MULLITE GLASS CERAMICS DERIVED FROM SILLIMANITE … · 24 K. V. Kannan Nithin, N. Suriyanarayanan, Dr Antonella Sola. c = 2.884 Å. It consists of edge shared AlO. 6. octahedral chains

Journal of Non-Oxide Glasses Vol. 2, No 1, 2010, p. 23 - 34

MULLITE GLASS CERAMICS DERIVED FROM SILLIMANITE THROUGH HOLLOW CATHODE PLASMA(HCP) AND TRANSFERRED ARC PLASMA

(TAP) TECHNIQUES

K. V. KANNAN NITHIN, N. SURIYANARAYANANa, DR ANTONELLA SOLAb

Department of Physics, Kathir College of Engineering, Coimbatore 641 062, India aDepartment of Physics, Govt. College of Technology, Coimbatore 641 013, India

bDepartment of Materials and Environmental Engineering, Università di Modena e Reggio Emilia, Italy

The production of low cost Mullite glass ceramics from silimanite is demonstrated through Hollow Cathode Plasma and Transferred Arc Plasma Processes. The XRD and SEM patterns show that the single phase mullite obtained from TAP and HCP processes are crystalline. The TAP process requires low power input than HCP process. Both processes are superior compared to conventional sintering methods since the processing time and cost of production are found to be lower. (Received December 28, 2009; accepted February 19, 2010) Keywords: Mullite, Glass ceramics, Transferred arc plasma, Hollow cathode plasma, Sillimanite, Alumina,Chemical resistance

1. Introduction Mullite is a stable intermediate compound (3Al2O3:2SiO2) formed with Al2O3- SiO2

system. Mullite is the only aluminosilicate which is stable at high temperatures. The Al2O3: SiO2 molar ratio in mullite may vary from 1.5 to 2. A lot of efforts have been made to prepare mullite ceramics using various starting materials such as industrial grade and chemical methods, all of which follow difference mullitization routes on heating. The path ways can be classified into two types.1.Mullite formation above 12000C through Al2O3 precursor. Mullite is observed in the mixture of Kaolinite and alumina sol mixtures and diphasic gels. 2. Direct formation of Aluminum (Al) rich mullite at about 900 – 10000C [1] is observed in monophonic gels and glasses.

The occurrence of different mullitization routes is due to the different degrees of mixing of Al2O3 and SiO2 with grains and on a molecular scale. In all these cases, mullitization occurs by nucleation growth mechanisms. The relatively low bending strength of mullite has led materials scientists to search for ways of improving its mechanical properties and reducing its synthesis and sintering temperatures. Some headway has been made on the latter problem by using ultrafine alumina and silica powders [2].

Small amounts of Yttria uniformly distributed over the grain surfaces of mullite ceramics enhance the strength of the material. Larger Yttria additions improve the sinterability of mullite but reduce the strength of the resulting ceramics, because of the formation of intergranular Y2 Si O5 [3].

The formation of mullite is enhanced by the addition of CeO2 to the Zr SiO4 – Al2 O3 powder compacts. The addition of CeO2 causes the formation of mullite directly from the reaction of Zr Si O4 with Al2 O3 and suppresses the decomposition of Zr Si O4 into Zr O2 and Si O2 [4].

Mullite glass ceramics possesses low thermal expansion, good high temperature strength, creep resistance and excellent stability which are suitable for high temperature applications [5]. Mullite has an orthorhombic structure with lattice constant a= 7.545 Å, b = 7.689 Å and

24 K. V. Kannan Nithin, N. Suriyanarayanan, Dr Antonella Sola

c = 2.884 Å. It consists of edge shared AlO6 octahedral chains aligned in the c – direction and cross shared (Si, Al) O4 tetrahedra.

The sintering of mullite – corundum ceramics of various compositions has been studied in detail [6, 7, 8]. Impurities added to the original materials during the manufacture of mullite – corundum ceramics improve the sintering but reduce its high temperature strength and thermal shock resistance [7].

