Fabrication and Characterization of Al₂O₃-Siloxane Composite Thermal Pads for Thermal Interface Materials

Abstract

In this study, Al₂O₃–siloxane composite thermal pads were fabricated using a tape–casting technique, and the thermal conductivity effect of the Al₂O₃ nanoparticle powder synthesized using a flame fusion process on siloxane composite thermal pads was investigated. Furthermore, various case studies were implemented, wherein the synthesized Al₂O₃ nanoparticle powder was subjected to different surface treatments, including dehydration, decarbonization, and silylation, to obtain Al₂O₃–siloxane composite thermal pads with high thermal conductivity. The experimental results confirmed that the thermal conductivity of the Al₂O₃–siloxane composite pads improved when fabricated using surface–treated Al₂O₃ nanoparticle powder synthesized with an optimally spheroidized crystal structure compared to that produced using non–treated Al₂O₃ nanoparticle powder. Therefore, this study provides guidelines for fabricating Al₂O₃–siloxane composite thermal pads with high thermal conductivity in the field of thermal interface materials.

1. Introduction

The rapid development of microelectronics technology has led to an increase in the integration and miniaturization of electronic components. However, there might be an increase in the heat generated by electronic devices during operation because of these innovations. Thermally conductive materials (TCMs) are used to ensure good heat transfer characteristics in electronic devices [1]. Among the different types of TCMs, thermally conductive adhesives, such as epoxy resins, are widely used in electronic devices because of their advantages, such as ease of processing, simple fabrication, low cost, and thermal stability [1]. The thermal conductivity of epoxy resins is very low, and it may be improved by filling the epoxy resin with ceramic, carbon, and metal particles [1]. However, incorporating metal particles deteriorates the electrical insulating and dielectric properties of these composites, thereby limiting their application to electronic packaging. Although carbon materials offer advantages such as high thermal conductivity and lightweight characteristics, their high cost and low electrical insulation properties limit their practical applications in industry. Ceramic particles provide excellent thermal conductivity and mechanical properties compared to carbon materials and metal particles, and, therefore, they have been extensively studied [1]. Ceramic materials are widely used in various fields of engineering, such as electronics, optics, metallurgy, and biomedicine [1,2]. Among them, Al₂O₃ is an attractive material for fabricating thermally conductive composite thermal pads because of its advantages, including high thermal conductivity, low cost, and stable chemical properties. In epoxy-resin composites, Al₂O₃ fillers increase the strength and elastic modulus of materials for product molding [1,2,3,4]. However, there are several challenges that are encountered when adding Al₂O₃ powder as a filler during the fabrication of thermal pads, such as molding failure and decreased rheological behavior of the Al₂O₃ paste. Therefore, numerous studies have investigated the material properties of the Al₂O₃ filling powder, including the particle size, shape, crystalline phase, polydispersity of particle size distribution, and elemental composition, which significantly influence the characteristics of Al₂O₃ pastes [5,6,7,8].

Given this context, this study investigated various surface treatments of Al₂O₃, including dehydration, decarbonization, and silylation. Furthermore, the structural properties were studied to overcome the Al₂O₃ nanoparticle agglomeration problem in trimodal distributions and improve the miscibility with siloxane to obtain highly conductive Al₂O₃–siloxane composite thermal pads. We focused on the surface modification of the spherical Al₂O₃ nanoparticle powder to fabricate Al₂O₃–siloxane composite thermal pads. The fabricated Al₂O₃-siloxane composite thermal pads were characterized using X–ray diffraction (XRD), field emission scanning electron microscopy (FE–SEM), Fourier transform infrared (FT–IR) spectroscopy, inductively coupled plasma-optical emission spectrometry (ICP–OES), nuclear magnetic resonance (NMR), and rheology, thermomechanical, and electrical properties.

2. Materials and Experimental Methods

2.1. Preparation of Al₂O₃ Nanoparticle Powder

Figure 1 shows the experimental setup of the flame fusion process for synthesizing spherical Al₂O₃ nanoparticle powders. An Al₂O₃ nanoparticle powder with a particle size and purity of 300 nm and 99.9%, respectively, was used as the starting raw material.

Al₂O₃ powders were purified using deionized (DI) water and an acidic solution. The Al₂O₃ powders were spheroidized using three different liquefied natural gas (LNG) flow rates (60, 105, and 120 Nm³/h) to obtain the desired particle size. Propane and butane gases were primarily used. The Al₂O₃ raw powder was melted and spheroidized at 2500 to 3000 °C via the flame fusion process. The Al₂O₃ nanoparticle powders were synthesized by using a flame fusion process with three different LNG flow rates (60, 105, and 120 Nm³/h), and the obtained by–products had a high purity (>99.9%) and a mean diameter (D50) of 800 nm. The Al₂O₃ powders were continuously injected during the spheroidization process, and oxygen and LNG were flowed concurrently into the powder feeder. These combustion reactions occurred under the LNG flow as the following reactions [9]:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

(1)

2C₄H₁₀ + 13O₂ → 3CO₂ + 10H₂O

(2)

The main parameters for spheroidization include three different LNG flow rates of 60, 105, and 120 Nm³/h at a fixed oxygen gas flow rate with a ratio of oxygen to LNG of 70:30. The Al₂O₃ powders in the reaction chamber treated under high–temperature conditions were quickly cooled after the synthesis process. In addition, when incomplete combustion occurs in the flame fusion spheroidization process using LNG, the generated CO₂ or H₂O is adsorbed on the surface of the Al₂O₃ nanoparticle powder and affects its physical properties.

