MoSi2 heater

Abstract

The present invention addresses the problem of providing an MoSi2 heater which is prevented from separation of a metal film that is formed on an electrode part, and is capable of achieving a long service life. The present invention provides an MoSi2 heater which comprises a heat generation part, a terminal part and an electrode part that is provided on a part of the terminal part. With respect to this MoSi2 heater, the electrode part is provided with a metal film; and an oxide film that has a film thickness of 2.5 µm or less is arranged between the metal film and an MoSi2 base material.

Description

Technical Field

[0001] The present invention relates to a MoSi2 heater.

Background Art

[0002] Since MoSi2 (molybdenum disilicide) heaters have excellent oxidation resistance characteristics, they have been used for a long time as ultra-high temperature heaters used under atmospheric or oxidizing atmospheres and are currently used in a wide range of applications. This heater contains MoSi2 as a main component, and insulating oxides such as SiO2 may be added in order to increase the electrical resistance.

[0003] Currently, MoSi2 heaters used in the glass industry, ceramics firing, and many other fields have various shapes and sizes as shown in Fig. 1, such as U-shaped (Fig. 1(a)), L-shaped (Fig. 1(b)), W-shaped or multi-U-shaped (Fig. 1(c)), three-dimensional U-shaped (Fig. 1(d)), multi-circular U-shaped (Fig. 1(e)), heat-insulating material incorporated type (Fig. 1(f)), etc., and these are used properly according to the purpose and application.

[0004] Fig. 2 shows a schematic diagram of a U-shaped heater. Although heaters have various shapes, basically, they are composed of a heating part 23 and terminal parts 21 at both ends, and an electrode part 22 connected to an external power source is provided in a part of the terminal part. When the heater is energized, a thin and high-resistance part becomes high temperature and plays the role of a heating part, and a thick and low-resistance part suppresses heat generation and plays the role of a terminal part for keeping the power supply part at low temperature. The diameter of the heating part and the diameter of the terminal part are in a relationship of about 1:2.

[0005] Currently, commercially available MoSi2 heaters have heating element and terminal diameters of φ3mm / φ6mm, φ4mm / φ9mm, φ6mm / φ12mm, φ9mm / φ18mm, φ12mm / φ24mm, etc. The electrode length is 25mm (terminal diameter: φ6mm, φ9mm), 45mm (terminal diameter: φ12mm), 75mm (terminal diameter: φ18mm), 100mm (terminal diameter: φ18mm), etc. There are industry standard values ​​for the combinations of heating element diameter and terminal diameter, as well as for the electrode length.

[0006] Figure 3 shows an example of how to connect the MoSi2 heater to an external power supply. An Al braided wire 31 is connected to the electrode section 33 and fixed with a metal fitting (clamp) 32, and power is supplied from the external power supply to the terminal sections 34 at both ends. The Al braided wire and clamp bundled together is also called a connecting band. Figure 3(a) is a photograph of the heater with the connecting band attached. For illustrative purposes, the photograph shows the connecting band attached to only one side; in actual use, connecting bands are attached to both sides. Figure 3(b) is a schematic diagram (front view) of the heater with the connecting band attached to the electrode section. Similar to Figure 3(a), for illustrative purposes, the connecting band is attached to only one side. Figure 3(c) is a top view (plan view) of Figure 3(b).

[0007] Generally, in order to improve conductivity, a metal film (such as an Al thermal spray film) is formed on the MoSi2 base material of the electrode portion of a MoSi2 heater. The process of forming the metal film involves blasting to remove the surface oxide film (protective film) and improve adhesion, followed by plasma spraying. For example, Patent Document 1 describes forming a metal film on the portion of the electrode where the oxide film has been removed, in order to prevent deterioration of the heater electrode portion, and also overlapping the metal film with the portion of the terminal where the oxide film has not been removed. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2000-48937 [Disclosure of the Invention] [Problems that the invention aims to solve]

[0009] The object of this disclosure is to provide a MoSi2 heater that prevents the peeling of the metal film formed on the MoSi2 base material of the electrode portion and enables a longer heater lifespan. [Means for solving the problem]

[0010] One aspect of the present invention is a MoSi2 heater having a heating element, a terminal element, and an electrode element provided on a part of the terminal element, wherein the electrode element comprises a metal film, and there is an oxide film with a thickness of 2.5 μm or less between the metal film and the MoSi2 base material. [Effects of the Invention]

