THERMAL MATERIAL WITH HIGH CAPACITY AND HIGH CONDUCTIVITY, METHOD FOR PREPARING SAME AND THE COMPONENTS THAT COMPRISE SAME

Abstract

The present invention relates to a boron nitride (BN(C)) composite material in the form of a continuous structure, and a phase change material (PCM) included inside said continuous structure of (BN(C)), the method for manufacturing same and the components that comprise same.

Claims

1. Composite material comprising: Boron nitride BN(C) in the form of a continuous structure; and A phase change material (PCM) incorporated in said continuous BN(C) structure, said composite material comprising at least one face; and characterized in that said composite material, underneath all or part of said face, comprises a surface portion formed of the continuous BN(C) structure free PCM.

2. The composite material according to claim 1 such that said surface portion is a layer of nonzero thickness E underneath the entirety of said face.

3. The composite material according to claim 1, such that the continuous BN(C) structure is a BN(C) foam.

4. The composite material according to claim 1, such that the continuous BN(C) structure is a continuous BNC structure.

5. The composite material according to claim 1, such that said composite material comprises a lower face and an upper face, and such that underneath each of the two faces it comprises at least one surface, portion of the continuous structure free of PCM.

6. The composite material according to claim 1, such that the composite material comprises a lower face, an upper face and one or more side faces, such that a surface portion of the continuous BN(C) structure free of PCM is present underneath each of these faces.

7. Method for preparing a composite material according to claim 1, said method comprising: infusion of the PCM in liquid form in the continuous BN(C) structure, and protection/deprotection of the surface portion(s) and/or removal of any PCM infused in the surface portion(s).

8. The method according to claim 7 comprising following steps: Prior protection of at least one surface portion of the continuous BN(C) structure by impregnating a protective material; Impregnating the continuous BN(C) structure with a PCM in liquid form; Selective deprotection of the protected portion(s); Thereby forming a continuous BN(C) structure in which the PCM is incorporated, with the exception of the surface portion(s) free of PCM.

9. The method according to claim 8, such that impregnation with the PCM is conducted at a temperature higher than the melting temperature of the PCM, and is either lower than the melting temperature of the protective material or at a temperature generating a melting time of the protective material that is less than the infusion time of the PCM.

10. The method according to claim 8 such that said protective material has a melting temperature higher than the melting temperature of the PCM.

11. The method according to claim 7 comprising the following steps: Impregnation of the continuous BN(C) structure with a PCM in form; Selective etching of the PCM in at least one surface portion.

12. The method according to claim 7 comprising: Prior protection of at least one surface portion of the continuous BN(C) structure with a protective material having an etch rate differing from that of the PCM; Impregnation of the continuous BN(C) structure with a PCM in liquid form; and Etching the PCM by immersing the surface area(s) impregnated with the material in an etching solvent.

13. Electronic component comprising a composite material according to claim 1.

14. Method for fabricating the component according to claim 13 comprising the step to apply the composite to the component.

Description

[0114] The invention and its advantages will be better understood on examining the following description given solely as an example and with reference to the appended drawings in which:

[0115] FIGS. 1 to 6 are diagrams illustrating the fabrication steps of a composite material of the invention according to the first embodiment;

[0116] FIGS. 7 to 9 are diagrams illustrating the fabrication steps of a composite material of the invention according to the second embodiment; and

[0117] FIGS. 10 and 11 are diagrams illustrating the fabrication steps of an electronic component comprising a composite material of the invention.

[0118] As illustrated in FIGS. 1 to 6, in a first embodiment, the composite material of the invention can be prepared in several protection/deprotection steps detailed below.

[0119] The fabrication of the composite comprises forming of the BN(C) foam 1, followed by protection of surface portions 1 of the foam 1 with a protective material 2 (FIGS. 1 to 3) to prevent the presence of PCM 5 on the surface, infusion of the PCM 5 in the foam 1 (FIG. 4) and finally removal of the protection 2 (FIG. 5) to free the surface portions 1 of the foam.

[0120] More specifically, as illustrated in FIGS. 1 and 2, a protective material 2 such as a polymer in solution in a solvent is prepared to obtain the desired viscosity (which impacts the thickness E of the surface portion 1 of the foam impregnated with said material 2) and to limit the presence of bubbles on solidification thereof. Bubbles would make the material 2 fragile in some areas and would allow the entry of liquid PCM 5. This viscosity is dependent on the polymer and level of dilution thereof in a solvent.

