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Infrared transmission and properties of glasses in the PbO–B2O3–SiO2–GeO2 system

A study of transparent materials for infrared emission is conditioned by the need for development and improvement of the optical and optoelectronic devices for space, aviation and medical purposes. Easily fusible optical glasses working in the nearand mid-infrared spectral region being characterized by a high refractive index are of great scientific and practical interest. The objective of these studies was to study influence of the chemical composition on the infrared transmission and properties of glasses in the PbO–B2O3–SiO2–GeO2 system. Conducted studies have testified to the fact that the transmission cutoff of glasses in the infrared region varies from 2.7 to 5.5 μm depending on the content of basic glass forming oxides B2O3, SiO2 and GeO2. The additive of fluoride to the compositions of glasses under study makes for the expansion of the transmission cutoff of infrared rays and the decrease in the intensity of absorption bands of the hydroxyl groups.


Introduction
Easily fusible glasses based on heavy metal oxides (PbO, Bi 2 O 3 , TeO 2 , GeO 2 and others) are characterized by a number of valuable properties making these materials attractive and promising for many areas of applied optics. The important properties among them are: high refractive index; low softening point (makes it possible to make complex-shape articles by the extrusion or moulding); wide range of spectral transmission (from the near ultraviolet to the middle infrared band). The glasses, which are characterized by a high transmission in the near-and mid-infrared spectral region (up to λ = 5.5 µm), are of special scientific and practical interest [1][2][3][4][5][6]. This is due to the fact that some windows of the atmospheric spectral transparency are in this region, which is critically important when transferring the emission over large distances, especially for the aeronautical and aerospace engineering.
However, regardless of the prospects, a high level of optical losses, first of all conditioned by the impurity absorption of hydroxyl groups in the near-and mid-infrared spectral region, still prevents wide use of easily fusible glasses in the infrared optics [6][7][8][9][10]. The nature of the influence exerted by these impurities on the optical properties of glasses is firstly determined by the chemical composition and structure of initial glass.
The objective of these studies is to examine the influence of the chemical composition on the infrared transmission and properties of glasses in the PbO-B 2 O 3 -SiO 2 -GeO 2 system.

Materials and methods
Finely ground quartz sand, high purity and chemically pure chemical reagents (H 3 BO 3 , BaCO 3 , ZnO, Pb 3 O 4 , La 2 O 3 , Al 2 O 3 , and GeO 2 ) were used to prepare batches of test glasses, which chemical composition is set forth in Table 1 The glasses were melted in platinum crucibles with a volume of 50 ml in an electric furnace with the silicon carbide heating elements at a temperature of 1050 °C within 60 minutes. The samples of glasses for the determination of their properties were manufactured by the glass melt casting into the steel moulds followed by their muffle furnace annealing at a temperature of 320 °C.
The properties of glasses were determined according to the standard procedures: the density (d) of glasses by hydrostatic weighing according to GOST 9553-74; the thermal coefficient of linear expansion (TCLE) in the temperature range of 20-200 °C and the glass transition temperature (t g ) with the quartz dilatometer according to GOST 10978-2014.
The refractive index of the glasses have been measured with ellipsometric method [11]. The ellipsometric measurements were carried out at seven different incident angles (for error minimization) using LEM-3M ellipsometer equipped with low-intensity He-Ne laser (λ = 632.8 nm).
The transmission spectra of plane-parallel glass samples 2.0 mm thick were measured at room temperature with Spectrometer Thermo-Nicolet Avatar 370 FT-IR (in the range of 2500-25000 nm wavelengths at a pitch of 1 nm).