2.2. Surface Treatment of the Synthesized Al₂O₃ Nanoparticle Powders

In Case I (before treatment), since the obtained spherical Al₂O₃ nanoparticle powder contains a large amount of impurities adsorbed in the reaction chamber during synthesis, it is necessary to remove impurities from the surface of the Al₂O₃ nanoparticle powder in order to apply thermal pads with high thermal conductivity. Three different surface treatments were used for the synthesized Al₂O₃ powders, including dehydration (Case II), decarbonization (Case III), and silylation (Case IV). The surface of the Al₂O₃ powder was produced with a small amount of residual impurities, including acetaldehyde (CH₃COH), hydrocarbons, and carbon-based components (CO₂, CO), because of combustion reactions. Therefore, it was necessary to perform dehydration (Case II) and decarbonization (Case III) to remove these carbon compounds from the Al₂O₃ powder. For the dehydration reaction (Case II), the surface of the spherical Al₂O₃ powder was treated using a methanol solution. In this step (Case II), 50 wt% Al₂O₃ powder was added to the methanol solution and stirred for 30 min. After that, the treated Al₂O₃ powders were filtered by separating the processed powder and methanol via centrifugation. After filtration, the dehydrated Al₂O₃ powder was dried in a vacuum oven at 110°C under a N₂ atmosphere for 24 h. In the case of decarbonization (Case III), the spherical Al₂O₃ powder was heated in a furnace at 750 °C for 1 h under ambient air conditions to remove the carbon compounds.

For silylation (Case IV) with an aryl functional group (diphenyl–methoxy silane, molecular weight of 214.33 g/mol), the Al₂O₃ powder was added to a solution of methanol and 2 wt% silane and stirred for 30 min. The treated powders were filtered by separating the processed powder and methanol via centrifugation. After filtration, the dehydrated Al₂O₃ powder was dried in the oven at 110 °C for 24 h under ambient air conditions.

2.3. Fabrication of Al₂O₃-Siloxane Composite Thermal Pads Using the Synthesized Al₂O₃ Nanoparticle Powder

When using Al₂O₃ loading above 60 wt% of large particles in this experiment, the decrease in viscosity occurs in the high content of large particles by the Farris effect [10]. In general, the particle size of the Al₂O₃ powder will affect the thermal conductivity of the composite pads. This relationship between the thermal conductivity of the composite pad and its particle sizes is described as follows: the smaller the particle size, the better the thermal conductivity. This trend suggests that smaller particles can transfer heat more efficiently due to their high specific surface area [1]. However, the nanoparticles continuously thicken by aggregation in the liquid matrix, and then the next size-up particles could also be thicken this size distribution. For this reason, to prevent an increase in viscosity and create a composite thermal pad with high thermal conductivity, the synthesized spherical Al₂O₃ powder was applied in a trimodal system with three particle size distributions to fabricate Al₂O₃-siloxane composite thermal pads with high packing density.

Figure 2 shows the mixing ratio of the synthesized spherical Al₂O₃ powder. The trimodal mixture of the Al₂O₃ powder was blended using a solution blending process based on the mixing ratio, as indicated in

Table 1. Mixing ratio of the synthesized spherical Al₂O₃ powder for Al₂O₃–siloxane composite thermal pads.

The Synthesized Spherical Al2O3 Powder	Particle Size of the Al2O3 Powder
	70 μm	10 μm	800 nm
Mixing ratio (wt. %)	55	36	9

. The blended Al₂O₃ powder mixtures were stirred for 24 h via wet mixing to achieve a homogeneous mixture.

Figure 3 shows the fabrication procedure of an Al₂O₃–siloxane composite thermal pad using a tape–casting technique with respect to various surface–treated nanoparticle powders (Cases I, II, III, and IV). For preparing Al₂O₃–siloxane composite pastes, the Al₂O₃ powder mixture was blended with vinyl–terminated polydimethylsiloxane (PDMS) combined with a silicone–hydride–terminated crosslinker (1:1 molar ratio with PDMS/silicone). The produced Al₂O₃–siloxane composite paste was homogeneously mixed for 1 h and maintained in a deformed state for 3 min to completely remove the air bubbles (AR–100, Thinky mixer, Tokyo, Japan). Finally, to fabricate the Al₂O₃–siloxane composite thermal pads, the Al₂O₃–siloxane composite paste was poured onto a Teflon substrate after mixing with a Pt catalyst (5 μL, 11.5 ppm). It was then laminated using a tape–casting technique and compressed at 25 °C under a pressure of 130 kg/m² for 1 min. The laminated Al₂O₃–siloxane composite thermal pads were cured in an oven at 120° C for 8 h under ambient air conditions. The laminated Al₂O₃–siloxane composite thermal pads were removed from the Teflon substrate to obtain the Al₂O₃–siloxane composite thermal pads. Various Al₂O₃–siloxane composite thermal pad samples were systematically prepared using different volume fractions ranging from 60 to 79 vol% of Al₂O₃ powder to investigate the effects of the high contents of the Al₂O₃ powder.

