HEATER FOR AEROSOL-FORMING SUBSTRATE COMPRISING A POSITIVE TEMPERATURE COEFFICIENT THERMISTOR

Abstract

A heater is provided for heating an aerosol-forming substrate, the heater including: a heating element to heat the aerosol-forming substrate, the heating element including at least one positive temperature coefficient (PTC) thermistor, the PTC thermistor being configured to be supplied with an electric current so as to heat the PTC thermistor, in which a resistance of the PTC thermistor increases when a temperature of the PTC thermistor increases within a stabilised temperature range, a lower end of the stabilised temperature range being, when a constant voltage is applied to the PTC thermistor, a reference temperature at which the resistance of the PTC thermistor is twice a value of a minimum resistance of the PTC thermistor, and in which the reference temperature is between about 100° C. and about 350° C. when a constant voltage of 3.3V is applied to the PTC thermistor.

Claims

1.-15. (canceled)

16. A heater for heating an aerosol-forming substrate, the heater comprising: a heating element configured to heat the aerosol-forming substrate, the heating element comprising at least one positive temperature coefficient (PTC) thermistor, the at least one PTC thermistor being configured to be supplied with an electric current so as to heat the at least one PTC thermistor, wherein a resistance of the at least one PTC thermistor increases when a temperature of the at least one PTC thermistor increases within a stabilised temperature range, a lower end of the stabilised temperature range being, when a constant voltage is applied to the at least one PTC thermistor, a reference temperature at which the resistance of the at least one PTC thermistor is twice a value of a minimum resistance of the at least one PTC thermistor, and wherein the reference temperature is between about 100 degrees Celsius and about 350 degrees Celsius when a constant voltage of 3.3 Volts is applied to the at least one PTC thermistor.

17. The heater according to claim 16, wherein the reference temperature is between about 200 degrees Celsius and about 250 degrees Celsius when a constant voltage of 3.3 Volts is applied to the at least one PTC thermistor.

18. The heater according to claim 16, wherein the heating element is further configured to be inserted into the aerosol-forming substrate.

19. The heater according to claim 16, wherein the heating element is further configured to heat an outer surface of the aerosol-forming substrate.

20. The heater according to claim 16, further comprising a cavity configured to receive the aerosol-forming substrate, wherein the heater is further configured to heat the aerosol-forming substrate when the aerosol-forming substrate is received in the cavity.

21. The heater according to claim 20, further comprising a heater housing, the heater housing comprising a peripheral portion extending in a transversal direction between a peripheral inner wall and a peripheral outer wall, and a bottom portion extending in a longitudinal direction between a bottom inner wall and a bottom outer wall, wherein the cavity extends longitudinally between an open end and the bottom inner wall, the cavity being delimited in the transversal direction by the peripheral inner wall.

22. The heater according to claim 21, wherein the at least one PTC thermistor comprises a PTC disk arranged within the bottom portion.

23. The heater according to claim 21, wherein the at least one PTC thermistor comprises a PTC tube arranged within the peripheral portion so as to circumscribe the peripheral inner wall.

24. The heater according to claim 21, wherein the peripheral outer wall comprises at least three planar sections, and wherein the at least one PTC thermistor comprises at least one PTC plate arranged on at least one of the at least three planar sections.

25. The heater according to claim 16, wherein the at least one PTC thermistor comprises a ceramic semiconductor.

26. The heater according to claim 25, wherein the ceramic semiconductor is barium titanate.

27. The heater according to claim 16, wherein the at least one PTC thermistor comprises a polymeric material.

28. An aerosol-generating device, comprising: the heater according to claim 16; a device housing; and a power supply electrically connected to the heating element to supply an electric current to the at least one PTC thermistor.

29. An aerosol-generating system, comprising: the aerosol-generating device according to claim 28; and an aerosol-generating article comprising the aerosol-forming substrate.

30. A method of operating the aerosol-generating system according to claim 29, the method comprising the steps of: determining a maximum operating temperature for the aerosol-forming substrate comprised in the aerosol-generating article; and supplying an electric current to the at least one PTC thermistor, by means of the power supply, the electric current having a constant voltage, such that a resistance of the at least one PTC thermistor increases when a temperature of the at least one PTC thermistor increases within a stabilised temperature range, a lower end of the stabilised temperature range being a reference temperature at which the resistance of the at least one PTC thermistor is twice a value of the minimum resistance of the at least one PTC thermistor, the constant voltage being such that the reference temperature of the PTC thermistor is substantially the maximum operating temperature for the aerosol-forming substrate.

