RU2369046C1 - Heating element and method of its manufacturing - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is related to the field of electric, in particular resistive heating, namely to monolithic self-controlling metal ceramic heating elements and method of its manufacturing. Invention comprises heating element with resistive element with positive temperature coefficient of resistance made of composite material on the basis of iron, and is installed in electric insulating layer made of glass ceramics on the basis of periclase (MgO), heat insulation layer and protective layer, and also comprises at least two current-collecting elements connected to resistive element with width d, and axis of symmetry of resistive element is installed perpendicular to cross section of resistive element, besides in the area of resistive element junction with current-collecting element axis of symmetry of resistive element relative to surface of protective layer, providing contact with heated object, creates angle a, which takes values from 0° to 180°. Composition of electric insulating layer glass ceramics is added with oxide glass, barium flint, bentonite clay, microtalc and polyvinyl acetate, and periclase (MgO) composition is used from components with various dispersion. Method for manufacturing of heating element includes manufacturing of resistive element with positive temperature coefficient of resistance, formation of electric insulating layer around it, pressing and sintering of heating element, besides, sintering of produced multi-layer structure is carried out in air at temperature of 850÷900°C with delay at specified temperature for 0.5 - 1.5 hours and further cooling together with furnace.

EFFECT: higher level of thermal expansion coefficient matching in material layers that make heating element.

9 cl, 6 dwg

Description

The invention relates to the field of electric, in particular resistive, heating, namely to monolithic self-regulating cermet heating elements, and is intended for use in various electric heating devices, both industrial and domestic. The invention also includes a method of manufacturing a heating element.

It is known (V.P. Isachenko, V.A. Osipova, A.S. Sukomol. Heat transfer, Moscow, Energy, 1975, p. 486) that the most effective process of heat transfer from a heated body to a cold one in comparison with heating by radiation and other types of heat transfer is the contact thermal conductivity method. In this regard, the development of fast heating elements with a uniform temperature distribution over the surface and an efficiency above η> 0.9 is an urgent task.

To date, the basic requirements for heating elements have been identified, the main of which are the following: high efficiency, reliability and low cost.

The high efficiency of the heating element - heat transfer from the heating structure to the surface through the insulating structure, is determined by the thermal conductivity of the latter. Maximum thermal conductivity can be achieved in monolithic heating elements, where as a result of diffusion synthesis processes a single structure of the heating and insulating layers is formed (US 6328913 B1, 12/11/2001, H01B 1/00; US 4596922, 06.24.1986, F27B 5/14; US 6143238 A, 11/7/2000, C04B 33/32; US 6025579 A, 02.15.2000, H05B 3/44, US 5948306 A, 09/07/1999, H05B 3/10).

The reliability of the heating element is determined by the properties of the heating structure. In this regard, metal heating structures susceptible to oxidation have lower reliability compared to ceramic or composite materials, where the oxidation process is not observed or is observed to a much lesser extent (US 6328913 B1, 12/11/2001, H01B 1/00; US 4596922, 24.06 .1986, F27B 5/14; US 6143238 A, 11/7/2000, C04B 33/32; US 6025579 A, 02/15/2000, H05B 3/44, US 5948306 A, 09/07/1999, H05B 3/10).

In addition, the reliability of the element largely depends on the nature of the temperature dependence of the resistivity of the heating structure. The positive temperature coefficient (RTS - characteristic) of the resistance eliminates spontaneous heating of the element and, as a result, its failure (US 6350969 B1, 02.26.2002, H05B 1/02).

An important factor is the availability of components of the heating, insulation and other structures, expressed, as a result, in the cost of the heating element, as well as the simplicity of the manufacturing technology of the heating element (RU 2311742 C2, 11.27.2007, H05B 3/14, D1).

And finally, the uniformity of the temperature distribution over the surface of the heating element depends on the shape and symmetry of the heating structure (D1).

All the above data corresponds to the heating element presented in (D1), which is the closest analogue of the claimed invention.

However, the durability, and with it the reliability of the heating element, are strongly dependent on the isotropy of its expansion during heating, i.e. from the coordination of the values of thermal expansion coefficients (CTE) that make up the heating element of materials. Satisfying the criteria of high efficiency and ease of manufacture, the prototype is far from perfect in terms of matching KTR materials that make up its layers.

