Thermocouple sensor

Abstract

This utility model relates to measurement technology, specifically to methods for manufacturing planar thermocouple sensors (thermopiles). The technical result is achieved by using a thermocouple sensor constructed from alternating monometallic constantan and bimetallic copper-constantan sections, one of which forms the "hot" junction zones, while the second, coated with a protective layer of epoxy adhesive, forms the "cold" junction zones. The bimetallic copper-constantan sections are made of a material produced by explosive welding of 20-µm-thick copper foil and 180-µm-thick constantan foil. The technical result is increased sensitivity and metrological reliability of the thermocouple sensor due to improved bonding quality between the constantan and copper in the bimetallic sections, resulting in the formation of a non-porous, durable metallic bond.

Description

[0001] This utility model relates to measurement technology, specifically to methods for manufacturing planar thermocouple sensors (thermopiles). The sensor provides measurements with high temporal and spatial resolution and can be used to measure high-speed heat flows, diagnose, and study thermal processes occurring during intense dynamic processes, such as explosive welding.

[0002] A known design of a temperature sensor is made in the form of a battery of planar thermocouples consisting of a monometallic (constantan) and a three-layer (copper-constantan-copper) parts, where the hot junction of the thermocouple is the junction of the monometallic and three-layer parts of the thermocouple. The sensor is made of constantan foil 185 μm thick, which serves as a supporting frame. The three-layer sections are made with two electrolytically deposited layers of copper 2.5 ... 3.5 μm thick (Measuring the temperature of liquid media under shock compression / M.A. Gulevich, V.V. Pai, I.V. Yakovlev [et al.] / / Bulletin of the Volgograd State Technical University. - 2014. - No. 20 (147). - P. 16-20).

[0003] A planar thermopile for recording high-intensity heat fluxes is known. It is a strip with alternating monometallic (constantan only) and bimetallic (copper-constantan) sections. It consists of a substrate with an 8-µm-thick constantan foil placed on its surface. Part of the foil is electrolytically coated with 2.5-µm-thick copper. The "cold" junction is every other junction between the monometallic and bimetallic sections, coated with a 30-µm-thick protective layer, which protects the junction from the thermal effects of the shock-compressed gas. The open joint between the monometallic and bimetallic parts acts as a hot seal (Measuring the temperature of preheating the surface of colliding plates with shock-compressed gas in the welding gap during explosion welding / S.V. Khaustov, V.V. Pai, Ya.L. Lukyanov, S.V. Kuzmin, V.I. Lysak, A.M. Andresyan / / Bulletin of VolGTU. Series. Explosion welding and properties of welded joints. - Volgograd, 2021. - No. 11 (258). - P.5-11). The effective value of the thermoelectric coefficient for one thermocouple is ~37 μV / deg, and for the entire thermopile - ~9.4 mV / deg.

[0004] A disadvantage of these designs is the high sensitivity of the signal to interference, requiring the use of measuring instruments with high input impedance. Furthermore, the sensor designs can limit their response speed and ability to respond to very rapid changes in heat flow.

[0005] The closest design is a planar thermocouple sensor (thermopile), constructed as alternating elementary thermocouples on a single substrate, consisting of bimetallic and monometallic sections. The monometallic sections are made of constantan foil with a thickness of 180 to 300 µm, while the bimetallic sections are made of constantan foil with a galvanic copper coating 2.5-3.5 µm thick. Open joints of bimetallic and monometallic sections form zones of active "hot" soldering, and joints of bimetallic and monometallic sections covered with epoxy glue form zones of "cold" soldering (Methodology for determining the heat flow from shock-compressed gas in front of the contact point to the surface of the plates during explosion welding / S.V. Khaustov, V.V. Pai, S.V. Kuzmin, V.I. Lysak, A.D. Kochkalov / / Bulletin of VolGTU. Series. Explosion welding and properties of welded joints. - Volgograd, 2022. - No. 11 (270). - P. 11-16).

[0006] The disadvantage of the design is the low strength and impact resistance of the adhesive bond between copper and constantan foil, the presence of pores and inclusions that impair the stability of the thermal EMF, as well as limited thermal stability, manifested in oxidation and peeling of the galvanic layer at high temperatures.

[0007] The technical problem that this utility model is aimed at solving is to increase the mechanical strength, thermal stability and metrological reliability of planar thermocouple sensors.

[0008] The technical result is an increase in the sensitivity and metrological reliability of the thermocouple sensor due to an improvement in the quality of the connection of constantan and copper in bimetallic sections, due to the formation of a non-porous, strong metallic bond.

[0009] The technical result is achieved by using a thermocouple sensor made in the form of alternating monometallic constantan and bimetallic copper-constantan sections, one line of joints of which forms the “hot” junction zones, and the second, covered with a protective layer of epoxy glue, forms the “cold” junction zones, while the bimetallic copper-constantan sections are made of a material manufactured by explosion welding of copper foil 20 μm thick and constantan foil 180 μm thick.

