Aerosol generating device for generating aerosols by microwave heating of an aerosol-forming substrate - Patent Application 20070233633

Abstract

The present invention relates to an aerosol generating device for generating an aerosol by microwave heating of an aerosol-forming substrate contained within a base portion of a cylindrical aerosol-generating article. The aerosol generating device comprises a cylindrical microwave cavity (30) configured to removably receive at least the base portion of the aerosol-generating article as well as a microwave generator configured to generate a microwave signal. The aerosol generating device further comprises a coaxial feed exciter (40) operatively connected to the microwave generator and coupled to the microwave cavity, such as for feeding a microwave signal into the microwave cavity and for exciting at least one specific transverse magnetic or transverse electric mode in the microwave cavity when the base portion of the article is received within the microwave cavity. The present invention further relates to an aerosol generating system comprising such a device and such an article.

Description

[Technical field]

[0001] The present disclosure relates to an aerosol-generating device for generating inhalable aerosols by microwave heating of an aerosol-forming substrate, and in particular to an aerosol-forming substrate contained within an aerosol-generating article with which the device is configured for use. The present invention further relates to an aerosol-generating system comprising such a device and such an article. [Background technology]

[0002] In general, microwave heating of an aerosol-forming substrate is based on the principle of dielectric heating, which occurs when high frequency electromagnetic radiation stimulates very rapid vibrations of dipole molecules within the substrate, such as water molecules in a surrounding medium. These rapid vibrations cause friction between the stimulated dipole molecules, which in turn generates heat within the substrate.

[0003] Aerosol generating devices for generating inhalable aerosols by microwave heating of an aerosol-forming substrate are generally known from the prior art. As an example, such devices may comprise a microwave generating unit, a cavity for receiving an aerosol-generating article containing the aerosol-forming substrate to be heated, and a microwave antenna located outside the cavity configured to transmit microwaves generated by the microwave generating unit to a range of a predefined effective area within the cavity to heat the substrate. While this configuration has proven to have sufficient aerosol generation capabilities even when implemented in a portable aerosol generating device, there is still an ongoing need to make the microwave heating process of the aerosol-forming substrate more efficient and homogeneous.

[0004] It would therefore be desirable to have an aerosol generating device and corresponding aerosol generating system for use with aerosol-generating articles that employ microwave heating that provides the advantages of the prior art solutions while mitigating their limitations. In particular, it would be desirable to have an aerosol generating device and corresponding system that, when received within the device, provides enhanced microwave heating of an aerosol-forming substrate within the article. Summary of the Invention

[0005] According to the present invention, there is provided an aerosol generating device for generating an aerosol by microwave heating of an aerosol-forming substrate contained within a base portion of a cylindrical aerosol-generating article. The aerosol generating device comprises a cylindrical microwave cavity configured to removably receive at least the base portion of the aerosol-generating article as well as a microwave generator configured to generate a microwave signal. The aerosol generating device further comprises a coaxial feed exciter operatively connected to the microwave generator and coupled to the microwave cavity, such as for feeding a microwave signal into the microwave cavity and for exciting at least one particular transverse magnetic or transverse electric mode in the microwave cavity when the base portion of the article is received within the microwave cavity.

[0006] Preferably, the microwave cavity has at least one closed axial end, in particular only one closed axial end, in particular the distal closed end. Similarly, the microwave cavity has at least one open axial end, in particular only one open axial end, in particular the proximal open end. In principle, it is also possible for the microwave cavity to have two open axial ends, such as a distal open end and a proximal open end.

[0007] According to the invention, it has been found that by implementing a microwave heating hollow resonator based on a waveguide structure supporting microwave propagation along the axial direction of a cylindrical microwave cavity, in particular the propagation of a microwave mode with a homogeneous electric field distribution, it is possible to enhance microwave heating of an aerosol-forming substrate, both in terms of efficiency and homogeneity. Such a waveguide structure may be provided when the inner surface of the cylindrical microwave cavity is reflective to microwaves, in particular when at least the inner surface along the inner circumference of the microwave cavity is, for example, electrically conductive. Alternatively, a similar waveguide structure may be provided by an article when it has a reflective shell within the substrate portion, for example a metallic wrapper circumferentially surrounding the aerosol-forming substrate.

[0008] Moreover, it has been found that in such a waveguide structure, the configuration of a hollow resonator can be easily realized when the substrate portion of the aerosol-generating article received in the cylindrical microwave cavity has a higher dielectric permeability along the length axis of the cylindrical aerosol-generating article than other portions of the article, e.g. more proximal portions of the article, such that along the length axis of the waveguide structure, the dielectric permeability of the medium in the microwave cavity changes abruptly at one or both axial ends of the substrate portion, e.g. at the proximal end of the substrate portion. Typically, this change may result from a higher air content in other portions of the article, such as in the filter portion. Depending on the frequency and dimensions of the waveguide structure, the change in dielectric permeability may therefore prevent microwaves from propagating through the substrate portion during use from propagating further beyond the axial end of the substrate portion (exponentially decaying evanescent waves). Instead, the microwaves are reflected back in the opposite direction. Details of this effect are further detailed below. This effect may be present in particular at the proximal end of the base part adjacent to where a more proximal part of the article, such as a filter element, may be disposed. The same reflection effect may be present at the distal end of the base part, where a similar change in dielectric permeability may occur, for example down to that of air, when the distal end of the base part forms the distal end of the article. This is particularly the case when the distal end of the base part faces an air volume in the distal direction, for example when there is a small air pocket between the possibly closed distal end of the microwave cavity and the distal end of the article received therein. Alternatively or additionally, the inner surface of the microwave cavity may be reflective, in particular conductive, at least one closed axial end, in particular the closed distal end of the microwave cavity, in order to provide a reflection of the microwaves back in the opposite direction, in particular in the proximal direction. As a result, microwaves fed into the base part when the article is received in the microwave cavity may undergo reflection at both ends of the base part, which effectively corresponds to a resonator configuration.

[0009] Similarly, in an empty microwave cavity (one that does not receive any object therein, but is only filled with air), the low dielectric permeability of air may also prevent the propagation of waves through the empty waveguide structure when the frequency of the microwave signal is appropriately selected relative to the dimensions of the cavity. Hence, leakage of the microwave field from a possibly open axial end of the microwave cavity, such as the proximal open end of the microwave cavity, may also be sufficiently suppressed.

[0010] As mentioned above, a specific spatial distribution of the microwave field within the substrate portion is important for homogeneous heating. For this reason, a proper geometric match between the microwave cavity and the substrate portion is essential to ensure close interaction of the aerosol-forming substrate with the microwave field established within the microwave cavity during operation of the device. Accordingly, it is preferred that the internal cross-sectional shape of the cylindrical microwave cavity matches the external cross-sectional shape of the cylindrical aerosol-generating article, specifically the substrate portion.

[0011] Preferably, the cylindrical microwave cavity is a circular cylindrical microwave cavity, i.e. the internal cross-sectional shape of the microwave cavity is preferably circular. This is particularly advantageous for matching the external shape of an aerosol-generating article having a circular cylindrical external shape, such as an aerosol-generating article resembling a conventional cigarette. As a result, the waveguide structure that can be provided by a circular cylindrical microwave cavity is a circular waveguide structure.

[0012] Alternatively, the internal cross-sectional shape of the microwave cavity may be rectangular, in particular square, i.e. the cylindrical microwave cavity is a rectangular cylindrical, in particular square cylindrical microwave cavity. The waveguide structure that can be provided by the rectangular cylindrical, in particular square cylindrical microwave cavity is a rectangular, in particular square waveguide structure.

[0013] In addition to the internal cross-sectional shape of the microwave cavity, the spatial distribution of the microwave field within the substrate portion may also be influenced by proper selection of the shape and location of the coaxial feed exciter, which determines the build-up of the modal spectrum within the cavity during operation of the device. This is exploited in the present invention by configuring the coaxial feed exciter to excite at least one specific transverse magnetic or transverse electric mode within the microwave cavity when the substrate portion of the article is received within the microwave cavity, and coupling the coaxial feed exciter to the microwave cavity in such a way that a microwave signal provided by a microwave generator is fed into the microwave cavity.