The technical result of the present invention is to increase the level of coordination of the KTP constituting the heating element of the layer materials and, as a result, the creation of a heating element that combines high efficiency with reliability, low cost of its components and the method of its manufacture.

The technical result is achieved by the fact that in the heating element (9) (Fig. 1), containing a resistive element (1) with a positive temperature coefficient of resistance, made of a composite material based on iron and placed in an insulating layer (2), a heat-insulating layer (4 ) and the protective layer (3), contains at least two collector elements (7) connected to the resistive element (1) with a width d (Figs. 2, 5), and the axis of symmetry from the resistive element (1) is perpendicular to the cross section resistive element (1) (figure 3), moreover, at the junction of the resistive element (1) with the collector element (7), the axis of symmetry from the resistive element with respect to the surface of the protective layer providing contact with the heated object (8) forms an angle α, taking values from 0 ° to 180 ° (figure 4). In order to match the values of KTP layers, oxide glass, barite flint, bentonite clay, microtalc and polyvinyl acetate (PVA) are added to the glass-ceramic composition of the insulating layer (2) and the composition of periclase (MgO) from components with different dispersion is used.

Also in the heating element (9), to locally reduce the temperature of the resistive element (1) at the junction with the collector element (7), a contact pad of width d and length 1 (FIG. 2) is made from the material of the resistive element (1), while the cross section of the resistive element (1) at the indicated location has an area S n times larger than the average over the entire length of the resistive element (1), where n is a positive number greater than unity, and the value of n is continuously changing from 1, at a distance L from the junction of the resistive element (1) with collector element (7), up to n at L = 0. Values of lengths 1 and L can take any values that are limited only by technological capabilities and tasks. If the scope of these tasks includes the execution of the heating element (9) with ready-made metal leads (not shown), then they are laid at the first stage of the technological process for the production of the heating element (9) - the stage of compacting the resistive element (1) with collector elements (7 )

The resistive element (1) in the form of a winding strip of resistive material of width d, consisting of linear sections that have a length not exceeding 20 d and connected in series by rounded sections, and the specified winding strip of the resistive element (1) is placed inside the insulating layer (2), which repeats its form. To achieve a linear section length of 20 d, metal compensation inserts (6) are used, through which sections of the resistive element (1) are connected in series. Compensation insert (6) is a metal conductor, for example nichrome, whose cross section is 10-15% of the cross section of the resistive element (1) with a ratio of the lengths of the compensation insert and the linear part of the resistive element from 1:10 to 1:20.

The heating element (9) is additionally provided with a protective layer (3) based on iron powder 0.5-1 d thick, which has a shape that covers an electrical insulating layer (2) on three sides, containing a resistive element (1).

The insulating layer (2) is preferably made of MgO-based glass ceramics with the following ratio of components, wt.%:

Periclase (MgO) 60-90 Ground quartz glass 2-6 Boric acid 1-6 Ground optical glass BF-112 1-6 Bentonite clay 2-6 Microtalc 1-6 Polyvinyl Acetate (PVA) 3-10

using at the same time periclase (MgO) of the following composition, wt.%:

Periclase (MgO) with a dispersion of 40 to 150 microns 20-35 Periclase (MgO) with a dispersion of less than 40 microns 65-80

The protective layer (3) based on iron powder is preferably made of a mixture with a ratio of weight parts, wt.%:

Fe 70-95 Cu 22-3 Sn 8-1

the shape of the protective layer (3) has a configuration that provides the maximum contact surface with the heated object (8) (figure 5).

The technical result for a method of manufacturing a heating element (9), including the manufacture of a resistive element (1) with a positive temperature coefficient of resistance with collector elements (7) and compensating metal inserts (6), the formation of an insulating layer around it (2), pressing and sintering of the heating element (9), it is decided that before sintering the product, at least one additional protective (3) or heat-insulating layer (4) is applied to predefined sections of the formed electrical insulation layer (2) with additional prepressing after applying each layer, and sintering is carried out after compression of the obtained multilayer structure under normal conditions in air with exposure for 0.5 to 2 hours at a temperature in the range from 850-900 ° C.

A method of manufacturing the proposed heating element is as follows.

If necessary, then metal compensation inserts (6) and leads (not shown) are placed in the mold for the manufacture of the resistive element (1).

