SELF-SINTERED THERMAL INTERFACE MATERIALS

Abstract

Self-sintering thermal interface material is prepared from a polymer matrix and a filler. The polymer matrix may be a water or hydrogen peroxide or other water-contained solvent solution such as ammonia or alcohol containing a water soluble resin and fumed silica or an alcohol or other solvent solution containing at least one water insoluble resin and fumed silica. The filler contains i) gallium alkali metal or gallium metal, ii) one or more micro/nano-sized metallic fillers, and iii) dielectric fillers.

Claims

1. A self-sintering thermal interface material comprising a polymer matrix and a filler; wherein the polymer matrix comprises a water or hydrogen peroxide solution comprising at least one water soluble resin and fumed silica; wherein the filler comprises i) gallium-alkali metal or gallium metal and ii) one or more micro/nano-sized metallic fillers.

2. The self-sintering thermal interface material of claim 1 wherein the self-sintering thermal interface material self-sinters when a temperature is raised above the eutectic reaction temperature of the gallium-alkali metal or the melting point of the gallium metal.

3. The self-sintering thermal interface material of claim 1 wherein the water soluble resin comprises water-soluble poly(ethylene oxide) resin.

4. The self-sintering thermal interface material of claim 1 wherein the thermal interface material is electrically conductive, the polymer matrix comprises 3-20 wt. % water soluble resin and 0-10 wt. % fumed silica, and the filler comprises 5-35 wt. % gallium-alkali metal alloy or gallium metal and 30-95 wt. % micro/nano-sized metallic fillers.

5. The self-sintering thermal interface material of claim 1 wherein the thermal interface material is dielectric, the polymer matrix comprises 3-20 wt. % water soluble resin and 0-10 wt. % fumed silica, and the filler comprises 5-35 wt. % gallium-alkali metal alloy or gallium metal, 5-15 wt. % micro/nano-sized metallic fillers, and 10-90 wt. % dielectric fillers.

6. The self-sintering thermal interface material of claim 1 wherein the gallium-alkali metal is selected from the group consisting of Ga—Al, Ga—AlTiC, Ga—Al—Ti—B, Ga—Mg, Ga—In—Al, Ga—In—Sn—Al, Ga—Zn, Ga—Fe, Ga—Li, Ga—K, Ga—Ba, Ga—Ca, and Ga—Na.

7. The self-sintering thermal interface material of claim 6 wherein the gallium-alkali metal further comprises at least one selected from the group consisting of In, Sn, Ti, B, C, Ag, Cu, Fe, Si, Pb, Zn, Ni, Cr, Bi, and rare earth elements.

8. The self-sintering thermal interface material of claim 1 wherein the micro/nano-sized metallic fillers include at least one selected from the group consisting of BN, AN, Al.sub.2O.sub.3, BeO, SiC, Si, diamond, and hybrids thereof.

9. The self-sintering thermal interface material of claim 1 wherein the micro/nano-sized metallic fillers include at least one selected from the group consisting of silver, copper, aluminum, gold, zinc, nickel and alloys thereof.

10. The self-sintering thermal interface material of claim 1 wherein the filler comprises gallium alkali metal.

11. The self-sintering thermal interface material of claim 1 wherein the filler comprises gallium metal.

12. The self-sintering thermal interface material of claim 1 further comprising at least one selected from the group consisting of KOH, KCl, NaCl, HCl, Ba.sub.2Cl.sub.2, BiOCl, NaBH.sub.4, NaMgH.sub.3, Al(OH).sub.3.

13. A method of forming a self-sintering thermal interface material comprising: mixing a water-soluble resin and fumed silica with water, hydrogen peroxide, or water-contained ammonia or alcohol to form a water absorbing gel; and mixing the water absorbing gel with micro/nano-sized metallic fillers and gallium-alkali metal or gallium metal, at a temperature below the eutectic reaction temperature of the gallium-alkali metal or below the melting point of the gallium metal, to form a mixture.

14. The method of claim 13 further comprising applying the mixture to a substrate and then raising the temperature above the eutectic reaction temperature of the gallium-alkali metal or above the melting point of the gallium metal until the mixture self-sinters.

15. A self-sintering thermal interface material comprising a polymer matrix and a filler; wherein the polymer matrix comprises an alcohol solution comprising at least one water insoluble resin and fumed silica; wherein the filler comprises i) gallium-alkali metal or gallium metal and ii) one or more micro/nano-sized metallic fillers.

16. The self-sintering thermal interface material of claim 15 wherein the self-sintering thermal interface material self-sinters when a temperature is raised above the eutectic reaction temperature of the gallium-alkali metal or the below the melting point of the gallium metal.

