Film heater and its manufacturing method

Abstract

FIELD: film heaters.SUBSTANCE: heater is made in the form of a metal blank, onto which a three-layer coating is successively applied using the detonation spraying method. The total thickness of the layers does not exceed 500 mcm. Moreover, the thickness of the first electrical insulating layer made of aluminium oxide Al2O3, is no more than 250 mcm, the thickness of the second electrically conductive layer, made in the form of a solid spiral of nichrome, is no more than 100 mcm, and the thickness of the third heat- and electrically insulating outer layer made of Al2O3 is from 150 to 200 mcm. One of the current-carrying contacts is adapted for shorting to a metal workpiece, and the second contact is brought out to the end of the workpiece.EFFECT: ability of the heater to operate in vacuum conditions up to 10-4 Pa and high temperatures up to 600 °C, adjustable uniform heating, resistance to thermal cycling and mechanical loads, as well as the ability to apply the heater to surfaces of complex shapes without losing its properties.2 cl, 5 dwg

Description

[0001] The invention relates to electric heating elements, specifically resistive heating devices containing an electrically conductive layer. Such a heating element can be used as laboratory equipment for uniform heating with precise temperature control, for thermal spraying in a vacuum, and for localized heating with electrical and thermal insulation in experimental setups.

[0002] Boron neutron capture therapy for malignant tumors requires accelerator-based neutron sources, which generate neutrons by irradiating a lithium target with protons. The most optimal method for producing lithium targets is thermal evaporation in a vacuum. This method produces the purest and most uniform lithium layer with crystalline density, which is critical for neutron generation. A lithium target for an accelerator-based neutron source is a vacuum chamber containing a cooled substrate onto which a thin layer of lithium is deposited.

[0003] Heaters based on metal coils deposited on ceramic substrates, as well as heating elements manufactured using plasma or flame spraying methods, are known. The disadvantages of these solutions include limited coating adhesion, high porosity, low resistance to thermal cycling, and insufficient electrical insulation under extreme conditions.

[0004] A prior art discloses a multilayer heating element RU 2094957C1 comprising a heat-reflecting and electrically conductive layer, insulated on both sides by electrically insulating layers. The heat-reflecting layer is made of metal or a metallized polymer film on one surface of the element. The electrically conductive layer is made of carbon fiber paper, with two main conductive busbars attached along opposite ends. The electrically insulating layers are made of a thermoplastic polymer film.

[0005] A wide-range heating element, RU 2713729C1, is also known. The heating element comprises a substrate, a heating layer formed by a topological pattern of resistive paste, and conductive busbars located along the edges of the substrate. The heating layer is made from a mixture of finely dispersed graphite powders, carbon black, and a mixture of metal powders in a polymer binder.

[0006] The heating elements described above and similar ones are not designed for temperatures above 200°C and cannot be used in vacuum conditions, since the electrically insulating polymer materials used, especially when heated, have a high gas emission rate.

[0007] The prototype of the claimed product is a heating plate for vacuum sputtering equipment, CN219227851U. The heating plate consists of a nickel-alloy steel heating tube with a nickel-chromium alloy coil, which heats an AISI 310S stainless steel plate when powered through the power connectors. However, the proposed plate-based design does not provide uniform heating, and the coil itself takes up a lot of space. For sputtering systems with limited heating element size and the need to effectively transfer heat to a specific area, such designs are impractical. This solution involves volumetric heating due to radiation from the heated plate, but localized heating is often required, where heat is transferred, in part, through contact between the heating element and the heated sample.Therefore, each specific sample requires a heater of a corresponding shape. This heating plate does not meet this requirement.

[0008] The technical result of the claimed invention is the possibility of its use in vacuum conditions (up to 10 -4 Pa) and relatively high temperatures (~600 °C); the ability to apply the film heater to complex surfaces without losing its properties; multi-layer construction providing electrical and thermal insulation; adjustable uniform heating up to 600 °C, which depends on the applied voltage; variability of the product shape, which allows it to be integrated into various designs; resistance to thermal cycling and mechanical loads.

