Leucite Crystals Research Articles

The crystallization of an aluminosilicate glass in the K 2O–Al 2O 3–SiO 2 system

The aims of the study were to explore the nucleation and crystallization kinetics of an aluminosilicate glass in K2O-Al2O3-SiO2 system and to characterize it. A starting glass composition of wt%; 64.2% SiO2, 16.1% Al2O3, 10.9% K2O, 4.3% Na2O, 1.7% CaO, 0.5% LiO and 0.4% TiO2 was heated in an electric furnace and later quenched to produce glasses. The glass powders were heat treated using differing heat treatment schedules and quenched. Dta, Xrd, Eds and Sem analyses were used to characterize and explore the crystallization kinetics of the glasses. Phase separation of the glasses was identified and characterized in the glasses. Tetragonal leucite, cubic leucite and sanadine glass-ceramics were produced. Fine leucite crystals (1 microm2) were crystallized with minimal matrix microcracking. Amorphous phase separation appeared to be an important precursor to nucleation and crystal growth in the alkali aluminosilicate glasses explored. It was possible to control the crystallization of tetragonal leucite and sanidine phases by selected heat treatment of glass powders and monoliths, resulting in the production of fine grained tetragonal leucite glass-ceramics.

Low-temperature synthesis of leucite crystals using kaolin

Synthesis of leucite crystals below 1000 °C using natural kaolin as the primary raw material was investigated. Spherical leucite crystals having a diameter of approximately 50 μm were prepared by heating a powder mixture of Al 2(SO 4) 3, kaolin and K 2SO 4 (in mass ratios of 3:3:15) at 900 °C for 3 h. Quartz, the main accessory phase in kaolin, and the amorphous metakaolin formed upon heating kaolin were found to be responsible for the decreased synthesis temperature.

Influence of hot pressing on the microstructure and fracture toughness of two pressable dental glass-ceramics.

Empress 1 and Empress 2 are well-known pressable all-ceramic dental materials that have generated substantial interest for many clinicians and patients. These two materials are reputed to benefit from heat pressing during the laboratory fabrication procedures, leading to better crystal distribution within a glass matrix, and hence an improved strength. The present study aimed to evaluate the effect of heat pressing on fracture toughness, microstructural features, and porosity. Results showed that Empress 1 had similar fracture toughness values before the pressing procedure, after it, and after the repressing procedure. The microstructural features were also similar among these specimens, but a more uniform distribution of leucite crystals was observed following the pressing and repressing procedures. Empress 2 demonstrated two different fracture toughness values. This was associated with the alignment of lithium disilicate crystals that occurred after the pressing and repressing procedures, which led to different indentation induced crack lengths, depending upon whether cracks propagated parallel to or perpendicular to the aligned crystals, the former having lower toughness than those that propagated in the perpendicular direction. Porosity, in terms of both the size and number of pores, was found to decrease after the pressing and repressing procedures for both materials. Repressing resulted in significant growth of the lithium disilicate crystals in Empress 2, but there was no change for the leucite crystals in Empress 1. The change in the lithium disilicate crystals' size did not have a noticeable effect on the fracture toughness of Empress 2. It was concluded that heat pressing did not significantly affect the fracture toughness of Empress 1, but resulted in two different values for Empress 2. It also decreased the size and number of pores for both materials, which could contribute to the strength improvement found after heat pressing, which has been reported in previous studies.

Synthesis of Leucite Crystals Using Potash Feldspar

Natural potash feldspar was used in an attempt to synthesize leucite crystals. The conditions necessary for synthesizing leucite crystals were examined and the crystal morphology were investigated. In addition, the formation mechanisms of leucite crystals were discussed. When a mixture of Al2(SO4)3: Potash feldspar: K2SO4=3:3:15 was heated at 1100°C for 3 h, 50-60 μm diameter leucite crystals formed. Potash feldspar was stable up to 1000°C by this synthetic method, therefore a heating temperature of 1100°C was needed to form leucite crystals.

