Tailoring of K0.8Al0.7Fe0.15Si2.25O6 Leucite Based Dental Ceramic Material

Potassium based ceramic materials composed from leucite in which 5 % of Al is exchanged with Fe and 4 % of hematite was synthesized by mechanochemical homogenization and annealing of K2O-SiO2-Al2O3-Fe2O3 mixtures. Synthesized material was characterized by X-ray Powder Diffraction (XRPD) and Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM/EDX). The two methods are in good agreement in regard to the specimen chemical composition suggesting that a leucite chemical formula is K0.8Al0.7Fe0.15Si2.25O6. Rietveld structure refinement results reveal that about 20 % of vacancies exist in the position of K atoms.


INTRODUCTION
N OPTIMIZATION of a material properties i.e. material tailoring for industrial applications is an imperative for successful applications. Detailed knowledge of a material structure is one step ahead to the final solution. Alkaline metal based ceramic materials are widely used in industry as: electroceramic components, [1] matrixes for fluorescent screens, [2,3] thermo-refractory materials, [4,5] electromagnetic windows, [1,5] dental ceramics [6] etc. So far, investigations of many alkaline ceramic materials, although started from the beginning of the last century, reveal unexplained properties and unsolved parts of a material structure.
Leucite, KAlSi2O6 is common mineral in some volcanic rocks in which it crystallizes with a cubic crystal structure at high temperature (ca. 900 °C). Upon cooling to 700-600 °C, it transforms into a tetragonal modification which is stable at room temperature, and forms characteristic polysynthetic twin lamellae. The transformation is reversible. [7] Structurally it belongs to feldspathoids -tectosilicates characterized with Al-Si framework structure. Voids within framework are partly filled with K atoms, Figure 1. Leucite crystalizes as euhedral pseudocubic crystals in tetragonal I41/a space group. Characteristic unit cell parameters are: a = 13.056 Å, c = 13.751 Å (a : c = 1 : 1.053). Inside the unit cell there are 16 asymmetric units (Z = 16). Common impurities in natural leucites are: Ti, Fe, Mg, Ca, Ba, Na, Rb, and Cs. Impurities concentrations are rather small in natural leucites. However, leucite structures exist even after complete exchange of Al with Fe. [7] The optical properties of the leucite glass-ceramic make it one of the most appropriate materials for the fabrication of dental restorations. The leucite has almost the same refractive index as the glass. Therefore, the translucency is never hindered by the crystallization of the leucite in the glass. Leucite based glass-ceramics has ability to match the colour of the natural tooth. Addition of other chemical elements, like iron, could slightly change the colour of the dental ceramic to desirable hue. Another advantage of the leucite based glass-ceramic materials in dental industry is that due to low thermal expansion coefficient its stability during fusion is remarkable. [6] In order to tailor potassium based ceramic materials with good properties for dental industry we have synthesized them by mechanochemical homogenization and annealing of K2O-SiO2-Al2O3-Fe2O3 mixtures. Our main goal was to synthesize leucite in which 10 % of Al is exchanged with Fe and to characterize it by X-ray Powder Diffraction (XRPD) and Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM/EDX).

Sample Preparation
The starting compounds were SiO2, Al2O3, K2CO3 and Fe2O3. They were mixed in appropriate molar ratio according to the stoichiometric formula KAl0.9Fe0.1Si2O6. Mechanochemical treatment was performed during one hour in a planetary ball mill (Fritsch Pulverisette 5) equipped with tungsten carbide bowls (250 ml in volume) and balls (10 mm in diameter). The mass of the powder was 10 g and the balls-topowder mass ratio was 20 : 1. The milling was done in air atmosphere without any additives. The angular velocity of the supporting disc and vial was 32.2 and 40.3 rad s -1 , respectively. The intensity of milling corresponded to an acceleration of about 10 times the gravitational acceleration. The milling vessels were opened for removing of the CO2 which evaporate during milling. After milling the obtained powders were pressed in pellets under pressure of 50 MPa and sintered at temperature of 1100 °C for 24 hours. Than specimens were milled again but not opened for removing of the CO2. At the end specimens were pressed in pellets under pressure of 50 MPa and sintered at 1100 °C for 24 hours.

Experimental Techniques and Methods
For the collection of the XRPD data a D8 Advance (Bruker, Germany) X-ray powder diffractometer was used. The diffractometer was equipped with a Cu-tube and a Xe-filed proportional counter. The divergence and receiving slits were 1 ° and 0.1 mm, respectively. The scanning range was 4-90 ° in 2θ, with a step of 0.03 ° and a scanning time of 22 s per step. The determination of the structural parameters of the K0.8Al0.7Fe0.15Si2.25O6 was carried out using the Rietveld method implemented in the FullProf program package. [8] SEM images were recorded using a MIRA3 FE-SEM microscope (TESCAN, Czech Republic) equipped with an EDX detector (Oxford Instruments, UK). No. of restraints 10 Computing details: Data collection: D8 Software; [11] program used to refine structure: FULLPROF; [8] molecular graphics: DIAMOND. [12] DOI: 10.5562/cca2860 Croat. Chem. Acta 2016, 89(1), 101-104

RESULTS AND DISCUSSION
Collected XRPD pattern corresponds to reference leucite KAlSi2 O6. [9] Few low intensity peaks in the pattern belong to hematite. [10] Quantitative phase analysis indicated that only 4(1) % of hematite is in the specimen. An amorphous phase could not be recognized by XPRD analysis. Therefore, it is reasonable to assume that part of Fe is incorporated into leucite structure. Obtained unit cell parameters for leucite (a = 13.1334 ( Table 2. However, estimated standard deviations of site occupation parameters are relatively high suggesting that they are not reliable for recalculation of leucite chemical formula. Ionic radii for Si 4+ , Al 3+ and Fe 3+ are 0.26, 0.39, and 0.49 Å respectively. Therefore, interatomic distance values are more reliable than site occupation parameters indicating that T1 atomic site is mostly occupied with Si while T2 and T3 are occupied with Si, Al and Fe. Atomic site T3 contains more Al and Fe than other two T sites, as given in Table 3. Interatomic distances, as well as overall temperature parameters, are in good agreement with literature data. [9] Obtained accuracy parameter values are reasonable, indicating reliable refinement, as shown in Table 1.