UDK 621.762.5:666.3/.7
Original scientific article/Izvirni znanstveni članek
ISSN 1580-2949 MTAEC9, 48(2)171(2014)
SYNTHESIZING Si3N4 FROM A MIXTURE OF SiO2-CaO
SINTEZA Si3N4 IN MEŠANICE SiO2-CaO
Nuray Karaku^, Hüseyin Özkan Toplan
Sakarya University, Engineering Faculty, Dept. of Metallurgy and Materials Engineering, 54187 Sakarya, Turkey
nurayc@sakarya.edu.tr
Prejem rokopisa - received: 2012-08-16; sprejem za objavo - accepted for publication: 2013-07-05
In this study a-phase-rich Si3N4 powders were synthesized containing the sintering additive (CaO) using carbothermal reduction and nitridation. The starting agent for the silicon source was high-purity (99 %) synthetic silica. Carbon was added to the high-purity SiO2 above the stoichiometric amount of oxygen. CaCO3 (for the silicon nitride containing the mass fraction of CaO w = 10 %) was premixed in the starting reactants depending on the final powder composition and the type and amount of the secondary phases required for sintering. The synthesis was carried out in a tube furnace in different temperature ranges (1400 °C, 1450 °C and 1475 °C for 3h) under a nitrogen-gas atmosphere. Having completed the synthesis process, the powder properties were examined using standard characterization techniques (XRD, SEM, etc.).
Keywords: silicon nitride, carbothermal reduction, CaO, SiO2
V tej študiji so bili sintetizirani s karbotermično redukcijo in nitridacijo z a-fazo bogati prahovi Si3N4 z dodatkom za sintranje (CaO). Začetni izvir silicija je bil zelo čisti (99 %) sintetični SiO2. Ogljik je bil dodan zelo čistemu SiO2 nad stehiometrično količino kisika. CaCO3 (za silicijev nitrid je vseboval 10 % CaO) je bil primešan začetnim reaktantom, odvisno od končne sestave prahu in vrste ter količine sekundarnih faz, potrebne za sintranje. Sinteza je bila izvršena v cevni peči pri različnih temperaturah (1400 °C, 1450 °C in 1475 °C, 3 h) v atmosferi dušika. Po sintranju so bile lastnosti prahu ugotovljene z uporabo navadnih tehnik za karakterizacijo (XRD, SEM itd.).
Ključne besede: silicijev nitrid, karbotermična redukcija, CaO, SiO2
1 INTRODUCTION
Silicon nitride (Si3N4) ceramics have a range of structural applications, such as engine components, heat exchangers, pump-seal materials, ball bearings, cutting tools, etc., owing to their excellent mechanical properties at both room and elevated temperatures.1 The material properties of Si3N4 have led to speculation that it may also have a role in biomedical fields, since it is biocom-patible and is visible on plain radiographs as a partially radiolucent material.2
The most common methods for silicon nitride preparation are the carbothermal reduction and nitridation (CRN) of silica, the direct nitridation of silicon and the thermal decomposition of silicon diimide. The carbother-mal reduction used in this study takes place according to the following overall reaction:3
3SiO2 + 6C + 2N2 ^ Si3N4 + 6CO (1)
One of the difficulties found in the fabrication process is the sintering applied to attain high relative densities. Therefore, the use of additives to form a liquid phase is required.4 Sintering aids such as MgO, Y2O3 and Al2O3 added to a-Si3N4 powders must be homogeneously distributed and possess the desired powder composition before shaping and sintering.5 In this study a-phase-rich Si3N4 powders were synthesized containing a sintering additive using carbothermal reduction and nitridation. Thus, Si3N4 powder is ready for sintering and in addition is produced in a single step. Calcium oxide (CaO) was chosen as a metal oxide additive and it was mixed before
synthesizing the Si3N4. Silicon nitride ceramic powders synthesized using this method might therefore be readily sintered because homogeneously distributed sintering additives were present in the starting materials. For this reason, the processing parameters are described in terms of the powder-synthesis conditions.
2 EXPERIMENTAL
For the CRN process, the raw material was high-purity synthetic silica of nearly colloidal range and it was supplied from EGE Kimya A. S. Activated charcoal was used as a reducing agent and it was supplied by TÜ-PRA§ (Turkish Petroleum Refineries Co). The properties of the silica are given in Table 1. CaCO3 was used as the CaO source; it was supplied by ^elvit Company. The CaCO3 (for the silicon nitride containing w = 10 % CaO) was premixed in the starting reactants, depending on the final powder composition and the type and amount of the secondary phases desired for sintering. Carbon was added to the high-purity SiO2 above the stoichiometric amount of oxygen (w(C)/w(SiO2) ratio of 3). Dry mixing was performed by ball milling for 10 h with alumina
Table 1: Properties of silica (with firm data) Tabela 1: Lastnosti SiO2 (podatki dobavitelja)
Properties	SiO2
Purity (%) Grain size (^m) Specific surface area (BET) (m2 g-1)	99 14 139
balls for the composition. The nitrogen gas (99.99 % in purity) that was used for the nitridation process was supplied by BOS.
