1. INTRODUCTION
Cordierite ceramics (2MgO·2Al2O3·5SiO2, Mg2A14Si5Ol8) with a ring framework have long been esteemed for their exceptional thermal stability, low thermal expansion coefficient, and advantageous dielectric properties, making them crucially important materials in high-temperature applications ranging from catalytic converters to refractory linings [1-5]. Despite these advantageous attributes, there is persistent demand to further enhance their mechanical properties, particularly flexural strength, to meet the escalating rigors of modern engineering environments. Moreover, the need to fabricate cordierite components with intricate shapes (e.g., as radomes, catalyst carriers, and electric equipment) has become increasingly important. To accomplish this, developing a robust process like slip casting, known for its versatility and capability to produce complex forms with impeccable surface finishes, has become imperative [6-8]. With its ability to precisely control slurry composition and processing parameters, slip casting offers a practical solution and an effective avenue for fabricating complex cordierite components.
At the same time, integrating ceramic particles with high mechanical reliability, such as BN, SiC, ZrO2, into cordierite matrixes has emerged as a promising strategy to enhance mechanical performance while preserving other essential material attributes [9-11].
This study centers on the incorporation of h-BN nanoparticles, known for their exceptional mechanical properties and superior dielectric behavior (out-of-plane dielectric constant = 3.29 – 3.76, in-plane dielectric constant = 6.82 – 6.93) [12], as reinforcing agents within cordierite ceramics using the slip casting process. The unique combination of cordierite's thermal (thermal conductivity = 3.5 W/m·K [13], coefficient of thermal expansion (CTE) = 1.5 × 10-6 – 4.0 × 10-6 K-1 [2]) and dielectric properties (dielectric constant = 4.9 at 1 MHz [14]) with h-BN's exceptional mechanical characteristics makes this composite material a compelling candidate for a wide array of applications, such as radomes, that require low dielectric constant and low thermal conductivity. By strategically adjusting the concentration and dispersion of the h-BN nanoparticles, we were able to simultaneously optimize the flexural strength, dielectric constants, and thermal conductivity of the resulting composite material. Through systematic experimentation and comprehensive analysis, this research seeks to elucidate the synergistic effects of slip casting and BN nanoparticle reinforcement on the mechanical, electrical, and thermal properties of cordierite ceramics.
In the present study, we optimized the slip casting process to fabricate a composite material consisting of cordierite and h-BN nanoparticles. Through the combination of pressureless sintering, we achieved a significantly enhanced 3-point bending strength of ~174.8 MPa, exceeding that of pristine cordierite by more than 46%, while maintaining the dielectric and thermal characteristics of the original cordierite ceramics.
2. MATERIALS AND METHODS
2.1 Synthesis of the sintered bulks of cordierite and h-BN nanoparticles introduced cordierite
Commercial cordierite powders (325 mesh, XINGTAI, China) with an average particle size of 15 µm were used as the starting materials. Prior to use, the commercial powders underwent dry ball milling with ZrO2 balls (10 mm in diameter, weight ratio of cordierite powders to ZrO2 balls = 1 : 1) as milling media to obtain powders with reduced size and a more uniform distribution (narrow particle size distribution) for the slip casting process. The h-BN nanoparticles (US Research Nanomaterials, USA) with an average particle size of 80 nm were employed as the reinforcing phase. The slurry was formulated with a total weight of 100 g, not including the dispersant. A measure of 33 g DI (deionized) water was utilized as the solvent, and 67 g of the powders (either cordierite or a mixture of cordierite and h-BN nanoparticles) were introduced to the DI water. Subsequently, 5 g of dispersant (Darvan 821A, Vanderbilt Minerals LLC, USA) was added, and the mixture was stirred for 2 h to complete the slurry’s formation. The slurry was cast into two types of gypsum molds (45 mm 5 mm × 4 mm and 20 mm × 10 mm × 7 mm) and kept at room temperature for 15 min to form specimens. Afterwards, the specimens were demolded and stored in an oven at 60 °C for 5 h as a drying process, to obtain the green bodies. The sintered bulks were fabricated via pressureless sintering at 1410 °C for 4 h in an air atmosphere. The resulting sintered bulks achieved a relative density of approximately 91%.
2.2 Characterization and evaluation of properties
X-ray diffraction (XRD; Smart Lab, Rigaku, Japan) using Cu-Kα radiation (λ = 1.5418 Å) was carried out to characterize the phase and crystal structure of both the powders and sintered bulks. The scans were performed in a 2θ range from 10° to 80°, with a scan speed of 5°/min and a step size of 0.02°. Particle size analysis (PSA; ELS-Z1000, Otsuka Electronics, Japan) was used to assess the particle size and particle size distribution of the ball milled cordierite powders. Microstructural characterization was conducted with a scanning electron microscope (SEM; JEOL-7800F, JEOL, Japan). For sintered bulks, the fractured surface was sputter-coated with a conductive layer and observed at various magnifications to assess the distribution of h-BN nanoparticles within the cordierite matrix.