17. The self-sintering thermal interface material of claim 15 wherein the water insoluble resin comprises ethyl cellulose.

18. The self-sintering thermal interface material of claim 15 wherein the alcohol solution is 70% isopropyl alcohol water solution or 91% isopropyl alcohol water solution.

19. The self-sintering thermal interface material of claim 15 further comprising a water-soluble resin.

20. The self-sintering thermal interface material of claim 15 wherein the thermal interface material is electrically conductive, the polymer matrix comprises 65-100 wt. % water-insoluble resin, 0-30 wt. % water soluble resin, and 0-10 wt. % fumed silica, and the filler comprises 5-35 wt. % gallium-alkali metal or gallium metal and 30-90 wt. % micro/nano-sized metallic fillers.

21. The self-sintering thermal interface material of claim 15 wherein the thermal interface material is dielectric, the polymer matrix comprises 3-20 wt. % water soluble resin and 0-10 wt. % fumed silica, and the filler comprises 5-35 wt. % gallium-alkali metal or gallium metal, 5-15 wt. % micro/nano-sized metallic fillers, and 10-90 wt. % dielectric fillers.

22. The self-sintering thermal interface material of claim 21 wherein the gallium-alkali metal is selected from the group consisting of Ga—Al, Ga—AlTiC, Ga—Al—Ti—B, Ga—Mg, Ga—In—Al, Ga—In—Sn—Al, Ga—Zn, Ga—Fe, Ga—Li, Ga—K, Ga—Ba, Ga—Ca, and Ga—Na.

23. The self-sintering thermal interface material of claim 22 wherein the gallium-alkali metal further comprises at least one selected from the group consisting of In, Sn, Ti, B, C, Ag, Cu, Fe, Si, Pb, Zn, Ni, Cr, Bi, and rare earth elements.

24. The self-sintering thermal interface material of claim 21 wherein the micro/nano-sized metallic fillers include at least one selected from the group consisting of BN, AN, Al.sub.2O.sub.3, BeO, SiC, Si, diamond, and hybrids thereof.

25. The self-sintering thermal interface material of claim 21 wherein the micro/nano-sized metallic fillers include at least one selected from the group consisting of silver, copper, aluminum, gold, zinc, nickel and alloys thereof.

26. The self-sintering thermal interface material of claim 21 wherein the filler comprises gallium alkali metal.

27. The self-sintering thermal interface material of claim 21 wherein the filler comprises gallium metal.

28. The self-sintering thermal interface material of claim 21 further comprising at least one selected from the group consisting of KOH, KCl, NaCl, HCl, Ba.sub.2Cl.sub.2, BiOCl, NaBH.sub.4, NaMgH.sub.3, Al(OH).sub.3.

29. A method of forming a self-sintering thermal interface material comprising: mixing a water-insoluble resin and fumed silica, dielectric filler, and micro/nano-sized metallic fillers with an alcohol solution or an equivalent solvent such as acetone, ionic liquids, NaOH/urea, ammonium hydroxide, phosphoric acid, acetic acid, formic acid, pyridine, aromatic hydrocarbons, halogenated hydrocarbons, and ketones; and mixing the alcohol solution with micro/nano-sized metallic fillers and gallium-alkali metal or gallium metal, at a temperature below the eutectic reaction temperature of the gallium-alkali metal or below the melting point of the gallium metal, to form a mixture.

30. The method of claim 29 further comprising applying the mixture and then raising the temperature above the eutectic reaction temperature of the gallium-alkali metal or above the melting point of the gallium metal until the mixture self-sinters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 shows an example illustrated flow chart of a method of preparing self-sintered dielectric thermal interface materials via exothermic reaction in accordance with aspects of the disclosure.

[0019] FIG. 2A depicts thermal expansion versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials.

[0020] FIG. 2B depicts electric field strength versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials.

[0021] FIG. 2C depicts glass transition temperature versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials.

[0022] FIG. 2D depicts electrical resistivity versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials.

[0023] FIG. 2E depicts Young's modulus versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials.

[0024] FIG. 3A depicts self-sintered thermal interface material of Sample 8 in accordance with an aspect of the disclosure.

[0025] FIG. 3B depicts self-sintered thermal interface material of Sample 9 in accordance with an aspect of the disclosure.

[0026] FIG. 4A depicts a water soluble gel mixed with dielectric and bonder particles prepared in accordance with the flow chart of FIG. 1.

[0027] FIG. 4B depicts a water soluble gel mixed with dielectric and bonder particles and Ga-alkali metal prepared in accordance with the flow chart of FIG. 1.

