METHOD FOR ACTIVATING A GAS, ELECTROTHERMAL GAS ACTUATOR, AND USE OF A GAS ACTUATOR

Abstract

A method for activating a gas, wherein an electrically conductive aeromaterial having a pore space comprising the gas is electrically contacted and at least one electric current, which varies over time, flows through the aeromaterial, wherein the aeromaterial exhales gas from the pore space when the electrical power consumption is increased and inhales gas from the surroundings of the aeromaterial when the power consumption is decreased, and wherein a temporally pulsed current having predefined pulse power levels, pulse durations and pulse spacings is fed through the aeromaterial and the temperature of the aeromaterial is changed by the time-varying current by 100° C. or more within one second or less. The invention also relates to an electrothermal gas actuator and to uses of a gas actuator.

Claims

1. A method for activating a gas, comprising electrically contacting an electrically conductive aeromaterial having a pore space comprising the gas and flowing at least one electric current, which varies over time, through the aeromaterial, wherein the aeromaterial exhales gas from the pore space when an electric power consumption is increased and inhales gas from the surroundings of the aeromaterial when the power consumption is decreased, wherein a time-varying pulsed current having predetermined pulse power levels, pulse durations and pulse spacings is fed through the aeromaterial and the temperature of the aeromaterial is changed by the time-varying current by 100° C. or more within one second or less.

2. The method according to claim 1, wherein a guided activation of the gas is performed by arranging the aeromaterial in the form of a shaped body in a housing with gas inlet and gas outlet and check valves.

3. The method according to any one of the preceding claims, wherein a current, constant in time and different from zero, is superimposed on the current, that varies with time.

4. The method according to claim 1, wherein the pulse intervals are set up at least one order of magnitude greater than the pulse durations.

5. The method according to claim 1, wherein the variation in time of the current is performed at a frequency of 10 Hz or more.

6. An electrothermal gas actuator for gas activation of defined volumes with defined energy quantities by means of aeromaterials, comprising: an electric current source, a control device designed to control the current source to feed a time-varying pulsed current having predetermined pulse power levels, pulse durations and pulse spacings through the aeromaterial and change the temperature of the aeromaterial by the time-varying current by 100° C. or more within one second or less, a shaped body made of an electrically conductive aeromaterial, wherein the shaped body has open-pored sides with accesses for a gas and/or ambient gas to the pore space of the aeromaterial and is contacted for electrical energization, wherein the electrically conductive aeromaterial is an aerocarbon or aerographene, and wherein the volume of the pore space of the shaped body has a value from the interval of 1 nanolitre to 1 liter.

7. A pumping unit of a micro-metering pump for gases comprising the gas actuator according to claim 6.

8. A device for destroying organic structures and microorganisms entrained in ambient air comprising the gas actuator according to claim 6.

9. An emitter of audible sound or a loudspeaker comprising the gas actuator according to claim 6.

Description

[0058] The invention is first explained in further detail with reference to an embodiment of a gas actuator with a molded aerographene body, and with reference to figures. Thereby shows:

[0059] FIG. 1 Pyrometrically measured temperature curves and final temperatures of the molded body from aerographene for different electrical powers of energization of the molded body;

[0060] FIG. 2 one of the plots as in FIG. 1 acquired in parallel with a pyrometer and an IR camera;

[0061] FIG. 3 shows the temperature profile of the molded part during a current pulse of 15 W for a pulse duration of 60 ms recorded with a pyrometer and IR camera;

[0062] FIG. 4 shows the pyrometrically measured temperature profile of the molded part during a sequence of current pulses of 109 W with pulse durations of 1.7 ms;

[0063] FIG. 5 shows the temperature profile of the molded part during a first current pulse of 7 W for a pulse duration of 10 ms in comparison with the temperature profiles after a further 50,000 or 100,000 applied current pulses with the same pulse parameters;

[0064] FIG. 6 the pyrometrically measured temperature profiles for a template of T-ZnO coated with graphene compared to the temperature profile of the remaining aerographene after removal of the T-ZnO;

[0065] FIG. 7 shows the pyrometrically measured temperature profile of the molded part made of aerographene during a temperature-timed pulse streaming with a frequency of approx. 10 Hz.

[0066] An experimental set-up with a shaped body of aerographene contacted for current flow has already been described in the work of Rasch et al. It is useful for the realization of the gas actuator proposed here. However, Rasch et al. do not make any statements on the ambient gas, on the temperatures of the shaped body or on a pulsed current flow. Only a conductance determination is made, from which it can be concluded that aerographene according to the manufacturing process described there have percolating current paths above a density of 1 mg/cm..sup.3

[0067] In the following, for abbreviation purposes, a molded body made of aerographene is always meant by way of example. It can be assumed that all electrically conductive aeromaterials exhibit qualitatively similar behavior, although there are of course differences between carbon networks and metallic microgrids which must be taken into account appropriately in individual cases. For example, aerocarbon (aerokohlenstoff) can be heated to more than 3000° C. in a chemically inert ambient gas without damage, which is more likely to be avoided for the microgrids. For the purposes of the gas actuator presented here, however, these differences are considered insignificant because extreme conditions are not required.