Growth of leucite crystals from a K2Mo2O7 flux

Growth of leucite crystals from a K2Mo2O7 flux

Microstructure effect on the properties of a commercial low-fusing dental porcelain.

The aim of this study was to investigate the effect of firing cycle on a dental porcelain microstructure in order to correlate microstructure changes with mechanical and thermal properties. A commercial low-fusing dental porcelain powder (Omega 900, Vita) was investigated for this purpose. The powder was treated at different temperatures in the range 750-1000 degrees C. The fired samples were characterized in terms of their morphology and microstructure, and their mechanical and thermal properties were evaluated. The results showed that firing temperature affects porcelain microstructure influencing significantly in this way both the mechanical properties and the thermal expansion coefficient of the fired objects. Firing at 800 degrees C led to a homogeneous structure. After treatment at this temperature, the leucite crystals exhibit their maximum concentration and they are well dispersed into the glassy phase. As a consequence the optimum mechanical strength and the maximum thermal expansion coefficient are observed in these samples.

Size Control of Spherical Leucite Crystals.

Size Control of Spherical Leucite Crystals.

Flexural strength optimisation of a leucite reinforced glass ceramic

Objectives: The aims of the study were to process a ceramic material with a fine leucite particle size using hot pressing techniques, to increase the flexural strength, reliability and ease of use. Methods: A starting glass composition of wt%; 64.2% SiO 2, 16.1% Al 2O 3, 10.9% K 2O, 4.3% Na 2O, 1.7% CaO, 0.5% LiO and 0.4% TiO 2 was used to produce a leucite reinforced ceramic material. Twenty-one porcelain discs were produced by sintering the ceramic frit (group 1) and sixty-three discs by heat pressing the frit (groups 2, 3 and 4). Twenty-one Empress 1 ceramic discs were also heat pressed (group 5). Disc specimens were tested using the biaxial flexure test at a crosshead speed of 0.15 mm/min and the data analysed using the Scheffé F multiple comparison test and Weibull statistics. Specimens were characterised using X-ray diffraction (XRD), secondary electron imaging and energy dispersive X-ray analysis where applicable. Results: The heat pressed groups (2, 3 and 4) had higher mean biaxial flexural strengths and characteristic strength values than groups 1 and 5 ( p<0.05). XRD revealed the presence of tetragonal leucite in all test groups. Fine leucite crystals, tabular platelets and minimal matrix microcracking were found in the microstructure of test groups (1–4) with a more uniform leucite distribution in the heat pressed specimen groups (2, 3 and 4), which were associated with a significant increase in the biaxial flexural strength and reliability. Significance: Optimisation of the microstructure by producing a fine microstructure and controlling the distribution via the correct pressing parameters may be extremely advantageous in these systems.

Structure, chemical durability and microhardness of glass-ceramics containing apatite and leucite crystals

Glass-ceramics based on SiO 2–Al 2O 3–K 2O–Na 2O–CaO–P 2O 5 prepared by controlled crystallization of glass are studied. Glass melted from a batch of high purity raw materials was quenched and milled. The powder was heat treated at 850°C, 900°C and 1150°C for 1 h, quenched in water, milled, formed and sintered at 1000°C for 1 min. The study focused on the phases of crystallization and microstructure of both heat treated and sintered samples. Microhardness and chemical durability were measured of glass-ceramics produced by sintering. Leucite and needle-shaped fluorapatite crystals were developed. The microstructure changed depending on heat-treatment and sintering temperatures. The chemical durability and microhardness of glass-ceramics prepared for restorative dental applications are influenced by the properties of the bulk glass.

Synthesis of Needlelike Leucite Crystals Using Potassium Sulfate Flux.