31. A method of operating the aerosol-generating system according to claim 29, the method comprising the steps of: measuring puff intensity when a puff is drawn during use of the aerosol-generating system; and determining a puff intensity threshold, such that, when the puff intensity is equal to or above the puff intensity threshold, the method further comprises the additional steps of: determining a first maximum operating temperature and a second maximum operating temperature for the aerosol-forming substrate comprised in the aerosol-generating article, selecting the first maximum operating temperature or the second maximum operating temperature, if the first maximum operating temperature is selected, supplying an electric current to the at least one PTC thermistor, by means of the power supply, the electric current having a first constant voltage, such that a resistance of the at least one PTC thermistor increases when a temperature of the at least one PTC thermistor increases within a stabilised temperature range, a lower end of the stabilised temperature range being a reference temperature at which the resistance of the at least one PTC thermistor is twice a value of the minimum resistance of the at least one PTC thermistor, the first constant voltage being such that the reference temperature of the PTC thermistor is substantially the first maximum operating temperature for the aerosol-forming substrate, and if the second maximum operating temperature is selected, supplying an electric current to the at least one PTC thermistor, by means of the power supply, the electric current having a second constant voltage, such that the resistance of the at least one PTC thermistor increases when the temperature of the at least one PTC thermistor increases within a stabilised temperature range, the lower end of the stabilised temperature range being a reference temperature at which the resistance of the at least one PTC thermistor is twice the value of the minimum resistance of the at least one PTC thermistor, the second constant voltage being such that the reference temperature of the PTC thermistor is substantially the second maximum operating temperature for the aerosol-forming substrate.

Description

[0116] These and other features and advantages of the invention will become more evident in the light of the following detailed description of preferred embodiments, given only by way of illustrative and non-limiting example, in reference to the attached figures:

[0117] FIG. 1 shows a temperature/resistance graphic of a PTC thermistor comprised in a heating element.

[0118] FIG. 2 illustrates a longitudinal section of a heater comprising a heater housing and a PTC disk.

[0119] FIG. 3 depicts a longitudinal section of a heater comprising a heater housing and a PTC tube.

[0120] FIG. 4 represents a longitudinal section of a heater comprising a heater housing and an internal heating element.

[0121] FIG. 5 shows a perspective view of a heater housing which in turn comprises six planar sections.

[0122] FIG. 6 depicts a cross section of the heater housing of FIG. 5.

[0123] FIG. 7 is a representation of plurality of external electrical contacts.

[0124] FIG. 8 illustrates a perspective view of a heater comprising the heater housing of FIG. 5 and the plurality of external electrical contacts of FIG. 7.

[0125] FIG. 9 represents the temperature of a peripheral inner wall for four examples of the heater of FIG. 8.

[0126] FIG. 10 depicts an aerosol-generating system comprising an aerosol-generating article and an aerosol-generating device which in turn comprises the heater of FIG. 3.

[0127] FIG. 11 shows the aerosol-generating system of FIG. 10, in which the aerosol-generating-article is received in the cavity of the heater housing.

[0128] FIG. 12 illustrates an embodiment of aerosol-generating article.

[0129] FIG. 13 represents the evolution of the temperature of the peripheral inner wall and the temperature of the PTC tube for the heater of FIG. 3.

[0130] FIG. 14 depicts the evolution of the temperature of the peripheral inner wall and the temperature of the PTC disk for the heater of FIG. 2.

[0131] FIG. 15 shows three temperature/resistance graphics of a PTC thermistor comprised in a heating element when three different constant voltages are applied to the PTC thermistor.

[0132] FIG. 1 shows a temperature T/resistance R graphic of a PTC thermistor comprised in a heating element of a heater for heating an aerosol-forming substrate.

[0133] The PTC thermistor is heated when an electric current is supplied to the PTC thermistor. When the PTC thermistor is heated, the temperature T and the resistance R of the PTC thermistor vary according to the function represented in FIG. 1.

[0134] In particular, the PTC thermistor may be heated to a temperature TMR which corresponds to a minimum resistance MR of the PTC thermistor.

[0135] When the PTC thermistor is heated to temperatures T below the temperature corresponding to the minimum resistance TMR, the resistance R of the PTC thermistor slightly decreases when the temperature T of the PTC thermistor increases, according to the function of FIG. 1. In some PTC thermistors, the resistance R of the PTC thermistor remains substantially constant, at a resistance slightly above the minimum resistance MR of the PTC thermistor, until the minimum resistance MR of the PTC thermistor is reached at the temperature corresponding to the minimum resistance TMR.

[0136] Likewise, if the PTC thermistor is heated to a temperature T beyond the temperature corresponding to the minimum resistance TMR, the resistance R of the PTC thermistor increases when the temperature T of the PTC thermistor increases, according to the function of FIG. 1.

[0137] If the PTC thermistor is heated to a temperature beyond the temperature corresponding to twice the minimum resistance TMR, the increase in the resistance of the PTC thermistor when the temperature of the PTC thermistor increases is so significant that the temperature of the PTC thermistor is substantially stabilised at the temperature T corresponding to twice the minimum resistance MR. Such temperature is normally referred to as reference temperature CT of the PTC thermistor. Put another way, the PTC thermistor has a high positive temperature coefficient α within a stabilised temperature range delimited at the lower end by the reference temperature CT. In dielectric materials, the reference temperature CT may substantially correspond to the Curie temperature of the dielectric material.

[0138] Temperatures T substantially beyond the reference temperature CT may be reached if the PTC thermistor is supplied with electric current for a period of time long enough to reach the maximum resistance of the PTC thermistor. However, it should be taken into account that FIG. 1 shows the resistance R in a logarithmic scale. Hence, the period of time needed to reach such maximum resistance is generally substantially longer than a conventional operating time of the heater for heating an aerosol-forming substrate. This may ensure that the PTC thermistor is effectively stabilised at a temperature which does not significantly exceed the reference temperature CT.

