THERMAL MATERIAL WITH HIGH CAPACITY AND HIGH CONDUCTIVITY, PREPARATION METHOD AND THE COMPONENTS COMPRISING THE SAME

The present invention relates to the thermal management, especially for technologies that require limiting the rise in temperature, as for example the temperature control of the electronic components. It seeks to effectively limit the increase in temperature of the components during their use during transient operation, while occupying a minimal space with reduced weight and reduced without shorting the circuit.

The compactness growing electronics makes the problems relating to the thermal management increasingly crucial. The miniaturization of the electronic request to techniques of heat management be compact and on energy. The problem is therefore to find a way of limiting the temperature rise of the components during their use, in order to ensure optimal functioning, and on the other hand be able to store and exchange heat efficiently, especially in the case of confined environments. This, without damaging components, by limiting the final weight and guaranteeing a minimum footprint.

The solution the more widespread for cooling is the use of a metal heat sink massive but it is generally cumbersome and non-negligible mass relative to that of the electronics. In addition to its ability to remove heat is limited.

Also, to facilitate extraction of heat, thermal interface materials are used to reduce all thermal resistances of contact between the heat source and the heat sink. These materials are typically pastes or adhesives which exhibit high compliance with the components. However, they are not capable of storing heat.

Finally, another solution is the use of phase change materials (PCM) for absorbing and storing heat during the period of use. Indeed, these materials have the ability to store the surrounding heat due to their high melting enthalpy (typically about 210 j/g), and by absorbing heat they reduce the elevation of temperature of their environment. LCM are passive components but they have a low thermal conductivity (the W/mK to about 0.15 - 0.25), and their stiffness generates a high contact thermal resistance which limits their ability to absorb heat.

The first point can be improved by incorporating a material having high thermal conductivity.

The first studies in an attempt to improve the thermal conductivity of the PCM were made in the years 1980. From several strategies have been implemented to try to increase their thermal conductivity without altering their properties of thermal storage. Ji and ai. The ENERGY about. IBS ., flight. 7, NO 3, PPs. 1185 - 1192, 2014 reports the various routes conductivity enhancing MCP.

Two strategies are utilized in order to improve the thermal conductivity of the PCM. Adding additives diffuse, such as carbon nanotubes, graphene sheets, carbon fibers thus better thermal conductivity with a gain based on the volume fraction of the additive less than 2. Better continuity in the conductive materials improves this gain. The first studies conducted with metal foams showed the interest (aluminum, carbon, nickel alloy). But again, only a maximum gain of 6 could be found.

However, graphite, and metal foams are also excellent electrical conductors which does not permit direct application to electronic components: the use of these materials on electrical components for thermal management would short the systems.

It is therefore a need to develop alternative composite material, based on the MCP and an additive having a high thermal conductivity but also provides modulation of its electrical conductivity by making the composite partially electrically conducting or insulating.

Boron nitride (CBN) in powder form has been proposed to improve the thermal properties of the LCM. The BN is an excellent electrical insulator while being an excellent heat conductor. By mixing it with LCM, increases the thermal conductivity of the PCM and therefore its storage capacity, while ensuring electrical insulation between the components. It is also possible to dope the bn with carbon (the BN (C.)) thus rendering slightly electrically conductive.

The BN is able to compete with the thermal properties of graphene, while further having a very high electrical resistance. 2D form, the BN is a chemically inert material, with a perfect thermal stability up to 1000 °c. Its thermal conductivity, although theoretically lower graphene, remains very high (on the order of 2000 to W/mK to in theory), compared to copper (the W/mK to 400) conventionally used.

Thus, Jeong and ai (Int.. A heat Transf. J on mass, flight. 71, pPs. 245 - 250, 2014) have disclosed a filling of MCP by the BN powder to 80% in LCM, BN is a fraction of 20%. A gain in thermal conductivity of 477% with respect to the thermal conductivity of the MCP is described, which thus corresponds to a gain by volume fraction BN of 0.24 only.

The foam of BN-doped carbon (the BN (C.)) is known (Loeblein and ai, small electric, v. 10, 15, 2992 - 2999, 2014) and also has shown its high thermal conductivity, comparable to the BN alone.

It remains therefore providing an improved composite for increased gain of the thermal conductivity.

Thus, it is suggested according to the invention a composite material comprising:

Boron nitride (BN on (C.)) as a continuous structure; and

A phase change material (PCM) embodied within said continuous structure of BN (C.),

said composite material comprising at least one face,

and characterized in that said composite material comprises in whole or part of said side surface portions of the continuous structure consisting of BN (C.) free of MCP.

