WO2014184146A1

WO2014184146A1 - Anti-icing system

Info

Publication number: WO2014184146A1
Application number: PCT/EP2014/059667
Authority: WO
Inventors: Sergey Goloviatinski; Sergueï MIKHAÏLOV; Mikhail LIFSHITS
Original assignee: Nci Swissnanocoat Sa
Priority date: 2013-05-13
Filing date: 2014-05-12
Publication date: 2014-11-20

Abstract

In one embodiment, a system for conductor surface heating and for removing ice or snow from surfaces. The system preferably includes a ferromagnetic coating or/and a wound conductor around the heated object and a AC source, e.g., a kHz-generator with matching box coupled to the object. Electromagnetic energy causes the coating or/and the wound conductor to generate heat, which melts snow and ice. In a preferred embodiment, a power conductor is coated by a magnetic composition having a Curie temperature close to 0 °C by a gas-dynamic cold spraying.

Description

Anti-Icing system

Field of the invention

[0001] The invention relates to methods, systems and structures for conductor surface heating and for removing ice and snow from surfaces, in particular, but not exclusively, from overhead power conductors and aircrafts.

Description of related art

[0002] The problem of ice build-up on surface is well known and is a source of considerable economic losses in several industries. In the case of air transportation, ice build-up is not only an economical, but also a major safety concern and is fought with several techniques that include

application of anti-stick fluids, vibrations, shocks, and radio energy. Other areas in which ice control is important are power transmission by overhead high voltage lines, railroads, road maintenance, shipping, and refrigerators, to name just a few. [0003] All known de-icing techniques have, while effective each in its own operational boundaries, weaknesses and shortcoming of economic, environmental, or practical nature.

[0004] In the case of overhead power lines, for example, the sheer size of the installation is an obstacle to the application of mechanical or chemical methods. It is known for example to use robots that travel along the power conductors and scrape, melt, or promote chemically the detachment, of ice. These time required to the clean a power line, however, grows together with the length of the same.

[0005] In aircrafts, anti-icing fluids lowering the ice melting point and hinder its adhesion to the fuselage are extensively used. These systems, cannot however always cope with severe icing conditions, such that accidents sometime happen despite them. [0006] It is also known to fight ice build-up on aircraft's surfaces, but also on land vehicles, and in buildings, by ohmic heating. In such cases a resistive circuit, for example a wire or a resistive compound, is applied on the surfaces according to a suitable pattern, and energized when necessary. Such assemblies, however, can fail easily if the circuit is scraped or broken, be it at a single point.

[0007] Inductive-based anti-icing systems for aircrafts have also been proposed. In these realizations, surfaces are heated by making them part of a magnetic circuit that includes transformer assemblies inside a wing or aerofoil. This scheme, albeit functional, requires bulky and heavy magnetic devices inside the wing profile, which is not always desirable.

[0008] Electric defrosting of glass windows and mirrors is also universally used in motor cars and street vehicles, and conventionally uses ohmic resistive tracks on the glass or, for the fore windshield, warm air heating. Such systems could be effective in vehicles powered by a conventional thermal plant. In of low-emission electric or hybrid vehicles, however, they draw their supply from the batteries, and their use can seriously limit performances and range.

[0009] Examples of inductive de-icing systems for aircrafts can be found in GB1306062, EP191 1673, and DE 102007026246.

[0010] CA2735341 and US2006272340 describe anti-icing devices based on the application high-current pulses;

[0011] US6723971 , WO0033614, US6427946, US2002017466 disclose anti- icing systems by which the interfacial ice is subject to a DC field that eventually leads to its detachment by a combination of electrolysis, ohmic heating, and sparking.

[0012] US2002092849, US2004149734, WO03062056, US20021 52762, US2005167427 describe systems in which the interfacial ice is subject to an AC electric field. [0013] WO2009059076, US2010084389 describe anti-icing systems based on resistive electrodes.

[0014] US2002175152 and WO0052966 disclose an anti-icing system for overhead conductors including a lossy coating that could be ferromagnetic. [0015] There is therefore a need for anti-icing devices free from the above shortcomings.

