WO2004062319A1

WO2004062319A1 - Monolithic self-regulating metal-ceramic heater

Info

Publication number: WO2004062319A1
Application number: PCT/BY2003/000001
Authority: WO
Inventors: Vladimir P. Lapin; Genadiy A. Govor; Alexandr L. Shitenkov
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-01-02
Filing date: 2003-01-02
Publication date: 2004-07-22
Anticipated expiration: 2005-07-02
Also published as: AU2003201228A1

Abstract

The invention relates to the field of electric heating and of resistive heating in particular, and namely to monolithic self-regulating metal-ceramic heating elements intended for use in various electric heating devices both for industrial and household purposes. In a heating element containing a heating resistive body (1) with PTC-characteristic positioned in an electric insulating layer (2), said heating resistive body is made of Fe-based composite material having the specific resistance increasing with the temperature rising in the range from 100 to 200 µOhm.m. The heating resistive body (1) is made in the shape of an irregular path having the width of d, said path being comprised of linear sections (11) having the maximum length of 10d, said sections being sequentially connected by rounded-off sections (12), while the symmetry axes of the heating resistive body (1) coincide with the symmetry axes of a heating element. Said irregular path of the heating resistive body is positioned inside an insulating layer (2), which follows its shape. A method of manufacturing of a heating element, said method comprising the steps of making a heating resistive body (1), forming around said heating resistive body (1) of an electric insulating layer (2), further pressing and sintering of the heating element. Prior to sintering at least one additional layer is applied on the pre-defined portions of the insulating layer (2) thus formed with subsequent additional pre-pressing following the application of each layer, while sintering is provided after compression of the multi-layer structure thus received.

Description

MONOLITHIC SELF-REGULATING METAL-CERAMIC HEATER

FIELD OF THE INVENTION

The invention relates to the field of electric heating and of resistive heating in particular, and namely to monolithic self-regulating metal-ceramic heating elements intended for use in various electric heating devices both for industrial and household purposes. It also includes the method of manufacturing thereof

PRIOR ART

It is known [V.P.Isachenko, N.A.Osipova, A.S.Sukomol. Heat-transfer process.

Moscow "Energiya" (1975), 486 pp.] that the most efficient method of heat transfer from a heated object to a cool one as compared to radiation heating and other types of heat transfer is a method of contact thermal conductivity. In connection with the above the development of fast operating heating elements featuring a uniform temperature distribution on the surface and minimum efficiency of η>0,9 becomes most urgent.

The basic requirements to the heating elements, which have been defined by now, are as follows: high efficiency, dependability and low cost.

High efficiency of a heating element, i.e. heat transfer from a heating body to the surface via an insulating body is defined by the heat conductivity of the latter. Maximum heat conductivity may be provided in such monolithic heating elements where the synthesis diffusion processes result in the formation of a unified structure of the heating and insulation layers. This aim has been achieved in a heating element disclosed in the US patent No. US6328913, dd. 11.12.2001.

The dependability of a heating element is defined by the characteristics of a heating body. In this connection a heating body made of metal, which can be subject to oxidation, is characterized by lesser dependability as compared to ceramic or composite materials, where the oxidation processes are not observed at all or are observed in much lesser degree. Ceramic heating elements for various purposes are offered in references [US5948306, dd. 07.09.1999; US6025579, dd. 15.02.2000; US6143238, dd. 07.11.2000].

Besides, the operating dependability of a heating element is largely influenced by the temperature dependence of the specific resistance of a heating body. The positive temperature coefficient (PTC- characteristic) makes it possible to eliminate spontaneous heating of the element and consequently its failure [see, for example, US6350969, dd. 26.02.2002]. The uniform temperature distribution on the heating element surface and the expansion isotropy under heating (which result in dependability and long lifetime) are determined by the form and symmetry of the heating body structure. Partly the symmetry of the heating body on the dependability of the heating element was considered in the US patent No. US4596922, dd. 24.06.1986]. The cost of a heating element is of no less importance, said cost being defined by the cost of components for heating, insulating and other bodies and the technology of manufacturing thereof. The latter excludes the use of rare-earth oxides and other costly components.