When a powder mixture consisting of Al2(SO4)3, SiO2 and K2SO4 was heated in a closed alumina crucible, a reactant mass remained at the bottom of the crucible. Needlelike leucite crystals were obtained by treating this reactant mass with hot HCl. The effect of the mixing ratio of the raw materials, the heating temperature and the duration of heating at the selected temperature on the formation of needlelike leucite crystals was investigated. When a powder mixture, consisting of Al2(SO4)3:SiO2:K2SO4=1.0:2.8:1.9 (molar ratio), was heated at 1200°C for 3h, the obtained needlelike leucite crystals were 1-2μm in length and 0.1-0.3μm in width. Needlelike leucite crystals were formed by the crystallization of leucite on small mullite crystals obtained by heating, followed by growth of the leucite crystals in a liquid phase of the K2SO4 flux as the main component.

Crystallization of leucite as the main phase in aluminosilicate glass with low fluorine content

The paper presents the results of the study of aluminosilicate glass that contains 0.77 wt% of F− anions. The results show that thecritical size of glass particles at which the surface mechanism ofcrystallization changes to volume crystallization is in the vicinityof 0.075 mm in this glass. The temperature of the maximum nucleationrate was determined to be Tn = 680°C, and it ishigher than the transformation temperature Tg. Atcrystallization temperatures up to Tc = 950°C,the crystal phase of Lucite is formed, alone. Above this temperature,besides the main phase of lucite, two other phases, diopside andphlogopite appear. The activation energy of crystallization Ea = 319 ± 23 kJ/mol was determined. The results alsoshow that the activation energy does not depend on thecrystallization mechanism in this glass. The value of the Avramiparameters is n = 1.45. Microstructural studies confirmed thefact that volume crystallization of leucite takes place in thisglass. The leucite crystals grow in form of equiaxial dendrites withpronounced anisotropy along the direction ‹001›.Kinetical and microstructural studies show that the process ofleucite crystal growth occurs at the smooth atomic scale-facetedcrystal/glass interfaces and that it is controlled by volumediffusion. During the isothermal heat treatment the structure ofdendrites changes.

Synthesis of Spherical Leucite Crystals.

Synthesis of Spherical Leucite Crystals.

The biaxial flexural strength of two pressable ceramic systems

The biaxial flexural strength of Optimal pressable ceramics and IPS Empress pressable ceramics were tested, compared and the microstructures and compositions explored. The materials evaluated were Optimal shaded (Opcs) and unshaded (Opcus) ceramics and IPS Empress shaded (Ems) and unshaded (Emus) ceramics. Twenty-one disc specimens per material were prepared, heat-treated and tested. The piston on three-ball test ASTM F394-78 (1991) was used to test the specimens in a universal testing machine at a crosshead speed of 0.15 mm/min. Specimens were viewed in a scanning electron microscope and X-ray diffraction used to determine the phases present. Mean strengths (MPa±SD) were: Ems 120.1±20.5, Emus 135.8±16.0, Opcs 139.1±14.3 and Opcus 138.0±11.5. There was no statistically significant difference between Optimal shaded, Optimal unshaded and Empress unshaded strength values ( p>0.05). Empress shaded strength values were significantly lower than the other materials tested ( p<0.05). Weibull analysis provided m values: Ems 6.1, Emus 10.2, Opcs 12.8 and Opcus 13.9 and 1% and 5% probabilities of failure. Secondary electron imaging revealed a dense dispersal of leucite crystals in the glassy matrix of the Optimal ceramics of an average size 5.5±9.7 μm 2 for the Optimal shaded ceramic and 6.6±13.3 μm 2 for the Optimal unshaded ceramic. Leucite crystal agglomerates were evident for the Empress shaded material and a uniform distribution of fine leucite crystals (1.9±1.8 μm 2) for the Empress unshaded ceramic. Crystal and matrix microcracking were present in most of the material microstructures, together with porosity and tabular alumina platelets in the Optimal ceramics. X-ray diffraction revealed the presence of tetragonal leucite and small amounts of cubic leucite. Optimal ceramics and Empress unshaded ceramic provided higher strength and Weibull m values compared with Empress shaded ceramic. Inherent material defects were characterised and cubic leucite was identified.

Reaction between .ALPHA.,.BETA.-Al2O3 Ceramics and Ba, Sr-Alkali Silicate Glass.

