Key operations in the production of a laboratory cryostat for a photodetector

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 JUSTIFICATION

It is unthinkable in the modern era to do without cooled infrared photodetectors. These devices have penetrated into many areas of human activity around the world, including in Russia. The main need for devices based on IR photodetectors has always been and remains the most relevant for such areas as the Armed Forces, the Ministry of Emergency Situations, intelligence, and security, especially in the current international situation. Today, active microcryogenic systems of thermomechanical action (according to the Stirling, Gifford-McMahon, Gifford-Longsworth cycle, etc.), vapor-liquid (Joule-Thomson effect), and thermoelectric action (Peltier effect) are widely used. However, conducting laboratory research in the infrared range: improving the photoelectric characteristics of existing photosensitive elements; studying the characteristics of compounds and solid solutions of new compositions; The search for transmission zones in areas of atmospheric absorption and other studies suggest the use of passive cooling systems for filling vacuum cryostats-dewars [5]. In Russia and abroad, fill laboratory cryostats of vertical design with horizontal orientation of the optical axis are used for research of IR photodetectors. purpose The aim of the study is to minimize the influx of atmospheric air into the pumped volume of the laboratory cryostat, which increases its reliability, prolongs its service life and preserves its thermal insulation ability under standard laboratory conditions.

METHODS

Research design The research was based on a correlation-experimental design based on monitoring the cryostat resource, identifying a way to increase it by reducing leakage, subsequent leak control, and estimating operating time. The scheme of the study is shown in Figure 1. Figure 1. Research scheme Figure 1. The scheme of the study Conditions of the event Specialists of NTC Cryonex LLC and Bauman Moscow State Technical University, who have sufficient experience and research level in the field of cryogenic vessels, have developed a new design of a filling research cryostat. The cryostat has a shape close to spherical, a high neck (item 8, Fig. 2) and a soldered structure, which provides reduced heat flow and increased reliability. The design of the cryostat is shown in Figure 2.

Figure 2. Dimensional drawing of the cryostat of the research infrared photodetector 1 – optical entrance window; 2 – aperture diaphragm; 3 – installation location of the IR photodetector; 4 – metal–ceramic assembly with electrical contacts; 5 – internal vessel; 6 – external vessel; 7 – thermal supports; 8 – neck; 9 - pumping rod; 10 - gas absorber Figure 2. Dimensional drawing of the cryostat of the research infrared photodetector 1 – an input optical window; 2 – an aperture diaphragm; 3 – the installation location of the IR photodetector; 4 – a metal-ceramic assembly with electrical contacts; 5 – an internal vessel; 6 – an external vessel; 7 – thermal supports; 8 – neck; 9 – pumping stem; 10 – getter

The internal elements of the cryostat of the IR photodetector - the photodetector holder, the internal vessel, etc. operate over a wide temperature range of more than 200 K. Therefore, in order to increase reliability and resource, special requirements must be placed on the materials of the main and connecting elements. The most suitable material for the inner vessel of the cryostat is high-alloy austenitic steel, which has low thermal conductivity in the required temperature range, increased toughness, yield strength and temporary tear resistance at cryogenic temperatures. Moreover, all these properties are improved by mechanical processing of such steel by the nagartovka method [4].

COMPLIANCE CRITERIA

The cryostat of the cooled photodetector is a high-vacuum product, and in order to acquire high thermal insulation properties, its inter-wall space is pumped out until a vacuum of at least 10^-5 Pa is reached. Providing a high service life at extreme temperatures and pressures, the most reliable connection is a solder joint [3]. The soldering seam formation process consists of several stages: heating of the soldered seam material to a temperature close to the melting point of the solder; melting of the solder; spreading of liquid solder over the surface of the solid material and filling of the soldered seam; cooling and crystallization of the solder in the soldered seam [1]. The strength of the solder joint is characterized, in addition to the mechanical properties of the solder material, by the presence of wettability of the base metal surface by the liquid-phase solder metal. All other things being equal, the wetting of the surface with a liquid is positively affected by the surface tension of the liquid , (1) where h is the lifting height of the liquid–phase solder in the capillary gap of the solder joint; g is the acceleration of gravity; r is the characteristic linear size of the solder joint; p’,p” is the density of the solder material and the surrounding gas, respectively; Θ is the wetting edge angle. The force of surface tension (1) also affects the spreading of solder over the surface of the base metal. With better wetting, the liquid spreads better over the surface of the solid. The spreading of a drop of liquid over the surface of a solid body from equilibrium conditions is determined by equation (2) of the balance of forces , (2) where Tg is the surface tension at the interface "solid–gas"; Tj is the surface tension at the interface "solid–liquid"; Tj is the surface tension at the interface "liquid–liquid".gas"; Θ is the wetting edge angle formed by the solid-liquid interface and tangent to the liquid-gas interface, counted towards the liquid phase. Hence, (3) Thus, according to (3), in the case when If (cos Θ is positive), then the solder moistens the metal surface. If ϬTg < If and (cos Θ is negative), then the solder, respectively, does not wet the surface [1]. The flowability of a liquid depends on many factors. The main factor influencing the spreadability is the ratio of the values of the surface tensions of the media interfaces: "base metal–ambient gas"; "base metal–solder"; "solder–base metal". The next factor affecting flowability is the viscosity of the liquid phase. Gravity also affects the flowability of a liquid. The higher the density of the liquid, the lower the drop height, the better the spreadability. Intermolecular adhesion takes place, as well as diffusion between a solid and a liquid, affecting wetting and, consequently, spreading. And the last of the well–founded factors affecting the spreadability of a liquid over a solid surface is the geometry of a solid: the shape, the amount of roughness, as well as inclusions in the material (surface) of the solid. When the area of the rough body is n times larger than the smooth one, expression (3) takes the form (4) Solving the identities (3) and (4) together, we obtain (5) Expression (5) establishes that the cosine of the wetting angle for a rough surface is greater than the cosine of the wetting angle for a smooth surface by as many times as the actual area of the first is greater than the second. Therefore, if the base metal is wetted with solder, the presence of roughness has a good effect on solder spreading. Thus, the wetting property and the spreading process have a common nature and vary depending on the chemical and physical properties and geometry of the base metal, all other things being equal. In a general sense, wetting and spreading depend on the properties of the base metal–solder metal pair, therefore, the same liquid metal behaves differently on the surface of different metals, all other things being equal. Therefore, the spreading of solder depends on its nature, as well as on the nature of the base metal. However, the nature of the spreading process also depends on the composition of the solder flux used. Therefore, the soldering process is a triumvirate – "base metal–solder–solder flux". With such a system, it is possible to select a flux that allows the use of solder that does not wet the metal well enough, but is optimally suited to the base metal due to other important characteristics (thermal conductivity, tear resistance, coefficient of thermal expansion, and other properties). Taking into account the above, the specialists of NTC Cryonex LLC and Bauman Moscow State Technical University have calculated and experimentally obtained the optimal metal-solder-flux composition for the manufacture of a high-vacuum cryostat that provides cryostating of a research photodetector.

