EP4252487A1

EP4252487A1 - Infrared radiator and component emitting infrared radiation

Info

Publication number: EP4252487A1
Application number: EP21819807.5A
Authority: EP
Inventors: Holger Zissing; Oliver Weiss; Daniel AMBS; Simone GANZ; Thomas Piela; Frank Diehl; Jürgen Weber
Original assignee: Heraeus Noblelight GmbH
Current assignee: Heraeus Noblelight GmbH
Priority date: 2020-11-26
Filing date: 2021-11-24
Publication date: 2023-10-04
Also published as: JP2023548025A; US12075531B2; US20230413391A1; DE102020131324A1; CN116438924A; WO2022112306A1

Abstract

Known infrared radiators have a moulded body, which has a radiation surface that emits short-wave or medium-wave infrared radiation with a first peak emission wavelength. In order to provide, proceeding therefrom, an infrared radiator with an emissions spectrum which is well matched to absorption characteristics with a maximum around 2750 nm, and which furthermore can be operated with a high electrical power density, and with which the warming-up time in industrial applications, such as, for example, drying of inks, joining of plastics or bending of glass, can be shortened, according to the invention a radiation converter material is applied to at least a part of the radiation surface and, as a consequence of heating by the infrared radiation of the first peak emission wavelength, emits infrared radiation with a second peak emission wavelength which is of a longer wavelength than the first peak emission wavelength.

Description

DESCRIPTION

Infrared radiator and infrared radiation emitting component

Technical background

The invention relates to an infrared emitter with a shaped body that has a radiation surface that emits short-wave or medium-wave infrared radiation with a first peak emission wavelength.

The invention also relates to a component that emits infrared radiation, with a base body made of a base body material, having an absorption surface for absorbing short-wave or medium-wave infrared primary radiation with a first peak emission wavelength, and an emission surface for emitting infrared secondary radiation with a second peak Emission wavelength longer than the first peak emission wavelength.

State of the art

In a wide variety of industrial manufacturing processes, short-wave, medium-wave and long-wave infrared emitters are used to heat up material to be heated. According to IEC 62798:2014 (p.11, Table 1) a distinction is made between the wavelength ranges

IR-A = 780-1400nm (corresponding to a temperature between 1800 and 3450°C),

IR-B = 1400-3000nm (corresponding to a temperature between 690°C and 1800°C), IR-C = 3000nm to 1mm (corresponding to a temperature lower than 690°C)

The working radiation of long-wave infrared radiators is usually absorbed particularly well and quickly by the heating material, so that the heating takes place with a high level of efficiency. However, the heating and cooling behavior is slow, so that rapid temperature changes cannot be managed.

Medium-wave infrared emitters (short: MWIR emitters) show a broadband infrared spectrum in the wavelength range from about 1400nm to 3000nm and are typically operated in the temperature range up to 1100°C. Medium-wave radiation is already absorbed in the upper layer of the material to be heated and primarily heats its surface. Medium-wave infrared emitters typically have an open cladding tube made of temperature-stable glass, metal or ceramics, which surrounds a heating filament made of an oxidation-stable resistance material. A disadvantage of medium wave radiators is their limited electrical power density of about 15W/cm and their thermal inertia and the associated slow response.

Short-wave radiation with emission wavelengths between 780nm and 1400nm penetrates deep into solid materials and ensures even heating. In the case of short-wave infrared emitters (in short: SWIR emitters), a heating filament made of carbon or tungsten in the form of a helix or ribbon is enclosed in an inert gas-filled emitter tube, which is usually made of quartz glass. The heating filaments are connected to electrical terminals that are inserted through one or both ends of the radiant tube. The heating filaments themselves have a low thermal mass and therefore a fast response time in the range of 1 to 2 seconds. A special feature of SWIR emitters is their high optical power density of up to 120 watts per centimeter of heating filament (hereinafter referred to as W/cm). SWIR emitters are used in particular for heating powder coatings, adhesive bonds or for rapid preheating.

US Pat. No. 1,02,013,104,577 B3, for example, discloses the use of short-wave infrared radiators for drying and sintering metal-containing ink.

DE 29905385 U1 describes a device for the homogeneous heating of semi-transparent and/or transparent glasses and/or glass ceramics with the aid of infrared radiation. A proportion of more than 50% of the short-wave infrared primary radiation with a color temperature greater than 1500°K that is not absorbed by the heating material is reflected or scattered by means of reflectors or diffusers and contributes to indirect heating.

DE 4202944 C2 describes a surface emitter consisting of several infrared radiators for the rapid heating of material to be heated, which has a high absorption above 2500 nm. A so-called radiation converter is arranged in the main propagation direction of the primary radiation emitted by the panel radiator ceramic fibers. The radiation converter serves as a secondary radiator, which, stimulated by the medium-wave or short-wave IR radiation of the surface radiator, emits secondary radiation in a longer-wave range, which overlaps more strongly with the optical absorption of the heating material. This enables rapid temperature changes with good efficiency.

DE 102015 119763 A1 discloses a tile-shaped infrared surface radiator in which a substrate is in contact with a conductor track made of a resistance material. The substrate material is preferably quartz glass, in which an additional component that absorbs infrared radiation is embedded in fine distribution. The additional component is preferably elemental silicon.