The composite material according to the invention based on the MCP and continuous structure of BN optionally carbonaceous, is such that the MCP is incorporated into the interstices, such that the pores of said continuous structure, and is characterized in that said composite comprises at least a portion of side under a non-zero thickness e free of MCP. The material therefore has at least one layer under all or part of one of its surfaces, which is free of MCP.

The "continuous structure of BN (C.)" refers to any porous material consisting of BN, continuous 3d structure, undispersed, optionally doped with carbon (i.e. polyethoxylated) carbon: it is now considered herein of BNC connectors. As a continuous structure, this in particular include foams, the gates.

The foams of BN (C.) and their method of manufacture are described by Loeblein and ai, small electric, v. 10, 15, 2992 - 2999, 2014.

Typically, the foam of BN (C.) can be grown on an iron metal skeleton of copper or nickel. After the growth BN (C.), the foam is coated with a polymer such as PMMA for stability and then, it is dipped in an acid bath to remove the metal skeleton. The foam of BN alone, is obtained by etching the polymer.

The foam of BN (C.) may be reinforced, for example providing long duration, or more growths to increase the thickness of BN (C.) or to retaining the PMMA on the foam (with a thickness PMMA sufficiently thin to maintain the desired heat conductivity, the PMMA having a low thermal conductivity of W/mK to about 0.2), and/or adding additives to increase thermal conductivity

Generally, the continuous structure of BN (C.) 5 and may include between 80% carbon (by weight). This percentage is dependent upon the intended applications. The carbon content can in particular be modulated uniformly or locally to vary the electrical properties, such as increasing the electrical conductivity on the whole structure or on certain areas, typically in applications such as electromagnetic shielding of the electronic components.

In one embodiment, the continuous structure of BN (C.) has a density of between 1 and 5 mg/cm.³ , and a porosity of between 5 and 120 pores per inch.

The thermal conductivity of the continuous structure is generally greater than the thermal conductivity of the PCM.

The term "side" as referred to herein refers to an outer surface of the composite material.

The "surface portion" or "section" corresponds to at least a portion of the face to be in contact with the electronics when the composite material is applied to a component. Said portion can correspond to all or part of the face of the composite, it being understood that the portion comprises the portion of the face in which it is located, as well as the layer of thickness e immediately below this portion of the face. In this portion, the continuous structure is devoid of MCP.

The composite material thus comprises at least one surface portion, of thickness e under all or part of the face, such that within said portion, the continuous structure is devoid of MCP.

In one embodiment, the material comprises a bottom face and a top face, and under each of the two faces at least surface portion of continuous structure devoid of MCP.

The thickness of said portion is substantially less than the thickness of the composite. Typically, the thickness e can depending on the roughness of the material on which the composite material will be applied. It is in general of thickness at least equal to the diameter of a pore of the continuous structure. The thickness e must be sufficient to minimize the contact resistance and perform the necessary thermal conduction. The thickness of the whole structure is generally limited due to requirements of the mass-electrical components. It is possible for example to cite a thickness e greater than 250 MW, in particular for a continuous structure of very high porosity.

The thickness e can be controlled by the manufacturing method of the composite material.

The portion of continuous structure devoid of MCP thus consists of continuous structure of BN (C.) which produces a good contact with the electronics and thus ensures good heat conduction from the circuit to the composite while monitoring the electrical impact of the composite on the rest of the circuit.

The continuous structure such that the foam of BN (C.) is flexible, it can adapt to the surface roughness of the electronic component and reduce the presence of air pockets, thereby reducing the contact thermal resistance. It may also enduring the changes in phase of the PCM which typically has a phase expansion of 10 to 15%.

In addition, the continuous structure such that the foam of BN (C.) has the advantage of an extremely low density, thereby maintaining the storage capacity of the PCM to constant weight. BN (C.) is a chemically inert material, hence it can complete the passivation/protection of electronic components against environment. This allows to use the compound directly on the electronics and improve heat absorption.

Is meant by MCP, any material capable of performing a phase transition at a temperature (or narrow temperature range) and storing and possibly yielding energy during this transition, . During the transition phase, the temperature of the PCM remains constant. LCM typically suitable for the invention involve a phase transition from solid/solid or liquid/solid.

To ensure optimal thermal storage, they typically have a melting latent energy density at least equal to 50 j/g.

In one embodiment, the PCM can be chosen from PCM type polymers, organic, organometallic, or inorganic eutectic.