[0011] According to this disclosure, the MoSi2 heater has the excellent effect of preventing the peeling of the metal film formed on the electrode portion and extending the lifespan of the heater. [Brief explanation of the drawing]

[0012] [Figure 1] This is an example of the shape of a MoSi2 heater. [Figure 2] This is an explanatory diagram of a MoSi2 heater (in the case of a U-shape). [Figure 3] This is an explanatory diagram showing how to attach the connection band to the heater electrode section. [Figure 4] This is an SEM image showing blasted and non-blasted areas. [Figure 5] This is an explanatory diagram showing an example of a SEM observation area of ​​a MoSi2 heater. [Figure 6] This is an explanatory diagram showing an example of the measurement location for oxide film thickness. [Figure 7] This is an explanatory diagram for determining the area ratio of the oxide film. [Figure 8]It is an explanatory diagram showing an example of the processing sequence of a MoSi2 heater (U-shaped). [Figure 9] The SEM image and EPMA element mapping of Example 1 are shown. [Figure 10] The SEM image and EPMA element mapping of Example 2 are shown. [Figure 11] The SEM image and EPMA element mapping of Comparative Example 1 are shown. [Figure 12] The SEM image and EPMA element mapping of Comparative Example 2 are shown. [Figure 13] It is a diagram showing the results of the peeling test of the metal film. [[Embodiments for Carrying Out the Invention]]

[0013] The terminal part of the MoSi2 heater hardly self-heats, but the electrode part provided in a part of the terminal part (hereinafter sometimes referred to as the terminal electrode part) is outside the heat insulating material and is away from the heating part. Nevertheless, the temperature rises by heat conduction. Also, depending on the structure of the furnace, the terminal electrode part may be in a closed space. In that case, since it is difficult to be cooled by the outside air, the temperature tends to rise. For the MoSi2 heater, it is desirable that the temperature of the terminal electrode part is 300°C or lower, but there is a possibility that it may be higher than that.

[0014] In MoSi2, selective oxidation of Mo occurs at 300°C to 800°C. This is commonly called the pest phenomenon. A metal such as Al is sprayed on the electrode part. However, in the case of spraying, since there is a limit to the density of the metal film, oxygen passes through the metal film and diffuses into MoSi2. At this time, selective oxidation of Mo occurs at the interface between MoSi2 and the metal film, gaps are generated, and the range spreads over time, and finally the metal film may peel off. When the metal film peels off, the electric resistance increases at that part, so abnormal heat generation or sparks may cause the terminal part to break or become non-conductive.

[0015] Not limited to MoSi2 heaters, before spraying, in order to activate the surface and improve adhesion, blast treatment is performed. However, even with blast treatment, the adhesion of the metal film is not sufficient. In particular, when exposed to high temperatures for a long time, such as in the electrode part of a MoSi2 heater, there is a problem that the metal film peels off due to pesting of the spraying interface.

[0016] To address such problems, when forming a metal film by spraying, since the metal film is composed of flat particles, attempts were made to densify the metal film by methods such as hitting the metal film with a hammer or hitting and compressing it with a ball. However, even though the metal film could be densified by these methods, the peeling of the metal film due to pesting could not be sufficiently suppressed, and it did not directly contribute to extending the product life.

[0017] During such trial and error, the conventional idea, that is, removing the oxide film, which is an insulator on the surface of the electrode part, by blast treatment to activate the surface of the electrode part and improve adhesion before metal spraying, was rethought. It was found that by controlling the oxide film to have an appropriate film thickness and allowing it to exist, an electrode part with excellent high-temperature durability could be obtained.

[0018] Based on such findings, the MoSi2 heater in an embodiment of the present invention is a MoSi2 heater having a heating part, a terminal part, and an electrode part provided on a part of the terminal part, wherein the electrode part includes a metal film, and an oxide film with a film thickness of 2.5 μm or less is provided between the metal film and the MoSi2 base material. The presence of an oxide film with a certain film thickness causes the oxide film and the metal film to bond, suppressing the peeling of the metal film even when exposed to high temperatures for a long time. And because the metal film is less likely to peel off, disconnection at the terminal electrode part is eliminated, enabling the heater itself to have a longer life.