[0139] FIG. 2 illustrates a heater 10 comprising a heater housing 20. The heater housing 20 comprises a peripheral portion 21 extending in the transversal direction between a peripheral inner wall 210 and a peripheral outer wall 211. The heater housing 20 comprises a bottom portion 22 extending in the longitudinal direction between a bottom inner wall 220 and a bottom outer wall 221. A cavity 23 for receiving the aerosol-forming substrate extends longitudinally between an open end 230 of the heater housing 20 and the bottom inner wall 220, the cavity 23 being delimited in the transversal direction by the peripheral inner wall 210. In the embodiment of FIG. 2, the heater comprises a heating element which is formed of a PTC disk 24 arranged within the bottom portion 22. When an electric current is supplied to the PTC disk 24, the temperature of the PTC disk 24 increases until it reaches the reference temperature of the PTC disk 24. If the supply of electric current is maintained after this instant, the temperature of the PTC disk 24 stabilises at a temperature which substantially corresponds to the reference temperature of the PTC disk 24. Thus, the peripheral inner wall 210 reaches a temperature that may not significantly differ from the temperature at which the PTC disk 24 stabilises. Hence, when the aerosol-forming substrate is received in the cavity 23, the aerosol-forming substrate may be heated to the temperature of the peripheral inner wall 210, such that an inhalable aerosol is formed.

[0140] FIG. 3 illustrates a heater 10 comprising a heater housing 20. The heater housing 20 comprises a peripheral portion 21 extending in the transversal direction between a peripheral inner wall 210 and a peripheral outer wall 211. The heater housing 20 comprises a bottom portion 22 extending in the longitudinal direction between a bottom inner wall 220 and a bottom outer wall 221. A cavity 23 for receiving the aerosol-forming substrate extends longitudinally between an open end 230 of the heater housing 20 and the bottom inner wall 220, the cavity 23 being delimited in the transversal direction by the peripheral inner wall 210. In the embodiment of FIG. 3, the heater comprises a heating element which is formed of a PTC tube 25 arranged within the peripheral portion 21. When an electric current is supplied to the PTC tube 25, the temperature of the PTC tube 25 increases until it reaches the reference temperature of the PTC tube 25. If the supply of electric current is maintained after this instant, the temperature of the PTC tube 25 stabilises at a temperature which substantially corresponds to the reference temperature of the PTC tube 25. Thus, the peripheral inner wall 210 reaches a temperature which substantially corresponds to the reference temperature of the PTC tube 25. Hence, when the aerosol-forming substrate is received in the cavity 23, the aerosol-forming substrate may be heated to the temperature which substantially corresponds to the reference temperature of the PTC tube 25, such that an inhalable aerosol is formed.

[0141] FIG. 4 illustrates a heater 10 comprising a heater housing 20. The heater housing 20 comprises a peripheral portion 21 extending in the transversal direction between a peripheral inner wall 210 and a peripheral outer wall 211. The heater housing 20 comprises a bottom portion 22 extending in the longitudinal direction between a bottom inner wall 220 and a bottom outer wall 221. A cavity 23 for receiving the aerosol-forming substrate extends longitudinally between an open end 230 of the heater housing 20 and the bottom inner wall 220, the cavity 23 being delimited in the transversal direction by the peripheral inner wall 210. In the embodiment of FIG. 4, the heater comprises a heating element which is formed of a PTC blade 27 extending longitudinally within the cavity 23, so that the PTC blade 27 is configured to pierce the aerosol-forming substrate when the substrate is received in the cavity 23. When electric current is supplied to the PTC blade 27, the temperature of the PTC blade 27 increases until it reaches the reference temperature of the PTC blade 27. If the supply of electric current is maintained after this instant, the temperature of the PTC blade 27 stabilises at a temperature which substantially corresponds to the reference temperature of the PTC blade 27. Thus, the PTC blade 27 may be used to heat the aerosol-forming substrate at substantially the reference temperature of the PTC blade, such that an inhalable aerosol is formed.

[0142] FIG. 5 depicts a perspective view of a heater housing 20. The heater housing 20 comprises a peripheral portion 21 extending in the transversal direction between a peripheral inner wall 210 and a peripheral outer wall 211. The peripheral outer wall 211 comprises six planar sections 2110, 2111, 2112, 2113, 2114, 2115, configured such that at least one PTC plate can be arranged on at least one planar section 2110, 2111, 2112, 2113, 2114, 2115. The PCT plate may be a round plate, square plate or a polygonal plate. The plates are planar. FIG. 6 is a representation of a cross section of the heater housing 20 of FIG. 5. A cavity 23 for receiving the aerosol-forming substrate extends longitudinally between an open end 230 and the bottom inner wall 220 (not represented in FIGS. 5 and 6), the cavity 23 being delimited in the transversal direction by the peripheral inner wall 210. In the embodiment of FIGS. 5 and 6, the cavity 23 delimited by the peripheral inner wall 210 is cylindrical, that is, the peripheral inner wall 23 has a circular cross section, as is shown in FIG. 5. Such shape may be convenient to receive a cylindrical aerosol-forming substrate.