On the grounds of the above it follows that the known heating elements as well as the technology of manufacturing thereof meet only some of the requirements as to their efficiency, dependability and cost. SUMMARY OF THE INVENTION

It is the aim the present invention to design a heating element characterized by high efficiency and dependability alongside with low cost of its constituent components as well as of the method of manufacturing thereof.

The aim set forth is achieved in a heating element containing a heating resistive PTC body placed in an insulating layer by means of making said heating resistive body of Fe-based composite material having the specific resistance increasing with the temperature rising in the range from 100 to 200 μOhm.m.

The net surface of the heating resistive body makes up from 40 to 60% of the heating element total area.

A heating resistive body is made in the shape of an irregular path having the width of d, said path being comprised of linear sections having the maximum length of lOd, said sections being sequentially connected by rounded-off sections, while the symmetry axes of the heating resistive body coincides with the symmetry axes of a heating element, and said irregular path of the heating resistive body is located inside an insulating layer, which follows its shape.

A heating element is additionally supplied with a Fe powder-based protective layer having the thickness of 0,5- Id, said protective layer being shaped such that to capsulate of the insulating layer on the three sides, said insulating layer comprising therein the heating resistive body.

An electric-insulating layer is preferably made of MgO-based glass ceramics with the following ratio of components, % by weight:

MgO 75 - 85% BN 10 - 15%

Alkali-free glass 5 - 10%

The magnesium oxide in the above-said ceramics has the following composition, % by weight:

MgO having the dispersion from 40 to 150 μk - 45-55% Mgo having maximum dispersion of 40 μk - 45-55%). A heating element is additionally supplied with a heat-insulating layer, which layer is preferably made of MgO-based glass ceramics with the following ratio of components, % by weight:

MgO 40 - 45% SiO₂ 40 - 45%

Borosilicate glass 10 - 20%.

The layer of heat-insulating glass ceramics is covered by at least one layer of the heat- insulating paste, e.g. on base of the Fire Sealant 1200.

The layer of heat-insulating glass ceramics and the layer of heat-insulating paste taken together are impregnated with high-temperature waterproofing silicon liquid.

The aim set forth in a method of manufacturing of a heating element, said method comprising the steps of making a heating resistive body, forming around said heating resistive body of an insulating layer, further compacting and sintering of the heating element, is achieved by means of that prior to sintering at least one additional layer is applied on the pre-defined portions of the insulating layer thus formed with subsequent additional pre-pressing following the application of each layer, while sintering is provided after compression of a multi-layer structure thus received.

A heating resistive body is made of Fe-based composite material having the specific resistance increasing with the temperature rising in the range from 100 to 200 . μOhm.m.

An electric insulating layer is made of MgO- based glass ceramics with the following ratio of components, % by weight:

MgO 75 - 80%

BN 10 - 15%

Alkali-free glass 10 - 20%.

The magnesium oxide in the above-said glass ceramics has the following ratio of components, % by weight:

MgO having the dispersion from 40 to 150 μk - 45-55% MgO having maximum dispersion of 40 μk - 45-55%.

A protective Fe-powder layer is placed in such a way that it covers an insulating layer on the three sides.

A heat-insulating layer of MgO-based glass ceramics is placed on the surface of the multi-layer structure thus received which is free from protective layer, said heat- insulating layer having the following ratio of components, % by weight:

MgO 40 - 45% SiO₂ 40 - 45%

Borosilicate glass 10 - 20%.

Compacting of a multi-layer structure of a heating element is provided in the pressure range from 3 to 4 kbar. Sintering of a multi-layer structure is carried out on the air with the time exposure period from 0,5 to two hours in the temperature range from 900 to 1150°C.

A heat-insulating layer is covered by at least one layer of heat-insulating paste, e.g. on base of Fire Sealant- 1200, and all these taken together are impregnated with a high temperature waterproofing silicon liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic cross-sectional view of a heating element.

Fig.2 shows the temperature dependence of the specific resistance of the conductive body. Fig.3 shows a schematic view of the heat transfer in a flat heating element.

Fig. 4 shows the heat distribution in a heat-absorbing surface Sa for various ratios of the radiant and heat-absorbing surfaces at different widths of a damping layer. Fig.5 shows an example of a conductive body for a circular-shaped heating element.

Fig.6 shows a method of manufacturing of a heating element in accordance with the present invention in its diagrammatic form.