Reaction between α, β-Al2O3 ceramics (produced by fusion cast method) and Ba-Sr-alkali-silicate glass was studied on samples from glass melting tank. This tank had been used for about 6.5 years. Dendritic leucite (KAlSi2O6) crystals were observed at the interface between ceramics and glass. These crystals were about 120μm long at the position of the glass meniscus line. Leucite was crystallized in Al2O3-rich liquid due to the corrosion of α, β-Al2O3 ceramics. β-Al2O3 crystals in the ceramics showed no morphological change but chemical substitution of K, Ba and Sr for Na occurred. Leucite and β-Al2O3-like crystals are occasionally found as a defect in glass production. It is clear that these crystals are formed at the interface between α, β-Al2O3 ceramics and molten glass.

Microstructure and homogeneity of dental porcelain frits.

The microstructure and homogeneity of three commercial dentin and incisal unfired porcelain frits (one conventional and two ultra-low fusing types, fused-to metal were analyzed by X-ray diffractometry, scanning electron microspectroscopy, and wavelength- and energy dispersive X-ray microspectroscopy. The average contents of tetragonal and cubic leucite for the conventional and one of the ultra-low fusing type frits were 20.1-22.6 wt% and 0-2.6 wt%, respectively, whereas those of another of the ultra-low fusing type frits were about 11.5-11.6 wt% and 2.9-4.6 wt%, respectively. The conventional type frits seemed to be admixtures of three kinds of glass frits. One of the ultra-low fusing type frits seemed to be an admixture of four kinds of glass frits. Another ultra-low fusing frits seemed to be only one kind of glass frit dispersed with small size, less than 1 micron, leucite crystals. There were no remarkable differences in microstructure and homogeneity between dentin and incisal porcelain frits in each brand.

Mechanical Properties of Porcelain Containing Leucite Ion-exchanged with Rubidium.

The purpose of this study was to investigate the effectiveness of leucite crystals ion-exchanged with rubidium on the mechanical properties of glassy porcelain. For ion-exchange, a mixture of high-pure natural leucite (KAlSi2O6) powder and RbNO3 were heat-treated at 450°C (RLa) or 650°C (RLb) for 1 hour. The density of leucite increased as a result of the rubidium ion-exchange treatment. Polymorphic transformation temperatures of the ion-exchanged leucites were low compared with the unmodified leucite. Composite porcelains were prepared by blending each leucite (30vol%) with “Component No.3” frit of Weinstein et al. patent. The flexural strength and fracture toughness of the glass-only and composite porcelains were measured by a three-point bending test and indentation technique. ANOVA and post hoc multiple comparisons test revealed that the flexural strength and fracture toughness of the composite porcelain containing RLa were significantly higher (p<.05 and p<.01, respectively) than those of the other groups. The results suggested that the rubidium-ion-exchanged leucite (RLa) could be a better reinforcing agent for dental porcelain.

The transverse strength, reliability and microstructural features of four dental ceramics — Part I

Objectives: The flexural strength, reliability and microstructure of a heat pressed ceramic system (Empress) was evaluated in comparison with three current frit materials used in ceramic crown and inlay construction. Methods: Twenty bar specimens per material were constructed, overglazed and transverse tested at a crosshead speed of 0.5 mm/min. Etched and fractured bar specimens were viewed under a scanning electron microscope using secondary electron imaging. Results: Mean strengths (MPa ± SD) were: Empress, 117.3 ± 31.7; Cerinate porcelain, 118.2 ± 8.7; Corum porcelain, 92.9 ± 13.3; and Alpha porcelain, 60.9 ± 10.1. Results were highly significant ( P < 0.001) with differences between all groups ( P < 0.05) except Cerinate porcelain and Empress ( P > 0.05). Weibull analysis of the results provided m-values and the predicted strength at the 1 and 5% probabilities of failure. Microstructural examination using secondary electron imaging showed leucite crystals 1.6–3.5 μm and associated microcracking in Corum porcelain and leucite crystal clusters (1.5 μm) in Empress. Cerinate porcelain exhibited a uniform distribution of fine leucite crystals (1 μm). Spherical porosity combined with crack pore combinations were noted in Vita Alpha porcelain. Conclusion: Empress glass ceramic was not stronger than Cerinate or more reliable than many of the frit materials. Uniform leucite distribution and fine crystal size may be associated with improved strength and reliability.