Technical task

Thermal radiation with a temperature of about. 700°C - corresponding to a medium-wave peak emission wavelength of around 2700nm - is particularly well absorbed by many plastics, glass and above all water and converted directly into heat. Medium-wave infrared radiation in this wavelength range is particularly well suited for drying applications in the printing industry, as the usual color selectivity when drying the different printing inks is avoided.

It has recently been proven that IR radiation in this wavelength range also avoids color selectivity when welding or heating and joining different plastics in the wavelength range of approx. 2700nm. In particular, the heating rates of different colored plastics are almost identical.

In addition, the heating of glasses - for example for thermally supported joining or forming - can be carried out quickly and homogeneously with infrared heaters with high emissivity.

In principle, SWIR and MWIR emitters can be considered for these applications. The higher the electrical connected load of the infrared heaters, the faster they reach the target temperature. Increasing the electrical connection power increases the optical output from the infrared heater Power density, but this can also lead to a shift in the peak emission wavelength of the emitted radiation in the direction of the short-wave spectral range.

However, it is desirable for the peak emission wavelength to match the absorption characteristics of the material to be heated, for example the printing inks, plastics or glasses, ie at around 2750 nm, for example. The previous commercial infrared emitters either have an emission spectrum adapted to it (MWIR emitters); but then they have a low electrical connected load and require a comparatively large radiation surface for a sufficiently high optical radiation output and accordingly a large thermal capacity, which in turn requires comparatively long heating and cooling times of the infrared radiator and thus inertness of the dryer system. Or the infrared emitters have a high electrical connected load and low reactivity (SWIR emitters); but then their emission spectrum is not optimally adapted to the absorption characteristics of the heating material.

The object of the invention is to provide an infrared emitter with an emission spectrum that is well adapted to heating material with an absorption characteristic in the medium-wave wavelength range, and which is also operated with a high electrical power density (e.g. with more than 50W/cm), and with which the Heating time can be reduced in industrial applications such as drying inks, joining plastics or bending glass.

In addition, the invention is based on the object of specifying a passive component which emits infrared radiation and whose emission spectrum is well adapted to a material to be heated with an absorption characteristic in the medium-wave range

Brief description of the invention

With regard to the infrared radiator, this object is achieved according to the invention, starting from an infrared radiator of the type mentioned at the outset, in that at least part of the radiating surface is covered with a radiation converter material, which, as a result of being heated by the infrared radiation of the first peak emission wavelength, emits infrared radiation with a second peak emission wavelength which is longer in wavelength than the first peak emission wavelength.

Typical shaped infrared radiator bodies have a cylindrical shape, for example a tube or tile shape. Tubular infrared radiators can be stretched or bent, for example in a U or ring shape. Plate-shaped moldings have two opposing plate sides, which can be flat or curved.

The radiating surface is the surface facing the meat product; it is part of the infrared radiator molding.

The infrared emitter has an electrical connection and generates medium-wave or preferably short-wave infrared radiation with the first peak emission wavelength, for example by thermally exciting an emitter that emits infrared radiation, such as a flexible coil, flexible ribbon or elemental silicon embedded in a quartz glass matrix. Short wave emitters have a slightly faster response time than medium wave emitters, but they are less expensive.

The short-wave or medium-wave infrared radiation of the first peak emission wavelength (hereinafter also referred to as "primary radiation") emerges from the radiating surface of the infrared radiator, is absorbed by the radiation converter material deposited there, which subsequently heats up and emits longer-wave infrared radiation. Their peak emission wavelength (also referred to below as “secondary radiation”) is preferably in the range from 2200 to 3100 nm, particularly preferably in the range from 2400 to 3000 nm and very particularly preferably in the range from 2600 to 2800 nm. The wavelength range around 2700 nm is also referred to below as the "relevant" wavelength range. The portion of the primary radiation that is diffusely or directionally transmitted by the radiation converter material is as small as possible and is preferably less than 20%, particularly preferably less than 10% of the emitted primary radiation.

In a first preferred embodiment, the radiation converter material is a coating material that contains a colored pigment or a precursor substance therefor. The coating material is, for example, a paste or a paint. The colored pigment is thermally stable and becomes, for example, by baking on the deposition surface fixed. The colored pigment can also be formed by thermal decomposition or chemical reaction of a precursor substance during or before baking.

The color pigment emits infrared radiation at least in the relevant wavelength range around 2750 nm with an emissivity of 0.8 or more, preferably at least 0.9. This emissivity is particularly adapted to a medium with high absorption in this wavelength range. Depending on the application and the goods to be processed, a color pigment can also be advantageous that has a high emissivity with an emissivity of, for example, 0.75 or higher, preferably at least 0.8, even in a broader wavelength range of, for example, 2000 nm to 8000 nm, in particular from 2000 nm to 4700 nm having.

Color pigments that appear black in the visible wavelength range usually also absorb (and emit) light in the relevant infrared wavelength range. It has proven useful if the color pigment contains black mineral particles, such as copper chromite black spinel or manganese ferrite black pigment, and if it is alkali-free. The fact that the coating material is alkali-free has the advantage that a radiating surface made of glass; made of quartz glass in particular, does not devitrify when heated in contact with the coating material, i.e. does not crystallize and thus lose its optical quality.

In a second preferred embodiment, the radiation converter material comprises an at least partially opaque quartz glass.