[0121] The prepared protective material 2 is placed in a container 3 (FIG. 1) and the BN(C) foam 1 is deposited on said material 2 (FIG. 2). The whole is heated over a hot plate 4 for example until the material 2 forms a thin layer on the surface of the foam 1. The thickness E of the protected surface portion 1 can be controlled by means of the viscosity of the material 2.

[0122] Optionally, and as illustrated in FIG. 3, this operation can be conducted in the same manner on another face of the BN(C) foam.

[0123] As illustrated in FIG. 4, once each face is protected on which it is desired to preserve a PCM-free surface portion 1, the PCM 5 is heated to change to the liquid state. The protected foam 1 is immersed in a bath of PCM 5. The PCM 5 is left to infuse only the core of the foam 2 and the foam thus impregnated is removed from the bath of PCM 5. The composite material obtained is left to cool so that the PCM 5 returns to the solid state. The shape of the mould for the PCM is arbitrarily shown to be square in the diagrams but can be modified to adapt to package and application restrictions.

[0124] Finally, as illustrated in FIG. 6, the composite material is immersed in a solvent bath 6 of the protective material 2, to solubilise the material 2 and thereby remove the material 2 from each surface portion 1.

[0125] As illustrated in FIGS. 7 to 9, in the second embodiment of the method of the invention, the composite material of the invention can be prepared by selective etching of PCM.

[0126] Fabrication of the composite first comprises immersion of the continuous BN(C) structure in a bath of PCM 5 contained in a container 3 placed over a hot plate 4. Full immersion is performed as in the second embodiment (FIG. 7). The material is then removed from the bath: it is composed of the BN(C) structure impregnated over its entire thickness with PCM 5. The surface portions of the material thus obtained are immersed in a bath of etching solution 6, allowing the PCM to be dissolved on the immersed portions. The lower and upper faces can be successively immersed to leave two lower and upper surface portions 1 of BN(C) free of PCM. In a variant of this embodiment, the material can be fully immersed, which means that all the surface portions are freed of PCM underneath all the faces of the material.

[0127] A third embodiment can be illustrated by combining the steps of FIGS. 1-6 and FIGS. 7-9.

[0128] The composite material thus formed can then be applied to an electronic component. In one variant illustrated in FIG. 10, the composite material is encapsulated between an aluminium cover 8 and an electronic component 7 e.g. a processor.

[0129] The component 7 has irregular surface relief. By compression, the surface portions 1 of the composite material fill the cavities and follow the contour of the roughness of the component 7.

[0130] Therefore, as illustrated in FIG. 11, the compressed surface portions 1 form layers 9 of BN(C) which are in contact with the component 7 and with the cover 8. This ensures electrical isolation, passivation of the component and reduced thermal contact resistance.

[0131] The following examples give a nonlimiting illustration of the present invention.

EXAMPLE 1: PREPARATION OF THE BN(C) FOAM

[0132] A BN foam was prepared by applying the methodology described by Loeblein et al., Small, vol. 10, n. 15, 2992-2999, 2014, without conducting the carbon growth step. PMMA was deposited just before etching the nickel for mechanical reinforcement of the BN. The PMMA can be removed or left in place after etching the nickel.

EXAMPLE 2: PREPARATION OF THE COMPOSITE

[0133] Strategy

[0134] To obtain the BNC foam infused with PCM (Phase Change Material) solely in the centre and not on the surface, the first strategy is to use a material which will protect the surfaces of the foam during infusion. This protective material is later removed.

[0135] Protection of the Foam Faces

[0136] PEO (Polyethylene Oxide) was used as protective material. It is first diluted in water in proportions allowing a polymer to be obtained with suitable viscosity, here between 20 and 25% PEO.

[0137] At a second step, the diluted polymer is placed in a vacuum at about 2.5 mTorr for 30 min. The purpose of this degassing step is to remove the air bubbles trapped in the polymer when mixing. Without this step, during the densification phase air bubbles could form, damage the foam and jeopardise the uniformity of polymer thickness.

[0138] At a third step, the polymer is deposited in an aluminium mould. The amount of polymer will depend on the size of the mould to reach a polymer thickness of about 3 mm. The foam is deposited on the polymer and will slightly penetrate the latter. The depth of penetration will depend on the viscosity of the polymer. Finally, the mould is placed over a hot plate to densify the polymer by gradually evaporating the solvent (here water). It was experimentally shown that a step at 80 C. for 40 min followed by a rise of 5 C. every 5 min to reach 120 C. is favourable. However, said temperature and time are dependent on the temperature probe of the hot plate and the laboratory environment, since everything takes place in air.