[0028] FIG. 4C depicts a self-sintered TIM prepared in accordance with the flow chart of FIG. 1.

DESCRIPTION

[0029] Aspects of the disclosure provide self-sintered thermal interface materials (TIMs) mechanized by adhesive polymer self-curing, liquid metal-thermal filler sintering, and exothermic reaction between alkali metal and water constituents. TIMs may be formulated for a broad range of electrically conductive (electromagnetic interference (EMI) noise protection) and dielectric (semi-conductive or electrical insulation) thermal interface materials.

[0030] The self-sintered TIMs can be processed and applied at a low temperature or ambient atmosphere, providing a wide ranges of tailored properties including extremely low thermal impedance, ultrahigh thermal conductivity, good conformability with minimum thermal expansion stress when joining two contact surfaces, great adhesion, and controlled sensitivity to moisture and temperature changes for different service life applications. By eliminating elevated temperature sintering, optical/ultra-violet curing, and/or pressure requirements associated with present common thermal interface materials, the self-sintered thermal interface materials provide low cost and good manufacturability advantages.

[0031] Aspects of the disclosure relate to composition formulations, fabrication processes, and suitable application scenarios of self-sintered thermal interface materials.

[0032] Electrically Conductive TIMs

[0033] Electrically conductive self-sintered thermal interface materials formed by non-exothermic reaction, which are sensitive to water/moisture or insensitive to water/moisture, may be prepared with a polymer matrix and fillers containing gallium metal as shown in, for example, Table I below. In particular, the gallium metal will melt when the temperature is increased to above room temperature.

TABLE-US-00001 TABLE I Thermal Adhesion Water Polymer Conductivity to moisture Matrix Filler (W/m-K) Substrate sensitivity MFRG Application (3-20) wt. % (5-35) ≥60-100 Strong High Simple Anti-temper WSR + wt. % liquid or easy to (0-10) wt. % metal Ga + repair; Fumed silica (30-95) electrical water/H.sub.2O.sub.2 wt. % connectivity; solution micro/nano EMI noise metallic protection fillers (0-30) wt. % (5-35) ≥60-100 Strong Low to Simple Long service WSR + (65- wt. % liquid none life under 100) wt. % metal Ga + water or high WIR + (0-10) (30-95) humidity wt. % Fumed wt. % atmosphere; silica + micro/nano electrical (0-5) wt. % metallic connectivity; KOH Alcohol fillers EMI noise solution protection (70-91% Isopropyl)

[0034] Electrically conductive self-sintered thermal interface materials formed by exothermic reaction, which are sensitive to water/moisture or insensitive to water/moisture, may be prepared with a polymer matrix and fillers containing gallium alkali metal as shown in, for example, Table II below. In particular, the gallium alkali metal will react with water when the temperature is raised above the eutectic reaction temperature of the gallium alkali metal.

TABLE-US-00002 TABLE II Thermal Adhesion Water Polymer Conductivity to moisture Matrix Filler (W/m-K) Substrate sensitivity MFRG Application (3-20) wt. % (5-35) ≥60-120 Strong High Complex Anti-temper WSR + wt. % Ga- or easy to (0-10 ) wt. % alkali metal + repair; Fumed silica (30-95) electrical water/H.sub.2O.sub.2 wt. % connectivity; solution micro/nano EMI noise metallic protection fillers (0-30) wt. % (5-35) ≥60-120 Strong Low to Complex Long service WSR + (65- wt. % Ga- none life under 100) wt. % alkali metal + water or high WIR + (0-10) (30-95) humidity wt. % Fumed wt. % atmosphere; silica + micro/nano electrical (0-5) wt. % metallic connectivity; KOH Alcohol fillers EMI noise solution protection (70-91% Isopropyl)

[0035] The thermal conductivity of self-sintered electrically conductive TIMs can be 60-100 W/m-K or more.

[0036] Dielectric TIMs

[0037] Dielectric self-sintered thermal interface materials formed by non-exothermic reaction, which are sensitive to water/moisture or insensitive to water/moisture, may be prepared with a polymer matrix and fillers containing gallium metal as shown in, for example, Table III below. In particular, the gallium metal will melt when the temperature is increased to above room temperature.