[0068] The energized molded part can be observed with a pyrometer and/or thermal imaging camera to determine its temperature from thermal radiation.

[0069] FIG. 1 shows a set of pyrometrically recorded temperature measurement data in which a molded part is heated to different temperature levels. The decisive factor for the extent of heating is the applied electrical power, as can be seen from the figure. The molded body reaches its final temperature when its energy absorption from the ohmic resistance when energized equilibrates with its energy losses from radiation and gas convection in the pore space. FIG. 1 clearly shows that the final temperature is approximately reached after less than one second. Cooling occurs almost as quickly when the power is removed, even from the equilibrated—i.e. not only briefly heated—state. Temperature increases of more than 100° C. can be achieved with just a few watts of power.

[0070] The base level of the temperature in FIG. 1 is about 120° C., but this is an artifact of the measurement. Temperatures below this can no longer be detected with the pyrometer. In FIG. 2, therefore, a temperature curve as in FIG. 1 has been recorded simultaneously with the pyrometer and with an IR camera, the camera having a larger measuring range. It can be seen from FIG. 2 that the return of the molded part to the ambient temperature actually takes place very quickly within about 1-2 seconds. This amazingly efficient cooling behavior of aeromaterials after ohmic heating has never been reported before to the knowledge of the inventors and shall be exploited here.

[0071] According to the invention, it is intended to allow time-variable currents, preferably current pulses with predetermined pulse powers, pulse durations and pulse spacings, to flow through aeromaterials, in particular shaped bodies of aeromaterials. For example, FIG. 3 shows the temperature curve of the said shaped body of aerographene for a current pulse of pulse power 15 W at a pulse duration of 60 ms. For clarification, the pulse power is, as usual, the product of voltage applied to the molded body and current generated in the molded body, and the pulse energy is the product of pulse power and pulse duration. The pulse energy can be varied by varying the pulse power and/or pulse duration. The pulse interval is the time interval between the onset of a first current pulse and the onset of a second current pulse immediately following in time the end of the first current pulse, regardless of the pulse duration. In a sequence of more than two current pulses, the pulse spacing may change over the course of the sequence.

[0072] From the comparison of FIG. 3 with FIG. 1, it is clear that the short pulse duration causes the heating of the molded part to stop long before a final temperature is reached (above 1000° C. for 15 W), i.e. still in the rising flank. It is also clear that the IR camera has measurement dynamics that are too slow to capture the rising temperature slope at higher pulse powers. Therefore, in the following only measurement data of the much faster pyrometer are shown with reference to the mentioned measurement artifact at 120° C.

[0073] In order to achieve a very steep heating edge, each current pulse of a sequence of pulses with a power of 109 W is applied for a pulse duration of 1.7 ms in the measurement data of FIG. 4. The cooling edge requires about 200 ms each (until leaving the pyrometer measuring range). With an exemplary selected current pulse interval of 1 s, a sequence of similar temperature curves can be produced.

[0074] It is considered generally advantageous to carry out pulsed energization of the aeromaterial in such a way that the pulse intervals are set up to be at least one order of magnitude greater than the pulse durations. In this way, a repetitive operation of the gas actuator can be ensured, i.e. each “breath” of the gas actuator has a clearly predictable effect on the gas. The gas actuator returns to its initial state after each pulse before the next pulse is applied.

[0075] FIG. 5 shows the temperature profile of the molded part during a first current pulse of 7 W for a pulse duration of 10 ms compared with the temperature profiles after a further 50,000 or 100,000 applied current pulses with the same pulse parameters. The pulse parameters were specifically selected here to heat the molded part to 400° C. each time. Thermal cycling recognizably does not affect the molded part, i.e. long sequences of current pulses are also possible and harmless.

[0076] FIG. 6 shows how important the low mass density and heat storage capacity of the aeromaterial are for its rapid cooling behavior. The curve labeled T-ZnO+AG represents the temperature profile of a T-ZnO template coated with exfoliated graphene flakes according to Rasch et al. When current is applied to the template, the current flows almost only through the graphene sheets, but the amount of heat generated is also transferred to the T-ZnO, where it is better stored. When the current is turned off, cooling below the measurement limit of the pyrometer takes more than a second, even though the crystalline T-ZnO is not a good heat storage material and itself has a density of only slightly more than 300 mg/cm.sup.3. The curve labeled AG shows, for identical current pulse parameters, the temperature profile of the same graphene layers after chemical dissolution of the T-ZnO, i.e. now as aerographene with a density of 4 mg/cm.sup.3. The cooling rate has increased by a whole order of magnitude.

[0077] It should be noted that the measurement data in FIGS. 1 to 5 were determined with aerographene of a mass density of 8 to 10 mg/cm.sup.3, which can be seen from the somewhat lower cooling rate compared with the aerographene in FIG. 6. As a rule of thumb, lower mass density is associated with faster cooling. The physical lower limit of the mass density is considered to be 0.2 mg/cm.sup.3, because below this level it is no longer possible to realize a coherent network for uniform flow. Technically, mass densities above 1 mg/cm.sup.3 are also preferable for reasons of mechanical properties.

