Chalcogenide compound glass and optical element

Abstract

This invention provides chalcogenide glass and optical components with excellent thermal stability. In the chalcogenide glass, the content of Ga is 2.0~40.0% by mass, the content of Sb is 20.0~75.0% by mass, the content of S is 15.0~40.0% by mass, the total content of Na, K, Rb and Cs R[Na+K+Rb+Cs] is 0.05% by mass or more, and the total content of Cl, Br and I X[Cl+Br+I] is 0.01% by mass or less.

Description

Technical Field

[0001] This invention relates to chalcogenide glass and optical components. Background Technology

[0002] In recent years, technologies and products utilizing infrared light have attracted considerable attention. For example, the demand for anti-theft and authentication equipment using infrared cameras and sensors is constantly growing, and far-infrared imaging is also highly anticipated for vehicle-mounted night vision devices. Infrared cameras and sensors can utilize optical elements that transmit infrared light. Such optical elements require excellent infrared transmittance as well as superior manufacturability to meet the expanding demands of recent years.

[0003] Previously, crystalline materials such as germanium, silicon, zinc sulfide, and selenium sulfide were used as materials for optical elements that transmit infrared light. However, these crystalline materials have poor machinability and are difficult to process into complex shapes such as aspherical lenses. In contrast, chalcogenide glasses, which contain chalcogen elements as the main component, have been proposed as infrared transmission materials with excellent machinability.

[0004] As a chalcogenide glass, Patent Document 1 discloses an infrared transmission glass suitable for mold forming.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: International Publication No. 2016 / 159289 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] Reference 1 (Journal of Optoelectronics and Advanced Materials Vol. 9, No. 12, December 2007, pp. 3751-3755) discloses that glasses containing cesium halides such as CsCl have poor water resistance. Therefore, it can be inferred that the glass disclosed in Patent Document 1, which contains both Cs (cesium) and halogens, has poor water resistance. Furthermore, such halogen-containing glasses pose a risk of corroding molten containers such as quartz glass during manufacturing, and of corroding production equipment when molten glass or volatiles leak out, suggesting poor manufacturability.

[0010] Then, the inventors conducted in-depth research and found that by introducing alkali metals without adding halogens, which pose risks to water resistance and manufacturability, the thermal stability of chalcogenide glasses can be improved. While Patent Document 1 discloses glasses containing both halogens and alkali metals, it does not propose glasses with a sufficiently reduced halogen content and containing alkali metals, and the thermal stability of such glasses has not been verified. Poor thermal stability of the glass leads to problems such as difficulty in shaping the molten glass, difficulty in reheat pressing, and difficulty in molding.

[0011] Therefore, the object of the present invention is to provide chalcogenide glass and optical components with excellent thermal stability.

[0012] Problem Solving Methods

[0013] The main points of this invention are as follows.

[0014] (1) A chalcogenide glass, wherein,

[0015] The Ga content is 2.0~40.0% by mass.

[0016] The Sb content is 20.0~75.0% by mass.

[0017] The sulfur content is 15.0~40.0% by mass.

[0018] The total content of Na, K, Rb, and Cs, R[Na+K+Rb+Cs], is ≥0.05% by mass.

[0019] The total content of Cl, Br and I, X[Cl+Br+I], is less than 0.01% by mass.

[0020] (2) A chalcogenide glass, wherein,

[0021] The Ga content is 0.5% to 30.0% by mass.

[0022] The Ge content is less than 5.0% by mass.

[0023] The sulfur content is 15.0~40.0% by mass.

[0024] The total content of Na, K, Rb, and Cs, R[Na+K+Rb+Cs], is ≥0.05% by mass.

[0025] The total content of Cl, Br and I, X[Cl+Br+I], is less than 0.01% by mass.

[0026] (3) An optical element made of the chalcogenide glass described in (1) or (2) above.

[0027] The effects of the invention

[0028] According to the present invention, chalcogenide glass and optical components with excellent thermal stability can be provided. Attached Figure Description

[0029] Figure 1 It is a schematic diagram of the differential scanning calorimetry (DSC) curve. Detailed Implementation

[0030] In this specification, the content of glass components can be identified and quantified by known methods such as ICP atomic emission spectrometry (ICP-AES), atomic absorption spectrometry (AAS), ICP mass spectrometry (ICP-MS), ion chromatography, and non-dispersive infrared absorption spectrometry (ND-IR).

[0031] In this specification, the glass composition is expressed as mass % (%). Mass % refers to the percentage by mass when the total content of all elements contained in the glass is set to 100%. Unless otherwise stated, the content and total content of glass components are based on mass percentages, and "%" means "mass %". Furthermore, a content of 0.00% for a constituent element means that the constituent element is not substantially present, but its presence at an unavoidable level of impurities is permissible.

[0032] In this specification, excellent thermal stability of glass refers to the resistance to crystallization during the forming of molten glass or during the solidification of softened glass. Specifically, this refers to a low liquidus temperature (LT) and a large difference between the crystallization peak temperature (Tc) and the glass transition temperature (Tg). A low liquidus temperature (LT) increases the viscosity of the liquid phase, making it easier to obtain glass without crystallization. Furthermore, a large difference between the crystallization peak temperature (Tc) and the glass transition temperature (Tg) widens the temperature range within which crystallization can be pressed without precipitation. Additionally, water resistance means that the transmittance of the glass does not easily decrease even when exposed to water.

[0033] Hereinafter, the chalcogenide glass of the present invention will be described in two embodiments: the first embodiment and the second embodiment.

[0034] Implementation Method 1

[0035] In the chalcogenide glass of the first embodiment,

[0036] The Ga content ranges from 2.0% to 40.0%.

[0037] The Sb content ranges from 20.0% to 75.0%.

[0038] The sulfur content is 15.0%~40.0%.

[0039] The total content of Na, K, Rb, and Cs, R[Na+K+Rb+Cs], is above 0.05%.

