DISTRIBUTED-ELEMENT RESISTANCE AND METHOD FOR REALIZING SUCH A

HEATING ELEMENT”

The invention refers to an electrical resistance heating element, more particularly distributed- element resistance, adapted to heat at predetermined temperatures surfaces or closed spaces by means of thermal conduction, thermal radiation or convection.

The invention relates also to a method for realizing such an electrical resistance heating element. Currently, there are used several electrical resistance heating element, that are elements able to generate heat is they are passed through by electric current.

Such heating elements are made of resistive or semiconductor materials, that as it is known, if passed through by electric current, they heat up thereby generating heat.

The materials used for realizing these heating elements are metal alloys, carbon agglomerate or resins mixed with semiconductor materials.

For reaching temperatures higher than 1300°C, it is not possible to use such materials, because these may have already surpassed the melting point, as for example metals, resins, etc., furthermore if these latter are heated in presence of oxygen, they may be subjected to redox phenomenon, that is they degrade gradually up to become ineffective, so unusable.

For overcoming this problem there are used ceramic-based semiconductor materials, as for example silicon carbide or other carbon-based materials, as for example graphite, because they are able to reach temperatures about 1600°C without been subjected to alterations of the form thereof and of heating functionalities thereof.

Heating elements made of these last materials are manufactured in the shape of round bar or tubular, or as a strip, usually obtained by extrusion, as it is disclosed for example by the prior European Patent EPl 645168 AL Said heating elements may also be constituted by a metallic rectilinear round bar to which, at least to a part thereof, a silicon carbide strip is wrapped to.

For heating a wide area it be also necessary to use a great plurality of these heating elements.

The object of the present invention is to realize an electrical resistance heating element able to reach high temperatures, at least of 1600°C and with constructive characteristics different from the known prior art and that solve the problems described above, and that produces additional advantages shown after in the following description.

Another object of the present invention is to realize an innovative method for realizing such an electrical resistance heating element.

One advantage is that the electrical resistance heating element may have dimensions equivalent to the ones of the object to be heated, also with important dimensions, as for example with a quadrangular shape with measures great than 2m * 2m.

One additional advantage is that the electrical resistance heating element may be heated in its entirety.

Another advantage is that the it is possible to determine in advance whether to distribute the produced heat in the area to be heated in a homogeneous way or in a heterogeneous way. Other possible advantages will shown in the following description.

The electrical resistance heating element is made with the constructive characteristics substantially described after by way of a not limiting example only, with reference to the accompanying Figures in which :

- Fig. 1 shows a perspective view from the top of a first example of electrical resistance heating element, according to the present invention, connected to electrical component elements for the power supply thereof and not comprised in the invention ;

- Fig. 2 shows a perspective view from the top of the heating element of Fig. 1, without the electric component elements, with a slightly different angle with respect to the one of Fig. 1 ;

- Fig. 3 shows a top view of the heating element of Fig. 2 ;

- Fig. 4 shows a top view of a second example of electrical resistance heating element, according to the invention ;

- Fig. 5 shows a schematic sectional view of a first example of a portion of the heating element to which one electric component element for its power supply is placed close ;

- Fig. 6 shows a schematic sectional view of a second example of a portion of the heating element to which one electric component element for its power supply is placed close ;

- Fig. 7 shows a schematic sectional view of portion of a heating element to which is coupled at least one electric component element of its power supply, in a first embodiment thereof ;

- Fig. 8 shows a schematic sectional view of portion of a heating element to which is coupled at least one electric component element of its power supply, in a second embodiment thereof.

The present invention refers to a heating element 10 operating as electrical resistance, more particularly distributed-element resistance, that may reach high temperatures, at least of 1600°C and which has excellent mechanical properties.

Said heating element 10 is made of stiff semiconductor material composed by a silica ceramic, constituting a first component element, obtained with per se known methods and constituted in particular by silicon carbide (SiC).

Such a silicon carbide has a level of purity such that it is in a quantity comprised between 60% and 99,99% in weight with respect to the total weight of the same heating element 10 and that is distributed in a homogeneous way.

The possible impurities contained with the silicon carbide (SiC), that is additional component elements, are as follows:

- Silicon (Si) in a quantity comprised between 0% and 20% in weight with respect of the total weight of the heating element 10 ;

- Silicon nitride (Si ₃ N ₄ ) in a quantity comprised between 0% and 30% in weight with respect of the total weight of the heating element 10 ;

- Oxides, in a quantity comprised between 0% and 7% in weight with respect of the total weight of the heating element 10.

As is known, the silicon carbide has excellent thermal properties, being able to reach temperatures at least of 1600°C without structural alterations thereof and has excellent mechanical properties, as for example a hardness of 9-9,5 according to Mohs scale, a reduced coefficient of thermal expansion,, in the order of 120 - 200 W/mK.

Furthermore, as is known, on the contrary of the other materials commonly used for the electrical resistances, increasing the temperature, up to almost 1000 - 1200°C, the silicon carbide presents a reduction of the electrical resistance, consequently the cold starting current is smaller than the nominal current with the element heated. Over the 1200°C the electrical resistance increases again. This allows to not oversize the control electric components with respect of the nominal currents, on the contrary of the electrical resistances using commonly materials.