A mold is filled with a powder of a composite material based on iron and preliminary compaction of the resistive element (1) with a specific pressure of 0.5-1 t / cm ^{2 is} performed.

To preserve the shape of the resistive element (1) after pressing, PVA is introduced into the powder composition of the composite material based on iron in an amount of 3-10%.

After pressing out the resistive element (1), a mold for the electrically insulating glass ceramics is installed and it is filled with powder of the electrically insulating glass ceramics.

On top of the mold for electrically insulating glass ceramics, a mold for the protective layer (3) is installed and it is filled with iron powder with anti-corrosion additives.

A powder layer of a decorative glass-ceramic coating is applied over an iron powder (5), and a powder layer of a heat-insulating glass-ceramic coating (4) is applied to the lower surface.

To coordinate the return efforts after pressing out the element from the matrix, the composition of all ceramics included PVA in an amount of 3-10%.

In addition, in order to ensure the complete closure of the ceramic by the metal, a circular strip of metal powder is applied to the surface of the thermal insulation layer (4).

The multilayer powder structure is compacted under normal conditions at a pressure in the range of 3 to 4 tons / cm ² .

The pressed billet of the heating element (9) is sintered under normal conditions in air with exposure for 0.5 to 2 hours at a temperature in the range from 850-900 ° C and subsequent cooling together with the furnace.

As a result of the high-temperature synthesis process, the billet of the heating element (9) is sintered to form its monolithic structure.

Changing the value of the angle α and the formation of current-collecting elements (7) with the given dimensions (Fig. 2) are carried out at the stages of formation of the resistive element (1) and the electrical insulating layer (2) by giving the layers the corresponding shape.

Coordination of KTP is carried out by choosing the appropriate components of glass ceramics and their mass ratios.

The resistive element (1) is made of a composite material based on iron with a resistivity that increases with increasing temperature. As the main material for the resistive element (1), for example, Distalloy AE iron powder from Höganäs AB (Sweden) can be used.

The insulating layer (2) is preferably made of MgO-based glass ceramics with the following ratio of components, wt.%:

Periclase (MgO) 60-90 Ground quartz glass 2-6 Boric acid 1-6 Ground optical glass BF-112 1-6 Bentonite clay 2-6 Microtalc 1-6 Polyvinyl Acetate (PVA) 3-10

using at the same time periclase (MgO) of the following composition, wt.%:

Periclase (MgO) with a dispersion of 40 to 150 microns 20-35 Periclase (MgO) with a dispersion of less than 40 microns 65-80

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. Sectional view of a heating element.

Figure 2. Schematic illustration of a resistive element with a collector element.

Figure 3. The location of the axis of symmetry of the resistive element.

Figure 4. An example of the implementation of the resistive element for the case when the angle α is not equal to zero.

Figure 5. Schematic representation of a resistive element with compensating metal inserts.

6. Examples of possible forms of a heating element.

Figure 1 shows a schematic representation of the inventive heating element (9) in the context of an example of its execution in the form of a flat heating element. The basis of the inventive heating element (9) is a resistive element (1), covered on all sides by an electrical insulating layer (2), repeating its shape. The protective layer (3) based on iron powder has a shape covering the electrical insulating layer (2) on three sides. The fourth side of the insulating layer is covered by a heat insulating layer (4), made in the form of a layer of insulating glass ceramics. On top of the protective layer (3), a decorative ceramic layer (5) can be made.

The resistive element (1) is made of a composite material based on iron with a resistivity that increases with increasing temperature. As a material for such a resistive element (1), for example, an iron-based composite material can be used, where each iron particle is coated with a thin dielectric layer, for example, SMC-500 from Höganäs AB (Sweden).

Figure 2 shows a schematic representation of a resistive element (1) with a collector element (7) for various lengths L. The symbols β and γ denote the secant planes, and S and S / n are the cross-sectional areas lying in these planes.

Figure 3 shows the location of the axis of symmetry of the resistive element (1). The axes a and b lie in the plane of the cross section, the axis c is perpendicular to the plane of the cross section and passes through the intersection point of the axes a and b.

Figure 4 shows an example of the implementation of the resistive element (1) for the case when the angle α is not equal to zero. The axis c forms with the surface (10) a protective layer that provides contact with the heated object (8), the angle α is not equal to zero.