2.4. Analysis and Characterization

2.4.1. X-ray Diffraction

The crystalline structure of the Al₂O₃ nanoparticle powder was measured using XRD (D/max–2500V/PC, Rigaku, Japan). To investigate the XRD peaks, XRD spectra were scanned over the θ–2θ range of 10° to 90° at 0.08 intervals using Cu Kα radiation (λ = 1.54 Å) at 40 mA and 100 kV.

2.4.2. Field Emission Scanning Electron Microscopy

The surface morphologies of the Al₂O₃ nanoparticle powder and Al₂O₃–siloxane composite thermal pads were evaluated using FE-SEM (JSM-7001F, JEOL, Tokyo, Japan) at a voltage and current of 20 kV and 0.8 nA, respectively. Before loading the samples into the chamber, they were made conductive by coating them with Pt.

2.4.3. Fourier Transform Infrared Spectroscopy

The main functional groups and crystalline phase of the Al₂O₃ nanoparticle powder were characterized using FT–IR spectroscopy (Vertex 70, Bruker, Billerica, MA, USA). The FT–IR spectrum was acquired using attenuated total reflection conditions at a wavenumber resolution interval of 0.6 cm⁻¹ in the region from 650 to 4000 cm⁻¹.

2.4.4. Inductively Coupled Plasma–Optical Emission Spectrometry

The elemental compositions and impurity quantifications of the Al₂O₃ powders were determined using ICP–OES (Optima 5300DV, Perkin Elmer, Waltham, MA, USA) for the cation analysis of the supernatant of the synthesized Al₂O₃ powders.

2.4.5. Nuclear Magnetic Resonance

The Al₂O₃ nanoparticle powder was investigated using an ²⁷Al solid–state NMR system (Avance 600 MHz, Bruker, Billerica, MA, USA) operated at a spinning speed of 22 kHz and 156.47 MHz for analyzing the aluminum (Al⁺) sites and structural transformations. All NMR spectra were calibrated using a 1M AlCl₃ aqueous solution at 0 ppm as a reference. The peak deconvolution was quantified by spectrum simulation using quadruple coupling constants presented in the reference journal.

2.4.6. Thermomechanical and Electrical Properties

The thermal resistance was measured at 50 °C in accordance with ASTM D5470 using a thermal resistance analyzer (T3Ster DynTIM, Siemens, München, Germany) to obtain the thermal conductivity of the Al₂O₃–siloxane composite thermal pads. For this measurement, the fabricated thermal pads were cut into 1 to 2 mm thick circles (diameter: 10 mm).

The thermal conductivity values of the Al₂O₃–siloxane composite thermal pads were calculated based on the changes in the thermal resistance at a fixed pressure of approximately 344.5 kPa. The thermal conductivity of the Al₂O₃–siloxane composite thermal pads was proportional to the slope of the curve obtained by plotting the measured thermal resistance values as a function of the distance between the measurement surfaces. These curves were obtained using Equations (3) and (4):

$$k=\frac{∆L}{∆R_{th}}\times \frac{1}{A}=\frac{1}{m\times A}$$

(3)

$$m=\frac{∆R_{th}}{∆L}$$

(4)

where $k$, $∆L$, $∆R_{th}$, A, and m represent the calculated thermal conductivity of the material, width of the sample holder (i.e., the bond line thickness (BLT)), $∆R_{th}$ measured thermal resistance, contact area, and slope, respectively. According to Equations (3) and (4), the observed thermal resistance of the Al₂O₃–siloxane composite pads linearly depends on the thickness of the composite pads [11,12].

The viscosity of the Al₂O₃–siloxane composite paste was measured using a Brookfield viscometer (TVB-10, Toki Sangyo Co., Ltd., Shinbashi, Japan) at 25 °C. The rheological characteristics of the Al₂O₃–siloxane composite pastes were compared by evaluating the viscosity values obtained at low shear rates (0.025 to 0.20 s⁻¹) to control the observed instability at high shear rates. This approach was used to investigate the effects of the high content ratio and surface treatment of Al₂O₃.

The hardness of the Al₂O₃–composite thermal pad was investigated as a mechanical performance indicator, which was used to characterize the degree of softness and hardness under specific conditions. Furthermore, the degree of hardness refers to the material’s resistance to local deformation, especially plastic deformation, and its indentation ability. The rubber hardness was evaluated using the ASTM D2240 method with a durometer. The hardness of the fabricated Al₂O₃–composite thermal pad was then measured using Shore 00–type indentor hardness.

Finally, a thermogravimetric (TG) loss curve was obtained using a TG/differential thermal analyzer (STA 409PC, NETZSCH, Selb, Germany) for the thermogravimetric analysis of the silane surface treatment. This analysis was conducted under the conditions of 1000 °C in an air atmosphere at a heating rate of 274.15 K/min. TGA was performed under a minimum ambient air flow of 40 mL/min in a temperature range of 25 to 1000 °C.

3. Results and Discussion

3.1. Synthesis of the Al₂O₃ Nanoparticle Powder

Figure 4 shows the XRD patterns of the synthesized Al₂O₃ powders with three different LNG flow rates. The experimental results indicate that all Al₂O₃ powder samples have three crystal phases corresponding to δ–Al₂O₃ (PDF#04–021–8097), θ–Al₂O₃ (PDF#01–086–1410), and α–Al₂O₃ (PDF#01–071–1123). The quantified crystal phase is analyzed using the Rietveld method in combination with the obtained XRD spectra. The increase in the proportion of the δ phase from 85% to 88% when the LNG flow rates changed from 60 to 120 Nm³/h can be associated with a change in the decrease in the proportion of the α phase from 7% to 2%. In general, the Al₂O₃ powder has a γ–Al₂O₃ phase when obtained below 700 to 800 °C [13]. However, the Al₂O₃ powder obtained in this study at approximately 3000 °C above 800 °C has a complex crystalline phase of δ–, θ–, and α–Al₂O₃ [13]. The detailed Rietveld analysis results are summarized in

Table 2. Quantitative phase analysis results of Al₂O₃ nanoparticle powders with three different LNG flow rates.