DURATION OF THE STUDY

The research work was carried out over two calendar weeks, of which four working days were devoted to soldering processes, the second week was required for a round-the-clock pumping operation with heating. One working day was allocated for performance testing and evaluation of the results of the study. The literature review and calculations are not included in this period.

THE PROCEDURES CARRIED OUT IN THE STUDY

The most loaded node of a high-vacuum cryostat is an internal vessel. The atmospheric pressure of normal boiling of liquefied nitrogen acts on its inner surface, and the outer side is additionally loaded with high vacuum. In addition, the inner vessel is in thermal cycling conditions with an amplitude of 77-300 K [2]. High-alloy corrosion-resistant heat-resistant steel of the ХН70ВМТЮ grade was determined as the base metal of the inner vessel of the cryostat. This steel has the minimum thermal conductivity for metals in the operating temperature range (about 7 W/mK) and has a sufficiently high tensile strength of at least 200 MPa, which is very important when working in such extreme conditions. By calculation, using, among other things, ratios (1-5) and practical verification of the result, solder and flux for soldering thin-walled elements of an internal vessel made of ХН70ВМТЮ steel were determined.

By calculation, using, among other things, ratios (1-5) and practical verification of the result, solder and flux for soldering thin-walled elements of an internal vessel made of ХН70ВМТЮ steel were determined. An alloy of the ПСр8КЦН grade was determined as the solder. This solder proved to be successful when soldering samples of the cryostat body, it was well wetted and spread. This solder contains: cadmium – 84%, silver – 8%, zinc – 6%, nickel – 2%. Literature data also confirm the use of silver solders containing cadmium, nickel, zinc and some other elements in soldering highly alloyed corrosion-resistant heat-resistant steels. Corrosion-resistant and heat-resistant steels are characterized by the formation of dense oxide films on their surfaces. These are oxides of titanium, aluminum, chromium and other elements with high chemical and thermal resistance, low vapor elasticity, which significantly reduces the wettability and spreadability of solders on their surface. Removal of the oxide film is achieved by using highly active fluxes. Concentrated acids (hydrochloric acid, orthophosphoric acid, etc.) are effectively used as a flux. Saturated zinc solution in hydrochloric acid is also effectively used. Figure 3 (a,b,c) illustrates the solder joints of sheet blanks made of steel ХН70ВМТЮ 0.3 mm thick during butt soldering using various solders. The quality of the seams and the presence of defects are shown.           

 Figure 3. Solder joints of sheet steel of the ХН70ВМТЮ  brand using various solders. a) solder ПСр8КЦН; b) solder brand Cd-5Ag; c) solder brand Pb-Ag2,5-Cu 

THE MAIN OUTCOME OF THE STUDY

The photographs show that the seam obtained by soldering with ПСр8КЦН solder (Figure 3a) has no erosion of the soldered material, is made with a minimum content of chemical compound interlayer and cracks in the seam, unlike with Cd-5Ag and Pb-Ag2.5-Cu solders. Pyrophosphoric acid (H4P2O7) was used as soldering flux in all three soldering operations. The amount of leakage into the heat-insulating inter-wall vacuum cavity for helium gas grade 6.0 TU 0271-001-45905715-02 was 3.2·10-13 Pa·m3/s, which corresponds to the continuous operation time of the product for at least 5 years. 

CONCLUSION

The cryostat, made of high-alloy, corrosion-resistant, heat-resistant steel of the ХН70ВМТЮ  grade with solder soldering of ПСр8КЦН , was tested on a helium leak detector. Based on the amount of leakage into the heat-insulating vacuum cavity of the developed laboratory high-vacuum dewar cryostat, it is determined by the recalculation method that the continuous operation time of the facility will be at least 5 years.

作者简介

Андрей Samvelov

编辑信件的主要联系方式.
Email: samv-andrej@yandex.ru
ORCID iD: 0000-0002-5840-7626
SPIN 代码: 9932-6353

 Candidate of Technical Sciences, General manager

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