Such at least partially opaque quartz glass is described in DE 102004051 846 A1 and has become known under the name “QRC” (Quartz Reflective Coating). To date, it has primarily been used as a material for producing diffusely reflecting reflector layers. The QRC reflector layer is produced using a slip process in which a highly filled, castable, aqueous Si0 ₂ slip containing amorphous Si0 ₂ particles is produced. This is applied as a layer of slip on a substrate, and then the layer of slip is dried and vitrified to form a more or less opaque layer of quartz glass.

In a preferred embodiment of the invention, the shaped body is designed as a cladding tube made of quartz glass, with the cladding tube having a power connection provided radiation emitter in the form of a heating coil or a heating strip surrounds, and wherein the radiating surface forms at least a part of the pipe jacket surface.

The cladding tube has, for example, a round, oval or polygonal cross section or it is designed as a so-called twin tube emitter, which has a cross section in the shape of a horizontal figure eight. The outer wall of the cladding tube is smooth, for example, or it is roughened. In particular, short-wave infrared emitters have a bulb-shaped enveloping tube that is closed on both sides, with the power supply coming out at one end or at both ends.

The cladding tube has a radiating surface, which is usually located on the outer surface of the tube. The cladding tube material is quartz glass, for example, and has a comparatively low inherent emissivity for infrared radiation, particularly in the wavelength range from 2200 to 3100 nm. The coating with a radiation converter material modifies the radiating surface with regard to a higher emissivity of, for example, more than 80%, preferably more than 90% in this wavelength range.

At least some of the infrared radiation emitted by the radiating surface reaches the radiation converter material and from there directly or indirectly—via a reflector—to the material to be heated. The radiating surface extends, for example, over a circumferential angle of between 20 and 360 degrees, preferably between 60 and 200 degrees, and particularly preferably between 90 and 180 degrees of the pipe jacket surface.

The proportion of the emission surface covered with the radiation converter material can be up to 100%; However, the area covered with the radiation converter material particularly preferably extends over a circumferential angle of between 20 and 360 degrees, preferably between 60 and 200 degrees, and particularly preferably between 90 and 180 degrees of the outer surface of the cladding tube.

In a particularly preferred first modification of the embodiment of the infrared radiator with a shaped body in the form of a cladding tube made of quartz glass, the radiation converter material comprises a lower layer made of opaque quartz glass and an upper layer made of the coating material containing color pigments applied to the lower layer, with at least part of the outer surface of the tube being cladding tube, preferably the entire outer tube surface of the cladding tube is covered by the lower layer, and at least a first peripheral section of the lower layer is covered by the upper layer.

On the one hand, the lower layer of opaque quartz glass can itself act as a radiation converter material and, on the other hand, it contributes to improving the adhesion of the upper layer of the coating material.

Although the quartz glass cladding tube and the lower layer of opaque quartz glass applied to it absorb a portion of the short-wave or medium-wave primary radiation, it takes some time to bring the infrared radiator up to operating temperature. On the one hand, the additional upper layer of the coating material causes an increase in the emissivity in the relevant wavelength range. In addition, it also causes a higher absorption of the short-wave or medium-wave primary radiation and thus enables the infrared heater to heat up more quickly (and thus make it ready for use earlier). In addition, the energy efficiency of the infrared radiator is increased, since a larger part of the electrical energy supplied is converted into infrared radiation in the relevant wavelength range.

It is advantageous for this if the upper layer with the coating material is suitable for absorbing at least 80% of the primary radiation in the wavelength range from 1000 to 2500 nm. The thickness of the top layer is less than 0.1 mm, preferably in the range of 30-50 μm.

On the one hand, the lower layer of opaque quartz glass shows a certain transmission for the short-wave or medium-wave primary radiation and, on the other hand, it can also act as a diffuse reflector for the primary radiation. In order to reduce the transmitted portion, the lower layer made of the opaque quartz glass is advantageously covered in a second peripheral section by a specularly reflecting reflector layer, preferably with a gold-containing reflector layer.

In order to improve the quality of the specularly reflecting reflector layer, it has proven useful if the lower layer of opaque quartz glass is first thermally compacted at least in the contact area of the specularly reflecting reflector layer in order to reduce or avoid open porosity there. It is advantageous if the first circumferential section and the second circumferential section do not overlap and preferably complement each other to form a circumferential angle of 360 degrees.

In this embodiment, the disgruent surface portion of the outer surface of the pipe left free by the upper layer of the coating material containing color pigments is covered with the specularly reflecting reflector layer.

In a particularly preferred second modification of the embodiment of the infrared radiator with a shaped body in the form of a cladding tube made of quartz glass, at least part of the outer surface of the cladding tube has a surface roughness—defined as the arithmetic mean roughness R _a , with R _a in the range from 0.5 to 5 μm , preferably in the range from 0.8 to 3.2 pm, of which a first peripheral section forms the emission surface covered with the radiation converter material.