Crystal Phase (%)	LNG Flow Rates (Nm3/h)
	60 Nm3/h	105 Nm3/h	120 Nm3/h
δ	85	87	88
θ	8	10	10
α	7	3	2

.

Figure 5 shows the high–resolution ²⁷Al solid NMR spectra of the synthesized Al₂O₃ powders obtained with respect to three different LNG flow rates. All spectra consist of three major peaks centered at 12, 35, and 67.2 ppm, which can be attributed to the Al³⁺ cations in the octa (Al_Oh), penta (Al_P), and tetrahedral (Al_Td) coordinations, respectively [14,15]. The surface of the Al₂O₃ crystal is regarded as a truncated plane of crystal consisting of coordinately unsaturated anion and cation sites. When the O–Al–O ratio of the basic Al₂O₃ structure increases, the Al₂O₃ structure is changed from a tetrahedral to an octahedral structure. In a stable octahedral structure, Al³⁺ cations have a strong ionic chemical bond and do not easily react with other ions. Meanwhile, Al³⁺ cations exhibit weak charge-balancing ionic interactions in the tetrahedral structure and easily react with other ions.

Because this reaction occurs easily within the tetrahedral structure, the viscosity of the Al₂O₃–siloxane composite paste may increase due to the resistance resulting from the free random movement and shear forces of the Al₂O₃ particles [15]. The ²⁷Al NMR analysis results indicate that the characteristics of the tetra–, penta–, and octa–coordinated Al⁺ sites can affect the surface properties of Al₂O₃. The crystal phase of the Al₂O₃ powder was dominant, as confirmed by the δ–Al₂O₃ phases of the octahedral and tetrahedral coordinations. The quantified octahedral and tetrahedral coordinations of the Al⁺ site in the Al₂O₃ powder were compared to the sum of the individual fitting lines. The ratios of the Al⁺ sites (Al_Oh/Al_Td) in the three samples obtained when the LNG flow rate was varied from 60, 105, and 120 Nm³/h are listed in

Table 3. Detailed octahedral and tetrahedral sites calculated from the ²⁷Al solid NMR spectra for Al₂O₃ powders with three different LNG flow rates.

LNG Flow Rates (Nm3/h)	AlTd (%)	AlP (%)	AlOh (%)	AlOh/AlTd
60 Nm3/h	33	2.6	100	3.0
105 Nm3/h	34	2.9	100	2.9
120 Nm3/h	31	2.7	100	3.2

. The detailed peak ratios of the octahedral and tetrahedral sites are summarized in Table 3, which were calculated from the ²⁷Al solid NMR spectra of the Al₂O₃ powders obtained under the three different LNG flow rates.

The ²⁷Al NMR results indicate that at an LNG flow rate of 120 Nm³/h, the ratio of Al⁺ sites (Al_Oh/Al_Td) slightly increased from 3.0% to 3.2% because of an increase in the ratio of octahedrons compared to that of tetrahedrons. Compared to the XRD results, the proportion of the δ–Al₂O₃ phase increased in the order of 85%, 87%, and 88% for LNG flow rates of 60, 105, and 120 Nm³/h, respectively. For the LNG flow rate of 105 Nm³/h, the synthesized Al₂O₃ powder showed a low peak ratio (Al_Oh/Al_Td) of the Al⁺ site. The comparatively more unstable tetrahedral structure became dominant on the surface of the systems, with a lower ratio of Al⁺ sites (Al_Oh/Al_Td). However, this is affected by the surface lattice distortions caused by molecular bonds such as O–Al–O and M–Al–O, owing to the inclusion of anionic impurities (M⁺) [16]. At an LNG flow rate of 120 Nm³/h, the Al₂O₃ powder has a high coordination of the octahedral phase with a high peak ratio (Al_Oh/Al_Td) of the Al⁺ site. The high coordination of the octahedral phase of the Al₂O₃ surface is more favorable toward the Al₂O₃ composite paste process for decreasing the resistance of the powder particle movement. This can help decrease the shear stress and viscosity, thereby increasing the powder packing density [16]. Therefore, we have established the optimum conditions for the LNG flow rate for spheriodization using frame fusion based on the ²⁷Al NMR results.

Figure 6 shows the FTIR spectra of the synthesized Al₂O₃ powders with three different LNG gas flow rates. Based on the experimental results of all samples, the strong absorption peaks of Al₂O₃ at 3430, 1637, and 1413 cm⁻¹ are the C–OH stretching vibration peak, –C=O stretching, and C–OH bending vibration peaks, respectively, thereby indicating the presence of oxygen–containing groups on carbonate [17]. These peaks are attributed to the residual carbonate formed via methane combustion due to incomplete flame fusion combustion. The peaks of the residual carbonate functional groups are reduced when the LNG flow rate is increased from 105 to 120 Nm³/h.

