rus
Issue #1/2023
A. S. Moskalenko, A. V. Samvelov, I. V. Libkind, A. V. Lobashov
Coating Technology in the Production of Aperture “Cold” Diaphragms for Photodetectors
Coating Technology in the Production of Aperture “Cold” Diaphragms for Photodetectors
DOI: 10.22184/1993-7296.FRos.2023.17.1.8.13
The most important optical element of the photodetector (PD) is the aperture “cold” diaphragm that protects the photodetector module against leakage radiation and flares. The article proposes a method to obtain and generate a coating on the aperture “cold” diaphragm surface being a PD component in the medium-wave and long-wave infrared band.
The most important optical element of the photodetector (PD) is the aperture “cold” diaphragm that protects the photodetector module against leakage radiation and flares. The article proposes a method to obtain and generate a coating on the aperture “cold” diaphragm surface being a PD component in the medium-wave and long-wave infrared band.
Теги: aperture diaphragm chemical coating cooled ir photodetectors grayness degree photodetector (pd) апертурная диафрагма охлаждаемые фотоприемники ик-диапазона степень серости фотоприемное устройство (фпу) химическое покрытие
Coating Technology in the Production of Aperture “Cold” Diaphragms for Photodetectors
A. S. Moskalenko 1, 2, A. V. Samvelov 1, 2, I. V. Libkind 1, A. V. Lobashov 1
Voskhod JSC – Kaluga Radiotube Plant, Kaluga, Russia
Scientific and Technical Center Cryonex LLC, Moscow, Russia
The most important optical element of the photodetector (PD) is the aperture “cold” diaphragm that protects the photodetector module against leakage radiation and flares. The article proposes a method to obtain and generate a coating on the aperture “cold” diaphragm surface being a PD component in the medium-wave and long-wave infrared band.
Keywords: photodetector (PD), cooled IR photodetectors, aperture diaphragm, chemical coating, grayness degree
Received on: 03.08.2022
Accepted on: 07.11.2022
In the current troubled political circumstances in the world, there is an increasing need to provide the armed forces with the cooled infrared photodetectors (PDs). The optoelectronic systems (OES), equipped with PDs, can operate in various modes according to the tasks performed and type of the analog-to-digital image processing, such as the target visualization, scanning, tracking, detection and following.
The most important optical element of the PD is the aperture “cold” diaphragm (diaphragm with an external cooling). It protects the infrared photodetector module against leakage radiation and flares that can get to the sensitive PD area during the OES operation process [1]. To ensure the standard technical specifications of the aperture “cold” diaphragm, it is necessary to apply various technological methods during its manufacturing process that provide the possible varying the optical properties and parameters of its working surfaces.
The article describes the main approach to the application technology for an optical absorbent coating of aperture diaphragms for various PDs manufactured by Voskhod JSC – Kaluga Radiotube Plant using the chemical oxidation methods. The photodetector aperture materials developed by Voskhod JSC – Kaluga Radiotube Plant include stainless steel, grade 07X17H16 and titanium alloy, grade VT6.
To ensure proper adhesion behavior, any type of coating requires a fairly thorough pre-treatment of the part. It is a common fact that prior to the chemical coating formation, chemical degreasing of the relevant surfaces is required [2]. We have developed a specialized solubilizing (facilitating the dissolution of poorly soluble ingredients in a liquid base) multicomponent highly dispersed and hyper-heterogeneous stable colloidal solution that provides the highly-efficient immediate degreasing of steels and a number of alloys. The solution assumes a single-stage degreasing procedure with a short-term cleaning cycle. For stainless steels, the cleaning duration is from 10 to 20 minutes, for titanium alloys – from 15 to 30 minutes at a temperature of (60±2) °C. The material for the bath used for cleaning can only be the grades of stainless steel. The solution shall be prepared in distilled water with a concentration of 60 g/l.
Further, after the cascade cleaning, the part surface is activated by deoxidation of active alloying elements. This procedure is performed under normal climatic conditions (NCC) and includes three stages:
In HCl solution (under NCC);
In a solution of a mixture of H2SO4 and HCl (with subsequent cooling to NCC);
In a solution of HOOC–COOH (under NCC).
The main oxidation procedure is performed in a compound developed by Voskhod JSC – Kaluga Radiotube Plant being a 60% NH4NO2 solution with the addition of barium nitrate. The procedure is performed at a temperature of (140±5) °C with occasional stirring. The holding time is 5–15 minutes (not more) depending on the weight of the parts, material grade and surface condition. The increased holding time can lead to the insufficient coating adhesion [3].