[0019] In this embodiment, the film thickness of the oxide film between the metal film and the MoSi2 base material in the electrode part is 2.5 μm or less. Thereby, the durability at high temperatures can be improved. The oxide film can be formed by heat treatment, and in the case of MoSi2, an oxide film is usually formed on the surface at temperatures above approximately 1000°C. On the other hand, above 1715°C, which is the melting point of the oxide film (cristobalite), the formation of the oxide film stops, and it peels off or volatilizes as a gas. Therefore, it is preferable to set the heat treatment temperature between 1000°C and 1715°C. The higher the heat treatment temperature, the faster the oxide film is formed, and the longer the treatment time, the thicker the oxide film becomes.

[0020] In this disclosure, the thickness of the oxide film is calculated as follows: The MoSi2 heater (electrode section) is divided into three sections perpendicular to its longitudinal direction at approximately equal intervals. The cross-sections are then observed at four locations, each 90° clockwise, using a scanning electron microscope (e.g., JEOL JXA-8500F) (magnification 1000x). An example of the observation locations is shown in Figure 5. Next, each of the three cross-sections is observed, for a total of 12 observation locations. Elemental analysis of these observation locations is performed using EPMA (electron beam microanalyzer) to identify the matrix material (MoSi2), oxide film, metal film, etc., that constitute the interface.

[0021] The oxide film is mainly composed of Si and O. The plaque formed after using a MoSi2 heater is composed of Mo, Si, and O, so the presence or absence of Mo allows us to distinguish between the two. For the oxide film identified as described above, the thickness of the oxide film is measured using image processing software (e.g., ImageJ: open source, developed by the National Institutes of Health, USA). For example, as shown in Figure 6, five thickness measurements are taken at 20 μm intervals for each image, and this is done for all 12 image data points. The thicknesses obtained from these 60 points are then averaged to determine the oxide film thickness for one sample.

[0022] The oxide film is formed by heat treatment of MoSi2, but if the heat treatment is insufficient, or if partial blasting is performed, or for some other reason, areas where the oxide film is absent may occur in a part of the electrode. However, even if there are areas where the oxide film is partially absent, the presence of a certain amount of oxide film can improve durability at high temperatures. There are no particular restrictions on the ratio of the surface area of ​​the electrode that the oxide film occupies, but it is preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more.

[0023] In this disclosure, the ratio of the surface area of ​​the electrode portion occupied by the oxide film is calculated as follows. The MoSi2 heater (electrode section) is divided into three sections perpendicular to its longitudinal direction at approximately equal intervals. The cross-sections of these sections are observed at four points, each 90° clockwise, using a scanning electron microscope (e.g., JEOL JXA-8500F) (magnification 1000x). This observation is performed on each of the three cross-sections, for a total of 12 observation points. Elemental analysis is then performed on these observation points using EPMA (electron beam microanalyzer) to identify the matrix material (MoSi2), oxide film, metal film, etc., that constitute the interface. For the oxide film identified in this way, image processing software (e.g., ImageJ: open source, developed by the National Institutes of Health, USA) is used to calculate the ratio of the oxide film's area (length) to the electrode area's area (length) in the observed image, as shown in Figure 7, for example. This calculation is performed for all 12 image data points, and the ratios for all 12 points are averaged to obtain the ratio of the oxide film's area to the electrode's surface area in one sample.

[0024] MoSi2 heaters preferably contain 35 wt% or more of MoSi2. It is preferable that they contain 50 wt% or more of MoSi2, and even more preferable that they contain 70 wt% or more of MoSi2.

[0025] Preferably, the oxide film is composed of an oxide containing Si. Since the surface of the base material MoSi2 is composed of an oxide film mainly of SiO2, unless special treatment is applied, the oxide film will be composed of an oxide containing Si, but the oxide film may also contain components other than Si. For example, Al, which diffuses from the metal film, may be partially included in the oxide film as an oxide of Al2O3. Also, if a metal film other than Al is formed, an oxide corresponding to that metal may be partially included. It is thought that the adhesion becomes stronger as Al, which constitutes the metal film, diffuses into the SiO2 that mainly constitutes the oxide film.

[0026] There are no particular restrictions on the method of forming the metal film, but it is preferable to form it using methods such as plasma spraying, plating, or ion plating. There are also no particular restrictions on the type of metal film, but it is preferable to use an alloy containing one or more of the following metals: Al, Ni, Cr, Ti, etc. Among these, it is preferable to use Al or an Al alloy. The thickness of the metal film is not particularly limited, as long as conductivity is ensured. For example, it can be 100 μm or more and 300 μm or less.

