Multi-layer susceptor assembly for inductively heating an aerosol-forming substrate

Abstract

The present invention relates to a multi-layer susceptor assembly for inductively heating an aerosol-forming substrate which comprises at least a first layer and a second layer intimately coupled to the first layer. The first layer comprises a first susceptor material. The second layer comprises a second susceptor material having a Curie temperature lower than 500° C. The susceptor assembly further comprises a third layer intimately coupled to the second layer which comprises a specific stress-compensating material and a specific layer thickness such that after a processing of the multi-layer susceptor assembly the third layer exerts a tensile or compressive stress onto the second layer at least in a compensation temperature range for counteracting a compressive or tensile stress exerted by the first layer onto the second layer. The compensation temperature range extends at least from 20 K below the Curie temperature of the second susceptor material up to the Curie temperature of the second susceptor material.

Claims

1. A multi-layer susceptor assembly for inductively heating an aerosol-forming substrate, the susceptor assembly comprising at least: a first layer comprising a first susceptor material; a second layer intimately coupled to the first layer, comprising a second susceptor material having a Curie temperature lower than 500° C.; a third layer intimately coupled to the second layer, comprising a specific stress-compensating material and a specific layer thickness such that after intimately coupling the layers to each other and/or after a heat treatment of the multi-layer susceptor assembly the third layer exerts a tensile or compressive stress onto the second layer at least in a compensation temperature range for counteracting a compressive or tensile stress exerted by the first layer onto the second layer, wherein the compensation temperature range extends at least from 20 K below the Curie temperature of the second susceptor material up to the Curie temperature of the second susceptor material.

2. The susceptor assembly according to claim 1, wherein a coefficient of thermal expansion of the second susceptor material is larger than a coefficient of thermal expansion of the first susceptor material and smaller than a coefficient of thermal expansion of the stress-compensating material.

3. The susceptor assembly according to claim 1, wherein the second susceptor material has a negative coefficient of magnetostriction and wherein the specific stress-compensating material and the specific layer thickness of the third layer is such that after intimately coupling the layers to each other and/or after a heat treatment of the multi-layer susceptor assembly the third layer exerts a compressive stress onto the second layer causing the second layer to be in a net compressive stress state at least in the compensation temperature range.

4. The susceptor assembly according to claim 1, wherein a coefficient of thermal expansion of the second susceptor material is smaller than a coefficient of thermal expansion of the first susceptor material and larger than a coefficient of thermal expansion of the stress-compensating material.

5. The susceptor assembly according to claim 1, wherein the second susceptor material has a positive coefficient of magnetostriction and wherein the specific stress-compensating material and the specific layer thickness of the third layer is such that after intimately coupling the layers to each other and/or after a heat treatment of the multi-layer susceptor assembly the third layer exerts a tensile stress onto the second layer causing the second layer to be in a net tensile stress state at least in the compensation temperature range.

6. The susceptor assembly according to claim 1, wherein the specific stress-compensating material and the specific layer thickness of the third layer is such that the third layer exerts a tensile or compressive stress onto the second layer after intimately coupling the layers to each other and/or after a heat treatment of the multi-layer susceptor assembly for enhancing a change of an electrical resistance of the second susceptor material at least when the temperature of the susceptor reaches the Curie temperature of the second susceptor material.

7. The susceptor assembly according to claim 1, wherein the specific stress-compensating material and the specific layer thickness of the third layer is such that the third layer exerts a tensile or compressive stress onto the second layer after intimately coupling the layers to each other and/or after a heat treatment of the multi-layer susceptor assembly for enhancing a change of a skin depth of the second susceptor material at least when the temperature of the susceptor reaches the Curie temperature of the second susceptor material.

8. The susceptor assembly according to claim 1, wherein the specific stress-compensating material and the specific layer thickness of the third layer is such that after intimately coupling the layers to each other and/or after a heat treatment of the multi-layer susceptor assembly the third layer exerts a tensile or compressive stress onto the second layer at least in the compensation temperature range for essentially compensating a compressive or tensile stress exerted by the first layer onto the second layer.

9. The susceptor assembly according to claim 1, wherein the first susceptor material includes aluminum, iron or an iron alloy, in particular a grade 410, grade 420, or grade 430 stainless steel.

