Ind. Eng. Chem. Res. 2006, 45, 1277-1280
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Synthesis and Sintering of Si3N4 Obtained by the SHS Process Irene G. Cano and Miguel A. Rodrı´guez* Instituto de Cera´ mica y Vı´drio (CSIC), Campus de Cantoblanco, Camino de Valdelatas, s/n 28049-Madrid, Spain
The synthesis of Si3N4 by the self-propagating high-temperature synthesis (SHS) process has been carried out to evaluate the effect of NH4Cl/NH4F as an additive. A higher R-phase content is obtained when the rate of NH4Cl/NH4F addition decreases. The powders obtained exhibit good sintering behavior. Their mechanical properties are comparable to those of a commercial sample, although the porosity of the SHS powders is higher. This behavior might be due to the greater particle size of SHS Si3N4 powders. Introduction
Table 1. Sintering Raw Materials
Many literature reports have described the production of silicon nitride powder using the self-propagating high-temperature synthesis (SHS) method in the past decade.1 With this method, the cost is lower,2 but control of the powder characteristics is more difficult. The nitridation of silicon powders takes place in a self-sustained regime when high pressures3-5 and dilution of the silicon reagent with fine powders of Si3N4 are used. Reaction characteristics include high rates of the combustion front (10-4-10-3 m/s), generation of high temperatures (1700-2000 °C), and rapid heating (103-104 K/s). Nevertheless, it is possible to achieve a final powder with the desired properties (phase content, morphologies, etc.) by controlling the initial parameters of the synthesis.6 Silicon nitride is considered to be a promising ceramic because of its exceptional chemical and mechanical properties, especially at high temperature.7 The ability to obtain these excellent final properties of sintered Si3N4 depends on the starting powder. Hence, a suitable morphology and size distribution are significant parameters to ensure a high quality of the final Si3N4 product. In the first studies on Si3N4, it was generally assumed that R-Si3N4 exhibits better sinterability. Nevertheless, it has been demonstrated that the addition of elongated β-Si3N4 fibers to R-Si3N4 powder can improve the final mechanical properties8,9 (specifically by toughening mechanisms). Therefore, it seems that a combination of R-Si3N4 and β-Si3N4 fibers might be optimal to achieve a good final product. Some works have been published in which R-Si3N410,11 and β-Si3N4 fibers12,13 were obtained by the SHS technique, but little research has been reported about sintering of SHS Si3N4.14 In this work, the main target was to obtain by SHS a Si3N4 powder whose final composition consisted a majority of R-phase (with a small part of β-Si3N4 fibers), ending with the sintering of the as-synthesized powders (without milling or any other treatment). First, we focused on the optimization of the synthesis parameters. The second step was to sinter the as-obtained powder through a standard process, for comparison with a commercial powder. The two powders were processed simultaneously and characterized. Experimental Procedure SHS Process. Silicon fine powders (Silgrain, 20 mm; Elkem, Oslo, Norway; >99.6 wt % purity, Al < 0.1 wt %, Fe < 0.05 * To whom correspondence should be addressed. E-mail: mar@ icv.csic.es.