[0040] The total content of Cl, Br and I, X[Cl+Br+I], is less than 0.01%.

[0041] In the chalcogenide glass of the first embodiment, the Ga content is 2.0% to 40.0%. The lower limit of the Ga content is preferably 2.2%, and more preferably 2.4%, 2.6%, 2.8%, 3.0%, 3.2%, 3.4%, 3.6%, 3.8%, 4.0%, 4.2%, 4.4%, 4.6%, 4.8%, 5.0%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, and 5.9%. Furthermore, the upper limit of the Ga content is preferably 38.0%, and more preferably 36.0%, 34.0%, 32.0%, 30.0%, 29.0%, 28.0%, 27.0%, 26.0%, 25.0%, 24.0%, 23.0%, 22.0%, 21.0%, 20.0%, 19.0%, and 18.0%.

[0042] Ga is a network-forming component of glass and also an expensive raw material. By setting the Ga content within the aforementioned range, the thermal stability of the glass can be improved, and the increase in raw material costs can be suppressed. On the other hand, if the Ga content is too low, there is a risk of reduced thermal stability of the glass and the inability to form the glass. If the Ga content is too high, there is a risk of the absorption end on the long wavelength side shifting to the short wavelength side, narrowing the wavelength range of light transmitted through the glass, as well as a risk of reduced liquid phase viscosity and increased raw material costs.

[0043] In the chalcogenide glass of the first embodiment, the Sb content is 20.0% to 75.0%. The lower limit of the Sb content is preferably 21.0%, and more preferably 22.0%, 23.0%, 24.0%, 25.0%, 26.0%, 27.0%, 28.0%, 29.0%, 30.0%, 31.0%, 32.0%, 33.0%, 34.0%, 35.0%, 36.0%, 37.0%, 38.0%, 39.0%, and 40.0%. Furthermore, the upper limit of the Sb content is preferably 74.0%, and more preferably 73.0%, 72.0%, 71.0%, 70.0%, 69.0%, 68.0%, 67.0%, 66.0%, 65.0%, 64.0%, 63.0%, 62.0%, and 61.0%.

[0044] Sb is a network-forming component in glass. By setting the Sb content within the aforementioned range, the thermal stability of the glass can be improved. On the other hand, if the Sb content is too low, there is a risk of reduced thermal stability of the glass, and there is also a risk that the glass cannot be formed.

[0045] In the chalcogenide glass of the first embodiment, the sulfur content is 15.0% to 40.0%. The lower limit of the sulfur content is preferably 16.0%, and more preferably 17.0%, 18.0%, 19.0%, 20.0%, 21.0%, 22.0%, 23.0%, 24.0%, and 25.0%. Furthermore, the upper limit of the sulfur content is preferably 39.0%, and more preferably 38.0%, 37.0%, 36.0%, 35.0%, 34.0%, 33.0%, 32.0%, and 31.0%.

[0046] By setting the sulfur content within the aforementioned range, the thermal stability of the glass can be improved. On the other hand, if the sulfur content is too low, there is a risk that the absorption end on the short wavelength side will shift to the long wavelength side and the absorption end on the long wavelength side will shift to the short wavelength side, thus narrowing the wavelength range of light transmitted through the glass. This could lead to a decrease in the thermal stability of the glass and make it impossible to form the glass.

[0047] In the chalcogenide glass of the first embodiment, the total content of Na, K, Rb and Cs, R[Na+K+Rb+Cs], is 0.05% or more. The lower limit of the total content R is preferably 0.06%, and more preferably 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%, 0.95%, 1.00%, 1.10%, 1.20%, 1.30%, 1.40%, 1.50%, 1.60%, 1.70%, 1.80%, 1.90%, 2.00%, 2.10%, 2.20%, 2.30%, 2.40%, 2.50%, 2.60%, 2.70%, 2.80%, 2.90%, and 3.00%. In addition, the upper limit of the total content R is preferably 35.0%, and more preferably 34.0%, 33.0%, 32.0%, 31.0%, 30.0%, 29.0%, 28.0%, 27.0%, 26.0%, 25.0%, 24.0%, 23.0%, 22.0%, 21.0%, and 20.0%.

[0048] Alkali metals such as Na, K, Rb, and Cs are components that lower the liquidus temperature (LT). By setting the total content R within the aforementioned range, the liquidus temperature (LT) decreases and the liquidus viscosity increases, thereby improving formability. Furthermore, it improves the thermal stability of the glass, allowing the absorption end on the short wavelength side to shift to even shorter wavelengths, thus expanding the wavelength range of light transmitted through the glass. If the total content R is too high, there is a risk of reduced thermal stability and the inability to obtain glass. Conversely, if the total content R is too low, there is a risk of insufficient reduction in the liquidus temperature (LT), insufficient increase in liquidus viscosity, failure to achieve the desired thermal stability, and failure to obtain the desired formability.

[0049] In the chalcogenide glass of the first embodiment, the total content X[Cl+Br+I] of Cl, Br, and I is 0.01% or less. The upper limit of the total content X is preferably 0.009%, and more preferably 0.008%, 0.007%, 0.006%, 0.005%, 0.004%, 0.003%, 0.002%, and 0.001%. Preferably, it is substantially free of Cl, Br, and I.

[0050] While halogens such as Cl, Br, and I shift the absorption end from the short wavelength side to even shorter wavelengths, they also reduce the water resistance of glass. Furthermore, there is a risk of corrosion during manufacturing of molten containers such as quartz glass, and corrosion of production equipment when molten glass or volatiles leak out, potentially leading to decreased productivity. By setting the total content X within the aforementioned range, the deterioration of the glass's water resistance can be suppressed, achieving the desired productivity.

[0051] In the chalcogenide glass of the first embodiment, the upper limit of the Ge content is preferably 2.0%, and more preferably 1.8%, 1.6%, 1.4%, 1.2%, 1.0%, 0.8%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, and 0.1%. In addition, the lower limit of the Ge content is preferably 0.0%.