To accelerate the coating growth and to obtain a greater thickness, our specialists have provided for introduction of special additives into the solution that promote the nitrate anion formation.
The direct chemical reaction in the NH4NO2 solutions leads to the generation of magnetite, Fe3O4, that increases the grayness degree (ε) of the formed coating:
3Fe + 4H2O = Fe3O4 + 8H+ + 8e. (1)
As a result of a series of reactions, a magnetite matting coating is formed on the basis of a supersaturated iron hydroxide solution:
2Fe(OH)3 + Fe(OH)2 = Fe3O4 + 4H2O. (2)
The addition of an iron dissipation activator (NH4)2S2O8 to the solution leads to the increased thickness and enhanced surface matting that significantly increases the surface grayness degree [4, 5]. Fig. 1–2 demonstrates the results of oxidizing coating application to the control samples and parts under various technological conditions.
When refining the coating technology with the properties of PD protection against any flares, it is necessary to measure the surface temperature of a “cold” diaphragm, for which the grayness degree is unknown (the emissive capacity uncertainty is due to the surface microstructure, chemical composition and phase state of the object). Therefore, after the coating application to the control samples, the latter are subject to the grayness degree measurements. The grayness degree of the samples is measured with a thermal imager using an iterative method.
A film with a well-known grayness degree ε is glued onto the sample surface. To adapt the film to the prototype surface, the required exposure time shall be 5–7 minutes. The film grayness degree value is entered into the measuring unit of the thermal imager. The surface temperature of the sample in the area covered with the film shall be measured. After that, the temperature of the sample surface, not covered with a film, shall be measured.
Next, it is necessary to measure the sample grayness degree using a thermal imager until its value stops changing (stabilizes). The resulting grayness factor will be the true factor for surface of the sample being measured.
According to the measurements results obtained by this method, the following ε values are obtained for the control samples: Fig.1 a (on the left): ε = 0.97; Fig.1a (on the right): ε = 0.92; Fig. 1b: ε = 0.98.
Fig. 2 (a, b) shows the aperture diaphragms of various PDs with the processing method adjustments. The measurement results for the grayness degree of the diaphragm surfaces are as follows: Fig. 2a (on the left and on the right): ε = 0.96...0.97 and (in the center): ε = 0.87; Fig. 2b (on the right and on the left): ε = 0.98 and (in the center): ε = 0.9.
Conclusion
In conclusion, it should be emphasized that the aperture “cold” diaphragm of the cooled photodetector, installed in the photodetector module case (Fig.3), is designed to protect the photodetector against the background noise and other leakage radiation that can interfere with the electrical signal received from the sensitive detector elements during its operation [3].
To improve the optical specifications of aperture diaphragms, the latter are usually polished on the outside and internally blackened. The aperture diaphragms currently used for the up-to-date photodetectors have a grayness degree in the range of ε = 0.95...0.98 (under NCC).
The results obtained by Voskhod JSC – Kaluga Radiotube Plant in relation to the surface coating of aperture “cold” diaphragms for the photodetector assemblies by the developed chemical oxidation method fall within the range of grayness degree ε = 0.97–0.98. This is a very representative result of works performed to develop the coating application technology on the PDs. The obtained emissivity factor value is quite consistent with the aperture diaphragm values of modern photodetectors.
ABOUT AUTHORS
Samvelov A.V., Cand.of Scien. (Engin.), General director STC CRyoNex Corresponding member of the International Academy of Cold, www.cryonex.ru , STC CRyoNex, Moscow, Russia.
ORCID: 0000-0001-5840-7626
Moskalenko A. S., Technical Director of STC Cryonex, Academic Advisor of the International Academy of Cold, www.cryonex.ru , STC CRyoNex, Moscow, Russia; eng.-constr. 1 cat., www.voshod-krlz.ru , Voskhod JSC – Kaluga Radiotube Plant, Kaluga, Russia.
ORCID: 0000-0002-1657-5015
Libkind I. V., technical director, Voskhod JSC – Kaluga Radiotube Plant, Kaluga, Russia
Lobashov A. V., chief DB-4 CDD, www.voshod-krlz.ru , Voskhod JSC – Kaluga Radio Tube Plant, Kaluga, Russia.