The carbothermal reduction and the nitridation process (CRN) were carried out in an atmosphere-controlled tube furnace. The synthesis was performed under a nitrogen gas flow (1 L/m) at 1400 °C, 1450 °C and 1475 °C for 3 h. After holding at these temperatures and this time, the furnace was allowed to cool to room temperature. The gas flow was stopped during cooling when the temperature reached 900 °C. After the CRN process, the products were heated in air for 1 h at 900 °C for residual carbon burning.
After the CRN process at different temperatures, the powder properties were examined by using standard characterization techniques (XRD, SEM, EDS).
3 RESULTS AND DISCUSSION
The silica was mixed with w(CaO) = 10 % (based on the final product) and carbon black for the CRN process. The prepared mixture was subjected to the CRN process for 3 h at 1400 °C, 1450 °C and 1475 °C. The XRD analyses of the produced powders are shown in Figure 1.
The a-Si3N4 was formed at a temperature of 1400 °C after 3h of reaction. In addition, small amounts of unconverted SiOi and CaN2O3 phase were found in the product powders. The temperature was increased to 1450 °C, but the /S-Si3N4 formation was not completed at this temperature, which indicates that the holding time and/or temperature were not enough to complete the reaction. The best result was obtained at 1475 °C after a 3 h reaction. At this temperature (1475 °C), the produced powder was converted to a-Si3N4 and /S-Si3N4, and in addition a minor amount of CaSi2N2O2 was present. The formation of the CaSi2O2N2 phase by a solid-state reaction was completed at 1300-1400 °C for 10 h.67
The complete formation of a-Si3N4 without using an additive (without CaO) was obtained at 1475 °C for a 6 h reaction, but the /3-phase was not dedicated.8 As seen from the XRD data, the carbothermal reduction and nitri-dation took place at 1475 °C. Comparing this with our previous work, the optimum temperature is the same for the product powders that used Y2O3 and Y2O3-Li2O and 25 °C higher than the products that used MgO.9 The powder produced at 1475 °C was fully converted to a and 3-Si3N4, and a minor amount of CaSiiNiOi phase, as expected, were present. Different types of additives have been employed for the pressureless and pressure-assisted sintering of Si3N4 ceramics.
The melting temperature of the CaO-SiO2 system is approximately 1436 °C, as indicated by the binary phase-equilibrium diagram.10 However, the presence of N lowers these eutectic temperatures further. The alkali and alkaline-earth oxides have a low melting point and the viscosity of the resulting liquid is also low. The solution-diffusion-precipitation processes are enhanced.11 Therefore, the CaSi2N2O2 phase obtained after the CRN process can be desirable as the sintering aids for later use in sintering. Studies on the sintering of these synthesizing powders will be the focus of future work.
SEM micrographs of the powder synthesized from SiOi - w(CaO) = 10 % at 1400 °C and 1475 °C for 3 h are given in Figure 2. The micron-sized Si3N4 grains
Figure 1: Phases formed at different temperatures (1400-1475 °C) after CRN of the SiOi - w(CaO) = 10 % mixture Slika 1: Faze, ki nastajajo pri različnih temperaturah (1400-1475 °C) po CRN mešanice SiO2 in w(CaO) = 10 %
Figure 2: SEM micrographs of the product powder synthesized at: a) 1400 °C, 3 h and b) 1475 °C, 3 h
Slika 2: SEM-posnetka prašnatega produkta, sintetiziranega pri: a) 1400 °C, 3 h in b) 1475 °C, 3 h
Figure 3: EDS analysis of the powders synthesized from SiO2 -w(CaO) = 10 % at 1475 °C for 3 h
Slika 3: EDS-analiza prahov, sintetiziranih iz SiO2 - w(CaO) = 10 % pri 1475 °C po 3 h
were formed after the CRN. SEM micrographs of the powder produced at 1400 °C for 3 h revealed different morphologies, ranging from irregular-shaped small particles to equiaxed small grains and long whiskers. The long whiskers had a cross-section of approximately 300 nm. SEM micrographs of the produced powder at 1475 °C for 3 h revealed the same morphologies as small particles and long whiskers.
It was clear that the grain size was increased with increasing temperature from 1400 °C to 1475 °C. The whiskers had a thickness of approximately 1 pm after the CRN process at 1475 °C. The EDS analysis of the pow-
ders synthesized from SiO2 - w(CaO) = 10 % at 1475 °C for 3 h is showed in Figure 3. According to the EDS analysis, the Si, N, O, and Ca elements were detected from the particles in a manner consistent with the XRD analysis (Figure 1).
4	CONCLUSIONS
a-Si3N4 powders containing CaO as an oxide additive were successfully synthesized by carbothermal reduction and nitridation. The best result was obtained at 1475 °C after a 3 h reaction. The Si3N4 powders showed two major grain morphologies: submicron equiaxed and long whiskers. The advantage of using pre-additive oxide (CaO) in the CRN process is in terms of the reaction temperature and time.
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