RESISISTIVE METAL OXIDE GAS SENSOR, MANUFACTURING METHOD THEREOF AND METHOD FOR OPERATING THE SENSOR

Abstract

A resistive metal oxide gas sensor comprises a support structure and a porous sensing layer (1) arranged on the support structure or partly housed therein. Electrodes (2) are in electrical communication with the porous sensing layer (1), and a heater (3) is in thermal communication with the porous sensing layer (1). The heater (3) can be operated to heat the porous sensing layer (1) to a target temperature for allowing a determination of the presence or the concentration of a target gas, i.e., ozone, based on a sensing signal supplied via the electrodes (2). The porous sensing layer (1) comprises a network of interconnected monocrystalline metal oxide nanoparticles (14) and a gas-selective coating (12) of the network. A thickness (t1) of the porous sensing layer (1) is at most 10 pm. The coating (12) comprises one or more of silicon oxide and silicon nitride, and is of a thickness (t12) of less than 5 nm.

Claims

1. A resistive metal oxide gas sensor, comprising a support structure; a porous sensing layer arranged on the support structure or partly housed therein; electrodes in electrical communication with the porous sensing layer; a heater in thermal communication with the porous sensing layer, the heater configured to heat the porous sensing layer to a target temperature so as to allow a determination of the presence or the concentration of a target gas based on a sensing signal supplied via the electrodes; wherein: the porous sensing layer comprises a network of interconnected monocrystalline metal oxide nanoparticles; and a gas-selective coating of the network; the coating comprises one or more of silicon oxide and silicon nitride; a thickness of the porous sensing layer is at most 10 μm; a thickness of the coating is less than 5 nm; and the target gas is ozone.

2. The resistive metal oxide gas sensor according to claim 1, wherein the porous sensing layer is configured to show a void fraction in a range between 30% and 60% denoting a porosity of the porous sensing layer.

3. The resistive metal oxide gas sensor according to claim 1, wherein the thickness of the coating is between 1 nm and 3 nm.

4. The resistive metal oxide gas sensor according to claim 1, wherein an average size of the monocrystalline metal oxide nanoparticles is at most 100 nm.

5. The resistive metal oxide gas sensor according to claim 1, wherein the thickness of the porous sensing layer is at most 5 μm; preferably wherein the thickness of the porous sensing layer is at most 1 μm.

6. The resistive metal oxide gas sensor according to claim 1, wherein: the sensor further comprises a controller configured to operate the heater for it to heat the porous sensing layer to said target temperature, the latter being in a temperature range between 400° C. and 600° C.; the sensor preferably comprises a temperature sensor integrated in the sensor and configured to provide a temperature feedback to the controller; and the controller is preferably integrated in the sensor.

7. The resistive metal oxide gas sensor according to claim 1, wherein the metal oxide material comprises one or more of SnO2, In2O3, WO3, TiO2, ZnO, Ga2O3, and Fe2O3.

8. The resistive metal oxide gas sensor according to claim 1, wherein the monocrystalline metal oxide nanoparticles comprise a noble metal doping of at most 10 wt %.

9. The resistive metal oxide gas sensor according to claim 1, wherein the electrodes are arranged on or in the support structure and are at least partly covered by the porous sensing layer.

10. The resistive metal oxide gas sensor according to claim 1, wherein the coating covers all surfaces of the network accessible by gaseous precursors of the coating material, preferably wherein the coating covers all surfaces of the network except for surfaces in direct contact with the support structure, the electrodes and the heater, if so, and except for surfaces facing voids in the porous sensing layer inaccessible to the gaseous precursors, preferably wherein the coating covers top surfaces of the network facing the environment prior to coating, and in addition covers buried surfaces within the porous sensing layer, preferably wherein the coating is uniform in its thickness including a tolerance of ±50%.

11. The resistive metal oxide gas sensor according to claim 1, wherein the target gas includes hydrogen in addition to ozone.

