WO2024085324A1 - Pickering emulsion composition for heat dissipation, heat dissipation paste using same, and method for manufacturing same - Google Patents

Abstract

The present invention relates to a Pickering emulsion composition for heat dissipation, a heat dissipation paste using same, and a method for manufacturing same and, more specifically, provides a Pickering emulsion composition for heat dissipation comprising: a silicone oil; an aliphatic alcohol; and a thermally conductive filler, and a heat dissipation paste based on the Pickering emulsion and a method for manufacturing same. The thermally conductive filler in the Pickering emulsion forms a segregated network structure, securing high thermal conductivity and stability, and thus the emulsion can be used as a heat dissipation material in various fields. In addition, the filler can be recovered from the heat dissipation paste by using a simple centrifugation method and recycled.

Description

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

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

Recently, as electronic components have become more integrated, the heat generated per unit area of the device, that is, heat density, is increasing. If the heat generated during use of the device is not dissipated within a short period of time, it may cause overheating of the device, reduced lifespan, reduced reliability, reduced performance, and even explosion. To solve this problem, it is important to dissipate unnecessary heat generated during equipment operation from the heat source to a heat sink using a thermal interface material (TIM) with high thermal conductivity.

Until now, TIM materials have been widely used as polymer composite materials that can demonstrate both the advantages of light and formable polymers and metal/ceramic fillers. Polymer composite materials are a type of polymer resin impregnated with a metal or ceramic filler, making use of both the formability and flexibility of the polymer and the thermal conductivity of the filler, making it suitable as a TIM material.

Polymer composite materials can be divided into metal-based and ceramic-based polymer composite materials depending on the filler used. In the case of metal-based polymer composite materials, gold (Au), silver (Ag), copper (Cu), etc. are used as metal fillers to achieve very high thermal conductivity, but they are also electrically conductive and are used in electronic devices that require insulation. It is unsuitable for use and it is difficult to secure lightness due to the high density of the metal. In the case of ceramic polymer composite materials, alumina, boron nitride, and silica are used as ceramic fillers, so unlike metal polymer composite materials, insulation properties can be secured, they are stable in the air, and the price is low. However, it has a disadvantage in that it has a relatively low thermal conductivity and requires a high filler content to ensure sufficient thermal conductivity.

In addition, TIM exists in various forms depending on the purpose, and the most representative forms are paste, phase change material (PCM), adhesive, and pad. TIM is applied between the heat source and the heat sink, and because the actual surfaces of the heat source and heat sink are very rough, it is very important to apply TIM with a minimum air gap (air layer) between the two interfaces. This is because the thermal conductivity of the voids (air) is very low, preventing effective heat transfer. In other words, it is very important to apply TIM with high thermal conductivity to effectively fill the space between two rough surfaces to maximize the actual contact area.

TIM that can well satisfy these conditions is a paste formulation. Generally, such thermal paste is a polymer composite material in which ceramic filler is dispersed in a low-molecular/polymer silicon matrix. The silicone matrix of silicone heat dissipation paste has good spreadability and excellent heat and weather resistance, but its thermal conductivity is very low at the level of 0.2 W/m·K, so even if a high content of filler is filled, it is impossible to secure thermal conductivity of 1.5 to 2 W/m·K or more. There is a difficult downside. Additionally, long-term stability is poor under general TIM application conditions such as heat and pressure.

Accordingly, there is a need for a heat dissipation paste manufacturing technology that has both high thermal conductivity and high stability.

The purpose of the present invention is to provide a heat dissipating Pickering emulsion composition that has both excellent thermal conductivity and stability.

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.

In order to achieve the above object, the present invention provides silicone oil; aliphatic alcohol; And it provides a Pickering emulsion composition for heat dissipation, comprising a thermally conductive filler.

The silicone oil may be selected from the group consisting of dimethyl silicone oil, methyl phenyl silicone oil, methyl hydrogen oil, fluorosilicone oil, amino-modified silicone oil, and epoxy-modified silicone oil.

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.

The silicone oil and aliphatic alcohol may be included in a volume ratio of 1:(0.1 to 2).

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

The thermally conductive filler may be a spherical filler with an average diameter of 0.1 to 100 μm.

The thermally conductive filler may be included in an amount of 10 to 60 parts by volume based on 100 parts by volume of the total composition.

The present invention provides a liquid matrix containing silicone oil and aliphatic alcohol; and a heat conductive filler dispersed in the matrix, providing a Pickering emulsion-based heat dissipation paste.

The thermally conductive filler may form a segregated network within the matrix.

The paste may have improved thermal conductivity.

The paste may have improved stability against heat and moisture.

The paste can be recycled or reused.

In addition, the present invention 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. A method for producing a Pickering emulsion-based heat dissipation paste is provided.

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 aliphatic alcohol to a suspension-type heat dissipation paste that was conventionally used as a simple mixture of silicone oil and alumina filler.

By forming a segregated network structure, the thermally conductive filler in the Pickering emulsion according to the present invention can perform heat transfer more effectively, and thus can be used as a heat dissipation material in various fields.

In addition, the heat dissipation paste manufacturing method according to the present invention is a low-cost, simple, and environmentally friendly method that can secure high thermal conductivity, stability, and applicability without the complicated manufacturing process of conventional heat dissipation pastes, expensive surfactants, or post-treatment processes. Paste can be manufactured, and filler can be recovered and recycled from heat dissipation paste using a simple centrifugation method.