The hot – pressing method makes it possible to obtain mullite glass ceramics without the impurities which degrade its high – temperature properties [9, 10, 11, 12]. Mullite fibers are synthesized from an aqueous solution of aluminumisopropoxide, aluminumnitrate and tetraethylorthosilicate by the Sol - Gel method [13].

The relationships between the solid – phase reactions and the grain – size compositions of the reactants using the formation of forsterite from magnesia and silica are studied [14].The same relationships are established during the synthesis of mullite from alumina and silica [15, 16]. Mullite is also synthesized by the action of high dynamic pressures generated by explosions [17].

Mullite Glass-ceramics are the polycrystalline materials with fine-grains, which can be formed when suitable glass compositions are heat treated and undergo controlled crystallization to a lower energy crystalline state. Glass-ceramics can be applied to kitchen wares, machinable ceramics, biological and optical materials, further it can be used as high-grade building materials due to their high flexural strength, wear resistant, and attractive visage [18, 19].Various wastes are used to produce glass-ceramics including coal ash [20, 21], incinerator fly ash [22, 23, 24, 25, 26, 27, 28, 29], blast furnace slag [30], and zinc- hydrometallurgy wastes [31, 32].

The present investigation is undertaken to synthesize mullite through sillimanite sand by thermal plasma processing. In this investigation, mullite is prepared using Hollow Cathode Plasma (HCP) processing torch and Transferred Arc Plasma (TAP) processing torch. Air is used as plasma forming gas in order to reduce the cost of the process. The phase formation, microstructure, chemical compositions of the processed samples are analyzed using XRD, SEM and EDAX respectively.

Sillimanite beach sand is a byproduct generated during the extraction of rare earth compounds from beach sand minerals. The estimated reserve of sillimanite beach sand in India is about 54 million tones [33]. Numerous attempts are made by many investigators to utilize this mineral [34, 35, 36, 37].

2. Methods and materials (Experimental) Two types of sillimanite sands are available from the Indian Rare Earths Limited,

OSCOM (O) variety, which is yellowish due to the presence of ferrogeneous coating on them, and QUILON (Q) containing some amounts of zircon. In this study, Q grade sillimanite sand is processed to form mullite through HCP and TAP torches. The operating parameters of the HCP and TAP torches are listed in table 1.

Table 1. Operating parameters for HCP and TAP torches.

OPERATING

PARAMETERS HCP TORCH TAP TORCH

Input power 20 and 25 KW 15,20 and 25 kW Plasma Gas and flow rate

Air; 12 lpm Air; 10 lpm

Cooling Air flow rate 5 minutes 5 minutes

Processing time 5 minutes 5 minutes

Cooling Time 10 minutes 10 minutes

Cooling Medium OpenAir(rapid cooling) OpenAir(rapid cooling)

Powder quantity (sillimanite)

100 grams 100 grams

Mullite glass ceramics derived from sillimanite through hollow cathode plasma(HCP)... 25

The formation of phase, microstructure and chemical compositions of raw, HCP and TAP processed samples are analyzed by X-Ray Diffractometer (XRD - PW Philips with Cu-Kα radiation), Scanning Electron Microscope SEM-Philips XL40 coupled with energy dispersion spectroscopy EDAX 9900 respectively. All the peak intensities of XRD are compared with XRD data files (JCPDS cards) for the identification of elements and compounds. Particle size of the sample is measured with laser beam diffraction with particle dispersion using the Malvern 2000 particle size analyzer.

3 Results and discussion 3.1 Chemical compositions EDAX, XRD, and PARTICLE SIZE of raw silimanite Chemical composition of raw silimanite is given in table 2. The XRD pattern, SEM image

EDAX spectrum and the particle size distribution plot of the raw sillimanite sand (Q) are shown in figures 3a, 3b, 3c and 3d respectively. The results show that the raw sillimanite sand is crystalline with irregular surface. The particle size distribution result depicts that the average particle size is 270 - 400 micrometers. The EDAX spectrum also confirms the presence of major compounds Al2O3, SiO2, and small quantities of other elements and compounds as mentioned in the table 2.