Such a heating element 10 is shaped with a flat quadrangular plate 11 provided with a plurality of rectilinear longitudinal cuts 12, arranged parallel and alternately opposed to each other, for forming a circuit 16 shaped as coil, furthermore the end portions 13 and 14 of such a coil are provided with fixing means, as for example through holes, for electric component elements 15, constituted for example by electrodes connected to a device for generating electric current of the per se known type and consequently adapted to power electrically the same heating element 10. in the above said example it is described a heating element 10 with a quadrangular shape and a circuit 16 constituted by quadrangular segments 17 parallel and spaced away laterally in partial and joined to each other at the end portions thereof, but the heating element 10 may be shaped with other shapes, as for example circular and/or with curvilinear segments, for forming for example a coil.

In the case in which the segments 17 have a curvilinear shape, it will be present only one curvilinear cut 12.

It should be noted the second Ohm's law, that is , where R = Resistance, p = resistivity of the material, 1 = length of the circuit 16 and S = Area of the cross-section of the segment 17.

Having a quadrangular plate with determined width and length, made of the above said ceramic material used for this invention, and knowing the resistivity of the same material, the length of the circuit 16 and the width of the segments 17 are dimensioned, through the realization of cuts 12 with determined length and spaced away to each other, in such a way that it is possible to know for each segment 17 the resistance obtained, so that it is possible to decide how much heat to be developed in a determined area.

Fig. 3 shows a heating element 10 with segments 17 having a homogeneous width, so with a homogeneous production of heat.

Fig. 4 shows a heating element 10 with segments 17 having different widths, specifically two central segments having the same width to each other and greater than the width of two side segments, so with a selective production of heat for each segment having different width, As visible in fig. 7 and 8, an electric component t element 15', acting as electrode for being connected to a power supply, mat be constitute by an upper plate 19 provided with at least a through hole 20 and a lower plate 21 provided with at least a through hole 22, coupled to one end portion 13 or 14 of a plate 11.

Such flat plates 19 and 21 are adapted to be coupled by means of the respective through holes 20 and 22 with the fixing means 23 of one end portion 13 or 14, also constituted by through holes, and locked in position by means of fixing devices 31 of the per se known type, constituted for example by bolts 31 (See Fig. 7).

Such flat plates 19 and 21 are made of conductor or semiconductor material, preferably of the metallic type, and are shaped for being connected to a power supply, as previously indicated, by means of electric connecting means of the per se known type, for transferring the electric current to the plate 11, so that this latter may be heated.

If it is required for technical requirements, it is possible to use a number of holes 20, 22 and 23 greater of one, as for example two, as shown in Fig. 8.

Now, we refer to Fig. 5, that shows a schematic sectional view of first example of an end portion 13 or 14 of a heating element 10, to which a flat plate 19 of an electric component element 15' is placed close.

The lower surface 24 and the upper surface 25 of the flat plate 11, for construction method, have intrinsically a roughness and a non-planarity characteristic of these plates.

Such a roughness may be of almost ± 10 pm.

As represented in fig. 5, between the contact surface 26 of the flat plate 19 of the electrode 15' and the upper surface 25 there contact points 27 alternated with non-contact areas 28.

The flow of the current, because of the non-contact areas 28, generates a non-uniform distribution of such a current between the surface 26 of the flat plate 19 and the only contact points 27 of the surface 25, that for strong current used, that is enough for reaching high heating temperatures of the plate 11, may cause increasing of temperatures in the contact points 27, being exposed to a greater concentration of current, with overheating, oxidation, damaging of the contact points 27, and fast deterioration of the heating element 10 up to make this latter unusable.

High heating temperatures means temperatures > 300°C.

Such a problem occurs also for the lower surface 24 of the plate 11 and the contact surface 28 of the lower flat plate 21.

Such a problem caused by the non-contact areas can be seen also between the screw 32 of the bolt 31 and the surfaces 33 of the through hole 23 of the plate 11.

For example, if in theory the contact surface 24 or 25 is without roughness, by applying a current density comprised between 8 and 10 A/mm ² to the flat plate 19 or 21, such a current density would remain unchanged during the passage from the electrode 15' and the plate 11.

Instead, if the contact surface 24 or 25 has a roughness, the current density may be subjected to a three-fold increase, so going from 8-10 A/mm ² to 24-30 A/mm ² in correspondence of the contact points 27, thanks to lack of a passage thereof in the non-contact areas 28 caused by the roughness. Furthermore, such contact surfaces 24 or 25, besides to be provided with e relevant roughness, may be non-planar, as shown in fig. 6, where the contact points 27 decrease further.

This increasing of non-contact areas in a uniform way causes 100-fold increase to the current density applied to the electrode 15', during its passage to plate 11, so that may reach values comprises between 800 and 1000 A/mm ² in the contact points 27.

It is intuitive that the increasing of oxidation phenomenon and deterioration phenomenon in the contact points 27 will be even more rapid and additional dangerous.