[0143] In a preferred embodiment, the heater housing 20 of FIGS. 5 and 6 is provided with six PTC plates, one on each planar section 2110, 2111, 2112, 2113, 2114, 2115, thus forming a heater 10.

[0144] In an embodiment, the heater housing 20 comprises an electrically conductive material, such as an electrically conductive metal. The heater housing 20 then forms a first electrode configured to be in electrical contact with the six PTC plates.

[0145] The heater 10 may also comprise at least one external electrical contact 30 comprising an electrically conductive material, such as an electrically conductive metal, and forming a second electrode configured to be in electrical contact with the six PTC plates. FIG. 7 depicts the at least one external electrical contact 30 comprising six elongate external electrical contacts 310, 311, 312, 313, 314, 315, each one configured to be in electrical contact with a PTC plate disposed on a planar section 2110, 2111, 2112, 2113, 2114, 2115.

[0146] FIG. 8 illustrates the heater 10 comprising the heater housing 20 of FIGS. 5 and 6 and the elongate external electrical contacts 310, 311, 312, 313, 314, 315 of FIG. 7. Six PTC plates 260, 261, 262, 263, 264, 265 are provided, one on each planar section 2110, 2111, 2112, 2113, 2114, 2115. The six PTC plates 260, 261, 262, 263, 264, 265 form the heating element of the heater 10. The heater housing 20 acts as first electrode for the PTC plates 260, 261, 262, 263, 264, 265, since the PTC plates 260, 261, 262, 263, 264, 265 are in electrical contact with the planar sections 2110, 2111, 2112, 2113, 2114, 2115 of the heater housing 20. The elongate external electrical contacts 310, 311, 312, 313, 314, 315, which are in electrical contact with the PTC plates 260, 261, 262, 263, 264, 265, act as a second electrode for the PTC plates 260, 261, 262, 263, 264, 265.

[0147] When electric current is supplied to the first and second electrodes, the temperature of the PTC plates 260, 261, 262, 263, 264, 265 increases until the reference temperature of the PTC plates 260, 261, 262, 263, 264, 265 is reached, as shown in FIG. 1. After such instant, the temperature of the PTC plates 260, 261, 262, 263, 264, 265 stabilises substantially at the reference temperature of the PTC plates 260, 261, 262, 263, 264, 265 (or at a temperature slightly above the reference temperature) for periods of time that are normally longer than the operating time of an aerosol-generating device. This allows for a consistent and predictable heating profile of the aerosol-forming substrate when the substrate is received in the cavity 23, in which the maximum temperature of each of the PTC plates 260, 261, 262, 263, 264, 265 during the operating time can be determined and controlled by selecting the reference temperature of the PTC plates 260, 261, 262, 263, 264, 265. The PTC plates 260, 261, 262, 263, 264, 265 may have the same or a different reference temperature.

[0148] FIG. 9 represents the temperature of the peripheral inner wall 210 for four examples of the heater 10 of FIG. 8 in which the reference temperature of the six PTC plates 260, 261, 262, 263, 264, 265 is identical.

[0149] In the first example CT190, the reference temperature of the six PTC plates 260, 261, 262, 263, 264, 265 is 190 degrees Celsius. When an electric current is supplied to the first and second electrodes, the six PTC plates 260, 261, 262, 263, 264, 265 reach their reference temperature of 190 degrees Celsius after approximately 30 seconds and stabilise at a temperature slightly above the reference temperature. Heat is transferred through the heater housing 20 in such a way that the temperature of the inner wall 210 is substantially the same as the temperature of the six PTC plates 260, 261, 262, 263, 264, 265, that is, slightly above 190 degrees Celsius, as shown in FIG. 9. When an aerosol-forming substrate is received in the cavity 23 after the inner wall 210 has reached the temperature of substantially 190 degrees Celsius, this temperature is consistently applied to the aerosol-forming substrate during the operating time of the heater 10, thus forming an inhalable aerosol.

[0150] In the second example CT200, the reference temperature of the six PTC plates 260, 261, 262, 263, 264, 265 is 200 degrees Celsius. When an electric current is supplied to the first and second electrodes, the six PTC plates 260, 261, 262, 263, 264, 265 reach their reference temperature of 200 degrees Celsius after approximately 30 seconds and stabilise at a temperature slightly above the reference temperature. Heat is transferred through the heater housing 20 in such a way that the temperature of the inner wall 210 is substantially the same as the temperature of the six PTC plates 260, 261, 262, 263, 264, 265, that is, slightly above 200 degrees Celsius, as shown in FIG. 9. When an aerosol-forming substrate is received in the cavity 23 after the inner wall 210 has reached the temperature of substantially 200 degrees Celsius, this temperature is consistently applied to the aerosol-forming substrate during the operating time of the heater 10, thus forming an inhalable aerosol.