[0052] Ge is a network-forming component of glass. From the viewpoint of improving the thermal stability of glass, it is preferable to set the Ge content within the aforementioned range. On the other hand, if the Ge content is too high, there is a risk that the absorption end on the long wavelength side will shift to the short wavelength side, thus narrowing the wavelength range of light transmitted through the glass. In addition, Ge is very expensive, so there is a risk of high raw material costs.

[0053] Regarding the content and ratio of glass components other than those described above in the chalcogenide glass of the first embodiment, non-limiting examples are shown below.

[0054] In the chalcogenide glass of the first embodiment, the lower limit of the Na content is preferably 0.00%, and more preferably 0.05%, 0.06%, 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%, and 0.95%. Furthermore, the upper limit of the Na content is preferably 35.0%, and more preferably 34.0%, 33.0%, 32.0%, 31.0%, 30.0%, 29.0%, 28.0%, 27.0%, 26.0%, 25.0%, 24.0%, 23.0%, 22.0%, 21.0%, 20.0%, 19.0%, 18.0%, 17.0%, 16.0%, 15.0%, 14.0%, 13.0%, 12.0%, 11.0%, 10.0%, 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, 4.0%, and 3.0%.

[0055] Sodium (Na) is a component that lowers the liquidus temperature (LT). From the viewpoints of improving formability by lowering the LT and increasing the liquidus viscosity, as well as improving the thermal stability of the glass and further shifting the absorption end from the short-wavelength side to the even shorter-wavelength side, the Na content is preferably set within the aforementioned range. If the Na content is too high, there is a risk of reduced thermal stability and the inability to obtain glass. If the Na content is too low, there is a risk of insufficient reduction in the LT, insufficient increase in liquidus viscosity, failure to obtain the desired thermal stability, and failure to obtain the desired formability.

[0056] In the chalcogenide glass of the first embodiment, the lower limit of the K content is preferably 0.00%, and more preferably 0.05%, 0.06%, 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%, and 0.95%. Furthermore, the upper limit of the K content is preferably 35.0%, and more preferably 34.0%, 33.0%, 32.0%, 31.0%, 30.0%, 29.0%, 28.0%, 27.0%, 26.0%, 25.0%, 24.0%, 23.0%, 22.0%, 21.0%, 20.0%, 19.0%, 18.0%, 17.0%, 16.0%, 15.0%, 14.0%, 13.0%, 12.0%, 11.0%, 10.0%, 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, and 4.0%.

[0057] K is a component that lowers the liquidus temperature LT. From the viewpoints of improving formability by lowering the liquidus temperature LT and increasing liquidus viscosity, as well as improving the thermal stability of the glass and further shifting the absorption end from the short-wavelength side to the even shorter-wavelength side, the K content is preferably set within the aforementioned range. If the K content is too high, there is a risk of reduced thermal stability and the inability to obtain glass. If the K content is too low, there is a risk of insufficient reduction in the liquidus temperature LT, insufficient increase in liquidus viscosity, failure to obtain the desired thermal stability, and failure to obtain the desired formability.

[0058] In the chalcogenide glass of the first embodiment, the lower limit of the Rb content is preferably 0.00%, and more preferably 0.05%, 0.06%, 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%, and 0.95%. Furthermore, the upper limit of the Rb content is preferably 35.0%, and more preferably 34.0%, 33.0%, 32.0%, 31.0%, 30.0%, 29.0%, 28.0%, 27.0%, 26.0%, 25.0%, 24.0%, 23.0%, 22.0%, 21.0%, 20.0%, 19.0%, 18.0%, 17.0%, 16.0%, 15.0%, 14.0%, 13.0%, 12.0%, 11.0%, and 10.0%.

[0059] Rb is a component that lowers the liquidus temperature (LT). From the viewpoints of improving formability by lowering the liquidus temperature (LT) and increasing liquidus viscosity, as well as improving the thermal stability of the glass and further shifting the absorption end from the short-wavelength side to a shorter wavelength side, the Rb content is preferably set within the aforementioned range. If the Rb content is too high, there is a risk of reduced thermal stability and the inability to obtain glass. If the Rb content is too low, there is a risk of insufficient reduction in the liquidus temperature (LT), insufficient increase in liquidus viscosity, failure to obtain the desired thermal stability, and failure to obtain the desired formability.

[0060] In the chalcogenide glass of the first embodiment, the lower limit of the Cs content is preferably 0.00%, and more preferably 0.05%, 0.06%, 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%, 0.95%, 1.00%, 1.10%, 1.20%, 1.30%, 1.40%, 1.50%, 1.60%, 1.70%, 1.80%, 1.90%, and 2.00%. Furthermore, the upper limit of the Cs content is preferably 35.0%, and more preferably 34.0%, 33.0%, 32.0%, 31.0%, 30.0%, 29.0%, 28.0%, 27.0%, 26.0%, 25.0%, 24.0%, 23.0%, 22.0%, 21.0%, 20.0%, 19.0%, and 18.0%.

[0061] Cs is a component that lowers the liquidus temperature (LT). From the viewpoints of improving formability by lowering the LT and increasing the liquidus viscosity, as well as improving the thermal stability of the glass and further shifting the absorption end from the short-wavelength side to a shorter wavelength side, the Cs content is preferably set within the aforementioned range. If the Cs content is too high, there is a risk of reduced thermal stability and the inability to obtain glass. If the Cs content is too low, there is a risk of insufficient reduction in the LT, insufficient increase in liquidus viscosity, failure to obtain the desired thermal stability, and failure to obtain the desired formability.