[0009] The technical result is achieved as follows:

[0010] 1. The possibility of applying a film heater to surfaces of complex shapes is ensured by detonation spraying of conductive and insulating layers;

[0011] 2. Uniform heating is achieved by means of a conductive spiral, which is formed on the outer cylindrical surface of the heater;

[0012] 3. To ensure localized heating and prevent overheating of structures located outside the heater, as well as to provide electrical insulation, the heater must be three-layered. The first layer is electrically insulating and consists of aluminum oxide; the second layer is electrically conductive and consists of a nickel-chromium alloy (nichrome); the third layer serves as thermal and electrical insulation and is also made of aluminum oxide. This coating application method allows for the formation of an electrically and thermally insulating coating with a total layer thickness of no more than 500 μm, while minimizing the external dimensions of the film heater.

[0013] 4. Detonation spraying ensures high adhesion of the coating to the substrate, which prolongs the service life and increases the efficiency of both the insulating layer and the conductive layer.

[0014] The essence of the invention is that the heater is made in the form of a metal blank (for example, a glass made of stainless steel), onto which a three-layer coating is successively applied using the detonation spraying method:

[0015] 1. The first layer—electrical insulating aluminum oxide (Al2O3)—is applied to the prepared metal workpiece.

[0016] 2. The second layer is an ohmic heater made in the form of a nichrome spiral.

[0017] 3. The third layer is an external electrical and thermal insulation layer made of aluminum oxide (Al2O3).

[0018] Fig. 1 shows a film heater applied to a stainless steel cup, where 1 is the terminals of the conductive contacts, 2 is the heater coil, 3 is the insulating coating made of aluminum oxide.

[0019] The ohmic heater formed in the second layer is powered through conductive copper contacts. One contact is connected to the metal workpiece, and the other is routed externally. To prevent the two contacts from shorting out, a first layer of aluminum oxide is applied to the metal workpiece. The third outer layer is necessary to prevent the ohmic heater from transferring heat to the outside of the part and to prevent breakdowns in the gap between the heater and the thermal spraying unit.

[0020] The method for manufacturing a film heater includes the following steps:

[0021] · Prepare the surface of the workpiece (sandblasting).

[0022] · The first layer of Al2O3 powder is applied using detonation spraying.

[0023] · A second layer of nichrome is applied using detonation spraying, followed by removal of part of the conductive layer by sandblasting or laser treatment to form a topological pattern (spiral).

[0024] · Form conductive contacts (copper tracks) using detonation spraying from copper powder.

[0025] · The third protective layer of Al2O3 is applied using detonation spraying.

[0026] Detonation spray technology provides low porosity (<1%) and strong layer cohesion, and enables the production of a film heater with minimal thickness for integration into complex thermal spray systems with a high heat transfer coefficient, which can be reused under vacuum conditions. Figure 2 shows a photograph of the fabricated film heater installed in a lithium spray system after several lithium spray cycles.

[0027] The film heater is manufactured as follows. A metal blank, in this case a cup, with an internal diameter of 92 mm and an external diameter of 95 mm (Fig. 3), is sandblasted on the outside to ensure high adhesion of subsequent coatings. To preserve the inner surface of the cup during processing and subsequent layer application, a fixture is used – an aluminum spacer ring for secure fixation and a fluoroplastic disk to protect the inner surface from dust.

[0028] The coating application process utilizes the new-generation CCDS2000 detonation system, designed primarily for applying electrically insulating and wear-resistant protective coatings from a wide range of powder materials. The detonation spraying process is fully automated and controlled by software, while the use of a manipulator allows for the processing of parts with complex geometries. The main unit of the system, called the detonation gun, consists of a barrel, a gas supply system (gas distributor), and one or two powder dispensers. It is mounted on a stand that allows the barrel to move vertically. The workpiece is secured to the manipulator, which enables horizontal movement and rotation. The movement of the barrel and the part during the spraying process, as well as the parameters of the detonation gun shots, is controlled by a control computer. Fig.Figure 4 shows a diagram of the detonation spraying system, where 4 is the gas supply system, 5 is the spark plug, 6 is the barrel, 7 is the gaseous explosive mixture, 8 is the powder dispenser, 9 is the powder particles, 10 is the coating being formed, and 11 is the substrate. With each shot from the detonation gun, the components of the explosive mixture (fuel and oxidizer) are fed into the mixing-ignition chamber, from where the prepared mixture enters the barrel of the detonation gun. Then, on command from the control computer, the powder dispenser delivers a portion of powder into the barrel, and the spark plug initiates detonation of the explosive mixture. The detonation front moves from the closed end of the barrel to the open end, and the detonation products accelerate and heat the particles inside the barrel to the melting point of the material and velocities of approximately 500–700 m / s. The particles then collide with a target located 250 mm from the muzzle of the barrel, forming a coating layer.By repeatedly scanning the surface of the part layer by layer, the required coating layer thickness is achieved.