Fig.7 shows the temperature distribution in the radiant Sr and heat-absorbing Sa surfaces with the ratio of the latter Sr/Sa = 0,5.

DETAILED DESCRIPTION AND MOST PREFERABLE EXAMPLES OF THE INVENTION

An electric heating element in accordance with the present invention is manufactured according to powder metallurgy technique and presents in itself a multi-layer structure comprising accordingly a metal-ceramic heating resistive body, an electric and heat insulating glass ceramic layers, a protective metal layer, all these taken together having been compacted and sintered to form a monolithic structure.

Fig.1 shows a schematic cross-sectional view of a heating element in accordance with the present invention embodying the latter in the form of a flat heating element. The basic constituent part of a heating element in accordance with the present invention is a metal-ceramic heating resistive body 1 capsulated by an electric insulating layer 2 which follows the shape of said heating resistive body 1. A protective Fe powder layer 3 is shaped such that to capsulate the insulating layer 2 on the three sides. A heat- insulating layer 4 is made in the form of a heat-insulating glass ceramic layer 5 with a heat-insulating paste layer 6. A decorative ceramic layer 7 is applied above the protective metallic layer 3.

A metal-ceramic heating resistive body 1 is made of Fe-based composite material having the specific resistance increasing from 100 to 200 μOlim.m with the temperature rising from 0 to 1000°C. A temperature dependence of the specific resistance of the metal-ceramic heating ^• resistive body 1 is shown in Fig.2. As it follows from Fig.2, the specific resistance of the metal-ceramic is by two to three orders higher than the specific resistance of the metal conductors known in the art while preserving the PTC-characteristic. The metal-ceramic heating resistive body 1 of the type described may be made of Fe-based composite material wherein each Fe particle is covered by a thin dielectric layer, e.g. SMC-500 of Hδganas AB Co. (Sweden).

The temperature dependence pecularity of the specific resistance of the metal-ceramic heating resistive body 1 consists in providing the heating of said body 1 within only a few seconds. Besides, high specific resistance makes it possible to considerably diminish the overall dimensions of the heating elements.

The net surface of the metal-ceramic heating resistive body of a heating element in accordance with the present invention makes up from 40 to 60%) of the total area of the heating element.

In the most common case the heat transfer from a radiant surface, i.e. from the heating resistive body 1, to a heat-absorbing surface, which may be considered as coinciding with the surface of a heating element, is defined by the following equation

Q = k . F . Δt (1), where

F is the net surface of a heating resistive body 1 ;

Δt = tπ - tβ is the temperature difference between a heating resistive body 1 and the environment. Since t_H » tβ, then the temperature of a heating resistive body 1 can be taken Δt as a first approximation;

1 k = - heat transfer coefficient, where. δi + δ λ] λ λl and λ2 are respectively heat conductivity values of a glass ceramic insulating layer 2 and of a protective metal layer 3, δl and δ2 present the width values of the glass ceramic insulating layer 2 and of the metal layer 3 accordingly.

The radiant energy losses Qr are defined as Qr = σ.T⁴ (2), where σ is a Stephen-Boltzmann Constant, and T is an absolute temperature of a heating body. It follows from (2) that to minimize losses the temperature of a heating element must be as minimum as possible. Besides, to meet fire-safety requirements the maximum permissible temperature rating of a heating element has been defined within 500- 600°C. In this case the heat transfer (Qmax) for maximum value of a heat transfer coefficient (kmax), maximum value the total surface of the heating resistive body (Fmax), and at some pre-defined limiting temperature rating of a heating resistive body (tconst), is determined by:

Q —> Qmax = kmax . Fmax . tconst (3)

As far as λ₂ >> λi, influence of λ₂ on the value of the heat transfer coefficient is negligibly small. The maximum value of the heat transfer coefficient is defined by the proper heat conductivity of insulating glass ceramics λi containing according to the present invention the magnesium oxide and the boron nitride:

i = C o • go + C B • λNB > (4) where C_Mgo and CN_B are respectively the concentrations of the magnesium oxide and the boron nitride, while λi igo and B are accordingly their heat conductivity values.

As a result the value of heat transfer k from a heating resistive body, to a heat- absorbing surface is also a definite one.

