Analysis on crystalline of glass ceramic in the K₂O-CaO-MgO-Al₂O₃-SiO₂-F system

Li Hong, Ran Junguo^#, Gou Li^#
(Department of Chemistry, Jinan University, Guangzhou 510632, China, ^#Department of inorganic materials, Sichuan University, Chengdu 610065, China)

Received Jul. 4, 2003.

Abstract The glass-ceramic mainly containing fluorophlogopite is one of widely used machinable ceramics. A new-type glass-ceramic containing fluorophlogopite has been synthesized in the K₂O-CaO-MgO-Al₂O₃-SiO₂-F system. Its crystalline was studied by XRD and EDS. The fluorophlogopite crystalline phase distinguishes from the family of fluoromica by K⁺ and Ca²⁺ acting as bonding ions altogether.
Keywords Fluorophlogopite, crystalline, glass ceramics .

1 INTRODUCTION      
Glass ceramics are produced by controlled crystallization of appropriate glass. Mica-containing glass ceramic， such as Dicor (Corning, Inc., Corning .NY)， shows good machinability. The material can be machined to close tolerance with conventional methods. Because of its excellent biocompatibility and natural esthetics, the material is used as veneer the metal framework of crown, bridge, or inlay and onlay in restorative dentistry ^[1]. Application of the glass ceramic is limited, for its intrinsic mechanical brittleness leads to high clinic failure. Generally, the bonding strength of interlayer ions is the weakest in mica crystal, so the mechanical properties apparently are depended on the bonding strength of these interlayer ions. It has been reported, fluorophlogopite-type Ca- or Ba-mica ^[2,3], which Ca²⁺ or Ba²⁺ take place of alkali ions between interlayers, exhibits higher strength.
    In the system, precipitation of fluorophlogopite-type Ca-mica with rod-shaped crystal from glass was formed. The composition and microstructure was studied with XRD, EDS and SEM.

2 EXPERIMENTAL
A bath mixture of nominal composition of SiO₂ 40%, Al₂O₃ 12%, MgO 10%, MgF₂ 24%, CaO 5%, K₂O 1%, ZrO₂ 8% in weight ratio was melt in a platinum crucible at 1550ºC for 2h. The melt was poured on to a graphite model. The cast glass nucleated at 660ºC for 1h and crystallized at 1100ºC for 4h. After heat treatment, the cast glass precipitated to form white glass-ceramic. The precipitated crystalline phases were identified by X-ray diffraction (D/MAX -II, Rigaku, Japan). Microstructure was characterized using scanning electron microscopy (X-650, Hitachi, Japan) and EDS (connected with SEM).

3 RESULTS AND DISCUSSION 
Figure 1 shows XRD pattern of the glass-ceramic. Fluorophlogopite (mica) and t-ZrO₂ are main crystalline phase. The microstructure of the glass-ceramic (Figure 2) displays typical feature of the machinable glass ceramic with rod like crystals isolated and interlocking. Tested by drilling and dental CAD/CAM system, the glass-ceramic showed excellent machinability^[4]. These rod-like crystals are 1-3mm thick and more than 10mm long, whose composition were analyzed by EDS (Figure 3). The atom concentration ratio of the rod like crystals is Mg/Al/Si/O/F=2.5/1.2/3/10/2 according to the EDS result, which is similar to that of fluorophlogopite. The formula of typical fluorophlogopite and fluorophlogopite Ca-mica is KMg₃AlSi₃O₁₀F₂, Ca_1/2Mg₃AlSi₃O₁₀F₂ respectively. According to EDS pattern (Figure 3), it is seen that not only Ca but also K was detected in rod like crystals. Therefore, we think that K⁺ and Ca²⁺ act as bonding ion between the double layers of Al-Si tetrahedral at the structural network of fluorophlogopite. The formula of the fluorophlogopite postulates K_1-XCa_X/2Mg₃AlSi₃O₁₀F₂ in our study. As we know, The ion field intensity of Ca²⁺ is much larger than that of K⁺, so its shield from electron field has a more effective influence on oxygen. This effect made the formation of [AlO_4/2]Ca difficult. Even if [AlO_4/2]Ca was formed, it has bad miscibility with [SiO_4/2]. Therefore it is deduced that Ca²⁺ has an unfavorably effect on the formation of the basic unit [AlSiO_4/2] in the system. It means that it is difficult to precipitate fluorophlogopite Ca-mica. The introduction to low content K₂O led to different result. The content of non-bridge oxygen increases with the introduction to K₂O, and so Al³⁺ tends to form [AlO_4/2] which carries excess negative charge. Potassium neighbors to [AlO_4/2] form K[AlO_4/2] compound^[5]. This compound possesses good miscibility with [SiO_4/2] tetrahedral. Then this effect results in the formation of [AlSiO_4/2], which is the basic structure of fluoromica. After the [AlSiO_4/2] groups were formed, Ca²⁺ works as bonding ion for its high electrovalence. At the same time, K⁺ still remains in the compound, and also act as bonding ion. As a result, a new-type fluorophlogopite precipitates from glass matrix.
    ZrO₂ are most commonly used as nucleating agent in glass ceramic procedure. Some of ZrO₂ dissolve in the glass system and Zr⁴⁺ substitutes one of Si⁴⁺ in [SiO₄] tetrahedral, therefore EDS can detect in the rod like crystal (Figure 3). The others precipitate as t- or m-ZrO₂ particles showed by XRD pattern (Figure 1).


Fig.1 XRD pattern of the glass-ceramic

Fig.2 SEM of the glass ceramic

REFERENCE
[1] Thompson JY, Ideymann HO, Bayne C. J. Prosthet. Dent., 1996, 76(6): 129.
[2] Uno T, Kasuga T, and Makajima K. J. Am. Ceram. Soc., 1991, 74(12): 3139.
[3] Uno T, Kasuga T, and Makajima K.J, et al. J. Am. Ceram. Soc., 1993, 76(6): 539.
[4] Li H. Doctoral dissertation. Sichuan University, Chengdu, Sichuan. 2002.
[5] Mazurine OV. Phase separation of glass, New York: North-Holland, 1984: 35.

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