The roughness with an R _a value of 0.8 pm corresponds to roughness class 6 and typically occurs during rough grinding, and the R _a value of 3.2 pm corresponds to roughness class 8, which defines roughed surfaces. The outer surface of the cladding tube is preferably only roughened where the coating material is to be applied, ie in the area of the radiating surface. The radiation converter material is applied to the roughened part of the outer surface of the tube. The roughening improves the adhesion of the radiation converter material, in particular in the case of a radiation converter material in the form of a coating material containing colored pigments, such as, for example, a lacquer or a paste. The surface is roughened, for example, mechanically or chemically, in particular by grinding,

sandblasting or etching. With a high surface roughness R _a of more than 5 pm, the optical quality of the radiating surface suffers without any appreciable gain in adhesion promotion. With a low surface roughness R _a of less than 0.5 μm, there is no appreciable contribution to the adhesion-promoting effect.

In this embodiment, it is particularly advantageous if a second peripheral section of the tubular surface of the cladding tube is covered by a reflector layer, the first peripheral section and the second peripheral section of the tubular surface not overlapping and preferably adding up to a circumferential angle of 360 degrees. In this embodiment, too, the second peripheral section, which is left free by the radiation converter material, in particular a coating material containing color pigments, is covered with the reflector layer. The reflector layer preferably comprises a layer of opaque quartz glass and/or a specularly reflecting, metal-based reflector layer, preferably a gold-containing layer.

In the case of a reflector layer composed of a layer of opaque quartz glass and a metal-based layer, the layer of opaque quartz glass forms the lower layer, on which the metal-based layer is applied as the upper layer. In order to improve the quality of the upper layer of the specularly reflecting reflector layer, the lower layer of the opaque quartz glass is first thermally compacted at least in the contact area of the specularly reflecting reflector layer in order to reduce or avoid open porosity there.

In another particularly preferred modification of the infrared radiator, the molded body is in the form of a tile made of a material that emits infrared radiation when heated, the tile having opposing flat sides, one flat side of which comprises the radiation surface at least partially covered with the radiation converter material, and on the On the other side of the plane, a heating conductor track made of a resistance material and connected to an electrical contact for the supply of a heating current is applied.

Tile-shaped infrared heaters are surface heaters with a predominantly two-dimensional radiation characteristic. In the following, the predominantly radiating plan side is also referred to as the front and the opposite plan side as the back. In the case of the infrared radiator according to the invention, at least the front side is completely or partially covered with the radiation converter material, for example at least 80%, at least 60% or at least 40%. The radiation converter material is, for example, opaque quartz glass or a pigment-containing coating material or a combination of the two radiation converter materials, with the opaque quartz glass forming a lower layer and the coating material forming an upper layer. The tile material is preferably ceramic, in particular Al2O3 or ZrC>2, or it comprises a composite material, in particular a quartz glass matrix in which elementary silicon or carbon is embedded.

The possible size of the tile surface depends on the properties of the material and the required dimensional stability.

When the temperature increases, some tile materials change their colour. That means their emissivity and thus the peak emission wavelength of the primary radiation becomes shorter. Because of the coating with the radiation converter material, the emissivity changes less or not at all when the temperature increases. In particular, the pigment-containing coating material and the opaque quartz glass lose little or no emissivity even at high temperatures up to, for example, 1100.degree.

With regard to the component emitting infrared radiation, starting from a component of the type mentioned at the outset, the above-mentioned technical object is achieved according to the invention in that at least part of the radiating surface is covered with a radiation converter material which comprises a coating material containing colored pigments.

The component emitting infrared radiation acts as a radiation converter. It is not an active, electrically operated fleizer element, but the base body is heated by the absorption of short or medium-wave infrared radiation from an active fleizer. The short- or medium-wave primary radiation allows comparatively rapid temperature changes. On the other hand, as a result of the heating of the base body, the component emits infrared secondary radiation in the longer-wave range, which is better adapted to the absorption characteristics of the material to be processed.

The base body is present, for example, in the form of a tube, piston, chamber, flask, spherical or ellipsoid segment, plate or the like.

The absorption surface for the absorption of short-wave or medium-wave infrared primary radiation can differ from the emission surface for the emission of infrared secondary radiation, or these surfaces can completely or partially coincide. Since color pigments that appear black in the visible wavelength range usually also absorb (and emit) light in the relevant infrared wavelength range, the color pigment-containing coating material of the radiation converter material preferably contains a color pigment with black mineral particles, such as copper chromite black spinel or manganese ferrite black pigment , and it is alkaline free.

The absence of alkali has advantages in particular in the case of a base body made of glass or quartz glass, since this prevents the base body surface from being changed and damaged by devitrification.

A component in which the radiation converter material comprises opaque quartz glass in addition to the coating material containing color pigments is particularly advantageous.

The two radiation converter materials complement each other in terms of their emissivity, and the opaque quartz glass can act as an adhesion promoter for the coating material, particularly in the case of a base body made of quartz glass. In this case, the radiation converter material is a combination of the two radiation converter materials, with the opaque quartz glass forming a lower layer and the coating material forming an upper layer.

Although the base body and the lower layer of opaque quartz glass applied to it absorb a portion of the short-wave or medium-wave primary radiation, it takes some time to bring the component up to operating temperature. On the one hand, the additional upper layer of the coating material causes an increase in the emissivity in the relevant infrared wavelength range. In addition, it also causes a higher absorption of the short-wave or medium-wave primary radiation and thus enables the component to heat up more quickly (and thus make it ready for use earlier).