[0014] By definition, a transverse mode of electromagnetic radiation is a particular electromagnetic field pattern of the radiation in a plane perpendicular (i.e. transverse) to the direction of propagation of the radiation. Transverse modes arise in microwaves constrained to a waveguide structure, such as the waveguide structure of the present invention, which can be realized either by the conductive inner surface of the microwave cavity itself, as described above, or by a reflective shell of an article within the substrate portion, for example, by a metallic wrapper surrounding the aerosol-forming substrate. More specifically, transverse modes arise due to the boundary conditions imposed on the microwave field by the waveguide structure. For example, in a cylindrical microwave guide as in the present invention, the tangential electric field amplitude of the microwave must be zero at the walls of the waveguide structure, and as a result the transverse pattern of the electric field is restricted to that which fits between the walls of the waveguide structure. For this reason, the modes supported by a waveguide are quantified. The allowed modes can be found by solving Maxwell's equations for the boundary conditions of a given waveguide structure.

[0015] In circular or rectangular waveguide structures, there are only two types of transverse modes: transverse magnetic modes (TM modes) and transverse electric modes (TE modes). Transverse magnetic modes (TM modes), also called E-waves, are characterized by the fact that the magnetic vector (H-vector) is always perpendicular (transverse) to the direction of propagation, while the electric field is perpendicular to the direction of propagation. That is, transverse magnetic modes have no magnetic field in the direction of propagation. Similarly, transverse electric modes (TE modes), also called H-waves, are characterized by the fact that the electric vector (E-vector) is always perpendicular (transverse) to the direction of propagation, while the magnetic field is perpendicular to the direction of propagation. That is, transverse electric modes have no electric field in the direction of propagation.

[0016] As mentioned above, the modes supported by the waveguide are quantified. As a result, the transverse magnetic and transverse electric modes TM in circular and rectangular waveguide structures are mn and T.E. mn is written with the subscripts m and n to represent the radial and axial field variations, respectively.

[0017] A waveguide only carries or propagates signals above a certain frequency, known as the cutoff frequency. The cutoff frequency of an electromagnetic waveguide is the lowest frequency at which a mode will propagate. Below the cutoff frequency, the waveguide structure cannot carry a signal. The cutoff frequency is specific to a particular waveguide mode that is considered to propagate in a waveguide of a given cross section, and determines the lower frequency of the waveguide operating frequency range.

[0018] Solving Maxwell's equations for the boundary conditions of a circular waveguide yields the following expression for the cutoff frequency of a waveguide with a circular internal cross-sectional shape:

number

[0019] where v represents the velocity of the microwaves in the medium filling the waveguide; JPEG2025513408000003.jpg7150 is TE according to the formula given below. mn and T.M. mn The cutoff phase constant calculated for the mode is:

number

number

[0020] During the ceremony, JPEG2025513408000006.jpg6150 is the nth root of the mth Bessel function, JPEG2025513408000007.jpg6150 is the n-th root of the m-th Bessel function derivative, and R is the radius of the circular waveguide structure. Some values ​​of the Bessel functions and their derivatives are shown in the table below. [Table 1]

[0021] The velocity of a microwave in a dielectric (non-magnetic) medium, ν, is given by the following equation:

number

[0022] where ε_r is the (static) relative permittivity of the dielectric medium (the real part of the frequency-dependent complex permittivity) and c is the speed of light in a vacuum. For a vacuum, the (static) relative permittivity ε_r is equal to 1, and therefore the microwave speed ν is equal to the speed of light in a vacuum, c.

[0023] The fundamental mode of a waveguide is the mode with the lowest cutoff frequency. This is because the fundamental mode of a circular waveguide is the TE 11The next higher mode is TM 01 It is a mode.

[0024] If the cylindrical microwave cavity is a circular cylindrical microwave cavity, the particular transverse magnetic mode excited by the coaxial feed exciter is preferably a TM 01 Mode. TM 01 The mode is rotationally symmetric, which proves particularly advantageous for homogeneous heating of the aerosol-forming substrate in the distal substrate portion of the aerosol-generating article.

[0025] TM 01 To ensure that the mode propagates into a “filled” circular cylindrical microwave cavity of a given internal diameter D, when an aerosol-generating article comprising an aerosol-forming substrate having a static relative dielectric constant ε_r is received therein, the microwave generator is preferably configured to generate a microwave signal having a frequency above a cut-off frequency f_cutoff defined by the following formula: [f_cutoff=(2.405*c) / (π*D*Sqrt(ε_r))].

number

[0026] Typically, known aerosol-forming substrates used to generate inhalable aerosols by heating (rather than by combustion), so-called "heat-non-burning" substrates, have a static relative dielectric constant ε_r in the range of 2 to 2.5, in particular 2.3 to 2.4, preferably 2.3 to 2.35. Consequently, in order to enable microwave coupling into a circular cylindrical microwave cavity, when an article comprising such an aerosol-forming substrate is received therein, the microwave generator is preferably configured to generate a microwave signal having a frequency above a cut-off frequency f_cutoff, the cut-off frequency f_cutoff being preferably outside the range defined by the following formula [f_cutoff=(2.405*c) / (π*D*Sqrt(ε_r))]:

number

[0027] In the formula, D is the inner diameter of the circular-cylindrical microwave cavity, c is the speed of light in a vacuum, and ε_r is a value within the range of 2 to 2.5, specifically 2.3 to 2.4, and preferably 2.3 to 2.35.

[0028] Conversely, to prevent or at least suppress microwave coupling into an empty circular-cylindrical microwave cavity of a given internal diameter D, the microwave generator is preferably configured to generate a microwave signal having a frequency below a frequency threshold f_thresh given by the following formula [f_thresh=(2.405*c) / (π*D)]:

number

[0029] where D is the inner diameter of the circular-cylindrical microwave cavity and c is the speed of light in a vacuum. For any value below the frequency threshold f_thresh defined above, the frequency of the microwave signal is below the cutoff frequency of the empty microwave cavity and therefore the TM 01The mode does not propagate into an "empty" microwave cavity.

[0030] Solving Maxwell's equations for the boundary conditions of a rectangular waveguide gives the TE in a rectangular waveguide with side lengths a and b. mn and T.M. mn This leads to the following equation for the cutoff frequency for the mode:

number

number

[0031] where v represents the velocity of the microwaves in the medium that fills the waveguide. As can be seen, TM mn The cutoff frequency for the mode is TE mn The cutoff frequency for the mode is the same as that for the mode. The only difference is that one cannot have m=0 or n=0. mn The mode is TM with m=1 and n=1. 11 Mode. The lowest TE mn The mode is TE with m=1 and n=0. 10 It is a mode.

[0032] TE in rectangular waveguide mn and T.M. mn Based on the cutoff frequency equation for the mode, similar conditions can be derived for a rectangular waveguide as for a circular cylindrical microwave cavity to ensure proper wave propagation into a "filled" rectangular cylindrical microwave cavity and to prevent or at least suppress microwave coupling into an empty rectangular cylindrical microwave cavity.

[0033] As a result, for example, TE 11To enable microwave coupling into the rectangular cylindrical microwave cavity of the mode, when the article including the aerosol-forming substrate is received therein, the microwave generator is preferably configured to generate a microwave signal having a frequency above a cut-off frequency f_cutoff, the cut-off frequency f_cutoff being a value outside the range defined by the following equation:

number

[0034] In the formula, a and b are the side lengths of the rectangular-cylindrical microwave cavity, c is the speed of light in a vacuum, and ε_r is a value within the range of 2 to 2.5, specifically 2.3 to 2.4, and preferably 2.3 to 2.35.

[0035] Similarly, for example, TE 11 In order to prevent or at least suppress microwave coupling of the mode into an empty rectangular-cylindrical microwave cavity with side lengths a and b, the microwave generator is preferably configured to generate a microwave signal having a frequency below a frequency threshold f_thresh given by the following formula:

number

[0036] where a and b are the side lengths of the rectangular cylindrical microwave cavity and c is the speed of light in a vacuum.

[0037] In general, the microwave generator may be configured to generate microwave signals within a frequency range of 5-50 GHz, in particular 10 GHz-40 GHz, preferably 12 GHz-18 GHz, or 20 GHz-30 GHz, most preferably 24 GHz-24.25 GHz. Advantageously, these frequency ranges have proven to be adequate to meet the above conditions for reasonable dimensions of the microwave cavity in the handheld device and for typical values ​​of the static relative dielectric constant ε_r of most aerosol-forming substrates. In particular, the latter frequency range of 24 GHz-24.25 GHz corresponds to the ISM (Industrial, Scientific, and Medical) radio band of 24-24.25 GHz, which is a part of the radio spectrum reserved internationally for Industrial, Scientific, and Medical (ISM) purposes (excluding telecommunications applications). Advantageously, the ISM radio band can be used by high-frequency devices in industry, science, medicine, in national and similar areas, license-free and with little authorization.

