EP4287877A1

EP4287877A1 - Heating assembly for an aerosol generating device

Info

Publication number: EP4287877A1
Application number: EP22700998.2A
Authority: EP
Inventors: Glenn Harrison; Alec WRIGHT; Dick Paul VOERMAN; Ernst Hupkes
Original assignee: JT International SA
Current assignee: JT International SA
Priority date: 2021-02-08
Filing date: 2022-01-25
Publication date: 2023-12-13
Also published as: TW202231199A; JP2024506517A; WO2022167260A1

Abstract

A heating assembly (200) for an aerosol generating device (100) is disclosed. The heating assembly (200) comprises a metal tubular heating chamber (202) comprising an opening (204) for receiving an aerosol substrate. A coating of electrically insulating material (206) at least partially surrounds an outer surface (203) of the heating chamber (202), wherein the coating of electrically insulating material (206) comprises one or more of: ceramic; glass; silicone; and carbon. A heating element (208) at least partially surrounds the heating chamber and is disposed against the coating of electrically insulating material (206), wherein the coating of electrically insulating material (206) prevents any contact between the heating element (208) and the heating chamber (202).

Description

Heating Assembly for an Aerosol Generating Device

The present invention relates to a heating assembly for an aerosol generating device and a method of manufacturing a heating assembly for an aerosol generating device. The disclosure is particularly applicable to a portable aerosol generation device, which may be self-contained and low temperature. Such devices may heat, rather than burn, tobacco or other suitable aerosol substrate materials by conduction, convection, and/or radiation, to generate an aerosol for inhalation.

BACKGROUND

The popularity and use of reduced-risk or modified-risk devices (also known as vaporisers) has grown rapidly in the past few years as an aid to assist habitual smokers wishing to quit using traditional tobacco products such as cigarettes, cigars, cigarillos, and rolling tobacco. Various devices and systems are available that heat or warm aerosolisable substances as opposed to burning tobacco in conventional tobacco products.

A commonly available reduced-risk or modified-risk device is the heated substrate aerosol generation device or heat-not-burn device. Devices of this type generate an aerosol or vapour by heating an aerosol substrate, i.e. consumable, that typically comprises moist leaf tobacco or other suitable aerosolisable material to a temperature typically in the range 150°C to 300°C. Heating an aerosol substrate, but not combusting or burning it, releases an aerosol that comprises the components sought by the user but not the undesirable by-products of combustion. In addition, the aerosol produced by heating the tobacco or other aerosolisable material does not typically comprise the burnt or bitter taste that may result from combustion that can be unpleasant for the user.

Within known heat-not-burn devices, it is desirable to improve the efficiency of the heating process, whilst also maintaining a compact device with reliable operation.

SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided a heating assembly for an aerosol generating device, comprising: a metal tubular heating chamber comprising an opening for receiving an aerosol substrate; a coating of electrically insulating material that at least partially surrounds an outer surface of the heating chamber, wherein the layer of electrically insulating comprises one or more of: ceramic; glass; silicone; and carbon; and a heating element that at least partially surrounds the heating chamber and is disposed against the coating of electrically insulating material, wherein the coating of electrically insulating material prevents any contact between the heating element and the heating chamber.

In this way, a more efficient heating assembly is provided compared with known aerosol generating devices. The arrangement and properties of the coating of electrically insulating material prevents a short circuit occurring between the metal heating chamber and the heating element, whilst allowing improved heat transfer from the heating element to the aerosol substrate received within the heating chamber. In particular, the use of ceramic, glass, silicone and/or carbon as the coating material provides a high electric breakdown voltage, e.g. 1000 V/mm, such that short circuiting may be prevented using only a thin layer of coating, e.g. 10 pm, whilst also providing high thermal conductivity to ensure an efficient transfer of heat through the coating of electrically insulating material to the heating chamber. In contrast, within known devices, a separate (i.e. distinct) electrically insulating film such as PEEK or polyimide is generally provided between the heating element and the heating chamber. This increases the bulkiness of the device and provides a lower thermal conductivity, thereby increasing the time required to heat up the heating chamber and decreasing the heating efficiency. The present invention eliminates the need for such a film, whilst also improving the heat transfer properties and thereby improving the efficacy of the heating assembly. Moreover, using a coating rather than a separate electrically insulating film greatly simplifies the manufacturing process.

