Universal Journal of Materials Science 6(1): 39-47, 2018 http://www.hrpub.org DOI: 10.13189/ujms.2018.060105 Effect of Cooling Cycle after Sintering on the Thermal Diffusivity of Y 2 O 3 Doped Si 3 N 4 Ceramics Pınar Uyan 1,* , Servet Turan 2 1 Vocational School, Metallurgy Program, Bilecik Seyh Edebali University, Turkey 2 Department of Materials Science and Engineering, Anadolu University, Turkey Copyright©2018 by authors, all rights reserved. Authors agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Abstract As a result of the studies that have been made for increasing the thermal conductivity of Si 3 N 4 , increasing thermal conductivity values have paved the way for the technologic applications such as using the Si 3 N 4 in electronic devices as heatsink and substrate. Two important parameters such as density and grain boundary phase that affect the thermal conductivity of the Si 3 N 4 ceramics are based on the sintering additives and techniques. Exposure time to gas applied in the sintering and cooling cycle after sintering are the critical factors that affect the intergranular phase crystallization. In this study, the effect of Y 2 O 3 doped Si 3 N 4 ceramics and different cooling cycle after gas-pressure sintering to the phase crystallization and thermal diffusivity was researched. The samples were subjected to two different cooling cycles after sintering. The thermal diffusivity value of the sample applied by slow cooling cycle is 17.79 mm 2 /sec, the sample applied by rapid cooling cycle is 16.2 mm 2 /sec. As a result of slow cooling cycle, the crystallization has increased but thermal diffusivity has decreased at the rate of ~ 8.94%. Keywords Thermal Conductivity, Crystallization, Microstructure Characterization 1. Introduction Si 3 N 4 ceramics have high thermal conductivity, electrical resistance, fracture toughness and strength properties [1]. Haggerty and Lightfoot calculated the [2] thermal conductivity of β-Si 3 N 4 at room temperature as 200-320 W/mK. The recent studies showed that the thermal conductivity of Si 3 N 4 has reached to the value of 100-155 W/mK [3-5]. These increasing thermal conductivity values have paved the way of new technologic applications of Si 3 N 4 as substrates for heat sinks and integrated circuits in electronic devices. Due to the strong covalent bonding structure of Si 3 N 4 , it has noticed that sintering at high density is difficult via traditional ceramic sintering methods and liquid phase sintering technique that is the alternative production technique was started to be used. Despite the fact that oxide additions during the sintering have generated a liquid phase having low eutectic, a liquid is occurred that could be dissolved without separating the Si 3 N 4 . It is more than important in which temperature the liquid phase is occurred, its viscosity and solubility of the Si 3 N 4 . For that reason, different additional materials and different compositions have been tried accordingly. It is important that such additions are distributed as uniform and those should pack around Si 3 N 4 grains [6, 7]. In order to increase the thermal conductivity of Si 3 N 4 ceramics, it is important to increase the density and to decrease the amount of grain boundary phase. These parameters are based on the sintering additions and sintering techniques. Y 2 O 3 is a suitable oxide additive to enhance the thermal conductivity of β-Si 3 N 4 ceramics [8]. When sintering additions are on the grain boundaries, it is the reason for decreasing the thermal conductivity. High N 2 pressure that is one of the sintering methods prevents the decomposition at Si 3 N 4 and thus, it allows increasing at high temperatures. When sintering temperature is increased, occurrence of big rod like grains is provided in thin matrix structure. As a result of this, the thermal conductivity is increased [5]. In Si 3 N 4 ceramics produced by GPS, by taking into the consideration of exposing to the nitrogen gas, it was found that the phases among the current grains have changed. Additionally, the cooling cycle after sintering has important effect in the occurrence of intergranular phases [9]. Type, amount, and distribution of grain boundary phases in microstructure effect thermal conductivity [10-15]. The aim of this study was to investigate the effects of different cooling cycles applied in gas pressure sintering on crystallization and thermal diffusivity at Y 2 O 3 added Si 3 N 4 .