This invention relates to an infra-red radiation emission arrangement.

Infra-red radiation emitters are known which comprise a tungsten filament in an envelope of vitreous silica. Such emitters have a fast response, in that the radiation emitted can change from full power to negligible values within one second of the emitter being switched off. However, such emitters emit radiation primarily in the short wavelength of infra-red radiation, ie. in the range of rom 0.8 to 2.5 microns or in the medium wavelength of infra-red radiation, from 1.2 to 4.0 microns.

It is also known to provide long wavelength infra-red radiation emitters, emitting radiation in the wavelength range from 2 to 10 microns, in which a substrate is maintained at a temperature which determines the wavelength of the emitted radiation. However, such emitters have a slow thermal response, of the order of 200 to 300 seconds.

It is an object of the present invention to provide an improved infra-red radiation emission arrangement.

According to the present invention there is provided an infra-red radiation emission arrangement comprising: a primary source of short or medium wavelength infra-red radiation; and a substrate of thermally insulating material having a surface spaced apart from said primary source and on which, in use, radiation from said primary source is incident, said surface absorbing the infra-red radiation whereby said surface is heated to re-radiate infra-red radiation at a longer wavelength.

In an arrangement provided in accordance with the present invention, radiation from said primary source is incident on, and absorbed by, said surface of the substrate. As the substrate is itself thermally insulating, little heat is conducted away from said surface and so it is maintained above ambient temperature. The temperature achieved by the infra-red absorptive material will depend upon the intensity of the short or medium wavelength infra-red radiation from said primary source incident thereon, and on the absorptivity/reflectivity of said surface. The intensity of radiation incident on said surface is itself dependent upon the positioning and shaping of said surface of the substrate relative to said primary source. Accordingly, said surface will be at a lower temperature, and so will emit radiation of a longer wavelength than, said primary source.

Advantageously, a part of said surface is reflective of radiation at infra-red wavelengths. The total radiation output of such an arrangement is a combination of the short or medium wavelength radiation from said primary source, reflected by said surface, and medium/long wavelength radiation emitted by said surface. In this way, the total radiation output of the arrangement can be further controlled.

Advantageously, an infra-red radiation absorptive material is provided as a coating on at least part of said substrate to absorb the infra-red radiation from said primary source. Advantageously, said coating has a low thermal mass, responding rapidly in temperature to variations in the incident infra-red radiation. The thermal response of the arrangement is dependent upon the thermal response of said primary source, which can be very

fast as indicated previously and the thermal mass of that part of the substrate which responds to the incident infra-red radiation. Accordingly, an arrangement according to this aspect of the present invention has a relatively fast thermal response and an emission spectrum in the long wavelength infra-red.

The arrangement of the invention has the advantages that it has a faster thermal response time when emitting in the long wavelength region of the infra-red spectrum than known emission arrangements, and also that it provides for controllable broadening of the useful wavelength limits of the total radiation output from the arrangement as compared with a conventional single hot body infra-red emitter.

Preferably said surface of the substrate re-radiates at least 50%, and preferably at least 85% of the radiation energy incident on said surface from the primary source.

Advantageously said primary source includes an envelope, a part of said envelope through which radiation is transmitted away from said surface being coated with an infra-red radiation absorptive material which, in operation, is heated to re-radiate infra-red radiation at a longer wavelength.

Alt ~e*rnatively, the arrangement can include a secondary reflector to direct radiation from said primary source towards ^" said surface.

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 shows a composite arrangement comprising three infra-red radiation emission arrangements provided in accordance with the present invention;

Figure 2 shows the spectral output of a tungsten quartz emitter for use with an arrangement according to the present invention;

Figures 3 and 4 show second and third embodiments of infra-red radiation emission arrangements provided in accordance with the present invention;

Figure 5 shows a substrate for use with a fourth embodiment of an infra-red radiation emission arrangement provided in accordance with the present invention;

Figures 6 and 7 show spectral outputs of arrangements provided in accordance with the present invention; and Figure 8 shows the reflectivity of a substrate for use with an arrangement according to the present invention.