Brief summary of the invention

[0016] According to the invention, these aims are achieved by means of the object of the appended claims. Brief Description of the Drawings

[0017] The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:

Figure 1 illustrates a ferromagnetic coating on a conductor (heated object) with a thickness comparable to a skin layer for this ferromagnetic material;

Figure 2 shows a wound ferromagnetic conductor (mechanically wound or coated in the form of a spiral) around a heated object;

Figure 3 depicts a ferromagnetic coating on a conductor together with a wound ferromagnetic conductor around the heated object;

Figure 4 illustrates a system for applying an AC source, e.g., a kHz-generator with matching circuit coupled to the heated object; Figures 5a-5d show heated objects with different forms and cross sections, single or multi conductors;

Figures 6 a-b illustrate a ferromagnetic coating forming a spiral- shaped circuit on a flat or quasi flat heated surface Figures 7-8 depict measured heating test results for 20 mm2 aluminum wires by using of 20kHz/8A generator and of 50Hz/16A line current with not coated original wire, with 100 microns nickel coating on aluminum and a mechanically wound Fe-Zn 0.6 mm wire around an aluminum wire with three different steps of winding.

Figure 9 shows measured heating test results for 20 mm2 aluminium wires by using of 50Hz line current, and compares temperatures reached by the uncoated original wire with those measured on an aluminium wire coated with 100 microns nickel, at three current values: 16, 80 and 100 A.

Figure 10 illustrates schematically in section a ferromagnetic-clad wire that can be used in the frame of the invention

Figure 1 1 shows schematically an aircraft having circuits of a ferromagnetic conductor on part of its surfaces, for de-icing. Detailed Description of possible embodiments of the Invention

[0018] Figure 1 shows schematically a conductor 1 that represents an object on which ice can accumulate, and on which ice build-up is countered by the system of the present invention. The conductor 1 could be a section of a power transmission line, as the drawing suggests, but, according to the application, may take other forms like for example, a fuselage or any external surface of an aircraft, a train railway, and so on. An important aspect of the present invention is that the conductor has an outer surface 2 on which circulate AC high-frequency currents generated by a suitable high-frequency power source. In this manner, the local surface temperature rises, thus negating ice build-up, or promoting the fall of existing ice.

Importantly, the currents are confined in an outer shell of the conductor due to the skin effect. [0019] The frequency generated may vary, according to the impedance and to the physical features of the object 1, but it may be loosely placed in a range of about 1 .0 kHz to 100 kHz.

[0020] According to an important aspect of the invention, the surface 2 of the conductor 1 is a magnetic layer, for example a thin film of a ferromagnetic compound, deposited, painted, glued, sprayed, or applied by any suitable means. In this manner, since the skin depth penetration is given by δ = (2p / ω μ)^Ί/2, the heating currents are sensibly confined in a thin layer at the surface of the object, and the local heating effect is enhanced.

[0021] Several materials, among the ferromagnetic substances can be usefully employed in the invention for the coating 2. In embodiments,

Nickel, deposited by an arc-plasma facility, was found particularly effective, but this is not a limitation of the invention.

[0022] Nickel can be layered effectively on a variety of known substrates by physical or galvanic techniques. Since it is highly resistant to corrosion, it forms a protective layer, and its properties of adhesion are excellent.

[0023] Advantageously, the conductive ferromagnetic layer of the invention could be part of a sandwiched structure, comprising a plurality of layers having different constitutions and functions.

[0024] Above the conductive magnetic coating 2 one could add, for example, a protection layer, in order to shield the magnetic coating from abrasion, and/or an hydrophobic coating, in order to prevent adhesion of ice and water, or/and an anti-slip coating, with the same purpose. Between the conductive magnetic coating 2 and the substrate 1 the invention could optionally foresee an adhesion-promoting layer, and/or a thermal isolation layer, such that the heating effect of the invention is directed preferably to the outside ice rather than to the substrate.

[0025] Another advantage of the present invention over other systems based, for example on ohmic heating by resistive circuits patterned on the surface, is that the system is robust against point failures. If the circuit is interrupted, in fact, High frequency currents continue circulating and can bypass the failure point either capacitively and via the substrate.

[0026] Figure 2 shows a variant of the invention in which the continuous coating 2 is replaced by a plurality of parallel ferromagnetic conductors 3, or by an helical winding 3. In the latter case, the AC high frequency current may be injected in the main conductor 1 , or directly in the winding 3. The structures of Figures 1 and 2 can also be combined together, as shown in figure 3.