Figure 5 shows a schematic representation of a resistive element (1) with compensating metal inserts (6).

Figure 6 shows examples of possible forms of the heating element (9), where the color indicates the heating objects (8).

The proposed monolithic heating element (9) for its reliable operation after sintering in its final form should have coordinated KTP of all layers of the element - resistive element (1), electrical insulating layer (2), protective metal (3) and thermally insulating (4) layers. The protective metal layer (3) is based on iron powder with anti-corrosion additives.

The inventive electric heating element (9) is made by powder metallurgy technology and is a multilayer structure of a conductive cermet, insulating glass-ceramic and protective metal layers, pressed and sintered into a single monoblock.

Ultrafast heating of the resistive element (1) leads to uneven mechanical stresses of the heating element (9) and, as a consequence, the possibility of its warping. To eliminate warpage, maintain integrity and, as a consequence, the reliability of the heating element (9), it is necessary to ensure uniformity of expansion of the latter, for which compensation inserts (6) can be used (Fig. 5).

Isotropic expansion is achieved by making the resistive element (1) in the form of a tortuous strip of resistive material of width d, consisting of linear sections that have a length not exceeding 20 d and connected in series by rounded sections, and the specified tortuous strip of the resistive element (1) is placed inside the electrical insulation layer (2), which repeats its shape. To achieve a linear section length of 20 d, metal compensation inserts (6) are used, through which sections of the resistive element (1) are connected in series.

Coordination of KTP is carried out by choosing the appropriate components of glass ceramics and their mass ratios.

To this end, the insulating glass-ceramic layer (2) with a density close to theoretical is made on the basis of periclase (crystalline magnesium oxide MgO) with the addition of oxide glass, barite flint, bentonite clay, microtalc and polyvinyl acetate in the following ratio:

Periclase (MgO) 60-90 Ground quartz glass 2-6 Boric acid 1-6 Ground optical glass BF-112 1-6 Bentonite clay 2-6 Microtalc 1-6 Polyvinyl Acetate (PVA) 3-10

using at the same time periclase (MgO) of the following composition, wt.%:

Periclase (MgO) with a dispersion of 40 to 150 microns 20-35 Periclase (MgO) with a dispersion of less than 40 microns 65-80

A method of manufacturing the proposed heating element (9) comprises the following steps.

If necessary, then metal compensation inserts (6) and leads (not shown) are first placed in the mold for the manufacture of the resistive element (1).

A mold is filled with a powder of a composite material based on iron and preliminary compaction of the resistive element (1) with a specific pressure of 0.5-1 t / cm ^{2 is} performed.

To preserve the shape of the resistive element (1) after pressing, PVA is introduced into the powder composition of the composite material based on iron in an amount of 3-10%.

After pressing out the resistive element (1), a mold for the electrically insulating glass ceramics is installed and it is filled with powder of the electrically insulating glass ceramics.

On top of the mold for electrically insulating glass ceramics, a mold for the protective layer (3) is installed and filled with iron powder with anti-corrosion additives.

A powder layer of a decorative glass-ceramic coating is applied over the iron powder, and a powder layer of a heat-insulating glass-ceramic coating is applied to the lower surface (4).

To coordinate the return efforts after pressing out the element from the matrix, the composition of all ceramics included PVA in an amount of 3-10%.

In addition, in order to ensure the complete closure of the ceramic by the metal, a circular strip of metal powder is applied to the surface of the thermal insulation layer (4).

The multilayer powder structure is compacted under normal conditions at a pressure in the range of 3 to 4 t / cm ² .

The pressed billet of the heating element (9) is sintered under normal conditions in air with exposure for 0.5 to 2 hours at a temperature in the range from 850-900 ° C and subsequent cooling together with the furnace.

As a result of the high-temperature synthesis process, the billet of the heating element (9) is sintered to form its monolithic structure.

The change in the value of the angle α and the formation of the collector elements (6) with the given dimensions (Fig. 2) are carried out at the stages of the formation of the resistive element (1) and the insulating layer (2) by giving the layers the corresponding shape.

The heating elements (9) can be made of any shape (Fig.6) and size with a thickness of the heating element from 1 mm and above.

From the foregoing, it follows that the developed flat electric heating element (9) rapid heating has several advantages in comparison with the known heating elements.

