Process and apparatus for producing silicon carbide by solid-state carbothermal reduction process

Abstract

The present invention relates to a process and apparatus for the production of silicon carbide. The process (900) for the production of silicon carbide includes the steps of: producing a mixture (901) comprising at least carbon powder and silica powder, the weight percentage of the carbon powder being at least about 25% based on the total weight of the mixture; placing the mixture in a crucible (6) (902); forming a substantially inert atmosphere in an area at least partially surrounding the crucible (6) (903); heating the crucible (6) at a use temperature (T) of at least 1500° C. and lower than the melting temperature of the silica powder contained in the mixture. W a step of heating (904) the crucible (6) with a heating device for a first time interval (Δt1) until the temperature reaches a temperature range (R ) comprised between 1500° C. and the melting temperature of the silica powder contained in the mixture; w ), for a second time interval (Δt2); and after the second time interval (Δt2), reducing the temperature of the crucible (6) (906), preferably by deactivating the heating device; wherein the step of heating the crucible (6) by the heating device (904) includes heating the crucible at an average heating rate of at least 75° C. / min.

Description

[Technical field]

[0001] The present invention relates to the field of silicon carbide production. In particular, the present invention relates to a process and an apparatus for producing silicon carbide by a solid-state carbothermal reduction process. [Background technology]

[0002] Silicon carbide (SiC) is a semiconductor with chemical and physical properties that make it suitable for use in hard coating applications or in abrasive machining processes such as grinding, honing, water jet cutting, sandblasting, etc. Currently, silicon carbide is also widely used in the semiconductor industry, as its electrical properties different from those of silicon make it suitable for applications in high power, high frequency, and high temperature devices.

[0003] Silicon carbide occurs naturally as moissanite, an extremely rare mineral found in trace amounts. Because natural moissanite is so rare, most silicon carbide is synthetic. Currently, many processes are known for producing silicon carbide, each with different methodologies for carrying out the reactions necessary to form the compound, and different purity levels for the final product.

[0004] The method is based on heating a mixture of silicon dioxide (SiO2), e.g. in the form of silica or quartz sand, and carbon, e.g. powdered coke, in a furnace. A graphite rod is embedded in the mixture, and when an electric current is passed through the graphite rod, the mixture is heated to approximately 1600 °C to 2500 °C. As a result, the silicon dioxide surrounding the graphite rod melts. As a result of the carbothermal reaction, a layer of silicon carbide forms around the rod and carbon monoxide (CO) is released. The final compound of silicon carbide varies in purity depending on the distance from the graphite resistor, with impurities, especially nitrogen and aluminum, adversely affecting the electrical conductivity of the compound.

[0005] Silicon carbide can also be produced by chemical vapor deposition (CVD), a synthesis process that allows to obtain a deposit on a solid support starting from precursors introduced in gaseous form and decomposing at the substrate surface. The main advantage of using CVD processes lies in the homogeneity and purity of the compounds obtained, but on the other hand, these processes are very expensive and involve the use of complex equipment for precise process control, especially with regard to temperature control.

[0006] Thus, a need is felt for efficient processes and apparatus that enable the production of silicon carbide characterized by high purity. Summary of the Invention

[0007] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to overcome the drawbacks of the prior art.

[0008] In particular, it is an object of the present invention to provide a process and an apparatus for the production of silicon carbide which makes it possible to reduce the reaction times compared to the prior art, while keeping the energy consumption of the process low.

[0009] It is a further object of the present invention to provide a process for producing silicon carbide which has a lower environmental impact than prior art processes.

[0010] A further object of the invention is to provide a process and an apparatus for the production of silicon carbide which makes it possible to obtain a high purity material, i.e. a material having a purity higher than 99.95% by weight.

[0011] A further object of the present invention is to obtain an apparatus for silicon carbide production in which the process parameters can be easily controlled and adjusted during the process steps.

[0012] Finally, it is an object of the present invention to provide a process for producing silicon carbide that is generally less expensive than prior art processes.

[0013] These and other objects of the present invention are achieved by processes and apparatus incorporating the features of the appended claims forming an integral part of this specification.

[0014] According to a first aspect, the invention relates to a process for the production of silicon carbide, comprising the steps of producing a mixture comprising at least carbon powder and silica powder, the weight percentage of the carbon powder being at least about 25% relative to the total weight of the mixture, and placing the mixture in a crucible. Advantageously, the process comprises the step of forming a substantially inert atmosphere in an area at least partially surrounding the crucible. Furthermore, the process is carried out at a working temperature T , which is at least 1500 °C and is lower than the melting temperature of the silica powder contained in the mixture. W heating the crucible by means of a heating device for a first time interval Δt1 until a temperature range R is reached between 1500° C. and the melting temperature of the silica powder contained in the mixture; w and after the second time interval Δt2, reducing the temperature of the crucible, preferably by deactivating the heating device. In particular, heating the crucible with the heating device includes heating the crucible at an average heating rate of at least 75° C. / min.