[0010] The essence of the technical solution lies in the thermocouple sensor, manufactured from constantan and a bimetallic copper-constantan material, produced by explosive welding of copper and constantan foils. The explosive energy creates a strong metallic bond between the dissimilar copper and constantan foils over their entire surface area, providing the key advantage of the claimed design: high-quality connections achieved by the formation of a monolithic metallic bond with minimal defects during explosion welding, ensuring reliable operation under shock wave conditions.

[0011] In the claimed design, the "hot" and "cold" junction zones form the transition boundaries between monometallic constantan and bimetallic (copper-constantan) sections. To ensure thermal compensation, the "cold" junction zones are additionally coated with a heat-insulating layer, which can be a 0.5 mm thick epoxy adhesive. The "hot" and "cold" junction zones are arranged alternately on the thermopile (thermocouple sensor), forming parallel "hot" and "cold" junction lines.

[0012] Thermocouple sensor is manufactured as follows.

[0013] A non-detachable bimetallic material is produced from copper and constantan foils using explosion welding. The bimetallic material is manufactured in accordance with patent RU 2833612 (IPC B23K 20 / 08, 2025) using 20-µm-thick commercially pure copper foil grade M1 and 180-µm-thick constantan alloy foil grade MNMts 40-1.5, ensuring an optimal balance of flexibility and mechanical strength.

[0014] The process is carried out by placing the source material foils in two parallel stacks—a fixed stack and a rolling stack. To ensure controlled bonding conditions and protect the materials from damage, an inert polymer layer of cellulose paper is placed between the solid base and the fixed stack, and a 3-5 mm thick damping layer of silicone rubber is placed between the rolling stack and the dummy stack. The key parameter is the use of a dummy stack with a specific gravity of 0.15-0.3 g / cm. 3, which optimizes the transfer of kinetic energy. Detonation of the explosive charge located on the surface of the false plate at a detonation velocity of 1900-2100 m / s imparts the necessary acceleration to the projectile, resulting in a high-speed impact, initiating the formation of a strong and defect-free bond zone across the entire contact area. During the high-speed impact, accompanied by intense plastic deformation, the original copper foil thins to 17.88 µm, ensuring the formation of a high-quality metallurgical contact.

[0015] High-precision electrical discharge cutting is used to create a complex thermopile geometry in the form of a continuous line winding at right angles (a simplified classical meander) consisting of multiple sequential thermocouples.

[0016] The next technological step is selective etching, which forms the functional structure of the "hot" and "cold" junctions. The surface of the workpiece, excluding areas that will later be used as monometallic, is protected with a durable varnish. Next, the workpiece undergoes chemical etching, which selectively removes copper from the unprotected areas. As a result, after etching, alternating monometallic and bimetallic areas are formed on a single substrate. After the etching process is complete, the protective coating is removed by rinsing in an organic solvent (acetone), ensuring a completely clean surface and preparing it for subsequent technological operations.

[0017] For final functionalization, the thermoelectrically passive "cold" junctions are coated with a protective layer of epoxy adhesive approximately 0.5 mm thick. This layer serves as an effective thermal barrier, confining the temperature effect exclusively to the "hot" junctions, ensuring high accuracy in measuring the generated thermal EMF.

[0018] A comparative table of the sensitivity of the claimed thermopile and the prototype is presented in the table.

[0019] Table

[0020] Parameter Thermopile of 25 thermocouples, according to the prototype Thermopile of 25 thermocouples Sensitivity of one thermocouple 12 µV / °C 22-29.6 μV / °C Sensitivity of the entire battery ~0.3 mV / °C 0.55-0.74 mV / °C

[0021] The thermoelectric coefficient of the entire array of 25 thermocouples (the temperature sensor may contain a different number of thermocouples, depending on the research objectives) is only ~0.3 mV / deg, while the sensitivity of a single element is 12 μV / deg. This indicates imperfect contact at the copper-constantan interface, likely caused by pore-like defects, which increases parasitic resistance and reduces the output thermal EMF.

[0022] A thermopile manufactured with bimetallic sections produced by explosion welding demonstrates significantly improved results. With the same number of thermocouples—25 (the temperature sensor may include a different number of thermocouples, depending on the research objectives)—the output signal of the entire thermopile reaches 0.55-0.74 mV / deg, more than twice the value of the prototype. The calculated sensitivity of a single thermocouple (22-29.6 μV / deg) also confirms that the bimetallic section material specified in the thermopile design ensures high-quality metallurgical contact.

[0023] The technical solution is illustrated by the images: Fig. 1 shows a fragment of a thermopile; Fig. 2 shows the zone of connection of copper and constantan obtained by explosion welding; Fig. 3 shows the zone of connection of copper and constantan obtained by electrolytic deposition.