[0151] In the third example CT210, the reference temperature of the six PTC plates 260, 261, 262, 263, 264, 265 is 210 degrees Celsius. When an electric current is supplied to the first and second electrodes, the six PTC plates 260, 261, 262, 263, 264, 265 reach their reference temperature of 210 degrees Celsius after approximately 30 seconds and stabilise at a temperature slightly above the reference temperature. Heat is transferred through the heater housing 20 in such a way that the temperature of the inner wall 210 is substantially the same as the temperature of the six PTC plates 260, 261, 262, 263, 264, 265, that is, slightly above 210 degrees Celsius, as shown in FIG. 9. When an aerosol-forming substrate is received in the cavity 23 after the inner wall 210 has reached the temperature of substantially 210 degrees Celsius, this temperature is consistently applied to the aerosol-forming substrate during the operating time of the heater 10, thus forming an inhalable aerosol.

[0152] In the fourth example CT220, the reference temperature of the six PTC plates 260, 261, 262, 263, 264, 265 is 220 degrees Celsius. When an electric current is supplied to the first and second electrodes, the six PTC plates 260, 261, 262, 263, 264, 265 reach their reference temperature of 220 degrees Celsius after approximately 30 seconds and stabilise at a temperature slightly above the reference temperature. Heat is transferred through the heater housing 20 in such a way that the temperature of the inner wall 210 is substantially the same as the temperature of the six PTC plates 260, 261, 262, 263, 264, 265, that is, slightly above 220 degrees Celsius, as shown in FIG. 9. When an aerosol-forming substrate is received in the cavity 23 after the inner wall 210 has reached the temperature of substantially 220 degrees Celsius, this temperature is consistently applied to the aerosol-forming substrate during the operating time of the heater 10, thus forming an inhalable aerosol.

[0153] FIGS. 10 and 11 show schematic cross-sections of an aerosol-generating device 200 and an aerosol-generating article 300. The aerosol-generating device 200 and the aerosol-generating article 300 form an aerosol-generating system.

[0154] The aerosol-generating device 200 comprises a substantially cylindrical device housing 202, with a shape and size similar to a conventional cigar.

[0155] The aerosol-generating device 200 further comprises a power supply 206, in the form of a rechargeable nickel-cadmium battery, a PCB (printed circuit board) controller 208 including a microprocessor and a memory, an electrical connector 209 and a heater 10. In the embodiment of FIGS. 10 and 11, the heater 10 is similar to that of FIG. 3. However, other heaters may be used. In particular, the heaters of FIGS. 2, 4 and 8 may be used.

[0156] The power supply 206, controller 208 and heater 10 are all housed within the device housing 202. The heater 10 of the aerosol-generating device 200 is arranged at the proximal end of the device 200. The electrical connector 209 is arranged at a distal end of the device housing 202.

[0157] As used herein, the term “proximal” refers to a user end, or mouth end of the aerosol-generating device or aerosol-generating article, that is, the end of the aerosol-generating device or aerosol-generating article configured to be the closest to the user's mouth during normal use of the aerosol-generating device or aerosol-generating system comprising the aerosol-generating device and the aerosol-generating article. The proximal end of a component of an aerosol-generating device or an aerosol-generating article is the end of the component the closest to the user end, or mouth end of the aerosol-generating device or the aerosol-generating article. As used herein, the term “distal” refers to the end opposite the proximal end.

[0158] The controller 208 is configured to control the supply of power from the power supply 206 to the heater 10. The controller 208 further comprises a DC/AC inverter, including a Class-D power amplifier. The controller 208 is also configured to control recharging of the power supply 206 from the electrical connector 209. The controller 208 further comprises a puff sensor (not shown) configured to sense when a user is drawing on an aerosol-generating article received in the cavity 23.

[0159] As explained for FIG. 3, the heater 10 comprises a heater housing 20. The heater housing 20 comprises a peripheral portion 21 extending in the transversal direction between a peripheral inner wall 210 and a peripheral outer wall 211. The heater housing 20 comprises a bottom portion 22 extending in the longitudinal direction between a bottom inner wall 220 and a bottom outer wall 221. A cavity 23 for receiving the aerosol-forming substrate extends longitudinally between an open end 230 and the bottom inner wall 220, the cavity 23 being delimited in the transversal direction by the peripheral inner wall 210. A PTC tube 25 is arranged within the peripheral portion 21 so as to circumscribe the peripheral inner wall 210.

[0160] The device housing 202 also defines an air inlet 280 in close proximity to the distal end of the cavity 23 for receiving the aerosol-forming substrate. The air inlet 280 is configured to enable ambient air to be drawn into the device housing 202. Airflow pathways (not represented) are defined through the device 200 to enable air to be drawn from the air inlet 280 into the cavity 23.

[0161] The aerosol-generating article 300 is generally in the form of a cylindrical rod, having a diameter similar to the diameter of the peripheral inner wall 210. The aerosol-generating article 300 comprises a cylindrical cellulose acetate filter plug 304 and a cylindrical aerosol-generating segment 310 wrapped together by an outer wrapper 320 of cigarette paper.

[0162] The filter plug 304 is arranged at a proximal end of the aerosol-generating article 300, and forms the mouthpiece of the aerosol-generating system, on which a user draws to receive aerosol generated by the system.