12. A method for manufacturing a resistive metal oxide gas sensor sensitive to ozone, the method comprising the following sequence of steps: manufacturing a support structure including electrodes and a heater; depositing monocrystalline metal oxide nanoparticles on the support structure; annealing or sintering the deposited monocrystalline metal oxide nanoparticles, thereby forming a network of interconnected monocrystalline metal oxide nanoparticles, applying a gas-selective coating of a thickness of less than 5 nm onto the network of interconnected monocrystalline metal oxide nanoparticles, the coating comprising one or more of silicon oxide and silicon nitride; the coated network of interconnected monocrystalline metal oxide nanoparticles contributing to a porous sensing layer of a thickness of at most 10 μm, which porous sensing layer is arranged on the support structure or partly housed therein such that the electrodes are in electrical communication with the porous sensing layer and such that the heater is in thermal communication with the porous sensing layer.

13. The method of claim 12, comprising providing noble metal particles and doping the monocrystalline metal oxide nanoparticles with the noble metal particles, the noble metal particles serving as nuclei for growing the coating on the network of interconnected monocrystalline metal oxide nanoparticles.

14. The method of claim 12, comprising applying the coating by means of a CVD process at a coating temperature in a coating temperature range between 400° C. and 700° C.

15. The method of claim 14, comprising generating the coating temperature by operating the heater.

16. The method of claim 14, applying the coating by means of the CVD process by using a gaseous oxygen and/or nitrogen species and a gaseous silicon species as gaseous precursors, preferably wherein the CVD process is applied at ambient pressure, in particular at a pressure of 1 atm, preferably wherein the gaseous oxygen species comprises one or more of O2 and O3 and/or the gaseous nitrogen species comprises ammonia, and preferably wherein the gaseous silicon species is one or more of: silane, dichlorosilane, tetraethylorthosilicate, or TEOS, hexamethyldisiloxane, or HMDSO, hexamethylcyclotrisiloxan, or D3, octamethylcyclotetrasiloxan, or D4, and decamethylcyclopentasiloxane, or D5.

17. A method for operating a resistive metal oxide gas sensor according to claim 1, the method comprising: taking a measurement including: operating the heater to heat the porous sensing layer to the target temperature; and determining a presence or a concentration of the target gas based on the sensing signal received from the electrodes, preferably while heating the porous sensing layer.

18. The method of claim 17, further comprising, prior to taking the measurement: placing the gas sensor in a gas mixture including hydrogen and calibrating the gas sensor by taking into account a hydrogen background concentration as expected in the measurement environment.

19. A method for operating a resistive metal oxide gas sensor as obtained in accordance with the method of claim 12, the method comprising: taking a measurement including: operating the heater to heat the porous sensing layer to the target temperature; and determining a presence or a concentration of the target gas based on the sensing signal received from the electrodes, preferably while heating the porous sensing layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Devices, apparatuses and methods embodying the present invention will now be described, by way of non-limiting examples, and in reference to the accompanying drawings, wherein:

[0051] FIG. 1 is a cross-sectional view of a gas sensor, according to an embodiment of the present invention;

[0052] FIG. 2 is a cross-sectional zoomed-in view of region 1z of FIG. 1, according to an embodiment of the present invention;

[0053] FIG. 3 is a cross-sectional zoomed-in view of region 1z of FIG. 1, according to another embodiment of the present invention;

[0054] FIG. 4 is a cross-sectional view of a gas sensor, according to another embodiment of the present invention;

[0055] FIG. 5 is a top view of the gas sensor of FIG. 4;

[0056] FIG. 6 is a top view of a gas sensor, according to a further embodiment of the present invention;

[0057] FIGS. 7 to 9 show plot representing measured resistances of a gas sensor according to an embodiment of the present invention after exposure to different types of gas molecules;

[0058] FIG. 10 shows a plot representing measured resistances of a gas sensor according to an embodiment of the present invention in a long term measurement; and

[0059] FIG. 11 is a flowchart illustrating high-level steps of a method of operating a gas sensor device, according to embodiments.

[0060] The accompanying drawings show simplified representations of devices or parts thereof, as involved in embodiments. Technical features depicted in the drawings are not necessarily to scale. Similar or functionally similar elements in the figures have been allocated the same numeral references, unless otherwise indicated.

MODES FOR CARRYING OUT THE INVENTION

[0061] Embodiments of the metal oxide (MOX) gas sensor according to the present invention are attractive because of their small footprint, fast response, and high sensitivity to the desired target gas/es. In addition, previous drawbacks of conventional MOX gas sensors are overcome by the present embodiments: The selectivity achieved for the target gas/es is raised, and long-term drift due to poisoning by silicone compounds is significantly reduced such that no or almost no deactivation of the catalytic activity occurs over time.