Conflict of interest
The authors claim that they have no conflict of interest. All authors took part in writing the article and supplemented the manuscript in part of their work.
AUTHOR CONTRIBUTIONS
A. S. Moskalenko – coating design and development of a specialized solubilizing multicomponent highly dispersed and hyper-heterogeneous stable colloidal solution; A. V. Samvelov – development of a control method for the optical properties of the diaphragm coating, arrangement of technology transfer; I. V. Libkind – development of a chemical oxidation method for the surfaces of aperture “cold” diaphragms; A. V. Lobashov – refinement of coating technology and measurements.
A. S. Moskalenko 1, 2, A. V. Samvelov 1, 2, I. V. Libkind 1, A. V. Lobashov 1
Voskhod JSC – Kaluga Radiotube Plant, Kaluga, Russia
Scientific and Technical Center Cryonex LLC, Moscow, Russia
The most important optical element of the photodetector (PD) is the aperture “cold” diaphragm that protects the photodetector module against leakage radiation and flares. The article proposes a method to obtain and generate a coating on the aperture “cold” diaphragm surface being a PD component in the medium-wave and long-wave infrared band.
Keywords: photodetector (PD), cooled IR photodetectors, aperture diaphragm, chemical coating, grayness degree
Received on: 03.08.2022
Accepted on: 07.11.2022
In the current troubled political circumstances in the world, there is an increasing need to provide the armed forces with the cooled infrared photodetectors (PDs). The optoelectronic systems (OES), equipped with PDs, can operate in various modes according to the tasks performed and type of the analog-to-digital image processing, such as the target visualization, scanning, tracking, detection and following.
The most important optical element of the PD is the aperture “cold” diaphragm (diaphragm with an external cooling). It protects the infrared photodetector module against leakage radiation and flares that can get to the sensitive PD area during the OES operation process [1]. To ensure the standard technical specifications of the aperture “cold” diaphragm, it is necessary to apply various technological methods during its manufacturing process that provide the possible varying the optical properties and parameters of its working surfaces.
The article describes the main approach to the application technology for an optical absorbent coating of aperture diaphragms for various PDs manufactured by Voskhod JSC – Kaluga Radiotube Plant using the chemical oxidation methods. The photodetector aperture materials developed by Voskhod JSC – Kaluga Radiotube Plant include stainless steel, grade 07X17H16 and titanium alloy, grade VT6.
To ensure proper adhesion behavior, any type of coating requires a fairly thorough pre-treatment of the part. It is a common fact that prior to the chemical coating formation, chemical degreasing of the relevant surfaces is required [2]. We have developed a specialized solubilizing (facilitating the dissolution of poorly soluble ingredients in a liquid base) multicomponent highly dispersed and hyper-heterogeneous stable colloidal solution that provides the highly-efficient immediate degreasing of steels and a number of alloys. The solution assumes a single-stage degreasing procedure with a short-term cleaning cycle. For stainless steels, the cleaning duration is from 10 to 20 minutes, for titanium alloys – from 15 to 30 minutes at a temperature of (60±2) °C. The material for the bath used for cleaning can only be the grades of stainless steel. The solution shall be prepared in distilled water with a concentration of 60 g/l.
Further, after the cascade cleaning, the part surface is activated by deoxidation of active alloying elements. This procedure is performed under normal climatic conditions (NCC) and includes three stages:
In HCl solution (under NCC);
In a solution of a mixture of H2SO4 and HCl (with subsequent cooling to NCC);
In a solution of HOOC–COOH (under NCC).
The main oxidation procedure is performed in a compound developed by Voskhod JSC – Kaluga Radiotube Plant being a 60% NH4NO2 solution with the addition of barium nitrate. The procedure is performed at a temperature of (140±5) °C with occasional stirring. The holding time is 5–15 minutes (not more) depending on the weight of the parts, material grade and surface condition. The increased holding time can lead to the insufficient coating adhesion [3].
To accelerate the coating growth and to obtain a greater thickness, our specialists have provided for introduction of special additives into the solution that promote the nitrate anion formation.