[0078] The temperature characteristics of a molded body made of an aeromaterial, preferably of an aerocarbon (aerokohlenstoff), particularly preferably of aerographene, shown so far are all associated with gas activation in the pore space of the aeromaterial. As mentioned earlier, the molded body can exhale and inhale in time with its time-varying current flow; in a sense, it behaves like an electric bellows, but without moving parts.

[0079] Thus, it is sufficient to add, for example, a control device for controlling the current source to the structure of Rasch et al. in order to arrive at a gas actuator according to the invention. The control device may be formed by a conventional microprocessor or PC, which issues programmed control commands to the current source for activation, deactivation or power control. The control device may be designed to allow user-selectable current pulse parameters to be implemented by the current source. The control device may read control commands from stored tables and execute them at predetermined times. In addition or alternatively, the control unit can also receive and process other measurement data and use it to issue control commands.

[0080] An example of feedback operation of the gas actuator may be to set a minimum pumping or breathing rate to the gas actuator with the proviso that it should ensure uniform exhalation and inhalation. To this end, the temperature of the gas actuator's molded body is monitored with a pyrometer, and the pyrometer measurement data is fed to the control device. The control device can then, for example, use the temperature curve to determine times at which it triggers further current pulses in order to generate uniform temperature cycles at the specified minimum frequency. Such a procedure generates an exemplary temperature curve as shown in FIG. 7, where a minimum frequency of 10 Hz is realized.

[0081] The control device can also vary the power and/or duration of repeated current pulses, adjusting to accurately achieve a fixed prescribed breathing rate and maintain uniformity of heating of the molded body for all pulses.

[0082] Although the cooling of the molded body—associated with its inhalation—is the slower process in gas activation, its speed can nevertheless also be influenced. In particular, it can be advantageous if a non-zero, time-constant current is superimposed on the time-varying current through the molded body. This has the effect that the molded body is basically at an elevated temperature relative to the ambient gas, so that its cooling by inhalation of ambient gas is faster. As a result, one sets oneself to a different operating point of the cooling curve. As a rule of thumb, the greater the temperature elevation of the molded part compared to the ambient temperature, the higher the breathing frequencies that the gas actuator can realize.

[0083] Preferably, it is provided that the temporal change of the current through the molded body is carried out with a frequency of 10 Hz or more. Particularly preferably, however, frequencies in the range greater than 100 Hz or even greater than 1 kHz are also realized. It is not always necessary for the temperature fluctuations of the molded body to be as pronounced as shown in the previous examples. Even less than 100° C. temperature variation produces gas activation, and at said frequencies the gas actuator then emits audible sound waves. A very interesting possible application of the gas actuator according to the invention is therefore seen in its use as a sound emitter or loudspeaker—also for the reproduction of speech or music. In principle, the radiation characteristic can be omnidirectional, or at least less restricted than in conventional loudspeakers. Moreover, the gas actuator is basically very small and space-saving.

[0084] Another application tested in the laboratory is to use the gas actuator, already understood as a bellows, directly as a pumping unit of a micro-metering pump for gases. For this purpose, the gas actuator can be arranged in a closed housing with a gas inlet and a gas outlet, both equipped with check valves. During electrothermal exhalation, the gas actuator generates an overpressure in the housing, and the gas outlet alone opens and allows a gas component to escape. During inhalation, a negative pressure is generated in the housing, which is compensated by the sole opening of the gas inlet. A sequence of current pulses thus conveys a precisely determinable quantity of gas through the housing and into a tube or the like arranged on the outlet side. By selecting the current pulse intervals, i.e. the repetition rate, the pump speed can be set up.

[0085] Under the impression of the Corona Pandemic 2020, one very important and forward-looking application of the gas actuator according to the invention is in air purification, particularly for destroying organic structures and microorganisms, such as viruses and fungal spores, which are also moved in the ambient air. In fact, it may even be assumed to be particularly effective here without any appreciable environmental pollution, which is especially important for use in hospitals.

[0086] Just like the gas molecules, entrained organic flight particles are inhaled into the pore space of the gas actuator's molded body during the repetitive pulsed current, where they are abruptly exposed to temperatures that reliably denature any biological organics. In this application, it is considered particularly advantageous if the temperature of the aeromaterial is changed by the time-varying current by 100° C. or more, preferably 300° C. or more, within one second or less. The remains thus destroyed or killed are largely exhaled, and if any remains do remain in the pore space, they can be thermally decomposed and outgassed by an occasional “service current pulse” that brings the molded body to a very high temperature for a short time. At the same time, it can be avoided that the room air as a whole experiences a noticeable heating, since the gas actuator usually supplies it with only a few joules of energy per second.

[0087] Experimental tests show that heating rates of 100,000° C. per second and more can easily be achieved. However, if a large volume of a molded part is heated up too quickly, the heated gas in the pore space can only escape quickly enough by blowing off parts of the molded part. It is therefore advisable to set up a limitation of the electrical power as a function of the volume of the aeromaterial being energized.