[0038] As already mentioned above, the microwave cavity preferably comprises an electrically conductive inner surface. More specifically, at least a part of the inner surface of the microwave cavity, or only a part of the inner surface, or the entire inner surface may be electrically conductive. Specifically, the microwave cavity may comprise an electrically conductive inner surface along the inner circumference of the microwave cavity, i.e. an electrically conductive circumferential inner surface, to provide a circular waveguide structure. In addition, the closed axial end, specifically the inner surface of the closed distal end of the microwave cavity, i.e. the axial end surface of the microwave cavity, may be electrically conductive.

[0039] However, the inner surface of the microwave cavity, in particular the inner surface along the inner circumference of the microwave cavity, does not necessarily have to be electrically conductive if the aerosol-generating article includes a reflective shell of the base portion. The shell may be disposed circumferentially around the aerosol-forming substrate, in particular exclusively circumferentially around the aerosol-forming substrate. For example, the aerosol-generating article may comprise a metallic wrapper, such as an aluminum foil, that circumferentially surrounds the aerosol-forming substrate. In this configuration, only the inner surface of the closed axial end, in particular the closed distal end of the microwave cavity, i.e. the axial end surface of the microwave cavity, may be electrically conductive. Alternatively, the shell may be disposed circumferentially around the aerosol-forming substrate, as well as at either one axial end of the substrate portion, in particular the distal end of the substrate portion, or both axial ends of the substrate portion. For example, the aerosol-generating article may comprise a metallic wrapper, such as an aluminum foil, that completely surrounds, in particular completely encapsulates, the aerosol-forming substrate.

[0040] From the above equations for the TE and TM modes in a circular or rectangular waveguide as follows, if the microwave frequency is not between the cut-off frequency of the mode with the lowest cut-off frequency and the cut-off frequency of the mode with the next lowest cut-off frequency, the waveguide structure will not support only one propagation mode. Thus, if an aerosol generating device according to the invention comprises, for example, a circular cylindrical microwave cavity of a given internal diameter and supports only one TM mode, then: 01 If the mode is considered to be a specific transverse magnetic mode, the fundamental mode of a circular waveguide, i.e., TE 11 The propagation of the mode is also 01The coaxial feed exciter has a cutoff frequency lower than the mode and is therefore also supported by the microwave cavity. The structure, shape and location of the coaxial feed exciter can be advantageously used to select and excite one, specifically only one, or selected, of the specific transverse magnetic or transverse electric modes in the microwave cavity to specifically determine the mode spectrum that is coupled into the microwave cavity. In general, there are different types and methods for coupling microwave signals into a waveguide structure, two of which are known as probe coupling and loop coupling. In probe coupling, a probe inside a coaxial line is used to distribute energy into the waveguide. When a current starts to flow in the probe, an electric field is set up and the field is separated from the probe to the waveguide. The probe (also called a probe antenna or excitation probe) radiates energy equally into the inserted waveguide. In loop coupling, a conductor is inserted into the waveguide and bent into a loop. The center of the loop may be equal distance from the top and bottom walls of the waveguide. When a current flows through the loop, the current generates a magnetic field component that couples with the waveguide field.

[0041] TM 01 For the sole purpose of exciting one or more particular transverse magnetic modes, such as a axial magnetic mode, into a circular cylindrical microwave cavity, a coaxial-feed exciter according to the present invention may comprise a coaxial line having an inner conductor surrounded by a concentric outer conductor shield, with a cavity-side end portion of the inner conductor extending beyond the cavity-side end of the outer conductor shield into the microwave cavity to form an excitation probe.

[0042] The coaxial feed exciter may be coupled to the microwave cavity via a feed-in opening at a closed axial end of the microwave cavity, specifically at the closed distal end of the microwave cavity.

[0043] Preferably, the coaxial feed exciter is disposed centrally at the closed axial end of the microwave cavity, specifically at the closed distal end of the microwave cavity opposite the proximal open end of the microwave cavity, i.e. at the axial end surface of the microwave cavity, specifically at the distal end surface of the microwave cavity. More specifically, the coaxial feed exciter is disposed coaxially into the cylindrical microwave cavity, i.e. the central axis of the coaxial feed exciter coincides with the central axis of the cylindrical microwave cavity.

[0044] If the microwave cavity has a conductive inner surface to provide a waveguide structure, the cavity side end of the outer conductor shield is preferably connected to the conductive inner surface of the microwave cavity. Advantageously, this increases the coupling efficiency.

[0045] The coupling efficiency and resonant frequency for optimal coupling of a microwave signal into a microwave cavity depends, among other things, on the dimensions of the coaxial feed exciter.

[0046] As a result, the length of the excitation probe extending beyond the cavity end of the outer conductor shield into the microwave cavity may be in the range of 1 millimeter to 8 millimeters, specifically 1 millimeter to 4 millimeters, and preferably 1 millimeter to 2 millimeters.

[0047] Similarly, the diameter of the excitation probe may be in the range of 1 mm to 2 mm, specifically 1.2 mm to 1.8 mm, and preferably 1.4 mm to 1.7 mm.

[0048] The above-mentioned dimensions of the coaxial feed exciter have been found to advantageously provide optimal conditions for coupling a microwave signal from a microwave generator into a microwave cavity. In particular, these dimensions help to reduce the reflection coefficient of a waveguide structure provided in the microwave cavity. Hence, higher heating temperatures may be achieved.

[0049] As mentioned above, the aerosol generating device according to the invention is preferably adapted for use with a cylindrical aerosol-generating article having a shape and dimensions similar to those of a conventional cigarette. The dimensions of the microwave cavity are therefore preferably selected such that they are compatible for receiving such types of aerosol-generating articles. As a result, the diameter of the microwave cavity may be in the range of 2 mm to 15 mm, in particular 4 mm to 12 mm, preferably 6 mm to 9 mm. Similarly, the length of the microwave cavity is preferably selected such that it completely receives at least the substrate portion of the aerosol-generating article containing the aerosol-forming substrate to be heated. As a result, the length of the distal part of the microwave cavity adapted to receive the distal substrate portion of the aerosol-generating article may be in the range of 5 mm to 20 mm, in particular 9 mm to 17 mm, preferably 10 mm to 14 mm.

[0050] As can be seen from the above formula, the specific values ​​of the cut-off frequency and frequency threshold for the TE and TM modes depend, among other things, on the lateral dimension, e.g., the diameter of the waveguide structure in which the substrate part to be heated is received snugly. If it is desired to operate at a frequency lower than the cut-off frequency for a given lateral dimension of the substrate part, e.g., a given diameter of the substrate part, it is suggested to increase the lateral dimension, e.g., the diameter of the microwave cavity, and to fill the empty space between the outer surface of the article and the inner surface of the microwave cavity with a filler. As a result, the aerosol generating device may comprise a non-conductive hollow cylindrical, specifically a circular hollow cylindrical filler, disposed in the microwave cavity, such that the outer peripheral surface of the hollow cylinder, specifically the circular hollow cylindrical filler, contacts the inner surface of the microwave cavity along the inner circumference of the microwave cavity, and said internal space of the hollow cylinder, specifically the circular hollow cylindrical filler, provides a receiving chamber configured to removably receive at least the substrate part of the aerosol generating article. Advantageously, this allows heating of an article of a given diameter at lower microwave frequencies by using a microwave cavity with a diameter larger than the given diameter of the article. Note that this is only applicable to devices in which the waveguide structure is provided by the reflective, specifically conductive, inner surface of the microwave cavity, but not to devices in which the waveguide structure is provided by a reflective outer shell of the article within the substrate portion, for example a metallic wrapper surrounding the aerosol-forming substrate.

[0051] To generate the microwave signal, the microwave generator preferably comprises at least one magnetron providing a source of the microwave signal, which may be coupled to a coaxial feed exciter.

[0052] The microwave generator may further comprise a microwave amplifier for providing a microwave signal of a desired power sufficient to heat the aerosol-forming substrate when the article is received within the microwave cavity.

[0053] The microwave generator may be configured to generate a microwave signal having a power output in the range of 1 Watt to 10 Watts, particularly 1.5 Watts to 10 Watts, and preferably 2 Watts to 7 Watts. Any power output within these ranges may be sufficient to heat the aerosol-forming substrate to the desired temperature.

[0054] As used herein, the term "aerosol generating device" refers to a device, typically electrically operated, capable of interacting with an aerosol-forming substrate provided in an aerosol-generating article, such as by heating the substrate, to generate an aerosol. The aerosol generating device is preferably a smoke extractor for generating an aerosol that can be directly inhaled by a user through the user's mouth. In particular, the aerosol generating device is a handheld aerosol generating device.