Thus as it follows from the heat transfer equation (3) the maximum value of heat transfer is defined by the net surface of a heating resistive body. Further there comes the description of the influence of the radiant and heat-absorbing surfaces ratio on the critical parameters of a heating element such as efficiency, the temperature of a radiant surface etc.

For the convenience of assessment there is represented (See Fig. 3) a heating resistive body 1 of a heating element comprised of m parallel flat conductors 8 of infinite length, having the width of d each, said conductors separated by insulation gaps 9 having the width of li.

Besides, a radiant surface Sr (which coincides with the surface of the heating resistive body 1) and a heat absorbing surface Sa are separated by a damping protective metallic layer 3 having the width of δ₂ and heat conductivity of λ₂, said damping layer providing on the one hand the temperature smoothing in a heat-absorbing surface and on the other hand the insulation and protection of said heat-absorbing surface.

While determining the quantity of heat passing through layer 3 each flat conductor 8 is considered as being comprised of n linear sources 10 of infinite length. The total heat quantity in a "A" point in a heat-absorbing surface is defined as a superposition from m flat conductors 8, each being comprised of n linear sources 10: n,m Q_A = ∑ q ij (5), ij = l where q ij is the heat quantity from i-numbered linear radiation source 10 of j- numbered flat conductor 8. In a steady-state condition the value of q ij is defined by the following equation: λ₂ q ij = - (tr -ta) (6), δ₂ where tr - ta is the temperature difference between the radiant surface Sr and the heat-absorbing surface Sa.

The calculations were made for various ratios of the radiant and heat-absorbing surfaces as well as for different thickness δ of the layer 3 at pre-defined temperature difference of tr - ta.

Fig. 4 shows the calculation results of the heat quantity transferred from the radiant surface to the heat-absorbing surface for various ratios of their areas Sr/Sa = 0,5 (Fig.4a), Sr/Sa = 0,25 (Fig. 4b) and Sr/Sa = 0,1 (Fig. 4c) through the layer 3 for various values of its thickness δ = 0.5d (curves 1, 4, 7), δ = d (curves 2, 5, 8) and δ = 2d (curves 3, 6, 9).

The study of the above-shown curves reveals that at the ratio of the radiant and heat- absorbing surfaces of Sr/Sa = 0,5 even with a thin damping layer δ = 0.5d (Fig. 4a, curve 1) being provided, a uniform heating of a heat-absorbing surface - ΔQ =0,1 Qm is achieved, where ΔQ is non-uniformity of the heat quantity in a heat-absorbing surface.

The above-stated value of heating non-uniformity in a heat-absorbing surface for the case when the ratio of the radiant and heat-absorbing surfaces makes up Sr/Sa = 0,1 is achieved only at the thickness of the damping layer 2δ = 2d.

The increase of the thickness of the protective metal layer 3 and the decrease of the ratio of the radiant and heat-absorbing surfaces from Sr/Sa = 0,5 to 0,1 results in the substantial decrease of the heat transfer to the heat-absorbing surface as in the case described (Fig. 4) Q,/Q₃ =7.

As it follows from the results of the above study the using of the heating elements with the ratio of the radiant and heat-absorbing surfaces of Sr/Sa = 0,5 and more leads to the increase of the efficiency of the heating element due to the fact that the predefined temperature of the heat-absorbing surface can be reached at a lower temperature of radiant surface (i.e. at a lower temperature of the heating resistive body 1), this resulting in the decrease of the radiant losses.

The decrease of the thickness of the damping layer and, hence,- of the weight of a heating element results in the decrease of initial heating time of the heating element up to several seconds and finally leads to the cost decrease of the heating element on the whole. Rapid initial heating of an element in accordance with the present invention imposes certain requirements to its structure. Should a heating resistive body 1 be fed with a supply voltage, the current through it will cause its heating. The temperature increase causes the resistance increase of the heating resistive body 1 (See Fig. 2), while the current flowing through it is decreased with the resulting temperature stabilization at the pre-set level. Since the temperature of the heating resistive body 1 is changed within the fractions of a second, this process can be considered as an adiabatic one not causing any heat- exchange with a protective metal layer 3. The initial heating of the latter is provided within few seconds.

Extra-rapid initial heating of a conductive body leads to non-uniform mechanical tensions of a heating element and, hence, to its possible hogging. To eliminate hogging and to preserve the integrity of a heating element and, hence, its dependability the expansion uniformity of the latter should be provided.