[0015] The applicant has found that by carrying out the process in an inert atmosphere, heating the crucible to the temperature of use and maintaining the crucible within the temperature range of use, it is possible to complete the carbo-reduction process in the solid phase, based on the reaction of the silica in the mixture with carbon. Thus, the silica molecules (SiO2) react with the carbon to form, as an intermediate product, silicon monoxide (SiO) in the gas phase at the reaction temperature. The silicon monoxide subsequently reacts with the remaining carbon to form silicon carbide (SiC). All these reactions generate gases such as CO and / or CO2 as by-products.

[0016] Amorphous silica has been proven to be the silicon dioxide phase with the highest reactivity with graphite in the reduction formation of SiC carbides. Meanwhile, it is difficult to obtain high-purity amorphous silica at a competitive price, but high-purity α-quartz powder is readily available at a reasonable price. During the heating process, the starting quartz powder undergoes several phase transformations to produce different polymorphs. In particular, at 573 °C, the phase transformation from α-quartz to β-quartz occurs reversibly. Further increasing the temperature around 1300 °C causes the transformation of β-quartz to cristobalite. This transformation is not reversible, and the bonds between the atoms are broken and rearranged to form a more open structure, with a density of about 2.53 g / cm. 3 which increases the volume by about 17%. In addition, it has been proven that there exists an intermediate phase transition from β-quartz to cristobalite. First, the β-quartz phase transforms into an intermediate amorphous phase, and then the intermediate amorphous phase transforms into the cristobalite phase. Compared to quartz, amorphous silica is characterized by a high reactivity to react with graphite to form SiC. This is because its unstructured surface can result in reactive heterogeneous energy surface sites.

[0017] The applicant has found that the heating rate strongly influences the formation and relative amounts of the different polymorphs. In particular, it has been shown that the transformation of quartz to amorphous silica is faster than that of amorphous silica to cristobalite. This suggests that faster heating rates favor the formation of amorphous silica over cristobalite, whereas slower heating rates favor cristobalite. The applicant has surprisingly found that in order to obtain an almost complete conversion of silica for the production of silicon carbide, it is advantageous to provide a heat lamp, preferably heating the crucible at an average heating rate of at least 75° C. / min.

[0018] Use of such lamps has been shown to provide high yields of SiC without the presence of residual cristobalite in the resulting SiC powder.

[0019] At the melting point of 1713°C, the cristobalite phase begins to melt into a highly viscous liquid. Thus, in the process according to the invention, the melting point is never reached and remains at a value below 1600°C. In particular, the steps of heating the crucible to the temperature of use and keeping it within the temperature range of use convert all the beta-quartz into amorphous silica, a very fast reaction that is completed within a few minutes. After this step, the temperature reaches about 1500°C, activating the reaction of SiO2 with the surrounding carbon.

[0020] The process according to the invention therefore makes it possible to significantly reduce the reaction times compared to the state of the art, whilst keeping the energy consumption of the process low.

[0021] In a second aspect, the present invention relates to an apparatus for the production of silicon carbide, comprising a crucible configured to receive a mixture comprising at least carbon powder and silica powder, the weight percentage of the carbon powder being at least about 25% relative to the total weight of the mixture, and comprising an inert atmosphere forming means configured to form an inert atmosphere in an area at least partially surrounding the crucible. The apparatus further comprises a heating device configured to regulate the temperature of the crucible. The apparatus comprises a programmable control unit configured to: activate the inert atmosphere forming means to form an inert atmosphere in an area around the crucible, activate the heating device to heat the crucible for a first time interval with an average heating rate of at least 75°C / min until a use temperature is reached that is at least 1500°C and is lower than the melting temperature of the silica powder contained in the mixture, maintain the crucible within the use temperature range for a second time interval; and reduce the temperature of the crucible after the second time interval, preferably by deactivating the heating device.

[0022] Advantageously, the device for producing silicon carbide thus constructed provides the same advantages as those explained with reference to the process for producing silicon carbide according to the invention. The present invention may have at least one of the following preferred features, which may be combined with each other as desired to meet specific application needs.

[0023] Preferably, the steps of heating the crucible and maintaining the crucible at the operating temperature include heating at least a surface of the crucible. This preferred configuration avoids contamination of the material contained in the crucible by radiation, for example carried out by a heating element (for example a resistive element) arranged within the crucible.

[0024] Preferably, the step of heating the crucible with a heating device comprises heating the crucible at an average heating rate of at least 95° C. / min.

[0025] More preferably, the step of heating the crucible with a heating device comprises heating the crucible at an average heating rate of about 100° C. / min.

[0026] Preferably, the periphery of the crucible is defined around a hot zone provided within the airtight vessel.

[0027] Preferably, the crucible has an elongated shape extending along an axis of elongation, and the crucible is preferably arranged in a region at least partially surrounding the crucible such that the axis of elongation is substantially aligned with the vertical direction.

[0028] Preferably, the shape of the crucible and its location within the hot zone are configured so that the temperature difference within the crucible volume is less than 50°C.

[0029] In this way it is advantageously achieved that the carbo-reduction reaction occurs efficiently and substantially throughout the entire internal volume defined by the crucible.