By way of MCP, this in particular include LCM selected from LCM marketed by RUBITHERM, the Polywax polythylene (marketed by Baker Hughes on), the Puretemp (marketed by Puretemp), paraffin, erythritol.

The choice of PCM is dependent upon the maximum temperature allowed by the use under consideration. Above are typically LCM whose phase transition temperature is less than or equal to the maximum rated temperature.

The composite material according to the invention thus has the following advantages:

- very good conformability with the surface to control the temperature

- versatility: the phase change temperature may be modulated between 50 and 200 °c, by modifying the MCP;

- thermal storage capacity raised easily;

- enhanced thermal conductivity relative to that of the PCM (the gain based on the volume fraction BN (C.) can reach 10);

- adaptable electric insulation generally or spatially, not to interfere with the electronic systems;

- inert and chemically stable, non-gassing in normal operation condition, avoiding all reactions with the environment;

low - density, for limiting the weight of the system;

τ of contact thermal resistance low, for providing good heat absorption from the electrical component.

According to another object, the present invention also relates to the process for preparing the composite material.

According to the invention, the method comprises infusing MCP in liquid form into the continuous structure of BN (C.), with the exception of the surface portion of said face of the continuous structure devoid of MCP.

In one embodiment, said method comprises the following steps:

- The protection previously said surface portion of the face of the continuous structure of BN (C.);

The impregnation of the continuous structure of BN (C.) by LCM in liquid form;

- The selective deprotection of the protected surface portion;

whereby a continuous structure of BN (C.) into which is incorporated the MCP, except for said surface portion, devoid of MCP.

The protection can be carried out by impregnation of a protective material in the thickness of said surface portion.

This impregnation may be performed by any method for applying in surface and the thickness of a liquid matrix material. The method of application depends on the nature of the material and its viscosity, and the matrix.

In one embodiment, the impregnation is carried out by infusion, hot. The infusion may be carried out by coating or dipping of the surface portion of the continuous structure of BN (C.) protecting on or in a solution of the protective material.

Generally, this impregnation is performed at a temperature above the melting temperature of the protective material, so that it is in liquid form with a viscosity adapted to the thickness desired.

The protective material is selected such that it can be:

- impregnated in the liquid state,

- maintained in the solid state during the impregnation of the PCM to the liquid state, and

- deprotected selectively formed composite.

Typically, the protective material is a polymer, optionally diluted in a solvent to adjust the viscosity of the protective material to the nature of the continuous structure of BN (C.) and of the desired thickness.

In one embodiment, the protective material may be particularly selected from polyethyleneoxide (PEO-) with water or isopropanol as solvent (API), polyvinylidene fluoride (PVDF membranes) with dimethylacetamide (DMAs) or n, n-dimethylformamide (Vilsmeier) as solvent, neopentyl glycol (OPL) with water as solvent.

In one embodiment, is used as the protective material the PEO, diluted in water, dilution rates of between 10 and 50%, in particular between 20 and 25%.

The PEO polymer is a very common and does not pose a particular problem in terms of handling and storage. Its solvent is water, which has the advantage of low cost and easy handling and storage again.

The protective material may be degassed prior to its use, to remove air bubbles and thus allow better impregnation.

When the screen is brought about by a protective material in the liquid state, the method comprises the intermediate step of attaching the protective material on the continuous structure, by increasing the viscosity of the protective material for example by evaporation of the solvent for example.

The protecting step can be performed as many times as necessary depending on the number of surface portions to be protected, before the step of impregnating the LCM.

The impregnation of the continuous structure of BN (C.) by MCP is performed with LCM liquid, after protecting the one or more surface portions to be protected.

The impregnation step is conducted at a temperature greater than the melting temperature of the PCM.

Typically, the protective material must possess either a melting temperature greater than the melting temperature of the PCM, either if the protective material has a lower melting temperature, have a melting time when immersed in liquid the MCP, much longer than the infusion time MCP in the continuous structure unprotected. In addition, typically, the protective material is not to be etched by the PCM in liquid form.

If necessary, it is possible to locally cool surface portions with the protective material during impregnation of the LCM.

Impregnating the PCM can be achieved by immersion of the entire continuous structure protected in a solution of MCP.

The deprotection can be carried out in particular by selective degradation of the protective material, for example chemically, typically by solvent action of deprotection wherein the protective material is soluble. This may be accomplished by soaking the entire structure continuous protected in a bath of the respective solvent.

