Pickering emulsion composition for heat dissipation, heat dissipation paste using the same, and method for producing the same

Abstract

The present invention relates to a Pickering emulsion composition for heat dissipation, a heat dissipation paste using the same, and a manufacturing method thereof. Specifically, the present invention provides a Pickering emulsion composition for heat dissipation containing silicone oil, an aliphatic alcohol, and a thermally conductive filler, a heat dissipation paste based on the Pickering emulsion, and a manufacturing method thereof. The thermally conductive filler in the Pickering emulsion forms a segregated network structure, ensuring high thermal conductivity and stability, making the paste suitable for use as a heat dissipation material in a variety of fields. Furthermore, the filler can be recovered and recycled from the heat dissipation paste using a simple centrifugation method.

Description

[Technical Field]

[0001] The present invention relates to a heat dissipation material technology, and more particularly to a Pickering emulsion composition for heat dissipation, a heat dissipation paste using the same, and a method for manufacturing the same. [Background technology]

[0002] In recent years, the integration of electronic components has led to an increase in the amount of heat generated per unit area of ​​equipment, i.e., heat density. If the heat generated during equipment use is not released in a short time, it can lead to overheating, reduced lifespan, reduced reliability, reduced performance, and even explosion. To solve this problem, it is important to use a thermal interface material (TIM) with high thermal conductivity to release the unnecessary heat generated during equipment operation from the heat source to the heat sink.

[0003] Until now, polymer composite materials have been widely used as TIM materials, as they combine the advantages of lightweight, easily moldable polymers with the advantages of metal / ceramic fillers. Polymer composite materials are made by impregnating polymer resins with metal or ceramic fillers, and are suitable as TIM materials because they can utilize the moldability and flexibility of polymers while exhibiting the thermal conductivity of the fillers.

[0004] Polymer composites can be divided into metal-based polymer composites and ceramic-based polymer composites depending on the filler used. Metal-based polymer composites use metal fillers such as gold (Au), silver (Ag), and copper (Cu), which can exhibit very high thermal conductivity. However, because they are electrically conductive, they are unsuitable for use in electronic devices that require insulation, and their high metal density makes it difficult to ensure lightweight construction. Ceramic-based polymer composites use ceramic fillers such as alumina, boron nitride, and silica. Unlike metal-based polymer composites, they can provide insulation, are stable in air, and are inexpensive, but their relatively low thermal conductivity requires a high filler content to ensure sufficient thermal conductivity.

[0005] TIMs come in various forms depending on the application, with typical forms including paste, phase change material (PCM), adhesive, and pad. TIMs are applied between the heat source and heat sink, but because the surfaces of the actual heat source and heat sink are very rough, it is very important to apply the TIM with minimal voids (air gaps) between the two interfaces. This is because the thermal conductivity of voids (air) is very low, preventing effective heat transfer. In other words, it is very important to apply a TIM with high thermal conductivity and effectively fill the gap between the two rough surfaces to maximize the actual contact area.

[0006] The TIM that fully meets these requirements is in paste form, and this thermal paste is typically a polymer composite material with ceramic filler dispersed in a low-molecular / high-molecular silicone matrix. The silicone matrix of silicone thermal paste has good application properties and excellent heat and weather resistance, but its thermal conductivity is very low at around 0.2 W / mK, making it difficult to achieve a thermal conductivity of 1.5-2 W / mK even with a high filler loading. Furthermore, its long-term stability under typical TIM application conditions, such as heat and pressure, is poor.

[0007] Therefore, there is a need for manufacturing technology for heat dissipation paste that has both high thermal conductivity and high stability. Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a heat-dissipating Pickering emulsion composition that has both excellent thermal conductivity and stability.

[0009] Another object of the present invention is to provide a heat dissipation paste based on the Pickering emulsion composition and a method for producing the same. [Means for solving the problem]

[0010] To achieve the above object, the present invention provides a Pickering emulsion composition for heat dissipation, which comprises silicone oil, aliphatic alcohol, and a thermally conductive filler.

[0011] The silicone oil may be selected from the group consisting of dimethyl silicone oil, methylphenyl silicone oil, methylhydrogen oil, fluorosilicone oil, amino-modified silicone oil, and epoxy-modified silicone oil.

[0012] The aliphatic alcohol may be selected from the group consisting of glycerol, ethylene glycol, propylene glycol, butylene glycol, neopentyl glycol, trimethylolethane, ditrimethylolethane, trimethylolpropane, and ditrimethylolpropane.

[0013] The silicone oil and the aliphatic alcohol may be contained in a volume ratio of 1:(0.1 to 2).

[0014] The thermally conductive filler may be selected from the group consisting of alumina, magnesia, boron nitride, silicon nitride, and silica.

[0015] The thermally conductive filler may be a spherical filler having an average diameter of 0.1 to 100 μm.

[0016] The thermally conductive filler may be contained in an amount of 10 to 60 parts by volume relative to 100 parts by volume of the total composition.

[0017] The present invention provides a Pickering emulsion-based thermal paste comprising a liquid matrix containing silicone oil and aliphatic alcohol, and a thermally conductive filler dispersed in the matrix.

[0018] The thermally conductive filler is Segregated networks can be formed within the matrix.

[0019] The paste may have improved thermal conductivity.

[0020] The paste may have improved stability to heat and moisture.

[0021] The paste can be recycled or reused.

[0022] The present invention also provides a method for preparing a heat dissipation paste based on Pickering emulsion, which includes the steps of mixing silicone oil and aliphatic alcohol to prepare a liquid matrix, and adding a thermally conductive filler to the prepared liquid matrix. [Effects of the Invention]

[0023] The heat dissipation composition and paste according to the present invention can have better thermal conductivity and stability by forming a Pickering emulsion by adding an aliphatic alcohol to the heat dissipation paste in the form of a suspension, which has conventionally been used by simply mixing silicone oil and alumina filler.