[0062] The gas sensor according to embodiments comprises a porous sensing layer of a semiconducting metal oxide, such as SnO2, including monocrystalline metal oxide nanoparticles, in following monocrystalline MOX NPs, with an average size typically in the tens of nm. The monocrystalline MOX NPs are assembled in network of interconnected monocrystalline MOX NPs, such that electrical conductivity is provided between the electrodes through the network of interconnected monocrystalline MOX NPs. The interconnectivity may be achieve in an annealing or sintering step applied after having deposited the monocrystalline MOX NPs on the support structure.

[0063] FIG. 1 illustrates a cross-sectional view of a gas sensor according to an embodiment of the present invention. The gas sensor comprises a support structure which presently is embodied as a substrate 5, and more preferably as a silicon substrate. The substrate 5 has a planar extension in x and y direction, and a height in z-direction.

[0064] A couple of elements are integrated in the substrate 5, which elements comprise one or more resistive heaters 3, and one or more temperature sensors 4. The various elements referencing a heating structure in one embodiment can represent a single heater 3 with multiple arms electrically interconnected, and hence controllable by a single controlling signal, or, in a different embodiment can represent multiple heaters individually controllable. Preferably, the heater 3 is made from tungsten. The temperature sensor 4 is preferably also made from tungsten and is provided for monitoring the temperature of the substrate 5 which may at the same time represent the temperature of a porous sensing layer 1 deposited on the substrate.

[0065] The porous sensing layer 1 is to be heated to a target temperature within a target temperature range. Accordingly, a controller (not shown, e.g., provided as part of the sensor or on a separate chip) controls the heater 3 to heat the porous sensing layer 1 to the target temperature while the temperature sensor 4 monitors the heating efforts. Subject to the temperature sensed by the temperature sensor 4, the heater is controlled e.g., to continue heating, or, e.g., to stop heating, in a feedback loop in which the target temperature is the set value to be compared to the actual value supplied by the temperature sensor 4.

[0066] Electrodes 2 are arranged on top of the substrate 5. Preferably, the electrodes 2 are made from platinum. Electrodes 2, heater 3, and temperature sensor 4, if any, may be manufactured within a conventional CMOS process for processing a CMOS layer stack on top of a Si wafer. In the present example, the heater 3 and the temperature sensor 4 may be manufactured from one or more conducting layers within a CMOS layer stack of conducting and insulating layers. The electrodes may also be made from one of the conducting layers of the layer stack, or may be made from a conducting material, such as Pt, applied on top of the CMOS layer stack. The electrodes 2, the heater 3 and/or the temperature sensor 4 may be shaped by lithography steps, for example. The porous sensing layer 1 is arranged on top of the support 5 and covers at least part of the electrodes 2. The porous sensing layer 1 shows an extension in x- and y-direction and has a thickness in z-direction referred to by t1. The thickness t1 of the porous sensing layer 1 is less than 10 μm, and in the present example e.g., is 5 μm.

[0067] The controller is configured to operate the heater 3, and to specifically heat the layer 1 to a target temperature in a temperature range between 450° C. and 550° C. In response to such heating, the electrodes supply an electrical sensing signal to the controller for further processing and/or evaluation. The sensing signal is representative of the electrical conductivity or the resistance of the porous sensing layer 1 in between the electrodes 2. In view of the heating of the porous sensing layer 1, ozone as a target gas may reach metal oxide material contributing to the porous sensing layer 1, interact there and lead to a change in the electrical conductivity thereof.

[0068] Reference sign 1z denotes an arbitrary region in the porous sensing layer 1, a zoomed view of which arbitrary region is illustrated in FIG. 2 in cross-section. As can be derived from FIG. 2, the sensing layer 1 is a porous layer given that the white spaces in FIG. 2 denote voids 11 while the dashed areas denote metal oxide material 10. The metal oxide material 10 preferably is SnO2 in the present example. The dotted areas refer to a coating 12 the metal oxide material 10 is covered with. In the present example, the coating 10 is made from SiO2. The metal oxide material 10 has the shape of monocrystalline metal oxide nanoparticles 14, referred to as monocrystalline MOX NPs. The different orientation of the dashes from MOX NP to MOX NP shall illustrate the monocrystalline property of each individual MOX NPs, in order to distinguish from a monocrystalline network which presently is not the case.

