DE102005025207B4 - Thermal insulation for a radiant heater, radiant heater and household electrical appliance - Google Patents

Abstract

Multi-layered thermal insulation for a radiant heater (6), which operates at least partially in the IR wavelength range, with a carrier layer (13) as the uppermost layer for fixing a heat conductor (5) as a heating element, a layer (11) below the carrier layer (13) with photonic Crystals and a thermal barrier coating (2) below the layer (11) with photonic crystals.

Description

The invention relates to a thermal insulation for a radiant heater, a radiant heater with such a thermal insulation and an electric household appliance with such a radiant heater.

Radiant heaters, such as those according to the DE 42 29 375 A1 used in cooktops, usually have a heating element, for example, a glowing Heizleiterband metal or a halogen heater. These are arranged or fastened on a plate-like or plate-like carrier body of heat-insulating material. This carrier body consists for example of various compressed materials, which have the best possible thermal insulation properties. The energy yield of the radiant heater up to a cooking vessel to be heated should be as large as possible and at the same time the heating below the radiant heater or below the carrier as low as possible. This warming down is particularly critical because underlying components, devices or the like. can be negatively affected or damaged.

The DE 101 54 887 A1 describes a further radiant heater. It has down towards a multi-layer reflector, which also has additional thermal barrier coatings.

The DE 203 11 942 U1 describes a method for producing an IR reflector or for the blocking of IR rays, wherein photonic crystals are provided for this purpose.

The DE 103 32 651 A1 describes the basic functional principle of photonic crystals. It also describes how photonic crystal layers can be produced by holographic lithography.

Task and solution

The invention has for its object to provide an aforementioned heat insulation, a radiation heater mentioned above and an aforementioned household electrical appliance with which problems of the prior art can be avoided and in particular the effect of heat insulation can be increased or a heating below the heat insulation during operation of the radiant heater can be reduced.

This object is achieved by a thermal insulation with the features of claim 1, a radiant heater with the features of claim 8 and an electric household appliance with the features of claim 11. Advantageous and preferred embodiments of the invention are the subject of further claims and will become more apparent in the following explained.

It is envisaged that the radiant heater operates at least partially in the IR wavelength range, ie at least partially generates and emits IR radiation, advantageously to a large extent. This also has the advantage that this wavelength range is particularly suitable for heat transfer by radiation. According to the invention, the material of which the thermal insulation consists or the thermal insulation itself has particles or structures which are arranged so regularly that the particles or structures form photonic crystals. The advantage of such photonic crystals or an arrangement as photonic crystals is that radiation in a certain wavelength range is thereby completely reflected and thus reflected. As a result, at least the propagation of heat via thermal radiation from the radiant heater in an undesired direction away is efficiently reduced or an undesired heating outside of the radiant heater avoided. By the process of reflection of the heat radiation so no warming takes place in the insulation itself, but the heat radiation is just as completely reflected. Photonic crystals are crystals for photons, hence their good reflector effect for photons.

The effect of the photonic crystals is that radiation in a certain wavelength range is largely or completely reflected.

In addition to the particles or structures in the nature of photonic crystals, the material of the thermal insulation u. a. have fumed silica and / or inorganic filler. Likewise, fiber material can be included, which can give the thermal insulation above all additional stability. Advantageously, fumed silica makes up the largest part of the thermal insulation. Such a thermal insulation material interrupts especially heat transfer by direct transfer, so heat flow.

In the invention, the photonic crystals are provided at least in a layer or layer of thermal insulation. This takes in the area of the thermal insulation or the radiant heater or extends over the entire surface, which is covered by the thermal insulation. According to the invention, on this layer of photonic crystals at least one further layer of conventional thermal insulation Material provided, advantageously both above and below. To the side, a kind of narrow, lateral cover can be provided, in particular as a mechanical protection to the outside.

A band gap of the photonic crystals is advantageously chosen so that it corresponds to the main wavelength range of the radiation of the heating element or the radiant heater. Thus, their greatest effectiveness or maximum reflection is designed for the wavelength range in which the radiant heater generates the most radiation and thus also the most heat. Since the band gap of the photonic crystals has a certain, albeit small, width, a certain wavelength range of the thermal radiation of the radiant heater can also be switched off or even reflected.