[0055] Preferably, the microwave cavity comprises at least one open axial end, in particular a proximal open end, which may function as an insertion opening through which the aerosol-generating article may be inserted into the microwave cavity. As used herein, the direction in which the aerosol-generating article is inserted is indicated as the insertion direction. Preferably, the insertion direction corresponds to the extension of the central axis of the cylindrical microwave cavity. After insertion into the microwave cavity, at least a portion of the aerosol-generating article may still extend outwardly through the open axial end, in particular the proximal open end. The outwardly extending portion may preferably be provided for interaction with a user, in particular for insertion into the user's mouth. Thus, during use of the device, the axial open end may be closer to the user's mouth and may therefore be indicated as the proximal open end. Accordingly, the section closer to the proximal open end or the section closer to the user's mouth during use of the device, respectively, is indicated with the prefix "proximal". Sections that are disposed further apart are indicated with the prefix "distal". Correspondingly, as used herein, "proximal" may connote a direction toward a user of the device, while "distal" may connote a direction opposite to the proximal direction, i.e., a direction away from the user of the device. The insertion direction may preferably be the distal direction.

[0056] The microwave cavity may be disposed or located within a proximal portion of the aerosol generating device, and similarly, the proximal open end may be disposed or located at the proximal end of the aerosol generating device.

[0057] The microwave cavity may comprise a sleeve or tube forming a circumferential sidewall of the cavity. Furthermore, the microwave cavity may comprise a closed axial end, specifically a bottom portion that may form the closed distal end of the microwave cavity. Similarly, the microwave cavity may be formed by or may include a barrel. The microwave cavity, specifically the sleeve, tube, bottom portion, and barrel, may be inserted into the device housing, specifically the proximal portion of the device housing of the aerosol generating device. The device housing is preferably made of plastic. The device may further comprise a thermal insulation material disposed between the microwave cavity and the device housing. The thermal insulation material may be configured to sustain heat within the microwave cavity and provide thermal insulation to prevent thermal conduction between the interior of the chamber and the device housing.

[0058] The microwave cavity may comprise or be made of a metal, such as stainless steel or aluminum, especially when the waveguide structure is realized by the microwave cavity itself. Alternatively, the microwave cavity may comprise or be made of a thermoplastic, such as a plastic, especially PEEK (polyetheretherketone). PEEK is a semi-crystalline thermoplastic with excellent mechanical and chemical resistance properties that are retained at high temperatures. When the waveguide structure is realized by the microwave cavity itself, the inner surface of the microwave cavity comprising or made of a plastic may be formed by a conductive coating, such as a metal coating.

[0059] The microwave cavity may also be integral with at least a portion of the coaxial feed exciter. In particular, the microwave cavity may be integral with at least an outer conductive shield of the coaxial feed exciter. Advantageously, this may facilitate the manufacture of the aerosol generating device.

[0060] The aerosol-generating device may further comprise a controller configured to control the operation of the device. In particular, the controller may be configured to control the microwave generator, preferably in a closed loop configuration, to control the heating of the aerosol-forming substrate to a predetermined operating temperature. The operating temperature used for heating the aerosol-forming substrate may be at least 180 degrees Celsius, in particular at least 300 degrees Celsius, preferably at least 350 degrees Celsius, more preferably at least 370 degrees Celsius, and most preferably at least 400 degrees Celsius.

[0061] The aerosol generating device may comprise a power source, in particular a DC power source configured to provide a DC supply voltage and a DC supply current for powering the microwave generator. Preferably, the power source is a battery, in particular a rechargeable battery such as a lithium iron phosphate battery.

[0062] The microwave generator, coaxial feed exciter and microwave cavity as described herein may together be part of or form a microwave heating arrangement for heating the aerosol-forming substrate, which may be an independent aspect of the invention.

[0063] According to the present invention there is also provided an aerosol generation system comprising an aerosol generating device according to the present invention and as described herein, the system further comprising a cylindrical aerosol-generating article comprising a distal substrate portion containing an aerosol-forming substrate which is heated by the device when the article is received within the device housing.

[0064] As used herein, the term "aerosol-generating article" refers to an article comprising at least one aerosol-forming substrate, which when heated releases a volatile compound capable of forming an aerosol. As a result, an aerosol-generating article may be indicated as a "heated aerosol-generating article", or an "aerosol-generating article for heating", or as a "heated non-combustion article". That is, an aerosol-generating article comprises at least one aerosol-forming substrate that is intended to be heated, rather than combusted, to release a volatile compound capable of forming an aerosol. An aerosol-generating article may be a consumable product, in particular a consumable product that is discarded after a single use.

[0065] As used herein, the term "aerosol-forming substrate" refers to a substrate capable of releasing volatile compounds capable of forming an aerosol when heated. The aerosol-forming substrate may be a solid aerosol-forming substrate or a gel-like aerosol-forming substrate or a liquid aerosol-forming substrate, or a combination thereof. The aerosol-forming substrate may comprise a tobacco-containing material containing volatile tobacco flavor compounds that are released from the substrate upon heating. Alternatively or additionally, the aerosol-forming substrate may comprise a non-tobacco material. The aerosol-forming substrate may further comprise an aerosol former. Examples of suitable aerosol formers are glycerin and propylene glycol. The aerosol-forming substrate may also comprise other additives and ingredients, such as nicotine or flavoring substances. In particular, the liquid aerosol-forming substrate may comprise water, solvents, ethanol, plant extracts, and natural or artificial flavors. The aerosol-forming substrate may also be a paste-like material, a sachet of porous material containing the aerosol-forming substrate, or, for example, loose tobacco mixed with a gelling or adhesive agent, which may include a common aerosol former such as glycerin, and then compressed or shaped into a plug. The aerosol-generating article may be a tobacco article.

[0066] As used herein, the term "cylindrical aerosol-generating article" refers to an aerosol-generating article in which at least a distal substrate portion of the cylindrical aerosol-generating article is cylindrical, i.e. has a cylindrical shape. The term "cylindrical aerosol-generating article" may also refer to an aerosol-generating article in which the entire article is cylindrical, i.e. has a cylindrical shape. In particular, the article may be a rod-shaped article, a rod-shaped article that may resemble a conventional cigarette.

[0067] Preferably, the external shape and diameter of at least the distal substrate portion, and in particular the entire aerosol-generating article, matches the shape and diameter of a circular waveguide structure provided in the microwave cavity. If the waveguide structure is realized by the microwave cavity itself, the external shape and diameter of at least the distal substrate portion, and in particular the entire aerosol-generating article, may match the shape and diameter within the microwave cavity. In particular, if the microwave cavity is a circular cylindrical microwave cavity, at least the distal substrate portion, and in particular the entire aerosol-generating article, has a circular cylindrical shape. Similarly, if the microwave cavity is a rectangular cylindrical microwave cavity, at least the distal substrate portion, and in particular the entire aerosol-generating article, has a rectangular cylindrical shape.

[0068] The distal substrate portion of the article may have a length (along the length axis of the cylindrical aerosol-generating article) in the range of 5 millimeters to 20 millimeters, particularly 9 millimeters to 17 millimeters, preferably 10 millimeters to 14 millimeters.

[0069] According to the invention, the cylindrical microwave cavity is configured to removably receive at least the distal substrate portion of the aerosol-generating article. Accordingly, the microwave cavity has a length at least equal to or greater than the length of the distal substrate portion. As a result, the microwave cavity, or at least the distal portion of the microwave cavity configured to receive the substrate portion of the aerosol-generating article, may have a length in the range of at least 5 mm to 20 mm, in particular 9 mm to 17 mm, preferably 10 mm to 14 mm.

[0070] As an example, the aerosol-generating article may comprise one or more of the following elements: a substrate element, a first tube element, a second tube element, and a filter element.

[0071] The substrate element preferably comprises at least one aerosol-forming substrate which is heated. In particular, the substrate element may be part of or form the distal substrate portion of the article. As a result, the substrate element may have a length which corresponds to the length of the distal substrate portion. With that in mind, the substrate element may have a length (along the length axis of the cylindrical aerosol-generating article) in the range of 5 mm to 20 mm, in particular 9 mm to 17 mm, preferably 10 mm to 14 mm.

[0072] The first tube element is distal to the second tube element. Preferably, the first tube element is proximal to the base element and the second tube element is proximal to the first tube element and distal to the filter element, i.e. between the first tube element and the filter element. At least one of the first tube element and the second tube element may comprise a central air passage. The cross section of the central air passage of the second tube element may be larger than the cross section of the central air passage of the first tube element. Preferably, at least one of the first tube element and the second tube element may comprise a hollow cellulose acetate tube. At least one of the first tube element and the second tube element may have a length of 6 mm to 10 mm, for example 8 mm.