In addition, the above manufacturing method can be immediately applied to industrial companies for efficient manufacturing and further enables mass production, which can contribute to the production and technology development of thermal interface materials in the future.

Figure 1 shows the results of measuring the thermal conductivity and yield stress of the heat dissipation paste according to the filler content according to an experimental example of the present invention. (a) shows the heat conduction according to the filler content of SA03, GA03, and SG5:5A03. Figure, (b) is the yield stress according to the filler content of SA03, GA03, and SG5:5A03, (c) is the thermal conductivity according to the filler content of SA90, GA90, and SG5:5A90, (d) is SA90, GA90, and SG5: This shows the yield stress according to the filler content of 5A90, where SA03, GA03, SA90, GA90, SG5:5A03, and SG5:5A90 are silicone oil + 3μm alumina, glycerol + 3μm alumina, silicon, respectively. This means oil + 90μm alumina, glycerol + 90μm alumina, a mixture of silicone oil and glycerol in a volume ratio of 5:5 + 3μm alumina, and a mixture of silicone oil and glycerol in a volume ratio of 5:5 + 90μm alumina. (e) and (f) are the thermal conductivity of the heat dissipation paste predicted by the Bruggeman model, (e) is the value predicted by fixing the thermal conductivity of the matrix at 0.2 W/m·K and changing the thermal conductivity of the filler, (f) is the value predicted by fixing the thermal conductivity of the filler at 100 W/m·K and changing the thermal conductivity of the matrix.

Figure 2 is an optical microscope (OM) image of a glycerol single matrix (GA03) containing 3 μm alumina filler, with filler contents of (a) 10 vol%, (b) 20 vol%, (c) 30 vol%, and (d), respectively. ) 40 vol%, and (e) 50 vol%, and the scale bar represents 20 μm.

Figure 3 is an optical microscope (OM) image of a silicone oil/glycerol emulsion (SG5:5A03) containing 3 μm alumina filler, with filler contents of (a) 0 vol%, (b) 10 vol%, and (c) 20 vol%, respectively. %, (d) 30 vol%, (e) 40 vol%, and (f) 50 vol%, and the scale bar represents 50 μm.

Figure 4 is an optical microscope (OM) image of a silicone oil/glycerol emulsion (SG5:5A90) containing 90 μm alumina filler, with filler contents of (a) 20 vol%, (b) 30 vol%, and (c) 40 vol, respectively. %, and (d) 50 vol%, and the scale bar represents 50 μm.

Figure 5 is a graph showing the change in thermal conductivity versus yield stress of heat dissipation paste samples according to another experimental example of the present invention.

Figure 6 schematically shows the heat transfer paths in different heat dissipation pastes, where (a) is a heat dissipation paste in the form of a suspension and (b) is a heat dissipation paste in the form of a Pickering emulsion.

Figure 7 shows changes in complex viscosity, yield stress, and thermal conductivity of heat dissipation paste samples under various temperature conditions according to another experimental example of the present invention. All samples were tested at 50, 75, and 100°C before measurement. It was heat treated (annealed) for a period of time and then cooled to room temperature and then measured. (a) is the complex viscosity of GA03_30 as a function of angular frequency, (b) is the complex viscosity of SG5:5A03_30 as a function of angular frequency, (c) is the yield stress of GA03_30 and SG5:5A03_30, (d) is the percentage change in yield stress shown in (c) relative to the value measured at 25°C, (e) is the thermal conductivity of GA03_30 and SG5:5A03_30, and (f) is the percent change in yield stress shown in (e) relative to the value measured at 25°C. It represents the percentage change in thermal conductivity.

Figure 8 shows the hygroscopicity of heat dissipation paste samples according to another experimental example of the present invention, showing (a) hygroscopicity and (b) normalized mass value of GA03 and SG5:5A03 by content at room temperature and 95% relative humidity. It represents.

Figure 9 shows the weight loss of alumina recovered from a silicone oil/glycerol emulsion (SG5:5A03) containing a 3μm alumina filler and pure alumina as the temperature rises in a nitrogen atmosphere.

Hereinafter, the present invention will be described in detail.

In order to manufacture a heat dissipation paste that has both high thermal conductivity and high stability, the present inventor prepared a paste in the form of a Pickering emulsion by including a thermally conductive filler in a matrix of a mixture of immiscible silicone oil and glycerol, and its excellent thermal conductivity and By confirming stability, the present invention was completed.

The present invention relates to silicone oil; aliphatic alcohol; And it provides a Pickering emulsion composition for heat dissipation, comprising a thermally conductive filler.

As used herein, “Pickering emulsion” refers to an emulsion in which two insoluble phases are stabilized using solid particles, unlike conventional emulsions that thermodynamically stabilize two immiscible phases through a surfactant. .

The silicone oil is a liquid silicone resin with a relatively low degree of polymerization and may include dimethyl silicone oil, methyl phenyl silicone oil, methyl hydrogen oil, fluorosilicone oil, amino-modified silicone oil, epoxy-modified silicone oil, etc. It is not limited to this.

The aliphatic alcohol may be an aliphatic polyhydric alcohol with relatively low volatility, for example, glycerol, ethylene glycol, propylene glycol, butylene glycol, neopentyl glycol, trimethylolethane, ditrimethylolethane, trimethylolpropane, ditrimethyl It may be selected from all-propane, etc., and preferably may be glycerol, but is not limited thereto.