After sintering, the acquired samples were cut and polished into bar-type (3 mm × 3 mm × 40 mm) and plate-type (10 mm × 6 mm × 3 mm and 6 mm × 6 mm × 1 mm) specimens to measure mechanical, dielectric, and thermal properties. Flexural strength (s) was determined using a 3-point bending test with a support span of 30 mm on a universal testing machine (UTM; AGX-VSTD, SHIMADZU, Japan). Room temperature dielectric response and complex impedance were measured at a frequency of 1 MHz using a precision LCR meter (E4980A, Keysight, USA). Eutectic InGa alloy is used as the electrode material. Room temperature thermal conductivity (κ) was determined using the equation of κ = DρCp, where D, ρ, and Cp are the thermal diffusivity, density, and the heat capacity, respectively. Archimedes' principle aided in the experimental determination of ρ. Cp and D were collected using differential scanning calorimetry (DSC, DSC8270, Rigaku, Japan) and a laser-flash instrument (LFA-457, NETZSCH, Germany).
3. RESULTS AND DISCUSSION
In Figure 1(a), the XRD patterns of both the commercial cordierite powders and the ball-milled powders derived from the same commercial material are presented. The commercial material was confirmed to be a complete single phase of cordierite (JCPDS 12-0303) without any impurities. This phase persisted after the ball milling process. Figure 1(b) shows the SEM image representing the morphology of the commercial cordierite powders with irregular shapes and sizes spanning a wide range of 1 – 25 μm. After using commercial cordierite powders to prepare a slurry, a good dispersion was initially achieved. However, it was observed that precipitation occurred within 10 min, rendering it unsuitable for the slip casting process. Therefore, we conducted a dry ball milling process to both reduce the size of the cordierite powders and achieve a narrower particle size distribution, aiming to enhance the dispersibility of the slurry. As shown in Figure 1(c), powders with reduced particle sizes within the range of 0.5 – 7 μm are clearly discernible after ball milling. PSA revealed that the average particle size was ~2.5 μm. We used the ball-milled powders to prepare slurries for both the pristine cordierite and cordierite with h-BN nanoparticles.
To attain a high relative density in the sintered bulk, we aimed to formulate a slurry capable of accommodating the highest possible solid content. By combining 33 g of DI water with 5 g of dispersant, and mixing 67 g of cordierite powders (or mixed powders of cordierite and h-BN) it was possible to produce a slurry. Slurries for slip casting were prepared for four types of materials, with 0, 1, 2, and 3 wt%- added h-BN nanoparticles to ball-milled cordierite powders, while maintaining a solid content of 67% (weight fraction of DI water : powder = 33 : 67).
Figure 2(a) presents photographs taken immediately after slurry preparation and after 100 h of aging. As depicted in Figure 2(a), it was confirmed that stability was maintained with no precipitation. Slip casting was performed with the four slurries to produce samples for the measurement of flexural strength (Figure 2(b)) and the measurements of dielectric constant and thermal diffusivity (Figure 2(c)). After a 15 min dwell in the gypsum mold, demolding was possible, as depicted in the insets of Figures 2(b) and 2(c). It is noted that with the addition of 3 wt% of h-BN nanoparticles, demolding was possible after about 10 min. This is considered to be related to the lubricating properties of h-BN [15], leading to a change in the slurry’s rheological characteristics.
Following the drying process, sintered bulk samples were fabricated through pressureless sintering.
Figure 3 shows the XRD patterns of the sintered bulk samples containing 0 and 2 wt% h-BN nanoparticles in the cordierite. The XRD pattern of the sintered bulk of pristine cordierite exhibited peaks almost identical to those observed in the powder form (Figure 1(a)), suggesting the formation of a single phase devoid of any secondary phases such as mullite (3Al2O3·2SiO2). Samples with 2 wt% h-BN nanoparticles showed similar XRD patterns, yet no discernible peaks for h-BN were observed. This can be attributed to the small size and low concentration of the introduced h-BN nanoparticles.
Figure 3(b) - 3(e) shows SEM images illustrating the fractured surfaces of the sintered bulks of the pristine cordierite and the cordierite with 1, 2, and 3 wt% h-BN nanoparticles, respectively. For cordierite with 1 and 2 wt% h-BN nanoparticles (Figure 3(c) and 3(d)), the monodispersed h-BN nanoparticles, ranging from 100 to 200 nm, are clearly discernible along the grain boundaries of the cordierite. This confirms the successful formation of a nanocomposite composed of cordierite and h-BN through a slip casting process. However, when 3 wt% of h-BN nanoparticles was introduced, agglomeration of the h-BN became evident (Figure 3(e)). This is attributed to the change in the slurry’s rheology, as explained in the demolding characteristics.