[0139] At step four, the foam with one protected face is removed from the mould. One of the faces is perforated with a needle. The purpose of these perforations is to promote later PCM infusion and only scarcely damage the foam.

[0140] The fifth step is the same as the third but on the opposite face of the foam.

[0141] Infusion of PCM in the Protected Foam

[0142] Paraffin was used as PCM.

[0143] The paraffin was heated to 110 C., i.e. slightly above the melting point of paraffin of 90 C., in the aluminium mould. Once the paraffin has changed to liquid phase, the foam with the two protected faces is immersed therein: the paraffin filters through the sides but also through the perforated face which is held in the upper position. The foam is left for between 3 and 5 min in the PCM to ensure full infusion whilst preventing melting of the protective polymer. Finally, it is left to cool naturally or in a refrigerator to accelerate cooling.

[0144] Removal of the Protective Polymer

[0145] To remove the polymer, the protected foam is immersed in water at ambient temperature. The compound is maintained vertically (to avoid damaging the surfaces) in a beaker of water overnight. The water bath is renewed and left to act overnight a further time to improve deprotection as a function of the thickness of the polymer, the size of the sample and amount of water. Finally, the sample is left to dry.

EXAMPLE 3: CHARACTERIZATIONS/PERFORMANCE OF THE COMPOSITE

[0146] Thermal Characterizations: [0147] Measurement of the density of the end compound. To show that the foam only scarcely modifies the weight of the PCM alone. [0148] Measurement of the latent heat of fusion of the compound. For the same reason, which is to show the low impact of the foam on the thermal storage capacity of the PCM. It is sought to maintain the latent heat of fusion of the PCM. [0149] Measurement of thermal conductivity to show the advantage and contribution made by the foam. [0150] Measurement of contact resistance to verify the capability of the compound to conform to surfaces.

[0151] Electrical Characterizations: [0152] To evaluate the electrical conductance of the compound and confirm its isolating nature for pure BN(C) and slightly conductive for BNC. Similarly, for validation of the isolating or slightly conductive areas in the event of localized doping. [0153] Radiofrequency measurements (losses, transmissions) to determine the impact of the presence of the compound in an electronic environment.

[0154] Physical Characterizations: [0155] Thermal expansion coefficient of the compound for future package design. [0156] Compressive and tensile mechanical strength. [0157] Visualisation of the conformability of the foam released on the surface.

EXAMPLE 4: PREPARATION OF THE COMPOSITE WITHOUT PROTECTIVE MATERIAL, BY ETCHING

[0158] For this method, the continuous BN(C) structure is first infused with PCM in liquid phase, at a temperature higher than the melting temperature of the PCM. The compound obtained is immersed in an ethanol bath at 65 C., allowing selective etching of the PCM in relation to the continuous structure. Each PCM face is etched at a rate of about 5 m/min. One bath hour allows the release of about 300 m of surface portion on each face of the compound. Thereafter, the compound is successively immersed in several ethanol baths at 65 C. for a few minutes to remove re-deposits of PCM on the surface portions. Finally, the compound is dried in an oven at 50 C. for 1 hour.

EXAMPLE 5: PREPARATION OF THE COMPOSITE WITH PROTECTIVE MATERIAL AND BY ETCHING

[0159] This method combines the two preceding preparations so that it is possible to obtain surface portions of different thicknesses.

[0160] First, the continuous BN(C) structure is infused on the surface with liquid NPG over a hot plate at a temperature of about 130 C. Typically, the NPG infuses the continuous structure over a thickness of 1 to 2 mm. This step is repeated on the two opposite faces of the structure. The structure thus protected is immersed in the liquid PCM at 110 C. since its melting temperature is 90 C. in this Example. After infusion and solidification of the PCM, the compound is immersed in an ethanol bath at 65 C. The NPG dissolves almost instantly on the two protected faces releasing the surface portion over a thickness of 1 to 2 mm on these faces, the other faces being etched at a rate of about 5 m/min. This makes it possible to obtain surface portions of different thicknesses.

EXAMPLE 6: FABRICATION OF A COMPONENT COMPRISING THE COMPOSITE

[0161] The invention can be applied to a power transistor dissipating 20 W for example when in cyclic use, e.g. for continuous operation of less than 15 min, with a cooling time of 15 min. The PCM is chosen as a function of the maximum critical temperature of the transistor: the melting temperature of the PCM must be equal to or lower than the critical temperature of the transistor. The material of the invention is applied directly onto the transistor, with one of the PCM-free faces in contact with the transistor to ensure good thermal contact. The surround of the PCM is encapsulated as well as the base of the processor to ensure sealing.