Combustion Synthesis of Sialon Powders

Title Combustion Synthesis of SiAlON Powders

Author(s) 牛, 晶

Citation 北海道大学. 博士(工学) 甲第11564号

Issue Date 2014-09-25

DOI 10.14943/doctoral.k11564

Doc URL http://hdl.handle.net/2115/57192

Type theses (doctoral)

File Information Niu_Jing.pdf

Instructions for use

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP

Combustion Synthesis of SiAlON Powders

サイアロン粉末の燃焼合成

Laboratory of Energy Media

Jing NIU

Hokkaido University 2014

Contents

Chapter 1

General Introduction ...... 1

1.1 SiAlON ceramics ...... 1

1.2 SiAlON phosphors ...... 3

1.3 Conventional methods to prepare SiAlON ...... 4

1.4 Combustion synthesis ...... 7

1.5 Scope of the present work ...... 11

References ...... 13

Chapter 2 Combustion synthesis of high purity β-SiAlON fine powders using natural kaolin ...... 17

2.1 Introduction ...... 17

2.2 Experimental procedure ...... 19

2.3 Results and discussion ...... 21

2.4 Conclusion ...... 28

References ...... 29

Chapter 3 Salt-assisted combustion synthesis of β-SiAlONs and its morphology control ...... 31

3.1 Introduction ...... 31

3.2 Combustion synthesis of β-SiAlONs (z=0.25-3) using NaCl as a diluent ...... 34

3.3 Effect of metal chlorides on the combustion synthesis of β-SiAlON ...... 50

3.4 Comparison between NaCl and SiAlON diluent on combustion synthesis .... 62

3.5 Conclusion ...... 79

References ...... 81

Chapter 4 Fabrication of mixed α/β-SiAlON powders via salt-assisted combustion synthesis ...... 84

4.1 Introduction ...... 84

4.2 Experimental procedure ...... 86

4.3 Results and discussion ...... 88

4.4 Conclusion ...... 98

References ...... 99

Chapter 5 Salt-assisted combustion synthesis of β-SiAlON:Eu2+ phosphors for white light-emitting diodes ...... 101

5.1 Introduction ...... 101

5.2 Experimental procedure ...... 103

5.3 Results and discussion ...... 104

5.4 Conclusion ...... 111

References ...... 112

Chapter 6

General Conclusions ...... 114

Acknowledgements...... 116

Chapter 1

General introduction

1.1 SiAlON ceramics SiAlONs is a general name for a large family of the so-called ceramic alloys based on silicon nitride, which were first discovered by Oyama et al. (1971) and Jack et al.

(1972) and have been widely studied since then [1, 2]. The structure of SiAlON is based

4+ 3− 3+ 2− on Si3N4, which Si and N being partially replaced by Al and O , respectively.

Therefore, its physical and mechanical properties are similar to silicon nitride, whereas chemically it is closer to aluminum oxide. Usually, SiAlON ceramics exhibit more excellent properties than that of Si3N4, such as lower thermal expansion coefficient, better oxidation resistance and corrosion resistance. Furthermore, they show good creep strength at high temperature due to the needless of sintering aids when the dense materials were produced [3, 4]. These features lead to economic and performance advantages over other ceramic candidate materials. SiAlON ceramics are extensively used in non-ferrous molten metal handing (especially aluminum and its alloys) as metal feeding tubes, protective tubes for immersion heaters and thermocouples, crucibles, and ladles. They are also used as cutting tools in machining of metal materials and fixtures in brazing or melding. Highly-dense SiAlON ceramics with reduced secondary phases are transparent and promising for some optical applications [5-7].

There are two SiAlON phases that are of interest as engineering ceramics, α and β, which are based on α- and β-Si3N4 structural modifications, respectively. β-SiAlON, which is of the form Si6−zAlzOzN8−z, where z denotes the number of Si–N bonds substituted by the Al–O bonds (0  z  4.2). α-SiAlON, which is of the form

v Mx Si12−(m+n)Alm+nOnN16−n (x = m/v, x ≤ 2; v is the valence of the metal, M; M represents

1 metals like Li, Mg, Ca, Y, and rare earth metals; and m (Al–N), n (Al–O) replace (m+n)

(Si–N) in each unit cell). α-SiAlON phase generally shows equiaxed grains in the microstructure of the material, while β-SiAlON phase exhibits elongated grains. The crystal structure of SiAlON is shown in Fig. 1-1 [8-10]. Due to the different grain structure, α-SiAlON shows a higher Vickers hardness, and greater oxidation and erosion resistance, but lower strength and toughness than β-SiAlON. The mechanical properties of these materials, therefore, can be controlled by varying the α-SiAlON: β-SiAlON phase ratio. Recent reports show that the mixed α/β-SiAlON ceramics providing superior mechanical properties in comparison to those of pure α-SiAlON and composites of α-SiAlON and polytypes because such materials combine the toughness of β-SiAlON and the wear resistance of α-SiAlON [11-13].

(a) α-SiAlON (b) β-SiAlON

Fig. 1-1 Crystal structure of SiAlON viewed along the [001] direction. The blue, red, and green spheres represent M, Si/Al, and O/N atoms, respectively [10].

1.2 SiAlON phosphors Recently, rare-earth cations (e.g., Eu2+, Ce3+, Tb3+, and Pr3+)-doped SiAlON phosphors have been considered as candidates for application in white LED due to their high photoluminescence intensity and outstanding thermal and chemical stability [10, 14-18].

Among them, Eu-doped β-SiAlON phosphor invented by Hirosaki et al. [19] in 2005, which emits strong green light under UV or blue excitation. Fig. 1-2 shows

β-SiAlON:Eu2+ phosphors produces intense green emission with a peak located at

538 nm, and the broad emission spectrum has a full-width at half-maximum of 55 nm.

Further studies by Xie et al. [20] revealed that with lower z-values (z < 1.0) has higher phase purity, more uniform particle size produced greater emission. In addition, the

β-SiAlON:Eu2+ phosphors has a small thermal quenching and high stability of chromaticity against temperature. Kimura et al. [21] successfully prepared a white LED with an ultrahigh color rendering index (Ra > 95) by utilizing β-SiAlON: Eu2+ phosphor in combination with other oxynitride phosphors and a blue LED chip.

Fig. 1-2 Excitation and emission spectra of β-SiAlON:Eu2+ with the

composition of β-Si5.5Al0.5O0.5N7.5:Eu0.03 [19].

1.3 Conventional methods to prepare SiAlON 1.3.1 Reaction sintering

SiAlON ceramics are typically produced by pressureless sintering [4, 22, 23] or hot-pressing (HP) [9, 24], which generally involves sintering of high-purity Si3N4, AlN, and Al2O3 at temperatures of 1700–2000 °C for many hours under nitrogen pressure.

Furthermore, Si3N4 and AlN must be synthesized prior to the sintering process as an additional step. Consequently, this synthesis process is limited by many disadvantages such as the need for multistep processes, long processing time at elevated temperatures and the contamination of the products introduced during pulverization of strong bulk samples. Fig. 1-3 shows a schematic diagram for a typical process for synthesizing

SiAlON.

Si3N4 + AlN

+ Al2O3 Reaction sintering + Y2O3 Ball milling, 1800℃， (Sintering aid ) several hours several hours

Fig. 1-3 Schematic diagram for a typical process for synthesizing SiAlON.

1.3.2 Carbonthermal reduction and nitridation (CRN)

The carbothermal reduction and nitridation (CRN) method invented by Lee and

Cutler in 1979 [25] has been reported for the synthesis of β-SiAlON using mineral kaolin (Al2O3·2SiO2·2H2O) and a carbon mixture as raw materials. The general reaction is given as:

3(Al2O3·2SiO2·2H2O) + 15C + 5N2(g)→2Si3Al3O3N5 + 15CO(g) + 6H2O (g) (1.1)

Other materials such as powder mixtures of SiO2–Al2O3 [26], SiO2–Al2O3–CaCO3 [27],

SiO2–AlN–CaF2–Dy2O3 [28, 29], or other SiO2 and Al2O3 rich minerals such as zeolites [30], and halloysite [31] that mixed with carbon have also been used as starting materials for synthesizing SiAlON powders. Because it uses low-cost raw material, this method is regarded as an economically attractive process for producing SiAlON.

However, the practical application of this method has been limited thus far due to the low purity of products, which usually contains a number of undesirable by-products, such as SiC and AlN. The formation of these by-products is caused by the sensitivity of the products to the experimental parameters, including the carbon content, nitrogen flow rate, temperature, and holding time [32, 33]. For example, lower carbon content stoichiometrically leads to the formation of Al2O3 or mullite impurities, whereas excess carbon leads to the formation of SiC or the 15R phase in the product.

1.3.3 Gas reduction and nitridation (GRN)

In the last decades, gas reduction and nitridation (GRN) method [34-36] was developed for synthesizing SiAlON powders by nitridating oxide precursors in a reduction-nitridation gas such as ammonia or an NH3-CH4 gas mixture. The homogeneous precursor is usually prepared by coprecipitation method or by mixing of nanosized alumina and colloidal silica. The nitridation reactions for conversion to

SiAlON are achieved at 1500 °C for several hours. Highly pure, uniform submicrometer-sized or nano-sized SiAlON particles can be obtained by this method.

1.3.4 Gas pressure sintering (GPS)

Gas pressure sintering (GPS) method is usually used to prepare SiAlON phosphors.

The starting materials include metal nitrides (e.g., Si3N4, AlN, Ca3N2, EuN), and metal oxides (e.g., Al2O3, CaCO3, CeO2, Li2CO3). The reactions involve high temperatures of

1500–2000 °C for many hours under a nitrogen pressure of 0.1–1.0 MPa [18, 37, 38].

The phosphor powders prepared by this method usually consist of hard agglomerates.

To obtain fine and well-dispersed powders, it is necessary to pulverize the fired products, which would introduce the contamination to the final products, hence reduces the luminescence.

These methods are not suited from the viewpoint of energy-saving because they have a drawback of having multistep pathways, and long processing time at elevated temperatures to produce the product.

1.4 Combustion synthesis Combustion synthesis (CS) or self-propagation high-temperature synthesis (SHS), established by Merzhanov et al. [39, 40] is a proven effective energy- and time-saving process for producing various industrial materials, particularly ceramics [41-47]. This process exploits a self-sustaining regime that utilizes the heat generated during a strongly exothermic reaction. No external energy is required, except for the ignition energy. Combustion synthesis is an attractive alternative to conventional methods of powders synthesis, owing to its demonstrated advantages, including (1) simplicity of the process, (2) relatively low energy requirement, (3) higher purity of the products, and (4) a short reaction time.

For it to be self-sustaining, the combustion synthesis process must be associated with high-temperature reactions. An important parameter in this regard is the adiabatic temperature of combustion, Tad. This thermodynamic parameter is the temperature to which the product is raised under adiabatic conditions as a consequence of the evolution of heat from the reaction. The adiabatic temperature can be calculated from the following equation:

T ad  0  niCip dT  H r 298   i  (1.2)

Here, ni expresses the stoichiometric number of a product (i), Cip denotes the molar heat

0 capacity of the respective product, and Hr is the standard enthalpy of reaction. It is assumed that there is no heat loss through the walls of the container during the combustion synthesis. In fact, the combustion temperature is frequently lower than the adiabatic temperature of the synthesis, which was mainly due to the heat losses to the surroundings. Nevertheless, Tad provides a useful estimate of the reaction temperature. It

7 also provides an indication of whether or not combustion synthesis can proceed via the self-propagating mode. It has been empirically suggested that the SHS mode can be sustained only if Tad ≥1800 [47].

1.4.1 Conventional combustion synthesis of SiAlON

Combustion synthesis of SiAlON powders is usually carried out in a N2 atmosphere, which requires high N2 pressure (2–150 MPa) to drive the reaction to completion [48]. The raw materials include Si, Al, SiO2, Al2O3, CaCO3, and rare-earth oxides. The raw materials are mixed according to the chemical compositions of

v Mx Si12−(m+n)Alm+nOnN16−n (x = m/v; v is the valence of M) for α-SiAlON and

Si6−zAlzOzN8−z (0  z  4.2) for β-SiAlON. Nevertheless, the combustion temperatures are extremely high and Si particles melt in the combustion front and coalesce during reaction, which inhibits the complete nitridation. To reduce the reaction heat or combustion temperature, and to facilitate the infiltration of N2, a large amount of diluents such as Si3N4, AlN, or SiAlON powders must be added to the starting materials to obtain a pure product [46, 49-52].

Aoyagi et al. [53, 54] successfully synthesized β-SiAlONs with different z values

(z = 1, 2, 3, and 4) under a low nitrogen pressure of 1 MPa with Si, Al, and SiO2 powders, and discussed the effect of SiAlON diluents on purity of products and adiabatic temperatures. It showed the purity of the β-SiAlON (z = 1) product increased steadily and adiabatic temperature decreased with increasing the amounts of diluents, the maximum purity of 91 mass% was achieved when 40 mass% of β-SiAlON diluents were added to raw material mixtures. Shahien et al. [55] synthesized single-phase

β-SiAlON powders (z = 2 to 4) with the addition of commercial β-SiAlON powders up to 50 mass% under N2 pressure of 1 MPa. However, the simple use of the diluent is an inefficient process by using a part of the product as the starting reactant. Furthermore, it

8 is difficult to obtain β-SiAlON with fine or submicron-size particles. The biggest obstacle for the wide use of β-SiAlON materials is the high cost for their synthesis, especially for their post-synthesis treatment, i.e., for obtaining a product with a fine grain size. Fig. 1-4 shows a schematic diagram for synthesizing highly pure β-SiAlON powders by combustion synthesis method.

Si + Al + SiO2 + SiAlON (45-50 mass%) Ball milling, Combustion Synthesis, several mins. a few mins. N2, 1MPa Fig. 1-4 Schematic diagram for the conventional combustion synthesis of β-SiAlON under nitrogen pressure of 1MPa.

1.4.2 Proposed novel combustion synthesis processes

1. Diluent-free combustion synthesis of high purity β-SiAlON using natural kaolin [56] (This research)

Kaolin as an aluminosilicate type of clay mineral provides a good source of the constituents necessary for the synthesis of SiAlON. Chemical formula of kaolin is

Al2O3∙2SiO2∙2H2O, where structural water is eliminated during the combustion reaction processes and absorbs part of the released heat. In this study, we use kaolin as the raw materials with addition of Si and Al powders to synthesize β-SiAlON without any diluents. Single-phase, submicron sized β-SiAlON particles were fabricated directly from natural kaolin by combustion synthesis at nitrogen pressure of 1 MPa.

The reaction for the dehydroxylation which occurred within the temperature range of 350–830 °C can be described by the following equation:

0 -1 Al2O3∙2SiO2∙2H2O = Al2O3∙2SiO2 + 2H2O (g) (ΔH 298K = 294.76 kJ mol ) (1.3)

By this endothermic reaction, the adiabatic temperature (Tad) of the combustion synthesis reaction using kaolin as a raw material was lowered by 350 °C compared with the Tad needed when using Al2O3∙2SiO2 as a raw material.

2. Salt-assisted combustion synthesis of SiAlON [57, 58] (This research)

NaCl is an efficient diluent during the combustion synthesis of SiAlON, the reaction heat is reduced through the latent heat of its phase transformation.

NaCl (solid) → NaCl (liquid) (ΔH = 82.1 kJ) (1.4)

NaCl (liquid) → NaCl (gas) (ΔH = 202.1 kJ) (1.5)

By reducing the amounts of heat, which ensures an appropriate combustion temperature, promotes the complete nitridation of Si. Furthermore, the melted NaCl can be regarded as a protective shield that prevents the agglomeration of the products. Single-phase

β-SiAlON powders with a submicron size were successfully synthesized with the addition of a small amount of NaCl.

The advantages of utilizing NaCl in this study are expected to be the following: (1)

It is inert to the initial components (Si, Al, and SiO2) and can be easily separated from the product due to its high solubility in water. (2) It plays a positive protection role against the melting of Si, because the melting point of NaCl is lower than that of Si. (3)

Product with fine particles in the submicron or nanometer range can be obtained.

1.5 Scope of the present work As mentioned in the previous sections, for obtaining high purity product, a large amount of the SiAlON product (up to 50 mass%) must be added into the starting materials as diluents to decrease reaction heat as well as to slow down the combustion temperature. However, using the product as diluents decreases the product yield.

Furthermore, it is difficult to gain a product with submicron particles although the combustion synthesis temperature decreases. The major obstacle to the wide use of

β-SiAlON materials is the high cost due to their complicated post-synthesis treatments, i.e., for obtaining a product with a fine grain size.

To solve this problem, we proposed two novel processes to fabricate highly pure, fine SiAlON particles: diluent-free combustion synthesis and salt-assisted combustion synthesis. In addition, the application of the new routes for synthesizing SiAlON phosphors could be of quite benefit in the field of white light-emitting diodes (LEDs).

This thesis includes six chapters:

Chapter 1 presents a general introduction of this study.

In chapter 2, we describe the combustion synthesis of β-Si6-zAlzOzN8-z (z = 1) by using natural kaolin, Si, and Al powders under a nitrogen pressure of 1 MPa without adding any diluent. The effect of structural water contained by kaolin was investigated by comparing the one without water after dehydrated treatment.

In chapter 3, salt-assisted combustion synthesis of β-Si6-zAlzOzN8-z (z = 0.25−3) powder with materials of Si, Al, SiO2 was investigated. The proper amount of NaCl addition for each z value was discussed. The effect of metal chlorides (KCl, MgCl2, and

CaCl2) on the synthesis of β-SiAlON and a detailed understanding of the endothermic processes occurring during the exothermic stages were investigated. This understanding supplies a new route for synthesizing a single-phase product with a morphology tailored by changing the type of metal chlorides to control the endothermic processes.

Furthermore, the characteristics of the reaction process and kinetic of the reaction on propagation were compared by using NaCl and SiAlON as the diluents.

In chapter 4, salt-assisted combustion synthesis of α/β-SiAlON composite ceramics and α-SiAlON were studied. The α/β-SiAlON ratio and morphology of the products were investigated using different type of metal chloride (NaCl and MgCl2).

In chapter 5, the novel synthesis route was applied to the preparation of

β-SiAlON:Eu2+ phosphors. The thermal quenching of the photoluminescence (PL) emission efficiency is compared with the commercial silicate green phosphor

2+ (Sr2SiO4:Eu ).

At the end, a summary of the work and concluding remarks were presented in chapter 6.

References

[1] Y. Oyama, O. Kamigaito, Solid Solubility of Some Oxides in Si3N4. Japanese Journal of Applied Physics 10 (1971) 1673. [2] K. Jack, W. Wilson, Ceramics based on the Si-Al-ON and related systems. Nature 238 (1972) 28-29. [3] K.H. Jack, Review Sialons and related nitrogen ceramics. Journal of Materials Science 11 (1976) 1135-1158. [4] J. Briggs, Pressureless sintering of SiAlON ceramics. Materials Research Bulletin 12 (1977) 1047-1055. [5] M.I. Jones, H. Hyuga, K. Hirao, Y. Yamauchi, Highly Transparent Lu-α-SiAlON. Journal of the American Ceramic Society 87 (2004) 714-716. [6] F. Ye, L. Liu, C. Liu, H. Zhang, Y. Zhou, J. Yu, High infrared transmission of Y3+–Yb3+-doped α-SiAlON. Materials Letters 62 (2008) 4535-4538. [7] X. Su, P. Wang, W. Chen, B. Zhu, Y. Cheng, D. Yan, Translucent α-Sialon Ceramics by Hot Pressing. Journal of the American Ceramic Society 87 (2004) 730-732. [8] T. Ekström, M. Nygren, SiAlON Ceramics. Journal of the American Ceramic Society 75 (1992) 259-276. [9] T. Ekström, P.O. Käll, M. Nygren, P.O. Olssen, Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing. Journal of Materials Science 24 (1989) 1853-1861. [10] R.-J. Xie, N. Hirosaki, Silicon-based oxynitride and nitride phosphors for white LEDs—A review. Science and Technology of Advanced Materials 8 (2007) 588-600. [11] S. Bandyopadhyay, M.J. Hoffmann, G. Petzow, Densification Behavior and Properties of

Y2O3-Containing α-SiAlON-Based Composites. Journal of the American Ceramic Society 79 (1996) 1537-1545.