TABLE-US-00003 TABLE III Thermal Adhesion Water Polymer Conductivity to moisture Matrix Filler (W/m-K) Substrate sensitivity MFRG Application (3-20) wt. % (5-35) wt. % ≥160-200 Strong High Simple Anti-temper WSR + liquid metal or easy to (0-10) wt. % Ga + (5-15) repair; Fumed silica wt. % electrical water/H.sub.2O.sub.2 micro/nano insulation solution metallic fillers + (10-90) wt. % dielectric fillers (0-30) wt. % (5-35) wt. % ≥160-200 Strong Low to Simple Long service WSR + (65- liquid metal none life under 100) wt. % Ga + (5-15) water or high WIR + (0-10) wt. % humidity wt. % Fumed micro/nano atmosphere; silica + metallic electrical (0-5) wt. % fillers + insulation KOH Alcohol (10-90) solution wt. % (70-91% dielectric Isopropyl) fillers

[0038] Dielectric self-sintered thermal interface materials formed by exothermic reaction, which are sensitive to water/moisture or insensitive to water/moisture, may be prepared with a polymer matrix and fillers containing gallium alkali metal as shown in, for example, Table IV below. In particular, the gallium alkali metal will react with water when the temperature is raised above the eutectic reaction temperature of the gallium alkali metal.

TABLE-US-00004 TABLE IV Thermal Adhesion Water Polymer Conductivity to moisture Matrix Filler (W/m-K) Substrate sensitivity MFRG Application (3-20) wt. % (5-35) wt. % ≥160-220 Strong High Complex Anti-temper WSR + Ga-alkali metal + or easy to (0-10) wt. % (5-15) wt. % repair; Fumed silica micro/nano electrical water/H.sub.2O.sub.2 metallic insulation solution fillers + (10-90) wt. % dielectric fillers (0-30) wt. % (5-35) wt. % ≥160-220 Strong Low to Complex Long service WSR + (65- Ga-alkali metal + none life under 100) wt. % (5-15) wt. % water or WIR + (0-10) micro/nano high wt. % Fumed metallic humidity silica + fillers + atmosphere; (0-5) wt. % (10-90) wt. % electrical KOH Alcohol dielectric insulation solution fillers (70-91% Isopropyl)

[0039] The thermal conductivity of self-sintered dielectric TIMs can be 160-200 W/K or more.

[0040] Exothermic Reaction: To prepare an electrically conductive TIM via exothermic reaction, the filler may contain 5-35 wt. % Ga-alkali metal and 30-95 wt. % micro/nano-sized metallic fillers. To prepare a dielectric TIM via exothermic reaction, the filler may contain 5-35 wt. % Ga-alkali metal, 5-15 wt. % micro/nano-sized metallic fillers, and 10-90 wt. % dielectric fillers. A polymer matrix containing water is mixed with the filler. An exothermic reaction results when the temperature is raised to above the eutectic reaction temperature of the gallium alkali metal.

[0041] Non-Exothermic Reaction: To prepare an electrically conductive TIM with a non-exothermic reaction, the filler may contain 5-35 wt. % gallium metal and 30-95 wt. % micro/nano-sized metallic fillers. To prepare a dielectric TIM with non-exothermic reaction, the filler may contain 5-35 wt. % gallium metal, 5-15 wt. % micro/nano-sized metallic fillers, and 10-90 wt. % dielectric fillers. A polymer matrix and filler is combined. Upon increasing the temperature to above room temperature, the gallium metal melts and reacts with other components.

[0042] Filler Materials

[0043] Filler materials may have physical dimensions small enough so that a consistent mixture may be formed within the TIM. The micro-/nano-sized conductive fillers may provide electrical and/or thermal conductivities. Conductive fillers may include, for example, nanoparticles, nanowires/whiskers, and/or micron size particles that are highly conductive. Micro-/Nano-sized dielectric fillers may be used to provide high thermal conductivities such as include boron nitride (BN), aluminum nitride (AlN), Al.sub.2O.sub.3, BeO, SiC, Si, diamond, graphite, carbon nanotubes, few-layer graphene (FLG) and hybrids comprising them, such as SiC/diamond. Micro/nano-sized metallic fillers may be used for electrically conductive TIMs or as a bonder for dielectric TIMs to enhance adhesion and bonding between dielectric fillers such as silver, copper, aluminum, gold, zinc, nickel; or their alloys; or a combination of them.

[0044] In general, particle sizes of micro/nano-sized metallic fillers may range from 20 nm to 2000 μm. The particle size is usually from 100 nm to 100 μm due to the combined considerations of material processing cost, performance, and easy operation. The minimum particle size may be as low as 4 nanometers or even lower, but at least 20 nm may be more common for commercial applications, mainly due to the consideration of material processing cost and operation difficulty.

