN-2) and for two batches of nanoparticles of boron nitride in accordance with the invention (BN-3) and (BN-4);

FIG. 3 is a graph showing the temperature cycle of the final step of manufacturing materials based on nanoparticles of boron nitride dispersed in a polyimide matrix;

FIG. 4 represents transmission electron microscopy images obtained for films of polyimide-based materials and particles of boron nitride not in accordance with the invention (PI-BN-1 and PI-BN-2) and for films of materials based on polyimide and particles of boron nitride according to the invention (PI-BN -3 and PI-BN-4 (2));

FIG. 5 is a graph representing the minimum dielectric breakdown field, obtained from 20 samples, as a function of temperature, for films of materials based on polyimide and boron nitride particles according to the invention (FIG. PI-BN-3 and PI-BN-4 (2)), for films of polyimide-based materials and boron nitride particles not in accordance with the invention (PI-BN-1 and PI-BN-2 ), and for a film of the same polyimide not loaded with nanoparticles (PI);

FIG. 6 shows a graph representing the minimum dielectric breakdown field, obtained from 20 samples, as a function of temperature, for films of materials based on polyimide and boron nitride particles according to the invention (FIG. PI-BN-4 (1), PI-BN-4 (2), PI-BN-4 (3)), of similar constitution but having loading rates of different nanoparticles;

FIG. 7 shows a graph representing the volume electrical resistivity as a function of temperature, for films of different conventional electrically insulating polymers; FIG. 8 shows a graph showing the volume-specific electrical resistivity as a function of temperature, for films of polyimide-based materials and boron nitride particles in accordance with the invention (Pl-BN-4 (1) and PI-BN-4 (2)), for films of polyimide materials and boron nitride particles not in accordance with the invention (PI-BN-1 and PI-BN-2), and for a film the same polyimide not loaded with nanoparticles (PI);

FIG. 9 shows a graph representing the evolution of the permittivity (ε) at 1 kHz, as a function of temperature, for a material according to the invention (PI-BN-4 (2)) and for the comparative material formed by the same polymer (PI) not loaded with nanoparticles; FIG. 10 shows a graph representing the evolution of the factor of dielectric losses (tanδ) at 1 kHz, as a function of temperature, for a material according to the invention (PI-BN-4 (2)) and for the comparative material formed by the same polymer (PI) not loaded with nanoparticles ;

FIG. 11 shows a graph showing the evolution of the leakage currents as a function of the electric field, for three different temperatures

(200 ° C., 250 ° C. and 300 ° C.), for a material according to the invention PI-BN-4 (2) and for the comparative material formed by the same polymer (PI) not loaded with nanoparticles;

FIG. 12 represents transmission electron microscopy images obtained for nanoparticles of aluminum nitride (AIN) and for nanoparticles of silicon nitride (SiN) according to the invention;

FIG. 13 shows a graph representing the size distribution of the nanoparticles, measured by laser granulometry at a wavelength of 633 nm, on a dispersion of 0.1 g of particles in 10 ml of ethanol, for particles of aluminum nitride (AIN) and for silicon nitride nanoparticles (SiN) according to the invention, as well as for a batch of boron nitride nanoparticles according to the invention (BN-4);

FIG. 14 shows a graph showing the evolution of the leakage currents as a function of the electric field, at the temperature of 250 ° C., for materials according to the invention PI-BN-4, PI-AIN and PI-SiN , and for the comparative material formed by the same polymer (PI) not loaded with nanoparticles;

FIG. 15 shows graphs showing the evolution of the leakage currents as a function of the electric field, for three different temperatures (200 ° C., 250 ° C. and 300 ° C.), for the comparative material formed by the same polymer (PI ) not loaded with nanoparticles and for a material according to the invention PI-AIN, at mass loading rates of nanoparticles respectively of (a) 3%, (b) 5%;

