Infrared radiating element

Abstract

An infrared emitter that comprises a cladding tube made of quartz glass that surrounds a heating filament as an infrared radiation-emitting element that is connected via current feedthroughs to an electrical connector outside the cladding tube. To improve the service life and power density, the heating filament comprises a carrier plate with a surface made of an electrically insulating material, whereby the surface is covered by a printed conductor made of a material that generates heat when current flows through it.

Claims

1. An infrared emitter comprising: a heating filament functioning as an infrared radiation-emitting element and including a carrier plate with a surface made of an electrically insulating material and a printed conductor covering the surface, the printed conductor being made of a material that generates heat when current flows through it and the carrier plate including a composite material that is formed by a matrix component and by an additional component in the form of a semiconductor material; a cladding tube made of quartz glass that surrounds the heating filament; and one or more current feedthroughs adapted to connect the heating filament to an electrical connector located outside the cladding tube.

2. The infrared emitter according to claim 1, wherein the material of the printed conductor is a non-precious metal.

3. The infrared emitter according to claim 1, wherein the material of the printed conductor contains one or more elements from the group of tungsten (W), molybdenum (Mo), silicon carbide (SiC), molybdenum disilicide (MoSi.sub.2), chromium suicide (Cr.sub.3Si), aluminum (Al), tantalum (Ta), polysilicon (Si), copper (Cu), and high temperature-resistant steel.

4. The infrared emitter according to claim 1, wherein the carrier plate is formed by at least two layers of material.

5. The infrared emitter according to, claim 1, wherein the matrix component is quartz glass and has a chemical purity of at least 99.99% SiO.sub.2 and a cristobalite content of at most 1%.

6. The infrared emitter according to claim 1, wherein the additional component contains a semiconductor material in elemental form.

7. The infrared emitter according to claim 1, wherein the additional component is present in a type and an amount such as to effect, in the carrier plate at a temperature of 600 C., an emissivity of at least 0.6 for wavelengths between 2 and 8 m.

8. The infrared emitter according to claim 1, wherein the carrier plate comprises a closed porosity of less than 0.5% and has a specific density of at least 2.19 g/cm.sup.3.

9. The infrared emitter according to claim 1, wherein the cladding tube surrounds the heating filament with a vacuum or in a protective gas atmosphere that comprises one or more gases from the series of nitrogen, argon, xenon, krypton, or deuterium.

10. The infrared emitter according to claim 1, wherein the printed conductor has a burnt-in thick film layer.

11. The infrared emitter according to claim 1, further comprising a coating made of opaque highly reflective quartz glass and wherein the cladding tube has a circumference with partial areas of the circumference being covered by the coating.

12. The infrared emitter according to claim 11, wherein the coating covers the circumference of the cladding tube over a range of angles from 180 to 330.

13. An infrared emitter comprising: a heating filament functioning as an infrared radiation-emitting element and including a carrier plate with a surface made of an electrically insulating material and a printed conductor covering the surface, the printed conductor being made of a material that generates heat when current flows through it; a cladding tube made of quartz glass that surrounds the heating filament; one or more current feedthroughs adapted to connect the heating filament to an electrical connector located outside the cladding tube; and a coating made of opaque highly reflective quartz glass, wherein the cladding tube has a circumference with partial areas of the circumference being covered by the coating.

14. The infrared emitter according to claim 13 wherein the carrier plate is formed by at least two layers of material, comprises a composite material that is formed by a matrix component and by an additional component in the form of a semiconductor material, or comprises a closed porosity of less than 0.5% and has a specific density of at least 2.19 g/cm.sup.3.

15. The infrared emitter according to claim 13, further comprising multiple printed conductors, which each can be electrically triggered individually, covering the surface of the carrier plate.

16. The infrared emitter according to claim 13, further comprising multiple carrier plates with printed conductors arranged in the cladding tube, whereby each of the carrier plates can be electrically triggered individually.