For solving the problem of non-planarity of the plate 11, at least the surface 24 and 25 of the respective end portions 13 and 14 of the same plate 11 are subjected to a treatment for obtaining a planarity by means of suitable machine tools, as for example machines for electrical discharge machining.

For solving the problem of the roughness of the plate 11, an antioxidant conductive composite 30 is applied in a uniform manner between the flat plates 19 and 21, in such a way to fill the above said non-contact areas 28, obtaining consequently an uniform contact surface between said plate 11 and the plates 19 and 21, thereby eliminating the problems described above.

Said antioxidant conductive composite 30 is constituted by at least a polymeric matrix, more particularly a synthetic resin, containing at least a dispersed suspension, constituted by a powder of a electrically conductive material.

Said electrically conductive powder is selected from metallic powder, as for example silver powder or copper powder, or carbon powder or powder of allotropes of carbon, as for example graphite powder.

Such a suspension is in a quantity such to allow a uniform passage of the current between the entire contact surface of the electrode 15' and the entire contact surface of the plate 11, more particularly it is present in a quantity greater than 50% in weight with respect to the total weight of the antioxidant conductive composite 30, more preferably in a quantity greater than 90%.

The polymeric matrix, over time, may vaporise, sublimate, polymerize, therefore detaching from the plate 11, leaving joined to this latter only the powder of electrically conductive material.

It is intuitive as it is advantageous and innovative the use only one heating element of this type in such a way to be able heat in a homogeneous or selective manner determined areas joined to each other, constituted by each segment 17 opportunely sized.

Therefore it will not necessary to use more heating elements separated for obtaining different heating of adjacent areas and also it will not areas not heated.

For knowing the resistivity of the material of the heating element there are used methods of the per se known type, as for example by knowing the quantity in percentage of the component elements of the material used or by detecting instrumentally said resistivity before the realization of the circuit. A heating element with quadrangular shape has been described, but it may have also other shape, as for example circular, with the segments 17 of the circuit 16 having desired dimensions and shapes for the aim to be obtained.

Such a heating element 10, with the exception of the end portions 13 and 14, may have its external surface electrically insulated, as for example may be ceramized or aluminized, or insulated by means of resistive resins of the per se known type.

More particularly, it is possible to use an electrically insulating composite constituted by resisitive resins containing dispersed zirconium powder, which is adapted to withstand temperatures up to at least of 1500°C. Such a zirconium powder is in a quantity greater than 50% in weight with respect the total weight of the electrically insulated composite, more preferably greater than 90%. Such resistive resins may vaporise, sublimate, polymerize, therefore detaching from the plate 11, leaving joined to this latter only the zirconium powder.

Such a surface electric insulation is essential and advantageous for using the heating element 10 in contact with products made externally of conductive materials, thereby avoiding dispersion of electric current with the consequent malfunction of the same heating element 10. Furthermore, advantageously, the exception of the end portions 13 and 14 from such a electric insulation is necessary for ensuring the passage of the electric current from the electrodes 15' of the plate 11. Now it is described at least a method for obtaining such a heating element 10.

The heating element 10 is obtained by using powders and/or grains of the above said component elements in the percentage indicated above, by means of molding methods of the per se known type. If the resistivity of the material constituting the heating element 10 is already known, the longitudinal cuts 12 may be realized during the above said molding step, by shaping opportunely the molding means, with a length and spaced away to each other of a predetermined distance, for the aim described above, or such cuts 12 may be realized after the molding step by means of suitable cutting or engraving means of the per se known type.

In the case the resistivity of the material used, it is carried out the molding of the plate and subsequently, after having detected said resistivity, the cuts 12 are realized.

The external surface of such a heating element 10, with the exception of the end portions 13 and 14, may be insulated electrically, particularly by subjecting the same external surface to surface treatments for the electric insulation of the per se known type, as for example ceramization, aluminization or covering by means of resistive resins of the per se known type.

Preferably, such resistive resins contains suspension of zirconium as electric insulation.

Finally, it is possible to couple integral electric component elements 15' to said heating element 10, by proceeding as described follows.

At least the surface 24 and 25 of the respective end portions 13 and 14 of the plate 11 are subjected to a treatment for obtaining a planarity thereof by means of suitable machine tools, as for example machines for electrical discharge machining.

Subsequently, for the coupling of the electric component elements 15' to the fixing means 23, we proceed as follows: - Application of a layer of antioxidant conductive composite 30, by means of application means of the per se known type, as for example a brush, a spatula, a pressure dispenser, to the surfaces 24 and 25 of the respective end portions 13 and 14 of the plate 11 ;

- joining of the electric component elements 15' to the fixing means 23 ;

- treatment in a oven of the assembly plate 11 - electric component elements 15', up to reaching the solidification of the antioxidant conductive composite 30, with the relative joining between this latter and the end portions 13 and 14 and the electric component elements 15'.

It is possible to insulate electrically with the plate 11 also the parts not in contact with the electrodes 15', thereby proceeding with the electric insulating step, described above, after the coupling step of the plate 11 to the electrodes 15'.