Glass synthesis
Compositions of glasses are designed to obtained thermo-mechanical and rheological properties similar to commercial Schott’s F2 and SF6 lead-silicate glasses. Initial compositions of glasses labeled NC-xx are shown in Table 1. All raw materials used for glass melting, grinded quartz glass and other chemicals, had a purity of 99.9 %. Designed compositions of substrates were melted in a resistance furnace, in a corundum crucible. This type of glass is not chemically aggressive, and did not damage the crucible made of alumina oxide. Despite this, after several melting processes, an etched ring in the inner side of the crucible could be clearly seen. These phenomena had to be taken into account in the considerations of real glass composition after melting. The melted glass, after pouring into preheated graphite mould, was 2 h thermal treated at 510 \(^{\circ }\hbox {C}\) and then annealed by slowly cooling at a rate of 0.5 \(^{\circ }\hbox {/min}\). Overall process was performed in the Glass Department at Institute of Electronic Materials Technology (ITME).
Table 1 Chemical composition of the synthesized high-alkaline borosilicate glasses
Characterization of glass
A dilatometer 801 type of Baehr Thermoanalyse GmbH was used to measure linear thermal expansion coefficient. Four characteristic glass temperatures—lower annealing point, transition temperature, upper annealing temperature, and dilatometric softening point—related to the dynamic viscosity \(\upeta \,=\,10^{14.6};\,10^{13.4};\, 10^{13}\,\hbox {and}\, 10^{11}\) P (Poise) were determined based on the obtained thermal expansion curves. The further specific glass temperatures were obtained with a Leitz heat microscope. A cube-shaped sample with dimensions \(4 \times 4 \times 4\,\hbox {mm}^{3}\) was inserted into a furnace and the shape of the sample was observed. At characteristic viscosities, glass changes its shape from cubic to oval \((\hbox {log}\upeta =9.0)\), sphere \((\hbox {log}\upeta =6),\) hemisphere \((\hbox {log}\upeta =4),\) and at very low viscosity \((\hbox {log}\upeta =2)\) the glass spreads. The final viscosity curves (log(Viscosity [P]) vs. temperature \([^{\circ }\hbox {C}]\)) were determined based on the measurements in the dilatometer and in the heat microscope.
Crystallization susceptibility can be investigated by three methods: differential scanning calorimetry (DSC), diffractive X-ray measurements (XRD) or by isothermal heating method. In this work we used the third method, which is simple and sufficient for our purposes, considering that the aim of our work was to avoid the crystallization in developed glasses. Characterization of the observed crystals formed in some of the samples was not intended to study. The glass sample with polished surface was heated for 2 h at the sphere creation temperature, determined in Leitz heat microscope, that corresponds to glass viscosity \(\upeta \,=\,10^{6}\,\hbox {P},\) and then after cooling the sample surface was inspected for presence of crystals using a microscope with attached polariscope. When the surface is changed or the crystals are visible, the glass has tendency to crystallization. The distribution and sizes of grown crystals can be compared with a composition changes in glass series, leading to design thermally stable glass. The examples of crystals grown on the glass surface appeared after isothermal heating test are shown in Fig. 1. Such a picture was observed only for glasses with clear tendency to crystallization. Lack of crystals testifies about high thermal stability of glass. The last test before choosing pair of glasses for joint thermal processing was the “sandwich” test. One polished glass plate was placed between two plates cast from different type of glass. Such stack is heated to the sphere creation temperature and slowly cooled. After cooling, this “sandwich” structure is cut and polished, and inspected under a polariscope. The observed difference between the colors of sample enables to identify stress induced by the difference in the thermal expansion during cooling. Images of prepared “sandwich” samples are shown on Fig. 1. Measured properties of melted glasses are summarized in Table 2.
Table 2 Basic thermal and optical properties of the investigated high-alkaline borosilicate glasses
In order to fabricate a stable optical structure comprising two jointly thermal-processed glasses, the difference in linear thermal coefficient has to be minimized, and viscosity of each of the glasses has to be matched. Excessive difference in linear thermal coefficient introduces stress at the interface of the two materials and results in cracking during cooling, especially in components of larger dimensions. From our experience the thermal expansion difference for two joined glasses should not exceed 10 %, which means about \(8\times 10^{-7}\,\hbox {K}^{-1}\) in our case, to maintain stable structures. In this work we select glasses for joint process of fiber drawing. The optimal viscosity \((\hbox {log}\upeta )\) for drawing process is between 7.6 and 8. For optimal selection of two glasses, their viscosities had to be in this range at the same temperature for stable process. Additionally, glasses should not crystallize; this will prevent multi-thermal processing like stack-and-draw method. Stack-and-draw involves typically between two and four consecutives steps of stacking of glass preform elements and drawing them at a fiber drawing tower. In this procedure, drawing of stacked elements is the thermal processing step. Considering glass parameters like viscosity, characteristic temperatures and crystallization resistance, the best candidate for joint thermal processing with lead-silicate glasses is our glass labelled NC-21A. The chosen glass does not have barium oxide in composition and is characterized by high thermal stability. This glass passed crystallization test without observable crystals on the surface of the thermally treated sample. Properties of the glasses chosen for further processing are summarized in Table 3. The viscosity-temperature relationship is presented in Fig. 2.
Table 3 Properties comparison of F2, SF6 glass (Schott), and NC-21A (ITME)
Group refractive index \(N\) was measured by the white light interferometric technique in the 400–1,700 nm range, and phase refractive index \(n\) for sodium line was measured with an Abbe refractometer. To obtain dispersion of the refractive index, classic Michelson interferometer setup was used (Fig. 3a). The cubic sample of the examined glass was placed in the signal arm of interferometer. The reference arm was equipped with a mirror mounted on the linear, manual stage, that enabled to measure the optical path difference (OPD) introduced by the glass in the signal arm. The interference patterns, as shown in Fig. 3b, was observed and analyzed by means of the spectrometers for visible and near infrared range. Measurement of the OPD as a function of the equalization wavelength allowed to determine the spectral dependence of group refractive index. Obtained curve was fitted by the function, that describes dispersion of group index of refraction \(N(\lambda )\) by including Sellmeier coefficients:
$$\begin{aligned} N(\lambda ) = n(\lambda ) + \frac{\lambda ^{2}}{n(\lambda )}\cdot \sum \limits _{i=1}^3 {\frac{B_i C_i }{(\lambda ^{2}-C_i )^{2}}} \end{aligned}$$
(1)
where phase index of refraction \(n(\lambda )\) is given by the Sellmeier formula:
$$\begin{aligned} n(\lambda ) = \sqrt{1 +\sum \nolimits _{i=1}^3 {\frac{B_i \lambda ^{2}}{\lambda ^{2}-C_i }} } \end{aligned}$$
(2)
The Sellmeier coefficients for NC-21A glass were: \(\hbox {B}_{1}\)—1.15702228, \(\hbox {B}_{2}\)—0.14959764, \(\hbox {B}_{3}\)—1.36007514, \(\hbox {C}_{1}\)—0.00614152, \(\hbox {C}_{2}\)—0.02521981, \(\hbox {C}_{3}\)—122.8441325. Measurements of glass transmittance were performed with the VARIAN Cary 500 spectrophotometer in the range of 200–3,300 nm; results are presented in Fig. 4.