[0030] Preferably, the internal pressure of the airtight vessel is controlled so as to be substantially maintained at a predetermined pressure value at least during the step of heating the crucible until it reaches the operating temperature and during the step of maintaining the crucible within the operating temperature range.

[0031] Preferably, the airtight container includes a first outlet port, and the step of controlling the internal pressure of the airtight container includes controlling the first outlet port to discharge gas when the internal pressure reaches a predetermined pressure value.

[0032] This makes it possible in a simple and economical way to ensure that the pressure remains below a given limit value while the temperature is increased.

[0033] Preferably, the process includes an additional step of forming an inert atmosphere in an area at least partially surrounding the crucible, performed after the step of reducing the temperature of the crucible after the second time interval.

[0034] This advantageously allows impurities in gaseous state to be removed from the working area by exhausting them via the first outlet port in order to further increase the purity of the silicon carbide obtained by the process, thereby obtaining a purity of greater than 99.95% by weight.

[0035] Preferably, the weight percentage of the carbon powder is at least about 25% by weight, based on the total weight of the mixture, and is less than about 55% by weight, based on the total weight of the mixture.

[0036] These carbon powder values ​​advantageously make it possible to reach a high purity of the final product whilst ensuring the efficiency of the process, particularly in terms of yield.

[0037] Preferably, the mixture comprises at least about 45% by weight, and preferably less than about 75% by weight, of silica powder based on the total weight of the mixture.

[0038] In a preferred embodiment of the present invention, the average size of the carbon granules contained in the premix is ​​comprised between about 5 μm and about 5 mm; more preferably between about 25 μm and about 3 mm, and even more preferably between about 50 μm and about 2 mm.

[0039] Advantageously, these values ​​ensure efficiency and reliability of the silicon carbide manufacturing process.

[0040] In a more preferred embodiment of the present invention, the silica granules contained in the premix have an average size of less than about 300 μm.

[0041] Advantageously, these values ​​are selected to keep the silica in the solid phase during the silicon carbide manufacturing process.

[0042] The above mentioned particle sizes advantageously allow the process to be carried out with an optimum yield, ie at least greater than 80%, preferably greater than 85%, more preferably greater than 90%.

[0043] In a preferred embodiment, the silica powder is selected from the group consisting of: quartz sand, Pure quartz, Agriculturally derived silica powder, Cristobalite, or Mixtures thereof, is selected from.

[0044] The use of this particular silica powder advantageously saves energy and therefore costs for carb-reductive activation compared to that required when using pure silica.

[0045] In a preferred embodiment of the invention, the process includes detecting the amount of CO and / or CO2 in the airtight container.

[0046] The concentrations of these gases change during the carbohydrate reduction process, and therefore the programmable control unit can be configured to analyze the detected values, monitor the progress of this reaction, and possibly adjust the heating device accordingly.

[0047] In a preferred embodiment of the invention, the process includes detecting the pressure within the airtight container.

[0048] In a preferred embodiment of the invention, the process includes the step of detecting the temperature within the airtight container.

[0049] This advantageous method makes it possible to obtain further information regarding the correct development of the reactions occurring during the manufacturing process and to adjust the heating device accordingly.

[0050] Preferably, the internal pressure of the airtight container is controlled by a programmable control unit.

[0051] Preferably, the programmable control unit is configured to operate the heating device to heat the crucible at an average heating rate of at least 75° C. / min, more preferably about 100° C. / min.

[0052] In a preferred embodiment of the present invention, the step of forming an inert atmosphere is carried out by introducing a flow of inert gas into the airtight container through the first inlet port.

[0053] Preferably, the inert gas is argon.

[0054] This advantageously makes it possible to achieve an appropriate reduction in oxygen within the airtight container in a simple and economical manner, while preventing deterioration of the container, for example due to oxidation phenomena.

[0055] In another embodiment, the step of forming an inert atmosphere is performed by sucking oxygen out of the airtight container using a vacuum pump.

[0056] In a preferred embodiment of the invention, the second time interval Δt2 is comprised between 5 minutes and 90 minutes.

[0057] Preferably, the crucible has a maximum transverse dimension D of about 350 mm or less. MAX has.

[0058] In this specification and the appended claims, the term "maximum transverse dimension D MAX " means the diameter of the equivalent cylindrical cross section containing the crucible.

[0059] Advantageously, a substantially uniform temperature is obtained inside the crucible, thereby achieving suitable conditions to ensure the efficiency of the reactions carried out in the manufacturing process steps.

[0060] In a preferred embodiment of the present invention, the apparatus includes a first sensor arranged and configured to detect the amount of CO and / or CO2 within the airtight container.

[0061] In a preferred embodiment of the invention, the apparatus includes a second sensor positioned and configured to detect pressure within the airtight container.

[0062] In a preferred embodiment of the invention, the apparatus includes a third sensor positioned and configured to detect a temperature within the airtight enclosure.

[0063] This advantageously makes it possible to find the most appropriate parameter values ​​for properly assessing the progress of the reactions occurring during the manufacturing process.