[0027] In the case of the power line application, the power source may be coupled to the line with the disposition represented schematically in figure 4, A matching circuit composed by capacitors 5, 6 is used to stop the low frequency mains component.

[0028] Advantageously, the invention can be applied to object having a variety of shapes and sections, and, particularly when the ferromagnetic coating is applied as a thin film, it can be particularly cost-effective. Figures 5a to 5d show schematically several different possible conductor sections, both solid and multi-wire to which the system of the invention can be applied.

[0001] In an extension of the inventive system, illustrated by figures 6a and 6b, the de-icing of non-conductive surfaces is also possible, In this case the invention foresees the creation of a suitable arrangement of a ferromagnetic conductor creating a circuit on the surface that must be protected from ice, and the circuit 3 is then connected, preferably

galvanically, to the AC frequency source 4. The frequency of the generator can range anywhere from 0.4 to 100 kHz, the higher frequencies generally providing a stronger heating and de-icing effect.

[0029] Preferably, the frequency generated by the source 4 is tuned to the impedance of the target system, to balance ferromagnetic surface heating and skin-effect heating. This can be designed in the system or, where appropriate, tuneable generators can be used.

[0030] The surface power density delivered by the system is determined by the amount of circulating current, and by the thickness of the

ferromagnetic layer 2, or by the dimension of the ferromagnetic circuit 3. Thus it is possible to design the system such as to attain a desired generated heat. Importantly, the power can be regulated according to the

environment temperature, humidity, wind, weather forecasts, or any other suitable control variables. It may be advisable, for example to change the amount of heat generated as a function of temperature, the amount being higher below freezing temperature than above freezing temperature.

[0031] According to the different use cases, the circuit 3 could consist in a patterned ferromagnetic thin layer deposed on the surface that must be protected, or else could be a wire or foil cladded with layer of

ferromagnetic layer that is applied, glued, inserted, bonded to the surface that must be protected, or otherwise put in thermal contact therewith.

[0032] Figure 10 shows a cross-section of a wire that could be used in this particular embodiment of the invention. The wire consist in a central core 40 with an outer cladding 45 of a ferromagnetic material. The section of the wire 39 need not be circular, as represented, nor must the cladding 35 completely encircle the core 40.

[0033] The inner core 40 can be metallic, for example steel, copper or aluminium, but also a synthetic material. Preferably the thickness of the cladding 45 is determined in relation to its magnetic properties and to the frequency of the current that the wire 39 should carry, such that the current is confined in the cladding. [0034] The cladding 35 of wire 39 could be realized by a Fe-Ni alloy, or a Fe-Co alloy, or comprise metal oxides, and be deposited by magnetron sputtering, gas dynamic cold spray system, as it will be explained in the following, arc, plasma, or any suitable technique. The thickness of the ferromagnetic cladding will be comprised from 0 and 1 mm, preferably higher than 300 μηη, but even thinner layers could be effective, dependent from the thermal power desired and the AC supply frequency.

[0035] Figure 1 1 shows a possible embodiment of the present invention in which an aircraft has circuits 37 on the aerodynamic surfaces like wings, ailerons, and the like that are realized with a ferromagnetic material. These circuits can be obtained by depositing a ferromagnetic layer directly on the aircraft surface, or by laying ferromagnetic-clad wires 39 as shown above. Importantly, this form of execution can easily be applied surfaces of composite materials: Fe-Ni layers can be coated directly on composite- reinforced resin, while ferromagnetic-clad wires can be adhesively bond thereupon or incorporated in the resin itself before curing.

[0036] The shape and dimension of the circuits 37 can be modified, depending from the size and geometry of the surface that ought to be protected from ice. The invention is not limited to the specific shape that is presented in the figure by way of example. It must be understood that the present invention is not limited to an airliner, as represented in the figure, but could be used in any airplane, drone, helicopter, balloon, or any other kind of aerial vehicle.