The term “coating” refers to a layer that is formed during application of the electrically insulating material to the outer surface of the heating chamber. The coating does not exist as a discrete layer prior to its application. In particular, the coating may be defined as a layer formed by the application of a liquid, vapour or gaseous material to the outer surface of the heating chamber. This contrasts with films such as PEEK or polyimide films which are pre-formed and exist as discrete layers prior to their application.

Preferably, the coating of electrically insulating is formed as a rigid layer on the outer surface of the heating chamber. In contrast, conventional electrically insulating films such as PEEK or polyimide are attached as a flexible layer on the outer surface of the heating chamber.

Preferably, the coating of electrically insulating material is formed using one of: vapour deposition; painting; dipping; or spraying. In this way, the coating of electrically insulating material exhibits advantageous mechanical and electrical properties compared to a film of electrically insulating material which is pre-formed (e.g. a polyimide film) and then attached to the outer surface of the heating chamber. For example, the coating of electrically insulating material conforms to specific morphology of the outer surface of the heating chamber, in contrast to a pre-formed film which does not form as closely to the surface, leading to suboptimal heat transfer.

Preferably, the coating of electrically insulating material comprises: ceramic and glass; ceramic and silicone; or glass and silicone. More preferably, the coating of electrically insulating material comprises ceramic; glass; and silicone. In this way, the coating of electrically insulating material provides significantly improved thermal conductivity compared to using a typical electrically insulating film comprising polyether ether ketone (PEEK) or polyimide. In particular, the inclusion of glass and ceramic within silicone provides high temperature resistance (e.g. a temperature rating of 482°C) and high thermal conductivity whilst being able to be formed as a thin coating.

Preferably, the coating of electrically insulating material consists of diamond-like- carbon, DLC. Preferably, the heating assembly further comprises: a flexible electrically insulating film that is wrapped around the heating chamber on an opposing side of the heating element to the coating of electrically insulating material. In this way, the heating element is electrically insulated from other components within the aerosol generating device. In one example, the heating element may be supported on the flexible electrically insulating film, and the heating element attached to the coating of electrically insulating material by wrapping the flexible electrically insulating film around the heating chamber with the heating element against the coating of electrically insulating material. In another example, the heating element may be supported on a carrier film, and the heating element may be wrapped around the coating of electrically insulating material using the carrier film. Once the heating element has been positioned, the carrier film may be removed and the flexible electrically insulating film may subsequently be wrapped around the heating chamber.

Preferably, the flexible electrically insulating film is configured to secure the heating element against the coating of electrically insulating material.

Preferably, the flexible electrically insulating film comprises a heat shrink film. In this way, the heat shrink film ensures that the heating element remains in contact with the coating of electrically insulating material, whilst also maintaining a compact arrangement of the heating assembly.

In some embodiments, the heating element is attached to the coating of electrically insulating material using an adhesive.

In some embodiments, the heating assembly further comprises: a thin film heater comprising the heating element and a flexible backing film on which the heating element is supported, wherein the thin film heater is wrapped around the heating chamber with the heating element against the coating of electrically insulating material.

Preferably, the coating of electrically insulating material surrounds the outer surface of the heating chamber, and the heating element surrounds the heating chamber. In particular, the the coating of electrically insulating material surrounds the circumferential outer surface of the heating chamber.

Preferably, the heating element comprises a resistive metal track.

Preferably, the coating of electrically insulating material has a thickness of between 0.8 and 20 microns, preferably between 3 and 18 microns. For example, the coating of electrically insulating material may have a thickness of 3 microns, 5 microns, 10 microns, 12.5 microns, 15 microns or 20 microns.

Preferably, the coating of electrically insulating material has a dielectric breakdown strength of at least 500 V/mm, and more preferably at least 1000 V/mm.

According to a second aspect of the present invention, there is provided a method of manufacturing a heating assembly according to any preceding claim, comprising: applying the coating of electrically insulating material to the outer surface of the heating chamber using one of: vapour deposition; painting; dip coating; or spraying.

Preferably, the method further comprises: curing the coating of electrically insulating material at a temperature of at least 80°C. In particular, the coating of electrically insulating material comprising ceramic, glass, and silicone may be thermally cured after it has been applied to the outer surface of the heating chamber. For example, the coating of electrically insulating material may be cured for 15 minutes at 95°C, then 15 minutes at 250°C, and then 10 minutes at 540°C.