[0064] Preferably, the programmable control unit is operatively associated with at least one among a first sensor arranged and configured to detect the amount of CO and / or CO2 in the airtight container, a second sensor arranged and configured to detect the pressure in the airtight container, and a third sensor arranged and configured to detect the temperature in the airtight container, and the programmable control is configured to control at least the heating device based on a value detected by at least one of the sensors.

[0065] In a preferred embodiment of the invention, the hot zone has an elongated shape extending along a vertical direction and is defined within a hot zone cover that includes at least one layer of refractory material.

[0066] Preferably, the crucible includes one or more sidewalls defining an exterior surface of the crucible, and the layer of refractory material is configured to at least partially contact the exterior surface of the crucible.

[0067] Preferably, the hot zone cover includes a mantle element exterior to at least one layer of refractory material.

[0068] In an alternative embodiment, the hot zone cover includes a single insulating layer, such as a layer of carbon or graphite felt.

[0069] The use of refractories advantageously allows for shielding of furnace components outside the hot zone, while at the same time reducing heat dispersion which increases heating efficiency.

[0070] Preferably, the heating device includes at least one heating element disposed within the airtight enclosure so as to contact an outer surface of the hot zone cover.

[0071] This preferred configuration advantageously achieves fairly uniform heating of the crucible.

[0072] More preferably, the heating device includes an inductor wrapped around the hot zone cover along substantially its entire extension.

[0073] Advantageously, the use of an inductor allows for the dynamic and rapid generation of the appropriate thermal gradient required to obtain a desired temperature in a relatively short period of time.

[0074] In a preferred embodiment of the invention, the inert atmosphere generating means is an oxygen removal device including an inert gas supply connected to at least a first inlet port of the airtight container and a relief valve connected to the first outlet port, the inert gas supply and the relief valve optionally being operatively connected to a programmable control unit.

[0075] In an alternative embodiment of the invention, the inert atmosphere generating means is an oxygen remover and includes a vacuum generating device connected to the first outlet of the airtight container and operably connected to an optionally programmable control unit.

[0076] In a preferred embodiment of the invention, the airtight container comprises a closed outer wall forming a first jacket and a closed inner wall forming a second jacket, the outer wall being positioned relative to the inner wall to form a gap between the outer and inner walls, the gap being configured to receive a cooling fluid flowing from a second inlet portion to a second outlet portion, both of which are obtained on the outer wall.

[0077] Preferably, the airtight container has an elongated shape extending along a vertical direction, and the gap extends along substantially the entire length of the airtight container.

[0078] This allows for more reliable temperature control of the container by providing suitable cooling of the container in a simple and inexpensive manner.

[0079] In a further preferred embodiment of the invention, the crucible comprises a bottom wall which together with one or more side walls defines an interior volume in which at least one separating element is disposed, the separating element extending substantially parallel to the bottom wall.

[0080] Advantageously, the use of a separating element makes it possible to create turbulence in the interior atmosphere of the crucible in order to control the outgassing phenomenon.

[0081] Further features and advantages of the present invention will be made apparent from the following detailed description of several preferred embodiments thereof, as illustrated in the accompanying drawings. [Brief description of the drawings]

[0082] The present invention will now be described with reference to some illustrative and non-limiting embodiments thereof as illustrated in the accompanying drawings which relate to different aspects of the invention.

[0083] [Figure 1a] FIG. 1 is a schematic diagram of an apparatus according to a first and second embodiment of the present invention. [Figure 1b] FIG. 1 is a schematic diagram of an apparatus according to a first and second embodiment of the present invention. [Figure 2a]1 shows a partial cross-sectional view of a detail of the device of FIG. 1a or 1b according to an alternative embodiment. [Figure 2b] 1 shows a partial cross-sectional view of a detail of the device of FIG. 1a or 1b according to an alternative embodiment. [Figure 2c] 1 shows a partial cross-sectional view of a detail of the device of FIG. 1a or 1b according to an alternative embodiment. [Figure 3a] 2 shows diagrammatically details of an apparatus according to an alternative embodiment of the present invention; [Figure 3b] 2 shows diagrammatically details of an apparatus according to an alternative embodiment of the present invention; [Figure 3c] 2 shows diagrammatically details of an apparatus according to an alternative embodiment of the present invention; [Figure 3d] 2 shows diagrammatically details of an apparatus according to an alternative embodiment of the present invention; [Figure 3e] 2 shows diagrammatically details of an apparatus according to an alternative embodiment of the present invention; [Figure 4] FIG. 1 is a flow diagram of a process for producing silicon carbide according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0084] While the present invention can be embodied in several alternative ways, certain preferred embodiments are shown in the drawings and will be described in detail below, It should be understood, however, that there is no intention to limit the invention to the particular disclosed embodiments, but on the contrary, the invention is intended to cover all modifications, alternative constructions and equivalents falling within the scope of the present invention as defined by the appended claims.

[0085] The use of "for example," "e.g.," and "or" indicates non-exclusive options without limitation, unless otherwise stated. The use of "including" means "including, but not limited to," unless otherwise stated.