[0073] The filter element preferably functions as a mouthpiece or as part of a mouthpiece together with a second tube element. As used herein, the term "mouthpiece" refers to the portion of the article through which the aerosol exits the aerosol-generating article. The filter element may have a length of 10 millimeters to 14 millimeters, for example 12 millimeters.

[0074] All of the aforementioned elements may be disposed sequentially along the length axis of the article in the order described above, with the substrate element preferably disposed at the distal end of the article and the filter element preferably disposed at the proximal end of the article. All of the aforementioned elements may have the same external cross-sectional shape and / or dimensions.

[0075] In addition, the elements may be surrounded by one or more outer wrappers, such as to hold the elements together and to maintain the desired cross-sectional shape of the rod-like article. Preferably, the wrapper is made of paper. Alternatively, the wrapper may be made of a metal foil, such as aluminum foil, to provide a cylindrical waveguide structure as described above. The wrapper may further comprise an adhesive that bonds the overlapping free ends of the wrapper to one another.

[0076] Further, as noted above, the distal substrate portion of the article preferably has a greater static dielectric constant than the more proximal portion of the aerosol-generating article to prevent or at least reduce leakage of the microwave field beyond the proximal end of the distal substrate portion.

[0077] As a result, the aerosol-generating article may comprise a proximal portion proximally adjacent to the distal portion, the average static dielectric constant of the proximal portion being lower than the static dielectric constant of the aerosol-forming substrate contained within the distal substrate portion. Similarly, the material of the proximal portion axially facing the distal substrate portion may have a static dielectric constant lower than the static dielectric constant of the aerosol-forming substrate contained within the distal substrate portion. For example, the proximal portion may comprise at least the first tube element described above. In addition, the proximal portion may comprise at least one of the second tube element and the filter element described above. The material of the proximal portion axially facing the distal substrate portion may be the material of the first tube element, e.g., cellulose acetate, and / or air within the interior space of the first tube element. Similarly, the average static dielectric constant of the proximal portion may be given by the average value of the static dielectric constants of cellulose acetate and air, preferably weighted by their respective mass or volume fractions within the first tube element.

[0078] The (weighted) average static dielectric constant of the proximal portion may be in the range of 1 to 1.5. Similarly, the material of the proximal portion axially facing the distal substrate portion may have a static dielectric constant in the range of 1 to 1.5.

[0079] As already mentioned above with respect to the aerosol-generating device, the aerosol-forming substrate may have a static dielectric constant in the range of 2 to 2.5, in particular 2.3 to 2.4, preferably 2.3 to 2.35.

[0080] Preferably, the aerosol-forming substrate may have a dielectric loss tangent (tan δ) in the range of 0.05 to 0.2, particularly 0.1 to 0.15, for example about 0.11. The dielectric loss tangent (tan δ) of a material quantitatively indicates the dissipation of electrical energy due to different physical processes such as electrical conduction, dielectric relaxation, dielectric resonance, and losses from nonlinear processes.

[0081] When the microwave cavity has a circular cross-sectional shape, the microwave generator may be configured to generate a microwave signal having a frequency above a cutoff frequency f_cutoff given by the following formula: [f_cutoff=(2.405*c) / (π*D*Sqrt(ε_r))];

[0082]

number

[0083] where D is the inner diameter of the microwave cavity, c is the speed of light in vacuum, and ε_r is the static relative dielectric constant of the aerosol-forming substrate. Advantageously, this condition corresponds to the TM mode, which is the preferred mode for microwave heating in a circular cylindrical microwave cavity due to its rotational symmetry. 01 This ensures that the mode can propagate into the "filled" microwave cavity.

[0084] Similarly, if the microwave cavity has a rectangular cross-sectional shape, the microwave generator may be configured to generate a microwave signal having a frequency above a cutoff frequency f_cutoff given by the following equation:

number

[0085] where a and b are the side lengths of the rectangular cylindrical microwave cavity, c is the speed of light in vacuum, and ε_r is the static relative dielectric constant of the aerosol-forming substrate. This condition can be expressed, for example, by the TE 11 This allows microwave coupling of the mode into a "filled" rectangular cylindrical microwave cavity.

[0086] Further features and advantages of the aerosol generating system and aerosol generating article according to the invention have already been described above in relation to the aerosol generating device and therefore apply equally.

[0087] The present invention is defined in the claims. However, below is provided a non-exhaustive list of non-limiting examples. Any one or more of the features of these examples may be combined with any one or more features of the other examples, embodiments, or aspects described herein.

[0088] Example 1: An aerosol generating apparatus for generating an aerosol by microwave heating of an aerosol-forming substrate contained within a distal substrate portion of a cylindrical aerosol-generating article, comprising: a microwave generator configured to generate a microwave signal; and a cylindrical microwave cavity configured to removably receive at least a distal substrate portion of an aerosol-generating article; and - an aerosol generating device comprising: a coaxial feed exciter operably connected to a microwave generator and coupled to the microwave cavity, such as to feed a microwave signal into the microwave cavity and excite at least one specific transverse magnetic mode or transverse electric mode within the microwave cavity (when a substrate portion of the article is received within the microwave cavity).

[0089] Example 2: An aerosol generating device according to Example 1, wherein the cylindrical microwave cavity is a circular cylindrical microwave cavity or a rectangular cylindrical microwave cavity.

[0090] Example 2a: An aerosol generating device according to any one of the preceding examples, wherein the microwave cavity has at least one closed axial end, in particular a distal closed end.

[0091] Example 2b: An aerosol generating device according to any one of the preceding examples, wherein the microwave cavity has at least one open axial end, in particular the proximal open end.

[0092] Example 3: An aerosol generating device according to any one of the preceding examples, wherein the specific transverse magnetic mode is a TM01 mode.

[0093] Example 4: An aerosol generation device according to any one of the preceding examples, wherein the microwave cavity is a circular cylindrical microwave cavity and the microwave generator is configured to generate a microwave signal having a frequency below a frequency threshold f_thresh given by the following formula: f_thresh=(2.405*c) / (π*D), where D is the inner diameter of the microwave cavity and c is the speed of light in vacuum.

[0094] Example 5: An aerosol generation device according to any one of the preceding examples, wherein the microwave cavity is a circular cylindrical microwave cavity and wherein the microwave generator is configured to generate a microwave signal having a frequency above a cut-off frequency, the cut-off frequency having a value outside the range defined by the following formula: f_cutoff=(2.405*c) / (π*D*Sqrt(ε_r)), where D is the inner radius of the microwave cavity, c is the speed of light in vacuum and ε_r is a value in the range of 2 to 2.5, in particular 2.3 to 2.4, and preferably 2.3 to 2.35.

[0095] Example 6: An aerosol generating device according to any one of the preceding examples, wherein the microwave generator is configured to generate a microwave signal within a frequency range of 5 to 50 GHz, in particular 10 GHz to 40 GHz, preferably 12 GHz to 18 GHz, or 20 GHz to 30 GHz, for example 24 GHz to 24.25 GHz.

[0096] Example 7: An aerosol generating device according to any one of the preceding examples, wherein the microwave cavity has a non-conductive inner surface or at least a portion of the inner surface of the microwave cavity, in particular the inner surface at the closed axial end of the microwave cavity, in particular the closed distal end, and at least one of the inner surfaces along the inner circumference of the microwave cavity, are conductive.

[0097] Example 8: An aerosol generating device according to any one of the preceding examples, further comprising a non-conductive hollow cylindrical, specifically a circular hollow cylindrical filler disposed within the microwave cavity, wherein the outer peripheral surface of the hollow cylindrical, specifically the circular hollow cylindrical filler, is in contact with the inner surface of the microwave cavity along the inner circumference of the microwave cavity, and said inner space of the hollow cylinder, specifically the circular hollow cylindrical filler, provides a receiving chamber configured to removably receive at least a base portion of an aerosol-generating article.

[0098] Example 9: An aerosol generating device according to any one of the preceding examples, wherein the coaxial feed exciter comprises a coaxial line having an inner conductor surrounded by a concentric outer conductor shield, the cavity side end portion of the inner conductor extending beyond the cavity side end of the outer conductor shield into the microwave cavity to form an excitation probe.

[0099] Example 10: An aerosol generating device according to Example 9, wherein the length of the excitation probe extending beyond the cavity side end of the outer conductor shield into the microwave cavity is in the range of 1 millimeter to 8 millimeters, specifically 1 millimeter to 4 millimeters, preferably 1 millimeter to 2 millimeters.

[0100] Example 11: The aerosol generating device according to Example 9 or Example 10, wherein the diameter of the excitation probe is within the range of 1 mm to 2 mm, specifically 1.2 mm to 1.8 mm, preferably 1.4 mm to 1.7 mm.