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

The thermally conductive filler is intended to improve the thermal conductivity of the composition, and may include an inorganic filler such as alumina, magnesia, boron nitride, silicon nitride, or silica, and preferably has an average diameter such as spherical alumina, spherical magnesia, or spherical silica. It may be selected from a spherical filler with an average diameter of 0.1 to 100 μm, more preferably a spherical filler with an average diameter of 0.1 to 10 μm, but is not limited thereto.

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 a total of 100 parts by volume of the heat dissipating paste composition, but is not limited thereto.

The present invention provides a liquid matrix containing silicone oil and aliphatic alcohol; and a heat conductive filler dispersed in the matrix, providing a Pickering emulsion-based heat dissipation paste.

The liquid matrix may include silicone oil and aliphatic alcohol, and the corresponding features may be substituted in the above-described portion.

The matrix may include 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.

In general, silicone-based matrices have good applicability due to low viscosity and surface energy, but have low thermal conductivity and low interfacial affinity between filler and matrix, so phase separation easily occurs within the paste. Accordingly, a stable emulsion-type matrix with sufficient viscosity and high thermal conductivity can be manufactured by mixing an aliphatic alcohol with a higher viscosity and thermal conductivity than a silicone-based matrix. However, aliphatic polyhydric alcohols such as glycerol have excellent hygroscopicity and have poor long-term stability, so 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.

Features corresponding to the thermally conductive filler can be substituted for the parts described above.

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 a total of 100 parts by volume of the heat dissipating paste composition, but is not limited thereto. As the content of the filler increases, it becomes easier to form a heat transfer path, thereby improving thermal conductivity. However, if the content of the filler is excessive, the density of the heat dissipation material increases, making it heavier, and the paste formulation cannot be maintained, resulting in lower processability. As far as possible, it is preferable that the volume content is within the above range.

As described above, thermal interface material (TIM) is applied between a heat source and a heat sink. It is very important to apply TIM with a minimum void (air layer) between the two interfaces, and the paste formulation provides this. can satisfy. The paste formulation can maintain a short distance between the interfaces (bond line thickness) even with small pressure, minimizing thermal resistance occurring at the interface and ensuring high thermal conductivity. No curing process is required and no external interference is required. It has the advantage of absorbing stress well and enabling dissipation (damping).

The thermally conductive filler can be adsorbed along the interface between the insoluble silicone oil and glycerol to form a Pickering emulsion that thermodynamically stabilizes the thermodynamically unstable liquid matrix. Additionally, 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 can further improve thermal conductivity by securing a heat transfer path by forming a segregated network on a thermodynamically stable Pickering emulsion, and can exhibit high yield stress and have excellent structural and dispersion stability. Additionally, this can improve stability against heat and moisture.

Accordingly, the paste can be used in various electronic devices as a thermal interface material to ensure heat dissipation.

Additionally, the paste can be recycled or reused.

According to an experimental example of the present invention, it was confirmed that when the manufactured paste is disassembled and recovered, high purity filler can be recovered and reused without residual solvent.

In addition, the present invention 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. A method for producing a Pickering emulsion-based heat dissipation paste is provided.

More details will be explained later by the examples below.

Hereinafter, the present invention will be described in detail through examples to aid understanding. However, the following examples only illustrate the content of the present invention and the scope of the present invention is not limited to the following examples. Examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

<Example 1> Preparation of a composite (heat dissipation paste) in the form of Pickering emulsion

1-1. experiment material

Silicone oil (KF-96, 350 cs) was purchased from Shinetsu chemical (Japan). Glycerol (≥99.0%) was purchased from Sigma-Aldrich (Republic of Korea). Globular alumina with different sizes (3 μm, 20 μm, 90 μm) was purchased from Denka (Japan). The chemicals were used as received.

1-2. heat dissipation paste manufacturing

To prepare a control suspension-type heat dissipation paste, silicone oil or glycerol, and alumina of various sizes were weighed. The volume content of alumina in the total heat dissipation paste was 10, 20, 30, 40, and 50 vol%, respectively. Afterwards, the weighed suspension was mixed evenly for 1 minute at 2,000 rpm in a Thinky mixer AR-100 (Thinky, Japan). To prevent bubbles from forming in the paste, defoaming was additionally performed at 2,200 rpm for 1 minute and mixing was performed at 2,000 rpm for 1 minute. The final paste obtained had flowability even though it was impregnated with 50 vol% of alumina.

To produce a heat dissipation paste in the form of a Pickering emulsion, silicone oil and glycerol were first weighed at a volume ratio of 1:1 and mixed with a Thinky mixer at 2,000 rpm for 1 minute. Afterwards, in order to stabilize the two unstable liquid matrices, 10, 20, 30, 40, and 50 vol% of alumina is impregnated relative to the total paste content, followed by a deforming and mixing process similar to the method of producing a heat dissipation paste in the form of a suspension described above. proceeded with. The produced pastes had stable formulations at all contents.

