Aluminum nitride shaping by non-aqueous gelcasting of low-viscosity and high solid-loading slurry

Abstract

Aluminum nitride (AlN) ceramics has been prepared by a modified non-aqueous gelcasting technique, and the ceramic slurries with low viscosity and high solid loading were obtained by using 1-methyl-2-pyrrolidinone as solvent and Solsperse® 24,000 as dispersant. The rheological behaviors of the AlN ceramic slurry and the densities of AlN ceramics were studied. Typically, the AlN ceramic slurry (Solsperse® 24,000 of 0.5 wt% and solid loading of 50 vol%) showed a viscosity of 0.09 Pa·s at shear rate of 100 s⁻¹. And more interestingly, when increasing solid loading to 55 vol%, the AlN slurry still kept a low viscosity of about 0.28 Pa·s, which was reduced by 30% compared to the reported values. As a result, the resultant green body exhibited a relative density of 65.5%, and a flexural strength of 42.3 MPa. After sintering at 1900 °C for 5 h, the AlN ceramics with an improved relative density of 99.4% was produced. Thus, this research would provide a new way to prepare low-viscosity and high solid-loading AlN slurry.

Introduction

Aluminum nitride (AlN) ceramic materials have many unique properties, such as low dielectric coefficient, high electrical resistance, high thermal conductivity, and thermal expansion coefficient well matched with silicon, which make them extensively be used in various engineering fields [1], [2], [3], [4]. To meet diverse needs, AlN ceramics have been prepared via various molding techniques, mainly including tape casting [5], [6] and gelcasting [7], [8]. Especially, gelcasting technique is an attractive way to mold ceramic with high quality and complex-shape, which was first developed by Janney et al., because the green bodies obtained by this technique have good homogeneity and high mechanical strength [9], [10]. The gelcasting techniques are mostly classified by natural and synthetic gelation system [11]. Derived from synthetic gelation system, a type of in-situ polymerization technique has been frequently used to gelcast ceramic materials [9], [11]. And the solvents used in this technique can generally be divided into two classes: aqueous and non-aqueous [12]. So far, aqueous gelcasting was employed to produce AlN ceramics in most case due to low cost and environmental friendliness [13]. However, it is reported that series of hydrolysis reactions between AlN powder and water often occurs during the aqueous gelcasting process, which leads to the formation of Al₂O₃ components and greatly influence the thermal conductivity of AlN ceramics [14].

Recently, non-aqueous gelcasting technique was successfully proposed to prepare many ceramic materials, including boron carbide (B₄C)–aluminum (Al) composites, Ba_0.6Sr_0.4TiO₃–MgO, and β-SiAlON [15], [16], [17]. Although the non-aqueous compounds are usually less environmentally friendly or more expensive compared to the aqueous ones, these compounds cannot react with the goal components that leads to high performances of the final products [14]. In view of this point, to avoid hydrolysis reactions between AlN powder and water, non-aqueous gelcasting technique has also been introduced to prepared AlN cermics [13], [14]. For instance, Xue et al. first prepared AlN ceramics by a non-aqueous gelcasting technique, in which ethanol was used as solvent and the polymerization was conducted under nucleophilic addition reaction [7]. In their work, they use a kind of epoxy resin (sorbitol polyglycidyl ether) as a gelling agent, tetraethylenepentamine as hardener, and polyethyleneimine as dispersant. Their obtained green body shows a relative density of 63.3%. However, due to low boiling point and high volatility of their used ethanol, the resultant samples were easy to be fragile during inconsistent drying rates of internal and external during drying [18]. And, the AlN ceramic slurry in their work showed the maximum solid loading of 53.8 vol% and a relatively high viscosity of 0.6 Pa s at shear rate of 100 s⁻¹, thus making the obtained green body have low-density, and accordingly reducing the flexural strength and the density of the sintered AlN ceramics [19]. On the other hand, the high viscosity of the ceramic slurry will hamper the large-scale and complex preparations of ceramic materials [20]. Therefore, it is still important to explore a rational strategy to optimize the non-aqueous gelcasting technique for the preparation of AlN ceramic slurry with low viscosity and high solid loading.

Here, our work demonstrated a modified non-aqueous gelcasting process of AlN ceramic slurry with high solid loading and low viscosity via using Solsperse® 24,000 as dispersant and 1-Methyl-2-pyrrolidinone as solvent. The mechanical properties and microstructures of AlN green bodies and the resultant ceramics were characterized. It is found that the viscosity of AlN slurry is reduced by over 30% compared to the Xue’s results. More interesting, the AlN slurry with high solid loading of 55 vol% kept very low viscosity (0.28 Pa s) at shear rate of 100 s⁻¹. This research might provide a new approach to prepare high-performance AlN ceramics.