[0086] In this specification and in the claims that follow, the term "area" is intended to indicate a portion of the space optionally defined within a container, e.g., a process chamber, which may be partially or totally closed off from the outside environment.

[0087] FIG. 1 a shows a part of an apparatus 100 according to an embodiment of the invention, which comprises a crucible 6 arranged in an area provided within and at least partially surrounding an airtight enclosure 1 .

[0088] In a further preferred embodiment of the invention (not shown), the crucible 6 can be placed in a tunnel furnace where it can be moved towards different zones having selected temperatures to obtain the desired thermal conditions for the hydrocarbon reduction reaction. Alternatively, the crucible can be considered as a tube in a rotary tube furnace, and the reagents (silica and carbon) are moved towards the heated tube section where the hydrocarbon thermal reaction takes place by adjusting the inclination and rotation speed of the tube.

[0089] According to both embodiments of Fig. 1a and 1b, the airtight container 1 may be formed by one or more walls configured to form a thermal barrier and configured to form several superimposed encapsulation layers, preferably made of metallic material. Preferably, the airtight container 1 comprises a closed outer wall 11 in which a first outlet section 14 and a first inlet section 15 are formed, the first inlet section 15 and the first outlet section 14 being configured to be able to supply a fluid to the interior of the airtight container 1 and to discharge a fluid therefrom. Preferably, the supply and discharge are performed in a controlled manner by suitable devices (not shown) connected to the first inlet section 15 and the first outlet section 14, respectively.

[0090] The airtight container 1 of Fig. 1 consists of a closed outer wall 11 forming a first jacket and a closed inner wall 12 forming a second jacket. In particular, the outer wall 11 is arranged relative to the inner wall 12 so as to form a gap 13 between the outer wall 11 and the inner wall 12. The gap 13 extends substantially along the entire extension of the airtight container 1. Preferably, the gap 13 is configured to receive a cooling fluid flowing between a second inlet 16 and a second outlet 17 provided in respective parts of the outer wall 11.

[0091] The airtight container 1 of Figures 1a and 1b defines a hot zone 20 therein configured to accommodate at least one crucible 6 having an elongated shape. When accommodated within the hot zone 20, the crucible 6 exhibits a shape that extends primarily along a vertically aligned extension axis A. The crucible 6 includes one or more side walls 61 that define a lateral inner surface 62 and a lateral outer surface 63. The crucible 6 further includes a bottom wall 64 that defines a bottom inner surface 64' and a bottom outer surface 64'', which together with the one or more side walls 61 define an interior volume configured to accommodate a volume of material to be processed.

[0092] The crucible 6 has a top opening 65 that allows access to the interior volume of the crucible and the introduction of the material to be processed. The top opening 65 is in fluid communication with the interior volume of the airtight vessel 1. The crucible optionally includes a cover element 66 that selectively closes the top opening 65.

[0093] The crucible 6 can be obtained according to various shapes and sizes, typically having a cylindrical shape as shown in Figures 2a-c. In alternative exemplary embodiments, the crucible 6 has the shape of a double cone (as shown in Figures 3a and 3b) or an inverted simple cone (as shown in Figures 3c and 3d), or is formed by a combination of a conical section and a cylindrical section (as shown in Figure 3e).

[0094] According to the embodiment of Fig. 2a, the hot zone 20 is defined in a hot zone cover comprising at least one layer 7 of refractory material. Preferably, the crucible 6 is formed so as to leave substantially no free space between the crucible lateral outer surface 63 and the hot zone cover. The hot zone cover of Fig. 2a further comprises a mantle element 8 extending longitudinally in the axial direction A outside the at least one layer 7 of refractory material. The layer 7 of refractory material is preferably made of zirconia sand, or zirconia wool, or alumina, and the mantle element 8 is advantageously made of a material that is substantially unchanged at high temperatures, such as quartz, zirconia, or derivatives thereof.

[0095] In a simplified embodiment (as shown in Figures 2b and 2c), the mantle element 8 and the refractory 7 can be replaced by a single element of insulating material such as a carbon or graphite felt layer 81. This solution also allows for a reduction in the time of the process, since it allows for very fast heating without damaging / cracking the refractory.

[0096] The shape and dimensions of the crucible 6 are advantageously selected so that the temperature difference within the volume of the crucible is no more than 50° C. with respect to a plane perpendicular to the axis of elongation.

[0097] Preferably, the maximum transverse dimension D of the crucible 6 MAX is about 400 mm or less, more preferably about 350 mm or less.

[0098] The apparatus 100 further includes a heating device, including a power supply 31 and at least one heating element 32 , disposed within the airtight enclosure 1 and configured to regulate the temperature of the hot zone 20 .

[0099] According to the embodiment of Figures 2a-2c, the power supply 31 comprises a high frequency generator and the heating element 32 comprises an inductor wound around the hot zone cover, preferably along substantially its entire vertical extension.

[0100] The inductor may be made of copper or a metal alloy containing copper and is preferably in the shape of a spiral disposed around the hot zone cover. The inductor generates an electromagnetic field that is applied to the hot zone cover, thereby generating heat at the crucible surfaces 62, 63, 64', 64''.