[0045] Gallium

[0046] Gallium metal melts just above room temperature (˜29.8° C.) and can be supercooled down to about 15° C. (still keeping liquid state). Gallium metal can be added as a liquid or solid but is easily melted by raising the temperature above room temperature. Thus the gallium metal melts without an exothermic reaction (i.e. a non-exothermic reaction.) Components within the mixture are wetted or reacted with liquid gallium metal (in a non-exothermic reaction) to enhance the connectivity between filler particles.

[0047] Gallium-alkali metal (aluminum for example) based low melting point alloys may be used to provide non-oxidized metal (fresh aluminum for example) for exothermic water reaction to trigger self-sintering and may be used for liquid metal fusion bonding for metallic binders and dielectric fillers. The gallium based alloy is also helpful to improve adhesion performance or bonding strength between the substrates.

[0048] The gallium-alkali metal alloy has a low melting point and provides non-oxidized metal for exothermic water reaction to trigger self-sintering and liquid metal fusion. The exothermic reaction provides the heat to allow the conductive fillers to melt and form liquid metal. The liquid metal can flow to fill voids and modify connectivity of the conductive network to enhance electrical and thermal conductivities. The resulting concentration and distribution of the liquid metal can be tuned to improve flexibility and stretchability. The gallium-alkali alloy may also improve adhesion performance or bonding strength between substrates.

[0049] The melting point range of the gallium-alkali metal may be −15° C. to 300° C. Commercially, the melting point of low melting alloys is usually below 150° C. In aspects described herein, the melting point can be below 85° C., or around room temperatures (e.g. 23 to 35° C.), especially for some polymer substrates with low glass transition temperatures.

[0050] The gallium-alkali metal generally contains 1-50 wt % alkali metal and additional elements, with a melting point range of −15° C. to 300° C. Examples of the gallium-alkali metal alloys include Ga—Al, Ga—AlTiC, Ga—Al—Ti—B, Ga—Mg, Ga—Zn, Ga—Fe, Ga—Li, Ga—K, Ga—Ba, Ga—Ca, and Ga—Na with or without a combination of other elements, such as In, Sn, Ti, B, C, Ag, Cu, Fe, Si, Pb, Zn, Ni, Cr, Bi, and rare earth elements etc.

[0051] Polymer Compositions

[0052] TIMs may be formulated from water-soluble resin (WSR), water-insoluble resin (WIR), or both as a matrix, to provide the desired properties. Properties may range from, for example, water/moisture sensitive to completely insensitive, and opaque to high optical transparency. The TIMs may be formulated for other properties, such as anti-temper or easy to repair, long service life under water or high humidity atmosphere, high electrical connectivity or insulation, and EMI noise protection.

[0053] Water Soluble Resins

[0054] Suitable water-soluble resins include water soluble poly(ethylene oxide). For instance, water-soluble poly(ethylene oxide) solution gel functions as a binder and suspending supporter for dielectric fillers, bonding additives, and gallium-alkali metal based low melting point alloys during molding/patterning/printing and provide water for the exothermic reaction during self-sintering. The ratio between the water or hydrogen peroxide and water-soluble poly(ethylene oxide) resin (WSR) in the solution gel, and the gel's concentration in the paste/ink can be tuned or manipulated to make the exothermic reaction provide high enough temperature for the self-sintering, meanwhile the formation of oxides, metal oxyhydrides, and H.sub.2 bubbles can be manipulated to minimize the volume of formed voids and remaining reactants after the self-sintering.

[0055] Water-soluble poly(ethylene oxide) solution gel may be a mixture of water (H.sub.2O) and/or hydrogen peroxide (H.sub.2O.sub.2) and/or alcohol (70% isopropyl) and/or ammonia (10% ammonium hydroxide), and 1-30 wt % soluble poly(ethylene oxide) polymer e.g., having a general composition of 95% to 100% poly(ethylene oxide) and up to 5% fumed silica. Industrial water soluble resin such as POLYOX™ WSR N750 or POLYOX™ WSR 301 may be used.

[0056] Other water-soluble materials such as phosphorus oxoacid compound, halogen compound, gelatin, polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone (PVP), carrageenan, carboxylmethyl cellulose, hydroxylpropyl cellulose, may be used as additional additives. Other additives such as KOH, KCl, NaCl, HCl, Ba.sub.2Cl.sub.2, BiOCl, NaBH.sub.4, NaMgH.sub.3, Al(OH).sub.3 may be added to the water-soluble poly(ethylene oxide) solution gel to enhance the exothermic reaction (changing the reaction strength and temperature) and promote the self-sintering at relatively low temperatures.

[0057] To form a moisture sensitive TIM, a polymer matrix may be formed by combining 3-20 wt. % water-soluble resin and 0-10 wt. % fumed silica in a water or hydrogen peroxide or alcohol or other water-contained solvent solution.