[0024] The thermally conductive filler in the Pickering emulsion according to the present invention forms a segregated network structure, which allows for more effective heat transfer, making it suitable for use as a heat dissipation material in various fields.

[0025] In addition, the method for manufacturing a heat dissipation paste according to the present invention does not require the complicated manufacturing process, expensive surfactants, or post-processing steps of conventional heat dissipation pastes, and can manufacture a heat dissipation paste that ensures high thermal conductivity, stability, and applicability in a low-cost, simple, and environmentally friendly manner, and the filler can be recovered and recycled from the heat dissipation paste using a simple centrifugation method.

[0026] Furthermore, the manufacturing method can be readily applied to industry, allowing for efficient manufacturing and mass production, and will contribute to the future production and technological development of thermal interface materials. [Brief explanation of the drawings]

[0027] [Figure 1]1 shows the measurement results of the thermal conductivity and yield stress of the thermal paste according to the filler content in one experimental example of the present invention. (a) shows the thermal conductivity according to the filler content of SA03, GA03, and SG5:5A03. (b) shows the yield stress according to the filler content of SA03, GA03, and SG5:5A03. (c) shows the thermal conductivity according to the filler content of SA90, GA90, and SG5:5A90. (d) shows the yield stress according to the filler content of SA90, GA90, and SG5:5A90. Here, SA03, GA03, SA90, GA90, SG5:5A03, and SG5:5A90 are silicone oils, respectively. (e) and (f) are the thermal conductivities of the thermal pastes predicted by the Bruggeman model. (e) is the predicted value when the matrix thermal conductivity is fixed at 0.2 W / m K and the filler thermal conductivity is varied. (f) is the predicted value when the filler thermal conductivity is fixed at 100 W / m K and the matrix thermal conductivity is varied. [Figure 2] Optical microscopy (OM) images of glycerol monomatrix (GA03) containing 3 μm alumina filler with filler contents of (a) 10 vol%, (b) 20 vol%, (c) 30 vol%, (d) 40 vol%, and (e) 50 vol%, respectively. Scale bar indicates 20 μm. [Figure 3] Optical microscope (OM) images of silicone oil / glycerol emulsions (SG5:5A03) containing 3 μm alumina filler with filler contents of (a) 0 vol%, (b) 10 vol%, (c) 20 vol%, (d) 30 vol%, (e) 40 vol%, and (f) 50 vol%, respectively. The scale bar indicates 50 μm. [Figure 4]Optical microscopy (OM) images of silicone oil / glycerol emulsions (SG5:5A90) containing 90 μm alumina filler with filler contents of (a) 20 vol%, (b) 30 vol%, (c) 40 vol%, and (d) 50 vol%, respectively. The scale bar indicates 50 μm. [Figure 5] 10 is a graph showing the change in thermal conductivity with respect to the yield stress of a thermal paste sample according to another example of the present invention. [Figure 6] 1A and 1B are schematic diagrams illustrating the heat transfer paths in different thermal pastes, (a) a thermal paste in the form of a suspension, and (b) a thermal paste in the form of a Pickering emulsion. [Figure 7] Figure 1 shows the complex viscosity, yield stress, and thermal conductivity of thermal paste samples according to another example of the present invention under various temperature conditions. All samples were annealed at 50°C, 75°C, and 100°C for 1 hour before measurement, then cooled to room temperature and measured. (a) shows the complex viscosity of GA03_30 as a function of angular frequency. (b) shows the complex viscosity of SG5:5A03_30 as a function of angular frequency. (c) shows the yield stress of GA03_30 and SG5:5A03_30. (d) shows the percent change in the yield stress shown in (c) relative to the value measured at 25°C. (e) shows the thermal conductivity of GA03_30 and SG5:5A03_30. (f) shows the percent change in the thermal conductivity shown in (e) relative to the value measured at 25°C. [Figure 8] This shows the hygroscopicity of thermal paste samples according to other experimental examples of the present invention, showing (a) hygroscopicity and (b) normalized mass values ​​of GA03 and SG5:5A03 at different content levels at room temperature and 95% relative humidity. [Figure 9] This shows the weight loss with increasing temperature under a nitrogen atmosphere for alumina recovered from a silicone oil / glycerol emulsion (SG5:5A03) containing 3 μm alumina filler and pure alumina. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention will be described in detail below.

[0029] In order to produce a thermal paste having both high thermal conductivity and high stability, the inventors prepared a paste in the form of a Pickering emulsion by incorporating a thermally conductive filler into a matrix made by mixing immiscible silicone oil and glycerol, and confirmed its excellent thermal conductivity and stability, thereby completing the present invention.

[0030] The present invention provides a heat-dissipating Pickering emulsion composition comprising a silicone oil, an aliphatic alcohol, and a thermally conductive filler.

[0031] As used herein, the term "Pickering emulsion" refers to an emulsion in which two insoluble phases are stabilized using solid particles, as opposed to conventional emulsions which generally use surfactants to thermodynamically stabilize two immiscible phases.

[0032] The silicone oil is a liquid silicone resin with a relatively low degree of polymerization, and may include, but is not limited to, dimethyl silicone oil, methylphenyl silicone oil, methylhydrogen oil, fluorosilicone oil, amino-modified silicone oil, epoxy-modified silicone oil, etc.

[0033] The aliphatic alcohol may be an aliphatic polyhydric alcohol having relatively low volatility, and may be selected from, for example, glycerol, ethylene glycol, propylene glycol, butylene glycol, neopentyl glycol, trimethylolethane, ditrimethylolethane, trimethylolpropane, ditrimethylolpropane, etc., and preferably glycerol, but is not limited thereto.