In one embodiment, the method further comprises: supporting the heating element on a carrier film; wrapping the heating element around the heating chamber using the carrier film such that the heating element is disposed against the coating of electrically insulating material; removing the carrier film; and wrapping a flexible electrically insulating film around the heating chamber such that the heating element is disposed between the flexible electrically insulating film and the coating of electrically insulating material. In another embodiment, the method further comprises: supporting the heating element on a flexible electrically insulating film; wrapping the flexible electrically insulating film around the heating chamber such that the heating element is disposed against the coating of electrically insulating material.

Preferably, the method further comprises: heating the flexible electrically insulating film which is a heat shrink film, such that the heating element is secured to the coating of electrically insulating material.

According to a third aspect of the invention, there is provided an aerosol generating device comprising a heating assembly according to the first aspect.

BRIED DESCRIPTION OF DRAWINGS

Embodiments of the invention are now described, by way of example, with reference to the drawings, in which:

Figure 1 is an exemplary aerosol generating device according to an embodiment of the invention;

Figure 2 is a perspective view of a heating assembly according to an embodiment of the invention;

Figure 3 is a schematic cross sectional view of the heating assembly of Figure 2 further comprising a flexible electrically insulating film;

Figure 4 is a flow diagram showing method steps for manufacturing the heating assembly according to an embodiment of the invention; and

Figure 5 is perspective view of the heating assembly of Figure 2 showing the application of the heating element using a carrier film.

DETAILED DESCRIPTION

Figure 1 illustrates an aerosol generating device 100 according to an embodiment of the invention. The aerosol generating device 100 is illustrated in an assembled configuration with the internal components visible. The aerosol generating device 100 is a heat-not-burn device, which may also be referred to as a tobacco-vapour device, and comprises a heating assembly 200 configured to receive an aerosol substrate such as a rod of aerosol generating material, e.g. tobacco. The heating assembly 200 is operable to heat, but not burn, the rod of aerosol generating material to produce a vapour or aerosol for inhalation by a user. Of course, the skilled person will appreciate that the aerosol generating device 100 depicted in Figure 1 is simply an exemplary aerosol generating device according to the invention. Other types and configurations of tobacco-vapour products, vaporisers, or electronic cigarettes may also be used as the aerosol generating device according to the invention.

Figure 2 shows a perspective view of a heating assembly 200 according to an embodiment of the invention. Similarly, Figure 3 shows a cross-sectional schematic view of the heating assembly 200, except the heating assembly 200 further comprises a flexible electrically insulating film 210 wrapped around the heating assembly 200. The heating assembly 200 comprises a heating chamber 202, also referred to as a thermally conductive shell, configured to hold an aerosol substrate, also referred to as a consumable, therein. In particular, the heating chamber 202 defines a cylindrical cavity in which a rod of aerosol substrate may be positioned. The heating chamber 202 is tubular, e.g. cylindrical, and has an opening 204 positioned at a longitudinal end of the heating chamber 202. In use, the user may insert the aerosol substrate through the opening 204 in the heating chamber 202 such that the aerosol substrate is positioned within the heating chamber 202 and interfaces with an inner surface 201 of the heating chamber 202. The length of the heating chamber 202 may be configured such that a portion of the aerosol substrate protrudes through the opening 204 in the heating chamber 202, i.e. out of the heating assembly 200, and can be received in the mouth of the user.

The heating chamber 202 comprises, and preferably consists of, metal such that an efficient transfer of heat is provided through a side wall of the heating chamber 202 to the aerosol substrate, whilst also ensuring that the heating chamber 202 has sufficient structural stability and durability. Examples of suitable metals include steel or stainless steel.

The thickness of the tubular side wall of the heating chamber is preferably 0.1 mm or less, or more preferably between 0.07 and 0.09 mm. This allows for efficient heat transfer through the side wall of the heating chamber 202 to a consumable while maintaining sufficient structural stability. The tubular member has a closed end opposite the opening 204, where preferably the thickness of the closed end is 0.2 to 0.6 mm, which adds further structural rigidity to the heating chamber. The method of manufacturing the heating chamber 202 is described in co-pending PCT/EP2020/074147.