FIG. 16 shows graphs representing the evolution of the leakage currents as a function of the electric field, for three temperatures different (200 ° C., 250 ° C. and 300 ° C.), for the comparative material formed by the same polymer (PI) not loaded with nanoparticles and for a material according to the invention PI-SiN, at mass loading rates. in nanoparticles respectively of (a) 3%, (b) 5%; FIG. 17 shows a graph representing the volume electrical resistivity as a function of temperature, for films of polyimide materials and particles in accordance with the invention PI-BN-4, PI-AIN and PI-SiN, mass loading rates in nanoparticles of 1%, 3% or 5%, and for a film of the same polyimide not loaded with nanoparticles (PI); FIG. 18 shows a graph representing the evolution of the permittivity (ε) at 1 kHz, as a function of temperature, for materials according to the invention PI-BN-4, PI-AIN and PI-SiN, mass loading rates in nanoparticles of 1%, 3% or 5%, and for the comparative material formed by the same polymer (PI) not loaded with nanoparticles; FIG. 19 shows a graph representing the evolution of the factor of dielectric losses (tanδ) at 1 kHz, as a function of temperature, for materials according to the invention PI-BN-4, PI-AIN and PI-SiN , at nanoparticle loading mass ratios of 1%, 3% or 5%, and for the comparative material formed by the same polymer (PI) not loaded with nanoparticles;

FIG. 20 is a graph showing the minimum dielectric breakdown field, at the respective temperatures of (a) 300 ° C., (b) 350 ° C., for films of polyimide materials and nitride particles conforming to FIG. PI-AIN and PI-SiN, at mass loading rates of nanoparticles of 1%, 3% or 5%, and for a film of the same polyimide not loaded with nanoparticles (PI);

and FIG. 21 shows a graph representing the volume electrical resistivity as a function of temperature, for a film of silicone gel material and boron nitride particles in accordance with the invention, at a mass loading rate of nanoparticles of 1%, and for a film of same silicone gel not loaded in nanoparticles.

EXPERIMENT A - Composite materials: polyimide matrix - nanoparticles of boron nitride

EXAMPLE 1 Preparation of Polymeric Matrix Film Materials

The polymer matrix used in this example is a biphenyltetracarboxylic acid (BPDA) / p-phenylenediamine (PDA) dianhydride type polyimide

Initially, the two precursor monomers of this polyimide are in liquid form, dissolved in the polar solvent N-methylpyrrolidone (NMP). This precursor solution is commonly called polyamic acid (

This PAA solution is obtained by the two-step synthesis method described in particular in the Sroog publication (1991), by dissolving the precursor monomers (in a 1: 1 ratio, representing 13.5% by weight) in the NMP ( 86.5% by weight). The viscosity of the PAA solution used is 1 10-135 poise at 25 ° C., and its density is 1.082 g / cm ³ .

The step of converting the PAA into the polyimide (PI) is carried out by a high temperature annealing step, causing an imidization reaction of the PAA. Inorganic nanoparticles

Different batches of nanoparticles of boron nitride (BN), the characteristics of which are indicated in table 1 below, are used to form several materials.

Batches of nanoparticles designated BN-1 and BN-2 are comparative examples, and do not meet the definition of the present invention.

Batches of nanoparticles named BN-3 and BN-4 are in accordance with the present invention.

In addition to the characteristics communicated by the suppliers, the actual size characteristics of the nanoparticles of each batch were established from observations by transmission electron microscopy (TEM) on the one hand, and by laser particle size on the other hand.

The TEM images were obtained using a Jeol JEM1400 transmission microscope with a voltage of 120 kV. An exemplary image obtained for each batch BN-1, BN-2, BN-3 and BN-4 is shown in FIG. It is observed that the nanoparticles BN-3 and BN-4 batches in accordance with the invention all have dimensions of less than 200 nm. Batches BN-1 and BN-2 all include nanoparticles having at least one dimension greater than 200 nm. The measurement by laser particle size, performed in a conventional manner in itself, consists of determining the particle size distribution by a light diffraction technique obtained from a laser (He-Ne), after suspending the particles by sonication in a liquid solvent. 0.1 g of each of the different batches of nanoparticles were introduced in 10 ml of ethanol and dispersed for 10 min in an ultrasonic bath at a power of 750 W. The measuring apparatus used is a Zetasizer NanoZS90 laser granulometer. The wavelength of the laser used is 633 nm. The device detects particles between 0.3 nm and 5 μιτι, with an uncertainty of +/- 2%.

The result obtained for each batch, in terms of size distribution of the nanoparticles, is shown in FIG. 2. It is clearly observed that lots BN-1 and BN-2 contain particles larger than 200 nm, and bimodal size distribution, unlike lots BN-3 and BN-4 which have a single distribution, with all particles smaller than 200 nm.