[0034] The silicone oil and the aliphatic alcohol may be contained in a volume ratio of 1:(0.1 to 2), preferably 1:(0.5 to 1.5), but are not limited thereto.

[0035] The thermally conductive filler is intended to improve the thermal conductivity of the composition and may include inorganic fillers such as alumina, magnesia, boron nitride, silicon nitride, and silica. Preferably, the thermally conductive filler may be selected from spherical fillers having an average diameter of 0.1 to 100 μm, such as spherical alumina, spherical magnesia, and spherical silica, and more preferably spherical fillers having an average diameter of 0.1 to 10 μm, but is not limited to these.

[0036] The thermally conductive filler may be contained in an amount of 10 to 60 parts by volume, preferably 10 to 50 parts by volume, relative to 100 parts by volume of the entire heat dissipation paste composition, but is not limited thereto.

[0037] The present invention provides a Pickering emulsion-based thermal paste comprising a liquid matrix containing silicone oil and aliphatic alcohol, and a thermally conductive filler dispersed in the matrix.

[0038] The liquid matrix may include silicone oils and fatty alcohols, the corresponding characteristics of which are described above.

[0039] The matrix may contain silicone oil and aliphatic alcohol in a volume ratio of 1:(0.1 to 2), preferably 1:(0.5 to 1.5), but is not limited thereto.

[0040] Generally, silicone-based matrices have good application properties due to their low viscosity and surface energy, but their low thermal conductivity and low interfacial affinity between the filler and the matrix make them prone to phase separation in the resulting paste. Therefore, by mixing an aliphatic alcohol with a higher viscosity and thermal conductivity than the silicone-based matrix, it is possible to produce a stable emulsion-type matrix with sufficient viscosity and high conductivity. However, aliphatic polyhydric alcohols such as glycerol have poor long-term stability due to their high hygroscopicity. Therefore, it is desirable to mix silicone oil and glycerol in the above volume ratio to produce a new matrix that can be used in the form of a Pickering emulsion.

[0041] For the corresponding characteristics of the thermally conductive filler, please refer to the above description.

[0042] The thermally conductive filler may be included in an amount of 10 to 60 parts by volume, preferably 10 to 50 parts by volume, based on 100 parts by volume of the total paste composition for heat dissipation, but is not limited thereto. As the filler content increases, the formation of heat transfer paths becomes easier and the thermal conductivity can be improved. However, if the filler content is too high, the density of the heat dissipation material increases, making it heavier, and the paste formulation may not be able to be maintained, resulting in reduced processability. Therefore, it is preferable that the filler content be within this range.

[0043] As mentioned above, thermal interface materials (TIMs) are applied between the heat source and heat sink, but it is very important to apply the TIM with a minimum of voids (air gaps) between the two interfaces, and a paste formulation can fulfill this requirement.Paste formulations have the advantages of being able to maintain a short bond line thickness even with low pressure, minimizing thermal resistance at the interface and ensuring high thermal conductivity, not requiring a curing process, and providing excellent external stress absorption and damping.

[0044] The thermally conductive filler can adsorb along the interface between the insoluble silicone oil and glycerol to form a thermodynamically stabilized Pickering emulsion, which thermodynamically stabilizes the thermodynamically unstable liquid matrix. The thermally conductive filler can effectively form a filler network within the liquid matrix, and this filler network structure is called a segregated network. The filler forms a segregated network within the thermodynamically stable Pickering emulsion phase, ensuring a heat transfer path, thereby improving thermal conductivity, exhibiting high yield stress, and providing excellent structural and dispersion stability. This also improves stability against heat and moisture.

[0045] Therefore, the paste is a thermal interface material for ensuring heat dissipation, and can be used in various electronic devices.

[0046] Additionally, the paste can be recycled or reused.

[0047] According to an experimental example of the present invention, it was confirmed that when the prepared paste was decomposed and recovered, a high-purity filler without residual solvent was recovered and could be reused.

[0048] The present invention also provides a method for producing a heat dissipation paste based on Pickering emulsion, which includes the steps of: mixing silicone oil and aliphatic alcohol to produce a liquid matrix; and adding a thermally conductive filler to the liquid matrix.

[0049] Further details are provided in the Examples section below.

[0050] <Example> The present invention will be described in detail below with reference to examples to aid in understanding the present invention. However, the following examples are merely illustrative of the content of the present invention, and the scope of the present invention is not limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0051] Example 1: Preparation of a composite (thermal paste) in the form of Pickering emulsion

[0052] 1-1. Experimental materials Silicone oil (KF-96, 350cs) was purchased from Shinetsu Chemical (Japan). Glycerol (≥99.0%) was purchased from Sigma-Aldrich (Korea). Spherical alumina of different sizes (3 μm, 20 μm, 90 μm) was purchased from Denka (Japan). The chemicals were used as received.

[0053] 1-2. Manufacturing of thermal paste To prepare a control thermal paste in suspension form, silicone oil or glycerol and various sizes of alumina were weighed. The volumetric content of alumina relative to the total thermal paste was 10, 20, 30, 40, and 50 vol%, respectively. The weighed suspension was then uniformly mixed at 2,000 rpm for 1 minute using a Thinky Mixer AR-100 (Thinky Corporation, Japan). To prevent air bubbles from forming in the paste, the mixture was defoamed at 2,200 rpm for 1 minute and then mixed at 2,000 rpm for 1 minute. The final paste had fluidity despite being impregnated with approximately 50 vol% alumina.