[0027] An example of the processing sequence for the MoSi2 heater (U-shaped) according to this embodiment will be explained using Figure 8 (right diagram) (process without blast treatment). Note that the present invention is not limited to this processing, and naturally includes cases where it is manufactured by other processing methods. Figure 8(a): A MoSi2 rod (rod-shaped base material) is produced by molding and firing a raw material mainly composed of MoSi2. This will later become a terminal component. The heat-generating section is omitted from the description, but a MoSi2 rod with a smaller diameter than this terminal component will be produced. There are no particular limitations on the raw materials or molding and firing conditions; general methods and conditions can be used. As an example, a powder sintering method is given, but it may also be produced by other methods such as melt casting. Subsequently, by heat-treating the base material, an oxide film with a predetermined thickness can be formed on the surface of the base material.

[0028] Figure 8(b): One end of the MoSi2 rod is cut to a thin, pen-tip shape. This is where the heating element will later be attached. Since there is an industry standard for the relationship between the diameter of the terminal and the diameter of the heating element, the diameter of the pen tip can be determined accordingly. The gradient ratio (height:length) from the terminal to the heating element can also be determined as appropriate.

[0029] Figure 8(c): Aluminum (Al) is sprayed onto the other end of the terminal to form the electrode. Here, Al spraying is shown as an example, but other metals or other film formation methods can be used as long as a metal film is formed. In the process with blasting (Figure 8 left), blasting is performed before aluminum spraying.

[0030] Figure 8(d): A MoSi2 rod is bent into a U-shape. This MoSi2 rod will ultimately become the heating element and has a smaller diameter than the MoSi2 rod (terminal member) described above. When creating a W-shaped heater, three U-shaped rods can be combined alternately in opposite directions. When creating a three-dimensional U-shaped or multi-circumferential U-shaped heater, multiple U-shaped rods can be combined three-dimensionally. Depending on the application, the shape may also be straight or curved instead of U-shaped.

[0031] Figure 8(e): A U-shaped MoSi2 heater is fabricated by welding an Al-sprayed terminal member (Figure 8(c)) to a U-shaped heating element (Figure 8(d)). There are no particular limitations on the welding method, and general welding methods can be used. Although a U-shape is shown here as an example, as long as terminal members (Figure 8(c)) are welded to both ends of the heating element, the shape of the heater can be designed relatively freely by combining one or more heating elements (excluding the ends) as appropriate and welding them together. [Examples]

[0032] The following explanation is based on examples and comparative examples. However, these examples are merely illustrative and do not limit the invention in any way. That is, the present invention is limited only by the claims and encompasses various modifications other than those included in the examples.

[0033] (Making rods) MoSi2 powder and SiO2 powder were weighed in a ratio of 94:6 wt%, mixed and ground in a pulverizer to an average particle size of 2-5 μm, then 10 wt% of a binder was added and mixed in a mixer. Next, this mixture was formed into a rod shape using an extruder, degreased under a nitrogen atmosphere, and sintered under an argon atmosphere. After that, it was removed from the furnace and electrically sintered in air to obtain a rod (diameter 9 mm).

[0034] (Example 1) A rod was cut to a length of approximately 40 mm and heat-treated (in air, 1500°C, 150 seconds) to create an oxide film. Without blasting, Al plasma spraying was applied to this rod to form a metal film 20 mm wide and 160-200 μm thick. A section of the rod was cut out, and its longitudinal cross-section was observed using a scanning electron microscope (SEM). The oxide film thickness was found to be 0.95 μm, and the oxide film completely covered the electrode area. The fabricated MoSi2 rods were kept in air at 450°C for 22 weeks (3696 hours), and observations were made to check for peeling of the metal film. No peeling or cracking of the metal film was observed. Furthermore, the longitudinal cross-section of the MoSi2 rod was observed using SEM, and the interface between the MoSi2 and the Al metal film was analyzed by elemental mapping using EPMA. The results are shown in Figure 9. As shown in Figure 9, no pests or other abnormalities were observed at the interface.