The expansion isotropy is achieved by making the heating resistive body 1 in the shape of an irregular path having the width of d, said path being comprised of linear sections having the maximum length of lOd, said sections being sequentially connected by rounded-off sections, while the symmetry axes of the heating resistive body 1 coincide with the symmetry axes of a heating element, and said irregular path of the heating resistive body 1 is positioned inside an insulating layer 2, which follows its shape. Fig. 5 shows an example of making a heating resistive body for the case of a circular- shaped heating element. The heating resistive body 1 is made in the shape of an irregular path having the width of d, said path being comprised of linear sections 11 having the maximum length of lOd, said sections being sequentially connected by rounded-off sections 12. It is obvious from Fig. 5 that the heating resistive body 1 in the example shown has a symmetry axis of the 6-th order. The uniformity of adiabatic expansion of the heating resistive body 1 is provided by high symmetry extent as well as by positioning the linear sections 11 both along the radii and parallel to the circumferences of the respective radii. To provide reliable operation of a monolithic heating element in accordance with the present invention in its finished form after sintering, the latter is to have coherent thermal expansion coefficients of all its layers, i.e. a heating resistive body 1, an insulating layer 2, a protective metal layer 3 and a heat-insulating layer 4. A protective metal layer 3 is Fe powder-based with anticorrosive additions.

The same values of thermal expansion coefficients are provided by means of the choice of appropriate glass ceramic components and their weight ratio.

In view of the above an electric insulating glass ceramic layer 2 having the density which approximates an apparent one is made on periclase-base (the crystalline magnesium oxide) with the addition of the hexagonal boron nitride and alkali-free glass to increase heat-conductivity and specific resistance, in the following ratio:

MgO 75 - 85% BN 10 - 15%

Alkali-free glass 5 - 10%

The magnesium oxide in above-said glass ceramics has the following composition, % by weight: MgO having the dispersion from 40 to 150 μk - 45-55% MgO having maximum dispersion of 40 μk - 45-55%).

A heat-insulating layer 4 contains the layer 5 which is made of MgO-based glass ceramics having the following ratio of components, % by weight:

MgO 40 - 45%

SiO₂ 40 - 45%

Borosilicate glass 10 - 20%

The above-said heat-insulating layer 5 is covered with at least one layer 6 of heat- insulating paste, e.g. on base Fire Sealant-1200.

The layer 5 of heat-insulating glass ceramics and the layer 6 of heat-insulating paste together are impregnated with a high temperature waterproofing silicon liquid.

A method of manufacturing of a heating element in accordance with the present invention can be described in more detail using the non-limiting examples presented below.

EXAMPLE 1. Manufacturing of a flat circular-shaped heating element having the power of 600 W and an outer diameter of 125 mm.

The steps of the process are shown in diagrammatic form in Fig.6.

A. A composite material having the specific resistance increasing in the range from 100 to 200 μOhm.m with a temperature increase up to 1000°C, e.g. a Fe-based composite material, wherein each Fe particle was covered by a thin dielectric layer, e.g. SMC-500 of Hδganas AB Co. (Sweden), was used in the form of the powder added with polyvinylacetate dispersion (PVA), 5% by weight.

A mold 13 for making a heating resistive body shown in Fig.5 was filled with 5 grams of said powder, and pre-compacting of the heating resistive body was carried out at the specific pressure of 0,3 kbar (Fig. 6a).

A blank of the heating resistive body 1 thus received was extracted from the mold 13 so that it turned out to be on the surface of said mold (Fig. 6b).

B. A face-mould 14 (Fig. 6c) was positioned above the mold 13, said face-mould having an internal cavity in the shape of an irregular path for making an insulating layer 2 following the shape of the heating resistive body 1.

The face-mould 14 was filled in full with 20 grams of electric isolation glass ceramic powder of the following composition: MgO 80%

BN 10%

Alkali-free glass 10% To provide the required density of electric isolation glass ceramics approximating an apparent one 11 grams of MgO having the dispersion from 40 to 150 μk and 9 grams of MgO having maximum dispersion of 40 μk were used.

Pre-pressing of insulating ceramic powder was carried out at the pressure of 0,3 kbar.