[0054] To prepare the thermal paste in the form of a Pickering emulsion, silicone oil and glycerol were first weighed out in a 1:1 volume ratio and then mixed using a Thinky mixer at 2,000 rpm for 1 minute. To stabilize the unstable two liquid matrices, 10, 20, 30, 40, and 50 vol% alumina was then added to the total paste content. The defoaming and mixing process was then carried out in the same manner as in the preparation of the suspension-type thermal paste described above. The pastes produced had a stable formulation at all content levels.

[0055] 1-3.Physical property analysis method The thermal conductivity of the paste was measured in isotropic mode using a transient plane source thermal analyzer TPS-2500S (Hot-disk AB, Sweden). Because the paste is fluid, to maintain its formulation, vial lids with a depth and diameter of 10 mm and 20 mm, respectively, were covered with vinyl wrap and then sealed with the paste. A sensor was then placed between two identical vial lids containing the paste, and the thermal conductivity was measured by measuring the contact resistance and temperature change. Rheological properties, such as complex viscosity and yield stress versus angular frequency, were measured using a rotational rheometer (HR-20, TA Instrument, USA) equipped with a 40 mm diameter parallel plate in small-amplitude oscillatory shear (SAOS) mode. The filler structure of the thermal paste was observed using an optical microscope Eclipse LV100ND (Nikon, Japan). Each sample was thinly spread on a glass slide and allowed to stabilize at room temperature for 5 minutes before being observed from above. To observe the thermal stability of the paste, the paste was placed on a heating plate and heated to 50, 75, and 100°C for 1 hour each, then cooled to room temperature. The rheological properties and thermal conductivity of the cooled samples were then measured using the same method as described above. To observe the moisture stability of the thermal paste, the weight change of the paste at room temperature was measured using an analytical balance AUW220D (Shimadzu Corporation, Japan). Four vials were filled with water and placed in the analytical balance. The weight change of the paste was recorded at 10-minute intervals, starting when the relative humidity reached 95%. To compare the alumina recovered from pure alumina and thermal pastes using 3 μm alumina, the weight change rate with increasing temperature was measured using a thermogravimetric analyzer (TGA) under a nitrogen atmosphere.

[0056] <Experimental Example 1> Confirmation of yield stress and thermal conductivity of thermal paste Yield stress and thermal conductivity are the most important characteristics required for a thermal paste. Thermal conductivity is a measure of an object's heat transfer ability, while yield stress refers to the minimum stress required to deform or flow the structure formed by the particles. The yield stress of a paste is due to the non-covalent interactions between the fillers dispersed in the matrix or the structure they form. Generally, the higher the viscosity of the complex, the higher the yield stress, and the higher the yield stress, the better the structural stability. In other words, a thermal paste that combines high thermal conductivity and high yield stress can be said to be the optimal type of thermal paste.

[0057] The thermal conductivity and yield stress of heat-dissipating paste composites were measured using emulsions of silicone oil, glycerol, and a 5:5 volumetric ratio mixture of silicone oil and glycerol, with alumina particles of varying sizes and content dispersed in them. The alumina used as filler had diameters of 3 μm and 90 μm. SA and GA represent suspension-type pastes composed of silicone oil and alumina, and glycerol and alumina, respectively, while SG5:5A represents an emulsion-type paste in which alumina is impregnated in a liquid matrix composed of silicone oil and glycerol in a 5:5 volumetric ratio. The number after the sample name indicates the diameter of the spherical alumina particles used.

[0058] The thermal conductivity and yield stress of thermal pastes prepared using silicone oil, glycerol, and glycerol / silicone oil as matrices were measured. As shown in Figures 1a to 1d, it was found that both the thermal conductivity and yield stress increased with increasing filler content, regardless of the type of matrix and filler.

[0059] First, referring to Figures 1b and 1d, the emulsion-type thermal paste containing a 5:5 mixture of silicone oil and glycerol consistently exhibited higher yield stress than the suspension-type thermal pastes (SA and GA) of silicone oil or glycerol at all content levels. This indicates that the emulsion-type thermal paste is always more stable than the suspension-type thermal paste based on a single matrix. Furthermore, regardless of the size of the impregnated alumina, the yield stress increased sharply when the filler was impregnated at 10 vol%, indicating that the emulsion-type paste exhibited a higher yield stress than the suspension-type thermal paste. This is thought to be because, unlike the suspension-type thermal paste in which alumina is randomly dispersed, the unstable interface between the silicone oil and glycerol is stabilized by the alumina, and at the same time, the interaction between the alumina particles is strengthened, forming a relatively strong structure. This strongly suggests the formation of a Pickering emulsion.

[0060] Thereafter, the yield stress steadily increases with increasing filler content up to 40 vol%, which is likely due to the alumina being continuously located at the silicone oil / glycerol interface, forming a stronger segregated network structure.

[0061] When the filler content is higher than 40 vol%, both the yield stress and thermal conductivity increase sharply (Fig. 1a and Fig. 1c). This is thought to be because alumina fills the spaces between the silicone oil droplets dispersed in the continuous phase glycerol, forming a segregated network filler structure and ensuring a heat transfer path.

[0062] As a result, SG5:5A03 [a mixture of silicone oil and glycerol in a volume ratio of 5:5 + 3 μm alumina] and SG5:5A90 [a mixture of silicone oil and glycerol in a volume ratio of 5:5 + 90 μm alumina] can be predicted to be Pickering emulsions in which the interface between two immiscible liquids is stabilized by solid particles.

[0063] Referring to Figure 1a, SG5:5A03 exhibited higher thermal conductivity than SA03 [silicone oil + 3 μm alumina] and GA03 [glycerol + 3 μm alumina], and showed significantly higher thermal conductivity especially at high filler contents.