Figure 1 shows three infra-red radiation emission arrangements 1, 2 and 3, each comprising a short wavelength infra-red radiation emitter 4, such as a tungsten quartz emitter. The spectral output of a known tungsten quartz emitter is shown in Figure 2. Each short wavelength infra-red radiation emitter 4 has an associated reflector 5 which is arranged to direct infra-red radiation from the emitter 4 towards a concave surface 6 of a high efficiency thermally insulating substrate 7. The substrate 7 may be made of a low thermal mass material, such as ceramic fibres or microporous thermally insulating material, which reflects radiation of infra-red wavelengths as well as being thermally insulating. The surface 6 has a coating 8 on which the radiation from the associated emitter 4 is incident. The coating 8 is of an infra-red radiation absorptive material, such as copper oxide, boron carbide or iron oxide. The coating 8 is heated by the incident radiation from the associated emitter 4 (indicated by arrow S) and emits infra-red

radiation in the medium/long wavelength part of the infra-red radiation spectrum (as indicated by the arrow M/L) . The coating is very thin, so that it has a low thermal mass and so responds quickly to variations in the infra-red radiation incident thereon.

The coating 8 may be made of any infra-red radiation absorptive material which has a high emissivity, preferably of at least 0.8. Table 1 shows a number of high emissivity materials and indicates the conditions at which this high emissivity was measured, namely the wavelength range of radiation emitted and the temperature of the material at which this emission took place.

TABLE 1

MATERIAL CONDITIONS FOR NORMAL COMMENTS

SPECTRAL EMITTANCE

E > 0.8

WAVELENGTH (μm) TEMP (K)

Carbon 1 to 6 1000 to 1400 In non- (graphite) oxidising condition

^Cr 2°3 1 to 10 900 to 1000

NiO 1 to 10 1273

TiB. 1 to 15 1273

ZrB. 1 to 15 873 to 1423

SiC 0.8 to 10 873 to 1375

MATERIAL CONDITIONS FOR NORMAL COMMENTS

SPECTRAL EMITTANCE

E > 0.8

WAVELENGTH (μ ) TEMP (K)

MoSi, 1 to 8 1273

TaSi, 1 to 15 1273

Be ₁₂ Ta, 1 to 7 1023 to 122 Be ₁₇ Ta ₂

NiAl 0.9 to 2 298

MATERIAL CONDITIONS FOR NORMAL COMMENTS

SPECTRAL EMITTANCE

E > 0.8

WAVELENGTH (μm) TEMP (K)

TaAl 1 to 6 1273

TiAl 1 to 15 1023

TiCr. 1 to 15 1023

NiAl/ 1 to 15 1023 78 to 88 wt ^A1 2°3 % NiAl

A1 ₂ 0 ₃ / 1 to 15 1023 88 wt % TiAl TiAl highest

Cr ₂ 0 ₃ / l to 10 1273 1 wt i TiCr.,

^Cr 2°3

TiCr ₂ /Ti0

NiO/NiAl 1 to 5 1273 28 wt % NiAl

NiO/NiAl/ 1 to 5 1273 A1 ₂ 0 ₃

1273

1023

MATERIAL CONDITIONS FOR NORMAL COMMENTS

SPECTRAL EMITTANCE

E > 0.8

WAVELENGTH (μm) TEMP (K)

1023 various compositions

Figure 3 shows a second embodiment of a radiation emission arrangement according to the present invention. In view of the similarity between this embodiment and the embodiment of Figure 1, like parts are designated by like references. In the embodiment of Figure 3, the surface 6 is provided as a planar surface. Surfaces having other geometries may also be used.

Figure 4 shows a third embodiment of an infra-red radiation emission arrangement prepared in accordance with the present invention. Like parts to those of Figures 1 and 3 are designated by like references. In this embodiment, the primary source 4 is shown in greater detail as a tungsten filament 10 in an envelope 12 of vitreous silica. Part of the envelope 12 through which radiation from the filament 10 is transmitted, away from the substrate 7, is coated with an infra-red radiation absorptive material having a high emissivity. Suitable

materials are those disclosed in table 1 previously. As with the coating 8 on the substrate 7, the coating 14 on the envelope 12 is heated by the incident radiation from the tungsten filament 10 and emits infra-red radiation in the medium/long wavelength part of the infra-red radiation spectrum. An advantage of this embodiment over the embodiments of Figures 1 and 3 is that the effect of shielding of radiation from the. coating 8 by the secondary reflector 5 of Figure 1 and 3 on the total radiation output of the embodiment is reduced. In Figure 4, the arrow s indicates short wavelength radiation from the primary source and the arrow M/L indicate longer wavelength radiation.

As discussed above, the total radiation output of each infra-red radiation emission arrangement l, 2 or 3 can be set by appropriate selection of the mass, ie. the thickness, and the material of the coating 8, and of the position of the emitter 4 and its reflector 5 relative to the coated surface 6 of the substrate 7, such selection determining the temperature of the coating 8 during operation.