3.2. Surface Treatment of the Al₂O₃ Powder

Surface treatments (Cases I, II, III, and IV) of the spherical Al₂O₃ powders were conducted to investigate their effect on the agglomeration of Al₂O₃ powders. Figure 7 shows photographs and FE-SEM images of the surface-treated Al₂O₃ powders with various treatment conditions (Cases I, II, III, and IV). All Al₂O₃ powders showed spherical morphologies according to the images obtained before and after surface treatment. In Cases II and III, the treated Al₂O₃ powders were well separated and dispersed without agglomeration after undergoing surface treatment. The FE–SEM image results show that in Case IV, the nanoparticles dispersed more effectively compared to those in the other treatment conditions. Furthermore, Case II has a disadvantage in that suspended nanoparticles (100 nm or less) can be removed by a solvent during the dehydration-washing process, as shown in Figure 7b. Case IV suggests that the treated Al₂O₃ powders show a more densified nanoparticle separation distribution when compared to Cases II and III after undergoing surface treatment.

Table 4. Elemental compositions obtained by ICP–OES quantification for the treated Al₂O₃ powders with the various treatment conditions of (a) Cases I, (b) II, and (c) III.

Case Study	Element Compositions (mg/L)
	Si	Fe	Ca	Mg	Na
Case I (before treatment)	0.16	<0.01	0.51	0.08	5.02
Case II	0.12	<0.01	0.65	0.04	0.74
Case III	0.11	<0.01	<0.01	<0.01	4.46

lists the element compositions analyzed using ICP–OES quantification of the treated Al₂O₃ powders to investigate the effects of decreasing cation compositions (Si, Fe, Ca, Mg, and Na) resulting from Case II and Case III on electrical conductivity. The samples were prepared by mixing Al₂O₃ powder and DI in a ratio of 1:10. The supernatant was prepared for extraction after sonication for 10 min. In Case III, the Si element concentration decreased from 0.16 to 0.1 mg/L, and Fe, Ca, and Mg were detected at levels below 0.01 mg/L. In Case II, the Na element decreased from 5.02 to 0.74 mg/L. The total amount of major cations (Si, Fe, Ca, Mg, and Na) that affect electrical conductivity in spherical Al₂O₃ powders could be reduced to less than 1 ppm by combining Cases II and III. Moreover, when fabricating Al₂O₃–siloxane composite thermal pads, these impurities easily bind to compounds with non–bonding electron pairs on the Pt catalyst through coordinate covalent bonds. Therefore, a low concentration of trace impurities can improve the crosslinking of siloxane polymerization in the presence of a Pt catalyst [18].

3.3. Fabrication of Al₂O₃–Siloxane Composite Thermal Pads

Figure 8 shows the XRD patterns of the crystalline phase of the spherical Al₂O₃ powders with coarse (70 µm), fine (10 µm), and ultrafine (800 nm) trimodal size distributions. The XRD quantitative analysis results of the spherical Al₂O₃ powders applied in this test are shown in Figure 8. For the Al₂O₃ powder, the main phase is the δ-Al₂O₃ phase (91%), accompanied by α-Al₂O₃ (7%) and θ–Al₂O₃ (2%). For a particle size of 10 μm, the primary phase is θ– Al₂O₃ (62%), whereas for a particle size of 70 μm, the main phase is α–Al₂O₃ (64%). The detailed quantitative phase results are presented in

Table 5. Relative XRD quantification results of the spherical Al₂O₃ powder with trimodal size distributions for Al₂O₃–siloxane composite thermal pads.

Crystal Phase (%)	Particle Size of Al2O3 Powder
	800 nm	10 µm	70 µm
δ	91	19	1
θ	2	62	35
α	7	19	64

. The crystal structure was quantified via Rietveld refinement analysis, and the peak shapes were fitted using a split pseudo–Voigt method.

Figure 9 shows the FT–IR spectra of the treated Al₂O₃ powder with various surface treatments (Cases I, II, III, and IV). In Case I, the surface of the untreated Al₂O₃ powder exhibited peaks at 840 to 784 cm⁻¹ and 735 to 725 cm⁻¹, which can be attributed to the stretching vibrations of Al–O in AlO₄ and bending vibrations of AlO–OH, respectively [19,20]. In Case II, the OH peak of the treated Al₂O₃ powder decreased compared to that in Case I. For Case III, the OH peak of the treated Al₂O₃ powder decreased because of the heat treatment conducted at 750 °C for 1 h. After the silane treatment (Case IV), the surface of the treated Al₂O₃ powder had aromatic functional groups (1472 and 1461 cm⁻¹), which corresponds to the C=C benzene ring vibrations [21]. It was confirmed that the aryl functional groups of silane were well attached to the surface of the Al₂O₃ powder.

Figure 10a shows the viscosities of the Al₂O₃–siloxane composite pastes with an Al₂O₃ concentration of 76 vol% according to the shear rate with various surface treatments (Cases I, II, III, and IV) measured using a viscometer at 25 °C. For Cases II and III, the viscosity of the Al₂O₃–siloxane composite pastes decreased due to the removal of the carbonate and hydroxyl groups in the Al₂O₃–siloxane composite paste compared to Case I.

When the absorbed hydroxyl of Al₂O₃ is removed, bonding with hydrophobic siloxanes can be promoted, and viscosity can be reduced in the Al₂O₃–siloxane pastes [22,23].