Table 2 Chemical compositions of raw sillimanite

Oxides in sillimanite sand

Oxides Al2O3 SiO2 TiO2 Fe2O3 P2O5 ZrO2 Na2O+K2O CaO Wt. % 59.30 37.10 0.40 0.50 0.05 1.50 0.50 0.60

20 30 40 50 60

Inte

nsity

(a.u

.)

2θ

s s s

ss

s

s

ss s

s

s

ss - Sillimanite

Fig. 3a XRD pattern of raw sillimanite powder (JCPDS Card # 22 – 18)


Fig. 3b SEM pattern of raw sillimanite powder

Figure 3c EDX spectrum of raw sillimanite powder

Fig. 3d Particle size distribution of raw sillimanite powder


4. Mullite glass ceramic through Hollow Cathode Plasma process. The HCP and TAP processes are better than conventional sintering and reaction sintering

methods because mullite produced by the HCP and TAP processes requires very short period of time and also cost effective when compared to conventional sintering methods. There are various conventional technologies including energy saving, clean energy, waste management and material recycling. Ceramics technology is one of the promising technologies to establish the practical environmental technology for better global environment.[38 ]. HCP and TAP processes are such a widely used technologies for melting and vetrification of hazardous wastes due to their high temperatures and simplicity of their generation and control. The HCP and TAP are assumed to be stable, steady, axi-symmetric, optically thin and in a local thermodynamic equilibrium in an atmospheric pressure environment [39, 40]. The HCP and TAP processes are characterized by extremely high temperatures (up to 20,000 - 30,000 OK), excellent arc stability and low environmental impact (low oxides emissions, low % of ultra fine powders).High density allows high production rate with evident time savings [41,42,43,44 ].

The schematic diagram of HCP torch is as shown in figure 4a and its typical operating parameters for sillimanite through HCP are given in table 1. The formation of phase ,microstructure and chemical compositions of HCP processed samples are analyzed by XRD , SEM and EDAX respectively.

The torch consists of graphite anode nozzle with 20 mm inner diameter and 100 mm length. The hollow cathode is made of graphite with 15 mm outer diameter and 70 mm length. Both the electrodes are insulated and partially water cooled. Vortex gas flow is established by a gas passing through the holes drilled in the insulator parallel to the walls to stabilize the plasma jet, while the centre hole with 5mm diameter in the cathode acts as powder injection port. Air is used as plasma forming gas.The melted sample is rapidly cooled for 10 minutes in openair.

Fig. 4a Schematic diagram of HCP torch

4.1 XRD, SEM and EDAX of Silimanite prepared through HCP process. Figures 4b and 4c show the XRD and SEM images of HCP torch processed sillimanite at

20 kW torch power with 5 minutes processing time. Generally sillimanite sand on heating in the temperature range from 1300 to 1700ºC decomposes to mullite [45, 46, 47, 48, 49]. Decomposition temperature depends on the particle size and impurity level of the raw materials [48].

3 (Al2O3.SiO2) → 3Al2 O3.2SiO2 + SiO2 (1)

The Fig.4b shows that the 20 kW torch power and processing time (5 min) is not sufficient for mullite formation however, the surface morphology and the structural orientation of sillimanite


are changed as shown in the Fig.4c due to the in-flight reaction with air plasma. Figures 4d and 4e and 4f show the XRD, SEM and EDAX spectrum of 25 kW HCP torch processed sillimanite. In this case, the complete phase change is occurred from sillimanite to mullite during the process. The XRD pattern depicts that the mullite formed is found to be crystalline. SEM reveals that the surface of the processed powders is found to be elongated bars due to the high input power and high temperature of the plasma arc [48]. The EDAX spectrum also confirms the major presence of Al2O3 ,SiO2, and very small amounts of Oxides, Ca due to a complete mullite transformation. Hence 25kW power is found to be the optimum plasma input power for a complete mullite transformation with 5 minutes processing time.