From these measurements are also extracted the minimum, maximum and average particle diameters for each batch and for each distribution.

The characteristics of the different batches are summarized in Table 1 below.

Table 1 - characteristics of boron nitride particles implemented

^* : measured values; ^# : values communicated by suppliers; NA: value not available The measured size values confirm that lots BN-3 and BN-4 are in accordance with the present invention, unlike lots BN-1 and BN-2.

In addition, the characteristics of lot BN-1 show that this lot is equivalent to the batch of particles described in Li et al. (201 1). Preparation of materials

Various composite materials, shown in Table 2 below, are prepared by a process comprising the following successive steps:

mechanical mixing of the nanoparticles in 10 to 15 g of the PAA solution in NMP. The masses of nanoparticles introduced into the solution, for each material, are indicated in Table 2 below;

dispersion of the particles in the composition thus obtained, by sonication at an amplitude of 300 W, for 1 h with a square exposure cycle (2 s ON and 12 s OFF);

centrifugation at 21,000 g (14,400 rpm) for 25 minutes; - Recovery of the supernatant and spin coating ("spin coating") on a stainless steel metal substrate at a speed between 2000 and 4000 rpm, depending on the viscosity of the solutions, for 30 s. An adhesion promoter (VM 652 from HD Microsystems) is previously deposited on the substrate before deposition to promote adhesion of the films; - annealing at 100 ° C for 1 min on a hotplate and in air, followed by annealing at 175 ° C for 3 min, so as to solidify the deposits;

- Annealing of the samples at 200 ° C for 20 min, then at 400 ° C for 1 h, in a controlled oven under nitrogen, in order to evaporate the solvent and then perform the imidization of the polyimide. The temperature cycle of this final annealing step is shown in FIG.

These steps are reproduced so as to obtain for each batch, by successive deposits, a multilayer film with a thickness of about 4 μιη.

Thus, films of materials are obtained in which nanoparticles of boron nitride are dispersed in the polyimide matrix, more particularly of materials in accordance with the invention (called PI-BN-3, PI-BN-4 (1), PI-BN-4 (2) and PI- BN-4 (3)), and comparative materials, not in accordance with the invention (called PI-BN-1 and PI-BN-2). For each of these materials, the exact volume loading rate of the nanoparticle matrix is determined by the helium pycnometry technique, using a Micromeritics Accupyc 1330 pyknometer. Calibrations were performed before each measurement. For measurements, a 0.1 cm ³ cell was used. The volume loading rates of the polyimide matrix into nanoparticles thus measured are shown in Table 2 below.

Table 2 - characteristics of the materials formed

It should be noted that, because of the high density of the particles of comparative batches BN-1 and BN-2, it is not possible with these batches to produce nanoparticle volume loading materials greater than 30% and which nanoparticles are properly dispersed.

TEM images of the films thus obtained are acquired by means of a Jeol JEM1400 transmission microscope, the voltage used being 120kV. For this purpose, the films were peeled off the substrates, cut by microtomy to obtain a strip of about 100 nm thick, and then fixed on a grid. The images obtained are shown in FIG. 4. It can be seen that the films of PI-BN-1 and PI-BN-2 not in accordance with the invention have numerous agglomerates of size greater than 0.5 μιτι, whereas the PI-BN-3 movie according to the invention has agglomerate sizes of less than 0.5 μιτι, and that the film of PI-BN-4 according to the invention has agglomerate sizes well below 0.3 μιτι.

As a further comparative example, a film of the same polyimide not loaded with nanoparticles, called PI, has also been formed on an identical metal substrate.

EXAMPLE 2 - Electrical Tests at High Temperatures

2.1 / Test structures

The electrical measurements are made on the material films formed in Example 1 above, using capacitive metal-insulator-metal (MIM) type structures.

To form these structures, metallization with a gold layer was carried out by evaporation under secondary vacuum at 10 ^-6 Torr, with a thickness of 150 nm, over the entire surface of the films of PI-BN materials formed in Example 1 on the metal substrate.

A step of etching through a photolithographed mask then made it possible to define the geometry of the upper gold electrodes. More specifically, these upper electrodes have been configured to have a substantially circular sectional shape, with a diameter of 5 mm.