It is advantageous for this if the upper layer with the coating material is suitable for absorbing at least 80% of the primary radiation in the wavelength range from 1000 to 2500 nm. The thickness of the top layer is less than 0.1 mm, preferably in the range of 30-50 μm. definitions

Mean roughness Ra

The arithmetic mean roughness R _a is determined according to EN ISO 25178. It is a line roughness parameter. To determine the measured value R _a , the surface of a defined measuring section is scanned (with a fine needle) and all differences in height and depth of the surface are recorded. After calculating the specific integral of this roughness curve on the measuring section, the result is divided by the length of the measuring section

radiating surface

From the radiating surface, the useful radiation reaches the heating material directly or indirectly - via a reflector.

Peak Emission Wavelength

It defines the maximum of the spectral distribution of the emitted radiation.

Detailed description of the invention

The invention is explained in more detail below using an exemplary embodiment and a calibration. It shows in detail:

FIG. 1 shows an embodiment of a short-wave quartz raw radiator, the enveloping tube of which is covered on the outside with a radiation converter material, in cross section and in a schematic representation,

FIG. 2a shows a further embodiment of a short-wave quartz raw radiator, based on the basic form shown in FIG. 1 in a schematic representation,

FIG. 2b shows a photo of the embodiment of the short-wave quartz raw radiator according to FIG. 2a,

FIGS. 3 to 5 further embodiments of a short-wave quartz raw radiator based on the basic form shown in FIG. FIGS. 6 and 7 show further embodiments of a short-wave quartz raw radiator, the cladding tube of which is covered on the outside with a radiation converter material, in cross section and in a schematic representation,

FIG. 8 shows an embodiment of a tile-shaped infrared radiator whose radiating surface is covered with a radiation converter material, in cross section and in a schematic representation,

FIG. 9 shows a diagram of the radial radiation of a short-wave quartz raw radiator according to FIG. 7,

FIG. 10 shows a diagram of the diffuse and directional transmission of a short-wave quartz raw radiator according to FIG. 1, and

FIG. 11 shows a diagram with the result of measurements over time

Irradiance with a short-wave quartz raw radiator according to Figure 1 compared to a short-wave quartz raw radiator according to Figure 4.

FIG. 1 schematically shows a first basic variant of the infrared radiator according to the invention. This is a short-wave infrared radiator with a lamp tube 1 made of quartz glass. The lamp tube 1 is closed on both sides and surrounds a tungsten heating wire (not shown), which is provided with an electrical connection and can be heated to temperatures of up to 2300°C.

The outer surface of the lamp tube is covered completely (360 degrees) with a QRC layer 2 made of opaque quartz glass, which acts as a radiation converter material.

The QRC layer 2 is produced on the outer lateral surface of the lamp tube 1 using the known slip method described in DE 102004051 846 A1. The castable, aqueous SiO ₂ slip is applied to the lamp tube 1 as a slip layer, the slip layer is then dried and vitrified to form the QRC layer 2 . This consists of porous, opaque quartz glass. It has a density of about 2.15 g/cm ³ and an average layer thickness in the range from 0.5 to 2 mm. Their surface is open-pored, as a dye penetration test shows. The QRC layer 2 converts the short-wave primary radiation of the infrared radiator into longer-wave secondary radiation with a peak emission wavelength of about 2750 nm.

This makes it possible to operate the short-wave infrared radiator with an emission spectrum that is well adapted to the heating application at 700 to 800°C, and to enable rapid temperature changes and still achieve a high electrical power density of more than 50W/cm, for example at least 120W/cm. cm.

The electrical power density in the unit “electrical power per heated length” (W/cm) is almost 100% converted into optical power (W/m ² ). A power density of a short-wave infrared radiator with, for example, 120W/cm is converted into primary radiation with a first peak emission wavelength and through the use of the radiation converter material, such as the QRC layer 2, into medium-wave infrared radiation with a longer-wave peak emission wavelength, for example coming at a distance of 200mm (from the heating filament) at the detector approx. 12kW/m ² in total.

The entire outer surface of the cladding tube acts as a radiation surface, resulting in a three-dimensional radiation characteristic of the infrared radiator.

Figures 2 to 5 show modifications of the basic variant of Figure 1 with additional layers. The illustrations are not to scale; in particular, the thicknesses of the additional layers can be shown thicker for reasons of better visibility.

In the modification of the basic variant shown in FIG. 2a, half (front) of the QRC layer 2 (180 degrees) is blackened by being covered with a lacquer layer 3 made of a temperature-stable black lacquer. In this case, the radiating surface corresponds to the outer surface of the lamp tube covered with the blackened lacquer layer 3 . The paint layer retains its black color - and thus also its emission spectrum - even when heated to 800°C and beyond.

The lacquer layer 3 is produced by spraying or brushing on a thermal paint. The thermal paint is alkali-free. It contains an aluminosilicate solution (10 to 20% by weight), Copper chromite black spinel as a mineral color pigment (25 to 35% by weight) and water (40 to 60% by weight). Suitable thermal paints are offered as oven paints, for example, by the companies ULFALUX Lackfabrikation GmbFI and Aremco Products Inc., with the following organic ingredients being specified: xylene, ethyl acetate, butyl acetate, ethylbenzene.

Multiple varnishing ensures a completely closed layer. After spraying, the thermal paint is dried at 250°C and is then touchable.