[0062] In the chalcogenide glass of the first embodiment, the upper limit of the content of each of In, Bi, and Te is preferably 30.0%, and more preferably 25.0%, 20.0%, 15.0%, 10.0%, and 5.0%, respectively. Furthermore, the lower limit of the content of each of In, Bi, and Te is preferably 0.0%. In, Bi, and Te are components that improve the thermal stability of the glass. From the viewpoint of improving the thermal stability of the glass, it is preferable to set the content of each of In, Bi, and Te within the above-mentioned ranges.

[0063] In the chalcogenide glass of the first embodiment, the upper limit of the Sn content is preferably 30.0%, and more preferably 28.0%, 26.0%, 24.0%, 22.0%, 20.0%, 18.0%, 16.0%, 14.0%, 12.0%, 10.0%, 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, 4.0%, 3.0%, and 2.0%. Furthermore, the lower limit of the Sn content is preferably 0.0%. Sn is a component that improves the thermal stability of the glass. From the viewpoint of improving the thermal stability of the glass, it is preferable to set the Sn content within the above-mentioned range.

[0064] In the chalcogenide glass of the first embodiment, the upper limit of the Li content is preferably 5.0%, and more preferably 4.0%, 3.0%, 2.0%, 1.0%, and 0.5%. Furthermore, the lower limit of the Li content is preferably 0.0%. If the Li content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Li content within the above-mentioned range.

[0065] In the chalcogenide glass of the first embodiment, the upper limit of the Zn content is preferably 10.0%, and more preferably 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, 4.0%, 3.0%, 2.0%, and 1.0%. Furthermore, the lower limit of the Zn content is preferably 0.0%. If the Zn content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Zn content within the above-mentioned range.

[0066] In the chalcogenide glass of the first embodiment, the upper limit of the La content is preferably 30.0%, and more preferably 25.0%, 20.0%, 15.0%, 10.0%, 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, 4.0%, 3.0%, 2.0%, and 1.0%. Furthermore, the lower limit of the La content is preferably 0.0%. La has the effect of shifting the absorption end on the short wavelength side to a shorter wavelength side and improving thermal stability. On the other hand, if the La content is too high, there is a risk that the thermal stability will decrease and the glass cannot be obtained. Therefore, it is preferable to set the La content within the above-mentioned range.

[0067] In the chalcogenide glass of the first embodiment, the upper limit of the Cu content is preferably 10.0%, and more preferably 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, 4.0%, 3.0%, 2.0%, and 1.0%. Furthermore, the lower limit of the Cu content is preferably 0.0%. If the Cu content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Cu content within the above-mentioned range.

[0068] In the chalcogenide glass of the first embodiment, the upper limit of the Ca content is preferably 10.0%, and more preferably 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, 4.0%, 3.0%, 2.0%, and 1.0%. Furthermore, the lower limit of the Ca content is preferably 0.0%. If the Ca content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Ca content within the above-mentioned range.

[0069] In the chalcogenide glass of the first embodiment, the upper limit of the Sr content is preferably 15.0%, and more preferably 14.0%, 13.0%, 12.0%, 11.0%, 10.0%, 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, 4.0%, 3.0%, 2.0%, and 1.0%. Furthermore, the lower limit of the Sr content is preferably 0.0%. If the Sr content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Sr content within the above-mentioned range.

[0070] In the chalcogenide glass of the first embodiment, the upper limit of the Ba content is preferably 15.0%, and more preferably 14.0%, 13.0%, 12.0%, 11.0%, 10.0%, 9.0%, 8.0%, 7.0%, 6.0%, 5.0%, 4.0%, 3.0%, 2.0%, and 1.0%. Furthermore, the lower limit of the Ba content is preferably 0.0%. If the Ba content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Ba content within the above-mentioned range.

[0071] In the chalcogenide glass of the first embodiment, the upper limit of the Ti content is preferably 1.0%, and more preferably 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, and 0.1%. Furthermore, the lower limit of the Ti content is preferably 0.0%. Ti has the function of bonding with oxygen within the glass and preventing other elements from bonding with oxygen. On the other hand, if the Ti content is too high, there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Ti content within the above-mentioned range.

[0072] In the chalcogenide glass of the first embodiment, the upper limit of the Si content is preferably 1.0%, and more preferably 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, and 0.1%. Furthermore, the lower limit of the Si content is preferably 0.0%. Si has the function of bonding with oxygen within the glass and preventing other elements from bonding with oxygen. However, Si-O has a strong absorption peak in the wavelength range of light transmitted through the chalcogenide glass. Therefore, it is preferable to set the Si content within the above-mentioned range.

[0073] In the chalcogenide glass of the first embodiment, the upper limit of the C content is preferably 5.0%, and more preferably 4.5%, 4.0%, 3.5%, 3.0%, 2.5%, 2.0%, 1.5%, 1.0%, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, and 0.1%. Furthermore, the lower limit of the C content is preferably 0.0%. C has the function of bonding with oxygen within the glass and preventing other elements from bonding with oxygen. However, if the C content is too high, there is a risk that the absorption end on the short wavelength side will shift to the long wavelength side, and the absorption end on the long wavelength side will shift to the short wavelength side, potentially reducing the maximum transmittance. Therefore, it is preferable to set the C content within the above-mentioned range.

[0074] In the chalcogenide glass of the first embodiment, the upper limit of the Al content is preferably 1.0%, and more preferably 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, and 0.1%. Furthermore, the lower limit of the Al content is preferably 0.0%. Al has the function of bonding with oxygen in the glass and preventing other elements from bonding with oxygen. On the other hand, if the Al content is too high, there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Al content within the above-mentioned range.

[0075] In the chalcogenide glass of the first embodiment, the upper limit of the Ag content is preferably 20.0%, and more preferably 15.0%, 10.0%, and 5.0%. Furthermore, the lower limit of the Ag content is preferably 0.0%. From the viewpoint of suppressing the rise in raw material costs, it is preferable to set the Ag content within the above-mentioned range.