The heating chamber 202 may comprise a plurality of elongate ridges protruding inwardly as shown in Figure 5. The elongate ridges may be produced by pressing into the outer surface of a tubular member as fluid is injected under pressure into the tubular member to provide a plurality of corresponding elongate protrusions running lengthwise on the inner surface of the tubular member.

The skilled person will appreciate that the heating chamber 202 is not limited to being tubular. For example, the heating chamber 202 may be formed as a cuboidal, conical, hemi-spherical or other shaped cavity, and be configured to receive a complementary shaped aerosol substrate. Moreover, in some embodiments, the heating chamber 202 may not entirely surround the aerosol substrate, but instead only contact a limited area of the aerosol substrate.

A coating of electrically insulating material 206, also referred to as an electrically insulating layer, surrounds an outer surface 203 of the heating chamber 202. In particular, the coating of electrically insulating material 206 lies adjacent to (i.e. abuts, contacts) the circumferential outer surface 203 of the heating chamber 202. That it, the coating of electrically insulating material 206 is directly bonded to the outer surface 203 of the heating chamber 202. In Figures 1 and 2, the coating of electrically insulating material 206 is depicted as only extending along a portion of the length of the outer surface 203 of the heating chamber 202. However, the skilled person will appreciate that, in other embodiments, the coating of electrically insulating material 206 may extend along the entire length of the heating chamber 202. Moreover, the skilled person will appreciate that the coating of electrically insulating material 206 may only partially surround the outer surface of the heating chamber 202.

The coating of electrically insulating material 206 comprises at least one of ceramic, glass, silicone, and carbon, or any combination thereof. Preferably, the coating of electrically insulating material 206 comprises (and optionally consists of) silicone, glass and ceramic. This combination of constituents provides a high electric breakdown voltage, e.g. between 100 and 1000 V/mm, and exhibits high thermal conductivity in comparison to, for example, polyimide which is used within typical electrically insulating films. Moreover, a thin coating, e.g. 10 pm, may be used which provides improved heat transfer to the aerosol substrate received within the heating chamber 202. In comparison polyimide has a thickness of around 25 microns. Such properties advantageously reduce the heat-up time and cool-down time of the heating chamber 202, and improve the energy efficiency of the heating assembly 200. Advantageously, such coating materials may also have a higher thermal stability than polyimide. For example, silicone glass is stable up to approximately 482°C, DLC up to 300°C, silicone modified alkyd up to 450°C, whereas polyimide is only stable up to 270°C. In other embodiments, the coating of electrically insulating material 206 may comprise (and optionally consists of) diamond-like-carbon (DLC) or silicone modified alkyd resin.

The coating of electrically insulating material 206 may be deposited using a variety of coating or deposition techniques as will be further discussed with reference to Figure 4.

A heating element 208 comprising one or more heater tracks surrounds the coating of electrically insulating material 206. In particular, the heating element 208 is wrapped around the heating chamber 202, e.g. in a circumferential direction, such that the heating element 208 lies adjacent to (i.e. abuts, contacts) the coating of electrically insulating material 206. The coating of electrically insulating material 206 acts as a barrier to separate the heating element 208 and the heating chamber 202 such that contact between the heating element 208 and the heating chamber 202 is prevented. The skilled person will appreciate that the heating element 208 may only partially surround the heating chamber 202 and/or one or more heating elements 208 may be arranged around the circumference of the heating chamber 202.

The heating element 208 comprises a heating material suitable for converting electrical energy into heat (such as stainless steel, titanium, nickel, Nichrome, nickel based alloy, silver etc.). In the depicted embodiment, the heating element 208 comprises one or more resistive heater tracks. However, in other embodiments, the heating element 208 may be formed in alternative configurations, e.g. as a heating sheet.

In use, power may be supplied to the heating element 208 from a power source such as a battery (not depicted) such that the temperature of the heating element 208 increases and heat energy is transferred across the coating of electrically insulating material 206 to the heating chamber 202. The aerosol substrate received within the heating chamber 202 is conductively heated by the heating chamber 202 to produce an aerosol for inhalation by the user.