By heating to 1200 ° C, the paint layer 3 gets its final state. This heating can take place when the infrared radiator is started up. In the process, ceramic components are sintered onto the surface of the lamp tube or onto the surface of the QRC layer and a firm, materially bonded connection is created, so that the layer of lacquer 3 is largely scratch-resistant. The thickness of the lacquer layer 3 is approximately 40 μm. The emissivity of the lacquer layer 3 at 800° C. is stated by the publisher to be over 90%.

The QRC layer 2 lying under the lacquer layer 3 generally shows open porosity and acts as a flattening agent. Fire polishing of the surface of the QRC layer 2 can prevent the paint from penetrating into the porous surface structure, as a result of which a more visually appealing surface structure is achieved. The fire polishing is carried out by heating the QRC layer 2 with an oxyhydrogen burner. This generates very high local temperatures of around 1800 °C, which makes it possible to create the thinnest possible glass film within a few seconds, which seals the porous surface.

During operation, the thin black lacquer layer 3 heats up to 700-750° C. in a few seconds and thus emits infrared radiation in a medium-wave range (preferably in the wavelength range from 2500 to 3500 nm). "Absorption = emission" applies, so that the short-wave radiation emitted by the lamp tube 1 and quickly absorbed in the lacquer layer 3 emits almost the same energy with high intensity just as quickly, but at a lower temperature (i.e. medium-wave). The black lacquer layer 3 acts as a radiation converter by converting high-energy short-wave radiation into high-intensity medium-wave radiation converts. The short-wave tungsten heating filament enables fast switching times of the energy supply in the range of seconds.

The photo from FIG. 2b shows the embodiment of the infrared radiator from FIG. 2a in a three-dimensional view. The electrical connections 1a brought out at one end of the lamp tube 1 can also be seen here.

In the modification of the basic variant shown in FIG. 3, half of the QRC layer 2 (180 degrees) is covered with a rear reflector layer in the form of a gold layer 4 .

The gold layer 4 is produced by applying a gold-containing emulsion (gold resinate) with a brush to the surface of the QRC layer 2 that is open-pored or sealed by thermal treatment. The emulsion is then burned in by heating. During firing, the gold resinate breaks down into metallic gold and resin acid, which in turn, like the other components of the paste, are volatilized by the high firing temperature. What remains is a closed, reflective gold layer 4, which acts as a reflector and whose thickness is preferably in the range from 50 to 300 nm, depending on the degree of reflection required. The thicker the layer, the higher the degree of reflection. In this case, the radiating surface corresponds to half (180 degrees) of the lateral surface of the lamp tube, which is covered by the QRC layer 2 but not by the gold layer 4 .

The gold layer 4 reduces the emissivity in the area of the back of the lamp tube and causes very good reflection of the radiation, which is reflected forwards onto the lacquer layer 3 and absorbed there. This proportion of radiation contributes significantly to the rapid heating of the black paint layer 3 .

In the modification of the basic variant shown in FIG. 4, the entire QRC layer 2 (360 degrees) is covered with a 0.04 mm thick layer 3 of thermal paint (manufacture and properties are explained with reference to FIG. 2). The entire enveloping surface acts as a radiation surface, resulting in a three-dimensional radiation characteristic of the infrared radiator.

In the modification of the basic variant shown in FIG. 5, one half of the surface of the QRC layer 2 (180 degrees) is covered with a 0.04 mm thick lacquer layer 3 Thermal paint covered (manufacture and properties are explained with reference to FIG. 2) and the disgruent half of the surface (180 degrees) with a 0.1 mm thick layer of gold 4 (manufacture and properties are explained with reference to FIG. 3). In this case, the radiating surface corresponds to half (180 degrees) of the outer surface of the lamp tube, which is covered by the lacquer layer 3 but not by the gold layer 4 .

FIG. 6 schematically shows a second basic variant of the infrared radiator according to the invention. This is also a short-wave infrared radiator with a lamp tube 1 made of quartz glass. The lamp tube 1 is closed on both sides and surrounds a tungsten heating wire (not shown), which is provided with an electrical connection and can be heated to temperatures of up to 2300° C., with halogen lamps up to 3000° C., and emits mainly short waves.

Conventional infrared radiators with a quartz tube have a gold reflector or a diffuse reflector made of QRC or ceramics on their rear side in order to bring the radiant energy forwards over the radiating surface of the transparent quartz glass lamp tube 1 into the material to be heated.

In this embodiment of the infrared radiator, one half of the outer surface of the lamp tube 1 (180 degrees) is blackened by being covered with a layer of lacquer 3 made of a temperature-stable black lacquer (manufacture and properties are explained with reference to FIG. 2). In this case, the radiating surface corresponds to the outer surface of the lamp tube covered with the blackened lacquer layer 3 . In another (not shown) embodiment of the infrared radiator, the entire lateral surface of the lamp tube 1 (360 degrees) is blackened.

On a smooth outer surface of the lamp tube, the black paint layer 3 can flake off over several hundred hours at high temperatures. To improve the adhesion of the paint layer 3, the lamp tube surface is roughened. The area of the roughening 6 is symbolized by a dashed line.

Roughening is done mechanically by sandblasting or grinding, or chemically: by treatment with an etching solution. A suitable etching solution (NH ₄ + HF + acetic acid) and its use to roughen a quartz glass surface is in DE 197 13014 C2. The average peak-to-valley height R _a is preferably in the range from 0.8 to 3.2 μm; in the exemplary embodiment it is 3pm.