[0101] The inductor is advantageously selected based on the shape of the crucible and / or the particular process to be performed through the apparatus 100; by way of example, it is possible to configure the spiral according to a cylindrical, rectangular, elliptical or square shape, and / or to provide a spiral of constant or variable pitch.

[0102] The device 100 of Fig. 1a further comprises several sensors arranged inside the airtight container 1 and suitable for measuring relevant parameters useful for the management of the process carried out, such as temperature, pressure or the amount of some elements (e.g. CO and / or CO2). More specifically, the device 100 of Fig. 1a comprises a first sensor 41 arranged and configured to detect the amount of CO and / or CO2 in the airtight container 1, a second sensor 42 arranged and configured to detect the pressure in the airtight container 1 and a third sensor 43 arranged and configured to detect the temperature in the airtight container 1. As an example, the amount of CO is an indicator of the reaction development. Therefore, based on the signal generated by the first sensor 41, the switching off of the device 100 can be controlled.

[0103] The apparatus 100 of Fig. 1a further comprises a programmable control unit 500 operatively connected to at least one of the first sensor 41, the second sensor 42 and the third sensor 43 and configured to control at least the heating device based on values ​​detected by at least one of the sensors, thereby advantageously allowing to automatically regulate the temperature of the material contained in the crucible 6 based on measurements performed by the sensors 41, 42, 43.

[0104] The apparatus 100 further comprises an inert atmosphere forming means configured to form an inert atmosphere in an area at least partially surrounding the crucible 6. According to the embodiment of Fig. 1a and 1b, the inert atmosphere forming means is constituted by an oxygen remover configured to evacuate oxygen from the gas-tight vessel 1. According to the embodiment of Fig. 1a, the oxygen remover comprises an inert gas supply 150 connected to the first inlet section 15 and a relief valve 90 connected to the first outlet section 14. The inert gas supply 150 and the relief valve 90 are preferably operated via a programmable control unit 500.

[0105] In an alternative embodiment shown in FIG. 1 b , the oxygen remover comprises a vacuum generator 9 connected to the first outlet 14 and preferably operated by a programmable control unit 500 .

[0106] A preferred embodiment of a process 900 for producing silicon carbide according to the present invention is described below.

[0107] First, a mixture is prepared that contains at least carbon powder and silica powder (step 901). The silica powder used in the mixture is preferably quartz sand, pure quartz, agriculturally derived silica powder, or silica powder from other silica phases such as cristobalite, or a mixture thereof. The carbon powder is preferably obtained from recyclable graphite or agriculturally derived carbon.

[0108] If desired, the above-mentioned materials are first crushed, preferably through an impact crusher, to obtain high quality granules. If desired, the granules are then dried in air, preferably at a temperature between 150° C. and 200° C., to remove residual moisture.

[0109] The powders thus obtained are then mixed according to a ratio that preferably comprises carbon powder in a proportion of at least about 25% by weight relative to the total weight of the mixture, and more preferably less than about 55% by weight relative to the total weight of the mixture, thus resulting in a mixture that comprises silica powder in a proportion of at least about 45% by weight relative to the total weight of the mixture, and preferably less than about 75% by weight.

[0110] According to an alternative embodiment, the mixture further comprises silicon carbide powder, which preferably originates from a previous industrial process. The use of silicon carbide in the initial mixture advantageously allows to considerably limit the cost of materials and to reduce the impact of the process on the surroundings, by using partly recycled materials.

[0111] The particle size of the powders used to make the initial mixture is specifically selected to achieve a particular result in the silicon carbide manufacturing process.

[0112] In particular, the average particle size of the carbon granules contained in the mixture is preferably selected to be between about 5 μm and about 5 mm, more preferably between about 25 μm and about 3 mm, and even more preferably between about 50 μm and about 2 mm.

[0113] The average size of the silica granules contained in the mixture is preferably less than about 300 μm, more preferably less than about 150 μm.

[0114] Alternatively, the silica and carbon powders can be mixed according to the weight ratio and particle size defined above, and then pressed to obtain granules or pellets with a diameter of 5 mm to 10 mm.

[0115] Furthermore, graphite is preferably used as a "matrix" (i.e., a kind of "seed" function for growing granules during the process) in order to properly define the particle size of the final product within the range of 5 μm to 5 mm.

[0116] Once the mixture is prepared, it is placed in a crucible 6 in the hot zone 20 of the airtight container 1 (step 902). The internal volume of the crucible 6 is preferably filled with the mixture starting from the bottom inner surface 64' of the bottom wall 64 until it reaches a maximum level equal to about 80% of its height, i.e., its dimension along its extension axis. More preferably, the internal volume of the crucible 6 is filled to a level equal to about 70% of its height.

[0117] Once the material is placed in the crucible, a substantially inert atmosphere is formed in the area at least partially surrounding the crucible 6 (step 903). Preferably, creating the substantially inert atmosphere is accomplished by evacuating oxygen from the airtight vessel 1 via the first outlet 14. This advantageously removes any residual oxygen in the chamber that may combine with carbon and reduce the amount available for reaction with the silica.