For Case IV, the viscosity of the Al₂O₃–siloxane composite paste was decreased at the fixed volume ratio of the Al₂O₃ powder when compared to Case I.

Figure 10b shows the TGA thermogram results of silane obtained using the treated Al₂O₃ powder (Case IV). The thermal weight loss of aryl silane and amine silane (3–aminopropyl-triethoxysilane, molecular weight of 221.37 g/mol) occurred at ~240 and 340 °C, respectively. The total weight losses of the aryl silane and amine silane were 2.07% and 0.44% at 1000 °C, respectively. The weight loss of aryl silane was observed in the range of 250 to 450 °C, which may have resulted from the removal of the double bond (sp²) in carbon. The gradient of the weight loss in this range can be attributed to the different oxidation reaction rates of carbon species (sp² and sp³) [24]. The double bond (sp²) of aryl silane can contribute to more stable thermal decomposition and improve the thermal resistance compared to that of amine silane [23].

Figure 11 shows photographs and FE-SEM cross-section images of Al₂O₃–siloxane composite pads with 76 vol% prepared by using Al₂O₃–siloxane composite pastes with various surface treatments (Cases I, II, III, and IV). Herein, to make the cross–sectional sample of thermal pads for the FE–SEM measurement, the thermal pad sample was prepared using physical fixation methods, such as immersion freezing and cryofixation. The Al₂O₃–siloxane composite thermal pad was plunged into liquid nitrogen (LN₂) and allowed to drop to −175 °C. In the cryofixation step, the pad was cut quickly to minimize cross–sectional damage to the sample fracture. After reaching room temperature, the cooled cross–sectional specimen was sufficiently dried in the oven at 100 °C for 24 h.

In all cases, the Al₂O₃ powder with a trimodal particle size was distributed in the siloxane matrix. In Case I, the cross-sectional surface of the Al₂O₃–siloxane composite thermal pads was relatively rough. The edges of the bare Al₂O₃ were partially exposed, and some regions of the Al₂O₃ particle were not covered by the siloxane. This can be attributed to the curing failure of the OH groups of siloxanes onto Al₂O₃ caused by the residual moisture prior to treatment [25]. The OH group from water may inhibit the catalysis of platinum (Pt) upon its addition to silicon. The OH group reacts with the unreacted vinyl (Vi) side–groups of the Si–O backbone that is cross–linked with the Pt catalyst via polymerization [18]. In Case IV, the silylated Al₂O₃ powder formed flexible bonds with the siloxane matrix. Al₂O₃ nanoparticles have a large specific surface area (adsorbent BET area of 2.47 m²/g at 800 nm, which is higher than that of 0.1 m²/g for 10–70 µm), and these particles were well distributed around the larger micrometer–sized particles, as shown in the FE-SEM cross–section images of the Al₂O₃–siloxane composite thermal pads.

As a generalized theory of heat conduction, Loeb’s model (also referred to as Sankar−Loeb’s model) is mainly used to understand the thermal energy transport in a polymer matrix. According to Loeb’s model, the thermal conductivity of the composite thermal pad depends on the size distribution and orientation of the solid Al₂O₃ [26].

In this study, we attempted to improve the thermal conductivity of Al₂O₃-siloxane composite thermal pads by improving the compatibility and adhesion between Al₂O₃ particles and the siloxane matrix using various surface treatments (Cases I, II, III, and IV).

In Case IV, the strong interactions caused by aryl silylation on the surface of the Al₂O₃ nanoparticles between the cross-linked Si–Vi and H–Si systems improved the bonding strength, thereby resulting in a higher mechanical strength. Considering the silylation mechanism, the silylated Al₂O₃ powder first reacts with the siloxane matrix.

The hydroxyl functional groups of the Al₂O₃ powder react with aryl silane through hydrolysis, which decreases the surface tension. The silane–treated Al₂O₃ becomes bonded to the non–polar siloxane, resulting in aryl groups in the polymeric network.

Finally, a reaction occurs in parallel between the siloxane vinyl monomers and the silicone hydride crosslinker via Pt-catalyzed hydrosilylation. The Al₂O₃–siloxane composite network includes physically and chemically linked species, improving the compatibility between Al₂O₃ and the siloxane resin matrix. The thermal energy transport in this system improved its mechanical and thermal conductivity properties. Furthermore, this fully connected network contributed to reducing the viscosity of the paste [27].

The FE-SEM results confirmed the densified distribution of the Al₂O₃ nanoparticles, and the siloxane coverage of the Al₂O₃ particles was improved after surface treatment.

Figure 12 shows the thermal conductivity of Al₂O₃–siloxane composite thermal pads prepared using an Al₂O₃ nanoparticle powder (800 nm) when the amount of Al₂O₃ nanoparticle powder is changed from 0 wt% to 10 wt% for Case I. In this experiment, the mass ratio of 70 and 10 μm micro–sized Al₂O₃ powder was tested by fixing it under total powder conditions of 76 vol% at a ratio of 6:4, respectively. The thermal conductivity improved noticeably as the Al₂O₃ nanoparticle powder content of the composite pad increased from 0 wt% to 10 wt%. Specifically, the thermal conductivity of the composite pads increased by more than 30% compared to the Al₂O₃–siloxane composite thermal pads without nanoparticles. This suggests that the addition of Al₂O₃ nanoparticle powder enhances the thermal conductivity of the composite material, potentially due to improved thermal pathways created by the nanoparticles within the matrix. In particular, this improvement in thermal conductivity properties is effective due to the Farris effect under conditions of high Al₂O₃ filler content of 50 vol% or more [10].