[0064] The thermal conductivity of a thermal paste is determined by the filler when the matrix is ​​assumed to be insulating. In particular, in the general case where spherical fillers are randomly dispersed in the matrix, the thermal conductivity of the thermal paste follows the Bruggeman model, as shown in Equation 1: <Expression 1> 1-V={(λ p -λ c ) / (λ p -λ m )}*{(λ m / λ c )^(1 / 3)} In the above formula, V is the volume fraction of the filler, and λ p , λ m and λ c are the thermal conductivities of the filler, matrix, and composite (thermal paste), respectively.

[0065] 1e and 1f, we can see that when the thermal conductivity of the filler increases for a matrix with the same thermal conductivity, the increase in the thermal conductivity of the entire composite is relatively small. However, when the thermal conductivity of the filler in the composite remains unchanged and only the thermal conductivity of the matrix increases, the increase in the thermal conductivity of the entire composite is very large. In other words, a higher thermal conductivity of the matrix is ​​more favorable for improving the thermal conductivity of the composite.

[0066] Analyzing Figures 1a and 1b from this perspective, we found that in the case of thermal pastes based on a single matrix, glycerol (0.299 W / mK) had a higher thermal conductivity than silicone oil (0.183 W / mK), and the composite also exhibited higher thermal conductivity. On the other hand, the thermal conductivity of the silicone oil and glycerol mixture in SG5:5A03 was 0.245 W / mK, which is intermediate between the thermal conductivities of glycerol and silicone oil. However, adding more than 20 vol% alumina resulted in a higher thermal conductivity than GA03, a result inconsistent with the Bruggeman model. This suggests that there is some other influence beyond the effect of the matrix that contributes to the improvement of thermal conductivity. Considering the results of the yield stress, it is expected that the alumina is located at the interface between the silicone oil and glycerol, forming a segregated network structure between the fillers, which increases the yield stress. This leads to the formation of a more effective heat transfer pathway, which significantly improves the thermal conductivity. In addition, it can be concluded that SG5:5A03 has the form of a Peakering emulsion.

[0067] On the other hand, Figure 1c shows that the increase in thermal conductivity of SG5:5A90 is not as significant as that of SG5:5A03 with increasing filler content. In the same emulsion system using 3 μm alumina, a segregated structure leads to a sharp increase in thermal conductivity (Figure 1a). However, when 90 μm alumina is added, the increase in thermal conductivity and yield stress are both small, despite the same matrix system. This is likely due to the lack of a segregated structure. This suggests that the alumina was too large compared to the size of the formed droplets, preventing the formation of a proper Pickering emulsion. Instead, the oil droplets and alumina were randomly dispersed.

[0068] In summary, it is generally known that in suspension-type thermal pastes using only silicone or glycerol, the larger the filler size used, the higher the thermal conductivity of the paste. However, in emulsion-type pastes, it was observed that the thermal conductivity of the paste increased when small-sized alumina was used, and it was confirmed that when 3 μm alumina was used, a Pickering emulsion and a segregated network structure were effectively formed.

[0069] To prove these facts, in the following Experimental Example 2, the morphology formed by the filler in the paste was confirmed using an optical microscope.

[0070] <Experimental Example 2> Confirmation of thermal paste morphology As mentioned above, within a thermal paste composite, the thermal conductivity of the matrix itself as well as the structure formed by the filler have a significant impact on the thermal conductivity of the entire composite. The yield stress of a composite refers to the minimum force required to deform the structure formed by the filler within the composite, and varies depending on the degree of structure formation.

[0071] Therefore, in order to understand the correlation between the yield stress and thermal conductivity and the structure formed by alumina, an optical microscope (OM) was used to observe the structure formed by the filler in the suspension-type thermal pastes SA03 / 90 and GA03 / 90, and the emulsion-type thermal paste SG5:5A03 / 90.

[0072] As a result, as shown in Figures 2 to 4, the filler dispersed in a suspension based on a single matrix using only silicone or glycerol and the filler in an emulsion showed different dosage forms.

[0073] First, referring to Figure 2, when fillers are dispersed in a single matrix like GA03, the fillers show a randomly dispersed morphology, and then as the filler content increases, the fillers tend to become densely impregnated. As a result, when the percolation threshold (>40 vol%) at which interconnection between fillers occurs is exceeded, thermal conductivity and yield stress increase sharply (Figures 1a to 1d).

[0074] On the other hand, emulsion-type composites (SG5:5A03, SG5:5A90) using a mixture of silicone oil and glycerol as the matrix exhibited behaviors different from those of suspension-type composites using only silicone or glycerol. Because alumina is a relatively hydrophilic particle, it exhibited behaviors in which it came into contact with glycerol more than silicone oil in the emulsion. In other words, it formed an oil-in-water (O / W) type emulsion in which glycerol was the continuous phase and silicone oil was the dispersed phase.

[0075] Therefore, as shown in Figure 3, the continuous phase is glycerol and the droplets are silicone oil. When silicone oil and glycerol are simply dispersed without alumina, the droplet size is unstable and not consistent (a). When alumina is impregnated at 10 vol%, the droplet size becomes relatively constant, and the alumina appears to surround the silicone oil droplets (b). The consistent droplet size is due to the filler surrounding the droplets to form a stable structure, which is considered strong evidence of a Pickering emulsion. Subsequently, as the filler content increases, the matrix ratio decreases, and the size of the oil droplets gradually decreases, and adjacent droplets are observed to be connected to each other by the filler. Finally, the droplets become smaller to a size comparable to that of the alumina, indicating dense packing of the oil droplets and alumina in the glycerol matrix.

[0076] This confirmed that SG5:5A03 forms a stable Pickering emulsion and a segregated network structure with alumina. SG5:5A03 also provides a more stable and stronger structure, with a higher yield stress than SA03 and GA03. Because of the well-formed, continuous heat transfer pathways, SG5:5A03 also exhibits higher thermal conductivity than SA03 and GA03.