[0076] In the chalcogenide glass of the first embodiment, the upper limit of the Gd content is preferably 20.0%, and more preferably 15.0%, 10.0%, and 5.0%. Furthermore, the lower limit of the Gd content is preferably 0.0%. From the viewpoint of suppressing the rise in raw material costs, it is preferable to set the Gd content within the above-mentioned range.

[0077] In the chalcogenide glass of the first embodiment, the upper limit of the Y content is preferably 20.0%, and more preferably 15.0%, 10.0%, and 5.0%. Furthermore, the lower limit of the Y content is preferably 0.0%. If the Y content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Y content within the above-mentioned range.

[0078] In the chalcogenide glass of the first embodiment, the upper limit of the Cr content is preferably 20.0%, and more preferably 15.0%, 10.0%, and 5.0%. Furthermore, the lower limit of the Cr content is preferably 0.0%. If the Cr content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Cr content within the above-mentioned range.

[0079] In the chalcogenide glass of the first embodiment, the upper limit of the Mn content is preferably 20.0%, and more preferably 15.0%, 10.0%, and 5.0%. Furthermore, the lower limit of the Mn content is preferably 0.0%. If the Mn content is too high, the thermal stability decreases, and there is a risk that the glass cannot be obtained. Therefore, it is preferable to set the Mn content within the above-mentioned range.

[0080] The chalcogenide glass of the first embodiment preferably consists mainly of the glass components described above, namely Ga, Sb, S, Na, K, Rb, Cs, Ge, In, Sn, Bi, Te, Zn, La, Cu, Ca, Sr, Ba, Ti, Al, Ag, Gd, Y, Cr, and Mn. The total content of the above glass components is preferably 95% or more, more preferably 98% or more, further preferably 99% or more, and particularly preferably 99.5% or more.

[0081] The chalcogenide glass of the first embodiment is preferably composed essentially of the above-described glass components, but may contain other components to the extent that it does not impair the effects of the present invention. Furthermore, the presence of unavoidable impurities is not excluded in the present invention.

[0082] (Glass properties)

[0083] <Light transmittance>

[0084] The chalcogenide glass of the first embodiment exhibits excellent light transmittance in the wavelength range of 0.4 to 14 μm. Specifically, the transmission-limiting wavelength (maximum transmission wavelength in the visible light region) on the short wavelength side is preferably 1.0 μm or less, more preferably 0.9 μm or less, and 0.8 μm or less. Furthermore, the transmission-limiting wavelength (maximum transmission wavelength in the infrared region) on the long wavelength side is preferably 11.0 μm or more, more preferably 11.5 μm or more, 12.0 μm or more, and 12.5 μm or more. The maximum transmission wavelength in the visible light region refers to the wavelength at which the maximum transmittance reaches 50% as measured by ultraviolet-visible spectrophotometry (UV-Vis) for a glass sample with a thickness of 1.5 mm. The maximum transmission wavelength in the infrared region refers to the wavelength at which the maximum transmittance reaches 50% as measured by Fourier transform infrared spectrophotometry (FT-IR) for a glass sample with a thickness of 1.5 mm.

[0085] <Glass transition temperature Tg>

[0086] In the chalcogenide glass of the first embodiment, the lower limit of the glass transition temperature Tg is preferably 150°C, and more preferably 160°C, 170°C, 180°C, 185°C, 190°C, 195°C, 200°C, 205°C, 210°C, 215°C, and 220°C. Furthermore, the upper limit of the glass transition temperature Tg is preferably 500°C, and more preferably 490°C, 480°C, 470°C, 460°C, 450°C, 440°C, 430°C, 420°C, 410°C, 400°C, 390°C, 380°C, 370°C, 360°C, 350°C, 340°C, 330°C, 320°C, 310°C, 300°C, and 290°C.

[0087] <Crystallization peak temperature Tc>

[0088] In the chalcogenide glass of the first embodiment, the lower limit of the crystallization peak temperature Tc is preferably 300°C, and even more preferably 310°C and 320°C.

[0089] The chalcogenide glass of the first embodiment also includes a glass that does not crystallize even when heated to 500°C and therefore does not exhibit a crystallization peak temperature Tc. Such a glass, which does not show a crystallization peak even when heated to 500°C, exhibits excellent thermal stability, excellent pressability, and excellent manufacturability.

[0090] Thermal stability

[0091] In the chalcogenide glass of the first embodiment, the thermal stability during pressing can be represented by the difference between the crystallization peak temperature Tc and the glass transition temperature Tg, [Tc-Tg]. In the pressing method, in order to soften the glass, it is necessary to heat it to a temperature above the glass transition temperature Tg. On the other hand, in order to prevent crystallization from precipitating in the glass, it is preferable to soften the glass at a temperature as low as possible below the crystallization peak temperature Tc. That is, the greater the difference between the crystallization peak temperature Tc and the glass transition temperature Tg, the wider the temperature range at which the glass can be softened without crystallization. Moreover, by increasing the difference between the crystallization peak temperature Tc and the glass transition temperature Tg, the viscosity range of the glass that can be pressed without crystallization when softening the glass can be expanded. Therefore, suitable pressing conditions with excellent productivity can be easily obtained. Regarding this point, for example, Patent Document 2 (Japanese Patent No. 6808543) describes that when [Tc-Tg] is high, crystallization can be sufficiently suppressed during mold forming, while when [Tc-Tg] is low, crystallization may occur during mold forming. Furthermore, Patent Document 1 states that a larger [Tc-Tg] indicates higher thermal stability and better moldability of the glass. In other words, these documents all state that the larger the [Tc-Tg], the better the thermal stability.

[0092] In the chalcogenide glass of the first embodiment, the difference between the crystallization peak temperature Tc and the glass transition temperature Tg [Tc-Tg] is preferably 100°C or higher, and more preferably 102°C or higher, 104°C or higher, 106°C or higher, 108°C or higher, 110°C or higher, 112°C or higher, 114°C or higher, 116°C or higher, 118°C or higher, 120°C or higher, 122°C or higher, 124°C or higher, 126°C or higher, and 128°C or higher. From the viewpoint of stably pressing and molding the glass, it is preferable to set the difference between the crystallization peak temperature Tc and the glass transition temperature Tg [Tc-Tg] within the above-mentioned range.

