UDK666.3/.7:546.28 Izvirni znanstveni članek
ISSN 1580-2949 MTAEC9, 37(1-2)25(2003)
G. SUČIKET AL.: THE CARBOTHERMAL PREPARATION OF SILICON NITRIDE WHISKERS
THE CARBOTHERMAL PREPARATION OF SILICON NITRIDE WHISKERS
KARBOTERMIČNA PRIPRAVA VISKERJEV IZ SILICIJEVEGA
NITRIDA
1Gabriel Sučik, 1Tarzicius Kuffa, 2Damir Hršak
1Faculty of Metallurgy, Technical University of Košice, Letna 9, Košice, Slovakia 2Faculty of Metallurgy, University of Zagreb, Aleja narodnih heroja 3, 44103, Sisak, Croatia
Prejem rokopisa - received: 2002-11-18; sprejem za objavo - accepted for publication: 2003-01-06
The type of silicon nitride (Si3N4) whiskers prepared in this study can be used for reinforcement in plastics, metals or ceramic composites, since silicon nitride has a high strength at high temperatures, high thermal shock resistance, and the whiskers have a tensile strength, close to the theoretical value. Condensed silica fume (CSF) and graphite was used as the precursors. The precursors must be in the amorphous phase and be particles that are as small as possible. Iron oxides from the precursors have an important role in the carbothermal preparation of silicon nitride whiskers, too. The optimum reaction conditions for preparing the silicon nitride whiskers were found to be: a reaction temperature in the interval 1380-1400 °C, a C/SiO2 molar ratio of 4 and a gas-phase composition (in vol. %) of 72 % N2 and 28 % NH3. The best efficiency rate was 40 mass. % of Si3N4 whiskers.
Key words: silicon nitride, whiskers, carbothermal preparation, reaction temperature, molar ratio C/SiO2, gas phase composition
Viskerji iz silicijevega nitrida (Si3N4), ki smo jih pripravili, se lahko uporabijo za utrditev kompozitov z matico iz kovine, plastike in keramike. Silicijev nitrid ima visoko trdnost pri visoki temperaturi, veliko odpornost proti toplotnim šokom in natezno trdnost, ki je blizu teoretične. Kondenzirani hlapi silicija (CSF) in grafit sta bila uporabljena kot prekursorja. Grafit mora biti amorfen in v čim manjših zrnih. Železov oksid iz prekursorja ima pomembno vlogo pri pripravi viskerjev iz silicijevega nitrida. Optimalni pogoji, določeni za pripravo viskerjev, so: reakcijska temperatura (1380-1400) °C, molsko razmerje C/SiO2 4 in volumski delež plinske faze 72 % N2 in 28 % NH4. Najboljši izkoristek je bil 40 % viskerjev Si3N4.
Ključne besede: viskerji silicijevega nitrida, karbotermična priprava, reakcijska temperatura, molsko razmerje C/SiO2, sestava plinske faze
1 INTRODUCTION
Silicon nitride is one of the most extensively studied structural ceramic materials, and the most important construction material for replacing steel in high-temperature and/or corrosive environments, for example in a gas turbine engine. It possesses high strength, fracture toughness, wear resistance, and thermal shock resistance, and is stable in oxidizing and other corrosive atmospheres, all of which are necessary for engineering applications at high temperatures1-4. Silicon nitride whiskers can be used for reinforcement in plastics, metals or ceramic composites5.
Carbothermal reduction and nitriding of silica is a more rapid process if inexpensive but pure reactants are used, and it is now established as one of the most promising alternatives to the silicon nitriding method in the production of silicon nitride of high purity6-11. The conditions for the preparation of silicon nitride from silica by carbothermal reduction and nitriding as well as the occurrence of other phases in the Si-C-O-N system can be predicted for different conditions of temperature and pressure12. Impurities that are present in the reactants, for example small concentrations of iron oxides, are also known to play an important role in determining the phase formed and their effects cannot be explained en-
tirely with equilibrium thermodynamics13. The silicon nitride can be in the form of powder, fibres or a needle-like shape. The shape of the silicon nitride particles is the result of a nucleation mechanism.
Condensed silica fume is a very fine non-crystalline silica produced by electric-arc furnaces as a by-product of the production of metallic silicon or ferrosilicon alloys. It is a powder with particles sized between 0.1 and 0.2 µm. From the economic point of view it is very important to find a good way of exploiting this waste material.
2 EXPERIMENTAL
All the experiments for preparing silicon nitride whiskers were realized in a laboratory semi-batch tube reactor with a controlled reaction temperature.
