Journal of Materials Science and Engineering A 1 (2011) 312-320 Formerly part of Journal of Materials Science and Engineering, ISSN 1934-8959
Properties of Zinc Silicate Glasses and Melts Doris Ehrt and Sylvia Flügel Otto-Schott-Institut, Friedrich-Schiller-Universität Jena, Fraunhoferstr. 6, D-07743 Jena, Germany Received: March 03, 2011 / Accepted: March 25, 2011 / Published: August 10, 2011. Abstract: Silicate glasses with high ZnO content are of interest for various applications, ranging from technical to optical glasses and glass ceramics. From structural effect ZnO and Al2O3 are intermediate oxides and can theoretically act as network modifier and also as network former depending on glass composition, but cannot form a glass per se. Zinc silicate glasses and melts were investigated beginning with the binary eutectic composition (in mol%) 50 ZnO-50 SiO2. The effect of Al2O3, Li2O, Na2O, K2O, BaO and B2O3 were studied. Glasses were prepared in a laboratory scale with high purity raw materials in high optical quality. Different methods were used for investigation of glasses and melts up to high temperature (about 1,500 °C). Large variations of the properties were found, refractive index ne: 1.53-1.66, Abbe-number νe: 44-58, transformation temperature Tg: 465-730 °C; linear thermal expansion coefficient α100-300 °C: 35-140 (×10-7 K-1); density: 2.7-3.60 g/cm3; viscosity temperature (η in d Pas) Tlogη = 4: 775-1,115 °C; Tlogη = 2: 1,065-1,460 °C; electrical conductivity by impedance measurements in the temperature range 200 to 1,500 °C (σ in Ω-1cm-1); Tlogσ = -8 : 100-500 °C; Tlogσ = -1 : 845-1,195 °C. The crystallization behaviour was studied and the following main crystal phases were detected: Zn2SiO4 willemite and variations, Li1.14Zn1.43SiO4, BaZn2Si2O7, Na, Zn-silicates of different types, NaAlSiO4, K2ZnSi3O8 and Na2ZnSi3O8 with 3D-framework structure, which form very stable glasses with extremely low crystallization tendency. The relations between structure and properties are discussed. Key words: Zinc silicate, glasses, melts, viscosity, electrical conductivity, optical properties, crystal phases.
1. Introduction Silicate glasses with high ZnO content are of interest for various applications, ranging from technical glasses and glass ceramics to high performance optical glasses [1-5]. From structural effect of glass formation theory ZnO and Al2O3 are so-called intermediate oxides. They can theoretically act as network modifier in mainly octahedral coordination and also as network former in tetrahedral coordination depending on glass composition, but cannot form a glass per se. ZnO is available as high purity raw material with low cost. It provides a high refractive index ~2.0 and is in opposite to PbO not toxic. The main goal of this work was to investigate the influence of composition on important properties of glasses and melts to prepare glasses with specific properties for applications as optical fibres in the automotive industry. Glass forming, crystallization, Corresponding author: Doris Ehrt, Ph.D., professor, research fields: material science, glass, glass-ceramic. E-mail:
[email protected].
optical properties, electrical conductivity of glasses and melts and viscosity behavior were studied. Starting with the simplest eutectic composition, 50 mol% ZnO-50 mol% SiO2, the effect of Al2O3 as a second intermediate oxide, of different network modifiers, Li2O, Na2O, K2O, BaO and of B2O3, as a second well-known network former, was investigated. Viscosity, electrical conductivity and crystallization behavior of the melts are important properties for application of the skull melting technique, a high-temperature high-frequency induction melting to prepare high purity optical materials. Due to strong cooling of the coil, a solid layer of the initial material is formed around the melt which protects the melt against impurities of the container material. A crucible is not necessary and the efficiency is very high [6].
2. Experiment The batch compositions and properties of the samples are shown in the tables and figures. High purity
Properties of Zinc Silicate Glasses and Melts
grade raw materials (Fe’ < 1 ppm), ZnO, SiO2, Al(OH)3, H3BO3, Li2CO3, Na2CO3, K2CO3, and BaCO3 were mixed and melted in 300 g-scale in Pt crucibles at 1,500-1,650 °C for 4-6 h. Glasses were obtained by pouring the melts into a mould. Only the melt of the eutectic composition 50ZnO-50SiO2 was quenched between brass plates because its crystallization tendency was very high. After annealing from Tg + 50 K to room temperature at 3-5 K/min, the glasses were cut, ground and polished to produce samples for different measurements. The refractive indices in the visible range were measured with a refractometer with an error ±2 × 10-5. The Abbe number, νe = (1-ne)/(nF’-nC’) with e 546 nm, F’ 480 nm, C’ 644 nm, was determined. The density was measured using Archimedes’ principle with an error ±0.002. DTA (10 K/min) and dilatometry measurements (5 K/min) were carried out to obtain values for the thermal properties, Tg, Tc and thermal expansion coefficient (TEC, 100-300 °C). The viscosities as function of temperature were determined with a rotating cylinder method in the η range, 101.5-105 dPa s, and with the beam bending method in the range 109-1011 dPa·s. The error of the viscosity values was