[12] T.-S. Sheu, Microstructure and Mechanical Properties of the in situ β-Si3N4/α′-SiAlON Composite. Journal of the American Ceramic Society 77 (1994) 2345-2353. [13] F. Ye, L. Liu, H. Zhang, Y. Zhou, Z. Zhang, Novel mixed α/β-SiAlONs with both elongated α and β grains. Scripta Materialia 60 (2009) 471-474. [14] R.-J. Xie, M. Mitomo, K. Uheda, F.-F. Xu, Y. Akimune, Preparation and Luminescence Spectra of Calcium- and Rare-Earth (R = Eu, Tb, and Pr)-Codoped α-SiAlON Ceramics. Journal of the American Ceramic Society 85 (2002) 1229-1234. [15] R.-J. Xie, N. Hirosaki, M. Mitomo, Y. Yamamoto, T. Suehiro, K. Sakuma, Optical Properties of Eu2+ in α-SiAlON. The Journal of Physical Chemistry B 108 (2004) 12027-12031. [16] R.-J. Xie, N. Hirosaki, M. Mitomo, K. Sakuma, N. Kimura, Wavelength-tunable and thermally stable Li-α-sialon: Eu2+ oxynitride phosphors for white light-emitting diodes. Applied Physics Letters

89 (2006) 241103-241103-241103. [17] T.-C. Liu, B.-M. Cheng, S.-F. Hu, R.-S. Liu, Highly Stable Red Oxynitride β-SiAlON:Pr3+ Phosphor for Light-Emitting Diodes. Chemistry of Materials 23 (2011) 3698-3705. [18] Y. -Q. Li, N. Hirosaki, R.-J. Xie, J. Li, T. Takeda, Y. Yamamoto, M. Mitomo, Structural and Photoluminescence Properties of Ce3+- and Tb3+-Activated Lu-α-Sialon. Journal of the American

Ceramic Society 92 (2009) 2738-2744. [19] N. Hirosaki, R.-J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro, M. Mitomo, Characterization and properties of green-emitting beta-SiAlON:Eu2+ powder phosphors for white light-emitting diodes. Applied Physics Letters 86 (2005) 211905. [20] R.-J. Xie, N. Hirosaki, H.-L. Li, Y.Q. Li, M. Mitomo, Synthesis and Photoluminescence Properties of 2+ 2+ β-sialon : Eu ( Si6−zAlzOzN8 − z: Eu ) : A Promising Green Oxynitride Phosphor for White Light-Emitting Diodes. Journal of The Electrochemical Society 154 (2007) J314-J319. [21] N. Kimura, K. Sakuma, S. Hirafune, K. Asano, N. Hirosaki, R.-J. Xie, Extrahigh color rendering white light-emitting diode lamps using oxynitride and nitride phosphors excited by blue light-emitting diode. Applied Physics Letters 90 (2007) 051109. [22] R.R. Wills, R.W. Stewart, J.M. Wimmer, Fabrication of Reaction-Sintered Sialon. Journal of the American Ceramic Society 60 (1977) 64-67. [23] M. Mitomo, N. Kuramoto, Y. Inomata, Fabrication of high strength β-sialon by reaction sintering. Journal of Materials Science 14 (1979) 2309-2316. [24] T. Ekström, P.-O. Olsson, β-Sialon Ceramics Prepared at 1700 °C by Hot Isostatic Pressing. Journal of the American Ceramic Society 72 (1989) 1722-1724. [25] J.G. Lee, I.B. Cutler, Sinterable SiAlON powder by reaction of clay with carbon and nitrogen. American Ceramic Society Bulletin 58 (1979) 869–871. [26] M. Sopicka-Lizer, R.A. Terpstra, R. Metselaar, Carbothermal production of β′-sialon from alumina, silica and carbon mixture. Journal of Materials Science 30 (1995) 6363-6369. [27] K. Komeya, C. Zhang, M. Hotta, J. Tatami, T. Meguro, Y.-B. Cheng, Hollow Beads Composed of Nanosize Ca α-SiAlON Grains. Journal of the American Ceramic Society 83 (2000) 995-997. [28] J.T. Huang, H.P. Zhou, Z.H. Huang, G.H. Liu, M.H. Fang, Y.G. Liu, Effect of Y-a-Sialon Seeding and Holding Time on the Formation of Elongated (Ca,Dy)-a-Sialon Crystals Prepared via Carbothermal Reduction and Nitridation. Journal of the American Ceramic Society 95 (2012) 2777-2781. [29] J. Huang, H. Zhou, Z. Huang, G. Liu, M. Fang, Y.g. Liu, Preparation and Formation Mechanism of Elongated (Ca,Dy)-α-Sialon Powder via Carbothermal Reduction and Nitridation. Journal of the American Ceramic Society 95 (2012) 1871-1877. [30] F.J. Li, T. Wakihara, J. Tatami, K. Komeya, T. Meguro, Synthesis of β-SiAlON powder by carbothermal reduction–nitridation of zeolites with different compositions. Journal of the European Ceramic Society 27 (2007) 2535-2540. [31] C. Zhang, K. Komeya, J. Tatami, T. Meguro, Y.B. Cheng, Synthesis of Mg-α SiAlON powders from talc and halloysite clay minerals. Journal of the European Ceramic Society 20 (2000) 1809-1814. [32] Z. Tatlı, A. Demir, R. Yılmaz, F. Çalışkan, A.O. Kurt, Effects of processing parameters on the production of β-SiAlON powder from kaolinite. Journal of the European Ceramic Society 27 (2007) 743-747. [33] X.J. Liu, X.W. Sun, J.J. Zhang, X.P. Pu, Q.M. Ge, L.P. Huang, Fabrication of β-sialon powder from kaolin. Materials Research Bulletin 38 (2003) 1939-1948.

[34] T. Suehiro, N. Hirosaki, R.-J. Xie, M. Mitomo, Powder Synthesis of Ca-α‘-SiAlON as a Host Material for Phosphors. Chemistry of Materials 17 (2004) 308-314. [35] Y. Masubuchi, M. Yoshikawa, T. Takeda, S. Kikkawa, Preparation of Eu-doped β- and 15R-SiAlONs by ammonia nitridation of the precursor obtained using aluminum glycine gel. Journal of Alloys and Compounds 487 (2009) 409-412. [36] L. Zhang, L. Zhang, Z. Lin, Y. Jiang, J. He, W. Cai, S. Li, Synthesis of β-SiAlON Nanopowder by Ammonolysis of Alumina–Silica Gel. Journal of the American Ceramic Society 97 (2014) 40-43. [37] R.-J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, M. Mitomo, Eu2+-doped Ca-alpha-SiAlON: A yellow phosphor for white light-emitting diodes. Applied Physics Letters 84 (2004) 5404-5406. [38] L. Gan, Z.-Y. Mao, Y.-F. Wang, F.-F. Xu, Y.-C. Zhu, Q. Huang, X.-J. Liu, Photoluminescence properties of Ca–α-SiAlON:Ce3+ phosphors as function of composition and microstructure. Ceramics International 39 (2013) 8319-8326. [39] A. Merzhanov, History and recent developments in SHS. Ceramics International 21 (1995) 371-379. [40] A. Merzhanov, Combustion processes that synthesize materials. Journal of Materials Processing Technology 56 (1996) 222-241. [41] I.P. Parkin, G. Elwin, L.F. Barquin, Q.T. Bui, Q.A. Pankhurst, A.V. Komarov, Y.G. Morozov,

Self-Propagating High Temperature Synthesis of Hexagonal Ferrites MFe12O19 (M = Sr, Ba). Advanced Materials 9 (1997) 643-645. [42] A.S. Mukasyan, A.S. Rogachev, A. Varma, Mechanisms of pulsating combustion during synthesis of advanced materials. AIChE Journal 45 (1999) 2580-2585. [43] G. Liu, J. Li, K. Chen, H. Zhou, Combustion synthesis of (TiC + SiC) composite powders by coupling strong and weak exothermic reactions. Journal of Alloys and Compounds 492 (2010) L82-L86. [44] I.G. Cano, M.A. Rodrı guez, Synthesis of β-Silicon nitride by SHS: fiber growth. Scripta Materialia 50 (2004) 383-386. [45] S.M. Bradshaw, J.L. Spicer, Combustion Synthesis of Aluminum Nitride Particles and Whiskers. Journal of the American Ceramic Society 82 (1999) 2293-2300. [46] Y. Wu, H. Zhuang, F. Wu, Mechanism of the formation of β–Sialon by self-propagating high-temperature synthesis. Journal of Materials Research 13 (1998) 166-172. [47] Z.A. Munir, U. Anselmi-Tamburini, Self-propagating exothermic reactions: The synthesis of high-temperature materials by combustion. Materials Science Reports 3 (1989) 277-365. [48] K. Smirnov, I. Borovinskaya, SHS and some properties of SiAlON ceramics. Key Engineering Materials 217 (2001) 159-164.

[49] J. Zeng, Y. Miyamoto, O. Yamada, Combustion synthesis of Sialon Powders (Si6-zAlzOzN8-z, z = 0.3, 0.6). Journal of the American Ceramic Society 73 (1990) 3700-3702. [50] J. Lis, S. Majorowski, J.A. Puszynski, V. Hlavacek, Dense β- and α/β-SiAlON materials by pressureless sintering of combustion-synthesized powders. American Ceramic Society bulletin 70 (1991) 1658-1664.

[51] G. Liu, K. Chen, H. Zhou, X. Ning, J.M.F. Ferreira, Effect of diluents and NH4F additive on the

combustion synthesis of Yb α-SiAlON. Journal of the European Ceramic Society 25 (2005) 3361-3366. [52] G. Liu, C. Pereira, K. Chen, H. Zhou, X. Ning, J.M.F. Ferreira, Fabrication of one-dimensional rod-like α-SiAlON powders in large scales by combustion synthesis. Journal of Alloys and Compounds 454 (2008) 476-482. [53] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, Mechanically Activated Combustion

Synthesis of β-Si6−zAlzOzN8−z (z=1–4). Journal of the American Ceramic Society 90 (2007) 626-628. [54] K. Aoyagi, R. Sivakumar, X. Yi, T. Watanabe, T. Akiyama, effect of diluents on high purity beta-SiAlONs by mechanically activated combustion synthesis. Journal of the Ceramic Society of Japan 117 (2009) 777-779. [55] M. Shahien, M. Radwan, S. Kirihara, Y. Miyamoto, T. Sakurai, Combustion synthesis of single-phase β-sialons (z = 2-4). Journal of the European Ceramic Society 30 (2010) 1925-1930. [56] J. Niu, T. Akiyama, X. Yi, I. Nakatsugawa, Combustion synthesis of high-purity β-SiAlON fine powders using natural kaolin. AIChE Journal 59 (2013) 19-22. [57] J. Niu, X. Yi, I. Nakatsugawa, T. Akiyama, Salt-assisted combustion synthesis of β-SiAlON fine powders. Intermetallics 35 (2013) 53-59. [58] J. Niu, K. Harada, I. Nakatsugawa, T. Akiyama, Morphology control of β-SiAlON via salt-assisted combustion synthesis. Ceramics International 40 (2014) 1815-1820.

Chapter 2

Combustion synthesis of high purity β-SiAlON fine powders

using natural kaolin

2.1 Introduction

Beta-SiAlONs are non-stoichiometric compounds, which derived from the β-Si3N4 structure by the equivalent substitution of Si–N with Al–O. They have a hexagonal crystal structure with a general formula of Si6-zAlzOzN8-z (0  z  4.2). β-SiAlON is one of the most promising ceramic materials for high temperature engineering applications due to its excellent properties, such as high strength and hardness, good thermal and chemical stability, superior wear resistance, and superior thermal shock resistance [1-3].

Moreover, the Eu2+-doped β-SiAlON is now considered as a candidate for application in white light-emitting diodes (LEDs) that have the characteristics of a green-emitting phosphor [4, 5].

Combustion synthesis (CS) or self-propagating high-temperature synthesis (SHS) is an economical, effective, and energy-saving process for synthesizing industrial materials [6-9], especially ceramics. This process utilizes the heat generated during a strong exothermic reaction in order to sustain the combustion process. No external energy is needed except for the ignition energy. Combustion synthesis has many advantages, including a low energy input, a short reaction time, simple equipment, and a high purity product. In conventional combustion synthesis for preparing β-SiAlON, it is difficult to obtain pure single-phase products except by adding a large amount of diluents to the reactants, such as β-SiAlON [10-12], or α/β-Si3N4 [13-15]. This is due to that the combustion temperature is extremely high and silicon particles melt in the

17 combustion front and coalesce during the reaction, which inhibits the complete nitridation of Si particles [16, 17]. Diluent plays a role of enthalpy absorber, which decreases the combustion temperature and simultaneously slows down the reaction speed to improve the reaction efficiency. However, the simple use of the diluent is an inefficient process by using a part of the product as the starting reactant. In fact, as much as 50 mass% of β-SiAlON diluent must be added to obtain a pure product.

Furthermore, it is still a challenge to obtain fine or submicron size particle product directly.

The biggest obstacle for the wide use of β-SiAlON materials is the high cost for their synthesis, especially for obtaining of fine product. How to prepare fine β-SiAlON products effectively is still a research hotspot. In this chapter, we report on the direct preparation of single-phase β-SiAlON with submicron size by combustion synthesis using natural kaolin as raw material without adding any diluents. The effects of the structural water within kaolin on the purity of the product as well as its microstructure were investigated.

2.2 Experimental procedure

The raw materials used in this work included the commercially available powders of Si (Soekawa Chemicals Co., Ltd., Tokyo, Japan; 99.9% purity; 1-2 µm) and Al

(Kojundo Chemical Laboratory Co., Ltd., Saitama, Japan; 99.9% purity; 3 µm) and natural kaolin. The kaolin clay was supplied by W. A. Kaolin Holdings Pty. Ltd. (LOT 3

Ward Rd, East Rockingham, WA). The chemical composition of the kaolin is given in

Table 2-1.

The clay was washed with distilled water and the suspension was passed through a

32-µm sieve to remove the large size impurities. The suspension was then dried at

110 °C. Thermogravimetric analyses (TGA, Mettler Toledo) were conducted using

8.7 mg of the sample, which was heated to 1200 °C at a rate of 10 °C/min under an argon flow of 50 ml/min. Based on the TGA result of kaolin, the dehydration treatment was carried out by calcining the sample at 800 °C for 5 h. The phases of kaolin before and after the dehydration treatment were characterized by an X-ray diffraction (XRD,

Rigaku Miniflex, Cu-Kα) analysis.

The reactions for the combustion synthesis of β-Si5AlON7 (β-Si6-zAlzOzN8-z, z = 1) using kaolin with and without dehydration treatment can be described by the following equations:

33 5 1 7 2 Si  Al (Al O 2SiO 2H O)  N β Si AlON  H O(g) (2.1) 7 7 7 2 3 2 2 2 2 5 7 7 2 33 5 1 7 Si  Al  (Al O 2SiO )  N  β Si 5AlON7 (2.2) 7 7 7 2 3 2 2 2 All reactants were weighed according to the stoichiometric ratio. The reactants were mixed and mechanically activated by a planetary ball milling using zirconia balls in a zirconia pot at a ball to sample mass ratio of 10:1. The activated mixture was subsequently charged into a cylindrical carbon crucible with vents, which were used for the intrusion of the nitrogen gas. Ignition agent of the Al powders was placed on the top

19 of the mixture. The combustion reaction was carried out at a nitrogen pressure of 1 MPa

(purity: 99.999%) by passing a current of 60 A for 10 s through a carbon foil. The conditions for the planetary milling and the equipment for combustion synthesis have been described in detail elsewhere [16].

XRD was used to for the phase composition analysis. A scanning electron microscope (FE-SEM, JEOL, JSM-7400F) was used to observe the microscopic morphology, and an energy dispersive X-ray spectrometer (EDS, JEOL, JED-2300) was used for the elemental analysis of the products. The particle size distribution was measured using a laser particle size analyzer (Mastersizer 2000).

Table 2-1 Chemical composition of kaolin for the combustion synthesis of

β-Si5AlON7

* Constituent SiO2 Al2O3 CaO Fe2O3 K2O MgO Na2O P2O5 SO3 TiO2 MnO LOI Content 46.7 38.7 0.01 0.12 0.3 0.06 0.06 0.017 0.06 0.16 0.01 13.8 (mass %) * Loss on ignition

2.3 Results and discussion

The characteristics of pretreated kaolin are presented in Fig. 2-1. The XRD patterns

revealed that kaolin contains a kaolinite phase before dehydration treatment, while

amorphous materials were formed after dehydration as a result of the loss of the

structural water. A mass loss of 12.81% was calculated from the TGA curve, based on

the initial mass of the sample, and this was in agreement with the results of the

dehydrated treatment.

The XRD patterns of the products are presented in Fig. 2-2. It can be clearly

observed that no other phase was indentified, except β-Si5AlON7; the diffraction pattern is in agreement with the JCPDS card (no.6000 48-1615).(a) However, strong Si peaks were Kaolinite detected as an impurity in the product when using dehydrated kaolin as(Al 2rawO3·2SiO material.2·2H2O) 4000

This can be explained by that Si particles were fused and subsequently hardened under

Before dehydration Intensity [a. u.]

the fast reaction with a high reaction temper2000 ature, which caused the remaining of Si in

the product. After dehydration

0 10 20 30 40 50 60 70 2 theta [degree]

102 6000 (a) (b) 100 Kaolinite 98 (Al2O3·2SiO2·2H2O) 96 4000

Before dehydration

Intensity [a. u.] Mass loss [%] 2000 90 88 Mass loss 12.81% After dehydration 86

0 200 400 600 800 1000 1200 10 20 30 40 50 60 70  2 theta [degree] Temperature [ C]

102 Fig.(b) 2-1 Characteristics of the washed kaolin used in this work. (a) XRD 100 patterns for the washed kaolin before and after dehydration treatment; the 98 96 dehydrated kaolin shows an amorphous phase due to the loss of hydroxyl

94 groups. (b) TGA curves for the washed kaolin heated to 1200 ºC at a rate of

92 10 ºC/min under argon flow of 50 ml/min.

Mass loss [%] 90 88 Mass loss 12.81% 21 86

200 400 600 800 1000 1200 Temperature [C]

(a) β-SiAlON Si

6000

3000

Intensity [a. u.] (b)

0 10 20 30 40 50 60 2 theta [degree]

Fig. 2-2 XRD patterns for the combustion synthesized β-Si5AlON7 powders obtained at nitrogen pressure of 1 MPa using kaolin as raw material, (a) using kaolin with structural water, (b) using kaolin without structural water.

Table 2-2 ICP analysis for the metal impurities of the combustion synthesized

β-Si5AlON7 powders at nitrogen pressure of 1 MPa using kaolin without dehydration treatment.

Impurity Ca Fe K Mg Ti Content 0.006 0.062 0.14 0.019 0.038 (mass %)

The energy dispersive X-ray spectrometry (EDS) analysis shows identified elements of Si, Al, O, and N; no impurities were detected in the product. This is because of the extremely high reaction temperature resulted in the evaporation of the minor impurities in kaolin. ICP was used for the analysis of metal impurities in the SiAlON product. The result was listed in Table 2-2. According to Table 2-2, the content of each metal element is lower than 0.3%, which suggests that the SiAlON prepared in this work is qualified for its application as engineering materials.

The SEM images of the combustion synthesized β-Si5AlON7 powders from kaolin are shown in Fig. 2-3. The β-Si5AlON7 product consisted of fine particles with diameter of around 500 nm as well as a small amount of irregular elongated particles. However, for the particles obtained using dehydrated kaolin, they exhibited a rod-like shape which were significantly agglomerated, with only very few tiny particles. Furthermore, large bulk morphologies with lengths between 4 and 6 µm were found in this product. This difference is owing to the untreated Si, as confirmed by the differences between the

XRD patterns in Fig. 2-2.

To obtain a precise estimation of the particle size and particle size distribution, particle size distribution measurement were performed by laser particle size analyzer.

Before particle size analysis, the sample suspension was deagglomerated by ultrasonic treatment for 5 min. Particle size distribution graph of β-SiAlON powders is given in

Fig. 2-4. The results are in fairly good in agreement with the direct observation by SEM.

The β-SiAlON powder was composed of particles with size ranging from 300 nm to

700 nm.

(a)

5μm

(b)

5μm

Fig. 2-3 SEM images for the combustion synthesized β-Si5AlON7 powders obtained at nitrogen pressure of 1 MPa using kaolin as raw material, (a) using kaolin with structural water, (b) using kaolin without structural water.

100 12 90

Cumulative percent finer [%] finer percent Cumulative 10 80 70 8 60

D50 = 470 nm 6 50 40

4 30 Frequency [%] 20 2 10 0 0 0 1 2 3 Particle size [m]

Fig. 2-4 Particle size distribution of the combustion synthesized β-Si5AlON7 powders obtained at nitrogen pressure of 1 MPa using natural kaolin as raw material.

The lattice parameters of the hexagonal β-Si5AlON7 phase were calculated from the XRD peaks, with the values of 7.631 Å for a and 2.937 Å for c. According to a and c, the calculated z value was 0.94 by the equations in the reference [2], which was in agreement with the expected value of 1 for the raw mixture.

Single-phase β-SiAlON was obtained when use kaolin without dehydration treatment, therefore it can be deduced that the structural water of kaolin plays a significant role in the synthesis process. The reaction for the dehydroxylation which occurred within the temperature range of 350–830 °C can be described by the following equation:

0 -1 Al2O3∙2SiO2∙2H2O = Al2O3∙2SiO2 + 2H2O (g) (ΔH 298K = 294.76 kJ mol ) (2.3)

According to the result confirmed above, it is clearly known that the structural water in kaolin is the key factor in the formation of single-phase β-SiAlON. In the conventional synthesis process of β-SiAlON using Si, Al, and SiO2 or Al2O3 as raw materials, at the pre-flame stage, a part of Al firstly react with N2 to form AlN; the remained Al react with SiO2 to form Al2O3 and Si; simultaneously, these two exothermic reactions release large amount of heat [18, 19]. With the huge amount of reaction heat generated instantaneously, silicon particles melt at the surface quickly, which makes it difficult for the diffusion of the N2 gas into the cores of powders through the liquid obstacle layer; consequently, un-reacted Si remain in the product within the very short reaction time of the whole combustion synthesis process. However, when kaolin

(Al2O3∙2SiO2∙2H2O) was used as the raw material in this study to substitute the use of pure SiO2 or Al2O3, the eliminating of structural water absorbed part of the released heat at the pre-flame stage; this reduced the temperature of the pre-flame and alleviated the melting of Si. Subsequently, the metakaolin (Al2O3∙2SiO2) transformed Al2O3∙3SiO2 and

SiO2 (amorphous), then the amorphous SiO2 reacted with Al to form Al2O3 and Si, accompanied by a mass of heat release, the temperature reached the ignition point of the

Si-N2 reaction, Si particles react with N2 to form Si3N4 crystals on Si surface.

Simultaneously, by the dissolution, mass transportation, and precipitation processed occurred between Si3N4, AlN, and Al2O3, β-SiAlON particles were formed [20].