[0024] The planar thermocouple sensor (thermopile) is designed as alternating monometallic 1 and bimetallic 2 sections. The monometallic sections 1 are made of constantan foil with a thickness of 180 to 300 µm. The bimetallic sections 2 are made of copper-constantan material produced by explosive welding of copper and constantan foils. The transition boundaries between the monometallic (constantan) and bimetallic (copper-constantan) sections form the "hot" 3 and "cold" 4 junctions. The "cold" junctions 4 are coated with a protective layer of epoxy adhesive approximately 0.5 mm thick.

[0025] Figure 2 shows the microstructure of the joint zone of a copper-constantan bimetallic material produced by explosion welding. The initial thickness of the copper foil was 20 μm, and that of the constantan foil was 180 μm. During explosion welding, high-speed impact and intense plastic deformation resulted in a thinning of the copper layer to 17.88 μm. Metallographic analysis performed using an optical microscope demonstrates the formation of a wave-free joint with a clear, smooth interface. Defects such as pores, microcracks, or delamination are absent in the joint zone, indicating the high quality of the metallurgical contact ensured by explosion welding. The use of optical microscopy in this case is justified by the significant thickness of the resulting bimetallic material (total thickness approximately 200 μm), which allows for examination of transverse sections using standard methods.

[0026] Figure 3 shows the microstructure of the copper-constantan bonding zone obtained by electrolytic deposition. The thickness of the copper layer deposited by electroplating on the constantan foil is 2.5–3.5 μm. The study was performed using a Versa 3D scanning electron microscope due to the thin coating thickness and the need for high resolution to reveal structural features. The micrograph clearly shows defects in the form of pores located both at the interface and within the copper layer. The presence of porosity is associated with the characteristics of the electrolytic deposition and can lead to an increase in the transient electrical resistance and a decrease in the stability of the thermoelectric characteristics.

[0027] Thus, the comparative analysis of Fig. 2 and Fig. 3 clearly demonstrates the advantage of the material obtained by explosion welding for the formation of high-quality bimetallic copper-constantan compounds used in thermocouple sensors, and ensuring the metrological reliability of the said thermocouple sensors.

[0028] The device operates as follows.

[0029] The device is designed to measure high-speed heat flows from shock-compressed gas that occur during intense dynamic processes, such as explosion welding.

[0030] A rectangular window was cut through the fixed plate. Its geometric dimensions ensured that the thermopile would be flush with the plate surface facing the welding gap. The rear side of the window was covered with a wooden insulator (wooden insert), which provided mechanical fixation for the sensor and served as a thermal insulating substrate, preventing parasitic heat transfer from the rear side of the thermopile into the fixed plate. A planar thermopile was directly mounted onto this wooden insert so that its working surface (the area where the hot junctions are located) was strictly in the same plane as the surface of the fixed plate facing the welding gap. This arrangement ensured direct convective heat exchange between the shock-compressed gas moving in the gap and the sensor's sensitive elements.

[0031] Alternating monometallic and bimetallic tracks in a planar thermopile creates a path for the sequential connection of multiple elementary thermocouples on a single substrate to sum the thermal EMF in a planar manufacturing process. When studying the explosion welding process under the influence of a shock wave front (5), in the active "hot" junction zone, when transitioning from a monometallic to a bimetallic region, where two different metals are in close physical and electrical contact, heating and the generation of a thermal EMF (Seebeck effect) occur at the copper-constantan interface.

[0032] Next, the current flows through the bimetallic region (copper-constantan) into the monometallic region (constantan). At the transition point (the "cold" junction), the copper layer is missing. For the thermoelectric effect, this point is the end of one thermocouple and the beginning of the other. It will have the temperature of the surrounding environment (the "cold" zone).

[0033] The thermal EMF generated in the “hot” zone moves to the “cold” junction zone, where it is fixed relative to the ambient temperature.

[0034] Thus, a thermocouple sensor made in the form of alternating monometallic constantan and bimetallic copper-constantan sections made of a material produced by explosive welding of copper foil 20 μm thick and constantan foil 180 μm thick, one line of joints of which forms the “hot” junction zones, and the second, covered with a protective layer of epoxy glue, forms the “cold” junction zones, provides an increase in the sensitivity and metrological reliability of the thermocouple sensor due to an increase in the quality of the connection of constantan and copper in the bimetallic sections, due to the formation of a non-porous, strong metallic bond.

Claims

A thermocouple sensor made in the form of alternating monometallic constantan and bimetallic copper-constantan sections, one line of joints of which forms the "hot" junction zones, and the second, covered with a protective layer of epoxy adhesive, forms the "cold" junction zones, characterized in that the bimetallic copper-constantan sections are made of a material produced by explosion welding of copper foil 20 µm thick and constantan foil 180 µm thick.

Images