[0077] As mentioned above, SG5:5A90, like SG5:5A03, has a higher yield stress and better stability than SA90 and GA90, but its thermal conductivity is not as high as that of GA90. Referring to Figure 4, this is because the diameter of the alumina is larger than the diameter of the silicone oil droplets, so the filler does not surround the droplets, but rather the droplets surround the filler.

[0078] That is, compared to when the fillers are randomly dispersed in a single matrix, SG5:90 behaves like solid particles due to the packing of droplets between the fillers, resulting in good stability (better yield stress than a single-matrix-based composite), but because the fillers are not well connected like in random packing, this does not lead to a significant improvement in thermal conductivity (no significant improvement in thermal conductivity is observed compared to a single-matrix-based composite).

[0079] Thus, it was found that the thermal conductivity and yield stress are significantly affected by the structure formed by the filler in the composite, and ultimately, only SG5:5A03 can be considered a Pickering emulsion.

[0080] <Experimental Example 3> Theoretical analysis of thermal paste These results show that thermal conductivity is significantly affected by the structure formed by the filler. In other words, the relationship between thermal conductivity and yield stress, which represents a measure of the strength of the structure formed by the filler, allows us to see how effectively the filler structure improves thermal conductivity. Furthermore, we used a thermal conductivity prediction model to analyze the extent to which the filler structure contributes to thermal conductivity.

[0081] The overall thermal resistance of a composite typically used as a TIM is given by Equation 2: <Expression 2> R bulk =BLT / k TIM In the above formula, R bulk is the overall thermal resistance of the TIM, BLT is the actual contact distance between the two solids, and k TIM is the thermal conductivity of the composite.

[0082] In order to analyze BLT, it is important to understand the type of fluid the composite is. Previous research has shown that the viscosity of TIM exhibits Herschel-Buckley (HB) fluid behavior, and BLT depends only on the yield stress of the composite in an equilibrium state. Therefore, experimentally, the relationship between the yield stress and thermal conductivity of TIM is shown in Equation 3: <Expression 3> k TIM =(1 / R bulk )C(τ y / P)^m In the above formula, C and m are constants, and τ y is the yield stress of the composite, and k TIM is the thermal conductivity of the composite and P is the pressure required to bond the two solid plates together.

[0083] As a result, R bulkAssuming that is minimum in all cases, i.e., close to 0 (zero), and P is constant in all cases, it can be inferred that the thermal conductivity of the composite is proportional to the mth power of the yield stress. When the value of m is large, the increase in thermal conductivity for each increase in yield stress is large, and when the value of m is small, the increase in thermal conductivity for each increase in yield stress is small. Therefore, even if the yield stress of the filler is the same, a larger value of m results in a structure that more efficiently improves thermal conductivity.

[0084] Generally, the thermal conductivity and yield stress of composites have percolation properties, so when a certain amount of filler is impregnated, physical properties tend to be maximized. Referring to Figures 1a to 1d, it can be seen that the slope of the increase in thermal conductivity and yield stress for all pastes increases at 40 vol%. In other words, since there are different sections depending on the filler content, it is appropriate to divide the relationship between conductivity and stress using 40 vol% as the base.

[0085] Table 1 below shows the calculated m values ​​for all pastes around 40 vol% based on Equation 3 above.

[0086] [Table 1]

[0087] Referring to Table 1 and FIG. 5, all pastes show lower m values ​​above 40 vol%, because above 40 vol%, the fillers exhibit percolation behavior in which they rapidly interconnect, resulting in a rapid increase in yield stress relative to thermal conductivity.

[0088] For all pastes, as the size of the filled filler increases, the interfacial thermal resistance that occurs at the filler interface where the fillers come into contact with each other decreases at all contents, and the thermal conductivity of the composite increases. As the size of the filler increases, the viscosity of the entire composite, i.e., the yield stress, decreases and m increases.

[0089] Compared to the suspension-type thermal pastes, SG5:5A03 and SG5:5A90 had relatively high m values ​​at concentrations below 40 vol%, but at concentrations above 40 vol%, the m values ​​decreased sharply compared to those at concentrations below 40 vol%. A high m value indicates that the increase in thermal conductivity is greater than the increase in yield stress. Below 40 vol%, unlike composites based on a single matrix, the filler forms a segregated structure, allowing for effective heat transfer. However, the rapid decrease in m values ​​above 40 vol% m indicates that the movement of the filler is significantly restricted by the silicone oil droplets, reaching the maximum limit of alumina impregnation, resulting in a significant increase in yield stress compared to the increase in thermal conductivity.

[0090] Judging from Figures 1 to 5 comprehensively, first, as shown in Figures 3 and 4, SG5:5A03 has a structure in which the filler surrounds the droplets, forming a segregated structure, whereas SG5:5A90 has a structure in which the droplets surround the filler because the filler size is larger than the droplet size, and it can be confirmed that only SG5:5A03 has a peaking emulsion structure.

[0091] However, both SG5:5A03 and SG5:5A90 showed larger m values ​​below 40 vol% compared to the random single-matrix composites because the filler had an ordered structure due to the oil droplets, resulting in a larger increase in thermal conductivity relative to the yield stress. Above 40 vol%, both SG5:5A03 and SG5:5A90 showed a significant decrease in m. That is, at high filler contents above 40 vol%, the increase in yield stress was greater than that of the single-matrix composites. This is because the oil droplets stabilized to a small size by the large amount of filler in the composite behaved like solid particles, significantly restricting the filler's movement and limiting the volume the filler could occupy, resulting in a rapid increase in yield stress.

[0092] As a result, it was confirmed that SG5:5A03 and SG5:5A90, which use silicone oil and glycerol as matrices, both theoretically have better stability and thermal conductivity than composites based on a single matrix, and that SG5:5A03, which has the form of a Pickering emulsion, has the best stability and thermal conductivity.