Moreover, with the loss of hydroxyl groups in kaolin during dehydration, a lot of pores formed which also enhanced the nitridation process to make a complete conversion to

β-SiAlON.

The microstructure of the product was also affected by reaction temperature [21].

In the case of using kaolin as the raw material, the reaction temperature was lowered attribute to the evaporation of structural water in kaolin. At low reaction temperature, high viscosity phase reduced the mass transport during the solution-reprecipitation process [22]. As a result, the agglomeration for β-SiAlON particles was limited, which showed small particles. On contrast, the high reaction temperature accelerated the mass transport during the reaction, hence, the β-SiAlON nuclei grew into elongated grains when using kaolin without structural water as raw material.

This new preparative process is of great importance for energy saving in fabricating β-SiAlON ceramics. Kaolin acts as a diluent to absorb the reaction enthalpy, and it thereby maintains a relatively low overall reaction temperature. In addition, the dehydration process requires significant amounts of heat from the combustion synthesis of β-SiAlON to eliminate the structural water. Consequently, the Si can be fully nitridated to form β-SiAlON. In any case, by the loss of hydroxyl groups in kaolin, the

26 pores that are formed in the structure further enhance the nitridation process. Moreover, in our previous research [23], experiments were carried out with Al2O3 as one of the sources of oxygen, instead of SiO2. The results showed that the reaction did not propagate to the synthesis of β-SiAlON owing to the low calorific values of the raw materials. In contrast with these results, kaolin, as the source of the oxygen, appropriately regulated the calorific values, and therefore, played an important role in the better control of the conversion of reactants to products and the production of a single-phase β-SiAlON.

2.4 Conclusion

Single-phase, submicron sized β-SiAlON particles were fabricated directly from natural kaolin by combustion synthesis at nitrogen pressure of 1 MPa without adding any diluent. The use of kaolin in this new preparative process has several notable advantages: it can produce pure products without requiring the addition of any diluent; the large-scale use of this material can greatly reduce the cost of the β-SiAlON production; provides fine particles with median diameter of about 470 nm. The major parameter affecting the process is the structure water in kaolin, which lowered the Tad by 350 °C.

Self-Propagating High Temperature Synthesis of Hexagonal Ferrites MFe12O19 (M = Sr, Ba). Advanced Materials 9 (1997) 643-645. [7] Z.A. Munir, U. Anselmi-Tamburini, Self-propagating exothermic reactions: The synthesis of high-temperature materials by combustion. Materials Science Reports 3 (1989) 277-365. [8] K.S. Martirosyan, D. Luss, Carbon combustion synthesis of complex oxides: Process demonstration and features. AIChE Journal 51 (2005) 2801-2810. [9] A.M. Locci, R. Licheri, R. Orrù, A. Cincotti, G. Cao, J. De Wilde, F. Lemoisson, L. Froyen, I.A. Beloki, A.E. Sytschev, A.S. Rogachev, D.J. Jarvis, Low-gravity combustion synthesis: Theoretical analysis of experimental evidences. AIChE Journal 52 (2006) 3744-3761. [10] X. Yi, K. Watanabe, T. Akiyama, Fabrication of dense β-SiAlON by a combination of combustion synthesis (CS) and spark plasma sintering (SPS). Intermetallics 18 (2010) 536-541. [11] K. Aoyagi, R. Sivakumar, X. Yi, T. Watanabe, T. Akiyama, effect of diluents on high purity beta-SiAlONs by mechanically activated combustion synthesis. Journal of the Ceramic Society of Japan 117 (2009) 777-779. [12] M. Shahien, M. Radwan, S. Kirihara, Y. Miyamoto, T. Sakurai, Combustion synthesis of single-phase β-sialons (z = 2-4). Journal of the European Ceramic Society 30 (2010) 1925-1930.

[13] J. Zeng, Y. Miyamoto, O. Yamada, Combustion synthesis of Sialon Powders (Si6-zAlzOzN8-z, z = 0.3, 0.6). Journal of the American Ceramic Society 73 (1990) 3700-3702. [14] Y. Wu, H. Zhuang, F. Wu, Mechanism of the formation of β–Sialon by self-propagating high-temperature synthesis. Journal of Materials Research 13 (1998) 166-172. [15] J. Lis, S. Majorowski, J.A. Puszynski, V. Hlavacek, Dense β- and α/β-SiAlON materials by pressureless sintering of combustion-synthesized powders. American Ceramic Society bulletin 70 (1991) 1658-1664.

[16] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, Mechanically Activated Combustion

Synthesis of β-Si6−zAlzOzN8−z (z=1–4). Journal of the American Ceramic Society 90 (2007) 626-628. [17] X. Yi, T. Akiyama, Mechanical-activated, combustion synthesis of β-SiAlON. Journal of Alloys and Compounds 495 (2010) 144-148. [18] J. Li, J. Wang, H. Chen, B. Sun, J. Jia, Synthesis of beta-SiAlON-AlN-Polytyppoid Ceramics from Aluminium Dross. Materials Transaction 51 (2010) 844-848. [19] D.H.L. Ng, T.L.Y. Cheung, F.L. Kwong, Y.-F. Li, R. Yang, Fabrication of single crystalline β'-SiAlON nanowires. Materials Letters 62 (2008) 1349-1352. [20] J.-F. Yang, Y. Beppu, G.-J. Zhang, T. Ohji, S. Kanzaki, Synthesis and Properties of Porous Single-Phase β′-SiAlON Ceramics. Journal of the American Ceramic Society 85 (2002) 1879-1881. [21] G. Madras, B.J. McCoy, Temperature effects for crystal growth: a distribution kinetics approach. Acta Materialia 51 (2003) 2031-2040. [22] E. He, J. Yue, L. Fan, C. Wang, H. Wang, Synthesis of single phase β-SiAlON ceramics by

reaction-bonded sintering using Si and Al2O3 as raw materials. Scripta Materialia 65 (2011) 155-158. [23] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, A new route to synthesize

β-Si6−zAlzOzN8−z powders. Journal of Alloys and Compounds 441 (2007) 236-240.

Chapter 3

Salt-assisted combustion synthesis of β-SiAlONs and its

morphology control

3.1 Introduction

β-SiAlON, with the general formula Si6-zAlzOzN8-z (0  z  4.2) has a hexagonal crystal structure that is derived from the β-Si3N4 structure by the equivalent substitution of Si–N with Al–O. β-SiAlON is an attractive material for applications in high-temperature engineering systems, automotive components, cutting tools, ball bearings, etc. It shows excellent properties such as high strength and hardness, good thermal and chemical stability, and superior wear and thermal shock resistance [1-3].

Recently, it has been considered an excellent host material for phosphors, with potential applications for white light-emitting diodes (LEDs) because of its chemical and high-temperature stability [4-6].

To date, various synthetic methods have been developed to obtain β-SiAlON products, including pressureless sintering [7], hot pressing [8], and carbothermal reduction and nitridation (CRN) [9, 10]. Most of these methods, however, involve complicated equipment and processes, which limit their further applications.

Combustion synthesis (CS) is an effective energy-saving method for the synthesis of a variety of advanced materials, such as metal oxides [11], ceramics [12, 13], and intermetallics [14-17]. The process is characterized by the fact that once the initial reaction mixture is ignited by means of an external thermal source, a rapid high-temperature reaction wave propagates through the mixture in a self-sustaining manner, leading to the formation of the final product without the need for any external energy. Combustion synthesis has many advantages, including a low energy input, a short reaction time, simple equipment, and a high-purity product.

However, in conventional combustion synthesis for preparing β-SiAlON, the purity of the product has always been low because of the existence of unreacted Si, which is because of the melting and subsequent agglomeration of Si during the combustion synthesis process with an extremely high reaction temperature and a very short reaction time [18]. In order to decrease the combustion temperature as well as to slow down the

reaction speed, diluents such as β-SiAlON [13, 19, 20] or α/β-Si3N4 [21-23] are added to the starting materials. In fact, as much as 50 mass% of the β-SiAlON diluent must be added to obtain a pure product. Furthermore, it is difficult to obtain combustion-synthesized β-SiAlON with fine or submicron-size particles. The second obstacle to the wide use of β-SiAlON materials is the high cost for their post-synthesis treatment, i.e., for obtaining a product with a fine grain size. It has been reported that

SiAlONs exhibit superplastic deformation when their grain size is reduced to the nanometer scale [24]. Thus, there is an urgent need to develop an alternative diluent with low cost and high efficiency for producing high-quality fine particles.

Recently, NaCl has attracted considerable attention for use as a diluent for fabricating many materials through combustion synthesis, such as MoSi2, tungsten, and

ZrB2 powders [25-28]. In these studies, NaCl decreased the adiabatic temperatures and wave velocity to facilitate a stable combustion synthesis reaction, and thus, reduced the rate of aggregation and grain growth of the particles. It is well known that many properties such as hardness, bending strength, and abrasive wear resistance are improved by decreasing the particle size of ceramics. However, to the best of our knowledge, there has been no investigation on preparing β-SiAlONs by using NaCl or other salts such as KCl, MgCl2, and CaCl2 as the diluent. Therefore, the purpose of this chapter is to synthesize high-purity, fine β-SiAlON particles using these salts as the diluents.

In section 3.2, combustion synthesis of β-Si6-zAlzOzN8-z (z = 0.25−3) powder was

32 carried out with the raw materials Si, SiO2, and Al powders, and varying amounts of

NaCl under a nitrogen pressure of 1 MPa. In section 3.3, the effect of metal chlorides

(KCl, MgCl2, and CaCl2) on the synthesis of β-SiAlON and a detailed understanding of the endothermic processes occurring during the exothermic stages were investigated. In section 3.4, the characteristics of the reaction process and kinetic of the reaction on propagation were compared by using NaCl and SiAlON as the diluents.

3.2 Combustion synthesis of β-SiAlONs (z = 0.25-3) using NaCl as a diluent 3.2.1 Objective of section 3.2 The conventional combustion synthesis of β-SiAlON powders usually requires a high combustion temperature. To decrease the reaction heat and thus the reaction temperature, it is necessary to add a certain amount of the β-SiAlON product as diluents.

In fact, as much as 50 mass% β-SiAlON powders are needed to obtain high-purity products [13, 19]. With the goal of using NaCl as the diluent in the combustion synthesis, this study first optimized the NaCl content for each z-value to obtain a high-purity product. The phase composition and particle morphologies of the synthesized powders were analyzed. The mechanism of the β-SiAlON formation in the presence of NaCl was discussed by analyzing the phase evaluation with a partially reacted sample. The advantages of utilizing NaCl in this study are expected to be the following: (1) It is inert to the initial components (Si, Al, and SiO2) and can be easily separated from the product due to its high solubility in water. (2) It plays a positive protection role against the melting of Si, because the melting point of NaCl is lower than that of Si. (3) Product with fine particles in the submicron or nanometer range can be obtained.

3.2.2 Experimental procedure

The starting materials used in this chapter were commercially available Si

(Soekawa Chemicals Co., Ltd., Tokyo, Japan; 99.9% purity, 1–2 µm), Al (Kojundo

Chemical Laboratory Co., Ltd., Saitama, Japan; 99.9% purity, 3 µm), and SiO2

(Kojundo Chemical Laboratory Co., Ltd.; 99.9% purity, 0.8 µm) powders, and NaCl

(Kojundo Chemical Laboratory Co., Ltd.; 99.9% purity). The raw materials were mixed according to a stoichiometric ratio to synthesize β-Si6-zAlzOzN8-z with z-values that varied from 0.25 to 3 with different amounts of NaCl as the diluent. The starting compositions for each sample are summarized in Table 3-2-1. The reaction can be expressed by the following equation:

(6 1.5z)Si  zAl 0.5zSiO 2  (4  0.5z)N2  β Si 6-z AlzOz N8-z (3.2.1)

The reactant powders were mechanically activated by planetary ball-milling

(Gokin Planetaring Inc., Japan) using zirconia balls in a zirconia pot at a ball-to-sample mass ratio of 10:1. The activated mixture was charged into a cylindrical carbon crucible with vents, through which the nitrogen gas was introduced. The ignition agent of the Al powders was placed on top of the mixture. The combustion reaction was carried out at a nitrogen pressure of 1 MPa (nitrogen purity: 99.999%) by passing a current of 60 A for

10 s through a carbon foil. The details for the planetary ball-milling and the equipment for combustion synthesis have been described elsewhere [18].

X-ray diffraction (XRD, Miniflex, Rigaku, Japan, Cu Kα, λ = 1.54056 nm) was used to assess the phase composition. A scanning electron microscope (FE-SEM, JEOL,

JSM-7400F) was used to observe the microscopic morphology, and an energy dispersive

X-ray spectrometer (EDS, JEOL, JED-2300) was used for the elemental analysis of the

35 product.

Table 3-2-1 The composition of the starting raw materials and combustion synthesized products obtained at different z values with NaCl addition

z value Compositions [mass%] Reaction Phase composition of characteristic products

Si Al SiO2 NaCl 90.79 3.89 4.32 1 ○ β-SiAlON+ Si 0.25 89.41 3.83 4.26 2.5 ○ β-SiAlON+ Si 88.03 3.77 4.19 4 ○ β-SiAlON 87.12 3.73 4.15 ≥5 ╳ -- 79.57 7.31 8.12 5 ○ β-SiAlON+ Si 0.5 77.90 7.15 7.95 7 ○ β-SiAlON 77.06 7.08 7.86 ≥8 ╳ -- 61.97 13.28 14.75 10 ○ β-SiAlON+ Si 1 60.59 12.98 14.42 12 ○ β-SiAlON 59.90 12.84 14.26 ≥13 ╳ -- 38.18 24.54 27.27 10 ○ β-SiAlON+ Si 33.94 21.82 24.24 20 ○ β-SiAlON+Si+NaCl 2 29.70 19.09 21.21 30 ○ β-SiAlON+Si+NaCl 28.85 18.54 20.61 32 ○ β-SiAlON+Si+NaCl 27.57 17.73 19.70 35 ○ β-SiAlON+Si+NaCl 27.15 17.45 19.39 ≥36 ╳ -- 13.80 26.62 29.58 30 ○ β-SiAlON+Si+NaCl 11.83 22.82 25.35 40 ○ β-SiAlON+Si+NaCl 3 11.44 22.06 24.51 42 ○ β-SiAlON+Si+NaCl 10.84 20.92 23.24 45 ○ β-SiAlON+Si+NaCl 10.65 20.54 22.82 ≥46 ╳ --

The ○ means that the combustion reaction could proceed, and ╳ means that the

combustion could not proceed.

3.2.3 Results and Discussion

Fig. 3-2-1 shows the characteristics for combustion-synthesized β-SiAlON with using NaCl (mass%) as the diluent at nitrogen pressure of 1 MPa. From Fig. 3-2-1 (a), it can be seen that for each z-value, there is a proper amount of NaCl added to obtain a high-purity product. Furthermore, the amount of NaCl added was dependent on the z-value; higher z-values required more NaCl. The effect of NaCl during combustion synthesis can be explained as follows: because of melting or sublimation, it absorbs the heat released during the reaction and lowers the combustion temperature. As a result, the melting and the agglomeration of Si particles of the raw materials were greatly reduced, which enhanced the infiltration of N2. As showed in this figure, as the z-value increases, the content of Al increases. As a result, the amount of heat released from the reactions (3.2.2) and (3.2.3) increased, and therefore, more amount of NaCl was needed to further reduce the generated heat in order to prevent the melting of Si as the z-value increases. Fig. 3-2-1 (b) shows the effect of NaCl addition on adiabatic temperature

(Tad) for z-value of 0.25, 0.5, and 1. The Tad for z = 2 and 3 cannot be calculated precisely due to the existence of NaCl in the products (see Fig. 3-2-2). The result shows that the addition of the NaCl decreased the Tad of the synthesis reaction for all z-values.

Moreover, Tad reduces as z-value increases due to the different starting compositions.

For example, for z-value of 0.25, Tad drops from 4344 °C to 4241 °C when 4% NaCl is added to the raw materials. In fact, the reaction temperature was significantly lower than the theoretical adiabatic temperature of the synthesis, which was mainly due to the heat losses to the surroundings. 4%, 7%, and 12% mass% NaCl addition are the proper content for z-values of 0.25, 0.5, and 1, respectively, with the proper reaction temperature, there is no Si remained in the product. However, when lesser amount of

NaCl and a higher reaction temperature, Si particles were fused and subsequently hardened under the fast reaction, which caused Si to remain in the product; with greater

37 amounts of NaCl added, the combustion reaction could not be maintained due to the insufficient heat released from the raw materials. Therefore, for each z-value, there is a proper amount of NaCl added and proper combustion temperature to obtain a high-purity product.

Fig. 3-2-2 shows the XRD patterns of the combustion-synthesized β-SiAlON powders with different z-values obtained using the optimized amount of NaCl diluent.

At z = 0.25, 0.5, and 1, the samples showed a single β-SiAlON phase and were free of secondary phases. However, when z = 2, Si and NaCl peaks were detected even with 35 mass% of NaCl, as shown in Fig. 3-2-1; the combustion reaction could not be completed with more than 35 mass% of NaCl, which indicates the difficulty in obtaining single-phase β-SiAlON with larger z-values. This was because with larger z-values, the corresponding larger amount of a liquid phase hindered the nitridation of

Si [29]. A similar trend was also seen with a z-value of 3.0, with as much as 45% NaCl addition.

Fig. 3-2-3 shows the SEM images of the combustion-synthesized β-Si6-zAlzOzN8-z powders with different z-values obtained at the optimized amount of NaCl diluent. NaCl was removed by washing with distilled water for (d) z = 2 and (e) z = 3. The grain shapes of the β-SiAlON varied with the z-value: for z = 0.25, particles were primarily rod-like particles with diameters of 0.25–0.65 µm and lengths of 0.8–1.3 µm. At z = 0.5, most particles tended to become rounder and larger, although a small number of them still showed a rod-like shape with smaller lengths and larger diameters. When the z-value increased to 1, the particles were quite uniform in size and shape, and they appeared round and equiaxed with diameters of approximately 0.6 µm. When the z-value increased to 2.0 and 3.0, the size of the particles decreased significantly, with diameters of around 0.3 µm. These changes in morphology and grain size were attributed to the variation in the composition of SiAlON and the amount of the NaCl

38 content, which greatly affects the growth of the β-SiAlON crystals.

(a) (b)

40 100 4400

4300 z=0.25 A 80 z=0.5 30 [mass%] content NaCl 4200 z=1 B 60 4100

4000 40 3900 10 C Al content [mass%] 20 C Combustion wave 3800 B propagation zone A Adiabatic temperature [degC] 0 0 3700 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 2 4 6 8 10 12 14 z-value NaCl content [mass%] Fig. 3-2-1 Characteristics for combustion-synthesized β-SiAlON with using NaCl (mass%) as the diluent at nitrogen pressure of 1 MPa. (a) Effect of z-values on Al for the raw materials and the amount of NaCl addition for obtaining a high purity product. (b) Effect of NaCl on adiabatic temperature for z-values of 0.25, 0.5, and 1, in which dotted lines indicate maximum NaCl content for the propagation of combustion wave.

β-SiAlON z = 3.0 NaCl 45% x Si NaCl x x z = 2.0 NaCl 35% x x x

z = 1.0 NaCl 12%

z = 0.5 NaCl 7%

Intensity [a. u.]

z = 0.25 NaCl 4%

10 20 30 40 50 60 2 theta [degree]

Fig. 3-2-2 XRD patterns for the combustion-synthesized β-SiAlON with z-values of 0.25, 0.5, 1.0, 2.0, and 3.0 obtained with optimized amount of NaCl diluent at nitrogen pressure of 1 MPa.

(a) (b) (c)

2 μm 2 μm 2 μm

(d) (e)

2 μm 2 μm

Fig. 3-2-3 SEM images of the combustion-synthesized β-SiAlON powders with z-values of (a) 0.25, (b) 0.5, (c) 1.0, (d) 2.0, and (e) 3.0 obtained with optimized amount of NaCl diluent at nitrogen pressure of 1 MPa. NaCl was removed by washing with distilled water for (d) and (e).

As shown in Fig. 3-2-2, no NaCl peaks were detected for the product in the case of z-values of 0.25, 0.5, and 1. It is therefore reasonable to assume that the entire amount of NaCl separated from the product by evaporation, due to the extremely high combustion temperature. An energy dispersive X-ray spectrometer (EDS) was used to evaluate the elements for the product with a z-value of 1. The results, presented in

Fig. 3-2-4, show the identified elements Si, Al, O, and N. The carbon peak was due to the substrate graphite; no Na or Cl was detected in the product. This confirmed that

NaCl did not remain in the product, due to its evaporation and diffusion outward at the boiling point of 1738 K, which was much lower than the reaction temperature.

Therefore, in the case of z-values of 0.25, 0.5, and 1, β-SiAlON product was free of

NaCl.

In fact, after combustion synthesis, the products with z-values of 0.25, 0.5, and 1 appeared loosely compacted, with a layered structure that could be easily pulverized into powders by hand. Taking the product with z = 1 as an example, as shown in

Fig. 3-2-5 (a), the product was divided into three parts: the ignition site, an outer layer, and an inner part (product). In this work, if not specifically mentioned, all samples for analysis were taken from the inner part of the product. To evaluate the role of NaCl, the phase compositions of products at both the ignition site and the outer layer were analyzed by XRD and the results are shown in Fig. 3-2-5 (c). It is clearly seen that NaCl peaks appear in both parts; moreover, the intensity of the NaCl peaks at the ignition site were much stronger than those at the outer layer, which was because NaCl vapor diffused upward to the post-combustion reaction and gathered in that region. Moreover, because of the vapor diffusion of NaCl, a product with a layered structure was formed.

The schematic diagram for the diffusion of NaCl during the combustion synthesis reaction is shown in Fig. 3-2-5 (b).