[0118] In a preferred embodiment, the removal of oxygen from the airtight container 1 is performed by introducing an inert gas, for example argon, into the airtight container 1 through the first inlet 15. Preferably, the inert gas is introduced into the airtight container 1 by a gas supply device 150.

[0119] In another preferred embodiment, the step of removing oxygen 903 is performed by a vacuum generator 9 connected to the first outlet 14 and operatively connected to a programmable control unit 500 .

[0120] Once an inert atmosphere is obtained in the airtight container 1, the process is started by activating the heating device to a working temperature T of at least 1500° C. and below the melting temperature of the silica powder contained in the mixture. W The crucible 6 is heated for a first time interval Δt1 until the melting temperature of the silica varies depending on its physical state (step 904). As is known, the melting temperature of silica varies depending on its physical state, for example, the melting temperature of amorphous silica is about 1630°C, and the melting temperature of cristobalite is about 1713°C.

[0121] According to the invention, the step of heating the crucible with a heating device comprises heating the crucible at an average heating rate of at least 75° C. / min, preferably less than 400° C. / min. In a preferred embodiment, the step of heating the crucible is carried out at an average heating rate of at least 95° C. / min, or more preferably at a heating rate of about 100° C. / min.

[0122] In a preferred embodiment of the present invention, the use temperature T W Heat the crucible until it reaches the operating temperature range R W The step of maintaining the interior of the crucible 6 includes heating the surfaces 62 , 63 , 64 ′, 64 ″ of the crucible 6 .

[0123] The process further comprises the steps of: during a second time interval Δt2, a use temperature range R comprised between 1500° C. and the melting temperature of the silica powder comprised in the mixture; W The method includes maintaining the crucible 6 within the heating chamber (step 905) and reducing the temperature of the crucible 6, preferably by deactivating the heating device after a second time interval Δt2 has elapsed (step 906).

[0124] In a preferred embodiment of the invention, the second time interval Δt2 is comprised between 5 and 90 minutes, depending on the amounts of reagents.

[0125] Preferably, at least the operating temperature T w The process of heating the crucible until it reaches the operating temperature range R w There is further provided a step of controlling the internal pressure of the airtight vessel 1 by gas exhausted through at least the first outlet 14 during the step of maintaining the crucible 6 therein (step 910).

[0126] Preferably, the step of controlling the internal pressure of the airtight container includes controlling the first outlet port 14 to discharge the gas when the internal pressure reaches a predetermined pressure value.

[0127] More specifically, the internal pressure of the airtight container 1 varies depending on the steps performed during the manufacturing process and can be maintained at a predetermined substantially constant value, for example, via a relief valve 90 arranged in communication with the first outlet portion 14.

[0128] According to a preferred embodiment, the step 910 of controlling the internal pressure of the airtight container 1 can be performed by the programmable control unit 500 by controlling the relief valve 90 based on the pressure value detected by the second sensor 42.

[0129] Preferably, the process of the invention further comprises the additional step of creating an inert atmosphere in a region at least partially surrounding the crucible 6, which additional step is carried out after the step of decreasing the temperature of the crucible 6 after the second time interval Δt2. Preferably, the additional step of creating an inert atmosphere is carried out at least until the crucible 6 reaches a temperature of about 600° C., more preferably about 400° C.

[0130] In a preferred embodiment, the process includes detecting the amount of CO and / or CO2 in the airtight container by a first sensor 41 operatively associated with a programmable control unit 500.

[0131] In a further preferred embodiment, the process includes detecting the temperature within the airtight container 1 by a third sensor 43 operatively associated with the programmable control unit 500 .

[0132] The invention thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the inventive concept characterizing it.

[0133] For example, the crucible 6 further comprises at least one separating element 67 (as shown in Figures 3a, 3b, 3c, 3d and 3e) extending substantially parallel to the bottom wall 64 of the crucible. Preferably, the separating element 67 has a partially open structure (for example made by a honeycomb structure or a lattice) and is configured to allow fluid communication between adjacent parts of the crucible and to generate turbulence in the internal atmosphere of the crucible 6 in order to avoid outgassing phenomena.

[0134] In another embodiment (not shown), the heating element 32 comprises at least a resistor, preferably connected directly to the crucible 6, and in a further possible embodiment, the heating element 32 comprises at least a resistor connected to the airtight vessel 1. According to both above-mentioned embodiments, the hot zone 20 is defined by the area contained within the perimeter determined by the elements connected to the heating resistor. More specifically, if the resistor is connected to the wall of the airtight vessel 1, the hot zone is defined by the gap surrounded by the airtight vessel.

[0135] Other technically equivalent details and materials may be used, and the shapes, dimensions and distances of the various components are arbitrary according to requirements.