1-3. Physical property analysis method

The thermal conductivity of the paste was measured at room temperature using a transient plain source method thermal analyzer TPS-2500S (Hot-disk AB, Sweden) in isotropic mode. Since the paste has flowability, to maintain the formulation, a vial lid with a depth and diameter of 10 mm and 20 mm, respectively, was covered with plastic wrap and the paste was placed and sealed. Afterwards, the sensor was placed between the lids of two identical vials containing paste, and thermal conductivity was measured by measuring contact resistance and temperature change. Rheological properties such as complex viscosity and yield stress with respect to angular frequency were measured using a HR-20 (TA Instrument, USA) rotational rheological property meter equipped with a parallel plate with a diameter of 40 mm, and the mode used was: was small-amplitude oscillatory shear (SAOS). The filler structure of the heat dissipation paste was observed using an optical microscope Eclipse LV100ND (Nikon, Japan). Each sample was spread thinly on a glass slide, stabilized at room temperature for 5 minutes, and then observed from above. To observe the temperature stability of the paste, the pastes were placed on a heating plate, heated at 50, 75, and 100°C for 1 hour each, and then cooled at room temperature. Afterwards, the rheological properties and thermal conductivity of the cooled sample were measured in the same manner as mentioned above. To observe the moisture stability of the heat dissipation paste, the weight change of the paste at room temperature was measured using an analytical balance AUW220D (Shimadzu, Japan). After filling each of the four vials with water, they were placed in an analytical balance, and the change in the weight of the paste was recorded at 10-minute intervals starting when the relative humidity reached 95%. In addition, to compare pure alumina and alumina recovered from a heat dissipation paste using 3 μm alumina, the weight change rate according to temperature increase was measured in a nitrogen atmosphere using thermogravimetric analysis (TGA).

<Experimental Example 1> Checking the yield stress and thermal conductivity of the heat dissipation paste

Yield stress and thermal conductivity are the most important characteristics that a heat dissipation paste must have. Thermal conductivity is a measure of the ability of an object to transfer heat, and yield stress refers to the minimum stress required to deform or flow the structure formed by particles. do. The yield stress of the paste results from noncovalent interactions between the fillers dispersed in the matrix or from the structure they form. In general, the higher the viscosity of the composite, the higher the yield stress, and the higher the yield stress, the better the structural stability. In other words, it can be said that the best type of heat dissipation paste is one that has high thermal conductivity and high yield stress at the same time.

The thermal conductivity and yield stress of heat-radiating paste composites in which different sizes and contents of alumina were dispersed in an emulsion of silicone oil, glycerol, and silicone oil and glycerol mixed at a volume ratio of 5:5 were measured. Alumina used as filler has a diameter of 3μm or 90μm. SA and GA represent suspension-type pastes composed of silicone oil, alumina, glycerol, and alumina, respectively, and SG5:5A represents an emulsion-type paste in which alumina is impregnated in a liquid matrix of silicone oil and glycerol in a 5:5 volume ratio. it means. The number after the sample name indicates the diameter of the spherical alumina used.

As a result of measuring the thermal conductivity and yield stress of heat dissipation paste made with silicone oil, glycerol, and glycerol/silicon oil as a matrix, as shown in Figures 1a to 1d, regardless of the type of matrix and filler, as the filler content increases, thermal conductivity and It was found that all yield stresses increased.

First, referring to Figures 1b and 1d, the heat dissipation paste in the form of an emulsion mixed with silicone oil and glycerol at a ratio of 5:5 always has a higher yield stress than the heat dissipation paste in the form of a suspension of silicone oil or glycerol (SA, GA) at all contents. indicated. This means that emulsion-type heat-dissipating pastes are always more stable than single-matrix-based suspension-type heat-dissipating pastes. In addition, the yield stress rapidly increased when 10 vol% of filler was impregnated regardless of the size of the impregnated alumina, and the emulsion type paste showed a higher yield stress than the suspension type heat dissipation paste. This is different from a heat dissipation paste in the form of a suspension in which alumina is randomly dispersed. When a liquid resin mixed with silicone oil and glycerol is used, alumina stabilizes the unstable interface between silicone oil and glycerol, and at the same time, the interaction between alumina becomes stronger, making the interaction between alumina relatively strong. This is thought to be because it forms a strong structure, and can also be seen as strong evidence that a Pickering emulsion has been formed.

Afterwards, the yield stress also steadily increases as the filler content increases up to 40 vol%, which is believed to be because alumina is continuously located at the silicon oil/glycerol interface to form a stronger segregated network structure.

At filler content higher than 40 vol%, both yield stress and thermal conductivity rapidly increase (Figures 1a and 1c), which forms a segregated network filler structure as alumina fills the space between silicone oil droplets dispersed in glycerol, a continuous phase. This is believed to be because a heat transfer path is secured.

As a result, SG5:5A03 [mixture of silicon oil and glycerol in a volume ratio of 5:5 + 3 μm alumina], SG5:5A90 [mixture of silicon oil and glycerol in a volume ratio of 5:5 + 90 μm alumina] were solid particles. It can be expected that the interface between the two insoluble liquids is a stabilized Pickering emulsion.

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

The thermal conductivity of the heat dissipation paste is determined by the filler assuming that the matrix is insulating. In particular, the thermal conductivity of the heat dissipation paste follows the Bruggeman model in Equation 1 below in the general case where spherical fillers are randomly distributed in the matrix:

<Equation 1>

1-V={(λ _p -λ _c )/(λ _p -λ _m )}*{(λ _m /λ _c )^(1/3)}

In the above equation, V is the volume fraction of the filler, λ _p , λ _m , and λ _c are the thermal conductivities of the filler, matrix, and composite (heat dissipation paste), respectively.

Referring to Figures 1e and 1f, when the thermal conductivity of the filler in a matrix having the same thermal conductivity increases, the increase in thermal conductivity of the entire composite is relatively not large, while the thermal conductivity of the filler in the composite does not change and only the thermal conductivity of the matrix increases. As it increases, it can be seen that the increase in thermal conductivity of the entire composite is very large. In other words, the greater the thermal conductivity of the matrix, the more advantageous it is to improve the thermal conductivity of the composite.