The thermal energy of the plasma arc at 25 kW torch power removes only a very small amount of silica from sillimanite through evaporation. Hence the complete mullite phase is easily formed due to the reaction of Al2O3 with residual silica in silimanite. The mullite diffraction peaks obtained in the XRD pattern is in good agreement with other reported works [49]

20 30 40 50 60

s - Sillimanite

sss sss s

sss

sss

ss

s

Inte

nsity

(a.u

.)

2θ

Fig. 4b XRD pattern of 5 minutes HCP torch processed sillimanite at 20 kW (JCPDS Card # 22 – 18).

Fig. 4c SEM image of 5 minutes HCP torch processed sillimanite at 20 kW


20 30 40 50 60

Inte

nsity

(a.u

.)

2θ

m - Mullite

mmm m

m

mm

m

mm

m

m

m

m

m

m

m

Fig. 4d XRD pattern of 5 minutes HCP torch processed sillimanite at 25 kW (JCPDS Card # 15 – 776).

Fig. 4e SEM image of 5 minutes HCP torch processed sillimanite at 25 kW

Fig. 4f. EDX spectrum of 5 minutes HCP torch processed silimanite at 25kW


5. Mullite glass ceramic prepared though Transferred Arc Plasma Process. The schematic diagram of TAP torch is as shown in figure 5a and the typical operating

parameters for sillimanite through TAP are given in table 1. The formation of phase and microstructure of TAP processed samples are analyzed by XRD and SEM respectively. The torch consists of graphite bowl with 100 mm depth and 50 mm inner diameter which serves as an anode and the melting bed. The cathode is made of graphite rod 250 mm long and 50 mm diameter. Its end tapers to a conical shape for better electron emission. The cathode is enclosed in a hollow brass cylinder and provisions are made for water circulation (for cooling) and gas flow. The system has multiple inlets for plasma gas at the cathode end.Air is used as plasma forming gas.The melted sample is rapidly cooled for 10 minutes in openair.

Fig. 5a Schematic diagram of TAP torch.

5.1 Phase analysis The XRD patterns of 5 minutes TAP processed sillimanite for selected input power levels

are shown in the figure 5b. The 15 kW TAP processed pattern shows the complete single phase mullite formation. Hence the 15kW torch power level and 5 minutes processing time are sufficient for a complete mullite phase transformation from sillimanite [49].The mullite obtained is found to be crystalline. To study the effect of increased power input, plasma torch power is further increased to 20kW and 25kW. 20 kW TAP processed pattern depicts, the presence of major mullite phase along with sillimanite and α-alumina peaks. The 20 kW power of the torch stimulates the mullite formation as a result of its high temperature. However, the power level and processing time are high for a complete mullite formation [49]. Partial mixing of SiO2 with Al2O3 occurs at this temperature.Mullite obtained in this case is found to be heterogeneous and polycrystalline. Generally mullite phase stability directly depends on the Al2O3/SiO2 concentration and it remains stable even though at very low concentration of SiO2 in Al2O3/SiO2 system [47,49]. The 25 kW TAP processed XRD pattern reveals the presence of predominant alumina with very small number of mullite peaks. This may be due to the incomplete removal of SiO2. During TAP process, mostly SiO2 is evaporated but, small amount of SiO2 settled in processed sample reacts with Al2O3 and forming small number of peaks of crystalline mullite.Hence the XRD results clearly show that the applied input power (25 kW) and processing time (5 min) is too high for a complete formation of mullite phase from silimanite. The mullite diffraction peaks obtained in the XRD pattern is in good agreement with other reported works [49]


20 30 40 50 60

25 kW

20 kW

15 kW

2θ

ma

aaamass

a

m mmm

mm

m

mmmm

mm

m

mmm mm

m

mm

m

m amm

a - Aluminas - Sillmanitem - Mullite

a

a

a

a

a

a

Inte

nsity

(a.u

.)