[0029] To form a three-layer coating, aluminum oxide powders (α-Al2O3) of the M28 "Super" grade (OOO NTC "Inoks", Russia) and nichrome of the GPNi80Cr20-4 grade (Luoyang Golden Egret Geotools Co., Ltd, China) were selected. An 850 mm long barrel with a caliber of 20 mm was selected through experimental selection for detonation spraying, the volume of which, together with the mixing-ignition chamber, is 286 cm 3 To form an aluminum oxide coating, an experimentally selected stoichiometric mixture of acetylene with oxygen C2H2+ 2.5O2 is used as an explosive mixture, filling 47% of the barrel volume (135 cm 3 ). To form a nichrome coating, a mixture of acetylene and oxygen with the addition of propylene is used, satisfying the composition 0.87C2H2 + 1.04C3H6 + 2.18O2 and filling the barrel by 33% (95 cm 3 ).

[0030] Next, the first insulating layer of aluminum oxide coating with a thickness of 250 ± 30 μm is applied using the Spiral scanning program. A subsequent layer of nichrome with a thickness of 100 μm is applied on top of the insulating layer. Using laser post-processing of the surface, a topological pattern is formed on the nichrome coating in the form of a spiral 4.6 m long (15.5 turns on a diameter of 95 mm). The topological pattern takes into account the length of the nichrome spiral (L) and the number of turns in the nichrome spiral (N). The length of the nichrome spiral (m) is calculated using the formula: , where S(mm 2 ) – cross-sectional area of ​​the spiral, R(Ohm) – resistance, ρ≈ 1.2 – the specific resistance of nichrome. Resistance is determined by the applied voltage and heating power , Where The number of turns in a nichrome spiral (N) is determined as follows: , Where ,D is the diameter of the workpiece, h is the spiral pitch. Heating power P (W) is calculated using the formula: , where m(kg) is the mass of the metal workpiece, c( ) – specific heat capacity, (°C) – the difference between the final and initial temperatures, – Heating efficiency, t(c) – heating time.

[0031] The cold coil resistance measured with a VC 9808+ digital multimeter (VICTOR, Shenzhen, China) was 29.9 ohms. This resistance corresponds to a 3 mm nichrome coil width with a 100 μm nichrome layer thickness. The inductive reactance for the specified length, diameter, and number of winding turns is estimated to be 0.007 ohms, which is negligible compared to the ohmic resistance and can be ignored. With an applied effective voltage of U = 100 V, the heater power W = U 2 / R= 334 W.

[0032] One end (contact) of the coil is connected to the top of the cup body, where there is no coating. The other end (contact) of the coil is brought to the bottom edge of the cup, where a contact pad is formed in the form of a thin conductive ring. An insulating layer of aluminum oxide ensures that there is no conductivity between the coil and the body.

[0033] The final layer is applied to insulate the conductive layers of the coating and is formed by spraying aluminum oxide with a thickness of 150 to 200 μm.

[0034] During the spraying process, the repeatability of the spraying parameters in each shot is ensured by the use of specialized software in the CCDS2000 detonation system, which monitors the spraying process in real time. The spraying mode is configured by the operator in a single-shot program—a cyclogram (the Al2O3 powder spraying cyclogram is shown in Fig. 5). The cyclogram specifies the opening and closing time of the corresponding electromagnetic valves used to supply gas components to the barrel, the injection of a powder portion into the barrel, the spark plug detonation initiation time, and the time for purging the barrel with inert gas at the end of the cycle.