2.2 / Test equipment and methods

The permittivity measurements (ε), dielectric loss factor (tanδ) and DC electrical resistivity (p) are performed by broadband dielectric spectroscopy using a Novocontrol Alpha-A device. The latter allows the characterization of samples over a temperature range of 25 ° C to 350 ° C under nitrogen, and for frequencies between 10 ^"1 and 10 ⁶ Hz, under an effective alternating voltage of 500 mV. The temperature and the resolution of the dielectric loss factor are ensured respectively at ± 0.1 ° C and ⁵ × 10 ^-5 . Leakage current and dielectric breakdown field measurements are carried out using a Signatone S-1 160 spike station with micrometric positioners and a sample holder which is temperature regulated between 25 and 350 ° C (± 1 ° C) by a heating system S-1 060R. The station is arranged in a Faraday cage. Electrical signals are applied using low noise coaxial spikes. In addition, the sample is electrically isolated, via an alumina plate, the sample holder itself connected to the ground. During measurements, the temperature of the sample is monitored using a K-type thermocouple placed in contact on the surface of the PI-BN material film.

Leakage current and DC dielectric field strength measurements are carried out using a Keithley SM 241 0 source with an internal voltage source (voltage ramp from 0 to 1100 V, 8 V / s) and a nanoamperemeter (0, 1 nA at 20 mA). At break, the voltage across the sample becomes zero, so the voltage source switches to a current source where a limiting current (or short-circuit current l _C c) has been pre-set to 20 mA. The tests are carried out according to the ASTM D149-97a standard for solid insulation rupture tests. The value of the breaking field E _B R is thus calculated through the relation: E _BR = V _br / d where V _B R is the breaking voltage and d is the thickness of the insulating material.

Since electrical breakdown is a random phenomenon resulting from a random distribution of the defects in the insulator, the experimental measurements are performed on a number of samples of capacitive structures for each temperature and for each material. Statistical processing is performed using the two-parameter Weibull distribution law.

2.3 / Measurement of the dielectric field of rupture The minimum field of dielectric breakdown, calculated for the 20 capacitive structures tested for each material, was determined, for different temperatures, for the following different materials: PI (uncharged polymer), materials not in accordance with the invention PI-BN-1 and PI-BN-2, and compliant materials to the invention PI-BN-3 and PI-BN-4 (2). The results obtained are shown in FIG. 5. They clearly show that the minimum dielectric breakdown field remains very high, around 4 MV / cm, at temperatures above 200 ° C. for the materials according to the invention, unlike the materials Comparative, that is to say the material not loaded nanoparticles and nanoparticle-loaded materials larger than the size recommended by the present invention, for which this dielectric breakdown field collapses with the temperature rise. This demonstrates the superior electrical insulation performance of the materials according to the invention at high temperature and in a strong electric field. The same experiment was carried out for the three materials according to the invention PI-BN-4 (1), PI-BN-4 (2) and PI-BN-4 (3), of similar constitution but having volume loading in different nanoparticles.

The results are shown in Figure 6. It is observed that all of these materials have a minimum dielectric breakdown field that remains high at high temperatures. The material PI-BN-4 (2), having a nanoparticle volume loading rate of 42.1%, has the best performance.

2.4 / Measurement of electrical resistivity

In order to clearly demonstrate the advantages of the present invention, the volume electrical resistivity was measured as a function of temperature, at temperatures above 200 ° C, for the following commercially available heat-insulating thermostable polymer films: Kapton® -HN (Goodfellow, 50 μιτι), polyaramide (PA) (Goodfellow, 50 μιτι reference T410), PEEK (Goodfellow, 50 μιτι amorphous) and polyamide-imide (PAI) (diphenyl methane diisocyanate and anhydride trimellitic, 5 μιτι).

For this purpose, a MIM structure containing a film of each of these polymers was formed, and the measured volume electrical resistivity. The results obtained are shown in FIG. 7. It is observed that the volume electrical resistivity of each of these polymers drops with the rise in temperature, these materials quickly becoming semi-insulating.

The volume electrical resistivity has also been measured, at different temperatures above 200 ° C., for the films of materials according to the invention PI-BN-4 (1) and PI-BN-4 (2), and for the comparative films PI-BN-1, PI-BN-2 and Pl.