Fig. 5b XRD pattern of 5 minutes TAP processed sillimanite at selected input power (15-25 kW) levels (s: JCPDS Card # 22 – 18, m: JCPDS Card # 15 – 776, a: JCPDS Card #

10 – 173)

The operation of TAP torch at more than 25 kW is more complex due to the vigorous rotation of plasma arc. The abnormal behavior of plasma arc cause higher erosion in graphite electrodes and ejection of feed stock materials from the anode crucible because of the random motion of plasma arc. For this reason the TAP torch was not operated at more than 25 kW power.

In order to remove mullite and to get a complete alumina phase (Fig.5b) the processing time is increased from 5 to 10 minutes with 25 kW torch power. The XRD pattern of 10 minutes TAP processed sillimanite is shown in figure 5c. The pattern shows only an alumina (α-alumina) phase because of the complete removal of SiO2 under high temperature and long processing time.

2 0 3 0 4 0 5 0 6 0

2 θ

Inte

nsity

(a.u

.)

a

a

a

a

a

a

a

a - A lu m in a

Fig. 5c. XRD pattern of 10 minutes TAP processed alumina from sillimanite at 25 kW (JCPDS Card # 10 – 173).

6. Microstructure analysis SEM image of the TAP processed sillimanite at selected input powers are shown in figures

6 a, b, c. It is seen that the microstructure is very much dependent on the torch input power. Needle shaped mullite formation is observed in 15 kW TAP (Fig.6a) processed sillimanite [49]. The needle structure of mullite is slightly disappeared in 20 kW TAP (Fig 6b) processing, due to the presence of sillimanite and alumina as shown in Fig 5b.Generally the reduction of silica ratio in


mullite has strong influence on the microstructure formation such as in the needle structure [49]. The abnormal grain growth with large number of pores is observed in 25 kW TAP (Fig 6c.) processed sillimanite due to the random growth of alumina (Fig 5b).

During the TAP process at higher power levels (>15 kW) the maximum amount of silica is evaporated because of high temperature of plasma arc and small amount of silica which is still being present in the alumina surfaces. Therefore the microstructure results confirm that the plasma torch power (> 15 kW) and processing time (5 min) are high for a complete mullite formation..

Fig. 6. SEM image of 5 minutes TAP processed sillimanite at : (a) 15 kW; (b) 20 kW and (c) 25 kW

Fig. 7 shows the SEM image of alumina surface derived from sillimanite dissociation through 10 minutes TAP processing at 25 kW. The increased processing time completely removes the silica in mullite through evaporation.

Fig. 7 SEM image of 10 minutes TAP processed sillimanite at 25 kW


7. Conclusions Low cost single phase Mullite glass ceramics production is demonstrated through HCP

and TAP Processes. The mullite obtained in both the processes is found to be crystalline. The input power (15 kW) and processing time (5min) for single phase mullite growth in TAP process is less when compared to HCP process. Needle shaped mullite is formed in TAP process and elongated bar- like mullite is formed in HCP process. The problems of low bulk, mullite grain boundary diffusion and processing time (>2hrs) in the conventional sintering methods are eliminated in HCP and TAP processes. The use of low cost raw material (silimanite) further reduces the cost of production of mullite and it may be the substitute for high cost industrial grades and chemicals. The reported results reveal that the HCP and TAP processes are competitive since the processing time is 40 times less than the conventional sintering methods.

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MULLITE GLASS CERAMICS DERIVED FROM SILLIMANITE … · 24 K. V. Kannan Nithin, N. Suriyanarayanan, Dr Antonella Sola. c = 2.884 Å. It consists of edge shared AlO. 6. octahedral chains