17. An infrared emitter comprising: a heating filament functioning as an infrared radiation-emitting element and including a carrier plate with a surface made of an electrically insulating material and a printed conductor covering the surface, the printed conductor being made of a material that generates heat when current flows through it and the carrier plate having a closed porosity of less than 0.5% and a specific density of at least 2.19 g/cm.sup.3; a cladding tube made of quartz glass that surrounds the heating filament; and one or more current feedthroughs adapted to connect the heating filament to an electrical connector located outside the cladding tube.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The disclosure is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:

(2) FIG. 1 shows a schematic partial view of the infrared emitter incorporated into a cladding tube made of quartz glass;

(3) FIG. 2 shows a cross-section through a cladding tube with an infrared emitter according to the invention; and

(4) FIG. 3 shows a diagram of the emission characteristics of the infrared emitter according to the invention compared to a conventional emitter with Kanthal coil.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) A preferred embodiment of the infrared emitter according to the invention consists of the material of the printed conductor covering the carrier plate being a non-precious metal.

(6) The material of the printed conductor being a non-precious metal is characterized by a high specific electrical resistance on a small surface area, which leads to high temperatures being attained even at relatively low current flows. Unlike printed conductors possessing high fractions of precious metals, for example platinum, gold or silver, the printed conductor material made of non-precious metal is significantly less expensive without this being associated with compromises in its electrical properties.

(7) The carrier plate with the heating conductor attached to it is incorporated into a cladding tube made of quartz glass, which prolongs the service life of the printed conductor since any corrosive attack, be it on a chemical and/or a mechanical basis, on the printed conductor by local ambient conditions is prevented. Printed conductors made of non-precious metals or non-precious metal alloys are particularly sensitive to this kind of corrosive attack.

(8) The material of the printed conductor advantageously contains one or more elements from the group of tungsten (W), molybdenum (Mo), silicon carbide (SiC), molybdenum disilicide (MoSi.sub.2), chromium silicide (Cr.sub.3Si), polysilicon (Si), aluminum (Al), tantalum (Ta), copper (Cu), and high temperature-resistant steel. Printed conductor materials of this type have a specific sheet resistance in the range of 50 to approximately 100 Ohm/sq. Due to their respective electrical and thermal properties, materials from this group fulfill their function of thermal excitation of the carrier plate of the infrared emitter according to the invention and can, in addition, be produced inexpensively.

(9) Moreover, it is time-proven for the carrier plate to be formed by at least two layers of material. In this context, the carrier plate can be formed by a basic material layer and a surface material layer, whereby the two material layers can differ in their electrical resistance or, if the electrical resistance is equal, can comprise different radiation emissivity. By this configuration, the optical and thermal properties of the carrier plate as the infrared radiation-emitting elementand therefore its emission characteristicscan be optimized for the individual application. Obviously, said advantageous embodiment is not limited to a two-layer system in a stack on top of one other. The material layers can just as well be arranged adjacent or next to each other.

(10) Referring to the material of the carrier plate, it is time-proven for the material to comprise a composite material that is formed by a matrix component and by an additional component in the form of a semiconductor material.

(11) The material of the carrier plate can be excited by thermal mechanisms and comprises a composite material that is formed by a matrix component and a semiconductor material as an additional component. The optical and thermal properties of the carrier plate result in absorption in the infrared spectral range. Conceivable matrix components include oxidic or nitridic materials, in which a semiconductor material is embedded as an additional component.

(12) In this context, it is advantageous for the matrix component to be quartz glass and to preferably possess a chemical purity of at least 99.99% SiO.sub.2 and a cristobalite content of at most 1%.

(13) Quartz glass possesses the aforementioned advantages of good corrosion, temperature, and temperature cycling resistance and is always available at high purity. It is therefore a conceivable substrate or carrier plate material even in high-temperature heating processes with temperatures of up to 1,100 C. Cooling is not required.

(14) The cristobalite content of the matrix being low, i.e. 1% or less, ensures that the devitrification tendency is low and, therefore, that the risk of crack formation during use is low. As a result, even the strict requirements concerning the absence of particles, purity, and inertness that are often evident in semiconductor fabrication processes are met.

(15) The heat absorption of the carrier plate material depends on the fraction of the additional component. The weight fraction of the additional component should therefore preferably be at least 0.1%. On the other hand, the volume fraction of the additional component being high can have an adverse effect on the chemical and mechanical properties of the matrix. Taking this into consideration, the weight fraction of the additional component is preferably in the range of 0.1% to 5%.

(16) In a preferred embodiment of the infrared emitter, the additional component contains a semiconductor material in elemental form, preferably elemental silicon.