CKa

Fig. 3-2-4 Energy dispersive X-ray spectrometer (EDS) analysis of the combustion-synthesized β-SiAlON (z = 1) powders with 12 mass% NaCl at nitrogen pressure of 1 MPa. The peak of carbon is due to substrate graphite. Inset shows the scanning image.

Ignition site (a) (b)

Product 5 mm

Outer layer NaCl

6000 (c) β-SiAlON NaCl x Si x x (ii) 4000 x

Ignition site

2000 (i) x Intensity [a. u.] Outer layer x

0 10 20 30 40 50 60 2 theta [degree] Fig. 3-2-5 β-SiAlON (z = 1) obtained with 12 mass% NaCl at nitrogen pressure of 1 MPa. (a) The surface, (b) longitudinal-section schematic diagram for the diffusion of NaCl during the combustion synthesis process, (c) XRD patterns of (i) The outer layer and (ii) the ignition site.

So far, no reports on the formation of β-SiAlONs from powders of Si, Al, and SiO2 have been reported. In this work, the reaction mechanism was first investigated by analyzing the quenched front with a partially reacted sample. The merit for analyzing the quenched front is that the phase and structural transformation during the combustion synthesis of β-SiAlON can be obtained by the combustion wave.

Fig. 3-2-6 (a) shows the appearance of the partially reacted β-SiAlON sample with a z-value of 2. After ignition at the top, the self-sustained reaction was not attained due to less heat, and the front extinguished before reaching the bottom of the core. Several zones were observed from the cone-shaped front, which are marked as “A,” “B,” and

“C,” in which “A” is the unreacted raw materials in the preheating zone, “B” is the intermediate product, and “C” is the product in the synthesizing zone. Fig. 3-2-6 (b) and

6 (c) show the evolution for the phase and the morphologies from raw materials to the final product. In the XRD patterns of part B, no Al peaks were detected and AlN peaks appeared; at the same time, the intensity of the SiO2 peaks weakened compared with those in part A (the unreacted raw materials). This indicates that at the early stage of the combustion reaction, a part of Al reacted with N2 to form AlN; the remaining Al reacted with SiO2 to form Al2O3 and Si. However, no Al2O3 peaks were detected in part B, while they were present in part C. It was demonstrated that Al2O3 was completely consumed in converting β-SiAlON when β-SiAlON grains initially formed, which is consistent with reports that the Al and O contents in the initially formed β-SiAlON grains were higher than those in the overall product [30-32]. The diffraction patterns for β-SiAlON in part C are in agreement with the patterns for β-Si3Al3O3N5, which showed a higher z-value than β-Si4Al2O2N6 (z = 2). The SEM image of B shows that Si particles are surrounded by some whiskers and agglomerated particles, which are speculated to be

AlN and another intermediate that eventually convert to β-SiAlON. C consists of irregular bulk material and small particles, which can be considered to be the unreacted

Si and the initially formed β-SiAlON, respectively, according to the XRD results.

Combustion wave

(a) C Product

C Intermediate B B product A Unreacted raw A materials

(b) (c)

AlN SiAlON Al2O3 C x Si Al SiO2 NaCl 15000 C x x x 5 μm

B 10000 x x

B x Intensity [a. u.] 5000 A x A x x

0 10 20 30 40 50 60 2 theta [degree]

Fig. 3-2-6 Characteristics of the partially combusted sample for β-SiAlON (z = 2) with NaCl as diluent: (a) Longitudinal section, (b) XRD patterns for parts A, B, and C, (c) SEM images of A, B, and C.

( II ): Product Si+N2→Si3N4

AlN+Al2O3+Si3N4→β’-SiAlON Reaction zone(II) above 1200 ºC β’-SiAlON + Si3N4 → β SiAlON NaCl(l) → NaCl(g) Reaction zone(I) 600–1100 ºC ( I ):

Combustion wave Combustion Al→ Al(l) Raw mixture Al(l) + N2 → AlN

β-SiAlON Al(l) + SiO2 → Al2O3+ Si β’-SiAlON NaCl → NaCl(l) Al2O3 AlN Si Al SiO2 NaCl Fig. 3-2-7 Proposed mechanism for the combustion synthesis of β-SiAlON

using the raw materials Si, Al, and SiO2 with NaCl as the diluent under N2 pressure of 1 MPa.

Therefore, based on the aforementioned XRD and SEM results, we propose a mechanism for the formation of β-SiAlON with submicron size in the presence of NaCl

(Fig. 3-2-7). In the synthesis process, the introduction of NaCl plays an important role in forming the fine particles. At the preheating zone, as the temperature reaches

600–1100 °C [33, 34], Al first melts due to its low melting point (660 °C). Then, by reactions (3.2.2) and (3.2.3), Al and SiO2 are transformed into AlN, Al2O3, and Si. NaCl melts (melting point: 800 °C) due to the heat released from reactions (3.2.2) and (3.2.3).

Reaction (3.2.4) indicates that NaCl absorbs the reaction enthalpy instantaneously that acts as an efficient diluent, which reduces the preheating temperature further to reduce the reaction temperature. The reduced reaction temperature prevents the melting and the agglomeration of Si particles in the raw materials before nitridation that enhanced the infiltration of N2.

According to reaction (3.2.1), as z-values increase, resulting in an increase in the amounts of Al and SiO2, the amount of heat released also increases. Therefore, more

45 amount of NaCl is needed to reduce the heat generated by reactions (3.2.2) and (3.2.3).

0 Al(l) + 0.5N2(g)→ AlN ΔH = −326.667 kJ (3.2.2)

0 2Al(l) + 1.5SiO2→ Al2O3 + 1.5Si ΔH = −326.775 kJ (3.2.3)

NaCl → NaCl(l) ΔH = 82.069 kJ (3.2.4)

The melted NaCl spreads the particles of the other materials, which can then be regarded as a wet NaCl–Al2O3–AlN system that surround Si particles due to capillary forces, as supported by the results for B in Fig. 3-2-6 (c). The system shows homogeneity for an instant before the nitridation of Si, which results in the formation of

Si3N4 crystals. As the temperature increases to around 1200 °C, the Si–N2 reaction starts: nitridation occurs on the surface of the exposed Si and a surface coating of α-Si3N4 is formed under the melting NaCl [35]. The compound α-Si3N4 converts to the more stable

β-Si3N4 as the temperature increases. Then, by dissolution, mass transportation, and a precipitation process occurring among β-Si3N4, AlN, and Al2O3, small β′-SiAlON nuclei that are rich in Al and O content are formed on the surfaces of the Si particles [36]. This implies that in the combustion process at low temperature, solid solution β′-SiAlON is formed by the simultaneous equivalent substitution amount of Al–O for Si–N. Then as the temperature increases, the nitridation of the inner Si occurs to form β-Si3N4, which ignites the reaction for formation β-SiAlON by providing the energy conditions. Then by the further dissolution between β-Si3N4 and the β′-SiAlON structure, the amounts of

Al–O are diluted by the solution and diffusion of Si–N in the β′-SiAlON nuclei.

Therefore, the solid solution β-SiAlON with the target z-value is formed by solution-reprecipitation mechanism. For smaller z-value, more Si–N is needed for diluting Al–O in β′-SiAlON structure, thus, morphologies of β-SiAlON with small z-value show longer and larger size than that with large z-values. The reactions can be presented as follows:

0 3Si + 2N2(g) → Si3N4 ΔH = −828.896 kJ (3.2.5)

β-Si3N4 + AlN + Al2O3 → β′-SiAlON (3.2.6)

β′-SiAlON + β-Si3N4 → β-SiAlON (3.2.7)

It is worth noting that due to the lack of data for reactions (3.2.6) and (3.2.7), we ignore the reaction heat for these by assuming that it was far less than that for reaction (3.2.5). At this stage, liquid NaCl acts as a diffusion barrier between the

β-SiAlON particles, which significantly restricts the crystal growth and agglomeration of β-SiAlON. On the other hand, NaCl is completely vaporized and separated out of the reaction system by the absorption of the significant amount of heat generated from these reactions, as shown in (3.2.8), and the melting and agglomeration of Si particles reduce.

Furthermore, the space formed as a result of NaCl evaporation is more favorable for nitridation.

NaCl(l)→NaCl(g) ΔH = 202.055 kJ (3.2.8)

In the case of a large amount of NaCl added to the raw materials, the nitridation of the inner Si cannot proceed due to insufficient energy prior to the formation of

β-SiAlON. As a result, the reaction is prevented from becoming self-sustaining, which leads to the final extinction of the combustion wave. Under this condition, the formed

β-SiAlON shows a higher z-value (β′-SiAlON) than does the overall product. Therefore the product consists of higher z-value β-SiAlON and un-reacted Si, as supported by the results for C in Fig. 3-2-6 (b, c).

The nitridation of the inner Si is governed by the diffusion of the N2 through the

β′-SiAlON layers [37], and the amount of wet NaCl–Al2O3–AlN system also plays an important role. As shown in reaction (3.2.1) and Fig. 3-2-2, with small z-values, Si particles of the raw materials are surrounded by a lesser amount of the

NaCl–Al2O3–AlN system; therefore, it is easier for N2 to filter into the core and react with Si. However, with large z-values, Si particles are covered by a greater amount of the liquid NaCl–Al2O3–AlN system, which make it difficult for N2 to diffuse into the

47 core of Si through the obstacle liquid layer. Consequently, unreacted Si remains in the product given the very short reaction time of the whole combustion synthesis process.

Therefore, it is tough to obtain single-phase β-SiAlON with larger z-values. However, the mass transport during the solution-reprecipitation process is reduced due to the larger amount of liquid NaCl and the lower temperature of the reaction system. Thus, the product consists of small particles that are completely covered with liquid NaCl. As a result, the product exhibit well-dispersed fine particles after the reaction.

Alkali chlorides including LiCl, NaCl, KCl, RbCl, CsCl, FrCl, have the similar physical properties, such as melting point, boiling point, and latent heat of melting and vaporization. However, NaCl is the low cost and abundant salt with low molar mass of molecules and high latent heat of melting and vaporization, which can be regarded as the best candidate for producing β-SiAlON to realize its industrialization. NaCl plays an important role by its melting and evaporating during the whole reaction processes for combustion synthesis β-SiAlON. At the preheating zone before the nitridation of Si, the melting of NaCl reduces the preheating temperature by absorbing the excessive reaction enthalpy, which provides the suitable energy for the nitridation of Si. Then the liquid

NaCl acts as a diffusion barrier between the β-SiAlON particles, and they cannot join to each other to grow further, which significantly restrict the crystal growth of β-SiAlON.

3.2.4 Summary

High-purity β-SiAlON (z = 0.25–3) powders with submicron size were successfully synthesized via combustion synthesis (CS) with NaCl as the diluent. The amount of NaCl needed to complete the combustion synthesis increased as the z-value increased, from 4 mass% with a z-value of 0.25 to 45 mass% with a z-value of 3. The particle size of the product significantly decreased as NaCl was added, with the smallest particles showing diameters of about 300 nm when z = 3. Single-phase β-SiAlON (with z-values of 0.25, 0.5, and 1) powders were obtained; however, with larger z-values, it was difficult to obtain the single-phase products due to the excessive liquid phase in the reaction process. NaCl served as an efficient diluent in the combustion synthesis reaction by absorbing the entropy. The resulting reduced reaction heat prevented the melting of Si, which greatly improved the conversion for nitridation. Furthermore, during the reaction, liquid NaCl acted as a diffusion barrier between β-SiAlON particles, which greatly limited the growth of β-SiAlON crystals.

3.3 Effect of metal chlorides on the combustion synthesis of β-SiAlON

3.3.1 Objective of section 3.3

In the preceding section, we proposed using NaCl as the diluent to combustion synthesize β-SiAlON. The proposed method provided the production of single-phase

β-SiAlON powders with a submicron size. NaCl effectively absorbed some of the reaction heat as the latent heat of its phase transformation [38]. Furthermore, melted

NaCl could be regarded as a protective shield that prevented the agglomeration of the products. Hence, fine particles were obtained. NaCl was also considered to be a suitable diluent for the synthesis of β-SiAlON because its melting point (800 °C) and boiling point (1413 °C) are just in the temperature ranges corresponding to the two major exothermic processes [39]. However, it was unclear whether other metal chlorides that have properties similar to NaCl could be used as diluents for the combustion synthesis of β-SiAlON with a different size or shape.

Therefore, the purpose of this section is to investigate the effect of metal chlorides

(KCl, MgCl2, and CaCl2) on the synthesis of β-SiAlON and to present a detailed understanding of the endothermic processes occurring during the exothermic stages.

This understanding supplies a new route for synthesizing a single-phase product with a morphology tailored by changing the type of metal chlorides to control the endothermic processes.

3.3.2 Experimental procedure The starting materials used in this chapter were Si (purity > 99.9%, 12 µm), Al

(purity > 99.9%, 14 µm), and SiO2 (purity > 99.9%, 12 µm) powders, along with KCl

(purity > 99.9%), MgCl2 (purity > 99.9%), and CaCl2 (purity > 99.9%). The effects of these different raw materials on the synthesis of β-Si5AlON7 were investigated.

Different amounts of KCl, MgCl2, and CaCl2 were added to the raw materials as diluents. A detailed explanation of both the apparatus and the preparation of samples are provided in section 3.2.2 [18, 38].

Phase analyses of the products were performed using X-ray diffraction (XRD,

Miniflex, Rigaku, Japan) with Cu Kα radiation (λ = 1.54056 nm). The microstructures were observed using a scanning electron microscope (FE-SEM, JSM-7400F, JEOL,

Japan). The data for calculating the latent heat and theoretical adiabatic temperature

(Tad) were taken from the database in the HSC Chemistry software (Ver. 5.11,

Outokumpu, Finland).

3.3.3 Results and discussion

Fig. 3-3-1 shows the XRD patterns of the combustion-synthesized β-Si5AlON7 powders with 12 mass% of KCl, MgCl2, and CaCl2 added. It is clearly seen that Si peaks appear for all of the products. The unreacted Si existed because of the melting and subsequent hardening occurring under the fast reaction conditions at a high reaction temperature. In section 3.2, when the same amount of NaCl was added, single-phase products could be obtained. The difference may be due to the different latent heats of the metal chloride phase transitions, which led to different reaction temperatures and resulted in different conversion rates for the nitridation of Si.

To determine the optimal amount of salt for obtaining complete nitridation of Si particles, a series of experiments was carried out in which varying amounts of salts were added to the raw materials. Fig. 3-3-2 shows the XRD patterns of the products synthesized with the optimized amounts of additives. No Si peaks were detected for any of the samples, which indicated that the complete nitridation of the Si was achieved with these amounts of additives. In fact, the intensity of the Si peaks gradually decreased as the additive content was increased. When the proper amount of additive was provided, a single-phase product could be obtained. Taking KCl as an example,

18 mass% was found to be the proper content to obtain a product with a high purity and no trace of Si. However, with more than 18 mass%, the combustion front flame was extinguished, and the reaction was not completed. This occurred because the reaction was not self-sustaining, since a lower amount of heat was released from the raw materials. The same results were obtained when MgCl2 and CaCl2 were used as the diluents, for which contents of 18 and 22 mass% were required to obtain single-phase products, respectively. The diffraction peaks were clearly observed, and they agreed with those of the standard β-SiAlON structure (JCPDS Card No. 48-1615). In addition, no metal chloride peaks were detected in the products, which indicated that their

52 complete evaporation was achieved under the high reaction temperature.

β-SiAlON Si NaCl 12%

CaCl 12%

MgCl 12%

2 Intensity [a. u.]

KCl 12%

10 20 30 40 50 60 2 theta [degree]

Fig. 3-3-1 XRD patterns of combustion-synthesized β-SiAlON (z = 1) obtained

with the addition of 12 mass% KCl, MgCl2, or CaCl2 (this work) or the addition of 12 mass% NaCl (section 3.2).

β-SiAlON CaCl 22% 2

MgCl 18%

2 Intensity [a. u.]

KCl 18%

10 20 30 40 50 60 2 theta [degree]

Fig. 3-3-2 XRD patterns of combustion-synthesized β-SiAlON (z = 1) obtained

with optimized amounts of KCl, MgCl2, and CaCl2 (mass%).

As is well known, the reaction heat from the raw materials is reduced by adding a certain amount of metal chloride. As a result, the melting and coalescence of Si particles is avoided, and the complete nitridation of Si can be achieved. Fig. 3-3-3 shows the characteristics of the phase transformations of the metal chlorides in terms of the absorbed heat value ΔH (kJ/kg). The data for calculating the specific heat capacities and latent heats of the phase transformations were taken from the database in the HSC

Chemistry software, Ver. 5.11. The change in the enthalpy of the raw materials during the reaction processes was estimated using ΔH1 and ΔH2, which correspond to the two major exothermic processes because their melting and boiling points are just in the corresponding ranges of the two-stage exothermic reaction [38, 39]. The first stage occurs at temperatures of ~660–1200 °C, and the reactions can be presented as follows:

0 Al(l) + 0.5N2(g)→ AlN ΔH = −326.667 kJ (3.3.1)

0 2Al(l) + 1.5SiO2→ Al2O3 + 1.5Si ΔH = −326.775 kJ (3.3.2)

The second process takes place at temperatures of ~1350–1930 °C, which includes the nitridation of Si and the simultaneous formation of β-SiAlON. This process is summarized as follows:

0 3Si + 2N2(g) → Si3N4 ΔH = −828.896 kJ (3.3.3)

Si3N4 + AlN + Al2O3→ β-SiAlON (3.3.4)

Therefore, the metal chlorides absorb the energy generated by reactions (3.3.1) and

(3.3.2) and subsequently melt, which provides an appropriate reaction temperature for reaction (3.3.3). Because reaction (3.3.3) releases a greater amount of energy, the melted metal chlorides vaporize when they absorb it.

Fig. 3-3-4 shows the changes in the enthalpy of the raw materials, which corresponds to the theoretical values of the energy absorbed in the raw materials, when the optimized amounts of KCl, MgCl2, and CaCl2 are added. The results from our previous work in which 12 mass% NaCl was added are also shown for comparison. It is

54 apparent that the calculated ΔH1 (kJ/kg) values are similar; that is, the reductions in the enthalpy during the first exothermic stage are almost the same for all added metal chlorides. In the second exothermic stage, the calculated values for ΔH2 are also comparable, except those for the CaCl2 additive, which shows a slightly higher value than the others do. In fact, in this calculation, the complete evaporation of the salts is taken into account. However, CaCl2 could not evaporate completely because of its high boiling point (1935 °C), although its peaks were not detected in the product. In our

XRD analysis, we found that reducing the reaction heat was a key factor for determining the conversion rate of nitridation. These metal chlorides decreased the reaction heat by comparable amounts in two steps, in accordance with the two exothermic processes, via their melting and evaporation. The reduction in the heat released effectively prevented the Si particles of the raw materials from melting and agglomerating, which enhanced the infiltration of N2. Thus, the results confirm that metal chlorides such as NaCl, KCl, and MgCl2 are suitable additives for achieving single-phase β-SiAlON products.

5000

NaCl 4000

3000 CaCl2 MgCl

ΔH2 2

KCl

H [kJ/kg] 2000 

1000 ΔH1 0 300 600 900 1200 1500 1800 2100 2400 Temperature [K]

Fig. 3-3-3 Characteristics of phase transformations of metal chlorides and changes in heat ΔH (kJ/kg).

700 676 H1 600 H2

500 474 429 413

400

300 H [kJ/kg]

 196 209 201 200 168

100

0 KCl MgCl2 CaCl2 NaCl [18%] [18%] [22%] [12%]

Fig. 3-3-4 Change in enthalpy of the raw materials (ΔH, kJ/kg) for the addition of optimized amounts of KCl (18 mass%), MgCl 2 (18 mass%), and

CaCl2 (22 mass%), along with ΔH for NaCl is listed here, which were calculated from their absorbed heat values corresponding to the two exothermic processes.

4213 °C 4000 3757 °C 3749 °C 3844 °C

] 3629 °C

 [ 3000

2000

1000 Adiabatic temperature 0 without KCl MgCl2 CaCl2 NaCl additives

Fig. 3-3-5 Calculated adiabatic temperatures (Tad) for the addition of optimized amounts of KCl (18 mass%), MgCl 2 (18 mass%), and

CaCl2 (22 mass%), along with Tad for addition of 12 mass% NaCl (section 3.2) and that obtained when no additives were used. These results were calculated according to the absorbed heat values.

Fig. 3-3-5 shows the calculated theoretical adiabatic temperatures (Tad) when the optimized amounts of the metal chlorides are added. For comparison, the Tad values in the case of NaCl addition and with no additives are also shown. Here, Tad was calculated using the HSC Chemistry software Ver. 5.11. The calculated Tad values for the different amounts of metal chlorides are also comparable, at approximately 360 °C lower than the

Tad obtained without the salt additives. It has been reported that the product contains a large amount of unreacted Si (44 mass%) when the raw materials are used without any additives [20]. In contrast, the use of a metal chloride appropriately reduces the reaction heat, ensures a proper reaction temperature, and thus efficiently controls the conversion of reactants to products. This theoretical calculation provides a principle for obtaining high-purity products with other, similar additives.

Fig. 3-3-6 shows SEM images of the β-SiAlON synthesized with 12 mass% added

KCl, MgCl2, and CaCl2. It is obvious that the sizes and shapes of the rodlike crystals have distinctive features when different types of additives are used. The crystals have diameters of ~1 µm, lengths of ~5 µm, and round tips when KCl is added. In contrast, when CaCl2 is added, the rodlike crystals are very large, with lengths of up to

~30–40 µm, diameters of ~10 µm, and regular hexagonal tips. Closer observations showed that cracks existed on the surfaces of the crystals, suggesting the incomplete growth of the crystals due to the rapid reaction rate. The crystals obtained by adding

MgCl2 were observed to have a shape that was between the small rod-like shapes obtained using KCl and the large shape obtained using CaCl2. This large variation in the morphologies demonstrates that the additives did not have the same effect on the growth of the β-SiAlON crystals. This can be explained by the different driving forces for grain growth arising for the different chemical compositions [40].