The direct chemical reaction in the NH4NO2 solutions leads to the generation of magnetite, Fe3O4, that increases the grayness degree (ε) of the formed coating:
3Fe + 4H2O = Fe3O4 + 8H+ + 8e. (1)
As a result of a series of reactions, a magnetite matting coating is formed on the basis of a supersaturated iron hydroxide solution:
2Fe(OH)3 + Fe(OH)2 = Fe3O4 + 4H2O. (2)
The addition of an iron dissipation activator (NH4)2S2O8 to the solution leads to the increased thickness and enhanced surface matting that significantly increases the surface grayness degree [4, 5]. Fig. 1–2 demonstrates the results of oxidizing coating application to the control samples and parts under various technological conditions.
When refining the coating technology with the properties of PD protection against any flares, it is necessary to measure the surface temperature of a “cold” diaphragm, for which the grayness degree is unknown (the emissive capacity uncertainty is due to the surface microstructure, chemical composition and phase state of the object). Therefore, after the coating application to the control samples, the latter are subject to the grayness degree measurements. The grayness degree of the samples is measured with a thermal imager using an iterative method.
A film with a well-known grayness degree ε is glued onto the sample surface. To adapt the film to the prototype surface, the required exposure time shall be 5–7 minutes. The film grayness degree value is entered into the measuring unit of the thermal imager. The surface temperature of the sample in the area covered with the film shall be measured. After that, the temperature of the sample surface, not covered with a film, shall be measured.
Next, it is necessary to measure the sample grayness degree using a thermal imager until its value stops changing (stabilizes). The resulting grayness factor will be the true factor for surface of the sample being measured.
According to the measurements results obtained by this method, the following ε values are obtained for the control samples: Fig.1 a (on the left): ε = 0.97; Fig.1a (on the right): ε = 0.92; Fig. 1b: ε = 0.98.
Fig. 2 (a, b) shows the aperture diaphragms of various PDs with the processing method adjustments. The measurement results for the grayness degree of the diaphragm surfaces are as follows: Fig. 2a (on the left and on the right): ε = 0.96...0.97 and (in the center): ε = 0.87; Fig. 2b (on the right and on the left): ε = 0.98 and (in the center): ε = 0.9.
Conclusion
In conclusion, it should be emphasized that the aperture “cold” diaphragm of the cooled photodetector, installed in the photodetector module case (Fig.3), is designed to protect the photodetector against the background noise and other leakage radiation that can interfere with the electrical signal received from the sensitive detector elements during its operation [3].
To improve the optical specifications of aperture diaphragms, the latter are usually polished on the outside and internally blackened. The aperture diaphragms currently used for the up-to-date photodetectors have a grayness degree in the range of ε = 0.95...0.98 (under NCC).
The results obtained by Voskhod JSC – Kaluga Radiotube Plant in relation to the surface coating of aperture “cold” diaphragms for the photodetector assemblies by the developed chemical oxidation method fall within the range of grayness degree ε = 0.97–0.98. This is a very representative result of works performed to develop the coating application technology on the PDs. The obtained emissivity factor value is quite consistent with the aperture diaphragm values of modern photodetectors.
ABOUT AUTHORS
Samvelov A.V., Cand.of Scien. (Engin.), General director STC CRyoNex Corresponding member of the International Academy of Cold, www.cryonex.ru , STC CRyoNex, Moscow, Russia.
ORCID: 0000-0001-5840-7626
Moskalenko A. S., Technical Director of STC Cryonex, Academic Advisor of the International Academy of Cold, www.cryonex.ru , STC CRyoNex, Moscow, Russia; eng.-constr. 1 cat., www.voshod-krlz.ru , Voskhod JSC – Kaluga Radiotube Plant, Kaluga, Russia.
ORCID: 0000-0002-1657-5015
Libkind I. V., technical director, Voskhod JSC – Kaluga Radiotube Plant, Kaluga, Russia
Lobashov A. V., chief DB-4 CDD, www.voshod-krlz.ru , Voskhod JSC – Kaluga Radio Tube Plant, Kaluga, Russia.
Conflict of interest
The authors claim that they have no conflict of interest. All authors took part in writing the article and supplemented the manuscript in part of their work.
AUTHOR CONTRIBUTIONS
A. S. Moskalenko – coating design and development of a specialized solubilizing multicomponent highly dispersed and hyper-heterogeneous stable colloidal solution; A. V. Samvelov – development of a control method for the optical properties of the diaphragm coating, arrangement of technology transfer; I. V. Libkind – development of a chemical oxidation method for the surfaces of aperture “cold” diaphragms; A. V. Lobashov – refinement of coating technology and measurements.
Readers feedback