The roughening 6 causes not only better adhesion of the paint layer 3 on the lamp tube surface, but also an even more homogeneous distribution of the medium-wave radiation by scattering the radiation on the roughened surface. The radial distribution of the converted radiation is very evenly distributed forwards over half the circumference of the quartz tube (see radial distribution according to FIG. 9).

The black lacquer layer 3 acts as a radiation converter and emits medium waves at temperatures in the range from 700 to 750.degree. Lifetime tests have shown that the paint layer or the infrared radiator can achieve a service life of up to 10,000 hours without visual or functional impairments.

The heating of the paint layer 3 to about 700° C., ie until medium-wave emission at about 3 pm, takes about 10 s. In comparison, standard medium-wave infrared emitters require about 5 minutes to reach thermal equilibrium.

In the modification of the second basic variant according to FIG. 6 shown in FIG. 7, the half of the lamp tube surface (180 degrees) not covered by the lacquer layer 3 has a 0.1 mm thick gold layer 4 (manufacture and properties are explained with reference to FIG. 3). In this case, the radiating surface corresponds to half (180 degrees) of the outer surface of the lamp tube, which is covered by the lacquer layer 3 but not by the gold layer 4 .

FIG. 8 shows a schematic of a flat, tile-shaped infrared radiator 8 made from a grain posit material made from quartz glass and elemental silicon embedded therein, as is described in DE 102015 119763 A1. The tile-shaped base body 9 of the infrared surface radiator 8 is covered with a heating conductor (not shown), which heats up the base body when current flows through it, so that it emits infrared radiation. At a temperature of 1000° C., such infrared radiators 8 achieve an emissivity of about 0.82 at a wavelength of 2.75 pm. This wavelength represents the peak wavelength at this temperature. As the temperature falls, the composite material loses its high emissivity. In order to counteract this, the radiating surface is covered with a layer 10 made of a radiation converter material. In one embodiment, this is a QRC layer 2. Its production and properties are explained with reference to FIG. Another embodiment is a lacquer layer 3. Its production and properties are explained with reference to FIG. Or, in a third embodiment, it is a combination of a lower layer, which is a QRC layer 2, and an upper layer, which is a lacquer layer 3. In FIG. 8, these possible combinations are symbolized by the combined reference number 2/3.

The coating of the emission surface with a layer of radiation converter material makes it possible for a high emissivity to be maintained independently of the temperature of the base body 9 . In this way, the already high efficiency of the tile-shaped infrared radiator is increased even further, since the emissivity is high even at low temperatures and the energy transfer can therefore take place in the best possible way.

The tile 9 has plate sizes of up to 400x400mm ² with a thickness of up to 2mm.

Alternatively, the tile 9 is made of a ceramic material, such as aluminum oxide or zirconium oxide. The thermal stimulation of the ceramic is made possible by means of a resistance heater. The radiating surface of the tile 9 is covered with a layer of lacquer 3 (manufacture and properties are explained with reference to FIG. 3). The lacquer layer 3 emits most of the absorbed energy by radiation. The temperature of the ceramic tile 8 determines the peak emission wavelength. Temperatures up to 1100 °C can be reached. Larger dimensions than those specified above, as well as curved geometries, are also particularly easy to implement with ceramic tiles.

To determine the effect of the radiation converter material on the radial distribution of the emitted optical power density (irradiance), a radial measurement was performed. This is done in the usual way using an infrared emitter mounted on a rotatable bracket, which rotates 360 degrees in 5 degree increments. A thermopile sensor mounted at a distance of 25 cm records the radiation emitted by the infrared heater. In the diagram of FIG. 9, the normalized irradiance (relative unit) is plotted against the circumferential angular position (in degrees) of the measurement points on the circle radius. The measurement curve shows this Result of the radial measurement for an infrared radiator according to FIG. 7 with a lacquer layer 3 on the front (radiating surface) and a mirror-reflecting gold layer 4 on the back.

In the rear radiator room 90, the measurement curve shows a small portion of the radiation intensity. This is made up of transmitted primary radiation and secondary radiation, which can be attributed to the heating of the gold layer 4 . In contrast, in the actual irradiation field 91, the measurement curve shows a high irradiance and a homogeneous distribution of the medium-wave radiation. The radial distribution of the converted radiation is evenly distributed over half the circumference of the quartz tube towards the front.

In the diagram of FIG. 10 for the transmission of a short-wave quartz raw radiator according to FIG. 1, the total transmission T (in %) determined using an integrating sphere is plotted against the wavelength l (in nm). The integrating sphere allows the measurement of the directional hemispheric spectral transmittance, which includes diffuse and directional transmittance.

It is found that after the infrared radiator has been switched on, a significant portion of the primary radiation is emitted by transmission through the QRC layer 2 due to multiple reflections. The non-transmitted radiation heats up the quartz glass jacket tube 1 together with the QRC layer 2 over time and thus additionally generates secondary radiation in the medium-wave range. After a few minutes of operation, thermal equilibrium is reached and the infrared heater emits a broad spectrum consisting of short-wave primary and medium-wave secondary radiation.

The diagram in FIG. 11 shows measurements over time for an infrared radiator according to FIG. ¹ compared to an infrared radiator according to FIG the x-axis.