The effect of microstructural features on the biaxial flexural strength of leucite reinforced glass-ceramics

In this study the effect of size and distribution of leucite crystals on the biaxial flexural strength of five leucite reinforced glass-ceramics used in dentistry for the construction of veneers was investigated. Three of these were found to have a non-uniform distribution of leucite crystals, which were associated with a considerable degree of microcracking in the glassy matrix around the leucite crystals. The mean values of biaxial flexural strength of these ceramics ranged from 56 to 70 MPa. The other two ceramics exhibited a more uniform distribution in their leucite crystals, which were also finer in size. The latter ceramics had a significantly higher biaxial flexural strength which ranged from 120 to 137 MPa. When the particle size of the starting powders of the three low strength ceramics was reduced, a more uniform microstructure with less evidence of microcracking was produced in the fired ceramics and the biaxial flexural strength values were significantly increased. We conclude that uniformity of microstructure and leucite crystal size are major factors in determining the biaxial flexural strength of leucite reinforced glass-ceramics.

High‐Thermal‐Expansion Polycrystalline Leucite Ceramic

A high‐thermal‐expansion ceramic consisting of leucite crystals was prepared by sintering leucite powder. The densification was promoted by adding Li2CO3 or Na2CO3. The leucite ceramic obtained had a thermal expansion coefficient of about 2.4 × 10−5/°C from room temperature to 600°C. Some of the Li+ and Na+ ions were incorporated into the leucite crystal lattice to form solid solutions, and the characteristic tetragonal‐cubic inversion point shifted from ca. 600° to 650°C.

The Brown Leucitic Tuff of Roccamonfina Volcano (Roman Region, Italy)

The Brown Leucitic Tuff (BLT) is a poorly to strongly lithified compositionally zoned pyroclastic-flow deposit with a minimum volume of 3 to 5 km3. It erupted from Roccamonfina Volcano about 385000 years ago, after formation of the summit caldera. Individual flow units are grouped into three facies (white, brown, and orange) which primarily differ in pumice color, lithic content, and matrix cementation. Pumices from the BLT range from phonolitic leucite-tephrites to leucite-trachytes (7.0 to 2.2 wt% CaO), covering over half of the total spectrum of High-K Series magmas known from Roman Region volcanoes. White-facies units dominate in lower stratigraphic levels and their pumices have the lowest CaO contents, indicating a general trend toward more basic compositions as the eruption evolved. At higher stratigraphic levels, however, orange- and brown-facies units are interbedded with other whitefacies units, indicating reversals in the dominant compositional progression. BLT pumices have crystal contents of 9.9 to 0.6 vol%, with green salite>plagioclase>sanidine>biotite>titanomagnetite>analcime (after leucite)>apatite>pyrrhotite. In most samples, plagioclase (An85–95) and sanidine (Or75–90) have much lower Na2O contents than usually found in coexisting feldspars, yet these are interpreted as equilibrium pairs. Primary leucite has been almost completely replaced by analcime. All samples also contain xenocrysts of colorless diopside and forsteritic olivine (Fo83–92). Recurrent alternations from colorless diopside to green salite are present in single clinopyroxene crystals and appear to reflect a complex history of magma mixing. Whole-rock BLT pumice compositions conform closely to High-K Series lavas from Roccamonfina for all elements except Na2O and K2O. The former is relatively enriched and the latter relatively depleted in mafic BLT pumices with >5.6% CaO; these differences reflect strong analcimization of abundant groundmass leucite crystals in these pumices. Otherwise, major and trace element data support fractionation of observed minerals in generating the compositional diversity among BLT pumices. Mineral assemblages and compositions of cumulate monzonite and syenite nodules carried to the surface during the BLT eruption correspond closely to the fractionated phases predicted by least-squares modeling.