[0101] Example 12: An aerosol generating device according to any one of Examples 9 to 11, wherein the cavity side end of the outer conductive shield is connected to the conductive inner surface of the microwave cavity.

[0102] Example 13: An aerosol generating device according to any one of the preceding examples, wherein the microwave cavity is a circular cylindrical microwave cavity and the diameter of the microwave cavity is in the range of 2 millimeters to 15 millimeters, particularly 4 millimeters to 12 millimeters, preferably 6 millimeters to 9 millimeters.

[0103] Example 14: An aerosol generating device according to any one of the preceding examples, wherein the length of the distal portion of the microwave cavity configured to receive the base portion of the aerosol-generating article is in the range of 5 millimeters to 20 millimeters, particularly 9 millimeters to 17 millimeters, preferably 10 millimeters to 14 millimeters.

[0104] Example 15: An aerosol generating device according to any one of the preceding examples, wherein the microwave generator comprises a magnetron.

[0105] Example 16: An aerosol generating device according to any one of the preceding examples, wherein the microwave generator comprises a microwave amplifier.

[0106] Example 17: An aerosol generating device according to any one of the preceding examples, wherein the microwave generator is configured to generate a microwave signal having a power output in the range of 1 Watt to 10 Watts, particularly 1.5 Watts to 10 Watts, preferably 2 Watts to 7 Watts.

[0107] Example 18: An aerosol generation system comprising an aerosol generating device according to any one of the preceding examples and a cylindrical aerosol-generating article comprising a distal substrate portion containing an aerosol-forming substrate.

[0108] Example 19: An aerosol-generating system according to Example 18, wherein the aerosol-generating article comprises a proximal portion proximally adjacent to the distal portion, and the material of the proximal portion axially facing the distal substrate portion has a static dielectric constant lower than the static dielectric constant of the aerosol-forming substrate contained within the distal substrate portion.

[0109] Example 20: An aerosol generating system according to example 19, wherein the material of the proximal portion axially facing the distal substrate portion has a static dielectric constant in the range of 1 to 1.5.

[0110] Example 22: An aerosol-generating system according to any one of Examples 18 to 20, wherein the aerosol-forming substrate has a static relative dielectric constant in the range of 2 to 2.5, specifically 2.3 to 2.4, preferably 2.3 to 2.35.

[0111] Example 22: An aerosol generation system according to any one of Examples 18 to 21, wherein the microwave generator is configured to generate a microwave signal having a frequency above a cut-off frequency f_cutoff given by the following formula: f_cutoff=(2.405*c) / (π*D*Sqrt(ε_r)), where D is the inner diameter of the microwave cavity, c is the speed of light in vacuum, and ε_r is the static dielectric constant of the aerosol-forming substrate.

[0112] Example 23: An aerosol generating system according to any one of Examples 18 to 22, wherein the distal substrate portion has a length (along the length axis of the cylindrical aerosol generating article) in the range of 5 millimeters to 20 millimeters, particularly 9 millimeters to 17 millimeters, preferably 10 millimeters to 14 millimeters.

[0113] The embodiments will now be further described with reference to the figures. [Brief description of the drawings]

[0114] [Figure 1] FIG. 1 illustrates generally in cross-section an exemplary embodiment of an aerosol generation system according to the present invention comprising an aerosol generating device and an aerosol-generating article. [Diagram 2] FIG. 2 shows an aerosol generating device of the aerosol generating system according to FIG. 1 without the aerosol-generating article. [Diagram 3] FIG. 3 shows details of the microwave cavity and coaxial feed exciter of the apparatus shown in FIGS. [Figure 4] FIG. 4 shows a schematic perspective view of the microwave cavity and coaxial feed exciter of the apparatus shown in FIGS. [Diagram 5] FIG. 5 shows the cutoff frequency f_cutoff of the TE11 mode in a circular waveguide structure as a function of the waveguide radius D / 2 (half the diameter D) for different values ​​of the dielectric permeability ε_r. [Figure 6] FIG. 6 shows the cutoff frequency f_cutoff of the TM01 mode in a circular waveguide structure as a function of the waveguide radius D / 2 (half the diameter D) for different values ​​of the dielectric permeability ε_r. [Figure 7] FIG. 7 illustrates an aerosol generation system according to an alternative embodiment of the present invention, comprising an aerosol generating device having a hollow cylindrical filler disposed within a microwave cavity. [Figure 8] FIG. 8 shows an aerosol generating system according to yet another embodiment of the invention comprising an aerosol generating device having an inner surface of a microwave cavity that is non-conductive, and an aerosol-generating article having an aerosol-forming substrate that includes a conductive outer shell. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0115] 1 and 2 illustrate generally an exemplary embodiment of an aerosol-generating system 1 according to the present invention configured to generate an inhalable aerosol by microwave heating of an aerosol-forming substrate 92. The system 1 comprises an aerosol-generating article 90 containing the aerosol-forming substrate 92 to be heated, and an aerosol-generating device 10 including a microwave heating arrangement for heating the substrate 92 upon engagement of the article 90 with the device 10.

[0116] As shown in FIG. 1, the aerosol-generating article 90 is a cylindrical article having a substantially rod-like shape resembling that of a conventional cigarette. In this embodiment, the article 90 comprises four elements: a substrate element 91, a first tube element 93, a second tube element 94, and a filter element 95, which are sequentially arranged along the length axis of the article 90 in a coaxial arrangement. The four elements 91, 93, 94, 95 are surrounded by an outer wrapper 99 to hold the four elements together and maintain the desired circular cross-sectional shape of the rod-like article 90. The wrapper 99 is preferably made of paper. Details of these elements have already been further described above. The filter element 95 is arranged at the proximal end, which functions as a mouthpiece. The substrate element 91 is arranged at the distal end of the article 90 and includes an aerosol-forming substrate 92, which is also heated. As a result, the substrate element 91 may be considered to form a distal substrate portion 97 of the article 90. Conversely, first tube element 93 , second tube element 94 , and filter element 95 may be considered to form a proximal portion 98 of article 90 .

[0117] The elongated aerosol generating device 10 comprises two sections: a proximal section 12 and a distal section 13. In the proximal section 12, the device 10 comprises a cylindrical microwave cavity 30 for receiving at least a distal substrate portion 97 of an aerosol-generating article 90. The microwave cavity 30 has a closed distal end 36 and a proximal open end 35 providing an insertion opening for inserting the article 90 into the microwave cavity 30. In the distal section 13, the device 10 comprises a DC power source 60, such as a rechargeable battery, for powering operation of the device, and a microwave generator 20 configured to generate a microwave signal. The microwave generator 20 preferably comprises at least one magnetron as a source of microwave signal, and a microwave amplifier for providing a desired output sufficient to heat the aerosol-forming substrate 92 when the article 90 is received within the microwave cavity 30.

[0118] To feed a microwave signal into the microwave cavity 30, the aerosol generating device 10 further comprises a coaxial feed exciter 40 operatively connected to the microwave generator 20 and coupled to the microwave cavity 30 via a feed-in opening 33 in the closed distal end 36 of the microwave cavity 30. The geometry and feed-in location of the coaxial feed exciter 40 are selected to feed a microwave signal into the microwave cavity 30 to excite at least one particular transverse magnetic or transverse electric mode in the microwave cavity 30 when the substrate portion 97 of the article 90 is received within the microwave cavity 30. Further details of the microwave generator 20, the coaxial feed exciter 40, and the microwave cavity 30 are described further below.

[0119] The microwave generator 20, the coaxial feed exciter 40, and the microwave cavity 30 together form part of a microwave heating arrangement for heating the aerosol-forming substrate 92 in the distal substrate portion 97 upon insertion of the article 90 into the device 10. Details of this microwave heating arrangement, specifically the coaxial feed exciter 40 and the microwave cavity 30, are illustrated in Figures 3 and 4. As can be seen specifically in Figures 2, 3, and 4, the microwave cavity 30 according to this embodiment has a circular cylindrical shape. The cavity 30 is formed by a circular cylindrical sleeve 31 and a bottom portion 32 disposed within a proximally open chamber formed within the proximal portion 12 of the device 10, more specifically within the device housing 11. The sleeve 31 forms a sidewall and the bottom portion 32 forms a closed distal end 36 of the microwave cavity 30. The surrounding device housing 11 is preferably made of plastic, while the microwave cavity 30, specifically the sleeve 31 and bottom portion 32, are made of a metal such as stainless steel or aluminum. Thus, the inner surface of the microwave cavity 30 at the closed distal end 36 and along the inner circumference of the microwave cavity 30 is electrically conductive and therefore reflective to microwaves. As a result, the microwave cavity 30, specifically the circular cylindrical sleeve 31, provides a circular waveguide structure that supports microwave propagation along the axial direction of the cylindrical microwave cavity 30.