[0037] The circuit 37 is connected to a suitable high-frequency generator that delivers current at a frequency comprised, preferably, between 0.4 and 100 kHz. Such generator can be comprised in the ordinary aircraft's equipment and need not be placed close to the surfaces that must be protected from ice. In this manner the invention achieves an effective de- icing, without adding weight to the aerodynamic surfaces, or using space inside the wings. [0038] Many airliners have electrical generators that generate 400 Hz AC current, and are driven by the engines through a constant speed drive gearbox. In other cases, however, the generators are directly coupled to the engine gearboxes and operate at variable frequency, for example between 360 and 800 Hz. Advantageously, the de-icing system of the invention can operate in an ample span of frequencies, and could therefore draw their supply from any constant or variable-frequency source in this range. In alternative, a fixed high AC supply, at any desired frequency between 1 .0 and 100 kHz, or even above, can be obtained by a suitable solid-state converter.

[0039] The invention is not limited to an airplane application, but could also be usefully employed in other vehicles. In particular, the circuit 37 could be applied to glass windows, windshields, external mirrors or other surfaces of vehicles. [0040] The inventors investigated the effect of AC current at frequencies of 20 kHz and 50 Hz on heating of 20 mm2 aluminium wires. The

configuration shown in figure. 4 was used. In the 20kHz tests, the current used was 8 A rms and by 50 Hz tests the current was 16 A rms.

[0041] The following conductors have been tested: uncoated aluminium wire; aluminium wire 1 with 100 microns of nickel coating 2 (figure 1 );

aluminium wire 1 with a mechanically wound Fe-Zn 0.6 mm wire 3 around (figure 2) with three various steps of winding.

[0042] Figures 6-7 show measured heating during 10 minutes at 20 kHz 8 Amp and 50 Hz 16 Amp correspondingly. The uniform nickel coating with a thickness 100 microns heats up to 35°C (temperature increase above ambient:1 5°C) after 10 minutes by 20 kHz current, while, by application of 50Hz current, the corresponding temperature rise was 7°C; the

mechanically wound Fe-Zn 0.6 mm wire 3 around aluminium wire with a winding 7 turns/cm heated up to 132°C (temperature rise 1 12°C) after during 10 minutes by 20 kHz current but also showed a temperature rise slightly higher (2°C) than the bare original aluminium wire at 50 Hz. [0043] The mechanically wound Fe-Zn 0.6 mm wire 3 around aluminium wire with a winding 1 turn/cm heated up to 45°C (AT=25°C) after 10 minutes by 20 kHz current and, when traversed by 50Hz current, behaved very similarly to the original aluminium wire. Thus, this system

configuration does not introduce additional losses at low frequency current and provides a satisfactory surface heating by 20 kHz current, that is enough to melt and remove ice and snow from a aluminium wire.

[0044] In a different embodiment, the present invention comprises a step of depositing a ferromagnetic layer on a surface that is treated for ice- removal with a Gas dynamic cold spray system.

[0045] Gas dynamic cold spray is a coating deposition method in which solid powders (comprising nano-particles and/or micro-particles with diameters ranging from sub-micrometre to 50 micrometres) are

accelerated in supersonic gas jets to velocities up to 500-1 500 m/s and directed to the surface that must be coated. During impact, particles undergo plastic deformation and adhere to the surface. Preferably, a spraying nozzle is scanned manually or automatically along the substrate in order to obtain a coat having a desired thickness ranging from few micrometres to some millimetre. The method is suitable to deposit coats of metals, polymers, and composite materials.

[0046] The kinetic energy of the particles, supplied by the expansion of the gas, is converted to plastic deformation energy during bonding. Unlike thermal spraying techniques e.g., plasma spraying, arc spraying, flame spraying, high velocity oxygen fuel (HVOF) the powders are not melted during the spraying process. Importantly, the mechanical properties of the sprayed object, for example the flexibility in the case of a conductor wire, are not significantly altered.

[0047] Without willing to be limited by theory, it is believed that, when particles impinge on the substrate with a velocity exceed a critical value, a dynamic phenomenon termed "adiabatic shear instability" occurs: a strong pressure field propagates spherically into the particle and substrate from the point of contact. As a result, a shear load is generated which

accelerates the material laterally and causes localized shear strain. The shear loading under critical conditions leads to adiabatic shear instability where thermal softening is locally dominant over work strain and strain rate hardening, which leads to a discontinuous jump in strain and temperature and breakdown of flow stresses. This adiabatic shear instability phenomenon results in an outwards viscous flow of material temperatures close to the melting point. Similar jet-like projections are also known in explosive welding of materials. [0048] Cold spray technology is used in a wide range of surfacing applications, manufacturing, repair and coating. They are particularly suitable for creating strong coatings in good electric contact with the substrate, even on metals with an impervious oxide barrier, like for example aluminium. If required, micro- or nano-particles of corundum, or a similar abrasive agent, can be added to the powder composition for surface cleaning.