Analyzing Figures 1a and 1b from this perspective, for the single matrix-based heat dissipation paste, glycerol (0.299 W/m·K) has a higher thermal conductivity than silicone oil (0.183 W/m·K), resulting in a higher thermal conductivity in the composite as well. showed the way. On the other hand, the thermal conductivity of the mixture of silicone oil and glycerol of SG5:5A03 is 0.245 W/m·K, which is an intermediate value between the conductivity of glycerol and silicone oil, but when more than 20 vol% of alumina is added, the thermal conductivity is higher than that of GA03. The diagram shows results that are inconsistent with the Bruggeman model. It is believed that there is some other influence that overcomes the effect of the matrix on improving thermal conductivity. When combined with the yield stress results, the yield stress increases as alumina is located at the interface between silicone oil and glycerol to form a segregated network structure between fillers, thereby forming a more effective heat transfer pathway and greatly improving thermal conductivity. It can be expected that it brings , and in addition, SG5:5A03 can be judged to have the form of a Pickering emulsion.

On the other hand, referring to Figure 1c, it can be seen that the increase in thermal conductivity of SG5:5A90 due to increase in filler content is not greater than that of SG5:5A03. In the same emulsion system using 3μm of alumina, it can be judged that the thermal conductivity increased rapidly due to the segregated structure (Figure 1a), but when 90μm of alumina was added, the increase in thermal conductivity was small and the increase in yield stress was also small despite having the same matrix system. It appeared to be small, which is thought to be because a segregated structure was not formed. In other words, the size of the alumina was very large compared to the size of the formed droplet, so the alumina did not effectively stabilize the interface between the two liquids. Therefore, it can be expected that the oil droplets and alumina were randomly dispersed rather than a normal Pickering emulsion being formed.

In summary, the thermal conductivity of the paste was found to be higher as the size of the filler used in the heat dissipation paste in the form of a suspension using only silicon or glycerol generally increased, but in the form of an emulsion, the thermal conductivity of the paste was higher when small-sized alumina was used. The thermal conductivity showed that Pickering emulsion and segregated network structures were effectively formed when 3μm alumina was used.

To prove these facts, in Experimental Example 2 below, the morphology formed by the filler in the paste was confirmed through an optical microscope.

<Experimental Example 2> Confirmation of morphology of heat dissipation paste

As mentioned above, within a heat dissipation paste composite, the structure formed by the filler has a greater impact on the thermal conductivity of the entire composite than the thermal conductivity of the matrix itself, and the yield stress of the composite is the minimum required to deform the structure formed by the filler within the composite. It refers to strength and varies depending on the degree of structure formation.

Therefore, in order to determine the relationship between yield stress and thermal conductivity and the structure formed by alumina, SA03/90 and GA03/90, which are suspension-type heat dissipation pastes, and emulsion-type heat dissipation paste were examined using an optical microscope (OM). The structure formed by the filler in SG5:5A03/90 was observed.

As a result, as shown in Figures 2 to 4, the filler dispersed in a single matrix-based suspension using only silicone or glycerol and the filler in the emulsion showed different modifications.

First, referring to Figure 2, when the filler was dispersed in a single matrix like GA03, the filler appeared to be randomly dispersed, and then, as the filler content increased, the filler appeared to be densely impregnated. As a result, thermal conductivity and yield stress rapidly increased after the percolation threshold (> 40 vol%), at which fillers were interconnected (FIGS. 1a to 1d).

On the other hand, the emulsion-type composites (SG5:5A03, SG5:5A90) using a mixture of silicone oil and glycerol as a matrix showed different aspects from the suspension-type composites using only silicone or glycerol. Because alumina is a relatively hydrophilic particle, it has more contact with glycerol than silicone oil within the emulsion. That is, an oil-in-water (O/W) emulsion is formed in which the continuous phase is glycerol and the dispersed phase is silicone oil.

Therefore, as shown in Figure 3, the continuous phase represents glycerol and the droplets represent silicone oil, and when silicone oil and glycerol are simply dispersed without alumina, it can be confirmed that the droplet size is unstable and is in an unstable state (a). As alumina was impregnated with 10 vol%, the droplet size became relatively constant and alumina appeared to surround the silicon oil droplet (b). The reason the droplet size is constant is because the filler surrounds the droplet and forms a stable structure, which can be judged as great evidence of a Pickering emulsion. Afterwards, as the filler content increased, the matrix ratio decreased, the oil droplet size gradually decreased, and adjacent droplets were observed to be connected to each other by the filler. Finally, the droplets were reduced to a level similar to the size of alumina, and oil droplets and alumina were observed to be densely packed in the glycerol matrix.

Through this, it can be confirmed that SG5:5A03 forms a stable Pickering emulsion and segregated network structure with alumina. In addition, SG5:5A03 provided a more stable and strong structure, and the yield stress was found to be higher than that of SA03 and GA03. The heat transfer path through which heat can pass was well formed continuously, and the thermal conductivity was also found to be higher than that of SA03 and GA03. .

Previously, it was confirmed that SG5:5A90, like SG5:5A03, has a higher yield stress than SA90 and GA90 and has good stability, but the thermal conductivity is not significantly higher than GA90. Referring to FIG. 4, the diameter of the alumina is larger than the diameter of the silicon oil droplet, so the filler does not surround the droplet, but rather the droplet surrounds the filler.

In other words, compared to fillers randomly dispersed in a single matrix, SG5:5A90 has good stability because the droplets between fillers are packed and behave like solid particles (better yield stress than single matrix-based composite), but the fillers are sufficiently connected like random packing. Therefore, it does not bring about a significant improvement in thermal conductivity (there is no significant improvement in thermal conductivity compared to a single matrix-based composite).