The results obtained are shown in FIG. 8. The materials according to the invention again show a better performance there than the comparative materials at high temperatures, including at a lower nanoparticle volume loading rate (20% for PI-BN- 4 (1)). This good preservation of the electrical resistivity of the materials according to the invention at high temperatures allows them to remain in the range of electrical insulators (volume resistivity greater than 10 ¹² Ω), well beyond 200 ° C.

2.5 / Measurement of the permittivity and the factor of dielectric losses The evolution of the permittivity (ε) and the factor of dielectric losses

(tanδ) at 1 kHz, as a function of temperature, was measured for the material according to the invention PI-BN-4 (2) and for the comparative material PI not loaded with nanoparticles.

The results obtained are shown in FIG. 9 for the permittivity, and in FIG. 10 for the dielectric loss factor. As can be seen in these figures, the material according to the invention PI-BN-4 (2) has a strong reduction in the level of dielectric loss, by a factor of about 10 to 250 ° C, of about 100 to 300 ° C and about 1000 to 350 ° C, relative to the comparative material PI, and a stabilization of the permittivity over the entire temperature range. In addition, the level of dielectric losses material according to the invention remains less than or equal to 1% over the entire temperature range up to 350 ° C, unlike the unfilled material.

2.6 / Measurement of leakage currents The evolution of the leakage currents as a function of the electric field, for three temperatures (200 ° C., 250 ° C. and 300 ° C.), was measured for the material according to the invention PI- BN-4 (2) and for the comparative material PI not loaded with nanoparticles.

The results obtained are shown in FIG. It is observed that the material according to the invention PI-BN-4 (2) shows a good maintenance of the levels of leakage currents, less than 100 nA / cm ² at 100 kV / cm and less than or equal to 1 μΑ / cm ² under 1 MV / cm, this up to 300 ° C. Thus, the leakage current densities of the material according to the invention are reduced by a factor of 10 to 100,000 compared to the comparative material PI at these high temperatures.

EXPERIMENT B - Composite materials: polyimide matrix - nanoparticles of aluminum nitride or silicon nitride

Preparation of the materials The following different materials were prepared in film form.

For each of these materials, the polymer matrix is identical to that described in Experiment A.

The inorganic nanoparticles, according to the present invention, are of two types: aluminum nitride nanoparticles (AIN), named AIN in the present description, and silicon nitride nanoparticles (Si ₃ N ₄ ), named SiN in the present description.

In addition to the characteristics communicated by the suppliers, the actual size characteristics of the nanoparticles of each batch were established from observations by transmission electron microscopy. (TEM) on the one hand, and by laser granulometry on the other hand.

The TEM images were obtained as described in Experiment A. An example image obtained for each type of AlN and SiN nanoparticles is shown in FIG. 12. It is observed that the nanoparticles all have dimensions of less than 200 nm. .

The measurement by laser granulometry was carried out as described in Experiment A. The result obtained for each of the nanoparticle types, in terms of size distribution of the nanoparticles, is shown in FIG. 13. It is clearly observed that the nanoparticles present a single distribution, with all particles smaller than 200 nm.

The characteristics of the different lots are summarized in Table 3 below.

Table 3 - characteristics of nitride particles implemented

^* : measured values; ^# : values communicated by suppliers; NA: value not available

The measured size values confirm that these nanoparticles are in accordance with the present invention. The preparation of the materials according to the invention, based on these different nanoparticles, was carried out as described in Experiment A.

For each type of nanoparticle, mass loading rates of 1%, 3% and 5% were achieved. There was thus obtained films of materials in which nitride nanoparticles are dispersed in the polyimide matrix, more particularly materials according to the invention respectively PI-AIN and PI-SiN.

By way of comparative examples, PI-BN-4 and PI-BN-1 materials, conforming to Experiment A, have also been produced with mass loading rates in nanoparticles of 1%, 3% and 5%.

A film of the same polyimide not loaded with nanoparticles, called PI, has also been formed on an identical metal substrate.

Measurement of leakage currents The evolution of the leakage currents as a function of the electric field, at a temperature of 250 ° C., was measured for the materials in accordance with the invention PI-BN-4 (at a mass loading rate of 1% nanoparticles), PI-AIN and PI-SiN (each at a mass loading rate of 3% of nanoparticles), as well as for the comparative material PI not loaded with nanoparticles.