[0163] The aerosol-generating segment 310 is arranged at a distal end of the aerosol-generating article 300, and has a length substantially equal to the length of the cavity 23. Although the aerosol-generating segment 310 of FIGS. 10 and 11 comprises only one aerosol-forming substrate, the aerosol-generating segment may equally include several aerosol-forming substrates. When there are multiple aerosol-forming substrates, the substrates may be arranged end-to-end with respect to one another in the longitudinal direction of the aerosol-generating article 300. However, it is envisaged that in other embodiments, a separation may be provided between the aerosol-forming substrates. It will be appreciated that in some embodiments two or more of the aerosol-forming substrates may be formed from the same materials, whereas in other embodiments, each of the aerosol-forming substrates is different. For example, one or more of the aerosol-forming substrates may comprise a gathered and crimped sheet of homogenised tobacco material including a flavouring in the form of menthol. One or more of the aerosol-forming substrates may also comprise a flavouring in the form of menthol, and not comprise tobacco material or any other source of nicotine. The one or more aerosol-forming substrates may also comprise further components, such as one or more aerosol formers and water, such that heating the aerosol-forming substrate generates an aerosol with desirable organoleptic properties.

[0164] The proximal end of the aerosol-generating segment 310 is exposed, as it is not covered by an outer wrapper 320. When the aerosol-generating segment 310 comprises several aerosol-forming substrates, the outer wrapper 320 may comprise a line of perforations circumscribing the aerosol-generating article 300 at the interface between the aerosol-forming substrates. The perforations enable air to be drawn into the aerosol-generating segment 310.

[0165] FIG. 12 shows an aerosol-generating article 300 similar to those of FIGS. 10 and 11. However, the filter plug 304 is a filter assembly 304 in the form of a rod. The filter assembly 304 includes three segments: a cooling segment 307, a filter segment 309 and a mouth end segment 311. In the embodiment of FIG. 12, the cooling segment 307 is located between the second aerosol-generating segment 310 and the filter segment 309, such that the cooling segment 307 is in an abutting relationship with the aerosol-generating segment 310 and the filter segment 309. In other examples, there may be a separation between aerosol-generating segment 310 and the cooling segment 307 and between the cooling segment 307 and the filter segment 309. The filter segment 309 is located in between the cooling segment 307 and the mouth end segment 311. The mouth end segment 311 is located towards the proximal end of the article 300, adjacent the filter segment 309. In the embodiment of FIG. 12, the filter segment 309 is in an abutting relationship with the mouth end segment 311. In one example, the total length of the filter assembly 304 is between 37 millimetres and 45 millimetres, more preferably, the total length of the filter assembly 304 is 41 millimetres.

[0166] In one example of the embodiment of FIG. 12, the aerosol-generating segment 310 is between 34 millimetres and 50 millimetres in length, more preferably, the aerosol-generating segment 310 is between 38 millimetres and 46 millimetres in length, more preferably still, the aerosol-generating segment 310 is 42 millimetres in length.

[0167] In one example of the embodiment of FIG. 12, the total length of the article 300 is between 71 millimetres and 95 millimetres, more preferably, the total length of the article 300 is between 79 millimetres and 87 millimetres, more preferably still, the total length of the article 300 is 83 millimetres.

[0168] In one example, the cooling segment 307 is an annular tube and defines an air gap within the cooling segment 307. The air gap provides a chamber for heated volatilised components generated from the aerosol-generating segment 310 to flow. The cooling segment 307 is hollow to provide a chamber for aerosol accumulation yet rigid enough to withstand axial compressive forces and bending moments that might arise during manufacture and whilst the article 300 is in use during insertion into the aerosol-generating device 200. In one example, the thickness of the wall of the cooling segment 307 is approximately 0.29 millimetres.

[0169] The cooling segment 307 provides a physical displacement between the aerosol-generating segment 310 and the filter segment 309. The physical displacement provided by the cooling segment 307 will provide a thermal gradient across the length of the cooling segment 307. In one example the cooling segment 307 is configured to provide a temperature differential of at least 40 degrees Celsius between a heated volatilised component entering a distal end of the cooling segment 307 and a heated volatilised component exiting a proximal end of the cooling segment 307. In one example, the cooling segment 307 is configured to provide a temperature differential of at least 60 degrees Celsius between a heated volatilised component entering a distal end of the cooling segment 307 and a heated volatilised component exiting a proximal end of the cooling segment 307. This temperature differential across the length of the cooling segment 307 protects the temperature sensitive filter segment 309 from the high temperatures of the aerosol formed from the aerosol-generating segment 310.

[0170] In one example of the article 300 of FIG. 12, the length of the cooling segment 307 is at least 15 millimetres. In one example, the length of the cooling segment 307 is between 20 millimetres and 30 millimetres, more particularly 23 millimetres to 27 millimetres, more particularly 25 millimetres to 27 millimetres and more particularly 25 millimetres.