[0093] Next, the Agari model, which is a predictive model for the thermal conductivity of composites, can be used to confirm the extent to which the internal structure formed by the filler is formed.

[0094] The Agari model is a model proposed to consider the influence of the filler network on thermal conductivity at high filler contents, and is expressed as Equation 4 below: <Expression 4> logλ c =v f C f logλ f +(1-v f )log(C p λ p ) In the above formula, λ c , λ f and λ p are the thermal conductivities of the composite, filler and matrix, respectively, and v f is the volume fraction of the filler.

[0095] C p is a constant that takes into account the effect of the filler on the secondary structure (crystallinity or crystal size) of the matrix, i.e., it is a measure of whether the filler affects the thermal conductivity of the matrix, and C p The fact that C is 1 means that under ideal conditions, the impregnation of the filler does not affect the thermal conductivity of the matrix. p is known to be close to 1, so in this experiment, C p =1.

[0096] Cf is a measure of how efficiently the filler forms a dense, airtight packing and network, in other words, a measure of the ability to form a heat conduction network. f has a value between 0 and 1, and C f The higher the filler volume ratio, the tighter the filler network structure, which leads to more effective heat transfer and higher thermal conductivity. As a result, Agari fitting was performed based on the thermal conductivity to filler volume ratio for all composites, and the C f By analyzing the values, it is possible to determine which composites form the most efficient filler network structure for heat transfer.

[0097] Table 2 below shows C p After fixing the C to 1, the C of all complex samples f The values ​​are calculated.

[0098] [Table 2]

[0099] Referring to Table 2, SG5:5A03, which actually formed a segregated structure by the filler, had the highest C compared to the other complexes. f In other words, it can be confirmed that SG5:5A03 has a strong network structure that is most advantageous for heat transfer.

[0100] SG5:5A90 also has a relatively high C f This is thought to be because the movement of the filler is restricted by the surrounding oil droplets and is relatively orderly compared to the composite based on a single matrix.

[0101] 6, it can be seen that SG5:5A03, unlike composites based on a single matrix, forms a segregated structure to form an effective heat transfer path, and also has high yield stress, i.e., structural stability and dispersion stability. That is, at <40 vol%, a segregated structure is formed, showing a high m value, and at >40 vol%, a dense structure due to oil droplets is formed, showing a rapid decrease in m value and the highest C f As a result, it was theoretically proven that the Pickering emulsion SG5:5A03 has high stability and high thermal conductivity, which is due to the segregated structure formed by the filler.

[0102] <Experimental Example 4> Evaluation of thermal paste stability Because thermal pastes are exposed to heat generated by electronic devices for extended periods of time, long-term stability at high temperatures is important. Complex fluid-type thermal pastes rapidly decrease in viscosity and begin to flow at high temperatures. When excessive heat is concentrated, the matrix with reduced viscosity flows out of the complex fluid, causing a pump-out phenomenon that reduces stability. Alternatively, air fills the gaps where the matrix disappears, reducing the heat transfer path, causing a dry-out phenomenon. Furthermore, a decrease in viscosity can cause filler sedimentation, which is the fundamental cause of thermal paste products' gradual decrease in thermal conductivity over long periods of use. Therefore, stable viscosity and minimal change in viscosity, even at high temperatures, are essential.

[0103] To observe the change in viscosity with temperature of thermal pastes in suspension and emulsion form, we observed the change in viscosity with temperature of GA03 and SG5:5A03, which have relatively similar thermal conductivities.

[0104] The temperature was raised from room temperature to 100°C, then lowered back to room temperature. The changes in complex viscosity, yield stress, and thermal conductivity of the high-temperature thermal paste as a function of angular frequency were examined. As shown in Figure 7, the complex viscosity of GA03 decreased rapidly with increasing temperature. The yield stress also decreased rapidly with the decrease in viscosity at high temperatures, dropping to approximately 0.5% of that at room temperature. The thermal conductivity measured at room temperature after high-temperature heat treatment also decreased by approximately 30% compared to that measured without heat treatment. This is due to the rapid flow of the matrix in the composite as the temperature increased, resulting in phase separation from the filler. These results indicate that GA03 has poor thermal stability.

[0105] On the other hand, in the case of SG5:5A03, the complex viscosity decreases with increasing temperature, but the decrease is much smaller than that of GA03. As the viscosity decreases at high temperatures, the yield stress also decreases to about 50% compared to room temperature, but this decrease is much smaller than that of GA03. The thermal conductivity at high temperatures also decreases by about 5% compared to room temperature, but the decrease is much smaller than that of GA03. In other words, the Pickering emulsion SG5:5A03 is structurally stable due to the formation of a segregated network structure in which the filler particles form, suppressing phase separation at high temperatures. This confirms that SG5:5A03 is extremely thermally stable.

[0106] Furthermore, in order to ensure efficient heat dissipation performance, the heat dissipation paste used in electronic devices must maintain a stable formulation even when exposed to harsh environments, such as high humidity.

[0107] Glycerol, one of the matrices used in the examples, has three -OH functional groups per molecule, which makes it hygroscopic. Therefore, although GA03 has a thermal conductivity comparable to that of the Pickering emulsion SG5:5A03, its hygroscopicity makes it unsuitable for use as a thermal paste.

[0108] To test the moisture absorption of GA03 and SG5:5A03, thermal pastes with different filler contents were used to measure the weight change over time under room temperature and 95% relative humidity conditions. The results were expressed using the following Equation 5: <Formula 5> Hygroscopicity (%)=[(W1-W0) / W0]×100

[0109] If each thermal paste has high hygroscopicity, it will absorb water vapor over time and the mass of the thermal paste will increase.