The skilled person will appreciate that the heating chamber 202 is not a resistive heater, and therefore should not receive a current. Thus, the coating of electrically insulating material 206 advantageously prevents a short circuit occurring between the heating element 208 and the heating chamber 202, whilst allowing an efficient transfer of heat from the heating element 208 to the heating chamber 202. That is, the coating of electrically insulating material 206 separates the heating element 208 and the heating chamber 202 and ensures that a current does not flow from the heating element 208 to the heating chamber 202. This eliminates the need for an additional layer of polyimide or PEEK, which is often wrapped between the heating chamber 202 and heating element 208 in conventional aerosol generating devices. Moreover, the skilled person will appreciate that, as the coating of electrically insulating material 206 is provided with a low thickness, the efficiency of heat transfer from the heating element 208 to the heating chamber 202 may be improved. As illustrated in Figure 3, a layer of electrically insulating film 210 is wrapped around the heating chamber 202 on an opposing side of the heating element 208 to the coating of electrically insulating material 206. That is, the heating element 208 is interposed between the coating of electrically insulating material 206 and the electrically insulating film 210. The electrically insulating film 210 comprises a flexible material preferably having a high dielectric capability and low thermal mass, such as polyimide, polyether ether ketone (PEEK) or polytetrafluorethylene (PTFE).

In some embodiments, the electrically insulating film 210 may be a heat shrink film, such that the heating element 208 is secured against the coating of electrically insulating material 206. In other words, the electrically insulating film 210, which surrounds the exterior of the heating assembly 200, consolidates the heating assembly 200 and ensures that the heating element 208 maintains contact with the coating of electrically insulating material 206. Additionally or alternatively, the heating element 208 may be secured to the coating of electrically insulating material 206 using an adhesive.

In Figure 3, the electrically insulating film 210 is illustrated as entirely enveloping the heating element 208 and the coating of electrically insulating material 206. That is, the electrically insulating film 210 extends beyond the extent of the heating element 208 and up to the longitudinal edges of the coating of electrically insulating material. However, it will be appreciated that the size and placement of the electrically insulating film 210 may vary.

In the depicted embodiment, the heating element 208 is a stand-alone heating element that is attached to the coating of electrically insulating material 206. However, in alternative embodiments, the heating element 208 may be comprised within a thin film heater (not illustrated), wherein the thin film heater comprises a flexible backing film on which the heating element 208 is disposed. For example, the heating element may be adhered to the flexible backing film, e.g. by a silicon adhesive. In this case, the thin film heater may be wrapped around the heating chamber 200 in a circumferential direction such that the heating element 208 lies adjacent to the coating of electrically insulating material 206. The flexible backing film lies on an opposing side of the heating element 208 to the coating of electrically insulating material 206, i.e. the heating element 208 is mounted to an inner surface of the flexible backing film with respect to the heating chamber 202.

Figure 4 illustrates a flow chart which is a method 300 of manufacturing a heating chamber according to an embodiment of the invention.

The method 300 begins at step 302, wherein a heating chamber 202 having an opening 204 for receiving an aerosol substrate within the heating chamber 202 is provided. The heating chamber 202 can be produced according to the method described in PCT/EP2020/074147 as aforementioned. At step 304, a coating of electrically insulating material 206 is applied onto an outer surface 203 of the heating chamber 202. That is, a coating of electrically insulating material 206 is bonded to the outer surface of the heating chamber 202 such that the coating at least partially surrounds, and preferably surrounds, the heating chamber 202 in a circumferential direction. The coating may be applied using a variety of coating processes and techniques, such as vapour deposition, chemical and electrochemical techniques, spraying, dipping or painting. For example, the coating of electrically insulating material 206 may be applied using plasma spraying, wire arc spraying, cathodic arc, sputtering, or ion beam deposition. The choice for these processes and techniques may depend on the coating material.

Optionally, at step 306, the coating of electrically insulating material 206 is cured in one or more curing stages, preferably at a temperature of above 80°C. In one example, the coating of electrically insulating material may be cured for 15 minutes at 95°C, then 15 minutes at 250°C, and finally 10 minutes at 250°C. This particular curing process is particularly applicable to the layer electrically insulating material 206 comprising silicone, and preferably silicone, glass, and ceramic. The skilled person will appreciate that the term “curing”, also known as thermal curing or heat curing, refers to a method of hardening or toughening polymers such as silicone by facilitating the cross-linking of polymer chains. The curing process increases the ability of the coating of electrically insulating material 206 to withstand corrosion and degradation. At step 308, a heating element 208 is attached to the coating of electrically insulating material 206. In particular, the heating element 208 is wrapped around the portion of the heater chamber 202 that is covered by the coating of electrically insulating material 206, such that the heating element 208 at least partially surrounds the heating chamber 202, and preferably surrounds the heating chamber 202, but does not contact the heating chamber 202 due to the physical barrier provided by the coating of electrically insulating material 206.