[0171] The cooling segment 307 is made of paper, which means that it is comprised of a material that does not generate compounds of concern. In one example of the article 300 of FIG. 12, the cooling segment 307 is manufactured from a spirally wound paper tube which provides a hollow internal chamber yet maintains mechanical rigidity. Spirally wound paper tubes are able to meet the tight dimensional accuracy requirements of high-speed manufacturing processes with respect to tube length, outer diameter, roundness and straightness. In another example, the cooling segment 307 is a recess created from stiff plug wrap or tipping paper. The stiff plug wrap or tipping paper is manufactured to have a rigidity that is sufficient to withstand the axial compressive forces and bending moments that might arise during manufacture and whilst the article 300 is in use during insertion into the aerosol-generating device 200.

[0172] For each of the examples of the cooling segment 307, the dimensional accuracy of the cooling segment is sufficient to meet the dimensional accuracy requirements of high-speed manufacturing process.

[0173] The filter segment 309 may be formed of any filter material sufficient to remove one or more volatilised compounds from heated volatilised components from the aerosol-generating segment 310. In one example of the article 300 of FIG. 12, the filter segment 309 is made of a mono-acetate material, such as cellulose acetate. The filter segment 309 provides cooling and irritation-reduction from the heated volatilised components without depleting the quantity of the heated volatilised components to an unsatisfactory level for a user.

[0174] The density of the cellulose acetate tow material of the filter segment 309 controls the pressure drop across the filter segment 309, which in turn controls the draw resistance of the article 300. Therefore the selection of the material of the filter segment 309 is important in controlling the resistance to draw of the article 300. In addition, the filter segment performs a filtration function in the article 300.

[0175] The presence of the filter segment 309 provides an insulating effect by providing further cooling to the heated volatilised components that exit the cooling segment 307. This further cooling effect reduces the contact temperature of the user's lips on the surface of the filter segment 309.

[0176] One or more flavours may be added to the filter segment 309 in the form of either direct injection of flavoured liquids into the filter segment 309 or by embedding or arranging one or more flavoured breakable capsules or other flavour carriers within the cellulose acetate tow of the filter segment 309. In one example of the article 300 of FIG. 12, the filter segment 309 is between 6 millimetres to 10 millimetres in length, more preferably 8 millimetres.

[0177] The mouth end segment 311 is an annular tube and defines an air gap within the mouth end segment 311. The air gap provides a chamber for heated volatilised components that flow from the filter segment 309. The mouth end segment 311 is hollow to provide a chamber for aerosol accumulation yet rigid enough to withstand axial compressive forces and bending moments that might arise during manufacture and whilst the article is in use during insertion into the aerosol-generating device 200. In one example, the thickness of the wall of the mouth end segment 311 is approximately 0.29 millimetres.

[0178] In one example, the length of the mouth end segment 311 is between 6 millimetres to 10 millimetres and more preferably 8 millimetres.

[0179] The mouth end segment 311 may be manufactured from a spirally wound paper tube which provides a hollow internal chamber yet maintains critical mechanical rigidity. Spirally wound paper tubes are able to meet the tight dimensional accuracy requirements of high-speed manufacturing processes with respect to tube length, outer diameter, roundness and straightness.

[0180] The mouth end segment 311 provides the function of preventing any liquid condensate that accumulates at the exit of the filter segment 309 from coming into direct contact with a user.

[0181] It should be appreciated that, in one example, the mouth end segment 311 and the cooling segment 307 may be formed of a single tube and the filter segment 309 is located within that tube separating the mouth end segment 311 and the cooling segment 307.

[0182] In the article 300 of FIG. 12, ventilation holes 317 are located in the cooling segment 307 to aid with the cooling of the article 300. In one example, the ventilation holes 317 comprise one or more rows of holes, and preferably, each row of holes is arranged circumferentially around the article 300 in a cross-section that is substantially perpendicular to a longitudinal axis of the article 300.

[0183] In one example of the article 300 of FIG. 12, there are between one to four rows of ventilation holes 317 to provide ventilation for the article 300. Each row of ventilation holes 317 may have between 12 to 36 ventilation holes 317. The ventilation holes 317 may, for example, be between 100 to 500 micrometres in diameter. In one example, an axial separation between rows of ventilation holes 317 is between 0.25 millimetres and 0.75 millimetres, more preferably, an axial separation between rows of ventilation holes 317 is 0.5 millimetres.

[0184] In one example of the article 300 of FIG. 12, the ventilation holes 317 are of uniform size. In another example, the ventilation holes 317 vary in size. The ventilation holes 317 can be made using any suitable technique, for example, one or more of the following techniques: laser technology, mechanical perforation of the cooling segment 307 or pre-perforation of the cooling segment 307 before it is formed into the article 300. The ventilation holes 317 are positioned so as to provide effective cooling to the article 300.

[0185] In one example of the article 300 of FIG. 12, the rows of ventilation holes 317 are located at least 11 millimetres from the proximal end of the article 300, more preferably the ventilation holes 317 are located between 17 millimetres and 20 millimetres from the proximal end of the article 300. The location of the ventilation holes 317 is positioned such that user does not block the ventilation holes 317 when the article 300 is in use.