Research of properties of glasses in the PbO-B 2 O 3 -SiO 2 -GeO 2 oxide system
Experimentally established values of the properties of oxide glasses are set forth in Table 2  Depending on the content of basic glass forming oxides B 2 O 3 , SiO 2 , and GeO 2 , the glasses have different transmission cutoff and transparency in the infrared region ( Fig. 1). A wavelength, at which the transmittance of glass 2 mm thick is greater than 50 % and 0 %, accordingly, is taken as the transmission cutoff (λ 50% ) and transparency (λ 0% ) in the infrared region. The PbB2 glass (Fig. 1) having B 2 O 3 in its content is characterized by a low transparency in the infrared region (λ 0% = 3.5 µm). The transmission cutoff of this glass is limited by a 2.7 µm wavelength; in the following, there begins a strong absorption band of free hydroxyl groups present in the glass.
Complete replacement of B 2 O 3 with SiO 2 makes it possible to expand transparency of the PbSi1 glass up to λ 0% = 5.0 µm. However, due to the absorption bands in the 2.8-3.2 µm and 4.25 µm region, which are associated with the stretch vibration of free and tightly bound hydroxyl groups, the transmission cutoff of the PbSi1 glass ends in the region λ 50% = 2.7 µm.
The PbGe1 and PbGe2 glasses based on the glass forming oxide GeO 2 , which are transparent up to λ 0% = 6.0 µm, have a high transparency in the infrared region. The transmission cutoff of these glasses ends in the region of absorption bands of free and tightly bound hydroxyl groups in the glass structure, which absorption maximum is in the region of 3.0 and 4.3 µm wavelengths, accordingly.
The analysis of IR transmission spectra of glasses in the PbO-B 2 O 3 -SiO 2 -GeO 2 system has proved that all glasses have a limited infrared transmission cutoff up to λ 50% = 2.7 µm due to the absorption bands in the 2.8-3.2 µm and 4.25 µm region, which are associated with the stretch vibration of free and tightly bound -OH groups.
The removal of hydroxyl groups from the glass structure is a rather complicated technological task, which requires the creation of special glass melting conditions. The optical glasses with the minimum amount of impurities of -OH groups are obtained: when glass melting in vacuum or dry atmosphere; when glass melt bubbling with dry oxygen or nitrogen; when fluoride or chloride additives are introduced to the glass batch [6-10].

Study of influence of fluoride additives on the infrared transmission and properties of glasses in the PbO-B 2 O 3 -SiO 2 -GeO 2 system
After a number of experiments carried out by us, we managed to attain high level of transparency and expand the transmission cutoff of oxide glasses with the help of fluoride glass dehydration technique [12]. The dehydration mechanism of this technique is based on a chemical reaction, which is going in the glass melt: (1) The study of influence of fluoride additives on the degree of dehydration of test glasses was conducted on the compositions (PbB2, PbSi1 and PbGe1) with different content of basic glass forming oxides. Lead, barium and zinc oxides were replaced with fluorides so that when fluorine was replaced with oxygen in the glass, its chemical composition (   The introduction of a small amount of fluorine additives to the composition of oxide glasses makes for a sharp decrease in the refractive index, density and glass transition temperature thereof as well as the increase of the TCLE and decrease in the glass colour intensity (Fig. 2).

Fig. 2 Samples of oxide and oxyfluoride glasses 2. ábra Oxid-és oxifluorid-üveg minták
Depending on the nature of the basic glass forming oxide in the composition of test glass, the introduction of fluorine additives exerts different influence on the change in the intensity of absorption bands of the hydroxyl groups.
When fluorine is introduced to the composition of the PbB2 glass (Fig. 3), there has been observed a decrease in content of free hydroxyl groups in the glass as evidenced by a decrease in the intensity of absorption band with the maximum in the 2.9 µm region and its shift to the longwave region. The introduction of fluorine to the composition of the PbSi1 glass (Fig. 4, line PbSiF1) leads to the decrease in the intensity of absorption bands of free and tightly bound hydroxyl groups with the maximum in the 3.0 µm and 4.25 µm wavelength region. Further increase of fluorine content in the glass does not exert influence on the intensity of absorption bands of the hydroxyl groups (Fig. 4, line PbSiF2), however, leads to a significant decrease in the refractive index, density and glass transition temperature (Table 4). The transparency of glasses based on glass forming oxide SiO 2 completely ends in the region λ 0% = 5.0 µm. We managed to attain a high level of transparency and expand the transmission cutoff by introduction of a small amount of fluorine additives to the composition of the PbGe1 glass ( Fig.  5) wherein GeO 2 is the glass forming oxide. The introduction of Fanion to the composition of the PbGe1 glass makes for the expansion of its transmission cutoff in the infrared region up to λ 50% = 5.5 µm and also increases its transparency up to λ 0% = 6.2 µm. Further increase of fluorine content in the glass (Fig. 5, line PbGeF2) does not exert significant influence on the intensity of absorption bands of the hydroxyl groups and the transmission cutoff of glass, however, leads to a decrease in the refractive index, density and glass transition temperature.

Conclusions
The influence of chemical composition on the physicochemical properties of glasses in the PbO-B 2 O 3 -SiO 2 -GeO 2 system is experimentally established. It is shown that the transmission cutoff of research glasses in the infrared region varies from 2.7 to 5.5 µm depending on the content and nature of the basic glass forming oxide. It is noted the efficiency of use of fluoride additives in the composition of oxide glasses in order to decrease the impurity absorptions of the hydroxyl groups in the infrared region.