[0110] As shown in Figure 8, all composites showed improved structural stability and reduced weight gain as the filler content increased, due to the decreased relative amount of matrix. Even with the highest filler content, GA03_50, showed a weight gain of approximately 20% compared to its initial weight. This is due to the high hygroscopicity of glycerol. On the other hand, SG5:5A03, even with the lowest filler loading (10 vol%), showed only a 7% weight gain compared to its initial weight, and SG5:5A03_50 showed only a 1% weight gain compared to its initial weight, suggesting almost no hygroscopicity. This is due to the segregated structure formed by the filler, despite the presence of some glycerol. As a result, SG5:5A03 is a Pickering emulsion, confirming that it is a thermal paste that is stable at high temperatures and in the presence of moisture.

[0111] <Experimental Example 5> Recovery of thermal paste filler In recent years, the demand for sustainable, environmentally friendly, and recyclable materials has increased significantly. Furthermore, fillers in polymer composites are typically expensive, so the recovery and recycling / reuse of fillers used in composites is of great industrial significance. For these reasons, we conducted a recovery experiment for recycling alumina fillers from the Pickering emulsion thermal paste produced according to the present invention.

[0112] Experiments were primarily conducted on the SG5:5A03_10 Pickering emulsion thermal paste, which was made with a low filler content. First, the mixture was centrifuged three times using a centrifuge (Labogene 1248R) at 4,000 rpm for approximately 10 minutes at room temperature. This induced phase separation between the filler and matrix, and the alumina was then recovered. The recovered alumina was washed several times with hexane and distilled water. Hexane was used as a solvent capable of dissolving silicone oil, and distilled water was used as a solvent capable of dissolving glycerol. The silicone oil and glycerol were then removed from the phase-separated alumina composite using a vacuum filtration device. The filtered alumina was then dried under vacuum at 100°C for approximately two days to remove residual solvent.

[0113] We also investigated whether filler recovery was possible in the case of the SG5:5A03_50 Pickering emulsion thermal paste, which is made with a high filler content. First, SGA03_50 was diluted with hexane and distilled water, and the filler was uniformly dispersed using a Thinky mixer and sonicator. This was then centrifuged, washed, filtered, and dried in the same manner as the SGA03_10 thermal paste described above, to separate the matrix and filler.

[0114] The recovered alumina was evaluated by thermogravimetric analysis (TGA). Referring to Figure 9, it can be seen that the alumina recovered from the Pickering emulsion thermal paste with a low filler content and the alumina recovered from the Pickering emulsion thermal paste with a high filler content both show no weight change due to the volatilization of distilled water, glycerol, or silicone oil even at high temperatures up to 800°C, just like pure alumina. This means that the recovered alumina is high-purity alumina with no residual solvent and can be reused in the future.

[0115] The Pickering emulsion thermal paste is simply a mixture of a glycerol / silicone oil matrix and alumina filler, which has the advantage that the filler can be recovered by physical decomposition. This means that filler separation methods can be applied to Pickering emulsion thermal paste systems, which have a wide range of applications. It is also expected that the recovered filler can be recycled for the production of new composite materials in the future.

[0116] The above describes specific embodiments of the present invention. Those skilled in the art will recognize that the present invention can be embodied in various modified forms without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments should be considered from an illustrative rather than a restrictive perspective. The scope of the present invention is defined by the claims, not the foregoing description, and all variations within the scope of the claims are intended to be encompassed within the scope of the present invention.

Claims

1. Contains silicone oil, aliphatic alcohol, and a thermally conductive filler A Pickering emulsion composition for heat dissipation, characterized by:

2. The silicone oil is One or more selected from the group consisting of dimethyl silicone oil, methylphenyl silicone oil, methylhydrogen oil, fluorosilicone oil, amino-modified silicone oil, and epoxy-modified silicone oil. The heat-dissipating Pickering emulsion composition according to claim 1 .

3. The fatty alcohol is One or more aliphatic polyhydric alcohols selected from the group consisting of glycerol, ethylene glycol, propylene glycol, butylene glycol, neopentyl glycol, trimethylolethane, ditrimethylolethane, trimethylolpropane, and ditrimethylolpropane. The heat-dissipating Pickering emulsion composition according to claim 1 .

4. The silicone oil and the fatty alcohol 1: (0.1 to 2) volume ratio The heat-dissipating Pickering emulsion composition according to claim 1 .

5. The thermally conductive filler is One or more selected from the group consisting of alumina, magnesia, boron nitride, silicon nitride, and silica. The heat-dissipating Pickering emulsion composition according to claim 1 .

6. The thermally conductive filler is It is a spherical filler with an average diameter of 0.1 to 100 μm. The heat-dissipating Pickering emulsion composition according to claim 1 .

7. The thermally conductive filler is The amount is 10 to 60 parts by volume relative to 100 parts by volume of the total composition. The heat-dissipating Pickering emulsion composition according to claim 1 .

8. a liquid matrix comprising a silicone oil and an aliphatic alcohol; and a thermally conductive filler dispersed in the matrix. A heat dissipation paste based on a Pickering emulsion, characterized in that:

9. The thermally conductive filler is Forming segregated networks within the matrix A thermal paste based on the Pickering emulsion of claim 8.

10. The paste is Improved thermal conductivity A heat dissipation paste based on the Pickering emulsion of claim 8.

11. The paste is Improved heat and moisture stability A heat dissipation paste based on the Pickering emulsion of claim 8.

12. The paste is Recyclable or reusable A heat dissipation paste based on the Pickering emulsion of claim 8.

13. mixing silicone oil and aliphatic alcohol to form a liquid matrix; adding a thermally conductive filler to the prepared liquid matrix. A method for producing a heat dissipation paste based on a Pickering emulsion.

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