C. The face-mould 14 having been removed, a mold 15 (Fig. 6d) for making a protective layer 3 was positioned, said mold being filled in full by 130 grams of Fe- powder with anticorrosive additions.

D. Above a multi-layer blank thus received a mold 16 (Fig. 6e) was positioned, said mold being used for making an decorative coating of glass ceramic powder having the following composition: MgO 80%

Borosilicate glass 20%)

5 grams of glass ceramic powder was added with 5% by weight of PVA. The layer 7 of decorative coating having the thickness 0,2-0,3 mm provides metal protective layer from oxidizing during a sintering step.

Pre-pressing of multi-layers powder was carried out at the pressure of 0,3 kbar. E. The mold 16 was turned upside-down, and on the bottom surface of a multi-layer blank thus received there was positioned a face-mould 17 (Fig. 6f) for making a heat- insulating coating. The face-mould. 17 was filled with 20 grams of glass ceramic powder of the following composition: MgO 40%

SiO₂ 40%

Borosilicate glass 20%

Glass ceramic powder was added with 5% by weight of PVA.

The thickness of a heat-insulating layer 5 of a glass ceramic coating usually makes up from 0,6 to 0,8 mm.

F. A multi-layers powder structure thus received was compacted under normal conditions at the pressure of 4 kbar.

G. A compacted blank of a heating element was sintered in the air at the temperature of 1150°C with the exposure time of 0,5 hrs. As a result of a high-temperature synthesis process the sintering of a heating element was carried out with the eventual formation of its monolithic structure.

H. The layer of the above-said heat-insulating glass ceramics was covered by one layer of 10 grams of Fire Sealant 1200 heat-insulating paste. I. The layer 5 of heat-insulating glass ceramics and the layer 6 of heat-insulating paste taken together were impregnated with high temperature waterproofing silicon liquid.

The weight of a heating element thus received made up 200 g.

The temperature distribution was measured experimentally on the radiant Sr and heat- absorbing Sa surfaces of a heating element thus manufactured with the ratio of the latter Sr/Sa = 0,5 (Fig. 7). The results shown in Fig.7 make it obvious that the temperature losses in the damping layer do not exceed Δt = 20°C.

Experimentally measured efficiency of a heating element devised is defined basically by the radiation losses and makes up the value of η = 0,95.

EXAMPLE 2. Manufacturing of a square-shaped flat heating element having the power of 600 W.

A pre-generated mold 13 for making a heating resistive body 1 was filled with 3 grams of the powder as stated at the Example 1, and pre-compacting of the heating resistive body 1 was carried out at the specific pressure of 0,4 kbar (Fig. 6a).

A blank of the heating resistive body 1 thus received was extracted from the mold 13 so that it turned out to be on the surface of said mold (Fig. 6a).

B. A face-mould 14 (Fig. 6b) was positioned above the mold 13, said face-mould having an internal cavity in the shape of an irregular path for making an insulating layer 2 following the shape of the heating resistive body 1. The face-mould 14 was filled in full 10 grams of with glass ceramic powder of the following composition:

MgO 70% BN 23%

Alkali-free glass 7%

To provide the required density of electric isolation glass ceramics approximating an apparent one 4 grams of MgO having the dispersion from 40 to 150 μk and 6 grams of MgO having maximum dispersion of 40 μk were used.

Pre-pressing of glass ceramic powder was carried out at the pressure of 0,5 kbar.

C. The face-mould 14 having been removed, a mold 15 (Fig. 6c) for making a protective layer 3 was positioned above a multi-layer blank thus received, said mold being filled by 100 grams of Fe-powder with anticorrosive additions.

D. Above a multi-layer blank thus received a mold 16 (Fig. 6d) was positioned, said mold being used for making an decorative coating 7 of glass ceramic powder having the following composition:

MgO 80%

Borosilicate glass 20%> 5 grams of glass ceramic powder was added with 5% by weight of PVA. Pre-pressing of the multilayers powder was carried out at the pressure of 0,5 kbar.

E. The mold 16 was turned upside-down and on the bottom surface of a multi-layer blank thus received there was positioned a face-mould 17 (Fig. 6e) for making a heat- insulating coating. The face-mould 17 was filled with 20 grams of glass ceramic powder of the following composition:

MgO 45% SiO₂ 40%

Borosilicate glass 15%

Glass ceramic powder was added with 5% by weight of PVA. F. A multi-layer powder structure thus received was compacted at room temperature with the pressure of 3,5 kbar.