The results obtained are shown in FIG. 14. It is observed that the materials in accordance with the invention all show good maintenance of the leakage current levels at 250 ° C., these leakage currents being much lower than those of the unfilled polyimide. in nanoparticles (PI). The evolution of the leakage currents as a function of the electric field, for three temperatures (200 ° C., 250 ° C. and 300 ° C.), was measured for each of the materials in accordance with the invention PI-AIN and PI-SiN, at mass loading rates in nanoparticles respectively of 3% and 5%, and for the comparative material PI not loaded with nanoparticles. The results obtained are shown in Figure 15 for PI-AIN, and in Figure 16 for PI-SiN. It is observed that the materials according to the invention show a good maintenance of the levels of leakage currents, these leakage currents being much lower than those of the polyimide not loaded with nanoparticles (PI), this up to 300 ° C.

Volumetric electrical resistivity measurement

The volume electrical resistivity of the materials according to the invention PI-AIN, PI-SiN, PI-BN-4, and polyimide alone (PI), was measured as a function of temperature at temperatures above 200 ° C. as described in Experiment A. The mass loading rates of nanoparticles were as follows: PI-BN-4: 1%; PI-AIN: 3% and 5%; PI-SiN: 3% and 5%.

The results obtained are shown in FIG. 17. The materials according to the invention again show a better performance than the comparative material at high temperatures.

Measurement of permittivity and dielectric loss factor

The evolution of the permittivity (ε) and the dielectric loss factor (tanδ) at 1 kHz, as a function of temperature, was measured as described in Experiment A, for the materials in accordance with the invention PI-AIN , PI-SiN, PI-BN-4, and for the comparison material PI not loaded with nanoparticles. The mass loading rates in nanoparticles were as follows: PI-BN-4: 1%; PI-AIN: 3% and 5%; PI-SiN: 3% and 5%.

The results obtained are shown in FIG. 18 for the permittivity, and in FIG. 19 for the dielectric loss factor. As can be seen in these figures, the materials according to the invention all have a strong reduction in the level of dielectric losses compared to the comparative material PI, and a stabilization of the permittivity over the entire temperature range.

Measurement of the dielectric breakdown field The minimum dielectric breakdown field was determined, as described in Experiment A, for respective temperatures of 300 ° C. and 350 ° C., for the following different materials: PI (unfilled polyimide), materials according to the invention PI-AIN and PI-SiN. The mass loading rates in nanoparticles were as follows: 1%, 3% and 5%.

The results obtained are shown in FIG. 20. They clearly show that, at temperatures as high as 300 ° C. and 350 ° C., the minimum dielectric breakdown field remains higher for the materials according to the invention than for the material not loaded with nanoparticles. This demonstrates the superior electrical insulation performance of the materials according to the invention at high temperature and in a strong electric field.

EXPERIMENT C - Composite Materials: Silicone Matrix - Boron Nitride Nanoparticles

Preparation of materials

In this experiment, the matrix is a silicone gel (Semicosil® 945 HT from Wacker Silicones). Its density is 0.97 g / cm ^3. Its viscosity at ambient temperature is 1000 mPa.s. It is a two-component material (ratio for 10: 1 mixing).

The nanoparticles are nanoparticles of BN-4 boron nitride described in Experiment A.

The material according to the invention was prepared in the following manner.

The nanoparticles, in an amount adequate to obtain a content of 1% by weight of nanoparticles in the matrix, were mixed in 10 g of silicone precursor, before being dispersed with an ultrasonic probe for 30 min at 225 W, using a square exposure cycle (2 s ON and 9 s OFF).

The hardener was then added (10: 100 ratio) with a pipette and the resulting mixture was stirred mechanically for 3 min. The mixture was degassed under vacuum and then poured between two stainless steel plates (33x33x1 mm) spaced 4 layers of thickness 50 μιτι each (ie a total thickness of 200 μιτι) of Kapton® adhesive tape placed at the level of four edges of the plates.

The crosslinking of the silicone matrix was carried out in an oven at 100 ° C. for 30 minutes in air.

Since the silicone gel obtained has no mechanical hardness, the metal plates of the mold have been used as electrodes for the electrical characterizations.