[0049] Importantly, the cold spray coating technology can be used advantageously to coat a ferromagnetic layer on a conductor, for example a copper or aluminium conductor's surface, or on an electric power transmission cable, that has a desired combination of magnetic properties and Curie temperature. This can be obtained by choosing the composition of the nano-particles that are introduced in the spraying torch. Preferably, the composition of the nano-particles that will form the coating on the conductor is chosen to give a Curie temperature lower than 20 °C, more preferably lower than 10 °C and, optimally, close to 0 °C. In this manner, the magnetic coating will produce heat to actively prevent ice build-up at temperatures close to the freezing point or lower, but will switch off, and not introduce appreciable losses at higher temperatures. Several known materials can be used to realize magnetic coating with a Curie temperature close to this value. In this embodiment, the mains current at 50 or 60 Hz transmitted by the conductor will generate the required thermal power and keep the cable free from ice without external intervention or additional power supplies. [0050] According to an aspect of the present invention, the magnetic coating comprises one or several Thermomagnetic Alloys with Curie point values are between-40 and 200 °C, for example copper-nickel alloys (30- 40% Cu), iron-nickel (30%-38% Ni), and iron-nickel alloys (30-38%

Ni)further comprising Cr (up to 14%), Al (for example up to 1 .5% ), Mn (for example up to 2%).

[0051 ] Copper-nickel alloys have proved effective in temperatures ranging from -50 to +80. °C; Iron-nickel alloys, depending on the

composition, can be used either in a narrow (-20 to 35 °C) or in a wide (60 to 170 ° C) temperature range. Fe-Ni-Cr alloys have proved effective at temperatures ranging from -70 °C to +70 °C

[0052] Several alloys, many of them including nickel and another magnetc or non-magnetic metal can be used in the frame of the invention, for example Ni-Cu alloys. They allow Curie Tc temperatures as low as -20 °C with 40% of Cu and +50°C with 30% of Cu.

[0053] Other possible alloys that can be used in the frame of the invention are Ni-Mn alloys. Pure nickel exhibits a Curie point Tc = 627 K (+354 °C); an alloy comprising 75% Ni and 25% Mn shows a Curie point Tc = 250 K (- 23 ° C) while an alloys with 70% Ni and 30% Mn have T_c = 180 K (- 93 ° C).

[0054] Other possible materials that could usefully be used in the frame of the invention are Ni-Fe alloys. This system shows a maximum of

Tc = 610 °C at 68% Ni, and then falls. An alloy with 36% of nickel (known as invar) has Tc = 230 °C. With a further decrease of nickel content Tc drops even lower and reaches a minimum of Tc ~ 300 K around 28-29% content. Even lower contents of nickel content give higher values of Tc up to the pure iron value T_c = 1043 K (770 ° C).

[0055] Cobalt can also be used, alone, or in alloy with other

ferromagnetic elements like iron and/or nickel, in the frame of the invention. [0056] Ternary alloys, like Fe-Cr-Ni or higher alloys, for example Fe-Cr-Ni- Mn can also be used in the frame of the invention. The invention could also comprise the deposition of a coating comprising metallic and/or non- metallic components, for example combinations of iron, or another ferromagnetic substance, and an oxide of Zn, Sn, In; ferrites could also be employed.

[0057] The density of heat losses below the Curie point will be

determined not only by the magnetic permeability μ, but also by the magnetic hysteresis cycle, the area bounded by the hysteresis cycle determining, as it is known, the magnetic reversal losses.

[0058] Preferably, the magnetic coating of the invention is a metallic alloy realized by direct gas dynamic cold spraying of a mixture of metallic micro- or nano-particles having the appropriate combination of

compositions. In alternative, an alloy having the desired magnetic

properties can be prepared and pulverized to nano- or micro-particles suitable for gas-dynamic cold spraying. The deposition temperature can vary, in function of the chosen material, but favourable results have been obtained with deposition temperatures lower than 100 °C.