In one embodiment, the heating element 208 may be wrapped around the heating chamber 202 using a carrier film 212, as illustrated in Figure 5. That is, the heating element 208 is supported on a surface of the carrier film 212 and the method 300 further comprises wrapping the carrier film 212 around the heating chamber 202 in a circumferential direction such that the heating element 208 lies against the coating of electrically insulating material 206. Once the heating element 208 is correctly positioned, the carrier film 212 may be removed.

In another embodiment, the heating element 208 may be comprised within a thin film heater, wherein the thin film heater comprises the heating element 208 and a flexible backing film on which the heating element 208 is supported. In this case, the method 300 comprises wrapping the thin film heater around the heating chamber 202 with the heating element 208 against the coating of electrically insulating material 206.

At step 310, a flexible electrically insulating film 210 is wrapped around the heating chamber 202. The flexible electrically insulating film 210 envelops the heating element 208 such that the heating element 208 is interposed between the flexible electrically insulating film 210 and the coating of electrically insulating material 206.

In one embodiment, the heating element 208 may be supported on the flexible electrically insulating film 210 prior to being attached to the coating of electrically insulating material 206. In this case, the heating element 206 is wrapped around the heating chamber 202 using the flexible electrically insulating film 210, i.e. steps 308 and 310 occur concurrently. In some embodiments, the flexible electrically insulating film 210 may be a heat shrink film. In this case, the method 300 further comprises applying heat to the flexible electrically insulating film 210 such that the flexible electrically insulating film 210 contracts around the heating element 208 and the heating element 208 is secured against the coating of electrically insulating material 206.

Claims

1 . A heating assembly for an aerosol generating device, comprising: a metal tubular heating chamber comprising an opening for receiving an aerosol substrate; a coating of electrically insulating material that at least partially surrounds an outer surface of the heating chamber, wherein the coating of electrically insulating material comprises one or more of: ceramic; glass; silicone; and carbon; and a heating element that at least partially surrounds the heating chamber and is disposed against the coating of electrically insulating material, wherein the coating of electrically insulating material prevents any contact between the heating element and the heating chamber.

2. The heating assembly of claim 1 , wherein the coating of electrically insulating material comprises: ceramic and glass; ceramic and silicone; or glass and silicone.

3. The heating assembly of claim 2, wherein the coating of electrically insulating material comprises ceramic; glass; and silicone.

4. The heating assembly of claim 1 , wherein the coating of electrically insulating material consists of diamond-like-carbon, DLC.

5. The heating assembly of any preceding claim, further comprising: a flexible electrically insulating film that is wrapped around the heating chamber on an opposing side of the heating element to the coating of electrically insulating material.

6. The heating assembly of claim 5, wherein the flexible electrically insulating film is configured to secure the heating element against the coating of electrically insulating material.

7. The heating assembly of claim 6, wherein the flexible electrically insulating film comprises a heat shrink film.

8. The heating assembly of any preceding claim, wherein the heating element is attached to the coating of electrically insulating material using an adhesive.

9. The heating assembly of any preceding claim, further comprising: a thin film heater comprising the heating element and a flexible backing film on which the heating element is supported, wherein the thin film heater is wrapped around the heating chamber with the heating element against the coating of electrically insulating material.

10. The heating assembly of any preceding claim, wherein the coating of electrically insulating material surrounds the outer surface of the heating chamber, and wherein the heating element surrounds the heating chamber.

11. The heating assembly of any preceding claim, wherein the heating element comprises a resistive metal track.

12. The heating assembly of any preceding claim, wherein the coating of electrically insulating material has a thickness of between 0.8 and 20 microns.

13. The heating assembly of any preceding claim, wherein the coating of electrically insulating material has a dielectric breakdown strength of at least 100 V/mm, and preferably at least 1000 V/mm.

14. A method of manufacturing a heating assembly according to any preceding claim, comprising: applying the coating of electrically insulating material to the outer surface of the heating chamber using one of: vapour deposition; painting; dipping; or spraying.

15. The method of claim 14, further comprising: curing the coating of electrically insulating material at a temperature of at least 80°C.