A training option of thermal insulation especially for a said radiant heater, as it can be used in an electric cooking appliance such as an oven or especially a hob, may be plate-shaped or plate-shaped. On its upper side or the side on which the radiant heater is arranged, it is advantageously substantially planar. An edge area can be pulled up a bit, so as to form a thermal insulation to the side as well as a stable investment opportunity, for example when applying to the underside of a hob plate. This edge region can be formed integrally on the thermal insulation or can also be a separate component, as is known to the person skilled in the art.

A further improvement of the thermal insulation properties or thermal insulating properties of the thermal insulation can be achieved by providing at least one further reflector layer. This can either also be designed in the manner of an arrangement or structure according to photonic crystals, but aligned for a slightly different wavelength range in order to achieve a broader overall reflection effect. Alternatively, it may be conventionally constructed, for example in the form of a metal layer or the like, in particular of polished or highly reflective metal. In principle, it is also possible in the material of the thermal insulation, in particular below the photonic crystals, known per se opacifiers or the like. to use.

For photonic crystals, it is important that areas of high transparency or translucency alternate with areas of low transparency. However, the transparency is physically linked to the dielectric constant ε _r and formally also to the magnetic inductance μ _{r of} a material. In order to obtain these regions of high and low transparency, it is therefore necessary to search as far as possible for a combination of materials which differs well with regard to the dielectric constants or which are far apart from one another.

As the material having a low dielectric constant, the mentioned fumed silica can be used. Materials with very high dielectric constants are usually the ferroelectrics. Ideally, the incorporation of ferroelectrics in the structure of fumed silica. This is attempted in the production of such photonic crystals or materials with their properties as possible.

It should also be noted that the dielectric constant is temperature-dependent. This temperature dependence should then be optimally selected, the relationships being known to the person skilled in the art. In particular, the temperature dependence is designed for maximum effectiveness for thermal insulation in the case of the operation of a radiation heater usually resulting temperatures.

Furthermore, the thermal insulation may also have material with ferroelectric properties. Also their dielectric constant can be adjusted in a desired range.

In a further advantageous embodiment of the invention, heat transfer by thermal radiation is largely reduced or avoided by a layer of photonic crystals. Heat transfer by direct heat conduction, ie within a solid material, can be prevented by a thermally insulating decoupling. On the one hand, such decoupling is advantageously provided between a layer of photonic crystals and the heating element. Thus, on the one hand, a mechanical attachment of the heating elements to the thermal insulation is possible and, on the other hand, there is already a certain temperature reduction at the layer with the photonic crystals. Alternatively or additionally, such a thermally insulating decoupling may be provided below a layer of photonic crystals, since then a residual heat conduction is again more strongly attenuated downwards. Furthermore, if above the photonic crystals only a small thermal insulation is arranged, the reflection or reflection of heat radiation is better possible. Such a thermally insulating decoupling may differ from the structure and structure of the other heat insulation, for example, have different material or be designed differently. It is also possible here larger air pockets or the like. provide, as they have a very good insulation effect.

A radiant heater with such a thermal insulation advantageously has a heating element which predominantly generates heating radiation or radiation in the IR wavelength range. In this wavelength range, the transfer of heat energy is particularly well possible. Such heating elements are, for example, Halogenheizeinrichtungen, which may be formed in the manner of an elongated halogen lamp. They can be laid or fixed in straight or even round form on the thermal insulation. Through the operation of halogen heaters with temperatures of well over 2000 ° C here is the proportion of heat radiation in the IR wavelength range particularly large. Similarly, conventional radiant heaters made of metal wire can be used, for example, upright laid flat metal strip.

Preferably, the largest part of the heating element, in particular with the area which generates the heat radiation or heating power, runs above and thus also outside the thermal insulation. Only spacers, support members or support feet can be plugged into the insulation. However, they should, if possible, not participate in the heat generation, or only to a limited extent. Advantageously, no parts coming into direct contact with a heating element will pass through the layer of photonic crystals.

Such thermal insulation can be produced by sedimenting polymer beads and / or quartz glass beads having a very small diameter, which should be less than 1 μm, out of a liquid. The liquid evaporates so slowly or is removed by the fact that the beads form a regular grid. Subsequently, a so-called infiltration material is introduced into this structure. Finally, the beads are removed and the result is said regular structure. For example, selenium or InP can be used as nanoparticles as infiltration material.