[0093] As described above, the chalcogenide glass of the first embodiment also includes glass in which no crystallization is observed even when heated to 500°C, that is, glass in which the crystallization peak temperature Tc cannot be measured even when heated to 500°C. For such glass, it can be considered that it has a wide temperature range in which it can be stably pressed without crystallization and has excellent thermal stability.

[0094] <Liquid phase temperature LT>

[0095] In the chalcogenide glass of the first embodiment, the upper limit of the liquidus temperature LT is preferably 700°C, and more preferably 690°C, 680°C, 670°C, 660°C, 650°C, 640°C, 630°C, 620°C, 610°C, 600°C, 590°C, 580°C, 570°C, 560°C, and 550°C. By setting the liquidus temperature LT within the above range, a glass with excellent thermal stability can be obtained. Furthermore, by setting the liquidus temperature LT of the glass within the above range, the liquid phase viscosity increases, allowing the glass to be easily obtained without crystallizing from the melt.

[0096] The "liquid phase temperature LT" in this invention and specification can be determined by the following method.

[0097] Using a differential scanning calorimeter, in a nitrogen atmosphere with a nitrogen flow rate of 50 mL / min, the glass sample was heated to 500~600℃ at a heating rate of 10℃ / min. The end of the endothermic peak generated when the crystals in the glass melted in a temperature region higher than the glass transition temperature Tg and the crystallization peak temperature Tc during the heating process was taken as the liquidus temperature LT. Figure 1 This is a schematic diagram of a differential scanning calorimetry (DSC) curve. The horizontal axis represents temperature; the temperature increases towards the right and decreases towards the left. The vertical axis corresponds to the exothermic / endothermic nature of the sample; the area above the baseline (dashed line) is exothermic, and the area below is endothermic. Crystallization during the heating process corresponds to an exothermic peak, and the melting of the precipitated crystals corresponds to an endothermic peak. The temperature at which all the crystals melt and liquefy is the liquidus temperature LT. The liquidus temperature LT is determined as the temperature at the intersection of the tangent of the high-temperature side of the endothermic peak and the baseline.

[0098] When the liquidus temperature LT is outside the measurement range of the differential scanning calorimeter, the glass can be vacuum-sealed in a quartz ampoule, heated to a given temperature by an electric furnace and held for 2 hours, then taken out and quenched. The sample is then observed by a microscope or other means, and the lowest temperature at which no crystals precipitate is determined as the liquidus temperature LT.

[0099] (Glass manufacturing)

[0100] The method for manufacturing the chalcogenide glass of the first embodiment is not particularly limited. For example, it can be manufactured by vacuum-sealing a given amount of raw material in a quartz ampoule to obtain the desired glass composition, and then vitrifying the contents by heat treatment. Alternatively, it can be obtained by filling a given amount of raw material into a crucible made of quartz, carbon, or alumina to obtain the desired glass composition, and then heating and melting it in an atmosphere containing an inert gas containing sulfur and a reducing gas. During vitrification, it is preferable to perform heat treatment at a heating temperature of 500 to 1000°C, and more preferably at a heating temperature of 700 to 950°C. The heat treatment temperature and time are only necessary to ensure that the contents are sufficiently vitrified.

[0101] There are no particular limitations on the raw materials used; elemental forms, sulfides, or other compounds of each glass component can be used. Since the total content X[Cl+Br+I] of Cl, Br, and I in the chalcogenide glass of the first embodiment is 0.01% by mass or less, it is preferable not to use raw materials whose main components are Cl, Br, and I.

[0102] (Manufacturing of optical components)

[0103] The chalcogenide glass of the first embodiment exhibits excellent thermal stability and is suitable for compression molding. During compression molding, the glass is heated to a temperature above its glass transition temperature Tg to achieve a desired softened state, and then pressed into a desired shape, for example, by clamping it between an upper and lower mold.

[0104] As optical elements that can be manufactured by pressing, especially by molding, there are no particular limitations. Examples include aspherical lenses, lens arrays, microlens arrays, and diffraction gratings, which require the property of transmitting infrared light. These are useful as optical elements used in infrared cameras and various sensors that utilize infrared light.

[0105] Implementation Method 2

[0106] In the chalcogenide glass of the second embodiment,

[0107] The Ga content is 0.5%~30.0%.

[0108] The Ge content is below 5.0%.

[0109] The sulfur content is 15.0%~40.0%.

[0110] The total content of Na, K, Rb, and Cs, R[Na+K+Rb+Cs], is above 0.05%.

[0111] The total content of Cl, Br and I, X[Cl+Br+I], is less than 0.01%.

[0112] In the chalcogenide glass of the second embodiment, the Ga content is 0.5% to 30.0%. The lower limit of the Ga content is preferably 0.6%, and more preferably 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, 3.0%, 3.2%, 3.4%, 3.6%, 3.8%, 4.0%, 4.2%, 4.4%, 4.6%, 4.8%, 5.0%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, and 5.9%. In addition, the upper limit of Ga content is preferably 29.0%, and more preferably 28.0%, 27.0%, 26.0%, 25.0%, 24.0%, 23.0%, 22.0%, 21.0%, 20.0%, 19.0%, 18.0%, and 17.0%.

[0113] Ga is a network-forming component of glass and also an expensive raw material. By setting the Ga content within the aforementioned range, the thermal stability of the glass can be improved, and the increase in raw material costs can be suppressed. On the other hand, if the Ga content is too low, there is a risk of reduced thermal stability of the glass and the inability to form the glass. If the Ga content is too high, there is a risk of the absorption end on the long wavelength side shifting to the short wavelength side, narrowing the wavelength range of light transmitted through the glass, as well as a risk of reduced liquid phase viscosity and increased raw material costs.