Figure 13 shows the thermal resistance (R_th) of Al₂O₃–siloxane composite thermal pads with various sample thicknesses (BLTs) obtained using Al₂O₃ powder samples with various surface treatments. The R_th values of the Al₂O₃-siloxane composite thermal pads were measured at approximately 275.6 to 344.5 kPa. As the Al₂O₃ content increased, the thermal resistances of the composites in Case I (76 vol%) and Cases II, III, and IV (79 vol%) decreased rapidly. In the Al₂O₃ powder samples for Cases II, III, and IV, the slope of R_th and BLT decreased to a greater extent than that of the sample before treatment (Case I). In Case IV, the R_th of the Al₂O₃–siloxane composite thermal pad (4.20 cm²·K/W at 1510 µm) decreased by almost 10% in comparison to that in Case I (4.67 cm²·K/W at 1510 µm). For Cases II and III, the R_th values of the Al₂O₃–siloxane composite thermal pads were 4.38 cm²·K/W at 1400 µm and 4.32 cm²·K/W at 1398 µm, respectively. These significantly decreased R_th values could be attributed to the high thermal conductivity and low BLT of the Al₂O₃–siloxane composite thermal pads achieved using an appropriate pressure.

Figure 14 shows the thermal conductivity of the prepared Al₂O₃–siloxane composite thermal pads with various volume fractions of Al₂O₃ powder from 60 to 78 vol%.

The experimental results indicated that the thermal conductivity of the Al₂O₃–siloxane composite thermal pads increased with an increase in the volume fraction of the Al₂O₃ filler in the siloxane matrix. At 78 vol% Al₂O₃ (4.15 W/m·K) in Case IV, the aryl silylation Al₂O₃ powder enhanced the heat transfer performance of the Al₂O₃–siloxane composite thermal pads. The thermal conductivity of the silylated Al₂O₃–siloxane composite thermal pads was considerably higher than that of the untreated Al₂O₃-siloxane composite thermal pads (3.05 W/m·K). For Cases II and III, the thermal conductivities of the 78 vol% Al₂O₃-siloxane composite were 3.93 and 3.96 W/m·K, respectively. The thermal conductivity of the Al₂O₃–siloxane composite was enhanced by 22% when compared to that of Case I. In Case IV, the thermal conductivity of the Al₂O₃–siloxane composite thermal pads improved by 26.5% at a high filler content compared to Case I. This improvement in thermal conductivity can be attributed to the thermal bridge effect at the interface between Al₂O₃ and siloxane. These surface treatments (Cases II, III, and IV) resulted in the formation of more interconnected thermal pathways and contributed to the enhancement of adhesive contact as the LNG flow rate increased via the relaxation of the internal friction between Al₂O₃ and siloxane [28]. The cross–sections of the Al₂O₃–siloxane composite thermal pads were improved considering the distribution of the Al₂O₃ particles and siloxane coverage, as shown in Figure 11.

Table 6. Thermal conductivity and mechanical hardness properties of Al₂O₃–siloxane composite thermal pads with various process parameters, such as volume fraction of Al₂O₃ nanoparticle powder, thickness of composite pads, and surface-treated Al₂O₃ nanoparticle powder (Cases II, III, and IV).

Case Studies	Volume Fraction (%)	Thickness (mm)	Hardness (HS)	Thermal Conductivity (W/m·K)
Case I	60	1.92	46	2.53
	69	2.40	55	2.62
	73	2.40	67	2.79
	76	2.43	69	3.05
Case II	60	2.20	59	2.41
	69	2.28	57	2.86
	73	2.21	72	3.44
	76	2.35	77	3.57
	79	2.38	80	3.93
Case III	60	2.52	53	2.47
	69	3.09	51	2.75
	73	3.02	71	3.11
	76	3.01	79	3.66
	79	3.13	80	3.96
Case IV	60	2.52	57	2.77
	69	2.92	72	3.15
	73	3.05	77	3.56
	76	2.90	79	3.71
	79	3.02	85	4.15

presents the thermomechanical and electrical properties of the Al₂O₃–siloxane composite thermal pads with various process parameters, such as volume fraction of Al₂O₃ nanoparticle powder, thickness of composite pads, and surface–treated Al₂O₃ nanoparticle powder (Cases II, III, and IV). The hardness of the Al₂O₃–siloxane composite pads was evaluated under various specific pressure conditions. According to the commercialized analysis standard reference, the thermal conductivity of the Al₂O₃–siloxane composite thermal pads had a range of 0.2 to 1.9 W/m·K, meaning that they showed a hardness range of 80 to 90 using Shore 00-type indentor hardness [29].