Fig. 3-3-7 shows SEM images of the β-SiAlON synthesized with the optimized amounts of the metal chlorides. The product obtained with 18 mass% KCl still consisted

57 of rodlike crystals that were very uniform in size, with diameters of 0.5 µm and lengths of 5 µm, although they were smaller than those obtained when 12 mass% KCl was added (see Fig. 3-3-6(a)). However, the crystals developed into small particles for the higher MgCl2 and CaCl2 contents, in contrast to the structures shown in Figs. 3-3-6(b) and (c), respectively. These distinct changes in morphology were mainly caused by the different reaction temperatures. At a higher temperature, the mass transport will be enhanced, and morphology evolution of the β-SiAlON grains will tend to occur more easily. Therefore, at a higher temperature, or with the addition of a smaller quantity of the additive, a dynamic ripening mechanism will allow the rapid growth of coarse rodlike crystals. In contrast, at a lower temperature, or with the addition of a larger quantity of the additive, the grain growth will be greatly restricted. Fig. 3-3-8 displays a series of SEM images showing the morphology evolution of samples produced using

CaCl2 as the diluent. These images support the aforementioned statement that the growth of the crystals is controlled by the reaction temperature. They also indicate that the dimensions of the crystal structure change during its evolution, with a reduction in the diameter of the rodlike structures and an increase in their length. These dimensional changes occur because the discontinuity of the rough surface morphology leads to evolution into an equilibrium crystal shape with the minimum total surface energy.

There are some large rodlike crystals with concave tips, as shown in Fig. 3-3-8(d).

The formation of this concave tip shape for rodlike β-SiAlON is discussed in detail by

Liu et al. [41].

Briefly, for the different kinds of additives, the morphology of the crystals is affected by the different chemical compositions and the different driving forces. For the same type of additives, the morphology of the crystals is controlled by the reaction temperature. These results will be helpful for designing the morphology of β-SiAlON crystals.

(a) (b)

Fig. 3-3-6 SEM images of combustion-synthesized β-SiAlON powders obtained with 12 mass% (a) KCl, (b) MgCl2, and (c) CaCl2. (d) Enlarged image of the area enclosed in a square in (c).

(a) (b)

5 μm 5 μm

(c)

5 μm

Fig. 3-3-7 SEM images of combustion-synthesized β-SiAlON powders obtained using optimized amounts of (a) KCl (18 mass%), (b) MgCl2 (18 mass%), and

Fig. 3-3-8 SEM images of combustion-synthesized β-SiAlON powders obtained using (a, b) 22 mass% CaCl2 and (c–f) 12 mass% CaCl2.

3.3.4 Summary

Single-phase β-SiAlON products with different morphologies were obtained by adding different amounts of KCl, MgCl2, and CaCl2 to the raw materials. The metal chlorides were considered to be suitable additives for the synthesis of single-phase

β-SiAlON for the following reasons. First, their melting absorbed the energy of the first exothermic process, and their evaporation absorbed that of the second exothermic process. Thus, the latent heat of their phase transitions could control the reaction heat and ensure an appropriate reaction temperature, which promoted the complete nitridation of Si. On the other hand, the morphology of the product was strongly affected by the type of chloride: the size of the rodlike crystals became smaller with increasing KCl content, while the coarse rodlike crystals developed into small particles when the amount of MgCl2 or CaCl2 was increased. These findings prove that this is a novel and facile method for fabricating single-phase β-SiAlON crystals with a tailored morphology by changing the type or amount of metal chlorides.

3.4 Reaction characteristic of combustion synthesis of β-SiAlON using different additives

3.4.1 Objective of section 3.4

In section 3.2 and 3.3, single-phase β-SiAlON products were obtained with the addition of different amounts of salt additives, and NaCl could be regarded as the best candidate for producing β-SiAlON because of its larger latent heat of melting and vaporization than other metal chlorides (KCl, MgCl2, and CaCl2) [38]. However, that work had merely been examined under laboratory conditions using a small amount of raw materials. Furthermore, the sample was longitudinally positioned and the top of the sample was ignited according to the classical combustion synthesis process, which was not suitable for commercial processing. To execution of this new combustion synthesis process under industrial conditions using a large sample size is still a challenge.

Therefore, in this section, to execute this new combustion synthesis process under industrial conditions, the sample was positioned horizontally. Besides, amount of

β-SiAlON powders were used to surround the sample, which could reduce the heat exchanges with the surroundings. In order to compare, the same experiment was carried out using the β-SiAlON product as diluents. This was the first execution of this method using a horizontal-type chamber and employing low-cost Si, Al, and SiO2 as raw materials, along with the comparison of the characteristics of the reaction process and kinetic of the reaction on propagation using Boddington’s model.

3.4.2 Experimental procedure

The starting materials used in this work were commercially available powders of

Si (6 µm, purity >98%, Kinseimatec Co., Ltd., Japan), Al (14 µm, purity >99.7%,

Minalco Co., Ltd., Japan), SiO2 (12 µm, purity >99%, Takeori. Co., Ltd., Japan),

β-SiAlON (0.5 µm, ISMAN J Corporation, Kawasaki, Japan), and NaCl (purity 99.9%,

Kojundo Chemical Laboratory Co., Ltd., Japan). The reaction characteristics of the combustion synthesis process were compared for the addition of either 45 mass%

β-SiAlON (denoted D45) or 12 mass% NaCl (denoted N12) to the starting materials.

The reactions for the combustion synthesis of β-Si5AlON7 (β-Si6-zAlzOzN8-z, z = 1) using β-SiAlON and NaCl as the diluent can be described by the following equations:

4.5Si  Al  0.5SiO2  3.5N2  0.427NaCl  β Si 5AlON7  0.427NaCl (3.4.1)

4.5Si  Al  0.5SiO2  3.5N2  0.533Si5 AlON7 1.533β Si 5AlON7 (3.4.2)

All reactants were weighed according to the stoichiometric ratio. The reactant powders were mechanically activated by planetary ball-milling at a ball-to-sample mass ratio of 10:1 for 15 min. Then, 120 g of the activated mixture was loosely packed into a graphite crucible with dimensions 80 ×130 × 45 mm. The experimental setup is shown in Fig. 3-4-1. Two W-Re type thermocouples protected by magnesia sheaths were inserted into the center of the sample to record the combustion temperature profiles. The propagation velocity of the combustion front was determined by measuring the time that lapsed between the two thermocouples, which were embedded at a distance of 40 mm.

Nitrogen (purity: 99.999%) was charged into the chamber up to a pressure of 1 MPa. To minimize the oxygen concentration in the chamber, the vacuuming/pressurizing with nitrogen was repeated twice. Combustion was triggered by igniting the Al and

β-SiAlON (Al/β-SiAlON = 50/50 mass%) powders by passing an electric current though the carbon foil placed on one end of the sample.

The experimental procedures employing both types of additives are compared in

Fig. 3-4-2. As shown in this figure, when using 45 mass% β-SiAlON product as diluents, the as-synthesized β-SiAlON product generated by addition of commercial β-SiAlON powder (ISMAN J, 0.5 µm) as the diluent was crushed in a mortar and then sieved through a 220-mesh sieve and milled to approximately 5 µm to be used as the diluent for the second combustion reaction under the same conditions. This process was repeated for several times. It is obvious that this approach has a drawback of having multistep pathways and a low product yield. Furthermore, the products were extremely hard and difficult to mill into powders. However, using 12 mass% NaCl as the diluent has the merit of high-efficiency and energy-saving process. In addition, the product obtained by addition of NaCl was readily pulverized by crushing.

The phase compositions of the combustion-synthesized products were analyzed via

X-ray diffraction (XRD, Cu Kα, Rigaku, Japan), the accelerating voltage and current are

30 kV and 15 mA, respectively, and the scanning angle of the detector was varied within the range 10–60° (2θ) with a step of 0.02° and fixed counting time of 0.5 s. The lattice parameters of the hexagonal β-SiAlON were calculated from the XRD peaks

(100), (110), (200), (101), (210), (201), (301), (320), (321), and (411) using pure Si as an internal standard. The microstructures were observed via scanning electron microscopy (FE-SEM, JSM-7400F, JEOL, Japan). The data for calculating the heat change caused by NaCl was taken from the database of the HSC Chemistry software

Ver. 5.11.

Top view Stainless steel

N2 outlet N2: 1 MPa

P T. C. 1, 2

N2 inlet Carbon paper T. C. 2 T. C. 1 Side view Electric pole Sample Ignition agent (Al + β-SiAlON) Carbon crucible Protective magnesia tube (80 ×130 ×45 mm) Fig. 3-4-1 Schematic diagram of horizontal-type combustion synthesis apparatus.

(a) Conventional method (b) Proposed method n ≥ 2 Diluent: 45 mass % NaCl: 12 mass%

n = 1 (initiate) Raw materials

SiAlON 45% Si, Al, SiO2 (commercial)

Milling to about 5 μm Mixing for 15 min by planetary ball milling

Sieving with 200 μm Combustion synthesis mesh nitrogen pressure of 1 MPa

Crusting with mortar and pestle Product Product (D45) (N12) Fig. 3-4-2 Experimental procedure for combustion synthesis of β-SiAlON using (a) 45 mass% β-SiAlON product as diluent, which was first combustion synthesized with addition of commercial SiAlON, and (b) 12 mass% NaCl.

3.4.3 Results and discussion

The morphology of the raw materials before combustion synthesis of β-SiAlON is presented in Fig. 3-4-3. When using 45 mass% β-SiAlON powders as diluents, Si grains are covered by some small ones, which are agglomerated together. In contrast, the raw materials exhibit well-distributed particles when using 12 mass% NaCl as diluents. The result indicates NaCl prevented the agglomeration of the raw materials during the milling. To obtain a precise estimation of the particle-size distribution for the two kinds of raw materials, particle-size distribution measurement was performed by laser particle-size analyzer. The results are given in Fig. 3-4-4. D45 is composed of particles with a median diameter of ~4.2 µm, which are smaller than that of N12 with a median diameter of ~6 µm. The differences are attributed to the addition of amount of small sized β-SiAlON diluents in D45.

(a) (b)

Si Si Si

Si 5 μm 5 μm

Fig. 3-4-3 SEM images of raw materials for CS of β-SiAlON: (a) D45, (b) N12.

(a) (b)

10 100 10 100

Cumulative percent finer [%] finer percent Cumulative

8 80 8 80

6 D50 = 4.2 μm 60 6 D50 = 6.1μm 60

4 40 4 40 Frequency [%] Frequency [%] 2 20 2 20

0 0 0 0

0 5 10 15 20 25 30 0 5 10 15 20 25 30 Particle size [m] Particle size [m] Fig. 3-4-4 Particle size distribution of the raw materials for CS of β-SiAlON: (a) D45, (b) N12.

Fig. 3-4-5 shows the heat contents of β-SiAlON and NaCl during the combustion synthesis processes, which were calculated using the data acquired from a laser-flash method and taken from the database of HSC Chemistry software Ver. 5.11, respectively.

The endothermic value (Q, kJ/kg) of β-SiAlON was estimated using the following equation:

Qβ-SiAlON = (3.4.3)

where Cp(T) is the specific heat capacity of β-SiAlON (z = 1) in kJ/(kg·K), and Q in kJ/kg, Cp = 0.731 kJ/(kg·K) was the measured value from the laser-flash method.

The endothermic value of NaCl was calculated using the specific heat and latent heat of its melting and evaporation, which could be determined from the following formula:

QNaCl = (3.4.4)

and Cp was 0.861 kJ/(kg·K), the data used in this calculation were taken from software

HSC chemistry Ver. 5.11. The values of specific heat capacity and heat capacity formula of β-SiAlON and NaCl can be found in Table 3-4-1. It is evident that the endothermic value of NaCl (1403 kJ/kg at 1073 K) is larger than that of β-SiAlON (938 kJ/kg at

1073 K) at temperatures of equal and higher than the NaCl melting point because of the high latent heat of phase transformation of NaCl.

Based on our previous studies [38, 39, 42], two endothermic processes can be defined with the addition of these additives, corresponding to a two-stage exothermic process for the combustion synthesis of β-SiAlON. The first stage exothermic process involving the reaction of Al and N2 to form AlN and Al and SiO2 to form Al2O3 and Si around 1473 K: 2Al + N2(g) → 2AlN, 2Al + 1.5SiO2 → Al2O3 + 1.5Si; and second exothermic occurs at 1473–1900 K, with the exothermic nitridation of Si and simultaneous formation of β-SiAlON: 3Si + 2N2(g) → Si3N4, Si3N4 + AlN + Al2O3 →

β-SiAlON. Therefore, we define the first and second endothermic processes as at

67 temperatures of 298–1473 K and 1473–1900 K, respectively. In fact, during the entire process for synthesis of β-SiAlON, there are several endothermic reactions, such as

Al–Si eutectic (eutectic melting point 570 °C), the melting of Al (melting point 660 °C) and Si (melting point 1414 °C), however, these endothermic reactions can be ignored due to small endothermic peaks observed from the temperature curves.

Fig. 3-4-6 shows the changes in the enthalpy of the raw materials (ΔH in kJ/kg), estimated using ΔH1 and ΔH2 that correspond to the two major exothermic processes. It is notable that the value of the total enthalpy change (ΔH1+ΔH2) of sample D45 is higher than that of N12, and the ΔH1 value is significantly higher than ΔH2, mainly because the endothermic region in the former located at temperatures of 298–1473 K, which is longer than the latter that at temperatures of 1473–1900 K .

Table 3-4-1 Specific heat capacity and heat capacity formula of β-SiAlON and

NaCl

Materials Cp (kJ/kg·K) Heat capacity formula

(T=298K) (kJ/kg·K)

β-SiAlON 0.731 -0.17304+3.99*10-3T-3.51967*10-6T2+1.05768*10-9T3

(298≤T≤1900K)

NaCl 0.861 A+B*10-3T+C*105T-2+D*10-6T2

A B C D T

0.96328 -0.2227 -0.05857 0.374074 (298≤T≤900K)

1.003319 -0.30512 -0.08372 0.42005 900≤T≤1073.

-0.43311 1.303886 6.810277 -0.30219 1073≤T≤1500

1.145458 0 0 0 1500≤T≤2500

5000 Zone I: First exothermic stage 0 Al + 0.5N2 → AlN, ΔH = −318 kJ 2Al + 1.5SiO → 1.5Si + Al O , ΔH0 = −309 kJ 4000 2 2 3 Zone II: Second exothermic stage

0 3000 3Si + 2N2(g) → Si3N4 , ΔH = −829 kJ

β-Si3N4 + AlN + Al2O3 → β-SiAlON

Q [kJ/kg] 2000 NaCl

β-SiAlON

1000 1738K

Zone I 1473K Zone II

0 1073K 400 600 800 1000 1200 1400 1600 1800 2000 Temperature [K]

Fig. 3-4-5 Heat content of β-SiAlON and NaCl during combustion synthesis. Zones I and II were determined based on previous studies.

800 1473 1900 0.45 Cp(T )dT ΔH1D45= 0.45 Cp(T )dT , ΔH2D45=  700 662 298 1473 1073 1473 ΔH1 = 0.12 Cp(T )dT  Q  Cp(T )dT  600 N12  sl  298 1073 1738 1900 ΔH2 = 0 .12 Cp(T )dT  Q  Cp(T )dT 500 N12   lg   1473 1738

400 370

317 H1 300 H2 H [kJ/kg] 224 200 100 0 D45 (-SiAlON 45 mass%) N12 (NaCl 12 mass%)

Fig. 3-4-6 Changes in enthalpy of raw materials ΔH (kJ/kg) for combustion synthesis of β-SiAlON with addition of β-SiAlON and NaCl, calculated from Fig. 3; “1” and “2” indicate first and second endothermic stages.

All the temperature profiles analyses were made from the thermocouple T. C. 1 data (see Fig. 3-4-1). In fact, the thermocouples are located at the level of the middle of the sample. The typical time-temperature profiles obtained during the combustion synthesis of β-SiAlON for the D45 and N12 samples by using raw reactants Si, Al, and

SiO2 under 1 MPa nitrogen pressure are shown in Fig. 3-4-7. The temperatures rapidly reached the maximum values after ignition, which then decreased at a slow rate for both samples; however, for N12, this process is followed by a longer period of after-burn, in which the temperature decreases, than with D45. This difference arises because of different reaction rates, as discussed in a subsequent section. The maximum measured temperature for D45 and N12 are 1681 °C and 1505 °C, respectively. The calculated wave propagation velocities by measuring the time that lapsed between the two thermocouples are presented in Table 3-4-2. Several times of the repeated experiments show the identical results although there was slightly variance in the maximum measured temperature.

Table 3-4-2 Characteristic parameters calculated from samples

Sample Wave velocity Lattice constant (Å) Calculated

(mm/s) a c z value

D45 0.168 7.6350 2.9331 1.05

N12 0.185 7.6310 2.9309 0.94

(a) (b)

1800 1800 1600 1600

D45 I: Preheating zone N12 ]

] 1400 1400

II: Reaction zone C

  1200 [ 1200 [ III: After burning zone 1000 1000

III

800 800 II 600 III 600 II

400 400 Temperature Temperature I 200 200 I 0 0 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 Time [s] Time [s] Fig. 3-4-7 Temperature profiles (T. C. 1) recorded during combustion synthesis of β-SiAlON: (a) D45, (b) N12.

(a) 20 (b) 1.5 D45 2

15 1.0

)

2 -

1 - s

10 s 0.5

C·

(º

5 2 0.0

T/dt dT/dt

0 2 -0.5 d

-5 -1.0

0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 Time [s] Time [s] (c) (d) 20 1.5 N12 16

1.0

)

2 1

1’ -

- s s 12

C· 0.5

C· 2’

(º

2 8

0.0

T/dt 2

dT/dt 4 d -0.5 0

-1.0 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 Time [s] Time [s] Fig. 3-4-8 First derivative dT/dt (a and c) and second derivative d2T/ dt2 (b and d) of temperature profiles during combustion synthesis of β-SiAlON, D45 (a and b) and N12 (c and d).

(a) (b) 5.2 D45 6.490 D45 5.1 slope = 0.05063 6.485 slope = -0.00275 5.0

6.480 ) ) t = 1/slope 0 d t = 1/slope 0 4.9 r 6.475 = 363.64

T-T = 19.75

T-T

( (

ln 4.8 ln 6.470 2 R2 = 0.97877 R = 0.99986 4.7 6.465

4.6 6.460 290 292 294 296 298 300 780 782 784 786 788 790 792 Time [s] Time [s] (c) (d)

4.6 N12 N12 slope = 0.08934 6.515 slope = -0.00207 4.4

tr = 1/slope 6.510 td = 1/slope

4.2

) )

0 = 11.19 0 = 483.09 6.505

4.0

T-T

(

( ln 2 ln 3.8 R = 0.9962 6.500 R2 = 0.9999

3.6 6.495

176 178 180 182 184 186 1198 1200 1202 1204 1206 1208 1210 Time [s] Time [s]

Fig. 3-4-9 Plots of ln (T-T0) vs. t for inert zone of temperature profiles, rise zone (a and c) and decay zone (b and d).

(a) D45 (b) N12

0.020 0.006 1.0 1.0 0.005 0.015

0.8 0.004 0.8 )

1 0.010 )

- 0.003

1 s

0.6 - 0.6

(

s (

0.002 η

0.005 η /dt

/dt 0.4

 0.4

 0.001 d

0.000 d 0.2 0.000 0.2 -0.005 -0.001 0.0 0.0 -0.010 -0.002 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 Time [s] Time [s] Fig. 3-4-10 Time dependence of reaction rate (d/dt) and fraction reacted () for combustion synthesis of β-SiAlON: (a) D45 and (b) N12.

Fig. 3-4-8 shows the first and second derivatives of the temperature profiles acquired during the combustion synthesis of β-SiAlON. The first derivatives ((a) and

(c)) of the temperature profiles for the two samples show similar trends, although the variation tendency and maximum value are remarkably different. The two peaks in the heating rate (dT/dt) curves indicate a two-stage exothermic process, consistent with the aforementioned reaction mechanism. For sample D45, the first peak in the dT/dt curves is observed at a maximum value of 11 °C/s, which is lower than that of N12 with a maximum value of 13 °C/s, as indicated by peaks of 1 and 1′ in (a) and (c), respectively; the second dT/dt peak of D45 rises sharply to reach a value as high as 18 °C /s (peak 2 of (a)). However, the second dT/dt peak for N12 (peak 2′ of (c)) approaches a stationary value of 10 °C/s and persists for a long period. The significant change in the profile of these curves is attributed to the change in the enthalpies (ΔH) of the raw materials.

Compared with Fig. 3-4-6, it is clear that a high value of ΔH results in a slow heating rate, and a low value contributes to a rapid heating rate.

The reaction rate,  , can generally be calculated from the temperature profile using the following equation [43]:

 (3.4.5)

where ; α is the effective thermal diffusivity, υ is the wave velocity, and τad denotes the temperature rise under adiabatic conditions. In general, the values of the

* parameters t , tx, and τad are determined from the profile as follows [44, 45]:

(3.4.6)

(3.4.7)

(3.4.8)

where tr and td are the rise and decay times in the remote inert zones of the temperature profile, respectively, and can be determined from the linear slopes of the plot of

73 ln (T-T0) vs. t in these two regions, as shown in Fig. 3-4-9. The parameter tx is the

* thermal relaxation time of the system. The calculated parameters tr, td, tx, and t are listed in Table 3-4-3. The values of t1 and t2 are arbitrarily chosen times located in the remote rise and decay zones, respectively. and are the first and second derivatives of the temperature profiles, as shown in Fig. 3-4-8. Therefore, the reaction rate is readily computed from the temperature profiles.