Si3N4 powder
average particle size (µm)
specific surface area (m2/g)
R-Si3N4 (wt %)
H. C. Starck SHS
1 20
11.5 1.3
84 63
wt %, d50 ≈ 4.1 µm, specific surface area ≈ 1.8 m2/g), homemade SHS silicon nitride (d50 ≈ 4.6 µm, specific surface area ≈ 2 m2/g, 65 wt % β-Si3N4), and ammonium salts NH4Cl and NH4F (both from Merck, Darmstadt, Germany; purity > 98%) were dry mixed for 1 h. Ammonium salts were selected as additives on the basis of a previous report where NH4Cl and NH4F were used to obtain R-Si3N4.11 From these data, the ammonium chloride quantity was progressively replaced by ammonium fluoride in different experiments performed to vary the final phase contents. The ammonium chloride and ammonium fluoride additions were initially 5 and 0 molar %, respectively. These additions were varied until reaching 1 molar % for NH4Cl and 4 molar % for NH4F. The experiments were performed in a stainless steel SHS reactor with a 2-L capacity and a useful length of 200 mm. The ignition was carried out at one end of the sample using a tungsten coil (0.5 mm in diameter) that was heated by a 20-V dc power supply. The initial nitrogen pressure in all experiments was 10 MPa, and preliminary outgassing was carried out in all cases. The crystalline phases of silicon, R-Si3N4 and β-Si3N4, in the final products were identified by X-ray powder diffraction (Siemens D-5000). Si3N4 phases were calculated according to the Gazzara and Messier method,15 and silicon content was quantified using the serial addition and Rietveld methods.16,17 Powder morphologies and microstructures of the sintered materials were observed by scanning electron microscopy (Carl Zeiss, DSM-950). Sintering of Si3N4. Both SHS-obtained and commercial (Hermann C. Starck, grade M11, Berlin, Germany) powders of Si3N4 were used to obtain sintered material. As mentioned above, SHS Si3N4 was not ground to avoid an increase of the process cost. The characteristics of both powders are detailed in Table 1, where the difference between average particle sizes is noted. The smaller size of the commercial powders is mainly due to the lower temperature of synthesis and also the postsynthesis grinding process. The more relevant characteristic difference is found in phase content. The β-Si3N4 content is higher in SHS powders than in commercial powders, as determined by XRD (Figure 1).
10.1021/ie050968+ CCC: $33.50 © 2006 American Chemical Society Published on Web 01/24/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 4, 2006
Figure 1. X-ray diffraction of SHS Si3N4 and commercial Si3N4.
Figure 3. Temperature profile in the case of 5 mol % NH4Cl addition (without NH4F addition). Figure 2. ([) Unreacted silicon and (2) β-Si3N4 content in final products versus NH4Cl/NH4F ratio.
The aim of part of the study was to compare results for the different starting powders. Therefore, common standard sintering conditions were selected. The additives used to improve Si3N4 densification were a mixture of Y2O3 and SiO2 (12 wt %), which are widely known.18 Si3N4 and the additives were mixed using attrition milling with Si3N4 balls (1 mm in diameter) in isopropyl alcohol for 2 h. The mixture was dried at 60 °C and sieved to 60 µm. The sintering process was carried out by the hot-press technique in a nitrogen atmosphere following a standard thermal cycle for this material: heating at 10 °C/min to 1750 °C, holding for 1 h at this plateau, and cooling at 10 °C/min. The maximum pressure was 50 MPa. The density of the as-obtained materials was analyzed by the Archimedes principle (using distilled water). Hardness and toughness were measured by Vickers indentation (loads, 1 and 50 kg; time, 15 s). Morphologies and crack propagation paths were observed by scanning electron microscopy (Carl Zeiss, DSM-950); to improve observations, after being polished, the samples were plasma etched. Results and Discusion As explained above, the initial composition was selected to favor high final R-Si3N4 contents. It is necessary to remark that, in addition to the reactive mixture, another parameter that also plays a significant role in this way is the initial mass. If the initial reactive mass is increased, the final products contain higher β-Si3N4 contents because of the lower energy dissipation. Hence, the initial mass used in this work was the lowest possible (35 g) for a successful combustion reaction in the reactor employed. The as-obtained products were mainly white in appearance with a thin exterior dark layer of unreacted silicon (about 1 mm) that was readily eliminated. In all cases, the final phases were silicon and silicon nitride (with different percentages of R-Si3N4 and β-Si3N4). It can be observed (Figure 2) that, when the NH4Cl/NH4F ratio decreased, unreacted silicon tended to be constant
Table 2. Maximum Combustion Pressure with Different NH4Cl/ NH4F Additions NH4Cl/NH4F
maximum pressure (MPa)
5/0 4/1 3/2 2/3 1/4
12.5 12.8 13 13.2 13.9
(