[0035] (Example 2) A rod was cut to a length of approximately 40 mm and heat-treated (in air, 1500°C, 450 seconds) to create an oxide film. Without blasting, Al plasma spraying was applied to this rod to form a metal film 20 mm wide and 160-200 μm thick. A section of the rod was cut out, and the longitudinal cross-section was observed using a scanning electron microscope (SEM). The oxide film thickness was found to be 2.5 μm, completely covering the electrode area. The fabricated MoSi2 rods were kept in air at 450°C for 22 weeks (3696 hours), and observations were made to check for peeling of the metal film. No peeling or cracking of the metal film was observed. Furthermore, the longitudinal cross-section of the MoSi2 rod was observed using SEM, and elemental mapping analysis was performed on the interface between the MoSi2 and the Al metal film using EPMA. The results are shown in Figure 10. As shown in Figure 10, no pests or other defects were observed at the interface.

[0036] (Comparative Example 1) A rod was cut to a length of approximately 40 mm and heat-treated (in air, 1500°C, 5 hours) to create an oxide film. Without blasting, Al plasma spraying was applied to this rod to form a metal film 20 mm wide and 160-200 μm thick. A section of the rod was cut out, and the longitudinal cross-section was observed using a scanning electron microscope (SEM). The oxide film thickness was found to be 2.6 μm, and the oxide film completely covered the electrode area. The fabricated MoSi2 rods were kept in air at 450°C for 22 weeks (3696 hours), and the presence or absence of metal film peeling was observed. As a result, peeling of the thermal spray coating was observed, and pests visible to the naked eye were found on the surface of the base material beneath the thermal spray. Furthermore, the longitudinal cross-section of the MoSi2 rod was observed using SEM, and elemental mapping analysis was performed on the interface between the MoSi2 and the Al metal film using EPMA. The results are shown in Figure 11. As shown in Figure 11, molybdenum containing molybdenum was confirmed at the interface.

[0037] (Comparative Example 2) A rod was cut to a length of approximately 40 mm, and heat treatment (in air, 1500°C, 150 seconds) was performed to create an oxide film. The oxide film was then removed by blasting the rod. Subsequently, an aluminum plasma spray was applied to form a metal film with a width of 20 mm and a thickness of 160-200 μm. A portion of the rod was cut out, and the cross-section in the longitudinal direction of the rod was observed with a scanning electron microscope (SEM), revealing that the oxide film had been almost completely removed (0 μm). The fabricated MoSi2 rods were kept in air at 450°C for 22 weeks (3696 hours), and the presence or absence of metal film peeling was observed. Furthermore, the longitudinal cross-section of the MoSi2 rod was observed using SEM, and elemental mapping of the interface between MoSi2 and the Al metal film was analyzed using EPMA. The results are shown in Figure 12. As shown in Figure 12, molybdenum containing molybdenum was confirmed at the interface.

[0038] Figure 13 summarizes the results of the metal film peeling test described above. When manufacturing a MoSi2 heater as a product, one end of a 9mm diameter rod (the side without the metal film) is shaped like a pen tip, and then it is welded to a U-shaped 4mm diameter MoSi2 material to create the U-shaped heater. However, since the presence or absence of welding is irrelevant to confirming the effects of the present invention, the device was not welded in this embodiment. [Industrial applicability]

[0039] According to the present invention, the MoSi2 heater has the excellent effect of preventing the peeling of the metal film formed on the electrode portion and extending the heater's lifespan. The MoSi2 heater according to the present invention is useful as an ultra-high temperature heater used in the glass industry, ceramic firing, and many other fields. [Explanation of Symbols]

[0040] 21 Terminal section 22 Electrode section 23 Heat-generating part 31 Al braided wire 32 Metal fittings (clamps) 33 Electrode section 34 terminals

Claims

1. MoSi has a heating element, a terminal element, and an electrode element provided on a part of the terminal element. ２ A heater wherein the electrode portion comprises a metal film, and the metal film and MoSi ２ It has an oxide film with a thickness of 2.5 μm or less between it and the base material, and the oxide film is SiO ２ MoSi is an oxide film mainly composed of ２ Heater.

2. (delete)

3. The MoSi according to claim 1, wherein the metal film comprises one or more metal elements selected from the group consisting of Al, Ni, Cr, and Ti. ２ Heater.

4. The MoSi according to claim 1 or 3, wherein the thickness of the metal film is 100 μm or more and 300 μm or less. ２ Heater.

5. MoSi ２ MoSi according to any one of claims 1, 3, or 4, containing 35 wt% or more of ２ Heater.

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