A control formed of the silicone gel alone, that is to say not containing nanoparticles, was also carried out.

Volumetric electrical resistivity measurement

The volume electrical resistivity of the material according to the invention and that of the silicone gel alone were measured as a function of temperature, at temperatures between 150 and 250 ° C., directly on the samples molded with the mold plates, and according to the protocol described in Experiment A.

The results obtained are shown in FIG. The material according to the invention shows a better performance than the comparative material throughout the temperature range tested, including at temperatures greater than or equal to 200 ° C.

The above description clearly illustrates that by its different characteristics and advantages, the present invention achieves the objectives it has set for itself. In particular, it provides an electrically insulating material which advantageously has, at high temperatures above 200 ° C and in a strong electric field, higher performance compared to the materials of the prior art. BIBLIOGRAPHIC REFERENCES

Feng et al. (2013) Material Letters, 96, 113-116

Li et al. (2011) Journal of Applied Polymer Sciences, 121, 916-922

Sato et al. (2010) J. Mater. Chem., 20, 2749-2752

Sroog (1991) Prog. Polym. Sci., 16, 561

Claims

An electrically insulating material, comprising a matrix of a thermostable and electrically insulating polymer, in which electrically insulating inorganic nanoparticles are dispersed, characterized in that the electrically insulating inorganic nanoparticles are chosen from electrically insulating metal nitrides, diamond, the oxides of at least one metal of columns 1 to 1 1 of the periodic table of electrically insulating elements, and mixtures thereof, and in that all of said electrically insulating inorganic nanoparticles have dimensions less than or equal to 200 nm.

2. Material according to claim 1, wherein said electrically insulating inorganic nanoparticles have a generally spherical shape.

The material of any one of claims 1 to 2, wherein said electrically insulating inorganic nanoparticles have a monomodal size distribution.

4. Material according to any one of claims 1 to 3, wherein said electrically insulating inorganic nanoparticles are present in said matrix in a volume ratio of 0.1 to 95%, preferably 1 to 95%, preferably 20 to 20% by weight. at 50%, and preferably from 35 to 45%.

5. Material according to any one of claims 1 to 4, wherein said inorganic electrically insulating nanoparticles are dispersed in said matrix so as to form no agglomerate of size greater than or equal to 2 μιτι, preferably greater than or equal to 1 μιτι .

The material of any one of claims 1 to 5, wherein said electrically insulating heat-stable polymer is a polyimide or a silicone.

7. Material according to any one of claims 1 to 6, wherein said electrically insulating inorganic nanoparticles are nanoparticles of metal nitride, including nanoparticles of boron nitride.

8. Material according to any one of claims 1 to 7, shaped in the form of a film, said film preferably having a thickness between 100 nm and 1 cm, preferably between 100 nm and 1 mm, and preferably between 1 and 100 μιτι.

9. A method of manufacturing a material according to any one of claims 1 to 8, comprising steps of:

dispersion of electrically insulating inorganic nanoparticles having dimensions of less than or equal to 200 nm in a liquid composition comprising one or more precursors of a thermostable and electrically insulating polymer, optionally in solution in a solvent,

- shaping of the dispersion thus obtained, in particular by deposition in the form of a film,

and heating under conditions capable of effecting the crosslinking of said polymer and the elimination of the solvent.

10. The method of claim 9, wherein the step of dispersing the nanoparticles in the liquid composition comprises the mechanical mixing of the nanoparticles in said liquid composition, and sonication of the mixture thus obtained.

11. A method according to any one of claims 9 to 10, wherein the step of dispersing the nanoparticles in the liquid composition is followed by a step of removing agglomerates of size greater than or equal to 2 μιτι, preferably greater than or equal to 1 μιτι, preferably by decantation by centrifugation.

12. Use of a material according to any one of claims 1 to 8 as an electrical insulator, especially in an electrical, electronic or electrotechnical system.

13. Use according to claim 12, at a temperature greater than

200 ° C.

14. Use according to any one of claims 12 to 13, wherein said material is applied to a support to be electrically insulated, in the form of a film of thickness between 100 nm and 1 cm, preferably between 100 nm and 1 mm, and preferably between 1 and 100 μιτι.

An electrical, electronic or electrotechnical system comprising as an electrical insulator a film of a material according to any one of claims 1 to 8.