[0059] Magnetic properties of the magnetic coating and the heating of this coating by the alternative current depend strongly from the coating density and on presence of emptiness in this coating. A prepared mixture of metallic micro- or nano-particles contains two or more groups of powders with different particles sizes. Particles with a smaller size fill spaces between a bigger particles. This special prepared mixture increases the coating density and improves the magnetic properties. The bigger size particles are useful to increase of the particles kinetic energy and for better adhesion of sprayed coating.

[0060] In an alloy, the Curie temperature depends strongly, as we have seen above, from the relative concentrations of its constituents. The difference of components material densities gives a different particles velocities and a different kinetic energies that can give a not identical Curie temperature on coating thickness. A prepared mixture of metallic micro- or nano-particles contains bigger size particles of easier components and smaller size of heavier components that gives the identical weight of all particles. [0061] When necessary, the invention can also include a step of melting, annealing, or applying a suitable heat treatment to the magnetic coating after it has been deposited to the substrate.

[0062] The ferromagnetic layer of the invention can also be deposited by a technique of Atmospheric Plasma Spray gun for thermal spray or plasma torch. The inventors find that these techniques allow to depose layers with higher magnetic quality. Preferably, the plasma torches employed are of the non-transferred DC type, with electrodes internal to the body/housing of the torch itself.

[0063] The thickness of the ferromagnetic layer will be chosen in function of the thermal power that is required to effectively protect the line from ice, and preferably will be higher than 300 μηη.

References used in the figures

[0064] heated object/conductor

2 ferromagnetic coating

3 ferromagnetic conductor wound around the heated object, 4 AC source

5 through-capacitors,

6 wires line electrical capacity.

70 aircraft

39 ferromagnetic-clad wire

37 circuit

40 core

45 ferromagnetic cladding

Claims

1 . A system of ice build-up and/or ice removal on a surface, comprising an electrically conductive ferromagnetic layer or circuit on said surface, and an AC power source to generate an AC current or an AC magnetic field through the electrically conductive ferromagnetic layer or circuit.

2. The system of the preceding claim, wherein the AC power source has a frequency lower than 100 Hz, for example 50 Hz or 60 Hz, and the surface is a surface of a power-line conductor, and wherein the ferromagnetic layer has a Curie temperature lower than 20 °C, preferably lower than 10°C.

3. The system of claim 1 , comprising a circuit of a ferromagnetic electric conductor wound or applied to the surface of an airplane wing.

4. The system of claim 1, comprising a circuit of a ferromagnetic electric conductor wound or applied to glass windows, windshields, external mirrors or other surfaces of vehicles.

5. The system of any of claims from 3 to 4, wherein the AC power source generates current having a frequency in a range of about 0.3 kHz to 1 kHz, or from 1 .0 kHz to 100 kHz.

6. The system of any of claims from 1 to 2, further comprising a coupling circuit stop power-line low frequencies.

7. The system of any of the preceding claims, further comprising a means for frequency-tuning high-frequency AC current to balance ferromagnetic surface heating and skin-effect heating.

8. The system of any of the preceding claims, wherein the ferromagnetic layer is deposited by a physical technique,

9. The system of the previous claim, wherein the ferromagnetic layer contains Nickel and/or Co and/or metal oxides.

10. The system of any of the preceding claims, wherein said ferromagnetic layer is deposited by Atmospheric Plasma Spray gun for thermal spray or plasma torch.

1 1 . The system of any of the preceding claims, wherein said ferromagnetic layer is deposited by gas-dynamic cold spraying of micro- or nano-particles.

12. The system of the previous claim, wherein the distribution of the sizes of the micro- or nano-particles includes two or more groups of powders with different particles sizes.

13. The system of the previous claim wherein one group of powders with a first particle size comprises micro- or nano-particles having a first density, and a second group of powders with a second particle size smaller than said first particle size comprises micro- or nano-particles having a second density higher than said first density, such that individual particle mass of particles in the first and in the second group is is essentially the same.

14. The system of any of claims from 1 1 to 13, wherein the mixture micro- or nano-particles include an abrasive.

1 5. The system of any of the preceding claim, comprising a circuit of a ferromagnetic electric wires that have a ferromagnetic outer cladding.

16. The system of any of the preceding claims, wherein the thickness of said ferromagnetic layer or of said ferromagnetic cladding is higher than 300 μηη.