[0035] During the detonation spraying process, when applying each of the three layers, the thickness is controlled using a multifunctional electromagnetic thickness gauge KONSTANTA K5 (OOO Konstanta, St. Petersburg, Russia).

[0036] The main coating volume in the manufactured film heater is Al2O3 ceramic, which provides electrical insulation for the heating coil. It is important to ensure sufficiently high density and adhesion of this ceramic layer to the substrate, which is achieved using the detonation spraying mode. The microstructure of the aluminum oxide coating was examined using a Carl Zeiss MERLIN Compact scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany). The coating has a dense structure without visible defects. The coating porosity was measured using computer image processing on an OLYMPUS GX-51 inverted metallographic microscope (Olympus Corporation, Tokyo, Japan) using the OLYMPUS Stream Image Analysis software Stream Essentials 1.9.1 (Japan) application; its values ​​​​ranged from 0.35 to 1.00%. The adhesion of the coating (the strength of the bond with the metal substrate) was measured using the pin method and was found to be 70 ± 5 MPa.Microhardness was measured on transverse sections of the coatings using a DuraScan 50 device (EMCO-TEST, Austria) under a load of 0.2 kgf; it was 1200 ± 90 HV0.2. The electrical conductivity of the coating was measured using an AKTAKOM AMM-2093 Hipot Tester AC / DC / IR device for measuring electrical insulating properties (ZAO NPP ELIKS, Moscow, Russia); it was (2.67 ± 0.53) 10. 13 Ohm cm at 20 °C and 55% humidity.

[0037] The declared heating power of the film heater is ~300 W for heating to 500 °C in 10 minutes with an efficiency of 0.7. The manufactured film heater was tested for its durability during thermal cycling. Thermal cycling was performed by heating by passing current through the heating coil with a voltage of U = 100 V from a SUNTEK 3000 VA variable-voltage transformer. Heating to 250 °C was carried out for 30 minutes, followed by cooling for another 30 minutes. The heater temperature during testing was monitored using a CEM DT-8861 infrared thermometer. A total of 5 thermal cycles were performed. After thermal cycling, no delamination or discontinuities of the coating were observed. Measurements showed that in a product heated to 250°C, at an effective voltage of U = 100 V, a current of 2.7 A flows through the coil, and the heating power, accordingly, is W = 270 W. The resistance of the heated coil under these conditions is R = 37.0 Ohms.

[0038] The manufactured film heater was installed in a lithium vacuum thermal spraying system, where it operates successfully (Fig. 2).

Claims

1. A film heater comprising a metal workpiece with a multilayer coating on the surface containing electrically insulating layers and an electrically conductive layer made in the form of a spiral, characterized in that the thickness of the first electrically insulating layer made of aluminum oxide (Al2O3) is no more than 250 μm, the thickness of the second electrically conductive layer made in the form of a solid spiral of nichrome is no more than 100 μm, and the thickness of the third heat- and electrically insulating (outer) layer made of aluminum oxide (Al2O3) is from 150 to 200 μm, while the total thickness of the layers does not exceed 500 μm; also, one of the current-carrying contacts is designed with the possibility of closing to the metal workpiece, and the second current-carrying contact is brought out to the end of the workpiece, wherein both contacts are made of copper.

2. A method for manufacturing a film heater, including the preparation of a metal blank and the sequential formation on its outer surface of a multilayer coating containing electrically insulating layers and an electrically conductive layer in the form of a spiral, characterized in that: - pre-prepare the surface of the workpiece by sandblasting; - the first electrically insulating layer of aluminum oxide powder (Al2O3) is applied using the detonation spraying method, using an explosive mixture of acetylene with oxygen of the stoichiometric composition C2H2+ 2.5O2; - a second continuous layer of nichrome powder is applied using the detonation spraying method, using an explosive mixture of acetylene with oxygen and propylene of the composition 0.87C2H2+ 1.04C3H6+ 2.18O2; - a resistive coil is formed from the second layer by removing part of the nichrome material using laser or sandblasting according to the topological pattern; - current-carrying contacts made of copper powder are applied using the detonation spraying method; - a third protective electrical and thermal insulation layer of aluminum oxide powder (Al2O3) is applied using the detonation spraying method.

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