(17) A semiconductor comprises a valence band and a conduction band that may be separated from each other by a forbidden band with a width of up to E3 eV. The conductivity of a semiconductor depends on how many electrons from the valence band cross the forbidden band to reach the conduction band. Basically, only a few electrons can cross the forbidden band and reach the conduction band at room temperature such that a semiconductor usually has only a low conductivity at room temperature. But the conductivity of a semiconductor depends essentially on its temperature. If the temperature of the semiconductor material rises, the probability that there is sufficient energy to elevate an electron from the valence band to the conduction band increases as well. Therefore, the conductivity of semiconductors increases with increasing temperature. Semiconductor materials show good electrical conductivity if the temperature is sufficiently high.

(18) The fine-particle areas of the semiconductor phase in the matrix act as optical defects and can cause the material of the carrier plate to look black or grey-blackish by eye at room temperature, depending on the thickness. On the other hand, the defects also impact the overall heat absorption of the material of the carrier plate. This is mainly due to the properties of the fine-distributed phases of the semiconductor that is present in elemental form, to the effect that, on the one hand, the energy between valence band and conduction band (bandgap energy) decreases with the temperature and, on the other hand, electrons are elevated from the valence band to the conduction band if the activation energy is sufficiently high, which is associated with a clear increase in the absorption coefficient. The thermally activated population of the conduction band leads to the semiconductor material being transparent to a certain degree at room temperature for certain wavelengths (such as from 1,000 nm) and becoming opaque at high temperatures. Accordingly, the absorption and the emissivity can increase abruptly with increasing temperature of the carrier plate. This effect depends, inter alia, on the structure (amorphous/crystalline) and doping of the semiconductor. For example pure silicon shows a notable increase in emission from approximately 600 C., reaching saturation from approximately 1,000 C.

(19) The spectral emissivity of the material of the carrier plate is at least 0.6 at a temperature of 600 C. for wavelengths between 2 m and 8 m.

(20) According to Kirchhoff's law of thermal radiation, the absorptivity .sub. and the spectral emissivity .sub. of a real body in thermal equilibrium are equal.
.sub.=.sub.(1)

(21) Accordingly, the semiconductor component leads to the emission of infrared radiation by the substrate material. The emissivity .sub. can be calculated as follows if the spectral hemispherical reflectance R.sub.gh and the transmittance T.sub.gh are known:
.sub.=1R.sub.ghT.sub.gh(2)

(22) In this context, the emissivity shall be understood to be the spectral normal degree of emission. The same is determined using a measuring principle that is known by the name of Black-Body Boundary Conditions (BBC) and is published in Determining The Transmittance And Emittance Of Transparent And Semitransparent Materials At Elevated Temperatures, J. Manara, M. Keller, D. Kraus, and M. Arduini-Schuster, 5th European Thermal-Sciences Conference, The Netherlands (2008).

(23) The semiconductor material, and specifically the elemental silicon that is preferably used, therefore have the effect to make the vitreous matrix material black and to do so at room temperature, but also at elevated temperature above, for example, 600 C., which results in good emission characteristics in terms of a high broadband emission at high temperatures being attained. In this context, the semiconductor material, preferably the elemental silicon, forms its own Si phase that is dispersed in the matrix. This phase can contain multiple metalloids or metals (but metals only up to 50% by weight, better no more than 20% by weight; relative to the weight fraction of the additional component). In this context, the carrier plate material shows no open porosity, but, at most, closed porosity of less than 0.5% and has a specific density of at least 2.19 g/cm.sup.3. It is therefore well-suited for infrared emitters, with regard to which the purity or gas tightness of the carrier plate are important.

(24) For use as infrared radiation-emitting material for an infrared emitter according to the present invention, the carrier plate material is covered by a printed conductor, which preferably is provided in the form of a burned-in thick film layer.

(25) The thick film layer can be formed from resistor pastes by screen printing or from metal-containing ink by inkjet printing, and is subsequently burned-in at high temperature.

(26) With regard to the temperature distribution being as homogeneous as possible, it has proven to be advantageous to provide the printed conductor as a line pattern covering a surface area of the carrier plate such that an intervening space of at least 1 mm, preferably at least 2 mm, remains between neighboring sections of the printed conductor.