G. A compacted blank of a heating element was sintered in the air at the temperature of 1050°C with the exposure time of 1 hrs.

As a result of a high-temperature synthesis process the sintering of a heating element was provided with the eventual formation of its monolithic structure.

H. The layer 5 of the above-said heat-insulating glass ceramics was covered by one layer 6 of 10 grams of Fire Sealant 1200 heat-insulating paste.

I. The layer 5 of heat-insulating glass ceramics and the layer 6 of glass cloth taken together were impregnated with waterproofing high temperature silicon liquid. The weight of a heating element thus received having the power of 600 W made up 150 g.

Experimentally measured efficiency of a heating element devised made up the value of η = 0,92.

EXAMPLE 3. Manufacturing of a flat circular-shaped heating element having the power of 300 W.

A pre-generated mold 13 for making a heating resistive body 1 shown in Fig.5 was filled with 2 grams of the above-said powder, and pre-compacting of the heating resistive body 1 was carried out at the pressure of 0,4 kbar.

B. A face-mould 14 (Fig. 6b) was positioned above the mold 13, said face-mould having an internal cavity in the shape of an irregular path for making an insulating layer 2 following the shape of the heating resistive body 1. The face-mould 14 was filled with 10 grams of electric isolation glass ceramic powder of the following composition: MgO 80%

BN 10%

Alkali-free glass 10%

To provide the required density of electric isolation glass ceramics approximating an apparent one 5 grams of MgO having the dispersion from 40 to 150 μk and 5 grams of MgO having maximum dispersion of 40 μk were used. Pre-pressing of electric isolation glass ceramic powder was carried out at the pressure of 0,4 kbar.

MgO 80%

Borosilicate glass 20%

5 grams of glass ceramic powder was added with 5% by weight of PVA.

Pre-pressing of glass ceramic powder was carried out at the pressure of 0,4 kbar.

E. The mold 16 was turned upside-down, and on the bottom surface of a multi-layer blank thus received there was positioned a face-mould 17 (Fig. 6e) for making a heat- insulating coating. The face-mould 17 was filled with 20 grams of glass ceramic powder of the following composition:

MgO 45%

SiO₂ 40% Borosilicate glass 15%

Glass ceramic powder was added with 5% by weight of PVA.

F. A multi-layer powder structure thus received was compacted at room temperature with the pressure of 3 ,0 kbar.

G. A compacted blank of a heating element was sintered in the air at the temperature of 950°C with the exposure time of 1,5 hrs. As a result of a high-temperature synthesis process the sintering of a heating element was provided with the eventual formation of its monolithic structure.

H. The layer 5 of the above-said heat-insulating glass ceramics was covered by one layer 6 of 10 grams of Fire Sealant 1200 heat-insulating paste. I. The layer 5 of heat-insulating glass ceramics and the layer 6 of heat-isolating paste taken together were impregnated with waterproofing high temperature silicon liquid.

The weight of a heating element thus received having the power of 300 W made up 150 g.

Experimentally measured efficiency of a heating element devised made up the value of η = 0,9.

It follows from the above that a flat-shaped fast-heating electrical element has a number of advantages as compared to the prior art volumetric Ni-Cr elements:

Efficiency - 0,9 - 0,95; ^■ Practically one-sided heating;

■ Initial heating time - max. 5-10sec;

■ Specific heat transfer - from 100 W/cm² in accordance environmental characteristics;

■ High degree of protection and high dependability; ■ Compact dimensions and low cost.

It follows from the above-shown basic characteristis that the claimed flat high speed heating elements i.e. express elements, considerably surpass the prior art elements as to their parameters.

The utilization of claimed high speed heating electical elements^' in various devices makes it possible to twice decrease the energy consumption as compared to traditional Ni-Cr elements used at present.

The heating elements in accordance with the present invention can be manufactured in various shapes and of dimensions with the thickness of a heating element from 1mm and more.

Claims

CLAIMS:

1. A heating element comprising a heating resistive body (1) with PTC characteristic positioned in an electric insulating layer (2) and a heat-insulating layer (4), wherein a heating resistive body (1) is made of Fe-based composite material having the specific resistance increasing with the temperature increase in the range from 100 to 200 μOhm.m.