Table 3-4-3 Parameters calculated from temperature profiles

Sample tr (s) td (s) tx (s) t* (s) ad (K)

D45 19.75 363.64 343.89 20.89 2040

N12 11.19 483.09 471.90 11.46 2271

tr : inert rise time

td : inert decay time

tx : thermal relaxation time of system

1/t* = 1/tr - 1/td

τad : temperature rise under adiabatic conditions

Fig. 3-4-10 shows the calculated reaction rate (dη/dt) and the reacted fraction () for the combustion synthesis of β-SiAlON using the two types of additives. For sample

D45, the maximum value of the reaction rate corresponds to a reacted fraction (η) of

70% at ~1587 °C; whereas η was only 46% for sample N12 for the maximum value of dη/dt at ~1370 °C. The results indicate that the maximum reaction rate is not achieved at the maximum temperature for both samples. At the maximum measured temperature,

74 for sample D45, η = 88%, which indicates that essentially complete conversion is achieved. In contrast, the reaction is only partially completed for sample N12 with η =

59%. The lower reaction rate was considered to result from significant heat reduction because of the evaporation of NaCl during the second exothermic process. As a result, η continued to increase as the reaction continued, well past the passage of the combustion wave, and a long after-burn time was required to reach completion of the reaction.

Fig. 3-4-11 shows the XRD patterns of the combustion-synthesized β-SiAlON powders. No impurity peaks were observed for either of the two samples, and all the observed peaks could be indexed to the pure β-SiAlON phase. The results indicate that complete nitridation of the Si was achieved with the respective quantities of β-SiAlON and NaCl additives. NaCl peaks were not detected in the product of N12, which indicates that their complete evaporation was achieved under the high reaction temperature and separated out of the reaction system [38]. The lattice parameters of the

β-SiAlON phase calculated from these XRD peaks are well matched with those documented in JCPDS 48-1615 (a = 7.635 Å, c = 2.934 Å). Furthermore, the z value of the synthesized β-SiAlON was calculated using the lattice constants a and c according to the reference method [2]. The calculated parameters are summarized in Table 3-4-2.

Fig. 3-4-12 shows the SEM images of the combustion-synthesized β-SiAlON powders obtained using the two types of additives. The sample D45 that was prepared by adding 45 mass% of β-SiAlON powder consisted of ball-like particles and columnar crystals of various sizes, most of which exhibited extensive agglomeration to form large blocks. Compared with sample D45, the rod-like crystals of sample N12, prepared by addition of 12 mass% NaCl, had larger sizes with lengths ~3 µm and diameters ~2 µm.

The formation of large crystals was induced by rapid mass diffusion, precipitation, and crystal growth of β-SiAlON nuclei caused by the long period of after-burning, as shown in the temperature profile presented in Fig. 3-4-7. The crystals of both samples had

75 rounded tips, typical of crystals formed by the vapor-liquid-solid (VLS) process. In this process, a liquid droplet is initially formed, and then reactant molecules in the vapor phase are transported by diffusion to the liquid–solid interface. With precipitation and crystal growth, the droplet is detached from the substrate [22, 46]. When the β-SiAlON powder was used as the additive, the vapor phase precipitated and grew on the surface of the powders. Solid-state diffusion between the newly formed β-SiAlON crystals and the added ones provides a source of atoms to fill up the concave areas to diminish the outer surface of the particle, resulting in self-sintering [47]. However, when NaCl is used as an additive, the melted NaCl creates a potential diffusion barrier between the

β-SiAlON crystals; hence, sintering of the β-SiAlON particles could be prevented. The melted NaCl may then be vaporized because of the high combustion temperature, followed by outward diffusion leaving space between the particles, thus generating well-isolated crystallized particles.

β-SiAlON

(a) D45

Intensity [a. u.] (b) N12

10 20 30 40 50 60 2 theta [degree]

Fig. 3-4-11 XRD patterns of combustion synthesized β-SiAlON powders, (a) D45 and (b) N12.

(a) (b)

5 μm 5 μm

Fig. 3-4-12 SEM images of combustion synthesized β-SiAlON powders: (a) D45 and (b) N12.

3.4.4 Summary

The reaction characteristics of the combustion synthesis of β-SiAlON in a horizontal-type chamber were compared using two types of additives (β-SiAlON and

NaCl). The results revealed that the various additives produced different changes in the enthalpies of the raw materials, which affected the reaction rate and morphology of the products. With addition of the β-SiAlON powder to the starting materials, the reaction was largely completed within the combustion front at the maximum temperature; in contrast, a relatively slow conversion rate was achieved (with η = 58.7%) when

12 mass% NaCl was used as the additive. This difference is attributed to the high latent heat of evaporation of NaCl, with consequent occurrence of a long after-burn effect that is beneficial for the formation of well-developed rod-like crystals.

3.5 Conclusion This chapter described salt-assisted combustion synthesis of β-SiAlON using raw materials of Si, Al, and SiO2 under nitrogen pressure of 1 MPa. In section 3.2, the effects of NaCl on combustion synthesis of β-Si6-zAlzOzN8-z (z = 0.25−3) powders were studied. As the z-values increased, the amount of NaCl needed to complete the reaction increased, which in turn decreased the particle size of the product. With small z-values, single-phase products were obtained; however, with large z-values, NaCl and unreacted

Si remained in the products due to the large amount of liquid phase. NaCl acted not only as a diluent by absorbing the heat generated by the reaction but also as a diffusion barrier between β-SiAlON particles, which greatly limited the growth of β-SiAlON crystals.

In section 3.3, the effect of metal chlorides (KCl, MgCl2, and CaCl2) on the synthesis of β-SiAlON was investigated. Single-phase products containing crystals with different shapes were obtained. The metal chlorides effectively absorbed the reaction heat via their melting and evaporation, which occurred in the same temperature ranges as the two stages of the exothermic processes. In addition, the products exhibited different morphologies when the type or amount of metal chlorides was changed.

In section 3.4, the characteristics of the reaction process and kinetic of the reaction on propagation were compared with the addition of NaCl additive and SiAlON diluents.

The reaction characteristics were influenced by the additives, which exerted a strong effect on the reaction rate and the morphology of the products. At the respective maximum combustion temperatures, a relatively slow conversion rate, with a reacted fraction (η) of 58.7%, was achieved with the addition of 12 mass% NaCl to the raw materials, whereas the addition of 45 mass% β-SiAlON powders afforded η = 88%. The

NaCl additive yielded rod-like crystals, which were well developed by a long after-burn time. However, self-sintering occurred in the presence of β-SiAlON powders.

These results demonstrated that NaCl is a potential additive for industrial-scale production of β-SiAlON powders, which is a more economic and energy-efficient synthesis process. This new process has several notable advantages: the production efficiency should be greatly improved because of the small amount of additive, a product with a fine grain size is obtained, and the high cost of postsynthesis treatments such as crushing and milling is avoided. Therefore, the overall production cost of

β-SiAlON can be significantly reduced, and the lower price of these β-SiAlON powders may open up new applications for these materials.

References [1] K.H. Jack, Review Sialons and related nitrogen ceramics. Journal of Materials Science 11 (1976) 1135-1158. [2] T. Ekström, P.O. Käll, M. Nygren, P.O. Olssen, Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing. Journal of Materials Science 24 (1989) 1853-1861. [3] T. Ekström, M. Nygren, SiAlON Ceramics. Journal of the American Ceramic Society 75 (1992) 259-276. [4] R.-J. Xie, N. Hirosaki, H.-L. Li, Y.Q. Li, M. Mitomo, Synthesis and Photoluminescence Properties of 2+ 2+ β-sialon : Eu (Si6−zAlzOzN8−z :Eu ) : A Promising Green Oxynitride Phosphor for White Light-Emitting Diodes. Journal of The Electrochemical Society 154 (2007) J314-J319. [5] T.-C. Liu, B.-M. Cheng, S.-F. Hu, R.-S. Liu, Highly Stable Red Oxynitride β-SiAlON:Pr3+ Phosphor for Light-Emitting Diodes. Chemistry of Materials 23 (2011) 3698-3705. [6] J.H. Chung, J.H. Ryu, Photoluminescence and LED application of beta-SiAlON:Eu2+ green phosphor. Ceramics International 38 (2012) 4601-4606. [7] C. Zhang, R. Janssen, N. Claussen, Pressureless sintering of β-sialon with improved green strength by using metallic Al powder. Materials Letters 57 (2003) 3352-3356. [8] S.-L. Hwang, I.-W. Chen, Reaction Hot Pressing of α’- and β’-SiAlON Ceramics. Journal of the American Ceramic Society 77 (1994) 165-171. [9] K.J.D. MacKenzie, D.V. Barneveld, Carbothermal synthesis of β-sialon from mechanochemically activated precursors. Journal of the European Ceramic Society 26 (2006) 209-215. [10] J. Li, H. Ma, Q. Fang, Synthesis of prismatic beta-sialon from the precursor of SBA-15 incorporated

with Al(NO3)3 via carbothermal reduction nitridation. Ceramics International 34 (2008) 1791-1795. [11] I.P. Parkin, G. Elwin, L.F. Barquin, Q.T. Bui, Q.A. Pankhurst, A.V. Komarov, Y.G. Morozov,

Self-Propagating High Temperature Synthesis of Hexagonal Ferrites MFe12O19 (M = Sr, Ba). Advanced Materials 9 (1997) 643-645. [12] Z.A. Munir, U. Anselmi-Tamburini, Self-propagating exothermic reactions: The synthesis of high-temperature materials by combustion. Materials Science Reports 3 (1989) 277-365. [13] X. Yi, K. Watanabe, T. Akiyama, Fabrication of dense β-SiAlON by a combination of combustion synthesis (CS) and spark plasma sintering (SPS). Intermetallics 18 (2010) 536-541.

[14] C.L. Yeh, R.F. Li, Formation of TiAl-Ti5Si3 and TiAl-Al2O3 in situ composites by combustion synthesis. Intermetallics 16 (2008) 64-70. [15] S. Gennari, F. Maglia, U. Anselmi-Tamburini, G. Spinolo, SHS (Self-sustained high-temperature synthesis) of intermetallic compounds: effect of process parameters by computer simulation. Intermetallics 11 (2003) 1355-1359.

[16] D.P. Riley, C.P. Oliver, E.H. Kisi, In-situ neutron diffraction of titanium silicide, Ti5Si3, during self-propagating high-temperature synthesis (SHS). Intermetallics 14 (2006) 33-38. [17] C.L. Yeh, H.J. Wang, A comparative study on combustion synthesis of Ta-Si compounds. Intermetallics 15 (2007) 1277-1284.

[18] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, Mechanically Activated Combustion

Synthesis of β-Si6−zAlzOzN8−z (z=1–4). Journal of the American Ceramic Society 90 (2007) 626-628. [19] M. Shahien, M. Radwan, S. Kirihara, Y. Miyamoto, T. Sakurai, Combustion synthesis of single-phase β-sialons (z = 2-4). Journal of the European Ceramic Society 30 (2010) 1925-1930. [20] K. Aoyagi, R. Sivakumar, X. Yi, T. Watanabe, T. Akiyama, effect of diluents on high purity beta-SiAlONs by mechanically activated combustion synthesis. Journal of the Ceramic Society of Japan 117 (2009) 777-779.

[21] J. Zeng, Y. Miyamoto, O. Yamada, Combustion synthesis of Sialon Powders (Si6-zAlzOzN8-z, z = 0.3, 0.6). Journal of the American Ceramic Society 73 (1990) 3700-3702. [22] Y. Wu, H. Zhuang, F. Wu, Mechanism of the formation of β–Sialon by self-propagating high-temperature synthesis. Journal of Materials Research 13 (1998) 166-172. [23] J. Lis, S. Majorowski, J.A. Puszynski, V. Hlavacek, Dense β- and α/β-SiAlON materials by pressureless sintering of combustion-synthesized powders. American Ceramic Society bulletin 70 (1991) 1658-1664. [24] Z. Shen, H. Peng, M. Nygren, Formidable Increase in the Superplasticity of Ceramics in the Presence of an Electric Field. Advanced Materials 15 (2003) 1006-1009.

[25] H.H. Nersisyan, H.I. Won, C.W. Won, Combustion Synthesis of Molybdenum Disilicide (MoSi2) Fine Powders. Journal of the American Ceramic Society 91 (2008) 2802-2807. [26] H.H. Nersisyan, H.I. Won, C.W. Won, K.C. Cho, Combustion synthesis of nanostructured tungsten and its morphological study. Powder Technology 189 (2009) 422-425. [27] K.V. Manukyan, S.V. Aydinyan, K.G. Kirakosyan, S.L. Kharatyan, G. Blugan, U. Müller, J. Kuebler,

Molten salt-assisted combustion synthesis and characterization of MoSi2 and MoSi2-Si3N4 composite powders. Chemical Engineering Journal 143 (2008) 331-336.

[28] H.E. Çamurlu, F. Maglia, Preparation of nano-size ZrB2 powder by self-propagating high-temperature synthesis. Journal of the European Ceramic Society 29 (2009) 1501-1506. [29] Y. Zhou, Y.-i. Yoshizawa, K. Hirao, Z. Lenčéš, P. Šajgalík, Preparation of Eu-Doped β-SiAlON Phosphors by Combustion Synthesis. Journal of the American Ceramic Society 91 (2008) 3082-3085. [30] X. Xu, T. Nishimur, N. Hirosaki, R.-J. Xie, Y. Yamamoto, H. Tanaka, Fabrication of β-sialon nanoceramics by high-energy mechanical milling and spark plasma sintering. Nanotechnology 16 (2005) 1569-1573. [31] S.-L. Hwang, I.W. Chen, Nucleation and Growth of β′-SiAlON. Journal of the American Ceramic Society 77 (1994) 1719-1728. [32] J.-F. Yang, Y. Beppu, G.-J. Zhang, T. Ohji, S. Kanzaki, Synthesis and Properties of Porous Single-Phase β′-SiAlON Ceramics. Journal of the American Ceramic Society 85 (2002) 1879-1881. [33] J. Li, J. Wang, H. Chen, B. Sun, J. Jia, Synthesis of beta-SiAlON-AlN-Polytyppoid Ceramics from Aluminium Dross. Materials Transaction 51 (2010) 844-848.

[34] D.H.L. Ng, T.L.Y. Cheung, F.L. Kwong, Y.-F. Li, R. Yang, Fabrication of single crystalline β'-SiAlON nanowires. Materials Letters 62 (2008) 1349-1352. [35] A. Markwitz, G.V. White, Nitridation of Silicon Oxide Layers Studied with Ion Beam Analysis on the Nanometer Scale. Advanced Materials 13 (2001) 1027-1030. [36] E. He, J. Yue, L. Fan, C. Wang, H. Wang, Synthesis of single phase β-SiAlON ceramics by

reaction-bonded sintering using Si and Al2O3 as raw materials. Scripta Materialia 65 (2011) 155-158. [37] F.-W. Chang, T.-H. Liou, F.-M. Tsai, The nitridation kinetics of silicon powder compacts. Thermochimica Acta 354 (2000) 71-80. [38] J. Niu, X. Yi, I. Nakatsugawa, T. Akiyama, Salt-assisted combustion synthesis of β-SiAlON fine powders. Intermetallics 35 (2013) 53-59. [39] X. Yi, J. Niu, T. Nakamura, T. Akiyama, Reaction mechanism for combustion synthesis of β-SiAlON

by using Si, Al, and SiO2 as raw materials. Journal of Alloys and Compounds 561 (2013) 1-4. [40] G. Liu, C. Pereira, K. Chen, H. Zhou, X. Ning, J.M.F. Ferreira, Fabrication of one-dimensional rod-like α-SiAlON powders in large scales by combustion synthesis. Journal of Alloys and Compounds 454 (2008) 476-482. [41] G. Liu, K. Chen, J. Li, Growth mechanism of crystalline SiAlON microtubes prepared by combustion synthesis. CrystEngComm 14 (2012) 5585-5588. [42] J. Niu, K. Harada, I. Nakatsugawa, T. Akiyama, Morphology control of β-SiAlON via salt-assisted combustion synthesis. Ceramics International 40 (2014) 1815-1820. [43] T. Boddington, P.G. Laye, J. Tipping, D. Whalley, Kinetic analysis of temperature profiles of pyrotechnic systems. Combustion and Flame 63 (1986) 359-368. [44] S.D. Dunmead, Z.A. Munir, J.B. Holt, Temperature Profile Analysis in Combustion Synthesis: I, Theory and Background. Journal of the American Ceramic Society 75 (1992) 175-179. [45] S.D. Dunmead, Z.A. Munir, J.B. Holt, Temperature Profile Analysis in Combustion Synthesis: II, Experimental Observations. Journal of the American Ceramic Society 75 (1992) 180-188. [46] J.H. Yang, L.S. Han, Y.X. Chen, G.H. Liu, Z.M. Lin, J.T. Li, Effects of pelletization of reactants and

diluents on the combustion synthesis of Si3N4 powder. Journal of Alloys and Compounds 511 (2012) 81-84. [47] J. Wiley, Sons, Sintering Theory and Practice. New York NY (1996) 225-213.

Chapter 4

Fabrication of mixed α/β-SiAlON powders via salt-assisted

combustion synthesis

4.1 Introduction SiAlON ceramics are widely regarded as promising structural candidate materials for high-temperature engineering applications owing to their exceptional mechanical,

4+ thermal, and chemical properties. The structure of SiAlON is based on Si3N4, with Si and N3− being partially replaced by Al3+ and O2−, respectively. The ceramic typically

v features two types of structures: (i) α, which is of the form Mx Si12−(m+n)Alm+nOnN16−n

(x = m/v; v is the valence of the metal, M; M represents metals like Li, Mg, Ca, Y, and rare earth metals); and (ii) β, which is of the form Si6−zAlzOzN8−z, where z denotes the number of Si–N bonds substituted by the Al–O bonds (0  z  4.2) [1-3].

α-SiAlON has a higher Vickers hardness, and greater oxidation and erosion resistance, but lower strength and toughness than β-SiAlON. These features are considered to be associated with their microstructures, with α-SiAlON generally exhibiting equiaxed grains while β-SiAlON exhibits elongated grains. Mixed

α/β-SiAlON ceramics have been extensively studied, because such materials combine the toughness of β-SiAlON and the wear resistance of α-SiAlON, providing superior mechanical properties in comparison to those of pure α-SiAlON and composites of

α-SiAlON and polytypes [4-6].

Typically, α/β-SiAlON composites or pure α-SiAlON have been prepared by hot-press (HP) [6], pressure-less sintering [7], reaction sintering [8], and carbothermal reduction and nitridation (CRN) [9]. Most of these methods, however, involve the use of expensive raw materials such as high-purity Si3N4 and AlN, which are sintered at

84 temperatures of 1700–1800 °C for several hours in a nitrogen atmosphere. The application of these approaches is limited because of their requirement of expensive raw materials or complicated equipment and processes. To this end, combustion synthesis

(CS) is a proven effective energy- and time-saving process for the synthesis of

SiAlONs [10-14]. It has multiple advantages over the above-mentioned methods, such as the requirement of simple equipment, short reaction times, and high-purity products.

However, the combustion synthesis of SiAlONs with Si, Al, SiO2, or other metal oxides under N2 at a specific applied pressure usually gives a low product yield owing to the existence of unreacted Si, caused by the melting and coalescence of silicon particles at the combustion front. To decrease the combustion temperature as well as to reduce the overall heat released from nitridation of Al and Si, it is necessary to add a large amount of Si3N4 and AlN instead of Si and Al [15, 16]. In addition, NH4Cl or NH4F additives are also required to be added to the starting materials in order to achieve complete nitridation [13, 17].

In chapter 3, single-phase β-SiAlON powders were successfully synthesized by adding a small amount of metal chlorides to the reaction mixture. The chlorides effectively absorbed some of the reaction heat as the latent heat of phase transformation, which promoting the complete nitridation of Si [18, 19]. Therefore, the focus of this chapter is to synthesize mixed α/β-SiAlON and α-SiAlON ceramics with the addition of

NaCl or MgCl2. The effect of salts additives on the phase composition and microstructure of the products was also investigated.

4.2 Experimental procedure

Si (purity >99.9 %, 12 µm), Al (purity >99.9 %, 14 µm), CaCO3 (purity >99.9 %), and Y2O3 (purity >99.9 %) powders were used as starting materials in this study. NaCl

(purity >99.9 %) and MgCl2 (purity >99.9 %) were used as the diluents at varying

v amounts. According to the general chemical formula of Mx Si12−(m+n)Alm+nOnN16−n for

α-SiAlON, two different compositions of α/β-SiAlON of the chemical formulae;

Y0.2Si11.1Al0.9O0.3N15.7 (m = 0.6, n = 0.3) and Ca0.3Si11.1Al0.9O0.3N15.7 (m = 0.6, n = 0.3) were fabricated, as described by the α-SiAlON phase diagram [20, 21]. α-SiAlON ceramics of the compositions of Y0.4Si10.2Al1.8O0.6N15.4 (m = 1.2, n = 0.6) and

Ca0.5Si10.5Al1.5O0.5N15.5 (m = 1, n = 0.5) were also prepared for analysis. The syntheses of Ca-α/β-SiAlON and Ca-α-SiAlON were carried out using CaCO3 and CaO (produced upon calcination of CaCO3 at 1000 °C).