[0114] In the chalcogenide glass of the second embodiment, the Ge content is 5.0% or less. The upper limit of the Ge content is preferably 4.8%, and more preferably 4.6%, 4.4%, 4.2%, 4.0%, 3.8%, 3.6%, 3.4%, 3.2%, 3.0%, 2.8%, 2.6%, 2.4%, 2.3%, 2.2%, 2.0%, 1.8%, 1.6%, 1.4%, 1.2%, and 1.0%. Furthermore, the lower limit of the Ge content is preferably 0.0%.

[0115] Ge is a network-forming component of glass. By setting the Ge content within the aforementioned range, the thermal stability of the glass can be improved. On the other hand, if the Ge content is too high, there is a risk that the absorption end on the long wavelength side will shift to the short wavelength side, thus narrowing the wavelength range of light transmitted through the glass. Moreover, since Ge is very expensive, there is a risk of increased raw material costs.

[0116] In the chalcogenide glass of the second embodiment, the sulfur content is 15.0% to 40.0%. The lower limit of the sulfur content is preferably 16.0%, and more preferably 17.0%, 18.0%, 19.0%, 20.0%, 21.0%, 22.0%, 23.0%, 24.0%, and 25.0%. Furthermore, the upper limit of the sulfur content is preferably 39.0%, and more preferably 38.0%, 37.0%, 36.0%, 35.0%, 34.0%, 33.0%, 32.0%, and 31.0%.

[0117] By setting the sulfur content within the aforementioned range, the thermal stability of the glass can be improved. On the other hand, if the sulfur content is too low, there is a risk that the absorption end on the short wavelength side will shift to the long wavelength side and the absorption end on the long wavelength side will shift to the short wavelength side, thus narrowing the wavelength range of light transmitted through the glass. This could lead to a decrease in the thermal stability of the glass and make it impossible to form the glass.

[0118] In the chalcogenide glass of the second embodiment, the total content of Na, K, Rb and Cs, R[Na+K+Rb+Cs], is 0.05% or more. The lower limit of the total content R is preferably 0.06%, and more preferably 0.08%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, 0.70%, 0.75%, 0.80%, 0.85%, 0.90%, 0.95%, 1.00%, 1.10%, 1.20%, 1.30%, 1.40%, 1.50%, 1.60%, 1.70%, 1.80%, 1.90%, 2.00%, 2.10%, 2.20%, 2.30%, 2.40%, 2.50%, 2.60%, 2.70%, 2.80%, 2.90%, and 3.00%. In addition, the upper limit of the total content R is preferably 35.0%, and more preferably 34.0%, 33.0%, 32.0%, 31.0%, 30.0%, 29.0%, 28.0%, 27.0%, 26.0%, 25.0%, 24.0%, 23.0%, 22.0%, 21.0%, and 20.0%.

[0119] Alkali metals such as Na, K, Rb, and Cs are components that lower the liquidus temperature (LT). By setting the total content R within the aforementioned range, the liquidus temperature (LT) decreases and the liquidus viscosity increases, thereby improving formability. Furthermore, it improves the thermal stability of the glass, allowing the absorption end on the short wavelength side to shift to even shorter wavelengths, thus expanding the wavelength range of light transmitted through the glass. If the total content R is too high, there is a risk of reduced thermal stability and the inability to obtain glass. Conversely, if the total content R is too low, there is a risk of insufficient reduction in the liquidus temperature (LT), insufficient increase in liquidus viscosity, failure to achieve the desired thermal stability, and failure to obtain the desired formability.

[0120] In the chalcogenide glass of the second embodiment, the total content X[Cl+Br+I] of Cl, Br, and I is 0.01% or less. The upper limit of the total content X is preferably 0.009%, and more preferably 0.008%, 0.007%, 0.006%, 0.005%, 0.004%, 0.003%, 0.002%, and 0.001%. Preferably, it is substantially free of Cl, Br, and I.

[0121] While halogens such as Cl, Br, and I shift the absorption end from the short wavelength side to even shorter wavelengths, they also reduce the water resistance of glass. Furthermore, there is a risk of corrosion during manufacturing of molten containers such as quartz glass, and corrosion of production equipment when molten glass or volatiles leak out, potentially leading to decreased productivity. By setting the total content X within the aforementioned range, the deterioration of the glass's water resistance can be suppressed, achieving the desired productivity.

[0122] In the chalcogenide glass of the second embodiment, the lower limit of the Sb content is preferably 0.0%, and more preferably 2.0%, 4.0%, 6.0%, 8.0%, 10.0%, 12.0%, 14.0%, 16.0%, 18.0%, 20.0%, 21.0%, 22.0%, 23.0%, 24.0%, 25.0%, 26.0%, 27.0%, 28.0%, 29.0%, 30.0%, 31.0%, 32.0%, 33.0%, 34.0%, 35.0%, 36.0%, 37.0%, 38.0%, 39.0%, and 40.0%. Furthermore, the upper limit of the Sb content is preferably 84.0%, and more preferably 82.0%, 80.0%, 78.0%, 76.0%, 75.0%, 74.0%, 73.0%, 72.0%, 71.0%, 70.0%, 69.0%, 68.0%, 67.0%, 66.0%, 65.0%, 64.0%, 63.0%, 62.0%, and 61.0%.

[0123] Sb is a network-forming component of glass. From the viewpoint of improving the thermal stability of glass, it is preferable to set the Sb content within the aforementioned range. On the other hand, if the Sb content is too low, there is a risk of reduced thermal stability of the glass, and there is also a risk that the glass cannot be formed.

[0124] In the chalcogenide glass of the second embodiment, the content of glass components other than those described above can be set to be the same as in the first embodiment described above.

[0125] In the chalcogenide glass of the second embodiment, the glass properties can be set to be the same as those of the first embodiment described above.