[0120] According to the invention, a waveguide structure is used to realize a hollow resonator configuration for heating the substrate 92 in the distal substrate portion 97 of the aerosol-generating article 90. For this purpose, the dimensions of the microwave cavity 30, the frequency of the microwave signal provided as well as the dielectric permeability of the aerosol-forming substrate 92 in the distal substrate portion 97 and the dielectric permeability of the material in the proximal portion 98 of the article 90, in particular the material of the first tube element 93, are chosen such that on the one hand the frequency of the microwave signal is above the cut-off frequency for microwave propagation in the distal part 37 of the microwave cavity 30, which is filled by the distal substrate portion 97, but on the other hand is below the cut-off frequency for microwave propagation in the proximal portion 38 of the microwave cavity 30, which receives a part of the proximal portion 98 of the article 90.

[0121] The two lowest transverse modes, i.e., TE, of a circular waveguide structure having a diameter D and filled with a medium having a dielectric permeability ε_r are 11 Modes and TMs 01 The cutoff frequency f_cutoff for a mode is given by the following equation:

[0122]

number

number

[0123] 5 and 6, on the one hand, TE 11 mode (see Fig. 5) on the one hand, and TM 01 5 and 6 show the cut-off frequency f_cutoff of a circular waveguide structure as a function of the waveguide radius D / 2 (half the diameter D) for different values ​​of the dielectric permeability ε_r for the mode (see FIG. 6). The value of ε_r=2.35 corresponds to a typical value of the dielectric permeability of the aerosol-forming substrate 92, while the dielectric permeability of the material in the proximal portion 98 of the article 90, specifically the material of the first tube element 93, is less than 1.5, specifically close to 1. With that in mind, the graphs of FIGS. 5 and 6 show, for example, a TE 11 Modes and TMs 01It is illustratively shown that both of the modes propagate through those portions of the microwave cavity 30 that are filled with the aerosol-forming substrate 92 in use, i.e., the distal portion 37 of the microwave cavity 30. This is because, for a dielectric permeability of ε_r=2.3 (the dielectric permeability of the aerosol-forming substrate 92), the operating frequency of 24 GHz is above the respective cutoff frequency, f_cutoff, of both modes. In contrast, in an empty microwave cavity 30, or those portions of the microwave cavity 30 that are filled with the proximal portion 98 of the article 90 in use, i.e., the proximal portion 38 of the microwave cavity 30, the waveguide structure has a TE 11 Modes and TMs 01 modes cannot be supported because the operating frequency of 24 GHz is below the respective cutoff frequencies, f_cutoff, of both modes for values ​​of ε_r close to 1, which corresponds to the dielectric permeability of air, and hence of the empty cavity 30, or of the proximal portion 98 of the article 90.

[0124] Thus, in use, when the aerosol-generating article 90 is received within the microwave cavity 30, TE 11 Modes and TMs 01can propagate through the distal substrate portion 97 of the article 90. Nevertheless, at the interface between the distal substrate portion 97 and the proximal portion 98 of the article 90, a change in dielectric permeability prevents the microwaves from propagating further beyond the proximal end of the substrate portion 97 (exponentially decaying evanescent waves). Instead, the microwaves are reflected back in the distal direction. The same effect occurs at the distal end of the substrate portion 97, where a similar change in dielectric permeability may occur due, for example, to a small air pocket between the closed distal end 36 of the microwave cavity 30 and the distal end of the article 90, which drops to a lower value when received within the microwave cavity 30, specifically to the dielectric permeability of air. Finally, the inner surface of the microwave cavity 30 at the closed distal end 36 is conductive and therefore causes the reflection of the microwaves back in the proximal direction. As a result, microwaves fed into the base portion 97 when the article 90 is received within the microwave cavity 30 experience reflections at both ends of the base portion 97 which effectively corresponds to a resonator configuration.

[0125] Conversely, when no article is received within the microwave cavity 30, the TE passes through an empty microwave cavity 30 (filled with air only). 11 Modes and TMs 01 propagation is unsupported, and therefore leakage of the microwave field from the proximal open end 35 of the microwave cavity 30 is substantially suppressed.

[0126] As mentioned above, the frequency of the microwave signal is preferably chosen to be within the 24-24.25 GHz ISM (Industrial, Scientific and Medical) radio band, which can be used by industrial, scientific and medical high frequency devices in domestic and similar areas, license-free and mostly without authorization. This frequency range is also well suited to meet the above conditions for reasonable dimensions of the microwave cavity in a handheld device and reasonable dimensions of the aerosol-generating article resembling the shape and dimensions of a conventional cigarette. In this embodiment, the diameter D of the microwave cavity 30 is about 7 millimeters, while the length of the distal portion 37 of the microwave cavity 30 for receiving the distal base portion 97 of the article 90 is about 12 millimeters. As a result, the diameter of the article 90 (at least within the base portion 97) is preferably slightly smaller than 7 millimeters, and the length of the base portion 97 is also about 12 millimeters. As shown in Figures 1 and 2, the microwave cavity 30 in this embodiment extends to the proximal end of the device 10. Alternatively, the microwave cavity may be shorter and may extend only to the proximal end of the distal portion 37 of the cavity 30 shown in Figures 1 and 2. That is, such a microwave cavity would not have a proximal portion 38, as does the microwave cavity 30 shown in Figures 1 and 2.

[0127] As explained above, the microwave cavity 30 of this embodiment has a TE of 100 MHz for a microwave signal frequency of about 24 GHz and a waveguide diameter D / 2 of 3.5 millimeters. 11 Modes and TMs 01 However, the TM 01The mode is more favorable for heating the aerosol-forming substrate 92 because it is rotationally symmetric and therefore provides more uniform heating. To select this particular mode, the structure, shape, and location of the coaxial feed exciter 40 can be advantageously used to determine the mode spectrum that is coupled into the microwave cavity 30. As a result, the coaxial feed exciter 40 of this embodiment comprises a coaxial line having an inner conductor 41 surrounded by a concentric outer conductor shield 42. A cavity end portion 44 of the inner conductor 41 extends beyond the cavity end 45 of the outer conductor shield 42 into the microwave cavity 30 to form an excitation probe 48. In this embodiment, the length 47 of the excitation probe 48 that extends beyond the cavity end 45 of the outer conductor shield 42 into the microwave cavity 30 is preferably in the range of 1 millimeter to 2 millimeters. Similarly, the diameter of the excitation probe 48 is preferably in the range of 1.4 millimeter to 1.7 millimeters. These dimensions of the coaxial feed exciter 40 have been found to advantageously provide optimal conditions for coupling the microwave signal from the microwave generator 20 into the microwave cavity 30. In particular, these dimensions help to reduce the reflection coefficient of the waveguide structure provided in the microwave cavity 30. Hence, higher heating temperatures may be achieved.

[0128] As can be seen from Figures 3 and 4, the coaxial feed exciter 40 is coupled to the microwave cavity 30 via a feed-in opening 33 in the closed distal end 36 of the microwave cavity 30, more specifically, via a feed-in opening 33 in the bottom portion 32 of the microwave cavity 30. The coaxial feed exciter 40 is disposed coaxially into the cylindrical microwave cavity 30. That is, the central axis of the coaxial feed exciter 40 coincides with the central axis 39 of the cylindrical microwave cavity 30. The coaxial feed exciter 40 may further comprise a flange 43 for mounting a distal outer surface of the bottom portion 32 of the microwave cavity 30. As can be further seen, the cavity side end 45 of the outer conductive shield 42 is connected to the conductive inner surface of the microwave cavity 30. Advantageously, this increases the coupling efficiency. Preferably, the coaxial feed exciter 40 , specifically the inner conductor 41 , the outer conductor shield 42 , and the flange 43 are made of the same material as the microwave cavity 30 , specifically the sleeve 31 and the bottom portion 32 .