Figure 4(a) presents the results of s obtained through 3-point bending measurements for the sintered bulk samples incorporating 0, 1, 2, and 3 wt% h-BN nanoparticles in the cordierite. To ensure measurement reliability, assessments were conducted on more than ten samples, with both the average and the standard deviation provided. The flexural strength of the sintered pristine cordierite was measured to be approximately 119.5 MPa, and it was enhanced by the addition of h-BN nanoparticles. When 1, 2, and 3 wt% of h-BN nanoparticles were introduced, the average flexural strengths showed enhanced values of 152.3, 174.8, and 147.8 MPa, respectively.
The enhancement in mechanical reliability can be attributed to several factors. The failure of the sintered bulk cordierite progresses along the grain boundaries, as evidenced by the fractured surface shown in Figure 3(b). As demonstrated in Figure 3(c) – 3(e), the incorporation of h-BN nanoparticles at these grain boundaries facilitates the following strengthening mechanisms. Firstly, the presence of h-BN nanoparticles serves as reinforcement within the material matrix, strengthening the overall structure. At the same time, the controlled microstructure and reduced grain size induced by the addition of h-BN contribute to the improved mechanical properties of the material. It is noted that the flexural strength reached its highest value in the sample with 2 wt% added h-BN nanoparticles, while the sample with 3 wt% addition showed rather lower flexural strength due to the agglomeration of h-BN nanoparticles [16], as depicted in Figure 3(e).
This result suggests that the change in the rheological characteristics of the slurry, which originates with the addition of nanoparticles, is a crucial factor in the manufacturing of the nanocomposite through slip casting.
To validate the enhanced mechanical property’s reliability, 28 samples of cordierite with 2 wt% h-BN nanoparticles were fabricated and subjected to flexural strengths measurements. Weibull statistics have been widely employed to describe the statistical behavior of mechanical properties in a variety of materials, including ceramics, glass, metallic matrix composites, ceramic matrix composite and polymeric matrix composites [17,18]. The s values of the cordierite with 2 wt% h-BN nanoparticles were subjected to Weibull distribution, as represented in Figure 4(b), and the Weibull modulus (m) and the scale parameter (s0) were determined through linear regression analysis using the following equation [17]:
where Pf is the probability of failure at stress σf. The parameter s0 represents the stress level at which failure occurred in 63.2% of the specimens, while m characterizes the width of the fracture distribution. The m value serves as an indicator of material homogeneity, reflecting the variation in flexural strength. A higher m value signifies a greater degree of homogeneity and less variability. With an m value of 11.2, it was confirmed that the measured strengths of the cordierites with 2 wt% h-BN nanoparticles exhibited a high degree of reliability.
The dielectric and thermal properties of sintered bulk samples (Table 1) containing 0 and 2 wt% h-BN nanoparticles in cordierite were also measured to evaluate the effect of adding h-BN nanoparticles. The room temperature dielectric constant of the sintered bulk pristine cordierite measured 4.72 (dielectric loss ~0.048) at 1 MHz. With the addition of 2 wt% h-BN nanoparticles, there was a modest change to 4.91 dielectric loss ~0.051). This relatively small increase confirms that the dielectric constant of the composite does not significantly change, thanks to the relatively low dielectric constant of h-BN (out-of-plane dielectric constant = 3.29 – 3.76, in-plane dielectric constant = 6.82 – 6.93) [12]. The κ value was also not significantly affected by the addition of h-BN nanoparticles. The room temperature κ of the sintered bulk cordierite was measured at 2.16 W/mK (D = 0.0113 cm2/s, ρ = 2.05 g/cm3, Cp = 0.917 J/g·K), and the sample with 2 wt% h-BN nanoparticles displayed a similar value of 2.23 W/mK (D = 0.0116 cm2/s, ρ = 2.09 g/cm3, Cp = 0.921 J/g·K). Consequently, the introduction of 2 wt% h-BN nanoparticles to cordierite resulted in an 46% improvement in flexural strength, while the dielectric constant and thermal conductivity could be maintained within a range of 5%.
4. CONCLUSIONS
We have developed a slip casting process for producing mechanically robust cordierite-based ceramics. A stable slurry with a maximum solid content of 67% was prepared by optimizing materials and processing parameters, and sintered bulks with a relative density of approximately 91% were fabricated through pressureless sintering. Through the homogeneous incorporation of 2 wt% h-BN nanoparticles at the grain boundary regions of the cordierite matrix, we achieved a remarkable 46% enhancement in flexural strength, while simultaneously maintaining dielectric constant and thermal conductivity within a narrow range. This process holds promise for manufacturing complex-shaped components using cordierite-based materials.
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