Figure 6 shows a substrate 7 for use with a fourth embodiment of an infra-red radiation emission arrangement provided in accordance with the present invention. Like- parts to tϋose of Figures 1 and 3 are designated by like references. In this embodiment, the surface 6 is partially not "covered by the coating 8. Accordingly, infra-red radiation incident on the uncoated parts 9 from the associated emitter 4 will be directly reflected therefrom. The total radiation output of this arrangement is a combination of radiation at medium/long wavelengths emitted from the coating 8 and infra-red radiation of short/medium wavelengths reflected from the uncoated part 9.

In the embodiments described previously, the coating 8 has a sufficient quantity of infra-red absorptive material per unit surface area of the part of the substrate 7 that substantially all of the incident infra-red radiation is absorbed and re-radiated at longer wavelengths. Alternatively, the coating 8 may include an insufficient quantity of infra-red radiation absorptive material to absorb all of the incident infra-red radiation. It is envisaged that this may be because the quantity of material is insufficient to coat the entire surface of the substrate 7 so that incident radiation is reflected from the uncoated parts. In this latter case, the infra-red radiation that is not absorbed by the coating 8 is transmitted to the substrate 7 and reflected therefrom. Accordingly, as with the embodiment of Figure 5, the total radiation output of such an arrangement is a combination of radiation at medium/long wavelength emitted from the coating 8 and infra-red radiation of short/medium wavelenghts reflected from the substrate 7.

Figures 6 and 7 show spectral output for infra-red emission arrangements provided in accordance with the present invention. In the examples, the substrate 7 was made of an alumino-silicate ceramic fibre board (for example "Kaowool 1600" manufactured by Morgan Ceramic Fibres Limited) of thickness 5 mm to 7 mm with a backing of microporous thermal insulation board (for example, "Microtherm" manufactured by Micropore Insulation Limited) of thickness 25 mm. Alternatively, a single board of ceramic fibre or Microtherm of thickness 25 mm to 30 mm may be used. The quoted thermal conductivities of the substrate material are 0.079 W/mK for Kaowool and 0.025 W/mK for Microtherm. The reflectivity of these two substrate materials at short wavelengths of infra-red radiation is high - a graph showing the reflectivity of Microtherm is provided as Figure 8.

In the examples, the material used for the absorptive coating is silicon carbide although successful emission arrangements have also been made using coatings of copper oxide, boron silicide and molybdenum disilicide. As indicated previously, silicon carbide is known to have an emissivity of at least 0.8. Its absorptivity is dependent on the quantity of material provided per unit surface area of substrate. Above a certain value of the coating thickness, the absorptivity equals the emissivity and the coating is opaque to the incident radiation. For the particular silicon carbide used, this critical, value was 150 grams per square metre.

In each example, the substrate 7 and coating 8 were planar and the substate and primary sources were arranged horizontally.

Figure 6 shows the effect of the amount of material in the coating on the spectral radiation output from the substrate. The details of the examples are as follows:

Example A - uncoated Kaowool;

Example B - Kaowool with a coating of silicon carbide of coat weight 50 grams per square metre;

Example C - Kaowool with a coating of silicon carbide of coat weight 150 grams per square metre.

It can be seen that the spectral radiation output for example C is primarily at wavelengths greater than 2 microns. The spectral radiation output for example B, however, includes a significant component at wavelengths less than 2 microns which is contributed by reflection from the Kaowool.

Figure 7 shows the effect of leaving part of the substrate surface uncoated, the remainder being coated with silicon carbide of coat weight 150 grams per square metre. The details are as follows:

Example A - uncoated Kaowool;

Example D - equal strips of coated and uncoated Kaowool;

Example E - surface area of coating is twice that of the uncoated Kaowool;

Example F - Kaowool completely coated;

It can be seen that as the area of uncoated Kaowool is increased, the proportion of infra-red radiation at wavelengths less than 2 microns increases.

Table 2 shows the thermal response of the infra-red radiation emission arrangements for the different examples. The thermal response of commercial long wave emitters, such as Pearlco 500 watt and Vulcan 400 watt is shown for comparison. It can be seen that the best thermal response is provided by example A which is, in fact, the uncoated substrate and so not an emission arrangement provided in accordance with the present invention. However, as shown in Figures 6 and 7, the spectral output of example A is primarily at the short wavelength end of the infra-red radiation spectrum and so the thermal response is primarily due to the fast thermal response of the primary source. The other examples B, C, D and E have a substantially faster thermal response than the commercial long wavelength radiation emitters.

Arrangements in accordance with the invention can be used as heat/curing sources in commercial process ovens, or as domestic heating sources.

Modifications to the embodiments described within the scope of the present invention will be apparent to those skilled in the art.