The mixtures were mechanically activated using a planetary ball mill (Gokin

Planetaring Inc., Japan) at a ball-to-sample mass ratio of 10:1. The activated powders were then charged into a porous carbon crucible, and placed in a combustion chamber.

The chamber was filled with N2 (purity: 99.999 %) up to a pressure of 1 MPa after evacuation. A tungsten–rhenium thermocouple protected by a layer of BN was inserted into the center of the powders to record the combustion temperature profiles. The combustion reaction was triggered by passing an electric current (60 A, 10 s) through a carbon foil to ignite the Al powder (ignition agent) that was placed on top of the mixture.

A detailed description of the planetary milling and equipment setup for combustion synthesis is provided in a previous report [11].

Phase composition of the products were analyzed using X-ray diffraction (XRD,

Miniflex, Rigaku, Japan) with CuKα radiation (λ = 1.54056 nm). The microstructures were observed using a scanning electron microscope (FE-SEM, JSM-7400F, JEOL,

Japan). The amounts of α- and β-SiAlON present were estimated by comparing the

86 intensities of the two strongest XRD peaks of the two phases, which are the (210) peaks for both α- and β-SiAlON [22].

4.3 Results and discussion

Fig. 4-1 displays the XRD patterns of combustion synthesized Y-α/β-SiAlON obtained with the addition of different amount of NaCl (mass %). The intensity of the Si peaks gradually decreased as the amount of NaCl added increased. When 12 mass %

NaCl was added, only α- and β-SiAlON were detected, which indicated the complete nitridation of Si. Similar experiments were carried out with varying amounts of MgCl2, as the diluent. An optimal 18 mass% MgCl2 was required to achieve complete nitridation of the Si particles and obtain products consisting of both the α- and

β-SiAlON crystalline phases, as shown in Fig. 4-2. Furthermore, the amount of

α-SiAlON obtained in the product when using MgCl2 was lower than that obtained using NaCl. This indicated that NaCl was more efficient at promoting the formation of

α-phase SiAlON. This result is consistent with results of a previous work, which reported that α-phase Si3N4 content increased upon addition of NaCl for the combustion synthesis of α/β-Si3N4 [23].

The qualitative estimation of the relative amounts of α- and β-SiAlON in the obtained samples was performed using the relative intensities of the (210) reflections, as seen in the X-ray diffraction patterns [22] for both α-SiAlON and β-SiAlON. The relative weight fractions of the α-phase SiAlON, obtained using optimal amount of

NaCl and MgCl2, were 81 and 40 mass %, respectively. It is important to note that the calculated values represent only approximated values as the amorphous phase fractions cannot be determined precisely using this method.

α-SiAlON β-SiAlON Si NaCl 12%

NaCl 10%

Intensity [a. u.]

NaCl 8%

10 20 30 40 50 60 2 theta [degree]

Fig. 4-1 XRD patterns of combustion synthesized Y-α/β-SiAlON with the addition of varied amounts of NaCl (mass %), for which powders of Si, Al,

Y2O3, and NaCl were mixed and a single end ignited.

α-SiAlON β-SiAlON

(b) MgCl2 18%

Intensity [a. u.] (a) NaCl 12%

10 20 30 40 50 60

2 theta [degree] Fig. 4-2 XRD patterns of combustion synthesized Y-α/β-SiAlON powders obtained using optimized amounts of (a) NaCl (12 mass %) and (b)

MgCl2 (18 mass %). The ratio of α-phase SiAlON is 81 mass % in sample (a) and is 40 mass % in sample (b).

SEM images of the combustion synthesized Y-α/β-SiAlON powders are shown in

Fig. 4-3. The α-SiAlON grains obtained using NaCl exhibited equiaxed morphology, although some agglomeration was detected, as is typical of such synthesis. In contrast, for MgCl2, the powder consisted of mainly rod-like grains, considered to be those of

β-SiAlON. The diverse morphologies observed in these products are attributed to the different initial α/β-SiAlON ratios.

Fig. 4-4 shows the typical temperature profiles during combustion synthesis of

Y-α/β-SiAlON, with the addition of either 12 mass % NaCl or 18 mass % MgCl2. The temperature rapidly reached the maximum value after the reaction was triggered. This was due to large amounts of heat release from the nitridation of Al and Si. The maximum measured temperature was higher in the case of NaCl addition (1912 °C) as opposed to MgCl2 addition (1792 °C), even though a slower heating rate was observed for the former. Due to the increased heating rate in the presence of MgCl2, the production of α-Si3N4 was extremely quick, which reduced the time for the formation of

α-SiAlON. The overabundant α-Si3N4 was thus found to transform into β-Si3N4 or

β-SiAlON, leading to an increase in the amount of β-phase SiAlON in the final product [14]. Moreover, the formation of β-SiAlON releases less energy compared to

α-SiAlON [15]; correspondingly, the overall energy released from the exothermic reaction was lower, and exhibited a lower combustion temperature.

Fig. 4-5 shows the XRD patterns of the combustion synthesized Ca-α/β-SiAlON, obtained with different salt additives. For the complete nitridation of Si, the same amounts of NaCl (12 mass %) and MgCl2 (18 mass %) were used. Correspondingly, the product obtained with the addition of NaCl exhibited a higher content of α-phase

SiAlON (95 mass %) than MgCl2 (20 mass %). The corresponding morphologies of the products are shown in Fig. 4-6. These were analogous to what was observed for the

Y-α/β-SiAlON, with the products exhibiting either equiaxed grains of α-SiAlON (for

NaCl) or rod-like and hexagonal β-SiAlON grains (for MgCl2).

(a) (b)

Fig. 4-3 SEM images of combustion synthesized Y-α/β-SiAlON powders using

(a) NaCl (12 mass %) and (b) MgCl2 (18 mass %), in which rod-like products are β-SiAlON. Scale bars provided.

Ignition agent 2000 1912 °C Carbon foil 1792 °C (a) Thermocouple (b) 23mm

1600 mm 65

] C

 Raw [ material

1200 ɸ40 mm

800 Temperature 400

0 0 100 200 300 400 500 600 700 800 Time [s]

Fig. 4-4 Typical temperature profiles recorded during combustion synthesis of Y-α/β-SiAlON with the addition of (a) NaCl (12 mass %) and (b)

MgCl2 (18 mass %), illustrating the dependence on the type and amount of chloride used. (inset) Schematic of the combustion synthesis chamber employed.

α-SiAlON β-SiAlON

(b) MgCl2 18%

Intensity [a. u.] (a) NaCl 12%

10 20 30 40 50 60 2 theta [degree]

Fig. 4-5 XRD patterns of combustion synthesized Ca-α/β-SiAlON powders, obtained using optimized amounts of (a) NaCl (12 mass %) and (b)

MgCl2 (18 mass %), for which powders of Si, Al, CaO were mixed with NaCl or MgCl2 and ignited. Major component of the product was either (a) α- or (b) β-SiAlON.

(a) (b)

Fig. 4-6 SEM images of synthesized Ca-α/β-SiAlON powders using (a) NaCl

(12 mass %) and (b) MgCl2 (18 mass %). Product (a) displays fewer rod-like SiAlON while an increase in density is observed in product (b).

Fig. 4-7 provides the XRD patterns of the combustion synthesized Y0.4-α-SiAlON powders obtained using different amounts of NaCl (mass %). The Si peaks weakened as

NaCl was increased; nevertheless, Si peaks were detected even at 28 mass % NaCl.

However, for more than 28 mass% of NaCl, the combustion reaction could not be completed, which indicated the difficulty in achieving nitridation of Si for the synthesis of Y0.4-α-SiAlON.

Fig. 4-8 shows the XRD patterns of the combustion synthesized Ca0.5-α-SiAlON powders obtained using different amounts of NaCl (in mass %). From these patterns, it is noticeable that residual Si and traces of β-SiAlON existed in all the products. No apparent differences in the XRD spectra and SEM images are observed when CaCO3 was used instead of CaO (see Figs. 4-10 (a) and (b)), although the decomposition of

CaCO3 into CaO and CO2 is expected to absorb a small amount of reaction heat. The

XRD patterns of Ca0.5-α-SiAlON products combustion synthesized with MgCl2 (in mass %), using CaCO3 as the Ca-source, are displayed in Fig. 4-9. Here, the phases detected were α- and β-SiAlON, along with residual Si. Compared with Fig. 4-8, the results demonstrate that the addition of MgCl2 increases the ratio of β-phase SiAlON in the final product; the corresponding rod-like β-SiAlON particles from this reaction are shown in Fig. 4-10 (c). Several reports have hypothesized that the stability of α-SiAlON can be precisely determined by the presence of oxygen, which leads to an increased amount of β-SiAlON [4]. In this work, the addition of MgCl2 to the starting materials may have promoted the increase of oxygen content during pre-treatment milling, as this salt is highly water-absorbent over NaCl.

α-SiAlON Si NaCl 28% NaCl

NaCl 25%

NaCl 20% Intensity [a. u.]

NaCl 10%

10 20 30 40 50 60 2 theta [degree]

Fig. 4-7 XRD patterns of combustion synthesized Y0.4-α-SiAlON powders using different amounts of NaCl (mass %). Note that all products consist of α-SiAlON and unreacted silicon. No peaks corresponding to the β-equivalent are observed.

α-SiAlON β-SiAlON Si NaCl 30% NaCl CaO

NaCl 20%

CaO

NaCl 30%

CaCO3 Intensity [a. u.] NaCl 20%

CaCO3

10 20 30 40 50 60

2 theta [degree]

Fig. 4-8 XRD patterns of the combustion synthesized Ca0.5-α-SiAlON powders obtained using different amounts of NaCl (mass %). With CaCO3 or CaO as the initial Ca-source, no significant differences are observed. All products consist of α-SiAlON with some unreacted silicon.

α-SiAlON β-SiAlON Si

MgCl2 30%

MgCl2 20% Intensity [a. u.]

MgCl2 10%

10 20 30 40 50 60 2 theta [degree]

Fig. 4-9 XRD patterns of combustion synthesized Ca0.5-α-SiAlON powders obtained with the addition of different amounts of MgCl2 (mass %) using

CaCO3 as the Ca source. The major component of the product was β-SiAlON with some unreacted silicon.

(a) (b)

5 μm 5 μm

(c)

10 μm

Fig. 4-10 SEM images of the synthesized Ca0.5-α-SiAlON powders using

30 mass % of various salt additives: (a) NaCl, CaCO3, (b) NaCl, CaO, and (c)

MgCl2, CaCO3. Note that products (a) & (b) are α-SiAlON, but the product (c) exhibits large rod-like β-equivalents. CaCO3 and CaO are the Ca sources in each case.

Based on these results, we concluded that achieving complete nitridation of Si in the presence of these salts for both Y- and Ca-α-SiAlON is difficult. According to the

α-SiAlON formula (MxSi12−(m+n)Alm+nOnN16−n), as m and n increase, the amount of the liquid phase M–Si–Al–O, which can form before nitridation of Si, also increases, thus inhibiting the infiltration of N2. Furthermore, the amount of chloride salt added also increased as m and n increases owing to the high reaction temperature [24]. As a consequence of the increase in m and n, nitridation of Si was greatly hindered, and Si remained in the product as an impurity.

The formation mechanism of α-SiAlON from Si, Al, SiO2, Si3N4, AlN, and metal oxides (MxOy) has been studied extensively [13, 14]. At the early stage of the reaction,

Al first reacts with nitrogen gas to form AlN, and the heat released from this reaction enhances the temperature of the sample. The increased temperature leads to the formation of a liquid phase, M–Si–Al–O. As the temperature rises close to the melting point of Si (1410 °C), Si nitridation occurs, eliciting the formation α-Si3N4. α-SiAlON is then precipitated from the liquid M–Si–Al–O–N mixture, formed from the dissolution of AlN and Si3N4 in the M–Si–Al–O system. When NaCl is used as an inert diluent, the reaction heat is reduced through the latent heat of phase transformation of NaCl. At the preheating zone, NaCl absorbs a portion of the reaction heat through the conversion reaction NaCl (solid) → NaCl (liquid) (ΔH = 82.1 kJ), and thereby reduces the excessive heat released from the reaction Al + 0.5N2 (g) → AlN (ΔH = −318.0 kJ).

Consequently, the melting and agglomeration of the Si particles in the starting materials are greatly reduced, and the infiltration of N2 is enhanced. Then, as the reaction, 3Si +

2N2 (g) →α-Si3N4 (ΔH = −828.9 kJ), proceeds, α-Si3N4 is generated, initiating the release of a large amount of heat. AlN and α-Si3N4 begin to dissolve in the liquid

M–Si–Al–O, and hence a new liquid phase, M–Si–Al–O–N system is formed.

Precipitation of α-SiAlON occurs simultaneously with the formation of this system. At

96 this stage, NaCl absorbs a large amount of heat through the reaction, NaCl (liquid) →

NaCl (gas) (ΔH = 202.1 kJ), and then separates out of the reaction system.

To summarize, the salt additives can be considered heat sinks given that they absorb and remove the excessive heat released upon nitridation of Al and Si in the form latent heat of melting and evaporation, respectively; this assumption ignores other small endo-/exothermic reactions such as Al and Si melting. The reduced heat provides for steady combustion wave propagation, thus improving the conversion rate. The basis for using varying amounts of NaCl and MgCl2 for total nitridation is the different endothermicity of each chloride, which has been detailed in our previous work [19]. The more rapid heating rate with MgCl2 as the diluent can be rationalized as follows: within the combustion front, the salt melts by absorbing the reaction heat to form a wet

MgCl2–AlN–MxOy system surrounding the Si particles through capillary forces. Raising the amount of melted MgCl2 increases the probability of particle–particle contact, and hence, accelerates the Si3N4–AlN–MxOy diffusion reaction that results in SiAlON nuclei formation, prompting a relatively higher heating rate as opposed to when NaCl is used.

The results in this work indicate NaCl or MgCl2 are potential additives for large-scale production of SiAlON powders. Note that the production efficiency can be greatly improved because of the small amount of additive (12 and 18 mass % for NaCl and MgCl2, respectively) based on latent heat and sensible heat, compared with the addition of Si3N4 (approximately 45 mass %) based on only sensible heat. Furthermore, the cost of post-synthesis treatments such as milling will be decreased by adding these salts, which effectively prevented the products from self-sintering. Moreover, the

α/β-SiAlON ratio and morphology of the products can be controlled by changing metal chloride content. Therefore, the overall production cost of SiAlON can be significantly reduced. The lower price and the designed phase of these products may open up new applications for these materials.

4.4 Conclusion High purity Y- and Ca-α/β-SiAlON composite ceramics and α-SiAlON were successfully synthesized by using a salt-assisted combustion method under 1 MPa N2 pressure. The latent heat of the salts permitted the absorption of the heat released from the starting materials, thereby promoting the complete nitridation of Si. The

α/β-SiAlON ratio and morphology of the products were strongly affected by the type of chloride employed: in the presence of NaCl, the amount of α-phase increased, while the amount of β-phase SiAlON increased when MgCl2 was used. Notably, the morphology of the product changed corresponding to the α/β-SiAlON ratio, equiaxed to rod-like grains as the α/β ratio decreased. These findings demonstrate a novel and facile approach for fabricating high-quality α/β-SiAlON ceramics or α-SiAlON via a salt-assisted combustion method.

Y2O3-Containing α-SiAlON-Based Composites. Journal of the American Ceramic Society 79 (1996) 1537-1545.

[5] T.-S. Sheu, Microstructure and Mechanical Properties of the in situ β-Si3N4/α′-SiAlON Composite. Journal of the American Ceramic Society 77 (1994) 2345-2353. [6] F. Ye, L. Liu, H. Zhang, Y. Zhou, Z. Zhang, Novel mixed α/β-SiAlONs with both elongated α and β grains. Scripta Materialia 60 (2009) 471-474. [7] Y.-W. Li, P.-L. Wang, W.-W. Chen, Y.-B. Cheng, D.-S. Yan, Phase formation and microstructural

evolution of Ca α-sialon using different Si3N4 starting powders. Journal of the European Ceramic Society 20 (2000) 1803-1808. [8] J.W.T. van Rutten, H.T. Hintzen, R. Metselaar, Phase formation of Ca-α-sialon by reaction sintering. Journal of the European Ceramic Society 16 (1996) 995-999. [9] J. Huang, H. Zhou, Z. Huang, G. Liu, M. Fang, Y.g. Liu, Preparation and Formation Mechanism of Elongated (Ca,Dy)-α-Sialon Powder via Carbothermal Reduction and Nitridation. Journal of the American Ceramic Society 95 (2012) 1871-1877. [10] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, A new route to synthesize

β-Si6−zAlzOzN8−z powders. Journal of Alloys and Compounds 441 (2007) 236-240. [11] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, Mechanically Activated Combustion

Synthesis of β-Si6−zAlzOzN8−z (z=1–4). Journal of the American Ceramic Society 90 (2007) 626-628. [12] X. Yi, K. Watanabe, T. Akiyama, Fabrication of dense β-SiAlON by a combination of combustion synthesis (CS) and spark plasma sintering (SPS). Intermetallics 18 (2010) 536-541. [13] G. Liu, C. Pereira, K. Chen, H. Zhou, X. Ning, J.M.F. Ferreira, Fabrication of one-dimensional rod-like α-SiAlON powders in large scales by combustion synthesis. Journal of Alloys and Compounds 454 (2008) 476-482. [14] G. Liu, K. Chen, H. Zhou, X. Ning, C. Pereira, J.M.F. Ferreira, Fabrication of yttrium-stabilized α-SiAlON powders with rod-like crystals by combustion synthesis. Journal of Materials Science 41 (2006) 6062-6068.

[15] C.L. Yeh, F.S. Wu, Y.L. Chen, Effects of alpha- and beta-Si3N4 as precursors on combustion synthesis of (alpha plus beta)-SiAlON composites. Journal of Alloys and Compounds 509 (2011) 3985-3990.

[16] C.L. Yeh, K.C. Sheng, Effects of alpha-Si3N4 and AlN addition on formation of alpha-SiAlON by

combustion synthesis. Journal of Alloys and Compounds 509 (2011) 529-534.

[17] G. Liu, K. Chen, H. Zhou, X. Ning, J.M.F. Ferreira, Effect of diluents and NH4F additive on the combustion synthesis of Yb α-SiAlON. Journal of the European Ceramic Society 25 (2005) 3361-3366. [18] J. Niu, X. Yi, I. Nakatsugawa, T. Akiyama, Salt-assisted combustion synthesis of β-SiAlON fine powders. Intermetallics 35 (2013) 53-59. [19] J. Niu, K. Harada, I. Nakatsugawa, T. Akiyama, Morphology control of β-SiAlON via salt-assisted combustion synthesis. Ceramics International 40 (2014) 1815-1820. [20] G.Z. Cao, R. Metselaar, .alpha.'-Sialon ceramics: a review. Chemistry of Materials 3 (1991) 242-252. [21] C.L. Hewett, Y.-B. Cheng, B.C. Muddle, M.B. Trigg, Phase Relationships and Related Microstructural Observations in the Ca-Si-Al-O-N System. Journal of the American Ceramic Society 81 (1998) 1781-1788.

[22] C. P.Gazzara, D.R. Messier, Determination of phase content of Si3N4 by X-Ray diffraction analysis. Ceramic Bulletin 56 (1977) 777-780. [23] H.I. Won, C.W. Won, H.H. Nersisyan, K.S. Yoon, Salt-assisted combustion synthesis of silicon nitride with high [alpha]-phase content. Journal of Alloys and Compounds 496 (2010) 656-659. [24] S. Bandyopadhyay, G. Petzow, Formation of multiphase SiAlON ceramic. Materials Chemistry Physics 61 (1999) 9-13.

100

Chapter 5

Salt-assisted combustion synthesis of β-SiAlON:Eu2+

phosphors for white light-emitting diodes

5.1 Introduction White light-emitting diodes (LEDs) are attractive devices for wide applications due to high light efficiency, low energy consumption, and long service lifetime promised by solid-state lighting. So far, one of the most common approaches to produce white light is to combine a blue LED chip with a green phosphor and a red phosphor. Eu-doped phosphors have a strong absorption in the UV to visible spectral region and exhibit broad emission bands covering the color from blue to red. Recently, the Eu-doped

β-SiAlON green phosphor has been considered as a candidate for application in white

LED due to its high photoluminescence intensity and outstanding thermal and chemical stability [1-4].

Usually, Eu-doped SiAlON phosphors are prepared via a solid-state reaction [5, 6], gas pressure sintering (GPS) [7, 8], or a gas-reduction nitrogen [9, 10] method. These methods involve the use of expensive raw materials such as high-purity Si3N4 and AlN, which are sintered at temperatures of 1900–2000 °C for many hours under nitrogen pressure. Single-phase, fine phosphor powder can be achieved followed by postsynthesis treatment to pulverize crude reaction products. However, these processes have a drawback of having multistep pathways, which are energy- and time-consuming.

These disadvantages significantly limited their widespread use in LED applications.

Thus, there is an urgent need to develop a highly efficient method of producing high-quality phosphors.

Recently, we have applied a combustion synthesis method for the synthesis of

101 ceramics materials [11-13]. This process effectively utilizes the heat generated by a strong exothermic reaction to sustain the reaction; therefore, no excess energy input is required beyond a small amount of electricity to initiate the reaction. This method has many advantages including a high-purity product, low energy investment, and short processing time. It has been reported that Eu-doped β-SiAlON phosphors can be prepared by combustion synthesis. However, expensive Si3N4 and AlN raw materials were still used under a very high nitrogen pressure [14]. It is still a challenge to obtain high-quality phosphors by a low-cost and high-efficiency process. In this chapter, we describe the preparation of Eu-doped β-SiAlON phosphors via a new synthesis route, which uses inexpensive Si, Al, and SiO2 raw materials and only a small amount of NaCl as the diluent under a relatively low nitrogen pressure. NaCl is an efficient diluent that decreases the heat generated by the reaction, ensures the production of a pure single-phase product, and offers fine powders with uniform particles [15].