[0129] As can be seen from the above formulas, the specific values ​​of the cut-off frequency and frequency threshold for the TE and TM modes depend, among other things, on the diameter of the circular waveguide structure in which the substrate part to be heated is received snugly. If it is desired to operate at a frequency lower than the cut-off frequency for a given diameter of the substrate part, the diameter of the microwave cavity can be increased and the empty space between the outer surface of the article and the inner surface of the microwave cavity can be filled with a filling agent. This is illustrated in FIG. 7, which shows an aerosol generating system 101 according to an alternative embodiment of the invention. The system 101 comprises an aerosol generating device 110 very similar to the device shown in FIGS. 1 and 2. Therefore, identical or similar features are indicated with the same reference numbers, but still with the addition of 100. In contrast to the device 10 shown in FIGS. 1 and 2, the microwave cavity 130 of the device 110 shown in FIG. 7 has a larger diameter, while the article 190 used with this device 110 is the same as the article 90 shown in FIG. 1. To fill the empty space between the outer surface of the article 190 and the inner surface of the microwave cavity 130 along the inner circumference of the sleeve 131, the apparatus 110 comprises a non-conductive circular hollow cylindrical filler 180 disposed within the microwave cavity 130. The outer circumferential surface of the circular hollow cylindrical filler 180 contacts the inner surface along the inner circumference of the sleeve 131. The inner space of the circular hollow cylindrical filler 180 provides a receiving chamber configured to removably receive at least the base portion 197 of the aerosol-generating article 190. Advantageously, this allows heating of an article of a given diameter at lower microwave frequencies by using a microwave cavity 130 having a diameter larger than the given diameter of the article 190.

[0130] FIG. 8 shows an aerosol-generating system 201 according to yet another embodiment of the invention. The system 201 comprises an aerosol-generating device 210, very similar to the device 10 and article 90 shown in FIG. 1, and an aerosol-generating article 290. Thus, identical or similar features are indicated with the same reference numbers, but with the addition of 200. In contrast to the device 10 shown in FIG. 1, the microwave cavity 230 of the device 210 shown in FIG. 8 has a non-conductive inner surface along the inner circumference of the microwave cavity 230. Instead, the article 290 comprises a conductive shell 296 (represented by a dotted line) on the base portion 297, which is circumferentially disposed around the aerosol-forming substrate 292, but not at the axial ends of the base portion 297. In this embodiment, the conductive shell 296 is a metallic wrapper wound around the aerosol-forming substrate 292. That is, in this embodiment, the waveguiding structure is provided by the conductive shell 296, rather than by the inner surface along the inner circumference of the microwave cavity 230. Nevertheless, the inner surface of the microwave cavity 230 at the closed distal end 236 is preferably conductive to support the reflection of microwaves in the proximal direction. In principle, it is also possible that the inner surface of the microwave cavity 230 along the inner circumference of the microwave cavity 230 is still non-conductive while having a conductive circumferential shell 296 around the aerosol-forming substrate 292. Instead of a conductive outer shell surrounding the aerosol-forming substrate 292 only along the circumference of the article 290, the article may comprise a conductive outer shell arranged circumferentially around the aerosol-forming substrate 292 and at both axial ends of the substrate portion 297, i.e. completely enclosing the aerosol-forming substrate 292 within the substrate portion 297. In this case, the entire inner surface of the microwave cavity 230 may be non-conductive. Nevertheless, the coaxial feed exciter should be arranged and configured to feed the microwave signal into the interior of the completely enclosing outer shell when the article is received in the microwave cavity.

[0131] For purposes of this specification and the appended claims, unless otherwise indicated, all numbers expressing amounts, quantities, percentages, and the like are understood to be modified in all instances by the term "about." Also, all ranges include the maximum and minimum points disclosed, and include any intermediate ranges therein, which may or may not be specifically recited herein. Thus, in this context, the number A is understood as A±5%. Within this context, the number A may be considered to include a numerical value that is within the general standard error for the measurement of the property that the number A modifies. The number A may, in some instances used in the appended claims, deviate by the percentages recited above, provided that the amount by which A deviates does not materially affect the basic and novel properties of the claimed invention. Also, all ranges include the maximum and minimum points disclosed, and include any intermediate ranges therein, which may or may not be specifically recited herein.

Claims

1. An aerosol generating device for generating aerosols by microwave heating of an aerosol-forming substrate contained within the base portion of a cylindrical aerosol generating article, - A microwave generator configured to generate microwave signals, - A cylindrical microwave cavity configured to removably receive at least the base portion of the aerosol generating article, - An aerosol generator comprising a coaxial feed exciter operably connected to and coupled to the microwave cavity, which feeds the microwave signal into the microwave cavity and excites at least one specific transverse magnetic mode or transverse electrical mode within the microwave cavity when the substrate portion of the article is received within the microwave cavity.

2. The aerosol generator according to claim 1, wherein the cylindrical microwave cavity is a circular cylindrical microwave cavity or a rectangular cylindrical microwave cavity.

3. The aerosol generator according to any one of claims 1 to 2, wherein the microwave cavity is a circular cylindrical microwave cavity, and the microwave generator is configured to generate a microwave signal having a frequency below a frequency threshold f_thresh given by the following formula: f_thresh = (2.405 * c) / (π * D), where D is the inner diameter of the microwave cavity and c is the speed of light in a vacuum.

4. The aerosol generator according to claim 1, wherein the microwave cavity is a circular cylindrical microwave cavity, and the microwave generator is configured to generate a microwave signal having a frequency above the cutoff frequency, and the cutoff frequency has a value outside the range defined by the following formula: f_cutoff = (2.405 * c) / (π * D * Sqrt(ε_r)), where D is the inner diameter of the microwave cavity, c is the speed of light in a vacuum, and ε_r is a value in the range of 2 to 2.5, specifically 2.3 to 2.4, and preferably 2.3 to 2.

35.

5. The aerosol generator according to claim 1, wherein the microwave generator is configured to generate microwave signals within a frequency range of 5 to 50 GHz, specifically 10 GHz to 40 GHz, preferably 12 GHz to 18 GHz, or 20 GHz to 30 GHz, for example, 24 GHz to 24.25 GHz.

6. The aerosol generator according to claim 1, wherein the microwave cavity has a non-conductive inner surface, or at least a portion of the inner surface of the microwave cavity, specifically the inner surface at the axial end of the closure of the microwave cavity, specifically the distal end of the closure, and the inner surface along the inner circumference of the microwave cavity are conductive.

7. The aerosol generating apparatus according to claim 1, further comprising a non-conductive hollow cylindrical filler, specifically a circular hollow cylindrical filler disposed within the microwave cavity, wherein the outer surface of the hollow cylindrical, specifically the circular hollow cylindrical filler, is in contact with the inner surface of the microwave cavity along the inner circumference of the microwave cavity, and the internal space of the hollow cylindrical, specifically the circular hollow cylindrical filler, is configured to removably receive at least the base portion of the aerosol generating article in a receiving chamber.

8. The aerosol generator according to claim 1, wherein the coaxial feed exciter comprises a coaxial cable having an internal conductor surrounded by a concentric external conductor shield, and the cavity-side end portion of the internal conductor extends beyond the cavity-side end of the external conductor shield into the microwave cavity to form an excitation probe.

9. The aerosol generator according to claim 8, wherein the length of the excitation probe extending into the microwave cavity beyond the cavity side end of the outer conductor shield is in the range of 1 mm to 8 mm, specifically 1 mm to 4 mm, preferably 1 mm to 2 mm, and / or the diameter of the excitation probe is in the range of 1 mm to 2 mm, specifically 1.2 mm to 1.8 mm, preferably 1.4 mm to 1.7 mm.

10. The aerosol generator according to claim 1, wherein the microwave cavity is a circular cylindrical microwave cavity, and the diameter of the microwave cavity is in the range of 2 mm to 15 mm, specifically 4 mm to 12 mm, preferably 6 mm to 9 mm.

11. An aerosol generating system comprising an aerosol generating device according to claim 1, and a cylindrical aerosol generating article having a distal substrate portion containing an aerosol forming substrate.

12. The aerosol generating system according to claim 11, wherein the aerosol generating article comprises a proximal portion adjacent to the distal portion, and the material of the proximal portion facing the distal substrate portion in the axial direction has a static dielectric constant lower than the static dielectric constant of the aerosol forming substrate contained in the distal substrate portion.

13. The aerosol generating system according to claim 12, wherein the material of the proximal portion facing the distal base portion in the axial direction has a static relative permittivity in the range of 1 to 1.

5.

14. The aerosol generating system according to claim 11, wherein the aerosol-forming substrate has a static relative permittivity in the range of 2 to 2.5, specifically 2.3 to 2.4, preferably 2.3 to 2.

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15. The aerosol generating system according to claim 11, wherein the microwave generator is configured to generate a microwave signal having a frequency exceeding the cutoff frequency f_cutoff, which is given by the following formula: f_cutoff = (2.405 * c) / (π * D * Sqrt(ε_r)), where D is the inner diameter of the microwave cavity, c is the speed of light in a vacuum, and ε_r is the static relative permittivity of the aerosol forming substrate.

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