[0058] Water Insoluble Resins

[0059] Water-insoluble resins (WIR) may be used for formulating electrically conductive and dielectric materials with long term chemical stability in a high humidity or wet environment. Water-insoluble resins may be a mixture of 91% isopropyl alcohol water solution and/or ammonium hydroxide and or NaOH/urea, and (65-100) wt % ETHOCEL™ Standard 200 Industrial Ethylcellulose (>98.0 cellulose, ethylether), (0-35) wt % soluble poly(ethylene oxide) polymer up to 3% fumed silica. ETHOCEL™ Standard 200. Industrial ethylcellulose can be used to fabricate TIMs with an optimal transparent matrix, and functions as (a) an adhesive to provide strength, viscosity, and rheology to alcohol solvent-based formulations of TIMs; (b) binder and rheology modifier (may be able to provide clean burn out or removal undesired constituents) in the dielectric TIMs; (c) surface coating for exposed surface of TIMs to provide waterproofing, toughness and flexibility.

[0060] To form a moisture insensitive TIM, a polymer matrix may be formed by combining 65-100 wt. % water-insoluble resin, 0-30 wt. % water-soluble resin, 0-10 wt. % fumed silica, and 0 to 5 wt. % potassium hydroxide in an alcohol or other solvent solution, such as acetone, ionic liquids, NaOH/urea, ammonium hydroxide, phosphoric acid, acetic acid, formic acid, pyridine, aromatic hydrocarbons, halogenated hydrocarbons, and ketones.

[0061] Additives

[0062] Other additives and solvents may be added to the water-soluble resin solution gel or water-insoluble resin solution gel, to adjust and obtain the desired rheological, wetting, healing, or stretching properties to the pastes/inks for different molding, patterning, and/or printing technologies. Other polymeric binders, such as acrylic, silicone, styrene, fluoroelastomers, or urethane backbones, may be added to help in homogeneous dispersion of the fillers and the Gallium based low melting point alloys (both liquid and solid) into the paste/ink, to hold the paste/ink components together upon solvent evaporation and also help bind the molded/printed patterns onto the substrate. In addition to water, other paste/ink solvents may be used to provide enhanced solubility to the water-soluble polymer or other polymeric binder and impart favorable viscosity, surface tension, and homogeneity. Other additives may be also included to further impart desired rheological, wetting, healing, or stretching properties to the inks. Additives in the form of surfactants, adhesion improvers, humectants, penetration promoters, and stabilizers are used to tailor the ink properties for specific applications.

[0063] Manufacturing Process

[0064] FIG. 1 is an illustrative flow chart illustrating an example of a process for making and using self-sintering dielectric thermal interface materials formulated through exothermic reaction and sensitive to water/moisture.

[0065] Water or hydrogen peroxide may be combined with a water soluble resin and mixed to form a water-soluble polymer gel. Fillers and bonding additives may be mixed in the water-soluble polymer gel. FIG. 4A depicts a water-soluble gel containing dielectric fillers and bonder particles. The gel is then mixed with gallium-alkali metal below the eutectic reaction temperature. FIG. 4B depicts a water-soluble gel containing dielectric fillers, bonder particles, and gallium-alkali metal. The constituents can be stably mixed and stored. The mixture can be molded, patterned, and/or printed below the gallium-alkali metal eutectic reaction temperature (˜26.8° C. for Ga—Al eutectic reaction) to avoid the exothermic reaction. The molded, patterned, and/or printed product may be self-sintered when the environmental temperature is above the gallium-alkali metal eutectic reaction temperature which triggers the exothermic reaction and liquid metal fusion. FIG. 4C depicts a self-sintered product. The relatively higher temperature (˜100° C. for example) can be used for the self-sintering to reduce the sintering time from about 1-3 hours (self-sintering at ˜30° C. for Ga—Al eutectic reaction) to several minutes or less for a 1-3 mm thick dielectric film, and even to seconds or less for a much thinner film on the order of 100 μm or less. The thinner the sample, the quicker the sintering. Different material constituents and the percentage of the concentrations can affect the sintering time.

Example 1

[0066] Self-sintered dielectric thermal interface materials (sensitive to water/moisture) are formulated via the following process including an exothermic reaction. POLYOX™ WSR N750 powder, dielectric filler, and nickel silver binder powder are mixed with water to form a uniform gel mixture at room temperature. Gallium-aluminum metal (Ga-2 wt % Al.sub.3Ti.sub.0.5C for example) is added to the gel mixture and mixed under a controlled temperature of 15-20° C. The temperature of the paste and substrate materials is controlled to less than 20° C. prior to and during loading of paste mixture into the syringe of the printer. As an example, a 1-3 mm thick sample is molded or printed and maintained at room temperature for 0.1-0.3 hours for self-sintering. Room temperature is generally 30-35° C. These materials may be much thinner, such as on the order of 100 um or less.