Deprotection as solvent, there may be mentioned solvents of protective materials cited higher.

Generally, the method further comprises the step intermediate fixing MCP on the continuous structure, by lowering the temperature to transition the PCM to the solid state, prior to the deprotection step. This can be done in molds of various shapes to suit the Packaging and application.

The method according to the invention may also comprise as previously preparing the continuous structure of BN (C.).

The foam of BN (C.) can be prepared by using or adapting the methodology described by Loeblein and Al, small electric, v. 10, no. 15, 2992 - 2999, 2014.

Thus, the foam of BN (C.) can in particular be prepared by growing a CVD (chemical vapor deposition) on a metal skeleton based on copper or nickel for example. After the growth BN (C.), the foam is coated with a polymer such as PMMA for stability, and then it is dipped in an acid bath to remove the metal skeleton. The foam of BN (C.) is then obtained by removing the polymer or, alternatively, the PMMA can be at least partially maintained for increasing strength of the foam.

The composite material thus formed can be applied to an electronic component.

The present invention therefore provides an electronic component comprising a composite according to the invention, in particular as the composite is applied on its face free of MCP in contact with the component.

Generally, the choice of the MCP is such that the melting temperature of the MCP is less than or equal to the maximum operating temperature of the component.

The foam of BN (C.) having high thermal conductivity and is flexible compliant./ in is compressing it complements all air holes and reduces the contact thermal resistance. This improves the transmission of heat from the electronic component to the PCM. In addition, the continuity of the foam to distribute this heat to the MCP for then the store. Also, the variation of the amount of carbon in the foam of BN (C.) globally or localized, makes it compatible with the electronics to which it is applied. This allows for the MCP closest to the hot spots.

According to another object, the present invention also includes the method of manufacturing an electronic component comprising a composite according to the invention

This may be accomplished by any conventional method such as by squeezing of the composite, such as by encapsulation of the composite in aluminum or other non-metallic encapsulating.

The invention and its advantages will be better understood to the study of the description which will follow, given solely by way of example, and made with reference to the accompanying drawings, on which:

figures 1 - 6 represent schemes to illustrative steps in the manufacture of a composite material according to the invention; and

figures 7 and 8 - represent diagrams illustrative steps of manufacturing an electronic component comprising a composite material according to the invention.

As shown in Figures 1 to 6, according to one embodiment, the composite material according to the invention can be prepared in several steps protection/deprotection, detailed hereinafter.

The manufacture of the composite comprises performing the foam of BN (C.) 1, then the protection of surface portions 1 'foam with protective material 1 2 (Figure 1 to 3), in order to avoid the presence of MCP surface 5, and then infusing MCP 5 in the foam 1 (fig. 4) and finally of shield 2 (fig. 5) to release the foam surface portions of 1'.

More specifically, as shown in Figures 1 and 2, a protective material 2 such as a polymer dissolved in a solvent is prepared so as to obtain the desired viscosity (which influences the thickness e of the surface portion 1' of foam impregnated with said material by 2) and limit the presence of bubbles upon solidification. In effect, the bubbles would the material 2 brittle by location and allow penetration of the LCM liquid 5. This viscosity depends on the polymer as well as its rate of solvent dilution.

Then, the protective material 2 is put in a container 3 (fig. 1) and the foam of BN (C.) 1 is deposited onto said material 2 (fig. 2). The whole is heated for example on a heating plate 4, until the material 2 forms a thin layer in the surface of the foam 1. The thickness of the surface portion 1' protected may be controlled by the viscosity of the material 2.

Optionally, and as shown in Figure 3, this operation may be performed similarly on another face of the foam of BN (C.).

As shown in Figure 4, once each side on which it is desired to preserve a surface portion 1' devoid of MCP is protected, the MCP 5 is heated from a liquid state. The foam 1 protected is immersed in a bath of the LCM 5. The MCP is allowed 5 infuse only the core of the foam 1 and then removing the impregnated foam bath of MCP 5. The composite material obtained is allowed to cool down for that MCP 5 into a solid state. The shape of the mold for the MCP is arbitrarily square on the schemes, but this can be modified to fit the constraints of the Packaging and application.

Finally, as shown in Figure 6 the composite material is immersed in a solvent bath 6 2 of the protective material, so as to solubilize the material 2, thereby releasing each surface portion 1' of the material 2.

The composite material thus formed may then be applied to an electronic component.

In one embodiment shown in Figure 7, the composite material is encapsulated between a hood 8 aluminum 7 and an electronic component, such as a processor.