Another possibility to produce a thermal insulation mentioned above, provides that a template or a starting material for the thermal insulation is produced by means of holographic lithography. This is based on a photosensitive photoresist on which four laser beams are superimposed at specific angles. As a result, the light intensity is modulated three-dimensionally or a three-dimensional modulation of the light intensity is generated. This modulation is approximately close to the wavelength of the lasers. Subsequently, the photoresist is exposed in this area and thus transferred this structure generated in the paint. As a result, one also obtains a very uniform and very regular structure, as required for the production of the photonic crystals.

Another possible use of photonic crystals or arrangements of particles or structures as photonic crystals are thus reflectors on radiant heaters, in particular on so-called halogen heaters. These can odgl directly to the radiating elements such as heating wires. be arranged. Thus, it is thus possible, for example, to arrange a reflector below a halogen heater within the glass tube in which the heating or glow wire extends, below this wire. This shields the area under the wire. The reflector has particles or structures that are formed as photonic crystals. As a result, it achieves a very good reflector effect and reflects heat radiation, which goes down from the wire, upwards. This in turn increases both the total achievable heating power and the heat transfer down, where it is considered to be disturbing reduced.

Alternatively, such a reflector may also be provided outside and below the glass tube. Advantageously, it is arranged directly under the glass tube, in particular even attached to the underside of the glass tube. Even so, a very good reflector effect can be achieved.

Brief description of the drawings

Embodiments of the invention are shown very schematically in the drawings and are explained in more detail below. In the drawings shows:

1 a section through a radiant heater with a thermal insulation according to the invention with a layer-like structure with photonic crystals;

2 a section through an alternative radiant heater with a halogen heater in a glass tube, under which a reflector is arranged with photonic crystals, this embodiment does not belong to the claimed invention,

3 a modification of the halogen heater 2 with the reflector in the glass tube directly below the filament, this embodiment does not belong to the claimed invention, and

4 a diagram of the transmission over the frequency.

Detailed description of the embodiments

In 1 is an electric radiant heater 6 shown at the bottom of a glass ceramic hob 8th is pressed. The radiant heater 6 has a receiving tray 1 on, preferably made of sheet metal. In this a thermal insulation molded body is inserted as a thermal insulation, on the top of known heating conductor 5 are arranged.

The thermal insulation is multi-layered. The topmost layer 13 is essentially a carrier layer. In possibly prefabricated grooves 9 becomes the heating conductor 5 inserted. Alternatively, it is directly in the carrier layer 13 pressed. At the heating conductor 5 can also protruding holding members or the like. be provided. In the middle region, the carrier layer 13 a frustoconical survey 4 on. This serves as a support for a temperature sensor 7 an overtemperature circuit breaker. Since this is sufficiently known from the prior art, it need not be discussed further here.

Outside at the edge of the carrier layer 13 is a circumferential ring 3 on. This in turn is directly to the bottom of the glass ceramic hob 8th created.

The layer of thermal insulation below the carrier layer 13 has a structure 11 with the above-described photonic crystals. Since these are known per se from the prior art, need not be discussed in more detail on their exact training. The layer is advantageous 11 as shown, throughout the entire surface below the heating conductors 5 or below the carrier layer 13 , Thus, an advantageous transfer of heat down is reduced or even prevented.

Below the reflective layer 11 with the photonic crystals is once again a thermal barrier layer as a base 2 educated. This serves on the one hand to adapt to the shape of the receiving tray 1 , Furthermore, it is possible for components of heat radiation or heat flow which pass through the layer 11 are not completely reflected, are insulated to the bottom or the underside shielded against it.

In 2 is another radiant heater 106 shown under a glass ceramic hob plate 108 is arranged. On a carrier layer 113 is a halogen heater 106 arranged. It consists of an elongated glass tube 115 , which by holding devices 116 on the carrier layer 113 is held with a certain distance to it. In the glass tube 115 , which is filled in a known manner with halogen gas, runs in a known manner, a heating or Glühdraht 118 , As can also be seen from the cross-sectional drawing of the halogen heater shown alongside, is below the glass tube 115 or the halogen heater 114 a reflector 120 intended. He can either be directly at the bottom of the glass tube 115 be attached, for example by gluing or mechanical fastening methods. Alternatively, it can be used as a coating directly on the glass tube 115 be applied. According to a further alternative, it may run at a certain distance underneath, possibly even on the carrier layer 113 aural.