[0067] Self-sintered thermal interface materials were prepared in accordance with the above process and compared with the prior art. FIG. 2A depicts thermal expansion versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials. The elastic nature of the inventive TIMs enables them to conformably coat contact surfaces, while simultaneously minimizing thermal expansion stress when joining two contact surfaces. FIG. 2B depicts electric field strength versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials. This provides quantitative insight on how the dielectric properties of the invented TIMs surpass other leading materials in dielectric strength and voltage breakdown resistance. FIG. 2C depicts glass transition temperature versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials. This provides quantitative insight on how the thermal stability of the invented TIMs can be is on par with other leading materials. FIG. 2D depicts electrical resistivity versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials. This quantifies how the invented dielectric TIMs can be customized with high electrical resistivity equivalent to or surpassing other leading materials. FIG. 2E depicts Young's modulus versus thermal conductivity of the self-sintered dielectric and conductive thermal interface materials. This provides quantitative insight on how the mechanical properties of the invented TIMs can be tuned from flexible to highly rigid versus the static properties of other leading materials.

Example 2

[0068] Eight samples were prepared in accordance with Tables V and VI below.

[0069] Table V depicts thermally conductive/electrically conductive TIMs.

TABLE-US-00005 TABLE V Pull test- Resin Composite Thermal bonding Matrix material conductivity Electrical strength Water Sample formulation formulation (W/m-K) resistivity on Cu soluble 1 15 wt. % WSR + 35 wt. % [Resin 91.77 0.3-50 ~20 Yes 5 wt. % Fumed Matrix] + Ω/10 mm lb/1 cm.sup.2 silica 25 wt. % Ga + on the block water solution 35 wt. % Ag sample (9 μm flake) surface 2 15 wt. % WSR + 5 wt. % [Resin 91.76 0.3-50 >30 No* 5 wt. % Fumed Matrix] + Ω/10 mm lb/1 cm.sup.2 silica + 80 wt. % 25 wt. % Ga + on the block (Cu foil Alcohol solution 35 wt. % Ag sample broken, (9 μm flake) surface sample not) 3 15 wt. % WIR + 35 wt. % [Resin 61.15 0 ~1 No 5 wt % Fumed Matrix] + (Rigid) Ω/10 mm lb/1 cm.sup.2 silica + 80 wt. % (70% 25 wt. % Ga + on the block Isopropyl alcohol) 35 wt. % Ag sample (9 μm flake) surface 8 15 wt. % WIR + 35 wt. % [Resin 117.20  0.02-0.03 >25 No 5 wt. % Fumed Matrix] + Ω/10 mm lb/1 cm.sup.2 silica + 80 wt. % (91% 15 wt. % Ga + on the block (Cu foil Isopropyl alcohol) 50 wt. % Ag sample broken) (9 μm flake) surface *Softened in water but hardened after drying

[0070] Table VI depicts thermally conductive/dielectric TIMs.

TABLE-US-00006 TABLE VI Pull test- Resin Composite Thermal bonding Matrix material conductivity Electrical strength Water Sample formulation formulation (W/m-K) resistivity on Cu soluble 9 15 wt. % WIR + 35 wt. % [Resin 197.44 >100,000 ~20 No 5 wt. % Fumed Matrix] + 5 wt. % Ω/10 mm lb/1 cm.sup.2 silica + 80 wt. % (91% Ga + 5 wt. % Ag on the block Isopropyl alcohol) (9 μm flake) + 30 sample wt. % h-BN (3M surface 012, 14 μm) + 20 wt. % h-BN (3M 500-3, 300 μm) 4 15 wt. % WSR + 40 wt. % [Resin 162.1 >100,000 ~20 Yes 5 wt. % Fumed Matrix] + 5 wt. % Ω/10 mm lb/1 cm.sup.2 silica Ga + 5 wt. % Ag on the block water solution flake (9 μm) + 30 sample wt. % h-BN (3M surface 012, 14 μm) + 20 wt. % h-BN (3M 500-3, 300 μm) 5 15 wt. % WSR + 40 wt. % [Resin 185.5 >100,000 >22 No* 5 wt. % Fumed Matrix] + 5 wt. % Ω/10 mm lb/1 cm.sup.2 silica + 80 wt. % Ga + 5 wt. % Ag on the block (Cu foil Alcohol solution flake(9 μm) + 30 sample broken) wt. % h-BN (3M surface 012, 14 μm) + 20 wt. % h-BN (3M 500-3, 300 μm) 6 15 wt. % WIR + 40 wt. % [Resin 174.38 >100,000 >24 No* 5 wt. % Fumed Matrix] + 5 wt. % Ω/10 mm lb/1 cm.sup.2 silica + 80 wt. % (70% Ga + 5 wt. % Ag on the block (Cu foil Isopropyl alcohol) flake(9 μm) + sample broken) 50 wt % h-BN surface (3M 075, 8 μm) 7 15 wt. % WIR + 40 wt. % [Resin 197.06 >100,000 >21 No* 5 wt. % Fumed Matrix] + 5 wt. % Ω/10 mm lb/1 cm.sup.2 silica + 80 wt. % (91% Ga + 5 wt. % Ag on the block (Cu foil Isopropyl alcohol) + flake (9 μm) + 30 sample broken) 5 wt % KOH solution wt. % h-BN (3M surface 075, 8 μm) + 20 wt. % h- BN(3M500-3, 300 μm)