[0069] Adjacent monocrystalline MOX NPs 14 are electrically connected to each other, i.e., they may contact each other, but are not necessarily mechanically linked to each other. The monocrystalline MOX NPs 14 thus form a network of interconnected monocrystalline MOX NPs 14. A coating 12 coats the network without coating the contacts between adjacent monocrystalline MOX NPs. The coating 12 is a gas-selective coating and presently is selective for ozone.

[0070] In the present example, the coating 12 is considered a uniform coating, i.e., a coating of rather uniform thickness including a tolerance of ±50%, and a conformal coating that follows the contour of the coated material by not changing its thickness within the tolerances. The coating of the present example may also be essentially continuous, i.e., free of holes or gaps other than the pores for the target gas. As can be derived from FIG. 2, all surfaces of the network of interconnected monocrystalline MOX NPs are covered by the coating.

[0071] A thickness t12 of the coating 12 preferably is less than 5 nm, and in the present example is 3 nm. A size of individual monocrystalline MOX NPs is referred to by s14. An average monocrystalline MOX NP size is determined by the arithmetic mean of the various monocrystalline MOX NP sizes s14. In the present example, the average monocrystalline MOX NP size is, for example, 13 nm. A fraction of the voids 11 relative to the total volume of the layer preferably is between 30% and 60% (±10%, e.g., between 30% and 40%). In the present example, the porosity is 35%.

[0072] FIG. 3 illustrates another zoomed in view of the arbitrary region 1z of FIG. 1, again in cross-section. In addition to the material compositions illustrated in the context of FIG. 2, a noble metal, such as one of Pt, Pd, Ir, is added to the metal oxide material. Adding in this context may preferably encompasses doping the metal oxide material by the noble metal and/or impregnating the metal oxide material by the noble metal. In the first variant, the noble metal may also appear within MOX NPs, while in the latter variant, the noble metal rather only deposits on the surfaces of the MOX NPs. In the present embodiment of FIG. 3, noble metal particles 13 are positioned on the surface of the MOX NPs 14. The noble metal particles 13 serve as nuclei for the material of the coating 12 during the CVD process, and hence act as support for the coating process. The result after coating is illustrated in FIG. 3: The coating 12 coats the network of monocrystalline MOC NPs including the noble metal particles 13 on the surfaces of the MOX NPs.

[0073] FIG. 4 illustrates a resistive metal oxide gas sensor according to another embodiment of the present invention.

[0074] The gas sensor comprises a support structure which presently includes a structured substrate 5, e.g., of silicon. The substrate 5 has a membrane configuration, i.e., the substrate 5 includes a recess 51 for providing a thinned portion of the substrate 5, on which electrodes 2 are arranged. In the thinned portion 51, a heater 3 is provided for heating a porous sensing layer 1 arranged on top of the thinned portion 51 of the substrate 5. The electrodes 2 are arranged on or in the gas sensor, so as to be in electrical communication with a porous sensing layer 1. They may be formed out of a platinum or gold layer, which metals are well suited for forming stable electrodes. The electrodes 2 may for instance be in an interdigitated configuration containing interdigitated fingers 21. Thus, the porous sensing layer 1 may advantageously have shape that spans a region that covers or includes the interdigitated fingers 21 of the electrodes 2 to ensure proper electrical communication therewith.

[0075] The heater 3 is in thermal communication with the porous sensing layer 1, to operate the metal oxide material at a target temperature. The heater 3 is a resistive heating element. For example, one may use a heater 3 of tungsten, i.e., a heater 3 comprising at least 50% (±1%), in particular at least 90% (±1%), of tungsten, to best withstand high temperatures. Several heaters may be provided, to heat the membrane, or a bridge, see FIG. 6, on which the porous sensing layer 1 is arranged. In variants, the heater 3 may be embodied as a hotplate, which is resistively heated, without additional resistive elements being needed. The heater 3 can be used to heat the porous sensing layer 1 and, if necessary, to furthermore control the temperature of the porous sensing layer 1. Thus, no additional temperature sensors need necessarily be provided.

[0076] The porous sensing layer 1 extends on an exposed surface of the thinned portion 51 of the substrate 2, i.e., the membrane, thereby covering the electrodes 2 to a large part. The porous sensing layer 2 may for instance have a disk, ovoid or, still, a rectangular shape. All other properties of the porous sensing layer 1 can be taken from the description of FIGS. 1 to 3.