A measurement of the irradiance over time shows that the infrared radiator (FIG. 1), which is only coated with a QRC layer 2, already generates about 50% of the maximum irradiance immediately after it is switched on. It then takes about 4 minutes until the full optical performance is reached. In the case of the additional coating layer 3 With a completely blackened infrared radiator (Figure 4), the irradiance increases more slowly, but due to the higher absorption, it reaches the maximum output earlier, after approx. 3 minutes. Above all, the rapid availability of part of the total optical power is advantageous for use in the printing industry, because the use of shutter systems to shade the paper web from the infrared radiators, which are still hot despite being switched off, can be dispensed with.

In the tile-shaped infrared radiators of the invention explained with reference to FIG. 8, a heating conductor track is provided on one of the plate sides of the tile, which generates heat when current flows and transfers it to the tile by thermal conduction, causing it to heat up. The tiles described without the heating conductor can be used as passive—currentless—heating elements if, instead of the heating conductor, they are heated by an external heat source that emits medium-wave or short-wave infrared radiation. The coatings with the radiation converter material may have the same effect as explained above, for example with reference to FIG.

Claims

EXPECTATIONS

1. Infrared radiator with a shaped body which has an emitting surface which emits short-wave or medium-wave infrared radiation with a first peak emission wavelength, characterized in that at least part of the emitting surface is covered with a radiation converter material (2; 3) which, as a result of heating by the infrared radiation of the first peak emission wavelength emits infrared radiation with a second peak emission wavelength longer in wavelength than the first peak emission wavelength.

2. Infrared radiator according to claim 1, characterized in that the radiation converter material (3) comprises a coating material containing colored pigment.

3. Infrared radiator according to claim 2, characterized in that the color pigment contains black mineral particles and is alkali-free.

4. Infrared radiator according to one of the preceding claims, characterized in that the radiation converter material (2) comprises opaque quartz glass.

5. Infrared radiator according to one of Claims 1 to 4, characterized in that the shaped body is designed as a cladding tube (1) made of quartz glass, which surrounds a radiation emitter provided with a power connection in the form of a heating coil or a heating strip, the radiating surface covering at least part of the Pipe shell surface forms.

6. Infrared radiator according to claim 5, characterized in that the radiating surface extends over a circumferential angle of between 20 and 360 degrees, preferably between 60 and 200 degrees, and particularly preferably between 90 and 180 degrees of the outer surface of the tube.

7. Infrared radiator according to claims 2 or 3 and 4 and according to one of claims 5 or 6, characterized in that the

Radiation converter material (2; 3) comprises a lower layer (2) made of the opaque quartz glass and an upper layer (3) applied to the lower layer (2) and made of the coating material containing color pigments, and that at least a part of the outer surface of the cladding tube (1) of of the lower layer (2), and at least a first peripheral portion of the lower layer (2) is covered by the upper layer (3).

8. Infrared radiator according to claim 7, characterized in that the lower layer (2) made of the opaque quartz glass is covered in a second peripheral section by a specularly reflecting reflector layer (4), preferably with a gold-containing reflector layer (4).

9. Infrared radiator according to claim 8, characterized in that the first peripheral section and the second peripheral section do not overlap and preferably complement each other to form a peripheral angle of 360 degrees.

10. Infrared radiator according to one of claims 5 or 6, characterized in that at least part of the outer surface of the cladding tube (1) has a surface roughness - defined as the arithmetic mean roughness R _a - with R _a in the range from 0.5 to 5 pm, preferably in Range from 0.8 to 3.2 pm - has, of which a first peripheral section forms the occupied with the radiation converter material emission surface.

11. Infrared radiator according to Claim 10, characterized in that a second peripheral section of the outer tube surface of the cladding tube (1) is covered by a reflector layer (4), the first peripheral section and the second peripheral section of the outer tube surface not overlapping and preferably forming a peripheral angle of 360° add degrees.

12. Infrared radiator according to claim 11, characterized in that the reflector layer (4) is formed by a layer of opaque quartz glass and/or by a layer containing metal, preferably a layer containing gold.

13. Infrared radiator according to one of claims 1 to 4, characterized in that the shaped body is designed in the form of a tile (9) made of a material that emits infrared radiation when heated, the tile (9) having opposite flat sides, of which one flat side comprising the radiation surface at least partially covered with the radiation converter material (2; 3), and on the other side of the plane a felt conductor track made of a resistance material and connected to an electrical contact for the supply of a felt current is applied.

14. Infrared radiator according to claim 13, characterized in that the tile material is a ceramic, in particular Al2O3 or Zr0 ₂ , or that the tile material comprises a composite material, in particular a matrix of quartz glass, in which elemental silicon or carbon is embedded.

15. Component emitting infrared radiation, with a base body (1; 9) made of a base body material, having an absorption surface for absorbing short-wave or medium-wave infrared primary radiation with a first peak emission wavelength, and an emission surface for emitting infrared secondary radiation with a second peak Peak emission wavelength which is longer than the first peak emission wavelength, characterized in that at least part of the emission surface is covered with a radiation converter material (2; 3) which comprises a coating material containing colored pigments.

16. The component according to claim 15, characterized in that the color pigment contains black mineral particles and is alkali-free.

17. The component according to claim 15 or 16, characterized in that the radiation converter material comprises opaque quartz glass.

18. The component according to any one of claims 15 to 17, characterized in that the base body material is quartz glass.