[0071] Both electrically conductive TIM Sample 8 and dielectric TIM Sample 9 were prepared with a water insoluble resin (WIR) matrix: 15 wt. % WIR+5 wt. % Fumed silica+80 wt. % (91% isopropyl alcohol) and tested. Two coupons of the electrically conductive TIM and three coupons of the dielectric TIM were molded and tested for thermal conductivity. FIG. 3A depicts self-sintered thermal interface material of Sample 8 in accordance with an aspect of the disclosure. FIG. 3B depicts self-sintered thermal interface material of Sample 9 in accordance with an aspect of the disclosure.

[0072] These samples were used to measure and evaluate the adhesion between the TIM and copper substrate, shear strength of the TIM, electrical and thermal conductivities.

[0073] Aspects of the present disclosure relate to formulation, process, and application of self-sintering thermal interface materials.

[0074] The self-sintered thermal interface materials can strongly adhere to many different substrates, such as metals or alloys (copper, aluminum, stainless steel, etc.), polymers (polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polyethersulfone (PES), polyimide (PI) and polyarylate (PAR), and polydimethylsiloxane (PDMS) as stretchable substrate, etc.), glasses, ceramics, papers, and textile, etc.

[0075] Self-sintered thermal interface materials achieve thermal conductivities greater than 150 W/m-K versus leading industry materials that are nominally 60 W/m-K without elevated temperature sintering, optical/ultra-violet curing and/or pressure requirements for formation and interface bonding resulting in better performance, ease of manufacturing, and lower cost.

[0076] Self-sintered thermal interface materials can be tailored to provide extremely low thermal impedance, ultrahigh thermal conductivity, good conformability with minimum thermal expansion stress when joining two contact surfaces, adequate adhesion, controlled sensitivity to moisture and temperature changes for different service life applications, and doable manufacturability under ambient atmosphere.

[0077] Self-sintered thermal interface materials can be used for thermal management technologies to be used in different applications such as the LED lighting, photovoltaics, lasers, telecommunications equipment, automotive electronics, industrial computing, defense and aerospace electronics, consumer and mobile handheld electronics, medical electronics, wireless sensor networks, PCB testing equipment, energy harvesting storage equipment, as well as 5G or beyond technologies.

[0078] Based on the tunable concentration of the gallium-alkali metal constituent, some type of the self-sintered thermal interface materials can be used for anti-tamper and emergency temperature response like bringing an auxiliary cooling unit online.

[0079] Since self-sintered thermal interface materials do not require elevated temperature sintering, optical/ultra-violet curing and/or pressure for formation and interface bonding, they enable high performance thermal management in applications where the physical sensitivity or opaque nature of the component elements precluded conventional high performance thermal management solutions that do require elevated temperature sintering, optical/ultra-violet curing and/or pressure for formation and interface bonding. While not required, elevated temperature sintering, optical/ultra-violet curing and/or pressure for formation and interface bonding may further improve the properties of self-sintered materials.

[0080] The foregoing has been presented for purposes of example. The foregoing is not intended to be exhaustive or to limit features to the precise form disclosed. The examples discussed herein were chosen and described in order to explain principles and the nature of various examples and their practical application to enable one skilled in the art to use these and other implementations with various modifications as are suited to the particular use contemplated. The scope of this disclosure encompasses, but is not limited to, any and all combinations, subcombinations, and permutations of structure, operations, and/or other features described herein and in the accompanying drawing figures.