The component 7 has a relief irregular surface. By compression, the surface portions 1' of the composite material fill the cavities and conform to the roughness of the component 7.

Thus, as shown in Figure 8, the surface portions 1' compressed form layers 9 of BN (C.) which are in contact with the component 7 on the one hand and the cover 8 on the other hand. This provides electrical isolation, passivation of the component and reducing the contact thermal resistance.

The following examples illustrate the invention without limitation.

A foam of BN has been prepared by applying the methodology described by Loeblein and Al, small electric, vol.10, no. 15, 2992 - 2999, 2014, without performing the step of growing the carbon. PMMA is deposited just prior to etching nickel to mechanically reinforce the BN. The PMMA can be removed or retained after etching nickel.

For performing the foam of BNC connectors brewed by the PCM (phase change material) only at its center and not surface, wherein the first strategy is to use a material that will protect the surfaces of the foam during the infusion. This protective material will then be removed.

The PEO (polyethylene oxide) was used as the protective material. In a first step, it is diluted in water in quantities that achieve a polymer with an appropriate viscosity, to between 25% and 20 PEO.

In the second step, the diluted polymer is stored in vacuum to about 2.5 mTorr for 30 minutes. The purpose of this step "degassing", is to remove the air bubbles that have been trapped in the polymer during mixing. Without this step, during the densification of the bubble could be formed, damaging the foam and compromising the uniformity of the thickness of the polymer.

Third step, the polymer is deposited in an aluminum mold. The amount of polymer will depend on the dimensions of the mold, so as to have a thickness of about 3 mm of polymer. Then the foam is deposited on the polymer that will slightly penetrate it. The penetration depth will depend on the viscosity of the polymer. Finally, the mold is placed on a heating plate, in order to densify the polymer by evaporating the solvent (here water) gradually. It has been established experimentally that a step of 80 °c during 40 min and then a rise 5 °c 5 min for all will reach 120 °c is favorable. However, these temperatures and time depend on the temperature sensor of the heating plate and surroundings of the laboratory because everything is made under air.

Fourth step, the foam with a face protected is removed from the mold. One of the faces is perforated by a needle. These perforations aim to help the infusing MCP subsequently and damaging that very little foam.

The fifth step is identical to the third but on the opposite side of the foam.

The paraffin has been used as of MCP.

The paraffin is heated to 110 °c, c'est to say slightly above the melting temperature of 90 °c paraffin, in the aluminum mold. Once paraffin liquid phase, the foam with both sides protected Y is diving: paraffin infiltrates the sides but also by the perforated face which is kept above. The foam remains between 3 and 5 min in the MCP to ensure that the infusion is complete, while avoiding melting of the protective polymer. Finally it is allowed to cool, naturally or in a refrigerator to accelerate cooling.

To remove the polymer, is dipped into the foam protected in water at room temperature. The compound is held upright (not to damage surfaces) in a beaker of water overnight. Renew the water bath and allowed to act overnight additional for improving deprotection according to the thickness of the polymer as well as the size of the sample and the amount of water. Finally, the sample is allowed to dry.

Thermal characterizations:

• Measuring the density of the final compound. To show that the foam little effect on the weight of the LCM alone.

• Measuring the energy latent fusion of the compound. For the same reason, that is to show the low impact of the foam to the thermal storage capacity of the PCM. Sought to keep energy latent melting MCP.

• Measuring thermal conductivity to show the supply and interest of foam.

• Measuring contact resistance to verify the ability of the compound to conform to the surfaces.

Electrical characterizations

• Evaluating the electrical conductivity of the compound to confirm its appearance insulator for BN (C.) neat, and slightly conductive for c-BN(C.). Similarly for the validation of insulating areas or slightly conductive in the case of localized doping.

• Radio frequency measurements (losses, transmission) in assessing the impact of the presence of the compound in an electronic environment.

Physical characterizations

• Coefficient of thermal expansion of the compound for the future design of the thinner.

• Mechanical strength in compression and tension

• Visualization of the conformability of the foam released surface.

The invention can be applied in the case of a power transistor, for example 20 W-dissipating, cyclic when used, for example for continuous operation less than 15 minutes, with 15 min of cooling. The MCP is selected as a function of the temperature of the transistor critical maximum: the melting temperature of the PCM must be equal to or lower than the critical temperature of the transistor. The inventive material is applied directly over the transistor, with one of the faces of the PCM in contact with stripped the transistor to ensure good thermal contact. An encapsulation is performed around the PCM and the base of the processor for sealing.