In this way, it can be seen that thermal conductivity and yield stress are greatly affected by the structure formed by the filler in the composite, and ultimately, only SG5:5A03 can be said to be a Pickering emulsion.

<Experimental Example 3> Theoretical analysis of heat dissipation paste

According to the previous results, it can be seen that thermal conductivity is greatly affected by the structure formed by the filler. In other words, it can be seen how effectively the structure of the filler has improved thermal conductivity through the relationship between the yield stress and thermal conductivity, which represents a measure of the strength of the structure formed by the filler. Additionally, the degree to which filler structure formation contributes to thermal conductivity was analyzed using a thermal conductivity prediction model.

The overall thermal resistance of composites commonly used as TIMs is given by Equation 2:

<Equation 2>

R _bulk = BLT/k _TIM

In the above equation, R _bulk refers to the total thermal resistance of the TIM, BLT refers to the actual contact distance between the two solids, and k _TIM refers to the thermal conductivity of the composite.

In order to analyze BLT, it is important to determine what type of fluid the composite is. Previous research has shown that the viscosity of TIM shows Herschel-Buckley (H-B) fluid behavior, and BLT depends only on the yield stress of the composite in equilibrium. Accordingly, it was experimentally reported that the yield stress and thermal conductivity of TIM have the following equation 3:

<Equation 3>

k _TIM = (1/R _bulk )C(τ _y /P)^m

In the above equation, C and m are constants, τ _y is the yield stress of the composite, k _TIM is the thermal conductivity of the composite, and P is the pressure required to join two solid plates.

As a result, under the assumption that R _bulk is the minimum in all cases, that is, close to 0 and that P is also constant in all cases, it can be inferred that the thermal conductivity of the composite is proportional to the m power of the yield stress. A high m value indicates a large increase in thermal conductivity compared to the increase in yield stress, and a low m value indicates a small increase in thermal conductivity compared to the increase in yield stress. The larger the m value, the more efficiently the thermal conductivity is improved even if the filler has the same yield stress. It can be judged that a structure is formed.

In general, the thermal conductivity and yield stress of the composite have percolation characteristics, so the physical properties tend to be maximized when the filler is impregnated with a certain amount or more. Referring to FIGS. 1A to 1D, it can be seen that the slope of increase in thermal conductivity and yield stress of all pastes increases based on 40 vol%. In other words, since the behavior of the section where percolation occurs and the section where percolation does not occur varies depending on the filler content, it is appropriate to divide the relationship between thermal conductivity and yield stress based on 40 vol%.

Table 1 below shows the calculated m values of all pastes around 40 vol% according to Equation 3 above.

Referring to Table 1 and Figure 5, all pastes show lower m values after 40 vol%, which shows percolation behavior in which fillers are rapidly interconnected after 40 vol%, resulting in a rapid increase in yield stress compared to thermal conductivity. Because it happens.

In all pastes, as the size of the filled filler increases, the interfacial thermal resistance that occurs at the interface of the fillers as the fillers come into contact with each other at all contents decreases, increasing the thermal conductivity of the composite, and as the size of the filler increases, the interfacial thermal resistance of the composite decreases. The overall viscosity, that is, the yield stress, decreases and m increases.

SG5:5A03 and SG5:5A90 had relatively high m values at <40 vol% compared to suspension-type heat dissipation pastes, and m decreased sharply at >40 vol% compared to <40 vol%. A high m value means that the increase in thermal conductivity is large compared to the increase in yield stress. This is because at 40 vol% or less, unlike single matrix-based composites, the filler forms a segregated structure, making heat transfer effective, whereas at 40 vol% or more. The rapid decrease in m means that the movement of the filler is greatly restricted by the silicone oil droplets and the maximum limit at which alumina can be impregnated is reached, and the yield stress increases significantly compared to the increase in thermal conductivity.

Comprehensively judging Figures 1 to 5, first, as shown in Figures 3 and 4, SG5:5A03 is a form in which the filler surrounds the droplet and a segregated structure is formed by the filler, but in SG5:5A90, the size of the filler is similar to that of the liquid droplet. Since it is larger than the size of the enemy, the droplet surrounds the filler, and only SG5:5A03 can be confirmed to be in the form of a Pickering emulsion.

However, at 40 vol% or less for both SG5:5A03 and SG5:5A90, the filler had a regular structure due to oil droplets compared to the random single matrix-based composite, and the increase in thermal conductivity compared to yield stress was large, showing a large m value. Above 40 vol%, the decrease in m value was significant for both SG5:5A03 and SG5:5A90. In other words, at a high content of 40 vol% or more, the increase in yield stress is greater than that of a single matrix composite. This means that oil droplets stabilized in small sizes by a large amount of filler within the composite behave like solid particles, greatly limiting the movement of the filler. This is because the yield stress rapidly increases by limiting the volume that the filler can occupy.

As a result, both SG5:5A03 and SG5:5A90, which used silicone oil and glycerol as matrices, theoretically showed better stability and thermal conductivity than single matrix-based composites. Among them, SG5:5A03 had the form of a Pickering emulsion and was the best. It can be confirmed that it has excellent stability and thermal conductivity.

Next, you can check how well the internal structure formed by the filler is formed through the Agari model, which is a thermal conductivity prediction model for the composite.