The reflector 120 also has particles or structures with photonic crystals or exhibits the functional properties of photonic crystals. It thus reflects heat radiation, wherein it is advantageously designed so that it reflects heat radiation in the wavelength range in which the halogen heater 114 mainly heat radiation generated. So can be achieved that of the glow wire 118 in all directions outgoing heat radiation downwards not only from the carrier layer 113 kept as completely as possible in the sense of improved thermal insulation down. In addition, this heat radiation is reflected upwards and thus improves the power output of the radiant heater 106 ,

In 3 is another variant of a halogen heater 214 as a radiant heater 206 similar to those from 2 illustrated, this embodiment does not belong to the claimed invention. In a glass tube 215 runs in a known manner, a filament 218 , Here, however, is a reflector 220 inside the glass tube 215 arranged, as can be seen from the sectional view. In this case, the reflector 220 either trough-like below the filament 218 run. It can also be used as a separate part in the lower part of the glass tube 115 be inserted, for example, as a narrow, curved strip. Furthermore, it is also conceivable, the glass tube 215 in the lower area with the reflector 220 to coat.

Again, should, as for 2 Please note, the reflector 220 be formed so that it reflects down radiated heat radiation substantially upwards. In this respect, curved or trough-like designs for the reflector are particularly advantageous. The curvature of a reflector should be designed so that of the somewhat linear filament 218 outgoing radiation is reflected directly and rectilinearly upward. However, this design is not an obstacle for the skilled person.

In 4 is a graph showing the transmission T over the frequency f in a photonic crystal and the characteristic band gap. The solid line is the theoretically calculated and the dotted line the measured. They apply to the so-called gamma-M direction as well as to the H-polarization of the photonic crystal.

The dotted lines illustrate where the transmission actually becomes zero, that is to say the range of f = 3000 per cm, which corresponds to a wavelength of approximately 3.3 μm. In this frequency interval, therefore, a complete band gap is actually given, in which no corresponding radiation is transmitted transversely to the photonic crystal or transversely to pores or tubular structures, of which the photonic crystal is constructed by arranging a plurality of parallel tubular structures next to each other or networked. This non-transmission is independent of the polarization. It is also noteworthy that this band gap has a certain width. It can therefore be reflected radiation of a certain bandwidth, which is particularly positive for the aforementioned purpose of thermal insulation, since the effectiveness in use is good.

Claims (11)

Multilayered thermal insulation for a radiant heater ( 6 ), which works at least partially in the IR wavelength range, with a carrier layer ( 13 ) as the topmost layer for defining a heating conductor ( 5 ) as a heating element, a layer ( 11 ) below the carrier layer ( 13 ) with photonic crystals and a thermal barrier coating ( 2 ) below the layer ( 11 ) with photonic crystals. Thermal insulation according to claim 1, characterized by fumed silica and / or inorganic filler and / or fibers in the thermal barrier coating ( 2 ). Heat insulation according to claim 1 or 2, characterized in that at least the layer ( 11 ) with photonic crystals is completely embedded in the rest of the material of thermal insulation ( 2 . 13 ). Heat insulation according to one of the preceding claims, characterized in that the band gap of the photonic crystals is the main wavelength range of the radiation of a heating element ( 5 ) of the radiant heater ( 6 ) corresponds. Heat insulation according to one of the preceding claims, characterized in that it is plate-shaped with a substantially flat surface and / or a raised edge region ( 3 ). Heat insulation according to one of the preceding claims, characterized in that it comprises at least one reflector layer ( 120 . 220 ) for reflecting heat back towards a heating element ( 5 ) having. Heat insulation according to one of the preceding claims, characterized in that the thermal barrier coating ( 2 ) Contains material with ferroelectric properties. Radiant heater with a thermal insulation according to one of the preceding claims, characterized in that the radiant heater ( 6 ) a heating element ( 5 ), which is designed for generating heating radiation substantially in the IR wavelength range. Radiant heater according to claim 8, characterized in that the heating element is a halogen heater. Radiant heater according to claim 8 or 9, characterized in that the heating element ( 5 ) with a portion generating the heat radiation or portion above the thermal insulation ( 13 ), wherein only spacers or carrier parts of the heating element in the thermal insulation and before the layer ( 11 ) end with photonic crystals. Electric household appliance as hob with a cooktop cover ( 8th ) and a radiation heating device arranged underneath ( 6 ) according to any one of claims 8 to 10.

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