10. The susceptor assembly according to claim 1, wherein the second susceptor material includes nickel or a nickel alloy, in particular a soft Fe—Ni—Cr alloy or a Fe—Ni—Cu—X alloy, wherein X is one or more elements taken from Cr, Mo, Mn, Si, Al, W, Nb, V and Ti.

11. The susceptor assembly according to claim 1, wherein the stress-compensating material includes austenitic a stainless steel.

12. The susceptor assembly according to claim 1, wherein the layer thickness of the third layer is in a range of 0.5 to 1.5, in particular 0.75 to 1.25, times a layer thickness of the first layer, preferably the layer thickness of the third layer is equal to a layer thickness of the first layer.

13. The susceptor assembly according to claim 1, wherein the first layer, the second layer and the third layer are adjacent layers of the multilayer susceptor assembly.

14. An aerosol-generating article comprising an aerosol-forming substrate and a susceptor assembly according to claim 1.

15. The aerosol-generating article according to claim 14, wherein the susceptor assembly is located in the aerosol-forming substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be further described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a schematic perspective illustration of an exemplary embodiment of a multilayer susceptor assembly according to the invention;

(3) FIG. 2 shows a schematic side-view illustration of the susceptor assembly according to FIG. 1; and

(4) FIG. 3 shows a schematic cross-sectional illustration of an exemplary embodiment of an aerosol-generating article according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

(5) FIG. 1 and FIG. 2 schematically illustrate an exemplary embodiment of a susceptor assembly 1 according to the present invention that is configured for inductively heating an aerosol-forming substrate. As will be explained below in more detail with regard to FIG. 3, the susceptor assembly 1 is preferably configured to be embedded in an aerosol-generating article, in direct contact with the aerosol-forming substrate to be heated. The article itself is adapted to be received within an aerosol-generating device which comprises an induction source configured for generating an alternating, in particular high-frequency electromagnetic field. The fluctuating field generates eddy currents and/or hysteresis losses within the susceptor assembly 1 causing it to heat up. The arrangement of the susceptor assembly 1 in the aerosol-generating article and the arrangement of the aerosol-generating article in the aerosol-generating device are such that the susceptor assembly 1 is accurately positioned within the fluctuating electromagnetic field generated by the induction source.

(6) The susceptor assembly 1 according to the embodiment shown in FIG. 1 and FIG. 2 is a three-layer susceptor assembly 1. The assembly comprises a first layer 10 as base layer comprising a first susceptor material. The first layer 10, that is, the first susceptor material is optimized with regard to heat loss and thus heating efficiency. In the present embodiment, the first layer 10 comprises ferromagnetic stainless steel having a Curie temperature in excess of 400° C. For controlling the heating temperature, the susceptor assembly 1 comprises a second layer 20 as intermediate or functional layer being arranged upon and intimately coupled to the first layer. The second layer 20 comprises a second susceptor material. In the present embodiment, the second susceptor material is nickel having a Curie temperature of in the range of about 354° C. to 360° C. or 627 K to 633 K, respectively (depending on the nature of impurities). This Curie temperature proves advantageous with regard to both, temperature control and controlled heating of aerosol-forming substrate. Once during heating the susceptor assembly 1 reaches the Curie temperature of nickel, the magnetic properties of the second susceptor material change from ferromagnetic to paramagnetic, accompanied by a temporary change of its electrical resistance. Thus, by monitoring a corresponding change of the electrical current absorbed by the induction source it can be detected when the second susceptor material has reached its Curie temperature and, thus, when the predefined heating temperature has been reached.

(7) However, the fact that the first and second layers 10, 20 are intimately coupled to each other may influence the change of the electrical resistance of the second susceptor material. This is mainly due to specific differences between the thermal expansion of the first and second susceptor materials as will be explained in the following. During processing of the susceptor assembly 1, the first and second layer 10, 20 are connected to each other at a given temperature, typically followed by a heat treatment, such as annealing. During a subsequent change of temperature, such as during a cooldown of the susceptor assembly 1, the individual layers 10, 20 cannot deform freely due to the conjoined nature of the assembly 1. Consequently, as the nickel material within the second layer 20 has a coefficient of thermal expansion larger than that one of the stainless steel within the first layer 10, a tensile stress state may develop in the second layer 20 upon cooldown. This tensile stress state in turn may reduce the magnetic susceptibility of nickel material due to magnetostriction because nickel has a negative coefficient of magnetostriction. In particular in the relevant temperature range around the Curie temperature of the nickel material, the reduced magnetic susceptibility may cause a change of the skin layer depth and, thus, a temporary change of the electrical resistance of the nickel material to be less pronounced. This in turn may undesirably impair the functionality of the second layer as temperature marker.