[0077] The gas sensor preferably includes circuitry (integrated therewith), which is electrically connected to the heater 3 and to the electrodes 2. The circuitry preferably forms a controller 6 to heat the heater 3 and perform resistive measurements, i.e., to measure an electrical conductivity and/or resistivity of the porous sensing layer 1 by means of the electrodes 2 that are electrically connected to the controller 6.

[0078] In FIG. 6, a gas sensor according to another embodiment is shown in top view, i.e., a multi-pixel gas.

[0079] A recess or opening 55 is arranged in the substrate 5, preferably silicon substrate. A bridge structure 56 extends over the recess or opening 55 and is anchored in the substrate 5. The bridge structure 56 includes individual bridges 52, 53, 54 spanning this opening or recess 55. Each bridge 52, 53, 54 comprises a central region 57 forming a hotplate 58 and two arms 59 extending between the central region 57 and the substrate 5, thereby suspending the hotplate 58 over the recess or opening 55.

[0080] Each of the bridge structures 56 forms a thin structure with a hotplate. By spanning the recess or opening 55 by means of a bridge and not a continuous thin film cover, the thermal conductance between the hotplate and the substrate 5 is reduced, thus allowing high temperatures to be achieved at low operating power. Further, the thermal mass is reduced, which allows to vary the temperature of the hotplate quickly.

[0081] The gas sensor comprises a set of sensing layers 1, 1a and 1b electrically separated from each other. Each sensing layer 1, 1a, 1b is arranged on a dedicated hotplate 58. Each of the sensing layers 1, 1a and 1b comprises a MOX material. The MOX material changes at least one electrical property as a function of the composition of the gas reaching it. The change of the property can be measured in order to obtain information on said composition.

[0082] Preferably, at least one of the sensing layers, e.g., the sensing layer 1, is the porous sensing layer 1 as introduced in the previous embodiments, while the other sensing layers 1a, 1b may, for instance, also show different properties as to the presence, the absence, the property of a coating, the porosity, a thickness, the material etc. Thus, several configurations can be contemplated, which enable selectivity of the sensed molecules.

[0083] The MOX material of each of the sensing layers 1, 1a, 1b is in electrical communication with a subset of electrodes, which are not visible in FIG. 6. Furthermore, one or more heaters (not visible in FIG. 6) are in thermal communication with the porous sensing layers 1, 1a, 1b. Furthermore, the substrate 5 carries an integrated CMOS circuitry contributing to a controller 6, e.g., including circuitry for driving heaters and processing signals from the electrodes and temperature sensors if any. Advantageously, the processing circuitry 6 is integrated in CMOS technology since the whole device described in this section preferably is compatible with current CMOS manufacturing processes.

[0084] The present approach makes it possible to clearly distinguish concentrations of the target gas/es from other, non-target gases to which the gas sensor according to the present invention may be exposed. For the following plots in FIGS. 7 to 10, the very same gas sensor was used in accordance with a given embodiment of the present invention, with SnO2 as metal oxide material, a Pt surface impregnation of 3% by weight %, a thickness of the porous sensing layer of 1.5 μm, an average MOX NP size of 13 nm, a SiO2 coating, and a target temperature of 500° C. FIG. 7 proves the sensitivity to the target gas hydrogen and the absence of cross-sensitivity to typical reducing gases, FIG. 8 proves the absence of cross-sensitivity to a typical oxidizing gas, FIG. 9 proves the sensitivity to the target gas ozone, and FIG. 10 proves the long-term stability.

[0085] In detail, FIG. 7 shows, in the upper chart, the resistances of a porous sensing layer of a gas sensor according to said given embodiment, after a sequential exposure to concentrations of various gas molecules over time in a gas mixing system, see lower chart. The gas molecules the gas sensor was exposed to are in the order as depicted in the chart: hydrogen, ethanol, acetone, toluene, cyclohexane and carbon monoxide in a carrier gas of humidified zero-air. As can be derived from the upper chart showing the resistance of the corresponding porous gas sensing layer when exposed to the various gases, only the exposure to hydrogen shows a significant response in the sensing signal taken by the electrodes, i.e., an increase in the electrical conductivity. The exposure of the same gas sensor to the other gases does not lead to any significant change in the sensing signal.