The Agari model is a model proposed to consider the effect on the thermal conductivity of the network formed by filler at high content, and can be expressed as Equation 4 below:

<Equation 4>

logλ _c = v _f C _f logλ _f + (1 - v _f ) log(C _p λ _p )

In the above equation, 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.

C _p is a constant that takes into account the influence of the secondary structure (crystallinity or crystal size) of the matrix depending on the filling of the filler, that is, it is a measure of whether the filler affects the thermal conductivity of the matrix, and C _p is 1 This means that under ideal conditions, filler impregnation does not affect the thermal conductivity of the matrix. In general, polymer composite materials are known to be close to C _p = 1, so in this experimental example, C _p = 1 was fixed.

C _f is a measure of how efficiently the filler is tightly packed and forms a network at high density. In other words, it is a measure of the ability to form a heat conduction network. Generally, C _f has a value between 0 and 1. The higher C _f , the more airtight the network structure of the filler is, so heat transfer occurs more effectively, resulting in higher thermal conductivity. As a result, analyzing the C _f value in each case through Agari fitting based on the thermal conductivity to the filler volume ratio of all composites determines which composite has the most efficient filler network structure for heat transfer. You can see how it is formed.

Table 2 below calculates the C _f values of all composite samples after fixing C _p at 1.

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

In addition, SG5:5A90 also shows a relatively high C _f value, which is believed to be because the movement of the filler is limited by the surrounding oil droplets and is relatively ordered compared to the single matrix-based composite.

Referring to Figure 6, it can be seen that, unlike single matrix-based composites, SG5:5A03 not only forms an effective heat transfer path by forming a segregated structure, but also has a high yield stress, that is, structural stability and dispersion stability. That is, at <40 vol%, it forms a segregated structure and shows a high m value, and at >40 vol%, it forms a dense structure with oil droplets, showing a rapid decrease in m, and has the highest C _f value. As a result, SG5:5A03, a Pickering emulsion, has high stability and high thermal conductivity, which has been theoretically proven to be due to the segregated structure formed by the filler.

<Experimental Example 4> Stability evaluation of heat dissipation paste

Because heat dissipation paste is exposed to heat generated from electronic devices for a long time, long-term stability under high temperatures is important. Under high temperatures, the heat dissipation paste in the form of a complex fluid rapidly decreases in viscosity and begins to flow. When excessive heat is concentrated in this way, a pump out phenomenon occurs in which the matrix with reduced viscosity flows out of the complex fluid and reduces stability, or a dry out phenomenon occurs in which air fills the area where the matrix disappears and the heat transfer path is reduced. A phenomenon occurs. In addition, the viscosity may decrease and problems with filler precipitation may occur, which is the fundamental cause of the disadvantage of heat dissipation paste products in which thermal conductivity gradually decreases during long-term use. In other words, it is important to be stable at high temperatures and have little change in viscosity.

To observe the change in viscosity according to temperature of the heat dissipation paste in the form of suspension and emulsion, the change in viscosity according to temperature was observed for GA03 and SG5:5A03, which have relatively similar thermal conductivities.

After sufficiently raising the temperature from room temperature to 100°C, the temperature was lowered back to room temperature, and the changes in complex viscosity, yield stress, and thermal conductivity according to the angular frequency of the high-temperature heat dissipation paste were examined. As a result, shown in Figure 7. As shown, the complex viscosity of GA03 was found to decrease rapidly as the temperature increased, and as the viscosity decreased at high temperature, the yield stress also decreased rapidly to 0.5% compared to room temperature. The thermal conductivity measured at room temperature after heat treatment at high temperature was also found to decrease by about 30% compared to the thermal conductivity measured without heat treatment. This is because as the temperature rises, the matrix within the composite flows rapidly and phase separation from the filler occurs. These results mean that GA03 has poor thermal stability.

On the other hand, in the case of SG5:5A03, the complex viscosity decreases as the temperature increases, but the decrease is very small compared to GA03. As the viscosity decreases at high temperature, the yield stress also decreases to 50% compared to room temperature, but in GA03 You can see that it is very small. At high temperatures, the thermal conductivity also decreases by about 5% compared to the thermal conductivity at room temperature, but it is still very small compared to GA03. In other words, it can be seen that SG5:5A03, a Pickering emulsion, is structurally stable as the filler forms a segregated network structure, suppressing phase separation at high temperatures. Through this, it can be confirmed that SG5:5A03 is thermally very stable.

In addition, the heat dissipation paste in electronic devices must maintain a stable formulation even when exposed to extreme environments with high humidity to ensure efficient heat dissipation performance.

Glycerol, one of the matrices used in the above example, has three -OH functional groups per molecule and has hygroscopic properties. Therefore, although GA03 has a thermal conductivity comparable to that of SG5:5A03, a Pickering emulsion, it is considered unsuitable for use as a heat dissipation paste due to its hygroscopicity.

To test the hygroscopicity of GA03 and SG5:5A03, the weight change over time of heat dissipating pastes with different filler contents was measured at room temperature and a relative humidity of 95%, and was expressed using Equation 5 below:

<Equation 5>

Hygroscopicity (%)=[(W1-W0)/W0]×100

If each heat dissipation paste has high hygroscopicity, the mass of the heat dissipation paste will increase as it absorbs water vapor over time.