[0186] Advantageously, providing the rows of ventilation holes between 17 millimetres and 20 millimetres from the proximal end of the article 300 enables the ventilation holes 317 to be located outside of the aerosol-generating device 200 when the article 300 is fully inserted in the aerosol-generating device 200. By locating the ventilation holes 317 outside of the device 200, non-heated air is able to enter the article 300 through the ventilation holes from outside the device 200 to aid with the cooling of the article 300.

[0187] The length of the cooling segment 307 is such that the cooling segment 307 will be partially inserted into the device 200 when the article 300 is fully inserted into the device 200.

[0188] In use, when an aerosol-generating article 300 is received in the cavity 23, a user may draw on the proximal end of the aerosol-generating article 300 to inhale aerosol generated by the aerosol-generating system. When a user draws on the proximal end of the aerosol-generating article 300, air is drawn into the device housing 202 at the air inlet 280, and is drawn into the aerosol-generating segment 310 of the aerosol-generating article 300.

[0189] In the embodiment of FIGS. 11 and 12, the controller 208 of the aerosol-generating device 200 is configured to supply electric current to the PTC tube 25 arranged within the peripheral portion 21 of the heater housing 20. The temperature of the PTC tube 25 increases until it reaches the reference temperature of the PTC tube 25. After such instant, the temperature of the PTC tube 25 stabilises at a temperature substantially equal to the reference temperature of the PTC tube 25 for a period of time that generally exceeds a user's session time for the aerosol-generating device 200. The heating profile of the aerosol-forming substrate contained in the aerosol-generating segment 310 of the aerosol-generating article 300 received in the cavity 23 can therefore be determined in function of the reference temperature of the PTC tube 25.

[0190] In the heater of FIGS. 3 and 10, the temperature of the PTC tube TE is substantially the same as the temperature of the peripheral inner wall TI, that is, substantially the same as the temperature that will be applied to the aerosol-forming substrate. This is represented in the graphic of FIG. 13. The reference temperature of the PTC tube 25 of the heater 10 of FIG. 13 is 200 degrees Celsius, which substantially corresponds to the temperature of the PTC tube TE and to the temperature of the peripheral inner wall TI after the stabilisation time.

[0191] In the case of the heater of FIG. 8, the temperature of the six PTC plates TE does also substantially corresponds to the temperature of the peripheral inner wall TI. However, differently to the case of FIG. 12, the stabilisation time may be inferior. In particular, the temperature of the six PTC plates TE and the temperature of the peripheral inner wall TI may stabilise at substantially the reference temperature of the six PTC plates at 30 seconds.

[0192] FIG. 14 represents the evolution of the temperature of the PTC disk TE and the temperature of the peripheral inner wall TI with time for the heater 10 of FIG. 2. In this embodiment, it can be appreciated that the temperature of the peripheral inner wall TI is lower than the temperature of the PTC disk TE. In particular, for a PTC disk 24 with a reference temperature of 220 degrees Celsius, the temperature of peripheral inner wall TI stabilises at 210.

[0193] FIG. 15 shows a temperature T/resistance R graphic of a PTC thermistor comprised in a heating element of a heater for heating an aerosol-forming substrate when different constant voltages V are supplied to the PTC thermistor. In FIG. 15, a first voltage V1 is greater than a second voltage V2, which is in turn greater than a third voltage V3. As can be appreciated in FIG. 15, the reference temperature CT of the PTC thermistor is dependent on the voltage V applied to the PTC thermistor. In particular, the first voltage V1 leads to a first reference temperature CT1, the second voltage V2 leads to a second reference temperature CT2 and the third voltage V3 leads to a third reference temperature CT3, such that the first reference temperature CT1 is greater than the second reference temperature CT2, which in turn is greater than the third reference temperature CT3.

[0194] A controller may control a power supply to supply an electric current to the PTC thermistor having the first voltage V1, the second voltage V2, the third voltage V3 or any other suitable voltage. The reference temperature of the PTC thermistor will therefore be adjusted to the first reference temperature CT1, the second reference temperature CT2, the third reference temperature CT3 or any other suitable temperature. The relationship between the supplied voltage V and the reference temperature CT for a particular PTC thermistor may be stored in the controller; in a preferred embodiment, such relationship may be stored in a memory comprised in the controller. Likewise, the first reference temperature CT1, the second reference temperature CT2, the third reference temperature CT3 or any other suitable temperature may be determined to correspond to the desired maximum operating temperatures for one or more aerosol-forming substrates. The controller may also store one or more maximum operating temperatures for a given aerosol-forming substrate; in a preferred embodiment, such maximum operating temperatures may be stored in a memory comprised in the controller.

[0195] The PTC thermistor of the aerosol-generating system may thus substantially stabilise at the maximum operating temperature which is determined by the controller for a given aerosol-forming substrate. The temperatures at which the PTC thermistor stabilises is substantially the same as, or sufficiently close to, the temperature applied to the aerosol-forming substrate when the aerosol-generating system is in use to heat the aerosol-forming substrate, as explained for the heaters of the above embodiments. Therefore, the temperatures at which the PTC thermistor stabilises may be chosen to optimise the formation of aerosol. This may be beneficial to provide an optimised aerosol experience.