Namely:

- a high degree of reliability due to the exact matching of the KTP layers, isotropic expansion of the resistive element when using metal compensation inserts and a local decrease in temperature at the point of supply of electric current due to an increase in the cross-sectional area of the resistive element.

Claims (9)

1. A heating element containing a resistive element with a positive temperature coefficient of resistance, made of a composite material based on iron and placed in an insulating layer made of glass ceramics based on periclase (MgO), a thermal insulation layer and a protective layer, characterized in that it contains at least two collector elements connected to a resistive element with a width of d, and the axis of symmetry of the resistive element is perpendicular to the cross section of the resistive electric ment, moreover, at the junction of the resistive element with the collector element, the axis of symmetry of the resistive element with respect to the surface of the protective layer providing contact with the heated object forms an angle α taking values from 0 ° to 180 °, while for matching thermal expansion coefficients (KTR) layers, oxide glass, barite flint, bentonite clay, microtalc and polyvinyl acetate (PVA) are added to the glass-ceramic composition of the insulating layer, and the composition of periclase (MgO) from Components with different dispersion. 2. The heating element according to claim 1, characterized in that the resistive element is made in the form of linear sections that have a length not exceeding 20 d (d is the width of the resistive element), connected in series by compensation metal inserts. 3. The heating element according to claim 1, characterized in that the insulating layer is made of glass ceramics based on periclase (MgO) with the following ratio of components, wt.%:
Periclase (MgO) 60-90 Ground quartz glass 2-6 Boric acid 1-6 Ground optical glass BF-112 1-6 Bentonite clay 2-6 Microtalc 1-6 Polyvinyl Acetate (PVA) 3-10,
using periclase (MgO) of the following composition, wt.%:
Periclase (MgO) with a dispersion of 40 to 150 microns 20-35 Periclase (MgO) with a dispersion of less than 40 microns 65-80 4. The heating element according to claim 1, characterized in that the protective layer based on iron powder is made of a mixture with a ratio of parts by weight, wt.%:
Fe 70-95 Cu 22-3 Sn 8-1,
the shape of the protective layer has a configuration that provides the maximum contact surface with the heated object. 5. A heating element according to any one of claims 1 to 4, characterized in that to locally reduce the temperature of the resistive element at the junction with the collector element from the material of the resistive element, a contact pad is made of width d and length l, and the cross section of the resistive element in the specified location has an area S n times larger than the average over the entire length of the resistive element, where n is a positive number greater than unity, and the value of n continuously changes from 1, at a distance L from the junction of the resistive element and with a collector element, up to n at L = 0. 6. A method of manufacturing a heating element with agreed values of the coefficient of thermal expansion (KTR) of a resistive element made of a composite material based on iron, an electrical insulating layer, a thermal insulating layer and a protective layer, including manufacturing a resistive element with a positive temperature coefficient of resistance, forming an electrical insulating layer around it pressing and sintering a heating element, characterized in that the sintering of the obtained multilayer the structures are carried out in air at a temperature of 850-900 ° C with exposure at the indicated temperature for 0.5 to 1.5 hours and subsequent cooling with the furnace, and oxide glass, barite flint, bentonite clay are introduced into the electric insulation layer, microtalc and polyvinyl acetate, and use the composition of periclase (MgO) from components with different dispersion. 7. The method according to claim 6, characterized in that the protective layer of iron powder is a pre-prepared mixture, with a ratio of weight parts, wt.%:
Fe 70-95 Cu 22-3 Sn 8-1 8. The method according to claim 6, characterized in that the insulating layer is made of glass ceramics based on periclase (MgO) with the following ratio of components, wt.%:
Periclase (MgO) 60-90 Ground quartz glass 2-6 Boric acid 1-6 Ground optical glass BF-112 1-6 Bentonite clay 2-6 Microtalc 1-6 Polyvinyl acetate 3-10,
using at the same time periclase of the following composition, wt.%:
Periclase (MgO) with a dispersion of 40 to 150 microns 20-35 Periclase (MgO) with a dispersion of less than 40 microns 65-80 9. The method according to any one of claims 6 to 8, characterized in that at the stage of manufacturing the resistive element, metal compensation inserts and leads are laid in the mold for manufacturing the resistive element and compacting the resistive element with a specific pressure of 0.5-1 t / cm ² .

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