The thermal conductivities of the Al₂O₃–siloxane composite pads were then calculated based on the variation in the thermal resistance at a pressure of 344.5 kPa and a temperature of 50 °C. For Case I, the thermal conductivity of the Al₂O₃–siloxane composite thermal pads was from 2.53 to 3.05 W/m·K, and the hardness ranged from 46H to 69H. In Case IV, the Al₂O₃ particles were treated with aryl silane before preparing the composite to enhance the interfacial contact between Al₂O₃ and the matrix. The thermal conductivity pads exhibited a thermal conductivity of 2.77 and a maximum value of 4.15 W/m·K at harnesses ranging from 57 HS to 85 HS. The Al₂O₃–siloxane composite thermal pads with high hardness values at high pressure exhibited a high compression rate, a short thermal conductivity path, a short heat transfer time, and better thermal conductivity compared to the low–hardness Al₂O₃–siloxane composite thermal pads. The Al₂O₃–siloxane composite thermal pads showed resistance to deformation when they exhibited high hardness. This value suggested that the hardness of the pads contributed to their structural integrity and ability to maintain their shape under applied pressure or load. High hardness often correlates with increased stiffness and resistance to compression, which are desirable properties for thermal interface materials like composite pads, as they need to maintain consistent thermal contact between surfaces without undergoing significant deformation. Further, the thermal conductivity of the Al₂O₃–siloxane composite pads was enhanced when the hardness was increased in the surface–treated Al₂O₃ nanoparticle powders (Cases II, III, and IV) compared to Case I.

Figure 15a shows the schematic diagram of the heat-interconnected pathway in the Al₂O₃–siloxane composite thermal pads prepared by using various surface–treated Al₂O₃ nanoparticle powders (Cases I, II, III, and IV). In all cases, the Al₂O₃ nanoparticle with a trimodal particle size was distributed in the siloxane polymer matrix. In Case I, the Al₂O₃ was partially exposed, and some regions of the Al₂O₃ particle did not cover the siloxane. This can be attributed to the curing failure of the OH groups of siloxanes onto Al₂O₃ caused by the residual moisture. The OH group from water may inhibit the catalysis of Pt upon its addition to silicon. The OH group reacts with the unreacted vinyl (Vi) side–groups of the cross–linked Si–O backbone by Pt catalyst polymerization [23,25]. In Case IV, the silylated Al₂O₃ nanoparticle formed flexible bonds with the siloxane matrix. Al₂O₃ nanoparticle powder has a large specific surface area, and these particles were well distributed around the larger micrometer–sized particles, as shown in the FE–SEM cross-section results (Figure 11d). Therefore, the silylated Al₂O₃ nanoparticles make it easy to form the thermal conductivity channel network, which is more stable through a dense stacking structure [1,10,18,29]. For this reason, the thermal conductivity of the composite thermal pad applied with the silylated Al₂O₃ nanoparticles was improved.

To understand the mechanism of the silylated Al₂O₃ nanoparticles for thermal conductivity, Figure 15b shows the chemical interaction of Al₂O₃–siloxane composite pads during both aryl functional silylation and hydrosilyation cross–linking reactions with a Pt-based catalyst. In Case IV, the Al₂O₃ particles showed a figure of the relations between structure and property performance with aryl silane before preparing the composite to enhance the interface contact between Al₂O₃ and the siloxane matrix. The silylated Al₂O₃ nanoparticle first reacts with the siloxane polymer matrix, and then the hydroxyl functional groups of the Al₂O₃ particle react with aryl silane through hydrolysis for surface functionality switching from hydrophilicity to hydrophobicity, which decreases the surface tension. The silylated Al₂O₃ nanoparticles become bonded to the non-polar siloxane, resulting in aryl groups in the polymeric network. This reaction occurs in parallel between the siloxane vinyl monomers and the silicone hydride crosslinker via Pt–catalyzed hydrosilylation. The Al₂O₃–siloxane composite network includes physically and chemically linked species, improving the compatibility between Al₂O₃ and the siloxane polymer matrix. The thermal energy transport in this system improved its mechanical and thermal conductivity properties. In addition, these reactions in the fully connected network contributed to reducing the viscosity of the paste [26].

4. Conclusions

This study investigated the structural properties of spherical Al₂O₃ powders prepared by flame fusion with three different LNG flow rates. When analyzing the Al₂O₃–siloxane composite thermal pads, the spherical Al₂O₃ powders were subjected to various surface treatments, such as before treatment (Case I), dehydration (Case II), decarbonization (Case III), and silylation (Case IV). In addition, the spherical Al₂O₃ powders and Al₂O₃-siloxane composite thermal pads were characterized using FE–SEM, XRD, FT–IR, NMR, ICP–OES, and thermal conductivity measurements.

In Cases II and III, the treated Al₂O₃ powders exhibited reduced carbonate and hydroxyl molecule components. The synthesized spherical Al₂O₃ nanoparticle powder was mainly composed of the δ–Al₂O₃ phase (88%) and exhibited a stable surface with a high octahedral ratio (A_Oh/A_Td) of 3.2%, which was confirmed by XRD and NMR analyses.

Moreover, Al₂O₃-siloxane composite thermal pads were fabricated using surface–treated spherical Al₂O₃ powders with trimodal size distributions, which resulted in high thermal conductivity. The Al₂O₃–siloxane composite thermal pads exhibited high hardness and excellent heat transfer at higher amounts of Al₂O₃ powder. For Case IV, the thermal conductivity of the Al₂O₃–siloxane composite thermal pads was enhanced by 26.5% compared to that of Case I. In this study, the spheriodized Al₂O₃ nanoparticles exhibiting an 88% δ–phase were optimally synthesized using the flame fusion process to achieve a trimodal–sized paste formulation. The surface functionalization of these systems improved their high thermal conductivity, indicating their potential for use as high-thermal–conductivity TCMs.