(27) The absorption capacity of the carrier plate material being high enables homogeneous emission even if the printed conductor occupation density of the heating surface is comparably low. A low occupation density is characterized in that the minimal distance between neighboring sections of the printed conductor is 1 mm or more, preferably 2 mm or more. The distance between sections of the printed conductor being large prevents flashover, which can occur, in particular, upon operation at high voltages in a vacuum. The printed conductor extends, for example, in a spiral-shaped or meandering line pattern.

(28) In order to reduce a possible corrosive attack on the material of the printed conductor, it is preferred to keep the carrier plate including the printed conductor applied to it in the cladding tube in a vacuum or in a protective gas atmosphere that comprises one or more gases from the series of nitrogen, argon, xenon, krypton or deuterium.

(29) The infrared emitter according to the invention is particularly well-suited for vacuum operation, but, in individual cases, it is sufficient to have a protective gas atmosphere surround the carrier plate in the quartz glass cladding tube to prevent oxidative changes to the printed conductor material.

(30) In a preferred refinement of the infrared emitter according to the invention, multiple printed conductors, which each can be electrically triggered individually, are applied to a carrier plate.

(31) The provision of multiple printed conductors makes feasible the individual triggering and adaptation of the irradiation intensity that can be attained with the infrared emitter. On the one hand, the radiation power of the carrier plate can be adjusted through suitable selection of the distances of neighboring sections of the printed conductor. In this context, sections of the carrier plate are heated to different degrees such that they emit infrared radiation at different irradiation intensities. Variation of the operating voltages and/or operating currents that are applied to the respective printed conductors allows for easy and rapid adjustment of the temperature distribution in the carrier plate.

(32) Moreover, an advantageous refinement of the invention consists of multiple carrier plates with printed conductors being arranged in a cladding tube, whereby each of the carrier plates can be electrically triggered individually. This embodiment of the invention enables emitter variants that are adapted to the geometry of the heating goods. Accordingly, for example by arranging multiple carrier plates in a row in a single cladding tube, a panel radiator can be implemented that comprises different radiation intensity in individual sub-areas due to the individual triggering of the carrier plates.

(33) It is also time-proven for the cladding tube to comprise, in sub-areas, a coating made of opaque, highly reflective, quartz glass. Specifically for formation of a slit-shaped radiator it is advantageous for the coating to be applied to the circumference of the cladding tube in a range of angles from 180 to 330. A coating of this type reflects the infrared radiation of the heating filament and thus improves the efficiency of the infrared radiation with respect to the heating goods. The coating, also called the reflector layer, consists of opaque quartz glass and has a mean layer thickness of approximately 1.1 mm. It is characterized by the absence of cracks and a high density of approximately 2.15 g/cm.sup.3 and is thermally stable at temperatures up to and above 1,100 C. The coating preferably covers a range of angles up to 330 of the circumference of the cladding tube and therefore leaves an elongated sub-area corresponding to the strip shape of the cladding tube unoccupied and transparent for the infrared radiation. This design renders the production of the so-called slit-shaped emitter easy.

(34) Referring now to the drawing, in which like reference numbers refer to like elements throughout the various figures that comprise the drawing, FIG. 1 shows a first embodiment of an infrared emitter according to the invention, which, in total, has reference number 100 assigned to it, incorporated into a cladding tube 101 made of quartz glass. The cladding tube 101 has a longitudinal axis L. FIG. 1 shows a partial view of the infrared emitter 100 with a carrier plate 102, a printed conductor 103, and two contacting regions 104a, 104b for electrical contacting of the printed conductor 103.

(35) The contacting regions 104a, 104b have thin wires 105a, 105b welded to them that lead to contact surfaces 106a, 106b in the crimping 107 in the connection base 108 of the cladding tube 101. The thin wires 105a, 105b comprise, on a longitudinal section of 5 mm, spring wire coils 115a, 115b to compensate for a thermal elongation of the thin wires 105a, 105b at high operating temperatures.

(36) In the connection base 108, contact wires 109a, 109b are guided outwards and are also connected by welding to the contact surfaces 106a, 106b in the crimping 107.

(37) There is a negative pressure (vacuum) established on the inside of the cladding tube 101 or an inert gas is used to produce a non-oxidizing atmosphere on the inside of the cladding tube 101 such that the printed conductors 103 made of non-precious metal are protected from oxidation.