2. A heating element as in claim 1, wherein the net surface of a heating resistive body (1) makes up from 40 to 60% of the total area of a heating element.

3. A heating element as in claims 1 or 2, wherein a heating resistive body (1) is made in the shape of an irregular path having the width of d, said path being comprised of linear sections (11) having the maximum length of lOd, said sections being sequentially connected by rounded-off sections (12), while the symmetry axes of the heating resistive body (1) coincide with the symmetry axes of a heating element.

4. A heating element as in claim 3, wherein said irregular path of the heating resistive body (1) is positioned inside an electric insulating layer (2), which follows its shape.

5. A heating element as in claims 1-4, wherein said heating element is additionally supplied with a Fe powder-based protective layer (3) having the thickness of 0,5- Id, said protective layer being shaped such that to enclose an electric insulating layer (2) on the three sides, said insulating layer (2) comprising therein a heating resistive body (1).

6. A heating element as in claims 1-5, wherein an electric insulating layer (2) is preferably made of MgO-based glass ceramics with the following ratio of components, % by weight:

MgO 75 - 85%

BN 10 - 15%

Alkali-free glass 5 - 10%

7. A heating element as in claim 6, wherein the magnesium oxide has the following composition, % by weight:

8. A heating element as in claims 1-7, wherein a heat-insulating layer (4) is positioned on the surface of an insulating layer (2) which is free from protective layer (3), said heat-insulating layer (4) contains a layer (5) made of MgO-based glass ceramics with the following ratio of components, % by weight:

MgO 40 - 45%

SiO₂ 40 - 45%

Borosilicate glass 10 - 20%.

9. A heating element as in claim 8, wherein said glass ceramics layer (5) is covered by at least one layer (6) of heat-insulating paste, e.g. on base Fire Sealant 1200.

10. A heating element as in claim 9, wherein the layer (5) of heat-insulating glass

5 ceramics and the layer (6) of a heat-insulating paste taken together are impregnated with high temperature waterproofing silicon liquid.

11. A method of manufacturing of a heating element comprising the steps of making a heating resistive body (1), forming around said heating resistive body (1) of an

10 insulating layer (2), further pressing and sintering of the heating element, wherein prior to sintering at least one additional layer is applied on the pre-defined portions of the insulating layer (2) thus formed with subsequent additional pre-pressing following the application of each layer, while sintering is provided after compression of a multilayer structure thus received.

15

12. A method as in claim 11, wherein a heating resistive body (1) is made of Fe-based composite material having the specific resistance increasing with the temperature increase in the range from 100 to 200 μOhm.m.

20 13. A method as in claims 11-12, wherein an electric insulating layer (2) is made of MgO- based glass ceramics with the following ratio of components, % by weight:

MgO 75 - 85%

BN 10 - 15%

25 Alkali-free glass 5 - 10%. using the magnesium oxide of the following composition, % by weight:

MgO having the dispersion from 40 to 150 μk - 45-55% 30 MgO having maximum dispersion of 40 μk - 45-55%.

14. A method as in claims 11-12, wherein a protective Fe-powder .layer (3) is applied as an additional layer in such a way that it covers an electric insulating layer (2) on the three sides.

-> 5

15. A method as in claims 11-14, wherein a heat-insulating layer (5) of MgO-based glass ceramics is applied on the surface of a multi-layer structure which is free from protective layer (3), said MgO-based glass ceramics having the following ratio of components, %> by weight:

40

MgO 40 - 45%

SiO₂ 40 - 45%

Borosilicate glass 10 - 20%.

45 16. A method as claims 11-16, wherein the compression of a multi-layer structure of a heating element is carried out in the pressure range from 3 to 4 kbar.

17. A method as in claims 11-17, wherein sintering of a multi-layer structure is carried out in the air with the time exposure period from 0,5 to 2 hrs. within the 50 temperature range from 900 to 1150°C.

18. A method as in claim 17, wherein a heat-insulating layer (5) is covered by at least one layer (6) of heat-insulating paste e.g. on base Fire Sealant- 1200.

19. A method as in claim 18, wherein a heat-insulating layer (5) and the layer (6) of heat-insulating paste taken together are impregnated with high temperature waterproofing silicon liquid.