Claims

1. 1. A process (900) for producing silicon carbide, comprising: preparing a mixture comprising at least carbon powder and silica powder, wherein the weight percentage of the carbon powder relative to the total weight of the mixture is at least about 25% (901); placing the mixture in a crucible (6) (902); forming (903) a substantially inert atmosphere in an area at least partially surrounding the crucible (6); A use temperature (T) of at least 1500° C. and lower than the melting temperature of the silica powder contained in the mixture Ｗ ) until the first time interval (Δt １ ) heating the crucible (6) by a heating device (904); The second time interval (Δt ２ ) the crucible (6) is heated in a temperature range (R ) comprised between 1500°C and the melting temperature of the silica powder contained in the mixture. ｗ ) (905); and The second time interval (Δt ２ ) followed by a step (906) of reducing the temperature of the crucible (6), preferably by deactivating the heating device; The process (900), wherein the step (904) of heating the crucible (6) with a heating device comprises heating the crucible at an average heating rate of at least 75°C / min.

2. 10. The process (900) of claim 1, wherein the step (904) of heating the crucible (6) with a heating device comprises heating the crucible at an average heating rate of at least 95°C / min, preferably about 100°C / min.

3. Operating temperature (T Ｗ ) and heating the crucible (6) until it reaches a temperature within the operating range (R Ｗ 3. The process (900) of claim 1 or 2, wherein the step (905) of maintaining the crucible (6) in the crucible (6) comprises heating at least a surface (62, 63, 64', 64'') of the crucible (6).

4. The average size of the carbon granules contained in the mixture is between about 5 μm and about 5 mm; and the average size of the silica granules contained in the mixture is less than about 300 μm; The process (900) of claim 1 further comprising at least one of:

5. 2. The process (900) of claim 1, wherein the area around the crucible (6) is defined around a hot zone (20) provided within the airtight vessel (1).

6. The internal pressure of the airtight container (1) is maintained at least at the operating temperature (T Ｗ ) and heating the crucible (904) until it reaches a working temperature range (R Ｗ 6. The process (900) of claim 5, wherein the pressure is controlled (910) to be substantially maintained at a predetermined pressure value during the step of maintaining (905) the crucible within the pressure range.

7. 7. The process (900) of claim 6, wherein the airtight container (1) includes a first outlet (14), and the step (910) of controlling the internal pressure of the airtight container (1) includes controlling the first outlet (14) to discharge the gas when the internal pressure reaches a predetermined pressure value.

8. CO and / or CO in the airtight container (1) ２ detecting the amount of A step of detecting the pressure inside the airtight container (1); and The process (900) according to any one of claims 5 to 7, further comprising at least one of the steps of: detecting the temperature inside the airtight container (1).

9. The process (900) according to any one of claims 5 to 7, wherein the step (903) of forming an inert atmosphere is carried out by sucking oxygen from the airtight container (1) by means of a vacuum pump and / or by introducing a flow of inert gas into the airtight container (1) via the first inlet (15).

10. An apparatus (100) for producing silicon carbide, comprising: a crucible (6) configured to receive a mixture including at least carbon powder and silica powder, the weight percentage of the carbon powder being at least about 25% of the total weight of the mixture; an inert atmosphere forming means configured to form an inert atmosphere in an area at least partially surrounding the crucible (6); a heating device configured to regulate the temperature of the crucible (6); and A programmable control unit (500) configured as follows: activating an inert atmosphere generating means to form an inert atmosphere in the area surrounding the crucible (6); The heating device is operated to heat the mixture to a working temperature (T) of at least 1500° C., below the melting temperature of the silica powder contained in the mixture, at an average heating rate of at least 75° C. / min. Ｗ ) until the first time interval (Δt １ ) the crucible (6) is heated to a temperature range (R ) comprised between 1500°C and the melting temperature of the silica powder contained in the mixture. ｗ ) within a second time interval (Δt ２ ) maintaining the crucible (6) during The second time interval (Δt ２ ) followed by reducing the temperature of the crucible (6), preferably by deactivating (906) the heating device; An apparatus (100) comprising:

11. 11. The apparatus (100) of claim 10, wherein the programmable control unit (500) is configured to operate the heating device to heat the crucible (6) at an average heating rate of at least 95°C / min, preferably about 100°C / min.

12. The apparatus (100) according to claim 10 or 11, wherein the area partially surrounding the crucible is an area partially surrounding a hot zone (20) provided inside the airtight container (1), and the hot zone (20) is preferably defined inside a hot zone cover (7; 8) including at least one layer of refractory material (7) and optionally including a mantle element (8) outside the at least one layer of refractory material (7).

13. the heating device comprises at least one heating element (32) arranged in the airtight container (1) so as to contact the lateral outer surface of the hot zone cover (7; 8); and / or 13. Apparatus (100) according to claim 12, wherein the heating device comprises at least one inductor wound around the hot zone cover (7; 8).

14. 12. Apparatus (100) according to claim 10 or 11, wherein the shape of the crucible (6) and its position within the hot zone (20) are configured to obtain a temperature difference of less than 50°C inside the volume of the crucible (6).

15. The crucible (6) has a maximum transverse dimension (D ＭＡＸ 12. The device (100) according to claim 10 or 11, comprising:

Images