(38) The carrier plate 102 comprises a composite material having a matrix component in the form of quartz glass. A phase of elemental silicon is homogeneously distributed in said matrix component in the form of non-spherical areas. The matrix looks translucent to transparent to the eye. Upon microscopic inspection, it shows no open pores and at most closed pores with maximum mean dimensions of less than 10 m. A phase of elemental silicon is homogeneously distributed in the matrix in the form of non-spherical areas. It accounts for a weight fraction of 5%. The maximum mean dimensions of the silicon phase areas (median) are in the range of approximately 1 m to 10 m. The composite material is gas-tight, it has a density of 2.19 g/cm.sup.3 and it is stable in air up to a temperature of approximately 1,150 C. The embedded silicon phase contributes not only to the overall opacity of the composite material, but also has an impact on the optical and thermal properties of the composite material. Said composite material shows high absorption of heat radiation and high emissivity at high temperature. The carrier plate 102 is black in appearance and has a length (I) of 100 mm, a width (b) of 15 mm, and a thickness (t) of 2 mm.

(39) The degree of emission measured on the composite material of the carrier plate 102 in the wavelength range of 2 m to approximately 4 m is a function of the temperature. The higher the temperature, the higher is the emission. At 600 C., the normal degree of emission in the wavelength range of 2 m to 4 m is above 0.6. At 1,000 C., the normal degree of emission in the entire wavelength range from 2 m to 8 m is above 0.75.

(40) The printed conductor 103 is provided to be meander-shaped. The material for the printed conductor 103 essentially comprises non-precious metals such as tungsten and molybdenum and also polysilicon, whereby the printed conductor 103 of a suitable layout is applied to the carrier plate 102 by a screen-printable paste, and is then burnt in.

(41) In an alternative embodiment of the infrared emitter 100 according to the invention, the carrier plate 102 comprises a material made of ceramics such as silicon nitride (Si.sub.3N.sub.4) or silicon carbide (SiC), both of which are dark grey to black in appearance. A carrier plate 102 with a base material layer made of SiC has a surface layer made of SiO.sub.2, which is electrically insulating with respect to the metallic printed conductor 103, applied to its surface.

(42) Glass ceramics that are dark brown or dark grey in appearance (for example NEXTREMA glass available from Schott AG of Germany) are also well suited as a carrier plate material, as are carrier plates made of glassy carbon, such as plates made of the SIGRADUR material (available from HTW Hochtemperatur-Werkstoffe GmbH of Germany).

(43) Another alternative material for the carrier plate 102 is a polyimide plastic material that can be heated to a temperature of up to 400 C. Especially in applications in which a particularly quick power-on time (of a few seconds) is required, a carrier plate made of a polyimide film with a low thermal mass is expedient. Said polyimide film, as the carrier plate 102, also has printed conductors 103 made of non-precious metal applied to it. Because it is incorporated into a cladding tube 101 made of quartz glass, it can be operated in a non-oxidizing atmosphere.

(44) FIG. 2 shows a cross-section perpendicular to the longitudinal axis L of the cladding tube 101 with the infrared emitter 100 arranged inside it. A reflector layer 200 made of quartz glass is applied to the external circumferential surface of the cladding tube 101 over a length that corresponds to the length of the carrier plate 102, and covers 330 of the circumference. This configuration results in a so-called slit-shaped emitter with a narrow elongated open surface on the cladding tube 101 that allows the infrared radiation emitted by the carrier plate 102 to exit.

(45) FIG. 3 shows the power spectrum of an infrared emitter 100 according to the invention (curve A) compared to an infrared emitter with a Kanthal coil (curve B). In this case, the carrier plate 102 of the infrared emitter 100 according to the invention is formed by a composite material made of a matrix component in the form of quartz glass and a phase of elemental silicon homogeneously distributed therein, of the type described in more detail above. The printed conductor material in this case is tungsten. The temperature of the printed conductor 103 of the carrier plate 102 of said IR emitter 100 is adjusted to 1,000 C. The reference emitter possessing a Kanthal coil is also operated at a temperature of approximately 1,000 C. It is evident that the infrared emitter 100 according to the invention has approximately 25% higher power in the wavelength range from 1,500 nm to approximately 5,000 nm in the peak of curve A than the reference emitter, represented by curve B.

(46) Although illustrated and described above with reference to certain specific embodiments and examples, the present disclosure is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the disclosure. It is expressly intended, for example, that all ranges broadly recited in this document include within their scope all narrower ranges which fall within the broader ranges.