The effect of silicon particle size on the characteristics of porous sintered reaction bonded silicon nitride
Introduction
High-speed vehicles, such as missiles, require radars to recognize certain interested targets or surrounding terrain. Since the radar is an electronic device, it should be covered with a protective material like a safety goggle for human eyes, as it may fail if exposed to strong stimulus such as mechanical stress or high temperature. Since the signal transmission and reception requires the use of a material capable of transmitting a long wavelength without loss, an insulating component with a low dielectric constant has been mainly used, so called as the radome. Typical materials include SiO2 and Al2O3 [1].
As the flight speed increases, the heat-generation due to stress, impact, and air friction increases correspondingly. To cope with such environmental changes, it is essential to upgrade the properties of the protective material. Since physical properties such as strength, hardness, and dielectric constant vary greatly depending on the type of material, it is difficult to use SiO2 which is weak in strength or Al2O3 which is vulnerable to thermal shock. Therefore, Si3N4 materials having suitable strength, hardness, thermal shock resistance and low dielectric constant are mainly used in a high-speed environment [2,3].
The reaction-bonding process between Si and N2, which is one of the methods of producing Si3N4, is known as a method advantageous for manufacturing near-net-shape products because of the small volume shrinkage from Si green body to final Si3N4 sintered body. Therefore, it is considered that the reaction bonded silicon nitride (RBSN) is suitable for manufacturing a radar protection component(i.e. radome) having a complex shape [[4], [5], [6]].
Although Si3N4 has a relatively low dielectric constant of ε = 8, lower dielectric constants are advantageous for the robust radome design. A way to lower the dielectric constant is to combine it with a material with a lower dielectric constant. The most effective composite material is porosity having the lowest dielectric constant, 1. When the RBSN containing a certain amount of porosity is produced, the dielectric constant can be tailored to low value appropriate for the radome design, but the strength is inevitably reduced [[7], [8], [9], [10], [11], [12], [13], [14]]. It is well-known that the dielectric constant of silicon nitride with porosity is predictable via Maxwell-Garnett effective medium theory [15]. So, what is more critical for porous silicon nitride, is to find out how to keep the strength high enough for the radome application.
Under the high velocity flying condition, the radome material can be heated over 1000 °C. The higher eutectic temperature of glassy phase in sintered Si3N4 is highly recommended not to be deformed under aerodynamic heating situation. Yb2O3 oxide is known as an efficient additive to reveal the refractory-like properties for Si3N4 at high temperature [[16], [17], [18], [19]]. As well, the previous study also presented that Yb2O3 was the effective additive for making strong porous Si3N4 [20].
In this study, to investigate how the initial particle size of raw Si powder affects the general characteristics of sintered reaction-bonded silicon nitride such as the densification, the α-β transformation, and the strength, the different condition of planetary milling process as well as the amount of sintering additive were applied to produce the porous Si3N4 ceramics. Besides, we tried to suggest the process guidelines of making strong porous sintered reaction-bonded silicon nitride (SRBSN) with consideration of additive ratio.
Section snippets
Experimental procedure
The size of starting Si powder (Sicomill Grade 4, Vesta Ceramics, Ljungaverk, Sweden, D50 = 4 μm) was controlled by the planetary milling with different processing duration of 0, 3 and 24 h. The ball media and pot used in the milling was made of Si3N4, and the liquid medium was anhydrous ethanol. Oxygen content was determined for dried powders by combustion-IR absorption method(ONH-2000, Eltra GmbH, Neuss, Germaby). Dried and sieved Si powders with 3 different size was mixed with Yb2O3 (Sigma
Results and discussions
Table 2 shows the results of controlling the Si particle size by planetary milling. The median particle size, D50, of pristine Si powder was 3.934 μm. After 3 h of milling, it decreased to 1.084 μm, which was about 1/4, and further decreased to 0.353 μm, which was about 1/10 after 24 h of milling. The oxygen content of each powder was 0.627, 1.538 and 4.226 wt%, respectively, as the particle size decreased. This was due to the natural increment of the surface oxygen content with increasing
Further discussions
An alternative parameter was introduced to clarify the relationship between the sample preparing conditions including sintering temperature, Yb2O3 and SiO2 content, and the characteristics of SRBSN such as α-β transformation behavior and flexural strength. If the sintering aids are simply set to wt%, the large difference in density between Yb2O3 and SiO2 results in a significant discrepancy in the volume of the eutectic liquid phase that can be formed. In addition, when the additive content is
Conclusions
In order to make the porous silicon nitride with high strength, the particle size of raw Si powder was controlled by planetary milling then reaction-nitrided and post-sintered at different sintering temperature. The apparent porosity of sintered RBSN was directly proportional to the total volumic amount of sintering additives, whereas the β-phase ratio were not dependent on the total amount of additives. The flexural strength was strongly dependent on whether the elongated β-grains evolved or
Declaration of Competing Interest
None
Acknowledgments
This work was financially supported by Fundamental Research Program of Korea Institute of Materials Science (Grant no. PNK7450).
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