The starting materials were Slovakian condensed silica fume and graphite. The chemical composition (in mass. %) of condensed silica fume was: amorphous SiO2 95.6 %, Fe2O3 0.2 %, Al2O3 0.1 %, MgO 0.9 %, CaO 0.1 % and Na2O + K2O 0.9 %. The ignition loss was 1.4 mass. %. A granulometric analysis showed that 90 % of particles were under 2 µm. The graphite used had a carbon mass portion over 97.9 %, and a particle size of 1-5 µm.
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G. SUČIKET AL.: THE CARBOTHERMAL PREPARATION OF SILICON NITRIDE WHISKERS
Figure 1: SEM photograph of two forms of ?-Si3N4 whiskers Slika 1: SEM-posnetek dveh oblik viskerjev ?-Si3N4
The total gas flow rate was 2.1 dm3/h, and the reaction time was 15 hours. The reaction temperature, the C/SiO2 molar ratio, and the gas-phase composition were varied.
3 RESULTS AND DISCUSSION
After the set of experiment was completed, the optimum reaction conditions for preparing the silicon nitride whiskers using condensed silica fume and graphite as precursors were found to be: reaction temperature, 1380-1400 °C; C/SiO2 molar ratio, 4; and gas phase composition (in vol. %), 72 % N2 and 28 % NH3. The best efficiency rate, using these parameters, was 40 mass. % of Si3N4 whiskers.
At temperatures lower than 1380 °C the reaction kinetics for the formation of silicon nitride whiskers is insufficient, and at temperatures higher than 1400 °C we
Figure 2: SEM photograph of broken Fe-Si droplets separated by
whiskers
Slika 2: SEM-posnetek prelomljenih kapljic Fe-Si, ločenih z viskerji
get unwanted silicon carbide as one of the reaction products. The highest efficiency rate was achieved when the molar ratio C/SiO2 was 4; a stoichiometric molar ratio of 2 is insufficient. Not surprisingly, a lot of excess carbon is needed because a full conversion at the stoichiometric coefficient can only be realized in the case of a full contact between the carbon and the silica particles, which is unlikely to occur under actual reaction conditions.
The gas-phase composition has a great influence on the formation of silicon nitride whiskers. For the preparation of ?-Si3N4 whiskers it is necessary to use a mixed nitrogen atmosphere because a pure N2 atmosphere results in the preferential formation of ß-Si3N4 whiskers. The best results are achieved with a gas-phase composition of 72 volume % of N2 and 28 volume % of NH3. In the carbothermal preparation of silicon nitride whiskers, as with the carbothermal
Figure 3: Presumed mechanism for the growth of silicon nitride whiskers Slika 3: Predpostavljeni mehanizem rasti viskerjev silicijevega nitrida
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G. SUČIKET AL.: THE CARBOTHERMAL PREPARATION OF SILICON NITRIDE WHISKERS
preparation of silicon nitride powder, the a-Si3N4 form is preferable.
In this paper two forms of silicon nitride whiskers were produced. Needle-shaped crystals of silicon nitride were produced on the surface of the reaction mixture and grew in the Š001] crystallographic direction. These crystals have FexSiy droplets on their upper ends and were formed via a VLS (vapor-liquid-solid) mechanism. The existence of a fibrous product formed on the internal surface of the tubular reactor proved the transport through the gaseous phase and the formation of silicon nitride whiskers via the heterogeneous nucleation of Si3N4(s) through the condensation of Si(s) from a supersaturated Si(g) vapor on a solid substrate and the consequent reaction of Si(s) with nitrogen from the gaseous phase. Figure 1 shows both forms of silicon nitride whiskers.
The critical size of the whiskers depends on the direction of their growth. The connection between the droplet and the whisker is interrupted and the whisker growth is finished when the critical size is exceeded. An SEM photograph of separated, broken Fe-Si droplets is shown in Figure 2.
Figure 3 presents the presumed mechanism for the growth of silicon nitride whiskers.
4 CONCLUSION
The optimum reaction conditions for preparing silicon nitride whiskers using condensed silica fume and graphite as the precursors in carbothermal reduction and subsequent nitridation of amorphous silica were found to
be: a reaction temperature between 1380-1400 °C, a C/SiO2 molar ratio of 4, and a gas-phase composition (in vol. %) of 72 % N2 and 28 % NH3. The best efficiency rate, using these parameters, was 40 mass. % of Si3N4 whiskers.
Two forms of silicon nitride whiskers were produced: needle-shaped crystals on the surface of the reaction mixture and fibre-shaped crystals on the internal surface of the tubular reactor.
The results presented in this paper will be useful in the further development of the process of silicon nitride whiskers preparation using Slovakian raw materials.
5 LITERATURE
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