[0126] The manufacturing of the chalcogenide glass and the optical element in the second embodiment can also be the same as in the first embodiment described above.

[0127] Example

[0128] The present invention will be further described in detail below through embodiments. However, the present invention is not limited to the embodiments shown.

[0129] (Example 1)

[0130] Glass samples with the glass compositions shown in Table 1 were prepared using the following steps, and various evaluations were performed. It should be noted that the contents of Cl, Br, and I in any glass sample were 0.00% by mass. Additionally, the content of Ge in any glass sample was 0.00% by mass. Furthermore, in Comparative Example 1, a glass sample was prepared with the contents of Na, K, Rb, and Cs all being 0.00% by mass.

[0131] [Glass Manufacturing]

[0132] Quartz ampoules were prepared and their interiors were cleaned with purified water. A rotary vacuum pump was operated, and the quartz ampoules were heated under vacuum using a burner to evaporate the water. The raw materials were mixed and added to the quartz ampoules to achieve the glass composition shown in Table 1. After evacuating the ampoules to a sufficient vacuum using the rotary vacuum pump, they were sealed using an H2-O2 burner. The sealed quartz ampoules were heated to 950°C at a rate of 20°C / hour and held at that temperature for 8 hours. They were then allowed to cool naturally to room temperature to vitrify the contents, thus producing a glass sample. Alternatively, raw materials were filled in a given amount into a carbon crucible to obtain the glass composition shown in Table 1, and heated and melted in an inert gas atmosphere containing sulfur gas to obtain a glass sample. The melting temperature was set to 500–1200°C, and the melting time was set to 1–12 hours. Glass samples were obtained by melting in quartz ampoules and by heating in an inert gas atmosphere containing sulfur gas.

[0133] [Light transmittance]

[0134] To evaluate transmittance from visible light to infrared light, the maximum transmission wavelengths in the visible and infrared regions were measured. The maximum transmission wavelength in the visible region was the wavelength at which 50% maximum transmittance was achieved by measuring a 1.5 mm thick glass sample using a UV-Vis-NIR spectrophotometer (Shimadzu Corporation, UV-3600i Plus, measurement wavelength range 350–2500 nm). The maximum transmission wavelength in the infrared region was the wavelength at which 50% maximum transmittance was achieved by measuring a 1.5 mm thick glass sample using a Fourier transform infrared spectrophotometer (Shimadzu Corporation, IRTracer-100, measurement wavelength range 2–16 μm). It should be noted that "maximum transmittance" as described in this specification refers to the transmittance at the wavelength where the transmittance is maximized, also known as the highest transmittance.

[0135] [Glass transition temperature Tg, peak crystallization temperature Tc]

[0136] The glass transition temperature Tg and crystallization peak temperature Tc were determined using a differential scanning calorimeter (DSC8271) manufactured by Rigaku Corporation, with a nitrogen flow rate of 50 mL / min and a heating rate of 10 °C / min, and the temperature was raised to 500 °C.

[0137] The difference between the crystallization peak temperature Tc and the glass transition temperature Tg [Tc-Tg] was calculated, confirming the thermal stability. It should be noted that glass samples that did not exhibit crystallization even when heated to 500℃ and therefore did not show a crystallization peak temperature Tc were evaluated as having excellent thermal stability.

[0138] [Liquid phase temperature LT]

[0139] Using a differential scanning calorimeter, a glass sample was heated to 500°C in a nitrogen atmosphere with nitrogen flowing at a rate of 10°C / min. The liquidus temperature LT was defined as the endpoint of the endothermic peak generated when the crystals precipitated in the glass during the heating process melt in a temperature region above the glass transition temperature Tg and the crystallization peak temperature Tc. The liquidus temperature LT was determined as the temperature at the intersection of the tangent on the high-temperature side of the endothermic peak and the baseline.

[0140] [Water resistance]

[0141] Glass samples were processed to a thickness of 1.5 mm and immersed in sufficient water at room temperature for 1 hour. The transmittance of light with wavelengths from 2 to 16 μm was measured in the glass samples before and after immersion in water using a Fourier transform infrared spectrophotometer (Shimadzu Corporation, IRTracer-100). Good water resistance was assessed as long as the maximum transmittance did not decrease by more than 10% before and after immersion in water. Good water resistance was confirmed for all glass samples.

[0142]

[0143] (Example 2)

[0144] The glass sample prepared in Example 14 was molded at 300°C in a nitrogen atmosphere to produce aspherical lenses, lens arrays, and microlens arrays. The glass sample exhibited excellent thermal stability and good machinability during molding, resulting in well-made aspherical lenses, lens arrays, and microlens arrays.

[0145] It should be understood that the embodiments disclosed herein are exemplary in all respects and do not constitute a limitation. The scope of the invention is defined by the claims, not by the foregoing description, and is intended to include all modifications within the meaning and scope of the claims.

[0146] For example, by adjusting the composition described in the specification, a chalcogenide glass of one aspect of the present invention can be prepared for the glass composition exemplified above.

[0147] In addition, of course, any combination of two or more items described in the specification as examples or preferred options is possible.

Claims

1. A chalcogenide glass, wherein, The Ga content is 2.0~40.0% by mass. The Sb content is 20.0~75.0% by mass. The sulfur content is 15.0~40.0% by mass. The total content of Na, K, Rb, and Cs, R[Na+K+Rb+Cs], is ≥0.05% by mass. The total content of Cl, Br and I, X[Cl+Br+I], is less than 0.01% by mass.

2. A chalcogenide glass, wherein, The Ga content is 0.5% to 30.0% by mass. The Ge content is less than 5.0% by mass. The sulfur content is 15.0~40.0% by mass. The total content of Na, K, Rb, and Cs, R[Na+K+Rb+Cs], is ≥0.05% by mass. The total content of Cl, Br and I, X[Cl+Br+I], is less than 0.01% by mass.

3. An optical element made of the chalcogenide glass of claim 1 or 2.

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