Referring to Figure 8, all composites showed a small increase in weight due to improved structural stability and a decrease in the relative amount of matrix as the filler content increased. Even at the highest content, GA03_50, GA03 increased by about 20% compared to the initial weight, which is due to the high hygroscopicity of glycerol. On the other hand, SG5:5A03 showed an increase of only 7% compared to the initial weight even at SG5:5A03_10 (10 vol%) with the lowest filler loading, and in SG5:5A03_50, it increased only 1% compared to the initial weight, so it can be said to have almost no hygroscopicity. there is. This is due to the segregated structure formed by the filler, although it contains some glycerol. As a result, it can be confirmed that SG5:5A03 is a heat dissipation paste that is stable even at high temperatures and moisture as a Pickering emulsion.

<Experimental Example 5> Heat dissipation paste filler recovery

Recently, the demand for sustainability, eco-friendly and recyclable materials has increased significantly. In addition, because fillers in polymer composite materials are usually expensive, recovery and recycling/reuse of fillers used in composite materials are of great industrial importance. For this reason, a recovery experiment was conducted for recycling of alumina filler from the Pickering imulsion heat dissipation paste produced in the present invention.

Experiments were first conducted on the Pickering imulsion heat dissipation paste of SG5:5A03_10, which was manufactured with a low content of filler. First, centrifugation was repeated three times for about 10 minutes at room temperature at a rotation speed of 4,000 rpm using a centrifuge (Labogene 1248R). Through this, phase separation of the filler and matrix was induced and alumina was recovered. The recovered alumina was washed several times with hexane and distilled water. Hexane was used as a solvent that could dissolve silicone oil, and distilled water was used as a solvent that could dissolve glycerol. Afterwards, silicone oil and glycerol were removed from the phase-separated alumina composite material through a vacuum filtration device. Additionally, to remove residual solvent, the filtered alumina was dried at 100°C under vacuum for about 2 days.

In addition, it was confirmed whether recovery of filler was possible in the case of Pickering emulsion heat dissipation paste of SG5:5A03_50, which was manufactured with a high content of filler. First, SGA03_50 was diluted with hexane and distilled water, and then the filler was evenly dispersed using a Thinky mixer and sonicator. As with the above-mentioned SGA03_10 heat dissipation paste, the matrix and filler were separated by centrifugation, washing, filtration, and drying.

Evaluation of the recovered alumina was conducted through thermogravimetric analysis (TGA). Referring to Figure 9, it can be seen that the alumina recovered from the Pickering imulsion heat dissipation paste of low and high content fillers, like pure alumina, does not change in weight due to volatilization of distilled water, glycerol, and silicone oil even at high temperatures up to 800°C. You can. This means that the recovered alumina can be reused in the future as high purity alumina without residual solvents.

The Pickering emulsion heat dissipation paste has the advantage of simply mixing a glycerol/silicon oil matrix and alumina filler, so the filler could be recovered through physical decomposition. This means that the filler separation method can be applied to the Pickering imulsion heat dissipation paste system, which can be applied in a variety of ways. Additionally, the recovered filler is expected to be recyclable in the manufacture of new composite materials in the future.

So far, we have looked at specific embodiments of the present invention. A person skilled in the art to which the present invention pertains will understand that the present invention may be implemented in a modified form 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 indicated in the claims rather than the foregoing description, and all differences within the equivalent scope should be construed as being included in the present invention.

Claims (13)

1. silicone oil; aliphatic alcohol; And a Pickering emulsion composition for heat dissipation, comprising a thermally conductive filler.
2. According to claim 1,

   The silicone oil is,

   A Pickering emulsion composition for heat dissipation, characterized in that it is at least one selected from the group consisting of dimethyl silicone oil, methyl phenyl silicone oil, methyl hydrogen oil, fluorosilicone oil, amino-modified silicone oil, and epoxy-modified silicone oil.
3. According to claim 1,

   The aliphatic alcohol is

   Characterized in that it 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. , Pickering emulsion composition for heat dissipation.
4. According to claim 1,

   The silicone oil and aliphatic alcohol are,

   A Pickering emulsion composition for heat dissipation, characterized in that it is contained in a volume ratio of 1: (0.1 to 2).
5. According to claim 1,

   The thermally conductive filler is,

   A Pickering emulsion composition for heat dissipation, characterized in that it is at least one selected from the group consisting of alumina, magnesia, boron nitride, silicon nitride, and silica.
6. According to claim 1,

   The thermally conductive filler is,

   A Pickering emulsion composition for heat dissipation, characterized in that it is a spherical filler with an average diameter of 0.1 to 100 μm.
7. According to claim 1,

   The thermally conductive filler is,

   A Pickering emulsion composition for heat dissipation, characterized in that it contains 10 to 60 parts by volume based on a total of 100 parts by volume of the composition.
8. A liquid matrix containing silicone oil and aliphatic alcohol; and

   A Pickering emulsion-based heat dissipation paste comprising a thermally conductive filler dispersed in the matrix.
9. According to claim 8,

   The thermally conductive filler is,

   A Pickering emulsion-based heat dissipation paste, characterized in that it forms a separated network within the matrix.
10. According to claim 8,

    The paste is,

    Pickering emulsion-based heat dissipation paste, characterized by improved thermal conductivity.
11. According to claim 8,

    The paste is,

    A Pickering emulsion-based heat dissipation paste characterized by improved stability against heat and moisture.
12. According to claim 8,

    The paste is,

    Pickering emulsion-based heat dissipation paste, characterized in that it can be recycled or reused.
13. Preparing a liquid matrix by mixing silicone oil and aliphatic alcohol; and

    A method for producing a Pickering emulsion-based heat dissipation paste, comprising adding a thermally conductive filler to the prepared liquid matrix.

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