[0086] In terms of quantitatively describing the effect, the selectivity of hydrogen versus the other gases is more than 100:1, meaning that when exposed to a given concentration, hydrogen effects a bigger change in the resistance of the porous sensing layer represented by the sensing signal than when exposed to a hundredfold increased concentration of any of the other gases listed. For instance, the dashed line shows that 0.3 ppm of hydrogen effect a larger signal (more resistance drop) than 30 ppm of any of the other gases. A cross-sensitivity to the other reducing gases is thus designed to be very low.

[0087] In the upper chart of FIG. 8 resistances of a porous sensing layer of the same sample as in FIG. 7 are shown, after an exposure to various concentrations of nitrogen dioxide over time in a carrier gas of humidified zero-air and 0.5 ppm hydrogen, see lower chart. The hydrogen is meant to reflect ambient atmospheric hydrogen concentration. As can be derived from the upper chart, the present gas sensor is also not sensitive to nitrogen dioxide given that an increased concentration of nitrogen dioxide does not lead to a change in the resistance of the porous sensing layer.

[0088] Presently, the resistance measured in the upper chart of roughly 700 kOhm rather stems from the continuous background concentration of H2 in the gas mixing system, i.e., 0.5 ppm (see FIG. 7 for the same resistance level at 0.5 ppm H2).

[0089] The upper chart of FIG. 9 shows the resistances of the same sample as in FIG. 7 and FIG. 8, after an exposure to various concentrations of ozone over time in a carrier gas of humidified zero-air and 0.5 ppm hydrogen, see lower chart. The ozone concentrations of the lower chart were measured by a UV-optical reference device. As can be derived from the upper chart, the present gas sensor is sensitive to ozone and shows fast reaction times. In addition, it can be derived that the present gas sensor has a limit of detection of much less than 20 ppb ozone in view of the stepwise increase of ozone in the atmosphere around the gas sensor of around 20 ppb, which perfectly are reflected in the change of resistance of the porous sensing layer. Note that the change in resistance is positive for oxidizing gases such as ozone. Note also that 20 ppb of ozone effect a much larger change in resistance than 750 ppb of nitrogen dioxide, which again suggests a selectivity of better than 100:1 in favour of ozone vs nitrogen dioxide.

[0090] FIG. 10 shows a plot representing an outdoor field measurement of ozone measured by a calibrated gas sensor according to that same given embodiment as used for FIGS. 7 to 9. The time period shown corresponds to a period of 24 hours, taken six months after the gas sensor was calibrated in a gas mixing system with humidified zero air and 0.5 ppm hydrogen and ozone steps such as shown in FIG. 9. In the six months after calibration and, hence, prior to the shown measurement, the gas sensor was randomly switched on and off with a duty cycle of roughly 50%. The grey signal in the 24-hour measurement represents a continuous monitoring by a UV-optical reference device of the actual ozone concentration, while the dark line represents the ozone concentration measured by the gas sensor, after randomly switching on the gas sensor four times for a random duration. Every time after switching on the gas sensor, the sensing signal quickly supplies the correct ozone concentration present at that point in time and follows any subsequent variation of the concentration of ozone. Hence, the gas sensor is proven as long term stable.

[0091] According to another aspect, the invention can further be embodied as a method of operating a resistive MOX gas sensor. Main aspects are briefly discussed in reference to FIG. 11. Basically, in this method, a porous sensing layer 1 is heated to an operating temperature, step S10, FIG. 11. Meanwhile, values indicative of an electrical conductivity of the porous sensing layer are determined (step S50), e.g., thanks to an evaluation unit), and based on signals received S40 from the electrodes. Steps S40, S50 may be continuously (i.e., repeatedly) performed, while heating the coated patch.

[0092] As discussed earlier, the porous sensing layer is preferably heated to a temperature that is between 400° C. and 600° C. In particular, one may, while heating the coated patch, want to maintain a temperature of the coated patch at a desired value. This can notably be achieved thanks to a feedback loop mechanism. This, of course, does not preclude the possibility to occasionally heat the sensor for shorter periods to the target temperature, i.e., intermittent heating.

[0093] The above embodiments have been succinctly described in reference to the accompanying drawings and may accommodate a number of variants. Several combinations of the above features may be contemplated.