102

5.2 Experimental procedure

Si6−zAlzOzN8−z:Eux (z = 0.25–0.75, x = 0.02) were prepared from Si (99.9% purity),

Al (99.9% purity), SiO2 (99.9% purity), and Eu2O3 (99.9% purity) with different amounts of NaCl (99.9% purity) as the diluent. To obtain phosphors with varying z values, the starting materials were mixed and mechanically activated via planetary ball milling with different amounts of NaCl. The activated mixture was charged into a cylindrical carbon crucible with vents, through which nitrogen gas was introduced. The combustion reaction was carried out at a nitrogen (purity: 99.999%) pressure of 1 MPa by passing a current through a carbon foil. The details of the planetary ball milling and equipment for the combustion synthesis are described elsewhere [11]. After the reaction, the products were washed with distilled water to remove any remaining NaCl and then dried at 110 °C.

Powder X-ray diffraction (XRD, Rigaku, Cu Kα), field-emission scanning electron microscopy (FE-SEM, JSM-7400F, JEOL), transmission electron microscopy (TEM,

JEM-2010F), and selected area electron diffraction (SAED) patterns were used to determine the phase composition, microscopic morphology, and crystal structure. The photoluminescence (PL) properties were measured using a fluorescence spectrometer

(FP-6400, JASCO) at room temperature with an excitation wavelength of 405 nm.

Temperature-dependent PL was measured using a multichannel spectrometer (Maya

2000 pro, Ocean Optics) in the range of 25–260 °C with a 1 mw LED as the excitation source.

103

5.3 Results and discussion 2+ Fig. 5-1 shows XRD patterns of the β-Si6-zAlzOzN8-z:Eu (z = 0.25–0.75) powders that were synthesized with NaCl as the diluent. The prepared powders consist of a single β-SiAlON crystalline phase and are free of secondary phases at all z values, indicating that NaCl is an efficient diluent for obtaining high-quality products. During combustion, NaCl absorbs the heat released by the two major exothermic reactions through its latent heat of melting and vaporization [15]. The first endothermic process takes place between 600–1200 °C; NaCl absorbs the heat released from this process, as shown by the following reactions:

Al + 0.5N2(g)→ AlN + Q1, (5.1)

2Al + 1.5SiO2→ Al2O3 + 1.5Si + Q2, (5.2)

NaCl → NaCl(l) – Q3. (5.3)

The second major endothermic process occurs at about 1350 °C, which is attributed to the nitridation of Si and the formation of β-SiAlON synchronously [16].

NaCl absorbs a significant amount of heat generated from this process and is completely vaporized, further reducing the reaction temperature, which prevents the melting and agglomeration of Si particles in the raw materials that enhanced the infiltration of N2.

The reactions can be presented as follows:

3Si + 2N2(g) → Si3N4 + Q4, (5.4)

2+ Si3N4 + AlN + Al2O3 + Eu2O3 → β-SiAlON:Eu , (5.5)

NaCl(l)→NaCl(g) – Q5. (5.6)

With the proper combustion temperature, no Si remained in the product. In the case of adding greater amounts of NaCl, the combustion reaction could not be maintained due to the insufficient heat released from the raw materials.

104

β-SiAlON z = 0.25

z = 0.35

z = 0.5 Intensity [a. u.]

z = 0.75

10 20 30 40 50 60 2 theta [degree]

Fig. 5-1 XRD patterns of combustion-synthesized β-Si6-zAlzOzN8-z:Eu0.02 powders with z values of 0.25, 0.35, 0.50, and 0.75 obtained under 1 MPa of nitrogen using NaCl as a diluent (1 mass% for z values of 0.25 and 0.35, and 3 and 5 mass% for z values of 0.50 and 0.75, respectively ).

(a) (b)

2μm 2μm (c) (d)

2μm 2μm

Fig. 5-2 SEM images of combustion-synthesized β-Si6-zAlzOzN8-z:Eu0.02: (a) z = 0.25, (b) z = 0.35, (c) z = 0.50, and (d) z = 0.75 obtained under 1 MPa of nitrogen using NaCl as a diluent (1 mass% for z values of 0.25 and 0.35, and 3 and 5 mass% for z values of 0.50 and 0.75, respectively).

105

SEM images of the synthesized β-SiAlON:Eu2+ powders with different z values are presented in Fig. 5-2. All the products show a uniform rodlike shape that changes gradually with changing z values. At z values of 0.25 and 0.35, the powders are rodlike and agglomerated, at z = 0.5, the powders are relatively uniform and have well-dispersed rodlike shapes with lengths of ~2 µm and diameters of 1 µm, and when z increases to 0.75, the rodlike shapes become shorter with larger diameters. These changes are thought to be associated with the composition of each sample and amount of NaCl added. Fig. 5-3 (a) shows a typical TEM image of a β-SiAlON particle (z =

0.5); the inset shows an SAED pattern of this particle. These results confirm that a single crystal of β-SiAlON is grown along the [0001] direction. Furthermore, the

HR-TEM image shown in Fig. 5-3 (b) reveals a lattice fringe of approximately 0.29 nm, which corresponds to the [0001] plane of β-SiAlON. This result is in good agreement with the refined value.

(a) (b) 0001 1121

1120

[0001]

Fig. 5-3 (a) TEM image of a particle of combustion-synthesized

β-Si6-zAlzOzN8-z:Eu0.02 (z = 0.5) and its corresponding SAED pattern. (b) High-resolution TEM image.

106

Fig. 5-4 shows PL emission spectra of the prepared β-SiAlON:Eu2+ phosphors with different z values under excitation at 405 nm at room temperature. The phosphors exhibit an intense green emission that consists of a single broad band, which is attributed to the 4f65d → 4f7 transition of Eu2+. No line emission peaks of Eu3+

(580–650 nm) are observed, indicating that Eu ions dissolved in the β-SiAlON lattice in nitrogen atmosphere even in the presence of NaCl (diluent) in this new synthesis process. The broad band with a full width at half maximum (FWHM) of ∼55 nm observed in β-SiAlON:Eu2+ phosphors is quite smaller than that of cerium-doped

3+ 2+ yttrium aluminum garnet (YAG: Ce , ~120 nm [17] ) or SrSi2O2N2:Eu (~82 nm [18] ).

This indicates higher color purity of the synthesized β-SiAlON:Eu2+ phosphors compared with other phosphor materials. With increasing z value, the emission intensity of the powders increases and the emission bands undergo a redshift to a longer wavelength. The emission peaks of the powders range from 526 (z value of 0.25) to

537 nm (z value of 0.75). The redshift is caused by lattice expansion, which occurs due to the replacement of Si–N (1.74 Å) bonds with longer Al–O (1.75 Å) and

Al–N (1.87 Å) bonds. Accordingly, the Stokes shift increases; hence, the emission bands redshift with increasing z value. In addition, gradual broadening of bandwidths is observed when the z value increases, which is ascribed to the changes in the crystal field around the Eu2+ ions. This is also caused by the lattice expansion.

Previous studies showed that the PL emission intensity of the powders decreases with increasing z value and was highest at z = 0.20 [14, 19]. The compositional dependence of PL is mainly attributed to the phase purity, crystallinity, and particle morphology of the powders. In this experiment, single-phase β-SiAlON:Eu2+ powders were obtained at all z values using NaCl as the diluent. However, at the smaller z values of 0.25 and 0.35, the particles did not crystallize well because some crystal grains agglomerated, as shown in Fig. 5-2, which causes strong light scattering, thereby

107 reducing the luminescence intensity. In contrast, at higher z values, the grains crystallize better with less agglomeration, which leads to higher PL emission intensity. In this salt-assisted synthetic method, NaCl plays a key role in determining the morphologies of the products. During the synthesis, melted NaCl can be regarded as a protective cell that prevents agglomeration of the products. For smaller z values with only a small amount of NaCl added, the particles of the products are not well isolated and therefore agglomerate.

2.0 Ex = 405 nm 1.8 z = 0.25 1.6 z = 0.35 z = 0.5 1.4 z = 0.75 1.2

1.0 0.8

0.6 PL Intensity [a. u.] 0.4 0.2 0.0 350 400 450 500 550 600 650 700 750 Wavelength [nm]

Fig. 5-4 Room-temperature emission spectra of the prepared

β-Si6-zAlzOzNz:Eu0.02 with z = 0.25, 0.35, 0.50, and 0.75 obtained at nitrogen pressure of 1 MPa using NaCl as diluent.

108

Fig. 5-5 (a) shows the temperature dependence of the emission spectrum of the prepared β-SiAlON:Eu2+ (z = 0.5) phosphor from 25 to 260 °C. For comparison, the inset of Fig. 5-5 (a) shows the PL emission spectra of a commercial silicate green

2+ phosphor (Sr2SiO4:Eu ) in the same temperature range. The emission intensity of the

β-SiAlON:Eu2+ phosphor decreases very slowly as the temperature increases. Fig. 5-5

(b) shows the gradual decrease of the normalized PL emission intensity of the prepared

β-SiAlON:Eu2+ (z = 0.5) phosphor and silicate green phosphor with increasing temperature [see Fig. 5-5 (a)]. At 160 °C, the PL emission intensity of the prepared

Eu2+-doped β-SiAlON phosphor is 84% of that measured at room temperature, which

2+ is ~30% higher than that of the silicate green phosphors (Sr2SiO4:Eu ). When the temperature was increased to 260 °C, the remaining PL emission intensity of the silicate green phosphors was only about 10%, which is far less than the 73% that remained for the prepared Eu2+-doped β-SiAlON phosphor. This minimal thermal quenching of the

PL emission efficiency is attributed to the rigid crystal structure of the host lattice that comprises networks of linked (Si, Al)(O, N)4. For practical white or high-power LED applications, the phosphors must sustain emission efficiency at temperatures of ~150 °C over a long period. At 160 °C, the prepared Eu2+-doped β-SiAlON phosphor maintains

84% of the intensity measured at room temperature. This value is comparable to those of Eu2+-doped β-SiAlON phosphors prepared via a gas-pressured solid-state reaction, which was about 85% of the intensity measured at room temperature [1, 6].

109

(a)

25C 60C 100C

140C 180C

220C Intensity [a. u.] 260C

450 500 550 600 650 700

Wavelength [nm] Intensity [a. u.]

450 500 550 600 650 700 Wavelength [nm]

(b)

1.0

0.8

0.6

0.4 -SiAlON:Eu2+

0.2 2+

Silicate (Sr2SiO4:Eu ) Normalized PL-Int [a. u.]

0.0 0 50 100 150 200 250 300

Temperature [C]

Fig. 5-5 (a) PL emission spectra of the prepared β-Si5.5Al0.5O0.5N7.5:Eu0.02 phosphors measured at increasing temperature from 25 to 260 ºC. For comparison, the PL emission spectra of a commercial silicate green phosphor 2+ (Sr2SiO4:Eu ) measured in the same temperature range are shown in the inset. (b) Temperature dependences of the relative PL peak intensities of the prepared β-Si5.5Al0.5O0.5N7.5:Eu0.02 phosphor and a commercial silicate green 2+ phosphor (Sr2SiO4:Eu ).

110

5.4 Conclusion

We report a simple, and highly efficient method of synthesizing Si6−zAlzOzN8−z:Eux

(z = 0.25–0.5, x = 0.02) phosphors. The new combustion synthesis method using low-cost raw materials with NaCl as a diluent offers many advantages such as the production of uniform, single-phase, rodlike particles and minimal thermal quenching of PL emission efficiency for white or high-power LEDs.

111

References [1] R.-J. Xie, N. Hirosaki, H.-L. Li, Y.Q. Li, M. Mitomo, Synthesis and Photoluminescence Properties of 2+ 2+ β-sialon:Eu (Si6−zAlzOzN8−z:Eu ): A Promising Green Oxynitride Phosphor for White Light-Emitting Diodes. Journal of The Electrochemical Society 154 (2007) J314-J319. [2] R.J. Xie, N. Hirosaki, T. Takeda, Wide Color Gamut Backlight for Liquid Crystal Displays Using Three-Band Phosphor-Converted White Light-Emitting Diodes. Applied Physics Express 2 (2009). [3] Y. Fukuda, A. Okada, A.K. Albessard, Luminescence Properties of Eu2+-Doped Red-Emitting Sr-Containing Sialon Phosphor. Applied Physics Express 5 (2012). [4] T. Suehiro, N. Hirosaki, R.-J. Xie, K. Sakuma, M. Mitomo, M. Ibukiyama, S. Yamada, One-step preparation of Ca-alpha-SiAlON:Eu2+ fine powder phosphors for white light-emitting diodes. Applied Physics Letters 92 (2008) 191904. [5] N. Hirosaki, R.-J. Xie, K. Kimoto, T. Sekiguchi, Y. Yamamoto, T. Suehiro, M. Mitomo, Characterization and properties of green-emitting beta-SiAlON:Eu2+ powder phosphors for white light-emitting diodes. Applied Physics Letters 86 (2005) 211905-211903.

[6] J. Ryu, H. Won, Y.-G. Park, S. Kim, W. Song, H. Suzuki, C. Yoon, Synthesis of EuxSi6−zAlz OzN8−z green phosphor and its luminescent properties. Applied Physics A 95 (2009) 747-752. [7] R.-J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, M. Mitomo, Eu2+-doped Ca-alpha-SiAlON: A yellow phosphor for white light-emitting diodes. Applied Physics Letters 84 (2004) 5404-5406. [8] Y.-Q. Li, N. Hirosaki, R.-J. Xie, J. Li, T. Takeda, Y. Yamamoto, M. Mitomo, Structural and Photoluminescence Properties of Ce3+- and Tb3+-Activated Lu-α-Sialon. Journal of the American Ceramic Society 92 (2009) 2738-2744. [9] H.-L. Li, R.-J. Xie, N. Hirosaki, T. Suehiro, Y. Yajima, Phase Purity and Luminescence Properties of Fine Ca-α-SiAlON:Eu Phosphors Synthesized by Gas Reduction Nitridation Method. Journal of The Electrochemical Society 155 (2008) J175-J179. [10] T. Suehiro, N. Hirosaki, R.-J. Xie, M. Mitomo, Powder Synthesis of Ca-α-SiAlON as a Host Material for Phosphors. Chemistry of Materials 17 (2004) 308-314. [11] K. Aoyagi, T. Hiraki, R. Sivakumar, T. Watanabe, T. Akiyama, Mechanically Activated Combustion

Synthesis of β-Si6−zAlzOzN8−z (z=1–4). Journal of the American Ceramic Society 90 (2007) 626-628.

[12] A. Kikuchi, D. Tran, S. Lin, N. Okinaka, T. Akiyama, Novel Combustion Route for SrTiO3 Powders. Applied Physics Express 5 (2012). [13] X. Yi, K. Watanabe, T. Akiyama, Fabrication of dense β-SiAlON by a combination of combustion synthesis (CS) and spark plasma sintering (SPS). Intermetallics 18 (2010) 536-541. [14] Y. Zhou, Y.-i. Yoshizawa, K. Hirao, Z. Lenčéš, P. Šajgalík, Preparation of Eu-Doped β-SiAlON Phosphors by Combustion Synthesis. Journal of the American Ceramic Society 91 (2008) 3082-3085. [15] J. Niu, X. Yi, I. Nakatsugawa, T. Akiyama, Salt-assisted combustion synthesis of β-SiAlON fine powders. Intermetallics 35 (2013) 53-59.

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[16] X. Yi, J. Niu, T. Nakamura, T. Akiyama, Reaction mechanism for combustion synthesis of β-SiAlON

by using Si, Al, and SiO2 as raw materials. Journal of Alloys and Compounds 561 (2013) 1-4. [17] Y.S. Lin, R.S. Liu, B.-M. Cheng, Investigation of the Luminescent Properties of Tb3+-Substituted YAG:Ce, Gd Phosphors. Journal of The Electrochemical Society 152 (2005) J41-J45. [18] Y.Q. Li, A.C.A. Delsing, G. de With, H.T. Hintzen, Luminescence Properties of Eu2+-Activated

Alkaline-Earth Silicon-Oxynitride MSi2O2-δN2+2/3δ (M = Ca, Sr, Ba): A Promising Class of Novel LED Conversion Phosphors. Chemistry of Materials 17 (2005) 3242-3248. [19] J. Ho Ryu, Y.-G. Park, H. Sik Won, S. Hyun Kim, H. Suzuki, C. Yoon, Luminescence properties of 2+ Eu -doped β-Si6-zAlzOzN8-z microcrystals fabricated by gas pressured reaction. Journal of Crystal Growth 311 (2009) 878-882.

113

Chapter 6

General conclusions

SiAlON ceramics are widely regarded as promising structural candidate materials for high-temperature engineering applications owing to their exceptional mechanical, thermal, and chemical properties. Combustion synthesis (CS) has been proved an effective energy- and time-saving process for the synthesis of SiAlON. However, the combustion synthesis of SiAlON with Si, Al, and SiO2 under N2 pressure usually gives a low product yield owing to the existence of unreacted Si, caused by the melting and coalescence of silicon particles at the combustion front. To decrease the combustion temperature as well as to reduce the reaction heat, a large amount of diluents such as

SiAlON or Si3N4 are added to the starting materials. The obstacle to the wide use of

SiAlON materials is the high cost for their production and post-synthesis treatment, i.e., for obtaining a product with a fine grain size. Thus, there is an urgent need to develop an alternative diluent with low cost and high efficiency for producing high-quality products. In this thesis, we prepared high-purity SiAlON powders by using two new routes: kaolin addition and salt-assisted combustion synthesis method using Si, Al, and

SiO2 under a low N2 pressure. The application of salt-assisted combustion synthesis in fabrication of SiAlON phosphors was also investigated.

Chapter 1 is the general introduction and a description of the purpose of this thesis.

In chapter 2, we described the synthesis of β-SiAlON powders using natural kaolin,

Si, and Al powders. Pure, single-phase β-SiAlON powders were synthesized when kaolin was used without first being dehydrated. The water contained by kaolin lowers the adiabatic temperature by 350 °C. The proposed method is simple and cost-efficient comparing with the conventional combustion synthesis, it is expected that this method would contribute to the wider industrial application of β-SiAlON.

114

In chapter 3, we produced high-purity β-SiAlONs (z = 0.25–3) powders via salt-assisted combustion synthesis with the addition of NaCl. NaCl served as efficient diluents in combustion synthesis process by absorbing the excessive amounts of heat generated by the exothermic reaction, but also as a diffusion barrier between β-SiAlON particles, which greatly limited the growth of β-SiAlON crystals. Furthermore, as the z-values increased, the amount of NaCl needed to complete the reaction increased, which in turn decreased the particle size of the product. The effect of metal chlorides on the combustion synthesis of β-SiAlON was investigated with the addition of KCl,

MgCl2, and CaCl2 to the raw materials, respectively. Single-phase products containing crystals with different shapes were obtained by adding different type of metal chlorides.

Their melting and vaporization absorb the excessive reaction enthalpy corresponding to the two-stage exothermic reaction including nitridation of Al and Si. The comparison between SiAlON diluents and NaCl additives on combustion synthesis was investigated.

Different additives give rise to different reaction rate and degree of conversion, and the morphology of the products.

In chapter 4, high purity α/β-SiAlON composite ceramics and α-SiAlON were successfully synthesized by using a salt-assisted combustion method. The α/β-SiAlON ratio and morphology of the product were strongly affected by the type of chloride employed: in the presence of NaCl, the dominant phase is α-SiAlON, while the β-phase

SiAlON is the major product when MgCl2 was used. The morphology of the product changes corresponding to the α/β-SiAlON ratio, equiaxed to rod-like grains as the α/β ratio decreased.

In chapter 5, we applied the salt-assisted combustion synthesis method to the synthesis of β-SiAlON:Eu2+ phosphors. The synthesized β-SiAlON:Eu2+ powders, which comprise rodlike crystals, exhibit high thermal stability minimal thermal quenching of PL emission efficiency for white or high-power LEDs.

115

Acknowledgements

This research had been carried out from 2010 to 2014 under the supervision of Dr.

T. Akiyama, Professor of the Center for Advanced Research of Energy and Materials,

Hokkaido University, Sapporo, Japan. I would like to express my deepest thanks and respect to his constant guidance, support encouragements and patience throughout this work. I also would like to express my sincere gratitude to the members of examining committee: Prof. K. Matsuura and Prof. S. Miura. Their adjustment, comments and advices are quite useful on improving the dissertation.

I appreciate Chinese government and the Ministry of Education, Culture, Sports,

Science and Technology (MEXT) of Japanese government for providing the chance and scholarship for my study in Hokkaido University, Japan.

I would like to express my great thanks to Combustion Synthesis Co., Ltd., Japan, for providing Si and Al raw materials and the use of experimental apparatus; the support and corporation during the reaserch offered by Dr. I. Nakatsugawa and Mr. K. Harada are greatly appreciated. This research was partly supported by a Grant-in-Aid for

Scientific Research on Priority Areas (B) (24360313, Salt-assisted Combustion

Synthesis of SiAlON nano-powders) and by the 2013 Strategic Foundational

Technology Improvement Support Operation, Ministry of Economy, Trade and Industry

(No. 24110105008, Development of high-quality and economical SiAlON powders by combustion synthesis method).

I appreciate the great help on my research work and daily life to Associate Prof. N.

Okinaka, Secretary M. Watanabe, Dr. C. Zhu, Dr. T. Nomura, Dr. X. Yi, Dr. N. Yasuda,

Dr. G. Saitou, and all members of Akiyama Lab.

Finally, I would like to express my sincere thanks to my family for their encouragement and understanding.

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