(8) In order to counteract the undesired tensile stress exerted by the first layer 10 onto the second layer 20, the susceptor assembly 1 according to the present invention further comprises a third layer 30 that is intimately coupled to the second layer 20. The third layer comprises a specific stress-compensating material and a specific layer thickness T30 which is specifically chosen such that after a processing of the multi-layer susceptor assembly 1, for example after a heat treatment, the third layer 30 exerts a specific compressive stress onto the second layer 20 at least in a certain compensation temperature range. The compensation temperature range extends at least from 20 K below the Curie temperature of nickel up to the Curie temperature of nickel. Accordingly, the third layer 30 advantageously allows for preserving the originally desired properties and functionalities of the second layer 20.

(9) In the present embodiment, the third layer comprises an austenitic stainless steel as stress-compensating material, for example V2a or V24 steel. Advantageously, austenitic stainless steel has a larger coefficient of thermal expansion larger than the nickel material of the second layer 20 and the ferromagnetic stainless steel of the first layer 10. Furthermore, due to its paramagnetic characteristics and high electrical resistance, austenitic stainless steel only weakly shields the nickel material of the second layer 20 from the electromagnetic field to be applied thereto.

(10) With regard to the embodiment shown in FIG. 1 and FIG. 2, the susceptor assembly 1 is in the form of an elongate strip having a length L of 12 mm and a width W of 4 mm. All layers have a length L of 12 mm and a width W of 4 mm. The first layer 10 is a strip of grade 430 stainless steel having a thickness T10 of 35 μm. The second layer 20 is a strip of nickel having a thickness T20 of 10 μm. The layer 30 is a strip of austenitic stainless steel having a thickness T30 of 35 μm. The total thickness T of the susceptor assembly 1 is 80 μm. The susceptor assembly 1 is formed by cladding the strip of nickel 20 to the strip of stainless steel 10. After that, the austenitic stainless steel strip 30 is cladded on top of the nickel strip 20.

(11) As the first and third layer 10, 30 are made of stainless steel, they advantageously provide an anti-corrosion covering for the nickel material in the second layer 20.

(12) FIG. 3 schematically illustrates an exemplary embodiment of an aerosol-generating article 100 according to the invention. The aerosol-generating article 100 comprises four elements arranged in coaxial alignment: an aerosol-forming substrate 102, a support element 103, an aerosol-cooling element 104, and a mouthpiece 105. Each of these four elements is a substantially cylindrical element, each having substantially the same diameter. These four elements are arranged sequentially and are circumscribed by an outer wrapper 106 to form a cylindrical rod. Further details of this specific aerosol-generating article, in particular of the four elements, are disclosed in WO 2015/176898 A1.

(13) An elongate susceptor assembly 1 is located within the aerosol-forming substrate 102, in contact with the aerosol-forming substrate 102. The susceptor assembly 1 as shown in FIG. 3 corresponds to the susceptor assembly 1 according to FIGS. 1 and 2. The layer structure of the susceptor assembly as shown in FIG. 3 is illustrated oversized, but not true to scale with regard to the other elements of the aerosol-generating article. The susceptor assembly 1 has a length that is approximately the same as the length of the aerosol-forming substrate 102, and is located along a radially central axis of the aerosol-forming substrate 102. The aerosol-forming substrate 102 comprises a gathered sheet of crimped homogenized tobacco material circumscribed by a wrapper. The crimped sheet of homogenized tobacco material comprises glycerin as an aerosol-former.

(14) The susceptor assembly 1 may be inserted into the aerosol-forming substrate 102 during the process used to form the aerosol-forming substrate, prior to the assembly of the plurality of elements to form the aerosol-generating article.

(15) The aerosol-generating article 100 illustrated in FIG. 3 is designed to engage with an electrically-operated aerosol-generating device. The aerosol-generating device may comprise an induction source having an induction coil or inductor for generating an alternating, in particular high-frequency electromagnetic field in which the susceptor assembly of the aerosol-generating article is located in upon engaging the aerosol-generating article with the aerosol-generating device.